Municipal Waste Incineration Fly Ash Research Articles

Degradation of dioxins in chelated municipal solid waste incineration fly ash by low-temperature pyrolysis

In this study, low-temperature pyrolysis is applied to raw and chelated municipal solid waste incinerator fly ash to degrade and remove PCDD/F (polychlorinated dibenzo-p-dioxins, and dibenzofurans) and corresponding I-TEQs (international toxic equivalents), respectively. Additionally, PCDD/F degradation pathways are identified based on PCDD/F signatures. From the analysis of the average signal intensity of dioxin isomers in thermally treated fly ashes, the PCDD/F degradation rate was between 89.97 and 99.80 %, and the I-TEQs removal rate was 98.96 and 99.90% across all samples compared to untreated raw fly ash. The chelating agent EDTA enhanced the thermal degradation of PCDD/F, but the addition of Na2HPO4 in chelated fly ash as an inhibitor promoted PCDD/F regeneration. Further, there is a variable output of dioxins and corresponding I-TEQs under different temperatures and reaction times; however, longer reaction duration and higher temperatures proved highly effective. The overall toxicity of all thermally tested fly ashes was lesser than the permissible value of 50 ng I-TEQ/kg for resource utilization and observed in the range of 1.84 ± 0.15 to 19.45 ± 0.89 ng I-TEQ/kg. The investigation of the PCDD and PCDF signatures suggests that thermal destruction was a major pathway of degradation by breakage of the C-O bond, compared to dechlorination as observed by a high percentage contribution of HpCDD/F and OCDD/F congeners in thermally treated fly ashes. Low-temperature pyrolysis is an auspicious disposal technology that requires additional studies for large-scale applications.

Immobilization mechanism of heavy metals by crystals and liquid phase during melting treatment of MSWI fly ash

Melting treatment has emerged as a promising technology for managing municipal solid waste incineration (MSWI) fly ash owing to its advantageous features of effective detoxification and volume reduction. The melting treatment of MSWI fly ash involves the immobilization of heavy metals by crystals and liquid phase. Herein, the immobilization mechanism of heavy metals (Cu, Pb and Cd) by the crystals and the liquid phase was investigated using melting experiments, thermodynamic calculations and density functional theory (DFT) calculations. Results demonstrate that the immobilization of heavy metals is influenced by a combination of factors: the reaction of heavy metals, physical encapsulation ability of the liquid phase and chemical fixation ability of both the crystals and the liquid phase. An increase in the content of SiO2 and Al2O3 promotes the conversion of heavy metals oxides into heavy metals chlorides. Furthermore, an increase in the content and polymerization degree of the liquid phase facilitates the physical encapsulation of heavy metals chlorides. The chemical fixation ability of the crystals and the liquid phase differs for Cu and Pb, while Cd cannot be immobilized through chemical fixation. To enhance the immobilization of heavy metals during melting treatment, the chemical composition of MSWI fly ash should be adjusted within the anorthite region of the ternary phase diagram. This study provides valuable insights into the immobilization mechanism of heavy metals by the crystals and the liquid phase during the melting treatment of MSWI fly ash.

Research on the Preparation of Dry Mixed Mortar Using Waste Incineration Fly Ash

This study investigated the effect of water-washed municipal solid waste incineration fly ash (MSWI FA) as an admixture on the performance of dry mixed mortar and used X-ray diffraction (XRD) and scanning electron microscope (SEM) detection methods to conduct microscopic analysis. The experiment investigated the effects of the amount and water content of washed municipal solid waste incineration fly ash, cement, additives, sand and gravel, and curing time on the compressive flexural strength of dry mixed mortar at 28 days. The results show that when the content of water-washed MSWI FA is 9.80%, the content of sand and gravel is 73.50%, the content of ordinary Portland cement (PO42.5) is 16.66%, the content of water-reducing agent is 1.47‰, the content of cellulose is 0.03‰, the content of the expansion agent is 0.49‰, the addition of water is 130–160 mL/kg, the consistency of the sample can reach 91.8 mm, and the water retention rate can reach 93.6%. The flexural strength of the sample at 28 days can reach 7.5 MPa, and the compressive strength at 28 days can reach 28.30 MPa. Metal ions, such as Pb2+ and Gd2+ in MSWI FA, under the combined action of silicate cement in dry mixed mortar and fibers in cellulose, crisscross and form a solidified material, which will not be leached out. This quality meets the requirements of dry mixed mortar for ordinary plastering and masonry mortar (GB-T 25181-2019), and the leaching toxicity of the sample meets the “Identification Standard for Hazardous Waste” (GB5085.3-2007). This work provides a meaningful exploration of the resource utilization of water-washed MSWI FA.

Efficient fluoride recovery from wastewater using mesoporous Ca-based nanomaterials derived from municipal solid waste incineration fly ash via fluidized bed crystallization

This study introduces an innovative approach utilizing mesoporous Ca-based nanomaterial (MCN) derived from municipal solid waste incineration (MSWI) fly ash as carriers in a fluidized bed crystallization (FBC) process for the recovery of fluoride from semiconductor wastewater. The transformation of nonporous MSWI fly ash into mesoporous structures was achieved through facile treatments, including ultrasonic-assisted leaching with ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA–Na) and surface modification with sodium silicate, enhancing surface functionalities and active site formation. The resulting MCN exhibited a significant surface area of 222.0 m2/g, ideal for fluoride recovery. When tested with synthetic wastewater, the recovery efficiency and crystallization ratio of fluoride using MCN in the FBC system reached 99.0 % and 93.3 %, respectively. Further investigation revealed that the unique surface properties and large surface area of the MCN promoted the crystallization of fluoride-containing precipitates, resulting in homogeneously nucleated cubic CaF2 crystals and facilitating layer-by-layer crystal growth. Testing the FBC process with real semiconductor wastewater resulted in a crystallization efficiency of 94.5 %, with the CaF2 content in the recovered precipitates reaching 84.3±0.7 %. These findings highlight the potential of MCN as an innovative carrier for fluoride recovery, demonstrating a novel and environmentally friendly method for converting waste into valuable materials. This study offers a promising solution for treating fluoride-contaminated wastewater, with high potential to revolutionize industrial wastewater management and promote a circular economy.

Revolutionizing soil and waste treatment: MoS2 nanosheets turbocharge geopolymerization for eco-friendly remediation and self-cementation of arsenic-contaminated soils and municipal solid waste incineration fly ashes

In the face of escalating arsenic pollution and the mounting challenge of municipal solid waste incineration (MSWI) fly ashes, cutting-edge technology has emerged as a game-changer. By harnessing the power of MoS2 nanosheets, a revolutionary approach to double-self-cementation has been unlocked, addressing the limitations of previous methods. Through heterogeneous adsorption, MoS2 nanosheets bolstered the stability of the matrix, leading to a remarkable surge in compressive strengths and a substantial reduction in leaching toxicities of heavy metals. The compressive strengths of the cemented cubes increased as the dosage of nanosheets increased from 0.3 % to 2.4 %, with compressive strengths ranging from 24.34 to 34.21 MPa. The leaching toxicities of Zn, Cu, Cr, Cd, Pb, and As from the solidified matrix correspondingly decreased to 0.32, 0.21, 0.04, 0.01, 0.05, and 0.53 mg/L, respectively. The role played by nanosheets in inhibiting the release and transport of chloride species further underscored their pivotal contribution. Moreover, the study delved into the intricate mechanisms underlying the impact caused by nanoscale materials cementation and geopolymerization systems, shedding light on their dual function as solidification intensifiers and chemical stabilizers. The nanosheet-enhanced double-self process can be appropriately fitted by the JMAK model. The linear nuclei growth of gels controlled the solidification and geopolymerization of the precursor materials and the stabilization of HM contaminants. This groundbreaking research not only elucidates the transformative potential of MoS2 nanosheets but also paves the way for a paradigm shift in the treatment of contaminated soils and waste materials.

Municipal Solid Waste Incineration (MSWI) Fly Ash as a Potential Adsorbent for Phosphate Removal

Phosphorus, in the form of phosphate, is an essential nutrient used in agriculture, but in excess, it becomes harmful to the environment by promoting the eutrophication of aquatic ecosystems, leading to oxygen depletion and phytoplankton overgrowth. This study aims to repurpose municipal solid waste incineration (MSWI) fly ash for phosphate removal by adsorption and develop sustainable, cost-effective MSWI fly ash-based mitigation strategies for phosphorus pollution. Batch experiments were conducted to examine the effects of contact time, phosphate concentration, MSWI fly ash dosage, and pH on phosphate removal efficiency. The results indicate that the phosphate removal efficiency significantly improved with a longer contact time, pH of 2, increased MSWI fly ash dosage, and higher initial phosphate concentrations. These findings highlight the potential of repurposing MSWI fly ash as an economical and sustainable adsorbent to mitigate the impacts of phosphate pollution on aquatic ecosystems, a strategy that promotes not only waste reduction and the circular economy but also environmental protection and conservation.

Insights into the environmental risk variation of heavy metals from MSWI fly ash after thermal plasma vitrification

Thermal plasma vitrification of municipal solid waste incineration fly ash (MSWI FA) shows potential for effective dioxin degradation and heavy metal immobilization. However, the environmental risks posed by heavy metals released after vitrification remain understudied. This work investigates two types of MSWI FA—circulated fluidized bed fly ash (CFA) and mechanical grate furnace fly ash (GFA)—blended with waste glass and vitrified via thermal plasma treatment. The physical and chemical properties of the original FA and vitrified glasses (CFA-glass and GFA-glass) were analyzed. Environmental risks were assessed using the risk assessment code (RAC) and the overall pollution toxicity index (OPTI). Results showed that CFA, with higher Si and Al content, exhibited better vitrification than GFA. Plasma vitrification reduced chlorine content from 5.79 % and 10.59 % to 0.22 % and 0.16% due to Cl volatilization. Heavy metals with low boiling points, such as Cd and Pb, were significantly reduced, while high-boiling-point Cr, Ni remained and the acid resistance of vitrificated FA significantly deteriorated, which increased the potential leaching of heavy metals. In alkaline conditions, the OPTI value of CFA-glass dropped to 2, indicating a strong immobilization effect. This study provides critical insights for managing heavy metals in fly ash through thermal plasma vitrification.

Synergistic effect of rhamnolipid and Bacillus mucilaginosus on detoxification and activation of municipal solid waste incineration fly ash

In recent years, the substantial increase in municipal solid waste incineration fly ash (MSWIFA) production has made its treatment a critical issue. However, the high toxicity of MSWIFA makes its utilization still in the exploratory stage. In this study, the heavy metal leaching rate, particle size distribution and activity index of MSWIFA were tested to investigate the detoxification and activation effects of Bacillus mucilaginosus on MSWIFA by introducing rhamnolipid. The results show that the microbial treatment can greatly increase the leaching rate of heavy metals in MSWIFA and its 28-day and 90-day activity indexes, especially by the synergistic treatment. For the Bacillus mucilaginosus and rhamnolipid treated MSWIFA (BR-MSWIFA), the maximum leaching rates follow the order from high to low of 85.57% Cr, 78% Cu, 76.38% Zn, 62.78% Pb and 37.65% Mn, while having a low mass loss of less than 5%, which is important for its reuse. Moreover, compared with the untreated MSWIFA (U-MSWIFA), the average particle sizes of Bacillus mucilaginosus treated MSWIFA (B-MSWIFA) and BR-MSWIFA decrease from 8.39 μm to 7.80 μm and 4.31 μm, and the 28-day activity indexes increase from 80.19% to 101.59% and 110.61%. The effect mechanism is that rhamnolipid not only can promote the growth, reproduction, and metabolic acid production of Bacillus mucilaginosus, but also disperse the extracellular polymeric substance (EPS) and MSWIFA particles, thus increase their contact area and the bioleaching. This study provided a theoretical foundation for the harmless treatment and resource utilization of solid wastes containing heavy metals.

Nanosilica-enhanced geopolymer cementitious materials: Mechanistic insights from experimental and computational simulations

To mitigate the pollution associated with municipal solid waste incineration fly ash (MSW-IFA), solidification/stabilization (S/S) techniques have garnered considerable attention. This study systematically investigates the modification effects of nanosilica (NS) on the properties of geopolymer functional cementitious materials (GFCM) derived from waste fly ash. By integrating experimental data with simulation-based calculations, the influence of NS on the macroscopic properties and microstructure of GFCM, including mechanical performance, heavy metal immobilization mechanisms, and hydration processes, was examined. The results indicate that NS enhances mineral phase and pore structure through pozzolanic and filling effects, promoting additional C-S-H gel formation and pore filling, which significantly accelerates the hydration process and improves mechanical properties. Furthermore, NS increases the proportion of heavy metals in a stable state, thereby reducing their leaching potential. Density functional theory (DFT) calculations further revealed that NS's modification effects primarily occur via filling, nucleation, and pozzolanic activity, which supported the construction of a geopolymer reaction network. This study offers a validated approach for advancing the understanding of NS modification mechanisms, the development of geopolymer reaction networks, and the mechanisms governing heavy metal S/S.

Impacts of alkaline washing pretreatment on municipal solid waste incineration fly ash (MSWI FA) and resulting geopolymers

The safe disposal and utilization of municipal solid waste incineration fly ash (MSWI FA) has emerged as a crucial challenge, primarily due to elevated levels of heavy metals and chlorides. In this study, the influence of key factors in different pretreatment methods on the physicochemical properties of MSWI FA was analyzed, and the effects and mechanism of different pretreatment methods on the resulting geopolymers were studied. Under the conditions of a liquid-solid ratio (L/S) of 7 mL/g, a rotational speed of 500 r/min, and a stirring time of 20 min, the alkali washing with 1 mol/L NaOH solution exhibited better effect than water washing. The total content of chlorides and the content of soluble chloride in AFA (MSWI FA with alkali washing) were 1.53 % and 0.85 %, respectively. Alkali washing had a greater influence on the migration of heavy metals from MSWI FA to filtrate than water washing. Meanwhile, alkaline washing exhibited better effect in calcium retention and sulfur removal. The pretreatment process enhanced the gel formation in geopolymers, subsequently exerting a significant influence on the compressive strength and leaching concentration of heavy metals. Thermogravimetric analysis showed the bound water content in AG (8.46 %) was the greatest in all simples, indicating that exerted the most pronounced impact on the gel formation during geopolymerization. And, the lower ratio of Si-O-Na to Si-O-T in AG (0.20) indicated a longer geopolymeric chains, which explained the higher compressive strength of geopolymers during the initial curing period. This article provides a new pretreatment method to promote the resource utilization of MSWI FA.

Utilization of Municipal Solid Waste Incineration (MSWIFA) in Geopolymer Concrete: A Study on Compressive Strength and Leaching Characteristics.

This study explores the utilization of municipal solid waste incineration fly ash (MSWIFA) in geopolymer concrete, focusing on compressive strength and heavy metal leachability. MSWIFA was sourced from a Shenzhen waste incineration plant and pretreated by washing to remove soluble salts. Geopolymer concrete was prepared incorporate with washed or unwashed MSWIFA and tested under different pH conditions (2.88, 4.20, and 10.0). Optimal compressive strength was achieved with a Si/Al ratio of 1.5, water/Na ratio of 10, and sand-binder ratio of 0.6. The washing pretreatment significantly enhanced compressive strength, particularly under alkaline conditions, with GP-WFA (washed MSWIFA) exhibiting a 49.6% increase in compressive strength, compared to a 21.3% increase in GP-FA (unwashed MSWIFA). Additionally, GP-WFA's compressive strength reached 41.7 MPa, comparable to that of the control (GP-control) at 43.7 MPa. Leaching tests showed that acidic conditions (pH 2.88) promoted heavy metal leaching, which increased over the leaching time, while an alkaline environment significantly reduced the leachability of heavy metals. These findings highlight the potential of using washed MSWIFA in geopolymer concrete, promoting sustainable construction practices, particularly in alkaline conditions.

Production of high-strength eco-conscious ceramics exclusively from municipal solid waste

This research investigated the application of municipal solid waste incineration fly ash (MSWIFA), municipal solid waste incineration bottom ash (MSWIBA), and construction waste residue (CWR) as raw materials for the comprehensive conversion into municipal solid waste ceramics employing the SiO2-Al2O3-CaO-MgO (8 wt%) phase diagram. This study aimed to evaluate the influence of the addition of MSWIFA and sintering temperature on important ceramics properties, including linear shrinkage, water absorption, sintering range, and flexural strength. Additionally, relationships were established among these physical parameters using Pearson's correlation coefficient. The thermal behavior of the mixture was analyzed through automatic slag melting point tester and TG-DSC techniques. Furthermore, characterization of the crystalline phase transition and microstructure of sintered samples was performed by XRD, Factsage, and SEM. The results showed that both the addition of MSWIFA and the sintering temperature significantly influenced the crystal phase composition of the sintered ceramics. Moreover, the addition of MSWIFA and the sintering temperature had a significant influence on the pore structure of the ceramics. These ceramics exhibited exceptional properties, such as the extremely low water absorption rate of 0.08 % and the remarkable flexural strength of 124.78 MPa. Ceramics performance indicators were far higher than the requirements of China's national standard GB/T4100-2015. The sintering range had the capability to attain 30 °C, guaranteeing the feasibility of practical manufacturing processes. Furthermore, leaching concentration tests conducted on additive-free ceramic samples reveal a low risk of heavy metal contamination, as the heavy metals were effectively solidified within the crystalline and amorphous phases of the ceramics. The comprehensive utilization of MSWIFA, MSWIBA, and CWR for the production of fully solid waste ceramics not only yields cost reduction benefits but also promotes efficient utilization, presenting a feasible and highly promising approach to sustainable waste management.

The promoting effect of Fe on the immobilization of heavy metals in MSWI FA–lead–zinc tailings-based backfill materials under field application: Dynamic simulation leaching and immobilization mechanisms

Currently, research on solid waste-based backfill materials containing municipal solid waste incineration fly ash (MSWI FA) was primarily concentrated at the laboratory stage, with insufficient studies on their leaching safety and heavy metal immobilization mechanisms under field application. This study demonstrated that MLBM (Binder weight ratio: MSWI FA: steel slag (SS): blast furnace slag (BFS): flue gas desulfurized gypsum (FGDG): coal fly ash (CFA) = 10 %:33.6 %:33.6 %:12.8 %:20 %; binder-to-lead–zinc tailings (LZT) ratio was 1:8; mass concentration was 60 %) exhibited excellent leaching safety in actual backfilling. The study found that MLBM posed no risk of heavy metal and anion leaching under various leaching standards. Furthermore, MLBM showed no risk of harmful element leaching within a simulated percolation period of 100 years. The release mechanisms of Pb, As, Zn, and Cr in MLBM were diffusion, diffusion, flushing, and dissolution, respectively. The immobilization efficiency of Pb, Zn, Cr, As, Cl−, and SO42− positively correlated with the pH value (pH>7.5). The immobilization mechanism included: 1) Formation of insoluble heavy metal compounds. In the MLBM system (The pH ranged from 7.89 to 8.87), Fe(III) (Fe(OH)3(aq) and Fe(OH)4−) contributed to the immobilization of Pb2+ and SO42− and oxidized As(III) (H2AsO3−) and Cr(III) (Cr(OH)3(aq)), forming As(V) (HAsO42−) and Cr(VI) (CrO42−), respectively. 2) Adsorption. The negatively charged surface of C–(A)–S–H gel could immobilize Zn2+ and Pb2+ through adsorption, and the ionic forms of Pb(II) and Zn(II) were PbOH+ and ZnOH+, respectively. This study provided theoretical support for the leaching safety and immobilization mechanisms of solid waste-based backfill materials containing MSWI FA in field applications.

The Effect of chlorine and glass additives on heavy metal volatilization during vitrification of simulated Municipal Solid Waste Incineration fly ash

This study examines the impact of chlorine and glass additives (SiO2 and B2O3) on heavy metal volatilization during vitrification of simulated Municipal Solid Waste Incineration (MSWI) fly ashes. Different simulated MSWI fly ash compositions were subjected to chloride volatilization, revealing that treating MSWI fly ash with Cl increases heavy metal volatilization especially Pb and Cu to about 40% and 20% respectively. Incorporating glass additives into fly ash with chlorine increases Pb volatilization by almost 80%, and Cu by 50%, and enables non-volatile Cd and Zn to vaporize.

Converting Municipal Solid Waste Incineration Fly Ash and Municipal Sludge into Environmentally Compatible Alkali-Activated Material

Converting Municipal Solid Waste Incineration Fly Ash and Municipal Sludge into Environmentally Compatible Alkali-Activated Material

Study on impact resistance performance of MSWIFA-based low-carbon fiber-reinforced concrete

The accumulation of carbide slag (CS), red mud (RM), and municipal solid waste incineration fly ash (MSWIFA) has led to significant environmental pollution issues, necessitating urgent resource utilization. Building upon the previously developed CS-MSWIFA synergistic activation RM-slag cementitious system, this study aims to further incorporate iron tailings sand and polypropylene fibers to prepare fiber-reinforced concrete. The systematic investigation focuses on the influence of cementitious material types, aggregate types, fiber shapes, lengths, and dosages on the impact resistance of concrete. Furthermore, an impact damage evolution equation and life prediction model were developed based on the two-parameter Weibull distribution. Results indicate that the type of cementitious material and aggregate has minimal influence on the impact resistance of concrete, while the addition of fibers significantly enhances its impact resistance, shifting the failure mode from brittle to ductile. Mesh polypropylene fibers with a length of 12 mm and a volume dosage of 1.0 % demonstrate excellent impact resistance, with initial and final crack numbers reaching 78 and 105, respectively. This represents an increase of 136.4 % and 200.0 % compared to the control concrete without fibers. The findings of this study offer new avenues for the resource utilization of solid waste and provide theoretical foundations and technical support for the potential application of solid waste based fiber-reinforced concrete in structural engineering.

Preparation and characterisation of sodium chloride-doped CaO-Al2O3-SiO2 system glass-ceramics

ABSTRACT It is known that the treatments of chlorine-containing wastes, such as municipal solid waste incineration fly ash are particularly challenging. One method for treating chloride is to entrap it in glass by melting. It has been demonstrated that CaCl2 can be retained in CAS system glass-ceramics. However, as the main chloride, NaCl exhibits a low boiling point and a high vapour pressure. The fixation of NaCl and its effect on glass-ceramics remain unclear. In this study, 2.17% Cl− was incorporated into the base glass by limiting NaCl volatilisation during melting. Glass-ceramics were successfully prepared using the Cl-containing glass powder as the raw material by sintering and the chloride was evenly distributed between the glass phase and crystalline phase. The primary crystal phases observed were wollastonite and anorthite. Furthermore, when the sample was sintered at 825 °C for 4 hours, it exhibited a bending strength of 155 MPa and a hardness of 6.29 GPa. This method can effectively address the resourceful treatment of chlorine-containing wastes.

Immobilization of heavy metals with chlorine and liquid phase formation during thermal treatment of MSWI fly ash

Due to effective detoxification, thermal treatment is a preferred treatment technology of municipal solid waste incineration (MSWI) fly ash. However, immobilization behavior of heavy metals during thermal treatment is unclear. In this work, a circulated fluidized bed fly ash with SiO2 addition and a grate furnace fly ash with high silica-aluminum coal ash addition were used to investigate combined effect of active chlorine content, liquid polymerization degree, temperature of initial liquid phase formation and liquid phase formation rate on the immobilization ratios of Zn, Pb and Cd by principle component analysis (PCA). The results show that the decrease in active chlorine content favors the immobilization of Zn, Pb and Cd because chlorination volatilization is reduced. Both the increase in liquid polymerization degree and the formation of initial liquid phase at low temperature promote the immobilization of Zn, Pb and Cd for physical encapsulation. However, a rapid formation of liquid phase is not favorable for the immobilization of Zn, Pb and Cd, because transfer of heavy metals between crystals and liquid phase is impeded by high viscosity of liquid phase during fusion. The results can provide a reference for how to achieve the immobilization of heavy metals by adjusting the chemical composition of MSWI fly ash during thermal treatment.

Adsorption behavior and solidification mechanism of Pb(II) on synthetic C-S-H gel with low and high Ca/Si ratios in highly alkaline environments

Adsorption behavior is rarely investigated as a heavy metal solidification mechanism in alkali-activated municipal solid waste incineration (MSWI) fly ash compacts. This study simulates the adsorption behavior and solidification mechanism of Pb(II) on synthetic C-S-H gel with low and high Ca/Si ratios in high alkaline internal pore solution environments of alkali-activated MSWI fly ash compacts. The study examines the fraction of Pb(OH)3- in a Pb(II) solution (100 mg/L) at pH 11.8 and 12.4, with percentages of 85.9–90.6 % and 93.4–95.1 %, respectively. pH values are identified as the key parameter affecting the adsorption capacity of C-S-H for Pb(II). The equilibrium adsorption capacities (Qe) values of Pb(OH)3- (pH = 11.8) for C-S-H with Ca/Si ratios 1.20 and 0.60 are approximately 104 mg/g and 110 mg/g. At pH 12.4, the C-S-H gel with a Ca/Si ratio = 1.20 performs better in terms of adsorption. The adsorption of Pb(OH)3- by C-S-H is predominantly a chemisorption process at pH 11.8. Based on the ΔG0 data, the adsorption process is determined to be spontaneous. The 1.16 eV LUMO and HOMO difference for Si2O(OH)6 + Pb(OH)3-, compared with the 3.92 eV difference for Si3O2(OH)8 + Pb(OH)3-, indicates a lower difficulty of electron transition in the reaction between Si2O(OH)6 and Pb(OH)3-. XPS and FTIR results confirm the dehydration and combination of Pb(OH)3- and Si-OH in a silicate tetrahedron (Q1 or Q2) on the C-S-H surface to create Si-O-Pb. Partial Ca(II) in C-S-H gel can be replaced or released during the Pb(II) adsorption process. These findings hold significant theoretical importance for the adsorption and solidification mechanism of Pb(II) in alkali-activated MSWI fly ash specimens.

Study on hydration and heavy metal leaching of washed MSWI FA as a green cementitious material

Despite municipal solid waste incineration fly ash (MSWI FA) being classified as hazardous waste, there is a consensus on its significant potential for resource utilization. The high content of soluble chlorides in MSWI FA not only limits its co-disposal in cement kilns but also hinders its application in the field of construction materials. The research aimed to investigate the viability of MSWI FA as a binding material in the construction engineering field. Through examining the impact of liquid/solid ratio and pH levels on the chemical composition of MSWI FA during the washing procedure, the study delved into the mechanisms affecting hydration and the characteristics of heavy metal contamination before and after washing pretreatment. The results indicate the feasibility of developing MSWI FA into construction engineering materials. The liquid/solid ratio is a significant factor affecting the removal of chlorides, with an optimal ratio of 30. pH plays a crucial role in the mineral composition during the water-washing process of MSWI FA. An acidic environment can remove a higher proportion of chlorides, while an alkaline environment can form more calcium salts. The compressive strength of pure MSWI FA at 28 days is 3.92 MPa, which can be increased by 0.99 MPa after washing. The washed MSWI FA not only exhibits heightened reactivity but also demonstrates reduced leaching concentrations of heavy metals, and its leaching toxicity was much lower than the safe landfill threshold. Therefore, MSWI FA, after washing, is a very promising alternative material for construction engineering.