Mechanistic and kinetic investigation of pH-dependent fluoride removal by lanthanum hydroxide precipitation: Validation in ammonium sulfate solution

High-fluoride acidic wastewater generated from industrial processes poses significant environmental and health risks. In this study, lanthanum hydroxide [La(OH)3] was evaluated as a precipitant for fluoride removal through thermodynamic analysis, batch experiments, product characterization, and kinetic modelling. Its performance was further compared with that of conventional CaO precipitation. Among the investigated parameters, pH was identified as the dominant factor governing fluoride removal, whereas temperature exhibited a comparatively minor effect. Under the optimum conditions (pH 2.5), La(OH)3 achieved a fluoride removal efficiency of 96.9%, reducing the residual fluoride concentration to 6.9 mg/L. In contrast, CaO showed markedly lower fluoride removal efficiency under acidic conditions, demonstrating the superior applicability of La(OH)3 in acidic fluoride-bearing solutions. Characterization results revealed that decreasing pH promoted the transformation of La(OH)3 into stable LaF3 precipitates, thereby enhancing fluoride removal. Kinetic analysis indicated that solid-product layer diffusion was the predominant rate-controlling step, while interfacial chemical reaction also contributed to the precipitation process. The effectiveness of La(OH)3 was further validated in an actual ammonium sulfate solution, indicating its potential applicability to sulfate-rich fluoride-bearing industrial streams with similar chemical characteristics. In addition, the resulting LaF3 precipitate has potential for further utilization as a fluorine-containing polishing material precursor, enabling simultaneous fluoride removal and resource recovery. These findings provide mechanistic insights and technical guidance for the development of efficient and sustainable fluoride-removal processes under acidic sulfate-rich conditions, although further validation in more diverse wastewater matrices is required before broader application can be established.