The Mechanism Of Soil Humin With Cr(Ⅵ)/As(Ⅲ) And Its Ability To Alleviate Wheat Stress

Chromium（Cr）and arsenic（As）cause serious soil pollution problems and have already posed a serious threat to the environment and human health.Cr（Ⅵ）and As（Ⅲ）are highly toxic and have strong carcinogenic effects,which have attracted widespread attention.Soil humin（HM）,as one of the main components of soil humus,contains a large number of reactive functional groups and structures such as phenolic hydroxyl,carboxyl,persistent free radicals（PFRs）and can also promote the production of reactive oxygen species（ROS）.Therefore,the impact of HM on the environmental behavior of Cr（Ⅵ）/As（Ⅲ）has received widespread attention.It is revealed that the impact of HM on the environmental behavior of Cr（Ⅵ）/As（Ⅲ）is important for understanding the ecological risks of Cr/As in the soil and repairing pollution.This work explored the adsorption-reaction process of HM and Cr（Ⅵ）/As（Ⅲ）extracted from different types of soil.First,The effects of reaction conditions,the types of functional groups on HM and the generation of PFRs and ROS on the conversion of Cr（Ⅵ）/As（Ⅲ）were investigated.Then,the mechanism of HM promoting the conversion of Cr（Ⅵ）/As（Ⅲ）was revealed and the effect of HM on the growth and development of wheat seedlings under Cr/As stress was further analyzed in this work.Finally,the potential of HM in soil remediation was explored,which provides scientific basis for the remediation of soil heavy metal pollution.The main conclusions are as follows:（1）The active functional groups（carboxyl,phenolic hydroxyl）and PFRs content of HM from different sources are quite different,which determines that HM from different sources has different reactivity.In the range of pH 2-7,the surface of HM exhibits electronegativity.As the pH increases,the amount of negative charge on the surface of HM gradually increases.（2）The reaction mode of HM and Cr（Ⅵ）is adsorption and reduction reaction.Cr（Ⅵ）follows an adsorption-reduction mechanism on the HM solid phase.HM mainly adsorbs Cr（Ⅵ）in monolayer.The phenolic hydroxyl and carboxyl groups of HM can complex Cr.The phenolic hydroxyl group and PFRs of HM participate in the reduction process of Cr（Ⅵ）.The phenolic hydroxyl group and PFRs can directly transfer electrons to Cr（Ⅵ）to reduce Cr（Ⅵ）,and can also transfer electrons to dissolved oxygen to generate O₂^·-Indirect reduction Cr（Ⅵ）.After suppressing the production of O₂^·-in the reaction,the ability of HM to reduce Cr（Ⅵ）is reduced by about 30%.As the pH of the reaction increases,the adsorption and reduction of HM on Cr（Ⅵ）weaken.（3）HM can directly adsorb As（Ⅲ）and indirectly oxidize As（Ⅲ）.The binding sites of HM to As are mostly weak binding sites.As（Ⅲ）and As（Ⅴ）have different binding modes on the HM solid phase.Phenolic hydroxyl and carboxyl are the main structures for complexing and adsorbing As.The binding strength of As（Ⅴ）and HM is stronger than that of As（Ⅲ）.As（Ⅲ）adsorbed on the HM solid phase can no longer be oxidized to As（Ⅴ）.Phenolic hydroxyl and PFRs can indirectly oxidize As（Ⅲ）by producing·OH,and the ability of HM to oxidize As（Ⅲ）after·OH is reduced by 70%-75%.The increase of reaction pH will weaken the adsorption and oxidation of As（Ⅲ）by HM.（4）Cr/As can significantly inhibit the growth and development of wheat roots,reducing the average root length,average root surface area,average root volume and root tip number of wheat by more than 80%compared with normal growth and development.The presence of HM can effectively reduce the content of free Cr/As in the culture solution,relieve the inhibition of Cr/As on the growth and development of wheat roots,and reduce the content of accumulated Cr/As in wheat.