Adsorption-Reduction Of Hexavalent Chromium At The Solid-water Interface And Its Soil Environmental Significance

Affected by improper treatment of industrial activity wastewater and waste residue such as electroplating,printing and dyeing,tanning and non-ferrous metal smelting,chromium(Cr)has become a common heavy metal pollution element in farmland soil.In the soil,Cr mainly exists in two valence states of +6 and +3.Studies have confirmed that hexavalent chromium is both highly biotoxic and easy to leaching-migrating.However,trivalent chromium is relatively less biotoxic,which is easily passivated by precipitation reaction.Therefore,the reduction of hexavalent chromium to trivalent chromium is the main idea to control the risk of soil chromium pollution.Hexavalent chromium can undergo chemical reduction and photocatalytic reduction in soil,but the microscopic mechanism of the above two types of reactions on soil solid-water interface has not been clarified for a long time.In addition,with the wide application of industrial nanomaterials,a considerable amount of carbon nanomaterials may be released into the soil,and how they will affect the reduction process of hexavalent chromium remains to be elucidated.Therefore,this paper systematically studied the reduction process of hexavalent chromium in solid-liquid systems composed of three model materials such as goethite,carbon nanotubes and graphene and water or low molecular weight organic acids(tartaric acid,citric acid).The effects of pH,time,and reactant concentration were investigated,and the related reaction mechanism was revealed by surface analysis techniques such as XPS and FTIR.The main findings are as follows:1.The adsorption amount of goethite on tartaric acid decreases with the increase of pH.This is mainly because the increase of the negative charge on the surface of goethite increases with the increase of pH,and the electrostatic repulsion of hexavalent chromium anion increases.The kinetic experiments showed that adsorption of tartaric acid and Cr(VI)onto the goethite surfaces was rapid during the initial stage,followed by a slow process until equilibrium state was obtained.And the kinetic data can be fitted using a pseudo second order reaction kinetics model.FTIR analysis found that the adsorption mechanism of tartaric acid on the surface of goethite is mainly composed of monodentate or bidentate surface complexes.The surface of goethite can catalyze the reduction of hexavalent chromium by tartaric acid,but the specific reaction mechanism is related to the initial concentration of tartaric acid.When the ratio of the initial concentration of tartaric acid to hexavalent chromium is 10:1,the adsorption of hexavalent chromium is completely inhibited,and under this condition,hexavalent chromium is mainly reduced by surface-adsorbed tartaric acid.When the initial concentration ratio of tartaric acid to hexavalent chromium is 1:1,the adjacent adsorption state of tartaric acid and hexavalent chromium on the goethite surface can directly undergo electron transfer.Alternatively,goethite acts as a conductor to transfer electrons to hexavalent chromium adsorbed on the surface of goethite.In addition,the catalytic effect of the ferric ion released by the dissolution of goethite on the reduction of hexavalent chromium by tartaric acid is negligible.2.In this study,MWCNT-COOH possesses a higher adsorption capacity for Cr(VI)than MWCNT-OH,which was more evident as the pH decreased.A low pH was favorable for Cr(VI)adsorption by both MWCNTs.The pure solution system citric acid has a very slow reduction rate of Cr(VI),but when the carbon nanotubes are present,the reduction rate of hexavalent chromium is significantly accelerated.The main reason is that the citric acid is adsorbed onto the surface of MWCNT,and the reduction activity is enhanced.In addition,the simulation experiment further found that when MWCNTCOOH and MWCNT-OH are present in the oxisol,the rate of hexavalent chromium reduction by citric acid is also significant,and the trivalent chromium fraction formed in the solution can be resorbed to the surface of the solid phase.3.Under dark conditions,low pH conditions favor the adsorption of hexavalent chromium on the surface of graphene.Similarly,as the pH increases,the amount of negative charge on the graphene surface increases,and thus the electrostatic repulsion of hexavalent chromium is enhanced,so that the adsorption amount of hexavalent chromium is reduced.The Cr(VI)reduction by GO with UV-light irradiation was observed,which was more efficient at low pH.The intrinsic mechanism is that after graphene absorbs ultraviolet light,electrons(e-)can transition from the valence band to the conduction band and form holes(h+)in the valence band.The conduction band electrons are captured by the surface-adsorbed hexavalent chromium and the hexavalent chromium is reduced to trivalent chromium.The above mechanism can be confirmed by the influence of EDTA and the degradation test of tetrachlorophenol: First,the presence of EDTA will promote this photocatalytic reaction because EDTA can capture valence band holes;second,graphene can also photooxidize tetrachlorophenol.And directly explain the existence of holes.Moreover,this kind of UV-light catalyzed Cr(VI)reduction could also occur in an oxisol with the presence of GO,indicating the GO released into soil environments would promote the transformation of Cr(VI)to Cr(III).The above results not only further enhance the understanding of the mechanism of hexavalent chromium reduction at the solid-water interface,but also verify that the release of carbon nanomaterials into the soil environment can promote the conversion of hexavalent chromium to trivalent chromium.It provides theoretical evidence for the establishment of soil hexavalent chromium pollution control methods.