EFFECTS OF ALUMINUM AND PHOSPHATE CONCENTRATIONS AND ACIDITY ON THE CRYSTALLIZATION OF VARISCITE AT 90-DEGREES-C

Two series of aluminum phosphate solutions were incubated at 90-degree s-C for 60 days to examine the processes of crystallization. Series A solutions were 0.05 M in AlCl3, 0.15 M in NaH2PO4, and had varying aci dity ranging from HCl/Al molar ratio = 5 to NaOH/Al molar ratio = 3. S eries 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 wa s greatly affected by acidity and the concentrations of aluminum and p hosphate. Variscite was observed only in Series A solutions of strong acidity. In a solution of HCI/Al molar ratio = 5 (pH 0.87), the reacti on product was soluble aluminum phosphate complex. In solutions of HCI /Al = 2 (pH 1.36) to 3 (pH 1.17), well-crystallized variscite was obse rved 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 inc ubation. 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 afte r 60 days of incubation. No variscite was found in any sample from the Series B solutions. With increasing NaOH/Al ratio, the reaction produ cts changed from soluble aluminum phosphate complexes to positively ch arged basic aluminum phosphate particles, then to a neutral precipitat e, and finally to negatively charged particles. All precipitates or su spended 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 con centrations 0.001 M in Al and 0.0006 M in P can be considered the uppe r limit of Al and P concentrations in ordinary soil solutions. Therefo re, the conditions favorable for variscite formation are not compatibl e with those found in the ordinary soil environment.