Evaluation of the Phosphorus Adsorption Capacity of Natural Diopside in Aqueous Phosphoric Acid

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In this study, the phosphorus adsorption ability of natural diopside in an aqueous solution of phosphoric acid was evaluated. The influence of calcium ions present in the aqueous solution and pH on the phosphorus adsorption capacity of diopside was investigated. Diopside was found to adsorb a significant amount of phosphorus in the calcium-rich solution. In addition, its phosphorus adsorption ability tended to improve in aqueous solutions with lower pHs. In such solutions, calcium was eluted to a large extent from diopside. The adsorption rate of phosphorus in an aqueous solution of diopside was highly correlated to calcium elution. For solutions with low pH, the calcium bound to the OH group interacts with H+ and the charge on diopside becomes more positive. With this increasing positive charge, it can more effectively adsorb the negatively charged phosphate ions. It was assumed that calcium reacts with the phosphate ions and forms calcium phosphate on the diopside surface. The amount of phosphorus adsorption is related to the elution of calcium from diopside. We also found that natural diopside could be used as a phosphorus adsorbent in aqueous solutions without the need for adding calcium or adjusting pH.