Valorization of coffee husks for the sustainable removal of pharmaceuticals from aqueous solutions

Pollution caused by contaminants of emerging concern (CECs) is one of the most critical environmental problems nowadays due to their continuous introduction into the environment (Aguilar-Aguilar et al. 2023). A wide variety of CECs, such as human and veterinary pharmaceutical compounds, have been widely reported in freshwater sources because a significant proportion (30–90%) of the pharmaceutical products cannot be absorbed or metabolized by organisms (Kümmerer 2009; Aus der Beek et al. 2016; Acelas et al. 2021). Although these compounds are found in low environmental concentrations, pharmaceutical contaminants cause adverse effects on plants and animals (Boxall et al. 2012). Unfortunately, wastewater treatment plants using conventional methods are inefficient in removing these compounds, and these contaminants may remain in wastewater effluents at similar or even higher concentrations than in wastewater influents (Gros et al. 2010; Gracia-Lor et al. 2012). In addition to these problems, several countries still need to establish legal requirements for their discharge.

Fluoroquinolones and analgesics are among the most reported pharmaceutical compounds in the effluents of wastewater treatment using conventional methods (Botero-coy et al. 2018). The presence of antibiotics in the environment has garnered attention due to their toxicity, ubiquity, persistence, and biological activity (Garcia-Ivars et al. 2017). This concern is particularly heightened by the potential risks associated with the development of microbial resistance (Botero-coy et al. 2018). Therefore, the removal of pharmaceutics from water streams is one of the main challenges around the world. Ciprofloxacin (CIP) and acetaminophen (ACE) are used extensively in human and veterinary healthcare. The CIP belongs to the fluoroquinolone group, and it is categorized as a third-generation antibiotic (Nguyen et al. 2022), while the ACE is classified as an antipyretic and analgesic (Thanh et al. 2020). Botero-coy et al. reported CIP levels in wastewater treatment plants across Colombia ranging from 0.98 to 2.29 μg/L, and these concentrations have shown an upward trend over time (Botero-coy et al. 2018). Other studies have reported that wastewater from pharmaceutical industries often contains significant amounts of CIP, even exceeding 30 mg/L (Khavari Kashani et al. 2022). Furthermore, ACE has been detected at peak concentrations from 24.53 to 112.78 μg/L (McLain et al. 2023). Among the different processes implemented for wastewater treatment, adsorption has been widely used because it is a fast, efficient, and low-cost method (Liu et al. 2019). The most used adsorbent is commercial activated carbon due to its high surface area, good pore volume, and presence of different surface-active functional groups. However, synthesizing activated carbon from nonrenewable sources has high associated production costs (Sun et al. 2014). Then, it has motivated the search for alternative adsorbents.

Another crucial environmental problem is related to agroindustrial waste accumulation. This issue arises from the inherent generation of solid, liquid, or gaseous waste during agroindustrial processing and production (Freitas et al. 2021). Coffee husks are one of the wastes generated in high coffee-producing countries such as Colombia (Czekała et al. 2023). Hence, large volumes of this waste are openly discarded, harming the environment (Czekała et al. 2023). Coffee husk (CH) is particularly interesting as it is one of the most important agricultural commodities worldwide and the most consumed beverage in the world after water (Oliveira & Franca 2015). It is grown in around 80 countries, with a global production of approximately 10.2 million tons in 2019 (I. C. Organization 2021). Colombia specifically produced 820,600 tons of coffee grown on approximately 839,700 ha. Of this production, only 9.5% by weight of the total fruit is consumable (C. for commodities Fund 2017). The remaining 90.5% are diverse residues, such as parchment, pericarp, pulp, mucilage, and husk (Jaramillo et al. 2021). These residues are often thrown away for further decomposition since these are lignin-rich materials (21%) and degrade slowly, causing methane emissions and negatively impacting the environment (Oosterkamp 2014). Hence, using agricultural wastes like adsorbent materials for wastewater treatment is an environmentally friendly option, which reduces the environmental charge and contributes to a circular economy, giving added value to waste materials.

These biomass residues have a high content of lignin, cellulose, hemicellulose, pectin, proteins, lipids, starch, and simple sugars (Ahmad 2023). This composition allows them to adsorb a wide range of contaminants, owing to the abundance of oxygenated functional groups on their surface. These functional groups include alcohols, carboxylic acids, phenols, and ethers, which enable various interactions, such as π-π electron donor-acceptor interactions, electrostatic interactions, and hydrogen bonds. These interactions have been established as the main mechanisms for removing contaminants in adsorption processes (Acelas et al. 2021). Several studies have reported the use of agroindustrial wastes as adsorbents of CIP and ACE without any treatment, whether physical or chemical modification (Tran et al. 2020; Acelas et al. 2021; Dou et al. 2022; Hamadeen & Elkhatib 2022; Nguyen et al. 2022). However, several unsolved aspects must be addressed to enhance our comprehension of pharmaceutical adsorption processes using bioadsorbents. This study is crucial in pursuing more efficient and economically sustainable adsorbent materials, which play a fundamental role in mitigating agroindustrial waste disposal. Thus, the primary objective of this research is to develop an adsorbent from CH aimed at removing two widely used pharmaceutical compounds (CIP and ACE) in Colombia. The investigation encompasses a comprehensive analysis of the adsorption system, including assessments of maximum adsorption capacities, kinetic, and thermodynamic parameters, the potential to remove contaminants from the complex matrix effectively, and reusability.

All the chemicals used in this study were of analytical grade. The following reagents were employed for solution preparation: CIP and ACE from source from Acros Organics, with a purity of 98%. In addition, HCl (Merck), NaOH (PanReac AppliChem, 98%), Urea (Merck, 99.5%), (PanReac AppliChem, 99%), (PanReac AppliChem, 99.5%), (PanReac AppliChem, 98%), KCl (Supelco Suprapur, 99.9%), (Supelco EMSURE, 99%), (Supelco EMSURE, 99%), (PanReac AppliChem, 99.7%), NaCl (PanReac AppliChem, 99%), (ASLO reactivos, >99%), (Supelco EMSURE, >99.5%), and (PanReac AppliChem, 98%) were utilized.

CHs are generated as a byproduct when processing coffee berries by wet methods, the most common procedure in Colombia. This method produces a more significant amount of CH residues due to the elimination of the skin and pulp during the initial processing phase. The CH used in this study was collected from a crop 1,150 m above sea level in the Colombian municipality of Fredonia, Antioquia. In this region, crops experience two annual blooms, resulting in a main and secondary crop (Echeverria & Nuti 2017). Batches of 5 kg of residues were collected for 6 months from which random samples were taken for subsequent homogenization. These samples were stored in airtight containers at room temperature until analysis in the laboratory. After collection, the CH was washed with distilled water to remove impurities. Subsequently, it was dried at 100 °C for 24 h in an oven (Memmert). The dried material was sieved to a particle size of 0.45 μm, stored, and labeled as CH for future use as an adsorbent.

The dry biomass was thermally treated by pyrolysis using a horizontal tubular furnace (model OTP-01, Resistencias y equipos, Colombia) with three distinct heating zones. The adsorbents were produced at three different temperatures: 300, 500, and 700 °C, utilizing a heating rate of 5 °C/min under a N₂ atmosphere. Each temperature level was held isothermally for 1 h. The materials produced were labeled as follows: CHP-300, CHP-500, and CHP-700 (Forgionny et al. 2022).

The materials' surface functional groups were identified before and after heat treatment using Fourier-transform infrared spectroscopy (FTIR) with attenuated total reflectance, in the range from 4,000 to 450 cm⁻¹, using a spectrum two spectrophotometer (PerkinElmer, Waltham, MA, USA). To evaluate the pH of the zero-charge point (pH_PZC), 0.05 g of CH was put in contact with 50 mL of deionized water. The pH was adjusted to 2, 4, 6, 8, and 10 by adding appropriate amounts of 2 M and 0.05 M HCl and 2 M and 0.01 M NaOH. The samples were shaken at 200 rpm for 24 h. The pH variation (ΔpH = initial pH – final pH) was plotted as a function of initial pH, and pH_PZC was identified at the pH when ΔpH = 0.

All final and initial concentrations of the experiments were measured by UV-VIS spectroscopy, and measurements were performed in duplicate. After conducting preliminary tests, the most effective adsorbent and contaminant were selected, and all subsequent experiments were performed using this system.

In the dose-effect experiments, the amount of adsorbent was systematically varied while maintaining a constant volume of the CIP solution (20 mg/L). The dose range explored encompassed values from 0.069 to 0.5004 g/L. The CIP solution-adsorbent systems were stirred for 3 h at 200 rpm. Then, the solution was filtered, and the final concentration of the antibiotic was determined using UV-Vis spectroscopy, as indicated in Section 2.4.1.

The effect of solution pH on CIP was evaluated in the pH range of 210. Therefore, CIP solutions with a concentration of 20 mg/L were prepared, and the pH was controlled by adding drops of HCl (2 and 0.1 M) and NaOH (2 and 0.005 M) solutions. Then, 0.15 g of adsorbent was added to the pH-controlled solutions. The pH was adjusted again, and the system was shaken for 180 min. The final pH was measured using a Hach potentiometer, and HQ2200 and CIP concentrations were measured by UV-Vis spectroscopy.

The kinetics of CIP adsorption were performed using the most effective material identified in preliminary tests.

Batch experiments were carried out using a propellant reactor (IKA EUROSTAR 40 digital), wherein 0.375 g of CH was introduced into contact with 250 mL of CIP solution, representing the optimal dose. The initial concentration of the CIP solution was in the range of 10–60 mg/g. The systems were stirred at 200 rpm for 3 h. The CIP concentration was determined by taking 2.0 mL aliquots over time, which were collected and filtered for subsequent analysis until equilibrium was reached. During all experiments, it was ensured that the total volume subtracted did not exceed 5% of the total volume. The experimental data were fitted to the pseudo-first-order (Equation (S1)) and pseudo-second-order (Equation (S2)) kinetic models (Lima et al. 2015), and the mathematical expressions are presented in Table S1 in the Supporting information.

Adsorption isotherms were obtained for each pollutant, varying the CIP concentration from 10 to 100 mg/L at the optimum contact time established (5 h). A 150 mg mass of CH was mixed with 100 mL of each CIP solution, and then, the systems were stirred at 200 rpm for 3 h. Subsequently, the data were fitted to the Langmuir (Langmuir 1918) and Freundlich (Freundlich 1907) isotherm models, according to Equations (S3) and (S4), respectively (Table S2 in the Supporting information).

Adsorption experiments were conducted using a propellant-type reactor to evaluate the thermodynamic behavior of CIP adsorption on CH. For these experiments, the optimum dose (1.5 g/L) was used with CIP solutions of 40 ppm concentration. The system was in agitation at 200 rpm for 120 min, and 2.0 mL aliquots was taken at different intervals. The temperatures evaluated were 5, 15, 25, 35 and 45 °C. The activation energy E_a (kJ/mol) was determined using the kinetic data obtained from the pseudo-second-order model, k₂, and the Arrhenius equation, as presented in Equation (S4) (see the Supporting information). The standard free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) thermodynamic parameters were calculated using Equations (S5) and (S6) in the Supporting Information (Table S3).

The effect of the complex matrices was studied by evaluating the adsorption of CIP in deionized water, wastewater, and synthetic urine. The synthetic urine and wastewater solutions were prepared based on the composition given by Paredes-Laverde et al. (2018). Table S4 (in the Supporting information) shows the matrices' composition. All solutions were prepared with deionized water. A CIP solution of 20 mg/L was added to each complex matrix. A volume of 250 mL of this solution was put in contact (200 rpm) with 250 mg of CH for 3 h. Finally, the percentage removal was determined using Equation (1).

The performance of the CH material was evaluated for three reuse cycles. The optimal dose was used in these experiments, and batch systems were exposed to a CIP solution (20 mg/L) for 3 h within Erlenmeyer flasks while agitated at 200 rpm. Following the adsorption phase, the material was filtered under vacuum conditions. It was subjected to ultrasound treatment using a Digital Pro (model: PS-30AL) for 30 min, operating at a power of 40 KHz. This ultrasound treatment was conducted in a mixture of 33.3 mL of methanol (CH₃OH) and 6.6 mL of a 3% sodium hydroxide (NaOH) solution. Subsequently, the material was washed under a vacuum using 700 mL of deionized water. Finally, the material was dried for 24 h at 100 °C and made ready to reuse in the subsequent adsorption cycle.

Figure 1(b) shows the FTIR spectra for the evaluated materials. For CH, different bands were found: a band around 3,325 cm⁻¹ is related to the vibration of the O-H groups, while the bands at 2,920 and 2,853 cm⁻¹ are associated with the -C-H bond stretching. On the other hand, the band at 1,736 cm⁻¹ suggests the presence of -C = O groups, and the band at 1,612 cm⁻¹ is associated with the vibrations of the aromatic ring's characteristic of lignin. Finally, the band at 1,000 cm⁻¹ corresponds to the stretching of the -C-O vibration in ether o lactones (Liang et al. 2015; Meseldzija et al. 2019; Ramirez et al. 2021). It can be observed that the CH sample exhibits bands associated with the presence of hydroxyl -O-H, carbonyl -C = O and -C-O groups in ether or lactone, which are absent in the carbonaceous materials thermally treated at higher temperatures (CHP-400 and CHP-700). In contrast, these bands present a low intensity in the spectrum of CHP-300. On the other hand, the signals associated with aromatic groups remain in the pyrolyzed materials because of the carbonization process experimented for the CH. These results suggest that chemical interactions between the functional groups in CH and the molecular structure of the contaminant govern the adsorption process of CIP.

The aforementioned information suggests that functional groups in CH may have partial charges, unshared electron pairs, or interact by Van der Waals' forces to promote interactions by electrostatic attraction, complexation, or hydrogen bonds, respectively (Petrovic et al. 2022). Both contaminants are heavily dependent on solution chemistry for the adsorption process (Li et al. 2011; Zide et al. 2018; Adaobi et al. 2021). For a comprehensive understanding of the mechanisms involved, it is necessary to have critical information about the solution pH, the point of zero charge (pH_PZC) of the adsorbent, and the pK_a of the adsorbate (Adaobi et al. 2021). In this sense, to properly evaluate the potential of CH in removing CIP and ACE, it is important to consider the ionic or neutral forms of the two contaminants and the material's zero-point charge. Then, the pH_PZC of CH is presented in Figure S1 (see the Supporting information), indicating a value of 4.7. Also, the pK_a values of CIP are 5.9 (pK_a1) and 8.9 (pK_a2), while ACE has a 9.5 pK_a value. Therefore, at a solution pH of 5.55 (Figure 1(a)) and pHpzc = 4.7 (Figure S1), the CIP is mainly in its cationic form (CIP⁺), and the material's surface presents a net negative charge since pH_solution > pH_PZC. As a result, the adsorption of this cationic adsorbate CIP⁺ onto the negative surface of CH through electrostatic attraction is highly expected as a primary adsorption mechanism (Figure 3(a)) (dos Santos et al. 2018; Yilimulati et al. 2021).

Regarding the removal of ACE, it is not preferred based on the experimental conditions described earlier. It is because the ACE molecule is in a neutral form at the natural pH of the working solution (pH = 5.5), preventing electrostatic attraction interactions. Also, it has been reported that the pore-filling mechanism is one of the predominant mechanisms for ACE adsorption, and it is typically observed in materials with a larger specific surface area and well-developed internal pore structure (Thanh et al. 2020). In contrast, biomass has low specific surface areas and poorly porous structures. Hence, neither of these two conditions is satisfied to favor ACE adsorption. CH was used as the adsorbent material for the subsequent experiments using CIP as the contaminant, and a comprehensive evaluation of all adsorption parameters was conducted.

The initial increase in removal percentage with adsorbent dosage can be attributed to the greater availability of active adsorption sites on the CH surface. As more adsorbent is added, there are more sites to bind the CIP molecules, increasing removal until saturation is reached (Nazari et al. 2016). The subsequent decline in removal percentage at higher dosages likely occurs due to remaining unsaturated sites after adsorption, reducing the total effective surface area (Bhattacharyya et al. 2023). In contrast to the removal percentage trend, the adsorption capacity decreased continuously from 91 mg/g at the lowest dosage tested to 0.5 mg/g at the highest dosage.

These results can be better understood by looking at Figure 3(a), which shows the CIP speciation curve. For CIP, the carbonyl group has a pK_a1 of 5.9, while the amine group has a pK_a2 of 8.9 (Adaobi et al. 2021). Other studies usually report 6.1 and 8.7 (Valdés et al. 2017; Adaobi et al. 2021). CIP can exist as a cation, anion, or Zwitterion (Adaobi et al. 2021), as shown in Figure 3(a). When the adsorbent surface has a net negative charge, CIP uptake could occur via cation exchange with the protonated amine group. Therefore, the good performance at pH 6 and 8 can be attributed to the material's surface acquiring a negative charge, a characteristic stemming from its pH_PZC at these pH values. According to the speciation curve (Figure 3(a)), at pH 6.44% of CIP is in cationic form, and 56% of CIP is in Zwitterion form, favoring adsorption by electrostatic attractions and hydrophobic interaction, respectively. Similarly, at pH 8, CIP is 89% in its Zwitterion form and 11% in its anionic form, favoring adsorption through hydrophobic interaction with its Zwitterion form. It has been reported that the CIP tends to get adsorbed due to its low solubility, where the adsorbate is electrostatically neutral and exists as a Zwitterion, which occurs at 5.90–8.89 pH region (Adaobi et al. 2021). Finally, at pH 2, where the lowest removal percentage was obtained, the material is positively charged, and the CIP is 1.6% in its anionic form and 98.4% in its cationic form, resulting in electrostatic repulsion and disfavoring adsorption. Ajala et al. found a similar behavior in CIP adsorption (Ajala et al. 2023). CIP removal increased with increasing pH until an equilibrium capacity of 11.6 mg/g was reached at pH 5.8 (Ajala et al. 2023).

Table 1 presents the parameters associated with the data fitting to the pseudo-first- and pseudo-second-order models. According to the R² value, the best fit of the experimental data was obtained with the pseudo-second-order kinetic model. It assumes that the rate of adsorption site occupancy is proportional to the square of the number of unoccupied sites (Paredes-Laverde et al. 2018), which suggests that the rate-limiting step is chemisorption (Mitrogiannis et al. 2017). This behavior is attributed to interactions between CIP and the CH surface functional groups such as C-H, C = O, C–O, and C–N, which includes electrostatic attraction, complexation, and hydrogen bonds (Wu et al. 2015). The occurrence of these mechanisms is extensively documented in the literature for the CIP adsorption process (Adaobi et al. 2021).

Concerning the Langmuir model, the Langmuir separation factor (K_L) shown in Table 2 indicates whether the adsorption is irreversible (K_L = 0), linear (K_L = 1), favorable (0 < K_L < 1), or unfavorable (K_L > 1). For the present study, a K_L = 0.0235 was obtained, indicating that the adsorption is favorable (Acelas et al. 2021). The Freundlich model uses ‘n_F’ to indicate the favorability of adsorption. A value of ‘n_F’ ranging from 2 to 10 indicates good adsorption characteristics, while a value between 1 and 2 suggests difficult adsorption, and less than 1 suggests poor adsorption (Treybal 1981). For CIP, the n_F value was 1.39, indicating that the physical adsorption of CIP onto CH is quite difficult, suggesting that the physical adsorption is not a favorable process in this system.

Adsorption is a complex process that involves several mechanisms. Identifying these mechanisms requires a systematic approach. Thermodynamics plays a crucial role in determining the adsorption mechanism by determining the nature of the interactions involved. By assessing the magnitude of ΔG°, we can determine whether the adsorption is favorable (Adaobi et al. 2021). Table S5 shows the thermodynamic adsorption parameters for CIP in CH. From the results, is positive, and its magnitude increases with increasing temperature. A chemical process is considered favorable if ΔG° < 0, indicating spontaneous reaction, and unfavorable if ΔG° > 0, indicating nonspontaneous reaction (Edet & Ifelebuegu 2020). Therefore, the adsorption of CIP onto the CH surface is unfavorable, nonspontaneous, and not favored by increasing temperature. The enthalpy values indicate that the adsorption of CIP by biosorbents is a physical and endothermic process (Adaobi et al. 2021). This physisorption is achieved through weak bonding forces between CIP molecules and biosorbent surfaces, as evidenced by the low values (<40 kJ/mol). This result suggests the occurrence of physisorption during the CIP adsorption process. The negative value of ΔS° indicates the ordered state of the system at the solid–solution interface during the removal process. The activation energy E_a of this adsorption process was 18.8 kJ/mol. A low E_a (5–50 kJ/mol) suggests physisorption, while E_a greater than 50 kJ/mol implies chemisorption (Acelas et al. 2021). Therefore, the value of E_a obtained indicates that physical adsorption takes place during the adsorption of CIP on CH, i.e., it also depends on weak forces such as Van der Waals forces.

These results highlight the potential of the CH bioadsorbent in removing cationic antibiotics like CIP from complex matrices, particularly wastewater. However, it is essential to investigate further the specific interactions between the matrix components and the adsorbent to gain a deeper understanding of the competitive adsorption mechanisms. Future research could focus on characterizing the wastewater and synthetic urine matrices and exploring the influence of various operational parameters, such as pH, temperature, and adsorbent dosage, on the removal efficiency in these complex matrices.

This study highlights the potential of untreated CH as a highly effective and sustainable bioadsorbent for removing pharmaceutical contaminants, specifically CIP, from wastewater. The results demonstrate that CH exhibits adequate adsorption capabilities, achieving up to 64% removal of CIP with a maximum adsorption capacity of 37.00 mg/g. Compared with most other adsorbents reported in the literature, CH in this study showed similar or better adsorption performance on CIP adsorption, demonstrating the potential of this bioadsorbent material for antibiotics removal.

The results of this investigation provide valuable insights into the adsorption mechanism. They suggest that chemisorption predominates, as evidenced by the excellent fit to the pseudo-second-order kinetic model. However, physisorption interactions also contribute to this adsorption, as supported by thermodynamic data and the reversibility of the adsorption process. In addition, the thermodynamic analysis indicates that the adsorption of CIP onto CH is an endothermic process, suggesting its suitability for removal under room temperature conditions. Furthermore, the nonspontaneous nature of the adsorption process, as indicated by positive ΔG° values, highlights the need for energy input in this removal mechanism.

This research extends the applicability of CH as a bioadsorbent by demonstrating its effectiveness in both synthetic wastewater and urine matrices, showing its potential for practical use in water treatment systems. Therefore, CH is a valuable alternative to mitigate the environmental impact of pharmaceutical contaminants in wastewater while advocating for sustainable and environmentally friendly solutions in adsorption research and wastewater treatment strategies. The results suggest that CH has potential as an eco-friendly and efficient option for large-scale adsorption processes since, despite its moderate maximum adsorption capacity, its stable performance across multiple reuse cycles and applicability in complex matrices make it a promising bioadsorbent for industrial use. Moreover, a renewable agricultural byproduct, such as CH, offers a sustainable alternative to costly commercial adsorbents, aligning with global environmental and economic sustainability goals.

The authors thank the University of Medellín for the financial support of Project 1241. Also, the authors thank the Delfín program for the participation of students from different Mexican universities within the framework of the XXVIII Pacific Scientific and Technological Research Summer 2023. J.P. thanks the Uniremington for the financial support of project 4000000389.

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

The authors declare there is no conflict.