Copper/Nickel-Decorated Olive Pit Biochar: One Pot Solid State Synthesis for Environmental Remediation

: Developing micro- and nanomaterials for environmental pollution remediation is currently a pertinent topic. Among the plethora of strategies, designing supported nanocatalysts for the degradation of pollutants has achieved prominence. In this context, we are addressing one of the UN Sustainable Development Goals by valorizing agrowaste as a source of biochar, which serves as a support for bimetallic nanocatalysts. Herein, olive pit powder particles were impregnated with copper and nickel nitrates and pyrolyzed at 400 ◦ C. The resulting material consists of bimetallic CuNi-decorated biochar. CuNi nanocatalysts were found to be as small as 10 nm and very well dispersed over biochar with zero valent copper and nickel and the formation of copper–nickel solid solutions. The biochar@CuNi (B@CuNi) exhibited typical soft ferromagnet hysteresis loops with zero remanence and zero coercivity. The biochar@CuNi was found to be an efﬁcient catalyst of the reduction in methyl orange (MO) dye, taken as a model pollutant. In sum, the one-pot method devised in this work provides unique CuNi-decorated biochar and broadens the horizons of the emerging topic of biochar-supported nanocatalysts.


Introduction
Recently, the materials science community witnessed a sky-rocketing number of research papers on biochar. A search on Web of Science with the keyword "biochar" returned the following results: 2 papers in 2001, 221 in 2011, 1413 in 2016, 4333 in 2020 and 3238 in 2021 (accessed on 12 August 2021), which means that over 5000 papers should be published on the topic by the end of 2021. This attests to the vitality of this branch of materials science.
The rationale for the rush in research on biochar lies in the availability of biomass worldwide in different forms, such as agrowastes, i.e., peels [1], leaves [2], palm fronds [3] and seeds [4]. The production of biochar depends on various parameters: the initial composition of the biomass [5] and the pyrolysis parameters [6]. Biochar production falls within the energy sector as it yields bio-oil and biogas in addition to the biochar itself. Biochar is employed in agriculture for soil remediation and as an adsorbent in Table 1. Biochar loads and weights with copper and nickel: from the olive pit (OP) to the CuNi-decorated biochar, including pyrolysis conditions. SEM images and EDX spectra were acquired using a Zeiss Merlin Field Emission Scanning Electron Microscope operating at 5 kV (Oberkochen, Germany) coupled with a SDD X-Max from Oxford Instruments. In order to avoid static charge on the surface, all samples were coated with a 3 nm-thin layer of palladium using a Cressington 208HR sputter-coater coupled with a Cressington MTM-20 thickness controller.

Materials OP
TGA measurements were performed with a Setaram apparatus (Setsys Evolution model). The ramp was from RT to 800 • C at a 10 • C/min heat rate.
The XRD patterns of all of the prepared samples were recorded using X'Pert-Pro Panalytical diffractometer equipped with a Cobalt X-ray source (λ = 1.7889 Å) and operating in the reflexion Bragg-Brentano geometry.
Magnetic measurements were conducted at room temperature on a Vibrating Sample Magnetometer (VSM) from QUANTUM DESIGN.
The biochar sample was characterized using a Horiba HR 800 apparatus fitted with a He−Ne laser beam, the wavelength of which was set at 514 nm.

Catalysis Assays
The catalytic reduction of methyl orange was conducted in the following conditions. In a quartz cuvette 1 cm in length, 4 mL of 20 ppm Methyl Orange solution was incubated with 1 mg of B@CuNi. The mixture was sonicated in a water bath for 2 min, then 30 mg of NaBH 4 was added to proceed with the catalysed discoloration reaction. The UV-vis spectra were recorded using a Varian Cary 50 Bio apparatus.

General Strategy of the Work
Several methods of making biochar from various sources have recently been summarized [6].The route for biochar is depicted in Figure 1a (upper panel): olive pit powder is the source of biomass, pyrolysis is the type of treatment and biochar stands for the main product. Slow and mild pyrolysis conditions were adopted: 30 • C/min heating under a nitrogen/H 2 (5%) atmosphere with a dwell time of 15 min at 400 • C prior to cooling at room temperature (RT). A temperature of 400 • C ensures a maximal biochar yield, whilst H 2 was employed in order to ensure metal ion reduction to the metallic state. The lower panel of Figure 1 displays the digital photographs of olive pit powder (Figure 1b), the same powder impregnated with the copper and nickel nitrates mixture (Figure 1c  It is interesting to note that, due to the nickel, the copper-nickel alloy-decorated biochar (B@CuNi) is magnetic and could be attracted by a magnet (Figure 1f, and Video S1, also available at https://vimeo.com/563456328, accessed on 10 September 2021).  Figure 2a depicts the surface morphology of CuNi-decorated biochar. It shows a porous, extending surface area for the nanocatalyst-decorated biochar. CuNi nanoparticles are spread along the top of the carbonaceous sample. Upon magnifying the SEM image to provide more comprehensive information, a clearer distribution of the metallic nanoparticles can be observed, as shown in Figure 2b. It is advantageous to notice CuNi nanoparticles covering the inner surface of the pores of biochar in the form of light spots on the inner denser surface surrounded by a circle and ellipse in Figure 2b. CuNi nanoparticles appear as distinct spheres, avoiding any aggregates that may arise in such cases, as illustrated in the inset of Figure 2c. They provide a spherical, undeformed structure for the provided nanoparticles. Upon comparing the B@CuNi surface to that of the pristine biochar, fewer pores can be visualized on the surface, as displayed in Figure 2d. These pores evidently manifest at higher magnification, as illustrated in Figure 2e, in a smaller proportion when compared to the CuNi-biochar sample. The inset in Figure 2f is mentioned here as a reference sample in the absence of the CuNi-supporting catalyst. The bare biochar micrograph denotes a plain surface without much informative morphology, and its porous structure cannot be easily recognized. It is interesting to note that, due to the nickel, the copper-nickel alloy-decorated biochar (B@CuNi) is magnetic and could be attracted by a magnet (Figure 1f, and Video S1, also available at https://vimeo.com/563456328, accessed on 10 September 2021). Figure 2 displays SEM images of pristine and decorated biochar samples. Figure 2a depicts the surface morphology of CuNi-decorated biochar. It shows a porous, extending surface area for the nanocatalyst-decorated biochar. CuNi nanoparticles are spread along the top of the carbonaceous sample. Upon magnifying the SEM image to provide more comprehensive information, a clearer distribution of the metallic nanoparticles can be observed, as shown in Figure 2b. It is advantageous to notice CuNi nanoparticles covering the inner surface of the pores of biochar in the form of light spots on the inner denser surface surrounded by a circle and ellipse in Figure 2b. CuNi nanoparticles appear as distinct spheres, avoiding any aggregates that may arise in such cases, as illustrated in the inset of Figure 2c. They provide a spherical, undeformed structure for the provided nanoparticles. Upon comparing the B@CuNi surface to that of the pristine biochar, fewer pores can be visualized on the surface, as displayed in Figure 2d. These pores evidently manifest at higher magnification, as illustrated in Figure  2e, in a smaller proportion when compared to the CuNi-biochar sample. The inset in Figure  2f is mentioned here as a reference sample in the absence of the CuNi-supporting catalyst. The bare biochar micrograph denotes a plain surface without much informative morphology, and its porous structure cannot be easily recognized. EDX was used to determine the surface composition of B@CuNi over several spots. For comparison, we have also recorded EDX spectra for OP impregnated with metal salts, and the EDX was used to determine the surface composition of B@CuNi over several spots. For comparison, we have also recorded EDX spectra for OP impregnated with metal salts, and the bare OPs. Figure 3a-c display the EDX spectra of the OPs, OP@metal salts and B@CuNi. The elemental composition of B@CuNi is displayed in Figure 3d, whereas the atomic ratios of (Cu+Ni)/(C+O) and Cu/Ni are compared in Figure 3e for OP@metal salts and B@CuNi. Figure 3a-c conclusively demonstrate the absence of any copper and nickel ions from the surface of the OPs, whereas the characteristic Cu and Ni peaks are noted on the impregnated OP particles with peak intensity increasing in the order Cu~Ni < O < C. The increase in the O/C peak intensity ratio is due to the hydrated metal nitrates. Upon pyrolysis, the metal nitrate-impregnated OPs are transformed into metallic alloy-decorated biochar B@CuNi with relative peak intensity increasing in the order O < Cu~Ni < C. Quantitatively, Figure 3d displays the C, O, Cu and Ni atomic % at 15 spots from the B@CuNi powder sample; the composition is fairly similar from one spot to another, which indicates even decoration of the biochar with metallic nanoparticles. This is due to homogenous impregnation of the OPs with metal nitrates. The (Cu+Ni)/(C+O) atomic ratio ( Figure 3d) increases continuously from OP@metal salts to B@CuNi due to the pyrolysis of the olive stones and subsequent release of water and nitrates. Figure 3e shows that the Cu/Ni atomic ratio remains the same within the standard deviation for OP@metal salts (1.00 ± 0.10) and B@CuNi (1.125 ± 0.277). The Cu/Ni ratio obtained for B@CuNi accounts for the equimolar impregnation of olive pits by metal ions.

Phase Analysis
The prepared catalysts were analysed using both XRD and Raman to identify the nature of the metal and carbon contents, respectively. Figure 4 displays XRD patterns of B@CuNi and of the reference materials Biochar and B@Ni. Those of B@Ni and B@Cu (not shown) correspond very well to the structures of nickel and copper-face-centered cubic metals (ICDD n • 98-004-1508 and 98-005-2256), respectively. The XRD pattern of B@CuNi exhibits the same type of diffraction lines but at 2θ positions located between those of pure Cu and pure Ni phases, suggesting that the biochar serves as a support of a Ni 1-x Cu x metallic solid solution. This is a strong supporting evidence that copper and nickel were alloyed during the one-step pyrolysis process. Moreover, it is noteworthy that all peaks are broadened, which is in line with the production of small crystals that were imaged by SEM (see Figure 2b,c). However, as the technique probes the bulk, no surface oxidation of the nanocatalysts was detected. This would best be accomplished using the more specific technique X-ray photoelectron spectroscopy (XPS). Indeed, elsewhere, surface oxidation of nickel [21], copper [21,22] and iron [14] loaded on biochar was detected, as well as the partial oxidation of palladium nanoparticles loaded on organo-functionalized mesoporous silica [23].
The formation of metallic Ni and CuNi particles was also confirmed by measuring the variation in the sample magnetization M as a function of the magnetic field H at room temperature. Indeed, by cycling the magnetic field H between −20.0 and +20.0 kOe, typical soft ferromagnet hysteresis loops with zero remanence and zero coercivity [24] were recorded on both B@CuNi and B@Ni samples ( Figure 5). This feature must be underlined since it accounts for the ability of the designed catalysts to be reversibly attracted by an external magnetic field, making their recovery and reuse easy to achieve. Interestingly, as the copper alloyed nickel particles are usually less magnetic than the pure nickel ones, it may be noticed on the recorded M(H) plots that the saturation magnetization of B@Ni is much more quickly reached than that of B@CuNi.
To complete these structural characterizations, Raman spectroscopy was carried out on the prepared sample, with a special emphasis on their carbon signature. The spectrum recorded in the 1000-2000 cm −1 energy region for the metal-free Biochar is shown in Figure 6 to illustrate this purpose. It is fitted with five components assigned to S (1262 cm −1 ; sp 2 -sp 3 ), D (1380 cm −1 ; sp 2 , C-C between aromatic cycles and highly ordered carbon material), V (1506 cm −1 ; sp 2 -sp 3 , amorphous carbon structures), G (1596 cm −1 ; graphite, sp 2 ) and G L (1696 cm −1 ; sp 2 , C=O) [25,26]. Particularly, the D and G bands are assigned to disordered and ideal graphitic lattice, respectively. The D/G peak height and area ratios equal 0.66 and 0.87, respectively. The spectrum indicates, therefore, the presence of C sp 2 atoms but no excess of amorphous sp 3 hydrocarbons. It is noteworthy that the D/G ratio, referring to the degree of defects in the carbon material, is much lower than those reported for hydrothermally synthesized graphene quantum dots [27]. This is also lower than the values reported for pruning wastes of apple trees post-treated at 400 • C (D/G height ratio~0.72) [28], ferrate-activated porous biochar prepared at 900 • C (D/G height ratio = 1.07) [29], and (D/G height ratio = 1.14) [30]. This is important when considering the low pyrolysis temperature and high yield we have obtained, as well as the relatively higher graphitization. ture of the metal and carbon contents, respectively. Figure 4 displays XRD patterns of B@CuNi and of the reference materials Biochar and B@Ni. Those of B@Ni and B@Cu (not shown) correspond very well to the structures of nickel and copper-face-centered cubic metals (ICDD n°98-004-1508 and 98-005-2256), respectively. The XRD pattern of B@CuNi exhibits the same type of diffraction lines but at 2θ positions located between those of pure Cu and pure Ni phases, suggesting that the biochar serves as a support of a Ni1-xCux metallic solid solution. This is a strong supporting evidence that copper and nickel were alloyed during the one-step pyrolysis process. Moreover, it is noteworthy that all peaks are broadened, which is in line with the production of small crystals that were imaged by SEM (see Figure 2b,c). However, as the technique probes the bulk, no surface oxidation of the nanocatalysts was detected. This would best be accomplished using the more specific technique X-ray photoelectron spectroscopy (XPS). Indeed, elsewhere, surface oxidation of nickel [21], copper [21,22] and iron [14] loaded on biochar was detected, as well as the partial oxidation of palladium nanoparticles loaded on organo-functionalized mesoporous silica [23]. The formation of metallic Ni and CuNi particles was also confirmed by measuring the variation in the sample magnetization M as a function of the magnetic field H at room temperature. Indeed, by cycling the magnetic field H between −20.0 and +20.0 kOe, typical soft ferromagnet hysteresis loops with zero remanence and zero coercivity [24] were recorded on both B@CuNi and B@Ni samples ( Figure 5). This feature must be underlined since it accounts for the ability of the designed catalysts to be reversibly attracted by an external magnetic field, making their recovery and reuse easy to achieve. Interestingly, as the copper alloyed nickel particles are usually less magnetic than the pure nickel ones, it may be noticed on the recorded M(H) plots that the saturation magnetization of B@Ni is much more quickly reached than that of B@CuNi. To complete these structural characterizations, Raman spectroscopy was carried ou on the prepared sample, with a special emphasis on their carbon signature. The spectrum recorded in the 1000-2000 cm −1 energy region for the metal-free Biochar is shown in Figure   − the degree of defects in the carbon material, is much lower than those reported for hydrothermally synthesized graphene quantum dots [27]. This is also lower than the values reported for pruning wastes of apple trees post-treated at 400 °C (D/G height ratio ~ 0.72) [28], ferrate-activated porous biochar prepared at 900 °C (D/G height ratio = 1.07) [29], and (D/G height ratio = 1.14) [30]. This is important when considering the low pyrolysis temperature and high yield we have obtained, as well as the relatively higher graphitization.

Thermal Stability
Thermogravimetric analysis (TGA) was utilized as a substantial technique to monitor the thermal stability of biochar and match it with CuNi-decorated biochar. For comparison, we have also plotted the thermograms of B@Cu and B@Ni prepared with single metal ion nitrate. The sequence of residual weight % at a high temperature is Biochar < B@Ni < B@Cu < B@CuNi. This is a logical sequence as, under air, biochar is burnt out, whereas, with nickel, the final biochar is loaded with metallic nickel, which converts to NiO. Copper converts to CuO, and, as both copper and nickel nitrates were impregnated at an equal number of moles per gram of biomass, it is logical to obtain a slightly higher weight % for B@Cu compared to B@Ni. With an increased final number of moles per gram of biochar (see Table 1), biochar burns out, but not the CuNi alloy, which converts to oxides, hence the higher weight % for B@CuNi (lower weight loss). In the literature, carbon@Cu and carbon@Ni prepared by the annealing of the corresponding metal organic frameworks showed only a slight decrease in weight loss because TGA was conducted under an inert atmosphere [20]. In contrast, biochar loaded with a copper complex catalyst exhibited substantial weight loss under air, with a steady state reached at around 500 • C and a final 50% weight [31]. As far as the nickel-biochar-based magnetic catalyst is concerned, a weight loss of 49.8% was reported at 800 • C, presumably under air, but this was not specified [32].
Returning to the main hybrid B@CuNi and reference material Biochar, both samples show a comparable thermal stability behaviour until reaching 240 • C for steady samples withstanding temperature, as shown in Figure 7. Thermogravimetric analysis was carried out under air. A slight increase in the weight of the pristine biochar sample is observed at 280 • C; this is correlated with the oxidation of both biochar and the metal alloy. It might have influenced the biochar specimen and led to a slight increase in its weight %. At higher temperatures, biochar started to decompose at 330-340 • C until reaching 500 • C.
For B@CuNi, a plateau region was reached and corresponds to 42% residual weight of the original sample. The sample retained its thermal stability up to 270-280 • C, then a decomposition step occurred at elevated temperatures to reach a steady state at 455 • C with a residual weight of 45% of the original weight, higher than that of biochar due to loaded metallic nanoparticles [20].

Potential Application: Proof of Concept of Heterogeneous Catalyzed Degradation of Methyl Orange Dye
Dye removal can be achieved through various routes comprising adsorption [34], fi tration [35] or catalyzed degradation [36,37]. Here, we focus on the catalyzed degradatio of MO in aqueous solution in order to assess the catalytic activity of the actual CuN decorated biochar. This process was carried out in the presence of NaBH4 as a reducin agent. Upon starting at time = 0 s, the investigated samples did not show any significan changes in the color of the tested vials, as illustrated in the upper row in Figure 8a. Afte 30 min, the color of the solution loaded with decorated biochar turned colorless, referrin to decomposed MO (Figure 8b). The UV-vis spectra of MO with different reactants in th presence and absence of the decorated biochar are illustrated in Figure 8c. Starting wit 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 firs 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 deion ized water, the same behaviour for absorbance bands arose but with lower peak intens ties. This may be related to the dilution effect resulting from introducing water to the pre viously tested MO aqueous solution. The dilution step was implemented to confirm th presence of MO in the solution even at a lower concentration with weaker absorptio bands. Upon investigating the samples of MO with either CuNi-decorated biochar o NaBH4 separately, the absorption peaks showed neither significant changes nor shift Upon testing MO with B@CuNi and NaBH4, the solution became transparent; this attest 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 too place to generate the -NH2 group, hence newly formed species, namely, sulfanilic acid an p-amino dimethylaniline, respectively [39,40] exist. It is noteworthy that no centrifugatio was required to split the biochar and the supernatant; the latter was sedimented easil using a magnet, which facilitated sampling of the supernatant for UV-vis analysis.
These preliminary results are encouraging, as the test dye aqueous solution contain ing ~0.24 μmol MO was discolored in less than 30 min using ~5.5 μmol of alloyed Cu an Ni metals, but the turnover number should be limited to the copper and nickel atom 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 From a quantitative viewpoint, the thermograms were obtained under air, which suggests the formation of CuO and NiO, and the weight % differences between pristine and decorated biochar should account for the oxides. B@Cu has a residual weight of 32.6%, and that of biochar is 12.5%. The difference of 20.1% accounts for CuO, which is 0.201 g CuO for 1 g of hybrid biochar B@C, which corresponds to 2.53 mmol Cu per gram of hybrid biochar. This matches the value of 2.489 mmol/g reported in Table 1, corresponding to an error of 1.6%. With the same reasoning, and by subtracting the residual mass of biochar from that of B@Ni, one obtains 2.45 mmol nickel per gram of hybrid biochar, which is 6% less than the value of 2.608 mmol nickel/g B@Ni. Finally, concerning B@CuNi and assuming the initial exact number of moles of nickel and copper nitrates (they differ by only 1%, see Table 1), the weight % difference with biochar is 44.5−12.5 = 32%, i.e., 0.32 g of the NiO/CuO mixture (for Ni/Cu equimolar ratio) per 1 g of hybrid biochar. Assuming a hypothetical oxide CuNiO 2 (molar mass = 154.2 g), one would receive 4.15 mmol of copper-nickel mixture, which is 25% less than the total number of moles compared to the expected value reported in Table 1 (5.521 mmol/g). This apparent discrepancy could be due to the incomplete oxidation of larger nanoparticles and possible segregation of one of the metals to the surface of the alloy nanoparticles [33]. This hypothesis requires a full investigation that is beyond the scope of this paper.

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 CuNidecorated 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 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.
Appl. Sci. 2021, 11, x FOR PEER REVIEW 11 of 13 freshly prepared MO solution. This is due to the presence of NBH4, which could reduce NiO and CuO if they were ever present at the surface of B@CuNi.
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.

Conclusions
We have composed a boosted biochar derived from an agrowaste: olive pit (OP) powder. Metallic salts of hydrated nickel and/or copper nitrate(s), as precursors, were mixed with OP, followed by pyrolysis, in one step, at 400 °C to generate biochar@CuNi (B@CuNi). The resulting biochar was characterized by SEM to show a homogenous surface with well-distributed CuNi nanoparticles on the surface of the biochar. EDX and XRD asserted the formation of CuNi nanoparticles as a metallic alloy decorating the biochar. Magnetization experiments accounted for the presence of nickel and demonstrated that nanocatalyst-decorated biochars behave as ferromagnets. B@CuNi showed a high catalytic performance in the decomposition of methyl orange (MO) dye.
The significance of this work lies in the valorization of agrowaste in general and the production of an enhanced biochar decorated with bimetallic nanocatalyst (CuNi) in a These preliminary results are encouraging, as the test dye aqueous solution containing 0.24 µmol MO was discolored in less than 30 min using~5.5 µmol 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.
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.

Conclusions
We have composed a boosted biochar derived from an agrowaste: olive pit (OP) powder. Metallic salts of hydrated nickel and/or copper nitrate(s), as precursors, were mixed with OP, followed by pyrolysis, in one step, at 400 • C to generate biochar@CuNi (B@CuNi). The resulting biochar was characterized by SEM to show a homogenous surface with well-distributed CuNi nanoparticles on the surface of the biochar. EDX and XRD asserted the formation of CuNi nanoparticles as a metallic alloy decorating the biochar. Magnetization experiments accounted for the presence of nickel and demonstrated that nanocatalyst-decorated biochars behave as ferromagnets. B@CuNi showed a high catalytic performance in the decomposition of methyl orange (MO) dye.
The significance of this work lies in the valorization of agrowaste in general and the production of an enhanced biochar decorated with bimetallic nanocatalyst (CuNi) in a facile process in particular. This strategy of making nanocatlyst-doped biochar opens new avenues for water treatment and, therefore, attempts to elegantly contribute to environmental remediation, and thus to address UN Sustainable Development Goal 6 pertaining to clean water and sanitation.