1,4-dioxane degradation using a pulsed switching peroxi-coagulation process

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 H 2 O 2 on cathode and Fe 2 þ production on iron sacri ﬁ ce anode were optimized to enhance the 1,4-dioxane degradation. Under current densities of 5 mA/cm 2 (H 2 O 2 ) and 1 mA/cm 2 (Fe 2 þ ), 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 H 2 O 2 and Fe 2 þ 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.


INTRODUCTION
As an important solvent stabilizer, reaction agent, and reaction media, 1,4-dioxane (C 4 H 8 O 2 ) has been widely used in the manufacturing processes of chemical products such as paints, varnishes, lacquers, cosmetics, resins, and deodorants (Clercq et al. ; Barndõk et al. a). Being possibly carcinogenic to humans and chemically stable, 1,4-dioxane can pose a great risk to human health and the ecosystem 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 ), and thus are a fast and efficient alternative to 1,4-dioxane degradation (Clercq et al. ; Barndõk et al. a). Because of its good performance, easy control, and environmental friendliness (Wang et al. ), 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. ). However, the high risk of H 2 O 2 transport limits application of the Fenton process (Brillas & Martínez-Huitle ; Gao et al. ; Wang et al. ). The risks of H 2 O 2 transport can be avoided by in-situ H 2 O 2 production using the electro-Fenton (EF) process (Brillas & Martínez-Huitle ). As a combined EF process, the peroxicoagulation (PC) process utilizes a sacrificial iron anode and a gas diffusion cathode (GDC) for H 2 O 2 generation (Brillas et al. ). Fe 2þ ions can release from the sacrificial iron anode to catalyze H 2 O 2 for hydroxyl radicals (·OH) production. Excess Fe 2þ dissolution from the iron anode can form Fe(OH) 3 precipitation as a coagulant. However, excess Fe 2þ may also result in severe OH scavenger and high iron sludge production (Sun & Pignatello ; Benhadji et al. ).
Recently, a pulsed switching peroxi-coagulation (PSPC) process was developed for effectively controlling Fe 2þ consumption (Lu et al. ). The controllable Fe 2þ release in the auxiliary anode and H 2 O 2 electrosynthesis in the cathode of PSPC was useful for maximum ·OH production and minimum iron sludge (Lu et al. ). Under current densities of 5.0 mA/cm 2 (H 2 O 2 ) and 0.5 mA/cm 2 (Fe 2þ ), and the pulsed switching frequency of 1.0 s (H 2 O 2 ): 0.3 s (Fe 2þ ), 500 mg/L 2,4-dichlorophenoxyacetic acid was completely removed in the PSPC within 240 min (Lu et al. ). 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. ). The iron consumption in the PSPC was only ∼5% of that in the conventional PC process. Although PSPC showed great potential in the 2,4dichlorophenoxyacetic 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. ; Lu et al. ).
For example, 1,4-dioxane has higher solubility and toxicity than 2,4-dichlorophenoxyacetic acid (Clercq et al. ; Lu et al. ). The optimal operational conditions for 1,4dioxane 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 H 2 O 2 and Fe 2þ 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 Fe 2þ and H 2 O 2 were identified to discuss the mechanism of efficient 1,4-dioxane degradation in the PSPC process.

PSPC setup
The PSPC process was investigated in an undivided electrochemical cell with a cylindrical chamber (diameter × length ¼ 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. ). Carbon black powder (EC-300 J, Hesen, Shanghai, China) was used as a catalyst for H 2 O 2 production. The effective surface area of the GDC was 7 cm 2 .
The distance between the GDC and Pt anode was 3.5 cm.
The distance between the GDC and Pt anode was 2 cm. Insitu H 2 O 2 and Fe 2þ production was driven by the power supply (IT6700, ITECH Electronic Co., Ltd, Nanjing, China) with constant current. Two time relays with an accuracy of

Experimental procedure
The current density for H 2 O 2 was set at 5.0 mA/cm 2 to produce H 2 O 2 efficiently with low energy consumption based on the results of our preliminary tests. In order to avoid excess iron oxidization into Fe 3þ , the current density for Fe 2þ production was controlled at 1 mA/cm 2 . The molar concentration ratio of Fe 2þ to total Fe in the solution was >95% under the iron anode operation with 1 mA/cm 2 .
Thus, the pulsed switching ratio of H 2 O 2 and Fe 2þ productions were tested under the current densities of 5 mA/cm 2 for H 2 O 2 and 1 mA/cm 2 for Fe 2þ , including 4 s (H 2 O 2 ): 4 s (Fe 2þ ) (i.e.  (Table 1). Control 1 was a conventional PC process to keep a continuous production of H 2 O 2 and Fe 2þ without the pulsed switching frequency in the cell under the current densities of 5 mA/cm 2 (H 2 O 2 ) and 1 mA/cm 2 (Fe 2þ ).
Control 2 was a conventional EF process to keep a continuous H 2 O 2 production of 5 mA/cm 2 , and add 7.9 mM Fe 2þ into the cell without iron anode operation. Control 3 was to keep a pulsed H 2 O 2 production at 5 mA/cm 2 with the pulsed switching frequency of 0.4:0.3 s (1.43 Hz), and 7.9 mM Fe 2þ added into the cell without iron anode operation. Control 4 was to  keep a continuous H 2 O 2 production of 5 mA/cm 2 , and add 7.9 mM Fe 2þ into the cell without iron anode operation. The addition of 7.9 mM Fe 2þ was equal to the total Fe 2þ production in the PSPC process under the pulsed switching frequency ratio of 0.4:0.3 s, and current densities of 5 mA/ cm 2 (H 2 O 2 ) and 1 mA/cm 2 (Fe 2þ ) within 120 min.

Analysis and calculation
The H 2 O 2 concentration was measured using a spectropho- The energy consumption in the PSPC process was calculated based on the electricity consumption for H 2 O 2 and Fe 2þ 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: where V s is the solution volume (L); I 1 and I 2 are the currents for H 2 O 2 and Fe 2þ production, respectively (A); U 1 and U 2 are the voltages for H 2 O 2 and Fe 2þ production, respectively (V); a is the ratio between the running time for H 2 O 2 production and the whole operation time (h); b is the ratio between the running time for Fe 2þ 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 H 2 O 2 production was estimated as previously described (Luo et al. ).

RESULTS
Effect of the pulsed switching ratio of H 2 O 2 and Fe 2þ productions on 1,4-dioxane degradation In the PSPC process, the molar ratio of H 2 O 2 and Fe 2þ concentrations were determined by the pulsed switching ratio of H 2 O 2 and Fe 2þ production. For example, when the pulsed switching ratio of H 2 O 2 and Fe 2þ production increased from 1.00 to 8.00, the molar ratio of H 2 O 2 and Fe 2þ 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 H 2 O 2 and Fe 2þ 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)). 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   In terms of 1,4-dioxane removal within 120 min, the different processes were in the order as follows: PSPC ≈

Intermediates in the 1,4-dioxane degradation
Under the pulsed switching frequency of 1.43 Hz and current densities of 5 mA/cm 2 (H 2 O 2 ) and 1 mA/cm 2 (Fe 2þ ), 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. a). 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/cm 2 (H 2 O 2 ) and 1 mA/cm 2 (Fe 2þ ), 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.

DISCUSSION
During the hydroxyl radical production, many secondary reactions can occur in the Fenton process as follows: The excess of Fe 2þ or H 2 O 2 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 (k 2 ) of about 10 7 -10 9 M À1 s À1 (Brillas & Martínez-Huitle ; Gao et al. ), which is comparable to that in the side reactions (4), (5), (6), and (7). The reaction rate of 63 M À1 ·s À1 in the ·OH production (Equation (3)) was much lower than those in Equations (4)

CONCLUSIONS
The pulsed switching peroxi-coagulation (PSPC) process was used to remove 1,4-dioxane in this study. The pulsed switching ratio of H 2 O 2 to Fe 2þ production was optimized