Influence of shaking or stirring dynamic methods in the defluoridation behavior of activated tamarind fruit shell carbon

Abstract

Fluoride is essential when it is present less than 0.5 mg L⁻¹ in water but its excess presence causes dental and skeletal fluorosis. In the present work, Tamarindus indica Fruit Shells (TIFSs) were activated by ammonium carbonate and then carbonized leading to carbon abbreviated as ACA–TIFSC. This material with a BET surface area of 473 m² g⁻¹ was used for the defluoridation studies by shaking and stirring dynamic experiments. For the two dynamic studies, the fluoride removal efficiency of ACA–TIFSC as a function of pH, initial fluoride concentration, sorbent dose and co-ionic interference was investigated. The kinetic and isotherm models were used to interpret the nature of the fluoride sorption onto ACA–TIFSC. From the experimental results, it may be evaluated that the fluoride removal efficiency of ACA–TIFSC during stirring was greater than with the shaking dynamics. The results suggest that the active sites of the adsorbent are more easily reached by fluoride anions under stirring agitation. Characterization of ACA–TIFSC through SEM and XRD studies was done before and after the fluoride loading process. The prominent feature of ACA–TIFSC is the presence of CaCO₃ compounds in the carbon matrix which is evident from the results of defluoridation studies. Groundwater samples with fluoride more than 1.5 mg L⁻¹ were efficiently treated by ACA–TIFSC. The fluoride scavenging capacity of ACA–TIFSC was improved to a maximum of 8% and 11% in the bicarbonate-free groundwater during shaking and stirring sorption methods, respectively.

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.