KINETICS OF ARSENITE AND ARSENATE DESORPTION FROM GOETHITE | ||||
| Journal of Soil Sciences and Agricultural Engineering | ||||
| Article 5, Volume 27, Issue 10, October 2002, Page 7149-7161 PDF (3.29 MB) | ||||
| Document Type: Original Article | ||||
| DOI: 10.21608/jssae.2002.255235 | ||||
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| Authors | ||||
| A. A. Abd EI-Haleem1; R. H. Loeppert2; L. R. Hossner2 | ||||
| 1Soils Department, College of Agriculture, Moshtohor, Zagazig University, Egypt. | ||||
| 2Soil & Crop Sciences Department, Texas A&M University, College Station, Texas, USA | ||||
| Abstract | ||||
| Arsenic is a toxin that can occur in the environment as a result of either natural processes or anthropogenic activities. The accumulation of arsenite and arsenate in soils and sediments threatens the health of plants, wildlife, and humans. Goethite can play an important role in controlling the concentration of soluble arsenic in pore water, by the formation of inner sphere surface complexes with arsenate and arsenite. Extraction, e.g., with phosphate, oxalate, and Hel has been suggested as a procedure to assess the amount and speciation of arsenic, yet desorption is not always quantitative. Arsenite and arsenate were equilibrated with goethite at low (0.02 mol kg-1) surface coverage and extracted with phosphate at P:As molar ratio of 100: 1, as well as with 0.1 M oxalate at pH values ranging from 1 to 13. Phosphate desorbed only 10-50% of the adsorbed arsenite, with the largest amount of arsenic desorbed at pH 4. At the same P:As ratio, phosphate extracted only 10-30% of adsorbed arsenate, with the greatest desorption at pH 1.5. Desorption of arsenite by phosphate at pH 5 was faster than that at pH 9 and reached an approximate maximum after 5 h. Desorption of arsenate was slow and increased gradually over the 200 h reaction time. Oxalate (0.1 M) was able to extract from 1 to 22% of adsorbed arsenate at pH<5 in the dark. With exposure to light, 100% of adsorbed arsenate was extracted at pH <3, due to the photo-induced dissolution of the solid phase. At pH <4 and >10, a small proportion of arsenate was reduced to arsenite during the reaction with 0.1 M oxalate. Arsenic speciation after the release of As(lll) from goethite by 6M Hel was affected greatly by the reaction time and the exposure to light. As(llI) was readily transformed to As0l) in the light in both oxalic acid (0.1 M) and 2-6 M Hel. In the dark, oxidation of As(lIl) to As0l) was reduced but not totally eliminated. | ||||
| Keywords | ||||
| Arsenite; arsenate desorption; kinetics; goethite | ||||
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