A comprehensive evaluation of the treatment of lead in MSWI fly ash by the combined cement solidification and phosphate stabilization process

Eliminating the stabilizer antagonistic effects for efficiently stabilizing Pb and As co-contaminated soil by innovative stepwise steam flash heating

Chemical stabilization is frequently used to stabilize lead (Pb) or arsenate (As), but faces challenges in Pb-As co-contaminated soils because of the antagonistic reactions between chemical stabilizers and contaminants. In this work, we innovated an effective and cost-efficient stepwise steam flash heating (SSFH) strategy to simultaneously immobilize Pb and As, and unraveled the underlying mechanisms. The combination of 1.5% Ca(H2PO4)2 and 2% Fe2(SO4)3 only decreased 1.99% Pb by toxicity characteristic leaching procedure (TCLP-Pb) but increased 17.8% of TCLP-As due to the antagonistic effects. SSFH with Ca(H2PO4)2 in the first step and Fe2(SO4)3 in the second step achieved the minimal TCLP-Pb and TCLP-As of 0.778 and 0.327 mg/L, respectively. It also reduced 69.8% of leachable As in 100-year acid rain simulation, indicating a favorable long-term stabilization performance. Additionally, SSFH approach reduced 43.2% stabilizer dosage and 14.9% cost. X-ray absorption near edge structure (XANES) documented that the stepwise SFH promoted the transformation of Pb(NO3)2 and NaAsO2/NaAsO3/As2O3/As2O5 into stable Pb3(PO4)2 and FeAsO4, preventing the formation of AsO43- and FePO4. Our findings proved the state-of-the-art SSFH approach and unraveled its mechanisms to stabilize Pb and As co-contamination in soils, offering a green and sustainable remediation alternative for the management of heavy metal contaminated sites. ENVIRONMENTAL IMPLICATION: A novel stepwise SFH approach can be applied to overcome the stabilizer antagonist effects by separately immobilizing Pb and As in two sequential steps. It also decreased 43.2% of stabilizer dosage and 14.9% of cost comparing to conventional chemical stabilization. This approach can be used for other metal co-contaminated soils facing similar antagonistic challenges, and our work raises a state-of-the-art solution for cost-effective, green and sustainable remediation practices.

Mechanism and effectiveness of enzymatically induced phosphate precipitation (EIPP) in stabilizing coexisting lead, zinc, and cadmium in tailings

Lead-zinc (Pb-Zn) tailings ponds carry the risk of multiple heavy metals (HMs) contamination and pile destabilization. This poses requirements for in-situ applicable, low-distribution, and effective stabilization/solidification (S/S) methods. For this, the novel enzymatically induced phosphate precipitation (EIPP) method was implemented in this study. Its mechanism and performance on stabilization of composite Pb, Zn, and cadmium (Cd) in tailings were explored and evaluated under typical erosion conditions for the first time. Results show that the EIPP stabilized HMs by chemically transforming the unstable carbonate-bound HMs to stable phosphate precipitates and by physically encapsulating tailings particles with newberyite precipitates. The stabilization effect on the three HMs was ranked as Pb > Zn > Cd. Comparing magnesium resources for the EIPP reactants, the EIPP utilizing Mg(CH3COO)2 was more effective at decontamination than MgCl2 because its special pre-activation and re-precipitation function enhanced the chemical transformation function of EIPP. The EIPP stabilization was confirmed to reduce simulated acid rain-leachable and bio-extractive HMs by about 90% and 60%, respectively. Under the prolonged acid attack, treated HMs were ultimately leached through the dissolution mechanism. Zn exhibited significant instability in highly acidic conditions (pH = 2.5-3.5), where its cumulative leaching toxicity after long-term dissolution warrants attention. Overall, EIPP presents a novel and effective strategy for on-site mitigation of composite HMs pollution.

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

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

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.

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.

Experimental study on solidification/stabilization of leachate sludge by sulfoaluminate cement and MSWI by-products

Leachate sludge is generated from the biochemical treatment sludge tank for disposing the leachate from landfill municipal solid waste (MSW). It has the characteristics of high water content and high organic matter content. Sulfoaluminate cement (SAC) is used as the main curing agent, and municipal solid waste incineration (MSWI) by-products are used as auxiliary curing agents to solidify/stabilize the leachate sludge. The influences of SAC content and MSWI by-products content on the strength and solidification mechanism of the leachate sludge are investigated by unconfined compressive strength (UCS) test and micro-observation tests. Moreover, the leaching concentration of heavy metals of the solidified samples is analyzed by leaching toxicity test. The results show that the UCS of the solidified samples increases with an increase in cement content. When the cement content is larger than 20%, the UCS of the solidified samples satisfies the strength requirement of landfill. The enhancing effect of bottom ash on the cement-solidified samples is slight. The fly ash is a good auxiliary curing agent for improving the UCS of cement-solidified samples, and the optimal dosage of fly ash is 5% and 15% for the solidified samples with 10 ~ 30% and 40 ~ 50% cement content, respectively. Ten percent fly ash can replace 10% cement to achieve better solidification effect for the solidified samples. The leaching concentration of heavy metals in the solidified sample with 30%/40% cement and 15% fly ash/bottom ash can satisfy the strength and leaching toxicity requirements of landfill. The immobilization of heavy metal of the cement and MSWI by-products solidified samples is mainly achieved through physical adsorption, physical encapsulation, ion exchange, and chemical precipitation.

Analysis of heavy metal, rare, precious, and metallic element content in bottom ash from municipal solid waste incineration in Tehran based on particle size

Waste incineration is increasingly used worldwide for better municipal solid waste management and energy recovery. However, residues resulting from waste incineration, such as Bottom Ash (BA) and Fly Ash (FA), can pose environmental and human health risks due to their physicochemical properties if not managed appropriately. On the other hand, with proper utilization, these residues can be turned into valuable Municipal metal mines. In this study, BA was granulated in various size ranges (< 0.075 mm, 0.075-0.125 mm, 0.125-0.5 mm, 0.5-1 mm, 1-2 mm, 2-4 mm, 4-16 mm, and > 16 mm). The physicochemical properties, heavy metal elements, environmental hazards, and other rare and precious metal elements in each Granulated Bottom Ash (GBA) group from Tehran's waste incineration were examined using ICP-MASS. Additionally, each GBA group's mineralogical properties and elemental composition were determined using X-ray fluorescence (XRF) and X-ray diffraction (XRD). The results showed that the average concentration of heavy metals in GBA, including Zn (1974 mg/kg), Cu, and Ba (790 mg/kg), Pb (145 mg/kg), Cr (106 mg/kg), Ni (25 mg/kg), Sn (24 mg/kg), V (25 mg/kg), As (11 mg/kg), and Sb (29 mg/kg), was higher in particles smaller than 4 mm. Precious metals such as gold (average 0.3 mg/kg) and silver (average 11 mg/kg) were significantly higher in GBA particles smaller than 0.5 mm, making their extraction economically feasible. Moreover, rare metals such as Ce, Nd, La, and Y were detected in GBA, with average concentrations of 24, 8, 11, and 7 mg/kg, respectively. The results of this study indicated that BA contains environmentally concerning metals, as well as rare and precious metals, with high concentrations, especially in particles smaller than 4 mm. This highlights the need for proper pre-treatment before using these materials in civil and municipal applications or even landfilling.

Comprehensive study of recycling municipal solid waste incineration fly ash in lightweight aggregate with bloating agent: Effects of water washing and bloating mechanism

Recycling into lightweight aggregate (LWA) by sintering is a promising technology for disposal of municipal solid waste incineration fly ash (FA). In this study, FA and washed FA (WFA) were combined with bentonite and SiC (bloating agent) to make LWA. The performance was comprehensively studied by hot-stage microscopy and laboratory preparation experiments. Water washing and increased FA/WFA improved LWA bloating extent, while shorten the bloating temperature range. Water washing also increased the 1 h-water absorption rate of LWA, making it harder to meet the standard. Excessive FA /WFA usage (70 wt%) will prevent LWA from bloating. For the goal of recycling more FA, mixture with 50 wt% WFA could prepare LWA that meet standard GB/T 17431 at 1140-1160 °C. After water washing, the ratio of Pb, Cd, Zn, and Cu stabilized in LWA increased by 279 %, 410 %, 458 %, and 109 % for 30 wt% FA/WFA addition, and 364 %, 554 %, 717 %, and 697 % for 50 wt% FA/WFA addition, respectively. The change of liquid phase content and viscosity at high temperature were determined using the thermodynamic calculations and chemical compositions. The bloating mechanism was further investigated by integrating these two properties. To obtain accurate results of the bloat viscosity range (2.75-4.44 log Pa·s) for high CaO systems, the composition of the liquid phase should be taken into account. The liquid phase viscosity required for bloating start was proportional to the liquid phase content. With temperature increasing, bloating would end when viscosity drops to 2.75 log Pa·s or liquid phase content reach 95 %. These findings provided further understanding of the heavy metal stabilization during LWA production and the bloating mechanism of high CaO content systems, and could contribute to the feasibility and sustainability of recycling FA and other CaO-rich solid wastes into LWA.

Submicron tourmaline enhanced the solidification of municipal solid waste incineration fly ash by chemical structure reorganization and stabilized heavy metals

Municipal solid waste incineration fly ash (MSWIFA) exsits in large quantitities and contains pollutants such as heavy metal. While solidification is one of the most effective methods for treating MSWIFA, this application is limited by cost, subsequent treatment, and simultaneous immobilization of anions and cations. This research demonstrated that under a certain initial pressure (20 MPa), a gelation reaction involving ball milling-modified tourmaline powder, a small amount of cement clinker, and MSWIFA forms a stable consolidated body and significantly reduces the risk of heavy metal dissolution. The consolidated MSWIFA can easily be formed into unfired bricks in large-scale pilot production, and a response surface model was used to optimize the experimental parameters. When the mass ratio of tourmaline: cement clinker: MSWIFA was 15:15:200 (mixed with a moisture content of 13 to 15 %), the compressive strength of the consolidated body reached 13 MPa, and the amounts of Cr and Pb leached decreased from 12 mg/L to 0.1 mg/L and 25 mg/L to 0.3 mg/L, respectively. The consolidated form contained a new mineral phase (Ca₃Si₂O₇·3H₂O, Ca₁₀Mg_0.8(SiO₄)_0.6O₂Cl, and CaCl₂∙Ca(OH)₂·H₂O) with a high compressive strength. Notably, the soluble PbSO₄ in the MSWIFA was converted into relatively stable PbSiO₃, and Cr(VI) was lattice-wrapped. This study was the first to demonstrate that tourmaline synchronously passivates Pb(II) and Cr(VI) in fly ash in the solid phase, with a low cost and requires no subsequent treatment. This study provided a novel technical path for recycling MSWIFA. Eventually, leaching of the heavy metals Pb, Cr, Cu, Cd, and Zn from the solids achieved concentrations less than 0.25, 1.5, 0.5, 0.15, and 100 mg/L.

Human carcinogenic risk analysis and utilization of shale gas water-based drilling cuttings in road materials

Water-based drilling cuttings (WDC) generated during shale gas development will endanger human health and ecological security. The modern analytical techniques are used to analyze the organic pollutants in WDC, and the human health and ecological security risks of harmful pollutants in WDC under specific scenarios are evaluated. The results showed that the content of organic pollutants in WDC was evaluated by human health and safety risk assessment. The comprehensive carcinogenic risks of all exposure pathways of single pollutant benzo(a)anthracene, benzo(a)pyrene, benzo(k)fluoranthene, and indeno(1,2,3-cd)pyrene were acceptable. However, the cumulative carcinogenic risk of exposure to dibenzo(a,h)anthracene particles via skin exposure was not acceptable. It was considered that only dibenzo(a,h)anthracene had carcinogenic effect, and the risk control limit of dibenzo(a,h)anthracene in WDC was 1.8700 mg/kg by calculation. As well as, the "WDC-cement" gel composite structure was deeply analyzed, and the physical and chemical properties and mechanism of organic pollutants in cement solidified WDC were analyzed, which provided theoretical support for the study of WDC pavement cushion formula. Based on the above conclusions and combined with the actual site, by studying and adjusting the formula of WDC pavement cushion, the WDC pavement cushion was finally designed by 6% cement + 50% WDC + 44% crushed stone. The 7d unconfined compressive strength met the requirements of the Chinese standard "Technical Guidelines for Construction of Highway Roadbases" (JTG/T F20-2015). Also, the process route of WDC as road cushion product was sampled and analyzed. In addition, the leaching concentration of main pollutants all met the relevant standards of China. Therefore, this study can provide a favorable way for the efficient, safe, and environmentally friendly utilization of WDC, and ensure the ecological environment safety and human health safety of WDC in resource utilization.

Coupling effect of superfine zeolite and fiber on enhancing the long-term performance of stabilized/solidified Pb-contaminated clayey soils

The focus of the present study was on the applicability of superfine zeolite (SZ) and polypropylene fibers in improving the geo-environmental parameters as well as the durability of cement-based stabilized/solidified low plasticity clay containing different dosages of Pb. The leaching data revealed that while adding a low range (≤ 7.5%) of sole cement even in the severely polluted soils could fully eliminate the Pb bioavailability, the metal retention capacity might portray a marked sensitivity to the acid-washing process. A major reduction was also observed in the mechanical/leaching performance of those samples after undergoing the wetting-drying (w-d) cycle, especially at a high proportion of Pb, which could weaken the cementation bonding dramatically; hence, much more cement was needed to pass the required stabilization/solidification (S/S) standards. Besides, the micro level tests indicated that the application of SZ (with 25% cement replacement) would alleviate the Pb declining impact on the S/S reactions and modify the porous network of soil. As a result, the specimens amended by cement-SZ (CSZ) were more functional (~ 1.4 times) in immobilizing the toxic ions than the cement alone was. However, the CSZ admixture might not perfectly restrain the w-d forces/deteriorations. Such a potential drawback was found to be solvable by the insertion of fiber, in which case, an enhancement in the ductility and the metal capsulation could be also manifested. In fact, the CSZ/fiber treatment could form a well-intertwined matrix, showing high success rates in stabilizing/solidifying the contaminated soils alongside a significant decrease (~ 2-folds) in the quantity of needed amount of cement to give the S/S satisfactory operation under the harsh environmental conditions.

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.

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.

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.

Immobilization of Cd and Pb in soil facilitated by magnetic biochar: metal speciation and microbial community evolution

The preparation of magnetic biochar from sewage sludge and rice straw for heavy metal contaminated soil remediation has greater application prospects, but its remediation mechanism was rarely considered by combining soil physicochemical properties with microbial community. In this study, the effects of magnetic sewage sludge biochar (SSB) and rice straw biochar (RSB) on Cd and Pb immobilization in paddy soil were compared and analyzed by 60-day soil incubation experiments. The results illustrated that DTPA-Cd and DTPA-Pb were reduced by 51.53% (43.07%) and 53.57% (50.47%), while the percentage of residual fraction of the BCR procedure was enhanced by 31.27% (30.78%) of Cd and 27.25% (23.22%) of Pb in the SSB (RSB) treatment, respectively. Fe was detected on both SSB and RSB surfaces, but SSB had rougher and a larger specific surface area compared to RSB. The addition of SSB and RSB in paddy soil increased soil pH and TOC content, and affected the diversity and species of soil microbial community. Compared with the CK group, the relative abundance of Proteobacteria, Bacteroidota, and Lysobacter decreased, and the relative abundance of Actinobacteriota, Pontibacter, and Alkaliphilus increased with SSB and RSB treatments, all of which reflected the bioavailability of Cd and Pb reduction.

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.

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.

Preparation of municipal solid waste incineration fly ash-based ceramsite and its mechanisms of heavy metal immobilization

With an increase in municipal solid waste incineration (MSWI) fly ash and its dangerous characteristics, the manner of its disposal has caused widespread concerns. In this study, ceramsite was prepared by using MSWI fly ash, civil sludge, and contaminated soil as the main raw materials; then, a certain proportion of clay was added as an additive. The optimum MSWI fly ash content and sintering conditions were investigated, and the immobilization mechanisms of heavy metals were explored. Based on the obtained results, the optimum preparation conditions were a preheating temperature of 400 °C, a preheating time of 10 min, a sintering temperature of 1150 °C, and a sintering time of 20 min. Moreover, the optimal raw material ratio of MSWI fly ash, civil sludge, contaminated soil, and flint clay was 30%:40%:15%:15%. Under these optimum preparation conditions, the obtained ceramsite showed the following excellent performance parameters: a 1-h water absorption of 0.97%, bulk density of 998.7 kg/m³, and cylindrical compressive strength of 37.84 MPa. Furthermore, the leaching of heavy metals was far less than the standard GB5085.3-2007. The immobilization of heavy metals in the ceramsite was mainly caused by the glass phase encapsulation and the formation of new crystal phase with the heavy metals. In addition, the generation of aluminosilicates played a positive role in the immobilization of heavy metals. Thus, the reuse of MSWI fly ash by preparing fly ash-based ceramsite is one of the effective methods for reducing solid wastes.

Risk assessment of lead and cadmium leaching from solidified/stabilized MSWI fly ash under long-term landfill simulation test

The long-term effectiveness concern of municipal solid waste incineration (MSWI) fly ash (FA) disposal has been placed more emphatic recently, however, few studies worked on the control of leaching risk of heavy metals under the long-term stability. In this study, the leaching properties and risk assessment of two representative solidified/stabilized (S/S) FA wastes, i.e., sodium dithiocarbamate (DTC) chelator treated and Portland cement + chelator combining treated, were evaluated by a long-term cycles assessment method which coupled multifaceted environmental stresses (e.g., freezing-thawing, drying-wetting, accelerated carbonation). The results showed that the cement/chelator had a better long-term stability and exhibited ~55% lower cumulative overall pollution toxicity index (OPTI) than chelator treatment after the test, which was always rated as "low risk" during the cycles. In addition, the cement/chelator exhibited ~23.3% smaller cumulative mass release rate than the chelator treatment after 6 cycles and restrained the transformation of Pb and Cd from stable states to removable fractions, which attributes to its great erosion resistance and compact pore structure. Under the cumulative external factors and carbon dioxide attacks, the decalcification of hydrate products (e.g., C-S-H, hydrocalumite), as well as deterioration of pore structure are the critical factors increasing the local erosion, cracking and heavy metals release. Thus, the optimization of S/S waste microstructure (e.g., enhancing binder system) and landfill site conditions (e.g., reducing rainfall impact) could be propitious to the S/S waste risk control and management.

Stabilization and passivation of multiple heavy metals in soil facilitating by pinecone-based biochar: Mechanisms and microbial community evolution

Soil contamination by multiple heavy metals and As is one of the major environmental hazards recognized worldwide. In this study, pinecone-biochar was used for stabilization and passivation of Pb, Cu, Zn, Cr, and As in contaminated soil around a smelter in Hubei province, China. The stabilization rate of heavy metals in soil can exceed 99%, and the leaching amount can meet the national standard of China (GB/T 5085.3-2007, less than 5, 100, 100, 15, and 5 mg/L, respectively.) within 90 days. The study confirmed that the addition of pinecone-biochar and the coexistence of indigenous microorganisms can effectively reduce the bioavailability of heavy metals. Among the heavy metals, As(III) can be oxidized to As(V) and then stabilized, and other heavy metals can be stabilized in a complex and chelated state characterized by X-ray photoelectron spectroscopy. After pinecone-biochar was added, the abundance of microbial community and intensity of metabolic activities became vigorous, the types and contents of dissolved organic matter increased significantly. A novel innovation is that the addition of pinecone-biochar increased the Bacillus and Acinetobacter in soil, which enhanced the function of inorganic ion transport and metabolism to promote the passivation and stabilization of heavy metals throughout the remediation process.

Selective recovery of manganese from electrolytic manganese residue by using water as extractant under mechanochemical ball grinding: Mechanism and kinetics

This research aimed to address the issue of residual manganese in electrolytic manganese residue (EMR), which is difficult to recycle and can easily become an environmental hazard and resource waste. This research developed a method for the efficient and selective recovery of manganese from EMR and the removal of ammonia nitrogen (ammonium sulfate) under the combined action of ball milling and oxalic acid. The optimum process parameters of this method were obtained through single-factor experiment and response-surface model. Results showed that the recovery rate of manganese can exceed 98%, the leaching rate of iron was much lower than 2%, and the leaching rates of manganese and ammonia nitrogen after EMR ball grinding were 1.01 and 13.65 mg/L, respectively. Kinetics and mechanism studies revealed that ammonium salts were primarily removed in the form of ammonia, and that insoluble manganese (MnO₂) was recovered by the reduction of FeS and FeS₂ in EMR under the action of oxalic acid. Iron was solidified in the form of Fe₂O₃ and Fe₂(SiO₃)₃. The technology proposed in this research has great industrial application value for the recycling and harmless treatment of EMR.

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

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

Chloride removal from municipal solid waste incineration fly ash using lactic acid fermentation broth

As far as improvement of chlorine removal from fly ash by lactic acid fermentation broth (LAFB) was concerned, it is particularly important to explore the instinct mechanism and understand how leaching protocols (i.e. lactic acid addition amount and timing) affect the dechlorination efficiency. Results revealed that the WLL leaching protocol yielded the highest dechlorination efficiency (i.e. removed 98.7% of the total chlorine content of fly ash). The undissolved chlorine in fly ash residue might wrap inside the crystal structure of CaAlSiO₄(OH). Given that the chlorine removal from fly ash might prohibit by the newly formed calcium salt precipitation, exclusively increase the addition amount of LAFB (i.e. LLL protocol) did not necessarily stimulate the dechlorination efficiency. Conversely, it might accelerate the fly ash mass reduction (compared with WLL protocol), resulting in a high chlorine content in fly ash residue. Therefore, instead of increasing lactic acid strength, reducing the thickness of the calcium salt precipitation layer or breaking the crystal structure of CaAlSiO₄(OH) during the leaching process was suggested for efficient fly ash dechlorination.

Electrolytic manganese residue-based cement for manganese ore pit backfilling: Performance and mechanism

Slag backfilling with electrolytic manganese residue (EMR) is an economical and environmentally-friendly method. However, high ammonium-nitrogen and manganese ions in EMRs limit this practice. In this study, a method of highly efficient simultaneous stabilization/solidification of ultrafine EMR by making EMR-based cementitious material (named EMR-P) was proposed and tested via single-factor and response surface optimization experiments. Results show that the stabilization efficiency of NH₄⁺ and Mn²⁺ were above 95%, and the unconfined compressive strength of the EMR-P was 18.85 MPa (megapascal = N/mm²). The mechanistic study concluded that the soluble manganese sulfate and ammonium sulfate in EMR were converted into the insoluble precipitates of manganite (MnOOH), gypsum (CaSO₄), MnNH₄PO₄·H₂O, and struvite (MgNH₄PO₄∙6 H₂O), leading to the stabilization of NH₄⁺ and Mn²⁺ in the EMR-P. Leaching tests of EMR-P indicated that NH₄⁺, Mn²⁺, and others heavy metals in the leachate were within the permitted level of the GB/T8978-1996. The novelty of this study includes the addition of phosphate and magnesium ions to precipitate ammonium-nitrogen and the combination between calcium ions (from CaHPO₄∙2 H₂O) and sulfate (from the EMR) to form calcium sulfate to improve the stability and unconfined compressive strength of cementitious materials (EMR-P).

Stabilisation/solidification of municipal solid waste incineration fly ash by phosphate-enhanced calcium aluminate cement

Landfill disposal of municipal solid waste incineration fly ash (MIFA) presents significant environmental and economic burden. This study proposed a novel and high-efficiency approach for stabilisation/solidification (S/S) of MIFA by phosphate-modified calcium aluminate cement (CAC). Experimental results showed that the presence of Pb (the most leachable metal contaminant in the MIFA) retarded the early-stage reaction of CAC, resulting in an extension of setting time and a significant decline of compressive strength of CAC pastes. The incorporation of phosphate additives (10 wt% of binder), especially for trisodium phosphate, in CAC system effectively mitigated the negative impact of Pb on the CAC reaction and reduced the Pb leachability. Elemental mapping results illustrated that Pb²⁺ coordinated with phosphate to generate insoluble precipitates (e.g., Pb₃(PO₄)₂). The S/S treated MIFA samples fulfilled the compressive strength and leachability requirements for on-site reuse. Overall, this study demonstrated that phosphate-modified CAC is a promising binder for S/S of hazardous MIFA.

Effect of the direct use of biomass in agricultural soil on heavy metals __ activation or immobilization?

In recent years, the biomass was directly used extensively in agriculture due to its low cost and convenience. Increasingly serious soil pollution of heavy metals may pose threats and risks to human health. Directly addition of biomass to soil may affect the bioavailability and content of heavy metals. Here, we reviewed the impact of direct application of oil cake, manure, sewage sludge, straw and municipal waste to soil on the form and concentration of heavy metals in soil, and also emphasized the role of biomass in soil heavy metals remediation. Heavy metals can be activated in a short term by the content of heavy metals in biomass, the production of low-molecular-weight organic acids by biomass application, and the oxidation of sulfides (except for ammoniation). However, heavy metals in soil can be immobilized by humic substances. These can be produced by biomass during a long-term application to soil. Moreover, the degree of immobilization depended on the kind of biomass. Biomass contaminated by heavy metals cannot be returned to the field directly. Therefore, Mitigating the activation of heavy metals in the early stage of biomass application is meaningful, especially for application of these biomass such as straw, sewage sludge and municipal waste. Future researches should focus on the heavy metal control on direct use of biomass in agricultural.

Municipal solid waste incineration fly ash to produce eco-friendly binders for sustainable building construction

Municipal solid waste incinerator (MSWI) fly ash is a residue of municipal solid waste incineration whose recycling is currently a worldwide problem. Therefore, considerable efforts are being made to establish effective recovery techniques so MSWI can be used as a substitute for natural resources in construction, as in masonry blocks, roads and so on, or in the manufacture of new materials. MSWI fly ashes contain elements such as Ca, Si and Al, which make it possible for them to be used as raw material to manufacture cements. This paper presents the results obtained from the physicochemical characterization of two MSWI fly ashes from two Spanish cities. The research aims to explore the feasibility of using MSWI fly ash as raw material for sintering belite cements. The results show that MSWI fly ashes have a suitable composition. However, appropriate pre-treatment will be required to eliminate chloride and possible traces of heavy metals and to improve pozzolanic activity. Furthermore, the addition of vitreous silica in the proper proportions is required. The phases generated after calcination of the blend at 800 °C are not those corresponding to pure belite cements. Nevertheless, the possibility of using these ashes as supplementary cementitious material in the manufacture of eco-cements should be contemplated.