## 3 Results
### Potential Application: Proof of Concept of Heterogeneous Catalyzed Degradation of Methyl Orange Dye  
Dye removal can be achieved through various routes comprising adsorption [34], filtration [35] or catalyzed degradation [36; 37]. Here, we focus on the catalyzed degradation of MO in aqueous solution in order to assess the catalytic activity of the actual CuNi-decorated biochar. This process was carried out in the presence of NaBH\({}_{4}\) as a reducing agent. Upon starting at time = 0 s, the investigated samples did not show any significant changes in the color of the tested vials, as illustrated in the upper row in Figure 8a. After 30 min, the color of the solution loaded with decorated biochar turned colorless, referring to decomposed MO (Figure 8b). The UV-vis spectra of MO with different reactants in the presence and absence of the decorated biochar are illustrated in Figure 8c. Starting with the spectrum showing peaks with the highest intensity among the explored samples, MO displays a typical intense peak at 464 nm, accompanied by a lower one at 270 nm. The first peak can be correlated with the conjugation of electron donor N=N bond. Meanwhile, the latter peak is related to aromatic pi-pi* transition [38]. After diluting MO with deionized  
Figure 7: TGA plots recorded under air for biochar and B@CuNi.  
water, the same behaviour for absorbance bands arose but with lower peak intensities. This may be related to the dilution effect resulting from introducing water to the previously tested MO aqueous solution. The dilution step was implemented to confirm the presence of MO in the solution even at a lower concentration with weaker absorption bands. Upon investigating the samples of MO with either CuNi-decorated biochar or NaBH\({}_{4}\) separately, the absorption peaks showed neither significant changes nor shifts. Upon testing MO with B@CuNi and NaBH\({}_{4}\), the solution became transparent; this attests to the degradation process of MO. The previously mentioned peaks vanished and new bands appeared at 245 and 248 nm. Accordingly, a cleavage of the -N=N- bond took place to generate the -NH\({}_{2}\) group, hence newly formed species, namely, sulfanilic acid and p-amino dimethylaniline, respectively [39, 40] exist. It is noteworthy that no centrifugation was required to split the biochar and the supernatant; the latter was sedimented easily using a magnet, which facilitated sampling of the supernatant for UV-vis analysis.  
These preliminary results are encouraging, as the test dye aqueous solution containing -0.24 umol MO was discolored in less than 30 min using \(\sim\)5.5 umol of alloyed Cu and Ni metals, but the turnover number should be limited to the copper and nickel atoms present at the surface of the bimetallic nanoparticles, thus resulting in a higher TON value. The recovered B@CuNi hybrid catalyst allowed for catalyzing the discoloration of a new, freshly prepared MO solution. This is due to the presence of NBH\({}_{4}\), which could reduce NiO and CuO if they were ever present at the surface of B@CuNi.  
Figure 8: (**a,b**) Digital photographs and (**c**) UV-vis spectra for MO vials before and after catalytic reaction. (**d**) A proposed scheme for MO before and after the catalytic degradation with decorated CuNi-biochar.  
To finish, we are presently testing new processes for making biochar@CuNi that permit a faster discoloration of MO solutions (within 1-10 min). The new studies need to be double-checked and confirmed prior to future publication.