Article pubs.acs.org/est
Enhanced Chlorine Dioxide Decay in the Presence of Metal Oxides: Relevance to Drinking Water Distribution Systems Chao Liu,† Urs von Gunten,‡,§ and Jean-Philippe Croué*,† †
Water Desalination and Reuse Center, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia ‡ Eawag, Swiss Federal Institute of Aquatic Science and Technology, Ueberlandstrasse 133, CH-8600 Dübendorf, Switzerland § School of Architecture, Civil and Environmental Engineering (ENAC), Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland S Supporting Information *
ABSTRACT: Chlorine dioxide (ClO2) decay in the presence of typical metal oxides occurring in distribution systems was investigated. Metal oxides generally enhanced ClO2 decay in a second-order process via three pathways: (1) catalytic disproportionation with equimolar formation of chlorite and chlorate, (2) reaction to chlorite and oxygen, and (3) oxidation of a metal in a reduced form (e.g., cuprous oxide) to a higher oxidation state. Cupric oxide (CuO) and nickel oxide (NiO) showed significantly stronger abilities than goethite (αFeOOH) to catalyze the ClO2 disproportionation (pathway 1), which predominated at higher initial ClO2 concentrations (56−81 μM). At lower initial ClO2 concentrations (13−31 μM), pathway 2 also contributed. The CuO-enhanced ClO2 decay is a base-assisted reaction with a third-order rate constant of 1.5 × 106 M−2 s−1 in the presence of 0.1 g L−1 CuO at 21 ± 1 °C, which is 4−5 orders of magnitude higher than in the absence of CuO. The presence of natural organic matter (NOM) significantly enhanced the formation of chlorite and decreased the ClO2 disproportionation in the CuO−ClO2 system, probably because of a higher reactivity of CuO-activated ClO2 with NOM. Furthermore, a kinetic model was developed to simulate CuO-enhanced ClO2 decay at various pH values. Model simulations that agree well with the experimental data include a pre-equilibrium step with the rapid formation of a complex, namely, CuOactivated Cl2O4. The reaction of this complex with OH− is the rate-limiting and pH-dependent step for the overall reaction, producing chlorite and an intermediate that further forms chlorate and oxygen in parallel. These novel findings suggest that the possible ClO2 loss and the formation of chlorite/chlorate should be carefully considered in drinking water distribution systems containing copper pipes.
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odor-causing compounds.5 Its disinfection efficiency is equal or even superior to chlorine,6 which makes ClO2 a suitable alternative. Moreover, ClO2 is effective over a wide pH range,5 and the formation of halogenated DBPs is typically much lower when compared to free chlorine. This is primarily attributed to the difference in oxidation reaction mechanisms. ClO2 reacts via electron transfer, whereas chlorine reacts partially via chlorine substitution, leading to chlorinated compounds.7 Furthermore, chlorine quickly oxidizes bromide to HOBr, which further reacts with NOM to produce brominated compounds,8 whereas the rate constant between ClO2 and bromide is very low ( bicarbonate/carbonate.28 If the relative affinities of the different anions to CuO are similar to goethite, it may be concluded that the inhibition efficiency on ClO2 decay by anions is related to blocking of the active sites. It should be pointed out that metal oxides and anions were dosed almost at the same time in this work. Previous studies indicated that silicate exhibits a strong time dependency to form multiple layers on oxide surfaces and longer term exposure to silicate can almost completely coat CuO, which dramatically reduces its effect on chlorine stability.29−31 If these results apply to ClO2, it can be expected that the catalytic activity of CuO for ClO2 decay might also be affected by extended exposures to silicate. However, for freshly generated CuO in new copper pipes, enhanced ClO2 decay should be observed even in the presence of silicate.
Figure 3. Experimental data (symbols) and model simulations (lines) for the effect of pH on (A) ClO2 decay and (B) chlorite and (C) chlorate formation. Experimental conditions: T, 21 ± 1 °C; [CuO], 0.1 g L−1; and [ClO2]0, 31.5 μM. The ClO2 decay via volatilization at pH values of 6.6 and 7.6 was subtracted according to Figure 1A. Symbols represent the mean of duplicate experiments. Error bars represent the standard deviations of duplicate experiments.
ClO2 decay follows second-order kinetics (see Figure S7 of the Supporting Information), with the rate constants increasing from pH 6.6 to 9.6 and k values ranging from 0.8 ± 0.3 to 59.0 ± 4.5 M−1 s−1 (see runs 9−13 in Table S2 of the Supporting Information). It was previously reported that in homogeneous solution ClO2 decay is enhanced in basic solutions.24 A plot of the second-order rate constants versus [OH−] (see Figure S8 of the Supporting Information) is characterized by a nearly linear correlation, indicating that ClO2 decay in the presence of CuO is a base-assisted process. The corresponding third-order rate constant was fitted as 1.5 × 106 M−2 s−1 at 21 ± 1 °C. In comparison to a value of 21.8 M−2 s−1 at 25 °C in homogeneous solutions reported by Odeh et al.,24 an enhancement of 4−5 orders of magnitude can be observed in the presence of 0.1 g L−1 CuO. Because of the higher ClO2 decay rate, the formation rates of chlorite and chlorate were higher at higher pH (see panels B E
dx.doi.org/10.1021/es4015103 | Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
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Figure 4. Effect of NOM on (A) ClO2 decay, (B) chlorite and (C) chlorate formation, and (D) sum of ClO2, chlorite, and chlorate in the absence or presence of CuO. Experimental conditions: T, 21 ± 1 °C; pH, 8.6. Symbols represent the mean of duplicate experiments. Error bars represent the standard deviations of duplicate experiments.
As mentioned above, CuO enhanced the decay of ClO2 to form chlorite and chlorate via reactions 1 and 3. When NOM was introduced in the CuO−ClO2 system, the product distribution was significantly modified (panels B and C of Figure 4). Chlorate formation decreased, while chlorite formation was significantly enhanced. For example, at a reaction time of 120 min, chlorate concentrations were 2.8 ± 0.5, 3.8 ± 0.1, and 9.5 ± 1.0 μM, corresponding to a NOM concentration of 5, 2.5, and 0 mg L−1, respectively. The corresponding chlorite concentrations were 16.6 ± 1.4, 17.5 ± 0.1, and 11.1 ± 0.7 μM, respectively. Therefore, the chlorite concentration increased by approximately 6 μM at 120 min in the presence of NOM. The expected chlorite production from the homogeneous reaction between ClO2 and NOM (i.e., around 2 μM at a reaction time of 120 min) was lower than the observed enhanced chlorite formation (i.e., around 6 μM). Preliminary results indicated insignificant NOM adsorption onto the CuO surface at pH 8.6 within 120 min. It was reported that the reactivity of hypohalous acids (HOCl/HOBr4 and HOI,33 respectively) can be enhanced when adsorbed onto the surface of metal oxides, such as CuO and δ-MnO2, respectively. Therefore, it can be hypothesized that ClO2 can be activated by CuO and exhibits enhanced electrophilic character and reactivity toward NOM moieties, leading to an enhanced chlorite formation. The sum of ClO 2 − and ClO 3 − formed (as molar concentrations was generally in agreement with the ClO2 consumed (Figure 4D). Additional analyses showed that the production of AOCl was low (less than 1 μM; see Figure S12 of the Supporting Information). It is known that the reaction between ClO2 and NOM does not lead to high halogenated organic compound formation.34 These results further confirm that the ClO2 consumption preferentially leads to the formation of inorganic products (chlorite and chlorate).
The effect of bromide on ClO2 decay and product formation is plotted in Figure S11 of the Supporting Information. The presence of 10 μM Br− did not significantly affect the ClO2 decay and chlorite/chlorate formation. After 120 min of reaction, bromate was not detected in the solution because of the very low rate constant for the reaction between ClO2 and bromide (
