Lanthanum-Modified Sludge Biochar for Geothermal Water Fluoride Removal

Municipal sludge was pyrolyzed to produce sludge-derived biochar (SBC), which was subsequently modified with lanthanum nitrate. Through orthogonal experiments, the optimal preparation conditions for La-SBC-700 were determined. The morphological and textural properties of the biochar material, such as specific surface area, were characterized and batch adsorption experiments simulating fluoride-containing wastewater were conducted to investigate the effects of pH, fluoride concentration, biochar dosage, and coexisting ions on the fluoride removal performance of La-SBC-700. The application potential of the biochar material in real geothermal water was also assessed. The results indicated that La-SBC-700 prepared under optimal conditions exhibited an adsorption capacity approximately 10 times higher than that of the pristine biochar (SBC). The adsorption process was stable within the pH range of 5.0 to 8.0 and conformed to the Quasi-secondary-order kinetic and Langmuir isotherm models, with a maximum theoretical adsorption capacity of 40.338 mg/g. The adsorption process was spontaneous and endothermic. NO3- and Cl^- had negligible effects on fluoride removal, whereas CO32-, SO42-, and HCO3- exerted varying degrees of influence on the adsorption process. La-SBC-700 demonstrated excellent performance in removing fluoride from geothermal water, providing a reference method for the resourceful utilization of sludge and the removal of fluoride from geothermal water.