Municipal Waste Incineration Fly Ash Research Articles

A novel approach to heavy metal immobilization in municipal solid waste incineration fly ash: Investigating the use of chicken eggshell waste and CaO addition

A novel approach to heavy metal immobilization in municipal solid waste incineration fly ash: Investigating the use of chicken eggshell waste and CaO addition

The long-term leaching behavior of heavy metals in geopolymers and Portland cement bricks containing MSWI fly ash: a comparative study

Geopolymers was considered as an ideal alternative to cement solidification/stabilization for municipal solid waste incineration fly ash (MSWI FA). However, the long-term leaching behavior of heavy metals in geopolymers had attracted less attention. In this study, the long-term leaching behavior of heavy metals in geopolymers containing MSWI FA was investigated and compared with Portland cement bricks under various corrosion scenarios. The findings revealed a consistent trend of "slowly increasing - slowly decreasing - gradually stabilizing" in the pH changes in the effluent of all samples and the absence of corrosive risk in the utilization of geopolymers. The maximum pH values of the effluent were G2 (12.00) > G1 (11.90) > G3 (10.83) > C1 (9.67), suggesting that geopolymers were not corrosive in resource utilization. The long-term leaching behavior of heavy metals under different corrosion conditions could be divided into two stages, which included quick release in stage Ⅰ and gradual stabilization in stage Ⅱ. Compared with Portland cement bricks, the cumulative leaching concentration of heavy metals in geopolymers was lower, indicating less environmental risk. Due to the strongest corrosiveness from simulated acid rain (SAR), C2 and G2 exhibited significant compressive strength loss rates of 20.2% and 9.25%, respectively. In the leaching of CO2-saturated water (CSW), the leaching behavior of heavy metals was mainly regulated by the nucleation-dissolution trade-off of carbonates, however the dissolution mechanism of heavy metals in SAR could be summarized as follows: (1) corrosion/replacement of H+, and (2) counter-ion effect. this study will promote the resource utilization of MSWI FA. In a word, this work systematically studied the long-term environmental risk of geopolymers, which was beneficial to the resource utilization of MSWI FA.

Preparation and characterization of cementitious materials from dedioxinized and dechlorinated municipal solid waste incineration fly ash and blast furnace slag

To address the resource utilization issue of MSWI fly ash, a method has been proposed to prepare cementitious materials using harmlessly processed MSWI fly ash (HPFA). After washing and thermal treatment, The physicochemical properties of HPFA were characterized using QXRD, ICP, XRF, and GC-MS. Cementitious materials were prepared using HPFA, gypsum and blast furnace slag (BFS) as raw materials, Through XRD, TG, FTIR, TEM, and SEM-EDS methods, the hydration and hardening characteristics were investigated, and the leaching of heavy metals from solidified materials in simulated surface water and landfill environments was evaluated using ICP-MS. The results indicate that the harmless treatment can effectively reduce the chloride content and dioxins in MSWI FA but still contains heavy metals. The addition of HPFA (10%-50%) can enhance the compressive strength of cementitious materials, accelerate the exothermic process of cement hydration, and increase the total heat release of hydration, implying an improvement in the degree of hydration of the system.HPFA could effectively promote the hydration of gypsum and BFS, resulting in the higher formation of ettringite and C-(A)-S-H（C/S=1，Al/Si=0.3）.The formation of C(Mg/Fe)-(A)-S-H suggests that the Fe-Mg solid solution phase in HPFA may have participated in the hydration reaction. The increased stability of highly cross-linked (Q4) silicate tetrahedral structure with time implies that the material may have good durability. The leaching amount of heavy metals met the standards of Class I surface water in GB3838–2002 and GB5085.3–2007 which implies that the material has environmental safety. This study innovatively provides a preparation path for harmless MSWI fly ash and cementitious material ratios, reveals the hydration mechanism of HPFA in cementitious materials, evaluates the solidification ability of heavy metals, and provides important ideas for the resource utilization of MSWI fly ash.

Preparation and Properties of Expansive Backfill Material Based on Municipal Solid Waste Incineration Fly Ash and Coal Gangue

To realize the large-scale utilization of municipal solid waste incineration (MSWI) fly ash in the field of building materials and to reduce the cost of coal mine backfill mining, the effects of the mixing ratio of cementitious materials, the particle size distribution of aggregates, and the amount and mass concentration of cementitious materials on the properties of backfill materials were experimentally investigated, and the microstructure of the hydration products was analyzed. The results showed that as the mass ratio of MSWI fly ash to bottom ash increased, the rate of expansion of the cementitious system continued to increase, and the compressive strength of the cementitious system continued to decrease. The Al (aluminum) and AlN (aluminum nitride) in the fly ash reacted with water to generate gas, causing the expansion of the cementitious materials; NaOH increased the alkalinity of the solution, which promoted the formation of more bubbles, thereby improving the expansion performance of the cementitious material. When the content of NaOH was 0.9%, the sample rate of expansion could reach 15.9%. The addition of CaCl2 promoted the early hydration reaction of the cementitious material, forming a dense microstructure, thus improving the early strength and rate of expansion of the cementitious material. The compressive strength of the backfill body increased as the fractal dimension of the aggregate particles increased, and the particle grading scheme of group S1 was optimal. The 1-day, 3-day, and 28-day strengths of the backfill body of group S1 reached 0.72 MPa, 1.43 MPa, and 3.26 MPa, respectively. It is recommended to choose a backfill paste concentration ranging between 78.5% and 80% and a reasonable amount of cementitious material between 20% and 25%. After the MSWI fly ash was prepared as a backfill material, the leaching of potentially harmful elements in the fly ash was greatly reduced, and the concentration of dioxin was reduced to 13 ng TEQ/kg. This was attributed to the dilution of the cement, the physical encapsulation of gel products, and the isomorphous replacement of Ca2+ in calcium aluminate chloride hydrate.

Immobilization properties and reaction mechanism of municipal solid waste incineration fly ash with alkali activation technology

Municipal solid waste incineration fly ash (MSWIFA) needs proper treatment prior to security landfilling because of considerable amounts of heavy metals and soluble chloride. Nowadays, alkali activation technology has become an effective solidification/stabilization (S/S) technology for MSWIFA. But few studies have been systematically carried out on the preparation techniques and reaction mechanism of MSWIFA S/S body. The effects of the technical parameters (alkali activator, sodium silicate modulus, alkali equivalent, water-solid ratio, curing regime) on the immobilization properties (compressive strength and heavy metal leaching toxicity) of MSWIFA S/S bodies were studied in this work. The hydration characteristics and reaction mechanism were also explored in depth based on the structure properties and microstructure characterization. Ca2+, Na+, SiO42−, [Al(OH)4]-, OH− dissociated from MSWIFA and alkali activator solution chemical reaction to form the silicates, aluminates and aluminosilicates with lower polymerization, and promoted the gel hydration products formation via depolycondensation. Lower Ca/Si molar ratio was conductive to the formation of amorphous substance, compactness of matrix pore structure and immobilization properties. Compared to the hydration products including halite, sylvine, glauberite, gypsum, calcite crystal, C–S–H gel contributed much more to the pore structure and strength improvement of MSWIFA S/S bodies. These findings will provide valuable theoretical basis for MSWIFA management and resource utilization promotion.

High-efficient stabilization and solidification of municipal solid waste incineration fly ash by synergy of alkali treatment and supersulfated cement

Municipal solid waste incineration fly ash (IFA) designated as hazardous waste poses risks to environment and human health. This study introduces a novel approach for the stabilization and solidification (S/S) of IFA: a combined approach involving alkali treatment and immobilization in low-carbon supersulfated cement (SSC). The impact of varying temperatures of alkali solution on the chemical and mineralogical compositions, as well as the pozzolanic reactivity of IFA, and the removal efficiency of heavy metals and metallic aluminum (Al) were examined. The physical characteristics, hydration kinetics and effectiveness of SSC in immobilizing IFA were also analyzed. Results showed that alkali treatment at 25 °C effectively eliminated heavy metals like manganese (Mn), barium (Ba), nickel (Ni), and chromium (Cr) to safe levels and totally removed the metallic Al, while enhancing the pozzolanic reactivity of IFA. By incorporating the alkali-treated IFA and filtrate, the density, compressive strength and hydration reaction of SSC were improved, resulting in higher hydration degree, finer pore structure, and denser microstructure compared to untreated IFA. The rich presence of calcium-aluminosilicate-hydrate (C-(A)-S-H) and ettringite (AFt) in SSC facilitated the efficient stabilization and solidification of heavy metals, leading to a significant decrease in their leaching potential. The use of SSC for treating Ca(OH)2- and 25°C-treated IFA could achieve high strength and high-efficient immobilization.

Adsorption behavior and solidification mechanism of Pb(II) on synthetic C-A-S-H gels with different Ca/Si and Al/Si ratios in high alkaline conditions

Calcium silicate aluminate hydrate (C-A-S-H) is the main hydration product and its porous adsorption property can effectively inhibit Pb(Ⅱ) leaching from alkaline pore solution of alkali-activated municipal solid waste incineration (MSWI) fly ash compacts. The study examines how different ratios of Ca/Si and Al/Si in C-(A)-S-H affect Pb(Ⅱ) adsorption behavior and mechanism in high alkaline conditions. The results showed that Al-doped C-A-S-H gels have increased specific surface area and cumulative pore volume, and slightly decreased average pore size. Pb(OH)3- adsorption by C-A-S-H was mainly chemisorption with spontaneity. Temperature, pH value, Al/Si and Ca/Si ratios of C-A-S-H affected Pb(Ⅱ) adsorption. Equilibrium adsorption capacity (Qe) initially increased then decreased with higher Al/Si for C-A-S-H with Ca/Si 0.60 but decreased with higher Al/Si for Ca/Si 1.20. Active Si-OH (Q1 or Q2b) and Al-OH (Q2(1Al)) in aluminosilicate tetrahedra chains dehydrate and condense with Pb(OH)3- to form Si-O-Pb or Al-O-Pb bonds. Higher interlayer water content in low Ca/Si and Al/Si C-A-S-H facilitated ion exchange during Pb(Ⅱ) adsorption. After adsorption, aluminosilicate tetrahedra chain length increased while interlayer water content decreased. Ca(Ⅱ) and Pb(Ⅱ) in C-A-S-H underwent partial ion exchange during the adsorption process. Simulation results indicate Pb(OH)3- was more likely to polycondensate with silica tetrahedral bridging oxygen in Si2AlO2(OH)8 to form Si-O-Pb than Al-O-Pb bonds. These findings provide theoretical support for designing dosages of alkali activators and doped aluminosilicate minerals to improve the solidification efficiency of Pb(Ⅱ) in alkali-activated MSWI fly ash compacts.

Resource utilization of MSWI fly ash supporting TiO2/BiOCl nanocomposite for enhanced photocatalytic degradation of sodium isopropyl xanthate: Mechanism and performance evaluation

The removal of organic pollutants in water environments and the resource utilization of solid waste are two pressing issues around the world. Facing the increasing pollution induced by discharge of mining effluents containing sodium isopropyl xanthate (SIPX), in this work, municipal solid waste incineration fly ash (MSWI FA) was pretreated by hydrothermal method to produce stabilized FA, which was then innovatively used as support for the construction of FA/TiO2/BiOCl nanocomposite (FTB) with promoted photocatalytic activity under visible light and natural sunlight. When the content of FA was 20 wt% and the mass ratio of TiO2 to BiOCl was 4:6, a remarkable performance for the optimal FTB (20-FTB-2) was achieved. Characterizations demonstrated that TiO2 and BiOCl uniformly dispersed on FA contributing to high surface area and broad light adsorption of FTB, which exhibits excellent adsorption capacity and light response ability. Build in electric field formed in the interface of TiO2/BiOCl heterojunction revealed by density functional theory calculations accelerated the separation of photoinduced e− and h+, leading to high efficiency for SIPX degradation. The synergetic effect combined with adsorption and photocatalytic degradation endowed 20-FTB-2 superior SIPX removal efficiency over 99% within 30 min under visible light and natural sunlight irradiation. The photocatalytic degradation pathways of SIPX were determined through theoretical calculations and characterizations, and the toxic byproduct CS2 was effectively eliminated through oxidation of •O2−. For 20-FTB-2, reusability of photocatalyst was showed by cycle tests, also the concentrations of main heavy metals (Pb, Zn, Cu, Cr, and Cd) in the liquid phases released during photocatalyst preparation process (< 1 mg/L) and photodegradation process (< 8.5 μg/L) proved the satisfactory stability with low toxicity. This work proposed a novel strategy to develop efficient and stable support-based photocatalysts by utilizing MSWI FA and realize its resource utilization.

Towards a low-emission resource circulation of valuable metals from municipal solid waste incineration fly ash

The incineration fly ash (IFA) resulting from municipal solid waste combustion is laden with heavy metals, necessitating proper treatment not only for environmental management but also to reclaim the metal values. The surge in non-traditional metals like cobalt as emerging contaminant within IFA samples further attracts to address this issue. In response, the hydrometallurgical recycling of a cobalt-bearing IFA has been studied. Thereby, approximately 98 % zinc and 96 % cobalt were leached using a 1.0 mol/L H2SO4 solution at 90 °C and 1 h of leaching time. In-depth analysis of the leaching process unveiled metals' dissolution primarily via the ion-exclusion mechanism, as evidenced by lower diffusion coefficients (between 10−9 and 10−11 m2/s) and activation energies (9.6–14.9 kJ/mol). Above 99 % separation of zinc from the cobalt-bearing leach liquor was achieved by extraction with 1.0 mol/L D2EHPA at an equilibrium pH below 3.0, followed by stripping with a 2.0 mol/L H2SO4 solution. Cobalt, remained in the raffinate was efficiently precipitated by adding a 20 % excess dosage of oxalic acid to the stoichiometric ratio of C2O42−:Co2+, resulting in only 5 mg/L cobalt left in the solution when precipitation occurred at a pH of 2.8. Additionally, the conversion of CoC2O4 to high-purity Co3O4 was conducted through heat-treatment at 600 °C. The resulting Co3O4 was mixed with Li2CO3 at a Li/Co molar ratio of 1.1, yielding a LiCoO2 precursor that exhibited good electrochemical properties with a capacity of 128 mAh/g, thus affirming the high quality of the recycled cobalt. A comprehensive life-cycle assessment of the recycling process revealed that cobalt precipitation alone contributes approximately 50 % of the total global warming potential (GWP = 4.2624 kg CO2-eq). Notably, this value is remarkably lower than the GWP reported for primary cobalt production, highlighting the environmentally-friendly approach of this recycling endeavor.

Role of municipal solid waste incineration fly ash components in co-pyrolysis of oily sludge: Pyrolysis products and catalytic mechanism

The co-pyrolysis of oily sludge (OS) and municipal solid waste incineration fly ash (IFA) is a promising strategy for sustainable waste management. This study delves into the distinct catalytic roles of individual IFA components during co-pyrolysis and assesses their impact on the inherent Fe species in OS, highlighting their contributions to overall catalytic activity. Notably, in comparison to IFA, CaCl2 and KCl significantly enhance pyrolysis oil upcycling, while IFA components collectively exhibit a positive catalytic effect on pyrolysis gas and coke production. Ca(OH)2 notably boosts H2 yield by 137.16 %. Alkali chlorides facilitate gaseous hydrocarbon formation and convert oxygen-containing compounds to CO and CO2 which are subsequently consumed and absorbed by CaO and Ca(OH)2. CaCl2 and KCl promote heavy compound decomposition and alkane aromatization, reducing coke formation and increasing light aromatic production. Conversely, NaCl increases alkane proportions. However, CaSO4 and CaCO3 hinder catalytic reactions, promoting carbon conversion to coke. Importantly, IFA compounds aid the dispersion of inherent Fe-based species from OS on char surface, enhancing in-situ catalytic pyrolysis. Additionally, the augmented H2 production accelerates the reduction of Fe-based species. The findings expand waste utilization possibilities and provide insights for co-processing solid wastes.

Insights into microstructural alterations in alkali-activated materials incorporating municipal solid waste incineration fly ash

Municipal solid waste incineration fly ash (MSWI FA) is categorized as a hazardous waste with significant implications on environmental safety and human health. This study tailored three types of alkali-activated materials (AAMs) by using ground granulated blast-furnace slag (GGBS) and metakaolin (MK) for the low-carbon stabilization/solidification (S/S) of MSWI FA. The intricate interactions between MSWI FA and AAMs synthesized from Al-rich precursors and Ca-rich precursors were investigated to elucidate the microstructural alterations of AAM-MSWI FA systems. Comprehensive characterization techniques were conducted, including isothermal calorimeter, thermal gravimetric analysis, Fourier-transform infrared (FTIR), X-ray diffraction (XRD), and 27Al and 29Si nuclear magnetic resonance (NMR). The results revealed that the incorporation of MSWI FA significantly delayed alkali activation reactions, hindering the formation of aluminosilicate gel due to high contents of Cl and potentially toxic elements (PTEs) in the MSWI FA. High volume of dissolved Cl from MSWI FA would react with available Ca from MSWI FA and GGBS as well as Al from GGBS and MK to form Friedel’s salt, and this reaction was found to be more dominant in the Al-rich environment. Meanwhile, the C-(N-)A-S-H gel exhibited longer chain length in the Al-rich environment than in the Ca-rich environment. Overall, this study delivers microstructural insights into AAM-MSWI FA systems utilizing GGBS and MK as precursors. The results provide scientific groundwork for diverse practical applications of AAM-MSWI FA materials, thereby supporting the development of resource-efficient and low-carbon hazardous waste treatment strategies.

Solidification/stabilization and optimization of municipal solid waste incineration fly ash with aluminosilicate solid wastes

Alkali-activation is an effective municipal solid waste incineration fly ash (MSWIFA) solidification/stabilization (S/S) technology. However, the characteristics of calcium-rich silica-poor aluminum phase in MSWIFA easily cause the structural instability and contamination of alkali activated MSWIFA S/S bodies. Therefore, the aluminosilicate solid wastes are used in this work to optimize the immobilization and structural properties. Results showed that incorporation of aluminosilicate solid wastes significantly improved the compressive strength and heavy metals pollution toxicity of MSWIFA S/S bodies. Compared to alkali activated MSWIFA, the compressive strength of S/S bodies with addition of coal fly ash, silica fume and granulated blast furnace slag improved by 31.0%, 47.6% and 50.8% when the curing time was 28 days, respectively. Leachability of Pb, Zn and Cd in these alkali activated MSWIFA S/S bodies was far below the threshold value specified in Standard GB16889. Aluminosilicate solid wastes provided abundant Si/Al structural units, and some new phases such as ettringite(AFt, 3CaO⋅Al2O3⋅3CaSO4⋅32H2O), calcium sulfoaluminate hydrate (3CaO⋅Al2O3⋅CaSO4⋅12H2O) and Friedel's salt (CaO⋅Al2O3⋅CaCl2⋅10H2O) can be detected in S/S matrix with aluminosilicate solid wastes, along comes increased the amount of the amorphous phases. Lower Ca/Si molar ratio tended to form the network structure gel similar to tobermorite with higher polymerization degree. Meanwhile, the silica tetrahedron of the gels changed from the oligomerization state like island to the hyperomerization state like chain, layer network or three-dimensional structure, and average molecular chain length increased. These findings provide theoretical basis for structural properties optimization and resource utilization of MSWIFA S/S matrices.

The hydration and heavy metal immobilization performance of the CO2-active incineration fly ash-calcined clay-cement (CFC3) composite low-carbon cementitious system

CO2-active enhances the performance of municipal solid waste incineration fly ash as a supplementary cementitious material, increasing its utilization in cementitious systems. CFC3, a low-carbon cementitious material, effectively utilizes CO2-active incineration fly ash (CIFA). To minimize cement usage, the component effects on hydration need to be studied. In this study, hydration, mechanical and environmental properties of CFC3 were evaluated based on cement dosage and calcined clay/CIFA values. The maximum compressive strength of the samples reached 18.0 MPa at 3d and 47.6 MPa at 56d. The introduction of CIFA delayed the reactions of C3S and aluminates. Calcined clay contributes to improved gel polymerization, optimizes the Si/AlCASH ratio, and refines pore structure. A decrease in cement content increased in the aluminate phases, making monocarbonate the primary hydration product. The results of leaching tests indicated that Cd and Pb in R2C50 were transformed into non-leachable, inactive substances. Demonstrating the environmental friendliness of CFC3.

Promotion of chloride removal from MSWI fly ash by an accelerated wet-carbonation process to enhance ash recycling in cement manufacture

Chlorides, sulfates, and heavy metals are the three main factors preventing the recycling of municipal solid waste incineration (MSWI) fly ash as an alternative material for cement clinker production, of which chloride plays the main role due to its extremelly hign content in fly ash. This study evaluated three wet-based pretreatment methods (water-washing, CO2-aided washing, and flue gas-aided washing) to remove chlorides using MSWI fly ashes from 13 incineration plants in China. Water washing was suitable for removing Cl- from MSWI fly ash containing a limited amount of less-soluble Cl-containing salts (Ca(OH)Cl, Ca2Al(OH)6(H2O)2Cl, and Ca6(CO3)2(OH)7Cl). However, for fly ash with a significant proportion of these less-soluble Cl-containing salts, injection of CO2/flue gas during washing promoted their decomposition and increased the chloride removal rates by 2–12%. Compared with chloride, the removal rate of sulfate was lower under all treatment scenarios and ranged from 7% to 47%. Therefore, caution should be taken when exploring techniques for the deep removal of chloride from MSWI fly ash, as sulfates may be the limiting factor in fly ash samples with low chloride contents when utilizing the fly ash for cement clinker production. Heavy metals in fly ash were not a limiting factor for any ash samples (either raw ash or treated ash) when utilizing the fly ash for clinker production. However, the dissolution of heavy metals in the washing solution deserves more attention, as a large amount of wastewater will be generated during wet-based pretreatment methods. This study confirmed that CO2/flue gas injection during washing promoted the removal of the less-soluble harmful components of fly ash, expanding the applications of ash materials.

A novel combined process for enhancing soluble salt recovery and reducing pollutant diffusion in municipal solid waste incineration fly ash

There is a limited body of research on the recovery of soluble salts from municipal solid waste incineration fly ash (MSWI-FA), with challenges stemming from the effective management of residual heavy metals and dioxins. In this investigation, we propose using water-washing treatment for fly ash dechlorination and using CO2 aeration carbonation combined with ceramic membrane filtration to recover soluble salt resources from fly ash. This study investigated the impact of combined processes on fly ash soluble salt recovery, carbon dioxide capture and sequestration, heavy metal removal, and dioxin diffusion reduction. The findings revealed that the combined process can significantly enhance the rate of carbonation and the removal of heavy metals. Specifically, the removal rates of Pb and Zn reach 100%. The resulting CaCO3 precipitation particle size is smaller, averaging only approximately 4 μm, with greater surface porosity, higher heavy metal and dioxin content, and dioxin toxic equivalents as high as 8.11 ng TEQ/kg. Moreover, the dioxin content in the recovered mixed salt decreased, and its dioxin toxic equivalent was only 3.228.11 ng TEQ/kg. Consequently, the combined process of CO2 aeration combined with ceramic membrane filtration was used in conjunction to significantly reduce pollutants (heavy metals and dioxins) in the MSWI-FA recovered salt. This approach enhances the recyclable resource utilization of MSWI-FA and reduces the risk of pollution dispersal during MSWI-FA disposal and resource utilization.

BEHAVIOR OF LANDFILLED STABILIZED FLY ASH TREATED BY DIFFERENT TREATMENT METHODS FOR THE PAST 27 YEARS

Municipal solid waste incineration (MSWI) fly ash (FA) contains toxic substances and hence, direct landfilling of MSWI FA is prohibited in Japan Complying with Japan's stringent Waste Disposal and Public Cleansing Law, which demands adherence to landfill disposal and effluent standards, is crucial for ensuring environmental safety. To address the issue, various treatment methods including treatment by chemical, phosphoric acid, and melting and solidification can be used to stabilize the MSWI FA through the immobilization of heavy metals. However, the long-term stability of these treatment methods has not been thoroughly investigated. Therefore, the purpose of this study is to evaluate the long-term effectiveness of these three treatment methods for MSWI FA respectively, both independently and in conjunction with compost. This study involves the monitoring of changes in the concentration of heavy metals, including lead (Pb), cadmium (Cd), total-chromium (T-Cr), copper (Cu), and zinc (Zn), that have been leached over a period of 27 years. This study aims to conduct a detailed analysis of the impact of various treatment methods on the concentration levels of these individual heavy metals. Notably, the results suggest that independently, phosphoric acid treatment outperforms chelate treatment in the long run, but when combined with compost, both treatments exhibit enhanced efficacy, suggesting a promising approach for immobilizing heavy metals.

Investigation of the cyclic separation of dioxins from municipal solid waste incineration fly ash by using fat

It is necessary to detoxify municipal solid waste incineration fly ash (MSWIFA) to facilitate resource utilization. The current research focused on the treatment of heavy metals in MSWIFA which is relatively well-developed, while dioxins (PCDD/Fs) need to be treated with more referable programs. Based on the excellent lipophilicities of dioxins, readily available, nontoxic, harmless, and easily treatable animal fats have been used to separate the dioxins from MSWIFA samples, thereby reducing the toxic equivalence of the MSWIFA. The experimental results indicated that animal fats significantly reduced the toxic equivalence of untreated, water-washed, and acid-washed MSWIFA samples, with reductions in the dioxin toxicity equivalence of up to 70%. Furthermore, in studies of heavy metal leaching, water washing followed by pig fat (FP) separation was identified as the optimal approach for the separation of dioxins from MSWIFA. Under the optimal conditions, the toxicity due to dioxins was reduced by over 70% after separation from MSWIFA with recycled animal fats. The environmental and economic benefits of this treatment option were analyzed on this basis.

Removal of heavy metals in water-extracted solution through adsorption by palygorskite and stabilization by comilling.

Removing water-soluble chlorides (WSCs) through water extraction is a common pretreatment technology for recycling municipal solid waste incineration (MSWI) fly ash (FA). However, the extracted solution often contains heavy metals, the concentrations of which exceed standards for effluent. This study aims to investigate the adsorption of heavy metals by palygorskite in water-extracted solution and explore the feasibility of stabilizing heavy metals through comilling palygorskite-adsorbed heavy metals (PAHMs) with water-extracted fly ash (WFA). The experimental parameters include: two-stage water extraction with a liquid-to-solid ratio of 5, adding 0, 0.125, 0.25, 0.5, 1, 2 or 3 g of palygorskite to 100 mL of water-extracted solution, and comilling the mixture of PAHMs and WFA for 0, 0.5, 1, 2, 4, 8, 12, 24 or 96 hours. The experimental results revealed that 3 g of palygorskite in 100 mL of extracted solution could absorb Pb, Cd, Cr, Cu and Zn, meeting the effluent standards. The total amount of Pb, Cd, Cr, Cu and Zn removal rate reached 99.7%. Moreover, 98.44% of the WSCs were not adsorbed, the water extraction process for removing WSCs was not compromised. After the comilling of PAHMs and WFA, the distribution of the heavy metals in the milled blended powder was greater than 99.44%; moreover, toxicity characteristic leaching procedure concentrations were determined to conform to regulatory standards, and the sequential extraction procedure revealed that the heavy metals tended to be in stable fractions. This achieves the goal of preventing secondary pollution from heavy metals during the MSWI FA recycling process.

Environmental performance of unfired bricks produced from co-disposal of mine tailings and municipal solid waste incineration fly ash based on comprehensive leaching tests

The potential leaching of heavy metals is a crucial concern for construction materials produced from solidification/stabilization (S/S) treatment of wastes. This study comprehensively evaluated the leaching characteristics of heavy metals from the unfired bricks produced from co-disposal of Pb–Zn mine tailings and municipal solid waste incineration fly ash using batch, sequential, and semi-dynamic leaching tests. The results show that S/S treatment drastically reduced the leachability of heavy metals from the unfired bricks through lowering their distribution in the acid-soluble fraction. The effective diffusion coefficients of heavy metals within unfired bricks were all below 1.55 × 10−13 cm2/s, which is indicative of low mobility in the environment. The release of heavy metals from the unfired bricks was primarily governed by diffusion and dissolution. Slaking treatment of fly ash significantly reduced the leaching of heavy metals from the unfired bricks due to their improved structural integrity and compactness, which minimizes the surface area in the solid matrix accessible by the leaching medium. The leachability indices of heavy metals within the unfired bricks ranged from 13.12 to 18.10, suggesting that they are suitable for “controlled utilization” in specific scenarios. Compared to untreated mine tailings, converting them into unfired bricks could reduce the releases of heavy metals by several to hundreds of folds. These findings demonstrate that S/S can be an effective and sustainable strategy for co-disposal of mining tailings and incineration fly ash to produce construction materials with sound long-term environmental performance.

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 9.38MPa, 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+.