Removal of Phosphate from Aqueous Solution by Crystallization Using Coal Fly Ash

The removal of phosphate from aqueous solutions by crystallization using coal fly ash has been studied in order to establish a simple and inexpensive wastewater treatment system. Coal fly ash samples obtained from coal fired power plants in Japan, Philippines and Thailand were used in this study. Phosphate ion is removed from an aqueous alkaline solution as hydroxyapatite by crystallization reaction with calcium and hydroxide ions when seed crystalline substance coexists in the solution. All the coal fly ashes used conducted itself as calcium ion supplier, pH controller and seed crystal when added into the aqueous phosphate ion solution. To determine the optimal conditions for an effective removal of phosphate, sets of 50-ml of 10 mg L -1 phosphate solutions were stirred with 1 g of each coal fly ash. Water-soluble calcium content of coal fly ash, pH of the solution, reaction time and reaction temperature was major operating parameters in removal of phosphate. A phosphate removal efficiency of 90 % was easily achieved at pH 10 for all the coal fly ashes studied. The rate of phosphate removal depended on the kind of coal fly ash. The phosphate removal capacity of the ash was calculated between 3.2 mg g -1 and 15.6 mg g -1 from the adsorption isothermal plots according to the Langmuir adsorption equation at 25 o C. Although, most of diverse ions such as Cl - , NO3 - , SO4 2- did not affect the removal efficiency of phosphate, the interference of CO3 2- ion was significant. Typically, the phosphate removal efficiency of 99.6 % at pH 11.29 with Philippine fly ash was decreased to 83.9 % when 120 mg L -1 of carbonate ion coexisted. Phosphate removal efficiency was enhanced by increase of the reaction temperature. Continuous removal of phosphate by a column method has also been performed with some coal fly ashes.