## 5 Conclusions  
The pollution of natural waters caused by toxic metals is a global issue, and various treatment technologies have been developed to remove these inorganic pollutants from water sources (mostly from groundwater). The application of most important novel nanostructured materials was summarized, regarding the removal of arsenic (As(III) and As(V)), chromium ((Cr(III) and Cr(VI)) and uranium (U(VI)) from aqueous systems. As it was shown, the use of nanostructured materials, exhibiting generally higher surface areas and average structural sizes below 100 nm, could be very promising in this direction. Specifically, in this review, the most certain of the novel and recently applied materials for the removal of certain inorganic pollutants from water sources were summarized, providing their major structure characteristics and application conditions (Table 1), according to relevant recent literature. In particular, the removal of arsenic was examined by the application of hierarchically porous CeO\({}_{2}\)-ZrO\({}_{2}\) nanospheres material, as well as by the incorporation of other metal ions into \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Sorbent Material**} & \multirow{2}{*}{**pH**} & \multicolumn{2}{c}{**Sorption Capacity (mg g\({}^{-1}\))**} & \multirow{2}{*}{**Reference**} \\ \cline{3-3} \cline{5-5}  & & & **Cr (VI)** & \\ \hline
**Manganese oxide coated zeolite** & **4.0** & 15 & [PERSON] et al. (2007) [87] \\ Natural sepiolite & 3.0 & 35 & [PERSON] (2009) [88] \\ Modified clays with titanium oxide & 3.5 & 0.6 & [PERSON] et al. (2009) [89] \\ Magnetite nanoparticles & 7.0 & 5 & [PERSON] et al. (2010) [75] \\ Ethylenediamine-modified chitosan & 3.0 & 83 & [PERSON] et al. (2011) [90] \\ Goethite (\(\alpha\)-FeOOH) & 6.0 & 34 & [PERSON] et al. (2011) [91] \\ Binary iron–manganese & 6.5 & 133 & [PERSON] et al. (2015) [92] \\ oxy-hydroxides (FMHO) & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Comparison of sorption capacities between various sorbents towards the removal of U(VI) ions. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Novel** & **Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\)** & **GO** & **GO-AC** & \begin{tabular}{c} **rGO/** \\ **NiAl-LDH** \\ \end{tabular} & **GO-NH\({}_{2}\)** & **SA-GO** & **COOH-GO** & **GO-DTPAA** & \begin{tabular}{c} **rGO/Fe\({}_{3}\)O/** \\ **TW** \\ \end{tabular} \\ \hline \begin{tabular}{c} Dose (g L\({}^{-1}\)) \\ C initial \\ (mg L\({}^{-1}\)) \\ pH \\ \end{tabular} & 2.5 & 0.4 & 0.2 & 0.5 & 0.2 & 0.1 & 1.0 & 0.1 & 0.5 \\ \begin{tabular}{c} C initial \\ (mg L\({}^{-1}\)) \\ pH \\ \end{tabular} & 50 & 119 & 50 & 130 & 60 & 35.6 & 10 & 50 & 10 \\ \begin{tabular}{c} Adsorption \\ (mg g\({}^{-1}\)) \\ \end{tabular} & 6.0 & 4.0 & 5.5 & 4.0 & 5.5 & 5.0 & 4.0 & 6.5 & 5.0 \\ \begin{tabular}{c} Acpacity \\ (mg g\({}^{-1}\)) \\ \end{tabular} & 52 & 299 & 298 & 278 & 215 & 162 & 169 & 485 & 105 \\ \begin{tabular}{c} [PERSON] et al. \\ (2012) \\ \end{tabular} & \begin{tabular}{c} [PERSON] et al. \\ (2012) \\ \end{tabular} & \begin{tabular}{c} [PERSON] et al. \\ (2013) \\ \end{tabular} & \begin{tabular}{c} [PERSON] et al. \\ (2015) \\ \end{tabular} & \begin{tabular}{c} [PERSON] et al. \\ (2016) \\ \end{tabular} & \begin{tabular}{c} [PERSON] \\ (2016) \\ \end{tabular} & \
\begin{tabular}{c} [PERSON] \\ (2018) \\ \end{tabular} &
\begin{tabular}{c} [PERSON] et al. \\ (2019) \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Novel nanostructured materials applied for the removal of U(VI) from waste/water sources. the lattice structure of iron(III) oxide, such as the hydrous titanium(IV), the bimetallic mixed oxide (NHITO), and the copper(II) binary oxide nanomaterials. Of particular interest was the application of graphene and a series of novel composite materials based on graphene oxide (GO), such as graphene oxide-ferric hydroxide GO/Fe(OH)\({}_{3}\) composites, magnetite Fe\({}_{3}\)O\({}_{4}\)-reduced graphite oxide-MnO\({}_{2}\) and graphene oxide-hydrated zirconium oxide.