English

• Over the past decades, photocatalysis technology has garnered significant attention in hydrogen production, CO₂ reduction, and environmental remediation, particularly for water purification, due to its economical, efficient, and eco-friendly nature [1,2]. However, achieving high photocatalytic efficiency remains a major challenge, primarily due to the recombination of photogenerated electron-hole pairs. Zinc oxide (ZnO), a group Ⅱ-Ⅵ wide band gap semiconductor [3], has been widely studied for its photocatalytic properties. Its asymmetric crystal structure gives ZnO excellent piezoelectric properties due to the accumulation of zinc and oxygen atoms [4]. Additionally, ZnO is less toxic, highly reactive, and more cost-effective than other photocatalysts such as TiO₂. Consequently, ZnO has become a prominent semiconductor in photocatalysis research [5].

  However, the band gap of pure ZnO is relatively high (~3.37 eV), necessitating an excitation wavelength of 370 nm, which means longer wavelengths of visible light are insufficient to stimulate its photocatalytic activity [6]. Furthermore, the rapid recombination of photoelectron-hole pairs in ZnO leads to low light quantum utilization efficiency and unsatisfactory photocatalytic performance [7]. These limitations have hindered the practical application of ZnO in photocatalysis. To address these issues, various methods have been developed. For instance, Quero Jimenez et al. [8] synthesized a more efficient photocatalyst, NH₂-MOF(Fe)-derived α-Fe₂O₃/ZnO, using a microwave-assisted method, significantly improving the mineralization of endocrine-disrupting compounds. Similarly, Louis et al. [9] enhanced ZnO's photocatalytic performance by loading it onto reduced graphene oxide (rGO) using a hydrothermal method. However, the complexity and expense of rGO and MOF materials limit their practical application. Thus, exploring cost-effective strategies to enhance ZnO's photocatalytic properties is essential.

  Halloysite nanotubes (Al₂Si₂O₅(OH)₄·nH₂O (n = 0 or 2, HNTs), a variant of kaolin, are aluminosilicate clay minerals with meso‑tubular structures [10]. HNTs exhibit a large specific surface area, high porosity, abundant hydroxyl groups, good dispersion, and strong adsorption properties, making them promising candidates for enhancing ZnO's photocatalytic performance. HNTs have been used as modifiers to improve the properties of other photocatalysts for contaminant elimination. For example, Chen et al. [11] developed a novel photocatalyst, Bi/BaSnO₃@HNTs, using a precipitation-photoreduction method, achieving a 90.20% removal rate of methylene blue under visible light irradiation. They found that HNTs not only provided a large specific surface area for pollutant adsorption but also improved mass transfer conditions on the photocatalyst surface, reduced agglomeration, suppressed carrier recombination, and ultimately enhanced photocatalytic performance. Given these benefits, incorporating HNTs into ZnO could potentially improve its photocatalytic performance, warranting further investigation.

  Accordingly, the objectives of this study were to (1) synthesize ZnO/HNTs composites and evaluate their photocatalytic performance for removing common organic contaminants, with a specific focus on tetracycline hydrochloride (TCH). TCH, a widely used antibiotic, was notably challenging to biodegrade; (2) investigate critical parameters influencing the performance of TCH degradation in the constructed ZnO/HNTs/vis system, including the intrinsic properties of the materials and the external conditions of the system; (3) elucidate the degradation mechanisms of TCH, identify the primary reactive oxygen species responsible for TCH removal, and assess the toxicity of TCH before and after degradation in the ZnO/HNTs/vis system.

  Chemicals and reagents, materials preparation methods, materials characterization, and analytical methods are detailed in Texts S1-S4 (Supporting information). Batch experiments were conducted in 250 mL conical flasks containing 100 mL of contaminant solution, maintained at 25 ± 1 ℃ under a xenon lamp light source simulating sunlight (10 A, 25 cm). The solutions were continuously stirred with a magnetic stirrer at approximately 150 rpm. The pH of the solution was monitored using a pH meter and initially adjusted with 2 mol/L NaOH or 2 mol/L HCl solutions.

  In a typical experiment, a specific mass of HNTs or ZnO/HNTs was added to the contaminant solution. Periodic sampling of the aqueous suspensions was performed, followed by filtration through a 0.22 µm micro-pore membrane filter. Subsequently, 2 mL samples were immediately quenched with 100 µL of pure methanol and analyzed. The adsorption experiments followed a similar procedure without the xenon lamp light. All experiments were conducted in triplicate and the mean values, along with average deviations were reported.

  As illustrated in Fig. 1a, HNTs exhibited a distinctive hollow tubular structure with a smooth wall surface. In contrast, the surface of ZnO/HNTs (Figs. 1b and c) appeared markedly rough due to the deposition of ZnO particles. Despite this, the tubular structure of HNTs remained intact, though the distribution of ZnO was uneven. Fig. 1c shows that some ZnO particles had infiltrated the HNTs' nanotube cavities. This penetration likely could enhance the mass transfer process between ZnO and HNTs and prevent ZnO aggregation, which could significantly improve the catalytic performance of ZnO alone in contaminant degradation. Fig. 1d presents the X-ray diffraction (XRD) patterns of HNTs, ZnO, and ZnO/HNTs. The characteristic diffraction peaks of HNTs were nearly absent, while the diffraction patterns of ZnO/HNTs closely resembled those of ZnO, albeit with stronger crystallinity. Further analysis revealed that HNTs, as a crystal, exhibited characteristic diffraction peaks at 2θ = 11.80°, 20.00°, 24.70°, 35.00°, 54.50°, and 62.60°, corresponding to the (001), (100), (002), (110), (210), and (300) crystal facets, respectively (JCPDS No. 29-1487, Fig. S1 in Supporting information), as previous reported [12]. The absence of these characteristic peaks in Fig. 1d could be attributed to the significantly higher crystallinity of ZnO, which overshadowed the peaks of HNTs. This also explained why the diffraction peaks of ZnO/HNTs closely resembled those of ZnO, a typical wurtzite crystal, with peaks at 2θ = 31.80°, 34.40°, 36.30°, 47.40°, 56.60°, 62.90°, and 66.40° (JCPDS No. 79-2205, Fig. 1d). The transmission electron microscopy (TEM) and XRD results preliminarily indicated the successful synthesis of ZnO/HNTs. To further confirm this, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FT-IR) analyses were conducted. As shown in Figs. 1e and f (XPS full spectrum of ZnO, HNTs, and ZnO/HNTs), the main chemical elements of ZnO/HNTs were Zn, O, Si, and Al. The elemental composition of HNTs and ZnO was similar to that of ZnO/HNTs, except for the absence of Zn and Si in HNTs and ZnO, respectively. Fig. S2 (Supporting information) displays the FT-IR spectra of HNTs, ZnO, and ZnO/HNTs composites. The absorption bands at 3700 cm^-1 and 3621 cm^-1 in the HNTs spectrum corresponded to the inner surface -OH stretching attributed to the hydroxyl groups on the HNTs surface. The absorption bands at 532 cm^-1 corresponded to the stretching vibration peak of Zn-O in pure ZnO [13], while the FT-IR spectra of ZnO/HNTs showed the vibration peaks of both ZnO and HNTs. These findings confirmed the successful synthesis of ZnO/HNTs composites [14].

  Figure 1

  

  Figure 1.  TEM images of HNTs (a), ZnO/HNTs (b, c). XRD pattern of HNTs, ZnO and ZnO/HNTs (d). XPS full spectrum of ZnO, HNTs, and ZnO/HNTs (e, f).

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  As shown in Fig. S3 (Supporting information), the individual adsorption effect of the materials was not effective in removing TCH, with ZnO/HNTs achieving the highest adsorption removal efficiency of only 29.35%. This value was significantly lower than the ZnO/HNTs/vis system, which achieved an efficiency of 83.74% (Fig. 2a). This suggested that the removal of TCH in the ZnO/HNTs/vis system was primarily driven by photocatalysis. Figs. 2a and b demonstrate that ZnO/HNTs composites significantly enhanced the photocatalytic properties of both ZnO and HNTs. The removal rate (k_obs) of TCH in the ZnO/HNTs/vis system reached 1.90 × 10^–2 min^-1, compared to only 1.25 × 10^–3 min^-1 and 1.13 × 10^–2 min^-1 in the HNTs/vis and ZnO/vis systems, respectively (Text S5 and Table S1 in Supporting information).

  Figure 2

  

  Figure 2.  The degradation effect of different materials on TCH (a), pseudo-first-order kinetic models of the degradation of different materials on TCH (b). Under different calcination temperatures (c) ZnO/HNTs (1:1) and (d) ZnO/HNTs with varying ratios of mass at 350 ℃ degradation effect on TCH. ([TCH] = 20 mg/L, HNTs = ZnO = 0.005 g, ZnO/HNTs = 0.01 g, pH 5.00).

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  To investigate why ZnO/HNTs composites exhibited superior photoactivity in the degradation of TCH, UV–vis diffuse reflection spectra analyses were conducted for ZnO and ZnO/HNTs. The light absorption bands of ZnO/HNTs did not shift significantly compared to ZnO, but the absorbance in the visible light region increased substantially (Fig. S4a in Supporting information). Further calculation using the Tauc plot method (Fig. S4b in Supporting information) revealed that the bandgap of ZnO/HNTs decreased to 3.12 eV from 3.21 eV for ZnO. This decrease might be attributed to the ZnO loaded on the HNTs surface or inserted in the nanotube cavity of HNTs, which inhibited the agglomeration of ZnO, leading to smaller ZnO particle size and the manifestation of the small size effect in ZnO/HNTs semiconductors. The widening of the band gap could hinder the recombination of photogenerated carriers, thereby improving the photocatalytic performance of the catalyst [15]. Additionally, the large specific surface area of HNTs allowed ZnO to disperse effectively on its surface (Figs. 1b and c), providing abundant reactive sites for light utilization [16]. Consequently, the composite material exhibited stronger visible light absorption intensity and higher photocatalytic activity than ZnO alone.

  To further optimize the catalytic properties of ZnO/HNTs, the intrinsic material properties, such as calcination temperature and ZnO-to-HNTs load-mass ratio, along with external conditions including initial pH, contaminant concentration, catalyst dosage, natural organic matter (NOM), and common concomitant ions, were investigated.

  Fig. 2c shows that varying the calcination temperature of ZnO/HNTs from 250 ℃ to 800 ℃ had a minimal impact on TCH degradation efficiency in the ZnO/HNTs/vis system. Specifically, TCH removal increased from 85.57% to 89.13% as the calcination temperature rose initially but slightly decreased to 80.56% when the temperature was further increased to 800 ℃. This decrease might be due to the destruction of hydroxyl groups on the HNTs surface at higher temperatures, diminishing the photoactivity of ZnO/HNTs. Thermogravimetric (TG) and differential thermogravimetric (DTG) analyses of HNTs and ZnO/HNTs precursors (Figs. S4c and d in Supporting information) support these observations. For HNTs (Fig. S4c), the mass loss was only 2.00% before the temperature reached 400 ℃, primarily due to the removal of crystal water between the layers of the double-layer pipe wall structure. As the temperature increased, the mass of HNTs precursors decreased sharply, with the DTG value reaching an extreme at 474.8 ℃, corresponding to the loss of hydroxyl groups on the HNTs surface. This irreversible removal of chemical water at high temperatures led to the transformation of HNTs from a crystalline to an amorphous structure [17]. Fig. S4d shows a significant mass loss of ZnO/HNTs precursors at 110.3 ℃, attributed to water loss, with a second stage of mass loss occurring as the temperature rose to 149.5 ℃. The DTG value peaked at 288.5 ℃, corresponding to the transformation of ZnO precursors in ZnO/HNTs to ZnO via de-hydroxylation [18]. Consequently, the crystal structure of HNTs might change after 400 ℃, affecting the ZnO/HNTs properties, and ZnO/HNTs precursors could transform to ZnO/HNTs between 149.5 ℃ and 288.5 ℃. Considering energy costs and contaminant removal efficiency, 350 ℃ was selected as the optimal calcination temperature for subsequent research. Additionally, the effect of the load-mass ratio of ZnO to HNTs on TCH removal in the ZnO/HNTs/vis system was explored (Fig. 2d). Results indicated that the TCH degradation efficiency remained relatively constant, at 85.65%–86.50%, when the mass ratio of ZnO to HNTs was higher than 1:1. However, when the mass ratio decreased to 1:5 or 1:10, the degradation rate of TCH reduced to 77.08%−77.85%, revealing that ZnO played a major role in TCH photocatalytic degradation.

  As shown in Fig. 3a, the performance of the ZnO/HNTs/vis system was evaluated across a range of initial TCH concentrations (10–50 mg/L). The results indicated that the ZnO/HNTs/vis system demonstrated satisfactory resistance to varying loads, suggesting its potential for effective TCH removal across different concentrations. Fig. 3b illustrates the effect of ZnO/HNTs dosage on TCH removal. Increasing the ZnO/HNTs dosage initially led to a significant increase in TCH degradation efficiency (0.05–0.20 g/L), which then plateaued (0.20–0.40 g/L). This behavior could be due to reaching an upper limit in light utilization rate at higher photocatalyst dosages. Excessive material might also form a barrier, preventing light penetration and thus reducing degradation efficiency [19].

  Figure 3

  

  Figure 3.  Influence of the operational parameters, including TCH initial concentration (a), ZnO/HNTs dosage (b), and pH (c) on the TCH removal in ZnO/HNTs/vis system. Effects of different anions and NOMs on TCH removal by the degradation of in ZnO/HNTs/vis system, including Cl^- (d), NO₃^- (e), CO₃^2- (f), PO₄^3- (g), SO₄^2- (h), and NOM (i). [TCH] = 20 mg/L, ZnO/HNTs = 0.01 g, pH 5.00.

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  Solution pH was a crucial factor affecting contaminant photodegradation efficiency. TCH molecules exhibited amphoteric behavior, existing in different forms depending on the solution's pH. Specifically, TCH predominantly existed as a mono-cationic form (TC⁺) under pH < 3.30, as molecular TC between pH 3.30 and 7.70, and as a mono-anionic form (TC⁻) under pH 7.70 to 9.70 [20]. As shown in Fig. 3c, the highest removal efficiency of TCH (84.04%) in the ZnO/HNTs/vis system was achieved at pH 5.00, with efficiency decreasing at higher or lower pH levels. To understand this phenomenon, the zero point of charge (pH_PZC) of the ZnO/HNTs composite was determined. Fig. S5 (Supporting information) shows that the ZnO/HNTs surface was positively charged below pH 7.48 and negatively charged above pH 7.48. At pH 3.00, TC⁺ was the predominant species, and electrostatic repulsion with the positively charged ZnO/HNTs surface hinders the catalytic reaction. Similarly, at pH above 7.70, TC⁻ experienced repulsion from the negatively charged ZnO/HNTs surface, reducing removal efficiency under near-neutral pH conditions.

  The effects of inorganic salt ions and natural organic matter (NOMs), commonly present in natural water, on TCH degradation in the ZnO/HNTs/vis system, were also examined (Figs. 3d-i). CO₃^2- and PO₄^3- had minimal impact on TCH degradation, while Cl^-, NO₃^-, SO₄^2-, and humic acid (HA) slightly inhibited TCH removal. The reasons might include: (1) Anions and HA competing with TCH for active sites on the ZnO/HNTs composite surface, thus hindering the generation of active species and electron-hole pairs; (2) Formation of low-oxidation active substances due to interactions between anions and active species [21]. For instance, Cl^- and NO₃^- can capture ^•OH to form lower oxidation potential species, and Cl^- could also compete with h⁺ to form less reactive species (Eqs. 1–3) [22]. With its large macromolecules and abundant electrons, excessive HA might also neutralize active species [23]. Despite these interactions, the ZnO/HNTs/vis system maintained satisfactory TCH degradation performance, indicating its robust anti-interference capability.

  $ \mathrm{Cl}^{-}+{ }^{\bullet} \mathrm{OH}=\mathrm{ClOH}^{\bullet-} $

  (1)

  $ \mathrm{Cl}^{-}+\mathrm{h}^{+}={ }^{\bullet} \mathrm{Cl} $

  (2)

  $ {\mathrm{NO}_3}^{-}+{ }^{\bullet} \mathrm{OH}={\mathrm{NO}_3} ^\bullet+\mathrm{OH}^{-} $

  (3)

  A scavenging experiment was conducted to identify the major reactive species involved in the ZnO/HNTs/vis system. As shown in Fig. S6 (Supporting information), the quenching intensities of the scavengers followed the order: EDTA-2Na > L-histidine > TEMPOL > IPA, targeting h⁺, ¹O₂, ^•O₂^- and ^•OH, respectively [24-26]. This result indicated that h⁺ played a major role in TCH degradation, while ¹O₂ and ^•O₂^- and ^•OH played minor roles. Additionally, Figs. S7a-d (Supporting information) presents the characteristic signals of TEMPO-h⁺ (strong) and DMPO-^•OH (slight) in the ZnO/HNTs/vis system, whereas DMPO (blank), DMPO (^•O₂^-), and TEMP (¹O₂) signals were absent, confirming the primary role of h⁺ and ^•OH as an ancillary.

  The possible degradation pathways of TCH in the ZnO/HNTs/vis system were also explored. Eleven intermediates were identified using liquid chromatography-mass spectrometry (LC-MS) (Fig. S8 in Supporting information). Initially, TCH underwent hydroxylation by ^•OH, forming two hydroxylation products: A (m/z 431) and D (m/z 462). Compound B (m/z 488) was produced via unsaturated bond addition during hydroxylation [27], while compound C (m/z 415) underwent demethylation and subsequent amino group oxidation. The aromatic ring of compound B, known for its strong ultraviolet absorption, likely generated compound E (m/z 554) through C—C bond cleavage and simultaneous hydroxylation under simulated sunlight [28]. Further oxidation of active groups in compounds A and C led to the production of compound F (m/z 208) and phenylpropionic acid I (m/z 150) by chain opening and oxidation. Compounds D and E underwent additional oxidation and demethylation to yield compound G (m/z 339), followed by functional group dissociation and ring opening to produce compounds J (m/z 227) and K (m/z 140). These intermediates eventually underwent ring-opening and partial oxidation to CO₂ and H₂O [29].

  The acute toxicity and mutagenicity of TCH and its degradation intermediates were predicted using the quantitative structural activity relationship (QSAR) tool within the Toxicity Estimation Software Tool (TEST). As shown in Figs. 4a–d, the acute toxicity of TCH degradation intermediates was generally lower than that of the original TCH, except for intermediate C, whose toxicity was similar to TCH. The acute toxicity of small molecule degradation products H, I, and K was significantly reduced. Additionally, the mutagenicity of TCH and most intermediates decreased or was predicted to be non-mutagenic (e.g., small molecule degradation products I and K). These results indicate that the toxicity of TCH can be significantly reduced following photocatalytic degradation, making the ZnO/HNTs/vis system a promising method for practical applications.

  Figure 4

  

  Figure 4.  Acute toxicity LC₅₀ of Fathead minnow (a), Daphnia magna (b), and Rat (c). Mutagenicity of TCH and its degradation products (d).

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  To further explore the potential of practical application, the performance of the ZnO/HNTs/vis system was tested in different water bodies. Fig. S9a (Supporting information) shows that actual water only slightly affected TCH degradation, consistent with the finding that HA did not significantly inhibit TCH removal. Additionally, the system was tested with other organic pollutants, including methylene blue (MB) and ciprofloxacin (CIP), to assess its general applicability. As shown in Fig. S9b (Supporting information), these contaminants were efficiently removed alongside TCH, demonstrating the potential of the ZnO/HNTs/vis system for practical organic wastewater treatment.

  The stability and reusability of ZnO/HNTs composites in photocatalytic reactions were verified through cycle tests. Fig. S10 (Supporting information) shows that the TCH degradation efficiency remained high at 79.10% after three cycles. However, by the fourth cycle, the efficiency decreased to 65.57%, likely due to ZnO loss caused by photo-corrosion during the recycling [30], leading to a decline in photocatalytic performance.

  This study synthesized ZnO/HNTs composites using a high-temperature calcination method. The intrinsic properties of the materials, including calcination temperature and the load-mass ratio of ZnO to HNTs, were investigated, along with external conditions such as initial pH, contaminant concentration, catalyst dosage, natural organic matter, and common concomitant ions. The ZnO/HNTs composites successfully alleviated ZnO's agglomeration, enhancing its photocatalytic properties. The degradation efficiency of TCH in the ZnO/HNTs/vis system reached 98.32% within 90 min, with stability evaluations indicating satisfactory reusability. The ZnO/HNTs/vis system significantly improved by removing various organic contaminants, including TCH, CIP, and MB. Additionally, this system effectively degraded contaminants under natural pH conditions and exhibited excellent resistance to NOM, NO₃^-, SO₄^2-, Cl^-, CO₃^2-, and PO₄^2-. Scavenging experiments and electron paramagnetic resonance (EPR) characterization confirmed that h⁺ was the primary species, with ^•OH also contributing to TCH removal. The pathways of TCH in the ZnO/HNTs/vis system involved hydroxylation, ring opening, and oxidation reactions. Toxicity predictions for TCH and its degradation intermediates indicated a significant reduction in toxicity in the intermediates. This study provides important insights for developing ZnO-based photocatalysts for treating water contaminated by antibiotics and other recalcitrant organic matter.

  CRediT authorship contribution statement

  Liangbo Zhang: Writing – original draft, Validation, Software, Resources, Investigation, Formal analysis, Conceptualization. Jun Cheng: Visualization, Validation. Yahui Shi: Visualization, Validation. Kunjie Hou: Writing – original draft, Visualization, Validation, Investigation, Funding acquisition, Data curation, Conceptualization. Qi An: Visualization, Validation. Jingyi Li: Visualization, Validation. Baohui Cui: Visualization, Validation. Fei Chen: Writing – review & editing, Supervision, Project administration, Funding acquisition.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This research was financially supported by the National Natural Science Foundation of China (Nos. 52270149, 51908528, 2200013). Natural Science Foundation of Henan Province, China (No. 242300421443). The Science and Technology Key Project of Henan Province, China (No. 242102321073). Doctoral Fund Project of Henan University of Technology, China (Nos. 2020BS005, 2023BS004).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110400.

  
  
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  Figure 1  TEM images of HNTs (a), ZnO/HNTs (b, c). XRD pattern of HNTs, ZnO and ZnO/HNTs (d). XPS full spectrum of ZnO, HNTs, and ZnO/HNTs (e, f).

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  Figure 2  The degradation effect of different materials on TCH (a), pseudo-first-order kinetic models of the degradation of different materials on TCH (b). Under different calcination temperatures (c) ZnO/HNTs (1:1) and (d) ZnO/HNTs with varying ratios of mass at 350 ℃ degradation effect on TCH. ([TCH] = 20 mg/L, HNTs = ZnO = 0.005 g, ZnO/HNTs = 0.01 g, pH 5.00).

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  Figure 3  Influence of the operational parameters, including TCH initial concentration (a), ZnO/HNTs dosage (b), and pH (c) on the TCH removal in ZnO/HNTs/vis system. Effects of different anions and NOMs on TCH removal by the degradation of in ZnO/HNTs/vis system, including Cl^- (d), NO₃^- (e), CO₃^2- (f), PO₄^3- (g), SO₄^2- (h), and NOM (i). [TCH] = 20 mg/L, ZnO/HNTs = 0.01 g, pH 5.00.

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  Figure 4  Acute toxicity LC₅₀ of Fathead minnow (a), Daphnia magna (b), and Rat (c). Mutagenicity of TCH and its degradation products (d).

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