Impact of physiochemical properties, microbes and biochar on bioavailability of toxic elements in the soil: a review

Lead (Pb2+) biosorption and bioaccumulation efficiency of Enterobacter chuandaensis DGI-2: Isotherm, kinetics and mechanistic study for bioremediation

Heavy metal (HM) contamination, particularly lead (Pb²⁺), threatens environmental and agricultural sustainability, necessitating effective remediation strategies. This study evaluates the Pb²⁺ sequestration potential of Enterobacter chuandaensis DGI-2, an HM-tolerant rhizobacterium isolated from a Pb-contaminated rhizosphere. DGI-2 exhibited high Pb²⁺ removal efficiency, achieving 94.73 % removal at 100 µg/mL and 69.09 % at 750 µg/mL over 96 h, primarily through cell surface and exopolysaccharide (EPS) adsorption. Biosorption studies demonstrated higher Pb²⁺ uptake in living biomass (102.95 mg/g, 68.63 %) than in dead biomass (98.61 mg/g, 65.74 %) under controlled conditions (0.5 g/L biomass, pH-6.5, 720 min). Mechanistic analyses revealed that Pb²⁺ adsorption primarily involved interactions with -OH, -COOH, and -PO₄³ ⁻ functional groups, facilitated by multilayer sorption, complexation, and ion exchange. Moreover, a 210.66 % increase in phosphatase activity promoted Pb²⁺ precipitation, forming stable Pb-phosphate minerals (e.g., Pb₅(PO₄)₃Cl, Pb₁₀(PO₄)₆(OH)₂), as confirmed by X-ray diffraction (XRD), significantly contributing to Pb sequestration. Regeneration studies demonstrated the biomass' reusability over four cycles. Soil microcosm experiments showed an 11.7-13.1 % reduction in bioavailable Pb, with greater stabilization in non-sterile soils, suggesting synergistic effects with native microbiota. Additionally, DGI-2 exhibited plant growth-promoting (PGP) traits, reducing phytotoxicity, enhancing soil health and phytostabilization potential, positioning it as a sustainable biosorbent for Pb²⁺ remediation.

Exploring geochemical distribution of potentially toxic elements (PTEs) in wetland and agricultural soils and associated health risks

This study is carried out to understand the degree of soil pollution, transport mechanism, and distribution pattern of potentially toxic elements (PTEs), including the exposure effects on human health. Towards this, topsoil samples were collected from the Saman wetland and surrounding agricultural fields in the Gangetic plain, India. The results show that the mean concentration of Cu, Hg, Zn, Pb, Th, As, U, and Cd of both soil types exceed the natural background values. The multivariate analysis suggests the soils are moderately contaminated with As, Cd, Zn, Pb, and Hg (possibly from anthropogenic sources) and heavily contaminated with Th and U, likely ascended from geogenic sources. The GIS-based geostatistical plots coupled with principal component analysis (PCA) and hierarchical cluster analysis (HCA) apportion the sources of these toxic elements, which vary greatly and are closely correlated to the geogenic processes and local anthropogenic sources like pesticides and agrochemicals. The health risk assessment revealed that the cumulative hazard index (HI) values of PTEs are lower than the safe level, suggesting no significant noncarcinogenic effect for adults and children. However, excess cancer risk (ECR) values exceed the permissible limit (1 × 10-6), signifying that exposure to the toxic element concentration may cause cancer in the exposed population, most probably in the children subpopulation. Thus, this study highlights the importance of local compliance, ensuring the quality checks and management policies in using pesticides and other agrochemicals containing PTEs to control the imposed cancer risks.

Accumulation characteristics, driving factors, and model prediction of cadmium in soil-highland barley system on the Tibetan Plateau

Cadmium (Cd) poses major human health problems due to its high toxicity and organ bioaccumulation potential. This study collected and analysed 130 pairs of representative soil-highland barley samples on the Tibetan Plateau. The total soil Cd content (Cd-soil), available soil Cd (Cd-ava), and highland barley Cd contents (Cd-barley) ranged from 0.03 to 0.46 mg kg-1, 0.006-0.185 mg kg-1, and 0.57-13.62 μg kg-1, with mean values of 0.19 ± 0.01 mg kg-1, 0.045 ± 0.003 mg kg-1, and 4.57 ± 0.17 μg kg-1, respectively. Redundancy analysis (RDA) demonstrated that geographic factors and soil properties explained 28.46% of the variation in Cd-soil and Cd-ava, and precipitation (14.6%) and pH (9.1%) were the dominant factors. The structural equation model (SEM) indicated that Cd-soil and Cd-ava were predominantly controlled by pH. Furthermore, the Cd-soil, Cd-ava, and Cd-barley with significantly different environmental conditions were more accurately predicted by conditional inference trees-multiple linear regression (CITs-MLR). When Cd-soil is more than 0.376 mg kg-1, Cd-ava obtains the most accurate predictor (R2 =0.64, P < 0.01). This study provides new scientific insight into understanding the environmental biogeochemical nexus of Cd in the complex and fragile plateau environment and evaluating food security on the Tibetan Plateau under the self-sufficiency model of highland barley.

Extraction of Metals from Polluted Soils by Bioleaching in Relation to Environmental Risk Assessment

Environmental pollution has particular implications for the whole geosystem and increases the global risk to human and ecological health. In this regard, investigations were carried out on soil samples to perform the quality status assessment by determining: pH, texture, structure and metal concentration, as well as carrying out an assessment of anthropogenic activity by determining pollution indices: C_f (contamination factor), C_d (degree of contamination), PLI (pollution load index), E_r (ecological risk index) and PERI (potential ecological risk index). Analyses on soil samples showed high concentrations of metals (Cu: 113-2996 mg kg^-1; Pb: 665-5466 mg kg^-1; Cr: 40-187 mg kg^-1; Ni: 221-1708 mg kg^-1). The metal extraction experiments were carried out by bioleaching using Thiobacillusferrooxidans, microorganisms at different amounts of bioleaching solution (20 mL and 40 mL 9K medium) and a stirring time of up to 12 h. The results on the degree of contamination, pollution loading index PLI (2.03-57.23) and potential ecological risk index PERI (165-2298) indicate that the soils in the studied area have a very high degree of pollution. The decontamination procedure by bioleaching showed a decrease, but at the end of the test (12 h), the followed indices indicate high values, suggesting that bioleaching should continue. The depollution yield after 12 h of treatment is, however, encouraging: Cu 29-76%, Pb: 10-32%, Cr: 39-72% and Ni 44-68%. The use of yield-time correlation equations allows the identification of the optimal exposure time on the bioleaching extraction process to obtain optimal results. The aim of the research is to determine the soil quality, soil environmental risk, extraction of metals from polluted soils by bioleaching and to identify influencing factors in achieving high remediation yields.