- Email: [email protected]
Enhanced organic removal for shale gas fracturing flowback water by electrocoagulation and simultaneous electro-peroxone process
Accepted Manuscript Enhanced organics removal for shale gas fracturing flowback water with electrocoagulation and simultaneous electro-peroxone process Fan-xin Kong, Xiao-feng Lin, Guang-dong Sun, Jin-fu Chen, Chun-mei Guo, Yuefeng F. Xie PII:
S0045-6535(18)32158-1
DOI:
https://doi.org/10.1016/j.chemosphere.2018.11.055
Reference:
CHEM 22539
To appear in:
ECSN
Received Date: 17 August 2018 Revised Date:
24 October 2018
Accepted Date: 8 November 2018
Please cite this article as: Kong, F.-x., Lin, X.-f., Sun, G.-d., Chen, J.-f., Guo, C.-m., Xie, Y.F., Enhanced organics removal for shale gas fracturing flowback water with electrocoagulation and simultaneous electro-peroxone process, Chemosphere (2018), doi: https://doi.org/10.1016/ j.chemosphere.2018.11.055. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
ACCEPTED MANUSCRIPT
Enhanced organics removal for shale gas fracturing
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flowback water with electrocoagulation and simultaneous
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electro-peroxone process
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Fan-xin Kong a*, Xiao-feng Lin a, Guang-dong Sun b, Jin-fu Chen a*, Chun-mei Guo a, Yuefeng F. Xie d
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a
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Control, China University of Petroleum, Beijing 102249, China
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State Key Laboratory of Heavy Oil Processing, Beijing Key Laboratory of Oil & Gas Pollution
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b
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Institute of Water Resources and Hydropower Research Beijing, 100038, China
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c
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17057, USA
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State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin, China
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Environmental Engineering Programs, The Pennsylvania State University, Middletown, PA
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Re-Submitted to
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Chemosphere
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* Corresponding author: Dr. Fan-xin Kong and Dr. Jin-fu Chen, Tel.: +86-10-8973 3637; Email: [email protected] and [email protected] Abstract:
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Abstract The colloids and organics of shale gas fracturing flowback water (SGFFW) during
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shale gas extraction is of primary concern for its treatment. Coagulation combined with
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oxidation might be a promising process for SGFFW treatment. In this study, a novel
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electrocoagulation-peroxone (ECP) process was developed for SGFFW treatment by
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simultaneous coagulation and oxidation process with Al plate as the anode and a
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carbon-PTFE gas diffusion electrode as the cathode, realizing the simultaneous
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processes of coagulation, H2O2 generation and activation by O3 at the cathode.
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Compared with electrocoagulation (EC) and peroxi-electrocoagulation (PEC), the
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COD removal efficiency mainly followed the declining order of ECP, PEC and EC
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under the optimal current density of 50 mA cm-2. The appearance of medium MW
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fraction (1919 Da) during ozonation and PEC but disappearance in ECP indicated
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these intermediate products can’t be degraded in ozonation and PEC but can be
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further oxidized and mineralized by the hydroxyl radical produced by the cathode in
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ECP, demonstrating the hydroxyl radical are responsible for the significant
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enhancement of the COD removal. The pseudo-first order kinetic model can well fit
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ozonation and coagulation process but not the PEC and ECP process due to the
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synthetic effect of coagulation and oxidation. However, the proposed mechanism
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based model can generally fit ECP satisfactorily. The average current efficiency for
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PEC was 35.4% and 12% higher than that of ozonation and EC, respectively. This
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study demonstrated the feasibility of establishing a high efficiency and space-saving
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with
integrated
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electrochemical
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electro-peroxone for SGFFW treatment.
anodic
coagulation
and
cathodic
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Keywords:
Electrocoagulation,
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Electrocoagulation-peroxone, Ozonation.
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Peroxi-electrocoagulation,
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1. Introduction Shale gas fracturing flowback water (SGFFW) generated by shale gas exploration
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can pose potential environmental risk due to its high total dissolved solids (TDS),
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organic matter and radioactive elements, which is a growing concern around the
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world with accelerated production of shale gas using horizontal drilling and hydraulic
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fracturing (Haluszczak et al., 2013; Shaffer et al., 2013). Currently, most of SGFFW is
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reused for another fracking event. Prior to reuse, SGFFW is typically treated on-site
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to remove suspended solids or specific constituents that may not be compatible with
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fracturing fluids chemistry. Colloids and organics in the SGFFWs is the primary
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concern for SGFFW reuse and discharge.
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Coagulation and oxidation has been shown to be an effective and low-cost
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approach for colloids and organics removal to improve overall water quality (Howe et
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al., 2006; Huang et al., 2009; Yang and Kim, 2009; Zularisam et al., 2009; Yu et al.,
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2016). Esmaeilirad et al. (2015) found electrocoagulation (EC) was one of the
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available technologies to treat produced water for reuse in fluids, eliminating solids
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and TOC but also inorganics such as Boron (Esmaeilirad et al., 2015; Sari and
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Chellam, 2015).Our previous study (Kong et al., 2017) indicated that coagulation can
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effectively remove colloids and organics in the SGFFWs and thus effectively mitigate
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the UF fouling in hybrid coagulation-ultrafiltration process. However, some studies
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(Miller et al., 2013; Xiong et al., 2016; Kong et al., 2018) further indicated that even
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pretreated coagulation-UF process, the subsequent nanofiltration and reverse osmosis
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still suffered high fouling propensity, clearly indicating the inefficient of coagulation
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sub-micron colloidal particles to organic coating at high ionic strength in the flowback
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was highly stable and was hard to be removed by coagulation or EC, which
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highlighted the importance of oxidation to destroy the organic coating to aid
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coagulation process. The oxidation process alone was used for dissolved organic
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removal in SGFFW. Medium removal efficiencies were achieved with COD (68.20%),
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color (88.48%) and total phenol (92.65%) removal by using Fenton processes (Erkan et al.,
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2017). Recent study (Xiong et al., 2018) indicated that radical-induced degradation (i.e.,
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hydroxyl radical and Fe2+) of PAM was reduced by two orders of magnitude, from roughly
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10 MDa to 200 kDa under typical HPT fracturing conditions, which indicated oxidation is
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an effective option for SGFFW treatment. The maximum removal efficiencies by
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Electro-Fenton process were found to be around 87.35%, 89.15%, and 91.75% for COD,
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color, and total phenol under the optimum conditions, respectively, indicating that
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electro-Fenton or Fenton-like process seem to be an efficient treatment and low cost
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method for shale gas wastewater (Chen et al., 2016; Turan et al., 2017). In contrast, some
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studies indicated that the potential of various AOPs (i.e., UV/H2O2, O3/H2O2, Solar
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light/Chlorine, and photo-Fenton) only marginally reduced the DOC of the flowback
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water despite for high doses of light and oxidants (e.g., 10% DOC removal by
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O3/H2O2, at O3 and H2O2 doses of 500 mg/L and 350 mg/L respectively) (Cui et al.,
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2017). In conclusion, coagulation can only partially remove the colloids and organics in
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SGFFWs which needs the oxidation process to enhance the coagulation efficiency, and the
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oxidation alone for the organics removal in SGFFW is high enough. It seems that
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Compared to conventional process, electrochemical process is an effective platform
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that is capable of achieving coagulation and electrochemical oxidation without adding
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any chemicals to eliminate colloids and contaminants, which has the potential to develope
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a cost-effective method to realize high efficientwasterwater treatment. Recently, a
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combined methodology called ‘‘peroxi-electrocoagulation (PEC) method’’ using a
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synergistic combination of anodic electrocoagulation and cathodic oxidation (Zarei et
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al., 2009; Esfandyari et al., 2015; do Vale-Júnior et al., 2018), which is very effective
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in removing colloids,suspended particles, oil or other contaminants due to the
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dissolution of coagulant ions (i.e., Al3+ or Fe2+) from the anode, while H2O2 was
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effectively electro-generated on cathode, realizing the simultaneous processes of
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coagulation and H2O2 oxidation. However, the oxidation rate and capacity of H2O2
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was relatively low, which is relatively ineffective in eliminating some refractory
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organic compounds (Zarei et al., 2009; Vasudevan, 2014). Fortunately, H2O2
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generated in the cathode can be in suit activated to a high oxidation potential hydroxyl
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radical (·OH, E0 = 2.8 V) with by O3 called peroxone (Bakheet et al., 2013; Fischbacher
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et al., 2013), which is very effective in breaking down some refractory organic
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compounds (Bokare and Choi, 2014; Asghar et al., 2015). Recently, E-peroxone has
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also been developed as an effective electrochemical oxidation process by using the
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carbon-PTFE gas diffusion electrode as the cathode to in-suit generation of generation
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and activation by O3 at the cathode, which only highlight the enhancement of
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oxidation process by cathodic peroxone or their combination with DSA anode
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Wang et al., 2018). However, the PEC and E-peroxone process imply that it might be
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possible to fulfill the anodic coagulation and cathodic E-peroxone by simply changing the
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anode and cahode for simultaneously electrocoagulation and E-peroxone with only two
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electrodes.
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In this study, a novel electrocoagulation-peroxone (ECP) process was proposed
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with Al plate as anode and a carbon-PTFE gas diffusion electrode as the cathode,
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realizing the simultaneous coagulation and H2O2 generation and activation by O3 at
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the cathode for the SGFFW treatment. The objective of this study was to verify the
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feasibility of this novel ECP for COD removal and better understand removal
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mechanisms. The performance of the novel process, the effect of current density, the
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dissolved organics removal and kinetic model were systematically elucidated. The
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results of this study allowed to create a novel high efficient ECP process for SGFFW
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treatment and develop solutions that enable the use of ECP for SGFFW reuse during
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shale gas exploration.
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2. Materials and methods
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2.1. Raw water
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The characteristic of the SGFFW was reported to be complicated, which was
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mainly consisted of dissolved salts, chemical additives and solid particles. The raw
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water in this study was sampled from one of the reservoir in Fuling shale gas play,
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Chongqing, China. It was turbid with a light yellow color with some small floccules,
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which is similar to the previous report on Marcellus shale gas play (Jiang et al., 2013). 7
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The detailed water quality will be comprehensively discussed in the following
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session.
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2.2. Experimental apparatus and procedures Bench scale experiments were conducted in a 150 mL custom-made Perspex cell
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with two electrode holders and some necessary accessories (Fig. 1). A DC power
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supply (KXN-305D, Zhaoxin, China) under galvano-static conditions was used for
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EC, PEC and ECP process. All the electrodes had an exposed area of 10 cm2 with the
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distance between the anode and cathode of 2 cm. For ozonation, an ozone generator
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(CF-YG5 Shanmei Shuimei Co., China) was used to produce O3 from high purity O2
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gas. The O3 concentration in the ozone generator effluent (mixture O2 and O3) can be
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adjusted by the power of the generator.
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The electrode material will determine the electrochemical reaction and removal
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efficiency of the processes. For EC, the anode was an aluminum (Al) sheet, while the
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cathode was an Al plate. For PEC and ECP, the anode was an aluminum (Al) sheet,
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while the cathode was an carbon-PTFE electrode, which was prepared with Vulcan
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XC-72 carbon powder (Cabot Corp., USA), PTFE dispersion, and anhydrous alcohol
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coated on the Ti plate (Yao et al., 2016). In addition, 0.6 L/min O2 was sparged into
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the reactor and thus H2O2 can be formed at the cathode for both PEC and ECP, since
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reaction (4) needs O2 presence which may be produced by reaction (2). Besides 0.6 L
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min-1 O2 addition, 0.6 L min-1 ozone (8.25 mgL-1 O3) was continuously sparged into
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the reactor at a constant flow by using a fine bubble diffuser for ECP. The operational
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parameters under different scenarios were shown in Table 1.
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sand paper, submerged in dilute HNO3 solution for 1 h and thoroughly rinsed with
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deionized water. Occasionally, the whole unit was cleaned with dilute HNO3. By
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adjusting the operating current of 200 to 600 mA, the desired Al3+ concentration and
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H2O2 dosage was obtained. The concentration of coagulant indicated by Al3+ at a
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specific current (I) and period of (t) can be calculated using Faraday’s Law of the
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following expression m = ItM VzF , where z is the number of electrons transferred (eq
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mol-1), M is the molecular weight (g mol-1), V is the volume of the treated water, and
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F is the Faraday’s constant (96485 C eq-1). The current efficiency (CE%) for H2O2
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production can be calculated by
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CE % =
nFc H 2 O2 V
∫
t
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Idt
× 100 , where n is 2 for H2O2
production (O2+ 2H+ + 2e-→ H2O2), cH2O2 is the concentration of H2O2 measured in
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the solution, V is the volume of the solution and t is the time of electrolysis. At
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specified time intervals, samples were taken from the reactors for analysis.
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2.4. Analytical methods
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The O3 concentration in the sparged gas was monitored using an ozone analyzer
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(UV-300, Sumsun EP Hi-Tech Co., Beijing). The ozone concentration in the water
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was determined by the indigo method (Bader and Hoigné, 1981). The H2O2
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concentration was measured using potassium titanium (IV) oxalate method (Sellers,
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1980). Chemical analyses for COD were carried out using HACH DR/3900
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spectrophotometer, following the testing procedure of each parameter. The
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interference of H2O2 for the measurement of COD was measured (Wu and Englehardt,
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2012).
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double beam PerkinElmer 25 spectrophotometer. The scan rate was 960 ms−1 within
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200–900nm wavelength range. The samples were scanned in quartz cells with a 1cm
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optical path. The molecular weight of the hybrid process was measured by the GPC
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gel permeation chromatography (Waters 1515 GPC).
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2.3. Current efficiency evaluation
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Current efficiency of the electrochemical oxidation for COD removal can be expressed using general current efficiency (GCECOD) (Radjenovic and Sedlak, 2015).
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GCECOD = no2 FV
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COD0 − CODt M o2 It
(1)
where nO2 is the number of electrons required for water oxidation (n = 4, 2H2O → O2
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+ 4H+ + 4e−), F is the Faraday constant (96487 C mol−1), V the electrolyte volume (L),
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COD0 and CODt are COD values measured at time t = 0 and time t (in g O2 L−1), MO2
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is the molecular weight of oxygen (32 g mol−1), I is the applied current (A), and t is
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the time over which treatment occurs (s).
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3. Results and discussion
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3.1. Removal efficiency of various process under different current density
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COD removal efficiency was measured as a function of time under different current
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density from 20 to 60 mA cm-2 for EC, PEC and ECP to optimize the operating
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parameters of these electrochemical processes (Fig. 2).
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For EC, Al3+ were produced by dissolution from the anode, which further
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hydrolyzed and produced hydroxyl complex in solution as the coagulant (SM-1 in the 10
ACCEPTED MANUSCRIPT supporting information). By using the Faraday’s law, aluminum irons were generated
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with a rate of 8.61 to 25.83 mg L-1 min-1 with the increase of current density from 20
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to 60 mA/cm2 (SM-1 in the supporting information). In all current density, there was a
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rapid drop in COD for the first 90 min of treatment with no limiting value reached.
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With the increase of current density from 20 to 50 mA cm-2, the COD removal
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efficiency drastically decreased from 27.5% to 60.8 % after 1.5 h (Fig. 2 a). In
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contrast, further increase of current density to 60 mA cm-2 led to the decrease of COD
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removal efficiency to 57.5% after 1.5 h (Fig. 2 a), which may result from the side
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reaction of H2O2 reduction to H2O due to cathodic O2 reduction to H2O2 is limited by
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O2 mass transfer to the cathode under the high current density (Xia et al., 2017).
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As for PEC, COD can be removed by coagulation through the coagulants produced
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at the anode and oxidation through H2O2 produced at the cathode (SM-2 in the
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supporting information). The COD removal efficiency drastically increased from
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40.26% to 65.6 % with the increase of current density from 20 to 50 mA cm-2.
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Nevertheless, the COD removal efficiency decreased to 57.7% with further increase
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of current density to 60 mA cm-2. Compared to the EC, the enhancement rate of COD
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removal efficiency decreased from 12.8% to 0.2% with the increase of current density
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from 20 to 50 mAcm2. It seemed that the synthetic effect of coagulation and H2O2
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oxidation was not obvious under high current density, which might be ascribed to the
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low oxidation ability of produced H2O2 in cathode. The decrease in the enhancement
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rate of COD removal efficiency with an increase in the current density (i.e. the higher
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the current density was, the less pronounced of the enhancement rate was),
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demonstrated it was also plausible that the organics oxidized by H2O2 can also be
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easily removed by electrocoagulation. Ozonation was inefficient for organic removal in SGFFWs (FigSM-3 in supporting
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information), which was consistent with our previous studies (Estrada and Rao, 2016;
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Butkovskyi et al., 2018). When the mixture of O2 and O3 were sparged in to the
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reactor with Al plate as the anode and carbon-PTFE as the cathode, the organics can
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be mainly degraded by the following three mechanisms: (1) coagulation due to the
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dissolution of Al3+ from the anode (2) • OH produced by the peroxone reaction of
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ozone and the H2O2 produced in the cathode (3) oxidation of the ozone due to the
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excess O3 molecules or H2O2 (S1 in the supporting information). With the increase of
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current density from 20 to 50 mA/cm2, the COD removal efficiency drastically
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increased from 61.0% to 82.4%. Nevertheless, further increase of current density to
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60 mA/cm2 led to the decrease of the COD removal efficiency to 77.4%. The higher
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COD removal efficiency of ECP than that of PEC under the same current density
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indicated that the·OH might be responsible for the significant enhancement of the
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COD removal.
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For all the three processes, the optimal condition for COD removal was under the
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current density of 50 mA cm-2. The COD removal efficiency mainly followed the
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declining order of ECP, PEC and EC.
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3.2. Comparison of the degradation mechanism of the different processes.
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The removal of organic compounds by different processes was comprehensively
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indicated by molecular weight (MW) change to provide an evidence of the 12
ACCEPTED MANUSCRIPT degradation mechanisms for EC, PEC and ECP (Fig.SM. 4 in the supporting
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information and Table 2). The raw water mainly contained 50.3% high
272
weight-average MW (57036 Da) biopolymers and 49.7% of the low weight-average
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MW compounds (651 Da). The fraction of the high weight-average MW (57036 Da) is
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far from 1, indicating relatively broad of MW distribution (i.e., guar gum or
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polyacrylamide), while the PD value was close to 1 for the compounds with the
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average MW of 651 Da, indicating that MW distribution of these fractions (such as
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small neutrals and acids) was relatively uniform. The weight average MW was 2071
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and 716 Da, and the relative content was 1.1% and 98.9%, respectively after EC. PD
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of both medium and small MW components were close to 1, indicating their MW
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distribution is relatively uniform. High MW fractions were not detected, since EC
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mainly remove the organics through coagulation. After PEC, the sample mainly
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composed of a small portion (1.3%) of medium MW (1920 Da) fraction and a large
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portion (98.7%) of small MW (673 Da) fraction. Both of MW distribution was
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relatively uniform (PD ≈1). Compared with EC, the mean MW slightly decreased due
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to the oxidation by H2O2 produced in the cathode. After ECP, the SGFFW was mainly
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composed of the fraction with the MW of 626 Da, and MW was evenly distributed
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with the PD value of approximately 1. Compared with ECP, PEC enhanced the
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degradation of medium MW (1920 Da) compounds. However, the average-weight
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MW of water sample treated by ozonation was 46546, 1919 and 640 Da, with relative
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contents of 21.0%, 2.0% and 77.0%, respectively. These results indicated the
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importance of EC in removing the fractions with the MW of 46546 Da. The
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disappearance in ECP indicated these intermediate products can’t be degraded in these
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processes but can be further oxidized and mineralized by the hydroxyl radical
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produced by the cathode in ECP. These findings demonstrated the ECP system with a
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simple and cost-effective Al anode and PTFE-carbon cathode integrated O3 is a
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feasible and efficient process for SGFFW treatment.
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In addition to MW distribution measurement, the intensity of the aromatic
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compounds which indicated by the peaks at 220 nm of UV-vis spectra (Fig. SM. 5 in
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the supporting information) significantly declined, demonstrating compounds were
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effectively decomposed but the intermediate compounds were not fully degrade.
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3.3. Kinetic modeling of the COD degradation
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Under the optimal condition of 50 mA cm-2, the degradation rate of COD for all the
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processes were fitted by the pseudo-first order kinetic model. Linear relationships
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obtained from a pseudo first-order analysis was shown in Fig. 3. Pseudo first-order
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rate constant (k) were determined to be 2.47×10-3 (R2=0.99) min-1 for ozonation,
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9.52×10-3 (R2=0.98) min-1 for EC, 1.0×10-2 min-1 (R2=0.65) for PEC and 2.47×10-3
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min-1 (R2=0.71) for ECP, which indicated that the pseudo-first order kinetic model
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can well fit the ozonation and EC but not the PEC and ECP due to the combination
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effect of coagulation and oxidation.
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Electrode materials determine the efficiency of electrochemical treatment processes.
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Both coagulation and oxidation were involved in ECP and PEC process, and thus
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COD removal can proceed via electrocoagulation and electrochemical oxidation. EC 14
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on the production of oxidizing species at the electrode that mediate the transformation
316
of contaminants, the oxidation product of which is affected by the electrode properties.
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The persdo-first order kinetic model can well fit the ozonation and coagulation
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process. It was assumed that coagulation can completely remove the organics (R)
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form the water, while the organics (R) were first oxidized into intermediates (I) and
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then partially oxidized into the final products P (i.e., CO2, H2O) by O3, H2O2 or ·OH.
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The rates of these electrochemical processes are affected by the species produced in
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the electrode. A kinetic model incorporated the mechanism of oxidation and
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coagulation were proposed. R denoted the initial compounds in SGFFWs, I denoted
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the intermediates of the oxidation process and P was the mineralized products CO2,
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H2O and flocculated sediments.
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R
k1
k2
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I
k3
Suppose the reaction rate follow persdo-first order kinetic model for I and P, then
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the degradation rate of R and the degradation rate of the I can be described as
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dCODR = ( k1 + k2 ) CODR dt dCODI − = k3CODs − k2CODI dt −
(2) (3)
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where k1 is the COD removal rate constant by coagulation or direct mineralization, k2
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is the oxidation rate constant to produce I, k3 is the mineralization rate constant of the
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intermediate product (I). The initial COD0 can be measured and the initial COD
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contributed by I was assumed to be zero at t=0, and thus the concentration of M and S
335
can be integrated as follow.
CODR = COD0e−( k1 + k2 )t
CODI =
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(5)
Both R and I contribute to COD. Then the COD degradation rate can expressed as −k t −( k + k )t CODt CODR + CODI k2 e 3 + ( k1 − k3 ) e 1 2 = = COD0 CODR 0 k1 + k2 − k3
(6)
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k2COD0 −k3t −( k1 +k2 )t e −e k1 + k2 − k3
(4)
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In Eq. (6), k1 for O3 oxidation was zero due to the assumption that two steps was
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involved in oxidation process. k2 and k3 for electrocoagulation was zero, because it
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was hypothesized that oxidation were not possible for EC. Therefore, the fitted kinetic
343
parameters can be obtained by using Eq. (6) for the various processes involving EC,
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O3, PEC and ECP under different cases (Fig. 4). In general, the fitted results matched
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the experimental data well for ozonation, PEC and ECP, while substantial
346
over-prediction of the rejection ratios was observed for EC. However, the relatively
347
good fitting results (R2=0.895-0.999) for EPC indicated that the developed chemical
348
kinetic model can satisfactorily model the abatements of COD during all the
349
mentioned processes in SGFFW treatment. The k1 value of ECP is much higher than
350
that of PEC plus O3, while the k2 and k3 is also much higher than other processes.
351
These indicated that ECP could substantially increase the COD abatement rate. These
352
were consistent with the MW distribution results that the ECP products were mainly
353
composed of low MW compounds (i.e., 626 Da).
354
3.4. Current efficiency evaluation and implications
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356
desirable to evaluate the current efficiency of these processes. The average current
357
efficiency for EC, PEC and ECP was 34.8%, 37.7% and 46.8%, respectively (Fig. 5).
358
Assuming the COD removed by ozonation was achieved by electrochemical process,
359
the equivalent average current efficiency is 11.4% within 90 min. Both kinetic model
360
and current efficiency evaluation suggested that the combination of EC and peroxone
361
can effectively enhance anodic coagulation and cathodic oxidation ability via
362
simultaneous generation of coagulant at anode and enhance oxidation by O3 and
363
carbon-PTFE cathode, which suggests ECP is a promising process for SGFFW
364
treatment. EC can only partially remove the colloids and organics in SGFFWs which
365
needs the oxidation process to enhance the COD removal, while the oxidation alone
366
for the organics removal in SGFFW is not high enough (Cui et al., 2017).
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The appearance of medium MW fraction (1919 Da) during ozonation and PEC but
368
disappearance in ECP indicated these intermediate products can’t be degraded in these
369
processes but can be further oxidized and mineralized by the hydroxyl radical
370
produced by the cathode in ECP. Although this study confirmed that the ECP process
371
could enhance the COD abatement, a qualitative and quantitative understanding ·OH
372
formation by peroxone reaction is still absence due to the complicated composition of
373
the SGFFWs and competive utilization of ozone by organics and hydrogen peroxide
374
(Pocostales et al., 2010; Fischbacher et al., 2013). Therefore, two aspects should be
375
considered in the future study. For one hand, the optimal parameters for the process
376
(i.e., O3 dosage, the influence of pH, and organics) should be considered in the future.
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especially for the competitive consumption of ozone by the peroxone process and
379
organics oxidation.
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4. Conclusions
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The ECP process was developed for SGFFW treatment by simultaneous
382
coagulation and oxidation process with Al plate as the anode and a carbon-PTFE gas
383
diffusion electrode as the cathode, realizing the simultaneous processes of coagulation,
384
H2O2 generation and activation by O3 at the cathode. Under the optimal current
385
density of 50 mA/cm2, the COD removal efficiency mainly followed the declining
386
order of ECP, PEC and EC. The appearance of medium MW fraction (1919 Da)
387
during ozonation and PEC but disappearance in ECP indicated these intermediate
388
products can’t be degraded in these processes but can be further oxidized and
389
mineralized by the hydroxyl radical produced by the cathode in ECP, demonstrating
390
the critical role of hydroxyl radical are responsible for the significant enhancement of
391
the COD removal. The pseudo-first order kinetic model can well fit the ozonation and
392
coagulation process but not the PEC and ECP due to the combination effect of
393
coagulation and oxidation. However, the proposed degradation mechanism based
394
model can satisfactorily fit ECP. The average current efficiency for PEC was as high
395
as 35.4% and 12% higher than that of ozonation and EC, respectively. These results
396
demonstrated the feasibility of establishing an efficient EF system with a simple and
397
cost-effective integrated anodic coagulation and enhanced cathodic oxidation by O3
398
for SGFFW treatment.
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Acknowledgments
401
The authors acknowledge the financial support provided by the National Natural
402
Science Foundation of China (No. 51708556), Young Backbone Individuals for
403
Outstanding Talents Project of Beijing (No. 2017000020124G102), special fund of
404
State Key Joint Laboratory of Environment Simulation and Pollution Control
405
(No.18K03ESPCT), State Key Laboratory of Pollution Control and Resource Reuse
406
Foundation, (No. PCRRF17011), and Science Foundation of China University of
407
Petroleum, Beijing (No. 2462015YJRC030).
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Appendix A. Supplementary material
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Supplementary data associated with this article can be found, in the online version.
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ACCEPTED MANUSCRIPT Tables Table 1. Specific operation parameters for the different scenarios. Ozonation
EC
PEC
ECP
Anode
_
Al
Al
Al
Cathode
_
Al
Carbon-PTFE
Carbon-PTFE
_
2.5
2.5
2.5
_
200-600
200-600
0.6 L/min O3
_
0.6 L/min O2
Electrode distance (cm) Currency (mA)
200-600
0.6L/min O2 and O3
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Parameter
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Table 2. The MW distribution of raw SGFFWs and treated by different processes.
Ozonation
EC PEC
26594 633
57036 651
42646 685
1.83 1.02
50.3 49.7
20.61
25965
46546
28455
1.98
21.0
25.06
1732
1919
1854
1.03
2.0
27.27
621
640
657
1.03
77.0
24.80
2014
2071
2210
1.03
1.1
27.0
693
716
760
1.03
98.9
24.95
1873
1920
2034
1.02
1.3
27.15
652
673
712
1.03
98.7
627
626
681
1.03
100
27.25
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Area(%)
20.02 27.24
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Raw Water
PD
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Dist Name Retention Time (min) Mn (Da) Mw(Da) Mp(Da)
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DC power supply
Reactor Ozone quenche r
Flowmeter
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O2
Anode
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Ozone Generator
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Fig. 1. Schematical diagram of the experimental apparatus
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20 mA/cm2 30 mA/cm2 40 mA/cm2 50 mA/cm2 60 mA/cm2
40
20
(b)
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60
80
100
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COD Removal efficiency (%)
80
20 mA/cm2 30 mA/cm2 40 mA/cm2 50 mA/cm2 60 mA/cm2
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0
20
40 Time (min)
3
60
80
100
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2.0
Ozonation EC PEC ECP
1.2 0.8 0.4
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-lnCODt/COD0
1.6
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Fig. 2. COD removal efficiency by EC, PEC and ECP under different current density.
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20
40
60
80
100
Time (min)
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Fig. 3. Pseudo-first order curves of COD degradation by different processes under the current density of 50 mA cm-2.
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1.0
0.6
Ozonation EC PEC ECP
0.2 0.0
0
20
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CODt/COD0
0.8
40
60
80
100
Time (min)
Fig. 4. The kinetic modelling of the COD degradation by different processes under the
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k1 k2 k3
40
30
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k (min-1)
0.15
50
Current Efficiency
Current Efficiency (%)
0.20
0.10
20
0.05
0.00 Ozonation
EC
PEC
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10
ECP
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6
ACCEPTED MANUSCRIPT Highlights
A novel electrocoagulation-peroxone (ECP) process was proposed.
COD removal efficiency mainly followed the declining order of ECP, PEC and EC. Hydroxyl radical are responsible for the enhancement of the COD removal.
The mechanism based model can well describe the ECP process
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S0045-6535(18)32158-1
DOI:
https://doi.org/10.1016/j.chemosphere.2018.11.055
Reference:
CHEM 22539
To appear in:
ECSN
Received Date: 17 August 2018 Revised Date:
24 October 2018
Accepted Date: 8 November 2018
Please cite this article as: Kong, F.-x., Lin, X.-f., Sun, G.-d., Chen, J.-f., Guo, C.-m., Xie, Y.F., Enhanced organics removal for shale gas fracturing flowback water with electrocoagulation and simultaneous electro-peroxone process, Chemosphere (2018), doi: https://doi.org/10.1016/ j.chemosphere.2018.11.055. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. 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.
ACCEPTED MANUSCRIPT
Enhanced organics removal for shale gas fracturing
1 2
flowback water with electrocoagulation and simultaneous
3
electro-peroxone process
4
Fan-xin Kong a*, Xiao-feng Lin a, Guang-dong Sun b, Jin-fu Chen a*, Chun-mei Guo a, Yuefeng F. Xie d
8
a
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Control, China University of Petroleum, Beijing 102249, China
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State Key Laboratory of Heavy Oil Processing, Beijing Key Laboratory of Oil & Gas Pollution
10
b
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Institute of Water Resources and Hydropower Research Beijing, 100038, China
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c
13
17057, USA
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State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin, China
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Environmental Engineering Programs, The Pennsylvania State University, Middletown, PA
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Re-Submitted to
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Chemosphere
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(No. CHEM56195) October 2018
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________________________________________
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* Corresponding author: Dr. Fan-xin Kong and Dr. Jin-fu Chen, Tel.: +86-10-8973 3637; Email: [email protected] and [email protected] Abstract:
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ACCEPTED MANUSCRIPT 37 38
Abstract The colloids and organics of shale gas fracturing flowback water (SGFFW) during
40
shale gas extraction is of primary concern for its treatment. Coagulation combined with
41
oxidation might be a promising process for SGFFW treatment. In this study, a novel
42
electrocoagulation-peroxone (ECP) process was developed for SGFFW treatment by
43
simultaneous coagulation and oxidation process with Al plate as the anode and a
44
carbon-PTFE gas diffusion electrode as the cathode, realizing the simultaneous
45
processes of coagulation, H2O2 generation and activation by O3 at the cathode.
46
Compared with electrocoagulation (EC) and peroxi-electrocoagulation (PEC), the
47
COD removal efficiency mainly followed the declining order of ECP, PEC and EC
48
under the optimal current density of 50 mA cm-2. The appearance of medium MW
49
fraction (1919 Da) during ozonation and PEC but disappearance in ECP indicated
50
these intermediate products can’t be degraded in ozonation and PEC but can be
51
further oxidized and mineralized by the hydroxyl radical produced by the cathode in
52
ECP, demonstrating the hydroxyl radical are responsible for the significant
53
enhancement of the COD removal. The pseudo-first order kinetic model can well fit
54
ozonation and coagulation process but not the PEC and ECP process due to the
55
synthetic effect of coagulation and oxidation. However, the proposed mechanism
56
based model can generally fit ECP satisfactorily. The average current efficiency for
57
PEC was 35.4% and 12% higher than that of ozonation and EC, respectively. This
58
study demonstrated the feasibility of establishing a high efficiency and space-saving
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2
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with
integrated
59
electrochemical
60
electro-peroxone for SGFFW treatment.
anodic
coagulation
and
cathodic
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Keywords:
Electrocoagulation,
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Electrocoagulation-peroxone, Ozonation.
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Peroxi-electrocoagulation,
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1. Introduction Shale gas fracturing flowback water (SGFFW) generated by shale gas exploration
74
can pose potential environmental risk due to its high total dissolved solids (TDS),
75
organic matter and radioactive elements, which is a growing concern around the
76
world with accelerated production of shale gas using horizontal drilling and hydraulic
77
fracturing (Haluszczak et al., 2013; Shaffer et al., 2013). Currently, most of SGFFW is
78
reused for another fracking event. Prior to reuse, SGFFW is typically treated on-site
79
to remove suspended solids or specific constituents that may not be compatible with
80
fracturing fluids chemistry. Colloids and organics in the SGFFWs is the primary
81
concern for SGFFW reuse and discharge.
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Coagulation and oxidation has been shown to be an effective and low-cost
83
approach for colloids and organics removal to improve overall water quality (Howe et
84
al., 2006; Huang et al., 2009; Yang and Kim, 2009; Zularisam et al., 2009; Yu et al.,
85
2016). Esmaeilirad et al. (2015) found electrocoagulation (EC) was one of the
86
available technologies to treat produced water for reuse in fluids, eliminating solids
87
and TOC but also inorganics such as Boron (Esmaeilirad et al., 2015; Sari and
88
Chellam, 2015).Our previous study (Kong et al., 2017) indicated that coagulation can
89
effectively remove colloids and organics in the SGFFWs and thus effectively mitigate
90
the UF fouling in hybrid coagulation-ultrafiltration process. However, some studies
91
(Miller et al., 2013; Xiong et al., 2016; Kong et al., 2018) further indicated that even
92
pretreated coagulation-UF process, the subsequent nanofiltration and reverse osmosis
93
still suffered high fouling propensity, clearly indicating the inefficient of coagulation
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95
sub-micron colloidal particles to organic coating at high ionic strength in the flowback
96
was highly stable and was hard to be removed by coagulation or EC, which
97
highlighted the importance of oxidation to destroy the organic coating to aid
98
coagulation process. The oxidation process alone was used for dissolved organic
99
removal in SGFFW. Medium removal efficiencies were achieved with COD (68.20%),
100
color (88.48%) and total phenol (92.65%) removal by using Fenton processes (Erkan et al.,
101
2017). Recent study (Xiong et al., 2018) indicated that radical-induced degradation (i.e.,
102
hydroxyl radical and Fe2+) of PAM was reduced by two orders of magnitude, from roughly
103
10 MDa to 200 kDa under typical HPT fracturing conditions, which indicated oxidation is
104
an effective option for SGFFW treatment. The maximum removal efficiencies by
105
Electro-Fenton process were found to be around 87.35%, 89.15%, and 91.75% for COD,
106
color, and total phenol under the optimum conditions, respectively, indicating that
107
electro-Fenton or Fenton-like process seem to be an efficient treatment and low cost
108
method for shale gas wastewater (Chen et al., 2016; Turan et al., 2017). In contrast, some
109
studies indicated that the potential of various AOPs (i.e., UV/H2O2, O3/H2O2, Solar
110
light/Chlorine, and photo-Fenton) only marginally reduced the DOC of the flowback
111
water despite for high doses of light and oxidants (e.g., 10% DOC removal by
112
O3/H2O2, at O3 and H2O2 doses of 500 mg/L and 350 mg/L respectively) (Cui et al.,
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2017). In conclusion, coagulation can only partially remove the colloids and organics in
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SGFFWs which needs the oxidation process to enhance the coagulation efficiency, and the
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oxidation alone for the organics removal in SGFFW is high enough. It seems that
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ACCEPTED MANUSCRIPT coagulation combined with oxidation might be a promising process for SGFFW treatment.
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Compared to conventional process, electrochemical process is an effective platform
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that is capable of achieving coagulation and electrochemical oxidation without adding
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any chemicals to eliminate colloids and contaminants, which has the potential to develope
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a cost-effective method to realize high efficientwasterwater treatment. Recently, a
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combined methodology called ‘‘peroxi-electrocoagulation (PEC) method’’ using a
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synergistic combination of anodic electrocoagulation and cathodic oxidation (Zarei et
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al., 2009; Esfandyari et al., 2015; do Vale-Júnior et al., 2018), which is very effective
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in removing colloids,suspended particles, oil or other contaminants due to the
125
dissolution of coagulant ions (i.e., Al3+ or Fe2+) from the anode, while H2O2 was
126
effectively electro-generated on cathode, realizing the simultaneous processes of
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coagulation and H2O2 oxidation. However, the oxidation rate and capacity of H2O2
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was relatively low, which is relatively ineffective in eliminating some refractory
129
organic compounds (Zarei et al., 2009; Vasudevan, 2014). Fortunately, H2O2
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generated in the cathode can be in suit activated to a high oxidation potential hydroxyl
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radical (·OH, E0 = 2.8 V) with by O3 called peroxone (Bakheet et al., 2013; Fischbacher
132
et al., 2013), which is very effective in breaking down some refractory organic
133
compounds (Bokare and Choi, 2014; Asghar et al., 2015). Recently, E-peroxone has
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also been developed as an effective electrochemical oxidation process by using the
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carbon-PTFE gas diffusion electrode as the cathode to in-suit generation of generation
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and activation by O3 at the cathode, which only highlight the enhancement of
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oxidation process by cathodic peroxone or their combination with DSA anode
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Wang et al., 2018). However, the PEC and E-peroxone process imply that it might be
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possible to fulfill the anodic coagulation and cathodic E-peroxone by simply changing the
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anode and cahode for simultaneously electrocoagulation and E-peroxone with only two
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electrodes.
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In this study, a novel electrocoagulation-peroxone (ECP) process was proposed
144
with Al plate as anode and a carbon-PTFE gas diffusion electrode as the cathode,
145
realizing the simultaneous coagulation and H2O2 generation and activation by O3 at
146
the cathode for the SGFFW treatment. The objective of this study was to verify the
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feasibility of this novel ECP for COD removal and better understand removal
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mechanisms. The performance of the novel process, the effect of current density, the
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dissolved organics removal and kinetic model were systematically elucidated. The
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results of this study allowed to create a novel high efficient ECP process for SGFFW
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treatment and develop solutions that enable the use of ECP for SGFFW reuse during
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shale gas exploration.
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2. Materials and methods
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2.1. Raw water
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The characteristic of the SGFFW was reported to be complicated, which was
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mainly consisted of dissolved salts, chemical additives and solid particles. The raw
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water in this study was sampled from one of the reservoir in Fuling shale gas play,
158
Chongqing, China. It was turbid with a light yellow color with some small floccules,
159
which is similar to the previous report on Marcellus shale gas play (Jiang et al., 2013). 7
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The detailed water quality will be comprehensively discussed in the following
161
session.
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2.2. Experimental apparatus and procedures Bench scale experiments were conducted in a 150 mL custom-made Perspex cell
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with two electrode holders and some necessary accessories (Fig. 1). A DC power
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supply (KXN-305D, Zhaoxin, China) under galvano-static conditions was used for
166
EC, PEC and ECP process. All the electrodes had an exposed area of 10 cm2 with the
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distance between the anode and cathode of 2 cm. For ozonation, an ozone generator
168
(CF-YG5 Shanmei Shuimei Co., China) was used to produce O3 from high purity O2
169
gas. The O3 concentration in the ozone generator effluent (mixture O2 and O3) can be
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adjusted by the power of the generator.
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The electrode material will determine the electrochemical reaction and removal
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efficiency of the processes. For EC, the anode was an aluminum (Al) sheet, while the
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cathode was an Al plate. For PEC and ECP, the anode was an aluminum (Al) sheet,
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while the cathode was an carbon-PTFE electrode, which was prepared with Vulcan
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XC-72 carbon powder (Cabot Corp., USA), PTFE dispersion, and anhydrous alcohol
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coated on the Ti plate (Yao et al., 2016). In addition, 0.6 L/min O2 was sparged into
177
the reactor and thus H2O2 can be formed at the cathode for both PEC and ECP, since
178
reaction (4) needs O2 presence which may be produced by reaction (2). Besides 0.6 L
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min-1 O2 addition, 0.6 L min-1 ozone (8.25 mgL-1 O3) was continuously sparged into
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the reactor at a constant flow by using a fine bubble diffuser for ECP. The operational
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parameters under different scenarios were shown in Table 1.
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sand paper, submerged in dilute HNO3 solution for 1 h and thoroughly rinsed with
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deionized water. Occasionally, the whole unit was cleaned with dilute HNO3. By
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adjusting the operating current of 200 to 600 mA, the desired Al3+ concentration and
186
H2O2 dosage was obtained. The concentration of coagulant indicated by Al3+ at a
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specific current (I) and period of (t) can be calculated using Faraday’s Law of the
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following expression m = ItM VzF , where z is the number of electrons transferred (eq
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mol-1), M is the molecular weight (g mol-1), V is the volume of the treated water, and
190
F is the Faraday’s constant (96485 C eq-1). The current efficiency (CE%) for H2O2
191
production can be calculated by
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CE % =
nFc H 2 O2 V
∫
t
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Idt
× 100 , where n is 2 for H2O2
production (O2+ 2H+ + 2e-→ H2O2), cH2O2 is the concentration of H2O2 measured in
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the solution, V is the volume of the solution and t is the time of electrolysis. At
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specified time intervals, samples were taken from the reactors for analysis.
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2.4. Analytical methods
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The O3 concentration in the sparged gas was monitored using an ozone analyzer
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(UV-300, Sumsun EP Hi-Tech Co., Beijing). The ozone concentration in the water
198
was determined by the indigo method (Bader and Hoigné, 1981). The H2O2
199
concentration was measured using potassium titanium (IV) oxalate method (Sellers,
200
1980). Chemical analyses for COD were carried out using HACH DR/3900
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spectrophotometer, following the testing procedure of each parameter. The
202
interference of H2O2 for the measurement of COD was measured (Wu and Englehardt,
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2012).
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double beam PerkinElmer 25 spectrophotometer. The scan rate was 960 ms−1 within
206
200–900nm wavelength range. The samples were scanned in quartz cells with a 1cm
207
optical path. The molecular weight of the hybrid process was measured by the GPC
208
gel permeation chromatography (Waters 1515 GPC).
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2.3. Current efficiency evaluation
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Current efficiency of the electrochemical oxidation for COD removal can be expressed using general current efficiency (GCECOD) (Radjenovic and Sedlak, 2015).
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GCECOD = no2 FV
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COD0 − CODt M o2 It
(1)
where nO2 is the number of electrons required for water oxidation (n = 4, 2H2O → O2
215
+ 4H+ + 4e−), F is the Faraday constant (96487 C mol−1), V the electrolyte volume (L),
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COD0 and CODt are COD values measured at time t = 0 and time t (in g O2 L−1), MO2
217
is the molecular weight of oxygen (32 g mol−1), I is the applied current (A), and t is
218
the time over which treatment occurs (s).
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3. Results and discussion
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3.1. Removal efficiency of various process under different current density
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COD removal efficiency was measured as a function of time under different current
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density from 20 to 60 mA cm-2 for EC, PEC and ECP to optimize the operating
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parameters of these electrochemical processes (Fig. 2).
224
For EC, Al3+ were produced by dissolution from the anode, which further
225
hydrolyzed and produced hydroxyl complex in solution as the coagulant (SM-1 in the 10
ACCEPTED MANUSCRIPT supporting information). By using the Faraday’s law, aluminum irons were generated
227
with a rate of 8.61 to 25.83 mg L-1 min-1 with the increase of current density from 20
228
to 60 mA/cm2 (SM-1 in the supporting information). In all current density, there was a
229
rapid drop in COD for the first 90 min of treatment with no limiting value reached.
230
With the increase of current density from 20 to 50 mA cm-2, the COD removal
231
efficiency drastically decreased from 27.5% to 60.8 % after 1.5 h (Fig. 2 a). In
232
contrast, further increase of current density to 60 mA cm-2 led to the decrease of COD
233
removal efficiency to 57.5% after 1.5 h (Fig. 2 a), which may result from the side
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reaction of H2O2 reduction to H2O due to cathodic O2 reduction to H2O2 is limited by
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O2 mass transfer to the cathode under the high current density (Xia et al., 2017).
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As for PEC, COD can be removed by coagulation through the coagulants produced
237
at the anode and oxidation through H2O2 produced at the cathode (SM-2 in the
238
supporting information). The COD removal efficiency drastically increased from
239
40.26% to 65.6 % with the increase of current density from 20 to 50 mA cm-2.
240
Nevertheless, the COD removal efficiency decreased to 57.7% with further increase
241
of current density to 60 mA cm-2. Compared to the EC, the enhancement rate of COD
242
removal efficiency decreased from 12.8% to 0.2% with the increase of current density
243
from 20 to 50 mAcm2. It seemed that the synthetic effect of coagulation and H2O2
244
oxidation was not obvious under high current density, which might be ascribed to the
245
low oxidation ability of produced H2O2 in cathode. The decrease in the enhancement
246
rate of COD removal efficiency with an increase in the current density (i.e. the higher
247
the current density was, the less pronounced of the enhancement rate was),
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demonstrated it was also plausible that the organics oxidized by H2O2 can also be
249
easily removed by electrocoagulation. Ozonation was inefficient for organic removal in SGFFWs (FigSM-3 in supporting
251
information), which was consistent with our previous studies (Estrada and Rao, 2016;
252
Butkovskyi et al., 2018). When the mixture of O2 and O3 were sparged in to the
253
reactor with Al plate as the anode and carbon-PTFE as the cathode, the organics can
254
be mainly degraded by the following three mechanisms: (1) coagulation due to the
255
dissolution of Al3+ from the anode (2) • OH produced by the peroxone reaction of
256
ozone and the H2O2 produced in the cathode (3) oxidation of the ozone due to the
257
excess O3 molecules or H2O2 (S1 in the supporting information). With the increase of
258
current density from 20 to 50 mA/cm2, the COD removal efficiency drastically
259
increased from 61.0% to 82.4%. Nevertheless, further increase of current density to
260
60 mA/cm2 led to the decrease of the COD removal efficiency to 77.4%. The higher
261
COD removal efficiency of ECP than that of PEC under the same current density
262
indicated that the·OH might be responsible for the significant enhancement of the
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COD removal.
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For all the three processes, the optimal condition for COD removal was under the
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current density of 50 mA cm-2. The COD removal efficiency mainly followed the
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declining order of ECP, PEC and EC.
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3.2. Comparison of the degradation mechanism of the different processes.
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The removal of organic compounds by different processes was comprehensively
269
indicated by molecular weight (MW) change to provide an evidence of the 12
ACCEPTED MANUSCRIPT degradation mechanisms for EC, PEC and ECP (Fig.SM. 4 in the supporting
271
information and Table 2). The raw water mainly contained 50.3% high
272
weight-average MW (57036 Da) biopolymers and 49.7% of the low weight-average
273
MW compounds (651 Da). The fraction of the high weight-average MW (57036 Da) is
274
far from 1, indicating relatively broad of MW distribution (i.e., guar gum or
275
polyacrylamide), while the PD value was close to 1 for the compounds with the
276
average MW of 651 Da, indicating that MW distribution of these fractions (such as
277
small neutrals and acids) was relatively uniform. The weight average MW was 2071
278
and 716 Da, and the relative content was 1.1% and 98.9%, respectively after EC. PD
279
of both medium and small MW components were close to 1, indicating their MW
280
distribution is relatively uniform. High MW fractions were not detected, since EC
281
mainly remove the organics through coagulation. After PEC, the sample mainly
282
composed of a small portion (1.3%) of medium MW (1920 Da) fraction and a large
283
portion (98.7%) of small MW (673 Da) fraction. Both of MW distribution was
284
relatively uniform (PD ≈1). Compared with EC, the mean MW slightly decreased due
285
to the oxidation by H2O2 produced in the cathode. After ECP, the SGFFW was mainly
286
composed of the fraction with the MW of 626 Da, and MW was evenly distributed
287
with the PD value of approximately 1. Compared with ECP, PEC enhanced the
288
degradation of medium MW (1920 Da) compounds. However, the average-weight
289
MW of water sample treated by ozonation was 46546, 1919 and 640 Da, with relative
290
contents of 21.0%, 2.0% and 77.0%, respectively. These results indicated the
291
importance of EC in removing the fractions with the MW of 46546 Da. The
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ACCEPTED MANUSCRIPT appearance of medium MW fraction (1919 Da) during ozonation and PEC but
293
disappearance in ECP indicated these intermediate products can’t be degraded in these
294
processes but can be further oxidized and mineralized by the hydroxyl radical
295
produced by the cathode in ECP. These findings demonstrated the ECP system with a
296
simple and cost-effective Al anode and PTFE-carbon cathode integrated O3 is a
297
feasible and efficient process for SGFFW treatment.
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In addition to MW distribution measurement, the intensity of the aromatic
299
compounds which indicated by the peaks at 220 nm of UV-vis spectra (Fig. SM. 5 in
300
the supporting information) significantly declined, demonstrating compounds were
301
effectively decomposed but the intermediate compounds were not fully degrade.
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3.3. Kinetic modeling of the COD degradation
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Under the optimal condition of 50 mA cm-2, the degradation rate of COD for all the
304
processes were fitted by the pseudo-first order kinetic model. Linear relationships
305
obtained from a pseudo first-order analysis was shown in Fig. 3. Pseudo first-order
306
rate constant (k) were determined to be 2.47×10-3 (R2=0.99) min-1 for ozonation,
307
9.52×10-3 (R2=0.98) min-1 for EC, 1.0×10-2 min-1 (R2=0.65) for PEC and 2.47×10-3
308
min-1 (R2=0.71) for ECP, which indicated that the pseudo-first order kinetic model
309
can well fit the ozonation and EC but not the PEC and ECP due to the combination
310
effect of coagulation and oxidation.
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Electrode materials determine the efficiency of electrochemical treatment processes.
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Both coagulation and oxidation were involved in ECP and PEC process, and thus
313
COD removal can proceed via electrocoagulation and electrochemical oxidation. EC 14
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315
on the production of oxidizing species at the electrode that mediate the transformation
316
of contaminants, the oxidation product of which is affected by the electrode properties.
317
The persdo-first order kinetic model can well fit the ozonation and coagulation
318
process. It was assumed that coagulation can completely remove the organics (R)
319
form the water, while the organics (R) were first oxidized into intermediates (I) and
320
then partially oxidized into the final products P (i.e., CO2, H2O) by O3, H2O2 or ·OH.
321
The rates of these electrochemical processes are affected by the species produced in
322
the electrode. A kinetic model incorporated the mechanism of oxidation and
323
coagulation were proposed. R denoted the initial compounds in SGFFWs, I denoted
324
the intermediates of the oxidation process and P was the mineralized products CO2,
325
H2O and flocculated sediments.
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R
k1
k2
328 329 330
I
k3
Suppose the reaction rate follow persdo-first order kinetic model for I and P, then
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the degradation rate of R and the degradation rate of the I can be described as
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P
dCODR = ( k1 + k2 ) CODR dt dCODI − = k3CODs − k2CODI dt −
(2) (3)
331
where k1 is the COD removal rate constant by coagulation or direct mineralization, k2
332
is the oxidation rate constant to produce I, k3 is the mineralization rate constant of the
333
intermediate product (I). The initial COD0 can be measured and the initial COD
15
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contributed by I was assumed to be zero at t=0, and thus the concentration of M and S
335
can be integrated as follow.
CODR = COD0e−( k1 + k2 )t
CODI =
337
339
(5)
Both R and I contribute to COD. Then the COD degradation rate can expressed as −k t −( k + k )t CODt CODR + CODI k2 e 3 + ( k1 − k3 ) e 1 2 = = COD0 CODR 0 k1 + k2 − k3
(6)
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k2COD0 −k3t −( k1 +k2 )t e −e k1 + k2 − k3
(4)
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In Eq. (6), k1 for O3 oxidation was zero due to the assumption that two steps was
341
involved in oxidation process. k2 and k3 for electrocoagulation was zero, because it
342
was hypothesized that oxidation were not possible for EC. Therefore, the fitted kinetic
343
parameters can be obtained by using Eq. (6) for the various processes involving EC,
344
O3, PEC and ECP under different cases (Fig. 4). In general, the fitted results matched
345
the experimental data well for ozonation, PEC and ECP, while substantial
346
over-prediction of the rejection ratios was observed for EC. However, the relatively
347
good fitting results (R2=0.895-0.999) for EPC indicated that the developed chemical
348
kinetic model can satisfactorily model the abatements of COD during all the
349
mentioned processes in SGFFW treatment. The k1 value of ECP is much higher than
350
that of PEC plus O3, while the k2 and k3 is also much higher than other processes.
351
These indicated that ECP could substantially increase the COD abatement rate. These
352
were consistent with the MW distribution results that the ECP products were mainly
353
composed of low MW compounds (i.e., 626 Da).
354
3.4. Current efficiency evaluation and implications
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356
desirable to evaluate the current efficiency of these processes. The average current
357
efficiency for EC, PEC and ECP was 34.8%, 37.7% and 46.8%, respectively (Fig. 5).
358
Assuming the COD removed by ozonation was achieved by electrochemical process,
359
the equivalent average current efficiency is 11.4% within 90 min. Both kinetic model
360
and current efficiency evaluation suggested that the combination of EC and peroxone
361
can effectively enhance anodic coagulation and cathodic oxidation ability via
362
simultaneous generation of coagulant at anode and enhance oxidation by O3 and
363
carbon-PTFE cathode, which suggests ECP is a promising process for SGFFW
364
treatment. EC can only partially remove the colloids and organics in SGFFWs which
365
needs the oxidation process to enhance the COD removal, while the oxidation alone
366
for the organics removal in SGFFW is not high enough (Cui et al., 2017).
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The appearance of medium MW fraction (1919 Da) during ozonation and PEC but
368
disappearance in ECP indicated these intermediate products can’t be degraded in these
369
processes but can be further oxidized and mineralized by the hydroxyl radical
370
produced by the cathode in ECP. Although this study confirmed that the ECP process
371
could enhance the COD abatement, a qualitative and quantitative understanding ·OH
372
formation by peroxone reaction is still absence due to the complicated composition of
373
the SGFFWs and competive utilization of ozone by organics and hydrogen peroxide
374
(Pocostales et al., 2010; Fischbacher et al., 2013). Therefore, two aspects should be
375
considered in the future study. For one hand, the optimal parameters for the process
376
(i.e., O3 dosage, the influence of pH, and organics) should be considered in the future.
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378
especially for the competitive consumption of ozone by the peroxone process and
379
organics oxidation.
380
4. Conclusions
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The ECP process was developed for SGFFW treatment by simultaneous
382
coagulation and oxidation process with Al plate as the anode and a carbon-PTFE gas
383
diffusion electrode as the cathode, realizing the simultaneous processes of coagulation,
384
H2O2 generation and activation by O3 at the cathode. Under the optimal current
385
density of 50 mA/cm2, the COD removal efficiency mainly followed the declining
386
order of ECP, PEC and EC. The appearance of medium MW fraction (1919 Da)
387
during ozonation and PEC but disappearance in ECP indicated these intermediate
388
products can’t be degraded in these processes but can be further oxidized and
389
mineralized by the hydroxyl radical produced by the cathode in ECP, demonstrating
390
the critical role of hydroxyl radical are responsible for the significant enhancement of
391
the COD removal. The pseudo-first order kinetic model can well fit the ozonation and
392
coagulation process but not the PEC and ECP due to the combination effect of
393
coagulation and oxidation. However, the proposed degradation mechanism based
394
model can satisfactorily fit ECP. The average current efficiency for PEC was as high
395
as 35.4% and 12% higher than that of ozonation and EC, respectively. These results
396
demonstrated the feasibility of establishing an efficient EF system with a simple and
397
cost-effective integrated anodic coagulation and enhanced cathodic oxidation by O3
398
for SGFFW treatment.
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Acknowledgments
401
The authors acknowledge the financial support provided by the National Natural
402
Science Foundation of China (No. 51708556), Young Backbone Individuals for
403
Outstanding Talents Project of Beijing (No. 2017000020124G102), special fund of
404
State Key Joint Laboratory of Environment Simulation and Pollution Control
405
(No.18K03ESPCT), State Key Laboratory of Pollution Control and Resource Reuse
406
Foundation, (No. PCRRF17011), and Science Foundation of China University of
407
Petroleum, Beijing (No. 2462015YJRC030).
408
Appendix A. Supplementary material
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Supplementary data associated with this article can be found, in the online version.
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ACCEPTED MANUSCRIPT Tables Table 1. Specific operation parameters for the different scenarios. Ozonation
EC
PEC
ECP
Anode
_
Al
Al
Al
Cathode
_
Al
Carbon-PTFE
Carbon-PTFE
_
2.5
2.5
2.5
_
200-600
200-600
0.6 L/min O3
_
0.6 L/min O2
Electrode distance (cm) Currency (mA)
200-600
0.6L/min O2 and O3
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Parameter
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Table 2. The MW distribution of raw SGFFWs and treated by different processes.
Ozonation
EC PEC
26594 633
57036 651
42646 685
1.83 1.02
50.3 49.7
20.61
25965
46546
28455
1.98
21.0
25.06
1732
1919
1854
1.03
2.0
27.27
621
640
657
1.03
77.0
24.80
2014
2071
2210
1.03
1.1
27.0
693
716
760
1.03
98.9
24.95
1873
1920
2034
1.02
1.3
27.15
652
673
712
1.03
98.7
627
626
681
1.03
100
27.25
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ECP
Area(%)
20.02 27.24
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Raw Water
PD
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Dist Name Retention Time (min) Mn (Da) Mw(Da) Mp(Da)
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+
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DC power supply
Reactor Ozone quenche r
Flowmeter
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O2
Anode
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Cathode
Aerator
Ozone Generator
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Fig. 1. Schematical diagram of the experimental apparatus
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20 mA/cm2 30 mA/cm2 40 mA/cm2 50 mA/cm2 60 mA/cm2
40
20
(b)
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COD removal efficiency (%)
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COD Removal efficiency (%)
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20 mA/cm2 30 mA/cm2 40 mA/cm2 50 mA/cm2 60 mA/cm2
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80
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2.0
Ozonation EC PEC ECP
1.2 0.8 0.4
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-lnCODt/COD0
1.6
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Fig. 2. COD removal efficiency by EC, PEC and ECP under different current density.
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20
40
60
80
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Time (min)
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Fig. 3. Pseudo-first order curves of COD degradation by different processes under the current density of 50 mA cm-2.
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1.0
0.6
Ozonation EC PEC ECP
0.2 0.0
0
20
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CODt/COD0
0.8
40
60
80
100
Time (min)
Fig. 4. The kinetic modelling of the COD degradation by different processes under the
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k1 k2 k3
40
30
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k (min-1)
0.15
50
Current Efficiency
Current Efficiency (%)
0.20
0.10
20
0.05
0.00 Ozonation
EC
PEC
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ECP
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Fig. 5. The kinetic parameter of the COD degradation based on the degradation mechanism and the equivalent current efficiency under the current density of 50 mA cm-2.
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ACCEPTED MANUSCRIPT Highlights
A novel electrocoagulation-peroxone (ECP) process was proposed.
COD removal efficiency mainly followed the declining order of ECP, PEC and EC. Hydroxyl radical are responsible for the enhancement of the COD removal.
The mechanism based model can well describe the ECP process
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