## 2 Nanostructure Materials for Arsenic Removal
### Application of Iron(III)-Copper(II) Binary Oxide for As(V) and As(III) Removal  
Recently, it was reported that cupric oxide was an effective sorbent for both As(V) and As(III) removals over a wide pH range values, relevant to drinking water treatment (i.e., for pH = 6.6-8.6), even in the presence of other competing anions, such as silica, sulfate and phosphate [23]. [PERSON] et al. (2013) [24] synthesized Fe-Cu binary oxides and evaluated their arsenic adsorption capacity. The results showed that the prepared Fe-Cu binary oxide particles are actually formed by smaller nano-sized particles and are very effective for the elimination of As(V) and As(III) species from waters. The highest adsorption capacities for As(V) and As(III) are 83 and 122 mg g\({}^{-1}\) at pH 7, correspondingly. Owing to its exceptional arsenic sorptive performance, straightforward and efficient synthesis process, and effective regeneration of depleted adsorbent materials, the Fe-Cu binary oxide could serve as an adsorbent with promising application for both As(V) and As(III) species removal from water and wastewater [24]. Furthermore, this material shows increased capacity for As(III) sorption, which is very promising, because the problem with arsenic removal is that As(III) is usually less efficiently removed from waters by the application of conventional methods. Therefore, the application of such materials, showing increased As(III) sorption capacity, especially at the pilot-plant scale, would be very interesting and could show the potential of such methods to remove As(III) by circumventing the application of pre-oxidation step. Very recently, [PERSON] et al. (2020) [25] used these oxides (ratio Fe:Cu = 2:1) by depositing them onto polymer beads and showed that As(III) removal takes place through a two-step mechanism, namely catalytic oxidation and adsorption. The adsorptive ability was measured as 92 mg As g\({}^{-1}\) of adsorbent, close to that previously referred by [PERSON] et al. (2013). Due to the formation of polymer beads, column experiments were performed, showing efficient removal of As(III) (reaching final concentrations below 10 ug L\({}^{-1}\)) from quite high initial concentrations (up to 500 ug L\({}^{-1}\)). Application of Hierarchically Porous CeO\({}_{2}\)-ZrO\({}_{2}\) Nanospheres for As(V) and As(III) Removal  
Hierarchically porous CeO\({}_{2}\)-ZrO\({}_{2}\) nanospheres were synthesized by [PERSON] et al. (2013) [26], and their suitability as arsenic sorbents was examined. The CeO\({}_{2}\)-ZrO\({}_{2}\) hollow nanospheres showed strong affinity and selectivity to arsenic with an adsorption capacity of 27 and 9 mg g\({}^{-1}\) for As(V) and As(III), respectively. These results were obtained at the equilibrium arsenic concentration of 0.01 mg L\({}^{-1}\) under circumneutral pH values (pH 6.9 \(\pm\) 0.2). As(V) removal could be so efficient as to achieve residual concentration below 10 ug L\({}^{-1}\) (from starting As(V) concentrations 12 mg L\({}^{-1}\)) by the CeO\({}_{2}\)-ZrO\({}_{2}\) nanospheric materials with an adsorbent dose of 0.2 mg mL\({}^{-1}\), while for the case of As(III), this could be also reduced to below 10 ug L\({}^{-1}\), but from As(III) concentrations of 1 mg L\({}^{-1}\). Both cases are considered as very efficient, since the usual arsenic concentrations in groundwaters are much lower, i.e., between 20-500 ug L\({}^{-1}\). The significant adsorption capacity of CeO\({}_{2}\)-ZrO\({}_{2}\) nanospheres is mainly due to the respective better pore accessibility and the abundant surface hydroxyl groups, participating to their substitution with arsenic species [26].