Green synthesis of iron nanoparticles from the baru (Dipteryx alata) endocarp extract for the efficient removal of rhodamine B and caffeine from water through the heterogeneous Fenton process Open Access

Nanotechnology has witnessed remarkable advancements in recent decades, leading to the exploration of various nanomaterials, including metallic nanoparticles (MNPs). MNPs possess exceptional properties that make them highly desirable for numerous applications across diverse fields such as medicine, microbiology, agriculture, materials science, energy, and the environment (Madubuonu et al. 2019; Selvaraj et al. 2022; Suppiah et al. 2023). These properties include a high surface area per unit volume, remarkable mechanical and thermal strength, distinct optical and magnetic characteristics, as well as antimicrobial and catalytic properties (Fahmy et al. 2018; Priya et al. 2021; Narayanan et al. 2021). As a result, MNPs have garnered significant attention and research interest.

However, traditional methods of synthesizing MNPs using chemical and physical approaches have certain limitations. These methods, including coprecipitation, hydrothermal synthesis, sol–gel processes, and microemulsion, often involve the use of toxic-stabilizing and -dispersing agents that can be harmful to living organisms and the environment (Selvaraj et al. 2022). Additionally, they require substantial amounts of energy, making the synthesis process costly (Anchan et al. 2019). Hence, there is a need for sustainable and cost-effective alternatives that overcome these drawbacks.

One promising approach is the utilization of biological systems for MNP synthesis. Among the various biological sources, plant tissues have gained attention due to their abundance and accessibility (Jadoun et al. 2021). Extracts derived from different plant species contain phytochemicals that act as reducing and stabilizing agents for the nanoparticles (Madubuonu et al. 2019). This biofabrication method offers numerous advantages over traditional techniques. Firstly, plant extracts provide a sustainable and eco-friendly alternative, as they eliminate the need for toxic chemicals and reduce environmental impacts (Suppiah et al. 2023). Additionally, plant extracts are cost-effective and readily available, making them accessible for large-scale production. Moreover, the biofabrication process using plant extracts is relatively simple, requiring less energy and time compared to complex chemical and physical methods (Demirezen et al. 2019; Jadoun et al. 2021).

Among the metals used for MNP synthesis, iron nanoparticles (FeNPs) have emerged as particularly important due to their wide-ranging applications. FeNPs exhibit antioxidant and antimicrobial properties, making them useful in medicine and microbiology (Bharathi et al. 2020; Suppiah et al. 2023). In agriculture, FeNPs promote plant growth and serve as a micronutrient fertilizer (Alabdallah et al. 2021; Haydar et al. 2022). They also serve as effective adsorbents for various pollutants and can be utilized in advanced oxidative processes (AOPs) for water treatment (Guo et al. 2020; Pai et al. 2021; Khoshkalam et al. 2023). In recent studies, different plant tissues have been explored for the synthesis of FeNPs, including leaves, roots, and fruits of various plant species (Demirezen et al. 2019; Ting & Chin 2020; Pai et al. 2021; Ningthoujam et al. 2023). However, the synthesis of FeNPs using the endocarp of the baru fruit (EB) (Dipteryx alata) has not been investigated before. The baru fruit, a tree species from the Fabaceae family, is native to Brazil and widely distributed in the Cerrado biome (Niedack et al. 2021). While the mesocarp and nuts of the fruit are used for food purposes, the endocarp remains an underutilized by-product (Rambo et al. 2021). Exploring the potential of baru fruit endocarp for FeNPs synthesis could provide new opportunities for sustainable and value-added utilization of this biomass.

Organic dyes play a crucial role in the manufacturing of paints, plastics, leather, rubber, and cosmetics, contributing to the development of products that are vivid and aesthetically pleasing (Briton et al. 2019). However, the discharge of these dyes as waste in industrial effluents has emerged as a significant environmental challenge. Annually, approximately 700,000 tons of organic dyes are released into water bodies, impacting sunlight penetration, inhibiting photosynthesis, and adversely affecting the aquatic life (Jain et al. 2021). Pharmaceutical compounds constitute another group of potential contaminants in aquatic ecosystems. The improper disposal of unused medications and the incomplete removal of pharmaceutical residues during wastewater treatment contribute to the presence of these substances in water bodies (Talib & Randhir 2017). These pharmaceutical compounds can adversely affect aquatic ecosystems and pose potential risks to human health. The continuous exposure of aquatic organisms to low concentrations of pharmaceuticals has been linked to various negative effects, including altered behavior, reproductive disruptions, and developmental abnormalities (Valdez-Carrillo et al. 2020; Jijie et al. 2021; Ríos et al. 2022). Moreover, the potential for these substances to accumulate in the food chain raises concerns about their impact on human health (Osuoha et al. 2023).

Rhodamine B (RhB) and caffeine are two common organic pollutants in water and wastewater, which represent the class of dyes and pharmaceuticals, respectively. RhB is a xanthene dye that is widely utilized in the textile, paint, leather, and paper industries (Wang et al. 2019). Despite its widespread application, experimental studies have confirmed its carcinogenicity, genotoxicity, and chronic toxicity (Golin et al. 2022). Caffeine is an alkaloid belonging to the class of methylxanthines, and it is found in over 60 plant species. It holds significant pharmaceutical relevance, as it is used to reduce fatigue, increase alertness, and enhance the effectiveness of analgesics used to treat colds and headaches (Bachmann et al. 2021). Caffeine is also a compound commonly found in beverages and foods such as teas, coffees, chocolates, and energy drinks (Korekar et al. 2020). Numerous studies have identified the presence of caffeine in wastewater, surface water, and groundwater (Li et al. 2020; Rani 2022), with anthropogenic contamination being the primary source of this pollution. Despite the impacts of the presence of caffeine in water bodies still being unknown, adverse effects such as enzyme alterations, lipid peroxidation, reduction in energy reserves, and oxidative stress have already been demonstrated in mollusk and fish (Cruz et al. 2016; Muñoz-Peñuela et al. 2021).

The negative environmental consequences of water pollution caused by dyes and pharmaceuticals emphasize the need for sustainable practices and innovative solutions in industrial processes (Ismail et al. 2022; Aneke & Adu 2023). AOPs emerge as a viable alternative for eliminating organic contaminants from water. These processes involve the generation of highly reactive species, such as hydroxyl radicals (·OH) or sulfate radicals ⁠, which, as potent oxidizing agents, can lead to the breakdown of organic pollutants (Guo et al. 2020). Among AOPs, the homogeneous Fenton reaction is a traditional method that involves the generation of hydroxyl radicals through the catalytic decomposition of H₂O₂ by iron salts (Fe²⁺, Fe³⁺) (Briton et al. 2019). However, there are some drawbacks that can limit its application. It is highly pH-dependent, requiring acidic conditions for optimal performance (typically pH 2.5–4), which may restrict its use in certain environments (Wu et al. 2015). The generation of large amounts of iron sludge as a byproduct, inefficient utilization of H₂O₂, and the inability to recover the catalyst also pose challenges, necessitating careful control and management (Wang et al. 2019). On the other hand, the heterogeneous Fenton process using a solid catalyst offers an alternative to address these drawbacks. Among these solid catalysts, green FeNPs have been successfully employed for removing bisphenol A (Guo et al. 2020), lindane (Ningthoujam et al. 2023), monochlorobenzene (Kuang et al. 2013), malachite green (Ting & Chin 2020), and methylene blue (Madubuonu et al. 2019) from water and wastewater.

In this study, we present the first-time synthesis of FeNPs using an aqueous extract obtained from the baru fruit endocarp. The FeNPs were characterized using various techniques, including Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), dynamic light scattering (DLS), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). Furthermore, FeNPs were employed as catalysts in a heterogeneous Fenton-like system for the removal of RhB and caffeine from an aqueous solution, with an investigation into the impact of initial solution pH, FeNPs and hydrogen peroxide dosages, and temperature on the removal process. Additionally, we assessed the phytotoxicity of the degradation products of RhB and caffeine to confirm their non-toxic nature.

All chemical reagents were of analytical grade. Ferrous sulfate heptahydrate (FeSO₄·7H₂O), RhB (C₂₈H₃₁ClN₂O₃), and hydrogen peroxide (H₂O₂, 29%) were purchased from Synth, Brazil. Caffeine (C₈H₁₀N₄O₂) was obtained from Sigma-Aldrich. All solutions were prepared using distilled water.

The synthesis of FeNPs was adapted from Bharathi et al. (2020). A solution of ferrous sulfate (0.1 M) was mixed with the EB extract at a 1:1 (v/v) ratio and stirred at 80 °C for 2 h. The change in color from light yellow to brownish-black indicated the formation of FeNPs (Figure 1(b)). The synthesized nanoparticles were collected by centrifugation (8,000 rpm, 15 min), washed twice with distilled water to remove ions and residues from the EB extract, and dried in an oven at 80 °C for 24 h.

The FeNPs were characterized using techniques such as FTIR, SEM, DLS, XRD, and TGA. For FTIR analysis, a Shimadzu IRAffinity-1 spectrometer was utilized. Samples were prepared as KBr discs, and the spectrum was recorded in the range of 4,000–400 cm⁻¹. SEM images were captured using a Zeiss EVO-MA10 instrument with an acceleration voltage of 20 kV. Particle size and frequency distribution were measured by the laser light scattering technique using DLS (Malvern Zetasizer Nano ZS, Malvern, UK). XRD analysis was conducted using a Bruker D8 Advance diffractometer with Cu Kα radiation, operating at a voltage of 40 kV and a current of 40 mA. TGA data were obtained using a Shimadzu DTG-60H thermogravimetric analyzer. The TGA curve was obtained using a synthetic air atmosphere and a heating rate of 20 °C min⁻¹, ranging from room temperature to 1,000 °C.

In the process of degrading organic contaminants, it is crucial that the resulting degradation products demonstrate non-toxic properties. Therefore, toxicity bioassays were carried out using Cucumis sativus seeds (cucumber) to evaluate the potential toxic effects of RhB and caffeine degradation products. Ten seeds were placed on 90 mm diameter Petri dishes containing filter paper moistened with 5 mL of the solution to be tested. The plates were then incubated at 23 °C for 5 days in the dark, with distilled water used as the negative control. Tests were conducted in triplicate. Subsequently, the germinated seeds, along with the lengths of the radicle and hypocotyl, were recorded for comparison with the control.

The decrease in the degradation of RhB and caffeine observed at higher pH values is attributed to the reduction in the oxidation potential of ·OH and the precipitation of iron oxides and hydroxides, leading to a decrease in the concentration of Fe²⁺ and Fe³⁺ ions in the solution (Wu et al. 2015). Furthermore, the degradation of RhB and caffeine declined to 89.03 and 59.59%, respectively, when the pH was reduced to 2.0. This decline is associated with hydrogen ions acting as scavengers for hydroxyl radicals (Babuponnusami & Muthukumar 2012) and the solvation of hydrogen peroxide, resulting in the formation of a stable oxonium ion [H₃O₂]⁺ in the presence of highly concentrated H⁺ ions (Rusevova et al. 2012). The identified optimum pH value (pH = 3.0) aligns with other studies that employed FeNPs as catalysts to degrade various pollutants, such as phenol (Babuponnusami & Muthukumar (2012)), monochlorobenzene (Kuang et al. 2013), bisphenol A (Guo et al. 2020), and methyl orange (Yuan et al. 2020).

In Figures 6(a) and 6(b), it is also possible to observe that in the system containing only H₂O₂, the degradation of RhB and caffeine remained minimal across all evaluated pH levels (1.94–4.77 and 1.95–3.88%, respectively). In contrast, in the system containing only FeNPs, RhB removal ranged from 28.71 to 36.14%, and caffeine removal ranged from 16.46 to 16.20%. These results can likely be attributed to the adsorption of these compounds by the nanoparticles. This affirms the excellent catalytic activity of FeNPs synthesized from the baru fruit endocarp extract in the Fenton oxidation of RhB and caffeine.

Despite the decrease in RhB and caffeine degradation at higher pH values, the efficacy of FeNP-mediated degradation remains satisfactory in the pH range of 4–6, particularly for RhB. This is advantageous for the practical application of the heterogeneous Fenton process in wastewater treatment. In contrast, the homogeneous Fenton process typically exhibits high efficiency only at pH values around 3.0. Dhahir et al. (2014) achieved close to 100% degradation rates for RhB in a UV/H₂O₂/Fe²⁺ system at pH 2.0 and 3.0; however, at pH 6.0, the rates declined to around 20%. Similarly, Oliveira et al. (2015) reported caffeine degradation rates of up to 95% using the homogeneous Fenton reaction at pH 3.0, but degradation sharply declined at pH values exceeding 4.0.

For RhB degradation, the determination coefficients (R²) for the pseudo-first-order model (0.917–0.968) and the pseudo-second-order model (0.954–0.985) indicated good fits for both models (Table 1), suggesting that RhB degradation involves prompt adsorption and simultaneous oxidation processes. However, the higher R² values for the pseudo-second-order model imply that the degradation of RhB is predominantly governed by a redox process (Guo et al. 2020). Similar conclusions were obtained for the degradation of methylene blue (Anchan et al. 2019) and reactive blue 238 (Hassan et al. 2020) using the Fenton system with green FeNPs. The reaction rate (k₂) calculated using the pseudo-second-order model at 25, 35, and 45 °C is 0.0194, 0.0212, and 0.0303 L mg⁻¹ min⁻¹, respectively.

The fittings for caffeine degradation were also satisfactory for both models. The R² values ranged from 0.933 to 0.993 for the pseudo-first-order kinetic model and from 0.912 to 0.964 for the pseudo-second-order kinetic model. Nevertheless, the pseudo-first-order model showed better agreement with the experimental data. The reaction rate (k₁) increased from 0.0114 to 0.0195 with the temperature rising from 25 to 45 °C.

The E_a for the oxidative degradation of RhB and caffeine was 17.46 and 20.98 kJ mol⁻¹, respectively. Typically, surface-controlled reactions exhibit higher E_a (>29 kJ mol⁻¹) compared to diffusion-controlled reactions (8–21 kJ mol⁻¹) in a solution. The calculated E_a values in this study indicate that the degradation of RhB and caffeine by FeNPs/H₂O₂ is a diffusion-controlled reaction. This conclusion aligns with those reported by Wu et al. (2015) and Li et al. (2015) who used green FeNPs for degrading malachite green and 2,4-dichlorophenol, respectively.

In this study, FeNPs were successfully synthesized using an aqueous extract from the EB. The synthesis method employed is characterized by its simplicity, efficiency, and sustainability. The characterization of FeNPs revealed their spherical morphology with an average diameter of 419.2 nm. The nanoparticles exhibited an amorphous nature, as observed through XRD analysis. FTIR analysis confirmed the role of phytochemicals in the baru endocarp extract, capping, and stabilizing the nanoparticles. The FeNPs demonstrated excellent catalytic activity, achieving removal efficiencies of 99.14% for RhB and 92.01% for caffeine. This was observed at an initial solution pH of 3.0, after 180 min, using 1.0 g L⁻¹ of FeNPs, 1% H₂O₂, and a temperature of 45 °C. Effective degradation of RhB and caffeine was also noted in a higher pH range (4–6), highlighting significant practical applications. The catalytic reaction kinetics were modeled as pseudo-first-order for caffeine and pseudo-second-order for RhB. The activation energy for the degradation of RhB and caffeine was calculated as 17.46 and 20.98 kJ mol⁻¹, respectively, indicating that the degradation of these organic compounds by FeNPs/H₂O₂ is a diffusion-controlled reaction. Studies on the phytotoxicity of the degraded products of RhB and caffeine with C. sativus confirmed their non-toxic nature, supporting the feasibility of employing FeNPs as an environmentally friendly solution for water treatment. This approach offers a sustainable method for pollutant remediation in aquatic environments.

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

The authors declare there is no conflict.