| Mercury（Hg）is a widely distributed pollutant and has volatility and extreme toxicity. Many domestic and international researches have confirmed that reservoirs formed by damming the environment are conducive to the activation and methylation of mercury, so the reservoir is a typical "Mercury sensitive ecosystem."Three Gorges Reservoir is the largest regulating reservoir in China. Because the water level of the Three Gorges Reservoir fluctuates from 145 m in summer to 175 m in winter, the water-level-fluctuation zone（WLFZ） with area of 350 km2 were formed.The water-level-fluctuation zone on the one hand is an effective sink of mercury and other pollutants, because it collects runoff. On the other hand, as last buffer zone of the reservoir, WLFZ is a source of mercury, because mercury in WLFZ will be released to the overlying water of the reservoir. Therefore, studying mercury in WLFZ can lay a foundation for understanding the biogeochemical cycle of mercury in Three Gorges Reservoir.Although the total mercury（THg） concentration in soil could reflect the overall pollution levels, it is insufficient to estimate the environmental impact exactly based simply on the THg concentration. Because in soil Hg is present in different forms that can be bound to various matrix phases, these matrix phases have different mobilities,bioavailabilities, and potential toxicities.Water-level-fluctuation zone in each year from April to August is exposed. There is abundant rain, so large area will grow herbaceous vegetation lush. Plants will secrete large amounts of organic matters, these secretions accumulate in the surrounding soil.After reaching a certain concentration, they can reduce the p H of rhizosphere soil,improve microbial activity. These organic matters contain a large number of small organic acids which could complex, adsorb, oxide-reduce mercury to change its speciation.Therefore, the objectives of this study are soils from Water-level-fluctuation zone of the mainstream and tributary of the Three Gorges Reservoir. By simulationexperiments, we could analyze the effect of the two low molecular weight organic acids（citric acid, oxalic acid） on the speciation of mercury under different experimental conditions（incubation time, concentration, p H value）. The results showed that:（1） As no addition of low molecular weight organic acids, mercury in soil were mainly transform from loosely bounded phases to strongly bounded phases.After adding organic acids, mercury in the soil would transform from strongly bounded phases to loosely bounded phases. In each experiment, the value of F5 was basically unchanged.（2） After adding various concentrations of citric acid, the value of exchangeable mercury（F12） and bioavailable mercury（F123） all increased at varying degrees. This effect strengthened with the increase of the added citric acid concentration, and peaked at 15mmol·L-1. F12 in mainstream soil increased from 9.10% to 16.79%, F123 increased from 29.12% to 44.82%; F12 in tributary soil increased from 10.21% to 23.89%, F123 increased from 50.84%to 70.17%.（3） Although adding oxalic acid can improve the bioavailability of mercury, the effect was not as obvious as citric acid. After the addition of oxalic acid, the migration of mercury in soil had been inhibited. Effect of oxalic acid on the speciation peaked at 10mmol·L-1. F12 in mainstream soil was reduced from9.10% to 4.09%, F123 increased from 29.12% to 29.68%; F12 in tributary soil decreased from 10.21% to 4.05%, F123 increased from 50.84% to a 52.60%.（4） The activation effect of combined organic acids was greater than every single low molecular weight organic acids. For mainstream soil, when added5mmol·L-1 oxalic acid and 10 mmol·L-1 citric acid, the value of F12, F123 reached maximum, is 27.41%, 53.66%, respectively. The value of F12, F123 in tributaries soil peaked at 19.59%, 49.36%, respectively, under the culture conditions of adding 7.5mmol·L-1 oxalic acid + 7.5mmol·L-1 citric acid.（5） Acid and alkaline environment were conducive to increase the mobility and bioavailability of mercury in soil. When the p H of added oxalic acid reduced from 6 to 3, F12 increased from 17.90% to 92.02%, F123 increased from45.34% to a 92.66%. When the p H increased from 6 to 9, F12 increased from17.90% to 19.59%, F123 increased from 45.34% to a 49.36%. Under acid conditions, the activation of 10mmol·L-1 oxalic acid was more significant. In the process of rising p H, the value of F12 declined from 95.2% to 4.11 %,F123 decreased from 95.87% to 29.67%. |
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