Electrochemically Catalytic Degradation of Phenol ... - ACS Publications

Mar 19, 2018 - and Han-Qing Yu*. CAS Key Laboratory of Urban Pollutant Conversion, Department of Chemistry, University of Science & Technology of Chin...
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Electrochemically Catalytic Degradation of Phenol with Hydrogen Peroxide in-situ Generated and Activated by a Municipal Sludge-derived Catalyst Bao-Cheng Huang, Jun Jiang, Wei-Kang Wang, Wen-Wei Li, Feng Zhang, Hong Jiang, and Han-Qing Yu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00416 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 21, 2018

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ACS Sustainable Chemistry & Engineering

Electrochemically Catalytic Degradation of Phenol with Hydrogen Peroxide in-situ Generated and Activated by a Municipal Sludge-derived Catalyst

Bao-Cheng Huang†, Jun Jiang†, Wei-Kang Wang, Wen-Wei Li, Feng Zhang, Hong Jiang, 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



These authors contributed equally to this work.

*Corresponding Author: Prof. Han-Qing Yu, Fax: +86-551-63601592. E-mail: [email protected]

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ABSTRACT

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With the rapid global urbanization, today’s cities are facing an increasing pressure

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for the treatment of both domestic and industrial wastewaters. In this work, a

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proof-of-concept of “treating industrial wastewater using the sludge originating from

5

domestic wastewater treatment for urban pollution control” was proposed. After

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one-step pyrolysis of the excess sludge from domestic wastewater treatment, a

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metal-carbon composite catalyst with a high H2O2-producting capacity (432 mg/h/g)

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was successfully synthesized. By applying the prepared material as a cathode catalyst

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in an electro-Fenton system, phenol (40 mg/L), a model pollutant in industrial

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wastewaters, was completely degraded within 40 min at a potential of 0.15-0.35 V (vs.

11

reversible

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approximately 60% of total organic carbon was efficiently removed by the

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electro-Fenton system within 4 h at 0.25 V, and the hydroxyl radicals were found to be

14

the main oxidation agent for the phenol degradation. More importantly, the phenol

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removal efficiency remained at a high level (87%) and the released iron was low (0.8

16

mg/L) even after 10 cycles of reuse. Thus, an efficient and cost-effective integrated

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system for the treatment of both domestic and industrial wastewaters was successfully

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developed and validated. The results from this work are useful to establish a new

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sustainable pollution control scenario.

hydrogen

electrode)

without

dosing

external

iron.

Meanwhile,

20 21

Keywords: Catalyst; electro-Fenton; H2O2 production; municipal sludge; pollutant

22

degradation

23

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INTRODUCTION

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A large amount of municipal wastewater, i.e., domestic and industrial wastewaters, is

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increasingly produced in today’s urban cities. Taking China as an example,

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approximately 73.5 billion m3 wastewater was produced in 2015,1 which places

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tremendous pressure on the environment and calls for efficient and cost-effective

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municipal wastewater pollutant control approaches. Electro-Fenton (EF) process, in

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which H2O2 can be continually generated on cathode via a two-electron oxygen

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reduction reaction (ORR), has recently received great interests because of its

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numerous merits.2-4 The efficiency of the EF for pollutant degradation depends

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heavily on the generation rate and cumulative concentration of H2O2.5,6 Among the

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studied

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H2O2-generating electrocatalyst due to its nontoxic, high overpotential for H2

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evolution and good stability.2 Up to date now graphite felt,7 carbon sponge,8 and

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activated carbon fiber9 have been widely tested for their feasibilities of using as

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cathode catalysts. However, the catalytic activity of ORR by pure carbon materials is

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low.10 Thus, doping of heteroatoms such as nitrogen and sulfur10,11 and synthesis of

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metal-carbon composites6,12 have also been used to improve the catalytic performance.

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Nevertheless, the fabrication of a composite H2O2-generating electrocatalyst is a

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multi-step process and external chemical reagents are generally required, which often

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makes it environmentally unfriendly. Thus, green synthesis of electrochemical

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catalysts with high H2O2-generating capacity at a low cost is crucial to facilitate the

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practical application of EF for wastewater treatment.

cathodes,

carbonaceous

material

is

recognized

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promising

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In fact, domestic sewage is embodied with a high content of organics and such an

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organic complex could be used as low-cost precursor to synthesize electro catalysts

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for H2O2 generation after appropriate treatment. Previous studies have shown that

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sewage sludge, which is formed during wastewater treatment, could exhibit an

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electrochemical activity for 4-e- ORR via pyrolysis.13,14 However, how to separate and

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obtain organic precursor from domestic wastewater in a cost-effective way is

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challenging. Coincidently, iron-based coagulant is widely used for capturing organic

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carbon in domestic wastewater treatment process and its application is found to be

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essential in upcoming wastewater treatment plants.15 Fe-enriched sludge would be

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produced accordingly from domestic wastewater treatment process with a pursuit of

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utmost recovery of energy and resource.16 Therefore, it is hypothesized that the

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metal-carbon composite (MCC) originating from the domestic wastewater treatment

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process exhibits an ORR activity to produce H2O2 after appropriate treatment.

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Furthermore, the produced H2O2 could be in-situ activated by MCC to degrade

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industrial pollutants. Thus, a new sustainable scenario for the treatment of both

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domestic and industrial wastewaters can be developed. In such a scenario, the sludge

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originating from the coagulation treatment of domestic wastewater is reused as

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electro-catalysts in an EF system to further treat industrial wastewaters.

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Therefore, to demonstrate the above concept, a composition-directed pyrolysis

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strategy was designed to prepare the MCC with a high electrochemical activity. The

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impact of doping Fe and N, which are the two elements found to be effective in

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improving electrochemical activity of the material,11,17 on the H2O2 production 4

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performance by MCC was explored. Also, an MCC-based EF system was established

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and its applied potential, mineralization efficiency, degradation kinetics, and cycling

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use stability for the degradation of phenol, a model pollutant in industrial wastewaters,

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were evaluated. Furthermore, the mechanism for the phenol degradation in the EF

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system was elucidated. In this way, an alternative sludge waste reuse approach was

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proposed and an efficient EF system to degrade phenol was developed. Moreover, a

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concept “treating industrial wastewater using the sludge originating from domestic

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wastewater treatment for urban pollution control” could be architected.

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MATERIALS AND METHODS

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Chemicals. Analytical grade phenol, sodium sulfate, iron (III) chloride (FeCl3•6H2O)

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were purchased from Sinopharm Chemical Reagent Co., China. Nafion 117 Proton

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exchange membrane was purchased from DuPont Co., USA. Nafion solution and

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5,5-dimethylpyrroline-N-oxide (DMPO) were obtained from Sigma Co., USA.

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Carbon papers (Toray Co., Japan) used in this study were sequentially rinsed with

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acetone, HCl (1 M), and ethanol for grease and other impurities removal. Milli-Q

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water was used to remove the residual chemicals from each rinsing step.

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MCC Preparation and Cathode Fabrication. MCC was obtained by

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carbonation of the coagulated sludge under a NH3/Ar atmosphere (V%=1:9) (Fig. S1).

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In brief, domestic wastewater after grit settling pretreatment was taken from the

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Wangtang Wastewater Treatment Plant (Hefei, China). FeCl3•6H2O of 0.5 mM was 5

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added to the wastewater, followed by 130-rpm rapid agitation for 3 min and 40-rpm

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slow agitation for 25 min. Then, the precipitates were freeze-dried for 24 h to obtain

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the precursor. Afterwards, MCCs were prepared by carbonization of the precursor at

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800 °C for 4 h (MCC800-4), 6 h (MCC800-6), and 8 h (MCC800-8) under a NH3/Ar

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atmosphere (detailed information in SI). Moreover, the MCCs at the other two

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carbonization temperatures, 600 °C (MCC600-6) and 1000 °C (MCC1000-6), were

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additionally prepared. All of the prepared MCCs were immersed in 1 M HCl solution

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three times to remove soluble ash, washed with milli-Q water to pH=7.0, and then

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dried at 105 °C overnight. As a comparison, the precursor was also annealed under Ar

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for 6 h, and the resulting product was named as MCC800-6Ar. In addition, the

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precursor with HCl pre-rinsing was carbonized under NH3 and named as MCCH800-6.

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The main preparation conditions and the characteristics of the different catalysts are

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summarized in Table S1.

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To prepare an MCC cathode, 10 mg of MCC was mixed with 0.5 ml of 75%

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isopropanol and 10 µL of a Nafion solution. Then, the mixture was sonicated for 30

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min and dropped onto a 3×3 cm carbon paper. The MCC electrode was obtained after

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fully drying at room temperature (25 °C).

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Electrochemical Experiment Setup. All of the electrochemical experiments

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were conducted on a CHI 760E potentiostat (Chenhua Instrument Co., China) via the

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potentiostatic method. The potentials reported in this study were referenced to the

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reversible hydrogen electrode (EVS RHE) according to the following equation: EVS RHE=EVS Ag/AgCl+Eθ Ag/AgCl+0.059pH

(1) 6

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where EVS Ag/AgCl (V) is the applied potential referenced to the saturated Ag/AgCl

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electrode, Eθ Ag/AgCl (V) is the potential of Ag/AgCl under the standard conditions.

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The H2O2 generation performance via ORR was firstly evaluated in a

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two-compartment cell with a Nafion 117 membrane as separator. Pt wire and Ag/AgCl

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(in 3.0 M KCl) were respectively used as the counter and reference electrodes. Both

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the anode and cathode cells were filled with 75 ml of electrolyte (0.05 M H2SO4 +

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0.05 M Na2SO4, pH=1.0), and oxygen was continuously flushed with a flowrate of 50

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ml/min.

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The ORR activity was measured with a rotating ring-disk electrode (5.5 mm

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diameter; Pine Research Instrumentation, Inc., USA) using a three-electrode mode. To

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prepare the working electrode, MCC was dispersed on the 75% isopropanol solution

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with a final concentration of 10 g/L. Nafion solution with 2% content was supplied to

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the above dispersion and sonication was then applied to form a homogeneous ink.

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Then, 10 µL of ink was loaded onto a rotating ring-disk electrode and dried at room

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temperature. Linear sweep voltammograms (LSV) were tested by sweeping the

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potential from 0.8 to -0.1 V (vs. RHE) at a rate of 5 mV/s with a rotating speed of

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1600 rpm. To detect H2O2, the ring potential was kept constant at 1.2 V, and the H2O2

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selectivity (H2O2%) was calculated as follows:17

‫ܪ‬ଶ ܱଶ % =

200݅௥ ݅௥ + ܰ݅ௗ

(2)

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where ir and id are the ring current and disk current, respectively, and N is the current

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collection efficiency of the Pt ring in the rotating ring-disk electrode (N=0.4). The

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electrochemical impedance spectroscopy measurements were conducted at a potential 7

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of 0.25 V with a frequency ranging from 100 kHz to 0.1 Hz and an amplitude of 10

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mV.

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In order to examine the feasibility of MCC for the treatment of industrial

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wastewater, the EF system was established to degrade phenol as a representative

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organic pollutant. The degradation of phenol (40 mg/L) was conducted in a single

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compartment cell with the MCC electrode as the working electrode and Pt wire as the

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counter electrode. Phenol was dissolved in 0.1 M Na2SO4, and the pH was adjusted to

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3.0 with 0.1 M H2SO4. A constant potential of 0.05~0.45 V was applied to the

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working electrode in the EF system. Moreover, an electrochemical cell with an open

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circuit was used to eliminate the adsorption impact. As a comparison, O2 was replaced

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with N2 to exclude electrical oxidation. In the EF degradation process, methanol (20%,

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v/v) was used as a quenching agent to determine the radical species.

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The mineralization current efficiency (MCE, %) at a given time was calculated as follows to evaluate the current utilization efficiency of the EF system:2 MCE=[△(TOC)tnFV/(4.32×107mIt)]×100

(3)

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where △(TOC)t is the TOC decay (mg C/L), F is faraday constant (96485 C/mol), n is

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the number of electrons exchanged, V is the electrolyte volume (L), m is the number

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of the carbon atoms of the pollutants, I is the applied current (A), t is the electrolysis

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time (h), and 4.32×107 is the conversion factor for units homogenization (= 3600

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s/h×12000 mg C/mol).

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Analysis. The H2O2 concentration was measured using a colorimetric method.18

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Phenol was detected by high performance liquid chromatography (HPLC, 1260 8

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Infinity, Agilent, Inc., USA) with 50% methanol as the mobile phase. Samples were

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taken at set intervals and mixed with methanol immediately to stop the reaction.

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However, in order to avoid the interference of methanol to the total organic carbon

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(TOC) detection, sodium nitrite was dosed to terminate the reaction. TOC was

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detected with a TOC analyzer (Muti N/C 2100, Analytik AG, Germany). The product

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morphology was observed with a field emission scanning electron microscopy (SEM,

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Zeiss Co., Germany). The chemical compositions and valences of the elements on the

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MCC surface were analyzed by X-ray photoelectron spectroscopy (XPS,

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EXCALAB250, Thermo Fisher, Inc., USA). The surface areas of the samples were

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measured by the Brunauer-Emmett-Teller (BET) method with a Builder 4200

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instrument (Tristar II 3020 M, Micromeritics Co., USA). Raman spectra were excited

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by radiation at 514.5 nm from a confocal laser micro-Raman spectrometer

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(LABRAM-HR, Jobin-Yvon Co., France). The radicals formed in the EF process were

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examined by electron spin resonance (ESR, JES-FA200, JEOL Co., Japan). Before the

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ESR detection, DMPO was immediately mixed with the sample to form DMPO-•OH.

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The metal element was detected by atomic absorption spectroscopy (AA800, Perkin

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Elmer Co., USA).

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RESULTS AND DISCUSSION

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Optimized H2O2 Production with MCC. The cumulative H2O2 concentration is vital

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to the overall EF performance, thus, the MCC preparation conditions were optimized 9

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to improve the H2O2 yields. It was found that the H2O2 concentration increased almost

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linearly over time, and the highest H2O2 concentration (116 mg/L) was achieved by

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MCC800-6 after 120-min electrolysis (Fig. 1a). Further increase in pyrolysis

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temperature or prolonging the carbonation time would cause the deterioration of

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catalytic activity. The average H2O2 production efficiency of the MCC reached 432

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mg/h/g, which is superior to most of the other carbon-based non-noble metal

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electrocatalysts under similar conditions (Table S2). Since the material obtained at

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800 °C for 6 h exhibited the best performance, such a carbonation condition was

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selected to further prepare MCC800-6Ar and MCCH800-6 to investigate the pyrolysis

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atmosphere and precursor composition influences.

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The electro-catalytic H2O2-producing abilities of MCC800-6, MCC800-6Ar, and

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MCCH800-6 were further evaluated. It was observed that Ar gas atmosphere

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pyrolysis strategy could greatly prevent the 2 e- oxygen reduction and no obvious

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H2O2 accumulated for the obtained product (Fig. 1b). In comparison, acid pickling of

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the precursor reduced the H2O2 level with accumulation concentration of 58 mg/L

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after 120-min electrolysis, which was only half of that for the MCC800-6. A

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comparison of the XRD patterns and SEM images between the three MCCs shows

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that no obvious differences were observed (Fig. S2 and S3), implying that the

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material’s morphology is not the key factor governing the performance.

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Electrochemical Properties of MCC. The electrochemical properties of MCC

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were further investigated by rotating ring-disk electrode measurements. Carbonization

197

under Ar gas atmosphere greatly decreased the electrochemical activity, as both the 10

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ring and disk currents were the lowest among the three studied materials (Fig. 2a).

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Although the calculated H2O2 selectivity of MCC800-Ar was close to 100%, the high

200

impedance resulted in a low current intensity (Fig. 2b and 2c), which further inhibited

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the oxygen reduction and induced the decline of H2O2 accumulation level. In

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comparison, the precursor with acid pretreatment changed the primary ORR on the

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surface from 2 e- to 4 e-, as evidenced by the decline of selectivity (10-18%) for H2O2

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production at 0.2-0.4 V.

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To further understand the electro-activity of the materials, the Raman spectrum of

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the as-obtained MCC was recorded (Fig. 3a). The obtained materials showed two

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main Raman peaks at ~1360 cm-1 (D band) and 1600 cm-1 (G band). D bands are

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known to be characteristic of disordered graphite with structural defects, while the G

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band is associated with graphitic carbon and the D/G band intensity ratio (ID/IG) is

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commonly used to characterize the graphited degree (the degree of graphitization) of

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carbonaceous materials.19,20 In the current case, the ID/IG of MCC800-6 (0.89) was

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clearly lower than those of MCC800-6Ar (1.02) and MCCH800-6 (0.98), which

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indicates a higher graphitization degree and results in better electrical conductivity.

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XPS analysis was further performed to explore the differences in the electronic

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structure and surface chemical composition among the three materials. The signals of

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C, N, O, and Fe were clearly observed on the MCC800-6 (Fig. 3b). Interestingly, the

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N1s signal vanished on MCC800-6Ar, implying that a NH3 atmosphere is essential for

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N doping. The high-resolution N 1s spectra of the MCC800-6 and MCCH800-6

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catalysts suggest that pyridinic N, pyrrolic N, graphitic N, and quaternary N21,22 11

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existed (Fig. S4). It has been widely reported that pyridinic N and pyrrolic N can form

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Fe-Nx moieties with Fe due to their long-pair electrons,17,23 and the graphitic N

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combined with Fe-Nx moieties was the efficient active sites for ORR.17,24 For

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comparison, since the N content of the MCC800-Ar was quite low (0.3 %, Table S3),

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the lack of ORR sites resulted in a negligible H2O2 accumulation. Although the N

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content of MCCH800-6 was comparable to that of MCC800-6, the low Fe content

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(Fig. 3b) might reduce the activity and induce a reduction in the H2O2 level due to the

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less Fe-Nx active sites. Therefore, N and Fe might play an important role in the

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electrosynthesis of H2O2.

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Phenol Degradation by MCC in EF. To examine the application potential of the

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prepared MCC catalysts, phenol degradation by these catalysts in EF was investigated.

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It was found that the phenol was efficiently removed within 40 min in EF system by

232

loading MCC as cathode catalyst and no external supply of iron was required (Fig. 4a).

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Moreover, the contribution of the pure carbon paper, which was not coated with

234

catalyst, to the overall phenol degradation was low. The phenol degradation kinetics

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in the EF with the MCC electrode was further analyzed. Although the initial

236

degradation rate was slow, which might be due to the insufficient accumulation of

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H2O2, the overall phenol degradation was found to follow the pseudo-first order (Fig.

238

S5). The apparent rate constant (k) for the system with the carbon paper was only

239

0.001 min-1. The degradation rate was greatly improved with an increase in k to 0.078

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min-1 when MCC was used as the working electrode.

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To optimize the phenol removal performance of the system, the impacts of 12

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several operational parameters on the system were investigated. Since the potential

243

applied to the cathode affected the H2O2 accumulation level, the impact of the

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potential on the phenol removal without the addition of iron was initially explored.

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The phenol concentration rapidly declined within 40 min when the potential was

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increased from 0.15 V to 0.35 V (Fig. 4b). However, a lower (0.015 V) or higher

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potential (0.45 V) resulted in a poorer performance. A lower potential might further

248

reduce H2O2 to H2O and 4 e- ORR process became predominated. The TOC removal

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efficiency increased continuously over time for each investigated potential (Fig. 4c).

250

With the same tendency as the phenol removal efficiency, the highest TOC removal

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efficiency was achieved at 0.25 V. Approximately 60% of TOC was removed after 4-h

252

electrolysis at 0.25 V. Such results imply a high efficiency of the EF system. In

253

addition to the applied potential, pH is another crucial factor to greatly affect overall

254

Fenton reaction performance. In this work, the phenol removal performances of the

255

EF system at raised pHs were further investigated. Phenol was found to be completely

256

removed at pH 4.0, while only 30% removal efficiency was achieved by raising the

257

pH to 5.0 (Fig. S6a). The poor performance at pH 5.0 might be due to the low H2O2

258

concentration (Fig. S6b). As discussed above, Fe might play a key role in catalyst’s

259

activity. Hence, FeCl3•6H2O at reduced concentrations (0.2 mM and nil) was dosed in

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the coagulation process to explore the impact of Fe content on MCC’s performance. It

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was observed that by reducing the Fe dosage to 0.2 mM, only 20% of phenol could be

262

removed within 60-min (Fig. S7a). If the domestic wastewater precipitate was directly

263

used as a precursor to carbonize without dose of Fe, the obtained product showed no 13

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H2O2 producing ability (Fig. S7b). The above results again proved that Fe was

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essential to achieve a high performance of H2O2 synthesis for MCC.

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To exclude the influences of anode oxidation and cathode material adsorption,

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electrical oxidation (aeration N2 to inhibit the H2O2 generation) and electrode

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adsorption (with open-circuit) experiments were conducted as references at 0.25 V. As

269

shown in Fig. 5d, 97% phenol was degraded in the EF system after 40-min treatment.

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In comparison, only 6% phenol was adsorbed by the electrode and 12% was removed

271

via electric oxidation. These results clearly demonstrate that H2O2 was in-situ

272

electro-synthesized and activated by the MCC electrode effectively to mineralize

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phenol.

274

In the Fenton reaction, intermediate free radical formation via H2O2 activation is

275

the key step to achieve effective pollutant degradation. Thus, the presence of hydroxyl

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radicals in the EF was verified using the DMPO spin-trapping method. Fig. 5a shows

277

the typical ESR spectrum obtained after a 30-min reaction. A spectrum consisting of

278

quartet-lines with a peak height ratio of 1:2:2:1 was clearly observed. Such an ESR

279

spectrum is characterized as a hydroxyl radical.25 Moreover, methanol was used as the

280

radical scavenger to quench the hydroxyl radicals. The results indicate that phenol

281

degradation was greatly inhibited by the dose of methanol (Fig. 5b). Therefore, the

282

hydroxyl radicals formed in the EF system was the main reactive species to

283

mineralize phenol.

284

From an economic perspective, the cycling reuse stability is another issue for the

285

practical application of catalysts. In our work, the recycling capability of the catalysts 14

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was evaluated by using a reused MCC electrode in the EF system. The MCC

287

electrode could achieve a high phenol removal efficiency (91%) after 7 repeated uses

288

(Fig. 5c). The phenol removal was slightly reduced to 87% even after 10 cycles. Since

289

the MCC contained iron, its release into the solution needs to be evaluated. Although

290

the released iron concentration in solution was slightly high after the third cycle (4

291

mg/L), this value was below 1 mg/L after seven-cycle use. H2O2 may be activated by

292

the released Fe for phenol degradation. As a result, H2O2 with a cumulative

293

concentration (135 mg/L) was mixed with ferrous iron at the released concentration

294

level for phenol degradation by the conventional Fenton reaction. It was observed that

295

4 mg/L iron resulted in excellent phenol removal (Fig. 5d). Reduction in iron dose to

296

1.9 mg/L decreased the degradation efficiency to 76%. When the iron dose was

297

continually reduced to 0.8 mg/L, the overall phenol removal sharply declined to 10%

298

within 60 min. However, the above experimental result shows that 87% of phenol was

299

removed by EF in the tenth cycle. Such a comparison indicates that MCC might be

300

able to activate the generated H2O2 in-situ, resulting in a better pollutant removal

301

performance compared to the conventional Fenton process.

302

The morphology and structure of reused material were further examined to prove

303

its stability. The SEM images indicate that the morphology of material remained

304

unchanged after recycling use (Fig. S8). In addition, the pyridinic N, pyrrolic N,

305

graphitic N, and quaternary N were observed in the XPS spectra (Fig. S9a). By

306

comparing to the pristine material, additional C-F structure (292 eV) was observed at

307

the reused catalyst’s surface (Fig. S9b), which is ascribed to the added Nafion 15

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substance. In all, both the morphology and chemical composition of the catalysts were

309

proven to be stable.

310

Other Pollutants Degradation in MCC Fabricated EF System. In addition to

311

phenol, the feasibility of the EF system on other industrial wastewater treatment

312

would of great importance to the proposed concept. As a result, the degradation of

313

various typical refractory pollutants, e.g., dye (rhodamine B), pesticide (atrazine), and

314

bisphenol-A by the fabricated EF system was evaluated. All of these three types of

315

pollutants were efficiently removed within 40 min at 0.25 V applied potential (Fig. 6).

316

These results further validate the universality of our proposed strategy. Although the

317

domestic wastewater quality is site-specific and the activity of the obtained MCC

318

might vary, the high activity of the material can be achieved by post chemical

319

modifications.

320

One of the major concerns for EF degradation of pollutants is the electric energy

321

consumption. Here, the mineralization current efficiency of EF system for phenol

322

degradation was only 5.9%, which is low. However, the current density (1.67 mA/cm2)

323

is much lower than those reported in other studies (10-30 mA/cm2).26,27 In this work,

324

only two factors, i.e., carbonation atmosphere and pyrolysis time, which affect the

325

product performance, were taken into account. Considering that the BET surface area

326

of the material (89 m2/g) was low (Fig. S10), other strategies such as tailoring pore

327

structure and tuning surface hydrophilicity,28 could be adopted to further improve the

328

MCC performance and current efficiency.

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CONCLUSIONS

331 332

In this work, a green waste pollution control strategy for urban cities’ sustainable

333

development, i.e., treating industrial wastewater with the sludge originating from

334

domestic wastewater treatment process was proposed. After one-step pyrolysis of

335

excess sludge formed in the domestic wastewater treatment process, an efficient H2O2

336

electrochemical synthesis material was successfully fabricated. Electrochemical tests

337

revealed that the obtained MCC was able to catalyze O2 reduction to H2O2 with an

338

accumulation rate of 432 mg/h/g under acidic condition. Benefitting from its high

339

activity and selectivity for H2O2 production, the MCC-fabricated EF system was

340

efficient in complete removal of 40 mg/L phenol within 40 min at a potential of

341

0.15-0.35 V without the need of dosing external iron. The hydroxyl radicals were

342

found to be the main reactive species and approximate 60% of total organic carbon

343

removal efficiency was achieved within 4 h at 0.25 V. This work shows a great

344

potential of such an integrated approach for efficient and cost-effectively urban water

345

pollution control. Further optimization of the MCC material and operation system

346

could also improve its performance.

347 348

ACKNOWLEDGEMENTS

349

We thank the National Natural Science Foundation of China (51538011), the

350

Collaborative Innovation Center of Suzhou Nano Science and Technology of the

351

Ministry of Education of China, and the Anhui S&T Key Project (1501041118) for the 17

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support of this work.

353 354

ASSOCIATED CONTENT

355

Supporting Information Available. The detail description of pyrolysis

356

procedure; comparison of catalytic activities of various catalysts; elemental analysis

357

of the precursor and MCC800-6; MCC preparation process; XRD patterns; SEM

358

images; XPS spectra of C1s, Fe 2p, and N1s; kinetic analysis of the phenol

359

degradation; impacts of pH and Fe content on phenol degradation and BET surface

360

area results are available free of charge via the Internet at http://pubs.acs.org/.

361 362

REFERENCES

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Figure Captions 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). 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. Figure 3. Raman spectra (a) and XPS surveys (b) of MCCs obtained under different conditions. 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 assistant 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 DMPO-trapped 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). 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.

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H2O2 concentration (mg/L)

a

MCC800-4 MCC800-6 MCC800-8 MCC600-6 MCC1000-6 C paper

120 100 80 60 40 20 0 0

b H2O2 concentration (mg/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

20

40

60

80

100

120

100

120

Time (min) 120

MCC800-6 MCC800-6Ar MCCH800-6

100 80 60 40 20 0 0

20

40

60

80

Time (min)

Figure 1

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Page 25 of 30

a

0.06

Current (mA)

0.04 0.02

Ring

0.00 0.0 -0.2 Disk -0.4 MCC800-6 MCC800-6Ar MCCH800-6

-0.6 -0.8 -1.0 0.0

0.2

0.4

0.6

0.8

Potential vs. RHE (V)

b

MCC800-6

H2O2 selectivity (%)

100

MCC800-6Ar

MCCH800-6

80 60 40 20 0

0.2

0.3

0.4

Potential vs. RHE (V)

c

MCC800-6 MCC800-6Ar

1600

-Z'' (ohm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1200 800 400 0 0

2000

4000

6000

Z' (ohm)

Figure 2

25

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8000

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a

1.0

1.03

d b

0.98

0.89

O 1s

D band G band

0.4 0.2 MCC800-6

MCC800-6Ar MCCH800-6

MCCH800-6

MCC800-6Ar

MCC800-6Ar MCC800-6 500

750

MCCH800-6

N 1s

Intensity (a.u.)

ID /I G

0.6

0.0

Fe 2p

C 1s

0.8

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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MCC800-6

1000

1250

1500

1750

0

200

-1

400

600

800

Binding energy (eV)

Raman shift (cm )

Figure 3

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Page 27 of 30

a

b

1.0

0.6

C paper MCC

0.4 0.2

0.6 0.4 0.2

0.0

0.0

0

10

0.05 V 0.15 V 0.25 V

60 50

20

30

40

50

60

0

Time (min) 0.35 V 0.45 V

d

10

20

30

40

50

60

Time (min) 1.0 0.8

40

C/C 0

c

0.25 V 0.35 V 0.45 V

0.05 V 0.15 V

1.0 0.8

C/C 0

C/C 0

0.8

TOC removal efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Electrode adsorption Electrical oxidation Electrical Fenton

0.6 0.4

20 0.2

10

0.0

0 1

2

3

0

4

10

20

30

40

Time (min)

Time (h)

Figure 4

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50

60

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a

b 0 min

1.0

C/C0

Intensity

0.8

30 min

0.6 0.4 0.2

320

322

324

326

0.0

328

0

10

20

Magnetic intensity (mT) Cycle 3

1.0

Cycle 5

Cycle 10

d

4

3

0.6 2 0.4 1 0.2 0

0.0 30

60

90

40

50

60

Relased Fe concentration Cycle 7

0.8

0

30

Time (min)

Leaching Fe (mg/L) C/C0

Phenol concentration

c

C/C0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1.0 0.8 0.6 0.4 4.0 mg/L Fe 1.9 mg/L Fe 1.0 mg/L Fe 0.8 mg/L Fe

0.2 0.0

120 150 180 210 240

0

30

60

Time (min)

90

120 150 180 210 240

Time (min)

Figure 5

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Figure 6

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Electro-Fenton cathode catalyst has been synthesized by using sludge originating from domestic wastewater treatment in a green way and applied for the industrial pollution control.

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