The adsorptive removal of antibiotics from aqueous solutions is recognized as the most suitable approach due to its easy operation, low cost, nontoxic properties, and high efficiency. However, the conventional regeneration of saturated adsorbents is an expensive and time-consuming process in practical wastewater treatment. Herein, a scalable adsorbent of magnetic Fe3O4@chitosan carbon microbeads (MCM) was successfully prepared by embedding Fe3O4 nanoparticles into chitosan hydrogel via an alkali gelation-thermal cracking process. The application of MCM composites for the adsorptive removal of doxycycline (DC) was evaluated using a fixed-bed column. The results showed that pH, initial concentration, flow rate, and bed depth are found to be important factors to control the adsorption capacity of DC. The Thomas and Yoon-Nelson models showed a good agreement with the experimental data and could be applied for the prediction of the fixed-bed column properties and breakthrough curves. More importantly, the saturated fixed bed can be easily recycled by H2O2 which shows excellent reusability for the removal of doxycycline. Thus, the combination of the adsorption advantage of chitosan carbon with catalytic properties of magnetic Fe3O4 nanoparticles might provide a new tool for addressing water treatment challenges.
In the past years, doxycycline (DC) has gradually become one of the most widely used antibiotics in the world especially in human therapy and livestock industry because of its specific antimicrobial properties and minor adverse side effects [
The Fenton oxidation is the most powerful method to decompose organic pollutants because the ⋅OH generated in the Fenton system (⋅OH, oxidation potential: 2.8 V) is highly oxidative and nonselective, which can destruct many hazardous organic pollutants easily and effectively like azo dye [
Herein, we synthesized magnetic Fe3O4@chitosan carbon microbeads (MCM) by a simple thermal cracking process under a nitrogen atmosphere at 350°C, aimed at combining the catalytic property of Fe3O4 with the adsorption capacity of chitosan carbon microbeads. Based on the XRD, SEM, and FT-IR characterization results, a possible mechanism for the formation of MCM was proposed. A fixed-bed column was employed to investigate the removal efficiency of doxycycline. The effects of pH, initial concentration, flow rate, and bed depth were also analyzed. Afterward, the regeneration/recycling tests were carried out by triggering the Fenton oxidation in the presence of H2O2 solution.
Chitosan, Fe3O4 nanoparticles, glutaraldehyde (25% (
0.3 gram of chitosan was dissolved in 10 mL of 1% aqueous acetic acid solution to prepare the chitosan solution. Then, 0.9 gram of Fe3O4 nanoparticles was slowly added into the prepared chitosan solution. The homogenous solution was injected into a gently stirred sodium hydroxide solution using a syringe to form magnetic microspheres. After magnetic stirring for 4 h, the products were harvested by filtration, followed by washing with a copious amount of distilled water until the pH value was 7. Then, these magnetic microbeads were immersed in a cross-linking agent, 5% glutaraldehyde, for 8 h at room temperature. The cross-linked microbeads were filtered, washed, and dried at 60°C for 12 h. Finally, the resulting products were pyrolyzed in a tubular reactor at 350°C for 40 min under a nitrogen atmosphere. The synthesis of chitosan carbon microbeads (CSM) was also carried out in accordance with the above steps but without the adding of Fe3O4.
The size and surface morphology of the samples were determined by a Philips XL 30 field emission scanning electron microscope (FE-SEM). The crystallographic structure of the samples was measured by X-ray diffraction (XRD) on a Rigaku D/MAX-3C diffractometer operated at a voltage of 40 kV and a current of 20 mA, at a 0.028 scan rate with Cu K
The fixed-bed adsorption experiments were performed in up-flow columns with an internal diameter of 0.6 cm and a length of 15 cm. The influences of different initial concentrations of DC (20, 25, and 30 mg/L), flow rates (1.1, 2.1, and 3.1 mL/min), pH (2, 4, 7, 9, and 11), and bed depths (0.8, 1.2, and 1.6 cm) were studied. The pH of the DC solutions was adjusted by the addition of NaOH (0.1 mol/L) and HCl (0.1 mol/L). In order to exclude the trapped air and wet the porosity of activated carbon, water was pumped into the column at the flow rate of 8 mL/min for 10 min before starting the experiment. The column effluent samples were taken out at regular time intervals, and the concentration was analyzed by a Jenway 6405 UV-vis spectrophotometer at 351 nm.
The dynamic adsorption behavior of fixed-bed columns was investigated in terms of analyzing the shape of breakthrough curves. And the experimental breakthrough curves determined as the ratio of
The volume of the effluent,
The area above the breakthrough curve means the total mass of DC adsorbed, and
Equilibrium DC uptake per unit mass of adsorbent,
The total amount of DC passed through the column
After each adsorption cycle, the sorbent bed was washed by distilled water in the upward direction at a suitable flow rate (2.1 mL/min) to remove the residual DC. Then, the regeneration experiment was carried out by injecting various aqueous solutions through the column bed in an upward direction at a flow rate of 1.1 mL/min for 1.5 h. After the heterogeneous Fenton-like reaction, the column was rinsed again to remove the residual H2O2. Finally, the bed was reused for the next adsorption/regeneration cycle, up to three consecutive cycles.
The formation of magnetic Fe3O4@chitosan carbon microbeads by alkali gelation-thermal cracking route was proposed in Scheme
Schematic of the adsorption and in situ regeneration mechanism of MCM in the removal of DC.
According to previous literatures, chitosan is insoluble in water but soluble in diluted acidic solution below its pKa (~6.3), in which the amine groups (–NH2) of chitosan can be facilely converted into the soluble protonated form (–NH3+) [
From the above analysis, chitosan polymers play a significant trifunctional role in the formation of the Fe3O4@chitosan composite. Firstly, the native entanglement or cross-linking property of chitosan chains helps Fe3O4 nanoparticles reunite together tightly and become a stable three-dimensional network due to the strong hydrogen bonding or Van der Waals forces. Secondly, the water absorption capacity of the three-dimensional network is partly retained, which is beneficial to the formation of pores or channels in the MCM during the thermal cracking process. Thirdly, chitosan polymers can act as a carbon source, providing lots of active sites for the adsorption of pollutants. Similarly, the Fe3O4 nanoparticles play two vital roles in the magnetic Fe3O4@chitosan carbon microbeads. On the one hand, the mechanical stability of MCM can be strengthened by embedding hard Fe3O4 nanoparticles into chitosan. On the other hand, part of chitosan is replaced by Fe3O4 nanoparticles, offering additional catalytic active sites on the MCM substance. From this point of view, the prepared MCMs are demonstrated to not only preserve the adsorption performance of chitosan carbon but also possess an in situ regeneration ability of Fe3O4 nanoparticles.
The incorporation of Fe3O4 nanoparticles into the CSM matrix can be observed straightforwardly from the microscopy photos. The optical microscopy photos of MCM and CSM are presented in Figures
Optical microscopy photos (40×) of MCM (a) and CSM (b); FE-SEM images: (c, d) Fe3O4 nanoparticles, (e) dissected CSM, (f) CSM under high magnification, (g) dissected MCM, and (h) MCM under high magnification.
The incorporation of Fe3O4 nanoparticles into the CSM matrix can be further verified from the SEM images. SEM images of Fe3O4 nanoparticles, CSM, and MCM are shown in Figures
To explain chemical binding between Fe3O4 and CSM, the Fourier transform infrared (FTIR) spectra of the pure Fe3O4, CSM, and MCM were recorded. In Figure
FT-IR spectra of Fe3O4 nanoparticles, CSM, and MCM.
Figure
XRD spectra of Fe3O4 nanoparticles, CSM, and MCM.
The nitrogen adsorption isotherm of MCM is shown in Figure
Nitrogen adsorption isotherm of the MCM.
The fixed-bed adsorption requires very few devices and low operational cost. The fixed-bed columns are essential towards the industrial scale design and scale-up of the continuous system for required applications and performances [
Parameters of Thomas and Yoon-Nelson models.
pH | Thomas model | Yoon-Nelson model | |||||||
---|---|---|---|---|---|---|---|---|---|
25 | 1.2 | 1.1 | 11 | 2.725 | 5.974 | 0.994 | 0.068 | 55.623 | 0.994 |
25 | 1.2 | 1.1 | 9 | 2.823 | 4.568 | 0.982 | 0.071 | 42.529 | 0.982 |
25 | 1.2 | 1.1 | 7 | 3.749 | 3.432 | 0.984 | 0.094 | 32.289 | 0.984 |
25 | 1.2 | 1.1 | 4 | 4.518 | 2.239 | 0.977 | 0.113 | 20.845 | 0.977 |
25 | 1.2 | 1.1 | 2 | 7.487 | 0.910 | 0.962 | 0.187 | 8.4716 | 0.962 |
20 | 1.2 | 1.1 | 7 | 4.107 | 4.168 | 0.983 | 0.082 | 48.139 | 0.983 |
25 | 1.2 | 1.1 | 7 | 3.749 | 3.432 | 0.984 | 0.094 | 32.289 | 0.984 |
30 | 1.2 | 1.1 | 7 | 2.834 | 2.168 | 0.985 | 0.095 | 16.806 | 0.985 |
25 | 0.8 | 1.1 | 7 | 4.647 | 2.487 | 0.998 | 0.116 | 15.286 | 0.998 |
25 | 1.2 | 1.1 | 7 | 3.749 | 3.432 | 0.984 | 0.094 | 32.289 | 0.984 |
25 | 1.6 | 1.1 | 7 | 2.817 | 3.538 | 0.982 | 0.070 | 45.545 | 0.982 |
25 | 1.2 | 1.1 | 7 | 3.749 | 3.432 | 0.984 | 0.094 | 32.289 | 0.984 |
25 | 1.2 | 2.1 | 7 | 4.108 | 2.959 | 0.996 | 0.103 | 14.429 | 0.996 |
25 | 1.2 | 3.1 | 7 | 5.518 | 0.840 | 0.929 | 0.138 | 2.775 | 0.929 |
pH is one of the dominant factors that affect the removal efficiency of antibiotic, because pH can affect the speciation of a DC molecule in solution and the surface charge of the adsorbent. The pH affects the prevalent chemical mechanisms by controlling the electrostatic interaction between the adsorbent and organic molecules, consequently changing the removal rate of DC from the aqueous solution and the breakthrough curves [
Breakthrough curve for adsorption of DC at different column conditions: (a) pH (
Two vital factors are likely responsible for the remarkable effect of pH on DC adsorption. Firstly, pH can change the surface charge of ionizable organic compounds in chemical speciation [
As the DC concentration decreases, the adsorption capacity of MCM increases, reaching saturation in the fixed bed only when the DC concentration is 30 mg/L, while at other concentrations, especially at 20 mg/L, the saturated time has been prolonged to achieve adsorption equilibrium (Figure
Figure
The effect of flow rate on DC biosorption by MCM was studied by varying it from 1.1 to 3.1 mL/min, keeping the bed height and initial solution concentration constant at 1.2 cm and 25 mg/L, respectively. A decrease in adsorption capacity from 2.448 to 2.282 mg/g by increasing the flow rate from 1.1 to 3.1 mL/min is shown in Table
The design and scale-up of fixed-bed adsorption columns require the prediction of the concentration-time profile or breakthrough curve for the effluent [
Experimental parameters of adsorptive removal of DC in a fixed bed.
pH | EBCT (min) | ||||||||
---|---|---|---|---|---|---|---|---|---|
25 | 1.2 | 1.1 | 11 | 108 | 2.970 | 1.233 | 4.816 | 118.8 | 0.308 |
25 | 1.2 | 1.1 | 9 | 93 | 2.558 | 0.839 | 3.281 | 102.3 | 0.308 |
25 | 1.2 | 1.1 | 7 | 72 | 1.980 | 0.627 | 2.448 | 79.2 | 0.308 |
25 | 1.2 | 1.1 | 4 | 54 | 1.485 | 0.426 | 1.666 | 59.4 | 0.308 |
25 | 1.2 | 1.1 | 2 | 30 | 0.825 | 0.239 | 0.934 | 33.0 | 0.308 |
20 | 1.2 | 1.1 | 7 | 96 | 2.112 | 0.801 | 3.128 | 105.6 | 0.308 |
25 | 1.2 | 1.1 | 7 | 72 | 1.980 | 0.627 | 2.448 | 79.2 | 0.308 |
30 | 1.2 | 1.1 | 7 | 63 | 2.079 | 0.523 | 2.045 | 69.3 | 0.308 |
25 | 0.8 | 1.1 | 7 | 48 | 1.320 | 0.396 | 2.333 | 52.8 | 0.206 |
25 | 1.2 | 1.1 | 7 | 72 | 1.980 | 0.627 | 2.448 | 79.2 | 0.308 |
25 | 1.6 | 1.1 | 7 | 96 | 2.640 | 0.989 | 2.787 | 105.6 | 0.411 |
25 | 1.2 | 1.1 | 7 | 72 | 1.980 | 0.627 | 2.448 | 79.2 | 0.308 |
25 | 1.2 | 2.1 | 7 | 51 | 2.678 | 0.598 | 2.334 | 107.1 | 0.162 |
25 | 1.2 | 3.1 | 7 | 30 | 2.325 | 0.585 | 2.282 | 93.0 | 0.109 |
The Thomas model is one of the most popular and widely used to represent the behavior of the adsorption process. This model assumes that the main reason limiting the adsorption in a fixed-bed column is the mass transfer at the interface rather than the chemical reaction [
The adsorption capacity of the column
The linearized Yoon-Nelson model is presented by the following equation [
Table
From an economic standpoint, the reusability of the adsorbents is one of the most important features to exemplify the potential of MCM for real applications. Moreover, both the magnetic and oxidation properties imparted by the mixing Fe3O4 nanoparticles are important for practical application. In this study, the regeneration experiments were carried out by using H2O2 solution to trigger the heterogeneous Fenton oxidation. According to the contrast experiments, the DC was hardly degraded in the absence of H2O2 (or Fe ion).
During the adsorption/regeneration process, the DC molecules are first removed from polluted water by the adsorption process and preconcentrated on the surface of MCM adsorbent. Then, the regeneration processes are carried out by triggering the heterogeneous Fenton oxidation. Namely, the decomposition of H2O2 using Fe3O4 nanoparticles as Fenton catalysts was able to generate principal oxidizing species like ⋅OH radicals according to the following reactions:
In these ways, the saturated DC molecules on the surface of MCM adsorbent could be oxidized to degradation products.
To investigate the effect of H2O2 doses on the regeneration efficiency of MCM, the regeneration process was conducted with different amounts of H2O2. And the performance of the continuous column was evaluated for three consecutive adsorption/regeneration cycles. The DC concentration, flow rate, bed depth, and pH were kept constant at 25 mg/L, 1.1 mL/min, 1.2 cm, and 7, respectively. The experimental results are shown in Figure
Regeneration efficiency of the saturated MCM at different H2O2 concentrations and reusability of MCM with optimal H2O2 concentration.
In Figure
As shown in Figure
As seen in Figure
The embedding of Fe3O4 nanoparticles into the CSM substrate is beneficial to the easy separation of catalysts from the aqueous system. In order to verify magnetic separation of MCM, an external magnet was used to separate the samples from mixture solution. Figure
Photographs of separation of MCM by gravity (a) and magnetism (b).
In summary, a low-cost magnetic chitosan@Fe3O4 composite material, which integrates the adsorption features of chitosan with the magnetic and catalytic properties of Fe3O4 nanoparticles, was synthesized via an alkali gelation-thermal cracking process. The mechanism for the formation of the product was discussed in detail. These composites exhibited excellent properties for the effective removal and oxidative destruction of DC from aqueous solution via a fixed-bed column method. The optimum conditions were observed at an initial DC concentration of 20 mg/L, a flow rate of 1.1 mL/min, and a bed depth of 1.6 cm. The Thomas and Yoon-Nelson models showed a good agreement with the experimental data and could be applied for the prediction of the fixed-bed column properties and breakthrough curves. The MCM possesses a ferromagnetic characteristic allowing them to be easily separated from the aqueous system by an external magnet. The regeneration of the saturated adsorbent after the adsorption process can be realized using H2O2 solution and could be controlled easily by adjusting the H2O2 dosage. Considering the abundant and low-cost raw materials, the facile fabrication route, the easy recovery and separation process, and the excellent reusability, the magnetic Fe3O4@chitosan carbon microbeads may serve as a promising and scalable adsorbent for the removal and oxidation of organic compounds in practical applications.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
This work was financially supported by the Fund Project of Shaanxi Key Laboratory of Land Consolidation and Fundamental Research Funds for the Central Universities (nos. 310829162014, 310829161015, 310829175001, and 310829165007).