Sulfuric acid (H₂SO₄), sodium hydroxide (NaOH) and sodium sulfate (Na₂SO₄) were of analytical grade. The textile effluent after biochemical treatment was obtained from a textile enterprise in Zhejiang province. The commercial UV lamp at 254 nm (GHO36T5H/80W(UK)) was purchased from Suzhou Hemingway Environmental Protection Equipment Co., Ltd. The platinum-plated titanium mesh electrode was purchased from Baoji Shuangliheng Titanium Technology Co., Ltd. The cylinder electrolyzer were purchased from Shaanxi Utron Environmental Protection Technology Co., Ltd.

As shown in Fig. 1, the UV lamp with the diameter of 254 mm, the length of 843 mm and UV intensity of 320 μm cm⁻² was located in the center of the platinum-plated titanium mesh cylinder anode with the diameter of 40 mm and the height of 680 mm. Platinum with the thickness of 0.5 mm was electrodeposited on titanium cylinder with the mesh of 5. The cathode was the titanium cylinder with the diameter of 70 mm, the thickness of 2 mm and the height of 680 mm. The space between the anode and the cathode was 13.5 mm. A magnetic pump (MP-55R, Baoding Qili Precision Pump Co., Ltd) and a DC power supply (JK3060K, Shenzhen Junke Instrument Technology Co., Ltd) were connected with the cylinder electrolyzer.

As shown in Fig. 1, the textile effluent of 25 L continually flowed between the cylinder electrolyzer and the effluent reservoir by pump during experiments. The experiments of electrochemical degradation with UV were carried out with the original textile effluent at the constant cell voltage when the DC power supply and the UV lamp were linked to electricity. To further evaluate the effectiveness of the electrochemical degradation with UV for textile effluent, the experiments of UV alone and electrochemical degradation alone were conducted. When the experiments of electrochemical degradation alone were performed in the cylinder electrolyzer, the UV lamp cut off. The experiments of UV alone were performed when the DC power supply cut off. 0.1 mol L⁻¹ H₂SO₄ solution and 0.1 mol L⁻¹ NaOH solution were used to adjust the pH of textile effluent with initial pH of 7.6. The experiments were operated during the temperature from 30°C to 35°C. The analysis samples were extracted from the effluent reservoir when each experiment was finished. The UV-visible spectrums of the samples were measured by an ultraviolet-visible spectrophotometer (UV-2600, Shimadzu, Japan). The changes of functional groups and chemical compositions of textile effluent were analyzed by in infrared spectrometer (8400S, Shimadzu, Japan).

The COD of textile effluent was measured by a spectrophotometer (DR3900, Hach, USA). Current efficiency (η, %) that presents the utilization rate of the charge during electrochemical degradation, is calculated using Eq. 1.

where COD_o and COD_t are the COD of the textile effluent before degradation and after degradation at time of t, respectively, mg L⁻¹; V is the volume of textile effluent, 25 L; F is the Faraday constant, 96485 C mol⁻¹; I is the current of the power supply, A; t is the time when the current is supplied, s.

Since the electrical consumption consists of cylinder electrolyzer, UV lamp and magnetic pump, the electrical consumption per kilogram of COD (W, kWh kg⁻¹) is calculated using Eq. 2:

where U is the cell voltage of the cylinder electrolyzer, V; P_u is the power of the UV lamp, 80 W; P_m is the power of the magnetic pump, 15 W.

The carbon emissions are due to the electrical consumption in present work. The carbon emissions per kilogram of COD (We, kg CO₂ kg⁻¹ COD) is calculated using Eq. 3:

where f is the carbon emissions factor in China, 0.5703 kg CO₂/kWh.

Fig. 2–4 displays the degradation performances of the textile effluent by different degradation methods with initial pH of 5.0, flow rate of 120 L h⁻¹, initial Na₂S₂O₈ concentration of 0.1 mol L⁻¹ and degradation time of 10 min. In addition, the electrochemical degradation with UV (EU+UV) and electrochemical degradation alone (EU) were performed at cell voltage of 5.0 V. It can be observed from Fig. 2 that there existed the aromatic structure was characterized by the peaks at 281–285 nm [19] in the original textile effluent. The peaks at 281–285 nm presented a rapid reduction after electrochemical degradation or UV. Compared with electrochemical degradation alone (EU) and UV alone, the peak exhibited more reduction by electrochemical degradation with UV (EU+UV), which the demonstrated that the good degradation performances of the textile effluent by electrochemical degradation with UV using platinum-plated titanium anode.

As shown in Fig. 3, the peak at 3446 cm⁻¹ and 617 cm⁻¹ was due to the –OH stretching vibration of hydroxyl groups and C–Cl stretching vibration, respectively [20]. The peak at 875 cm⁻¹ was due to the vibration of C=C bending [21]. A strong peak was observed at 1127 cm⁻¹, which belonged to S–O band [22]. The peak at 1477 cm⁻¹ was caused by the presence of –CH₂ [23]. The peaks vibration of N=C=N stretching of carbodiimidd appeared at 2120–2176 cm⁻¹ and the peak at 2010 cm⁻¹ was attributed to the vibration of C≡C stretching alkyne in the FTIR spectras of EU+UV and EU. It can be observed that the functional groups and chemical compositions of textile effluent after UV treatment almost were same as original textile effluent. The peaks at 875 cm⁻¹ and 1477 cm⁻¹ disappeared after electrochemical degradation with UV (EU+UV) and electrochemical degradation alone (EU) compared with original textile effluent, which indicated that the chemical compositions with C=C got removed effectively after electrochemical degradation with (EU+UV) and electrochemical degradation alone (EU).

As shown in Fig. 4, the COD after electrochemical degradation with UV (EU+UV), electrochemical degradation alone (EU) and UV alone (UV) were 80.3 mg L⁻¹, 95.3 mg L⁻¹ and 110.9 mg L⁻¹ with the corresponding electrical consumption of 34.1 kWh kg⁻¹, 33.2 kWh kg⁻¹ and 66.7 kWh kg⁻¹, respectively. It was obvious that the degradation performances of electrochemical degradation with UV (EU+UV) were better compared with UV alone (UV). On one hand, the organic contaminants could be electrochemically degraded on the surface of platinum-plated titanium anode under electrical voltage. On the other hand, the persulfate radical could be generated on the platinum-plated titanium anode under electrical voltage shown in Reaction (1) with the oxygen evolution shown in Reaction (2). Persulfate radical as an oxidizing ion could achieve the degradation of organic contaminants. Therefore, the direct degradation of organic contaminants and the generation of persulfate radical were the main effects of electrical voltage. As for the UV lamp, persulfate ion could be activated by UV at 254 nm with sulfate radical (SO₄⁻) generated shown in Reaction (3) [24]. The SO₄⁻ could achieve the degradation of textile effluent. Considering the degradation performance of UV for textile effluent to a certain extent shown in Fig. 4, activation of persulfate ion and degradation of organic contaminants were main effects of UV lamp.

Although the electrical consumption of electrochemical degradation alone (EU) with quite high COD reduction was lower than that of electrochemical degradation with UV (EU+UV), activation of persulfate ion and degradation of organic contaminants by UV lamp improved the COD reduction of electrochemical degradation with UV (EU+UV) compared with electrochemical degradation alone (EU), which achieved the higher COD reduction than that of electrochemical degradation alone (EU) plus UV alone (UV). Therefore, electrochemical degradation with UV (EU+UV) adopting UV lamp has better degradation performances of textile effluent than that of electrochemical degradation alone (EU).

The activation of persulfate ion and degradation of organic contaminants could be enhanced with the increase of intensity of UV lamp [25]. Intensity of commercial UV lamp at 254 nm was proportional to the length of UV lamp and the longest UV lamp was utilized in the present cylinder electrolyzer. Therefore, the degradation performance of electrochemical degradation with UV was not investigated in different intensity of UV lamp in this work.

Fig. 5 presents that COD and current efficiency under different initial pH at flow rate of 120 L h⁻¹, initial Na₂S₂O₈ concentration of 0.1 mol L⁻¹, degradation time of 10 min and cell voltage of 5.0 V. As shown in Fig. 5, the COD reduced at first and then gradually increased when the current efficiency increased slightly at first and then gradually reduced. The solution conductivity was decreased due to the reduction of corresponding added H₂SO₄ concentration as the initial pH increased from 4.0 to 5.0, which lowered the current at the same cell voltage and increased the current efficiency according to Eq. 1. In addition, the reduction of H₂SO₄ concentration lowered the persulfate generation in Reaction (1), which caused the increase of COD from 80.3 mg L⁻¹ to 87.2 mg L⁻¹ and the reduction of current efficiency from 91.60% to 87.81% at initial pH from 5.0 to 7.0. As the initial pH increased from 7.0 to 9.0, the generation of persulfate ion could be lowered by the alkaline as shown in Reaction (4) [26], which caused the decrease of COD from 87.20 mg L⁻¹ to 90.40 mg L⁻¹. The increase of COD and solution conductivity were attributed to the drop of current efficiency from 87.81% to 73.54% at initial pH from 7.0 to 9.0. Considering the higher current efficiency and the lower COD, initial pH of 5.0 was selected for the following experiments.

As shown in Fig. 6, the COD was gradually reduced while the current efficiency was gradually increased with flow rate from 80 L h⁻¹ to 130 L h⁻¹ at initial pH of 5.0, initial Na₂S₂O₈ concentration of 0.1 mol L⁻¹, degradation time of 10 min and cell voltage of 5.0 V. As flow rate increased from 80 L h⁻¹ to 120 L h⁻¹, the mass transfer between the textile effluent and the anode was gradually enhanced, which resulted in the reduction of COD and the increase of current efficiency. When the flow rate reached 120 L h⁻¹, the mass transfer was sufficient in the cylinder electrolyzer, which caused that COD and current efficiency remained relatively stable at flow rate from 120 L h⁻¹ to 130 L h⁻¹. Therefore flow rate of 120 L h⁻¹ was selected for the following experiments.

As shown in Fig. 7, the COD gradually decreased while the current efficiency was increased at first and then gradually reduced with initial Na₂SO₄ concentration from 0 to 0.5 mol L⁻¹ at initial pH of 5.0, flow rate of 120 L h⁻¹, degradation time of 10 min and cell voltage of 5.0 V. With the increase of initial Na₂SO₄ concentration, the generation of persulfate ion was enhanced, which improved the degradation of organic contaminants in textile effluent and reduced the COD. As the initial Na₂SO₄ concentration increased from 0 to 0.1 mol L⁻¹, the reduction of COD was attributed to the increase of current efficiency from 84.42% to 91.60%. When the initial Na₂SO₄ concentration was beyond 0.1 mol L⁻¹, the solution conductivity was further increased due to the further increase of initial Na₂SO₄ concentration, which resulted in the increase of electricity and the reduction of current efficiency at the same cell voltage according to Eq. 2. Although the COD at initial Na₂SO₄ concentration of 1.0 mol L⁻¹ was higher than that at initial Na₂SO₄ concentration beyond 0.1 mol L⁻¹, the current efficiency at initial Na₂SO₄ concentration of 1.0 mol L⁻¹ was much higher than that at initial Na₂SO₄ concentration beyond 0.1 mol L⁻¹. Therefore, initial Na₂SO₄ concentration of 0.1 mol L⁻¹ was selected for the following experiments.

Fig. 8 displays the degradation performances under degradation time from 10 min to 60 min at initial pH of 5.0, flow rate of 120 L h⁻¹, initial Na₂S₂O₈ concentration of 0.1 mol L⁻¹ and cell voltage of 5.0 V. It could be observed from Fig. 8 that the COD and the current efficiency decreased with the increasing degradation time. On one hand, the accelerated degradation time resulted in the increase of degraded organic contaminants in the textile effluent, which was attributed to the reduction of COD. On the other hand, as degradation time increased from 10 min to 60 min, more byproducts were generated with less degradable organic contaminants that could hinder the mass transfer between the organic contaminants in textile effluent and the anode, which resulted in the reduction of current efficiency from 91.60% to 38.97%. As shown in Fig. 8, the COD at degradation time of 60 min reduced to 54.1 mg L⁻¹, which met the first-order discharge standard for municipal wastewater treatment plant in China [27].

Fig. 9 shows the degradation performances under cell voltage from 4.0 to 6.0 at initial pH of 5.0, flow rate of 120 L h⁻¹, initial Na₂S₂O₈ concentration of 0.1 mol L⁻¹ and degradation time of 10 min. It can be seen from Fig. 8(a) that the COD gradually reduced and the current efficiency increased at first and then reduced. With the increase of cell voltage from 4.0 to 6.0, the direct electrochemical oxidation of organic contaminants on the anode and the persulfate ion generation shown in Reaction (1) were enhanced, which improved the degradat ion of organic contaminants and the reduction of COD from 97.3 mg L⁻¹ to 71.6 mg L⁻¹. The increase of COD reduction contributed to the increase of current efficiency at cell voltage from 4.0 to 5.0 as shown in Fig. 8(a) according to Eq. 2. However, with the further increase of cell voltage from 5.0 to 6.0, the oxygen evolution reaction shown in Reaction (2) on the anode was enhanced, which lowered the effective reaction surface of the anode and caused the reduction of current efficiency.

As shown in Fig. 9(b), electrical consumption and carbon emissions decreased at first and then increased. With the increase of cell voltage from 4.0 V to 5.0 V, the enhancement of direct electrochemical oxidation of organic contaminants on the anode and persulfate ion generation shown in Reaction (1) improved the reduction of COD and the increase of current efficiency, which was attributed to the decrease of electrical consumption from 43.6 kWh kg⁻¹ to 34.1 kWh kg⁻¹. When the cell voltage was beyond 5.0 V, the enhancement oxygen evolution reaction shown in Reaction (2) on the anode lowered the current efficiency, which caused the increase of electrical consumption from 34.7 kWh kg⁻¹ to 37.6 kWh kg⁻¹ according to Eq. 2. It could be seen that the change tendency of carbon emissions with the increase of cell voltage was consistent with electrical consumption because the carbon emissions were calculated by electrical consumption multiplied by the carbon emissions factor according to Eq. 3. The carbon emissions was 19.4 kg CO₂/kg COD at cell voltage of 5.0 V. In addition, it could be calculated that the carbon emissions by the electrical consumption of electrochemical degradation alone and UV lone were 10.4 kg CO₂ kg⁻¹ COD and 7.6 kg CO₂ kg⁻¹ COD, respectively, which were the main sources of carbon emissions in the present experiments of electrochemical degradation with UV. Consequently, the optimization of experimental parameters for electrochemical degradation with UV was an effective method to reduce the carbon emissions.