The mechanistic insights into the leaching behaviors of potentially toxic elements from the indigenous zinc smelting slags under the slag dumping site scenario

Driving factors for distribution and transformation of heavy metals speciation in a zinc smelting site

Heavy metal pollution at an abandoned smelter pose a significant risk to environmental health. However, remediation strategies are constrained by inadequate knowledge of the polymetallic distribution, speciation patterns, and transformation factors at these sites. This study investigates the influence of soil minerals, heavy metal occurrence forms, and environmental factors on heavy metal migration behaviors and speciation transformations. X-ray diffraction analysis revealed that the minerals associated with heavy metals are mainly hematite, franklinite, sphalerite, and galena. Sequential extraction results suggest that lead and zinc are primarily present in the organic-sulfide fractions (F4) and residual form (F5) in the soil, accounting for over 70% of the total heavy metal content. Zinc displayed greater instability in carbonate-bound (16%) and exchangeable (2%) forms. The migration and diffusion patterns of heavy metals in the subsurface environment were visualized through the simulation of labile state heavy metals, demonstrating high congruence with groundwater pollution distribution patterns. The key environmental factors influencing heavy metal stable states (F4 and F5) were assessed by integrating random forest models and redundancy analysis. Primary factors facilitating Pb transformation into stable states were available phosphorus, clay content, depth, and soil organic matter. For Zn, the principal drivers were Mn oxides, soil organic matter, clay content, and inorganic sulfur ions. These findings enhance understanding of the distribution and transformation of heavy metal speciation and can provide valuable insights into controlling heavy metal pollution at non-ferrous smelting sites.

Pollution levels and probability risk assessment of potential toxic elements in soil of Pb-Zn smelting areas

Soil pollution around Pb-Zn smelters has attracted widespread attention around the world. In this study, we compiled a database of eight potentially toxic elements (PTEs) Pb, Zn, Cd, As, Cr, Ni, Cu, and Mn in the soil of Pb-Zn smelting areas by screening the published research papers from 2000 to 2023. The pollution assessment and risk screening of eight PTEs were carried out by geo-accumulation index (Igeo), potential ecological risk index (PERI) and health risk assessment model, and Monte Carlo simulation employed to further evaluate the probabilistic health risks. The results suggested that the mean values of the eight PTEs all exceeded the corresponding values in the upper crust, and more than 60% of the study sites had serious Pb and Cd pollution (Igeo > 4), with Brazil, Belgium, China, France and Slovenia having higher levels of pollution than other regions. Besides, PTEs in smelting area caused serious ecological risk (PERI = 10912.12), in which Cd was the main contributor to PREI (86.02%). The average hazard index (HI) of the eight PTEs for adults and children was 7.19 and 9.73, respectively, and the average value of total carcinogenic risk (TCR) was 4.20 × 10-3 and 8.05 × 10-4, respectively. Pb and As are the main contributors to non-carcinogenic risk, while Cu and As are the main contributors to carcinogenic risk. The probability of non-carcinogenic risk in adults and children was 84.05% and 97.57%, while carcinogenic risk was 92.56% and 79.73%, respectively. In summary, there are high ecological and health risks of PTEs in the soil of Pb-Zn smelting areas, and Pb, Cd, As and Cu are the key elements that cause contamination and risk, which need to be paid attention to and controlled. This study is expected to provide guidance for soil remediation in Pb-Zn smelting areas.

The longevity evaluation of multi-metal stabilization by MgO in Pb/Zn smelter-contaminated soils

The re-mobilization risks of potentially toxic elements (PTEs) during stabilization deserve to be considered. In this study, artificial simulation evaluation methods based on the environmental stress of freeze-thaw (F-T), acidification and variable pH were conducted to assess the long-term effectiveness of PTEs stabilized by MgO in Pb/Zn smelter contaminated soils. Among common stabilizing materials, MgO was considered as the best remediation material, since PTEs bioavailability reduced by 55.48% for As, 19.58% for Cd, 10.57% for Cu, and 26.33% for Mn, respectively. The stabilization effects of PTEs by MgO were best at the dosage of 5 wt%, but these studied PTEs would re-mobilize after 30 times F-T cycles. Acid and base buffering capacity results indicated that the basicity of contaminated soils with MgO treatment reduced under F-T action, and the leached PTEs concentrations would exceed the safety limits of surface water quality standard in China (GB3838-2002) after acidification of 2325 years. No significant changes were found in the pH-dependent patterns of PTEs before and after F-T cycles. However, after F-T cycles, the leaching concentrations of PTEs increased due to the destruction of soil microstructure and the functionality of hydration products formed by MgO, as indicated by scanning electron microscopy (SEM) coupled with energydispersive Xray spectroscopy (EDS) results. Hence, these findings would provide beneficial references for soil remediation assessments of contaminated soils under multi-environmental stress.

Organic amendment application affects the release behaviour, bioavailability, and speciation of heavy metals in zinc smelting slag: Insight into dissolved organic matter

Organic amendments are commonly used in assisted phytostabilization of mine wastes by improving their physicochemical and biological properties. These amendments are susceptible to leaching and degradation, resulting in the generation of dissolved organic matter (DOM), which significantly influences the geochemical behaviour of heavy metals (HMs). However, the geochemical behaviour of HMs in metal smelting slag driven by organic amendment-derived DOM remains unclear. In this study, we investigated the impact of cow manure-derived DOM on the release behaviour, bioavailability, and speciation of HMs (Cu, Pb, Zn, and Cd) in zinc smelting slag using a multidisciplinary approach. The results showed that DOM enhanced the weathering of the slag, with a minimal impact on the slag's mineral phases, except for causing gypsum dissolution. The DOM addition resulted in a slight increase in HM release from the slag during the initial inoculation period, followed by a reduction in HM release during the later period. Furthermore, the DOM addition increased the diversity and relative abundance of the bacterial community. This, in turn, led to a decrease in the dissolved organic carbon (DOC) content and enhanced the transformation of labile DOM compounds into recalcitrant compounds. The variation in HM release during various inoculation periods can be attributed to the bacterial decomposition and transformation of DOM, which further enhanced the transformation of HM fractions. Specifically, during the later period, DOM promoted the conversion of a portion of the reducible and oxidizable fractions of Cu, Pb, and Zn into the acid-soluble and residual fractions. Moreover, it partially transformed the reducible, oxidizable, and residual fractions of Cd into the acid-soluble fraction. Overall, this study provides new insights into the geochemical behaviour of HMs in slag governed by the coupling effect of DOM and the bacterial community. These findings have implications for the use of organic amendments in assisted phytostabilization of metal smelting slag. ENVIRONMENTAL IMPLICATION: Metal smelting slag is hazardous due to its high levels of HMs, and its improper disposal has serious consequences for the ecosystem. Organic amendments are employed in assisted phytostabilization of the slag site by improving its microecological properties. However, the impact of organic amendment-derived DOM on HM migration and transformation in slag remains unclear. This study indicated that the coupling effects of DOM and microbes governed the geochemical behaviour of HMs in slag. These findings provide new insights into how organic amendments impact the geochemical behaviour of HMs in slag, contributing to the development of phytostabilization technology.

Inhibition of high sulfur on functional microorganisms and genes in slightly contaminated soil by cadmium and chromium

It is generally accepted that sulfur can passivate the bioavailability of heavy metals in soil, but it is not clear whether high sulfur in cadmium (Cd) and chromium (Cr) contaminated soil has negative effect on soil microbial community and ecological function. In this study, total sulfur (TS) inhibited the Chao 1, Shannon, Phylogenetic diversity (Pd) of bacterial and Pd of fungi in slightly contaminated soil by Cd and Cr around pyrite. TS, total potassium, pH, total chromium, total cadmium, total nitrogen, soil organic matter were the predominant factors for soil microbial community; the contribution of TS in shaping bacterial and fungal communities ranked at first and fifth, respectively. Compared with the low sulfur group, the abundance of sulfur sensitive microorganisms Gemmatimonas, Pseudolabrys, MND1, and Schizothecium were decreased by 68.79-97.22% (p < 0.01) at high sulfur one; the carbon fixation, nitrogen cycling, phosphorus cycling and resistance genes abundance were significantly lower (p < 0.01) at the latter. Such variations were strongly and closely correlated to the suppression of energy metabolism (M00009, M00011, M00086) and carbon fixation (M00173, M00376) functional module genes abundance in the high sulfur group. Collectively, high sulfur significantly suppressed the abundances of functional microorganisms and functional genes in slightly contaminated soil with Cd and Cr, possibly through inhibition of energy metabolism and carbon fixation of functional microorganisms. This study provided new insights into the environmental behavior of sulfur in slightly contaminated soil with Cd and Cr.

An integrated overview of metals contamination, source-specific risks investigation in coal mining vicinity soils

Heavy metals in soil are harmful to natural biodiversity and human health, and it is difficult to estimate the effects accurately. To reduce pollution and manage risk in coal-mining regions, it is essential to evaluate risks for heavy metals in soil. The present study reviews the levels of 21 metals (Nb, Zr, Ag, Ni, Na, K, Mg, Rb, Zn, Ca, Sr, As, Cr, Fe, Pb, Cd, Co, Hg, Cu, Mn and Ti) in soils around Barapukuria coal-mining vicinity, Bangladesh which were reported in literature. An integrated approach for risk assessments with the positive matrix factorization (PMF) model, source-oriented ecological and health hazards were applied for the study. The contents of Rb, Ca, Zn, Pb, As, Ti, Mn, Co, Ag, Zr, and Nb were 1.63, 1.10, 1.97, 14.12, 1.20, 3.13, 1.22, 3.05, 3.85, 5.48, and 7.21 times greater than shale value. About 37%, 67%, 12%, and 85% of sampling sites posed higher risks according to the modified contamination factor, Nemerow pollution index, Nemerow integrated risk index, and mean effect range median quotient, respectively. Five probable metal sources were computed, including industrial activities to coal mining (17%), agricultural activities (33%), atmospheric deposition (19%), traffic emission (16%), and natural sources (15%). Modified Nemerow integrated risk index reported that agricultural activities, industrial coal mining activities, and atmospheric deposition showed moderate risk. Health hazards revealed that cancer risk values computed by the PMF-HHR model with identified sources were higher than the standard value (1.0E-04) for children, adult male, and female. Agricultural activities showed higher cancer risks to adult male (39%) and children (32%) whereas traffic emission contributed to female (25%). These findings highlight the ecological and health issues connected to potential sources of metal contamination and provide useful information to policymakers on how to reduce such risks.

Effect of simulated root exudates on the distribution, bioavailability, and fractionation of potentially toxic elements (PTEs) in various particle size fractions of zinc smelting slag: Implication of direct revegetation

Direct revegetation is a promising strategy for phytostabilization of metal smelting slag sites. Slag comes into direct contact with root exudates when slag sites undergo direct revegetation. The slag particle size fractions are considered the key factor influencing the geochemical behaviour of potentially toxic elements (PTEs). However, the effects of root exudates on the geochemical behaviours of PTEs in various slag particle size fractions remain unclear. Here, the effects of simulated root exudates of perennial ryegrass (Lolium perenne) directly revegetated at a zinc smelting slag site on the distribution, bioavailability, and fractionation of PTEs (Cu, Pb, Zn, and Cd) in various slag particle size fractions were investigated. The results showed that PTEs mainly occurred in the <1 mm slag particles; the mass loads of PTEs in the <1 mm slag particles were higher than those in the >1 mm slag particles. The bioavailability of Cu, Zn, and Cd rather than Pb in the slag increased as the particle size decreased. There was a decrease in the <0.25 and 1-2 mm slag particles and an increase in the 0.25-0.5, 0.5-1, and >2 mm slag particles in the presence of root exudates. Root exudates enhanced the transformation of acid-soluble PTEs into other more stable fractions in various slag particle size fractions. Root exudates enhanced the aggregation of slag particles associated with the migration of PTEs, causing differences in the geochemical behaviour of PTEs in various slag particle size fractions. These findings are beneficial for understanding the geochemical behaviour of PTEs in metal smelting slags undergoing direct revegetation and provide an important basis for the guidance of environmental risk management of the revegetated metal smelting slag sites.

Vertical distribution of nutrients, enzyme activities, microbial properties, and heavy metals in zinc smelting slag site revegetated with two herb species: Implications for direct revegetation

Direct revegetation is an important measure to immobilize heavy metals and improve the microecological properties of metal smelting slag sites. However, the vertical distribution of nutrients, microecological properties, and heavy metals at a directly revegetated metal smelting slag site remains unclear. Here, the distribution characteristics of nutrients, enzyme activities, microbial properties, and heavy metals in the vertical profile at a zinc smelting slag site directly revegetated with two herb species (Lolium perenne and Trifolium repens) for 5 years were investigated. The results showed that the nutrient contents, enzyme activities, and microbial properties decreased with increasing slag depth after revegetation with the two herb species. The nutrient contents, enzyme activities, and microbial properties of the surface slag revegetated with Trifolium repens were better than those in the surface slag revegetated with Lolium perenne_. The higher root activity in the surface slag (0-30 cm) resulted in relatively higher contents of pseudo-total and available heavy metals in the surface slag. Moreover, the contents of pseudo-total heavy metals (except for Zn) and available heavy metals in the slag revegetated with Trifolium repens were lower than those in the slag revegetated with Lolium perenne at most slag depths. Overall, the greater phytoremediation efficiency of the two herb species occurred mainly in the surface slag (0-30 cm), and the phytoremediation efficiency of Trifolium repens was higher than that of Lolium perenne. The findings are beneficial for understanding the phytoremediation efficiency of direct revegetation strategies for metal smelting slag sites.

Shewanella oneidensis MR-1 and oxalic acid mediated vanadium reduction and redistribution in vanadium-containing tailings

The microbially- and chemically-mediated redox process is critical in controlling the fate of vanadium (V) in tailing environment. Although the microbial reduction of V has been widely studied, the coupled biotic reduction mediated by beneficiation reagents and the underlying mechanism remain unclear. Herein, the reduction and redistribution of V in V-containing tailings and Fe/Mn oxide aggregates mediated by Shewanella oneidensis MR-1 and oxalic acid were explored. The dissolution of Fe-(hydr)oxides by oxalic acid promoted the microbe-mediated V release from solid-phase. After 48-day of reaction, the dissolved V concentrations in the bio-oxalic acid treatment reached maximum values of 1.72 ± 0.36 mg L^-1 and 0.42 ± 0.15 mg L^-1 in the tailing system and the aggregate system, respectively, significantly higher than those in control (0.63 ± 0.14 mg L^-1 and 0.08 ± 0.02 mg L^-1). As the electron donor, oxalic acid enhanced the electron transfer process of S. oneidensis MR-1 for V(V) reduction. The mineralogical characterization of final products indicates that S. oneidensis MR-1 and oxalic acid promoted solid-state conversion from V₂O₅ to NaV₆O₁₅. Collectively, this study demonstrates that microbe-mediated V release and redistribution in solid-phase were promoted by oxalic acid, suggesting that the role of organic agents for the V biogeochemical cycle in natural systems deserves greater attention.

Environmental and health impacts assessment of long-term naturally-weathered municipal solid waste incineration ashes deposited in soil-old burden in Bratislava city, Slovakia

Municipal solid waste incineration (MSWI) is an effective method for reducing the volume/mass of waste. However, MSWI ashes contain high concentrations of many substances, including trace metal (loid)s, that could be released into the environment and contaminate soils and groundwater. In this study, attention was focused on the site near the municipal solid waste incinerator where MSWI ashes are deposited on the surface without any control. Here, combined results (chemical and mineralogical analyses, leaching tests, speciation modelling, groundwater chemistry and human health risk assessment) are presented to assess the impact of MSWI ash on the surrounding environment. The mineralogy of ∼forty years old MSWI ash was diverse, and quartz, calcite, mullite, apatite, hematite, goethite, amorphous glasses and several Cu-bearing minerals (e.g. malachite, brochantite) were commonly detected. In general, the total concentrations of metal (loid)s in MSWI ashes were high, following the order: Zn (6731 mg/kg) > Ba (1969 mg/kg) ≈ Mn (1824 mg/kg) > Cu (1697 mg/kg) > Pb (1453 mg/kg) > Cr (247 mg/kg) > Ni (132 mg/kg) > Sb (59.4 mg/kg) > As (22.9 mg/kg) ≈ Cd (20.6 mg/kg). Cadmium, Cr, Cu, Pb, Sb and Zn exceeded the indication or even intervention criteria for industrial soils defined by the Slovak legislation. Batch leaching experiments with diluted citric and oxalic acids that simulate the leaching of chemical elements under rhizosphere conditions documented low dissolved fractions of metals (0.00-2.48%) in MSWI ash samples, showing their high geochemical stability. Non-carcinogenic and carcinogenic risks were below the threshold values of 1.0 and 1 × 10^-6, respectively, with soil ingestion being the most important exposure route for workers. The groundwater chemistry was unaffected by deposited MSWI ashes. This study may be useful in determining the environmental risks of trace metal (loid)s in weathered MSWI ashes that are loosely deposited on the soil surface.

Enrichment and distribution characteristics of heavy metal(loid)s in native plants of abandoned farmlands in sewage irrigation area

Soil samples and native plants were collected from abandoned farmlands with a long history of sewage irrigation along Dongdagou stream, Baiyin City. We investigated the concentrations of heavy metal(loid)s (HMMs) in soil-plant system to evaluate the accumulation and transportation ability of HMMs in native plants. Results indicated that soils in study area were severely polluted by Cd, Pb, and As. With the exception of Cd, the correlation between total HMM concentrations in soil and plant tissues was poor. Among all investigated plants, no one was close to the criteria for the HMM concentrations of hyperaccumulators. The concentrations of HMMs in most plants were reached the phytotoxic level and the abandoned farmlands could not be used as forages, which showed that native plants may possess resistance capabilities or high tolerance for As, Cu, Cd, Pb, and Zn. The FTIR (Fourier transform infrared spectrometer) results suggested that the detoxification of HMMs in plants may depend on the functional groups (-OH, C-H, C-O, and N-H) of some compounds. Bioaccumulation factor (BAF), bioconcentration factor (BCF), and biological transfer factor (BTF) were used to identify the accumulation and translocation characteristics of HMMs by native plants. S. glauca had the highest mean values of BTF for Cd (8.07) and Zn (4.75). C. virgata showed the highest mean BAFs for Cd (2.76) and Zn (9.43). P. harmala, A. tataricus, and A. anethifolia also presented high accumulation and translocation abilities for Cd and Zn. High HMMs (As, Cu, Cd, Pb, and Zn) accumulation in the aerial parts of plants may lead to increased accumulation of HMMs in the food chain; additional research is desperately required. This study demonstrated the HM enrichment characteristics of weeds and provided a basis for the management of abandoned farmlands.

The geochemical and mineralogical controls on the release characteristics of potentially toxic elements from lead/zinc (Pb/Zn) mine tailings

Large quantities of lead/zinc (Pb/Zn) mine tailings were deposited at tailings impoundments without proper management, which have posed considerable risks to the local ecosystem and residents in mining areas worldwide. Therefore, the geochemical behaviors of potentially toxic elements (PTEs) in tailings were in-depth investigated in this study by a coupled use of batch kinetic tests, statistical analysis and mineralogical characterization. The results indicated that among these studied PTEs, Cd concentration fluctuated within a wide range of 0.83-6.91 mg/kg, and showed the highest spatial heterogeneity. The mean Cd concentrations generally increased with depth. Cd were mainly partitioned in the exchangeable and carbonate fractions. The release potential of PTEs from tailings was ranged as: Cd > Mn > Zn > Pb > As, Cd > Pb > Zn > Mn > As and Cd > Pb > Mn > Zn > As, respectively, under the assumed environmental scenarios, i.e. acid rain, vegetation restoration, human gastrointestinal digestion. The results from mineralogical characterization indicated that quartz, sericite, calcite and pyrite were typical minerals, cumulatively accounting for over 80% of the tailings. Sulfides (arsenopyrite, galena, and sphalerite), carbonates (calcite, dolomite, cerussite and kutnahorite), oxides (limonite) were identified as the most relevant PTEs-bearing phases, which significantly contributed to PTEs release from tailings. A combined result of statistical, geochemical and mineralogical approaches would be provided valuable information for the alteration characteristics and contaminant release of Pb/Zn mine tailings.