Article Open Access
Received: 20 March 2024 Accepted: 06 June 2024 Published: 11 June 2024
© 2024 The authors. This is an open access article under the Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/).
Our environment has been impacted by man-made pollutants from numerous manufacturing industries, such as rubber products, plastics, paper products, and leather goods. These manufacturing industries make substantial use of synthetic dyes such as methylene blue, Congo red, acid black 1, acid black 234, and acid black 210 [1,2]. These dyes exceed authorized limits and exhibit cytotoxicity. Repeated release of these dyes without proper removal or conversion into less dangerous compounds can have significant environmental consequences, ultimately affecting all living creatures [3,4,5,6,7,8]. Consequently, there is considerable interest in the search for simple, adaptable, and effective photocatalyst-based technologies for degrading synthetic dyes into less toxic chemical waste. Metal (Ag, Au, Pt, Cu, Zn, etc.) and the corresponding metal oxide (MgO, NiO, CuO, ZnO, TiO2, etc.) nanoparticles (NPs) are considered promising materials due to their distinct physical properties, including a high surface area to volume ratio, regulated morphology, smaller dimensions, and light-absorbing properties [9,10,11,12]. These properties make them suitable for various applications, including biomedicine, biosensing, and optoelectronics. Metal and metal oxide NPs are extensively utilized in biomedicine due to their antibacterial, antimycotic, anticancer, antilarvicidal, and antidiabetic properties [9,10,11,12]. MnO NPs, in particular, have gained popularity in synthesis and manufacturing due to their reduced toxicity [13]. Nanoparticles are typically synthesized using chemical and physical methods, both of which are energy-intensive, involve hazardous chemicals for reduction and capping, and are difficult to scale. The use of harmful chemicals compromises the biocompatibility of NPs, as these compounds may remain on the NPs surface even after multiple washings, limiting their use in biological applications [14,15]. Therefore, synthesizing NPs using biological approaches, particularly plant-based methodologies, serves as an alternative to traditional methods. Plant-based biological approaches offer several advantages, including one-pot synthesis, robustness, cost-effectiveness, and eco-friendliness. The biological molecules present in plants act as reduction and capping agents, decreasing the risk of NPs losing biocompatibility during synthesis [6,7,8]. Moreover, the use of plant-based methods is considered safe and environmentally friendly. In our current research study, we synthesized manganese oxide nanoparticles (MnO NPs) utilizing an innovative approach involving the leaf extract of Lathyrus aphaca. Lathyrus aphaca, renowned for its medicinal properties, has historically been employed in the treatment of various diseases. In this investigation, we have examined the MnO NPs with the Lathyrus aphaca, which acted as a reducing and stabilizing agent. Subsequently, the synthesized MnO NPs were tested to evaluate their efficacy as a photocatalyst against methylene blue.
All chemicals utilized in the experiment, including MnSO4·H2O, NaOH, methylene blue dye, ethanol, methanol, and distilled water, were of analytical grade and were employed without additional purification.
Approximately 50 g of dried and ground plant sample is placed in a sealed Pyrex glass jar, to which 400 mL of absolute methanol is added. The jar is left undisturbed for up to 3 weeks at room temperature, with daily agitation to facilitate the release of plant-soluble phytochemicals. The resulting soaked extract is filtered using a standard filter paper (Whatman filter paper) to obtain a concentrated methanolic extract through solvent evaporation. Subsequently, the extract is further concentrated to achieve the desired dry extract for subsequent processing [16,17,18,19,20].
A 100 mL solution of 0.1 M MnSO4·H2O was placed into a 500 mL beaker, followed by the addition of 100 mL of plant extract were carried based on literature [21,22]. Subsequently, a 0.1 M NaOH solution was carefully added dropwise to the beaker while stirring constantly for approximately 1 h at a pH of 8.0 and a temperature of 50 °C. Afterward, the mixture was filtered and washed with methanol to eliminate impurities. The resulting precipitates were then dried in an oven at 90 °C for 1 h before being subjected to calcination in a muffle furnace at 150 °C for 2 h. These materials were subsequently utilized for further characterization and the photocatalytic degradation of methylene blue dye.
The photocatalytic degradation activity of green-synthesized MnO nanoparticles was assessed using methylene blue dye as a model system, adopted from [23,24]. Initially, 0.05 g of green-synthesized MnO nanoparticles were introduced into 100 mL of methylene blue dye solution and the resulting suspension was left in a dark for about 1 h to achieve adsorption-desorption equilibrium. Subsequently, the suspensions were exposed to solar irradiation without external pressure or pH adjustment, with the temperature maintained at 30 °C. Throughout the irradiation process, continuous stirring was maintained using a magnetic stirrer to ensure uniform mixing. At regular intervals (every 20 minutes for 2 h), 5 mL samples were withdrawn for analysis. These samples were centrifuged to separate the photocatalyst (MnO nanoparticles), and the remaining dye content was determined using a UV spectrophotometer at the specific absorption peak (664 nm) of methylene blue. The percentage degradation of the dye under solar light exposure was calculated using the Equation (1):
where Co is the initial dye concentration and Ct is the dye concentration at time t in min.
A UV light box was used to carry out all of the photochemical processes. The wavelength distribution of the emission ranged from 200 to 800 nm. The lamp was placed approximately 10 cm away from the dye suspension. The dye concentration was monitored using a spectrophotometer (UV-1601, Shimadzu, Japan). Three consecutive experiments were carried out under sunlight irradiation to assess the recovery and stability of MnO nanoparticles as a photocatalyst. Following each run, the MnO nanoparticles photocatalyst was extracted, cleaned with deionized water and ethanol, dried, and subsequently reused. The degradation efficiency of the nanoparticles was then evaluated.
The green-synthesized MnO nanoparticles were then characterized using various spectroscopic techniques. The band gap energy was determined by employing the absorption spectra of a UV-Visible spectrophotometer. X-ray diffraction (XRD) analysis was performed to detect the crystalline structure of the synthesized nanoparticles. Energy-dispersive X-ray spectroscopy (EDX) analysis was conducted to confirm the chemical composition of the green-synthesized MnO nanoparticles. Morphology determination of the synthesized nanoparticles was carried out using a scanning electron microscope (SEM). Fourier transform infrared (FTIR) analysis of the synthesized nanoparticles was performed to highlight the involvement of biological moieties from the plant extract.
The X-ray diffraction (XRD) pattern depicted in [Figure 1] illustrates the analysis of manganese oxide (MnO) nanoparticles synthesized through a green method. The XRD pattern closely aligns with existing literature, revealing distinct diffraction peaks at specific angles corresponding to crystal planes. These angles are recorded as follows: (6.4°, 10.24°, 14.88°, 18.66°, 26.22°, 29.22°, 36.14°, 38.72°, 44.18°, 47.18°, 49.92°, 53.24°, 62.44°, and 71.32°). Notably, the peaks of greatest prominence emerge at approximately 10.24°, 18.66°, 26.22°, 47.18°, and 62.44°. These prominent peaks signify the presence of specific crystallographic planes within the MnO2 nanoparticles, showing their crystalline nature. The synthesis process involves utilizing plant extract as a dual agent for both reduction and oxidation. The resultant XRD pattern confirms the crystallinity of the synthesized nanoparticles. Moreover, the synthesized nanoparticles exhibit heightened peak intensity and sharpness, indicative of their extremely crystalline nature. Any observed impurity peaks within the XRD pattern are minimal, further affirming the purity of the synthesized MnO nanoparticles. The XRD graph refinement was performed, and the hkl Miller indices were assigned based on the literature. Manjula et al. [25] assigned their XRD patterns to a tetragonal phase and compared them with standard data from the JCPDS card (File no: 14-644). They proposed that the synthesized MnO2 nanoparticles are of poor crystalline nature, hence the diffraction peaks are indexed as a tetragonal phase [26]. No additional diffraction peaks related to impurities were detected, indicating the good quality of the synthesized MnO2 [27]. While, Zheng et al. [28] found that their XRD reflections readily indexed to cubic MnO with a lattice constant of 4.443 Å, in good accordance with the literature (JCPDS 89–4835). They observed no other phases of manganese oxide, indicating the monophase of cubic MnO. Similarly, Mansoor et al. [29] revealed that the synthesized MnO nanoparticles exhibited a good crystalline nature with a wurtzite hexagonal structure, consistent with the JCPDS card number (01-075-1533) [30].
The size and shape of manganese oxide (MnO) nanoparticles extracted from green plants were studied using a scanning electron microscope (SEM). The nanoparticles were examined within a range of 20 micrometers, using an electron beam accelerated to 50 kilovolts. The SEM images, shown in [Figure 2], revealed the morphology and average size of the nanoparticles. The analysis confirmed successful synthesis of MnO nanoparticles, which had sizes in the nanometer range. The particles appeared randomly arranged and asymmetrical, with closely spaced arrangements. [Figure 2] showed zoomed-in views of the nanoparticles, indicating an average size of approximately ≥ ± 50–70 nanometers, with clear boundaries visible upon further magnification. However, SEM scans indicated that the produced MnO nanoparticles were dispersed randomly. EDX analysis was employed to examine the formation and chemical composition of manganese oxide (MnO) nanoparticles synthesized from green sources. The presence of elemental manganese (Mn) in the samples was confirmed through EDX analysis, as showed in [Figure 3], showcasing the EDX pattern and mapping. This verification underscores the effective synthesis of nanoparticles using biological components extracted from Lathyrus aphaca leaf extract.
EDX peaks in the spectrum indicated the presence of Mn at lower energy levels (around 1 kilo electron volts, keV) with the highest peak representing MnO, and at higher energy levels (above 5 keV) representing elemental manganese. Furthermore, peaks corresponding to oxygen (O) were observed, confirming the adsorption of biological molecules from the plant extract onto the nanoparticle surfaces. The presence of oxygen confirms the oxide form of the Mn nanoparticles. The absence of impurities and the high concentration of Mn were confirmed by the EDX analysis. Consequently, it can be inferred that the desired nanoparticles were successfully synthesized using plant leaf extract. [Figure 3] presents the observed atomic percentages for all elements, with Mn accounting for 70.40% and O for 29.60% by weight. The EDX pattern of the green-synthesized MnO nanoparticles was consistent with previously published literature [23,26,27,28,29,30,31,32,33]. The UV-visible graph, depicted as [Figure 4], illustrates significant optical properties of the synthesized nanoparticles and their interaction with ultraviolet (UV) and visible light. This graph provides insights into the absorbance and reflection characteristics of the nanoparticles across different wavelengths. UV-visible spectroscopy is instrumental in identifying the electronic transitions of nanoparticles and assessing their potential applications, including catalytic activity. Additionally, the X-ray diffraction (XRD) profile reveals distinct peaks characteristic of the synthesized nanoparticles, which align precisely with data from existing literature. This confirms the structural properties of the nanoparticles as consistent with the literature data [25,33,34,35,36,37].
Fourier Transform Infrared (FTIR) spectroscopy is essential for the identification of chemical functional groups and the characterization of intermolecular forces resulting from the interaction between two or more mixtures of different functional groups or compounds. In this study, FTIR analysis was employed, as showed in [Figure 5]. Plants contain a variety of phytochemicals, constituting a rich source of various functional groups. Consequently, FTIR analysis holds promise in predicting the presence of additional functional groups containing heteroatoms, predominantly involving carbon bonded with H, O, N, OH, and X, when compared to manganese oxide synthesized nanoparticles. Through extensive review of the literature [38,39,40,41,42,43,44], manganese oxide nanoparticles (MnO NPs) were examined using FTIR analysis within the frequency range of 719.80 to 494.81 cm−1.
Our findings, illustrated in [Figure 5], revealed the presence of the O-O stretching mode at a frequency of 719 cm−1, and showed the presence of MnO6 oxides of manganese. Furthermore, a peak observed at 548 cm−1 corresponded to the Mn-O stretching mode, aligning closely with existing literature [45,46,47]. Consequently, these findings suggest that the green-synthesized MnO NPs are coated with biologically active molecules derived from plant leaf extracts, encompassing diverse functional groups. The peaks observed at wavenumbers above 3400 cm−1 correspond to the stretching vibrations of hydroxyl groups OH, while the peak around 1720 cm−1 is associated with the stretching vibration of carbonyl groups CO showed carbonyl and other carbon functional groups. These peaks indicate the presence of O containing functional groups attached to the carbon structure. The Fourier-transform infrared (FTIR) spectra also reveal the presence of other fundamental functional groups commonly found in plant-derived nanoparticles, as illustrated in [Figure 5].
We have examined the photocatalytic degradation of methylene blue (MB) dye employing green-synthesized manganese oxide nanoparticles (MnO NPs) under ultraviolet (UV) light irradiation as illustrated in [Figure 6]. The experimental setup utilized a UV lamp as the radiation source. Initially, the self-degradation behavior of MB dye was assessed in the absence of synthetized photocatalyst (MnO NPs), yielding no detection of self-degradation [44,45,46,47]. Subsequently, the photocatalytic activity of green-synthesized MnO NPs was evaluated, demonstrating efficient degradation of MB dye under UV light irradiation for duration of 120 minutes and noted that the observed photocatalytic activity corresponds to the average efficiency within the UV lamp irradiation spectrum. Furthermore [Figure 7] illustrates the percentage degradation of methylene blue (MB) over time. A single sample with a concentration of 40 parts per million (ppm) was employed in the experiment. The results indicate a clear direct correlation between time and the percentage degradation of methylene blue dye. At a maximum duration of 120 min, MB experienced approximately 93% degradation when treated with green synthesized plant extracted MnO nanoparticles. This underscores the efficiency and potential of the synthesized nanoparticles in degrading methylene blue. In our analysis, the process of catalytic degradation of MB follows the pseudo-first-order kinetic model. We conducted both linear and non-linear fitting of the curve, as illustrated in Figure 8.
Table 1 provides a comprehensive analysis of the efficacy of the MB dye removal between MnOx NPs synthesized utilizing Lathyrus aphaca extract with other environmentally friendly synthesized NPs [22,48,49,50,51,52,53,54]. The current study reveals that the MnOx NPs employed exhibit superior performance compared to the relevant literature. Lathyrus aphaca extract MnOx NPs demonstrate efficient ability to remove dye at maximum concentrations, requiring minimal time and concentration. Remarkably, this superior performance is achieved without the reaction environment control parameters such as temperature and pH. This underscores the promising potential of MnOx NPs synthesized with Lathyrus aphaca extract for robust and efficient dye removal applications.
In this investigation, we employed a green synthesis method to produce manganese oxide nanoparticles (MnO NPs) for the first time, utilizing leaf extract from the Lathyrus aphaca plant. Our findings of FTIR, revealed the presence of the O-O stretching mode at a frequency of 719 cm−1, and showed the presence of MnO6 oxides of manganese, and peak observed at 548 cm−1 corresponded to the Mn-O stretching mode, aligning closely with existing literature. Furthermore, the green-synthesized manganese oxide nanoparticles exhibited promising photocatalytic and adsorption capabilities against Methylene Blue (MB) dye, leading to approximately 93% degradation of MB when treated with the green-synthesized MnO nanoparticles derived from plant extract. This highlights the efficacy and potential of these nanoparticles in environmental remediation applications, particularly in the degradation of methylene blue contaminants.
The author acknowledges the affiliated institute for providing research facilities and financial support for conducting research and publication support.
Not applicable.
Financial support provided by institutional affiliations.
The authors declare no conflicts of interest.
