Catalytic removal of aqueous contaminants on N-doped graphitic. biochars: Inherent roles of adsorption and nonradical mechanism

Transcription

1 Supporting Information Catalytic removal of aqueous contaminants on N-doped graphitic biochars: Inherent roles of adsorption and nonradical mechanism Shishu Zhu,,, Xiaochen Huang, Fang Ma, Li Wang, *, Xiaoguang Duan, *, Shaobin Wang State Key Laboratory of Urban Water Resource and Environment, School of Environment, Harbin Institute of Technology, 73 Huanghe Road, Harbin , P. R. China Department of Chemical Engineering, Curtin University, GPO Box U1987, Perth, Western Australia 6845, Australia * Corresponding author wanglihit@hotmail.com; xiaoguang.duan@curtin.edu.au The Supporting Information included: 2 Tables 19 Figures 18 Pages S1

2 Text 1 Materials and methods Adsorption study The typical adsorption experiments were conducted as following. The certain volume stock solution of the pollutant (1000 mg L -1 ) was injected into sonicated N-BCs suspension (200 mg L -1 ). The ph was fixed at 9.5±0.8 by adding 1 mm bicarbonate buffer. The final 100 ml solution was mixed using a stirring paddle. At certain time intervals, 1 ml solution was filtered through a 0.22 µm filter for further detection. Pseudo first order kinetic model and Langmuir isotherm model were applied. The commercial common catalysts Reduced graphene oxide (rgo) and carbon nanotubes (CNT) were purchased from Timesnano, Chengdu, China. A zero-valent iron (ZVI) was purchased from Sinopharm Chemical Reagent Co., China. Co 3 O 4 and Fe 3 O 4 were purchased from Sigma-Aldrich Co. Determination of free radical species An EPR instrument (Bruker EMX/plus spectrometer, Germany) was serviced to detect the reactive free radicals generating from the N-doped biochar suspension with parameters as follows: center field 3460 G, sweep width 100 G, microwave frequency 9.71 GHz, microwave power mw, modulation frequency 100 khz, modulation amplitude of 1.0 G, sweep width of 100 G, sweep time of s. Two spin-trapping agents, 5,5-dimethyl-1-pyrroline N-oxide (DMPO, > 99%, Aladdin) and S2

3 2,2,6,6-tetramethyl-4-piperidinol (TEMP, > 99%, Aladdin), were employed to capture free radicals. Briefly, DMPO and TEMP were dissolved in phosphate buffer solutions (ph=7.4) to final concentrations of 80 mm and 50 mm, respectively. Then, 1 ml sample was withdrawn from N-doped biochar suspension reaction system (organic pollutant: 20 ppm, catalyst: 200 mg/l, and PDS: 2 mm) and was filtered using 0.22 µm filter. DMPO or TEMP solution (40 µl) was mixed with 40 µl filtered sample. A quartz capillary tube was used to suck up the mixed DMPO or TEMP solution to detect corresponding signals from spin-trapping adducts in the EPR. Quenching experiments A classical quenching experiment was conducted to further determine free radical species and nonradical reaction. Before N-doped biochar suspension reaction started, different quenching agents were added into the reaction solutions at a constant concentration (molar ratios of ethanol to OG from 500 to solvent, DMSO (dimethyl sulfoxide) 10 mm, KI 5 mm, benzoquinone 5 mm, and NaN 3 10 mm). Electrochemical measurements A carbon paper electrode loaded with N-B900 powder was first prepared. Nafion solution (5.0 wt% isopropanol in water, 0.2 ml) and N-B900 powder (20 mg) were mixed with EtOH (2 ml) for 4 h. The mixed suspension was dropped onto the surface of the carbon paper electrode. The carbon paper electrode was vacuum dried at 60 C for 8 h. The standard three electrodes configuration was set up with the carbon paper S3

4 electrode, a Pt wire, and a Ag/AgCl (4 M KCl) electrode as the working, counter, and reference electrodes, respectively. The cell reactor contained an electrolyte of 0.5 M Na 2 SO 4 (or 2 mm PDS and 50 ppm OG). The electrochemical measurements were performed by a VMP 3 instrument (Bio-Logic SA, France). Electrochemical impedance spectroscopy was measured by a 5 mv amplitude AC voltage with a 1.0 V (vs. Ag/AgCl). Linear sweep voltammograms were recorded as the potential was swept from 0.3 to 1.0 V at a scan rate of 5 mv s 1. Analysis of organic pollutants The concentration of OG dye was determined by a UV vis spectrophotometer at a wavelength of 504 nm (T6, Purkinje General, China). Bisphenol A and sulfamethoxazole concentrations were determined using a HPLC (2690, Waters, USA) with a C-18 column and a mobile phase of acetonitrile and methane acid (60:40, v/v). Phenol concentration was determined using the above HPLC with a mobile phase of acetonitrile and ultrapure water (60:40, v/v). An HPLC system (with a C18 column, waters, USA) coupled to mass spectrometer with an ESI source in a positive ion mode was used for HPLC-MS analysis. Text 2 Results and discussion Probing the generated free radicals with DMPO as the spin trapping The EPR instrument was further employed to probe the generated free radicals with DMPO as the spin trapping. During OG or phenol degradation, DMPO-OH signals S4

5 with four characteristic peaks (Figure 3) were both presented at an intensity ratio of 1:2:2:1 and hyperfine splitting constants, a H =a N =14.7 G. DMPO-SO 4 was discovered with six peaks (1:1:1:1:1:1) and hyperfine splitting constants of a N = 13.3 G, a H = 9.5 G, a H = 1.46 G, and a H = 0.77 G. PDS showed a low intensity, however, higher intensities occurred when N-BC900 was introduced, suggesting that the free radicals ( OH and SO 4 ) were generated in N-B900/PDS system. The carbon-dmpo adduct with the sextet signal (1:1:1:1:1:1) was also observed during OG degradation. The intensities increased as the reaction proceeded (Figure 3a), which could be ascribed to accumulation of intermediates during dye oxidation. 1-3 HPLC-MS analysis From the data of HPLC-MS (Figure S15), the main oxidation byproducts were aniline and naphthol derivatives that were similar to the previous studies. 4, 5 First pathway: The singlet oxygen generation ( 1 O 2 ) was proposed as one of oxidation mechanisms involved in the graphitic biochars/pds system. As a selective oxidation pathway, the electron-rich structure was more sensitive to be attacked by 1 O 2. 6, 7 Thus the electron-rich sulfonic groups were firstly attacked to form the azo-intermediate. The formation of hydroxyl-additional naphthol derivative was resulted from the reaction between 1 O 2 and aniline radicals, or the transformation of an endoperoxide intermediate (or quinine derivative). 8, 9 Second pathway: The direct electron transfer occurring on the surface of N-B900 has been identified. Thus the azo-bonds with low bond energy were broken, forming the naphthol derivative intermediate (m/z=307) with sulfonic groups. Another molecular intermediate was oxidized by 1 O 2 to form the S5

6 nitrosobenzene (m/z=103) by intramolecular reaction. The above oxidation byproducts and degradation pathways also demonstrated the proposed oxidation mechanisms ( 1 O 2 and electron transfer). S6

7 Table S1. Surface porosity of pristine and N-doped biochars. SSA (m 2 /g) Pore volume (m 3 /g) Pore size (nm) Atomic percentage (%) C 1s O 1s N 1s BC N-BC N-BC N-BC N-BC Table S2. C 1s and N 1s state percentages of N-doped biochars. Pyridinic N N 1s states percentage (%) Pyrrolic N Graphitic N N oxides N-BC N-BC N-BC C 1s states percentage (%) C-C/C=C C-O C=O/C-N N-BC N-BC N-BC S7

8 Figure S1. SEM images of N-BC700 (a) and N-BC800 (b), HR-TEM images of N-BC400 (c, d). Figure S2. XPS full survey of N-doping biochars. S8

9 Figure S3. XPS C 1s spectra of N-BC700 (a), N-BC800 (b), and N-BC900 (c). Figure S4. XRD patterns of N-doped graphitic biochars. S9

10 Figure S5. Raman spectra of different biochars. Figure S6. PDS activation for catalytic degradation of OG by ash, N-BC950, and N-BC1000. Inset figure shows the relative contents of the elements in the ash. S10

11 Figure S7. OG removal efficiencies by PDS only, N-BC800 and N-BC900. Figure S8. FT-IR spectra of N-B900 (a) and OG sorption after the different pretreatments (b). S11

12 Figure S9. Effect of catalyst concentration on OG removal by oxidative degradation ([PDS] 0 = 2 mm, [OG] 0 = 50 ppm). Figure S10. Effect of PDS concentration on OG removal by oxidative degradation ([N-BC] 0 = 0.2 g L -1, [OG] 0 = 50 ppm) S12

13 Figure S11. The scavenge effect of ethanol on OG removal with different addition concentrations. Figure S12. EPR spectra of N-BC900 in the presence of DMPO during oxidative degradation of OG at different reaction time. [PDS] 0 = 2 mm, [OG] 0 = 50 ppm, catalyst dose = 0.2 g L -1, [DMPO] 0 = 40 mm. Gray square: DMPO-OH, Black circle: DMPO-SO 4, Gray circle: DMPO adducts of carbon-based radicals. S13

14 Figure S13. EPR spectra of N-BC900 in the presence of DMPO during oxidative degradation of phenol at different reaction time. [PDS] 0 = 2 mm, [Phenol] 0 = 50 ppm, catalyst dose = 0.2 g L -1, [DMPO] 0 = 40 mm. Gray square: DMPO-OH, Black circle: DMPO-SO 4, Gray circle: DMPO adducts of carbon-based radicals. Figure S14. The catalytic stability of N-BC900 during PDS activation for oxidative degradation of OG in five cycles and after recovering by hexane extraction. S14

15 Figure S15. HPLC-MS results of OG degradation intermediates after PDS activation by N-B900. S15

16 Figure S16. The adsorption kinetics fitted by pseudo-first-order kinetic model of N-doped biochars. Figure S17. Pseudo-first-order kinetic models for OG, phenol, BPA, and SMX oxidative degradation. [PDS] 0 = 2 mm, [OG] 0 = 50 ppm, catalyst dose = 0.2 g L -1. S16

17 Figure S18. Langmuir isotherm models for OG, phenol, BPA, and SMX adsorption. Figure S19. Adsorption capacity and oxidative degradation rate normalized by specific surface area (m 2 ). S17

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