## 3 Applications
### Catalysis  
It is known that the presence of toxic metals in water poses a serious threat to living organisms. Examples of the most dangerous toxic metal ions found in wastewater are Hg\({}^{2+}\), Cr\({}^{6+}\), As\({}^{3+}\), Cd\({}^{2+}\), Pb\({}^{2+}\), Mn\({}^{2+}\), Ni\({}^{2+}\)[64; 65]. They cause renal failure, lung impairment, lung cancer, dermatitis, dyspnea, and damage to the reproductive system, liver, and central nervous system [65; 66]. In addition, the excess of the above-mentioned metals adversely affects agriculture and the environment [65; 66]. CuFe nanoparticles are also examined as materials for the elimination of heavy metals--such as Cr or As--and metalloids from metallic ones in which Cu prevents the oxidation processes of corrosive metals [42; 49]. [PERSON] et al. [42] reported that for the efficiency of As\({}^{3+}\) removal from smelting wastewater (wastewater produced from smelting, the extraction of metal from its ore by a process involving heating and melting), the copper content in Cu doped Fe@Fe\({}_{2}\)O\({}_{3}\) nanoparticles is crucial and must not exceed 20%. Additionally, it was investigated how pH, temperatureand the initial concentration of As\({}^{3+}\) affect the efficiency and rate of As\({}^{3+}\) removal. It has been shown that the maximum removal capacity of As\({}^{3+}\) was achieved at a broad pH range from 3 to 9, suggesting that Cu doped Fe@Fe\({}_{2}\)O\({}_{3}\) particles have the potential to treat As\({}^{3+}\) in most surface and groundwater under natural conditions. The removal capacity increased with the temperature increase from 298 K to 308 K, which can be explained by the mobility of the ions as the reaction temperature increased. The adsorption capacity of obtained particles increased with an increase in the initial As\({}^{3+}\) concentration, but with an increase in the initial As\({}^{3+}\) concentration, the removal rate was almost halved. [PERSON] et al. [49] investigated how the morphology of the nanoparticles and copper content influenced the capacity, speed, and intensity of adsorption in As\({}^{5+}\) removal. They found that the low content of Cu (Fe\({}_{0.9}\)Cu\({}_{0.1}\)) favored the core@shell structure and increased the removal of As\({}^{5+}\) compared to Fe\({}_{0.5}\)Cu\({}_{0.5}\) favoring the segregated (Janus) structure. They proposed the mechanism of As\({}^{5+}\) removal with the CuFe bimetal, indicating that Cu is responsible for the initial reduction of As\({}^{5+}\) to As\({}^{3+}\), and the removal of impurities is favored by the transfer of electrons from the Fe\({}^{0}\)(nZVI) layer to the Cu oxide layer (Figure 10). It should also be noted that the modification of NP CuFe surface with another material, such as chitosan (CS), improved their adsorption capacity [43]. The authors show that  
Figure 10: Proposed mechanism of As\({}^{5+}\) removal using bimetallic CuFe materials as the adsorbent. Reprinted with permission from [49]. Copyright 2018, with permission from Elsevier. Figure 9: Electrocatalytic activity of the CuFe alloy. **(a)** Linear sweep voltammograms of CuFe, Cu, Fe, and Pt/C measured by a RRDE system. **(b)** Tafel plots of each catalyst. **(c)** Kinetic current at 0.9 V of each catalyst (except Fe in **(b,c)** due to low on-set potential of Fe). **(d)** Calculated number of transferred electrons of CuFe and Pt/C. Reproduced with permission from ref. [50]. Copyright 2015 American Chemical Society. CS-FeCu has the highest Cr\({}^{6+}\) removal efficiency in comparison with nZVI, FeCu, and CS-Fe. They reported that the highest stabilization and the reactivity of nZVI, because of the effect of surface coating, were obtained for 2 wt% of chitosan and 3 wt% of Cu in composites. Additionally, the maximum absorption capacities were achieved with pH in the range of 3-9, and CS-FeCu nanoparticles are active even when the initial concentrations of Cr\({}^{6+}\) are high. To remove Cr\({}^{6+}\) from water, the composite of GT and nZVICu was also tested.