Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 5540−5546
pubs.acs.org/journal/ascecg
Electrochemically Catalytic Degradation of Phenol with Hydrogen Peroxide in Situ Generated and Activated by a Municipal SludgeDerived Catalyst Bao-Cheng Huang,† Jun Jiang,† Wei-Kang Wang, Wen-Wei Li, Feng Zhang, Hong Jiang, and Han-Qing Yu* CAS Key Laboratory of Urban Pollutant Conversion, Department of Chemistry, University of Science & Technology of China, No. 96, Jinzhai Road, Baohe District, Hefei 230026, P.R. China S Supporting Information *
ABSTRACT: With the rapid global urbanization, today’s cities are facing an increasing pressure for the treatment of both domestic and industrial wastewaters. In this work, a proof-of-concept of “treating industrial wastewater using the sludge originating from domestic wastewater treatment for urban pollution control” was proposed. After one-step pyrolysis of the excess sludge from domestic wastewater treatment, a metal−carbon composite catalyst with a high H2O2-producing capacity (432 mg/h/g) was successfully synthesized. By application of the prepared material as a cathode catalyst in an electro-Fenton system, phenol (40 mg/ L), a model pollutant in industrial wastewaters, was completely degraded within 40 min at a potential of 0.15−0.35 V (vs reversible hydrogen electrode) without dosing external iron. Meanwhile, approximately 60% of total organic carbon was efficiently removed by the electro-Fenton system within 4 h at 0.25 V, and the hydroxyl radicals were found to be the main oxidation agent for the phenol degradation. More importantly, the phenol removal efficiency remained at a high level (87%) and the released iron was low (0.8 mg/L) even after 10 cycles of reuse. Thus, an efficient and cost-effective integrated system for the treatment of both domestic and industrial wastewaters was successfully developed and validated. The results from this work are useful to establish a new sustainable pollution control scenario. KEYWORDS: Catalyst, Electro-Fenton, H2O2 production, Municipal sludge, Pollutant degradation
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INTRODUCTION A large amount of municipal wastewater, that is, domestic and industrial wastewaters, is increasingly produced in today’s urban cities. Taking China as an example, approximately 73.5 billion m 3 wastewater was produced in 2015,1 which places tremendous pressure on the environment and calls for efficient and cost-effective municipal wastewater pollutant control approaches. Electro-Fenton (EF) process, in which H2O2 can be continually generated on cathode via a two-electron oxygen reduction reaction (ORR), has recently received great interest because of its numerous merits.2−4 The efficiency of the EF for pollutant degradation depends heavily on the generation rate and cumulative concentration of H2O2.5,6 Among the studied cathodes, carbonaceous material is recognized as a promising H2O2-generating electrocatalyst because of its nontoxic, high overpotential for H2 evolution and good stability.2 To date, graphite felt,7 carbon sponge,8 and activated carbon fiber9 have been widely tested for their feasibility of being used as cathode catalysts. However, the catalytic activity of ORR by pure carbon materials is low.10 Thus, doping of heteroatoms such as nitrogen and sulfur10,11 and synthesis of metal−carbon composites6,12 have also been used to improve the catalytic © 2018 American Chemical Society
performance. Nevertheless, the fabrication of a composite H2O2-generating electrocatalyst is a multistep process and external chemical reagents are generally required, which often makes it environmentally unfriendly. Thus, green synthesis of electrochemical catalysts with high H2O2-generating capacity at a low cost is crucial to facilitate the practical application of EF for wastewater treatment. In fact, domestic sewage is embodied with a high content of organics and such an organic complex could be used as low-cost precursor to synthesize electro-catalysts for H2O2 generation after appropriate treatment. Previous studies have shown that sewage sludge, which is formed during wastewater treatment, could exhibit electrochemical activity for 4-e− ORR via pyrolysis.13,14 However, how to separate and obtain organic precursor from domestic wastewater in a cost-effective way is challenging. Coincidently, iron-based coagulant is widely used for capturing organic carbon in domestic wastewater treatment process and its application is found to be essential in upcoming Received: January 26, 2018 Revised: March 9, 2018 Published: March 19, 2018 5540
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ACS Sustainable Chemistry & Engineering wastewater treatment plants.15 Fe-enriched sludge would be produced accordingly from domestic wastewater treatment process with a pursuit of utmost recovery of energy and resources.16 Therefore, it is hypothesized that the metal− carbon composite (MCC) originating from the domestic wastewater treatment process exhibits ORR activity to produce H2O2 after appropriate treatment. Furthermore, the produced H2O2 could be in situ activated by MCC to degrade industrial pollutants. Thus, a new sustainable scenario for the treatment of both domestic and industrial wastewaters can be developed. In such a scenario, the sludge originating from the coagulation treatment of domestic wastewater is reused as electro-catalysts in an EF system to further treat industrial wastewaters. Therefore, to demonstrate the above concept, a compositiondirected pyrolysis strategy was designed to prepare the MCC with high electrochemical activity. The impact of doping Fe and N, which are the two elements found to be effective in improving electrochemical activity of the material,11,17 on the H2O2 production performance by MCC was explored. Also, an MCC-based EF system was established and its applied potential, mineralization efficiency, degradation kinetics, and cycling use stability for the degradation of phenol, a model pollutant in industrial wastewaters, were evaluated. Furthermore, the mechanism for the phenol degradation in the EF system was elucidated. In this way, an alternative sludge waste reuse approach was proposed and an efficient EF system to degrade phenol was developed. Moreover, a concept “treating industrial wastewater using the sludge originating from domestic wastewater treatment for urban pollution control” could be architected.
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paper. The MCC electrode was obtained after fully drying at room temperature (25 °C). Electrochemical Experiment Setup. All of the electrochemical experiments were conducted on a CHI 760E potentiostat (Chenhua Instrument Co., China) via the potentiostatic method. The potentials reported in this study were referenced to the reversible hydrogen electrode (EVS_RHE) according to the following equation:
E VS_RHE = E VS_Ag/AgCl + Eθ _Ag/AgCl + 0.059 pH
(1)
where EVS_Ag/AgCl (V) is the applied potential referenced to the saturated Ag/AgCl electrode and Eθ_Ag/AgCl (V) is the potential of Ag/ AgCl under the standard conditions. The H2O2 generation performance via ORR was first evaluated in a two-compartment cell with a Nafion 117 membrane as separator. Pt wire and Ag/AgCl (in 3.0 M KCl) were respectively used as the counter and reference electrodes. Both the anode and cathode cells were filled with 75 mL of electrolyte (0.05 M H2SO4 + 0.05 M Na2SO4, pH 1.0), and oxygen was continuously flushed with a flow rate of 50 mL/min. The ORR activity was measured with a rotating ring-disk electrode (5.5 mm diameter; Pine Research Instrumentation, Inc., USA) using a three-electrode mode. To prepare the working electrode, MCC was dispersed on the 75% isopropanol solution with a final concentration of 10 g/L. Nafion solution with 2% content was supplied to the above dispersion and sonication was then applied to form a homogeneous ink. Then 10 μL of ink was loaded onto a rotating ring-disk electrode and dried at room temperature. Linear sweep voltammograms (LSV) were tested by sweeping the potential from 0.8 to −0.1 V (vs RHE) at a rate of 5 mV/s with a rotating speed of 1600 rpm. To detect H2O2, the ring potential was kept constant at 1.2 V, and the H2O2 selectivity (H2O2%) was calculated as follows:17 H 2O2 % =
200ir ir + Nid
(2)
where ir and id are the ring current and disk current, respectively, and N is the current collection efficiency of the Pt ring in the rotating ringdisk electrode (N = 0.4). The electrochemical impedance spectroscopy measurements were conducted at a potential of 0.25 V with a frequency ranging from 100 kHz to 0.1 Hz and an amplitude of 10 mV. To examine the feasibility of MCC for the treatment of industrial wastewater, the EF system was established to degrade phenol as a representative organic pollutant. The degradation of phenol (40 mg/ L) was conducted in a single-compartment cell with the MCC electrode as the working electrode and Pt wire as the counter electrode. Phenol was dissolved in 0.1 M Na2SO4, and the pH was adjusted to 3.0 with 0.1 M H2SO4. A constant potential of 0.05−0.45 V was applied to the working electrode in the EF system. Moreover, an electrochemical cell with an open circuit was used to eliminate the adsorption impact. As a comparison, O2 was replaced with N2 to exclude electrical oxidation. In the EF degradation process, methanol (20%, v/v) was used as a quenching agent to determine the radical species. The mineralization current efficiency (MCE, %) at a given time was calculated as follows to evaluate the current utilization efficiency of the EF system:2
MATERIALS AND METHODS
Chemicals. Analytical-grade phenol, sodium sulfate, iron(III) chloride (FeCl3·6H2O) were purchased from Sinopharm Chemical Reagent Co., China. Nafion 117 proton exchange membrane was purchased from DuPont Co., USA. Nafion solution and 5,5dimethylpyrroline-N-oxide (DMPO) were obtained from Sigma Co., USA. Carbon papers (Toray Co., Japan) used in this study were sequentially rinsed with acetone, HCl (1 M), and ethanol for grease and other impurities removal. Milli-Q water was used to remove the residual chemicals from each rinsing step. MCC Preparation and Cathode Fabrication. MCC was obtained by carbonation of the coagulated sludge under a NH3/Ar atmosphere (V% = 1:9) (Figure S1, Supporting Information). In brief, domestic wastewater after grit settling pretreatment was taken from the Wangtang Wastewater Treatment Plant (Hefei, China). FeCl3· 6H2O (0.5 mM) was added to the wastewater, followed by 130 rpm rapid agitation for 3 min and 40 rpm slow agitation for 25 min. Then the precipitates were freeze-dried for 24 h to obtain the precursor. Afterward, MCCs were prepared by carbonization of the precursor at 800 °C for 4 h (MCC800-4), 6 h (MCC800-6), and 8 h (MCC800-8) under a NH3/Ar atmosphere (detailed information in Supporting Information). Moreover, the MCCs at the other two carbonization temperatures, 600 °C (MCC600-6) and 1000 °C (MCC1000-6), were additionally prepared. All of the prepared MCCs were immersed in 1 M HCl solution three times to remove soluble ash, washed with MilliQ water to pH 7.0, and then dried at 105 °C overnight. As a comparison, the precursor was also annealed under Ar for 6 h, and the resulting product was named as MCC800-6Ar. In addition, the precursor with HCl prerinsing was carbonized under NH3 and named MCCH800-6. The main preparation conditions and the characteristics of the different catalysts are summarized in Table S1. To prepare an MCC cathode, 10 mg of MCC was mixed with 0.5 mL of 75% isopropanol and 10 μL of a Nafion solution. Then the mixture was sonicated for 30 min and dropped onto a 3 × 3 cm carbon
MCE = [△(TOC)t nFV /(4.32 × 107mIt )] × 100
(3)
where Δ(TOC)t is the TOC decay (mg C/L), F is the Faraday constant (96485 C/mol), n is the number of electrons exchanged, V is the electrolyte volume (L), m is the number of carbon atoms of the pollutants, I is the applied current (A), t is the electrolysis time (h), and 4.32 × 107 is the conversion factor for units of homogenization (=3600 s/h × 12000 mg of C/mol). Analysis. The H2O2 concentration was measured using a colorimetric method.18 Phenol was detected by high-performance liquid chromatography (HPLC, 1260 Infinity, Agilent, Inc., USA) with 50% methanol as the mobile phase. Samples were taken at set intervals and mixed with methanol immediately to stop the reaction. However, 5541
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ACS Sustainable Chemistry & Engineering to avoid the interference of methanol to the total organic carbon (TOC) detection, sodium nitrite was dosed to terminate the reaction. TOC was detected with a TOC analyzer (Muti N/C 2100, Analytik AG, Germany). The product morphology was observed with a field emission scanning electron microscope (SEM, Zeiss Co., Germany). The chemical compositions and valences of the elements on the MCC surface were analyzed by X-ray photoelectron spectroscopy (XPS, EXCALAB250, Thermo Fisher, Inc., USA). The surface areas of the samples were measured by the Brunauer−Emmett−Teller (BET) method with a Builder 4200 instrument (Tristar II 3020 M, Micromeritics Co., USA). Raman spectra were excited by radiation at 514.5 nm from a confocal laser micro-Raman spectrometer (LABRAM-HR, Jobin-Yvon Co., France). The radicals formed in the EF process were examined by electron spin resonance (ESR, JESFA200, JEOL Co., Japan). Before the ESR detection, DMPO was immediately mixed with the sample to form DMPO−•OH. The metal element was detected by atomic absorption spectroscopy (AA800, PerkinElmer Co., USA).
metal electrocatalysts under similar conditions (Table S2). Since the material obtained at 800 °C for 6 h exhibited the best performance, such a carbonation condition was selected to further prepare MCC800-6Ar and MCCH800-6 to investigate the pyrolysis atmosphere and precursor composition influences. The electrocatalytic H2O2-producing abilities of MCC800-6, MCC800-6Ar, and MCCH800-6 were further evaluated. It was observed that the Ar gas atmosphere pyrolysis strategy could greatly prevent the 2 e− oxygen reduction and no obvious H2O2 accumulated for the obtained product (Figure 1b). In comparison, acid pickling of the precursor reduced the H2O2 level with accumulation concentration of 58 mg/L after 120 min electrolysis, which was only half of that for the MCC800-6. A comparison of the XRD patterns and SEM images between the three MCCs shows that no obvious differences were observed (Figures S2 and S3), implying that the material’s morphology is not the key factor governing the performance. Electrochemical Properties of MCC. The electrochemical properties of MCC were further investigated by rotating ringdisk electrode measurements. Carbonization under Ar gas atmosphere greatly decreased the electrochemical activity, as both the ring and disk currents were the lowest among the three studied materials (Figure 2a). Although the calculated H2O2 selectivity of MCC800-Ar was close to 100%, the high impedance resulted in a low current intensity (Figure 2b,c), which further inhibited the oxygen reduction and induced the decline of H2O2 accumulation level. In comparison, the precursor with acid pretreatment changed the primary ORR on the surface from 2 e− to 4 e−, as evidenced by the decline of selectivity (10−18%) for H2O2 production at 0.2−0.4 V. To further understand the electro-activity of the materials, the Raman spectrum of the as-obtained MCC was recorded (Figure 3a). The obtained materials showed two main Raman peaks at ∼1360 cm−1 (D band) and 1600 cm−1 (G band). D bands are known to be characteristic of disordered graphite with structural defects, while the G band is associated with graphitic carbon and the D/G band intensity ratio (ID/IG) is commonly used to characterize the graphited degree (the degree of graphitization) of carbonaceous materials.19,20 In the current case, the ID/IG of MCC800-6 (0.89) was clearly lower than those of MCC800-6Ar (1.02) and MCCH800-6 (0.98), which indicates a higher graphitization degree and results in better electrical conductivity. XPS analysis was further performed to explore the differences in the electronic structure and surface chemical composition among the three materials. The signals of C, N, O, and Fe were clearly observed on the MCC800-6 (Figure 3b). Interestingly, the N 1s signal vanished on MCC800-6Ar, implying that a NH3 atmosphere is essential for N doping. The high-resolution N 1s spectra of the MCC800-6 and MCCH800-6 catalysts suggest that pyridinic N, pyrrolic N, graphitic N, and quaternary N21,22 existed (Figure S4). It has been widely reported that pyridinic N and pyrrolic N can form Fe−Nx moieties with Fe due to their lone-pair electrons,17,23 and the graphitic N combined with Fe−Nx moieties was the efficient active site for ORR.17,24 For comparison, since the N content of the MCC800-Ar was quite low (0.3%, Table S3), the lack of ORR sites resulted in a negligible H2O2 accumulation. Although the N content of MCCH800-6 was comparable to that of MCC800-6, the low Fe content (Figure 3b) might reduce the activity and induce a reduction in the H2O2 level because of the less Fe−Nx active sites. Therefore, N and Fe might play an important role in the electrosynthesis of H2O2.
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RESULTS AND DISCUSSION Optimized H2O2 Production with MCC. The cumulative H2O2 concentration is vital to the overall EF performance; thus, the MCC preparation conditions were optimized to improve the H2O2 yields. It was found that the H2O2 concentration increased almost linearly over time, and the highest H2O2 concentration (116 mg/L) was achieved by MCC800-6 after 120 min electrolysis (Figure 1a). Further increase in pyrolysis temperature or prolonging of the carbonation time would cause the deterioration of catalytic activity. The average H2O2 production efficiency of the MCC reached 432 mg/h/g, which is superior to most of the other carbon-based non-noble-
Figure 1. Optimization of the MCC fabrication for the improved H2O2 generation performance. Impacts of pyrolysis temperature and time on cumulative H2O2 production by MCCs (a). Comparison of carbonation atmosphere and acid pickling pretreatment of precursor for H2O2 production (b). The applied potential was 0.25 V and the experiment was conducted in triple trials (n = 3). 5542
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the EF system by loading MCC as cathode catalyst and no external supply of iron was required (Figure 4a). Moreover, the contribution of the pure carbon paper, which was not coated with catalyst, to the overall phenol degradation was low. The phenol degradation kinetics in the EF with the MCC electrode was further analyzed. Although the initial degradation rate was slow, which might be due to the insufficient accumulation of H2O2, the overall phenol degradation was found to follow the pseudo first order (Figure S5). The apparent rate constant (k) for the system with the carbon paper was only 0.001 min−1. The degradation rate was greatly improved with an increase in k to 0.078 min−1 when MCC was used as the working electrode. To optimize the phenol removal performance of the system, the impacts of several operational parameters on the system were investigated. Since the potential applied to the cathode affected the H2O2 accumulation level, the impact of the potential on the phenol removal without the addition of iron was initially explored. The phenol concentration rapidly declined within 40 min when the potential was increased from 0.15 to 0.35 V (Figure 4b). However, a lower (0.015 V) or higher potential (0.45 V) resulted in a poorer performance. A lower potential might further reduce H2O2 to H2O and 4 e− ORR process became predominated. The TOC removal efficiency increased continuously over time for each investigated potential (Figure 4c). With the same tendency as the phenol removal efficiency, the highest TOC removal efficiency was achieved at 0.25 V. Approximately 60% of TOC was removed after 4-h electrolysis at 0.25 V. Such results imply a high efficiency of the EF system. In addition to the applied potential, pH is another crucial factor to greatly affect overall Fenton reaction performance. In this work, the phenol removal performances of the EF system at raised pHs were further investigated. Phenol was found to be completely removed at pH 4.0, while only 30% removal efficiency was achieved by raising the pH to 5.0 (Figure S6a). The poor performance at pH 5.0 might be due to the low H2O2 concentration (Figure S6b). As discussed above, Fe might play a key role in catalyst’s activity. Hence, FeCl3·6H2O at reduced concentrations (0.2 mM and nil) was dosed in the coagulation process to explore the impact of Fe content on MCC’s performance. It was observed that by reduction of the Fe dosage to 0.2 mM, only 20% of phenol could be removed within 60 min (Figure S7a). If the domestic wastewater precipitate was directly used as a precursor to carbonize without a dose of Fe, the obtained product showed no H2O2-producing ability (Figure S7b). The above results again proved that Fe was essential to achieve a high performance of H2O2 synthesis for MCC.
Figure 2. Electrochemical performance of the as-prepared catalyst. Rotating ring-disk electrode measurements (a) show the oxygen reduction currents. The calculated H2O2 selectivity (b) at different potentials and EIS analysis (c) of MCCs under different carbonation atmospheres.
Phenol Degradation by MCC in EF. To examine the application potential of the prepared MCC catalysts, phenol degradation by these catalysts in EF was investigated. It was found that the phenol was efficiently removed within 40 min in
Figure 3. Raman spectra (a) and XPS surveys (b) of MCCs obtained under different conditions. 5543
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Figure 4. Phenol degradation performance by the loading MCC as cathode catalyst in an EF system. Phenol removal in the EF system with MCC as working electrode at 0.25 V (a). The impacts of applied cathode potential on phenol degradation (b) and TOC removal efficiency (c) in the EF system without the external assistance of iron. Phenol removal efficiency over time for the EF, electrode adsorption, and electrical oxidation at 0.25 V condition (d).
Figure 5. Reactive species responsible for phenol degradation in EF system and the cycling reuse stability of MCC. ESR spectrum of the DMPOtrapped hydroxyl radical after 30 min reaction (a), phenol removal as a function of time by using methanol as radical scavenger (b), phenol degradation in different batch runs in the EF system (c), and conventional Fenton system by dosing with 135 mg/L H2O2 (d).
oxidation. These results clearly demonstrate that H2O2 was in situ electro-synthesized and activated by the MCC electrode effectively to mineralize phenol. In the Fenton reaction, intermediate free radical formation via H2O2 activation is the key step to achieve effective pollutant degradation. Thus, the presence of hydroxyl radicals in the EF was verified using the DMPO spin-trapping method. Figure 5a
To exclude the influences of anode oxidation and cathode material adsorption, electrical oxidation (aeration N2 to inhibit the H2O2 generation) and electrode adsorption (with opencircuit) experiments were conducted as references at 0.25 V. As shown in Figure 4d, 97% phenol was degraded in the EF system after 40 min treatment. In comparison, only 6% phenol was adsorbed by the electrode and 12% was removed via electric 5544
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ACS Sustainable Chemistry & Engineering shows the typical ESR spectrum obtained after a 30 min reaction. A spectrum consisting of quartet lines with a peak height ratio of 1:2:2:1 was clearly observed. Such an ESR spectrum is characterized as an hydroxyl radical.25 Moreover, methanol was used as the radical scavenger to quench the hydroxyl radicals. The results indicate that phenol degradation was greatly inhibited by the dose of methanol (Figure 5b). Therefore, the hydroxyl radicals formed in the EF system was the main reactive species to mineralize phenol. From an economic perspective, the cycling reuse stability is another issue for the practical application of catalysts. In our work, the recycling capability of the catalysts was evaluated by using a reused MCC electrode in the EF system. The MCC electrode could achieve a high phenol removal efficiency (91%) after seven repeated uses (Figure 5c). The phenol removal was slightly reduced to 87% even after ten cycles. Since the MCC contained iron, its release into the solution needs to be evaluated. Although the released iron concentration in solution was slightly high after the third cycle (4 mg/L), this value was below 1 mg/L after seven-cycle use. H2O2 may be activated by the released Fe for phenol degradation. As a result, H2O2 with a cumulative concentration (135 mg/L) was mixed with ferrous iron at the released concentration level for phenol degradation by the conventional Fenton reaction. It was observed that 4 mg/L iron resulted in excellent phenol removal (Figure 5d). Reduction in iron dose to 1.9 mg/L decreased the degradation efficiency to 76%. When the iron dose was continually reduced to 0.8 mg/L, the overall phenol removal sharply declined to 10% within 60 min. However, the above experimental result shows that 87% of phenol was removed by EF in the tenth cycle. Such a comparison indicates that MCC might be able to activate the generated H2O2 in situ, resulting in a better pollutant removal performance compared to the conventional Fenton process. The morphology and structure of reused material were further examined to prove its stability. The SEM images indicate that the morphology of material remained unchanged after recycling use (Figure S8). In addition, the pyridinic N, pyrrolic N, graphitic N, and quaternary N were observed in the XPS spectra (Figure S9a). By comparison to the pristine material, additional C−F structure (292 eV) was observed at the reused catalyst’s surface (Figure S9b), which is ascribed to the added Nafion substance. In all, both the morphology and chemical composition of the catalysts were proven to be stable. Other Pollutants Degradation in MCC-Fabricated EF System. In addition to phenol, the feasibility of the EF system on other industrial wastewater treatment would be of great importance to the proposed concept. As a result, the degradation of various typical refractory pollutants, for example, dye (rhodamine B), pesticide (atrazine), and bisphenol A, by the fabricated EF system was evaluated. All of these three types of pollutants were efficiently removed within 40 min at 0.25 V applied potential (Figure 6). These results further validate the universality of our proposed strategy. Although the domestic wastewater quality is site-specific and the activity of the obtained MCC might vary, the high activity of the material can be achieved by post chemical modifications. One of the major concerns for EF degradation of pollutants is the electric energy consumption. Here, the mineralization current efficiency of the EF system for phenol degradation was only 5.9%, which is low. However, the current density (1.67 mA/cm2) is much lower than those reported in other studies (10−30 mA/cm2).26,27 In this work, only two factors, that is,
Figure 6. Performance of the MCC-based EF system for the degradation of other pollutants. Inert is the color change of rhodamine B after EF treatment.
carbonation atmosphere and pyrolysis time, which affect the product performance, were taken into account. Considering that the BET surface area of the material (89 m2/g) was low (Figure S10), other strategies such as tailoring pore structure and tuning surface hydrophilicity28 could be adopted to further improve the MCC performance and current efficiency.
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CONCLUSIONS In this work, a green waste pollution control strategy for urban cities’ sustainable development, that is, treating industrial wastewater with the sludge originating from domestic wastewater treatment process, was proposed. After one-step pyrolysis of excess sludge formed in the domestic wastewater treatment process, an efficient H2O2 electrochemical synthesis material was successfully fabricated. Electrochemical tests revealed that the obtained MCC was able to catalyze O2 reduction to H2O2 with an accumulation rate of 432 mg/h/g under acidic conditions. Benefiting from its high activity and selectivity for H2O2 production, the MCC-fabricated EF system was efficient in complete removal of 40 mg/L phenol within 40 min at a potential of 0.15−0.35 V without the need of dosing external iron. The hydroxyl radicals were found to be the main reactive species and approximately 60% of total organic carbon removal efficiency was achieved within 4 h at 0.25 V. This work shows a great potential of such an integrated approach for efficient and cost-effective urban water pollution control. Further optimization of the MCC material and operation system could also improve its performance.
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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.8b00416. Detailed description of pyrolysis procedure; comparison of catalytic activities of various catalysts; elemental analysis of the precursor and MCC800-6; MCC preparation process; XRD patterns; SEM images; XPS spectra of C 1s, Fe 2p, and N 1s; kinetic analysis of the phenol degradation; impacts of pH and Fe content on phenol degradation and BET surface area results (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Fax: +86-551-63601592. E-mail:
[email protected] (H.-Q.Y.). 5545
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ACS Sustainable Chemistry & Engineering ORCID
(14) Yuan, S. J.; Dai, X. H. An efficient sewage sludge-derived bifunctional electrocatalyst for oxygen reduction and evolution reaction. Green Chem. 2016, 18 (14), 4004. (15) Wilfert, P.; Kumar, P. S.; Korving, L.; Witkamp, G. J.; van Loosdrecht, M. C. M. The relevance of phosphorus and iron chemistry to the recovery of phosphorus from wastewater: A review. Environ. Sci. Technol. 2015, 49 (16), 9400. (16) McCarty, P. L.; Bae, J.; Kim. Domestic wastewater treatment as a net energy producer−can this be achieved? Environ. Sci. Technol. 2011, 45, 7100. (17) Lin, L.; Zhu, Q.; Xu, A. W. Noble-metal-free Fe−N/C catalyst for highly efficient oxygen reduction reaction under both alkaline and acidic conditions. J. Am. Chem. Soc. 2014, 136 (31), 11027. (18) Eisenberg, G. M. Colorimetric determination of hydrogen peroxide. Ind. Eng. Chem., Anal. Ed. 1943, 15 (5), 327. (19) Wang, Y.; Alsmeyer, D. C.; Mccreery, R. L. Raman spectroscopy of carbon materials: Structural basis of observed spectra. Chem. Mater. 1990, 2 (5), 557. (20) Tuinstra, F.; Koenig, J. L. Raman spectrum of graphite. J. Chem. Phys. 1970, 53 (3), 1126. (21) Niu, W. H.; Li, L. G.; Liu, X. J.; Wang, N.; Liu, J.; Zhou, W. J.; Tang, Z. H.; Chen, S. W. Mesoporous N-doped carbons prepared with thermally removable nanoparticle templates: An efficient electrocatalyst for oxygen reduction reaction. J. Am. Chem. Soc. 2015, 137 (16), 5555. (22) Song, L. T.; Wu, Z. Y.; Zhou, F.; Liang, H. W.; Yu, Z. Y.; Yu, S. H. Sustainable hydrothermal carbonization synthesis of iron/nitrogendoped carbon nanofiber aerogels as electrocatalysts for oxygen reduction. Small 2016, 12 (46), 6398. (23) Liu, R. L.; Wu, D. Q.; Feng, X. L.; Mullen, K. Nitrogen-doped ordered mesoporous graphitic arrays with high electrocatalytic activity for oxygen reduction. Angew. Chem., Int. Ed. 2010, 49 (14), 2565. (24) Chung, H. T.; Won, J. H.; Zelenay, P. Active and stable carbon nanotube/nanoparticle composite electrocatalyst for oxygen reduction. Nat. Commun. 2013, 4, 1922. (25) Utsumi, H.; Han, Y. H.; Ichikawa, K. A kinetic study of 3chlorophenol enhanced hydroxyl radical generation during ozonation. Water Res. 2003, 37 (20), 4924. (26) Babuponnusami, A.; Muthukumar, K. Advanced oxidation of phenol: A comparison between Fenton, electro-Fenton, sono-electroFenton and photo-electro-Fenton processes. Chem. Eng. J. 2012, 183, 1. (27) Montes, I. J. S.; Silva, B. F.; Aquino, J. M. On the performance of a hybrid process to mineralize the herbicide tebuthiuron using a DSA (R) anode and UVC light: A mechanistic study. Appl. Catal., B 2017, 200, 237. (28) Liu, W. J.; Jiang, H.; Yu, H. Q. Development of biochar-based functional materials: Toward a sustainable platform carbon material. Chem. Rev. 2015, 115 (22), 12251.
Wen-Wei Li: 0000-0001-9280-0045 Feng Zhang: 0000-0002-8809-6910 Hong Jiang: 0000-0002-4261-7987 Han-Qing Yu: 0000-0001-5247-6244 Author Contributions †
These authors contributed equally to this work (B.C.H. and J.J.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (51538011), the Collaborative Innovation Center of Suzhou Nano Science and Technology of the Ministry of Education of China, and the Anhui S&T Key Project (1501041118) for the support of this work.
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DOI: 10.1021/acssuschemeng.8b00416 ACS Sustainable Chem. Eng. 2018, 6, 5540−5546
