- Email: [email protected]
Degradation of bisphenol A by electrocatalytic wet air oxidation process: Kinetic modeling, degradation pathway and performance assessment
Journal Pre-proofs Degradation of bisphenol A by electrocatalytic wet air oxidation process: kinetic modeling, degradation pathway and performance assessment Min Sun, Hui-Hui Liu, Yu Zhang, Lin-Feng Zhai PII: DOI: Reference:
S1385-8947(20)30115-7 https://doi.org/10.1016/j.cej.2020.124124 CEJ 124124
To appear in:
Chemical Engineering Journal
Received Date: Accepted Date:
6 September 2019 12 January 2020
Please cite this article as: M. Sun, H-H. Liu, Y. Zhang, L-F. Zhai, Degradation of bisphenol A by electrocatalytic wet air oxidation process: kinetic modeling, degradation pathway and performance assessment, Chemical Engineering Journal (2020), doi: https://doi.org/10.1016/j.cej.2020.124124
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier B.V.
Degradation of bisphenol A by electrocatalytic wet air oxidation process: kinetic modeling, degradation pathway and performance assessment Min Sun, Hui-Hui Liu, Yu Zhang, Lin-Feng Zhai* Anhui Province Key Laboratory of Advanced Catalytic Materials and Reaction Engineering, School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, China
* Corresponding author: Dr. Lin-Feng Zhai Tel: +086-551-2901450 E-mail: [email protected]
Abstract: Pharmaceuticals and personal care products (PPCPs) are becoming ubiquitous in environment, and effective removal of PPCPs from contaminated water bodies is urgently needed to avoid their potential hazards to human health. Electrocatalytic wet air oxidation (ECWAO) process is a highly efficient technology for the rapid and deep mineralization of PPCPs under room condition. In this work, the degradation behavior of the bisphenol A (BPA), a typical PPCPs compound, in the ECWAO process is comprehensively investigated. A simple kinetic model is developed to describe the removal rate of BPA as a response to the initial concentration of BPA and intensity of electric field. BPA removal in the ECWAO process is accompanied by its partial mineralization, with p-isopropenyl phenol, benzoquinone and propanedioic acid as intermediate products. At last, the performances of the ECWAO process in treating BPA and other two PPCPs compounds, i.e. triclosan (TCS) and sulfamethoxazole (SMX), are assessed under a condition optimized by the kinetic model. The results demonstrate the ECWAO process is able to totally eliminate the toxicity of PPCPs-contaminated waters at low energy consumption. Notably, the treated-waters show good aerobic biodegradabilities, enabling such a process a prospective pretreament for coupling with aerobic biological process.
Keywords: Electro-assisted catalytic wet air oxidation; Bisphenol A; Degradation pathway; Kinetic model; Performance assessment
1. Introduction Pharmaceuticals and personal care products (PPCPs) are recognized as pseudo persistent organic pollutants due to their toxicity and low biodegradability in environment [1]. The discharge of PPCPs into environment results in their ubiquitous distribution in a wide range of natural water bodies, posing potential threat to human health. Since conventional biological treatments are inefficient to eliminate PPCPs, physicochemical and/or chemical technologies are necessary for the remediation of PPCPs-contaminated water bodies. Wet air oxidation (WAO) is an aqueous phase oxidation process which makes use of oxygen molecules (O2) in air to oxidize water contaminants. Such a process is especially effective for the treatment of refractory organic pollutants such as PPCPs [2,3]. However, since the reaction between O2 and organic compounds needs very high activation energy, WAO systems are usually operated at high temperatures (125-320 °C) and pressures (0.520 MPa) in order to achieve intensive oxidation of organic pollutants within a short time [4]. The presence of a catalyst has largely decreased the temperature and pressure of the WAO, but the catalytic wet air oxidation (CWAO) still needs an operation temperature quite higher than room temperature [5]. The severe operation condition has been the major obstacle that limits the practical application of WAO/CWAO. Recently, we have found the air oxidation reaction can be triggered by an anodic electric field under room condition, based on which electrocatalytic wet air oxidation (ECWAO) process has been developed for treating PPCPs in wastewater [6-8]. The ECWAO process is not only eco-friendly but also energy-saving as compared to conventional WAO/CWAO processes [7]. Mechanistic study has indicated a series of elementary steps in such a process, involving the dissolution of O2 from bulk gas phase to bulk liquid phase, the adsorption of
O2 and organic molecules on the active sites of catalyst, the activation of O2 on the active sites via electro-oxidation reaction, and the surface chemical reactions between active oxygen species and organic molecules [7]. Obviously, reaction parameters such as strength of electric field, concentration of dissolved O2 and concentration of pollutants influence the reaction rate. A sufficient knowledge of ECWAO kinetics is essential for the proper design of an ECWAO reactor as well as the process optimization. In this work, the oxidation of bisphenol A (BPA) is chosen as a model reaction to gain kinetic data about the ECWAO process. BPA is a typical PPCPs compound utilized in the manufacture of food and drink storage containers, polycarbonate baby bottles, tableware, white dental fillings and sealants [9-11]. While the leaching of BPA from products has led to its occurrence in various environmental matrices [12,13], the remediation methods of such a chemical is required due to its strong estrogenic endocrine disrupting effect [14]. Herein, an ECWAO system is constructed on partially oxidized cobalt (Co-CoO)/graphite felt (GF) electrode for BPA treatment under room condition. A kinetic model is developed to describe the removal of BPA regarding the effect of various operating parameters, and the degradation pathway of BPA is elucidated. At last, the performances of ECWAO process in treating BPA and other PPCPs are assessed in terms of treatment efficiency and energy consumption.
2. Experimental methods 2.1 . Operation of the ECWAO process for BPA degradation The ECWAO process was performed in a 175 mL cylindrical glass-made reactor with a liquid volume of 120 mL. The reactor was equipped with a Co-CoO/GF composite (2.5×4
cm2) as the anode electrode and a platinum wire (Φ 0.5 mm × 37 mm) as the cathode electrode. The Co-CoO/GF electrode was prepared by a combined impregnation and pyrolysis approach, as previously described [7]. The electrolyte was 50 mM Na2SO4 solution and the concentration of BPA in the electrolysis was 5 to 100 mg L-1. The other PPCPs treated by the ECWAO process include triclosan (TCS) and sulfamethoxazol (SMX) at a concentration of 20 mg L-1. The initial solution pH was 7.0 except for the TCS, whose solution pH was set at 9.0 to ensure its dissolution. The electric field was applied in galvanostatic mode at a constant current ranged from 1 to 20 mA, and air was bubbled from the bottom of the reactor. The pH was monitored during the process showing that only marginal change had occurred between the initial and final solutions. The concentration of BPA, TCS and SMX were determined on a high performance liquid chromatography system (Waters, UAS) equipped with a multiple wavelength UV detector and a SunFireTM C18 column (150 mm × 4.6 mm, 5μm). The mobile phase was wateracetonitrile mixture (40/60, v/v) at a flow rate of 1 mL min-1. The concentration of total organic carbon (TOC) was determined on a HTM-CT1000M TOC analyzer (Tailin, China), and the concentration of dissolved O2 (DO) was determined using an AZ8403 DO meter (HengXin, China). The chemical oxygen demand (COD) was determined using the dichromate reflux method, and the 5-day biological oxygen demand (BOD5) was determined from the decrease of DO after 5 days of incubation in the dark at 20 °C [15]. All experiments were conducted in triplicate, and the average values with standard deviations were presented.
2.2 Gas chromatography-mass spectrometry (GC-MS)
Intermediate products generated from the degradation of BPA were identified by GC-MS on a Thermo Trace 1300 system equipped with a TR-V1 capillary column (30 m × 0.25 mm ID × 1.4 μm film, USA). High-purity helium was chosen as the carrier gas, and injection temperature was set at 280 °C. Column temperature was gradually increased from 80 to 280 ºC at a rate of 12.5 ºC min-1. Electron impact (EI) mass spectra were monitored from 50 to 500 m/z. The ion source and inlet line temperatures were set at 220 and 280 °C, respectively. Solid-phase extraction was conducted to preconcentrate the products prior to GC-MS analysis. 1000 mL of sample was loaded on an Oasis HLB SPE cartridge (3 mL, 60 mg, Waters, USA) at 10 mL min-1, and then elution was performed with 5 mL of methanol.
2.3. Toxicity Analysis The toxic effects of PPCPs and their degradation products were evaluated by the inhibition growth of bacterium Escherichia coli in Luria-Bertani (LB) medium. The assays were carried out in a 96-well microtiter plate. Samples were sterilized using 0.45 mm membrane filters and, then, added into the 96-well microtiter plate at 40 μL. For each sample duplicate wells were inoculated with 160 μL of LB culture of E. coli. The specimen with 50 mM Na2SO4 was used as a control. The plate was incubated at 37 °C and the growth of E. coli was monitored by reading the absorption at 600 nm in a microtiter plate reader.
3. Results and discussion 3.1 Effect of operation parameters on BPA removal
The effect of the initial concentration of BPA on its removal rate is studied at a constant current of 7 mA and aeration rate of 0.1 L min-1. As shown in Fig.1a, the removal efficiency of BPA was gradually increased along with the reaction time t until the utmost value of 100%. The experimental results regarding to t (min) and concentration of BPA [BPA] (mg L-1) are fitted to a pseudo-first-order kinetic equation: d[BPA] d𝑡
= ― 𝑘′[BPA]
(1)
where k′ is a pseudo-first-order rate constant. As shown in Fig. 1b, a linear relationship between -ln([BPA]t /[BPA]0) and t is observed, suggesting a pseudo-first-order kinetics for BPA oxidation. Fig. 1c demonstrates the value of k′ is significantly dependent upon [BPA]0. The k′ gradually increases with the increment of [BPA]0 from 5 to 15 mg L-1, and the k′ versus [BPA]0 plot fits an equation as k′ = 0.05132[BPA]00.07998. The k′ keeps constantly at 0.06358 min-1 as [BPA]0 is between 15 and 20 mg L-1, indicating the oxidation rate of BPA is no longer limited to its initial concentration. Then, a continuous decrease of k′ is observed with the further increment of [BPA]0 from 20 to 100 mg L-1, giving a relationship expressed as k′ = 0.14736[BPA]0-0.27486. The oxidation of BPA by O2 on the electrode surface is a typical heterogeneous reaction dependent upon both the surface oxidation kinetics and mass transfer kinetics [16-17]. High concentration of the reactant BPA tends to clog the pores of the Co-CoO/GF electrode and impede the transportation of O2 to the active sites of the electrode, thus delay the reaction kinetics. Figs. 2a and 2b show the oxidation rate of BPA is positively dependent upon circuit current I (mA). The k′ with respect to I is determined to be k′ = 0.01872I0.41901 when I is in the range of 1 to 10 mA (Fig. 2c) . However, the further rise of I beyond 10 mA does not
yield proportional increase in k′, suggesting the intensity of electric field is high enough that it is not the rate limiting factor in BPA oxidation. Finally, the effect of aeration rate on BPA removal kinetics is investigated. As shown in Fig. 3, the k′ is kept at 0.04012 min-1 as the aeration rate increases from 0.1 to 0.9 L min1.
It seems the aeration rate plays an insignificant role in determining the BPA removal
rate, which can be explained by comparing the O2 demand and supply in the process. Assuming the BPA is completely mineralized into CO2 and H2O in the ECWAO process, the oxidation of 0.0526 mM (100 mg L-1) BPA consumes at most 0.789 mM O2, which equals to 0.019 L O2 under the experiment condition of this work, according to eq. 2: 𝑃𝑉 = 𝑛𝑅𝑇
(2)
where P refers to gas pressure (101325 Pa), V to gas volume (L), T to the temperature (298.15 K), n is the molar of O2 (0.789×10-3 mol) and R is the gas constant 8.314×10-3 Pa L mol-1 K-1. In comparison, a total amount of as high as 3.78 L of oxygen is supplied during the 180 min of ECWAO process even at the lowest aeration rate of 0.1 L min-1, assuming O2 content in air is 21%. Therefore, bubbling rate of oxygen is not considered to be a critical parameter for practical application of the ECWAO process.
3.2 Kinetic model describing the oxidation of BPA in the ECWAO process The generalized kinetic model for the oxidation of BPA in the ECWAO process is constructed on the basis of a reaction scheme involving the formation and activation of chemisorbed oxygen species on the Co-CoO catalyst [7]. As depicted by eqs. 3-6, at the first step, the O2 is adsorbed onto the catalytically active site of CoO (CoII[]) , and then undergoes dissociation to form chemisorbed oxygen species O-/O2-, which are
subsequently electro-oxidized to their active states O-/O2-* in the electric field. The activated O-/O2-* species possess potent oxidizing power at room temperature, and they react with BPA in successive steps yielding the final oxidation products. CoII[] + O2→CoII[O2]
(3)
CoII[O2]→CoIII[O ― /O2― ] -e
CoIII[O - /O2- ] CoIII[O - /O2CoIII[O - /O2-
∗
(4) ∗
]
(5)
] + [BPA]→CoII[] +oxidation products
(6)
Ageneral rate equation for BPA removal is expressed as: d[BPA] d𝑡
= ―𝑘𝑎𝑝𝑝[BPA]a[O - /O2-
∗ b
]
(7)
where kapp is an apparent oxidation rate constant, [O-/O2-*] denotes concentration of O-/O2-*, and a and b are orders of the reaction with respect to BPA and O-/O2-*, respectively. The [O-/O2-*] is dependent upon the amount of CoII[] ([CoII[]]), concentration of dissolved oxygen ([O2]) and circuit current I. Thus, eq. 7 is expressed as: d[BPA] d𝑡
𝑛
= ―𝑘𝑎𝑝𝑝[BPA]𝑎[O2]𝑚[CoII[]] 𝐼𝑐
(8)
where m, n and b are orders of the reaction with respect to O2, CoII[] and I, respectively. With a given electrode, the [CoII[] can be viewed as a constant. Moreover, since the [O2] is not the rate limiting factor in the oxidation of BPA, it is also regarded as a constant. Hence, eq. 8 is further simplified as: d[BPA] dt
= ―𝑘𝑜𝑏𝑠[BPA]a𝐼c
(9) 𝑛
Where kobs = kapp[CoII[]] [O2]𝑚 is the observed rate constant. Next, the relationship between k′ and kobs can be deduced from eqs. 1 and 9 as:
𝑘′ = 𝑘𝑜𝑏𝑠[BPA]a ― 1𝐼c
(10)
By alternation of the variable one at a time and linearization of the data, the dependence of k′ on [BPA]0 and I in eq. 10 can be determined. Given all the parameters and coefficients calculated, three different rate equations are established to describe the oxidation of BPA in the ECWAO process: 0.41901
𝑘′ = 0.02279 × 𝐼
× [BPA]0.07998 0
0.41901
= 0.02655 × 𝐼
0.41901
= 0.06242 × 𝐼
× [BPA]0―0.27486
1 ≤ 𝐼 ≤ 10 mA, 5 ≤ [BPA] 0 ≤ 15 mg L ―1
(11) 𝑘′
1 ≤ 𝐼 ≤ 10 mA , 15 ≤ [BPA]0 ≤ 20 mg L ―1
(12) 𝑘′
1 ≤ 𝐼 ≤ 10 mA, 20 ≤ [BPA] 0 ≤ 100 mg L ―1 (13) Fig.
4 graphically displays the kinetic model in term of k ′ as a function of [BPA]0 and [I]. In order to demonstrate accuracy of the kinetic model, seven different reaction conditions are selected within the kinetic model for verification. As shown in Fig. 4, minor discrepancies are found between the predicted k ′ and the experimental measurements, suggesting the kinetic model describes reasonably well the removal of BPA in EWAO process.
3.3 Degradation pathway of BPA in the ECWAO process GC-MS analysis identifies three new compounds generated from BPA degradation, which are p-isopropenyl phenol (m/z=134), benzoquinone (m/z=78) and propanedioic acid (m/z=104). The three compounds are regarded to be the intermediate products, because their peaks eventually disappear from the GC chromatogram. Therefore, a possible degradation pathway of BPA in the ECWAO process is proposed and depicted in Fig. 5. For the BPA, the two electron-donating hydroxyl groups increase the electron densities of phenyl rings, making the C-C bond between them vulnerable to be attacked. Therefore, the
degradation is initiated by the cleavage of C-C bond linking the two phenyl rings, yielding the p-isopropenyl phenol and benzoquinone as intermediate products. Next, the pisopropenyl phenol and benzoquinone undergo ring cleavage and produce propanedioic acid, which is then further oxidized to CO2, H2O and small organic molecules, as generally occurring in conversional wet air oxidation or electrochemical oxidation systems [18-20].
3.4 Performance assessment of the ECWAO process for PPCPs treatment The efficiency of the ECWAO process on PPCPs removal is evaluated using three representative compounds, i.e. BPA, TCS, and SMX. The ECWAO process is operated with current set at 10 mA and concentration of PPCPs at 20 mg L-1. According to the kinetic model, the BPA obtains relatively higher removal rate under such a condition. As shown in Figs. 6a and 6b, it requires 80 min for the complete removal of BPA and SMX, and 120 min for SMX removal. Accompanied with their total elimination, the TOC is reduced by 71.9%, 73.9% and 60.4% for the BPA, TCS, and SMX, respectively, Next, the efficiency of the ECWAO process for utilizing electric energy is evaluated by calculating specific energy consumption (SEC, kW h kg-TOC-1) on degrading the PPCPs. The SEC is calculated in terms of the removal of 1 kg-TOC as eq. 14: 𝑡
∫0𝑈𝐼
𝑆𝐸𝐶 = ∆𝑇𝑂𝐶
(14)
where t represents the time required for the removal of TOC (h), U is the applied voltage (kV), I is the circuit current (A), and ΔTOC is the decrease of TOC (kg). As shown in Fig. 6c, the SEC of the ECWAO process gradually increases along with the electrolysis time. Such a phenomenon has also been observed in other electrochemical oxidation processes
[17, 21]. However, In comparison to conventional electrochemical oxidation processes with SEC values generally between 160 and 1476.5 kW h kg-TOC-1 (60 and 553.7 kW h kg-COD-1) [17, 21, 22], the ECWAO process is quite energy saving. The SECs required for the total removal of BPA and TCS are as low as 1.43 and 2.72 kW h kg-TOC-1, and in case of SMX removal, it is only 3.64 kW h kg-TOC-1. In the ECWAO process, the PPCPs are oxidized via air oxidation, and the electric field is just used to initiate air oxidation reaction under room condition [7]. Therefore, the ECWAO process is energetically advantageous as compared with conversional electrochemical oxidation processes, in which the oxidizing power is exclusively supplied by the electric field. Moreover, the operation condition of room temperature and atmospheric pressure makes the ECWAO energy efficient than conversional WAO or CWAO processes as well. In case of complex organic pollutants, their partial oxidation results in products which might be more toxic than parent compounds [23]. Hence, toxicity tests are necessary to evaluate whether detoxification takes place. The toxic effects of treated waters are estimated by monitoring the growth of selected bacterium E. coli. As shown in Fig. 6d, the bacterium E. coli can hardly grow in the presence of the broad-spectrum antimicrobial agent TCS, and BPA and SMX also show severe inhibition on bacterial growth. In contrast, bactieral growth is obviously accelerated by the treated waters. This promoting effect suggests that the ECWAO process not only totally eliminates the toxicities of PPCPs, but, more importantly, converts them to certain compounds that can be used as nutrients by E. coli. The BOD5/COD ratio is a commonly-used indicator of biodegradablity. A value of 0 indicates nonbiodegradability and an increase of the ratio reflects a greater availability of
organic constituent for aerobic microbial degradation. For any wastewater, a minimum BOD5/COD of 0.30 is desirable for aerobic treatment [24]. The effluents from the ECWAO reactors used to treat BPA, TCS and SMX give BOD5/COD values of 0.46, 0.59 and 0.30, respectively, suggesting good biodegradabilities of the treated waters. This is a fact of remarkable importance enabling the coupling of ECWAO and aerobic biological processes for the treatment of PPCPs. Present wastewater treatment plants usually employ the costeffective bioremediation, usually under aerobic condition, as an ultimate step before the release of effluent. Unfortunately, most PPCPs show strong resistance to microbial degradation due to their toxicity and biorefractory nature [1]. Therefore, advanced oxidation processes are usually necessary to improve biodegradability of PPCPs. The twostep combined advanced oxidation and biological process has been accepted as a reliable and cost-effective way for the treatment of biorefractory pollutants [25-27]. Notably, the advantage of low energy consumption endows the ECWAO process with great potential to be widely used for pretreating the PPCPs ahead of aerobic biological treatments.
4. Conclusions The oxidation of BPA in the ECWAO process follows a pseudo-first order kinetics. The removal rate of BPA is significantly affected by initial concentration of BPA and circuit current. A kinetic model is constructed that is able to adequately describe the BPA oxidation kinetics as a function of initial concentration of BPA and circuit current. The BPA is degraded in the ECWAO process with p-isopropenyl phenol, benzoquinone and propanedioic acid as intermediate products. Under a condition optimized by the kinetic model, the ECWAO process totally removes the BPA and TCS in 80 min, and the SMX in
120 min. Meanwhile, TOC is reduced by 71.9%, 73.9% and 60.4% for the BPA, TCS, and SMX, respectively. SECs of the ECWAO process are as low as 1.43 and 2.72 kW h kgTOC-1 for removing the BPA and TCS, and in case of SMX removal, it is only 3.64 kW h kg-TOC-1. During such a process, the PPCPs-contaminated waters are totally detoxified and the treated-waters show good aerobic biodegradabilities. The ECWAO process shows a great potential to couple with aerobic biological process for effective and energy-efficient treatment of PPCPs.
Acknowledgments The authors wish to thank the National Natural Science Foundation of China (51478157) and the Program for New Century Excellent Talents in University (NCET-130767) for partial support of this work.
References [1] J. Wang, S. Wang, Removal of pharmaceuticals and personal care products (PPCPs) from wastewater: a review, J. Environ. Manage. 182 (2016) 620-640. [2] F.J. Benitez, J. García, J.L. Acero, F.J. Real, G. Roldan, Non-catalytic and catalytic wet air oxidation of pharmaceuticals in ultra-pure and natural waters, Process Saf. Environ. 89 (2011) 334-341. [3] I. Quesada-Penate, C. Julcour-Lebigue, U.J. Jauregui-Haza, A.M. Wilhelm, H. Delmas, Degradation of paracetamol by catalytic wet air oxidation and sequential adsorption–catalytic wet air oxidation on activated carbons, J. Hazard. Mater. 221 (2012) 131-138. [4] V.S. Mishra, V.V. Mahajani, J.B. Joshi, Wet air oxidation, Ind. Eng. Chem. Res. 34 (1995) 2-48. [5] M. Kumari, A.K. Saroha, Performance of various catalysts on treatment of refractory pollutants in industrial wastewater by catalytic wet air oxidation: A review, J. Environ. Manage. 228 (2018) 169-188.
[6] L.F. Zhai, M.F. Duan, M.X. Qiao, M. Sun, S. Wang, Electro-assisted catalytic wet air oxidation of organic pollutants on a MnO@ C/GF anode under room condition, Appl. Catal. B-Environ. 256 (2019) 117822. [7] M. Sun, Y. Zhang, H.H. Liu, F. Zhang, L.F. Zhai, S. Wang, Room-temperature air oxidation of organic pollutants via electrocatalysis by nanoscaled Co-CoO on graphite felt anode, Environ. Int. 131 (2019a) 104977. [8] M. Sun, Y. Zhang, S.Y. Kong, L.F. Zhai, S. Wang Excellent performance of electro-assisted catalytic wet air oxidation of refractory organic pollutants, Water Res. 158 (2019b) 313-321. [9] J. Du, J. Bao, Y. Liu, S.H. Kim, D.D. Dionysiou. Facile preparation of porous Mn/Fe3O4 cubes as peroxymonosulfate activating catalyst for effective bisphenol A degradation, Chem. Eng. J. 2018. https://doi.org/10.1016/j.cej.2018.05.177 [10] X. Dong, B. Ren, Z. Sun, C. Li, X. Zhang, M. Kong, D. D. Dionysiou. Monodispersed CuFe2O4 nanoparticles anchored on natural kaolinite as highly efficient peroxymonosulfate catalyst for bisphenol A degradation, Appl. Catal. B- Environ. 253 (2019) 206-217. [11] C. Barrios-Estrada, M. de Jesús Rostro-Alanis, A.L. Parra, M.P. Belleville, J. Sanchez-Marcano, H. M. Iqbal, R. Parra-Saldívar, Potentialities of active membranes with immobilized laccase for Bisphenol A degradation, Int. J. Biol. Macromol. 108 (2018) 837-844. [12] J. Sajiki, J. Yonekubo, Leaching of bisphenol A (BPA) from polycarbonate plastic to water containing amino acids and its degradation by radical oxygen species, Chemosphere 55 (2004) 861-867. [13] B.S. Rubin, Bisphenol A: an endocrine disruptor with widespread exposure and multiple effects. J. Steroid Biochem. 127 (2011) 27-34. [14] A. Bhatnagar, I. Anastopoulos. Adsorptive removal of bisphenol A (BPA) from aqueous solution: a review, Chemosphere 168 (2017) 885-902. [15] R. B. Baird, A. D. Eaton, L. S. Clesceri, Standard methods for the examination of water and wastewater, Washington, DC: American Public Health Association, 2012. [16] C.A. Martinez-Huitle, S. Ferro, Electrochemical oxidation of organic pollutants for the wastewater treatment: direct and indirect processes, Chem. Soc. Rev. 35 (2006) 1324-1340. [17] S. Kumar, S. Singh, V. C. Srivastava, Electro-oxidation of nitrophenol by ruthenium oxide coated titanium electrode: parametric, kinetic and mechanistic study, Chem. Eng. J. 263 (2015) 135-143.
[18] C. Li, X. Z. Li, N. Graham, N. Y. Gao, The aqueous degradation of bisphenol A and steroid estrogens by ferrate, Water Res. 42 (2008) 109-120. [19] G. Jing, M. Luan, T.Chen, Progress of catalytic wet air oxidation technology, Arab. J. Chem. 9 (2016) 1208-1213. [20] X. Duan, F. Ma, Z. Yuan, L. Chang, X. Jin, Electrochemical degradation of phenol in aqueous solution using PbO2 anode, J. Taiwan Inst. Chem. E. 44 (2013) 95-102. [21] H. Wang, B. Bakheet, S. Yuan, X. Li, G. Yu, S. Murayama, Y. Wang, Kinetics and energy efficiency for the degradation of 1, 4-dioxane by electro-peroxone process, J. Hazard. Mater. 294 (2015) 90-98. [22] L. Zhang, L. Xu, J. He, J. Zhang, Preparation of Ti/SnO2-Sb electrodes modified by carbon nanotube for anodic oxidation of dye wastewater and combination with nanofiltration, Electrochim. Acta 117 (2014) 192-201. [23] L. Rizzo, Bioassays as a tool for evaluating advanced oxidation processes in water and wastewater treatment,Water Res. 45 (2011) 4311-4340. [24] K.V. Padoley, V.K. Saharan, S.N. Mudliar, R.A. Pandey, A.B. Pandit, Cavitationally induced biodegradability enhancement of a distillery wastewater, J. Hazard. Mater. 219 (2012) 69-74. [25] I. Oller, S. Malato, J.A. Sánchez-Pérez, Combination of advanced oxidation processes and biological treatments for wastewater decontamination-a review, Sci. Total Environ. 409 (2011) 4141-4166. [26] W.K. Lafi, Z. Al-Qodah, Combined advanced oxidation and biological treatment processes for the removal of pesticides from aqueous solutions, J. Hazard. Mater. 137 (2006) 489-497. [27] O. Ganzenko, D. Huguenot, E.D. Van Hullebusch, G. Esposito, M.A. Oturan. Electrochemical advanced oxidation and biological processes for wastewater treatment: a review of the combined approaches, Environ. Sci. Pollut. R. 21 (2014) 8493-8524.
Figure Captions Fig.1. BPA removals at different initial concentrations of BPA (a), pseudo first-order kinetics fitting curves (b), and rate constant k’ as a function of [BAP]0 (c). Circuit current was at 7 mA and aeration rate was at 0.1 L min-1. Fig.2. BPA removals at different circuit currents (a), pseudo first-order kinetics fitting curves (b), and rate constant k’ as a function of I (c). Initial concentration of BPA was set at 100 mg L-1 and aeration rate was at 0.1 L min-1. Fig.3 Pseudo first-order rate constant k’ at different aeration rates. Initial concentration of BPA was set at 100 mg L-1 and circuit current was at 7 mA. Fig 4. The kinetic model in term of pseudo first-order rate rate constant k’ as a function of [BAP]0 and I. Fig.5. Proposed degradation pathway of PBA in the ECWAO process. Fig.6 Removals of PPCPs (a) and TOC (b), growth curves of E.coli in the presence of PPCPs and their degradation products (c), and SECs of the ECWAO process in treating different PPCPs.
Fig.1. BPA removals at different initial concentrations of BPA (a), pseudo first-order kinetics fitting curves (b), and rate constant k’ as a function of [BAP]0 (c). Circuit current was at 7 mA and aeration rate was at 0.1 L min-1.
Fig.2. BPA removals at different circuit currents (a), pseudo first-order kinetics fitting curves (b), and rate constant k’ as a function of I (c). Initial concentration of BPA was set at 100 mg L-1 and aeration rate was at 0.1 L min-1.
Fig.3 Pseudo first-order rate constant k’ at different aeration rates. Initial concentration of BPA was set at 100 mg L-1 and circuit current was at 7 mA.
Fig 4. The kinetic model in term of pseudo first-order rate rate constant k’ as a function of [BAP]0 and I.
Fig.5. Proposed degradation pathway of PBA in the ECWAO process.
Fig.6 Removals of PPCPs (a) and TOC (b), growth curves of E.coli in the presence of PPCPs and their degradation products (c), and SECs of the ECWAO process in treating different PPCPs.
Highlights
An electrocatalytic wet air oxidation process for PPCPs removal at room temperature.
A kinetic model is developed to describe removal of BPA at different [BPA]0 and I.
P-isopropenyl phenol, benzoquinone and propanedioic acid are intermediate products.
Toxicities of PPCPs-contaminated waters are eliminated at low energy consumptions.
The treated-waters show good aerobic biodegradabilities.
S1385-8947(20)30115-7 https://doi.org/10.1016/j.cej.2020.124124 CEJ 124124
To appear in:
Chemical Engineering Journal
Received Date: Accepted Date:
6 September 2019 12 January 2020
Please cite this article as: M. Sun, H-H. Liu, Y. Zhang, L-F. Zhai, Degradation of bisphenol A by electrocatalytic wet air oxidation process: kinetic modeling, degradation pathway and performance assessment, Chemical Engineering Journal (2020), doi: https://doi.org/10.1016/j.cej.2020.124124
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier B.V.
Degradation of bisphenol A by electrocatalytic wet air oxidation process: kinetic modeling, degradation pathway and performance assessment Min Sun, Hui-Hui Liu, Yu Zhang, Lin-Feng Zhai* Anhui Province Key Laboratory of Advanced Catalytic Materials and Reaction Engineering, School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei 230009, China
* Corresponding author: Dr. Lin-Feng Zhai Tel: +086-551-2901450 E-mail: [email protected]
Abstract: Pharmaceuticals and personal care products (PPCPs) are becoming ubiquitous in environment, and effective removal of PPCPs from contaminated water bodies is urgently needed to avoid their potential hazards to human health. Electrocatalytic wet air oxidation (ECWAO) process is a highly efficient technology for the rapid and deep mineralization of PPCPs under room condition. In this work, the degradation behavior of the bisphenol A (BPA), a typical PPCPs compound, in the ECWAO process is comprehensively investigated. A simple kinetic model is developed to describe the removal rate of BPA as a response to the initial concentration of BPA and intensity of electric field. BPA removal in the ECWAO process is accompanied by its partial mineralization, with p-isopropenyl phenol, benzoquinone and propanedioic acid as intermediate products. At last, the performances of the ECWAO process in treating BPA and other two PPCPs compounds, i.e. triclosan (TCS) and sulfamethoxazole (SMX), are assessed under a condition optimized by the kinetic model. The results demonstrate the ECWAO process is able to totally eliminate the toxicity of PPCPs-contaminated waters at low energy consumption. Notably, the treated-waters show good aerobic biodegradabilities, enabling such a process a prospective pretreament for coupling with aerobic biological process.
Keywords: Electro-assisted catalytic wet air oxidation; Bisphenol A; Degradation pathway; Kinetic model; Performance assessment
1. Introduction Pharmaceuticals and personal care products (PPCPs) are recognized as pseudo persistent organic pollutants due to their toxicity and low biodegradability in environment [1]. The discharge of PPCPs into environment results in their ubiquitous distribution in a wide range of natural water bodies, posing potential threat to human health. Since conventional biological treatments are inefficient to eliminate PPCPs, physicochemical and/or chemical technologies are necessary for the remediation of PPCPs-contaminated water bodies. Wet air oxidation (WAO) is an aqueous phase oxidation process which makes use of oxygen molecules (O2) in air to oxidize water contaminants. Such a process is especially effective for the treatment of refractory organic pollutants such as PPCPs [2,3]. However, since the reaction between O2 and organic compounds needs very high activation energy, WAO systems are usually operated at high temperatures (125-320 °C) and pressures (0.520 MPa) in order to achieve intensive oxidation of organic pollutants within a short time [4]. The presence of a catalyst has largely decreased the temperature and pressure of the WAO, but the catalytic wet air oxidation (CWAO) still needs an operation temperature quite higher than room temperature [5]. The severe operation condition has been the major obstacle that limits the practical application of WAO/CWAO. Recently, we have found the air oxidation reaction can be triggered by an anodic electric field under room condition, based on which electrocatalytic wet air oxidation (ECWAO) process has been developed for treating PPCPs in wastewater [6-8]. The ECWAO process is not only eco-friendly but also energy-saving as compared to conventional WAO/CWAO processes [7]. Mechanistic study has indicated a series of elementary steps in such a process, involving the dissolution of O2 from bulk gas phase to bulk liquid phase, the adsorption of
O2 and organic molecules on the active sites of catalyst, the activation of O2 on the active sites via electro-oxidation reaction, and the surface chemical reactions between active oxygen species and organic molecules [7]. Obviously, reaction parameters such as strength of electric field, concentration of dissolved O2 and concentration of pollutants influence the reaction rate. A sufficient knowledge of ECWAO kinetics is essential for the proper design of an ECWAO reactor as well as the process optimization. In this work, the oxidation of bisphenol A (BPA) is chosen as a model reaction to gain kinetic data about the ECWAO process. BPA is a typical PPCPs compound utilized in the manufacture of food and drink storage containers, polycarbonate baby bottles, tableware, white dental fillings and sealants [9-11]. While the leaching of BPA from products has led to its occurrence in various environmental matrices [12,13], the remediation methods of such a chemical is required due to its strong estrogenic endocrine disrupting effect [14]. Herein, an ECWAO system is constructed on partially oxidized cobalt (Co-CoO)/graphite felt (GF) electrode for BPA treatment under room condition. A kinetic model is developed to describe the removal of BPA regarding the effect of various operating parameters, and the degradation pathway of BPA is elucidated. At last, the performances of ECWAO process in treating BPA and other PPCPs are assessed in terms of treatment efficiency and energy consumption.
2. Experimental methods 2.1 . Operation of the ECWAO process for BPA degradation The ECWAO process was performed in a 175 mL cylindrical glass-made reactor with a liquid volume of 120 mL. The reactor was equipped with a Co-CoO/GF composite (2.5×4
cm2) as the anode electrode and a platinum wire (Φ 0.5 mm × 37 mm) as the cathode electrode. The Co-CoO/GF electrode was prepared by a combined impregnation and pyrolysis approach, as previously described [7]. The electrolyte was 50 mM Na2SO4 solution and the concentration of BPA in the electrolysis was 5 to 100 mg L-1. The other PPCPs treated by the ECWAO process include triclosan (TCS) and sulfamethoxazol (SMX) at a concentration of 20 mg L-1. The initial solution pH was 7.0 except for the TCS, whose solution pH was set at 9.0 to ensure its dissolution. The electric field was applied in galvanostatic mode at a constant current ranged from 1 to 20 mA, and air was bubbled from the bottom of the reactor. The pH was monitored during the process showing that only marginal change had occurred between the initial and final solutions. The concentration of BPA, TCS and SMX were determined on a high performance liquid chromatography system (Waters, UAS) equipped with a multiple wavelength UV detector and a SunFireTM C18 column (150 mm × 4.6 mm, 5μm). The mobile phase was wateracetonitrile mixture (40/60, v/v) at a flow rate of 1 mL min-1. The concentration of total organic carbon (TOC) was determined on a HTM-CT1000M TOC analyzer (Tailin, China), and the concentration of dissolved O2 (DO) was determined using an AZ8403 DO meter (HengXin, China). The chemical oxygen demand (COD) was determined using the dichromate reflux method, and the 5-day biological oxygen demand (BOD5) was determined from the decrease of DO after 5 days of incubation in the dark at 20 °C [15]. All experiments were conducted in triplicate, and the average values with standard deviations were presented.
2.2 Gas chromatography-mass spectrometry (GC-MS)
Intermediate products generated from the degradation of BPA were identified by GC-MS on a Thermo Trace 1300 system equipped with a TR-V1 capillary column (30 m × 0.25 mm ID × 1.4 μm film, USA). High-purity helium was chosen as the carrier gas, and injection temperature was set at 280 °C. Column temperature was gradually increased from 80 to 280 ºC at a rate of 12.5 ºC min-1. Electron impact (EI) mass spectra were monitored from 50 to 500 m/z. The ion source and inlet line temperatures were set at 220 and 280 °C, respectively. Solid-phase extraction was conducted to preconcentrate the products prior to GC-MS analysis. 1000 mL of sample was loaded on an Oasis HLB SPE cartridge (3 mL, 60 mg, Waters, USA) at 10 mL min-1, and then elution was performed with 5 mL of methanol.
2.3. Toxicity Analysis The toxic effects of PPCPs and their degradation products were evaluated by the inhibition growth of bacterium Escherichia coli in Luria-Bertani (LB) medium. The assays were carried out in a 96-well microtiter plate. Samples were sterilized using 0.45 mm membrane filters and, then, added into the 96-well microtiter plate at 40 μL. For each sample duplicate wells were inoculated with 160 μL of LB culture of E. coli. The specimen with 50 mM Na2SO4 was used as a control. The plate was incubated at 37 °C and the growth of E. coli was monitored by reading the absorption at 600 nm in a microtiter plate reader.
3. Results and discussion 3.1 Effect of operation parameters on BPA removal
The effect of the initial concentration of BPA on its removal rate is studied at a constant current of 7 mA and aeration rate of 0.1 L min-1. As shown in Fig.1a, the removal efficiency of BPA was gradually increased along with the reaction time t until the utmost value of 100%. The experimental results regarding to t (min) and concentration of BPA [BPA] (mg L-1) are fitted to a pseudo-first-order kinetic equation: d[BPA] d𝑡
= ― 𝑘′[BPA]
(1)
where k′ is a pseudo-first-order rate constant. As shown in Fig. 1b, a linear relationship between -ln([BPA]t /[BPA]0) and t is observed, suggesting a pseudo-first-order kinetics for BPA oxidation. Fig. 1c demonstrates the value of k′ is significantly dependent upon [BPA]0. The k′ gradually increases with the increment of [BPA]0 from 5 to 15 mg L-1, and the k′ versus [BPA]0 plot fits an equation as k′ = 0.05132[BPA]00.07998. The k′ keeps constantly at 0.06358 min-1 as [BPA]0 is between 15 and 20 mg L-1, indicating the oxidation rate of BPA is no longer limited to its initial concentration. Then, a continuous decrease of k′ is observed with the further increment of [BPA]0 from 20 to 100 mg L-1, giving a relationship expressed as k′ = 0.14736[BPA]0-0.27486. The oxidation of BPA by O2 on the electrode surface is a typical heterogeneous reaction dependent upon both the surface oxidation kinetics and mass transfer kinetics [16-17]. High concentration of the reactant BPA tends to clog the pores of the Co-CoO/GF electrode and impede the transportation of O2 to the active sites of the electrode, thus delay the reaction kinetics. Figs. 2a and 2b show the oxidation rate of BPA is positively dependent upon circuit current I (mA). The k′ with respect to I is determined to be k′ = 0.01872I0.41901 when I is in the range of 1 to 10 mA (Fig. 2c) . However, the further rise of I beyond 10 mA does not
yield proportional increase in k′, suggesting the intensity of electric field is high enough that it is not the rate limiting factor in BPA oxidation. Finally, the effect of aeration rate on BPA removal kinetics is investigated. As shown in Fig. 3, the k′ is kept at 0.04012 min-1 as the aeration rate increases from 0.1 to 0.9 L min1.
It seems the aeration rate plays an insignificant role in determining the BPA removal
rate, which can be explained by comparing the O2 demand and supply in the process. Assuming the BPA is completely mineralized into CO2 and H2O in the ECWAO process, the oxidation of 0.0526 mM (100 mg L-1) BPA consumes at most 0.789 mM O2, which equals to 0.019 L O2 under the experiment condition of this work, according to eq. 2: 𝑃𝑉 = 𝑛𝑅𝑇
(2)
where P refers to gas pressure (101325 Pa), V to gas volume (L), T to the temperature (298.15 K), n is the molar of O2 (0.789×10-3 mol) and R is the gas constant 8.314×10-3 Pa L mol-1 K-1. In comparison, a total amount of as high as 3.78 L of oxygen is supplied during the 180 min of ECWAO process even at the lowest aeration rate of 0.1 L min-1, assuming O2 content in air is 21%. Therefore, bubbling rate of oxygen is not considered to be a critical parameter for practical application of the ECWAO process.
3.2 Kinetic model describing the oxidation of BPA in the ECWAO process The generalized kinetic model for the oxidation of BPA in the ECWAO process is constructed on the basis of a reaction scheme involving the formation and activation of chemisorbed oxygen species on the Co-CoO catalyst [7]. As depicted by eqs. 3-6, at the first step, the O2 is adsorbed onto the catalytically active site of CoO (CoII[]) , and then undergoes dissociation to form chemisorbed oxygen species O-/O2-, which are
subsequently electro-oxidized to their active states O-/O2-* in the electric field. The activated O-/O2-* species possess potent oxidizing power at room temperature, and they react with BPA in successive steps yielding the final oxidation products. CoII[] + O2→CoII[O2]
(3)
CoII[O2]→CoIII[O ― /O2― ] -e
CoIII[O - /O2- ] CoIII[O - /O2CoIII[O - /O2-
∗
(4) ∗
]
(5)
] + [BPA]→CoII[] +oxidation products
(6)
Ageneral rate equation for BPA removal is expressed as: d[BPA] d𝑡
= ―𝑘𝑎𝑝𝑝[BPA]a[O - /O2-
∗ b
]
(7)
where kapp is an apparent oxidation rate constant, [O-/O2-*] denotes concentration of O-/O2-*, and a and b are orders of the reaction with respect to BPA and O-/O2-*, respectively. The [O-/O2-*] is dependent upon the amount of CoII[] ([CoII[]]), concentration of dissolved oxygen ([O2]) and circuit current I. Thus, eq. 7 is expressed as: d[BPA] d𝑡
𝑛
= ―𝑘𝑎𝑝𝑝[BPA]𝑎[O2]𝑚[CoII[]] 𝐼𝑐
(8)
where m, n and b are orders of the reaction with respect to O2, CoII[] and I, respectively. With a given electrode, the [CoII[] can be viewed as a constant. Moreover, since the [O2] is not the rate limiting factor in the oxidation of BPA, it is also regarded as a constant. Hence, eq. 8 is further simplified as: d[BPA] dt
= ―𝑘𝑜𝑏𝑠[BPA]a𝐼c
(9) 𝑛
Where kobs = kapp[CoII[]] [O2]𝑚 is the observed rate constant. Next, the relationship between k′ and kobs can be deduced from eqs. 1 and 9 as:
𝑘′ = 𝑘𝑜𝑏𝑠[BPA]a ― 1𝐼c
(10)
By alternation of the variable one at a time and linearization of the data, the dependence of k′ on [BPA]0 and I in eq. 10 can be determined. Given all the parameters and coefficients calculated, three different rate equations are established to describe the oxidation of BPA in the ECWAO process: 0.41901
𝑘′ = 0.02279 × 𝐼
× [BPA]0.07998 0
0.41901
= 0.02655 × 𝐼
0.41901
= 0.06242 × 𝐼
× [BPA]0―0.27486
1 ≤ 𝐼 ≤ 10 mA, 5 ≤ [BPA] 0 ≤ 15 mg L ―1
(11) 𝑘′
1 ≤ 𝐼 ≤ 10 mA , 15 ≤ [BPA]0 ≤ 20 mg L ―1
(12) 𝑘′
1 ≤ 𝐼 ≤ 10 mA, 20 ≤ [BPA] 0 ≤ 100 mg L ―1 (13) Fig.
4 graphically displays the kinetic model in term of k ′ as a function of [BPA]0 and [I]. In order to demonstrate accuracy of the kinetic model, seven different reaction conditions are selected within the kinetic model for verification. As shown in Fig. 4, minor discrepancies are found between the predicted k ′ and the experimental measurements, suggesting the kinetic model describes reasonably well the removal of BPA in EWAO process.
3.3 Degradation pathway of BPA in the ECWAO process GC-MS analysis identifies three new compounds generated from BPA degradation, which are p-isopropenyl phenol (m/z=134), benzoquinone (m/z=78) and propanedioic acid (m/z=104). The three compounds are regarded to be the intermediate products, because their peaks eventually disappear from the GC chromatogram. Therefore, a possible degradation pathway of BPA in the ECWAO process is proposed and depicted in Fig. 5. For the BPA, the two electron-donating hydroxyl groups increase the electron densities of phenyl rings, making the C-C bond between them vulnerable to be attacked. Therefore, the
degradation is initiated by the cleavage of C-C bond linking the two phenyl rings, yielding the p-isopropenyl phenol and benzoquinone as intermediate products. Next, the pisopropenyl phenol and benzoquinone undergo ring cleavage and produce propanedioic acid, which is then further oxidized to CO2, H2O and small organic molecules, as generally occurring in conversional wet air oxidation or electrochemical oxidation systems [18-20].
3.4 Performance assessment of the ECWAO process for PPCPs treatment The efficiency of the ECWAO process on PPCPs removal is evaluated using three representative compounds, i.e. BPA, TCS, and SMX. The ECWAO process is operated with current set at 10 mA and concentration of PPCPs at 20 mg L-1. According to the kinetic model, the BPA obtains relatively higher removal rate under such a condition. As shown in Figs. 6a and 6b, it requires 80 min for the complete removal of BPA and SMX, and 120 min for SMX removal. Accompanied with their total elimination, the TOC is reduced by 71.9%, 73.9% and 60.4% for the BPA, TCS, and SMX, respectively, Next, the efficiency of the ECWAO process for utilizing electric energy is evaluated by calculating specific energy consumption (SEC, kW h kg-TOC-1) on degrading the PPCPs. The SEC is calculated in terms of the removal of 1 kg-TOC as eq. 14: 𝑡
∫0𝑈𝐼
𝑆𝐸𝐶 = ∆𝑇𝑂𝐶
(14)
where t represents the time required for the removal of TOC (h), U is the applied voltage (kV), I is the circuit current (A), and ΔTOC is the decrease of TOC (kg). As shown in Fig. 6c, the SEC of the ECWAO process gradually increases along with the electrolysis time. Such a phenomenon has also been observed in other electrochemical oxidation processes
[17, 21]. However, In comparison to conventional electrochemical oxidation processes with SEC values generally between 160 and 1476.5 kW h kg-TOC-1 (60 and 553.7 kW h kg-COD-1) [17, 21, 22], the ECWAO process is quite energy saving. The SECs required for the total removal of BPA and TCS are as low as 1.43 and 2.72 kW h kg-TOC-1, and in case of SMX removal, it is only 3.64 kW h kg-TOC-1. In the ECWAO process, the PPCPs are oxidized via air oxidation, and the electric field is just used to initiate air oxidation reaction under room condition [7]. Therefore, the ECWAO process is energetically advantageous as compared with conversional electrochemical oxidation processes, in which the oxidizing power is exclusively supplied by the electric field. Moreover, the operation condition of room temperature and atmospheric pressure makes the ECWAO energy efficient than conversional WAO or CWAO processes as well. In case of complex organic pollutants, their partial oxidation results in products which might be more toxic than parent compounds [23]. Hence, toxicity tests are necessary to evaluate whether detoxification takes place. The toxic effects of treated waters are estimated by monitoring the growth of selected bacterium E. coli. As shown in Fig. 6d, the bacterium E. coli can hardly grow in the presence of the broad-spectrum antimicrobial agent TCS, and BPA and SMX also show severe inhibition on bacterial growth. In contrast, bactieral growth is obviously accelerated by the treated waters. This promoting effect suggests that the ECWAO process not only totally eliminates the toxicities of PPCPs, but, more importantly, converts them to certain compounds that can be used as nutrients by E. coli. The BOD5/COD ratio is a commonly-used indicator of biodegradablity. A value of 0 indicates nonbiodegradability and an increase of the ratio reflects a greater availability of
organic constituent for aerobic microbial degradation. For any wastewater, a minimum BOD5/COD of 0.30 is desirable for aerobic treatment [24]. The effluents from the ECWAO reactors used to treat BPA, TCS and SMX give BOD5/COD values of 0.46, 0.59 and 0.30, respectively, suggesting good biodegradabilities of the treated waters. This is a fact of remarkable importance enabling the coupling of ECWAO and aerobic biological processes for the treatment of PPCPs. Present wastewater treatment plants usually employ the costeffective bioremediation, usually under aerobic condition, as an ultimate step before the release of effluent. Unfortunately, most PPCPs show strong resistance to microbial degradation due to their toxicity and biorefractory nature [1]. Therefore, advanced oxidation processes are usually necessary to improve biodegradability of PPCPs. The twostep combined advanced oxidation and biological process has been accepted as a reliable and cost-effective way for the treatment of biorefractory pollutants [25-27]. Notably, the advantage of low energy consumption endows the ECWAO process with great potential to be widely used for pretreating the PPCPs ahead of aerobic biological treatments.
4. Conclusions The oxidation of BPA in the ECWAO process follows a pseudo-first order kinetics. The removal rate of BPA is significantly affected by initial concentration of BPA and circuit current. A kinetic model is constructed that is able to adequately describe the BPA oxidation kinetics as a function of initial concentration of BPA and circuit current. The BPA is degraded in the ECWAO process with p-isopropenyl phenol, benzoquinone and propanedioic acid as intermediate products. Under a condition optimized by the kinetic model, the ECWAO process totally removes the BPA and TCS in 80 min, and the SMX in
120 min. Meanwhile, TOC is reduced by 71.9%, 73.9% and 60.4% for the BPA, TCS, and SMX, respectively. SECs of the ECWAO process are as low as 1.43 and 2.72 kW h kgTOC-1 for removing the BPA and TCS, and in case of SMX removal, it is only 3.64 kW h kg-TOC-1. During such a process, the PPCPs-contaminated waters are totally detoxified and the treated-waters show good aerobic biodegradabilities. The ECWAO process shows a great potential to couple with aerobic biological process for effective and energy-efficient treatment of PPCPs.
Acknowledgments The authors wish to thank the National Natural Science Foundation of China (51478157) and the Program for New Century Excellent Talents in University (NCET-130767) for partial support of this work.
References [1] J. Wang, S. Wang, Removal of pharmaceuticals and personal care products (PPCPs) from wastewater: a review, J. Environ. Manage. 182 (2016) 620-640. [2] F.J. Benitez, J. García, J.L. Acero, F.J. Real, G. Roldan, Non-catalytic and catalytic wet air oxidation of pharmaceuticals in ultra-pure and natural waters, Process Saf. Environ. 89 (2011) 334-341. [3] I. Quesada-Penate, C. Julcour-Lebigue, U.J. Jauregui-Haza, A.M. Wilhelm, H. Delmas, Degradation of paracetamol by catalytic wet air oxidation and sequential adsorption–catalytic wet air oxidation on activated carbons, J. Hazard. Mater. 221 (2012) 131-138. [4] V.S. Mishra, V.V. Mahajani, J.B. Joshi, Wet air oxidation, Ind. Eng. Chem. Res. 34 (1995) 2-48. [5] M. Kumari, A.K. Saroha, Performance of various catalysts on treatment of refractory pollutants in industrial wastewater by catalytic wet air oxidation: A review, J. Environ. Manage. 228 (2018) 169-188.
[6] L.F. Zhai, M.F. Duan, M.X. Qiao, M. Sun, S. Wang, Electro-assisted catalytic wet air oxidation of organic pollutants on a MnO@ C/GF anode under room condition, Appl. Catal. B-Environ. 256 (2019) 117822. [7] M. Sun, Y. Zhang, H.H. Liu, F. Zhang, L.F. Zhai, S. Wang, Room-temperature air oxidation of organic pollutants via electrocatalysis by nanoscaled Co-CoO on graphite felt anode, Environ. Int. 131 (2019a) 104977. [8] M. Sun, Y. Zhang, S.Y. Kong, L.F. Zhai, S. Wang Excellent performance of electro-assisted catalytic wet air oxidation of refractory organic pollutants, Water Res. 158 (2019b) 313-321. [9] J. Du, J. Bao, Y. Liu, S.H. Kim, D.D. Dionysiou. Facile preparation of porous Mn/Fe3O4 cubes as peroxymonosulfate activating catalyst for effective bisphenol A degradation, Chem. Eng. J. 2018. https://doi.org/10.1016/j.cej.2018.05.177 [10] X. Dong, B. Ren, Z. Sun, C. Li, X. Zhang, M. Kong, D. D. Dionysiou. Monodispersed CuFe2O4 nanoparticles anchored on natural kaolinite as highly efficient peroxymonosulfate catalyst for bisphenol A degradation, Appl. Catal. B- Environ. 253 (2019) 206-217. [11] C. Barrios-Estrada, M. de Jesús Rostro-Alanis, A.L. Parra, M.P. Belleville, J. Sanchez-Marcano, H. M. Iqbal, R. Parra-Saldívar, Potentialities of active membranes with immobilized laccase for Bisphenol A degradation, Int. J. Biol. Macromol. 108 (2018) 837-844. [12] J. Sajiki, J. Yonekubo, Leaching of bisphenol A (BPA) from polycarbonate plastic to water containing amino acids and its degradation by radical oxygen species, Chemosphere 55 (2004) 861-867. [13] B.S. Rubin, Bisphenol A: an endocrine disruptor with widespread exposure and multiple effects. J. Steroid Biochem. 127 (2011) 27-34. [14] A. Bhatnagar, I. Anastopoulos. Adsorptive removal of bisphenol A (BPA) from aqueous solution: a review, Chemosphere 168 (2017) 885-902. [15] R. B. Baird, A. D. Eaton, L. S. Clesceri, Standard methods for the examination of water and wastewater, Washington, DC: American Public Health Association, 2012. [16] C.A. Martinez-Huitle, S. Ferro, Electrochemical oxidation of organic pollutants for the wastewater treatment: direct and indirect processes, Chem. Soc. Rev. 35 (2006) 1324-1340. [17] S. Kumar, S. Singh, V. C. Srivastava, Electro-oxidation of nitrophenol by ruthenium oxide coated titanium electrode: parametric, kinetic and mechanistic study, Chem. Eng. J. 263 (2015) 135-143.
[18] C. Li, X. Z. Li, N. Graham, N. Y. Gao, The aqueous degradation of bisphenol A and steroid estrogens by ferrate, Water Res. 42 (2008) 109-120. [19] G. Jing, M. Luan, T.Chen, Progress of catalytic wet air oxidation technology, Arab. J. Chem. 9 (2016) 1208-1213. [20] X. Duan, F. Ma, Z. Yuan, L. Chang, X. Jin, Electrochemical degradation of phenol in aqueous solution using PbO2 anode, J. Taiwan Inst. Chem. E. 44 (2013) 95-102. [21] H. Wang, B. Bakheet, S. Yuan, X. Li, G. Yu, S. Murayama, Y. Wang, Kinetics and energy efficiency for the degradation of 1, 4-dioxane by electro-peroxone process, J. Hazard. Mater. 294 (2015) 90-98. [22] L. Zhang, L. Xu, J. He, J. Zhang, Preparation of Ti/SnO2-Sb electrodes modified by carbon nanotube for anodic oxidation of dye wastewater and combination with nanofiltration, Electrochim. Acta 117 (2014) 192-201. [23] L. Rizzo, Bioassays as a tool for evaluating advanced oxidation processes in water and wastewater treatment,Water Res. 45 (2011) 4311-4340. [24] K.V. Padoley, V.K. Saharan, S.N. Mudliar, R.A. Pandey, A.B. Pandit, Cavitationally induced biodegradability enhancement of a distillery wastewater, J. Hazard. Mater. 219 (2012) 69-74. [25] I. Oller, S. Malato, J.A. Sánchez-Pérez, Combination of advanced oxidation processes and biological treatments for wastewater decontamination-a review, Sci. Total Environ. 409 (2011) 4141-4166. [26] W.K. Lafi, Z. Al-Qodah, Combined advanced oxidation and biological treatment processes for the removal of pesticides from aqueous solutions, J. Hazard. Mater. 137 (2006) 489-497. [27] O. Ganzenko, D. Huguenot, E.D. Van Hullebusch, G. Esposito, M.A. Oturan. Electrochemical advanced oxidation and biological processes for wastewater treatment: a review of the combined approaches, Environ. Sci. Pollut. R. 21 (2014) 8493-8524.
Figure Captions Fig.1. BPA removals at different initial concentrations of BPA (a), pseudo first-order kinetics fitting curves (b), and rate constant k’ as a function of [BAP]0 (c). Circuit current was at 7 mA and aeration rate was at 0.1 L min-1. Fig.2. BPA removals at different circuit currents (a), pseudo first-order kinetics fitting curves (b), and rate constant k’ as a function of I (c). Initial concentration of BPA was set at 100 mg L-1 and aeration rate was at 0.1 L min-1. Fig.3 Pseudo first-order rate constant k’ at different aeration rates. Initial concentration of BPA was set at 100 mg L-1 and circuit current was at 7 mA. Fig 4. The kinetic model in term of pseudo first-order rate rate constant k’ as a function of [BAP]0 and I. Fig.5. Proposed degradation pathway of PBA in the ECWAO process. Fig.6 Removals of PPCPs (a) and TOC (b), growth curves of E.coli in the presence of PPCPs and their degradation products (c), and SECs of the ECWAO process in treating different PPCPs.
Fig.1. BPA removals at different initial concentrations of BPA (a), pseudo first-order kinetics fitting curves (b), and rate constant k’ as a function of [BAP]0 (c). Circuit current was at 7 mA and aeration rate was at 0.1 L min-1.
Fig.2. BPA removals at different circuit currents (a), pseudo first-order kinetics fitting curves (b), and rate constant k’ as a function of I (c). Initial concentration of BPA was set at 100 mg L-1 and aeration rate was at 0.1 L min-1.
Fig.3 Pseudo first-order rate constant k’ at different aeration rates. Initial concentration of BPA was set at 100 mg L-1 and circuit current was at 7 mA.
Fig 4. The kinetic model in term of pseudo first-order rate rate constant k’ as a function of [BAP]0 and I.
Fig.5. Proposed degradation pathway of PBA in the ECWAO process.
Fig.6 Removals of PPCPs (a) and TOC (b), growth curves of E.coli in the presence of PPCPs and their degradation products (c), and SECs of the ECWAO process in treating different PPCPs.
Highlights
An electrocatalytic wet air oxidation process for PPCPs removal at room temperature.
A kinetic model is developed to describe removal of BPA at different [BPA]0 and I.
P-isopropenyl phenol, benzoquinone and propanedioic acid are intermediate products.
Toxicities of PPCPs-contaminated waters are eliminated at low energy consumptions.
The treated-waters show good aerobic biodegradabilities.