## 2 Nanostructure Materials for Arsenic Removal
### Application of Iron(III)-Copper(II) Binary Oxide for As(V) and As(III) Removal
Application of Graphene Oxide-Ferric Hydroxide GO/Fe(OH)\({}_{3}\) (GO-Fe) Composites for As(V) Removal  
Graphene is a two-dimensional (2-D) nanosheet of graphite with one atom thickness; it was firstly found in 2004 by [PERSON] and [PERSON] [27], and since then, a wide range of applications has been exploited, e.g., in transparent conductors, field-effect transistors, fuel cells, batteries, solar cells, and water purifiers [28; 29; 30], as well as in the food industry [31]. Graphene holds some exceptional properties, for instance, extremely large specific surface area (\(\sim\)2600 m\({}^{2}\) g\({}^{-1}\)) and very strong chemical stability [27; 32], making it an efficient material for use in the water industry, mainly as a support material, e.g., for the nano-scaled iron oxides, which were proven to be excellent adsorbents for the removal of arsenic [6]. [PERSON] et al. (2010) [33] developed a series of innovative composites, based on graphene oxide (GO) cross-linked with Fe-hydroxides with several GO/FeSO\({}_{4}\):7H\({}_{2}\)O ratios, i.e., GO-Fe-1, GO-Fe-2, for the successful treatment of arsenic contaminated drinking water. GO was mainly used as a supporting substrate of ferric hydroxide, due to its several interesting properties, such as high mechanical strength, large surface area and simple preparation using inexpensive natural graphite. The efficient As(V) sorption, using GO-Fe-5, was identified over a broad pH range, i.e., for pH 4-9, and different initial As(V) concentrations (0.5-20.0 mg L\({}^{-1}\)), but the efficiency was decreased at pH values higher than 8, where iron oxides exhibit a negative overall surface charge, which repulses the negatively charged arsenate species, such as HAsO\({}_{3}\)\({}^{2-}\)[19; 34]. However, in all examined cases, the remaining Fe levels in the finished water were rather low (<1.0 mg L\({}^{-1}\)), indicating that iron stays mostly stable on the support material and can be further tested in pilot-scale applications, e.g., using continuous operating adsorption columns [33]. Application of Magnetic Fe\({}_{3}\)O\({}_{4}\)-Reduced Graphite Oxide-MnO\({}_{2}\) Nanocomposites for As(V) and As(III) Removal  
[PERSON] et al. (2012) [35], in their study, combined the oxidizing ability of manganese dioxide, the large surface area of GO and the magnetic characteristics of Fe\({}_{3}\)O\({}_{4}\), aiming at synthesizing a new adsorbent for arsenic, i.e., Fe\({}_{3}\)O\({}_{4}\) and MnO\({}_{2}\) nanoparticles modified GO (Fe\({}_{3}\)O\({}_{4}\)-RGO-MnO\({}_{2}\)) nanocomposites. The preparation of this material was carried out by a two-stage precipitation reaction, starting with graphite oxide (GO) as the raw material, followed by the reduced Fe\({}_{3}\)O\({}_{4}\)-RGO as intermediate product, and forming finally a material with a specific area of 114 m\({}^{2}\) g\({}^{-1}\). This adsorbent presents a high removal capacity towards both As(III) and As(V), i.e., 14 mg g\({}^{-1}\) and 12 mg g\({}^{-1}\), respectively (using a solution containing NaAsO\({}_{2}\) and Na\({}_{2}\)HAsO\({}_{4}\)-7H\({}_{2}\)O), with specific emphasis given to the higher adsorption capacity of As(III). In addition, arsenic adsorption remained rather constant for a broad pH span between 2-10. The aforementioned adsorbent provides a prominent advantage that can be further separated from the treated water sample by the application of magnetic separation [36]. Consequently, from the pragmatic point of view, the Fe\({}_{3}\)O\({}_{4}\)-RGO-MnO\({}_{2}\) nanocomposites could serve as excellent adsorbents for treating arsenic polluted waters in the future [35].