There has been great interest in developing cost-effective and high-performance catalysts for the ozonation treatment of biologically refractory wastewaters. This study developed a novel copper-cerium oxide supported alumina (Cu-Ce/Al2O3) catalyst for the catalytic ozonation of pulp and paper mill wastewater. The evenly distributed composite metal oxides on the surface of catalysts evidently improved the catalytic degradation efficiency. The Cu-Ce/Al2O3/O3 process increased the total organic carbon (TOC) removal by 6.5%, 9.5%, 24.5%, and 35.5%, compared with Ce/Al2O3/O3, Cu/Al2O3/O3, Al2O3/O3, and ozone alone processes, respectively. The enhanced catalytic ozonation efficiency was mainly ascribed to an increased hydroxyl radical (·OH)-mediated ozonation, both in the bulk solution and on the surface of catalysts. The surface hydroxyl groups (-OHs) of Al2O3 along with the deposited Cu-Ce oxides greatly enhanced the catalytic performance. This work illustrated potential applications of Cu-Ce/Al2O3 catalyzed ozonation for the advanced treatment of biologically recalcitrant wastewaters.
Mineralization of Recalcitrant Organic Pollutants in Pulp and Paper Mill Wastewaters through Ozonation Catalyzed by Cu-Ce Supported on Al2O3
Shuaiming He,a Pengcheng Luan,a Lihuan Mo,a,* Jun Xu,a Jun Li,a,* Liqi Zhu,b and Jinsong Zeng a
There has been great interest in developing cost-effective and high-performance catalysts for the ozonation treatment of biologically refractory wastewaters. This study developed a novel copper-cerium oxide supported alumina (Cu-Ce/Al2O3) catalyst for the catalytic ozonation of pulp and paper mill wastewater. The evenly distributed composite metal oxides on the surface of catalysts evidently improved the catalytic degradation efficiency. The Cu-Ce/Al2O3/O3process increased the total organic carbon (TOC) removal by 6.5%, 9.5%, 24.5%, and 35.5%, compared with Ce/Al2O3/O3, Cu/Al2O3/O3, Al2O3/O3, and ozone alone processes, respectively. The enhanced catalytic ozonation efficiency was mainly ascribed to an increased hydroxyl radical (·OH)-mediated ozonation, both in the bulk solution and on the surface of catalysts. The surface hydroxyl groups (-OHs) of Al2O3 along with the deposited Cu-Ce oxides greatly enhanced the catalytic performance. This work illustrated potential applications of Cu-Ce/Al2O3 catalyzed ozonation for the advanced treatment of biologically recalcitrant wastewaters.
Keywords: Catalytic ozonation; γ-Al2O3; CeO2; CuO; Papermaking wastewater; TOC
Contact information: a: State Key Laboratory of Pulp and Paper-making Engineering, South China University of Technology, Guangzhou City, Guangdong Province, 510640, China; b: Department of Materials Science and Engineering, University of Maryland College Park, College Park, Maryland 20742, USA; *Corresponding authors: firstname.lastname@example.org (L. Mo); email@example.com
Effluents of the pulp and paper industry contain many recalcitrant organic pollutants, such as carboxylic acids, phenolic compounds, saccharides, and surfactants. These pollutants are either formed during lignin decomposition or introduced by adding needed chemicals during pulping and paper-making processes (Thompson et al. 2001; Malaviya and Rathore 2007; Kamali and Khodaparast 2015; Song et al. 2017; Jia et al. 2018). These toxic compounds may cause deleterious ecological and environmental impacts upon direct discharge to receiving waters (Wu et al. 2005; Hubbe et al. 2016; Sun et al. 2017). Recently, great efforts have been made to remove these recalcitrant organic pollutants (Jaafarzadeh et al. 2017; Marques et al.2017; Yadav and Garg 2018). However, conventional physico-chemical and biological treatments are not sufficient for the complete degradation of these recalcitrant organic pollutants (Rintala and Puhakka 1994; Liu et al. 2011; Krishna et al. 2014). Heterogeneous catalytic ozonation is one of the most promising treatment methods for the removal of recalcitrant organic pollutants due to its ease of operation and high efficiency (Roncero et al. 2003; Chen et al.2017b). The catalysts adopted in the process of catalytic ozonation can facilitate the generation of the strong oxidant •OH. The •OH-mediated oxidation can effectively oxidize these recalcitrant organic pollutants.
A variety of support materials, such as carbon nanofibers (Restivo et al. 2012; Restivo et al. 2013; Yang et al. 2017), activated carbon (Faria et al. 2008; He et al. 2016; Gümüş and Akbal 2017; He et al.2017), graphene (Wang et al. 2016b; Yin et al. 2017), carbon nanotube (Fan et al. 2014; Wang et al. 2016b), silica (Afzal et al.2016), resins (Liotta et al. 2009), zeolites (Ikhlaq and Kasprzyk-Hordern 2017), and alumina (Al2O3) (Ikhlaq et al. 2013; He et al.2017a; Ziylan-Yavaş and Ince 2017), have been widely investigated for catalytic applications. Among these, Al2O3 has been extensively studied as a catalyst or a catalyst support material due to its excellent catalytic performance and low cost. The surface basic sites (Al-OH) of Al2O3 can promote the generation of •OH, and the relatively high surface area of Al2O3 benefits the adsorption of recalcitrant organic pollutants (Ernst et al. 2004; Trueba and Trasatti 2005). Vittenet et al.(2014) reported that in comparison to the ozone alone process, the ozonation process catalyzed by Al2O3 enhances total organic carbon (TOC) removal of the 2,4-dimethylphenol solution by two-fold due to the interactions between the aqueous ozone and the surface basic sites (Al-OH) of Al2O3. Active components, such as metal oxides loaded on Al2O3, can further increase the catalytic degradation efficiency of recalcitrant organic pollutants. Metal oxides, including copper (Cu) (Qu et al. 2004), manganese (Mn) (Roshani et al. 2014), iron (Fe) (Nie et al. 2014), cerium (Ce) (Nie et al. 2014), and ruthenium (Ru) (Wang et al. 2013), were found to greatly increase the degradation efficiency of recalcitrant organic pollutants in effluents when they were deposited onto Al2O3. In comparison to single metal oxides, composite metal oxides loaded on Al2O3 can increase the efficiency of ozonation for the treatment of organic pollutants (Tong et al.2011). Copper oxide (CuO) particles have rich surface basic sites, where the ozonation process catalyzed by CuO mainly follows OH-mediated oxidation causing the high mineralization of recalcitrant organic pollutants, such as alachlor (Qu et al. 2004) and substituted phenols (Udrea and Bradu 2003), to occur. By doping into or loading on activated carbon (Qin et al. 2014), SBA-15 (Yan et al. 2013), and zeolites (Lan et al. 2013), the surface of cerium oxides form abundant Lewis acid sites as well as –OH groups that can promote the transformation of aqueous ozone into •OH. Hence, ozonation systems catalyzed by cerium oxides show high degradation efficiency of recalcitrant organic pollutants such as fulvic acids (Qin et al. 2014), dimethyl phthalate (Yan et al. 2013), and p-chlorobenzoic acid (Lan et al. 2013). In the past, studies of catalytic ozonation have been focused on model compounds. Only a few studies have used actual papermaking wastewater.
In this study, the composite Cu-Ce/Al2O3 catalysts were prepared and characterized. The catalytic degradation efficiency, mechanism, and potential of the prepared catalysts for catalytic ozonation of recalcitrant and complex organic pollutants in papermaking wastewater were investigated.
Copper (II) nitratetrihydrate [Cu(NO3)2·3H2O], Cerium (III) nitrate hexahydrate [Ce(NO3)·6H2O], tert-butanol (TBA), and phosphate were purchased from Fuchen Chemical Reagent Co., Ltd. (Tianjin, China). The commercial γ-Al2O3, with a diameter range of 3 mm to 5 mm, was obtained from Jiangsu Jingjing New Material Co., Ltd. (Jiangsu, China). The ozone generator was supplied by Guangzhou Weigu Equipment Co., Ltd. (Guangdong, China).
Pulp-making wastewater after secondary biological treatment was collected at an integrated pulp and paper mill in Guangdong Province, China. The main properties of the effluent are as follows: 152 mg/L TOC, 206 mg/L chemical oxygen demand (COD), 35 mg/L biochemical oxygen demand (BOD5), 438 Color Unit (C.U.) color, and pH 7.9.
Preparation of catalysts
The Cu-Ce/γ-Al2O3 catalyst was prepared via an incipient wetness impregnation method. The Cu(NO3)2·3H2O and Ce(NO3)3·6H2O were used as precursors, and the γ-Al2O3 (φ = 3 mm to 5 mm) was used as catalyst support. Commercial γ-Al2O3 was ground and passed through an 80-mesh sieve, then 4 g γ-Al2O3 powder was dispersed in a mixture solution of 30 mL (0.3 g Cu(NO3)2·3H2O and 0.28 g Ce(NO3)3·6H2O dissolved in DI water) for 12 h at room temperature. The samples then underwent filtration and drying at 105 °C for 12 h. Finally, the dried samples were heated to 550 °C for 4 h in air to obtain Cu-Ce/γ-Al2O3 catalysts. The Cu/γ-Al2O3 and Ce/γ-Al2O3catalysts were prepared according to a similar method.
Characterization of catalysts
X-ray diffraction (XRD) was performed using a Bruker D8 advance X-ray diffractometer (Bruker Corporation, Karlsruhe, Germany) with a scanning range of 10° to 80° and a Cu Kα radiation source at 40 kV and 40 mA.
The Brunauer-Emmett-Teller (BET) specific surface areas and pore volumes of the catalysts were determined by nitrogen adsorption at 77 K using a Micromeritics ASAP 2000 BET surface area analyzer (Norcross, Georgia, USA). First, the porous catalysts samples were degassed at 110 to 115 °C on the instrument, then they were transferred into the PCT Pro 2000 sample holder (Hy-Energy, Newark, CA, USA) in a glove box. Measurements were conducted with the pressure up to 3.0 MPa in a microdoser with the sample holder placed in a Dewar containing liquid nitrogen. The Brunauer-Emmett-Teller (BET) equation was adopted to compute specific surface areas based on nitrogen adsorption isotherms obtained at 77 K and relative pressures approaching 1.0.
The surface morphologies of the as-prepared catalysts were observed by a scanning electron microscope (EVO18, Zeiss Corporation, Oberkochen, Germany). The chemical composition concentrations of the catalysts were determined by X-ray fluorescence (XRF) with a PW 4000 X-ray spectrometer (Axios, Groningen, Holland). The pH of point of zero charge (pHpzc) of catalysts was measured according to the pH-drift method (Newcombe et al. 1993; Bessekhouad et al. 2004).
Ozonation reaction and analytical methods
The schematic of experimental apparatus for ozonation and catalytic ozonation of the effluent has been reported in the authors’ former work (He et al. 2016). All catalytic ozonation experiments were performed in a 1-L glass column reactor at room temperature. First, 500 mL papermaking wastewater and 1 g catalyst were added in the column reactor. Then, 0.3 g/h O3 was introduced into the reactor through a porous diffuser at the bottom of the glass reactor. The operation was performed using the effect of different catalyst types, doses, and initial pH (adjusted with 1 N NaOH or 1 N HCl). Wastewater samples were withdrawn at regular intervals and centrifuged for 5 min to separate solid catalysts before further analyses. Radical quenching experiments were conducted to investigate the mechanism of catalytic ozonation. As usually done, 1 g/L hydroxyl radical scavenger tert-butanol (TBA) and phosphate were added to the effluent before the catalytic ozonation reaction. The excess ozone in the off-gas stream was immediately quenched with a 5% potassium iodide (KI) solution. The stability of the as-prepared catalyst was investigated by reusing it up to 10 times.
Analyses of papermaking wastewater
The TOC of the effluent was determined via Hach® TOC Method 10173 Direct (Mid-Range TOC 15-150 mg·L−1). The COD was tested by the dichromate closed reflux colorimetric method using a Hach DR2800 model spectrophotometer (Hach, Loveland, CO, USA) and the BOD5 was determined on a BOD Trak II meter (Hach, Loveland, CO, USA). The ratio of BOD5/COD represented the biodegradability of the effluent. The color of the wastewater before and after treatment was measured according to the Platinum-Cobalt method using a HACH DR2800 model UV-Vis spectrophotometer (Hach, Loveland, CO, USA). The leaching of metal ions from the solid catalysts into the effluent after treatment was determined through inductively coupled plasma mass spectroscopy (ICP/MS 7500a, Agilent, Salt Lake City, UT, USA).
RESULTS AND DISCUSSION
Characterization of Catalysts
To confirm the effective loading of Cu and Ce on the Al2O3 catalyst carriers and to characterize the morphologies and physical properties of the as-prepared catalyst, XRF, SEM, BET, and XRD analyses were conducted. As shown in Fig. 1a, the surface of Al2O3 was very rough. Copper oxides and cerium oxides were deposited on the surface of Al2O3 creating micro-agglomerates in irregular sizes and shapes (Fig. 1b). The micro-agglomerates of copper oxides and cerium oxides in irregular shapes and sizes can be observed on the surface of γ-Al2O3by transmission electron micrographs (TEM) analyses (Fig. 1c). These micro-agglomerates may facilitate the interaction between the recalcitrant organic pollutants and catalysts.
Fig. 1. SEM images of (a) γ-Al2O3; (b) Cu-Ce/γ-Al2O3; (c) transmission electron micrographs (TEM) of Cu-Ce/γ-Al2O3catalysts; SEM-EDX spectrum (e) of the area indicated in (d). Inset: the table shows the weight percentage and atomic ratio of elements in the Cu-Ce/γ-Al2O3 catalysts
Selected area SEM images and energy dispersive X-ray (EDX) patterns of the Cu-Ce/Al2O3 catalysts are illustrated in Figs. 1d and e. Note that Cu, Ce, and O elements can be found on the catalysts surfaces. The elemental contents of Cu, Ce, and O were further obtained via the EDX patterns analyses. As can be seen from the inset of Fig. 1e, the weight percentages of Cu and Ce in the Cu-Ce/Al2O3catalyst were 2.92% and 1.37%, respectively.
The XRD diffraction patterns of γ-Al2O3 and Cu-Ce/γ-Al2O3catalysts are shown in Fig. 2. Three typical diffraction peaks at 2θ = 38.7°, 46.2°, and 67.3° corresponding to γ-Al2O3 were observed (Liu et al. 2015) (Fig. 2a). In comparison to the XRD pattern of γ-Al2O3, newly emerged diffraction peaks at 2θ = 28.6°, 33.1°, 47.6°, and 56.5° in the patterns of Cu-Ce/γ-Al2O3 catalysts were due to the presence of CeO2 (JCPDS File No. 81-0792). Other newly emerged diffraction peaks at 2θ = 35°, 38°, and 50° were attributed to the presence of CuO (JCPDS File No. 48-1548). The binding energies of Cu2p3/2 of Cu-Ce/Al2O3 were centered at 932.5, 932.6, and 933.4 eV, respectively (Fig. 2b) (Biesinger et al. 2010). The binding energies at 882.1 eV, 888.1 eV, 898 eV, 900.9 eV, 906.4 eV, and 916.4 eV correspond to Ce4+ oxide (CeO2) (Fig. 2c) (Nikolaev et al. 2015). The adsorption-deadsorption isotherms varied among different kinds of catalysts (Fig. 2d). These isotherms indicate the existence of representative type IV mesoporous structures within catalysts (Chenet al. 2017a).
Fig. 2. (a) XRD patterns of γ-Al2O3 and Cu-Ce/γ-Al2O3; X-ray photoelectron spectroscopy (XPS) spectra of Cu2p (b) and Ce3d (c); (d) N2 adsorption-desorption isotherm curves of γ-Al2O3, Cu/γ-Al2O3, Ce/γ-Al2O3, and Cu-Ce/γ-Al2O3 catalysts
As shown in Table 1, the loadings of CuO and CeO2 on γ-Al2O3reduced the BET surface area, average pore diameter, and pore volume. The decrease of BET surface area and pore volume may be due to the partial capping of the pores on the γ-Al2O3 catalyst after loading metal oxides. The weight contents of CuO and CeO2 in Cu-Ce /γ-Al2O3 catalysts were 3.7% and 1.7%, respectively, and the pHpzc of γ-Al2O3, Cu/Al2O3, Ce/Al2O3, and Cu-Ce/Al2O3 were 8.01, 8.06, 8.07, and 8.15, respectively.
Table 1. BET Analyses and Metal Oxide Contents of Catalysts
Enhanced Catalytic Performance
Figure 3 shows the treatment results of the effluent in O3 alone, Cu-Al2O3/O3, Ce-Al2O3/O3, and Cu-Ce/Al2O3 processes. The effect of adsorption of the catalysts on the TOC removal of the effluent is depicted in Fig. 3a. Results indicated that the adsorption of Cu-Al2O3/O3, Ce-Al2O3/O3, and Cu-Ce/Al2O3 catalysts reached saturation in 60 min. The TOC removals of the effluent by adsorption after 60 min on Al2O3, Cu/Al2O3, Ce/Al2O3, and Cu-Ce/Al2O3 were 5.2%, 3.9%, 3.6%, and 2.8%, respectively (Fig. 3a). These catalysts showed weak adsorption capabilities toward the recalcitrant organic pollutants in the effluent.
Note that the adsorption capacity of Cu-Ce/Al2O3 was the lowest among these catalysts. This may have been due to the decreased surface area (Table 1). However, the TOC removal rates were all enhanced in the Cu/Al2O3/O3, Ce/Al2O3/O3, and Cu-Ce/Al2O3processes. The Cu-Ce/Al2O3 process achieved the best performance in terms of TOC removal rate (Fig. 3b). The TOC removal rate was evidently enhanced by the introduction of the catalyst into the ozonation system. For example, after a reaction time of 60 min, the TOC removal rate remained at 26% in the ozone alone process. After adding Al2O3, Cu/Al2O3, Ce/Al2O3/O3, and Cu-Ce/Al2O3 catalysts to the ozonation system, the TOC removal rates were enhanced 11% (37%), 26% (52%), 29% (55%), and 35.5% (61.5%), respectively.
The enhancement of the TOC removal rate was mainly due to the generation of strong oxidant radical species (such as •OH) induced by these catalysts (Zhao et al. 2009; Rosal et al. 2010; Tong et al. 2010, Thibault-Starzyk et al. 2014; Bing et al. 2015; Chen et al. 2017b). It is worth noting that the TOC removal rate was much faster in the first 30 min of the degradation reaction compared to the second 30 min of the reaction. This may have been due to the fast oxidation of easily degraded organic pollutants in the initial 30 min of treatment and the slow degradation of the refractory organic pollutants remaining after 30 min of treatment. The results of the analyses showed that the effect of adsorption of the catalysts on TOC removal of the effluent was negligible when compared to catalytic ozonation during these processes.