Research On Adsorption-desorption And Abiotic Methylation Of Mercury By Humic Acid In Soils

The environment problems which are caused by the heavy metal mercury into soilsare always the key of governance and prevention the mercury pollution. To treat andrestore the soil polluted by mercury should firstly study the mobility of mercury in soils,while its mobility mostly relates to the adsorption-desorption process. Besides, themercury absorbed by the soil is not static and can be activated, generated with organicmatter such as fulvic acid, converting to organic mercury. Therefore, investigating theadsorption-desorption behaviors in soils, as well as the methylation of inorganic mercurywith humic acid, are important to control its migration, transformation and bioavailability.And these researches will be very useful to control the pollution of mercury in soils. Themain contents as follows:1. Adsorption-desorption of inorganic mercury （HgCl₂） and Methyl mercury （MeHg） infour different soils were studied using batch equilibration techniques. The results showedthat the adsorption-desorption isotherms of HgCl₂and MeHg in soils were fitted by theFreundlich empirical equation （r>0.98）. The adsorption coefficients Kfof HgCl₂inpaddy soil, red soil, shajiang black soil, yellow brown soil were122.25,86.60,70.95,76.60, respectively; The Kfvalues of MeHg were55.99,45.00,41.94, and40.43,respectively. The soils had the great adsorption capacity for HgCl₂and MeHg, and soiladsorption for HgCl₂was stronger than that for MeHg. The adsorption of HgCl₂andMeHg in soils increased with increase in the organic matter contents of the soil. And thechanges of soil initial pH could influence the adsorption of HgCl₂, and the largest amountof adsorption took place under neutral conditions, but it had little effect on the adsorptionof MeHg.2. The desorption of HgCl₂and MeHg from soils showed significant hysteretic effectswhich was indicated by the higher adsorption slope （1/nads） compared to the desorptionslope （1/ndes）. Soils that adsorbed more mercury also desorbed less mercury. That is: theadsorption of HgCl₂in paddy soils is the strongest while its hysteresis coefficient is thebiggest; the adsorption of MeHg in shajiang black soil is the weakest and hysteresiscoefficient is the smallest.3. The reduction and abiotic methylation was able to occur between mercury and HA.And the results showed that the initial concentration of Hg²⁺, HA concentration, the reaction system temperature, pH and light conditions played an important role for itsmethylation and restore process while the reaction system in the presence of a sufficientmethyl donor （vitamin B12）. During the impact factors, the higher concentration of Hg²⁺and HA, the more MeHg would be generated, However, the reduction would be reducedwhen these concentration increase(＞40mg·L^-1). Besides, temperature could promote thegeneration of MeHg and Hg0; the lower pH conditions were more conducive to thegeneration of MeHg, and as the pH increases, the methylation and the reduction would beinhibited. In addition, the lights conditions also significantly contributed to themethylation, and the photosensitization of the UV lamp was most obvious.