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Sequential bioreduction - bioleaching and bioreduction - chemical leaching hybrid tests for enhanced copper recovery from a concentrator ball mill reject sample
Abstract
Dumping of poor but metal containing industrial waste is associated with several environmental issues. Exposure of these wastes to the natural environment offers serious concerns for the mineral processing industries to utilize them for metal recovery and check environmental pollution. In the present study, a novel sequential bioreduction-bioleaching and bioreduction-chemical leaching route as a hybrid process is compared and discussed for the enhanced recovery of copper from an industrial concentrator plant ball milling unit rejected sample. A mixed consortium of metal reducing bacteria (DMRB) initially adapted to high Fe(III) concentrations was found to cause mineralogical/matrix alteration (possibly silicate weathering) including Fe(III) bioreduction in the sample and dissolute 29.73% copper during the first 35 days under facultative anaerobic conditions. Sequential leaching of the bioreduced waste sample (generated from the first step) using a mixed meso-acidophilic bacterial consortium predominantly Acidithiobacillus ferrooxidans showed additional 28.72% copper dissolution within 2 days using 1 gL- 1 Fe(II). On the other hand, a comparative chemical leaching of the same bioreduced sample using 0.5 M H2SO4 yielded additional 32.17% copper within 4 days of leaching and indicated better performance than the bioleaching tests.
... In addition, the reported potentialities of occurred microbial community for toxic metal(loid)s bioremediation were summarized in Table S6. The main functions of microbes for promoting toxic metal (loid)s leach and mobility were sulfide oxidation, iron-manganese oxide reduction, arsenic reduction, siderophores, biosurfactant and organic acid-promoted dissolution (Din et al., 2020;Osman et al., 2019;Panda et al., 2015;Vera et al., 2013;Juwarkar et al., 2008;Makita et al., 2004). Besides, microbial function including sulfate reduction, oxidation of iron and manganese, As oxidation and biomineralization mediated by phosphatase and urease could all promote the immobilization of toxic metal(loid)s (Zeng et al., 2021;Jiang et al., 2019b;Jong and Parry, 2005;Gadd, 2000). ...
Over the past few decades, nonferrous mining has produced numerous waste rock and part of the waste that has not been properly treated was generally dumped at roadsides and hill slopes. However, the vertical distributions of toxic metal(loid)s and composition of microbial communities in waste heap and the under-laid pristine soil are rarely studied. In this work, the fraction-related distributions of toxic metal(loid)s were investigated at a waste heap profile and the indigenous microbial assemblages were also analyzed by Illumina sequencing of 16 s rRNA genes. Results showed that compared to the under-laid pristine soil, content of toxic metal(loid)s, especially Cd, As and Pb, in waste rock layer were higher. Most of As in subsoil existed as non-specifically sorbed and specifically-sorbed fractions, which could be ascribed to the migration from the upper layer. The mobility was significantly correlated with Eh, EC, clay content, CEC and the total content of metal(loid)s. Phyla Proteobacteria, Acidobacteria and Firmicutes dominated the microbial communities. The microbial community compositions at the genus level were similar, but their relative abundances were mainly influenced by pH, CEC, Eh, SOM, and bioavailability content of toxic metal(loid)s. Besides, microbial functions of elements (S, Fe, Mn and As) oxidation/reduction and metabolites (siderophore, biosurfactant, organic acid, phosphatase and urease) potentially were used for pollutants bioremediation.
... Cases today, for example, focus on the utilization of copper (Cu) and/or zinc (Zn) resources to meet and balance the ever-growing demand and supply of the respective metals (Watling 2014). A lot of advanced studies are being available for utilization of copper ore reserves with many commercial plants existing today to process the oxide, sulfide, and mixed copper ores (Panda et al. 2015;Brierley and Brierley 2009;Domic 2007). However, it becomes very essential, on the other hand, to look at the zinc resources which are at the borderline of getting depleted as per the recent predictions. ...
In this study, the effect of graphite on sphalerite dissolution was investigated using a Leptospirillum ferrooxidans (L. ferrooxidans) dominated meso-acidophilic bacterial consortium. Results of the studies indicated that Zn dissolution enhanced from 18.75% (in control tests) to 78.46% (using L. ferrooxidans) without graphite in the system (the conventional approach). Addition of graphite further improved sphalerite bio-oxidation up to 10% i.e. from 78.46 to 87.83% indicating bio-catalytic effect and a possible galvanic interaction between sphalerite and graphite. Through the conventional route, the system’s pH eventually lowered from 1.8 to 1.4 and ORP increased from 300 to 700 mV. In presence of graphite, the pH lowered to nearly 1.0 and ORP reached 600 mV. This indicated efficient bacterial activity and stabilized leaching conditions in both the cases. Analytical characterization studies through XRD and SEM-EDAX indicated the presence of sulfur and K-type jarosite (KFe3+3(OH)6(SO4)2), ascribed to the passivation layers formed during sphalerite bioleaching.
... Acid mine drainage (AMD) can be the major source of various heavy metals. Ferric hydroxide precipitation enables selective separation of other metals present in the solution [3,4]. Such opportunities offer an industry implemented BioSulphide ® process [5]. ...
Reductive dissolution is like “biomining in reverse gear”. Selected sulfur reducing bacteria (SRB) can be used under anaerobic conditions – as in bioleaching – for metal recovery from oxide minerals and laterite. General assumptions of the BioSulphide ® and Ferredox processes for nickel recovery from laterite ores and copper from acid mine drainage or waste water were presented.
... Pant et al. (2014) used a combination of Serratia plymuthica and EDTA for metal bioleaching from CRT waste. Panda et al. (2015) estimated that a combination of bio-reduction and chemical leaching can extract about 61.9% of copper metal after 39 days-and suggested that this approach is feasible for treatment. Qu and Lian (2013) found that, citric and oxalic acids were the main lixiviants for bioleaching of rare earth elements using Penicillium tricolor. ...
Most waste electrical and electronic equipment (WEEE or E-waste) is recycle in the informal sector, even though—many more sophisticated recycling technologies are available. Mechanical recycling, usually the first step, can achieve maximum metal extraction, however has direct or indirect health effects, higher capital costs, and results in the loss of numerous secondary metal resources. The second most commonly used approach is the chemical method, which is much faster than the biological one, but chemical hydrometallurgy is metal-specific and also causes secondary pollution. The third approach—microbial metal recovery—is an eco-friendly and promising technology for WEEE processing. This comprehensive review evaluates and provides details of recent trends and developments in various technologies, including: physical, chemical and biological methods for the recycling and recovery of secondary resource materials, such as valuable metals. Although no single approach by itself is adequate, and as it cannot achieve ecofriendly high extraction rates of metals from WEEE, it is possible to develop a combined process, such as the initial use of mechanical treatment followed by leaching with either easily biodegradable reagents or organic acids produced by microbes—this would be more ecofriendly and economically feasible. Therefore, this article highlights specific gaps in the available technology for solving the e-waste issue, and recommends a hybrid strategy as the best available approach.
... It is important to note that Acidithiobacillus ferrooxidans, an iron and sulfuroxidizing microbe belonging to the c-proteobacteria group, is the well studied microorganism ( [35] and references therein). Acidithiobacillus ferrooxidans along with Leptospirillum ferrooxidans and Acidithiobacillus thiooxidans constitute a remarkably important consortium for metal sulfide oxidation and in particular finds a number of applications for bioleaching of metals, biodesulfurization of coal, etc., [3,[31][32][33][36][37][38][39][40][41]. The chemical activity of biogenic Fe ?3 also contribute towards acid generation. ...
Acid mine drainage (AMD) is recognised as a serious and global environmental problem. The major producer of these toxic effluents is the mining industry. Owing to the severe effects of these effluents, their prevention and treatment have been a primary focus of research over several decades. The problems have invited the attention of a large group of researchers, governmental bodies, educational and research establishments, mining industries, general public and environmental specialists. A preferable option is to prevent the formation and movement of AMD from its source of origin; however, it is not possible in many locations. It, therefore, becomes essential to collect and treat AMD to which a number of treatment techniques are available. Despite the extreme environmental conditions, several communities of autotrophic and heterotrophic bacteria and archaea are seen to flourish that mainly drive the rate of release of sulfur and toxic metals into the environment. The present review briefly discusses the cause and occurrence of AMD and the microbial diversity observed in such ecosystems. In addition, the bioremediation options are briefly presented with a discussion on the role of sulfidogenic biosystems in the bioremediation of the AMD.
... Bioleaching is another green alternative to extract metals from electronic and industrial waste and several authors have reported significant extraction efficiency using bioleaching methods (Bas et al., 2013;Willner and Fornalczyk, 2013;Madrigal-Arias et al., 2015;Panda et al., 2015a). Sequential bioreduction-bioleaching and bioreduction-chemical leaching hybrid experiments were performed to recover copper from an industrial waste (Panda et al., 2015b). Recovery of copper was observed to be higher in case of chemical leaching methods, still eco-friendly attribute substantiates the applicability of bioleaching methods. ...
Technology innovations resulted into a major move from agricultural to industrial economy in last few decades. Consequently, generation of waste electronic and electrical equipments (WEEE) has been increased at a significant rate. WEEE contain large amount of precious and heavy metals and therefore, can be considered a potential secondary resource to overcome the scarcity of metals. Also, presence of these metals may affect the ecosystem due to lack of adequate management of WEEE. Building upon our previous experimental investigations for metal extraction from spent catalyst, present study explores the concept of Green Technology for WEEE management. Efforts have been made to recover base metal from a printed circuit board using eco-friendly chelation technology and results were compared with the conventional acid leaching method. 83.8% recovery of copper metal was achieved using chelation technology whereas only 27% could be recovered using acid leaching method in absence of any oxidant at optimum reaction conditions. Various characterization studies (energy dispersive X-ray analysis, scanning electron microscopy, X-ray Diffraction, inductive coupled plasma spectrophotometry) of printed circuit board (PCB) and residues were performed for qualitative and quantitative analysis of samples. Significant metal extraction, more than 96% recovery of chelating agent, recycling of reactant in next chelation cycle and nearly zero discharge to the environment are the major advantages of the proposed green process which articulate the transcendency of chelation technology over other conventional approaches. Kinetic investigation suggests diffusion controlled process as the rate determining step for the chelate assisted recovery of copper from WEEE with activation energy of 22kJ/mol.
Developed economies such as the USA and European Union (EU) have classified antimony as a critical raw material. China leads the global antimony production (67% on an average from 2015 to 2019) followed by Russia and Tajikistan. Antimony has been applied in the industry (plastics, etc.) and new/emerging technologies (cell panels, infrared, etc.) where antimony trioxide (Sb2O3) is its most produced and used compound. With technological advances, recent trends indicate a growing demand for this metal; however, with the on-going production rate, antimony is anticipated to become one of the scarcest metals by 2050. Several minerals of antimony exist; nevertheless, stibnite (Sb2S3) is the primary mineral. Extractive metallurgical routes such as pyro and hydro-metallurgy have found industrial applications for stibnite processing; however, bio-hydrometallurgy is slowly gaining momentum. In this piece of review, the world-wide scenario of antimony production, recent market trends along with the common and current research advances related to applications of antimony in emerging technologies is presented. Comprehensive details along with the recent advances related to stibnite processing through the aforementioned extractive metallurgy routes, their technological improvements and antimony purification/recovery methods from leach solutions are also discussed. Furthermore, the future perspectives in terms of research and industrial needs are discussed and summarized.
Large amounts of waste electric and electronic equipment (WEEE) has been produced due to continuous technological innovation and the improvement of living standards. Based on the rich metal compositions, waste printed circuit boards (WPCBs) are considered as the most valuable component among WEEE. The recycling of metals from WPCBs with inappropriate methods or in informal sectors has been a profitable business, which, however, wastes resources and is harmful to environment and human health. Proper methods that are eco-friendly to recover metals from WPCBs are therefore urgent and essential for both industries and policy makers. In this article, the technology details of metal recycling processes are reviewed based on past research efforts to provide a complete image of the current status. The advantages and problems of several conventional methods are summarized and compared according to cost, environmental impact, and recovery efficiency. Possible approaches for improving mechanical treatment and bioleaching methods, are accordingly analyzed. Novel hybrid processes, such as chelation technology & bioleaching process, green adsorption & chemical leaching process, etc., are put forward to open up possibilities of multi-technology coupling processes for metal recycling from WPCBs.
Pyrite ash, a waste by-product formed during roasting of pyrite ores, is a good source of valuable metals. The waste is associated with several environmental issues due to its dumping in sea and/or land filling. Although several other management practices are available for its utilization, the waste still awaits and calls for an eco-friendly biotechnological application for metal recovery. In the present study, chemolithotrophic meso-acidophilic iron and sulphur oxidisers were evaluated for the first time towards simultaneous mutli-metal recovery from pyrite ash. XRD and XRF analysis indicated higher amount of Hematite (Fe2O3) in the sample. ICP–OES analysis indicated concentrations of Cu > Zn > Co> As that were considered for bioleaching. Optimization studies indicated Cu −95%, Co −97%, Zn − 78% and As −60% recovery within 8 days at 10% pulp density, pH −1.75, 10% (v/v) inoculum and 9 g/L Fe²⁺. The productivity of the bioleaching system was found to be Cu −1696 ppm/d (12% dissolution/d), Co −338 ppm/d (12.2% dissolution/d), Zn −576 ppm/d (9.8% dissolution/d) and As −75 ppm/d (7.5% dissolution/d). Synergistic actions for Fe²⁺ −S° oxidation by iron and sulphur oxidisers were identified as the key drivers for enhanced metal dissolution from pyrite ash sample.
Multiplicity of the chemical, biological, electrochemical and operational variables and nonlinear behavior of metal extraction in bioleaching environments complicate the mathematical modeling of these systems. This research was done to predict copper and iron recovery from a copper flotation concentrate in a stirred tank bioreactor using a fuzzy logic model. Experiments were carried out in the presence of a mixed culture of mesophilic bacteria at 35˚C, and a mixed culture of moderately thermophilic bacteria at 50˚C. Input variables were method of operation (bioleaching or electrobioleaching), the type of bacteria and time (day), while the recoveries of copper and iron were the outputs. A relationship was developed between stated inputs and the outputs by means of “if-then” rules. The resulting fuzzy model showed a satisfactory prediction of the copper and iron extraction and had a good correlation of experimental data with R-squared more than 0.97. The results of this study suggested that fuzzy logic provided a powerful and reliable tool for predicting the nonlinear and time variant bioleaching processes.
Aluminum toxicity is widely considered as the most important limiting factor for plants growing in acid sulfate soils. A study was conducted in laboratory and in field to ameliorate Al toxicity using plant growth promoting bacteria (PGPB), ground magnesium limestone (GML) and ground basalt. Five-day-old rice seedlings were inoculated by Bacillus sp., Stenotrophomonas maltophila, Burkholderia thailandensis and Burkholderia seminalis and grown for 21 days in Hoagland solution (pH 4.0) at various Al concentrations (0, 50 and 100 μM). Toxicity symptoms in root and leaf were studied using scanning electron microscope. In the field, biofertilizer (PGPB), GML and basalt were applied (4 t·ha-1 each). Results showed that Al severely affected the growth of rice. At high concentrations, the root surface was ruptured, leading to cell collapse; however, no damages were observed in the PGPB inoculated seedlings. After 21 days of inoculation, solution pH increased to >6.0, while the control treatment remained same. Field study showed that the highest rice growth and yield were obtained in the bio-fertilizer and GML treatments. This study showed that Al toxicity was reduced by PGPB via production of organic acids that were able to chelate the Al and the production of polysaccharides that increased solution pH. The release of phytohormones further enhanced rice growth that resulted in yield increase.
An exopolysaccharide (EPS) reaching a maximum of 13 g/L was isolated from Micrococcusluteus by ethanol precipitation. The crude EPS was purified by chromatography on DEAE-cellulose and Sephacryl S-200, affording a polysaccharide active fraction (AEP) with a molecular weight of ∼137 kDa. AEP was investigated by a combination of chemical and chromatographic methods including FTIR, HPLC, periodate oxidation, methylation and GC–MS. Data obtained indicated that AEP was composed of mannose, arabinose, glucose and glucuronic acid in a molar ratio of 3.6:2.7:2.1:1.0, respectively. The main backbone consists of mannose units linked with (1→6)-glycosidic bonds and arabinose units linked with (1→5)-glycosidic bonds. There is a side chain consisting of mannose units linked with (1→6)-glycosidic bonds at C3, when all glucose and most of glucuronic acid are found in the side chain. The in vitro antioxidant assay showed that AEP possesses DPPH radical-scavenging activity, with an EC50 value of 180 μg/mL.
The effect of an adapted Dissimilatory Iron Reducing bacterial consortium (DIRB) towards bio-reduction of Sukinda Chromite overburden (COB) with enhanced recovery of nickel and cobalt is being reported for the first time. The remarkable ability of DIRB to utilize Fe (III) as terminal electron acceptor reducing it to Fe (II) proved beneficial for treatment of COB as compared to previous reports for nickel leaching. XRD studies showed goethite as the major iron-bearing phase in COB. Under facultative anaerobic conditions, goethite was reduced to hematite and magnetite with the exposure of nickel oxide. FESEM studies showed DIRB to be associated with COB through biofilm formation with secondary mineral precipitates of magnetite deposited as tiny globular clusters on the extra polymeric substances. The morphological and mineralogical changes in COB, post DIRB application, yielded a maximum of 68.5% Nickel and 80.98% Cobalt in 10 days using 8M H2SO4.
A Micrococcus strain occurring frequently and isolated multiple times from stones was studied for its ability to adhere to substrates and to produce exopolysaccharides or extracellular polymeric substances, one of the essential prerequisites for biofilm formation. Measurements of the biodeterioration activity of this strain in terms of (i) pH drop, (ii) organic acid production, and (iii) weight loss of marble slabs demonstrated the highly aggressive biodeteriorating potential of the strain in question.
The majority of the bacteria tested were able to reduce both ferric iron and nitrate. However, ferric iron may be reduced by bacteria which do not possess a nitrate reductase and, conversely, the possession of a nitrate reductase does not confer the ability to reduce ferric iron. A glucose-fermenting Vibrio and an anaerobic lithotroph were tested for their ability to reduce a greater variety of metals. The Vibrio reduced the wider range of metals and to a greater degree. Lithotrophic bacteria not only reduce a more restricted range of metals, they also appear not to possess the capacity of fermentative bacteria to use metals as minor electron sinks.
Thermophiles have been shown to be the only micro-organisms to leach chalcopyrite successfully. Heap leaching may be a feasible alternative to conventional bio-reactors, providing a high temperature environment can be maintained within the heap without external heating.In the present study thermophilic heap leaching of a chalcopyrite concentrate coated onto inert support rocks (the GEOCOAT™ process) was studied in sets of small heated columns. The temperature was gradually increased to 70 °C, while successively introducing various mesophile and thermophile cultures. Individual columns were dismantled after progressively longer leach periods and the residual concentrates analysed. Copper extractions in excess of 90% were achieved within 100 days.On the basis of head and residue analyses the rate of reaction heat generated was calculated. A comprehensive heap heat conservation model was used to determine whether the experimental temperatures can be achieved and maintained in a full scale heap. Results indicate that operating hot heaps successfully is possible within a certain range of process parameters.
Geochemical characteristics and the increase in the total reactive surface area of the sulphide-bearing rock due to weathering processes at dumping area, are expected to enhance the oxidation of sulphides, leading to the generation of acid mine drainage (AMD). The study of the changes of geochemical characteristics of sulphide-bearing rocks showed that the oxidation uniformly occurred at the surface layer of 2- and 10-year-old waste rock dumps. The changes of mineralogical composition were also investigated, particularly for the cases where significant formation of clay minerals occurred. The study showed that the formation of clay minerals after the weathering processes took place at the dumping area, suggested that the generation of AMD would be minimised because the interior of the waste rock dump provided a barrier to oxygen and water.
The phosphate solubilizing characteristics of a strain YC, which was isolated from phosphate mines (Hubei, China), were studied
in National Botanical Research Institute’s phosphate (NBRIP) growth medium containing tricalcium phosphate (TCP) as sole phosphorus
(P) source. The strain YC is identified as Stenotrophomonas maltophilia (S. maltophilia) based upon the results of morphologic, physiological and biochemical characteristics and 16S rRNA sequences analysis. The
results show that the strain S. maltophilia YC can solubilize TCP and release soluble P in NBRIP growth medium. A positive correlation between concentration of soluble
P and population of the isolate and a negative correlation between concentration of soluble P and pH in the culture medium
are observed from statistical analysis results. Moreover, gluconic acid is detected in the culture medium by HPLC analysis.
It indicates that the isolate can release gluconic acid during the solubilizing experiment, which causes acidification of
the culture medium and then TCP solubilization. S. maltophilia YC has a maximal TCP solubilizing capability when using maltose as carbon source and ammonium nitrate as nitrogen source,
respectively, in NBRIP growth medium.
Neutralization by carbonate of acidification generated by pyrite (FeS2) oxidation was investigated by both solution (iron and sulfur speciation, pH and Eh) and solid (FT-IR) characterizations.
Batch dissolution experiments were carried out in contact with atmospheric oxygen (20 %) in four different bicarbonated solutions
([NaHCO3]=10–3, 1,12.10−2, 10−1 and 1 mol/L). Five different contact duration were selected : 6 hours, 1, 3, 8 and 30 days. Ferrous carbonate complexes (FeOHCO3 − and Fe(CO3 )22− ) tend to maintain iron in solution (up to 152.2 μmol/L in [NaHCO3]=1 mol/L solution) and to increase pyrite oxidation rate by preventing surface coating. Acidification is thus more intense
in diluted and concentrated carbonate medium ([NaHCO3− ] =10–3 and 1 mol/L) with respectively ΔpH=5.06 and ΔpH=1.99 at 30 days whereas pH remains buffered in [NaHCO3 − ] =1.12.10−2 and 0.1 mol/L solutions. Siderite appears to be the first solid precipitating, transforming into gœthite, oxyhydroxy ferric
sulfate incorporating sulfite and thiosulfate, and then lepidocrocite. Sulfur chemistry controls the acidification observed.
Thiosulfate is the first sulfoxyanion released in solution and its oxidation into sulfite then sulfate seems to be the key
of acidification production. Thus, carbonate pH buffer properties seem to be limited and effective for moderated carbonate
concentrations.
Long terminal repeat (LTR) retrotransposons are a class of mobile genetic element capable of autonomous transposition via an RNA intermediate. Their large size and proliferative ability make them important contributors to genome size evolution, especially in plants, where they can reach exceptionally high copy numbers and contribute substantially to variation in genome size even among closely related taxa. Using a phylogenetic approach, we characterize dynamics of proliferation events of Ty3/gypsy-like LTR retrotransposons that led to massive genomic expansion in three Helianthus (sunflower) species of ancient hybrid origin. The three hybrid species are independently derived from the same two parental species, offering a unique opportunity to explore patterns of retrotransposon proliferation in light of reticulate evolutionary events in this species group.
We demonstrate that Ty3/gypsy-like retrotransposons exist as multiple well supported sublineages in both the parental and hybrid derivative species and that the same element sublineage served as the source lineage of proliferation in each hybrid species' genome. This inference is based on patterns of species-specific element numerical abundance within different phylogenetic sublineages as well as through signals of proliferation events present in the distributions of element divergence values. Employing methods to date paralogous sequences within a genome, proliferation events in the hybrid species' genomes are estimated to have occurred approximately 0.5 to 1 million years ago.
Proliferation of the same retrotransposon major sublineage in each hybrid species indicates that similar dynamics of element derepression and amplification likely occurred in each hybrid taxon during their formation. Temporal estimates of these proliferation events suggest an earlier origin for these hybrid species than previously supposed.
The recently-developed statistical method known as the "bootstrap" can be used to place confidence intervals on phylogenies. It involves resampling points from one's own data, with replacement, to create a series of bootstrap samples of the same size as the original data. Each of these is analyzed, and the variation among the resulting estimates taken to indicate the size of the error involved in making estimates from the original data. In the case of phylogenies, it is argued that the proper method of resampling is to keep all of the original species while sampling characters with replacement, under the assumption that the characters have been independently drawn by the systematist and have evolved independently. Majority-rule consensus trees can be used to construct a phylogeny showing all of the inferred monophyletic groups that occurred in a majority of the bootstrap samples. If a group shows up 95% of the time or more, the evidence for it is taken to be statistically significant. Existing computer programs can be used to analyze different bootstrap samples by using weights on the characters, the weight of a character being how many times it was drawn in bootstrap sampling. When all characters are perfectly compatible, as envisioned by Hennig, bootstrap sampling becomes unnecessary; the bootstrap method would show significant evidence for a group if it is defined by three or more characters.
A new method called the neighbor-joining method is proposed for reconstructing phylogenetic trees from evolutionary distance data. The principle of this method is to find pairs of operational taxonomic units (OTUs [= neighbors]) that minimize the total branch length at each stage of clustering of OTUs starting with a starlike tree. The branch lengths as well as the topology of a parsimonious tree can quickly be obtained by using this method. Using computer simulation, we studied the efficiency of this method in obtaining the correct unrooted tree in comparison with that of five other tree-making methods: the unweighted pair group method of analysis, Farris's method, Sattath and Tversky's method, Li's method, and Tateno et al.'s modified Farris method. The new, neighbor-joining method and Sattath and Tversky's method are shown to be generally better than the other methods.
Extracellular polymeric substances (EPS) play a pivotal role in many processes (Costerton 1985). Often their importance is not fully known. One of these processes, in which we recently started to appreciate the role of EPS for its function, is the biological leaching of precious metals and its detrimental effect on acid mine/rock drainage.
Coal collected from Meghalaya state situated in northeastern (NE) region of India contains about 6.17% total sulphur (4.7% organic) with a gross calorific value of 26,208 J/g. In the present study, the coal sample was subjected to biodesulphurization along with optimization of various process parameters using a bacteria isolated from the native coal mine site. The native bacterium was identified to be Sinomonas flava 1C and reported first time for the capability to remove about 40% of sulphur from the coal. An innovative two-step sequential leaching using S. flava 1C followed by Acidithiobacillus ferrooxidans was used in the present study to enhance desulphurization. Sequential leaching resulted in 50-53% of sulphur removal from the coal sample. Sequential leaching reduced the sulphur content to 3.08% (organic 2.15% + pyritic 0.1% and 0.85% of sulphate sulphur) in -500+300 mu m size fraction of coal. Analytical characterization indicated that the energy value of coal was not affected adversely due to the biodesulphurization process rather its calorific value increased from 26,208 J/g to 29,481 J/g.
Dumping of low-grade chalcopyrite encompasses several environmental problems. Despite slow dissolution rate, meso-acidophilic bioleaching is preferred for the extraction of copper from such ores. In the present study, meso-acidophilic bioleaching of a low-grade chalcopyrite in presence of an acid-processed waste newspaper (PWp) is discussed for the first time. The study illustrated a strong catalytic response of PWp with enhanced bio-recovery of copper from acid-conditioned chalcopyrite. A maximum of 99.13% copper recovery (0.36% Cu dissolution/day) was obtained in 6 days of bioleaching in presence of 2 gL- 1 PWp in contrast to only 5.7% copper in its absence. FTIR analysis of bioleached residues revealed similar spectral patterns to the original acid-conditioned ore in presence of PWp, thus indicating less development of passivation layer which was also confirmed through a complementary raman characterization of the bioleached residues. Further, a reaction mechanism (chemistry) was proposed suggesting the possible role of PWp as the electron donor under oxygen limiting conditions which facilitated microbial reduction of Fe (III). The resulting biochemical changes provided an energy source for the bacteria, thus allowing free flow of electrons through the ore surface, thus contributing towards enhanced bioleaching of copper.
The effects of activated carbon and/or surfactants on the bioleaching behavior of cobalt ore have been investigated in this paper. It was demonstrated that activated carbon and surfactants (Tween-20 and Tween-80) can significantly promote the dissolution rate of carrollite, either individually or in combination when coupled with ZY101 bacteria strain. The cobalt leaching efficiency increased from 71.3% to 90.8% when 1.0 g/L activated carbon was added, and such an improvement was ascribed to the galvanic interaction between activated carbon and carrollite. In addition, the leaching efficiency of cobalt increased from 71.3% to 92.4% by 0.1 g/L Tween-20 or to 93.2% by 0.1 g/L Tween-80, by changing the surface condition of the mineral and accelerating the bio-oxidation of formed elemental sulfur. When both activated carbon and surfactant were added, the leaching efficiency of cobalt increased by more than 22%, and meanwhile the leaching time decreased by more than 30%. It is shown that the catalytic effect of the combined catalyst was much more significant than that when only using a single catalyst. A similar trend was observed in the case of elemental copper extraction during the bioleaching process.
Infrared spectra of two minerals; Urkut quartz (Hungary) and a Swedish feldspar, of different origin with dif-ferent physical characteristics and crystallinity were studied. Samples were taken after appropriate grinding, and infrared spectra, X-ray powder diffraction and water vapor absorption measurements were made along with electronmicrographs. Quantitative conclusions were drawn from changes of particle size and the ratio of bands of the SilO groups and also the degree of crystallinity and changes of the particle size, respectively. INTRODUCTION Some problems of the quantitative analysis of min-erals by infrared spectrophotometry were reported in Part I. It was stated that the intensity of the character-istic absorption bands of the kaolinites depends con-siderably on the shape and crystalline state of the par-ticles, as well as on their actual chemical composition. Quantitative determination of quartz, by infrared spectrophotometry, in dust, soil, other silicates and powdered coal was reported by Larsen (1972), Small-wood (1963), Hlavay (1976) and Tuddenham (1960). It was established that accurate and reproducible deter-minations can be carried out only on samples of strictly identical particle size and crystallinity. No literature data was found relating to a quantitative infrared anal-ysis of feldspar. In the present work infrared and X-ray spectra were recorded, as well as water vapor absorption and scan-ning electron microscopic measurements of the original samples and of those subjected to grinding, in order to reduce the particle size and the degree of crystallinity.
Copper slag was subjected to in-depth mineralogical characterization by integrated instrumental techniques and evaluated for the efficacy of physical beneficiation and mixed meso-acidophilic bioleaching tests towards recovery of copper. Point-to-point mineral chemistry of the copper slag is discussed in detail to give better insight into the association of copper in slag. Characterization studies of the representative sample revealed the presence of fayalite and magnetite along with metallic copper disseminated within the iron and silicate phases. Physical beneficiation of the feed slag (~0.6% Cu) in a 2 L working volume flotation cell using sodium isopropyl xanthate resulted in Cu beneficiation up to 2-4% and final recovery within 42-46%. On the other hand, a mixed meso-acidophilic bacterial consortium comprised of a group of iron and/or sulfur oxidizing bacteria resulted in enhanced recovery of Cu (~92-96%) from the slag sample. SEM characterization of the bioleached slag residue also showed massive coagulated texture with severe weathered structures. FE-SEM elemental mapping with EDS analysis indicated that the bioleached residues were devoid of copper.
The advances in the area of applied and industrial microbial biotechnology have opened up many new avenues for application of several microorganisms in the industrial sector. A group of certain metal reducers such as the Dissimilatory Iron Reducing Microorganisms possess an inherent potential to reduce oxidized metals under strict anaerobic/facultative anaerobic condition, thereby opening possibilities to combat environmental pollution. This unique property has invited researchers towards understanding the metabolic regulatory pathways that enables the microbes to thrive under extreme environmental conditions. Currently, Dissimilitory Iron Reducing Bacteria (DIRB) is in the limelight of researchers to elucidate the specific mechanisms responsible for microbial metal reduction. The recent advances towards understanding the metabolism of iron reduction in Shewanella and Geobacter, the model DIRB has been covered in this review. It is believed that the metabolic insights into the Fe (III) reduction systems of the model DIRB; Shewanella and Geobacter (as discussed in the review) can be a basis for metabolic engineering to provide improved practical applications. With the advancement of our existing knowledge on the metabolic processes of the model iron reducers, application ranging from laboratory to field scale practices can be carried out. Interestingly, DIRB has gained immense interest for its application in the field of bioremediation, electrobiosynthesis, and bioelectronics in this decade. It can therefore be anticipated that the forthcoming years will see more applications of microbial iron reducers based on the existing as well as advanced metabolic informations available in open source literature.
Bioleaching studies for chalcopyrite contained ball mill spillages are very scarce in the literature. We developed a process flow sheet for the recovery of copper metal from surface activated (600 A degrees C, 15 min) ball mill spillage through bio-hydrometallurgical processing route. Bioleaching of the activated sample using a mixed meso-acidophilic bacterial consortium predominantly A. ferrooxidans strains was found to be effective at a lixiviant flow rate of 1.5 L/h, enabling a maximum 72.36% copper recovery in 20 days. Mineralogical as well as morphological changes over the sample surface were seen to trigger the bioleaching efficiency of meso-acidophiles, thereby contributing towards an enhanced copper recovery from the ball mill spillage. The bio-leach liquor containing 1.84 g/L Cu was purified through solvent extraction using LIX 84I in kerosene prior to the recovery of copper metal by electrowinning. Purity of the copper produced through this process was 99.99%.
Using IR absorption data from polarized measurements on single-crystal minerals with stoichiometric water contents (in the
form of H 2 O or OH groups in the structure), a linear calibration curve (r 2 nearly equal 0.98) for water in minerals is established in the form: epsilon i , (the integrated molar absorption coefficient in units of cm (super -2) per mol H 2 O/L) = 246.6(3753 - upsilon ) (upsilon = the mean wavenumber of the OH stretching band [in cm (super -1) ]). The investigated
minerals include hydrogrossular, analcime, hemimorphite and its dehydrated phase, lawsonite, goethite, diaspore, manganite,
mozartite, and pectolite. The influence of hydrogen bonding, leading to increased absorption values with lower OH stretching
band energies, is confirmed. It is further shown that only the use of integrated absorbance values (band areas) results in
a linear correlation with water content, whereas linear absorption data (peak heights) are not correlated. The calibration
agrees with previously published quantitative IR data on staurolite and trace H in pyroxenes. It is also close to the frequently
used trend of Paterson (1982). However, some of the previous calibrations of trace H in nominally anhydrous minerals, e.g.,
kyanite and pyrope, differ appreciably from the correlation derived from stoichiometrically hydrous minerals.
Liming is a known forest management procedure used to amend nutrient-poor soils such as soils of acidic forests to rectify cation deficiencies and to restore soil pH. However, although this procedure is well known for its beneficial effect on the forest trees, its relative impact on the functional and taxonomic diversity of the soil bacterial communities has been poorly investigated. In this study, we characterized the ability of the soil bacteria to weather soil minerals and to hydrolyze chitin. A collection of 80 bacterial strains was isolated from the Scleroderma citrinum ectomycorrhizosphere and the adjacent bulk soil in two stands of mature beeches (Fagus sylvatica) developed on very acidic soil and presenting two levels of calcium (Ca) availability: a control plot as well as a plot amended with Ca in 1973. All the bacterial isolates were identified by partial 16S rRNA gene sequence analysis as members of the genera Burkholderia, Bacillus, Dyella, Kitasatospora, Micrococcus, Paenibacillus, Pseudomonas, and Rhodanobacter. Using a microplate assay for quantifying the production of protons and the quantity of iron released from biotite, we demonstrated that the bacterial strains from the amended plot harbored a significant higher mineral weathering potential that the ones isolated from the control plot. Notably, the weathering efficacy of the ectomycorrhizosphere bacterial isolates was significantly greater than that of the bulk soil isolates in the control treatment but not in the amended plot. These data reveal that forest management, here mineral amendment, can strongly affect the structure of bacterial communities even over the long term.
This study investigates the biooxidation of a refractory gold concentrate using a mixed culture of acidophilic mesophiles, moderate thermophiles and extreme thermophiles and their effect on the subsequent cyanidation and gold recovery. The experiments with high % solids using mixed mesophiles showed better oxidation potential compared to moderate thermophiles and extreme thermophiles. However, the extreme thermophiles performed better than mesophiles and moderate thermophiles during the biooxidation with
Heap bioleaching of low grade chalcopyrite (CuFeS2) was carried out in 1000 tons scale over a time period of
383 days. Bacterial solution was prepared and re‐circulated in specially designed BACFOX (BACterial Film OXidation)
tank. The micro‐organisms used in the pilot scale were a mixed culture of acidophilic bacteria predominantly
of the Acidithiobacillus ferrooxidans strain. Effect of rest period, solution recirculation, acid concentration, frequency
of solution transfers and seasonal effects on copper recoveryweremonitored. The leaching studies showed a cumulative
copper dissolution rate of 0.14% per day (2.37 kg d−1). The overall recovery of the heapwas 30%. Variation in
leaching efficiency and some of the precautionary measures to improve performance of heap bioleaching are also
discussed.
Copper silica catalyst precursors have been characterized by diffuse reflectance infrared Fourier transform spectroscopy. The measurements were performed both under atmospheric conditions and under controlled gasatmospheres at elevated temperatures. Two reflection bands at 3615 and 690 cm-1 have been attributed to hydroxyl vibrations associated with the presence of copper ions highly dispersed over the silica surface. Calcination of a catalyst precursor at 800 K led to the disappearance of the copper-related surface hydroxyl groups.
Biomining is an increasingly applied biotechnological procedure for processing of ores in the mining industry (biohydrometallurgy). Nowadays the production of copper from low-grade ores is the most important industrial application and a significant part of world copper production already originates from heap or dump/stockpile bioleaching. Conceptual differences exist between the industrial processes of bioleaching and biooxidation. Bioleaching is a conversion of an insoluble valuable metal into a soluble form by means of microorganisms. In biooxidation, on the other hand, gold is predominantly unlocked from refractory ores in large-scale stirred-tank biooxidation arrangements for further processing steps. In addition to copper and gold production, biomining is also used to produce cobalt, nickel, zinc, and uranium. Up to now, biomining has merely been used as a procedure in the processing of sulfide ores and uranium ore, but laboratory and pilot procedures already exist for the processing of silicate and oxide ores (e.g., laterites), for leaching of processing residues or mine waste dumps (mine tailings), as well as for the extraction of metals from industrial residues and waste (recycling). This chapter estimates the world production of copper, gold, and other metals by means of biomining and chemical leaching (bio-/hydrometallurgy) compared with metal production by pyrometallurgical procedures, and describes new developments in biomining. In addition, an overview is given about metal sulfide oxidizing microorganisms, fundamentals of biomining including bioleaching mechanisms and interface processes, as well as anaerobic bioleaching and bioleaching with heterotrophic microorganisms.
A simple computer model for the dissolution kinetics of crystalline matter governed by etch-pit formation predicts different development, paths, and states for geometric, total (BET), and reactive surface area during the dissolution process. The model also explores the dynamics of the dissolution rate of a given model crystal surface as a function of the development of surface area. Because the surface area term is used in the normalization of bulk dissolution rates, results of this normalization reflect the large differences explored. Based on this evaluation, we discuss the application of the diversely defined surface area terms. In the light of this discussion, the likelihood of an unambiguous definition or application of reactive surface area is problematic.
The model focuses on the relationship between the variation in total surface area and the global dissolution rate, and thus is independent of specific surface reaction mechanisms. The actual model calculations presented as an example in this paper utilize experimentally determined dissolution data of three dolomite [CaMg(CO3)2] cleavage surfaces obtained by vertical scanning interferometry (VSI). Similar data from minerals such as calcite, feldspars, and barite can be used and make this model applicable to a range of different crystalline phases.
Abstract Bioleaching of a low grade chalcopyrite (ball
mill spillage material) was tested for copper recovery in
shake flasks. The original samples (as received) were
thermally activated (600°C, 30 min) to notice the change in
physico-chemical and mineralogical characteristics of the
host rock and subsequently its effect on copper recovery. A
mixed culture of acidophilic chemolithotrophic bacterial
consortium predominantly entailing Acidithiobacillus fer-
rooxidans strain was used for bioleaching studies and
optimization of process parameters of both original and
thermally activated samples. Mineralogical characteriza-
tion studies indicated the presence of chalcopyrite, pyrite
in the silicate matrix of the granitic rock. Field emission
scanning electron microscopy coupled with Energy
dispersive spectroscopy (FESEM-EDS) and X-ray Fluor-
escence (XRF) analysis indicated mostly SiO2. With pH 2,
pulp density 10% w/v, inoculum 10% v/v, temperature
30°C, 150 r$min–1
,49% copper could be recovered in 30
days from the finest particle size ( – 1 + 0.75 mm) of the
original spillage sample. Under similar conditions 95%
copper could be recovered from the thermally activated
sample with the same size fraction in 10 days. The study
revealed that thermal activation leads to volume expansion
in the rock with the development of cracks, micro and
macro pores on its surface, thereby enabling bacterial
solution to penetrate more easily into the body, facilitating
enhanced copper dissolution.
Keywords ball mill spillage, thermal activation, bioleach-
ing, copper
The recently-developed statistical method known as the "bootstrap" can be used to place confidence intervals on phylogenies. It involves resampling points from one's own data, with replacement, to create a series of bootstrap samples of the same size as the original data. Each of these is analyzed, and the variation among the resulting estimates taken to indicate the size of the error involved in making estimates from the original data, In the case of phylogenies, it is argued that the proper method of resampling is to keep all of the original species while sampling characters with replacement, under the assumption that the characters have been independently drawn by the systematist and have evolved independently. Majority-rule consensus trees can be used to construct a phylogeny showing all of the inferred monophyletic groups that occurred in a majority of the bootstrap samples. If a group shows up 95% of the time or more, the evidence for it is taken to be statistically significant. Existing computer programs can be used to analyze different bootstrap samples by using weights on the characters, the weight of a character being how many times it was drawn in bootstrap sampling. When all characters are perfectly compatible, as envisioned by Hennig, bootstrap sampling becomes unnecessary; the bootstrap method would show significant evidence for a group if it is defined by three or more characters.
Meso-acidophilic bacterial leaching of ball mill spillage (containing chalcopyrite >80%) was carried out in an innovative two-step bioleaching method. The major drawback of meso-acidophilic bioleaching limiting industrial application is the passivation phenomenon over the ore surfaces in iron-sulfur rich environments. In the present study, we present a novel wash solution that efficiently removed the passivation layer. FTIR characterization of the bioleached sample indicated that the residues could be further leached to recover extra copper after wash solution application. XRD study indicated accumulation of sulfates (SO(4)(-)) of Na, K, Fe and oxy hydroxides of iron [FeO(OH)] in the form of jarosite outlining the passivation layer. SEM, FESEM-EDS studies indicated severe corrosion effects of the wash solution on the passivation layer. Two step bioleaching of the ore sample yielded 32.6% copper in 68days in the first interlude and post wash solution application yielded 10.8% additional copper.
Silico-antimonate as inorganic ion exchange material has been synthesized and characterized using different available tools (X-ray diffraction (XRD) pattern, X-ray fluorescence, infrared spectroscopy and differential thermal analysis). From the analysis data, the empirical formula of silico-antimonate was obtained to be H2SiSb4O13·10H2O. Sorption kinetics for Cu2+, Zn2+, Cd2+ and Ni2+ ions on silico-antimonate were studied and found to be follow the first order kinetics obeying the Freundlich isotherm over the entire range for the bulk concentration of the metal ions. Thermodynamic parameters (i.e. ΔG°, ΔS° and ΔH°) have also been calculated for the adsorption of Cu2+, Zn2+, Cd2+ and Ni2+ ions on silico-antimonate showing that the overall adsorption process is spontaneous and exothermic.
Quantitative aspects of microbial crystalline iron(III) oxide reduction were examined using a dissimilatory iron(III) oxide-reducing bacterium (Shewanella alga strain BrY). The initial rate and long-term extent of reduction of a range of synthetic iron(III) oxides were linearly correlated with oxide surface area. Oxide reduction rates reached an asymptote at cell concentrations in excess of ≈1 × 109/m2 of oxide surface. Experiments with microbially reduced goethite that had been washed with pH 5 sodium acetate to remove adsorbed Fe(II) suggested that formation of a Fe(II) surface phase (adsorbed or precipitated) limited the extent of iron(III) oxide reduction. These results demonstrated explicitly that the rate and extent of microbial iron(III) oxide reduction is controlled by the surface area and site concentration of the solid phase. Strain BrY grew in media with synthetic goethite as the sole electron acceptor. The quantity of cells produced per micromole of goethite reduced (2.5 × 106) was comparable to that determined previously for growth of BrY and other dissimilatory Fe(III)-reducing bacteria coupled to amorphous iron(III) oxide reduction. BrY reduced a substantial fraction (8−18%) of the crystalline iron(III) oxide content of a variety of soil and subsurface materials, and several cultures containing these materials were transferred repeatedly with continued active Fe(III) reduction. These findings indicate that Fe(III)-reducing bacteria may be able to survive and produce significant quantities of Fe(II) in anaerobic soil and subsurface environments where crystalline iron(III) oxides (e.g., goethite) are the dominant forms of Fe(III) available for microbial reduction. Results suggest that the potential for cell growth and Fe(II) generation will be determined by the iron(III) oxide surface site concentration in the soil or sediment matrix.
Fourier Transform Infrared (FTIR) spectroscopy, thermal analysis -Differential Scanning Calorimetry (DSC) and Thermo-Gravimetry (TG-DTG) were used to study the dehydration behavior of synthetic goethite and two naturally occurring goethite samples (Natural 1 and Natural 2) from Banded Iron Formation (BIF), at C.S. Halli, Chitradurg district, Karnataka, India. Goethites and its dehydration products were also identified by powder X-ray Diffraction (XRD) method. The dehydration temperatures were at 538, 567 and 578 K for synthetic, Natural 1 and 2 goethite, respectively. On approaching the dehydration temperature, infrared active modes of the hydroxyl groups have shown distinct variations. The peak position for the stretching mode around 3150 cm−1 was shifted upwards, while that for in-plane- deformation mode around 890 cm−1 was down shifted indicating weakening of strength of the hydrogen bonding. No intermediate phase, so called hydro-hematite, was observed in these studies. The total absorbance (area under the peak) of these modes have shown the Arrehenius type behavior in the temperature range 500–600 K, using which the activation energy for the dehydration process was estimated as 71, 103 and 85 kJ/mol for synthetic, Natural 1 and 2 goethites respectively.
Zn(II) and Pb(II) from Nigerian sphalerite and galena ores were bioleached by a mixed culture of acidophilic bacteria. The influences of pH and ferric ion on the bioleaching rates of sphalerite and galena were examined. The result shows that pH 2.1 and 2.7 are favourable for the leaching of Zn(II) and Pb(II) from sphalerite and galena, respectively. It was observed that the use of agarose-simulated media caused cells to excrete exopolymers containing ferric ions which enhanced oxidation. The oxidation equilibrium for sphalerite and galena took 3 and 4 d, respectively. About 38.3% sphalerite and 34.2% galena were leached within 1 d and approximately 92.0% Zn(II) and 89.0% Pb(II) were recovered in 5 d, respectively. The unleached residual products were examined by X-ray diffraction for sphalerite, revealing the presence of elemental sulphur(S), zinc sulphate (ZnSO4) and few traces of calcium aluminate (Ca3Al2O6). The XRD pattern also indicates the presence of elemental sulphur (S), lead sulphate (PbSO4) and few traces of itoite [Pb(S,Ge)(O,OH)4] and cobalt lead silicate [Pb8Co(Si2O7)3] in the unleached galena ore.
Low grade copper ore (ball mill spillage) obtained from Malanjkhand Copper Mine was processed through heap bioleaching at pilot scale. Bioleach liquor (Pregnant Leach Solution) from the heap contains (g/L of) Cu (II) 0.45, Fe (III) 0.838, Zn (II) 0.006, Ni (II) 0.0014, Mn (II) 0.011 and Pb (IV) 0.004. Solvent extraction of copper from Pregnant Leach Solution was carried out using LIX 984N-C. Effect of different operational factors such as equilibrium pH (pHe), extractant concentration, strip solution concentration, phase ratio was examined to optimize the condition for selective and quantitative extraction of copper. Based on the results of extraction as well as stripping isotherm, a 6-cycle counter current simulation study (CCS) was conducted for the conformational study. The extraction of copper was quantitative in 2-stages using 1.5% (v/v) LIX 984N-C at A: O ratio of 1:2 and pHe 1.85. The CCS condition (two stages, phase ratio A:O = 1:4) obtained from the stripping isotherm study, further attributes to the enrichment (4 folds) of copper concentration in the strip solution.
A process flowsheet was developed to recover copper metal from the lean sulfide ore of copper available at Malanjkhand, Hindustan Copper Limited (HCL), India. Copper pregnant leach solution (PLS) obtained from bio-heap leaching of chalcopyrite containing 0.3% copper was purified through solvent extraction (SX) and the copper recovered by electrowinning (EW). The copper-free raffinate obtained from SX stripping unit was returned back to the bioleaching circuit. The purity of the electrolytic copper produced at pilot scale was found to be 99.96%. During electrowinning, the effect of flow rate of electrolyte on current efficiency and energy consumption was also studied.
Mineral dissolution experiments using batch cultures of soil and groundwater bacteria were monitored with solution chemistry and various microscopic techniques to determine the effects of these organisms on weathering reactions. Several strains of bacteria produced organic and inorganic acids and extracellular polymers in culture, increasing the release of cations from biotite (Si, Fe, Al) and plagioclase feldspar (Si, Al) by up to two orders of magnitude compared to abiotic controls. Microbial colonies on mineral grains were examined by cryo-scanning electron microscopy (cryo-SEM), confocal scanning laser mi-croscopy (CSLM), and epifluorescence microscopy. Bacteria colonized all mineral sur-faces, often preferentially along cleavage steps and edges of mineral grains. Low-voltage high-resolution cryo-SEM of high-pressure cryofixed and partially freeze-dried colonized minerals showed many bacteria attached by extracellular polymers of unknown composi-tion. These biofilms covered much larger areas of the mineral surfaces than bacterial cells alone. Mineral surfaces where bacteria and extracellular polymers occurred appeared more extensively etched than surrounding uncolonized surfaces. CSLM was used to observe microbial colonization of biotite and to measure pH in microenvironments surrounding living microcolonies using a ratiometric pH-sensitive fluorescent dye set. A strain of bac-teria (B0693 from the U.S. Department of Energy Subsurface Microbial Culture Collection) formed large attached microcolonies, both on the outer (001) surface and within interlayer spaces as narrow as 1 m. Solution pH decreased from near neutral at the mineral surface to 3–4 around microcolonies living within confined spaces of interior colonized cleavage planes. However, no evidence of pH microgradients surrounding exterior microcolonies was noted.
The present understanding of the surface chemistry of acidic ferric sulphate dissolution of chalcopyrite is critically reviewed with regard to hindered dissolution and how the hydrometallurgical limitations, especially for microbial heap leaching operations, might be overcome. In particular the surface science investigations of what surface phases might be responsible for hindered dissolution are reviewed. Some other mechanistic issues are also considered which require further investigation. The possible phase candidates for hindered dissolution are examined, with most discussion focussed on elemental sulphur and jarosites. Phases such as polysulphides are rejected as candidates. The physical reality of metal-deficient sulphides is also questioned.A conceptual 4-stage model is proposed which explains all the general dissolution behaviour that is widely observed, i.e. of an induction period and a parabolic rate curve that may or may not be followed by linear rate behaviour. The general conclusion is that thick over-layers of sulphur cause the initial parabolic behaviour, and a thin systemic sulphur layer is responsible for the rate-limiting step, even in the linear region. Depending upon solution conditions, either unhindered near linear dissolution may occur, or jarosite precipitation that will cause a second parabolic region. Sulphur formation remains a systemic phase in the context of heap bioleaching but is not a problem of any consequence for mixed culture systems unlike jarosites. Suggestions are made as to a low-cost jarosite precipitation pond for iron removal and advantages that could result.
Differences in pH between silicate–biofilm interfaces and bulk medium (ΔpH=pHinterface−pHbulk) were detectable with commercial microelectrodes in cultures grown in unbuffered medium (|ΔpH|=0.27–1.08) for an arthrobacter species, but were generally beneath detection (ΔpH<0.04) for a streptomyces species. Biofilm half-thicknesses developed by Arthrobacter ranged from 1.2 to 11.5 mm, and were highly variable even for replicates. In buffered medium, neither bacterium produced a measurable ΔpH across the biofilms grown on silicates. The silicates consisted of polished hornblende, synthetic Fe-rich glass similar to hornblende in bulk composition, and two commercially available “float glasses,” one low-Fe and one high-Fe. The two species of soil bacteria investigated are both known to accelerate release of Fe from hornblende. For the Arthrobacter, values of |ΔpH| developed on hornblende crystal or glass substrates were generally larger than those developed on either float glass. Differences in ΔpH developed on different substrates could not be related simply to relative rates of dissolution of substrates.Differences between the two bacterial species are probably related to differences in (1) rates of growth, (2) production of low-molecular-weight organic acids, (3) physical characteristics of polysaccharide slimes excreted and/or (4) production of siderophores. Although values of |ΔpH| developed at mineral–water interfaces in natural systems may not be as large as those measured here except for water-saturated systems, it is probable that significant values of ΔpH (≤pH unit) develop where fast-growing, acid-producing microbes colonize slow-dissolving phases in the presence of unbuffered solutions.
Bioleaching of metal sulfides is effected by bacteria, like Thiobacillus ferrooxidans, Leptospirillum ferrooxidans, Sulfolobus/Acidianus, etc., via the (re)generation of iron(III) ions and sulfuric acid.According to the new integral model for bioleaching presented here, metal sulfides are degraded by a chemical attack of iron(III) ions and/or protons on the crystal lattice. The primary iron(III) ions are supplied by the bacterial extracellular polymeric substances, where they are complexed to glucuronic acid residues. The mechanism and chemistry of the degradation is determined by the mineral structure.The disulfides pyrite (FeS2), molybdenite (MoS2), and tungstenite (WS2) are degraded via the main intermediate thiosulfate. Exclusively iron(III) ions are the oxidizing agents for the dissolution. Thiosulfate is, consequently, degraded in a cyclic process to sulfate, with elemental sulfur being a side product. This explains, why only iron(II) ion-oxidizing bacteria are able to oxidize these metal sulfides.The metal sulfides galena (PbS), sphalerite (ZnS), chalcopyrite (CuFeS2), hauerite (MnS2), orpiment (As2S3), and realgar (As4S4) are degradable by iron(III) ion and proton attack. Consequently, the main intermediates are polysulfides and elemental sulfur (thiosulfate is only a by-product of further degradation steps). The dissolution proceeds via a H2S*+-radical and polysulfides to elemental sulfur. Thus, these metal sulfides are degradable by all bacteria able to oxidize sulfur compounds (like T. thiooxidans, etc.). The kinetics of these processes are dependent on the concentration of the iron(III) ions and, in the latter case, on the solubility product of the metal sulfide.
This study compares the capacity of pure and mixed cultures of mesophilic bacteria for bioleaching of a low grade, pyritic chalcopyrite concentrate. In pure culture form, Acidithiobacillus ferrooxidans was found to have a higher bioleaching capacity than Leptospirillum ferrooxidans and Acidithiobacillus thiooxidans with the capability of the latter to bioleach copper being very limited. Mixed cultures, MixA (At. ferrooxidans, L. ferrooxidans and At. thiooxidans) and MixB (L. ferrooxidans and At. thiooxidans) were shown to perform better than the pure cultures with the highest extraction of copper (62.1% Cu) being achieved by MixA. Copper bioleaching performances of the cultures were observed to agree with their respective growth pattern. The results also indicated that the increase in the pulp density (1–5% wt/vol) adversely affected bioleaching process regardless of the pure and mixed cultures used having led to the decrease in the extent of final copper extraction i.e. 50.3% Cu recovery at 1% wt/vol for At. ferrooxidans compared with 38.6% Cu at 5% wt/vol. This study underlines the importance of mixed cultures and, iron and sulphur-oxidising activity of a bacterial culture to efficiently oxidise chalcopyrite.