Effect Of Sulfhydryl-modified Bentonite On Transformation Of Cadmium In Rice Soil

China is a country encompassing a large population,comparatively limited soil resources.Simultaneously,the rapid process of urbanization and industrialization have made the heavy metals contamination of agricultural soil increasing seriousness.Cadmium is a carcinogen classified by the International Agency for Research on Cancer?IARC?.The cadmium in soil is extremely easy enriched by rice,which causes harmful effects to human.However,the absorption of cadmium by rice depends not only on the total amount of cadmium,but also on the forms of cadmium in the soil.In this study,a cadmium-contaminated farmland soil in Danling,Sichuan,was used as a remediation object.Field and pot experiments were conducted to investigate the influences of sulfhydryl-modified bentonite on the transformation of cadmium and the absorption by rice as well as its growth.It also estimates the passivation effect of sulfhydryl-modified bentonite on cadmium,which provides a foundation for the remediation of cadmium-contaminated soil.The primary research contents and results are as follows:?1?The extraction and determination methods of seven forms of cadmium in soil in"Technical Requirements for Analysis of Ecological Geochemical Evaluation Samples"?DD2005-03?were optimized by selecting experimental water,blank sample and oscillation mode.?2?The structure of sulfhydryl-modified bentonite and it's adsorption to cadmium were characterized,further experiments were carried out to obtain a pilot product with the same performance of prime products,and it was applied to passivation remediation experiment of rice cultivation in pot and the field.?3?The results of the field experiment show:after adding materials Bent-1 and Bent-2,the available cadmium in the field soil decreased significantly with the increase of the amount of materials,which decreased from 0.7 mg/kg and 0.48 mg/kg to0.36 mg/kg and 0.31 mg/kg,respectively.The changes of carbonate bound cadmium,humic acid bound cadmium and organic matter bound cadmium were not obvious,and exchangeable cadmium and Fe-Mn oxides bound cadmium changed significantly.After adding Bent-1,the proportion of exchangeable cadmium decreased from 30%to 20%,and the proportion of Fe-Mn oxides bound cadmium increased from 15%to 24%.After adding Bent-2 to soil,the proportion of exchangeable cadmium in soil decreased from39%to 27%,and the proportion of Fe-Mn oxides bound cadmium increased from 11%to 23%.Adding Bent-1,the cadmium content of rice and straw decreased gradually with the increase of material addition.The cadmium content of rice and straw decreased by 70.48%and 63.08%at most,and the material Bent-2 had no obvious effect on rice.Correlation analysis of cadmium form results in field experiments showed that the amount of material addition was significantly or extremely significantly negatively correlated with available cadmium,water soluble and exchangeable cadmium in soil.The water soluble and exchangeable cadmium in soil contributed greatly to the available cadmium content.Sulfhydryl-modified bentonite passivated cadmium in soil by transforming water-soluble and exchangeable cadmium into the Fe-Mn oxide bound cadmium.?4?The results of pot experiment indicated that:the available cadmium decreased significantly with the increase of Bent-1 and Bent-2,from 3.55 mg/kg to 1.38 mg/kg and 3.00 mg/kg to 1.79 mg/kg,respectively.After adding Bent-1 and Bent-2,except for water soluble cadmium and residual cadmium,the other five forms of cadmium had changed.The proportion of exchangeable cadmium decreased from 34%to 17%and13%,and the proportion of carbonate bound cadmium decreased from 24%to 22%and15%,respectively.The proportion of Fe-Mn oxides bound cadmium increased from 8%to 24%and 30%,and the proportion of organic matter bound cadmium increased from1%to 4%and 11%,respectively.The proportion of humic acid bound cadmium increased from 20%to 24%after adding Bent-1,while adding Bent-2 it decreased from20%to 17%.The Bent-1 effectively reduced the oxidative stress of rice under cadmium-contaminated,then reduced the activity of enzymes in rice leaves.At the same time,the CAT activity decreased gradually with the increase of material addition from 168 U·g^-1FW·min^-1 to 49 U·g^-1FW·min^-1;the SOD activity increased slightly when 12.5 g material was added,and then decreased from 111 U·g^-1FW·h^-1 to62 U·g^-1FW·h^-1 with the increase of amount;with the increase of Bent-1,the POD activity decreased from 199?g·g^-1FW·min^-1 to 141?g·g^-1FW·min^-1 gradually.The results of form analysis of cadmium in soil indicated that the passivation mechanism of the two materials was similar,meant that the materials reduced the available cadmium by transforming exchangeable cadmium,carbonate bound cadmium into Fe-Mn oxides bound cadmium and the organic matter bound cadmium.The cadmium content of rice and straw declined with the increase of Bent-1 and Bent-2 in pot experiment.The maximum content descended to 0.39 and 1.31 mg/kg by Bent-1 and 0.45 and1.54 mg/kg by Bent-2.The exchangeable cadmium and carbonate bound cadmium were significantly correlated with cadmium content of rice and straw,indicating that the two forms contributed most to cadmium content in rice and straw.