# Efficacy of magnetic APTES-modified hydrochar for remediation of Cr(VI) from water-base solutions

T. Matamela, A. Pholosi, Julien Vieillard, V. Pakade

► To cite this version:

T. Matamela, A. Pholosi, Julien Vieillard, V. Pakade. Efficacy of magnetic APTES-modified hydrochar for remediation of Cr(VI) from water-base solutions. MRS Advances, 2025, 10, pp.2486-2492. 10.1557/s43580-025-01343-3. hal-05421808

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# Efficacy of magnetic APTES-modified hydrochar for remediation of Cr(VI) from water-base solutions

T. Matamela<fnref n="1" />☐A. Pholosi<fnref n="1" />·J. Vieillard<fnref n="2" />·V. E. Pakade<fnref n="3" />

Received: 31 January 2025 / Accepted: 7 July 2025 / Published online: 25 July 2025

© The Author(s) 2025

## Abstract

This study investigates an innovative approach for chromium removal using a novel adsorbent synthesized from macadamia nutshell biomass. Hydrochar derived from macadamia nuts was functionalized with magnetite ($Fe_3O_4$) nanoparticles and further grafted with 3-aminopropyltriethoxysilane (APTES) to enhance its adsorption capacity. Characterization was conducted using techniques such as FTIR, XRD, and XPS to confirm the functional groups on the adsorbent. The batch adsorption experiments showed that the material was highly efficient in removing Cr(VI) under different pH, contact time, adsorbent dosage, and initial chromium concentration conditions. The results showed that APTES-modified hydrochar had a higher chromium adsorption capacity than unmodified hydrochar, reaching 420.8 mg/g as the highest adsorption capacity at pH 2. The kinetic adsorption data fitted best in a rate model of pseudo-second order. This study shows that APTES-modified hydrochar has the potential to be used as a sustainable and cost-effective adsorbent for chromium remediation in aqueous solutions.

## Introduction

Hexavalent chromium [Cr(VI)] represents the highly dangerous chromium compound which creates major environmental threats and health hazards. The industrial sectors of production of dyes, leather tanning, and electroplating produce Cr(VI) as a priority pollutant through their industrial wastewater. [1, 2]. The toxicity of chromium varies depending on its oxidation states, with Cr(VI) identified as the most hazardous form [3]. Its high solubility and capacity to cross biological membranes make it a significant concern [2]. Cr(VI) poses risks of carcinogenic and mutagenic effects on living organisms [4]. The human body can develop lung and kidney cancers and gastrointestinal problems when exposed to Cr(VI) [1]. The World Health Organization sets a maximum drinking water concentration of 0.05 mg/L [5].

Different methods have been investigated to extract Cr(VI) from polluted water systems. The methods for Cr(VI) removal from contaminated water systems include chemical precipitation, membrane filtration, ion exchange resins, photocatalytic reduction, adsorption, and biological systems [6]. Among these methods, adsorption has become increasingly popular for the removal of Cr(VI) from contaminated water environments because adsorption is simple, effective, and affordable [7–11]. The adsorption process cost-effectiveness relies on the adsorbent material selection [12] so researchers have employed traditional adsorbents including biosorbents [13], activated carbon [14], silica [15], and alumina [16] to extract Cr(VI) from water solutions. To improve the efficiency of the adsorbents, various surface modification strategies have been adopted, and the incorporation of magnetite nanoparticles is one such strategy [1, 17, 18]. Magnetite nanoparticles ($Fe_3O_4$) offer remarkable properties such as superparamagnetism [19], low toxicity to humans [20], large surface area-to-volume ratio [20], ease of production [20], and ease of separation [21]. However, magnetite can oxidize to other forms of iron and agglomerate; therefore, stabilization is necessary.

Hydrochar is an emerging carbon-rich adsorbent prepared from hydrothermal carbonization of biomass at low temperatures [22–24]. Along with several other group kinds, the surface of hydrochar comprises compounds that contain oxygen functional groups, for instance carboxyl, hydroxyl,

<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>

<footnote>T. Matamela
tshifhiwa@vut.ac.za</footnote>

<footnote n="1">¹ Department of Natural Sciences, Vaal University of Technology, Vanderbijlpark, South Africa</footnote>

<footnote n="2">² Univ Rouen Normandie, INSA Rouen Normandie, Univ Caen Normandie, ENSICAEN, CNRS, Institut CARMEN UMR 6064, 55 Rue Saint Germain, 27000 Evreux, France</footnote>

<footnote n="3">³ Department of Chemistry, University of South Africa, UNISA, Florida 1709, South Africa</footnote>
and carbonyl groups [5]. In addition, hydrochar has a low ash content and is hydrophobic [25]. The hydrophobicity character of hydrochar limits its application as an adsorbent in aqueous solutions. The surface functional properties of hydrochar receive attention through multiple strategies which include heteroatom doping [25], organic substance grafting [26, 27], and immobilization of Fe₃O₄ nanoparticles [19]. The surface functionality and morphology of hydrochar determine its surface modification behavior which affects the adsorption performance[23]. For instance, functionalization of hydrochar with nitrogen-containing groups has yielded high efficient adsorption of Cr(VI) with reported adsorption capacities of 424.09 mg/g [28] and 323.25 mg/g [25]. It is against this backdrop that the present study explores the grafting of 3-aminopropylethoxysilane (APTES) onto the surface of hydrochar containing magnetic for evaluation in the adsorptive removal of Cr(VI). APTES is a non-toxic, commercially available and high reactive source of aminoalkylsilane groups which covalently bonds to hydroxyl groups of the substrate during its grafting [29]. The amino group of the APTES may bond with Cr(VI) through ion exchange and surface complexation [30]. Several studies have reported the efficacy of APTES-modified adsorbents to sequestrate various metal ions [17, 31]. Fotsing et al. [31] have demonstrated that grafting APTES onto cocoa shell-derived hydrochar is a potential adsorbent for the elimination of Cr(VI) from wastewater. The current research demonstrates that hydrochar derived from macadamia nutshell modified with APTES and magnetite ions functions as an effective adsorbent for Cr(VI) removal.

## Experimental

### Reagents and chemicals used

Without any additional purification, all of the chemicals and reagents utilized in this investigation were of analytical quality. Potassium dichromate (≥ 99.0%), 1,5'-diphenylcarbazide (≥ 99.0%), hydrochloric acid (37%), ferrous chloride hexahydrate (98%), ferric chloride tetrahydrate (97%), potassium hydroxide (≥ 85.0%), sodium hydroxide (≥ 97.0%), and 3-aminopropyltriethoxysilane (≥ 98.0%) were all purchased from Merck Chemical Co (Johannesburg, South Africa).

### Collection of macadamia nutshell (MNS)

Shells of macadamia nuts (MNS) that had been dried and cracked were donated by Danroc (Pty) (Ltd) from Barberton, South Africa. The collected shells were soaked in a solution of 1 mol/L HCl for 24 h and then washed with ultrapure water to eliminate contaminants and debris. After cleaning, the shells

were dried for 24 h at 80 °C in an oven to obtain a dry and stable MNS sample. After drying, the MNS were crushed and sieved into distinct size fractions to obtain a biomass. The biomass of MNS samples were stored in closed containers when not in use.

### Preparation of a hydrochar

Hydrochar was produced using a hydrothermal carbonization process adapted from Vieillard et al. [26]. Briefly, the Teflon-lined autoclave received 5 g of MNS biomass which was submerged in 50 mL of ultrapure water. The Teflon-lined autoclave was sealed tight and positioned in a furnace at 240 °C for 24 h. The solid material underwent filtration followed by ultrapure water washing and 24-h drying at 70 °C in an oven. Hydrochar (HC) was stored in a closed container for further use.

### Preparation of HC-Fe₃O₄

The synthesis of magnetized hydrochar occurred through Fe(III) and Fe(II) salt co-precipitation using a method described by Fatehi et al. 2017 [17] with slight modifications. Specifically, 200 mL of ultrapure water was used to dissolve the mixture of 5.4 g ferrous chloride hexahydrate (FeCl₃·6H₂O) and 4 g ferric chloride tetrahydrate (FeCl₂·4H₂O), which was then refluxed for 2 h at 70 °C in an inert nitrogen environment. Approximately, the Fe²⁺/Fe³⁺ solution received 5 g of HC to establish a 1:1 ratio before undergoing 30 min of stirring. The black precipitate HC-Fe₃O₄ formed best at pH levels between 10 and 11 through the addition of 5 mol/L NaOH. The HC-Fe₃O₄ precipitate was separated by magnet after 24 h and then washed multiple times with ultrapure water and ethanol before drying at 60 °C overnight. The magnetite nanoparticles formed through the following chemical reaction:

$$2\text{FeCl}_3 \cdot 6\text{H}_2\text{O} + \text{FeCl}_2 \cdot 4\text{H}_2\text{O} + 8\text{NaOH} \\ \rightarrow \text{Fe}_3\text{O}_4 + 8\text{NaCl} + 14\text{H}_2\text{O}$$

### Grafting of HC-Fe₃O₄ with APTES

Briefly, the surface modification of HC-Fe₃O₄ with 3-aminopropyltriethoxysilane (APTES) required adding 0.3 wt% of APTES to a water–ethanol mixture (1:3 V/V) containing pre-weighed mass of HC-Fe₃O₄ [31]. The mixture underwent stirring at 80 °C for 6 h. The resulting HC-Fe₃O₄-APTES underwent cleaning and filtration before drying at 80 °C for 12 h.
## Characterization of hydrochar and modified hydrochar

The FTIR spectra of synthesized hydrochar (HC) and modified hydrochar (HC-Fe₃O₄-APTES) were recorded using a Perkin-Elmer infrared spectrophotometer (Thermo Scientific, Smart iTX, Nicolet 1550), covering a wavelength range of 4000 cm⁻¹ to 400 cm⁻¹. The analysis revealed the fundamental functional groups present in the materials. The X-ray diffraction (XRD) analysis of crystallographic structure was performed using an X'Pert PRO diffractometer (XRD-700-XRAY Diffractometer) at 45 kV and 40 mA over a 2θ range of 10° to 80° for 20 min with a step size of 0.02°. The instrument was used with a wide-angle goniometer, a high-intensity Cu X-ray tube with a wavelength of 1.54 Å, and a scintillation counter detector. The surface chemical oxidation was determined using X-ray photoelectron spectroscopy (XPS) (PHI 5000 Scanning ESCA Microscope).

## Batch adsorption

The effectiveness of hydrochar and its modified versions—HC-Fe₃O₄ and HC-Fe₃O₄-APTES—was evaluated through batch adsorption experiments for chromium (VI) ion removal from aqueous solutions. The research investigated the best conditions for adsorption including adsorbent amount and pH level and temperature and solution amount and time to reach equilibrium. The stock solution of 1000 mg/L Cr(VI) was created through the dissolution of potassium dichromate (K₂Cr₂O₇) in ultrapure water after drying. The research evaluated Cr(VI) adsorption at different initial solution pH values ranging from 2.0 to 9.0. The solution pH received adjustments using 0.1 mol/L HCl or NaOH solutions. The adsorbent dose ranged between 0.01 g and 0.10 g, while the

initial Cr(VI) solution concentrations spanned from 10 mg/L to 150 mg/L. The amount of Cr(VI) adsorbed at equilibrium concentration, qe (mg/g), was calculated using Eq. 1. The concentration of Cr(VI) was measured using the calorimetric method described in Pakade et al. 2016 [32].

$$qe = \frac{Co - Ce}{m} \times V \quad (1)$$

The equilibrium and initial concentrations of Cr(VI) are represented by Ce and Co (mg/L). The variable m (g) represents the mass of the adsorbent and V (L) represents the volume of the solution.

## Results and discussion

### Adsorbent characterization

The FTIR spectra of HC, HC-Fe₃O₄, and HC-Fe₃O₄-APTES are presented in Fig. 1a. The HC spectrum exhibits a broad absorption peak at 3352 cm⁻¹ attributed to the O-H stretching vibrations of cellulose, hemicellulose, and lignin structures. The C-H stretching vibrations of aliphatic C-H bonds produce absorption peaks at 2911 and 2800 cm⁻¹ [5, 26]. The absorption peaks at 1686, 1588, and 1046 cm⁻¹ correspond to carboxyl C=O [19], C=C stretching vibration in aromatic rings [23], and lignin C-O-C groups [12, 33]. The Fe₃O₄ addition to HC resulted in two distinct peaks at 577 cm⁻¹ and 3322 cm⁻¹ which correspond to Fe-O and O-H stretching vibrations. The C=O peak at 1686 cm⁻¹ in HC shifted to 1697 cm⁻¹ in HC-Fe₃O₄, while its intensity decreased. The C=O group participated in the immobilization process of Fe₃O₄ onto HC substrate. The inclusion of -NH₂ groups resulted in a new peak at 3665 cm⁻¹ in the HC-Fe₃O₄-APTES spectrum. An increase in the intensity and a

<figure><img src="image_4.png" /><figcaption>Fig. 1 a FTIR spectra of HC, HC-Fe₃O₄, and HC-Fe₃O₄-APTES; b XRD spectra of HC, HC-Fe₃O₄, and HC-Fe₃O₄-APTES</figcaption></figure>
slight shift of C–O–C peak (1046 cm⁻¹) were also observed due to the presence of Si–O–HC bond which may have overlapped with the C–O–C bond of the backbone structure [6].

Figure 1b shows the XRD diffraction patterns of the HC, HC-Fe₃O₄, and HC-Fe₃O₄-APTES. In the HC material, a broad diffraction peak at 21.7° was observed. This diffraction peak was attributed to the presence of amorphous cellulose structure [25] due to a very low degree of carbonization used. HC-Fe₃O₄ displays distinct diffraction peaks at 2θ = 37.6°, 43.9°, 57.64, and 64.4°, corresponding to the crystalline planes with indices (311), (400), (440), and (444), respectively [27]. The diffraction peaks of HC-Fe₃O₄-APTES were found to be similar to those of HC-Fe₃O₄, but with a lower intensity. The addition of APTES did not change the crystal structure of the HC-Fe₃O₄.

Figure 2 displays the XPS data for HC-Fe₃O₄. Three peaks that correspond to C 1 s, O 1 s, and Fe 2 p were visible in the survey scan spectrum (Fig. 2a) of HC-Fe₃O₄, suggesting that modified HC contained the elements of C, O, and Fe. The deconvoluted high-resolution C 1 s spectra (Fig. 2b) of HC-Fe₃O₄ show three peaks at 284.8, 286.3, and 288.7 eV, corresponding to C–C, C–O, and C=O bonds, respectively [5, 33]. Figure 2c shows the O 1 s spectra of HC-Fe₃O₄-APTES. The peaks at 533.3 eV and 531.9 eV signified the presence of oxygen-bearing functional groups containing C=O and C-O bands [28].

Figure 2d displays the Fe2p spectrum of HC-Fe₃O₄-APTES and the deconvoluted spectrum which shows a peak at 710 eV attributed to FeO [33].

## Adsorption studies

Figure 3 depicts the batch adsorption data for Cr(VI) removal by HC, HC-Fe₃O₄, and HC-Fe₃O₄-APTES, while varying the pH solution, amount of adsorbent used, initial concentration of Cr(VI), and duration of contact time. In Fig. 3a, it can be noted that the effectiveness of removal or adsorption capacity (qe) decreased when the pH was raised from pH 2 to pH 9. Thus, the removal efficiency of hexavalent chromium was observed to be maximized at lower pH levels (pH 2). Under low pH conditions, hexavalent chromium (Cr(VI)) mainly exists as potassium dichromate (Cr₂O₇²⁻) and hydrogen chromate (HCrO₄⁻) [5, 25]. The elevated concentration of hydrogen ions (H⁺) leads to the protonation of the adsorbent surface, increasing its positive charge. This enhancement facilitates effective electrostatic attraction [25], allowing the negatively charged chromate ions to easily bind to the protonated surface. As pH increases, Cr(VI) predominantly exists as chromate ion (CrO₄²⁻), carrying a negative charge, while the adsorbent surface also becomes increasingly negatively charged, leading to greater repulsion [3]. Figure 3b shows the impact of dosage of the adsorbent. When the adsorbent dosage increases, it tend to decrease adsorption capacity.

<figure><img src="image_5.png" /><figcaption>Fig. 2 a XPS spectra of HC-Fe3O4-APTES, b XPS spectra of C1s, c XPS spectra of O1s, d XPS spectra of FeO</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 3** Batch adsorption results; a effect of pH, b effect of concentration, c effect of dosage, d effect of contact time on HC, HC-Fe₃O₄, and HC-Fe₃O₄-APTES</figcaption></figure>

According to Eq. 1, the adsorption capacity (qe) and mass (m) are inversely proportional, meaning that using higher dosages of adsorbents in low concentrated solutions could be a waste of resources. This is because, as the dosage concentration is increased, the amount of adsorption sites also increases but the amount of adsorbate in solution is fixed. Thus, the adsorption capacity decreases because there is not enough adsorbate to saturate all active sites.

Figure 3c shows the results illustrating the impact of the initial concentration of Cr(VI). As the concentration of Cr(VI) rose from 25 mg/L to 200 mg/L, the qe increased as well, appearing to plateau between 200 and 300 mg/L. It is possible to explain the rise in qe at lower concentrations by assuming that the concentration gradient is what drives the Cr(VI) ions to the adsorbent's active sites[17, 33]. Nevertheless, at elevated concentrations (between 200 and 300 mg/L), the adsorption capacity stabilized as most active sites became saturated. The impact of contact time on Cr(VI) adsorption by HC, HC-Fe₃O₄, and HC-Fe₃O₄-APTES is depicted in Fig. 3d. The influence of chromium (VI) adsorption was measured from 5 to 360 min of contact time. Early

in the adsorption process, there is a noticeable sharp rise in the amount of Cr(VI) adsorbed, which is followed by a gradual increase until a constant stage was reached. This is because, initially, the adsorbent sites were unoccupied, and there was rapid uptake. However, as the adsorption progressed with time, the surface becomes saturated with chromium molecules on the adsorbents, leading to a slower uptake and eventually saturation.

To further understand how quickly adsorption occurs, pseudo-first-order (PFO) and pseudo-second-order (PSO), two popular kinetic models, were employed to analyze the experimental data for Cr(VI) adsorption onto HC, HC-Fe₃O₄, and HC-Fe₃O₄-APTES adsorbents. Table 1 displays the correlation coefficient data (R²) and adsorption kinetic parameters acquired from the adsorption of Cr(VI) onto HC, HC-Fe₃O₄, and HC-Fe₃O₄-APTES adsorbents. From Table 1, the R² values for all three adsorbents were greater in the PSO model than in the PFO model. The estimated capacity for equilibrium values of the PSO rate model was closest to the experimental equilibrium capacities. Additionally, for every adsorbent under study, the R² values from the PSO
<table><caption>**Table 1** Kinetic data for Cr (VI) adsorption onto HC, HC-Fe₃O₄, HC-Fe₃O₄-APTES</caption><thead><tr><th>Kinetic model</th><th>HC</th><th>HC-Fe3O4</th><th>HC-Fe3O4-APTES</th><th>100 mg/L-HC-Fe3O4-APTES</th><th>150 mg/L-HC-Fe3O4-APTES</th></tr></thead><tbody><tr><td>Pseudo-first order</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Exp. q (mg/g)</td><td>31.99</td><td>36.79</td><td>40.89</td><td>78.47</td><td>100.63</td></tr><tr><td>Model q (mg/g)</td><td>29.19</td><td>31.927</td><td>36.137</td><td>70.34</td><td>91.64</td></tr><tr><td>k1 (g/mg min)</td><td>0.1518</td><td>0.1113</td><td>0.117</td><td>1.827</td><td>2.5082</td></tr><tr><td>R<sup>2</sup></td><td>0.919</td><td>0.811</td><td>0.818</td><td>0.8993</td><td>0.9240</td></tr><tr><td>Pseudo-second order</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Exp. q (mg/g)</td><td>31.99</td><td>36.79</td><td>40.89</td><td>78.47</td><td>100.63</td></tr><tr><td>Model q (mg/g)</td><td>30.97</td><td>35.42</td><td>40.22</td><td>74.56</td><td>96.25</td></tr><tr><td>k2 (g/mg min)</td><td>0.01814</td><td>0.00181</td><td>0.00696</td><td>0.0377</td><td>0.0425</td></tr><tr><td>h</td><td>17.39</td><td>22.70</td><td>11.58</td><td>49.14</td><td>120.33</td></tr><tr><td>R<sup>2</sup></td><td>0.9865</td><td>0.9754</td><td>0.987</td><td>0.9658</td><td>0.9722</td></tr></tbody></table>

model were greater than those from the PFO model. This indicates a better fit between the kinetic data and the PSO kinetic model.

## Conclusion

The hydrochar was prepared from macadamia nutshell using a hydrothermal carbonization method. The hydrochar was subsequently modified with magnetite and APTES to obtain a nanocomposite, HC-Fe₃O₄-APTES, and it was utilized to remove Cr(VI) from water-based solutions. Grafting APTES plus incorporation of Fe₃O₄ nanoparticles on the HC was validated by FTIR, XPS, and XRD studies. Peaks attributed to crystalline planes of Fe₃O₄ were observed at 2θ = 37.6°, 43.9°, and 64.4° across the HC-Fe₃O₄ and HC-Fe₃O₄-APTES XRD patterns. Additionally, it was demonstrated that the addition of APTES had no effect on the crystal structure of HC-Fe₃O₄.

Functional groups C-O, C=O, O-H, Fe-O, Si-O, and NH₂ were detected on the modified hydrochar by FTIR and XPS analysis.

At pH 2, all adsorbents reached their maximal adsorption capabilities, with a mass of 0.06 g and a contact time of 120 min. The pseudo-second-order model fit the adsorption data quite well. For environmentally friendly water treatment applications, the HC-Fe₃O₄-APTES adsorbent is a viable option.

**Acknowledgments** This research was funded by the National Research Foundation (NRF) of South Africa Grant No: (138388 and 150491) and Vaal University of Technology Research Directorate.

**Author contributions** All authors have contributed to the creation of the concept, the draft, the revision, and the accepted submission.

**Funding** Open access funding provided by Vaal University of Technology. This research was funded by the National Research Foundation (NRF) of South Africa Grant No: (138388, 150491) and Vaal University of Technology Research Directorate. This work was also supported by the University of Rouen Normandie, INSA Rouen Normandie, the Centre National de la Recherche Scientifique (CNRS), European Regional Development Fund (ERDF), Labex SynOrg (ANR-11-LABX-0029), Carnot Institut I2C, the graduate school for research XI-Chem (ANR-18-EURE-0020 XL CHEM), and the PHC program Protea (48919XL) supported by Region Normandie and the Grand Evreux Agglomeration.

**Data availability** The corresponding author is prepared to provide the datasets generated and analyzed during this study upon reasonable request.

## Declarations

**Conflict of interest** The authors affirm their commitment to transparency, declaring that they have no conflicts of interest concerning this article.

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