FLUORIDE REMOVAL FROM WATER USING NANOADSORBENTS AND MIXED MATRIX MEMBRANE

Abstract

Fluoride contamination in water occurs either through the natural weathering of rocks or anthropogenic activities. Wastewater from various industries like glass manufacturing, fertilizer, electroplating, and steel producing industries can also contribute excess fluoride in water bodies. In general, fluoride concentrations in industrial wastewater found to be in the range of few hundred to thousands mg/L while in groundwater, it is found only in the range of 1 to 40 mg/L. Fluoride in drinking water is not completely forbidden. At lower concentrations (0.5 to 1.5 mg/L), fluoride is beneficial for bone health but concentration more than 1.5 mg/L can have severe health effects such as dental and skeletal fluorosis. This research work deals with fluoride removal from contaminated water having very high and low fluoride content. Initially, calcium hydroxide nanorods (CHN) were synthesized through the ultrasonic-assisted chemical method to remove fluoride from wastewater having high fluoride concentration. The BET surface area, Barrett-Joyner-Halenda (BJH) surface area, pore volume and average pore diameter of CHN were found to be 367 ± 10 m2/g, 501 ± 8 m2/g, 0.54 ± 0.02 mL/g, and 4.4 ± 0.1 nm, respectively. Experimental data was applied to Freundlich, Langmuir, Temkin, and Two-step adsorption isotherm models, but neither model fits well to the experimental data. Dissolution and precipitation of calcium ions (from CHN) with fluoride ions under various experimental conditions along with the little extent of adsorption were found the main mechanism for F- removal. Batch study of fluoride removal using CHN reveals that with an initial fluoride concentration of 50–250 mg/L and fixed CHN dose of 0.12 g/L, fluoride removal was found to be 450 ± 10 mg/g within the 45 min of contact time at pH ∼6.5. The fluoride adsorption kinetic data was applied to pseudo first order and pseudo second order kinetic models. Fluoride removal follows pseudo second order kinetics in this study. Pseudosecond order rate constants at initial acidic and basic pHs of 4.5 and 9.5 were found to be 0.00041 (g/mg-min) and 0.00116 (g/mg-min), respectively. Surface complexation model was applied at equilibrium pHs of fluoride contaminated solutions, and mainly precipitation of calcium and fluoride ions was found here as a governing mechanism for removal of fluoride ions. Fluoride removal study was performed successfully using synthetic F- spiked solutions and electroplating industrial wastewater in a wide range of pHs (2–12). The aqueous solution with an initial fluoride concentration of 550 mg/L was treated with 2 g/L of CHN and 99.30 % fluoride removal was achieved. It is commonly known that nanoparticles are highly efficient fluoride adsorbents, but agglomerate easily due to their high surface activity and are difficult to separate from the ii aqueous medium after use. Mixed-metals oxyhydroxides nanoparticles were prepared into a natural polymeric matrix of chitosan to overcome such a problem of agglomeration. Hydrous mixed-metal oxyhydroxides, loaded chitosan composite (Fe-Al-Mn@chitosan) was prepared using abundantly available laterite clay and waste from the steel industry via co-precipitation method. Fe-Al-Mn@chitosan exhibited maximum F- adsorption capacity of 40±0.5 mg/g, while if only the inorganic mass fraction of composite is considered, the same value could be reached to 55±0.5 mg/g. Fluoride adsorption on Fe-Al-Mn@chitosan follows the pseudo second order kinetics with rapid adsorption. No significant effect of other competitive ions was observed on F− adsorption using Fe-Al-Mn@chitosan composite. The composite adsorbent is found to be effective in producing drinking water from fluoride contaminated groundwater. Afterward, shrinking core model is applied to predict the adsorption kinetics of fluoride ions on mixed metal oxyhydroxide and mixed metal oxyhydroxide loaded chitosan composite to investigate the effect of polymeric barrier’s resistance on mass transfer of fluoride ions from the external surface to interior pore surface of the adsorbent. For nanoparticles and nanocomposites, the reaction between adsorbate and adsorbent is much faster near the external surface than in the interior pores surfaces of the particles. Adsorption kinetic study of both adsorbents reveals that Fe-Al-Mn@chitosan composite is more efficient than mixed metal oxide nanoparticles alone. The unknown parameters of external mass transfer coefficient (Kf) and pore diffusion coefficient (De) were estimated by comparing the simulated concentration profile with the experimental data using a nonlinear optimization technique. Adsorption kinetic modeling using shrinking core model reveals that 28 % of total adsorption front radius was accessible to fluoride ions in the case of Fe-Al-Mn mixed metal oxyhydroxide while for Fe-Al- Mn@chitosan; more than 80 % of total adsorption front radius was accessible for fluoride ions. The value of De is found to be almost in the same order for both the adsorbents proving that the polymer layer provides minimum resistance barrier to the mass transfer of F- ions from external surfaces to interior pore surfaces. Further, faster and more extent of adsorption for composite is observed due to smaller particle sizes of nanoparticles present inside of composite than nanoparticles alone (due to the natural tendency of agglomeration of nanoparticles). The practical applicability of developed composite material was explored through Fe-Al- Mn@chitosan composite loaded cellulose acetate mixed matrix membrane (MMM). The compatibility of the two polymers namely chitosan and cellulose acetate, helps to obtain homogeneous dispersion of the nanoparticles inside the MMM. Fe-Al-Mn@chitosan loaded cellulose acetate-based MMM (optimum membrane: M8) showed significantly higher treatment capacity (4000 and 2000 L water per m2 for low ionic strength fluoride contaminated distilled iii and groundwater as feed, respectively) compared to performance of other MMM reported in the literature. It is commonly known that the performance of any MMM is limited by adsorption process and shows low sustenance time. This drawback of MMM has been addressed in the present study. In this study, fluoride ions rejection happened through adsorption at the initial time of operation. Subsequently, the development of negative charges on membrane surface through adsorption of F- ions might cause the electrostatic repulsion based F- rejection mechanism; as a result, MMM showed much higher fluoride rejection compared to the performance expected based on adsorption phenomenon. The infectious test reveals that the MMM membrane is less susceptible to microbial attack compared to the cellulose acetate membrane. Fluoride rejection through membrane confirms that mixed oxide nanoparticlespolymer- composite loaded MMM is capable of removing F- from water quite efficiently and has shown very interesting fluoride rejection behavior through electrostatic repulsion during ultrafiltration mode of operation.