Changes in the mechanism of the reaction between phosphate and magnesium ions: Effect of initial concentration and contact time on removal of phosphate ions from aqueous media

In this study, the mechanism of the reaction between phosphate and magnesium hydroxide ions was assessed. In particular, the effect of initial concentration and contact time on the removal of phosphate ions from aqueous media using magnesium hydroxide was demonstrated. The evaluations were conducted after analyses entailing scanning electron microscopy, powder X-ray diffraction, and elemental distribution. The rate of removal of phosphate ions using magnesium hydroxide increased with increase in the initial phosphate ion concentration. Under our experimental conditions, the products after the removal of phosphate ions were magnesium hydroxide, magnesium potassium phosphate hexahydrate, and magnesium hydrogen phosphate trihydrate when the initial concentration was low (5 to 2000 mg/L), medium (from 4000 to 8000 mg/L), and high (15,000 mg/L), respectively. When the initial concentration was 15,000 mg/L, the product changed with elapsed time. Our findings indicated that the crystalline structure changed from that of magnesium potassium phosphate hexahydrate to that of magnesium hydrogen phosphate trihydrate (after 6 h). These results serve as useful information regarding the interaction between phosphate and magnesium ions at high concentrations of phosphate ions in aqueous media.