Remediation Of Cadmium-contaminated Soil By Different Passivators

Industrial and agricultural production activities such as mining,smelting,farmland irrigation,and fertilization lead to serious heavy metal（HM）pollution in soil.Cadmium（Cd）is one of the most important soil HM pollutants.Chemical stabilization is a remediation technique that uses passivators to reduce the mobility and bioavailability of HMs in soil.There are various types of passivators with different remediation mechanisms and application conditions.Therefore,more research is needed to compare the remediation effects of different passivators on Cd-contaminated soils.Solanum nigrum L.,as a hyperaccumulator of Cd,can be used for ecological restoration of Cd-contaminated soils.However,the growth of S.nigrum is still suppressed in highly Cd-contaminated soils.Chemical stabilization can alleviate Cd stress and assist in restoring the ecology of highly Cd-contaminated soils by S.nigrum.However,there are few studies on the joint remediation of highly Cd-contaminated soils by chemical stabilization and S.nigrum.The effects of common passivation agents and compound passivation agents on soil heavy metals stabilization and soil ecology need further study.In the first study,highly Cd-contaminated soil(50 mg kg^-1)was simulated and S.nigrum was used as an indicator plant for soil remediation.The remediation effects of hydroxyapatite（HAP）,zero-valent iron（ZVI）,calcined eggshell powder（ES）,polyacrylate water-retaining agent（SDK）,and biochar（BC）at different doses（0.1%,1%,w/w）were compared and studied.Under Cd stress,the photosynthesis and growth of S.nigrum were inhibited,while peroxidase（POD）activity and soluble protein（SP）content in leaves were significantly increased.The application of passivators reduced the DTPA-Cd content in soil and the uptake of Cd by S.nigrum,as well as alleviated the stress of Cd on S.nigrum.The treatments of 0.1%dose were generally inferior to the treatments of 1%dose,SDK,ES,HAP,ZVI,and BC at 1%dose reduced the DTPA-Cd content in soil by 16.58%,22.74%,19.43%,17.11%,and 3.25%,respectively.DTPA-Cd content in soil was positively correlated with the Cd concentration in S.nigrum.ES and HAP had the best stabilization effects,which significantly increased soil p H,improved soil nutrition（available phosphorus and available potassium）,and increased the biomass of S.nigrum,but did not significantly reduce TCLP-Cd content in soil.However,ZVI and SDK at 1%dose significantly reduced TCLP-Cd content by 14.89%and 35.60%,respectively.ES was the best passivator in screening test and BC had poor effect,but BC was a hotspot material in passivators,so we chose another BC material in second study.We investigated the effects of BC,ES,and their combined treatment（EB）on Cd availability,S.nigrum growth and soil bacterial communities in soils with low,medium,and high Cd pollution(1,10,and 25 mg kg^-1 Cd,respectively).The results showed that only high Cd stress reduced the biomass and photosynthetic pigment content,increased the malondialdehyde（MDA）content and POD activity,while all three treatments alleviated the effect of Cd on S.nigrum.ES and EB treatments increased catalase（CAT）activity in leaves under high Cd stress.The ES treatment had the most pronounced effect on the reduction of DTPA-Cd content and Cd concentration in S.nigrum,followed by EB and BC.BC treatment had the minimal effect on the reduction of Cd concentration in S.nigrum,but improved the growth condition of S.nigrum under high Cd stress.EB and ES treatments significantly increased soil p H and Alpha（α）diversity of soil bacteria（Ace,Sobs,and Chao）,and effectively alleviated the inhibition of Cd on soil CAT activity,while BC treatment had little effect on these indexes.EB and ES enriched a variety of nitrate-reducing bacteria in the soil,such as Pseudomonas,f＿＿Bacillaceae,Microvirga,norank＿f＿＿JG30-KF-CM45,and Devosia,which are beneficial for stabilizing soil HMs through carbonate precipitation.In summary,all five passivators had certain remediation effects on Cd-contaminated soil,and ES was the best passivator.ES reduced soil available Cd,improved soil nutrients,increased bacterial community richness and microbial activity,reduced Cd absorption of S.nigrum,regulated the plant’s antioxidant enzyme system,and alleviated oxidative stress of S.nigrum.ES was not suitable for combination with BC,as the combination reduced the stabilization effect.This study explored the possibility of co-remediation of Cd-contaminated soil by chemical stabilization and S.nigrum,providing an important basis for the selection of passivators.