Abiotic Attenuation of Nitrobenzene Controlled by Reduction Capacity Shifts during Fe(II)aq–Catalyzed Ferrihydrite Transformation

During the Fe(II)_aq-catalyzed transformation of ferrihydrite, the abiotic reduction of organic contaminants is an important process in the natural attenuation of groundwater pollutants. However, during this transformation, the reduction capacity of the Fe(II)_aq-Fe(III) oxyhydroxide system evolves substantially over time and its impacts on contaminant attenuation processes remain unclear. In this study, a single quantitative relationship between contaminant reduction rates (k_sa) and shifts in the reduction capacity during the ferrihydrite transformation (log(k_sa) = 0.91·(-E_H/59 mV - pH) + 2.96) was established using nitrobenzene as a representative contaminant, despite variations in transformation pathways and product assemblages at different pH values and Fe(II)_aq concentrations. The reduction capacity of such systems was dependent on the iron oxide composition when ferrihydrite was transformed to more thermodynamically stable Fe(III) oxyhydroxides. E_H decreased from 1.5 and - 46.8 mV to -113.1 and -136.5 mV (all measurements vs. a standard hydrogen electrode) as ferrihydrite transformed to lepidocrocite and goethite, respectively. When structural Fe(II)-containing magnetite was formed, decreases in the pH and Fe(II)_aq concentration increased E_H from -159.7 to -79.9 mV. This work provides an effective method for predicting the abiotic attenuation of organic contaminants during complex mineral transformation processes and deepens our understanding of the natural transformation behavior of contaminants in aquifers.