## 3 Nanostructured Materials for Chromium Removal
### Application of Poly-Pyrrole Graphene Oxide Nanocomposite (PPy-GO NC) for Cr(VI) Removal  
The sorption of Cr(VI) onto poly-pyrrole graphene oxide nanocomposite (PPy-GO NC) from waters was examined by [PERSON] et al. (2015) [57], using batch and packed-bed column operational modes. The batch sorption isotherm data at the optimal solution pH value 2 were adequately described by the Langmuir isotherm model with a maximum sorption capacity 625 mg g\({}^{-1}\) at 25 \({}^{\circ}\)C. The sorption of Cr(VI) onto the PPy-GO NC adsorbent from binary ions systems was also assessed and the results revealed that the existence of ions had no significant effect on Cr(VI) removal. In addition, a volume of Cr(VI) contaminated water (64 L), with 10 mg L\({}^{-1}\), was effectively treated in column experiments, using 2 g of PPy-GO NC at a flow rate of 3 mL min\({}^{-1}\), effectively achieving the maximum allowable discharge limit.  
Application of a Reductive and Magnetic Graphene/Fe\({}_{3}\)O\({}_{4}\) Composite for Cr(VI) Removal  
A highly reductive and magnetic graphene/Fe\({}_{3}\)O\({}_{4}\) composite (HR-M-GO/Fe\({}_{3}\)O\({}_{4}\)) was created via the graphene oxide (GO) in situ oxidation of FeCl\({}_{2}\) by [PERSON] et al. (2016) [58]. This super-paramagnetic composite material could be utilized for the exceedingly efficient removal of Cr(VI) from wastewaters, appropriately separated by employing an external magnet. The maximum adsorption capacity of the HR-M-GO/Fe\({}_{3}\)O\({}_{4}\) for Cr(VI) achieves 32 mg g\({}^{-1}\), which is bigger than the simple graphene/\(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) composite. The probable mechanism of HR-M-GO/Fe\({}_{3}\)O\({}_{4}\), removing Cr(VI) efficiently was that Cr(VI) reduced to Cr(III) by the occurrence of ferrous hydroxide in the graphene structure and the resulting Cr(III) ions were simply captured by the negatively charged composite materials (HR-M-GO/Fe\({}_{3}\)O\({}_{4}\)).  
Application of Graphene Oxide/Poly-Amido-Amine Dendriner (GO/ParamAMs) Composites for Cr(VI) Removal  
Graphene oxide/poly-amido-amine dendrimer (GO/ParamAMs) composites [59] were utilized to remove Cr(VI) from mimicked effluents. The results exhibited that the optimal pH value was 2.5, the removal percentage reached 91% for 30 mg L\({}^{-1}\) initial concentration of Cr(VI) within 120 min and the maximum adsorption capacity reached 211 mg g\({}^{-1}\) at 40 \({}^{\circ}\)C; while in the pH range of values 2.5-7.0, there was a sharp decline, regarding the removal efficiency of Cr(VI), i.e., it decreased as the pH value increased.  
Table 3 shows the reviewed nanomaterials applied for Cr(VI) removal from water sources, providing information about their major structural characteristics and applications in water treatment. A comparison between the sorption capacities of selected adsorbents, recently reported in literature for the removal of Cr(VI) from aqueous solutions, is presented in Table 4, indicating that the previously presented nanomaterials (i.e., NiO and CCGO) showed enhanced sorption capacities (Table 3).  
Table 3 displays comparative results obtained from the different materials presented in this paper. Regarding Cr(VI) adsorption, it was shown that the most efficient adsorption takes place at pH values between 2 and 5 (i.e., acidic conditions) for almost all reviewed materials. Poly-pyrrole graphene oxide nanocomposite (PPy-GO NC) presents maximum sorption capacity 625 mg g\({}^{-1}\) at 25 \({}^{\circ}\)C. Higher adsorption capacity was observed by graphene based materials, compared especially to nickel oxide nanoparticles (NiO), which contain no graphene in its structure and provide the relatively lower absorption capacity (5 mg g\({}^{-1}\)). The low (acidic) pH values is the main reason why adsorption is not considered as a favorable method for the removal of Cr(VI) at pH values more relevant to drinking water sources (i.e., 6.5-8.5). However, these adsorbents could be useful for the treatment of acidic wastewaters.