The investigation of kinetic and isotherm of fluoride adsorption onto functionalize pumice stone
Highlights
► Removal of fluoride from drinking water. ► Pumice as adsorbent functionalized by the cationic surfactant. ► Effects of HDTMA loading. ► Best fit of equilibrium expressions. ► Maximum amount of adsorption.
Introduction
Fluoride is the first element of the halogen family in the periodic table that does not occur in the element state in the environment due to its high reactivity [1]. The presence of fluoride in drinking water in acceptable concentrations is known as an essential constituent for human health, especially in children below 8 years of age [2]. However, when fluoride concentration exceed the acceptable level (1.5 mg L−1), it leads to serious health problems such as skeletal fluorosis, mottling of teeth and lesions of endocrine glands, thyroid, liver and some other organs. Fluoride compounds are used in industry for a wide range of applications, such as: aluminum production, glass fiber [3], phosphate fertilizers, bricks, tiles, ceramics [3], [4], drinking water fluoridation and toothpaste [5]. Weathering of rocks and industrial discharges are the main sources of fluoride in water, air, and soil [1]. In some provinces of Iran, the level of fluoride in drinking water is greater than 1.5 mg L−1, which can lead to endemic fluorosis. This problem is visible in several countries, including China, the United States, Tanzania, Mexico, Kenya, Poland and Pakistan [2]. The levels of human daily exposure/intake of fluoride mainly depend on the geographical conditions and lifestyles [6]. Potable water is the main source of fluoride intake in humans. As previously stated, water in many places of the world contains high concentrations of permissible fluoride ions. Therefore, treatment of fluoride-contaminated water to a level below the permissible value that is recommended by the WHO has become a critical health issue.
Several methods, such as reverse osmosis, ion exchange/adsorption, coagulation, precipitation, and electro coagulation have been used for the removal of excess fluoride from drinking water [7], [8], [9]. Among these methods, adsorption is the most extensively used and is a promising technique for the removal of fluoride. A large number of materials such as activated alumina, red mud, quartz, and fly ash have been suggested for the adsorption of fluoride from water [2], [10], [11], [12]. However, in recent years, studies have been devoted to low-cost materials such as local mineral sorbents for the elimination of pollutants from water. These sorbents can be used in natural or modified forms. One such low-cost material is pumice [13], [14], [15], [16].
Pumice is a light, porous, volcanic stone with a large surface area. It is easily and cheaply found in nature or some kinds of waste. Pumice is composed of highly microvesicular glass pyroclastic with very thin, translucent bubble walls of extrusive igneous rock. Pumice is commonly pale in color, ranging from white, cream, blue, or grey, to green-brown or black. It is formed when volcanic gases exsolving from viscous magma nucleate bubbles, which cannot readily decouple from the viscous magma prior to chilling to glass [16], [17]. It is a common product of explosive eruptions (plinian and ignimbrite-forming) and commonly forms zones in upper parts of silicic lavas. Pumice has an average porosity of 90%, and initially floats on water [18], [19], [20], [21].
Pumice has been widely tested and used in water treatment as an adsorbent, filter bed and support media [17], [18], [20]. Akbal studied the adsorption of phenol and 4-chlorophenol onto surfactant-modified and unmodified pumice from an aqueous solution. Experimental results showed that unmodified pumice cannot adsorb the phenol compound, but modified pumice is an excellent adsorbent of phenol and 4-chlorophenol [22]. Since pumice is a low-cost material with a porous structure and a large surface area widely available and easily processed and modified, thus modified pumice stone would be a suitable candidate as an adsorbent.
Due to the aforementioned advantage of original and modified pumice, to develop its application for removal of pollutant, it is very beneficial to study the performance of surfactant-modified pumice in eliminating other contaminates, such as fluoride. As mentioned above, several methods and many adsorbents in literature have been used to remove fluoride from water; however, to our knowledge from reviewing the literature, pumice and surfactant-modified pumice used for fluoride removal have not yet been reported. Accordingly, in this work, the capabilities of pumice and surfactant-modified pumice were evaluated in the removal of fluoride from water. The main aim of this study is to demonstrate the performance capacity of modified pumice to adsorb fluoride from drinking water. A series of experiments such as surfactant loading, pH, adsorbent dosage, contact time, and environmental water quality was carried out to investigate their effects on the fluoride adsorption capacity of modified pumice. The kinetics and isotherms of fluoride adsorption with modified pumice stone were also studied.
Section snippets
Sorbent
Pumice stone was supplied from Tikmadash mine, located in the south of East Azerbaijan province, in the northwestern area of Iran. It was washed with distilled water several times and dried out at room temperature. The desired particle size (mesh 80–100) of pumice was obtained from sieve pumice, which had been grinded previously. The characteristics of the natural pumice and its modified form were determined by evaluating surface morphology, specific surface area, pore size and volume, and
Pumice characterization
SEM was used to observe the natural and modified pumice morphology, and micrographs are shown in Fig. 1, Fig. 2. The image of the original pumice (Fig. 1) indicated that the pumice surface had a porous surface. The SEM modified pumice (SMP2) is given in Fig. 2. As seen in Fig. 2, the surfaces of original pumice clearly changed after modification, and porous surface could not be seen clearly. The reason is that the external surface of pumice was covered by surfactant (Fig. 2). In other words,
Conclusion
The findings of this study revealed that functionalized original Iranian pumice caused a decrease in its surface area, due to pore blocking. The adsorption process is pH dependent, and the optimum pH was 6. The kinetic studies showed that the adsorption data are fitted well to the pseudo-second order model (type 1). Furthermore, the isotherm equilibrium studies confirmed that the Langmuir-1 form is the best-fitted model for the adsorption process of fluoride by modified pumice. The maximum
Acknowledgements
The authors are grateful to the Hamadan University of Medical Sciences for its technical and financial support of this research.
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