Temperature Dependence of the Reaction between the Hydroxyl

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Temperature Dependence of the Reaction between the Hydroxyl Radical and Organic Matter Garrett McKay,† Mei Mei Dong,‡ Jonathan L. Kleinman,† Stephen P. Mezyk,† and Fernando L. Rosario-Ortiz*,‡ †

Department of Chemistry and Biochemistry, California State University at Long Beach, 1250 Bellflower Blvd., Long Beach, California 90840, United States ‡ Department of Civil, Environmental, and Architectural Engineering, 428 UCB, University of Colorado at Boulder, Boulder, Colorado 80309, United States

bS Supporting Information ABSTRACT: The temperature-dependent bimolecular rate constants for the reaction of the hydroxyl radical (HO•) with organic matter (OM) (kOM
HO•) have been measured for three natural organic matter (NOM) isolates and three bulk effluent organic matter (EfOM) samples using electron pulse radiolysis and thiocyanate competition kinetics. The range of values for the room temperature kOM
HO• was 1.21
9.37  108 MC
1s
1, with NOM isolates generally reacting slower than EfOM samples. The NOM isolates had an average apparent activation energy of 19.8 kJ mol
1, while the EfOM samples had an average value slightly lower (14.3 kJ mol
1), although one NOM isolate (Elliot Soil Humic Acid, 29.9 kJ mol
1) was a factor of 2 times greater than other samples studied. These apparent activation energies are the first determined for OM and HO•, and the Arrhenius plots obtained for NOM isolates (lowest R2 > 0.993) suggest that no significant structural changes are occurring over the temperature range 8
41 °C. In contrast, the greater scatter (lowest R2 > 0.903) observed for the EfOM samples suggests that some structural changes may be occurring. These results provide a deeper fundamental understanding of the reaction between OM and HO• and will be useful in quantifying HO• reactions in natural and engineered systems.

’ INTRODUCTION The reactions between the hydroxyl radical (HO•) and organic compounds in water are of great importance in natural and engineered systems. For example, HO• is one of the most important reactive species in the indirect photolytic decay of organic contaminants in natural systems.1
4 In these processes, HO• is formed via many photolytic pathways, including photolysis of nitrate and organic matter.5 The HO• radical is also the main reactive species in the application of advanced oxidation processes (AOPs) for treatment of water and wastewater.6
10 The source of HO• in these systems depends on the specific technology being used. AOPs that generate HO• radicals include O3/UV, O3/H2O2, TiO2/hν, ZnO/hν, H2O2/UV, pulsed UV, and Photo-Fentons, while other processes such as electron-beam irradiation, supercritical water, electrohydraulic cavitation, and sonolysis generate a mixture of hydroxyl radicals, hydrated electrons (e
aq) and hydrogen atoms (H•). The reactions between HO• and organic contaminants occur mainly by hydrogen abstraction or addition to an aromatic ring and are generally very fast (kHO• ∼ 108
1010 M
1 s
1).11 For low molecular weight organic molecules, the magnitude of the reaction rate constant is known to depend on the reaction mechanism, the molecule’s electronic properties, and steric effects.12 In natural and engineered systems one of the most important r 2011 American Chemical Society

reactions for HO• is its reaction with organic matter (OM), which results in significant scavenging of this radical. Historically, the focus of studies on the reactivity of OM with HO• have been focused on natural organic matter (NOM), although recent efforts have been made on effluent organic matter (EfOM). Brezonic and Fulkerson-Brekken reported values for the second order reaction rate constant with NOM (kNOM
HO•) ranging from 1.7
4.0  108 MC
1s
1 for five surface water sources.1 Goldstone and co-workers reported kNOM
HO• values of 3.24 and 2.28  108 MC
1s
1 for Suwannee River fulvic acid and Suwannee River humic acid, respectively.13 Westerhoff et al. examined multiple NOM isolates, yielding kNOM
HO• values ranging 1.39
2.18  108 MC
1s
1.14 Considering EfOM, Hoigne and Bader15 reported rate constants between EfOM and HO• (kEfOM-HO•) for two diluted, nonisolated samples to be 2.3
2.8  108 MC
1s
1. A recent report measured a kEfOM
HO• value of 4.1  108 MC
1s
1 for one EfOM sample.16 One additional study reported kEfOM
HO• for eight nonisolated EfOM samples ranging over 4.5
12.1  108 MC
1s
1.17 Received: April 21, 2011 Accepted: June 10, 2011 Revised: June 9, 2011 Published: June 10, 2011 6932

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Table 1. Water Quality Data for EfOM Samples DOC

alkalinity

NH3

sample

pH

(mgC/L)

(as mg CaCO3/L)

(mgN/L)

sample A

6.5

6.9

20

0.02

sample B

7.6

6.7

37

0.07

sample C

6.5

8.5

20

0.02

Overall, it is expected that kNOM
HO• and kEfOM
HO• represent an averaged value for the reaction rate constants between HO• and multiple OM sites. Since OM has a much higher average molecular weight (AMW) than simple organic molecules, some additional dependence on three-dimensional structure is expected.18 Though much research has been done on the reactivity of OM with HO•, there are no reported studies that assess the temperature dependence of these reactions. Therefore, the purpose of this study was to evaluate the temperature dependence of the reaction between HO• and both NOM and EfOM. The main value of measuring the temperature dependence of kOM
HO• is the ability to determine Arrhenius parameters, which allows calculation of rate constants at any given temperature. This would be valuable for both natural and engineered systems, for example, in estimating the scavenging of EfOM in AOP systems that deviate from room temperature, and for NOM in natural systems where the waters can be significantly below room temperature. To this end, three NOM isolates, including Elliot Soil humic acid (ESHA), Pony Lake fulvic acid (PLFA), Suwannee River standard fulvic acid I (SRFA), and three EfOM samples were examined in this work. These OMs were characterized by size exclusion chromatography (SEC). From the temperature-dependent kinetic data, apparent activation energies (Ea) and transition-state entropic parameters (ΔS‡) were calculated. This is the first report of these fundamental parameters for the reaction between HO• and OM, which increases the basic understanding of these reactions.

’ MATERIALS AND METHODS Chemicals. All NOM isolate standards were obtained from International Humic Substance Society (IHSS). Potassium thiocyanate (KSCN) was purchased from Aldrich Chemical Co. (St. Louis, MO) at the highest purity available, and used as received. Samples Studied. The three NOM samples were prepared by dissolving IHSS standards in 5 mM K2HPO4 buffer, each of which was adjusted to pH 7.0 ( 0.1 using HClO4 or NaOH (5 M). Three EfOM samples (samples A
C) were collected from a series of pilot membrane bioreactors operating at different retention times. Upon collection, these wastewater samples were filtered through 0.7 μm glass fiber filters to remove particulate matter and stored at 4 °C until analysis. Table 1 presents the concentration of carbonates, ammonia, and pH values for samples A
C (pH ranged from 6.5 to 7.6). Samples for kinetic studies were shipped overnight to the Radiation Laboratory, University of Notre Dame, in icechilled coolers and kept cold until analysis. Analytical Methods. Dissolved organic carbon (DOC) (minimum detection level (MDL) = 0.2 mgC/L) was measured using a TOC-VSCH (Shimadzu Corp., Japan) analyzer. Ammonia (NH3, MDL = 0.02 mgN/L) was measured with method TNT830 using Hach DR-5000 (Hach Corp.). Alkalinity and pH were determined via standard methods.19

OM Characterization. EfOM size characterization was performed using SEC with UV and DOC quantification. An Agilent 1200 LC system (Palo Alto, CA) with a Toyopearl HW-50 S 250  20 mm2 column (Grace, Rottenburg, Germany) was used with an injection volume of 2.0 mL. A diode array from Agilent was used as a detector (model 1200 Palo Alto, CA) monitoring at 254 nm. The mobile phase consisted of phosphate buffer (0.0024 M NaH2PO4, 0.0016 M Na2HPO4) and 0.025 M Na2SO4 adjusted to pH of 6.8 ( 0.1 for EfOM samples and pH 7.0 ( 0.1 for NOM samples. The flow rate was held at 1.0 mL/min. A modified commercially available Sievers-800 total organic carbon (TOC) analyzer (General Electric, CO) with 1.5 μL/min acid and oxidizer flow rates was used to monitor the DOC eluting from the SEC column. Temperature-Dependent Kinetic Experiments. The linear accelerator electron pulse radiolysis facility at the University of Notre Dame Radiation Laboratory was used for the quantification of the reaction rate constants. This irradiation and transient absorption detection system has been previously described in detail.20 Procedures for the reaction rate constant determination of OM samples have been presented previously.18 Briefly, the NOM and wastewater solutions were presaturated with N2O gas to isolate the HO• upon radiolysis. Standard dosimetry was performed using N2O-saturated, 1.00  10
2 M KSCN solutions at λ = 475 nm, (Gε = 5.2  10
4 m2 J
1) with average doses of 2
5 Gy per 3
4 ns pulse21 resulting in initial HO• concentrations of 1
3 μM. During kinetic measurements, solutions were continuously sparged with the minimum amount of N2O necessary to prevent air ingress, and experiments were carried out at four or five controlled temperatures (8
41 °C) for each sample in triplicate. The quoted errors for the reaction rate constants are a combination of the measurement precision and concentration errors.

’ RESULTS AND DISCUSSION Characterization of OM Samples. Elemental and selected functional group compositions for IHSS purchased NOM isolates are shown in Table SI-1 of the Supporting Information, while basic water quality data for the EfOM samples are shown in Table 1. Figure 1 shows the SEC for NOM isolates and Figure SI-2 of the Supporting Information shows the corresponding EfOM samples. The NOM isolates exhibited mainly a single peak with a Gaussian-type distribution while the SEC results for the EfOM samples were more heterogeneous. The EfOM samples had a peak at ∼30 kDa, which is characterized by a low absorbance and has been attributed to polysaccharides and organic colloids, both of which have low aromatic content.22 The amount of aliphatic and aromatic character of OM is of interest as Rosario-Ortiz et al. has shown a correlation between these parameters and kEfOM
HO•.17 Values for AMW were estimated from the SEC by integrating the curve from 100 to 18 000 Da (further details are discussed in the Supporting Information). In general, humic acids had higher AMWs than fulvic acids (see Table SI-2 of the Supporting Information). Rate Constant Measurements. The radiolysis of pure water or low concentration solutions produces a suite of radicals and other species according to the following stoichiometry:11

H2 O ' ½0:28HO• + ½0:06H• + ½0:27e
aq + ½0:05H2 + ½0:07H2 O2 + ½0:27H+ 6933

ð1Þ

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ARTICLE

whereas for wastewater samples the following solution reactions occurred (the levels of nitrite in the samples were below detection limit and hence no reactivity with HO• was included in the analysis): HO• + SCN
ð + SCN
Þ f ðSCNÞ•
2 kSCN
¼ 1:05  1010 M
1 s
1

HO• + EfOM f products

kEfOM
HO•

ð4Þ ð6Þ

HO• + HCO3
f H2 O + CO3
• kHCO3
¼ 8:5  106 M
1 s
1

ð7Þ

HO• + CO3 2
f HO
+ CO3
• kCO3 2
¼ 3:9  108 M
1 s
1

ð8Þ

HO• + NH3 f H2 O + • NH2 kNH3 ¼ 9:0  107 M
1 s
1

ð9Þ




Figure 1. SEC chromatograms for NOM isolates (see SI-1 of the Supporting Information for EfOM SECs). Lines represent the DOC (bottom) and UV254 (top) response of SRFA (475 μMC), ESHA (417 μMC), and PLFA (467 μMC) (units of molarity of carbon assuming 12.01 g mol
1). Conditions: buffer (2.4 mM NaH2PO4, 1.6 mM Na2HPO4, and 25 mM Na2SO4 adjusted to pH 7.0 ( 0.1), 1.0 mL/min flow rate, 2.0 μL/min acid and oxidizer flow rate.

where the bracketed numbers are G values (yields in μmol Gy
1) for each species produced. Prior to the kinetic experiments, samples were presaturated with N2 O in order to quantitatively convert the e
aq and H• radicals to HO•:11 •
e
aq + N2 O + H2 O f N2 + HO + OH

k2 ¼ 9:1  109 M
1 s
1 H• + N2 O f HO• + N2

k3 ¼ 2:1  106 M
1 s
1

ð2Þ ð3Þ

•

The rate constants for the reaction of HO with phosphate species are many orders of magnitude less than other reactions in the system 24 and were therefore ignored. For wastewater samples, the reactivity between HO• and other species, including NH3, HCO3
, and CO3 2
, was accounted for. Previous work has shown that kNOM
HO• values measured via thiocyanate competition kinetics are in excellent agreement with those measured directly via transient absorbance;14 therefore, kOM
HO• values for all samples in this study were measured by the competitive method. The overall HO• loss rate was due to multiple competing reactions. For NOM isolates, the standard competition was present: HO• + SCN
ð + SCN
Þ f ðSCNÞ•
2 kSCN
¼ 1:05  1010 M
1 s
1 HO• + NOM f products kNOM
HO•

ð4Þ ð5Þ

The SCN concentrations were varied between 40 and 230 μM by injecting known volumes of stock 0.100 M KSCN solution. The thiocyanate dimer radical (SCN)2•
has a strong absorbance at 475 nm, which was the basis for these measurements (see Figure 2a). Rearrangement of the standard competition kinetics equation yields the following expression:    1 1 1 kX ½X 1 ¼ + AbsðSCNÞ2 •
Abs°ðSCNÞ2 •
Abs°ðSCNÞ2 •
kSCN
½SCN
 ð10Þ where Abs°(SCN)2
• is the limiting (SCN)2
• absorbance in the absence of any OM, and Abs(SCN)2
• is the corresponding decreased absorbance in the presence of all competitors. In this equation, the reactions of OM, HCO3
, CO32
, and NH3 with HO• are summed together with the total pseudo-first-order contribution of these species assigned as kX[X]. Plotting 1/Abs(SCN)2
• against 1/[SCN
] yields a straight line, the slope being equal to kX[X]/(kSCN
 Abs°SCN
3 ), the y-intercept being equal to 1/Abso(SCN)2
•, and the quotient kX[X]/kSCN
obtained by the ratio of these two. To determine accurate Abs°(SCN)2
• values, data were collected for the full radical decay of the (SCN)2
• species (Figure 2a) and fitted by a mixed first- and second-order exponential decay in order to extrapolate to zero-time intensity. As shown in Figure 2a, the extrapolated value of Abs°(SCN)2
• increased with increasing [SCN
], which shows that SCN
in solution is accessible to HO• and suggests that it is not significantly complexed with OM. In addition, a correction was required for the dimer radical anion formation equilibrium at the low experimental SCN
concentrations used:25 SCN
+ SCN
h ðSCNÞ2
•

K ¼ 2:0  105

ð11Þ

Figure 2b presents the transformed competition
kinetic plot for wastewater sample C at 36.6 °C. From the ratio of the slope (2.25 ( 0.05)  10
3 to intercept (49.6 ( 0.5), a value of kX[X] = (7.06 ( 0.22)  105 s
1 is obtained. To determine the specific kOM
HO•[OM] value, the corresponding pseudofirst-order contributions from HO• scavenging by HCO3
(kHCO3
[HCO3] = 3.83  103 s
1), CO32
(kCO32
[CO32
] = 28.90 s
1), and NH3 (kNH3[NH3] = 1.54  102 s
1) were then 6934

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Table 2. Room Temperature Data for All OM Samples sample

Figure 2. (a) Decay kinetics of (SCN)2
• for N2O-saturated sample C at pH 6.5 and 36.6 °C. Symbols represent 45.85 μM (r), 93.45 μM ()), 140.30 μM (Δ), and 224.18 μM (0) added SCN
. Mixed order decays were fitted to each kinetic trace in order to obtain Abs (SCN)2
• values. (b) Transformed competition
kinetics plot for this sample based on extrapolated Abs(SCN)2
• values obtained from (a). Error bars correspond to one standard deviation from repeat value intensities. Solid line is a weighted linear fit, with a slope of (2.25 ( 0.05)  10
3 and an intercept of 49.6 ( 0.5 (R2 = 0.998), corresponding to kX[X] = 7.06 ( 0.22  105 s
1, determined as detailed in the text.

subtracted from the observed pseudo-first-order rate constant of 7.06  105 s
1 to yield a value of 7.02  105 s
1, which was then divided by the molar carbon concentration of OM (assuming 12.01 g of carbon per mole). This gave a final rate constant of kOM
HO• = 9.91  108 MC
1s
1 under these conditions. In addition, the temperature dependencies of reactions 4 and 6
9 are well described in the literature and quantitatively taken into account in our analysis. For a detailed description of these procedures please see the Supporting Information. The measured temperature-dependent kOM
HO• values were plotted according to the Arrhenius equation, which allowed for determination of the apparent activation energy: lnðkÞ ¼


Ea + lnðAÞ RT

ð12Þ

This relationship is well established for reactions of HO• with compounds having lower molecular weights than OM.12 However, since OM is relatively undefined compared to other chemical species, we use the term apparent activation energies to highlight the complexity of the reaction between OM and HO•. Based on the fitted pre-exponential factor terms, the change in entropy between the reactants and transition state (ΔS‡) can be

temperature (°C)

kOM
HO•  108 (MC
1s-1)

ESHA

24.4

1.21 ( 0.09

SRFA

20.6

2.06 ( 0.09

PLFA

20.5

6.90 ( 0.53

sample A

19.2

7.39 ( 0.06

sample B

19.4

9.37 ( 0.07

sample C

19.4

6.79 ( 0.11

Figure 3. Arrhenius plots for ESHA (0), SRFA ()), and PLFA (Δ) at pH 7.0 ( 0.1. Solid lines correspond to a weighted linear corresponding to activation energies of 29.93 ( 1.73, 14.42 ( 0.67, and 15.18 ( 0.49 kJ mol
1.

obtained via the following equation: A¼e

kB T ΔS‡ =R e h

ð13Þ

where kB is Boltzman’s constant and h is Planck’s constant. The usefulness of ΔS‡ is in its ability to compare the change in entropy due to reactant organization (necessary for a reaction to occur) and the change in entropy due to any dissagregation of OM that occurs at the transition state. Temperature Dependence for OM Isolates. Table SI-3 of the Supporting Information presents the temperature-dependent kinetic data measured for all OM samples in this study. Table 2 presents the kNOM
HO• values at room temperature (19.2
24.4 °C). The room temperature kNOM
HO• values varied markedly among the NOM isolates (1.21
6.90  108 MC
1s
1), which further confirms the finding that NOM reactivity is dependent on source and its intrinsic properties (i.e., distribution of functional groups and AMW) influence its reactivity toward HO•.15 The kNOM
HO• value measured for SRFA in this study (2.06 ( 0.09  108 MC
1s
1) is slightly higher than previously measured (1.60 ( 0.24  108 MC
1s
1) by Westerhoff et al.14 using the same methodology. Figure 3 presents Arrhenius plots for ESHA, SRFA, and PLFA. These samples exhibited excellent linearity (R2 > 0.993) in the range of temperatures studied, indicating that varying the temperature did not result in significant configurational changes to the structure of the NOM isolates. It has been hypothesized that the kOM
HO• values are impacted by the three-dimensional configuration of OM.18 For example, if not all carbon atoms are accessible to HO• due to the formation of a macrostructure, then the average kOM
HO• value for these units would be lowered. However, the lack of deviation from Arrhenius behavior indicates 6935

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Table 3. Arrhenius Activation Energies and Thermodynamic Data for Selected NOM and EfOM Samples sample

Ea (kJ mol
1)

ln(A)

ΔS‡ (J K
1 mol
1)a

NOM Isolates ESHA

29.93 ( 1.73

30.67 ( 0.69

1.21 ( 0.03

SRFA PLFA

14.42 ( 0.67 15.18 ( 0.49

25.04 ( 0.67 26.55 ( 0.49


4.43 ( 0.05
2.91 ( 0.02

EfOM

a

sample A

15.16 ( 1.32

26.61 ( 0.53


2.85 ( 0.06

sample B

10.71 ( 2.02

25.04 ( 0.82


4.42 ( 0.14

sample C

16.95 ( 1.67

27.33 ( 0.67


2.14 ( 0.05

ΔS‡ values calculated at 300 K (see eq 13).

that, as the temperature changed, no significant configurational changes occurred, which is consistent with current theories23 that suggest that the interactions between OM building blocks are quite strong. Any OM disaggregation, which would present more surface area per carbon available to HO•, would be expected to exhibit an increased (greater than Arrhenius) reactivity with increasing temperature. Table 3 presents the Arrhenius parameters for all NOM isolates. The apparent activation energies ranged 15.18
29.93 kJ mol
1, with PLFA (15.18 ( 0.49 kJ mol
1) and SRFA (14.42 ( 0.67 kJ mol
1) about a factor of 2 less than that of ESHA (29.93 ( 1.73 kJ mol
1). This difference was interesting considering that ESHA has the greatest aromatic carbon content, has the highest AMW of the NOM isolates, and is a humic acid while SRFA and PLFA are fulvic acids (which differ in oxidation state). If some disaggregation of ESHA occurs, perhaps it does so more than PLFA and SRFA, resulting in a greater apparent activation energy. However, any increased reactivity due to disaggregation did not result in a deviation from Arrhenius behavior. Temperature Dependence for EfOM. On average, the values for kEfOM
HO• were greater than kNOM
HO• (see Table SI-3 in the Supporting Information) consistent with previous findings.17,18 The obtained room temperature rate constants ranged 6.79
9.37  108 MC
1 s
1. The corresponding apparent activation energies for EfOM ranged from 10.71
16.95 kJ mol
1 (Arrhenius plots shown in Figure 4, values given in Table 3). In general, the values for the EfOM samples had larger standard deviations than for the NOM isolates, suggesting that EfOM could be disaggregating at higher temperatures (and aggregating at low temperatures), which might be expected when considering that EfOM is relatively more heterogeneous than NOM isolates (see SEC plots in Figure 1 and Figure SI-1 of the Supporting Information). Furthermore, the observed differences in apparent activation energies among all OM samples may simply be due to differences in the proton dissociation enthalpy of different humic and fulvic acids; further research is being done to investigate this possibility. To gain further insight into these temperature-dependent kOM
HO• values, the transition-state entropies (ΔS‡) were calculated at 300 K based on the fitted pre-exponential factors (see eq 13) (Table 3). Negative ΔS‡ values would be expected based on the more ordered transition state as compared to the reactants, and the values of this study for most of the OM samples ranged from
4.43 to
2.14 J mol
1 K
1. In contrast, the single positive value for ESHA (1.21 J mol
1 K
1) suggests that some

Figure 4. Arrhenius plots for wastewater samples A (0, blue), B (O, red), and C (Δ, orange). Samples A, B, and C pH values were 6.5, 7.6, and 6.5, respectively. Solid lines correspond to a weighted linear fit corresponding to activation energies of 15.16 ( 1.32, 10.71 ( 2.02, and 16.95 ( 1.67 kJ mol
1 for samples A, B, and C, respectively.

additional entropic increase occurs in the HO• oxidation. Although hypothetical, we attribute this additional increase to ESHA having a more aggregated structure than PLFA or SRFA. This could be due to the higher molecular weight and greater aromatic character of ESHA (see Table SI-1 of the Supporting Information). Assuming a similar transition state for all NOM reactions with HO•, the higher ESHA molecular weight, combined with strong intermolecular forces between aromatic moieties, might produce a more ordered reactants’ structure for ESHA therefore resulting in a greater ΔS‡ value. These results increase the fundamental understanding of the reactivity between HO• and OM and will be practically useful in engineered and natural systems that deviate from room temperature.

’ ASSOCIATED CONTENT

bS

Supporting Information. SEC characterization, AMW estimations, and full kinetic data for the temperature dependence. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: (303) 492-7607; e-mail: [email protected].

’ ACKNOWLEDGMENT The kinetic experiments were performed at the Radiation Laboratory, University of Notre Dame, which is supported by the Office of Basic Energy Sciences, U.S. Department of Energy. The authors gratefully acknowledge support from the U.S. National Science Foundation through grant no. 0926396 and through the Research Experience for Undergraduates program (project no. 1004122) at the University of Colorado, Boulder. ’ REFERENCES (1) Brezonik, P. L.; Fulkerson-Brekken, J. Nitrate-induced photolysis in natural waters: Controls on concentrations of hydroxyl radical photo-intermediates by natural scavenging agents. Environ. Sci. Technol. 1998, 32 (19), 3004–3010. (2) Brekken, J. F.; Brezonik, P. L. Indirect photolysis of acetochlor: Rate constant of a nitrate-mediated hydroxyl radical reaction. Chemosphere 1998, 36 (12), 2699–2704. 6936

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