Effects of mercury binding by humic acid and humic acid resistance on mercury stress in rice plants under high Hg/humic acid concentration ratios.

Abstract

Due to the nonsystematic nature of previous studies on mercury (Hg) mobility with humic substances (HS) in terrestrial ecosystems and the uncertainty of Hg accumulation in plants, oxygen-rich humic acid (HA), which is the main component of HS, was used as the target in this study. Batch sorption tests and a series of pot experiments were designed to investigate the effect of HS on Hg binding and therefore Hg uptake in rice plants under extreme conditions, i.e., a high Hg/HS concentration ratio. The results showed that HA was eligible for Hg binding, though it has a tiny proportion of sulfur according to its characteristics analysis. The binding of HA and Hg was a chemisorption process in a single layer that followed the pseudo-second order and Langmuir models, and it was also verified that the pH was dependent on the ion strength associated with high Hg/HA concentration ratios. Based on the pot experiments, the performance of HA with Hg was investigated. The Hg in the toxicity characteristic leaching procedure (TCLP) leachate under high Hg/HA concentration ratios declined significantly, and accordingly, all treatments met the concentration criteria of 0.1mg/l (GB 5085.3-2007) for wastes after 30days of exposure. At concentration ratios of 50, 25, and 10μgHg/mg HA, we observed that HA application promoted rice plant growth, as reflected in the increase of fresh weight of different organs. Regarding accumulation in the soil-plant system, the degradation of HA to smaller molecules by rhizosphere microorganisms and organic acids in roots made HA available for plant uptake through the vascular bundle in roots, thus promoting Hg transformation in plants to a certain extent. However, considering the decline in available Hg in the soil, the Hg concentrations of roots, straw, and grains in the ripening stage were found to be lower than those in the standalone Hg treatments. HA clearly has a direct effect on Hg and an indirect influence on plants exposed to Hg under extreme conditions (very high Hg/HA concentration ratios); thus, the biogeochemical behavior of Hg at high Hg/HA concentration ratios should be considered and further investigated.