Synthesis of silicon nitride from rice husks and sugarcane bagasse ashes for adsorption of flouride ions from aqueous solutions
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High concentrations of fluoride in drinking water pose serious problems associated with dental and skeletal fluorosis to humans in several parts of the world. Among various methods used for defluoridation such as electrochemical degradation, ion exchange, precipitation coagulation, adsorption offers an attractive method for the removal of fluoride in terms of cost, simplicity of design and operations. Silicon nitride is a material that is characterized by high mechanical strength, thermal and chemical resistance. It also has high surface area with positive sites emanating from the silamine functional groups. Despite these excellent properties that make it a potential adsorbent for fluoride ions, no reports have been made on its use for this purpose. Therefore, the aim of this study was to synthesize, characterize silicon nitride from rice husk ash (RHA) and sugarcane bagasse ash (SBA) and to determine its adsorption capacity and efficiency for fluoride ions from aqueous media. The silicon nitride was synthesized by hydrothermal synthesis technique at four different temperatures, of 100oC, 150oC, 200oC and 300oC and labeled, SNA, SNB, SNC and SND respectively. The resultant silicon nitride samples were characterized by X-ray diffraction, X-ray fluorescence (XRF) and FTIR spectroscopy. FTIR spectra suggested that the compounds prepared had similar functional groups. The X-ray diffractogram show that SNA contained 64.6% α-phase and 27.8% β-phase silicon nitride. The adsorptions capacities and efficiencies were determined through batch experiments under various conditions of temperature, pH, adsorbent dose, initial fluoride ions concentration, contact time and shaking speed. The adsorptions capacities of silicon nitride samples were found to be 7.9, 8.9, 13.9 and 49.8 mg/g for SNA, SNB, SNC and SND, respectively. Optimum adsorptions capacities were achieved at pH 2 for SNA, SNB, SNC and pH 5 for SND, contact time of 100 minutes, temperature 25oC and shaking speed of 600 rpm. The adsorption data best fitted the Langmuir isotherm. This results indicate that the silicon nitride constitute a promising material for the development of an adsorption technology for the removal of fluoride ions from aqueous systems.