ARSENATE SORPTION/DESORPTION KINETICS AND COMPETITION REACTIONS ON GOETHITE

O'Reilly, Susan Erin

MS

University Of Delaware

1998

To make sound assessment and remediation decisions regarding arsenate contamination of soils and water, it is necessary to have a thorough understanding of As sorption/desorption reactions over time and how they are affected by common soil nutrients. Research has shown that sorption of metals and metalloids on natural materials can proceed for a long time; however, the mechanisms for this slow sorption are not well understood. Yet, few studies have examined the effects of time on sorption, desorption, and competition. Adsorption of anions onto soil components is strongly influenced by competition from other adsorbing counter anions. Potential competing anions for arsenate include sulfate and phosphate which are both commonly present in nature. Thus, the major objectives of this study were to determine the effects of: 1) time on arsenate sorption mechanisms on goethite, 2) residence time or aging on the kinetics and mechanisms of arsenate desorption on goethite, in the presence of phosphate and sulfate, and 3) competition on the sorption of As on goethite, in the presence of phosphate and sulfate at various concentration ratios.

Extended X-ray absorption fine structure spectroscopy (EXAFS) analyses and macroscopic kinetic studies (at pH 6 and 4) were employed to provide much needed information on the mobility and fate of As in the environment over long time periods (4 m to about 12 months). Total arsenate sorption on goethite increased with time. Sorption was initially rapid, with over 93% arsenate being sorbed in a 24 h period at pH 6. Similar results were found at pH 4, but sorption was slightly more rapid since the sorption reached a maximum at about pH 4. EXAFS analyses revealed two coordination shells. The first shell was due to As-O contributions and the second shell backscatterers were ascribed to As-Fe contributions. The As-Fe bond distance of 3.30 Å, derived from XAFS data, indicates bidentate binuclear bonding. There were no significant differences in the arsenate bonding mechanism on goethite with time. In short, arsenate appears to be strongly and rapidly bound as a bidentate binuclear (bridging) complex that does not change over time.

Desorption kinetic and spectroscopic studies showed that when aging was increased, there was no significant change in the amount of arsenate desorbed from goethite by phosphate. Initially desorption was quite rapid with greater than 35% of the arsenate desorbed within 24 h at pH 6. After the initial rapid desorption, a plateau was reached with desorption slowly continuing. A significant amount of arsenate remained bound to the goethite after five months of desorption even though the phosphate desorptive solution was three times stronger than the initial arsenate sorption solution. Overall, little sorbed arsenate was desorbed by sulfate. At pH 6, no more than 2.5% of the total sorbed arsenate desorbed over a five month period. Similar results were also found at pH 4. XAFS analyses of the arsenate remaining on the goethite surface after desorption with phosphate were similar to the analyses of the arsenate adsorption studies and indicated bidentate binuclear bonding. There were no differences in the bonding mechanism of arsenate on goethite as it continued to undergo desorption by phosphate. Therefore, arsenate appears to remain strongly bound as a bidentate binuclear (bridging) complex even when a counter anion, phosphate, is present in a much higher concentration, and the type of surface complex does not change over time.

The competition studies showed that, regardless of the concentration ratio, there was less sorption of the oxyanion in a competitive situation than there was for that oxyanion alone. Decreases ranged from approximately 25 to 75% for phosphate and 30 to 60% for arsenate depending on the concentration ratio. Sulfate adsorption was initially so low that additional decreases in sorption were difficult to accurately measure. For equal concentrations of arsenate and phosphate (at pH 6), phosphate sorption was initially greater than arsenate sorption, but, after five hours, arsenate sorption was greater than phosphate sorption.