Methylene blue adsorption by chemical-activated Trichanthera gigantea leaf

The issue of wastewater pollution has garnered significant concern due to the harmful discharge of various contaminants, including dyes, into the environment. Industries engaged in manufacturing, printing, coloring, and textiles are among the major contributors to this pollution as these dyes pose a significant threat to environmental ecosystems and human health due to their high toxicity and impact on marine life (Han et al. 2015). This pollution necessitates the development of sustainable solutions to address the growing environmental concern (Grabi et al. 2021).

One dye of particular concern is methylene blue (MB), a common contaminant in textile industry wastewater. MB is widely used for dyeing cotton, silk, and wool fabrics (Alghamdi & El Mannoubi 2021; Choudhary et al. 2021; Giraldo et al. 2021). However, beyond its industrial applications, the use of MB also extends into medical contexts for treating methemoglobinemia, cyanide poisoning, and diagnostic procedures as a staining agent (Shakoor & Nasar 2016; Alvarez-Torrellas et al. 2019). Despite its diverse applications, exposure to this substance poses health risks, including eye irritation, gastrointestinal discomfort, increased heart rate, and respiratory difficulties (Shakoor & Nasar 2016; Alvarez-Torrellas et al. 2019).

Given the environmental and health risks associated with MB, practical methods for its removal from wastewater are crucial. A diverse array of physicochemical methods, encompassing adsorption, reverse osmosis, coagulation/flocculation, electrochemical processes, ion exchange, membrane filtration processes, and advanced oxidation processes, have been actively explored to remediate water pollution (Salleh et al. 2011; Zhou et al. 2019; Javed et al. 2024). However, a critical evaluation of these methods reveals inherent limitations, including financial considerations, technical complexities, and treatment efficacy (Dasgupta et al. 2015; Merine et al. 2024). Within this context, adsorption emerged as a frontrunner due to its numerous environmental benefits (Bello et al. 2020; Lin et al. 2020). Specifically, adsorption is recognized as a sustainable approach due to its operational simplicity, minimal design complexity, and remarkable ability to deliver high-quality treated water (De Gisi et al. 2016; Katheresan et al. 2018). Its deployment for effluent treatment has demonstrated superior efficacy in eliminating harmful dye compounds and potentially transforming effluents into both environmentally safe and reusable forms (Mittal & Mittal 2015; Bulgariu et al. 2019; Danyliuk et al. 2020).

The wide variety of potential adsorbents, including naturally abundant and low-cost biomaterials, contributes to the flexibility of adsorption as a water treatment technique, leading to a substantial cost reduction compared to conventional options (Reddy et al. 2016; Setiabudi et al. 2016; Filho et al. 2017; Mokhtar et al. 2017; Georgin et al. 2020; Sharma et al. 2021). Studies have shown that various sustainable, plant-derived materials and agricultural byproducts, such as tea leaves (Wong et al. 2019), elephant grass (Menkiti et al. 2018), bilberry leaves (Mosoarca et al. 2022), lemon grass leaf (Ahmad et al. 2021), and Citrullus colocynthis seeds (Alghamdi & El Mannoubi 2021) can effectively remove organic and inorganic pollutants. This effectiveness stems from functional groups within these materials, which possess inherent adsorptive properties (De Gisi et al. 2016).

Recognizing the potential of naturally abundant resources for wastewater treatment, this study investigates Trichanthera gigantea leaf (TGL) powder as a novel biosorbent for the adsorptive removal of the MB dye from an aqueous solution. TGL, commonly called Nacedero, is a versatile tree native to Colombia's Andean foothills and thrives in the lush and biodiverse rainforests of central and northern South America. This tree has garnered significant attention for its impressive ethnomedicinal properties, offering edible sprouts and traditional uses as a blood purifier, nephritis remedy, and even a milk-boosting drink (Rosales 1997; Cook et al. 2005).

This study aims to explore the adsorption capacity of MB onto TGL, establishing its potential as a novel and economically viable biosorbent. Such an objective was achieved by analyzing the impact of concentration, contact time, pH, and adsorbent dosage on MB adsorption. We conducted a detailed characterization of the leaf material using Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET), and an energy-dispersive X-ray spectrometer (EDX) to elucidate its morphology, functional groups, surface area, and elemental composition. This study also explored the mathematical models of adsorption isotherms to gain additional insight into the adsorption process.

TGL leaves were collected from Kuala Nerus, Terengganu, Malaysia and thoroughly washed with distilled water to remove impurities. They were cut into smaller pieces and dried in an air oven at 80 °C for 12 h to remove excess moisture and inhibit microbial growth. A portion of the dried leaves was pulverized into fine powder using a blender and passed through a 200 μm stainless-steel sieve to ensure a uniform particle. The untreated TGL was then stored in an airtight container for further use. The remaining dried leaves were immersed in a 1 M aqueous sodium hydroxide (NaOH) solution for 24 h to prepare the chemically activated leaves. The resulting mixture was separated from the leaves and thoroughly rinsed with distilled water until reaching a neutral pH. The NaOH-impregnated leaves were later dried in an oven at 80 °C for 12 h. Finally, the leaves were re-pulverized into fine powder labeled ‘NaOH-TGL’ and stored for subsequent experimentation.

The adsorbent's specific surface area, pore volume, and pore diameter were determined using the BET method on an ASAP 2020 Micromeritics instrument. Before analysis, the samples were degassed at 300 °C under nitrogen flow for 3 h to remove any adsorbed moisture or other impurities that might be present on the surface. Following degassing, the samples were transferred to the BET analyzer for analysis.

The surface chemistry of the prepared adsorbents was investigated using FTIR. The analyses were performed on a Brucker INVENIO S spectrometer equipped with a platinum attenuated total reflectance (ATR) module. The ATR sampling technique facilitated direct analysis of the solid adsorbents onto the spectroscopy detector as the preparation method for this spectroscopy does not necessitate using a potassium bromide pellet preparation. A small amount of the adsorbent powder was placed directly onto the ATR crystal and held in place with a pressure clamp to ensure optimal contact. Infrared spectra were recorded for the untreated and treated TGL biosorbents before and after dye adsorption over a spectral range of 4,000–400 cm⁻¹.

The surface morphology and porosity of the prepared adsorbents were examined using a JEOL JSM-6360LA SEM. Before imaging, the samples were mounted onto an aluminum stub using double-sided carbon tape. The stubs were then placed in an auto fine coater (JFC-1600) where a thin layer of gold was deposited onto the adsorbent surface under vacuum. This gold coating improves the sample's conductivity and prevents charging during imaging, thus enhancing image quality. The gold-coated samples were carefully transferred to the SEM chamber and the microscope was operated in high vacuum mode with an accelerating voltage of 10 kV. The micrographs were captured at 850× and 1,500× magnifications to visualize the microstructure and assess the porosity of the adsorbent materials.

The adsorbent's surface elemental composition and chemical characterization were determined using EDX on a TESCAN, Bruker Quantax Compact system equipped with an XFlash 600 Mini detector. Before analysis, the adsorbent sample was mounted on a thin carbon film to minimize charging effects that could arise from the interaction of the electron beam with the sample during EDX analysis. Additionally, the sample was flattened and polished to ensure a smooth and uniform surface for interaction with the electron beam before being placed directly onto the spectroscopy detector for analysis.

The effect of operating parameters on dye removal efficacy was investigated through batch adsorption experiments. The influence of pH (2–10), biosorbent dosage (2–10 g/L), contact time (10–90 min), and initial dye concentration (5–30 mg/L) on the adsorption process was systematically evaluated. The pH of the test solutions was adjusted using 1 M NaOH or 1 M H₂SO₄ as required.

BET surface area analysis (Table 1) revealed a significant increase in surface area from 5.06 m²/g for untreated TGL to 6.04 m²/g after NaOH treatment. This enhancement aligns with the established understanding (Jain & Gogate 2017) that NaOH treatment facilitates lignin removal, mitigating its interference with dye adsorption on the biosorbent. Additionally, such lignin removal may promote pore development, increase porosity, and further contribute to the higher surface area.

Following MB adsorption (Figures 2(b) and 2(c)), untreated and treated TGLs displayed minimal peak intensity and location changes, indicating weak interactions between the dye and adsorbent. However, no new peaks emerged, suggesting the absence of covalent bonding. Nevertheless, a slight wavenumber shift in some of the peaks indicates the potential interaction of functional groups during adsorption. Similar observations were reported in a study of alkaline-treated avocado shells (Ait Haki et al. 2022).

However, further extending contact time beyond the equilibrium point leads to a slight decrease in removal efficiency. This likely results from a combination of factors, primarily the saturation of active sites on the adsorbent surface, which limits further MB uptake as evidenced by the findings by Bharathi & Ramesh (2013). Additionally, some desorption, where adsorbed MB molecules detach and return to the solution, also contributes to the decreased efficiency. Even at the relatively short contact time of 80 minutes, desorption could occur after equilibrium, leading to a slight increase in the liquid-phase MB concentration. This observation of decreased efficiency after equilibrium due to potential desorption aligns with previous findings by Kurniawati et al. (2021).

Therefore, considering the adsorbent's efficiency and capacity, 8 g/L was selected as the ideal condition for further experimentation. Similar trends in the relationship between adsorbent usage, dye removal effectiveness, and the dye adsorption per unit of adsorbent have been observed in previous research (Suyamboo & Srikrishnaperumal 2014; Quansah et al. 2020).

Both adsorbents displayed a general trend of increasing MB adsorption within the pH range of 2–8, followed by a subtle decrease at higher pH values (8–10). This trend can be explained by the increasing concentration of negatively charged hydroxyl groups at higher pH. These negatively charged groups electrostatically attract the positively charged MB molecules, facilitating adsorption onto the TGL surface. Such a phenomenon explains the increased adsorption efficiency and capacity at higher pH values. Consequently, the notable efficacy of MB adsorption in a basic solution led to the selection of pH 8 as the optimal condition for further experiments. Similar observations were reported for MB adsorption using thypa stems and leaves (Orozco et al. 2018) and walnut shell powder (Uddin & Nasar 2020), highlighting the potential of various biomass materials for dye removal.

where K_L (L/mg) represents the Langmuir constant and C_o (mg/L) denotes the initial dye concentration. Based on R_L values, adsorption behavior can be categorized as favorable (0 < R_L < 1), unfavorable (R_L > 1), linear (R_L = 1), or irreversible (R_L = 0). This study calculated R_L values ranging from 0 to 1, confirming favorable blue MB adsorption onto TGL under the investigated conditions. This suggests favorable interactions between the TGL surface and MB molecules, facilitating efficient dye removal from the aqueous solution.

The influence of various physicochemical parameters on the adsorption efficiency of TGL powder for dye removal was investigated using a one-way analysis of variance (ANOVA) by focusing on adsorbent dosage, contact time, initial dye concentration, and pH. The ANOVA procedure relied on comparing the F statistic with the F critical value and evaluating the p-value. We reject the null hypothesis and accept the alternative hypothesis if the F-value exceeds the F critical and the p-value is less than 0.05, indicating a statistically significant difference between the mean removal efficiencies. Conversely, a p-value greater than 0.05 implies no significant difference among the means, thus the null hypothesis is accepted.

The ANOVA results in Tables 3 and 4 revealed a statistically significant impact of adsorbent dosage on the removal efficiency for both untreated and treated TGL (p < 0.05). It indicates that the amount of TGL significantly impacted its dye removal capacity. Conversely, no statistically significant differences were observed for contact time, initial dye concentration, and pH (p > 0.05). These findings suggest that within the investigated range, these parameters did not substantially influence the adsorption performance of TGL.

This study demonstrates the potential of TGL treated with NaOH as an effective, cost-efficient, and high-performance adsorbent for removing MB from wastewater. The treated leaves significantly increased MB uptake capacity with higher initial dye concentrations. The optimal contact time for adsorption was 80 min while the ideal biosorbent dosage and pH were 8.0 g/L and 8.0, respectively. Langmuir isotherm analysis revealed the presence of homogeneous adsorption sites on the treated TGL, which is further supported by the favorable separation factor (R_L) indicative of efficient MB sorption. Notably, the maximum monolayer adsorption capacity of the leaf powder reached 0.822 mg/g, highlighting the potential of NaOH-treated TGL as a cost-effective and practical solution for MB removal for further study.

The authors gratefully acknowledge the financial support from the Ministry of Higher Education (MoHE) through the Malaysian International Scholarship. The authors also thank the Faculty of Ocean Engineering Technology, Universiti Malaysia Terengganu for providing essential facilities and Mr Raji Ibrahim Olayemi for designing the graphical abstract.

This research was funded by the Ministry of Higher Education (MoHE), Malaysia through the Malaysian International Scholarship.

A.A.F. conceived the study, conducted the investigation, wrote the manuscript, and reviewed and edited the article. A.A. and S.H. supervised and reviewed the research and edited the article. A.A.F., A.A., S.H., and F.A. read and approved the final manuscript.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.