Effect of Carbonate Ions on the Adsorption of Phosphorus by Diopside in Aqueous Solution

Abstract

In order to use diopside as a phosphorous absorption material which does not require a decarboxylation step, its phosphorus removal ability was evaluated by dipping it in an aqueous solution containing 40.0 to 200 mg/L of carbonic acid and 2.50 mg/L of phosphorus. When diopside was immersed in phosphate solutions containing a low concentration of carbonic acid, phosphorus concentration was under 0.46 mg /L. However, in a solution containing over 150 mg/L of carbonic acid, even after mixing, phosphorus concentration was over 0.50 mg/L. On the other hand, when 40.0 mg/L of calcium was added to the solution containing 200 mg/L of carbonic acid, phosphorus concentration was 0.24 mg/L and well adsorbed. Calcite and magnesian calcite were newly precipitated in the residue of the aqueous solution after the treatment. In addition, by energy dispersive X-ray spectroscopy surface analysis it was possible to confirm the presence of phosphorus on the diopside surface. Elution of magnesium from diopside may accelerate the formation of soluble magnesian calcite and may prevent the precipitation of insoluble calcite, the cause of phosphorus suppression. Thus, diopside could be used as a phosphorus absorption material capable of adsorbing phosphorus even in an aqueous solution containing carbonic acid.