Effect of iron plaque and selenium on mercury uptake and translocation in rice seedlings grown in solution culture

A hydroponics experiment was conducted to investigate the effect of iron plaque on root surfaces and selenium (Se) on the uptake and transfer of mercury (Hg) in rice seedlings by adding in the EDTA-Fe (0, 10, 30, 50, 70 mg Fe l^-1) into the solution to produce a different amount of iron plaque outside rice root. After 24 h, the red-brown iron plaque was formed on the root surface, and the amount of iron plaques was positively correlated with the amount of Fe in the solution. The iron plaque deposited on the root surface has a strong adsorption effect on the inorganic Hg. The addition of Se could promote the adsorption of Hg²⁺ on the iron plaque of rice, and the introduction of Se could increase the adsorption capacity of Hg on iron plaque on average by 1.42 times. The Hg was extracted by DCB (Dithionite-citrate-bicarbonate) up to between 66.2 and 67.8% of the total Hg when the roots with iron plaque (Fe70) were incubated with the combination of 5 μmol L^-1 of HgCl₂ and 5 μmol L^-1 of Na₂SeO₃ for an hour. After 3 days, the content of Hg in the iron plaque decreased to 6.3-33.9%, indicating that part of the inorganic Hg adsorbed by the iron plaque could be reabsorbed and used. Besides that, the iron plaque allowed the Hg to stay longer in the iron plaque, which hindered the transfer of Hg to the shoot significantly. Hg adsorbed in the iron plaque can be desorbed by low-molecular-weight organic acids, which was equivalent to desorption of Hg from ferric hydroxide oxides. Hg adsorbed on the iron plaque can be moved back to the rest of the plant. These results suggest that the iron plaque and Se in the root surface might play a role as "physical buffer" in the absorption and transfer of Hg.