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Article

Highly Efficient and Environmentally Friendly Walnut Shell Carbon for the Removal of Ciprofloxacin, Diclofenac, and Sulfamethoxazole from Aqueous Solutions and Real Wastewater

by
Seda Tunay
1,
Rabia Koklu
1,* and
Mustafa Imamoglu
2,*
1
Environmental Engineering Department, Faculty of Engineering, Sakarya University, 54050 Sakarya, Türkiye
2
Chemistry Department, Faculty of Sciences, Sakarya University, 54050 Sakarya, Türkiye
*
Authors to whom correspondence should be addressed.
Processes 2024, 12(12), 2766; https://doi.org/10.3390/pr12122766
Submission received: 12 November 2024 / Revised: 26 November 2024 / Accepted: 3 December 2024 / Published: 5 December 2024
(This article belongs to the Section Chemical Processes and Systems)
Browse Figures
Versions Notes

Abstract

:
The objective of this study is to assess the efficacy of walnut shell-derived activated carbon with phosphoric acid (WSAC) in the removal of ciprofloxacin (CIP), diclofenac (DC), and sulfamethoxazole (SMX) from aqueous solutions and real wastewater. WSAC was characterized using various analytical techniques such as specific surface area and pore size distribution determination, elemental analysis, SEM images, and FT-IR spectroscopy. The BET-specific surface area of WSAC was determined to be 1428 m2 g−1. The surface is characterized by the presence of irregular pits of varying dimensions and shapes. The adsorption of SMX, CIP, and DC from aqueous solutions using WSAC was tested under various parameters, including contact time, adsorbent dosage, initial concentration, pH, and temperature. The adsorption of SMX, CIP, and DC was found to be in accordance with the Langmuir isotherm model, which suggests that monomolecular adsorption is the predominant mechanism. The maximum adsorption capacities of WSAC towards SMX, CIP, and DC were calculated to be 476.2, 185.2, and 135.1 mg g−1, respectively. The adsorption of SMX, CIP, and DC were found to be consistent with the pseudo-second-order model. Thermodynamic analyses demonstrated the spontaneous and endothermic nature of SMX, CIP, and DC adsorption onto WSAC. The adsorption performances of SMX, CIP, and DC on WSAC were found to be 60.2%, 77.4%, and 74.2%, respectively in the effluent from the municipal wastewater treatment plant. In conclusion, WSAC may be regarded as a readily available, eco-friendly, and efficient substance for the extraction of SMX, CIP, and DC from wastewater and aqueous solutions.

1. Introduction

Fresh water resources have been decreasing due to many factors such as increased household and industrial usage, decrease in precipitation due to global warming, population growth, and pollution. The increasing use of micro-pollutants, driven by technological advancements and increasing human needs, poses a potential threat to shrinking water resources and is detected in various environmental matrices, including water bodies, soil, air, and the food chain [1]. Among the micro-pollutants, pharmaceuticals pose a significant threat to aquatic organisms and human health even at very low concentrations due to their resistance, persistence, biological activities, and toxicities in aquatic ecosystems, leading to irreversible harmful effects [2,3]. Pharmaceuticals that humans and animals cannot completely metabolize may transform into other molecules in a variety of environmental settings. When released into soil, surface waters, and groundwater, these molecules have potential risks to biota [4]. Due to their inability to biodegrade, weak adsorption capacity, and complex structures, pharmaceuticals have limited removal efficiency by conventional wastewater treatment methods [5]. Ciprofloxacin (CIP) as an antibiotic, diclofenac sodium (DC) as a nonsteroidal anti-inflammatory drug, and sulfamethoxazole (SMX) as an antimicrobial are among the significant pharmaceuticals observed in wastewater due to incomplete treatment of wastewater by conventional techniques [6,7].
Commonly employed methods used to remove micropollutants from wastewater comprise biodegradation [8], electrochemical catalysis [9], advanced oxidation [10], membrane filtration [11], membrane bioreactors [12], and degradation by ultrasonic irradiation [13]. However, these methods have significant disadvantages, including the complex chemical structures of pharmaceuticals, production of hazardous byproducts, and operation and handling costs [14]. Recent scientific studies have focused on balancing technological advancement and environmental sustainability.
Adsorption’s affordability, efficacy, sustainability, and environmental friendliness make it a potential method for removing various contaminants from wastewater. Therefore, the development of cost-effective, efficient, readily available, and environmentally friendly adsorbents has become a primary focus of research in the field of adsorption methods. Various adsorbents such as activated carbon (AC) [15], silica [16], clay [17], and carbon nanotubes [18] have been employed for the elimination of micropollutants from water samples. AC has benefits like a high pore volume, broad specific surface area, and dense porous structure that lead to better performance for the removal of pharmaceuticals from wastewater. Consequently, AC is often preferred as an adsorbent in wastewater treatment applications [19]. The biggest disadvantage of commercial AC’s is their high cost. To reduce the cost of AC production, various materials, primarily from agricultural wastes like orange peel [20], pumpkin peel [21], cherry seeds [22], teff husk [23], date palm shell [24], coconut shell [25], walnut shell [26,27], hazelnut husk [28], tea leaves [29], peanut shell [30], groundnut shell [31], and watermelon rind [32] have been used as agricultural raw materials for AC production. Using agricultural waste for AC production not only ensures the effective and efficient removal of pollutants from wastewater but also helps mitigate environmental issues, such as solid waste accumulation, which leads to air and water pollution during their degradation in nature.
Walnuts are produced widely across many countries, with Turkey ranking fourth in global production after China, the USA, and Iran, contributing 5% to the world’s total, amounting to 215,000 tons in 2019 [33,34]. Walnut shell (WS), a by-product of walnut, constitutes 67% of the total mass of the walnut fruit [35]. Annual walnut shell production in Turkey is estimated to be 150,000 tons. Traditionally, WS is either discarded unused or used as fuel. Thus, due to its low cost, abundant local availability, and renewable nature every year, WS could be considered as a very suitable material for the production of AC for the removal of SMX, CIP, and DC from wastewater.
Walnut shell-based activated carbons (ACs) have been used for the adsorption of various pollutants, including naphthalene and phenanthrene [36], Congo red and methylene blue [37], diazinon [38], Ni(II) [39], sulfonamides [40], diclofenac (DC) and sulfamethoxazole (SMX) (AC prepared using a mixture of H3PO4 and H2SO4 for activation) [41], metronidazole and SMX (AC synthesized with K2CO3 activation) [42], and ciprofloxacin (CIP) (AC prepared by different methods such as KOH activation via wet and dry processes, CO2 activation, steam activation, and combined CO2 and steam activation) [43].
There is no study in the literature about adsorption of SMX, CIP, and DC from aqueous solutions and real wastewater using walnut shell activated carbon prepared by H3PO4 activation. In the present study, H3PO4 was selected to be a chemical activating agent due to its commercial usage in the AC production process. The use of phosphoric acid offers a number of advantages including low environmental impact, ease of recovery, and high carbon yield [44]. The objective of this research is to evaluate the adsorption performance of phosphoric acid-activated walnut shell activated carbon (WSAC) for the removal of some pharmaceuticals (SMX, CIP, DC) from solutions and wastewater. The characterization of WSAC was performed using various analyses. The effects of contact time, pH, WSAC dosage, temperature, and initial concentration on the adsorption of SMX, CIP, and DC were examined through batch adsorption experiments. SMX, CIP, and DC adsorption kinetics, isotherms, and thermodynamic modeling were examined. Following the optimization of the batch adsorption procedure using WSAC, the developed method was applied to wastewater samples collected from urban wastewater treatment plant effluents in Sakarya, Turkey.

2. Materials and Methods

2.1. Chemicals and Instruments

The reagents and chemicals employed in this study were of analytical grade. SMX and DC were procured from Sigma-Aldrich (St Louis, MO, USA), while CIP was obtained from Deva Holding, Istanbul, Turkey. Solutions of SMX solutions at 200 mg L−1 and CIP, DC were prepared at concentrations of 500 mg L−1. The solutions utilized in the study were prepared daily through the dilution of the aforementioned stock solutions. The phosphoric acid (H3PO4, 85%) was obtained from Sigma-Aldrich.
An electrically driven sieve shaker (Retsch type AS200, Haan, Germany) was used to sieve the WS and WSAC. The working solutions with WSAC were stirred using an orbital shaker with temperature control (KS4000i, Staufen, Germany). Using a UV spectrophotometer (Shimadzu UV 2600, Kyoto, Japan), the balance concentrations of SMX and CIP, DC were determined at wavelengths of 285 nm and 276 nm, respectively. A Schott CG 840 pH meter (Schott AG, Mainz, Germany) was used to measure the pH of the solutions. The pH values of the solutions were adjusted to the proper amounts using 0.1 M HCl and 0.1 M NaOH solutions. WSAC was synthesized in a tube furnace (Protherm PTF 12, Ankara, Turkey). Elemental analysis (C, H, and N) was made using Leco CHNS 932 analyzer (Leco Corp., St. Joseph, MI, USA). The surface morphology of WSAC was characterized using a scanning electron microscope (SEM Quanta FEG 250, Hillsboro, OR, USA) at Düzce University, Turkey. At the Middle East Technical University in Ankara, Turkey, BET-specific surface area was determined, and NLDFT pore structure analysis was carried out using Quantachrome Autosorb-6B (Quantachrome, Boynton Beach, FL, USA). The simultaneous experimental studies for solutions and wastewater were conducted using liquid chromatography, as detailed in our previous study [45].

2.2. Preparation of WSAC

After washing with deionized water, walnut shells were dried overnight at 105 °C, and grinded using a stainless-steel blender. The walnut shell particles, between 1000 and 1500 μm, were used for AC preparation. Concentrated phosphoric acid H3PO4 (150 g) was mixed with 150 mL of purified water, then 150 g of walnut shell (WS) was added to the solution. The suspension was maintained in a water bath at 80 °C for 24 h to allow for the phosphoric acid impregnation into the adsorbent, after which it was dried at 105 °C for another 24 h. The material was then pyrolyzed in a tube furnace under a nitrogen (N2) flow (100 mL min−1) at 600 °C for 4 h, and then cooled to ambient temperature under N2 atmosphere. The resulting AC was washed with hot deionized water and then soaked in a 1% NaHCO3 solution overnight to neutralize any remaining acid residues. The AC was rinsed with deionized water and then dried at 105 °C for 24 h. After the AC was sieved, the study used the particles that were between 250 and 500 μm [46].

2.3. Characterization of WSAC

Various techniques were employed to characterize the walnut shell activated carbon (WSAC). Proximate analyses, including volatile matter, moisture, ash, and fixed carbon content, as well as iodine number determinations, were conducted in accordance with the standards set by the American Society for Testing and Materials (ASTM) [47]. Surface functional groups were determined by Fourier Transform Infrared Spectroscopy (FT-IR) in the wavenumber range of 450 cm−1 to 4000 cm−1. The specific surface area was calculated using the BET approach and N2 adsorption–desorption isotherms. Furthermore, the surface morphology of WSAC was examined through the utilization of scanning electron microscopy (SEM) images. To determine the point of zero charge (pHPZC), 0.1 M NaCl solutions with initial pH values ranging from 2 to 12 were suspended with 0.1 g of WSAC, and the suspensions were shaken for 24 h. Then, 0.1 M NaOH or 0.1 M HNO3 solution was used to change the pH of the NaCl solutions. After shaking the solutions, the equilibrium pH was found, and the ΔpH values (the difference between the initial and equilibrium pH values) were calculated. Plotting the initial pH values against ΔpH values yielded a curve, and the pHPZC value was the point where the curve intersected the x-axis [48]. Boehm titration was used to determine the amounts of carboxylic, phenolic, and lactonic groups for WSAC. Then, 50 milliliters of 0.1 M NaOH, Na2CO3, and NaHCO3 solutions were mixed with 1 g of WSAC and shaken for 24 h at 400 rpm. Following vacuum filtration of the samples, 0.1 N HCl was used to titrate 20 mL of the filtrate. Based on the titration results, the quantities of carboxylic, phenolic, and lactonic groups in WSAC were computed [49]. Three distinct quantities of adsorbent (0.5, 1, and 2 g) were introduced to 10 mL of a 5% HCl solution with the objective of ascertaining the iodine number of WSAC. Subsequently, the solutions were heated to boiling point for a period of 30 ± 2 s. Following the cooling of the mixture, 0.1 N iodine solution (100 mL) was added, and the contents were agitated for a period of 30 ± 1 s. Subsequently, the solutions were filtered, and 0.1 N sodium thiosulfate was employed for the titration of the filtrates [49].

2.4. Adsorption of SMX, CIP, and DC onto WSAC

Batch experimentations were conducted to investigate the adsorption of SMX, CIP, and DC onto WSAC. The effect of various factors such as temperature, pH, WSAC dosage, initial concentration, and contact time on the adsorption process was investigated. Individual SMX, CIP, and DC aqueous solutions (50 mL each) were used in all batch studies. The suspensions were stirred for the specified allotted amount of time at 25 °C (apart from temperature experiments). The temperature effects on the adsorption of SMX, CIP, and DC onto WSAC were assessed at 25 °C, 35 °C, and 45 °C. Centrifugation was used to separate the suspensions after agitation. A UV-Vis spectrophotometer was used to quantify the analyte equilibrium concentrations at 285 nm for SMX and 276 nm for DC and CIP. To ensure the reliability of the results, three replicates of each experiment were conducted, and the mean values were subsequently calculated and shared. Equations (1) and (2) were used to calculate the amounts of pharmaceutical adsorbed onto WSAC and the adsorption percentages, respectively.
q e = ( ( C 0 − C e ) × V ) / m
R e m o v a l   ( % ) = ( C 0 − C e ) / C e ) × 100
where, qe (mg g−1) is the amount of the pharmaceutical adsorbed by WSAC at equilibrium, C0 and Ce are the pharmaceutical concentration values at the initial and final stage (mg L−1), V is the solution volume (L), and m is the mass of WSAC (g) [50,51].

3. Results

3.1. WSAC Characterization

Table 1 presents the elemental analysis results, water solubility, iodine number, pHPZC, BET-specific surface area, ash and moisture content, fixed carbon, volatile matter, and certain surface functional groups of WSAC. The high ash concentration of WSAC (11.9%) that has been found may cause the activated carbon’s pores to get clogged, which would reduce its specific surface area [52]. This phenomenon could be attributed to the high concentration of phosphoric acid used for activation and the inorganic content of WS. ACs obtained from corn cob [53] and the Rumex abyssinicus plant [54] have 13.2% and 9.8% ash contents, respectively. The ACs having a low ash content exhibit a better adsorption capacity, while the high ash content of AC’s significantly reduces the specific surface area and porosity of ACs [55].
The moisture content of WSAC was found to be 10.4%. The moisture contents of ACs from sesame seed husk [56], red seaweed [57], pumpkin [58] and peanut shells [31] have been reported as 8.53%, 18.9%, 15.9%, and 8%, respectively. WSAC’s volatile matter content was found to be 17.8%. The release of volatile compounds increases with the pyrolysis temperature, leading to less carbon production [59]. Additionally, an increase in the activator amount leads to a decrease in the volatile matter content of ACs. AC quality increases with low moisture, low ash content, and low volatile matter [60]. The volatile matter of hyacinth plant [61] and soap nut [62] activated carbons were reported to be 18.2% and 18%, respectively.
The carbon content of WSAC (71.6%) is higher than that of pumpkin shell biochar (63% C) [58], peanut shell activated carbon (51.7% C) [31], and rice husk AC (30.7% C) [63]. The pHPZC of WSAC was defined to be 4.26 (Figure S1). Accordingly, WSAC is cationic at pH < 4.26, anionic at pH > 4.26, and neutral at pH 4.26. The surface morphology of WSAC was determined through SEM analysis. Figure 1 displays the SEM images of WSAC at 1000× and 5000× magnifications. The WSAC surface is randomly scattered with a large number of irregular pits of different sizes and forms [64,65]. The findings of the multi-point BET study showed that WSAC’s specific surface area was 1428 m2 g−1. In the reviewed studies, specific surface areas of ACs gained from various sources were reported as follows: 239.2 m2 g−1 from castor oil seed husk [56], 616 m2 g−1 from peanut shell [31], 329 m2 g−1 from rice husk [63], and 980.9 m2 g−1 from hazelnut shell [66]. Additionally, the cumulative specific surface area defined by the NLDFT method for WSAC was found to be 1372 m2 g−1.
The N2 adsorption/desorption isotherm of WSAC is presented in Figure 2. The isotherms display hysteresis of kinds I (b) and H4, per the IUPAC classification [67]. Microporous solids and materials with incredibly small exterior surfaces are commonly observed to exhibit type I isotherms. Materials with a wider variation of pore sizes, including both narrow mesopores (<∼2.5 nm) and wider micropores, are specified by type I (b) isotherms. H4-type hysteresis is commonly reported in micro-mesoporous carbons [67]. Accordingly, the WSAC isotherm is consistent with the type I isotherm, where micropore filling occurs at low P/P0. The pore size and average pore size were observed to range from 8.6 to 229 Å and 70.83 Å, respectively. It indicated that WSAC has a microporous structure (Figure 3).
FT-IR analysis was used to identify the functional groups on the WSAC surface (Figure 4). The O-H stretching of acidic functional and phenolic alcoholic groups was identified as the peak detected at 3424 cm−1 [68,69,70]. The lignin structure’s aromatic C=C stretching [68] and the C-H stretching vibration of alkyl groups [69,70,71] may be responsible for the peak at 2924 cm−1. The C-H asymmetric deformation and aromatic C-C functional group may be the cause of the peak at 1388 cm−1, whereas the C-O stretching is represented by the peak at 1565 cm−1 [71,72,73]. The C-O stretching of primary alcohols may be the cause of the peaks at 1158 and 1064 cm−1 [71,74,75].

3.2. Optimization of Adsorption of SMX, CIP, and DC onto WSAC

Adsorbent dosage, contact time, initial concentration, temperature, and pH were among the variables examined in order to improve the adsorption of SMX, CIP, and DC onto WSAC. To examine the effect of contact time, CIP, DC, and SMX solutions were stirred for periods ranging from 5 to 1440 min, and the equilibrium time was determined to be 1440 min. Figure 5A shows the variation of the mass of SMX, CIP, and DC adsorbed per gram of WSAC with contact time. Initially, CIP, DC, and SMX adsorption occurred rapidly but slowed down after 240 min. This could be attributed to the initial adsorption sites being vacant and easily accessible, which became occupied by CIP, DC, and SMX over time. Consequently, the adsorption continued within the internal pores of WSAC, requiring more time to reach the internal pores [76].
In order to examine the impact of the adsorbent dose, varying dosages of WSAC (0.025 to 0.3 g) were added to 50 mL volumes of DC and CIP solutions at a concentration of 300 mg L−1 and SMX solution at a concentration of 200 mg L−1. The solutions were agitated for 24 h, and the results are shown in Figure 5B1,B2. As the WSAC amount was increased from 0.025 g to 0.3 g, the adsorption percentage increased from 20.5% to 98.7% for DC, from 30.3% to 99.9% for CIP, and from 67.3% to 98.6% for SMX. The elevated dosage of WSAC resulted in a greater number of available adsorption sites, thereby facilitating a higher percentage removal of the pharmaceuticals [77]. However, the adsorption capacity decreased from 269.2 mg g−1 to 32.8 mg g−1 for SMX, from 182 mg g−1 to 53.9 mg g−1 for CIP, and from 123 mg g−1 to 49.3 mg g−1 for DC. The amount of SMX, CIP, and DC adsorbed per gram of WSAC (qe) decreased as a result of the inverse relationship between the mass of WSAC and the amount of pharmaceuticals adsorbed [46,78].
The effect of initial concentration on the adsorption of SMX, CIP, and DC onto WSAC is illustrated in Figure 5C1,C2. The quantities of SMX, CIP, and DC adsorbed onto WSAC demonstrated a notable increase in correlation with the initial concentrations. The initial concentrations of SMX up to 175 mg L−1 and DC and CIP up to 400 mg L−1 exhibited a rapid increase, which subsequently slowed. Due to the adsorption sites’ existence on the WSAC surface at low initial concentrations of SMX, CIP, and DC, greater amounts of WSAC were adsorbed per gram as SMX, CIP, and DC concentrations increased. But after the WSAC surface’s adsorption sites are completely occupied, there are no more appropriate sites for adsorption [66,79]. As the initial concentrations increased, the adsorption percentages of SMX, CIP, and DC by WSAC were shown to have decreased.
The solution pH plays a pivotal role in the adsorption performance of activated carbon, as it affects both the ionic forms of the adsorbed species and ionization of the surface functional groups of the WSAC. It is important to examine the effect of the pH parameter since it affects the reaction kinetics and equilibrium properties of the adsorption process. Additionally, determining the pKa value of the pharmaceutical is important to accurately describe the interaction between the surface functional groups of the WSAC and the pharmaceuticals. The adsorption capacities of SMX, CIP, and DC onto WSAC at different pH values are represented in Figure 5D. As the pH increased from 6.0 to 10.0, a slight reduction in DC adsorption onto WSAC was observed. The decrease could be ascribed to the augmented negative surface charge of WSAC and the deprotonation of DC molecules with rising pH levels [80]. SMX adsorption onto WSAC exhibited a more pronounced decrease as the pH rose from 6.0 to 7.0, with a continued but slower decrease up to pH 10.0. The observed decrease can be attributed to a reduction in the electrostatic attraction between SMX and the WSAC surface [80]. On the other hand, as the pH rose from 6.0 to 10.0, the amount of CIP adsorbed onto WSAC progressively increased. CIP exists in a cationic form at pH < pKa1 = 6.1, due to deprotonation of the carboxylic group, and in an anionic form at pH > pKa2 = 8.7. As the pH level rises, the partial positive form undergoes a conversion to the zwitterionic form. This results in a reduction in competition and an increase in electrostatic attraction, thereby enhancing the capacity of CIP adsorption [81]. In all experiments, except for the pH effect, the adsorption of SMX, DC, and CIP was conducted at the original pH values.
To ascertain the effect of temperature on the adsorption of SMX, DC, and CIP onto WSAC from an aqueous solution, adsorption experiments were conducted at 25 °C, 35 °C, and 45 °C. The results are presented in Figure 5E. The data indicated that the adsorption capacity (mg g−1) of WSAC increased with rising temperature for all three pharmaceuticals. The adsorption efficiency increased from 60.6% to 65.1% for DC, from 77.7% to 93.95% for CIP, and from 89.4% to 90.2% for SMX as the temperature rose from 25 °C to 45 °C. The contribution of the temperature increasing from 25 °C to 45 °C to the adsorption efficiency was determined to be 4.5% for DC, 16.3% for CIP, and 0.8% for SMX. According to the results, the increase in temperature had a greater effect on CIP adsorption compared to DC and SMX. This temperature-dependent increase in adsorption indicates that the adsorption process is endothermic [82,83].

3.3. Adsorption Equilibriums of SMX, CIP, and DC onto WSAC

The experimental curves predicted by Freundlich, Langmuir, Dubinin–Radushkevich, and Temkin equations for the adsorption of SMX, CIP, and DC onto WSAC are presented in Figure 6. When compared to the curves obtained with other models, the experimental curves showed good agreement with those predicted by the Langmuir equation. The constants obtained from Freundlich, Langmuir, Dubinin–Radushkevich, and Temkin isotherm models are presented in Table 2. It was noted that the regression coefficients obtained for the Langmuir isotherm model were higher than those for the Freundlich isotherm model, suggesting that the adsorption of SMX, CIP, and DC with WSAC occurs as single-layer adsorption on the homogeneous surface of WSAC [84].
The adsorption capacities of various adsorbents for SMX, CIP, and DC were evaluated with WSAC and are presented in Table 3. The DC adsorption capacity of WSAC was found to be higher than that of cocoa shell AC. The adsorption capacity for CIP is superior to that of activated carbons derived from bamboo and sugarcane bagasse, while the SMX adsorption capacity is more effective than that of activated carbons derived from rice husk and pine wood.

3.4. Adsorption Kinetics of SMX, CIP, and DC onto WSAC

Intraparticle diffusion models, pseudo-second-order kinetics, and pseudo-first-order kinetics were used to characterize the SMX, CIP, and DC adsorption rate onto WSAC. The computed kinetic parameters are compiled in Table 4.
The r2 values CIP, SMX, and DC adsorption onto WSAC were higher and closer to unity when the correlation coefficients from the pseudo-second-order model and the pseudo-first-order model were compared. The calculated qe values from the pseudo-second-order model were in better agreement with the experimentally determined values. Consequently, it was found that the pseudo-second-order kinetic model is better suited for utilizing WSAC to define the SMX, CIP, and DC adsorption kinetics (Figure S2). The better representation of SMX, CIP, and DC adsorption onto WSAC using the pseudo-second-order model suggests that chemisorption occurs between the adsorbates and WSAC [93,94,95]. Previous studies have reported that the adsorption of DC with AC from sugar cane bagasse [84] and the Moringa oleifera plant [96], CIP adsorption with activated carbon obtained from Sterculia villosa Roxb shell [97], and SMX adsorption with activated carbon produced from pine wood [86] and some plant wastes [77] are more reliable with the pseudo-second-order kinetic model.
The mass transfer resistance observed at the boundary layer plays a significant role in the adsorption mechanism [96]. Therefore, the intraparticle diffusion model was used to investigate the adsorption mechanism and characterize the rate-controlling step. The adsorption of SMX, CIP, and DC onto WSAC occurs in two steps: film diffusion and intraparticle diffusion (Figure S3). Two regions that reflect two distinct processes in the adsorption process are present in the curve plotted for qt against t 0.5 for the adsorption of SMX, CIP, and DC on WSAC, which does not pass through the origin [66]. The first step is very short, occurs rapidly, and is externally diffusion-controlled. The second step is a complex adsorption stage, controlled by intraparticle diffusion, where saturation occurs [86]. This indicates that intraparticle diffusion is not the sole step limiting adsorption and that film diffusion also affects the adsorptions [29]. In the literature, two-stage kinetics have been reported for the adsorption of Cr(III) with activated carbon obtained from tea industry waste, methylene blue adsorption using hazelnut-derived activated carbon, and sulfamethoxazole adsorption with activated carbon obtained from pine wood [29,86,98].

3.5. Adsorption Thermodynamics of SMX, CIP, and DC onto WSAC

Distribution coefficients (Kd) were calculated at three different temperatures for the adsorption of SMX, CIP, and DC onto WSAC. The ΔG values were then calculated using the Kd values at each temperature. Moreover, the slope and intercept of the lnKd against 1/T plots (Figure S4) were employed to ascertain the ΔH and ΔS values. The results of these calculations are presented in Table 5. Adsorption is a spontaneous process when the Gibbs free energy change (ΔG) is negative [96]. The observed increase in ΔG with rising temperature suggests that a greater number of active sites become accessible in the WSAC at elevated temperatures [75]. The total enthalpy changes (ΔH) for SMX, CIP, and DC adsorption onto WSAC were calculated as 7.6, 58.7, and 3.4 kJ mol−1, respectively. A positive ΔH value indicates an endothermic adsorption process [31], while positive ΔS values suggest an increase in randomness at the solid–liquid interface during adsorption [99].

3.6. Removal of SMX, CIP, and DC from Wastewater Samples

A solution including 10 mg L−1 of SMX, 10 mg L−1 of CIP, and 10 mg L−1 of DC was made in order to study the competitive adsorption of SMX, CIP, and DC onto WSAC. HPLC was then used to determine the remaining concentrations of SMX, CIP, and DC in the solutions after a variety of WSAC amounts, from 0.025 to 0.1 g, were added to the solutions (100 mL). The results are presented in Figure 7. As the quantity of WSAC increased from 0.025 g to 0.1 g, the adsorption percentage rose from 74.4% to 93.1% for DC, from 25.0% to 100% for CIP, and from 59.5% to 100% for SMX.
The wastewater sample was obtained from the municipal wastewater treatment plant in Sakarya, Turkey, and then spiked with SMX, CIP, and DC (10 mg L−1). The initial levels of SMX, CIP, and DC were measured using HPLC and determined to be 11.2 mg L−1, 10.5 mg L−1, and 9.5 mg L−1, respectively. WSAC was added to 100 mL of wastewater in quantities ranging from 0.025 g to 0.1 g, and the solutions were stirred for 24 h at room temperature. Following the adsorption procedure, HPLC was used to quantify the remaining amounts of SMX, CIP, and DC. As the WSAC amount rose from 0.025 g to 0.1 g, the findings, which are shown in Figure 7, demonstrate a significant rise in the adsorption percentages of SMX, CIP, and DC. The adsorption of DC increased from 38% to 93.2%, CIP from 39.6% to 92.3%, and SMX from 29.9% to 99.2%. Thus, it can be said that WSAC is a very good adsorbent for removing SMX, CIP, and DC from actual wastewater.

3.7. Desorption and Reusability

The desorption of SMX, CIP, and DC from WSAC was evaluated using 0.1 M NaOH, 0.1 M HCl, acetone, and ethanol. In a series of flasks, 50 mL of solutions containing 200 mg L−1 SMX and 400 mg L−1 DC and CP was mixed with 0.05 g of WSAC. After adsorption equilibrium was reached, the concentration of pharmaceuticals remaining in the solutions was measured, and their adsorbed amounts were calculated. For the desorption of SMX, DC, and CIP from the adsorbent, pharmaceutical-loaded WSAC was mixed with 50 mL of 0.1 M NaOH, 0.1 M HCl, acetone, and ethanol solvents and shaken for 24 h. Then, the concentration of pharmaceuticals in the solvents was measured to calculate the desorption percentages. The desorption–regeneration cycle was carried out three times using the same WSAC.
According to the findings in Figure 8A, the 0.1 M NaOH solution demonstrated good desorption performance, providing 39.0% for SMX, 50.0% for DC, and 77.6% for CIP from WSAC. Ethanol also showed a maximum desorption for DC (49.3%) and SMX (41.4%), but desorption of CIP was low (11.6%). Acetone and 0.1 M HCl provided a low desorption of all the pharmaceuticals.
Reusing WSAC after desorption was also studied and the findings were illustrated in Figure 8B. Adsorption of SMX, DC, and CIP were gradually decreased step by step. The maximum adsorption of the micropollutants was observed in the first step by using fresh WSAC. Due to not completing their desorption, the adsorption of SMX, DC, and CIP in the second step decreased, and this decrease continued in the third step. Similar trends about incomplete desorption of micropollutants and decreasing adsorption capacities in reusing the adsorbents were reported in the literature [21,100,101]. Although WSAC has low adsorption for SMX, DC, and CIP, it could also adsorb after regeneration, which can be considered as an advantage of WSAC.

4. Conclusions

This study demonstrates the effectiveness of activated carbon derived from walnut shells, activated with phosphoric acid, in the removal of SMX, CIP, and DC from both solutions and wastewater. The removal capacities (mg g−1) and efficiencies (%) of SMX, CIP, and DC adsorption onto WSAC were optimized with respect to a number of parameters, including adsorbent dosage, pH, contact time, temperature, and initial pharmaceutical concentration. The optimal contact time for SMX, CIP, and DC was identified as 1440 min. The amounts adsorbed per gram of WSAC increased in proportion to an increase in the initial concentrations of SMX, CIP, and DC. The adsorption equilibriums for SMX, CIP, and DC were found to be consistent with the Langmuir isotherm. It was shown that the maximum adsorption capacities of SMX, CIP, and DC with WSAC were determined to be 476.2, 185.2, and 135.1 mg g−1, respectively. The pseudo-second-order kinetic model was consistent with the kinetic behavior of CIP, DC, and SMX adsorption onto WSAC. Positive ΔH values verified that the process is endothermic. The developed adsorption method was tested for the removal of SMX, CIP, and DC from real wastewater, achieving removal efficiencies of 60.2%, 77.4%, and 74.2% for SMX, CIP, and DC, respectively. The 0.1 M NaOH solution demonstrated good desorption performance. These findings demonstrate that WSAC works well as an adsorbent to remove DC, CIP, and SMX from wastewater.
In conclusion, a full understanding of the fate of antibiotics and their potential environmental impact requires consideration of their presence in wastewater. Therefore, WSAC can be considered as an effective, affordable, widely available, and environmentally benign adsorbent for the removal of SMX, CIP, and DC from wastewater.

Supplementary Materials

The following Supporting Information can be downloaded at: https://www.mdpi.com/article/10.3390/pr12122766/s1, Figure S1: pHpzc measurement for WSAC. Figure S2: Pseudo-first-order and pseudo-second-order kinetic models for the adsorption of DC (a.d), CIP (b.e) and SMX (c.f) onto WSAC, respectively. Figure S3: Intraparticle diffusion model for the adsorption of DC, CIP, and SMX onto WSAC. Figure S4: The plot of lnKd and 1/T for thermodynamic parameters determination.

Author Contributions

All authors contributed to the study conception and design. R.K. founded resources. M.I. provided laboratory facilities. WSAC preparation was carried out by S.T. and M.I. Adsorption studies, characterization analyses, and data collection were performed by S.T. Calculations and graphs were made by S.T. and R.K. The first draft of the manuscript was written by S.T.; R.K. and M.I. edited the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Sakarya University Scientific Research Projects Unit, Project Number: 2019-7-25-260.

Data Availability Statement

The detailed datasets used during the current study are available from the corresponding author upon reasonable request. The manuscript has Supplementary Material that includes some of the data.

Acknowledgments

The authors express their gratitude to Abdil Özdemir (Sakarya University, Department of Chemistry) for his help with the HPLC measurements and to Deva Holding (Istanbul, Türkiye) for providing ciprofloxacin. We also thank the Scientific Research Coordination Office of Sakarya University.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. SEM images of WSAC at various magnification: (a) 1000 and (b) 5000.
Figure 1. SEM images of WSAC at various magnification: (a) 1000 and (b) 5000.
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Figure 2. N2 adsorption/desorption isotherms of WSAC.
Figure 2. N2 adsorption/desorption isotherms of WSAC.
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Figure 3. Pore size distribution of WSAC.
Figure 3. Pore size distribution of WSAC.
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Figure 4. FT-IR spectrum of WSAC.
Figure 4. FT-IR spectrum of WSAC.
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Figure 5. The effect of contact time on the adsorption of SMX, CIP, and DC (A); the effect of WSAC dosage on the adsorption of CIP, DC (B1) and SMX (B2); the effect of initial concentration on the adsorption of CIP, DC (C1) and SMX (C2); the effect of initial pH on the adsorption of SMX, CIP, and DC (D); the effect of temperature on the adsorption of SMX, CIP, and DC (E).
Figure 5. The effect of contact time on the adsorption of SMX, CIP, and DC (A); the effect of WSAC dosage on the adsorption of CIP, DC (B1) and SMX (B2); the effect of initial concentration on the adsorption of CIP, DC (C1) and SMX (C2); the effect of initial pH on the adsorption of SMX, CIP, and DC (D); the effect of temperature on the adsorption of SMX, CIP, and DC (E).
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Figure 6. Freundlich, Langmuir, Dubinin–Radushkevich and Temkin models for SMX, CIP, and DC adsorption onto WSAC.
Figure 6. Freundlich, Langmuir, Dubinin–Radushkevich and Temkin models for SMX, CIP, and DC adsorption onto WSAC.
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Figure 7. Adsorption of SMX, CIP, and DC from solutions (A) and wastewater (B) by WSAC.
Figure 7. Adsorption of SMX, CIP, and DC from solutions (A) and wastewater (B) by WSAC.
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Figure 8. Desorption (A) and recyclability (B) of WSAC for SMX, DC, and CIP.
Figure 8. Desorption (A) and recyclability (B) of WSAC for SMX, DC, and CIP.
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Table 1. WSAC characterization results (* Calculated by difference).
Table 1. WSAC characterization results (* Calculated by difference).
FactorsConditionsValue
Elemental analysis (%) WSAC
 C 71.6
 H 1.6
 N 1.1
 O *
 C/H		 25.8
3.7
Surface functional groups (mmol g−1)
 Lactonic 0.22
 Phenolic 0.66
 Carboxylic 0.34
 Total acidic groups 1.21
Proximate analysis (%)
 Moisture150 °C. 3 h10.4
 Ash650 ± 25 °C. 6 h11.9
 Volatile matter950 ± 25 °C. 7 min.17.8
 Fixed carbon * 70.3
 Water solubility 1.42
pHpzc 4.26
Iodine number (mg g−1) 602.7
BET-specific surface area (m2 g−1) 1428
Table 2. Isotherm constants for SMX, CIP, and DC adsorption onto WSAC.
Table 2. Isotherm constants for SMX, CIP, and DC adsorption onto WSAC.
Isotherm ModelParameterAdsorbate
DCCIPSMX
Freundlich KF (mg g−1) (mg L−1)1/n19.0162.6336.27
n3. 014.691.85
r20.990.740.98
Langmuir qmax (mg g−1)135.1185.2476.2
KL (L mg−1)0.030.220.04
r20.990.990.99
Dubinin–Radushkevich qm (mg g−1)89.07168.12277.82
β (m mol2 J−2)21 × 0−61 × 10−71 × 10−6
r20.780.730.80
Temkin b20.7519.7992.6
KT1.0749.781.57
r20.960.900.95
Table 3. Comparing the adsorption capacity of WSAC with that of other adsorbents reported in the literature with respect to SMX, CIP, and DC.
Table 3. Comparing the adsorption capacity of WSAC with that of other adsorbents reported in the literature with respect to SMX, CIP, and DC.
Adsorbentqmax (mg g−1)Reference
DCCIPSMX
Rice straw AC 3.7[51]
Cocoa pod husks AC5.5 [85]
Pine tree AC 131[86]
Bamboo AC 36.0 [87]
Commercial AC487 [88]
Anthriscus sylvestris AC392.9 [89]
Sugarcane bagasse AC 9.5 [90]
Cynodon dactylon AC 211.7 [91]
Activated sludge biomass 50.6[92]
Hazelnut shell AC (HSAC)12595.2285.7[45]
Walnut shell AC (WSAC)135.1185.2476.2This study
Table 4. Kinetic model constants for SMX, CIP, and DC adsorption onto WSAC.
Table 4. Kinetic model constants for SMX, CIP, and DC adsorption onto WSAC.
Kinetic Model ParameterAdsorbate
DCCIPSMX
qe exp (mg g−1)122.8197.8169.6
Pseudo-first-order
k1 (dk−1)2.1 × 10−32.1 × 10−33.2 × 10−3
qe,cal (mg g−1)99.0133.966.7
r20.990.980.93
Pseudo-second-order
k2 (g mg−1 dk−1)6.5 × 10−56.5 × 10−52.4 × 10−4
qe, cal (mg g−1) 131.6204.1172.4
r20.990.990.99
Intraparticle diffusion
kid,1 (mg g−1 dk−1/2)3.937.6311.87
r20.950.990.90
C1 (mg g−1)12.0927.9224.39
kid,2 (mg g−1 dk−1/2)1.841.990.66
r20.980.990.96
C2 (mg g−1)52.59121.03146.38
Table 5. Thermodynamic parameters for the adsorption of SMX, CIP, and DC onto WSAC.
Table 5. Thermodynamic parameters for the adsorption of SMX, CIP, and DC onto WSAC.
Adsorbate t (°C)T (K)KdΔG (kj mol−1)ΔH (kj mol−1)ΔS (j mol−1 K−1)
25298.158.43−5.29
SMX35308.158.80−5.573.429.3
45318.159.20−5.87
25298.153.48−3.09
CIP35308.155.60−4.4158.7206.4
45318.1515.53−7.26
25298.151.54−1.07
DC35308.151.66−1.307.629.0
45318.151.87−1.65
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Tunay, S.; Koklu, R.; Imamoglu, M. Highly Efficient and Environmentally Friendly Walnut Shell Carbon for the Removal of Ciprofloxacin, Diclofenac, and Sulfamethoxazole from Aqueous Solutions and Real Wastewater. Processes 2024, 12, 2766. https://doi.org/10.3390/pr12122766

AMA Style

Tunay S, Koklu R, Imamoglu M. Highly Efficient and Environmentally Friendly Walnut Shell Carbon for the Removal of Ciprofloxacin, Diclofenac, and Sulfamethoxazole from Aqueous Solutions and Real Wastewater. Processes. 2024; 12(12):2766. https://doi.org/10.3390/pr12122766

Chicago/Turabian Style

Tunay, Seda, Rabia Koklu, and Mustafa Imamoglu. 2024. "Highly Efficient and Environmentally Friendly Walnut Shell Carbon for the Removal of Ciprofloxacin, Diclofenac, and Sulfamethoxazole from Aqueous Solutions and Real Wastewater" Processes 12, no. 12: 2766. https://doi.org/10.3390/pr12122766

APA Style

Tunay, S., Koklu, R., & Imamoglu, M. (2024). Highly Efficient and Environmentally Friendly Walnut Shell Carbon for the Removal of Ciprofloxacin, Diclofenac, and Sulfamethoxazole from Aqueous Solutions and Real Wastewater. Processes, 12(12), 2766. https://doi.org/10.3390/pr12122766

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