Conditions and mechanisms of the transformation of gibbsite into taranakite in phosphate solutions

Structural changes in gibbsite (an Al-containing soil mineral) in NH4H2PO4 solutions of different concentrations (from 10−6 to 2 M) were studied. As the concentration of the solution was increased, increasing release of aluminum from gibbsite to the liquid phase was observed along with binding of phosphate anions. The resulting soluble complex (aluminophosphate) anions were responsible for gibbsite dissolution. The most intense gibbsite dissolution occurred in 1 M and 2 M phosphate solutions. Evidence was obtained to show that this phenomenon is associated with the formation of aluminum complexes with pyro(poly)phosphate ligands. Along with intense gibbsite dissolution, a phosphate mineral (ammonium taranakite) formed, which was confirmed instrumentally. The key role of anionic aluminum pyro(poly)phosphate complexes in the formation of taranakite crystals was revealed. Sequential changes in the ligands in the complexes, associated with increasing phosphate loading, are discussed.