Influence of shaking or stirring dynamic methods in the defluoridation behavior of activated tamarind fruit shell carbon
Graphical abstract
Highlights
► The carbonization of tamarind fruit shell improved its defluoridation efficiency. ► Calcium carbonate particles in the carbonized tamarind fruit shell were involved in the defluoridation process. ► The defluoridation efficiency of stirring dynamics was greater than shaking dynamics. ► Maximum fluoride removal was achieved at pH 7–8. ► Fluoride removal was greater in bicarbonate-free groundwater.
Introduction
Fluoride is often called a two-edge sword whose higher and lower concentration in water is not safe [1]. A preponderance of evidence indicates that moderate levels of fluoride (0.8–1.0 mg L−1) ingestion can reduce the incidence of dental caries and, under certain conditions, promote the development of strong bones [2], [3], [4], [5]. When the concentration of fluoride exceeds 1.5 mg L−1, it adversely affects the dental system by causing dental fluorosis. Some reports evidenced that the adverse health effects of fluoride are enhanced by a lack of Ca, vitamins, and protein in the diet [6], [7], [8]. Around 200 million people from 25 nations have health risks because of high fluoride in groundwater [9]. In India, there has been an increase in incidence of dental and skeletal fluorosis with about 62 million people at risk [10]. High concentrations of fluoride in groundwater are common in some of the semi-arid areas of Rajasthan, southern Punjab, Gujarat, Karnataka, Tamil Nadu, Madhya Pradesh and Southern Haryana [9], [11], [12]. In India, the maximum permissible limit of fluoride in drinking water is 1.5 mg L−1 [13] which is the same as the WHO guideline value [14].
Current methods used to remove fluoride from water typically include chemical precipitation [15], membrane processes [16], electrolysis and electrodialysis [17]. Due to the factors like high operational costs and formation of sludge, these technologies are found to have limited access. Among the existing methods, adsorption is one of the most extensively used methods for the removal of fluoride because of its ease of operation and cost effectiveness. In adsorption techniques, biomaterials are employed in carbonized, activated and chemically modified forms. The chemically modified bio-sorbents such as glutaraldehyde cross linked calcium alginate [18], calcium pretreated algal biomass [19], zirconium(IV) impregnated – collagen fiber [20], aluminum hydroxide coated rice husk [21] and tamarind indica fruit shell [22] are reported as fluoride scavengers in water. The defluoridation capacity of activated carbons prepared from cashew nut sheath [23], cashew nut shell [24], Eichhornia crassipes biomass [25] and zirconium impregnated coconut shell [26] was also studied.
In the way of exploring an abundant and efficient adsorbent, an interest was focused on the biomaterial called tamarind fruit (Tamarindus indica Linn.) shell. In India, tamarind is an economically important and tropical evergreen tree which grows abundantly in the dry tracts of Central and South Indian States. Indian production of tamarind is about 0.3 million tonnes per year. The hard pod shell is removed and discarded when the fruit is ripe, and the fruit is the chief acidulant used in the preparation of foods. The removal of cadmium [27] and arsenic [28] by carbonized tamarind materials was already reported.
In continuation to our previous work on fluoride removal with Tamarindus indica Fruit Shell (TIFS) [22], we were interested by an improvement of this material. So, we investigated the preparation and uses in defluoridation of modified materials arising from TIFS. Tamarind fruits contain naturally high content of calcium compounds [29]. We found that tamarind fruit shell contains also calcium compounds which could lead mainly to calcium carbonate particles during the carbonization process. These calcium compounds in the carbon matrix were studied to be good candidates for fluoride removal. In order to improve the efficiency of TIFS carbon we investigated the effect of activation by impregnation of the starting TIFS with ammonium carbonate solution. The efficacy of TIFS carbon in fluoride removal was studied both by shaking and stirring dynamic experiments, and the results are compared.
Section snippets
Preparation of carbon adsorbent
The precursor, Tamarindus indica Fruit Shells (TIFSs) were collected from a local area near Madurai District and dried under the shadow. The dried fruit shells were successively washed by acid solution, base solution and then distilled water, and finally air dried. The air dried TIFS was pulverized and sieved for the particle size of 600–300 μm. An amount of 50 g of the above TIFS was stirred in a 200 mL solution of 0.1 M ammonium carbonate. TIFS was maintained immersed in solution for 24 h, at room
Characterization studies
The ACA–TIFSC was initially characterized for its physical parameters. The physical parameters analyzed are Bulk Density (0.64 g mL−1), Moisture (5.72%), Ash (4.48%), Solubility in water (1.69%), Solubility in acid (6.34%), pH (6.97) and BET surface area of 473 m2 g−1. The morphological data (Table 1) have shown that the roughness of fluoride loaded ACA–TIFSC was reduced when compared to the unloaded adsorbent. This may be attributed to the fluoride sorption onto the surface active sites of
Conclusion
TIFS were impregnated with ammonium carbonate solution before being dried and then carbonized, leading to ACA–TIFSC. SEM and XRD studies revealed that the prepared carbon contained calcium carbonate which was the potential scavenger of fluoride ions. Defluoridation experiments were performed under stirring and shaking dynamic methods. The stirring method appeared more efficient than the shaking method in the case of pH, ACA–TIFSC dose and initial fluoride concentration variations. Whatever the
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2018, Journal of Environmental Chemical EngineeringCitation Excerpt :The equilibrium sorption data (lnCe against qe at 303 K) also gave linear plots with a mean value of r = 0.992 for Temkin isotherm, indicating the applicability of this model for fluoride – SLP sorption system, with AT having values from 1.12 to 8.03 L/mg (mean 4.68 L/mg) and B from 0.60 to 2.10 mg/g (mean 1.08 mg/g). Similar values of AT and B were earlier reported for activated tamarind fruit shell carbon (AT: 3.25 L/mg and B: 1.36 mg/g) [14], Neem Leaf powder (AT: 5.70 L/mg and B: 0.98 mg/g) [15]. The good applicability of Temkin model to SLP – fluoride sorption system suggested that the adsorption of fluoride onto SLP occurred by physiorption followed by chemisorptions [45].