Biomass waste-derived porous carbon efficient for simultaneous removal of chlortetracycline and hexavalent chromium.

The simultaneous removal of mixed containments of antibiotics and heavy metals is still a big challenge in wastewater treatment. Herein, we report the successful synthesis of N-doped porous carbon (abbreviated as NC) from straw waste through the Maillard reaction to activate sp3-sp2 conversion efficient for the simultaneous removal of chlortetracycline (CTC) and hexavalent chromium (Cr(VI)). In 200 min, 96.9% of Cr(VI) was reduced into Cr(III) and 93.1% of CTC was oxidatively degraded. Reactive substances (e.g., h+, e-1, ⋅OH, and ⋅O2 -) were verified for the photocatalytic reactions. Besides, the possible degradation intermediates of CTC were analyzed with ultra performance liquid chromatography-mass spectrometry (UPLC-MS/MS), and the mechanism of photocatalytic degradation of CTC was then proposed. The synthesized bifunctional NC materials could also be applied for the similar system; this will open the door for promising practical applications.

Introduction

The misuse of antibiotics and the arbitrary release of the sewage containing heavy metal ions have deteriorated the case of n class="Chemical">water quality (Ye et al., 2019; Zhou et al., 2019; Sun et al., 2020; Yang et al., 2020), which severely menaces the subsistence of aquicolous and terrestrial living body. Chlortetracycline (CTC) with the formula of C22H23ClN2O8 is a typical antibiotic applied in treating many bacterial and rickettsial infections, which is also frequently utilized in the area of livestock and fish due to its low cost and high efficiency (Wu et al., 2015; Wang et al., 2017, 2020a; Huang et al., 2020). However, due to the very low metabolism and high water solubility of CTC, long-term or a large number use will make an accumulation of CTC in the organism, resulting damage for human or animals (Nguyen et al., 2019; Weng et al., 2019; Zhao et al., 2020; Zhong et al., 2020). Also, because of the random discharge of industrial wastewater, hexavalent chromium (Cr(VI)) has become a main source of heavy metal ions in wastewater pollution (Jin et al., 2016; Zhao et al., 2019; Li et al., 2020). Compared with Cr(III), Cr (VI) with very high toxicity is easy to accumulate in the living body, causing illness or even death in human or animals (Sun et al., 2013; Thacher et al., 2016). More seriously, CTC can combine with Cr(VI) to generate a more complex pollutant that is hard to be degraded (Zhao et al., 2019). Chromium is an essential trace element for animal breeding (promoting metabolism and improving survival rate) (Levina et al., 2016). However, excessive use is extremely harmful, and the abuse of antibiotics in the breeding process adds fuel to the fire. Therefore, how to effectively remove aquaculture wastewater containing both Cr(VI) and CTC is a crucial step in improving the water quality.

More than 10 billion tons of straw-based cellulosic biomass is produced annually in China. Most of them are burned or discarded, generating a large amount of waste and environmental pollution. Rational utilization of straw waste to fabricate carbon materials offers the benefits of abundant renewable sources, eco-friendly property, rich fun class="Chemical">nctional groups, and good stability (Hu et al., 2019; Wang et al., 2019; Chen et al., 2020; Huang et al., 2021). So far, carbon-based photocatalysts with the improved photocatalytic activities have been frequently studied (Li et al., 2019a; Huang et al., 2021). With a typical porous structure, N-doped carbon exhibits a large surface area, much improved electrical conductivity, and rich reaction active sites (Achour et al., 2014; Chen et al., 2020). In addition, a number of studies have shown that the introduction of N into porous frameworks promotes the electron migration and improves the adsorption of pollutants, thus significantly boosting the removal efficiency (Nie et al., 2019; Wang et al., 2020a; Zhang et al., 2020a). Moreover, rich functional groups of N-doped C materials enhance the interaction between semiconductor catalysts and pollutants to enhance the stability (Lu et al., 2004; Wang et al., 2020b). Albeit C-based materials and the other catalysts (e.g., MoC/C3N4) have made great progress (Huang et al., 2020; Wang et al., 2020a; Lu et al., 2004), it is still a big challenge to rationally design higher efficient, visible light photocatalysts with multifunctional roles in the photocatalytic oxidative degradation of CTC and removal of Cr(VI).

In this study, we have successfully fabricated two-dimensional N-doped hierarchically porous n class="Chemical">carbon (NC) using a facile hydrothermal strategy (Figure 1 and supplemental information) from biomass waste straw (cellulose and corn cobs; Figure S1). These bifunctional NC materials are efficient for the selective removal of the mixed CTC and Cr(VI) with high efficiency. The catalytic system has several advantages: (i) high specific surface area as well as hierarchically porous structure can facilitate the contact and provides more access to the internal active space; (ii) the utilization of biomass waste offers a sustainable route to fabricate carbon materials and N doping can improve the electronic conductivity, increasing the amount of active sites; (iii) different types of nitrogen species (e.g, pyridinic-, pyrrolic-, and graphitic-N) can facilitate the adsorption capacity of CTC and Cr(VI) through hydrogen bonding and π-π stacking interactions. (iv) NC materials exhibit high efficiency and stability for the simultaneous removal of CTC and Cr(VI).

Figure 1

Schematic of preparing two-dimensional N-doping porous carbon (NC).

Results and discussion

Characterizations of the as-prepared photocatalyst

X-ray diffraction (XRD) patterns of the synthesized NC from cellulose and corn stalk are shown in Figures 2A and 2B, respectively. The broad peak position located at 2θ of ∼23° and the weak peak sat at 2θ of ∼43° can be assigned to the degree of stacking order of the layered n class="Chemical">carbon structure (002) and ordered hexagonal carbon structure (100) planes, respectively. This indicated the low crystallinity degree and the presence of amorphous carbons. Besides, the weak peak at 17.5° was highly possible due to the formation of C3N4 (Wang et al., 2019; Hu et al., 2020). Fourier transform infrared spectroscopy (FTIR) spectra were then applied to explore the chemical bond and functional groups on the surface. The fabricated both NC samples contain rich oxygen functional groups. Figures 2C and 2D showed that the broad peak at 3300–3550 cm−1 was O–H stretching from the water molecules adsorbed and phenolic hydroxyl. Peaks at 1060–1170 cm−1, 1380 cm−1, 1620–1720 cm−1, and 2350 cm−1 were the vibrations induced by C-O, O=C-O, C=O, and O=C=O bonds, respectively (Huang et al., 2010; Yan et al., 2013; Cheng et al., 2019), suggesting sp2 -hybridized carbon in oxygen functional groups. Besides, NC materials fabricated from corn cobs exhibit weak peaks at 1320 cm−1 and 1434 cm−1 from C-N vibration, confirming that the N-(C)3 structure existed.

Figure 2

FTIR of NC synthesized from (C) cellulose and (D) corn cobs.

The Brunauer-Emmett-Teller (BET) surface area and porosity information of the synthesized NC samples are analyzed by n class="Chemical">N2 adsorption-desorption tests (Figure 3). The BET specific surface area of NC from cellulose is calculated to be ∼178 m2/g. In comparison, the NC synthesized from corn cobs has much larger surface area of ∼325 m2/g. This was possible as the plant itself has a naturally optimized multi-channel structure and abundant functional groups, and at the same time, it promotes pore formation with the aid of a KOH activator (Li et al., 2014). Thus, a porous carbon material with a larger specific surface area was obtained. The isotherm curve of NC synthesized from corn cobs shows the typical IV curve with a hysteresis loop, confirming that the NC possesses a number of mesoporous structures which existed in the NC samples. Pore size dispersion calculated from the Barrett-Joiner-Halenda method indicates a relatively narrow distribution (mainly around 5 nm). The synthesized NC materials with large surface area and high porosity are considered as attractive candidates for adsorption or photocatalysis. Besides, the large surface area and pore structure are beneficial for electrolyte permeation and enhancement of ion diffusion, improving the photocatalytic activities.

Figure 3

N2 adsorption-desorption isotherm curves and pore size dispersion of NC from (A, B) cellulose and (C, D) corn cobs.

Raman spectra were then utilized to further study the defect structure of N-doped n class="Chemical">carbon samples. As shown in Figures 4A and 4B, the clear D and G bands are observed. It can be seen that both D band peak (∼1345 cm−1) and G band peak (∼1593 cm−1) are presented in the samples. The D band can be assigned to the disorder originating from sp3 carbons which indicates successful introduction of defects by N doping. The G band was associated with a hexagonal carbon structure, and the intensity ratio of the D band (∼1345 cm−1) to the G band (∼1593 cm−1) (ID/IG) was around 1.10 which was very similar to that of graphene oxide reported by the literature (Tang et al., 2015; Wang et al., 2019). In addition, Raman spectrum (Figure 4B) indicates that NC fabricated from corn cobs has lots of defects, which is likely due to the internal structure of corn cobs. The distance between D and G bands of NC fabricated from corn cobs (Figure 4B) has larger distance than that of the material prepared from cellulose (Figure 4A). X-ray photoelectron spectroscopy (XPS) analysis was further carried out to investigate the covalent state of N and C elements in the NC samples (Figures 4C and 4D). The N1s spectra hold two peaks (398.6 ± 0.1 and 400.3 ± 0.2 eV), assigning to pyridinic N (28.8 at%) as well as pyrrolic N (14.7 at%) and corresponding to C=N-C and N-(C)3, respectively. This information suggested that the nitrogen atoms were successfully doped into the porous carbon and N-(C)3 was dominant compared to the ratio of these two structures. Carbon and nitrogen have similar atom diameters, favoring the atom replacement. Besides, pyridinic N and pyrrolic N are often reported as active centers for the photo-enhanced adsorption. Therefore, the high amount of pyridinic N and pyrrolic N in NC samples synthesized from carbon cobs could contribute to the desirable photocatalytic activities.

Figure 4

Raman spectra and high-resolution N 1s XPS analysis spectra of NC fabricated from (A, C) cellulose and (B, D) corn cobs.

The surface morphology of the two NC samples is shown in Figure S2. Scanning electron min class="Chemical">croscopy (SEM) images show the interconnected porosities existing in two carbon samples. In Figures S2A and S2B, samples prepared using cellulose showed clear aggregation of layers, while in Figures S2C and S2D, NC samples synthesized from corn cobs presented the enlarging porosities. The pore size ranged around tens of nanometers. As previously reported (Dong et al., 2021; Wang et al., 2019), carbon particles would gradually fuse together and then form larger aggregates at the pyrolysis temperature. The typically textural structure of NC samples is studied by transmission electron microscopy (TEM), as shown in Figure 5. The obtained NC exhibits large porosity, and the pore sizes ranged from dozens to hundreds of nanometers. Figure 5A displays the layers with the clear aggregation, and the amorphous structure was observed at high magnification (Figure 5B). The typical amorphous carbon existed as the distorted lattice fringes, in which there were plentiful micropore structures. Figure 5C shows the thin carbon layers with wrinkles. The higher magnification TEM image (Figure 5D) shows the interconnected porosity that is made of abundant 2–5-nm mesopores linked by 3–5 graphitic layers with the obscure worm-like fringes. These porous structures can not only provide high specific surface area but also offer a 3D interconnected conductive substance (matrix) which is beneficial for ion and electron transport.

Figure 5

TEM images of NC materials fabricated from (A and B) cellulose and (C and D) corn cobs.

Photocatalytic reduction of Cr (VI) using NC fabricated from cellulose

The experimental design was performed using analysis of variance (Yan et al., 2013; Brown and Wright, 2014; Zarrinbakhsh et al., 2014; Li et al., 2019b). The details of model parameters are shown in supplemental information. Important reaction parameters containing two-factor interactions can be verified through the measured p values. Besides, the gaps between the predictions and actual values can be generated to show the quality of the model. To better evaluate many reaction parameters to identify the key factor and their interactions, a two-level factorial model was utilized according to the previous tests. In the two-level factorial design, one upper and one lower value of each factor was chosen: initial con class="Chemical">ncentration of Cr(VI), catalyst amount, and intensity of visible light. The actual composite design was shown in Figure S3, and the actual experimental runs were shown in Table S2.

Based on the 20 runs, Figure 6 shows the experimental values compared with the theoretically predicted data. A very small gap between predicted and practical values was observed, which indicated that the model can predict the experiment very well. Figure 7 describes the interaction of two factors on the removal efficiency of Cr(VI). Figures 7A and 7B show the influence of the initial concentration of Cr(VI) and catalyst amount on the total removal efficiency. When the light intensity was 30 mW/cm2, the removal amount increased when the catalyst amount decreased (Figure 7A). Besides, the initial concentration has more significant effect on the total removal efficiency of Cr(VI). The contour map is an approximate ellipse, indicating that the interaction of the initial concentration of Cr(VI) and catalyst amount is strong on the total removal efficiency. Figures 7C and 7D display the effect of the initial concentration of Cr(VI) and light intensity on the removal efficiency. When the catalyst amount was 15 mg, the light intensity can promote the reaction and enhance the removal of more Cr(VI). Figures 7E and 7F describe the mutual effect of catalyst amount and visible light intensity on the reaction. The model identified that the light intensity has less effect in comparison with the catalyst amount.

Figure 6

The comparison between the predicted and actually experimental values

(A and B) (A) The theory predicted the total removal capacity vs the actual values from experiments and (B) the photoremoval capacity vs the actual values.

Figure 7

Effect of (A) X1X2, (C) X1X3, and (E) X2X3 on Qtot; Effect of (B) X1X2, (D) X1X3, and (F) X2X3 on the removal efficiency of Cr(VI). Reaction note: X1: initial concentration of Cr(VI); X2: catalyst amount; X3: light intensity.

Effect of (A) X1X2, (C) X1X3, and (E) X2X3 on Qtot; Effect of (B) X1X2, (D) X1X3, and (F) X2X3 on the removal efficiency of n class="Chemical">Cr(VI). Reaction note: X1: initial concentration of Cr(VI); X2: catalyst amount; X3: light intensity.

Photocatalytic removal of Cr(VI) by NC from corn cobs

Based on the aforementioned work, we utilize the optimal conditions (e.g., 40 mg/L n class="Chemical">Cr(VI), 50 mL, pH of 2.0, 20 mg carbon catalyst, the adsorption time of 120 min, and the illumination time of 200 min) to further investigate the photocatalytic removal of Cr(VI) using the NC fabricated from corn cobs. As shown in Figure S4, it is clear to see that NC fabricated from corn cobs could efficiently remove the Cr(VI), and ∼88% was removed in 5 hr. The difference between adsorption and photoreduction was shown in Figure 8A. The concentration of Cr(VI) has clear decrease between the adsorption and photoreduction parts. The color change over time was shown in Figure 8B. At 0 min, the black color mainly from the carbon and the transparent solution was reached after 320 min.

Figure 8

Adsorption and photoreduction of Cr(VI) by NC fabricated from (A) corn cobs; (B) pictures of solution color change over time.

Simultaneous removal of CTC and Cr(VI) by NC fabricated from corn cobs

The photocatalytic evaluation of the synthesized n class="Chemical">NC fabricated from corn cobs was studied by photocatalytic oxidative degradation of CTC and photocatalytic reduction of Cr(VI). As shown in Figure S5, 6 mg/L CTC reached the adsorption-desorption equilibrium in the dark for 120 min. Before the light irradiation, adsorption experiments were performed under dark conditions, and the data were shown in Figure 9. It was clear to see that the concentrations of CTC and Cr(VI) changed after 30 min, and nearly no obvious decrease was observed after 120 min, indicating that an adsorption-desorption equilibrium reached in the dark for 120 min. It was worth noting that different concentrations of CTC and Cr(VI) showed the similar trend. The irradiated experiments of CTC and Cr(VI) solution were then continuously carried out for 200 min. Figure 9A shows the change of Cr(VI) concentration versus reaction time at pH of 2.0 in 50 mL solution using 5 mg NC catalyst. The adsorption capacity of Cr(VI) for 6 samples containing different CTC concentrations (0, 6, 8, 10, 12, 14 mg/L) using NC reached 36.3–40.4% in 120 min, indicating that the adsorption of Cr(VI) was not significantly influenced by different CTC concentrations. The photoreduction of Cr(VI) was greatly enhanced, and the concentration of Cr(VI) decreased greatly under light irradiation. When no CTC was added, the removal efficiency of Cr(VI) was 71.4% at 320 min. When the CTC concentration was increased to 6 mg/L, the efficiency increased to 87.6%. Further increasing CTC, more Cr(VI) was removed. When the CTC concentration was 14 mg/L, the efficiency reached 96.9% in 320 min. Reaction constant was then calculated to be 0.0010, 0.0015, 0.0016, 0.0017, 0.0018, and 0.0019 min−1 when the concentration of CTC was increased from 0 to 14 mg/L, suggesting that more photoreduction of Cr(VI) occurred when CTC increased. This was due to the fact that CTC would consume the photogenerated hole and enhance the separation of photogenerated hole vs electron, promoting the photoreduction of Cr(VI). Figure 9B depicted the change of CTC with reaction time at the fixed amount of Cr(VI) with the concentration of 12 mg/L. When the CTC concentration was 6 mg/L, the adsorption efficiency was 37.8%. While the CTC concentration was increased to 8, 10, and 12 mg/L, the adsorption efficiency decreased to 35.9%, 31.8%, and 21.8%, respectively, in 120 min. When light irradiation started, the various concentrations of CTC started to degrade. After 200 min, the removal efficiency was 93.1%, 81.6%, 79.6%, 68.5%, and 50.4%.

Figure 9

Simultaneous removal of different concentrations of CTC with Cr(VI) by NC fabricated from corn cobs under the conditions of pH = 2.0, 50 mL, 5 mg catalyst.

Simultaneous removal of different concentrations of CTC with n class="Chemical">Cr(VI) by NC fabricated from corn cobs under the conditions of pH = 2.0, 50 mL, 5 mg catalyst.

Photocatalytic degradation intermediates of CTC

After light irradiation, a series of intermediates could be produced as the photocatalytic degradation of CTC ocn class="Chemical">curred (Figure S6). According to previous works (Wang et al., 2020a; Zhang et al., 2020b), the decomposing pathway of CTC was divided into three parts in this work, as shown in Figure 10. In pathway I, the CTC molecule was converted to C1 (m/z 362) via a series of reactions including deamination, dechlorination, and dealcoholization. The as-produced C1 would be further degraded to C2 (m/z 318) due to the C-N bond cleavage resulting in the dimethylamino group removed. Under the attack of ⋅OH or ⋅O2-, C3 (m/z 274) and C4 (m/z 246) could further be obtained by the demethylation and deformaldehyde process of C2 (Zheng and Zhang, 2018). For pathway II, the similar degradation process to pathway I of CTC occurred to form smaller molecules (C5 m/z 318) with a tricyclic structure via the open-ring reaction, dichlorination, etc. After that, the ethyl and carboxyl of C5 are further oxidized to produce C6 (m/z 230) (Kong et al., 2020). In addition, the ketone substitution of the dimethylamino moiety in the CTC molecule was pathway III, which could form C7 (m/z 465) by this way. Subsequently, C7 was oxidized to C8 via the hydroxylation process to replace the Cl atom. Ultimately, all the above intermediate products would be decomposed to the small molecule intermediates such as (C9 m/z 194, C10 m/z 170, C11 m/z 214, C12m/z 218) and further mineralized to CO2, H2O, and other inorganic anions.

Figure 10

The degradation mechanism of CTC using NC fabricated from corn cobs.

Total organic carbon (n class="Chemical">TOC) concentration was detected after the photocatalysis process was completed. According to TOC results, 54.6% of the carbon content in the CTC antibiotic solution could be mineralized on NC catalyst under 190 min of the simulated sunlight irradiation. To study the stability of the prepared NC catalyst from corn cobs, the spent NC catalyst was recycled, washed by pure ethanol, dried in the Ar protecting oven, and then utilized for the next run. Overall, the stable performance (Figure S7) was obtained, and very slight decrease in degradation performance was mainly due to the inevitable catalyst loss during the recovery process and the potential deactivation of the catalyst surface caused by the degraded intermediate residues. The XRD pattern of the spent NC showed the similar diffraction peak at 2θ = 26° and 44° meaning the stable crystal structure of the NC (Figure S8). XPS spectrum analysis of the used NC from corn cobs illustrates that it consisted of C, O, and N. Moreover, the XPS spectrum of the NC after the reaction proved the existence of Cr (Figure S9). The N1s spectra (Figure S10) confirmed the pyridinic N and pyrrolic N with the close values of the fresh NC sample. Moreover, after the reaction, the SEM morphology of the NC still maintained obvious porous structure (Figure S11), indicating the stable structure.

Photocatalytic degradation mechanisms

In order to identify the main active substances that affect CTC degradation and n class="Chemical">Cr(VI) reduction, we used p-benzoquinone (BPQ), EDTA disodium salt dihydrate (EDTA-2Na), and tert-butanol (TBA) as quenchers for ⋅O2-, h+, and ⋅OH to perform quenching experiments. Compared with the blank control, the process of CTC removal was hindered to varying degrees after the scavengers were added (Figure S12). Among them, the BPQ exhibited the intensive inhibition of 44% for CTC degradation, which might ascribe to the large amount of ⋅O2- formed in the photocatalytic process. Moreover, both of EDTA-2Na and TBA performed the hindrance effect for CTC degradation resulting ∼16% degradation efficiency decreasing. These results indicated the existence of ⋅OH and photogenerated h+, both of which could make some contribution for CTC removal. In addition, the photogenerated holes (h+) and electrons (e−) existed in pairs. The generation of h+ was normally accompanied by e− generation, which was conductive to reducing Cr(VI) to Cr(III). The synergetic photocatalytic degradation and reduction mechanism of CTC and Cr(VI) was depicted in Figure S13.

To further confirm the reaction mechanism, electron paramagnetic resonance (EPR) was applied to prove the presen class="Chemical">nce of active substances using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as the capture agent. As shown in Figures 11A and 11B, there were no radical characteristic peak appearances without light irradiation. On the contrary, the standard quadruple and sextuple characteristic peaks of DMPO-⋅OH and DMPO-⋅O2- appeared under the visible light irradiation, in which the peak height ratio of DMPO-⋅OH was 1:2:2:1, and the main peak height ratio of DMPO-⋅O2- was 1:1:1:1. Furthermore, the peak intensity could reflect the amount of corresponding radicals. From the obviously higher peak intensity in Figure 11, it indicated that a large amount of ⋅OH and ⋅O2- were produced. All of the above results were consistent with the scavenger experiment and further confirmed the dominant contribution of ⋅OH and ⋅O2- for CTC degradation. From the photocurrent curve of NC catalyst (Figures 11C), the current density increased significantly under the simulated natural light compared with the dark condition, which further illustrated the generation of photogenerated carriers (Equation (1)). The as-generated h+ and e− could directly oxide pollutants and reduce Cr(VI), respectively (Equations (2) and (3)). Furthermore, the h+ and e− could react with H2O, OH−, and O2 to form ·OH and ·O2- (Equations (4), (5), and (6)). Depending on these reactive substances, the CTC and Cr (VI) could be effectively oxidized and reduced (Equation (7)).

Figure 11

EPR spectra for (A) ⋅O2- and (B) ⋅OH in the presence of DMPO, (C) the photocurrent curve under dark/light irradiation condition. Operational parameters: 10 ppm DMPO, 0.1 g L−1 catalyst, 6 mg/L CTC, pH of 7, 25°C.

EPR spectra for (A) ⋅O2- and (B) ⋅OH in the presen class="Chemical">nce of DMPO, (C) the photocurrent curve under dark/light irradiation condition. Operational parameters: 10 ppm DMPO, 0.1 g L−1 catalyst, 6 mg/L CTC, pH of 7, 25°C.

Conclusions

In conclusion, we have successfully fabricated two-dimensional n class="Chemical">N-doped hierarchically porous carbon from biomass waste (cellulose and corn cobs) via a facile route. The N-doped hierarchically porous carbon has large surface area, better visible light absorption capacity, the enhanced separation efficiency of e− and h+, and improved lifetime. NC materials thus presented superior photocatalytic performance for the simultaneous removal of CTC and Cr(VI) compared with the original carbon. After 90 min of visible light irradiation, the removal efficiencies of CTC and Cr(VI) were up to 93.1% and 96.9% by N-doped C semiconductors, respectively. Besides, it can keep the removal efficiency over >90% for both CTC and Cr6+ after 5 runs, showing promising prospect in the actual treatment of the simulated wastewater. Free radical capture tests and EPR results demonstrated that main active materials in the photocatalytic oxidation reaction are h+, ⋅OH, and ⋅O2-, while main active species in the reduction reaction of Cr(VI) is e−. UPLC-MS/MS analysis revealed that CTC molecules can be effectively degraded by photocatalysts NC documenting their excellent photocatalytic performance.

Limitations of the study

We had investigated the porous carbon synthesized from biomass waste for phon class="Chemical">tocatalytic oxidation of CTC and reduction of Cr(VI). Various reaction parameters and mechanism have been studied in this work. However, the actual wastewater containing various antibiotics and heavy metals has not been studied. We will perform the study in the future work.

Methods

All methods can be found in the accompanying transparent methods supplemental file.