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Article

Development of Vitamin B6-Mediated Biochar with Nano Zero-Valent Iron Coating for Oxytetracycline Removal through Adsorption and Degradation under Harsh Acidic Conditions

1
School of Environmental Science and Engineering, Shandong University, Qingdao 266237, China
2
Institute for Energy, Environment and Sustainable Communities, University of Regina, Regina, SK S4S 0A2, Canada
*
Author to whom correspondence should be addressed.
Water 2022, 14(17), 2734; https://doi.org/10.3390/w14172734
Received: 30 June 2022 / Revised: 26 July 2022 / Accepted: 29 August 2022 / Published: 1 September 2022
(This article belongs to the Special Issue Future Water Resources and Air Pollution Management and Innovation)

Abstract

:
Oxytetracycline-containing wastewater, particularly produced by pharmaceutical industries, is too acidic to treat with iron-assisted materials. In order to tackle this issue, vitamin B6-mediated biochar with nano zero-valent iron coating (nZVI/[email protected]) was developed. Oxytetracycline (OTC) removal performance of biochar (BC), vitamin B6-coated biochar ([email protected]), nZVI-coated biochar ([email protected]), and vitamin B6-mediated biochar with nano zero-valent iron coating (nZVI/[email protected]) were investigated to analyze contributions and mechanisms of adsorption and degradation. Through modification, the adsorption capacity of [email protected] was slightly increased from 81.38 mg/g of BC to 85.64 mg/g. In the removal test, the 5-min OTC removal efficiencies with [email protected] and nZVI/[email protected] were 52.25% and 59.05%, yet the BC and [email protected] were limited to 5.61% and 8.54%. The distinct difference may be attributed to the existence of nZVI on biochar strongly improving the reactivity from adsorption to chemical reaction. Moreover, 98.28% of OTC was removed within 60 min in the nZVI/[email protected] suspension. The adsorption of OTC on BC fitted the Freundlich isotherm, Temkin isotherm, and intramolecular diffusion model, whereas that on [email protected] fitted Langmuir isotherm and pseudo-second-order better. Based on HPLC-MS analyses, there were three pathways proposed for OTC degradation in nZVI/[email protected] suspension. nZVI provided active sites on biochar for OTC degradation through oxidization, de-hydroxylation, ring-opening, reduction, addition, demethylation, and alkylation reactions. B6 as a mediate helped improve the stabilization and distribution of nZVI on biochar, which facilitates the capability of nZVI/[email protected] for OTC removal through adsorption and degradation under acidic conditions. The OTC can not only be captured on biochar but also be metabolized to achieve complete removal from aquatic systems.

1. Introduction

Oxytetracycline (OTC), a broad-spectrum tetracycline antibiotic, is widely used to treat bacterial infections in human beings, livestock, and fish, and OTC release is inevitable during its production and consumption [1]. A large amount of OTC-containing wastewater has been produced from healthcare, agricultural, and industrial sectors worldwide, resulting in severe OTC pollution in aquatic ecosystems [1,2,3,4,5]. In winter, the mean levels of OTC in water and sediment have been reported at 265.35 ng/L and 23.52 ng/g, respectively [6]. Consequently, long-term OTC exposure can not only lead to serious damage to organisms but can also cause resistance gene contamination in the biosphere [7,8,9]. Therefore, it is desired to develop green and effective technologies to treat OTC wastewater.
Traditional strategies to solve antibiotic pollution include advanced oxidation, filtration, and electrochemistry [3,10,11]. Compared to these, adsorption is a high-efficiency, cost-saving, and environment-friendly technique to remove OTC [12,13]. Recently, biochar materials derived from agricultural wastes through pyrolysis have been investigated to adsorb OTC [14]. For example, macro-, colloidal- and nano-biochar materials have been examined for OTC adsorption. The colloidal biochar has the highest maximum adsorption capacity of 136.7 mg/g, while the nano one has the lowest value of 113.2 mg/L [15]. Although biochar has a remarkable specific surface area, surface modification studies on biochar have further been investigated to enhance OTC adsorption. Magnetic montmorillonite biochar has maximum OTC adsorption of 58.85 mg/L, which is 2.63 times as large as the original biochar [16]. In addition, vitamin B6 (B6) modification has been undertaken to improve the OTC adsorption of biochar through the increase in hydroxyls on biochar [17]. Additionally, nano zero-valent iron (nZVI) has been composited to biochar to enhance OTC removal through degradation followed by adsorption. Environmental remediation applications of nZVI-BC composites applied to different aspects have shown the reactivity of nZVI composites and oxidative removal capacity of contaminants, such as nitro and chlorinated organics, heavy metals, and antibiotics [18,19]. The OTC adsorption capacity of nZVI-pickling biochar is 196.70 mg/g, and the degradation has also been found through mass spectrometry analysis [20]. The OTC can be partially converted through the Fenton reaction and further degraded to smaller molecules through a ring-opening reaction [21].
However, the pH range of pharmaceutical wastewater is 3.9 to 9.2, which is not suitable for the application of nZVI-biochar, particularly under harsh acidic conditions [22,23]. The coated nZVI would be dissolved and inactivated under such conditions. In addition, OTC has three acid dissociation constants (pKa1 = 3.30, pKa2 = 7.68, pKa3 = 9.69), which can be dissociated to multiple moieties, including tricarbonyl system, phenolic diketone system, and dimethylammonium group under different pH conditions [24,25,26]. The adsorption capacities of these moieties on biochar would be various due to the electrostatic effect. Although previous studies have focused on OTC adsorption and degradation of iron-modified biochar, there has been no report on such OTC removal under harsh acidic conditions due to the instability of iron. Depending on the pKa = 5.6, vitamin B6 can exist in several different stable forms, which help improve the stability of the material [27]. It provides a solution for developing stable iron-modified biochar. In addition, the mechanisms of adsorption and degradation of OTC moieties in biochar suspension are not clear under harsh acidic conditions. Therefore, as an extension, B6-mediated biochar with nZVI coating (nZVI/[email protected]) is developed to remove OTC through adsorption and degradation under harsh acidic conditions. In detail, the objective of this study entails (1) fabrication of nZVI/[email protected] through liquid-phase reduction, (2) investigation of chemical properties of nZVI/[email protected] through SEM, FTIR, and X-ray analyses, (3) examination of OTC removal performance of nZVI/[email protected], and (4) exploration of OTC removal mechanism through analyses of adsorption capacities of raw biochar (BC) and B6-coated BC([email protected]) and degradation pathways in nZVI/[email protected] suspension. The results of this study are expected to help vigorously remove emerging antibiotics through the application of stable modified biochar materials in the real world.

2. Materials and Methods

2.1. Materials

Oxytetracycline Hydrochloride (OTC-HCl ≥ 95%) was purchased from Aladdin Chemistry (Shanghai, China). Vitamin B6, NaBH4, and FeSO4·7H2O were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). These reagents were above analytical grade.

2.2. nZVI/[email protected] Fabrication

Biochar was derived from corn straw particles pyrolyzed at 350 °C for 2 h (filled in a crucible with a lid in a muffle furnace), which was ground and passed through a 200-mesh sieve. The raw biochar was then washed with HCl (0.1 mol/L) to remove acid-soluble impurities at 200 rpm for 2 h at room temperature. Collected biochar was further washed with deionized water repeatedly until the pH of leachate was about neutral. These biochar particles were dried at 60 °C for 12 h (i.e., BC).
The BC (0.5 g) was immersed into the B6 solution (0.25 wt%) and stirred for 24 h at room temperature in the dark. These particles were washed and dried in a vacuum oven at 40 °C for 12 h (i.e., [email protected]). The nZVI/[email protected] was fabricated based on the [email protected] through the liquid-phase reduction method. In detail, the [email protected] was further immersed into 250 mL 0.054 M FeSO4 solution with magnetic stirring, and then 250 mL 0.138 M NaBH4 solution was added by drops [27]. After 30 min reaction, the nZVI could be synthesized according to the following equation:
FeSO4 + 2NaBH4 + 6H2O = Fe0↓ + 2B(OH)3 + 7H2↑ + Na2SO4
The collected biochar was washed several times with deionized water and absolute ethanol 3 times, then dried in a vacuum drying oven under 40 ℃ for 12 h (i.e., nZVI/[email protected]). Then, this nZVI/[email protected] was stored in the centrifugal tube (kept at a low O2 concentration) for further experiments. The complete preparation process was shown in Figure 1. In order to compare with nZVI/[email protected], nZVI was also coated on BC to form nZVI-biochar composites (i.e., [email protected]) through the aforementioned liquid-phase reduction method.

2.3. Biochar Characterization

Functional groups of nZVI/[email protected] were analyzed by a Fourier transform infrared spectroscope (EQUINOX55, Bruker, Coventry, Germany) with the KBr pellet technique. Surface morphologies of BC, [email protected], [email protected], and nZVI/[email protected] were observed by scanning electron microscopy (SUPRA55, ZEISS, Heidenheim, Germany). The surface element properties of the nZVI/[email protected] before and after the experiment were acquired by an X-ray photoelectron spectrometer (K-Alpha+, Thermo Scientific, USA) with a source of Al K-alpha.

2.4. OTC Removal Performance

Generally, positively charged (OTCH3+) and zwitterionic forms (OTCH20(+/−)) are half/half dominants in OTC solution at a pH level of 3.5. To identify the contributions of adsorption and degradation of nZVI/[email protected], this pH was chosen in this study. The OTC removal experiment was conducted in multiple 50 mL polyethylene centrifuge tubes containing nZVI/[email protected] suspension composed of 500 mg/L OTC and 1.0 g/L nZVI/[email protected] under a pH level of 3.5. These tubes were shaken at 200 rpm at room temperature in the dark. OTC concentrations were analyzed at different times (5 min, 10 min, 15min, 20 min, 30 min, 45 min, 60 min, 90min, 120 min, 360 min, and 720 min), which were measured by HPLC (Thermo Fisher Scientific, Thermo, CA, USA) coupled a UV detector (355 nm) with a C18 column (150 mm × 4.6 mm × 5 µm). The detailed method is described in the SI.
The kinetics were modelled through pseudo-first order and pseudo-second order models, and their coefficients were estimated for analyzing the removal mechanism of OTC in nZVI/[email protected] suspension.

2.5. OTC Adsorption on BC and [email protected]

OTC adsorption on BC and [email protected] was undertaken to investigate adsorption contribution and mechanism in terms of non-degradation assumption on biochar without nZVI. Different initial OTC concentrations from 100 mg/L to 500 mg/L were carried out until they reached the adsorption balance with 1.0 g/L BC or [email protected] All experiments were conducted in the dark at 25 °C with pH = 3.5. In addition, adsorption kinetics were also conducted at different time intervals (5 min, 10 min, 15 min, 20 min, 30 min, 45 min, 60 min, 90 min, and 120 min). The adsorption isotherms were fitted by three thermodynamic models: Langmuir isotherm, Freundlich isotherm, and Temkin model, while the kinetics were modelled through pseudo-first-order (a), pseudo-second-order, and intramolecular diffusion models. The model equations fitted in a linear relationship were given in the supporting information. Parallel blank experiments were conducted under the same conditions.

2.6. OTC Degradation Pathway Analysis

Potential degradation pathways of OTC in nZVI/[email protected] suspension were analyzed through HPLC-MS analysis. After the removal tests, degradation products were determined by an HPLC-system (Ultimate 3000, Thermo, Waltham, MA, USA) liquid chromatography with was for OTC and its hydrolysis and degradation products, the column Agilent ZORBAX SB-C18 column (150mm × 4.6mm × 5 μm) column was used to separate OTC and its hydrolysis and degradation products. The mobile phases were acetonitrile (A) and water (formic acid adjusted pH of 2.5, B), flow rate: 0.8 m L/min. High-resolution mass spectrometry (LCQ Fleet, Thermo, Waltham, MA, USA) was equipped with an electrospray ionization source (ESI source). OTC and its hydrolysis and degradation products were identified with positive ion monitoring mode analysis, full scan mode set with a scanning range: 50 to 500 Da, ionization voltage: 4 KV, and capillary temperature: 335 °C. The data of HPLC-MS was analyzed on Thermo Xcalibur.

3. Results and Discussion

3.1. Characterization of Biochar Composite

As shown in Figure 2a, it can be observed that BC has a porous, smooth surface on the fractured layer of biochar with a sharp and flaky structure. Scaled to 200 nm, BC presented a simple and blocky form and had little spatial structure. Compared with BC, Figure 2b shows that the structure of B6-coated biochar had a rough surface, which may indicate that the B6-coating had successfully changed the surface, perhaps because of the B6 adherence to the biochar. The nanoscale nZVI particles observed in Figure 2c indicated that the surface of BC was embedded with nZVI. The aggregation was observed at 500 nm. However, on nZVI/[email protected], little nZVI aggregation appeared. In Figure 2d, nZVI/[email protected] shows the fine nanosheet structure and small nZVI particle balls, which were well dispersed and attached to [email protected], indicating that the B6 as an intermediate material can effectively improve the biochar properties as a substrate and provide a wider surface attachment point. The successful B6-layer coating and immobilization of [email protected] tend to reduce the particle size of zero-valent iron particles and the aggregation issue onto unmodified substrates such as raw biochar.
Figure 3 shows the FTIR spectra of BC and nZVI/[email protected] The differences in the chemical structure of these materials were presented in the range of 400 to 4000 cm–1. Typically, the broad peak around 3430 cm–1 was attributed to the O-H [28], and the band around 3550 cm–1 was amplified as O-H(free) [29]. The band around 2923 cm–1 was assigned to aliphatic hydrocarbon C-H. In addition, the peaks at 1630 cm–1 and 1430 cm–1 were attributed to C=O vibration (diaryl ketone, quinone, and/or carboxyl groups), while that at 1560 cm–1 was aromatic C=C. The band at wavenumber near 1050 cm–1 was assigned to the C-OH bending vibration, implying the existence of large numbers of hydroxyl (-OH) and carboxylate (-COOH) groups on the BC. Compared with the original BC, [email protected] decreased the transmittance at 3430 cm–1, indicating that the O-H and N-H bands of B6 succeeded in sticking in the process. In addition, nZVI/[email protected] exhibited that the intensities of the peak at 3430 cm–1 and band of 1000 to 1500 cm–1 were lower than those of BC due to the deposition of iron on carbon spheres. Besides, the peaks around 800 cm–1, 781 cm–1, and 866 cm–1 are intended to be the aromatic C-H [29], illustrating that both the B6-coating and nZVI attaching processes reduced the amount of aromatic carbon. There were differences between [email protected] and nZVI/[email protected] in peaks at 1035 and 1430, which are supposed to be the C=C, C=O, and C-O bands. These differences indicated the formation of C-O–Fe bonds on nZVI/[email protected]
Figure 4a shows the XRD patterns of BC, [email protected], [email protected], and nZVI/[email protected] For BC and [email protected], there was a strong peak (2θ = 26.69°) and two small peaks (2θ = 20.77° and 27.79°), which indicated that the amorphous carbon matrix was the main structure. A specific Fe peak at 42.26° was detected obviously in [email protected] and nZVI/[email protected], indicating the presence of nZVI. In addition, the B6-coating process decreased the intensity of this peak in nZVI/[email protected] Figure 4b–d show the surface element and high-resolution Fe XPS spectra. Peaks of Fe2p were observed in both [email protected] and nZVI/[email protected] in Figure 4b. As shown in the high-resolution Fe spectra (Figure 4c,d), there are two dominant peaks of Fe2p1/2 and Fe2p3/2 in terms of Fe(II) and Fe(III) in both [email protected] and nZVI/[email protected] except for Fe0. In addition, the intensities of Fe(II) and Fe(III) presented more similarity in individual orbitals of nZVI/[email protected] than those of [email protected], which were attributed to improvement in stabilization and distribution of Fe through B6 loading. After the OTC removal experiment, peaks of Fe0 could hardly be observed in both [email protected] and nZVI/[email protected], while peaks of Fe(II) were increased significantly in these materials. This indicated that Fe reduction was a crucial contribution to OTC degradation.

3.2. OTC Removal Performance

The removal efficiencies were displayed as the Figure 5 shown in the following order: nZVI/[email protected] > [email protected] > [email protected] > BC. The 24-h OTC removal efficiencies in BC and [email protected] suspensions only approached 16.51% and 17.49%, respectively, and no significant change was observed between 2 h and 24 h. It is assumed that there is only adsorption on BC and [email protected], and the adsorption equilibrium was reached after 2 h. Without the iron modification, the adsorption capacity of [email protected] compared to BC was slightly increased from 81.38 mg/g to 85.64 mg/g only via B6-coating. Meanwhile, the 24-h efficiencies in nZVI/BC and nZVI/[email protected] suspensions approached 98.40% and 98.55%, illustrating that OTC had been almost completely removed. Moreover, the 5-min OTC removal efficiencies in [email protected] and nZVI/[email protected] suspensions were 52.25% and 59.05%, respectively. Such dramatically rapid removal rates indicate the great removal performance, which may be due to the strong oxidation of ·OH radicals generated through the Fenton reaction triggered by iron on biochar. Moreover, 98.28% of OTC was removed within 60 min in the nZVI/[email protected] suspension. It may be attributed to the amino and hydroxyl groups of B6 as a mediate between BC and nZVI, improving the stability of nZVI attachment and generating more active sites on nZVI/[email protected] for OTC removal than [email protected] Furthermore, the reusable performance of [email protected] and nZVI/[email protected] were also exanimated through 4-time cycles of OTC removal (Figure S1). Although the removal efficiencies of these two materials declined after multiple cycles, nZVI/[email protected] exhibited a significantly higher removal capacity than [email protected] due to the B6-associated stabilization of nZVI.

3.3. Adsorption Contribution and Mechanism of BC and [email protected]

In order to investigate the assumption that adsorption is the critical effect of BC and [email protected], modelling analyses were based on the batch OTC adsorption experiments on BC and [email protected] to fitting adsorption isotherm and kinetic processes non-degradation assumption on biochar without nZVI. Figure 6 shows the adsorption isotherms of OTC on BC and [email protected], while the isotherm parameters are shown in Table S1. [email protected] exhibited a stronger adsorption capacity than BC.
According to the modelling results, the adsorption on BC fitted the Freundlich isotherm (Kf = 11.945 mg/g·(L/mg)1/n, n = 2.235, R2 = 0.9693) and Temkin one (KT = 0.6747 L/mg, B = 11.158 J/mol, R2 = 0.9699), while that on [email protected] fitted Langmuir isotherm (Qm = 52.7869 mg/g, Kl = 0.0349 L/mg, R2 = 0.9973) better. The heterogenic surface of BC provides diverse surface tension and surface energy which help adsorb OTC [30]. While B6 coating converted to homogenous surface on [email protected], leading monolayer adsorption on [email protected] In addition, B6 coating also facilitated OTC physical adsorption through hydrogen bonds and Van der Waals’ force [31,32].
On the other hand, the adsorption kinetics are shown in Figure 7, and the kinetic parameters are described in Table S3. The intramolecular diffusion model was the best for BC (Kint = 6.0202 mg/(mg·min1/2), C = 17.182 mg/g, R2 = 0.9885), indicating that the adsorption behaviour on the raw biochar mainly depended on physical adsorption. Due to the porous structure of biochar, a long diffusion time was required for OTC to achieve inner adsorption sites to reach saturation. While pseudo-second order was the best model for [email protected] (K2 = 0.0104 g/(mg·min), Qe = 90.09 mg/g, R2 = 0.9928), indicating that there is chemical reaction between [email protected] and OTC. The alkaline pyridine ring of B6 on biochar promoted such a reaction with acidic groups of OTC. In addition, ionic OTC may also facilitate the adsorption on B6-associated hydrophilic biochar. Hence, the interactions between [email protected] and OTC may be electrostatic interaction, acid–base reaction, and ionic exchange.

3.4. Degradation Contribution and Mechanism of [email protected] and nZVI/[email protected]

According to the kinetic modelling pattern in Figure 8 and the kinetic parameters shown in Table S4, the degradation kinetics of both [email protected] and nZVI/[email protected] follow the pseudo-second-order model rather than the pseudo-first-order one, indicating that the chemical reaction between the modified materials and the OTC is the main effect. In addition, there was no significant difference in OTC removal capacities of [email protected] and nZVI/[email protected] during 700 min. Meanwhile, the nZVI/[email protected] presented a faster reaction rate with a larger K2 than [email protected], which is attributed to better stabilization and distribution of nZVI on biochar through B6 coating.
In this study, OTC was rapidly removed in the first 5 min and showed apparent equilibrium at 120 min. The MS spectra of OTC degradation products are exhibited in Figure S2. The OTC peak sharply decreased or disappeared with the increase in the reaction time in nZVI/[email protected] suspension, and three anions with m/z of 477, 430, and 192 were observed at a relatively high level, which may be the main intermediate products. While dominant ionic fragments were m/z of 460 and 461 (two forms of OTC) in blank, BC, and [email protected] suspensions, indicating that there is not any degradation product in these systems.
Based on the MS spectrum, three pathways of OTC degradation in nZVI/[email protected] suspension are proposed in Figure 9. In pathway A, OTC was metabolized A1 (m/z 477) and A2 (m/z 494), corresponding to a previous study [33]. In this pathway, Fe0 reacted with the O2 in suspension to generate H2O2, as shown in the following equation:
Fe0 + O2 + 2H+ = Fe2+ + H2O2
Then, the OH radicals were generated from H2O2 and attacked benzene ring on OTC to form phenolic hydroxyl groups, which was similar to the Fenton reaction.
While pathways B and C were opening processes of benzene ring, OTC was metabolized to B1 (m/z of 430) through conversion of dimethylamine to amine and C1 (m/z of 383) through loss of hydroxyl, methyl, carboxyl, and amide groups to initialize pathways B and C, respectively [20,33]. On the one hand, B1 (m/z of 430) converted into B2 (m/z of 391) and B3 (m/z of 360) in a two-step reaction of amide loss, followed by a one-step ring-opening reaction, and B4 (m/z of 322) was generated. Then, hydroxyl groups were detached from the ring to form B5 (m/z of 276). Finally, the B6 (m/z of 192) was generated through a ring-opening reaction. On the other hand, the OTC is converted into C1 (m/z of 383) through the loss of a hydroxyl group and an amide group. C2 (m/z of 357) was further generated through the conversion of dimethylamine to amine. Consequently, the amine group was off from the carbon ring to form C3 (m/z of 342). Finally, C9 (m/z of 163) was produced through multiple reactions, including de-hydroxylation, ring-opening, reduction, addition, demethylation, and alkylation. In nZVI/[email protected] suspension, the OTC was adsorbed on biochar particles, followed by multiple chemical reactions initiated by nZVI on the biochar particles, particularly in acidic conditions.

4. Conclusions

The composite of nZVI/[email protected] was successfully synthesized via liquid-phase coating and reduction reactions. The B6 coating process improved the distribution and stabilization of ZVI nanosheets on biochar due to the mediation of the B6-associated hydrophilic layer between ZVI and biochar. This composite exhibited excellent OTC removal performance through adsorption and degradation. Compared to BC, the adsorption capacity of [email protected] was slightly increased from 81.38 mg/g to 85.64 mg/g through the B6-coating process. In the removal test, the 5-min OTC removal efficiencies in [email protected] and nZVI/[email protected] suspensions were 52.25% and 59.05%, while in BC and [email protected], they were limited to 5.61% and 8.54%. The distinct difference illustrated the existence of nZVI on biochar strongly improving the reactivity of nZVI/[email protected] Moreover, 98.28% of OTC was removed within 60 min in the nZVI/[email protected] suspension, indicating an excellent removal effect. Through modelling analysis, the adsorption on BC fitted the Freundlich and Temkin isotherms, while that on [email protected] fitted the Langmuir isotherm better. At the same time, the intramolecular diffusion model was the best for BC, while the pseudo-second-order was best fitted on [email protected] Based on HPLC-MS analyses, there were three pathways proposed for OTC degradation by nZVI/[email protected] B6 as a mediate helped improve stability and distribution of nZVI on biochar, providing active sites on biochar for OTC degradation through chemical oxidization and reduction, ring-opening, etc., which facilitates nZVI/[email protected] for OTC removal through adsorption and degradation under acidic conditions. The OTC can not only be captured on biochar but also be metabolized to achieve removal from aquatic systems. The degradation products were consistent with the previous literature, which may still be very toxic and require subsequent treatment. The developed nZVI/[email protected] is a modified, high-efficiency and cost-saving material which is expected to be applied to pharmaceutical wastewater treatment.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/w14172734/s1. The relevant adsorption isotherm models and kinetic models was referred to the previous articles [17,34]. Figure S1: The removal rate in four cycles OTC exposure experiment by nZVI @BC and nZVI/[email protected]; Figure S2: Dissolution of iron of nZVI/[email protected] and [email protected] in acid conditions (pH = 3.5) exposure cycles under a) pure deionized water adjusted by hydrochloric acid and b) 500 mg/L OTC solutions; Figure S3: The HPLC-MS spectra of (a) OTC blank and residues in suspensions of (b) BC after 120 min, (c) [email protected] after 120 min, (d) [email protected] after 5 min, (e) nZVI/[email protected] after 5 min, (f) [email protected] after 120 min and (g) nZVI/[email protected] after 120 min; Table S1: Zeta potential of different composites in aqueous water; Table S2: The isotherm parameters of oxytetracycline adsorption onto BC and [email protected] at 25 °C and pH = 3.5; Table S3: The kinetic parameters of oxytetracycline adsorption onto BC and [email protected] at 25 °C and pH = 3.5; Table S4: The kinetic parameters of oxytetracycline removal onto [email protected] and nZVI/[email protected] at 25 °C and pH = 3.5.

Author Contributions

Investigation, Y.X.; Data curation, Y.X.; methodology, P.Z. & S.R.; validation, P.Z.; formal analysis, Y.X.; writing—original draft preparation, Y.X.; writing—review and editing, P.Z., J.S., & S.R.; visualization, J.S.; supervision, P.Z.; project administration, J.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy.

Acknowledgments

We are very grateful for the helpful input from the editor and anonymous reviewers.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Flow chart of nZVI/[email protected] fabrication.
Figure 1. Flow chart of nZVI/[email protected] fabrication.
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Figure 2. SEM imagines of biochar materials (a) BC, (b) [email protected], (c) [email protected], and (d) nZVI/[email protected]
Figure 2. SEM imagines of biochar materials (a) BC, (b) [email protected], (c) [email protected], and (d) nZVI/[email protected]
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Figure 3. FTIR spectra of biochar materials: BC, [email protected], [email protected], and nZVI/[email protected]
Figure 3. FTIR spectra of biochar materials: BC, [email protected], [email protected], and nZVI/[email protected]
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Figure 4. X-ray analyses of biochar materials (a) XRD, (b) XPS, (c) high-resolution Fe spectra of [email protected] and nZVI/[email protected] before OTC removal application, and (d) high-resolution Fe spectra of [email protected] and nZVI/[email protected] after OTC removal application.
Figure 4. X-ray analyses of biochar materials (a) XRD, (b) XPS, (c) high-resolution Fe spectra of [email protected] and nZVI/[email protected] before OTC removal application, and (d) high-resolution Fe spectra of [email protected] and nZVI/[email protected] after OTC removal application.
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Figure 5. OTC removal performance of biochar materials at 25 °C, pH = 3.5.
Figure 5. OTC removal performance of biochar materials at 25 °C, pH = 3.5.
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Figure 6. The equilibrium of (a) original adsorption capacity and adsorption isotherm modelling (b) Langmuir isotherm, (c) Freundlich isotherm, and (d) Temkin isotherm of OTC on BC and [email protected]
Figure 6. The equilibrium of (a) original adsorption capacity and adsorption isotherm modelling (b) Langmuir isotherm, (c) Freundlich isotherm, and (d) Temkin isotherm of OTC on BC and [email protected]
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Figure 7. Adsorption kinetic modelling including (a) pseudo-first order, (b) pseudo-second order, and (c) intramolecular diffusion of OTC on BC and [email protected]
Figure 7. Adsorption kinetic modelling including (a) pseudo-first order, (b) pseudo-second order, and (c) intramolecular diffusion of OTC on BC and [email protected]
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Figure 8. Degradation kinetic modelling (a) pseudo-first order, and (b) pseudo-second order of OTC on [email protected] and nZVI/[email protected] (initial concentration = 500 mg/L, adsorbent concentration = 1.0 g/L, temperature = 25 °C, pH = 3.5).
Figure 8. Degradation kinetic modelling (a) pseudo-first order, and (b) pseudo-second order of OTC on [email protected] and nZVI/[email protected] (initial concentration = 500 mg/L, adsorbent concentration = 1.0 g/L, temperature = 25 °C, pH = 3.5).
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Figure 9. Proposed degradation pathways of OTC in nZVI/[email protected] suspension.
Figure 9. Proposed degradation pathways of OTC in nZVI/[email protected] suspension.
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Xin, Y.; Zhang, P.; Shen, J.; Ren, S. Development of Vitamin B6-Mediated Biochar with Nano Zero-Valent Iron Coating for Oxytetracycline Removal through Adsorption and Degradation under Harsh Acidic Conditions. Water 2022, 14, 2734. https://doi.org/10.3390/w14172734

AMA Style

Xin Y, Zhang P, Shen J, Ren S. Development of Vitamin B6-Mediated Biochar with Nano Zero-Valent Iron Coating for Oxytetracycline Removal through Adsorption and Degradation under Harsh Acidic Conditions. Water. 2022; 14(17):2734. https://doi.org/10.3390/w14172734

Chicago/Turabian Style

Xin, Yuelin, Peng Zhang, Jian Shen, and Shaojie Ren. 2022. "Development of Vitamin B6-Mediated Biochar with Nano Zero-Valent Iron Coating for Oxytetracycline Removal through Adsorption and Degradation under Harsh Acidic Conditions" Water 14, no. 17: 2734. https://doi.org/10.3390/w14172734

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