| Mercury(Hg), the sole toxic heavy metal in liquid under the condition of normal temperature, is a persistent and global distributed contaminant because of its volatility and long-range transport in the atmosphere. Its fate and environmental chemical behaviors in the ecosystem therefore have continued to be a hot issue in the related field of environmental science. In the aquatic ecosystem, Hg is apt to be activated,resulting in higher migration ability and bioavailability. Simultaneously, it can also be converted into methyl mercury(Me Hg) which has neurotoxicity through methylation and then accumulates in aquatic organism(like fish). New built reservoir is a representative eco-sensitive region owing to its effect of mercury activation. Three Gorges Reservoir is an oversize annual regulation reservoir in China and its special regulation mode has generated a huge water-level-fluctuating zone(WLFZ) around it with the periodic characteristic of water-flooding and non-flooding. As soil in this zone conducts dry-wet alternation, plentiful plants also present a renewal period from growth to submersion-decomposition and then to regrowth. During this period, the roots of vegetation can secrete relevant organic matters(OM) provided that there are some force of environmental factors so as to improve rhizosphere environment. When the concentrations of these OM(including some low-molecular-weight organic acids and amino acids) increase and reach to a certain amount, the activation and methylation of Hg in soil may be influenced by these OM through acidifying, chelating,oxidizing and reducing. Therefore, Shibaozhai, located in Zhongxian in Three Gorges Reservoir area, was chose to as the sampling site to get the original soil and plants samples. On the basis of surveying the dominant plant species, biomass as well as Hg levels in their bodies ahead of time, simulation experiments were conducted to analysis the impact of the main components of vegetation root exudates, low-molecular-weight organic acids(LMWOAs, including citric acid, oxalic acid and tartaric acid), on the activation of Hg in the soil of this zone under different experimental condition(like concentration and incubation time). On the other hand, in order to get autoclaved soil,a certain amount of original soil was removed and disposed through sterilization. After that original soil and autoclaved soil were incubated under different dark conditions(with oxygen or without oxygen) to study the variation characteristics of Me Hg content in these two soils and the factors influencing methylation of Hg in soils. The purpose is to provide fundamental data for the research of migration and transformation of Hg in vegetation and even to more accurately understand the fate and geochemical characteristics of Hg in Three Gorges Reservoir. The research results as follows:1. Effects of LMWOAs on the activation of Hg in soils of WLFZ(1) The LMWOAs had the capacity to strengthen the desorption of Hg in soils of WLFZ of Three Gorges Reservoir. Under the affection of low concentration of LMWOAs(1mmol·L-1 for citric acid, 1mmol·L-1and 2mmol·L-1 for oxalic acid and tartaric acid), the content of total mercury(THg) and reactive mercury(RHg) in extracts presented an upward trend through the whole extraction period, but they both increased initially and then decreased under other higher concentrations of LMWOAs,and the time reaching the peak values reduced with the increase of LMWOAs concentrations. While all of the dissolved mercury(DHg) were roughly extracted within 5 minutes and its value remained at 15ng·L-1 or so, the maximal extractive amount of DHg was not affected by the type and concentration of LMWOAs.(2) After extracting with LMWOAs, the amount of complexing Hg(A) showed a significant positive correlation with the content of THg in corresponding leaching solution, and there was a parallel variation tendency with the change of LMWOAs concentration and oscillating time for the value of A and the content of THg. Increasing the concentration of LMWOAs could accelerate the activation of Hg in soils, and the maximum figure of A increased initially and then reached plateaus with the increase of LMWOAs concentrations. Under the disposition of citric acid, oxalic acid and tartaricacid, the maximum value of A were 62ng·L-1, 58ng·L-1 and 52ng·L-1 with the percentage against the total mercury in soil of 2.73%, 2.57% and 2.31%, respectively.The activation ability of LMWOAs to Hg were in the descending order: citric acid >oxalic acid > tartaric acid.(3) The changing curve of A varied with extracting time could be divided into two stages and the first was ascent stage, which could be fitted by Elovich Equation, and the complexing rate of this stage increased with the increase of LMWOAs concentrations. The next stage was declining, which was consistent with the feature of the second-order kinetics, and its rate changed slightly or held steady. Besides, the effects of oxalic acid and tartaric acid were similar on the adsorption and desorption of Hg in soils, while there was a little different with the impact of citric acid.2. Effects of LMWOAs on the methylation of Hg in soils of WLFZ(1) During a certain cultured period, LMWOAs could promote the transformation process from inorganic mercury to methyl mercury in soils of WLFZ and this promotability became increasingly significant with the extension of incubation time.Increasing the concentration of LMWOAs appropriately could enhance its ability to strengthen methylation of Hg, which differ from organic acid to organic acid. This promotability followed the trend: citric acid > oxalic acid ≈ tartaric acid, and the effect of the mixture of these three acids was similar with that for 8mmol·L-1 citric acid.(2) The condition of cultivation could influence the capacity of LMWOAs to promote methylaytion of Hg, oxygen and microorganism in soil were two main factors affecting this capacity. Anaerobic environment was conducive to methylation of Hg in soil with bacteria but there were no discernible effect on autoclaved soil, which was apt to conduct methylation in aerobic environment. After cultivation using LMWOAs, the increase of Me Hg in original soil(with bacteria) was more remarkable than that in autoclaved soil, so microorganism made a dominant contribution to methylation of Hg in soils of WLFZ. Additionally, the time when the content of Me Hg in soil reached its peak levels had nothing to do with the concentration of LMWOAs but with the kind of LMWOAs, oxygen and microorganism in soils.(3) The maximum of net rate of methylation(F) among all processing was2.718%, which discovered in the original roil incubated under anaerobic condition using 8mmol·L-1 citric acid. The values of F in original roil incubated under aerobic condition and in autoclaved soil cultured in anaerobic environment did not show anobvious fluctuation. There were many simultaneous approaches to conduct methylation of Hg in soils, while the predominant one was varying under different conditions. |
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