Stabilization of heavy metals in municipal solid waste incineration fly ash via hydrothermal treatment with coal fly ash

Hydrothermally-targeted synthesis of Al-substituted tobermorite using MSWI fly ash with industrial SiO2 for potentially high-quality utilization

Al-substituted tobermorite (TOB) can be synthesized as a value-added product through hydrothermal treatment (HT) using incineration fly ash (IFA) with an exogenous additive. However, extreme conditions and low purity limit its further use. To solve these issues, we explored the potential of industrial SiO2 (i-SiO2) in synthesizing Al-substituted TOB using IFA from two megacities: Shanghai (IFA-H) and Shenzhen (IFA-Z). Results showed that vast majority of Ca, Si, and Al in IFA and i-SiO2 can be converted into Al-substituted TOB, allowing for targeted synthesis with low i-SiO2 mass. Besides, the experimental condition, e.g., 0.4 mol/L of NaOH concentration, 160 °C of reaction temperature, and 24 h of reaction time, appears to be milder than the previous relevant studies. Both Al-substituted TOBs had Cl- contents below 5.0 wt%, and total immobilization efficiencies for typical heavy metals exceeded 98.0 wt%. The cation exchange capacities (CECs) for Al-substituted TOBs from IFA-H and IFA-Z were 424.0 cmol/kg and 241.3 cmol/kg, respectively, comparable to commercial zeolite and indicating potential for high-quality use. The higher CEC of the Al-substituted TOB from IFA-H may be due to its higher Cl- content. The HT solution, rich in Cl-, can be reused. The HT process utilizing i-SiO2 can reduce economic cost by nearly half compared to current resource utilization approach. This study aims to enhance the resource utilization of IFA.

Utilization of municipal solid waste incineration fly ash with different pretreatments with gold tailings and coal fly ash for environmentally friendly geopolymers

Municipal solid waste incineration fly ash (MSWIFA) is considered a hazardous solid waste, traditionally disposed by solidified landfill methods. However, solidified landfills present challenges with leaching heavy metals, polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs). To address this issue, this study examined two pretreatment methods for MSWIFA: sintering at 850℃ for 30 min and washing with three water baths (20 min each) at a 3:1 liquid-solid ratio. Then, the pretreated MSWIFA was employed in geopolymer synthesis, along with gold tailings (GT) and coal fly ash (CFA). The optimal raw material ratio was GT:CFA:MSWIFA = 5:4:1 after pretreatment, which resulted in maximum compressive strengths of 20.95 and 25.87 MPa for the sintered and washed samples, respectively. Washing removed 87.3 % of soluble chlorides from MSWIFA, enhancing the compressive strength of the geopolymer. High-temperature treatment effectively reduced the leaching concentrations of heavy metals and the toxic equivalent quantity (TEQ) of PCDD/Fs. The leaching concentrations of heavy metals (Cu, Zn, Cd, Pb) and the TEQ of PCDD/Fs were all below the limits established by Chinese standards GB 5085.3-2007 and HJ 1134-2020. X-ray Diffraction, Fourier transform infrared spectrometry, scanning electron microscopy, and energy dispersive spectroscopy analyses revealed that the primary hydration products of the geopolymer are C-(A)-S-H gels. Washing treatment facilitated the formation of ettringite and Ca(OH)2, which enhanced the pore structure and optimized the performance of the geopolymer. Thermogravimetric analysis revealed the good thermal stability of the geopolymer, indicating that the high-temperature and washing pretreatments minimally impacted its thermal stability.

Cu-Ion Hybrid Porous Carbon with Nanoarchitectonics Derived from Heavy-Metal-Contaminated Biomass as Ultrahigh-Performance Supercapacitor

It is challenging to handle heavy-metal-rich plants that grow in contaminated soil. The role of heavy metals in biomass on the physicochemical structure and electrochemical properties of their derived carbon has not been considered in previous research. In this study, Cu-ion hybrid nanoporous carbon (CHNC) is prepared from Cu content-contaminated biomass through subcritical hydrocharization (HTC) coupling pyrolytic activation processes. The CHNCs are used as advanced electrode material for energy storage applications, exhibiting an impressively ultrahigh capacitance of 562 F g-1 at a current density of 1 A g-1 (CHNC-700-4-25), excellent energy density of 26.15 W h kg-1, and only 7.59% capacitance loss after enduring 10,000 cycles at a current density of 10 A g-1, making CHNCs rank in the forefront of previously known carbon-based supercapacitor materials. These comprehensive characterizations demonstrate that copper ions introduce new electrochemically active sites and enhance the conductivity and charge transport performance of the electrode material, elevating the specific capacitance of CHNC from 463 to 562 F g-1. These findings offer valuable insights into the effective energy storage application of heavy-metal-contaminated biomass wastes.

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

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

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

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

Opportunities, challenges and modification methods of coal gangue as a sustainable soil conditioner-a review

The persistent reliance on coal has resulted in the accumulation of substantial coal gangue, a globally recognized problematic solid waste with environmental risks. Given the coal gangue properties and global land degradation severity, the resourceful utilization of coal gangue as soil conditioners is believed to be a universally applicable, cost-effective, high-demand and environment-friendly model with broad application prospect. The direct application of raw coal gangue faces challenges of low active beneficial ingredients, inadequate water and fertilizer retention, presence of potentially toxic elements, resulting in limited efficacy and environmental contamination. This paper provided a comprehensive review of various modification methods (including mechanical, chemical, microbiological, thermal, hydrothermal and composite modifications) employed to enhance the soil improvement performance and reduce the environmental pollution of coal gangue. Furthermore, an analysis was conducted on the potential application of modified coal gangue as a muti-function soil conditioner based on its altered properties. The modified coal gangue is anticipated to effectively enhance soil quality, exhibiting significant potential in mitigating carbon emissions and facilitating soil carbon sequestration. This paper provided innovative ideas for future research on the comprehensive treatment of coal gangue and restoration of degraded soil in order to achieve the dual goals of zero-coal gangue waste and sustainable agriculture.

Comparison and mechanism analysis of MgO, CaO, and Portland cement for immobilization of heavy metals in MSWI fly ash

The significant production of municipal solid waste incineration fly ash (MSWI FA) underscores the importance of developing efficient solidification materials. This study employed MgO and CaO for immobilizing MSWI FA (with a 70% fly ash incorporation), and the immobilization effect was compared with that of Portland cement (PC). Experimental findings revealed that MgO exhibited the most effective stabilization for heavy metals (Cd, Cu, Pb, and Zn) compared to CaO and PC. XRD, FTIR, TG, and SEM analysis indicated that the principal hydration products in MSWI FA binders solidified with MgO, CaO, and PC were Mg(OH)2, CaCO3, and C-S-H gel, respectively. Mg(OH)2 efficiently immobilized heavy metals through chemical complexation and surface adsorption mechanisms. The MgO-treated MSWI FA demonstrated the highest residual fractions and the lowest easily leachable fractions. Moreover, the leaching characteristics of heavy metals were significantly influenced by the pH level, so MgO-treated MSWI FA with a leachate pH of 9.18 achieved the precipitation and stabilization of most heavy metals. In summary, this study provided an effective material selection for MSWI FA immobilization and presented a novel strategy for MSWI FA management.

Mechanistic insights into temperature-driven retention and speciation changes of heavy metals (HMs) in ash residues from Co-combustion of refuse-derived fuel (RDF) and red mud

Red mud is a promising candidate for promoting the incineration of Refuse Derived Fuel (RDF) and stabilizing the resulting incineration ash. The combustion conditions, notably temperature, significantly steers the migration and transformation of harmful metal components during combustion, and ultimately affect their retention and speciation in the ash residue. The study attempted to investigate the effect of co-combustion temperature on the enrichment and stability of Cr, Ni, Cu, Zn, Cd and Pb within bottom ashes, and to reveal the underlined promotion mechanism of red mud addition. As temperature increased, red mud's active components formed a robust matrix, helping the formation, melting, and vitrification of silicates and aluminosilicates in the bottom ashes. The process significantly contributed to the encapsulation and stabilization of heavy metals such as Ni, Cu, Zn, Cd, and Pb, with their residual fractions ascending to 71.37%, 55.75%, 74.78%, 84.24%, and 93.54%, respectively. Conversely, high temperatures led to an increase in the proportion of Cr in the extremely unstable acid-soluble fraction of the bottom ashes, reaching 31.52%, posing a heightened risk of environmental migration. Considering the stability of heavy metals in the bottom ashes and the combustion characteristics, 800 °C is identified as the optimal temperature for the co-combustion of RDF and red mud, balancing efficiency and environmental safety. The findings will provide valuable insights for the co-utilization strategy of RDF and red mud, contributing to more informed decision-making in waste-to-energy processes.

Preparation of geopolymer based on municipal solid waste incineration fly ash-phosphorus slag and its function for solidification of heavy metals

Municipal solid waste incineration fly ash (MSWIFA) contains potential contaminants and needs to be efficiently solidified/stablized and so should be managed properly. To achieve this goal, alkali-activated MSWIFA and phosphorus slag (PS) based geopolymer solidified bodies were investigated. Therefore, the mechanical properties of the solidified body, heavy metal leaching characteristics, heavy metal chemical forms, and heavy metal solidification/stabilization mechanisms were also analyzed. The results show that: The addition of an appropriate amount of PS can promote the strength development of a solidified body. When the mass ratio of MSWIFA to PS is 7:3, the strength of the solidified body reaches 22.8 MPa at 90d curing age, which is 5.3 times higher than that of the unmodified material. The MSWIFA/PS immobilized Zn 99.9 %, Pb 99.4 % and Cd 99.8 % in 60 day leaching tests. Meanwhile, PS can significantly increase the proportion of chemically stabilized forms of heavy metals in the solidified body. PS affects on the hydration process of the solidified body. When the mass fraction of PS doping was 30 %, the main hydration products of the solidified body were calcium silicate hydrate (C-S-H) and calcium alumina (AFt). When the mass fraction of PS is 50 %, the main hydration products are calcium aluminosilicate hydrate (C-A-S-H), sodium aluminosilicate hydrate (N-A-S-H), and AFt. These hydration products have good solidification effects on heavy metals. Therefore, it can be concluded that the MSWIFA/PS solidified body is an environmentally friendly and efficient binder.

Leaching behavior and comprehensive toxicity evaluation of heavy metals in MSWI fly ash from grate and fluidized bed incinerators using various leaching methods: A comparative study

Municipal solid waste incineration fly ash (MSWI FA) is a kind of hazardous waste that contains a substantial amount of heavy metals. To facilitate the appropriate treatment of MSWI FA, the leaching behavior of heavy metals was evaluated in MSWI FA from various sources using different leaching methods. Nine kinds of MSWI FA were investigated using three kinds of batch leaching tests (TCLP, HJ/T 300, and EN12457-2). The chemical form distributions of heavy metals in MSWI FA were obtained by sequential extraction procedures (SEPs) and the environmental risk posed by MSWI FA was comprehensively evaluated. The results showed that the grate and fluidized bed MSWI FA performed differently in various leaching methods, which was mainly dependent on the leachate pH and the chemical form distributions of the heavy metals. In addition, the BCR SEP was more suitable for the fractionation of heavy metals and the environmental risk assessment of MSWI FA when compared with Tessier's SEP. The overall pollution toxicity index allowed a comprehensive risk assessment specific to the leaching environment, thereby offering valuable guidelines for the stabilization or resource-based treatment of MSWI FA.

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

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

Preparation of functional geopolymers from municipal solid waste incineration fly ash: An approach combining experimental and computational simulation studies

This study aims to evaluate the feasibility and safety of using municipal solid waste incineration fly ash (MSW-IFA) in the development of geopolymer-based solidification/stabilization (S/S) treatments. Geopolymers have garnered attention as a sustainable alternative to traditional cement, owing to their high strength, stability, and minimal CO2 emissions. In this study, a combination of experimental and simulation calculations was used to investigate the setting time, mechanical properties, environmental risks, hydration mechanisms and processes of municipal solid waste incineration fly ash-based polymeric functional cementitious materials (GFCM). The results demonstrate that the mechanical properties of GFCM are related to the changes in the mineral phases and the degree of compactness. Quantum chemical calculations indicate that the hydration products may be [Si(OH)4], [Al(OH)3(OH2)] and [Al(OH)4]-. It is possible that the heavy metals are embedded in the hydrated silica-aluminate by electrostatic interaction or chemisorption. Heavy metals may be embedded in hydrated silica-aluminate by electrostatic action or chemisorption. This study provides a feasible method for resource utilization and heavy metal stabilization mechanism of MSW-IFA.

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

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

Characterization and stabilization of incineration fly ash from a new multi-source hazardous waste co-disposal system: field-scale study on solidification and stabilization

The multi-source hazardous waste co-disposal system, a recent innovation in the industry, offers an efficient approach for hazardous waste disposal. The incineration fly ash (HFA) produced by this system exhibits characteristics distinct from those of typical incineration fly ash, necessitating the use of adjusted disposal methods. This study examined the physicochemical properties, heavy metal content, heavy metal leaching concentration, and dioxin content of HFA generated by the new co-disposal system and compared them with those of conventional municipal waste incineration fly ash. This study investigated the solidification and stabilization of HFA disposal using the organic agent sodium diethyl dithiocarbamate combined with cement on a field scale. The findings revealed significant differences in the structure, composition, and dioxin content of HFA and FA; HFA contained substantially lower levels of dioxins than FA did. Concerning the heavy metal content and leaching; HFA exhibited an unusually high concentration of zinc, surpassing the permitted emission limits, making zinc content a critical consideration in HFA disposal. After stabilization and disposal, the heavy metal leaching and dioxin content of HFA can meet landfill disposal emission standards when a 1% concentration of 10% sodium diethyldithiocarbamate (DDTC) and 150% silicate cement were employed. These results offer valuable insights into the disposal of fly ash resulting from incineration of mixed hazardous waste.

A novel amine functionalized porous geopolymer spheres from municipal solid waste incineration fly ash for CO2 capture

Municipal solid waste (MSW) incineration fly ash (FA) is classified as hazardous waste, and strategies for recycling FA have attracted attention. In this study, the porous geopolymer spheres (PGS) were prepared from FA by the foaming-suspension-solidification method, and then the PGS were functionalized with tetraethylenepentamine (TEPA) to capture CO2. The results showed that washing pretreatment and the addition of H2O2 foaming agent enhanced the pore volume and specific surface area of PGS. The CO2 adsorption capacity of amine-functionalized PGS exhibited a trend of increasing and then decreasing in the range of 35-80 °C. The maximum adsorption capacity of TEPA-WPGS3 was 2.55 mmol/g at 65 °C higher than expected for the average of TEPA and PGS. This was because PGS improved the dispersion of TEPA, thus exposing more active sites of TEPA and making it more likely to interact with CO2. The adsorption efficiency of amine-functionalized PGS decreased by only 2.4% after 10 cycles, indicating that it has excellent regeneration performance. In addition, amine-functionalized PGS, which showed excellent CO2 adsorption capacity, had a significant ability to selectively adsorb CO2 and the adsorption capacity of the rapid stage accounted for approximately 80% of the saturated adsorption capacity. This study shows that FA-derived geopolymers have excellent CO2 adsorption properties and provides a new method for the resource utilization of FA.

Novel cost-effective oxygen-enriched melting method for MSWI fly ash

Herein, a novel oxygen- enriched melting process for fly ash, which uses the biogas produced from the leachate of municipal solid waste incineration (MSWI) plants, is proposed to reduce the high cost of conventional fly ash - melting technology. The fly ash composition was estimated via X-ray fluorescence analysis; the six constituent elements detected in fly ash in the decreasing order of their content were calcium, chlorine, silicon, sulfur, sodium, and potassium. Based on literature and actual production data, the average yield of the leachate was 15% of the total waste entering the MSWI plants and the COD of leachate was 30,000-75,000 mg/L. The amount of biogas that can be used per ton of fly ash was calculated to be 62.0-157.0 m3. The analysis of melting thermal equilibrium revealed the amount of biogas required per ton of fly ash as 57.8 m3. The aforementioned research findings indicate that the biogas produced by MSWI plants can successfully meet the demands of the oxygen- enriched melting of fly ash produced in these plants. By establishing an oxygen- enriched- melting pilot platform, the pilot tests of melting were conducted on fly ash; the results revealed the good melting effects of fly ash. The X-ray diffraction analysis of the slag demonstrated that the content of the vitreous body met the technical requirements for glassy substances. Furthermore, the leaching toxicity test results revealed that heavy metals were well solidified in the slag. This study presents a novel fly ash - melting scheme for MSWI fly ash, namely, biogas oxygen- enriched melting strategy, which has the advantages of technical feasibility and cost- effectiveness. The proposed technique exhibits considerable prospects for widespread application in MSWI plants in China and can play an important role in the safe disposal of fly ash.Implications: In this paper, a low- cost melting method of municipal solid waste incineration(MSWI) fly ash is proposed. This method uses the biogas generated by MSWI plant itself as fuel for melting. Through research, it has been found that the production of biogas can meet the demand for fly ash melting. Adopting biogas as a molten fuel can significantly reduce the cost of melting, thereby significantly reducing the cost of fly ash melting. This study established a pilot scale platform for the melting of biogas and conducted pilot scale experiments on fly ash and additives. The experimental results showed that the melting system operated well and achieved the vitrification of fly ash. The leaching test results of the molten slag showed that heavy metals were well solidified in the slag. The research results can be extended to the MSWI plant for application, which can significantly reduce the cost of fly ash melting disposal, and has broad application prospects.

A novel and economical approach for the synthesis of short rod-shaped mesoporous silica nanoparticles from coal fly ash waste by Bacillus circulans MTCC 6811

Coal fly ash (CFA) is an industrial byproduct produced during the production of electricity in thermal power plants from the burning of pulverized coal. It is considered hazardous due to the presence of toxic heavy metals while it is also considered valuable due to the presence of value-added minerals like silicates, alumina, and iron oxides. Silica nanoparticles' demands and application have increased drastically in the last decade due to their mesoporous nature, high surface area to volume ratio, etc. Here in the present research work, short rod-shaped, mesoporous silica nanoparticles (MSN) have been synthesized from coal fly ash by using Bacillus circulans MTCC 6811 in two steps. Firstly, CFA was kept with the bacterial culture for bioleaching for 25 days in an incubator shaker at 120 rpm. Secondly, the dissolved silica in the medium was precipitated with the 4 M sodium hydroxide to obtain a short rod-shaped MSN. The purification of the synthesized silica particle was done by treating them with 1 M HCl at 120 °C, for 90 min. The synthesized short rod-shaped MSN were characterized by UV-vis spectroscopy (UV-Vis), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Particle size analyzer (PSA), Field emission scanning electron microscopy (FESEM), and transmission electron microscope. The microscopic techniques revealed the short rod-shaped mesoporous silica nanoparticles (MSN) for the final nano-silica, whose size varies from 40 to 80 nm, with an average size of 36 ± 5 nm. The XRD shows the crystalline nature of the synthesized MSN having a crystallite size of 36 nm. The FTIR showed the three characteristic bands in the range of 400-1100 cm-1, indicating the purity of the sample. The energy dispersive X-ray (EDX) showed 53.04 wt% oxygen and 43.42% Si along with 3.54% carbon in the final MSN. The particle size analyzer revealed that the average particle size is 368.7 nm in radius and the polydispersity index (PDI) is 0.667. Such a novel and economical approach could be helpful in the synthesis of silica in high yield with high purity from coal fly ash and other similar waste.

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

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

Synergistic influence of diatomite and MoS2 nanosheets on the self-alkali-activated cementation of the municipal solid waste incineration fly ash and mechanisms

The solidification/stabilization technique recommended for the disposal of municipal solid waste incineration (MSWI) fly ashes in developed countries was inappropriate for the treatment in most developing counterparts. In this study, the diatomite and MoS₂ nanosheets were synergistically employed to activate the self-alkali-activated cementation of the MSWI fly ashes to achieve efficient solidification, the immobilization of heavy metals (HMs), and the inhibition of chloride release. The compressive strength of 28.61 MPa and the leaching toxicities (mg/L) of Zn, Pb, Cu, Cd, and Cr of 2.26, 0.87, 0.5, 0.06, and 0.22 were obtained from the hardened mortars. Diatomite significantly influenced the self-alkali-activated cementation of the MSWI fly ashes while MoS₂ nanosheets played both roles in intensifying the stabilization of HMs and strengthening the binding process by inducing the formation of sodalite and kaolinite, enhancing the growth rates of nucleation, and transforming the layered cementation to the partial and full three-dimensional cementation in the hardened matrix. This study not only verified the feasibility of diatomite and MoS₂ in activating the self-alkali-activated cementation of the MSWI fly ashes but also supplied a reliable technique for the harmless disposal and efficient utilization of MSWI fly ashes in developing countries.

Numerical simulation on the deposition characteristics of MSWI fly ash particles in a cyclone furnace

In melting municipal solid waste incineration (MSWI) fly ash by cyclone furnace, the deposition characteristics of particles affect the slag flow and the secondary MSWI fly ash formation. In this study, the composition mechanism based on critical viscosity is selected as the particle deposition model to predict the deposition and rebound of particles on the furnace wall. The Riboud model with an accurate viscosity prediction performance is selected, then the particle deposition model is integrated into a commercial computational fluid dynamics (CFD) solver through the user-defined function (UDF) to realize the coupling of particle motion and deposition process. The results show that under the same case, the deposition rate decreases obviously with the increase of MSWI fly ash particle size. And the escape rate reaches a maximum at particle size 120 μm. Controlling the particle size of fly ash particles within 60 μm can effectively reduce the generation of secondary MSWI fly ash. During the forward movement of the fly ash inlet position, the escape of MSWI fly ash particles with large particle sizes has been significantly weakened. This measure not only lowers the post-treatment cost but also dramatically reduces the pretreatment step of MSWI fly ash before the melting and solidification process. In addition, the deposition rate and quality will reach the maximum values, respectively, along with gradually increasing MSWI fly ash input flow. Overall, this study has the guiding significance for reducing the pretreatment steps and post-treatment costs of MSWI fly ash by melting in the cyclone furnace.

Synthesis of high-quality NaP1 zeolite from municipal solid waste incineration fly ash by microwave-assisted hydrothermal method and its adsorption capacity

The Si and Al in municipal solid waste incineration fly ash (MSWI FA) can be utilized for zeolite fabrication, which can improve the application value of the products. This study focuses on the fabrication of zeolite from MSWI FA by microwave-assisted hydrothermal (MH) treatment. The effects of magnetic stirring time, Na₂SiO₃ dosage, MH time, and NaOH solution concentration on the crystallization of zeolite NaP1 from MSWI FA are systematically analyzed. The synthetic products are analyzed through spectroscopic and mineralogical methods. The results show that zeolite NaP1 with high crystallinity (51.68 %) can be fabricated by magnetic stirring and MH treatment, and the cation exchange capacity (CEC) of the product can reach a value of 2.58 meq/g, which is approximately 133 times that of the CEC of MSWI FA. The Si/Al ratio plays a decisive role in the zeolite NaP1 synthesis, and a Na₂SiO₃ dosage of 30 wt% is adopted for zeolite NaP1 fabrication. A NaOH concentration of 1 M is sufficient for zeolite NaP1 synthesis. Additionally, the zeolite NaP1 content is found to obviously increase with increasing MH time from 0.5 h to 2 h. To demonstrate the feasibility of the method provided in this study, the optimal experimental condition is employed for various MSWI FAs, and zeolite NaP1 and analcime are fabricated successfully. The leachability of heavy metals for the synthetic products was evaluated, which met the requirements for pollution control. The BET surface area and total pore volume of zeolite NaP1 fabricated at optimal condition are 61.42 m²/g and 0.44 cm³/g, respectively. The adsorption capacity of zeolite NaP1 for Cu²⁺ ion and methylene blue are determined to be 84.65 mg/g and 84.55 mg/g, respectively, indicating zeolite NaP1 is a potential adsorbent for cation ion and dyes. This study provides an environmentally friendly scheme for the utilization of MSWI FA.

Transformation and leaching behavior of Pb in hazardous waste incineration fly ash after thermal treatment with addition of Fe2O3

This study investigated the leaching behavior of Pb in hazardous waste incineration fly ash (HWIFA) after adding Fe₂O₃ thermal treatment and revealed the leaching mechanism of Pb from the perspective of phase transformation. The static leaching results showed that at 600 °C-1300 °C, with the addition of Fe₂O₃ increased, the Pb leaching toxicity continued to decrease. The dynamic results indicated that as the thermal treatment temperature was higher than 1100 °C, the addition of Fe₂O₃ can effectively inhibit the dynamic leaching of Pb in HWIFA. Meanwhile, the inhibition effect was not very closely related to the amount of Fe₂O₃. The addition of Fe₂O₃ can react with PbO to form PbFe₁₂O₁₉, which has a better stability. The appearance of PbFe₁₂O₁₉ was the main reason for adding Fe₂O₃ to enhanced the immobilization of Pb. However, the amount of Fe₂O₃ should be carefully controlled to avoid an excessive reducible fraction of Pb in the thermal treated HWIFA, which will affect the long-term stability of Pb.