Long-term dissolution experiments from waste dumps that includes marine sediment under both anoxic and aerobic conditions were performed to elucidate the As dissolution mechanism and assess As dissolution risk. Core samples drilled at a waste disposal site in Sendai were collected for the experiment. Almost completely oxidized and partially oxidized portions of a drilling core were both characterized by naked-eye observation and were used for the elution experiments. XANES analyses reveal that the chemical state of As in the partially oxidized samples is As (V) and As (III) or sulfide form, and acidic solutions are formed by a reaction with a hydrogen peroxide solution, indicating the existence of sulfide minerals. The arsenic state in the almost completely oxidized sample is mainly As (V). There is no significant difference in As concentrations in both anoxic (maximum concentration: 4.30 μg/l) and aerobic extracts (4.16 μg/l) from the almost completely oxidized sample, and the predominant As species in both solutions are arsenate. On the other hand, As concentrations in extracts from partially oxidized samples under an anoxic condition (maximum concentrations of two samples: 25.9 and 24.7 μg/l) tend to be higher than those from aerobic samples (10.9 and 4.96 μg/l). Experiments to separate aqueous arsenic speciation show that arsenate is the predominant As species in the extracts under an anoxic condition, whereas both arsenate and arsenite coexist in an aerobic solution. Water samples from wells and pond at the waste disposal site were also analyzed to clarify water quality (pH and toxic element concentrations) before release into a natural river near the waste disposal site. These water samples are, in general, characterized by low dissolved oxygen (less than 1 mg/l), and experimental results are similar to those under an anoxic condition. Arsenate is the predominant aqueous As species. Dissolution of As is enhanced under more alkaline conditions (greater than pH 8.5), indicating that arsenate sorbed onto a mineral surface is possibly released.
