Diffusive phosphate transport in Al-rich acidic porous cation exchange system

Abstract

One-dimensional diffusive phosphate transport in a porous cation-exchange resin model system was investigated to examine the effect of exchangeable Al on the movement of phosphate. This study presumes that Al interacts with phosphate and H+ is produced as a result of deprotonation of H2PO4−. The production of H+ due to interactions between positively charged Al ions and phosphate ions was confirmed by measuring the pH change during the titration of phosphate by aluminum or aluminum by phosphate ions. A set of cylindrical wax columns packed with: (A) a mixture of sand and solid Al2O3, (B) a mixture of sand and Al3+-saturated cation-exchange resin, (C) a mixture of sand, solid Al2O3, and Al3+-saturated cation-exchange resin, and (D) sand as a control was used. A 0.2-g sample of 32P-KH2PO4 was applied on the surface of the mixture in each column simulating one-dimensional diffusion from an instantaneous planar source into a semi-finite system. Retardation of phosphate transport by exchangeable Al3+ was characterized by lowering of pH. Phosphate transport was not retarded in the presence of solid Al2O3 alone. Phosphate movement was further retarded in the presence of exchangeable Al3+ with solid Al2O3. Under this situation, H+ ion produced from precipitation reactions dissolved solid Al2O3, giving rise to more Al3+ in solution. Aluminum ion in the solution phase moved towards the site where phosphate was precipitated. From a mass balance of P, a slight amount of P was not recovered in the system whose pH was kept around 2.0 or below, indicating a probable formation of K-taranakite. Either K-taranakite or K2Al5H7(PO4)8−14H2O was identified as a final precipitation product from a series of titration experiments. However, at least two unidentified morphologically different amorphous aluminum phosphates were observed. Key words: Aluminum, cation-exchange, phosphate, precipitation, retardation, transport