A typical case study from smelter-contaminated soil: new insights into the environmental availability of heavy metals using an integrated mineralogy characterization

Integrating HYDRUS-2D and Bayesian Networks for simulating long-term sludge land application: Uncovering heavy metal mobility and pollution risk in the soil-groundwater environment

The release of sludge-derived heavy metals (HMs) to soil and their subsequent migration into groundwater poses a significant challenge for safe and low-carbon sludge land application. This study developed a predictive framework to simulate 60-year sludge land application, evaluating the risk of HMs pollution in the soil-groundwater environment and assessing the influence of soil and water properties. HYDRUS-2D simulations revealed that highly mobile Cu, Ni, and Zn penetrated a 10 m soil layer over a 60-year period, contributing to groundwater pollution. In contrast, Cr was easily sequestered within the topsoil layer after 5-years continuous operation. The non-equilibrium parameter α could serve as an indicator for assessing their potential risk. Furthermore, the limited soil adsorption sites for Pb (f = 0.02772) led to short-term (1-year) groundwater pollution at a 0.5 m-depth. Bayesian Networks model outcomes indicated that humic-like organics crucially influenced HMs transformation, enhancing the desorption of Cd, Cu, Ni, Pb, and Zn, while inhibiting the desorption for Cr. Additionally, electrical conductivity promoted the release of most HMs, in contrast to the Mn mineralogy in soil. This study bridges the gap between the macro-level HMs migration trends and the micro-level adsorption-desorption characteristics, providing guidance for the safe land application of sewage sludge. ENVIRONMENTAL IMPLICATION: This study introduces a framework integrating HYDRUS-2D simulations with Bayesian Networks to assess the risks of groundwater pollution by heavy metals (HMs) over a 60-year sludge application. Sludge-derived Cu, Ni, and Zn are found to penetrate soil up to 10 m and exceed safety limits, with the non-equilibrium parameter α serving as an indicator for pollution risk. The importance of nutrients from sludge-amended soil for the transformation of HMs in the subsurface environment highlights the need for enhanced sludge management, specifically through more detailed regulation of nutrient composition. These findings contribute to developing precise strategies for the long-term sludge land application.

Distinguishing the contributions of different smelting emissions to the spatial risk footprints of toxic elements in soil using PMF, Bayesian isotope mixing models, and distance-based regression

Toxic element pollution of soils emanating from smelting operations is an escalating global concern due to its severe impact on ecosystems and human health. In this study, soil samples were collected and analyzed to quantify the risk contributions and delineate the spatial risk footprints from smelting emissions for 8 toxic elements. A comprehensive health risk contribution and delineation framework was utilized, consisting of Positive matrix factorization (PMF), spatial interpolation, an advanced Bayesian isotope mixing model via Mixing Stable Isotope Analysis in R (MixSIAR), and distance-based regression. The results showed that the mean concentrations of As, Cd, Cu, Hg, Pb, and Zn exceeded the background levels, indicating substantial contamination. Three sources were identified using the PMF model and confirmed by spatial interpolation and MixSIAR, with contributions ranked as follows: industrial wastewater discharge and slag runoff from the smelter site (48.9 %) > natural geogenic inputs from soil parent materials (26.7 %) > atmospheric deposition of dust particles from smelting operations (24.5 %). Among the identified sources, smelter runoff posed the most significant risk, accounting for 97.9 % of the non-carcinogenic risk (NCR) and 59.9 % of the carcinogenic risk (CR). Runoff also drove NCR and CR exceedances at 7.8 % and 4.7 % of sites near the smelter, respectively. However, atmospheric deposition from smelting emissions affected soils across a larger 0.8 km radius. Although it posed lower risks, contributing just 1.1 % to NCR and 22.6 % to CR due to the limited elevation of toxic elements, deposition reached more distant soils. Spatial interpolation and distance-based regression delineated high NCR and CR exposure hotspots within 1.4 km for runoff and 0.8 km for deposition, with exponentially diminishing risks at further distances. These findings highlight the need for pathway-specific interventions that prioritize localized wastewater containment and drainage controls near the smelter while implementing broader regional air pollution mitigation measures.

Heavy metals pollution from smelting activities: A threat to soil and groundwater

Throughout the literature, the word "heavy metal" (HM) has been utilized to describe soil contamination; in this context, we characterize it as those elements with a density greater than 5 g per cubic centimeter. Contamination is one of the major global health concerns, especially in China. China's rapid urbanization over the past decades has caused widespread urban water, air, and soil degradation. This study provides a complete assessment of the soil contamination caused by heavy metals in China's mining and smelting regions. The study of heavy metals (HMs) includes an examination of their potential adverse impacts, their origins, and strategies for the remediation of soil contaminated by heavy metals. The presence of heavy metals in soil can be linked to both natural and anthropogenic processes. Studies have demonstrated that soils contaminated with heavy metals present potential health risks to individuals. Children are more vulnerable to the effects of heavy metal pollution than adults. The results highlight the significance of heavy metal pollution caused by mining and smelting operations in China. Soil contaminated with heavy metals poses significant health concerns, both carcinogenic and non-carcinogenic, particularly to children and individuals living in heavily polluted mining and smelting areas. Implementing physical, chemical, and biological remediation techniques is the most productive approach for addressing heavy metal-contaminated soil. Among these methods, phytoremediation has emerged as a particularly advantageous option due to its cost-effectiveness and environmentally favorable characteristics. Monitoring heavy metals in soils is of utmost importance to facilitate the implementation of improved management and remediation techniques for contaminated soils.

Effects of coal-fired power plants on soil microbial diversity and community structures

Long-term deposition of atmospheric pollutants emitted from coal combustion and their effects on the eco-environment have been extensively studied around coal-fired power plants. However, the effects of coal-fired power plants on soil microbial communities have received little attention through atmospheric pollutant deposition and coal-stacking. Here, we collected the samples of power plant soils (PS), coal-stacking soils (CSS) and agricultural soils (AS) around three coal-fired power plants and background control soils (BG) in Huainan, a typical mineral resource-based city in East China, and investigated the microbial diversity and community structures through a high-throughput sequencing technique. Coal-stacking significantly increased (p < 0.05) the contents of total carbon, total nitrogen, total sulfur and Mo in the soils, whereas the deposition of atmospheric pollutants enhanced the levels of V, Cu, Zn and Pb. Proteobacteria, Actinobacteria, Thaumarchaeota, Thermoplasmata, Ascomycota and Basidiomycota were the dominant taxa in all soils. The bacterial community showed significant differences (p < 0.05) among PS, CSS, AS and BG, whereas archaeal and fungal communities showed significant differences (p < 0.01) according to soil samples around three coal-fired power plants. The predominant environmental variables affecting soil bacterial, archaeal and fungal communities were Mo-TN-TS, Cu-V-Mo, and organic matter (OM)-Mo, respectively. Certain soil microbial genera were closely related to multiple key factors associated with stacking coal and heavy metal deposition from power plants. This study provided useful insight into better understanding of the relationships between soil microbial communities and long-term disturbances from coal-fired power plants.

Source identification and health risk assessment of heavy metals with mineralogy: the case of soils from a Chinese industrial and mining city

Understanding the precise sources of heavy metals (HMs) in soil and the contribution of these sources to health risks has positive effects in terms of risk management. This study focused on the HMs in the soil of five land uses in an industrial and mining city. The sources of HMs in soils were identified, and the soil mineralogical characteristics and health risks of HMs were discussed. The results showed that the HMs (Cu, Zn, Ni, Cd, Pb) found in the soil of the five land uses were affected by human activities. For example, the Cu in grassland, gobi beach, woodland, green belt, and farmland is 22.3, 3.5, 22.5, 16.7, and 21.3 times higher than the soil background values in Gansu Province, respectively. The Positive Matrix Factorization model (PMF) results revealed that traffic emissions and industrial and agricultural activities were the primary sources of HMs in the soil, with industrial sources accounting for the largest share at 55.79%. Furthermore, various characteristics proved that the studied HMs were closely related to smelting products. Concentration-oriented health risk assessments showed that HMs in the different soil types held non-carcinogenic and carcinogenic risks for children and adults. Contamination source-oriented health risk assessments of children and adults found that industrial activities controlled non-carcinogenic and carcinogenic risks. This study highlighted the critical effects of smelting on urban soil and the contribution of pollution sources to health risks. Furthermore, this work is significant in respect of the risk control of HMs in urban soils.

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 changes in the physicochemical properties of calcareous soils and the factors of arsenic (As) uptake by wheat were investigated after the cessation of effluent irrigation for nearly 20 years

It is not known what the buffering capacity of soils and arsenic (As) enrichment by crops is for calcareous agricultural soils after the end of long-term effluent irrigation. In this study, changes in soil physicochemical properties and factors of influencing As uptake by wheat were investigated in agricultural soils where sewage irrigation had been ceased for nearly 20 years. The results showed that the content of CaCO₃ and pH in soil increased compared to the period before the cessation of sewage irrigation, but remained below the soil background value. Furthermore, CaCO₃ is by far the main buffering substance in agricultural soils and indirectly contributes to the increase in pH. The As concentration in the soil was 36.4 ± 34.8 mg/kg, which was 0.56-10.28 times and 0.28-5.18 times higher than the soil background and risk screening values, respectively, but showed a decreasing trend. pH and Fe dissolution were the main reasons for the lower As concentration in the soil. Total As in soil was a better predictor of As in wheat, and soil electrical conductivity (EC) and soil organic matter (SOM) promoted As uptake by wheat. The competitive uptake of As by dissolved Si was an important reason for the mismatch between As concentrations in soil and wheat. This study highlighted the key issues of As transport transformation in soil-wheat systems after cessation of effluent irrigation, using agricultural soils, and provided a reference for soil risk management in agricultural soils in mining areas.