## 1 Introduction  
Arsenic (As) is a chemical element which occurs naturally and is commonly present in the earth's crust [1]. It has been found in air, biota, water, soil and rocks [1, 2]. In particular, arsenic present in an aqueous medium is of most concern because of its likely detrimental impact on plants, animals and humans. When present in high concentrations in drinking water, arsenic has been found to adversely affect human health [3] and this topic has been the subject of several recent reviews [4, 5, 6]. In this comprehensive review, we have additionally provided a detailed evaluation of treatment options for arsenic removal from water, with due consideration for affordable small scale technologies that can be easily implemented in developing countries such as Bangladesh, India, Nepal and Pakistan. Arsenic has been introduced to the environment through various natural and anthropogenic sources [7]. The primary natural source of arsenic is from As-enriched minerals [2]. Typical examples of natural sources include volcanoes and eroded arsenic bearing rocks, such as arsenopyrite (FeAsS), lollingite (FeAs\({}_{2}\)), orpiment (As\({}_{2}\)S\({}_{3}\)) and realgar (AsS) [8]. Under oxidizing and reducing conditions, arsenic can be mobilized at pH 6.5-8.5 [3; 9] which is a common pH range in groundwater [9]. On the other hand, the common anthropogenic sources include agriculture, livestock and industrial manufacturing [10]. It is also widely used for the manufacture of glassware, industrial chemicals, copper, lead alloys and pharmaceuticals [10]. Industrial processes, such as smelting of iron ores, mining, pulp and paper production, cement manufacture, burning of fuels and wastes are known sources for the release of arsenic into the environment [8]. However, the focus of this review is on the contamination of groundwater by arsenic from predominantly natural sources. Arsenic is a group Va element on the periodic table and is classified as a metalloid. In the environment, it exists in several oxidation states as As(3-) (arsine), As(0) (arsenic), As(3+) (arsenite) and As(5+) (arsenate) [11]. In natural waters, the common soluble arsenic species are the inorganic oxyanions of As(III) or As(V) [12]. As(III) species include As(OH)\({}_{3}\), H\({}_{2}\)AsO\({}_{3}\)\({}^{-}\), HAsO\({}_{3}\)\({}^{2-}\) and AsO\({}_{3}\)\({}^{3-}\)[13; 14]. As(V) species are AsO\({}_{4}\)\({}^{3-}\), HAsO\({}_{4}\)\({}^{2-}\), H\({}_{2}\)AsO\({}_{4}\)\({}^{-}\) and H\({}_{3}\)AsO\({}_{4}\)[13; 14]. Arsenic (III) forms complexes preferentially with oxides and nitrogen [14]. On the other hand, As(V) forms complexes with sulfides [14]. There are also organic arsenic species such as arseno-sugars dimethyl arsenic acid (DMAA) and monomethyl arsenic acid (MMAA) [15]. However, in drinking water treatment, these species are not of significant concern due to the limited effect they have on human health, and their ease of elimination from the body [15]. Under reducing anaerobic conditions, the dominant species are trivalent arsenic [9], while in oxygen rich aerobic conditions pentavalent species are dominant [9; 11; 12]. However, the key considerations for controlling the arsenic species present under various conditions are redox potential (Eh) and pH [9; 16; 17; 18]. Figure 1 shows that, under oxidizing conditions, H\({}_{2}\)AsO\({}_{4}\)\({}^{-}\) is the dominant arsenate species at low pH (pH \(<\) 6.9), whereas HAsO\({}_{4}\)\({}^{2-}\) is the dominant arsenate species at a higher pH, as illustrated in Figure 1c [9; 16; 17]. On the other hand, H\({}_{3}\)AsO\({}_{3}\)\({}^{0}\) is the dominant arsenite species at pH \(<\) 9.2 when operating in reducing conditions, as illustrated in Figure 1b. The presence of nitrate in groundwater has been proposed as a likely significant contributing mechanism for the oxidation of arsenic to As(V) [18]. Nitrate can act as a terminal electron acceptor and this mechanism is likely to be more prevalent under anoxic conditions. Figure 1: pH dependence of the aqueous speciation of As(III) and As(V) species.