## 4 Nanostructured Materials for Uranium Removal  
### Uranium (U)  
Identified as a harmful element, uranium (U) is plentiful in nuclear waste disposal facilities (and testing sites), as well as in uranium mining, processing and milling sites. In groundwaters, \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Sorbent Material**} & \multicolumn{2}{c}{**Sorption Capacity (mg g\({}^{-1}\))**} & \multirow{2}{*}{**Reference**} \\ \cline{2-2} \cline{4-4}  & \multicolumn{2}{c}{**Cr (VI)**} & & \\ \hline Amino starch & 12 & [PERSON] et al. (2010) [60] \\ Silica matrices & 18 & [PERSON] et al. (2011) [61] \\ Mesoporous TiO\({}_{2}\) & 33 & [PERSON] et al. (2010) [62] \\ Alumina & 4 & [PERSON] et al. (2010) [63] \\ Alumina/chitosan composite & 9 & [PERSON] et al. (2010) [63] \\ Activated Alumina & 2 & [PERSON] et al. (2004) [64] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparison of the sorption capacities of various sorbents towards Cr(VI) ions. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Novel** & & & & & & \\
**Nano-Material** & **NiO** & **CCGO** & **CS-GO** & **PPy–GO NC** & **HR-M-GO/Fe\({}_{3}\)O\({}_{4}\)** & **GO/PAMAMs** \\ \hline Cr species & Cr(VI) & Cr(VI) & Cr(VI) & Cr(VI) & Cr(VI) & Cr(VI) \\ \multicolumn{1}{l}{Dose (g L\({}^{-1}\))} & 0.6 & 1 & 2 & 2 & 0.5 & 0.5 \\ C initial & 20 & 50 & 50 & 10 & 10 & 50 \\ (mg L\({}^{-1}\)) & 4.7 & 3.0 & 2.0 & 2.0 & 3.0 & 2.5 \\ \multicolumn{1}{l}{Adsorption} & & & & & & \\ capacity & 5 & 68 & 104 & 625 & 32 & 211 \\ (mg g\({}^{-1}\)) & & & & & & \\ \multicolumn{1}{l}{[PERSON] and} & [PERSON] et al. & [PERSON] et al. & [PERSON] et al. & [PERSON] et al. (2016) & [PERSON] et al. \\ \multicolumn{1}{l}{[PERSON]} & (2013) & [PERSON] et al. & [PERSON] et al. & [PERSON] et al. (2016) & [PERSON] et al. \\ \multicolumn{1}{l}{(2014) [53]} & [54] & (2019) [56] & (2015) [57] & (2015) [58] & (2019) [59] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Novel nanostructured materials applied for the removal of chromium mainly from wastewater sources. it is usually present in the form of U(VI). Uranium contamination poses a threat to both surface and groundwaters [65; 66]. Uranium disposed into the environment can ultimately reach the top of food chain and be consumed by humans, initiating severe kidney or liver damage and even death [67]. Thus, the WHO and US EPA recognized U(VI) as a human carcinogen and recommended 30 \(\upmu\)g L\({}^{-1}\) as the temporary guideline level for its presence in drinking water, after the 2012 relevant revision [68]. The contamination of groundwaters with uranium is a subject of concern in several countries around the world, such as in the USA, Canada, Germany, Finland, Norway, and Greece [68]. Therefore, it is very important to choose a suitable and effective method to remove uranium from water sources. U(VI) is the most ubiquitous species of uranium in aerated waters, whereas in anoxic groundwaters, uranium is usually absent, because the respective reduced form (U(IV)) is insoluble in water [69]. The major uranium species in water supplies are the anionic carbonate complexes, i.e., the UO\({}_{2}\) (CO\({}_{3}\))\({}_{2}^{2-}\) at pH values lower than 7 and the UO\({}_{2}\) (CO\({}_{3}\))\({}_{3}^{4-}\) at pH greater than 8, whereas at pH values between 5 and 6.5 the neutral UO\({}_{2}\)CO\({}_{3}\) species may also make up an important part, which depending on pH, could vary between 20%-90% [69]. The creation of the aforementioned complexes with carbonates determines its removal from water, depending on the application of specific treatment processes. For example, conventional methods, such as coagulation or lime softening, can remove uranium from water, but they are very sensitive to pH changes and water composition [8]. On the contrary, ion-exchange [70; 71] is the most efficient removal method, because it can remove about 98% of uranium from water, mainly through the removal of anionic uranium carbonate species; the ion exchange resins used in this case are usually selective strong base (anionic) [8].