AN INVESTIGATION OF ARSENATE AND ARSENITE BONDING STRUCTURES ON GOETHITE BY FTIR

The bonding structures of arsenite (As(III)) and arsenate (As(V)) on goethite (α-FeOOH) were studied by Transmission-Fourier Transform Infrared (T-FTIR) and Attenuated Total Reflectance-FTIR (ATR-FTIR) spectroscopy. ATR-FTIR spectra of deuterated goethite showed three bands of OD stretch, corresponding to singly, doubly, and triply coordinated OH hydroxyls on the goethite surface. Both FTIR modes provided similar structural information for arsenic adsorption. Most arsenate and arsenite oxyanions replaced two singly coordinated surface OH groups (A-type) to form binuclear bridging complexes Fe-O-AsO(OH)-O-Fe and Fe-O-As(OH)-O-Fe. However, their effects on doubly (C-type) and triply (B-type) coordinated surface OH groups are different. As(III) mainly reacted with C-type OH groups, whereas As(V) reacted with B-type OH groups. These reactions may be attributed to hydrogen-bonding and/or chemical bonding. Finally, we applied a sequential extraction method to evaluate the fractions with different bonding strengths. Relatively strong chemical extraction was necessary to remove 75 to 80% of adsorbed arsenite or arsenate from goethite. The strong retention of As(III) and As(V) is most likely caused by the formation of binuclear complexes with goethite.