Heavy Metal Migration and Potential Environmental Risk Assessment During the Washing Process of MSW Incineration Fly Ash and Molten Slag

A systematic review on the bioremediation of metal contaminated soils using biochar and slag: current status and future outlook

Heavy metals contaminated soils are posing severe threats to food safety worldwide. Heavy metals absorbed by plant roots from contaminated soils lead to severe plant development issues and a reduction in crop yield and growth. The global population is growing, and the demand for food is increasing. Therefore, it is critical to identify soil remediation strategies that are efficient, economical, and environment friendly. The use of biochar and slag as passivators represents a promising approach among various physicochemical and biological strategies due to their efficiency, cost-effectiveness, and low environmental impact. These passivators employ diverse mechanisms to reduce the bioavailability of metals in contaminated soils, thereby improving crop growth and productivity. Although studies have shown the effectiveness of different passivators, further research is needed globally as this field is still in its early stages. This review sheds light on the innovative utilization of biochar and slag as sustainable strategies for heavy metal remediation, emphasizing their novelty and potential for practical applications. Based on the findings, research gaps have been identified and future research directions proposed to enable the full potential of passivators to be utilized effectively and efficiently under controlled and field conditions.

Transport of dissolved organic matters derived from biomass-pyrogenic smoke (SDOMs) and their effects on mobility of heavy metal ions in saturated porous media

Biomass-pyrogenic smoke-derived dissolved organic matter (SDOMs) percolating into the underground environment profoundly impacts the transport and fate of environmental pollutants in groundwater systems. Herein, SDOMs were produced by pyrolyzing wheat straw at 300-900 °C to explore their transport properties and effects on Cu²⁺ mobility in quartz sand porous media. The results indicated that SDOMs exhibited high mobility in saturated sand. Meanwhile, the mobility of SDOMs was enhanced at a higher pyrolysis temperature due to the decrease in their molecular sizes and the declined H-bonding interactions between SDOM molecules and sand grains. Furthermore, the transport of SDOMs was elevated as pH values were raised from 5.0 to 9.0, which resulted from the strengthened electrostatic repulsion between SDOMs and quartz sand particles. More importantly, SDOMs could facilitate Cu²⁺ transport in the quartz sand, which stemmed from forming soluble Cu-SDOM complexes. Intriguingly, the promotional function of SDOMs for the mobility of Cu²⁺ was strongly dependent on the pyrolysis temperature. Generally, SDOMs generated at higher temperatures exhibited superior effects. The phenomenon was mainly due to the differences in the Cu-binding capacities of various SDOMs (e.g., cation-π attractive interactions). Our findings highlight that the high-mobility SDOM can considerably affect heavy metal ions' environmental fate and transport.

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.

A study of the solidification and stability mechanisms of heavy metals in electrolytic manganese slag-based glass-ceramics

To better solve the waste pollution problem generated by the electrolytic manganese industry, electrolytic manganese slag as the main raw material, chromium iron slag, and pure chemical reagents containing heavy metal elements mixed with electrolytic manganese slag doping. A parent glass was formed by melting the slag mixture at 1,250°C, which was, thereafter, heat-treated at 900°C to obtain the glass-ceramic. The results from characterizations showed that the heavy metal elements in the glass-ceramic system were well solidified and isolated, with a leakage concentration at a relatively low level. After crystallization, the curing rates of harmful heavy metals all exceed 99.9%. The mechanisms of heavy metal migration, transformation, and solidification/isolation in glass-ceramic curing bodies were investigated by using characterization methods such as chemical elemental morphological analysis, transmission electron microscopy, and electron microprobe. The most toxic Cr and Mn elements were found to be mainly kept in their residual state in the glass-ceramic system. It was concluded that the curing mechanism of the heavy metals in a glass-ceramic can either be explained by the chemical curing induced by bonding (or interaction) during phase formation, or by physical encapsulation. Characterization by using both Transmission electron microscopy and EPMA confirmed that Cr and Mn were mainly present in the newly formed spinel phase, while the diopside phase contained a small amount of Mn. Zn, Cd, and Pb are not found to be concentrated and uniformly dispersed in the system, which is speculated to be physical coating and curing.

Leaching behavior and environmental risk assessment of toxic metals in municipal solid waste incineration fly ash exposed to mature landfill leachate environment

Solidification/stabilization pretreatment + landfill disposal in municipal solid waste (MSW) landfill sites is a widely accepted MSW incineration (MSWI) fly ash (FA) management strategy in China. However, in reality, the stability of FA disposed in MSW landfill sites may be affected by the organic landfill leachate environment. The purpose of this study was to explore the mobility and environmental risks of six toxic metals (Mⁿ⁺, Pb/Zn/Cu/Cd/Cr/Ni), from raw and solidified/stabilized FA, by simulating a leaching environment with mature landfill leachate (MLL). The leaching of Mⁿ⁺ mainly occurred in the early leaching stage, and their leaching behavior was controlled by the diffusion of surface Mⁿ⁺ in the FA matrix. The destructive effect of dissolved organic matter (DOM) on the local precipitation-dissolution equilibrium of FA-leachate interface, the formation of non-adsorptive DOM-Mⁿ⁺ complex (easy to migrate), and the competitive effect of DOM on the binding sites of Mⁿ⁺ on the surface of the FA matrix may play an important role in increasing the leaching level of most Mⁿ⁺. By contrast, the potential of solidified FA in reducing the environmental risk level of leached Mⁿ⁺ was better than that of stabilized FA. However, the immobilization capability of solidification/stabilization pretreatment on various types of Mⁿ⁺ in FA should be judged according to their practical disposal environment. Compared to MLL leaching tests, Acetic Acid Buffer Solution Method (HJ/T300-2007) can effectively strengthen the exposure environment and provide a reliable reference level of environmental risk for MSWI FA disposed in MSW landfill sites.

Municipal solid waste incineration fly ash exposed to carbonation and acid rain corrosion scenarios: Release behavior, environmental risk, and dissolution mechanism of toxic metals

This study investigated the leaching behavior, environmental risk, and dissolution mechanism of toxic metals (TMs) in solidified/stabilized municipal solid waste incineration fly ash (MSWI FA) exposed to alternative "carbonation + acid rain corrosion" disposal scenarios. The content of TMs (mg/kg) showed a trend of Zn (12,187.10 ± 168.60) > Pb (3374.43 ± 66.12) > Cu (1055.14 ± 32.52) > Cr (127.95 ± 8.12) > Cd (119.05 ± 6.26) > Ni (49.50 ± 3.20). Initial leaching of CO₂-saturated water (CSW) and replacement of simulated acid rain (SAR) increased the environmental risk of leached TMs. The results of "average release rate" (mg/(kg·d)) of TMs indicated that Zn (0.8307)/Cu (0.0278)/Cd (0.0109) and Cu (0.0581)/Cr (0.001176)/Ni (0.004339) in phosphoric acid stabilized FA and Pb (0.0753)/Cr (0.001921)/Ni (0.00111) and Pb (0.0656)/Zn (1.0560)/Cd (0.0050) in Portland cement solidified FA were the key "problem TMs" during carbonation and acid rain corrosion, respectively. CSW leaching increased the independent environmental risk of most TMs in residual FA (especially Zn/Cd) due to the increased carbonate-bound fraction. Compared with independent carbonation, alternative "carbonation + acid rain corrosion" contributed to a higher comprehensive environmental risk for TMs in residual FA. CSW leaching system was an indirect carbonation based on CO₂-water and FA matrix, in which "nucleation and dissolution" of carbonates and "immobilization and dissolution" of TMs coexisted. The dissolution mechanism of TMs was mainly controlled by reaction equilibrium of nucleation and dissolution of carbonates containing TMs. Dissolution and nucleation were the dominant mechanism in the early and later periods of CSW leaching, respectively. Carbonate layer dissolution, H⁺ corrosion/displacement, and counter-ion effect (SO₄^2- > NO₃^- > Cl^-) were the main mechanisms affecting TM dissolution during SAR leaching.

Geoaccumulation and Ecological Risk Indexes in Papaya Cultivation Due to the Presence of Trace Metals

The aim of this research was to evaluate the impact of heavy metals on Maradol papaya cultivation soil, through the geoaccumulation index (Igeo) and the ecological potential risk index (RI). Soil samples from 15 locations in the Cotaxtla municipality of Veracruz, Mexico were tested for pH, soil texture, and concentrations of lead (Pb), chromium (Cr), cadmium (Cd), zinc (Zn), copper (Cu), and organic matter (MO). The pH varied between values of 5.5 ± 0.10 and 7.7 ± 0.22, while the MO varied from 1.57% ± 0.97% to 13.1% ± 1.342%. The type of soil texture represented 48% sandy loam, 40% loam, 8% clay loam, and 4% silt-loam. For heavy metals, average levels were found in the following order Cr (0.695 ± 0.018) > Zn (0.615 ± 0.016) > Pb (0.323 ± 0.012) > Cu (0.983 ± 0.011) > Cd (0.196 ± 0.011) mg kg−1. The Igeo values from 96% of the analyzed sampling points were below zero and were considered uncontaminated. The other 4% of samples, from the Potrerillo1 (PT) site, had Igeo values of 1.13, where the highest concentration of Cd was found, which indicates moderate contamination levels. The RI index at the PT site was in the category of moderate contamination, and the rest of the points correspond to the category of low pollution.

Environmental and Health Risks of Heavy Metals in Farmland Soils of Drinking Water Protection Areas and a Contaminated Paddy Field in Taiwan

This study assessed heavy metal contents and their mobility, bioaccessibility, environmental risk, and health effects in the farmland soils of Drinking Water Source Quality Protection (DWSQP) areas contaminated by livestock manure and a paddy field contaminated by co-use of irrigation and drainage canals in Taiwan. The risk assessment code (RAC) and synthesis toxicity index (STI) for the soils were obtained. The potential health effects caused from soil direct ingestion by hand-to-mouth activity and dermal contact frequently occurring to farmers were further evaluated. The Cu, Zn, and Cr levels in DWSQP areas and the Changhwa (CH) paddy field exceeded the standards promulgated by Taiwan Environmental Protection Administration (EPA). Nevertheless, RAC in DWSQP areas was in low risk levels. In contrast, RAC from Cu and Zn in CH paddy soils was in medium levels. Non-carcinogenic risks for farmers based on the total and bioaccessible metals in DWSQP areas and CH soils were all <1. However, carcinogenic risks based on bioaccessible Cr still exceeded 10−6 in several soils, indicating that the potential impacts on environmental and human health due to direct and indirect exposures to these contaminated soils should be concerned.