## 3 Nanostructured Materials for Chromium Removal
### Application of Graphene Oxide Functionalized with Magnetic Cyclodextrin-Clitosan for Cr(VI) Removal  
A simple chemical bonding method to synthesize magnetic cyclodextrin-chitosan/graphene oxide (CCGO) was published in 2013 by [PERSON] et al. [54]. Cyclodextrins are cyclic oligosaccharides endued with a hydrophilic outer surface and a hydrophobic inner cavity, capable of developing inclusion complexes with a broad variety of guest molecules, certainly affecting their physicochemical properties [55]. Chitosan-based sorbents have exhibited relatively elevated sorption capacities for heavy metals, due to their high nitrogen content and porosity. On the other hand, graphene oxide (GO), comprising several oxygen-containing functional groups on its surface, shows high-level adsorption performance for metal ions, but cannot be easily separated from the finished water. Magnetic \(\beta\)-cyclodextrin-chitosan nanoparticles have attracted attention, due to their excellent properties, such as easy separation and elevated adsorption capability.  
Thus, the magnetic \(\beta\)-cyclodextrin-chitosan/graphene oxide materials were formulated via a staged process, where the carboxyl groups of GO chemically react with the amine groups of magnetic \(\beta\)-cyclodextrintrim-chitosan with the subsequent creation of chemical bonds between GO and chitosan. It was found that the Cr(VI) adsorptive capacity of CCGO strongly depends on the surface charge concentration, as well as on the specific surface area, and the respective adsorption capacity was found to be 68 mg g\({}^{-1}\) of Cr(VI) using the CCGO material. The observable differences between graphene oxide and CCGO originated mainly from the greater surface area in case of CCGO (446 m\({}^{2}\) g\({}^{-1}\)) in contrast with the pure graphene oxide (342 m\({}^{2}\) g\({}^{-1}\)), enhancing the availability of effective surfaces for sorption. Thus, the more effective deployment of relevant adsorption sites can be accomplished by the new adsorbent [54].  
pH is an extremely significant variable affecting both the adsorption characteristics and the speciation of chromium. There are several forms of Cr(VI), namely chromate (CrO\({}_{4}\)\({}^{2-}\)), dichromate (Cr\({}_{2}\)O\({}_{7}\)\({}^{2-}\)) and hydrogen chromate (HCrO\({}_{4}\)\({}^{-}\)) [46]. These ionic forms are related to the solution pH, as well as to the total chromate concentration. During the adsorption, Cr(VI) is partially reduced to Cr(III) by the reductive surface hydroxyl groups, existing on the CCGO surface. According to the obtained results, when increasing the pH value, the uptake of Cr(VI) decreases, which is due to the higher concentration of OH\({}^{-}\) ions present in the solution, competing with the Cr(VI) species. On this basis, when the pH is low (i.e., pH < 4), the uptake of Cr(VI) is about 55 mg g\({}^{-1}\), but at higher pH levels (i.e., pH > 6.5), the uptake of Cr(VI) decreases to 38 mg g\({}^{-1}\), when the initial Cr(VI) concentration is 50 mg L\({}^{-1}\) and the dosage of CCGO is 1 g L\({}^{-1}\). These results show the high potential of CCGO as an example of new composite materials, to remove effectively metal ions from wastewaters [54; 55].  
A recent study [56] involves the adsorption of hexavalent chromium, using chitosan grafted graphene oxide nanocomposite (CS-GO) in batch mode, providing an adsorption capacity of 104 mg g\({}^{-1}\), achieved at pH 2.0 and for the contact time of 420 min, noting also that the CS-GO material is recyclable up to 10 cycles with the minimum loss of adsorption capacity.