Accurate description of the fate of metals and metalloids in natural systems requires determination of the bonding mechanisms related to those of other ions on mineral surfaces. Andisols of Southern Chile are characterized by low pH, high phosphate adsorption ability and the leaching of anions weakly fixed to the mineral surfaces, like sulfate, especially in winter by heavy rainfall (Mora et al., 2005). These conditions influence the mobility and bioavailability of microelements as selenium (Se) and molybdenum (Mo) in soils. Surface complexation models have been employed to predict anion adsorption on different mineral surfaces and it has been proposed that selenate and sulfate form a weakly bonded, outer-sphere surface complex and that selenite, molybdate and phosphate form a strongly bonded, inner-sphere complex (Goldberg, 1992; Goldberg et al., 2002). The aim of this research is to compare the adsorption behavior of selenite, selenate, molybdate, phosphate and sulfate on Andisols by the Triple Layer Model (TLM). Anion adsorption experiments were made in batch systems for 24 h at 25 ºC using soil samples of Andisols of Southern Chile (37º - 44º S, 70º - 71º W) with increasing organic matter content from 140 to 240 g kg^-1, allophane from 28.5 to 96.5 g kg^-1 and ferrihydrite from 1.51 to 2.02 g kg^-1 for Vilcún, Pemehue, Corte Alto and Piedras Negras Series, respectively. The initial concentrations varied between 0 and 5 mmol L^-1. The initial pH of the solution was adjusted to 5.0 and the background electrolyte was 0.1 M KCl. Data were fitted by the TLM assuming the formation of monodentate inner sphere complexes for selenite, molybdate, phosphate and monodentate outer sphere complexes for selenate and sulfate. The results showed that anion adsorption was positively correlated with the organic matter content, allophane and Al/Fe oxides, and the adsorption ability was in the follow order: Piedras Negras > Corte Alto > Pemehue > Vilcún Series. According to the model, the surface complexation constants (logK values) varied between 2.43 and 3.01 for log K{XHSeO₃}, -0.48 and 0.22 for logK{XSeO₃[-]}, 5.21 and 7.24 for logK{XHMoO₄}, 2.30 and 3.92 for logK{XMoO₄[-]}, 3.45 and 4.68 for logK{XH₂PO₄}, 0.64 and 1.99 for logK{XHPO₄[-]}, 0.98 and 1.91 for logK{XOH₂[+]-SeO₄[2-]}, 9.48 and 9.72 for logK{XOH₂[+]-HSeO₄[-]}, and 1.23 and 1.49 for logK{XOH₂[+]-SO₄[2-]}, 10.59 and 12.22 for logK{XOH₂[+]-HSO₄[-]}. The complexation values of inner sphere complexes were in decreased order molybdate > phosphate > selenite. Other researches have been reported lower constant complexation values for phosphate than those for molybdate, which can be explained by the higher organic matter content of Andisols compared with other soils previously studied (i.e. Goldberg et al., 2002). The molybdate interactions with the Al/Fe oxides can be determined by the different electronic structure and polarity of the molybdate compared with phosphate and selenite related whit the organic matter. On the other hand, sulfate and selenate showed similar values of the no-protonated complexes while for protonated complexes sulfate presented a slightly higher value than selenate for all the soils. References: (1) Goldberg, S. 1992. Use of surface complexation models in soils chemical systems. Adv. Agrom., 47, 233-329. (2) Goldberg, S., Lesch, S.M. and Suarez, D.L. 2002. Predicting molybdenum adsorption by soils using soil chemical parameters in the constant capacitance model. Soil Sci. Soc. Am. J. 66, 1836-1842. (3) Mora, M.L., Shene, C., Violante, A., Demanet, R. y Bolan, N.S. 2005. The effect of organic matter and soil chemical properties on sulfate sorption in Chilean volcanic soils. p. 444. In Huang, Violante, Bollag and Vityakon (eds) Soil Abiotic and Biotic Interactions and the impact on the ecosystem and human welfare.

Acknowledgement. FONDECYT 1020934 and MECESUP FRO-0309.