## 2 Results
In a next study [5] arsenic, manganese, zinc, cadmium and lead removal from aqueous solutions was investigated by the use of sludge, resulting from an iron-ore processing area. The adsorption capacity of target adsorbates was investigated in batch experiments of both single- and mixed-metal solutions. The results demonstrated that the maximum Langmuir adsorption capacities of the heavy metals onto the adsorbent occurred in the order Pb > As > Cd > Zn > Mn, and ranged from 0.710 mg/g to 1.113 mg/g in the single-metal solutions and from 0.370 mg/g to 1.059 mg/g in the mixed-metal solutions. Adsorption performances indicated that iron-ore sludge can simultaneously adsorb multiple metal ions and is a promising adsorbent for the removal of toxic pollutants from water, since there are several cases, where multiple heavy metal pollution of waters occurs and to remove them from waters, there is a need for adsorbents able to remove several contaminants simultaneously. The article by [PERSON] et al., [6] summarizes the recent findings regarding the removal of arsenic, chromium and uranium by novel composite hybrid materials, with emphasis in the use of novel composite materials, based mainly on hybrid metallic oxide nanoparticles and on composites based on graphene oxide (GO) (i.e., graphene-based hybrids). Particularly, for the removal of arsenic, it was reviewed the application of hierarchically porous CeOZrO\({}_{2}\) nanospheres, the bimetallic mixed oxide (NHITO), and the copper(II) binary oxide nanomaterials and graphene based, 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. Among these materials, the iron(III)-copper(II) binary oxide, showed particularly high adsorption capacities for both As(III) and As(V) of 122 and 83 mg/g respectively. This material was more efficient for As(III) removal than for As(V), which suggests that it might be very useful in the treatment of anoxic arsenic contaminated groundwaters, which contain mostly As(III) [11]. In these cases usually a peroxidation step is required, unless if As(III) efficient adsorbent are developed, such as the one in this study, or others, such as the use of polyferric sulfate [18] or Aquaszero [19,20]. Regarding chromium removal, the application composite graphene based materials, such as Graphene Oxide Functionalized with Magnetic Cyclodextrin-Chitosan (CS-GO), Poly-Pyrrole Graphene Oxide Nanocomposite (PPy-GO-NC), and Graphene Oxide/Poly-Amido-Amine Dendrimer (GO/PAMAMs) were found very efficient for the adsorption/removal of Cr(VI), presenting adsorption capacities of 104, 625, and 211 mg/g respectively [6]. However, these values were recorded only at pH values below 5 and, therefore, they could find applications for the treatment of acidic wastewaters, avoiding the usually applied preliminary reduction step towards the Cr(III) formation. With respect to uranium removal, the review highlighted the use of magnetic Fe\({}_{3}\)O\({}_{4}\)@SiO\({}_{2}\) composite particles, as well as off several novel graphene modified composite materials, such as Three-Dimensional Layered Double Hydroxide-Graphene Hybrid Material (rGO/LDH), Novel Graphene Oxide-Activated Carbon Felt Composite (GO-ACF), Graphene Oxide and Its Amine-Functionalized Composite (GO-NH\({}_{2}\)) which showed promising results with reported adsorption capacities as high as 278, 298 and 215 mg/g, which are very good results when compared to conventional adsorbents such as iron oxides [21], which show capacities between 100 and 135 mg/g. The next paper [7] focused on the contents of the five most bio-toxic heavy metals, As, Cd, Hg, Cr, and Pb in samples from 26 municipal solid waste (MSW) from the Eastern Guangdong Area.