Sustainable Bio-Adsorbent Generated from Coffee Waste for Dual Application in Heavy Metal and Dye Removal

Abstract

Heavy metal and dye contamination from industrial wastewater present substantial dangers to both ecological systems and human well-being. This study explores the upcycling of Coffee Powder Trimmings (CPT), a biomass waste rich in oxygen-containing functional groups, for water remediation. CPT was first used to adsorb Cu²⁺ and Fe³⁺ ions, then pyrolyzed at 750 °C to form metal oxide biochar composites (Cu/CB and Fe/CB). Characterization confirmed the formation of CuO and Fe₃O₄ particles and the retention of key adsorption functionalities. The materials were evaluated for methylene blue (MB) removal across pH levels, various water bodies, and multiple reuse cycles. CPT effectively removed >95% of Cu²⁺ and Fe³⁺ via chelation, while Fe/CB achieved up to 97.8% MB removal due to synergistic π–π, hydrogen bonding, and coordination interactions. Both biochars retained high performance after five cycles, with Fe/CB maintaining 86.88% efficiency. These results highlight CPT-derived biochar as a sustainable, low-cost adsorbent for dual removal of heavy metals and dyes.

1. Introduction

Environmental and human health face severe repercussions from heavy metal pollution, which has emerged as a major global concern due to its high accumulation potential and toxicity [1,2]. Among the heavy metals, copper (Cu) and iron (Fe) are some of the most prevalent and significant contaminants. These metals are widely used across various industries, including electronics, construction, and transportation, leading to increased environmental discharge through industrial wastewater, mining, and agricultural runoff [3]. Excess copper in aquatic systems can disrupt enzyme functions [4,5] and damage proteins and cellular structures within organisms. In contrast, excess iron can reduce oxygen levels in water bodies, triggering anaerobic conditions and destabilizing ecological balance.

In addition to heavy metals, dye pollution from the textile industry poses a serious environmental concern [6]. Textile dyes, often complex organic compounds, are discharged into water sources during production and dyeing processes [7,8], where they persist due to their high stability and resistance to degradation. These dyes not only cause visible pollution by imparting color to water bodies, but also introduce toxic, carcinogenic, and mutagenic chemicals that can harm aquatic life and accumulate in the food chain [9,10,11]. Furthermore, some dyes block sunlight penetration through water, inhibiting photosynthesis in aquatic plants and thus disrupting entire aquatic ecosystems [12].

In order to protect ecological and human health, numerous nations have implemented regulatory limits on the levels of heavy metals and specific industrial dyes permitted in wastewater discharges [13]. For example, the United States Environmental Protection Agency (EPA) sets the maximum allowable concentration for copper in drinking water at 1.3 ppm [14,15]. In comparison, the recommended limit for iron is no more than 0.3 ppm [16,17]. However, despite these regulations, concentrations of heavy metals and dye pollutants in many water sources continue to exceed permissible limits, underscoring the urgent need for effective, low-cost, and environmentally friendly solutions to substantially remove these contaminants. Globally, the coffee industry generates over 6 million tons of coffee waste annually, primarily in the form of spent coffee grounds (SCGs) discarded after the brewing process. In this study, we use the broader term Coffee Powder Trimmings (CPT) to refer to a heterogeneous mixture of coffee-derived residues, including SCGs and dried coffee solids collected from industrial processing lines. While SCGs are the most widely studied fraction in the literature, CPT is used here to represent a more generalized category of coffee-based biomass waste suitable for upcycling into functional materials [18,19]. These materials are rich in functional groups, such as polyhydroxy and polyphenolic groups, making them ideal candidates for the adsorption of heavy metals and dyes [20,21].

Despite the promising potential of CPT as a sustainable, low-cost adsorbent, to the best of our knowledge, its dual application for heavy metal and dye removal remains underexplored [22]. In this study, CPT was initially applied for the removal of copper and iron, two of the most prevalent and impactful heavy metal pollutants [1,23]. Following adsorption, the CPT was pyrolyzed to form a biochar composite containing CuO and iron oxides, which was subsequently used for dye removal [1,24], demonstrating efficient adsorption capabilities for a range of industrial dyes. Given the similar need for effective and low-cost adsorbents for both heavy metal and dye pollutants, a single material capable of addressing both issues would be highly advantageous. This innovative approach provides a new approach to valorizing discarded coffee materials, showcasing CPT’s potential as a sustainable and versatile adsorbent for both heavy metal and dye contaminants. This study introduces an innovative approach where CPT is first utilized for heavy metal removal, followed by pyrolysis to produce a biochar composite containing CuO and iron oxides. This composite demonstrates high efficiency in removing industrial dyes, showcasing the dual application of a single material for environmental remediation.

2. Materials and Methods

2.1. Reagents and Chemicals

The Coffee Powder Trimmings (CPT) used in this study were provided by CH Biotech (Nantou, Taiwan). Iron(III) chloride hexahydrate (FeCl₃·6H₂O, ≥97.0%) was purchased from Echo Chemical (Miaoli, Taiwan), and copper(II) chloride dihydrate (CuCl₂·2H₂O, 98%) and sodium hydroxide (NaOH) were obtained from Emperor Chemical (New Taipei City, Taiwan). Methylene blue (MB, 0.1% aqueous solution) was purchased from Choneye Pure Chemicals (Taichung, Taiwan), and hydrochloric acid (HCl, <35%) was purchased from Union Chemical Works (Taoyuan, Taiwan). Ultrapure water and deionized (DI) water were used throughout all aqueous experiments.

2.2. Preparation of Metal-Loaded Biochar Composites (Fe/CB and Cu/CB) via Adsorption and Carbonization

CPT was first used to adsorb iron (Fe³⁺) and copper (Cu²⁺) ions before being converted into Fe/CB and Cu/CB through a carbonization process, as illustrated in Figure 1a. To prepare the material, CPT was rinsed several times with deionized (DI) water, dried, and stored in an oven at 80 °C. To facilitate the adsorption of Fe³⁺ and Cu²⁺, 6 g of dried CPT was mixed with either a FeCl₃·6H₂O (0.05 g·L⁻¹ in 100 mL DI water) or CuCl₂·2H₂O (0.03 g·L⁻¹ in 100 mL DI water). The mixtures were stirred at room temperature for 3 h to allow efficient adsorption of metal ions onto the CPT surface. The samples were then dried again at 80 °C until completely desiccated.

Following the adsorption process, the metal-loaded CPT was subjected to carbonization. The dried samples were placed in a muffle furnace and heated to 750 °C at a rate of 5 °C/min. The temperature was maintained at 750 °C for 3 h to ensure complete conversion of the CPT into biochar composites. The resulting materials, referred to as Fe/CB and Cu/CB, were then cooled to room temperature for further characterization and application. This process demonstrates the dual functionality of CPT as both an adsorbent for metal ions and a precursor for the synthesis of functional biochar materials.

2.3. Characterization of Fe/CB and Cu/CB

In this work, the concentration of metal ions was quantitatively estimated using an Inductively Coupled Plasma Mass Spectrometer (ICP-MS, NexION 2200, PerkinElmer, Waltham, MA, USA). The concentrations of organic dyes were determined at a wavelength of 664 nm using an ultraviolet-visible (UV-Vis) spectrophotometer (Tecan Austria GmbH SPARK, Grödig, Austria). The crystalline structure of all prepared materials was analyzed using X-ray diffraction (XRD) with a Bruker X-ray diffractometer (Bruker, Billerica, MA, USA). The morphologies and microstructures of the materials were characterized using a scanning electron microscope (SEM, JEOL, Tokyo, Japan) and high-resolution transmission electron microscopy (TEM, JEOL, Tokyo, Japan).

Thermogravimetric analysis (TGA) of the prepared materials was performed using a TGA analyzer (TGA 4000, PerkinElmer, USA). The specific surface area and pore size distribution of the material were determined using a gas adsorption analyzer (NOVAtouch, Anton Paar, Graz, Austria). Additionally, the chemical compositions of CPT, CB, Fe/CB, and Cu/CB were analyzed using Fourier-transform infrared (FT-IR) spectroscopy (iS5, Thermo Fisher Scientific, Waltham, MA, USA) with potassium bromide (KBr) pellets prepared at a 1:100 ratio (biochar: KBr, w/w). FT-IR measurements confirmed the functional group compositions of CPT, CB, Fe/CB, and Cu/CB within the wavenumber range of 4000–400 cm⁻¹.

To confirm the chemical state of materials, X-ray photoelectron spectroscopy (XPS, PHI 5000, ULVAC-PHI, Chigasaki, Japan) was employed to investigate the valence states of active surface elements. Additionally, the vibrational properties of the materials were characterized using Raman spectroscopy (MRID, ProTrusTech, Tainan City, Taiwan).

2.4. Adsorption Experiments for Organic Dyes

The adsorption of methylene blue (MB) was investigated at initial concentrations of 5, 10, 25, and 50 mg L⁻¹, prepared by dilution from a 1 g L⁻¹ stock solution. A 0.1 g portion of the adsorbent was added to 100 mL of MB solution and stirred at room temperature until adsorption equilibrium was reached. Samples were collected at regular intervals using a 1.0 mL syringe and centrifuged to separate the supernatant from the solid.

The adsorption performance was evaluated using the removal efficiency (RE, %), which quantifies the percentage of MB removed from solution and is calculated using the following equation:

$$RE (\%)={\displaystyle \frac{(C_0-C_e)}{C_0}}\times 100\%$$

(1)

where C₀ and C_e are the initial and equilibrium concentrations of MB (mg/L).

The MB concentration was determined using a UV-Vis spectrophotometer by measuring absorbance at its maximum wavelength (λₘₐₓ = 664 ± 1 nm). A linear regression equation obtained from a standard calibration curve was used to calculate the concentration in all samples.

The adsorption experiments were conducted under controlled operating conditions, including a fixed adsorbent dosage of 0.1 g and a reaction time of 120 min, at room temperature (RT). To investigate the effect of pH on the efficiency of methylene blue (MB) removal using Fe/CB, the pH of the solution was adjusted and maintained within the range of 3 to 11 using 0.1 M NaOH or 0.1 M HCl pH was selected as a key variable because it influences both the surface charge of the adsorbent and the ionization state of MB molecules, thereby affecting the overall adsorption behavior.

To determine the adsorption isotherms, a 0.1 g sample of each adsorbent (CB, Cu/CB, and Fe/CB) was used. The experiments were conducted at a fixed MB concentration of 50 mg/L, a pH of 7.5, and a stirring speed of 450 rpm, at temperatures of room temperature (RT), 40 °C, and 50 °C. Additionally, since Fe/CB exhibited superior adsorption performance, further temperature-dependent experiments were conducted at RT, 40 °C, 45 °C, and 50 °C to better understand and confirm the temperature effect on the adsorption process.

In order to elucidate the adsorption kinetics of methylene blue (MB) on the synthesized adsorbents, the experimental data were analyzed using both pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic models, as shown in Equation (2) [25,26].

The PFO model, which assumes that the rate of occupation of adsorption sites is proportional to the number of unoccupied sites, is expressed as:

$$\mathrm{l}\mathrm{n}(q_e-q_t)=ln{q}_e-k_{1}t$$

(2)

where q_e and q_t (mg/g) are the adsorption capacities at equilibrium and at time t, respectively, and k₁ (min⁻¹) is the rate constant.

The PSO model shown in Equation (3) [25,26], which assumes that the adsorption rate is determined by chemisorption involving valence forces, is expressed as:

$${\displaystyle \frac{t}{q_t}}={\displaystyle \frac{1}{k_2{q}_e^2}}+{\displaystyle \frac{t}{q_t}}$$

(3)

where k₂ (g/mg⁻¹min⁻¹) is the rate constant of the pseudo-second-order model.

By comparing the correlation coefficients (R²) and the consistency between the calculated and experimental q_e values, the dominant kinetic mechanism can be evaluated.

In addition to fitting the adsorption kinetics with the pseudo-first-order (PFO) and pseudo-second-order (PSO) models, the activation energy (E_a) of the adsorption process was also calculated to further evaluate the adsorption mechanism, as shown in Equation (4) [25]. The activation energy provides insight into the nature of the interaction between the adsorbate and adsorbent.

The Arrhenius equation was applied to estimate E_a:

$$k=A·e^{{\displaystyle \frac{{-E}_a}{RT}}}$$

(4)

where k is the rate constant (obtained from kinetic fitting), A is the frequency factor, E_a is the activation energy (kJ/mol), R is the universal gas constant (8.314 J/mol·K), and T is the absolute temperature (K).

The adsorption kinetics were investigated by measuring the adsorption capacity at various time intervals to evaluate the relationship between adsorption capacity and contact time. This analysis provides insight into the mass transfer behavior of pollutants between the adsorbent and the solution phase, serving as a key method for elucidating the underlying adsorption mechanism.

Two commonly used kinetic models were applied: the pseudo-first-order model (Equation (2)) and the pseudo-second-order model (Equation (3)). In these equations, q_e and q_t (mg/g) represent the adsorption capacities at equilibrium and at time t (min), respectively. The rate constants are denoted as k₁ and k₂ for each corresponding model.

2.5. Desorption and Reusability

Among the tested adsorbents, Fe/CB exhibited the highest removal efficiency for methylene blue (MB) and was therefore selected for regeneration and reusability evaluation. The desorption of MB was carried out using ethanol (EtOH) as the regenerating agent due to its proven effectiveness in disrupting dye–adsorbent interactions while preserving the biochar structure.

After each adsorption cycle, the used Fe/CB was immersed in ethanol and stirred until the filtrate became clear, indicating effective dye desorption. The sample was then rinsed thoroughly with deionized water to remove residual ethanol and dried at 80 °C prior to reuse. This adsorption–desorption cycle was repeated five times to assess the stability and reusability of the Fe/CB composite.

3. Results

3.1. Characterization of CPT, CB, Cu/CB, and Fe/CB

To understand the morphology and elemental composition of the prepared materials, SEM and EDS analyses were conducted and are shown in Figures S1 and S2. As presented in Figure S1a, the original CPT shows an irregular and porous structure with abundant cavities that are beneficial for adsorption. EDS analysis confirms that CPT mainly consists of carbon (51.58 wt%) and oxygen (47.56 wt%), with minor sulfur (0.39 wt%) and calcium (0.47 wt%) contents, indicating its organic and carbon-rich nature.

After pyrolysis, the SEM image of CB (Figure S1b) displays a more structured porous surface, and EDS shows an increase in carbon content (up to 87.30 wt%), demonstrating successful carbonization. The SEM image of Cu/CB (Figure S2a) reveals some surface modification, and EDS confirms the presence of Cu at a trace level of 0.02 wt%. For Fe/CB (Figure S2c), EDS indicates the incorporation of iron at 0.10 wt%, supporting the formation of iron oxides during pyrolysis. Used samples (Figure S2b,d) exhibit slight increases in Cu and Fe content—0.12 wt% and 2.34 wt%, respectively—due to adsorbed MB–metal complexes.

The synthesis and structural evolution of the materials are further illustrated in Figure 1a. The SEM images (Figure 1b–d) show that both Cu/CB and Fe/CB retain a porous architecture, with visible changes in surface morphology following metal incorporation. TEM images (Figure 1e–g) confirm the dispersion of CuO and Fe₃O₄ nanoparticles within the biochar matrix. These results demonstrate that CPT, after metal adsorption and pyrolysis, can be effectively converted into multifunctional biochar composites with trace metal oxide loading sufficient to enhance their reactivity while preserving their carbon-dominant structure.

Figure 2 illustrates the particle size distribution of CPT, CB, Cu/CB, and Fe/CB, showing the transformation of the materials through pyrolysis and metal incorporation. Raw CPT exhibits a broad particle size range of 50–450 µm, with a frequency peak around 150–200 µm, reflecting its unprocessed nature. After pyrolysis, CB demonstrates a significant size reduction, with particles ranging from 10 to 90 µm and a peak around 40 µm, indicating the breakdown of organic material and increased size uniformity. The incorporation of metals further reduces particle size to the micrometer scale. For Cu/CB, particle sizes range from 1000 to 5000 nm (1 to 5 µm), with a frequency peak at 3000 nm, attributed to the formation of CuO particles during pyrolysis. Fe/CB exhibits an even smaller size distribution, ranging from 1000 to 2500 nm (1–2.5 µm), with a peak at 1500 nm, likely due to the generation of iron oxides particles. These results highlight the progressive size reduction and improved uniformity of CPT-derived materials, enhancing their functionality for adsorption applications.

To elucidate the structural and textural properties of CPT-derived materials after pyrolysis and metal incorporation,

Table 1. Textural properties of prepared samples.

Sample	SBET (m2 g−1)	Vpore (cc g−1)	ID/IG
CPT	15.230	0.015	-
CB	450.493	0.025	1.18
Cu/CB	559.213	0.029	1.15
Fe/CB	492.097	0.020	1.19

and Figure 3 provide detailed insights. Table 1 summarizes the textural properties of the prepared materials, specifically CPT, CB, Cu/CB, and Fe/CB. The BET surface area (S_BET) of CPT is low at 15.230 m²/g, indicating minimal porosity. In contrast, CB shows a significant increase in surface area to 450.493 m²/g, attributed to the creation of mesoporous structures during pyrolysis. Cu/CB and Fe/CB exhibit even higher surface areas of 559.213 m²/g and 492.097 m²/g, respectively, demonstrating that metal incorporation further enhances the porosity and active surface area.

The pore volume (V_pore) also follows a similar trend, with CPT exhibiting a low value of 0.015 cc/g, increasing to 0.025 cc/g for CB. Cu/CB reaches the highest pore volume at 0.029 cc/g, while Fe/CB shows a slightly lower value of 0.020 cc/g, indicating variations in the structural modifications induced by Cu and Fe incorporation. The N₂ adsorption–desorption isotherms (Figure 3a) confirm these results, showing that CB, Cu/CB, and Fe/CB exhibit type IV isotherms, characteristic of mesoporous materials with enhanced adsorption capacities at higher relative pressures.

The pore size distribution (Figure 3b) reveals that CB, Cu/CB, and Fe/CB primarily possess mesopores in the range of 2 to 6 nm, with Cu/CB demonstrating a more pronounced mesoporous profile.

Figure 3c presents the XRD patterns of CB, Cu/CB, and Fe/CB, highlighting the crystalline structures of the biochar materials. The CB sample shows two broad diffraction peaks centered at approximately 2θ (Theta) = 25° (002) and 2θ = 43° (100), which correspond to the amorphous carbon structure commonly observed in biochar materials. These peaks indicate the presence of disordered graphitic layers, characteristic of pyrolyzed carbon-based materials. The broad nature of the peaks reflects the low degree of crystallinity typical of biochar derived from biomass precursors [18,27].

Despite the incorporation of Cu and Fe, no distinct peaks corresponding to copper oxide (CuO) or iron oxides (Fe₂O₃ or Fe₃O₄) are observed in the XRD patterns of Cu/CB and Fe/CB, as exhibited in Figure 3c. This absence can be attributed to two factors. Firstly, the atomic weights of copper and iron are significantly higher than that of carbon, making their relative atomic concentrations in the material very low, at less than 1 wt%. Secondly, the atomic fraction of these metals is even smaller due to their low loading levels [28]. As a result, any crystalline phases of CuO or iron oxides are either too minimal to produce detectable peaks or are highly dispersed within the carbon matrix, leading to a lack of distinct diffraction signals. The absence of metal oxide peaks is consistent with the low loading levels and high dispersion of Cu and Fe within the carbon matrix. This suggests that the biochar effectively incorporates these metals without significantly altering its overall crystalline structure.

Additionally, the Raman spectra (Figure 3d) provide insights into the carbon structure. The intensity ratio of the D to G bands (I_D/I_G) is 1.18 for CB, reflecting a moderate level of graphitization. Cu/CB exhibits a lower I_D/I_G ratio of 1.15, suggesting a higher degree of graphitization, which could enhance electrical conductivity and structural stability. In contrast, Fe/CB shows a slightly higher I_D/I_G ratio of 1.19, indicating a more disordered carbon structure, potentially providing more active sites for adsorption.

To investigate the functional groups and chemical changes resulting from pyrolysis and metal incorporation, the FTIR spectra of CPT, CB, Cu/CB, and Fe/CB were analyzed (Figure 3e). The spectrum of CPT exhibits a broad band at 3400–3500 cm⁻¹ corresponding to –OH stretching, as well as peaks at 2920–2850 cm⁻¹ and 1720 cm⁻¹ assigned to C–H and C=O vibrations, respectively. These features confirm the presence of hydroxyl, aliphatic, and carbonyl groups derived from cellulose, hemicellulose, and lignin in the raw biomass [29,30]. After pyrolysis, significant changes are observed in the CB spectrum. The reduction in the intensity of the –OH and C=O bands indicate the decomposition of organic matter and the dehydration of the biochar surface, characteristic of the carbonization process. Additionally, the C–O band around 1100–1000 cm⁻¹ remains, suggesting that oxygen-containing functional groups persist in the biochar, providing active sites for subsequent metal binding.

Furthermore, Cu/CB exhibits a new absorption peak emerging at 619 cm⁻¹, corresponding to Cu–O stretching vibrations [31]. This confirms the successful incorporation of CuO onto the biochar surface during the modification process. Moreover, a slight shift and reduction in the intensity of the C–O band indicate interactions between Cu²⁺ ions and oxygen-containing groups, such as –OH and C=O, during synthesis. And Fe/CB exhibits distinctive features that further validate the successful incorporation of iron. A broader –OH band around 3400–3500 cm⁻¹ is observed, indicating a higher abundance of hydroxyl groups compared to CB, which can be attributed to interactions with Fe ions. Additionally, new absorption bands at 620 cm⁻¹ and 817 cm⁻¹ correspond to Fe–O stretching vibrations of Fe₂O₃ and Fe₃O₄, respectively, confirming the presence of iron oxides on the biochar surface [32]. These results demonstrate the effective chemical modification of biochar and the integration of iron oxides during synthesis.

Figure 3f presents the TGA and derivative thermogravimetric (DTG) profiles of CPT, CB, Cu/CB, and Fe/CB, showing the weight loss and thermal degradation behavior of the samples as a function of temperature. In the TGA curves, raw CPT shows significant weight loss between 200 and 500 °C, attributed to the decomposition of cellulose, hemicellulose, and lignin. In contrast, CB exhibits enhanced thermal stability with reduced weight loss, while Cu/CB and Fe/CB show further delayed weight loss and higher degradation temperatures due to the stabilization effect of CuO and iron oxide particles. The DTG curves reveal that CPT has two major weight loss peaks (~300 °C and ~450 °C), corresponding to the decomposition of biomass components. After pyrolysis, CB shows a single peak at 450 °C, indicating a more stable carbon structure. Cu/CB and Fe/CB exhibit similar trends but with peaks shifted to 470–500 °C, confirming improved thermal resistance. These results demonstrate that pyrolysis and metal incorporation enhance the thermal stability of biochar, making it suitable for high-temperature applications.

3.2. Effect of Contact Time on Cu²⁺ and Fe²⁺ Removal from Aqueous Solutions Using CPT

The effect of contact time on the removal efficiency of Cu²⁺ and Fe²⁺ from aqueous solutions using CPT is examined, as shown in Figure 4a–c. The data reveal that the removal efficiency of both Cu²⁺ and Fe²⁺ increases rapidly within the initial 20 min, reaching a plateau as equilibrium is approached. Fe²⁺ shows a slightly higher removal efficiency than Cu²⁺, suggesting that Fe ions are adsorbed at a faster rate onto CPT. The adsorption kinetics were evaluated using both the pseudo-first-order (PFO) and pseudo-second-order (PSO) models. The R² values indicate that the PSO model provides a better fit for both Cu²⁺ (R² = 0.998) and Fe²⁺ (R² = 0.999), suggesting that chemical interactions likely dominate the adsorption mechanism.

Figure 4b presents the PFO kinetic model plots for the adsorption of Cu²⁺ and Fe²⁺ onto CPT. The linear fit for Cu²⁺ (R² = 0.883) is weaker than that for Fe²⁺ (R² = 0.997), indicating that the PFO model does not adequately describe the adsorption process, particularly for Cu²⁺. This finding is consistent with the preference for the PSO model in describing the adsorption kinetics. In Figure 4c, the PSO kinetic model plots for Cu²⁺ and Fe²⁺ adsorption show strong linearity, with high R² values (R² = 0.998 for Cu²⁺ and R² = 0.999 for Fe²⁺), confirming that the PSO model is a superior fit. This suggests that the adsorption of both ions onto CPT is likely governed by chemisorption, where the rate-limiting step involves electron sharing or exchange between the adsorbent and the adsorbate. These results demonstrate that CPT is an effective adsorbent for Cu²⁺ and Fe²⁺, with rapid adsorption kinetics and strong alignment with the PSO model. This reveals its potential as a low-cost, sustainable option for heavy metal removal in water treatment applications.

3.3. Effect of Contact Time and Temperature on MB Removal from Aqueous Solutions Using CB, Cu/CB, and Fe/CB

Figure 5 illustrates the effect of contact time and temperature on the removal of MB using CB, Cu/CB, and Fe/CB, along with the corresponding kinetic models and thermodynamic analysis. The adsorption capacity (q_t) of MB increases rapidly in the first 40 min for all materials, after which it gradually approaches equilibrium. Fe/CB shows the highest adsorption capacity, followed by Cu/CB and CB, indicating that the incorporation of Fe and Cu significantly enhances MB removal efficiency. The pseudo-first-order (PFO) and pseudo-second-order (PSO) models were applied to describe the adsorption kinetics. The R² values reveal that the PSO model fits the data better (R² = 0.990 for Fe/CB), suggesting that chemisorption dominates the adsorption process. Figure 5b shows the PFO plots, where the fit is less accurate, particularly for Fe/CB and Cu/CB. In contrast, the PSO model in Figure 5c provides an excellent fit for all three materials, with R² values of 0.990 for Fe/CB, 0.985 for Cu/CB, and 0.948 for CB as summarized in

Table 2. Kinetic parameters for the adsorption of methylene blue (MB) onto CB, Cu/CB, and Fe/CB at 30 °C, with an initial dye concentration (C₀) of 10 mg L⁻¹, adsorbent dosage of 6 g, and solution volume of 0.1 L. Both pseudo-first-order and pseudo-second-order models are applied to evaluate adsorption performance.

Sample	Experiment qe (mg/g)	Pseudo-First Order Kinetics			Pseudo-Second Order Kinetics
		qe (mg/g)	k1 (min−1)	R2	qe (mg/g)	k2 (g/mg·min)	R2
CB	41.086	47.050	−0.032	0.973	44.316	0.002	0.948
Cu/CB	52.520	64.846	−0.046	0.978	55.431	0.002	0.985
Fe/CB	55.242	46.610	−0.035	0.976	57.607	0.003	0.990

. These results indicate that the MB adsorption process primarily follows chemisorption pathways, consistent with PSO kinetics, and the improved performance of Cu/CB and Fe/CB can be attributed to coordination interactions and π–π stacking with MB molecules on the carbonaceous matrix. The kinetic behavior highlights the superior efficiency and reactivity of metal-modified CPT-derived biochar for dye removal.

In addition, the reaction temperature effect is essential for understanding thermodynamic behavior, adsorption mechanisms, and the practical applicability of adsorbents under varying environmental conditions. The removal efficiency of MB at different temperatures (40 °C in Figure 5d and 50 °C in Figure 5e) increases with temperature, indicating an endothermic adsorption process. Fe/CB consistently exhibits the highest removal efficiency, achieving near-complete MB removal within 60 min, followed by Cu/CB and CB. The enhanced performance of Fe/CB can be attributed to the presence of iron oxide particles, which provide additional active sites for adsorption. The Arrhenius plots (lnK vs 1/T) in Figure 5f reveal a linear relationship for all three materials, with Fe/CB exhibiting the highest activation energy (E_a) of 48.4 kJ/mol and the highest linearity (R² = 0.990). This indicates that the adsorption process on Fe/CB is more thermodynamically favorable and efficient at higher temperatures compared to Cu/CB (E_a = 38.2 kJ/mol, R² = 0.871) and CB (E_a = 39.8 kJ/mol, R² = 0.839). The higher activation energy of Fe/CB suggests a stronger interaction between MB molecules and the adsorbent surface, likely due to the presence of iron oxide particles that provide additional active adsorption sites.

As summarized in

Table 3. Comparison of adsorption capacity (qₑ) and activation energy (Eₐ) for methylene blue (MB) adsorption onto various materials, including CB, Cu/CB, and Fe/CB from this study. Adsorption capacity was measured at room temperature.

Materials	k (min−1) at Room Temperature	qe (mg/g)	Ea (kJ/mol)	Reference
CB	0.032	41.086	53.7	This study
Cu/CB	0.046	52.520	35.6	This study
Fe/CB	0.035	55.242	48.4	This study
FRB/O71-H89—N4	0.02	280.61	54.21	[33]
SU-KOH	0.039	384	92.4	[34]
AC-GS	0.027	208.29	94.65
SS+TW biochar	0.008	5.8871	86.3	[35]
Sludge-derived biochar	0.057	1.083	72.93	[36]

, Cu/CB and Fe/CB show higher adsorption rate constants (0.046 and 0.035 min⁻¹, respectively) than CB (0.032 min⁻¹), indicating enhanced adsorption kinetics after metal incorporation. Both also exhibit improved adsorption capacities (qe = 52.52 and 55.24 mg/g, respectively) compared to CB (41.09 mg/g), due to additional active sites from metal oxides. In terms of activation energy, Cu/CB has the lowest E_a (35.6 kJ/mol), suggesting more favorable adsorption, while Fe/CB shows a moderate E_a (48.4 kJ/mol) but superior q_e. Compared to previous studies, the CPT-derived adsorbents achieve a balanced performance in terms of kinetics and adsorption capacity, supporting their effectiveness for dye removal.

3.4. Influence of Dosage, pH, and Water Bodies on MB Removal from Aqueous Solutions Using Fe/CB

Fe/CB was selected for further testing due to its superior adsorption performance, as evidenced by its high removal efficiency, thermal stability, and favorable activation energy values compared to CB and Cu/CB. Investigating the effects of dosage, pH, and water bodies is critical to understanding the practical applicability and performance of Fe/CB under various conditions.

Figure 6a shows the effect of dosage on MB removal efficiency. The removal efficiency increases with increasing Fe/CB dosage, from 34.5% at 30 mg to 97.8% at 150 mg. This trend occurs because higher dosages provide more active sites for adsorption, enhancing the adsorption capacity. These results highlight the importance of optimizing the adsorbent dosage to achieve maximum removal efficiency while minimizing material usage.

The effect of pH on MB removal is shown in Figure 6b. Fe/CB achieves high removal efficiencies over a wide pH range, with a maximum removal of 95.2% at pH 4. As pH increases, removal efficiency decreases slightly, reaching 79.3% at pH 9. The results suggest that acidic to neutral conditions favor MB adsorption, likely due to electrostatic attraction between the positively charged MB molecules and the Fe/CB surface at lower pH values. This demonstrates the importance of pH optimization for adsorption in practical water treatment scenarios.

Figure 6c explores the effect of different water bodies (lake, rain, tap, and seawater) on MB removal. Fe/CB achieves 100% removal efficiency in all tested water types, confirming its robust performance in varying water matrices. This indicates that Fe/CB is highly adaptable and effective for real-world applications, even in complex water environments with potential interference from other ions or organic matter.

3.5. Recyclability of Fe/CB

Recyclability is a crucial parameter for assessing the cost-effectiveness and long-term sustainability of an adsorbent. Figure 6d shows the recyclability of Fe/CB over five adsorption–desorption cycles. Fe/CB maintains a 100% removal efficiency for the first three cycles and decreases slightly to 97.98% in the fourth cycle and 86.88% after the fifth cycle. This minor decline in performance is likely due to partial saturation or loss of active sites during repeated use. Nevertheless, the results demonstrate that Fe/CB possesses excellent reusability, making it a promising and sustainable material for practical water treatment applications.

3.6. The Proposed Reaction Mechanism for Metal Ion and MB Removal Using CPT

In order to characterize the reaction mechanism for metal ions using CPT and MB removal with Fe/CB, XPS analysis was conducted. As shown in Figure 7a, the survey spectra of fresh CPT confirm the abundant presence of oxygen- and nitrogen-containing functional groups, including carboxyl (–COOH), hydroxyl (–OH), and carbonyl (C=O) groups. These groups provide active sites for metal ion adsorption through electrostatic interactions and chelation mechanisms. Further analysis of the C 1s and O 1s spectra (Figure 8) demonstrates the critical role of these functional groups in metal ion binding. After adsorption, the peaks corresponding to C=O, C–O, and –COOH groups in the O 1s spectra (Figure 8d–f) decrease significantly in intensity, while new peaks associated with Fe–O and Cu–O bonds appear, indicating the formation of stable coordination complexes between the metal ions and surface functional groups on CPT [37,38].

After carbonization, CB exhibits excellent MB removal capabilities through π-π interactions, coordinate covalent bonding, and precipitation mechanisms [39]. The highly porous structure of CB, as shown in the SEM images (Figure 9a), facilitates the diffusion of MB molecules, while its graphitized carbon regions enable π-π stacking interactions with the aromatic rings of MB. The C_1s spectra in Figure 8a highlight a decrease in sp² hybridized carbon (C=C) after MB adsorption, confirming the π-π interactions. Additionally, the presence of oxygen-containing groups such as C=O and C–O supports the formation of coordinate covalent bonds with MB molecules, as evidenced by their decreased intensity in the XPS spectra post-adsorption [40].

On the other hand, Fe/CB, due to the incorporation of Fe, demonstrates enhanced adsorption efficiency compared to CB. In addition to the mechanisms observed in CB, Fe/CB introduces hydrogen bonding as an additional interaction mode. This is supported by the O_1s spectra in Figure 8f, which show a distinct contribution from -OH groups that facilitate hydrogen bonding with MB. The mapping images in Figure 9e confirm the uniform distribution of Fe on the biochar surface, providing additional active sites for adsorption. Furthermore, Figure 10 illustrates the combined mechanisms, including π-π stacking, hydrogen bonding, and the precipitation of MB facilitated by Fe/CB. The addition of Fe further improves the adsorption performance by enabling hydrogen bonding, making Fe/CB a highly efficient multifunctional adsorbent for environmental remediation.

4. Conclusions

In this study, Coffee Powder Trimmings (CPT) demonstrated excellent efficacy in removing heavy metals such as Fe³⁺ and Cu²⁺ due to their rich functional groups, including carboxyl (-COOH), hydroxyl (-OH), and carbonyl (C=O). Post adsorption, the pyrolyzed materials, Fe/CB and Cu/CB, showed remarkable capabilities for methylene blue (MB) removal through mechanisms like π-π interactions, coordinate covalent bonding, and precipitation. Fe/CB, benefiting from the integration of iron oxides, exhibited additional hydrogen bonding interactions, achieving up to 97.8% removal efficiency under optimized conditions. Further experiments revealed that Fe/CB maintained high performance across a wide pH range and various water types (lake, rain, tap, and seawater), achieving near 100% removal efficiency. Moreover, recyclability tests demonstrated the material’s sustainability, retaining over 86.88% efficiency after five cycles. These results confirm that CPT effectively adsorbs heavy metals and can be further utilized as an adsorbent for dye removal, presenting a green and sustainable pathway for circular reuse.