Degradation of Nonylphenol Ethoxylate-40 in High Saline Wastewater by Electrochemical Oxidation

Abstract

Nonylphenol ethoxylate-n (NP_nEO) is a nonionic surfactant, and can be degraded in chemical and biological ways for short-chain NP_nEO with n ≤ 10. However, when n is >10, the degradation of NPEO in wastewater was seldom reported. In this study, the electrochemical oxidation of long-chain NP₄₀EO at high saline wastewater was investigated. The main influencing factors such as current density, plate distance, initial pH, salt type, and salt concentration on electrochemical degradation of high saline NP₄₀EO wastewater were evaluated. The chemical oxygen demand (COD) removal efficiency increased with the increase of current density and reached 73.4% at 45 mA/cm² after 6 h electrolysis. Plate distance had no effect on the COD removal. As compared with sodium sulfate (Na₂SO₄), sodium chloride (NaCl) solution led to better COD removal. A high COD removal efficiency was obtained at lower initial pH value. A possible electrochemical degradation pathway of NP₄₀EO was proposed. The energy consumption increased with increasing current density, plate distance, initial pH value, and decreasing NaCl concentration. Based on the energy consumption analysis, the optimal operating parameters were obtained at current density of 30 mA/cm², plate distance of 2.0 cm, NaCl concentration of 0.50 M, and initial pH value of 3.0. This result indicated that electrochemical advanced oxidation process is a feasible way for the treatment of long-chain NP₄₀EO wastewater at high saline conditions.

Introduction

Nonylphenol ethoxylate (NP_nEO) is a nonionic surfactant and widely used as dispersant, wetting agent, emulsifier, detergent, and penetration agent in various industries, for example, washing, dyeing, and chemical industries (Zgola-Grzeskowiak et al., 2009). Salt, for example, nitrate, chloride, sulfate, etc., is also widely used in chemical industry. Hence, high saline NP_nEO wastewater is often discharged from many industrial factories.

In the biological process, NP_nEO is slowly degraded to nonylphenol and NP_mEOs (m = 1–4), which are considered as endocrine disrupting compounds and more toxic than their parent compound NP_nEO (Hou and Sun, 2007; da Silva et al., 2015a; Iqbal and Bhatti, 2015). Meanwhile, the high salinity of wastewater increases the difficulty of NP_nEO biodegradation. High concentration of salt inhibits the activity of activated sludge because of bacterial plasmolysis (Klontza et al., 2010). Therefore, physicochemical treatment is the primary selection for high saline wastewater, including evaporation, reverse osmosis, ion exchange, and electrodialysis (Sundarapandiyan et al., 2010; da Silva et al., 2015b).

Advanced oxidation processes have been widely used for the decomposition of refractory organic contaminants (Lefebvre and Moletta, 2006; Abou-Elela et al., 2010) and the treatment of NP_nEO wastewater with high salinity. Iqbal and Bhatti investigated the degradation of NP_nEO by gamma radiation/H₂O₂, and obtained >90% removal and the reduction in cytotoxicity and mutagenicity (Huang et al., 2020). Nearly 90% NP₉EO degradation was achieved by ultraviolet (UV) and solar irradiation (Meng et al., 2019), and 98% NP₁₀EO was degraded in UV/H₂O₂ process at low concentrations of NP_nEO (50–150 mg/L) (Iqbal and Bhatti, 2015). The salts in the wastewater improved the mineralization of organic pollutants (Ashar et al., 2016). With the increase of the chain length, the degradation of NP_nEO decreased. However, the electrochemical oxidation process of NP_nEO with n > 10 has seldom been reported. Therefore, electrochemical oxidation may be considered as an alternative technique for the treatment of high saline long-chain NP₄₀EO wastewater.

Electrochemical advanced oxidation processes (EAOPs) have many advantages, such as strong oxidation performance, mild condition, and environmental compatibility, for the treatment of refractory and complex organic wastewater (Chaplin, 2014; Moreira et al., 2017; Carotenuto et al., 2019). Electrochemical oxidation is divided into direct electrochemical oxidation (anodic oxidation) and indirect electrochemical oxidation (Tang et al., 2014). Anodic oxidation is based on the direct electron transfer to the anode surface, resulting in the generation of the powerful physically adsorbed hydroxyl radical (^·OH) (Zhuo et al., 2011; Feng et al., 2013). The ^·OH group is a nonselective oxidant with oxidation potential of 2.70 V, enhancing the degradation of organic matter (Panizza and Cerisola, 2009; Moreira et al., 2017). Indirect electrochemical oxidation is based on other weaker oxidant agents such as active chlorine (Deng et al., 2019; Meng et al., 2019). When chloride ions are presented in wastewater, active chlorine can be generated in the electrochemical process (Brillas and Martinez-Huitle, 2015), which enhances the oxidation of organic matter (Nidheesh et al., 2018). The chlorine evolution potential depends on the types of anodes, with the potential of the graphite anode being 1.36 V (Anglada et al., 2011). The electrochemical oxidative degradation of NP₁₀EO in synthetic wastewater and actual wastewater was investigated, showing higher degradation efficiency (>85%) (Sivri et al., 2020), but longer NP_nEO (n > 10) was also not reported in the previous studies.

In this work, the feasibility of NP₄₀EO removal in high saline wastewater by electrochemical oxidation using graphite electrode was investigated. The influencing factors, including current density, plate distance, salt type and concentration, and initial pH on NP₄₀EO removal, were explored. The possible degradation intermediates of NP₄₀EO were identified, and then a possible electrochemical degradation pathway was proposed. The energy consumption of electrochemical wastewater treatment was also evaluated.

Materials and Methods

Materials

Nonylphenol ethoxylate (Tergitol NP-40) (NP₄₀EO) was purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd (Shanghai, China) with a chemical oxygen demand (COD) value of 155,000 mg/L. Other chemicals were obtained from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China) with reagent grade. All solutions were prepared using ultrapure water (resistivity ≥18.2 MΩ·cm). The graphite electrode plate was purchased from Jinglongtetan Tech Co., Ltd (Beijing, China). The stainless steel electrode plate was purchased from Quanfu metal Co., Ltd (Shenzhen, China). NP₄₀EO wastewater was prepared by diluting the original NP₄₀EO solution using ultrapure water. The pH value was adjusted by adding H₂SO₄ (1.00 M) or NaOH (1.00 M) solution. In this experiment, pH buffer was not used as it was causing a large change in the conductivity of the reaction system. The original NP₄₀EO synthetic wastewater contained 1,550 mg/L COD and 0.50 M sodium chloride (NaCl), except the assays investigating the influence of different NaCl concentrations.

Electrochemical experiment setup

The electrochemical degradation of NP₄₀EO was carried out in a 250 mL glass beaker with 200 mL working volume, and the solution was mixed using a magnetic stirrer. The graphite electrode plate (dimensions: 5.0 cm × 4.0 cm; thickness: 0.1 cm) was used as the anode, while stainless steel electrode plate of the same size was used as the cathode with immersed area of 16 cm². The distance between the two electrodes was 2.0 cm. A regulated DC power supply (APS3005S-3D; ATTEN, China) was used as a power source.

The influence of current density (15, 30, and 45 mA/cm²), plate distance (2.0, 3.0, 4.0, and 5.0 cm), and initial pH (3.0, 5.0, 7.0, and 9.0) on NP₄₀EO degradation was investigated. For investigating the effect of salt type on the degradation, sodium sulfate (Na₂SO₄) and NaCl were separately prepared as electrolytes. The effect of salt concentration on the NP₄₀EO degradation was also explored at different NaCl concentrations (0.10, 0.15, 0.20, 0.25, and 0.50 M). The NP₄₀EO degradation pathway was investigated at the conditions of current density 30 mA/cm², plate distance 2.0 cm, initial pH value 3.0, and NaCl concentration 0.50 M. The experiment was carried out at ambient temperature (15–20°C) for 6 h. The liquid samples were collected every 1 h and filtered through 0.22 μm filters for analysis. All assays were performed in duplicate.

Analytical methods

Because of the long chain, it is very difficult to directly measure the NP₄₀EO concentration. Thus, the degradation of NP₄₀EO was assessed by COD reduction according to the standards (Fernandes et al., 2014) because COD change can easily quantify the NP₄₀EO removal in the wastewater during oxidation process although the change of the structure cannot be identified. Active chlorine was determined by N,N-diethyl-1,4-phenylenediamine spectrophotometry according to the standard methods (Hine et al., 1974). The UV-vis spectra of NP₄₀EO wastewater were measured by UV-vis spectrophotometer (UV2600; Unicosh, Shanghai). The solution pH was measured by pH meters (SG68; Mettler Toledo, Switzerland).

Degradation intermediates of NP₄₀EO were identified using gas chromatography mass spectrometry (GC-MS, Elite-1; Perkin Elmer) and liquid chromatography mass spectrometry (LC-MS, Ultimate 3000; Dionex). In GC-MS analysis, the sample was first subjected to liquid–liquid extraction, 3.0 mL dichloromethane was added to the sample to perform the extraction three times, the three extracted organic phases obtained were then mixed together, dried by adding a little Na₂SO₄, blown to dryness under a nitrogen stream, and finally dissolved by adding 1.0 mL of dichloromethane. Then, a HP-5MS-fused silica capillary column (30 m × 0.25 mm, 0.25 μm; Perkin Elmer) was equipped for the separation, with helium as carrier gas at flow rate of 50 mL/min. Injection volume was 1 μL. Inlet temperature was 280°C. The temperature was programmed at 65°C for 2 min, ramping up to 160°C at 14°C/min, to 240°C at 5°C/min, and then to 290°C at 10°C/min and holding for 10 min. The range of m/z values was 40–500 in mass spectrometry. In LC-MS analysis, a BEH amide column (2.1 mm × 100 mm, 1.7 μm; Waters) was equipped for the separation. Gradient elution was used for the separation with mobile phase A (ultrapure water) and mobile phase B (acetonitrile). The flow rate of the mobile phase was first maintained at 0.30 mL/min for 1 min with 100% B; second, the flow rate was increased to 1.47 mL/min, and the mobile phase was changed to 75%B/25% water (v/v) over 29 min linearly; third, the flow rate was changed to 1.00 mL/min and the mobile phase was changed to 100% B for 5 min. Finally, the flow rate was returned to 0.30 mL/min for 10 min according to the method described by Shao et al. (2001). The injection volume was 10 μL. Mass spectrometry was Q Exactive™ Plus Hybrid Quadrupole-Orbitrap™ Mass Spectrometer with spray voltage 3,500.00 V, capillary temperature 300.00°C, and positive MS mode (+).

Results and Discussions

Electrochemical degradation of NP₄₀EO

The electrochemical degradation of high saline NP₄₀EO wastewater was assessed according to the change of COD in solution. As shown in Fig. 1, at current density of 30 mA/cm², plate distance of 2.0 cm, NaCl concentration of 0.50 M, and initial pH value of 7.0, 64.3% COD removal was achieved after 6 h electrolysis, indicating that EAOP was an effective way for treating NP₄₀EO wastewater under saline conditions. Meanwhile, active chlorine was also detected in the solution, implying that indirect electrochemical oxidation might participate in the degradation process. Therefore, the effects of factors such as current density, plate distance, salt type and concentration, and initial pH on electrochemical oxidation of NP₄₀EO were further investigated.

FIG. 1.

Effect of electrochemical degradation of NP₄₀EO and variation of active chlorine in solution (Experiment condition: current density: 30 mA/cm²; plate distance: 2.0 cm; initial pH value: 7.0; and [Cl⁻]₀ = 0.50 M). NP₄₀EO, nonylphenol ethoxylate-40.

Factors affecting the electrochemical degradation of NP₄₀EO

Current density

The effect of current density on NP₄₀EO removal is shown in Fig. 2a. The COD removal efficiency of NP₄₀EO wastewater after 6 h electrolysis was 33.1%, 64.3%, and 73.4% at 15, 30, and 45 mA/cm², respectively, indicating that higher current density caused higher COD removal efficiency. The possible reasons were that current density affects the generation of ·OH and other electrogenerated oxidants, and thus affects the removal of NP₄₀EO in high salinity wastewater (APHA, 2005). However, when current density increased from 30 to 45 mA/cm², the COD removal only slightly increased ∼9.1%, which might be due to the parasitic reaction, resulting in the oxidation of ·OH to oxygen and the dimerization of •OH to H₂O₂ (APHA, 2005).

FIG. 2.

Effect of current density (a), plate distance (b), supporting electrolyte species (c) and concentration (d), and initial pH (e) on the COD removal, and the pH variation with respect to the electrolysis times (f). COD, chemical oxygen demand.

Plate distance

It has been reported that plate distance is an important parameter in the electrochemical oxidation influencing mass transfer (Moreira et al., 2017). As presented in Fig. 2b, the COD removal efficiency of NP₄₀EO was 64.3%, 61.9%, 63.5%, and 63.8% after 6 h electrolysis at plate distance of 2.0, 3.0, 4.0, and 5.0 cm, respectively, indicating that plate distance had no obvious effect on the COD removal, which was inconsistent with previous results. Longer plate distance resulted in lower degradation efficiency of perfluorooctanoic acid, which was due to the decrease of mass transfer with the increase of plate distance (Moreira et al., 2017). In this case, the mass transfer was not affected by the plate distances, which might be due to the high concentration of salt in the solution.

Salt type and concentration

NaNO₃, Na₂SO₄, and NaCl are the most used salts in the industry. The differences in the electrochemical degradation of NP₄₀EO by NaNO₃, Na₂SO_4, and NaCl were studied. Correspondingly, the conductivity of the solution containing 0.50 M NaCl, 0.50 M Na₂SO₄, 0.50 M NaNO₃, and 0.25 M Na₂SO₄ were 70.0, 133.0, 64.7, and 69.1 ms/cm, respectively. As indicated in Fig. 2c, NaCl as an electrolyte led to better removal efficiency (64.3%), which might be related to the generation of active chlorine in NaCl solution (Niu et al., 2016).

The effect of NaCl on COD removal was further investigated, as shown in Fig. 2d. The COD removal efficiency of NP₄₀EO was 57.8%, 67.2%, 63.1%, 66.7%, and 64.3% after 6 h electrolysis at NaCl concentrations of 0.10, 0.15, 0.20, 0.25, and 0.50 M, respectively. At 0.10 M NaCl concentration, the COD removal efficiency was relatively low. High conductivity of the solution resulted in faster electron transfer and better degradation of organics (Lin et al., 2012). The low NaCl concentration was anticipated to decrease the conductivity of the solution and, subsequently, decrease the rate of electron transfer, which further led to a reduction in the COD removal efficiency. With the further increase of NaCl concentration, the COD removal also increased. However, when the concentration of NaCl was >0.15 M, the COD removal efficiency did not change a lot, which might be due to the generation of ClO₂⁻, ClO₃⁻, and ClO₄⁻. These species had lower oxidation power than ClO⁻ (Fajardo et al., 2017).

Initial pH value

Figure 2e shows the effect of initial pH on the electrochemical degradation of NP₄₀EO. The COD removal efficiency of NP₄₀EO wastewater was 72.4%, 65.2%, 64.3%, and 59.6% after 6 h electrolysis at initial pH 3.0, 5.0, 7.0, and 9.0, respectively. A higher COD removal efficiency was observed at lower pH value, suggesting that acidic medium was beneficial to the electrochemical degradation of NP₄₀EO. The possible reasons were as follows: on the one hand, the oxidation performance of active chlorine was better in acidic solution than in alkaline solution (Niu et al., 2016). This could be explained by the preponderant active of chlorine species [Cl2 (E^° = 1.36 V/SHE) at pH <3.0, HClO, HClO (E^° = 1.49 V/SHE) at pH 3.0–8.0, and ClO⁻ (E^° = 0.89 V/SHE) at pH >9.0] (Niu et al., 2016). It has been reported that the release of Cl₂O seems to be minimal compared with the Cl₂ evolution (Martinez-Huitle et al., 2015). Therefore, the effect of Cl₂O on NP₄₀EO removal was ignored in this experiment. On the contrary, under alkaline condition, more OH^- was absorbed on the surface of anode [Eq. (1)], which competes with pollutants and chlorine for active sites on the surface (Martinez-Huitle and Brillas, 2009; Mostafa et al., 2018). $4 O H^{-} \to O_{2} + 2 H_{2} O + 4 e^{-} .$ (1)

NP₄₀EO degradation pathway

The UV-vis spectra of NP₄₀EO wastewater during electrochemical degradation are presented in Fig. 3. The UV-vis spectra of the original NP₄₀EO wastewater showed a large peak between 210 and 240 nm, and a small peak between 260 and 300 nm, which was attributed to the absorbance of benzene ring (Niu et al., 2016). During the electrochemical degradation, both peaks decreased gradually, indicating the breakdown of the benzene ring.

FIG. 3.

UV-vis spectra of the wastewater under different electrolyte times (Experiment condition: current density: 30 mA/cm²; plate distance: 2.0 cm; initial pH value: 3.0; [Cl⁻]₀ = 0.50 M).

The degradation of organic pollutants in advanced oxidation processes usually proceeded through the following two stages: degradation of the parent pollutants into intermediates and mineralization of the intermediates to CO₂ and H₂O (Zhuo et al., 2012). Therefore, the identification of intermediates was necessary. And the degradation intermediates of NP₄₀EO during electrochemical oxidation might be potential environmental pollutants. Thus, these degradation intermediates were further analyzed by GC-MS and LC-MS. As shown in the Supporting Information (Supplementary Figs. S1–S3), P1 was a possible intermediate that is inferred from the substance detected by GC-MS analysis and P6 was identified in GC-MS analysis. Supplementary Figure S3 presents the selected MS spectra of the degradation intermediates as hydrogen adduct ions [M+H] ⁺. The compounds could be recognized by their equally spaced ions of Δm/z 44 or 14 and parent compound. The possible structure for the m/z 255 and 283 was NaOOC-(CH₂) _m-C₆H₄O-COONa (m = 1 and 3), and that for m/z 97 and 83 was CH₃CH₂COONa and CH₃COONa.

Based on the results of the identified degradation intermediates, the degradation pathway was further proposed, as shown in Fig. 4. First, the breakdown of C-O bond of ethoxylate chain led to the shortening of the ethoxylate chain. Short-chain nonylphenol ethoxylate intermediates were formed in this process. This was consistent with the report that the ethoxylate chain was first attacked to break down during the NP₄₀EO photoelectrooxidation (Chen et al., 2007). Then, shortening of the alkyl chain and further shortening of ethoxylate chain resulted in the formation of low molecular organic matters. In the research by Armijos-Alcocer et al., the alkyl chain and ethoxylate were also shortened during the electrochemical degradation of NP₇EO (Liao et al., 2016). In the degradation process of the chain, low molecular organic matters were produced, such as P4 and P5. It was believed that these small molecules of organic matter are generated by the oxidation of NP_nEO (da Silva et al., 2015a). Finally, these low molecular organic matters were further degraded to CO₂ and H₂O.

FIG. 4.

Proposed degradation pathway of NP₄₀EO treated by electrochemical oxidation (Experiment condition: current density: 30 mA/cm²; plate distance: 2.0 cm; initial pH value: 3.0; [Cl⁻]₀ = 0.50 M).

Energy consumption

Energy consumption is a key factor for evaluating the feasibility of electrochemical wastewater treatment. Thus, the effect of current density, plate distance, salt type, salt concentration, and initial pH value on the energy consumption was assessed. The energy consumption E_sec (kWh/kg COD) was calculated in Equation (2): $E_{s e c} = \frac{U \cdot I \cdot Δ t}{1000 \cdot V \cdot Δ C O D},$ (2)

where U is the average cell voltage (V), I is the applied current (A), Δt is the electrolytic time (h), V represents the volume of electrochemical wastewater treatment (L), and ΔCOD is the COD removed during the time Δt (kg/L).

The energy consumption under different experiment conditions is listed in Table 1. The increase of current density resulted in the increase of E_sec, from 40.8 kWh/kg COD at 15 mA/cm² to 81.0 kWh/kg COD at 45 mA/cm², suggesting that excess energy was consumed in side reaction at high current density. For the plate distance, the COD removal efficiency did not change significantly with the increase of plate distance, but the applied voltage increased, resulting in the increase of energy consumption. For the salt type, NaCl as electrolyte had lower energy consumption. The faster oxidation kinetics and higher current efficiency could be achieved using NaCl as supporting electrolyte species (Armijos-Alcocer et al., 2017). Increasing NaCl concentration improved the wastewater conductivity and thus reduced the voltage (6.7 V for NaCl 0.10 M, 6.3 V for NaCl 0.15 M, 5.3 V for NaCl 0.20 M, 4.2 V for NaCl 0.25 M, and 4 V for NaCl 0.50 M). Regarding the effect of initial pH value, as the initial pH increases, the energy consumption also increased, as presented in Table 1.

Table 1.

The Specific Energy Consumption Under Different Experiment Conditions

Group	Current density (mA/cm²)	Plate distance (cm)	Salt type	Salt concentration (M)	Initial pH value	Esec (kWh/kg COD)
1	15	2.0	NaCl	0.50	7.0	40.8
2	30	2.0	NaCl	0.50	7.0	55.0
3	45	2.0	NaCl	0.50	7.0	81.0
4	30	3.0	NaCl	0.50	7.0	68.0
5	30	4.0	NaCl	0.50	7.0	70.7
6	30	5.0	NaCl	0.50	7.0	81.4
7	30	2.0	Na₂SO₄	0.25	7.0	86.4
8	30	2.0	Na₂SO₄	0.50	7.0	73.1
9	30	2.0	NaNO₃	0.50	7.0	65.9
10	30	2.0	NaCl	0.25	7.0	62.4
11	30	2.0	NaCl	0.20	7.0	74.3
12	30	2.0	NaCl	0.15	7.0	83.4
13	30	2.0	NaCl	0.10	7.0	106.7
14	30	2.0	NaCl	0.50	3.0	47.5
15	30	2.0	NaCl	0.50	5.0	57.4
16	30	2.0	NaCl	0.50	9.0	60.4

COD, chemical oxygen demand; Na₂SO₄, sodium sulfate; NaCl, sodium chloride.

The analysis of variance for energy consumption is shown in Supplementary Table S1. Through single-factor analysis of variance, the p-value was <0.01 under different conditions, indicating that current density, plate distance, salt type and concentration, and initial pH value have a significant impact on energy consumption. It could be seen that, under different single-factor conditions, the random errors were much smaller than the systematic errors, showing that different sources have great impact on energy consumption.

Based on the above information, the application of optimal operating parameters could further reduce energy consumption and achieve higher COD removal. To obtain the optimal operating parameter, an additional experiment was carried out under the following conditions: applied current density of 30 mA/cm², plate distance of 2.0 cm, NaCl concentration of 0.50 M, and pH of 3.0. Under this condition, 72.4% of COD removal efficiency was obtained and E_sec was reduced to 47.5 kWh/kg COD.

Conclusions

The electrochemical oxidation of NP₄₀EO with high salinity using graphite as anode material was investigated. The COD removal efficiency of NP₄₀EO wastewater increased with increasing current density and reached 73.4% at 45 mA/cm² after 6 h electrolysis. The plate distance had negligible effect on the COD removal efficiency. Addition of NaCl led to better removal efficiency than Na₂SO₄, and the COD removal efficiency of NP₄₀EO wastewater was highest when the concentration of NaCl was 0.25 M. The COD removal efficiency increased with the decline of pH from 9.0 to 3.0. The degradation intermediates of NP₅EO, NaOOC-(CH₂)₃-C₆H₄O-COONa (m/z = 283), NaOOC-CH₂-C₆H₄O-COONa (m/z = 255), sodium acetate (m/z = 83), sodium propionate (m/z = 97), and 2,4,6-trichlorophenol were identified. Based on the intermediates, the degradation pathway was proposed. The energy consumption increased with increasing current density, plate distance, initial pH value, and decreasing NaCl concentration. Based on the energy consumption analysis, the optimal operating parameters were obtained at current density of 30 mA/cm², plate distance of 2.0 cm, NaCl concentration of 0.50 M, and initial pH value of 3.0.

Footnotes

Author Disclosure Statement

No competing financial interests exist.

Funding Information

This research was supported by National Key R&D Program of China (2019YFC0408502), the National Natural Science Foundation of China (Grant No. 51578205, 51728801, 51538012), and the Science and Technology Major Project of Anhui Province (18030801102).

Supplementary Material

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Supplementary Material

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Degradation of Nonylphenol Ethoxylate-40 in High Saline Wastewater by Electrochemical Oxidation

Abstract

Introduction

Materials and Methods

Materials

Electrochemical experiment setup

Analytical methods

Results and Discussions

Electrochemical degradation of NP40EO

Factors affecting the electrochemical degradation of NP40EO

Current density

Plate distance

Salt type and concentration

Initial pH value

NP40EO degradation pathway

Energy consumption

Conclusions

Footnotes

Author Disclosure Statement

Funding Information

Supplementary Material

References

Supplementary Material

Electrochemical degradation of NP₄₀EO

Factors affecting the electrochemical degradation of NP₄₀EO

NP₄₀EO degradation pathway