Screening And Repairing Effects Of Cd-contaminated Farmland Soil Passivators

Soil environment is an important carrier of natural environment and agricultural production on which mankind depends.With the intensification of human activities and the expansion of urban scale,soil pollution is becoming more and more serious.The area of cultivated land contaminated by heavy metals such as Cd,As,Cr and Pb in our country is nearly 20 million hectares,accounting for about 1/5 of the total area of cultivated land,especially the most serious pollution situation of cadmium.In April 2014,the survey bulletin of soil pollution in China pointed out that the exceeding standard rate of Cd reached 7.0%,ranking first in the list of inorganic pollutants.Cd is one of the most toxic heavy metal elements,and has strong chemical activity in the soil and long-lasting toxicity.It is easily accumulated in the human body through the food chain and endangers human health.If the cadmium contaminated soil can not be treated timely and effectively,it will have an enduring and far-reaching impact on the environment and biology.Therefore,it is necessary to rehabilitate and manage timely.In this paper,the effect of different passivators on Cd in Guangxi yellow soil,Jiangxi red soil and Jiangsu brown soil was studied.The four common passivators were CaSiO3,Ca(H2PO4)2,CaCO3 andNa2S.Through the pot experiments in wheat,the impact of different passivators on bio-availability of Cd-polluted soils was also studied,aiming to screen out cost-effective and environment-friendly passivators for the different types of soils to effectively control crop heavy metals.The main conclusions are as follows:(1)After adding four passivators in three soils,the soil pH value changed to some extent in Guangxi yellow soil,Jiangxi red soil and Jiangsu brown soil.The pH values of these three soils were all decreased with the addition of Ca(H2PO4)2 reagent,while increased to some extent with the addition of the other three passivators.Futhermore,the pH value of Jiangsu brown soil reached the highest after adding 20 Na2S reagent,about 10.53,which showed a strong alkalinity.And the pH value of Jiangxi red soil reached the lowest value after adding 20‰Ca(H2PO4)2 reagent,about 5.66,which showed a weak acidity.The changes of organic matter content in the three soils added with four different passivators were similar,the organic matter content of the soils decreased when the passivators of CaSiO3,CaCO3,Na2S were added,while increased when Ca(H2PO4)2 reagent was added.When 20‰ Ca(H2PO4)2 reagent When 20‰Na2S reagent was added,the organic matter content of Jiangsu brown soil reached the minimum,about 8.29g.(2)After adding four passivators in three soils respectively,the available state of Cd in soils all showed a decreasing trend.For three soils,the best passivation effect was achieved at 20‰addition amount of Na2S reagent,the contents of Cd with available state in yellow soil,red soil and brown soil respectively decreased 16.96%,12.16%and 12.71%,followed by 20‰Ca(H2PO4)2,which decreased 16.12%,11.11%and 12.32%.Passivation effect of CaSiO3 and CaCO3 reagents on three soils were similar,which were inferior to those of Na2S and Ca(H2PO4)2 reagents.However,the analysis of the effective state with various single morphologies showed that morphological changes of Cd in the three soils differed.After the addition of four passivators,contents of Cd with exchangeable,reducible and oxidizable state in Guangxi yellow soil and Jiangxi red soil both decreased and the residual ones increased,while contents of Cd in Jiangsu brown soil with exchangeable and reducible state decreased and the oxidizable and residual contents increased.When the addition amounts of four passivators were 20‰,the exchangeable,reducible and oxidizable contents of yellow and red soils were the lowest,and the residual content was highest;the exchangeable and reducible contents of brown soil were the lowest,the oxidizable and residual contents were highest.The results showed that Na2S reagent had the most significant effect on contents of Cd with all forms in three soils.Under the addition of 20‰ Na2S,the contents of exchangeable Cd in yellow soil,red soil and brown soil decreased by 29.54%,21.61%and 33.02%respectively,and the contents of reducible Cd decreased by 30%,22%and 60%respectively.The contents of oxidizable Cd in yellow soil and red soil decreased by 6.4%and 26%respectively,and the content of oxidizable Cd in brown soil increased by 67%.The passivation effect of the four passivators in three soils was the best for Na2S reagent,followed by Ca(H2PO4)2 reagent and the worst among CaCO3 and CaSiO3 reagents.(3)The research of Jiangsu brown soil found that passivators of CaSiO3,CaCO3 and Ca(H2PO4)2 had similar effects on aboveground biomass,aboveground Cd content,grain yield and grain Cd content of wheat.Under the same addition amount of passivators,the treatment of Ca(H2PO4)2 reagent showed better biomass and grain yield,and the maximum of biomass and grain yield with the addition of 20‰ Ca(H2PO4)2 were 3.42 g/pot and 0.97 g/pot respectively.While the decrease of Cd content was lower.Under the addition amount of 20 ‰ Ca(H2PO4)2 reagent,Cd content of wheat aboveground and grain were the lowest,achieved 0.28 mg/kg and 0.047 mg/kg respectively,which was better than CaSiO3 and CaCO3 reagents.The effect of Ca(H2PO4)2 reagents on the control of wheat biomass and heavy metal Cd was better,probably due to the better passivation effect of phosphates to Cd in soil.Under the addition amount of 20‰ passivators,the contents of Cd with available state in Jiangsu brown soil decreased 7.83%(CaSiO3),12.32%(Ca(H2PO4)2),4.88%(CaCO3)and 12.71%(Na2S),of which Na2S reagent exhibited better passivation effect,followed by Ca(H2PO4)2,and CaSiO3 and CaCO3 showed the worst.All in all,among the three passivators(CaSiO3,Ca(H2PO4)2,CaCO3),Ca(H2PO4)2 had the best control effect on wheat biomass and Cd content,while Na2S was not conducive to the growth of wheat.With the consideration of wheat biomass and Cd controll effect,Ca(H2PO4)2 reagent seemed to be the best passivator,and its best addition amount was 20‰.