Effects Of Typical Hydrogeochemical Factors On Mechanism Of Sorption And Transformation Of Cr(?) On Ferrihydrite

Hexavalent chromium Cr(VI)is one of the most common heavy metal pollutants in soil and water environment.It has high biological toxicity and strong migration,posing a threat to human health and environmental safety.There are large amounts of iron oxide in vadose zone and aquifer media,which are important carriers of heavy metals and other contaminates.Ferrihydrite(Fh)is a weak crystalline iron oxide,which widely presents in soil and groundwater.It has a large specific surface area and high surface activity.It plays a higher adsorption capacity for Cr(VI)during a common p H value rang(5.0-7.0)in the environment.However,a great variety of hydrogeochemical factors often play affects on the Cr(VI)retention by Fh.In this study,Si?Fe(II)and citric acid which are typical hydrogeochemical factors were chosen and the influence on the mechanism of sorption and transformation of Cr(VI)on Fh were investigated by batch experiments of adsorption-desorption and microscopic characterization techniques.The main conclusions obtained are as follows:(1)The molar ratio of Si/Fe significantly affects the properties of the mineral and their sorption behaviors for Cr(VI).With the iron oxide Si/Fe molar ratio increased from 0 to 0.5,the adsorption capacity of Cr(VI)decreased by 34.0%,47.7% and 61.4% under the conditions of p H values of 5.0,6.0 and 7.0 in the reaction system.Interfacial microscopic characterization techniques such as X-ray photoelectron spectroscopy(XPS)and density functional theory(DFT)theoretical calculation were used to analysis results of experiments,they are proved that with the increase of Si content,the more electrostatic repulsion between Cr(VI)and the mineral.At the same time,the content of adsorbed hydroxyl(A-OH-)in the iron oxide gradually increases,and then the complex sorption capacity for Cr(VI)is enhanced.The two effects play opposite results on the sorption of Cr(VI)on Fh.In general,The increase in electrostatic repulsion is greater than the increase in complexing effect.The analysis of the microscopic mechanism further explains the macroscopic phenomenon that the sorption of Cr(VI)on Fh decreases with the increase of Si content.(2)The presence of Fe(II)has a significant effect on the sorption of Cr(VI)on Fh.During the reaction between Fe(II)and Fh,the total sorption capacity of Cr(VI)on the solid phase(Fh)is reduced by 65%.,And the proportion of the total Cr that could not desorbed from the solid phase has increased by about 40%.When Fe(II)is adsorbed on the surface of Fh,the amount of Cr(VI)sorption decreases by about 70% with the increase of p H value range(5.0-7.5),and the Cr(VI)desorption rate increases by 30%.After Fe(II)catalyzes the transformation of Fh crystal phase,the desorption rate of Cr(VI)is only about 30%.Fourier transform infrared spectroscopy(FTIR)and XPS characterization analysis further prove that the undesorbable Cr on the solid phase includes partial Cr(VI)which were wrapped by mineral besides Cr(III)which reduced from Cr(VI).(3)Citrate inhibits the sorption of Fh on Cr(VI)by competitive adsorption when citrate and Cr(VI)co-adsorbed on Fh,and then induced Fh to dissolve and released Fe(III)and adsorbed Cr(VI).While at the same time the Cr(VI)were reduced which is the same amount of adsorbed on the initial Fh.Citrate desorbs a part of Cr(VI)on Fh by ligand exchange and releases them into the liquid phase when citrate interact with Fh with adsorbed Cr(VI).As citrate induced Fh to dissolve and release Fe(III)with the remaining Cr(VI)on Fh.Under the p H=5.23,6.15,7.09 in solution the ratio of reducing Cr(VI)is 38.6%,29.2%,10.8% respectively.Electron paramagnetic resonance(EPR)analysis results indicate that Cr(III),which is the reduction product of Cr(VI)in the above reaction,exists in the liquid phase in an organic complex state.According to the calculation result of DFT,Fe(III)in the ferric-citrate complex induces citrate as an electron donor by the electron donating effect,and then promotes the reduction of Cr(VI)by the mechanism of organic ligand-metal ion electron transfer(LMCT).