Effects Of Key Geochemical And Biological Factors On Bioavailability And Bioaccumulation Of Mercury In Paddy Soils

Previous studies have shown the consumption of fish and shellfish is the main pathway for human exposure to methylmercury(MeHg).However,recent studies indicated that rice consumption could be the major pathway of MeHg exposure in inland Hg mining areas.In soil-rice systems,inoganic mercury could be transferred to methylmercury under anaerobic condition.Rice could accumulate high levels of MeHg and most of MeHg in rice plantsstable food crops in South and Eastern Asia,and MeHg is the most toxic Hg species is distributed in grain.Rice is one of the major and could biomagnify through the food chain.Therefore,the accumulation of MeHg in rice could pose health risk to human.This study mainly focused on investigating mercury bioavailability in rice paddy field,the methylation of inorganic mercury,and the subsequent accumulation of MeHg in rice grain.The influence of key geochemical factors(i.e.,clay and organic matter in soils)on the partitioning and bioavailability of inorganic mercury or MeHg was studied by doing batch experiments in controlled laboratory conditions.Key biological factor,i.e.,MeHg uptake by rice plant and its translocation within plant(affected by rice varieties),on MeHg accumulation in rice grain was also explored.Besides,the uptake of MeHg from soil by rice root was investigated to choose a suitable extraction method to predict MeHg accumulation in rice grain.And the translocation of MeHg from rice root to rice grain was studied to explore key factors controlling MeHg accumulation in rice grain.The conclusions were listed below:(1)Humus coating increased partitioning of IHg and MeHg in clay.Furthermore,humus coating led to a decrease in IHg and MeHg bioavailability,due to strong binding between mercury and clay-sorbed humus.(2)MeHg extracted by(NH4)2S2O3(highly affinitive to MeHg)correlated well with grain MeHg levels.Thus(NH4)2S2O3 extraction,solubilizing not only weakly-bound and but also strongly-bound MeHg,may provide a measure of'phytoavailable MeHg pool' for rice plants.However,MeHg extracted by CaCl2 or DTPA,possibly quantifying 'exchangeable MeHg pool' or 'weakly-complexed MeHg pool' in soils,may not predict grain MeHg levels.(3)Chemical extraction method simulated MeHg desorption from soil and the subsequent uptake by rice root.Considering that(NH4)2S2O3 extracted MeHg was correlated with grain MeHg levels,it was possible that MeHg desorption from soil and MeHg uptake by root could be the limiting step of MeHg accumulation in rice grain.(4)For different rice varieties,MeHg concentrations in grain were found to be correlated to amino acid or protein concentrations.This is probably because that MeHg could be co-transported with amino acids or proteins during rice in the grouting stage.Therefore,differences in production and transportation of amino acid or protein among different rice varieties could be a key factor controlling variations in MeHg accumulation in grain.