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Azo dye removal in a membrane-free up-flow biocatalyzed electrolysis reactor coupled with an aerobic bio-contact oxidation reactor
Journal of Hazardous Materials 239–240 (2012) 257–264
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Azo dye removal in a membrane-free up-flow biocatalyzed electrolysis reactor coupled with an aerobic bio-contact oxidation reactor Dan Cui a , Yu-Qi Guo a , Hao-Yi Cheng a , Bin Liang a , Fan-Ying Kong a , Hyung-Sool Lee b , Ai-Jie Wang a,∗ a b
State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, No. 202 Haihe Road, Harbin 150090, PR China Department of Civil and Environmental Engineering, University of Waterloo, 200 University Avenue West Waterloo, Ontario, Canada N2L 3G1
h i g h l i g h t s A membrane-free up-flow biocatalyzed electrolysis reactor coupled with an aerobic bio-contact oxidation reactor was developed. Alizarin Yellow R as the mode of azo dyes was efficiently converted to p-phenylenediamine (PPD) and 5-aminosalicylic acid (5-ASA). PPD and 5-ASA were further oxidized in a bio-contact oxidation reactor. The mechanism of UBER for azo dye removal was discussed.
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Article history: Received 19 June 2012 Received in revised form 20 August 2012 Accepted 28 August 2012 Available online 4 September 2012 Keywords: Up-flow biocatalyzed electrolysis reactor (UBER) Alizarin Yellow R (AYR) Bioelectrochemical systems (BESs) Aerobic bio-contact oxidation reactor (ABOR)
a b s t r a c t Azo dyes that consist of a large quantity of dye wastewater are toxic and persistent to biodegradation, while they should be removed before being discharged to water body. In this study, Alizarin Yellow R (AYR) as a model azo dye was decolorized in a combined bio-system of membrane-free, continuous up-flow bio-catalyzed electrolysis reactor (UBER) and subsequent aerobic bio-contact oxidation reactor (ABOR). With the supply of external power source 0.5 V in the UBER, AYR decolorization efficiency increased up to 94.8 ± 1.5%. Products formation efficiencies of p-phenylenediamine (PPD) and 5-aminosalicylic acid (5-ASA) were above 90% and 60%, respectively. Electron recovery efficiency based on AYR removal in cathode zone was nearly 100% at HRTs longer than 6 h. Relatively high concentration of AYR accumulated at higher AYR loading rates (>780 g m−3 d−1 ) likely inhibited acetate oxidation of anode-respiring bacteria on the anode, which decreased current density in the UBER; optimal AYR loading rate for the UBER was 680 g m−3 d−1 (HRT 2.5 h). The subsequent ABOR further improved effluent quality. Overall the Chroma decreased from 320 times to 80 times in the combined bio-system to meet the textile wastewater discharge standard II in China. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Azo dyes have been widely used in textiles, leathers, plastics, cosmetics, and food industry for tinting [1]. They make up approximately 70% of all dyestuffs used worldwide by weight [2], making them the largest group of synthetic colorants and the most common synthetic dyes released into the environment [3–5]. Azo dyes are characterized by containing one or more azo groups ( N N ), with most of them being in red, orange or yellow color with a high Chroma. Azo dye-containing wastewater typically leads to sever esthetic problems and obstructs light penetration and oxygen transfer into water bodies [6]. Thus, the wastewater is usually toxic and mutagenic [7–10], and causes a serious environmental problem and public health concern.
∗ Corresponding author. Tel.: +86 451 86282195; fax: +86 451 86282195. E-mail addresses: [email protected], [email protected] (A.-J. Wang). 0304-3894/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2012.08.072
Physical–chemical methods that include adsorption, coagulation and advanced oxidation have been used to remove the dyes in wastewaters successfully [11–14], but there are some challenges, such as high costs of reagents or large sludge formation. It also needs additional steps for solvent extraction to separate pollutants from adsorption materials, which causes increase of operating costs and system complexity [15,16]. In comparison, biological reductive methods are attractive, due to less expense and high decolorization efficiency of azo dyes; azo dyes are typically recalcitrant to microbial aerobic oxidation. Anaerobic treatment processes are able to reductively cleave azo linkages and form aromatic amines, which result in decolorization [17]. However, anaerobic reduction processes are usually slow and requires an exogenous electron donor for reducing azo dyes [18,19]. Bio-electrochemical systems (BESs), one of the anaerobic biological processes, have been studied to reductively transform several contaminants into less toxic forms (e.g., nitrobenzene, copper (II), chloroethenes, 2-chlorophenol, X-ray contrast media, and azo dyes)
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on the abiotic cathode [19–29]. As microbial oxidation of organics (e.g., acetate) on an anode is able to provide the electron for cathodic reduction of toxic contaminants, both of energy consumption and electron donor requirements would be much less than conventional electrochemical or anaerobic biological processes [19,20]. Although significant enhancement of reduction efficiency may need to switch operation of BESs from power generation to power supply mode, BESs would be still economical, as compared to other anaerobic options [20]. Previous works employed two-chamber BESs using membrane that can be a serious bottleneck for energy losses [19,30]. These studies suggested that azo dyes were reduced to less toxic or readily degradable forms at abiotic cathode in two-chamber BESs [19,31]. However, there are no solid evidences on biodegradability or toxicity of daughter reductive products from azo dyes in these BESs. Those daughter products would be still toxic or recalcitrant to natural biodegradation in ecosystem, which means requirement of post-treatment [32]. There are several studies for improving reduction efficiency of azo dyes in BESs using anode biofilm in which anode-respiring bacteria utilize azo dyes as the electron acceptor [6,29]. However, this approach would not be efficient because of mass transport limitation in azo dyes from bulk to anode biofilm and their inhibition to anode-respiring bacteria [29]. BESs would reduce azo dyes into less toxic forms economically, but reductive byproducts accumulated by incomplete anaerobic reduction would be challenging. Membrane resistance in two-chamber BESs would attenuate economic benefits of BESs, due to significant energy losses. In this work we developed an economical BES lacking membrane (an up-flow biocatalyzed electrolysis reactor (UBER)), evaluated reduction efficiency of azo dyes, and characterized reductive daughter products in the UBER. We also assessed the effects of hydraulic retention time (HRT) on decolorization efficiency of azo dyes in the UBER. Then, we finally investigated aerobic degradation of reductive daughter products accumulated in the UBER using a subsequent aerobic biological oxidation reactor (ABOR).
2. Materials and methods 2.1. Bioreactor configurations Lab scale UBERs were constructed as a cylinder style (ID 4 cm × H 20 cm) with a total empty volume (TV) of 250 mL and an effective liquid volume of 180 mL after installation of cathodes and an anode (Fig. 1A). A carbon brush (ID 4.5 cm × L 4 cm, TOHOTENAX Co. Ltd., USA) was pre-colonized by electrochemical active bacteria in a microbial fuel cell as described in Wang et al. [23], and then was fixed on the upper portion of the UBER (1 cm below the outlet) as anode. Graphite granules (ID from 3.0 mm to 5.0 mm, Harbin Northern Electrical Carbon Co. Ltd., China) were filled in the bottom of the UBER as cathodes that were connected to external wires and a power supply using an inserted graphite rod (Harbin Northern Electrical Carbon Co. Ltd., China) as current collector. We did not introduce microorganisms into cathode zone to avoid microbial growth on cathodes because electrochemical reactions on the cathodes are mainly responsible for AYR reduction in the UBERs; microorganism growth on the surface of cathodes could attenuate electrochemical reduction of AYR. To mitigate the transport of microorganisms in the anode zone to the cathode we fed medium from the bottom to the top of the UBERs. Cathode granules also help preventing anodic microorganisms from moving to the cathode zone. Two plates with evenly spaced holes were installed at 2.0 cm above the bottom of the reactor and the top of cathode zone, respectively, to allow even distribution of medium flow. The volumes of anode and cathode zones were 45 cm−3 and 90 cm−3 ,
respectively. Three samples ports (SPs) were built at 1.0, 6.0 and 12.0 cm above the bottom (designated as SP1#, SP2# and SP3#) to monitor Alizarin Yellow R (ARY) and its reductive products in the UBER. Effluent from the UBER flowed to an aerobic biological oxidation reactor (ABOR) as post-treatment (Fig. 1B). A lab scale ABOR was constructed as a cylinder type (ID 10 cm × H 12 cm) with a total empty volume (TV) of 942 mL and an effective liquid volume of 600 mL after filling carriers (Fig. 1A). The carriers for supporting microbial growth in the aerobic reactor were made up of porous plastic fiber. They were cubic with the side of 1 cm. An air-diffuser was placed on the bottom of the ABOR for evenly providing air throughout the reactor. A final effluent was collected from a second settling tank of 400 mL (see Fig. 1B). 2.2. Operations Sodium acetate was used as sole electron donor for the UBER, and AYR was used as a representative to azo dyes that can be reduced on the surface of cathodes abiotically. The chemical composition of medium is: potassium phosphate (50 mM, pH 7.0), sodium acetate (1.0 g L−1 ), AYR (100 mg L−1 ), KCl (0.13 g L−1 ), NH4 Cl (0.31 g L−1 ,), Wolf’s trace element solution (0.5 ml L−1 ) and vitamin solution (0.5 ml L−1 ). We continuously fed the medium into the UBER with a peristaltic pump (BT100-1L, Longer Pump Co. Ltd., China). We changed hydraulic retention time (HRT) in the UBER from 8 h to 1.5 h, which correspond to AYR loading rate from 200 g m−3 TV d−1 to 900 g m−3 TV d−1 . Each HRT lasted about 10 days to allow steady state conditions. One UBER was operated in open circuit as control experiment, while the other UBERs were operated in closed circuit. This comparison test allowed us to separately evaluate cathodic AYR reduction for which anodic reactions provide electrons. The medium pH was adjusted to 7.0 with NaOH (1 mol L−1 ). A saturated calomel electrode (SCE, +247 m V vs. standard hydrogen electrode) (Shanghai Precision Scientific Instruments Co. Ltd., China) was placed between the anode and cathodes to measure electrode potentials; we reported electrode potential against SCE here. The UBER was operated with external resistance of 20 and an applied voltage to the UBER was 0.5 V using a DC power supply (IT6921, Itech Co. Ltd., USA). The anode and cathode rods, and the reference electrode were connected to a data acquisition (Keithley 2700, Keithley Co. Ltd., USA), which recorded electrode potentials and currents at every 10 min. Effluent from the UBER with HRT of 2.5 h was continuously fed into the ABOR in sequence using a peristaltic pump (BT100-1L, Longer Pump Co. Ltd., China) in a flow rate of 0.1 mL min−1 and the HRT of ABOR was maintained at 6 h. A final effluent was collected for chemical analyzes after a secondary settling tank (Fig. 1B). All bioreactors were operated at 25 ◦ C. 2.3. Analytical methods 2.3.1. Chemical analyzes Liquid samples taken from bioreactors were immediately filtered through a 0.22 m filter. AYR concentration was measured using a UV–vis spectrophotometer (Shimadzu UV2550, Japan) at a wavelength of 374 nm. The UV–vis absorption spectra of AYR and decolorized samples were recorded over a wavelength range from 190 nm to 900 nm with the spectrophotometry. Reduced products of AYR were analyzed using a liquid chromatography–mass spectrometer (LC–MS, ThermoFinnigan LCQ Deca XP Max LC/MS, Germany), which was equipped with an electrospray ionization source and operated in the positive/negative polarity mode. Separation was performed with a C18 column (5 m; 5 mm × 250 mm, Waters Co., USA) and mobile phase was methanol/H2 O (9:1) at a
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Fig. 1. (A) Schematic diagram of UBER. (B) Schematic diagram of the process of UBER coupled with ABOR.
flow rate of 1 mL min−1 . Then the confirmed products were quantified using a high performance liquid chromatography (HPLC, model e2695, Waters Co., USA) under the same condition as described above. The reduced products of AYR were detected by a UV detector at 288 nm in the HPLC. Two main reductive products of AYR were pphenylenediamine (PPD) and 5-aminosalicylic acid (5-ASA) in this study, and their retention times were 2.1 and 3.2 min, respectively, in the HPLC. AYR decolorization efficiencies in the UBER (DE) and cathode zone (DEcathode ), decolorization rate (DR: mol m−3 TV d−1 ) and products formation efficiency (FE) were calculated with the difference between influent and effluent concentrations and empty total volume (TV) of the UBER, according to Eqs. (1)–(4): DE =
Cin-AYR − Cef-AYR × 100% Cin-AYR
(1)
DEcathode = DR = FE =
Cin-AYR − Cef-cathode × 100% Cin-AYR
(Cin-AYR − Cef-AYR ) × QA × 1000 TV Cef-product Cin-AYR − Cef-AYR
× 100%
(2) (3) (4)
where Cin-AYR is influent AYR concentration, mM. Cef-AYR is effluent AYR concentration, mM. Cef-cathode is effluent AYR concentration in cathode zone, mM. QA is influent flow rate, m3 d−1 . Cef-product is effluent product concentration, mM. COD was measured according to standard method (APHA, 1998) [33]. Chroma was measured according to Chinese standard method (GB1190-85, China). All samples were filtered through filter paper prior to COD and Chroma measurements.
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Fig. 2. Cyclic voltammograms (scan rate: 10 mV/s) carried out on the cathode in a dual-chamber MFC.
2.3.2. Electrochemical monitoring and calculation Current was calculated from the external resistance using Ohm’s law. Electron recovery efficiency based on the cathode AYR reduction (ERE-AYR) was computed with Eq. (5): ERE-AYR =
n × (Cin-AYR − Ccathode-AYR ) × (QA × 103 )/(24 × 3600) × F × 100% I
(5)
where F is Faraday’s constant (96,485 C mol−1 e− ); I is the current (mA). Ccathode-AYR was the effluent AYR concentration of cathode zone (mM). n = 10, the theoretical number of electrons for the complete reduction of AYR to PPD (mol mol−1 ). 2.3.3. Cyclic voltammetry Cyclic voltammetry (CV) tests were carried out in a dualchamber abiotic cathode BES to determine the AYR redox potentials on the cathode using an Auto lab potentiostat (PGSTAT 30, EcoChemie). A graphite rod was the working electrode, SCE was the reference electrode, and the anode (carbon brush) was the counter electrode during CV experiments. Culture medium was used for anolyte and catholyte in CV tests where the catholyte only contained 100 mg L−1 AYR. The cathode and solution were kept under anaerobic conditions by purging the electrochemical solution with pure nitrogen gas (99.9%) for 30 min. In control CV tests, we used the culture medium lacking AYR for the catholyte.
Fig. 3. AYR decolorization efficiencies (DE) with the variation of HRT and the LR-ARY.
3.2. AYR decolorization in the continuous UBER During open circuit operation, decolorization efficiency (DE) of AYR decreased from 50.0 ± 6.1% to 18.9 ± 3.0% when the influent AYR loading rate increased from 200 g m−3 TV d−1 to 900 g m−3 TV d−1 (HRT decreased from 8 h to 1.5 h) (see Fig. 3). Under the open circuit condition, AYR would be reduced mainly by anaerobic microorganisms in the anode zone (DE of anode zone: 48–17.6%), as the decolorization efficiency in cathode zone was as trivial as less than 6%, which indicates that anaerobic microorganisms in anode zone primarily reduced AYR using acetate as the electron donor. In relatively long HRTs, the open circuit operation allowed moderate reduction of AYR, but its decolorization efficiency substantially dropped down to 18.9 ± 3.0% (DE 17.7% in the anode zone and 1.2% in the cathode zone) at shorter HRT. These results well agree to slow kinetic features of anaerobic reduction of azo dye [35]. During closed circuit operation, decolorization efficiency of AYR was significantly improved from 81.3 ± 5.9 to 94.8 ±1.7% at AYR loading rates of 200 g m−3 TV d−1 to 900 g m−3 TV d−1 (HRTs from 8 h to 1.5 h), as shown in Fig. 3. Decolorization efficiency of AYR in cathode zone was 87.0 ± 1.5% at HRT of 8 h, which was 57.2 ±1.8% even at HRT of 1.5 h; decolorization efficiency in anode zone was
3. Results and discussion 3.1. Cyclic voltammetry showed catalytic behavior for AYR reduction At the beginning of experiments, the reduction behavior of AYR on the cathode was tested using cyclic voltammetry (CV) in a dual-chamber BES. As shown in Fig. 2, a couple of distinguishable reduction peaks appeared on the cathode: the first peak in the forward scan at −0.30 V and the second in the forward scan at −0.75 V. Adversely, no catalytic behavior was found in control CV tests using culture medium lacking AYR, which indicates that soluble redox-mediators did not existed in the medium (Fig. 2). These results support that AYR could be reduced in BESs at cathode potential below −0.8 V. At least 0.4 V voltage should be supplied to BESs to facilitate AYR reduction on the cathode given that the anode potential of BESs would be stable at about −0.5 V vs. SCE [34]. The anode potential in the UBER was stable at −0.4 V during continuous experiments.
Fig. 4. Variation of current density, cathode and anode potentials with the decrease of HRT.
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increased from 7.5% to 24.1%. These results clearly indicate that AYR was primarily reduced in cathode zone in the UBER operated under closed circuit. Fig. 4 presents current density with HRT. Current density increased from 23.7 ±0.2 A m−3 to 36.8 ±0.02 A m−3 (total cathode volume) as HRT decreased from 8 h to 2.5 h. However, current density slightly dropped from HRT 2 h, which was
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34.3 ±1.4 A m−3 at HRT 1.5 h. A short HRT that increases acetateloading and AYR-loading rates can increase current density until substrate saturation [36,37], since current density is proportional to substrate-utilization rate of anode-respiring bacteria on the anode and AYR reduction on the cathode [36]. A complete reduction of AYR to amine requires 1.25 mol acetate per mole of AYR (0.26 g acetic acid per g AYR) (see Supplementary Information for details). In our
Fig. 5. (A). UV–vis scan of the influent and effluent samples of UBER when HRT was 2.5 h. (B) LC–MS ambipolar ion chromatogram of the effluent samples of UBER when HRT was 2.5 h. (C) HPLC chromatogram of the effluent samples of UBER when HRT was 2.5 h. (D) Concentration of different products and product formation efficiencies under different HRTs. (E) The possible AYR reduction pathway in UBER.
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study, acetate concentration is 10 times larger than AYR concentration, much higher than the stoichiometric ratio of 0.26 (acetate acid/AYR) for complete AYR reduction. Thus, the decrease of current density after HRT 2.5 h implies kinetic limitations on the cathode. Shorter HRT with higher AYR loading rate could improve cathodic reduction with current density increasing. Simultaneously, anode and cathode potentials were stable during this period, as illustrated in Fig. 5: anode potential −370 ± 9.6 to −350 ± 1.5 mV and cathode potential −850 ± 17.7 to −814.4 ± 7.6 mV. We found that the current density started to decrease from AYR loading rate of 680 g m−3 TV d−1 (HRT = 2.5 h). This was well consistent with a sharp drop of AYR decolorization efficiency in cathode zone (57.2 ±1.8%) at AYR loading rate of 780 g m−3 TV d−1 in HRT 2 h (see Fig. 3). Decolorization efficiency in anode zone was 24%. The anode and cathode potentials were also increased to −274 ± 1.1 mV and −739 ± 5.4 mV, respectively. As the effluent concentration of the reductive products of AYR (PPD and 5-ASA) was almost constant or even decrease, whereas 10% and 13% increase of AYR concentration was observed at HRT of 2 h and 1.5 h, respectively (Fig. 5C). The decreasing current density and increasing anode potential suggested that higher AYR loading rate (>680 g m−3 TV d−1 ) was beyond of the treatment capability of cathode. The cathodic effluent with more residual AYR flew into anode zone would seriously inhibit anode-respiring bacteria for acetate oxidation in the UBER. This also implies that AYR would be more toxic than its daughter products (PPD and 5-ASA) to anode-respiring bacteria. 3.3. Products analysis and electron recovery efficiency Fig. 5A shows UV absorbance spectra for AYR and effluent from the UBER. The absorbance band at 374 nm for AYR standard (25 mg L−1 ) indicates double azo bond ( N N ), which almost disappeared in UBER effluent. Instead, the effluent showed intensive absorbance at 318 nm that would indicate reductive products of AYR having aminobenzene. The products were further analyzed by LC–MS in positive polarity mode (Fig. 5B). The ambipolar MS results indicated that two main peaks emerged with m/z ratio of 108.84 and 153.89, respectively. The former one was in accordance with PPD adding a proton. The later one corresponded to 5-ASA without a hydroxyl. Thus, we concluded that the main products in AYR reduction were PPD and 5-ASA. The two main reductive products were quantified using HPLC. As shown in Fig. 5C, two distinctive peaks in UBER effluent were identified as PPD (2.09 min) and 5-ASA (3.05 min), respectively. Formation efficiencies of PPD and 5-ASA ranged from 92% to 100% and from 60% to 70%, respectively, with a variety of ARY loading rates (Fig. 5D). In theory, nitrogen double bond (azo group) in AYR is reduced to hydrazo compounds in the first step, which are reduced to 5-ASA (4 moles of electrons per mole of 5-ASA) and p-nitroaniline, respectively. In the next step, nitroaniline can be further reduced to PPD (10 moles of electrons per mole of PPD). Fig. 5E shows a schematic diagram of the feasible reduction pathway of AYR. During experiments we did not detect p-nitroaniline from UBER effluent and the formation efficiency of PPD was nearly 100%, which indicates that the reduction of nitro-aniline would readily occur in the UBER [38]. Stoichiometric equivalency essential for 5-ASA formation from AYR is equal to PPD formation. However, we found relatively low formation efficiency for 5-ASA in experiments, as compared to PPD (Fig. 5D), which would be due to instability of 5-ASA. Anaerobic bacteria are able to transform 5-ASA into simple acids (e.g., pyruvate, or malate) and ammonia [35,39]. Electron recovery efficiency based on AYR removal in cathode zone (ERE-AYR) was employed to evaluate the contribution of cathodic electrons for AYR reduction. As previous results, the
Fig. 6. UV–vis scan of the AYR solution of 25 mg L−1 and the effluent samples of UBER, ABOR and the second settling tank. (B) HPLC chromatogram of the effluent samples of UBER, ABOR and the second settling tank.
formation efficiency of PPD was nearly 100%, indicating that AYR in cathode zone was completely reduced and 10 mol electrons were accepted. Thus, the electron recovery efficiency based on the AYR removal in cathode zone could be calculated according to Eq. (5). ERE-AYRs were 96.8 ± 2.8% and 94.7 ±6.6%, respectively, for HRTs of 8 h and 6 h. These results support that cathodic AYR reduction was very effective and AYR reduction was mainly due to electrochemical reaction. However, ERE-AYR was higher than 100% at shorter HRT (<6 h), which implied that a small part of AYR was reduced by non-electrochemial process. Biological reduction was a probably alternative pathway for AYR removal. Although we attempted to mitigate biofilm formation in cathode zone by up-flow feed and graphite granules used for the cathode in the UBERs, some biofilm formation in cathode zone would be unavoidable during long operation period (longer than five months) and probably contributed to higher electron recovery efficiency than 100% (see Fig. S1 in Supplementary Information). Tracking electron distributions to electrochemical and biological reduction respectively in cathode zone is quite challenging, due to inaccessibility of microorganism in the cathodes. However, constant cathode potentials from −850 ± 17.7 mV to −814.4 ± 7.6 mV (except HRTs less than 2 h showing kinetic limitation on the cathodes) indirectly support that microbial growth in cathode zone would rarely affect electrochemical reduction of AYR. Further investigations are essential to quantify electrons for biological AYR reduction in the UBERs.
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3.4. UBER coupled with an aerobic bio-contact oxidation reactor
References
To further stabilize the reduction products of AYR from the UBER we subsequently oxidized these reduction products with an aerobic bio-contact oxidation reactor (ABOR). A UV–vis spectrophotometry revealed the degradation of reduction products of AYR in the ABOR, as shown in Fig. 6A. The peak at 318 nm that indicates AYR reduction products disappeared in ABOR effluent, which supports that the major reduction products of 5-ASA and PPD would be oxidized by aerobic bacteria. HPLC analyzes showed that there were no obviously peaks for PPD (2.09 min) and 5-ASA (3.05 min) in ABOR effluent (Fig. 6B). It was concluded that the main accumulated products of 5-ASA and PPD in the UBER would be oxidized to simple acids and alcohols that the HPLC cannot detect or carbon dioxide through the ABOR. COD concentrations in UBER effluent were 591 ± 72 mg L−1 (influent COD concentrations 890 ± 109 mg L−1 ), and they were decreased down to 175 ± 25 mg L−1 (COD removal efficiency 63 ± 6%) via aerobic oxidation in the subsequent ABOR. The Chroma decreased from 320 times to 80 times, which meet the textile wastewater discharge standard II in China. Therefore, AYR was efficiently removed in the combined process of the UBER with the ABOR.
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Wu, A membrane-free, continuously feeding, single chamber up-flow biocatalyzed electrolysis reactor for nitrobenzene reduction, J. Hazard. Mater. 199–200 (2012) 401–409. [24] H.C. Tao, M. Liang, N. Xu, J.R. Ni, W.M. Wu, A membrane-free baffled microbial fuel cell for cathodic reduction of Cu (II) with electricity generation, Bioresour. Technol. 102 (2011) 4774–4778. [25] S.T. Lohner, D. Becker, K.M. Mangold, A. Tiehm, Sequential reductive and oxidative biodegradation of chloroethenes stimulated in a coupled bioelectroprocess, Environ. Sci. Technol. (2011), doi:es200801r. [26] S.M. Strycharz, T.L. Woodard, J.P. Johnson, K.P. Nevin, R.A. Sanford, F.E. Loffler, D.R. Lovley, Graphite electrode as a sole electron donor for reductive dechlorination of tetrachlorethene by Geobacter lovleyi, Appl. Environ. Microbiol. 74 (2008) 5943–5947. [27] S.M. Strycharz, S.M. Gannon, A.R. Boles, A.E. Franks, K.P. Nevin, D.R. Lovley, Reductive dechlorination of 2-chlorophenol by Anaeromyxobacter dehalogenans with an electrode serving as the electron donor, Environ. Microbiol. Rep. 2 (2010) 289–294. [28] Y. Mu, J. Radjenovic, J.Y. Shen, R.A. Rozendal, K. Rabaey, J. Keller, Dehalogenation of iodinated X-ray contrast media in a bioelectrochemical process, Environ. Sci. Technol. 45 (2011) 782–788. [29] Y. Cao, Y. Hu, J. Sun, B. Hou, Explore various co-substrates for simultaneous electricity generation and Congo red degradation in air-cathode single-chamber microbial fuel cell, Bioelectrochemistry 79 (2010) 71–76. [30] H. Liu, R. Ramnarayanan, B.E. Logan, Production of electricity during wastewater treatment using a single chamber microbial fuel cell, Environ. Sci. Technol. 38 (2003) 2281–2285.
4. Conclusion This study demonstrated the feasibility of the UBER for decolorization of azo dye. AYR was efficiently removed (94.8 ± 1.5%) in a short HRT of 2 h with the AYR loading rate of 780 g m−3 TV d−1 . PPD and 5-ASA were major reductive products in the UBER. Electron recovery efficiency based on AYR removal in cathode zone indicated that AYR decolorization reaction was mainly due to electrochemical reduction on the cathode, while bio-reduction in cathode zone would slightly contribute to AYR removal. Remaining AYR at higher AYR loading rates in anode zone likely inhibited metabolism of anode-respiring bacteria, which decreased current density. Hence, optimal AYR loading rate for the UBER was 680 g m−3 TV d−1 (HRT 2.5 h and ARY concentration 100 mg L−1 ). The up-flow membrane-less design as well as the flexible placement of anode and cathode not only reduced the UBER’s internal resistance and applied voltage, but also would enhance its adaptability to various wastewaters and pollutants. When the UBER was coupled with ABOR, the decolorization byproducts of AYR, PPD, and 5-ASA were further mineralized. The integrated processes were able to treat azo dye wastewater treatment in more economical and efficient manners. Acknowledgments This research was supported by the National Natural Science Foundation of China (Grant No. 51078100; No. 51178140), by National Creative Research Groups Project (Grant No. 51121062), by the National High-tech R&D Program of China (863 Program, Grant No. 2011AA060904), by The Ph.D. Programs Foundation of Ministry of Education of China (20102302110055), by State Key Laboratory of Urban Water Resource and Environment, HIT (Grant No. HCK201019), and by Heilongjiang Science Foundation for Distinguished Young Scholars (Grant No. JC201003). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat. 2012.08.072.
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[31] L. Liu, F.B. Li, C.H. Feng, X.Z. Li, Microbial fuel cell with an azo-dye-feeding cathode, Appl. Microbiol. Biotechnol. 85 (2009) 175–183. [32] J. Sun, Z. Bi, B. Hou, Y.Q. Cao, Y.Y. Hu, Further treatment of decolorization liquid of azo dye coupled with increased power production using microbial fuel cell equipped with an aerobic biocathode, Water Res. 45 (2011) 283–291. [33] American Public Health Association (APHA), American Water Works Association and Water Environment Federation. (1998). Standard Methods for the Examination of Water and Wastewater, twentieth ed. Washington, DC. [34] B.E. Logan, Microbial Fuel Cell, John Wiley & Sons, Inc., Hoboken, New Jersey, USA, 2007. [35] E.R. Flores, M. Luijten, B.A. Donlon, G. Letinga, J.A. Field, Complete biodegradation of the azo dye azo disalicylate under anaerobic conditions, Environ. Sci. Technol. 31 (1997) 2098–2103.
[36] H.S. Lee, C.I. Torres, B. Rittmann, Effects of substrate diffusion and anode potential on kinetic parameters for anode-respiring bacteria, Environ. Sci. Technol. 43 (2009) 7571–7577. [37] C.I. Torres, A.K. Marcus, B.E. Rittmann, Kinetics of consumption of fermentation products by anode-respiring bacteria, Appl. Microbiol. Biotechnol. 77 (2007) 689–697. [38] D. Cui, F.Y. Kong, B. Liang, D. Liu, Q. Sun, A.J. Wang, Decolorization of azo dyes in a dual-chamber biocatalyzed electrolysis system seeding with enhanced inoculum, J. Environ. Analytic Toxicol. (2011), http://dx.doi.org/10.4172/2161 -0525.S3-001. [39] B. Donlon, F.E. Razo, E. Luijten, H. Swarts, G. Lettinga, J. Field, Detoxification and partial mineralization of the azo dye mordant orange 1 in a continuous upflow anaerobic sludge-blanket reactor, Appl. Microbiol. Biotechnol. 47 (1997) 83–90.
Contents lists available at SciVerse ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Azo dye removal in a membrane-free up-flow biocatalyzed electrolysis reactor coupled with an aerobic bio-contact oxidation reactor Dan Cui a , Yu-Qi Guo a , Hao-Yi Cheng a , Bin Liang a , Fan-Ying Kong a , Hyung-Sool Lee b , Ai-Jie Wang a,∗ a b
State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, No. 202 Haihe Road, Harbin 150090, PR China Department of Civil and Environmental Engineering, University of Waterloo, 200 University Avenue West Waterloo, Ontario, Canada N2L 3G1
h i g h l i g h t s A membrane-free up-flow biocatalyzed electrolysis reactor coupled with an aerobic bio-contact oxidation reactor was developed. Alizarin Yellow R as the mode of azo dyes was efficiently converted to p-phenylenediamine (PPD) and 5-aminosalicylic acid (5-ASA). PPD and 5-ASA were further oxidized in a bio-contact oxidation reactor. The mechanism of UBER for azo dye removal was discussed.
a r t i c l e
i n f o
Article history: Received 19 June 2012 Received in revised form 20 August 2012 Accepted 28 August 2012 Available online 4 September 2012 Keywords: Up-flow biocatalyzed electrolysis reactor (UBER) Alizarin Yellow R (AYR) Bioelectrochemical systems (BESs) Aerobic bio-contact oxidation reactor (ABOR)
a b s t r a c t Azo dyes that consist of a large quantity of dye wastewater are toxic and persistent to biodegradation, while they should be removed before being discharged to water body. In this study, Alizarin Yellow R (AYR) as a model azo dye was decolorized in a combined bio-system of membrane-free, continuous up-flow bio-catalyzed electrolysis reactor (UBER) and subsequent aerobic bio-contact oxidation reactor (ABOR). With the supply of external power source 0.5 V in the UBER, AYR decolorization efficiency increased up to 94.8 ± 1.5%. Products formation efficiencies of p-phenylenediamine (PPD) and 5-aminosalicylic acid (5-ASA) were above 90% and 60%, respectively. Electron recovery efficiency based on AYR removal in cathode zone was nearly 100% at HRTs longer than 6 h. Relatively high concentration of AYR accumulated at higher AYR loading rates (>780 g m−3 d−1 ) likely inhibited acetate oxidation of anode-respiring bacteria on the anode, which decreased current density in the UBER; optimal AYR loading rate for the UBER was 680 g m−3 d−1 (HRT 2.5 h). The subsequent ABOR further improved effluent quality. Overall the Chroma decreased from 320 times to 80 times in the combined bio-system to meet the textile wastewater discharge standard II in China. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Azo dyes have been widely used in textiles, leathers, plastics, cosmetics, and food industry for tinting [1]. They make up approximately 70% of all dyestuffs used worldwide by weight [2], making them the largest group of synthetic colorants and the most common synthetic dyes released into the environment [3–5]. Azo dyes are characterized by containing one or more azo groups ( N N ), with most of them being in red, orange or yellow color with a high Chroma. Azo dye-containing wastewater typically leads to sever esthetic problems and obstructs light penetration and oxygen transfer into water bodies [6]. Thus, the wastewater is usually toxic and mutagenic [7–10], and causes a serious environmental problem and public health concern.
∗ Corresponding author. Tel.: +86 451 86282195; fax: +86 451 86282195. E-mail addresses: [email protected], [email protected] (A.-J. Wang). 0304-3894/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2012.08.072
Physical–chemical methods that include adsorption, coagulation and advanced oxidation have been used to remove the dyes in wastewaters successfully [11–14], but there are some challenges, such as high costs of reagents or large sludge formation. It also needs additional steps for solvent extraction to separate pollutants from adsorption materials, which causes increase of operating costs and system complexity [15,16]. In comparison, biological reductive methods are attractive, due to less expense and high decolorization efficiency of azo dyes; azo dyes are typically recalcitrant to microbial aerobic oxidation. Anaerobic treatment processes are able to reductively cleave azo linkages and form aromatic amines, which result in decolorization [17]. However, anaerobic reduction processes are usually slow and requires an exogenous electron donor for reducing azo dyes [18,19]. Bio-electrochemical systems (BESs), one of the anaerobic biological processes, have been studied to reductively transform several contaminants into less toxic forms (e.g., nitrobenzene, copper (II), chloroethenes, 2-chlorophenol, X-ray contrast media, and azo dyes)
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on the abiotic cathode [19–29]. As microbial oxidation of organics (e.g., acetate) on an anode is able to provide the electron for cathodic reduction of toxic contaminants, both of energy consumption and electron donor requirements would be much less than conventional electrochemical or anaerobic biological processes [19,20]. Although significant enhancement of reduction efficiency may need to switch operation of BESs from power generation to power supply mode, BESs would be still economical, as compared to other anaerobic options [20]. Previous works employed two-chamber BESs using membrane that can be a serious bottleneck for energy losses [19,30]. These studies suggested that azo dyes were reduced to less toxic or readily degradable forms at abiotic cathode in two-chamber BESs [19,31]. However, there are no solid evidences on biodegradability or toxicity of daughter reductive products from azo dyes in these BESs. Those daughter products would be still toxic or recalcitrant to natural biodegradation in ecosystem, which means requirement of post-treatment [32]. There are several studies for improving reduction efficiency of azo dyes in BESs using anode biofilm in which anode-respiring bacteria utilize azo dyes as the electron acceptor [6,29]. However, this approach would not be efficient because of mass transport limitation in azo dyes from bulk to anode biofilm and their inhibition to anode-respiring bacteria [29]. BESs would reduce azo dyes into less toxic forms economically, but reductive byproducts accumulated by incomplete anaerobic reduction would be challenging. Membrane resistance in two-chamber BESs would attenuate economic benefits of BESs, due to significant energy losses. In this work we developed an economical BES lacking membrane (an up-flow biocatalyzed electrolysis reactor (UBER)), evaluated reduction efficiency of azo dyes, and characterized reductive daughter products in the UBER. We also assessed the effects of hydraulic retention time (HRT) on decolorization efficiency of azo dyes in the UBER. Then, we finally investigated aerobic degradation of reductive daughter products accumulated in the UBER using a subsequent aerobic biological oxidation reactor (ABOR).
2. Materials and methods 2.1. Bioreactor configurations Lab scale UBERs were constructed as a cylinder style (ID 4 cm × H 20 cm) with a total empty volume (TV) of 250 mL and an effective liquid volume of 180 mL after installation of cathodes and an anode (Fig. 1A). A carbon brush (ID 4.5 cm × L 4 cm, TOHOTENAX Co. Ltd., USA) was pre-colonized by electrochemical active bacteria in a microbial fuel cell as described in Wang et al. [23], and then was fixed on the upper portion of the UBER (1 cm below the outlet) as anode. Graphite granules (ID from 3.0 mm to 5.0 mm, Harbin Northern Electrical Carbon Co. Ltd., China) were filled in the bottom of the UBER as cathodes that were connected to external wires and a power supply using an inserted graphite rod (Harbin Northern Electrical Carbon Co. Ltd., China) as current collector. We did not introduce microorganisms into cathode zone to avoid microbial growth on cathodes because electrochemical reactions on the cathodes are mainly responsible for AYR reduction in the UBERs; microorganism growth on the surface of cathodes could attenuate electrochemical reduction of AYR. To mitigate the transport of microorganisms in the anode zone to the cathode we fed medium from the bottom to the top of the UBERs. Cathode granules also help preventing anodic microorganisms from moving to the cathode zone. Two plates with evenly spaced holes were installed at 2.0 cm above the bottom of the reactor and the top of cathode zone, respectively, to allow even distribution of medium flow. The volumes of anode and cathode zones were 45 cm−3 and 90 cm−3 ,
respectively. Three samples ports (SPs) were built at 1.0, 6.0 and 12.0 cm above the bottom (designated as SP1#, SP2# and SP3#) to monitor Alizarin Yellow R (ARY) and its reductive products in the UBER. Effluent from the UBER flowed to an aerobic biological oxidation reactor (ABOR) as post-treatment (Fig. 1B). A lab scale ABOR was constructed as a cylinder type (ID 10 cm × H 12 cm) with a total empty volume (TV) of 942 mL and an effective liquid volume of 600 mL after filling carriers (Fig. 1A). The carriers for supporting microbial growth in the aerobic reactor were made up of porous plastic fiber. They were cubic with the side of 1 cm. An air-diffuser was placed on the bottom of the ABOR for evenly providing air throughout the reactor. A final effluent was collected from a second settling tank of 400 mL (see Fig. 1B). 2.2. Operations Sodium acetate was used as sole electron donor for the UBER, and AYR was used as a representative to azo dyes that can be reduced on the surface of cathodes abiotically. The chemical composition of medium is: potassium phosphate (50 mM, pH 7.0), sodium acetate (1.0 g L−1 ), AYR (100 mg L−1 ), KCl (0.13 g L−1 ), NH4 Cl (0.31 g L−1 ,), Wolf’s trace element solution (0.5 ml L−1 ) and vitamin solution (0.5 ml L−1 ). We continuously fed the medium into the UBER with a peristaltic pump (BT100-1L, Longer Pump Co. Ltd., China). We changed hydraulic retention time (HRT) in the UBER from 8 h to 1.5 h, which correspond to AYR loading rate from 200 g m−3 TV d−1 to 900 g m−3 TV d−1 . Each HRT lasted about 10 days to allow steady state conditions. One UBER was operated in open circuit as control experiment, while the other UBERs were operated in closed circuit. This comparison test allowed us to separately evaluate cathodic AYR reduction for which anodic reactions provide electrons. The medium pH was adjusted to 7.0 with NaOH (1 mol L−1 ). A saturated calomel electrode (SCE, +247 m V vs. standard hydrogen electrode) (Shanghai Precision Scientific Instruments Co. Ltd., China) was placed between the anode and cathodes to measure electrode potentials; we reported electrode potential against SCE here. The UBER was operated with external resistance of 20 and an applied voltage to the UBER was 0.5 V using a DC power supply (IT6921, Itech Co. Ltd., USA). The anode and cathode rods, and the reference electrode were connected to a data acquisition (Keithley 2700, Keithley Co. Ltd., USA), which recorded electrode potentials and currents at every 10 min. Effluent from the UBER with HRT of 2.5 h was continuously fed into the ABOR in sequence using a peristaltic pump (BT100-1L, Longer Pump Co. Ltd., China) in a flow rate of 0.1 mL min−1 and the HRT of ABOR was maintained at 6 h. A final effluent was collected for chemical analyzes after a secondary settling tank (Fig. 1B). All bioreactors were operated at 25 ◦ C. 2.3. Analytical methods 2.3.1. Chemical analyzes Liquid samples taken from bioreactors were immediately filtered through a 0.22 m filter. AYR concentration was measured using a UV–vis spectrophotometer (Shimadzu UV2550, Japan) at a wavelength of 374 nm. The UV–vis absorption spectra of AYR and decolorized samples were recorded over a wavelength range from 190 nm to 900 nm with the spectrophotometry. Reduced products of AYR were analyzed using a liquid chromatography–mass spectrometer (LC–MS, ThermoFinnigan LCQ Deca XP Max LC/MS, Germany), which was equipped with an electrospray ionization source and operated in the positive/negative polarity mode. Separation was performed with a C18 column (5 m; 5 mm × 250 mm, Waters Co., USA) and mobile phase was methanol/H2 O (9:1) at a
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Fig. 1. (A) Schematic diagram of UBER. (B) Schematic diagram of the process of UBER coupled with ABOR.
flow rate of 1 mL min−1 . Then the confirmed products were quantified using a high performance liquid chromatography (HPLC, model e2695, Waters Co., USA) under the same condition as described above. The reduced products of AYR were detected by a UV detector at 288 nm in the HPLC. Two main reductive products of AYR were pphenylenediamine (PPD) and 5-aminosalicylic acid (5-ASA) in this study, and their retention times were 2.1 and 3.2 min, respectively, in the HPLC. AYR decolorization efficiencies in the UBER (DE) and cathode zone (DEcathode ), decolorization rate (DR: mol m−3 TV d−1 ) and products formation efficiency (FE) were calculated with the difference between influent and effluent concentrations and empty total volume (TV) of the UBER, according to Eqs. (1)–(4): DE =
Cin-AYR − Cef-AYR × 100% Cin-AYR
(1)
DEcathode = DR = FE =
Cin-AYR − Cef-cathode × 100% Cin-AYR
(Cin-AYR − Cef-AYR ) × QA × 1000 TV Cef-product Cin-AYR − Cef-AYR
× 100%
(2) (3) (4)
where Cin-AYR is influent AYR concentration, mM. Cef-AYR is effluent AYR concentration, mM. Cef-cathode is effluent AYR concentration in cathode zone, mM. QA is influent flow rate, m3 d−1 . Cef-product is effluent product concentration, mM. COD was measured according to standard method (APHA, 1998) [33]. Chroma was measured according to Chinese standard method (GB1190-85, China). All samples were filtered through filter paper prior to COD and Chroma measurements.
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Fig. 2. Cyclic voltammograms (scan rate: 10 mV/s) carried out on the cathode in a dual-chamber MFC.
2.3.2. Electrochemical monitoring and calculation Current was calculated from the external resistance using Ohm’s law. Electron recovery efficiency based on the cathode AYR reduction (ERE-AYR) was computed with Eq. (5): ERE-AYR =
n × (Cin-AYR − Ccathode-AYR ) × (QA × 103 )/(24 × 3600) × F × 100% I
(5)
where F is Faraday’s constant (96,485 C mol−1 e− ); I is the current (mA). Ccathode-AYR was the effluent AYR concentration of cathode zone (mM). n = 10, the theoretical number of electrons for the complete reduction of AYR to PPD (mol mol−1 ). 2.3.3. Cyclic voltammetry Cyclic voltammetry (CV) tests were carried out in a dualchamber abiotic cathode BES to determine the AYR redox potentials on the cathode using an Auto lab potentiostat (PGSTAT 30, EcoChemie). A graphite rod was the working electrode, SCE was the reference electrode, and the anode (carbon brush) was the counter electrode during CV experiments. Culture medium was used for anolyte and catholyte in CV tests where the catholyte only contained 100 mg L−1 AYR. The cathode and solution were kept under anaerobic conditions by purging the electrochemical solution with pure nitrogen gas (99.9%) for 30 min. In control CV tests, we used the culture medium lacking AYR for the catholyte.
Fig. 3. AYR decolorization efficiencies (DE) with the variation of HRT and the LR-ARY.
3.2. AYR decolorization in the continuous UBER During open circuit operation, decolorization efficiency (DE) of AYR decreased from 50.0 ± 6.1% to 18.9 ± 3.0% when the influent AYR loading rate increased from 200 g m−3 TV d−1 to 900 g m−3 TV d−1 (HRT decreased from 8 h to 1.5 h) (see Fig. 3). Under the open circuit condition, AYR would be reduced mainly by anaerobic microorganisms in the anode zone (DE of anode zone: 48–17.6%), as the decolorization efficiency in cathode zone was as trivial as less than 6%, which indicates that anaerobic microorganisms in anode zone primarily reduced AYR using acetate as the electron donor. In relatively long HRTs, the open circuit operation allowed moderate reduction of AYR, but its decolorization efficiency substantially dropped down to 18.9 ± 3.0% (DE 17.7% in the anode zone and 1.2% in the cathode zone) at shorter HRT. These results well agree to slow kinetic features of anaerobic reduction of azo dye [35]. During closed circuit operation, decolorization efficiency of AYR was significantly improved from 81.3 ± 5.9 to 94.8 ±1.7% at AYR loading rates of 200 g m−3 TV d−1 to 900 g m−3 TV d−1 (HRTs from 8 h to 1.5 h), as shown in Fig. 3. Decolorization efficiency of AYR in cathode zone was 87.0 ± 1.5% at HRT of 8 h, which was 57.2 ±1.8% even at HRT of 1.5 h; decolorization efficiency in anode zone was
3. Results and discussion 3.1. Cyclic voltammetry showed catalytic behavior for AYR reduction At the beginning of experiments, the reduction behavior of AYR on the cathode was tested using cyclic voltammetry (CV) in a dual-chamber BES. As shown in Fig. 2, a couple of distinguishable reduction peaks appeared on the cathode: the first peak in the forward scan at −0.30 V and the second in the forward scan at −0.75 V. Adversely, no catalytic behavior was found in control CV tests using culture medium lacking AYR, which indicates that soluble redox-mediators did not existed in the medium (Fig. 2). These results support that AYR could be reduced in BESs at cathode potential below −0.8 V. At least 0.4 V voltage should be supplied to BESs to facilitate AYR reduction on the cathode given that the anode potential of BESs would be stable at about −0.5 V vs. SCE [34]. The anode potential in the UBER was stable at −0.4 V during continuous experiments.
Fig. 4. Variation of current density, cathode and anode potentials with the decrease of HRT.
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increased from 7.5% to 24.1%. These results clearly indicate that AYR was primarily reduced in cathode zone in the UBER operated under closed circuit. Fig. 4 presents current density with HRT. Current density increased from 23.7 ±0.2 A m−3 to 36.8 ±0.02 A m−3 (total cathode volume) as HRT decreased from 8 h to 2.5 h. However, current density slightly dropped from HRT 2 h, which was
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34.3 ±1.4 A m−3 at HRT 1.5 h. A short HRT that increases acetateloading and AYR-loading rates can increase current density until substrate saturation [36,37], since current density is proportional to substrate-utilization rate of anode-respiring bacteria on the anode and AYR reduction on the cathode [36]. A complete reduction of AYR to amine requires 1.25 mol acetate per mole of AYR (0.26 g acetic acid per g AYR) (see Supplementary Information for details). In our
Fig. 5. (A). UV–vis scan of the influent and effluent samples of UBER when HRT was 2.5 h. (B) LC–MS ambipolar ion chromatogram of the effluent samples of UBER when HRT was 2.5 h. (C) HPLC chromatogram of the effluent samples of UBER when HRT was 2.5 h. (D) Concentration of different products and product formation efficiencies under different HRTs. (E) The possible AYR reduction pathway in UBER.
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study, acetate concentration is 10 times larger than AYR concentration, much higher than the stoichiometric ratio of 0.26 (acetate acid/AYR) for complete AYR reduction. Thus, the decrease of current density after HRT 2.5 h implies kinetic limitations on the cathode. Shorter HRT with higher AYR loading rate could improve cathodic reduction with current density increasing. Simultaneously, anode and cathode potentials were stable during this period, as illustrated in Fig. 5: anode potential −370 ± 9.6 to −350 ± 1.5 mV and cathode potential −850 ± 17.7 to −814.4 ± 7.6 mV. We found that the current density started to decrease from AYR loading rate of 680 g m−3 TV d−1 (HRT = 2.5 h). This was well consistent with a sharp drop of AYR decolorization efficiency in cathode zone (57.2 ±1.8%) at AYR loading rate of 780 g m−3 TV d−1 in HRT 2 h (see Fig. 3). Decolorization efficiency in anode zone was 24%. The anode and cathode potentials were also increased to −274 ± 1.1 mV and −739 ± 5.4 mV, respectively. As the effluent concentration of the reductive products of AYR (PPD and 5-ASA) was almost constant or even decrease, whereas 10% and 13% increase of AYR concentration was observed at HRT of 2 h and 1.5 h, respectively (Fig. 5C). The decreasing current density and increasing anode potential suggested that higher AYR loading rate (>680 g m−3 TV d−1 ) was beyond of the treatment capability of cathode. The cathodic effluent with more residual AYR flew into anode zone would seriously inhibit anode-respiring bacteria for acetate oxidation in the UBER. This also implies that AYR would be more toxic than its daughter products (PPD and 5-ASA) to anode-respiring bacteria. 3.3. Products analysis and electron recovery efficiency Fig. 5A shows UV absorbance spectra for AYR and effluent from the UBER. The absorbance band at 374 nm for AYR standard (25 mg L−1 ) indicates double azo bond ( N N ), which almost disappeared in UBER effluent. Instead, the effluent showed intensive absorbance at 318 nm that would indicate reductive products of AYR having aminobenzene. The products were further analyzed by LC–MS in positive polarity mode (Fig. 5B). The ambipolar MS results indicated that two main peaks emerged with m/z ratio of 108.84 and 153.89, respectively. The former one was in accordance with PPD adding a proton. The later one corresponded to 5-ASA without a hydroxyl. Thus, we concluded that the main products in AYR reduction were PPD and 5-ASA. The two main reductive products were quantified using HPLC. As shown in Fig. 5C, two distinctive peaks in UBER effluent were identified as PPD (2.09 min) and 5-ASA (3.05 min), respectively. Formation efficiencies of PPD and 5-ASA ranged from 92% to 100% and from 60% to 70%, respectively, with a variety of ARY loading rates (Fig. 5D). In theory, nitrogen double bond (azo group) in AYR is reduced to hydrazo compounds in the first step, which are reduced to 5-ASA (4 moles of electrons per mole of 5-ASA) and p-nitroaniline, respectively. In the next step, nitroaniline can be further reduced to PPD (10 moles of electrons per mole of PPD). Fig. 5E shows a schematic diagram of the feasible reduction pathway of AYR. During experiments we did not detect p-nitroaniline from UBER effluent and the formation efficiency of PPD was nearly 100%, which indicates that the reduction of nitro-aniline would readily occur in the UBER [38]. Stoichiometric equivalency essential for 5-ASA formation from AYR is equal to PPD formation. However, we found relatively low formation efficiency for 5-ASA in experiments, as compared to PPD (Fig. 5D), which would be due to instability of 5-ASA. Anaerobic bacteria are able to transform 5-ASA into simple acids (e.g., pyruvate, or malate) and ammonia [35,39]. Electron recovery efficiency based on AYR removal in cathode zone (ERE-AYR) was employed to evaluate the contribution of cathodic electrons for AYR reduction. As previous results, the
Fig. 6. UV–vis scan of the AYR solution of 25 mg L−1 and the effluent samples of UBER, ABOR and the second settling tank. (B) HPLC chromatogram of the effluent samples of UBER, ABOR and the second settling tank.
formation efficiency of PPD was nearly 100%, indicating that AYR in cathode zone was completely reduced and 10 mol electrons were accepted. Thus, the electron recovery efficiency based on the AYR removal in cathode zone could be calculated according to Eq. (5). ERE-AYRs were 96.8 ± 2.8% and 94.7 ±6.6%, respectively, for HRTs of 8 h and 6 h. These results support that cathodic AYR reduction was very effective and AYR reduction was mainly due to electrochemical reaction. However, ERE-AYR was higher than 100% at shorter HRT (<6 h), which implied that a small part of AYR was reduced by non-electrochemial process. Biological reduction was a probably alternative pathway for AYR removal. Although we attempted to mitigate biofilm formation in cathode zone by up-flow feed and graphite granules used for the cathode in the UBERs, some biofilm formation in cathode zone would be unavoidable during long operation period (longer than five months) and probably contributed to higher electron recovery efficiency than 100% (see Fig. S1 in Supplementary Information). Tracking electron distributions to electrochemical and biological reduction respectively in cathode zone is quite challenging, due to inaccessibility of microorganism in the cathodes. However, constant cathode potentials from −850 ± 17.7 mV to −814.4 ± 7.6 mV (except HRTs less than 2 h showing kinetic limitation on the cathodes) indirectly support that microbial growth in cathode zone would rarely affect electrochemical reduction of AYR. Further investigations are essential to quantify electrons for biological AYR reduction in the UBERs.
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3.4. UBER coupled with an aerobic bio-contact oxidation reactor
References
To further stabilize the reduction products of AYR from the UBER we subsequently oxidized these reduction products with an aerobic bio-contact oxidation reactor (ABOR). A UV–vis spectrophotometry revealed the degradation of reduction products of AYR in the ABOR, as shown in Fig. 6A. The peak at 318 nm that indicates AYR reduction products disappeared in ABOR effluent, which supports that the major reduction products of 5-ASA and PPD would be oxidized by aerobic bacteria. HPLC analyzes showed that there were no obviously peaks for PPD (2.09 min) and 5-ASA (3.05 min) in ABOR effluent (Fig. 6B). It was concluded that the main accumulated products of 5-ASA and PPD in the UBER would be oxidized to simple acids and alcohols that the HPLC cannot detect or carbon dioxide through the ABOR. COD concentrations in UBER effluent were 591 ± 72 mg L−1 (influent COD concentrations 890 ± 109 mg L−1 ), and they were decreased down to 175 ± 25 mg L−1 (COD removal efficiency 63 ± 6%) via aerobic oxidation in the subsequent ABOR. The Chroma decreased from 320 times to 80 times, which meet the textile wastewater discharge standard II in China. Therefore, AYR was efficiently removed in the combined process of the UBER with the ABOR.
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4. Conclusion This study demonstrated the feasibility of the UBER for decolorization of azo dye. AYR was efficiently removed (94.8 ± 1.5%) in a short HRT of 2 h with the AYR loading rate of 780 g m−3 TV d−1 . PPD and 5-ASA were major reductive products in the UBER. Electron recovery efficiency based on AYR removal in cathode zone indicated that AYR decolorization reaction was mainly due to electrochemical reduction on the cathode, while bio-reduction in cathode zone would slightly contribute to AYR removal. Remaining AYR at higher AYR loading rates in anode zone likely inhibited metabolism of anode-respiring bacteria, which decreased current density. Hence, optimal AYR loading rate for the UBER was 680 g m−3 TV d−1 (HRT 2.5 h and ARY concentration 100 mg L−1 ). The up-flow membrane-less design as well as the flexible placement of anode and cathode not only reduced the UBER’s internal resistance and applied voltage, but also would enhance its adaptability to various wastewaters and pollutants. When the UBER was coupled with ABOR, the decolorization byproducts of AYR, PPD, and 5-ASA were further mineralized. The integrated processes were able to treat azo dye wastewater treatment in more economical and efficient manners. Acknowledgments This research was supported by the National Natural Science Foundation of China (Grant No. 51078100; No. 51178140), by National Creative Research Groups Project (Grant No. 51121062), by the National High-tech R&D Program of China (863 Program, Grant No. 2011AA060904), by The Ph.D. Programs Foundation of Ministry of Education of China (20102302110055), by State Key Laboratory of Urban Water Resource and Environment, HIT (Grant No. HCK201019), and by Heilongjiang Science Foundation for Distinguished Young Scholars (Grant No. JC201003). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jhazmat. 2012.08.072.
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