The Effect Of Acid Rain On The Stabilization Of Cadmium In Contaminated Agricultural Soils

Acid rain and agricultural soils contaminated with Cd are two major environmental problems for Chinese agriculture,and they overlap in majority area.Stabilization technology is one of widely used remediation technologies for cadmium(Cd)contaminated agricultural soils,which promote the transformation from Cd with high mobility to Cd with less mobility,however,it doesn't decrease the total concentration of Cd.Stabilization often had good short-term performance,whereas it had bad long-term performance due to the stabilization performance is vulnerable to external environment.The stabilized Cd may be reactivated again in condition of acid rain,freeze thawing,etc.Therefore,it is necessary to study the effect of acid rain on the stabilization of Cd contaminated agricultural soils when the stabilization technology is applied in the Cd contaminated agricultural soils in the acid rain area.In this study,we collected the monitoring data of acid rain in China in recent years,analyzed the characteristics and changing trends of acid rain in several typical regions of China,and study the effect of simulated acid rain on the stabilization of Cd in contaminated agricultural soils treated with stabilizing agents.Three widely used stabilizing agents,mono-calcium phosphate(MCP),mono-ammonium phosphate(MAP)and artificial zeolite(AZ)were used for Cd contaminated agricultural soil that collected from the middle region of Jiangsu province with 3-month incubation.Two types of simulated acid rain with three levels of acidity(pH = 3.0,4.0 and 5.6)were simulated by sulfuric acid and nitric acid.The three stabilized soils were soaked with different simulated acid rain to study the long-term affect of simulated acid rain on stabilized soil with different stabilizing agent,respectively.The chemical forms and the leaching toxicities of Cd in the soils were determined by Tessier's sequential extraction procedure and toxicity characteristic leaching procedure(TCLP),respectively.The results as follow:(1)In recent years,acid rain in China had a substantial improvement on the whole,whereas the type of acid rain is changing from the sulfuric type to the mixed type.(2)There is a large spatial overlap between acid rain pollution and agricultural soils contaminated with Cd.(3)The three stabilizing agents can reduce the mobility of Cd in soil to varying degrees,and their stabilization performances followed the order of AZ<MAP<MCP.(4)Continuous acid rain soaking treatment could promote the transformation of Cd with less mobility to Cd with high mobility,and cause the stabilization performance weakened to some extent.(5)Both anion composition and pH of acid rain were two major factors that influenced the chemical forms and leachability of Cd in soil.In general,the environmental risk of Cd reactivation will be enhanced with the decrease of pH and the increase of the ratio of SO42-/NO3-.(6)Compared with MAP and AZ,MCP is a more suitable stabilizing agent for the stabilization of Cd contaminated soil in the acid rain area.(7)The quantitive relationship of the leachability of Cd in stabilized soil with the above three stabilizing agents and the type and pH of acid rain could be well simulated through the models of power exponential function.