CHARACTERIZATION OF ARSENATE (ASV) AND ARSENITE (ASIII) ADSORPTION ONTO HEMATITE IN THE PRESENCE OF NATURAL ORGANIC MATTER (NOM)

Arsenic contamination poses a threat to many drinking water sources around the world. An important control on arsenic mobility in the environment is adsorption onto metal oxide surfaces. However, in previous work it was shown that natural organic matter (NOM), a ubiquitous substance in natural waters, influences arsenic adsorption onto hematite. It was found that in the presence of NOM, less arsenic adsorbed to the oxide surface. In order to be able to accurately predict arsenic mobility it is important to understand why this occurs. The current work was undertaken to define more rigorously arsenate (AsV), arsenite (AsIII), and NOM adsorption to hematite, and to determine how the adsorption complexes are influenced by each other’s presence at the oxide surface. Presented here are some preliminary results towards meeting these goals.

Arsenate and arsenite adsorption onto hematite were studied at ca. pH 5.5 with Fourier transform infrared (FTIR) spectroscopy. Using attenuated total reflectance (ATR)-FTIR measurements, arsenate adsorption was detected by bands at ca. 910 cm-1 and ca. 869 cm-1. Arsenite adsorption was indicated by a band at ca. 847 cm-1. Upon contact with Suwannee River NOM, the arsenate and arsenite adsorption peaks decreased, consistent with less As adsorption occurring when NOM was present. ATR measurements, complemented by KBr pellet measurements, also showed a decrease in the stretching (ca. 2700-3600 cm-1 ), bending (ca.1250 – 1750 cm-1 ), and deformation (ca. 700 – 1000 cm-1 ) modes of hematite surface hydroxyls in the presence of AsV and AsIII. These results are consistent with AsV and AsIII forming adsorption complexes with hematite by replacing hematite surface hydroxyl groups. More detailed studies will be conducted in order to further elucidate the structure of these complexes. The relative importance of specific NOM functional groups with respect to their influence on arsenic adsorption will also be studied using model compounds such as salicylic acid and citric acid.