Zn-Doped MnCO3/CS Composite Photocatalyst for Visible-Light-Driven Decomposition of Organic Pollutants

Zn-doped MnCO3/carbon sphere (Zn-doped MnCO3/CS) composites were synthesized using a simple hydrothermal procedure. Among various samples (ZM-50, ZM-05, and ZMC-0), the ternary Zn-doped MnCO3/CS (ZMC-2) catalyst demonstrated excellent visible light-induced photocatalytic activity. This improvement comes from the Zn addition and the conductive CS, which facilitate electron movement and charge transport. The catalyst exhibited efficient degradation of methylene blue (MB) over a wide pH range, achieving a removal efficiency of 99.6% under visible light. Radical trapping experiments suggested that •OH and •O2− played essential roles in the mechanism of organic pollutant degradation. Moreover, the catalyst maintained good degradation performance after five cycles. This study offers valuable perspectives into the fabrication of carbon-based composites with promising photocatalytic activity.


Introduction
As industries grow, global concerns about environmental pollution are on the rise.One significant issue involves the release of organic dyes into wastewater during manufacturing and processing activities [1,2].Among these organic dyes, methylene blue (MB) has been widely used in various industrial applications and medicine, which could be causing serious environmental and health problems.Various technologies such as physical adsorption, biodegradation, and advanced oxidation were developed for decomposing or degrading organic dyes in wastewater [3,4].Compared with other traditional methods, photocatalysis technology has garnered more attention recently for eliminating various organic pollutants, because of its mild reaction conditions and ability to degrade thoroughly [3].
Heterogeneous photocatalysis based on inorganic semiconductors stands out as one of the most promising technologies for solving this issue due to its environmental friendliness, low energy consumption, and mild reaction conditions [3][4][5].Semiconductor materials including TiO 2 , BiOBr, C 3 N 4 , CuO, ZnO, etc., have been extensively explored as photocatalysts for organic contaminant degradation [6][7][8][9][10].It has been reported that highly porous TiO 2 nanofibers produced in high humidity exhibit a MB degradation efficiency of 90% in 30 min, benefiting by its large surface area (128 m 2 /g) [6].The introduction of carbon quantum dots could improve the band structure significantly.Zhong et al. prepared a carbon dot-doping C 3 N 4 /BiOBr heterostructure in order to enhance band regulation and electron transfer, resulting in 98.48% of rhodamine B (RhB) degradation within 15 min [7].Xing et al. reported that CuO synthesized via the hydrothermal method displayed significant catalytic efficiencies in activating peroxymonosulfate for ciprofloxacin degradation.The presence of inorganic ions (Ca 2+ , Mg 2+ and Cl − ) had little effect on ciprofloxacin degradation and, under optimal conditions, ciprofloxacin would be completely degraded [9].Sometimes, the selection of the components has led to miraculous synergies in removal of organic pollutants.As reported in Tian's work [10], a ternary magnetic photocatalyst (ZnO@CoFe 2 O 4 @carbon nanotube) with excellent photocatalytic activity was synthesized for PMS activation under UVC light.The porous structure, separation of photogenerated electron-hole (e --h + ) pairs, and surface area are beneficial for enhancing the ternary photocatalyst activity on degradation of cefixime.Additionally, ZnO, CoFe 2 O 4 , and carbon nanotubes displayed high synergies during the degradation process; thus, 100% of cefixime was removed at 30 min.Despite this, many photocatalysts face challenges such as low solar utilization efficiency or a high recombination rate of photo-generated carriers [11,12].Therefore, there is a widespread interest in exploring and preparing photocatalysts with broadened light absorption and enhanced carrier separation efficiency.
Manganese carbonate (MnCO 3 ) is a well-known transition metal salt with high abundance, negligible toxicity, and a simple preparation procedure.However, MnCO 3 has received limited attention in photocatalysis research owing to its low solar utilization efficiency and conductivity [13,14].In the literature, the range of light absorption of the catalyst could be extended by introducing other metal ions [15].In addition, it has been reported that doping with elements such as Zn 2+ , Bi 3+ , Fe 3+ , Ni 2+ , and Cu 2+ can increase photocatalytic activity owing to enhanced electron exchange rates and a reduced band gap [16][17][18].For instance, Zn doping enhances the charge separation and the utilization of visible light, and changes band gaps; therefore, porous Zn doped Zr 3+ -ZrO 2 composite showed excellent photocatalytic activity for tetracycline degradation due to its narrow band gaps and broaden visible light [16].Bhatia et al. reported that a TiO 2 photocatalyst co-doped with Bi and Ni displayed higher photoactivity in ofloxacin degradation compared with TiO 2 catalyst under visible light, and the concentration of Bi and Ni played an important role in photocatalytic activity.Moreover, metal ions act as trapping centers for photoinduced e --h + pairs, resulting in suppressed recombination of e --h + pairs, and the trapped charges are transferred to the surface of the semiconductor, thereby enhancing photocatalytic performance [17].It has been reported that Cu-and Fe-doped TiO 2 nanocatalysts reveal more lattice defects and a higher concentration of surface-bound hydroxyl groups/chemically adsorbed oxygen.Consequently, Cu-Fe-TiO 2 composite has demonstrated significant photocatalytic activity, which can be attributed to its reduced band gap [18].
Recently, carbon-based materials have emerged as the preferred additive for enhancing the conductivity of catalysts.Previous studies have reported that carbon-based catalysts reduce the photo-generated e --h + recombination, extend excitation wavelengths, and increase the adsorption of reactants [19][20][21].For instance, carbon dots/Ag nanoparticles/TiO 2 nanocomposites demonstrated widened visible light absorption and reduced e --h + recombination rate.This effect was attributed to the synergism between the photoelectrical properties of the catalyst and carbon dots [22].Acting as the electron transfer bridge, the incorporation of CS into the BiOBr/g-C 3 N 4 composite improved the charge separation efficiency [23].Studies have shown that Co-doped carbon aerogels, possessing large specific surface areas and pore volumes, efficiently eliminate organic contaminants by activating peroxymonosulfate species [24].The synergy between the materials has been shown to improve the photocatalytic process.After the combination of Ag, TiO 2 , and carbon nanotubes, the photocatalytic activity was increased significantly because of the surface plasmon resonance effect of Ag [25].Moreover, graphene, carbon foams, and other carbonaceous catalysts have been reported to significantly improve photocatalysts [26,27].Among these carbon materials, CS stands out as a carrier due to its high surface area, excellent stability, low cost, and rich pattern of functional groups.
In this work, we report a straightforward hydrothermal synthesis of Zn-doped MnCO 3 /CS composite and explore its efficiency as a photocatalyst for the visible light decomposition of organic pollutants.The obtained composites were measured by XRD, SEM, TEM, and XPS, and the results reveal that the porous structure of the photocatalyst ensures a high contact area and a considerable number of active sites.Additionally, the integration of CS into the Zn 2+ /MnCO 3 material enhances charge transport, resulting in outstanding photocatalytic degradation activity.In addition, the effect of various photocatalysts (tuned to the molar ratio of Zn/Mn and the amount of glucose, Table S1), initial pH, and recyclability of catalyst on MB degradation was explored.Based on the results of the free radical trapping experiment, a possible photocatalytic degradation mechanism has been proposed.
CS stands out as a carrier due to its high surface area, excellent stability, low cost, and rich pattern of functional groups.
In this work, we report a straightforward hydrothermal synthesis of Zn-doped MnCO3/CS composite and explore its efficiency as a photocatalyst for the visible light decomposition of organic pollutants.The obtained composites were measured by XRD, SEM, TEM, and XPS, and the results reveal that the porous structure of the photocatalyst ensures a high contact area and a considerable number of active sites.Additionally, the integration of CS into the Zn 2+ /MnCO3 material enhances charge transport, resulting in outstanding photocatalytic degradation activity.In addition, the effect of various photocatalysts (tuned to the molar ratio of Zn/Mn and the amount of glucose, Table S1), initial pH, and recyclability of catalyst on MB degradation was explored.Based on the results of the free radical trapping experiment, a possible photocatalytic degradation mechanism has been proposed.
The morphology and elemental distribution of ZMC-2 were investigated using SEM and TEM. Figure 2a reveals that the sample was made of microspheres having diameters between 1 and 5 µm, featuring numerous smaller particles on their surface (Figure 2b).Moreover, a portion of the sample exhibited a hydrangea-like morphology, comprised of loosely arranged, hierarchical nanosheets with a thickness of ~20 nm (Figure 2c,d).
Elemental mapping of the ZMC-2 catalyst (Figure 2e-i) reveals the evenly dispersed C, O, Zn, and Mn elements.TEM imaging (Figure 2j-l) further confirmed that the ZMC-2 photocatalyst comprised both solid microspheres and hydrangea-like structures.
The morphology and elemental distribution of ZMC-2 were investigated using SEM and TEM. Figure 2a reveals that the sample was made of microspheres having diameters between 1 and 5 μm, featuring numerous smaller particles on their surface (Figure 2b).Moreover, a portion of the sample exhibited a hydrangea-like morphology, comprised of loosely arranged, hierarchical nanosheets with a thickness of ~20 nm (Figure 2c,d).Elemental mapping of the ZMC-2 catalyst (Figure 2e-i) reveals the evenly dispersed C, O, Zn, and Mn elements.TEM imaging (Figure 2j-l) further confirmed that the ZMC-2 photocatalyst comprised both solid microspheres and hydrangea-like structures.[30][31][32].In the O 1s XPS spectrum (Figure 3c), the main maximum at 531.7 eV corresponded to surface -OH groups, while two fitted peaks at 532.9 and 533.9 eV indicated the presence of carbonate ions [32,33].
The XPS data for the Zn 2p energy level (Figure 3d) indicated two maxima at binding energies of 1022 and 1045.6 eV.According to the literature, these peaks originate from the Zn 2p3/2 and Zn 2p1/2 levels of the Zn 2+ ion [34,35].Additionally, the Mn 2p XPS spectrum (Figure 3e) showed two characteristic maxima at 641.8 and 655.2 eV, associated  [30][31][32].In the O 1s XPS spectrum (Figure 3c), the main maximum at 531.7 eV corresponded to surface -OH groups, while two fitted peaks at 532.9 and 533.9 eV indicated the presence of carbonate ions [32,33].
The pH of the solution has a large influence on the photocatalytic decomposition of organic pollutants [42][43][44].Additionally, previous studies have shown the substantial impact of pH on the adsorption of small organic molecules onto catalyst surfaces [45,46].In this experiment, the studied pH range was from 5 to 9, and the influence of initial pH on MB decomposition is illustrated in Figure 5.The adsorption performance of the catalyst increased gradually with decreasing pH (Figure 5a) and reached its peak at pH 6.However, a further decrease to pH 5 significantly diminished MB adsorption.
Molecules 2024, 29, x FOR PEER REVIEW 7 of 14 respectively.Moreover, the MB absorption peak gradually decreased with irradiation time, while the peak position showed no shift during degradation (Figure 4d).This consistent trend in the degradation process highlights the stability of the photocatalyst and the effective removal of MB from the solution over time.
The pH of the solution has a large influence on the photocatalytic decomposition of organic pollutants [42][43][44].Additionally, previous studies have shown the substantial impact of pH on the adsorption of small organic molecules onto catalyst surfaces [45,46].In this experiment, the studied pH range was from 5 to 9, and the influence of initial pH on MB decomposition is illustrated in Figure 5.The adsorption performance of the catalyst increased gradually with decreasing pH (Figure 5a) and reached its peak at pH 6.However, a further decrease to pH 5 significantly diminished MB adsorption.The zeta potential of ZMC-2 is shown in Figure 5d, and the isoelectric point was 5.8, indicating that ZMC-2 was positively charged at pH > 5.8.If the electrostatic interaction is the only predominant adsorption mechanism, the removal of MB (cationic dyes) by ZMC-2 at pH 5 should be higher than that at the other pH values, because the ZMC-2 catalyst carries negative charge at this pH based on the zeta potential results.However, the highest MB removal was attained at pH 6, as observed in Figure 5a, indicating that other potential mechanisms played a role on the adsorption process, such as coordination interaction, π-π stacking, and H-bonding, etc. [19,47].When the pH > 6, the adsorption efficiency of ZMC-2 decreased significantly, which could be attributed to the electrostatic repulsion between the positive charges of catalyst and the positively charged MB.In conclusion, the adsorption of MB on ZMC-2 was the result of the synergistic action of multiple mechanisms.The zeta potential of ZMC-2 is shown in Figure 5d, and the isoelectric point was 5.8, indicating that ZMC-2 was positively charged at pH > 5.8.If the electrostatic interaction is the only predominant adsorption mechanism, the removal of MB (cationic dyes) by ZMC-2 at pH 5 should be higher than that at the other pH values, because the ZMC-2 catalyst carries negative charge at this pH based on the zeta potential results.However, the highest MB removal was attained at pH 6, as observed in Figure 5a, indicating that other potential mechanisms played a role on the adsorption process, such as coordination interaction, π-π stacking, and H-bonding, etc. [19,47].When the pH > 6, the adsorption efficiency of ZMC-2 decreased significantly, which could be attributed to the electrostatic repulsion between the positive charges of catalyst and the positively charged MB.In conclusion, the adsorption of MB on ZMC-2 was the result of the synergistic action of multiple mechanisms.
As depicted in Figure 5b,c, the order of the degradation rate constants was found to be pH 9 > 5 > 7 > 8 > 6.This indicates that the PCA exhibited irregular variations as the pH decreased.Notably, after 75 min of illumination, the removal efficiency of MB at various pH values became quite similar (94.2%, 96.5%, 96.0%, 94.2%, and 96.4%), suggesting that the pH of the starting solution had almost no impact on the long-term decomposition of MB, offering strong evidence for its practical application.
To broaden the range of potential applications, we tested the photocatalytic activity (PCA) of the ZMC-2 catalyst on many organic pollutants other than MB under an identical experimental setup.Figure 6 summarizes the photocatalytic experiments and gives fitting lines for the removal of six structurally diverse organic pollutants.After 30 min of adsorption in the dark and 90 min of irradiation, the removal efficiencies for MB, BPA, MO, CR, MG, and RhB were found to be 99.06%,62.76%, 4.14%, 10.77%, 98.86%, and 47.82%, respectively (Figure 6a).The data for each of these dyes aligned well into the first-order kinetic model, showing the corresponding k values were 0.04219, 0.00559, 0.00014, 0.00003, 0.03419, and 0.00436 min −1 (Figure 6c).be pH 9 > 5 > 7 > 8 > 6.This indicates that the PCA exhibited irregular variations as the pH decreased.Notably, after 75 min of illumination, the removal efficiency of MB at various pH values became quite similar (94.2%, 96.5%, 96.0%, 94.2%, and 96.4%), suggesting that the pH of the starting solution had almost no impact on the long-term decomposition of MB, offering strong evidence for its practical application.
To broaden the range of potential applications, we tested the photocatalytic activity (PCA) of the ZMC-2 catalyst on many organic pollutants other than MB under an identical experimental setup.Figure 6 summarizes the photocatalytic experiments and gives fitting lines for the removal of six structurally diverse organic pollutants.After 30 min of adsorption in the dark and 90 min of irradiation, the removal efficiencies for MB, BPA, MO, CR, MG, and RhB were found to be 99.06%,62.76%, 4.14%, 10.77%, 98.86%, and 47.82%, respectively (Figure 6a).The data for each of these dyes aligned well into the first-order kinetic model, showing the corresponding k values were 0.04219, 0.00559, 0.00014, 0.00003, 0.03419, and 0.00436 min −1 (Figure 6c).
The notable variations in removal efficiency across these different pollutants were expected, considering that photocatalytic degradation is highly influenced by the structural differences among organic pollutants [48][49][50].This wide-ranging efficacy against various pollutants highlights the versatility and potential utility of the Zn-doped MnCO3/CS composite photocatalyst in treating diverse organic contaminants.Both the reproducibility and stability of a catalyst are essential parameters that limit its application [51].The photoactivity of ZMC-2 showed only a slight reduction, from 99.06% to 96.88%, after undergoing five repeated runs (Figure 7a).This negligible The notable variations in removal efficiency across these different pollutants were expected, considering that photocatalytic degradation is highly influenced by the structural differences among organic pollutants [48][49][50].This wide-ranging efficacy against various pollutants highlights the versatility and potential utility of the Zn-doped MnCO 3 /CS composite photocatalyst in treating diverse organic contaminants.
Both the reproducibility and stability of a catalyst are essential parameters that limit its application [51].The photoactivity of ZMC-2 showed only a slight reduction, from 99.06% to 96.88%, after undergoing five repeated runs (Figure 7a).This negligible decrease may be attributed to catalyst weight loss during the experiments.Moreover, Figure 7b illustrates that the crystal phase of the composites remained unchanged, indicating its exceptional photocatalytic stability.To elucidate the mechanism of MB photodegradation over ZMC-2, we conducted radical trapping experiments to identify the main photoactive species.Benzoquinone (BQ), isopropanol (IPA), and sodium oxalate (Na2C2O4) were utilized as sacrificial agents for •O2 − , •OH, and h + , respectively [52,53].The photodegradation performance of ZMC-2 toward MB in the presence of various radical quenchers is displayed in Figure 8.When BQ and IPA were introduced, the initial MB removal rate (99.06%) decreased to 81.88% and 77.91%, respectively.These results suggest the participation of •OH and •O2 − radicals in the photodegradation mechanism of MB [54].In addition, it was reported that reactive oxygen species (•OH and •O2 − ) played an important role not only in degradation process under visible light illumination but also in the dark (•OH and •O2 − species also can be produced under dark condition) [55,56].However, in this work, the MB removal efficiency was slightly decreased (58.19% to 56.56%, 54.35%) in the dark in the presence of BQ and IPA (Figure S3), which indicated the primary reason for the abatement of MB was absorption rather than reactive oxygen species.In addition, it is worth noting that the addition of Na2C2O4 triggered a fierce promotion of MB degradation, as shown in Figure 8a, and the removal rate of MB was significantly higher compared with other conditions within 60 min of irradiation.The possible reason is that the introduction of Na2C2O4 led to the capture of holes, thus improving the separation efficiency of electron-hole pairs, which is beneficial to the To elucidate the mechanism of MB photodegradation over ZMC-2, we conducted radical trapping experiments to identify the main photoactive species.Benzoquinone (BQ), isopropanol (IPA), and sodium oxalate (Na 2 C 2 O 4 ) were utilized as sacrificial agents for •O 2 − , •OH, and h + , respectively [52,53].The photodegradation performance of ZMC-2 toward MB in the presence of various radical quenchers is displayed in Figure 8.When BQ and IPA were introduced, the initial MB removal rate (99.06%) decreased to 81.88% and 77.91%, respectively.These results suggest the participation of •OH and •O 2 − radicals in the photodegradation mechanism of MB [54].In addition, it was reported that reactive oxygen species (•OH and •O 2 − ) played an important role not only in degradation process under visible light illumination but also in the dark (•OH and •O 2 − species also can be produced under dark condition) [55,56].However, in this work, the MB removal efficiency was slightly decreased (58.19% to 56.56%, 54.35%) in the dark in the presence of BQ and IPA (Figure S3), which indicated the primary reason for the abatement of MB was absorption rather than reactive oxygen species.
Molecules 2024, 29, x FOR PEER REVIEW 9 of 14 decrease may be attributed to catalyst weight loss during the experiments.Moreover, Figure 7b illustrates that the crystal phase of the composites remained unchanged, indicating its exceptional photocatalytic stability.To elucidate the mechanism of MB photodegradation over ZMC-2, we conducted radical trapping experiments to identify the main photoactive species.Benzoquinone (BQ), isopropanol (IPA), and sodium oxalate (Na2C2O4) were utilized as sacrificial agents for •O2 − , •OH, and h + , respectively [52,53].The photodegradation performance of ZMC-2 toward MB in the presence of various radical quenchers is displayed in Figure 8.When BQ and IPA were introduced, the initial MB removal rate (99.06%) decreased to 81.88% and 77.91%, respectively.These results suggest the participation of •OH and •O2 − radicals in the photodegradation mechanism of MB [54].In addition, it was reported that reactive oxygen species (•OH and •O2 − ) played an important role not only in degradation process under visible light illumination but also in the dark (•OH and •O2 − species also can be produced under dark condition) [55,56].However, in this work, the MB removal efficiency was slightly decreased (58.19% to 56.56%, 54.35%) in the dark in the presence of BQ and IPA (Figure S3), which indicated the primary reason for the abatement of MB was absorption rather than reactive oxygen species.In addition, it is worth noting that the addition of Na2C2O4 triggered a fierce promotion of MB degradation, as shown in Figure 8a, and the removal rate of MB was significantly higher compared with other conditions within 60 min of irradiation.The possible reason is that the introduction of Na2C2O4 led to the capture of holes, thus improving the separation efficiency of electron-hole pairs, which is beneficial to the In addition, it is worth noting that the addition of Na 2 C 2 O 4 triggered a fierce promotion of MB degradation, as shown in Figure 8a, and the removal rate of MB was significantly higher compared with other conditions within 60 min of irradiation.The possible reason is that the introduction of Na 2 C 2 O 4 led to the capture of holes, thus improving the separation efficiency of electron-hole pairs, which is beneficial to the degradation of MB [57].However, the promoting effect of Na  degradation of MB [57].However, the promoting effect of Na2C2O4 decreased markedly after 75 min of irradiation, and the MB removal rate only increased to 99.80%, implying the dominant role of •OH and •O2 − in the degradation process.
Starting from the aforementioned discussion, we propose a plausible degradation mechanism for organic pollutants by ZMC-2 (Figure 9).During the adsorption stage, O2 and organic pollutants attach to the surface of ZMC-2 in the dark.Subsequently, visible light excitation drives e − from the valence band (VB) to the conduction band (CB) of Zn-doped MnCO3, leaving abundant h + in the VB.The good conductivity of the CS component of the photocatalyst facilitates the electron transfer to the surface, where they combine with the adsorbed O2 to yield •O2 − .After that, a partial amount of •O2 − reacted with H2O to produce •OH.These radicals are identified as the primary species that participate in organic pollutant removal via the ZMC-2 catalyst.
All chemicals were of p.a. grade and were utilized without additional purification.

Fabrication of Zn-Doped MnCO3
In a typical synthesis, 0.0878 g (0.4 mmol) of zinc acetate, 0.1188 g (0.6 mmol) of manganese chloride dihydrate, and 0.0901 g (1.5 mmol) of urea were added in 20 mL of deionized (DI) water and stirred for 30 min.The resulting solution was placed into a 25-mL Teflon-coated stainless steel autoclave and subjected to solvothermal synthesis at 180 °C for 4 h.After slowly cooling to RT, the resulting solid was centrifuged and subjected to three cycles of washing with DI water and EtOH.Following this, the solid was dried for 12 h at 60 °C and ground into powder for subsequent use.

Fabrication of Zn-Doped MnCO3/CS Composites
For the synthesis, 0.1 g of Zn-doped MnCO3 and 2 g of glucose (C6H12O6) were dispersed in 40 mL of DI water.Next, the suspension was placed into a 50 mL Teflon-coated autoclave and allowed to react for 4 h at 180 °C.Subsequently, the products
All chemicals were of p.a. grade and were utilized without additional purification.

Fabrication of Zn-Doped MnCO 3
In a typical synthesis, 0.0878 g (0.4 mmol) of zinc acetate, 0.1188 g (0.6 mmol) of manganese chloride dihydrate, and 0.0901 g (1.5 mmol) of urea were added in 20 mL of deionized (DI) water and stirred for 30 min.The resulting solution was placed into a 25-mL Teflon-coated stainless steel autoclave and subjected to solvothermal synthesis at 180 • C for 4 h.After slowly cooling to RT, the resulting solid was centrifuged and subjected to three cycles of washing with DI water and EtOH.Following this, the solid was dried for 12 h at 60 • C and ground into powder for subsequent use.

Fabrication of Zn-Doped MnCO 3 /CS Composites
For the synthesis, 0.1 g of Zn-doped MnCO 3 and 2 g of glucose (C 6 H 12 O 6 ) were dispersed in 40 mL of DI water.Next, the suspension was placed into a 50 mL Teflon-coated autoclave and allowed to react for 4 h at 180 • C. Subsequently, the products were collected once the system cooled down naturally to RT, and underwent multiple washes with DI water and absolute EtOH.The final product was gathered and dried at 60 • C for 1 day.
For comparative analysis, the samples were prepared by adjusting the quantity of C 6 H 12 O 6 to 0, 2, and 5 g, denoted as ZMC-0, ZMC-2, and ZMC-5, respectively.Moreover, additional samples were synthesized according to the same procedure by varying the molar ratio of Zn(CH 3 COO) 2 /MnCl 2 from 0.5:0 to 0:0.5, and labeled as ZM-50 and ZM-05.Further information on the experimental procedure is available in Table S1.

Photocatalytic Activity (PCA) Measurements
To assess the PCA of Zn-doped MnCO 3 /CS composites, we monitored the decomposition rate of methylene blue (MB).A visible light source (Xe lamp, 300 W, PerfectLight, Beijing, China) in a wavelength range above 420 nm was used, and the UV part of the spectrum was filtered out.In the standard experiment for monitoring the PCA, the solid catalyst (30 mg) was introduced into a 60 mL of 50 mg/L MB aqueous solution in a glass beaker.The resulting mixture was subjected to stirring in the dark for 30 min to equilibrate.Following this, the suspension underwent irradiation with white light (λ > 420 nm) under continuous stirring.During irradiation, 3 mL aliquots were taken at certain time steps, and the residual concentration of MB was monitored spectrophotometrically at 663 nm after removing solid particles through centrifugation.The solution's pH was tuned with 0.1 M HCl and/or 0.1 M NaOH.
The degradation rate (η) of MB was analyzed by applying the equation η = C t /C 0 × 100%, where C 0 denotes the starting molarity of MB (before irradiation) and C t is the molarity of MB at reaction time t.

Conclusions
In this work, we successfully prepared a Zn-doped MnCO 3 /CS material for the visiblelight-driven removal of organic pollutants.Various characterization results revealed that the obtained composites were composed of microspheres and hydrangea-like morphology.Moreover, C, O, Zn, and Mn elements were uniformly dispersed throughout the materials.Among the prepared composites, ZMC-2 exhibited the most excellent performance in degradation of MB under visible light.The possible reason may be as follows.On the one hand, the inclusion of CS in the Zn-doped MnCO 3 composite significantly enhanced charge transport, resulting in exceptional photocatalytic activity.On the other hand, the porous structure of the product not only increased the contact area but also increased the abundance of active sites, resulting in the photocatalytic removal efficiency of MB up to 99.6% within 120 min.Furthermore, this catalyst demonstrated stability over a wide pH range (5-9) and exhibited excellent recyclability, with an MB removal rate of 96.88% after four cycles.Quench experiments revealed that •O 2 − and•OH were the major reactive species during the MB photocatalytic reaction process.These findings offer valuable insights for the complete removal of MB and structurally versatile pollutants from wastewater.Further research could explore the application of this composite in wastewater treatment and environmental remediation.

Supplementary Materials:
The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules29051094/s1,Table S1: The detailed experimental parameters; Figure S1: XRD pattern of as-prepared MnCO 3 /CS sample; Figure S2a,c: Photocatalytic activities of the as-prepared photocatalysts for the degradation of MB under simulated sunlight irradiation, respectively; Figure S2b,d: the corresponding reaction kinetics of the samples.Figure S3: Free radical trapping experiments for MB removal over ZMC-2 in dark.

Figure 1 .
Figure 1.XRD spectra of the as-prepared materials.

Figure 1 .
Figure 1.XRD spectra of the as-prepared materials.

Figure 2 .
Figure 2. (a-d) SEM images, (e-i) elemental mapping, and (j-l) TEM images of ZMC-2 photocatalyst.To analyze the chemical composition of ZMC-2, XPS spectra were recorded, as depicted in Figure3a.The characteristic maxima for C 1s, O 1s, Zn 2p, and Mn 2p electrons indicated the presence of C, O, Zn, and Mn in this material.Deconvolution of the C 1s peak (Figure3b) revealed three Gaussians at 284.5, 285.8, and 288.4 eV that originated from C−C, C−O, and O−C=O groups, respectively[30][31][32].In the O 1s XPS spectrum (Figure3c), the main maximum at 531.7 eV corresponded to surface -OH groups, while two fitted peaks at 532.9 and 533.9 eV indicated the presence of carbonate ions[32,33].The XPS data for the Zn 2p energy level (Figure3d) indicated two maxima at binding energies of 1022 and 1045.6 eV.According to the literature, these peaks originate from the Zn 2p3/2 and Zn 2p1/2 levels of the Zn 2+ ion[34,35].Additionally, the Mn 2p XPS spectrum (Figure3e) showed two characteristic maxima at 641.8 and 655.2 eV, associated

Figure 2 .
Figure 2. (a-d) SEM images, (e-i) elemental mapping, and (j-l) TEM images of ZMC-2 photocatalyst.To analyze the chemical composition of ZMC-2, XPS spectra were recorded, as depicted in Figure3a.The characteristic maxima for C 1s, O 1s, Zn 2p, and Mn 2p electrons indicated the presence of C, O, Zn, and Mn in this material.Deconvolution of the C 1s peak (Figure3b) revealed three Gaussians at 284.5, 285.8, and 288.4 eV that originated from C−C, C−O, and O−C=O groups, respectively[30][31][32].In the O 1s XPS spectrum (Figure3c), the main maximum at 531.7 eV corresponded to surface -OH groups, while two fitted peaks at 532.9 and 533.9 eV indicated the presence of carbonate ions[32,33].The XPS data for the Zn 2p energy level (Figure3d) indicated two maxima at binding energies of 1022 and 1045.6 eV.According to the literature, these peaks originate from the Zn 2p 3/2 and Zn 2p 1/2 levels of the Zn 2+ ion[34,35].Additionally, the Mn 2p XPS spectrum (Figure3e) showed two characteristic maxima at 641.8 and 655.2 eV, associated with Mn 2p 3/2 and Mn 2p 1/2 states in the Mn 2+ ion from MnCO 3[36,37].These findings confirm the successful preparation of Zn-doped MnCO 3 /CS material.

Figure 4 .
Figure 4. (a) Photocatalytic degradation curves of MB.(b) Fitted line for pseudo-first-order ( kinetics, (c) the apparent rate constants (k), and (d) MB absorption spectra at different rea times using the ZMC-2 photocatalyst.Furthermore, the PCA data were fitted by the PFO kinetics model: ln (C0/C) = kt

Figure 4 .
Figure 4. (a) Photocatalytic degradation curves of MB.(b) Fitted line for pseudo-first-order (PFO) kinetics, (c) the apparent rate constants (k), and (d) MB absorption spectra at different reaction times using the ZMC-2 photocatalyst.

Figure 5 .
Figure 5. (a) Effect of pH values on the photocatalytic decomposition of MB; (b) kinetic profiles; (c) rate constant k; and (d) zeta potential of ZMC-2 at different pH values.

Figure 5 .
Figure 5. (a) Effect of pH values on the photocatalytic decomposition of MB; (b) kinetic profiles; (c) rate constant k; and (d) zeta potential of ZMC-2 at different pH values.
decrease may be attributed to catalyst weight loss during the experiments.Moreover, Figureillustratesthat the crystal phase of the composites remained unchanged, indicating its exceptional photocatalytic stability.

Figure 7 .
Figure 7. (a) Photodegradation of MB over ZMC-2 under visible light illumination and (b) XRD spectrum of ZMC-2 before and after 5 successive cycles.

Figure 7 .
Figure 7. (a) Photodegradation of MB over ZMC-2 under visible light illumination and (b) XRD spectrum of ZMC-2 before and after 5 successive cycles.

Figure 7 .
Figure 7. (a) Photodegradation of MB over ZMC-2 under visible light illumination and (b) XRD spectrum of ZMC-2 before and after 5 successive cycles.
2 C 2 O 4 decreased markedly after 75 min of irradiation, and the MB removal rate only increased to 99.80%, implying the dominant role of •OH and •O 2 − in the degradation process.
aforementioned discussion, we propose a plausible degradation mechanism for organic pollutants by ZMC-2 (Figure9).During the adsorption stage, O 2 and organic pollutants attach to the surface of ZMC-2 in the dark.Subsequently, visible light excitation drives e from the valence band (VB) to the conduction band (CB) of Zndoped MnCO 3 , leaving abundant h + in the VB.The good conductivity of the CS component of the photocatalyst facilitates the electron transfer to the surface, where they combine with the adsorbed O 2 to yield •O 2 − .After that, a partial amount of •O 2 − reacted with H 2 O to produce •OH.These radicals are identified as the primary species that participate in organic pollutant removal via the ZMC-2 catalyst.Molecules 2024, 29, x FOR PEER REVIEW 10 of 14

Table 1 .
Preparation details of the main photocatalytic materials 1 .