Separations 2023 , 10 , 148
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In order to reduce the pollution and emissions caused by pulp and paper mill wastew- ater, 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 pro- cessing 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 signifi- cantly 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 im- prove 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 Fe 2+ ,Mn 2+ , and Cu 2+ 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 TiO 2 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 spec- troscopy 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.
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