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The effect of irradiance and temperature on the role of photolysis in the removal of organic micropollutants under Antarctic conditions

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The effect of irradiance and temperature on the role of photolysis in the removal of organic micropollutants under Antarctic conditions

Abstract

Knowledge of the environmental fate of organic micropollutants in Antarctica is limited, especially with respect to photolysis. The Antarctic is characterised by extreme light conditions of either continuous sunshine or darkness depending on the season. The photolytic degradation of benzophenone-3 (BP-3), bisphenol A (BPA), 17 alpha-ethinylestradiol (EE2), methyl paraben (mParaben), 4-t-octylphenol (4-t-OP) and triclosan in MilliQ and seawater was investigated over a range of irradiance levels and temperatures. Photodegradation was compound specific. Up to 20% of BPA, BP-3 and EE2 was degraded over a 7-h irradiance period. Triclosan and 4-t-OP degraded to below the limit of detection in all experiments whereas mParaben was not degraded. The degradation of triclosan increased with irradiance in both MilliQ (P = 2.2 x 10(-16)) and seawater (P-2.2 x 10(-16)). The degradation of 4-t-OP increased with irradiance in MilliQ (P = 8.5 x 10(-9)) and seawater (P = 1.1 x 10(-5)), and with temperature in MilliQ (P = 8.5 x 10(-9)) and seawater (P = 1.0 x 10(-5)). Similar relationships could not be established for BPA, BP-3, EE2 and mParaben due to the limited extent of degradation observed. The photolysis of triclosan was enhanced 4-fold in seawater compared to MilliQ water. Results from this study indicate that micropollutants may persist for extended periods of time in Antarctic coastal waters, particularly with ice cover, above and beyond that exhibited in temperate seawater.
The effect of irradiance and temperature on the role
of photolysis in the removal of organic micropollutants
under Antarctic conditions
Philipp Emnet,
A
Rai S. Kookana,
B
Ali Shareef,
B
Sally Gaw,
A
,
D
Mike Williams,
B
Deborah Crittenden
A
and Grant L. Northcott
C
A
Department of Chemistry, University of Canterbury, Christchurch 8140, New Zealand.
B
CSIRO Water for a Healthy Country Flagship, PMB 2, Glen Osmond, SA 5064, Australia.
C
Northcott Research Consultants Limited, Hamilton 3200, New Zealand.
D
Corresponding author: sally.gaw@canterbury.ac.nz
Environmental context. Antarctica has several scientific research stations located along its coast, where they
discharge often untreated sewage containing organic micropollutants. Although degradation of these pollutants
by microorganisms is limited by the cold conditions, other pathways such as photodegradation may be
significant. Our results indicate that, during the summer, photolysis is a potentially significant degradation
pathway for organic micropollutants in Antarctic surface waters, although the rate of loss would depend on ice
cover and water depth.
Abstract. Knowledge of the environmental fate of organic micropollutants in Antarctica is limited, especially with
respect to photolysis. The Antarctic is characterised by extreme light conditions of either continuous sunshine or darkness
depending on the season. The photolytic degradation of benzophenone-3 (BP-3), bisphenol A (BPA), 17a-ethinylestradiol
(EE2), methyl paraben (mParaben), 4-t-octylphenol (4-t-OP) and triclosan in MilliQ and seawater was investigated over a
range of irradiance levels and temperatures. Photodegradation was compound specific. Up to 20 % of BPA, BP-3 and EE2
was degraded over a 7-h irradiance period. Triclosan and 4-t-OP degraded to below the limit of detection in all experiments
whereas mParaben was not degraded. The degradation of triclosan increased with irradiance in both MilliQ (P¼2.2 10
–16
)
and seawater (P¼2.2 10
–16
). The degradation of 4-t-OP increased with irradiance in MilliQ (P¼8.5 10
9
) and
seawater (P¼1.1 10
5
), and with temperature in MilliQ (P¼8.5 10
9
) and seawater (P¼1.0 10
5
). Similar
relationships could not be established for BPA, BP-3, EE2 and mParaben due to the limited extent of degradation observed.
The photolysis of triclosan was enhanced 4-fold in seawater compared to MilliQ water. Results from this study indicate
that micropollutants may persist for extended periods of time in Antarctic coastal waters, particularly with ice cover, above
and beyond that exhibited in temperate seawater.
Received 29 June 2013, accepted 4 October 2013, published online 25 October 2013
Introduction
Organic micropollutants are of increasing concern with respect
to environmental and human health because of their reported
physiological effects, including endocrine disruption.
[1]
The
main inputs of organic micropollutants into the environment
are the release of industrial and municipal wastewaters into
aquatic receiving waters.
[1]
The environmental fate and behav-
iour of organic micropollutants in aquatic systems in temperate
climatic regions is currently an area of intense research. In
comparison there is limited research on the fate and behaviour
of organic micropollutants in the Antarctic aquatic environ-
ment. Antarctica is hailed as one of the last places on earth
remaining relatively untouched by humans. However, most
Antarctic scientific research stations are located along the
coast, into which their often untreated sewage effluent is dis-
charged.
[2,3]
A recent survey of wastewater management prac-
tices at Antarctic research stations reported that 37 % of
permanent research stations and 69 % of summer stations lack
any kind of sewage treatment.
[4]
Wastewater treatment facilities
at many other stations regularly encounter operational problems
and malfunctions during summer or throughout the year as a
result of fluctuating wastewater inflow, frozen pipes or reduced
microbial activity because of low temperatures.
[4]
Photochemical degradation, or photolysis, is a potentially
significant removal mechanism for many organic micropollu-
tants within aquatic environments.
[5]
There are two forms of
photolysis that can occur; direct and indirect photolysis.
[6]
For
direct photolysis an organic micropollutant containing a chro-
mophore must absorb a photon, causing the compound to become
excited and degrade.
[6]
Indirect photolysis involves the reaction
of the micropollutant with a reactive species such as hydroxyl,
superoxide, or carbonate radicals. These species are mainly
produced by the photolysis of dissolved organic matter (DOM),
dissolved oxygen or dissolved nitrate ions.
[6]
Many organic
compounds that are not readily susceptible to direct photolysis
can degrade by indirect photolysis.
[7]
The concentrations of
nitrate and carbonate ions, as well as the type of organic matter,
have been shown to affect the photolysis of organic micropollu-
tants.
[8,9]
The kinetic energy (i.e. temperature) of the system is
potentially important as indirect photolysis requires a reaction
CSIRO PUBLISHING
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http://dx.doi.org/10.1071/EN12089
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Research Paper
between at least two chemical species. Changes in temperature
may therefore also change the degradation rates of compounds
that are highly susceptible to indirect photolysis processes.
A range of hydrophilic and hydrophobic organic micropol-
lutants can undergo photolysis, including pharmaceuticals,
[10]
bisphenol A (BPA),
[7]
natural and synthetic steroid hormones
(e.g. 17aethinylestradiol (EE2),
[11]
nonylphenol (NP),
[12]
4-tert-octylphenol (4-t-OP),
[13]
triclosan,
[14]
UV-filters,
[15]
phthalates
[16,17]
and paraben preservatives.
[18]
Various studies
have investigated photolysis at temperature and irradiance con-
ditions common to temperate climates, where they demonstrated
half-lives of hours to days.
[7,1215,19]
Despite their relatively fast
degradation the continuous release and replenishment of organic
micropollutants into aquatic ecosystems produces a state of
‘pseudo-persistence’. This can result in more significant effects
than would otherwise be predicted from chemical risk assess-
ments. In addition, very few studies have investigated the
photolytic degradation of organic pollutants in seawater, focus-
sing instead on freshwater systems such as rivers and lakes.
There are limited data for the photolytic degradation of
organic micropollutants in the Antarctic environment. Within
temperate regions, photolytic processes in aquatic systems are
significant degradation mechanisms for organic micropollu-
tants. Direct and indirect photolysis of organic micropollutants
sourced from personal care products released into Antarctic
coastal waters by sewage discharges are likely to be affected by
the extreme cold, semi-permanent ocean ice cover and radically
changing light conditions.
The temperature of ocean waters in the proximity of the US
Antarctic McMurdo Research Station ranges between 1.9 and
0.35 8C.
[20]
Levels of UV-B radiation increase during the
summer season because of the hole in the ozone layer.
[21]
The
22-year averaged monthly solar irradiance levels measured by
NASA at 778S, 1668E, the location of New Zealand’s Antarctic
Scott Base and USA’s McMurdo research stations, range
between 30 W m
2
in early spring to 735 W m
2
day
1
in mid-
summer (NASA database, NASA Earth Science Enterprise
(ESE) Program, see http://eosweb.larc.nasa.gov/cgi-bin/sse/
sse.cgi?þs01#s01, accessed 22 October 2013). During cloud
free skies the monthly averaged solar irradiance increases from
50 W m
2
day
1
in early spring to up to 1010 W m
2
day
1
in
mid-summer. It is completely dark over the Antarctic Winter
(May to July).
The photolysis experiments reported in the present study
represent a component of a larger project assessing the pre-
sence and fate of sewage derived organic micropollutants in
Antarctica. Preliminary results from the first monitoring season
identified residues of BPA, 2,4-dihydroxybenzophenone
(BP-1), 2-hydroxy-4-methoxybenzophenone (BP-3), ethyl
paraben (eParaben), 4-methyl-benzylidene camphor (4-MBC),
methyl paraben (mParaben), 4-t-OP, octyl-methoxycinnamate
(OMC), propyl paraben (pParaben) and triclosan in ocean water
samples collected from Erebus Bay, Antarctica. Six compounds
representing various classes and sources of emerging organic
micropollutants previously detected in aquatic environments
receiving sewage effluents were selected for investigation: BP-3
(UV-filter), BPA (industrial and plastic products), EE2 (phar-
maceutical), mParaben, (preservative), 4-t-OP (industrial use
and detergent metabolite) and triclosan (antimicrobial). The
objectives of this study were to:
(1) investigate the photolytic degradation of BPA, BP-3, EE2,
mParaben, 4-t-OP and triclosan in MilliQ water and
seawater to allow for comparisons to be made with predom-
inantly freshwater focussed studies,
(2) investigate the photolytic degradation of BPA, BP-3, EE2,
mParaben, 4-t-OP and triclosan over a range of tempera-
tures and irradiance levels in ultrapure (MilliQ) water and
seawater, and
(3) to predict how photolysis rates for these chemicals may
change under Antarctic climatic conditions.
Experimental
Materials
Solid standards of BPA (.99 % purity), EE2 (.98 %), triclosan
(97 %) and 4-t-OP (97 %) were purchased from Sigma–Aldrich
(Sydney, NSW, Australia), BP-3 (98 %) was purchased from
Wako Pure Chemical Industries, Inc. (Chuo-ku, Osaka, Japan)
and mParaben (97.7 %) from AccuStandard (New Haven, CT).
Individual stock solutions were prepared at a concentration of
1000 mg L
1
in methanol. High performance liquid chromatog-
raphy (HPLC) grade methanol, was purchased from Biolab
(Melbourne, Vic., Australia). Pesticide grade acetonitrile was
purchased from Optima. Formic acid (puriss p.a. ,98 %) was
purchased from Fluka. GF/C filter papers were purchased from
Whatman (Sydney). Working solutions of the target compounds
were prepared in ultrapure (18 MOcm
1
,pH6.0)MilliQwater
(Millipore, Billerica, MA, USA) and coastal seawater (Adelaide,
Australia, pH 7.8, total organic carbon 5.3mg L
1
). Coastal
seawater was filtered through a GF/C filter paper, followed by
filtration through 0.22-mm syringe filters (ChromTech, Apple
Valley, MN, USA, 25-mm filter size, nylon membrane) to
remove bacteria, before use in experiments.
Preparation of aqueous solutions for
photolysis experiments
Target compounds were combined into a mixed aqueous solu-
tion. The nominal concentrations of target compounds in the
mixed aqueous solution were 500 mgL
1
for mParaben, BPA,
BP-3, triclosan and 4-t-OP, and 1000 mgL
1
for EE2.
Aqueous solutions (3.5 L) were prepared by addition of
appropriate volumes of the mParaben, BPA, EE2, BP-3, triclo-
san and 4-t-OP stock solutions into empty 5-L Schott bottles.
Methanol had to be removed from the aqueous solutions as it has
been shown to affect photolysis at very low concentrations.
[10,22]
Methanol was evaporated from the Schott bottles under a gentle
stream of nitrogen and slight heating until dry. The bottles were
made up to volume with MilliQ or seawater, and sonicated for
45 min to ensure full dissolution of the chemicals. The solutions
were left to stand overnight, and subsequently stored at 4 8C until
and during use. The concentrations of target compounds in the
aqueous solutions were monitored to confirm their stability over
the duration of the experiments.
Photochemical experiments
Irradiation experiments were performed in a Suntest Solar
Simulator (Atlas Material Testing Technology, Chicago, IL,
USA) fitted with a 1500-W xenon lamp and filter to remove light
below 300 nm. A water bath was installed into the solar chamber
to provide improved control of exposure temperatures. Experi-
ments were conducted at varying temperatures and irradiance
levels (27, 21, 14 or 7 8C at either 330, 500 or 650 W m
2
).
The experimental temperature of 7 8C was achieved by lining
the solar chamber with ice, which was regularly replaced
during the course of the irradiation period.
P. Emnet et al.
418
Irradiance levels were monitored with a SLIK SBH-60
StellarNet Inc. Radiometer (Tampa, FL, USA). The tempera-
ture of the water bath was monitored with a HOBO Pendant
Temperature/Light Data logger (Onset, Bourne, MA, USA) and
remained within 28C of the desired temperature. Fifty-milli-
litre borosilicate glass beakers were used as reaction vessels,
and were wrapped in aluminium foil to ensure light entered the
beaker through the surface of the test solutions and not through
the sides of the beaker. Aliquots of test solution (40 mL) were
irradiated in triplicate along with a dark control (beaker with
test solution completely wrapped in aluminium foil). Subsam-
ples of one MilliQ water and one seawater beaker (1 mL) were
collected at 0, 30, 60, 90, 120, 180, 300 and 420 min. The
remaining replicate and dark control beakers were sampled at
0, 60, 180 and 420 min to reduce analysis time. Beakers were
weighed after each sample collection to correct for any volume
loss as a result of evaporation during the irradiation period.
Samples were stored in amber HPLC vials at 4 8Cuntilanalysis
(6 days at most).
HPLC analysis
Samples were analysed on an Agilent 1100 Series HPLC
(Santa Clara, CA, USA) fitted with a photodiode array
(PDA) detector, quaternary pump, mobile phase degasser and
autosampler. Aliquots of the test solutions were directly
injected (40 mL) onto an Alltima C18 reverse phase column
(250 4.6-mm internal diameter, 5 mm). Separation was carried
out using an isocratic solvent mix (40 % MilliQ water (0.1 %
formic acid) and 60 % acetonitrile) at ambient temperature and a
flow rate of 1 mL min
1
, with a total run time of 20 min
per sample. UV PDA detection was at 256 nm for mParaben,
228 nm for BPA, EE2, triclosan and OP and 277 nm for
BP-3. Retention times were as follows: mParaben (3.9 min),
BPA (4.7 min), EE2 (5.3 min), BP-3 (10.2 min), triclosan
(15.3 min) and 4-t-OP (18.1 min).
A six-point external standard calibration curve (0, 100,
200, 300, 400 and 500 mgL
1
) was freshly prepared from the
500 mgL
1
MilliQ solution for each analysis run for mParaben,
BPA, BP-3, triclosan and 4-t-OP. The six-point calibration
range for EE2 was 0 to 1000 mgL
1
. The calibration standards
were run before and after each batch of samples. The 300 mgL
1
standard and a MilliQ blank were analysed after every set of
12 samples to confirm the stability and reproducibility of
the calibration and assess if carryover occurred. Calibration
curves were linear (R
2
.0.9741) over the concentration
range. The intra-day variability of the calibration curve slopes
for all compounds was below 10 %, except for 4-t-OP, which
was 14.1 %.
Data evaluation
Rate constants (k) were determined from concentration (X) and
time (t) data by least-squares regressions fitted to the linearised
first-order rate equation in Microsoft Excel (2008 for Mac,
Version 12.1.0, Microsoft, Redmond, WA, USA).
Statistical analyses were conducted using R(Version 2.14.1
for Mac, see www.r-project.org). The complete non-averaged
triplicate data were used in the analyses. The irradiance and
temperature data obtained from the radiometer and temperature
logger measurements were averaged over the 7-h experimental
period and used for the data analysis instead of the nominally
defined irradiance and temperature settings. Differences in
photolysis rates between MilliQ water and seawater were tested
with a Welch Two Sample t-test. The effects of irradiance and
temperature on the degradation rate of the target analytes were
modelled using multilinear regression, assuming that the rate
constant (k) depends linearly on the total integrated irradiance
(I) and ln(k) depends inversely on temperature (T) as outlined in
the Supplementary material.
Results and discussion
Only triclosan and 4-t-OP exhibited significant degradation
under the experimental conditions, with the concentration of
both analytes in MilliQ and seawater decreasing below the
limits of detection over the 7-h irradiation period (Fig. 1). The
degradation rates for triclosan were greater than those calculated
for 4-t-OP (Figs 2,3). The calculated half-lives of triclosan and
4-t-OP for the three replicates at all 12 irradiance and temper-
ature conditions are provided in Table 1. The concentrations of
BPA, BP-3 and EE2 in MilliQ and seawater were observed to
decrease by as much as 20 % (depending on experimental
parameters). However the resulting degradation curves exhib-
ited poor regression fits, and the calculated half-lives were
highly variable with up to a 5-fold difference being observed
between the three replicates. In comparison the concentration of
mParaben remained stable over the exposure period with no
degradation occurring in either MilliQ or seawater. Because of
the limited degradation of BPA, BP-3, EE2 and mParaben, the
photochemical degradation of triclosan and 4-t-OP will be the
main focus of discussion in the following sections.
0
0.2
0.4
0.6
0.8
1.0
1.2
0 100 200 300 400 500
[triclosan]
t
/[triclosan]
0
Time (min)
0
0.2
0.4
0.6
0.8
1.0
1.2
0 100 200 300 400 500
[4-t-OP]
t
/[4-t-OP]
0
Fig. 1. Degradation of triclosan and 4-tert-octylphenol (4-t-OP) over the 7-h irradiance exposure period at 330 W m
2
and at 14 8C
(triclosan) and 27 8C (4-t-OP).
Photolysis of organic micropollutants
419
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
330 500
7C14C21C27C
650 330 500 650 330 500 650 330 500 650
MilliQ water
Seawater
Irradiance (W m2) and temperature (C)
Rate (min
1
)
Fig. 2. Average (standard deviation, n¼3) degradation rates (min
1
) of triclosan in MilliQ water
and seawater from the 12 environmental condition settings.
0
0.002
0.004
0.006
0.008
0.01
0.012
330 500 650 330 500 650 330 500 650 330 500 650
MilliQ water
Seawater
Rate (min
1
)
7C14C21C27C
Irradiance (W m2) and temperature (°C)
Fig. 3. Average (standard deviation, n¼3) degradation rates (min
1
)of4-tert-octylphenol in MilliQ
water and seawater from the 12 environmental condition settings.
Table 1. Mean photolysis half-lives (s.d., n53) for triclosan and 4-tert-octylphenol (4-t-OP) degradation
over the range of investigated irradiance and temperature conditions
Temperature MilliQ water Seawater
330 W m
2
500 W m
2
650 W m
2
330 W m
2
500 W m
2
650 W m
2
Triclosan
78C 3.0 (0.8) 1.7 (0.3) 2.2 (0.9) 0.8 (0.2) 0.5 (0.03) 0.4 (0.2)
14 8C 2.9 (1.5) 1.9 (0.2) 1.2 (0.4) 0.6 (0.2) 0.5 (0.1) 0.2 (0.1)
21 8C 2.8 (0.5) 2.2 (0.7) 1.6 (0.6) 0.6 (0.3) 0.4 (0.1) 0.2 (0.1)
27 8C 3.4 (1.1) 2.1 (0.4) 1.4 (0.4) 0.7 (0.2) 0.4 (0.1) 0.5 (0.1)
4-t-OP
78C 4.3 (1.0) 3.8 (0.6) 4.2 (0.3) 2.6 (0.7) 2.6 (0.5) 3.3 (0.5)
14 8C 3.7 (1.0) 2.8 (0.5) 2.5 (0.6) 2.5 (0.3) 2.3 (0.2) 2.0 (0.5)
21 8C 4.2 (0.4) 1.8 (0.3) 1.5 (0.3) 3.2 (1.2) 1.7 (0.1) 1.4 (0.2)
27 8C 3.4 (0.3) 1.4 (0.2) 1.4 (0.3) 2.2 (0.4) 1.6 (0.04) 1.5 (0.1)
P. Emnet et al.
420
Effect of irradiance and temperature on photolysis
Triclosan
The photodegradation rate of triclosan depended significant-
ly on irradiance in MilliQ water (P¼2.2 10
–16
) and seawater
(P¼2.2 10
–16
)(Fig. 2). Temperature did not affect the photo-
degradation rate of triclosan in either MilliQ water or seawater.
Irradiance and temperature were also tested for covariance but
did not interact with each other.
Previously published laboratory and field studies on the
photolytic degradation of triclosan have reported decreasing
degradation rates with decreasing irradiance.
[23]
The photolytic
degradation of triclosan decreased with increasing depth in lake
water (95 % reduction at 50 cm below the surface)
[24]
and
displayed large seasonal changes in response to seasonal
changes of sunlight intensity.
[25]
The temperature independence
we observed for triclosan photodegradation suggests direct
photolysis is the major degradation pathway, as this only
involves the absorption of photons, which does not depend on
the kinetic energy of the aqueous system. Previous studies
have similarly identified direct photolysis as the likely major
degradation mechanism for triclosan.
[2426]
The singlet excited
state of triclosan is considered to be most prone to photo-
degradation
[26]
and identified degradation products include
dichlorophenol,
[23,2628]
chlorophenol,
[23,27]
phenol
[23,27]
and
dioxins.
[14,23,26,27]
Cleavage of the ether bond is considered to
be the main photodegradation mechanism for triclosan.
[23,27]
Indirect photodegradation pathways for triclosan are thought to
play only a minimal role in its degradation within aquatic
ecosystems.
[26]
4-t-OP
The photodegradation rate of 4-t-OP increased with irradi-
ance in MilliQ water (P¼8.5 10
9
) and seawater (P¼
1.1 10
5
), and with temperature in MilliQ water (P¼8.5
10
9
) and seawater (P¼1.1 10
5
)(Fig. 3). Degradation rates
were not enhanced in seawater compared to MilliQ water and
irradiance and temperature effects were statistically unrelated.
Only one previous study investigating the effects of irradi-
ance on 4-t-OP degradation is available for comparison.
This study also reported decreased degradation of 4-t-OP at
decreased irradiance.
[29]
Reduced degradation of 4-t-OP and
nonylphenol with decreasing temperature has also been reported
in previous studies. For example, the photodegradation of
4-t-OP increased from 14 to 30 % as the temperature increased
from 15 to 25 8C over 8 h irradiation
[30]
and the photodegrada-
tion rate of nonylphenol increased from 11 % at 10 8Cto41%
at 25 8C over 10 h.
[31]
The TiO
2
-assisted photodegradation of
4-t-OP similarly increased over the experimental temperature
range of 30 to 60 8C.
[32]
A combination of direct and indirect photolysis pathways
have been proposed for 4-t-OP. The main degradation mecha-
nism of 4-t-OP proposed by Mazellier et al.
[33]
and Huang
et al.
[34]
involves the production of an 4-t-OP
radical on the
phenol ring through direct photolysis, which subsequently reacts
with dissolved oxygen to form 4-t-octylcatechol. According to
this mechanism the photolysis of 4-t-OP is highly dependent
upon the presence of oxygen. This mechanism was confirmed by
the enhanced degradation of 4-t-OP in oxygen-saturated water
compared with argon-flushed
[33]
or nitrogen-flushed
[34]
water,
with decreased generation of 4-t-octylcatechol observed in the
nitrogen-saturated water.
[34]
Continuous bubbling of oxygen
through water increased the photodegradation of 4-t-OP
compared to oxygen-saturated water.
[33]
The 4-t-OP radical
formed by photolysis has been observed to dimerise under high
initial concentrations of 4-t-OP (,9mgL
1
),
[34]
which were
,15 times higher than the initial 4-t-OP concentrations used in
the present study. Other reported photoproducts of 4-t-OP are
4-t-octylcatechol,
[29,33]
phenol,
[30]
1,4-dihydroxybenzene
[30]
and 1,4-benzoquinone.
[30]
Photolysis in seawater
The photodegradation of triclosan was significantly enhanced in
seawater compared to the MilliQ water (P¼6.6 10
–11
, Welch
Two Sample t-test), increasing between 3- to 4-fold. The
majority of previous experiments reporting the photolysis
behaviour of triclosan in fresh and seawater observed enhanced
photodegradation of triclosan in seawater compared to fresh-
water
[2426]
but one study reported enhanced photodegradation
of triclosan in fresh and seawaters compared to pure water.
[14]
The explanation proposed for the reduced photodegradation of
triclosan in freshwater compared to seawater is the absorption,
and therefore reduction, of light by the organic matter and the
potential for it to scavenge reaction intermediates.
[2426]
How-
ever, the presence of low concentrationsof DOM (#0.18 mg L
1
)
in aqueous solution has also been reported to enhance the deg-
radation of structurally related polychlorinated biphenyls
(PCBs) at low concentrations of DOM.
[35]
At higher con-
centrations, the light filtering effect became more dominant and
the photolysis of PCBs was reduced.
[35]
A more likely explanation is the effect of pH on the ionisation
state of triclosan and relative susceptibility to photolysis. The
UV spectrum of the ionised anionic form of triclosan (pK
a
¼8.1)
more readily absorbs light than the neutral form
[36]
and has been
shown to degrade 19 times faster than the neutral form.
[24]
Other
studies have also observed pH to be an influencing factor on
triclosan degradation.
[25]
In this study the pH of the MilliQ water
and seawater were ,5 and ,7.8. At pH 7.8 ,20 % of triclosan
will be present in the ionised form.
[36]
If ,20 % of the total
triclosan is present in the ionised form, which degrades
,19 times faster than the unionised form,
[24]
a 4-fold increase
in the photodegradation rate could be expected.
The photolysis of 4-t-OP was not enhanced in the seawater
(P¼0.0780, Welch Two Sample t-test). As observed for triclo-
san, previous studies of the photodegradation of 4-t-OP have
observed reduced degradation in natural waters.
[13,33]
This
reduction is speculated to result from the radical quenching
properties of DOM in rivers and lakes, even though DOM can
also provide a significant source of radical species.
[7,13,37]
DOM
(such as humic acids) are also postulated to attenuate the
incident irradiation penetrating freshwaters and reduce 4-t-OP
degradation rates.
[29]
However, enhanced instead of reduced
degradation has also been reported in natural waters for
4-t-OP
[30]
as well as nonylphenol.
[12]
Nitrates and Fe
3þ
have
been shown to enhance the photolysis of 4-t-OP and nonylphe-
nol as a result of their capacity to produce OH
radicals through
photolytic processes.
[31,34]
The presence of sulfates does not
affect the photolysis of 4-t-OP.
[34]
Bicarbonate, in contrast, was
shown to decrease the photolysis of 4-t-OP and nonylphenol
because of its capacity to increase the pH of solution.
[30,31,34]
Similar to triclosan the photolysis of 4-t-OP is enhanced at high
pH as the deprotonated phenol group shows a greater photolysis
potential than the protonated phenol group.
[34]
However, the pK
a
of 4-t-OP is 10.33
[34]
and at the pH of seawater used in this study
(pH ,7.8) only ,0.2 % of 4-t-OP is present in the deprotonated
form. The enhanced photolysis of deprotonated phenols will
Photolysis of organic micropollutants
421
only have a limited influence on the photodegradation of 4-t-OP
under the environmental conditions in this study.
Implications for Antarctica
In this study only two (triclosan and 4-t-OP) of the six com-
pounds tested showed rapid loss as a result of photolysis. The
photodegradation rate for triclosan was dependent on irradiance
whereas both temperature and irradiance influence the photo-
degradation rate for 4-t-OP. These results indicate that photo-
lysis is a potentially significant degradation pathway during
summer. Although quantitative estimates of photodegradation
rates and half-lives for triclosan and 4-t-OP under Antarctic
conditions (extreme cold and varying irradiance levels) were not
feasible, rates obtained for 7 8C and 330 W m
2
(Figs 2,3)
provide upper estimates of the photodegradation rates for tri-
closan and 4-t-OP that could occur at the surface of Antarctic
coastal waters. These photodegradation rates are expected to be
significantly reduced by the presence of sea ice as well as with
increasing water depths.
Coastal sea ice in Antarctica can reach up to 2 m thick and
remains present for much of the summer season.
[21]
The chemi-
cal and physical composition of sea ice results in it absorbing
and scattering a larger proportion of sunlight than pure ice,
[38,39]
preferentially allowing light in the blue-green spectrum
(450–550 nm) to pass through the ice.
[38,40]
Experimentally
measured under-ice irradiance levels have been determined to
be less than 1 % of that measured at the surface.
[38,39,41]
This
reduction in irradiance will reduce the photochemical degrada-
tion potential of organic micropollutants in seawater beneath sea
ice. Sea ice thickness decreases throughout the Antarctic sum-
mer from bottom melting, until the ice breaks away.
[42]
During
this period irradiance levels also increase as summer progresses
(NASA ESE Program, see http://eosweb.larc.nasa.gov/cgi-bin/
sse/sse.cgi?+s01#s01). These processes occur in parallel, with
the result that irradiance increases in the water column, until the
ice eventually breaks away in late summer. Photolysis processes
may become a significant removal mechanism for organic
micropollutants within Antarctic seawater some time before
the ice breaks away and for the duration of the ice-free period.
The photodegradation rates presented here are only applica-
ble to the surface of seawater, whereas micropollutants are
expected to mix to a much greater extent in the receiving
environment. Previous studies have shown that photodegrada-
tion rates of micropollutants can decrease with increasing water
depth. For example decreased degradation of nonylphenol with
increasing water column depth has been reported.
[12]
Factors
that determine the spectroscopic irradiance and light spectrum at
a specific water depth include the light intensity reaching the
surface, transmission through the air–water interface and the
composition and optical properties of the water.
[43]
A water
attenuation coefficient of 0.148 m
1
for light transmission
through ocean water in McMurdo Sound can be calculated
using data from Lesser et al.
[41]
These combined factors indicate that micropollutants con-
sidered to be readily photodegradable in more temperate envir-
onments may persist for longer under Antarctic conditions. The
current understanding relating to the fate and behaviour of
organic micropollutants derived from investigations in temper-
ate climates may have limited applicability when predicting
effects of micropollutants in the Antarctic environment.
Antarctic conditions, being unfavourable to biodegradation
and allowing only limited potential for photodegradation
(even for susceptible compounds like triclosan), are likely to
result in a greater persistence of micropollutants compared with
temperate environments. Clearly more work is required to
quantify the attenuation potential of micropollutants under
Antarctic conditions especially in the presence of mitigating
factors, such as ice cover at extremely low temperatures and at
depth in seawater.
Conclusions
The photolytic behaviour of the tested compounds was highly
compound specific. Triclosan and 4-t-OP were quickly photo-
degraded under the experimental conditions whereas limited
photolytic degradation was observed for BPA, EE2 and BP-3.
Methyl paraben was found to be photostable. The photolysis of
triclosan was highly dependent on irradiance in both MilliQ and
seawater, but was independent of temperature. The enhanced
degradation of triclosan in seawater compared to MilliQ water
was attributed to the increased degradation potential of the
anionic form of triclosan, which occurs at the higher solution pH
common in seawater. 4-t-OP degradation was significantly
affected by irradiance and temperature in both MilliQ and sea-
water, suggesting that it may follow complex photodegradation
dynamics in the environment.
Photodegradation is potentially an important degradation
process for micropollutants in the surface coastal waters of
Antarctica depending on the time of year (e.g. summer with no
ice cover). Our data indicate that micropollutants may persist
for extended periods of time in Antarctic coastal waters, parti-
cularly with ice cover, above and beyond that exhibited in
temperate sea water.
Acknowledgements
The authors thank CSIRO for providing the facilities for this study, and
Elena Moltchanova from the University of Canterbury for assistance
with statistical analysis. The authors also acknowledge the University of
Canterbury for providing financial support for this project following the
February 2011 earthquake disaster.
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Photolysis of organic micropollutants
423
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Chapter
Microbial production in the Southern Ocean is not limited to benthic and planktonic habitats. The annual sea-ice provides a growth substratum and refugium for a complex microbial community composed primarily of micro-algae, bacteria and protozoans. We assessed growth and development of the sea-ice microbial community (SIMCO) in McMurdo Sound and obtained evidence that light was a major limiting factor. A light perturbation experiment was set up on the annual sea-ice of McMurdo Sound near Cape Armitage during October-December 1981 in which 2 experimental quadrats of 100 m2 each were constructed. On 1 quadrat snow cover of 15–70 mm was maintained, while the adjacent quadrat received 0.7 m of snow to provide 2 different under-ice irradiances. Significant growth of ice algae occurred at irradiance of 0.2–2.9 μE m−2 s−1. Estimates of in situ algal and bacterial growth rates indicated doubling times of 7 and 14 d, respectively. The growth of heterotrophic ice bacteria appeared to be coupled to growth of ice algae. At least 20 × 106 kg new carbon per yr is contributed to McMurdo Sound by SIMCOs. We conclude that ecosystem models of Southern Ocean food webs must consider not only total C input but also the dynamics of primary and secondary production derived from sea-ice microbial communities.
Article
4-tert-octylphenol (OP), the endocrine disrupting compound, in aqueous solution was degraded by polychromatic UV lamp. It was found that the rate of OP depletion depends on light intensity, pH of the reaction mixture, initial compound concentration and the presence of natural water constituents. Nitrate addition seemed to give the best results in terms of OP decay rate. The time of 50% OP degradation in the presence of NO(3)(-) (2 mg l(-1)) was about 8 times shorter in comparison to degradation without any additives.
Article
Stratospheric ozone depletion over Antarctica is expected to continue for the next 50 years, with increases in ecologically damaging ultraviolet radiation (UVR: 290-400 nm), specifically the ultraviolet-B (UVB: 290-320 nm) portion of the spectrum. Most of coastal Antarctica is covered with 2-3 m of annual sea ice during the occurrence of the ''ozone hole.'' This physical barrier to UVR transmission has long been assumed to provide complete protection from the biologically damaging effects of UVR, especially for the planktonic developmental stages of the benthic invertebrate fauna. We found that short-wavelength UVB (down to 304 nm) is transmitted through the Austral spring annual ice of McMurdo Sound, and causes significant mortality and DNA damage in the embryos of the sea urchin Sterechinus neumayeri.Although mortality of sea urchin embryos has been reported for the open waters of the Antarctic, this is the first documentation of mortality and DNA damage for embryos under the annual sea ice. The degree of mortality and DNA damage was dependent on both year and depth, with higher mortality and DNA damage at 1 m depth below the ice compared to 3 m and 5 m. Greater DNA damage occurred in 2003 compared to 2002 despite the thicker annual ice (3.1 m vs. 2.5 m). Although the thickness of the annual ice was greater, the severity of the ozone hole, 230 Dobson units (DU) versus 320 DU, and the ratio of UVB to visible radiation was greater in 2003. Embryo and larval mortality from exposure to UVR under the annual ice should be considered as another abiotic factor potentially affecting the temporally episodic recruitment of invertebrates that occur in this benthic ecosystem.
Article
Spectral transmission data in the 400-1,000-nm range were obtained from about 60 sites beneath first-year sea ice near Point Barrow, Alaska. The amount of energy reaching the ocean depended strongly on the nature of the upper surface. Maximum transmission oc- curred in the 450-550-nm region, regardless of surface conditions or ice thickness. Initial results were influenced by the presence of interstitial algae in the lower part of the ice. The characteristic signature of thcsc algae was a secondary peak at about 540 nm. Results are generalized to provide estimates of the magnitude and composition of downwelling irradiance beneath the types of ice typically encountered in coastal portions of the Arctic Ocean. The penetration of solar radiation through the arctic ice pack is important to the existence of photosynthetic organisms in the Arctic Ocean and is significant in the regional heat and mass balance of the ice cover itself (Maykut and Untersteiner 1971). The amount of light entering the Arctic Ocean undergoes large spatial and temporal variations due not only to the high latitude but also to changes in the ice thickness and the character of its upper surface. Despite the critical role of the ice in determining primary production levels, direct observational data are sparse and only crude estimates are available on the ,optical properties of sea ice. Primary production in an ice-covered ,ocean is not strictly limited to the growth *of phytoplankton since the ice provides a supporting medium on which algae can .grow, a phenomenon first described by Ehrenberg ( 1841). Algae lining the bot- tom of the ice have been observed in both polar oceans. Bunt ( 1963)) Bunt and Wood ( 1963), and Bunt and Lee (1970) reported considerable growth in the brash layer below the 1-2-m-thick ice in Mc- Murdo Sound, Antarctica. Apollonio (1961) Sobscrved a brownish layer of algae at the bottom of sea ice near Devon Island in the Arctic Ocean. Clasby et al. (1972) have observed the spring buildup of a %cm-thick layer of algae (most commonly Nitzschia frigida Grunow) on the underside of sea- sonal sea ice near Point Barrow, Alaska. The bloom was characterized by intense growth during April and May which was abruptly terminated as the algae were de- tached from their support by the onset of bottom ablation in early June. The growth of similar species, although not nearly as vigorous as in the coastal regions, has also been reported beneath the thicker (34 m) multiycar ice in the central arctic (English 1961). An algal layer intercepts a substan- tial portion of the light passing through the ice cover-energy that would otherwise be available for primary production in the un- derlying water column.
Article
The sea ice in McMurdo Sound, Antarctica, is extensively utilized for runways, travel, freight hauling and docking facilities. The safety and efficiency of these operations depend upon a better understanding of the sea-ice bearing strength. Variations in shear and tensile strength, decreased thickness, salinity changes, internal deterioration and changes of the temperature gradient are all related to and dependent upon snow cover, ambient temperatures and solar radiation. During the austral summer of 1964–65, shear strength decreased from 9.8 kg./cm. ² in October to 6.3 kg./cm. ² in late January and then increased to 8.0 kg./cm ² . by 10 February. The salinity of collected brine decreased from 125 p.p.t. in November to 43 p.p.t. in January. Thickness increased until mid-December, then decreased rapidly by bottom melting until break-out in February. In the Cape Armitage shoal area, thickness decreased from 2.5 m. in mid-December to 36 cm. in late January. Snow cover significantly affects the degree of internal deterioration and the amount of strength loss during the summer. Sea ice with more than 6 cm. snow cover is consistently stronger than unprotected ice and deterioration is less. Bearing strength of the sea ice is sufficient for most ordinary loads throughout the period of most extensive use.
Article
Emerging contaminants (ECs) have been wildly distributed in the environment and attracted increasing attention over the past decades. In this paper, the contaminants including pharmaceuticals and personal care products, surfactants, and their degradation products, plasticizers, pesticides, and fire retardants are comprehensively reviewed. Their main categories, properties, followed by their occurrences and behavior (fate and transport) in natural and engineered systems are discussed. The fate of the nanomaterial in different environmental compartments as well as their effects on human health and other fauna are also presented in this paper. Furthermore, the emerging molecular biology techniques to enumerate microbes capable of degrading ECs are introduced. The study has clearly showed the presence of complex mixtures of ECs, having various origins, and raised concern about their potential interactive effects in the environment.
Article
Photocatalytic degradation of 4-(tert-octyl)phenol (4-OP) has been investigated by recirculating the aqueous solution through a packed bed reactor with TiO2. The first-order rate constant k for the degradation of 4-OP was evaluated to be 5.40×10−3min−1 and an activation energy of 18.6kJmol−1 was obtained. The rate constant k was not dependent of the flowrate but a decrease in total organic carbon (TOC) became smaller as the flowrate increased. Under the illumination for 6h at the flowrate of 28.5mlmin−1, 83.2% of 4-OP was degraded but 60.7% of the initial TOC was remained. Measurements of LC/MS using electrospray ionization revealed the formation of byproducts having molecular weights of 136, 178, 192, 220 and 222. Possible candidates for these byproducts were proposed. The degradation rate of 4-OP was remarkably accelerated by addition of K2S2O8: 4-OP was completely disappeared under irradiation of 4 or 2h, respectively, in the presence of 4×10−3 or 2×10−2moldm−3 K2S2O8. In the latter case, the TOC decreased to 34.6% by continuing the irradiation even after 4-OP was disappeared. In the presence of S2O82−, 4-OP was degraded without TiO2, which is attributable to the reaction by SO4− radicals. We demonstrated that the UV/TiO2/S2O82− system was more appropriate than the UV/S2O82− for decontamination of 4-OP in water.