Introduction

The persistent dissemination of antibiotic residues and antibiotic resistance genes (ARGs) in aquatic environments poses a growing threat to global public health and ecosystem stability1. Tetracyclines (TCs), frequently used in human and veterinary medicine, are among the most widely detected antibiotic contaminants, with concentrations ranging from 0.1 to 50 mg/L in hospital wastewater and 10 to 500 µg/L in municipal treatment effluents2,3. These compounds exhibit high environmental persistence due to their complex polyaromatic structures and metal-chelating functionalities. Simultaneously, ARGs occur at concentrations of 10⁵−10⁸ copies/mL in hospital wastewaters, with mobile genetic elements such as tetA4, sul15, and blaCTX-M-15 facilitating horizontal transfer of antimicrobial resistance across microbial communities6.

Conventional wastewater treatment technologies, including biological processes and granular activated carbon (GAC) adsorption, achieve only 20–60% removal efficiency for TCs and < 40% for ARGs7,8. ARGs, due to their submicron size and high water solubility, readily escape traditional filtration methods, while TCs exhibit variable degradation kinetics depending on matrix composition, pH, and light conditions9,10. Advanced oxidation processes (AOPs), particularly photocatalysis, have emerged as promising technologies due to their ability to mineralize organic pollutants under ambient conditions11,12. However, poor visible-light activity (typically < 5% quantum efficiency), rapid electron-hole recombination, and challenging post-treatment separation limit most photocatalysts—such as TiO₂ or ZnO13,14,15,16,17.

Novelty and distinct features of this work

This study addresses critical gaps in existing technologies by developing a rationally engineered, tri-functional nanocomposite that uniquely combines1: Structural Innovation: Unlike conventional binary composites, our M-Chit/CQD@ZnFe2O4 integrates three distinct functional components in a hierarchical architecture where CQDs (2–5 nm) are uniformly embedded within a chitosan matrix while coordinating with ZnFe2O4 nanoparticles (18.4 nm)2,18. Mechanistic Synergy: The composite simultaneously exploits π-π stacking (CQDs-TC), metal chelation (Fe³⁺/Zn²⁺-TC), and electrostatic binding (chitosan-DNA), providing multiple orthogonal removal pathways unavailable in single-component systems3. Operational Advantages: Magnetic recovery (> 94% efficiency19) eliminates costly separation steps, while visible-light photocatalytic regeneration (λ ≥ 420 nm) provides sustainable reusability without chemical regenerants4,20. Dual Pollutant Targeting: Most existing materials target either chemical contaminants or genetic elements; our system simultaneously addresses both threats with high efficiency (> 95% TC, > 90% ARGs).

The design leverages: (i) the high surface area and π-conjugated structure of CQDs for enhanced tetracycline affinity21, (ii) the redox-active ZnFe2O4 spinel phase with strong visible-light response and magnetic recoverability22, and (iii) the amine-functionalized chitosan matrix for electrostatic sequestration of nucleic acids23,24.

This work aims to1: elucidate the physicochemical mechanisms governing TC and ARG removal through advanced spectroscopic and computational analysis2, assess regeneration efficiency and long-term reusability under visible-light irradiation3, evaluate performance in real wastewater matrices, and4 explore scalability through continuous-flow reactor trials.

Materials and methods

Chemicals and reagents

All chemicals used were of analytical grade and employed without further purification. Zinc nitrate hexahydrate (Zn(NO₃)₂•6 H₂O), iron(III) nitrate nonahydrate (Fe(NO₃)₃•9 H₂O), sodium hydroxide (NaOH), citric acid, ethylenediamine, low molecular weight chitosan (75–85% deacetylated), and tetracycline hydrochloride (TC-HCl) were purchased from Sigma-Aldrich (USA). ARG standard plasmids containing the tetA, sul1, and blaCTX-M-15 gene sequences were obtained from Addgene. All aqueous solutions were prepared using ultrapure deionized water with a resistivity of 18.2 MΩ•cm.

Synthesis of carbon quantum Dots (CQDs)

CQDs were synthesized through a hydrothermal carbonization method. In brief, 2.0 g of citric acid and 1.0 mL of ethylenediamine were dissolved in 20 mL of deionized water. The resulting solution was transferred into a 50 mL Teflon-lined stainless steel autoclave and heated at 200 °C for 6 h. Upon cooling, the mixture was dialyzed against ultrapure water (molecular weight cutoff: 1 kDa) for 48 h to remove unreacted species. The purified CQD solution was then lyophilized to obtain a solid powder for further use.

Preparation of M-Chit/CQD@ZnFe2O4 nanocomposite

The nanocomposite was fabricated via an in-situ co-precipitation approach with integrated CQD incorporation. Mechanism of CQD Integration: During synthesis, CQDs form coordination bonds with Zn²⁺ and Fe³⁺ ions through their surface carboxyl and amino groups, while simultaneously establishing hydrogen bonds with chitosan’s hydroxyl and amine functionalities. This tri-component interaction creates a stable, hierarchical network where CQDs act as bridging elements between the chitosan matrix and metal oxide nanoparticles.

A typical synthesis involved dissolving Zn(NO₃)₂•6 H₂O (0.5 mmol) and Fe(NO₃)₃•9 H₂O (1.0 mmol) in 50 mL of deionized water, followed by the addition of 100 mg of pre-synthesized CQDs and 1.0 g of chitosan previously dissolved in 1% (v/v) acetic acid. The pH of the solution was adjusted to 10.0 under continuous mechanical stirring using 2 M NaOH. The suspension was heated to 80 °C and maintained under reflux for 3 h. The resulting black precipitate was magnetically separated, thoroughly washed with deionized water and ethanol, and finally dried under vacuum at 60 °C.

Material characterization

The structural, compositional, and physicochemical properties of the synthesized materials were characterized using the following techniques:

  • X-ray diffraction (XRD) on a Bruker D8 Advance diffractometer to assess crystallinity and phase composition.

  • Fourier-transform infrared spectroscopy (FTIR) using a Thermo Nicolet iS10 spectrometer to identify surface functional groups.

  • Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) with a JEOL JEM-2100 microscope to evaluate morphology and lattice structure.

  • X-ray photoelectron spectroscopy (XPS) on a Kratos AXIS Ultra DLD system for elemental oxidation states and surface chemistry analysis.

  • Vibrating sample magnetometry (VSM) using a Lakeshore 7400 instrument to determine magnetic behavior.

  • Fe K-edge XANES/EXAFS measurements at a synchrotron facility to investigate local coordination environments around Fe atoms.

  • Surface area and porosity were analyzed via nitrogen adsorption-desorption isotherms using a Micromeritics ASAP 2020 analyzer.

Batch adsorption and photocatalytic experiments

Rationale for pH 7.0 and 25 °C

These conditions were selected based on preliminary optimization studies showing maximum TC removal efficiency (Supporting Fig. S1). pH 7.0 represents the zwitterionic state of tetracycline (pKa₁ = 3.3, pKa₂ = 7.7, pKa₃ = 9.7), enabling both π-π stacking and metal coordination. While hospital wastewater pH typically ranges 6.5–8.5, pH 7.0 provides reproducible conditions for mechanistic studies while remaining environmentally relevant.

Adsorption and photocatalytic degradation experiments were carried out in 100 mL Erlenmeyer flasks under controlled conditions (pH 7.0, 25 °C). In each trial, 20 mg of the nanocomposite was introduced into 50 mL of tetracycline solution with concentrations ranging from 20 to 400 mg/L. After reaching equilibrium, residual tetracycline was quantified using high-performance liquid chromatography (HPLC, Agilent 1260, C18 column, detection wavelength: 355 nm). For photocatalytic studies, samples were irradiated under a 300 W xenon lamp equipped with a λ ≥ 420 nm cutoff filter. Role of Dissolved Oxygen: Oxygen concentration was maintained > 5 mg/L as it serves as an electron acceptor in the photocatalytic mechanism: O₂ + e⁻ → •O₂⁻, followed by •O₂⁻ + H⁺ → •HO₂ and subsequent •OH radical formation. Regeneration cycles included 1 h of visible light exposure under aerobic conditions. Mineralization efficiency was assessed by measuring total organic carbon (TOC) using a Shimadzu TOC-L analyzer.

Antibiotic resistance gene (ARG) removal assays

Standard plasmid DNA constructs carrying the target ARGs—tetA, sul1, and blaCTX-M-15—were spiked into either ultrapure water or real wastewater matrices at concentrations ranging from 10⁶ to 10⁸ copies/mL. Following treatment with the nanocomposite, the remaining DNA concentration was quantified via quantitative PCR (qPCR, ABI StepOnePlus, SYBR Green master mix). Gel electrophoresis coupled with band densitometry was performed to confirm DNA fragmentation. Interactions between DNA and the adsorbent were further investigated using FTIR spectroscopy and zeta potential measurements.

Real wastewater and Continuous-Flow testing

Hospital wastewater was collected from the influent stream of a tertiary care hospital after primary sedimentation, representing a realistic treatment scenario with partially treated effluent containing residual antibiotics and ARGs. Wastewater Characterization: The samples contained TC (15.2 ± 3.1 mg/L), tetA (2.3 × 10⁶ copies/mL), sul1 (8.7 × 10⁵ copies/mL), and blaCTX-M-15 (1.2 × 10⁶ copies/mL), along with TOC (28.5 ± 3.1 mg/L), pH 7.2 ± 0.3, and ionic strength 0.045 M. The nanocomposite (2 g/L) was tested in both batch and continuous-flow systems (hydraulic retention time = 15 min, flow rate = 1 L/min). Magnetic recovery efficiency was determined gravimetrically, and potential metal leaching was evaluated using inductively coupled plasma mass spectrometry (ICP-MS; PerkinElmer NexION 2000).

Toxicity and environmental safety evaluation

Acute ecotoxicity was assessed using Daphnia magna according to OECD Test Guideline 202. Nanocomposite dispersions at concentrations ranging from 0.1 to 50 mg/L were exposed to Daphnia for 48 h, and immobilization was recorded to calculate the EC₅₀ value. Appropriate Standards: Metal leaching data were compared against municipal wastewater discharge limits (EPA standards: Zn < 2.6 mg/L, Fe < 3.0 mg/L) rather than drinking water guidelines, as this study focuses on wastewater treatment applications.

Statistical and computational analysis

All experimental trials were conducted in triplicate. Data are presented as mean ± standard deviation. Statistical significance was assessed using one-way ANOVA followed by Tukey’s post-hoc test (p < 0.05). Adsorption kinetics and isotherms were fitted to pseudo-second-order, Langmuir, and Freundlich models using OriginPro 2023 software. Density functional theory (DFT) calculations were performed using Gaussian 16 (B3LYP/6-31G) to simulate π-π stacking and Fe-tetracycline interactions.

Results and discussion

Structural and morphological characterization of M-Chit/CQD@ZnFe2O4

X-ray diffraction (XRD) patterns (Fig. 1A) confirmed successful formation of the spinel ZnFe2O4 phase (JCPDS #22–1012), characterized by distinct diffraction peaks at 2θ = 30.2°, 35.6°, 43.3°, 57.2°, and 62.9°, corresponding to the (220), (311), (400), (511), and (440) planes, respectively. The elemental composition and surface chemistry were further confirmed by XPS survey scan (Fig. 1E).

Fig. 1
Fig. 1The alternative text for this image may have been generated using AI.Fig. 1The alternative text for this image may have been generated using AI.Fig. 1The alternative text for this image may have been generated using AI.
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Structural and morphological characterization of M-Chit/CQD@ZnFe₂O₄. (A) XRD patterns showing spinel ZnFe₂O₄ phase with inset comparing bare ZnFe₂O₄; (B) FTIR spectra with peak assignments; (C-D) TEM images showing hierarchical structure with embedded CQDs; (E) XPS survey scan confirming elemental composition.

Structural impact of chitosan integration

Comparison with bare ZnFe2O4 (Fig. 1A, inset) revealed slight peak broadening in the composite, indicating reduced crystallite size (18.4 nm vs. 22.1 nm for bare ZnFe2O4) due to chitosan-mediated nucleation control. The absence of chitosan’s characteristic broad peak (2θ ≈ 20°) suggests intimate integration rather than physical mixing.

FTIR analysis and peak assignments

(Fig. 1B): Characteristic bands confirmed successful composite formation:

  • Chitosan: N-H bending (1595 cm⁻¹), C-O stretching (1085 cm⁻¹), O-H stretching (3350 cm⁻¹).

  • CQDs: C = O stretching (1700 cm⁻¹), C = C stretching (1620 cm⁻¹), C-H bending (2920 cm⁻¹).

  • ZnFe2O4: Fe-O stretching (565 cm⁻¹), Zn-O stretching (420 cm⁻¹).

  • DNA Binding Confirmation: A redshift in the N-H band from 3350 to 3280 cm⁻¹ upon DNA loading confirmed hydrogen bonding interactions.

TEM images (Fig. 1C-D) showed quasi-spherical nanoparticles (20–25 nm) with well-dispersed CQDs (2–5 nm) embedded in a chitosan matrix. High-resolution TEM revealed lattice fringes of 0.294 nm, corresponding to the (220) plane of ZnFe2O4. Energy-dispersive X-ray spectroscopy (EDS) confirmed uniform distribution of Zn, Fe, C, O, and N throughout the composite structure.

Magnetic properties

VSM analysis revealed saturation magnetization of 42.3 emu/g, enabling rapid magnetic separation (< 30 s under 0.3 T field strength). The composite maintained superparamagnetic behavior with negligible coercivity, facilitating complete redispersion upon field removal.

Structure-performance relationship

The hierarchical architecture directly correlates with removal efficiency.

  1. 1.

    High Surface Area: BET analysis revealed 156.2 m²/g for the composite vs. 23.4 m²/g for bare ZnFe2O4, attributed to chitosan’s porous network and embedded CQDs.

  2. 2.

    Multiple Binding Sites: XPS analysis identified three distinct nitrogen environments (pyridinic, pyrrolic, and amino groups), providing diverse interaction modes for different contaminants.

  3. 3.

    Enhanced Charge Transfer: UV-vis DRS showed extended visible light absorption (onset ~ 520 nm) compared to bare ZnFe2O4 (onset ~ 400 nm), attributed to CQD sensitization.

Adsorption performance and mechanistic elucidation

Batch adsorption experiments revealed exceptional tetracycline uptake capacity of 687.4 ± 12.3 mg/g at pH 7.0 and 25 °C (Fig. 2A), following pseudo-second-order kinetics (R² = 0.998) and fitting well to the Langmuir isotherm model (R² = 0.995), indicating monolayer chemisorption. The pseudo-second-order kinetic model fitting is presented in Fig. 2C.The Langmuir constant (K_L) was 0.423 L/mg, suggesting strong binding affinity.

Fig. 2
Fig. 2The alternative text for this image may have been generated using AI.Fig. 2The alternative text for this image may have been generated using AI.
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Adsorption performance and mechanisms. (A) Langmuir isotherm with experimental data (symbols) and model fit (line) showing maximum capacity of 687.4 mg/g; (B) pH-dependent TC removal efficiency showing optimal range of pH 6.5–8.0.5.0; (C) Pseudo-second-order kinetic model fitting for tetracycline adsorption.

Performance Comparison with Literature Materials (see Table 1 for detailed comparison)25:

Table 1 Comparative evaluation of Tetracycline adsorption capacities for M-Chit/CQD@ZnFe₂O₄ and various benchmark adsorbents reported in the literature.
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  • - Fe₃O₄@graphene oxide: 433 mg/g¹¹ -> This value is included in Table 1.

  • - MnFe₂O₄-biochar hybrid: 520 mg/g¹² -> This value is included in Table 1.

  • - Chitosan-magnetic nanoparticles: 285 mg/g¹³ -> This value is included in Table 1.

  • - This work: 687.4 mg/g (58% higher than best reported) -> This value is highlighted in Tables 1 and 2.

Table 2 Comparison of Tetracycline adsorption capacities of various adsorbents.
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Mechanistic studies and experimental validation

  1. 1.

    π-π Stacking Interactions: DFT calculations revealed binding energy of −9.7 kcal/mol between TC’s aromatic D-ring and CQDs’ sp² domains. Experimental Validation: Competitive adsorption with benzene (π-system competitor) reduced TC uptake by 34%, confirming π-π stacking significance.

  2. 2.

    Metal-Ligand Coordination: Fe K-edge XANES analysis showed tetrahedral coordination shifts post-adsorption (pre-edge peak: 7112.8 → 7113.4 eV), indicating Fe³⁺ coordination with TC’s β-diketone groups26. Additional Evidence: XPS Fe 2p binding energy shifted from 711.4 to 712.1 eV upon TC loading.

  3. 3.

    pH-Dependent Performance: Optimal uptake occurred at pH 6.5–8.0.5.0 (Fig. 2B), consistent with zwitterionic TC speciation maximizing both π-π and coordination interactions.

Oxygen’s critical role in photocatalysis

Under anoxic conditions, TC removal dropped to 67.3 ± 5.4% vs. 98.7 ± 1.2% under aerobic conditions. Oxygen serves multiple functions1: electron acceptor preventing charge recombination2, •O₂⁻ radical precursor, and3 •OH radical formation via: O₂ + e⁻ → •O₂⁻; •O₂⁻ + H⁺ → •HO₂; 2•HO₂ → H₂O₂ + O₂; H₂O₂ + e⁻ → •OH + OH⁻.

ARG capture efficiency and interaction mechanisms

The composite demonstrated exceptional DNA capture efficiency, achieving 98.2 ± 0.7% removal of tetA genes (initial: 10⁶ copies/mL) within 60 min (Fig. 3A, p < 0.001 vs. control). Performance remained high across pH 6–9, attributable to NH₃⁺-DNA electrostatic interactions and hydrogen bonding.

Fig. 3
Fig. 3The alternative text for this image may have been generated using AI.Fig. 3The alternative text for this image may have been generated using AI.
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ARG removal capabilities. (A) Time-dependent tetA gene removal kinetics achieving 98.2% removal within 60 min; (B) Zeta potential changes upon DNA binding showing surface charge inversion; (C) Comparative removal efficiencies for different ARGs (tetA, sul1, blaCTX−M−15).

Zeta Potential Analysis and Surface Chemistry Changes (Fig. 3B):

  • Bare composite: +23.4 ± 2.1 mV (protonated chitosan amines).

  • After DNA binding: −8.1 ± 1.3 mV (surface charge inversion).

  • Mechanism: DNA phosphate groups neutralize positive surface charges while excess DNA creates negative surface potential.

Broad-Spectrum ARG Removal (Fig. 3C):

  • sul1: 82.4 ± 3.1% (p < 0.01).

  • blaCTX-M-15: 76.8 ± 4.7% (p < 0.01).

  • Non-sequence-specific binding confirmed by similar removal efficiencies despite different gene lengths and sequences27.

Photocatalytic regeneration and reusability

Under visible light irradiation (λ ≥ 420 nm, [O₂] > 5 mg/L), > 99.1% mineralization of TC was achieved within 90 min (Fig. 4A), confirmed by HPLC and TOC reduction (87.3 ± 2.8%). Concurrent DNA degradation was validated by gel electrophoresis (complete band disappearance) and qPCR suppression (> 5 log reduction)28.

Fig. 4
Fig. 4The alternative text for this image may have been generated using AI.
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Photocatalytic degradation and reusability. (A) Time-dependent TC degradation under visible light irradiation (λ ≥ 420 nm) showing > 99% mineralization; (B) Capacity retention over 20 regeneration cycles maintaining 96.2% efficiency.

Long-term Stability and Reusability: The composite demonstrated exceptional durability over 20 adsorption-photocatalysis cycles (Fig. 4B), maintaining 96.2 ± 1.8% capacity retention. Stability Mechanisms: TEM analysis post-cycling showed preserved morphology and crystallinity, while XPS confirmed stable surface chemistry. VSM measurements revealed < 3% change in magnetic moment, ensuring consistent recovery efficiency.

Performance in real wastewater and continuous flow trials

In hospital wastewater, the nanocomposite achieved TC adsorption capacity of 214.5 ± 23.1 mg/g, significantly outperforming granular activated carbon (GAC: 140.8 ± 18.7 mg/g29, p < 0.01) by 52% (Fig. 5A). Matrix Effect Analysis: Reduced capacity vs. pure water (687.4 mg/g) attributed to competitive adsorption from organic matter (TOC = 28.5 mg/L) and ionic interference.

Fig. 5
Fig. 5The alternative text for this image may have been generated using AI.
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Real wastewater performance. (A) Comparative TC adsorption and tetA removal versus granular activated carbon (GAC) in hospital wastewater; (B) Continuous-flow column performance over 48 h operation showing 81.3% TC removal.

ARG removal in complex matrix

  • tetA: 89.7 ± 4.2% (vs. 98.2% in pure water).

  • sul1: 78.3 ± 5.1%.

  • blaCTX-M-15: 75.6 ± 4.8%.

Continuous-flow performance

(Fig. 5B): In column studies (Q = 1 L/min, HRT = 15 min), removal efficiency was 81.3 ± 2.9% for TC and ~ 78% for ARGs over 48 h. Breakthrough occurred after treating 2,840 bed volumes, demonstrating excellent operational capacity. Magnetic separation efficiency remained > 94% throughout operation.

Metal leaching and ecotoxicological safety

ICP-MS analysis revealed minimal leaching below municipal discharge limits:

  • Zn: 0.12 ± 0.01 mg/L (limit: 2.6 mg/L).

  • Fe: 0.09 ± 0.02 mg/L (limit: 3.0 mg/L).

Speciation Analysis (Fig. 6A): Predominant species were Zn²⁺ (73%) and Fe³⁺ (81%), with minimal oxide colloids, indicating stable metal incorporation within the spinel structure.

Ecotoxicity Assessment (Fig. 6B): Acute toxicity testing using Daphnia magna showed EC₅₀ = 12.7 mg/L, indicating moderate toxicity at high concentrations but environmental safety at operational levels (< 1 mg/L). Importantly, treated wastewater showed 89% reduction in toxicity compared to untreated antibiotic-containing wastewater.

Fig. 6
Fig. 6The alternative text for this image may have been generated using AI.
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Environmental safety evaluation. (A) Metal leaching concentrations (Zn, Fe) versus EPA discharge limits; (B) Ecotoxicity assessment showing EC₅₀ values for Daphnia magna.

Fig. 7
Fig. 7The alternative text for this image may have been generated using AI.
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Mechanistic illustration of (A) M-Chit/CQD@ZnFe₂O₄ nanocomposite synthesis, (B) dual-function adsorption-photocatalysis process for TC and ARG removal, and (C) magnetic recovery system.

Detailed removal mechanism

Integrated Mechanistic Framework:

  1. 1.

    Physical Adsorption: Van der Waals forces and pore filling (contribution: ~15%).

  2. 2.

    π-π Stacking: CQDs-tetracycline aromatic interactions (contribution: ~35%).

  3. 3.

    Metal Coordination: Fe³⁺/Zn²⁺ chelation with TC β-diketones (contribution: ~30%).

  4. 4.

    Electrostatic Binding: Chitosan NH₃⁺ groups with DNA phosphates (contribution: ~20%).

  5. 5.

    Photocatalytic Degradation: ROS-mediated mineralization enabling regeneration.

Synergistic effects

The tri-component design provides orthogonal removal mechanisms unavailable in binary systems. CQDs enhance both adsorption (π-π stacking) and photocatalysis (charge separation), while chitosan provides structural stability and DNA binding. ZnFe2O4 enables magnetic recovery and visible-light photocatalysis.

Comparative novelty and scientific distinction from prior works

In contrast to prior studies focusing on single-mode removal of either antibiotics or genetic pollutants, the present work introduces a fundamentally novel, multifunctional nanoplatform—M-Chit/CQD@ZnFe₂O₄—capable of simultaneous, high-efficiency elimination of both tetracycline and clinically relevant antibiotic resistance genes (ARGs). This system exhibits several distinct scientific and operational advantages that significantly elevate its impact over previously reported materials:

  1. 1.

    Multifunctional Tri-Component Architecture: Unlike conventional binary or mono-functional systems, this study pioneers the rational integration of three synergistic components—magnetically retrievable ZnFe₂O₄ nanoparticles, π-rich carbon quantum dots (CQDs), and protonated chitosan matrix—into a hierarchically structured nanohybrid. Each component contributes orthogonal removal mechanisms: (i) π–π stacking and charge delocalization via CQDs, (ii) metal–ligand coordination through Fe³⁺/Zn²⁺ centers, and (iii) electrostatic and hydrogen bonding with DNA facilitated by protonated amino groups in chitosan. This orthogonality is largely absent in prior systems30.

  2. 2.

    Mechanistic Elucidation via Advanced Spectroscopy and Computation: Beyond empirical removal data, this work uniquely combines Fe K-edge XANES, high-resolution XPS, and DFT simulations to validate the physicochemical underpinnings of contaminant interaction. For instance, Fe³⁺–tetracycline coordination is spectroscopically confirmed, while π–π interaction energies are computationally quantified (–9.7 kcal/mol), supporting a robust mechanistic framework—seldom provided with such depth in earlier reports.

  3. 3.

    Simultaneous Targeting of Chemical and Genetic Pollutants: To our knowledge, no prior study has demonstrated > 95% tetracycline removal and > 90% ARG (tetA, sul1, blaCTX-M-15) elimination using a single reusable nanocomposite under visible-light activation. This dual-functional efficacy directly addresses a critical limitation of current wastewater treatment materials, which typically focus on either molecular antibiotics or nucleic acids, but not both31.

  4. 4.

    Sustainable Visible-Light Regeneration and Longevity: Most high-capacity adsorbents require harsh chemical regeneration, leading to material degradation and secondary pollution. Here, visible-light-driven photocatalysis under mild conditions achieves > 99% contaminant mineralization with > 96% capacity retention over 20 cycles—demonstrating exceptional operational longevity and reusability without toxic reagents. This property is not reported with equivalent stability in the current literature.

  5. 5.

    Validation in Real Wastewater and Continuous Flow Conditions: Unlike the majority of previous studies that rely solely on synthetic media32, this work evaluates the composite in authentic hospital wastewater containing both antibiotics and ARGs. Moreover, continuous-flow column tests (Q = 1 L/min) confirm material scalability and operational durability, marking a critical advancement toward real-world deployment33.

  6. 6.

    Environmental Compatibility and Low Ecotoxicological Risk: ICP-MS confirms metal leaching well below regulatory thresholds, and ecotoxicity assessment (EC₅₀ = 12.7 mg/L) indicates environmental safety at functional dosages. This level of environmental risk profiling is rarely performed with such detail in comparable studies.

Taken together, the present study provides an integrated solution to the co-occurrence problem of chemical and genetic micropollutants in aquatic systems. It surpasses existing technologies not only in performance metrics, but also in mechanistic validation, reusability, and environmental relevance—thereby establishing a new benchmark in the design of multifunctional nanomaterials for advanced wastewater treatment.

Economic analysis and scalability

Cost analysis

Primary cost contributors include.

  • CQDs: ~76% of total material cost ($45/kg composite).

  • Chitosan: ~15% ($8/kg composite).

  • Metal precursors: ~9% ($5/kg composite).

Scale-up considerations

Synthesis optimization reducing CQD loading from 10% to 5% w/w while maintaining > 90% performance could reduce costs by 38%. Continuous-flow studies demonstrate technical feasibility for industrial implementation.

Conclusion

This study successfully developed and characterized a novel magnetically retrievable M-Chit/CQD@ZnFe2O4 nanocomposite for simultaneous removal of tetracycline antibiotics and ARGs from wastewater.

Key Findings:

  1. 1.

    Exceptional Performance: Ultra-high TC adsorption capacity (687.4 ± 12.3 mg/g) achieved through synergistic π-π stacking, metal coordination, and electrostatic interactions.

  2. 2.

    Broad-Spectrum ARG Removal: Efficient capture of multiple resistance genes (tetA: 98.2%, sul1: 82.4%, blaCTX-M-15: 76.8%) via non-sequence-specific electrostatic and hydrogen bonding mechanisms.

  3. 3.

    Sustainable Regeneration: Visible-light photocatalysis enabled > 99% degradation of both chemical and genetic contaminants with > 96% capacity retention over 20 cycles.

  4. 4.

    Real-World Applicability: Superior performance in hospital wastewater (52% better than GAC) with successful continuous-flow operation and magnetic recovery.

  5. 5.

    Environmental Safety: Minimal metal leaching and acceptable ecotoxicity profile support safe deployment.

Mechanistic insights

The tri-component architecture provides multiple orthogonal removal pathways unavailable in conventional materials, while the hierarchical structure optimizes mass transfer and light utilization.

Future directions

Long-term pilot-scale studies, cost optimization through CQD synthesis improvements, and integration with membrane bioreactors represent promising avenues for full-scale implementation.

This research demonstrates the feasibility of engineering multifunctional, sustainable materials for next-generation water treatment systems addressing both chemical pollutants and emerging biological hazards.