Free-Radical Induced Chain Degradation of High-Molecular-Weight

Research Article pubs.acs.org/journal/ascecg

Free-Radical Induced Chain Degradation of High-Molecular-Weight Polyacrylamide in a Heterogeneous Electro-Fenton System Min Sun,* Meng-Xia Qiao, Jun Wang, and Lin-Feng Zhai* Anhui Province Key Laboratory of Advanced Catalytic Materials and Reaction Engineering, School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei, 230009, China S Supporting Information *

ABSTRACT: A heterogeneous electro-Fenton (EF) process is applied for degrading high-molecular-weight polyacrylamides (PAMs). Degradation efficiencies of the PAM compound with molecular weight of 15 000 kDa at different cathodic potentials are compared in terms of solution viscosity, NH4+−N concentration, chemical oxygen demand (COD), and total organic carbon (TOC). The degradation products of PAM are identified, and anaerobic biodegradability of the products is evaluated. Partial mineralization of PAM is observed in the heterogeneous EF process, producing polymeric products with newly generated ether and ketone groups. As the cathodic potential changes from −0.3 to −1.1 V (vs SCE), the mineralization efficiency of PAM increases from 20.8% to 32.9%. The PAM degradation in the heterogeneous EF process is proposed to follow a novel pathway starting with the cleavage of polymer backbone at the head-to-head linkage and ending with a termination reaction between the carbon-centered radical and oxygen-centered radical. The polymeric products from PAM degradation demonstrate favorable anaerobic biodegradability. Thus, the heterogeneous EF process has great potential as a pretreatment unit before anaerobic digestion for the treatment of nonbiodegradable high-molecular-weight PAMs. KEYWORDS: Heterogeneous electro-Fenton process, Polyacrylamide, Catalysis, Radical termination, Chain recombination

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ion (Fe2+).9−11 The electro-Fenton (EF) process is a typical Fenton process in which hydrogen peroxide is produced from the electrochemical reduction of oxygen (O2).12,13 So far, the EF process has demonstrated great efficiency in degrading recalcitrant organic pollutants including dyes, drugs, herbicides and insecticides, and polymers like cellulose.13−16 While the homogeneous EF process with Fe2+ catalyst encounters the limitations of rigid acidic operation conditions and catalyst loss, research has focused more on the heterogeneous EF process which adopts a solid iron catalyst instead of Fe2+.17 The heterogeneous Fenton catalysts, such as magnetite (Fe3O4), wustite (FeO), pyrite (FeS2), hematite (Fe2O3), and Fe−Co/ Fe−Cu bimetals, have shown satisfactory activity and reusability in wide pH ranges.17−22 While the heterogeneous EF process has shown effectiveness in mineralizing small-molecule pollutants, its potential in treating high-molecular-weight synthetic polymers, like the PAM, is not adequately evaluated. Although several possible reaction modes have been proposed to predict structure changes of PAM in various radical-dominated processes,5 independent evidence involving the detection and identification of intermediates and final products is needed to substantiate them. In the present work, the performance of the heterogeneous EF process on degrading PAM is comprehensively investigated. Structure change of PAM during the process

INTRODUCTION High-molecular-weight polyacrylamide (PAM) and its derivatives are important commercial flocculants widely used in water treatment, mineral extraction, oil recovery, and paper manufacturing. The discharge of PAMs into surface and ground waters is arising as an environmental issue related to flocculant safety.1 The treatment of wastewater using a biological approach is environmentally and economically beneficial for water recycling. However, PAMs have long been regarded as biorefractory compounds due to their rigid carbon backbones and huge molecular volumes making them unable to pass through microbial membranes and be metabolized in vivo.2,3 In particular, PAM compounds with molecular weights higher than 10 000 kDa present high viscosity and flocculability and are hardly degraded by biological processes.4 Backbone cleavage of PAMs can occur in mechanical shear, photo-oxidation, or advanced oxidation processes which rely on free radicals, mainly the hydroxyl radical HO•, to cause chain bond scission.1,5 In some cases, radical attack instigates consecutive scission of C−C bonds until total depolymerization of PAM.6,7 Whereas the C−C bond cleaved by radical attack might undergo recombination or disproportionation, creating polymeric products with reduced molecular weights.3 The HO• generated by the Fenton reaction has been known to cause chain bond scission of the PAM, resulting in the reduction of its molecular weight.8 Fenton process is a catalytic process based on the formation of HO• from hydrogen peroxide under the catalysis of ferrous © XXXX American Chemical Society

Received: April 26, 2017 Revised: August 2, 2017 Published: August 15, 2017 A

DOI: 10.1021/acssuschemeng.7b01311 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 1. Time courses of (A) relative viscosity of solution. (B) NH4+−N concentration. (C) COD. (D) HO• generating rate in the heterogeneous EF process treating the PAM compound at different cathodic potentials. The molecular weight of PAM is 15 000 kDa.

is elucidated, and final products are identified. The prospect of heterogeneous EF process as a pretreatment to enhance anaerobic biodegradability of high-molecular-weight PAMs is envisaged.

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the heterogeneous EF process was quantified spectrophotometrically at λ = 440 nm with p-nitrosodimethylaniline as an indicator.26 Toxicity Analysis of the PAM and Its Degradation Products. The toxic effects of PAM and its degradation products were evaluated by the inhibition growth of selected bacteria E. coli in Luria−Bertani (LB) medium.27 The assays were carried out in a 96-well microtiter plate. Samples taken from the heterogeneous EF cell were sterilized using 0.45 mm membrane filters and, then, added into the microtiter plate at 40 μL. For each sample triplicate wells were inoculated with 160 μL of LB culture of E. coli. Controls consisted of 160 μL of LB broth and 40 μL of sterile water. The plate was incubatd at 37 °C and the growth of E. coli was monitored by reading the absorption at 600 nm in a microtiter plate reader. Structural Characterization of PAM and Its Degradation Products. Samples taken from the heterogeneous EF cell were centrifuged at 12 000 rpm for 15 min. The supernatant was dialyzed using a dialysis membrane (Biosharp MD44, 3.5 kDa) against deionized water to separate the polymeric products from inorganic salts. The solutions of polymeric products were subjected to lyophilization to obtain solid powder samples. Molecular weight change of PAM during the heterogeneous EF process was monitored by gel permeation chromatography (GPC; 1515, Waters Co., USA) equipped with Waters Ultrahydrogel 250, 500, and 2000 columns in series and refractive index detector. The 5 mg of solid sample was dissolved in 5 mL of deionized water and then injected into the GPC system at 40 μL. Deionized water was used as eluent at a flow rate of 0.5 mL min−1. The structures of polymeric products were identified by nuclear magnetic resonance spectrometry (NMR), infrared spectrometry (IR), and X-ray photoelectron spectrometry (XPS). The 1H NMR spectroscopy was performed using a VNMRS 600 MHz spectrometer (Agilent Corp., USA). The samples were dissolved in 5 mL of deuterium oxide (D2O) for 1H NMR analysis. The IR spectra were obtained on a KBr disk with a VERTEX 70 Fourier transform infrared spectroscopy (Bruker Co., Germany), and XPS was performed on an ESCALAB 250 spectrometer (Thermo, USA) equipped with a monochromatic Mg Kα X-ray source (1253.6 eV). Anaerobic Digestion of the Polymeric Products of PAM. Batch anaerobic digestion test was carried out to assess anaerobic biodegradability of the polymeric products from heterogeneous EF degradation of PAM. The sludge system was applied in a cylindrical anaerobic reactor with a liquid volume of 400 mL. The medium was 50 mM phosphate buffer (pH 7.0) containing KCl, 130 mg L−1; CaCl2,

MATERIALS AND METHODS

Operation of Heterogeneous EF Process for PAM Degradation. The heterogeneous EF process was carried out in a 400 mL glass made three-electrode cell-assembly, using a Fe3O4/graphite felt (GF) composite (3 cm × 3 cm) as working electrode, Pt wire as counter electrode and saturated calomel electrode (SCE) as the reference. The Fe3O4 in Fe3O4/GF composite serves as the heterogeneous EF catalyst. The Fe3O4/GF composite was prepared as previously described.23 Briefly, iron oxide was loaded on the GF in an air− cathode fuel cell and then calcinated at 700 °C in nitrogen gas to obtain the Fe3O4/GF composite. Iron content in the Fe3O4/GF composite was determined by thermogravimetry (TG) on a TGA DT50 apparatus (Sahimadzu Co., Japan) in an air atmosphere. The electrodes were immersed into the electrolyte solution which was 300 mL of 50 mM Na2SO4 containing 500 mg L−1 PAM at pH 7.0. The linear PAM compound with viscosity-average molecular weight of 15 000 kDa and hydrolysis degree of 25% was used unless otherwise indicated. Electrolysis was performed by imposing a cathodic potential on the working electrode through a CHI 1000C electrochemical workstation (CH Instruments Inc., USA). The potentials of working electrode were set at −1.1, −0.7, or −0.3 V (vs SCE), according to experimental design. Air was bubbled at a rate of 1.5 L min−1 from the bottom of the cell to ensure the electrolyte was saturated with O2. A control reactor was operated with only air bubbling. All experiments were conducted in triplicate, and the average values with standard deviations were presented. Chemical Analysis. Relative viscosity of PAM solution was determined using an Ubbelohde capillary viscometer (type 1836-A, Shanghai Glass Instruments Factory, China) at 30 °C, with 50 mM Na2SO4 as in ref 24. Concentration of ammonia nitrogen (NH4+−N) was determined by Nesslerization, and chemical oxygen demand (COD) was determined using the dichromate reflux method. The fiveday biological oxygen demand (BOD5) was determined from the decrease of O2 concentration after 5 days of incubation in the dark at 20 °C.25 Total organic carbon (TOC) was measured on a HTMCT1000 M TOC analyzer (Tailin Co., China). The HO• produced in B


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Figure 2. (A) GPC chromatograms of PAM compound and its polymeric products obtained at different cathodic potentials. (B) 1H NMR spectra of PAM and its polymeric products obtained at cathodic potential of −1.1 V. (C) IR spectra of (a) PAM compound and its polymeric products obtained at (b) −0.3, (c) −0.7, and (d) −1.1 V (vs SCE). The molecular weight of PAM is 15 000 kDa.

from 721 ± 1 to 663 ± 0 mg L−1. Such a result suggests both the reaction on the side chains and the scission of C−C bond are enhanced by promoting cathodic potential, which is in good accordance with the accelerated HO• production at increased cathodic potential (Figure 1D). Then, PAM is treated at −1.1 V with isopropanol as a radical scavenger of HO•.28 The concentration of isopropanol is 350 mM to ensure the HO• is totally quenched. Figure S1 (see the Supporting Information) shows the molecular weight of PAM is not reduced when the HO• has been quenched by isopropanol. Thus, the degradation of PAM in the heterogeneous EF process should be attributed to the action of HO•. The PAM degradation at the GF electrode is also evaluated to highlight the catalytic role Fe3O4 plays in the heterogeneous EF system. The •OH production is negligible at the GF, and the molecular weight of PAM is hardly influenced in the process (Figure S1, see the Supporting Information). Therefore, the Fe3O4 as the catalyst plays an indispensible role in inducing the PAM degradation in the heterogeneous EF process. At the end of the heterogeneous EF process, solution TOC is measured to evaluate the mineralization efficiency of PAM. TOC is removed by 20.8%, 28.1%, and 32.9% in the process operated at −0.3, −0.7, and −1.1 V, respectively. Next, the toxicity of the unmineralized fraction is estimated by investigating its effect on the growth of E. coli. As shown in Figure S2 (see the Supporting Information), the PAM presents negligible inhibition on the growth of E. coli. In comparison, bactieral growth is obviously accelerated by the samples obtained at the end of heterogeneous EF process. Therefore, the degradation products of PAM are deemed to be nontoxic. Molecular weight change of PAM during the heterogeneous EF process is monitored by GPC. The PAM demonstrates one peak at retention time of 23.28 min on the GPC chromatogram (Figure 2A). The shift of such a peak to higher retention times during the heterogeneous EF process indicates reduced

10 mg L−1; MgCl2, 20 mg L−1; NaCl, 2 mg L−1; FeCl2, 5 mg L−1; CoCl2 , 1 mg L−1 ; MnCl2 , 1 mg L−1 ; AlCl3, 0.5 mg L −1; (NH4)6Mo7O24, 3 mg L−1; H3BO3, 1 mg L−1; NiCl2, 0.1 mg L−1; CuSO4, 1 mg L−1 and ZnCl2, 1 mg −1. Polymeric products obtained from the heterogeneous EF process operated at −1.1 V were used as the substrates. The initial COD in the medium was equivalent to 350 mg L−1. The reactor was inoculated with anaerobic sludge collected from a cultivation reactor at a total solid concentration of 12 g L−1. For biomass acclimation, the cultivation reactor had continuously run over six months with anaerobic sludge from a domestic wastewater plant as innoculum and the polymeric products as substrates. The sludge was washed with 50 mM phosphate buffer (pH 7.0) before inoculation. Anaerobic biodegradation of the polymeric products was evaluated by monitoring the COD and GPC chromatogram of solution. The PAM was also treated in the anaerobic reactor under the same conditions as used for the polymeric products.

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RESULTS Evaluating Degradation Efficiency of PAM in the Heterogeneous EF Process. Degradation efficiencies of the PAM at the Fe3O4/GF composite electrode with different potentials are evaluated in terms of solution viscosity, NH4+−N concentration, and COD. No obvious change of PAM solution is observed in the control reactor without cathodic potential application, indicating the PAM removal due to adsoprtion or mechanical degradation is negligible. As shown in Figure 1A, relative viscosity of solution decreases from the initial value of 1.6 to approximately 1.1 after the PAM is treated in the heterogeneous EF process for 120 h. Meanwhile, the increase of NH4+−N concentration in solution implies the reaction on the side chains of PAM, and the decrease of solution COD implies the scission of C−C bond (Figure 1B and C). Solution viscosity and COD remain unchanged after 120 h, which means the PAM cannot be degraded further. As the cathodic potential changes from −0.3 to −1.1 V, the NH4+−N concentration rises from 19.1 ± 0.7 to 23.5 ± 0.3 mg L−1 while the COD drops C


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Figure 3. C 1s XPS spectra of (A) PAM compound and its polymeric products obtained from the heterogeneous EF process operated at (B) −0.3, (C) −0.7, and (D) −1.1 V (vs SCE). The molecular weight of PAM is 15 000 kDa.

electrolyte. The slight Fe3O4 detachment and neglectable iron leaching ensure good reusability of the Fe3O4/GF composite electrode in heterogeneous EF process. Identifying the Degradation Products of PAM. At the end of heterogeneous EF process, the polymeric products are extracted. The result of TOC analysis suggests these polymeric products contribute to more than 96% of the total solution TOC. Thus, after the PAM is degraded in the heterogeneous EF process, the unmineralized fraction is almost all presented as polymers. Next, functional groups in the polymeric products of PAM are identified by IR and XPS analyses. Figure 2C shows the IR spectra of PAM and its polymeric products. Characteristic bands at 2940 and 1451 cm−1 due to −CH2 vibrations exhibit relatively lower intensities for the products than for the PAM, implying the shortening of backbone during the degradation of PAM. The bands located at 3400 and 3190 cm−1 are attributed to O−H and N−H stretching vibrations in COOH and CONH2, respectively. Relative intensities of the two bands to others on the IR spectra are weakened after PAM degradation, suggesting the decrease of side chain groups in the PAM. The bands at 1608 and 1554 cm−1 are assigned to vibrations of −NH2, and the bands at 1667 and 1412 cm−1 are attributed to vibrations of CO and C−O in COOH, respectively. Relative intensities of the latter two bands to the former two ones are enhanced after PAM degradation, suggesting the hydrolysis of side chains of PAM. Noticeably, The band at 1667 cm−1 due to CO vibration displays remarkably higher intensity for the polymeric products than for the PAM, indicating the formation of ketone or ester groups in the polymeric products. Moreover, a strong band due to C− O−C stretching vibration emerges at 1113 cm−1 for the polymeric products, suggesting ether or ester structures are introduced into the polymer chain during the degradation of PAM. Figure 3A shows three peaks on the C 1s XPS spectrum of PAM, which are assigned to C−(C, H) (284.8 eV), CONH2 (287.9 eV), and COOH (288.6 eV), respectively. The

molecular weight of polymer. This reduction in molecular weight should be resulted from the shortening of backbone ranther than the hydrolysis of amide group in PAM, because the conversion from PAM to poly(acrylic acid) hardly influences the molecular weight due to their approximately same molecular weights.3 Such a conclusion is also drawn from the result of 1H NMR analysis. The polymers demonstrate two characteristic peaks at δ = 2.15 and 1.58 ppm on the NMR spectra (Figure 2B), corresponding to protons in the −CH and −CH2 of carbon backbone, respectively.29 Peak intensities for the polymeric products are visibly weaker than those for the PAM, suggesting the shortening of PAM backbone in the heterogeneous EF process. Noticing the peak belonging to the degradation products is visibly broader than that belonging to the PAM on the GPC chromatogram (Figure 2A), the products from PAM degradation should be a mixture of polymers with close molecular weights. The polymeric products obtained at higher cathodic potential present lower molecular weights, evidenced by the movement of corresponding peak from 25.21 to 28.67 min when the cathodic potential changes from −0.3 to −1.1 V. The homogeneous EF process is also used to treat the PAM for performance comparison. Unfortunately, such a process fails to totally degrade the PAM because a large amount of PAM interacts with iron ions and forms precipitate (See section S1 and Figure S3 in the Supporting Information). From this point of view, the heterogeneous EF process is more applicable than its homogeneous counterpart for treating PAM-like polymers. Notably, the heterogeneous EF process exhibits satisfactory stability toward the PAM degradation as evidenced by the wellmaintained mineralization efficiencies of PAM in four successive batch cycles (Figure S4A, see the Supporting Information). TG analysis indicates that iron content in the Fe3O4/GF composite electrode slightly reduces from 15.0 to 13.9 wt % in recycling operation (Figure S4B, see the Supporting Information). Solution pH is always kept in the range of 6.98−7.00, and neither Fe2+ nor Fe3+ is detected in the D


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molecular-weight PAMs. Taken together, the 19.2% of COD removal in heterogeneous EF process and 46.3% of COD removal in anaerobic digestion, a total COD removal efficiency of 65.6% is achieved in the coupled process. In particular, the peak attributed to polymer disappears from the GPC chromatogram after anaerobic digestion, suggesting the polymeric products are totally depolymerized (Figure S8B, see the Supporting Information). Thus, complete depolymerization of high-molecular-weight PAMs is anticipated in the coupled process.

assignment and quantification of these peaks are listed in Table S1 (see the Supporting Information). Relative area ratio of COOH versus CONH2 for the polymeric products is higher than that for the PAM and increases with the increase of cathodic potential. Such a result provides evidence for the hydrolysis of amide group at the side chains of PAM, which is positively dependent upon the cathodic potential. The shortening of polymer backbone is also suggested by the XPS spectra, giving the decreased area percentage of peak associated with C−(C, H) after PAM degradation. Notably, there are two new peaks emerging on the XPS spectra of polymeric products, the peak at 285.9 eV being attributed to C−O in ether, and the one at 287.6 eV belonging to CO in ketone. Hence, the PAM degradation in the heterogeneous EF process is proposed to follow a pathway composed of chain cleavage and chain recombination, the former causing the shortening of backbone and detachment of side chains and the latter introducing ether and ketone groups into the polymer chain. While area percentage of the peak due to C−(C, H) is decreased by increasing cathodic potential, area percentages of the peaks due to C−O and CO are increased meanwhile. It is thus deduced increasing cathodic potential is able to promote the degradation of PAM but unable to totally depolymerize the PAM because of chain recombination. The PAM compounds with molecular weights of 1750 and 6600 kDa are also treated in the heterogeneous EF process. The results show that lowering the molecular weight is not able to improve the degradation efficiency of PAM due to the chain cleavage and recombination pathway (See Section S2 in the Supporting Information). Intermediates in the heterogeneous degradation of PAM are also characterized. While the PAM is gradually mineralized in the process, almost all the TOC is embodied in the polymeric intermediates (Figure S6, see the Supporting Information). XPS analysis suggests the C−(C, H) groups in the polymeric intermediates is decreased along with the process, accompanied by the increase of both C−O and CO groups (Figure S7 and Table S1, see the Supporting Information). Therefore, the PAM depolymerization is proposed to be synchronized with its mineralization in the heterogeneous EF process. Determining the Biodegradability of Polymeric Products. The BOD5/COD ratio for the PAM and its polymeric products is determined to be 0.0036 and 0.065, respectively. Such a result suggests the aerobic biodegradability of PAM is increased in the heterogeneous EF process. However, the polymeric products remain poorly degradable in aerobic treatment. While anaerobic digestion shows great potential in degrading complex organic wastes, in our previous work we have demonstrated the PAM compounds with molecular weights higher than 10 000 kDa are nondegradable by anaerobic microorganisms.4 Likewise, here we find the PAM compound with molecular weight of 15 000 kDa fails to be degraded in anaerobic digestion. In comparison, the polymeric products from the heterogeneous EF degradation of PAM are easily decomposed by anaerobic digestion, giving the COD decrease from 350 ± 3 to 188 ± 1 mg L−1 after 12 d of anaerobic digestion (Figure S8A, see the Supporting Information). Therefore, the heterogeneous EF process is able to convert the nonbiodegradable PAM to anaerobically biodegradable species. Notably, the effluent from anaerobic digestion displays a moderate aerobic biodegradability as evidenced by the BOD5/COD ratio of 0.36. This is a fact of remarkable importance enabling the coupling of heterogeneous EF and anaerobic digestion processes for the treatment of high-

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DISCUSSION For reduction in molecular weight, degradation of PAM might occur through exocleavage from the terminal ends or endocleavage on the backbone. As displayed by thermal degradation of PAM between 200 and 300 °C,5,30 the cleavage limited to the terminal sites of side chains reduces the molecular weight of PAM, but the backbone stays intact. When the cleavage is initiated at the ends of the main chain, consecutive degradation of the backbone would be induced until total depolymerization of PAM.31 Quite different from above, the degradation of PAM in the heterogeneous EF process causes cleavage of polymer backbone and yields ketone and ether groups in the polymer chain. Thus, the degradation should follow an endocleavage pathway starting at the C−C bond within the backbone of PAM. As illustrated by eqs 1−4, the cleavage of polymer backbone (PH with H representing the extractable proton) upon attack of HO• in the presence of O2 produces fragment polymer radical (F•) and stable fragment polymer molecule (F). PH + HO• → P • + H 2O (1) P • + O2 → PO2 •

(2)

2PO2 • → 2PO • + O2

(3)

PO• → F • + F

(4)

In some cases, consecutive attack of HO• on the produced polymer fragments leads to small molecules as the end products.6,7 However, polymer radicals are also capable of undergoing termination reaction, resulting in chain recombination that creates new polymer structures.5,31−33 Taking into consideration the different chain structures of PAM and its polymeric products, the degradation of PAM in the heterogeneous EF process is proposed to be initiated by the cleavage of polymer backbone, followed by chain shortening and detachment of side chains, and ended with chain recombination through radial termination reaction. Random bond scission and nonrandom bond scission are the two ways by which free radicals cause the cleavage of polymer backbone. Through the first option PAM is randomly split into fragment polymer radicals. Termination reaction among these radicals produces polymeric products with varied molecular weights. In contrast, the products from the heterogeneous EF degradation of PAM are a mixture of polymers with close molecular weights. Therefore, the backbone of PAM is thought to be cleaved through nonrandom bond scission in which the bond scission is confined to specific sites of the polymer chain. The PAM has three sites vulnerable to attack of free radicals, i.e. the nitrogen atom in amide group, and the secondary and tertiary carbon atoms in polymer backbone. For chemically produced radicals, their susceptibilities to attack are in the ratio 1:2:8.3 In particular, within the chemical structure of PAM E


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Figure 4. Proposed chain cleavage and recombination pathway of PAM under action of HO• and O2 in the heterogeneous EF process.

effluent. Unfortunately, high-molecular-weight PAMs show strong resistance to microbial degradation because of their large molecular volumes and rigid chain structures. While chain recombination of PAM might happen through termination reaction between carbon-centered radicals in most radicalinduced processes in the absence of O2,1,5 the resulting polymers with C−C bond are still recalcitrant to microbial degradation. In this work, we find the presence of O2 in heterogeneous EF process can induce oxygen-centered radical which reacts with carbon-centered radical to form C−O−C bond within the polymer chain. The polymeric products with such C−O−C bond show favorable anaerobic biodegradability, enabling the heterogeneous EF process a prospective approach to pretreat high-molecular-weight PAMs that are otherwise unable to be degraded by biological processes. As a fact of matter, since the EF process is a potent pretreatment for increasing the biodegadability of biorecalcitrant pollutants,37,38 the coupling of EF and biological processes has been accepted as a reliable and cost-effective way for the treatment of biorafractory pollutants.39−41 To become a economically feasible pretreatment unit, further effort is anticipated to improve the heterogenerous EF process. Noticing the time reqired for the heterogeneous EF degradation of PAM is as long as 120 h, rate-limiting step in the process should be clarified and measures should be taken to accelerate the PAM degradation kinetics. Strategies aiming at promoting the process efficiency, such as design of appropriate reactor configuration, selection of effective electrodes, and optimization of process parameters, should be pursued in the future.

there are a few head-to-head units inserted into the normal head-to-tail repeats. Such abnormal head-to-head unit is inherently weak linkage susceptible to radical attack. Previous study has demonstrated the backbone cleavage of PAM started primarily from the head-to-head linkage upon free radicals attack.32 Bond dissociation theory indicates the more substituted C−C bond in head-to-head linkage requires less energy to be broken than the C−C bond in head-to-tail linkage.34,35 Here, we assume the head-to-head linkage would be given attack priority when the PAM is degraded under the action of HO•. The chain cleavage and recombination pathway of PAM in heterogeneous EF process is therefore depicted in Figure 4. After attack by HO•, the C−C bond in the backbone is broken at the head-to-head linkage and carbon-centered radicals are formed. This can detach the adjacent side chains and liberate small organic molecules which are subsequently mineralized. The carbon-centered radical is unstable and incline to react with O2 to afford a peroxy macromolecular radical.5 Peroxy radicals have a strong propensity to abstract hydrogen atoms available on the polymer backbone, resulting in a new carboncentered radical and a hydroperoxide species. This macromolecular hydroperoxide end group subsequently decomposes to form an oxygen-centered radical and a HO•. Finally, combination between the oxygen-centered radical and the carbon-centered radical terminates the chain reaction through forming an ether bond. Excessive HO• might have the opportunity to attack other C−C bonds outside the head-tohead linkage. Sequential hydrogen abstraction after radical attack introduces ketone group into the polymer chain. So far, the EF process has been used to mineralize various organic pollutants such as the dyes, pesticides, drugs, and phenolic compounds.13,14 However, our practice in this work demonstrates the total mineralization of PAM-like polymers is not easy in a single EF process, due to the occurrence of a radical termination reaction. Notably, the initiating system that produces a higher concentration of free radicals is not necessarily going to cause more polymer degradation, as the probability of radical−radical termination will also be increased.5 Although deep degradation of PAM might happen in an extremely strong electric field, it is not recommended because of the high economic cost and electrode damage caused by high current density.36 Nevertheless, the heterogeneous EF process is a desirable choice for pretreating the PAM ahead of biological treatments. Present wastewater treatment plants usually employ bioremediation, under either aerobic or anaerobic conditions, as an ultimate step before the release of

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CONCLUSION The potential of heterogeneous EF process in degrading the PAM compound with molecular weight of 15 000 kDa is adequately evaluated. The PAM is partially mineralized in the heterogeneous EF process, producing polymeric products with reduced molecular weights. Structure analysis identifies ether and ketone groups in the chains of polymeric products. The PAM degradation in the heterogeneous EF process is proposed to follow a novel free-radical induced pathway starting from the backbone cleavage at the head-to-head linkage, followed by chain shortening and detachment of side chains, and ending with termination reaction between carbon-centered radical and oxygen-centered radical. The polymeric products from heterogeneous EF degradation of PAM show favorable anaerobic biodegradability, enabling such a process a prospective pretreament before anaerobic digestion for the treatment of nonbiodegradable high-molecular-weight PAMs. F


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01311. Table with assignment and quantification of peaks on the C 1s XPS spectra of PAM compounds and their polymeric products; GPC chromatograms of the PAM before and after treated at different electrodes; growth of E. coli in the presence of PAM and its degradation products; experiments on the homogeneous degradation of PAM and the results; stability of PAM in four successive batch cycles; TOC and C 1s XPS spectra of polymeric intermediates; degradation efficiencies of PAM compounds with different molecular weights; COD evolution in anaerobic digestion of the polymeric products and GPC chromatograms of polymeric products before and after anaerobic digestion (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Fax: +086-551-62901450. E-mail: [email protected] (M.S.). *E-mail: [email protected] (L.-F.Z.). ORCID

Min Sun: 0000-0001-7527-6712 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank the NSFC (51478157, 51378166), the Program for New Century Excellent Talents in University (NCET-13-0767), and the Natural Science Foundation of Anhui Province (1508085ME75) for the financial support of this work.

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REFERENCES

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