Stepwise removal and recovery of phosphate and fluoride from wastewater via pH-dependent precipitation: Thermodynamics, experiment and mechanism investigation. (20th October 2021) - Xia, Liu
Abstract: 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 lg c -pH diagram of the La–F–P–H2 O system. Lab-scale experiments were performed using La 3+ 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. Graphical abstract: Image 1 Highlights: Stepwise precipitation of F and P was realized through controlling pH value. Optimum pH range for F and P separation was predicted by thermodynamic analysis. A high removal efficiency of F by La 3+ was obtained at pH 1.0–4.0. High removal efficiency of P by La 3+ was reached in wide pH regions of 5.0–11.0. It realizes the recycling of precipitant and recovery of F and P.