Removing Arsenic from Synthetic Groundwater with Iron Electrocoagulation: An Fe and As K-Edge EXAFS Study
Electrocoagulation (EC) using iron electrodes is a promising arsenic removal strategy for Bangladesh groundwater drinking supplies. EC is based on the rapid in situ dissolution of a sacrificial Fe(0) anode to generate iron precipitates with a high arsenic sorption affinity. We used X-ray absorption spectroscopy (XAS) to investigate the local coordination environment (<4.0 Å) of Fe and As in EC precipitates generated in synthetic Bangladesh groundwater (SBGW). Fe and As K-edge EXAFS spectra were found to be similar between samples regardless of the large range of current density (0.02, 1.1, 5.0, 100 mA/cm2) used to generate samples. Shell-by-shell fits of the Fe K-edge EXAFS spectra indicated that EC precipitates consist of primarily edge-sharing FeO6 octahedra. The absence of corner-sharing FeO6 octahedra implies that EC precipitates resemble nanoscale clusters (polymers) of edge-sharing octahedra that efficiently bind arsenic. Shell-by-shell fits of As K-edge EXAFS spectra show that arsenic, initially present as a mixture of As(III) and As(V), forms primarily binuclear, corner-sharing As(V) surface complexes on EC precipitates. This specific coordination geometry prevents the formation of FeO6 corner-sharing linkages. Phosphate and silicate, abundant in SBGW, likely influence the structure of EC precipitates in a similar way by preventing FeO6 corner-sharing linkages. This study provides a better understanding of the structure, reactivity, and colloidal stability of EC precipitates and the behavior of arsenic during EC. The results also offer useful constraints for predicting arsenic remobilization during the long-term disposal of EC sludge.