Municipal Solid Waste Incineration Fly Ash Research Articles

Effect of magnetite on the catalytic oxidation of polycyclic aromatic hydrocarbons in fly ash from MSW incineration: A comparative study of one-step and two-step hydrothermal processes

Polycyclic aromatic hydrocarbons (PAHs), many of which are carcinogenic, teratogenic, and mutagenic, exist in fly ash (FA) produced from municipal solid waste incineration (MSWI). Hydrothermal treatment (HT) is an efficient approach to remove PAHs from MSWI FA. Here, magnetite (Fe3O4) was used as the catalyst and hydrogen peroxide (H2O2) as the oxidant for one-step and two-step catalytic hydrothermal methods. When the magnetite dosage increased to 15 wt%, the maximum degradation rates of PAHs were 84.36% and 92.51%, respectively; however, the toxicity equivalent quantity (TEQ) degradation rates of the PAHs both increased upon increasing the magnetite dose. At 20 wt% Fe3O4, the maximum TEQ degradation rates of the PAHs were 93.29% and 97.76%, respectively. The reaction between OH and PAHs is non-selective, which means that LMW, MMW, and HMW PAHs were all degraded. The decrease in TEQ was mainly due to the degradation of HMW PAHs, i.e., those with five rings. Under the same Fe3O4 dose, oxidant dose, and reaction time, the detoxification of PAHs by the two-step method was significantly better than that of the one-step method, possibly because the two-step method more effectively produced OH. The first step degraded more than 90% of PAHs, and the residual PAHs in the HT products of the first step limited the utilization of the oxidant during the second step. The minerals in the HT products implied that the two-step hydrothermal method not only produced more OH, which reacted with PAHs, but also generated metal-magnetite substitution, which affected its surface reactivity during heavy metal adsorption and catalysis. These results revealed that both magnetite and the two-step hydrothermal treatment degraded PAHs. 20 wt% magnetite was the optimal amount during the two-step hydrothermal catalytic oxidation of MSWI FA.

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.

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.

Roles of biochar in cement-based stabilization/solidification of municipal solid waste incineration fly ash

Low-carbon stabilization/solidification (S/S) is of increasing importance for sustainable treatment of hazardous wastes. In this study, we integrated carbon-negative rice husk biochar (RBC) and yard waste biochar (YBC) as green additives into the binder for S/S of municipal solid waste incineration (MSWI) fly ash. Experimental results illustrated that the addition of both biochars promoted cement hydration reaction via pozzolanic reaction and internal curing. In particular, the incorporation of 10 wt% RBC (rich in activated Si) significantly increased the content of C-S-H gel from 41.6 wt% (control sample) to 52.0 wt% and increased the average degree of connectivity of C-S-H gel from 1.43 to 1.52 as indicated by quantitative X-ray diffraction and 29Si nuclear magnetic resonance analysis. The incorporation of RBC and YBC (20 or 30 wt%) enhanced the immobilization efficiency of potentially toxic elements in MSWI fly ash due to the additional hydration products and high adsorption ability of biochar. For instance, in R-80FA and Y-80FA samples (namely, 20 wt% binder dosage, of which RBC or YBC accounted for 10 wt% of binder), the immobilization efficiency for Pb could reach 96.2% and comply with the leachability limit. The biochar-modified S/S blocks achieved comparable strength to the cement-based S/S blocks, presenting a mechanically stable solidified matrix for engineering application. Therefore, this study expands the emerging application of biochar and demonstrates that biochar-augmented binder can ensure low-carbon and high-performance S/S of hazardous materials.

Impact of electrodialytic remediation of MSWI fly ash on hydration and mechanical properties of blends with Portland cement

Municipal solid waste incineration (MSWI) residues such as fly ash and air pollution control (APC) residues may serve as new supplementary cementitious materials (SCM). These SCMs may, however, require a pre-treatment to improve material properties. This study investigates the impact of electrodialytically remediated (EDR) MSWI residues to remove heavy metals and salts, on the phase development, setting, and compressive strength development of composite cements with 10 wt% cement replacement, compared to inert quartz or untreated (raw) MSWI residues. All treated MSWI residues showed reactivity in mortar, resulting in higher compressive strength than inert quartz, attributed to additional ettringite and monocarbonate formation. The results indicate that electrodialytical remediation improves the performance of MSWI fly ash in blends with Portland cement, while MSWI APC residues might be used without pre-treatment.

CO2 pretreatment of municipal solid waste incineration fly ash and its feasible use as supplementary cementitious material

In this study, municipal solid waste incineration fly ash (MSWIFA) was pretreated with CO2 via slurry carbonation (SC) and dry carbonation coupled with subsequent water washing (DCW). Both the treated MSWIFAs were then used as cement replacement in cement pastes by weight of 10%, 20% and 30% to investigate the influence on hydration mechanisms, physico-mechanical characteristics and leaching properties. The results showed that carbonates formed on the surface of SC-MSWIFA particles were finer (primarily 20–50 nm calcite) than those from the corresponding DCW-MSWIFA (mostly 130–200 nm vaterite). Hence, SC-MSWIFA blended cement pastes led to shorter setting time and higher early compressive strength than the DCW-MSWIFA pastes. In contrast, the presence of vaterite-rich DCW-MSWIFA in the blended cement pastes could accelerate the cement hydration after 24 h. Both the CO2-pretreated MSWIFA can replace cement up to 30% without sacrificing the long-term strength and mechanical properties of cement pastes, demonstrating excellent performance as a supplementary cementitious material. Moreover, volume stability in terms of expansion and lead leaching of CO2-pretreated MSWIFA cement pastes were far below the regulatory limits.

Purification of landfill gas by extracted calcium ions from municipal solid waste incineration fly ash

This study proposes the utilization of CO2 based on the purification of landfill gas (LFG). The process included absorption of CO2 from LFG using monoethanolamine (MEA) absorbent, extraction of calcium ions from municipal solid waste incineration (MSWI) fly ash using various acids, and formation of calcium carbonate using the extracted calcium ions. During LFG purification, the concentration of CH4 in the gas after absorption was time dependent. The pH swing method was used for the extraction of calcium ions and comprised three phases: calcium ion leaching from MSWI fly ash phase, removal of cations from the supernatant, and calcium ion recovery. Hydrochloric and nitric acids, known as strong acids, and citric, acetic, and formic acids, which are weak acids, were used as extraction agents. Hydrochloric acid, nitric acid, acetic acid, and formic acid showed significant calcium ion recovery rates of 99.32%, 99.18%, 98.35%, and 97.99%, respectively, whereas citric acid showed a relatively low recovery rate of 82.82%. The extracted calcium ions were converted into calcium carbonate by reacting with ionic CO2 in the saturated MEA. The calcium carbonate formed showed different crystal structures based on the extraction agent used: aragonite for hydrochloric acid and nitric acid, amorphous CaCO3 for citric acid, vaterite for acetic acid, and calcite for formic acid. The results of this study can be applied to various CO2 utilization processes based on LFG and MSWI fly ash.

Cytotoxicity of stabilized/solidified municipal solid waste incineration fly ash

Low-carbon stabilization/solidification (S/S) is of increasing importance as an option for the treatment of municipal solid waste incineration fly ash (MIFA). This study tailored four binders (e.g., ordinary Portland cement (OPC), calcium aluminate cement (CAC), phosphate-modified OPC, and phosphate-modified CAC) for S/S of MIFA and evaluated the cytotoxicity of treated MIFA by using A549 cell-based in-vitro assay. After S/S treatment, the leachability of Cr, Cu, Zn and Pb from MIFA decreased by 76.1%, 93.4%, 69.6%, and 85.5%, respectively. Spectroscopic analysis indicated that the hydration products determined the immobilization efficiencies of various binders, and strong bonding between metallic cations and phosphate enhanced the immobilization efficiency. The treated MIFA showed significantly lower cellular reactive oxygen species (ROS)-inducing abilities than original MIFA, in which with phosphate-modified OPC treated MIFA showed the lowest ROS levels. Intracellular ROS and multicytotoxicity results also revealed that the treated MIFA not only decreased the cytotoxicity-inducing capability but also enhanced the tolerant dosage of cytotoxicity, in which phosphate-modified S/S treatments showed more effective mitigation (25% less cytotoxicity) than plain cement treatments due to the high-efficiency immobilization of potentially toxic elements. This study develops a pioneering assessment protocol to measure the success of sustainable treatment of MIFA in human health perspective.

A systematic approach to evaluating environmental-economic benefits of high-gravity technology for flue gas purification and municipal solid waste incineration fly ash utilization

This study proposed a systematic approach to evaluating the environmental-economic benefits of multiple air pollution control, CO2 mineralization, and municipal solid waste incineration (MSWI) fly ash stabilization-utilization with high-gravity (HiGee) technology. MSWI fly ash due to its high alkalinity and calcium-rich compositions could be used to absorb acid gas pollutants and mineralize CO2 simultaneously. HiGee system can be deployed to eliminate the mass-transfer limit to accelerate the reaction kinetics. Technical-economic analysis (TEA) and life cycle analysis (LCA) were integrated with the technique to order preference by similarity to ideal solution (TOPSIS) approach and comprehensively evaluate the prioritized alternatives. Compared to existing wet-scrubber and fly ash landfill, the combination of fly ash slurry and further utilization of cementitious material can generate additional profits of direct/indirect carbon credit, calcium carbonate byproduct, reduction of air pollution fees and fly ash utilization revenue. In terms of environmental benefits, the installed HiGee system can reduce water and energy consumption to avoid water depletion and greenhouse gas emissions. TOPSIS results prioritized the various alternatives of HiGee operation modes and provided a technological evaluation for substituting the existing scrubber in an MSWI plant.

Evaluation of various microwave-assisted acid digestion procedures for the determination of major and heavy metal elements in municipal solid waste incineration fly ash

Municipal solid waste incineration (MSWI) generates large amounts of fly ash, which requires proper disposal due to containing high levels of toxic elements. Accurate determination of the contents of these elements is crucial to its further treatment and utilization. Microwave-assisted acid digestion is a widely used approach for elemental determination in MSWI fly ash. The digestion procedure affects the analytical results of element contents. This study compares various procedures for microwave-assisted acid digestion of MSWI fly ash in the literature and standard methods. Factors including four common acid combinations and sample mass (from 25 to 250 mg) were investigated, and the analytical results for major and heavy metal element contents were evaluated. The effects of HF and HClO4 in acid combinations were studied in detail. The results showed that acid combinations and sample mass significantly affect analytical results for Ca, Mg, Al, Ba, and Sr. HF is essential to the total digestion of MSWI fly ash, but can lead to loss of Ca, Mg, Al, Ba, and Sr. Loss of elements was caused by the precipitation of Ca and HF as well as complex co-precipitation reactions during digestion. Reducing sample mass and using HClO4 can mitigate loss of element and facilitate the determination of elements in MSWI fly ash.

Early solidification/stabilization mechanism of heavy metals (Pb, Cr and Zn) in Shell coal gasification fly ash based geopolymer

Immobilizing heavy metals (HMs) from municipal solid waste incineration fly ash (MSWIFA) using shell coal gasification fly ash (SFA)-based geopolymer can solve the energy and environmental challenges simultaneously. In this study, we synthesized a geopolymer with SFA, metakaolin (MK), and steel slag (SS) to solidify and stabilize HMs (Pb, Cr, and Zn) and investigated the early immobilization mechanisms. The results show that the prepared geopolymer possessed high early-age mechanical strength and immobilization efficiency to HMs (>90%), even under the effect of excess HMs. The early immobilization mechanism of the geopolymer for the HMs could be described as follows. (1) Most of HMs were remained in the aluminosilicate. (2) The presence of amorphous zeolite precursor and clay minerals may contribute to restrain the HMs leaching; (3) Pb and Zn were trapped by the gel structure in M-O-Al and M-O-Si forms (M = Pb or Zn), whereas Cr (VI) was reduced to Cr (III). (4) Cr might involve in the geopolymerization of [SiO4] and [AlO4]− units. (5) The immobilization process of Pb and Zn in the geopolymer could be described as crystal growth (NG) - phase boundary reaction (I) - NG - I - diffusion (D), whereas that of Cr is prolonged to NG-I-NG-I-NG-I-D.

Designing novel magnesium oxysulfate cement for stabilization/solidification of municipal solid waste incineration fly ash

Municipal solid waste incineration (MSWI) fly ash is a typical hazardous waste worldwide. In this study, an innovative magnesium oxysulfate cement (MOSC) binder was designed for stabilization/solidification (S/S) of MSWI fly ash, focusing on the interactions between MOSC binder and typical metallic cations (Pb2+)/oxyanions (AsO33-). Experimental results showed that Pb and As slightly inhibited the reaction of high-sulfate 5MS system but significantly suppressed the reaction process of low-sulfate 10MS system. The 5MS binder system exhibited excellent immobilization efficiencies (99.8%) for both Pb and As. The extended X-ray absorption fine structure spectra suggested that Pb2+ coordinated with SO42-/OH- in the MOSC system and substituted Mg2+ ion sites in the internal structure of 5Mg(OH)2·MgSO4.7H2O (5−1−7) phase. In contrast, the AsO33- substituted SO42- sites with the formation of inner-sphere complexes with Mg2+ in the large interlayer space of the 5–1–7 structure. Subsequent MSWI fly ash S/S experiments showed that a small amount of reactive Si in MSWI fly ash interfered with the MOSC reaction and adversely influenced the immobilization efficiencies of Pb, As, and other elements. Through the use of 33 wt% tailored MOSC binder for MSWI fly ash treatment, a satisfying S/S performance could be achieved.

A comparative study on solidification/stabilization characteristics of coal fly ash-based geopolymer and Portland cement on heavy metals in MSWI fly ash

Cement-based solidification/stabilization (S/S) is a widely approach applied for municipal solid waste incineration (MSWI) fly ash. Although Portland cement on MSWI fly ash S/S has been widely developed, the economical and environment-friendly binders should be explored to reduce the energy and improve MSWI fly ash S/S performances. A comparative study about MSWI fly ash S/S treatment with coal fly ash-based geopolymer and Portland cement was carried out in this work. Specifically, physical and chemical characteristics of MSWI fly ash, compressive strength, leachability and chemical speciation transformation of heavy metals, microstructures and morphology of S/S samples were investigated. Results showed that leaching concentration of Pb, Cd and Zn in MSWI fly ash was 5.36 mg/L, 40.3 mg/L and 129.0 mg/L respectively. It was 34.7, 160.2 and 0.29 times higher than Standard GB16889, far exceeding the requirements for municipal solid waste landfill. Leaching tests indicated that immobilization rates of Pb, Zn and Cd with geopolymer S/S treatment were exceeding 99%. Compressive strength of geopolymer and Portland cement S/S samples curing for 28 days varied from 14.3 to 22.4 MPa and 2.5–10.8 MPa, respectively. In summary, geopolymer exhibited better immobilization performances on MSWI fly ash than Portland cement. In geopolymer S/S samples, active aluminosilicate in MSWI fly ash participated the geopolymerization reaction and generated new phases such as Friedel’ salts (3CaO•Al2O3•CaCl2•10H2O) and hydrocalumite (Ca2Al(OH)6Cl2H2O). It not only promoted the bulk resource utilization and disposal of fly ash, but also realized the effective encapsulation of heavy metals and chloride ions. Therefore, geopolymer is a more promising candidate for MSWI fly ash S/S treatment.

Emission characteristics of dioxins during iron ore Co-sintering with municipal solid waste incinerator fly ash in a sintering pot

The disposing of municipal solid waste incineration(MSWI) fly ashes containing dioxins is an intractable problems. Co-sintering is one of the most ideal methods to dispose MSWI fly ash, because it not only degrades the dioxins but also makes it possible to re-utilize MSWI fly ashes. In the present study, MSWI fly ash(FA) and water washed MSWI fly ash(WFA) were added into the sinter raw mixture in a lab-scale sintering pot. Different effects of fly ash and water washed fly ash on emission characteristics of dioxins were studied, and possible pathways to form dioxins were discussed in detail. During co-sintering, at least 88.9%, 99.1% of dioxins brought in by FA, WFA was decomposed, respectively, and re-synthesis with a significant distribution characteristic of dioxins originated from sintering process. In the preheating and dry zone, the recombination and condensation reaction of precursors were the main formation pathways of 2,3,7,8-PCDDs in the sintering process and 2,3,7,8-PCDFs were formed by de novo synthesis. In addition, the resynthesis process was affected by chlorine and calcium brought in by fly ash. Sufficient chlorine boosted the chlorination of reactants while calcium increased the residence time, causing the emission concentration of toxic dioxins and the adding ratio were nonlinearly dependent. Therefore, the maximum proportion of water washed MSWI fly ash that can be added into the sintering process was 0.5 wt.%. Besides, the possible pathways to form dioxins were proposed.

Efficiently sintering of MSWI fly ash at a low temperature enhanced by in-situ pressure assistant: Process performance and product characterization

Municipal solid waste incineration (MSWI) fly ash disposal is an urgent task with some technical bottlenecks. In this study, a novel pressure-assisted sintering method was employed to treat the MSWI fly ash. A series of pressure-assisted sintering experiments were carried out by varying mechanical pressures and sintering temperatures, and their properties of compressive strength, density and heavy metals leaching behavior were determined to screen out the optimal conditions. Instrumental analysis of XRF, SEM, XRD and TEM-EDX and calculation kinetics were conducted to explore the enhancement mechanism of pressure-assisted sintering. With the help of mechanical pressure, a high-strength ceramic product was produced from MSWI fly ash sintered at a low temperature (400 °C), which never occurred in the conventional low-temperature sintering process. Maximum compressive strength of 218.30 ± 4.08 MPa was obtained at 400 °C and 100 MPa, which was much higher than conventional construction materials of brick and cement. In addition, the leaching concentrations of heavy metals obtained from pressure-assisted sintering process were lower than the standard limitation. The SEM and XRD results revealed that the increased mechanical properties and the decreased heavy metals leaching concentration were mainly attributed to the increased density and crystalline degree. The kinetics calculation results indicated that the sintering activation energy was much lower than the sintering process without pressure, suggesting surface diffusion and grain boundary diffusion were main sintering mechanisms in the pressure-assisted sintering process. These findings proved that pressure-assisted sintering could be a promising method to treat fly ash together with producing high-value building materials.

Coupled effects of superplasticizers and glazed hollow beads on the fluidy performance of cemented paste backfill containing alkali-activated slag and MSWI fly ash

To acquire solid knowledge on the coupled effects of superplasticizers and glazed hollow beads (GHB) on the fluidity of backfill materials, the impact of municipal solid waste incineration (MSWI) fly ash content, superplasticizer type and content, GHB content and solid concentration were studied. Additionally, scanning electron microscopy-energy dispersive spectroscopy and methods for determining total organic carbon (TOC) were used to study the mechanism of action of superplasticizers. The TOC test results indicated that the adsorption between naphthalene sulfonate (NS) and MSWI fly ash was the weakest. After adding NS, the morphology of ettringite changed from slender acicular to short columnar. NS has the best effect on improving fluidity. Adding 0.5% NS and 6% GHB increased the solid concentration by 4%, and the compressive strength increased by 55%. The synergistic action of NS and GHB can significantly improve the fluidity, improve the engineering efficiency and reduce the backfill cost.

Characteristics of MSWI fly ash with acid leaching treatment

The chemical and mineralogical characteristics of fly ash from a municipal solid waste incineration (MSWI) in China and the influence of processing parameters on heavy metals removal during leaching were investigated in this work. The fly ash particles had complex surface structure with limited specific surface area. The alkali chloride and metal salts in MSWI fly ash showed evidently impact on leaching efficiency. Metal leachability was related to their properties and speciation in fly ash. Water-soluble salts such as KCl, NaCl and CaCl2 in fly ash were easily washed out. In this study, removal efficiency by water washing was achieved to 93.1% for Cl, 41.4% for Na, 48.5% for K and 24.8% for Ca, respectively. Mineralogical analysis also revealed change of fly ash mineral phases and specification distribution after water washing. Under liquid to solid ratio of 40:1 L/kg and treatment time of 120 min, the leaching process achieved high dropping yields of toxicity characteristic leaching procedure (TCLP) concentrations for Cu, Zn Cd and Pb (80%–100%), moderate dropping yields for As (30%–80%) and relatively low dropping yields of Ni (< 30%). In addition, heavy metals such as Pb and Zn in fly ash with twice water washing treatment at a low liquid-solid ratio could reach lower TCLP concentrations. The result indicated that the combination process of twice water washing and one acid washing could significantly reduce the environmental risk of MSWI fly ash.

Investigation of properties change in the reacted molten salts after molten chlorides cyclic thermal treatment of toxic MSWI fly ash

To realize the thermal detoxification of municipal solid waste incineration (MSWI) fly ash in a relatively mild environment, molten salts thermal treatment technology was proposed in our previous research, which showed good effects. To investigate the properties of molten salts (NaCl-CaCl2) during cycling reusing, the change of the main components and the physical properties of the used molten salts were estimated. Results showed that the salts in fly ash would dissolve into molten salts. During this process, the concentration of K+, SO42- kept increasing while Cl- was decreased. The changing trend of Na+ and Ca2+ was dependent on the ratio of Ca/Na in raw fly ash. Ca(OH)2 in fly ash would react with CaCl2 to form CaClOH. Moreover, the introduction of the salt components on the thermal properties of molten salts were also studied. The melting point hardly changed by NaCl, CaSO4, and SiO2. Nevertheless, it was lowered to 431 °C with 15% CaCO3 addition, while increased to 523 °C with 20% KCl. Besides, there were no significant influences on the viscosity, stability, and thermal diffusivity of molten salts. KCl had the greatest influence on the specific heat capacity of molten salt, with an increase of about 20%.

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.

Characteristics and leaching behavior of MSWI fly ash in novel solidification/stabilization binders.

Municipal solid waste incineration (MSWI) fly ash has become a challenge for waste management because it is rich in heavy metals and its output is increasing sharply year by year. A novel binder system for MSWI fly ash solidification/stabilization was produced by blending MgO and silica fume in this work. And several tests including compressive strength, toxicity leaching behavior, hydration products, pore structure and micromorphology of MgO-SiO2-H2O (M-S-H) binders without and with MSWI fly ash were conducted. Results showed that the compressive strength of MSWI fly ash-blended samples varied from 1.8MPa to 29.3MPa, and the maximum leaching concentration of Pb, Zn and Cd were 0.052mg/L, 1.02mg/L, 0.098mg/L, which all met the national standard limitation on the landfill site. Besides, M-S-H binders could buffer against the harsh acid environment at a certain degree, which would provide a proper low alkalinity environment for immobilization of heavy metals such as Pb, Zn and Cd in MSWI fly ash. Meanwhile, the microstructure and morphology characteristics analysis results demonstrated that the favorable ion exchange and high specific surface area of hydration product contributed much to the excellent immobilization and adsorption characteristics on heavy metals, so it can be deduced that M-S-H binders mainly fixed the heavy metals by the physical encapsulation and chemical bonding. Overall, this work demonstrated that the application potential of M-S-H as immobilization binders in the field of sustainable construction materials and hazardous waste management, and providing a new way for the development and utilization of magnesium resources.