Optimization of Soil Washing to Reduce the Selenium Levels of Seleniferous Soil from Punjab, Northwestern India

New Insights into Selenium Enrichment in the Soil of Northwestern Guizhou, Southwest China

Selenium (Se) is an essential trace element for animals and plants. Se in soil has an important influence on the Se intake by animals and plants. To explore the source of Se in soils of the zinc-smelting area in northwest Guizhou, China, 271 topsoils and 50 deep soil samples were collected, and the concentration, speciation and distribution of Se in soils were analysed. The results showed that the concentration of Se in topsoils ranged from 0.2 mg/kg to 1.79 mg/kg, with an average of 0.84 mg/kg, which was more than 2 times of that in deep soil. These observations indicated that Se was enriched in the surface layer of soil. In terms of spatial distribution, high-Se topsoils (> 1.0 mg/kg) were mainly distributed near the zinc smelting area, and topsoil samples with relatively low content of Se were mainly distributed in areas with less human activities influence. The Se occurrence species in topsoils were in the order of residue, organic-binding, humic-acid binding, water-soluble, Fe/Mn/Al oxide-binding, carbonate-binding and ion-exchange. The contribution of residual Se to total Se in topsoil was decisive, and the content of other species of selenium changed slightly. The Se speciation that was residual in soil is difficult for plants to utilize, which is consistent with previous studies on seleniferous soils caused by zinc smelting. These results indicated that the main reason for Se enrichment in the topsoils of northwest Guizhou Province was indigenous zinc smelting.

Selenium Toxicity in Plants and Environment: Biogeochemistry and Remediation Possibilities

Selenium (Se) is a widely distributed trace element with dual (beneficial or toxic) effects for humans, animals, and plants. The availability of Se in the soil is reliant on the structure of the parental material and the procedures succeeding to soil formation. Anthropogenic activities affect the content of Se in the environment. Although plants are the core source of Se in animal and human diet, the role of Se in plants is still debatable. A low concentration of Se can be beneficial for plant growth, development, and ecophysiology both under optimum and unfavorable environmental conditions. However, excess Se results in toxic effects, especially in Se sensitive plants, due to changing structure and function of proteins and induce oxidative/nitrosative stress, which disrupts several metabolic processes. Contrary, Se hyperaccumulators absorb and tolerate exceedingly large amounts of Se, could be potentially used to remediate, i.e., remove, transfer, stabilize, and/or detoxify Se-contaminants in the soil and groundwater. Thereby, Se-hyperaccumulators can play a dynamic role in overcoming global problem Se-inadequacy and toxicity. However, the knowledge of Se uptake and metabolism is essential for the effective phytoremediation to remove this element. Moreover, selecting the most efficient species accumulating Se is crucial for successful phytoremediation of a particular Se-contaminated area. This review emphasizes Se toxicity in plants and the environment with regards to Se biogeochemistry and phytoremediation aspects. This review follows a critical approach and stimulates thought for future research avenues.

Selenium Toxicity in Plants and Environment: Biogeochemistry and Remediation Possibilities

Selenium (Se) is a widely distributed trace element with dual (beneficial or toxic) effects for humans, animals, and plants. The availability of Se in the soil is reliant on the structure of the parental material and the procedures succeeding to soil formation. Anthropogenic activities affect the content of Se in the environment. Although plants are the core source of Se in animal and human diet, the role of Se in plants is still debatable. A low concentration of Se can be beneficial for plant growth, development, and ecophysiology both under optimum and unfavorable environmental conditions. However, excess Se results in toxic effects, especially in Se sensitive plants, due to changing structure and function of proteins and induce oxidative/nitrosative stress, which disrupts several metabolic processes. Contrary, Se hyperaccumulators absorb and tolerate exceedingly large amounts of Se, could be potentially used to remediate, i.e., remove, transfer, stabilize, and/or detoxify Se-contaminants in the soil and groundwater. Thereby, Se-hyperaccumulators can play a dynamic role in overcoming global problem Se-inadequacy and toxicity. However, the knowledge of Se uptake and metabolism is essential for the effective phytoremediation to remove this element. Moreover, selecting the most efficient species accumulating Se is crucial for successful phytoremediation of a particular Se-contaminated area. This review emphasizes Se toxicity in plants and the environment with regards to Se biogeochemistry and phytoremediation aspects. This review follows a critical approach and stimulates thought for future research avenues.

Sorption Mechanism and Optimization Study for the Bioremediation of Pb(II) and Cd(II) Contamination by Two Novel Isolated Strains Q3 and Q5 of Bacillus sp

The use of bacterial strains as agents in bioremediation processes could reduce the harmfulness of potential toxic elements (PTEs) from water and soil with low or even no impact on the natural ecosystems. In this study, two new metal resistant-bacterial strains (Q3 and Q5) of Bacillus sp. were isolated from a sulfurous spring and their potential (as pure cultures or mixed) to remove Pb(II) and Cd(II) from an aqueous matrix was evaluated and optimized using response surface methodology (RSM). The optimal conditions for Cd(II) removal from all tested strains combinations were observed at an initial pH 5, a temperature of 38 °C, and an initial Cd(II) concentration of 50 mg L-1, while the performance of bacterial strains on Pb(II) removal was strongly correlated to initial pH and temperature conditions. Moreover, the efficiency of bacterial strains in removing both PTEs, Pb(II) and Cd(II), from an aqueous matrix was considerably higher when they were used as a mixed culture rather than pure. According to field emission SEM (FESEM) and EDS analysis, the two bacterial strains showed different mechanisms in removing Cd(II): Bacillus sp. Q5 bio-accumulated Cd(II) in its periplasmic space, whereas Bacillus sp. Q3 bio-accumulated Cd(II) on its cell surface. On the other hand, Pb(II) is removed by chemical precipitation (lead sulfide) induced by both Bacillus sp. Q3 and Q5. This study discloses new aspects of Pb(II) and Cd(II) bioremediation mechanisms in Bacillus species that can be extremely useful for designing and operating novel PTEs bioremediation processes.