Interplay of transport processes and interfacial chemistry affecting chromium reduction and reoxidation with iron and manganese

Hexavalent chromium(Cr(Ⅵ))is a water-soluble pollutant in soil and groundwater,the mobility,bioavailability,and toxicity of which can be controlled by transforming to less mobile and more environmentally benign Cr(Ⅲ)by ways of reduction.This review focused on recent advances in identifying the reaction pathways,kinetics,and products of iron-based techniques for Cr(Ⅵ)removal.It also examines new information regarding remobilization of Cr(Ⅲ)in the existence of complexing ligands and manganese(Mn)of different oxidation states.A range of iron-based techniques can remove Cr(Ⅵ)from water by adsorption or reduction-coprecipitation processes.However,the success of a chromium treatment or remediation strategy requires the stability of the Cr(Ⅲ)-containing solids with respect to solubilization or reoxidation in the settings they are generated.Manganese is ubiquitous in aquatic and terrestrial environments,and the redox cycling of manganese may greatly influence the fate,transport,and distribution of chromium.Coupling of redox reactions of chromium,iron,and manganese involves reaction pathways not only in the aqueous phase but also at solid-aqueous interfaces.To provide a quantitative understanding of these processes,it is essential to develop mechanistically based kinetic and transport models.Continued research should be made on iron-based treatment of Cr(Ⅵ)-contaminated water and soils and the stability of the subsequently produced Cr(Ⅲ)-containing solids at environmentally relevant conditions,which will support improved predictions of chromium's environmental fate and transport and aid in decision-making for remediation and treatment of Cr contamination.