Experimental evaluation and optimization of sorptive removal of Fluoride from water using iron ore

Abstract

Excessive fluoride in drinking water is a worldwide problem. Now a days, many millions of people rely on groundwater with fluoride concentrations above the World Health Organization (WHO) guideline value where Ethiopian people living in Rift Valley area are the case in point. Therefore, in this research, the capacity of locally available Iron ore (IO) to remove fluoride from synthetic and natural groundwater was investigated. Series of batch adsorption experiments were carried out to assess the effect of contact time, pH of the solution, agitation rate, particle size of the adsorbent, mass of the adsorbent, initial fluoride concentration and the presence of competing anions in the solution. The adsorption of fluoride in the first 120 min was fast and equilibrium time was achieved within 120 min. The adsorption kinetic was found to follow a pseudo-first-order rate equation. The equilibrium adsorption data were well described with the Freundlich (R2 = 0.993) and Langmuir isotherm models (R2 =0.983), giving a sorption capacities of 1.465 and 4.433 mg/g, respectively. However, from the values of coefficient of determination and chi-square, the Freundlich isotherm equation best described the equilibrium adsorption of fluoride onto IO. The obtained mean free energy (10.206 kJ/mol) suggested that chemisorptions should responsible for the adsorption of fluoride onto IO. The presence of carbonate, bicarbonate and phosphate significantly affected fluoride removal efficiency while chloride, sulfate and nitrate did not significantly affect fluoride removal efficiency within the concentration range tested (5.0-200 mg/L). The higher percentage of fluoride removal from laboratory prepared fluoride solution than from natural groundwater which could be due to the presence of other multiple factors that are not considered in this study. About 52% of fluoride was desorbed from the fluoride loaded IO indicating that strong bond formed between adsorbed fluoride and the adsorbent. The results of this study provided important information for the further evaluation of IO for the treatment of water contaminated with fluoride. Further investigation of IO for the treatment of fluoride from ground water should be considered to assess its practical application.