Uptake of Phosphate by Some Layered Silicate Minerals in Relation to Their Titration Curves with Acid and Base

Uptake of phosphate by some layer silicate minerals, viz., pyrophyllite, kaolinite (both in natural forms), montmorillonite and mica (both as Na-systems) has been studied in relation to their titration curves with NaOH, HCl, H₃PO₄ and Na₃PO₄. When titrated, each of the four minerals showed inflection/break in the pH/conductometric titration curve with the consumption of an equivalent amount of either acid or alkali. More significantly, an equal amount of phosphate, was taken up for an addition of H₃PO₄ where the inflection/break in the titration curve with this acid occurred.

According to the pH titration curves, the titratable acid groups had pK = 7.5. The basic groups had pK = 6.0. Broken bonds exposed on the lateral surfaces gave rise to groups having such acidic and basic strengths. Since the systems studied had a pH > 6 to start with, the exposed Al ions and the corresponding negative centres were already hydrolysed, OH⁻ ions being taken up by the exposed Al ions, H+ ions balancing the negative centres. On titration with a base, the H+ ions balancing the negative centres were neutralised and replaced by the cations of the base. In an acid titration, the OH⁻ ions attached to the edge Al were neutralised by the acid and replaced by the anions of the acid. In titrations both with acid and base, equal amounts of the two were consumed showing that equal amounts of positive and negative centres are created on the lateral surface as a result of lattice termination.

H₂PO₄, like HCl, neutralised OH groups attached to Al ions exposed on the lateral surface. Phosphate ions replaced the OH groups and a kind of surface precipitation of aluminium phosphate tooke place. Since the other product of the reaction with H₃PO₄ was, water, the specific conductance remained constant till this reaction was complete.

The time allowed for the reaction with H₃PO₄ was found to play an important part. From a given solution of H₃PO₄, the same amount of phosphate was taken up by any of the four minerals in two hours and twenty four hours. More phosphate was taken up when a longer time of interaction was given.

In titrations with Na₃PO₄ in place of NaOH, about three times more (m.e./100g) of the former had to be added, to reach the inflection/break in the titration curve. Also, the amount of phosphate taken up from Na₃PO₄ was three times larger than that from H₃PO₄. These results have been rationalized on the assumption that (a) hydrolysis of one molecule of Na₂PO₄ gave one molecule of NaOH which was used up in the titration and (b) PO₄⁻³ ion was taken up from the solution of Na₃PO₄ in place of the H₂PO₄⁻¹ ion which was immobilized from solutions of H₃PO₄.