Mercury methylation and its accumulation in rice and paddy soil in degraded lands: A critical review

Abstract

Rice and paddies soil in the mining degraded areas are mostly contaminated with toxic metals like mercury (Hg) Hg is a global pollutant which can change into methylmercury (MeHg), the highly toxic and bio-accumulative organic form. This review paper highlights various aspects of Hg including its methylation and uptake by rice (a staple food) in degraded soil and its associated human health effects. It has been found that MeHg in rice grown in unrestrained MeHg areas close to mining degraded sites, contain over (100 μ g/kg) in the edible part (grain) and shown to be over ten times higher than other food crops. This manuscript examines Hg speciation in paddy field soils with various levels of Hg contamination. Many factors including types of soils, pH, organic matter, sulfur, light, oxygen availability, manganese (Mn) and iron oxides, temperature etc. affect the methylation and bioavailability of Hg. In Hg mining regions rice intake appears to be the primary MeHg exposure pathway. On health side the studies regarding MeHg gastrointestinal absorption have indicated a huge percentage of MeHg absorption in the body. Rice is considered as the second major source of MeHg for human exposure and health risk. Furthermore the various factors and mechanisms responsible for Hg speciation in degraded soil, Hg bioavailability, rice plant uptake, health risk of human, future research gaps and needs are also addressed and identified in this review paper. • Industrialization/urbanization have caused Hg contamination of paddy soil and rice. • Soil physico-chemical properties and bacteria affect Hg methylation and availability. • Various mechanisms responsible for Hg speciation are discussed. • Hg-methylation and uptake by rice leads to human health risk such as cancer.