# Maple wood biochar-supported nickel nanoparticles: One pot synthesis for simultaneous removal of cationic and anionic dye mixture

Arvind K Bhakta, Lamia Znaidi, Esther Blanc, Nabil Challab, Philippe Decorse, Arlette Vega Gonzalez, Dominique Vrel, Souad Ammar, Swaminathan Prasanna

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Arvind K Bhakta, Lamia Znaidi, Esther Blanc, Nabil Challab, Philippe Decorse, et al.. Maple wood biochar-supported nickel nanoparticles: One pot synthesis for simultaneous removal of cationic and anionic dye mixture. Next Sustainability, 2025, 6, pp.100124. 10.1016/j.nxsust.2025.100124. hal-05368250

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Research article

# Maple wood biochar-supported nickel nanoparticles: One pot synthesis for simultaneous removal of cationic and anionic dye mixture

Arvind K. Bhakta<sup>a,b,*</sup>, Lamia Znaidi<sup>a</sup>, Esther Blanc<sup>a</sup>, Nabil Challab<sup>b</sup>, Philippe Decorse<sup>b</sup>, Arlette Vega Gonzalez<sup>a</sup>, Dominique Vrel<sup>a,b</sup>, Souad Ammar<sup>b,c</sup>, Swaminathan Prasanna<sup>a,b</sup>

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## ARTICLE INFO

Keywords:

Magnetic nanoparticles

Sustainability

Biochar

Pyrolysis

Circular economy

## ABSTRACT

This work reports on a facile and efficient technique to synthesize nickel particles supported on maple wood biochar to form magnetic nanocomposite able to remove dyes like methyl orange (MO) and rhodamine B (RhB) from water. Methodology involves wet impregnation with aqueous nickel nitrate solution followed by a pyrolysis at 500 °C. This resulted in homogeneously and uniformly distributed irregular shaped Ni particles in the size range of 16 – 68 nm on the biochar surface. X-ray diffraction characterization confirms the presence of Ni particles and X-ray photoelectron spectroscopy reveals their slight surface oxidation. Their ability to answer to a magnetic field has been confirmed by recording the variation of their magnetization as a function of the magnetic field at room temperature using a Vibrating Sample Magnetometer. The G Raman band of the biochar appeared to be less intense in the nanocomposite, compared to pristine biochar prepared as a reference, meaning Ni nanoparticles hinder carbonaceous graphitization during pyrolysis. This can be attributed to the porosity creation, as suggested by SEM observations. Ultimately, magnetic nanocomposites were successfully applied for the removal of RhB and MO from aqueous solutions. The adsorption capacity for the RhB is found to be 54.21 mg/g. Thanks to the magnetic properties of the engineered nanocomposites; they can be separated by a magnet and reused. Reusability test of adsorbent have shown 99.41 %, 74.58 %, and 49.78 %, adsorption efficiency after first, second and third cycles, respectively. This work opens up a future perspective to create filters for treating waste water coming from textile industries. This work well aligns with UN Sustainable Development Goals 6 (clean water and sanitation), 11 (sustainable cities and communities) and 13 (climate action).

## 1. Introduction

Population growth compounded with extensive industrialization have played significant role in polluting worldwide water. As a result, utilization of synthetic dyes such as Rhodamine B (RhB) and methyl orange (MO), have become very common in printing industry, dyeing, textile, tannery and paint industry, cosmetics, food industry, dye manufacturing, paper and pulp industry and pharmaceuticals. Of all, textile industry is responsible for maximum pollution of water due to heavy consumption of water and discharge of coloring materials in effluent water, often without any treatment. Recent reports have shown that, 10,000 tons of dyes are used of which 100 s of tons end up in ecosystem through wastewater every year [1-3]. Most of these dyes are carcinogenic and cause skin irritation, allergic reactions and respiratory issues in humans [4-6]. Therefore much of the effort in recent years has

been devoted towards waste water treatment by efficiently removing these pollutants [7-9].

Among different strategies, adsorption is an effective way to remove water pollutants due to cost-effectiveness, ease of methodology and high efficiency [10-16]. There are different materials employed for the adsorption of dyes including pyrimidine based hydroxyl azo polymer [17], clays [18], graphene oxide/carbon nanotube nanostructures [19], Fe₂O₃ nanocrystals/MWCNTs [20], clew like Cu₂O/CuO microsphere [21], to mention few. Out of different adsorbent and catalyst, zero valent metals (ZVM) are very promising for water decontamination through the interplay of their synergic chemical, physical and catalytic mechanisms [22]. Further, materials with magnetic properties have certain advantage of being easy to separate after treatment through an external magnet, which is crucial for reusability [23,24]. Considering previous aspects, the use of ZVM magnetic nickel nanoparticles [25-27] would be

<footnote><sup>a</sup> Université Sorbonne Paris Nord, CNRS, LSPM UPR-3407, Villetaneuse 93430, France</footnote>

<footnote><sup>b</sup> Université Paris Cité, CNRS, ITODYS UMR-7086, Paris 75013, France</footnote>

<footnote>* Corresponding author at: Université Sorbonne Paris Nord, CNRS, LSPM UPR-3407, Villetaneuse 93430, France
E-mail address: arvind.bhakta@lspm.cnrs.fr (A.K. Bhakta).</footnote>
<table><caption>**Table 1**
Percentage yield of biochar and Ni modified biochar.</caption><thead><tr><th>Sample</th><th>Weight before pyrolysis (g)</th><th>Weight after pyrolysis (g)</th><th>% yield</th></tr></thead><tbody><tr><td>Maple wood</td><td>1.0140</td><td>0.2085</td><td>20.56 %</td></tr><tr><td>Maple wood impregnated with Nickel (II) nitrate hexahydrate</td><td>1.1530</td><td>0.1980</td><td>17.17 %</td></tr></tbody></table>

valuable for the target application. However, like any other nano-particles, due to high surface charge energy, they are prone to agglomeration. Thus, making a composite with suitable low cost supports, such as biochars [28], could solve this.

Biochars are carbon materials made by thermochemical process called pyrolysis of biomass which are mainly derived from plant or animal origin. It possesses high surface area, surface charge, tunable functional groups (-OH, C=O, -R-OH and -COOH), water holding capacity, porosity, etc [29–31]. This makes biochars also as suitable candidates for the plenty of applications including water treatment [14, 32–34].

So focusing on nickel cations wet impregnation technique of low-cost maple wood and its pyrolysis we were able to produce biochar-supported nickel nanoparticles composites to test them as adsorbents to remove two different classes of model pollutants, namely RhB (a cationic dye) and MO (an anionic dye) from contaminated water. These dyes have been chosen as they share similar chemical groups with major pollutants; Moreover, use of dyes in a study is extremely convenient as their suppression is very easy to follow [35] and the effectiveness of the engineered composites can be tested. To the best of our knowledge there are no such work reported in the literature.

## 2. Materials and methods

### 2.1. Chemicals

All the aqueous solutions are prepared from double distilled water. Maple wood waste was recuperated from the workshop of LSPM-CNRS, USPN, Villetaneuse, France. Nickel (II) nitrate hexahydrate was purchased from Aldrich.

### 2.2. Apparatus

Thermo-Scientific K Alpha + instrument was used for the X-ray photoelectron spectroscopy (XPS) analysis with pass energy maintained

<figure><img src="image_4.png" /><figcaption>Fig. 2. XRD patterns of (bottom) MWB-500 and (top) MWB@Ni-500.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 3. Raman spectra of (a) MWB-500, and (b) MWB@Ni-500.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 1. Overall steps involved for synthesis of biochar and Ni coated biochar for dye adsorption.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 4. SEM images of (a,b) MWB-500 and (c,d) MWB@Ni-500.</figcaption></figure>

<table><caption>**Table 2**
XPS atomic % composition of biochar and modified biochar.</caption><thead><tr><th></th><th>% C</th><th>% O</th><th>% N</th><th>% Ni</th></tr></thead><tbody><tr><td>MWB-500</td><td>90.04</td><td>8.93</td><td>1.03</td><td>0</td></tr><tr><td>MWB@Ni-500</td><td>88.59</td><td>8.60</td><td>1.98</td><td>0.83</td></tr></tbody></table>

at 80 eV and 200 eV for high-resolution spectra and survey scan, respectively. GEMINI ZEISS SUPRA 40VP microscope was used for Scanning Electron Microscopy (SEM) imaging. Raman spectroscopy was carried out on a HORIBA Spectrophotometer (ANDOR Technology model no. DU420-OE-322) equipped with a 473 nm laser and a OLYMPUS U-5RE-2 objective lens to focus on the surface sample. UV-Vis absorption spectra were recorded on a JENWAY 6850 UV/Vis Spectrophotometer and X-ray diffraction (XRD) patterns were collected on a INEL Equinox 1000 diffractometer equipped with a copper K-α₁ X-ray source.

## 2.3. Preparation of Nickel nanoparticles coated biochar

Firstly, maple wood pieces (MW) were cut and the waste generated while cutting (powder sample) were allowed to soak in Nickel (II) nitrate hexahydrate aqueous solution (1 mmol/g of biomass), then dried over night at 60 °C. This dried impregnated sample and pure maple wood powder were subjected to pyrolysis with the following conditions: ramp-20 °C/min, pyrolysis temperature- 500 °C, nitrogen gas flow rate- 1 L/min, residence time- 1 hour and cooling time- 1 h [36,37]. The obtained material was referenced as MWB@Ni-500 and MWB-500, respectively. The percentage yield is reported in the Table 1 and the synthesis scheme is shown in Fig. 1.

## 2.4. Dye removal applications

2 mg of MWB@Ni-500 (adsorbent) is added to 10 mL of RhB aqueous solution (5 ppm). Further, the solution is subjected to magnetic stirring (it was made sure that the magnetic field of the stirring was almost insufficient to prevent the attraction of Ni). The process is monitored by measuring the absorbance values at 553 nm using a UV-Vis spectrophotometer for 0, 2.5, 5, 7.5, and 10 min reactions to understand the kinetics of the reaction and to observe if it correlates with the Beer-Lambert law. Concentration variation studies have been also carried out whose details are given in the relevant section.

## 2.5. Overall steps involved

To summarize, in this work, the maple wood powders were impregnated with nickel nitrate solution, drying and followed by pyrolysis and applied for dye adsorption. The overall steps in the current methods have been given the Fig. 1.
<figure><img src="image_8.png" /><figcaption>Fig. 5. (a) XPS survey spectra of MWB-500 and MWB@Ni-500, and (b) Ni2p XPS high resolution spectrum of MWW@Ni-500.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 6. Hyteresis loop recorded on MWB@Ni-500 sample with a close to zero coercivity and remanence, which is typical of soft ferromagnet.</figcaption></figure>

# 3. Results and discussions

## 3.1. Crystal structure

XRD analysis was carried out to check the crystal structure of the produced samples. The 5 intense peaks, Fig. 2, are a clear indication of the presence of nickel in its classical fcc crystal structure. The peaks at 44.4°, 51.7°, 76.5°, 92.9° and 98.3° corresponds to (111), (200), (220), (311) and (222) planes of Ni (ICDD No.: 98-005-3808) [38]. The one remaining broad peak in the low 2θ range left corresponds to poorly crystalized material: the char.

To complete these investigations, Raman spectroscopy was performed to check the structural feature of the biochar part of the composite, focusing on the peaks at 1360 and 1593 cm⁻¹ (Fig. 3) corresponding to the D and G carbon bands [39], respectively. D band is referred for doubly resonant disorder induced mode. It arises from the C-C bonds stretching or k-point phonons breading mode of A₁g symmetry. G bond signifies the graphitic nature (sp² carbon) [40]. G band is very prominent in pristine biochar sample (Fig. 3a) while its intensity decreases after Ni nanoparticles incorporation. This gives an indication

that pure biochar was more graphitic structures with fewer defects but after surface modification and with development of new pores, a sit will be evidenced by SEM observations (Fig. 4), the graphitic phase has been ruptured.

## 3.2. Surface morphology

SEM observations show that biochar sample is slightly porous as seen in Fig. 4a, with in given regions, some particles like structures. This could be possible as the raw biomass powder was contaminated with small metal chips produced from numerous machining activities in the laboratory workshop. The porosity of the biochar is enhanced significantly after impregnation with nickel (Fig. 4). This reinforces that influence of the nickel on the pyrolysis of the wood is to create porosity [41]. As a catalyst, it promotes organic matter decomposition into volatile gas, which departs from the treated matter, leaving voids. This porosity increase explains the lower yield of the nickel-impregnated biochar as reported in Table 1. The Ni nanoparticles are homogeneously distributed on the biochar surface with size ranging from 16 to 68 nm. Fig. 4d shows that the particles have mixed morphology consisting of spherical, semispherical, cuboidal, pebbles and irregular shaped. The nanoparticles are both embedded in the biochar matrix as well as deposited on the surface.

## 3.3. Chemical composition

The chemical composition of the produced samples (Table 2) and the oxidation state of nickel in the composites were investigated using XPS. The pristine biochar mostly contains C, O and N, while its composite counterpart contains also Ni, indicating its successful incorporation (Fig. 5a). Further, XPS analysis shows this typical shape (Fig. 5b) signature of the surface oxidation of Ni nanoparticles [42–44].

The main metallic nature of the formed Ni nanoparticles is confirmed by the recorded variation magnetization as a function of the magnetic field at room temperature, with its typical sigmoidal shape (Fig. 6). It is much more consistent with soft ferromagnetic behavior of Ni, with a coercive field of about 11 Oe, than with the antiferromagnetic behavior of NiO (Fig. 6).

Note that the total magnetization of the produced composite remains low, but high enough to make it able to respond to an external magnetic field. This feature would be useful for the target application in the sorbent separation and reuse processing.
<figure><img src="image_10.png" /><figcaption>Fig. 7. (a) UV-Vis spectra of initial RhB solution and those contacted with MWB-500 and MWB@Ni-500 for 10 min. (b) Results of reusability tests conducted on MWB@Ni-500.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 8. Spectra of a dye mixture solution of Rh B and MO before and after contacting with MWB@Ni-500 of 60 min.</figcaption></figure>

## 3.4. Application of Ni coated biochar nanocomposite

### 3.4.1. Dye adsorption

The dye adsorption application was carried out using RhB as a pollutant model. UV-Vis spectra of RhB solution in presence of pristine biochar and its nickel based nanocomposite, for different contacting times (0–10 min) are shown in Fig. 7. It is found that Ni coated biochar adsorption compared to pristine biochar is almost 3.5 times faster (Fig. 7a). Reusability of the adsorbent are reported in Fig. 7(b). The adsorption capacity after first, second and third cycles were found to be 99.41 %, 74.58 %, and 49.78 %, respectively for this composite precisely.

Further, till focusing on MWB@Ni-500 sample, we have carried out dye mixture analysis (a mix of RhB and MO). We have performed UV-Vis analysis over time (Fig. 8), evidencing a better affinity for RhB dye, as the peak corresponding to the maximum absorbance of RhB completely disappears after 60 min of experiment, while the one corresponding to MO is only lowered. This result is also visible from the color of the solution before and after the trial: in the beginning the solution is a mix of pink and orange, while in the end only the orange color remains. This

difference in affinity for the adsorption of dye by modified-biochar nanocomposites can be explained by the surface charge of RhB dye: with the variety of functional groups involved in the chemical surface composition of the biochar part of the considered composite, leading to a negative surface charge making it more likely attracting cationic dyes, such as RhB, instead of anionic ones like MO.

### 3.4.2. Kinetics

Recording the UV-Vis absorption spectra of a RhB aqueous solution contacted with MWB@Ni-500 powder over time allows studying the adsorption kinetics. Obviously, the intensity of RhB UV-Vis absorption band decreases with the time (Fig. 9a), in agreement with the adsorption of the dye on the MWB@Ni-500 surface. The amount of adsorbed RhB on the modified biochar was obtained using the mass balance equation [20]:

$$q_t = V (C_0 - C_t) / W \quad (1)$$

V (L) = Volume of the RhB solution, W (g) = Mass of involved MWB@Ni-500 powder, q_t (mg/g) = amount of adsorbed RhB at time t, C_0 and C_t (mg/L) = initial concentration of RhB in the liquid-phase as measured from UV-Vis spectroscopic analysis at time t, respectively.

$$t/q_t = 1/K_2 q_e^2 + t/q_e \quad (2)$$

Where q_e (mg/g) = equilibrium concentration of adsorbate

K_2 (g/mg/min) = pseudo second order rate constant of adsorption.

The plot of t/q_t vs. t gives linear relationships (Fig. 9b) indicating pseudo-second order kinetics.

### 3.4.3. Adsorption capacity

RhB adsorption on MWB@Ni-500 assays were performed for different aqueous solutions with different concentrations: 5 ppm, 10 ppm, 15 ppm, 20 ppm, and 25 ppm. The corresponding adsorption capacity was found to be 24.85 mg/g, 39.88 mg/g, 54.21 mg/g, 38.08 mg/g, and 22.62 mg/g, respectively. At low concentration of dye, there is huge availability of active site at adsorbent surface and within the pores to adsorb the dye. With increase in dye concentration, the adsorption capacity increases and reaches a certain point where adsorbent get saturated with dye and further adsorption not possible. Usually, the adsorption capacity value should remain same after reaching saturation as no further adsorption possible due to the repulsive forces
<figure><img src="image_12.png" /><figcaption>Fig. 9. (a) %Rh B removed with time (inset: photograph of the cuvette containing solution after adsorption), (b) Kinetic study (plot of t/q<sub>t</sub> vs. t).</figcaption></figure>

<caption>**Table 3**</caption>

Comparison of performances of different materials in literature from current one.

<table><thead><tr><td>Name of adsorbant</td><td>Synthesis Method</td><td>Adsorption capacity (mg/g)</td><td>Reference</td></tr></thead><tbody><tr><td>Egg shell</td><td>Drying and grinding</td><td>20.70 mg/g</td><td>2025 [46]</td></tr><tr><td>Punica granatum peel</td><td>Peel drying and grinding</td><td>44.28 mg/g</td><td>2025 [12]</td></tr><tr><td>ZnO nanoparticles on graphene oxide nanosheets</td><td>Hummer's method + Ultra-sonication</td><td>47.62 mg/g</td><td>2024 [47]</td></tr><tr><td>nAg-AC (cellulosic biochar-based nanocomposite)</td><td>Pyrolysis + ball milling + impregnation</td><td>46.38 mg/g</td><td>2022 [48]</td></tr><tr><td>Polyaniline/tea waste char nanocomposites</td><td>Coating of polyaniline onto carbonized tea waste material</td><td>34.93 mg/g</td><td>2024 [49]</td></tr><tr><td>Zeolite/Bi<sub>2</sub>O<sub>3</sub> nanocomposite</td><td>Sol-gel</td><td>0.009 mg/g</td><td>2024 [50]</td></tr><tr><td>MWW@Ni-500</td><td>Wet impregnation + Pyrolysis</td><td>54.21 mg/g</td><td>This work</td></tr></tbody></table>

between the dyes but as there may possibility of mass transfer limitation. In this scenario, high dose of dye generate concentration gradients, this phenomenon resists the diffusion of dye molecule into the pores of adsorbent and creates competition on the adsorbent surface. This explains the reason for decrease in adsorption capacity in our case after reaching saturation which is in accordance with literature [45].

### 3.4.4. Comparison of the adsorption efficiency of the nanocomposite with the literature

The present method was compared with the ones reported in the literature (Table 3), and found that it exhibits an adsorption capacity higher or similar to those reported. Besides, this method has another advantage that of cost-effectiveness and simplicity with a wider applicable potential (catalysis...). The unique limitation of the present work is that the prepared nanocomposite is more selective to cationic than anionic dyes. This could be tricky for simultaneous removal of cationic and anionic dyes in real aqueous samples. This problem may be overcome by playing with the pH which will affect the surface charge and allows tuning the selectivity.

## 4. Conclusions

Porous biochar based nanocomposite, loading magnetic Ni nano-particles on their surface have successfully been prepared with the combination of wet impregnation followed by pyrolysis at moderate temperature. This one pot synthesis resulted in irregular shaped small particles, whose size ranges between 16 and 68 nm, homogenously distributed over the biochar surface. Valorization of trash to treasure principle is demonstrated by utilizing current magnetic composite as an efficient adsorbent for pollutant models like RhB (cationic) and MO (anionic) dyes. Interestingly, contacted to an aqueous solution of a mixture of the two dyes, the prepared composites exhibit a higher adsorption affinity towards cationic dye. Besides, these adsorbents can be reusable, after magnetic separation, up to two times with descent adsorption. After two cycles, a drastic reduction in adsorption efficiency has been noticed. This work also helps in achieving UN Sustainable Development Goals 6, 11 and 13. It opens up new perspective in the field of water treatment of real aqueous samples in real world condition.

## Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

The authors would like to acknowledge of Labex SEAM (Science and Engineering for Advanced Materials and devices), ANR-10-LABX-096 and ANR-18-IDEX-0001 grant for its financial support. They want to acknowledge too Rendy Prastiko for giving the formation on UV-Vis spectrophotometer, Nicolas Fagnon for providing wood powder sample, Maria Konstantakopoulou for assisting in obtaining SEM images.

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