Transformation and Products of Organic Micropollutant in Water during

Chapter 10

Downloaded by COLUMBIA UNIV on December 12, 2016 | http://pubs.acs.org Publication Date (Web): December 9, 2016 | doi: 10.1021/bk-2016-1241.ch010

Transformation and Products of Organic Micropollutant in Water during Electro-Enzymatic Catalysis He Zhao,*,1 Penghui Du,1 Di Zhang,1 Hongbin Cao,1 and Laura Mast2 1Beijing

Engineering ResearchCenter of Process Pollution Control, Division of Environment Technology and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China 2Department of Civil Environmental and Sciences, Georgia Institute of Technology, Atlanta 30318, United States *E-mail:[email protected].

The transformation and products of micropollutant in waterbodies and water treatment have recently raised great concerns. In this chapter, micropollutants with the presence and absence of humic acid in the electro-enzymatic system were studied. Organic pollutants and products were analyzed by ultra performance liquid chromatography coupled with time-of-flight mass spectrometry (UPLC-TOF-MS). Possible enzymatic catalyzed transformation intermediates and products in electro-system were assessed by non-target and suspect screening. Then, the mechanism of self-polymerization and cross coupling of micropollutant during electro-enzymatic oxidation was proposed.

Introduction Due to the potential damage on human health and the environment, the transformation and products of micropollutant during water treatment have attracted great concerns. To remove micropollutant, several potential technologies have been studied. One such method, enzyme catalysis, is promising due to its high reactivity and selectivity (1–3). Peroxidases, such as horseradish peroxidase (HRP) or polyphenol oxidase, e.g., laccase, can catalyze the conversion of © 2016 American Chemical Society Drewes and Letzel; Assessing Transformation Products of Chemicals by Non-Target and Suspect Screening Strategies and ... ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


phenolic moieties into phenoxy free radicals with the aid of hydrogen peroxide or oxygen. These radicals can couple to form low-solubility polymers that can be removed from water through precipitation or coagulation (4, 6). The catalytic cycle of HRP is shown in Scheme 1. This process has been widely studied for the treatment of phenolic emerging micropollutant in waters due to the high efficiency and specificity of enzyme (7). Humic acid (HA) molecules also can be oxidized for polymerization by enzyme-catalyzed oxidative coupling reaction (8).

Scheme 1. The Catalytic Cycle of HRP with Ferulate as Reducing Substrate However, the practical application of enzyme catalysis is limited by stylishic choice challenges in continuous external H2O2 supply. Given this, an electro-enzymatic process was proposed, which combines the catalysis of oxidoreductases and the electrogeneration of in situ H2O2 (9, 10). In electroenzymatic systems, H2O2 is continuously supplied by the two-electron reduction of dioxygen on cathode, which does not require additional chemicals, and electricity is readily available (11), following reaction (1):

In this chapter, two model micropollutant, bisphenol A (BPA) and 2,4-dichlorophenol (DCP) were studied in the electro-enzymatic system. Possible enzymatic catalyzed intermediates and products in electro-system were identified by Ultra performance liquid chromatography coupled with time-of-flight mass spectrometry (UPLC-TOF-MS). Targeted and hidden-targeted screening 148 Drewes and Letzel; Assessing Transformation Products of Chemicals by Non-Target and Suspect Screening Strategies and ... ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

methods were used to verify the possible products. The transformation and products of BPA and DCP in the presence of HA were examined. Then, the mechanism of self-polymerization and cross coupling of micropollutant during electro-enzymatic oxidation was proposed.

Experimental


Materials HRP (EC 1.11.1.7) was obtained from Sigma (USA). BPA (97% purity) and DCP (97% purity) was obtained from J&K Scientific Ltd. (China). HA was purchased from Sinopharm Chemical Reagent Beijing Co., Ltd (China). All other chemicals used in this study were analytical reagent grade and were obtained from Sinopharm Chemical Reagent Beijing Co., Ltd (China). HRP activity was measured via the 4-amino antipyrine method. Preparation of Electro-Enzymatic System For the experiments of self-polymerization, DCP and free enzyme were added in an electro-system. The electrochemical reactor was arranged in a membrane system with a pair of Ti electrodes. The effective volumes of the two cells were both 150 mL. Oxygen gas was supplied into the bottom of cathode cell at 0.5 L/min for the saturation of the dissolved oxygen. CO2 was also provided as a buffer reagent at 0.1 L/min. Effect of free enzyme dose was also examined. For the experiments of cross-coupling, BPA with the presence of HA was conducted in an electro-system with immobilized enzyme as cathode. HRP was immobilized on graphite felt (GF) as a reported method (12). Briefly, oxidized GF was fixed onto Ti electrode surface using silver conducting resin (GF/Ti). Then, partially oxidized HRP was added to hydrazine pretreated GF/Ti to prepared HRP-GF/Ti electrode. The electrochemical reactor was constructed in a membrane system with a HRP-GF/Ti electrode as cathode and a bare Ti electrode as anode. oxygen gas and CO2 were supplied to the bottom of the cathode cell. All samples were filtrated by 0.22 μm polytetrafluoroethylene (PTFE) membrane filters (Millipore, Billerica, MA) before analysis. The concentrations of BPA and DCP residues were determined using an Agilent 1100 HPLC (Agilent, USA) equipped with a UV detector and a C18 reversed-phase (RP) column (150 mm × 2.1 mm, 3.5 μm particle, Agilent). Isocratic elution with 50% Milli-Q water and 50% methanol at a flow rate of 0.25 mL min-1 was used as the mobile phase. LC-MS Method and Data Processing Workflows for the Analysis of Intermediates and Products To identify the intermediates and reaction products, UPLC-TOF-MS analysis was performed . In a Waters ACQUITY UPLC (Waters, Milford, MA) system, separation was performed on a C18 RP column (130 Å, 1.7 µm, 50 mm × 2.1 mm, 3/pkg, Waters), and the injection volume was 5 μL. Initially, methanol component was 10%, increased to 50% at 1.5 min, and then maintained 50% for 1 min, then 149 Drewes and Letzel; Assessing Transformation Products of Chemicals by Non-Target and Suspect Screening Strategies and ... ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


increased to 100% at 4 min, reverted to 10% at 4.1 min, and maintained 10% for 1 min. The flow-rate was 0.3 mL min-1. In a Waters Xevo G2-XS TOF mass spectrometer, full scan mass spectra (m/z 100−800) were recorded in negative ion mode. Capillary voltage was 4500 V of the cone voltage was 40 V and source temperatures was 80 °C. Desolvation gas was nitrogen (Airgas, >99.999% purity) and flow rate was 600 L/h. Based on these MS analyses, the tentative identification of reaction products was done according to the following strategy: The molecular formulas of each species present in reaction mixtures and not in control samples were derived from the accurate measured mass and isotope patterns. Targeted m/z was selected to form a new peak by Masslynx software, then MS spectrum of the specific peak was collected. Using Elemental Composition program, the corresponding possible chemical formula of specific m/z ions could be calculated, with the help of the general reaction mechanisms, we chose the best matched formula and proposed corresponding structures and reaction pathways.

Results and Discussion Generally, enzyme catalyzed oxidation of phenolic compounds involves several major processes (13): (i) enzymes are activated by O2 or H2O2, (ii) electrons transfer from phenolic substrates to the activated enzyme, forming radicals, and (iii) radical coupling and other reactions occur. In order to identify clearly the products and transformation of micropollutants in electro-enzymatic catalysis, we focus on self-polymerization and cross-coupling process of micropollutants. Firstly, DCP and free enzyme were added into an electro-system to explore the products and pathways of enzymatic catalyzed self-polymerization in the electro-system. Furthermore, we investigated the mechanism of BPA with the presence of HA in the electro-enzyme system, where enzyme immobilized on oxygenated cathode.

Enzymatic Catalyzed Products and Transformation of DCP in Electro-Chemical System In electro-system, with H2O2 serving as an electron acceptor for the activation of HRP, self-polymerization reaction was induced between organic pollutants. Therefore, enzyme dosage and electric current were important factors affecting DCP transformation and products. In the previous study, as shown in Figure 1, DCP removal and transformation was remarkably accelerated with increasing enzyme dosage or current. For example, Increases in HRP dosage provide more active sites (7, 14). For 75 U higher HRP addtion, DCP depetion achieved as high as 97.4% after only 4 min of reaction. Furthermore, H 2O2 can continuously generate rapidly via O2 reduction with higher electric current. When 8 mA of electric current was applied, DCP was removed as high as 98.3% in 6 min of reaction. 150

Drewes and Letzel; Assessing Transformation Products of Chemicals by Non-Target and Suspect Screening Strategies and ... ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 1. Effects of experimental parameters on removal of DCP ( [DCP]0 = 1 mg L-1) by enzymatic catalysis in electro-system. (a) HRP dosage (current = 5 mA); (b) current (HRP dosage = 50 U). (Reproduced with permission from reference (15). Copyright 2016 Elsevier.)

To evaluate the possible reaction pathways for enzymatic catalyzed DCP transformation in the electro-system, the reaction products were analyzed by UPLC-TOF-MS technique, and targeted and hidden-targeted screening method were used. Detailed possible reaction pathways for the transformation of DCP via electro-enzymatic catalytic oxidation are presented in Figure 2. In this study, 2-chlorohydroxyquinone (2-CHQ, species A, m/z 143) and DCP were identified by the targeted screening method. The peaks were well matched with the authentic standard based on with retention time and mass spectrum. Other products were detected by hidden-targeted screening method. Based on the identification of DCP products, we found a large pool of CHQ products in the electro-system. As reaction I in Figure 2 presents, DCP was firstly hydroxylated into 2-CHQ and 2-CBQ through the hydroxide substitution under neutral or alkaline pH condition. The hydroxyl substitution of chlorophenols with the release of chlorine ions is the first step of detoxifying process. Then, 2-CHQ could generate corresponding semiquinone radicals, which further coupled with other different radicals to produce different CHQs . The radical coupling reaction is the second type of dehalogenation reactions. Parts of CHQs were readily oxididzed to form corresponding CBQs. Due to electron-poor sites, CBQs easily underwent nucleophilic reaction and were structurally transformed (16). With the presence of nucleophiles OH- and H2O2 (17–19) in this study, OH-CBQs readily formed via CBQ hydroxylation in the electro-system. Thus, the hydroxyl substitution of CBQs into OH-CBQs provides another type of dehalogenation process. 151



Figure 2. Possible reaction pathways of DCP removal in electro-enzymatic system. (Reproduced with permission from reference (15). Copyright 2016 Elsevier.)

With increasing incubation time, further enzymatic oxidation and coupling reactions continued due to the presence of phenolic groups in the products. Solid products analyzed by using mass spectrometry (Figure 3), suggests a transformation of DCP dimer to hexamer and their related by-products. With radical coupling continuing, more hydrophobic oligomer products with high molecular weights could form, which is similar to natural humification processes. 152 Drewes and Letzel; Assessing Transformation Products of Chemicals by Non-Target and Suspect Screening Strategies and ... ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 3. Tof-MS spectrum of precipitated sample taken after 30 min ([DCP]0 = 10 mg L-1, HRP amount = 150 U, 30 °C, current = 10 mA). (Reproduced with permission from reference (15). Copyright 2016 Elsevier.)

In the electro-system, CO2 and currents may directionally change the concentrations of hydroxyl anions and H2O2, and then influence the HRP-driven oxidation or substitution/addition reactions. An adequate supply of CO2 can provide favorable pH conditions and facilitate enzymatic steps, such as substrate oxidation and radical coupling, to generate precipitable polymerized products. Higher currents facilitated pathway B in our electro-enzymatic system. Electron-rich radical anions, generated by deprotonation under alkaline environments, are more likely to undergo nucleophilic aromatic substitution reactions (18, 20). More H2O2 was generated under higher currents. Besides accelerating HRP-driven oxidation, H2O2 also encouraged the transformation of CBQs into less-toxic OH-CBQs. Thus, proper adjustment of the parameters in enzymatic catalysis of electro-system may greatly change the process of DCP transformation, and facilitate the generation of products with less toxicity. In this study, 99.7% DCP can be removed in 10-min enzymatic catalyzed oxidation of electro-system. Most of products were 2-CHQ, dimers, oligomers, and the related quinone derivatives. According to the products identified, the mechanism of DCP removal via enzymatic catalyzed oxidation in the electro-system is proposed (Figure 4), including C-O-C or C-C coupling of chlorophenoxy radicals, hydroxyl substitution, further oxidation, CBQ nucleophilic reactions and oligomer formation. Current variations and the presence of CO2 could significantly affect these reaction pathways. In particular, higher currents enhance the hydroxylation process by promoting alkaline conditions and abundant H2O2 formation. These findings are useful for understanding the mechanism of HRP-driven DCP removal in the electro-system. 153



154 Figure 4. Summary mechanism of DCP transformation by electro-enzymatic catalyzed oxidation. (Reproduced with permission from reference (15). Copyright 2016 Elsevier.)


Transformation and Products of BPA with the Prensence of HA during Elecro-Enzymatic Process


In order to reveal the cross-coupling mechanism of micropollutants in electro-enzymatic system, we further analyzed the polymeric products in the electro-system with enzyme immobilized on cathode (Figure 5). The study found that, H2O2 continuously generated on the cathode through a two-electron reaction. When organic matter was added in the system, the amount of H2O2 was influenced at the beginning. However, H2O2 formation increased quickly with the time increasing.

Figure 5. Concentration of H2O2 as a function of the electrolytic time in cathode cell of immobilized enzyme system (30 °C, O2 flow-rate = 0.5 L min-1, current = 10 mA).

HA, with abundant phenolic moieties, existed widely in nature and water treatment process. It can also be oxidized for polymerization by enzyme-catalyzed oxidative coupling reaction. Therefore, the effect of HA on the transformation and product of BPA in enzymatic catalysis of electro-system was investigated. As shown in Figure 6, HA can significantly influence the transformation and products of micropollutant BPA during enzymatic catalysis in electro-system (21). After addition of HA, the BPA and dissolved organic carbon (TOC) removal rate increased 47.6% and 15% in 10min enzymatic catalysis of electro-system, respectively. 155 Drewes and Letzel; Assessing Transformation Products of Chemicals by Non-Target and Suspect Screening Strategies and ... ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 6. Removal of BPA and TOC in the enzymatic catalysis of electro-system with HA absent or present.

We have attempted to identify products formed in the electro-enzymatic system using UPLC-Tof-MS. As shown in Figure 7, untreated BPA and self-polymerized products (with star) were analyzed by the targeted screening method. With the presence of HA in the electro-enzyme system, the distribution of BPA and its self-polymerized products changed significantly. The obvious difference was due to the cross-coupling between BPA/BPA self-polymers and HA during the enzymatic catalysis of electro-system. Like BPA free-radical formation, phenolic moieties in HA can also produce one phenoxyl radical through HRP-driven oxidation (22). It is noted that in addition to self-polymerization, HA radicals also react with BPA radical via cross-coupling (23). Therefore, the competition between HA radicals and BPA radicals is inhibited the BPA self-coupling, resulting in less BPA dimers formed. Based on the targeted screening method, BPA and its self-polymerized products can be identified. However, HA and cross-coupling products were too complex, it is difficult to analyze them by non-target and suspect screening method.Thus, the chemical and structural features of HA analyzed by combined LC-MS and FTIR methods. The mechanisms of self-polymerization and cross-coupling processes of BPA in electro-enzymatic system were proposed (Figure 8). Firstly, H2O2 was generated in situ on the cathode by O2 reduction. Then, the HRP on cathode was activated by H2O2 to form intermediate products and mediated BPA into phenoxygen free radicals followed by self-polymerization and/or incorporation into HA, leading to 100% BPA removal during the enzymatic catalysis in electro-system. This process also greatly altered the chemical and structural features of HA, where hydrophilic moieties transformed into hydrophobic forms. 156



Figure 7. TOF-MS spectra of samples after 10-min electro-enzymatic catalysis. (a) Untreated BPA solution (5 mg/L) as a reference control, (b) 5 mg/L BPA but absence of HA sample, (c) 200 μg/L BPA with the presence of HA (TOC=20 mg/L) sample, (d) 50μg/L BPA with the presence of HA (TOC=20 mg/L) sample and (e) 5 μg/L BPA with the presence of HA (TOC=20 mg/L) sample. (Reproduced with permission from reference (21). Copyright 2015 Elsevier.)

Development and Prospect of Electro-Enzymatic Catalysis Electro-enzymatic technology was first proposed by Moon S.H.’s group (12). In our study, we summarized the role of electro-enzymatic in promoting organic pollutants removal. Firstly, enzymatic catalysis can be accelerated through enhanced electron transfer on the immobilized HRP in electro-chemical process. In situ H2O2 also could generate rapidly via reduction of dissolved O2 on a cathode. With the aid of the new formed H2O2, HRP can immediately catalyze the conversion of phenolic 157 Drewes and Letzel; Assessing Transformation Products of Chemicals by Non-Target and Suspect Screening Strategies and ... ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


moieties into phenoxy free radicals or other phenoxy active intermediates. In subsequent reactions, the pathways of organic pollutants in electro-enzymatic catalysis are similar with those in enzymatic process. On one hand, phenoxy free radicals of one phenolic micropollutant can self-polymerize into dimmer, trimer, etc. More important, phenoxy active intermediates of different phenolic micropollutant also can react with each other via cross-coupling. With polymerization and coupling continuing, more hydrophobic oligomer products with high molecular weights could form, which is similar with the natural humification process. In fact, the electro-enzymatic catalysis is similar with an enhanced process of natural humification reaction.

Figure 8. The proposed mechanism of BPA removal process in electro-enzymatic system in the presence of humic acid. (Reproduced with permission from reference (24). Copyright 2013 Elsevier.)

Secondly, hydroxylation enhanced by electro-system also affects the products and pathway of organic pollutants in electro-enzymatic catalysis. In this work, we detected more hydroxyl intermediates and products than enzymatic catalysis, including OH-CBQs and hydroxyl polymers and so on. That indicates electroprocess may promote hydroxylation of intermediates during enzymatic catalyzed oxidation. Due to the halides of micropollutants substituted by hydroxyl, electroenzymatic technology provides a way for detoxifying halogen containing organic micropollutants. 158 Drewes and Letzel; Assessing Transformation Products of Chemicals by Non-Target and Suspect Screening Strategies and ... ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Lastly, the electro-enzymatic catalysis was not only effective to reduce phenol pollutants, but also could be applied in combined pollutants system. In addition to remove phenolic pollutants, electro-enzymatic catalysis could also reduce organic micropollutants containing aniline and thiol groups. The aniline and thiol micropollutants as nucleophiles could attack the intermediates of HA via nucleophilic addition reactions, forming C-S-C/C-N-C covalent bonds in the electro-enzymatic system. It is an similar cross-coupling mechanism with the C-O-C bond between different phenolic micropollutants. Furthermore, research on the cross-coupling mechanism of multiple pollutants, especially for different kinds of pollutants, can broaden the applications of electro-enzymatic technology in water/wastewater treatment. In our study (21), micropollutants with small molecules like BPA can be enzymatic catalyzed oxidized and integrated into HA structure through cross coupling reaction. The hydrophobicity and molecular size of HA increased in enzyme-catalyzed polymerization and prone to removal via precipitation separation. Therefore, the electro-enzyme catalysis could change the properties of dissolved organic matter (DOM) of water/wastewater. It can provide an effective way for DOM removal in combination with other techniques like coagulation or filtration process, in which the polymerized micropollutant products can be precipitated along with DOM. For example, the authors had done the research on the micropollutants removal approach combining electro-enzyme and electrocoagulation process. Wherein, HRP was immobilized on the cathode and aluminum plate was used as anode to establish a pair of working electrodes. Compared to individual electrocoagulation, the combined process showed a higher TOC removal and less energy consumption to control certain contaminants (21). In addition, DOM with higher molecular weight could not only reduce the membrane fouling (25). Thus, it may be potential research on electro-enzyme process to control membrane fouling.

Acknowledgments The authors thank support from the National Natural Science Foundation of China (Grant No. 51378487) and Youth Innovation Promotion Association, CAS (2014037).

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161 Drewes and Letzel; Assessing Transformation Products of Chemicals by Non-Target and Suspect Screening Strategies and ... ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Transformation and Products of Organic Micropollutant in Water during

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