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Cyanide and Removal Options from Effluents in Gold Mining and Metallurgical Processes

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... Different conventional physicochemical processes such as alkaline chlorination, adsorption with activated carbon, Degussa process, biological treatments, acidification-recovery, and sulfur dioxide injection have been used to treat gold mining wastewater contaminated with cyanocomplexes and simple cyanide-based compounds [16,17]. However, most of those techniques are not able to degrade cyanide substances efficiently (such as [Fe(CN) 6 ] 3-). ...
... UV/S 2 O 8 2fulfills the requirements for mining wastewater treatment since the pH of mining tailings usually is above 10 to preclude the conversion of CNinto HCN (Eq. (2)) [17]. Elevated pH conditions is convenient for the UV/S 2 O 8 2process since at alkaline pH, PS is decomposed by hydroxyl anions to form sulfate radicals and superoxide radical anion (O 2 •-) (Eq. ...
... K 3 [Fe(CN) 6 ] was used to prepare 1 L of 50 mg L − 1 [Fe(CN) 6 ] 3as the contaminant solution, which stoichiometrically corresponds to 36.58 mg L − 1 of CN -. This concentration was fixed in all the tests, and it was obtained by dissolving 79.25 mg of K 3 [Fe (CN) 6 ] in deionized water type II at pH 11 or 13, previously adjusted with 10 M NaOH to avoid the volatilization of free cyanide [17,34,35]. The contaminant concentration chosen is above the [Fe(CN) 6 ] 3concentration in some mining tailings [35]. ...
Article
The ultraviolet light activation of persulfate (PS) under alkaline conditions was evaluated for treating Hexacyanoferrate (III) ion ([Fe(CN)6]³⁻). The effect of the wavelength type (i.e., UVA and UVC), initial PS concentration (0.3, 0.6, 0.9 g L⁻¹), and pH value (11 and 13) on the degradation of 50 mg L⁻¹ of [Fe(CN)6]³⁻ were studied. Finally, the role of the main degrading agents (SO4•-, O2•-, or HO•) involved in the degradation process was determined using scavengers and a degradation pathway of [Fe(CN)6]³⁻ was proposed. Results show that [Fe(CN)6]³⁻ can be decomposed by UVC light, while UVA is not effective neither in [Fe(CN)6]³⁻ photolysis nor in the activation of PS. Alkaline activation alone at pH 13 can also not degrade the cyanocomplex. However, the combination of UVC with PS (0.3 g L⁻¹) at pH 13 showed high efficiency in the elimination of [Fe(CN)6]³⁻, achieving 93.3% of removal after 125 min of treatment. The highest CN⁻ release and Fe dissolved removal also occurs at pH 13 and 0.3 g L⁻¹ PS. Further increases in initial PS concentrations may lead to an excess of radicals in solution, resulting in detrimental recombination reactions that affect the efficiency of the process. Quenching tests showed that the importance of radicals involved in the degradation of [Fe(CN)6]³⁻ follows the order: HO• > O2•- >>> ¹O2 or SO4•-, and allowed to demonstrate that singlet oxygen could participate in the UVC photolysis of [Fe(CN)6]³⁻. All these results suggest the feasibility of this technology to treat this type of industrial wastewater efficiently.
... For example, it is used to leach gold and silver from their ores. These processes generate significant amounts of cyanide waste [1,2]. Cyanide wastewaters also come from, e.g., electroplating, jewelry industry, (petro)chemical industry and cokemaking [1,[3][4][5][6][7]. ...
... These processes generate significant amounts of cyanide waste [1,2]. Cyanide wastewaters also come from, e.g., electroplating, jewelry industry, (petro)chemical industry and cokemaking [1,[3][4][5][6][7]. ...
... Cyanide occurs as free cyanide (fCN), i.e., CN − and HCN, or as metal complexes. fCN is the most toxic; at pH ≤8.5, it occurs predominantly as volatile HCN, which is very hazardous [1]. The above industrial effluents also contain other chemicals such as sulfide, thiocyanate, ammonia, phenols or metals [4][5][6][7][8][9], which can complicate their treatment. ...
Article
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Industries such as mining, cokemaking, (petro)chemical and electroplating produce effluents that contain free cyanide (fCN = HCN + CN−). Currently, fCN is mainly removed by (physico)chemical methods or by biotreatment with activated sludge. Cyanide hydratases (CynHs) (EC 4.2.1.66), which convert fCN to the much less toxic formamide, have been considered for a mild approach to wastewater decyanation. However, few data are available to evaluate the application potential of CynHs. In this study, we used a new CynH from Exidia glandulosa (protein KZV92691.1 designated NitEg by us), which was overproduced in Escherichia coli. The purified NitEg was highly active for fCN with 784 U/mg protein, kcat 927/s and kcat/KM 42/s/mM. It exhibited optimal activities at pH approximately 6–9 and 40–45 °C. It was quite stable in this pH range, and retained approximately 40% activity at 37 °C after 1 day. Silver and copper ions (1 mM) decreased its activity by 30–40%. The removal of 98–100% fCN was achieved for 0.6–100 mM fCN. Moreover, thiocyanate, sulfide, ammonia or phenol added in amounts typical of industrial effluents did not significantly reduce the fCN conversion, while electroplating effluents may need to be diluted due to high fCN and metal content. The ease of preparation of NitEg, its high specific activity, robustness and long shelf life make it a promising biocatalyst for the detoxification of fCN.
... Cyanidation is a leaching process where CN − is used to dissolve the gold or silver from the ore to form metal complexes. This complexation is typically carried out at strong alkaline conditions (pH > 10) to avoid the formation of HCN from CN − (Johnson, 2015;Kuyucak and Akcil, 2013). Elsner's reactions (Eqs. ...
... Elsner's reactions (Eqs. (1) and (2)) summarize the cyanidation process (Kuyucak and Akcil, 2013). ...
... The slow photolysis at uncontrolled environmental conditions leads to the release of CN − from these metal cyanocomplexes (e.g., Eq. (5)) (Johnson et al., 2008), and of CN − into CNO − which is also toxic (Eqs. (6) and (7)) (Dzombak et al., 2006;Kuyucak and Akcil, 2013;Tian et al., 2016). Therefore, the open-air storage of cyanidation wastes without any additional treatment generates toxic emissions, representing a risk for surrounding ecosystems and water bodies (González-Ipia et al., 2020;Johnson et al., 2008;Joven-Quintero et al., 2020;Kuyucak and Akcil, 2013). ...
Article
This work studied the influence of several parameters on free cyanide (CN −) degradation (50 mg L − 1) by the UVC-activated persulfate (PS) at alkaline conditions (UVC/PS). Firstly, photolysis and alkaline activation of PS were evaluated. Then, the effect of initial PS concentration (0.2, 0.4, and 0.6 g L − 1) and dissolved oxygen in solution (absence/presence) were studied. Lastly, the influence of phosphate, carbonate, and nitrate presence at different concentrations (50, 150, 350, and 500 mg L − 1) on CN − elimination was tested. Additionally, the electric energy per order (E EO), a measure of the energy consumption in the process was determined, and a mechanistic view of CN − degradation was proposed. The results show that photolysis and alkaline activation of PS degraded 8 and 11% of CN − , respectively, whereas their combination presented a synergistic effect on CN − pollutant elimination. While oxygen had a vital role in photolysis due to the formation of 1 O 2 to oxidize CN − to CNO − , HO • and SO 4 •− were primarily responsible for CN − degradation by UVC/PS. It was also found that cyanide removal followed a pseudo-first-order kinetics whose apparent reaction rate constant (k) increased from 0.0104 to 0.0297 min − 1 as the initial concentration of PS increased from 0.2 to 0.6 g L − 1 , indicating a strong dependency of the removal efficiency on the PS amount. Remarkably, cyanide degradation by the combined UVC/PS showed a high CN − conversion and selectivity even in the presence of high concentrations of phosphate, carbonate, and nitrate ions (500 mg L − 1), which resulted in CN − removals higher than 80% after 60 min of degradation treatment. Furthermore, the E EO values were similar in the presence and absence of phosphate or carbonate; however, they decreased slightly with nitrate presence. All these results suggest the feasibility of the combined UVC/PS process for the elimination of cyanide such as that found in mining wastewater.
... Cyanide is usually treated with an oxidation process under alkaline conditions using oxidants, such as hypochlorite [2]. Although alkaline chlorination is an efficient process for the removal of cyanide to reach the discharge limit of 1 mg CN − /L in Taiwan, the process is highly pH-dependent and associated with high chemical costs [3,4]. ...
... According to the modeling, the excess iron precipitates as ferric hydroxides (see Supplementary Figure S2). At pH > 10, the Fe(OH) 4 − complex forms. Based on the modeling result, Fe II (CN) 6 4− complex forms at 6.5 < pH < 10.5, and Fe(II):CN − molar ratio of >0.2. ...
Article
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Alkaline chlorination, an efficient but high chemical cost process, is commonly employed for cyanide (CN−) removal from CN-rich wastewater streams. CN− removal and recovery through the precipitation of Prussian Blue (Fe4III[FeII(CN)6]3, PB) or Turnbull’s Blue (Fe3II[FeIII(CN)6]2, TB) were realized using iron salts, leading to a cost-effective and sustainable process producing a valuable recovery product. However, the precipitation of PB and TB is highly affected by pH and dissolved oxygen (DO). CN− removal and recovery from CN-containing water by crystallization of PB and/or TB were investigated using dissolved iron that was electrochemically generated from a sacrificial iron anode under various pH values, initial CN− levels (10 to100 mg/L) and DO levels (aeration, mechanical mixing, and N2 purging). It was shown that the complexation of CN− with Fe ions prevented the vaporization of HCN under acidic pH. At pH of 7 and initial CN− concentration of 10 mg/L, CN− removal efficiency increases linearly with increasing Fe:CN− molar ratios, reaching 80% at the Fe:CN− molar ratio of 5. A clear blue precipitate was observed between the pH range of 5–7. CN− removal increases with increasing initial CN− concentration, resulting in residual CN− concentrations of 8, 7.5 and 12 mg/L in the effluent with the Fe:CN− molar ratio of 0.8 for initial concentrations of 10, 50 and 100 mg CN−/L, respectively. A polishing treatment with H2O2 oxidation was employed to lower the residual CN− concentration to meet the discharge limit of <1 mg CN−/L.
... Cyanide is a highly toxic compound used in several industrial processes (Mudder et al.,40 2004) given its capacity to form tight complexes with different metals (Dash et al., 2009) 41 (Hendry-Hofer et al., 2019; Leavesley et al., 2008). Industries that generate cyanide-42 containing wastes must reduce its concentration before discarding them to the 43 environment, and as such proper strategies have to be implemented for cyanide 44 remediation (Kuyucak & Akcil, 2013). Cyanide bioremediation by bacteria that express 45 nitrilases is one possible low-cost and environmental-friendly approach (Dash et al., 2009). ...
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Cyanide is widely used in industry as a potent lixiviant due to its capacity to tightly bind metals. This property imparts cyanide enormous toxicity to all known organisms. Thus, industries that utilize this compound must reduce its concentration in recycled or waste waters. Physical, chemical, and biological treatments have been used for cyanide remediation; however, none of them meet all the desired characteristics: efficiency, low cost and low environmental impact. A better understanding of metabolic pathways and biochemistry of enzymes involved in cyanide degradation is a necessary step to improve cyanide bioremediation efficacy to satisfy the industry requirements. Here, we used several approaches to explore this topic. We have isolated three cyanide-degrading Bacillus strains from water in contact with mine tailings from Lima, Peru, and classified them as Bacillus safensis PER-URP-08, Bacillus licheniformis PER-URP-12, and Bacillus subtilis PER-URP-17 based on 16S rRNA gene sequencing and core genome analyses. Additionally, core genome analyses of 132 publicly available genomes of Bacillus pumilus group including B. safensis and B. altitudinis allowed us to reclassify some strains and identify two strains that did not match with any known species of the Bacillus pumilus group. We searched for possible routes of cyanide-degradation in the genomes of these three strains and identified putative B. licheniformis PER-URP-12 and B. subtilis PER-URP-17 rhodaneses and B. safensis PER-URP-08 cyanide dihydratase (CynD) sequences possibly involved cyanide degradation. We identified characteristic C-terminal residues that differentiate CynD from B. pumilus and B. safensis, and showed that, differently from CynD from B. pumilus C1, recombinant CynD from the Bacillus safensis PER-URP-08 strain remains active up to pH 9 and presents a distinct oligomerization pattern at pH 8 and 9. Moreover, transcripts of B. safensis PER-URP-08 CynD (CynDPER-URP-08) are strongly induced in the presence of cyanide. Our results warrant further investigation of B. safensis PER-URP-08 and CynDPER-URP-08 as potential tools for cyanide-bioremediation.
... Among various biologically important anions, cyanide has witnessed a major interest in the development of efficient sensor as it considered to be a deadly xenobiotic [3,4], a chemical warfare agent [5][6][7] and largest threat to human health. Despite its extreme toxic effects [8], cyanide is widely used as an essential raw material in several industries including textile [9], plastics [10], paints [11] , [12], electroplating [13], agriculture [14], food [15], medicine and metallurgy [16] , [17]. A low dosage of cyanide exposure causes headache, dizziness, mild confusion, abdominal cramping, nausea, whereas the high dosages may leading to dyspnoea, respiratory depression, apnoea, hypotension, arrhythmias, coma and even Figure S8). ...
Article
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A pyrene functionalized oxacalix[4]arene architecture (DPOC) was utilized as a fluorescence probe for selective recognition of cyanide ions. The receptor DPOC shows excellent selectivity towards cyanide ion with a red shift of 108 nm in absorption band along with a significant change in colour from light yellow to pink. The fluorescence titration experiments further confirm the lower limit of detection as 1.7µM with no significant influences of competing anions. ¹ H-NMR titration experiments support the deprotonation phenomena, as the -NH proton disappears upon successive addition of cyanide ions. The DFT calculation also indicates a certain increment of -NH bond length upon interaction with cyanide ions. The spectral properties as well as colour of DPOC-CN⁻ system may be reversed upon the addition of Ag⁺/ Cu²⁺ ions up to 5 consecutive cycles. Moreover, DPOC coated “test strips” were prepared for visual detection of cyanide ions.
... These reductions are related to the increase in IC and metal ions such as Al, Fe and Mn (Tables 3 and 4), which contributed to a greater retention of anions in more stable fractions . The greater bioavailability of As in the cyanidation mining tailings may be related to the use of Na cyanide solution during gold extraction (Kuyucak and Akcil, 2013), which promotes partial dissolution of Fe, Al and Mn minerals (Rubinos et al., 2011), releasing As. After dissolution, the metalloid tends to be even more desorbed with increasing pH, due to competition with OH − ions Vodyanitskii, 2006;Xue et al., 2019;Yin et al., 2015). ...
Article
Artisanal gold mining has generated tailings highly contaminated by arsenic (As) in Cachoeira do Piriá, eastern Amazon, leading to severe risks to the environment. Such risks should be mitigated considering the bioavailable concentration of the element, since it implies immediate damage to the ecosystem. The objective of this study was to evaluate the potential of biochars in mitigating the environmental risks of bioavailable As concentrations in gold mining tailings from underground and cyanidation exploration. The biochar addition increased mineral components, cation retention, phosphorus in all fractions, and organic and inorganic carbon. The bioavailability of As was reduced after adding the biochars, following the order palm kernel cake biochar > Brazil nut shell biochar > açaí seed biochar, with reductions of up to 13 mg kg⁻¹ in the underground mining tailings and 17 mg kg⁻¹ in the cyanidation mining tailings. These results contributed to the statistically significant reduction of the environmental risks in both mining tailings (6–17% in the underground mining tailings and 9–20% in the cyanidation mining tailings), which was emphasized by Pearson's correlation and multivariate analyzes. The incorporation of the bioavailable fractions of As (from sequential extraction) in the environmental risk assessment was a promising method for evaluating the efficiency of biochars in mitigating the damage caused by this metalloid in gold mining tailings.
... It also occurs naturally in foods such as cashew nuts, lima beans, and certain root tubers such as cassava and coco-yam (Dash et al. 2008(Dash et al. , 2009). The concentration of cyanide per 100 g of plant tissue increases can be as high as 104 mg (Kuyucak and Akcil 2013). Although, this may depend on various factors such as; the plant's age, the cassava cultivar, harvest period, etc. (Oliveira et al. 2001). ...
Article
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The continuous generation of waste resulting from the industrial activities of humans has significantly been on the rise, especially liquid wastes emanating from cassava processing mills, which is a major cause for concern in developing countries. This study focused on the preparation of H3PO4 activated periwinkle carbon (APSC) and use in the removal of cyanide in cassava wastewater. The influence of variables such as pH, adsorbent dosage, contact time, and different cyanide concentrations was investigated in batch procedures. Results from the batch studies reveal a strong pH-dependent adsorption process with optimum cyanide removal occurring at pH 8. An equilibrium time of 80 min and adsorbent dosage of 3.0 g gave the highest percentage of cyanide adsorbed at 83.93%. The Pseudo-first-order, Pseudo-second-order, and Elovich kinetic models were used for the analysis of experimental data while equilibrium data analysis using Langmuir, Freundlich, and Redlich–Peterson was carried out to determine the best-fit isotherm model. The Coefficient of determination (r2), Sum of square error (SSE), and Chi-square (χ2) were used to estimate the error deviations between the predicted and the experimental models using nonlinear regression analysis to determine models that best explain the adsorption process. Kinetic data fitted well to the Elovich and Pseudo-second-order kinetic model which implies chemisorption as the dominant adsorption process. The Redlich–Peterson and Langmuir model best describes the adsorption process suggesting mono-layer adsorption with the monolayer adsorption capacity of APSC found to be 2.856 mg.g−1.
... The contamination by cyanide contributes the most to the calculation of the GWF because the cyanidation technique is used in the Su arez plants with greater crushing capacity. It is important to note that cyanide contamination has a short-term effect and is not bioaccumulable (with the exception of some complexes) because it can be partially degraded by the sun's ultraviolet radiation [39]; however, the possible formation of soluble complexes of cyanide with other metals, such as mercury, and the possible breaking of storage dams and tailings spills to water sources can generate the production of hydrocyanic acid (when the water pH reaches values below 11), constituting a serious risk to the environment and human health because cyanide inhibits the transfer of oxygen to cells causing suffocation and death [40]. ...
Article
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This research deepens the analysis of the mineral water footprint, especially that of gold, in regions that are understudied and where mining has been an intensified extractive activity since the colonial era, as is the case in the northern part of department of Cauca in Colombia. Thus, the purpose was to estimate the water footprint indicators in gold mining in Suárez (Cauca, Colombia), to quantify the impacts generated by the non-returned water in the production process and the levels of pollutants in the wastewater, aimed to strength public policies, control strategies and mitigation that generates reductions in the impacts from mining activities on the environment. The blue water footprint was estimated in 79.91 m³ per kg of gold extracted and the gray water footprint was found to be in the range of 272,125.39 to 404,825.11 m³ per kg of gold extracted. The water footprint values obtained were compared with other mines with similar operations. These results generate a baseline for decision making, providing elements for environmental strategic planning, regulations and showing the great environmental pressure that gold activity exerts on water resources and the territories.
... Natural attenuation reactions and/or physical-chemical treatment processes using, for example: hydrogen peroxide; SO 2 plus air; Caro's Acid; alkaline chlorination; and iron / copper precipitation, can convert cyanide to less harmful forms, decreasing its reactivity and lowering its toxicological characteristic (Dash et al., 2009;Kim et al., 2003;Kuyucak and Akcil, 2013;Parga et al., 2003;Teixeira et al., 2013) so treated effluents can be disposed in the environment. Although natural degradation bears low operating costs, it is a slow and efficiency-limited process that depends largely on climatic conditions, so only partially useful. ...
Article
The main purpose of this research has been to evaluate and optimize the application of hydrodynamic cavitation (HC), combined with hydrogen peroxide, as a promising process for the effective degradation of cyanide in aqueous effluents. The experimental work was carried out using cavitation equipment with a venturi device connected to a tubular circuit which allowed a closed-cycle flow to run for 120 min, in which the effect of control parameters as inlet pressure, H2O2:CN─ ratio, pH, and temperature have been evaluated for the treatment of solutions with initial cyanide concentration in range 100 to 550 mg L─1. The results showed that in optimal conditions cyanide degradation using only HC reached 70% and, using solely H2O2 as oxidizing agent it reached 63%. Efficiency of the combined treatment process was evaluated on the basis of their synergetic effect as it turned out to be more effective showing a 99.9 % cyanide degradation in less than 120 min. The optimum set of conditions that produced the highest degradation rate and efficiency was: inlet pressure 4 bar; pH 9.5; and H2O2:CN─ ratio = 1.5:1. The process was also evaluated on the basis of cavitational yield and in terms of energy and chemical treatment costs. The results have demonstrated that the combined treatment technology of HC + H2O2 can be effectively used as a fast and highly efficient treatment of wastewater containing cyanide.
... Sulfide minerals in gold-containing ores can generate thiocyanate with cyanide, which is formed faster under low alkalinity and inadequate aeration conditions (Kuyucak and Akcil 2013). There is a variety of chemical and biological treatments for the destruction and removal of cyanide (Akcil 2003) and thiocyanate (Gould et al. 2012) in industrial effluents. ...
Article
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In nature, microorganisms developed at various places and adapted to the various weather and geological conditions. Microorganisms participate in geological transformations leading to the dissolution of some minerals and conversion to others. While some microorganisms with their metabolic activity increase the mobility of metals, others cause precipitation of metals and the formation of new minerals. These biogeochemical interactions found practical application in the recovery of metals. In the article, the proposals for improvement of existing engineering commercial processes for recovery of metals are given which can enable the formation of nanogold and nanogold compounds. Key points • Amino acids in pretreatment can increase the dissolution of the layer around the gold.• Amino acids in the complexing stage can increase gold leaching.• After the complexing stage, the bionanosynthesis of gold and its compounds is possible.Graphical abstract
... To minimize environmental disasters, cyanide containing liquid must be properly treated before releasing into the water bodies. One of the challenges cyanide industries face in meeting their standards and recycling wastewater is the removal of cyanide in the effluent (Kuyucak and Akcil 2013). Several chemical and physical cyanide bioremediation methods have been developed; however, these methods are expensive, and the by-products of these processes are hazardous, thus, requiring additional treatment before disposal (Saarela and Kuokkanen 2004). ...
Article
Rhodanese, a cyanide detoxifying enzyme synthesized by Klebsiella oxytoca was immobilized on alginate-glutaraldehyde beads. K. oxytoca was isolated from industrial wastewater and identified using 16S rRNA gene sequencing with gene bank accession number MN590525. Rhodanese was produced from the bacterium through submerged fermentation. The rhodanese produced was immobilized on alginate-glutaraldehyde beads; its physicochemical properties and cyanide bio-remediation potential were compared with the free enzyme. In this study, the optimum concentrations of glutaraldehyde and sodium alginate that resulted in the highest enzyme immobilization yield (89.71%) and lower leakage (1.45 ± 0.2%) were 4.5% (v/v) and 2.5 (%) respectively. The optimum temperature for free and immobilized rhodanese was observed at 50 C and 60 C respectively while the optimum pH for both preparations was 6.0. The free rhodanese retained 31% and 23% of relative activity at 60 C and 70 C respectively after 30 minutes of incubation while immobilized rhodanese retained about 95% and 70% at the same condition. The entrapped rhodanese showed activity until the 10th cycle and maintained about 70% of its activity after the fifth cycle. After 180 minutes of incubation, the free and immobilized rhodanese was able to biodegrade 115 mg/L cyanide to 77 mg/L and 45 mg/L respectively with degradation efficiency of 33 and 64.34%. These results suggest that immobilized K. oxytoca rhodanese may be profitably exploited in bioremediation of cyanide polluted environment due to its thermal stability and its reusability. available online at https://www.tandfonline.com/eprint/G7TN7ZKE6STWM3MWAZXY/full?target=10.1080/10242422.2022.2087510
... In the process of gold mining, cyanide (such as sodium cyanide) is usually used as the solvent to extract gold from minerals. Thus, cyanide wastewater containing heavy metal ions (Cu 2+ , Pb 2+ , Zn 2+ , Hg 2+ ) will be generated, which is toxic and non-degradable (Kim et al. 2019;Kuyucak and Akcil 2013). When the wastewater enters into the gold extraction process, residual Cu 2+ , Pb 2+ , Zn 2+ , and Hg 2+ in the wastewater will increase the consumption of sodium cyanide in the cyanidation process and the production cost. ...
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As a promising solar energy conversion technology, solar water evaporation has been regarded as an energy-efficient approach to alleviate the freshwater shortage caused by industrial water pollution. In this paper, we developed a straightforward method with a solar-driven steam generator (SSG) based on the carbonized green algae (CGA) as a light-to-heat conversion material (LHCM) to deal with the industrial wastewater of gold smelting. CGA SSG exhibited excellent light absorption, hydrophilicity, and water evaporation rate (1.66 kg·m⁻²·h⁻¹). It accomplished the non-selective removal of heavy metal ions (Cu²⁺, Pb²⁺, Zn²⁺, Hg²⁺) and CN– in the treatment of gold smelting wastewater, and the ion removal rate was 99%. Compared with traditional and complex wastewater treatment technologies, the solar-driven CGA SSG presented many advantages (low cost, simple preparation, and high performance) in water purification, which could be employed in backward areas to obtain clean water.
... Cyanide leaching is acknowledged as an efficient metal leaching method since the 1880s which can extract 1-3 g/t of gold from WEEE (Tuncuk et al. 2012). Under alkaline conditions (pH is greater than 10.5), most of the free cyanide exists in the form of CN -, which is conducive to gold being combined to form complex (Kuyucak and Akcil 2013), while cyanide has high toxicity and poses great risks to human and environmental health (Quinet et al. 2005). Thus, in recent years, the non-cyanide method, which is an alternative method with low toxicity and great leaching potential, has received widespread attention (Abdelbasir et al. 2018b). ...
Article
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In the twenty-first century, the increasing demand for electrical and electronic equipment (EEE) has caused its quick update and the shortening of its service life span. As a consequence, a large number of waste electrical and electronic equipment (WEEE) needs to be processed and recycled. As an environmentally friendly method, biometallurgy has received extensive attention in the disposal of WEEE in recent years. Aspergillus niger is an acid-producing fungus with a potential applicability to improve metals’ recycling efficiency. This review article describes the latest statistical status of WEEE and presents the latest progress of various metallurgical methods involved in WEEE recycling for metal recovery. Moreover, based on the summary and comparison towards studies have been reported for bioleaching metals from WEEE by A. niger, the bioleaching mechanisms and the bioleaching methods are explained, as well as the effects of process parameters on the performance of the bioleaching process are also discussed. Some insights and perspectives are provided for A. niger to be applied to industrial processing scale.
... To minimize environmental disasters, cyanide-containing liquid must be properly treated before releasing into the water bodies. The removal of cyanide from the effluent is one of the challenges in the cyanidebased industries to fulfil their standards and recycling of the water (Kuyucak and Akcil 2013). Despite the toxicity of cyanide, cyanotrophic microorganisms such as Pseudomonas sp. ...
Article
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Microorganisms are increasingly being used in cyanide bioremediation. Several organisms have been reported to thrive in cyanide contaminated wastewater due to their ability to produce cyanide detoxifying enzymes. However, to improve the production efficiency of these enzymes combinations of process variables need to be optimized. In this study, Klebsiella oxytoca JCM 1665 was isolated from industrial wastewater, identified by sequencing its 16S rRNA gene and subjected to rhodanese production using submerged fermentation. The conditions for production were optimized using response surface methodology (RSM). Central composite design was employed to evaluate the effects of three production parameters-peptone (1-5 %), KCN (0.1-0.5 %), and time of incubation (1-24 h). Second-order polynomial model was used to predict the response. Rhodanese activity in the experiments varied from 0.05 to 7.5 RU.mg-1. Under the optimum conditions of 4.35 % peptone, 0.4 % KCN and incubation time of 13 hr., the value for rhodanese yield was 7.810 U.mL-1. The R 2 value for the model was 0.9925 (R 2 = 0.9925). Also, the experimental values are in accordance with those predicted, indicating the suitability of the employed model and the success of RSM in optimizing the production conditions.
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The construction of a low-cost and green copper recovery method in gold plants is extremely important for treating CuSCN-containing refractory acidified sediments (ASs). In this work, an effective and environmentally-friendly hydrometallurgical process, consisting of thiosulfate leaching and protective electrodeposition, was proposed for selective copper recovering from ASs. The effects of leaching parameters on the leaching efficiency were systematically investigated. Subsequently, cyclic voltammetry, linear sweep voltammetry, and chronoamperometry were utilized to determine the predominant speciation and investigate the electrochemical behavior of the leachate. Eventually, the optimum electrolysis parameters for copper recovery were considered. The results demonstrated that the selective leaching efficiency of copper reached 99% under optimum leaching conditions, while no zinc and iron were leached, which coincides with the results of speciation calculations. Simultaneously, the kinetic analysis indicated that the leaching process exhibited a diffusion-controlled step with an apparent activation energy of 13.64 kJ/mol. Na2Zn3Fe(CN)6 as the main ingredient of the leaching residue was formed, which played a vital role in facilitating the copper leaching process. The electrochemical measurements indicated that the [Cu(S2O3)3]⁵⁻ complex was the dominant species in the pH range from 7.0 to 11.0, and the mass transfer diffusion was the main resistance element for the cathodic reduction of the [Cu(S2O3)3]⁵⁻ complex. Regarding the S2O3²⁻ stability, it decomposed more easily into S, S²⁻, S4O6²⁻, and SO4²⁻ in the presence of Cu²⁺. However, the presence of SO3²⁻ significantly alleviated thiosulfate decomposition. The electrodeposition experiments showed that metallic copper was obtained with a recovery efficiency of 95%, current efficiency of 50%, and energy consumption of 1.25 kW h/kg Cu. This research developed an approach for the efficiently recycling ASs, yielding great economic results and more favorable environmental performance.
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Cyanide wastewater contains highly toxic substances such as cyanide and metals. A simple and efficient photoelectro-catalytic oxidation method was proposed. Effect and mechanism of cyanide degradation and Cu/Zn recovery by photoelectro-catalytic oxidation were investigated. The experimental results show the degradation efficiency of total cyanide and removal efficiencies of copper and zinc reached 96.90%, 99.76% and 90.56%, respectively, under the optimum conditions of 2.5 V external bias voltage and 4.0 h with air and lighting. An external electric field can significantly improve the migration and separation of photo-generated electrons (e–) and holes (h⁺), thereby increasing the photocatalytic efficiency. Simultaneously, the surface potential of semiconductor is lower than the body under the action of bias voltage and leads to energy band bending. XPS analysis indicates that cyanide is finally degraded into carbon oxide and nitrogen. Cu and Zn are mainly recovered as elementary substance.
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Traditional methods of cyanides’ (CN⁻) mineralization cannot overcome the contradiction between the high alkalinity required for the inhibition of hydrogen cyanide evolution and the low alkalinity required for the efficient hydrolysis of cyanate (CNO⁻) intermediates. Thus, in this study, a novel Electro-Fenton system was constructed, in which the free cyanides released from ferricyanide photolysis can be efficiently mineralized by the synergy of •OH and •O2⁻. The complex bonds in ferricyanide (100 mL, 0.25 mM) were completely broken within 80 min under ultraviolet radiation, releasing free cyanides. Subsequently, in combination with the heterogeneous Electro-Fenton process, •OH and •O2⁻ were simultaneously generated and 92.9% of free cyanides were transformed into NO3⁻ within 120 min. No low-toxic CNO⁻ intermediates were accumulated during the Electro-Fenton process. A new conversion mechanism was proposed that CN⁻ was activated into electron-deficient cyanide radical (•CN) by •OH, and then the •CN intermediates reacted with •O2⁻ via nucleophilic addition to quickly form NO3⁻, preventing the formation of CNO⁻ and promoting the mineralization of cyanide. Furthermore, this new strategy was used to treat the actual cyanide residue eluent, achieving rapid recovery of irons and efficient mineralization of cyanides. In conclusion, this study proposes a new approach for the mineralization treatment of cyanide-containing wastewater.
Article
Cyanidation is considered the most employed process to recover precious metals such as silver and gold. However, cyanide containing wastes can be hazardous for the environment. Therefore, the soil electroremediation technique was systematically studied to eliminate/degrade the cyanide contained in mining tailing dams using an electrolytic cell with cationic separator and cheap-stable carbon electrodes. The extent and kinetics of cyanide degradation was firstly analyzed in synthetic solutions using the electrolytic cell at different current density, ionic conductivity, and cyanide concentration. The electrolytic system was also evaluated for the treatment of a sample obtained from a mining tailing dam containing cyanide. The results revealed that 100 % of cyanide can be degraded from synthetic solutions containing 200 ppm CN– at 60 A/m², 0.25 M NaOH and room temperature. It was also found that 100 % of cyanide can be degraded from the real samples obtained from a Mexican mining tailing dam. The kinetic analysis revealed that cyanide degradation can occur through two pathways: a) in the case of tests containing 500 and 800 ppm NaCN, the rate determining step of the cyanide degradation to cyanate species follows a first order reaction with respect to the cyanide ions, and b) when the tests are carried out with 200 ppm NaCN, the same first order reaction controls the process kinetics from 0 to 30 min, while at t > 60 min, the rate determining step for cyanide degradation is modified and limited by the occurrence of parasite reactions. The proposed mechanism for cyanide degradation is also consistent with the thermodynamic calculations. The results also revealed that complete cyanide electro-degradation can be achieved with a current efficiency of 100 %.
Article
Merrill-Crowe is the primary process used for the concentration and purification of silver in a cyanide solution. The Langmuir Adsorption model for silver recovery in a cyanide solution was used for the study. The maximum adsorption capacity was 6.19 mmol/g for silver. This model properly adjusts the experimental results of the adsorption equilibrium with high correlation coefficients, which also favors the formation of a single layer of molecules adsorbed for the silver in the electrocoagulation-generated species. The thermodynamic parameters ΔG, ΔH, and ΔS were estimated, and it was found that the adsorption process is exothermic and spontaneous. For the kinetic study, the Lagergren second-order equation was used to study the adsorption speed. To characterize the solids coming from the electrocoagulation, X-ray diffraction (XRD), and scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX) were used; the results of this study suggest that silver is present in aluminite, hydroniumjarosite, and alunogen.
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The cyanidation process is the most common method applied for the extraction of gold and silver in the hydrometallurgy industry, in which, sodium cyanide is used as a leaching agent. Therefore, the wastewater of gold mines contains a wide variety of cyanide ions needing to be removed before these wastewaters can be discharged to the receiving environments. In this study, a fuzzy multi-attribute decision-making approach (Fuzzy Delphi AHP and Fuzzy TOPSIS) was used for selecting the best cyanide removal method from the wastewater of Muteh gold mine. According to the experts' opinion, three methods including calcium hypochlorite, hydrogen peroxide and sodium hypochlorite were selected as alternatives. Then, by introducing the criteria influencing decision making, including cyanide removal ability, cost of process, amount of material consumed, time, pH, ease of performance and safety, and performing separated experiments, the criteria for each of three methods were determined. Finally, sodium hypochlorite was proposed as the best method for eliminating cyanide from wastewater. It was found that the rank of methods was as sodium hypochlorite (0.517) > calcium hypochlorite (0.474) > hydrogen peroxide (0.463).
Article
Numerous non-cyanide leaching lixiviants have been developed, among which thiosulfate is considered the most promising alternative to cyanide due to its non-toxicity, low price, high leaching rate and excellent characteristics in dealing with carbonaceous and copper-bearing gold ores. The traditional copper−ammonia−thiosulfate system has been studied extensively. However, with many years of process development, there are still some problems and challenges with this gold leaching system. A series of studies using nickel-, cobalt- and ferric-based catalyst to substitute copper have been conducted with the purpose of reducing the consumption of thiosulfate. A variety of non-ammonia thiosulfate leaching systems including oxygen−thiosulfate, copper−thiosulfate, copper−EDA−thiosulfate, ferric− EDTA−thiosulfate, and ferric−oxalate−thiosulfate leaching systems have been also developed to eliminate the potential side-effect of ammonia. In this review, the basic theory and process development of some main gold leaching systems based on thiosulfate solutions were systematically summarized to illustrate the research status on thiosulfate leaching process. The potential effects of various additives such as organic ligands containing amino, carboxyl or hydroxy functional groups on gold thiosulfate leaching were described in detail. The potential opportunity and challenge for promoting the industrial development of thiosulfate-based gold leaching systems were also discussed.
Article
At present, the cyanide gold extraction process is still the main technology for gold production. Generated cyanide tailings containing highly toxic substances exhibit potential environmental risks. These tailings are in urgent need of purification treatment, especially after being classified as hazardous waste. In this study, the impacts of elution methods, operating time, tailings/water ratios, reagent types on the elution rates of cyanide were investigated. Furthermore, the composite elution method developed in this research was extended for engineering. Results showed that the optimum elution conditions were determined to be: stirring elution, tailings/water ratio (M/V; 1:1) and operating time (10-20 min). Besides, 4 reagents (sodium dodecyl benzene sulfonate, cyclodextrin, sodium silicate and calcium hydroxide) were selected from four categories of 21 reagents for further composite elution. The cyanide elution rate was the highest (90.7%±0.1%) while the molar ratio of these 4 reagents was 5:2:2:1. Moreover, the combination of reagent elution and positive pressure filtration improved the elution efficiency of cyanide (92.6%±0.8%). And the cyanide content in the toxic leaching solution was lower than the standard value (5.0 mg/L). Furthermore, the composite elution method developed in this study was also extended for engineering. The concentration of cyanide in the leachate was < 5.0 mg/L, and was stable during 189 days of detection. Notably, the effluent can be reused directly, or reused after further treatment. The zero discharge of effluents and solid wastes was realized in the processes. The above results provided supports for the engineering treatment of cyanide tailings.
Article
Cyanide wastewater contains highly toxic contaminants such as free cyanide and metal-cyanide complex, it is necessary to find the appropriate degradation technologies. In this study, a simple and effective photocatalyst with enhanced adsorption and photocatalytic activity was developed. A new composite photocatalyst was successfully prepared by solid-state dispersion (SSD) method, which combined graphene oxide (GO), nano titanium dioxide (nano-TiO2) and silicon-based zeolite (ZSM-5). The techniques of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), N2 adsorption–desorption test, X-ray diffraction (XRD), ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) were used to characterize the samples. The experiments with different content of nano-TiO2, different wavelengths of light source, different catalyst dosage, control experiments of different materials, and the determination of pH value and zeta potential were also carried out. The results showed that the 3.0 g/L of GTZ-60 (GO/TiO2/ZSM-5 composite photocatalyst with a content of 60 wt% TiO2) has the highest photocatalytic degradation efficiency under ultraviolet wavelength which the degradation efficiency of total cyanide and copper could reach 98.73% and 98.96%, respectively. The degradation of total cyanide followed the pseudo-first-order kinetics. The photoelectric properties of GTZ-60 obviously improved the migration and separation of photogenerated electrons (e⁻) and holes (h⁺), so that high photocatalytic efficiency was obtained. XPS analysis showed that cyanide was finally degraded to carbon oxide and nitrogen oxide. Copper and zinc are removed in the form of copper (Ⅱ) and zinc (Ⅱ).
Article
Cyanide tailings(CT) are a typical hazardous waste, containing large amounts of heavy metals and highly toxic cyanide. As so far, it is difficult to solve this problem at the same time. In this study, a “two-step” process was proposed for the first time to treat CT, first using microorganisms to degrade cyanide and then using microbially induced carbonate precipitation (MICP) to solidify the CT. We isolated a bifunctional bacterium which exhibited cyanide degradation and high urease activity from CT, identified as Aneurinibacillus tyrosinisolvens strain (named JK-1). We used JK-1 bacteria to treat the CT in a “two-step” process. The results showed that the degradation of free cyanide (F–CN) and total cyanide (T-CN) in CT by JK-1 bacteria reached 94.54% and 88.13%. After the MICP treatment, the spindle-shaped CaCO3 solidified the CT into a block of calcite and sphalerite crystals, and the uncompressed compressive strength (UCS) reached 0.74 MPa; the morphology of heavy metals in the CT changed from the exchangeable state to the carbonate-bound state, and the mobility was significantly reduced. Compared with chemical treatment, the treatment of CT by the new process is highly efficient and green, which can realize the solidification of CT, degradation of cyanide and immobilization of heavy metals at the same time. Compared with chemical treatment, the new process is efficient and green, which can realize the solidification of CT, the degradation of cyanide and the immobilization of heavy metals at the same time; it is of great significance to the harmless treatment of CT and the sustainable development of gold smelting industry.
Article
Efficient destruction of cyanide by thermal decomposition with ferric oxide addition was proposed. The mechanism of destruction of sodium cyanide with or without ferric oxide addition under various conditions was examined by XRD, DSC−TG, and chemical analysis technologies. In the absence of ferric oxide, sodium cyanide decomposes at 587.4 °C in air and 879.2 °C in argon atmosphere. In the presence of ferric oxide, about 60% of sodium cyanide decomposes at 350 °C for 30 min in argon, while almost all sodium cyanide decomposes within 30 min in air or O2 with mass ratio of ferric oxide to sodium cyanide of 1:1. The increase of ferric oxide addition, temperature, and heating time facilitates the destruction of sodium cyanide. It is believed that with ferric oxide addition, NaCN reacts with Fe2O3 to form Na4Fe(CN)6, Na2CO3, NaNO2 and Fe3O4 in argon. NaCN decomposes into NaCNO, Na4Fe(CN)6, minor NaNO2, and the formed NaCNO and Na4Fe(CN)6 further decompose into Na2CO3, CO2, N2, FeOx, and minor NOx in air or O2.
Article
An affinity membrane was prepared by coaxial electrospinning and amidoxime (AONFA), and it was applied to selectively recovery Au (III) from an aqueous solution. The static adsorption results showed that, when pH at 5, the maximum adsorption capacity of AONFA membrane for Au (III) was 509.3 mg·g⁻¹. AONFA membrane exhibit much higher affinity and selectivity towards Au (III) than other metal cations. The membrane could be regenerated effectively by mixture solution of thiourea and HCl, and the desorption ratio reached almost 100% after 4 hours desorption. The dead-end filtration results showed that, the membrane utilization efficiency and adsorption capacity can be improved by increasing the flow rate, while increasing the concentration shorted the breakthrough process and had little impact to adsorption capacity. We can flexibly adjust the flow rate and concentration according to the situation to obtain the maximum utilization efficiency of the membrane in filtration process. The dynamic adsorption capacity is higher than the static adsorption capacity. The adsorption mechanism for Au (III) is electrostatic adsorption and reduction. Thus, AONFA membrane filtration was demonstrated to be a promising method for continuous recover Au (III) from wastewater.
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Mexico is the top producer of silver and is on the eighth place from producing gold in the world. For instance, the hydrometallurgical extraction process produces wastewater (mining tailing) characterized by being composed with varying concentrations of cyanide and heavy metals. The purpose of this research was to study the biodegradation of cyanide contained in mining tailings by means of a bacterial consortium isolated from a tailings dam. For this purpose, three types of Eckendfelder reactors were used, one with suspended biomass (BS) and two moving bed biofilter reactors, one with biomass immobilized on Kaldnes (BK) supports, and the other on polyurethane cubes (BCP). Three experimental stages were worked; in each of them, the concentrations of total cyanide were varied. In the first one, it was 26 ± 2 mg·L⁻¹; in the second one 40 ± 4 mg·L⁻¹; and the third one 55 ± 4 mg·L⁻¹. During the whole operation, the pH and temperature were maintained at 9.5 units and 25 °C. After 141 days of operation, biodegradation of the total cyanide contained in the mining tailings was 69% (17 mg·L⁻¹) in the BS reactor, while in the BK reactor, it was 93% (3.9 mg·L⁻¹) and in the BCP reactor 95% (2.5 mg·L⁻¹). The predominant families in each of the reactors, as well as their respective relative abundances, were for the BS and for the BK of Cyclobacteriaceae (20.65% and 24.64%) and Rhizobiaceae (18.48% and 14.01%) and Halomonadaceae (46.97%) and Hyphomonadaceae (24.94%) in the BCP.
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In mining, membranes are used in different areas such as treatment of mining wastewater, precious metal recovery from wastewater, the concentration of loaded leach solutions and recovery of precious metals, Acid Mine Drainage (AMD) treatment, acid, caustic, and cyanide recovery. In mining, membranes are preferred because of their low cost, easy operation, environmentally friendly, selective, high removal efficiency, and take up less space. In this study, the membrane applications used by the mining industry in the world and our country have been evaluated in light of the latest developments in membrane technology. According to these evaluations, critical properties of membranes such as cost and clogging problems have been improved with the development of innovative membrane processes. Membrane costs and operating pressures have decreased significantly, and the problem of clogging and contamination has become controllable. Thus, its use for different purposes in the mining industry on a global scale has become widespread and an ideal method. In our country, along with ongoing studies, it is foreseen that this technology has recently become widespread in mining.
Article
An ecofriendly, efficient, inorganic salt catalyzed facile method has been developed for the synthesis of potential ligands azomethines and α-aminonitriles employing aqueous medium. This procedure involves the use of Zn(CN)2 an inexpensive, less toxic as compared to KCN, ecofriendly and readily available effective cyanide source. Cyanated products specially have been isolated in high yield on usual work-up procedure. In methanol and DMF solvents, all of the produced compounds were evaluated for in vitro antibacterial activity against certain bacterial and fungal strains. Among the compounds tested, b and g had the most promising antibacterial action against Bacillus subtilis. Furthermore, compounds a and c were discovered to be the most effective antifungal agents against Candida albicans.
Article
Since the birth of cyanidation, it has been dominant in the gold extraction industry. Recently, with the increasing awareness of environmental hazards and potential risks posed by the severe toxicity of cyanide, attempts to seek alternative lixiviants have arisen. Over the past three decades, a significant amount of literature has examined alternative lixiviants to cyanide for recovering gold, while few industrial applications have been reported due to various obstacles, such as toxicity, excessive consumption, or low leaching efficiency. These obstacles are progressively overcome in multiple ways, including process improvement, system optimization, use of co-intensifying systems, and development of additives. In this paper, related studies about alternative lixiviants and methods such as cyanide, thiosulfate, thiourea, thiocyanate, polysulfides, halides, and microbial leaching are summarized. The history, fundamentals, advancements, and challenges of alternative lixiviants were fully concluded to provide a reference for cleaner gold production. In addition, the comprehensive performance of lixiviants was evaluated according to a novel evaluation criterion proposed in terms of economy, efficiency, and environment.
Article
In the present study, the waste part of the banana tree was used as a precursor, and copper chloride salt was used as an impregnating agent for the preparation of adsorbent to remove both cyanide and phenol from synthetic wastewater. Initially, thermogravimetric analysis was used to determine the rate of carbonization of the material with temperature, and thus, the optimum temperature (370 °C) and time of carbonization (35 min) were assessed. Different samples of adsorbents were prepared next by varying the weight ratio of pseudo-stem of waste banana tree to copper salt from 1:1 to 30:1. All the samples were then tested for removal of both the pollutants, and the ratio (20:1) corresponding to maximum removal of both the pollutants was considered as optimum. Therefore, further studies were conducted with the adsorbent prepared at optimum ratio, temperature and time and such adsorbent was termed as copper impregnated activated banana tree (CIABT). One variable at a time approach was followed to find out the most effective condition based on the maximum removal of pollutants. Maximum removal of 95.99 ± 1.03% and 97.33 ± 0.04% was achieved for cyanide (initial concentration: 100 ppm) and phenol (initial concentration: 450 ppm), respectively, at an optimum contact time of 150 min, the particle size of 90 μ, the adsorbent dosage of 10 g/L, pH 8.0 using CIABT at 25 °C. Hybrid artificial neural network–particle swarm optimization were employed for modelling-optimization of removal of both the pollutants while achieving 91.4–99.99% and 86.43–99.99% removal of cyanide and phenol, respectively, from simulated wastewater.
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Processing of cassava into garri is a multi-staged process that causes adverse environmental impacts. This study aimed at assessing the environmental impact of various stages of cassava processing into garri flakes. A detailed sanitary study of the processing plants was undertaken to ascertain their immediate and remote environmental impacts. Wastewater and soil samples were collected from four garri processing plants and analysed for physico-chemical parameters. The data was subjected to descriptive statistical analysis and various pollution indices were calculated. The results indicate that hydrogen cyanide, total suspended solids and electrical conductivity, which ranged from 8.6 to 42.8 mg/L, 11,900–40,800 mg/L and 4006–5240 µS/cm, respectively, were above the Federal Environmental Protection Agency (FEPA) effluent discharge guideline values. The concentrations of Cr, Cu, Ni, Pb, Cd, Hg and As were: 0.23, 0.06, 0.03, 0.15, 0.09, 0.75 and 2.09 mg/L in the wastewater sample; 0.07, 0.04, 0.33, 0.15, 0.09, 3.75 and 4.85 mg/L in sludge; and 0.10, 0.14, 0.31, 0.15, 0.03, 2.25 and 3.05 mg/L in soil. The average pollution load index of the recipient soils ranged from moderately polluted (2.48) to highly polluted (4.56). Nemerow’s pollution index (NPI) indicated that soil samples from three locations were severely polluted (> 3.0), while one was moderately polluted (2 < NPI < 3). The geo-accumulation indices ranged from − 0.03 (uncontaminated) to 2.59 (heavy contamination). The order of ecological risk of various metals was: As > Cd > Cu > Cr > Pb > Ni with corresponding ecological risk values of 50.63 (moderate risk), 34.43 (low risk), 15.93 (low risk), 10.51 (low risk), 8.93 (low risk) and 5.63 (low risk). Hence, cassava effluent exerts adverse impacts on the immediate premises of cassava processing plants.
Article
This study develops an efficient technique to purify the highly concentrated cyanide-containing industrial wastewater by recovering the valuable components. Firstly, sodium sulfide, sodium polysulfide and ferrous sulfate were respectively used as the precipitating agents to retrieve zinc, copper and iron from wastewater stepwise. In the meantime, cyanide presented as free cyanide and copper–cyanide complexes were transformed into thiocyanate by polysulfide. Afterwards, the solvent extraction of thiocyanate was systematically conducted with trioctylamine (TOA) as the extraction agent and sodium hydroxide solution as the stripping agent. The results showed that the contents of zinc, copper and iron in the solution after precipitation were less than 1 mg·L⁻¹. Focusing on the reaction between copper–cyanide complex and polysulfide, three stages were established for the first time. In the first stage, Cu(CN)3²⁻ was gradually converted to CuCN⁰ and thiocyanate was rapidly generated. In the second stage, in addition to the formation of thiocyanate, Cu(I) was oxidized by polysulfide and precipitated as CuS ultimately. With an apparent activation energy of 70.9 kJ∙mol⁻¹, kinetic analysis of the second stage suggested a significant effect of temperature on the copper–cyanide complex conversion rate. The apparent reaction orders of zero-valent sulfur and cyanide was 1.34 and 2.33, respectively. After the extraction and stripping process, the extraction efficiency of thiocyanate was nearly 100 % and the concentration of it can be enriched to 125 g·L⁻¹ in the stripping solution. TOA and thiocyanic acid formed an extraction complex in a 1:1 ratio. This approach achieves the simultaneous recovery of valuable metallic ions (zinc, copper and iron) and cyanide conversion under the alkaline condition, which provides a new idea for the treatment of highly concentrated cyanide-containing wastewater with a complicated composition.
Article
In this report, three oxidized diindolylarylmethane (DIAM) based chromogenic probes (designated as 1, 2, and 3) have been developed for the simultaneous and dual-channel detection of cyanide (LOD: 6.2 ppb) and bisulfate (LOD: 8.7 ppb) ions in water medium. The orange-colored solution of 1 turns red in the presence of bisulfate ions, while the addition of cyanide leads to yellow. Concurrently, the blue fluorescence of the compound gets quenched in the presence of bisulfate ions, while faint cyan color appears upon the addition of cyanide ions. Thus, it is evident that the present system can achieve naked-eye, ratiometric sensing of both cyanide and bisulfate ions in the pure aqueous medium. Further, it is observed that the presence of electron-withdrawing group on the central phenyl ring improves the sensitivity towards cyanide ion, whereas the electron-donating group favors the interaction with bisulfate ion. Detailed computational investigations have been performed to rationalize such ion-specific distinct chromogenic responses and intramolecular charge transfer (ICT) properties of the probes. Considering the high selectivity and sensitivity, 1 is employed for the determination of excess cyanide in natural water resources and the presence of bisulfate in real-life samples. Finally, low-cost, reusable paper strips have been designed for rapid, on-location detection of both the cyanide and bisulfate ions.
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Understanding the biochemistry and metabolic pathways of cyanide degradation is necessary to improve the efficacy of cyanide bioremediation processes and industrial requirements. We have isolated and sequenced the genome of a cyanide-degrading Bacillus strain from water in contact with mine tailings from Lima, Peru. This strain was classified as Bacillus safensis based on 16S rRNA gene sequencing and core genome analyses and named B. safensis PER-URP-08. We searched for possible cyanide-degradation enzymes in the genome of this strain and identified a putative cyanide dihydratase (CynD) gene similar to a previously characterized CynD from Bacillus pumilus C1. Sequence analysis of CynD from B. safensis and B. pumilus allow us to identify C-terminal residues that differentiate both CynDs. We then cloned, expressed in Escherichia coli, and purified recombinant CynD from B. safensis PER-URP-08 (CynDPER-URP-08) and showed that in contrast to CynD from B. pumilus C1, this recombinant CynD remains active at up to pH 9. We also showed that oligomerization of CynDPER-URP-08 decreases as a function of increased pH. Finally, we demonstrated that transcripts of CynDPER-URP-08 in B. safensis PER-URP-08 are strongly induced in the presence of cyanide. Our results suggest that the use of B. safensis PER-URP-08 and CynDPER-URP-08 as potential tool for cyanide bioremediation warrants further investigation. IMPORTANCE Despite being of environmental concern around the world due to its toxicity, cyanide continues to be used in many important industrial processes. Thus, searching for cyanide bioremediation methods is a matter of societal concern and must be present on the political agenda of all governments. Here, we report the isolation, genome sequencing and characterization of cyanide degradation capacity of a bacterial strain isolated from an industrial mining site in Peru. We characterize a cyanide dehydratase (CynD) homolog from one of these bacteria, Bacillus safensis PER-URP-08.
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Cyanide residues weighing many millions of tons are disposed of in cyanide residue monofills (CRMs) worldwide. The degradation characteristics of cyanide in the anoxic environments of closed landfills may have been overestimated, leading to an underestimation of the long-term risk of cyanide residue landfills. To study the effect, a total of 387 cyanide residue samples were collected for analysis from nine closed CRMs in northen China that have been closed for more than 10 years. The study shows that the probability of achieving the target cyanide concentration (5 mg/L) in the nine sites was only 2.9%. And there is no significant reduction in the overall concentrations compared to the pre-closure period. The effectiveness of the CRM containment barrier needs to be maintained for at least 220 years to allow cyanide concentrations to degrade to harmless levels. Nine CRMs sites, except for CRMs A and B, had a low short-term risk, but in the long term exposure concentrations can exceed the groundwater Class III water quality limit by a factor of 1.64–30, posing a risk of groundwater contamination. This study reveals the risk of cyanide residue degradation in CRMs and its long-term evolution, providing theoretical support for site management and risk control.
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Non-glucosidic cyanogen, which easily break down to liberate toxic cyanide ions, present in sorghum, wheat and fermented grains involved in Chinese liquor fermentation affect its quality and safety, and may endanger public health. In this study, a simple and practical method for the determination of non-glucosidic cyanogen was developed using QuEChERS sample-preparation, combined with spectrophotometry. The QuEChERS extraction solvent, buffer and SPE adsorbents were varied to optimize analyte recovery from spiked samples. Under the optimum conditions, the linear range for cyanide determination was 0.02–0.44 μg/mL, with a correlation coefficient of 0.99 and an LOD of 0.0023 μg/mL. The recoveries of spiked sample (82.11–115.12%) and the relative standard deviations (RSDs; 4.87–9.62%) were considered satisfactory. The method was successfully applied to real samples and was simple, efficient, inexpensive, practical and reliable.
Article
In this study, a silver-containing zinc sulfide concentrate was characterized and then leached to investigate the solvent extraction behavior and recovery of silver. Chemical and phase analyses were performed using X-ray fluorescence (XRF), atomic absorption (AA), and X-ray diffraction (XRD) methods. The microstructure of concentrate was characterized by using optical (OM) and scanning electron microscopes (SEM). The amount of silver in the concentrate equals 580 ppm as an Ag2S compound. About 77% of silver sulfide was free. Leaching of Ag2S was performed in the presence of sodium metabisulfite (0.2 M), copper sulfate (0.01 M), sodium thiosulfate (0.01 M), and ascorbic acid (2.8 mM). The dissolution efficiency in this condition reached 90.5%. The obtained solution was used to conduct solvent extraction tests. According to the leaching conditions, Cyanex302, TMTDS (Tetramethylthiuram disulfide) + 5 %TBP (Tributyl phosphate), and D2EHPA (Di-(2-Ethylhexyl)phosphoric acid) were used as organic extractants. The results showed that the maximum extraction was reached in about 15 min at pH above 1.5 and extractant concentration above 0.1 M. The presence of metabisulfite ions in high values can reduce the efficiency. The organic solvent released one H⁺ ion for each silver ion, and 2 mol of the extractant participated in the extraction reaction. The extraction and recovery of silver ions by all three organic solvents were similar.
Article
With the decline of gold grades of primary resources in many parts of the world, gold recovery from primary resources may become unprofitable. Therefore, the identification of secondary gold resources (such as mine tailings) to prolong the existence of the gold industry has developed growing interest. Life cycle assessment (LCA) was used to design two process flowsheets (cyanide and thiosulphate) for recovering gold from a hypothetical tailings dam scenario in the West Witwatersrand region of South Africa while reducing environmental impacts of processes. The thiosulphate flowsheet reduced environmental impacts for 18 out of 19 impact categories compared to the cyanide flowsheet, apart from the freshwater consumption. The flowsheets proposed reduced sulphide, uranium, and arsenic in mine tailings by 82%, 75% and 94% respectively. Therefore, the solutions presented reduce environmental impacts associated with acid mine drainage, radionuclides and arsenic while producing gold as a value stream. The LCA model and corresponding sensitivity analysis showed that South Africa’s dependency on coal for electricity generation was a major contributor to environmental impacts for both flowsheets and transition to renewable energy can reduce environmental impact of mining operations.
Article
La cianuración ha sido por años el proceso más empleado para la extracción de oro, sin embargo, las soluciones resultantes afectan al medio ambiente ya que son tóxicas y complejas. Algunos de los procesos convencionales para la eliminación de cianuro es la oxidación, no obstante, esta puede resultar lenta. Debido a lo anterior el presente trabajo se enfoca en la eliminación de cianuro con procesos de oxidación avanzada catalítica como lo son el ozono y los óxidos de hierro. Las pruebas experimentales se desarrollaron bajo condiciones controladas a nivel laboratorio, usando óxidos de hierro como catalizadores. Los resultados mostraron que al agregar 1 g/L de óxidos de hierro, la concentración inicial de cianuro (250 ppm) se lograron disminuir a 11.9 ppm en un promedio de 11 minutos, por otro lado, en las pruebas que no usaron dichos óxidos a este tiempo se tenían 150 ppm de cianuro en la solución.
Article
Artisanal and small-scale mining are informal activities used as a source of family income in many countries, mostly in developing regions. They also represent rudimentary practices that generate environmental and health impacts. In these activities, gold is the main mineral extracted commonly using cyanide leaching as the main technique in their production process, despite its environmental impacts. Biodegradation is an alternative technology that has shown advantages over other techniques; however, most microorganisms studied in cyanide biodegradation processes do not tolerate alkaline environments. Thus, the present study assessed the cyanide biodegradation potential under alkaline conditions of a native strain isolated from an artisanal gold mine. The methodology used consisted in the following steps: isolation of bacteria, identification of the isolated strain, adaptation to alkaline environments, and cyanide degradation tests. The strain was identified using the mass spectrometry technique (MALDI-TOF) and it was subsequently compared with the 16S rDNA sequencing technique. Degradation assays were performed with adapted bacteria in an agitated flask containing a synthetic solution with 500 mg.L−1 of free-cyanide (CN−) and initial cell concentration of 2.5 × 1011 CFU.mL−1. Incubation was performed in orbital agitation at 27 °C and 190 rpm for 120 h. In conclusion, the identification techniques elucidated that the isolated strain probably belongs to the Bacillus subtilis species. Finally, cyanide degradation assays showed that the B. subtilis strain adapted to alkaline environments was able to degrade 100% of the free-cyanide in the solution in three days.
Article
During the gold extraction in opencast mining, many hazardous substances, such as cyanide, are spilled into the water bodies. This study's aim was to develop a novel rotary photocatalytic TiO2-based reactor to remove cyanide from polluted water using a rotary concentrator photoreactor (RCPR). This pilot-scale reactor was tested with synthetic cyanide water at concentrations from 0.05 to 50 ppm, varying the pH and commercial TiO2 load. The optimal conditions from experimental data were 87.4% of cyanide removal and catalyst load of 0.30 g/L at pH 9.5. Further, samples of cyanide water from an opencast gold mine were treated, achieving removal of 68.7% after 240 min. Our value-added is the rotary motion of the set of four glass tubes, achieving satisfactory performance, which is promising for cyanide wastewater treatment with a more compact footprint than a standard CPC solar photoreactor. Thus, it was possible to reduce mass and heat transfer limitations with a simple design by considering this photoreactor as a photocatalytic process intensifier.
Article
Fluorescent chemosensors are highly valuable in a variety of fields including environmental chemistry, analytical chemistry and biomedical science. They have provided accurate, online and low-cost detection of toxic heavy metal ions, anions and enzymes with high selectivity and sensitivity. Coumarins, with the structure of benzopyrone, have many advantages including high fluorescence quantum yield, large Stokes shift, excellent light stability and less toxicity. By far coumarin has been used as fluorescent probes of pH, for detection of nitric oxide, nitroxide and hydrogen peroxide. Moreover, coumarin derivatives have served as good chemosensors of anions including cyanide, fluoride, pyrophosphate, acetate, benzoate and dihydrogenphosphate as well as various metal ions comprised of Hg(II), Cu(II), Zn(II), Ni(II), Ca(II), Pb(II), Mg(II), Fe(III), Al(III), Cr(III) and Ag(I). Several systems containing coumarin exhibited simultaneous sensitivity toward two or more different metal ions, e.g., Ca(II) and Mg(II), Ni(II) and Co(II), Cu(II) and Hg(II), Na(I) and K(I), Cu(II) and Ni(II), Hg(II) and Ag(I), Cu(II)/Ni(II)/Cd(II), Zn(II)/Cd(II)/Pb(II) or Ni(II)/Pd(II)/Ag(I). The sensing approach toward the ion has been discussed in detail along their detection limit to find the efficacy of the probe easier. Recently, scientists have been working on synthesizing novel fluorescence chemosensors with multiple-ion recognition ability to make the fluorescence sensing more effective and prolific.
Article
In the present work, thermal decomposition of ASs in air was characterized by a combination of TG-DSC, XRD, and TG-FTIR. The treatment of generated toxic (CN)2 gas was investigated as well. The result showed that the decomposition of Zn2Fe(CN)6 in ASs preferentially reacted with CuSCN leading to the early decomposition of ASs, in which a part of CuSCN decomposed into Cu5FeS4 or Cu2S followed by being oxidized to sulfates and oxides as the temperature increased to 420°C. For Zn2Fe(CN)6·3H2O in ASs, the decomposition products below 500°C include ZnS, ZnSO4, CuxFeySz, iron oxides and Zn(CN)2; instead, Fe3O4, ZnSO4 and ZnFe2O4 were formed. The FTIR and chemical quantitative analysis showed that nitrogen-containing gaseous products predominately contained (CN)2, HCN and small amounts of NH3 and NOx. In view of toxic gases released, catalytic oxidation employing in-situ generation of roasting slag at 600°C (AS1) can be effectively used for the conversion of (CN)2 to N2 under the optimal conditions of airflow rate of 0.7 L/min and AS1/ASs mass ratio of 0.5. Significantly, the ZnFe2O4 phase in AS1 completely disappeared and was converted to ZnSO4 after the experiment, which facilitated the subsequent acid leaching, thereby achieving the synergistic treatment of exhaust gases and slag.
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Cyanide compounds are hazardous compounds which are extremely toxic to living organisms, especially free cyanide in the form of hydrogen cyanide gas (HCN) and cyanide ion (CN−). These cyanide compounds are metabolic inhibitors since they can tightly bind to the metals of metalloenzymes. Anthropogenic sources contribute significantly to CN− contamination in the environment, more specifically to surface and underground waters. The treatment processes, such as chemical and physical treatment processes, have been implemented. However, these processes have drawbacks since they generate additional contaminants which further exacerbates the environmental pollution. The biological treatment techniques are mostly overlooked as an alternative to the conventional physical and chemical methods. However, the recent research has focused substantially on this method, with different reactor configurations that were proposed. However, minimal attention was given to the emerging technologies that sought to accelerate the treatment with a subsequent resource recovery from the process. Hence, this review focuses on the recent emerging tools that can be used to accelerate cyanide biodegradation. These tools include, amongst others, electro-bioremediation, anaerobic biodegradation and the use of microbial fuel cell technology. These processes were demonstrated to have the possibility of producing value-added products, such as biogas, co-factors of neurotransmitters and electricity from the treatment process.
Article
Electrical and electronic equipment waste (WEEE) problems should be set as a priority in Thailand due to unpleasant operations. E-waste mismanagement caused by illegal transport of WEEE has intensified public health and environmental concerns. Computers are processed in recycling industries, but they still contain vulnerable metal remnants, such as gold. High-grade components, such as Random-Access Module (RAM), for example, are often extracted by hydrometallurgical processes through different leaching chemicals along with their related methods. Gold leaching agents, including aqua regia and cyanide-based solutions, were considered to extract gold from computer RAM in this study. Environmental impacts of the two extracting processes were investigated using material flow analysis and life cycle assessment, based on 1 kg of recovered gold. The effect of pulp density was also measured. The comparison between the two leaching processes without waste management demonstrated that the cyanide-based solution provided 8.5 times lower in terrestrial ecotoxicity and 6.4 times lower in human carcinogenic toxicity than aqua regia due to the lower overall chemical consumption. Nevertheless, proper wastewater treatments and a combination of incineration and landfill for solid waste treatment are potential options to minimize these impacts.
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Enzyme technology has been used in industrial applications since ancient Greece. The present industrial applications of enzymes are, however, the result of a rapid development seen primarily over the past four decades thanks, first of all, to the evolution of modern biotechnology. The result is a highly diversified market that is still growing both in terms of size and complexity.
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Natural oxidation of sulphide mineral wastes during their disposal and storage at the mining sites may result in generation of acid rock drainage (ARD) that may contain high acidity and high concentrations of dissolved metals and sulphates. Due to its chemical composition, ARD poses potential harm to the environment and significant challenges to many mining industries requiring development and application of effective ARD management plans to the mining sites during the operational and post-closure periods. If the generation of ARD cannot be prevented or controlled, it must be collected and treated to eliminate acidity and reduce the concentration of heavy metals and suspended solids before its release to the environment. As a group of microorganisms may contribute to the generation of ARD, different types of microorganisms may play a vital role in the development of microbiological prevention, control and treatment technologies. The role of microorganisms in the generation of ARD and their role in the methods used for prevention, control and treatment are discussed in this paper. Benefits, limitations and design criteria for passive biological processes are presented using examples of passive and biological treatment processes that have been developed recently and implemented to mine sites.
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Cyanide is the reagent of choice for gold and silver extraction, but also a toxic chemical that may cause severe environmental pollution problems. Vascular plants possess an enzyme system that detoxifies cyanide by converting it to the amino acid asparagine. The phytotoxicity of cyanide is indirectly connected to the efficiency of this enzyme system: Plants only survive cyanide exposure up to a dosage they can metabolize. Cyanide phyto-toxicity was measured for the subtropical grass Sorghum bicolor. Potassium cyanide was not toxic when added to the irrigation water at up to 125mg KCN/l (50mg CN/l). In a degradation test, cyanide was efficiently degraded by sorghum roots and leaves. Cyanide elimination using plants seems to be a feasible option for gold and silver mine waste and wastewater. Theoretical estimates indicate that a large area of land is needed. But the process is cost effective, sustainable, and has less critical emissions than any competing technology. Until now, phytotreatment of gold mining wastewater has only been tested on a lab scale. With the current knowledge, a pilot-scale demonstration could be implemented immediately
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Summary Cadmium and zinc biosorption, alone or in combination, was investigated with sodium alginate immobilizedChlorella homosphaera cells. Concentrations ranging from 20.0 to 41.0mg/l cadmium, 75.0 and 720.0mg/l zinc were tested and, in all cases, the metal removal achieved values near 100%. When these metals were put in combination a decrease in the rate of absorption was detected. Gold was also tested in the immobilized system and 90% of the initial metal added was recovered in a solution containing 213.0mg/l of the metal, the alginic matrix being responsible for 40% gold uptake.
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Sulphate-reducing passive bioreactors have received much attention lately as alternating technologies for acid mine drainage (AMD) treatment. The long-term efficiency is sometimes limited because they rely on the activity of sulphate- reducing bacteria (SRB) which is primarily controlled by the reactive mixture composition. Our review shows that the most important mixture component is the organic carbon source. Several studies conducted to find the best mixture of natural organic substrates for SRB showed that a combination of organic carbon sources is preferable than a unique source. In addition to metal sulphide precipitation, which is the main metal removal mechanism, other mechanisms including adsorption and precipitation of metal carbonates and hydroxides occur in passive bioreactors. The more successful designs allow the flow to be reversed between upward and downward modes. Pilot and field-scale passive bioreactors filled with mixtures of organic and cellulosic wastes were installed in Canada and USA and efficiently removed sulphate and metals for periods up to 5 years. Additional work needs to be done to properly assess the long- term efficiency of reactive mixtures and the metal removal mechanisms. Furthermore, toxicity assessment of treated effluent from on-site passive bioreactors has yet to be performed on a regular basis.
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Applications of biotechnology are in use or have been proposed for almost all sectors of the mining and minerals industries for metal extraction, metal recovery, and environmental control. A recently completed study in Canada reviewed the status of biotechnological process development in different sectors of the industry and by commodity. This paper provides an overview of the findings of the study including a discussion of the sectors of the industry in which biotechnology enjoys commercial success and those for which future applications are indicated. Special emphasis is given to the commercial metal extraction processes and to applications for environmental control for which future technical and economic advantages are likely as environmental regulations become more stringent.
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Heavy metals, depending on their oxidation states, can be highly reactive and, as a consequence, toxic to most organisms. They are produced by an expanding variety of anthropogenic sources suggesting an increasingly important role for this form of pollution. The toxic effect of heavy metals appears to be related to production of reactive oxygen species (ROS) and the resulting unbalanced cellular redox status. Algae respond to heavy metals by induction of several antioxidants, including diverse enzymes such as superoxide dismutase, catalase, glutathione peroxidase and ascorbate peroxidase, and the synthesis of low molecular weight compounds such as carotenoids and glutathione. At high, or acute, levels of metal pollutants, damage to algal cells occurs because ROS levels exceed the capacity of the cell to cope. At lower, or chronic, levels algae accumulate heavy metals and can pass them on to organisms of other trophic levels such as mollusks, crustaceans, and fishes. We review here the evidence linking metal accumulation, cellular toxicity, and the generation of ROS in aquatic environments.
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The present note refers the results about the isolation of an Aspergillus fumigatus strain able to grow on an industrial cyanide waste as nitrogen source. The fungus was selected from an alkaline unpolluted soil in enrichment cultures in 50 ml of Minimal Medium added with 20 mmol glucose and supplemented initially with 0.1 mmol KCN and then with 70 μl of a waste solution from a jewelry industry containing free cyanide and cyanide complexes of heavy metal ions including copper, silver, nickel, and others. The cyanide content of the waste was 1,500 ppm. The fungal growth was monitored determining dry weight, protein content and glucose consumption. The fungus efficiently utilized the cyanide as evidenced by the decrease in the inoculated medium of the compound under detection limits within 24 h and the concomitant growth within 15 days during which periodical additions of the waste to the cultures were made. The amount of the cyanide in the biomass of the fungus grown in presence of the waste was very scarce and comparable to that in absence of the pollutant. Furthermore the fungus was able to sequestrate metals such Ag, Cu, and Ni as a resistance mechanism against heavy metals. In conclusion our results are of interest for biodegradation plans of electroplating industrial wastes containing cyanide based pollutants.
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Cladosporium cladosporioides biomass was a highly efficient biosorbent of copper cyanide and nickel cyanide from aqueous solutions. A 32–38 fold concentration of initial 0.5mM metal cyanides could be achieved when biosorption process was carried out under standardised conditions. Residual, unrecoverable metal cyanide could be completely biodegraded in 5–6h. The solution treated with the combined biosorption-biodegradation process was fit for discharge in the environment.
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Biosorbent materials are a potential alternative to conventional processes of metal recovery from industrial solutions. Algal biomass ofSargassum natans andAscophyllum nodosum outperformed ion exchange resins in sequestering respectively gold and cobalt from solutions. Non-living biomass ofSaccharomyces cerevisiae andRhizopus arrhizus exhibited higher metal-uptake capacity than the living biomass for the uptake of copper, zinc, cadmium, uranium. The solution pH affected the metal-uptake capacity of the biomass whereas the equilibrium biosorption isotherms were independent of the initial concentration of the metal in the solution. Desorption of the metal from the biosorbent and recycle of the biosorbent have also been demonstrated.
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process of asset pricing, which has developed dramatically in the last few years due to advances in financial theory and econometrics. This book covers the science of asset pricing by concentrating on the most widely used modelling technique called: Linear Factor Modelling. Linear Factor Models covers an important area for Quantitative Analysts/Investment Managers who are developing Quantitative Investment Strategies. Linear factor models (LFM) are part of modern investment processes that include asset valuation, portfolio theory and applications, linear factor models and applications, dynamic asset allocation strategies, portfolio performance measurement, risk management, international perspectives, and the use of derivatives. The book develops the building blocks for one of the most important theories of asset pricing - Linear Factor Modelling. Within this framework, we can include other asset pricing theories such as the Capital Asset Pricing Model (CAPM), arbitrage pricing theory and various pricing formulae for derivatives and option prices. As a bare minimum, the reader of this book must have a working knowledge of basic calculus, simple optimisation and elementary statistics. In particular, the reader must be comfortable with the algebraic manipulation of means, variances (and covariances) of linear combination(s) of random variables. Some topics may require a greater mathematical sophistication.
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A Central American gold mine aims to install an advanced three-phase cyanide and metals removal system comprised of high-rate ballasted flocculation technology, cyanide oxidation, and carbon adsorption. The advanced cyanide and metals removal system is being implemented at the Marlin gold mine in the western highlands of Guatemala. The proposed effluent treatment plant is expected to comply with Guatemalan and World Bank discharge standards along with another standards required by Goldcorp. Crown Solutions is working on engineering, procurement, and construction management process for the cyanide oxidation and metal removal plant. The new treatment plant is comprised of cyanide oxidation, clarification, and carbon adsorption process to reduce the cyanide levels to meet the requirement of various standards. The plant is expected to treat water from Marlin's tailings impoundment at a rate of 2,200 gal/min.
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The Handbook promotes the concepts of sustainable development by focusing attention on the benefits - both environmental and economic - of pollution prevention, including cleaner production and good management techniques. The Handbook consists of three parts. Part I contains a summary of key policy lessons in pollution management, derived from practical experience inside and outside the World Bank Group over the past decade. Although Part I is aimed primarily at government decision-makers, other readers will derive considerable benefit from a better understanding of the issues facing government agencies. Part II presents good-practice notes on implementation of policy objectives, based on experience with World Bank Group projects and on lessons from the policies and practices of other agencies and organizations in this field. Part III provides detailed guidelines to be applied in the preparation of World Bank Group projects. The guidelines, which cover almost 40 industrial sectors, represent state- of-the-art thinking on how to reduce pollution emissions from the production process. In many cases, the guide-lines provide numerical targets for reducing pollution, as well as maximum emissions levels that are normally achievable through a combination of cleaner production and end-of-pipe treatment. The guidelines are designed to protect human health; reduce mass loadings to the environment; draw on commercially proven technologies; be cost-effective; follow current regulatory trends; and promote good industrial practices, which offer greater productivity and increased energy efficiency. The application of the guidelines set out in Part III can minimize the use of resources and reduce the quantity of wastes requiring treatment and disposal. The guidelines represent good environmental management practices that can be implemented and maintained with the skills and resources typically available in countries in which the World Bank Group operates. The World Bank Group is committed to strengthening management and technical skills and to supporting the development of the necessary institutions in these countries. Where relevant national regulations do not exist, the guidelines may provide a basis for negotiating site-specific agreements between regulators and enterprises.
Article
Cyanidation processes for the extraction of gold or silver frequently involve the processing of sulfur-containing solids that react with cyanide to form thiocyanate. The formation of thiocyanate is an undesired side reaction, and in many cases thiocyanate formation results in a prohibitively high cyanide consumption. With the increasing number of gold and silver mining projects that involve the processing of sulfide ores or concentrates or that utilize a biological sulfur oxidation process, more attention is being focused on minimizing thiocyanate formation and in treating solutions to eliminate toxicity associated with thiocyanate. Though not yet commercially applied, there are several processes available to recover cyanide from thiocyanate to reduce tailings toxicity and to allow the reuse of cyanide otherwise lost to thiocyanate formation. This paper presents an overview of processes available for recovering cyanide from thiocyanate along with estimated operating costs for the processes. Pilot testing data suggests that in some cases cyanide can be recovered from thiocyanate for approximately the same cost as newly purchased cyanide.
Article
During operations, mining and metallurgical processes may generate effluents such as tailings water, acid mine drainage (AMD) and seepage and process acid streams. Depending on the type of ore and the metallurgical process, these effluents may contain one or more toxic compounds (eg acidity or alkalinity, cyanide, ammonia and/or nitrate, heavy metals, total suspended solids (TSS), sulfate) in elevated concentrations requiring treatment before their discharge to the environment or recycle/reuse in the process. Natural oxidation of sulfide minerals present in mining wastes (eg tailings and waste rock) at mining sites may generate AMD, which is characterised as a low pH, high acidity effluent containing heavy metals and sulfate. If generation of AMD cannot be controlled and/or prevented, AMD needs to be collected and treated for neutralisation of acidity and reduction of metals and TSS to meet regulated water quality standards. It is important to note that some of these compounds may persist in run-off from mine sites, even after the mine and processing facilities are decommissioned. Treatment of mining and metallurgical process effluents can be accomplished by means of various physical, chemical and/or biological methods. The mode of process application may vary from the use of either specifically designed, controlled and automated facilities or passive systems. Lime neutralisation and precipitation is the most common method used in the mining industry to treat AMD. To reduce the problems associated with disposal and long-term storage of the resulting sludge, the use of a high-density sludge process (HDS) has become a preferred option. The use of other chemical reagents, waste or by-products from other industries, and biological sulfate reduction methods can also be considered as viable options for site-specific situations. Recently, several passive treatment systems have been designed and successfully operated, even in the cold North American climate. This paper will discuss available options and provide insights for selecting a suitable method for a given situation using case studies from projects conducted by Golder Associates around the world over the last decade.
Article
Chemical replacements for cyanide have been investigated for decades; however cyanide remains the exclusive lixiviant of choice in the mining industry due to a combination of its availability, effectiveness, economics and ability to use it with acceptable risk to humans and the environment. About 90% of the significant gold producing operations worldwide currently utilize cyanide for gold and silver extraction. Despite the number of cyanide-related mining operations, there have been no documented accounts during the previous three decades of the death of humans due to cyanide as a direct consequence of major mining-related environmental incidents. Major mining-related environmental incidents have not been concentrated in any geographic location, may occur regardless of the size of the company and do not occur more frequently with a specific type of mining activity. The main aspects of cyanide management that should be addressed at mining operations include transportation of cyanide to site, process solution conveyance, worker health and safety training, water management and treatment, emergency response and preparedness, workplace and environmental monitoring, and community relations. If these aspects of cyanide management are integrated into an overall cyanide management plan, dramatic reductions in risk and potential incidents at mine sites will be realized.
Article
Cyanide (CN -) is a toxic species that is found predominantly in industrial effluents generated by metallurgical operations. Cyanide's strong affinity for metals makes it favorable as an agent for metal finishing and treatment and as a lixivant for metal leaching, particularly gold. These technologies are environmentally sound but require safeguards to prevent accidental spills from contaminating soils as well as surface and ground waters. Various methods of cyanide remediation by separation and oxidation are therefore reviewed. Reaction mechanisms are given throughout. The methods are compared in regard to their effectiveness in treating various cyanide species: free cyanide, thiocyanate, weak-acid dissociables and strong-acid dissociables.
Article
Cyanidation tailings which are disposed in a surface impoundment experience a loss of cyanide due to natural attenuation, frequently reducing the cyanide concentration to very low levels. Quantifying cyanide losses in terms of impoundment geometry, local weather conditions and feed solution chemistry has been largely empirical though in many cases mining operations rely on surface impoundments to reduce cyanide to below an internally regulated concentration or below an effluent limitation. To permit a more quantitative evaluation of cyanide losses in an impoundment, a computer simulation was developed to estimate the losses of free, weak acid dissociable (WAD) and total cyanide due to dissociation, photolysis and volatilization. Results of the model are compared against data collected for a North American tailings impoundment during 1998.
Article
The potential of sewage sludge for decomposition of cyanide has been investigated at different temperatures, ratio sewage sludge to cyanide, pH and also at prolonged and at four cycle repeated processes. Along with the kinetics of cyanide decomposition, the consumption of reagents necessary to maintain pH of the biosystem and the releasing of volatile cyanide have been examined. The positive effect of the activation of sewage sludge by means of aeration and its correlation to the kinetics of the bacterial growth have been also studied. Along with aeration, “carrier biology” has been employed to improve the characteristics of sewage sludge, using wood peels as a carrier material.
Article
This work is devoted to the removal of free cyanide from aqueous solution by oxidation with hydrogen peroxide H2O2 catalyzed by copper-impregnated activated carbon. Effects of initial molar ratio [H2O2]0/[CN−]0, copper-impregnated activated carbon amount, pH and the temperature on cyanide removal have been investigated.The presence of copper-impregnated activated carbon has increased the reaction rate showing thus a catalytic activity. The rate of cyanides removal increases with the raise of the initial molar ratio [H2O2]0/[CN−]0 and decreases with the increase in the pH from 8 to 12. The increase in the copper-impregnated activated carbon amount from 1.5 to 10g/L in reaction solution has a beneficial effect. Beyond this value, the impact of activated carbon amount is not anymore significant. The temperature does not have a significant effect between 20 and 35°C. The four successive times re-use of catalyst shows a good stability. The kinetics of cyanide removal has been found to be of pseudo-second-order with respect to cyanide and the rate constants have been determined. This process seems very interesting because the rate of cyanides removal is very fast, the reaction does not use soluble metal catalyst and it consumes only hydrogen peroxide as chemical product.
Article
Results of a bench scale study of the behaviour of cyanide in soil and groundwater are reported and discussed. Effects of chlorination on cyanide were examined and retention capacity of soil was measured. Decay reates for cyanide were determined.
Article
Experiments were carried out to develop the technological basis for the integrated biodegradation of cyanide and formamide. The ability of both Fusarium oxysporum CCMI 876 and Methylobacterium sp. RXM CCMI 908 to transform cyanide and formamide was analysed and characterised. The Kmi for cyanide hydratase in the immobilised Fusarium oxysporum CCMI 876 was 19±4mM of cyanide and the Vmaxi was 21±5μmolmin−1g−1 (DW) cells. For the entrapped Methylobacterium sp. RXM CCMI 908 the values of the apparent reaction kinetics were Kmi=1.7±0.3mM of cyanide and the Vmaxi=20±2μmolmin−1g−1 (DW) cells. These data were used to design and operate a two-catalyst system to balance cyanide and formamide transformations and assess the system stability. The cyanide was degraded by Fusarium oxysporum CCMI 876 at a rate of 0.059mMh−1 leading to 96% cyanide conversion and leaving a residual 0.21mM. Average 3.76mM of formamide and 0.34mM of formate were formed. This effluent is contacted with Methylobacterium sp. RXM CCMI 908 which used 84% of formamide. Then this compound drops to an average value of 0.62mM. In parallel, formate builds up to nearly 12-fold the initial concentration, correlating with the amount of formamide used. The two systems were merged in a single system which design involved a scale-up criterion and the degradation profiles along the system were determined.
Article
Former gasworks sites are sometimes be heavily contaminated with spent oxide which contains cyanide complexed to metals (especially iron). In this study, mixed fungal cultures have been isolated from acidic gasworks soil by their ability to utilize iron or nickel cyanide as the sole source of nitrogen at acidic or neutral pH, respectively. A mixed culture comprising Fusarium solani and Trichoderma polysporum was obtained by enrichment on tetracyanonickelate [K2Ni(CN)4] at pH 4. A second mixed culture consisting of Fusarium oxysporum, Scytalidium thermophilum, and Penicillium miczynski was isolated on hexacyanoferrate [K4Fe(CN)6] also at pH 4. Both consortia were able to grow on K4Fe(CN)6 as the sole source of nitrogen under acidic conditions. Growth was associated with progressive removal of cyanide from the culture supernatant. After the termination of growth, at least 50% of the total cyanide had been degraded. Growth of the fungi on K2Ni(14CN)4 as a source of nitrogen at pH 7 yielded 14C-labelled carbon dioxide. Growth of the Fusarium isolates on K2Ni(CN)4 at pH 7, associated with the removal of cyanide, required 5 days as compared to 28 days on K4Fe(CN)6 at pH 4. Cyanide uptake by the fungi on K4Fe(CN)6 at pH 4 occurred simultaneously with removal of iron from the biomass-free medium. Pure cultures of F. solani and F. oxysporum were grown on K2Ni(CN)4 or K4Fe(CN)6 in pure culture at pH 7 or 4, respectively.
Article
Cyanide is used extensively in the electroplating and mining industries due to its strong affinity to metal cations. Commercial granular activated carbon (GAC) was used as adsorbent for the adsorptive study of sodium, zinc and iron, cyanide complexes in the present study. The effect of process parameters such as pH, temperature, adsorbent size and dose, contact time on the performance of adsorption was investigated. Optimum pH was found to be 9, 7 and 5 for sodium, zinc and iron cyanides respectively. In the higher temperature range more percentage removal was observed for iron cyanides, whereas, sodium and zinc cyanides were removed optimally at 25–35°C. Although particle size did not show any major influence on the percentage removal, but optimum size was taken as 2–4mm. The percentage removal of cyanide compounds increased with the increase in adsorbent (GAC) concentration. However, specific uptake did not increase at GAC concentration above 20–25g/L. Hence 20g/L was considered as the optimum dose of adsorbent. Higher removal efficiency was achieved for metal cyanides as compared to sodium cyanide at optimal conditions.
Article
Water-soluble iron cyanide compounds are widely used as anticaking agents in road salt, which creates potential contamination of surface and groundwater with these compounds when the salt dissolves and is washed off roads in runoff. This paper presents a summary of available information on iron cyanide use in road salt and its potential effects on water quality. Also, estimates of total cyanide concentrations in snow-melt runoff from roadways are presented as simple mass-balance calculations. Although available information does not indicate a widespread problem, it also is clear that the water-quality effects of cyanide in road salt have not been examined much. Considering the large, and increasing, volume of road salt used for deicing, studies are needed to determine levels of total and free cyanide in surface and groundwater adjacent to salt storage facilities and along roads with open drainage ditches. Results could be combined with current knowledge of the fate and transport of cyanide to assess water-quality effects of iron cyanide anticaking agents used in road salt.
Article
Biological treatment of a synthetic leachate containing cyanide was accomplished in a sequencing batch biofilm reactor (SBBR). A mixed culture of organisms growing on silicone tubing were provided with cyanide as a sole carbon and nitrogen source. Organisms consumed cyanide (20 mg/liter CN−WAD) and produced ammonia in an approximate 1:1 molar yield. The SBBR was operated on a 24-h cycle. Over the course of each cycle, 20 mg/liter of cyanide was degraded to below 0.5 mg/liter. Results from four track studies are presented. It was demonstrated that, when supplied with glucose, the organisms would readily consume excess ammonia. For each mole of glucose added, 10 moles of NH3-N were removed from solution. The SBBR can be used as a mobile system for treatment of leachate from gold-mining operations. Large volumes of low concentration wastewater can be treated in the SBBR since it is not necessary to maintain a consortium of settling organisms. © 1998 Elsevier Science Ltd. All rights reserved
Chapter
There are several water and tailings treatment processes that have been successfully used worldwide for cyanide removal at mining operations. The key to successful implementation of these processes involves consideration of the following:•Site water and cyanide balances under both average and extreme climate conditions.•Goals to be adopted for cyanide levels in treated effluent, including the form of cyanide to be regulated (free vs. WAD vs. total cyanide).•The range of cyanide treatment processes available and their ability to be used individually or in combination to achieve treatment objectives.•Proper treatability testing, design, construction, maintenance and monitoring of both water- and cyanide-management facilities.By carefully considering these aspects of water and cyanide management before, during and after mine operation, operators can reduce the potential for environmental impacts associated with the use of cyanide. Another aspect of cyanide treatment to be considered is the potential environmental impact of the cyanide-related compounds - cyanate, thiocyanate, ammonia, nitrate and nitrite. These compounds may be present in mining solutions to varying extents and may require treatment if water is to be discharged. Each of these cyanide-related compounds is affected differently in the treatment processes discussed, and this should be considered when evaluating cyanide-treatment alternatives for a given site. Table 13 provides a simplified summary of the general applications of various treatment technologies for the removal of iron cyanide and WAD cyanide. This table represents a very simplified summary, but can be used as a conceptual screening tool when evaluating cyanide-treatment processes.
Article
The International Cyanide Management Code was developed to improve the management of cyanide at gold mines. Spills and other incidents involving cyanide solutions at gold mines such as the January 2000 incident at a Romanian gold mine (Baia Mare) demonstrated to the gold mining industry, governments, and the public that better management of cyanide was needed, particularly at operations with limited experience or in countries lacking adequate regulatory programs. In 2001, the Code was underwritten by a group of gold companies and cyanide producers from around the world. The Gold Institute was instrumental in organizing this financial and technical support and provided the administrative and logistical support necessary to successfully complete the project. This effort represents one of practical action that an industry has worked with mining companies and producers including UNEP, World Banks, and Environmental Groups to develop an international voluntary industry Code of Practice.The regional and national environmental management criteria of cyanide are different in various countries. Thus, a common language should be applied as a global perspective which is provided by the Cyanide Code Management. In this review article, a general and brief introduction on Cyanide Code Management is presented.
Article
Tests were conducted at the Ryan Lode Mine near Fairbanks, Alaska, to determine the comparative costs of chemical and biological destruction of cyanide in mine wastewater. The main body of pond and rinse water was treated by the patented, INCO Air-SO2 process. A 250 ton test heap was built and inoculated with a cyanide-reducing bacterium Pseudomonas pseudoalcaligenes (UA7). The capital and operating costs for both processes were carefully recorded during the treatment. These costs were used as the basis for an analysis of the comparative costs of rinsing and detoxifying a hypothetical, two-million ton heap using each method. Four scenarios were analyzed. The biological method had a higher capital cost, but a significantly lower operating cost, so that the present-worth cost was significantly lower for the biological method.
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The presence of cyanide in industrial effluent waste presents a major environmental and ecological hazard. Although chemical methods of treating this compound are known, bacterial detoxification of cyanide is of interest both in order to understand how cyanide may be dealt with in the environment and to evaluate the economic viability of bacterial systems for cyanide detoxification. The enzyme rhodanese, which catalyzes the formation of thiocyanate and sulfite from cyanide and thiosulfate, has been found in various organisms including Bacillus subtilis and E. coli. Thiobacillus denitrificans was shown to have the highest levels of this enzyme, but growth conditions in continuous culture on defined media have recently been developed for the production of equally high rhodanese levels in the thermophile Bacillus stearothermophilus. Purified rhodanese from this latter organism has already proved to be of value as an antidote in experimental cyanide poisoning in small mammals. This communication reports on the use of a culture of B. stearothermophilus in a small chemical reactor for the continuous removal of cyanide in the form of thiocyanate. The capacity of B. stearothermophilus to remove cyanide in the form of thiocyanate in the process described is high (5 to 8 g NaCN/l culture/hr at 27°C); furthermore, both the rate of cyanide removal and the half life of the process were unaffected by the presence of 5x10-5M Zn2+, Cu2+, Ni2+, or Al3+ over a 12 day period. By running the process at temperatures at which B. stearothermophilus is capable of growth in normal media (i.e. above 35°C) higher rates of cyanide detoxification are possible (14 to 25 g NaCN/l culture/hr at 50°C), although preliminary evidence indicates a reduction in half life at higher temperature.
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Cyanide compounds are widely used in gold ore processing plants in order to facilitate the extraction and subsequent concentration of the precious metal. Owing to the high cyanide concentrations employed in gold processing, effluents generated have high contents of free cyanide as well as metallic cyanide complexes, which lend them a high degree of toxicity. The process under study, developed in laboratory scale with the use of a distillation apparatus, consists of highly decreasing the pH of the solution by adding sulfuric acid. Thus, the cyanide present in either free form or as a metallic complex is made volatile and the resulting cyanide gas is absorbed in an alkaline solution for reutilization. This work aims at recognizing the chemical relations between the cyanide and metals during distillation. The regeneration of cyanide from gold processing proved to be a viable procedure. Cyanide recoveries pointed to the fact that if a method for reutilization of cyanide contained in mining effluents is employed, the precious metal processing will become more efficient. Also, the environmental conditions in the area of the operation will be improved.