Enhanced Removal of Organic Compounds Assisted by Activated Carbon/Copper (II) Complex Composite

Abstract

Industrial wastewater treatment is essential to mitigate pollution and address global water scarcity. In this study, an activated carbon/copper (II) complex (AC/CuL) composite was obtained for enhanced removal of organic compounds by coupling adsorption and oxidation. Tests were performed using Drimaren Red X-6BN (DRX-6BN) and oily effluent at pH~6.0. Tests to obtain the adsorption kinetics of DRX-6BN (20 mg/L) were performed at 25 °C and using an amount of 0.42 g/200 mL of solution. The data were well fitted by several models, suggesting a complex adsorption process; however, the best fit was achieved by the pseudo-second-order (PSO) model (R² = 0.9996). The adsorption data best fit the Freundlich model. The addition of hydrogen peroxide to the system reduced the need for adsorbent, removing approximately 100% of the chemical oxygen demand (COD) from the emulsion in 120 min using only 0.04 g of AC/CuL. The material exhibited high storage stability and maintained its effectiveness in removing oil and grease (O&G) content and COD for at least 12 months. These results indicate that AC/CuL is promising for the removal of complex organic compounds, such as that from the textile and petroleum industries. These findings offer a sustainable, economical, and safe approach for wastewater treatment.

1. Introduction

Modern industry has been expanding rapidly and continuously, leading to several environmental challenges [1]. Among them, the disposal of wastewater generated by intensive industries contains large amounts of chemical pollutants that, without proper adequacy, pose a widespread threat throughout the world due to the potential for environmental pollution, especially of water [2,3].

Water scarcity is a major concern. By 2025, it is estimated that two-thirds of the population will live in regions with scarcity owing to use, growth, and climate change [4]. The United Nations (UN) and its partners have been working to achieve the Sustainable Development Goals (SDG), which are related to ensuring safe drinking water and sanitation (SDG 6), health and well-being (SDG 3), sustainable cities and communities (SDG 11), action against global climate change (SDG 13), life below water (SDG 14), and on land (SDG 15) [5].

Despite its economic importance, the textile industry generates effluents containing compounds that are difficult to remove. Among them, dyes are the main ones, such as the Drimaren Red X-6BN (DRX-6BN) reagent, which contains azo groups (-N=N-) in its structure, which are responsible for its characteristic color [6]. The presence of -N=N- in synthetic dyes makes them highly stable, toxic, and non-biodegradable, leading to a decrease in the concentration of dissolved oxygen in water when released into a receiving water body [1,7].

Oil is the main source of energy and revenue worldwide, being an important strategic resource and known as the “blood” of the industry [8,9]. In the field, its occurrence is associated with gas and water, which, due to the pressure drop that occurs during extraction, generates emulsions [10,11]. Combined with the increase in oil production, the generation of a complex effluent containing oil and, often, chemical additives that increase the chemical oxygen demand (COD) deserves attention [12,13].

Over the years, several technologies have been developed for the removal of organic compounds. Among them are physical–chemical processes such as coagulation/flocculation, chemical precipitation, advanced oxidation, electrochemical processes, ion exchange, adsorption, and membrane separation processes, as well as biological treatments [7]. Of these, adsorption is a widely used technology owing to its easy implementation, low cost, and lack of toxic byproduct formation [14].

During adsorption in aqueous media, pollutants are attracted to the surface of a porous adsorbent through mass transfer and diffusion from the medium to the active sites of the material, and activated carbon (AC) is recognized as a highly efficient adsorbent [15,16]. However, as the pollutant will only be phase-transferred, problems of secondary environmental contamination may occur [1]. In particular, the development of technologies to degrade adsorbed contaminants is a point of attention.

Advanced oxidative processes (AOPs) are efficient in the degradation of a range of compounds because of the production of reactive radicals with high oxidation potential, such as hydroxyl radicals (•OH), which are non-selective and can promote the degradation of several contaminants through rapid reactions [17,18]. Of these, the Fenton reaction is a typical AOP that aims to produce •OH from the catalysis of the disproportionation of hydrogen peroxide (H₂O₂) by ferrous ions (Fe²⁺), which has been combined with adsorptive processes [19].

The acidic operating pH (2.5–4.0) and the generation of chemical sludge at the end due to the precipitation of iron hydroxide [20,21] inhibit the application of the Fenton process to a certain extent. However, a Fenton-like reaction can be carried out using copper coordination compounds as a substitute for iron to work over a wide pH range, including near-neutral conditions, with less critical deactivation by precipitation as oxide/hydroxide [22,23]. The degradation of organic compounds such as methyl orange, triclosan, humic acid, and phenol by homogeneous copper complexes has been reported [24,25,26,27,28].

In a previous study, [bis-N-(2-hydroxyethyl)salicylaldiminate]copper(II) (CuL), used in the present work, was used to degrade the reactive dye Drimaren Red X-6BN (DRX-6BN) in the absence of light [29]. The highest reaction rate (k_app = 4.05 × 10^–2 min⁻¹) and removal of DRX-6BN (90.17 ± 0.52%) were achieved at pH 8.82 ± 0.05, and the initial concentrations of H₂O₂ and CuL were 1.17 × 10^–2 and 2.67 × 10^–4 mol/L, respectively.

AC composites with iron oxides have been reported in the literature [30,31,32,33,34]. With regard to copper oxides, Li and co-authors (2019) used an AC composite with Cu-Ni bimetallic oxides at a dosage of 4 g/L for the removal of 100 mg/L of quinoline. The results showed a removal of ~90.84% in 18 min at a pH range between 6.5 and 8.0, when employing 0.066 mol/L H₂O₂, 60 °C, and microwaves, demonstrating that this technology is viable and promising [35]. Recently, Wang et al. (2025) developed a copper and nitrogen co-doped biochar via pyrolysis, which efficiently removed tetracycline in the presence of ultraviolet light [36].

Wastewater from the textile and petroleum industries presents unique challenges owing to its complex composition and high pollutant loads. In the case of the former, effluents are usually colored and have high COD and dissolved solid concentrations [6]. On the other hand, oil wastewater contains high levels of hydrocarbons, O&G, COD, aromatic compounds, and many other compounds resistant to biodegradation [12,13,37]. The approach combining the adsorptive capacity of coal associated with the catalytic ability of CuL is innovative and particularly advantageous, especially where there is a need to simultaneously treat different classes of pollutants, offering superior efficiency and operational viability compared to technologies that rely exclusively on conventional processes.

Although the use of new adsorbents with high adsorption capacity associated with Fenton oxidation (or Fenton-like) may represent effective technology for the treatment of pollutants in aquatic systems [31], to the best of our knowledge, studies on the enhanced removal of organic compounds by activated carbon composites with copper (II) complexes have been neglected, especially when using oily effluent as a model substrate and in the absence of light. Thus, this study aims to (1) characterize the obtained AC/CuL composite; (2) investigate the adsorption kinetics and isotherms of AC/CuL for DRX-6BN; (3) evaluate the association between the adsorption and oxidation of DRX-6BN in the presence of H₂O₂ in the AC/CuL composite; (4) evaluate the enhanced removal promoted by the composite in the treatment of oily effluent; and (5) evaluate the stability of the material during storage. This study demonstrated that the AC/CuL is promising for the enhanced removal of organic compounds, such as DRX-6BN, and those responsible for the increase in COD and oil and grease (O&G) present in oily effluents, with high removal efficiency and storage stability, even in the absence of light.

2. Materials and Methods

2.1. Reagents and Supplies

Salicylaldehyde (C₇H₆O₂, 98.00%) and ethanolamine (C₂H₇NO, ≥99.00%) were obtained from Sigma-Aldrich (Rio de Janeiro, Brazil). Methanol (MeOH, CH₃OH, ≥99.80%) and copper (II) acetate monohydrate (Cu(OAc)₂.H₂O, ≥ 98.00%) were obtained from Vetec (Rio de Janeiro, Brazil). The carbon used was activated carbon (AC) powder C (Carbomafra C-117), of vegetable origin, purchased from Indústrias Químicas Carbomafra (Curitiba, Brazil). Sodium hydroxide (NaOH) was obtained from Isofar (Rio de Janeiro, Brazil). Hydrogen peroxide (H₂O₂, 50% v/v) used in the degradation tests was obtained from Sumatex Produtos Químicos Ltd (Rio de Janeiro, Brazil). Drimaren Red X-6BN (DRX-6BN, CI Reactive Red 243), obtained as a powder, supplied by Clariant (Rio de Janeiro, Brazil). The oil-in-water (o/w) emulsions were prepared using oil from a Brazilian offshore oil production unit (28° API), aiming to reproduce the characteristics of real production water (PW), using a procedure similar to that of Ferreira et al. (2021) [37], with the aid of a Turrax mixer (Ultra-Turrax T-25, IKA^®Works, Staufen, Germany) under continuous stirring (15,000 rpm) and heating (60 °C). All reagents were used without prior purification, and the solutions were prepared in deionized water.

2.2. Activated Carbon/Copper (II) Complex (AC/CuL) Composite Obtaining

The synthesis of [Bis-N-(2-hydroxyethyl)salicylaldiminato]copper(II) (molar mass of 391.91 g/mol) (CuL) was carried out in situ following the methodology described by da Silva et al. (2024) [29]. Ligand (L) was prepared by the equimolar reaction of salicylaldehyde with ethanolamine in methanol under magnetic stirring for 120 min. A methanol solution of Cu(OAc)₂.H₂O was added to form a dark-green solution, which was stirred for another 120 min. After filtration and standing, olive green crystals were obtained in a 90% yield. The stoichiometry of the reaction between the Cu(OAc)₂.H₂O and L is 1:2. The ligand and CuL were synthesized at room temperature (~25 °C).

The AC/CuL composite was obtained using the methodology described by Oliveira et al. (2002) and Jain et al. (2018), with some modifications [33,38]. The AC was impregnated in a 1000 mL beaker with a useful volume of 400 mL under mechanical stirring at 60 °C. The mass of CuL in the beaker was equal to that of AC (AC:CuL ratio of 1:1). After approximately 10 h of impregnation, 100 mL of a 5 mol/L NaOH solution was added to the material. The composite was filtered, washed with deionized water to a neutral pH, and dried for 24 h at 60 °C. The material was macerated in a grail and pistil for better homogenization. The material was then transferred to a conical polypropylene tube (Falcon tubes) for storage to avoid contamination.

2.3. Characterization Methods

The functional groups present on the surfaces of CuL, AC, and AC/CuL were analyzed by Fourier transform infrared spectroscopy (FT-IR) using a Shimadzu IRSpirit FT-IR spectrophotometer (Kyoto, Japan) in the spectral range of 4000–400 cm⁻¹. The X-ray analysis (XRD) technique, using the powder method, was employed to determine the crystalline nature of the samples using a Rigaku Ultima IV Diffractometer (Tokyo, Japan) with a high-frequency X-ray generator (3 kW), Cu X-ray tube (λ = 1.5418), normal focus (2 kW), Universal Theta-2Theta goniometer with a radius of 185 mm, fixed grooves, and Kβ Ni filter, with a voltage of 40 kV and current of 20 mA, varying the 2θ angle in 5° increments, using a step of 0.05° and a counting time of 1 s for each step.

The materials were sputtered with a thin layer of gold, and their morphologies and surface compositions were analyzed using a scanning electron microscope (SEM) equipped with an energy dispersive X-ray (EDX) using an FEI TESCAN VEGA 3 (Brno, Czech Republic). Textural characteristics were determined by nitrogen (N₂) physisorption analysis performed at −196 °C using a Micromeritics^® TriStar II 3020 surface area and porosity analyzer (Norcross, GA, USA). Pretreatment of the samples was performed 24 h before degassing at 300 °C. Surface area and pore volume/diameter were calculated using the Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods, respectively.

The pH at the point of zero charge (pH_pzc) analysis was performed according to the methodology described by Mustafa et al. (2024), with modifications [39]. To 50 mL of a 0.1 mol/L NaCl solution adjusted to pH 2, 4, 6, 8, 10, and 12 using 0.1 mol/L HCl or NaOH, 0.050 g of AC or AC/CuL was added. The flasks were shaken for 24 h at 200 rpm. The final pH values were plotted against those of the initial pH, and the pH_pzc was determined.

2.4. Adsorption and Degradation Assays of Organic Compounds

The tests were carried out on a shaking table using 250 mL Erlenmeyer flasks with a useful volume of 200 mL of adsorbate. The adsorbates used were DRX-6BN at an initial concentration (C_DRX0) of 20 mg/L, and an o/w emulsion with an initial O&G concentration (C_O&G0) of 100 ± 5 mg/L. A pH of ~6.0, varied doses of adsorbent (0.01–0.60 mg/200 mL), a temperature of 25 °C, and a stirring rate of 200 rpm were used. In summary, 200 mL of the adsorbate and different masses of AC or the AC/CuL composite were added to a 250 mL Erlenmeyer flask and then transferred to a shaking table. The mass of the adsorbent was based on a study by Khader et al. (2022) [40]. For the tests using DRX-6BN, the concentration of DRX-6BN was evaluated. For tests using oily effluent, the concentrations of O&G and chemical oxygen demand (COD) were evaluated. The tests to evaluate the degradation of organic compounds were carried out under the same conditions described above in the presence of 20 mg/L of H₂O₂ added to Erlenmeyer flasks after the addition of the adsorbent. H₂O₂ concentration was based on a previous study by Silva et al. (2019) [41]. In both cases, the removal efficiency was calculated using Equation (1). The adsorption capacity (q) was calculated using Equation (2).

(1)$\%Efficiency=\left(1-{\displaystyle \frac{C}{C_0}}\right)\times 100$

(2)$q=\left({\displaystyle \frac{C_0-C_e}{m}}\right)\times V$

where C₀ represents the initial concentration (mg/L) of DRX-6BN (C_DRX-6BN0), COD (COD₀), or O&G (C_O&G0); C represents the concentration over time (mg/L) of DRX-6BN, or the COD or O&G of the o/w emulsion; C_e is the adsorbate concentration at equilibrium (mg/L); V is the volume of the working solution (L); and m is the mass of the adsorbent (g).

Storage stability tests were performed in similar experiments, using o/w emulsions with an O&G concentration of 100 ± 5 mg/L, AC/CuL mass of 0.04 g, pH~6.0, 25 °C, 200 rpm, 120 min. In the presence of H₂O₂, its H₂O₂ concentration was 20 mg/L.

2.5. Adsorption Isotherms and Adsorption Kinetics

To obtain information about the adsorption mechanism, adsorption isotherms and kinetics were obtained.

To evaluate the process kinetics, linearized pseudo-first-order (PFO) and pseudo-second-order (PSO) models, Equations (3) and (4), respectively, were used. The intraparticle diffusion model (Equation (5)) and the Elovich Equation (Equation (6)) were also considered.

(3)$\mathit{ln}\left(q_e-q_t\right)=\mathit{ln}\left(q_e\right)-k_1\times t$

(4)${\displaystyle \frac{t}{q_t}}={\displaystyle \frac{1}{k_2\times q_e^2}}+{\displaystyle \frac{t}{q_e}}$

(5)${\displaystyle \frac{q_e-q_t}{q_e}}={\displaystyle \frac{6}{{\pi }^2}}\times {\displaystyle \frac{1}{\sqrt{\tau }}}\times t^{{\displaystyle \frac{1}{2}}}$

(6)$q_t={\displaystyle \frac{1}{\beta }}\mathit{ln}\left(\alpha \times \beta \right)+{\displaystyle \frac{1}{\beta }}\mathit{ln}\left(t\right)$

where k₁ is the PFO adsorption rate constant (min⁻¹), q_t is the amount adsorbed per gram of adsorbent at time t (mg/g), k₂ is the PSO adsorption rate constant (g/mg · min), τ is the characteristic intraparticle diffusion time, α is the initial adsorption rate (mg/g · min), and β is the desorption constant (mg/g).

The PSO model was used to obtain the initial adsorption rate (h) using Equation (7).

(7)$h=k_2\times q_e^2$

The adsorption isotherms studied were the linear Langmuir, Freundlich, and Temkin and Henry equations, using Equations (8)–(11), respectively [42].

(8)${\displaystyle \frac{1}{q_e}}={\displaystyle \frac{1}{q_{max}}}+{\displaystyle \frac{1}{{K_L\times q}_{max}\times C_e}}$

(9)$\mathit{log}\left(q_e\right)=\mathit{log}(K_F)+{\displaystyle \frac{1}{n}}\mathit{log}(C_e)$

(10)$q_e={\displaystyle \frac{R\times T}{b}}\mathit{ln}\left({\alpha }_T\times C_e\right)\to q_e=B\times \mathit{ln}\left(K_T\right)+B\times \mathit{ln}\left(C_e\right)$

(11)$q_e=K_H\times C_e$

where q_max is the maximum adsorption capacity (mg/g), K_L is the adsorbate/adsorbent interaction constant (L/mg), 1/n is a constant related to surface heterogeneity, K_F is the Freundlich adsorption capacity constant (mg/g × (L/mg)^1/n), R is the universal gas constant, T is the temperature, B is the Temkin constant in relation to the heat of sorption, a_T is the Temkin isotherm constant, and K_H is the Henry constant (g/L).

2.6. Analytical Determinations

DRX-6BN concentration was evaluated using a Shimadzu UV-1800 UV spectrophotometer (Kyoto, Japan) at 516 nm. COD was evaluated using a Hack DR-2800 spectrophotometer (Loveland, CO, USA) following the colorimetric method (5220 D) [43]. The O&G concentration was determined following the standard extraction method (5520 D), with readings at 257 and 300 nm on a Shimadzu UV-1800 UV spectrophotometer [44,45].

3. Results and Discussion

3.1. Characterization

The FT-IR spectrum of CuL (Figure S1A) showed the characteristic bands of this complex, which is in agreement with previous reports [29,46]. The functional groups in AC may be the result of the origin and activation methods used for the material. In general, it is possible to observe that AC/CuL presented a spectrum quite similar to that of AC, with differences that were possibly due to modification by CuL. Bands in the range of 2360–2330 cm⁻¹, characteristic of AC, are present in AC/CuL. Peaks caused by the asymmetric and symmetric stretching vibrations of carbon dioxide (CO₂) have been previously reported in this range [47].

The bands around 1620 cm⁻¹ are characteristic of the stretching of the imine functional group (–C=N–), frequently observed in complexes containing Schiff base-type ligands, such as that used in this study [48]. In the CuL spectrum, the corresponding band appeared at 1615 cm⁻¹, with a shift to 1635 cm⁻¹ in the composite, possibly due to interactions between the –C=N– group and the support [49,50,51]. Typical stretching and deformation of –C=C– bonds in aromatic compounds were identified at 1599 cm⁻¹ and 645 cm⁻¹ in CuL, with slight shifts to 1596 cm⁻¹ and 642 cm⁻¹, respectively, in the composite. The elongation of the carbon–oxygen bond (–C–O–) associated with phenols and the axial deformation of the same group associated with alcohols were found at 1204 cm⁻¹ and 1056 cm⁻¹ in CuL, respectively, shifting to 1190 cm⁻¹ and 1051 cm⁻¹ in the composite, as well as possible interactions [51,52]. Typical deformations of the carbon–hydrogen bonds (–C–H–) of methylenes (CH₂) and aromatic groups were detected at 1450 cm⁻¹ and 756 cm⁻¹, respectively, for both materials. The stretching vibrations of the copper–oxygen (Cu-O) and copper–nitrogen (Cu-N) bands, previously reported in the literature [53,54], can be observed at 577 and 475 cm⁻¹ in CuL and 573 and 466 cm⁻¹ in AC/CuL.

The XRD pattern of CuL (Figure S1B) presented fine and intense peaks, mainly at 2θ angles of 9.0° and 9.7°, confirming its crystalline nature [55], in agreement with a previous report [29]. Similar peaks are observed in the diffractograms of AC/CuL at 8.7° and 9.8°. AC is a typically amorphous material, with a narrow and sharp peak around a 2θ angle of 26°, associated with the presence of a graphitized fraction in the AC sample [56,57], 29° and 43°, which also appears in AC/CuL [58,59,60,61]. The increased crystallinity of the composite can be attributed to modification with CuL. The diffractograms were compared with the XRD standards of copper propionate [62] and showed agreement regarding the peaks in the 2θ range of 10.8° (110), 12.2° (111), 14.2° (021), 15.9° (020), 19.4° (111), 21.5° (040), 22.8° (311), 24.5° (512), and 30.5° (603) for CuL, with a slight shift towards AC/CuL.

A comparison of the CuL diffractogram with those of other copper complexes is challenging to undertake. Despite this, similarities were observed between the peaks observed in the works of Khalaji et al. (2011) and Inac et al. (2024) with copper complexes with Schiff base-type ligands [63,64]. However, the structure of CuL has been confirmed previously [46].

As can be seen in Figure S2A,B, the CuL particles have different sizes and irregular shapes that mostly resemble quadrangular prisms. AC also consists of irregular particles without uniform size, depending on the carbonization and activation processes used. Kang et al. (2020) studied the feasibility of producing AC from coffee ground waste using a carbon dioxide (CO₂) plasma jet [65]. SEM analyses implied that the treatment conditions were responsible for the variations in the pore size and distributions. Despite this, AC has a rough surface and porosity resulting from activation, which increases its surface area, which is essential for the adsorption process [65,66,67]. These characteristics are reflected in AC/CuL (Figure S2C,D), which presents the characteristic CuL particles associated with AC. Figure S2D shows the porous structure of this material. The EDX spectra of the samples (Figure S2E,F) confirmed the presence of copper in CuL and AC/CuL, with a normalized mass (%) of 17.29% in the first and 9.44% in the second. The normalized mass (%) calculated for CuL, disregarding hydrogen, was 17.22%. Therefore, the results are adequate.

The characterization using N₂ physisorption (Figure S3) showed that the BET surface area (A_BET) of AC was 535.24 m²/g, with a micropore area and volume of 437.24 m²/g and 0.21 cm³/g, respectively. These values agree with those reported in the literature for AC. Brazil et al. (2020) carried out studies to obtain AC from Kraft lignin by conventional and microwave routes, obtaining AC with A_BET between 580 and 1220 m²/g [68]. Other studies reported AC with a lower A_BET [69,70]. CuL presented A_BET of 0.53 m³/g. The modification of AC with CuL (Figure S3) increased the A_BET to 44.11 m²/g. There are reports of the application of adsorbents with lower A_BET in the removal of pollutants, such as AC obtained from water treatment sludge with chemical activation by phosphoric acid [71], biochar obtained from rice husk sludge [72], and steam-activated biochar [73], with A_BET of 23.72 m²/g, 29.18 m²/g, and 2.12 m²/g, respectively.

According to the BET classification, both isotherms (Figure S3) can be classified as type IV [74], since the adsorption curve reaches a maximum and then presents a desorption pattern that does not follow exactly the same path. This isotherm shape is generally observed in mesoporous materials (with intermediate pore sizes between 2 and 50 nm). Since the pore sizes of AC and AC/CuL are 2.51 and 5.93 nm, respectively, the mesoporous nature is confirmed [75].

Figure S4 shows that the pH_pzc values of AC and AC/CuL are similar, with that of AC being 7.37 and that of AC/CuL being 7.44. A pH lower than pH_pzc causes protonation of the functional groups on the adsorbent surface, making the surface positively charged, which is favorable for the adsorption of anions. A pH value above pH_pzc causes deprotonation of the surface owing to the presence of hydroxyl ions (OH⁻) in the solution, resulting in a negatively charged surface, which favors the adsorption of cations [39,76]. Thus, the removal efficiency of DRX-6BN is expected to be higher at pH values below pH_pzc because it has negative functional groups in its structure.

3.2. Adsorption Experiments

3.2.1. Adsorption Kinetics

In studies involving adsorption, the equilibrium time can be understood as the period required for a system to reach a state in which the amount of adsorbate adsorbed on the surface of the adsorbent stabilizes. In general, at this point, the adsorption and desorption rates are equal, and the concentration of the adsorbate in the liquid and on the surface remains constant. Figure 1 illustrates the results obtained from the experiments performed with AC and AC/CuL in contact with DRX-6BN.

The equilibrium time was 120 min, with DRX-6BN removals of 72.00 ± 0.19% and 60.56 ± 0.62% using 0.42 g of AC and AC/CuL, respectively. Owing to the higher surface area, removal at the beginning of contact is greater, and is reduced as the process progresses due to the decrease in available pores and repulsion of already-adsorbed molecules in relation to those still in solution [77].

The lower adsorption of DRX-6BN on AC/CuL can be explained by the lower A_BET (Figure S3B) compared to that of AC (Figure S3A). This phenomenon was also observed by Ruey-Shin et al. (2018), who reported the preparation of AC nanocomposites with Fe₃O₄, attaching Fe₃O₄ nanoparticles to the AC surface, with a drop in adsorption capacity from 384 to 60–114 mg/g [34]. Furthermore, pore blocking with consequent inaccessibility for adsorption of dyes and other pollutants due to impregnation is a known effect [31,78,79]. Functional groups were inserted into the AC (Figure S1A). Such additions can alter the dynamics of the interactions with pollutants and decrease the adsorption of specific adsorbates. Figure 2 illustrates the fit of the observed results to kinetic models.

As shown in Figure 2, although the PFO (Figure 2A) and Elovich (Figure 2C) models presented adequate adjustments to the adsorption results (R² > 0.94), the PSO model (Figure 2B) was the most suitable for describing adsorption kinetics (R² > 0.99). The PSO model suggests that chemisorption (chemical bonds between adsorbent and pollutant) may be the rate-limiting step of the adsorption kinetics [80,81].

Adequate adjustments to both the PSO and PFO models at the same time suggest that the adsorption dynamics are complex, with the influence of multiple mechanisms. In addition, adjustment to the Elovich model may indicate that the adsorbents have an irregular energy distribution.

The intraparticle diffusion model (Figure 2D) shows that the adsorption can be divided into two stages for both AC and AC/CuL. In general, because the value of the constant related to the diffusion resistance (linear coefficient of the line) is greater than zero, the initial stage is film diffusion with rapid diffusion on the surface of the adsorbents. The effect of the boundary layer was more pronounced for AC/CuL, with a constant value of 3.1062 mg/g, whereas AC was 1.1685 mg/g. The second stage is the gradual adsorption of DRX-6BN on the surface of the adsorbents, which denotes intraparticle diffusion. The diffusion constant value (k_d) of the film was greater than that of the intraparticle diffusion stage in both cases, indicating that the main adsorption was acquired in the first adsorption stage [82]. Thus, it is possible to suggest that the energy disseminated on the surface of the adsorbents was irregular, with the mechanism involving chemisorption, with film diffusion characterizing the first stage and intraparticle diffusion characterizing the last.

The effect of temperature on adsorption was investigated between 25 and 40 °C, at an initial concentration of 20 mg/L DRX-6BN, adsorbent dose of 0.42 g/200 mL solution, stirring rate of 200 rpm, and for a contact time of 120 min. The removal profile and fit of the data to the PSO kinetics are presented in Figure 3.

Figure 3 shows that the amount of adsorbed dye increases with increasing temperature (Figure 3A). From a kinetic point of view, the increase in temperature is responsible for the increase in the diffusion rate of adsorbate molecules, mainly by decreasing the viscosity of the solution [83,84], as well as indicating the increase in the mobility of molecules and their greater interaction with the active sites [85]. It is worth mentioning that, although temperature is a relevant parameter for adsorption, in the present study, the removals obtained in 120 min were similar in the range used. The maximum removal was obtained at 40 °C and corresponded to 63.94 ± 0.28%. The fit to the PSO model was adequate for both conditions (R² > 0.9945) (Figure 3B).

3.2.2. Adsorption Isotherms

The adsorption isotherm describes the equilibrium performance of the adsorbents at a constant temperature [86]. Isotherm analysis is a strategy used to determine the ratio between adsorbent and adsorbate, aiding in understanding the adsorption mechanism and obtaining the adsorption capacity of the adsorbent [84,87,88]. The experimental adsorption data were fitted using the linear Langmuir, Freundlich, Temkin isotherms, as well as the Henry model. Figure 4 illustrates these results.

As shown in Figure 4, according to the R² obtained, the experimental adsorption data in AC were best fitted to the Freundlich model (R² = 0.9385, Figure 3B), while those of AC/CuL were best fitted by the linear Langmuir model (R² = 0.9919, Figure 4A). This model is based on the generation of adsorbate molecules on the homogeneous surface of the adsorbent. However, treatment of the data to obtain the parameters of the model revealed a negative maximum adsorptive capacity, which has no physical sense.

Thus, the adequate fitting of the adsorption data in AC/CuL by the linear Freundlich model favors a better comparison between the adsorbents. The attempt to fit the data using other models (Figure 4C,D) did not result in a good fit. The fit to the Freundlich model suggests heterogeneity of the adsorbent surfaces [89]. The Freundlich adsorption capacity constant (K_F) obtained for AC was 0.0578 mg^1−1/n*L^1/n/g, which decreased to 0.021 mg^1−1/n*L^1/n/g upon modification by CuL. The constant related to surface heterogeneity (n) was 0.4377 for AC and 0.3573 for AC/CuL. Values of n between 0 and 1 indicate strong sorption on the surface of the adsorbents due to active forces [90]. Values of n below unity may indicate chemical adsorption [85,91].

The Temkin model examines the interaction between the adsorbate and adsorbent and assumes a linear decrease in the adsorption energy with increasing surface coverage of the adsorbent. The Henry model is one of the simplest and is used for adsorption systems at low adsorbate concentrations, where the interaction between the adsorbed molecules is negligible and adsorption sites are widely available.

The experimental data were fitted to the Freundlich model in its linearized form, presenting a high coefficient of determination, which suggests a plausible logarithmic relationship between the adsorption and adsorption sites. Since linearizations introduce distortion of the model to fully describe the adsorption system studied, additionally, average relative error deviation (ARED) and Marquardt’s percent standard deviation (MPSED) were calculated [92]. The ARED and MPSED calculations resulted in values of 3.14 and 5.23. Thus, a linearized fit can be used, but more robust models may be better suited to fit the adsorption data.

3.3. Coupled Adsorption and Oxidation

The degradation kinetics was studied using the PSO model. Figure 5 shows the removal by adsorption coupled with oxidation using H₂O₂, fitting of the adsorption coupled with oxidation data to the model and without oxidation, and the removal of DRX-6BN at 120 min. Table 1 shows the values of the initial adsorption rate in systems without H₂O₂ (h_ads) and with (h_{ads + oxi}), calculated for all systems. As reported in a previous study, the removal of DRX-6BN using only H₂O₂ is unsatisfactory [29].

As shown in Figure 5A,D, the removal of DRX-6BN was continuous over time under all conditions employed. At 120 min, more than 70% of the pollutant was removed using 0.24 g of AC/CuL with H₂O₂ activation (Figure 5D), which was followed by removal of more than 80% at 240 min. The activation of H₂O₂ by 0.04 g and 0.06 g of AC/CuL showed higher removal values, justifying the use of lower masses in future studies. This result is expected, since in AOP, the presence of a catalyst mass higher than the optimum leads to a decrease in activity.

The data in Table 1 were calculated using the data shown in Figure 5B,C. The experiments without addition of H₂O₂ showed an increase in h with an increase in the mass of AC/CuL from 0.02 to 0.04 g, but it subsequently decreased until 0.24 g. The explanation for the increase in h at an initial moment lies in the greater number of active sites available for adsorption and the greater initial concentration gradient, which accelerates the initial diffusion of the adsorbate to the sites [93,94]. However, the results suggest that factors such as surface saturation due to the relative concentration of available sites decrease or even the lower use of the sites, as well as adsorbent agglomeration and dilution effects, whether combined or not, begin to prevail at a second moment, decreasing h.

It can be observed that the initial adsorption rate decreased for all systems in which H₂O₂ was added (Table 1). This evidence, combined with the higher removal in systems containing H₂O₂ (Figure 5D), indicates that oxidation is responsible for part of the removal, which decreases the amount of pollutants available for adsorption in the system. Thus, oxidation contributes to the removal of pollutants, promoting a more efficient removal of pollutants with the combination of both functions in the same material.

The electrochemical behavior of CuL was previously reported using cyclic voltammetry, and the anodic and cathodic peaks of the Cu⁺/Cu²⁺ couple were verified [46]. CuL is an efficient catalyst for the decomposition of H₂O₂ into •OH [29]. Regarding the combined action of adsorption and oxidation using AC/CuL in the presence of H₂O₂, initially, owing to its high surface area and porosity, AC/CuL adsorbs pollutants (P) on its surface (sup) (Equation (12)). Subsequently, when H₂O₂ is added, it interacts with copper(II) (Cu²⁺) present in CuL, generating highly reactive •OH (Equations (13)–(15)), which degrade the pollutants adsorbed or present in the solution (Equation (16)) [95,96].

(12)$P_{solution}+sup\to P_{sup}$

(13)${\mathrm{C}\mathrm{u}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{C}\mathrm{u}}^{+}+{\mathrm{H}\mathrm{O}}_2•+{\mathrm{O}\mathrm{H}}^{-}$

(14)${\mathrm{C}\mathrm{u}}^{+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{C}\mathrm{u}}^{2+}+•\mathrm{O}\mathrm{H}+{\mathrm{O}\mathrm{H}}^{-}$

(15)${\mathrm{C}\mathrm{u}}^{2+}+{\mathrm{H}\mathrm{O}}_2•\to {\mathrm{C}\mathrm{u}}^{+}+{\mathrm{O}}_2+{\mathrm{H}}^{\mp }$

(16)$P+•\mathrm{O}\mathrm{H}\to \mathrm{o}\mathrm{x}\mathrm{i}\mathrm{d}\mathrm{i}\mathrm{z}\mathrm{e}\mathrm{d}\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{s}$

As can be seen in Equations (13)–(16), free radicals are generated, such as •OH, which are not selective and can promote the degradation of several contaminants through rapid reactions [17,18], in addition to oxidized products that, in general, can be toxic [97,98]. Furthermore, copper can be toxic to aquatic and terrestrial organisms [99,100], which may pose a challenge for the use of unsupported CuL in conjunction with H₂O₂. AC increases the efficiency and sustainability of the process in the presence of H₂O₂, thereby ensuring greater environmental safety for the application of this technology. Control of copper concentration, adequate neutralization of H₂O_2, and monitoring of byproducts, in any case, are essential for the application of advanced oxidative processes.

3.4. Enhanced Removal of Organic Compounds of from Oily Effluent Aided by AC/CuL

The primary challenges associated with the management of oily effluents in the oil industry are the concentration of O&G and the associated organic matter, as indicated by COD. This can result in environmental impacts when such effluents are discharged into a receiving body without adequate treatment. In Brazil, CONAMA Resolutions 393/2007 and 430/2011 established limits of 29 mg/L and 20 mg/L for discharges on offshore platforms and near the coast, respectively [101,102]. Furthermore, considering the water stress experienced by many countries, adjusting effluents for reuse justifies the adoption of treated effluents of this type. Therefore, most countries in world have adopted regulations aimed at near-zero discharge [103,104,105].

The removal behavior of DRX-6BN with AC/CuL was used as a probe for subsequent experiments involving the treatment of oily effluents. Therefore, we used the same concentration of H₂O₂ and a mass of AC/CuL of 0.04 g. The results are shown in Figure 6.

As shown in Figure 6A, the O&G removal rate for AC/CuL in the absence of H₂O₂ was approximately 73% at 120 min. In the presence of the oxidant, the removal was approximately 96%, which was very close to that obtained for AC, demonstrating an advantage in the association. It is worth mentioning that the presence of CuL in the AC structure, as discussed with the N₂ physisorption data, decreased the A_BET for the material (Figure S3), justifying the lower removal results.

Regarding COD removal (Figure 6B), less than 50% removal was observed at 120 min for both AC and AC/CuL in the absence of an oxidant. However, the association of AC/CuL with H₂O₂ led to the removal of approximately 100%, indicating this is a valid strategy for the improved removal of organic compounds present in oily effluents from the oil industry.

A direct comparison of the results obtained in this study with those reported in the literature concerning copper-containing AC for this application is challenging. However, Yousef et al. (2020) have reported that adsorption technology is a viable process for treating oil-production water [106]. The primary objective of such treatment was the removal of hydrocarbons, a presence that was noted in more than 70% of the studies verified.

Okiel et al. (2011) studied the removal of oil from o/w emulsions in bentonite, AC, and deposited coal. Among the best results with AC, the removal of 93.54% of oil stands out when using 1.0 g/200 mL of emulsion with a stirring time of 4 h at 400 rpm [107].

Recently, Khader et al. (2022) conducted batch adsorption experiments using AC. According to the results, the best oil and COD removal efficiencies were 99.58% and 95.87%, respectively, at a dosage of 0.25 g/100 mL of emulsion [40].

Given the improvement in removal promoted by using small masses of the adsorbent, the use of the composite is promising, especially in the presence of an oxidant.

3.5. Storage Stability

The stability of materials during storage is essential for industrial applications that have increasingly high requirements regarding efficiency, reliability, and sustainability. In the present study, the removal of O&G and COD from oily effluent was investigated for 120 min, at 25 °C, for 15 months, both in the presence and absence of H₂O₂. The results are shown in Figure 7.

As can be seen in Figure 7, the removal of both the concentration of O&G and COD remained practically unchanged in the first 12 months, both for the tests in which the material was used only as an adsorbent and for those in which oxidation was coupled. However, a slight decrease was observed in the removal of the O&G concentration from the 12th to 15th month. In terms of values, there is a drop in removal from 95.27 ± 1.36% to 85.94 ± 1.82% in tests with the aid of oxidation and from 43.57 ± 0.59% to 42.23 ± 3.97%. These drops represented less than 10% of the initial values. Statistical analysis of the data was performed using a t-test to compare the range of means with 95% confidence (Figures S5–S8). Based on the results of this test, COD removal by adsorption alone is equal between the 1st week and 12th month (Figure S5), while COD removal with the aid of oxidation (Figure S6) and O&G removals (Figures S7 and S8) did not show a change in removal over time. These results suggest that this material does preserve its critical properties for at least 12 months.

In this study, the synthesis and application of AC/CuL in the treatment of effluents were investigated, with characterization, kinetics, and adsorption isotherms, as well as the coupling of oxidation in the presence of H₂O₂ to the adsorptive function of the adsorbent and stability during storage. Future research will focus mainly on scalability, application to other pollutants, parameter optimization, and cost–benefit analysis.

3.6. Comparison Between AC/CuL and Other Technologies for Wastewater Treatment

Regarding the use of conventional adsorptive technologies, the evaluation of several parameters is important for ensuring efficiency. Among them, adsorption capacity, contact time, adsorption kinetics, and temperature stand out. In the present study, the evaluation of enhanced removal owing to the coupling of adsorption with oxidation is relevant.

The adsorption capacity indicates the number of pollutants that the adsorbent can retain on its surface. Therefore, a higher capacity is desirable [87]. As for the contact time, this determines how fast the material reaches maximum adsorption; a shorter time is desirable with high adsorptions for more economical processes. Adsorption kinetics helps in understanding how an adsorbate interacts with the adsorbent over time, allowing the evaluation of the practical efficiency of the material under real conditions. Temperature is a parameter that affects the kinetics and thermodynamics of the adsorption process by increasing the diffusion of pollutants or reducing the affinity of the adsorbent [83,84]. The improved removal due to the coupling of adsorption with oxidation provides a comprehensive metric of water purification capacity. Table 2 summarizes the advantages and disadvantages, and makes a comparison with other existing effluent treatment technologies.

4. Conclusions

An AC/CuL composite was obtained and characterized by FT-IR, XRD, SEM-EDX, N₂ physisorption, and pH_pzc. In the FT-IR spectra, typical groups of Schiff base were observed, such as the one corresponding to the imine functional group, both in CuL and AC/CuL, as well as the presence of elongation of the Cu-O and Cu-N bands. XRD by the powder method revealed that CuL presents high crystallinity, while AC/CuL has intermediate crystallinity between that of CuL and AC, which is amorphous. SEM-EDX revealed the presence of CuL in AC/CuL, whereas N₂ physisorption revealed that the A_BET of AC decreased considerably with CuL modification. The pH_pzc remained unchanged at approximately 7.5.

The adsorption of DRX-6BN on AC/CuL was lower than that on AC because of the lower availability of the active sites in the adsorbent. In 120 min, using 0.42 g of adsorbents, the removals were 72.00 ± 0.19% and 61.56 ± 0.62% for AC and AC/CuL, respectively. The study of the adsorption kinetics revealed a better fit to the PSO model. Despite this, the fit to the PFO model suggests that the adsorption dynamics are more complex than chemisorption owing to the influence of multiple mechanisms. In addition, the fit was adequate for the Elovich model. The intraparticle diffusion model describes adsorption in two stages: film diffusion and intraparticle diffusion. Regarding the adsorption isotherm, the Freundlich model was the most appropriate, confirming the heterogeneity of the adsorbent surface.

The combination of adsorption with chemical oxidation showed that it was feasible to use masses between 0.04 and 0.06 g of AC/CuL in the presence of H₂O₂. The decrease in the initial adsorption rate in the experiments in which H₂O₂ was added, combined with the higher removal rates in systems containing the oxidant, confirmed its participation in promoting improved removal of the pollutant.

The tests for the removal of O&G and organic compounds from oily effluent showed removals of approximately 100% and 96% of COD and O&G, respectively, for AC/CuL in the presence of H₂O₂, in 120 min, using 0.04 g of adsorbent and 20 mg/L of oxidant. COD removal was twice that obtained for AC. Furthermore, the AC/CuL showed high storage stability, maintaining virtually unchanged COD removal throughout the 15 months of the experiment and a decrease of less than 10% in O&G removal activity between months 12 and 15.

Therefore, the results suggest that the AC/CuL composite is a promising solution for the enhanced removal of complex organics present in effluents, such as those from textile and petroleum industries. The combination of initial adsorption, coupling with oxidation, and storage stability allows this system to handle challenging pollutants in a sustainable, economical, and safe manner.

Owing to its high efficiency, even at pH levels close to neutral and in the absence of light, combined with its high storage stability, this material can have practical applications in the treatment of industrial effluents, purification of drinking water, and continuous reactor systems. It has the potential for advanced solutions and is an alternative to meet the increasingly demanding environmental demands.

Author Contributions

F.P.d.S.: conceptualization, data curation, methodology, formal analysis, investigation, visualization, writing—original draft; L.C.F.: investigation, writing—review and editing; C.P.B.: conceptualization, project administration, supervision, validation, visualization, writing—review and editing; A.C.: conceptualization, resources, project administration, supervision, validation, visualization, writing—review and editing; F.V.d.F.: conceptualization, resources, funding acquisition, project administration, supervision, validation, visualization, writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The original contributions presented in this study are included in the article/supplementary material. Further inquiries can be directed to the corresponding author.

Acknowledgments

The authors would like to thank the Green Technologies Laboratory (Greentec) and the Electron Microscopy Center of COPPE at the Federal University of Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil, for their support in using their equipment. The authors would also like to thank the financial support of the ANP and the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), through the ANP Human Resources Program for the Petroleum, Natural Gas and Biofuels Sector (PRH-ANP), in particular PRH 3.1, of the School of Chemistry–Training in Processes and Systems of the Petroleum and Biofuels Industry.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Table 1

Initial adsorption rate for systems with and without oxidation.

Mass (g)	hads(mg/g/min)	hads + oxi(mg/g/min)
0.02	9.26	5.90
0.04	17.83	1.66
0.06	4.01	1.38
0.24	1.08	0.85

Table 2

Comparison between AC/CuL and other technologies for wastewater treatment [108,109,110,111].

Method	Advantages	Disadvantages	Comparison
Chemical precipitation	Operational simplicity; effective for heavy metal removal and pH adjustment.	Does not remove organic pollutants; may generate toxic waste; requires additional chemical reagents.	AC/CuL offers more complete solutions for organic compounds in addition to metals.
Coagulation/flocculation	Low cost; simple operation; and stable effect.	High dosage of coagulants required for high concentration oily wastewater; high pH sensitivity; large amount of sludge containing metal hydroxide.	AC/CuL is more suitable for dissolved pollutants and resistant organics.
Biological treatment	Color removal ability of various dyes; inexpensive; no foaming.	Long residence time; large area; and microrganisms might be intolerant to some pollutants.	AC/CuL is most effective for non-biodegradable compounds such as dyes and hydrocarbons.
Adsorption	High removal capacity with suitable adsorbents; well established; and widely available.	Only performs mass transfer; problems with material saturation; and requires frequent regeneration or disposal.	AC/CuL excels in efficiency and has the ability to treat persistent pollutants due to catalytic activity.
Membrane filtration	Excellent removal of particles, dissolved solids, and some organic pollutants; continuous operation; compact.	Susceptible to fouling; effluent often needs pretreatment.	AC/CuL is efficient for organic compounds; membranes offer better removal of dissolved solids.
Oxidation	Direct application; can completely degrade the pollutant; and common method of pollutant removal.	May form toxic byproducts; pH-dependent; and sludge generation may occur.	AC/CuL in the presence of oxidant incorporates oxidation with lower energy cost and controlled use of reagents.
Irradiation	Effective on a laboratory scale.	Requires enormous amounts of oxygen; and expansive.	AC/CuL is most effective for non-photosensitive pollutants and complex effluents.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/pr13020447/s1, Figure S1: FT-IR spectra (A) and XRD patterns for CuL, AC, and AC/CuL (B); Figure S2. Micrographs of CuL 500x (A), CuL 5000x (B), AC/CuL 500x (C), AC/CuL 5000x (D), and EDX spectra of CuL (E) and AC/CuL (F); Figure S3. N₂ adsorption–desorption isotherms of AC (A) and AC/CuL (B); Figure S4. pH_pzc of AC and AC/CuL; Figure S5. T-test for COD removal (%) via adsorption; Figure S6. T-test for COD removal via adsorption coupled with oxidation; Figure S7. T-test for O&G content removal (%) via adsorption; Figure S8. T-test for O&G content removal via adsorption coupled with oxidation.

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