Tuesday, November 6, 2007
227-2
Inorganic Phosphate Interactions with Hydroxy-Interlayered Soil Minerals.
Paul Shumaker, Yvonne Thompson, and A.D. Karathanasis. University of Kentucky, Department of Plant and Soil Sciences, Ag. Science Center North N-129, Lexington, KY 40546-0091
Excess Phosphorus (P) can cause fresh water eutrophication, thus requiring appropriate P management to prevent water pollution. Soil clay mineralogy, particularly aluminum (Al) and iron (Fe) hydroxides, has been shown to influence inorganic P adsorption, desorption, and movement through soils. This study was designed to determine sorption and desorption characteristics of inorganic P with soil hydroxy-interlayered vermiculites (HIV) and smectites (HIS), and compare the findings to those of reference Al and Fe hydroxide minerals. Inorganic P adsorption and desorption experiments were conducted with clay size fractions of six soils containing abundant HIV or HIS quantities and two reference gibbsite and goethite samples. The experiments were repeated on the samples following removal of amorphous Al and Fe using pH 3, 0.2M ammonium oxalate. With a loading rate of 10 mg P/g of sample, adsorption capacities ranged from 0.55-1.45 mg P/g for reference minerals, 2.68-3.96 mg P/g for HIS samples, and 1.49-2.36 mg P/g for HIV samples. Phosphorus desorption following four sequential extractions with 0.01M CaCl2 ranged from 0.08-0.19 mg P/g for reference minerals, 0.60-0.81 mg P/g for HIS samples, and 0.30-0.64 mg P/g for HIV samples. Following ammonium oxalate extractions, P adsorption capacities ranged from 0.37-1.89 mg P/g for reference minerals, 1.00-2.08 mg P/g for HIS samples, and 0.85-2.51 mg P/g for HIV samples. The respective desorption ranges for ammonium oxalate treated samples were 0.02-0.24 mg P/g for reference minerals, 0.03-0.57 mg P/g for HIS samples, and 0.07-0.62 mg P/g for HIV samples. The results showed that some HIV and HIS minerals may exhibit equal or greater P sorption potential compared to reference or soil Fe or Al hydroxides, but the sorbed P can be much more easily released to soil environments. Sorption-desorption characteristics of hydroxy-interlayered minerals with organic and mixed P sources are currently being investigated.