Effect of hydrogen peroxide on the destruction of organic contaminants synergism and inhibition in a con
Applied Catalysis B: Environmental 50 (2004) 259–269 Effect of hydrogen peroxide on the destruction of organic contaminants-synergism and inhibition in a continuous-mode photocatalytic reactor Dionysios D Dionysiou a,∗ , Makram T Suidan a , Isabelle Baudin b , Jean-Michel Laˆné b ı a b Department of Civil and Environmental Engineering, Drinking Water, Water Supply, Quality and Treatment Laboratories, University of Cincinnati, 765 Baldwin Hall, Mail Stop #0071, Cincinnati, OH 45221-0071, USA Centre International de Recherche Sur l’Eau et l’Environnement, CIRSEE, ONDEO Services, 38 rue du President Wilson, 78230 Le Pecq, France Received 18 September 2003; received in revised form 16 January 2004; accepted 27 January 2004 Available online April 2004 Abstract The effect of hydrogen peroxide on the photocatalytic degradation of organic contaminants in water was investigated using a TiO2 -rotating disk photocatalytic reactor (RDPR) operated in a continuous-mode and at steady state The experiments were performed at pH 3.0, in the presence of near-UV radiation, and using 4-chlorobenzoic acid (4-CBA) as a model non-volatile organic contaminant at influent concentration of 300 mol l−1 Experiments were performed at concentrations of hydrogen peroxide in the range 0–10.74 mmol l−1 Addition of hydrogen peroxide at small concentrations ( 320 nm)/TiO2 system [35] The authors hypothesized that this effect could be due to the competition of H2 O2 with the nitroaromatic contaminants for conduction band electrons They explained that these reactions generate • OH and the corresponding nitroaromatic radical anion (reductive pathway), respectively The latter is not an efficient pathway for the photocatalytic degradation of nitroaromatics In general the rates of the heterogeneous system (UV/TiO2 /H2 O2 ) were higher than those of the homogeneous system (UV/H2 O2 ) [35] However, most of the previous photocatalytic studies on the effect of hydrogen peroxide reported the existence of an optimum concentration Mengyue et al studied the effect of hydrogen peroxide on the photocatalytic degradation of monocrotophos and parathion organophosphorus pesticides [46] They found that a concentration of mM hydrogen peroxide enhanced the degradation efficiency (i.e., expressed as the fraction of organophosphate that was mineralized as soluble phosphate ion) by approximately four times for both pesticides The enhancement effect was slightly lower at hydrogen peroxide concentrations of and 10 mM with a gradually reducing trend (i.e., 3.76 and 3.54 for monocrotophos; 3.78 and 3.60 for parathion, respectively) Kumar and Davis studied the effect of hydrogen peroxide on the photocatalytic degradation of 2,4-dinitrotoluene (DNT) in batch slurry reactors [56] They used initial 2,4-DNT concentration of 0.6 mM and varied the concentration of hydrogen peroxide from zero to 100 mM They observed a slight enhancement of the degradation rate by 10% at hydrogen peroxide concentrations of 1–10 mM At 100 mM of hydrogen peroxide, the rate decreased to that in the absence of hydrogen peroxide Haarstrick et al studied the photocatalytic degradation of 4-chlorophenol and p-toluenesulfonic acid in a fluidized bed photocatalytic reactor operated in a batch mode [47] In experiments dealing with the effect of hydrogen peroxide concentration on the degradation rates, they used equimolar mixtures of the two contaminants with total organic carbon (TOC) of 140 mg l−1 and hydrogen peroxide molar concentrations varied in the range 0–9 mM The degradation rates increased with increasing hydrogen peroxide in this range The enhancement factor at hydrogen peroxide concentrations of 2–3 mM was approximately However, this factor did not considerably increase at higher concentrations For this reason, and considering the cost of hydrogen peroxide, the authors suggested that the optimum hydrogen peroxide was at mM Cornish et al reported an optimum concentration of hydrogen peroxide on the photocatalytic destruction 261 of microcystin-LR toxin with a maximum enhancement factor of approximately [52] The authors also observed that during dark adsorption, hydrogen peroxide competed with microcystin-LR for active sites and that hydrogen peroxide at concentration of 0.6% (v/v) in water caused coagulation of the suspension, an effect that was attributed to catalyst surface charge modifications by adsorbed hydrogen peroxide molecules This hypothesis was further supported by the formation of a yellow color in the suspension solution, which was attributed to the Ti(IV)–peroxo complexes [52] In summary, previous studies dealing with the role of hydrogen peroxide on the photocatalytic degradation of organic contaminants reported positive, neutral, or negative effect Most studies reported that hydrogen peroxide could increase the reaction rates or cause inhibition effects depending on its concentration in the reaction solution The results of all these studies suggest that the effect of hydrogen peroxide is a function of many interrelated parameters including the properties of radiation (i.e., wavelength, intensity), solution pH, physicochemical properties of the contaminant, type of catalyst (i.e., surface characteristics) and the oxidant to contaminant molar ratio However, in most previous studies, the effect of hydrogen peroxide was investigated in batch or semi-batch photocatalytic systems in which both the concentrations of contaminant and hydrogen peroxide were changing with time Few studies dealt with addition of hydrogen peroxide at constant rate [29] but again in batch or semi-batch systems As explained above, synergistic or inhibitive effects are a function of the magnitude of hydrogen peroxide concentration relative to other reaction conditions (i.e., contaminant concentration, total organic carbon, UV light flux) When such conditions are time-dependent, the task to elucidate the effect of a certain parameter becomes more difficult Our approach was to investigate the effect of hydrogen peroxide in a continuous-mode photocatalytic reactor operated at steady state The reactor used in this study is the rotating disk photocatalytic reactor (RDPR) In addition, the RDPR has mixing characteristics similar to a continuously stirred tank reactor (CSTR), meaning that the concentration of chemical species and the conditions of the effluent are equal to those inside the reactor vessel This is very important for assessing the effect of hydrogen peroxide concentration in the reactor solution, during a process in which all conditions in the reactor (i.e., concentrations, pH, temperature) remain stable with time when a steady state operation is achieved Experimental procedures 2.1 Rotating disk photocatalytic reactor (RDPR) Details on the development, characterization, evaluation, and mechanism of operation of the RDPR for the destruction of pesticides and other organic pollutants in water at small 262 D.D Dionysiou et al / Applied Catalysis B: Environmental 50 (2004) 259–269 concentrations (i.e., 295 nm) [33] They compared three different systems: UV/TiO2 , UV/oxidant, and UV/TiO2 /oxidant Their study showed that addition of 10 mM H2 O2 significantly increased the reaction rate compared to the UV/TiO2 system However, the degradation rate was somewhat slower to that of UV/H2 O2 system Similar results were observed for the other oxidants The authors suggested that the enhancement effect of the oxidants could be due to homogeneous photolysis However, they also pointed out that shading and scattering effects in the heterogeneous system will result in less actual photon flux input in this system [33] Positive effects of hydrogen peroxide were observed by Suárez-Parra et al in the visible light (λ > 400 nm)-induced photodegradation of a blue azo dye and using composite TiO2 /CdO–ZnO nanoporous film as the catalyst and acidic pH = 3.0 [78] The authors suggested that hydrogen peroxide plays a significant role in this process since it scavenges the photoinjected electrons from the dye, preventing thus the recombination of the electrons with the cation radical of the dye Several previous studies have reported on several reactions that take place in a photocatalytic process and involve H2 O2 , • OH and O2 •− The following pertinent reactions have appeared in the literature dealing with TiO2 photocatalysis and the effect of hydrogen peroxide ([46,47,73,74,76,77] and references therein): TiO2 + hv → TiO2 (h+ + e− ) hv (1) → H2 O2 − 2• OH (2) H2 O(ads) + h+ → • OH + H+ (3) OH− (ads) (4) + + h → HO• O2 + e− → O2 •− (5) O2(ads) + e− + H+ → HO2 • (6) HO2 • + HO2 • → H2 O2 + O2 (7) O2 •− + HO2 • → HO2 − + O2 (8) HO2 − + H+ → H2 O2 (9) H2 O2(ads) + e− → • OH + OH− (10) H2 O2 + O2 •− → • OH + OH− + O2 (11) H2 O2(ads) + 2h+ → O2 + 2H+ (12) + − H2 O2(ads) + 2H + 2e → 2H2 O (13) H2 O2(ads) + h+ → HO2 • + H+ (14) O2(ads) + 2e− + 2H+ → H2 O2(ads) (15) O2 •− + H+ ⇔ HO2 • (16) O2 •− + htr + → O2 (17) H2 O2 + • OH → H2 O + HO2 • (18) HO2 • + • OH → H2 O2 + O2 (19) • OH (20) + • OH → H2 O2 HO2 • + H2 O2 → • OH + H2 O + O2 (21) In these reactions e− and h+ refer to conduction band electron and valence band hole, respectively, generated during the photoexcitation process (reaction (1)) On the other hand, htr + (see reaction (17)) refers to a trapped hole and has redox potential of 1.5 V [73] Although several of these reactions D.D Dionysiou et al / Applied Catalysis B: Environmental 50 (2004) 259–269 Table Reaction rate constants of radicals in aqueous solutions (obtained from [81]) k Value (l mol−1 s−1 ) pH k7 k8/9 k11 × 106 9.7 × 107 × 10−4 –2.3 ≤2.0 k18 2.7 × 107 (average)∗ 2.7 2.0 3.8 2.4 × × × × 107 107 107 107 Neutral to basic 7.8 7.0 7.7–11.0 7.0 Original reference [82] [83] [85–88] 5.4–9.9 [94] 1.0 × 1010 k20 4.2 × 109 5.2 × 109 3.7 × 10−2 –5 0.5–3.5 2.0 293 [82] [103] [104] [98] 298 [97] 298 k19 k21 T (K) [99] [98] [85] [89] [90] [91] [92] are considered to take place at the surface of the catalyst, a short discussion on some relevant reaction rate constants in aqueous solutions will be helpful to further rationalize the existence of an optimum concentration of hydrogen peroxide and its inhibition effect at higher concentrations A summary of the reaction rate constants of certain of these reactions is provided in Table Redox potentials of many of these reactions in homogeneous solutions and at various pH ranges (acidic, neutral, basic) are provided elsewhere [79,80] As explained in a previous section, reaction (2) occurs mainly at wavelengths lower than 300 nm, where H2 O2 absorbs more strongly and it is unlikely to have a significant effect at the wavelengths employed in this study Dimerization of perhydroxyl radical (reaction (7)) has k value of × 106 l mol−1 s−1 at pH ≤ 2.0 [82] while reaction between HO2 • and O2 •− (reactions (8) and (9)) has a k value of 9.7 × 107 l mol−1 s−1 [83] Reaction between H2 O2 with HO2 • /O2 •− (reactions (11) and (16)) has been reported to have a very small reaction rate constant, k = 1.1 l mol−1 s−1 (T = 273 K) and 3.7 l mol−1 s−1 (T = 298 K) [84] Small k value was reported for reaction (11) (1 × 10−4 l mol−1 s−1 to 2.3 l mol−1 s−1 ) at high pH (5.4–9.9) [85–88] and for reaction (21) (1 × 10−2 l mol−1 s−1 to