## 2 Nanostructure Materials for Arsenic Removal  
### Arsenic (As)  
Arsenic (As) is considered as a contaminant of major concern, due to its extreme toxicity at relatively low doses and its worldwide occurrence in groundwaters. It is mostly naturally present in groundwaters, but several industrial processes and products, such as wood preservatives, semi-conductors and agricultural applications (pesticides), may also introduce arsenic into the environment [10; 11]. Increased arsenic concentration levels in groundwaters cause significant issues in the supply of secured drinking water [12], because long-term arsenic contaminated water consumption has been linked with various diseases such as several types of cancers [13]. As a result, the WHO [12] has reduced the acceptable concentration value for arsenic in drinking water from 50 to 10 \(\mu\)g L\({}^{-1}\) and most countries also apply the same maximum allowed concentration [14]. Arsenic in natural waters is mainly found with its inorganic forms of As(III) (H\({}_{3}\)AsO\({}_{3}\)) and/or As(V) (H\({}_{3}\)AsO\({}_{4}\), H\({}_{2}\)AsO\({}^{4-}\) and HAsO\({}_{3}\)\({}^{2-}\)) [15; 16]. The prevalence of these species in natural waters is mainly dependent on the existing oxidation-reduction conditions and pH values [17]. Under oxic conditions, the pentavalent form of As(V) is predominantly present as H\({}_{2}\)AsO\({}^{4-}\), HAsO\({}_{4}\)\({}^{2-}\) with pKa\({}_{1}\) = 2.19 and pKa\({}_{2}\) = 6.94, respectively. Conversely, when anoxic conditions prevail, e.g., in anaerobic groundwaters,As(III) is the main species found, which at pH values relevant to groundwaters is present as the non-ionic arsenous acid (H\({}_{3}\)AsO\({}_{3}\), pKa\({}_{1}\) = 9.22) [7]. As(III) is less adsorbed than the As(V), when interacting with solid surfaces and, therefore, As(III) is less efficiently removed by the use of common treatment processes, such as adsorption and precipitation. To handle this problem, an oxidation stage is usually incorporated in the treatment train to convert As(III) to As(V) [15; 16]. Several studies have indicated that As(III) removal to concentrations less than 10 \(\mu\)g L\({}^{-1}\) by adsorption onto suitable adsorbent materials, such as iron oxides, is feasible [16; 17], but requires relatively large quantities of the used adsorbent, which makes this method rather inadequate for full-scale applications. Consequently, the development of more effective technologies to remove (particularly) the As(III) species from water sources is important. The use of nano-materials could serve as a viable alternative to provide more sustainable solutions, by offering extensive surface areas and using similar material quantity, leading potentially to enhanced As(III) removal without the need for the pre-oxidation step. In the following, the recent applications of newly synthesized nanomaterials, as applied for arsenic removal [17; 18], will be presented, showing--in certain cases--improved sorption capacity for As(III). Application of Hydrous Nanostructure Iron(III)-Titanium(IV) Binary Oxide for As(III) and As(V) Removal  
Iron oxides are widely applied for arsenic removal from aqueous solutions [19]. The most recent developments include the incorporation of other metals into the structure of Fe(III) oxides to achieve increased efficiency, regarding arsenic removal. [PERSON] et al. (2008) [20] synthesized and used nano-structured hydrous Fe(III)-Ti(IV) bimetallic mixed oxides (NHITO) for the sorption of arsenic, because the incorporation of Ti(IV) was found to present enhanced material properties, namely surface sorption and photo-induced catalysis. The ability of NHITO to remove arsenic from solution containing 50 mg As(III) L\({}^{-1}\) to the permissible concentration value (i.e., 0.01 mg L\({}^{-1}\)) was investigated with batch experiments at the pH value 7, by varying the dose of solid/adsorbent, while maintaining the other parameters constant.