Efficiency And Mechanisms Of Cr(Ⅵ) Removal By Ferrihydrite Containing Aluminum/Silicate And Its Behaviors During Aging

Chromium,as a rich element in the earth’s crust,is widely present in water environments.The main forms of chromium are Cr(Ⅲ) and Cr(Ⅵ),while Cr(Ⅵ) is more mobile,toxic and carcinogenic.Cr(Ⅵ) is discharged from industrial production,sewage irrigation,sludge return and the use of chromium-containing pesticides and fertilizers in agriculture.The disposal of Cr(Ⅵ)can make it migrate and spread in the aqueous environment,endangering biological and human health,and destroying ecology system.Ferrihydrite,as a common iron hydroxide,is also ubiquitous in sediments and soils.Because of its large specific surface areas and high surface activity,it is widely used in the removal of pollutants in wastewater.Ferrihydrite and the compounds of Al and Si often coexist in nature in the form of co-precipitates.Compared to pure ferrihydrite,Al-or Si-containing ferrihydrite is relatively difficult to be affected by environmental factors and will not be easily transformed into more stable crystalline iron(hydro)oxides.The aging behaviors of Al-or Si-containing ferrihydrite in the environment can also regulate the migration of heavy metals.Therefore,it is of great significance to explore the removal of heavy metals in wastewater by Al/Si-containing ferrihydrite and the environmental behavior of heavy metals during aging.In this study,two types of ferrihydrite containing Al and Si,which were prepared by a simple chemical precipitation method,were applied to remove Cr(Ⅵ).The effects of various experimental factors on the removal of Cr(Ⅵ)by Al/Si-containing ferrihydrite and the behaviors of Cr(Ⅵ)during the aging process were discussed.X-ray diffraction(XRD),Brunauer-Emmett-Teller analysis(BET),transmission electron microscopy(TEM),energy dispersion X-ray spectrometer(EDS),Fourier transform infrared spectroscopy(FTIR),and X-ray photoelectron spectroscopy(XPS)were used to study the structure,morphology,and composition of the samples before and after the reaction,and also the mechanisms of Cr(Ⅵ) removal.The main conclusions obtained are as follows:(1)Al-containing ferrihydrite had outstanding removal properties for Cr(Ⅵ) and phosphate.The existence of phosphate negatively affected Cr(Ⅵ) removal by Al-containing ferrihydrite.When the initial phosphate concentration increased from 3.1 mg/L to 31.0 mg/L,the removal efficiency of Cr(Ⅵ) decreased by 71.5%,and the removal efficiency of phosphate merely decreased by 9.4%.Phosphate could affect the adsorption behavior of Cr(Ⅵ).Cr(Ⅵ) and Fh-Al may form outer-sphere complexation rather than inner-sphere complexation in the presence of phosphate.(2)The mechanisms of Cr(Ⅵ)removal by Al-containing ferrihydrite in the presence of phosphate can be mainly explained as follows.Firstly,Al-containing ferrihydrite adsorbed Cr(Ⅵ) through electrostatic interaction.Secondly,-OH on the surface of Al-containing ferrihydrite directly interacted with Cr(Ⅵ) to form Fe-O-Cr complexes.Finally,Al-containing ferrihydrite may form Fe-O-P complexes with phosphate first,and then Cr(Ⅵ) combined with the complexes on the surface of Al-containing ferrihydrite by intermolecular hydrogen bonding.(3)Cr(Ⅵ) was co-precipitated with ferrihydrite during the preparation of Si-containing ferrihydrite.With the transformation process of Si-containing ferrihydrite to goethite and hematite,Cr(Ⅵ) adsorbed on the surface of the particles would be wrapped inside the particles or would be released to the solution again due to the reduction of the specific surface area of the particles.(4)pH had an extremely significant effect on Cr(Ⅵ)removal,and with the increase of pH,the residual concentrations of Cr(Ⅵ) increased.The pH was also related to the transformation process.Under acidic conditions,Si-containing ferrihydrite transformed into hematite,and it transformed into hematite and goethite under neutral and alkaline conditions.(5)Silicate directly affected the migration of Cr(Ⅵ) in the solution and adsorbed state by competing for the reaction sites of Si-containing ferrihydrite.Silicate also acted on the migration of the adsorbed and non-adsorbed Cr(Ⅵ)during the transformation of ferrihydrite which was beneficial to the stability and fixation of Cr(Ⅵ) on iron(hydro)oxides or other iron minerals.