International Journal of Engineering Research and Development e-ISSN: 2278-067X, p-ISSN : 2278-800X, www.ijerd.com Volume 5, Issue 4 (December 2012), PP. 69-76 69 Evaluation of Guava Seeds (Psidium Guajava) As a Low- Cost Biosorbent for the Removal of Fluoride from Aqueous Solutions S. A. Valencia-Leal 1 , R. Cortés-Martínez 2 , R. Alfaro-Cuevas-Villanueva 1 1 Instituto de Investigaciones Químico-Biológicas, Universidad Michoacana de San Nicolás de Hidalgo, Edificio B1, C.U. Morelia, Michoacán. México. 2 Facultad de Químico Farmacobiología, Universidad Michoacana de San Nicolás de Hidalgo. Tzintzuntzan 173 Col. Matamoros, C.P. 58240, Morelia, Michoacán. México. Abstract: - The sorption of fluoride ions by guava seeds (GS) from aqueous solutions in batch systems was investigated. The kinetic parameters, as well as the sorption capacities, of GS at different temperatures were evaluated. It was found that the pseudo second-order kinetic model describes the sorption kinetic experimental data and that the Langmuir model describes the isotherms. It was also found that pH has an important effect on the adsorption of fluoride ions by GS, as more were removed at pH values between 5 and 8. Sorption capacities of fluoride increase as temperature rise; suggesting an endothermic nature of the process. GS showed a high affinity for fluoride ions compared with conventional adsorbents; therefore, it can be considered as a good low-cost biosorbent for defluoridation of water. Keywords: - Fluoride, biosorption, guava seeds, kinetics, isotherms. I. INTRODUCTION Fluoride is present in ground water in many areas of the world in concentrations ranging from 0.1 to 10 mg/L. Fluoride concentration levels between 0.5 and 1 mg/L are beneficial to health, providing substantial protection against dental caries, but higher concentrations of fluoride (1.5-2 mg/L) may lead to dental fluorosis. At 3-6 mg/L, skeletal fluorosis may be observed, and crippling skeletal fluorosis can develop with fluoride concentrations in drinking water that are higher than 10 mg/L [1], [2]. The dental and skeletal fluorosis is irreversible and no treatment exists. The only remedy is prevention by keeping fluoride intake within the safe limits [3], [4]. Fluoride is a persistent and non-degradable poison that accumulates in soil, plants, wild life and in humans. Fluoride can be enriched in natural waters by geological processes. Besides, there can also be contributions from industries. High fluoride containing wastewaters are generated by thermal power plants, rubber, fertilizer and semiconductor manufacturing, glass and ceramic production and electroplating industries. According to the World Health Organization guidelines and recommendations in the areas with a warm climate, the optimal fluoride concentration in drinking water should remain below 1 mg/L, while in cooler climates it could go up to 1.2 mg/L [1]. The differentiation derives from the fact that people perspire more in hot weather and consequently drink more water. The problem is more acute in rural and small urban communities particularly in the third world countries [3]. In Mexico, the prevalence and incidence of fluorosis have increased during the past years, as a result of ingestion of fluorine during dental formation by means of water, food prepared with fluorinated salt, and inadvertent ingestion of this element during dental brushing with fluorinated tooth paste, as well as hidden fluorine in beverages [5], [6]. Fluoride concentrations above drinking water standards have been detected in various Mexican aquifers. The geological characteristics of the Mexican territory indicate that fluoride may be above drinking water standards in many areas of the country. The exposed population may be larger than that already identified [7]. Defluoridation of water was normally carried out by adsorption, chemical treatment, electro-chemical methods, dialysis and ion exchange processes. Among various processes, adsorption process is found to be effective, environmental friendly and economical [8]. Many studies have reported the possible utilization of conventional adsorbents for fluoride removal, like modified activated carbon, zeolites, clays, activated alumina, etc., with different degrees of success [9]-[12]. It has also been reported that removal of fluoride can be achieved by different types of biomass, such as algae [8], [13], Tamarind fruit shell [14], rice husk, Moringa oleifera seeds extract [15], protonated chitosan beads [16], zirconium-loaded garlic peel [17], among others. Guava (Psidium guajava) seeds are considered as a solid waste produced at great quantities by agricultural and industrial activities in Mexico, particularly on the State of Michoacán. Guava seeds have