## 1 Introduction  
Water pollution by toxic metals and metalloids (e.g., arsenic, copper, mercury, cadmium, lead, uranium, chromium) is nowadays considered a serious global environmental problem, especially for groundwaters. Even in concentrations in the \(\upmu\)g L\({}^{-1}\) range, their presence in various water bodies is considered particularly dangerous for human health and hence, their maximum allowable concentration in waters is strictly regulated by international organizations (World Health Organization (WHO), US Environmental Protection Agency (EPA), EU). Therefore, the efficient treatment of contaminated waters for the removal of toxic contaminants is necessary to provide people with safe drinking water. In order to detoxify the polluted waters, various treatment techniques, such as coagulation-flocculation, lime softening, photocatalytic, chemical or biological oxidation, bioremediation, ion-exchange, reverse osmosis, and adsorption have been employed with different efficiency degrees, depending upon the specific applicable conditions [1]. Considerable interest has attracted the preparation and use of novel nanostructured adsorbent materials with average particle sizes below 100 nm, expecting that their properties will be superior to those of conventional materials with structures of bigger size [2]. The application of nano-scaled adsorbents comprises a novel and promising technology, especially for toxic metals removal from water sources, because of their respective substantially higher surface areas, which may provoke higher reactivity and increased affinity, capacity and selectivity for different metals [3]. Recently, promising materials, i.e., activated carbons [4] and bamboo-based biochar/montmorillonite composites [5] have been used for oxoanions (i.e., pertechnate) and anionic pollutant (i.e., nitrates) removal from aqueous solutions, respectively. The objective of this review paper is to summarize the recent developments of nano-structured materials, including graphene-based modified/hybrid adsorbents, applied particularly in the water treatment of arsenic, uranium or chromium removal from water sources, with specific emphasis given to groundwater treatment. All the relevant compounds of the examined toxic metals have the characteristic of being present in waters as oxyanions, in pH relevant to ground/waters, i.e., 6.5-8.5. Arsenic is present in waters with two major valency states, i.e., either as trivalent As(III) or pentavalent As(V). As(III) cannot be easily removed by the application of conventional water treatment technologies; therefore, it is usually pre-oxidized to As(V), which in turn is usually present as the respective oxyanions of arsenic acid (H\({}_{2}\)AsO\({}_{4}\)\({}^{-}\) or HAsO\({}_{4}\)\({}^{2-}\)) [6]. Cr(VI) is present in waters mainly as chromate (CrO\({}_{4}\)\({}^{2-}\)) or dichromate (Cr\({}_{2}\)O\(\gamma\)\({}^{2-}\)). Cr(III) is rather insoluble in aqueous solutions, easily forming precipitates and interacting with other co-existing constituents; therefore, it is quite seldom found in considerable concentrations [7]. U(VI) is present mainly as the uranyl cation (UO\({}_{2}\)\({}^{2+}\)), which, in waters containing bicarbonate anions however, is most probably complexed and presented as UO\({}_{2}\)(CO\({}_{3}\))\({}_{2}\)\({}^{2-}\) [8]. These oxyanions can usually be removed from waters by the application of different technologies, due to differences in their aquatic chemistry. Arsenic, in most cases, is firstly oxidized by means of chemical or biological oxidation and then removed by conventional coagulation (with Fe or Al based salts), or by adsorption on Fe-oxides or activated alumina, which is also a widely applied technology [1]. Cr(VI) is mostly removed from water by reduction using Fe(II), which reduces to Cr(III), which is subsequently removed usually by precipitation as insoluble hydrous hydroxides [9]. U(VI) is mostly removed by lime softening or adsorption onto iron oxides [8].