Efficient Absorption of Antibiotic from Aqueous Solutions over MnO2@SA/Mn Beads and Their In Situ Regeneration by Heterogeneous Fenton-Like Reaction
Alginate has been extensively used as absorbents due to its excellent properties. However, the practical application of pure alginate has been restricted since the saturated adsorbent has weak physical structure and could not be regenerated easily. In this study, a low-cost and renewable composite MnO2@alginate/Mn adsorbent has been prepared facilely for the absorptive removal of antibiotic wastewater. FE-SEM, FTIR, and XRD analyses were used to characterize the samples. The norfloxacin (NOR) was used as an index of antibiotics. More specifically, the batch absorption efficiency of the adsorbents was evaluated by pH, contact time with different NOR concentration, and the temperature. Thus, the performance of absorption kinetic dynamics and isotherm equations were estimated for the adsorptive removal process. Parameters including , , and were utilized to describe the feasible adsorption process. To regenerate the saturated absorptive sites of the adsorbent, the heterogeneous Fenton-like reactions were trigged by introduction of H2O2. The results showed that the in situ regenerating has exhibited an excellent recycling stability. The high activity and the simple fabrication of the adsorbents make them attractive for the treatment of wastewater containing refractory organic compound and also provide fundamental basis and technology for further practical application.
Extensive usages of chemical antibiotics have played an important role in the health care for human and animals . However, about 50~90% of ingested antibiotics are excreted into domestic sewage without being metabolized due to the incomplete metabolism [2, 3]. As a result, residual chemical antibiotic has been frequently found in the ground and drinking waters, which inevitably lead to some unfriendly environmental influences such as the effects of the toxicity to animalcule and the threats to human health [4, 5]. In recent years, compared with the traditional strategies for removal of toxic antibiotics from wastewater, using absorptive approaches for the highly efficient degradation of stable antibiotics from wastewater treatment systems has been regarded as most efficient route owing to their interested features of simple operation, low budget, nontoxicity, and efficient removal rate. Particularly, the application of low-cost, regeneration, and environmentally friendly adsorbent attracts much more attentions. For example, some wastes for economic crops, such as rice-bran , cacao-shell , maize-stalk , and peanut-shell , have been widely used as adsorbents to deal with various antibiotics wastewater. Sodium alginate (SA), a natural polysaccharide extracted from seaweed and comprised homopolymeric blocks of guluronate blocks and mannuronate blocks, was used extensively as supported adsorbent owing to its advantages of biocompatibility, low toxic properties and budget, and ease of gelation [10, 11]. Several alginate-based adsorbents including alginate nanohybrids , alginate nanohydrogel , alginate fiber , alginate bead , and alginate film  have already been explored for organic pollution adsorption applications. In contrast, the alginate beads have been more widely used than the fiber, nanoparticles, or film forms because of their simple fabrication and recovery process, controllable particle dimension, and excellent dispersion stability . Unfortunately, practical application of pure alginate bead as adsorbent has been restricted since the adsorbent has weak physical structure and could not be regenerated easily, which needs extra process or complete replacement . Therefore, the exploration of developing an effective and easy regeneration route for alginate bead is of particular significance in contemporary industry.
The degradation of contaminated organism from wastewater by traditional Fenton oxidation process using slurry suspensions of iron oxide as catalysts is considered as an expensive process since longer reaction times are usually required to entirely oxidize the pollutants due to their inefficient hydroxyl radicals’ concentration inside reactions [19–21]. To overcome this dilemma, utilizing an enrichment method or prethickening adsorption way prior to the oxidation process has been ascertained that the removal efficiency of pollutants through the Fenton-like reaction has been significantly increased. Typically, the embedding of Fe3O4 nanoparticles of yeast has integrated biosorption properties from pure yeast cells under the Fenton performance from Fe3O4 nanoparticles, resulting in the high-effective degradation of cationic azo dye in wastewater treatment . The enhanced efficiency of wastewater treatment is ascribed to the successive and synergistic effect on yeast biosorption and Fe3O4 nanoparticles over heterogeneous Fenton catalyzes performance and regeneration process. Compared with traditional iron oxide Fenton catalysts, MnO2 exhibit most attractive transition metal oxides which predisposed to coordinate with oxidant forming the Fenton-like process reagent to remove the contaminates in water treatment. Such kind of catalyst possessed intriguing features such as widening operative pH ranges and high positive catalyst performance . Furthermore, researches prove that decomposition of H2O2 can be catalyzed by MnO2 nanoparticles to generate reactive oxygen species, like hydroxyl radicals, carboxyl radicals, and single oxygen, to remove organic pollutants [24–26]. As a consequence, the simultaneous utilization of MnO2 and H2O2 has been considered as Fenton-like process reagent to oxidize the low-biodegradable organism. For instance, methylene blue , methyl orange , and azo dyes  have been removed using MnO2-involved hybrid composite as a catalyst in heterogeneous MnO2/H2O2 Fenton-like system.
Thus, in this work, novel absorbents MnO2@alginate/Mn (MnO2@SA/Mn) beads were first fabricated. The adsorbents were characterized by a scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and powder X-ray diffraction (XRD), respectively. Norfloxacin (NOR), frequently detected in wastewater and surface water with high concentrations, was used on purpose as an index of antibiotics to evaluate the adsorption properties of MnO2@SA/Mn beads. The detailed absorptive removal processes for NOR antibiotic from aqueous solutions over MnO2@SA/Mn beads were investigated. Afterward, the in situ regeneration of the saturated MnO2@SA/Mn absorbents was trigged by introduction of H2O2. The possible mechanism for in situ regenerating norfloxacin-loaded MnO2@SA/Mn was discussed.
Sodium alginate (SA) with the purity 0.95 was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Norfloxacin (C16H18FN3O3) was purchased from Sigma-Aldrich Co., Ltd. (Shanghai, China), and the purity is 0.98. The reagents in this experiment are analytical grade with mass fraction purity 0.99 and used as received. Manganese sulfate (MnSO4) and potassium permanganate (KMnO4) were purchased from Tianjin Yonghao Jingxi Chemical Co., Ltd. (Tianjin, China). Ethanol (C2H5OH) was purchased from Tianjin Fuyu Jingxi Chemical Co., Ltd. (Tianjin, China). Distilled water (18.3 MΩ cm) was used to make required aqueous solutions.
2.2. MnO2@SA/Mn Beads Preparation
MnO2 nanoparticles were prepared via a hydrothermal method. Typically, 30 ml 0.2 mol·L−1 MnSO4 solution was added dropwise into 30 ml, 0.3 M KMnO4 solution with continuous stirring for at least 30 min. Then the mixture was treated at 180°C with a Teflon-lined autocave for 10 h. After cooling down to room temperature, the suspension was centrifuged and washed the precipitate with distilled water. After that, the precipitate was dried at 60°C in air for 6 h; the final MnO2 nanoparticles were obtained. The reaction equation is as follows:
Subsequently, 1.000 g of MnO2 nanoparticles was dispersed into 50 ml of 2.5 (w)% sodium alginate solutions at 298.15 K with continuous stirring for 2 h. After that, the sodium alginate solution was dropped into MnSO4 solution (0.05 mg/L) with a peristaltic pump. After the hydrogel beads were stored at 4°C for 10 h to form MnO2@SA/Mn hydrogel beads, the obtained hydrogels were finally washed and stored at 4°C until use.
2.3. Adsorbents Characterization
The morphology of the MnO2@SA/Mn adsorbent was observed by a scanning electron microscopy (SEM, Hitachi S-4800, Japan). Element content and line-scanning analysis were investigated by energy-dispersive spectroscopy (EDS) analysis. To study the chemical structures, Fourier transform infrared spectra (FTIR, BiO-RAD FTS135, America) of the adsorbents were monitored by a Bio-Rad FTS135 spectrometer in the range 500–4500 cm−1 using KBr. And the crystal phase was investigated by powder X-ray diffraction (XRD, Rigaku D/MAX-3C diffractometer, Japan) patterns which were conducted on X. Pert Pro diffractometer at a scanning rate of 100 per min using Cu Kα radiations (λ = 0.15418).
2.4. Adsorption Experiments
In a typical run, the experiment was conducted in 200 mL conical flasks containing 100 mL of the desired NOR concentration at pH 4. Since the adsorbents were primarily responsible for the adsorption, 2.0 g·L−1 MnO2@SA/Mn adsorbents were added in each flask. Then the solution was stirred using a magnetic stirrer for 85 min. 5 ml samples in solution were picked up at regular intervals to centrifuge in order to separate the absorbents from the liquid, and then the supernatant was analyzed by a wavelength of 273 nm using an Evolution 201 ultraviolet-vis (Jenway, Cambridge, UK) spectrophotometer to confirm the residual concentration of NOR and the loading efficiency. After that, the samples were immediately reverted to the flask. The adsorption capacity (, mg·g−1) and loading efficiency (, %) of NOR were determined as follows:where (mg·L−1) and (mg·L−1) are the NOR concentration before and after adsorption, respectively. (L) is the volume of the solution and (g) is the weight of the absorbent.
2.5. In Situ Regeneration of Absorbents
Typically in an adsorptive removal process, 100 mL 10 mg·L−1 NOR aqueous solution was conducted in conical flask, and 2.0 g·L−1 of MnO2@SA/Mn adsorbents was used as the absorbents, then analyzing the supernatant of the solution to determine the NOR loading efficiency when the adsorption equilibrium finished. After that, the solution was added by 5 mL 1 (w/v)% H2O2 and irradiated for 4 h using two ultraviolet lamps fixed directly above the flask. Then the absorbents were collected by centrifugation, washed thoroughly, and dried to be reused in the next run. Another cycle of sorption-regeneration process was repeated in the same manner as mentioned above.
3. Results and Discussion
3.1. Characterization of MnO2@SA/Mn Beads
An estimated formation pathway of MnO2@SA/Mn beads is shown in Scheme 1.
In this work, sodium alginate was dissolved ahead in distilled water with magnetic stirring for 10 h to get a homogeneous expansive solution which would intertwine each together forming a three-dimensional network through covalent and noncovalent interactions like hydrogen bond, electrostatic interactions and van der Waals forces [30, 31]. Then, the MnO2 nanoparticles were introduced into the solution gradually with continuous stirring during the intertwined processes. Thus, the expansive solution of sodium alginate would provide a certain possibility for MnO2 nanoparticles uniformly dispersing and embedding into the polymer network to obtain MnO2@SA gel solution. After that, the composite was dropwise injected into MnSO4 solution. When MnO2@SA gel droplets were immersed into MnSO4 aqueous solution, MnO2@SA/Mn hydrogel beads were achieved due to the ionic cross-linking interactions between carboxyl group of the alginate chains and Mn2+ by chelation and covalent and noncovalent forces. It was conceivable that the impregnation of MnO2 nanoparticles is an effective approach to enhance the physicochemical properties of hydrogel beads, and the physical MnO2@SA/Mn hydrogel beads acquired would be comprised of water, MnO2, Mn2+ ions, and alginate with abundant free carboxyl groups and hydroxyl groups.
Based on the previous analysis, we can infer that alginate molecule, MnO2 nanoparticles, and Mn2+ ions have had their own extremely important contribution for the formation of MnO2@SA/Mn hydrogel beads, respectively. For the alginate material, the physical cross-linked property of the alginate polymer chains have assembled the dispersive MnO2 nanoparticles into hydrogel beads, helping to fabricate the uniform and scattered spherical alginate gel microspheres. Besides, the generated alginate hydrogels have reserved sufficiently original groups including hydroxyl and carboxyl groups on the surface of alginate substrates, which can be used for capturing the chemical antibiotics compound from aqueous solutions by covalent and noncovalent forces. And thus, the enrichment or preconcentration adsorption of chemical antibiotics compound prior to the heterogeneous oxidation process could be fulfilled. As for the MnO2 nanoparticles, they have acted as the inner skeleton to strengthen the mechanical stability of MnO2@SA/Mn hydrogel beads. Also, the MnO2 nanoparticles within the inner and surface of MnO2@SA/Mn hydrogel beads can provide abundant catalytic active sites for the heterogeneous Fenton-like reaction, speeding up the degradation rate of norfloxacin antibiotics. Owing to the fact that coordination for divalent ions is permitted by the structure of guluronate blocks of alginate, the ample Mn2+ ions within the system hereby have functioned as a cross-linking agent to help to form alginate hydrogels in solution by binding solely with guluronate blocks from polymer chains. By these means, one polymer guluronate block could form junctions with the adjacent polymer guluronate chains, and finally the MnO2@SA/Mn beads with complicated cross-linking hydrogel and unique egg-box shape were constructed successfully .
To examine the chemical bonding structure and possible interactions of the MnO2@SA/Mn beads, FTIR analysis was employed to detect the chemical bonds transformation in parallel to MnO2, pure alginate hydrogel beads, and MnO2@SA/Mn beads. For the alginate hydrogel beads in Figure 1(a), the peaks at 3428, 2925, 2854, 1632, 1426, and 1163 cm−1 are ascribed to the -OH, -CH2-, -CH3, the asymmetric and symmetric stretching vibration of -COOH, and -CO vibrations, respectively [33–37]. In the case of the MnO2 nanoparticles in Figure 1(b), the peak at 3429 cm−1 is mainly generated by -OH of the water molecule adhering to the surface of MnO2 nanoparticles which indicates that the surface of synthesized MnO2 nanoparticles has sufficient hydroxyl groups . The two peaks around 716 and 523 cm−1 are aroused by Mn-O-Mn and Mn-O stretching vibration [39, 40]. In the FTIR spectrum of MnO2@SA/Mn beads (Figure 1(c)), the characteristic adsorption peaks of pure alginate hydrogel beads and MnO2 were approximately maintained or even stronger. Moreover, an original peak at 1740 cm−1 probably caused by the absorption vibrations of the –COOH in alginate molecule [41, 42]. However, the peaks at 3428 cm−1, 1632 cm−1, 1426 cm−1, and 1163 cm−1 of pure alginate hydrogel beads and 3429 cm−1, 716 cm−1, and 523 cm−1 of MnO2 were changed to 3429 cm−1, 1636 cm−1, 1410 cm−1, 1178 cm−1, 670 cm−1, and 594 cm−1 in MnO2@SA/Mn beads, respectively. These changes implied that the alginate network and MnO2/Mn in the MnO2@SA/Mn hydrogel beads were linked with the strong intermolecular forces of hydrogen bonding and chelation between Mn and hydroxyl groups, carboxyl groups, and such linkages resulted in forming a stable three-dimensional network.
Optical photographs and FE-SEM images of MnO2@SA/Mn beads are presented in Figure 2, respectively. Figure 2(a) indicates that the surface of MnO2@SA/Mn hydrogel beads in a gel state is smooth and the bead has a black spherical shape with a diameter approximately 2.0 ± 0.2 mm; simultaneously the average diameter after freezing drying is about 1.5 ± 0.2 mm. The shrinkage of the MnO2@SA/Mn hydrogel beads indicated that the obtained hydrogel beads have a definite adsorption capacity for water molecule. Theoretically, the water-holding capacity (WHC) can be calculated as follows:where is the density of water, is the volume of MnO2@SA/Mn hydrogel beads, and and are the radius of the gelatinous beads (eq) and dried () beads. After calculation, its water-holding capacity was theoretically about 73.27 mg·bead−1.
Figure 2(b) is the SEM photograph of a freezing dried MnO2@SA/Mn hydrogel bead. It shows that the surface of MnO2@SA/Mn hydrogel bead is relatively sags and crests with small concave depressions on; the reason should be attributed to two cases. For the one side, it is due to rough shrinkage of MnO2@SA/Mn hydrogels over complete freezing drying process. For another side, it is due to the impregnation of MnO2 nanoparticles into alginate hydrogel beads in the aqueous solution. Figures 2(c) and 2(d) show that the local surface magnified and cross section magnified SEM photographs of samples. The photographs indicated that the MnO2 nanoparticles, with an average size of 20.0 ± 2.0 μm widths and 400.0 ± 2.0 μm length, were successfully embedded on/in the three-dimensional cross-linked alginate hydrogel beads heterogeneously and deeply due to the intermolecular forces of hydrogen bond between MnO2 nanoparticles and alginate polymers, and the chelation between Mn2+ ions and hydroxyl, carboxyl groups. Briefly, the stable and tight MnO2@SA/Mn hydrogel beads were acquired with abundant carboxyl and hydroxyl groups for NOR adsorption.
The distribution of elements of selected MnO2@SA/Mn zone was further investigated by EDS analysis and line-scanning. The experimental results were shown in Figure 3. As shown in Figure 3(a), the EDS analysis of MnO2@SA/Mn beads inferred that C, O, and Mn elementals were the main components with weight percentage of 44.10%, 26.36%, and 29.54%, respectively, confirming the purity of MnO2@SA/Mn materials. Moreover, from the graph in Figure 3(b), the line-scanning for selected surface zone of MnO2@SA/Mn bead presented high homogeneity that, during the area with MnO2 impregnation, the intensities of Mn and O are notable much stronger than C, while, in the other area, the intensities of C, O, and Mn trended towards stable values. These results further prove the successful immobilization of MnO2 nanoparticles on/in the SA beads and lead to an enhancement of amount of available adsorption/regeneration sites.
The structural characterization of MnO2@SA/Mn beads are further determined by powder X-ray diffraction technology. The typical XRD analyses are shown in Figure 4. The diffraction of alginate gel beads (Figure 4(c)) shows typical peaks around 13.1° and 20.6° . Moreover, in Figure 4(b), the composites had similar diffraction peaks at 2θ = 28.7°, 37.4°, 41.0°, 42.8°, 56.7°, and 59.4°, giving an index to a β-MnO2 phase . Therefore, Figure 4(a) exhibits the XRD pattern of MnO2@SA/Mn hydrogel bead including both the diffraction peaks of MnO2 nanoparticles and alginate gel beads, correspondingly. And the successful impregnating of MnO2 on/in the MnO2@SA/Mn beads was confirmed since no further impurities peaks were found.
3.2. Adsorptive Removal of NOR Antibiotic from Aqueous Solutions
In this study, the applications of MnO2@SA/Mn beads were evaluated by the adsorptive removal of NOR compound from the simulated solution using batch reactor. The parameters of the adsorptive removal process determined in the experiments were pH (2–10), NOR concentration (2–30 mg·L−1), contact time (0–85 min), and temperature (288.15–318.15 K). In addition, parameters of adsorptive kinetic and thermodynamic were utilized to confirm efficiency for the adsorptive removal process.
3.2.1. Effect of pH in Solution
The effect of pH is a significant factor in adsorptive removal process since it is closely integrated with the surface charge of the adsorbent and the chemical structure of organic materials . Herein, parameters of the NOR adsorptive removal experiments were performed with 10 mg/L NOR, 2 mM H2O2 solution, and 2.0 g/L adsorbent at 30°C, and the adsorption properties including removal efficiency and adsorptive removal capacity were investigated by MnO2@SA/Mn and pure AB, respectively (Figure 5). The value of the maximum NOR adsorptive rate by catalyst MnO2@SA/Mn beads is 98.3%, while by catalyst AB the value is 55.2% at pH 4.0. However, with the pH of the solution (pH 4.0–10.0) further increasing, the efficiency of NOR adsorption decreases from 98.3% to 37.2% and 55.2% to 11.7%, respectively. The maximum adsorption capacities are 4.9 mg/g by catalyst MnO2@SA/Mn beads and 2.8 mg/g by AB beads individually. This observation gives the evidence that MnO2 and Mn2+ ions can improve the NOR adsorption capacity in Fenton-like system.
Such phenomenon is probably ascribed to the causes below. For MnO2@SA/Mn beads adsorbents, their surface is apt to be adjusted at different pH since the of MnO2 is 2.25  and the of pure alginate bead is 4.2. For NOR adsorbate, each NOR molecule has two functional groups including acid carboxyl and basic piperazine. Therefore, the acid-base equilibrium of the NOR molecule was inevitably influenced by the solution pH. More specifically, the piperazine group of the NOR exists as a protonated cation form in an acidic environment when pH is less than 6. On the contrary, a basic environment could cause a deprotonation when pH is greater than 9. Namely, the NOR performs as deprotonated anion form at a basic environment. While the solution pH is varied from 6 to 9, the NOR exists as zwitterions form [47, 48].
3.2.2. Effects of NOR Concentration
Effects of adsorption contact time with different initial NOR concentration (2–30 mg·L−1) were carried out for 85 min at 30°C with 2.0 g·L−1 adsorbent and 2 mM H2O2 in pH 4 solution in Figures 5(b) and 5(c). Figure 5(b) showed that the kinetics of NOR included two stages of behaviors during the adsorption process: a fast adsorption period over a short time followed by a slower adsorption period for a longer time. In the first stage, it is due to the adsorptive sites on the MnO2@SA/Mn adsorbent surface through the initial period of adsorption behavior. For the second stage, adsorption rate was probably by the molecular-repulsive interactions between NOR molecule on the absorbent surface and the solution. As time goes on, some adsorbed NOR molecules were desorbed from the adsorbent dispersing to the bulk phase again. Equilibrium adsorption capacity increased notably from 9.93 to 96.85 mg·g−1 with the increasing of NOR concentration varying from 2 to 30 mg·L−1. Such phenomenon can be probably contributed to the strong interactions of driving force among NOR ions during high concentration. From Figure 5(c), it indicates that a lower initial NOR concentration results in a higher adsorption efficiency. With NOR concentration varying from 2 to 30 mg·L−1, the final degradation of NOR by MnO2@SA/Mn beads and pure AB beads at 85 min decreases from 99.3% to 57.9% and from 46.1% to 11.5%, respectively. The NOR removal efficiency and adsorption capacity by MnO2@SA/Mn beads are higher than those values by pure AB beads. These results indicate that MnO2@SA/Mn beads have more abundant adsorption sites than pure AB beads.
3.2.3. Kinetics of NOR
The kinetics is significant for the removal efficiency since it can provide the mechanism of adsorption process. For the reason that the adsorbate residence time is controlled by the kinetics, removal rate of the adsorption is so important that the design parameters of the process can be better optimized. Therefore, predicting removal rate in which adsorption behavior occurs is considered as the most significant element for the design of the adsorptive removal process .
To study the adsorption mechanism, linearized adsorptive kinetic equation is utilized to find the adsorption kinetics of NOR adsorbed by the MnO2@SA/Mn hydrogel beads . The pseudo-first-order equation is as follows:where and represent the amount of NOR adsorbed on the adsorbent at the equilibrium and at time , separately. (min−1) is rate coefficient for the pseudo-first-order.
The pseudo-second-order equation is as follows [51, 52]”where and are the capacities at equilibrium and at time (s), respectively. (g·mg−1·min−1) is rate coefficient for the second-order equation. The fitted curves for both two kinetics are presented in Figure 6; curve-fitting data and correlation parameters are shown in Table 1.
Figure 6 shows the plots of two kinetics of NOR adsorption on MnO2@SA/Mn with varied NOR concentrations. The rate coefficients and correlation coefficient for adsorption were simulated and summarized in Table 1. Table 1 revealed that the practical adsorption amount values () disagree with the theoretical values () although the constant values ( for the pseudo-first and pseudo-second equations were in the range of 0.910~0.989 and 0.994~0.998, respectively. However, the values of correlation coefficient () for the pseudo-second-order are much closer to 1.0 than the values for pseudo-first-order, confirming that the adsorption process of MnO2@SA/Mn hydrogel beads for simulated NOR-solution fits the pseudo-second-order equation better.
3.2.4. Isothermal and Thermodynamic Experiments of Adsorption
Thermodynamic experiments on NOR adsorption were investigated in a temperature range of 15°C to 45°C with pH 4, 10 mg·L−1 NOR, 2.0 g·L−1 adsorbent, and 2 mM H2O2 solution. In order to study whether the adsorption process might take place spontaneously, parameters including the changes of enthalpy (), the entropy (), and the Gibbs free energy () associated with adsorptive removal process were calculated as follows:where and (K) are the equilibrium adsorption constant and temperature, respectively. Constant (8.314 J·mol−1·K−1) is the ideal-gas coefficient. In addition, parameters and are obtained from the plots of versus .
Table 2 exhibits the data of , , and for the adsorptive removal process. From Table 2, the negative indicates that, at temperature ranging from 15 to 45°C, the spontaneous nature of adsorption occurs relatively easier at 35°C. Considering is in the range −20 to 0 kJ·mol−1, the processes are dominated by the physical adsorption [53, 54]. Moreover, the positive (20.52 kJ·mol−1) reveals that such adsorptive removal process is the result of endothermic nature of adsorption and physical interactions, including van der Waals interactions, hydrogen-bonding forces, and electrostatic force . Furthermore, the positive value of (88.87 kJ·mol−1) confirms that excellent affinity of NOR molecule towards adsorbent and randomness increases at the solid-liquid interface at 15–45°C .
The Langmuir, Freundlich, and Dubinin-Radushkevich isotherms are selected to describe how adsorbents interact with adsorbate during the adsorption behavior. The Langmuir isotherm assumes that uniform adsorptive process occurs with the monolayer at the adsorbent surface . While the Freundlich model is an empirical expression which describes the multilayer sorption behaviors that occurred in the heterogeneous system . Compared with the isotherm equations above, Dubinin-Radushkevich isotherm model is used to distinguish the adsorption mechanics as chemical and physical adsorption of NOR .
The Langmuir equation has the following form:where (mg·L−1) and (mg·g−1) are the equilibrium NOR concentration and maximum adsorptive capacity; and (L·mg−1) is the coefficient of adsorption.
Moreover, the Freundlich isotherm equation is as follows:where and are the constants indicating the adsorption capacity and adsorption intensity, respectively.
Additionally, a dimensionless constant () can reflect the significant performance of Langmuir and is given bywhere the coefficient implies the type of isotherm based on the following ranges: = 0, irreversible; , favorable; = 1, linear; > 1, unfavorable .
Furthermore, the Dubinin-Radushkevich isotherm model for the linear form is where (mol·J−1) is the constant related to the mean free energy of adsorption; is Polanyi potential which can be calculated from (11). Constant (J·mol−1·K−1) and (K) are the gas constant and absolute temperature. (kJ·mol−1) is the mean energy of the adsorption.
The simulated data on the basis of the aforementioned models are shown in Table 3. It reveals the determined coefficient constant for the two isotherm models at different temperature. The result proves that the Langmuir model better fits the practical values of MnO2@SA/Mn beads. From Table 3, we can find that the Langmuir constant indicating the maximum adsorption capacity of MnO2@SA/Mn beads was increased, while the values of b indicating the energy of the adsorption were decreased with increasing temperature till 45°C. And the values of range within 0.06–0.63 for different initial NOR concentrations at different temperatures. These phenomena once again confirm that the adsorption for NOR on MnO2@SA/Mn beads was favorable.
Freundlich model could not explain the adsorption behavior as the Langmuir theory did since the constant was lower than the values in Langmuir model. The Freundlich constant ranged from 2.28 to 4.57 at different temperatures, also revealing that the adsorption was favorable . It confirmed that values of increased with the temperature of the solution up to 308.15 K, proving that high NOR adsorption capacity easily occurred at relatively temperature.
From Table 3, it can be seen that the values of mean energy simulated are all smaller than 2 kJ·mol−1, confirming that the adsorption of NOR by MnO2@SA/Mn absorbent was dominated by physical adsorption during the process .
3.3. Regeneration of NOR-Loaded MnO2@SA/Mn by Heterogeneous Fenton-Like Reaction
The migration of NOR antibiotics from aqueous solutions have been achieved by absorptive enrichment or preconcentration approach over MnO2@SA/Mn beads. Then, the regeneration of the saturated absorptive sites by subsequent destruction of the adsorbed organic NOR pollutants are extremely crucial for the economic cost. In the present study, the reuse and regeneration of saturated absorbent were performed by trigging UV assisted-heterogeneous Fenton-like reaction. The experimental results were shown in Figure 7.
Figure 7 showed the reuse performance of in situ regenerated alginate and MnO2@SA/Mn. The cycling properties of alginate and MnO2@SA/Mn used for catalytic reaction were evaluated at [NOR] = 10 mg·L−1, [catalyst] = 2.0 g·L−1, pH = 4, and = 30°C. In Figure 7, the removal rate of pure AB after UV/H2O2 regeneration over four cycles was apparently dropped from 58% to 35%. This contributed to the fact that the removal of NOR in solution by pure alginate beads mainly depended on the adsorption properties. Herein, it is well known that decomposition of H2O2 could be catalyzed by UV radiation to create oxidizing radicals. The significant mechanisms include that NOR and H2O2 molecules occupied the bare active sites on the surface of the alginate; then NOR was removed by the oxidizing radicals created by the decomposition of H2O2 and dissolved into the solution, similar results were acquired by Tunç et al. . Although NOR in solution could be removed by the pure alginate beads in the UV/H2O2 system, the adsorbed NOR could block the activated sites and decrease catalytic efficiency on the surface of the alginate towards the H2O2 decomposition . However, compared with the pure alginate beads, the MnO2@SA/Mn has achieved a higher removal rate in the UV-Fenton-like system. The removal rate of MnO2@SA/Mn was 98%, 95%, 91% and 86%, respectively (Figure 7). It indicated that the MnO2@SA/Mn catalyst retained excellent activity and stability after recycle for four times. This probably can be attributed to the effects of alginate adsorption property, UV photolysis, and MnO2/Mn-triggered heterogeneous Fenton-like oxidation. Comparing with pure alginate beads, the MnO2/Mn-triggered heterogeneous Fenton-like oxidation process directs an ascendant position for the contributor during regeneration process. The phenomena give a firm evidence that MnO2@SA/Mn catalyst can be reused at least four times without losing much effectiveness to remove NOR pollutant, which is significant in practical and long-term applications.
For UV/Fenton-like reaction system, ∙OH, HOO∙, and 1O2 were generated by decomposition of H2O2, and they can powerfully and nonselectively oxidize or destroy the molecules structure of organic pollutants . Nevertheless, organic pollutant, as NOR, can be removed from aqueous solutions only they were adsorbed on the catalyst surface . Based on this assumption, possible formation mechanism for in situ regenerating NOR-loaded MnO2@SA/Mn is proposed to be a process of adsorption-decomposition-desorption. Firstly, H2O2 and NOR are adsorbed on the catalyst surface; secondly, under UV irradiation photolysis and MnO2/Mn-triggered heterogeneous Fenton-like oxidation, H2O2 is decomposed into ∙OH, HOO∙ and 1O2 radicals ((13)–(21)). Part of the newly generated radicals diffuses on the surface and reacts directly with the adsorbed NOR molecules and decompose them into small organic molecules and inorganic substances. And the other radicals are desorbed from the surface, dispersed into the solution, and decomposed the NOR in the solution. Finally, the degraded small units of NOR are desorbed from the catalyst surface and enter into the solution, recovering the active potential site of the catalyst surface. Therefore, MnO2@SA/Mn could be in situ regenerated for the next catalytic reaction.
In this study, the present research attempted to develop a simple and ecofriendly approach to prepare a superabsorbent composite material via the modification of alginate hydrogel beads impregnating with MnO2 nanoparticles. The abundant hydroxyl radicals and hydroperoxyl radicals derived from H2O2 and distinctive chemical/physical performance inherited from alginate have guaranteed the strengthened MnO2@SA/Mn composites with enhanced NOR adsorption and pH sensitivity. FE-SEM photographs displayed that the catalyst has a surface of relative sags and crests with small concave depressions. And FTIR analysis confirmed that the composites have abundant carboxyl and hydroxyl groups for adsorption. The batch experiment was investigated by pH, contact time with different initial NOR concentration, and temperature. Moreover, the performance of kinetic dynamics and the kinetic data revealed that the adsorption of NOR onto MnO2@SA/Mn fitted pseudo-second-order kinetic model when compared with the pseudo-first-order kinetic equation confirming the rate determining step dominated by the chemical forces of attraction. The adsorption process was evaluated by Langmuir isotherm equation and Freundlich isotherm model, and it was found that the adsorption followed Langmuir isotherm equation well. This revealed that the adsorption process obeyed the monolayer sorption process. Thermodynamic parameters, such as negative value of , indicated the spontaneous adsorption process. More importantly, the in situ regenerating tests justified the excellent recycling stability, reusability, and renewable ability. This study confirmed that NOR-containing solutions demonstrated high removal efficiency in the heterogeneous Fenton-like process over MnO2@SA/Mn, the high activity of MnO2@SA/Mn, and their simple preparation make them attractive for the treatment of antibiotics in wastewater treatment and provide fundamental basis and technology for further practical application.
Conflicts of Interest
The authors declare that they have no potential or actual conflicts of interest pertaining to this submission.
This work was financially supported by National Natural Science Foundation of China (no. 21176031), Shanxi Provincial Natural Science Foundation of China (no. 2015JM2071), and Fundamental Research Funds for the Central Universities (no. 310829165027, no. 310829162014, and no. 310829175001).
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