A critical review on adsorption and recovery of fluoride from wastewater by metal-based adsorbents

Approaches for the Efficient Removal of Fluoride from Groundwater: A Comprehensive Review

Contamination of groundwater with fluoride represents a significant global issue, with high concentrations posing serious public health threats. While fluoride is a critical element in water, excessive levels can be detrimental to human health and potentially life-threatening. Addressing the challenge of removing fluoride from underground water sources via nanotechnological approaches is a pressing concern in environmental science. To collate relevant information, extensive literature searches were conducted across multiple databases, including Google Scholar, PubMed, Scopus, Web of Science, the American Chemical Society, Elsevier, Springer, and the Royal Society of Chemistry. VOS Viewer software version 1.6.20 was employed for a systematic review. This article delivers an exhaustive evaluation of various groundwater fluoride removal techniques, such as adsorption, membrane filtration, electrocoagulation, photocatalysis, and ion exchange. Among these, the application of nanoparticles emerges as a notable method. The article delves into nano-compounds, optimizing conditions for the fluoride removal process and benchmarking their efficacy against other techniques. Studies demonstrate that advanced nanotechnologies-owing to their rapid reaction times and potent oxidation capabilities-can remove fluoride effectively. The implementation of nanotechnologies in fluoride removal not only enhances water quality but also contributes to the safeguarding of human health.

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.

Hybrid lignin particles via ion-crosslinked for selective removal of anionic dyes from water

Hybrid lignin (HL) particles were synthesized by compounding lignosulfonate and carboxylated chitosan through a simple ionic cross-linking method, and modifying by polyvinylpolyamine. Due to the synergistic effect of recombination and modification, the material exhibits excellent adsorption performance for anionic dyes in water. The structural characteristics and adsorptive behavior were systematically investigated. The pseudo-second-order kinetic model and the Langmuir model were revealed to well describe the sorption procedure of HL for anionic dyes. The results exhibited that the sorption capacities of HL on sodium indigo disulfonate and tartrazine were 1099.01 mg/g and 436.68 mg/g, respectively. Simultaneously, the adsorbent behaved no significant adsorption capacity loss after five adsorption-desorption cycles, indicating its superb stability and recyclability. Additionally, the HL exhibited excellent selective adsorption of anionic dyes form binary dye adsorption systems. The interaction forces between adsorbent and dye molecules, such as hydrogen bonding, π-π stacking, electrostatic attraction and cation bonding bridge, are discussed in detail. The facile preparation process and superior dyes removal performance of HL were considered a potential adsorbent to remove anionic dyes from wastewater.