Catalytic Ozonation for Pulp and Paper Mill Wastewater Treatment: COD Reduction and Organic Matter Degradation Mechanism

Abstract

Rapid degradation of pulping and papermaking wastewater in a pulp and paper mill is crucial for recycling purposes yet challenging to achieve. The purpose of this research is to provide a technical guide for the ozone degradation treatment process of pulp and paper mill wastewater and to explore the reaction mechanism of dissolved and colloidal substances (DCSs). This study is vital for effectively treating pulp and paper mill wastewater through ozonation. In the catalytic ozonation process to treat pulp and paper mill wastewater, a polyurethane sponge loaded with titanium dioxide was used as a catalyst. The optimal process conditions were determined to be 8 min of treatment time, a 16 mg/L ozone concentration, pH 9, and a 7.5% catalyst filling ratio. The COD reduction under these conditions is approximately 52%. The catalytic ozonation system, according to the FI-IR and GC-MS analyses, could degrade the large-molecule volatile organic compounds in the raw wastewater into small-molecule substances. Furthermore, the relative content of common DCSs in paper wastewater, such as palmitic acid and stilbene, could be reduced. The catalytic ozonation system is more effective for treating refractory organic compounds and has a higher COD reduction than the ozonation system.

1. Introduction

The paper industry is an industrial sector with a high consumption of water resources. With the development of the pulp and paper industry in recent years, more and more pollutants are being produced and the types are complex. More than 250 organic pollutants have been detected in pulp and pulp and paper mill wastewater [1]. They come from the wood itself and with various paper-making additives. These pollutants are more diverse and include fatty acids, resin acids, alcohols, phenols, etc. They are also usually complicated to degrade. According to data from the Food and Agriculture Organization of the United Nations Statistics (FAOSTAT), global paper production in 2015 exceeded 390 million tons [2]. China’s total pulp consumption was 100.51 million tons, of which 63.02 million tons of waste pulp accounted for 63% in 2019. In 2017, imported wastepaper accounted for 36% of the raw material composition of waste paper in papermaking. The release of the discharge standard of water pollutants for the pulp and paper industry in 2008 (GB354-2008) also reflects the trend of increasingly strict requirements for pulp and paper mill wastewater in China. Therefore, more and more enterprises have the demand of a closed water cycle. DCSs generally refer to adhesives with a size of less than 5 μm. The main sources of DCSs are adhesives and resin components (fatty acids and esters) in adhesives and printing inks [3]. Although the DCS content is not much, it is difficult to remove, and easy to gather again with the change of system environmental conditions [4], which greatly impacts the reuse of wastewater from the paper mill [5]. As a result, more and more recycled pulp and paper mill wastewater in China needs to be treated.

In order to reduce the pollution and emissions caused by pulp and paper mill wastewater, more and more treatment methods have emerged, such as coagulation, flocculation, chemical precipitation, membrane separation, biological treatment, advanced oxidation technology, etc. However, applying a single method is often difficult to achieve better processing results. Chemical coagulation sedimentation is a physical and chemical treatment process that can separate solid and liquid phases under the action of a coagulant. However, the separated solid will produce chemical sludge, and the treatment and disposal of this sludge will bring new problems [6]. Membrane separation is also an effective physical technology. Its separation mechanism is to remove macromolecular pollutants through the small pore size of the membrane. It has been reported that ultrafiltration can significantly reduce 54% of COD [7]. Although the treatment effect of the membrane separation method is excellent, it will reduce the service life of the membrane due to the deposition of pollutants. With the membrane pore size becoming smaller, the operating costs will increase more and more [8]. Biological treatment is widely used as a treatment technology with low economic cost. The activated sludge method forms a mixed solution by mixing wastewater with activated sludge and continuously injects oxygen into the mixed solution to maintain the aerobic respiration conditions of microorganisms. This results in lower BOD levels in the water. However, it also has the disadvantage of having a low ability to remove refractory organic matter.

Ozonation treatment has a shorter reaction time, smaller equipment footprint, and no secondary pollution generation; ozonation technology is gradually gaining the attention of many enterprises. Ozone oxidizes organic compounds in two ways: direct oxidation by ozone and indirect oxidation by hydroxyl radicals, which can oxidize many large molecules of organic compounds, thus degrading large molecules into small molecules [9]. Because ozone has a good oxidation ability, more and more research on catalysts is appearing. Catalysts can be divided into two categories: homogeneous catalysts and heterogeneous catalysts [10]. It is not appropriate to use homogeneous catalysts in wastewater treatment because they will introduce new impurities into the water. The main research direction of catalysts in the wastewater treatment field is applications of heterogeneous catalysts. He et al. [11] used a Fe–Al composite catalyst for catalytic ozonation, which could improve the TOC reduction efficiency of pulp and paper mill wastewater by 25%. Zhuang [12] used activated carbon loaded with manganese oxide as a catalyst for ozone-catalyzed deep treatment of pulp and paper mill wastewater with special reduction of 58.5 and 77.5% for chromaticity and COD, respectively. In addition, catalytic ozonation technology can improve the biochemical properties of pulp and paper mill wastewater. Ramos et al. [13] found that the biochemical properties of pulp and paper mill wastewater increased (0.067–0.29) during ozonation of pulp and paper mill wastewater. Yan et al. [14] used Fe²⁺, Mn²⁺, and Cu²⁺ homogeneous catalytic ozonation to degrade polyvinyl alcohol (PVA) in water. The PVA removal rate could reach about 85%. Yang et al. [15] used rutile-type titanium dioxide nanoparticles as a catalyst for the degradation of nitrobenzene. About 55% of nitrobenzene in water solution could be oxidized by ozone in the presence of TiO₂ calcined at 500 °C. Due to ozonation has good performance in water treatment, the research on catalyst has become increasingly hot in recent years.

Titanium dioxide often has good catalytic ozonation activity at a relatively low price. It is non-toxic and insoluble [16], and its ozone catalytic performance in pulp and paper mill wastewater needs to be explored. Because of the large specific surface area of polyurethane sponge, it can be loaded with sufficient titanium dioxide catalyst and is conducive to its recycling. In this study, titanium dioxide nanoparticles were loaded on a polyurethane sponge to treat pulp and paper mill wastewater to reduce COD. This study explored the optimal reaction conditions of the process and investigated the degradation mechanism of organic compounds in pulp and paper mill wastewater by Fourier infrared transform spectroscopy and gas chromatography–mass spectrometry under optimal reaction conditions. It provides a technical reference for titanium-dioxide-catalyzed degradation of pulp and paper mill wastewater.

2. Materials and Methods

2.1. Materials and Natural Wastewater

The experimental water was obtained from a paper mill in Dongguan after anaerobic biological treatment. As shown in

Table 1. Pulp and paper mill wastewater quality indicators.

	COD mg/L	BOD5 mg/L	NH3-N mg/L	TN mg/L	TP mg/L	Chromaticity	SS mg/L
Wastewater	953	254	14.23	13.55	0.61	3841	2439

, the water had COD 953 mg/L, pH 7.8, ammonia nitrogen (NH₃-N) 14.23 mg/L, total nitrogen (TN) 13.55 mg/L, total phosphorus (TP) 0.61 mg/L, chromaticity 3841, and suspended solid (SS) 2439 mg/L. Guangdong Guanghua Technology Co. Ltd (Shantou, China) The supplied methylene chloride, and Shanghai Maclean Biochemical Technology Co supplied the potassium bromide. Chengdu Kolon Chemical Co., Ltd (Chengdu, China) provided the hydrochloric acid, and Nanjing Chemical Reagent Co. provided the ethanol. An ozone generator (Anseros, Anshan, China, COM-AD-02: the concentration ranges from 10 mg to 100 mg), a pH meter, Fourier transform infrared spectrophotometer (Shimadzu, Kyoto, Japan, IRTracer-100), magnetic stirrer, and gas chromatography-mass spectrometry were the main instruments used (GC-MS).

2.2. Sponge Cubes and Titanium Dioxide Coating Procedure

For the polyurethane sponge purchased from Shandong Banhor Environmental Protection Technology Co., Ltd. (Linyi, China), according to the information provided by the company, the polyurethane sponge carrier is a 1 cm³ cube (98% porosity, 1.01–1.02 g/cm³ wet density, 422.2% water absorption, 100% retention, 4000 specific surface area, and 40PPI average aperture). The average particle size of titanium dioxide is 15 nm. Titanium dioxide was fixed on the surface of the sponge carrier, and a layer of TiO₂ film was formed on the surface of the sponge carrier. It is a heterogeneous catalyst, and was characterized in our previous publication by SEM-EDS (1000×) [17].

Figure 1 depicts the specific preparation procedure. Forty polyurethane sponge carriers were placed in beakers and ultrasonically treated with ultra-pure water, ethanol, and acetone in that order. One g of TiO₂ powder was weighed and placed in a beaker, followed by 10 mL and 0.3 g/L dioctyl sodium sulfosuccinate (DSSC) solution, which was magnetically stirred to form solution A. Approximately 0.3 g of hydroxypropylmethyl cellulose (HPMC) powder was weighed and placed in a beaker, followed by 20 mL of DSSC solution and mechanical stirring to form solution B. Solution A and B were mixed and mechanically stirred to form TiO₂ slurry; the sponge carrier, which had been cleaned, was placed in the TiO₂ slurry and mechanically stirred; to obtain the TiO₂/sponge composite material, the polyurethane sponge carrier was removed and heated in an oven at 70 °C for 8 h. The material was then ultrasonically cleaned with ultra-pure water to remove excess TiO₂ on the surface. Subsequently, the TiO₂/sponge composite material cleaned to produce no TiO₂ shedding was placed in the oven at 70 °C and dried to a constant weight to obtain TiO₂-coated sponge composite carriers.

2.3. Experimental Procedure

At a temperature of 25 °C, the experiment was carried out by adding 500 mL of actual wastewater to a 1000 mL beaker. The raw material for the ozone generators was oxygen. The ozone generator was preheated for 10 min while the ozone flow rate was adjusted using the flowmeter. The catalyst was then added at the start of the reaction. Following the reaction, the supernatant was collected for analysis.

2.4. Analysis Methods

HJ 828-2017, “Water quality—determination of the chemical oxygen demand—dichromate method” was used to calculate COD. HJ 535-2009, “Water quality—determination of ammonia nitrogen reagent—Nessler’s spectrophotometry,” was used to determine ammonia nitrogen. HJ 670-2013, “Water quality—determination of orthophosphate and total phosphorus—continuous flow analysis (CFA) and ammonium molybdate spectrophotometry,” was used to calculate total phosphorus. In addition, the determination of total nitrogen was based on GB11894-89 “Water quality—determination of total nitrogen—alkaline potassium persulfate digestion UV spectrophotometric method”. The suspended solids were determined according to GB11901-89, “Water quality—determination of suspended substance—gravimetric method”. The pH value was calculated using the GB/T 6920-1986 glass electrode method. The above test methods were developed by the Chinese government as national or industrial standards. On each sample, three repeated tests were performed, and the results were reported with an average value of less than 5%.

The wastewater functional group changes were investigated using Fourier transform infrared (FT-IR). To prepare the sample, 300 mL was filtered through a 0.45 μm membrane, concentrated to 15 mL by rotary evaporation of the solvent at 50 °C, and freeze-dried to obtain a solid powder. For FT-IR detection, the product was combined with a potassium bromide tablet. The scanning wavelength was set to 400–4000 cm⁻¹, the resolution was set to 4 cm⁻¹, and the scanning number was set to 32 [18]. Gas chromatography–mass spectrometry (GC-MS) is an accurate analytical tool for determining water quality and organic matter. The following are the specific steps: a total of 100 mL of wastewater was filtered through a 0.45 μm microporous membrane before being transferred to a partition funnel. Following that, 10 g sodium chloride was added and shaken to dissolve. Then, 40 mL of dichloromethane was added and shaken to release gas, followed by another shake. The organic phase was extracted for 10 min and then left for 10 min before being collected. The organic phase was extracted for 10 min and left for 10 min, and the organic phase was collected. The aqueous phase was extracted again with 20 mL of dichloromethane, and the organic phase was combined and dehydrated by adding anhydrous sodium sulfate to the organic phase. After that, the organic phase was concentrated to 12 mL using a rotary evaporator at 36 °C for analysis by GC-MS. The heating procedure: 40 °C (hold for 3 min) → 200 °C (5 °C/min hold for 3 min) → 280 °C (10 °C/min, hold for 10 min) with a solvent delay of 4 min.

3. Results and Discussion

3.1. Exploration of Optimum Process Conditions

3.1.1. Effect of Ozone Concentration and Time

Under the experimental conditions of a 10 min reaction time, 25 °C, a pH 7.8, and a catalyst filling rate of 7.5%, the effect of the ozone concentration on the COD reduction was examined. Figure 2 shows the effect of different ozone concentrations and time on COD reduction; it can be seen that the COD reduction increased with time before 8 min. The COD reduction of 16 mg/L and 20 mg/L reached the plateau after 8 min and the COD reductions of 12 mg/L, 16 mg/L, and 20 mg/L decreased after 8 min [19], which proves that the reaction time plays a role in COD reduction. This may be due to the oxidation of the suspended solids in the water with the increase in the reaction time, thus releasing a large number of soluble compounds [20,21]. For this reason, we chose 8 min as the optimal reaction time.

Figure 2 also shows that the COD reduction is high at ozone concentrations of 16 mg/L and 20 mg/L, while the COD reduction at an ozone concentration of 4 mg/L is low. It can therefore be assumed that the reduction in COD is proportional to the ozone concentration. The COD reduction increases by 11% at 8 min as the ozone concentration is increased from 4 mg/L to 8 mg/L. However, the COD reduction only increases by 3% at 8 min as the ozone concentration is increased from 16 mg/L to 20 mg/L. Although the COD reduction efficiency is better at 20 mg/L, the best experimental condition should be 16 mg/L ozone concentration considering the limited improvement of reduction efficiency and the treatment cost.

3.1.2. Effect of pH

The efficiency of the COD reduction also varies significantly at different pH conditions. Figure 3 shows the effect of pH on COD reduction at a reaction time of 8 min, a temperature of 25 °C, and an ozone concentration of 12 mg/min during catalytic ozonation. It can be seen from Figure 3 that the COD reduction increases as the initial pH is increased from 3 to 9 in the catalytic ozonation system. This is mainly due to the fact that the decomposition ratio of ozone in the water body is slow in acidic conditions, so ozone reacts directly with reducing substances in water in molecular form, while under alkaline conditions, it can enhance the decomposition of ozone and promote the generation of hydroxyl radicals [20,21]. It also can be seen from Figure 3 that the COD reduction at pH 11 is higher than pH 9 before 6 min. However, after 6 min, the COD reduction at pH 11 was lower than that at pH 9. This is mainly due to the quenching of hydroxyl radicals caused by too high pH [22]. Therefore, pH 9 was selected as the condition for the treatment of pulp and paper mill wastewater by catalytic ozonation.

3.1.3. Effect of the Catalyst Filling Ratio

The titanium dioxide composite carriers were used as a catalyst, and the catalyst filling ratio was the ratio of the volume of the carrier to the volume of water. Furthermore, the catalyst filling ratio also has a significant effect on the COD reduction [23]. The effect of the catalyst filling ratio on the COD reduction was investigated at a reaction time of 8 min, an ozone concentration of 12 mg/L, and pH 9. As shown in Figure 4, when the catalyst filling ratio is 10%, the COD reduction reaches the highest of 37%. It can therefore be assumed that the catalyst filling ratio of 10% should be selected as the best experimental condition. However, the COD reduction of the 10% catalyst filling ratio was only 1% different from that of the 7.5% catalyst filling ratio. It meant that similar results can be obtained under the condition of 7.5% COD reduction. Therefore, considering the problem of treatment costs, a 7.5% catalyst filling ratio was selected as the optimal experimental condition.

3.2. Kinetic Study

The kinetic of COD reduction in ozonation and catalytic ozonation were carried out. The equation of COD reduction in ozonation and catalytic ozonation with respect to time can be written as Equations (1) and (2) [24]. Where [COD₀], [COD_t], [O₃] [TiO₂], [K₁], and [K₂] are the initial COD, final COD, ozone, catalyst filling rate, and rate constants of ozonation and catalytic ozonation processes, respectively. In Equation (1), the COD concentration is always much higher than the ozone concentration. In Equation (2), the rate of COD reduction is dependent on the reaction between O₃ concentrations and [TiO₂]; both O₃ and [TiO₂] were nearly constant, while the COD concentration varies with time. In fact, both ozonation and catalytic ozonation are regarded as pseudo-first-order reactions. Therefore, Equation (2) can be reduced to Equation (3). Finally, Equation (3) was integrated to obtain Equation (4).

$$\frac{d\left(CO{D}_0\right)}{dt}=-K_1\left(CO{D}_t\right)\left(O_3\right)$$

(1)

$$\frac{d\left(CO{D}_0\right)}{dt}=-K_2\left(CO{D}_t\right)\left(O_3\right)\left(Ti{O}_2\right)$$

(2)

$$\frac{d\left(CO{D}_0\right)}{dt}=-K\left(CO{D}_t\right)$$

(3)

$$ln\frac{(CO{D}_0)}{\left(COD\right)}=-Kt$$

(4)

The solution of Equation (4) is obtained. As shown in Figure 5, K is the quasi-primary kinetic rate constant. In the experiments with different catalyst filling ratios, the rate constants of COD reduction are 0.03202, 0.03986, 0.04572, 0.05045, and 0.05307, and the correlation coefficient is large (R₂ > 0.9), so the quasi-first-order kinetic model fits the experimental data well. With the increase in the catalyst filling ratio, the K value increases, and the reaction rate constant increases from 0.03202 to 0.05307 when the catalyst filling ratio increases from 0 to 7.5%. Therefore, the use of a catalyst affects the initial kinetics of COD reduction.

3.3. Reusability Test

Three rounds of continuous degradation experiments of pulping and papermaking wastewater were set up to test the reusability of TiO₂/sponge composite materials. At the end of each round of experiments, the gravity sedimentation method was used to separate the composite carrier, and the composite carrier was washed three times with ultra-pure water, and then put into the oven for drying before being used. The results of repeated cycle experiment are shown in Figure 6. The results show that catalyzed ozonation can degrade 52% of COD within 8 min. After three rounds of repeated experiments, THE COD reduction effect decreased slightly, but was basically stable, proving that the TiO₂/sponge composite carrier has good reusability.

3.4. Water Quality Indicators under Optimal Process Conditions

The pulp and paper mill wastewater was treated under the experimental conditions determined according to the single-factor experiment; the conditions were a reaction time of 8 min, an ozone concentration of 16 mg/L, a pH 9, and a catalyst filling ratio of 7.5%.

Table 2. Water quality indicators of wastewater before and after treatment.

	COD mg/L	B/C	NH3-N mg/L	TN mg/L	TN mg/L	Chromaticity	SS mg/L
Wastewater	953	0.26	24.23	13.55	0.61	3841	2439.4
Ozone oxidation	715	0.33	23.05	12.31	0.63	184	982.2
Ozone-catalyzed oxidation	453	0.37	25.03	11.97	0.67	153	849.4

shows the water quality indicators of wastewater before and after treatment. It can be seen that the BOD₅/COD(B/C) value increases from 0.26 to 0.33 after ozonation treatment. In contrast, the B/C value is 0.37 after catalytic ozonation treatment, which means that catalytic ozonation treatment can improve biodegradability more effectively. Furthermore, the contents of NH₃-N, TN, and TP change slightly after different treatments, which can be inferred that these treatments are insufficient to effectively remove NH₃-N, TN, and TP. However, ozonation treatment and catalytic ozonation treatment have good reduction effects on the chromaticity and SS (suspended solid). After ozonation treatment, chromaticity decreases from 3481 to 184, and after catalytic ozonation treatment, chroma decreases from 3481 to 153. Compared with ozonation, the reduction efficiency of COD and SS by catalytic ozonation is better, and the reduction efficiency of COD and SS by catalytic ozonation reaches 52% and 65%, respectively.

In general, ozone treatment cannot effectively remove nitrogen and phosphorus, but the reduction of COD, chroma, and SS is efficient.

3.5. FT-IR Analysis of Effluent Water Quality in Different Systems

Figure 7 shows the Fourier transform infrared spectra measured before and after the treatment of the ozonation system and catalytic ozonation system. The absorption peak at 616 cm-1 may be due to the presence of halides [25]. The absorption peak at 1116 cm⁻¹ represents the O-H in-plane bending vibration of phenols or alcohols and the C-O-C stretching vibration of ethers. The absorption peaks at 1465 cm⁻¹ and 1567 cm⁻¹ are caused by the C=C stretching vibration on the benzene ring [26]. The peaks generated at 1410 cm-1 and 1663 cm⁻¹ represent the presence of amide in the water, which may come from the printing ink residue in the paper mill wastewater. The absorption peak at 1796 cm⁻¹ can be inferred as the characteristic peak of the carbonyl group, which is the primary source of chromaticity of paper mill wastewater.

Furthermore, it can be seen from Figure 7 that the absorption peak here disappears after treatment, which also explains the reduction of the chroma of wastewater. A broadband and strong absorption spectrum of O-H stretching vibration at 3450 cm⁻¹ indicate that the raw water may contain carboxylic acids, phenols and alcohols [27]. The Figure 7 shows the peak formed by the C-H stretching vibration of methyl at 2965 cm⁻¹ [28]. Most of the absorption peaks of the samples treated by initial ozonation and catalytic ozonation are the same. After being treated by the ozonation system and the catalytic ozonation system, their absorption peak intensities are reduced, which indicates that they have a good effect on removing organic matter from papermaking wastewater. The absorption peak intensity of wastewater treated by catalytic ozonation at 3450 cm⁻¹, 1410 cm⁻¹, 1465 cm⁻¹, 1567 cm⁻¹, and 1663 cm⁻¹ decreases more than that of wastewater treated by ozonation, indicating that catalytic ozonation has a relatively strong degradation effect on organic substances in papermaking wastewater. In addition, a new benzene ring substitution peak was formed at 785 cm⁻¹, indicating that new aromatic organic compounds were produced during ozonation.

3.6. GC-MS Analysis of Pulp and Paper Mill Wastewater before and after Degradation by Different Systems

Table 3. GC-MS analysis of pulp and paper mill wastewater before and after treatment.

RT	Organic Compound	Relative Content (%)
		Wastewater	Ozonation	Catalytic Ozonation	Pure TiO2/O3
4.344	2-Methyl-2-pentenoic acid	0.11	-	-	-
6.502	2-Aminoanthracene	-	0.06	0.18	0.12
6.634	Benzaldehyde	-	0.12	-	0.06
6.934	2,2-Diethoxyacetophenone	-	0.72	0.6	0.52
7.741	2-Ethylhexanol	-	0.34	0.55	0.42
8.145	Nineteen alkane	0.13	-	-	-
8.154	Dodecane	-	0.05	0.13	0.07
8.241	Octadecane	-	0.72	0.03	0.34
8.379	Acetophenone	-	-	0.03	-
8.867	3,6-Dimethyldecane	-	-	0.05	-
8.911	Nonyl aldehyde	-	0.82	-	0.12
9.133	Phenethyl alcohol	0.02	-	-	-
9.416	p-Ethylbenzoic acid	0.01	-	-	-
9.68	Diethoxymethane	-	0.1	0.13	0.11
9.968	3,4-Dimethylaniline	-	-	0.03	0.01
10.103	Myristyl alcohol	-	-	-	0.27
10.218	Dodecene	0.13	-	-	-
10.44	1,13-Tetradecanediene	0.01	-	-	-
10.52	Styrene	-	-	-	0.61
10.687	2,4-Dimethylbenzaldehyde	0.08	-	-	-
11.267	2-Ethylhexylhexyl ester	0.01	-	-	-
11.4	Nonanoic acid	-	0.05	0.13	0.27
11.506	Pentadecane	-	0.13	0.12	0.11
12.324	Myristic aldehyde	-	-	-	0.02
12.582	1,3-Diisocyanato-2-methylbenzene	-	-	0.22	0.13
13.107	Tetradecane	0.03	0.12	0.08	0.09
13.252	Tetrahydro-alpha-naphthol	-	-	0.07	0.01
13.366	3-Isopropylaniline	0.04	-	-	-
13.905	Dimethyl phthalate	0.04	-	-	-
14.621	3,5-Di-tert-butylphenol	0.77	1.89	2.56	2.04
14.765	4-Tetradecyl methoxyacetate	-	0.12	0.05	0.07
14.888	Tetradecane	0.05	-	-	-
15.178	Dodecanoic acid	-	-	0.03	0.52
15.386	Dihexadecane	0.04	-	-	-
15.603	Diethyl phthalate	0.03	-	-	-
15.746	Octadecyl chloride	0.04	-	-	-
15.918	stilbene	2.09	-	-	-
15.996	4-Methyloctane	0.05	-	-	-
16.156	Dodecylbenzene	0.07	-	-	-
16.428	Triallyl isocyanurate	0.1	-	-	-
16.701	Heptadecane	0.09	-	-	-
16.841	n-hexadecane	0.11	-	-	-
16.849	Tridecane	0.25	0.19	0.14	0.17
16.931	4-Methyldodecane	0.11	-	-	-
17.141	5-Phenyldodecane	0.07	-	-	-
17.271	Eighteen methyl carbonate	0.07	-	-	-
17.318	Octadecane	-	-	0.12	0.14
17.477	2,4,6-Trimethyl-N-(2,4,6-trimethylphenyl)-aniline	0.12	-	-	-
17.574	3,5-Di-tert-butyl-4-hydroxybenzaldehyde	0.17	-	-	-
17.854	n-octacosane	0.45	0.32	0.12	0.22
18.15	2-Methyl-benzo[f]quinoline	-	-	0.13	-
18.213	7,11-Hexadecadienyl acetate	0.64	-	-	-
18.453	Oleic acid amide	6.58	-	-	-
18.581	Dimethyl phthalate	-	-	0.11	0.09
18.841	2-Pentadecanone	2.46	1.35	0.21	0.65
19.082	Eicosane	-	0.21	0.37	0.32
19.186	Stearamide	0.19	-	-	-
19.448	Palmitic acid	5.67	2.21	-	0.09
19.502	Dibutyl phthalate	0.24	-	-	-
19.726	Ethyl oleate	0.48	-	-	-
19.856	N-Pentadecanenitrile	-	-	0.05	-
20.092	2,2′-Methylenebis(4-methyl-6-tert-butylphenol)	1.45	-	-	-
20.607	1-21 Alcohol	0.59	-	-	-
20.871	N,N-Dimethyldodecanamide	0.84	-	-	-
21.096	9,12-Octadecadienoic acid (Z,Z)-	-	1.82	2.11	1.76
21.133	Oleic acid	4.17	2.32	1.73	2.11
21.311	Pentadecanoic acid	0.5	-	-	-
21.448	Stearyl Nitrile	2.15	-	-	-
21.615	1-Chlorononane	1.22	0.54	0.14	0.22
21.696	(5E)-5-Octadecene	1.07	-	-	-
21.703	Cyclohexadecane	-	-	0.33	0.14
22.147	N,N-Dimethyloctanamide	-	0.25	0.34	0.17
23.188	Oleic acid amide	1.45	-	-	-
23.51	Ditetradecane	-	-	0.4	0.43
23.548	Eruvic acid amide	7.51	3.25	0.64	1.23
23.655	6-Ethyl-3-octanol	2.34	-	-	-
24.699	Nonyl hexanoate	1.1	0.64	-	0.12
24.724	Dihexadecanoic acid	-	1.15	1.64	3.22
26.219	Nineteen alkane	-	0.32	0.48	0.34

shows GC-MS analysis of pulp and paper mill wastewater before and after treatment of the pulp and paper mill wastewater. The wastewater contains a lot of organic compounds and its composition is complex. There are two organic compounds in DCSs [28], namely stilbene and palmitic acid. Stilbene is derived from styrene–butadiene–styrene copolymers and styrene–butadiene rubber, and palmitic acid is usually derived from dispersant, surfactant, or in-pulp sizing agents in the papermaking process [29]. The relative content of stilbene is 2.09%, and it can be degraded in two different treatments. The relative content of palmitic acid decreases from 5.67% to 2.11% after ozonation. In contrast, the catalytic ozonation system completely degrades palmitic acid [30,31]. Meanwhile, nine kinds of ester are detected in pulp and paper mill wastewater, such as dibutyl phthalate and diethyl phthalate, which are common toxic and hazardous organics in pulp and paper mill wastewater and are also included in Annex C of the USA EPA Priority Pollutant Control “Protocol Order” [32]. There is also an ester after ozonation, and the ester is completely degraded after catalytic ozonation, mainly due to the ester having an unsaturated bond, which is easily oxidized by catalytic ozonation. In addition, many of the saturated alkanes, such as dodecane, octadecane, tetradecane, etc., generated after treatment may be due to the oxidation of ample molecular weight organics by ozone with hydroxyl radicals to generate smaller molecular weight alkanes. In general, the ozonation technique is suitable for esters, aromatic rings, long-chain alkanes, halogenates, fatty acids, and other large-molecule volatile organic compounds, which can be decomposed into smaller alkanes, alcohols, esters, and acids (e.g., dodecane, dimethoxyethane, pentadecanoic acid, methoxy acetic acid-tetradecyl ester, and 2-ethyl hexanol) by ozone molecules and hydroxyl radicals.

When pure titanium dioxide powder was used to catalyze the ozonation treatment, intermediates produced by the degradation of fatty acids such as tetradecanol and tetradecyl aldehyde were detected. At the same time, palmitic acid was not completely degraded after treatment, and styrene was completely degraded, but the intermediate styrene still existed. This shows that the degree of mineralization of pure titanium dioxide powder catalytic ozonation treatment is not high, and some compounds cannot be completely degraded, which may be caused by the fact that titanium dioxide powder is easy to agglomerate in water, resulting in a serious decline in activity.

3.7. Analysis of Degradation Mechanism of Two Kinds of DCSs

As shown in Figure 8, according to the results of FT-IR and GC-MS, the degradation mechanisms of two kinds of DCSs are analyzed and the possible reaction paths are deduced. Under the strong oxidation of hydroxyl radicals, the palmitic acid is first decarboxylated to a radical with the release of the first C atom to CO; the C₁₄H₂₉ radical is then oxidized to pentadecanol (C₁₅H₃₂O). The alcohol C₁₅H₃₂O is then oxidized to pentadecanal (C₁₅H₃₀O), followed by oxidation to pentadecanoic acid (C₁₅H₃₀O₂) [33]. Subsequently, the acid undergoes chain reactions to release the second and additional CO₂ until the pentadecanoic acid achieves complete mineralization into CO₂ and H₂O [29]. Alcohols and aldehydes, the intermediate products of pentadecanoic acid, are not detected by GC-MS, which may be attributed to their rapid decomposition and oxidation.

As shown in Figure 9, the degradation of stilbene may follow as: ozone and C=C on styrene generate unstable primary ozonate-4-phenyl-1,2,3-trioxane through the dipolar cycloaddition reaction. Due to the asymmetric structure of styrene, the primary ozonides 4-phenyl-1,2,3-Trioxolane recombine to form new epoxides 3-phenyl-1,2,4-Trioxolane; the epoxides are decomposed to form intermediate benzaldehyde, which is then oxidized to benzoic acid by ozone; subsequently, the aromatic ring in benzoic acid is destroyed by ozone; then, it opens to form maleic anhydride and formaldehyde, which are eventually oxidized and degraded into carbon dioxide and water by ozone.

4. Conclusions

In this study, the polyurethane sponge/TiO₂ composite carrier was used as a catalyst to catalyze the oxidation of papermaking wastewater. The highest COD reduction of 52% was observed at pH 9, an ozone concentration of 16 mg/L, a catalyst filling ratio rate of 7.5%, and a reaction time of 8 min, meanwhile, the color decreased from 3481 to 153 and the B/C value increased from 0.26 to 0.37. Compared with the ozonation system, the catalytic ozonation system had a better effect on COD reduction, chroma, B/C value and SS. Kinetic studies showed that both ozonation and catalytic ozonation conformed to pseudo-first-order kinetics. The reaction rate constant of catalytic ozonation was significantly higher than that of ozonation. The GC-MS results showed that the catalytic ozonation system was effective in degrading DCS, as stilbene and palmitic acid were completely degraded; it could be concluded that it is suitable for treating water that can be reused in different papermaking processes. Furthermore, the degradation path and mechanism of stilbene and palmitic acid were deduced.

In sum, this study improves our understanding of catalytic ozonation mechanism of DCS and provides technical references for closed water cycle paper mills. All nine esters in the raw water were removed, and many small molecules were generated, which indicated that the large molecules were effectively decomposed. There is also a good treatment effect for organic substances such as palmitic acid and styrene that affect wastewater reuse. That is, palmitic acid and styrene were decomposed entirely at the end of the reaction. This proved that ozone catalysis could treat pulp and paper mill wastewater efficiently and provide a new idea for wastewater treatment technology.