Arsenic Adsorption Onto Cr-substituted Ferrihydrite:Performance And Mechanisms

Iron(Fe)(hydr)oxides directly influence the fate of arsenic(As)through adsorption.However,the As adsorption capacities of Fe(hydr)oxides vary with their types and structures.Structure-substitution is a common phenomenon in the natural formation of Fe(hydr)oxides,during which similar elements,like chromium(Cr)and aluminium(Al),could replace a part of Fe to form substituted Fe(hydr)oxides.Nowadays,there is a lack of knowledge about the role of substituted Fe(hydr)oxides,especially Cr-substituted Fe(hydr)oxides,in the fate of arsenic.Cr-substituted Fe(hydr)oxides are widely found in Cr-rich areas with chromite mines.Studying the adsorption of As onto Cr-substituted Fe(hydr)oxides for arsenic is helpful for the prediction and prevention of As pollution in Cr-rich areas.In this study,the adsorption performance and mechanisms of Cr-substituted ferrihydrites(Cr-Fhs)for As(III)and As(V)were systematically investigated.The main contents were summarized as follows:(1)Cr-Fhs with Cr/Fe molar ratio of 1/3,1/1 and 3/1 were synthesized via coprecipitation of Cr³⁺and Fe³⁺.The morphology,structure,surface reactivity,and mineral stability of Cr-Fhs were analyzed through chemical methods,mineralogical characterization and model fitting.Cr-Fhs,like Fh,consisted of nanocrystalline particles,which tended to aggregate.The crystalline structures of Cr-Fhs were similar with that of ferrihydrite(Fh).However,with increasing ratios of Cr-substitution,the crystallinity of Cr-Fhs gradually decreased,while their surface areas increased.The surface Fe sites were partially replaced by Cr sites,due to the homogeneous substitution of Fe atoms by Cr atoms in the structures of Cr-Fhs.The changes of types and densities of surface sites resulted in the rise of surface charge densities as the ratios of Cr substitution in Cr-Fhs increased.In addition,dissolution kinetics results indicated that Cr-Fhs were more insoluble than Fh.(2)The As adsorption performance of Cr-Fhs was measured as a function of pH(3-11),solution concentrations(0.1-3 m M),and background electrolyte levels(0.05and 0.1 M).The results of adsorption kinetics showed that the sorption rates and densities of As decreased with increasing Cr content in Cr-Fhs,and the decreasing extent of As(V)was greater than that of As(III).According to the adsorption experiments under different pH conditions,the As adsorption densities on Cr-Fhs surfaces were pH-dependent.The sorption densities of As(III)were gained maximum values under near-neutral(pH 6-8)conditions,and dropped as pH decreased or increased.The sorption densities of As(V)were higher under acidic conditions,and gradually decreased with increasing pH.Cr substitution did not change the type of pH-dependent adsorption behavior.Based on isothermal adsorption experiments under different ionic strengths,elevated ionic strengths led to a decrease of As adsorption on Cr-Fhs,but had no effect on As adsorption on Fh,which indicated that the substitution of Cr possibly induced outer-sphere complexation of As on the surface of Cr-Fhs.(3)The As adsorption mechanisms of Cr-Fhs were quantitatively analyzed by CD-MUSIC model fitting.The main mechanisms of As adsorption on Fh were bidentate and monodentate inner-sphere complexation between?Fe OH^-0.5 sites and As.Cr substitution altered the site density and arrangement of?Fe OH^-0.5 in Cr-Fhs,affecting the reactions related to?Fe OH^-0.5 sites.Especially,the amount of adjacent?Fe OH^-0.5sites was reduced by Cr substitution,which inhibited bidentate complexation but promoted monodentate complexation.Meanwhile,outer-sphere complexation of As was verified to be an added adsorption mechanism for Cr-Fhs.Generally,increasing ratios of Cr substitution could result in a decrease of bidentate complexes and a growth of monodentate and outer-sphere complexes.