Mercury Speciation in Soils and Its Influence on Bioaccumulation in Crops at a 33-Year-Old Solid Waste Dumping Site

Mercury (Hg) pollution garnered global attention after the Minamata disaster. The complex biogeochemical processes governing soil Hg distribution, speciation, and its accumulation in crops have remained elusive. This study investigates the soil Hg distribution, speciation, and bioaccumulation in crops at Dhapa, a 33-year-old solid waste dumping station of the Kolkata Municipal Corporation. Vegetable cultivation in this 35-hectare area rely on industrial wastewater of the city for irrigation, and these sewage-irrigated vegetables find their way into markets across the Kolkata suburbs. Whole plant of vegetable crops and paired soil samples were collected from 92 locations in the study area. The findings unveiled a wide range of soil total Hg (THg) concentrations, spanning from 48 ± 2.4 to 8108 ± 405.4 µg/kg, with an average of 1697 µg/kg. 45% of the total samples surpassed the Maximum Permissible Concentration of Hg in agricultural soil (1500 µg/kg), and 91% exceeded the background concentration of world soils (400 µg/kg), indicating significant ecological risks. The chemical speciation study revealed that soil organic matter (SOM) was the dominant hosting phase for Hg in the studied soil. According to relative abundance, the different species could be arranged as follows: Hg bound to SOM> elemental Hg > Hg bound to sulphides > Hg bound to reducing binding phase > residual fraction of Hg. High elemental Hg contents (average: 21.86 ± 1.1% of THg) underscore prominent Hg (II) reduction in the soils. Among edible parts of the vegetables examined, the highest accumulation was observed in leafy vegetables. The edible parts, as per the bioaccumulation factor (Hg concentration in edible part / soil THg), could be sorted as Cauliflower < Bottlegourd < Malabar spinach (Poi) < Ipomea< Brinjal < Radish (root) < Amaranthus< Spinach. According to Polish, Serbian, and Chinese regulations, all vegetable crops (besides Cauliflower) exceeded the maximum tolerable limit (300 µg/kg). SOM-bound was identified as the chemical form of Hg primarily responsible for its root uptake. Interestingly, shoot Hg contents were significantly correlated to soil elemental Hg contents, which implies that soil Hg evasion contributes to Hg uptake in plant shoots. The findings of this study suggest that high organic matter input in polluted soils could enhance the phytoaccumulation of Hg. The outcomes of this study will contribute to assisting policymakers in minimizing human exposure to mercury (Hg) by providing comprehensive guidelines on vegetable intake from this area. Additionally, the study's results will be valuable for farmers, helping them select crops that demonstrate low uptake of mercury.