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INTRODUCTION
The increased industrial, agricultural, and urban activities have generated an increase of polluting waste (Guerrero-Pérez et al. 2012). Antibiotics, such as enrofloxacin (ENRO) and oxytetracycline (OTC) used in veterinary industry are not eliminated or degraded during wastewater treatment processes and remain in water bodies (Babić et al. 2006), generating potential risk to human health and environmental damage (Martín et al. 2023).
ENRO and OTC are widely used in agriculture, aquaculture, and livestock to treat many infections. These drugs are used as food additives or in water to maintain the optimal health of food-producing animals. Its use will depend on the animal treated, production volume, and pathogens to be eliminated. Some are excreted in their active form via urine or feces after administration. Due to its recent detection in low concentrations (µg/L, ng/L), knowledge about its occurrence is still generated, fate, behavior, risk assessment, and ecological and human effects (Vargas-Berrones et al. 2020). Both antibiotics have been related to the development of resistance in human and animal pathogens, contributing to increased morbidity (Alanazi et al. 2021). The presence of OTC residues can produce damage to the liver and toxic effects (Yang et al. 2013), as well as allergic reactions, bioaccumulation, and intestinal flora imbalance in the case of ENRO (Wu et al. 2023). For the above reasons, the development of strategies to eliminate or reduce the impacts of these substances become a relevant issue. There are various physicochemical methods and technologies in water treatment that include advance oxidation, photocatalytic degradation, membrane filtration, electrochemical oxidation, and biocomposites adsorption for removal drugs in water (Rahman et al. 2022).
Due to their low cost and local availability, natural materials such as chitosan, zeolites, clay, oxides, and carbon are classified as good options for synthesis adsorbents (Babel et al. 2003). However, like any other technology, biocomposite removal has disadvantages such as a decrease in efficiency due to operating without pressure, pH variations or need high operating temperatures (Esfahani et al. 2019).
Attention has been paid to the development of biocomposites formed by a combination of materials, in which one of them acting as a matrix, merging the best intrinsic characteristics of both materials (mechanical, thermal, electrical, and magnetic properties) to improve pollutants removal from water (Nisticò et al. 2020).
The precursors used for the synthesis of activated carbon (AC) must have a high carbon content. Agricultural and industrial wastes are suitable materials for producing AC because they contain high carbon contents and are generated in large quantities at low cost. Several studies have reported that it is possible to produce high-quality AC from lignocellulosic waste (Lewoyehu 2021). There are many residual lignocellulosic materials from the agroindustry that could be used as precursors to produce AC (González-García. 2018). Mixtures of agriculture wastes have also been used as feedstock to synthetize AC used in pesticide removal (Hussain et al. 2023). Some of them are Eucalyptus biomass (Shi et al. 2023), fruit shell (Anita et al. 2023), peanut hulls, and corn hulls (Neme et al. 2022). One biomass option for AC synthesis is bamboo biomass, which is rich in lignocellulose content, making it a good raw material (Suhaimi et al. 2022). Research has been done on methylene blue removal using AC synthetized from Acacia erioloba (Daniel et al. 2023).
Other studies have shown that tarap residues (Artocarpus odoratissimus) have been successfully used as adsorbents for the removal of heavy metals and dyes (Mohamad Zaidi et al. 2021). Studies have also been made on the synthesis of AC from sheep manure for the adsorption of drugs in water (Dilekoglu et al. 2023).
Agave bagasse is one of the main wastes generated by the tequila and mezcal industries. Total production was 788.2 thousand tons in 2014 to 941.8 tons in 2016. Due to its low cost, availability, and avoidance of contamination problems, it can be used for the synthesis of AC (Robles et al. 2018).
AC is a microcrystalline and non-graphitic material, prepared by carbonizing of organic materials, especially of lignocellulosic origin, which are subjected to a physical (thermal treatment) or chemical (ZnCl2, H3PO4, KOH, or NaOH) activation process. Activation process increase its porosity, internal surface, active sites, and diversify its superficial chemical functional groups, resulting in a material with a higher capacity to adsorb different molecules (Dizbay-Onat et al. 2017), such as heavy metals, phenolics, drugs, and organic matter (Teow 2019).
The transformation of a material on a nanometric scale of carbon crystals increases its thermal, mechanical, and chemical properties, improving its potential for application in medicine, electronics, chemistry, and engineering. This is why the synthesis analysis and testing of these materials are necessary (Kulkarni 2015).
At present, biopolymers are valued for their adsorptive capacity and as a support material. Chitosan is a biodegradable polysaccharide with a high adsorption capacity due to its amine and hydroxyl surface functional groups. However, its low mechanical stability is a significant disadvantage. One solution to these drawbacks is the preparation of chitosan biocomposites, combining it with an adsorbent with mechanical and chemical resistance (Sharififard et al. 2018).
The union of both materials increases the contact surface and modifies the surface charges, which increases the adsorption capacity and promotes linkage mechanisms (Tee et al. 2023).
The current study aimed to physicochemically characterize the AC, chitosan, and biocomposite formed by both materials to evaluate the removal potential of ENRO and OXY solved in water.
MATERIALS AND METHODS
Preparation of AC
Samples of agave bagasse resulting from the mezcal manufacturing industry were donated by producers of Durango, Mexico (23º51′N 104º15′W). Samples were collected during the mezcal production cycle in August 2019. The agave bagasse was washed with distilled water to avoid impurities and dried at 60 ºC to a constant weight. Different temperatures were used for the physical activation of the agave bagasse (400-800 ºC), according to Pallarés et al. (2018). Whereas, for the chemical activation, the range of impregnation ratio (IR) of H3PO4 was evaluated considering the acid reduction criteria reported by Üner and Bayrak (2018). The AC preparation was carried out using previously reported procedures with some modifications. Then, the samples were activated with H3PO4 in a muffle furnace with an IR between 0.3-1.25 (w/w), carbonation times 30-120 min. Finally, the AC were cooled to room temperature, washed with 1 M NaOH until pH reached 6-7. The AC was dried in an oven at 80 ºC for 12 h and stored in plastic containers (Neme et al. 2022).
Nanometric grinding
AC was conditioned by hand grinding (50 g for one hour). After this, a sample was taken, and the average particle size was determined. A sample of AC was analyzed with dynamic light scattering (DLS) equipment (Horiba LA-910, Japan). The result of the particle sizes was statistically expressed as a mean value derived from the intensity distribution of the scattered light called the Z average. The average particle size achieved was 31.1256 µm. To reach a nanometric scale particle size of 500 nm, 2 g of the grounded material was combined with 1.5 L of distilled water; the dispersion was subjected to an ultrasound bath for 30 minutes to avoid agglomeration and submitted to a high-energy grinding process for four h in a Neos System LabStar equipment (Netzsch - Feinmahltechnik GmbH, Germany) at 3600 RPM. This procedure was repeated thrice to obtain a final sample of 5 g of AC.
Preparation of biocomposites
For the preparation of the biocomposites, mixtures of 25 mL of low molecular weight chitosan (Sigma-Aldrich, St. Louis, MO, USA) and 0, 0.5, 2, and 5% (w/v) of the selected AC were prepared. Chitosan and AC were mixed in acetic acid (4% v/v) (Betancourt-Galindo et al. 2014). Samples were prepared in a Branson brand fixed frequency ultrasonic bath (Branson M1800 Bransonic, CT, USA) to ensure the homogeneity of the mixture (López-Muñoz 2022). The solution was prepared to form a viscous gel and stirred for two h at 40 kHz at a temperature of 60 ºC and precipitated in 1 M NaOH. The beads were filtered from the NaOH bath and washed with tri-distilled water to neutral pH, dried at room temperature and measured until obtained an average diameter of 1 mm (Sharififard et al. 2012).
Biocomposite pH measurement was made using the potentiometric method. 5 g of sample was mixed in water, and the pH was measured using a pH meter Hannah-18424. Electrical conductivity was measured using the electrometric method with the help of a Con5 LaMotte conductivity meter (Tee et al. 2023).
Elemental characterization
The organic matter content (C2) of AC, chitosan, and biocomposites was determined by the AS-7 method Walkley and Black based on the Official Mexican Standard NOM-021-RECNAT-2000 (SEMARNAT 2002). The micro Kjeldhal method AOAC Official Method 960.52-1961 (AOAC 2010) was used to determine the total nitrogen (N2) content.
Scanning electron microscopy (SEM)
To obtain the images of the surface morphology of AC and biocomposites, samples were analyzed by SEM (FEI Nova NanoSEM 200, USA). Two-electron SEM signals and a backscattered electron detector were used. The instrument was operated on the parameters on mode Surface Specific (SS) 5.0 kV, pressure 0.60 mbar with magnifications of 500, 1000, 2000 and 2500 times for the AC samples, and 2000 for biocomposites, according to Benedetti et al. (2017).
Fourier transform infrared spectroscopy (FTIR)
FTIR was used to identify the chemical groups present on the surface of samples, which was revealed by identifiable bands within the infrared absorption (Fathizadeh et al. 2019). Samples of the solid and dry materials (AC and chitosan biocomposites) were placed on the diamond crystal of attenuated total reflectance (ATR) accessory of the FTIR/NIR equipment (Perkin Elmer FTIR/NIR Frontier, Mexico). The spectra were collected between the interval of wavenumbers of 4000-400 cm-1, with 256 scans, at a resolution of 4 cm-1 (Baig et al. 2019).
Preparation of antibiotic solutions and removal treatments
To evaluate the removal capacity of prepared biocomposites, antibiotic solutions in high concentration were prepared by dissolving the corresponding solid standards in 1 L of tri-distilled water and homogenized manually for 2 min (Esfahani et al. 2019), in the case of ENRO the pH was adjusted to 6 with HCl to increase its solubility (Babić et al. 2006) and stored in refrigerator at -4 ºC in the dark. Samples were mixed in tri-distilled water (10 mg/mL of Enrofloxacin and Oxytetracycline standards reagent grade Sigma-Aldrich, St, Louis, MO, USA) for removal treatments (Du et al. 2017, Esfahani et al. 2019).
Presence of ENRO and OTC in water
The concentration of ENRO and OTC in the model water samples was determined by a method (Babić et al. 2006) of high-performance liquid chromatography/diode array detector (HPLC/DAD) using HPLC/DAD Perkin Elmer (Flexar PDA Plus Detector, Shelton, CT, USA) and the Chromera software version 4.2.0.6415. The column (Spheri-5 RP-18 5 µM 250x4.5 mm, Perkin Elmer, USA) temperature was set to 30 ºC and the injection volume was 20 µL. The mobile phase consisted of a binary mixture of solvents (A: 0.01 M oxalic acid and B: acetonitrile).
The elution started with 100% of A which was maintained for 6 min, followed by 19 min linear gradient to 50% of B and 5 min linear gradient back to 100% of A. Flow rate gradient started with 1.0 mL/min maintained for 6 min, followed by 19 min linear gradient to 0.8 mL/min, and 5 min linear gradient back to 1.0 mL/min. A 30 min post time allowed re-equilibration of the column. Linearity limit of detection (LOD) for ENRO was (0.1 µg/L) and for OTC (10 µg/L), limit of quantification (LOQ) for ENRO was (1.5 µg/L) and for OTC (30 µg/L), recovery was evaluated for quantitative purposes. Calibration curves were prepared from standard solutions of ENRO and OTC, to calculate the recovery of these antibiotics from the respective solutions before and after removal treatments.
The linearity was evaluated for both ENRO and OTC in the mixture using four concentrations in the range 3.625-25 mg/mL for ENRO, and 0.625-5 mg/mL for OTC. The results were subjected to linear regression analysis to obtain the respective standard equations for Pearson correlation (r) for both drugs (r = 0.9991 for ENRO and r = 0.9992 for OTC) to estimate the ENRO and OTC concentration in the samples and confirm the linearity of the method.
The estimation of the antibiotic concentration in the samples of prepared water of each treatment was made by an external standard method, with the commercial references of ENRO and OTC, by area measurements, using standard curves. Removal treatments were made by preparing 1 L of water and ENRO (10 mg/mL) and 4 g of biocomposite, that was put in constant agitation at 100 rpm during different contact times (0, 2, 6 and 24 h) at pH 6 and temperature of 30 ºC. For samples with OTC, the same operating conditions were maintained, only at pH 7.
The removal percentage (% R) of both antibiotics was calculated using the equation 1 with the data of the initial concentration of the drug Ci (mg/mL) and the final concentration Cf (mg/mL), calculated after each treatment (Jiménez et al. 2017).
RESULTS AND DISCUSSION
Characterization of AC and chitosan
Different temperatures, times, and H3PO4 impregnation ratio (0.6 w/v) were assayed to obtain the AC. Treatments 1-5 conditions had an incomplete carbonization because the activation temperatures were not high enough. These results agree with Neolaka et al. (2023), who reported that temperatures above 400 ºC are required to carbonize some materials. For this reason, the products from treatments 1-5 were discarded for further analysis. ACs from bamboo (Suhaimi et al. 2020) and sugar cane bagasse (Chatterjee et al. 2020) obtained at 500 ºC drastically increased their adsorption performances, which were references to select the product of treatment 6 of the current study.
AC with high carbon content and lower ash level (not more than 7% w for the last) are considered suitable as adsorbent material because high ash contents are related to the blocking of the contact surface and porosity of ACs (Mariah et al. 2023). Due to treatments 7-10 showing the highest ash content (7.3-9.7% w/w), they were discarded for further analysis. Treatment 6 (CA6) had a total yield of 22.85% (w/w) of AC, similar to that reported by Neme et al. (2023) of 6-19% (w/w) obtained from castor seed hull. AC from treatment 6 (500 ºC, IR 0.6, 60 min) was selected to prepare the biocomposites, as it represented the highest carbonization (average yield of 22.85% w/w) and the lowest ash content (6.9% w/w).
The time required to prepare AC was the same than reported (60 min) by Abdel-Ghani et al. (2016) from olive tree residues. The impregnation ratio (IR) of treatment 6 was 0.6% H3PO4 (w/v) and the average yield obtained (28% w/w) were similar to those reported by Neme et al. (2022), who prepared AC from Castor seed hull (IR = 0.8 H3PO4 w/v, 18-24% average yield). Other studies showed a yield of 30.2% for AC obtained from Cassava stem at temperatures 450-900 ºC for 150 min under of its own combustion gas (Sulaiman et al. 2018). Yield values of 35% were found for AC synthetized from sunflowers under a N2 environment at 600 ºC for 150 min (Zhao et al. 2023), which represents more drastic conditions than those used in the present study. Synthesis of AC from Acacia reported by Gulfam Alam et al. (2023) required a higher temperature (900 ºC) than the required in the current study, but less time of pyrolysis (45 min); however, these authors reported no data for the IR and yield, so no comparison can be done.
The contents of C2 (46%) and N2 (0.37%) in the AC selected are shown in table I. The values are similar to those reported in the AC from the agave bagasse generated from the tequila production (45% and 0.3%, respectively), pine wood (48% and 0.01%, respectively), sugar cane bagasse (45.5% and 0.1%, respectively) by Nieto-Delgado (2011). However, the values were lower than those reported for the AC from Eucalyptus (48% and 0.5%, respectively) by Heidari et al. (2014) and also lower than those reported for AC from Durian shell (60% and 3%, respectively) reported by Chandra (2009). Many biomasses contain different elemental compositions depending on their nature and the pretreatments used.
TABLE I CARBON (C2) AND NITROGEN (N2) PRESENT IN ACTIVATED CARBON (AC) AND COMMERCIAL CHITOSAN.
| Sample | C2 (%) | N2 (%) |
| AC | 46.54 ± 0.20 a | 0.37 ± 0.02 a |
| Commercial chitosan | 37.43 ± 0.2 b | 7.22 ± 0.15 b |
Different letters in the same column mean significant differences according to the Tukey test (p < 0.05).
The content of C2 found here for chitosan agrees with the content reported for commercial shrimp chitosan (39.2%) by Padilla-Quintero et al. (2017). In contrast, the content of N2 was similar to 7% found for commercial chitosan by Rodríguez-Pedrozo et al. (2009) but less than (40% of C2 and similar at 7.2% N2) found by (Muñoz et al. 2023). The amount of C2 and N2 is related to the deacetylation degree and the number of amine groups available that determine the reactivity and adsorption power of chitosan (Muñoz et al. 2023). This group type can potentially adsorb anions by electrostatic attraction (Sharififard et al. 2012).
Scanning electron microscopy (SEM)
The SEM images of the AC particles at different magnification levels are shown in figure 1. The images of electrons reveal the presence of a porous matrix constituted by organic matter, observed by atomic contrast. Macro, meso, and micro porosities were observed, openings and hollow structures were also observed. If activation temperature is higher than 600 ºC, irregularity in shape and size of pore increases (Yorgun et al. 2015).
Fig. 1 Scanning electron microscope (SEM) micrographs of the activated carbon (AC) particles (Table I) at a: 500X, b:1000X, c: 2000X, d:2500X.
Microporosity is a sought attribute because it indicates efficacy during the synthesis and activation process; the high formation of microporosity in ACs is related to an increase in efficiency because the available contact surface increases (Sizirici et al. 2021). Another way to increase the surface is by decreasing the size of the AC particle (Saman et al. 2018). These changes occur during physical and chemical treatments during carbonization, as Thileep Kumar et al. (2018) reported.
The control of the H3PO4 impregnation ratio maintained a relationship with the amount and heterogeneous porosity size and agreed with the waste tea AC reported by Mariana et al. (2023). Chemical activators (such as H3PO4) contribute to thermal decomposition and increase the amount of porosity present in the carbonized material (Gómez-Delgado et al. 2022). The quantity and pore size are related to the impregnation ratio of the activator agent, which agrees with the reported by Lim et al. (2010) for palm shell AC.
Nanometric grinding
The average size (time 0) found for the AC, as revealed by a DLS analysis, was 31.12 µm. This size contrasted with the obtained after two h of grinding (time 1), which was 1.28 µm, as revealed by the respective DLS analysis. The average size obtained after four h (time 2) was 0.57 µm. Bassyouni et al. (2019) state that particles between 1 nm and 0.1 µm are considered nanoparticles. Thus, after four hours of grinding, AC acquired the condition of a nanoparticle.
AC is frequently used as a standard filler in the preparation of biocomposites due to its physical and chemical properties, which are considered to improve mechanical properties (Rahmi et al. 2018).
According to Kwiatkowski et al. (2022), molecules containing N2 atoms will not be retained by macro, meso, or microporosities due to kinetic size restrictions of the material. For these reasons, the AC was prepared to nanometric scale and added in the matrix of chitosan.
Biocomposites of chitosan and AC
The success of gelation is related to the degree of deacetylation and molecular weight of chitosan, having a more significant number of amino groups in acid solution due to a higher molecular weight, increasing the number of positive charges (Rodríguez et al. 2010). The increase could have produced a high degree of crosslinking with the AC, as observed in the SEM images of the different synthesized biocomposites, as shown in figure 2. There is a relationship between decreasing the quantity and size of pores in chitosan biocomposite particles after adding nanometric scale compounds, so it is essential to control the addition of said materials. An adequate balance allows for obtaining a biocomposite with greater removal efficiency (Al-Salman et al. 2023).
Physical and chemical characterization
Table II shows the pH values registered for the different biocomposites prepared with chitosan mixed with different amounts (0, 0.5, 2, and 5% w/w) of AC. The electrical conductivity values ranged from 0.331 to 0.387 S/m. These electrical conductivity values can be attributed to the 2% acetic acid content in the medium, which releases H+, and were higher than those reported by Padilla-Quintero et al. (2017) for a chitosan-glutaraldehyde gel (0.203-0.256 S/m). According to Esfahani et al. (2019) the formation of sponge structures within the biocomposite of chitosan and AC is due to the addition of acid during the gelation process, increasing the number of negative charges of the biocomposite surface, which increases the adsorption capacity of chemical species.
Fourier transform infrared spectroscopy (FTIR)
Figure 3 shows the infrared spectra of biocomposites formed with different concentrations of AC (0, 0.5, 2, and 5% w/w) and chitosan. It was possible to identify characteristic functional groups and bands for the AC and the biocomposites formed with the chitosan.
Fig. 3 Fourier transform infrared spectroscopy (FTIR) spectra of biocomposites formed of chitosan (CHITO) with several amounts of activated carbon (AC): 0% AC, 0.5% AC, 2% AC, 5% AC (w/w).
The signals in the spectral region 3700 cm-1 correspond to the stretching vibrations of the OH bonds (Morales-Olán et al. 2021). Another band around 1626 cm-1 responds to the stretching vibration C=O bond (Luo et al. 2010). The signal at 1562 cm-1 represents the bending vibration of the NH bond (Barbosa et al. 2019). The vibration in 1372 cm-1 arises from the stretching NH2 bond (Mauricio-Sánchez et al. 2018). The vibration in 1370 cm-1 is related to the stretching and symmetrical deformation in the CH3 bond (Branca et al. 2016). Fingerprint bands were between 1500-900 cm-1, which were helpful in evidencing chitosan presence. The little peak at the region of 1146 cm-1 is assigned to the stretching of CO (Corazzari et al. 2015). Another band at 1058 cm-1 corresponds to the stretching vibration of the P-O-P bond, reported by Neme et al. (2022). The vibration peak in the frequency 1028 cm-1 corresponds to the stretching C=C of the aromatic ring (Wibawa et al. 2020). The absorption band at 888 cm-1 is related to the stretching vibration of the C-O-C bond characteristic in the union between AC and chitosan, as described by (Rahmi et al. 2018). The peak in the region 656 cm-1 is attributed to the CH stretching (Cui et al. 2021). Finally, the absorption band at 584 cm-1 corresponds to the CH3 binding in activated carbons (Mariana et al. 2023).
This vibrational characterization indicated that the nanoparticles of AC were attached to the chitosan biocomposite, and a stable chemical crosslink was formed during the polymerization process. The functional groups of each material were preserved after the polymerization process according to what Ang et al. (2019) described, improving the ability to bind with other molecules according to what was described by Kumar et al. (2019).
Presence of ENRO and OTC in water
Separation and identification of both drugs were done using the method proposed in this work. The HPLC chromatograms of ENRO and OTC are shown in figures 4a and 4b, respectively. The retention times for the drugs were 25 min for ENRO, which agreed with reported by Babić et al. (2006), and 21 min for OTC, a lower value than 25.9 reported by Sanderson et al. (2005). The values found for the removal of drugs at times of 0, 2, 6, and 24 h are shown in table III and IV. The values found for the removal of drugs at different times (0, 2, 6, and 24 h) are shown in figures 5 and 6. A concentration of 10 mg/mL (10 000 ppm) was based on other contaminants removal biocomposites studies, where 100 000 ppm concentration was used (Esfahani et al. 2019).
Fig. 4 High-performance liquid chromatography-diode array detector (HPLC-DAD) chromatograms of a: enrofloxacin (ENRO), and b: oxytetracycline (OTC) standard solutions.
TABLE III REMOVAL PERCENTAGES (%R) OF ENROFLOXACIN (ENRO) FROM AN AQUEOUS SOLUTION (10 mg/mL) WITH THE DIFFERENT CHITOSAN (CHITO) AND ACTIVATED CARBON (AC) BIOCOMPOSITES AFTER DIFFERENT CONTACT TIMES (0, 2, 6, 24 h).
| Biocomposites containing different amount of AC (%, w/w) | %R (0 h) | %R (2 h) | %R (6 h) | %R (24 h) |
| CHITO / 0.0% AC | 0 | 25.66 ± 0.5 a | 27.60 ± 1.4 a | 28.33 ± 1.5 a |
| CHITO / 0.5% AC | 0 | 34.33 ± 1.5 a | 39.66 ± 1.5 b | 40.31 ± 2.0 b |
| CHITO / 2% AC | 0 | 40.33 ± 1.5 b | 48.66 ± 1.1 c | 50.30 ± 2.0 c |
| CHITO / 5% AC | 0 | 45.44 ± 1.1 b | 49.37 ± 2.0 c | 48.66 ± 0.5 c |
Different letters in the same column mean significant differences according to the Tukey test (p ˂ 0.05).
TABLE IV REMOVAL PERCENTAGES (%R) OF OXYTETRACYCLINE (OTC) FROM AQUEOUS SOLUTION (10 mg/mL) WITH THE DIFFERENT CHITOSAN (CHITO) AND ACTIVATED CARBON (AC) BIOCOMPOSITES AFTER DIFFERENT CONTACT TIMES (0, 2, 6, 24 h).
| Biocomposites containing different amount of AC (%, w/w) | %R (0 h) | %R (2 h) | %R (6 h) | %R (24 h) |
| CHITO / 0.0% AC | 0 | 40.33 ± 0.5 a | 41.19 ± 1.1 a | 44.54 ± 1.2 a |
| CHITO / 0.5% AC | 0 | 50.11 ± 2.6 b | 56.49 ± 1.0 b | 59.38 ± 1.0 b |
| CHITO / 2% AC | 0 | 56.66 ± 1.5 c | 53.22 ± 1.0 b | 54.62 ± 1.5 a |
| CHITO / 5% AC | 0 | 57.05 ± 1.0 c | 57.66 ± 1.1 c | 58.33 ± 1.5 c |
Different letters in the same column mean significant differences according to the Tukey test (p ˂ 0.05).
Fig. 5 High-performance liquid chromatography-diode array detector (HPLC-DAD) chromatograms of removal treatments of enrofloxacin (ENRO) in aqueous solution (10 mg/mL) with the different chitosan (CHITO) and activated carbon (AC) biocomposites. a: CHITO / 0.0% AC, b: CHITO / 0.5% AC, c: CHITO / 2% AC, and d: CHITO / 5% AC (w/w) at different contact times (0, 2, 6, and 24 h).
Fig. 6 High-performance liquid chromatography-diode array detector (HPLC-DAD) chromatograms of removal treatments of oxytetracycline (OTC) in aqueous solution (10 mg/mL) with the different chitosan (CHITO) and activated carbon (AC) biocomposites. a: CHITO / 0.0% AC (w/w), b: CHTO / 0.5% AC, c: CHITO / 2% AC, and d: CHITO / 5% AC (w/w) at different contact times (0, 2, 6, and 24 h).
The removal capacity was dependent on both AC biocomposite concentration and time. Our results, which are consistent with those of Wu et al. (2021), underscore the importance of time increments in improving removal capacity. However, it’s crucial to note that after more than an hour, the adsorption ability can decrease due to the reduction of functional groups on the surface of the biocomposite. Our study, in comparison to existing research on the treatment and removal of ENRO and OTC, demonstrates the adsorption capacities of different biocomposites under various operating conditions. For instance, grafted chitosan hydrogel in water with an ENRO concentration of 600 mg/L showed a removal capacity of 388 mg/g (Karmi-Maleh et al. 2021), which is in line with our study’s findings.
Studies based on ZSM nanosized zeolite-carbon composite showed an adsorption capacity of 88 mg/g of adsorbate, a low performance due to the low affinity of the adsorbent material with the drug (Al-Jubouri et al. 2022). To remove OTC, reduced graphene oxide nanocomposites have been used at a concentration of 600 mg/L of OTC solved in water with a removal capacity of 177 mg/g of adsorbent. It was shown that the adsorption of biocomposites of chitosan and AC performed better (Hao et al. 2021). Nanocomposite formed by Zeolite/Fe3O4 showed an adsorption capacity of 83 mg/g adsorbate in a solution of OTC in water, less than other treatments (Baskan et al. 2022). Studies carried out with BiO2/BiOCl composites supported on graphene-chitosan and photocatalysis showed a 63% OTC removal capacity (Priya et al. 2016).
The results for removal capacity for different drugs by a composite with nanometric scale material is 500-900 mg/g of adsorbent material in low-concentration solutions (12 mg/L). This capacity is more significant than each component separately (Nayak et al. 2023).
There are other biocomposites or nanocomposites that have been manufactured with different materials and whose removal capacity is function of materials which biocomposite was synthetized and affinity with contaminant to be removed. Some of these materials and their removal capacity are magnetic CS nanoparticles (27.5 mg/g of adsorbent), glutaraldehyde crosslinked CS (180 mg/g) for removal heavy metals (Chang et al. 2006), MWCNT-PDA-CS-GO (150 mg/g) of Li removal (Zhang et al. 2021), and magnetic crosslinked CS (171 mg/g) for removal Pt (IV) from water (Zhou et al. 2010).
The performance of biocomposites also depends on drug concentration and its affinity with the adsorbent (Li et al. 2021). A greater removal capacity of AC is due to a higher electron density in the planes of aromatic rings, establishing H2 bridges between OH- groups of the oxytetracycline and the oxygenated groups of carbon. Saturation of porosities and active sites of biocomposite particles can produce blockages, decreasing efficiency by reducing the available surface area. To avoid this problem, washing, recovery, and reuse treatments are necessary (Rivera et al. 2013).
Some reports suggest that waste from antibiotic-treated animals can be composted, resulting in drug degradation in approximately 30 days (Arikan et al. 2009). In this study, we propose a scheme that could be tested once the antibiotics are captured and concentrated in the biocomposite. The use of these biocomposites represent a good option due to low cost to obtain a raw material. However, it’s crucial to emphasize the need for further systematic studies on the effect of AC-chitosan biocomposites to optimize performance and efficiency of drug removal, implementing variations in parameters such as pH, temperature and pressure that could increase the percentage of drug removal, taking into account the price-performance ratio.
AC particle type with specific physical and chemical properties (surface charge, composition, contact surface) and amount of chitosan can improve the performance of biocomposites in other water treatment processes, such as desalination, removal of drugs, heavy metals, and organic and inorganic contaminants.
CONCLUSIONS
The synthesis conditions (500 ºC, IR 0.6, 60 min) allowed to obtain, after grinding, 571 nm AC particles.
The addition of AC to the chitosan formed a biocomposite with high porosity. The characteristic functional groups observed by FTIR spectroscopy on AC and the chitosan biocomposites suggest that the nanoparticles of AC were attached to the chitosan biocomposite, and a stable chemical crosslinking was formed during the polymerization process.
The results showed that the addition of AC to chitosan formed a biocomposite with the capacity to remove 59.3% of OTC (contact time two h at 100 rpm, 4 g biocomposite/L solution, pH 6-7, 30 ºC) and, in a lower percentage, 50.3% of ENRO (contact time two h at 100 rpm, 4 g biocomposite/L solution, pH 6, 30 ºC).
The AC and chitosan biocomposite synthesis method is simple, fast, and degradable. The continuum development of techniques for synthesizing biocomposites will provide new knowledge on the objects under investigation.
