| The adsorption-desorption characteristics in soil after Cd inputed into the soils influenced the Cd behavior, and detemined the extent of Cd pollution in a way. The adsorption-desorption was affected by many factores, on which many studies have been done.Freeze/thawing may be the important factor influenced the Cd adsorption-desorption in soil in north clod area because that freeze/thawing could affected the mineral,charge type and quantity in the soil directly and also affected the organics quantity and form.However, so far the studied on effect of freeze/thawing were less,and the studied on the mechamism of effect of freeze/thawing on Cd adorption-desorption in soil were more less.In this paper,the brown soils distributing in Shenyang area were collected for tested soils, the soil samples of various water content were treated with various freeze/thawing time by artificial temperature control method. The cadmium static isothermal adsorption-desorption isotherms and kinetics isothermal adsorption-desorption isotherm, the adsorbed Cd fractionation in the soil were assayed, then contected the Cd adsorption-desorption characteristics with the soil physical and chemical properties by statistical analysis, and discussed the the mechamism of effect of freeze/thawing on Cd adorption-desorption in soil.The soil samples were incubated at-30℃and-20℃for24h, respectively. Soil samples of different water content (such as10%,40%,70%,100%and120%of the field capacity) were subsequently refrozen and thawed for0,1,3,6,9cycles. The results were as below:1. Freeze/thawing time (FTT) and soil water content affected the soil pH, free iron oxide, amorphous iron oxide, iron oxide activation degree, the specific surface significantly, and the FTT also affected CEC, CECp, DOC and the fractal dimension(D)significantly. Most of the soil physical and chemical characteristics were fluctuant change with the FTT, and these phenomena were demonstrated by the SEM pictures of the soil particles and the FTIR spectra when the soil water content equaled to the field capacity. Compared with soil water content and its interaction with FTT, FTT had more obvious effects on soil physical and chemical characteristics.2. FTT affected the Cd adsorption-desorption in the soil. The Cd adsorption quantity increased firstly and then decreased with the increase of the FTT, the max was at3FTT, and the freeze/thawing improved the Cd adsorption in the soils in the extent of this experiment. The Cd adsorption amount was less when the soil water content was70-100%of the field capacity. The effect of the interaction of the FTT and water content on Cd adsorption quantity in soil was significant, while that of the water content was not. The Cd adsorption ability of tested soils was higher, the mean Cd adsorption ratio reached90.15%increasing firstly and then decreasing with the increase of the FTT. The change in free energy ΔG<0, and ΔG value ranged from-30～0kJ·mol-1,so the adsorption progress belonged to spontaneous physical-chemical adsorption. Initial and average adsorption speed firstly increased and then decreased with the FTT increasing, and appeared adverse change with the water content.The Cd desorption in soils treated with the different freeze/thawing presented hysteresis quality, and the extent of which firstly enhanced and then receded with the FTT increasing. The Cd desorption rate decreased firstly and then increased with the increase of FTT. Initial and average desorption speed firstly decreased and then increased with the FTT and soil water content increasing.The Cd desorption process in soil can be divided into two sections:fast reaction and slow reaction stage, while the desorption rate of the two reaction stage was different depending on the FTT. The desorption speed of the two reaction stage and the turn time of fast reaction to slow reaction decreased firstly and then increased with the the FTT increasing.3. The fractionation of the absorbed Cd ordered in:exchangeable from>residual form> bound to carbonates form>bound to Fe-Mn oxides form>bound to organic matter form after adsorbing Cd of soils under various freeze/thawing treatments.The FTT and the soil water content affected the exchangeable from Cd, Carbonates form Cd, Fe-Mn oxides form Cd significantly, and the FTT also affected the residual Cd significantly.The availability index (K) and mobility index (M) of the adsorbed Cd in soils increased firstly then decreased with the freezing and thawing frequency increase,and the mean of them were66%and63%respectively.4.The adsorption capacity, hysteresis coefficient,fast desorption speed, slow desorption speed and the various fractionation of the Cd in soils were connected with the pH, CEC, CECp, DOC, amorphous iron oxide, the specific surface and the fractal dimension significantly at0.05or0.01level during the progress of the Cd adsorption-desorption in freeze/thawed soils, while the exchangeable Cd and CECp were significant positive correlation at0.01level. And the statistical analysis results showed that the facilitation of pH and CECp on kF was produced through the direct action, that of CEC on kF was produced through the indirect action; the inhibition of DOC on kF was produced through direct action.To sum up, freeze/thawing treatments made soil experience frost heave-ablation process, which lead to soil structure and mainly pure montmorillanite destruct, organic matter component and soil solution components change, caused the soil pH, CEC, CECP, DOC, amorphous iron oxide, specific surface area and D changed, so that caused the change of the soil Cd adsorption quantity,adsorption-desorption speed and the fractionation of the adsorbed Cd in the soil under different freeze/thawing treatments. The change of the Cd adsorption-desorption characteristics of soil under different freeze/thawing treatments was resulted by integrated effects of physical and chemical properties. |
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