Performance of chemical chelating agent stabilization and cement solidification on heavy metals in MSWI fly ash: A comparative study

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.

Co-melting mechanisms for municipal solid waste incineration fly ash with fine slag from coal gasification and coal gangue

Vitrification is a promising treatment for municipal solid waste incineration fly ash (MSWI-FA); however, high energy consumption due to the high MSWI-FA fusion temperature limits the development and application of this technique. In this study, fine slag ash (FSA) derived from coal gasification and coal gangue ash (CGA) were mixed with MSWI-FA to reduce the ash fusion temperature. The transformation of minerals in ash during thermal treatment was examined via X-ray diffraction and thermodynamic equilibrium calculations. The ash flow behaviour was observed using a thermal platform microscope, and the silicate structure was quantified using Raman spectra. The co-melting mechanisms for the mixed ash were systematically investigated. Results indicate that the flow temperature (FT) of the mixed ash exhibited an initial decrease and subsequent increase as a function of the addition ratio of FSA or CGA. Lowest ash FT of 1215 °C and 1223 °C were recorded for addition of 50% FSA and 50% CGA, respectively; further, these temperatures were lowered by > 285 °C and >277 °C respectively, relative to FT of the MSWI-FA. The transformation of minerals and silicate structure during mixed ash heating was responsible for the variation in the ash fusion temperature. CaO in MSWI-FA tended to react with mullite, quartz and haematite in FSA and CGA, forming minerals such as anorthite, gehlenite, and andradite with relatively low melting points. The addition of FSA or CGA caused changes in the silicate network structure of the mixed ash. In particular, 50% FSA incorporation caused the transformation of Q4 and Q3 to Q2, whereas 50% CGA introduction resulted in the conversion of Q4 and Q2 into Q3 and Q1 + Q0, respectively. The silicate network depolymerised, causing reduction in the ash fusion temperature and increasing the melting rate.

Solidification and utilization of municipal solid waste incineration ashes: Advancements in alkali-activated materials and stabilization techniques, a review

Researchers are actively investigating methodologies for the detoxification and utilization of Municipal Solid Waste Incineration Bottom Ash (MSWIBA) and Fly Ash (MSWIFA), given their potential as alkali-activated materials (AAMs) with low energy consumption. Recent studies highlight that AAMs from MSWIFA and MSWIBA demonstrate significant durability in both acidic and alkaline environments. This article provides a comprehensive overview of the processes for producing MSWIFA and MSWIBA, evaluating innovative engineering stabilization techniques such as graphene nano-platelets and lightweight artificial cold-bonded aggregates, along with their respective advantages and limitations. Additionally, this review meticulously incorporates relevant reactions. Recommendations are also presented to guide future research endeavors aimed at refining these methodologies.

Applicability assessment of a plasma melting technology for a fly ash recycling system in the Republic of Korea

ABSTRACTThe Ministry of Environment of Korea has proposed a ban on landfill disposal of municipal solid waste (MSW) from 2026. Thus, it is inferred that the amount of incineration ash will increase drastically. Against this backdrop, this study assessed the applicability of a plasma melting process to fly ash. Fly ash was collected from 14 incineration facilities to analyze its basic properties and perform melting experiments. Furthermore, scanning electron microscope (SEM) analysis and economic feasibility assessment were conducted. The molten fly ash slag exhibited a pH value of 9.9, and the ignition loss of fly ash was found to range from 14.5 to 25.7 wt.%. None of seven toxic elements (arsenic (As), cadmium (Cd), cyanide (CN), mercury (Hg), hexavalent chromium (Cr(VI)), copper, and lead (Pb)) was detected from the molten slag. In addition, 99.3 wt.% of chloride ion (Cl-), 97.9 wt.% of fluoride ion (F-), and 98.1 wt.% of sulphate ion (SO42-) were removed. The contents in the molten slag were found to be 0.19, 7.8, 27.8, 33.1, and 38 mg/kg for Cd, Pb, zinc, nickel, and F, respectively, and none of CN, Hg, and As was detected, thereby meeting the criteria for soil pollution. All of the environmental standards were met, and SEM analysis confirmed stable quality with high density and no surface pore. In the economic feasibility assessment, a profit of approximately 152.4 $/ton was also estimated compared to landfill disposal.

Synergistic utilization, critical mechanisms, and environmental suitability of bauxite residue (red mud) based multi-solid wastes cementitious materials and special concrete

The energy consumption and carbon emissions in the construction field, coupled with the accumulation of various industrial solid wastes, particularly bauxite residue (red mud), represent formidable barriers to sustainable development. The synergistic utilization of bauxite residue (red mud) in cementitious materials and special concrete is widely considered one of the most practical approaches for these issues. In this comprehensive review, characteristics and composition of red mud worldwide were investigated. By comparing and reviewing the latest research, the current achievements in applying red mud with various solid wastes in cementitious materials and special concrete were discussed. In addition, critical mechanisms and environmental suitability issues are emphasized. In conclusion, the present work culminates in identifying the challenges faced and opportunities for progressing in synergizing red mud and multi-solid wastes, which will contribute to the international research community for sustainable development in the industry.

A robust acid-resistant chelating polymer for enhanced stabilization of lead ions in fly ash

Fly ash derived from municipal solid waste incinerators (MSWIs) harbors significant quantities of heavy metals with high leaching toxicity, resulting in detrimental environmental effects. Pb2+ in fly ash is the ion most likely to exceed permissible levels. However, chemical stabilization methods demonstrate poor efficacy in stabilizing Pb2+ under acidic conditions. Herein, we have developed a robust acid-resistant chelating polymer (25DTF) for enhanced stabilization of Pb2+ in fly ash. 25DTF was synthesized through the reaction of formaldehyde with 2,5-dithiourea. 25DTF exhibited remarkable chelation efficiency, nearing 100%, for Pb2+ in fly ash. 25DTF demonstrated exceptional chelation efficiency, surpassing 99.9%, when interacting with Pb2+ in fly ash at pH ≤ 7. Even under acidic conditions, 25DTF effectively prevented the secondary dissolution of Pb2+. Additionally, it indicated outstanding Pb2+ chelation efficiency across diverse regions of China. The 25DTF chelating agent shows considerable potential in alleviating metal ion contamination in soil, wastewater, and urban environmental management, thereby fostering advancements in environmental stewardship.

A new strategy to stabilize the heavy metals in carbonized MSWI-fly ash using an acid-resistant oligomeric dithiocarbamate chelator

Fly ash (FA) derived from municipal solid waste incineration (MSWI) requires safe handling before landfilling due to its extremely high salt content and the risk of leaching heavy metals (HMs) under acidic conditions. Herein, aimed at improving the acid stability of dithiocarbamates, a cost-effective oligomeric dithiocarbamate (ODTC) was developed to stabilize HMs from carbonated MSWI-FA. Spiking of 3.6 wt% ODTC reduced the HM leaching below landfill standards in China, even across the pH range of 2.0-13.0 or 8-week exposure to the natural environment. Stabilization decreased the acid-soluble/exchangeable fractions of Cd, Pb, and Zn from 22.2%, 4.49%, and 21.9% to 0.14%, 0.11%, and 12.2%, respectively, resulting in safe levels for Pb and Cd with risk assessments. Compared to DDTC and SDD, ODTC exhibited higher stability under acidic conditions after chelation with the HMs, minimized the risk of HM leaching, and significantly reduced stabilization costs. In-depth studies proved that the stabilization mechanism involved the ability of ODTC to chelate HMs strongly and form acid-resistant ODTC-HM complexes, agglomeration of the MSWI-FA grains to encapsulate the ODTC-HM complexes, transformations of the HMs from acid-soluble species to stable oxidizable and residual species, and specifically ODTC reducing high-valent Pb to more stable Pb(II) species.

Analysis of composition characteristics and treatment techniques of municipal solid waste incineration fly ash in China

The rapid development of the economy and society is causing an increase in the amount of municipal solid waste (MSW) produced by people's daily lives. With the strong support of the Chinese government, incineration power generation has steadily become the primary method of treating MSW, accounting for 79.86%. However, burning produces a significant amount of municipal solid waste incineration fly ash (MSWI-FA), which contains heavy metals, soluble chlorine salts, and dioxins. China's MSWI-FA yield increased by 8.23% annually to 7.80 million tons in 2022. Besides, the eastern region, especially the southeastern coastal region, has the highest yield of MSWI-FA. There are certain similarities in the chemical characteristics of MSWI-FA samples from Northeast, North, East, and South China. Zn and CaO have the largest amounts of metals and oxides, respectively. The Cl content is about 20 wt%. This study provides an overview of the techniques used in the thermal treatment method, solidification and stabilization, and separation and extraction of MSWI-FA and compares their benefits and drawbacks. In addition, the industrial applications and standard requirements of landfill treatment and resource utilization of MSWI-FA in China are analyzed. It is discovered that China's resource utilization of MSWI-FA is insufficient through the study on the fly ash disposal procedures at a few MSW incineration facilities located in the economically developed Guangdong Province and the traditional industrial city of Tianjin. Finally, the prospects for the disposal of MSWI-FA were discussed.

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

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

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.

Use of municipal solid waste incineration fly ash as a supplementary cementitious material: CO2 mineralization coupled with mechanochemical pretreatment

The use of municipal solid waste incineration fly ash, commonly referred to as "fly ash", as a supplementary cementitious material (SCM), has been explored to mitigate the CO2 emissions resulting from cement production. Nevertheless, the incorporation of fly ash as an SCM in mortar has been shown to weaken its compressive strength and increase the risk of heavy metal leaching. In light of these challenges, this study aims to comprehensively evaluate the influence of CO2 pressure, temperature, and residual water/binder ratio on the CO2 uptake and compressive strength of mortar when combined with fly ash. Additionally, this study systematically examines the feasibility of mechanochemical pretreatment, which enhances the homogenization of fly ash and augments the density of the mortar's microstructure. The results indicate that the use of mechanochemical pretreatment leads to a notable 43.6% increase in 28-day compressive strength and diminishes the leaching of As, Ba, Ni, Pb, Se, and Zn by 17.9-77.8%. Finally, a reaction kinetics model is proposed to elucidate the CO2 sequestration process under varying conditions. These findings offer valuable guidance for incorporating fly ash as an SCM and CO2 sequestrator in mortar.

Mixing uniformity effect on leaching behaviour of cement-based solidified contaminated clay

Mixing uniformity is essential for the quality control of the contaminated clay's solidification. To investigate the effect of the mixing uniformity on the leaching behaviour of the cement-based solidified contaminated clay, this study proposed a quantitative method to characterize the mixing uniformity and investigated the leaching behaviour by the leaching toxicity tests and semi-dynamic leaching tests. X-ray computed tomography (X-CT) was employed to reveal the internal mesoscopic structure. In this case, Pb2+ was selected as a tagged pollutant because of the widespread attention at heavy metal-contaminated sites. The leaching toxicity indicates the significant Pb2+ concentration deviation among the parallel specimens and non-association with the mixing uniformity. However, the Pb2+ cumulative leaching mass and observed diffusion coefficient by the semi-dynamic leaching tests both decrease with the mixing uniformity. X-CT image analysis reveals that the high cement zones wrap the low cement zones with different dimensions in the heterogeneous solidified matrix. Moreover, the specimen pretreatment method in the existing leaching toxicity standards may be inadequate because of the overall encapsulation destruction by the crushing process and representativeness uncertainty when sampling. However, for semi-dynamic leaching, the Pb2+ migration depends on the uniformity, reflecting the continuous distribution of high cement zones.

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.

Research on leakage environmental risk assessment and risk prevention and control measures in the long-term landfill process of ultra-alkaline fly ash

In this study, we simulated the actual landfill disposal process using accelerated carbonization experiments, based on the leaching characteristics of heavy metals from "alkaline" fly ash, and used the LandSim-HELP coupling model to assess the environmental risk of the leaching. The results showed that the leaching data of "alkaline" fly ash before carbonization showed the illusion of admission to landfill with only a small amount of chemical addition or even without curing/stabilization. The leached concentrations of Zn and Cd from "alkaline" fly ash after carbonation were significantly higher. The risk assessment of the leakage of heavy metals in the case of a single artificial composite liner system showed that the exposure concentrations of Pb, Zn, and Cd in samples exceeded Standard for groundwater quality (GB/T 14848-2017) the Class III permissible limits after carbonation; exposure risk for Cd was exceeded in all samples. However, although the use of a double-layer artificial composite liner to improve the level of impermeability effectively reduced the risk of Cd leaching, so that none of the non-carcinogenic risks exceeded the standard, the carcinogenic risk of Cd in the carbonized samples exceeded the factor of 1.1-4.5 of the acceptable hazard quotient, and the contamination characteristics of the alkaline fly ash still need to be kept in view.

Hospital waste incinerator ash: characteristics, treatment techniques, and applications (A review)

The amount of medical waste generated has increased enormously since the COVID-19 outbreak. An incineration process is the main method that is usually used to treat this waste, causing an increase in both medical waste bottom ash (MWBA) and medical waste fly ash (MWFA). In this work, the physical and chemical characteristics of MWFA and MWBA were reviewed. This ash contains high levels of polychlorinated dibenzo-p-dioxin (PCDD), dibenzofurans (PCDFs), and heavy metals. Furthermore, medical waste ash appears to have high leachability in the toxicity characteristics leaching procedure (TCLP) test and the European standard test (EN 12457). Owing to its toxicity, medical ash can be treated using various methods prior to disposal based on the covered review. These techniques include chemical, supercritical fluid, cement-based, melting, microwave, and mechanochemical techniques. The shortcomings of some of these treatment methods have been identified, such as the emission of high levels of chlorine from the melting technique, limited applications of the flotation method on the industrial scale, long-term stability of leachate treated by cement-based methods that have not been confirmed yet, and high energy consumption in the supercritical technique. This review also covers possible applications of medical waste ash in cement production, agriculture, and road construction.

Stabilization and solidification of arsenic contaminated silty sand using alkaline activated slag

Contamination of soils by arsenic represents a significant environmental and public health risk, making effective remediation strategies a pressing concern. One commonly employed technique is stabilization and solidification, which involves the addition of stabilizing binders such as cement to immobilize arsenic. This study investigates the potential of alkaline activated slag for stabilization and solidification of arsenic-contaminated soil, employing the toxic characteristic leaching procedure (TCLP) and unconfined compressive strength (UCS) tests. To assess the strength and leachability behavior, the research examines the effect of several factors, including binder content, curing time, curing conditions, alkaline activator solution to slag ratio, sodium silicate to sodium hydroxide ratio, and sodium hydroxide concentration. Additionally, field emission scanning electron microscopes (FE-SEM) in combination with energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) tests are employed to analyze the stabilization and solidification mechanism. The study shows that increasing the slag content to 20% by weight after 28 days of curing at ambient temperature leads to a decrease of almost 92% in the concentration of leached arsenic and an increase in UCS from 80 kPa to approximately 19 MPa. The formation of albite and anorthite crystals, along with gels such as (N, C)-A-S-H and C-S-H, contributes to enhanced strength and reduced leachability. As a result, the use of alkaline activated slag is identified as an effective and environmentally friendly approach for the stabilization/solidification of arsenic-contaminated soils. This study highlights the use of alkaline activated slag as an effective solution for remediating arsenic-contaminated soils while simultaneously reducing waste, greenhouse gas emissions, and energy consumption. Slag, a byproduct of metal production, is often wasted due to a lack of value and landfill space. However, alkaline activated slag demonstrates the potential to stabilize soil, immobilize heavy metals, and provide efficient and sustainable soil remediation.

Synthesis of metakaolin-based geopolymer foamed materials using municipal solid waste incineration fly ash as a foaming agent

The existence of metallic aluminum in municipal solid waste incineration fly ash (MSWIFA) makes it challenging to recycle MSWIFA into cement materials because expansion occurs in the resultant matrices. Geopolymer-foamed materials (GFMs) are gaining attention in the field of porous materials due to their high-temperature stability, low thermal conductivity and low CO2 emission. This work aimed to utilize MSWIFA as a foaming agent to synthesize GFMs. The physical properties, pore structure, compressive strength and thermal conductivity were analyzed to assess different GFMs which were synthesized with various MSWIFA and stabilizing agent dosages. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) analysis were conducted to characterize the phase transformation of the GFMs. Results showed that when MSWIFA content was increased from 20 to 50%, the porosity of GFMs increased from 63.5 to 73.7%, and bulk density decreased from 890 to 690 kg/m3. The addition of stabilizing agent could trap the foam, refine the cell size, and homogenize the cell size range. With the stabilizing agent increase from 0 to 4%, the porosity increased from 69.9 to 76.8%, and the bulk density decreased from 800 to 620 kg/m3. The thermal conductivity decreased with increasing MSWIFA from 20 to 50%, and stabilizing agent dosage from 0 to 4%. Compared with the collected data from references, a higher compressive strength can be obtained at the same level of thermal conductivity for GFMs synthesized with MSWIFA as a foaming agent. Additionally, the foaming effect of MSWIFA results from the H2 release. The addition of MSWIFA changed both the crystal phase and gel composition, whereas the stabilizing agent dosage had little impact on the phase composition.

Designing low-carbon cement-free binders for stabilization/solidification of MSWI fly ash

Low-carbon and high-efficiency binder is desirable for sustainable treatment of municipal solid waste incineration fly ash (MSWI FA). In this study, CaO or MgO was used to activate ground granulated blast furnace slag (GGBS) to form calcium silicate hydrate and magnesium silica hydrate gel for stabilization/solidification of hazardous MSWI FA. Experimental results showed that potential toxic elements (PTEs), such as Pb and Zn, significantly inhibited the formation of reaction products in CaO-GGBS system due to the complexation between Ca(OH)₂ and PTEs, whereas PTEs only had insignificant inhibition on transformation from MgO to Mg(OH)₂ in MgO-GGBS system, resulting in lower leachabilities of PTEs and higher mechanical strengths. Stabilization/solidification experiments demonstrated that MSWI FA (70 wt%) could be recycled by MgO-GGBS binder (30 wt%) into blocks with desirable 28-day compressive strengths (3.9 MPa) and PTEs immobilization efficiencies (99.8% for Zn and 99.7% for Pb). This work provides mechanistic insights on the immobilization mechanisms of PTEs in CaO/MgO-GGBS systems and suggests a promising MgO-GGBS binder for low-carbon treatment of MSWI FA.

Regional CO2 accounting and market layout of incinerator fly ash management in China

The reduction of greenhouse gas (GHG) emissions from solid waste incinerator fly ash (IFA) management attracts growing interests since China's zero-waste plan and carbon peak/neutral goals. Herein, provincial GHG emissions from four demonstrated IFA reutilization technologies in China were estimated after analyzing IFA spatial-temporal distribution. Results indicate that technologies transition (landfilling-to-reutilization) could reduce GHG except for glassy slag production. IFA to cement option could potentially realize negative GHG emissions. Spatial GHG variation drivers in IFA management were recognized as provincial-different IFA composition and power emission factors. IFA management options were recommended provincially after weighting local development goals related to GHG reduction and economic benefits. Baseline scenario analysis shows that China's IFA industry would reach carbon peak in 2025 (5.02 Mt). 2030's GHG reduction potential (6.12 Mt) is equivalent to that of absorbed CO₂ by 340 million trees annually. Overall, this research could contribute to illustrating future market layout complying with carbon peaking.

Evaluating the recycling potential of ashes discharged from waste incineration facilities and its dependency on pretreatment efficiency in Korea

The amount of incineration ash (IA) is expected to increase in South Korea from the rapidly rising numbers and operation capacities of incineration facilities; therefore, it remains necessary to establish measurements for the enhanced recycling and circularity of IA. This study established a database of hazardous substances in IA by compiling discharge data and survey results from domestic incineration facilities, along with literature survey values. The recycling potential of IA was assessed considering leaching reduction efficiency of various pretreatment methods. In particular, 98.2% of bottom ash and 49.0% of fly ash satisfied the IA recycling criteria after melting. Also, when mixed at a ratio of ∼ 78:22 natural soil to IA, the resulting material was usable for media-contact recycling by meeting the heavy metal content criteria of the Soil Environment Conservation Act.

The physical encapsulation and chemical fixation of Zn during thermal treatment process of municipal solid waste incineration (MSWI) fly ash

Thermal treatment is a promising treatment technology of municipal solid waste incineration (MSWI) fly ash because of its detoxication and volume reduction. However, the relationship between immobilization of heavy metals and mineral transformation during thermal treatment remains unclear. In this study, the immobilization mechanism of Zn during thermal treatment process of MSWI fly ash was investigated by experiment and calculation. The results show that addition of SiO₂ facilitates transition of dominant minerals from melilite to anorthite during sintering, increases liquid content during melting and improves liquid polymerization degree during vitrification. ZnCl₂ tends to be physically encapsulated by liquid phase, and ZnO is mainly chemically fixed into minerals at high temperature. Increase in both liquid content and liquid polymerization degree favors the physical encapsulation of ZnCl₂. The decreasing order of chemical fixation ability of minerals to ZnO is spinel > melilite > liquid > anorthite. To better immobilize Zn during sintering and vitrification process chemical composition of MSWI fly ash should be located in melilite and anorthite primary phases of pseudo-ternary phase diagram, respectively. The results are helpful to understand immobilization mechanism of heavy metals and avoid volatilization of heavy metals during thermal treatment process of MSWI fly ash.

Contaminant containment for sustainable remediation of persistent contaminants in soil and groundwater

Contaminant containment measures are often necessary to prevent or minimize offsite movement of contaminated materials for disposal or other purposes when they can be buried or left in place due to extensive subsurface contamination. These measures can include physical, chemical, and biological technologies such as impermeable and permeable barriers, stabilization and solidification, and phytostabilization. Contaminant containment is advantageous because it can stop contaminant plumes from migrating further and allow for pollutant reduction at sites where the source is inaccessible or cannot be removed. Moreover, unlike other options, contaminant containment measures do not require the excavation of contaminated substrates. However, contaminant containment measures require regular inspections to monitor for contaminant mobilization and migration. This review critically evaluates the sources of persistent contaminants, the different approaches to contaminant remediation, and the various physical-chemical-biological processes of contaminant containment. Additionally, the review provides case studies of contaminant containment operations under real or simulated field conditions. In summary, contaminant containment measures are essential for preventing further contamination and reducing risks to public health and the environment. While periodic monitoring is necessary, the benefits of contaminant containment make it a valuable remediation option when other methods are not feasible.

Subcritical hydrothermal treatment of municipal solid waste incineration fly ash: A review

Municipal solid waste incineration fly ash (MSWI-FA) is a hazardous waste generated from the incineration process, and the harmless treatment of MSWI-FA has attracted widespread attention. Subcritical hydrothermal treatment is competitive in achieving the harmless treatment and resource recycling of MSWI-FA. It exhibits excellent performance in degrading dioxins, stabilizing heavy metals, and converting MSWI-FA into zeolite or tobermorite at low temperatures. This paper clearly introduces the characteristics of MSWI-FA, roundly summarizes the current research status of treating MSWI-FA by subcritical hydrothermal methods, and deeply clarifies the mechanisms of dioxins degradation, zeolite/tobermorite synthesis, and heavy metals stabilization. Considering the research status of handling MSWI-FA by subcritical hydrothermal methods, future research directions are proposed. Owing to the advantages of high efficiency, energy-saving, and environmental sustainability, subcritical hydrothermal treatment of MSWI-FA exhibits promising prospects for industrialization.

Leaching characteristics and solidification strategy of heavy metals in solid waste from natural graphite purification

The tailings and fluorine-containing sludge were produced during the physical and chemical purification of natural crystalline graphite, containing heavy metals in different occurrence forms. To evaluate the threat of different heavy metals to the environment, this work uses the modified sequential extraction method (BCR) to study the presence of heavy metals in two solid wastes and their dissolution characteristics in different environments. The results show that the pollution risk of heavy metals in graphite tailings to the environment is ranked as Mn > Cr > Ni > Zn, and the pollution risk of Mn in fluorine-containing sludge is higher than that of Cr. This is because the Mn in the two solid wastes mainly exists in the form of weak acid extraction. The leaching number of heavy metals in the two solid wastes is directly proportional to the soaking time and soaking temperature, and inversely proportional to the pH value and the solid-to-liquid ratio. The number of heavy metals dissolved in solid waste landfills is significantly higher than that of acid rain and surface water environments. Based on the above results and the distribution of graphite solid waste, solidification agent was suggested to prevent heavy metal dissolution and reduce environmental risks.

Zeolite NaP1 synthesized from municipal solid waste incineration fly ash for photocatalytic degradation of methylene blue

The disposal of hazardous municipal solid waste incineration (MSWI) fly ash is a challenge nowadays. Recently, the re-utilization of MSWI fly ash by converting it to useful zeolite-containing materials has attracted attention. However, the zeolitic products fabricated from MSWI fly ash are usually of low quality and rarely reported to be applied for photocatalysis. In this study, valuable zeolites (e.g., NaP1) are synthesized from MSWI fly ash via a modified microwave-assisted hydrothermal method. The key parameters for the hydrothermal method including temperature, duration, the amount of additive, and water volume, are investigated and optimized. Specifically, increasing the hydrothermal temperature can promote the synthesis of zeolitic materials; a relatively long hydrothermal duration is essential to accomplish the assembly of zeolites; the addition of Na₂SiO₃ can increase the precursor for the fabrication of zeolites; the water volume makes little influence on the crystal style of products. Eventually, the hydrothermal condition of 180 °C, 1 h, 0.5 g Na₂SiO₃, and 10 mL water is suggested based on the energy consumption and the quality of zeolites. The product containing zeolite NaP1 from such a condition is further applied to degrade methylene blue by photocatalysis. The removal rate has reached 96% within 12 h, which dramatically surpasses that of the raw fly ash (38%). Such excellent photocatalytic performance is attributed to the 10-fold increased surface area (24.864 m² g^-1) and active metal elements embedding in the zeolite structures.

Safe disposal of hazardous waste incineration fly ash: Stabilization/solidification of heavy metals and removal of soluble salts

Hazardous waste incineration fly ash (HFA) is considered a hazardous waste owing to the high associated concentrations of heavy metals and soluble salts. Hence, cost effective methods are urgently needed to properly dispose HFA. In this study, geopolymers were prepared by alkali-activation technology to stabilize and solidify heavy metals in HFA. In addition, the effects of three different aluminosilicates (metakaolin, fly ash, and glass powder) on the heavy metal immobilization efficiency were investigated. Because the soluble salt content of HFA is too high for their direct placement in flexible landfill sites and water washing can lead to heavy metal leaching, water-washing experiments were conducted after alkali-activation treatment to remove soluble salts. The results suggest that the concentrations of heavy metals leached from geopolymers can satisfy the Chinese Standard limits (GB18598-2019) when the addition of aluminosilicates exceeds 20 wt%. More than 77% of Cl^- and >64% of SO₄^2- in geopolymers could be removed via water-washing treatment. The Zn leaching concentration was maintained below approximately 0.52 ppm. After alkali-activation treatment, the water-washing process could efficiently remove soluble salts while inhibiting heavy metal leaching. Sodium-aluminosilicate-hydrate (N-A-S-H) gel, a product of the geopolymerization process in this study, was demonstrated to act as a protective shell that inhibited heavy metal leaching. Hence, HFA-based geopolymers are considered suitable for disposal in flexible landfills.

Resource utilization of municipal solid waste incineration fly ash - cement and alkali-activated cementitious materials: A review

The increase in municipal solid waste (MSW) production has led to an increase in MSW incineration fly ash (MSWIFA) production. MSWIFA contains toxic and harmful substances such as heavy metals and dioxins, which can cause harm to the environment if not treated properly. Only a few MSWIFAs will be landfilled directly, and the rest will need to be treated by other methods. The treatment of MSWIFA can be divided into three types: separation, stabilization/solidification (S/S), and thermal treatment, which are either not fully developed or too costly. Resource utilization is a sustainable means of treating MSWIFA. MSWIFA is used in the production of cement and alkali-activated cementitious materials as a means of resource utilization with significant advantages. This can alleviate the consumption of nature and reduce greenhouse gas emissions in conventional cement production. Compared with MSWIFA cement, MSWIFA alkali-activated cementitious material can be achieved with almost no consumption of natural resources, which is worthy of further research to realize the large-scale application of MSWIFA. At the end of the paper, the perspective of separation of dioxins from MSWIFA, co-processing of MSWI ash, and production of "MSWIFA green materials" is presented.

Basic Research on Co-treatment of Municipal Solid Waste Incineration Fly Ash and Municipal Sludge for Energy-Saving Melting

MSWI fly ash and municipal sludge are solid wastes. Melting vitrification treatment was a resource utilization method. However, the flow temperature of grate furnace MSWI fly ash and municipal sludge was high (>1325 °C), which increased the energy consumption in the melting process. MSWI fly ash contained a large amount of CaO, and municipal sludge contained a large amount of SiO₂, Al₂O₃, and Fe₂O_3. The temperature of melting vitrification can be reduced using these two kinds of CITY garbage as raw materials to change the proportion of ingredients. The eutectic characteristics of MSWI fly ash and municipal sludge and the phase diagrams of CaO-SiO₂-Al₂O₃ (C-S-A) and CaO-SiO₂-Al₂O₃-Fe₂O₃ (C-S-A-F) were analyzed in this paper. It established a low melting point mixing system. The results showed that when the amount of municipal sludge was 50-70%, the flow temperature of the mixtures was <1215 °C, which was significantly lower than that of MSWI fly ash (1490 °C) and municipal sludge (1325 °C). The optimal range of low melting point components was 14.1-36.3% CaO, 21.6-40.4% SiO₂, 6.7-12.6% Al₂O₃, and 6.3-11.4% Fe₂O₃. At 400-1400 °C, the minerals in the mixtures mainly changed as follows: CaCO₃ + SiO₂ + Al₂O₃ → Ca₂SiO₄ + Ca₃SiO₅ + Ca₂Al₂SiO₇ + Ca₃Al₂O₆ + Ca₁₂Al₁₄O₃₃ → CaAl₂Si₂O₈. In the melting experiment, with the increase in temperature, most of the phases in the mixtures might become amorphous. Therefore, the low melting point phase anorthite (CaAl₂Si₂O₈) only accounted for a small part of the final molten product.

Effects and mechanism of the conditions of sintering on heavy metal leaching characteristic in municipal solid waste incineration fly ash

Municipal solid waste incineration (MSWI) fly ash treated with toxicity metals holds enormous potential for constructure use to economize on resources and protect environment. To reach the goal, this study investigated the effects of sintering conditions on leaching characteristic of heavy metals for MSWI fly ash, especially Cr, Cr⁶⁺, Ag, and Ba, with the orthogonal and Box-Benhnken design experiment, which considered grain size (D₅₀ = 30, 45, and 60 μm), fluxing agent (CaO = 0, 2.5, and 5%), setting temperature (1000, 1050, and 1100 °C), and setting time (120, 180, and 240 min). The mechanism of immobilization for heavy metals was also discussed through the analyses of morphological characterizations, mineral phases, chemical composition, and leaching values of metals. The results indicated that changing grain size and adding fluxing agent of CaO have positive influence on reducing the leaching of heavy metals compared with direct sintering. The leaching values of As, Pb, Cd, Cu, Ni, Zn, Mn, Hg, Be, Se, and fluoride are not detected after sintering. Ideal sintering condition with desirability of 1.00 was predicted and optimized by the Box-Benhnken response method in grain size of D₅₀ = 30 μm, fluxing agent of CaO = 5%, setting temperature = 1050 °C, and setting time = 180 min, which immobilized Cr, Cr⁶⁺, Ag, and Ba lower than the limitation of standards. Actual experiment was consistent with numerical optimization. Furthermore, the model of leaching characteristic for heavy metals in MSWI fly ash was established with the discussion on species distribution of heavy metals to better explain the mechanism during sintering.

PCDD/Fs and heavy metals in the vicinity of landfill used for MSWI fly ash disposal: Pollutant distribution and environmental impact assessment

This study focused on the syngenetic control of polychlorinated-ρ-dibenzodioxins and dibenzofurans (PCDD/Fs) and heavy metals by field stabilization/solidification (S/S) treatment for municipal solid waste incineration fly ash (MSWIFA) and multi-step leachate treatment. Modified European Community Bureau of Reference (BCR) speciation analysis and risk assessment code (RAC) revealed the medium environment risk of Cd and Mn, indicating the necessity of S/S treatment for MSWIFA. S/S treatment significantly declined the mass/toxic concentrations of PCDD/Fs (i.e., from 7.21 to 4.25 μg/kg; from 0.32 to 0.20 μg I-TEQ/kg) and heavy metals in MSWIFA due to chemical fixation and dilution effect. The S/S mechanism of sodium dimethyldithiocarbamate (SDD) and cement was decreasing heavy metals in the mild acid-soluble fraction to reduce their mobility and bioavailability. Oxidation treatment of leachate reduced the PCDD/F concentration from 49.10 to 28.71 pg/L (i.e., from 1.60 to 0.98 pg I-TEQ/L) by suspension absorption or NaClO oxidation decomposition, whereas a so-called "memory effect" phenomena in the subsequent procedures (adsorption, press filtration, flocculating settling, slurry separation, and carbon filtration) increased it back to 38.60 pg/L (1.66 pg I-TEQ/L). Moreover, the multi-step leachate treatment also effectively reduced the concentrations of heavy metals to 1-4 orders of magnitude lower than the national emission standards. Furthermore, the PCDD/Fs and heavy metals in other multiple media (soil, landfill leachate, groundwater, and river water) and their spatial distribution characteristics site were also investigated. No evidence showed any influence of the landfill on the surrounding liquid media. The slightly higher concentration of PCDD/Fs in the soil samples was ascribed to other waste management processes (transportation and unloading) or other local source (hazardous incineration plant). Therefore, proper management of landfills and leachate has a negligible effect on the surrounding environment.

Co-stabilization/solidification of heavy metals in municipal solid waste incineration fly ash and electrolytic manganese residue based on self-bonding characteristics

Municipal solid waste incineration (MSWI) fly ash and electrolytic manganese residue (EMR) were classified as hazardous waste, must be harmlessly processed prior to subsequent treatment or disposal. The competition between massive free manganese ions of raw EMR and other heavy metals was found, thus raw EMR was pretreated by calcining to eliminate competition of manganese with other heavy metals for stabilizer complexation. MSWI fly ash was successfully solidified with 6% NaH₂PO₄, 6% H₂NCSNH₂ and 20% sintered EMR (800 °C). The addition of sintered EMR enhanced solidification/stabilization of heavy metals in fly ash and the resulting product had a higher compressive strength for further reutilization like trench backfilling, structural fill and void filling. The stabilization/solidification mechanism of heavy metals was attributed to the combined interaction of heavy metal precipitation in stabilizers and ion exchange or physical encapsulation in silicate compounds like calcium silicate, which is a feasible and valuable approach to co-disposal of MSWI fly ash and EMR.

Preparation of Alkali Activated Cementitious Material by Upgraded Fly Ash from MSW Incineration

Utilization of municipal solid waste incineration fly ash (MSWI-FA) can avoid land occupation and environmental risks of landfill. In this paper, MSWI-FA was used to prepare alkali activated cementitious materials (AACMs) after two-step pretreatment. The ash calcination at 450 °C removed 93% of dioxins. The alkali washing with 0.2 g NaOH/g ash removed 89% of chlorine and retained almost 100% of calcium. The initial setting time of AACMs was too short to detect for 20% of MSWI-FA addition, and the prepared block had extensive cracks and expansion for CaClOH and CaSO₄ inside. Alkaline washing pretreatment increased the initial setting time by longer than 3 min with 30% ash addition and eliminated the cracks and expansion. The significance of the factors for compressive strength followed the modulus of alkali activator > silica fume amount > alkaline washing MSWI fly ash (AW-MSWI-FA) amount. When the activator modulus was 1.2, 1.4 and 1.6, the blocks with 30% of AW-MSWI-FA had a compressive strength of up to 36.73, 32.61 and 16.06 MPa, meeting MU15 grade. The leaching test shows that these AACM blocks were not hazardous waste and almost no Zn, Cu, Cd, Pb, Ba, Ni, Be and Ag were released in the leaching solution.

Disposal technology and new progress for dioxins and heavy metals in fly ash from municipal solid waste incineration: A critical review

Incineration has gradually become the most effective way to deal with MSW due to its obvious volume reduction and weight reduction effects. However, since heavy metals and organic pollutants carried by municipal solid waste incinerator fly ash (MSWI FA) pose a serious threat to the ecological environment and human health, they need to be handled carefully. In this study, the current status of MSWI FA disposal was first reviewed, and the harmless and resourceful disposal technologies of heavy metals and organic pollutants in MSWI FA are summarized as well. A summary of the advantages and disadvantages of each technology, including sintering, melting/vitrification, hydrothermal treatment, mechanochemistry, solidification/stabilization of MSWI FA, is compared. Finally, the research work that needs to be strengthened in the future (such as codisposal of multiple wastes, long-term stability research of disposal products, etc.) was proposed. Through comprehensive analysis, some reasonable and feasible suggestions were provided for the effective and safe disposal of MSWI FA in the future.

Identification of heavy metal leaching patterns in municipal solid waste incineration fly ash based on an explainable machine learning approach

The leaching risk of heavy metal (HM) in municipal solid waste incineration fly ash (MSWI-FA) leads to a big challenge for FA landfilling. In this work, the HM leaching patterns were identified via 6 highly available indices of FA, in which 160 stabilized FA samples were collected from 4 incineration plants in eastern China and an explainable machine learning approach based on boosting and game analysis was conducted to assess the leaching potentials of 6 HMs (Cr, Cd, Cu, Ni, Pb and Zn). We found that, there remained high exceeding risks of Cd and Pb in stabilized FA. In addition, the S-Cl (soluble chlorine) content was the most influential factor of the leaching behaviors of Cd, Cu, Pb and Zn, more important than pH in regard to Cu, Pb and Zn. We quantified the influence of S-Cl on the HM leaching of Cd, Cu, Pb and Zn, whereby their leaching concentrations would increase by 223.5%, 215.2%, 216.5% and 222.6%, respectively, for every 0.5 mol/L order increase in S-Cl concentration. Finally, a fast determination criterion for the FA landfill was proposed, that is, FA of which S-Cl was less than 0.412 mol/L was acceptable.

Hydrothermal oxidation degradation of dioxins in fly ash with water-washing and added Ce-Mn catalyst

A comprehensive analysis of the effects of the temperature, reaction time, liquid-solid ratio (L/S), and initial pH on the hydrothermal degradation of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) (which are both PCDD/Fs) in municipal solid waste incineration (MSWI) fly ash is presented. Consequently, the hydrothermal degradation reaction is catalyzed using Ce-Mn catalyst under low-temperature conditions to study the effect of the catalyst on the degradation efficiency of PCDD/Fs. The experimental results show that temperature is the most critical factor for the reaction. When the hydrothermal oxidation temperature reaches 280 °C (reaction time = 120 min, original pH = 8.5, L/S = 4 mL/g), the toxicity equivalent (I-TEQ) of PCDD/Fs is only 5.4 ng TEQ/kg, and the degradation efficiency reaches 99.71%. Under these conditions, 2,3,4,7,8-P₅CDF makes the highest contribution to I-TEQ degradation, reaching 37.4%. There are four main pathways for the reaction of 2,3,4,7,8-P₅CDF with hydroxyl radicals. A comparison of the PCDD/F concentrations of different products shows that the addition of 0.5%, 1.0%, and 1.5% of the Ce-Mn catalyst reduces the degradation efficiency by 8.79%, 1.40%, and 0.07%, respectively, which indicates that the addition of a small quantity of Ce-Mn catalyst does not facilitate the degradation of PCDD/Fs. The addition of the catalyst significantly decreases the degradation efficiency of low-chlorinated homologs but has a relatively small effect on that of high-chlorinated homologs. Therefore, it is concluded that Ce-Mn catalysts are more likely to promote resynthesis than degradation of PCDD/Fs.

Stabilization of Waste-to-Energy (WTE) fly ash for disposal in landfills or use as cement substitute

This study investigated two approaches for managing Waste-to-Energy (WTE) fly ash (FA): (i) phosphoric acid stabilization of FA and disposal in non-hazardous landfills, so that it can pass the U.S. TCLP procedure and meet the U.S. Resource Conservation and Recovery Act (RCRA) standards; (ii) use of FA or phosphoric acid stabilized fly ash (PFA) as cement substitute in construction for avoiding disposal in landfills and reducing the consumption of Portland cement. The effect of stabilization was identified by TCLP tests and XRD quantification (QXRD), which showed that the economically optimal concentration for PFA to pass the RCRA was 1 mol/L H₃PO₄ (equivalent to 0.4 mol of H₃PO₄/kg of FA). Zn/Pb-phosphates were formed in treated ash by using high concentration H₃PO₄ (e.g., 3 mol/L). Thus, the hazardous FA was chemically stabilized to PFA, that were both discussed as cement substitute. QXRD and SEM results showed that both FA and PFA (1 mol/L H₃PO₄) chemically reacted with cement and water. Up to 25 vol% of the cement can be replaced by FA or PFA, with similar mechanical performance of cement mortars than that of reference. Testing by LEAF Method 1313-pH dependence showed that the FA and PFA cement mortars exhibited the same leachability of heavy metals; therefore, this study demonstrated the technical feasibility of utilizing either raw FA or stabilized PFA as supplementary cementitious material. The leachability of heavy metals in optimal FA or PFA 25 vol% cement mortar was under the U.K. WAC non-hazardous limits.

Heavy metal removal using an advanced removal method to obtain recyclable paper incineration ash

Various agents, including ethylenediaminetetraacetic acid, oxalic acid, citric acid, and HCl, were applied to remove heavy metals from raw paper incineration ash and render the ash recyclable. Among these prepared agent solutions, ethylenediaminetetraacetic acid showed the highest efficiency for Pb removal, while oxalic acid showed the highest efficiencies for Cu, Cd, and As removal. Additionally, three modes of an advanced removal method, which involved the use of both ethylenediaminetetraacetic acid and oxalic acid, were considered for use at the end of the rendering process. Among these three modes of the advanced removal method, that which involved the simultaneous use of ethylenediaminetetraacetic acid and oxalic acid, i.e., a mixture of both solutions, showed the best heavy metal removal efficiencies. In detail, 11.9% of Cd, 10% of Hg, 28.42% of As, 31.29% of Cu, and 49.19% of Pb were removed when this method was used. Furthermore, the application of these three modes of the advanced removal method resulted in a decrease in the amounts of heavy metals eluted and brought about an increase in the CaO content of the treated incineration ash, while decreasing its Cl content. These combined results enhanced the solidification effect of the treated incineration ash. Thus, it was confirmed that the advanced removal method is a promising strategy by which recyclable paper incineration ash can be obtained.

Dissolved organic matter (DOM) was detected in MSWI plant: An investigation of DOM and potential toxic elements variation in the bottom ash and fly ash

The content of dissolved organic matter (DOM) and potentially toxic elements (PTEs) were investigated in the bottom ash (BA) and fly ash (FA) of different sections of the municipal solid waste incineration (MSWI) plant. BA and FA were collected from the dry (BA1-BA2), burn (BA3-BA4), and burn-out (BA5) sections of the grate incinerator; FA was collected after denitration (DNFA), and from the deacidification tower (FA1) and bag-type dust remover (FA2), respectively. The DOM concentration in BA was higher than that in FA, the highest concentration was in BA3 (556.18 mg/kg), while the lowest concentration was in DNFA (17.53 mg/kg). DOM in BA was mainly composed of protein-like, fulvic-like, tryptophan-like, and humic-like substances, of which humic-like substances accounted for more than 40%. DOM in FA consisted of tryptophan-like and humic-like substances, of which humic-like substances accounted for more than 80%. DOM still existed in BA which may be related to the incomplete combustion, and the influence of microbes, while DOM was increased in FA1, which might be due to the addition of lime slurry. PTEs were analyzed by the Tessier extraction method, Fe-Mn hydroxide-bound fraction of PTEs increased in FA1 in which DOM concentration (137.22 mg/kg) was 7.83 times that in DNFA. The increase of DOM may lead to a higher risk of PTEs in FA. FTIR results indicated that DOM can bond to PTEs in BA and FA. The contents of humus-like substances in DOM were positively correlated with the effective fraction of As, Cu, Pb, Cr, and Cd. This paper investigated the risk of DOM existing in BA and FA in MSWI plant, which can provide a new perspective on how to deal with BA and FA, and reduce their environmental risks.

Chemical stabilization of heavy metals in municipal solid waste incineration fly ash: a review

Sufficient attention should be attached to the large amount of fly ash containing high levels of toxic heavy metals generated after municipal solid waste incineration. Because heavy metals could be leached out of the fly ash under specific conditions, it is necessary to stabilize the heavy metals in fly ash before landfill disposal. Processing technologies of incineration fly ash include solidification/stabilization technology, thermal treatments, and separation processes. This study reviewed the current treatment technologies of municipal solid waste incineration (MSWI) fly ash, with the main focus on the treatment of heavy metals in fly ash with chemical stabilization. Chemical stabilization processes involve chemical precipitation of heavy metal and chelation of heavy metals. In multiple studies, chemical stabilization technology has shown practical feasibility in terms of technology, economy, and effect. In addition, the combination of two or more stabilization agents broadens the general applicability of the agents to heavy metals and reduces the cost. The application of joint processing technology realizes the remove of soluble salt from fly ash. To minimize pollutants while increase their usable value, effective use of waste and co-disposal of several kinds of wastes have gradually become the research hotspots. New developments in chemical stabilization are progressively moving towards the sustainable direction of harmlessness and resource utilization of MSWI fly ash.

Management of Solid Waste Containing Fluoride-A Review

Technological and economic development have influenced the amount of post-production waste. Post-industrial waste, generated in the most considerable amount, includes, among others, waste related to the mining, metallurgical, and energy industries. Various non-hazardous or hazardous wastes can be used to produce new construction materials after the “solidification/stabilization” processes. They can be used as admixtures or raw materials. However, the production of construction materials from various non-hazardous or hazardous waste materials is still very limited. In our opinion, special attention should be paid to waste containing fluoride, and the reuse of solid waste containing fluoride is a high priority today. Fluoride is one of the few trace elements that has received much attention due to its harmful effects on the environment and human and animal health. In addition to natural sources, industry, which discharges wastewater containing F− ions into surface waters, also increases fluoride concentration in waters and pollutes the environment. Therefore, developing effective and robust technologies to remove fluoride excess from the aquatic environment is becoming extremely important. This review aims to cover a wide variety of procedures that have been used to remove fluoride from drinking water and industrial wastewater. In addition, the ability to absorb fluoride, among others, by industrial by-products, agricultural waste, and biomass materials were reviewed.

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

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

Propositure of maximum permissible limits for environmental assessment and classification of materials stabilized by solidification incorporated with oil waste

In the present work, oil sludge was subjected to the process of stabilization by solidification, the objective was to indicate maximum permissible limits of chemical demand for oxygen and oils and greases to evaluate the resulting material. A factorial design was used with the addition of three repetitions at the central point, to evaluate the performance of different percentage of residue and different curing times. The factors adopted were the percentage of oil sludge and the curing time. This material was evaluated using contaminants immobilization tests. From the leached and solubilized liquid, the concentration of the contaminants was determined and the environmental assessment was also carried out through the analysis of chemical demand for oxygen and oils and greases. One of the contributions of this work was to propose the maximum allowable limit for chemical oxygen demand, which is 1,000 (one thousand) milligrams per litre and, for oil and grease content, 100 (one hundred) milligrams per litre, both for the leaching test. For the solubilization test, 10 (ten) milligrams per litre were proposed for chemical oxygen demand and 1 (one) milligrams per litre for the oil and grease content. It was observed that the best results are obtained, when lower values of percentage of oil sludge were used and longer cure time. This work allows to affirm that the proposal of limits of environmental evaluation contributes to assure an adequate disposition and use of the cement matrix, that is the final product of the oily residue stabilized by solidification.

Optimization of Solidification and Stabilization Efficiency of Heavy Metal Contaminated Sediment Based on Response Surface Methodology

Solidification and stabilization (S/S) by agents and stabilizers is an effective way to treat heavy metal-contaminated sediments. Optimization of curing condition is crucial to minimize the consumption of reagents on the base of effective S/S. In this work, the synergistic effects of cement and stabilizer on mechanical strength and leaching toxicity of contaminated sediments were investigated, and the S/S conditions were optimized using response surface methodology. On the basis of a single-factor test, multi-factor experiments were conducted to fit the relationship between the S/S effect of contaminated sediments and the amount of cement and stabilizer. The mechanism of stabilization was investigated by the results from the revised BCR method. The results indicate that the optimal curing conditions were 44.29% of cement content with 2.05% of trimercapto-s-triazine trisodium salt (TMT). After 28 days of curing, the compressive strength reached 2.07 MPa and the leaching concentrations of Cd, Cu, and Pb were 0.094 mg/L, 0.031 mg/L, and 0.173 mg/L, respectively, which met the requirement of in-situ resource recycling standard. The stability of heavy metals was significantly improved as a result of the removal of acid extractable fraction (15.58~69.92%) and an increase in the residual fraction (18.27~49.07%).

Effects of medical waste incineration fly ash on the promotion of heavy metal chlorination volatilization from incineration residues

High contents of heavy metals and Cl are major challenges for incineration residue disposal. Classification by the Chinese government and the coronavirus disease 2019 pandemic have changed the characteristics of incineration residues, thereby increasing the difficulty of disposal. In this study, medical waste incineration fly ash (MWI FA) was proposed as an additive to promote chlorination volatilization of heavy metals from municipal solid waste incineration fly ash (MSWI FA) and medical waste incineration slag (MWI S). When the mixing ratio of MWI FA to MSWI FA was 1:3, the chlorination volatilization efficiencies of Cu, Zn, Pb, and Cd at 1000 °C for 60 min were 50.2%, 99.4%, 99.7%, and 97.9%, respectively. When MWI FA was mixed with MWI S at a ratio of 1:1, the chlorination volatilization efficiencies of Cu, Zn, Pb, and Cd at 1200 °C for 40 min were 88.9%, 99.7%, 97.3%, and 100%, respectively. Adding MWI FA can replenish Cl in MSWI FA and MWI S while increasing the surface area and forming pore structures by sublimation of NaCl and decomposition of CaSO₄, or can reduce the melting point and viscosity by Na₂O destroying the glass matrix. Therefore, MWI FA can be co-disposed with MSWI FA and MWI S respectively to enhance the chlorination volatilization of heavy metals.

Physiochemical properties, heavy metal leaching characteristics and reutilization evaluations of solid ashes from municipal solid waste incinerator plants

A clear understanding of the physicochemical, compositional, morphological properties and heavy metal leaching behaviours of municipal solid waste (MSW) incinerated fly ash (FA) and bottom ash (BA) are essential to guide their respective re-utilizations. In this study, FA and BA collected from three MSW incinerator plants located in Xiamen were systematically exploited. Results indicated that FA in the three plants exhibited more porous structures than BA, and the particle sizes of FA and BA were 45-295 μm and >3000 μm, respectively. However, both ashes showed similar main mineralogical crystalline phases of Ca(OH)₂, CaCO₃ and SiO₂, indicative of high feasibilities in manufacturing cement, bricks or construction materials. Additionally, the heavy metal migration of MSW into leachate, flue gas, FA and BA were all specifically measured in this study to provide full data analyses and in-depth understandings of heavy metal migrations, manifesting that the heavy metals of MSW majorly migrated into the FA and BA with clearly discrepant metal ratios and only a very small fraction migrated into the leachate and flue gas. To maximumly reuse both FA and BA, importantly, the green degree and cost-benefit analysis methods were integrated into this study to evaluate their re-utilization alternatives on environmental impacts and economic benefits, and results implied that FA was beneficial for re-utilizing as aggregates in bricks while BA was optimum as paving materials. This study provides overall systematic perspectives on guiding the re-utilization of FA/BA from the MSW incinerators and also considers their environmental and economic benefits for future long-term management.

Stability evaluation of potentially toxic elements in MSWI fly ash during carbonation in view of two leaching scenarios

Carbonation treatment (CT) by alkaline fly ash (FA) affects the stability of potentially toxic elements (PTEs). This study investigated the leachability and environmental risk of six PTEs contained in FA during natural and accelerated carbonation (NC, AC) using two typical leaching scenarios with distilled water (DW) and acetic acid (AA). The leaching of Pb/Cu/Cr/Ni in solidified/stabilized FA decreased due to CT in DW leaching, but the leaching of Pb/Zn/Cu/Cd increased due to CT in AA leaching. The leaching of the six PTEs (especially Pb/Cd) in AA leaching was significantly higher than that in DW leaching. CT was a promoting factor to increase the environmental risk level of PTEs in FA leachate, especially in AA leaching with H⁺ input. In the early stage of NC, under DW leaching tests, the environmental risk level of PTEs in FA leachate can be weakened due to the formation of carbonate minerals in the FA matrix. However, excessive NC increases the environmental risk of leached PTEs due to the decalcification of carbonate minerals. Both NC and AC increased the potential environmental risk of PTEs contained in the carbonated FA matrix. The nucleation and dissolution of carbonate minerals were interdependent with the immobilization and leaching of PTEs, which played a dominant role in the CT and leaching tests respectively. They jointly affected the occurrence behavior of PTEs in the FA matrix in CT tests and the leachability of PTEs in leaching tests. This study demonstrates that it is more scientific to evaluate the leachability of PTEs in carbonated FA according to the actual disposal scenarios.

Removal of heavy metals in municipal solid waste incineration fly ash using lactic acid fermentation broth

Municipal solid waste incineration fly ash (MSWIFA) is considered as a hazardous solid waste because of the high mobility of heavy metals. In this study, the removal of heavy metals in MSWIFA using lactic acid fermentation broth (LAFB) under various leaching protocols (i.e. LAFB addition amount and timing) was investigated. Results revealed that compared with that in pure lactic acid solution, the synergistic effect of various substances in LAFB was more favourable to the dissolution of heavy metals. Although the content of acid-soluble heavy metals in MSWIFA decreased after leaching with LAFB, the leaching toxicity measured by acetic acid buffer solution method increased to varying degrees (except that of Cr). Moreover, the maximum leaching concentration of Pb was 14.1 mg/L (standard limit, 0.25 mg/L), which was not conducive to the landfill treatment of MSWIFA. However, if the LAFB-treated MSWIFA was used in cement kiln for co-disposal, the amount of MSWIFA entering the kiln was 6.0 percentage points higher than that in pure water leaching. Therefore, LAFB leaching instead of water leaching is expected to be an effective pre-treatment method for the utilisation of MSWIFA.

Phase changes during various treatment processes for incineration bottom ash from municipal solid wastes: A review in the application-environment nexus

Incineration technology has been widely employed, as an effective method to decrease the volume of waste disposal. In this review, relationships between municipal solid waste (MSW) inputs and residues after combustion-specifically, the incineration bottom ashes (IBA) of MSW, were discussed, with an emphasis on the geoenvironmental impacts of IBA associated with the complex crystal and amorphous phase reactions and changes during combustion and from their downstream treatments, whereas, their influences on IBA leaching behaviors are considered as another focus. This review summarizes the IBA leaching behaviors based on literature, showing the leaching variabilities induced by natural weathering and artificial intervention conditions, such as accelerated carbonation, washing treatment, stabilization/solidification, and thermal treatments, all of which can be attributed to changes of mineral phases and microstructure. It helps to understand IBA characteristics and transitions in application-environment nexus, and better reuse it for multiple applications.

Effect of freezing-thawing cycles on the strength and pollutant leachability of stabilized/solidified metallurgical waste

Stabilized/solidified hazardous wastes may lose their structural integrity and stability when exposed to long-term extreme weather conditions, such as repeated wetting-drying and freezing-thawing (F-T). This situation causes wastes to become environmentally risky again even if they are stabilized and solidified. In this study, the effects of F-T cycles on the strength and pollutant leachability of stabilization/solidification (S/S) products of zinc extraction residue (ZER) stabilized/solidified by Portland cement (PC), fly ash (FA), and alkaline phosphate (AP) were comprehensively investigated. S/S efficiency and F-T resistance of the samples were determined by unconfined compressive strength (UCS), and leaching tests widely applied in the USA (TCLP), European Union (EN-12457/1-4 2020), and Turkey (TS-EN 12457/4). In addition, X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses and visual inspection were performed to examine the variations in mineralogical and morphological structures. S/S efficiency and F-T resistance changed depending on the type and amount of S/S reagent used and decreased in the order of PC+AP>PC>>FA. All applied S/S procedures, except for 10% PC addition, were found to be suitable for the S/S of ZER from the viewpoint of the achievement of the minimum required strength and pollutant leachability. F-T events did not change the mineralogical structures of the S/S products, but significantly reduced the UCS value by causing structural deformation. The pollutant concentrations released from the samples that lost their monolithic structure by crumbling after certain cycle increased with the increasing F-T cycle, causing a change in the storage criteria of the samples. Therefore, it is important to consider and apply the F-T cycle as a standard test in addition to the existing leaching and strength tests for S/S products of the wastes in terms of minimizing environmental risks.