Stepwise removal and recovery of phosphate and fluoride from wastewater via pH-dependent precipitation: Thermodynamics, experiment and mechanism investigation

Phosphate and fluoride are the two most crucial contaminants in water resources. In this work, fluoride and phosphate were separately removed and recovered from wastewater through the step-by-step precipitation method. The thermodynamic analysis proved that the process could be achieved by controlling the pH value and the optimal operating window was predicted by constructing the lgc-pH diagram of the La–F–P–H2O system. Lab-scale experiments were performed using La3+ as a precipitant and the results were well matched with thermodynamic analysis. It indicated that the precipitation of LaF3(s) was predominated at 1.0< pH < 4.0 with slight precipitation of phosphate, while LaPO4(s) was primarily formed at a pH range of 5.0–11.0 with fluoride left in the solution. Furtherly, the generated LaF3 and LaPO4 were treated with NaOH solution to regenerate the precipitant and recycle fluorine and phosphorus, and the regeneration efficiency reached 97.4% and 74.3%, respectively. Eventually, the continuous experiment for the simulated wastewater revealed that 97.7% of fluoride and 99.9% of phosphate could be removed and recovered by the proposed stepwise precipitation method. The process is eco-friendly and sustainable, not only realizing the removal and recovery of F and P efficiently but also making them separated.