FIXATION OF PHOSPHATE BY IRON AND ALUMINUM AND REPLACEMENT BY ORGANIC AND INORGANIC IONS

Abstract

Potentiometric titrations of Fe and Al chlorides in the presence of phosphate showed inflection points in the titration curves which indicated that for each metal ion, two hydroxyls and one phosphate ion reacted to give a basic phosphate represented by the formulas A1(H2O)3(OH)2H2PO4 and Fe(H2O)3(OH)2H2PO4. The pH of maximum precipitation of basic Fe phosphate was 2.5 to 3.5, whereas for basic Al phosphate it was from 3.5 to 4. The amt. of phosphate which would combine chemically with one Fe or Al ion increased as the phosphate increased, to a value where one phosphate was combined with one Fe or one Al ion, and 2 hydroxyls were required to complete the precipitation. In no instance was the ratio of phosphate to Fe or Al in the precipitated compound greater than unity even when the amount of phosphate present was 9 times that of the Fe or Al. Hydroxyl, fluoride, and arsenate will replace chemically combined phosphate. The effectiveness of replacement depends upon the relative stability of the compounds formed and on the concn. of the replacing anion. Phosphate is several times as effective in replacing arsenate and fluoride from Al as these anions are in replacing phosphate, except where the ratio of fluoride to Al exceeds 6. Several organic anions were effective in preventing phosphate from combining chemically with Al and Fe or replacing chemically combined phosphate. Only those anions which form stable complexes with the Fe and Al were effective. Aliphatic hydroxy-organic acids and aromatic hydroxy acids were effective. A lyotropic series of these organic acids is presented. Humus and lignin were also effective in replacing chemically combined phosphate.