Effects of incubation time and arsenic load on arsenic bioaccessibility in three Florida soils amended with sodium arsenate

Abstract

The potential for human exposure to arsenic (As), a group A carcinogen, has increased tremendously due to the encroachment of suburban areas into former agricultural lands, where arsenical pesticides were used extensively prior to the 1990s. Soil ingestion is the no. 1 route of As exposure, attributable to incidental hand-to-mouth activities by children in playgrounds or house yards having As-contaminated soil. Previous studies have shown that As bioaccessibility in soils is mostly a function of their physicochemical properties. We selected three Florida soils covering a wide range in chemical properties, such as, Fe/Al hydroxides, organic matter, and Ca/Mg content, which are most likely to influence As bioavailability. The soils were amended with sodium arsenate pesticide at loads ranging from 45 to 450 mg As per kg, and subsequently incubated up to 12 months. The overall objectives of this study were to evaluate the effects of incubation time and As load on soil speciation, hence bioaccessibility of As. Results showed a reduction in the water-soluble, and plant-available (water+NH4Cl-extractable) As fractions of the three soils after four months of incubation, which remained unchanged up to 12 months. This reduction with time was accompanied by a concurrent increase in the NaOH-extractable As fraction, suggesting As sorption by amorphous Fe/Al hydroxides could decrease As bioaccessibility. The effect was most pronounced for the Pahokee Muck soil, which had the greatest amount of amorphous Fe/Al hydroxides (2000 mg kg−1) of all soils. Arsenic associated with the water- and plant-available As fractions is most bioaccessible as indicated by the significant (α=0.05) correlation between the water-soluble, plant-available, and the in-vitro bioaccessible As fraction in the soils. The NaOH-extractable As fraction was negatively correlated with the in-vitro bioaccessible As fraction, suggesting that the presence of Fe/Al hydroxides could decrease As lability in soils.