Stabilization of heavy metals in MSWI fly ash with a novel dithiocarboxylate-functionalized polyaminoamide dendrimer

Solidification/stabilization pre-treatment coupled with landfill disposal of heavy metals in MSWI fly ash in China: A systematic review

The growth in municipal solid waste incineration (MSWI) has resulted in a substantial rise in the production of fly ash in China. It is anticipated that during the "14th Five-Year Plan", the accumulated amount of fly ash stocked and disposed of at landfills will surpass 100 million tons. With the development of the economy and the implementation of garbage classification relevant policies, the pollution characteristics of heavy metal change in spatiotemporal distribution. Solidification/stabilization (S/S) pre-treatment coupled with landfill disposal is the mainstream method for fly ash. This study provides a systematic overview and comparison of the current application status and research on the mechanism of S/S technology, and the long-term stability of solidified/stabilized fly ash is a crucial factor in controlling the risks of landfills. Subsequently, it examines the influencing factors and mechanisms associated with heavy metals leaching under different environmental scenarios (meteorological factors, leachate and acid rain erosion, and carbonation, etc.), and concludes that single stabilization technology is difficult to meet long-term landfill requirements. Finally, the limits of heavy metal leaching toxicity evaluation methods and landfilled fly ash supervision were discussed, and relevant suggestions for future development were proposed. This study can provide theoretical instruction and technical support for the risk control of potential environmental risks of heavy metals in solidified/stabilized fly ash from landfills in China.

Geopolymer Materials from Fly Ash-A Sustainable Approach to Hazardous Waste Management

This study explores the utilisation challenges of fly ash from municipal waste incineration, specifically focusing on ash from a dry desulphurisation plant (DDS), which is categorised as hazardous due to its high heavy metal content. The ash's low silicon and calcium contents restrict its standalone utility. Laboratory investigations initially revealed that geopolymers derived solely from fly ash after flue gas treatment (FGT), in combination with coal combustion fly ash, exhibited low compressive strength (below 0.6 MPa). However, the study demonstrated significant improvements by modifying the FGT ash through water leaching. This process enhanced its performance when mixed with high-silica and -aluminium fly ash, resulting in geopolymers achieving compressive strengths of up to 18 MPa. Comparable strength outcomes were observed when the modified ash was blended with commercial cement. Leachability tests conducted for heavy metals (HMs) such as copper, zinc, lead, cadmium, and nickel indicated that their concentrations fell below the regulatory limits for landfill disposal: 2, 4, 0.5, 0.04, and 0.4 mg/kg, respectively. These results underscore the effectiveness of water-washing FGT ash in conjunction with other materials for producing geopolymers, contributing to sustainable waste management practices.

Highly Efficient Recovery of Au(I) from Gold Leaching Solution Using Sodium Dimethyldithiocarbamate

As a sustainable, nontoxic and environmentally friendly cyanide-free gold leaching agent, thiosulfate has been applied to some extent in the field of hydrometallurgy. However, the difficult recovery of gold ions in gold leaching solutions limits further application of thiosulfate gold leaching technology. This study demonstrated the feasibility of gold recovery by sodium dimethyldithiocarbamate (SDD) precipitation and recycling of ammonia and a lixiviant in solution. SDD achieved the purpose of recovering gold by forming granular precipitates with gold ions in solution. It can almost completely recover gold ions in 2.5-17.34 mg/L of gold leaching solution within 1 min at 25 °C, in which a gold recovery capacity of 7.99 kg/t is achieved. The leaching rate of gold ore did not change significantly after recycling the residual ammonia and thiosulfate in the leaching solution after gold recovery by SDD, and its leaching rate basically remained at 81%. The mechanism of SDD recovering Au was determined to involve the ligand exchange of SDD- and Au[(S2O3)2]3-. Moreover, the interaction mechanism between SDD and Au(I) was further validated by density functional theory calculations. Considering its low cost, simple technology, and environmental friendliness, the SDD precipitation process has the potential for large-scale application in gold recovery from thiosulfate gold leaching solutions.

A new strategy to stabilize the heavy metals in carbonized MSWI-fly ash using an acid-resistant oligomeric dithiocarbamate chelator

Fly ash (FA) derived from municipal solid waste incineration (MSWI) requires safe handling before landfilling due to its extremely high salt content and the risk of leaching heavy metals (HMs) under acidic conditions. Herein, aimed at improving the acid stability of dithiocarbamates, a cost-effective oligomeric dithiocarbamate (ODTC) was developed to stabilize HMs from carbonated MSWI-FA. Spiking of 3.6 wt% ODTC reduced the HM leaching below landfill standards in China, even across the pH range of 2.0-13.0 or 8-week exposure to the natural environment. Stabilization decreased the acid-soluble/exchangeable fractions of Cd, Pb, and Zn from 22.2%, 4.49%, and 21.9% to 0.14%, 0.11%, and 12.2%, respectively, resulting in safe levels for Pb and Cd with risk assessments. Compared to DDTC and SDD, ODTC exhibited higher stability under acidic conditions after chelation with the HMs, minimized the risk of HM leaching, and significantly reduced stabilization costs. In-depth studies proved that the stabilization mechanism involved the ability of ODTC to chelate HMs strongly and form acid-resistant ODTC-HM complexes, agglomeration of the MSWI-FA grains to encapsulate the ODTC-HM complexes, transformations of the HMs from acid-soluble species to stable oxidizable and residual species, and specifically ODTC reducing high-valent Pb to more stable Pb(II) species.

Stabilization of heavy metals in municipal solid waste incineration fly ash using organic chelating agents: Insight into risk assessment and function mechanism

Landfill treatment of municipal solid waste incineration fly ash (MSWI FA) after stabilization is the primary disposal technology. However, only few studies have assessed the stability of MSWI-FA-chelated products in different landfill scenarios. In this study, three commonly used dithiocarbamate (DTC)-based organic chelating agents (CAs) (TS-300, SDD, and PD) were selected to stabilize heavy metals (HMs) in MSWI FA. In addition, the leaching toxicity and environmental risks of the chelated products were assessed in different disposal environments. The results demonstrate that the HM leaching concentrations of the chelated products met the concentration limits of the sanitary landfill standard (GB16889-2008; mixed Landfill Scenario) for the three CAs at a low additive level (0.3 %). However, in the compartmentalized landfill scenario (the leaching agent was acid rain), the leaching of HMs from the chelated products met the standard when TS-300, SDD, and PD were added at 1.5 %, 6.0 %, and 8.0 %, respectively. Additionally, Pb, Zn, and Cd in the chelated products from the 1.5 %-TS-300 and 6.0 %-SDD groups met the leaching limits within the pH ranges 6-12 and 7-12, 6-12 and 7-12, and 8-12 and 8-12, respectively. This was primarily due of TS-300's multiple DTC groups forming stable chain-like macromolecular chelates with Pb. However, although the environmental risks associated with Pb, Zn, and Cd in the initial (0-d) chelated products of the 1.5 %-TS-300 and 6.0 %-SDD groups were minimized to low and negligible levels, there was a significant increase in the leaching of the three HMs after 28 d of storage. Therefore, with appropriate CA addition, although the leaching concentration of HMs in the chelated product may comply with the GB16889-2008 standards, it remains essential to consider its environmental risk, particularly in highly acidic or alkaline environments and during prolonged storage of the product.

Transformation behavior of heavy metal during Co-thermal treatment of hazardous waste incineration fly ash and slag/electroplating sludge

In this study, the behavior of heavy metal transformation during the co-thermal treatment of hazardous waste incineration fly ash (HWIFA) and Fe-containing hazardous waste (including hazardous waste incineration bottom slag (HWIBS) and electroplating sludge (ES)) was investigated. The findings demonstrated that such a treatment effectively reduced the static leaching toxicity of Cr and Pb. Moreover, when the treatment temperature exceeded 1000 °C, the co-thermal treated sample exhibited low concentrations of dynamically leached Cr, Pb, and Zn, indicating that these heavy metals were successful detoxified. Thermodynamic analyses and phase transformation results suggested that the formation of spinel and the gradual disappearance of chromium dioxide in the presence of Fe-containing hazardous wastes contributed to the solidification of chromium. Additionally, the efficient detoxification of Pb and Zn was attributed to their volatilization and entry into the liquid phase during the co-thermal treatment process. Therefore, this study sets an excellent example of the co-thermal treatment of hazardous wastes and the control of heavy metal pollution during the treatment process.

Factors affecting the durability of dimethyl dithiocarbamate-stabilized air pollution control (APC) residues derived from municipal solid waste incineration

Sodium dimethyl dithiocarbamate (SDD) is widely used for stabilizing heavy metals to minimize pollution from air pollution control (APC) residues derived from municipal solid waste incineration. However, the effect of environmental conditions on heavy metal leaching from SDD-stabilized APC residues remains unknown. Therefore, this study aimed to evaluate the durability of SDD-stabilized APC residues and determine the relationship between heavy metal leaching and environmental factors, including pH, temperature, and oxygen. The results revealed that accelerated SDD decomposition and the decline in durability of SDD-stabilized APC residues were caused by acidic and aerated conditions and temperatures above 40 °C. A decrease in pH from 12.25 to 4.69 increased the Cd and Pb concentrations in SDD-stabilized APC residue leachate from below detection (0.002 mg/L) to 1.32 mg/L and 0.04 mg/L to 3.79 mg/L, respectively. Heating at 100 °C for 2 d increased the Cd and Pb concentrations from below detection (0.002 mg/L and 0.01 mg/L) to 2.96 mg/L and 0.47 mg/L, respectively. Aeration for 5 d increased the Cd and Pb concentrations from below detection to 0.09 mg/L and 0.49 mg/L, respectively. The decline in durability was attributed to acid hydrolysis, thermal decomposition, and oxidative damage of SDD, resulting in breakage of the chelated sulfur-metal bond, which was confirmed by the decrease in the oxidizable fraction of heavy metals and the SDD content. This study improves the understanding of the factors contributing to the decline in durability of heavy metals in SDD-stabilized APC residues, which is important for ensuring the long-term stabilization and environmental safety of these residues.

Leaching of Cr, Cu, Ni, and Zn from different solid wastes: Effects of adding adsorbents and using different leaching solutions

The leaching of potentially toxic elements from different industrial solid wastes (ISWs) must be understood to manage the environmental concerns they pose. The objective of this research was to investigate the effect of clay mineral (bentonite) and nanoparticle (MgO) on potentially toxic elements (Cr, Cu, Ni, Zn) leaching in some ISWs, when they leached with different leaching solutions. The highest amount of Zn and Ni was leached from ceramic factory waste (CFW) and stone cutting wastes (SCW), respectively, while the highest amount of Cr was leached from leather factory waste (LFW). In ISWs, the leaching percentage of Cu, Ni, and Zn were up to 11.2%, whereas the greatest leaching percentage of Cr was 26.7% of the total content. The addition of bentonite and MgO decreased potentially toxic element leaching. The results of effluents speciation of SFW indicated that at the beginning of leaching with CaCl2, nitric acid, and citric acid, 75.1%, 84.1%, and 39.6% of Cr were in different forms of Cr (III), respectively, while at the end of leaching the percentage of Cr (III) species were decreased and Cr (VI) species were increased to 83.6%, 88.4%, and 93.4%, respectively. The addition of bentonite and especially MgO to the ISWs reduced the leaching of potentially toxic elements as well as reduced the percentage of Cr (VI) in the effluents of SFW. The findings suggested that bentonite has the potential to be a low-cost and environmentally acceptable adsorbent for minimizing the leaching of Cr and other potentially toxic elements from ISWs.

Mineral phase transition characteristics and its effects on the stabilization of heavy metals in industrial hazardous wastes incineration (IHWI) fly ash via microwave-assisted hydrothermal treatment

Toxic heavy metals in industrial hazardous waste incineration (IHWI) fly ash can be effectively stabilized by using microwave-assisted hydrothermal technology. However, few works have focused on the relationship between mineralogical conversion and stability of heavy metals of fly ash during hydrothermal process. This study investigated the effect of mineral phase transition process on the stabilization and migration behavior of heavy metals in IHWI fly ash using coal fly ash as silicon‑aluminum additive. Mineral composition analysis reveals that after microwave-assisted hydrothermal treatment (MAHT) of IHWI fly ash, zeolite-like minerals (e.g., tobermorite, katoite and sodalite), secondary aluminosilicate minerals (e.g., prehnite and anorthite) and other newly-formed minerals (e.g., wollastonite, pectolite and larnite) were found. The leaching concentrations of heavy metals (Cr, Ni, Cu, Zn, Cd and Pb) in IHWI fly ash decrease sharply after MAHT with the most obvious decreases in Cu, Pb and Zn. Spearman correlation analysis show significantly negative correlation between the content of zeolite-like minerals and the leaching concentrations of most heavy metals (e.g., Ni, Cu, Zn, Cd and Pb). These results suggest that the immobilization effects of heavy metals in IHWI fly ash can be effectively enhanced by promoting the formation of zeolite-like minerals during the MAHT. This study is expected to further promote the development of IHWI fly ash harmless treatment technology.

An all-in-one strategy for municipal solid waste incineration fly ash full resource utilization by heat treatment with added kaolin

Resourcization has become a popular research topic for the final disposal of municipal solid waste incineration fly ash (MSWI FA). However, the current research is limited to building material preparation or valuable chloride recovery, which may cause resource waste and secondary pollution. A unique process, heat treatment with the addition of kaolin (KL), was presented to achieve complete resource utilization of MSWI FA. The physical properties of ceramsite could be improved by adding KL, and dioxin removal, heavy metals, and valuable chloride separation could be achieved via sintering at 1150 °C. The separation and purification of dust carried by the flue gas during thermal treatment (secondary fly ash) was achieved via wet separation. A building ceramsite with a compressive strength of 24.8 MPa was obtained, whereas dioxin and heavy metal toxicity were far below the standard limits. Heavy metal content was enriched by 12 times, approximately 59.6%, achieved after secondary fly ash separation and purification. A heavy metal product containing 39.5% Zn, 19.1% Pb, and chloride salt containing 41.8% KCl were obtained. This showed a high potential for the developed process to separate multiple valuable elements from ashes. This novel process will further promote the development and application of harmless and resourceful technologies for MSWI FA.

Submicron tourmaline enhanced the solidification of municipal solid waste incineration fly ash by chemical structure reorganization and stabilized heavy metals

Municipal solid waste incineration fly ash (MSWIFA) exsits in large quantitities and contains pollutants such as heavy metal. While solidification is one of the most effective methods for treating MSWIFA, this application is limited by cost, subsequent treatment, and simultaneous immobilization of anions and cations. This research demonstrated that under a certain initial pressure (20 MPa), a gelation reaction involving ball milling-modified tourmaline powder, a small amount of cement clinker, and MSWIFA forms a stable consolidated body and significantly reduces the risk of heavy metal dissolution. The consolidated MSWIFA can easily be formed into unfired bricks in large-scale pilot production, and a response surface model was used to optimize the experimental parameters. When the mass ratio of tourmaline: cement clinker: MSWIFA was 15:15:200 (mixed with a moisture content of 13 to 15 %), the compressive strength of the consolidated body reached 13 MPa, and the amounts of Cr and Pb leached decreased from 12 mg/L to 0.1 mg/L and 25 mg/L to 0.3 mg/L, respectively. The consolidated form contained a new mineral phase (Ca₃Si₂O₇·3H₂O, Ca₁₀Mg_0.8(SiO₄)_0.6O₂Cl, and CaCl₂∙Ca(OH)₂·H₂O) with a high compressive strength. Notably, the soluble PbSO₄ in the MSWIFA was converted into relatively stable PbSiO₃, and Cr(VI) was lattice-wrapped. This study was the first to demonstrate that tourmaline synchronously passivates Pb(II) and Cr(VI) in fly ash in the solid phase, with a low cost and requires no subsequent treatment. This study provided a novel technical path for recycling MSWIFA. Eventually, leaching of the heavy metals Pb, Cr, Cu, Cd, and Zn from the solids achieved concentrations less than 0.25, 1.5, 0.5, 0.15, and 100 mg/L.

Utilization of gasification slag and petrochemical incineration fly ash for glass ceramic production

This study investigated glass ceramics produced using coal gasification slag (CGS) and petrochemical incineration fly ash (PIFA) to immobilize hazardous heavy metals such as Cr and As. However, the crystallization kinetics and stabilization behavior mechanism of different heavy metals in the petrochemical incineration fly ash-derived glass-ceramics remains unclear. And X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, and inductively coupled plasma mass spectrometry were used to characterize glass and crystalline products. In this paper, we reported the crystallization kinetics and chemical leaching characteristics of the glass ceramic. A low crystallization activation energy of 121.49 kJ/mol was achieved from crystallization peak of several different heating rates around 850°C, implying that it is easier to produce the glass ceramics at that temperature. The Avrami parameter of the former crystallization was determined to be 1.23 ± .12, which indicated two-dimensional crystal growth with heterogeneous nucleation. The toxicity characteristic leaching procedure results indicated that the heavy metals were well solidified, and that the leaching concentration was significantly lower than the limit specified by governmental agencies. The potentially toxic element index of the parent glass and the two glass ceramics were 11.7, 5.8, and 3.6, respectively. Therefore, the conversion of hazardous petrochemical incineration fly ash and other solid waste into environmentally friendly glass ceramics shows considerable potential and reliability.

PCDD/Fs and heavy metals in the vicinity of landfill used for MSWI fly ash disposal: Pollutant distribution and environmental impact assessment

This study focused on the syngenetic control of polychlorinated-ρ-dibenzodioxins and dibenzofurans (PCDD/Fs) and heavy metals by field stabilization/solidification (S/S) treatment for municipal solid waste incineration fly ash (MSWIFA) and multi-step leachate treatment. Modified European Community Bureau of Reference (BCR) speciation analysis and risk assessment code (RAC) revealed the medium environment risk of Cd and Mn, indicating the necessity of S/S treatment for MSWIFA. S/S treatment significantly declined the mass/toxic concentrations of PCDD/Fs (i.e., from 7.21 to 4.25 μg/kg; from 0.32 to 0.20 μg I-TEQ/kg) and heavy metals in MSWIFA due to chemical fixation and dilution effect. The S/S mechanism of sodium dimethyldithiocarbamate (SDD) and cement was decreasing heavy metals in the mild acid-soluble fraction to reduce their mobility and bioavailability. Oxidation treatment of leachate reduced the PCDD/F concentration from 49.10 to 28.71 pg/L (i.e., from 1.60 to 0.98 pg I-TEQ/L) by suspension absorption or NaClO oxidation decomposition, whereas a so-called "memory effect" phenomena in the subsequent procedures (adsorption, press filtration, flocculating settling, slurry separation, and carbon filtration) increased it back to 38.60 pg/L (1.66 pg I-TEQ/L). Moreover, the multi-step leachate treatment also effectively reduced the concentrations of heavy metals to 1-4 orders of magnitude lower than the national emission standards. Furthermore, the PCDD/Fs and heavy metals in other multiple media (soil, landfill leachate, groundwater, and river water) and their spatial distribution characteristics site were also investigated. No evidence showed any influence of the landfill on the surrounding liquid media. The slightly higher concentration of PCDD/Fs in the soil samples was ascribed to other waste management processes (transportation and unloading) or other local source (hazardous incineration plant). Therefore, proper management of landfills and leachate has a negligible effect on the surrounding environment.

Co-stabilization/solidification of heavy metals in municipal solid waste incineration fly ash and electrolytic manganese residue based on self-bonding characteristics

Municipal solid waste incineration (MSWI) fly ash and electrolytic manganese residue (EMR) were classified as hazardous waste, must be harmlessly processed prior to subsequent treatment or disposal. The competition between massive free manganese ions of raw EMR and other heavy metals was found, thus raw EMR was pretreated by calcining to eliminate competition of manganese with other heavy metals for stabilizer complexation. MSWI fly ash was successfully solidified with 6% NaH₂PO₄, 6% H₂NCSNH₂ and 20% sintered EMR (800 °C). The addition of sintered EMR enhanced solidification/stabilization of heavy metals in fly ash and the resulting product had a higher compressive strength for further reutilization like trench backfilling, structural fill and void filling. The stabilization/solidification mechanism of heavy metals was attributed to the combined interaction of heavy metal precipitation in stabilizers and ion exchange or physical encapsulation in silicate compounds like calcium silicate, which is a feasible and valuable approach to co-disposal of MSWI fly ash and EMR.

Disposal technology and new progress for dioxins and heavy metals in fly ash from municipal solid waste incineration: A critical review

Incineration has gradually become the most effective way to deal with MSW due to its obvious volume reduction and weight reduction effects. However, since heavy metals and organic pollutants carried by municipal solid waste incinerator fly ash (MSWI FA) pose a serious threat to the ecological environment and human health, they need to be handled carefully. In this study, the current status of MSWI FA disposal was first reviewed, and the harmless and resourceful disposal technologies of heavy metals and organic pollutants in MSWI FA are summarized as well. A summary of the advantages and disadvantages of each technology, including sintering, melting/vitrification, hydrothermal treatment, mechanochemistry, solidification/stabilization of MSWI FA, is compared. Finally, the research work that needs to be strengthened in the future (such as codisposal of multiple wastes, long-term stability research of disposal products, etc.) was proposed. Through comprehensive analysis, some reasonable and feasible suggestions were provided for the effective and safe disposal of MSWI FA in the future.

Stabilization of Waste-to-Energy (WTE) fly ash for disposal in landfills or use as cement substitute

This study investigated two approaches for managing Waste-to-Energy (WTE) fly ash (FA): (i) phosphoric acid stabilization of FA and disposal in non-hazardous landfills, so that it can pass the U.S. TCLP procedure and meet the U.S. Resource Conservation and Recovery Act (RCRA) standards; (ii) use of FA or phosphoric acid stabilized fly ash (PFA) as cement substitute in construction for avoiding disposal in landfills and reducing the consumption of Portland cement. The effect of stabilization was identified by TCLP tests and XRD quantification (QXRD), which showed that the economically optimal concentration for PFA to pass the RCRA was 1 mol/L H₃PO₄ (equivalent to 0.4 mol of H₃PO₄/kg of FA). Zn/Pb-phosphates were formed in treated ash by using high concentration H₃PO₄ (e.g., 3 mol/L). Thus, the hazardous FA was chemically stabilized to PFA, that were both discussed as cement substitute. QXRD and SEM results showed that both FA and PFA (1 mol/L H₃PO₄) chemically reacted with cement and water. Up to 25 vol% of the cement can be replaced by FA or PFA, with similar mechanical performance of cement mortars than that of reference. Testing by LEAF Method 1313-pH dependence showed that the FA and PFA cement mortars exhibited the same leachability of heavy metals; therefore, this study demonstrated the technical feasibility of utilizing either raw FA or stabilized PFA as supplementary cementitious material. The leachability of heavy metals in optimal FA or PFA 25 vol% cement mortar was under the U.K. WAC non-hazardous limits.

Chemical stabilization of heavy metals in municipal solid waste incineration fly ash: a review

Sufficient attention should be attached to the large amount of fly ash containing high levels of toxic heavy metals generated after municipal solid waste incineration. Because heavy metals could be leached out of the fly ash under specific conditions, it is necessary to stabilize the heavy metals in fly ash before landfill disposal. Processing technologies of incineration fly ash include solidification/stabilization technology, thermal treatments, and separation processes. This study reviewed the current treatment technologies of municipal solid waste incineration (MSWI) fly ash, with the main focus on the treatment of heavy metals in fly ash with chemical stabilization. Chemical stabilization processes involve chemical precipitation of heavy metal and chelation of heavy metals. In multiple studies, chemical stabilization technology has shown practical feasibility in terms of technology, economy, and effect. In addition, the combination of two or more stabilization agents broadens the general applicability of the agents to heavy metals and reduces the cost. The application of joint processing technology realizes the remove of soluble salt from fly ash. To minimize pollutants while increase their usable value, effective use of waste and co-disposal of several kinds of wastes have gradually become the research hotspots. New developments in chemical stabilization are progressively moving towards the sustainable direction of harmlessness and resource utilization of MSWI fly ash.

Risk assessment of lead and cadmium leaching from solidified/stabilized MSWI fly ash under long-term landfill simulation test

The long-term effectiveness concern of municipal solid waste incineration (MSWI) fly ash (FA) disposal has been placed more emphatic recently, however, few studies worked on the control of leaching risk of heavy metals under the long-term stability. In this study, the leaching properties and risk assessment of two representative solidified/stabilized (S/S) FA wastes, i.e., sodium dithiocarbamate (DTC) chelator treated and Portland cement + chelator combining treated, were evaluated by a long-term cycles assessment method which coupled multifaceted environmental stresses (e.g., freezing-thawing, drying-wetting, accelerated carbonation). The results showed that the cement/chelator had a better long-term stability and exhibited ~55% lower cumulative overall pollution toxicity index (OPTI) than chelator treatment after the test, which was always rated as "low risk" during the cycles. In addition, the cement/chelator exhibited ~23.3% smaller cumulative mass release rate than the chelator treatment after 6 cycles and restrained the transformation of Pb and Cd from stable states to removable fractions, which attributes to its great erosion resistance and compact pore structure. Under the cumulative external factors and carbon dioxide attacks, the decalcification of hydrate products (e.g., C-S-H, hydrocalumite), as well as deterioration of pore structure are the critical factors increasing the local erosion, cracking and heavy metals release. Thus, the optimization of S/S waste microstructure (e.g., enhancing binder system) and landfill site conditions (e.g., reducing rainfall impact) could be propitious to the S/S waste risk control and management.

A novel waste-recycled chelating agent for the stabilization of lead in municipal solid waste incineration fly ash: Preparation, feasibility, and mechanism analysis

There are two key issues in the area of waste management: one is stabilizing heavy metal, lead (Pb), in municipal solid waste incineration fly ash (MSWI-FA), the other is enhancing nitrogen utilization efficiency from organic waste. However, the connection between both issues was limited. In this study, a novel organic chelating agent based on food waste, FW-CA, was produced for immobilizing Pb. A maximum 86.22% of polypeptide in hydrolysis liquid of FW was utilized for preparing FW-CA with a yield of 8.22 g/L. Results indicated that FW-CA-stabilized fly ash satisfied the criteria of GB 18598-2019 with a dosage of 4.6%, lower than the demand of pure chemicals and industrially applied chelating agents. After treating with FW-CA, the exchangeable and bound to carbonate fraction of Pb decreased by 5.08% and 18.57%, respectively, contributing to a low environmental risk class of the Pb assessment code. FW-CA effectively chelated Pb at a wider range of leaching pH (3.19-11.24), and the leachability was hardly affected by curing time, which were attributed to the presence of dithiocarbamate group and formation of cross-linked structure between Pb and sulfur. Overall, the unique waste-utilized chelating agent was a suitable alternative for Pb stabilization in MSWI-FA.

Stability evaluation of potentially toxic elements in MSWI fly ash during carbonation in view of two leaching scenarios

Carbonation treatment (CT) by alkaline fly ash (FA) affects the stability of potentially toxic elements (PTEs). This study investigated the leachability and environmental risk of six PTEs contained in FA during natural and accelerated carbonation (NC, AC) using two typical leaching scenarios with distilled water (DW) and acetic acid (AA). The leaching of Pb/Cu/Cr/Ni in solidified/stabilized FA decreased due to CT in DW leaching, but the leaching of Pb/Zn/Cu/Cd increased due to CT in AA leaching. The leaching of the six PTEs (especially Pb/Cd) in AA leaching was significantly higher than that in DW leaching. CT was a promoting factor to increase the environmental risk level of PTEs in FA leachate, especially in AA leaching with H⁺ input. In the early stage of NC, under DW leaching tests, the environmental risk level of PTEs in FA leachate can be weakened due to the formation of carbonate minerals in the FA matrix. However, excessive NC increases the environmental risk of leached PTEs due to the decalcification of carbonate minerals. Both NC and AC increased the potential environmental risk of PTEs contained in the carbonated FA matrix. The nucleation and dissolution of carbonate minerals were interdependent with the immobilization and leaching of PTEs, which played a dominant role in the CT and leaching tests respectively. They jointly affected the occurrence behavior of PTEs in the FA matrix in CT tests and the leachability of PTEs in leaching tests. This study demonstrates that it is more scientific to evaluate the leachability of PTEs in carbonated FA according to the actual disposal scenarios.

Removal of heavy metals in municipal solid waste incineration fly ash using lactic acid fermentation broth

Municipal solid waste incineration fly ash (MSWIFA) is considered as a hazardous solid waste because of the high mobility of heavy metals. In this study, the removal of heavy metals in MSWIFA using lactic acid fermentation broth (LAFB) under various leaching protocols (i.e. LAFB addition amount and timing) was investigated. Results revealed that compared with that in pure lactic acid solution, the synergistic effect of various substances in LAFB was more favourable to the dissolution of heavy metals. Although the content of acid-soluble heavy metals in MSWIFA decreased after leaching with LAFB, the leaching toxicity measured by acetic acid buffer solution method increased to varying degrees (except that of Cr). Moreover, the maximum leaching concentration of Pb was 14.1 mg/L (standard limit, 0.25 mg/L), which was not conducive to the landfill treatment of MSWIFA. However, if the LAFB-treated MSWIFA was used in cement kiln for co-disposal, the amount of MSWIFA entering the kiln was 6.0 percentage points higher than that in pure water leaching. Therefore, LAFB leaching instead of water leaching is expected to be an effective pre-treatment method for the utilisation of MSWIFA.

Chloride removal from municipal solid waste incineration fly ash using lactic acid fermentation broth

As far as improvement of chlorine removal from fly ash by lactic acid fermentation broth (LAFB) was concerned, it is particularly important to explore the instinct mechanism and understand how leaching protocols (i.e. lactic acid addition amount and timing) affect the dechlorination efficiency. Results revealed that the WLL leaching protocol yielded the highest dechlorination efficiency (i.e. removed 98.7% of the total chlorine content of fly ash). The undissolved chlorine in fly ash residue might wrap inside the crystal structure of CaAlSiO₄(OH). Given that the chlorine removal from fly ash might prohibit by the newly formed calcium salt precipitation, exclusively increase the addition amount of LAFB (i.e. LLL protocol) did not necessarily stimulate the dechlorination efficiency. Conversely, it might accelerate the fly ash mass reduction (compared with WLL protocol), resulting in a high chlorine content in fly ash residue. Therefore, instead of increasing lactic acid strength, reducing the thickness of the calcium salt precipitation layer or breaking the crystal structure of CaAlSiO₄(OH) during the leaching process was suggested for efficient fly ash dechlorination.

Performance and mechanism of mold-pressing alkali-activated material from MSWI fly ash for its heavy metals solidification

The safe disposal of municipal solid waste incineration fly ash (MSWIFA) has become the weakest link of the circular economy of MSW due to its hazardous nature. In this study, we focused on the heavy metals solidification of MSWIFA by using alkali-activation technology and introducing a mold-pressing method. The influence of alkaline activator (AA) including alkali concentration and dosage of sodium silicate solution were well designed and studied. MSWIFA before and after alkali-activation, as well as the sample treated by commercial chelating agent (CA), were contrastively studied the performance of heavy metals solidification. The results show that the alkali-activated MSWIFA exhibits superior solidification for heavy metals than the blank control and the CA treated ones. With mold-pressing technology, the alkali-activated MSWIFA shows a core-shell structure, in which a thin layer that is composed of mainly N-A-S-H gel is as the shell and acts as a protective layer to inhibit the leaching of heavy metals. Besides, the introduced mold-pressing technology is beneficial for the improvement of materials strength and the reduction of AA dosage. The optimal AA composition is that the net concentration of NaOH is ∼4 M and sodium silicate dosage is ∼65 wt% in alkaline activator, and the total alkaline activator requirement is only 32 wt% of MSWIFA, yielding 7.9 MPa compressive strength at 10.2 MPa molding pressure. In summary, this work paves a potential new way for safe and recycling use of hazardous MSWIFA, which will be of great significance to environmental sustainability.