THE INFLUENCE OF POLYMER ARCHITECTURE ON INHIBITION OF AMORPHOUS CALCIUM PHOSPHATE PRECIPITATION

Abstract

The influence of polymer architecture on the precipitation of amorphous calcium phosphate from supersaturated solutions has been investigated using the pH-stat technique. It has been found that low concentration of acrylic acid based copolymers and terpolymers markedly inhibit the precipitation of calcium phosphate. Homopolymers containing anionic, non-ionic, and cationic groups under similar experimental conditions are inefficient as calcium phosphate inhibitors. The inhibition data of several polymers indicate that the performance of polymers depends upon the functional group, ionic charge, and the molecular weight. In addition, it has been found that cationic flocculants and biocides when present at low concentrations exhibit strong antagonistic effect on the performance of calcium phosphate inhibiting polymers. INTRODUCTION In recent years many studies have been undertaken concerning the precipitation of sparingly soluble alkaline earth metal phosphates from their supersaturated solutions. Precipitation of calcium phosphates is of particular interest because of its importance in many areas. Common examples include dental calculus, calcified plaques of arteriosclerosis, kidney calcinosis, etc. Calcium phosphates deposits have been encountered during pasteurization of milk. In industrial water systems (i.e., desalination, boiler and cooling) precipitation and deposition of calcium phosphates on membrane and heat exchanger surfaces continue to pose serious operational challenges. In addition, calcium phosphates are widely produced in industry, in such forms as, ceramics, nutrient supplements, medicines, dentifrices, and stabilizers for plastic. They are utilized in solid state chemistry as fluorescent lamp phosphors and play a role in waste water treatment processes. It is generally agreed that initially precipitated calcium phosphate from aqueous solution of high supersaturation and at pH >7, is an amorphous calcium phosphate, ACP. The formation and stability of ACP phase in aqueous solution has been the subject of numerous investigations. It has been suggested that if ACP is allowed to remain in contact with the solution, it transforms to the thermodynamically stable hydroxyapatite, HAP, perhaps via the hydrolysis of octacalcium phosphate. The metastability of the precursor is very sensitive to the conditions of precipitation such as pH, temperature, and degree of supersaturation. The presence of additives and impurities, ranging from simple ions to small molecules and polymers, including proteins at crystal surfaces, is frequently found to have a profound effect on crystallization properties. Some impurities can suppress growth entirely; others may actually enhance nucleation and crystal growth. An important aspect in industrial and biological systems is the role that various macromolecules play as precipitation and crystal growth inhibitors for a variety of sparingly soluble salts such as calcium oxalate, calcium carbonate, calcium fluoride, barium sulfate, etc. It has been reported that the presence of both dissolved and suspended impurities in supersaturated solution exhibit marked influence on the performance of polymers as precipitation inhibitors. In the present work, we have investigated the effect of polymer architecture (i.e., polymer composition, ionic charge, molecular weight, etc.) on the precipitation of calcium phosphate from supersaturated solution. This study also reports the impact of cationic polymeric coagulants/flocculants and biocides on the performance of calcium phosphate precipitation inhibitors.