Drinking water has been contaminated over decades with some very detrimental compounds such as fluoride. Exposure to fluoride through drinking water above the permissible limit (1.0-1.5 mg/L) causes severe dental and skeletal fluorosis. Adsorption technique which deals with adsorbents for fluoride removal from an aqueous solution is a highly efficient and selective process. This review paper provides insights on adsorbents used and developed by researchers for defluoridation of drinking water. It includes various categories of adsorbents used and parameters affecting the whole process. Adsorbents studied by researchers are enlisted with their adsorption capacity, optimum pH, temperature, equilibrium isotherm, kinetics, interfering ions, thermodynamic studies and regeneration procedure adapted. Efforts are needed to develop low cost reusable adsorbents with high adsorption capacity. Although, some adsorbents are reported to show remarkable capacity for fluoride removal; still there is an urgent need for development of more novel adsorbents holding both economic and technological benefits.

No abstract

This research work presents the synthesis of hydroxyapatite (Hap) nanorods for defluoridation of drinking water by using both conventional (CM) and ultrasonication with precipitation (USPM) methods. Calcium nitrate was reacted with potassium phosphate in presence of ammonia for controlled pH to synthesize Hap nanorods, which was characterized using FTIR, XRD, SEM, TG-DTA, and TEM/EDS for determining its phase composition, structural and thermal decomposition behavior. When USPM method was used for synthesis, the yield of the Hap nanorods was improved from 83.24±1.0% to 90.2±1.0%, and complete phase transformation occurred with formation of elongated Hap nanorods. Effects of process parameters such as solution pH, contact time and adsorbent dose were studied through response surface methodology (RSM). A simple quadratic model was developed using Central Composite Design (CCD) and optimum parameters for fluoride adsorption process were determined to be pH 7, contact time 3h and adsorbent dose 7g/L for maximum removal capacity. Fluoride removal efficiency was predicted to be 93.64% which was very close to the experimental value obtained at 92.86% using ultrasonically prepared Hap. Fluoride adsorption isotherms fitted the Freundlich isotherm with an adsorption capacity of 1.49mg/g, while the kinetic studies revealed that the process followed pseudo-second order model. The treated water quality parameters such as residual fluoride, calcium leached, total hardness and alkalinity was investigated, and it was observed that all these parameters were within the permissible limits as per WHO and BIS standards.

In many parts of the world, fluoride in drinking water is responsible for notable public health issues. The present study is aimed to prepare a new adsorbent magnesia-hydroxyapatite (Mg-HAP) that can serve as a valuable defluoridating agent. Characterization of the synthesized adsorbent was done by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Transmission electron microscope (TEM), and Scanning electron microscope (SEM)/Energy-dispersive X-ray spectroscopy (EDX) analysis to reveal the bonding patterns, phase characteristics, and microstructural and morphological details. The influences of pH, adsorbent dose, contact time, and initial fluoride concentration and the effect of interfering anions were studied. The defluoridation capacity was evaluated to be 1.4 mg/g, and the adsorbent showed very good capability to remove fluoride from contaminated water over a wide range of pH. Equilibrium modeling was done, and the experimental data was fitted into Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich isotherms. Study of the kinetic data for the adsorption process revealed that it follows pseudo-second-order reaction. It also indicated that the intraparticle diffusion contributes to the rate-determining step in the process. The quality of treated water was analyzed for total dissolved solids (TDS), turbidity, residual calcium, residual phosphorus content, electrical conductivity, hardness, and total alkalinity. The results obtained were very promising and confirmed the prospects of usage of Mg-HAP in defluoridation of drinking water.Abbreviations q e Amount of adsorption at equilibrium (mg/g) q t Amount of adsorption at time t (mg/g) C 0 Initial concentrations of fluoride (mg/L) C e Equilibrium concentrations of fluoride (mg/L) C t Concentration of fluoride at time t (mg/L) vVolume of the aqueous solution (L) m Adsorbent mass (g) Q 0 Maximum adsorption capacity reflecting complete monolayer (mg/g) R L Separation factor depicting favorability of the process bLangmuir constant related to energy K f Freundlich constants (mg/g) N Heterogeneity factor R Universal gas constant (8.314 J/mol K) T Temperature (K) A T Temkin isotherm equilibrium binding constant (L/g) B T Temkin isotherm constant b 1 Adsorption rate constant of first order reaction (min −1 ) b 2 Adsorption rate constant of first order reaction (g mg −1 min −1 ) k i Intraparticle diffusion rate constant (mg g −1 min −1/2 ) a Initial adsorption rate (mg g −1 min −1 ) α Desorption rate (g mg −1 ) Water Air Soil Pollut (2015) 226:241

Marble waste powder consisting of calcium and magnesium compounds was used to synthesize a novel biocompatible product, marble apatite (MA) primarily hydroxyapatite (Hap) for applications in defluoridation of drinking water. Synthesis of marble apatite was carried out by using calcium compounds (mixture of hydroxide and nitrate) extracted from marble waste powder which was treated with potassium dihydrogen phosphate at 80°C under alkaline conditions using conventional precipitation method (CM) and ultrasonication method (USM). Qualitative analysis of synthesized marble apatite from both the methods was carried out using FTIR, phase analysis by XRD and microstructure analysis by SEM and TEM. When ultrasonication (USM) method was used, the yield of marble apatite was improved from 67.5% to 78.4%, with reduction in crystallite size (58.46nm), lesser agglomeration and comparatively well-defined spherical morphology compared to the CM method. Studies also include estimation of the defluoridation capacity of MA as an adsorbent for drinking water treatment and effects of process parameters such as pH, contact time, initial fluoride concentration, dosage and presence of other co-ions on fluoride removal capacity. The results showed that the experimental adsorption capacity of the marble apatite synthesized using USM method was significantly higher (1.826mg/g) than marble apatite synthesized using conventional method (0.96mg/g) at pH 7 with a contact time of 90min. The mechanism of adsorption was studied, and it was observed that Langmuir isotherm model fitted best to the experimental data, while the kinetic studies revealed that the process followed pseudo-second order model. This novel compound, marble apatite synthesized from marble waste powder is found to be promising for defluoridation of drinking water and will help in alleviating the problems of fluorosis as well as reduce the problems of disposal of marble waste.