The Adsorption Characteristics Of Cadmium On Clay Mineralsand Their Capacity In Alleviating The Toxicity Of Cadmium To Myriophyllum Verticillatum

Sepiolite and montmorillonite are two kinds of typical clay minerals,which had been widely applied in heavy metal pollution controlling areas for their good adsorption performance,low cost,abundance,and environmental compatibility.In recent years,the use of clay minerals to reduce the migration and bioavailability of the heavy metals in the environment by adsorption,coordination,and coprecipitation has become the hot research topics.Therefore,alleviating the toxic effects of heavy metals on plants by clay minerals has shown great potential applications.On the other hand,the adsorption performance and mechanism of heavy metal ions are different due to the different types and amounts of surface active functional groups,and the ability of alleviating the plant toxicity may also be different,especially the ability of alleviating poison in environment containing a variety of co-existing ions;while the relevant studies have been rarely reported.Therefore,it is necessary to investigate the effect of clay mineral surface structure and coexisting ions on the adsorption behavior of heavy metals,and their effects in alleviating the toxicity of heavy metals to aquatic plants.In this paper,natural sepiolite and montmorillonite were used as an representative to compare and study the effects of coexisting cations(Ca²⁺、K⁺、Na⁺)and coexisting anions(H₂PO₄^-、SO₄^2-、NO₃^-)on Cd²⁺adsorption and analyzing their differences on adsorption performance and mechanisms.On this basis,we compared the effects of two kinds of clay minerals on biomass,chlorophyll,soluble protein and malondialdehyde of Myriophyllum spicatum under different concentrations of Cd²⁺stress,and investigated the mechanisms of clay mineral in alleviating the toxicity of cadmium to Myriophyllum verticillatum.This study has come to the following conclusions:（1）The structure characteristics of natural sepiolite and montmorillonite were characterized with XRD,XRF,FT-IR,and TG-DSC,and the results showed that sepiolite and montmorillonite had the similar chemical composition.Sepiolite mainly dehydrated the hydroxy water in the structure,which accounted for up to 71.6%of the total lost mass;while montmorillonite mainly lost the surface adsorbed water and pore water,which accounted for90.9%of the lost mass.As such,sepiolite should contain a large amount of structural hydroxyl groups（Si-OH,Mg-OH）,which was further comfirmed by FT-IR.（2）The studies have revealed the relative mechanism that the adsorption of Cd²⁺by montmorillonite and sepiolite was statistically significant.Montmorillonite showed better Cd²⁺adsorption capacity than sepiolite in the single contaminant system,but sepiolite showed better adsorption capacity in the nutrient solution that contains multiple ions.This is because the coexistence of cations and oxoanions can strongly inhibit the adsorption of Cd²⁺on montmorillonite;while for sepiolite,although the presence of cation and NO₃^-are not conducive to the adsorption of Cd²⁺,H₂PO₄^-and SO₄^2-could undergo ligand exchange with Si-OH on sepiolite,resulting in the formation of a ternary complex compound with Cd²⁺,which then can promote the adsorption of Cd²⁺on sepiolite.（3）Both sepiolite and montmorillonite can alleviate the toxic effects of Cd²⁺on Myriophyllum verticillatum,and the relationship between Cd²⁺adsorption and the alleviation on their toxicity has been established.It can also reduce the destruction of chloroplast structure and the membrane peroxidation of Cd²⁺.Due to the difference in structure,sepiolite is a better adsorbent for heavy metals when multiple ions were coexistenced,and it showed better capacity in alleviating the toxic effect than montmorillonite.