Influence Factors on the Composition and Structure of CaP during Electrodeposition of Simulated Wastewater Containing Phosphorus

Electrodeposition (ED) of calcium phosphate (CaP) is a promising method for phosphorus (P) recovery, as P-containing wastewater typically contains abundant calcium ions. To further clarify the influencing factors and cathodic electrochemical behavior of ED reactions, an undivided electrolytic cell was used to investigate the effects of various cathode materials (metal-based electrode of Ti mesh, stainless steel, carbon-based electrodes of polyacrylonitrile (PAN)-based carbon felt), Ca/P molar ratio, initial pH of wastewater, and temperature on P recovery efficiency. When PAN-based carbon felt was used as the cathode, a Ca/P molar ratio of 2 resulted in the shortest nucleation induction time for CaP, approximately 2 h, and the highest P removal rate, at 34.35%. The initial pH value of the wastewater influenced the induction time of CaP on the cathode and the solubility product constant (Ksp) of the solid in the wastewater, thereby affecting the P removal rate in the system. The temperature of 35°C has promotes the formation of cathodic deposits, increasing the growth rate of CaP and shortening the nucleation induction time. Saturation index (SI) analysis found that when the Ca/P molar ratio was between 1 and 2.5, and pH>8, the SI of hydroxyapatite (HAP) was the highest, and HAP and amorphous calcium phosphate (ACP) were the most likely ED products. Voltammetry experiments conducted under different initial pH values of the electrolyte solution and varying scan rates revealed that PAN-based carbon felt exhibits better stability and electronic transfer properties. The electrolysis reaction of water, producing OH− near the cathode, facilitates the production of electrodeposited CaP products such as HAP and ACP, and the electron transfer process controls the deposition to nucleate on the cathode surface.