Effect Of Freeze-Thawing Cycles On Cadmium Transformation And Transference In Soil

Freeze-thawing cycles is a kind of non-biological impact on soil.Its effects on soil physical-chemical properties have been studied comprehensively but its effects on soil pollution and its ecological significance was not concerned enough,particularly the mechanism of Cd element transfer and transformation in soil under freezing-thawing cycles was still remained elusive.This paper used burozem which is a typical soil in the northeast region of China to determine the effects of freezing-thawing cycles on Cd transfer and transformation in soil with indoor culture under different soil water content and different frequency of freeze-thawing cycles.These results provide a scientific basis for establishing of Cd environmental capacity in soil,assessing of Cd eco-environmental risk,evaluating of soil ability to buffer exogenous Cd and diagnosing of soil pollution in the northeast region.The condition of the freezing-thawing cycles was adopted as the temperature of freezing and thawing being-30℃and 30℃,respectively,both freezing and thawing were lasted for 3 days (as 6 days for one freeze-thawing frequency),the frequency of freeze-thawing was 0,1,3,6, 9,12,respectively.The main results of this study are presented:1.Effect of freeze-thawing cycles on the form of Cd.The difference of concentration among exchangeable Cd,Fe/Mn-Oxide binding Cd,bound-organic Cd,and residual Cd was big under freeze-thawing cycles after added Cd 10mg kg^-1to soil with water content 0.21kg kg^-1～0.45kg kg^-1,and exchangeable Cd was the main form,second was residural Cd,the amount of bound-organic Cd and Fe/Mn-Oxide binding Cd were relatively low.The content of soil water and the frequency of freezing-thawing cycles significantly influenced the content of each Cd form(p＜0.01).The optimum regression equations for the relationship among the content of soil water(X₁),the frequency of freezing-thawing cycles (X₂),and the content of each Cd form(Y)were as followed: For exchangeable Cd,Y=18.04-62.86X₁-0.48X₂+75.19X₁²+1.49X₁X₁;for Fe/Mn-Oxide binding Cd,Y=0.46+0.13X₁X₂;for bound-organic Cd,Y=0.74+8.3l×10^-2X₂²;and for residural Cd,Y=2.91-0.45X₁X₂.This means that the effects of soil water content and freeze-thawing frequency on Cd concentration of four forms were not the same.The results showed that soil water content has significant effect on exchangeable Cd,frequency of freeze-thawing has significant effect on bound-organic Cd,the interaction effect of soil water content and frequency of freeze-thawing has significant effect on Fe/Mn-Oxide binding Cd and residural Cd.2.Effect of Freeze-thawing cycles on Cd release from soil.Freezing-thawing cycles increased Cd release from soil under these conditions of soil water content equal to field moisture capacity and soil Cd content between 10mg kg^-1～50mg kg^-1.The release rate of ion-state Cd from soil under freeze-thawing conditions was larger than that observed under no freeze-thawing conditions but the release rate of unknown-state Cd was smaller than that observed under no freezing-thawing conditions.However,the release rate of complex-state Cd was not significant different with or without freezing-thawing cycles.Those results demonstrated that some unknown-state Cd originally bound firmly to soil colloids could be transformed into ion-state Cd under freeze-thawing cycles so that it was easily to release from soil.3.Effect of Freeze-thawing cycles on the ability of Cd absorbed to soil and desorbed from soil.The ability of soils treated with freeze-thawing cycles to absorb Cd was smaller and their ability to desorb Cd became larger as compared to the untreated soil at the same content of added Cd under field capacity water of soil.The treated soils with freeze-thawing cycles exhibited less ability to absorb Cd and more ability to desorb Cd with the increase of freeze-thawing frequency.These results suggest that freeze-thawing cycles can reduce the ability of soil to keep Cd.Langmuir and Freundlich equations could fit well to the ability of soils to absorb Cd with or without freeze-thawing cycles(p＜0.01).The desorption capacity of Cd in soil with or without freeze-thawing cycles could be well described by the Freundlich equation(p＜0.01). 4.Effect of Freeze-thawing cycles on Cd transferability.Soil porosity was determined after original soil sampled by cutting ring and treated by Freeze-thawing cycles.Results showed that freeze-thawing cycles increased the soil porosity.Experiments showed that deionized water transferred faster in the earth column filled with Cd polluted soil under freezing-thawing conditions than that of no freezing-thawing cycles when use the same amount deionized water to leach the earth column.Similarly,Cd ion transferred faster in the earth column with freeze-thawing cycles treatment as compared to that of no freeze-thawing cycles treatment.The concentration of Cd ion in leacheate from the column with freeze-thawing cycles treatment was higher than that observed in the untreated soil.The total amount of Cd ion leaching from the column increased with the increase of freeze-thawing frequency.Those data suggest that freeze-thawing cycles increase the ability of Cd to transfer in soil so that enhance the harm of Cd to soil-underground water system.