Effects of Aluminum and Phosphate Concentrations and Acidity on the Crystallization of Variscite at 90°c

Two series of aluminum phosphate solutions were incubated at 90°C for 60 days to examine the processes of crystallization. Series A solutions were 0.05 M in AlCI3, 0.15 M in NaH2PO4, and had varying acidity ranging from HCl/Al molar ratio = 5 to NaOH/Al molar ratio = 3. Series B solutions were 0.001 M in AlCl3, 0.0006 M in NaH2PO4, and had a NaOH/Al molar ratio = 0 to 3. The nature of the reaction products was greatly affected by acidity and the concentrations of aluminum and phosphate. Variscite was observed only in Series A solutions of strong acidity. In a solution of HCl/Al molar ratio = 5 (pH 0.87), the reaction product was soluble aluminum phosphate complex. In solutions of HCl/Al = 2 (pH 1.36) to 3 (pH 1.17), well-crystallized variscite was observed at the very beginning of precipitation, with the amount and size increasing slowly with time. In solutions of NaOH/Al molar ratio = 2 (pH 3.29), the product was initially fine particles that were amorphous to X-ray diffraction, but gradually converted to variscite during incubation. In weakly acid solutions (NaOH/Al = 2.5 to 3.0; pH 5.2 to 6.0), the products remained X-ray-amorphous aluminum phosphates even after 60 days of incubation. No variscite was found in any sample from the Series B solutions. With increasing NaOH/Al ratio, the reaction products changed from soluble aluminum phosphate complexes to positively charged basic aluminum phosphate particles, then to a neutral precipitate, and finally to negatively charged particles. All precipitates or suspended particles were amorphous to X-ray diffraction and consisted of both OH− and PO43− linkages. These products did not show any tendency toward variscite crystallization up to at least 60 days. Solution concentrations 0.001 M in Al and 0.0006 M in P can be considered the upper limit of Al and P concentrations in ordinary soil solutions. Therefore, the conditions favorable for variscite formation are not compatible with those found in the ordinary soil environment.