Neutralization of an extremely acidic sludge and stabilization of heavy metals in flyash aggregates

An extremely acidic, heavy metal-rich sludge (pH=-1.2) was scrubbed with a Class-F fly ash in order to simultaneously neutralize the acidity and stabilize the heavy metals contained in both wastes. This paper outlines the leaching behavior of the aggregate material generated by scrubbing. For proper fly ash/sludge ratios, the fly ash acted as an outstanding neutralizer for the acidic waste. Leaching of heavy metals from the aggregate samples was below the environmental limits within a pH range between 3 and 9. Subsequent washing of the leached aggregate with acidic CALWET solutions did not result in an additional release of heavy metals. It is proposed that coordinative bonding of the metal cations onto neutral surface sites and electrostatic adsorption led to stabilization of the heavy metals within the aggregate structure below hydrolysis pHs.     

Reducing volatilization of heavy metals in phosphate-pretreated municipal solid waste incineration fly ash by forming pyromorphite-like minerals

This research investigated the feasibility of reducing volatilization of heavy metals (lead, zinc and cadmium) in municipal solid waste incineration (MSWI) fly ash by forming pyromorphite-like minerals via phosphate pre-treatment. To evaluate the evaporation characteristics of three heavy metals from phosphate-pretreated MSWI fly ash, volatilization tests have been performed by means of a dedicated apparatus in the 100-1000 °C range. The toxicity characteristic leaching procedure (TCLP) test and BCR sequential extraction procedure were applied to assess phosphate stabilization process. The results showed that the volatilization behavior in phosphate-pretreated MSWI fly ash could be reduced effectively. Pyromorphite-like minerals formed in phosphate-pretreated MSWI fly ash were mainly responsible for the volatilization reduction of heavy metals in MSWI fly ash at higher temperature, due to their chemical fixation and thermal stabilization for heavy metals. The stabilization effects were encouraging for the potential reuse of MSWI fly ash.     

Thermal and hydrometallurgical recovery methods of heavy metals from municipal solid waste fly ash

Heavy metals in fly ash from municipal solid waste incinerators are present in high concentrations. Therefore fly ash must be treated as a hazardous material. On the other hand, it may be a potential source of heavy metals. Zinc, lead, cadmium, and copper can be relatively easily removed during the thermal treatment of fly ash, e.g. in the form of chlorides. In return, wet extraction methods could provide promising results for these elements including chromium and nickel. The aim of this study was to investigate and compare thermal and hydrometallurgical treatment of municipal solid waste fly ash. Thermal treatment of fly ash was performed in a rotary reactor at temperatures between 950 and 1050 °C and in a muffle oven at temperatures from 500 to 1200 °C. The removal more than 90% was reached by easy volatile heavy metals such as cadmium and lead and also by copper, however at higher temperature in the muffle oven. The alkaline (sodium hydroxide) and acid (sulphuric acid) leaching of the fly ash was carried out while the influence of temperature, time, concentration, and liquid/solid ratio were investigated. The combination of alkaline-acidic leaching enhanced the removal of, namely, zinc, chromium and nickel.     

Comparison of leaching characteristics of heavy metals from bottom and fly ashes in Korea and Japan

The objective of this research was to compare the leaching characteristics of heavy metals such as cadmium, chromium, copper, nickel, lead, etc., in Korean and Japanese municipal solid waste incineration (MSWI) ash. The rate of leaching of heavy metal was measured by KSLT and JTL-13, and the amount of heavy metals leached was compared with the metal content in each waste component. Finally, bio-availability testing was performed to assess the risks associated with heavy metals leached from bottom ash and fly ash. From the results, the value of neutralization ability in Japanese fly ash was four times higher than that in Korean fly ash. The reason was the difference in the content of Ca(OH)(2) in fly ash. The amount of lead leached exceeded the regulatory level in both Japanese and Korean fly ash. The rate of leaching was relatively low in ash with a pH in the range of 6-10. The bio-availability test in fly ash demonstrated that the amount of heavy metals leached was Pb>Cd>Cr, but the order was changed to Pb>Cr>Cd in the bottom ash. The leaching concentration of lead exceeded the Japanese risk level in all fly ashes from the two countries, but the leaching concentration of cadmium exceeded the regulatory level in Korean fly ash only.     

Literature review: Electrodialytic remediation and potential environmental applications of oil fly ash

Oil fly ash (OFA) contains environmentally toxic heavy metal and substituted polycyclic aromatic hydrocarbons. This review discusses the physical and chemical properties of OFA and presents information from other types of fly ash that can be used as concepts for the remediation and uses of OFA. Electrokinetic remediation is useful to remove some of the heavy metals for broader uses of the fly ash in agriculture, for making construction material, for contaminated wastewater treatment, and also for carbon dioxide sequestration. This review can be useful to develop approaches for the remediation and environmental management of OFA. © 2012 Wiley Periodicals, Inc.     

Heavy metal speciation in various types of fly ash from municipal solid waste incinerator

Fly ash (FA) from municipal solid waste incinerators has been known as hazardous waste, which is mostly because of the high content of heavy metal and dioxins. Besides the content, the form of the heavy metals in fly ash is also very important, because it is tightly related with the leaching behavior of fly ash in diverse circumstances. To evaluate the environment potential risk of fly ash, the Tessier chemical extraction method was adopted. In this study, four kinds of fly ash were examined, one sample from China (CFA) and the other three from Japan (RFA, CaFA and NaFA). Five metal elements were detected and they were Ni, Cr, Cd, Pb, and Cu. The result of total heavy metals’ concentration showed CFA has the lowest content. As to the Tessier chemical extraction experiments, the results show that Cd, Pb, and Cu have higher leaching risk in the environment than other heavy metals. The result of leaching test experiment showed that the more exchangeable speciation of Cd, Cr and Pb in FA, the more it could leach out in natural environment.     

Leaching toxicity and heavy metal bioavailability of medical waste incineration fly ash

The aim of this study was to provide a comprehensive risk assessment for medical waste incineration fly ash from another aspect through various leaching methods. The differences and connections between leaching concentrations achieved via the toxicity characteristic leaching procedure (TCLP), the physiologically based extraction test (PBET) and the sequential extraction procedure were also described. Heavy metal contents of the used medical waste incineration fly ash were 1.7–31 times higher than that from Japan, indicating poor medical waste management in China. The fly ash leaching concentration in the TCLP test exceeded the regulation value and can be characterized as hazardous waste under current regulations. However, the PBET concentrations were only 1/10 of the TCLP value or even lower, and the calculated ingested contents of all heavy metals were lower than tolerable daily intake, demonstrating that TCLP might have overestimated the environment risk to some degree. The leaching metal content of TCLP ranged from exchangeable to residual forms, and the leaching percentage varied from 7.75 to 92.55 %, while the content for PBET was equal to or lower than the exchangeable form.     

Leaching characteristics of residual lateritic soils stabilised with fly ash and lime for geotechnical applications

The effective use of residual lateritic soils as fill material for various construction activities is often limited by the difficulty in handling them. Attempt to improve their workability with fly ash and lime has shown promising results, although accompanied by risk of release of heavy metal leachates to the groundwater. In the present work, the leaching properties from residual lateritic soils from a part of Northeast India stabilised with fly ash and lime (CaO) was investigated with the help of single batch leaching test and column leaching test for different soil-fly ash-lime mixes. Test results show that the high pH induced by lime treatment of the mixes helps in keeping most of the metals within the stabilised soil matrix. Although the heavy metal concentrations in the leachates were generally within permissible limits, the release response for different metals was different suggesting implications for permeate solutions having metal pre-concentrations, such as those emanating from hazardous landfill sites. The observed characteristics provide insights towards the potential and realistic estimates of leaching of metals and its variation due to change in fly ash and lime content in the stabilised mix. Many of these constituents found in the stabilised soil had a first-flush phenomenon. But, as they occur only for short duration (about 5 pore volumes over 5 days) and at low concentration, dilution effect may eliminate them. The effect of continuous permeation on the flow parameters and the leaching pattern of the mixes have also been highlighted.     

Leaching behaviour of elements and evaluation of pre-treatment methods for municipal solid waste incinerator residues in column leaching tests.

Two new pre-treatment methods (water-washing/carbonation and carbonation/phosphate stabilization) of municipal solid waste (MSW) incinerator residues were evaluated by column leaching tests under aerobic conditions and anaerobic conditions (which were changed to aerobic conditions after 10 months). A mixture of bottom ash and fly ash (5:1 ratio) was pre-treated using each method. Shredded incombustible residues (SIR) were added to each ash preparation in proportions similar to the ratios present in landfills. For comparison, landfill wastes typical of Japan, namely, a mixture of bottom ash, chelating-pre-treated fly ash, and SIR, were also examined. Leachate samples were collected periodically and analysed over a 15-month period. When compared with chelating pretreatment, both water-washing/carbonation and carbonation/ phosphate stabilization reduced the leaching of Pb, Al, and Cu by about one to two orders of magnitude. Moreover, the initial concentrations of Ca and Pb in leachates from column of water-washing/carbonation were 56-57% and 84-96% less than those from the column of carbonation/phosphate stabilization. Therefore, water-washing/carbonation was considered to be a promising approach to obtain early waste stabilization and to reduce the release of heavy metals to near-negligible levels. The leaching behaviour of elements was also discussed.     

Experimental and numerical analysis of metal leaching from fly ash-amended highway bases

A study was conducted to evaluate the leaching potential of unpaved road materials (URM) mixed with lime activated high carbon fly ashes and to evaluate groundwater impacts of barium, boron, copper, and zinc leaching. This objective was met by a combination of batch water leach tests, column leach tests, and computer modeling. The laboratory tests were conducted on soil alone, fly ash alone, and URM-fly ash-lime kiln dust mixtures. The results indicated that an increase in fly ash and lime content has significant effects on leaching behavior of heavy metals from URM-fly ash mixture. An increase in fly ash content and a decrease in lime content promoted leaching of Ba, B and Cu whereas Zn leaching was primarily affected by the fly ash content. Numerically predicted field metal concentrations were significantly lower than the peak metal concentrations obtained in laboratory column leach tests, and field concentrations decreased with time and distance due to dispersion in soil vadose zone.     

Solidification/stabilization of ash from medical waste incineration into geopolymers

In the present work, bottom and fly ash, generated from incinerated medical waste, was used as a raw material for the production of geopolymers. The stabilization (S/S) process studied in this paper has been evaluated by means of the leaching and mechanical properties of the S/S solids obtained. Hospital waste ash, sodium hydroxide, sodium silicate solution and metakaolin were mixed. Geopolymers were cured at 50 °C for 24 h. After a certain aging time of 7 and 28 days, the strength of the geopolymer specimens, the leachability of heavy metals and the mineralogical phase of the produced geopolymers were studied. The effects of the additions of fly ash and calcium compounds were also investigated. The results showed that hospital waste ash can be utilized as source material for the production of geopolymers. The addition of fly ash and calcium compounds considerably improves the strength of the geopolymer specimens (2–8 MPa). Finally, the solidified matrices indicated that geopolymerization process is able to reduce the amount of the heavy metals found in the leachate of the hospital waste ash.     

Sustainable alkali-activated binders with municipal solid waste incineration ashes as sand or fly ash replacement

The present study investigates the feasibility of using two types of municipality solid wastes incineration ashes, namely, fly ash and bottom ash in the production of sustainable alkali-activated binder. The ashes are collected from the incineration plant and characterized to determine their particle size distribution, specific gravity, chemical composition, and heavy metals content. The ashes are then used as either fly ash or sand replacement with five replacement ratios 0%, 5%, 10%, 15%, and 20% to produce the binder. The produced binder are characterized in terms of strength, workability, density, water absorption, thermal conductivity and stability, chemical composition, and heavy metals content. The results reflect the ability of producing sustainable alkali-activated binder with small dosage of MSWI ashes as either fly ash or sand replacement without negatively affecting its strength, workability, density, and water absorption. The ashes enhance the thermal insulation capability of the binder.

     

Evaluation of the use of a sequential extraction procedure for the characterization and treatment of metal containing solid waste

Metal containing wastes like MSWI fly ashes and blast furnace sludge form a major environmental problem as they are polluted with heavy metals. The ash has to be landfilled or can be used as a construction material, but a pretreatment is in general necessary. Washing of the ashes with water in order to dissolve soluble salts or extracting the heavy metals with chemicals are possibilities. Blast furnace sludge contains large quantities of iron and carbon and could be recycled in the blast furnace, if the zinc content were not that high. Using a hydrometallurgical process the zinc can be removed from the sludge particles. In order to evaluate such treatment methods knowledge of the leaching behaviour of the studied material is very important. One of the factors influencing the leaching behaviour is the composition and mineralogy of the solids. A sequential extraction procedure, whereby the material is sequentially leached with different leaching solutions, can be used as an aid to characterize the material and to determine which chemical conditions are needed to obtain a sufficient extraction efficiency. To verify the accuracy of the sequential extraction procedure, a method is tested on MSWI fly ash and evaluated by comparing the results with those of leaching experiments whereby the final pH of the leaching solutions is varied over a wide range. Based upon this evaluation some suggestions for the use of the sequential extraction procedure are made and an adapted procedure is suggested, and applied to a blast furnace sludge.     

Sintering characteristics of CaO-rich municipal solid waste incineration fly ash through the addition of Si/Al-rich ash residues

Thermal treatment is a promising technology for the fast disposal of hazardous municipal solid waste incineration (MSWI) fly ash in China. However, fly ash produced in grate incinerator (GFA) is rich in CaO and chlorides, which promote the formation of toxic hexavalent chromium [Cr(VI)] and ash agglomeration during the thermal process, inhibiting the thermal disposal of GFA. In this study, sintering characteristics of CaO-rich GFA were improved by adding Si/Al-rich MSWI ash residues. According to the results, ash agglomeration was well suppressed during thermal treatment of the mixed ash. Si/Al/Fe-compounds competed with un-oxidized Cr-compounds to react with CaO and suppressed Cr(VI) formation. Meanwhile, chlorides in GFA facilitated heavy metal volatilization from added ashes to the secondary fly ash, favoring the recovery of these metals. Ca-aluminosilicates was found as the main mineral phase in the thermally treated mixed ash, which has attractive potential for applications. The formation of the aluminosilicates made the heavy metals that remained in the treated mixed ash more stable than the thermally treated single ash.     

Strength, leachability and microstructure characterisation of Na2SiO3-activated ground granulated blast-furnace slag solidified MSWI fly ash.

The chemical composition and the leachability of heavy metals in municipal solid waste incinerator (MSWI) fly ash were measured and analysed. For the leachability of unstabilized MSWI fly ash it was found that the concentrations of Pb and Cr exceeded the leaching toxicity standard. Cementitious solidification of the MSWI fly ash by Na2SiO3-activated ground granulated blast-furnace slag (NS) was investigated. Results show that all solidified MSWI fly ash can meet the landfill standards after 28 days of curing. The heavy metals were immobilized within the hydration products such as C-S-H gel and ettringite through physical encapsulation, substitution, precipitation or adsorption mechanisms.     

Lead immobilization in mechanochemical fly ash recycling

In previous studies, we focused on a mechanochemical process for recycling fly ash for use in cement; this process was expected to immobilize heavy metals in the fly ash, a desirable outcome in light of the fact that recycled fly ash is commonly used in the synthesis of inorganic materials. Here, we investigated the leaching of lead (Pb) from fly ash treated by a mechanochemical process and from cement prepared from the treated fly ash. We used lead oxide (PbO), a typical Pb compound in fly ash, as a model substance. Mechanochemical treatment of the fly ash inhibited Pb leaching by 93%, and further inhibition (more than 99.9%) was observed in cement produced from the treated fly ash. During the mechanochemical treatment, PbO was reduced to Pb by iron from the stainless-steel mill used for processing, and the lower solubility of Pb in water resulted in immobilization of the Pb.     

Leaching of heavy metals by citric acid from fly ash generated in municipal waste incineration plants

The leaching behavior of heavy metals from municipal waste incineration (MWI) fly ash was investigated in this study. The leaching process includes two steps, i.e., fly ash was firstly washed with water, and then subjected to citric acid leaching. The main parameters of the washing process such as liquid/solid ratio, washing time, and number of washing were tested. The optimum conditions for water washing were found as follows: washing time 5–10 min, liquid/solid ratio 10:1 (ml:g), and number of washing was twice; under these conditions, 86% Na, 70% K, 12% Ca, 1.2% Al, and 0.5% Pb were removed from the fly ash in the prewashing. From the results of screening tests of leaching lixiviants, citric acid was found to be the most effective leaching agent, taking account of its environmentally benign characteristics. Optimum metal extraction can be achieved with citric acid under the following conditions: pH 3.0, liquid/solid ratio 40 (ml:g), citric acid concentration 0.10 mol/dm³, contact time 20 min at room temperature.     

Utilization of municipal solid waste incineration (MSWI) fly ash in ceramic brick: product characterization and environmental toxicity

In this study, municipal solid waste incineration (MSWI) fly ash was used as a blending in making ceramic brick based on its characterization and an orthogonal test was performed to determine the optimal mixture ratio of the materials. Besides, the fired bricks made in accordance with the optimal mixture ratio were characterized for performance, phase transformation, microstructure, leaching toxicity of the heavy metals in accordance with GB/T 2542-92 (Detection methods for bricks analysis, China) and by means of XRD, SEM and leaching toxicity analysis. It was found that the optimal mixture ratio of materials (MSWI fly ash:red ceramic clay:feldspar:gang sand) was 20:60:10:10 by mass, and the optimal sintering temperature was 950 °C. Leaching results of heavy metals from sintered bricks were reduced considerably in comparison with those from green bricks prior to sintering process. The results as a whole suggested that utilization of MSWI fly ash in ceramic brick constituted a potential means of adding value.     

Influence of treatment techniques on Cu leaching and different organic fractions in MSWI bottom ash leachate

The leaching of heavy metals, such as copper, from municipal solid waste incinerator (MSWI) bottom ash is of concern in many countries and may inhibit the beneficial reuse of this secondary material. Previous studies have focused on the role of dissolved organic carbon (DOC) on the leaching of copper. Recently, a study of the Energy Research Centre of The Netherlands (ECN) showed fulvic acid-type components to exist in the MSWI bottom ash leachates and to be likely responsible for the generally observed enhanced copper leaching. These findings were verified for a MSWI bottom ash (slashed circle 0.1-2 mm) fraction from an incinerator in Flanders. The filtered leachates were subjected to the IHSS fractionation procedure to identify and quantify the fractions of humic acid (HA), fulvic acid (FA) and hydrophilic organic carbon (Hi). The possible complexation of fulvic acid with other heavy metals (e.g., lead) was also investigated. The identified role of fulvic acids in the leaching of copper and other heavy metals can be used in the development of techniques to improve the environmental quality of MSWI bottom ash. Thermal treatment and extraction with a 0.2 M ammonium-citrate solution were optimized to reduce the leaching of copper and other heavy metals. The effect of these techniques on the different fractions of organic matter (HA, FA, Hi) was studied. However, due to the obvious drawbacks of the two techniques, research is focused on finding other (new) techniques to treat MSWI bottom ash. In view of this, particle size-based separation was performed to evaluate its effect on heavy metal leaching and on HA, FA and Hi in MSWI bottom ash leachates.     

Environmental impact of fly ash utilization in roadway embankments

The leaching potential of heavy metals from a roadway embankment constructed of fly ash and soil mixture was studied. Leaching of eight environmentally concerned metals Ag, As, Ba, Cd, Cr, Hg, Pb, and Se from the fly ash–soil mixtures was examined through batch leaching test and column leaching test. The batch leaching test results showed that the fly ash meets the local regulatory standards for beneficial use of nonhazardous wastes. The column leach test revealed that only Ba, Cr, and Se were detectable in the effluents. The peak concentration of Ba in the effluents was much lower than the US EPA Primary Drinking Water Regulations’ maximum contaminant level (MCL). The peak concentrations of Cr and Se exceeded the MCLs only in the initial flush stage and quickly decreased to below the MCLs. Results of this study suggest a great potential for fly ash to be used in roadway embankments to enhance their mechanical properties, reduce the use of soil, and avoid the disposal of fly ash as waste.