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Comparing the UV/monochloramine and UV/free chlorine Advanced Oxidation Processes (AOPs) to the UV/hydrogen peroxide AOP Under Scenarios Relevant to Potable Reuse Yi-Hsueh Chuang, Serena Chen, Curtis Chinn, and William A. Mitch Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03570 • Publication Date (Web): 09 Nov 2017 Downloaded from http://pubs.acs.org on November 11, 2017

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Environmental Science & Technology

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Comparing the UV/monochloramine and UV/free chlorine Advanced Oxidation Processes

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(AOPs) to the UV/hydrogen peroxide AOP Under Scenarios Relevant to Potable Reuse

7 8 9 10 11

Yi-Hsueh Chuang1, Serena Chen2, Curtis J. Chinn2, and William A. Mitch1, *

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1

16 17 18

California 94305, United States

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2

Department of Civil and Environmental Engineering, Stanford University, 473 Via Ortega, Stanford,

Galileo Academy of Science and Technology, 1150 Francisco Street, San Francisco, California,

94109, United States *Corresponding author: email: [email protected], Phone: 650-725-9298, Fax: 650-723-7058

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1

ACS Paragon Plus Environment


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Abstract

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Utilities incorporating the potable reuse of municipal wastewater are interested in converting

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from the UV/H2O2 to the UV/free chlorine advanced oxidation process (AOP). The AOP treatment of

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reverse osmosis (RO) permeate often includes the de facto UV/chloramine AOP because chloramines

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applied upstream permeate RO membranes. Models are needed that accurately predict oxidant

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photolysis and subsequent radical reactions. By combining radical scavengers and kinetic modeling,

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we have derived quantum yields for radical generation by the UV photolysis of HOCl, OCl-, and

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NH2Cl of 0.62, 0.55, and 0.20, respectively, far below previous estimates that incorporated

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subsequent free chlorine or chloramine scavenging by the •Cl and •OH daughter radicals. The

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observed quantum yield for free chlorine loss actually decreased with increasing free chlorine

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concentration, suggesting scavenging of radicals participating in free chlorine chain decomposition

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and even free chlorine reformation. Consideration of reactions of •ClO and its daughter products

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(e.g., ClO2-), not included in previous models, were critical for modeling free chlorine loss. Radical

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reactions (indirect photolysis) accounted for ~50% of chloramine decay and ~80% of free chlorine

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loss or reformation.

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AOP for degradation of 1,4-dioxane, benzoate and carbamazepine across pH 5.5-8.3. The UV/free

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chlorine AOP was more efficient at pH 5.5, but only by 30% for 1,4-dioxane. At pH 7.0-8.3, the

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UV/free chlorine AOP was less efficient. •Cl converts to •OH. The modeled •Cl:•OH ratio was ~20%

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for the UV/free chlorine AOP and ~35% for the UV/chloramine AOP such that •OH was generally

The performance of the UV/chloramine AOP was comparable to the UV/H2O2

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more important for contaminant degradation.

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Introduction

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Highly-purified municipal wastewater effluents are increasingly considered as a local, reliable

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supply of potable water.1, 2 Potable reuse facilities frequently employ Full Advanced Treatment (FAT)

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trains, incorporating microfiltration (MF), reverse osmosis (RO), and the UV/hydrogen peroxide

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(H2O2) advanced oxidation process (AOP).2 Following the broad-screen physical barrier of RO, the

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UV-based AOP represents a broad-screen chemical barrier by producing hydroxyl radical (•OH;

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equation 1) to destroy contaminants passing through RO. The California Department of Public

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Health regulations require that the AOP achieve 0.5-log destruction of 1,4-dioxane or 1.2-log

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destruction of N-nitrosodimethylamine (NDMA) for potable reuse operations.3

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H2O2 + hν → 2 •OH

[1]

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While the UV/H2O2 AOP has been the most widely implemented and studied for potable reuse,

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there is growing interest in two other AOPs. The UV/free chlorine AOP produces both •OH and

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chlorine radical (•Cl) by UV photolysis of HOCl or OCl- (equations 2-4)4-6; •O- from OCl- photolysis

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converts to •OH below its 11.9 pKa.7 This AOP is attractive for two reasons. First, where AOP

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effluents require a chlorine or chloramine residual (e.g., direct potable reuse), the UV/free chlorine

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AOP avoids the additional chlorine demand required to quench residual H2O2 (equation 5). Second,

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for low pressure mercury lamps emitting at 254 nm, radical production from HOCl and OCl- may be

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more efficient than from H2O2 due to higher molar absorption coefficients and quantum yields for 3



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oxidant (e.g., HOCl) photolysis, reducing the energy requirement for contaminant degradation by

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30-75%.6,

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photolysis of NH2Cl produces •Cl and the amidogen radical (•NH2; equation 6).9

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relatively unreactive,14 •Cl could degrade contaminants directly or form •OH via equations 7-9.

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Chloramines are commonly applied upstream of MF to control biofouling in FAT trains, and pass

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through RO membranes. Accordingly, many UV/H2O2 systems at such facilities are de facto

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combinations of UV/H2O2 and UV/NH2Cl systems.

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The UV/chloramine (UV/NH2Cl) AOP has received some attention.9-13 The UV

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HOCl + hν → •OH + •Cl

[2]

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OCl- + hν → •O- + •Cl

[3]

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•O- + H+ → •OH

[4]

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HOCl + H2O2 → O2 + H2O + Cl- + H+

[5]

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NH2Cl + hν → •NH2 + •Cl

[6]

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•Cl + OH- ↔ ClOH•-

[7]

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•Cl + H2O ↔ ClOH•- + H+

[8]

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ClOH•- ↔ •OH + Cl-

[9]

While •NH2 is

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Previous research has characterized the degradation of specific compounds by the UV/free

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chlorine and, to a lesser degree, the UV/NH2Cl AOPs; there are several examples of such studies.4,

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15-21

However, there is significant uncertainty regarding their radical production efficiency. For 4


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example, previous research suggested that quantum yields for HOCl photolysis range from

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~1.0-1.5.4-6, 15, 22 However, these were observed quantum yields based on total oxidant loss rates,

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incorporating both the innate quantum yield from direct photolysis (equations 2 and 3), and

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subsequent scavenging by the •Cl and •OH daughter radicals; such chain decomposition reactions

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account for observed quantum yields above 1. Daughter radical reactions are more important for

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OCl- than HOCl; the resultant scavenging of both radicals and free chlorine reduces treatment

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efficiency by the UV/free chlorine AOP above the 7.5 pKa of HOCl.6 The importance of •Cl for

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contaminant degradation is also debated. For the UV/free chlorine AOP, Nowell and Hoigné23 used

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probe molecules to argue that •OH was the only significant contributor to contaminant degradation,

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while Fang et al.4 used a model to indicate that •Cl dominated benzoic acid degradation. Lastly,

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research directly comparing the efficacy of these AOPs has been limited to the comparison of the

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UV/H2O2 and UV/HOCl AOPs for treating 8 contaminants by Sichel et al.8 and nitrobenzene by

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Watts et al.6 The Sichel et al. study highlighted the energy savings associated with the UV/HOCl

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AOP, but direct comparison of the AOPs was hindered by the use of different oxidant concentrations.

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The first objective of this study was to determine parameters (e.g., molar absorption

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coefficients and innate quantum yields at 254 nm) needed to accurately model both radical

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generation and oxidant loss in the UV/free chlorine and UV/NH2Cl AOPs. Separating direct

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photolysis of oxidants from subsequent radical reactions is critical because the competition between

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contaminants, oxidants and matrix components to react with these daughter radicals varies with the 5



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contaminants. The second objective was to use the kinetic model to compare the contribution of •Cl

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and •OH to contaminant degradation. The third objective was to compare under comparable

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conditions (e.g., UV fluence and oxidant dose) all three AOPs for contaminant degradation. These

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evaluations were conducted at pH 5.5, relevant to RO permeates, but also at pH 7.0 and 8.3,

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potentially relevant to drinking waters or RO-free advanced treatment trains for potable reuse.

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Utilities are increasingly interested in such advanced trains to minimize energy usage and avoid

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issues associated with brine disposal.24

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MATERIALS AND METHODS

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Materials: Stock solutions of monochloramine were made and standardized as described

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previously.25 Briefly, sodium hypochlorite was added to ammonium chloride at a 1:1.2 molar ratio in

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deionized water adjusted to pH 8 with sodium hydroxide. Hydrogen peroxide, free chlorine and

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monochloramine stock solutions in deionized water were standardized using a Cary 60 UV-visible

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spectrophotometer at 254 nm (ε254nm = 18.6 M-1 cm-1),26

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245 nm (ε245nm = 445 M-1 cm-1),25 respectively. Sources for other reagents are provided in the

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Supporting Information.

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Experimental procedures: The UV absorbance of diluted oxidant stock solutions was measured at

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254 nm to determine their molar absorption coefficients. Because the pKa for HOCl is 7.5, diluted

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HOCl stock solutions were adjusted to pH 5 using hydrochloric acid or pH 10 using sodium

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hydroxide to evaluate ε254nm for HOCl and OCl-, respectively.

292 nm (ε292nm = 365 M-1 cm-1),22 and at

6


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UV irradiation was conducted using a semi-collimated beam apparatus with three 15 W Philips

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low pressure mercury lamps emitting at 254 nm, as described previously.27 Briefly, light shone down

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through a shutter onto an open-top 750 mL crystallization dish that was stirred by a magnetic stir bar.

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Incident irradiance (0.60 mW cm-2) was determined by iodide-iodate actinometry with the quantum

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yield (0.67) corrected for temperature (20±1 ºC).28, 29 In general, the direct photolysis rate of a

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compound (C) at 254 nm can be described by equation 10, assuming the compound is the only

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significant absorber of light: ௗ஼

− ௗ௧ =

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஍ூబ (ଵିଵ଴షഄ಴೗ )

[10]

௭

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where Ф is the photolysis quantum yield in mol/Einstein, I0 is the incident light intensity (mEin cm-2

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s-1), l is the light pathlength (cm), and z is the solution depth (cm). However, under conditions of

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minimal light absorbance (εCl < 0.02), and assuming a collimated beam (i.e., l = z), equation 10

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reduces to equation 11.

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−

ௗ஼ ௗ௧

= 2.303Φ‫ܫ‬଴ ߝ‫݇ = ܥ‬௢௕௦ ‫ܥ‬

[11]

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To determine direct photolysis quantum yields at 254 nm (Ф254nm), the depth of HOCl, OCl- or

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monochloramine solutions were reduced to satisfy the condition εCl < 0.02 for equation 11 and were

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exposed to UV light. Aliquots withdrawn over time were analyzed for total residual chlorine by the

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DPD colorimetric method.30 Plots of ln(C/C0) vs. t demonstrated first order decay over two half-lives

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(e.g., Figure S1). Combining the slopes of these plots (kobs) with the measured incident light intensity

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and molar absorption coefficients provided the quantum yields. 7



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Benzoate, nitrobenzene, and carbamazepine were analyzed by HPLC-UV. 1,4-Dioxane was

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extracted with 3 mL methyl tert-butyl-ether containing 300 µg/L 1,2-dibromopropane as the internal

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standard and analyzed by GC-MS (further details in the SI).

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Kinetic modeling: A kinetics model combining 88 elementary reactions obtained from the literature

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or estimated by analogy with similar reactions (Table S1) was implemented with Kintecus 4.55.31

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This model was based on our previous model,32 which was validated against experimental data from

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the UV/H2O2 AOP.32-34 The model was updated with recent modifications by Sun et al.15 and Fang et

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al.4 for the UV/free chlorine AOP. The model included reactions with chloride, which can occur in

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hypochlorite stock solutions or form as an end product of the photolysis of free chlorine and NH2Cl.

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Additional reactions added to refine the model are discussed below.

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RESULTS AND DISCUSSION

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Oxidant loss and radical production. Kinetic models for contaminant degradation must accurately

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1) predict loss of the oxidant serving as the radical source (i.e., H2O2, HOCl, OCl- and NH2Cl) and 2)

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differentiate radical production rates by direct oxidant photolysis from the subsequent radical

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reactions to characterize the competition between the target contaminants, oxidants, and other matrix

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components for reaction with these radicals. The tendency of oxidants to absorb photons (molar

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absorption

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quantum yields; Φ) are the key parameters needed to predict radical production (equation 11).

coefficients) and to fragment to radicals upon photon absorption (direct photolysis

8


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Quantum yields at 254 nm found in previous research ranged widely from ~1.0-1.5 for HOCl,4-6, 15, 22

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0.9-1.08 for OCl-,4, 15, 22 and 0.26-0.62 for NH2Cl.6, 10, 35, 36 However, these observed quantum yields

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(Φobs) incorporated reactions of the daughter radicals formed upon oxidant photolysis (•R; i.e., •OH

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and •Cl) with the oxidant (i.e., indirect photolysis), leading to its chain decomposition (equation 12).

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Unfortunately, Φobs is condition-specific, increasing with oxidant concentrations by enhancing chain

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decomposition.22

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݇௢௕௦ = 2.303Φ‫ܫ‬଴ ߝ + ݇∙ோ [∙ ܴ] = 2.303Φ௢௕௦ ‫ܫ‬଴ ߝ

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We measured the molar absorption

[12]

coefficients and quantum yields at 254 nm for HOCl, OCl-,

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and NH2Cl to compare with literature values for H2O2 (Table 1). Monochloramine features the

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highest ɛ254nm. The ε254nm for HOCl is comparable to that for OCl-, and similar to those reported

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previously6, 22; both are 3-fold higher than for H2O2.37

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Observed quantum yields (Φ obs) were determined in 2 mM phosphate buffer for loss of 2

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µM−250 µM NH2Cl (pH 7), 4-500 µM HOCl (pH 5) or 2-500 µM OCl- (pH 10) (Figure 1); the

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