Journal of Water and Environment Technology, Vol. 10, No.2, 2012

Catalytic Effect of Several Iron Species on Ozonation

Naoyuki KISHIMOTO, Shouhei UENO

Faculty of Science and Technology, Ryukoku University, Otsu 520-2194, Japan

ABSTRACT Catalytic degradation of 1,4-dioxane in a synthetic wastewater by ozonation with several iron species was investigated. Iron species discussed were zero-valent iron (Fe), ferrous ion (Fe2+), 3+ ferric ion (Fe ), hematite (α-Fe2O3) and magnetite (Fe3O4). The addition of Fe accelerated the degradation rate of 1,4-dioxane at pH ranging from 3 to 9. The addition of Fe2+ and Fe3+ also enhanced the 1,4-dioxane degradation at pH 3, but the enhancement effects disappeared at higher pH. Iron oxides (α-Fe2O3 and Fe3O4) did not affect the degradation rate of 1,4-dioxane. Hydroxyl radical (·OH) production via the reaction of ozone with Fe2+/Fe3+ was believed to be responsible 2+ 3+ 2+ – for the enhancement effects of Fe, Fe and Fe . The production of Fe and ozonide ion (·O3 ) was expected via Fe corrosion by ozone. Therefore, ·OH generated via both Fe2+ oxidation by – ozone and ·O3 /water reaction was also believed to contribute to the 1,4-dioxane degradation in ozonation with Fe addition, especially at pH 5 or higher.

Keywords: advanced oxidation, catalytic ozonation, 1,4-dioxane, iron catalyst

INTRODUCTION Ozone is a powerful oxidant with standard potentials of 2.07 V vs. standard hydrogen electrode (SHE) in acid solution and 1.25 V vs. SHE in basic solution (Bard et al., 1985). Ozone easily reacts with a carbon double bond and cuts off the bond via well-known Criegee mechanism (Gottschalk et al., 2000). As a result of the ozone reactivity with organic molecules, ozone is often applied to the decolorization of wastewater and transformation of biorefractory organic matter into a more biodegradable one (Wu and Wang, 2001). However, some kinds of biorefractoy organic matter such as 1,4-dioxane resists ozone treatment because of the low reactivity of ozone with a carbon single bond. An applicable technology to decompose persistent organic matter such as 1,4-dioxane is the advanced oxidation technology, which is a technology using hydroxyl radical (·OH) as an oxidant. Several types of ozone-based advanced oxidation technologies have been proposed like ozonation with UV irradiation (Peyton and Glaze, 1988), ozonation with the addition of hydrogen peroxide (Nakayama et al., 1979) and ozonation combined with electrolysis (Kishimoto et al., 2005, 2010a). These processes successfully enhance the oxidation potential of ozone treatment.

Catalytic ozonation is also one of the technologies enhancing the oxidation potential of ozone treatment. Catalytic ozonation technologies are mainly classified into two cases. One case involves ·OH generation via the catalytic ozone decomposition (Park et al., 2004) and the other case involves direct ozone reaction with intermediates formed by reactions of pollutants and a catalyst (Beltrán et al., 2005). The former case is classified into one of the advanced oxidation processes while the latter case shows high selectivity and is very effective to remove specific pollutants forming intermediates with the catalyst. Many compounds such as activated carbon (Beltrán et al., 2002), titanium dioxide, cobalt oxide, nickel oxide and copper oxide (Pines and Reckhow, 2003), have been considered as catalysts for ozonation. However, Pines and Reckhow (2003)

Address correspondence to Naoyuki Kishimoto, Department of Environmental Solution Technology, Faculty of Science and Technology, Ryukoku University, Email: [email protected] Received January 13, 2012, Accepted March 27, 2012. - 205 - Journal of Water and Environment Technology, Vol. 10, No.2, 2012 reported that the aforementioned metal oxides had no catalytic effect. Granular activated carbon (GAC) is a catalyst of ozone decomposition and had a catalytic effect on ozonation (Oh et al., 2004). The catalytic effect of GAC was enhanced by manganese doping (Ma et al., 2004; Oh et al., 2004). Manganese oxide and manganese ion also acted as a catalyst in ozonation (Cortés et al., 2000; Zaloznaya et al., 2009). Thus, manganese is thought to have high catalytic activity in ozonation. However, manganese is a suspiciously toxic heavy metal with 0.4 mg/L guideline value for drinking water (World Health Organization, 2011). Therefore, a safer catalyst than manganese is desired. Iron is one of the candidates for a cheap and relatively safe catalyst. Park et al. (2004) demonstrated that goethite (α-FeOOH) had a catalytic effect on ozonation.

In this study, the catalytic effect of several iron species, zero-valent iron(Fe), ferrous 2+ 3+ ion(Fe ), ferric ion(Fe ), hematite(α-Fe2O3) and magnetite(Fe3O4), on ozonation was investigated, especially from the viewpoint of enhancing the hydroxyl radical generation.

MATERIALS AND METHODS Wastewater and Catalyst Table 1 summarizes the composition of synthetic wastewater used in this study. The chemical oxygen demand (COD), biochemical oxygen demand (BOD), total nitrogen and total phosphorus concentrations were 528 mg/L, 127 mg/L, 25 mgN/L and 1.7 mgP/L, respectively. We added 1,4-dioxane as a ·OH probe, because it shows high reactivity with ·OH (Moriarty et al., 2003) and low reactivity with ozone (Hoigné and Bader, 1983). The pH of the wastewater was adjusted by the addition of adequate portions of 1 M sulfuric acid or 1 M sodium hydroxide.

Iron species used in this study were Fe (iron powder, Wako Chemical, Japan), Fe2+ as iron(II) sulfate heptahydrate (GR grade, Nacalai Tesque, Japan), Fe3+ as iron(III) chloride tetrahydrate (GR grade, Nacalai Tesque, Japan), α-Fe2O3 (CP grade, Nacalai Tesque, Japan) and Fe3O4 (CP grade, Nacalai Tesque, Japan). The Fe, α-Fe2O3 and Fe3O4 were ground before the experiments.

Experimental Equipment Figure 1 shows our experimental setup. The reactor was a bubble column made of glass with 3.0 cm in diameter and 55.5 cm in height. Ozone was generated with a silent discharge ozonizer with a pressure-swing adsorption system (Fuji Electric, Japan). Ozone gas was injected into the bubble column via a flat diffuser, which covered the bottom of the bubble column. The exhausted gas was dried with a gas dryer (DH106-1,

Table 1 - Composition of synthetic wastewater.

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Fig. 1 - Schematic diagram of the experimental setup. 1: bubble column, 2: peristaltic pump, 3: pressure-swing adsorption equipment, 4: ozone generator, 5: gas dryer, 6: ozone gas monitor, 7: KI trap, 8: ozone decomposer, 9: sampling tap

Komatsu Electronics, Japan). Then, the ozone concentration in the exhausted gas was monitored with an ozone monitor (EG-550UA, Ebara Jitsugyo, Japan). The residual ozone in the exhausted gas was finally absorbed into a potassium iodide (KI) solution and was decomposed with an ozone decomposer (ED-MD9-500S, EcoDesign, Japan). Water in the bubble column was circulated with a peristaltic pump (RP-NB3, Furue Science, Japan) for sampling.

Experimental Procedure The bubble column was filled with 350 mL of the synthetic wastewater with the addition of an iron species at the final concentration of 3 mM. The ozone generator was turned on and operated for more than 30 minutes before an experiment. Then, 40 mg/L of ozone gas was injected into the bubble column at a flow rate of 1.0 L/min and an experimental run started. The peristaltic pump was operated during the experiment. Each run was continued for 60 minutes, and the wastewater was sampled every ten 2+ 3+ minutes. Dissolved ozone concentration (DO3), Fe and Fe concentrations, 1,4-dioxane concentration, COD and pH were measured. Analytical methods applied were as follows: indigo colorimetric method for DO3, phenanthroline method for ferrous and ferric ion concentration, closed reflux-colorimetric method for COD and electrometric method for pH (APHA/AWWA/WEF, 1998). The concentration of 1,4-dioxane was determined by high performance liquid chromatography following the method by Kishimoto et al. (2010b).

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RESULTS AND DISCUSSION Effect of pH on Ozonation Figure 2 shows the changes in 1,4-dioxane concentration at various pH. A decrease in 1,4-dioxane concentration followed the zero-order reaction. A zero-order degradation rate was evaluated as a slope of a fitting line determined by the least squares method and plotted in Fig. 3. The zero-order degradation rate increased with an increase in pH. Hoigné and Bader (1983) reported that the second-order reaction rate constant of 1,4-dioxane with ozone was 0.32 M–1s–1. Accordingly, 1,4-dioxane degradation rates by direct ozone reaction were estimated from the aforementioned reaction rate constant, observed 1,4-dioxane concentration and observed DO3. The estimated rates are shown in Fig. 3. The estimated 1,4-dioxane degradation rate by direct ozone reaction decreased with an increase in pH resulting from DO3 decrease at higher pH (data not shown). This differed from the observed 1,4-dioxane degradation rate. It is well known that ozone generates ·OH via self-decomposition of ozone as follows (Tomiyasu et al., 1985):

– – O3 + OH → O2 + HO2 (1) – – O3 + HO2 →·O3 + HO2· (2) + – HO2· ↔ H + ·O2 (3) – – O3 + ·O2 →·O3 + O2 (4) – – ·O3 + H2O → O2 + ·OH + OH (5)

The self-decomposition of ozone is accelerated at higher pH resulting from the enhancement of reaction (1). Therefore, the acceleration of self-decomposition of ozone at higher pH was believed to induce the enhancement of 1,4-dioxane degradation. The contributions of ·OH to 1,4-dioxane degradation at various pH were estimated to be 6% at pH 3, 30% at pH 5, 52% at pH 7 and 66% at pH 9.

Fig. 2 - Changes in 1,4-dioxane concentration during ozonation without the addition of iron species.

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Fig. 3 - Observed overall 1,4-dioxane degradation rates and estimated 1,4-dioxane degradation rates by direct ozone reaction. Error bars mean the 95% confidence limits.

Catalytic Effect of Iron Species Figure 3 demonstrates that ·OH generation via self-decomposition of ozone was observed at pH 5 or higher. Accordingly, ozonation with the addition of iron species was preliminary performed at pH 3 to demonstrate the catalytic effect of each iron species clearly. Figure 4 shows the changes in 1,4-dioxane concentration at pH 3. The addition 2+ 3+ of Fe, Fe and Fe showed higher degradation rates than that of α-Fe2O3 and Fe3O4, in which the degradation rates did not differ from that in ozonation alone. Beltrán et al. (2005) reported that Fe2O3 did not influence ozone decomposition and ·OH did not participate in the catalyzed degradation of oxalic acid. Our result accorded with their 2+ 3+ report. Thus, the catalytic effects of Fe, Fe and Fe were higher than those of α-Fe2O3 and Fe3O4. Therefore, the following discussion focuses on the catalytic effects of Fe, Fe2+ and Fe3+.

Figure 5 shows the changes in 1,4-dioxane concentration during ozonation with the addition of Fe, Fe2+ or Fe3+ at different pH. The addition of Fe, Fe2+ or Fe3+ enhanced 1,4-dioxane degradation. However, the influence of pH on the catalytic effects of Fe, Fe2+ and Fe3+ was different among the three species.

To clarify the contribution of the catalytic degradation of 1,4-dioxane, the observed and estimated 1,4-dioxane degradation rates are summarized in Fig. 6. The observed degradation rates in Fig. 6 were evaluated as the slope of the graph in Fig. 5. As the rapid 1,4-dioxane degradation was observed at the beginning of treatment in the cases of Fe2+ addition and Fe or Fe3+ addition at pH 3, the experimental data after 10 minutes of treatment were used for the evaluation. The 1,4-dioxane degradation rates by direct O3 reaction were calculated by the same method used in Fig. 3. Although the observed rate 3+ with Fe addition at pH 5 was below the estimated rate by direct O3 reaction, the features of the catalytic effects of iron species are depicted in Fig. 6, in which Fe shows

- 209 - Journal of Water and Environment Technology, Vol. 10, No.2, 2012 the catalytic degradation of 1,4-dioxane at all pH, whereas the catalytic effects of Fe2+ and Fe3+ are observed only at pH 3.

Fig. 4 - Changes in 1,4-dioxane concentration at pH 3 with and without the addition of iron species.

Fig. 5 - Changes in 1,4-dioxane concentration during ozonation with the addition of Fe, Fe2+ or Fe3+ at various pH.

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Fig. 6 - Observed overall 1,4-dioxane degradation rates and estimated 1,4-dioxane degradation rates by direct ozone reaction in the cases of Fe, Fe2+ or Fe3+ addition. Error bars mean the 95% confidence limits.

Løgager et al. (1992) proposed the following reactions and rate constants:

3+ 2+ + –3 –1 –1 Fe + O3 + (H2O) → FeO + ·OH + O2 + H k=1.5×10 M s (6) 2+ 2+ 5 –1 –1 Fe + O3 → FeO + O2 k=8.2×10 M s (7) 2+ 3+ – –2 –1 FeO + (H2O) → Fe + ·OH + OH k=1.3×10 s (8) 2+ 2+ + 3+ 5 –1 –1 FeO + Fe + (2H ) → 2Fe + H2O k=1.4×10 M s (9)

+ where, the concentrations of H2O and H on the left hand side of the equations were ignored in the estimation of reaction rate constants. These reactions indicate that both Fe2+ and Fe3+ can generate ·OH via reactions (6) and (8). Thus, catalytic effects of Fe2+ and Fe3+ were believed to emerge by ·OH generation initiated by the reaction of ozone and Fe2+/Fe3+. Ozone oxidizes Fe2+ into Fe3+ as suggested by the reactions (7), (8) and (9). Therefore, the oxidation of Fe2+ into Fe3+ was expected to proceed during ozonation. Figure 7 shows the changes in Fe2+ and Fe3+ concentrations at pH 3 and 9 during ozonation with Fe2+ addition. Figure 7 demonstrates the rapid oxidation of Fe2+ into Fe3+ at pH 3. Consequently, Fe2+ initially contributed to the catalytic degradation of 1,4-dioxane in ozonation with Fe2+ addition at pH 3, but Fe3+ was responsible for the catalytic degradation after 10 minutes of treatment. Figure 7 also demonstrates the rapid removal of Fe2+ at pH 9 without an increase in Fe3+ concentration. It is known that Fe3+ forms precipitates of ferric hydroxide (Fe(OH)3) at pH higher than 3 (Kitamura et al., 2011) and brownish precipitates were observed during treatment at pH 5, 7 and 9 in this 2+ 3+ study. Therefore, the oxidation of Fe to Fe followed by the formation of Fe(OH)3 precipitates was believed to be responsible for the removal of Fe2+ without the Fe3+ increase at pH 5, 7 and 9, which resulted in the loss of the catalytic effects of Fe2+ and Fe3+.

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Fig. 7 - Changes in Fe2+ and Fe3+ concentrations during ozonation with Fe2+ addition at (a) pH 3 and (b) pH 9.

Figure 8 shows the changes in Fe2+ and Fe3+ concentrations at pH 3 and 9 during ozonation with Fe addition. The Fe added supplies Fe2+ by corrosion, which is further oxidized into Fe3+ by ozone. As a result of the oxidation of Fe2+ to Fe3+ by ozone, Fe3+ concentration continuously increased at pH 3 and soluble iron was removed at pH 9 as Fe(OH)3 precipitates. Aqueous half reactions related to Fe and ozone were summarized as follows:

Fe2+ + 2e– → Fe -0.44V vs. SHE (10) – – O3 + e → ·O3 (aq) 1.23V vs. SHE (11) – – O3 + H2O + 2e → O2 + 2OH 1.25V vs. SHE (12) + – O3 + 2H + 2e → O2 + H2O 2.07V vs. SHE (13)

The predominant half reaction of ozone at pH 3 is thought to be reaction (13) because of high H+ concentration and the higher standard potential than reactions (11) and (12). Therefore, the overall reaction of Fe corrosion at pH 3 is expressed by the combination of reactions (10) and (13) as follows:

+ 2+ Fe + O3 + 2H → Fe + O2 + H2O (14)

Accordingly, continuous supply of Fe2+ by reaction (14) was thought to enhance the ·OH production via reactions (7) and (8), in addition to that via Fe3+ (reactions (6) and (8)). Contrary to pH 3, the predominant half reactions of ozone are reactions (11) and (12) at pH 5 or higher because of low H+ concentration. Therefore, the overall reactions of Fe corrosion at pH 5 or higher are as follows:

2+ – Fe + 2O3 → Fe + 2·O3 (15) 2+ – Fe + O3 + H2O → Fe + O2 + 2OH (16)

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Fig. 8 - Changes in Fe2+ and Fe3+ concentrations during ozonation with Fe addition at (a) pH 3 and (b) pH 9.

– The ozonide ions (·O3 ) in equation (15) can produce ·OH by equation (5). Thus, both 2+ – the Fe and ·O3 generated by equations (15) and (16) were thought to contribute to 1,4-dioxane degradation at pH 5 or higher.

CONCLUSIONS 2+ 3+ Catalytic effects of several iron species, Fe, Fe , Fe , α-Fe2O3 and Fe3O4, on ozonation using 1,4-dioxane as a hydroxyl radical probe, were investigated in this study. The obtained results are summarized as follows:

(1) No catalytic effect of α-Fe2O3 and Fe3O4 was observed at pH ranging from 3 to 9.

(2) The catalytic effects of Fe2+ and Fe3+ disappeared at pH higher than 3 because of the 2+ 3+ rapid oxidation of Fe into Fe and the precipitation of Fe(OH)3.

2+ – (3) The catalytic effect of Fe was observed in the pH range of 3 to 9. The Fe and ·O3 generated through Fe corrosion by ozone were thought to contribute to the enhancement of 1,4-dioxane degradation.

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