Heavy Metals In Fly Ash Research Articles

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

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

Application and mechanism of carbonate material in the treatment of heavy metal pollution: a review.

Among the many heavy metal pollution treatment agents, carbonate materials show strong flexibility and versatility by virtue of their high adsorption capacity for heavy metals and the characteristics of multiple and simple modification methods. It shows good potential for development. This review summarizes the application of carbonate materials in the treatment of heavy metal pollution according to the research of other scholars. It mainly relates to the application of surface-modified, activated, and nano-sized carbonate materials in the treatment of heavy metal pollution in water. Natural carbonate minerals and composite carbonate minerals solidify and stabilize heavy metals in soil. Solidification of heavy metals in hazardous waste solids is by MICP. There are four aspects of calcium carbonate oligomers curing heavy metals in fly ash from waste incineration. The mechanism of treating heavy metals by carbonate in different media was discussed. However, in the complex environment where multiple types of pollutants coexist, questions on how to maintain the efficient processing capacity of carbonate materials and how to use MICP to integrate heavy metal fixation and seepage prevention in solid waste base under complex and changeable natural environment deserve our further consideration. In addition, the use of carbonate materials for the purification of trace radioactive wastewater and the safe treatment of trace radioactive solid waste are also worthy of further exploration.

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

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

Mitigating heavy metal volatilization during thermal treatment of MSWI fly ash by using iron(III) sulfate as a chlorine depleting agent

In the thermal treatment of municipal solid waste incineration fly ash (FA), the presence of chlorides leads to the pronounced volatilization of heavy metals at high temperature, making heavy metals stabilization challenging. Conventional washing processes struggle to remove chlorides completely, and even minor residual chlorides can lead to significant heavy metal volatilization. This study innovatively applied iron(III) sulfate as a chlorine depleting agent, which can form FeCl3 (boiling point 316 °C) and volatilize to remove the residual chlorides at below 500 °C, thus preventing the chlorination and volatilization of heavy metals at 600–1000 °C. Using water-washed FA to produce lightweight aggregate (LWA) preparation, after adding iron(III) sulfate, the volatilization rates of Pb and Cd at 1140 °C decreased to 5.4% and 9.3%, respectively, a reduction of 82.8% and 84.1% compared to before its addition. The LWA met standard requirements in both performance and heavy metal leaching toxicity. The mechanism was further studied through thermodynamic equilibrium calculations and heating experiments of pure chemicals. This study presents novel approaches and insights for suppressing the volatilization of heavy metals in FA at high temperature, thereby promoting the advancement of thermal treatment techniques and the safe, resourceful disposal of FA.

Stabilization effect of chelating agents on heavy metals in two types of municipal solid waste incineration fly ash

Municipal solid waste incineration (MSWI) fly ash is complex in composition and contains large amounts of heavy metals. To investigate the effects of different passivating agents on Cd, Pb, Cr, Ni and Zn in different types of fly ash, 7 reagents were selected as passivating agents, including 3 inorganic reagents (Na2S, Na3PO4, NaH2PO4) and 4 organic reagents (SDD, DDTC, TMT, thiourea). The results showed that at low addition levels, sulfur-containing organic chemicals and phosphates were more effective in chelating these metals in both grate furnace incineration fly ash (GF FA) and circulating fluidised bed incineration fly ash (CFB FA), while Na2S was more effective in chelating heavy metals in CFB FA. In addition, agents containing -N-CS groups showed better chelation of heavy metals in GF FA, while heavy metals in CFB FA were more affinity to agents containing -SH groups. The ternary compound TMT-NaH2PO4-SDD (m1:m2:m3 = 2:3:1) was an excellent chelating agent for Pb, Cd in the GF FA, and TMT-Na2S-thiourea (m1:m2:m3 = 2:1:3) was a suitable chelating agent for these metals in CFB FA. The chelation rate of Cd and Pb after the treatments mixed ternary reagents was above 99%, and the leaching amount was much lower than the limit value of pollution control standards for domestic landfills. Moreover, under different pH leaching conditions, the leaching concentrations of Cd and Pb in the fly ashes treated with the ternary mixture were lower than the standard limits. In general, the ternary compound can effectively chelate heavy metals in fly ash, which provides a reference for selecting a suitable stabilizer combination for heavy metal in fly ash.

Mineral phase evolution and heavy metals migration during the hydrothermal treatment of municipal solid waste incineration fly ash

The solidification of heavy metals in the municipal solid waste incineration fly ash was realized by the hydrothermal treatment with mineral additives added. The experimental results showed that the addition of SiO2 at 200 °C with the NaOH concentration was 0.5 mol/L attributed to the formation of fibrous tobermorite and effectively reduced the heavy metals toxicity of hydrothermal products, especially Pb and Cu. When SiO2 and Al2O3 were both introduced, the generated katoite and sodalite showed a better curing effect on Zn at lower NaOH concentrations. Further, the dynamic correlation between the tobermorite formation and heavy metals solidification was explored. It was found that the heavy metals in fly ash were first dissolved into the alkaline liquid phase and then transferred into the solid phase as the treatment proceed. The solidification of Pb which finished before 8 h was mainly due to the encapsulation of the tobermorite precursor before crystallization, while that of Cu which lasted over 48 h mainly relied on the adsorption and cation exchange effect caused by the formed tobermorite. Therefore, selecting the appropriate mineral addition scheme and hydrothermal time according to the fly ash components and heavy metals categories can effectively improve the curing cost performance.

Novel and sustainable silica-assisted low-temperature roasting method for the solidification of lead in municipal solid waste incineration fly ash: Process and mechanism

During the high-temperature treatment process of municipal solid waste incineration (MSWI) fly ash (FA), heavy metals and chlorine are severely volatilized, leading to serious secondary pollution. Therefore, developing low-temperature solidification technology for heavy metals in FA is of great significance. This article proposes a process for silica-assisted low-temperature sintering to solidify Pb in FA. The effects of roasting temperature (100–800 °C), roasting time (30–180 min) and curing agent dosage (0–12.5 wt%) on the leaching toxicity and immobilization rate of Pb were investigated. The optimum conditions for Pb solidification were calcination temperature of 600 °C, roasting time of 180 min and silica proportion of 10 wt%. The Pb leaching concentration, Pb immobilization rate and comprehensive strength of cured FA were 0.086 mg/L, 99% and 1.65 MPa, respectively, which satisfied the requirements of hazardous waste landfill (GB 18598–2019). The solidification mechanism indicates that most of the Pb2+ reacts with SiO2 to form a stable mineral phase PbSiO3, which is the main reason for the reduction of Pb leachability. In addition, at high temperatures, the added silicon as well as the Al2O3 in the FA reacted with CaO to form aluminosilicate skeletons, which can also solidify a small portion of the Pb2+ into the crystal lattice. As the roasting temperature increases, the chemical speciation of Pb gradually changes to the oxidizable and residual states. When the temperature is higher than 700 °C, a large amount of CaO reacts with silica to form CaSiO3, which hinders the reaction between PbO and SiO2 and reduces the solidification rate of Pb. Finally, the feasibility and economy of the process were analyzed. The findings can provide theoretical guidance for the development of low-temperature detoxification process of FA.

Utilization of size-fractionated pozzolanic bottom ash to immobilize heavy metals in MSW incinerator fly ash

Fly ash (FA) from Municipal solid waste incineration (MSWI) is disposed of in landfills after the immobilization of hazardous heavy metals. Pozzolans found in the fine-fraction of MSWI bottom ash (BA) have the potential to immobilize heavy metals. The aim of this study was to examine the effect of size-fractionated BA in immobilizing heavy metals in FA, in order to enhance the effective utilization of resources. BA was applied at two distinct fractions (d<0.25 and d<2.0 mm) when considerable amorphous reactive pozzolans (i.e., siliceous and aluminous) were present. In order to conduct the experiments, FA, BA, and water were mixed in a ratio of 10:1:10, dried at room temperature and in an oven at 120 °C for 4 h, and settled for 4, 8, 16, 30, and 60 days. The results showed that under the air-dried conditions, both BA fractions significantly immobilized Pb and Zn. Pb and Zn leaching stabilized at about 99.9% and 95.6%, respectively, over 16 days with a BA fraction of d< 0.25 mm in an air-dry environment. The pH reduction, the calcite, C-S-H gel, and several new secondary minerals formation demonstrated that the carbonation and pozzolanic reactions were the immobilization mechanisms. Despite the method's simplicity, the findings suggested a potential approach of utilizing residual BA fractions that can be reused directly in the incineration facility. Therefore, this technique can minimize the amount of raw materials required as a metal stabilizer, reduce the amount of waste sent to landfills, and create a new material that can be reused as fillers.

Heavy Metals in Fly Ash as a Factor Limiting Its Use in Fertilizing Composts

Composting enables the recycling of significant amounts of biodegradable waste, while ensuring its disposal. The addition of fly ash increases the concentration of fertilizing substances in the compost. Compost is a source of many nutrients for plants. The addition of fly ash might also cause a slower release of these nutrients into an aqueous solution, meeting the fertilization needs of plants over a longer period of time. Tests were carried out using sewage sludge (WWTP Piaseczno, 163,500 P.E.), straw as a structure-forming material and fly ash (WWTP Krakow, 780,000 P.E.). The compost obtained in the research was analyzed in terms of environmental conditions specified in legal regulations. The most favorable compost mass density (520 kg/L) and the amount of air supplied to the process (5.0 L/(h∙kg d.m.)) were determined. The addition of fly ash to the compost mass did not significantly affect the temperature distribution obtained in the process. The increase in fly ash content increased the pH of the compost mass and was associated with higher nitrogen losses. It was found that the factor which may limit the possibility of using fly ash as a compost substrate is the presence of higher concentrations of heavy metals, especially chromium. It was determined that the maximum addition of fly ash to the compost mass was 154 kg d.m.ash/(ton d.m.straw+sludge).

Comprehensive assessment of thermal characteristics, kinetics and environmental impacts of municipal solid waste incineration fly ash during thermal treatment

The volatilization of heavy metals and the release of harmful gases during the thermal treatment of municipal solid waste incineration (MSWI) fly ash (FA) can cause secondary pollution. Understanding the thermal characteristics, heavy metal behaviors and environmental emissions in FA roasting is significant for pollution control and heavy metal elimination. In this work, the thermodynamics, kinetics, environmental impacts, carbon emission, phase transition, leaching toxicity, transition behavior of chlorine and heavy metals in FA thermal treatment were investigated comprehensively for the first time through TG-FTIR, XRD, ICP-OES and IC. The apparent activation energy and thermodynamic parameters were obtained according to the kinetic modeling method. The average apparent activation energy values of the first, second, third, and fourth stages were 60.21, 114.91, 190.23, and 226.67 kJ/mol, respectively, and the reaction control models were chemical reaction model (No.3), assumed random nucleation and subsequent growth model (No.21), assumed random nucleation and subsequent growth model (No.14), and nucleation model (No.10). The environmental impacts illustrate that large amounts of gaseous products, including CO2, H2O, NO and SO2, are released during the thermal treatment of FA, and the emission of CO2 sharply increases as the temperature exceeds 550 °C. When treating 1 kg of FA at 1000 °C, the release of CO2 in stages I, II, III and IV is 0.18, 7.06, 116.34 and 177.21 g, respectively, and the total carbon emission is approximately 300 g. The leaching toxicity of heavy metals in FA is higher below 800 °C, and it is mainly caused by Pb. In addition, the volatilization rate of heavy metals and chlorine increased significantly as the calcination temperature exceeded 800 °C, among which, the volatilization rates of Pb, Cu, and Cl can reach 99.87%, 90.25% and 73.85%, respectively. At high temperatures, heavy metals in FA exist primarily in oxidizable and residual states. The obtained findings can provide theoretical guidance for pollution control and process optimization of FA thermal treatment.

Mechanochemical treatment with red mud added for heavy metals solidification in municipal solid waste incineration fly ash

The effective stabilization of heavy metals municipal solid waste incineration fly ash (FA) is very important for the disposal of FA, due to that heavy metals leaching from FA will induce environmental risks to a content. In this study, Red mud (RM) was used as an additive for the stabilization of heavy metals in FA during the mechanochemical process. The results demonstrated that the addition of RM significantly reduce the leaching of Pb, Cr and Zn from FA. In the conditions of 30 wt% of red mud, 0.1 g L−1 of solid-to-liquid ratio and 24 h of ball-milling time, the leaching concentration of Pb, Cr and Zn decreased by 99%, 81.18% and 100%, respectively. Meanwhile, the residue fraction of Pb, Cr and Zn increased, part of the reducible Cr is changed into acid-soluble fraction of Cr due to oxidation. Moreover, FA was broken into small particles and then agglomerated into larger ones after mechanochemical treatment. The main reasons accounting for the solidification of heavy metals were ascribed to the precipitation and sealing by the fine particles, capture by new phases, dissolution and elution of soluble salt. RM play a key role in forming new phases (include hydrocalumite, andradite and aluminosilicate minerals) which in favor of solidification of heavy metals. This work provides a good method for the effective heavy metals stabilization in FA and presents a new sight into the reuse of RM.

Research Progress on CO2 Capture, Utilization, and Storage of Fly Ash

The research progress of different technologies of fly ash for CO2 capture, utilization, and storage at home and abroad was summarized, and the research opportunities were discussed. Fly ash could mineralize, capture, and store CO2 through direct dry, semi dry, wet, and indirect methods, reducing the leaching of heavy metals in fly ash while mineralizing CO2. The mineralized fly ash was more suitable for making concrete additives because it could effectively reduce the content of free CaO and MgO. Fly ash could also be made into activated carbon, zeolite, porous silica, and other products for physical adsorption and capture of CO2. The type of products depended mainly on the composition and physical/chemical properties of fly ash. In terms of CO2 utilization, fly ash could not only expand the utilization of building materials but also be made as catalysts or catalyst carriers required for various chemical processes of CO2 and new materials such as pseudo boehmite. The proposal of "double carbon" in China and the physical/chemical characteristics of fly ash from coal-fired power plants provide a new method for comprehensive utilization of fly ash.

Study on the reduction of chlorine and heavy metals in municipal solid waste incineration fly ash by organic acid and microwave treatment and the variation of environmental risk of heavy metals

Municipal solid waste incineration (MSWI) fly ash usually needs to undergo dechlorination or heavy metal stabilization pretreatment for further treatment, recycling or disposal. In this paper, the removal effect of chlorine in fly ash by water washing, lactic acid, citric acid and microwave treatment was studied, and XANES was used to analyze chlorine chemical form in fly ash. In addition, the heavy metals in fly ash were also checked. The results indicated that double washing and triple washing could remove 88.0 % and 95.5 % of chlorine from fly ash respectively. The “double water washing + microwave/organic acid” could remove about 96.6 % of chlorine, and 42.9 % and 47.2 % of insoluble chloride respectively. The microwave treatment could maximize the stabilization of heavy metals with a BI value of 39.1 %, 0.11 %, 1.65 %, 15.4 % and 3.98 % for Cd, Cr, Cu, Pb and Zn. The elution of heavy metals by citric acid was obvious. “Double water washing + citric acid” removed 87.0 % of Cd, 17.2 % of Cr, 11.9 % of Cu, 39.6 % of Pb and 43.6 % of Zn, but the environmental risk of Cu and Cr increased about 2–3 % after the treatment. The results of this study provide guidance for the pretreatment of fly ash before resource utilization.

Particle Size Distribution and Enrichment of Alkali and Heavy Metals in Fly Ash on Air and Oxy-Fuel Conditions from Sludge Combustion

Comparative tests in air and oxy-fuel combustion were conducted in a 30 kWth circulating fluidized bed (CFB) pilot plant for waste sludge combustion. General combustion characteristics of the CFB, such as pressure profiles, temperatures along the bed, and flue gas composition, were different under the air and oxy-fuel conditions. At the bottom and in the fly ash, alkali and heavy metals had different distributions under the air and oxy-fuel combustion conditions. The particle size distribution in fly ash from air combustion was dominated by coarse particles, over 2.5 μm in size, whereas with oxy-fuel combustion, most particles were submicron in size, approximately 0.1 μm, and a smaller quantity of coarse particles, over 2.5 μm in size, formed than with air combustion. Mass fractions of Al, Ca, and K, below 2.5 μm in size, were found in the ashes from oxy-fuel combustion and in higher quantity than those found in air combustion. Submicron particle formation from Cr, Ni, Cu, and Zn in the fly ash occurred more during oxy-fuel combustion than it did in air combustion.

Utilization of mechanochemically pretreated municipal solid waste incineration fly ash for supplementary cementitious material

With the rapid development of municipal solid waste (MSW) incineration, the effective disposal and recycling utilization of fly ash is attracting increasing attention. The mechanochemical (MC) treatment as a non-thermal method shows good stabilization of heavy metals and degradation of dioxins in fly ash. The inhibition effect and long-term safety of CaSO4, Ca3(PO4)2, NaH2PO4, and composite additives (50 %NaH2PO4 +50 %CaO) as additives for heavy metals stabilization were investigated. The composite additive showed the best inhibitory effect on heavy metals, and the leaching concentration of Cd, Cr, Cu, Ni, Pb, and Zn were decreased by 95.21 %, 46.84 %, 99.42 %, 49.91 %, 99.83 %, and 99.55 %, respectively. In addition, the modification by MC treatment reduced the particle size and increased the specific surface area of fly ash and improved the activity for resource utilization. Therefore, MC treated fly ash is used as supplementary cementitious material (SCM), which will produce a filling effect and pozzolanic effect improving the performance of fly ash concrete blocks. The compressive strength of the MC-treated fly ash concrete blocks curing for 28 days increased by 35.5 % and exceeded 40 MPa, close to OPC’s strength. The concrete block also had a secondary inhibition on potentially toxic elements, making the resource utilization of fly ash more stable and safer. In general, the MC treatment and the utilization of MC-treated fly ash as SCM effectively inhibit the leaching of heavy metals in fly ash, delivering a promising method for green fly ash disposal.

Mechanistic Investigation for Solidification of Pb in Fly Ash by Alkali Mineral Slag—Calcium Chloroaluminate as an Example

With the increase in municipal solid waste incineration, fly ash, its heavy metal content, and its disposal methods have attracted wide attention. This work investigates if the alkali-activated mineral slag gel solidification of heavy metals in fly ash has positive significance in promoting the harmless treatment of fly ash. This study obtained the optimal solidification conditions of fly ash from a grate incinerator, which are mineral slag content of 40%, activator content of 4%, and water content of 27.5%. Furthermore, the stability of synthesized calcium chloroaluminate is systematically investigated. The solidification effect of calcium chloroaluminate on Pb at pH = 10–13 was conducted at ambient temperatures from 15 °C to 35 °C to simulate the solidification environment of fly ash. The results show that the adsorption capacity of calcium chloroaluminate to Pb in a strongly alkaline environment is 0.1–3.5 mg/g. Pb is mainly solidified as lead-acid calcium chloroaluminate. This work provides a novel treatment strategy for fly ash.

Characterization of heavy metals in fly ash stabilized by carbonation with supercritical CO2 coupling mechanical force

Fly ash carbonation is a feasible alternative method for CO2 reduction and heavy metal stabilization. There are four main mechanisms for fly ash carbonation to stabilize heavy metals. However, due to differences in heavy metals and fly ash, the stability of heavy metals is also different. Especially, the properties of supercritical CO2 are greatly changed compared with those of non-supercritical CO2, and the influence of this change on the four mechanisms is still unclear. In this paper, carbonation experiments of different fly ash were conducted in the transition from non-supercritical to supercritical, and leaching characteristics of Cu, Zn, Pb, Cr, Cd, and Ni were investigated to explore stabilization mechanisms. Our research showed that physical encapsulation was a physical phenomenon that had stabilizing effects on all heavy metals. Chemical reaction was aimed at heavy metals that reacted with CO32-. Pb, Zn, and Cu belonged to the isomorphism with Ca, and adsorption precipitation was formed by the isomorphism substitution during carbonation process. Supercritical CO2 enhances the three mechanisms. And mechanical force broke up sintered surface of fly ash, exposing more active sites to enhance the three mechanisms. On the other hand, under supercritical state, the solubility of CO2 increased, resulting in the speciation transformation of heavy metals. Under the above mechanisms, there were two trends regarding stability of heavy metals in fly ash. One trend was a gradual increase in stability, and the other trend an initial increase in stability followed by a peak at the critical point and then a decrease.

Concentrations, Speciation, and Potential Release of Hazardous Heavy Metals from the Solid Combustion Residues of Coal-Fired Power Plants.

Hazardous heavy metal-laden coal combustion byproducts exposed to precipitation or surface/groundwater are of environmental concern. This study analyzed fly ash (FA) and desulfurization gypsum (FGD gypsum) samples obtained from 16 coal-fired power plants in Guizhou Province, China. A combination of field and laboratory studies was used to investigate the binding forms of lead (Pb), cadmium (Cd), and chromium (Cr) and their leaching characteristics under natural storage conditions. The results showed that there were significant proportions of residual states of these elements in FA (84–99% for Pb, 83–91% for Cd, and 73–97% for Cr), indicating a lack of migration to other environmental media. FGD gypsum contained high proportions of metals in migratable states, but the environmental risks were low due to their very low concentrations. The release of Pb, Cd, and Cr from FA and FGD gypsum into extracts varied according to pH. This is related to the form of each element in the solid and the series of reactions that occurs during the leaching process. However, aside from Cr in FA, all heavy metals in FA and FGD gypsum samples were present in concentrations below the relevant standards for landfill leachate, indicating very low release rates. The Cr levels (206–273 μg/L) in some of the FA extracts were higher than the limits for water pollution from domestic landfill, indicating that Cr in FA poses a leaching risk. The results of field leachate sampling and indoor simulated rainfall experiments further validated these findings, indicating that the release of Cr from stockpiled coal FA is a cause for concern.

Stabilization of Heavy Metals in Fly Ash and Its Effect on Strength

This study investigated the effect of Ethylenediaminetetraacetic acid (EDTA) and Triple Super Phosphate (TSP) as stabilization agents to immobilize various heavy metals to enhance the use of fly ash without any environmental concern. The effect for the application of stabilization agents on the strength on fly ash is also studied. First, the various dosages of TSP (0–8%) or EDTA (0–2%) were applied to Class C and F fly ash to identify the appropriate dosages of these two stabilization agents using Toxicity Characteristic Leaching Procedure (TCLP) tests. Thereafter, the two fly ashes were mixed with the determined dosages (i.e., 6% TSP and 1% EDTA) to assess the impact on time-depended leaching and the strength of the hardened fly ash. The column leaching tests disclosed that TSP immobilized well for all the heavy metals except As and EDTA also immobilized quite well for all the heavy metals except Hg and Pb for Class C fly Ash. The combined usage of EDTA and TSP showed mutually supplementary effects: EDTA responded well to some metals and TSP responded well to others. In addition, combination of the two agents further enhanced the stabilization of Cd, Cr, Zn, and As. For Class C fly ash, adding either EDTA or TSP decreased the compressive strength of 28 days cured specimens by 38%. The combination of both TSP and EDTA decreased the strength by 36%. For Class F fly ash, EDTA did not noticeably affect the compressive strength of the cured specimen, but TSP increased the strength significantly.

Utilization of fly ash as building material admixture: Basic properties and heavy metal leaching

The potential harm to the environment and human body caused by heavy metal elements such as Cd, Cr, Pb in fly ash cannot be ignored. In this study, microscopic analysis and heavy metal hazard analysis were performed on fly ash samples from four regions in China. XRF, XRD, FTIR and SEM/EDS test results showed that the main components of fly ash are Si, Ca, Al, O and other elements, and the main mineral components are mullite, quartz and amorphous aluminosilicate. The results showed that six heavy metal elements of As, Hg, Cd, Cr, Pb and Ni from the EDS test charts of these four types of fly ash. Based on the detection of the six heavy metal contents in fly ash, the results of the pollution evaluation of these four types of fly ash were consistent according to the single factor index, potential ecological risk index, and index of geo-accumulation. This indicated that Cd/Hg in FA 1, FA 2 and FA 3 has a high risk of environmental pollution. In the meantime, Cr and As in fly ash need to be controlled, because the human health risk assessment showed that they will bring carcinogenic risk to the human body through oral intake. The study on the migration law of characteristic heavy metals by taking FA 4 as an example showed that the leaching rate of Cr, Hg and Cd decreased with the increase of grinding particle size. Among the three characteristic heavy metals, mercury had the highest leaching rate. From the XPS detection results of FA 4, it was found that the valence states of these three heavy metals in FA 4 are Cr3+, Cr6+, Cd2+, Hg2+ respectively. These harmful heavy metal ions gradually enter the deep soil and groundwater through osmosis. Finally, based on the control model constructed in this study, the contents of these three heavy metals in fly ash in different application scenarios were limited.