Is UV/Ce(IV) process a chloride-resistant AOPs for organic pollutants decontamination?

Abstract

Most of the current advanced oxidation processes (AOPs) are vulnerable to the presence of chloride in saline wastewater treatment because chloride not only affects the degradation kinetics but also probably leads to absorbable organic halogen (AOX) formation. Here we report an UV/Ce(IV) process can efficiently oxidize organic pollutants such as Acid Orange 7, even in the presence of chloride. Fluorescent probe technology suggests hydroxyl radicals were generated in UV/Ce(IV) process, but not in UV/Ce(IV)/Cl⁻ system. In the presence of chloride, Ce(IV)–chloride complex was formed, which can directly oxidize dyes or generate reactive oxygen species by chlorine activation. Although degradation and mineralization rates of dyes were still inhibited to some extents by large amounts of chloride, but negligible AOX was generated. Therefore, UV/Ce(IV) process can be recommended as an alternative AOPs when treating acidic saline wastewater.

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1 Introduction

In recent years, advanced oxidation processes (AOPs) have attracted much attention^1–3 and have been widely used for removal of organic contaminants from wastewaters.³ Highly reactive radicals such as HO˙ and SO₄˙⁻ generated in AOPs are responsible for rapid oxidation of organic pollutants. The most widely used process to produce HO˙ is the homogeneous Fenton reaction (Fe²⁺/H₂O₂) or UV-Fenton reaction.^4,5 Hydroxyl radicals are substances with high oxidizing abilities (E = 2.80 V), and are capable of degrading most of organics to form small molecules and even carbon dioxide and water.⁶

Several studies have shown that treatment of refractory organic pollutants with UV-Fenton reaction is more efficient,^6,7 due to UV irradiation can drive the catalyst (Fe³⁺/Fe²⁺) cycling in reaction process. Briefly, Fe²⁺ ion react with H₂O₂ to form Fe³⁺ ion and HO˙. UV-generated Fe³⁺ ion also undergoes reduction to form Fe²⁺ ion that participates in Fenton reaction generating additional radical thereby accelerating the oxidation process.^6–8 Similar to typical UV-Fenton reaction, UV irradiation can also promote Ce(IV)/Ce(III) circulation.^9–13 Upon UV mercury lamp, cerium salts are supposed to play an important role in production of highly reactive radicals.¹⁴ Under irradiation high reactive oxygen species and Ce(III) ion are produced by Ce(IV) ion. In addition, some researchers have confirmed that Ce(IV) is one of the strongest one-electron oxidant.¹⁵ It can substantively oxidize organic matter to generate Ce(III). The Ce(IV)-generated Ce(III) ion can also convert to Ce(IV) with a release of electron, which can combine with oxygen to form active radicals.^14,15 Although there have been some scattered reports on the catalytic mechanism of cerium salt, however, to the best of our knowledge, there are no related studies about the effects of salinity on cerium-induced degradation of organic pollutants, while effects of chloride ion in treatment of high salinity wastewater by other redox-active metal ions such as Fe(III)/Fe(II) and Co(III)/Co(II)have been widely studied.^16,17

Dyeing wastewater contains large amounts of inorganic salts, particularly chloride ion because it is a common additive used during dyeing processes.¹⁸ Cl⁻ not only influences the degradation efficiency of the organic pollutants, but also possibly lead to the formation of carcinogenic chlorinated organic compounds.^19,20 In UV/H₂O₂ system catalytic process, acid dyes decolorization rate and mineralization rate were reduced with high concentrations of chlorine ion. More seriously, it also produces carcinogenic chlorinated organic compounds.^20–22 Furthermore, compared to hydroxyl radicals, highly reactive sulfate radicals have strong oxidizing capacity.²³ However, high salinity wastewater treatment by a couple of advanced oxidation technology based on sulfate radicals (SO₄˙⁻, 2.5–3.1 V) was found to have a dual effect on both the dye decoloration.^17,24 For the Co/PMS system, Co²⁺ ion can effectively activate PMS and generate SO₄˙⁻, high concentrations of chloride (>5 mM) can significantly enhance the degradation efficiency of the reaction system, but did greatly inhibit the dye mineralization rate and produce more carcinogenic chlorinated organic compounds.¹⁷ Therefore, it is desirable to develop a new AOPs which produces less and even no chlorinated byproducts when it works in saline wastewater treatment. Our preliminary experimental data indicates Ce(IV)-based AOPs generates less AOX in the presence of 300 mM.

In this study, the catalytic mechanism of Ce(IV) ion with Cl⁻ under UV irradiation in salinity wastewater treatment was studied. AO7 was used as a model pollutant to examine the performance of this new process. The effects of initial pH on AO7 degradation in UV/Ce(IV) and Ce(IV) system were investigated. In addition, reactive radicals were identified and reaction mechanism was discussed.

2 Materials and methods

2.1 Chemicals

Acid orange 7 (AO7: C₁₆H₁₁N₂O₄SNa) and coumarin (COU: C₉H₆O₂) were purchased from Sigma-Aldrich. Ce(SO₄)₂·4H₂O, Ce(NO₃)₃·6H₂O, methanol, iso-propyl alcohol (IPA), NaNO₂, NaNO₃, NaCl, H₂SO₄, HCl and NaOH were of reagent grade and used without further purification. The aqueous solutions were prepared using ultra-pure water throughout the experiments. All reagent stock solutions were freshly prepared.

2.2 Experimental procedures

The degradation experiments were conducted in 50 mL of quartz tube. For the UV/Ce(IV) system, experiments were performed in a 100 W mercury lamp (maximal emission: 365.0–366.3 nm) photochemical reactor (Xujiang Electromechanical Plant, Nanjing, China).²⁵ Samples were taken for analysis in all experiments at specific time intervals, and quenched with methanol (for UV analysis), NaNO₂ (for TOC analysis) or NaNO₃ (for AOX analysis) immediately. The reaction solutions were always freshly prepared with an initial pH of 3.0 ± 0.5 (adjusted with NaOH and H₂SO₄) unless specified, and the pH was not controlled during the reaction process.

2.3 Methods and analysis

The AO7 degradation was monitored by measuring the absorbance at 484 nm with a UV-vis spectrophotometer (Hitachi Model U-2910). The pH of the samples were measured with a Multi WTW 340i instrument (Germany) and PHS-3C. A Shimadzu TOC-VCPH analyzer was employed for TOC (Total Organic Carbon) measurement. COU can be used as fluorescent probe to detect the hydroxyl radicals generated by UV irradiation,^25,26 so the detection of HO˙ was carried out on a luminescence spectrometer (LS55, Perkin Elmer precisely). In addition, Ce(III) itself has a strong fluorescence,¹⁴ which can be also detected by a Perkin Elmer LS55 luminescence spectrometer. By concentrating on activated carbon, AOX determination was carried out by AOX analytical instruments (Multi x 2500, Jena, Germany).²⁷ All experiments were performed in duplicate.

3 Results and discussion

3.1 Catalytic activity of Ce(IV) and UV/Ce(IV)

According to the spectral data (Fig. S1†), Ce(IV) and UV/Ce(IV) can significantly lead to the rapid degradation of AO7. Fig. 1 presents the effect of chloride concentration on AO7 decolorization in Ce(IV) system and UV/Ce(IV) system. The decolorization rate of AO7 decreased with an increase in chloride ion concentration from 0 to 300 mM (Fig. 1(a) and S2(a)†). The decolorization rate of dye by Ce(IV) only was 45% within 60 min in the absence of chloride and was further down to 25% when adding 300 mM chloride ion. Upon UV irradiation, AO7 degradation rate was greatly enhanced. However, the AO7 degradation rate declined from 85% to 45% within 30 min when the chloride concentration increased from 0 to 300 mM (Fig. 1(b) and S2(b)†). Apparently, Cl⁻ showed an inhibition effect, but UV promoted AO7 degradation.¹⁴ Chloride ion combines with Ce(IV) to form a complex, H₂CeCl₆, which result in a decrease of the oxidation ability of Ce(IV).²⁸ However, Yuan et al.²¹ have found the degradation rates of dyes decreased significantly with addition of Cl⁻ (<5 mM) in a Co/PMS system, but further addition of Cl⁻ (>50 mM) accelerated phenols or azo compound degradation. This behavior was also observed in other similar systems, such as UV/H₂O₂ system, UV/TiO₂ system.^29,30

Fig. 1 The effect of chloride concentration on AO7 decolorization in Ce(IV) system (a) and UV/Ce(IV) system (b). Conditions: [AO7]₀ = 0.04 mM, [Ce(IV)]₀ = 0.25 mM.

The decolorization rate of dye wastewater cannot be a sufficient criterion for water treatment.³¹ In order to further assess the influence of chloride ion on AO7 decolorization, TOC after 8 h reaction was generally monitored, as depicted in Table S1.† UV light can significantly promote AO7 mineralization rate. However, chloride ion was found to reduce the AO7 mineralization efficiency. In UV/Ce(IV) system, the relatively higher TOC removal was achieved, about 11.9%. Without UV irradiation, the highest mineralization rate of AO7 was only 5.6%. Furthermore, in UV/Ce(IV)/Cl⁻ system, TOC removal was reduced with increasing chloride concentrations, the corresponding mineralization rate decreased from 11.9% to 7.5%. Therefore, TOC results confirm that only a small amount of AO7 was mineralized into inorganics during UV/Ce(IV) system and UV/Ce(IV)/Cl⁻ system.

3.2 The effect of pH

The pH value is considered one of the most important factors for Ce(IV) system, because the concentration and speciation of tetravalent cerium ion are pH-dependent.³² Accordingly, a series of experiments were performed at pH values in the range 2.0–6.0. Fig. 2 shows the effect of pH on the catalytic activity of Ce(IV)/Cl⁻ and UV/Ce(IV)/Cl⁻ systems, where k/min⁻¹ is the initial catalytic reaction rate calculated as pseudo-first-order kinetics. In general, degradation rates in UV/Ce(IV) were higher than those in Ce(IV) system. In the absence of chloride, the catalytic activity declined significantly with the increasing pH. When pH was higher than 4.0, the decoloration rate constants rapidly declined to 0.002 min⁻¹ (Fig. 2(a)) and 0.11 min⁻¹ (Fig. 2(b)). However, in the presence of Cl⁻, the decoloration rate constants also rapidly declined when pH was from 2.0 to 6.0.

Fig. 2 The effect of pH on the catalytic activity of Ce(IV)/Cl⁻ (a) and UV/Ce(IV)/Cl⁻ (b) system. Conditions: [AO7]₀ = 0.04 mM, [Ce(IV)]₀ = 0.25 mM.

The variation of degradation rates at different pH can be attributed to changes of pH-dependent Ce(IV) speciation. At very low pH (pH < 0.7), the species Ce⁴⁺ and Ce(OH)³⁺ are dominant. Under weak acidic and neutral conditions, Ce(IV) ion is hydrolyzed to Ce(OH)³⁺, Ce(OH)₂²⁺, Ce(OH)₃⁺ and Ce(OH)₄ compound. Ce(OH)₄ is the predominant substance and may precipitate spontaneously at pH > 4.³² As reported in the literature, a higher degradation rate for methyl orange, p-nitrophenol and benzoquinone with Ce(IV) can be achieved at low pH. In addition, the Ce(OH)₄ species are more stable, which cannot be photoreduced under UV irradiation.¹⁴ It suggests with the increase of pH, more Ce(IV) species are formed with less oxidizing capacity and less photoactivity. This can explain why the degradation rates diminished at a higher pH in Fig. 2.

3.3 Radicals identification

It is known that Ce(IV) ion itself can oxidize organics under acidic conditions, and produced hydroxyl radicals under UV irradiation.¹⁴ To identify the production of hydroxyl radicals in the Ce(IV) system, methanol as the hydroxyl radical trapping agent was used in this experiment.^14,33 As indicated in Fig. S3,† the AO7 decolorization rate decreased after the addition of methanol and IPA, indicating that the UV/Ce(IV) system and UV/Ce(IV)/Cl⁻ system were able to produce HO˙. In order to further confirm its involvement, COU photoluminescence probing technology was applied in Ce(IV)-based processes. COU is known to be an effective trapping agent for hydroxyl radical,²⁶ so it could produce strong luminescent compounds (7-hydroxycoumarin, 7-HC) with hydroxyl radical (emission wavelength is 450 nm).²⁶ In addition, UV/H₂O₂/COU system and UV/H₂O₂/COU/Cl⁻ system had similar fluorescence spectrum (Fig. S4†). Consequently, it is confirmed that chloride ion could not affect the reaction between coumarin and hydroxyl radical.

The photoluminescence spectral (excited at 330 nm) changes of 7-HC under UV/Ce(IV) and UV/Ce(IV)/Cl⁻ system are shown in Fig. 3(a) and (b). Compared with Fig. 3(c) and (d) the fluorescence intensity sharply increased within 40 min as shown in Fig. 3(a). HO˙ were indeed generated in the system, which could rapidly react with COU.²⁶ Moreover, when Cl⁻ were simultaneously added into the solution, there was almost no fluorescence peaks at 450 nm (Fig. 3(b) and (d)). In summary, hydroxyl radicals could not be formed during UV/Ce(IV)/Cl⁻ system, Ce(IV) and Ce(IV)/Cl⁻ system.

Fig. 3 Photoluminescence spectral changes of 7-HC in (a) UV/Ce(IV) system, (b) UV/Ce(IV)/Cl⁻ system, (c) Ce(IV) system and (d) Ce(IV)/Cl⁻ system. Conditions: [AO7]₀ = 0.04 mM, [COU]₀ = 0.25 mM, [Ce(IV)]₀ = 0.25 mM, [Cl⁻]₀ = 300 mM.

The fluorescence spectra of 7-HC in UV/Ce(IV)/Cl⁻ system and UV/Ce(III)/Cl⁻ system have also been detected (Fig. 4). Almost no fluorescence peaks at 450 nm were observed in Ce(IV)/UV/Cl⁻ system (Fig. 4(a)). Ce(III) ion could produce O₂˙⁻ under UV light irradiation, which could produce HO˙.¹¹ As shown in Fig. 4(b), UV irradiation can lead to an increase in fluorescence intensity at 450 nm in UV/Ce(III)/Cl⁻ system. In addition, as a control, UV/Ce(III) and UV/Ce(III)/Cl⁻ photoluminescence spectral changes have been detected (Fig. S5†). It suggests that hydroxyl radical could be formed in UV/Ce(III) and UV/Ce(III)/Cl⁻ system, however, it could not be formed in UV/Ce(IV)/Cl⁻ system.

Fig. 4 Photoluminescence spectral changes of 7-HC in UV/Ce(IV)/Cl⁻ system (a), UV/Ce(III)/Cl⁻ system (b). Conditions: [COU]₀ = 0.25 mM, [Ce(IV)]₀ = [Ce(III)]₀ = 0.25 mM, [Cl⁻]₀ = 300 mM.

Ce(III) ion inherently has strong fluorescence (emission wavelength is 353 nm).¹⁴ In order to confirm the redox cycling of Ce(IV)/Ce(III), the measurement of fluorescence spectroscopy of the Ce(III) ion (excited at 303 nm) at different irradiation time was performed. As shown in Fig. 5(a) and b, the fluorescence intensity of Ce(III) increased sharply within 50 min both in UV/Ce(IV) and UV/Ce(IV)/Cl⁻ systems. As shown in Fig. 5(c) and (d) the fluorescence intensity increased within 90 min in Ce(IV) system and Ce(IV)/Cl⁻ system. It suggests that Ce(IV) participate in AO7 degradation and Ce(IV) was reduced to Ce(III). It has been reported that chloride and Ce(IV) could form CeCl₆²⁻.^34–39 Under heat or light conditions, cerium(IV) complexes were reduced to cerium(III).³⁴

Fig. 5 The fluorescence intensity of Ce(III) ion in (a) UV/Ce(IV), (b) UV/Ce(IV)/Cl⁻, (c) Ce(IV) and (d) Ce(IV)/Cl⁻ system. Conditions: [AO7]₀ = 0.04 mM, [COU]₀ = 0.25 mM, [Ce(IV)]₀ = 0.25 mM, [Cl⁻]₀ = 300 mM.

3.4 AOX formation

Absorbable organic halogen (AOX) is an important index for wastewaters from cleaning agents.¹⁷ Chloride ion in the solution can scavenge hydroxyl radicals and sulfate radicals rapidly to generate chlorine species, leading to the generation of AOX.⁴⁰ According to literature data, AOX could be produced in the degradation of dyes wastewater with AOPs.^19,40 We made a comparison between UV/Ce(IV)/Cl⁻ and UV/Fe(III)/Cl⁻ systems under the similar conditions ([AO7]₀ = 0.04 mM, [Ce(IV)]₀ = [Fe(III)]₀ = 0.25 mM, [Cl⁻]₀ = 300 mM) because both UV/Ce(IV) and UV/Fe(III) are known to produce HO˙ radicals driven by irradiation and metals redox cycling. However, we found the AOX value in UV/Ce(IV)/Cl⁻ system was much lower in comparison with UV/Fe(III)/Cl⁻ system. In Ce(IV)/UV/Cl⁻ system, the AOX value was 4.4 μg L⁻¹ at 30 min, which increased to 17.6 μg L⁻¹ after 60 min. In contrast, the AOX value in a well-known UV/Fe(III)/Cl⁻ system was 231 μg L⁻¹ at 90 min and 163.6 μg L⁻¹ at 180 min, respectively. This difference in AOX formation may be ascribed to the variation of their reaction mechanisms in these two photochemical systems. Cl⁻ can readily complex with Fe(III) or Ce(IV) at acidic pH. Ferric chloride complex is known to produce Cl˙ and HO˙ radicals, which are important for AOX formation. In contrast, ceric chloride complex is supposed not to generate HO˙ radicals, but produce reactive [O] species (as verified in Fig. 3). Hence, it is believed that Ce(IV)/UV/Cl⁻ system only generated negligible AOX and was superior to UV/Fe(III)/Cl⁻ system in this regard.

3.5 Proposed catalytic mechanism

Based on the obtained experimental results, a possible pathway was proposed for oxidative degradation of AO7 catalyzed by Ce(IV) ion under different conditions (eqn (1)–(14)). At acidic pH, the Ce(IV) ions were the predominant species³² (eqn (1)). Without UV lamp irradiation the Ce(IV) ion acts as a strong one-electron oxidant.¹² Cerium(IV) itself can oxidize AO7 to produce Ce(III) ion and other oxidation products (eqn (2)). In the presence of chloride, an unstable H₂CeCl₆ is formed (eqn (3)).^34–41 To prove the formation of ceric chloride complex, a high concentration of tetravalent cerium and sodium chloride was mixed under strongly acidic conditions. As shown in Fig. S6,† a dark orange color was observed in Ce(IV)/Cl⁻ solution, which is ascribed to the formation of H₂CeCl₆. This complex was not stable and faded to a colorless solution. This color change offers an indirect evidence for the formation of H₂CeCl₆ and its transformation to colorless Ce(III) compounds. H₂CeCl₆ can be converted to CeCl₄ (eqn (4)). CeCl₄ could directly react with AO7 to produce Ce(III) and oxidation products (eqn (5)). Upon UV irradiation and in the absence of chloride, Ce(IV) is photoactive and can produce the active HO˙ and Ce(III) ion with UV irradiation (eqn (7)). Dioxygen can be photoactivated by Ce(III) and generate superoxide, even H₂O₂ and HO˙ (eqn (8) and (9)). All these reactive species can oxidize dye effectively.

Ce⁴⁺ + H₂O ↔ Ce(OH)³⁺ + H⁺ + H₂O ↔ Ce(OH)₂²⁺ + H⁺ (1)

Ce⁴⁺ + AO7 → Ce³⁺ + oxidation products (2)

Ce⁴⁺ + 6Cl⁻ → [CeCl₆]²⁻ + 2H⁺ → H₂CeCl₆ (3)

H₂CeCl₆ ↔ CeCl₄ + 2HCl (4)

CeCl₄ + AO7 → CeCl₃ + oxidation products (5)

[CeCl₆]²⁻ + AO7 ↔ [CeCl₆(AO7)]²⁻ → [CeCl₆]³⁻ + oxidation products (6)

UV/Ce(IV)/Cl⁻ reaction process was more complex. The present study indicates that chloride can diminish the generation of hydroxyl radicals in UV/Ce(IV) system. It is likely that CeCl₄ reacts with H₂O to form CeCl₃ and Cl₂ under UV irradiation (eqn (12)), and Cl₂ could directly react with H₂O to generate HOCl.^39,41,42 However, there should be only a small amount of Cl₂ to produce HOCl³⁴ in a solution containing large amounts of Cl⁻.³⁹ HOCl could produce [O] with UV irradiation (13) and (14).³⁹ Thus, in UV/Ce(IV)/Cl⁻ process, although HO˙ was not detected (Fig. 3), AO7 was still degraded by CeCl₄ and [O] under UV irradiation.

Ce⁴⁺ + H₂O + hv (UV) → Ce³⁺ + HO˙ + H⁺ (7)

Ce³⁺ + O₂ + hv → O₂˙⁻ (8)

O₂˙⁻ + H⁺ + hv → H₂O₂ + hv → 2HO˙ (9)

HO˙ + AO7 → oxidation products (10)

CeCl₄ + AO7 + hv → CeCl₃ + oxidation product (11)

CeCl₄ + hv → CeCl₃ + Cl₂ + H₂O ↔ HCl + HOCl (12)

HOCl + hv → HCl + [O] (13)

[O] + AO7 → oxidation products (14)

4 Conclusions

In this study, AO7 degradation efficiency was examined in Ce(IV)-based catalytic oxidation systems, considering the impacts of pH, UV irradiation and chloride ion concentration. Acidic pH and UV irradiation can largely promote the degradation performance, while chloride inhibited the oxidation and mineralization rates of dyes to some extents. According to the results of HO˙ probing fluorescence technology, hydroxyl radicals should be the main reactive oxidants in UV/Ce(IV) system, whereas Ce(IV)–chloride complex and [O] should be responsible for the dye degradation in UV/Ce(IV)/Cl⁻ solution, leading to negligible AOX formation in the presence of chloride. Therefore, UV/Ce(IV) process may be used as a new AOPs which is suitable for working in the high salinity wastewater treatment.

Acknowledgements

This work was supported by the National Science Foundation of China (NSFC) (No. 41273108 and 21377023), DHU Distinguished Young Professor Program and Shanghai Pujiang Program. The authors also appreciate the financial support from the Fundamental Research funds for Central Universities Central (15D311312).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra21682j

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