Article

Effect Heating Dwell Time Has on the Retention of Heavy Metals in the Structure of Lightweight Aggregates Manufactured From Wastes

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Abstract

Abbreviations: BCR-SEP: optimized BCR sequential extraction procedure; b.d.l: below the detection limit; F1: weakly adsorbed, exchangeable and water- and acid- soluble fraction; F2: reducible fraction; F3: oxidable fraction; F4: residual fraction; HM: heavy metal; ICP-MS: inductively coupled plasma-mass spectroscopy; LOI: loss on ignition; LWA: lightweight aggregate; LWA-5: lightweight aggregate sintered for 5 min; LWA-10: lightweight aggregate sintered for 10 min; LWA-20: lightweight aggregate sintered for 20 min; LWA-30: lightweight aggregate sintered for 30 min; LRx,y: leaching ratio of the element x in the fraction y; n.e: not established; S: compressive strength; SS: sewage sludge; WA24h: water absorption after 24 hours; WAS: washing aggregate sludge; W75S25: mixture of 75% (wt) of the dried washing aggregate sludge and 25% (wt) of the dried sewage sludge; ρb: loose bulk density; ρd: dry particle density; ∑1 + 2 + 3: non-residual fraction; ∑1 + 2 + 3 + 4: total concentration; ∑2 + 3: reducible and oxidable fractions.

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This study aims to develop a particulate model combining solid waste particle combustion and heavy metal vaporization from burning particles during MSW incineration in a fluidized bed. The original approach for this model combines an asymptotic combustion model for the carbonaceous solid combustion and a shrinking core model to describe the heavy metal vaporization. A parametric study is presented. The global metal vaporization process is strongly influenced by temperature. Internal mass transfer controls the metal vaporization rate at low temperatures. At high temperatures, the chemical reactions associated with particle combustion control the metal vaporization rate. A comparison between the simulation results and experimental data obtained with a laboratory-scale fluid bed incinerator and Cd-spiked particles shows that the heavy metal vaporization is correctly predicted by the model. The predictions are better at higher temperatures because of the temperature gradient inside the particle. Future development of the model will take this into account.
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Sixteen soil samples were collected from the vicinity of an abandoned lead–zinc mine in Shangyu City, eastern China, and the heavy-metal speciation and wheat phytotoxicity in the soils were studied. The results showed that the concentrations of free Cu2+, Zn2+, Cd2+ and Pb2+ were highly variable and ranged from <0.01 to 0.32, 0.06 to 10.62, <0.01 to 1.40 and 0.02 to 37.10μmoll−1, respectively. The concentrations of soluble Cu, Zn, Cd and Pb ranged from 0.38 to 3.24, 0.72 to 78.74, <0.01 to 1.95 and 0.15 to 639.34μmoll−1, respectively. The general trend of mean solid/liquid partition coefficient and percentage of free metal ion to total soluble metal concentration were Cu>Pb>Zn>Cd and Cd>Zn>Cu>Pb, respectively. Stepwise multiple linear regression with pH, log(total metal) and log(organic matter) showed that log(total metal) was an important factor that controlled log(free metal ion) and log(soluble metal). Of the variability in log(free Cu2+), log(free Cd2+) and log(free Pb2+), 55.2, 58.6 and 64.3% could be explained by log(total Cu), log(total Cd) and log(total Pb) alone, respectively. Of the variability in log(soluble Cu) and log(soluble Cd), 77.1 and 72.5% could be explained by log(total Cu) and log(total Cd) alone, respectively. Wheat root length was controlled by the various metals with different free and soluble concentrations, and 99.2% of the variability in root length could be explained by concentrations of free and soluble Pb, soluble Cu and total Zn in the soils. KeywordsHeavy metals–Speciation–Phytotoxicity–Mining area
Article
The Superfund dumpsites are frequently composed of soils contaminated with hazardous organic constituents and toxic heavy metals. While thermal treatment is an effective method of remediating the contaminated soils, the major environmental concerns are the emissions of toxic metal fumes during the treatment and the leaching of metals from the treated soil. The US EPA has reported that metals can account for almost all of the identified cancer risks from waste incineration systems. Research leading to better understanding of their behavior and better controlling of their emissions is urgently needed. In this study, the behavior of metals during the fluidized bed thermal treatment of artificially prepared metal-contaminated clay was experimentally and theoretically investigated. The objective of the study was to evaluate the effects of operating conditions on metal volatilization and metal leachability associated with the process. Metal experiments were carried out in a well instrumented 76 mm (3 inch) i.d. fluidized bed incinerator. The metals involved were compounds of lead and cadmium and the operating parameters included metal concentration, air flow rate, treatment temperature and treatment duration. The observed results indicated that metal volatilization is mainly a function of treatment temperature and treatment duration. The degree of volatilization was observed to range from 5 to 40% depending on the operating conditions. Cadmium leachability was observed to be relatively high compared to that of lead. In addition to the experimental study, a theoretical model based on the laws of heat and mass transfer operations and reaction kinetics was derived to simulate the metal volatilization process. The derived model was found to predict reasonably well the experimental observations.
Article
A hydrochemical study on a 630 km stretch of river Gomti, a tributary of the river Ganges examined the distribution of heavy metals in sediments and the partitioning of their chemical species between five geochemical phases (exchangeable fraction, carbonate fraction, Fe/Mn oxide fraction, and organic fraction) using Tessier's analytical sequential extraction technique. Most fractions in the sediments associated with the carbonate and the exchangeable fractions were between 11 and 30% except in a few cases where it was more than 50%. According to the Risk Assessment Code (RAC), the sediments having 11–30% carbonate and exchangeable fractions are at medium risk. The concentrations of cadmium and lead at mid Lucknow, Pipraghat, Sultanpur U/S and Sulthanpur D/S are between 31 and 50%. They thus pose a high risk to the environment. Since the concentrations of cadmium and lead at Neemsar (Cd 56.79%; Pb 51%) are higher than 50%, the RAC as very high. In most cases, the average metal concentrations were lower than the standard shale values. Various physicochemical parameters such as pH, total solids, total dissolved solids, total suspended solids, COD, BOD, DO, conductivity, chloride, sulphate, phosphate, fluoride, total alkalinity, total hardness, etc. were also reported.
Article
The revised (four-step) BCR sequential extraction procedure has been applied to fractionate the chromium, copper, iron, manganese, nickel, lead and zinc contents in urban soil samples from public-access areas in five European cities. A preliminary inter-laboratory comparison was conducted and showed that data obtained by different laboratories participating in the study were sufficiently harmonious for comparisons to be made between cities and land types (e.g. parks, roadside, riverbanks, etc.). Analyte recoveries by sequential extraction, with respect to direct aqua regia digestion, were generally acceptable (100 ± 15%). Iron, nickel and, at most sites, chromium were found mainly in association with the residual phase of the soil matrix. Copper was present in the reducible, oxidisable and residual fractions, whilst zinc was found in all four sequential extracts. Manganese was strongly associated with reducible material as, in some cities, was lead. This is of concern because high lead concentrations were present in some soils (>500 mg kg−1) and the potential exists for remobilisation under reducing conditions. As would be expected, extractable metal contents were generally highest in older, more heavily industrialised cities. Copper, lead and zinc showed marked (and often correlated) variations in concentrations between sites within the same city whereas manganese and, especially, iron, did not. No overall relationships were, however, found between analyte concentrations and land use, nor between analyte partitioning and land use.
Article
Heavy metals contained in municipal solid waste (MSW), after combustion in modern waste-to-energy facilities, are collected in bottom and fly ash, only a small quantity being discharged from the stack as particulate or vapor. These metals are found to be broadly distributed throughout the constituents, limiting the potential for reducing them by targeting specific components. The many factors which determine metals partitioning to bottom ash, boiler hopper and emission control flyash, and stack emissions, shows that the complex relationship between feed composition and emissions makes it difficult if not impossible to assign cause and effect on their quantities and concentrations in these discharges. Data showing the relationship between particulate matter, emission controls, and emission factors for the heavy metals is examined. A finding that substantial spiking of lead and cadmium in the feed resulted in only marginal changes in stack emissions indicates that efforts to remove these metals from the waste would not produce a significant change in stack emissions. The range of trace metal emissions from a single waste-to-energy (WTE) facility over a period of three to four years is compared with the range reported from individual tests of about twenty facilities also having acid gas controls and fabric filters, indicating that the waste composition and the combustion and emission control technology employed all contribute to the variability of metals and particulate emissions. The relationship between annual averages and probable maximum values which may be anticipated from periodic testing is examined. Special attention is given to mercury, its various species, chemical reactions, and the effectiveness of various carbon-based reagents used for emission control. Emissions from WTE facilities are compared with those from oil and coal-fired utility boilers on a mass per kWh generated.
Article
The goal of this study was to develop an understanding of metals behavior during thermal treatment. Clay samples, contaminated with metals to obtain a surrogate waste, were analyzed prior to and following thermal treatment using nitric acid and/or hydrogen fluoride digestion, followed by inductively coupled plasma emission spectrophotometry analysis. Techniques were used to examine particle surface and metal distribution within cross sections. Lead, cadmium, and chromium results are discussed. With hydrogen fluoride-digested samples, the results indicated that vaporization increased slightly with increasing temperature for cadmium and lead. Chromium did not show increased vaporization. At higher temperatures, the nitric acid digestions did not completely remove the metals. Scanning electron microscope pictures showed that, at higher temperatures, the particle structure became compact and glassy; the electron microprobe results indicated that lead and cadmium were located in regions with high silicon, suggesting reactions with the silicon. Chromium distribution remained uniform, suggesting that chromium was immobilized due to structural changes not reactions. 40 refs., 11 figs., 2 tabs.
Article
The accumulation of toxic metals generated by coal-fired power stations presents a serious threat to the environment. The volatilization behavior of two representative metals (Cd and Zn), and the influence of temperature were investigated during coal combustion. An inductively coupled plasma atomic emission spectrometric (ICP-AES) method was developed to continuously measure the heavy metal concentrations quantitatively in flue gas under combustion conditions in order to track the metal release process. This continuous heavy metal analysis system was implemented by coupling it to two types of high temperature reactors: a bubbling fluidized bed reactor and a fixed bed reactor with diameter of 0.1 m and 0.08 m respectively. For the two metals considered in this study (Cd and Zn), the experimental setup was successfully used to continuously monitor the metal vaporization process during coal combustion independent of reactor design, and at different temperatures. Cd is more easily vaporized than Zn during coal combustion. Temperature significantly influences the metal vaporization process. In general, the higher the temperature, the higher the metal vaporization, although the vaporization is not proportional to temperature. In addition to the experimental study, a thermodynamic calculation was carried out to simulate the heavy metal speciation during coal combustion process. The theoretical volatilization tendency is consistent with the experiment. The thermodynamic calculation identified the formation of binary oxides retarding heavy metal vaporization.
Article
The environmental impact and possibilities for remediation and reuse of contaminated sediments was investigated. Water-soluble and (NH4)2-EDTA-soluble metal concentrations in sediments from three different rivers were monitored for five months after dredging. Furthermore, the pH-dependent leaching behaviour of heavy metals and the influence of elevated temperatures (900-1100 degrees C) on the release of metals were investigated in order to evaluate possibilities for remediation, through thermal treatment, and reuse of the sediments, such as for the production of bricks and lightweight aggregates. In general, the sediments investigated in this study present a relatively low risk to the environment when exposed to upland conditions. If the sediments were to be used for the production of bricks or lightweight aggregates, the increased mobility of As, Cr and V following thermal treatment of the sediments should be taken into account.
Article
Washing aggregate sludge from a gravel pit, sewage sludge from a wastewater treatment plant (WWTP) and a clay-rich sediment have been physically, chemically and mineralogically characterized. They were mixed, milled and formed into pellets, pre-heated for 5 min and sintered in a rotary kiln at 1150 degrees C, 1175 degrees C, 1200 degrees C and 1225 degrees C for 10 and 15 min at each temperature. The effects of the raw material characteristics, heating temperatures and dwell times on the loss on ignition (LOI), bloating index (BI), bulk density (rho(b)), apparent and dry particle densities (rho(a), rho(d)), voids (H), water absorption (WA(24h)) and compressive strength (S) were determined. All the mixtures presented a bloating potential taking into consideration the gases released at high temperatures. The products obtained were lightweight aggregates (LWAs) in accordance with Standard UNE-EN-13055-1 (rho(b)<or=1.20 g/cm(3) or particle density<or=2.00 g/cm(3)). LWAs manufactured with 50% washing aggregate sludge and 50% clay-rich sediment were expanded LWAs (BI>0) and showed the lowest apparent particle density, the lowest water absorption and the highest compressive strength. It was possible to establish three groups of LWAs on the basis of their properties in comparison to Arlita G3, F3 and F5, commercially available lightweight aggregates manufactured in Spain. Our LWAs may have the same or similar applications as these commercial products, such as horticulture, prefabricated lightweight structures and building structures.
Article
Chlorides derived from plastics and food residue content in MSW will affect the formation and partitioning of metal chlorides in the incineration discharges. Our study investigated the effects of waste-derived chlorides on the partitioning of heavy metals in a single-metal combustion system. The results indicate that the heavy metal partitioning behaviors are mainly affected by the presence of chloride, alkaline metals (i.e., Na, K) and moisture in the wastes. The configuration of the metal partitioning is determined by the availability of chlorine, hydrogen, and alkaline metals, or the extent to which the elements may divide from their compounds at a given combustion temperature. The effects of chlorides, including PVC, C2Cl4, FeCl3, NaCl and KCl, were also discussed.
Article
The leachability of heavy metals such as chromium (Cr), lead (Pb) and cadmium (Cd) from the ash material obtained from waste combustion was studied. The effects of ash surface topography and morphology on the leachability of these elements were examined using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The AFM (scan size 10 x 10 microns) and SEM images of the simulated ash pellet obtained at various operating temperatures (1000, 1400 and 1500 degrees C) showed significant microstructural and topographical changes. Ash pellets treated at 1000 degrees C contain porous and non-continuous surface. On the other hand, the ash pellet obtained at higher temperature (1500 degrees C) was found to contain a smooth, continuous and non-porous surface. The AFM height profile studies indicated that the top surface variation of the ash pellet at 1000, 1400 and 1500 degrees C were found to be -40.0 to 25.5, -3.7 to 4.7 and -0.10 to 0.66 nm respectively. The SEM analyses also confirmed the presence of smooth, non-porous outer surface of ash formed at 1500 degrees C. In addition, it also showed the presence of compact and rigid interior for the same ash pellet. The leachability of the heavy metals was determined using standard toxicity characteristic leaching procedure (TCLP) test and the samples were analysed using atomic absorption spectroscopy. The results showed that the TCLP leaching ratios of the heavy metals were Cr = 0.30, Pb = 0.05 and Cd = 0.09 at 1000 degrees C. However, the ash obtained at 1400 degrees C showed negligible heavy metals leaching ratio while at 1500 degrees C no leachability was detected (TCLP concentration dropped to nondetectable levels). The use of high temperature treatment enabled the immobilization of heavy metals in the ash preventing their leaching. Such ash can be considered as a non-hazardous material for reuse or safe disposal.
Article
Most of the municipal solid waste (MSW) in Japan is incinerated and the generated ash is landfilled. However, environmental pollution problems have increased and Japan has decreased final disposal sites for landfills. With the application of a melting system, the volume of incinerated ash can be reduced and the effective use of melted slag is being developed for use in civil engineering works. However, the low strength of melted slag as a vitreous structure has limited its effective use. As a solution for this deficiency, a technology to crystallize melted slag into higher strength produced stones was developed. With the joint cooperation of Chiba Prefecture and Kamagaya City, a demonstration plant for melting and stone production with a capacity of 4.8 tons of incinerator ash per day was constructed. The demonstration test was conducted from May 1998 to June 1999 with satisfactory results stated below. Long-term stable operation and performance of the plant have been confirmed and effective applications of produced stones have been demonstrated on a commercial scale. The results are as follows.
Article
Knowledge of chemical mobility of heavy metals is fundamental to understanding their toxicity, bioavailability, and geochemical behavior. In this paper, two different methods, i.e. mineralogical means and sequential extractions, were employed to analyze the total contents, existing states, and chemical forms of heavy metals in coal mine spoils. The results demonstrate that the mobility of heavy metals in coal mine spoils depends not only on their existing states and the stability of their host minerals but also on the properties of the coal mine spoils. In the process of coal mine spoils-water interaction, sulfides that contain heavy metals first break down and release metals, which are then adsorbed and complexed by the iron oxyhydroxide colloid resulting from pyrite oxidization and organic matter. During the natural weathering of coal mine spoils, only a small fraction of these metals are released to the environment, and most of them still remains in the residual material.
Article
The importance of heavy metal bioavailability on the bioconcentration in aquatic biota is examined. To this purpose, mono- and multivariate statistical techniques are applied to develop correlations between heavy metal bioconcentration factor and sediment characteristics, that are expected to affect bioavailability, using a database of heavy metal concentrations in biota and sediment along with the available physicochemical characteristics. The statistical analysis shows that satisfactory correlations are obtained only when factors that affect bioavailability, such as metal oxides concentration and organic carbon content in the sediment, are taken into account.
Article
The state of heavy metal pollution and the mobility of Cd, Cu, Ni, Cr, Pb and Zn were studied in three texturally different agricultural soil profiles near a Cu-Ni smelter in Harjavalta, Finland. The pseudo-total concentrations were determined by an aqua regia procedure. Metals were also determined after division into four fractions by sequential extraction with (1) acetic acid (exchangeable and specifically adsorbed metals), (2) a reducing agent (bound to Fe/Mn hydroxides), (3) an oxidizing agent (bound to soil organic matter) and (4) aqua regia (bound to mineral structures). Fallout from the smelter has increased the concentrations of Cd, Cu and Ni in the topsoil, where 75-90% of Cd, 49-72% of Cu and 22-52% of Ni occurred in the first two fractions. Slight Pb and Zn pollution was evident as well. High proportions of mobile Cd, Cu and Ni also deeper in the sandy soil, closest to the smelter, indicated some downward movement of metals. The hydroxide-bound fraction of Pb dominated in almost all soils and horizons, while Ni, Cr and Zn mostly occurred in mineral structures. Aqua regia extraction is usefully supplemented with sequential extraction, particularly in less polluted soils and in soils that exhibit substantial textural differences within the profiles.
Article
Bottom and fly ash collected from automobile shredder residue (ASR) incinerator have been characterized in terms of particle size, compositions, and heavy metal leaching by the standard TCLP method. Two alternative methods were also examined for the treatment of heavy metals in ASR incinerator ash from the aspect of recycling into construction or lightweight aggregate material. It was remarkable that the concentration of Cu was very high compared to common MSWI bottom and fly ash, which was probably originated from copper wires contained in ASR. As a whole, the results of characterization of ASR fly ash were in good agreement with common MSWI fly ash in terms of particle size, pH, and water-soluble compounds. It was clearly found that heavy metals could be removed thoroughly or partly from ASR fly ash through acid washing with dilute HCl solution so that the remaining fly ash could be landfilled or used as construction material. It was also found that the amount of heavy metal leachability of lightweight aggregate pellet prepared with ASR incineration ash could be significantly decreased so that the application of it to lightweight aggregate would be possible without pre-treatment for the removal of heavy metals.
Article
A sequential extraction method has been applied for the determination of binding forms of trace elements in the municipal solid waste incineration (MSWI) fly ash and evaluating their leaching behavior in view of their potential environmental impact. The elemental determinations in the different leachates are performed by ICP-AES and ICP-MS, respectively. The morphology and mineralogical phases after extraction step were performed by scanning electron microscopy (SEM). Total of 20 elements in the samples are investigated. A reference material of city waste incineration fly ash (BCR No. 176) is also tested to examine the applicability as well as accuracy of the proposed method. The sum of most elements present in the individual fractions shows a good agreement with the total elemental concentrations. The extraction efficiencies are generally higher than 80% except for that of Cr and V. The extractable data of most elements give information about the binding forms of various elements in both incineration fly ashes. It was found that the elements such as Ca, K, Na, Pb, Zn, Cd, Cu and Sr have exhibited a remarkable mobility in fly ash. More than half of them would be dissolved or exchanged under a mild leaching condition. The toxic elements such as Pb, Cd, Zn and Cu have a great potential to be released into the environment under normal conditions.
Article
Manufacturing the glass-ceramic has been proposed as a useful choice to recycle coal fly ash from power plants. In this work, a glass-ceramic of SiO2-Al2O3-Fe2O3-CaO family was synthesized by mixing 90 wt% of coal fly ash, from a power plant in west of China, with Na2O, and then melted at 1350 degrees C. The ceramization of the obtained glass was carried out at 770 degrees C for 2h. Esseneite and nepheline were found present as major crystal phases. The produced glass-ceramic exhibited good chemical durability as well as good mechanical properties. The toxicity characteristic leaching procedure (TCLP) method found that the glass-ceramic was non-hazardous.
The method standard for leaching toxicity of solid wastes-horizontal vibration extraction procedure (GB5086.2-1997)
  • Epa China
Reciclaje de residuos industriales: residuos sólidos urbanos y fangos de depuradora. Madrid: Editions Díaz de Santos
  • X E Castells