| Chromium,a common heavy metal element in the environment,usually exists in the form of trivalent(Cr(ⅡI))and hexavalent(Cr(Ⅵ)).Cr(Ⅵ)has attracted the attention of many scholars,because it has higher solubility,mobility and toxicity than Cr(ⅡI).Iron oxides are widely present in natural environments,such as soil,groundwater,and sediments,and play an important role in the immobilization and migration of pollutants in the natural environment.However,the process is easily affected by factors such as p H,coexisting ions,and common low-molecular organic acids(oxalic acid,citric acid,etc.).Moreover,iron oxides rarely exist in the form of a single phase in the environment,and are often doped with some elements,such as Mn,Al,and Si.This will also affect the immobilization of iron oxides on pollutants to a certain extent.Therefore,laboratory-synthesized goethite and Mn-incorporated ferrihydrite are used to remove Cr(Ⅵ)wastewater.The effects of Mn(Ⅱ),oxalic acid,initial p H,Mn/Fe molar ratios,and other variables on the experimental results were explored,and X-ray diffraction,scanning electron microscopy,Fourier transform infrared spectroscopy,X-ray photoelectron spectroscopy and other spectroscopic instruments are used to characterize the samples.According to the experimental results,the fixation mechanisms of Cr(Ⅵ)by goethite and Mn-incorporated ferrihydrite during the aging process are proposed.The main conclusions of this study are as follows:(1)Goethite had a good removal efficiency for Cr(Ⅵ).After 12 h of reaction,the removal efficiency of 20 mg/L Cr(Ⅵ)reached 81%.Subsequently,the addition of Mn(Ⅱ)improved the removal of Cr(Ⅵ)by goethite.This was because a small amount of Mn(Ⅱ)was adsorbed on the surface of the goethite,which increased the positive charge on the surface of the goethite,thereby improving the adsorption of Cr(Ⅵ)by the goethite..(2)Oxalic acid complexed with the surface of the goethite,causing the Cr(Ⅵ)originally adsorbed on the surface of the goethite to be released into the solution,increasing the migration of Cr(Ⅵ).Moreover,goethite could act as a medium for conducting electrons,causing oxidation-reduction reactions between oxalic acid and residual Cr(Ⅵ)on the surface to generate Cr(ⅡI).Due to the electrostatic repulsion,Cr(ⅡI)was released into the solution.As Cr(ⅡI)was released into the solution,the adsorption sites on the surface of the goethite were exposed again,and the Cr(Ⅵ)in the solution was adsorbed to the surface of the goethite,reducing the concentration of Cr(Ⅵ)in the solution.(3)When Mn(Ⅱ)and oxalic acid were added at the same time,oxalic acid and Mn(Ⅱ)formed a complex in the solution,and the formed complex would form an ester compound with Cr(Ⅵ).However,the ester compound was unstable and decomposed to produce Cr(ⅡI),increasing the concentration of Cr(ⅡI)in the solution.This weakened the influence of oxalic acid on the migration of adsorbed Cr(Ⅵ)on the surface of goethite.(4)The adsorption efficiency of ferrihydrite on Cr(Ⅵ)enchanted due to the incorporation of Mn and increased with the increase of Mn content.When the initial p H was3.0,the maximum adsorption capacity of pure ferrihydrite on Cr(Ⅵ)was 36.1 mg/g,and when the Mn/Fe molar ratio was 5%,it reached 48.5 mg/g.In addition,the p H of the solution also affected the removal efficiency of Cr(Ⅵ)by ferrihydrite.With the increase of p H,the adsorption efficiency of ferrihydrite on Cr(Ⅵ)gradually decreased.(5)During the aging process,ferrihydrite transformed into hematite and goethite,and adsorbed Cr(Ⅵ)on the surface of ferrihydrite was released into the solution.This was because compared with ferrihydrite,hematite and goethite had a smaller specific surface area and weaker surface activity,so that adsorbed Cr(Ⅵ)was released into the solution.(6)The incorporation of Mn inhibited the transformation behavior of ferrihydrite.This was because the incorporation of Mn modified the internal structure of ferrihydrite,which affected the dehydration and recrystallization process of ferrihydrite.Therefore,the transformation of ferrihydrite was inhibited.This process also inhibited the release of adsorbed Cr(Ⅵ)on the surface of ferrihydrite,but the incorporation of Mn converted the adsorbed Cr(Ⅵ)on the surface into non-desorbed Cr(Ⅵ),thereby enhancing the immobilization of Cr(Ⅵ). |
