## 3 Nanostructured Materials for Chromium Removal  
### Chromium (Cr)  
Chromium is an odorless and tasteless metallic element, and it can be found naturally in rocks, plants, soil and volcanic dust. The most widespread forms of chromium that appear in real waters are the trivalent and hexavalent species. Cr(VI) is 100 times more toxic than Cr(III), showing high carcinogenic, mutagenic and teratogenic action to biological systems [46]. Therefore, the Environmental Protection Agency (EPA) has set the maximum concentration limit of 0.1 mg L\({}^{-1}\) for the case of total chromium in water, whereas the World Health Organization (WHO) has set a lower respective limit (0.05 mg L\({}^{-1}\)) in drinking water for Cr(VI) [47].  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sorbent Material** & **pH** & \begin{tabular}{c} **Langmuir Q\({}_{\text{max}}\) Capacity (mg g\({}^{-1}\))** \\ **As(III)** \\ \end{tabular} & **Reference** \\ \hline \begin{tabular}{c} Crystalline hydrous ferric oxide \\ \end{tabular} & 7.0 & 33 & 25 & [PERSON] et al. (2003) [40] \\ \begin{tabular}{c} Crystalline hydrous titanium oxide \\ \end{tabular} & 7.0 & 32 & - & [PERSON] et al. (2004) [41] \\ \begin{tabular}{c} Nanoscale zero valent iron \\ \end{tabular} & 7.0 & 3 & - & [PERSON] et al. (2005) [42] \\ \begin{tabular}{c} Nano-TiO\({}_{2}\) \\ \end{tabular} & 7.0 & 60 & 37 & [PERSON] et al. (2005) [43] \\ \
\begin{tabular}{c} Hydrous stannic oxide \\ \end{tabular} & 7.0 & 16 & 4 & [PERSON] et al. (2007) [44] \\
\begin{tabular}{c} Akaganeite nanocrystal \\ \end{tabular} & 7.5 & - & 134 & [PERSON] et al. (2003) [45] \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparative results of Langmuir monolayer sorption capacity for arsenic using the aforementioned materials.  
Chromium has been applied traditionally in the chrome plating of metallic surfaces, and also as an element in dyes and pigments, in steel, in the leather tanning process and as a wood preservative. It is frequently discharged to the environment, after the discarding of chromium-containing materials, or as a by-product of the industrial processes that use it. There are also demonstrated cases of chromium being released to the environment by leakage, poor storage or inadequate industrial waste disposal practices, causing severe anthropogenic groundwater contamination and human health problems [48]. The serious toxicity of Cr(VI) has attracted the attention of the scientific community and environmental associations, provoking an growing number of headlines and a general increase of public awareness. In that direction, on 1 July 2014, the US State of California adopted a lower Maximum Concentration Level (MCL) of 0.01 mg L\({}^{-1}\) for the presence of hexavalent chromium in drinking water [49].  
To reduce Cr concentration from contaminated water sources, various treatment methods, such as coagulation followed by filtration [9], adsorption and membrane filtration, have been successfully applied. Nevertheless, for each of these treatment methods, some constraints exist, and therefore, it is challenging to address the newly imposed concentration limits [50].  
The effective removal of Cr(VI), usually involves its preliminary reduction to Cr(III). In the trivalent state, chromium instantly forms Cr(OH)\({}_{3}\) or (Cr,Fe)(OH)\({}_{3}\), which are both very insoluble. The fairly lower solubility of Cr(OH)\({}_{3}\) and (Cr,Fe)(OH)\({}_{3}\) is mainly responsible for the generally low Cr(III) concentrations in waters, lower than the drinking water standards over the entire pH range relevant to drinking water [51].  
However, it would be very useful for the drinking water treatment plants if Cr(VI) could be efficiently directly removed by sorption onto suitable materials, without the need for preliminary Cr(VI) reduction. Therefore, the current research is focusing on the development of novel adsorbent materials, with superior performance/competence against the removal of Cr(VI).