Factors affecting the degree of phosphate-removal in the system FeCl3-orthophosphate and nature of the precipitates

In this batch type parametric study the influence of phosphate concentrations on the extent of its removal by constant amount of FeCl₃ at different pH, temperature and aging time is evaluated. The efficiency of FeCl₃ in removing the phosphate is strongly dependent on the pH of the precipitation. At P/Fe molar ratios of higher than 0.5 the pH of maximum phosphate removal is found to be at pH 2.5 and is followed by pH 4, 6 and 9. The optimum pH for maximal phosphate removal greatly varies with the amount of phosphate present relative to that of iron present. It is also observed that maximal phosphate removal by FeCl₃-solution is achieved at P/Fe ratios of more than 1, 0 suggesting that in removal process not only Fe³⁺ but also colloidal hydroxide particles could play an important role. The samples of non-aged synthesized precipitates obtained at P/Fe ratios of 8, 4 and 2 at pH 2.5 are found to have characteristic lines of strengite. The formation of strengite at room temperature from fresh solutions has not previously been reported. Thermally treated precipitates with the characteristic lines of strengite transform into a quartz-like phase which suggests that the loss of the coordinated water leads to a compound like ABO₄ where A and B both are in tetrahedral coordination. Upon heat treatment, the amorphous precipitates, having P/Fe ratios > 0.5, obtained between pH 2.5 and 6 result in a quartzlike phase. However, the characteristic lines of the quartz phase are not of the same intensity. In contrast, the precipitates having P/Fe ratios < 0.5 transform into hematite after heat treatment. The thermally treated products of precipitates obtained at pH 9 are found to have η-Fe₂O₃ phase which indicates that basic phosphates formed at this pH are structurally similar to that of basic sulfates.