SSe = aSecSe /(cSe+ kSe,PcP)1-bSe and SP = aPcP /(cP+ kP,SecSe)1-bP,
where aSe and aP represent anion sorption when concentration (cSe or cP) in solution is 1, bP is the index term of the Freundlich equation, bSe = b1cSe b2 is the Sibbesen term in which the index term of the Freundlich equation is a function of concentration, aSe = aSe,0 – mSe,P SP, mSe,P represents the decrease of aSe with increasing levels of sorption of phosphate (SP) by effects on the surface charge, aSe,0 is the value of aSe at zero sorption of phosphate, and kSe,P and kP,Se are competition coefficients of phosphate on selenite and selenite on phosphate, respectively. The ability for sorption of selenite (aSe) was lower than those of phosphate (aP) in the Andisols evaluated (Table 1). The addition of equimolar concentrations of selenite and phosphate decreased the amount of selenite adsorbed between 5 and 28%, while selenite decreased the sorption of phosphate in lesser extent. For every soil, competition coefficients (Ki,j) were not symmetrical, which means that selenite and phosphate not only competed for sorption sites, being phosphate a stronger competitor than selenite as shown by Barrow et al. (2005). Furthermore, there was evidence of feed back of phosphate on the sorption of selenite (mSe,P), which was attributed to a decrease of the surface electric potential as a consequence of diffusive penetration of phosphate as described by mechanistic models (Barrow, 1992). Acknowledgements: FONDECYT-1020934 and MECESUP FRO-0309 References: (1) Barrow, N. J. 1992. The effect of time on the competition between anions for sorption. J. Soil Sci. 43: 421–428. (2) Barrow, N. J., Cartes, P. and Mora, M. L. 2005. Modifications to the Freundlich equation to describe anion sorption over a large range and to describe competition between pairs of ions. Eur. J. Soil Sci. 56: 601–606.
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