Controlled Precipitation of Sparingly Soluble Phosphate Salts Using Enzymes. I. Controlled Development of Solution Supersaturation in Situ

The formation of phosphorus containing sparingly soluble salts from supersaturated solutions which may be formed at locations such as subsoil or in subsediment formations consisting of loose sand is a promising alternative solution for attaining stabilization of loose particulate systems. In the present work, we present a novel methodology for the development of supersaturation by the release of phosphate through the hydrolysis of polyphosphate substrates by phosphatase enzymes (acid and alkaline). The phosphate released through the enzymic activity in the presence of excess of alkaline earth metals concentrations are critical in developing the supersaturation needed for the precipitation of the respective phosphate salt. In the present work, the methodology was demonstrated by the precipitation of calcium phosphate and magnesium ammonium phosphate in silicic sand and topsoil particulate samples. The rate of the enrichment of the solutions with phosphates and the extent of decomposition of the substrates (kinetic parameters) were investigated as a function of solution pH, amount of enzyme, substrate concentration, and temperature. pH increase was found to decrease the rate of total phosphates released and the extent of hydrolytic decomposition in the presence of acid phosphatase. The kinetic parameters were increased with increasing enzyme concentration. At pH = 7.50, kinetics of polyphosphate hydrolysis increased with increasing enzyme concentration in the presence of acid phosphatase. Above this pH, the enzyme activity was insignificant. Hydrolysis rates increased with increasing substrate concentration for the case of alkaline phosphatase only. Alkaline phosphatase was employed in solutions with pH values between 9 and 10. Both the rates and the extent of hydrolysis increased with temperature for both enzymes. In the presence of calcium ions in the hydrolysis medium, a mixture of calcium phosphate dihydrate (CaHPO 4 ·H 2 O, DCPD) and hydroxyapatite (Ca 5 (PO 4 ) 3 OH, HAP) were obtained at the alkaline pH, which is optimal for the alkaline phosphatase. In the presence of magnesium and ammonium ions, struvite crystals formed as soon as critical supersaturation was achieved. In the presence of silicate sand, the rate of substrate hydrolysis increased only in the presence of the acid phosphatase. Higher sand concentrations however resulted in reduction of the rates and of the extent of hydrolysis. In the presence of soil rich in silicates the activity of both enzymes tested was reduced.