EFFECT OF ORGANIC ANIONS ON PHOSPHATE PRECIPITATION BY IRON AND ALUMINUM AS INFLUENCED BY pH

Abstract

Nine organic acids, some of which are found extensively in soils, were investigated to determine their effectiveness in preventing phosphate precipitation by Fe and Al at various pH values. The 9 acids used were: DL-alpha-alanine, DL-aspartic, citric, DL-lactic, DL-malic, malonic, oxalic, succinic, and DL-tartaric. The specific effect of each acid with Fe was different from that with Al although there was a general similarity of effect in some cases. Some of the acids were most active at low pH levels, some in moderately acid solns., and others at the neutral point or above. The most effective were citric, oxalic, tartaric, malonic, malic, and lactic and their greatest effectiveness was in the more acid pH range, although some were active also at higher pH values. These organic acids occur in soils in considerable quantity as a result of the action of microorganisms on organic matter. They are therefore operative in forming stable complexes with Fe and Al in a pH range where these elements are most active in phosphate fixation, thus helping to prevent fixation. Citrate was the most active anion in preventing phosphate precipitation with both Fe and Al; 1 millimol completely complexed 1 millimol of Al, and no phosphate was precipitated in the pH range 4 to 9. With Fe, citrate was most effective between pH 4 and 6 where 1 millimol released more than 90% of the phosphate from 1 millimol of basic Fe phosphate. The structure of the organic anion is of importance in determining its efficiency in preventing phosphate precipitation. The general observations concerning the effect of structure are as follow: the functional amino group has no significant effect; the functional hydroxyl group increases the effectiveness as the number of hydroxyl groups is increased; the effectiveness of the organic anion is increased as the number of carboxyl groups is increased; and the effectiveness increases as the chain-length decreases.