Soil mercury speciation and accumulation in rice (Oryza sativa L.) grown in wastewater-irrigated farms

Abstract

Forty whole rice plants and corresponding rhizospheric soil samples were collected from rice paddy fields along four rivers in Tianjin, including three wastewater-irrigated areas [Beijing (BJR), Beitang (BTR) and Dagu (DGR) Wastewater Drainage River) and a control area (Haihe River (HHR)]. A sequential extraction technique was used for quantification of five mercury (Hg) fractions (soluble and exchangeable, specifically sorbed, oxide-bound, organic-bound and residual fractions) in the soil samples. Additionally, total Hg (THg) and methylmercury (MeHg) concentrations in soils and plants were measured using a DMA-80 direct Hg analyser and gas chromatography-cold vapour atomic fluorescence spectrometry (GC-CVAFS). The inorganic Hg (IHg) was calculated by subtracting the MeHg value from the THg value. Soil THg and MeHg concentrations were significantly higher in the wastewater-irrigated soils than those in the control area. Soil Hg was mainly found in the oxide-bound, organic-bound and residual fractions, the sum of which exceeded 98%. The average concentrations of THg and MeHg in seeds collected from the wastewater-irrigated areas ranged from 13 ± 7.3 to 28 ± 13 ng g−1 and from 4.5 ± 2.3 to 19 ± 11 ng g−1, respectively. Rice grain accumulated the highest amount of MeHg rather than IHg. About 21% of MeHg intake through rice consumption presented a health risk to the local population in the four areas. Soluble and exchangeable Hg can be used to predict the THg and MeHg concentrations in the rice grains. If soluble and exchangeable Hg in paddy soil exceeds 0.0091 ng g−1, the MeHg dietary intake may be above the permissible limit recommended by the United States Environmental Protection Agency.