Treatment of acidic wastewater via fluoride ions removal by SiO₂ particles followed by phosphate ions recovery using flow-electrode capacitive deionization

A new process for the treatment of phosphoric acid industry wastewater is proposed. The technique comprises the following major steps: (1) selective removal of fluoride ions via reaction/adsorption with SiO₂-containing particles; (2) microfiltration; and (3) flow electrode capacitive deionization (FCDI) for selective separation of phosphate ions. Three different silica-based materials were tested (quartz send, glass beads and fly ash) and F^- removal from initial concentration of 4.1 gr/l to undetectable level was achieved using fly ash & quartz sand mixtures (FA-QS). Fluoride adsorption by FA-QS resulted in pH reduction to pH≈1.0, at which the phosphate system in the wastewater shifts towards the H₃PO₄ form as a dominating species. These uncharged species could be efficiently separated from Cl^- and SO₄^2- ions electrochemically using a single-electrode FCDI process. FCDI operated with a single electrode (5 %wt activated charcoal powder) at a constant voltage of 1.2 V resulted in 90% selective separation of phosphorous with the complete removal of sulfate and chloride ions. The results, therefore, indicated that the new process can be effectively used to minimize the risk posed by the wastewater, while enabling the recovery of valuable phosphorus.