A review on bio-electro-Fenton systems as environmentally friendly methods for degradation of environmental organic pollutants in wastewater

Bio-electro-Fenton (BEF) systems have been potentially studied as a promising technology to achieve environmental organic pollutants degradation and bioelectricity generation. The BEF systems are interesting and constantly expanding fields of science and technology. These emerging technologies, coupled with anodic microbial metabolisms and electrochemical Fenton''s reactions, are considered suitable alternatives. Recently, great attention has been paid to BEFs due to special features such as hydrogen peroxide generation, energy saving, high efficiency and energy production, that these features make BEFs outstanding compared with the existing technologies. Despite the advantages of this technology, there are still problems to consider including low production of current density, chemical requirement for pH adjustment, iron sludge formation due to the addition of iron catalysts and costly materials used. This review has described the general features of BEF system, and introduced some operational parameters affecting the performance of BEF system. In addition, the results of published researches about the degradation of persistent organic pollutants and real wastewaters treatment in BEF system are presented. Some challenges and possible future prospects such as suitable methods for improving current generation, selection of electrode materials, and methods for reducing iron residues and application over a wide pH range are also given. Thus, the present review mainly revealed that BEF system is an environmental friendly technology for integrated wastewater treatment and clean energy production.

Bio-electro-Fenton system is a promising technology for the environmental organic pollutants degradation and bioelectricity generation.     

Synthesis and characterization of a novel CNT-FeNi3/DFNS/Cu(ii) magnetic nanocomposite for the photocatalytic degradation of tetracycline in wastewater

Herein, Cu(ii) complexes were anchored within the nanospaces of a magnetic fibrous silicate with a high surface area and easily accessible active sites via a facile approach, leading to the successful synthesis of a novel potent nanocatalyst (FeNi₃/DFNS/Cu). Furthermore, FeNi₃/DFNS/Cu was supported on carbon nanotubes (CNTs) via an usual nozzle electrospinning method (CNT-FeNi₃/DFNS/Cu). In addition, its performance as a photocatalyst for the degradation of tetracycline was tested in a batch reactor. Tetracycline is an antibiotic that is commonly utilized in veterinary medicine and in the treatment of human infections, but is hazardous to aquatic environments. However, the usual processes for the removal of tetracycline are not efficient. The eco-friendly attributes of this catalytic system include high catalytic activity and ease of recovery from the reaction mixture using an external magnet, and it can be reused several times without significant loss in its performance. Also, protocols such as hot filtration, and mercury poisoning provided complete insight into the nature of this heterogeneous catalyst.

Herein, Cu(ii) complexes were anchored within the nanospaces of a magnetic fibrous silicate with a high surface area and easily accessible active sites via a facile approach, leading to the successful synthesis of a novel potent nanocatalyst (FeNi₃/DFNS/Cu).     

Synthesis of Cu3P/SnO2 composites for degradation of tetracycline hydrochloride in wastewater

Antibiotic drugs have become dominating organic pollutants in water resources, and efficient removal of antibiotic drugs is the priority task to protect the water environment. Cu₃P/SnO₂ photocatalysts of various Cu₃P loadings (10–40 wt% Cu₃P) were synthesized using a combination of hydrothermal synthesis and a partial annealing method. Their photocatalytic activity was tested for tetracycline hydrochloride (TC-HCl) degradation under visible light irradiation. Cu₃P/SnO₂ samples were characterized by X-ray diffraction (XRD), N₂-adsorption, ultraviolet-visible diffuse reflectance spectra (UV-vis DRS), scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS). The results showed that the p–n type heterostructure between Cu₃P and SnO₂ was successfully constructed, and addition of Cu₃P to SnO₂ could improve its photocatalytic activity at an optimized loading of 30 wt% Cu₃P. In photocatalytic degradation studies, removal rates of around 80% were found in 30 minutes of dark reaction and 140 min of photodegradation. The removal rate was superior to that of Cu₃P and SnO₂ alone under the same experimental conditions. According to trapping experiments and electron spin resonance (ESR) measurements, photogenerated holes (h⁺) and superoxide radicals ˙O₂⁻ were considered as the main oxidation species in the present system. Finally, the reuse experiments showed high stability of Cu₃P/SnO₂. This study reports Cu₃P as a cocatalyst combined with semiconductor SnO₂ to form a highly efficient heterogeneous photocatalyst for the degradation of tetracycline hydrochloride for the first time.

Cu₃P@SnO₂ was synthesized with Cu₃P nanoparticles loaded on the surface of hollow SnO₂ microspheres via a simple annealing process.     

Study of catalytic ozonation for tetracycline hydrochloride degradation in water by silicate ore supported Co3O4

Tetracycline hydrochloride (TCH) degradation by cobalt modified silicate ore (CoSO) catalytic ozonation in aqueous solution was investigated. CoSO catalyst was synthesized by an impregnation method using Co(NO₃)₂ as the precursor and natural silicon ore (SO) as the support. The key catalyst preparation conditions (i.e., impregnation concentration, calcination temperature and time) were optimized. The activity and stability of CoSO catalyst and its catalytic ozonation mechanism for TCH degradation were studied. The results showed that Co₃O₄ was successfully coated on the silicon ore and the CoSO catalyst was highly efficient in catalytic ozonation for TCH degradation. The TCH removal by CoSO/O₃ could reach 93.2%, while only 69.3% by SO/O₃ and only 46.0% by O₃ alone at 25 min. The reaction of TCH degradation followed pseudo-first order kinetics. TOC removal rate by CoSO/O₃ was 2.0 times higher than that by SO/O₃, and 3.5 times higher than that by O₃ alone. The reaction conditions (TCH initial concentration, catalyst concentration, pH and temperature) for catalytic ozonation were systematically investigated. The possible mechanism for the CoSO catalytic ozonation process was proposed, where hydroxyl radical oxidation mainly accounted for the substantial TCH degradation. Furthermore, CoSO showed great durability and stability after seven reaction cycles.

Antibiotic tetracycline hydrochloride was efficiently degraded by CoSO-catalytic ozonation and the catalytic oxidation mechanism was elucidated.     

Preparation of Cu2O@TiOF2/TiO2 and its photocatalytic degradation of tetracycline hydrochloride wastewater

A new high-efficiency photocatalyst Cu₂O@TiOF₂/TiO₂ was synthesized by a hydrothermal method and applied to the degradation of tetracycline hydrochloride (TTCH). The samples were analyzed by SEM, EDS, XRD, BET, UV-vis DRS, Raman, PL, FT-IR. The Cu : Ti = 1 : 8 catalyst showed a narrow band gap of 2.10 eV, indicating that it can degrade TTCH as a novel photocatalyst capable of responding to sunlight. The average particle diameter is (2–6) nm, and the particle size distribution is narrow. When the reaction was carried out under simulated solar light for 3 hours, the efficiency for degrading 10 mg L⁻¹ tetracycline hydrochloride was as high as 96.83% when the catalyst dosage was 40 mg. It is shown from the capture experiments that ·O₂⁻ and ·OH play a major role in this reaction. In addition, it was found that the degradation of TTCH conforms to the first-order kinetic model.

Cu₂O@TiOF₂/TiO₂ composites with large surfaces were prepared by a hydrothermal method and exhibited excellent activity under simulated solar light, showing high efficiency for tetracycline hydrochloride photocatalytic degradation, and reusability.     

A catalytic ozonation process using MgO/persulfate for degradation of cyanide in industrial wastewater: mechanistic interpretation,kinetics and by-products

Cyanide-laden wastewaters generated from mining and electroplating industries are extremely toxic and it is of vital importance to treat them prior to discharge to receiving water resources. The present study aims to oxidize cyanide using an ozonation process catalyzed by MgO and persulfate (PS). A MgO nanocatalyst was synthesized using the sol–gel method and characterized. The results show that the synthesized catalyst had a BET surface area of 198.3 m² g⁻¹ with a nanocrystalline particle size of 7.42 nm. In the present study, the effects of different operational parameters were investigated, and it was found that the MgO/O₃/PS process is able to oxidize 100 mg L⁻¹ of cyanide after 30 min under optimum operational conditions. Cyanide degradation mechanisms in the MgO/O₃/PS process were completely investigated and the main radical species were identified using scavenging experiments. It was found that sulfate and hydroxyl radicals both contributed to the cyanide degradation in the MgO/O₃/PS process. Cyanide degradation by-products were also tracked and it was found that cyanate and ammonium species are primarily generated during the oxidation, but increase of reaction time allowed their conversion to much less toxic compounds such as nitrate and bicarbonate. Cyanide degradation was also conducted in real industrial wastewater containing 173 mg L⁻¹ of cyanide. Although there was a reduction in cyanide removal rate, the MgO/O₃/PS process was able to completely oxidize cyanide within 70 min. Finally, it can be concluded that the ozonation process catalyzed by MgO and persulfate is an efficient and reliable advanced oxidation process for removal of cyanide from industrial wastewater.

Cyanide-laden wastewaters are extremely toxic and it is of vital importance to treat them prior to discharge to receiving water resources.     

Comparing the efficacy of various methods for sulfate radical generation for antibiotics degradation in synthetic wastewater: degradation mechanism,kinetics study,and toxicity assessment

In the present study the aim was to investigate and compare various activation processes for amoxicillin degradation. UV radiation, ultrasound, heat, and hydrogen peroxide were selected as the persulfate activation methods. The effects of various parameters such as pH, persulfate concentration, reaction time, AMX concentration, radical scavengers, and anions were thoroughly investigated. The results showed that AMX degradation was following the pseudo-first order kinetic model. The reaction rate of 0.114 min⁻¹ was calculated for the UV/PS process, which was higher than that of the other investigated processes. The AMX degradation mechanism and pathway investigations revealed that sulfate and hydroxyl radicals were responsible for the degradation of AMX by two degradation pathways of hydroxylation and the opening of the β-lactam ring. Competition kinetic analysis showed that the second-order rate constant of AMX with sulfate radicals was 8.56 × 10⁹ L mol⁻¹ s⁻¹ in the UV/PS process. Cost analysis was conducted for the four investigated processes and it was found that 1.9 $m⁻³ per order is required in the UV/PS process for the complete destruction of AMX. Finally, cytotoxic assessment of the treated effluent on human embryonic kidney cells showed a considerable reduction in AMX-induced cell cytotoxicity, proving that the investigated process is sufficiently capable of completely destroying AMX molecules to nontoxic compounds. Therefore, it can be concluded that UV radiation is much more effective than other methods for persulfate activation and can be considered as a reliable technique for antibiotic removal.

In the present study the aim was to investigate and compare various methods of persulfate activation for amoxicillin degradation.     

Improving the efficiency of Fenton reactions and their application in the degradation of benzimidazole in wastewater

Reducing the quantity of sludge produced in Fenton reactions can be partly achieved by improving their efficiency. This paper firstly studies the effect of uniform deceleration feeding (ferrous iron and hydrogen peroxide) on the efficiency of a Fenton reaction by measuring the yield of hydroxyl radicals (˙OH) and chemical oxygen demand (COD) removal rate. The dynamic behavior of ˙OH was also investigated. The results indicated that uniform deceleration feeding was the best feeding method compared with one-time feeding and uniform feeding methods when the same amount of Fenton reagents and the same reaction times were used. Besides, it was found the COD removal rate reached 79.3% when this method was applied to degrade 2-(a-hydroxyethyl)benzimidazole (HEBZ); this COD removal rate is larger than those when the other two modes were used (they reached 60.7% and 72.1%, respectively). The degradation pathway of HEBZ was determined using PL, UV-vis, FTIR, HPLC and GC-MS. Ultimately, HEBZ was decomposed into three small molecules (2-hydroxypropylamine, oxalic acid, and 2-hydroxypropamide). This research is of great significance for the application of Fenton reactions in wastewater treatment.

Improving the efficiency of Fenton reactions and their application in the degradation of benzimidazole in wastewater.     

Enhanced degradation of dimethyl phthalate in wastewater via heterogeneous catalytic ozonation process: performances and mechanisms

Ozonation process is a promising yet challenging method for the removal of refractory organic matter due to the sluggish reaction for generating hydroxyl radical (˙OH) at a neutral pH condition. Herein, an efficient heterogeneous catalytic ozonation system using CeO₂/Al₂O₃ catalyst was developed to remove dimethyl phthalate (DMP) from wastewater. Under a neutral condition of pH = 6, this system achieved almost 100% DMP removal within 15 min at an optimized catalyst dosage of 30 g L⁻¹ and the ozone flow rate of 22.5 mg min⁻¹. Moreover, the catalytic ozonation system exhibited a stable degradation performance of DMP in a wider pH range (pH = 5–10). The results of electron paramagnetic resonance (EPR) and quantitative tests confirmed the ultrafast conversion of O₃ to ˙OH (0.774 μM min⁻¹) on the surface of CeO₂ based ceramic catalyst. The quenching experiments further supported the predominant role of ˙OH in the mineralization of DMP. These results highlight the potential of using the heterogeneous catalytic ozonation system for the efficient removal of refractory organic matter from wastewater.

An efficient heterogeneous catalytic ozonation system using CeO₂/Al₂O₃ catalyst was developed to remove dimethyl phthalate (DMP) from wastewater.     

Facile microwave-assisted synthesis of In2O3 nanocubes and their application in photocatalytic degradation of tetracycline

Novel In₂O₃ nanocube photocatalysts were successfully prepared by a facile microwave-assisted synthesis method. The obtained products are nano-sized with square corners and high crystallinity. The In₂O₃ nanocubes possessed high efficiency of electron–hole separation, resulting in high photocatalytic activities for the degradation of tetracycline under both visible light (λ > 420 nm) and full-range light irradiation (360–760 nm), the ratios of which are 39.3% and 61.0%, respectively. Moreover, the calculated positions of the CB and VB under our experimental conditions at the point of zero for In₂O₃ nanocubes are −0.60 V and +2.17 V, respectively. Note that the redox potentials of [O₂/·O₂⁻] and [·OH/OH⁻] are −0.33 V and +2.38 V, respectively, which means that the irradiated In₂O₃ nanocubes can reduce O₂ into ·O₂⁻ without oxidizing OH⁻ into ·OH. It can be concluded that ·O₂⁻ and h⁺ are the main active species of In₂O₃ in aqueous solution under visible light irradiation and full-range light irradiation.

Novel In₂O₃ nanocube photocatalysts were successfully prepared by a facile microwave-assisted synthesis method.     

Nanorod bundle-like silver cyanamide nanocrystals for the high-efficiency photocatalytic degradation of tetracycline

Silver cyanamide (Ag₂NCN) is a type of functional semiconductor material with a visible-light response. Ag₂NCN nanocrystals with nanorod bundle-like (RB) or straw bundle-like (SB) assemblies were successfully prepared, and it was found that the as-prepared Ag₂NCN nanorod bundle (RB) samples had a narrower bandgap of 2.16 eV, which was lower than those reported. As a result, RB samples demonstrated a higher photocatalytic activity towards tetracycline (TC) degradation. The analyses of active species confirmed that both the photo-generated holes and ˙O₂⁻ radicals of the RB sample played significant roles during the process of photocatalytic degradation of TC, and the holes were the main active species. These results indicated that effective charge separation could be achieved by adjusting the morphologies of Ag₂NCN nanocrystals. This study provides a new approach to prepare Ag₂NCN nanocrystals with a narrower bandgap and strong visible-light response towards antibiotic degradation.

Ag₂NCN nanoparticles with nanorod bundle-like assemblies and a narrower bandgap were synthesized and demonstrated a high photocatalytic activity towards TC degradation.     

Enzymic degradation of a beta-lactam antibiotic,ampicillin, in the gut: a novel treatment modality

Antibiotics can cause severe alterations in the gut microflora and promote diarrhoea and overgrowth of pathogenic bacteria. The present study investigated the potency of targeted recombinant beta-lactamase (TRBL) to degrade a beta-lactam antibiotic in the jejunum of fistula-operated beagles. We used different peroral doses of purified beta-lactamase (PenP) of Bacillus licheniformis in enteric-coated pellets together with intravenous ampicillin. Serum and jejunal samples were collected for ampicillin and beta-lactamase analysis. A dose-response effect of TRBL on ampicillin concentrations in the jejunal samples could be observed. The highest doses applied decreased the jejunal ampicillin concentrations to undetectable levels. In the serum samples, the ampicillin concentrations were not affected by the beta-lactamase dose used. Our results indicate that it may be possible to evolve a targeted treatment to degrade beta-lactam antibiotics intestinally and, thus, decrease antibiotic-induced adverse effects on the gut microflora.     

Green synthesis of ZnO coated hybrid biochar for the synchronous removal of ciprofloxacin and tetracycline in wastewater

Preparation of biochar from kaolinite and coconut husk (KCB) and further activated with HCl (KCB-A) and KOH (KCB-B) via a microwave technique for the remediation of ciprofloxacin (CIP) and tetracycline (TET) from water was carried out. Characterization using scanning electron microscopy, energy dispersive X-ray, Fourier transform infrared spectroscopy and X-ray diffraction showed the successful synthesis of functionalized biochars. Batch adsorption experiments demonstrated the potential of the adsorbents for fast and efficient removal of CIP and TET from solution. The adsorption capacities were found to be 71, 140 and 229 mg g⁻¹ for CIP and 118, 117 and 232 mg g⁻¹ for TET removal on KCB, KCB-A and KCB-B, respectively. For KCB, KCB-B and KCB-B, CIP adsorption best followed the pseudo second order kinetic model (PSOM), pseudo first order kinetic model (PFOM) and intraparticle diffusion (IDP) respectively. TET adsorption followed PSOM for KCB, IPD for KCB-B and PFOM for KCB-A. CIP adsorption on KCB, KCB-A and KCB-B best fit the Temkin, Langmuir and Brouers–Sotolongo isotherms, respectively, and TET adsorption on KCB best fit Brouers–Sotolongo while KCB-A and KCB-B best fit Langmuir–Freundlich. Adsorption of both contaminants was thermodynamically feasible showing that these materials are excellent adsorbents for the treatment of pharmaceuticals in water.

Preparation of biochar from kaolinite and coconut husk (KCB) and further activated with HCl (KCB-A) and KOH (KCB-B) via a microwave technique for the remediation of ciprofloxacin (CIP) and tetracycline (TET) from water was carried out.     

Development and application of novel bio-magnetic membrane capsules for the removal of the cationic dye malachite green in wastewater treatment

Novel bio-magnetic membrane capsules (BMMCs) were prepared by a simple two-step titration-gel cross-linking method using a polyvinyl alcohol (PVA) and sodium alginate (SA) matrix to control the disintegration of phytogenic magnetic nanoparticles (PMNPs) in an aqueous environment, and their performance was investigated for adsorbing cationic malachite green (MG) dye from water. The prepared BMMCs were characterized by FTIR, powder XRD, SEM, EDX, XPS, VSM and TGA techniques. The findings revealed that the hysteresis loops had an excellent superparamagnetic nature with saturation magnetization values of 11.02 emu g⁻¹. The prepared BMMCs not only controlled the oxidation of PMNPs but also improved the adsorptive performance with respect to MG dye (500 mg g⁻¹ at 298.15 K and pH 6.5) due to the presence of a large amount of hydrophilic functional groups (hydroxyl/–OH and carboxyl/–COOH) on/in the BMMCs. The smooth encapsulation of PMNPs into the PVA–SA matrix established additional hydrogen bonding among polymer molecular chains, with improved stability, and adsorptive performance was maintained over a wide range of pH values (3–12). Importantly, the prepared BMMCs were easily regenerated just by washing with water, and they could be re-utilized for up to four (4) consecutive treatment cycles without observing any apparent dissolution of iron/Fe⁰ or damage to the morphology. According to the mass balance approach, an estimated amount of 100 mL of treated effluent can be obtained from 160 mL of MG dye solution (25 mg L⁻¹) just by employing a 0.02 g L⁻¹ adsorbent dosage. Finally, a model of BMMCs based on zero-effluent discharge was also proposed for commercial or industrial applications. The prepared BMMCs are greatly needed for improving the water/wastewater treatment process and they can be utilized as an excellent adsorbent to remove cationic pollutants for various environmental applications.

Novel bio-magnetic membrane capsules were prepared by a simple two-step titration-gel cross-linking method using a polyvinyl alcohol and sodium alginate matrix to control the disintegration of phytogenic magnetic nanoparticles in aqueous media.     

Visible-light-driven CQDs@MIL-125(Ti) nanocomposite photocatalyst with enhanced photocatalytic activity for the degradation of tetracycline

In the present study, a novel photocatalyst, CQDs@MIL-125(Ti) (CQDs = carbon quantum dots), was prepared via a solvothermal procedure. The photocatalytic properties were tested by the degradation of tetracycline (TC) with a 250 W Xe lamp (λ > 420 nm). Compared with pure MIL-125(Ti), the 10 wt% CQDs@MIL-125(Ti) photocatalyst can significantly improve the degradation process of TC, and the degradation efficiency can reach 90% within 4 h. The enhancement in the photocatalytic performance is due to the CQDs, which can promote the absorption of visible light and also efficiently accelerate the separation of photogenerated electron–hole pairs. We have also demonstrated that superoxide radicals (·O₂⁻) and holes (h⁺) play crucial roles in the photocatalytic degradation of TC through capture experiments. The current work provides a new idea for constructing high-efficiency photocatalysts based on MIL-125(Ti).

A novel CQDs@MIL-125(Ti) photocatalyst had been synthesized by a facile solvothermal process for the degradation of TC. The possible degradation mechanism was proposed based on the active species trapping experiments.     

A new cobalt(ii) meso-porphyrin: synthesis,characterization, electric properties and application in the catalytic degradation of dyes

In this work, a new porphyrin, 5,10,15,20-tetrakis{4-[((4-methoxyphenyl)acetyl)oxy]phenyl}porphyrin (H₂TMAPP) (1), and its cobalt complex [Co^II(TMAPP)] (2) were synthesized in good and quantitative yields, respectively. The chemical structures of these synthesized compounds were confirmed by FT-IR, ¹H NMR, MS, UV-visible, and fluorescence spectroscopy. Their photophysical properties, namely their molar extinction coefficients (∑), fluorescence quantum yields (Φ_f) and lifetimes (τ_f), were determined and compared with those of meso-tetraphenylporphyrin. Furthermore, their electrochemical behaviours were examined using cyclic voltammetry (CV). Dielectric properties such as the conductivity (σ) and the real (M′) and imaginary (M′′) parts of the dielectric modulus were investigated as a function of temperature and frequency. The impedance analysis was carried out using Cole–Cole plots to elucidate the electrical conduction mechanism. The catalytic power and the adsorption properties of the prepared compounds were studied for methylene blue (MB) and crystal violet (CV) degradation. The results reveal that the studied compound [Co^II(TMAPP)] can be used as a catalyst for the decolourisation of dyes in the presence of H₂O₂.

In this work, a new porphyrin, 5,10,15,20-tetrakis{4-[((4-methoxyphenyl)acetyl)oxy]phenyl}porphyrin (H₂TMAPP) (1), and its cobalt complex [Co^II(TMAPP)] (2) were synthesized in good and quantitative yields, respectively.     

A simple hydrothermal route for the preparation of novel Na–Y–W nano-oxides and their application in dye degradation

The fabricated NaY(WO₄)₂ was identified through diverse analysis methods. Therefore, to optimize NaY(WO₄)₂ morphology, saccharide carbohydrates were manipulated as a capping agent. In this study, glucose, fructose, lactose, cellulose, and starch were utilized as the capping agents. SEM images show that fructose was the optimal capping agent for achieving uniform and well-shaped nanoparticles. The photodegradation of organic dyes such as M.O and Rd.B by NaY(WO₄)₂ was evaluated under UV and Vis light. The bandgap energy of the as-prepared sample was measured by the Tauc plot, and was found to be nearly 3.85 eV. To study the photocatalytic characteristics, the influence of dye dosage and reusability on photodegradation behavior were investigated.

NaY(WO₄)₂ nanoparticles were fabricated via a simple hydrothermal method using saccharide carbohydrates as capping agents. The photocatalytic behavior of the as-prepared NaY(WO₄)₂ nanostructures was studied.     

Facile assembly of novel g-C3N4@expanded graphite and surface loading of nano zero-valent iron for enhanced synergistic degradation of tetracycline

The two-stage removal process of tetracycline (TC) in aqueous solutions using a novel photocatalyst based on nano-zero-valent iron (NZVI), g-C₃N₄ and expanded graphite by carbon layer (EGC) is reported for the first time. The composite (NZVI/g-C₃N₄@EGC) exhibits remarkable adsorption, reduction ability and visible light activity over the reaction course. Compared with pristine g-C₃N₄ (25.9%) and pure NZVI (45.9%), NZVI/g-C₃N₄@EGC achieves high degradation efficiency of TC (98.5%) due to the formation of a heterogeneous photo-Fenton system. This study shows that synergistic effects are achieved in the reaction system, including maintaining the reduction ability of NZVI and enhancing the photocatalytic activity of g-C₃N₄ by facilitating the separation of photogenerated electrons (e⁻) and holes (h⁺). TC removal involved a two-stage process of adsorption–reduction and photo-degradation. The quencher experiments determined that holes (h⁺) and superoxide radicals (˙O₂⁻) are the major reactive species in the degradation of TC. The degradation pathways of TC were proposed based on the analysis of the intermediates. In addition, NZVI/g-C₃N₄@EGC revealed a high stability in a five-cycle test and good magnetic properties for facile separation from aqueous solutions. From an application viewpoint, NZVI/g-C₃N₄@EGC has favorable prospects in the direction of the photocatalytic degradation of antibiotic wastewater.

The two-stage removal process of tetracycline (TC) in aqueous solutions using a novel photocatalyst based on nano-zero-valent iron (NZVI), g-C₃N₄ and expanded graphite by carbon layer (EGC) is reported for the first time.