DOI: 10.14457/tu.the.2015.6
title: INVESTIGATION ON THE EFFICIENCIES OF LOW COST ADSORBENTS FOR THE TREATMENT OF FLUORIDE CONTAMINATED WATER

description: Clean and safe drinking water is fundamental right of human. The physical, biological and chemical parameters of water is very useful for evaluating its usefulness for intended purpose. One of such contaminants is fluoride, which occurs naturally or used as an additive to municipal water supplies. Fluoride in drinking water is gaining wide attention because of its unusual behavior as regards to its health impact in the living beings. The motivation for carrying out this study was to address the problems related to high fluoride levels in water by looking for an environmentally-friendly and cost-effective method for its treatment. This thesis embodies results of the laboratory-based studies on defluoridation of fluoride contaminated water.  Agro-industrial wastes such as dewatered sewage sludge, sea shell, litchi peel and banana peel were selected for the fluoride adsorption experiments, and their performance was compared with the commercial granular activated carbon. An attempt was made to modify the litchi peel, both physically and chemically for the enhancement of adsorption efficiencies. The physicochemical characterization of the adsorbents was done by Brunauer–Emmett–Teller (BET) for surface area analysis, scanning electron microscopy (SEM) for surface morphological studies and Fourier transform infrared spectroscopy (FTIR) for functional group identification. The FTIR study highlighted the absorption bands pertaining to rich organic functional groups like hydroxyl, amine and carbonyl groups in the agro-based adsorbents like litchi peel and banana peel. The nature and morphology of the adsorbents were also clearly understood from the SEM images. Moreover, the influence of operating parameters such as pH, adsorbent dose, agitation speed, contact time and initial fluoride concentration was studied by a series of batch experiments at 25±3◦C. The interference of co-existing ions (F-/Cl- and F-/SO42-) on the fluoride uptake was also tested for 10 mg/l fluoride solution using the banana peel (BP) adsorbent. For the studied adsorbents, the percentage removal of fluoride was found to increase with the increasing dose of adsorbents, agitation speed and time for a given fluoride concentration, while the removal followed decreasing trend for the rise in fluoride concentration. Further, solid addition method was employed to determine the point of zero charge (pHpzc) of the adsorbents. The pH value for all the selected adsorbents was found to be lower than the pHpzc values, making the surface positively charged in favor of negatively charged fluoride ions. Although remarkable removal efficiency was observed in acidic range for most of the adsorbents, removal at neutral pH is considered for all adsorbents from drinking water viewpoint. Sludge and BP rendered the maximum removal efficiency of 88% and ULP showed the least efficiency of 70% for 5 mg/l F- solution at the optimum conditions. All the tested adsorbents showed good stability in lowering the fluoride level below the prescribed limit as specified by World Health Organization. The equilibrium was observed within 3 hours for most of the tested adsorbents. For BP there was no remarkable change in the fluoride removal in presence of monovalent chloride ions, while the presence of divalent sulfate at higher concentrations resulted in decrease of fluoride removal efficiency. The equilibrium adsorption data after the experiments were modeled with suitable isotherm and kinetic equations. The data fitted quite well with both Freundlich and Langmuir isotherms. Higher R2 values for Freundlich isotherm confirms that adsorbents surface were highly heterogeneous and the sites having higher energy level were occupied first. Based on the experimental results, pseudo-second order kinetic model seems to be more compatible, with less fluctuations in calculated and experimental uptake capacities (qe) for all the adsorbents. The adsorption process was found to be complex, with the involvement of more than one rate limiting mechanisms.