Characteristics Of Phosphate Controlled Release Glasses And Their Applications As Water Treatment Agents

Literatures of controlled release technology were reviewed. The Comvlete Erosion Controlled Release Systems are characterized with the special advantages of 0-order release kinetics and no need of operation to remove the carrier when the release is complete. Controlled release glasses (CRGs) are the most suitable carriers for the release of inorganic metal ions and have found applications in agriculture, veterinary pharmacology and medical treatment. The release characteristics of phosphate CRGs and their prospective applications as controlled release water treatment agents are investigated in this paper, which includes the following parts. The chemical durability of phosphate glass is highly dependent on its composition, which comprises the internal factor of release kinetics of phosphate CRGs. Leach tests were conducted in deionized water at 30C in which the dissolution rates of specimens were measured by weight loss method. Results show that the 40Na2O . 60P,05 glass has the lowest dissolution rate among the Na2O-P2O~ binaiy glasses. On the basis of this composition, the durability of CRGs can be further improved by substitution of divalent metal oxides such as CaO for Na20 or trivalent metal oxides such as Al203 for P205, but deteriorated by introducing B203 or Si02 into the glass composition. FTIR spectra shows that the structural change related to the improvement of durability caused by composition adjustment includes strengthened Mæ“® bond on the [P04] tetrahedron formed by the non-bridging oxygen with the metal ions introduced and the P-O-P bond formed by the bridging oxygen between 2 [P04] tetrahedrons. The nature of solvent and experimental conditions comprises the external factors of the release kinetics of phosphate CRGs, which depends on the dissolution mechanism of phosphate glass. Results of leach test under different conditions show that the 25Na2&25CaO5OP2O5 glass dissolves congruently in deionized water, with an activation energy of about 79 kJ/mol. Its dissolution rate increases dramatically as the pH of solution decreases, and decreases slowly as the pH of solution increases. Sf-IMP or CaCl2 in solution have accelerating or decelerating effect, respectively, on the dissolution of phosphate glasses. The fact that the dissolution rate is not obviously affected by the stirring speed of solution implies that the mass transfer rate of the dissolution process of phosphate glass is controlled by the chemical reaction of dissolution itself. Discussions on the mass transfer-reaction behavior of dissolution mechanisms show that the nature of the reaction taking place in the hydrated layer is the dissociation of Na~ ions from the phosphate chains under the effect of the penetrating water molecules, rather than the generally accepted reaction of Na-H ion exchange. The velocity of formation and development of hydrated layer depends on the diffusion rate of water molecules in glass. The dissolution of glass is realized by the breakage of P-0-P bonds in the phosphate network in the hydrated layer followed by the disentanglement of phosphate chains into water solutions. The chelating ability of polyphosphate with divalent cations has great influence on the dissolution rate of phosphate CRGs and is found responsible for the accelerated or decelerated dissolution of the glass in SHMP or CaCl2 solutions, respectively. The hydrolysis reaction has been one of the main restrictions of polyphosphates as corrosion and scale inhibitors in cooling water systems. Static hydrolysis test of SHMP and dissolution-hydrolysis test of 25Na2O?5CaO~50P2O5 were conducted. Results show that the hydrolysis...