Nanocomposites from spent coffee grounds and iron/zinc oxide: green synthesis, characterization, and application in textile wastewater treatment

The textile industry is one of the largest consumers of industrial water and producers of effluents in the world. The dye concentration of textile effluents varies widely (Azanaw et al. 2022). In addition, these industries have high concentrations of chemical oxygen demand (COD), biochemical oxygen demand (BOD), total suspended solids (TSS), temperature, and salts (Yaseen & Scholz 2019). The discharge of dyed effluents negatively affects ecosystems because they inhibit the penetration of sunlight, generating environmental problems (Ardila-leal et al. 2021). Indigo carmine dye, also known as 5,5′-indigodisulfonic acid disodium salt, is classified as a vat dye and is a basic organic blue dye that has been used in the textile industry (Benkhaya et al. 2020). It is a commercially available dye and is cheap and stable, but it is also hazardous to human health and highly soluble in aqueous solutions. Currently, its main consumption is in the blue jeans industry, with an annual production that exceeds 120,000 tons (Hevira et al. 2020; Cesur Özcan & Gürel 2023). Some treatment methods that have been used in the decolorization of indigo carmine such as ozonation (Peramune et al. 2022), electrocoagulation (Oliveira et al. 2020), and filtration, are efficient at removing dye, but present equipment problems and high energy consumption (Mousset et al. 2021).

The use of nanotechnology has gained prominence owing to technological advancements and the use of a variety of nanomaterials such as nanotubes, nanocomposites, nanofilters, and nanoparticles (NPs) to build systems and have significant applications in environmental issues. Some of these drawbacks, for example, carbon nanotubes are related to the difficulty of producing them without defects and their high cost of production (Aghababai Beni & Jabbari 2022; Elsehly et al. 2023).

Specifically, NPs are used due to their properties related to high surface area, which often makes them stand out from other bulk materials (Kumar et al. 2021). Furthermore, it is possible to overcome the limitations of conventional treatment technologies and employ safe, cheap, and non-toxic methods in their synthesis. Various physicochemical methods have been employed for the synthesis of metallic NPs, but the synthesis of these NPs through environmentally benign methods is more attractive and promising due to the production of non-toxic, cost-effective, and high-quality NPs (Nair et al. 2022). Plants have been considered the best choice to reduce metal ions and are ideal candidates for large-scale production; their phytochemicals like terpenes, flavones, ketones, aldehydes, amides, and carboxylic acids can be utilized as reducing and capping agents, minimizing the agglomeration and oxidation of NPs (Albeladi et al. 2020; Nasrollahzadeh et al. 2020). Leaf extracts from eucalyptus (Eucalyptus globulus Labill.) and trumpet (Cuphea aequipetala Cav.) contain phenolic compounds such as syrupic acid, epicatechin, quercetin, gallic acid, robustaol B, and eucalyptin, among other polyphenols (Martınez-Bonfil et al. 2014; Vitta et al. 2020). Polyphenols are powerful antioxidants and free radical scavengers. Iron NPs can be prepared using plant extracts (polyphenols and caffeine) as intrinsic reductants and dispersants (Xiao et al. 2020). Many researchers have employed a green synthesis process for the preparation of metal/metal oxide NPs, such as iron and zinc, via plant leaf extracts (Katata-Seru et al. 2018; Jiménez-Rosado et al. 2022).

Zinc oxide (ZnO) NPs with iron oxide allow for obtain a material with improved photocatalytic properties, due to the addition of iron oxide, which reduces the width of the energy gap (Długosz et al. 2021; Elsehly et al. 2023; Sun et al. 2023). Zinc and iron oxide NPs have the ability to oxidize organic compounds, which makes them applicable for wastewater treatment (Aragaw et al. 2021). The application of supported NPs enables the reaction to be fast due to increased surface area and active sites. Therefore, NPs are able to work at low concentrations, reducing the amount of catalyst compared to that applied in conventional treatment. In the case of ZnO NPs, modifying them with other elements like iron increases their photocatalytic performance under visible light irradiation. The combination of two different metallic elements effectively improves the efficiency of nanomaterials through their individual metallic properties and the new properties that arise due to the synergistic effect (Scaria et al. 2020; Ferrier et al. 2022; Liang et al. 2022). Therefore, they may be useful for water and wastewater treatment, especially for recalcitrant contaminants, and thus could contribute to sustainable solutions to the water crisis (Scaria et al. 2020).

Coffee is a global consumer product; the annual global coffee consumption is about 8 million tons, and this consumption grows at an annual rate of 1.5% (Jin et al. 2018). Spent coffee grounds (SCGs), which are composed primarily of macromolecular cellulose and lignocellulose, can be used in sustainable processes and bioproducts due to their large amount of stock and renewability (Mangindaan et al. 2020). Six million tons of such residues are produced every year, according to the coffee trade record from the International Coffee Organization. The coffee extract could be used as a green reductant for synthesizing metallic NPs and function as a support material for heterogeneous reactions (Chien et al. 2019; Wu et al. 2019). In this paper, a novel composite was obtained with iron/zinc oxide nanoparticles (FeO/ZnO NPs) supported by SCGs. This nanomaterial was assayed for the degradation of indigo carmine in wastewater using a Fenton-like process.

Zinc nitrate [Zn(NO₃)₂ • 6H₂O] purity 99.1%, ferrous sulfate [FeSO₄ • 7H₂O] purity 100%, and hydrogen peroxide 30% reagent grade were acquired from Fermont, indigo carmine dye from chemical reagents HYCEL, and for pH adjustment, a 4 M of sulfuric acid solution and a 2 M of NaOH solution were used. Eucalyptus (E. globulus Labill) and trumpet (C. aequipetala Cav) leaves were purchased at the local market from Metepec, State of Mexico. SCGs were donated by a local coffee shop. This is a residue with a fine particle size obtained during the treatment of raw coffee powder with hot water during the preparation of coffee. The coffee beans were of Mexican origin.

Eucalyptus and trumpet leaves were washed several times with tap water to remove dust and any additional waste and were subsequently dried in the oven at 100 °C for 2 h, after which they were finely crushed and powdered with a mortar and pestle. The plants' extract was prepared following the method reported by Nagar & Devra (2018) and Siripireddy & Mandal (2017). 2.5 g of each plant were weighed and combined to obtain a total of 5 g, which was subsequently added to a beaker with 100 mL of distilled water. This mixture was then brought to a boil for 10 min and mixed using a magnetic stirrer before being filtered with standard filter paper to obtain the extract. The filtrate was used immediately for the biosynthesis of FeO/ZnO NPs.

For the biosynthesis of Zn/Fe oxide, 20 mL of 0.05 M Zn(NO₃)₂ and 20 mL of 0.02 M FeSO₄ were prepared. The reduction reaction was carried out in two steps: first, 2 g of SCG were weighed, washed, and placed in a 50-mL beaker, to which 20 mL of the Zn(NO₃)₂ solution was added, before being constantly stirred for 2 h. Separately, 20 mL of infusion was separated, and its pH of 12 was adjusted. This infusion was added to the zinc solution with the SCG, and the temperature was raised to 100 °C for 1 h. Subsequently, another 20 mL of the eucalyptus–trumpet extract infusion were added without pH adjustment, immediately followed by 20 mL of iron solution. The mixture was then constantly stirred for 1 h before being left undisturbed for 24 h, and then filtered through a standard filter paper, rinsed with distilled water, dried in the oven at 100 °C, placed in an airtight container, and reserved for later use.

UV–vis spectral analysis was performed using the VELAB VE-5600UV PC UV–visible spectrophotometer, where the spectrum corresponding to the surface plasmon resonance (SPR) typical of FeO and ZnO NPs was identified. The absorption of the sample was recorded at wavelengths ranging from 200 to 900 nm, at a resolution of 1 nm, with 1 cm long quartz cuvettes.

The Fourier transform infrared (FTIR) analysis was performed using the Bruker Tensor 27 Fourier transform infrared spectrometer with an MIR source with the Bruker platinum ATR accessory with diamond glass to identify the functional groups present. The spectra were obtained in the infrared region between 4,000 and 400 cm⁻¹. The material surface analysis and morphology of the biosynthesized NPs were studied using the JEOL JSM-6510LV scanning electron microscope (SEM), which was coupled with an X-ray detector in order to perform chemical analysis by means of energy dispersion (EDS), Oxford, with a resolution of 137 eV. The NP sizes were determined using transmission electron microscopy (TEM) on evaporated drops from the reaction mixture placed over carbon-coated grids. TEM observations were performed on a JEOL 2100 TEM operated at a 200 kV accelerating voltage with a LaB6 source. X-ray powder diffraction (XRD) was performed using XRD Rigaku/Ultima-IV equipment with Cu-kα radiation (1.5406 Å) at 45 kV and a scanning speed of 2°/min. The pHpzc of the FeO/ZnO + SCG composite was determined using the procedure as follows: 50 mL of distilled water was taken in 100 mL of Erlenmeyer flasks, adjusting the pH of each solution between 2 and 8 units, adding the appropriate amounts of 0.1 M of HCl and 0.1 M NaOH, a fixed dose of 10 mg composite. The mixture was shaken at 170 rpm for 48 h. Then, the samples were centrifuged, and the pH of each solution was measured (Farrokhi et al. 2014).

Optimization is one of the essential steps in both the development of a synthesis protocol and the evaluation of the interaction of reaction parameters influencing the biosynthesis. An experimental response surface Box–Behnken design was realized using the Minitab Ver. 19.0 statistical package. In order to optimize the Fenton-like process with the FeO/ZnO + SCG catalyst, three factors were considered with two levels each and six central points in duplicate, obtaining a total of 30 runs; the dye concentration, amount of catalyst, and amount of hydrogen peroxide were considered independent variables, while the percentage of dye removal was considered a dependent or response variable. The values of the independent variables were selected based on the literature that was previously reviewed during research carried out for the removal of dyes through Fenton and Fenton-like processes (Hassani et al. 2018; Nadeem et al. 2020; Bel et al. 2021); the obtained matrix can be observed in Table 1. The initial conditions of the experiments were a volume of 10 mL and a pH of 3.0 adjusted with a 0.1 M sulfuric acid solution at a reaction time of 120 min.

The textile industry effluents used in the present study were collected from Mohammedia in Almoloya del Río, State of Mexico, and contain a mixture of compounds, with indigo carmine as the main dye.

Once the optimal conditions were obtained, they were tested with a sample of textile wastewater. The wastewater treatment was performed in a beaker with 0.5 g/L of FeO/ZnO + SWPS added to 100 mL of IC aqueous solution (pH = 3.1), H₂O₂ concentration 8.8 mM/L, under constant stirring for 60 min to achieve equilibrium. Subsequently, it was placed under direct sunlight (651–787 W/m² in Toluca, México) for 60 min of contact time.

The sample was characterized by determined parameters such as pH, conductivity, turbidity, and COD in accordance with the procedures established in the Standard Methods for the Examination of Water and Wastewater, 2012 edition. The dye removal was measured through UV–visible spectrophotometry with the VE-5600UV PC spectrophotometer, using Equation (1) at the maximum absorbance of the sample.

The presence and shifting peaks in the FeO/ZnO NPs after reduction indicate interaction among the functional groups of the plant extract and the metallic precursors. The reduction and stabilization of NPs are aided by this interaction (Liu et al. 2018; Patiño-Ruiz et al. 2020). All the peaks below 1,000 cm⁻¹ should be attributed to the bonds between inorganic elements and therefore show the presence or absence of NPs (Akbar Jan et al. 2021). In this case, one slight peak was observed at 589, indicating the formation of iron oxides due to interaction between the iron and oxygen in the organic compounds (Fe–O) (Devatha et al. 2016; Ouyang et al. 2019), while the capping layer on the surface of NPs explains the absence of signs of ZnO.

The FTIR shown in Figure 2(b) revealed that SCG is composed of lignocellulosic materials; the sharp band at 1,741 cm⁻¹ is associated with carbonyl (C = O) vibration, while an absorption band between 1,700 and 1,500 cm⁻¹ may be assigned to the stretching vibration of C–C and C–N (Chien et al. 2019).

Table 2 shows the ANOVA analysis corresponding to the optimization of the Box–Behnken experimental design. The factors considered were dye concentration (A), catalyst dose (B), and H₂O₂ dose (C). In the Fenton process, the dye concentration was significantly affected at a 5% level of significance. The interactions between dye concentration and catalyst were observed to be significant. The interaction between the catalyst and H₂O₂ was not significant, which suggests the formation of different degradation products when Fe³⁺ was used.

The results, in accordance with the design of the experiments, determine that the highest removal efficiencies (95%) for indigo carmine dye occur at pH = 3, H₂O₂ concentration 8.8 mM/L, contact time = 120 min, initial concentration = 69 mg/L, and with 0.5 g/L FeO/ZnO + SCG.

Several Fenton investigations have shown that acidic pH levels, close to 3, are normally optimal for Fe oxidations. At low pH levels and in the presence of organic substrates, hydroxyl radicals can abstract a hydrogen atom, initiating the oxidation of Fe. One of the advantages of the Fenton reaction with a solid catalyst is that it reduces sludge generation despite the reaction's reliance on acidic conditions (Gökçe Didar Değermenci 2018; Anil et al. 2022; Wakrim et al. 2022).

Batch experiments were conducted to compare the removal efficiencies of indigo carmine through various processes. As shown in Figure 8(b), the FeO/ZnO + SCG composite exhibited relatively low catalytic activity, with only 6.48 ± 1.49% decolorization after 120 min; this suggests that the capacity of the catalyst alone is not significant. The degradation efficiency was approximately 26.60 ± 2.4% for only H₂O₂, due to the access of H₂O₂ molecules to the active sites, resulting in the generation of OH^• radicals (Sun et al. 2019). The best decolorization yield (95%) was achieved using FeO/ZnO + SCG composites reacting with H₂O₂ for 120 min to form active hydroxyl radicals in the aqueous medium.

As illustrated in Figure 10(b), the COD measurement showed that the COD value decreased from 724.5 to 321 mg/L. The remaining COD value in the solution could be due to the presence of intermediates, simpler compounds due to the breakdown of dye, and especially the complex composition of wastewater containing a concentration of recalcitrant organics, metals content and non-soluble substances (Flores et al. 2022).

The highest decolorization percentage (approximately 50%) occurs within the first 40 min. As the time increases to 120 min, 88% is achieved, as shown in Figure 9(a).

The Fe–Zn metal ions tend to increase reactive sites, photon adsorption, and the production of hydroxyl radicals. It also had another specific role, as FeO could activate Fenton-like photochemistry, while Zn could delay the electron–hole pair recombination rate, resulting in greater efficiency in the degradation of organic compounds (Talreja et al. 2021).

Table 3 includes the relevant literature on different composites with metallic NPs that have been applied as catalysts in Fenton-like processes for the removal of dyes. The cases applied in the Fenton process to decolorization dyes show the influence of calcination for obtaining zinc NPs, which is different from the process of this work, reducing time and energy. It has been experimented with mainly in synthetic solutions, in addition to the fact that the dye concentrations are lower than reported in this work.

The efficiency obtained by this work in the Fenton-like experiments is above that reported and compared to the concentrations used by the authors.

The SCGs-supported FeO/ZnO catalyst proposed in this study has been successfully prepared using plant leaf for the first time (C. aequipetala Cav). This method is environmentally safer than conventional chemical methods, which is important in terms of process economy for the removal of pollutants from wastewater by heterogeneous Fenton-like processes. XRD, TEM, SEM, FTIR, and EDS validated the synthesis, indicating the obtention of ZnO with a wurtzite structure and a hematite structure (Fe₂O₃). The combination of FeO/ZnO + SCG and hydrogen peroxide (H₂O₂) in the treatment generated hydroxyl radicals capable of degrading organic matter. The best activity for degradation of indigo carmine was at pH 3, 0.8 g/L of catalyst dosage, and 8.8 mM/L of H₂O₂ for 120 min. This allowed the nanocomposite to be applied in the treatment of residual water from textile dyeing, reducing COD, and color intensity. Thus, nanocomposite is a promising catalyst for the removal of organic dye in the practical application of wastewater treatment.

The authors gratefully acknowledge Consejo Mexiquense de Ciencia y Tecnología (COMECYT) for the financial support for the research stay EESP2021-0040, and the Centro Conjunto en Química Sustentable UAEMex-UNAM for the infrastructure provided.

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

The authors declare there is no conflict.