Electrochemical degradation of chloramphenicol using Ti-based SnO₂–Sb–Ni electrode

Antibiotics have been detected in different water bodies and reported as a new environmental problem (Zhou et al. 2019). CAP is an effective broad-spectrum antibiotic that has been used for treating respiratory and bacterial infections in both humans and animals (Radko et al. 2019; Yu et al. 2019). CAP is effective against several Gram-positive, Gram-negative bacteria and groups of microorganisms. It has been detected in sewage treatment plant (STP) effluents (Amildon Ricardo et al. 2018; Yerabham et al. 2020) at concentrations of 47 μg L⁻¹ in China and 0.56 μg L⁻¹ in Germany. Concentrations of 0.6–11.2 μg L⁻¹ in China and 30–80 ng L⁻¹ in Kenya (Kimosop et al. 2016; Yerabham et al. 2020) have also been reported in surface waters (SW). CAP is toxic to human health. Its presence in the aquatic environment may have harmful effects on aquatic organisms (Sun et al. 2019; Wu et al. 2020).

Ti/SnO₂–Sb electrodes have been studied extensively in recent years (Stucki et al. 1991; Comninellis & Pulgarin 1993). To further improve the catalytic activity of the electrode, doping various metals on the surface of the electrode has become a research hotspot, such as Pt (Montilla et al. 2004; Berenguer et al. 2014; Fernández-Aguirre et al. 2020), Ru (Berenguer et al. 2014), Ni (Hamed & Amini 2011), etc. Transition metals with d-orbitals and unpaired d-electrons will form various characteristic adsorption bonds on these d-orbitals when contacting the electrons of reactant molecules to achieve the purpose of molecular activation, thus reducing the activation energy of complex reactions and promoting electrocatalytic reactions (Sharpe et al. 2018). Moreover, the cost of transition metals is lower than the cost of precious metals (Zhang et al. 2020a,2020b).

In this work, the Ti/SnO₂–Sb electrodes doped with nickel (transition metals) were first proposed to remove CAP. A detailed investigation of reaction kinetics of CAP degradation on the Ti-based SnO₂–Sb–Ni electrode was conducted. The influence factors (applied current, initial pH, initial concentration and electrolyte) on the removal ratio and mineralization ratio were systematically investigated, and an adequate explanation was provided accordingly. The total organic carbon (TOC) and antibacterial activity were used to evaluate the entire electrochemical process.

CAP with a purity of 98% was provided by Aladdin Industrial Corporation (Shanghai, China). Na₂SO₄, Sb₂O₃, Ni(NO₃)₂·6H₂O, NaOH, NaCl, phosphoric acid, oxalate, agar powder, beef paste, peptone, glycol, ethanol, methanol and acetonitrile were provided by Da Mao Chemical Reagent Factory, China. SnCl₄·5H₂O, citric acid and ammonia were provided by Dong Xing Chemical Reagent Factory, China. All chemicals were of analytical grade. Ultrapure water was obtained from a reverse osmosis control system.

The Ti-based SnO₂–Sb–Ni electrodes were prepared using a sol–gel method. Titanium plates (purity 99.9%, 40 mm × 5 mm × 2 mm) were polished with the increasing grits (120-grit, 240-grit and 360-grit) of abrasive papers to obtain a mirror-like finish, then soaked in 40 wt% NaOH solution at 80 °C for 1 h, and then etched in 10 wt% oxalic acid for 1 h. The pre-treated titanium plates were washed with distilled water and stored in ultrapure water. The precursor solutions were prepared as follows: the Sb₂O₃ was dissolved in concentrated HCl, the SnCl₄·5H₂O and a certain amount of Ni(NO₃)₂•6H₂O were dissolved in ethanol: the Sn:Sb:Ni molar ratio for the film preparation was 100:3:1. The total concentration of Sn ions was kept constant and was 0.5 mol L⁻¹. Citric acid and ethylene glycol were added to the metal precursor solutions. The molar ratio of metal, citric acid and ethylene glycol was 1:3:3 (Liang et al. 2015). The solution was stirred in a water bath (60 °C) for 60 min, and stood for 120 min to form a gel. The Ti-based SnO₂–Sb–Ni electrodes were prepared using a dip coating technique. The pre-treated Ti plates were dipped in the precursor solutions for 10 min, then pulled through a liquid level at 1 mm s⁻¹. The process of dipping and raising was carried out using a coating puller machine (ZR4200, Junray, Qingdao). After that, the electrodes were dried for solvent evaporation in an infrared oven at 120 °C for 10 min, and heated in a muffle furnace under air at 450 °C for 10 min. The above procedures were repeated 15 times. Finally, the electrodes were heated in a muffle furnace at 600 °C for 2 h.

The Ti-based SnO₂–Sb electrodes were prepared in a similar method, except that nickel was not added.

The morphologies of Ti/SnO₂–Sb–Ni electrodes were examined using a scanning electron microscopy (SEM,S8010, Hitachi, Japan). The microstructures of the electrodes were investigated by X-ray diffraction (XRD, D/max2500pc, Shimadzu, Japan), with CuKα radiation, an operating voltage of 40 kV and a current of 30 mA.

The CAP degradation experiments were carried out in a 400 mL single-chamber reactor equipped with a magnetic stirrer at room temperature. A DC power-supply (PS-303D, Zhao Xin, China) was used for the electrolysis experiment. The self-made Ti/SnO₂–Sb–Ni electrode was used as the anode; two stainless steel substrates with the same areas were used as cathodes on both sides of the anode. The distance between the anode and the cathode was 20 mm. The experiments were carried out for a period of 300 min under different conditions. In order to explore the role of anode and cathode in the process of electrocatalytic reaction, a double chamber reactor was designed. The cathode chamber (200 mL) and anode chamber (200 mL) were separated by a cation exchange membrane. The self-made Ti/SnO₂–Sb–Ni electrode was used as the anode; the stainless steel substrate with the same areas was used as the cathode. The experiments were carried out for a period of 300 min at 20 mA cm⁻². During the experiment, the concentrations of CAP and TOC were measured every 30 min.

A LC1200 high-performance liquid chromatograph (Agilent), equipped with a UV-DAD detector (Agilent) and a Phenomenex C-18 column (Luna; 5 μm, 250 mm × 4.6 mm) as the stationary phase, was used to monitor the CAP concentrations during the electrolysis experiments. Phosphoric acid/acetonitrile mixtures were used as the mobile phase during the measurements. In each assay, a 10 μL sample was eluted isocratically using a 68:32 phosphoric acid/acetonitrile mixture at a flow rate of 1 mL min⁻¹. Under these conditions, the retentiontime for CAP was 14 ± 0.5 min. TOC analyser (N/C 3100, Analytik Jena AG, Germany) was used to monitor the TOC concentrations during the electrolysis experiments. The residual antibacterial activity of CAP in the degradation process was tested by E. coli beef extract and peptone solid medium (LB medium). Here, 3 g of beef extract, 10 g of peptone, 18 g of agar and a certain amount of sodium chloride were dissolved in 1,000 mL distilled water. The pH of the solution was adjusted to 7.0–7.2 and the solution was sterilized at 121 °C for 20 min to obtain LB medium. The LB medium was poured into a surface dish, and the Oxford cup was placed in the center of the solidified LB medium; 20 μL of sample was injected into the Oxford cup. The dish was placed in a constant temperature incubator and incubated at a specified temperature for 24 h. Finally, the diameter of bacteriostatic ring was measured.

The morphology of the electrodes was observed in the SEM images (Figure 1). The rough and uniformed surface of the pre-treated Ti substrate was observed in Figure 1(a). This is beneficial to form coatings(Montilla et al. 2004). The SEM images of Ti/SnO₂–Sb and Ti/SnO₂–Sb–Ni electrodes are shown in Figure 1(b) and 1(c). The coating particles of Ti/SnO₂–Sb electrode doped with Ni were more compact and uniform than those of the Ti/SnO₂–Sb electrode. This was beneficial for catalysis. The surface morphology of the Ti/SnO₂–Sb–Ni electrode after electrolysis of CAP was similar to that of the unused electrode, as shown in Figure 1(d). After 30 rounds of CAP degradation in the laboratory, the surface morphology of the Ti/SnO₂–Sb–Ni electrode was almost unchanged, and the effect of electrocatalytic degradation of CAP was still good.

The XRD patterns of the Ti/SnO₂–Sb, Ti/SnO₂–Sb–Ni, used Ti/SnO₂–Sb–Ni and Ti substrate are shown in Figure 2. The intensity of the diffraction peak of Ti/SnO₂–Sb electrode doped with Ni element decreases and the peak broadens slightly. This indicates that the size of crystal particles on the surface of the electrode decreased, which was conducive to the formation of active substances for electrocatalytic oxidation. No significant peaks were obtained for the nickel oxide. Ni could be embedded in SnO₂ crystal lattice in the form of atomic Ni rather than in the form of oxide crystal (Liang et al. 2016).

The oxidation of CAP on Ti/SnO₂–Sb and Ti/SnO₂–Sb–Ni electrodes was investigated. Each data point was the average of three independent experiments. Here, 100 mg L⁻¹ CAP was electrolyzed at the current density of 20 mA cm⁻² in a neutral solution of 35.5 g L⁻¹ Na₂SO₄. The removal efficiencies of CAP and TOC on Ti/SnO₂–Sb and Ti/SnO₂–Sb–Ni electrodes were shown in Figure 3. The removal ratios of CAP on Ti/SnO₂–Sb and Ti/SnO₂–Sb–Ni electrodes were 92.50 and 100%, respectively (Figure 3(a)). The removal ratios of TOC on Ti/SnO₂–Sb and Ti/SnO₂–Sb–Ni electrodes were 45.00 and 56.78%, respectively (Figure 3(b)). The removal ratios of CAP and removal ratio of TOC on Ti/SnO₂–Sb–Ni electrode were higher than those on the Ti/SnO₂–Sb electrode. This could be because transition metal nickel has d-orbitals and unpaired d-electrons. When it contacts the electron of reactants, various characteristic adsorption bonds are formed on the d-orbitals (Sharpe et al. 2018). This is beneficial to the oxidation of CAP and by-products. Moreover, the surface morphology of the electrode has a certain effect, as shown by SEM. The influence of the production of active substances on CAP oxidation is discussed in the following sections. Therefore, the Ti/SnO₂–Sb–Ni electrode had a better performance for CAP oxidation than the Ti/SnO₂–Sb electrode. Additionally, the electrodes can be reused more than 30 times with good stability.

In this part, the effects of current density, initial pH, initial CAP concentration and electrolyte on CAP removal by using Ti/SnO₂–Sb–Ni as anode are discussed. Each data point was the average of three independent experiments.

The effect of applied current density (i = 10–30 mA cm⁻²) on EO process parameters by Ti/SnO₂–Sb–Ni electrodes was studied systematically under the conditions of pH 7, the initial concentration of CAP 100 mg L⁻¹(C₀ = 100 mg L⁻¹), and the initial concentration of Na₂SO₄ 35.5 g L⁻¹(S₀ = 35.5 g L⁻¹). Figure 4(a) shows the effect of different values of applied i (mA cm⁻²) on the electrolytic removal ratio of CAP. When i value increased from 10 to 20 mA cm⁻², the removal ratio of CAP increased from 39.98% to 100%. With the increase of applied current, the number of oxidant species (chemisorbed and physiosorbed ·OH) produced on the surface of the anode increased significantly, which improved the removal ratio of CAP (Rubí-Juárez et al. 2016; Kaur et al. 2018). When i value increased from 20 to 30 mA cm⁻², the removal ratio decreased from 100% to 81.19%. The inevitable reaction of oxygen evolution on an anode surface at high current intensity competed with the removal of CAP, which resulted in the decrease of the removal ratio of CAP (He et al. 2019; Kaur et al. 2019). The removal ratio of CAP at 300 min was the highest, approaching 100%, at the applied current density of 20 mA cm⁻².

TOC removal ratio was tested in the same range of current density (10–30 mA cm⁻²) (Figure 4(b)). When the current density was 20 mA cm⁻², the removal ratio of TOC was the highest and reached 56.78%, indicating that most of the organic matter was converted into inorganic matter, and the mineralization ratio was better without secondary pollution.

The removal ratio of organic pollutants by EO is highly correlated with the pH value. The property of various oxidation species generated on the anode surface and in the bulk solution largely depends on the pH of the treated wastewater. Effect of initial pH (3–11) of the solution on CAP removal ratio (Figure 5(a)) and TOC removal ratio (Figure 5(b)) was studied at i = 20 mA cm⁻², C₀ = 100 mg L⁻¹ and S₀ = 35.5 g L⁻¹. As shown in Figure 4(a), the removal ratio of CAP increased with time. With the increase of solution pH from 3 to 7, the removal ratio of CAP first decreased and then increased. CAP removal ratio weakened with the increase of the solution pH from 7 to 11. CAP removal ratio is the highest at pH 3 and 7, close to 100%. This is because the anodic oxygen evolution potential in acidic solution is high and competitive oxygen evolution reaction does not easily occur. With the increase of pH value, the oxidation potential of ·OH decreases and the oxidation ability decreases, which results in the decrease of CAP degradation ratio. When pH continues to rise to 7, H⁺ in the solution is neutralized, which is conducive to the formation of ·OH. When the pH value reaches 11, the solution is in a strong alkali state, and the oxygen evolution potential decreases, which is prone to oxygen evolution reaction (Qiang et al. 2010; Asim et al. 2017). As shown in Figure 4(b), TOC removal ratio was 60.60% and 56.68% at pH 3 and 7, respectively. This indicated that most of the organic matter was degraded into inorganic matter without secondary pollution. Therefore, for the neutral CAP wastewater, it was not necessary to adjust the initial pH to effectively remove CAP by electrocatalytic oxidation.

The effect of initial CAP concentration (50–200 mg L⁻¹) of the solution on CAP removal ratio (Figure 6(a)) and TOC removal ratio (Figure 6(b)) was studied at i = 20 mA cm⁻², pH = 7 and S₀ = 35.5 g L⁻¹. It is shown in the figure that the degradation ratio of CAP was as high as 90% with the initial concentration of CAP from 50 to 200 mg L⁻¹ and the electrolysis time was 300 min. Compared with other concentrations, the degradation ratio of CAP at an initial concentration of 200 mg L⁻¹ at 300 min was apparently lower. This may due to the degradation ratio of CAP, which decreases slightly as the concentration of CAP increases, and the intermediate product consumes part of ·OH to compete with CAP. Conversely, the concentration of CAP was too high to degrade CAP completely in the system. It is shown in the figure that the removal ratio of TOC by electrocatalytic oxidation of CAP with medium concentration (80 mg L⁻¹ and 100 mg L⁻¹) was approximately 60.0%.

Kinetics curve parameters of CAP removal on Ni²⁺-doped Ti/SnO₂–Sb electrode at different initial CAP concentrations are shown in Figure 7. In the figure, ln(C₀/C_t) was linear with time. The correlation coefficients were 0.982, 0.981, 0.997, 0.996 and 0.998 at different initial CAP concentrations (50 mg L⁻¹, 80 mg L⁻¹, 100 mg L⁻¹, 150 mg L⁻¹ and 200 mg L⁻¹), respectively. This indicated that the oxidation of CAP on the Ni²⁺-doped Ti/SnO₂–Sb electrode displayed pseudo-first-order kinetic behavior. The higher the initial concentration of CAP, the higher was the reaction rate constant and the higher was the reaction rate.

The electrolyte concentration has a significant effect on a conductance of the solution. CAP removal depends on a conductance of the solution too. The variation of CAP removal ratio with time is shown in Figure 9. The inner figure shows the variation of conductance with Na₂SO₄ concentration. Conductance of the solution was increasing with concentration of Na₂SO₄ when Na₂SO₄ concentration varied from 1.25 g L⁻¹ to 25 g L⁻¹. The CAP removal ratio was 98.0% at Na₂SO₄ concentration of 6.25 g L⁻¹. When the concentration of electrolyte in the solution was excessively high, the generation of free radicals was inhibited. Excessive ions affected the diffusion of free radicals to aqueous solution, which affected the degradation of organic pollutants in water.

Figure 10 shows the effects of ethanol and tertiary butanol (free radical quencher) on the removal of CAP (Figure 9(a)) and TOC (Figure 9(b)). Ethanol was used to quench ·OH and ·SO₄⁻, and tertiary butanol was used to quench ·OH (Tan et al. 2018). The removal ratio of CAP with ethanol (42.0%) was much lower than that without quencher(100.0%). The removal ratio of CAP with tertiary butanol was slightly lower than that without a quencher. This indicated that in this electrochemical system, ·SO₄⁻ and direct oxidation contributed much to the degradation of CAP. In the absence of quencher, the mineralization of CAP was highest. Therefore, the degradation of CAP on the Ti-based SnO₂–Sb–Ni electrode in a single chamber resulted from the synergistic effect of direct oxidation and indirect oxidation (·OH and ·SO₄⁻).

The treatment of CAP by electrochemical processes was conducted in a divided chamber. CAP was oxidized in an anode chamber and reduced in a cathode chamber. The removal ratio of CAP, the removal ratio of TOC and antibacterial activity by oxidation and reduction are compared in Table 1. Through the comparison, CAP was completely removed by reduction in the cathode chamber. Nevertheless, the removal ratio of TOC in the cathode chamber was low. The cathodic reduction can only remove halogen and nitro group from CAP and reduce toxicity. CAP cannot be completely mineralized by cathodic reduction. This was consistent with the results reported in the literature. For the CAP reduction, four main products, nitroso product (NO), aromatic amine product (AMCl₂) and dechlorinated AMCl₂ products (AMCl and AM) were detected using high performance liquid chromatography. The intermediate NO slightly accumulated initially and then quickly reduced to amine products under the cathode potentials between −0.75 and −1.25 V. AMCl₂ could be accumulated when the cathode potential was higher than −0.75 V. When the cathode potential was lower than −0.41 V, AMCl₂ could be fractional reduced to the dechlorinated product AMCl when the cathode potential was −0.45 V. AMCl could be further dechlorinated only when the cathode potential was lower than −0.95 V (Kong et al. 2015). The mineralization of CAP was improved by anodic oxidation in the anode chamber. In a single chamber, the effects of CAP removal and mineralization are enhanced, which were contributed to the synergism of oxidation and reduction. After 300 min reaction, CAP had no antimicrobial activity in a single chamber and anode chamber. CAP maintained certain antimicrobial activity in the cathode chamber. This may be due to the removal of functional groups with antimicrobial activity by anodic oxidation. Therefore, anodic oxidation was more effective for the removal of CAP. CAP was almost eliminated by cathodic reduction and the mineralization ratio was quite low. However, it may be easier to achieve complete mineralization by further treatment after cathodic reduction (Liang et al. 2012).

This study reported the preparation of a Ti-based SnO₂–Sb–Ni electrode and its application for the degradation of CAP. Due to the doping of transition metal nickel, the surface of the Ti-based SnO₂–Sb–Ni electrode was more compact and uniform than the Ti-based SnO₂–Sb electrode. Compared to the Ti-based SnO₂–Sb electrode, the addition of nickel improved the oxidation performance of the Ti-based SnO₂–Sb–Ni electrode and the mineralization of CAP. For 100 mg L⁻¹ CAP, 100% CAP removal ratio and 60% TOC removal ratio could be achieved for 300 min treatment at the current density of 20 mA cm⁻² and in neutral electrolyte (6.25 g L⁻¹ Na₂SO₄). The electro-oxidation of CAP on the Ti-based SnO₂–Sb–Ni electrode displayed a pseudo-first-order kinetic model. Free radical quenching experiments have shown that the oxidation of CAP on the Ti-based SnO₂–Sb–Ni electrode resulted from the synergistic effect of direct oxidation and indirect oxidation (·OH and ·SO₄⁻). Direct oxidation and ·SO₄⁻ contributed much to the degradation of CAP. In the process of cathodic reduction, the removal ratio of CAP was high, while the removal ratio of TOC was low. Anodic oxidation was more effective for the removal of CAP.

The project was supported by the ‘LiaoNing Revitalization Talents Program (XLYC1908034)’, ‘Liaoning BaiQianWan Talents Program’ and ‘Liaoning Technology Innovation Center of Water Treatment and Resource’.

All relevant data are included in the paper or its Supplementary Information.