Adsorption of Phosphate by Hydroxy-Aluminum Species on a Cation Exchange Resin1
- W. P. Robarge and
- R. B. Corey2
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.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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