Surface modification of sewage sludge derived carbonaceous catalyst for m-cresol catalytic wet peroxide oxidation and degradation mechanism

Abstract

Sewage sludge derived carbonaceous materials (SWs) treated with different kinds of acid were used as the catalysts for catalytic wet peroxide oxidation (CWPO) of m-cresol in batch reactor and continuous reactor, respectively. The results showed that SW treated with nitric acid (HNO₃-SW) and sulfuric acid (H₂SO₄-SW) exhibited high catalytic activity in a CWPO reaction. The conversion of m-cresol reached 100%, after CWPO reaction for 3 h at an initial pH of 7 and a temperature of 25 °C over a H₂SO₄-SW catalyst. The elements content and chemical state of SWs treated with different kinds of acids were measured by XRF, XPS and Mössbauer spectra, the results implied that ferric iron and surface functional groups might play the main role in m-cresol degradation. The intermediate oxidation products were identified by in situ NMR, GC-MS, IC and HRMS analyses. Based on a free-radical reaction mechanism, a m-cresol oxidation pathway in CWPO was proposed.

---

1. Introduction

The increasing amount of sewage sludge generated by municipal wastewater treatment plants has become an persistent environmental problem.^1,2 Common disposal methods of sewage sludge include landfill, sea dumping, application to farmland and incineration. Sewage sludge pyrolysis is considered an interesting disposal and has developed rapidly since it is effective in reducing the volume of sludge and producing bio-oil.^3–7 Sewage sludge derived carbonaceous material (SW) could be formed during pyrolysis of sewage sludge, which contains a high content of iron and could be employed as the catalyst in advanced oxidation processes (AOPs).

Recently, advanced oxidation processes (AOPs) have received remarkable attention owing to their high efficiency in pollutant degradation.⁸ Among the AOPs technologies, catalytic wet peroxide oxidation (CWPO), in which hydrogen peroxide is used as oxidant and metal ion as catalyst, is a Fenton-like oxidation reaction and has been widely used for the treatment of industrial wastewater containing non-biodegradable organic pollutants.⁹ In most previous studies, SW has been generally used for adsorbing air pollution, organic pollutants and heavy metals.^10–14 Only a few studies have been carried out for CWPO reaction with SW.^2,15,16 Lin Gu¹⁵ reported the optimum carbonization temperature of SW for a superior porous structure. Yuting Tu¹⁶ reported a co-catalytic effect of SiO₂ in SW for the CWPO reaction. However, a surface modified SW catalyst in a CWPO reaction has not yet been reported, while other carbon materials such as activated carbon, graphene and carbon nanotubes have been widely investigated as surface functional groups.^17–20

The coal chemical industry in China has grown rapidly, resulting in the generation of significant levels of cresol-containing wastewater. It is necessary to study the degradation pathways of cresol in the CWPO reaction in detail since the toxicity of some intermediates can be higher than that of the initial compound.²¹ With the development of analysis technologies, some intermediates can now be measured and the degradation process can be observed in situ. Nuclear magnetic resonance (NMR) spectroscopy is well established for investigating organic compounds. In many catalytic systems, it is invaluable to be able to monitor a reaction process in situ.²² High-resolution mass spectrometry (HRMS), equipped with a weak ionization source such as an electrospray ionization (ESI) source, is useful to identify the accurate molecular formula for mixed organic compounds in a reaction system. Furthermore, the study of m-cresol degradation could determine the ring-opening location with methyl existing in the intermediates.

In this study, the effect of SWs treated with different kinds of acid on the degradation of m-cresol was studied in batch and continuous CWPO reactions. Furthermore, the influence of surface functional groups and ferric iron on the catalytic activity was discussed. On the basis of intermediate products identified by in situ NMR, GC-MS, IC and HRMS analyses, the oxidation pathways of m-cresol in CWPO are proposed.

2. Materials and methods

2.1. Preparation of SWs

SWs were formed from municipal sewage sludge, from a wastewater treatment plant (WWTP) in Dalian (China). Sewage sludge was dried at 105 °C to constant weight. Then sludge was carbonized at 600 °C for 4 h with a heating rate of 3 °C min⁻¹ and high pure nitrogen (99.999 wt%) flow of 500 mL min⁻¹. After the furnace had cooled to room temperature, the SWs were obtained. In order to modify the surface of SWs, different acids were used to treat SWs. In the acid treatment process, 50 mL SWs were produced by immersing carbonized SWs with the same volume of HCl (20.5 wt%), H₂SO₄ (63.4 wt%) and HNO₃ (40.5 wt%) for 24 h, respectively. Then HCl-SW, H₂SO₄-SW and HNO₃-SW were washed with deionized water until the pH of the washing water reached 6–7 and the recovered solids were dried at room temperature.

2.2. Structural characterization

X-ray Fluorescence (XRF) was used to analyze elemental content in SW samples with Magix 601 equipment produced by PANalytical. Thermogravimetric analysis (TGA) (WRT-1D, China) was used to measure the amount of carbon in SWs. Functional groups on the SWs were analyzed by Fourier Transform Infrared Spectroscopy (FTIR, Tensor 27, Germany). ⁵⁷Fe Mössbauer spectra were recorded on a Topologic 500A spectrometer and a proportional counter at room temperature. Moving ⁵⁷Co(Rh) in a constant acceleration mode was used as the radioactive source. The spectra were fitted with appropriate superpositions of Lorentzian lines using the MossWinn 3.0i computer program. The Doppler velocity of the spectrometer was calibrated with respect to α-Fe foil. And the free recoil fraction was assumed to be the same for all iron species. In this way, Mössbauer parameters such as the isomer shift (IS), the electric quadrupole splitting (QS), the full line width at half maximum (line width), the magnetic hyperfine field (H), and the relative resonance areas of the different components of the absorption patterns (peak area) were determined. X-ray photoelectron spectroscopy (XPS) was performed to analyze the composition and chemical state of the surface elements of the SW catalysts. A Thermo ESCALAB 250Xi instrument, with an Al K X-ray source (1486.6 eV) and pass energy of 20 eV was used at a pressure of 6.5 × 10⁻⁵ Pa. The binding energies were calibrated with respect to the signal for contamination carbon (binding energy = 284.6 eV). The specific surface area of SW catalyst was measured by Brunauer–Emmett–Teller (BET) methods and was determined with N₂ adsorption/desorption isotherms at 77 K using QUADRASORB SI4 instrument produced by Quantachrome Company. Scanning electron microscopy (SEM) experiments were performed on a scanning electron microscope (Quanta 200F, FEI Company) with an accelerating voltage of 20 kV. Energy dispersive X-ray spectroscopy (EDAX’s Sapphire Si(Li) Detector, USA) was carried out on an EDAX silicon-drift detector, which enabled rapid determination of elemental compositions and acquisition of compositional maps of the catalysts.

2.3. Degradation experiments

The CWPO batch reaction of m-cresol over SWs was performed in a 1000 mL glass conical beaker, with a stopper shaken in a thermostatic bath at an equivalent stirring velocity around 200 rpm. The experiments were carried out in two stages: (1) adsorption and reduction of m-cresol (the first 210 min) and (2) oxidation via a heterogeneous CWPO reaction. The comparison was based on SWs treated with different kinds of acid (hydrochloric acid, sulfuric acid and nitric acid, respectively). In each run, 700 mL 100 mg L⁻¹ m-cresol solution was added with 0.5 g L⁻¹ SW catalyst powders. The solution pH was adjusted to 7.0 ± 0.1 with NaOH and H₂SO₄. After the adsorption/desorption equilibrium was established in 210 min, hydrogen peroxide solution was added into 600 mL suspension and the hydrogen peroxide concentration in the reactor was 15.7 mmol L⁻¹. Samples were taken at given time intervals and filtrated with 0.45 μm film to remove any catalyst particles or solid residue.

The catalytic activity and stability of SW catalysts in CWPO were tested by an up-flow fixed bed reactor (glass, inside diameter = 1 cm, height = 20 cm). SW catalyst was added into the reactor. The experiment was performed at room temperature with a liquid hourly space velocity (LHSV) of 1 h⁻¹. The solution containing 100 mg L⁻¹ m-cresol and 15.7 mmol L⁻¹ hydrogen peroxide was pumped to the reactor with an electronic diaphragmatic metering pump. The initial pH of the solution was 7.0 ± 0.1.

In the adsorption process, m-cresol concentration ranging from 10 to 200 mg L⁻¹ was prepared. 0.5 g SW treated with different acids, respectively, was added with 100 mL of the m-cresol solutions in a series of stopper bottles. The suspensions were then placed in an isothermal shaker (20 °C, 200 rpm) for 8 h to allow complete equilibration. Samples of the equilibrated solutions were filtered and analyzed.

2.4. Analysis methods

All solutions used in this study were prepared in deionized water of 18.25 MΩ cm⁻¹. m-Cresol, hydrogen peroxide, hydrochloric acid, sulfuric acid and nitric acid were of analytical grade.

m-Cresol concentration was analyzed using high performance liquid chromatography (HPLC). The HPLC system produced by Dalian Elite Analytic Instruments Co., Ltd was equipped with an Elite C18 chromatographic column (4.6 mm × 250 mm, 5 μm) and a UV-detector set at 272 nm. The mobile phase was methanol : water = 80 : 20 (V/V) and the flow rate was fixed at 1.0 mL min⁻¹. Total organic carbon (TOC) analyzer (TOC-V_CPH/CPN, Shimadzu, Japan) was employed to determine the residual amount of organic substances in the effluent. The pH of m-cresol solution was measured with a REX PHS-3C pH meter (Rex Instrument Factory, Shanghai, China), and the meter was calibrated daily before use. The leaching concentration of Fe in the solution was measured by 1,10-phenanthroline colorimetric method with absorbance at 510 nm.

2.5. Intermediates analysis

Gas chromatography-mass spectrometry (GC-MS) was utilized to identify the intermediates in treated solutions. In order to detect the intermediate of low molecular weight acids from m-cresol degradation in CWPO, an esterification reaction was used to prepare the sample. The esterification method was given as follows. First, a liquid sample was concentrated using a rotary evaporator. Secondly, 2 mL of 10% (v/v) H₂SO₄ in methanol was mixed with the residues in a vial and placed in an oven at 60 °C for 8 h. While the mixture was cooled, it was neutralized with 1 mL saturated Na₂CO₃. Then, 1 mL hexane was added to the mixture to extract the fatty acids. After stirring for 2 min, the mixture was left to settle, and a pipette was used to remove the top hexane layer into another vial. Finally, the hexane layer was dried with Na₂SO₄ (anhydrous) to remove any residual water and then filtered for GC-MS analyses. An Agilent model 7890A Gas Chromatograph (GC) equipped with a model 5975C Mass Selective Detector and a capillary column (HP-5ms, 30 m × 0.25 mm × 0.25 μm, Agilent J&W GC Columns) was used. The initial oven temperature of the gas chromatograph was kept at 50 °C for 5 min. The oven temperature increased at a rate of 15 °C min⁻¹ to reach a final temperature at 250 °C, and this temperature was held for 10 min. The injector temperature was 250 °C. The detector temperature was 280 °C.

Nuclear magnetic resonance (NMR) was used in situ to investigate the m-cresol degradation. m-Cresol and SW catalyst were dissolved by D₂O solution in the NMR sample tube, then H₂O₂ was added to the NMR sample tube and the CWPO reaction carried out. ¹H and ¹³C NMR spectra were recorded on a Bruker DRX-400 spectrometer and chemical shift values are relative to TMS (δ_TMS = 0.00 ppm) or D₂O (δ (¹H) = 4.79 ppm).

The carboxylic acids were analyzed via ion chromatography (IC). The ICS-3000 system produced by DIONEX equipped with an AS11-HC chromatographic column (4 mm × 250 mm). The temperature of column and cell heater was 30 °C and 35 °C, respectively. The mobile phase was 30 mol L⁻¹ NaOH and the flow rate was fixed at 1.2 mL min⁻¹.

The chemical formula of intermediates was determined by high-resolution mass spectrometry (HRMS) equipped with an electrospray ionization (ESI) source (Agilent Q-TOF 6540 mass spectrometer).

3. Results and discussion

3.1. Characterization of SWs

The elemental content of SWs is displayed in Fig. 1 and Table S2.† All samples had an extremely high content of SiO₂, Al₂O₃ and Fe₂O₃, which could be ascribed to the high content of inorganic compounds in the sludge. The content of Fe₂O₃ was 5.1%, 1.7%, 3.9% and 2.4% in the SW, HCl-SW, H₂SO₄-SW and HNO₃-SW, respectively. The primitive SW exhibited the highest content of iron in all SW catalysts, while after acid treatment the content of iron in SWs was reduced. The result confirmed that iron was removed by acid treatment. However, SiO₂ showed an opposite result compared to iron, and this is because SiO₂ did not dissolve in the acid.

Fig. 1 XRF and TG analysis of elements in SWs treated with different acid: *detected by TG, other elements detected by XRF.

The FTIR spectra of SWs treated with acid are displayed in Fig. 2. After SWs was treated with HNO₃ and H₂SO₄, respectively, the peak at 1633 cm⁻¹ related to C=O^23,24 was strong and sharp. Only HNO₃-SW exhibited a peak at 1384 cm⁻¹, this peak was broadly associated with nitrogen-containing compounds such as NO₂ groups. The FTIR spectra demonstrated that the surface functional groups on SWs were modified by acid treatment.

Fig. 2 FTIR spectra of SWs treated with different acid.

In order to confirm the Fe structure transformation after acid treatment, ⁵⁷Fe Mössbauer spectra were measured at room temperature as shown in Fig. 3. The Mössbauer parameters are listed in Table 1. The Mössbauer spectrum of SW shows the superimposition of two doublets. The doublet with IS = 0.44 mm s⁻¹ and QS = 0.96 mm s⁻¹ could be attributed to Fe³⁺, and the doublet accounted for 49.2% of the total spectral area. The doublet (IS = 1.03 mm s⁻¹, QS = 2.18 mm s⁻¹) could be attributed to Fe²⁺, which in good agreement with the literature,^25,26 and accounted for 50.8% of the total spectral area. For HNO₃-SW sample, the Mössbauer spectrum shows the superimposition of two doubles and two sextets. The doublet with IS = 0.40 mm s⁻¹ and QS = 0.93 mm s⁻¹ could be assigned to Fe³⁺, which accounted for about 45.9% of the total spectral area, while the doublet with IS = 1.06 mm s⁻¹ and QS = 2.27 mm s⁻¹ could be attribute to Fe²⁺, which accounted for 34.1% of the total spectral area. The two sestets with IS = 0.64 mm s⁻¹, QS = 0.04 mm s⁻¹, H = 45 T and IS = 0.30 mm s⁻¹, QS = −0.09 mm s⁻¹, H = 50 T could be attributed to Fe₃O₄. For H₂SO₄-SW, the Mössbauer spectrum shows the superimposition of two doublets. The doublet with IS = 0.32 mm s⁻¹ and QS = 0.69 mm s⁻¹ could again be ascribed to Fe³⁺, which accounted for 62.4% of the total spectral area, while the double with IS = 0.32 mm s⁻¹, QS = 0.69 mm s⁻¹ could be ascribed to Fe²⁺, which accounted for about 37.6% of the total spectral area. For HCl-SW, the Mössbauer spectrum shows the superimposition of three doublets, the doublet with IS = 0.32 mm s⁻¹, QS = 0.95 mm s⁻¹ could be attributed to Fe³⁺, and the doublet accounted for 34.4% of the total area. The two doublets (IS = 1.08 mm s⁻¹, QS = 2.24 mm s⁻¹ and IS = 0.76 mm s⁻¹, QS = 1.26 mm s⁻¹) could be attributed to Fe²⁺. Compared with SW sample, it was interesting to note that the concentration of Fe²⁺ decreased after oxidizing acid (H₂SO₄ and HNO₃) treatment. However, the concentration of Fe²⁺ increased after HCl treatment, and the ferric iron might be beneficial for CWPO reaction.

Fig. 3 ⁵⁷Fe Mössbauer spectra of SWs: SW (a), HCl-SW (b), H₂SO₄-SW (c) and HNO₃-SW (d).

Table 1 ⁵⁷Fe Mössbauer parameters of SWs

Sample	Component	IS (mm s^−1)	QS (mm s^−1)	LW (mm s^−1)	H (T)	RA (%)
SW	Fe(II)	1.03	2.18	0.76	—	50.8
	Fe(III)	0.44	0.96	0.81	—	49.2
HCl-SW	Fe(II)	1.08	2.24	0.78	—	60.4
	Fe(II)	0.76	1.26	0.28	—	5.2
	Fe(III)	0.32	0.95	0.70	—	34.4
H2SO4-SW	Fe(II)	1.25	1.88	0.78	—	37.6
	Fe(III)	0.32	0.69	0.74	—	62.4
HNO3-SW	Fe(II)	1.06	2.27	0.90	—	34.1
	Fe(III)	0.40	0.93	0.76	—	45.9
	Fe3O4	0.64	0.04	0.72	45	14.0
		0.30	−0.09	0.30	50	6.0

---

The XPS C1s peaks of SW, HCl-SW, H₂SO₄-SW and HNO₃-SW are presented in Fig. 4, which clearly shows that the oxygen-containing organic groups were prevalent on the outer surface of SWs. The main peak at 284.6 eV was assigned to aliphatic/aromatic carbon (C–H, C–C and C=C), then the peaks at 285.1–285.2 eV, 286.0–286.3 eV and 287.4–287.7 eV were assigned to the oxygen-containing moieties, that is C–O, C=O and COOH, respectively, from a previous study.¹⁹ Table 2 lists their relative percentages on SWs. After SW was treated with HNO₃, C–C declined from 64.3% for SW to 47.8% for HNO₃-SW, and the content of C=O/COOH increased. It could be observed that the COOH group appeared after H₂SO₄ and HNO₃ treatment, respectively, and the high catalytic activity might be caused by the COOH group. Presumably, these carboxylic groups on the surface of H₂SO₄-SW and HNO₃-SW were the sites for esterification with m-cresol. Other characterization of SWs such as SEM, EDAX and specific surface area is shown in Fig. S1, S2 and Table S3,† respectively.

Fig. 4 Deconvolution of XPS C1s for SWs.

Table 2 Deconvolution results of XPS C1s spectra of SWs

Sample	Peak	Position (eV)	FWHM (eV)	Area (%)
SW	C–C	284.6	1.02	64.3
	C–O	285.1	0.86	23.4
	C=O	286.3	1.69	12.3
HCl-SW	C–C	284.6	1.02	69.8
	C–O	285.1	0.82	23.1
	C=O	286.3	1.13	7.1
H2SO4-SW	C–C	284.6	1.02	65.7
	C–O	285.2	0.75	17.4
	C=O	286.0	1.23	10.8
	COOH	287.4	1.90	6.1
HNO3-SW	C–C	284.6	1.02	47.8
	C–O	285.2	0.82	20.3
	C=O	286.0	1.23	21.6
	COOH	287.7	1.90	10.3

---

3.2. m-Cresol degradation in CWPO batch reaction

The SWs were employed in CWPO batch tests and the effect of SWs on the m-cresol conversion was investigated. As shown in Fig. 5(a), in the adsorption phase, before the introduction of H₂O₂ (210 min), less than ca. 5% of m-cresol was removed by SWs. It indicated that the adsorption capacity of SWs was very low. After 210 min adsorption and 180 min oxidation period at 25 °C, the conversion of m-cresol was 3.2%, 4.6% and 1.9% for blank (without catalyst), SW and HCl-SW, respectively, much lower than the 100% and 95.9% removal efficiency obtained with the H₂SO₄-SW and HNO₃-SW catalyst, respectively, indicating that the catalytic activity of SW treated with oxidizing acid was highly improved. After oxidation for 180 min TOC removal was 4.4% and 0.8% for SW and HCl-SW, respectively, lower than 36.6% and 12.4% removal efficiency for H₂SO₄-SW and HNO₃-SW, respectively. The leaching concentration of Fe in the solution was 0.7 mg L⁻¹, 1.2 mg L⁻¹, 2.1 mg L⁻¹ and 1.9 mg L⁻¹ for SW, HCl-SW, H₂SO₄-SW and HNO₃-SW, respectively. With an increasing temperature the TOC removal by H₂SO₄-SW was highly increased (Fig. 5(c)). It indicated that high temperature could increase TOC removal with SWs in CWPO. However, when the temperature was above 60 °C, the TOC removal increased slowly, since the decomposition of H₂O₂ would be much faster at higher temperature. The result showed that m-cresol conversion and TOC removal were highly increased in CWPO by SWs treated with H₂SO₄ and HNO₃, respectively. The consumption of H₂O₂ is shown in Fig. 5(b). The presence of H₂O₂ after 140 min was, 97.1%, 68.8%, 85.9%, 66.5% and 79.9% for blank, SW, HCl-SW, H₂SO₄-SW and HNO₃-SW, respectively. The effect of SWs on the adsorption of m-cresol is shown in Fig. 5(d). The results indicate that the adsorption capacity (q_m) of m-cresol was 1.7 mg g⁻¹, 1.7 mg g⁻¹, 1.9 mg g⁻¹ and 1.8 mg g⁻¹ for SW, HCl-SW, H₂SO₄-SW and HNO₃-SW, respectively. It revealed that the SWs adsorption capacity of m-cresol was low and the effect of m-cresol adsorption in the batch reactor could be ignored.

Fig. 5 Degradation of m-cresol in batch reactor through adsorption and oxidation by hydrogen peroxide and different SW catalysts: m-cresol conversion at 25 °C (a), H₂O₂ consumption at 25 °C, (b) TOC removal at different temperatures by H₂SO₄-SW (c) and adsorption isotherms for m-cresol onto SWs, where q_e (mg g⁻¹) is the concentration of adsorbed m-cresol per gram of adsorbent, C_e (mg L⁻¹) is the concentration of m-cresol in equilibrium solution (d).

3.3. m-Cresol degradation in CWPO continuous reaction

The effect of SW catalysts on m-cresol conversion was studied in a CWPO continuous reaction. As seen in Fig. 6(a), the results showed that the concentration of iron in SWs might play a main role in the degradation of m-cresol. For SWs treated with HCl, H₂SO₄ and HNO₃, respectively, the content of Fe₂O₃ in SWs was 1.7%, 3.9% and 2.4%, respectively, and the conversion of m-cresol at 432 h was 26.3%, 100% and 85.1%, respectively. However, the content of Fe₂O₃ in SW was 5.1% which was the highest in SWs, but the conversion of m-cresol was only 7.1%. It indicated that SW treated with an oxidizing acid was another main influencing factor for the catalytic activity. The decrease of m-cresol conversion was due to adsorption equilibrium at the beginning of the reaction, while SWs treated with oxidizing acid had an increasing conversion of m-cresol after adsorption equilibrium. It might be caused by the SWs surface groups transformation during the continuous reaction in CWPO. The surface of SW was changed during the CWPO continuous reaction since the carbon on the catalyst was oxidized by ˙OH. Due to the pH of the washing water of the H₂SO₄-SW and HNO₃-SW catalyst being 6.0–7.0 and the initial pH of batch reaction being 7.0, it can concluded that the high catalytic efficiency was not caused by acidification of the solution. However, after H₂SO₄ and HNO₃ treatment, respectively, the surface of the H₂SO₄-SW and HNO₃-SW catalyst was modified and the surface groups on H₂SO₄-SW and HNO₃-SW might be beneficial for CWPO reaction. For SW and HCl-SW catalyst, the decrease of m-cresol conversion was due to adsorption equilibrium, and the catalytic activity of SW and HCl-SW was much lower than for H₂SO₄-SW and HNO₃-SW catalyst in CWPO.

Fig. 6 m-Cresol conversion and pH of effluent vs. running time in continuous reaction by different SWs. Reaction conditions: C_m-cresol = 100 mg L⁻¹, C_H2O2 = 15.7 mmol L⁻¹, LHSV = 1 h⁻¹, T = 25 °C, initial pH = 7.

The pH of the effluent of SWs was investigated in a continuous reaction. As is seen in Fig. 6(b), the effluent by SW catalyst had the highest pH value, whereas the effluent from SW treated with acid exhibited low pH. High pH could increase the invalid decomposition of hydrogen peroxide and the reduction of ˙OH content due to the formation of ferric hydroxide complexes as reported previously.²⁷ The low pH might be caused by the degradation products of m-cresol in a CWPO reaction such as low molecular weight acids. Furthermore, the result showed that the effluent by SW catalyst had a pH in the range 8.6–11.5. It indicated that a significant number of basic sites were on the surface of SW, which might influence the catalytic activity.

3.4. Activated carbon surface functional groups catalytic mechanism

The reaction was possibly initiated by carbon adsorption. After adsorption equilibrium is reached, CWPO played a main role in the degradation of m-cresol. The FTIR data (Fig. 2) and XPS spectrum (Fig. 4) provided direct evidence of the existence of functional groups. SWs were treated with oxidizing acid and formed C=O and –COOH. After treatment with oxidizing acid, H₂SO₄-SW and HNO₃-SW presented larger adsorption capacity of m-cresol than SW, as shown in Fig. 5(d). It was reported that the carboxyl group on the carbon could react with hydroxy groups and form esters.^28–30 Therefore it was easy for m-cresol to react with carboxyl gorups and form esters by SWs treated with oxidizing acid, as shown in Fig. 7. Since H₂O₂ could be absorbed on SiO₂ by hydrogen bonds with the oxygen in siloxane bridges, Si–O–Si, this leads to a higher concentration of H₂O₂ on the surface of SWs treated with acid than that in the bulk solution and hence would facilitate the CWPO reaction.¹⁶ Iron was also supported on the surface of SWs and this is beneficial to enhance the CWPO reaction. The high content of iron element in SWs had a beneficial effect on m-cresol conversion in CWPO. After the CWPO reaction, esters were oxidized forming carboxyl and m-cresol was transformed into intermediates. Subsequently, the surface functional groups of SWs treated with oxidizing acid were transformed into carboxyl groups and siloxane bridges again. In this cycle the concentration of m-cresol decreased.

Fig. 7 Possible reaction mechanism on the surface of HNO₃-SW for the oxidation of m-cresol in CWPO reaction: the surface of original SW catalyst (a), the surface of HNO₃-SW catalyst (b), esterification and CWPO degradation of m-cresol on the surface of HNO₃-SW catalyst (c) and catalytic cycle in CWPO reaction by HNO₃-SW catalyst (d) *black balls, blue balls and yellow balls represent carbon, silicon and iron atoms, respectively.

3.5. m-Cresol oxidation mechanism

The degradation of m-cresol was investigated for the first time using in situ NMR. It is shown in Fig. 8(a) that in the ¹H NMR (D₂O, 400 MHz) spectrum δ 7.1–6.5 peaks are due to aromatic H, δ 2.1 is due to methyl H connected to the phenyl ring and δ 4.7 is due to D₂O in solution. As shown in Fig. 8(b), the ¹³C NMR (D₂O, 400 MHz) spectrum shows δ 160–110 peaks are due to aromatic C, δ 21.5 is due to methyl C connected to the phenyl ring. Many small peaks were observed (δ 160–110 and δ 30–10) at 90 min by prolonged ¹³C NMR scanning times, which were identified as the intermediates. With the CWPO reaction carried out, the peaks of m-cresol in the ¹H NMR and ¹³C NMR spectra were weak. It indicated that m-cresol was degraded and that many intermediates were generated in the CWPO reaction.

Fig. 8 In situ NMR spectra of m-cresol in a CWPO reaction: in situ ¹H NMR spectra (a) and in situ ¹³C NMR (b).

In order to investigate the degradation pathway of m-cresol pollutants in the CWPO process by HNO₃-SW catalyst, GC-MS, IC and HRMS were applied to identify the organic products, and the results are shown in Fig. 9, S3, S4 and Table S1.† In the CWPO experiment of m-cresol, 12 kinds of intermediates were identified. It was proposed that m-cresol might be degraded via three pathways, similar to phenol in a previous report.²¹ However, with the methyl on the phenol, it was easier to determine the ring-opening location. In the first step, ˙OH attacked the ortho and para position of m-cresol and formed 2-methylbenzene-1,4-diol (A), 3-methylbenzene-1,2-diol (B) and 4-methylbenzene-1,2-diol (C), and then (A), (B) and (C) were quickly oxidized to quinones (D), (E) and (F), respectively. However, it was difficult to generate 5-methylbenzene-1,3-diol, because the hydroxy group on the benzene ring was the ortho and para directing group. Secondly, 2-methylcyclohexa-2,5-diene-1,4-dione (D) was oxidized to 2-oxopropanoic acid (O) and maleic acid (1), while for 3-methylcyclohexa-3,5-diene-1,2-dione (E) and 4-methylcyclohexa-3,5-diene-1,2-dione (F), the ring was opened and generated (2Z,4Z)-2-methyl-6-oxohexa-2,4-dienoic acid (H), (2Z,4Z)-5-methyl-6-oxohexa-2,4-dienoic acid (I) and (2Z,4Z)-3-methyl-6-oxohexa-2,4-dienoic acid (J). Then they were oxidized to (Z)-5-oxohex-3-enoic acid (L), (2Z,4Z)-2-methylhexa-2,4-dienedioic acid (M) and (Z)-2-methyl-5-oxopent-2-enoic acid (N), respectively. L was oxidized to (Z)-4-oxopent-2-enoic acid (6) and then maleic acid (1). M and N were oxidized to (Z)-2-methylpent-2-enedioic acid (7). After that the oxidation of high molecular weight dicarboxylic acids in CWPO proceeded with the consecutive oxidation of higher to lower molecular weight dicarboxylic acids through α-oxidations. Further oxidations of the intermediates were from C₆ compound to C₁ compound through α-oxidations and CO₂ released.

Fig. 9 Possible degradation pathway of m-cresol in CWPO by HNO₃-SW catalyst. No. 1–12 carboxylic acids were detected by GC-MS and HRMS in this work, A–N compounds were degradation intermediates, *compounds were reported previously.^31–36

3.6. Mechanism of oxidation at carbons near a –COOH group

For a hydroxyl radical mechanism, the weaker C–H bonds of the oxidized organic compounds were considered to be easily attacked. For high molecular weight carboxylic acids, it was reported that one of the most probable attacks of hydroxyl radicals was usually considered to occur easily at carbons near a –COOH group as these carbons had high acidity.^37,38 For high molecular weight dicarboxylic acids, the mechanism of the attack of hydroxyl radicals at an α-carbon may be proposed as shown in Fig. 10. First, a hydroxyl radical ˙OH attacks an α-hydrogen from a high molecular weight dicarboxylic acid and yields HOOC–(CH₂)_n−1–C(OH)HCOOH. Since an α-hydroxy acid is unstable, it quickly decomposes to aldehyde and formic acid. Subsequently the aldehyde is further oxidized to form a carboxylic acid with one less carbon atom, HOOC–(CH₂)_n−1COOH. Therefore, following α-oxidation, a mixture of all carboxylic acids and aldehydes form C₁ to C₇ existed in the solution. As aldehydes were oxidized easily, it was difficult to find aldehydes. Therefore various carboxylic acids were identified of all carbon numbers under 7.

Fig. 10 The reaction pathway proposed for the α-oxidation.

4. Conclusions

In this study, we have prepared surface modification of sewage sludge derived carbonaceous materials (SWs) by using different kinds of acid treatment. Further studies indicated that the surface modified SWs could be used as an efficient heterogeneous catalyst to degrade m-cresol in a CWPO reaction. The enhanced catalytic activity of SWs was ascribed to ferric iron and surface modified groups on SWs after oxyacid treatment. The degradation pathway of m-cresol by CWPO was investigated. It was proposed that the reaction of m-cresol in CWPO proceeds with consecutive oxidation of higher molecular weight carboxylic acids to lower molecular weight carboxylic acids.

References

1. R. R. N. Marques, F. Stüber, K. M. Smith, A. Fabregat, C. Bengoa, J. Font, A. Fortuny, S. Pullket, G. D. Fowler and N. J. D. Graham, Appl. Catal., B, 2011, 101, 306–316.
2. L. Gu, N. Zhu, D. Zhang, Z. Lou, H. Yuan and P. Zhou, Bioresour. Technol., 2013, 136, 719–724.
3. J. S. Cha, J.-C. Choi, J. H. Ko, Y.-K. Park, S. H. Park, K.-E. Jeong, S.-S. Kim and J.-K. Jeon, Chem. Eng. J., 2010, 156, 321–327.
4. C. Jindarom, V. Meeyoo, B. Kitiyanan, T. Rirksomboon and P. Rangsunvigit, Chem. Eng. J., 2007, 133, 239–246.
5. Y. Yang, J. G. Brammer, M. Ouadi, J. Samanya, A. Hornung, H. M. Xu and Y. Li, Fuel, 2013, 103, 247–257.
6. R. Luque, J. A. Menéndez, A. Arenillas and J. Cot, Energy Environ. Sci., 2012, 5, 5481.
7. T. J. Bandosz and K. Block, Appl. Catal., B, 2006, 67, 77–85.
8. W.-J. Liu, F.-X. Zeng, H. Jiang, X.-S. Zhang and W.-W. Li, Chem. Eng. J., 2012, 180, 9–18.
9. E. Neyens and J. Baeyens, J. Hazard. Mater., 2003, 98, 33–50.
10. K. M. Smith, G. D. Fowler, S. Pullket and N. J. D. Graham, Water Res., 2009, 43, 2569–2594.
11. X. G. Chen, S. Jeyaseelan and N. Graham, Waste Manag., 2002, 22, 755–760.
12. R. Pietrzak and T. J. Bandosz, J. Hazard. Mater., 2008, 154, 946–953.
13. A. Ros, M. A. Montes-Moran, E. Fuente, D. M. Nevskaia and M. J. Martin, Environ. Sci. Technol., 2006, 40, 302–309.
14. F. Rozada, M. Otero, A. Moran and A. I. Garcia, Bioresour. Technol., 2008, 99, 6332–6338.
15. L. Gu, N. Zhu and P. Zhou, Bioresour. Technol., 2012, 118, 638–642.
16. Y. Tu, S. Tian, L. Kong and Y. Xiong, Chem. Eng. J., 2012, 185, 44–51.
17. D. Chen, H. B. Feng and J. H. Li, Chem. Rev., 2012, 112, 6027–6053.
18. H. P. Boehm, Carbon, 2002, 40, 145–149.
19. Q. L. Fang, B. L. Chen, Y. J. Lin and Y. T. Guan, Environ. Sci. Technol., 2014, 48, 279–288.
20. K. Erickson, R. Erni, Z. Lee, N. Alem, W. Gannett and A. Zettl, Adv. Mater., 2010, 22, 4467–4472.
21. J. A. Zazo, J. A. Casas, A. F. Mohedano, M. A. Gilarranz and J. J. Rodriguez, Environ. Sci. Technol., 2005, 39, 9295–9302.
22. Y. Wu, C. D’Agostino, D. J. Holland and L. F. Gladden, Chem. Commun., 2014, 50, 14137–14140.
23. R. R. N. Marques, F. Stueber, K. M. Smith, A. Fabregat, C. Bengoa, J. Font, A. Fortuny, S. Pullket, G. D. Fowler and N. J. D. Graham, Appl. Catal., B, 2011, 101, 306–316.
24. L. Gu, N. Zhu, H. Guo, S. Huang, Z. Lou and H. Yuan, J. Hazard. Mater., 2013, 246, 145–153.
25. J. Wang, G. Li, X. Ju, H. Xia, F. Fan, J. Wang, Z. Feng and C. Li, J. Catal., 2013, 301, 77–82.
26. G. K. Reddy, P. Boolchand and P. G. Smirniotis, J. Phys. Chem. C, 2012, 116, 11019–11031.
27. P. V. Nidheesh, R. Gandhimathi and S. T. Ramesh, Environ. Sci. Pollut. Res., 2013, 20, 2099–2132.
28. H. J. Salavagione, M. A. Gomez and G. Martinez, Macromolecules, 2009, 42, 6331–6334.
29. M. M. Heravi, F. K. Behbahani, R. Hekmat Shoar and H. A. Oskooie, Catal. Commun., 2006, 7, 136–139.
30. S. Gowda and K. M. Lokanatha Rai, J. Mol. Catal. A: Chem., 2004, 217, 27–29.
31. Y. Chu, D. Zhang, L. Liu, Y. Qian and L. Li, J. Hazard. Mater., 2013, 252, 306–312.
32. A. Boshoff, M. H. Burton and S. G. Burton, Biotechnol. Bioeng., 2003, 83, 1–7.
33. H. Beiginejad, D. Nematollahi, F. Varmaghani, M. Bayat and H. Salehzadeh, J. Electrochem. Soc., 2013, 160, G3001–G3007.
34. F. Subrizi, M. Crucianelli, V. Grossi, M. Passacantando, L. Pesci and R. Saladino, ACS Catal., 2014, 4, 810–822.
35. M. Guazzaroni, M. Pasqualini, G. Botta and R. Saladino, ChemCatChem, 2012, 4, 89–99.
36. M. Nikolantonaki and A. L. Waterhouse, J. Agric. Food Chem., 2012, 60, 8484–8491.
37. F. M. Jin, T. Moriya and H. Enomoto, Environ. Sci. Technol., 2003, 37, 3220–3231.
38. F. Jin, J. Cao, H. Enomoto and T. Moriya, J. Supercrit. Fluids, 2006, 39, 80–88.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra00858a

---