Advances of selenium migration and microbial transformation in the environment and the application of seleniumnanoparticles

Abstract

As one of the indispensable trace elements for humans and animals, selenium (Se) plays an important role in the geochemical cycle. Se in nature is usually released from selenium-rich sources such as phosphorite and coal mine by weathering. Then it is migrated and transformed in ecosystems through sedimentation, volatilization, atmospheric circulation and biological action. Although Se occurs in environment naturally, about 37%-40% of Se in the atmosphere and water comes from human activities, which result in an uneven distribution of Se in environment. Studies have shown that human health can be affected easily by Se deficiency or excess, and the minimum demand of Se required by the body is 40 μg d-1. When the concentration of ingested Se exceeds 400 μg d-1, it will cause toxic effects on the human body. Therefore, it is necessary to study the global migration and transformation of Se. Selenium circulates globally through the atmospheric, oceanic and terrestrial systems and generally exists in four valence states: Se(-II), Se(0), Se(IV) and Se(VI), where the Se(IV) is the most toxic. Selenium compounds in the atmosphere mainly have three kinds of transformation pathways: volatile organic selenium compounds, volatile inorganic selenium compounds and elemental selenium that attached to particle surface. Atmospheric selenium enters the soil and plants through sedimentation. In soil, the most common forms of Se are SeO42- and SeO32-. When the soil is under oxidizing conditions, SeO42- is the main selenium-containing compound, while SeO32- is dominant in reducing conditions. On the one hand, Se in the soil can be ingested by plants, which depends on various factors including climatic and soil parameters, as well as the accumulation capacity. On the other hand, a part of Se can be transferred to natural water. The concentration of Se in natural water is usually less than 10 g L-1, but Se concentration are as high as 140-1400 g L-1 in some selenium-rich agricultural drainages. Among various pathways, the transformation of selenium mediated by microbes is particularly important, including dissimilatory reduction, assimilatory reduction, oxidation, methylation and demethylation, wherein the dissimilatory reduction is the main way to remove toxic SeO42- and SeO32- from environmental media. For SeO42- reduction mechanism, there exists significant differences in the reduction process due to the different cell structure between gram-negative and gram-positive bacteria. Additionally, the mechanism of SeO32- dissimilation reduction could be divided into enzyme promotion and non-enzymatic reaction. SeO42- and SeO32- can be transformed into Se2- by assimilatory reduction, and then involved in the generation of selenoprotein. The methylation process may also have great potential in soil selenium pollution control. It is generally considered that the toxify of Se will increase after methylated, but the new substances, such as trimethylated selenium ion, usually have low bioavailability. In addition, most volatile selenium compounds are produced by dimethyl diselenium, and this process can effectively remove Se from soil. Selenium nanoparticles (SeNPs), as the main reduction products, have been widely used in many fields such as electronic, medical, catalysis and environmental remediation, due to their unique physical and chemical properties. However, relevant studies on the synthesis of bio-SeNPs, process regulation and mechanism analysis are still in the preliminary exploration stage. Therefore, this article systematically reviewed the migration and transformation processes of selenium in the environment, focusing on the transformation mechanisms of selenium by microorganisms, and the applications of biosynthetic SeNPs in the fields of chemical sensors, anticancer and catalysis. Overall, this review aims to provide necessary information and theoretical references for revealing the geochemical cycle of selenium, mechanisms of microbial transformation and broadening the application fields of bio-SeNPs.