Industrial textile removal using date pit assisted CuO-MgO nanocomposite: Molecular dynamics and biosynthesis analysis

2.1 Methylene blue dye interactions with CuO-MgO nanocomposite: molecular dynamics simulation

Molecular dynamics simulation is a technique for exploring conformational space using various sized molecules, including proteins, active compounds, and nanoparticles (Akbari et al., 2020; Hirel, 2015). The energy surface is studied in molecular dynamics by solving Newton's laws of motion for the system. In this contribution, a molecular dynamics simulation is used to investigate the interactions of methylene blue (MB) dye with the CuO-MgO nanocomposite and water as the universal solvent.

To run the molecular dynamics simulations, the nanocomposite of CuO-MgO containing two CuO and MgO nanoparticles in cubic shapes with a long side of 15 A˚ for each, created by Atomsk (Abraham et al., 2019) (see Fig. 1). The nanocomposite placed in the centre of a simulation cubic box and 20 molecules of MB dye were put into the box at random positions with a minimum distance of 1 nm from the simulation box's edge sides (Fig. 1). The simulation box was then filled with 7000 water molecules. GROMACS 2019 software (Huang and MacKerell, 2013) was used to perform all MD simulations under periodic boundary conditions (PBCs), with the CHARMM36 force field (Jorgensen et al., 1983) and SPC water model (Adcock and McCammon, 2006). TIP3P water molecules dissolved the whole complex. The energy of the system was minimized using the steepest descent minimization algorithm for all atoms (Hess et al., 1997).

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Fig. 1

Schematic view of CuO-MgO nanocomposite surrounded with 20 MB dye molecule and water molecules in a simulation box.

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The system was equilibrated in the NVT ensemble (constant number of particles (N), volume (V), and temperature (T)) coupled to the V-rescale thermal bath at 300 K over 100 ps, as well as the NPT ensemble (constant number of particles (N), pressure (P), and temperature (T)) coupled to the Berendsen pressure bath at 1 atm over 200 ps. The system was then exposed to a 100 ns molecular dynamics (MD) simulation with a time step of 2 fs under constant circumstances of 1 atm, and 300 K. The LINCS algorithm (Essmann et al., 1995) was used to constrain H-bond lengths. The particle mesh Ewald (PME) (Humphrey et al., 1996) was used to apply long-range electrostatics. GROMACS tools were used to analyze the trajectory data, and VMD 1.9.3 was used for molecular graphics and visualization (Vanommeslaeghe and MacKerell, 2012). The force field parameters of MB dye were acquired using CHARMM CGenFF (Zhao et al., 2007), and the Lennard Jones parameters for CuO and MgO nanoparticles were obtained from previously published data (Zhao et al., 2018; Frisch and Trucks, 2009). The electronic structure calculations, including charge distribution and electrostatic potential maps of MB dye, were performed using the Gaussian program, version 09 (Bakayoko et al., 2020). The geometry of MB dye was optimized at the B3LYP/6-311++g(d,p) level of theory.

2.2 Photochemical interactions with CuO-MgO nanocomposite: Bio-synthesis process

Date pits, Magnesium acetate tetrahydrate (Mg(CH₃COO)₂·4H₂O), and cupric acetate (C₄H₈CuO₅) of analytical grade are purchased from ‘cash n carry’ shop (strand, South Africa), Sigma-Aldrich, and BDH Chemicals Ltd (UK, England) respectively. Extraction of the bioactive compounds in the agro-waste products was done following the method described in Jorgensen and Duffy (2002). Briefly, in a typical experimental set-up, 30.0 g of dried clean date pits were brought to boil in 150 ml of distilled water under constant magnetic stirring at temperature 70 °C for two hours. The obtained yellowish aqueous extracts with a pH of 4 were filtrated twice under 55 mm Whatman filter paper once cooled. The extracts were kept at ambient temperature, ready to be used for biosynthesis.

Green nanochemistry bio-synthesis of CuO-MgO nanocomposites is achieved by dissolving 5.0 g of each precursor into 10 ml distilled water. Subsequently, both solutions were immersed simultaneously in 100 ml agro-waste aqueous extracts. A noticeable deep green color was observed, and a pH of 6 was recorded. The mixture was allowed to stir for 2 h under ambient temperature and later left in the oven to dry. Following this step, the dried precipitates were thermally oxidized under air in a laboratory furnace for 2 h. The annealed samples are characterized instantly for structural, optical, functional, and catalytic properties.

2.3 Photocatalysis activity of CuO-MgO nanocomposites

The photocatalysis activity of date pit mediated CuO-MgO nanocomposite was studied with methylene blue (MB) under visible light irradiation. Initial 0.0027 g of methylene blue dye was added to 500 ml of distilled water and stirred in the dark to attain a uniform solution. Two sets of experiments were performed, one with MB only under visible irradiation and the other with the addition of catalysis to MB under visible light irradiation. A volume of 100 ml of MB was used for each set of experiments. UV–Vis absorbance measurements in wavelength ranging from 200 to 800 nm are recorded every 15 min.

The first experiment is conducted with 100 ml of MB solution exposed to visible light irradiation. A 4 ml of the solution was taken after every 15 min for 180 min, and UV–Vis absorbance data was recorded.

The second set of experiments describes the photocatalysis degradation of CuO-MgO nanocomposite with methyl blue under visible light irradiation. A volume of 100 ml of MB solution was added to a mass of 0.01 g of CuO-MgO nanocomposite. The mixture was allowed to stir at 120 rpm in the dark for 30 min to build adsorption–desorption equilibrium. Before the mixture is exposed to visible light irradiation, at t = 0, UV–Vis absorbance measurement was recorded. Subsequently, mixture aliquots (∼4 ml) in contact with visible light irradiation were taken every 15 min for 150 min.

4.1 Computational analysis: root mean square displacements (RMSD)

Fig. 2 shows the root mean square displacements (RMSD) of active compounds during simulations indicating whenever the analyzed systems have achieved their equilibrium states. The spectra demonstrate that in simulations with significantly reduced RMSD fluctuations, all systems attain their equilibrium state before 22 ns. As a result, the MD trajectories extracted after 22 ns are used for further analysis.

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Fig. 2

RMSD plot of MB dye with respect to CuO-MgO nanocomposite. The MD trajectories after 22 ns have been taken for further analysis.

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4.2 Computational analysis: charge distribution map

The charge distribution and electrostatic potential map of MB dye in Fig. 3 illustrate the active sites of this molecule to interact with the nanocomposite. The Hirshfeld point charges and electrostatic potential map for the optimized structure were produced using DFT calculations with water as the solvent. The results indicate that the most reddened region (vicinity of the atom N₁) in the charge distribution map corresponds to the active sites of molecules interacting with the nanoparticles.

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Fig. 3

Charge distribution (Hirshfeld point charges) of MB dye and its electrostatic potential map. The geometry optimization of MB dye was carried out at the B3LYP/6-311++g(d,p) level of theory.

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4.3 Computational analysis: Radial Distribution Function (RDF)

The left structure in Fig. 4(a) represents the initial structure of the system before simulation, whereas the right structure represents the structure of the system following a 100 ns molecular dynamics simulation. As shown in the figure, MB dye molecules are absorbed primarily by the surface of the CuO nanoparticle.

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Fig. 4

MD simulation showing (a) the structure of the system before and after simulation, (b–d) RDF plots of MB dye.

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The Radial Distribution Function (RDF), g(r), for the active sites of MB dye molecules with respect to the surface of the CuO nanoparticle is shown in Fig. 4(b), whereas the RDF for the active sites of molecules with respect to the surface of the MgO nanoparticle is shown in Fig. 4(c). As illustrated in Fig. 4(b,c), the g(r) has a maximum peak for atom N₁ at MB dye molecule, indicating that this atom has the highest affinity to interact with the surface of nanocomposite in comparison to other nitrogen atoms, which agrees with DFT calculations (Fig. 3).

The RDF of MB dye with respect to the surface of each nanoparticle and, in general, CuO-MgO nanocomposite is depicted in Fig. 4(d). These results demonstrated that MB dye has a substantially great affinity to CuO nanoparticles than MgO nanoparticles. In addition, the RDF plot of MB dye in relation to the surface of the nanocomposite shows the highest peak for g(r) in the vicinity of the nanocomposite's surface at around 2.5 Å, indicating the absorbs in of all MB dye molecules on the surface of the nanocomposite.

LogS is directly proportional to an active compound's water solubility and is defined as a common solubility unit corresponding to the 10-based logarithm of a molecule's solubility in mol/L. It is suggested that logS values lower than −5 indicate a low polar molecule, implying stronger hydrophobicity and a lower tendency to interact with water (Delmondes and Stefani, 2017). According to empirical evidence, molecules with logS values between −1 and −5 exhibit hydrophilicity, lipophilicity, and water solubility, enabling them to interact with hydrophobic surfaces (Tetko et al., 2005).

The logS value for the water solubility of MB dye was calculated to be −5.02 in this study using the ALOPGPS 2.1 software (Åsbrink and Norrby, 1970), indicating that the molecule is more hydrophobic and has a higher affinity for interacting with the nanocomposite than water.

The interaction energies (Van der Waals + electrostatic potential energies) shown in Table 1 indicate that the interaction energy between the MB dye and the nanocomposite is much higher than the interaction energy between the MB dye and water. The obtained interaction energies (Table 1), final structures after MD simulations in Fig. 4, all corroborate the discussion on log S values, which indicate that MB dye is significantly showing hydrophobic nature and consequently has a high affinity to interact with the nanocomposite and absorb on the surface than interacting with water molecules.

4.4 Experimental analysis: XRD

Fig. 5 shows experimental and calculated XRD spectra for date pit mediated CuO-MgO nanocomposites. Weight percentages calculated using the MAUD software recorded values of 42.53 % and 57.46 % for CuO and MgO, respectively. A cubic structure belonging to the Fm-3m space group proposed by Hazen R M (33) is used to fit the MgO portion of the nanocomposite. The corresponding reflections are presented in Table 2(a). Meanwhile, a monoclinic structure of CuO in the space group of c2/c:b1 proposed by Asbrink S (Sackey et al., 2021) is used to fit well the CuO portion of the date pits mediated CuO-MgO nanocomposite. Characteristic reflection from crystal plane with respective d-spacing is given in Table 2(b). From the MAUD analysis, it can be observed that such proposed structures of both MgO and CuO fit fairly well with the experimental XRD data. Some interesting parameters such as atomic positions, lattice parameters, crystallite size, and micro-strain are extracted from the refined Rietveld analysis using MAUD software. These parameters are presented in Tables 3 -4(a,b).

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Fig. 5

Observed and calculated XRD spectra for CuO-MgO nanocomposite. Redlines represent (--) the calculated peaks using MAUD software, while black diamond (**) illustrate experimental data.

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4.5 Experimental analysis: optical properties

Fig. 6 (a) shows the diffuse reflectance spectrum of CuO-MgO recorded in a 200–2500 nm wavelength. The spectrum displays low reflectance below 8% in the UV–Vis region, designated by R₁. A gradual rise in reflectance is observed in a wavelength range from 900 to 1400 nm (see R₂). In the NIR region designated by R₃, date pits mediated CuO-MgO nanocomposite display a steady-state reflectance of ∼25%. Such reflectance behaviour exhibited by CuO-MgO nanocomposite is identical to a solar absorber material.

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Fig. 6

Optical property of CuO-MgO nanocomposite showing (a) Diffuse reflectance spectrum, (b) energy band gap.

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Fig. 6(b) shows the corresponding energy bandgap estimated following the Kubelka-Munk equation expressed in Widiarti et al. (2017). A plot of $\alpha h{\upsilon }^2$ verses $h\nu$ gives a bandgap which is a drawn tangent line to touch the x-axis. The estimated bandgap 1.6 eV consistent with literature is caused by combined transition from O₂(2p) to Mg²⁺(2p⁶) and Cu²⁺(3d⁹) [37–39].

The UV–Vis absorbance spectra for aqueous date pit extracts and CuO–MgO nanocomposite are shown in Fig. 7. Aqueous date pits extracts show two strong absorbance peaks centred at 280 and 330 nm. Such peaks originate from phytochemicals contained in the date pits, which generally have absorbance peaks at 240–280 nm and 300–380 nm attributed to B-ring cinnamoyl and A-ring benzoyl systems, respectively. Therefore, observation of such prominent peaks confirms that the mechanism of the reaction between phytochemical in date pits extracts with CuO-MgO precursor is feasible, which would result in nanoparticle formation. To further validate that, UV–Vis absorbance was measured for CuO-MgO in date pit extracts. The result shows an absorbance peak at 700 nm.

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Fig. 7

UV–Vis absorbance spectra for date pit extracts and CuO-MgO nanocomposite.

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4.6 Experimental analysis: morphological and particle size analysis

Fig. 8 shows the High-Resolution Transmission Electron Microscope (HRTEM) images of the synthesized CuO-MgO nanocomposites. It is clear from the images shown in Fig. 8(a) that both bigger and smaller cubic-shaped nanoparticles are strongly agglomerated, which is evident in the presence of CuO-MgO nanocomposite. The average grain size was estimated via image j software and recorded a value of 12.1 nm. A histogram of the particle size distribution is shown in Fig. 8(b).

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Fig. 8

HTEM images showing (a) cubic shaped CuO-MgO nanocomposites, (b) histogram of particle size distribution, (c) singular nanogram with appropriated_hkldistances, (d) SAED images.

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A singular nanogram is shown in Fig. 8(c) reveal a series of distinguishable planes with appropriate d_hkl distances of 0.53 and 0.48 nm

The selected area electron diffraction (SAED) image displayed in Fig. 8(d)reveals bight diffraction spots with each spot arising from Bragg reflection from CuO-MgO nanocomposite. Precisely, the SAED images exhibit multiple rings, which confirms the polycrystalline nature of the CuO-MgO nanocomposite. The lattice fringes with d-spacing of 0.243 nm, 0.176 nm, 0.126 nm, and 0.102 nm correspond respectively to (1 1 1), (1 1 2), (3 1 1), and (4 0 0) reflection of the CuO-MgO.

SEM images of the date pit assisted CuO-MgO nanocomposite is shown in Fig. 9(a). The images reveal cubic agglomerated CuO nanoparticles decorated on the flake and sheet-shaped morphology of MgO.

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Fig. 9

showing (a) SEM images of CuO-MgO nanocomposite, (b) the EDS spectrum of CuO-MgO nanocomposite showing clearly the chemical elements. Inset is a table of the weight percentage of the chemical elements present in the sample.

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EDS analysis is performed to confirm the synthesis of pure CuO-MgO nanocomposite, and the result is shown in Fig. 9(b). The presence of Cis due to carbon coating samples before SEM analysis. Identified chemical elements are Cu, Mg, and O. This confirms the synthesis of pure CuO-MgO nanocomposite devoid of any impurities.

4.7 Experimental analysis: photocatalysis analysis

Fig. 10 shows the photocatalysis degradation of methyl blue (MB) under visible light irradiation. Before contact with visible light irradiation, samples were taken at the initial time (t = 0), and UV–Vis absorbance data was recorded. By visual observation (see insert), MB did not show any colour change from the beginning to the end of the experiment. In addition, the intensity of the UV–Vis absorbance spectra shown in Fig. 10 is contact throughout the experiment. This confirms the inability of photocatalysis degradation of MB in the presence of visible light only. As shown, peak levels from t = 0 to t = 180 min remains almost the same.

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Fig. 10

Photocatalysis activity of methylene blue under UV-light irradiation for 180 min, insert is an image showing no color change.

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Fig. 11 shows the photocatalysis degradation performance of synthesized CuO-MgO nanocomposite with methylene blue under visible light irradiation. The dye degradation is monitored by the colour changes from blue to transparent, as seen in the insert. In addition, the spectra show a reduction of the intensity of peaks w.r.t increase contact time of illumination. Furthermore, it is also observed that the shape of peak intensity changes with increased contact time. Initial sharp peaks gradually reduce to become broad at higher contact times, signifying complete dye degradation.

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Fig. 11

UV–Vis spectra for photocatalysis degradation of methylene blue under UV-light in contact time with CuO-MgO nanocomposites, insert is an image of the color change.

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4.8 Experimental analysis: photocatalysis degradation analysis

The Photocatalysis degradation efficiency is calculated using the equation given as

Fig. 12 shows the photocatalysis degradation rate of methylene blue and in contact with CuO-MgO nanocomposite in the presence of visible light irradiation. The degradation rate of MB in the presence of visible light is 10% which implies that MB in the presence of visible light does not show any significant degradation ability. Yet when in contact with CuO-MgO nanocomposite in the presence of visible light, the degradation rate increased to 77%.

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Fig. 12

Photocatalysis degradation (a) rate of methylene blue only and in contact with CuO-MgO in the presence of visible light irradiation, (b) reaction rate for MB and MB + CuO-MgO.

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4.9 Experimental analysis: removal kinetic of methylene blue

The photocatalysis degradation obeys the pseudo-first-order kinetics (see Fig. 12(a)), which follows the equation written as $$ln(\frac{A_t)}{A_o)}=kt$$ where $A_t$ is the absorbance at time t = t (sampling time), $A_o$ is the absorbance at t = 0 min, $k$ is the reaction rate constant, and $t$ is irradiation time. The reaction rate constant ( $k$ ) was calculated from the slope of $ln\frac{A_t}{A_o}$ versus irradiation time. From Fig. 12(b), the reaction rate constant values for MB and MB + CuO-MgO nanocomposite respectively are found as −0.000728 ± 7.5 min⁻¹ (R² = 0.9117) and −0.00929 ± 7.7 min⁻¹ (R² = 0.9405). The smaller $k$ value obtained for MB only in the presence of visible light compared to the higher $k$ value for MB + CuO-MgO nanocomposite in the presence of visible light further confirms the higher degradation ability of date pit mediated CuO-MgO nanocomposite. The higher photocatalytic degradation activity of CuO-MgO nanocomposite is caused by the smaller bandgap that favours the stepping of electrons from the valence band to the conduction band and thus enhancing the radical species.