Removing Steroids from Contaminated Waters ... - ACS Publications

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Chapter 9

Removing Steroids from Contaminated Waters Using Radical Reactions Stephen P. Mezyk,*,1 Edsel M. Abud,1 Katy L. Swancutt,1 Garrett McKay,1 and Dionysios D. Dionysiou2 1Department

of Chemistry and Biochemistry, California State University at Long Beach, 1250 Bellflower Blvd., Long Beach, CA 90840 2Department of Civil and Environmental Engineering, University of Cincinnati, 765 Baldwin Hall, Cincinnati, OH 45221 *[email protected]

Among the most important pharmaceutical contaminants in wastewater are the estrogenic steroids steroids. As quantitative removal of steroidal activity can be difficult, radical-based advanced oxidation/reduction processes (AO/RPs) are gaining interest for augmenting traditional water treatments. In support of the application of AO/RPs we have determined rate constants for the reactions of three representative steroids with the oxidizing hydroxyl (·OH) and sulfate (SO4-·) radicals. The fast ·OH rate constants for ethinylestradiol and estradiol suggest a common mechanism of initial radical addition to the common phenol ring, whereas the slower progesterone value is more consistent with hydrogen atom abstraction. The rate constants for SO4-· reaction with estradiol and progesterone are identical, indicating a common initial reaction, but the faster value for ethinylestradiol suggests significant initial oxidative reaction at the triple bond.

Introduction The adverse ecological impacts of endocrine-disrupting compounds, personal care products, antibiotics, and pesticides/herbicides in water supplies (1–3) and wastewater effluents are causing concern amongst regulatory groups and the public © 2010 American Chemical Society In Contaminants of Emerging Concern in the Environment: Ecological and Human Health Considerations; Halden, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


around the world. Traditional water treatment relies primarily upon adsorptive and chemical-physical processes to remove or transform these unwanted organic contaminants. However, nearly a decade ago, the U.S. Geological Survey (USGS) study first documented the occurrence of hormonally active chemicals, such as reproductive steroids, at trace concentrations in multiple water sources originating from residential, industrial, and agricultural environments throughout the United States (4). The relatively high frequency in which steroids were detected in this survey implied that these chemicals persisted throughout conventional water treatment methods, although their absolute concentrations did not exceed established drinking-water guidelines. The presence of synthetic and natural steroids such as estradiol, ethinylestradiol, and progesterone (Fig. 1) is attributed to human excretion as both natural and synthesized biological byproducts. However, human waste is not the only contributing factor in estrogen compound contamination; livestock have also been suggested as producers of similar steroid contaminants that pollute freshwater streams (5, 6). Concentrations of estrogenic compounds vary depending upon the season and location (7), but are nonetheless ever-present in the water cycle (8). Once in water, these steroid compounds interfere with normal endocrine function of various species of fish (9–13) and amphibians (14), as well a variety of invertebrates (15). Due to their negative environmental impact and the poorly-understood health concerns of human sensitivity to reproductive steroids, the removal of estrogens from water is necessary, and may warrant the use of special technologies. However, traditional treatment processes may not be sufficient, as quantitative removal of small (ng L-1) levels of steroids may be complicated by the presence of much higher levels of other water constituents such as dissolved organic matter (DOM) and carbonate. Moreover, the presence of DOM may further complicate removal strategies as these hydrophobic compounds may be preferentially adsorbed onto them. Therefore, the specific use of free radical species, such as the oxidizing hydroxyl radical (•OH) or reducing electron (eaq-), to degrade trace contaminants following standard water treatment processes could be a viable approach. These processes are generally referred to as advanced oxidation/reduction processes (AO/RPs) (16–20). Radicals can be created using a variety of techniques including a combination of O3/H2O2, O3/UV-C, H2O2/UV-C, UV irradiation of titanium dioxide, sonolysis, or the irradiation of water via electron beams or γ rays.

Figure 1. Structures (left to right) of estradiol, ethinylestradiol (EE2), and progesterone. 214 In Contaminants of Emerging Concern in the Environment: Ecological and Human Health Considerations; Halden, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Several studies have investigated the efficiency of radical-based oxidation to destroy estrogenic steroids (17, 21–24), and the results of this effort have encouraged further investigation for the large-scale implementation of AO/RPs for this purpose. It has been demonstrated that simple oxidation of the phenolic moiety of the steroidal backbone can decrease estrogenic activity by at least 13% (22). However, optimal quantitative removal through the use of these processes requires a thorough understanding of the redox chemistry occurring between free radicals and the chemicals of concern. This can be accomplished if absolute kinetic rate constants are determined for redox reactions occurring in the AO/RP systems. Unfortunately, there has only been a single quantitative measurement (25) for the reaction of the hydroxyl radical with ethinylestradiol (EE2) utilizing ozone competition kinetics, with a reported rate constant of 9.8 ± 1.2 x 109 M-1 s-1. This order-of-magnitude value is consistent with a subsequent attempt to predict these reaction rate constants, where Lee et al. estimated the overall rate oxidative constant for steroids to be in the range 109-1010 M-1s-1, based on the rate constants of •OH reacting with phenols, single carbon-hydrogen bonds, acetylene, and carbon-hydrogen bonds of aliphatic rings of various chemicals (22).

The direct measurement of absolute rate constants for estrogenic steroids (Reaction 1) reaction with oxidizing radicals is difficult due to their low aqueous solubility. In this work we describe our rate constant measurements for three representative estrogenic steroids (see Figure 1) with the oxidizing hydroxyl (•OH) radical and sulfate (SO4-•) radicals. These two oxidizing radicals were chosen for our study after taking into consideration that reducing AO/RP species, such as the hydrated electron (eaq-) and hydrogen atom (H•), will predominantly react with dissolved oxygen at the real-world concentrations of these steroids.

Experimental All chemicals used were purchased from Sigma-Aldrich Chemical Company at the highest purity available (hexafluoroacetone, >98%, steroids, >98%, KSCN, 99%, K2S2O8, 99%). All were used as received. There are many methods of producing radicals for AO/RP studies, but the use of an electron beam is optimal as it allows for the selective and quantitative production of •OH, hydrated electron (eaq-), and hydrogen atom (•H) from the direct decomposition of water (26):

The number preceding each species in Equation (2) is its absolute yield (G-value) in units of μmol J-1. The secondary reaction between the produced radicals and an added solute molecule typically occurs in microseconds, far faster than for hydrogen peroxide reaction, and so the latter oxidation does not interfere with our radical kinetic measurements. 215 In Contaminants of Emerging Concern in the Environment: Ecological and Human Health Considerations; Halden, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


All rate constant data were collected using the Linear Accelerator facilities at the Radiation Laboratory, University of Notre Dame. This irradiation and transient absorption detection system has been described in detail previously (27). Absolute radical concentrations (dosimetry) were based on the transient absorption of (SCN)2•- at 475 nm, using 10-2 M thiocyanate (KSCN) in N2O-saturated solution at natural pH with Gε = 5.2 x 10-4 m2 J-1 (28), performed daily. Steroid solutions were made in high quality, Millipore Milli-Q, charcoalfiltered (TOC 18.0 MΩ) water. To better mimic real-world AO/RP treatment conditions, these solutions were at natural water pH (measured as 6.8 – 7.2). However, under these conditions, the steroid solubility was not sufficient for its necessary concentration required for hydroxyl radical kinetic measurements (typically in the range 1-10 μM). To dissolve these steroids at this concentration, a small amount of co-solvent (hexafluoroacetone) was used. This perfluorinated species was chosen as its •OH radical reaction rate is very slow (k < 106 M-1 s-1) and therefore it does not interfere with the steroid oxidation reaction. No solubility problem occurred for our SO4-• measurements, as they were conducted in 1.0 – 2.0 M tert-butanol, which was used as a hydroxyl radical and hydrogen atom scavenger. Solution flow rates were adjusted so that each pulse irradiation was performed on a fresh sample, and multiple traces (5-15) were averaged to produce a single kinetic trace. Typically, 3-5 ns pulses of 8 MeV electrons generating radical concentrations of 1-5 µM per pulse were used in these experiments. All of these experiments were conducted at ambient room temperature (20 ± 2°C). Rate constant error limits reported here are the combination of experimental precision and compound purities.

Results and Discussion Hydroxyl Radical Measurements To minimize interference from the reducing species also produced by the pulsed electron irradiation, in these studies the •OH radical was isolated by pre-sparging our aerated water with N2O. At the relatively high (~25 mM) concentration of this gas, the following chemical conversions occur rapidly:

leading to quantitative conversion of the hydrated electron and some hydrogen atom to the •OH radical. The relatively low steroid concentrations (< 10 μM) used meant that no direct absorbance change upon its oxidation by the hydroxyl radical reaction (Reaction 1) was possible. Instead, we determined these rate constants using SCNcompetition kinetics (26). The reaction of the hydroxyl radical with thiocyanate in N2O-saturated solution is (26): 216 In Contaminants of Emerging Concern in the Environment: Ecological and Human Health Considerations; Halden, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


which occurs in competition with added steroid (such as EE2)

The transient (SCN)2-• species has a strong absorption whose maximum is at 475 nm. Upon addition of EE2 to a standard KSCN solution, Reactions (5) and (6) occur together, which lowers the total transient (SCN)2-• absorption intensity. The competition for the hydroxyl radicals follows the equation:

where Abso(SCN)2-• is the peak transient absorption measured for only the SCNsolution, and Abs(SCN)2-• is the reduced absorbance of the (SCN)2-• transient when EE2 is present. As this analysis depends upon the initial hydroxyl radical concentration being constant for all the measurements, low concentrations of SCN- (typically 30-40 μM) were deliberately used in this study, which minimized the impact of intra-spur scavenging of radicals (29). Typical data showing the reduction in (SCN)2-• absorbance are given in Figure 2. By taking the peak intensities of this transient absorption, and transforming them according to Equation (7), the competition-kinetics plot shown in Figure 3 is obtained. This plot shows an excellent straight line, with an intercept of unity and a slope corresponding to the rate constant ratio (k6/k5). At this temperature k5 = 1.1 x 1010 M-1 s-1, allowing calculation of k6 = (1.52 ± 0.23) x 1010 M-1 s-1. This value is slightly higher, but within combined experimental error, to the only previously determined rate constant of 9.8 ± 1.2 x 109 M-1 s-1 (25). This competition kinetics approach was also used for the other two estrogenic steroids of interest in this work, and the measured rate constants are summarized in Table 1. Within experimental error, the values for ethinylestradiol and estradiol are the same, indicating a common reaction mechanism. However, the measured value for progesterone is considerably slower, k = (8.5 ± 0.9) x 108 M-1 s-1. This suggests that the primary •OH oxidation occurs at the common phenolic ring in EE2 and estradiol (see Scheme 1), consistent with the hydroxyl radical oxidation of phenol (k = 0.7 – 1.8 x 1010 M-1 s-1) in aqueous solution (26, 30). For progesterone, which only has one C=C double bond, the slower rate constant instead suggests hydrogen atom abstraction from the saturated ring backbone.

217 In Contaminants of Emerging Concern in the Environment: Ecological and Human Health Considerations; Halden, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 2. Reduction of transient (SCN)2-• intensity at 475 nm for 33.50 μM SCNin N2O-saturated solution at natural pH and 22.7°C with zero ( ), 1.83 ( ), 3.80 ( ), 5.97 ( ) and 10.04 ( ) μM added EE2.

Figure 3. Transformed competition-kinetics plot for EE2. The weighted fit slope of this line corresponds to the ratio (k6/k5), which gives k6 = (1.52 ± 0.23) x 1010 M-1 s-1 (R2 = 0.998). 218 In Contaminants of Emerging Concern in the Environment: Ecological and Human Health Considerations; Halden, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

Table 1. Summary of determined rate constants for •OH and SO4-• oxidation of estrogenic steroids in aqueous solution


Estrogen M-1 s-1

M-1 s-1

Ethinylestradiol

(1.52 ± 0.23) x 1010

(3.01 ± 0.28) x 109

Estradiol

(1.15 ± 0.28) x 1010

(1.21 ± 0.16) x 109

(8.5 ± 0.9) x 108

(1.19 ± 0.16) x 109

Progesterone

Scheme 1. Suggested initial mechanism of hydroxyl radical oxidation of EE2 (and also estradiol).

Sulfate Radical Reactions Under real-world treatment conditions, the reactions of oxidizing radicals will be favored due to the relatively easy reduction of dissolved oxygen (concentration ~ 250 μM). However, one possibility is to deliberately add chemicals that can convert these reducing radicals to oxidizing ones. While saturation by N2O(g) is not feasible at large scale, adding persulfate (S2O82-) would allow the conversion of produced hydrated electrons to oxidizing sulfate radicals:

This allows the oxidation of estrogenic steroids (for example EE2) to occur, through



Figure 4. Decay of SO4-• radical at 450 nm for 38.3 ( ), 115.8 (intermediate solid line) and 200.0 μM (lowest solid line) added EE2. Solid line through data points corresponds to fitted first-order kinetics. Inset: Second order plot of first-order fitted values plotted against EE2 concentration. Solid line corresponds to second order rate constant, k9 = (3.01 ± 0.28) x 109 M-1 s-1. These kinetic measurements require the removal of hydroxyl radicals and hydrogen atoms in order to isolate the hydrated electron-persulfate reaction. Therefore, these experiments were conducted using a constant high concentration of tert-butanol, (CH3)3COH, as a co-solvent (1.0-2.0 M), which immediately scavenges the radiolytically produced hydroxyl radicals and hydrogen atoms (see Equations 10 and 11) to produce the relatively inert •CH2(CH3)2COH alcohol radical:



Scheme 2. Suggested initial mechanism of sulfate radical oxidation of EE2. The isolated hydrated electron will quantitatively react with added persulfate (in our experiments 5.0 mM) to give the oxidizing sulfate radical. This radical will slowly react with the added tert-butanol,

but a significant fraction will also react with added steroid. Under these relatively high alcohol concentrations, we were able to dissolve up to 200 μM of each of these steroids, allowing direct determination of the reaction kinetics. The sulfate radical has a broad absorption spectrum, with a maximum near 450 nm. The kinetics of the pure sulfate radical decay are shown in Figure 4. The initial spike corresponds to second-order SO4-• radical decay, but this is followed by a first-order component corresponding to this radical’s reaction with added steroid. By fitting the first-order tail with simple exponential kinetics, and then plotting these fitted values against the EE2 concentration (Figure 4, Inset) a second-order rate constant of k9 = (3.01 ± 0.28) x 109 M-1 s-1 is obtained. The sulfate radical oxidation is considerably slower than that for the hydroxyl radical, indicating different initial reaction mechanisms for these two oxidizing species. Following the same methodology, sulfate radical reaction rate constants were obtained for the other two estrogenic steroids. All rate constants are again summarized in Table 1. For both progesterone and estradiol the SO4-• rate constant (~1.2 x 109 M-1 s-1) was much slower than for EE2. While these kinetic data do not allow the specific mechanism of oxidation for progesterone and estradiol to be determined, the significantly faster rate constant for EE2 suggests that significant SO4-• oxidation occurs at the triple bond in this molecule (see Scheme 2). The data presented above provides fundamental information necessary to apply AO/RPs to treatment of aqueous waste streams containing estrogenic steroids. It appears that both hydroxyl radical and sulfate radical oxidation could result in, or help assist, the total removal of these chemicals. The kinetic data obtained in this study provides the fundamental information necessary to estimate the efficiency of using AO/RPs to remove estrogenic steroids from real-world waters containing high levels of dissolved organic matter 221 In Contaminants of Emerging Concern in the Environment: Ecological and Human Health Considerations; Halden, R.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


(DOM) and other hydroxyl radical scavengers through standard kinetic relative rates analyses. For example, in aerated wastewater containing 3.5 ppm DOM (290 μM DOM assuming 12 g C per mole C, k = 2.3 x 108 M-1 s-1 (31)), an assumed high level of 10 ppb ethinylestradiol (3.4 nM), and a pH of 8.0 and alkalinity of 100 mg/L (as CaCO3, giving ~1.0 mM HCO3-, k = 8.5 x 106 M-1 s-1 (26)), any hydrated electrons produced will be quantitatively scavenged by dissolved oxygen, and the hydroxyl radical reaction will be partitioned to DOM (88.63%), HCO3- (11.30%) and the steroid (0.07%). While some enhancement from sulfate radical reaction could also be achieved through high added persulfate levels, the low total efficiency of using these radicals to destroy this steroid under these conditions suggests that further optimization would be necessary.

Conclusions Rate constants for the reactions of oxidizing hydroxyl and sulfate radicals have been determined for three estrogenic steroids in water. The relatively fast and equivalent values for •OH reaction with estradiol, (1.15 ± 0.28) x 1010 M-1 s-1 and ethinylestradiol, (1.52 ± 0.23) x 1010 M-1 s-1, suggest a common mechanism believed to be initial addition to the phenol ring in these compounds. The much slower value for progesterone is consistent with this oxidation occurring by hydrogen atom abstraction from the steroid backbone. For the SO4-• oxidation kinetics, the faster rate constant for ethinylestradiol (3.01 ± 0.28) x 109 M-1 s-1, relative to the values obtained for estradiol and progesterone, k = 1.2 x 109 M-1 s-1, suggests significant sulfate radical reaction at the triple bond for the former. These kinetic and mechanistic data provide sufficient background information to calculate AO/RP efficiencies through standard competitive relative rates analyses for these radicals. The estimated efficiencies are low in real-waters, which suggests that additional AO/RP optimization would be required.

Acknowledgments Work was performed at the Radiation Laboratory, University of Notre Dame, which is supported by the Office of Basic Energy Sciences, U.S. Department of Energy. Some financial support for E.A. was also provided by the MARC-NIHNIGMS Grant T34-GM008074-22.

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Removing Steroids from Contaminated Waters ... - ACS Publications

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