In this paper, methods of effective removal of fluorine from rare earth chloride solution by adsorption, ion exchange and precipitation with lanthanum carbonate or CO₂ gas as fluorine-removal agent, respectively, were studied. The relevant parameters studied for fluorine-removal percentage were the effects of the type and dosage of fluorine-removal agent, the injection flow and mode of CO₂, the initial concentration of rare earth solution and initial pH value, contact time, temperature and stirring. XRD, SEM and EDS were used to analyze and characterize the filter slag obtained after fluorine removal. SEM and EDS results showed that RECO₃(OH) with a porous structure was formed in rare earth chloride solution when lanthanum carbonate was used as fluorine-removal agent, and it had strong selective adsorption for F⁻. The XRD spectra showed that F⁻ was removed in the form of REFCO₃ precipitates, which indicates that the adsorbed F⁻ replaced the OH⁻ group on the surface of RECO₃(OH) by ion exchange. The experimental results showed that a fluorine-removal percentage of 99.60% could be obtained under the following conditions: lanthanum carbonate dosage, 8%; initial conc. of rare earths, 240 g/L; initial pH, 1; reaction temperature, 90 ${}^{\circ }$C; reaction time, 2 h. Simultaneously, a fluorine-removal process by CO₂ precipitation was explored. In general, RE₂(CO₃)₃ precipitation is generated when CO₂ is injected into a rare earth chloride solution. Interestingly, the results of XRD, SEM and EDS showed that the sedimentation slag was composed of REFCO₃ and RE₂O₂CO₃. It was inferred that RE₂(CO₃)₃ obtained at the initial reaction stage had a certain adsorption effect on F⁻ in the solution, and then F⁻ replaced CO₃²⁻ on the surface of RE₂(CO₃)₃ by ion exchange. Therefore, F⁻ was finally removed by the high crystallization of REFCO₃ precipitation, and excess RE₂(CO₃)₃ was aged to precipitate RE₂O₂CO₃. The fluorine-removal percentage can reach 98.92% with CO₂ precipitation under the following conditions: venturi jet; CO₂ injection flow, 1000 L/h; reaction temperature, 70 ${}^{\circ }$C; initial pH, 1; reaction time, 1.5 h; initial conc. of rare earths, 240–300 g/L; without stirring. The above two methods achieve deep removal of fluorine in mixed fluorine-bearing rare earth chloride solution by exchanging different ionic groups. The negative influence of fluorine on subsequent rare earth extraction separation is eliminated. This technology is of great practical significance for the further development of the rare earth metallurgy industry and the protection of the environment.

We develop a highly active electrocatalyst composite based on BiVO ₄ @Ce–BDC–NH ₂ (B(VO) _1−δ @CeBN) nanocomposite including VO ₄ vacancies ((VO) _{1− δ} ) and heterojunction comprising bismuth vanadate (BiVO ₄ ) and a cerium-based metal–organic framework (Ce–BDC–NH ₂ ) for the ultrasensitive electrochemical detection of organic pollutant metol. The structural and compositional characteristics of B(VO) _{1− δ} @CeBN nanocomposite are analyzed by various microscopic and spectroscopic techniques, and confirmed that forming of the (VO) _{1− δ} defect and heterojunction structures. Furthermore, the B(VO) _{1− δ} @CeBN nanocomposite modified screen printed carbon electrode (B(VO) _{1− δ} @CeBN/SPCE) with exhibited enhanced electrochemical response toward metol including limit of detection of 1.15 nM in a broad linear range of 0.01–1000 µ M as well as a remarkable sensitivity of 0.2692 μ A · μ M ⁻¹ · cm ⁻² . This excellent detection sensitivity arises from the synergistic effect of heterojunction structures constructed between B(VO) _{1− δ} and Ce–BDC–NH ₂ , while VO ₄ vacancies defect enhances charge carrier concentration and conductivity. Additionally, B(VO) _{1− δ} @CeBN/SPCE sensor displays remarkable repeatability, reproducibility, stability, and selectivity toward metol detection. Finally, this work investigated the real-time practicability of metol over the B(VO) _{1− δ} @CeBN/SPCE electrode to detect spiked tape water, with good recovery ranges of 97.7%–98.6% achieved. These results highlight the potential of B(VO) _{1− δ} @CeBN/SPCE as a promising nanoplatform for trace-level detection of organic pollutants in environmental water bodies.