Internalization of Fluoride in Hydroxyapatite Nanoparticles

Sulfate Doping Promotes Agglomeration of Calcium Fluoride Crystals

Calcium salt precipitation is an effective solution to wastewater fluoride pollution. The purity and precipitation efficiency of calcium fluoride is critical for its removal and recovery. This study aimed to reveal the role of coexisting sulfates in the precipitation of calcium fluoride. A low sulfate concentration promoted calcium fluoride precipitation. The size of calcium fluoride-aggregated particle clusters increased from 750 to 2000 nm when the molar ratio of sulfate to fluoride was increased from 0 to 3:100. Sulfate doped in the calcium fluoride crystals neutralized the positive charge of the calcium fluoride. Online atomic force microscopy measurements showed that sulfate reduced the repulsive force between calcium fluoride crystals and increased the adhesion force from 1.62 to 2.46 nN, promoting the agglomeration of calcium fluoride crystals. Sulfate improved the precipitation efficiency of calcium fluoride by promoting agglomeration; however, the purity of calcium fluoride was reduced by doping. Sulfate reduced the induction time of calcium fluoride crystallization and improved the nucleation rate of calcium fluoride. Sulfate should be retained to improve the precipitation of calcium fluoride and to avoid its loss from the effluents. However, it is necessary to separate sulfate from fluoride to obtain high-purity calcium fluoride. Therefore, sulfate concentration regulation in high-fluoride wastewater is key to achieving the efficient removal and recovery of fluoride ions.

Relative extents, mechanisms, and kinetics of fluoride removal from synthetic groundwater by prevalent sorbents

Fluoride (F) contamination in groundwater affects millions of people across the world. Although several sorbents have been identified for low-cost F removal, the choice of the optimal sorbent is dictated by the specific chemistry of contaminated groundwater. In this contribution, eight prevalent sorbents-activated alumina (AA), calcite, hydroxyapatite-coated calcite (HCC), natural chitosan, chalk, Mg-Al-CO3 layered double hydroxide (LDH), calcined Mg-Al-CO3 LDH (cLDH), and hydrous ferric oxide (HFO)-were categorized on their relative F removal mechanisms, extents, and kinetics from a typical synthetic groundwater, representative of contaminated aquifers of North India. Initially, batch experiments containing sorbents at 4 g·L-1 were conducted at a high F concentration (2.9 mM). The dominant F removal processes were identified by aqueous- and solid-phase characterization of reaction by-products. While chalk and calcite removed F by secondary precipitation of fluorite, HCC removed F by fluorapatite precipitation, and other sorbents removed F by sorption. Depending on the immobilization mechanism identified, the F uptake kinetics on each sorbent was modeled with either pseudo-second order or generalized rate equations. Among sorptive F uptake, cLDH exhibited the highest (10-2.15 mg·g-1·h-1) and HFO showed the lowest (10-4.15 mg·g-1·h-1) rates. Fluoride removal by precipitation was the fastest with chalk at 10-1.3 (h-1) (0.16). Subsequent experiments with AA and HCC at lower initial F concentration (0.42 mM) suggested increased uptake by ∼30x and ∼7x, respectively, relative to uptake in 2.9 mM initial F systems. For AA, apart from the widely-accepted mechanism of adsorption, an unidentified F-containing surface precipitate was formed. HCC was identified as the most promising sorbent with no sludge generation.

Preparation of aluminium-hydroxide-modified diatomite and its fluoride adsorption mechanism

As the current excessive accumulation of fluoride (F^-) in the environment can be hazardous to human health, it is essential to remove fluoride from wastewater. In this study, diatomite (DA) was used as a raw material and modified using aluminum hydroxide (Al-DA) for use in the adsorption of F^- from water bodies. SEM, EDS, XRD, FTIR, and Zeta potential characterization analyses were carried out; adsorption tests and kinetic fitting were performed, and the effects of pH, dosing quantity, and presence of interfering ions on the adsorption of F^- by the materials were investigated. The results show that the Freundlich model effectively describes the adsorption process of F^- on DA, which therefore involves adsorption-complexation interactions; however, the Langmuir model effectively describes the adsorption process of F^- on Al-DA, corresponding to unimolecular layer adsorption mainly via ion-exchange interactions, that is, adsorption is dominated by chemisorption. Aluminum hydroxide was shown to be the main species involved in F^- adsorption. The efficiency of F^- removal by DA and Al-DA was over 91% and 97% for 2 h, and the adsorption kinetics were effectively fit by the quasi-secondary model, suggesting that chemical interactions between the absorbents and F^- control the adsorption process. The adsorption of F^- was highly dependent on the pH of the system, and the maximum adsorption performance was obtained at pH 6 and 4. The optimal dosage of DA and Al-DA was 4 g/L. Even in the presence of interfering ions, the removal of F^- on Al-DA reached 89%, showing good selectivity. XRD and FTIR studies showed that the mechanism of F^- adsorption on Al-DA involved ion exchange and the formation of F-Al bonds.

Mechanisms of fluoride uptake by surface-modified calcite: A 19F solid-state NMR and TEM study

Fluoride pollution in groundwater is a serious problem threatening millions of people worldwide. Calcite is considered an ideal adsorbent for defluoridation owing to its widespread presence and low cost. To further enhance its performance, we synthesize a series of phosphate-modified calcites with varying phosphate concentrations. The surface modification led to the formation of a nanosized hydroxyapatite (HAP) coating on the calcite surface. With increasing concentrations of phosphate used for modification, the BET specific surface area of the adsorbents was dramatically enhanced, resulting in a great enhancement of F uptake. At low F concentrations (i.e., <1 mM), surface-modified calcite can achieve up to 25 times higher F removal efficiency than calcite. The ¹⁹F solid-state MAS NMR spectra yielded three distint peaks at δ(¹⁹F) = -86 ppm, -99 ppm, and -122 ppm, representing the formation of carbonate fluorapatite (CFA), fluorapatite (FAP), and coprecipitated F, respectively. This provides strong evidence for the contribution of newly formed HAP to F removal. In contrast, at high F concentrations (e.g., >2 mM), surface modification did not enhance F uptake by calcite. The ¹⁹F solid-state MAS NMR analysis revealed that the predominant deflurodation mechanism is the formation of CaF₂ precipitates (δ(¹⁹F) = -108 ppm) for both pristine and modified calcite at high F concentrations. Under this condition, the contribution of the newly formed nanosized HAP to F uptake is insignificant. Taken together, our results demonstrated the potential of surface modification of calcite as a cost-effective technique for defluoridation for most F-rich groudwater.

Synergistic removal of fluoride from groundwater by seed crystals and bacteria based on microbially induced calcium precipitation

A new hypothesis that seed crystals (SC) and bacteria based on microbially induced calcium precipitation (MICP) synergistically remove fluoride (F^-) from groundwater was proposed, with a focus on evaluating the defluoridation potential of this method and revealing its F^- removal mechanism. The crucial conditions were optimized to reduce preparation and operation costs. SC furnished more available binding sites due to the existence of bacteria, and the reuse experiments showed that the defluoridation efficiency of SC still remained a high level after 14 cycles (70.10%), with a residual F^- concentration of 0.96 mg L^-1. The SEM-EDS, FTIR and XRD analyses indicated the predominant F^- removal mechanism of SC could be ascribed to the chemisorption, ion exchange, and co-precipitation. Moreover, ion exchange and co-precipitation (PO₄^3- involvement) were validated more contributive than chemisorption (CaCO₃ and CaSO₄ involvement). As a feasible, reusable, and eco-friendly technique, SC suggests promising applications in the treatment of fluoride-contaminated groundwater.