Adsorption of Phosphate by Hydroxy‐Aluminum Species on a Cation Exchange Resin

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Abstract
A study was conducted to provide a possible explanation for the reported decrease in the concentration of P in soil solutions with increasing pH and/or ionic strength. The assumption was made that P in solution decreases because of the formation of a new P‐adsorbing surface, the source of which would be exchangeable Al3+. An Al‐saturated resin was used as a model of an acid soil to study the effects of pH, charge and/or concentration of associated cation on the concentration of P in solution when the P‐adsorbing complex is affixed to a negatively charged surface. The hydroxy‐Al species formed by adding base was found to be the primary P‐adsorbing surface. The OH/Al molar ratio of the hydroxy‐Al species formed remained nearly constant with addition of base (2.0 to 2.1:1 in the presence of Ca(ClO4)2; 2.4:1 in the presence of NaClO4; µ = 0.003) until all the Al3+ present had been converted to the hydroxy‐Al form. The pH of formation of the hydroxy‐Al species was a function of µ and neutral salt cation.Phosphate was added at Al/P molar ratios of 12:1 or 6:1 in the presence of Ca(ClO4)2 or NaClO4 of equal ionic strengths (µ = 0.0003 or 0.003). The hydroxy‐Al species demonstrated a marked capacity to adsorb P. At an Al/P molar ratio of 2.0:1 (Ca(ClO4)2; µ = 0.003) the equilibrium concentration of P was < 0.2 mg/l. As % neutralization of exchangeable Al3+ increased, at constant total P concentrations, P in solution initially decreased, passed through a minimum, and then increased. The pH of minimum P in solution corresponds to the point where increased adsorption due to increasing adsorbing surface is balanced by increasing competition from hydroxyl ions for adsorption sites. Exchangeable Al3+ did not react with added P until the Al/P ratio of the hydroxy‐Al species equalled or became less than 2.0:1.The effects of µ and pH on P adsorption in the Al‐resin system are similar to many results reported in the literature for soil systems. Our studies suggest that the often observed correlation between exchangeable Al3+ and P fixation by soils is due to the relationship between Al3+ and the formation of the hydroxy‐Al species, unless the P addition rate is high enough to favor the precipitation of an aluminum‐phosphate compound. Hydroxy‐Al species formed by liming acid soils to above the pH at which all exchangeable Al3+ is converted to nonexchangeable forms appear to be stable enough to affect the concentration of P in solution over the short‐term, but may be unstable with respect to gibbsite or other Al minerals over long periods of time.