Characteristics of the cement-solidified municipal solid waste incineration fly ash

Co-treatment of municipal solid waste incineration fly ash and alumina-/silica-containing waste: A critical review

Municipal solid waste incineration fly ash (MSWI-FA) is a hazardous by-product of the incineration process, characterized by elevated levels of heavy metals, chlorides, and dioxins. With a composition high in calcium but low in silicon/aluminum, MSWI-FA exhibits a poor immobilization effect, high energy demands, and limited pozzolanic activity when it is disposed of or reutilized alone. Conversely, alumina-/silica-containing waste (ASW) presents a chemical composition rich in SiO2 and/or Al2O3, offering an opportunity for synergistic treatment with MSWI-FA to facilitate its harmless disposal and resource recovery. Despite the growing interest in co-treatment of MSWI-FA and ASW in recent years, a comprehensive evaluation of ASW's roles in this process remains absent from the existing literature. Therefore, this study endeavors to examine the advancement in the co-treatment of MSWI-FA and ASW, with the focus on three key aspects, i.e., elucidating the immobilization mechanisms by which ASW improves the solidification/stabilization of MSWI-FA, exploring the synergies between MSWI-FA and ASW in various thermal and mechanochemical treatments, and highlighting the benefits of incorporating ASW in the production of MSWI-FA-based building materials. Additionally, in the pursuit of sustainable solid waste management, this review identifies research gaps and delineates future prospects for the co-treatment of MSWI-FA and ASW.

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.

Effect of long-term dry-wet circulations on the Solidification/stabilization of Municipal solid waste incineration fly ash using a novel cementitious material

Solidification/stabilization (S/S) is a typical technique to immobilize toxic heavy metals in Municipal solid waste incineration fly ash (MSWI FA). This study utilized blast furnace slag, steel slag, desulfurization gypsum, and phosphoric acid sludge to develop a novel metallurgical slag based cementing material (MSCM). Its S/S effects of MSWI FA and long-term S/S effectiveness under dry-wet circulations (DWC) were evaluated and compared with ordinary Portland cement (OPC). The MSCM-FA block with 25 wt.% MSCM content achieved 28-day compressive strength of 9.38 MPa, indicating its high hydration reactivity. The leaching concentrations of Pb, Zn and Cd were just 51.4, 1895.8 and 36.1 μg/L, respectively, well below the limit standard of Municipal solid wastes in China (GB 16889-2008). After 30 times' DWC, leaching concentrations of Pb, Zn and Cd for MSCM-FA blocks increased up to 130.7, 9107.4 and 156.8 μg/L, respectively, but considerably lower than those for OPC-FA blocks (689, 11,870.6 and 185.2 μg/L, respectively). The XRD and chemical speciation analysis revealed the desorption of Pb, Zn and Cd attached to surface of C-S-H crystalline structure during the DWC. The XPS and SEM-EDS analysis confirmed the formation of Pb-O-Si and Zn-O-Si bonds via isomorphous replacement of C-A-S-H in binder-FA blocks. Ettringite crystalline structure in OPC-FA block was severely destructed during the DWC, resulting in the reduced contents of PbSO4 and CaZn2Si2O7·H2O and the higher leachability of Pb2+ and Zn2+.

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.

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.

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.

Incineration disposal of organic waste bio-residue via a deep dewatering process using refuse incineration bottom ash: moisture transfer and low calorific value improvement

Bio-residue is a by-product from organic waste anaerobic digestion process with high moisture, high organic matter, low calorific value and poor biological stability, and may be incinerated after deep dewatering. The moisture existence forms of bio-residue were clarified firstly, finding that adsorbed water, capillary water and bound water occupied 75.61%, 23.81% and 0.58%, respectively. Furthermore, refuse incineration bottom ash was used as a typical deep dewatering reagent, when compared with calcium oxide (CaO), refuse incineration fly ash and iron powder-potassium persulfate (Fe(0)-K₂S₂O₈). Results showed that the addition of bottom ash powder could decrease the bio-residue's moisture from 80.0% to 37.9% at the optimal dosage of 2.0%, and corresponding low calorific value reached 10.5 MJ/kg at the 10^th d. It was suggested that ZnCl₂ and ZnSO₄ present in bottom ash powder could react with moisture in bio-residue, forming hydrates of inorganic salt and breaking the polyacrylamide colloid, and lead to the release of adsorbed water in the bio-residue.

Study of Solidifying Surplus Sludge as Building Material Using Ordinary Portland Cement

In an attempt to effectively utilize a multitude of surplus sludge from sewage treatment plants, ordinary Portland cement was used to solidify the dry surplus sludge as a building material. The dry surplus sludge and cement were mixed at different proportions with a certain dosage of water and then cured for 3–60 days at room temperature. The unconfined compression strength (RC) of solidified blocks was investigated with respect to the effects of the ratio of liquid to solid (Rl/S), surplus sludge dosage (DS), the dosage of sodium silicate (DNa2SiO3), and the proportion of fly ash (WF). The fabricated solidified blocks were characterized by scanning electron microscopy (SEM), Fourier transform-infrared spectroscopy (FT-IR), and X-ray Diffraction Analysis (XRD). The results demonstrated that RC at 60 days reduced obviously with the increase in Rl/s when Ds was given, whereas RC reduced with DS increased to 15.0 wt% from 5.0 wt% for solidified blocks. When DS was 5.0 wt%, RC of 28 days was reduced from 20.87 MPa to 14.50 MPa, with an increase in Rl/s from 0.35 to 0.55. For the given Rl/s, such as Rl/s = 0.35, RC at 60 days was 23.75 MPa, 2.80 MPa, and 2.50 MPa when DS were 5.0 wt%, 10.0 wt%, and 15.0 wt%, respectively, which were relatively lower in comparison to that of Portland cement solidified blocks without surplus sludge (51.40 MPa). In addition, the addition of Na2SiO3 and fly ash was favorable in terms of improving the RC for solidified blocks. RC of 60 days increased initially and then reduced with the increase in DNa2SiO3 from 0.0 wt% to 9.0 wt% at Rl/s = 0.45 and DS = 5.0 wt%. At DNa2SiO3 = 7.5 wt%, Rl/s = 0.45, and DS = 5.0 wt%, the highest RC value of 34.70 MPa was achieved after being cured for 60 days. Furthermore, RC of 60 days increased initially and then reduced with WF increasing from 0.0 wt% to 25.0 wt%, and the highest RC value of 34.35 MPa was achieved at WF = 10.0 wt%, Rl/s = 0.45, and DS = 5.0 wt%. At the ratio of DNa2SiO3 = 7.50 wt%, Rl/S = 0.35, WF = 20 wt%, DS = 15.0 wt% and M = 1.00, RC of 28 days reached 26.70 MPa. With these values, the utilization of sludge utilized (DS = 15.0 wt%) was increased by double compared with DS = 5.0 wt% (20.87 MPa). To investigate the effect of environmental temperature on the mechanical properties and mass of solidified blocks, the freeze-thaw cycling experiment was carried out. The RC of 28 days and the mass of the solidified block reduced with the number of freeze-thaw cycles, increasing for solidified blocks with DS of 5.0 wt%, 10.0 wt%, and 15.0 wt%, manifesting a decrease of 25.60%, 32.30%, and 40.60% for RC and 3.40%, 4.10%, and 4.90% for mass, respectively. This work provides sufficient evidence that surplus sludge has a huge potential application for building materials from the perspective of improving their mechanical properties. It provides an important theoretical basis for the disposal as well as efficient utilization of sludge.

Solidification/Stabilization of MSWI Fly Ash Using a Novel Metallurgical Slag-Based Cementitious Material

Four industrial wastes, i.e., blast furnace slag, steel slag, desulfurization ash, and phosphoric acid sludge, were used to prepare a low-carbon binder, metallurgical slag-based cementitious material (MSCM). The feasibility of solidification/stabilization of municipal solid waste incineration (MSWI) fly ashes by MSCM were evaluated, and the immobilization mechanisms of heavy metals were proposed. The MSCM paste achieved 28-day strength of 35.2 MPa, showing its high-hydration reactivity. While the fly ash content was as high as 80 wt.%, the 28-day strength of MSCM-fly ash blocks reached 2.2 MPa, and the leaching concentrations of Pb, Zn, Cr, and Hg were much lower than the limit values of the Chinese landfill standard (GB 16889-2008). The immobilization rates of each heavy metal reached 98.75–99.99%, while four kinds of MSWI fly ashes were solidified by MSWI at fly ash content of 60 wt.%. The 28-day strength of binder-fly ash blocks had an increase of 104.92–127.96% by using MSCM to replace ordinary Portland cement (OPC). Correspondingly, the lower leachability of heavy metals was achieved by using MSCM compared to OPC. The mechanisms of solidification/stabilization treatment of MSWI fly ash by MSCM were investigated by XRD, SEM, and TG-DSC. Numerous hydrates, such as calcium silicate hydrate (C-S-H), ettringite (AFt), and Friedel’s salt, were observed in hardened MSCM-fly ash pastes. Heavy metals from both MSWI fly ash and MSCM could be effectively immobilized via adsorption, cation exchange, precipitation, and physical encapsulation.

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.

Municipal solid waste incineration residues recycled for typical construction materials-a review

Focusing on the great potential of municipal solid waste incineration (MSWI) residues in the construction sector, the applications of recycling MSWI residues in construction materials are discussed in this review. Incineration is a promising method for managing the great quantity of municipal solid waste (MSW). Careful handling of incineration residues including fly ash, air pollution control (APC) residues, and bottom ash is required for this approach. The yield of these residues is large, and they contain many toxic and harmful substances. On the other hand, these residues contain valuable components such as SiO2, CaO, Al2O3, MgO, which are important components of building materials. Therefore, MSWI residues present huge opportunities for potential recycling and reuse in the construction and building industry. This paper summarized and discussed the application of MSWI residues in four typical building materials including cast stone, glass-ceramic, cement, and concrete. Before utilization, three types of pretreatment methods can be used to reduce the toxicity of the residues and improve the performance of the products. In addition, the current issues and the prospects of this field, and the environmental impacts of this application were discussed. It was concluded that MSWI residues can be used to prepare building materials after proper treatment which can improve the mechanical and chemical properties of the residues. The recycling can gain significant economic and environmental benefits at the same time. However, further researches on treatment methods for fine particles are needed.

Solidification of heavy metals and PCDD/Fs from municipal solid waste incineration fly ash by the polymerization of calcium carbonate oligomers

Calcium carbonate oligomers are gel-state precursors that can be crystallized by low-temperature heat treatments to form an inorganic material with a monolithic and continuous structure, this material can effectively solidify/stabilize heavy metals in municipal solid waste incineration fly ash (MSWI FA). Calcium chloride addition achieves FA stabilization/solidification by the formation and polymerization of calcium carbonate oligomers. The effects of calcium, triethylamine (TEA), and water-washing pretreatment on the solidification of heavy metals by the polymer were studied. Consequently, as more calcium was added, the solidification improved. When the ratio of TEA/Ca²⁺ was increased from 2:1 to 3:1, the solidification efficiency of As and Cd increased, but it decreased when the ratio was continuously increased to 4:1. After the water-washing pre-treatment, the MSWI FA had a significantly improved solidification effect on the heavy metals, and the solidification efficiencies of zinc, copper, cadmium, chromium, lead, and arsenic were 81.9%, 90.0%, 93.5%, 91.8%, 99.6% and 95.5%, respectively. Additionally, the solidification efficiency of PCDD/Fs was 56.5%. The heavy metals and PCDD/Fs in MSWI FA solidified by physical adsorption, wrapping and chemical precipitation. The continuous calcium carbonate structure adsorbed and encased the MSWI FA, and the heavy metals in the MSWI FA were converted from a free state to carbonate precipitates through carbonation, and the carbonate precipitate was more likely to be physical solidification by calcium carbonate.

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.

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

A novel heavy metal chelating agent, dithiocarboxylate-functionalized polyaminoamide dendrimer (PAMAM-0G-DTC), was evaluated for the stabilization of heavy metals from municipal solids waste incineration (MSWI) fly ash. PAMAM-0G-DTC achieved overall stabilization performance at a lower dosage (3% w/w) and a wider pH range (2-13) compared to conventional chelating agents such as sodium dimethyl dithiocarbamate (SDD) and dithiocarboxylate-functionalized tetraethylenepentamine (TEPA-DTC). The leaching toxicity of Pb and Cd in the MSWI fly ash by PAMAM-0G-DTC stabilization met the landfill requirements but could not be achieved by SDD and TEPA-DTC even at a 10 wt% concentration. Sequential chemical extraction of fly ash before and after stabilization shows that PAMAM-0G-DTC can be combined with active heavy metals in water-soluble, interchangeable, and carbonate states to form more stable heavy metals in organic and residual states. Mechanistic studies show that multiple PAMAM-0G-DTC molecules can combine with multiple heavy metals to form three-dimensional network-like super-molecular compounds with an infinite extension of space size. This makes the heavy metals more stable and embedded in the network-like super-molecular structure, thus minimizing the potential risk of leaching. Overall, by forming more geochemically stable phases, the treatment of fly ash with PAMAM-0G-DTC has a strong ability to reduce the toxic leaching of heavy metals at a lower dosage and suppress the risk of secondary pollution in a landfill at a wide range of pH values (2-13).

Investigation of controlling factors on toxic metal leaching behavior in municipal solid wastes incineration fly ash

Municipal solid wastes incineration (MSWI) fly ash has drawn worldwide attention for its substantial annual generation capacity and high toxic metals leachability. Although many factors have been shown to affect the leachability of metals in fly ash, the controlling factors, which guide the selection of appropriate risk reduction method, remain unclear. The purpose of this study was to evaluate the effects of the two most important factors, total metal content, and remaining alkaline substances of MSWI fly ash, on the leaching behavior of toxic metals. In this work, a series of leaching tests and sequential extraction procedures were performed for seven fly ash samples collected from one MSWI plant. Results show that particulate size distribution, morphology, and mineralogy of all samples are similar, indicating the effects of these properties on metal leaching behavior can be ignored. In leaching tests, although the leaching behavior in terms of metal species and concentration levels vary as expected, only the leachate Pb concentration in four samples (up to 17.32 mg/L) exceeds the threshold in Chinese regulation (0.25 mg/L). The variation of the leachate Pb concentration is not consistent with the change of the total Pb concentration in fly ash. Further correlation analysis evidences that the acid-soluble Pb, which is highly correlated to the calcium content of fly ash, dominates the concentration of leachate Pb. Notably, when the addition of lime is about 1.5 times over the theoretical value, the concentration of leachate Pb would exceed the threshold regardless of the total Pb concentration in fly ash. Overall, this study demonstrates that the remaining alkaline substances (mainly calcium-bearing compounds), rather than the total content of metals, are the controlling factor of metal leaching behavior in fly ash. Thus, strategies to delicately optimize the quantity of lime addition in acid gas purification process should be considered to minimize MSWI fly ash environmental risks in the future.