# Removal of Fluoride and Arsenate from Aqueous Solutions by Aluminum-Modified Guava Seeds
The adsorption process is preferred when arsenic and fluoride removal from water is required in a small-scale system owing to its high efficiency, low cost, and the relative easiness in operation of the system. Moreover, the adsorption process is found to be environmentally friendly, economical, and suitable for single families and small communities. Many studies have reported the possible utilization of conventional adsorbents for arsenic or fluoride removal, separately, like modified activated carbon, zeolites, clays, activated alumina, titanium dioxide, etc., with different degrees of success [14,15,16,17]. Recently, more attention has been paid by the scientific community in using diverse naturally occurring biosorbents in sorption processes since this type of adsorbents offers advantages of sludge minimization, high efficiencies when diluted solutions are treated, regeneration of biosorbent, and low cost [18]. However, few studies have reported simultaneous removal of As(V) and F\({}^{-}\)[12,13,19,20]. Arsenate (0.9 mg/g) and fluoride (0.5 mg/g) adsorption capacities have been reported when goethite is used to remove these ions from groundwater simultaneously. Furthermore, a decrease in fluoride removal from 82 to 28% was reported when arsenic was present in the solution at a concentration of 1 mg/L [12]. It has been reported [13] that aluminum and iron binary oxide is suitable for simultaneous As and F removal and found that the adverse effects of competitive adsorption between As(V) and F were significantly reduced by using binary oxide rather than the single Al and Fe oxides. [PERSON] et al. [12]found that arsenate and fluoride adsorption capacities of an activated carbon modified with lanthanum and titanium oxides were pronouncedly higher than some aluminum-based adsorbents for synthetic water and real contaminated groundwater. Therefore, the purpose of Al modification of guava seeds is to enhance fluoride sorption capacity since it has been reported the affinity of these ions to Aluminum-type adsorbents. Furthermore, in our previous studies, we have reported a low fluoride sorption capacity of guava seeds [8]. The use of conventional adsorbents for arsenic and fluoride removal from groundwater could be expensive and ineffective. Consequently, the use of alternative adsorbents is of great relevance, such as products of biological origin, or wastes and by-products from industrial and agricultural activities [21]. These agricultural waste materials used as biosorbents have some advantages over conventional adsorbents because they are abundant, available, inexpensive, and may act as complexing agents due to their physicochemical characteristics and particular structure [21]. Particularly, lignocellulosic materials derived from agricultural wastes have been considered as efficient adsorbents due to their high cellulose, hemicellulose and lignin content and because of their large scale generation and low cost. Guava (_Psidium guajava_) seeds are considered as a solid waste produced in great quantities by agricultural and industrial activities in Mexico. Recent increases in guava production and consumption have led to the production of large amounts of guava seed waste that requires disposal; this could be have been mitigated by recycling this waste as an inexpensive source of biomass [22]. Guava seeds have structural properties, like high content of cellulose, lignin and protein that make them suitable for their use as a biosorbent, particularly for anionic contaminants present in water. These properties are due to the presence of various functional groups on its surface such as amines, hydroxyl, carbonyl, carboxyl, alkanes, etc. which have been involved on fluoride sorption as well as in the formation of different types of interactions with metals [22]. It has been reported the use of guava seeds as biosorbent for the removal of Cr(VI) from aqueous solutions with acceptable sorption capacity [23].