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.