Abstract

Widely used in chemical product manufacture, 1,4-dioxane is one of the emerging contaminants, and it poses great risk to human health and the ecosystem. The aim of this study was to degrade 1,4-dioxiane using a pulsed switching peroxi-coagulation (PSPC) process. The electrosynthesis of H2O2 on cathode and Fe2+ production on iron sacrifice anode were optimized to enhance the 1,4-dioxane degradation. Under current densities of 5 mA/cm2 (H2O2) and 1 mA/cm2 (Fe2+), 95.3 ± 2.2% of 200 mg/L 1,4-dioxane was removed at the end of 120 min operation with the optimal pulsed switching frequency of 1.43 Hz and pH of 5.0. The low residual H2O2 and Fe2+ concentrations were attributed to the high pulsed switching frequency in the PSPC process, resulting in effectively inhibiting the side reaction during the ·OH production and improving the 1,4-dioxane removal with low energy consumption. At 120 min, the minimum energy consumption in the PSPC process was less than 20% of that in the conventional electro-Fenton process (7.8 ± 0.1 vs. 47.0 ± 0.6 kWh/kg). The PSPC should be a promising alternative for enhancing 1,4-dioxane removal in the real wastewater treatment.

HIGHLIGHTS

  • 1,4-dioxiane was efficiently removed in the PSPC process.

  • Residual H2O2 and Fe2+ were minimized by optimizing pulsed switching circuits.

  • The energy consumption in the PSPC reduced to 20% of that in the EF.

Graphical Abstract

Graphical Abstract
Graphical Abstract

INTRODUCTION

As an important solvent stabilizer, reaction agent, and reaction media, 1,4-dioxane (C4H8O2) has been widely used in the manufacturing processes of chemical products such as paints, varnishes, lacquers, cosmetics, resins, and deodorants (Clercq et al. 2010; Barndõk et al. 2016a). Being possibly carcinogenic to humans and chemically stable, 1,4-dioxane can pose a great risk to human health and the ecosystem (Clercq et al. 2010; Takeuchi & Tanaka 2020; Zhao et al. 2020). Since 1,4-dioxane is bio-refractory, indicated by the low ratio of biochemical oxygen demand (BOD) to chemical oxygen demand (COD) (i.e. 0.06), non-volatility, and its miscibility with water (Nakagawa et al. 2016; Radcliffe & Page 2020; Rossum 2020; Somda et al. 2020), it is difficult to effectively remove it in typical biological wastewater treatments (Mahendra et al. 2013; Huang et al. 2018; Xu et al. 2020). For example, complete decomposition of 100 mg/L 1,4-dioxane in the activated sludge processes required 7 days (Sei et al. 2010). Even with pure culture, more than 25 h was needed for the 1,4-dioxane removal (Sun et al. 2010; Sei et al. 2013). The biological co-metabolism can be used to enhance the 1,4-dioxane degradation. However, additional nutrients, such as tetrahydrofuran and lactate (Sekar & DiChristina 2014), may increase the treatment cost (Hand et al. 2015; Zhang et al. 2016; Chen et al. 2020; Fan et al. 2020; Lin et al. 2020). Thus, it is necessary to develop an efficient method for 1,4-dioxane degradation.

The advanced oxidation processes (AOPs) have been developed for 1,4-dioxane removal in recent years. Compared with the biological treatment process, AOPs can produce high reactive and non-selective oxidant (i.e. ·OH) (Elkacmi & Bennajah 2019), and thus are a fast and efficient alternative to 1,4-dioxane degradation (Clercq et al. 2010; Barndõk et al. 2016a). Because of its good performance, easy control, and environmental friendliness (Wang et al. 2016), the Fenton process is one of the powerful AOPs for 1,4-dioxane removal. The Fenton process could reduce 1,4-dioxane from 100 to 0.5 mg/L within 6 h (Nakagawa et al. 2016). However, the high risk of H2O2 transport limits application of the Fenton process (Brillas & Martínez-Huitle 2015; Gao et al. 2015; Wang et al. 2016). The risks of H2O2 transport can be avoided by in-situ H2O2 production using the electro-Fenton (EF) process (Brillas & Martínez-Huitle 2015). As a combined EF process, the peroxi-coagulation (PC) process utilizes a sacrificial iron anode and a gas diffusion cathode (GDC) for H2O2 generation (Brillas et al. 2009). Fe2+ ions can release from the sacrificial iron anode to catalyze H2O2 for hydroxyl radicals (·OH) production. Excess Fe2+ dissolution from the iron anode can form Fe(OH)3 precipitation as a coagulant. However, excess Fe2+ may also result in severe OH scavenger and high iron sludge production (Sun & Pignatello 1993; Benhadji et al. 2016).

Recently, a pulsed switching peroxi-coagulation (PSPC) process was developed for effectively controlling Fe2+ consumption (Lu et al. 2018). The controllable Fe2+ release in the auxiliary anode and H2O2 electrosynthesis in the cathode of PSPC was useful for maximum ·OH production and minimum iron sludge (Lu et al. 2018). Under current densities of 5.0 mA/cm2 (H2O2) and 0.5 mA/cm2 (Fe2+), and the pulsed switching frequency of 1.0 s (H2O2): 0.3 s (Fe2+), 500 mg/L 2,4-dichlorophenoxyacetic acid was completely removed in the PSPC within 240 min (Lu et al. 2018). The energy consumption for 500 mg/L 2,4-dichlorophenoxyacetic acid removal in the PSPC was only 50% of that in the conventional EF process (68 ± 6 vs. 136 ± 10 kWh/kg TOC) (Lu et al. 2018). The iron consumption in the PSPC was only ∼5% of that in the conventional PC process. Although PSPC showed great potential in the 2,4-dichlorophenoxyacetic acid removal, the application of PSPC is still in its infancy and needs further testing for other refractory organics. Moreover, the physicochemical characteristics of different refractory organics can affect its removal in the AOPs (Clercq et al. 2010; Lu et al. 2018). For example, 1,4-dioxane has higher solubility and toxicity than 2,4-dichlorophenoxyacetic acid (Clercq et al. 2010; Lu et al. 2018). The optimal operational conditions for 1,4-dioxane removal in the PSPC may be different from those for 2,4-dichlorophenoxyacetic acid removal. The experiments on 1,4-dioxane removal in the PSPC should be not only important for enhancing 1,4-dioxane degradation in the wastewater treatment but also for expanding the potential application of PSPC. Thus, the objective of this study was to investigate the feasibility of the PSPC process for 1,4-dioxane degradation. The effect of H2O2 and Fe2+ production, pulsed switching frequency and current density were tested on the 1,4-dioxane removal. Five control experiments were carried out to distinguish the PSPC process from the EF and PC processes. The main intermediates of 1,4-dioxane degradation and the residual Fe2+ and H2O2 were identified to discuss the mechanism of efficient 1,4-dioxane degradation in the PSPC process.

MATERIALS AND METHODS

PSPC setup

The PSPC process was investigated in an undivided electrochemical cell with a cylindrical chamber (diameter × length = 3 × 4.5 cm). A platinum wire electrode (CHI115, CH Instrument, Inc., Shanghai, China) and an iron mesh (diameter of 3 cm, thickness of 0.3 mm, 90 meshes) were used as main and auxiliary anodes, respectively. A gas diffusion cathode (GDC) was constructed with a conductive gas diffusion layer, a catalyst layer, and a supporting layer of stainless steel mesh (90 meshes) (Wang et al. 2017). Carbon black powder (EC-300 J, Hesen, Shanghai, China) was used as a catalyst for H2O2 production. The effective surface area of the GDC was 7 cm2. The distance between the GDC and Pt anode was 3.5 cm. The distance between the GDC and Pt anode was 2 cm. In-situ H2O2 and Fe2+ production was driven by the power supply (IT6700, ITECH Electronic Co., Ltd, Nanjing, China) with constant current. Two time relays with an accuracy of 0.1 s (ZYS48-S, Zhuoyi Electronic Co., Ltd, Shanghai, China) were used to control the pulsed switching circuits and the running time of H2O2 and Fe2+ productions in PSPC. A solution of 0.1 M Na2SO4 and 200 mg/L 1,4-dioxane (99%, Merck) was used as the electrolyte. The pH of the electrolyte was adjusted to 5.0 with H2SO4 or NaOH. The electrolyte was recycled in the electrochemical cell using a peristaltic pump (BT-100, Qite, China) with a flow rate of 36 mL/min.

Experimental procedure

The current density for H2O2 was set at 5.0 mA/cm2 to produce H2O2 efficiently with low energy consumption based on the results of our preliminary tests. In order to avoid excess iron oxidization into Fe3+, the current density for Fe2+ production was controlled at 1 mA/cm2. The molar concentration ratio of Fe2+ to total Fe in the solution was >95% under the iron anode operation with 1 mA/cm2. Thus, the pulsed switching ratio of H2O2 and Fe2+ productions were tested under the current densities of 5 mA/cm2 for H2O2 and 1 mA/cm2 for Fe2+, including 4 s (H2O2): 4 s (Fe2+) (i.e. 1.00), 4:3 s (i.e. 1.33), 4:2 s (i.e. 2.00), 4:1 s (i.e. 4.00), and 4:0.5 s (i.e. 8.00). The pulsed switching frequency, including 4:3 s (0.14 Hz), 2:1.5 s (0.29 Hz), 1.2:0.9 s (0.48 Hz), 0.8:0.6 s (0.71 Hz), and 0.4:0.3 s (1.43 Hz), was tested under the fixed pulsed switching ratio between H2O2 and Fe2+ productions of 1.33 (4/3 s), respectively. Under the pulsed switching frequency of 1.43 Hz, the 1,4-dioxane removal was tested under different current densities, including 15 mA/cm2 (H2O2) +3 mA/cm2 (Fe2+), 10 mA/cm2 (H2O2) +2 mA/cm2 (Fe2+), 5 mA/cm2 (H2O2) +1 mA/cm2 (Fe2+), and 2.0 mA/cm2 (H2O2) +0.4 mA/cm2 (Fe2+), respectively.

Five controls were used to distinguish the PSPC process from others, such as EF and PC processes (Brillas et al. 2009; Ahangarnokolaei et al. 2017). Control 0 was to test the 1,4-dioxane adsorption in the cell without H2O2 and Fe2+ production (Table 1). Control 1 was a conventional PC process to keep a continuous production of H2O2 and Fe2+ without the pulsed switching frequency in the cell under the current densities of 5 mA/cm2 (H2O2) and 1 mA/cm2 (Fe2+). Control 2 was a conventional EF process to keep a continuous H2O2 production of 5 mA/cm2, and add 7.9 mM Fe2+ into the cell without iron anode operation. Control 3 was to keep a pulsed H2O2 production at 5 mA/cm2 with the pulsed switching frequency of 0.4:0.3 s (1.43 Hz), and 7.9 mM Fe2+ added into the cell without iron anode operation. Control 4 was to keep a continuous H2O2 production of 5 mA/cm2, and add 7.9 mM Fe2+ into the cell without iron anode operation. The addition of 7.9 mM Fe2+ was equal to the total Fe2+ production in the PSPC process under the pulsed switching frequency ratio of 0.4:0.3 s, and current densities of 5 mA/cm2 (H2O2) and 1 mA/cm2 (Fe2+) within 120 min.

Table 1

Operational conditions in five controls

Control experimentCurrent density for H2O2 production (mA/cm2)Current density for Fe2+ production (mA/cm2)
Control 0 
Control 1 
Control 2 0b 
Control 3 5a 0b 
Control 4 
Control experimentCurrent density for H2O2 production (mA/cm2)Current density for Fe2+ production (mA/cm2)
Control 0 
Control 1 
Control 2 0b 
Control 3 5a 0b 
Control 4 

aPulsed H2O2 production at 5 mA/cm2 and the pulsed switching frequency of 0.4:0.3 s (1.43 Hz).

b7.9 mM Fe2+ addition into cell without iron anode operation.

Analysis and calculation

The H2O2 concentration was measured using a spectrophotometer (T6, Persee, Beijing, China) according to the titanium (IV) sulfate method (Barazesh et al. 2015). Fe2+ and total iron concentrations were determined with the phenanthroline spectrophotometric method (Xu et al. 2013). The total organic carbon (TOC) was measured using a Shimadzu TOC-L CPH analyzer (Shimadzu Co., Japan). The pH was measured by a pH meter (FE20, Mettler-Toledo, Switzerland). The total COD (TCOD) and soluble COD (SCOD) was determined using the dichromate standard method according to the samples filtrated without or with a 0.22 μm filter, respectively (Liu et al. 2015; Ye et al. 2017). A high performance liquid chromatograph (HPLC, P230II, Dalian Yilite Analytic Instrument Co. Ltd, China) was used to determine 1,4-dioxane concentration. A 10:90 (v/v) acetonitrile/water (phosphate buffer of pH 3) solution at 1.0 mL/min was used as the mobile phase. The chromatograph was equipped with a SinoChrom ODS-BP column (5 μm, 4.6 mm × 25 cm, Dalian Yilite Analytic Instrument Co. Ltd, China). The UV detector was set at 190 nm and the temperature was maintained at 30°. The intermediates of 1,4-dioxane degradation, including oxalic, acetic and formic acids, were quantified by an ion chromatograph (IC, CIC-D100, SHINE IC Solution Experts, China).

The energy consumption in the PSPC process was calculated based on the electricity consumption for H2O2 and Fe2+ production, except for the energy for electrolyte recirculation in the cell. The energy input (P, W) and energy consumption per kg 1,4-dioxane removal (EC, kWh/kg) were calculated as follows:
formula
(1)
formula
(2)
where Vs is the solution volume (L); I1 and I2 are the currents for H2O2 and Fe2+ production, respectively (A); U1 and U2 are the voltages for H2O2 and Fe2+ production, respectively (V); a is the ratio between the running time for H2O2 production and the whole operation time (h); b is the ratio between the running time for Fe2+ production and the whole operation time (h); t is the operation time (h); and ΔC is the decrement of 1,4-dioxane concentration in the PSPC process during the experiment (mg/L). The current efficiency of H2O2 production was estimated as previously described (Luo et al. 2015).

RESULTS

Effect of the pulsed switching ratio of H2O2 and Fe2+ productions on 1,4-dioxane degradation

In the PSPC process, the molar ratio of H2O2 and Fe2+ concentrations were determined by the pulsed switching ratio of H2O2 and Fe2+ production. For example, when the pulsed switching ratio of H2O2 and Fe2+ production increased from 1.00 to 8.00, the molar ratio of H2O2 and Fe2+ increased from 1.49 to 12.0. As shown in Figure 1(a), more than 80% of 1,4-dioxane was removed under different pulsed switching ratios of H2O2 and Fe2+ production within 120 min. The 1,4-dioxane removal slightly increased from 80.1 ± 1.8 to 89.6 ± 1.1% with the pulsed switching ratio increasing from 1.00 to 8.00. The low pulsed switching ratio resulted in low energy consumption (Figure 1(b)). The energy consumption under the pulsed switching ratio of 1.33 was 25.6 kWh/kg, which was only 76% of that under the pulsed switching ratio of 8.00. The energy consumption was almost kept stable at the pulsed switching ratio of 1.33 and 1.00. Therefore, the pulsed switching ratio of 1.33 could be suitable for 1,4-dioxane removal in the PSPC process. Correspondingly, the molar ratio of H2O2 and Fe2+ was 1.99, and the 1,4-dioxane removal efficiency was 84 ± 2%.

Figure 1

(a) 1,4-dioxane removal and (b) energy consumption under different pulsed switching ratios of H2O2 and Fe2+ within 120 min (current densities of 5 mA/cm2 for H2O2 production and 1 mA/cm2 for Fe2+ production).

Figure 1

(a) 1,4-dioxane removal and (b) energy consumption under different pulsed switching ratios of H2O2 and Fe2+ within 120 min (current densities of 5 mA/cm2 for H2O2 production and 1 mA/cm2 for Fe2+ production).

Effect of pulsed switching frequency of H2O2 and Fe2+ production on 1,4-dioxane degradation

Under the fixed pulsed switching ratio of 1.33, different pulsed switching frequency, including 0.14, 0.29, 0.48, 0.71, and 1.43 Hz, was tested as shown in Figure 2. High pulsed switching frequency resulted in high 1,4-dioxane removal and low energy consumption. The 1,4-dioxane removal gradually increased from 84.8 ± 0.4 to 95.3 ± 2.2% with the pulsed switching frequency increasing from 0.14 to 1.43 Hz within 120 min. Based on Equation (1), the energy input for H2O2 and Fe2+ production was determined by the pulsed switching ratio of H2O2 and Fe2+ production. Under the pulsed switching ratio of 1.33, the energy input of the PSPC process was about 0.0718 W with 80% 1,4-dioxane removal, 96% of which was consumed by H2O2 generation. With the pulsed switching frequency increasing from 0.14 to 1.43 Hz, the energy consumption of the PSPC process with 80% 1,4-dioxane removal was decreased from 23 to 14 kWh/kg (Figure 2(b)). Therefore, the pulsed switching frequency of H2O2 and Fe2+ production was optimized to 0.4:0.3 s in the PSPC process. Correspondingly, 95.3 ± 2.2% of 1,4-dioxane was removed with the energy consumption of 22.1 ± 0.5 kWh/kg within 120 min.

Figure 2

Effect of pulsed switching frequency on (a) 1,4-dioxane removal and (b) energy consumption in the PSPC process (pulsed switching ratio of 1.33, current densities of 5 mA/cm2 for H2O2 production and 1 mA/cm2 for Fe2+ production).

Figure 2

Effect of pulsed switching frequency on (a) 1,4-dioxane removal and (b) energy consumption in the PSPC process (pulsed switching ratio of 1.33, current densities of 5 mA/cm2 for H2O2 production and 1 mA/cm2 for Fe2+ production).

Effect of current densities in the PSPC process on 1,4-dioxane degradation

Under the optimally pulsed switching frequency of 1.43 Hz, the effect of current densities in the PSPC process was tested as shown in Table 2. To keep the optimal molar ratio of H2O2 and Fe2+ production as described above, the current densities of H2O2 and Fe2+ production were changed proportionally, including 15 mA/cm2 (H2O2) +3 mA/cm2 (Fe2+), 10 mA/cm2 (H2O2) +2 mA/cm2 (Fe2+), 5 mA/cm2 (H2O2) +1 mA/cm2 (Fe2+), and 2.0 mA/cm2 (H2O2) +0.4 mA/cm2 (Fe2+). High current density of H2O2 and Fe2+ resulted in high 1,4-dioxane removal under the optimally pulsed switching frequency in the PSPC process (Table 2). However, even with low current densities such as 2.0 mA/cm2 (H2O2) and 0.4 mA/cm2 (Fe2+), 89.8 ± 2.2% of 1,4-dioxane was still removed within 120 min.

Table 2

The 1,4-dioxane removal in the PSPC processes and different controls

Time (min)1,4-Dioxane removal (%)
Current densities (mA/cm2) in the PSPC process (pulsed switching frequency = 1.43 Hz)
Control 0Control 1Control 2Control 3Control 4
15 (H2O2) + 3 (Fe2+)10 (H2O2) + 2 (Fe2+)5 (H2O2) + 1 (Fe2+)2.0 (H2O2) + 0.4 (Fe2+)
20 52.9 ± 1.7 50.4 ± 5.8 32.6 ± 9.9 28.2 ± 3.1 3.4 ± 3.1 33.0 ± 5.2 59.0 ± 0.2 57.3 ± 6.4 6.4 ± 3.8 
40 78.7 ± 1.1 77.4 ± 4.7 62.8 ± 0.2 49.2 ± 6.5 4.8 ± 2.5 66.0 ± 3.6 65.0 ± 2.4 63.2 ± 1.5 11.3 ± 2.7 
60 90.5 ± 1.7 88.7 ± 3.9 78.3 ± 4.8 64.2 ± 3.3 6.9 ± 3.3 82.4 ± 3.3 66.6 ± 2.1 65.4 ± 2.0 13.9 ± 3.9 
80 95.2 ± 1.4 94.1 ± 3.1 87.1 ± 4.2 77.4 ± 5.4 10.2 ± 1.4 92.0 ± 2.7 68.8 ± 2.2 67.9 ± 1.2 18.0 ± 3.1 
100 98.2 ± 0.1 96.6 ± 2.3 91.5 ± 4.0 83.9 ± 1.4 10.7 ± 1.4 96.5 ± 1.7 70.9 ± 1.7 69.8 ± 0.8 18.6 ± 2.3 
120 98.8 ± 0.5 97.7 ± 1.7 95.3 ± 2.2 89.8 ± 2.2 11.3 ± 2.2 97.2 ± 2.0 74.2 ± 0.5 71.7 ± 0.4 19.0 ± 1.7 
Time (min)1,4-Dioxane removal (%)
Current densities (mA/cm2) in the PSPC process (pulsed switching frequency = 1.43 Hz)
Control 0Control 1Control 2Control 3Control 4
15 (H2O2) + 3 (Fe2+)10 (H2O2) + 2 (Fe2+)5 (H2O2) + 1 (Fe2+)2.0 (H2O2) + 0.4 (Fe2+)
20 52.9 ± 1.7 50.4 ± 5.8 32.6 ± 9.9 28.2 ± 3.1 3.4 ± 3.1 33.0 ± 5.2 59.0 ± 0.2 57.3 ± 6.4 6.4 ± 3.8 
40 78.7 ± 1.1 77.4 ± 4.7 62.8 ± 0.2 49.2 ± 6.5 4.8 ± 2.5 66.0 ± 3.6 65.0 ± 2.4 63.2 ± 1.5 11.3 ± 2.7 
60 90.5 ± 1.7 88.7 ± 3.9 78.3 ± 4.8 64.2 ± 3.3 6.9 ± 3.3 82.4 ± 3.3 66.6 ± 2.1 65.4 ± 2.0 13.9 ± 3.9 
80 95.2 ± 1.4 94.1 ± 3.1 87.1 ± 4.2 77.4 ± 5.4 10.2 ± 1.4 92.0 ± 2.7 68.8 ± 2.2 67.9 ± 1.2 18.0 ± 3.1 
100 98.2 ± 0.1 96.6 ± 2.3 91.5 ± 4.0 83.9 ± 1.4 10.7 ± 1.4 96.5 ± 1.7 70.9 ± 1.7 69.8 ± 0.8 18.6 ± 2.3 
120 98.8 ± 0.5 97.7 ± 1.7 95.3 ± 2.2 89.8 ± 2.2 11.3 ± 2.2 97.2 ± 2.0 74.2 ± 0.5 71.7 ± 0.4 19.0 ± 1.7 

In terms of 1,4-dioxane removal within 120 min, the different processes were in the order as follows: PSPC ≈ Control 1 > Control 2 > Control 3≫Control 4 ≈ Control 0. Control 0 showed that the adsorption was not significant for 1,4-dioxane removal in the PSPC process. Without Fe2+ catalyst addition, 1,4-dioxane could be hardly removed by H2O2 oxidation (see Control 4). Control 1 (i.e. the conventional PC process) removed 1,4-dioxane efficiently but consumed ∼18.2 mM Fe2+ from the iron electrode within 120 min. Control 2 (i.e. the EF process) removed less 1,4-dioxane than PC and PSPC processes. Control 3 showed that only pulsed H2O2 production with 7.9 mM Fe2+ addition did not remove 1,4-dioxane as efficiently as that in the PSPC process within 120 min (71.7 ± 0.4% vs. 95.3 ± 2.2%).

High current densities of H2O2 and Fe2+ production resulted in high energy consumption in the PSPC process (Table 3). The energy consumption under 15 mA/cm2 (H2O2) +3 mA/cm2 (Fe2+) was 15 times higher than that under 2.0 mA/cm2 (H2O2) +0.4 mA/cm2 (Fe2+) within 120 min (121.0 ± 0.6 vs. 7.8 ± 0.1 kWh/kg). Nevertheless, the minimum energy consumption in the PSPC process under 2.0 mA/cm2 (H2O2) +0.4 mA/cm2 (Fe2+) was less than 20% of that in Control 1 and 2 within 120 min, respectively (Table 3).

Table 3

Energy consumption in the PSPC processes and different controls

Time (min)Energy consumption (kWh/kg)
Current density (mA/cm2) in the PSPC process (pulsed switching frequency = 1.43 Hz)
Control 1Control 2Control 3
15 (H2O2) + 3 (Fe2+)10 (H2O2) + 2 (Fe2+)5 (H2O2) + 1 (Fe2+)2.0 (H2O2) + 0.4 (Fe2+)
20 36.7 ± 1.2 20.4 ± 1.8 10.1 ± 1.2 4.5 ± 0.5 21.9 ± 2.9 9.8 ± 0.0 5.8 ± 0.6 
40 49.0 ± 0.7 26.6 ± 0.8 11.0 ± 0.5 5.0 ± 0.6 21.6 ± 1.3 17.9 ± 0.5 10.4 ± 0.2 
60 64.2 ± 1.2 34.8 ± 0.5 13.3 ± 0.5 5.6 ± 0.2 25.7 ± 1.2 26.2 ± 0.7 15.1 ± 0.4 
80 81.9 ± 1.2 44.2 ± 0.2 16.0 ± 0.5 6.1 ± 0.4 30.4 ± 0.8 33.8 ± 0.9 19.3 ± 0.3 
100 101.0 ± 0.0 52.5 ± 0.3 19.1 ± 0.7 7.0 ± 0.1 36.5 ± 0.8 41.0 ± 0.7 23.5 ± 0.2 
120 121.0 ± 0.6 62.9 ± 0.8 22.1 ± 0.5 7.8 ± 0.1 43.5 ± 1.1 47.0 ± 0.6 27.5 ± 0.3 
Time (min)Energy consumption (kWh/kg)
Current density (mA/cm2) in the PSPC process (pulsed switching frequency = 1.43 Hz)
Control 1Control 2Control 3
15 (H2O2) + 3 (Fe2+)10 (H2O2) + 2 (Fe2+)5 (H2O2) + 1 (Fe2+)2.0 (H2O2) + 0.4 (Fe2+)
20 36.7 ± 1.2 20.4 ± 1.8 10.1 ± 1.2 4.5 ± 0.5 21.9 ± 2.9 9.8 ± 0.0 5.8 ± 0.6 
40 49.0 ± 0.7 26.6 ± 0.8 11.0 ± 0.5 5.0 ± 0.6 21.6 ± 1.3 17.9 ± 0.5 10.4 ± 0.2 
60 64.2 ± 1.2 34.8 ± 0.5 13.3 ± 0.5 5.6 ± 0.2 25.7 ± 1.2 26.2 ± 0.7 15.1 ± 0.4 
80 81.9 ± 1.2 44.2 ± 0.2 16.0 ± 0.5 6.1 ± 0.4 30.4 ± 0.8 33.8 ± 0.9 19.3 ± 0.3 
100 101.0 ± 0.0 52.5 ± 0.3 19.1 ± 0.7 7.0 ± 0.1 36.5 ± 0.8 41.0 ± 0.7 23.5 ± 0.2 
120 121.0 ± 0.6 62.9 ± 0.8 22.1 ± 0.5 7.8 ± 0.1 43.5 ± 1.1 47.0 ± 0.6 27.5 ± 0.3 

Residual Fe2+ and H2O2 concentrations during 1,4-dioxane degradation in the PSPC process

Under the pulsed switching frequency of 1.43 Hz and current densities of 5 mA/cm2 (H2O2) and 1 mA/cm2 (Fe2+), the residual Fe2+ and H2O2 concentrations during 1,4-dioxane degradation was determined in the PSPC process within 120 min (Figure 3). The residual Fe2+ concentration was kept as low as 0.6 ± 0.5 mg/L within 120 min. The H2O2 concentration reached 100 mg/L and remained stable at 100 mg/L within 120 min. The residual Fe2+ and H2O2 concentrations in the PSPC process were different from that in Controls 2 and 3. With initial 7.9 mM Fe2+ addition, the residual Fe2+ concentration was decreased to <0.5 mg/L within 40 min in Controls 1 and 2. However, the residual H2O2 concentration was gradually increased to 280 and 250 mg/L in Controls 1 and 2, respectively. High residual Fe2+ concentration within 40 min and high residual H2O2 concentration within 40–120 min in Controls 1 and 2 may account for the lower 1,4-dioxane removal compared with that in the PSPC process. The residually soluble Fe3+ was not detected in all the tests, indicating that Fe(OH)3 precipitated in the cell (Brillas et al. 2009).

Figure 3

Residual (a) Fe2+ and (b) H2O2 concentrations in PSPC process (pulsed switching frequency of 1.43 Hz, current densities of 5 mA/cm2 for H2O2 production and 1 mA/cm2 for Fe2+ production) and controls within 120 min.

Figure 3

Residual (a) Fe2+ and (b) H2O2 concentrations in PSPC process (pulsed switching frequency of 1.43 Hz, current densities of 5 mA/cm2 for H2O2 production and 1 mA/cm2 for Fe2+ production) and controls within 120 min.

Intermediates in the 1,4-dioxane degradation

Under the pulsed switching frequency of 1.43 Hz and current densities of 5 mA/cm2 (H2O2) and 1 mA/cm2 (Fe2+), TCOD and SCOD in the cell were measured within 120 min as shown in Figure 4. The TCOD and SCOD concentrations decreased from 356 ± 28 to 119 ± 27 mg/L and from 356 ± 24 to 99 ± 10 mg/L within 120 min, respectively. Almost the same removals of TCOD and SCOD (67 vs. 72%) indicated that hydroxyl radical oxidation played the key role in the removal of 1,4-dioxane and its intermediates. Only 26.0% of TOC was removed in the PSPC system within 120 min, indicating that 1,4-dioxane was incompletely mineralized. High concentrations of the residual intermediates (e.g. small molecular organic acids) should account for low TOC removal in the 1,4-dioxane degradation in the PSPC process according to previous reports (Barndõk et al. 2016a). Acetic acid, formic acid, and oxalic acid were identified in the PSPC process (Figure 5). Under the pulsed switching frequency of 1.43 Hz and current densities of 5 mA/cm2 (H2O2) and 1 mA/cm2 (Fe2+), the maximum formic acid concentration of 51.2 ± 0.3 mg/L was determined within 80 min. The concentrations of acetic acid, formic acid and oxalic acid were 10.0 ± 4.6, 31.1 ± 9.1, and 2.7 ± 0.8 mg/L at 120 min, respectively.

Figure 4

Removal of TCOD, SCOD and TOC in the PSPC process within 120 min (pulsed switching frequency of 1.43 Hz, current densities of 5 mA/cm2 for H2O2 production and 1 mA/cm2 for Fe2+ production).

Figure 4

Removal of TCOD, SCOD and TOC in the PSPC process within 120 min (pulsed switching frequency of 1.43 Hz, current densities of 5 mA/cm2 for H2O2 production and 1 mA/cm2 for Fe2+ production).

Figure 5

Intermediate concentrations in the 1,4-dioxane degradation in the PSPC process (pulsed switching frequency of 1.43 Hz, current densities of 5 mA/cm2 for H2O2 production and 1 mA/cm2 for Fe2+ production).

Figure 5

Intermediate concentrations in the 1,4-dioxane degradation in the PSPC process (pulsed switching frequency of 1.43 Hz, current densities of 5 mA/cm2 for H2O2 production and 1 mA/cm2 for Fe2+ production).

DISCUSSION

During the hydroxyl radical production, many secondary reactions can occur in the Fenton process as follows:
formula
(3)
formula
(4)
formula
(5)
formula
(6)
formula
(7)
formula
(8)

The excess of Fe2+ or H2O2 may give rise to the side reactions (4), (5), (6), and (7). As previously reported, the ·OH attacks the organics to form hydroxylated derivatives with a reaction rate (k2) of about 107–109 M−1 s−1 (Brillas & Martínez-Huitle 2015; Gao et al. 2015), which is comparable to that in the side reactions (4), (5), (6), and (7). The reaction rate of 63 M−1·s−1 in the ·OH production (Equation (3)) was much lower than those in Equations (4)–(7). Excess of Fe2+ or H2O2 can act as powerful competitors to the organics such as 1,4-dioxane to react with the ·OH (Sun & Pignatello 1993; Muruganandham & Swaminathan 2004). Therefore, excess of Fe2+ or H2O2 can result in low 1,4-dioxane removal and high energy consumption in the EF and PC processes. However, with the pulsed switching circuits, the excess of Fe2+ and H2O2 production was minimized in the PSPC process (Figure 3, Tables 1 and 2). The low residual Fe2+ concentration in the PSPC process within 120 min could be attributed to the high reduction activity of Fe2+ released from the iron sacrifice electrode, which can be rapidly consumed as described in Equation (3). However, low residual Fe2+ concentration in Controls 2 and 3 within 40–120 min indicated that not enough Fe2+ catalyzed H2O2 to produce ·OH. The Fe2+ in Controls 2 and 3 may be regenerated via the side reaction such as Equation (8) with low reaction rate, inhibiting efficient ·OH production. Therefore, the PSPC could have higher 1,4-dioxane removal with lower energy consumption compared with other processes.

The optimal molar ratio of H2O2 to Fe2+ (summarized as [H2O2]/[Fe2+]) was greatly changed depending on the organics, solutions, etc. The optimal [H2O2]/[Fe2+] was in the range of 5–11 for the chlorinated aliphatic organic removal in the Fenton process (Tang & Huang 1997). [H2O2]/[Fe2+] = 1000:1 had been reported to produce the highest ·OH concentration in the Fenton process among different molar ratios (Fischbacher et al. 2017). The theoretically optimal [H2O2]/[Fe2+] was 2.0 based on the pulsed switching ratio of 1.33 in this study, which was different from [H2O2]/[Fe2+] = 6 for the 2,4-dichlorophenoxyacetic acid degradation in the PSPC process (Lu et al. 2018). It indicated that the pulsed switching ratio of H2O2 to Fe2+ should be further investigated to optimize [H2O2]/[Fe2+] during the PSPC application into the real wastewater treatment.

With a continuous consumption of H2O2 and Fe2+ as shown in Equation (3), lower H2O2 and Fe2+ concentrations were accumulated under higher pulsed switching frequency in the PSPC process. Similar results had been reported in the Fenton process with the stepwise addition of H2O2 to avoid excess H2O2 accumulation and decrease H2O2 scavenging ·OH (Zhang et al. 2012; Gan & Li 2013). With the pulsed switching frequency increasing from 0.71 to 1.43 Hz, the 1,4-dioxane removal was not significantly improved (93.1 ± 2.6 vs. 95.4 ± 2.2%), indicating the limited effect of high pulsed switching frequency on the 1,4-dioxane removal. In addition, higher pulsed switching frequency will require higher precision of time relays and higher investment. Although the highest pulsed switching frequency of 1.43 Hz had the highest 1,4-dioxane removal with the lowest energy consumption in our PSPC process, the optimal pulsed switching frequency still needs further study in the PSPC process in future.

Because the residual H2O2 concentration was much higher than that of the residual Fe2+ concentration in the PSPC process, the ·OH production and 1,4-dioxane removal should occur close to the sacrifice iron electrode. However, with 7.9 mM Fe2+ addition in Control 2 (i.e. the EF process), the ·OH production and 1,4-dioxane removal could occur close to the cathode. Compared with the PSPC, the EF process may suffer from Fe(OH)3 precipitation on the cathode surface, which can decrease the reaction area of H2O2 electro-generation in the cathode. Because of high solubility of 1,4-dioxane, the Fe(OH)3 coagulation had no apparent effect on the 1,4-dioxane removal. The 1,4-dioxane degradation in the PSPC process was proposed as shown in Figure 6, based on the quantified intermediates. Various short chain organic acids have been identified in the intermediates of 1,4-dioxane degradation in AOPs (Barndõk et al. 2016a). In the UV photo-Fenton process, the intermediates such as acetic, oxalic, methoxyacetic and glycolic acids were identified during 7.3 g/L 1,4-dioxane degradation (Barndõk et al. 2016a). The maximum acetic acid concentration reached ∼200 mg/L within 120 min (Barndõk et al. 2016a). In the solar photocatalysis using an NF-TiO2 composite with TiO2 nanoparticles, 140 mg/L 1,4-dioxane degradation resulted in the maximum formic acid of ∼60 mg/L within 7 h (Barndõk et al. 2016b). Different AOPs including the PSPC showed that complete mineralization of 1,4-dioxane was greatly postponed due to high concentrations of short chain organic acids as intermediates (Barndõk et al. 2016a, 2016b). The short chain organic acids can be easily degraded by microbiology (Huang et al. 2017; Wu et al. 2020), thus the PSPC combined with a biological treatment process may be a potential way to mineralize 1,4-dioxane efficiently in future.

Figure 6

Proposed degradation process of 1,4-dioxane in the PSPC.

Figure 6

Proposed degradation process of 1,4-dioxane in the PSPC.

CONCLUSIONS

The pulsed switching peroxi-coagulation (PSPC) process was used to remove 1,4-dioxane in this study. The pulsed switching ratio of H2O2 to Fe2+ production was optimized at 1.33. Under the optimized pulsed switching frequency of 1.43 Hz, the maximum 1,4-dioxane removal reached 98.8 ± 0.5% with the current densities of 15 mA/cm2 (H2O2) +3 mA/cm2 (Fe2+) within 120 min. The minimum energy consumption reached 7.8 ± 0.1 kWh/kg with the current densities of 2.0 mA/cm2 (H2O2) +0.4 mA/cm2 (Fe2+). At the end of 120 min operation, the residual Fe2+ concentration was kept as low as 0.6 ± 0.5 mg/L. Under the current densities of 5.0 mA/cm2 (H2O2) +1 mA/cm2 (Fe2+), the residual H2O2 concentration reached 100 mg/L and remained stable at 100 mg/L. Low residual H2O2 and Fe2+ concentrations were attributed to high pulsed switching frequency in the PSPC process, resulting in effectively inhibiting the side reaction during the ·OH production and improving the 1,4-dioxane removal with low energy consumption. The quantified intermediates showed that the formic acid reached the maximum concentration of 51.2 ± 0.3 mg/L within 80 min. High concentrations of intermediates significantly hindered the mineralization of 1,4-dioxane.

ACKNOWLEDGEMENTS

This work was partly supported by grants from the Science and Technology Program of Guangzhou, China (No. 201804010450), the National Natural Science Foundation of China (Nos. 51608547, 51978676 and 51308557), the Guangdong Provincial Key Laboratory Project (2019B121203011), and the Fundamental Research Funds for the Central Universities (No. 19lgjc08).

DATA AVAILABILITY STATEMENT

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

REFERENCES

REFERENCES
Ahangarnokolaei
M. A.
,
Ganjidoust
H.
&
Ayati
B.
2017
Optimization of parameters of electrocoagulation/flotation process for removal of Acid Red 14 with mesh stainless steel electrodes
.
Journal of Water Reuse and Desalination
8
(
2
),
278
292
.
Barazesh
J. M.
,
Hennebel
T.
,
Jasper
J. T.
&
Sedlak
D. L.
2015
Modular advanced oxidation process enabled by cathodic hydrogen peroxide production
.
Environmental Science & Technology
49
(
12
),
7391
7399
.
Barndõk
H.
,
Blanco
L.
,
Hermosilla
D.
&
Blanco
Á
, .
2016a
Heterogeneous photo-Fenton processes using zero valent iron microspheres for the treatment of wastewaters contaminated with 1,4-dioxane
.
Chemical Engineering Journal
284
,
112
121
.
Barndõk
H.
,
Hermosilla
D.
,
Han
C.
,
Dionysiou
D. D.
,
Negro
C.
&
Blanco
Á
, .
2016b
Degradation of 1,4-dioxane from industrial wastewater by solar photocatalysis using immobilized NF-TiO2 composite with monodisperse TiO2 nanoparticles
.
Applied Catalysis B Environmental
180
,
44
52
.
Benhadji
A.
,
Taleb Ahmed
M.
,
Djelal
H.
&
Maachi
R.
2016
Electrochemical treatment of spent tan bath solution for reuse
.
Journal of Water Reuse and Desalination
8
(
1
),
123
134
.
Brillas
E.
&
Martínez-Huitle
C. A.
2015
Decontamination of wastewaters containing synthetic organic dyes by electrochemical methods. An updated review
.
Applied Catalysis B: Environmental
166–167
,
603
643
.
Brillas
E.
,
Sirés
I.
&
Oturan
M. A.
2009
Electro-Fenton process and related electrochemical technologies based on Fenton's reaction chemistry
.
Chemical Reviews
109
(
12
),
6570
6631
.
Clercq
J.
,
Van de Steene
E.
,
Verbeken
K.
&
Verhaege
M.
2010
Electrochemical oxidation of 1,4-dioxane at boron-doped diamond electrode
.
Journal of Chemical Technology and Biotechnology
85
,
1162
1167
.
Elkacmi
R.
&
Bennajah
M.
2019
Advanced oxidation technologies for the treatment and detoxification of olive mill wastewater: a general review
.
Journal of Water Reuse and Desalination
9
(
4
),
463
505
.
Gao
G.
,
Zhang
Q.
,
Hao
Z.
&
Vecitis
C. D.
2015
Carbon nanotube membrane stack for flow-through sequential regenerative electro-Fenton
.
Environmental Science and Technology
49
(
4
),
2375
2383
.
Hand
S.
,
Wang
B.
&
Chu
K. H.
2015
Biodegradation of 1,4-dioxane: effects of enzyme inducers and trichloroethylene
.
Science of Total Environment
520
,
154
159
.
Huang
H.
,
Wu
J.
,
Ye
J.
,
Ye
T.
,
Deng
J.
,
Liang
Y.
&
Liu
W.
2018
Occurrence, removal, and environmental risks of pharmaceuticals in wastewater treatment plants in south China
.
Frontiers of Environmental Science & Engineering
12
(
6
),
69
79
.
Liu
G.
,
Zhou
Y.
,
Luo
H.
,
Cheng
X.
,
Zhang
R.
&
Teng
W.
2015
A comparative evaluation of different types of microbial electrolysis desalination cells for malic acid production
.
Bioresoure Technology
198
,
87
93
.
Lu
Y.
,
He
S.
,
Wang
D.
,
Luo
S.
,
Liu
A.
,
Luo
H.
,
Liu
G.
&
Zhang
R.
2018
A pulsed switching peroxi-coagulation process to control hydroxyl radical production and to enhance 2,4-Dichlorophenoxyacetic acid degradation
.
Frontiers of Environmental Science & Engineering
12
(
5
),
9
.
Mahendra
S.
,
Grostern
A.
&
Alvarez-Cohen
L.
2013
The impact of chlorinated solvent co-contaminants on the biodegradation kinetics of 1,4-dioxane
.
Chemosphere
91
(
1
),
88
92
.
Muruganandham
M.
&
Swaminathan
M.
2004
Decolourisation of Reactive Orange 4 by Fenton and photo-Fenton oxidation technology
.
Dyes & Pigments
63
(
3
),
315
321
.
Sei
K.
,
Kakinoki
T.
,
Inoue
D.
,
Soda
S.
,
Fujita
M.
&
Ike
M.
2010
Evaluation of the biodegradation potential of 1,4-dioxane in river, soil and activated sludge samples
.
Biodegradation
21
(
4
),
585
591
.
Sei
K.
,
Miyagaki
K.
,
Kakinoki
T.
,
Fukugasako
K.
,
Inoue
D.
&
Ike
M.
2013
Isolation and characterization of bacterial strains that have high ability to degrade 1,4-dioxane as a sole carbon and energy source
.
Biodegradation
24
(
5
),
665
674
.
Sekar
R.
&
DiChristina
T. J.
2014
Microbially driven Fenton reaction for degradation of the widespread environmental contaminant 1,4-dioxane
.
Environmental Science & Technology
48
(
21
),
12858
128567
.
Somda
W.
,
Tischbein
B.
&
Bogardi
J.
2020
Water use inside inland valleys agro-systems in the Dano Basin. Burkina Faso
.
Water Cycle
1
,
88
97
.
Sun
Y.
&
Pignatello
J. J.
1993
Photochemical reactions involved in the total mineralization of 2,4-D by iron(3+)/hydrogen peroxide/UV
.
Environmental Science & Technology
27
(
2
),
304
310
.
Sun
B.
,
Ko
K.
&
Ramsay
J. A.
2010
Biodegradation of 1,4-dioxane by a Flavobacterium
.
Biodegradation
22
(
3
),
651
659
.
Tang
W. Z.
&
Huang
C. P.
1997
Stochiometry of Fenton's reagent in the oxidation of chlorinated aliphatic organic pollutants
.
Environmental Technology
18
(
1
),
13
23
.
Wang
N.
,
Zheng
T.
,
Zhang
G.
&
Wang
P.
2016
A review on Fenton-like processes for organic wastewater treatment
.
Journal of Environmental Chemical Engineering
4
(
1
),
762
787
.
Wang
W.
,
Lu
Y.
,
Luo
H.
,
Liu
G.
&
Zhang
R.
2017
Effect of an improved gas diffusion cathode on carbamazepine removal using the electro-Fenton process
.
RSC Advances
7
(
41
),
25627
25633
.
Xu
H. Y.
,
Shi
T. N.
,
Wu
L. C.
&
Qi
S. Y.
2013
Discoloration of methyl orange in the presence of Schorl and HO: kinetics and mechanism
.
Water Air & Soil Pollution
224
(
10
),
1
11
.
Zhang
H.
,
Wu
X.
&
Li
X.
2012
Oxidation and coagulation removal of COD from landfill leachate by Fered–Fenton process
.
Chemical Engineering Journal
210
(
4
),
188
194
.
Zhang
S.
,
Gedalanga
P. B.
&
Mahendra
S.
2016
Biodegradation kinetics of 1,4-dioxane in chlorinated solvent mixtures
.
Environmental Science & Technology
50
(
17
),
9599
9607
.
Zhao
Y.
,
Ji
B.
,
Liu
R.
,
Ren
B.
&
Wei
T.
2020
Constructed treatment wetland: Glance of development and future perspectives
.
Water Cycle
1
,
104
112
.
This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/).