Effect Of Rhizosphere Microenvironment On Cadmium Adsorption/Desorption By Microplastics

Microplastics（MPs）are difficult to degrade and easily aged in environment,and have strong aggregation,migration and adsorption capacity in water and soil.These may change environmental behavior of existing pollutants in soil.Cd is one of the most widely existed heavy metals in Chinese soil,and its over standard rate in sites was the highest as 7.0%according to the national soil pollution survey.Therefore,MPs in soil can interact with Cd and affect the environmental behavior of Cd.Since the adsorption of heavy metals,such as Cd,by MPs is mainly through physical and chemical interactions,this process may be changed by environmental conditions,such as rhizosphere microenvironment.Rhizosphere is a special region formed near the root system of plants,which has abundant root exudates（such as small molecular organic acids,amino acids,proteins,etc.）.The interaction between MPs and heavy metal may be changed in this micro zone.However,little is known up to now.Therefore,in this study,firstly,the effects of MPs aging on the adsorption/desorption behavior of Cd²⁺was studied;Secondly,the adsorption/desorption of Cd²⁺by aging MPs in the presence of simulated root exudates in solution and soil was studied.Finally,the effect of real root fraction was studied.The main results are as follows:（1）The study on the effects of MPs aging on adsorption/desorption of showed that aging increased the Cd²⁺adsorption capacity of Polystyrene（PS）and Polylactic acid（PLA）MPs by 262.44 mg/kg and 187.99mg/kg respectively.MPs aging increased the oxygen-containing functional groups in PS-MPs and PLA-MPs.The decrease of p H and the increase of Na⁺concentration decreased the adsorption of Cd²⁺by aging PS-MPs and PLA-MPs.The increase of p H and Na⁺content inhibited the desorption of Cd²⁺（2）The study on the effects of simulated root exudates on Cd²⁺adsorption/desorption by aging MPs in solution environment showed that,single root exudate（such as glucose,serine,and citric acid）reduced the adsorption capacity of aging MPs on Cd²⁺,especially citric acid.Glucose and serine inhibited the desorption of Cd²⁺from aging MPs,but citric acid promoted the desorption rate of Cd²⁺as by 15.61%and 13.50%,respectively.The complex of root exudates decreased the adsorption capacity of aging PS-MPs and PLA-MPs for Cd²⁺,and the higher the concentration was,the stronger the inhibition effect was.The complex of root exudates inhibited the desorption of Cd²⁺from aging MPs,but root exudate concentration had little effect on Cd²⁺desorption.（3）Effects of simulated root exudates and aging MPs on Cd²⁺adsorption/desorption of soil showed that the simulated root exudates increased the Cd²⁺adsorption of black soil and red soil containing 1%（w/w）aging PS-MPs and PLA-MPs,and decreased the Cd²⁺adsorption of brown soil.The Cd²⁺adsorption of black soil and brown soil containing 10%aged PS-MPs and PLA-MPs was decreased,while the Cd²⁺adsorption of red soil was increased.The simulated root exudates decreased the Cd²⁺desorption of soil containing 1%aging PS-MPs and PLA-MPs,and decreased the Cd²⁺desorption of black soil containing 10%aging PS-MPs and PLA-MPs,but had no effect on the Cd²⁺desorption of red soil and brown soil.（4）The study on the effects of real root exudates on Cd²⁺adsorption/desorption by aging MPs showed that,root exudates inhibited the adsorption of Cd²⁺by aging MPs regardless of its concentrations.Low and medium concentration of root exudates did not affect the desorption of Cd²⁺from aging PS-MPs,but high concentration of root exudates significantly inhibited Cd²⁺desorption.Root exudates inhibited the desorption of Cd²⁺adsorbed from aging PLA-MPs regardless of their concentrations.