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Reaction mechanism and metal ion transformation in photocatalytic ozonation of phenol and oxalic acid with Ag+/TiO2
Release time:2023-02-06    Views:500

abstract

Photocatalytic ozonation of phenol and oxalic acid (OA) was conducted with a Ag+/TiO2 catalyst and different pathways were found for the degradation of different compounds. Ag+ greatly promoted the photocatalytic degradation of contaminants due to its role as an electron scavenger. It also accelerated the removal rate of OA in ozonation and the simultaneous process for its complex reaction with oxalate. Phenol could be degraded both in direct ozonation and photolysis, but the TOC removal rates were much higher in the simultaneous processes due to the oxidation of hydroxyl radicals resulting from synergetic effects. The sequence of photo-illumination and ozone exposure in the combined process showed quite different effects in phenol degradation and TOC removal. The synergetic effects in different combined processes were found to be highly related to the properties of the target pollutants. The color change of the solution and TEM result confirmed that Ag+ was easily reduced and deposited on the surface of TiO2 under photo-illumination, and dissolved again into solution in the presence of ozone. This simple cycle of enrichment and distribution of Ag+ can greatly benefit the design of advanced oxidation processes, in which the sequences of ozone and photo-illumination can be varied according to the needs for catalyst recycling and the different properties of pollutants.

Introduction

Advanced oxidation processes (AOPs) have attracted great interest in recent years due to their capacity for mineralizing organic pollutants. Among all the alternative procedures, photocatalytic oxidation has emerged as a promising means to degrade recalcitrant compounds and produce less toxic substances. This process involves the separation of UV-induced hole and electron couples and formation of hydroxyl radicals (·OH) (Liu et al., 2006). Oxygen is commonly used to prevent the recombination of the holes and electrons by eliminating the UV-induced electrons (Orlov et al., 2007), which inevitably leads to a complicated three-phase reaction system and high energy consumption. Inorganic anions and metal ions with suitable redox potential were reported to be good oxidant sacrificial agents (Selvam et al., 2007; Kashif and Ouyang, 2009), and the reaction rates and efficiencies of photocatalytic reactions are sensitive to the dissolved metal ions present in natural waters and industrial wastewaters (Litter, 1999). The transition metal ions were widely studied (Ni2+, Mn2+, Co2+, Cu2+ and Fe3+) as well as noble metal ions (Ag+, Au3+, Pt4+ and Pd2+). The type and concentration of the metal ions are found to be very important to the photocatalytic oxidation of organic contaminants (Li and Qu, 2009; Vinu and Madras, 2008). Although the effects of metal ions on photocatalysis have been investigated, very few studies have discussed the behavior of the metal ions in the system, especially the noble metal ions.

The relatively low efficiency of photocatalysis restricted its application to the laboratory scale. The combined AOPs such as UV/Fenton, UV/H2O2 and UV/O3 (KasprzykHordern, 2003) are attractive due to the promoted generation of ·OH and corresponding potential economic benefits. Ozone is a selective oxidant, and some aldehydes and saturated carboxylic acids were found to be very hard to decompose through ozonation (Nawrocki and KasprzykHordern, 2010). The highly complementary effect of the combination of UV and ozone deserves further research (Giri et al., 2008). Photocatalytic ozonation, with in-situproduced strong oxidant holes and ·OH, is considered to enhance the oxidation rate of photocatalysis and mineralization extent of ozonation in pollutant removal (Qi et al., 2007; Wang et al., 2002). Although photocatalytic ozonation has been applied in the treatment of endocrine disruptors (Oyama et al., 2009), the comparison of different synergetic effects in the combined processes is still largely lacking.

In this work, Ag+/TiO2-based photocatalytic ozonation was systematically studied in the degradation of phenol and oxalic acid (OA). Phenol is a simple aromatic pollutant in industrial wastewater, which can quickly react with molecular ozone but generates several organic intermediates (Busca et al., 2008). OA is a common intermediate product during oxidation of many recalcitrant pollutants, which can be directly oxidized to CO2 and H2O (Liu et al., 2008; Wu et al., 2008), but it has a very low reaction rate with ozone (Avramescu et al., 2008; Sanchez- ´ Polo et al., 2005). To better illustrate the mechanism and synergetic effects in the simultaneous processes, sequential photocatalysis-ozonation (SPO) and sequential ozonation-photocatalysis (SOP) were designed for phenol degradation. The transformation of Ag+, possible intermediate products, and influence of different radical quenchers were investigated. It was found that the reaction pathways and synergetic effects in phenol and OA degradation were quite distinct. Ag+ was flexibly interchangeable between homogeneous and heterogeneous phases in different processes, and this unique characteristic could be utilized in AOP design for removal of different pollutants.

1 Materials and methods

1.1 Materials and reagents

Phenol and OA were from Xilong Chemical Corporation (China). Copper nitrate, manganese nitrate, nickel nitrate, silver nitrate and t-butanol (t-BA) were obtained from National Chemical Corporation (China). Horseradish peroxidase (POD) was provided by Sigma Aldrich. A commercial TiO2 (Degussa P25, 80% anatase and 20% rutile) was used as catalyst or support without further treatment. All the experimental solutions were prepared with ultrapure water (Millipore Milli-Q).

1.2 Experimental procedure

All experiments were carried out in a cylindrical borosilicate glass reactor (120 mm long and 113 mm diameter) with a quartz cap (Fig. 1). The degradation of phenol and OA were performed in 0.5 L of un-buffered solution at atmospheric pressure and 25◦C. The initial concentration of contaminants was 100 mg/L and the dosage of TiO2 was 0.2 g/L. In the system with metal ion/TiO2 catalysts, the solution of metal nitrates was directly added to obtain a certain concentration of metal ions. Before reaction under UV or ozone, the suspension was sonicated for 5 min to uniformly disperse TiO2, and then magnetically stirred in darkness for 30 min to establish an adsorption/desorption equilibrium. Samples with constant volume were taken at fixed time intervals and filtered (Millipore, 0.22 μm) when catalysts were added.

In the photocatalytic processes, the reactor was irradiated under a 300 W Xenon lamp (CEL-HXUV300, Aulight Corporation, China), which emits light in the region of 200–400 nm. The intensity of the incident light on the solution surface was about 320 mW/cm2 as measured by a photometer (CEL-NP2000, Aulight Corporation, China). The distance between the vent of the lamp and the bottom of the reactor was about 23 cm.

In the ozone-based procedures, an oxygen-ozone mixture was continuously bubbled into the solution through a diffuser at the bottom of the reactor. Ozone was produced from pure oxygen using an ozone generator (COM-AD-01, Anseros Corporation, Germany). The gas flow was kept at 115 mL/min with an ozone concentration of (27.15 ± 1) mg/L.

1.3 Analysis and characterization methods 

Phenol and organic intermediates were analyzed by high-performance liquid chromatography (HPLC, Agilent series 1260, USA) with a CE column C18 and a UV-Vis detector qualified at 215, 245 and 278 nm, respectively. The concentration of OA was detected with ion chromatography (IC, Dionex DX500, USA) using 1.1 g/L NaOH solution as the eluent. Total organic carbon (TOC) and pH of the solution were determined with a TOC-V CPH analyzer (Shimadzu, Japan) and Five Easy pH meter (Mettler-Toledo, Switzerland), respectively.

The concentration of ozone in the gas phase was monitored in-situ with an ozone analyzer (Anseros ozomat GM, Germany), and the residual ozone in the solution was detected by spectrophotometer using the indigo method (Bader and Hoigne, 1981). The trace amount of H ´ 2O2 generated during the reaction was measured with the photometric method (Bader et al., 1988). The suspended Ag+/TiO2 catalyst in the photocatalytic processes was filtered and dried for transmission electron microscopy (TEM, JEM 2010 microscope, Japan) analysis, which was carried out at an acceleration voltage of 40 kV. The amount of Ag+ deposited on TiO2 was detected with inductively coupled plasma atomic emission spectroscopy (ICP-AES, OPTIMA 5300DV, PerkinElmer, USA).

2 Results and discussion

2.1 Degradation of phenol

2.1.1 Photocatalysis and catalytic ozonation

The recombination of UV-induced electrons and holes should be avoided during photocatalysis, and metal ions are possible choices to remove the electrons. Herein, the effect of several transition metal ions (Cu2+, Ni2+, Mn2+ and Ag+) on the photocatalytic degradation of phenol was investigated and the results are presented in Fig. 2. The addition of TiO2 and metal ions/TiO2 greatly enhanced the phenol removal rate in photocatalysis, and the order of promotion effect for these ions was: Ag+ > Cu2+ > Ni2+ > Mn2+. Furthermore, different concentrations of Ag+ were investigated, such as 25, 50, 75 and 100 μmol/L (data not shown here). It was found that 50 μmol/L was the optimal choice, and this was used in the following experiments if not otherwise mentioned. Direct ozonation and Ag+ catalytic ozonation were also carried out to degrade phenol. Phenol was totally degraded within 60 min in direct ozonation, while the addition of Ag+ did not show any improvement. This indicated that Ag+ played different roles in photocatalytic oxidation and catalytic ozonation of phenol.

2.1.2 Sequential photocatalysis-ozonation and sequential ozonation-photocatalysis

Ozonation was proved to be an effective pretreatment method for the decolorization and mineralization of in-dustrial wastewater (Shu, 2006; Tanaka et al., 1992). Ozone molecules can directly attack aromatic organics but not saturated aliphatic compounds (Beltran et al., ´ 2002), and this reaction selectivity restricts its ability to totally mineralize the target pollutants. UV and ozone are thus combined to get higher removal rates of organics. Sequential photocatalysis-ozonation (SPO) and sequential ozonation-photocatalysis (SOP) were then carried out in phenol degradation and the results are shown in Fig. 3

SOP was much more efficient than SPO in the first stage, because ozone was a stronger oxidant than UV in phenol degradation. TiO2 and Ag+/TiO2 both benefited the photocatalysis of phenol, but Ag+ was unfavorable for the TiO2 catalyzed ozonation of phenol in SOP and the second stage of SPO, as shown in Fig. 3, which may be attributed to its active role in ozone decomposition to oxygen (Lin and Lin, 2008). Figure 3 also shows that TiO2 and Ag+/TiO2 also promoted TOC removal in phenol degradation. The TOC removal rates in the SPO processes were 12.0%, 19.7% and 22.7% for the blank, TiO2 and Ag+/TiO2 systems, respectively. In the SOP processes, the corresponding TOC removal rates were 16.3%, 32.0% and 38.6%, respectively. The higher TOC removal rates in SOP indicated that the sequence of UV and ozone influenced the removal of intermediate products. Besides, the TOC removal rates in the photocatalysis and ozonation under the same conditions were 23.1% and 36.5% in 120 min, respectively. It was concluded that simple combination of UV and ozone can promote phenol degradation and TOC removal, and pre-ozonation can enhance the photocatalytic efficiency.

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