## 1 Introduction  
Groundwater is one of the major sources of water for domestic use especially in rural communities. Due to its source, groundwater contains several ions including sulphates (SO4\({}^{2-}\)), nitrates (NO3\({}^{-}\)), chlorides (CI\({}^{-}\)), carbonates (CO3\({}^{2-}\)), and phosphates (PO3\({}^{2-}\)), amongst others. These ions must remain within permissible limits to prevent negative impacts on human health. Fluoride, one of the impurities in groundwater has received much attention in recent times because of its health concerns. At low concentrations, fluoride is important to humans for the prevention of tooth decay, especially in children. However, excessive consumption of fluoride can lead to health complications such as dental, skeletal, and non-skeletal fluorosis [1]. Thus the World Health Organization (WHO) has specified the permissible limit of fluoride in potable water as 1.5 mg/L [2]. The occurrence of high fluoride in groundwater in particular has led to the development of defluoridation techniques based on the concepts of adsorption [3], precipitation [4], and membrane processes [5]. Adsorption is the most appropriate method because it has a combination of affordability and effectiveness [6]. Clay minerals such as montmorillonite [7], kaolinite [8], and bauxite [9] have been used as adsorbents for defluoridation. The effectiveness of these materials have been improved by modification processes such as acid treatment [8] and metal impregnation [10], amongst others. Vermiculite--one of the common clay minerals, which originates from the mica group--despite having its largest global deposit in the Phalaborwa area of Limpopo Province (South Africa), has not been exploited for its defluoridation potential. Thus this study sheds more light on the possible uses of this geological material. Vermiculite is generally composed of hydrous magnesium aluminium layers interpolated by layers of H\({}_{2}\)O [11]. The structure is composed of two tetrahedral sheets attached to a central octahedral sheet, hence its structure is referred to as \"2:1 phyllo-silicate\" [12]. Al\({}^{3+}\) substitution for Si\({}^{4+}\) in the tetrahedral layers and Mg\({}^{2+}\) or Fe\({}^{2+}\) substitutions for Al\({}^{3+}\) in the octahedral layers are responsible for the overall negative charge of the vermiculite structure [13]. This negative charge is balanced by the presence of exchangeable cations such as Na\({}^{+}\), K\({}^{+}\), Ca\({}^{2+}\), Mg\({}^{2+}\), and Al\({}^{3+}\) within the interlayer spaces [14]. Besides the use of inorganic modifying agents, organic compounds, such as surfactants, have also been used to modify clay minerals to form organoclay. An organoclay is a class of material formed when the exchangeable cations of a clay mineral are replaced by a quaternary ammonium cation (otherwise known as a surfactant) [15]. This process changes the overall surface charge of the clay mineral (i.e., vermiculite) from negative to positive, thus enhancing electrostatic attraction of negatively charged species in solution. Hexadecyltrimethylammonium bromide (C\({}_{19}\)H\({}_{42}\)BrN) is one such cationic surfactant used to prepare organoclays for adsorption purposes. Examples of such applications include the use of organosmectite for the removal of chromates from solution [16], organovermiculite for the removal of anionic dyes and herbicides from surface water [17; 18], and organokaolinite for the removal of oxyanions from wastewater [19]. Though these studies have shown that organoclays have been effectively used for the removal of such anions from solutions, organoclays have not been actively used for the removal of fluoride from solution, thus this study explains more on the subject area. The efficiency of vermiculite functionalised with hexadecyltrimethylammonium bromide (HDTMABr) for fluoride sorption was evaluated in this study using higher concentrations of the surfactants.