Persulfate activation by Fe(III) with bioelectricity at acidic and near-neutral pH regimes: Homogeneous versus heterogeneous mechanism

Journal of Hazardous Materials 374 (2019) 92–100

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Persulfate activation by Fe(III) with bioelectricity at acidic and near-neutral pH regimes: Homogeneous versus heterogeneous mechanism

T

Suding Yana,b, Xinping Zhangb, Hui Zhanga,

⁎

a

Department of Environmental Science and Engineering, Hubei Environmental Remediation Material Engineering Technology Research Center, Wuhan University, Wuhan 430079, China b Hubei Key Laboratory of Pollutant Analysis & Reuse Technology, Department of Environmental Engineering, Hubei Normal University, Huangshi 435002, China

GRAPHICAL ABSTRACT

ARTICLE INFO

ABSTRACT

Keywords: Peroxydisulfate Iron precipitate Bioelectricity Homogeneous activation Heterogeneous activation

The combination of persulfate (PS) activation by iron ions with electrochemical process (electro/Fe3+/PS) is a promising advanced oxidation process. However, almost all these systems were performed in an unbuffered solution and actually under acidic pH condition, with the electricity being frequently supplied by external power. Considering the high buffering capacity of wastewater and energy saving, peroxydisulfate (PDS) activation by Fe(III) species with bioelectricity provided by microbial fuel cell (MFC) for bisphenol A (BPA) oxidation was investigated at fixed near-neutral pH as well as acidic pH. The results indicate that 90.8% of BPA could be removed at pH 2.5. Though the iron existed in the form of precipitate, BPA could still be efficiently removed at pH 6.0. The precipitate formed in the system at pH 6.0 was identified as the amorphous iron oxyhydroxides. Sulfate radicals in the bulk solution and that adsorbed on the precipitate were the dominant reactive species responsible for the oxidation of BPA in the homogeneous and heterogeneous MFC/Fe(III)/PDS processes, respectively. The mechanisms of BPA degradation at both pH values were proposed via EPR and quenching tests as well as XPS analysis. The effects of operating parameters, the mineralization, the mineralization current efficiency and energy consumption were also explored.

⁎

Corresponding author at: Department of Environmental Science and Engineering, Wuhan University, P.O. Box C319, Luoyu Road 129#, Wuhan 430079, China. E-mail address: [email protected] (H. Zhang).

https://doi.org/10.1016/j.jhazmat.2019.03.068 Received 24 October 2018; Received in revised form 15 March 2019; Accepted 16 March 2019 Available online 02 April 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.

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1. Introduction

photoelectron spectroscopy (XPS). The reactive species present in the studied systems at acidic and near-neutral pH were determined through electron paramagnetic resonance (EPR) and XPS techniques, and quenching experiments. The influences of operational conditions, such as the dosage of Fe(III) and PDS, and current density on the decay of BPA were also examined.

Recently, persulfate (PS) activation technologies have been widely explored for water depollution [1–3]. Sulfate radical (SO4•−) formed in the PS activated system has a comparable oxidizing ability (SO4•−/ E0 = 2.5–3.1 VǀNHE) with hydroxyl radical (•OH, SO42−, E0 = 1.8–2.7 VǀNHE) [4], high selectivity at acidic condition and relatively long lifetime (t1/2 = 30–40 μs) [5–8]. As is reported that PS, either peroxydisulfate (PDS) or peroxymonosulfate (PMS), can be activated by energy (heat, ultrasound, ultraviolet, etc.) or chemicals (transition metals, carbonaceous materials, base, etc.) [9–11]. As one of transition metals, Fe2+ has been frequently employed to activate persulfate via Eq. (1) or (2) [12] (denoted as Fe2+/PS) for the destruction of refractory organic pollutants in water and wastewater [13–15] because of its nontoxicity, inexpensiveness and efficiency. However, the Fe2+/PS system suffers from the high consumption of iron, narrow pH range (∼3.0) and the formation of iron sludge in downstream treatment. Chelation of ferrous ion can maintain the Fe2+/PS process homogeneous and efficient in a wide pH range by keeping ferrous ion available to react with PS or sulfate radical [16–18]. Nevertheless, the important imperfection in the employment of the chelators is the possible interference of the chelators with the contaminant degradation process and/or the potential secondary pollution risk of the chelators [19]. Fe(II) + S2O82− → Fe(III) + SO4•− + SO42− Fe(II)

+ HSO5− →

Fe(III) +

SO4•–

−

+ OH

2. Experimental 2.1. Materials Sodium peroxydisulfate (Na2S2O8) and 5,5-dimethyl-1-pyrrolidine N-oxide (DMPO) were purchased from Aladdin Chemical Co., Ltd. (Shanghai, China). 2,2-Bis(4-hydroxyphenyl)propane (> 99%, C15H16O2, BPA) was supplied by Sigma-Aldrich (UK). All other chemical reagents were obtained from Sinopharm Chemical Reagent Co., Ltd., China and at least of analytical grade. Pure water was used to prepare aqueous solutions throughout the experiments. 2.2. Characterization of the sample A Bruker D8 advance X-ray diffractometer (Germany) was employed to collect the XRD pattern of the sample under Cu Kα radiation (λ = 1.5406 Å) at 40 kV of voltage and 80 mA of current. FTIR spectra on KBr pellets of the samples were obtained on a Nicolet FTIR 5700 (USA). The morphology of the sample was observed by an FESEM (Zeiss SIGMA, Germany). Surface electronic states of the precipitate were recorded on an XPS spectrometer (ESCALAB 250Xi, USA). TGA was performed on a TGA-7 analyzer (PE, USA) under nitrogen.

(1) (2)

Electrochemical process is a promising technology for water and wastewater treatment thanks to its high efficiency, environmentally friendly and versatility [20]. When it was combined with the Fe2+/PS system (denoted as electro/Fe2+/PS), organic pollutants could be degraded by direct oxidation at the anode and ferrous ions could be regenerated at the cathode as given in Eq. (3) [21,22]. This would reduce the consumption of Fe2+ in the persulfate activation process and the electro/Fe2+/PS system has been applied to degrade Acid Orange 7 [23] and landfill leachate [24]. Due to the instability of Fe2+ in aerobic circumstance, Fe2+ was replaced by Fe3+ (electro/Fe3+/PS) for the oxidation of organic pollutants, such as clofibric acid [25], phenol [26] and bisphenol A (BPA) [27]. Fe(III) + e− → Fe(II) E0 = 0.77 V

2.3. Degradation experiments The experimental setup was depicted in Fig. S1 and the detail was described in our previous reports [34,35]. In batch trials, 50 mM of Na2SO4 was used as the supporting electrolyte. A specific amount of PDS and Fe2(SO4)3·5H2O and 50 μM of BPA solution (250 mL) were added to a 500-mL glass beaker under vigorous agitation at 25 ± 2 °C. The reaction was initiated by connecting the circuit. The pH value of the solution was controlled at 2.5 ± 0.1 (natural pH of the solution with 0.2 mM of Fe(III), 4 mM of PDS, 50 μM of BPA and 50 mM of Na2SO4) or 6.0 ± 0.1 by dilute H2SO4 or NaOH throughout the reaction. At specific time intervals, 0.8 mL of the sample was taken from the reactor and then mixed with 0.4 mL of methanol (MeOH) to terminate the reaction. The mixture was filtered via 0.22 μm nylon filters before the assay.

(3)

Generally, most real wastewaters have a relatively high buffering capacity, usually in the form of bicarbonate [28]. However, nearly all the electro/Fe3+/PS processes were performed in an unbuffered solution [25–27]. In this case, the pH of the solution progressively would drop to acidic range as the reaction proceeded though the initial pH was adjusted at neutral or even alkaline [29,30]. Therefore, the electro/ Fe3+/PS system was actually carried out under the acidic condition during most of the reaction time, regardless of the initial pH. When the pH was kept at near-neutral during the reaction, the externally applied Fe3+ would be precipitated. In this case, it is wondering whether the electro/Fe3+/PS process still exhibits the similar behavior and efficiency for the removal of target pollutants. In this study, BPA, a well-known endocrine disruptor, was selected as a probe contaminant due to its estrogenic effects and significant biological toxicity [31]. To save the energy of the system, electricity was supplied by microbial fuel cell (MFC) rather than direct current power (DC power) since MFC is a sustainable electricity production technology [32,33]. The MFC/Fe(III)/PDS system was thus proposed to investigate the elimination of BPA at acidic and near-neutral pH regimes. The difference between the mechanisms of PDS activation at both pH conditions was explored. The precipitate formed under nearneutral condition was characterized by the field-emission scanning electron microscope (FESEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR), thermogravimetric analysis (TGA) and X-ray

2.4. Analyses The concentration of BPA was determined using a Shimadzu LC20AT HPLC system (Japan) consisting of a UV-vis diode array detector at 280 nm. A Shim-PackVP-ODS-C18 column was employed for the separation of the compounds while the mobile phase of 50% acetonitrile and 50% water was flowing through the column at a rate of 1.0 mL min−1. The residual PDS was analyzed using an iodometric titration [36]. The total soluble iron was determined via a TAS-990 atomic absorption spectrophotometer (Pgeneral, China). EPR tests were performed on a Bruker X-bond A-200 spectrometer (Germany) at room temperature, the parameters for EPR analysis were set according to our previous report [37]. The concentration of dissolved organic carbon (DOC) was detected on a TOC analyzer (Elementar, Germany). According to the following theoretical total mineralization reaction for BPA with the number of carbon atoms (m) of 15 and that of exchanged electrons (n) of 72 (Eq. (4)), C15H16O2 + 28H2O → 15CO2 + 72H+ + 72e− 93

(4)

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the reaction solution (L), and 7.2 × 105 is the conversion factor for units homogenization (60 s min−1 × 12,000 mg C mol−1). 3. Results and discussion 3.1. BPA degradation at acidic and near-neutral pH regimes As presented in Fig. 1a, only 7.3% of BPA was eliminated in the Fe (III)/PDS system at acidic pH (2.5 ± 0.1), and 6.7% of PDS was decomposed over 60 min reaction (Fig. 1c). This indicates the infeasible conversion of Fe(III) to Fe(II) for PDS activation without the introduction of MFC. As expected, the degradation of BPA was dramatically increased to 90.8% and 77.2% of PDS was concurrently decomposed when the bioelectricity was introduced to the Fe(III)/PDS system (MFC/ Fe(III)/PDS). It demonstrates that PDS was efficiently activated to oxidize BPA since Fe(III) was continuously converted into Fe(II) at the cathode (Eq. (3)) under acidic pH condition in the MFC/Fe(III)/PDS process. As can be seen in Fig. 1a, the contribution of individual PDS, Fe (III) or MFC to BPA removal could be neglected (< 5%) due to the stability of PDS at ambient conditions and the low-voltage bioelectricity. In the MFC/PDS process, the degradation of BPA and PDS loss reached 25.2% and 34.0%, respectively, which was attributed to that bioelectricity generated in the MFC could activate PDS to a certain extent via Eq. (6) [12]. S2O82− + e− → SO42− + SO4•−

(6)

When the reaction pH was risen to 6.0 and maintained at this value, Fig. 1b shows that 12.0% of BPA was removed in the Fe(III)/PDS system and 7.2% of PDS was decomposed. At near-neutral pH, the Fe(OH)3 precipitate would be formed according to the speciation distribution of Fe(III) (Fig. S2) via Eqs. (7)–(9) [39]. The flocculation of the iron precipitate would lead to the limited removal of BPA, which can be verified by the fact that BPA removal by Fe(III) feebly improved from 1.3% (Fig. 1a) to 7.5% (Fig. 1b) as pH rose from 2.5 to 6.0. In the meanwhile, the heterogeneous activation of PDS with the iron precipitate was responsible for a slight additional degradation of BPA. Surprisingly, when MFC was introduced into the Fe(III)/PDS system, 69.8% of PDS was decomposed accompanied by 82.2% of BPA removal, though the performance of MFC/Fe(III)/PDS system at near-neutral pH was a little inferior to that at acidic pH. Similarly, PDS activation by bioelectricity could only lead to 22.3% of BPA oxidation (Fig. 1b) and 31.0% of PDS depletion (Fig. 1c), and no obvious removal of BPA could be achieved by sole bioelectricity, Fe(III) or PDS system. Fe3+ + H2O → FeOH2+ + H+ k1 = 2.3 × 107 s−1

the mineralization current efficiency (MCE, %) at different reaction time (min) was obtained from DOC decay (ΔDOC, mg L−1) at given current I (mA) through Eq. (5) [38],

nFV DOC × 100 7.2 × 105mIt

Fe

3+

+ 2H2O

→ Fe(OH)2+

−

+ 2H

+

7 −1

k1 = 1.1 × 10 s −38

+ 3OH ⇌ Fe(OH)3 (s) ksp = 4.0 × 10

(7) (8) (9)

To further differentiate the contribution of Fe precipitate to BPA elimination in the MFC/Fe(III)/PDS system under near-neutral condition (pH 6.0), the concentration and percentage of soluble Fe was detected and analyzed (Fig. 2). The results show that the concentration of dissolved iron species was less than 0.4 mg L−1 and only accounted for about 4% of total iron. Therefore, the activation of PDS by soluble iron species for BPA degradation could be neglected when pH was fixed at 6.0. Based on the these results, it is considered that the oxidation of BPA in the tested process at acidic and near-neutral pH was mainly attributed to the homogeneous and heterogeneous activation of PDS by Fe2+ and ^Fe(II) (^represents the surface of the precipitate) species, respectively. The kinetics of the MFC/Fe(III)/PDS system at acidic and nearneutral pH was also examined (Fig. S3) and it can be observed that BPA removal at both pH conditions followed pseudo first-order kinetics (R2 > 0.98),

Fig. 1. BPA removal in various systems at pH (a) 2.5 ± 0.1, (b) 6.0 ± 0.1 and (c) PDS remaining after 60 min reaction. Conditions: C0 = 50 μM, [Fe (III)] = 0.2 mM, [PDS] = 4 mM, jA = 100 mA m−2, [Na2SO4] = 50 mM.

MCE (%) =

Fe

3+

(5)

where F is the Faraday constant (96,485 C mol−1), V is the volume of 94

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examine the chemical groups in the sample. The peak at 1630 cm−1 was designated to the HOH deformation vibration of H2O molecules [40] while those at 1128 and 1055 cm−1 responded to the v3 vibrations of SO42− [41] which stemmed from the precursor or the electrolyte. In the low wavenumber area, the absorption band at 980 cm−1 was assigned to the bending vibrations of the group eOH⋯O of iron oxyhydroxides [42]. The bands at 607 and 467 cm−1 were attributed to the FeeO bonds of iron oxides and oxyhydoxides according to the report by Durães et al. [43]. For comparison, the spectrum of the precipitate formed only by Fe2(SO4)3 at pH 6.0 ± 0.1 was also recorded. As shown in Fig. 3c, there is no significant difference between the spectra of the two samples in the range of 2200–400 cm−1, disclosing that the precipitate formed in the studied system was mainly composed of iron oxyhydroxide. TGA result indicates that the weight of the sample lost 27.6% when the temperature increased to 840 °C. It was even slightly higher than the theoretical value of the dehydration of Fe(OH)3 (25.2%) which was probably resulted from the presence of water on the surface of the sample which was obtained without heat treatment. XPS was used to further study the chemical components of the precipitate formed in the MFC/Fe(III)/PDS system (Fig. 4). It can be observed from Fig. 4a that the peaks of Fe 2p, O 1s, C 1s and S 2p were present in the survey spectrum of the precipitate and the C 1s peak is attributed to the residual carbon from the instrument itself while S 2p peak may stem from SO42−, which is consistent with the result of FTIR. However, from the peak table in the inset of Fig. 4a, the sulfur content only accounted for 1.9%, suggesting that the existence of sulfur in the precipitate could be neglected. Additionally, atomic ratio of O/Fe was calculated to be 2.9, which was between that of Fe(OH)3 (3.0) and Fe(OH)2 (2.0) and closer to that of Fe(OH)3, demonstrating that Fe(OH)3 could be the main composition of the precipitate. Fe 2p high-resolution spectrum of the precipitate is illustrated in Fig. 4b. The peaks of Fe 2p spectrum at 711 and 719 eV represented the binding energy of Fe 2p3/2 and its shake-up satellite, respectively, while 725 eV responded to that of Fe 2p1/2 [44]. Moreover, the peaks at 710.3 and 711.4 eV in Fe 2p3/2 were designated to the Fe(II) and Fe(III) species, respectively, confirming the existence of Fe(II) and Fe(III) species in the precipitate. Furthermore,

Fig. 2. The concentration and percentage of soluble Fe in the MFC/Fe (III)/PDS system at pH 6.0 ± 0.1. Conditions: C0 = 50 μM, [Fe(III)] = 0.2 mM, [PDS] = 4 mM, jA = 100 mA m−2, [Na2SO4] = 50 mM.

ln

C = k1 t C0

(10)

where C0 and C are the concentration of BPA at initial time and time t, respectively, and k1 is the reaction rate constant. Furthermore, the k1 value at pH 2.5 (0.0424 min−1) was higher than that at pH 6.0 (0.0326 min−1), showing the homogeneous activation of PDS was more efficient than the heterogeneous one in the tested system. 3.2. Characterization of the precipitate To identify the precipitate formed at pH 6.0, several characterization techniques were employed. It can be observed from Fig. 3a that the sample displays irregular shape and the size is mainly below 200 nm. No obvious diffraction peaks at 2θ from 10° to 80° could be observed from the XRD pattern of the precipitate (Fig. 3b), indicating the sample was amorphous. FTIR spectrum of the sample (Fig. 3c) was employed to

Fig. 3. (a) FESEM image, (b) XRD pattern, (c) FTIR spectra, and (d) TGA curve of the precipitate formed in the MFC/Fe (III)/PDS system at pH 6.0 ± 0.1. 95

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Fig. 5. EPR spectra trapped by DMPO in the MFC/Fe(III)/PDS system at pH 2.5 and 6.0. Conditions: [Fe(III)] = 0.2 mM, [PDS] = 4 mM, jA = 100 mA m−2, [Na2SO4] = 50 mM, [DMPO] = 10 mM, t = 2 min.

EPR spectra are assigned to the oxidized DMPO radical (aN = 15.5 G) [46]. It is noted that the intensities of DMPO-SO4 and DMPO-OH at pH 2.5 were slightly higher than that at 6.0, verifying that more sulfate radical could be generated at acidic pH than at near-neutral pH which was in line with the results of BPA oxidation in the MFC/Fe(III)/PDS process (Fig. 1). SO4•− + H2O → •OH + HSO4−

(11)

3.3.2. Quenching experiments To further differentiate the predominant reactive species present in the MFC/Fe(III)/PDS system at acidic and near-neutral pH, quenching experiments were conducted using different scavengers. Due to the fact that the rate constant of tert-butyl alcohol (TBA) and hydroxyl radical (3.8–7.6 × 108 M−1 s−1) is far faster than that of TBA and sulfate radical (4–9.1 × 105 M−1 s−1) [47], TBA was thus employed to trap hydroxyl radical while methanol (MeOH) and phenol were used to quench both the hydroxyl and sulfate radical based on the fact that the rate constant between these two quenchers and hydroxyl radical (kMeOH/ •OH = 9.7 × 108 M−1 s−1, kphenol/•OH = 8.8 × 109 M−1 s−1) is approximate to that of MeOH/phenol and sulfate radical (kMeOH/SO4• = 2.5 × 107 M 1 s 1, k phenol/SO4• = 6.6 × 109 M 1 s 1) [48,49]. Moreover, due to the difference in hydrophilicity, phenol is prone to quench the bound/adsorbed radicals (•OH and SO4•−) on the surface of solids while MeOHis frequently used to trap the radicals in the bulk solution [50–52]. As can be seen from Fig. 6a that at pH 2.5 ± 0.1, when 20 mM of TBA was added, BPA removal slightly declined to 79.3%, implying the presence of a small amount of hydroxyl radical in the MFC/Fe(III)/PDS system at acidic condition. However, the addition of MeOH (20 mM) led to a remarkable decrease of BPA removal, demonstrating that sulfate radical in the bulk solution was the dominant species for BPA elimination in the tested process at acidic condition. The effects of scavengers (20 mM) on the decay of BPA at pH 6.0 were demonstrated in Fig. 6b. It can be observed that 20 mM of TBA could induce 20.7% of inhibition efficiency on BPA decay at pH 6.0 ± 0.1 which was higher than that at pH 2.5 (12.9%). It may be attributed to that sulfate radical was more readily transformed to hydroxyl radical at near-neutral pH than that at acidic pH as reported previously. Interestingly, when MeOH was added into the system, no obvious change of BPA degradation could be observed, which could result from the tradeoff between sweeping the surface of the precipitate and scavenging the radicals by MeOH. However, with the presence of phenol, the elimination of BPA dramatically dropped to 23.7%,

Fig. 4. (a) Wide survey XPS spectrum and (b) Fe 2p high-resolution spectrum of the precipitate formed in the MFC/Fe(III)/PDS system at pH 6.0 ± 0.1.

the percentage of Fe(II) and Fe(III) in the precipitate was 32.6% and 67.4%, respectively, indicating that most of the iron in the precipitate was in the +3 valence state which was in accordance with the result of TGA. The Fe(II) species in the precipitate would be formed through the reduction of Fe(III) species on the cathode when bioelectricity was supplied. All these characterization results disclose that the precipitate produced in the MFC/Fe(III)/PDS process at near-neutral condition (pH 6.0) is mainly the amorphous iron oxyhydroxides. 3.3. Mechanistic study 3.3.1. Reactive species identification EPR spectra using DMPO as the trapper was firstly measured after 2 min reaction to identify the reactive species involved in the MFC/Fe (III)/PDS process at acidic and near-neutral pH (Fig. 5). Based on the hyperfine coupling constants, the characteristic peaks of the sulfate radical (aN = 13.2 G, aH = 9.6 G, aH = 1.48 G and aH = 0.78 G) could be observed at pH 2.5 and 6.0, demonstrating the generation of sulfate radical in the tested system at both pH conditions. In addition, the apparent signals of the adduct of DMPO and hydroxyl radical (DMPOOH, aN = aH = 14.9 G) with the intensity ratio of 1:2:2:1 was detected at acid and near-neutral conditions, which may be owing to the partial conversion of sulfate radical via Eq. (11) [45] and the transformation of DMPO-SO4 via nucleophilic substitution. The other three peaks in the 96

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Fig. 7. The possible mechanisms of BPA degradation in the MFC/Fe(III)/PDS system at acidic and near-neutral pH.

Fig. 6. The effects of scavengers on the degradation of BPA in the MFC/Fe(III)/ PDS system at pH (a) 2.5 ± 0.1 and (b) 6.0 ± 0.1. Conditions: C0 = 50 μM, [Fe(III)] = 0.2 mM, [PDS] = 4 mM, jA = 100 mA m−2, [Na2SO4] = 50 mM, [scavengers] = 20 mM.

inferring that bound/adsorbed sulfate radical on the surface of the precipitate was the dominant reactive species responsible for BPA oxidation in the studied process at near-neutral pH. Therefore, sulfate radical in the bulk solution and that adsorbed on the surface of the precipitate were the main reactive species in the homogeneous and heterogeneous MFC/Fe(III)/PDS system, respectively. According to the characterization of the precipitate, EPR spectra and the results of scavenging experiments, a probable mechanism of BPA oxidation in the MFC/Fe(III)/PDS process at acidic and near-neutral pH was proposed and plotted in Fig. 7. At pH 2.5, PDS was mainly activated by dissolved ferrous iron produced via cathodic reduction to generate sulfate radicals in the bulk solution. While at pH 6.0, ^Fe(II) formed through the reduction of the precipitate on the cathode was the principal activator for PDS decomposition to produce the bound/adsorbed sulfate radical on the surface of the precipitate. In the meantime, the bioelectricity provided by the MFC could also activate PDS to some extent at acidic and near-neutral pH. Sulfate radical could partially be converted to hydroxyl radical. The generated sulfate and hydroxyl radicals were responsible for the oxidation of BPA under acidic and nearneutral conditions in the MFC/Fe(III)/PDS system.

Fig. 8. (a) DOC decay and (b) MCE changes of the tested system at acidic and near-neutral pH. Conditions: C0 = 50 μM, [Fe(III)] = 0.2 mM, [PDS] = 4 mM, jA = 100 mA m−2, [Na2SO4] = 50 mM.

complete degradation, the mineralization of BPA in the MFC/Fe(III)/ PDS process was further investigated. As shown in Fig. 8a, during the reaction, the residual concentration of DOC gradually decreased at acidic and near-neutral pH. After 60 min reaction, 64.6% and 59.3% of DOC decay could be obtained at acidic and near-neutral pH,

3.4. BPA mineralization, mineralization current efficiency and energy consumption Since the removal of target pollutants does not represent its 97

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respectively. When the time was prolonged to 120 min, the removal of DOC increased to 81.8% and 72.9% at pH 2.5 and 6.0, respectively, manifesting the high mineralization efficiency of BPA in the homogeneous and heterogeneous systems. In comparison with DOC removal at pH 2.5, the lower removal efficiency of DOC was achieved at pH 6.0 which was attributed to the lower reaction rate in the heterogeneous system. Based on the results of DOC removal, the MCE as a function of reaction time in the homogeneous and heterogeneous MFC/Fe(III)/PDS systems was calculated and illustrated in Fig. 8b. After 20 min reaction, 99.7% and 94.0% of MCE could be achieved in the homogeneous and heterogeneous systems, respectively, indicating the high MCE at the initial phase of the reaction in the two systems. Within 120 min, MCE progressively decreased to 53.7% and 41.5% at pH 2.5 and 6.0, respectively, which may attribute to the formation of the intermediates of BPA. It can be also observed that the MCE in heterogeneous system was lower than that in homogeneous system during the whole reaction, which was owing to the lower DOC removal in the heterogeneous system. Besides the mineralization of the target contaminant, the energy consumption is another concern in the practical application, and consequently electrical energy per order (EE/O) [52,53] was employed to assess the energy consumption of the homogeneous and heterogeneous MFC/Fe(III)/PDS systems (Eq. (12)). Since the electricity used in the studied system was supplied by the MFC, only the electricity consumption of magnetic agitation was considered.

EE /O =

Pt 60V log (C0/ Cf )

literature on BPA treatment by various advanced oxidation processes (AOPs) are listed in Table 1. As can be seen, both of the present systems are more energy-saving than other ultrasound (US), ultraviolet (UV), visible light, or electrochemically (DC power or galvanostat) assisted AOPs. The slightly higher energy consumption in the heterogeneous MFC/Fe(III)/PDS system was resulted from the lower removal rate of the target pollutants as compared with the corresponding homogeneous system. 3.5. Effects of operating parameters To further demonstrate the performance of the MFC/Fe(III)/PDS process at acidic and near-neutral pH for BPA elimination, the effects of reaction parameters including the dosage of Fe(III), PDS concentration and current density were explored. The results (Fig. S4a) present that under acidic condition, BPA elimination markedly increased from 25.2% to 73.7% to 90.8% when the dosage of Fe(III) rose from 0 to 0.1 to 0.2 mM. However, when the dosage further increased to 0.4 mM, the degradation of BPA decreased to 84.0%, which was due to the fact that excessive Fe2+ would be generated via Eq. (3) and then the side reaction between Fe2+ and sulfate radical (Eq. (14)) would occur [10]. When the pH of the solution was maintained at 6.0, the degradation of BPA raised with the increase of Fe(III) dosage (Fig. S4b). Notably, no significant change of BPA oxidation could be observed when the dosage of Fe(III) was raised from 0.2 to 0.4 mM, which was different from that at pH 2.5, inferring that the side reaction induced by the same dosage of the activator was less readily taken place in the heterogeneous system than that in the homogeneous one. The influences of PDS concentration on BPA oxidation were similar to those of Fe(III) dosage (Fig. S5). Under both acidic and near-neutral conditions, BPA removal noticeably increased as PDS concentration rose from 2 to 4 mM. The further increase of PDS concentration from 4 to 8 mM caused an obvious drop in BPA degradation at pH 2.5 (Fig. S5a), while insignificant change of BPA removal was achieved at pH 6.0 (Fig. S5b). It was owing to the fact that more reactive radicals were generated in the homogeneous system at pH 2.5 than those in the heterogeneous one at pH 6.0, as shown in EPR spectra of Fig. 5. This led to the more severe scavenging of free radicals by overdosed PDS (Eq. (15)) [49] and the recombination reaction of the radicals (Eqs. (16) and (17)) [48,65] under acidic pH condition. Fig. S6 manifest that higher current density in the range of 50–150 mA m−2 was beneficial to the elimination of BPA in both

(12)

where P is the rated power (W) of the reaction system (3.0 W for magnetic agitation in this study); t is the reaction time (60 min in this study); V is the solution volume (L); C0 and Cf are the initial and final concentration of BPA (μM), respectively. Since BPA removal in both the homogeneous and heterogeneous systems followed pseudo first-order kinetics as given in Eq. (10), substituting this equation into Eq. (12) gives the following Eq. (13):

EE /O =

2.303P 60Vk1

(13)

The EE/O values of the homogeneous and heterogeneous MFC/Fe (III)/PDS systems were calculated to be 10.9 and 14.1 kWh m−3 order−1, respectively. These values and those in the Table 1 The energy consumption in various AOPs for BPA removal. AOP type

pH

[BPA]0 (mM)

P (W)

t (min)

V (L)

k1 (min−1)

EE/O (kWh m−3 order−1)

Ref.

US/Fe(II) US/UV/Fe(II) UV/H2O2 UV/H2O2 UV-C/PMS UV/PDS UV/PDS UV-PDS/H2O2-Fe(II, III) Cu, N-titanate nanotube/visible light Electro-Fenton Anodic oxidation (BDDd anode) Electro-oxidation (Nb/BDDd anode) Electro-oxidation (Ti/SnO2 anode) Electro/Mn0.6Zn0.4Fe2O4/PDS Homogeneous MFC/Fe(III)/PDS Heterogeneous MFC/Fe(III)/PDS

3 3 7 6 5.2 7 6 7 NAa ∼3 6 6 NAa 6.2 2.5 6.0

0.12 0.12 0.01 0.22 0.22 0.01 0.22 0.05 0.02 0.66 0.09 0.66 0.04 0.1 0.05 0.05

80 105 700 40 40c 700 40 15c 32 176 28 59 60 44 3 3

600 120 180 240 360 180 240 300 120 50 300 180 40 60 60 60

0.3 0.3 0.03 0.45 0.45 0.03 0.45 1 0.15 1.5 0.25 0.4 3.5 0.2 0.25 0.25

NAa NAa 0.061 0.005 0.025 0.200 0.009 0.038 0.163 0.016 0.008 0.017 NAa NAa 0.042 0.033

6010b 1033b 14682 509b 136 4478 307b 15.2 50.2 281 537 333 316e 335e 10.9 14.1

[54] [54] [55] [56] [57] [55] [56] [58] [59] [60] [61] [62] [63] [64] This study This study

a b c d e

NA: non-available. Provided by the authors. Only the power of UV or UV-C lamp(s) was calculated. Abbreviation of boron-doped diamond. Calculated according to Eq. (12). 98

Journal of Hazardous Materials 374 (2019) 92–100

S. Yan, et al.

homogeneous and heterogeneous systems owing to the accelerated (re) generation of Fe2+ or ^Fe(II) as given in Eq. (3), and the enhancement of sulfate radical production (Eq. (1)). It is noted that when the current density increased from 100 to 150 mA m−2, the increase of BPA degradation was greater at pH 6.0 (from 82.2% to 89.0%) than that at pH 2.5 (from 90.8% to 92.5%), revealing that higher current density would be needed in the heterogeneous system to achieve a comparable elimination of BPA as obtained under acidic condition. Fe2+ + SO4•− → Fe3+ + SO42− SO4

•−

+ S2O8

2−

→

SO42−

SO4•− + SO4•− → S2O82−

k2 = 4.9 × 109 M−1 s−1

+ S2O8•−

5

k2 = 3.8–8.1 × 108 M−1 s−1

SO4•− + •OH → HSO5− k2 = 1 × 109 M−1 s−1

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(14)

−1 −1

k2 = 6.1 × 10 M

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s

(15) (16) (17)

4. Conclusions In this work, PDS was activated by Fe(III) with low-voltage bioelectricity provided by MFC at acidic and near-neutral pH. The results demonstrate that in the MFC/Fe(III)/PDS system, when the dosage of Fe (III) was 0.2 mM, PDS concentration was 4 mM, and current density was 100 mA m−2, respectively, nearly 91% BPA was eliminated at acidic pH (2.5) over 60 min reaction. Under the same conditions except that current density was 150 mA m−2, 89.0% of BPA was still removed at near-neutral pH (6.0) though the iron existed in the form of precipitate. Through the XRD, FTIR, TGA and XPS characterization, the precipitate generated in the MFC/Fe(III)/PDS system at pH 6.0 was regarded as the amorphous iron oxyhydroxides. Sulfate radical in the bulk solution and that adsorbed on the surface of the precipitate were the predominant reactive species in the homogeneous and heterogeneous MFC/Fe(III)/ PDS system, respectively. The potential mechanisms of BPA degradation at acidic and near-neutral pH were also proposed via EPR, quenching tests and XPS analysis. The rate constant (k1) based on BPA removal, DOC decay and MCE at near-neutral pH were lower than those at acidic pH. Meanwhile, a slightly higher energy consumption was required for the heterogeneous MFC/Fe(III)/PDS system than the homogeneous one. This study revealed the superiority of electro-assisted transitional metal activated PDS system. Specifically, the homogeneous activation of PDS by transitional metal ion was efficient only under acidic pH condition. The introduction of bioelectricity could overcome this deficiency and extend pH range of the PDS activated process. In the near-neutral pH regime, the externally applied ferric ion would be precipitated, but the iron species in the precipitate could be reduced at the cathode and heterogeneously activated PDS to produce reactive radicals for the degradation of contaminants. This would make sense for the application of the MFC/Fe(III)/PDS system in the treatment of real wastewater with high buffering capacity. Acknowledgments This work was supported by National Natural Science Foundation of China (Grant No. 21547006) and Wuhan Applied Basic Research Project (Grant No. 2016060101010074). Suding Yan would like to acknowledge the financial support by Hubei Key Laboratory of Pollutant Analysis & Reuse Technology (Hubei Normal University) (Grant No. PA20170201). The analysis of FESEM and XPS was partially supported by the Large-scale Instrument and Equipment Sharing Foundation of Wuhan University. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jhazmat.2019.03.068. 99

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