The rapiddecrease in the arsenite concentration in the case of activatedcarbon was attributedto theincreasedoxidation of arsenite into arsenate in thepresence of carbon andtraces of oxygen. The arsenatewas partly foundin the solution, although after 5 h thediscrepancy between the total measured concentrationandthe initial concentration (500 mg l1 As(III), 0 mg l1As(V)) was about 150 mg l1, which must be sorbed. Thespeciation of the sorbedfraction is uncertain.