College of Environmental Science and Energy, South China University of Technology, Guangzhou, 510006, China.
Guizhou Academy of Sciences, Guiyang, 550001, China.
Environ Sci Pollut Res Int. 2020 May;27(15):18650-18660. doi: 10.1007/s11356-020-08328-9. Epub 2020 Mar 21.
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 0.1 mg/l (GB 5085.3-2007) for wastes after 30 days of exposure. At concentration ratios of 50, 25, and 10 μg Hg/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.
由于先前关于汞（Hg）与腐殖物质（HS）在陆地生态系统中迁移的研究具有非系统性，并且植物中 Hg 的积累存在不确定性，因此本研究选择富氧腐殖酸（HA）作为目标。设计了批量吸附试验和一系列盆栽试验，以研究 HS 对极端条件下（即高 Hg/HS 浓度比）水稻植株中 Hg 结合和吸收的影响。结果表明，尽管根据其特征分析，HA 中含有少量的硫，但它具有结合 Hg 的能力。HA 与 Hg 的结合是一个单层的化学吸附过程，符合准二级和 Langmuir 模型，还验证了 pH 值取决于与高 Hg/HA 浓度比相关的离子强度。基于盆栽试验，研究了 HA 与 Hg 的性能。在高 Hg/HA 浓度比下，毒性特征浸出程序（TCLP）浸出液中的 Hg 明显下降，因此，所有处理在暴露 30 天后均符合 0.1 mg/L（GB 5085.3-2007）的浓度标准。在浓度比为 50、25 和 10μg Hg/mg HA 时，我们观察到 HA 的应用促进了水稻植株的生长，这反映在不同器官的鲜重增加上。关于土壤-植物系统中的积累，根际微生物和根系中的有机酸将 HA 降解为小分子，使 HA 通过根中的维管束被植物吸收，从而在一定程度上促进了 Hg 在植物中的转化。然而，考虑到土壤中有效 Hg 的减少，在成熟阶段，根部、秸秆和谷物中的 Hg 浓度发现低于单独 Hg 处理的浓度。HA 对 Hg 具有直接影响，对处于极端条件（非常高的 Hg/HS 浓度比）下暴露于 Hg 的植物具有间接影响；因此，应考虑并进一步研究高 Hg/HA 浓度比下 Hg 的生物地球化学行为。
