A

Fine sand

elemental analyzer (Perkin Elmer 2400 Series II). Total Ca, Mg, Fe, Al, P, and As were extracted following USEPA method 3050B [22]. Phytoavailable As was obtained by shaking 1 g of soil with 50 ml of 1 M NH4Cl solution for 30 min. Particle size and cation exchange capacity were not measured; values were obtained from the soil characterization database of Florida Cooperative Soil Survey (1967-1989).

Phosphorus was measured colorimetrically using the molybdate-ascorbic acid method [23] using Varian Cary 50 Spectrophotometer. Iron was also determined colorimetrically according to Olson and Ellis [24] as a complex with 1,10-phenanthroline reagent. Ca, Mg, and Al were analyzed using flame atomic absorption spectrometry and As was determined using graphite furnace atomic absorption spectrometry (GFAAS).

5.2.2 Determination of Bioavailable Arsenic

Bioavailable As was estimated according to the in vitro gastrointestinal method of Sarkar and Datta [25]. The reactions were carried out in 250-ml beakers in a 37°C water bath to simulate body temperature. Anaerobic conditions were maintained by passing argon gas through the solutions. Constant mixing of the solution was maintained using a stirrer to simulate gastric mixing. The extractant used consisted of 0.15 M NaCl and 1% porcine pepsin. One gram of soil sample was added to 150 ml of gastric solution, and the pH of the solution was adjusted to 1.8 using 1 N HCl. The solution was incubated for 1 h.

The solution was then modified for the intestinal phase by adjusting the pH to 7.0 using a saturated solution of NaHCO3, followed by the addition of 525 mg of porcine bile extract and 52.5 mg of porcine pancreatin (Sigma Chemical Co., St. Louis, MO). In order to simulate absorption through the intestinal lining, a 40-cm2 filter paper strip coated with Fe oxide was used. The Fe oxide strip was placed in a square bag (sides 6.5 cm) made of nylon membrane filter of 8-^m pore size. The bag was tied with a string and suspended in the reaction vessel. The solution was incubated for 1 h, at the end of which 10 ml of solution was collected, centrifuged at 5000 rpm for 30 min, and analyzed by GFAAS. Arsenic adsorbed by the Fe oxide strip was desorbed by shaking it vigorously in 80 ml 1 N HNO3 for 1 h.

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