Methylmercury (MeHg) accumulation in rice is an emerging food safety issue in China and other countries; however, mitigation methods are scarce. A joint research group from School of the Environment and Key Laboratory of Geographic Information Science, Ministry of Education, East China Normal University has gained insight into a novel remediation strategy to mitigate MeHg accumulation in rice grown in mercury-contaminated paddy soil. Their research has been published on March 5th, 2019 in Journal of Hazardous Materials.

The team investigated the effects of selenium (Se) and multiple applications of Se and biochar on rice MeHg bioaccumulation by using pot and microcosm experiments. They found that Se amendment was still effective in reducing MeHg levels in paddy soil and rice grain after three years of aging. Biochar amendment (0.5%, w/w) further decreased grain (brown rice) MeHg levels by 82–87%. The grain MeHg level decrease following the combination of Se and biochar amendment could be partly attributed to inhibition of net MeHg production in soil by Se. In addition, biochar decreased not only net MeHg production but also MeHg bioavailability in the soil, which could be due to organosulfur compounds in the biochar.

The fitting results showed that the organosulfur compound (which has a structure similar to that of dibenzothiophene) represented ～23% of the total sulfur content in the biochar. The results are similar to those obtained for oak- and corn stover-derived biochar.

https://www.sciencedirect.com/science/article/pii/S0304389418310720

XANES spectrum of biochar (black line) generated from bamboo at 600 ℃ for 1 h and the linear combination fitting result (red line, R-factor = 0.0032; reduced chi-squared = 0.0064) from two S model compounds, dibenzothiophene (a) and potassium sulfate (b). The black dotted vertical lines indicate the dominant peaks at 2474 and 2482.5 eV.

The results suggest that multiple applications of Se and biochar could be a novel remediation strategy to mitigate MeHg accumulation in rice, synchrotron source 4B7A (BSRF) has helped the team to unveil the sulfur species in bamboo-derived biochar. "However, the application rate of Se should be carefully considered due to its potential environmental risk at high levels. Remediation strategies using biochars from different types of feedstocks and pyrolysis processes could be an alternative approach for Hg-contaminated soil or sediment. X-ray absorption near-edge structure (XANES) can provide direct speciation information of sulfur and other elements in biochars and improve the understanding of interactions between MeHg and biochar." explains Huan Zhong, the team leader (metal biogeochemistry and food safety lab) and the professor of School of the Environment, Nanjing University.

Article:

Yongjie Wang, Fei Dang, Xiangmin Zheng*, Huan Zhong* Biochar Amendment to Further Reduce Methylmercury Accumulation in Rice Grown in Selenium-amended Paddy Soil. Journal of Hazardous Materials. 365(2019), 590–596.