Chemical Decontamination of Outdoor Pool Water using Oxone® and
the Impact of Nanoparticles from Personal Care Products
A thesis submitted to the
Department of Biomedical, Chemical and Environmental Engineering (DBCEE)
University of Cincinnati
In partial fulfillment of the requirement for the degree of
MASTER OF SCIENCE
2013
By
Lijuan Sang
M.S. and B.S., Environmental Science and Engineering
Nanjing Normal University, China 2009
Committee:
Dionysios D. Dionysiou, PhD (Chair)
Margaret J. Kupferle, PhD
Ting Lu, PhD
i
Abstract
Swimming and other water activities are available for people of all ages and are considered as one of the most complete forms of exercise, representing a great benefit to human health. However, the use of chlorine as disinfectant in swimming pools and spas has brought a growing concern of adverse health effects for bathers, due to the contact and exposure to the formed disinfection by-products (DBPs) by chlorine disinfectant and organic precursors present in the swimming pool water.
The active nanomaterial-based ingredients, which could be introduced into pool water through the use of personal care products by bathers, has received relatively little attention as potentially environmental contaminants and hazardous health risks from the environmental authorities. These active ingredients, such as zinc oxide (ZnO) or titanium dioxide (TiO2) nanoparticles, present in sunscreens formulations, are known to be photocatalytically active upon solar light irradiation. Little information on their transport and fate in swimming pool is known and they may not behave in a predictable way. They might interact with the disinfectant; organic compounds introduced by the bathers especially some nitrogen-containing compounds excreted by human body, or even the bathers in swimming pools.
The main objective of this study is to evaluate, for the first time, the impact of TiO2 nanoparticles from personal care products on the performance of Oxone®, a DuPont product with peroxymonosulfate as active component, in swimming pool water. Oxone® is considered as an alternative non-chlorine shock oxidizer used in pool water treatment.
ii
The decontamination of pool water containing creatinine, an organic model human metabolite contaminant present in swimming pools and spas, with Oxone® and
® Oxone /TiO2 (including control tests in the absence of the simulated solar light irradiation) was evaluated. The effect of solar irradiance on the overall performance was addressed. The study was focused on the fundamental aspects of the oxidation with the disinfectants tested as well as on the understanding of the effects of TiO2 nanoparticles in pool water chemistry. The outcomes of this study make significant contributions in environmental chemistry and environmental nanotechnology fields. Moreover, the findings from this study serve as an important basis for future studies on potential human health risks from personal care products.
iii iv
Acknowledgement
Firstly I would like to express my sincere gratitude to my adviser, Dr. Dionysios D.
Dionysiou, for his guidance throughout my entire period of graduate study. Also thanked are the other committee members, Dr. Margaret Kupferle and Dr. Ting Lu, for their advice and comments for the thesis.
Many thanks also go to Swimming Pool Foundation for providing financial support for this study. Special thanks are given to UC colleague Xuexiang He for her instruction and help with the experimental work. I would also like to say thank you to other UC colleagues, Miguel Pelaez, Changseok Han, Chun Zhao, Guanglong Liu, Geshan Zhang and Xiaodi Duan for their help.
Thanks to many of my friends and roommates at Cincinnati for their friendship. Last but not least, I would like to thank my family and my husband Qingshi Tu for their love and support for me to accomplish the degree.
v
Disclaimer
The names of the brand and company mentioned in this thesis are only provided as examples of relevant industries and thereof do not reflect any preference or recommendation. Any opinions expressed in this thesis are those of the author based on the experimental conditions used and results obtained.
vi
Table of Contents
Abstract II Acknowledgement V Disclaimer VI List of Figures VIII List of Tables IX Chapter 1 Problem Statement and Motivation 1 1.1 Statement of the problem 1 1.2 Focus of the study 1 1.3 Structure of thesis 2 1.4 Hypotheses and rationale 2 Chapter 2 Literature Review 4 2.1 DBPs by chlorine disinfection in swimming pool water 4 2.2 The use of Oxone® as non-chlorine oxidizer in pool water 14 2.3 Nanoparticles from sunscreen products in swimming pool 18 Chapter 3 Methodology 24 3.1 Chemicals 24 3.2 Solar simulator 24 3.3 Photo-reactor 27 3.4 Degradation experiments 28 3.5 Chemical analysis 29 Chapter 4 Results and Future Work 31 4.1 Oxone® self-depletion 31 4.2 Oxidation of creatinine by Oxone® 34 4.3 Impact of TiO2 nanoparticles 36 4.4 Conclusions and future work 38 References 40
vii
List of Figures
Figure 2.1 Potential chemical contaminants in pools 6 Figure 2.2 Chemical structures of certain N-containing precursors 8 Figure 2.3 Cobalt activation of peroxymonosulfate (PMS) 17 Figure 2.4 UV activation of peroxymonosulfate (PMS) 17 Figure 3.1 Typical spectral irradiance of Mercury/Xenon lamp, showing % of 25 total irradiance in specific UV, Vis and NiR spectral ranges Figure 3.2 Quartz glass photo-reactor 28 Figure 3.3 Set-up for the light irradiation experiment 28 Figure 4.1 Effect of initial concentrations (mg/L) on the stability of Oxone® 31 solution Figure 4.2 Effect of initial concentrations on the self-depletion rate of Oxone® 32 solution Figure 4.3 Effect of initial pH on the stability of Oxone® solution 33 Figure 4.4 pH variation of Oxone® solution 34 Figure 4.5 Degradation of creatinine by Oxone® at different concentrations 35 under the dark condition Figure 4.6 Effect of Oxone® concentration and solar light irradiance on the 36 photochemical degradation of creatinine ® Figure 4.7 Impact of TiO2 on the degradation of creatinine by Oxone as 1.0 37 − mM [HSO5 ] under the dark condition Figure 4.8 Impact of TiO2 nanoparticles on the photochemical degradation of 37 ® − creatinine by Oxone as 1.0 mM [HSO5 ]
viii
List of Tables
Table 2.1 Body fluid analogue formulation (Judd and Bullock, 2003) 7 Table 2.2 Concentrations of chlorine and DBPs found in swimming pool water 9 Table 2.3 Concentrations of DBPs found in the air phase above swimming pool water 13 Table 2.4 List and quantity (wt.%) of active agents allowed in sunscreen products 19 under the US Federal Regulations Table 3.1 Simulated solar light irradiance in this study 26 Table 3.2 Solar light irradiance (mW/cm2) of the Gulf of Mexico area 26 Table 3.3 Solar light irradiance (mW/cm2) of Prince William Sound, Alaska 27 Table 4.1 Depletion rates (mg/L/h) of Oxone® solutions 32
ix
Chapter 1
Problem Statement and Motivation
1.1 Statement of the problem
The removal of pathogenic microorganisms, minimization of disinfection by-products
(DBPs) formation and monitoring additional inputs are the three major challenges for swimming pool management, which may pose health threat on swimmers. Oxone®, an inorganic salt manufactured product from DuPont Co., as an environmental friendly shock oxidizer, has been used to improve pool water treatment. There is null information available on the relation between the disinfectants used for pool water treatment and TiO2 or ZnO nanoparticles present in sunscreen formulations with or without solar light radiation. To our knowledge, most related publications were about the organic contaminants from lotions, but few about the inorganic chemicals in them. Moreover, no other study has been previously published on the DBPs formation of creatinine with
® Oxone under solar light irradiation, or on the influence of TiO2 and ZnO nanoparticles that are released from sunscreens to pool water.
1.2 Focus of the study
Swimming pools usually involve continuous recirculation and introduction of organic materials from several output sources, as well as the exposure to solar light. Two main tasks were carried out during the research study: (1) to establish the performance of
Oxone® as a shock oxidizer in pool water under simulated solar light irradiation. The decontamination of pool water containing creatinine (C4H7N3O), a break-down product of creatine in muscle produced at a fairly constant rate by human body and present in 1 swimming pools and spas, with Oxone®, has been evaluated. The effect of solar irradiation on the overall performance of those processes has been addressed; (2) to investigate the effects of TiO2 nanoparticles from personal care products on the decontamination procedure in swimming pool water. These inorganic ingredients can interact with the disinfectant present, other organic compounds or even the bathers. The photochemistry of these active ingredients, such as ZnO or TiO2 nanoparticles, present in sunscreens formulations, is expected in illuminated (e.g., considering the sun light) swimming pool water. Little information is known on the effects of the inorganic ingredients introduced to pool water from bathers.
1.3 Structure of thesis
The main body of this thesis consists of four chapters. Chapter 1 presents the problem statement, research objectives and hypotheses. Chapter 2 provides background information about the selected topics in this thesis through a review of existing literature, based on which the specific areas to be explored are stated. Methodology is presented in
Chapter 3 for the experiment set-up and analysis procedure. Chapter 4 serves as the
Results and Conclusions for this thesis. Future work is also included.
1.4 Hypotheses and rationale
With the increase use of nanomaterials, TiO2 and ZnO nanomaterials in sunscreen formulations are expected to make their way into swimming pools. These nanomaterials are activated by solar light, especially in outdoor swimming pools; they are expected to drive photochemical reactions that will affect the chemistry of the disinfectant and the
2 pathways of disinfection byproduct formation. The interaction of active ingredients, such
® as TiO2 nanoparticles, introduced in swimming pools by bathers with Oxone and creatinine under dark and solar conditions will give us an insight on the fate of these nanomaterials, either active or with no effects. Accordingly to previous experiments, the degradation of creatinine by Oxone® under visible light was about 50% within two days, while under the dark condition the reduction was only 21% in seven days. This indicates that potential positive effects may be achieved with the use of whole range solar light.
3
Chapter 2
Literature Review
2.1 DBPs by chlorine disinfection in swimming pool water
Recreational water activity is available for people of all ages, providing a substantial benefit to human health and some advantages over land-based activities. It becomes one of the most popular leisure and exercise activities among the population in the United
States and other countries around the world. With the development of water treatment technology, the swimming pools make swimming a year-round activity [1, 2].
Given the range of contamination, a variety of disinfectants or disinfection systems is used to achieve suitable hygienic and chemical water quality, inactivate pathogens and other nuisance microorganisms in water and avoid disease transmission between bathers, including flocculation, filtration, adsorption to activated carbon, chlorine, ozone, UV, bromine, iodine, hydrogen peroxide, silver/copper, biguanide. Ozone or UV is often used in conjunction with residual disinfectants (chlorine or bromine) to get a lasting disinfection effect. In public pools, a quick biocidal effect is necessary to prevent pathogen contamination between bathers. Consequently, although there are a lot of studies trying to find alternative disinfectants, none is as effective as chlorine-based chemicals. Chlorine is the most widely used disinfectant in swimming pools, usually in the form of gaseous chlorine, sodium, calcium and lithium hypochlorite, or chlorinated isocyanurates, forming hypochlorous acid, the so-called free available chlorine (FAC).
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4
According to U. S. Environmental Protection Agency (EPA) regulation, to achieve a sufficient disinfection, the concentration of FAC residual in pool water must be kept with a minimum value of 1.0 mg/L and maintained around 1.0-4.0 mg/L, the levels of which are the same as those used in drinking water regulation [3], although it is assumed by most people that the chlorine levels in swimming pool water are much higher than that in chlorinated drinking water [4]. The minimum 1.0 mg/L FAC concentration could achieve the inactivation time for E. coli bacterium within 1 minute [3].
There are other chemicals beside of the disinfectants found in pool water and they could derive from the following sources as shown in Fig 2.1 [5]: the source water, the additions from bathers, and management-derived chemicals from filtration, coagulation treatment and pH adjustment. Natural organic matter (NOM) such as humic acid, and DBPs from previous treatment could exist in the source water from municipal drinking water supplies
[5]. Bather-derived substances that are introduced into pool water include body fluid such as sweat and urine, skin particles, hair, microorganisms, and personal care products such as sunscreen, cosmetics, soap residue, etc. Continuous disinfectants can react with this constant organic load in the swimming pool water to produce high levels of DBPs.
5
Figure 2.1. Potential chemical contaminants in pools, adapted from [5]
It was assumed that about 50 mL urine and 200 mL sweat could be excreted from an average bather per hour [8]. About 25-77.5 mL of urine and 20-1000 mL of sweat may be emitted per bather in recreational waters [11, 40]. Judd and Bullock (2003) formulated an analogue of body fluid (BFA) as a surrogate mixture and added into simulated model pool water after calculation of bather loading rates and body fluid excretion rate per bather [7-8]. The body fluid analogue (as shown in Table 2.1) contains the primary endogeous organic amino compounds, with total absolute and relative concentrations of carbon and nitrogen similar to those in the mixture of 1:4 urine and sweat. The total nitrogen content could reach around 1 g/L in sweat and 12 g/L in urine [5, 9]. Given the high nitrogen-containing precursors in the substances eluted from bathers, primarily in
6 the form of urea, ammonia, creatinine and amino acids, with the chemical structures shown in Fig 2.2, the nitrogenated DBPs were found very common in swimming pools with chlorine disinfection [5, 7, 8, 10].
Table 2.1. Body fluid analogue formulation (Judd and Bullock, 2003) [7-8] Constituents Concentration (mg/L) Urea 14800
NaH2PO4 4300 Ammonium chloride 2000 Creatinine 1800 Hippuric acid 1710 L-Histidine 1210 Citric acid 640 Uric acid 490
L-Arginine and L-Histidine and are amino acids usually found in sweat. Urea is the predominant component in urine (about 20 g/L) and sweat (about 1.5 g/L), and the major final product of protein metabolism (about 0.8-1.5 g urea/bather) [11]. It is the main nitrogen compound introduced into swimming pools (84% and 68% of the total nitrogen in urine and sweat, respectively) and the concentration of urea measured from 17 indoor pools [11] was reported between 0.14 and 3.7 mg/L (2-62 µM) with the mean concentration around 1.08 mg/L (18.0 µM) with standard deviation 11.7. Creatinine is excreted in urine and sweat as metabolic waste product of creatine, which exists in muscle tissue and blood. Li and Blatchley (2007) [12] applied a target concentration 1.8 ×
10-5 M of nitrogen-containing precursors such as creatinine, urea, L-Histidine and L-
Arginine for the study of DBPs in swimming pool.
7
Creatinine Urea L-Arginine L-Histidine
Fig 2.2. Chemical structures of certain N-containing precursors [12]
Predominant chlorine-derived DBPs include trihalomethanes (THMs), trichloramine, chlorate, chloropicrin (trichloronitromethane), cyanogen chloride, halogenated derivatives like halogenated acetic acids (HAAs), haloacetonitriles, chloral hydrate
(trichloroacetaldehydes), haloketones. From chlorine disinfection, more than 700 halogenated DBPs with uncertain toxicological effects have been identified [13]. Among these by-products, THMs such as chloroform and HAAs such as di- and trichloroacetic acid are generally produced in the greatest quantities [6]. They are the only regulated chlorine DBPs in drinking water beside of chlorite [13], with the maximum contaminant level (MCL) 0.080 mg/L for total THMs, 0.060 mg/L for total HAAs and 1.0 mg/L for chlorite [14].
Few studies, most recently, have investigated the chemistry or the health significance of real swimming pool water. A complete chemical characterization of DBPs from large public swimming pool in Spain was conducted including both water and air phases [4] by using gas chromatography/mass spectrometry (GC/MS), GC with electron capture
8 detection, photometry and ion chromatography methods. Five DBPs (chloroform, bromodichloromethane, dichloroacetonitrile, chloral hydrate, and 1,1,1- trichloropropanone) were found by the chlorination of the human origin materials (hair, lotion, saliva, skin, and urine) in a model swimming pool system under sufficient supply of chlorine residuals between 0.84 to 6.0 mg/L, with the sum of 84 µg/L DBPs produced from the mixture of human origins [10]. The formation of dissolved organic carbon
(DOC), total organic halide (TOX) and total trihalomethanes (tTHMs) was found with the maximum values of 3.4 mg/L, 329 µg/L and 125 µg/L, respectively, in a public outdoor swimming pool [103]. The detection of DBPs is likely to be affected by temperature. THMs were not detected in outdoor pools, possibly due to their volatilization during hot summer season [105]. A summary of detected chlorine DBPs concentrations in swimming pool from several previous studies is shown in Tables 2.2 and 2.3 [4, 7, 12, 15-21, 22-25, 26-39, 103]. More information about the DBPs that have been detected could be found in the WHO guidelines for safe recreational water environments [5].
Table 2.2 Concentrations of chlorine and DBPs found in swimming pool water
Chemicals Concentration Reference
Free chlorine 1.28±0.43 Large public swimming pool in
(mg/L as Cl2) Spain (2011) [4]
0.68-6.52 7 indoor and outdoor municipal
swimming facilities (2007) [12]
1.8 Model pool system with BFA (2003)
[7]
9
1.4-2.0 17 indoor pools (2011) [11]
Combined chlorine 0.25-1.76 [12]
(mg/L as Cl2) 0.21-1.0 [11]
Monochloramine 0.29±0.11 [4]
(NH2Cl) (mg/L) 0.05-0.4 as Cl2 [7]
0.06-0.8 as Cl2 [18]
Dichloramine 0.38±0.14 [4]
(NHCl2) (mg/L) 0.25-5 as Cl2 [7]
0.18-2.58 as Cl2 [18]
Trichloramine < 0.10 [4]
(NCl3) (mg/L) 0.07-0.16 as Cl2 [12]
Chloroform 15.4±3.5 [4]
(CHCl3) (µg/L) 70-140 [12]
13.7 Indoor swimming pool in Spain
(2009) [19]
45-212 8 indoor swimming pools in UK
(2002) [20]
85 (1994) [21]
9-179 12 indoor swimming pools in Italy
(1995) [17]
68-73 One indoor natatorium (1997) [22]
3.04-27.8 Indoor swimming pool (1995) [24]
10
18.4-24.0 Indoor and outdoor pools (1993)
[25]
<2-62.3 Indoor pool in Hungary (1998) [26]
145-151 Indoor pool in Denmark (1987) [27]
7.1-24.8 Indoor pool in Germany (2004) [28]
0.51-69 Indoor pool in Germany (1998) [29]
67.1-313 Pools and spas (1990) [34]
3.8-6.4 Hot spring pools (1997) [35]
Bromodichloromethane 14.2±4.2 [4]
(CHCl2Br) (µg/L) 2.5-23 [20]
0.69-5.64 [24]
<1.0-11.4 [26]
0.12-15 [29]
Chlorodibromomethane 12.8±4.4 [4]
(CHClBr2) (µg/L) 0.67-7 [20]
0.03-6.51 [24]
0.03-4.9 [29]
Total THMs 49.6±10.6 [4]
(tTHMs) (µg/L) 5-65 [7]
43-543 Swimming pool (1983) [15]
19.5-31.1 Indoor swimming pool in Korea
(1994) [16]
11
16 [19]
57-222.5 [20]
29.7 Indoor pool (1999) [23]
17.8-70.8 Indoor pool (2001) [36]
125 One public outdoor pool [103]
N, N-Dichloromethylamine 10 One lap and high-level competitive
(CH3NCl2) (µg/L) swimming pool (2007) [12]
Dichloroacetonitrile 0.01-0.03 [12]
(CNCHCl2) (mg/L) <0.5-18.2 (1994) [33]
<0.01-148 [29]
N-Nitrosodimethylamine 5.3-429 27 different public pools (2008) [13]
(C2H6N2O) (ng/L)
Monochloacetic acid 2.5-174 [30] (ClCH2COOH) (µg/L) Dichloacetic acid 1.8-562 [30] (Cl2CHCOOH) (µg/L) 5.6-119.9 (1992) [38]
419 (1998) [39]
Trichloacetic acid 1.1-887 (1996) [30] (Cl3CCOOH) (µg/L) 25-136 (1984) [31]
2.3-100 (1995) [32]
Total haloacetic acid 5.8-1634 Indoor, hydrotherapy and outdoor (tHAA) (µg/L) pools (1996) [30]
12
Table 2.3 Concentrations of DBPs found in the air phase above swimming pool water
Chemicals Concentration Reference
Trichloramine 0.29±0.10 [4]
3 (NCl3) (mg/m )
Chloramine 0.84 Indoor swimming pool (1995)
(mg/m3) [37]
Chloroform (CHCl3) 32.1±11.9 [4]
(µg/m3) 22 [19]
16-853 [17]
87 [21]
145 [22]
7.77-191 [24]
35-195 Indoor pools (1998) [44-46]
0.33-9.7 Outdoor pools (1998) [29]
Bromodichloromethane 14.9±4.5 [4]
(CHCl2Br) (µg/L) 5.49-22.4 [24]
16-24 [44-46]
0.08-2.0 [29]
Chlorodibromomethane 14.0±4.2 [4]
(CHClBr2) (µg/L) 0.53-2.91 [24]
9-14 [44-46]
0.02-0.5 [29]
13
Total THMs 72.1±20.7 [4]
(tTHMs) (µg/L) 28.0-33.6 [16]
142 [23]
58.0±22.1 [36]
The exposure routes, epidemiology and toxicity of chlorinated DBPs in drinking water and swimming pool water have been studies extensively. With the presence of sunlight in outdoor settings, these chlorinated DBPs can be volatilized or photolyzed [40] with an enhanced decomposition or transformation to other organic compounds. DBPs and their derivatives bring adverse effects to human health in swimming pools and spas through ingestion, dermal contact, or inhalation when some of them are volatile enough to the gas phase [2, 12, 41-43]. For instance, chloramine and other volatile chemicals, especially trichloramine, are respiratory irritants in swimming pools; swimmers have been associated with asthma and other respiratory effects. Chronic exposure to DBPs has been proved to increase the risk for bladder cancer [106].
2.2 The use of Oxone® as non-chlorine oxidizer in pool water
Equivalent disinfection efficiency to that of hypochlorous acid must be achieved by
® alternative disinfectants in the United States [107]. Oxone (2KHSO5.KHSO4.K2SO4,
− 681 g/mole, containing 43% KHSO5), essentially HSO5 and also known as peroxymonosulfate (PMS), is an inorganic triple salt manufactured product by DuPont
Co. and widely used as an oxidizer in a variety of industrial and consumer applications such as the paper and pulp industries. It has been successfully used in dark and
14 photochemical reactions such as olefin epoxydations, diverse sulfate oxidation reactions or the photobleaching of dye under visible light irradiation [47].
Nowadays, it is the most widely used chlorine-free oxidizer in swimming pools and spas in North America [7-10] and could help relieve concerns of the production of DBPs by chlorine disinfection. Oxone® is sold as the active ingredient in several non-chlorine oxidizing shock commercial products. Instead of a replacement for chlorine sanitizer, it works as a weekly shock oxidizer in conjunction with chlorine, with the usage around 1-2 pounds per 10,000 gallons of pool water. In this role, Oxone® oxidizes organic matters that are introduced by swimmers and the environment as stated above, the presence of which will cause pool water to become cloudy or dull. Without regular oxidation, the organic levels will continue to increase; sanitizing chemicals are potentially consumed faster than they are being supplied and less amount of disinfectant is available for disinfection. Thereby, Oxone® could help increase the sanitizing efficiency by chlorine or other sanitizers, decrease the demand for sanitizer, and resolve the common pool water problems such as cloudy or colored water, eye, nose or skin irritation and chlorine odor caused by combined chlorine [10]. There might be some allergic reaction [52] concerning the health effects of the application of PMS [52]. However, no adverse human health effects would occur if following the recommendation [53]. The use of Oxone® could help avoid negative health effects by chlorine disinfection in pool water.
Other than oxygen, hydrogen peroxide or “free chlorine”, the use of peroxymonosulfate has not been fully investigated for environmental applications [56]. Though its
15 characterization is still incomplete, potassium peroxymonosulfate (PMS, KHSO5, also called as monopersulfate), the active ingredient of Oxone®, has a higher standard electrode oxidation potential (1.85 V) as given by the following half cell reaction compared to chlorine (1.48 V in the form of hypochlorite) at room temperature [54, 55].
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Similarly to hydrogen peroxide, if used alone, Oxone® is not as an effective disinfectant in swimming pools and spas as active chlorine compounds [58]. From previous studies,
PMS could be activated by radiolysis, photolysis, or thermal activation, forming both sulfate and hydroxyl radicals [48]. PMS could also be activated by coupling with a transition metal such as Co2+ via electron transfer [57-60], forming sulfate radical as the major oxidizing species as shown in Fig 2.3 [60], which is in a similar manner to the
2+ traditional Fenton Reagent Fe /H2O2 [47]. After their formation, sulfate radicals could effectively react with organic molecules and contribute to their degradation by electron transfer, hydrogen abstraction, or addition mechanisms [61-63].
16
Fig 2.3. Cobalt activation of peroxymonosulfate (PMS) [60]
The activation of PMS under UV light irradiation, solely or in combination with transition metals, has been investigated to achieve higher rates for the treatment of recalcitrant organic contaminants in water. The use of UV light induces the formation of hydroxyl and sulfate radical species as shown in Fig 2.4 [59]. The combined UV/PMS and UV/Co(II)/PMS systems both led to complete degradation of 2,4-dichlorophenol within 1 hour. The exposure to sunlight in outdoor swimming pool is expected to activate the introduced Oxone® and affect the degradation behavior of contaminants by Oxone®.
Fig 2.4. UV activation of peroxymonosulfate (PMS) [59, 110]
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2.3 Nanoparticles from sunscreen products in swimming pool
Engineered nanomaterials (ENMs) have been used in many consumer products. The increasing production volumes of ENMs consequently induce an exposure of environmental systems such as soil, sediment, biological tissue, sewage water plant effluent or sludge. The first study that reported environmental concentrations of ENMs was the detection of TiO2 nanoparticles in the leachate from exterior facades [65].
Characterization and quantification of ENMs in complex environmental systems is still a nascent endeavor [66] and the estimation of their concentrations in some studies is substituted by modeling [67]. Based on a probabilistic material flow analysis approach, nano-TiO2 was modeled and its concentration was calculated to increase between 2008 and 2012 from 0.2 to 0.6 mg/kg in sediments (U.S.) and 0.1 to 0.5 mg/kg in sludge- treated soil [68].
Sunscreens are complex formulations, usually based on water in oil emulsion, applying several active ingredients, both UV light absorbing organic chemicals (such as PABA p- aminobenzoic acid, oxybenzone, octocrylene, salicylates, cinnamates, etc.) up to 10% by weight, and light reflecting and scattering inorganic chemicals, e.g., TiO2 and ZnO [69,
70], of which the maximum quantity under the US Federal Register could be 25% (wt.%) as shown in Table 2.4 [93, 94].
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Table 2.4 List and quantity (wt.%) of active agents allowed in sunscreen products under
the US Federal Regulations [94]
PABA (15 %) Homosalate (15 %) Octyl salicylate (5 %) Sulisobenzone (10 %)
Avobenzone (3 %) Menthyl anthranilate (5 %) Oxybenzone (6 %) Titanium dioxide (25 %)
Cinoxate (3 %) Octocrylene (10 %) Padimate-O (8 %) Trolamine salicylate (12 %)
Dioxybenzone (3 %) Octyl methoxycinnamate Phenylbenzimidazole Zinc oxide (25 %) (OMC; 7.5 %) sulfonic acid (4 %)
The ability of TiO2 and ZnO to absorb solar UV radiation finds their application in the
cosmetic field. They provide an advantage beyond organic chemical ingredients in
sunscreen products. It was found that organic chemicals from sunscreens might
contribute to the endocrine contamination and some of them might have estrogenic
activity [76, 78]. Of these inorganic ingredients in sunscreens, TiO2 is the most
commonly used though ZnO also has its own advantages [71, 72, 78]. Recent study
estimates that 70% of the use of TiO2 in sunscreen is nanometric TiO2 because of its
effectiveness on UV reflection and absorption, and its adequate tolerance by human skin
[73, 96, 108].
However, these semiconductor oxides have been reported to be active photocatalysts to
destroy organic compounds. Upon ultra band gap photoexcitation, they generate highly
- oxidizing radicals such as hydroxyl radical •OH and superoxide anion radical O2• and
1 other reactive oxygen species such as H2O2 and singlet oxygen O2, which are known to
be genotoxic and/or cytotoxic [71, 77, 79-81]. A study found that several types of 0.5 g/L
inorganic pigments in commercial sunscreen products-TiO2 and ZnO with different
coatings (rutile TiO2 coated with alumina and stearic acid; rutile TiO2 coated with 19 alumina and dimethicone; ZnO coated with dimethicone; TiO2 coated with 1,3-butanediol, benzoic acid, and alumina) might photocatalytically degrade 1.1 mM phenol with half life t1/2 of 8.83, 160, 0.82, 128 hours, respectively [81]. Their photocatalytic activity could lead to the degradation of organic coating 1,3-butanediol of TiO2 in sunscreen products under similar energy flux to sunbathing times [102].
Most of the ENMs are surface modified by using varied methods to obtain specific surface properties in the industrial final products. As stated above, TiO2 nanoparticles in sunscreens are normally rutile phase TiO2 surface coated with silica, alumina and/or various polymers to increase its stability in the lotion or cream, avoid the access of particle surface to the surrounding environment and the formation of reactive oxygen species through its photo-activity. For instance, from the transmission electron microscopy (TEM) micrograph of the widely used TiO2 nanocomposite in sunscreen-T-
TM Lite SF (BASF, Germany), the TiO2 core of nanometric lattice short sticks with an average size of 10x50 nm is coated with a layer of aluminum hydroxide Al(OH)3 and polydimethylsiloxane (PDMS), also called methicone/dimethicone copolymer.
Sunscreens go through rigorous assessment before commercialization for the concern of safety, especially skin penetration, skin tolerance, acute and chronic toxicity, phototoxicity and photogenotoxicity [96]. Nano-sized and microfine TiO2 and ZnO were suggested not to penetrating through skin, nor having any health risks to human beings
[72, 74-75]. Nevertheless, these studies are based on direct exposure, i.e., the application of sunscreen formulations directly on skin, without considering the alteration of these
20 nanomaterials when they are discharged into the environment during their life cycle.
Depending on the aging processes, these nanomaterials might exhibit different surface properties from the initial nanomaterials [97]. The environmental risks of ENMs on the environment and human health are greatly dependent on their surface properties [86].
TiO2 specimens extracted from ten commercial sunscreen products were found to inflict strand breaks and DNA damage in nuclei of human skin cells after being photo-excited
[93, 95].
For recreational water activities, especially outdoor water sports, many people prefer to apply sunscreens that resist washing off. Although most of commercially available sunscreen formulations claim to be waterproof, it is believed that many of them could incorporate water [82, 83]. In the USA, many “water resistant” products can only remain stable for 40 minutes when immersed in water and many day-care products could be readily washed off in 20 minutes. A highest concentration of 40 µg/L of five common organic ingredients in sunscreen was detected in baby pools and several of their halogenated derivatives were identified [104]. Some tests also showed the temperature effects on the stability of sunscreen ingredients [84].
Thereby, the residual nanoparticles tend to accumulate under constant recirculation [2]. It was estimated that about 1000 tons of nanoparticles, mainly TiO2 and ZnO, were produced globally for sunscreen products during 2003/2004 [74, 84]. This represents a growing concern considering that in the United States around 368 million people visit public pools each year [2]. The concentrations of chemicals released from cosmetics,
21 soap residue, suntan oil, etc in actual swimming pools is not known well [6]. The occurrence, size distribution and behavior of Ti in a typical swimming pool was first reported and the concentration of total Ti in the pool water samples was found between
20 and 60 µg/L by using inductively coupled plasma mass spectrometry (ICP-MS), which is higher than the concentration in the water supply of swimming pool-the tap water samples (28 µg/L). Much higher (301 ± 236 µg/L) concentration was found in the backwash samples from sand filter of the swimming pool [85, 101].
The aging and depletion of coating of TiO2 nanocomposites in sunscreen was investigated in recent studies over the past decade [87-92]. The hydrophobic coating layer undergoes rapid desorption in aqueous media and gets dispersed as stable colloid or settles. 90% weight of the total Si from the organic layer of of T-LiteTM SF nanocomposite is desorbed
[92] and the remaining PDMS layer is oxidized. The Al(OH)3 mineral layer is affected but preserved at the nanocomposite surface suggesting that the TiO2 nanocomposite remains inert from the photocatalytic point of view under continuous mixing in ultrapure water and simulated natural sunlight irradiation [87]. The remaining Al(OH)3 layer at the surface prevents possible chemical interactions between the Ti atoms and the outside O2 and/or H2O molecules, inhibiting the electron/hole pair and the generation of reactive oxygen species [92].
However, the concern still remains when considering the possible media that might favor
Al(OH)3 dissolution. Different results were found that when in contact with pool water
2+ - ingredients, especially Ca and OCl ions, the protective Al(OH)3 layer present in the
22 sunscreen lotion formulation is compromised and leached [88, 89]. The hydroxyl radical production was ca. 1.34 and 54.7 nM/min for the original and the damaged nanoparticles after aging in synthetic swimming pool water with 5 mg/L chlorine, which is an intermediate amount of chlorine between most (7 mg/L) and less (3.5 mg/L) effective chlorine dosage found in swimming pool water. Production of reactive oxygen species
(superoxide anion radical, hydroxyl radical, alkoxyl radical and singlet oxygen) was also found when the nine over-the-counter sun care products with declared containing TiO2 were irradiated under light mainly between 300 and 400 nm monitored by EPR spectroscopy [91].
23
Chapter 3
Methodology
3.1. Chemicals
Ammonium sulfate (NH4)2SO4 was purchased from Fisher. Creatinine (C4H7N3O, 98%) as well as Oxone® (manufactured by DuPont) were purchased from Sigma-Aldrich. The synthetic pool water (SPW) was prepared by dissolving 0.7695 g of CaSO4.2H2O (98%,
Sigma-Aldrich), 0.3 g of NaHCO3 (Fisher) in 2 L of Milli-Q water and the pH was adjusted to 7.5 using 0.02 N H2SO4 or 0.1 N NaOH [58]. The resulting qualities of SPW were 85 mg/L CaCO3 as total alkalinity and 240 mg/L calcium as hardness. Stock
® solutions of creatinine, Oxone , rutile phase TiO2 in Milli-Q water were prepared respectively in advance. Solutions containing TiO2 nanoparticles were homogenized in ultra-sound bath for at least 10 minutes before spiking into the reactor.
3.2. Solar simulator
A 200W-500W Xenon & Mercury (Xenon) Research Source (model: 66485 with 500
Watt Hg(Xe) lamp model 66142, Newport-Oriel Instruments, Stratford, CT, USA) was used as the simulated solar light source with an air mass 1.5 global type square filter
(Newport-Oriel Instruments, Stratford, CT, USA) fitted into the light source’s filter holder to match the total (direct and diffuse) spectrum when the sun is at a zenith angle of 48.2°.
The light irradiation output was a collimated beam with a diameter of 3.3 cm, delivering stable, low to medium power UV to NiR radiation (200 to 2500 nm transmittance range) as shown in Fig 3.1 (66142) [98]. The usage of full spectrum simulated solar light source
24 in this study could provide scientific information on activation energies under realistic conditions.
Fig 3.1. Typical spectral irradiance of Hg/Xe Lamp, showing % of total irradiance in
specific UV, VIS and NiR spectral ranges [98]
A compact and durable power and energy meter (P/N 7Z01500, Nova-Ophir, North
Logan, UT, USA) and a high sensitivity thermal sensor (3A-P, 12mm aperture, 150-6000 nm, 60 µW-3W) (P/N 7Z02622, Nova-Ophir, North Logan, UT, USA) were utilized to measure the light irradiance in the unit of mW/cm2. The final averaged solar irradiance reported here = light irradiance measured in the center of the photo-reactor × Petri factor
[99]. Where, Petri factor (considering the non-uniform light irradiation of the 3.3 cm diameter irradiation area), was obtained by drawing a 0.5 cm × 0.5 cm grid and measuring the light irradiance every 0.5 cm in the x and y directions at the same level of
25 the top of the solution in the photo-reactor. Place all of the readings of the light irradiance in the worksheet and the Petri factor for the irradiated area in this study was calculated as
0.7.
The light irradiance was adjusted by the watts value of the solar simulator and photo- reactor position. Different levels of simulated solar light irradiance (as shown in Table
3.1) were chosen based on the calculated monthly averaged and midday insolation incident data (mW/cm2, as shown in Table 3.2 and Table 3.3 [100]) of Gulf of Mexico area (around Latitude 28 N, Longitude 86 W) and the area of Prince William Sound,
Alaska (around Latitude 60.8 N, Longitude 146.9 W), from the NASA Atmospheric
Science Data Center to simulate the realistic conditions.
Table 3.1. Simulated solar light irradiance in this study
Solar Solar light irradiance Petri Averaged solar simulator at the center factor light irradiance (Watts) (mW/cm2) (mW/cm2) 200 65 0.7 45 300 130 0.7 91
Table 3.2. Solar light irradiance (mW/cm2) of the Gulf of Mexico area [100]
Lat 28 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Long -86
Average 13.5 17.0 21.8 27.1 30.1 28.4 27.2 24.9 22.0 19.5 15.4 12.6
Midday 48.0 57.0 70.0 82.0 87.0 81.0 77.0 74.0 68.0 64.0 53.0 46.0
26
Table 3.3. Solar light irradiance (mW/cm2) of Prince William Sound, Alaska [100] Lat 60.8 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Long -146.9
Average 1.3 3.8 8.7 14.8 19.9 21.7 18.9 15.4 10.1 5.3 2.0 0.7 Midday 6.0 15.0 28.0 42.0 48.0 49.0 45.0 40.0 31.0 20.0 10.0 4.0
3.3. Photo-reactor
The photo-reactor used in all the experiments was a customized quartz glass reactor with the inner diameter 6 cm, inner height 1 cm as shown in Fig 3.2. The reactor was completely sealed so the evaporation during light irradiation could be avoided. The photo-reactor was placed under the output of the simulated solar light source for the irradiation experiments as shown in Fig 3.3. The light irradiation could reach the reactant by passing through the photo-reactor made of quartz glass, which can be transparent, depending on quality, even to vacuum UV wavelengths (200-10 nm) whereas the ordinary glass is partially transparent to UVA (400-315 nm) but opaque to shorter wavelength. The solution of reactants was spiked into the reactor chamber and well mixed with a stirring bar during irradiation. A fan was applied on the side to remove the heat caused by light irradiation.
27
Fig 3.2. Quartz glass photo-reactor
Fig 3.3. Set-up for the light irradiation experiment
3.4. Degradation experiments
Specific aliquots of stock solutions were spiked to the reactor vessel to get the concentrations reported here. The reactor was filled with total initial volume of 28.3 mL synthetic pool water. Chemical oxidation experiments for SPW containing an initial concentration of 10 μM (or 1.13 mg/L) creatinine were conducted; the concentration level was close to the measured concentration of nitrogen-containing precursors in indoor
28 pools [11, 12]. A sensitivity analysis was performed on several parameters such as
® − Oxone concentrations (0.25, 0.50 and 1.0 mM as [HSO5 ]), solar light irradiances (45
2 and 91 mW/cm ) and the concentrations of TiO2 nanoparticles (10, 50, 100 and 200
μg/L). In addition to the comparison of the “dark” reaction, this study also compares the photolytic reaction. For the dark control experiment, 35 mL of solution with different scenarios were kept in borosilicate 40 mL VOA vials with PTFE septa cap, covered with aluminum foil and mixed with rotator labquake under the constant rotating speed for several days and samples were taken at certain time intervals for the concentration analysis of creatinine and Oxone®. For the photo-related experiments, the solutions were irradiated for several hours and samples were taken from the sampling port (GL-14 screw thread cap with septum) on the side of the photo-reactor by using a gas-tight syringe at certain time intervals for the chemical analysis.
3.5. Chemical analysis
150 mL sample was taken and injected into the filter vial with 0.45μm membrane in
PTFE if the solution is TiO2 related to avoid the block of HPLC column. The decay of creatinine was monitored by an Agilent 1100 Series liquid chromatography (LC) with a
Quat-Pump and a UV-Vis diode array detector (DAD, set at a wavelength= 205.6 nm).
The analysis was conducted under a isocratic conditions with a WATERS-C18 5 μm (3.9 mm x 150 mm) reversed phase column. The mobile phase was 100% of 0.045 M ammonium sulfate (NH4)2SO4 at a flow rate of 0.32 mL/min, with an injection volume of
20 μL. Post time was set to be 10 minutes and the stop time was 2 minutes. The thermostat temperature was 25 °C.
29
The solution of Oxone® was analyzed with an iodometric titration method to determine the variance of residual Oxone® with time under dark condition, to check the self- depletion rate of Oxone® and its stability [109]. Solution of Oxone® was titrated with 5 mN thiosulfate. Add solution of potassium iodide into the sample, add 2-3 mL starch indicator solution, and then immediately titrate with 5 mN sodium thiosulfate solution to a colorless endpoint that persists for at least 30 seconds. Several Oxone® solutions with known concentrations (5, 10, 20, 50, 100, 150, 200, 300, 400 mg/L) were titrated respectively to get the calibration curve. The volume of thiosulfate solution used was in linear relationship with the concentration of Oxone® with the slope 0.0339 and R square value of 0.9999 when the concentration of Oxone® was between 5-400 mg/L. Thereby, the unknown concentrations of Oxone® could be calculated by the linear relationship.
30
Chapter 4
Results and Future Work
4.1 Oxone® self-depletion
Oxone® is very stable in the solid state, however, like other peroxygens, it undergoes very slow disproportionation with oxygen gas and heat. The stability of Oxone® using synthetic pool water as solvent was studied. Concentration variations of Oxone® solutions with different initial concentrations (17, 85, 170, 255, 340 mg/L as Oxone® or
− 0.05, 0.25, 0.5, 0.75, 1.0 mM as [HSO5 ]) under dark condition are shown in Fig 4.1. The initial pH was adjusted to 7.5.
Oxone® self-depletion 350
300
17 250 85 200 170 150 255
Concentration (mg/L) Concentration 100 340
50
0 0 48 96 144 192 Time (hr)
Fig 4.1. Effect of initial concentrations (mg/L) on the stability of Oxone® solution
31
The calculated depletion rates (slope k, mg/L/h) are shown in Table 4.1 and a linear relationship was found between the depletion rates and the initial concentrations of
Oxone® in Fig 4.2.
Table 4.1. Depletion rates (mg/L/h) of Oxone® solutions
Initial Conc. of Oxone® Rate of depletion R2 (mg/L) (k, mg/L/h) 17 -0.0091 0.743 85 -0.047 0.985 170 -0.1171 0.992 225 -0.1659 0.965 340 -0.2339 0.946
Oxone® self-depletion
0.00 0 100 200 300 400 -0.05
-0.10
-0.15
-0.20 y = -0.0007x + 0.0064 R² = 0.9965 Rates of depletion ( k, mg/L/h) k, ( ofdepletion Rates -0.25 Concentration (mg/L)
Fig 4.2. Effect of initial concentrations on the self-depletion rate of Oxone® solution
32
Under low pH, the aqueous solution of Oxone® is relatively stable. The stability is affected adversely by higher pH. With the same initial concentration, its depletion rate increased with higher initial pH value as shown in Fig 4.3.
Oxone® self-depletion 180
y = -0.0136x + 165.71 R² = 0.119
160
170 (7.5) 140 y = -0.1171x + 167.49 R² = 0.9918 170(6.0)
y = -0.1871x + 140.52
Concentration (mg/L) Concentration 170(9.0) 120 R² = 0.8184
100 0 48 96 144 192 Time (hr) Fig 4.3. Effect of initial pH on the stability of Oxone® solution
The variation of pH was also checked. The Oxone® solutions in synthetic pool water were adjusted to initial pH value around 7.5. Under dark condition, the pH value kept relatively stable as shown in Fig 4.4.
33
pH variation 9.0 17 8.5 85 8.0
170
7.5 pH
255 7.0
340 6.5
6.0 0 48 96 144 192 Time (hr)
Fig 4.4. pH variation of Oxone® solution
4.2 Oxidation of creatinine by Oxone®
The oxidation of 10 μM creatinine by Oxone® with different initial concentrations (0.06,
− 0.1, 0.3, 0.5 and 1.0 mM as [HSO5 ]) was first investigated under the dark condition. The solution with only creatinine in SPW was stable and almost no change in creatinine concentration was observed within several days. Creatinine was oxidized faster with higher concentration of Oxone® as shown in Fig 4.5. 70% of creatinine was degraded
® − under 1.0 mM Oxone as [HSO5 ] after 211 hrs.
34
Degradation of cretinine by Oxone® under dark condition
10.0
M)
µ 8.0
(
6.0
no Oxone 4.0 0.06 mM as [HSO5]
Conc. of Creatinine of Conc. 0.1 mM as [HSO5] 0.3 mM as [HSO5] 2.0 0.5 mM as [HSO5] 1.0 mM as [HSO5] 0.0 0 50 100 150 200 Time (hr)
Fig 4.5. Degradation of creatinine by Oxone® at different concentrations
under the dark condition
The degradation of creatinine by Oxone® was also conducted under simulated solar light irradiation as shown in Fig 4.6. Comparing to the dark controls, the degradation rates of creatinine increased significantly, with more than 80% degradation under 1.0 mM
® − 2 Oxone as [HSO5 ] after 10 hrs. The photolysis of creatinine under 91 mW/cm solar light irradiation (without the presence of Oxone®) showed almost no degradation after 10 hours. The significant increase of the degradation rates under light irradiation was probably due to the activation of Oxone® which was obviously affected by the increase of light intensities or the concentration of Oxone®.
35
Degradation of creatinine by Oxone® under solar light irradiation
1
0.8 of creatinine (C/Co) ofcreatinine
0.6 Conc.
0.4
0.5 mM [HSO5] + 45 mW/cm2 light 0.2 1.0 mM [HSO5] + 45 mW/cm2 light 1.0 mM [HSO5] + 91 mW/cm2 light no Oxone + 91 mW/cm2 0 0 2 4 Time (hr) 6 8 10
Fig 4.6. Effect of Oxone® concentration and solar light irradiance on the photochemical
degradation of creatinine
4.3 Impact of TiO2 nanoparticles
The impact of TiO2 nanoparticles was investigated under both of dark condition and light
® irradiation (Oxone /TiO2 system). There was no significant difference observed with the
2 presence of TiO2 nanoparticles under dark conditions (Fig 4.7), or under 91 mW/cm solar light irradiation with the presence of different concentrations TiO2 nanoparticles
(Fig 4.8).
36
Impact of TiO2 under dark condition
10.0
M)
µ ( No Oxone 9.0 1.0 mM Oxone 10 ug/L TiO2 8.0 50 ug/L TiO2 100 ug/L TiO2
7.0 200 ug/L TiO2 Conc. ofcreatinine Conc.
6.0
5.0
4.0 0 24 48 72 96 120 Time (hr)
® − Fig 4.7. Impact of TiO2 on the degradation of creatinine by Oxone as 1.0 mM [HSO5 ] under the dark condition
Impact of TiO2 under light irradiation
1 Without TiO2 With 10 ug/L TiO2 0.8 With 100 ug/L TiO2 With 200 ug/L TiO2
0.6 of creatinine (C/Co) ofcreatinine
0.4 Conc. 0.2
0 0 2 4 Time (hr) 6 8 10
Fig 4.8. Impact of TiO2 nanoparticles on the photochemical degradation of creatinine ® − by Oxone as 1.0 mM [HSO5 ]
37
4.4 Conclusions and future work
Under dark condition, Oxone® solutions undergo self-decomposition and the stability of
Oxone® solutions decreases with increasing initial concentrations. The pH of Oxone® solutions remains relatively stable under dark condition after adjusting the initial value to
7.5.
In outdoor swimming pool water, the presence of solar light irradiation could activate
Oxone® for the decontamination of organic nitrogen-containing compounds introduced by bathers. The degradation rates of creatinine, a model compound in this study, increase significantly with the Oxone®/light system comparing to Oxone®/dark system. Without light irradiation, Oxone® itself could still degrade creatinine but with a much slower degradation rate under the previous experimental conditions. The photochemical degradation of creatinine (Oxone®/light system) is significantly affected by the solar light irradiance and the concentration of Oxone®.
No obvious difference on the degradation of creatinine occurs when the rutile phase TiO2
® nanoparticles are introduced with the Oxone /light/TiO2 system or with the different concentration of TiO2 nanoparticles, which is possibly due to the low concentration existed in swimming pool water. For the future work, to study any impact caused by TiO2 nanoparticles on the photochemical degradation of organic compounds by Oxone®, the difference on the production of reactive oxygen species, such as hydroxyl and sulfate radicals, could be quantified by electron paramagnetic resonance (EPR) spectroscopy
[79].
38
The degradation intermediates of pool water containing nitrogen-containing compounds by Oxone® should be determined for the comparison of chlorine disinfection procedure.
Considering the weekly application of Oxone® as shock oxidizer under realistic condition to remove not only the organics introduced by bathers and water source in swimming pools, but also the DBPs formed by chlorine disinfection, the photochemical activity of
Oxone® should also be studied on the degradation of chlorinated DBPs in the future work.
Transition metal ions like cobalt, manganese and nickel are particularly strong catalysts for the activation of Oxone® in solution; the degree to which catalysis occurs is dependent on the concentrations of Oxone® and the metal ions. It should be noted that the possible presence of trace amount of transition metal ions released from water supply pipes or the source water itself [110] might cause the activation of PMS in swimming pool. Previous studies found that even with the amount of less than 100 ppb of cobalt,
PMS could be activated and produce sulfate radicals [56, 59]. Detrimental effect was found because of the presence of transition metals in the solution during Kraft pulp bleaching process, the coupling of which with PMS was proven to be too strong with respect to oxidizing power [56, 64]. The possible presence of metal ions such as copper and iron in swimming pool water is expected and their effects on the activity of Oxone® should be investigated in the future work.
39
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