Facet-dependent As(V) adsorption behavior on goethite: Molecular-scale coordination mechanisms and surface reactivity

Goethite is a major arsenic scavenger in natural waters, sediments, and soils, yet facet-dependent As(V) coordination remains poorly characterized. We studied As(V) adsorption on two well-defined goethite morphologies: plate-like GA (higher (021)/(110) ratio) and rod-like GB (lower (021)/(110) ratio), both dominated by the (110) facet. Results showed GA exhibits a ∼1.5-fold higher As(V) adsorption capacity (5.69 vs. 3.73 µmol/m²) and a ∼3.6-fold higher rate constant than GB (1.34 vs. 0.37 m²/(μmol·min). Area-normalized QCM-D revealed ∼1.1-fold faster As(V) deposition on GA, indicating higher loading and faster kinetics on the (021)-enriched GA. At 0.8 mM As(V), pH5.0, in-situ ATR-FTIR, supported by EXAFS and DFT, indicates bidentate inner-sphere complexes dominate on both goethites, with a minor monodentate fraction detectable on GA but not on GB, reflecting different (021)/(110) facet ratios. DFT confirmed the (021) facet has a higher density of singly-coordinated hydroxyl groups and more negative adsorption energies for bidentate and monodentate As(V) complexes (-1.97 to -1.39 eV) than the (110) facet (-1.09 to -0.86 eV), explaining its coordination versatility and higher reactivity. Overall, As(V) adsorption is strongly facet-dependent; enhancing (021) exposure is a promising strategy for tailoring goethite adsorbents, and findings provide a molecular-scale foundation for predicting As(V) mobility in oxic environments.