| The study was engaged in an investigation and analysis of the effects of paddy field soil components on the adsorption and desorption of mercury by micro-aggregates along the Mingshan River watershed. Theoretically speaking, we had already made clear that heavy metal contaminants, such as Hg2+, can strongly affect the migration of such heavy metal pollutants in the environment in the groundwater, the organisms and crops in the watershed. Therefore, in proceeding with our study, we had collected different-sized micro-aggregates samples and separated them by wet sieving-siphon sedimentation-freeze drying method from the paddy fields in the said watershed. Furthermore, we had used the isothermal adsorption method, batch method and NaN03 and HCl desorption method for tracing the actual kinetic and thermodynamics of mercury adsorption and desorption via the micro-aggregates. And, next, we had derived the micro-aggregates samples without organic matters (OM) and free iron oxides (Fed) respectively with the help of selective dissolution in hoping to disclose the effects of OM and Fed in the paddy soil components on the mercury adsorption and desorption. The results were as follows:Hg2+ adsorbance of the bulk soil and micro-aggregates tended to increase with the increase of equilibrium concentration, which was in significantly positive correlation with the cation exchange capacity (CEC), OM and Fed (R2=0.9746-0.9836). Moreover, the Hg2+ adsorption isotherms turned out well in accord with the Langmuir equations, nevertheless, the distribution coefficient (Kd) turned out in negative correlation with the initial concentration of Hg2+. On the other hand, though after removing OM and Fed from the soil, the Hg2+ adsorbance in the bulk soil and micro-aggregates tended to get decreased, there existed a significantly positive correlation with the residual OM and Fed. Such a fact indicates that the contributions of OM and Fed removal to the decrease of adsorbance turn out to vary among the different micro-aggregates of the particle sizes, with equivalent contribution (30%) in 0.053-0.25 mm micro-aggregates samples and 10-fold more of Fed than OM in<0.002 mm micro-aggregates samples. Therefore, the desorption study indicated that the desorption rate turned to increase with the adsorbance of Hg2+. In such a situation, the majority of desorption reaction belonged to the specific sorption, which can reach 50%, wheras the non-specific sorption was merely less than 1%. Thus, it can be seen that the specific adsorptions of Hg2+ by the bulk soil and the micro-aggregates tended to decrease after the removal of OM and Fed, whereas, the non-specific adsorption turned to increase simultaneously. This was likely to accelerate the Hg2+ mobility in the soil and lead to highly potential threated to the basin groundwater, the soil organisms and, in turn, the crops themselves, indeed.The reaction of adsorption and desorption of Hg2+ by the bulk soil and micro-aggregates can be divided into two types:a fast reaction and a slow reaction.The adsorption can be finished in about 1h～3h.The Hg2+ adsorption kinetic turn out well in accord with the Elovich equations. The reaction of adsorption and desorption of Hg2+ by the bulk soil and micro-aggregates can be divided into two types:a fast reaction and a slow reaction after the removal of OM and Fed. |
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