Bioavailability and speciation of arsenic in carrots grown in contaminated soil†

Abstract

Carrots were grown in seven experimental plots (A–G) containing mixtures of arsenic-contaminated and uncontaminated soil at concentrations ranging from 6.5 to 917 µg g^–1 (dry mass). The carrots harvested from plots A–D (6.5–338 µg g^–1 arsenic in the soil mixtures) showed a gradually increasing depression of growth with increasing level of contamination. At the experimental plots E–G with soil arsenic concentrations above 400 µg g^–1 no carrots developed. Whether this effect was caused by arsenic or the concomitant copper content which ranged from 11 to 810 µg g^–1 in the soil mixtures is unknown. The arsenic species extracted from the soils and carrots were separated and detected using anion-exchange HPLC coupled with ICP-MS. In the less contaminated soils from plots A and B arsenite (AsIII) was more abundant than arsenate (AsV) in the soil using 1 mmole l^–1 calcium nitrate as extractant. In the soils from plots C and D however, AsV dominated over AsIII whereas in the corresponding carrots AsV and AsIII were found at similar concentrations. Methylated arsenic species were sought after but not detected in any of the samples. The soil-to-carrot uptake rate (bioavailability) of arsenic was 0.47 ± 0.06% (average ± one standard deviation) of the arsenic content in the soils from plots A–D. In contrast to arsenic, the increasing copper content in the soils from plot A through D was not available to the carrots as the concentration of this element did not increase with increasing soil copper content. The ingestion of the potentially toxic inorganic arsenic via consumption of carrots grown in soil contaminated at 30 µg g^–1 in arsenic (plot B) was conservatively estimated at 37 µg week^–1. This was equivalent to only 4% of the provisional tolerable weekly intake (PTWI) for inorganic arsenic as suggested by the WHO and was therefore toxicologically safe. Consumption of carrots grown in more intensely arsenic-contaminated soils, however, would lead to a higher intake of inorganic arsenic and is therefore not recommended.

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