Emerging investigators series: ultraviolet and free chlorine aqueous-phase advanced oxidation process: kinetic simulations and experimental validation†

Divya Kamath and Daisuke Minakata *
Department of Civil and Environmental Engineering, Michigan Technological University, 1400 Townsend Drive, Houghton, Michigan 49931, USA. E-mail: dminakat@mtu.edu; Fax: +1 906 487 2943; Tel: +1 906 487 1830

First published on 21st June 2018

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Water impact An elementary reaction based kinetic model provides mechanistic insight into the reaction mechanisms induced by both hydroxyl and chlorine radicals and can be used as a comprehensive predictive model for any other compounds in the application of aqueous-phase ultraviolet combined with free chlorine advanced oxidation process for direct potable reuse of reclaimed wastewater.

Introduction

Ultraviolet (UV) light combined with free chlorine (UV/free chlorine) is an emerging advanced oxidation process (AOP) that produces active hydroxyl radicals (HO˙) and chlorine radicals (Cl˙) to degrade a variety of organic compounds in water.^1–3 The UV/free chlorine AOP is an attractive alternative to conventional UV or chlorination disinfection techniques because of the potential to degrade organic compounds via active radicals and the use of residual free chlorine as a secondary disinfectant.^4,5 The UV/free chlorine AOP has recently been shown to degrade some target organic compounds more efficiently than the UV/hydrogen peroxide AOP due to: (1) the larger molar absorptivity of HOCl/OCl⁻ (ε_HOCl = 59 M⁻¹ cm⁻¹ and ε_OCl⁻ = 66 M⁻¹ cm⁻¹ at 253.7 nm)^6,7 and (2) the contribution of chlorine-derived radicals (i.e., Cl˙: 2.34 V versus SHE; Cl₂˙⁻: 2.13 V; ClO˙: 2.39 V)⁸ to organic compound degradation. For example, UV/free chlorine AOP has been considered as an alternative AOP after RO in wastewater reclamation processes for potable reuse of treated wastewater aiming to degrade low molecular weight neutral trace organic compounds that may be present in the RO permeate. UV/free chlorine does not require the quenching of hydrogen peroxide residue in the current practice of UV/hydrogen peroxide AOP because free chlorine can be used as secondary disinfectant.

The use of free chlorine and the reactivity of chlorine-derived radicals with many organic compounds in UV/free chlorine AOP results in the potential formation of toxic degradation byproducts, such as chlorinated byproducts.^9,10 Consequently, experimental investigations on the formation of chlorinated byproducts from some organic compounds have become an active research area. Because a number of organic compounds are used and commercially produced,¹¹ a kinetic model that predicts the fate of these degradation products is needed to preliminarily screen organic compounds and AOP designs.

An elementary reaction pathway-based kinetic model is an attractive and promising model that predicts the degradation of a target organic compound and the degradation products and provides mechanistic insight into the reaction mechanisms.¹² Many experiment-based kinetic models have been developed for UV/hydrogen peroxide AOPs based on experimentally identified reaction pathways, and the rate constants were determined by fitting the experimentally determined concentration profiles.^13,14 However, this type of kinetic model often simplifies the reaction pathways and fails to predict the degradation products of other compounds. In contrast, an elementary reaction pathway-based kinetic model contains all possible elementary reactions and can comprehensively predict the degradation pathways of organic compounds.¹² This is particularly important for the UV/free chlorine AOP because chlorine-derived radicals are very selective and produce different products depending on the elementary reaction pathways.¹⁵ For example, Cl˙ reacts by abstracting a hydrogen (H) atom from a C–H bond and reacts with an alcohol functional group via a single electron transfer to produce an alkoxyl radical,¹⁶ while HO˙ favorably abstracts a H atom from a C–H bond to produce a carbon-centered radical. While the overall reactivities of Cl˙ and HO˙ are very similar (e.g., second-order reaction rate constant, k = 10⁸–10⁹ M⁻¹ s⁻¹),^17–19 their reaction products are different because of the different elementary reaction mechanisms. Thus, the potential formation of typical transformation products such as aldehydes, ketones and carboxylic compounds should be mechanistically understood because of the concern about their toxicity (e.g., halogenated acids).

Quantum mechanical calculations using ab initio and density functional theory (DFT) are attractive techniques to identify elementary reaction pathways by calculating the thermodynamic properties using statistical thermodynamics.²⁰ Our previous studies used this technique with an implicit solvation model [universal solvation model (SMD)] to identify thermodynamically favorable, aqueous-phase elementary reactions for a series of chlorine-derived inorganic reactions produced in the UV/free chlorine AOP.¹⁵ We also calculated the aqueous-phase free energies of activation for a series of Cl˙ reactions with approximately 30 aliphatic organic compounds and found the linear free energy relationships (LFERs) that relate the free energies of activation to the experimental k_exp values for H-atom abstraction and Cl-adduct formation.¹⁵

In this study, we developed an elementary reaction pathway-based kinetic model for a test compound, acetone, in the UV/free chlorine AOP. The HO˙-induced elementary reaction pathways for acetone and the reaction rate constants have been previously investigated.¹² Thus, we focused on the reactions of chlorine-derived radicals with acetone and the degradation products. The ordinary differential equations (ODEs) were developed based on the theoretically identified elementary reaction pathways and reaction rate constants predicted by the LFERs and numerically solved to obtain the time-dependent concentration profiles of acetone and the degradation products. We also performed batch experiments with the UV/free chlorine AOP to validate the model simulation results.

Materials and methods

Chemicals

All chemicals were ACS grade except for the chemicals that were used for the analytical measurements (HPLC grade). Acetone (>99%), sodium hypochlorite (available chlorine 10–15%), formic acid (>95%), sodium chlorate, and potassium chloride were purchased from Sigma Aldrich. Acetic acid (glacial) and sodium thiosulfate were purchased from Fisher Scientific. All solutions used during the experiment were prepared with ultrapure water (>18 Ω) generated from a MilliQ system.

Experimental procedures

The experiments were carried out using an apparatus equipped with a low-pressure UV lamp (Atlantic UV) emitting photons at 254 nm. The intensity of the measured light was 4.18 × 10⁻⁸ einstein L⁻¹ s⁻¹ using an actinometry procedure.²¹ The path length was determined to be 44.24 cm based on the photolysis of dilute H₂O₂.²² The UV lamp was housed in a double-walled quartz immersion well, and cooling water was passed through the system to control the temperature. The temperature of the reactors was monitored, and the temperature of the solutions did not change by more than 1 °C for the duration of the experiments. A detailed description of this photoreactor setup is available.²³ A 67.5 μM solution of acetone was prepared, and sodium hypochlorite (NaOCl) was added to obtain 150 μM (10.7 mg L⁻¹) of free chlorine. After initiating the experiment, the solutions were sampled at different time intervals. These samples were transferred to vials containing a sodium thiosulfate solution (approximately 220 μM) to quench the chlorine and terminate further reactions. All chemical analyses to measure the acetone and transformation byproducts were performed within 24 h of the experiment.

Analytical methods

Acetone was measured using direct aqueous injection on a gas chromatograph (GC) equipped with a flame ionization detector (FID) and column (8 ft × 0.1 in. ID, stainless-steel column) packed with 1% SP-1000 on Carbopak-B 60/80 mesh. The injector and detector temperatures were 200 °C and 220 °C, respectively. Helium was used as the carrier gas, and hydrogen and air were used for the flame. The analysis method was 60 °C for 2 min followed by a 60 min increase of 2 °C min⁻¹ and holding at 120 °C for 6 min. The retention time of acetone in this method was 4.6 min. The free chlorine in the aqueous solution was measured using a chlorine meter (Hach DPD colorimeter). Transformation byproducts were measured using an ion chromatograph (Dionex ICS 2100 series, IonPac AS17-C anion exchange column, 4 mm). The eluent was a potassium hydroxide (KOH) solution. The flow rate was 1.5 mL min⁻¹, and the flow conditions were set as follows: 0–15 min, 1 mM KOH (isocratic); 15–20 min, 1–10 mM KOH (ramp); 20–25 min, 10 mM KOH (isocratic); 25–30 min, 10–15 mM KOH (ramp). The retention times for acetate, formate, chloride and chlorate were 7, 9, 15 and 21 min, respectively.

Computational studies

All of the ab initio molecular orbital and DFT-based quantum mechanical calculations were performed with the Gaussian 09 revision D.02 program²⁴ using the Michigan Tech high-performance cluster “Superior”. The electronic structures of the molecules and radicals in the ground and transition states were optimized at the level of B3LYP/6-31G(2df,p) implemented in Gaussian-4 theory (G4)²⁵ in both the gaseous and aqueous phases. The aqueous-phase calculations were performed using a universal solvation model (SMD).²⁶ We previously verified the combination of G4 with the SMD model by successful applying the combination to other aqueous-phase, radical-involved reactions.²⁷ Finally, these elementary reactions and rate constants were used to generate the kinetic rate equations in the form of ODEs and were solved using a numerical solver based on the Adam–Gear method from IMSL Roguewave's solver suite²⁸ by modifying an original UV/H₂O₂ model without assuming constant pH at non-steady-state condition.²⁹

Results and discussion

Experimental product study

Fig. 1 shows the time-dependent concentration profiles of the acetone, free chlorine, and transformation byproducts measured in this experiment. While the free chlorine was completely consumed after 60 min of UV irradiation, only 53.6% of the acetone was degraded. The acetone degradation ceased upon complete photolysis of free chlorine, and no further degradation was observed. This indicates that acetone degradation occurred via acetone directly reacting with free chlorine and/or the photolysis caused by free chlorine. Thermal degradation of acetone by free chlorine is possible only for the enolate form, and the reported acetone enolization is insignificant (k = 0.173 M⁻¹ s⁻¹)³⁰ during the observed experimental time. In the dark, only 3% acetone degradation was observed after 2 h. Thus, the degradation of acetone results from the photolysis of free chlorine. The initial 150 μM sodium hypochlorite solution contained 15 μM chlorate, 300 μM chloride, and a few micromoles of acetic acid and formic acid to reach the given pH. As we eliminated all possible contamination from the vials, ultrapure water, source water, and ion chromatography measurements, the presence of chlorate, chloride, and trace organic acids seemed to result from the stock sodium hypochlorite chemical. For example, hypochlorite auto decomposes into chlorate,³¹ which is the method used to form chloride for bleach (NaOCl) production. Previous literature reported trace quantities of perchlorate measured by an ion chromatograph tandem mass spectrometer (0.0003 to 0.0005 μM for the method detection limit).³² However, we did not detect perchlorate in our free chlorine source due to the higher detection limit using ion chromatography (∼1 μM). Accordingly, the initial concentrations of these species were at some levels.

Elementary reaction pathways and reaction rate constants

The HO˙ and Cl˙ produced from the photolysis of free chlorine react with HOCl/OCL⁻ to generate Cl-derived radicals, such as Cl₂˙⁻, ClO˙, and ClO₂˙. The HO˙-induced elementary reaction pathways of acetone degradation have been previously identified, and an elementary reaction-based kinetic model has been proposed.¹²Table 1 summarizes the elementary reaction pathways that involve Cl-derived radicals with the theoretically calculated aqueous-phase free energy of reaction, ΔG^react_aq,calc, and free energy of activation, ΔG^act_aq,calc, values and the reaction rate constants. While the ΔG^react_aq,calc values indicate the thermodynamical feasibility of elementary reaction pathway (e.g., if the value is negative, the reaction is exothermic and thermodynamically favorable to occur), the ΔG^act_aq,calc values represent the kinetics. It should be noted that the kinetics overruns the thermodynamics for fast radical reactions. The HO˙ and Cl-derived radicals react with acetone via H-atom abstraction from a C–H bond in a methyl functional group to produce a carbon-centered radical.^15,17,18 The ΔG^act_aq,calc values for the reactions of HO˙ and Cl˙ with acetone were previously determined to be 7 kcal mol⁻¹ and 3.2 kcal mol⁻¹, respectively. Our new calculations using the same method obtained a ΔG^act_aq,calc value of 7.8 kcal mol⁻¹ for Cl₂˙⁻, 1.5 kcal mol⁻¹ for ClO˙, and 14.1 kcal mol⁻¹ for ClO₂˙. The second-order reaction rate constant of Cl₂˙⁻ with acetone was experimentally determined and is 1.4 × 10³ M⁻¹ s⁻¹,³³ which indicates the insignificant contribution of this reaction to the overall degradation of acetone. However, as indicated by the ΔG^act_aq,calc value and postulated by several other experimental studies,^34,35 the reaction of ClO˙ with acetone is not insignificant. More discussion on the reactivity of ClO˙ will be provided in the following section and kinetic simulation section.

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While Cl-derived radicals react with acetone initially, Cl-derived radicals also react with the other transformation byproducts formed during the degradation of acetone. For example, Cl˙ and ClO˙ abstract H atoms from a C–H bond in HCOOH and HCOO⁻ with ΔG^act_aq,calc values of 15.0 kcal mol⁻¹ and 21 kcal mol⁻¹, respectively. Additionally, Cl˙ reacts with the OH functional group of HCOOH with a ΔG^act_aq,calc value of 6.0 kcal mol⁻¹ by forming a Cl-adduct and then transferring a single electron to produce the alkoxyl radical HCOO˙. Using the previously developed LFERs: lnk = −0.50 ΔG^act_aq,calc + 20.53 for H-atom abstraction and lnk = −0.95 ΔG^act_aq,calc + 23.43 for Cl-adduct formation by Cl˙,¹⁵ the k_cal values were estimated to be 4.56 × 10⁵ M⁻¹ s⁻¹ for the H-atom abstraction and 5.01 × 10⁷ M⁻¹ s⁻¹ for the Cl-adduct formation. The k_exp value is (1.3 ± 0.1) × 10⁸ M⁻¹ s⁻¹, and Cl-adduct formation is the dominant reaction. We obtained a similar result for the reaction of Cl˙ with CH₃COOH.

The reactivity of ClO˙ with organic compounds is not well understood. A very limited number of k_exp values have been reported for ionized aromatic compounds, and these values range from 10⁷–10⁹ M⁻¹ s⁻¹. The upper limit of the k_exp values for aliphatic compounds (e.g., formate ion) is reported to be 1 × 10⁶ M⁻¹ s⁻¹. Our series of theoretical calculations results in a ΔG^act_aq,calc value of approximately 15–25 kcal mol⁻¹ (Table 1). We obtained a ΔG^act_aq,calc value of 1.5 kcal mol⁻¹ for the reaction of ClO˙ with acetone, but the reason for this abnormally low free energy of activation is not clear.

Acetone degradation simulation

Conclusions

This study highlights the importance of an elementary reaction-based kinetic model for the UV/free chlorine AOP. The elementary reaction pathways and reaction rate constants were predicted by quantum mechanical calculations. ODEs were numerically solved to predict the concentration profiles of a target organic compound, acetone, and the transformation products. ClO˙ was identified as a potential oxidant in this system because of its high concentration, and its reaction rate constant with acetone was determined to be 3 × 10⁴ M⁻¹ s⁻¹. Chlorate formation was in the range of 2.5 mg L⁻¹ with 10.7 mg L⁻¹ of free chlorine. Although chlorate is not yet regulated, this may be a potential cause for concern when using this treatment technology.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Science Foundation Award: CBET-1435926. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the supporting organization. The authors appreciate the support from the Michigan Tech HPC cluster ‘Superior’.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ew00196k

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