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

One-dimensional diffusive phosphate transport in a porous cation-exchange r esin model system was investigated to examine the effect of exchangeable Al on the movement of phosphate. This study presumes that Al interacts with p hosphate and H+ is produced as a result of deprotonation of H2PO4-. The pro duction of H+ due to interactions between positively charged Al ions and ph osphate ions was confirmed by measuring the pH change during the titration of phosphate by aluminum or aluminum by phosphate ions. A set of cylindrica l wax columns packed with: (A) a mixture of sand and solid Al2O3. (B) a mix ture Of sand and Al3+-saturated cation-exchange resin, (C) a mixture of san d, solid Al2O3, and Al3+-saturated cation-exchange resin, and (D) sand as a control was used. A 0.2-g sample of P-32-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 phos phate transport by exchangeable Al3+ was characterized by lowering of pH. P hosphate transport was not retarded in the presence of solid Al2O3 alone. P hosphate 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. Alum inum ion in the solution phase moved towards the site where phosphate was p recipitated. From a mass balance of P, a slight amount of P was not recover ed in the system whose pH was kept around 2.0 or below, indicating a probab le formation of K-taranakite. Either K-taranakite or K2Al5H7(PO4)(8). 14H(2 )O was identified as a final precipitation product from a series of titrati on experiments. However, at least two unidentified morphologically differen t amorphous aluminum phosphates were observed.