| In recent years, increased anthropogenic inputs of cadmium have aroused considerable concern relative to its long latent period, strong mobility, and chronic toxicity. Cadmium contaminated soils would not only inhibit the yield and quality of crops, but impose serious menace on human health through the food chain. The transportability and bioavailability of cadmium largely depends on its concentration and retention in the soil, which is essentially controlled by soil colloids, the most important determinant in this process. As a matter of fact, the intensity of retention, transport characteristics of cadmium in soils vary with the components and properties of soil colloids, therefore, soil colloids are closely related to environmental risk of cadmium.The water-level-fluctuating-zone in Chongqing consists 85% of all such land in the Three Gorges Reservoir area. Influenced by seasonal inundation, erosion and sedimentation, soils in this area would become both source and sink of various pollutants in the near future, therefore, characteristics of local soil colloids have significant meaning to the safety of water quality of the Yangtze River. Though fruitful achievements have been obtained from past researches on adsorption-desorption process of cadmium on different types of soil, most of these studies have been limited to soil matrix, and few from the aspect of soil colloid were reported. Meanwhile, evidences showed surface transport and soil-water interface migration of cadmium were dominated by metal-colloid binding phase. Therefore, to investigate the adsorption-desorption characteristics of cadmium by different soil colloids is vital to discover the transport mechanism of heavy metals in the soil, as well as to evaluate the environmental risk of soil related water contamination. Based on references and field investigations, four types of soil collected from the Three Gorges Reservoir area, namely the purple soil, yellow soil, purple alluvium, and brown alluvium, were selected as typical soil samples in this study. Consequently, isothermal and kinetic characteristics of adsorption-desorption processes of cadmium by the above four soil colloids, as well as the influences of phosphate and seasonal wet-dry phase, were investigated. Meanwhile, colloidal facilitated cadmium migrations were also studied.Results showed that isotherms of cadmium adsorption by soil colloids could be best described by Freundlich model, while Langmuir model came next. According to simulated maximum adsorption capacities, mineral colloid of the purple soil had much higher cadmium adsorption ability (8216.340 mg/kg) than that of mineral colloid of the yellow soil (6468.108 mg/kg). Meanwhile, extractable ferric oxide appeared to have significant contribution to cadmium adsorption by soil colloids. Soil 3 colloids deprived of extractable ferric oxide unanimously exhibited dramatically reduced adsorption abilities. The maximum adsorption capacity of de-ironed mineral colloid of the purple soil and the yellow soil were merely 4963.130,2342.831 mg/kg, respectively.Desorption isotherms indicated the intensity of adsorption gradually decreased with the raise of adsorption amount, and cadmium blocked by low energy exchange sites were likely to release into soil solution. Generally, mineral colloid of the purple soil had a higher desorption amount than that of mineral colloid of the yellow soil, moreover, desorption amount of cadmium significantly increased after extractable ferric oxides were removed. Desorption trend (logK_d) of the four colloids followed the order of mineral colloid of the yellow soil < mineral colloid of the purple soil < de-ironed mineral colloid of the yellow soil < de-ironed mineral colloid of the purple soil. On the surface, purple soil colloid had a higher adsorption capacity compared with that of yellow soil colloid, however, exchangeable adsorptions by low energy binding site were the dominant force here, in which cadmium were more easily to return to the environment when external condition changed. Therefore, yellow soil colloid, which is rich in extractable ferric oxides, had stronger retention for cadmium compared with purple soil colloid.Kinetics of cadmium adsorption by soil colloids could be best fitted by Double-constant model, and the reaction time of 90% adsorption equilibrium for mineral colloid of the purple soil, mineral colloid of the yellow soil, de-ironed mineral colloid of the purple soil, and de-ironed mineral colloid of the yellow soil were 35.80, 13.88, 307.94, 424.56 minuets, respectively. Adsorption process was dominated by the fast reaction, however, relatively long time would be needed for a complete equilibrium. Because the fast reaction was essentially influenced by specific adsorption through high energy exchange sites, yellow soil colloid, which has a high content of extractable ferric oxide, could reach the 90% adsorption equilibrium in much shorter period compared with purple soil colloid. The speed of adsorption sharply decreased when extractable ferric oxides were removed.Desorption of cadmium from soil colloids was a long process when compared with the adsorption reaction. No evident equilibrium appeared during the whole period (120 hours). Desorption kinetics could be best described by Double-constant model. Similar to adsorption reaction, desorption of cadmium could also be divided into two different stages: the first stage, which was from the beginning to the 12~th hour, and the second stage, from the end of the first stage to the 120~th hour. Desorption speed of the first stage was much faster than that of the second stage.The presence of phosphate in soil solution reduced the maximum adsorption capacity of cadmium of soil colloids, moreover, its inhibiting effect increased with the raising concentration of phosphate. According to Double-constant model, phosphate could also prolong the reaction time of adsorption equilibrium. With the raising concentration of phosphate, reaction time needed for 90% adsorption equilibrium increased dramatically at first, then sharply reduced.Phosphate could enhance desorption of cadmium from purple soil colloid, purple alluvium colloid, and brown alluvium colloid, however, this process was closely related to soil properties and...
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