Adsorption of fluoride from aqueous solution by acid treated spent bleaching earth $ M. Mahramanlioglu * , I. Kizilcikli, I.O. Bicer Chemistry Department, Engineering Faculty, Istanbul University, Avcilar, Istanbul 34850, Turkey Received 28 August 2001; received in revised form 7 January 2002; accepted 9 January 2002 Abstract Acid treated spent bleaching earth was studied to assess its capacity for the adsorption of fluoride from aqueous solutions. Adsorption isotherms have been modeled by Langmuir and Freundlich equations and isotherm constants for both isotherms were calculated. The effect of the adsorbent concentration on the adsorption was studied. The dependence of the adsorption of fluoride on the pH of the solution has been studied to achieve the optimum pH-value and a better understanding of the adsorption mechanism. It has been found that maximum adsorption of fluoride from aqueous solutions takes place at pH-value of 3.5. Second-order equation was used to describe the adsorption rate of fluoride and adsorption rate constant was calculated. Intraparticle and mass transfer coefficients were calculated. The influence of addition of the anions on the adsorption of fluoride was also studied to simulate industrial waste waters and the addition of anions decreased the adsorption of fluoride on the acid treated spent bleaching earth (SBE). # 2002 Elsevier Science B.V. All rights reserved. Keywords: Fluoride; Adsorption; Spent bleaching earth; Fluoride removal; Waste water treatment; Mass transfer; Intraparticle diffusion; Adsorption kinetics 1. Introduction Fluorine usually in combined fluoride forms is naturally present in varying amounts in the atmosphere, water and soils. A large amount of fluorides have been generated from glass industry, chemical industry, metal industry and high tech industry such as manufacturing semiconductors and integrated circuits. Fluoride is recognized as an essential constituent in the human diet. Skeletal and dental problems can be prevented by maintaining fluoride concentration of about 1 ppm in the dietary intake [1]. Drinking water is considered to be safe for human consumption, if fluoride concentration does not exceed 1.0 mg dm 3 . Although, it is necessary to take trace amounts of fluoride for the formation and conservation of teeth in children, large fluoride doses result in mottling as well as fluoride deposi- tion in bone leads to structural damages in ligaments. Several methods have been tried to remove fluorides from water, namely adsorption, precipitation, electrodialysis, ion exchange and reverse osmosis. Among these methods, adsorption is still one of the most extensively used methods for defluoridation of water and activated carbon prepared from various materials exhibits good capacity for removal of fluorides and other pollutants from waters. In recent years, attention of scientists have been devoted to the study of different types of low cost materials such as spent bleaching earth [2,3], wollastonite and chine clay [4], bentonite and activated bentonite [5–8], kaolinitic clay [9], agricultural by products [10], fly ash [11], carbon slurry [12], biogas residual slurry [13], zeolite [14], bone char [15] and flax shive [16] since activated carbon is expensive. Spent bleaching earth (SBE) that is a solid waste from edible oil processing industry was chosen as raw material [2,17,18]. SBE has two components: residual oil not removed by filter pressing and montmorillonite clay. This material is disposed of directly to landfill either dry or as a wet slurry. But the landfill option causes problems, the most dangerous being the pyrogenic nature of the unsaturated oil which rapidly oxidizes on the clay surface to the point of spontaneous ignition. The constant changes in environmen- tal legislation has led to restrictions in solid waste manage- ment. Therefore, it is necessary to study on the minimization of pollutant such as SBE from oil industry and a good solution from an environmental and economic standpoint is the application in the waste water industries as low cost adsorbent after suitable regeneration process. Therefore, in Journal of Fluorine Chemistry 115 (2002) 41–47 $ A new adsorbent was produced and used in the removal of fluoride. Activated bentonite ! SBE ! a new adsorbent for the removal of fluoride. * Corresponding author. E-mail address: mehmah@istanbul.edu.tr (M. Mahramanlioglu). 0022-1139/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0022-1139(02)00003-9