Cobalt in soils: sources, fate, bioavailability, plant uptake, remediation, and management

Cobalt is a trace metal in soil with a relatively low abundance in the earth’s crust. It is known to have some role in plant metabolism. The soil contamination of Co is mostly anthropogenic due to activities like burning of coal or oil, or the production of Co alloys. High concentrations of Co in soil cause an adverse impact on plant growth. The bioavailability of Co depends on soil pH, Co concentration, speciation and partitioning, presence of other cations and anions, Fe and Mn oxides and clay minerals, soil organic matter content, redox conditions, and aging. Its plant uptake is dependent on soil physicochemical properties, total and bioavailable Co concentration in soil, soil organic matter, mineralogy, plant species, and their growth stage, and microbial population. Once absorbed by the plant, Co is primarily accumulated in roots, from where it is translocated and distributed to other plant parts including shoot and leaves. At the subcellular level, it is accumulated in the vacuole, cytoplasm, cell wall, chloroplast, cortex, endodermis, and vasculature tissues. Its accumulation in vacuole and cell wall is regarded as a resistance mechanism for plants to combat Co toxicity. Various physical, chemical, and biological remediation, and management options, such as immobilization using chemical amendments, chemical extraction, electrokinetic remediation, phytostabilization, phytoextraction, biosolubilization, and mycoremediation using mycorrhizal fungal strains have been utilized to remediate Co-contaminated soils.