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Removal and transformation of polycyclic aromatic hydrocarbons during electrocoagulation treatment of an industrial wastewater
Chemosphere 168 (2017) 58e64
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Removal and transformation of polycyclic aromatic hydrocarbons during electrocoagulation treatment of an industrial wastewater Chenhao Gong a, b, Gang Shen a, Haiou Huang a, *, Peiran He b, Zhongguo Zhang b, **, Baoqing Ma c a b c
School of Environment, Beijing Normal University, No. 19 Xinjiekouwai Street, Beijing, 100875, China Environmental Protection Research Institute of Light Industry, Beijing Academy of Science and Technology, No.1 Gao Li Zhang Road, Beijing, 100095, China Shandong Century Sunshine Paper Group Co., Ltd., Changle Economic Development Zone, Weifang, 266400, China
h i g h l i g h t s PAHs with 2e4 aromatic rings were found in a paper-making wastewater. Electrocoagulation removed up to 86% of PAHs in the wastewater. Removal of PAHs resulted from electro-oxidation and adsorption to coagulation flocs. Two-ring PAHs were produced from humic- and fulvic-like organics during EC treatment. Transformation of PAHs and other wastewater organics was confirmed by EEM.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 July 2016 Received in revised form 26 September 2016 Accepted 12 October 2016 Available online 22 October 2016
Polycyclic aromatic hydrocarbons (PAHs) are an important class of water pollutants because of their known ecological and human toxicity. Electrocoagulation (EC) is a promising technology for mitigating industrial wastewater pollution, but the removal and transformation of PAHs during EC treatment has not yet been understood. Therefore, a paper-making wastewater effluent (PMWW) was employed in this study to investigate the relationship between PAHs’ removal and transformation during EC treatment. The results show that 86% of PAHs were effectively removed not only by the electro-oxidation reactions, but also by adsorption onto Fe hydroxide flocs. The removal and transformation of PAHs were related to the number of rings in their structures. Some PAHs composed of two aromatic rings (e.g., naphthaline and dimethylnaphthalene) were produced from humic acid-like and fulvic acid-like organics in PMWW, while PAHs with three to four rings were degraded, thus being removed efficiently. Therefore, PAH transformation during EC treatment exerted double-sided effects on the removal of PAHs; the net effect appeared to be positive. Overall, this study revealed the existence and importance of PAH transformation during EC treatment and provided useful guidance for pulp and paper mills to improve the design and operation of wastewater treatment facilities. © 2016 Elsevier Ltd. All rights reserved.
Handling Editor: E. Brillas Keywords: Polycyclic aromatic hydrocarbons Electrocoagulation Transformation Excitation-emission matrix Electrochemical degradation
1. Introduction Industrial production and human activities create large quantities and varieties of chemicals that are causing serious environmental pollution. One class of these chemicals is polycyclic
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Huang), [email protected] (Z. Zhang). http://dx.doi.org/10.1016/j.chemosphere.2016.10.044 0045-6535/© 2016 Elsevier Ltd. All rights reserved.
aromatic hydrocarbons (PAHs). PAHs have mutagenic and carcinogenic effects due to the generation of covalent DNA adducts and oxidative DNA lesions. Moreover, they are persistent in the environment and have long-term adverse effects (Cao et al., 2010; Tian et al., 2009; Jung et al., 2010; Arp et al., 2011; Timoney and Lee, 2011). Thus, many countries have established regulations and control strategies for PAHs. For example, the United States Environmental Protection Agency has categorized 16 PAHs in the priority contaminants list based upon their identified adverse impacts towards environment and human health (Zhang et al., 2004). The European Union, EEC Directive 98/83/EC has also set a limit of
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0.1 ng L1 for total PAHs in the drinking water (EU, 1998). The sources of PAHs are complex and can be divided into natural and anthropogenic ones. The anthropogenic sources, such as coal and biomass combustion, vehicular emissions and industrial wastewater, are the main contributors of PAHs in aquatic environments (Boonyatumanond et al., 2006). Currently, a few studies have reported the occurrence of PAHs in industrial wastewaters like coking and landfill wastewater (Zhang et al., 2012; Haddaoui et al., 2016), but the fate of PAHs existed in paper-making wastewater (PMWW) and their transformation process during treatment are still poorly understood. PMWW contains complex mixtures of organic and inorganic substances characterized by high levels of chemical oxygen demand (COD) and total organic carbon (TOC). In addition, each paper-making mill consumes a large amount of water, ranging from 5 to 300 m3 per ton of pulp products, with an average-sized mill generating about 2000 m3 of effluents per day (Boroski et al., 2008; Zodi et al., 2011). However, existing effluent emission standards for organics in industrial wastewaters only focus on color, COD, and other aggregate parameters, while neglecting specific classes of organic pollutants, including PAHs that are toxic even at relatively low levels. This results in unregulated discharge of PAHs into the environment. Consequently, suitable technologies need to be identified to control the discharge of PAHs by pulp and paper industries. The electrocoagulation (EC) treatment has recently attracted considerable attention because of its increasing applications to industrial wastewater treatment (Mollah et al., 2001; Wang et al., 2009; Labanowski et al., 2010; Katal and Pahlavanzadeh, 2011). Previous studies have shown that EC is an effective technique to remove organic matters from pulp and paper effluents by coagulation, redox reaction, and flotation (Ugurlu et al., 2008; Katal and Pahlavanzadeh, 2011; Zodi et al., 2011). During the EC process, iron or aluminum in the sacrificial anode is oxidized into divalent or trivalent metal ions, thereby dissolving into the solution and reacting with water to form metal hydroxides, such as Al(OH)3, Fe(OH)2 and Fe(OH)3. These metal hydroxide flocs are effective at removing aqueous pollutants either by complexation or electrostatic attraction, followed by precipitation. In addition, the oxidation reactions at the anode and the reduction reactions at the cathode can transform and/or remove pollutants (Mollah et al., 2004; Hua et al., 2015). Moreover, hydrogen and oxygen bubbles produced at the cathode and the anode, respectively, may separate agglomerated particles from water by flotation or stripping of volatile organic pollutants into the air (Ugurlu et al., 2008). Previous studies showed that the removal of PAHs during EC treatment of PMWW is attributed to the degradation of acids, phenolic compounds, sugars and lignin derivatives. However, different characteristics of organic pollutants and their effects on PAHs removal have not been thoroughly described to date (Platt and Nystrom, 2004; Amat et al., 2005). Moreover, full-scale applications of EC treatment exhibit that the removal efficiency of EC decreases as organic pollutants degrade into low molecular weight compounds, thus increase in current density cannot maintain continuous total organic carbon (TOC) removal or COD removal (Can et al., 2003). These findings suggest the relevance of organic transformation to the performance of EC in removing organic pollutants. However, the specific effect of PAHs transformation on their removal during EC treatment has not been reported in the literature. Lack of information in this regard makes it difficult to design and operate EC facilities so as to minimize the release of PAHs into the environment from pulp and paper industries. Therefore, this study aimed to examine the removal and transformation of PAHs during EC treatment and determine the relationship between PAHs transformation and removal. Consequently,
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complementary analytical approaches were employed to determine the chemical composition of organic pollutants in PMWW and quantify PAHs contents in water and coagulation sludge. Batch EC experiments were also conducted to explore the main reaction mechanisms that occurred during the EC treatment process. The results demonstrate the capability of EC treatment in removing PAHs in PMWW by electrochemical reactions and adsorption onto Fe hydroxide flocs. However, the production of partially degraded organics, such as naphthaline and dimethylnaphthalene, needs to be addressed in order to ensure safe discharge or reuse of industrial wastewater. These findings provided valuable insights into the removal and transformation of PAHs during EC treatment of PMWW; it was also instructive for future improvement in water treatment technologies for efficient removal of PAHs in PMWW and other industrial wastewaters. 2. Materials and methods 2.1. Paper-making wastewater The PMWW used in this study was collected from the secondary sedimentation tank of a biological wastewater treatment facility in a pulp and paper factory located in Shandong province, China. All samples received in the lab were immediately prefiltered by using 1.2 mm glass-fiber filters (Whatman GF/C) to remove small particles and then stored in the dark at 4 C in the laboratory. Table SM-1 (Supplementary materials) summarizes the characteristics of the prefiltered water samples. 2.2. Electrocoagulation experiments The experimental set-up for electrocoagulation of the wastewater consisted of four electrodes, a reactor, and a power supply (Dahua, MC-100/5) (Fig. 1). The iron electrodes used in this study were 10 cm long, 4 cm wide, and 2 mm thick, and the anode and the cathode were wired to the side electrodes, separately. The distance between each electrode was maintained at 1 cm. Prior to each experiment, 1 L of the prefiltered wastewater was transferred into the reactor. The experimental conditions selected in this work were based on our previous pre-experiments (Fig. SM-1). After the onset of the experiment, 15 mL of treated samples were collected at regular time intervals from the reactor and then stored in separate beakers to allow separation of the coagulation sludge by gravity settling. Subsequently, the supernatant from each sample was passed through a flat-sheet acetate fiber membrane with a nominal pore size of 0.22 mm (Navigator/13e0.22) to eliminate the unsettled sludge. 2.3. Analytical methods The PAHs and other low molecular weight organics in the raw and treated wastewater were measured using a gas
Fig. 1. Schematic diagram of the experimental set-up for electrocoagulation treatment.
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chromatography coupled with mass spectrometry (GC/MS) system (Agilent, 7890GC/5977A MS). The samples were injected at a rate of 1 mL min1 and helium was used as the carrier gas at a constant flow rate of 1.2 mL min1. The oven program was started at 40 C and held for 4 min, increased at a rate of 10 C min1 up to 160 and held for 1 min, and increased at rate of 10 C min1 up to 280 C and held for 4 min, and increased at rate of 10 C min1 up to 300 C and held for 10 min. The ion sources of mass detection was kept at 275 C. Quantification was performed using a seven-point calibration curve established using hexane-based internal standard for each individual PAH. The R2 values of the PAH calibration curves were all greater than 0.99. Fluorescence excitation-emission matrix (EEM) spectroscopic analysis of water samples was performed using a Hitachi F-4600 Fluorescence Spectrophotometer. Excitation and emission slit widths were set to 5 nm and the photo-multiplier tube voltage was controlled at 700 V. The excitation spectrum was scanned from 200 to 500 nm and the corresponding emission spectrum was recorded from 220 to 600 nm. The size distribution of flocs formed during the EC process were determined by using a light diffraction particle sizer (Malvern Mastersizer, 2000); this instrument is capable of determining particle size in a range from 0.02 to 2000 mm via the analysis of forward scattered light. Liquid and solid samples were analyzed for total organic carbon content by using an Elementar Liqui TOC II analyzer. Fe concentrations were analyzed with an inductively coupled plasma-mass spectrometer (Agilent, 7700).
3. Results and discussion 3.1. Total organic carbon removal during the EC treatment The relationship between the organic matter removal and PAHs transformation in the EC treatment was first investigated with comparative EC experiments conducted at current densities of 10, 20, and 40 mA cm2, with constant initial pH, inter-electrode distance (1 cm), and area/volume (A/V) ratio (0.16 cm1). Increasing the current density led to steady increases in the TOC removal efficiencies after 30 min of treatment (Fig. 2). The highest TOC removal efficiency was 38.7% at a current density of 40 mA cm2, while efficiencies of 33.3% and 26.5% were obtained at current densities of 20 and 10 mA cm2, respectively.
Fig. 2. TOC removal obtained with different applied current densities for electrocoagulation of the paper-making wastewater. (initial pH ¼ 7.75, area/volume ratio ¼ 0.16 cm1, temperature ¼ 25 C).
According to the Faraday's law, Fe ions are released from anodes at a certain rates depending upon the treatment time and current density. Thus, at a higher current density, anodic dissolution of Fe would increase to a greater extent, resulting in more precipitates and higher TOC removal. Moreover, the applied current density determines the coagulant dosage and the generation of hydrogen and oxygen bubbles on the anode and cathode surfaces. Thus, increasing the current density will increase air bubble density and decrease the bubble size, which also enhances organic matter degradation (Can et al., 2003). 3.2. Presence and removal of PAHs in the water A variety of PAHs composed of up to four aromatic rings were found in the EC-treated effluent at a current density of 40 mA cm2 where TOC removal was the highest (Fig. 3). PAHs with more than four rings were not detected either in PMWW or in the treated effluent. In total, EC treatment removed 86% of PAHs found in PMWW. Among these compounds, naphthaline and dimethylnaphthalene were not present in PMWW before treatment, but were measured at respective concentrations of 5 and 3 ng L1 in the treated effluent. For compounds that were present in PMWW, the concentrations of dichloronaphtalene and acenaphthylene remained almost unchanged during the EC treatment. However, the concentrations of acenaphthene, anthracene, phenanthrene, fluorene and fluoranthene decreased continuously over the 30-min treatment, especially during the first 10 min. For example, the concentration of fluorene decreased by approximately 67% after the treatment. PAHs have also been found in other industrial wastewaters. For example, Zhang et al. (2012) identified 18 types of PAHs in a coking wastewater, treated effluent, treatment sludge and gas samples. The fate of PAHs during the treatment process was determined by calculating the mass balance of each pollutant. However, the relationship between PAH transformation and removal was not evaluated. 3.3. Fluorescence EEM analyses Based on the operationally defined fluorescence boundaries of organic matters (Fig. 4), five EEM peaks, corresponding to five different types of fluorophores, were identified in PMWW: (1) small aromatic compounds at Ex. 230e235 nm/Em 340e360 nm (peak 1), including ethylbenzene, naphthaline, and xylene (Wei et al., 2013); (2) humic acid-like compounds at Ex. 250e260 nm/ Em 420e440 nm (peak 2) (Coble, 1996); (3) protein tryptophan at Ex. 275e285 nm/Em 340e360 nm (peak 3) (Coble, 1996); (4) humic acid-like compounds with small amount of fulvic acid-like compounds at Ex. 275e300 nm/Em 420e440 nm (peak 4) (Baker and Curry, 2004); (5) fulvic acid-like compounds at Ex. 325e350 nm/ Em 420e440 nm (peak 5) (Baker, 2002; Wei et al., 2013). Significant changes in fluorescent intensity were observed for all EEM peaks after EC treatment, but differed in extents (Fig. 4). Reductions in different types of fluorophores were calculated by summing the individual peak intensities (Janhom et al., 2009). Peak 2 and 4 were completely removed, in 30 min and 20 min, respectively. Comparatively, peak 5 was also removed but in a slower rate than the above two peaks, and the removal efficiency reached 50% in 30 min. The removal efficiencies for the three peaks are in the following order: Peak 4 > Peak 2 > Peak 5. Unlike the organics for peaks 2, 4, and 5, small bicyclic aromatic compounds (peak 1) and protein tryptophan (peak 3) appeared to be recalcitrant for EC treatment. Fluorescent organics corresponding to the two peaks increased after 30 min of EC treatment, almost by 500% for peak 1 and approximately 200% for peak 3.
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also in soil and sediments (Vane et al., 2014; Wang et al., 2015). During EC treatment, the Fe hydroxide flocs were formed due to hydrolysis of ionic Fe species released from the electrode and settled to the bottom of the reactor. As described by the Faraday's Law (Eq. (1)), different amount of Fe can be released from the electrode by varying either the applied current or the treatment time:
m¼
56It zF
(1)
where m is the mass of Fe (g) generated at a specific current I (A) over a time interval of t (s), z is the number of electrons transferred per Fe atom, 56 (g mol1) is the atomic weight of Fe, and F is the Faraday's constant (96,486 C eq1). In realistic applications, the number of electrons transferred per Fe atom varies with solution pH and the presence of organic matters (Sasson et al., 2009). Therefore, the theoretical Fe concentration as a function of treatment time can be estimated in Fig. SM-2, by assuming Z is 2, i.e., Fe2þ is the dominant species released from the electrode. During the experimental study, the Fe concentrations increased from 0 to 2439 mg L1, which were slightly lower than the theoretical values (Fig. SM-2). In the EC treatment, the pH value will increase along with the experiment time, at higher pH, the real iron dissolution could be lower than the value calculated by Faraday's law, indicating that there are other reactions (apart from iron dissolution) that are occurring simultaneously near the anode (Sasson et al., 2009), and this result was in conformity with findings reported by Harif (Harif et al., 2012). Water samples were collected from the reactor during EC treatment at a current density of 40 mA cm2 and analyzed for particle size distribution as a function of treatment time (Fig. 5). The size of the flocs formed during the first 5 min of EC treatment was mainly smaller than 10 mm (Fig. 5a). In comparison, the size of particles in PMWW was primarily around 0.89 mm. The increase in treatment time expanded the range of floc size distribution and caused the modal diameter to increase from 8.6 mm at 5 min to larger than 100 mm at 30 min of treatment (Fig. 5b). The broadening size distribution and increasing modal diameter of coagulation flocs were closely related to continuous formation and agglomeration of Fe hydroxide, since increasing amount of Fe were dissolved as measured previously. 3.5. PAHs in the coagulation flocs Fig. 3. Variations in PAH concentrations with EC treatment time: (a) Naphthaline and Dimethylnaphthalene, (b) Dichloronaphtalene and Acenaphthylene. (c) Acenaphthene, Anthracene, Phenanthrene, Fluorene and Fluoranthene. (initial pH ¼ 7.75, current density ¼ 40 mA cm2, area/volume ratio ¼ 0.16 cm1, temperature ¼ 25 C).
Based on the EEM results, EC treatment is capable of removing humic acid-like and fulvic acid-like fractions of the organics in PMWW (peak 2, 4, 5), leading to noticeable TOC removal (Fig. 2). However, the TOC removal was incomplete, possibly due to an increase in the concentrations of partially degraded organics, including small PAH compounds such as naphthaline and dimethylnaphthalene (Table 1). Formation of these small molecular weight organics should result from partial degradation of humiclike and fulvic-like organics as indicated by disappearance of related fluorophores (Fig. 4). 3.4. Formation of Fe hydroxide flocs during the EC treatment PAHs are known to exist not only in the water bodies and air, but
Particle size, specific surface area and amount of adsorbent are key factors for adsorption of PAHs (Alcantata et al., 2009). In this study, the amount of Fe hydroxide flocs increased with increasing time for EC treatment; this should have provided greater amount of adsorbents for PAH adsorption (Amstaetter et al., 2012). The surface area of Fe hydroxide flocs was not measured in this study, but a previous study found that the specific surface area of freshly prepared hydrous ferric oxide was 5500 ± 170 m2 g1 as determined by the dye adsorption method (Imre, 2008). Therefore, the large surface area of Fe hydroxide flocs should play an important role in the adsorption of PAHs and other organics during the EC treatment. In order to validate this assumption, the flocs formed during the EC treatment at a current density of 40 mA cm2 was collected after 30 min of treatment and analyzed for TOC and PAH concentrations. Based upon the mass balance for TOC, approximately 14.3 mg (27.1% of the TOC removed from the PMWW) of organics was transferred from the liquid phase into the flocs. These organics included five types of PAHs: naphthaline, dimethylnaphthalene, acenaphthene, anthracene, and phenanthrene, as measured by GC/
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Fig. 4. Fluorescence excitation emission matrix spectra of the wastewater effluents treated with EC for 0 min, 10 min, 20 min, and 30 min. P1, P2, P3, P4, P5 represent different peaks in the spectra.
Table 1 Organic pollutants in the raw and electrocoagulation-treated wastewater, identified by gas chromatography-mass spectrometry. Compounds
Raw water (mg L1)
Treated water (mg L1)
Removal () or increased (þ) percentage (%)
Attribution
Hexachlorobutadiene Dibutyl phthalate Pentadecanoic acid Diethylene glycol dibenzoate 1, 2 Benzenedicarbonic acid, dioctyl ester 1, 2,4-Trimethylbenzene 1, 3-Dichlorobenzene Nitrobenzene Ethylbenzene
20 38 27 24 38
6 6 5 9 5
70 84 81 63 86
Acid, esters, and long-chain organics containing benzene ring
1 2 3 4
12 9 15 20
þ1100 þ350 þ400 þ400
Peak 1 (EEM)
MS (Fig. 6). This finding is consistent with the aforementioned literature results.
3.6. Relationship between organic removal and PAH transformation EC treatment and organic characterization results suggest the occurrence of chemical transformation among different organics in PMWW (Fig. 4 and Table 1), coincident with significant decreases in TOC (Fig. 2) and total PAH concentration. Based upon the changes in concentration during EC treatment, the organic compounds identified in PMWW can be classified into two categories: (1) easily degradable compounds (alkene, ester and some long chains organics containing benzene rings), and (2) aromatic compounds and small compounds corresponding to peak 1 of the EEM diagram (Fig. 4). There was a noticeable reduction in the easily degradable compounds, with removal efficiencies up to 86% (Table 1), as a result of the reactions caused by the EC treatment. For chemicals related to peak 1 of the EEM diagram (Fig. 4), their concentrations
were noticeably increased after EC treatment, to a maximum level of 1100% (Table 1). In summary, PAH transformation, as well as the transformation of other organics, may either promote or reduce the removal efficiencies of PAHs by EC treatment. The mechanisms for the formation of PAHs in liquid phase during the EC treatment is herein proposed: the humic-like and fulvic-like organics were firstly degraded into small organics, including small PAH compounds, and accumulated in the treated effluent; among these organics, the easily degradable compounds containing benzene rings were transformed into naphthaline and dimethylnaphthalene by the oxidation effect of EC treatment.
4. Conclusions This study systematically investigated the removal and transformation of PAHs in a paper-making wastewater effluent by electrocoagulation. It was found that EC treatment effectively
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the treated effluent and the coagulation sludge in order to mitigate environmental pollution. These findings provide important instructions for paper-making factories to improve the design and operation of wastewater treatment and reuse facilities. Acknowledgement The authors greatly appreciate the financial support of the Special Funds for Technological Development of Research Institutes from the Ministry of Science and Technology of China (2013EG111129), the Program for Youth Scientists of the Beijing Academy of Science and Technology (201505), the 2016 BJAST Reform and Development Program and the Special Fund of the State Joint Key Laboratory for Environmental Simulation and Pollution Control (270403GK). Appendix A. Supplementary data
Fig. 5. Changes in particle size distribution during electrocoagulation of the papermaking wastewater: (a) first 5 min and (b) the remaining 25 min (current density: 40 mA cm2, initial pH: 7.75, A/V: 0.16 cm1, temperature: 25 C).
Fig. 6. The concentrations of PAHs measured in the flocs formed during EC treatment.
removed a total of 86% of PAHs by mass in the effluent. Transformation of PAHs had double-sided consequences on their removals during EC treatment. On the one hand, PAHs possessing three or four aromatic rings, including acenaphthene, anthracene, phenanthrene, fluorene and fluoranthene were significantly removed from the effluent, by transforming into smaller molecules or being mineralized (measured as TOC decreases). On the other hand, PAHs with two aromatic rings, including naphthaline and dimethylnaphthalene were generated by degradation of humic-like and fulvic-like organics, resulting in increased concentrations after EC treatment. In addition to chemical transformation, PAHs, including naphthaline, dimethylnaphthalene, acenaphthene, anthracene, and phenanthrene were also detected in the settled coagulation flocs. This suggests that Fe hydroxide flocs produced during EC process served as adsorbents for PAHs in the effluent. Overall, this study demonstrates that EC treatment is effective for the removal of many PAH compounds contained in papermaking wastewater. However, partial degradation of humic and fulvic substances may produce 2-ring PAH compounds. Therefore, post-treatment is needed to remove residual PAH compounds from
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Removal and transformation of polycyclic aromatic hydrocarbons during electrocoagulation treatment of an industrial wastewater Chenhao Gong a, b, Gang Shen a, Haiou Huang a, *, Peiran He b, Zhongguo Zhang b, **, Baoqing Ma c a b c
School of Environment, Beijing Normal University, No. 19 Xinjiekouwai Street, Beijing, 100875, China Environmental Protection Research Institute of Light Industry, Beijing Academy of Science and Technology, No.1 Gao Li Zhang Road, Beijing, 100095, China Shandong Century Sunshine Paper Group Co., Ltd., Changle Economic Development Zone, Weifang, 266400, China
h i g h l i g h t s PAHs with 2e4 aromatic rings were found in a paper-making wastewater. Electrocoagulation removed up to 86% of PAHs in the wastewater. Removal of PAHs resulted from electro-oxidation and adsorption to coagulation flocs. Two-ring PAHs were produced from humic- and fulvic-like organics during EC treatment. Transformation of PAHs and other wastewater organics was confirmed by EEM.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 July 2016 Received in revised form 26 September 2016 Accepted 12 October 2016 Available online 22 October 2016
Polycyclic aromatic hydrocarbons (PAHs) are an important class of water pollutants because of their known ecological and human toxicity. Electrocoagulation (EC) is a promising technology for mitigating industrial wastewater pollution, but the removal and transformation of PAHs during EC treatment has not yet been understood. Therefore, a paper-making wastewater effluent (PMWW) was employed in this study to investigate the relationship between PAHs’ removal and transformation during EC treatment. The results show that 86% of PAHs were effectively removed not only by the electro-oxidation reactions, but also by adsorption onto Fe hydroxide flocs. The removal and transformation of PAHs were related to the number of rings in their structures. Some PAHs composed of two aromatic rings (e.g., naphthaline and dimethylnaphthalene) were produced from humic acid-like and fulvic acid-like organics in PMWW, while PAHs with three to four rings were degraded, thus being removed efficiently. Therefore, PAH transformation during EC treatment exerted double-sided effects on the removal of PAHs; the net effect appeared to be positive. Overall, this study revealed the existence and importance of PAH transformation during EC treatment and provided useful guidance for pulp and paper mills to improve the design and operation of wastewater treatment facilities. © 2016 Elsevier Ltd. All rights reserved.
Handling Editor: E. Brillas Keywords: Polycyclic aromatic hydrocarbons Electrocoagulation Transformation Excitation-emission matrix Electrochemical degradation
1. Introduction Industrial production and human activities create large quantities and varieties of chemicals that are causing serious environmental pollution. One class of these chemicals is polycyclic
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Huang), [email protected] (Z. Zhang). http://dx.doi.org/10.1016/j.chemosphere.2016.10.044 0045-6535/© 2016 Elsevier Ltd. All rights reserved.
aromatic hydrocarbons (PAHs). PAHs have mutagenic and carcinogenic effects due to the generation of covalent DNA adducts and oxidative DNA lesions. Moreover, they are persistent in the environment and have long-term adverse effects (Cao et al., 2010; Tian et al., 2009; Jung et al., 2010; Arp et al., 2011; Timoney and Lee, 2011). Thus, many countries have established regulations and control strategies for PAHs. For example, the United States Environmental Protection Agency has categorized 16 PAHs in the priority contaminants list based upon their identified adverse impacts towards environment and human health (Zhang et al., 2004). The European Union, EEC Directive 98/83/EC has also set a limit of
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0.1 ng L1 for total PAHs in the drinking water (EU, 1998). The sources of PAHs are complex and can be divided into natural and anthropogenic ones. The anthropogenic sources, such as coal and biomass combustion, vehicular emissions and industrial wastewater, are the main contributors of PAHs in aquatic environments (Boonyatumanond et al., 2006). Currently, a few studies have reported the occurrence of PAHs in industrial wastewaters like coking and landfill wastewater (Zhang et al., 2012; Haddaoui et al., 2016), but the fate of PAHs existed in paper-making wastewater (PMWW) and their transformation process during treatment are still poorly understood. PMWW contains complex mixtures of organic and inorganic substances characterized by high levels of chemical oxygen demand (COD) and total organic carbon (TOC). In addition, each paper-making mill consumes a large amount of water, ranging from 5 to 300 m3 per ton of pulp products, with an average-sized mill generating about 2000 m3 of effluents per day (Boroski et al., 2008; Zodi et al., 2011). However, existing effluent emission standards for organics in industrial wastewaters only focus on color, COD, and other aggregate parameters, while neglecting specific classes of organic pollutants, including PAHs that are toxic even at relatively low levels. This results in unregulated discharge of PAHs into the environment. Consequently, suitable technologies need to be identified to control the discharge of PAHs by pulp and paper industries. The electrocoagulation (EC) treatment has recently attracted considerable attention because of its increasing applications to industrial wastewater treatment (Mollah et al., 2001; Wang et al., 2009; Labanowski et al., 2010; Katal and Pahlavanzadeh, 2011). Previous studies have shown that EC is an effective technique to remove organic matters from pulp and paper effluents by coagulation, redox reaction, and flotation (Ugurlu et al., 2008; Katal and Pahlavanzadeh, 2011; Zodi et al., 2011). During the EC process, iron or aluminum in the sacrificial anode is oxidized into divalent or trivalent metal ions, thereby dissolving into the solution and reacting with water to form metal hydroxides, such as Al(OH)3, Fe(OH)2 and Fe(OH)3. These metal hydroxide flocs are effective at removing aqueous pollutants either by complexation or electrostatic attraction, followed by precipitation. In addition, the oxidation reactions at the anode and the reduction reactions at the cathode can transform and/or remove pollutants (Mollah et al., 2004; Hua et al., 2015). Moreover, hydrogen and oxygen bubbles produced at the cathode and the anode, respectively, may separate agglomerated particles from water by flotation or stripping of volatile organic pollutants into the air (Ugurlu et al., 2008). Previous studies showed that the removal of PAHs during EC treatment of PMWW is attributed to the degradation of acids, phenolic compounds, sugars and lignin derivatives. However, different characteristics of organic pollutants and their effects on PAHs removal have not been thoroughly described to date (Platt and Nystrom, 2004; Amat et al., 2005). Moreover, full-scale applications of EC treatment exhibit that the removal efficiency of EC decreases as organic pollutants degrade into low molecular weight compounds, thus increase in current density cannot maintain continuous total organic carbon (TOC) removal or COD removal (Can et al., 2003). These findings suggest the relevance of organic transformation to the performance of EC in removing organic pollutants. However, the specific effect of PAHs transformation on their removal during EC treatment has not been reported in the literature. Lack of information in this regard makes it difficult to design and operate EC facilities so as to minimize the release of PAHs into the environment from pulp and paper industries. Therefore, this study aimed to examine the removal and transformation of PAHs during EC treatment and determine the relationship between PAHs transformation and removal. Consequently,
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complementary analytical approaches were employed to determine the chemical composition of organic pollutants in PMWW and quantify PAHs contents in water and coagulation sludge. Batch EC experiments were also conducted to explore the main reaction mechanisms that occurred during the EC treatment process. The results demonstrate the capability of EC treatment in removing PAHs in PMWW by electrochemical reactions and adsorption onto Fe hydroxide flocs. However, the production of partially degraded organics, such as naphthaline and dimethylnaphthalene, needs to be addressed in order to ensure safe discharge or reuse of industrial wastewater. These findings provided valuable insights into the removal and transformation of PAHs during EC treatment of PMWW; it was also instructive for future improvement in water treatment technologies for efficient removal of PAHs in PMWW and other industrial wastewaters. 2. Materials and methods 2.1. Paper-making wastewater The PMWW used in this study was collected from the secondary sedimentation tank of a biological wastewater treatment facility in a pulp and paper factory located in Shandong province, China. All samples received in the lab were immediately prefiltered by using 1.2 mm glass-fiber filters (Whatman GF/C) to remove small particles and then stored in the dark at 4 C in the laboratory. Table SM-1 (Supplementary materials) summarizes the characteristics of the prefiltered water samples. 2.2. Electrocoagulation experiments The experimental set-up for electrocoagulation of the wastewater consisted of four electrodes, a reactor, and a power supply (Dahua, MC-100/5) (Fig. 1). The iron electrodes used in this study were 10 cm long, 4 cm wide, and 2 mm thick, and the anode and the cathode were wired to the side electrodes, separately. The distance between each electrode was maintained at 1 cm. Prior to each experiment, 1 L of the prefiltered wastewater was transferred into the reactor. The experimental conditions selected in this work were based on our previous pre-experiments (Fig. SM-1). After the onset of the experiment, 15 mL of treated samples were collected at regular time intervals from the reactor and then stored in separate beakers to allow separation of the coagulation sludge by gravity settling. Subsequently, the supernatant from each sample was passed through a flat-sheet acetate fiber membrane with a nominal pore size of 0.22 mm (Navigator/13e0.22) to eliminate the unsettled sludge. 2.3. Analytical methods The PAHs and other low molecular weight organics in the raw and treated wastewater were measured using a gas
Fig. 1. Schematic diagram of the experimental set-up for electrocoagulation treatment.
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chromatography coupled with mass spectrometry (GC/MS) system (Agilent, 7890GC/5977A MS). The samples were injected at a rate of 1 mL min1 and helium was used as the carrier gas at a constant flow rate of 1.2 mL min1. The oven program was started at 40 C and held for 4 min, increased at a rate of 10 C min1 up to 160 and held for 1 min, and increased at rate of 10 C min1 up to 280 C and held for 4 min, and increased at rate of 10 C min1 up to 300 C and held for 10 min. The ion sources of mass detection was kept at 275 C. Quantification was performed using a seven-point calibration curve established using hexane-based internal standard for each individual PAH. The R2 values of the PAH calibration curves were all greater than 0.99. Fluorescence excitation-emission matrix (EEM) spectroscopic analysis of water samples was performed using a Hitachi F-4600 Fluorescence Spectrophotometer. Excitation and emission slit widths were set to 5 nm and the photo-multiplier tube voltage was controlled at 700 V. The excitation spectrum was scanned from 200 to 500 nm and the corresponding emission spectrum was recorded from 220 to 600 nm. The size distribution of flocs formed during the EC process were determined by using a light diffraction particle sizer (Malvern Mastersizer, 2000); this instrument is capable of determining particle size in a range from 0.02 to 2000 mm via the analysis of forward scattered light. Liquid and solid samples were analyzed for total organic carbon content by using an Elementar Liqui TOC II analyzer. Fe concentrations were analyzed with an inductively coupled plasma-mass spectrometer (Agilent, 7700).
3. Results and discussion 3.1. Total organic carbon removal during the EC treatment The relationship between the organic matter removal and PAHs transformation in the EC treatment was first investigated with comparative EC experiments conducted at current densities of 10, 20, and 40 mA cm2, with constant initial pH, inter-electrode distance (1 cm), and area/volume (A/V) ratio (0.16 cm1). Increasing the current density led to steady increases in the TOC removal efficiencies after 30 min of treatment (Fig. 2). The highest TOC removal efficiency was 38.7% at a current density of 40 mA cm2, while efficiencies of 33.3% and 26.5% were obtained at current densities of 20 and 10 mA cm2, respectively.
Fig. 2. TOC removal obtained with different applied current densities for electrocoagulation of the paper-making wastewater. (initial pH ¼ 7.75, area/volume ratio ¼ 0.16 cm1, temperature ¼ 25 C).
According to the Faraday's law, Fe ions are released from anodes at a certain rates depending upon the treatment time and current density. Thus, at a higher current density, anodic dissolution of Fe would increase to a greater extent, resulting in more precipitates and higher TOC removal. Moreover, the applied current density determines the coagulant dosage and the generation of hydrogen and oxygen bubbles on the anode and cathode surfaces. Thus, increasing the current density will increase air bubble density and decrease the bubble size, which also enhances organic matter degradation (Can et al., 2003). 3.2. Presence and removal of PAHs in the water A variety of PAHs composed of up to four aromatic rings were found in the EC-treated effluent at a current density of 40 mA cm2 where TOC removal was the highest (Fig. 3). PAHs with more than four rings were not detected either in PMWW or in the treated effluent. In total, EC treatment removed 86% of PAHs found in PMWW. Among these compounds, naphthaline and dimethylnaphthalene were not present in PMWW before treatment, but were measured at respective concentrations of 5 and 3 ng L1 in the treated effluent. For compounds that were present in PMWW, the concentrations of dichloronaphtalene and acenaphthylene remained almost unchanged during the EC treatment. However, the concentrations of acenaphthene, anthracene, phenanthrene, fluorene and fluoranthene decreased continuously over the 30-min treatment, especially during the first 10 min. For example, the concentration of fluorene decreased by approximately 67% after the treatment. PAHs have also been found in other industrial wastewaters. For example, Zhang et al. (2012) identified 18 types of PAHs in a coking wastewater, treated effluent, treatment sludge and gas samples. The fate of PAHs during the treatment process was determined by calculating the mass balance of each pollutant. However, the relationship between PAH transformation and removal was not evaluated. 3.3. Fluorescence EEM analyses Based on the operationally defined fluorescence boundaries of organic matters (Fig. 4), five EEM peaks, corresponding to five different types of fluorophores, were identified in PMWW: (1) small aromatic compounds at Ex. 230e235 nm/Em 340e360 nm (peak 1), including ethylbenzene, naphthaline, and xylene (Wei et al., 2013); (2) humic acid-like compounds at Ex. 250e260 nm/ Em 420e440 nm (peak 2) (Coble, 1996); (3) protein tryptophan at Ex. 275e285 nm/Em 340e360 nm (peak 3) (Coble, 1996); (4) humic acid-like compounds with small amount of fulvic acid-like compounds at Ex. 275e300 nm/Em 420e440 nm (peak 4) (Baker and Curry, 2004); (5) fulvic acid-like compounds at Ex. 325e350 nm/ Em 420e440 nm (peak 5) (Baker, 2002; Wei et al., 2013). Significant changes in fluorescent intensity were observed for all EEM peaks after EC treatment, but differed in extents (Fig. 4). Reductions in different types of fluorophores were calculated by summing the individual peak intensities (Janhom et al., 2009). Peak 2 and 4 were completely removed, in 30 min and 20 min, respectively. Comparatively, peak 5 was also removed but in a slower rate than the above two peaks, and the removal efficiency reached 50% in 30 min. The removal efficiencies for the three peaks are in the following order: Peak 4 > Peak 2 > Peak 5. Unlike the organics for peaks 2, 4, and 5, small bicyclic aromatic compounds (peak 1) and protein tryptophan (peak 3) appeared to be recalcitrant for EC treatment. Fluorescent organics corresponding to the two peaks increased after 30 min of EC treatment, almost by 500% for peak 1 and approximately 200% for peak 3.
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also in soil and sediments (Vane et al., 2014; Wang et al., 2015). During EC treatment, the Fe hydroxide flocs were formed due to hydrolysis of ionic Fe species released from the electrode and settled to the bottom of the reactor. As described by the Faraday's Law (Eq. (1)), different amount of Fe can be released from the electrode by varying either the applied current or the treatment time:
m¼
56It zF
(1)
where m is the mass of Fe (g) generated at a specific current I (A) over a time interval of t (s), z is the number of electrons transferred per Fe atom, 56 (g mol1) is the atomic weight of Fe, and F is the Faraday's constant (96,486 C eq1). In realistic applications, the number of electrons transferred per Fe atom varies with solution pH and the presence of organic matters (Sasson et al., 2009). Therefore, the theoretical Fe concentration as a function of treatment time can be estimated in Fig. SM-2, by assuming Z is 2, i.e., Fe2þ is the dominant species released from the electrode. During the experimental study, the Fe concentrations increased from 0 to 2439 mg L1, which were slightly lower than the theoretical values (Fig. SM-2). In the EC treatment, the pH value will increase along with the experiment time, at higher pH, the real iron dissolution could be lower than the value calculated by Faraday's law, indicating that there are other reactions (apart from iron dissolution) that are occurring simultaneously near the anode (Sasson et al., 2009), and this result was in conformity with findings reported by Harif (Harif et al., 2012). Water samples were collected from the reactor during EC treatment at a current density of 40 mA cm2 and analyzed for particle size distribution as a function of treatment time (Fig. 5). The size of the flocs formed during the first 5 min of EC treatment was mainly smaller than 10 mm (Fig. 5a). In comparison, the size of particles in PMWW was primarily around 0.89 mm. The increase in treatment time expanded the range of floc size distribution and caused the modal diameter to increase from 8.6 mm at 5 min to larger than 100 mm at 30 min of treatment (Fig. 5b). The broadening size distribution and increasing modal diameter of coagulation flocs were closely related to continuous formation and agglomeration of Fe hydroxide, since increasing amount of Fe were dissolved as measured previously. 3.5. PAHs in the coagulation flocs Fig. 3. Variations in PAH concentrations with EC treatment time: (a) Naphthaline and Dimethylnaphthalene, (b) Dichloronaphtalene and Acenaphthylene. (c) Acenaphthene, Anthracene, Phenanthrene, Fluorene and Fluoranthene. (initial pH ¼ 7.75, current density ¼ 40 mA cm2, area/volume ratio ¼ 0.16 cm1, temperature ¼ 25 C).
Based on the EEM results, EC treatment is capable of removing humic acid-like and fulvic acid-like fractions of the organics in PMWW (peak 2, 4, 5), leading to noticeable TOC removal (Fig. 2). However, the TOC removal was incomplete, possibly due to an increase in the concentrations of partially degraded organics, including small PAH compounds such as naphthaline and dimethylnaphthalene (Table 1). Formation of these small molecular weight organics should result from partial degradation of humiclike and fulvic-like organics as indicated by disappearance of related fluorophores (Fig. 4). 3.4. Formation of Fe hydroxide flocs during the EC treatment PAHs are known to exist not only in the water bodies and air, but
Particle size, specific surface area and amount of adsorbent are key factors for adsorption of PAHs (Alcantata et al., 2009). In this study, the amount of Fe hydroxide flocs increased with increasing time for EC treatment; this should have provided greater amount of adsorbents for PAH adsorption (Amstaetter et al., 2012). The surface area of Fe hydroxide flocs was not measured in this study, but a previous study found that the specific surface area of freshly prepared hydrous ferric oxide was 5500 ± 170 m2 g1 as determined by the dye adsorption method (Imre, 2008). Therefore, the large surface area of Fe hydroxide flocs should play an important role in the adsorption of PAHs and other organics during the EC treatment. In order to validate this assumption, the flocs formed during the EC treatment at a current density of 40 mA cm2 was collected after 30 min of treatment and analyzed for TOC and PAH concentrations. Based upon the mass balance for TOC, approximately 14.3 mg (27.1% of the TOC removed from the PMWW) of organics was transferred from the liquid phase into the flocs. These organics included five types of PAHs: naphthaline, dimethylnaphthalene, acenaphthene, anthracene, and phenanthrene, as measured by GC/
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Fig. 4. Fluorescence excitation emission matrix spectra of the wastewater effluents treated with EC for 0 min, 10 min, 20 min, and 30 min. P1, P2, P3, P4, P5 represent different peaks in the spectra.
Table 1 Organic pollutants in the raw and electrocoagulation-treated wastewater, identified by gas chromatography-mass spectrometry. Compounds
Raw water (mg L1)
Treated water (mg L1)
Removal () or increased (þ) percentage (%)
Attribution
Hexachlorobutadiene Dibutyl phthalate Pentadecanoic acid Diethylene glycol dibenzoate 1, 2 Benzenedicarbonic acid, dioctyl ester 1, 2,4-Trimethylbenzene 1, 3-Dichlorobenzene Nitrobenzene Ethylbenzene
20 38 27 24 38
6 6 5 9 5
70 84 81 63 86
Acid, esters, and long-chain organics containing benzene ring
1 2 3 4
12 9 15 20
þ1100 þ350 þ400 þ400
Peak 1 (EEM)
MS (Fig. 6). This finding is consistent with the aforementioned literature results.
3.6. Relationship between organic removal and PAH transformation EC treatment and organic characterization results suggest the occurrence of chemical transformation among different organics in PMWW (Fig. 4 and Table 1), coincident with significant decreases in TOC (Fig. 2) and total PAH concentration. Based upon the changes in concentration during EC treatment, the organic compounds identified in PMWW can be classified into two categories: (1) easily degradable compounds (alkene, ester and some long chains organics containing benzene rings), and (2) aromatic compounds and small compounds corresponding to peak 1 of the EEM diagram (Fig. 4). There was a noticeable reduction in the easily degradable compounds, with removal efficiencies up to 86% (Table 1), as a result of the reactions caused by the EC treatment. For chemicals related to peak 1 of the EEM diagram (Fig. 4), their concentrations
were noticeably increased after EC treatment, to a maximum level of 1100% (Table 1). In summary, PAH transformation, as well as the transformation of other organics, may either promote or reduce the removal efficiencies of PAHs by EC treatment. The mechanisms for the formation of PAHs in liquid phase during the EC treatment is herein proposed: the humic-like and fulvic-like organics were firstly degraded into small organics, including small PAH compounds, and accumulated in the treated effluent; among these organics, the easily degradable compounds containing benzene rings were transformed into naphthaline and dimethylnaphthalene by the oxidation effect of EC treatment.
4. Conclusions This study systematically investigated the removal and transformation of PAHs in a paper-making wastewater effluent by electrocoagulation. It was found that EC treatment effectively
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the treated effluent and the coagulation sludge in order to mitigate environmental pollution. These findings provide important instructions for paper-making factories to improve the design and operation of wastewater treatment and reuse facilities. Acknowledgement The authors greatly appreciate the financial support of the Special Funds for Technological Development of Research Institutes from the Ministry of Science and Technology of China (2013EG111129), the Program for Youth Scientists of the Beijing Academy of Science and Technology (201505), the 2016 BJAST Reform and Development Program and the Special Fund of the State Joint Key Laboratory for Environmental Simulation and Pollution Control (270403GK). Appendix A. Supplementary data
Fig. 5. Changes in particle size distribution during electrocoagulation of the papermaking wastewater: (a) first 5 min and (b) the remaining 25 min (current density: 40 mA cm2, initial pH: 7.75, A/V: 0.16 cm1, temperature: 25 C).
Fig. 6. The concentrations of PAHs measured in the flocs formed during EC treatment.
removed a total of 86% of PAHs by mass in the effluent. Transformation of PAHs had double-sided consequences on their removals during EC treatment. On the one hand, PAHs possessing three or four aromatic rings, including acenaphthene, anthracene, phenanthrene, fluorene and fluoranthene were significantly removed from the effluent, by transforming into smaller molecules or being mineralized (measured as TOC decreases). On the other hand, PAHs with two aromatic rings, including naphthaline and dimethylnaphthalene were generated by degradation of humic-like and fulvic-like organics, resulting in increased concentrations after EC treatment. In addition to chemical transformation, PAHs, including naphthaline, dimethylnaphthalene, acenaphthene, anthracene, and phenanthrene were also detected in the settled coagulation flocs. This suggests that Fe hydroxide flocs produced during EC process served as adsorbents for PAHs in the effluent. Overall, this study demonstrates that EC treatment is effective for the removal of many PAH compounds contained in papermaking wastewater. However, partial degradation of humic and fulvic substances may produce 2-ring PAH compounds. Therefore, post-treatment is needed to remove residual PAH compounds from
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