Adsorption And Reduction Of Cr(?) By Magnetite And The Role Of HA

Cr(?)is a common pollutant in soil and groundwater,mainly derived from industrial and anthropogenic activities.Since the toxicity of Cr(?)is one hundred times greater than that of Cr(III),the natural reduction process of Cr(?)in soil and sediment is of great significance to the migration and transformation of Cr(?).Magnetite is a natural inorganic mineral with adsorption and reduction capabilities commonly found in soils and sediments.It may have an important effect on the migration and transformation of Cr(?).However,the adsorption and reduction mechanism of Cr(?)on magnetite is unclear at present.In addition,it is necessary to investigate the effect of adsorbed HA on the magnetite adsorption and reduction of Cr(?)in order to deeply understand the effect of magnetite on Cr(?)migration and transformation in the actual environment.Therefore,this study explored the adsorption and reduction mechanism of Cr(?)on magnetite and the effect of HA on the reaction process.The main conclusions obtained are as follows:(1)The adsorption and reduction of Cr(?)on magnetite follows a coupling reaction mechanism of "adsorption-reduction",which consists of three reaction stages including "rapid adsorption,direct reduction and indirect reduction".Ligand exchange occurs during the rapid adsorption process,and the reaction reaches equilibrium within 6 h.The reduction takes place on the basis of the adsorption.The direct reduction occurs as a result of Cr(?)reduction by Fe(II)on the surface of magnetite.The limiting factor of the reaction is the amount of Fe(II)on the surface.The indirect reduction process is realized by electrons transferring from Fe(II)inside to Cr(?)through the surface "passivation layer".The reaction rate is relatively slower than the direct reduction,and the electron transfer rate in the passivation layer is the rate-limiting step of the reaction.(2)The multi-step kinetic model based on the second-order reaction overcomes the limitations of the pseudo first-order and pseudo second-order kinetic models,and well describes the multi-step reaction process of "quick adsorption,direct reduction and indirect reduction" of Cr(?)by magnetite with strong applicability.(3)The presence of adsorbed HA on the surface of magnetite inhibits the adsorption of Cr(?)and promotes the reduction of Cr(?)on the magnetite.The reason is that the adsorption of HA occupies the adsorption sites on the magnetite surface,thereby inhibiting the adsorption of Cr(?)on magnetite.In addition,when the reaction time is long enough(64 d),HA apparently promotes the Cr(?)reduction on magnetite greatly.