(AS2O5) is more soluble than trivalent. In aqueous systems, arsenic forms anions in solution, and thus it does not form complexes with simple anions such as Cl (Bodek et al. 1988). Anionic arsenic complexes behave like ligands in water. In aerobic aqueous systems, the arsenic acids H2AsO4-1 and HAsO4-2 are the dominant species in the pH range 2-11. Below pH 2, the arsenious acid, H2AsO3-1 can be the dominant species in reducing conditions and above pH 12, the arsenious acid, HAsO3-2 appears.
Bodek et al. (1988) have reviewed and compared a number of sorption measurements and sorption models for arsenic in soil-water and sediment-water systems. They report that, in soils and sediments estimated Kd values range from 15-5,500 mol/kg.
As a likely result of uptake in water, arsenic is absorbed by roots and readily translocated to other plant parts (Bodek et al. 1988). Plant-soil partition coefficients have been reported in the range 0.01-0.04 mg/kg on a dry-mass basis (Baes et al. 1984a), which corresponds to the approximate range of 0.002 to 0.008 on a fresh-mass basis.
According to Bodek et al. (1988), airborne deposition is believed to contribute to some of the concentrations of arsenic found in plant leaves. At low concentrations, the ratio of plant leaf concentration to air concentration when air and plant environments are in contact, can be estimated based on the balance of gains from wet and dry deposition versus losses by wash-off and plant decay. Bodek et al. (1988) report both ocean and freshwater fish bioconcentration factors in the range 10-500 L/kg.
Arsenic metal and arsenic compounds (with the exception of arsine gas) have low volatility and exist in air primarily incorporated as fine suspended particulate matter. When inhaled, some fraction of this particulate matter is deposited in the airways of the lung and the rest is exhaled. Mean concentrations of arsenic in ambient air are estimated to be usually in the range of less than 0.001-0.003 µg/m3 in remote areas and in the range of 0.02-0.03 µg/m3 in urban areas (ATSDR 1998b). Air concentrations of arsenic near nonferrous metal smelters were reported to reach 2.5 µg/m3 (Schroeder et al. 1987). Arsenic exposures can occur through contact with water, food, soil, and house dust in ways similar to those discussed for cadmium.
ATSDR (1998b) indicates that food is typically the greatest source of arsenic exposure for the general population. NRC (1999b) reports that one of the most comprehensive studies of arsenic in food was published in 1993 (Dabeka et al. 1993). The food groups containing the highest mean arsenic compounds were fish (1.66 µg/g), meat and poultry (0.024 µg/g), bakery goods and cereals (0.024 µg/g), and fats and oils (0.019 µg/g). The average daily dietary ingestion of total arsenic by Canadians was estimated to be 38.1 µg (48.5 µg for adults). Adams et al. (1994) estimates that food contributes 93% of total intake of arsenic in the United States, and seafood contributes 90% of that 93%.