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Defluoridation Performance of Nano-Hydroxyapatite/Stilbite

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Defluoridation Performance of Nano-Hydroxyapatite/Stilbite Separation and Purification Technology 157 (2016) 241–248 Contents lists available at ScienceDirect Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur Defluoridation performance of nano-hydroxyapatite/stilbite composite compared with bone char ⇑ Taju Sani a, Luis Gómez-Hortigüela b, Joaquín Pérez-Pariente b, Yonas Chebude a, Isabel Díaz a,b, a Chemistry Department, Addis Ababa University, Addis Ababa, Ethiopia b Instituto de Catálisis y Petroleoquímica, CSIC, c/Marie Curie 2, 28049 Madrid, Spain article info abstract Article history: Fluoride contamination of drinking water is a serious problem in the Rift Valley region of Ethiopia. In this Received 20 July 2015 work, a nano-hydroxyapatite/stilbite (nHAST) composite was prepared based on locally available stilbite Received in revised form 10 November 2015 zeolites and its fluoride removal performance was compared with that of Bone Char (BC). The reaction Accepted 12 November 2015 mechanism involving the fluoride adsorption is analyzed based on kinetics and isotherm studies. In both Available online 12 November 2015 nHAST and BC the kinetic data fitted well to a pseudo-second order kinetic model of similar characteris- tics. In contrast, the adsorption isotherm on the nHAST composite fitted best with the Freundlich model, Keywords: whereas on BC, it correlated well with the Langmuir model, suggesting a different mechanism: adsorp- Fluoride tion of fluoride on BC was homogeneous, whereas on the composite was heterogeneous, possibly related Zeolites Nano-hydroxyapatite to the higher load of fluoride on the hydroxyapatite component in the composite. At low concentrations, Defluoridation both adsorbents behave similarly; however, the maximum adsorption capacity, measured at high con- Bone char centrations, is higher in the nHAST composite than in BC. The intrinsic HAp capacity of nHAST, normal- À Fluorosis ized to the amount of HAp on the adsorbent, is significantly higher (9.15 mg F /gHAp) than that of BC (1.08 mg FÀ/gHAp) (measured at low FÀ initial concentrations where the OH/F-exchange mechanism pre- dominates), showing a much higher FÀ removal efficiency of HAp on the composite. These results foresee a high potential of nHAST composite toward FÀremoval implementation, specially taking into account the low-cost, ease of production, local availability and social acceptance. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction is especially beneficial for young children [2]. However, an exces- sive intake of fluoride can cause a serious health hazard, dental Fluorine, the most electronegative and extremely reactive or in extreme cases skeletal fluorosis [3]. Fluoride affects not only chemical element, is found in nature only in combined forms as teeth and skeleton but its accumulation over a long period of time À fluoride ions (F ), of which fluorite (CaF2), fluoroapatite can lead to cancer, osteosclerosis and it has been also related with (Ca5(PO4)3F) and cryolite (Na3AlF6) represent the most dominant neurological impairment and liver and kidney damages in human fluoride-containing minerals. Fluoride is one of the most abundant beings [4,5]. anions in groundwater all over the world, and the main source is More than 260 million people all over the world consume attributed to these fluoride-bearing rocks leaching out fluoride drinking water with a fluoride concentration higher than under certain favorable conditions. Another potential source of flu- 1.0 mg/L, the majority of which live in tropical regions. According oride comes from man-made activities related mainly to wastew- to UNICEF’s report, dental or skeletal fluorosis is a worldwide aters produced from industries applying fluorine-containing endemic problem, prevailing in at least 25 countries: fourteen in substances as raw materials, such as in the aluminum and steel Africa including Ethiopia, eight in Asia, and six in America [6,7]. industries as well as electroplating, glass and semiconductor In Ethiopia alone more than 8 million people, primarily in rural manufacturing, ore beneficiation, or fertilizer uses [1]. Fluoride is areas of the Main Ethiopian Rift, are at high risk due to a regular an important element to human health. As a trace element it is exposure to fluoride-contaminated groundwater, the largest essential for strengthening bones and preventing tooth decay; it source of water for public consumption [2]. Although a number of fluoride related health problems had been identified in this area, a tragic situation was reported in a study conducted by Haimanot ⇑ Corresponding author at: Instituto de Catálisis y Petroleoquímica, CSIC, c/Marie et al. that found dental fluorosis in more than 80% of the sampled Curie 2, 28049 Madrid, Spain. children resident in this area (1221 out of 1456) [8], where fluoride E-mail address: [email protected] (I. Díaz). http://dx.doi.org/10.1016/j.seppur.2015.11.014 1383-5866/Ó 2015 Elsevier B.V. All rights reserved. 242 T. Sani et al. / Separation and Purification Technology 157 (2016) 241–248 levels as high as 36 mg/L had been recorded in the groundwater high defluoridation capacities [30–34]. However, the potential that was used for drinking purpose [8,9]. However, according to application of these materials in the field is not easily achievable, World Health Organization (WHO) guidelines followed in most due to the cost associated with their production and significant nations, the acceptable fluoride concentration in drinking water pressure drops during filtration owing to the small particle size. must be in the range 0.5–1.5 mg/L [6], above which the occurrence To overcome such permeability problems and at the same time uti- of fluorosis is inevitable. Thus, it is essential to alleviate the sever- lize the advantages of n-HAp, it is essential to prepare adsorbents ity of the problem through the provision of drinking water with based on n-HAp in a usable form by combining them with other optimal fluoride concentration [10,11]. high-surface materials [35–37], on which surface the HAp Over the years, a large number of materials and methods have nanoparticles are supported. By having the HAp nanoparticles been proposed at least to minimize the effect of the fluoride prob- not free but supported on the external surface of a material with lem [12]. The methods may be classified as chemical precipitation, a large particle size, we can avoid pressure drops associated with ion-exchange, adsorption, and membrane-based mechanisms. the nanosize of n-HAp, as well as prevent the potential toxicity However, most of them present certain disadvantages: chemical typical of nanoparticles and possible sintering (and loss of effi- precipitation involves addition of soluble chemicals, such as cal- ciency) of the HAp nanoparticles during defluoridation treatments. cium and aluminum salts, to fluoride-contaminated water, and In our previous work we developed the synthesis of n-HAp defluoridation occurs by formation of fluoride precipitates or grown on the surface of natural zeolites from Ethiopia, in particular adsorption onto the formed precipitate. Its low treatment effi- on stilbite [38] (nHAST). In addition to the use of inexpensive raw ciency, requirement of a large chemical dosage and close monitor- materials with high surface areas as support, growth of HAp on the ing, adverse health effects of dissolved aluminum species in the external surface of the zeolites is carried out at room temperature treated water, and continuous production of a sludge that requires and autogenous pH (around 8), reducing the production costs, appropriate disposal, are the main limiting factors for its applica- unlike the most common precipitation methods of HAp which in tion [13]. In the case of ion-exchange, the presence of other anions most cases is carried out at high temperatures and at basic pHs such as sulfate, phosphate and bicarbonate results in ionic compe- of 9 or higher [26,27,30,34,35]. tition, it is relatively costly, and treated water sometimes has a low The aim of the present study is to compare the performance of pH and high levels of chloride. Membrane-based processes, which our newly developed zeolite composite, nHAST, with that of Bone are among the most advanced water treatment technologies, are Char (BC), which is currently applied as adsorbent in the field in highly effective for fluoride removal and produce high-quality Ethiopia, and whose fluoride-removal active phase (HAp) is similar water [14] that meets the WHO standards. However, these meth- to that of our composite. ods require skilled manpower, they are expensive and require a high and constant supply of electric energy, and may not be suit- 2. Materials and methods able for waters with high salinity and TDS (total dissolved solids). Instead, adsorption processes, which involve the passage of water 2.1. Adsorbent materials through a bed containing a defluoridating material (adsorbent) to retain fluoride either by physical, chemical or ion-exchange mech- The bone char sample was kindly donated by Oromo Self-Help anisms, are very attractive due to their effectiveness, convenience, Organization (OSHO) Defluoridation Plant located at Modjo, Ethio- easy availability, simplicity of operation, and economical as well as pia. The sample was manually grounded with a mortar and sieved environmental low impacts [15,16]. Many low cost adsorbent to an average particle size fraction between 75 and 105 lm. The materials have been tested for fluoride removal, including natural stilbite (STI) zeolite used in the preparation of the compos- locally-available materials like activated clay [17], waste residues ite is obtained from the Hashenge basalt formation, Tigray region from alum manufacturing process [1], ceramic materials [18], car- (Ethiopia). The STI was ground with a disc mill in the desired pow- bonaceous materials produced from coffee grounds [19] or natural der form and sieved into an average particle size between 75 lm zeolites [20,21]. However, their low adsorption capacity for fluo- and 105 lm.
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