A novel lanthanum ion-assisted fluorapatite precipitation approach for efficient fluoride removal from water

Excess amounts of fluoride ions (F-) in water/wastewater bring great hazards to human health and the environment. The traditional method such as precipitation by calcium ions does not work well for low-concentration F- removal. Herein, we developed a lanthanum ion (La3+)-assisted fluorapatite precipitation approach to treat F--containing water, aiming at high removal efficiency. Through an orthogonal experimental design and single-factor experiment, the optimized reaction conditions were obtained as the molar ratio of (Ca2++La3+): PO43-: F-=7.5: 4: 1, Ca2+: La3+ = 90: 10, and reaction pH of 9. Under such reaction conditions, F- in water (10 mg L−1) could be well controlled under 1 mg L−1, fulfilling relevant environmental standards. No remarkable F- removal efficiency loss was found in the presence of typical anions (e.g., halides, bicarbonate, sulfate). The developed F- removal approach also worked well in a wide F- concentration range (10–200 mg L−1) and reaction temperature range (5–45 °C). Further reaction mechanism study proved that the formation of LaFx(OH)3-x colloids was the key step, which could accumulate F- ions in water into colloids. The subsequently added Ca2+ and PO43- destroyed the stable colloids, and then promoted the formation of fluorapatite with simultaneous La3+ doping, which existed as precipitate and allowed the easy separation from the reaction solution. As a by-product, the as-produced La3+-doped fluorapatite was evaluated as an adsorbent for organic pollutant removal, which possessed a high adsorption capacity (763.4 mg g−1) for the model pollutant of Congo red. Considering the facile operation process, high F- removal efficiency, wide operation temperature window, and anti-inference for co-existing anions, the La3+-assisted fluorapatite precipitation approach possessed great potential for F--containing water/wastewater treatment.