PHOTODISSOLUTION OF COPPER FROM RESUSPENDED ESTUARINE SEDIMENTS
Alyssa M. Wetterauer
A Thesis Submitted to the
University of North Carolina Wilmington in Partial Fulfillment
of the Requirements for the Degree of
Master of Science
Center for Marine Science
University of North Carolina Wilmington
2012
Approved by
Advisory Committee
G. Brooks Avery
Larry B. Cahoon
Robert J. Kieber
Ralph N. Mead
Stephen A. Skrabal
Chair
Accepted by
Ron Vetter
______________________________
Digitally signed by Ron Vetter
DN: cn=Ron Vetter, o=UNCW, ou=Computer
Science, email=vetterr@uncw.edu, c=US
Date: 2013.06.12 20:08:08 -04'00'
Dean, Graduate School
This thesis has been prepared in the style and format
consistent with the journal
Marine Chemistry
ii
TABLE OF CONTENTS
ABSTRACT ................................................................................................................................... iv
ACKNOWLEDGEMENTS .............................................................................................................v
LIST OF TABLES ......................................................................................................................... vi
LIST OF FIGURES ...................................................................................................................... vii
INTRODUCTION ...........................................................................................................................1
METHODS ......................................................................................................................................3
Sampling ..............................................................................................................................3
Light and dark resuspension experiments ............................................................................3
Sediment and water characterization ...................................................................................4
Readsorption of Cu ..............................................................................................................4
Effects of TSS on Cu photorelease ......................................................................................5
Long term irradiation experiments.......................................................................................5
Sample preparation for anodic stripping voltammetry (ASV) analysis ...............................5
Electrode construction .........................................................................................................6
Analytical- Total dissolved Cu (TDCu) ...............................................................................6
Analytical- Resuspension characterization ..........................................................................7
Statistical treatment ..............................................................................................................8
RESULTS & DISCUSSION............................................................................................................9
IMPLICATIONS ...........................................................................................................................16
REFERENCES ..............................................................................................................................34
iii
ABSTRACT
Photolysis of resuspended aquatic sediments from coastal areas produces dissolved
organic carbon (DOC), nitrogen, and Cu. Estuarine and coastal sediments from varying
biogeochemical settings were collected and photolyzed with simulated sunlight to determine the
extent of photolytic dissolved Cu release. Significant photorelease was observed in sediments
from10 of 13 sites. The effect of total suspended solids (TSS) on Cu photodissolution was tested
using sediment from Bradley Creek, NC. TSS loads ranged from 38-318 mg/L and suspensions
were exposed to simulated sunlight for 6 hrs. Cu was photoproduced in all suspensions (18.799.5 nmol/g dry sed.); the highest net production occurred at the lowest TSS concentration
implying that particle shading or competitive photoreactions occur. Three sites varying in
anthropogenic impact were selected for 12 h and one 24 h photolysis. All three sediments
followed the same trend in that as sunlight exposure increased so did the concentrations of DOC
and Cu. In sediments from all three sites, no significant photoproduction occurred until 4 h of
irradiation had passed suggesting that a protective biofilm or other organic coatings needed to be
photodegraded before Cu release occurred or a critical concentration of photoproduced radicals
needed to be reached. A time course experiment established that photoreleased Cu remained in
the dissolved phase for up to 24 h after irradiation. Photolytic fluxes of Cu from resuspended
sediments are comparable to benthic and riverine fluxes and may be a significant episodic source
to coastal ecosystems. Photoproduced Cu may demonstrate a photoprotective effect of CDOM
by providing a sink for produced superoxide radicals.
iv
ACKNOWLEDGEMENTS
First and foremost I would to extend my gratitude to all of MARCL for being great
friends and researchers over the past two years. My graduate experience would not have been as
memorable. I would like to thank my committee, Dr. Kieber, Dr. Mead, Dr. Avery, Dr. Cahoon,
and Dr. Skrabal for the guidance and support throughout my project. I would especially like to
thank my advisor Dr. Skrabal for always being available to answer questions and encouraging
when research wasn’t behaving. I know we were missing something simple on Pb…
I’d like to thank my friends Nikki Byrd and Matt McCarthy for keeping me sane during
tough classes and making me take a break from research every now and then to relax and have a
couple of laughs. I’d like to thank Donna Glinski for being a mentor and friend in lab especially
through long photolysis days. A special thanks goes to Wil McBurney my incredible boyfriend.
Thank you for being there on both the good and bad days and always knowing what to say to
make me smile.
Lastly, I’d like to thank my family and friends from home. Without the support and love
from all of you, I’d still be waiting to take a chance on my dreams. I’d like to thank my sisters
for the frequent visits and sharing life through phone calls. I’d like to thank my grandparents Jay
and Kay Sharp for always making each of their grandchildren a priority. Last of all, my parents
deserve more than words can express. Thank you from the depths of my heart for the love and
encouragement that you have generously provided throughout my life. I would not be the person
I am today without you both in my life.
Financial support for this research was support by the National Science Foundation
Chemical Oceanography program (OCE0825538).
v
LIST OF TABLES
Table
Page
1. Site abbreviation, TSS, net photorelease of Cu, % organic carbon, leachable Cu, and
leachable Fe .............................................................................................................................18
2. Site abbreviation, TSS, irradiation time, net photorelease of Cu, % organic carbon, leachable
Cu, and leachable Fe ................................................................................................................19
vi
LIST OF FIGURES
Figure
Page
1. Map of study sites ....................................................................................................................23
2. Photorelease of Cu for all study sites .......................................................................................24
3. Photorealease of DOC for all study sites .................................................................................25
4. Net photoreleased Cu vs. leachable Cu....................................................................................26
5. a. Effect of TSS on Cu photorelease ......................................................................................27
b. Net photorelease DOC vs. net photorelease Cu with respect to TSS
6. Effect of readsorption on net Cu photorelease .........................................................................28
7. Jacob’s Creek 12 h irradiation .................................................................................................29
a. Photorelease of Cu
b. Net photorelease Cu vs. time
c. Photorelease of DOC
d. Net photorelease DOC vs. net photorelease Cu
8. Elizabeth River 2 12 h irradiation ............................................................................................30
a. Photorelease of Cu
b. Net photorelease Cu vs. time
c. Photorelease of DOC
d. Net photorelease DOC vs. net photorelease Cu
9. Bradley Creek 12 h irradiation .................................................................................................31
a. Photorelease of Cu
b. Net photorelease Cu vs. time
c. Photorelease of DOC
d. Net photorelease DOC vs. net photorelease Cu
10. Elizabeth River 2 24 h irradiation ............................................................................................32
a. Photorelease of Cu
b. Net photorelease Cu vs. time
c. Photorelease of DOC
d. Net photorelease DOC vs. net photorelease Cu
11. Absorbance vs. net photorelease Cu ........................................................................................33
vii
INTRODUCTION
Sediment remobilization can increase concentrations of nutrients, metals, and organics in
natural waters (Sondergaard et al. 1992; Simpson et al. 1998; Cantwell et al. 2002; Koelmans
and Prevo 2003). Coastal zones are particularly critical settings for studying sediment-water
interactions because they are the interface between the terrestrial and marine environments and
are greatly impacted by natural and anthropogenic sediment disturbances. Natural events like
river flow, tides, wind, storms, and human activities such as shipping and dredging remobilize
and reintroduce sediments to sunlit surface waters. These integrated resuspension events
contribute to prominent sediment plumes (e.g., the area of the Mississippi River plume is 1714139 km2; Walker et al. 1996) and discharges of creeks and estuaries of disparate size
worldwide.
Koelmans and Prevo (2003) demonstrated that resuspension of particulate organic carbon
(POC) causes a phase change in organic carbon from particulate to dissolved, although the role
of light was not examined. The effect of sunlight in this process was demonstrated by Mayer et
al. (2006), who irradiated sediment samples from the Mississippi River for 6 hrs per day for 7-9
days, observing that loss of POC was directly related to an increase in DOC concentrations.
Kieber et al. (2006) demonstrated photoproduction of DOC from sediment resuspensions of the
Cape Fear River (NC) using shorter irradiation lengths (9 h). Other studies have shown an
increase in concentrations of nutrients such as ammonium, phosphate, and dissolved nitrogen and
organic carbon due to photorelease from resuspended sediments (Kieber et al. 2006; Southwell et
al. 2010, 2011).
Sediment-water interactions are critical to trace element cycling in estuarine
environments especially metal enriched locations, which are typically near ports and cities
(Simpson et al. 1998; Kalnejais et al. 2007; Cantwell et al. 2008). Experiments using surface bay
sediments demonstrated that natural shear stresses are likely to resuspend the top 0.5 cm of
sediments (Kalnejais et al. 2007). Simpson et al. (1998, 2000) observed that resuspension of
anoxic sediments promoted oxidation of sulfide phases with resulting release of metals such as
Cu, Cd, Pb, and Zn. Organic complexation of trace metals influences both their sediment-water
reactivity and bioavailability in estuarine systems. Zafiriou (2002) determined that the
photoreactivity of metals is a result of chromophoric organic matter to POC in sediments.
Copper is a quintessential example of a micronutrient trace metal that is highly
complexed (typically >99%) by dissolved organic matter in natural waters, which controls its
biological lability (Coale and Bruland 1988; Donat et al. 1994; Moffet 1995) and photoreactivity
(Zafiriou et al. 2002). Shank et al. (2006) observed photodegradation of Cu-complexing ligands
in water samples of the Cape Fear River thus implying that with increased light exposure free
Cu2+ ions may also become more available in the water column. However, no studies have
focused on the role of photochemistry in the release of metals from resuspended sediments.
The purpose of this study was to determine the role of sediment resuspension coupled
with sunlight irradiation on the release of dissolved Cu. Resuspended sediments typically have
limited prior exposure to sunlight, and in these cases the remaining POC tends to be more
photolabile. As Cu is strongly organically complexed, the photodegradation of POC may
transform more particulate complexed Cu to the dissolved state. The implications of this research
are important for understanding micronutrient sources in the biologically active waters of
estuaries and coastal zones.
2
METHODS
Sampling
Sediment samples were collected from southeastern US tidal creeks and rivers with
varying biogeochemical properties and anthropogenic inputs (Figure 1). The top one centimeter
of sediments was collected and sealed for refrigerated storage in polyethylene cups or Ziploc
plastic bags.
Light and dark resuspension experiments
Sediment samples were removed from cold storage to acclimate to room temperature
overnight. Sediments were then homogenized; approximately 10g of wet sediment was added to
a 4 L high density polyethylene carboy filled with previously filtered seawater from Wrightsville
Beach, NC, (acid-washed 0.2 μm Supor membrane in Teflon holder). Sediment resuspensions
were prepared by shaking and rolling the carboy vigorously for 3 minutes. Only fine particles
that were likely to remain in an actual resuspension event were used in the experiments.
Resuspensions were allowed to sit for 45 minutes for settling of coarser particles. The top 20
centimeters of suspension, only containing fine particles (<ca. 10-20 µm, Jackson 1973), was
siphoned into another carboy. The carboy containing the fine particles was shaken again and
distributed evenly into six acid-washed 500 mL round bottom quartz flasks with Teflon magnetic
stir bars. A small amount of this resuspension was retained and filtered through three separate
tared 45 mm GF/F (Whatman) filters for quantification of total suspended solids (TSS) and
leachable metals of the sediments. Three flasks were wrapped in aluminum foil to serve at dark
controls. All flasks were placed in a constant 25 °C water bath and irradiated using a solar
simulator (Spectral Energy solar simulator LH lamp housing with a 1000 W Xe arc lamp)
equipped with a sun lens diffuser and an AM1 filter to remove wavelengths not in the solar
3
spectrum. Settings were adjusted to mimic midday midsummer sun at 40 °N latitude. Irradiation
lengths of six hours were chosen since it approximates one full day’s sunlight at that latitude.
During the irradiation flasks were rotated clockwise under the simulator every two hours for
even light exposure. At the end of the photolysis, resuspensions were filtered using acid-washed
0.2 μm Supor filters in Teflon holders using a peristaltic pump fitted with acid-washed C-flex
tubing. Filtered samples were stored in acid-washed polyethylene bottles and spiked with high
purity (double-distilled) HCl to ~pH 2.
Sediment and Water Characterization
Parameters such as dissolved organic carbon (DOC), particulate organic carbon (POC),
particulate organic nitrogen (PON), TSS, and leachable resuspended metal concentrations of Al,
Fe, Cu, and Pb were determined in all sediments used in irradiation experiments. Small volumes
of the filtered and acidified water from each light and dark flask were stored in combusted glass
vials for DOC analysis. TSS was determined by filtering 50-200 mL of the suspension through
precombusted, preweighed GF/F glass filters. Filters were freeze-dried and then reweighed.
Filter papers were cut into quarters using a razor blade; two quarters were used for organic
carbon (OC) and organic nitrogen (ON) analyses. Total leachable metals in resuspensions were
analyzed by leaching the remainder of the filter paper in 7 mL of high purity 1M HCl for 2 hours
in Teflon vials while being shaken vigorously (250 rpm) on the shaker table. The leachate was
filtered using VWR 25 mm syringe disc filters (0.45 µm polyethersulfone membrane) before
added to 15 mL acid washed centrifuge tubes.
Readsorption of Cu
To establish whether photoreleased Cu remains dissolved in the water column or is
readsorbed back on to sediment particles, one resuspension using Bradley Creek (Wilmington,
4
NC) sediment was set up as described above with six flasks. However, after six hours of
irradiation, all flasks were removed from the solar chamber. One pair, one light and one dark,
was filtered immediately. The remaining two pairs of flasks (lights were covered with aluminum
foil) remained on stir plates to maintain suspensions. The remaining pairs were filtered 4 and 24
h after irradiation.
Effects of TSS on Cu photorelease
The relationship between varying TSS and the concentration of photoreleased Cu and
DOC was addressed using Bradley Creek sediment. Three resuspensions were prepared as for
prior 6 h irradiations with the exception that 1 g, 9 g, and 50 g of wet sediment were weighed and
resuspended in Wrightsville Beach, NC, filtered seawater. Theses suspensions yielded 38, 138,
and 320 (mg/L) TSS concentrations, respectively.
Long-term irradiation experiments
To better understand the kinetics and mechanism of photolytic release of Cu from
sediment particles, three resuspensions were prepared using sediments from Bradley Creek (NC),
Jacob’s Creek (Neuse River basin, NC) and Elizabeth River site 2 (VA). All experiments had the
same design with two flasks designated as dark controls and four flasks for light exposure. A
small portion of the resuspension was retained and filtered for intial samples of DOC and
dissolved Cu. Time points were taken at 0.5, 1, 4, and 12 hours. At each time point one light
flask was removed and filtered and half the volume of one dark flask for DOC and dissolved
copper analysis. Small volumes were filtered for TSS, CHN analysis, light absorption, and
leachable resuspended metals. Following the same procedure, one 24 hour resuspension with
Elizabeth River site 2 was completed using time points of 2, 6, 15, and 24 hours.
5
Sample preparation for Anodic Stripping Voltammetry (ASV) Analysis
All samples were UV irradiated using an Ace Glass power supply and photochemical
lamp (1200 W) for a period of 2 hours to oxidize organics that may complex Cu and cause
interferences with voltammetric analyses. Filtered water from resuspension were added to preweighed 100 mL Teflon beakers and weighed after the addition of the sample. To prevent
contamination, beakers were placed on round glass platforms inside glass Petri dishes with
covered by an inverted quartz beaker. Milli-Q deionized water (MQ) (≥ 18 MΩ cm-1 Millipore
Corp.) was added to the petri dishes to control evaporation and contamination during irradiation.
Once removed and cooled to room temperature, the Teflon beakers were reweighed, and MQ
was added to refill to the initial volume. Volume corrections were calculated by dividing final
mass by the initial volume. Samples were stored in acid-washed 125 mL high density
polyethylene bottles for at least 24 hours to avoid interference with radicals before voltammetric
analysis.
Electrode Construction
Microwire electrodes were constructed by adapting methods used by Billon and van den
Berg (2004). High purity gold wire (25 μm diameter, >99.99% purity, [Goodfellow Cambridge
Limited, Huntington, England]) was attached to insulated polyvinylchloride coaxial Cu covered
steel cable ( 0.41 cm; Belden, US) covered with a conductive, silver epoxy resin (MG
Chemicals, Canada) and covered with polypropylene tubing (0.3 cm) extending just beyond the
epoxy resin. The tip was heat sealed and inspected by a digital microscope. At the other cable
end, an alligator clip was attached and sealed with Ag epoxy. Accuracy and proper functioning
of the electrode was validated with cyclic voltammetry scans in 0.5M H2SO4 (Billon and van
den Berg 2004; Larson 2011).
6
Analytical- Total dissolved Cu
The electroanalytical system used for dissolved copper analysis consisted of an Autolab
PGSTAT 10 potentiostat and a BAS cell stand with a platinum wire counter electrode, an
Ag/AgCl salt-bridged (high purity KCl) reference electrode, and a constructed gold microwire
working electrode. Parameters for Cu voltammetric analysis were adapted from Chapman and
van den Berg (2007). For analysis, 10 mL aliquots of acidified and UV irradiated sample was
added to the cell. Samples were purged for 5 min. with ultra high purity nitrogen while being
stirred before scans. An anodic stripping voltammetry (ASV) scan was run using the following
parameters: 0.65 V conditioning potential for 30 s, frequency 30 Hz, initial potential 0.0 V, end
potential 0.6 V, step potential 0.00705 V, amplitude 0.02505 V, equilibration time 10 s. The
deposition duration was adjusted for individual samples (usually between 30 and 90 s) depending
on Cu concentration. Samples were stirred at 625 rpm during deposition. Position, peak height,
and first derivatives were recorded, but only peak heights were used to determine Cu
concentrations. Accuracy of method was checked with National Research Council Canada’s
certified reference materials SLEW-3 (actual 24.4 ± 1.9 nM; determined 24.4 ± 3.0 nM, n = 18)
and NASS-5 (actual = 4.67 ± 0.7; determined = 4.42 ± 1.2 nM, n= 14). Standard addition curves
with three spikes were used to establish linearity, and then one spike analyses were used
thereafter to determine concentrations (Larson 2011).
Analytical- Resuspended sediment characterization
Resuspensions (2 replicates per treatment) were analyzed using a Shimadzu TOC-5050A
total organic carbon analyzer equipped with an ASI autosampler to determine concentrations of
DOC (μmol L-1). Standards were prepared from reagent grade potassium hydrogen phthalate
(KHP) dissolved in MQ and acidified to pH 2 with 6M HCl (Willey et al. 2000). Light
7
absorption was quantified according to Helms et al. (2008) using a Cary 1E UV-visible
spectrophotometer (Varian) and 1 cm quartz cells. MQ water was used as the blank and
reference. Organic nitrogen and carbon percentages were determined by CHN analysis. Filter
quarters were placed in a desiccator and exposed to HCL vapors for 24 h to remove inorganic
carbon from the sediment prior to analysis on a Thermo Quest EA2500 elemental analyzer
(Hedges and Stern 1984). Total leachable metals in the resuspended sediments were analyzed on
the inductively coupled plasma optical emission spectrometer (2100 DV Perkin Elmer, Waltham,
MA) for resuspended leachable metal concentrations. Known concentrations of Cu, Fe, Pb, and
Al were run as standards to create calibration curves with R2-values ≥ 0.999. Blanks containing
only filter paper and acid were run for blank corrections of analyte concentrations.
Statistical Treatment
Uncertainties between TDCu concentrations, TSS, total leachable Cu from resuspended
sediments, Fe, Al, DOC, %OC, %ON are expressed as ± 1 standard deviation (SD) for triplicate
analyses. Significance of correlation was used for comparisons (significance p < 0.1) and t-test
was used to test significance of net photoproduction between light and dark flasks for DOC and
TDCu.
8
RESULTS & DISCUSSION
Total dissolved Cu photorelease in 6 h irradiations
Net photoproduction of total dissolved Cu (TDCu) occurred in 10 of 13 irradiated
suspensions with sediments from a broad range of geographic and biogeochemical regimes
(Figure 1 and Table 1). Release of Cu ranged from 0 to 11.8 nM (0 to 46 nmol Cu/ g dry sed.) for
all sites (Figure 2). There was a significant correlation (R2 = 0.666; p < 0.05) between net TDCu
photorelease and the leachable Cu content of the sediments (Figure 4) suggesting that Cu content
was the dominant control on photoproduction. As expected, the greatest releases occurred from
sediments impacted by development or urbanization (ER 1 and 2, BC 1 and 2, PC) whereas more
pristine sites had low Cu photoproduction due to low initial leachable Cu concentrations.
Although it is well established that sediments containing relative higher organic content
have greater concentrations of released metals from desorption in dark processes (Cantwell et al.
2002; Audry et al. 2005), our sites did not show a significant correlation (p > 0.05) between
%OC and Cu photorelease. This suggests that the organic content of the sediment alone is not a
controlling factor on Cu photorelease. However, it is likely that the nature and reactivity of the
organic matter are important in determining photoreactivity of associated Cu. There was no
significant correlation between net TDCu photoproduction and leachable Fe in the sediments (p
> 0.05). Leachable Fe in resuspended sediments might drive a positive correlation between
dissolved Cu and Fe content because iron oxyhydroxides can absorb Cu and other trace metals
which might be released when the carrier phase is photoreduced (Waite et al 1998) via a reaction
such as
Fe(OOH)-Cu(s) + hv → Fe(II)(aq) + Cu(I)(aq) or Cu(II)(aq)
9
where Fe(OOH)-Cu(s) represents Cu sorbed to the Fe oxyhydroxide surface. However, no
significant correlation was observed suggesting either a lack of significant photoreduction of
oxidized Fe phases, possibly because of protection by organic coatings or relatively low Cu
concentrations associated with Fe phases.
Photoproduction of DOC in 6 h irradiations
Net photoproduction of DOC occurred in all irradiated suspensions and was consistent
with results of previous studies (Figure 3) (Kieber et al. 2006; Mayer et al. 2006). For 11 of 13
sites, significant light-induced production of DOC was observed ranging from 4.55 ± 17 µM to
69.3 ± 45 µM. The two sites that did not demonstrate DOC production were sites PC and LA.
Site PC is a heavily industrialized estuarine creek that has been undergoing restoration for
several years and lies south of the industrial and naval shipyard in Norfolk, VA. Site LA is in the
Atchafalaya River in the Mississippi River basin. Both sites experience recurrent sediment
mobilization and high amounts of anthropogenic input which may decrease the photoreactive
organic matter content of sediment particles. No significant correlation (p > 0.05) was observed
between net photoreleases of TDCu and DOC. Southwell et al. (2011) observed that molecular
markers such as the terrestrial-aquatic ratio (TAR) and carbon preference index (CPI) were better
for predicting DOC photoreactivity and photolytic release based on irradiation experiments using
Cape Fear estuarine sediments. Additionally, the research demonstrated that fresher terrestrially
derived material is more photolabile than diagenetically altered material. Although these indices
were not determined in this study, they might be useful for predicting the photoproduction of
dissolved Cu from estuarine sediments.
Role of sediment concentration on TDCu photorelease
10
The impact of TSS on metal release in resuspended sediments has been widely studied in
the absence of light, but the role of photolytic processes has not been addressed. Sediment from
site BC was resuspended in three irradiations of 6 h with varying TSS loads ranging from 38 ±
7.1 to 320 ± 62 mg/L, which are concentrations relevant to both relatively undisturbed (TSS ≈ 30
mg/L) and moderately disturbed environments (TSS ≈ 250 mg/L) (Schoellhamer 1996). Each
irradiation experiment demonstrated net photoproduction of TDCu with the greatest net release
of 99.4 ± 15.8 nmol/ g dry sed. at the lowest TSS. When TSS increased by a factor of ≈ 10, net
photorelease decreased three-fold (Figure 4a & b). The highest rate of release at low TSS
concentration suggests photoproduction may be controlled by the mass of particles resuspended;
as particle concentration increases, less light may penetrate the water column due to particle
shading effects and increased light attenuation. This may result in fewer photoreactions and less
overall photorelease. Shank et al. (2011) observed that a sizeable increase in photodegradation of
POC to DOC occurred at lower TSS of 100 mg/L than in a 1g/L suspension, and concluded that
lower suspension rates enhance solar DOC photoproduction.
TDCu readsorption experiment
Sediment from site BC was resuspended and irradiated for 6 h (TSS, 0.197 ± 0.01 mg/L),
and then pairs of flasks were filtered at 0, 4, and 24 h after irradiation. Continual photorelease of
TDCu was observed for up to 24 h after irradiation ended with the greatest net release of 7.4 nM
Cu (37.6 nmol/g dry sed.) at 4 h following irradiation. Increases in TDCu in dark controls may
be explained by the oxidation of metal sulfides (Simpson et al. 1998, 2000). For example in dark
experiments, Simpson et al. (1998) found that within 5 h of resuspending anoxic estuarine
sediments, 15% of CuS had been oxidized to soluble phases. Light release in our experiments
was always much greater than dark release implying that radicals produced during irradiation
11
may be present at significant levels in resuspensions for up to 24 h. Prolonged activity of radicals
produced during irradiation allows time for more light reactions thereby increasing the Cu
dissolved concentration in the suspension.
The role of light in these results is significant given that a previous study by Caetano et
al. (2003) observed that Cu and Pb readsorbed to a significant extent onto newly formed Fe
oxyhydroxides in anoxic sediments near a dredging site. The high concentration of metals (1801200 nmol of Cu/g sed.) in the latter study may have influenced Cu speciation in the suspensions
and therefore the adsorption behavior of Cu. Dissolved Cu in natural waters is typically strongly
organically complexed to at least two major classes of ligands. Complexes of Cu with the strong
ligand class (L1: Kcond ≥ 1013.5) are thought to be relatively soluble but are less abundant than
weaker class ligands (L2: Kcond ≥ 109.0-9.6, Donat et al. 1994). The high Cu concentrations found
in Caetano et al. (2002) may have surpassed the L1 concentration thus leaving only weaker, less
soluble ligands for Cu complexation accounting for the low concentration measured in the
dissolved phase. Since estuarine sites were examined, an excess of strong ligand L1 for Cu is
likely present (Skrabal et al. 1997; Shank et al. 2004). Although Shank et al. (2006) found that L1
type ligands do photodegrade (15-33% decrease in 1-day exposures to natural sunlight and much
larger degradation in 94 h irradiations in solar simulator), the likely excess of strong ligands that
remain may serve to keep organically complexed Cu dissolved and not readily readsorbed onto
sediment particles. Irradiation of sediments may produce more stable and soluble complexation
of Cu than dark resuspensions alone allowing a greater concentration of TDCu to remain.
Role of long irradiation exposures on TDCu photorelase
Suspensions of sediments at three sites of varying biogeochemical characteristics were
irradiated for 0, 0.5, 1, 4, and 12 h and one site, ER 2 was irradiated for 24 h (time points = 0, 2,
12
15, and 24 h). The sites and TSS concentrations (in mg/L) were: JC, 147 ± 53; ER 2, 238 ± 95;
BC 2, 231 ± 27. Each suspension demonstrated similar trends of increasing concentrations of
TDCu and DOC with irradiation time (Figures 6-8). Cantwell et al. (2002) observed that greater
total organic carbon (TOC) concentrations present in sediment co-occurred with increased metal
dissolution in dark resuspensions. A similar result was observed in the irradiation experiments.
Site BC 2 showed net photoproduction of 67.2 ± 7.5 nmol Cu/g dry sed. whereas the net
photolytic release for site ER 2 was 42.3 ± 12 nmol Cu/g dry sed. Although site ER 2 contained a
higher Cu concentration in the sediment, it demonstrated less net photorelease. This maybe a
result of having a lower %OC in the sediment (Table 2).
TDCu photoproduction was weakly significantly correlated (p < 0.10) to DOC
photoproduction for sediments subjected to 12-24 h irradiations (Figures 6-9). This demonstrates
that DOC and TDCu have significant relationships when tracked for a specific site, but they do
not correlate when all sites are considered. However, the significance of this relationship does
not convey much understanding, as slopes varied greatly among sites due to varying sediment
characteristics such as %OC and leachable Cu concentrations (Table 2). The variation of the data
implies that within a sediment net photoproduced DOC and TDCu may be correlated, but could
be a result of irradiation time. As irradiation time is increased, the concentrations of net
photoproduced DOC and TDCu increased.
Continual significant photoproduction of DOC and TDCu suggests that resuspended
particles may dissolve in layers of varying photochemical activity. As particles are stressed by
collisions and are subjected to photolytic reactions, relatively more labile outer layers may
photodegrade first followed by less labile inner layers. Each site began to demonstrate
photoproduction of DOC and TDCu at the 4 h time point (Figures 7-9) and 6 h for the 24 h
13
irradiation (Figure 10). This may suggest that a coating or encapsulation exists around sediment
particles and has to be degraded before DOC, and TDCu are photochemically released.
Microbial organisms are known to be abundant in sediments and secrete a gelatinous
layer known as extracellular polymeric substance (EPS) that can alter the biogeochemical and
surface properties of sediment (Patterson 1995). Decho and Kawaguchi et al. (2003) observed
that EPS biofilms scatter light and reduce spectral reflectance of Bahamas bottom sediments.
Biofilms may surround the resuspended sediment particles and act as a layer that needs to be
photodegraded to release both DOC and Cu. An additional explanation may be a hysteresis effect
in that photoproduced radicals need to accumulate to a critical level before photorelease can
occur. This may explain the increase of TDCu in light vessels observed from 0 to 4 h in the
readsorption experiment (Figure 6). TDCu increased in the irradiated flasks after being removed
from the solar simulator. Therefore, light reactions may have continued until remaining radicals
fell below some critical level that prevented further photorelease. Long irradiations may be
important for understanding sediment-water interactions for ecosystems such as bays, shallow
intertidal areas, and estuaries where sediment remobilization is a recurring event and particles are
prone to sunlight exposure.
Relationship of TDCu and absorbance
Optical properties of water are important to consider when studying coastal and estuarine
environments especially in the presence of sediment particles. Chromophoric dissolved organic
matter (CDOM) strongly absorbs UV light (Blough and Green 1995) and is abundant in coastal
waters influenced by terrestrial inputs (Blough et al. 1993). Absorbance measurements of
photoreleased CDOM from site ER 2 sediment resuspensions of both the 12 and 24 hr
irradiations were compared to net photoproduced TDCu. A significant natural log correlation
14
was found (p < 0.001, Figure 11). Both TDCu and absorbance increased until absorbance leveled
off at ~ 4 m-1 (15 h of irradiation) while net TDCu continued to increase. Overall net
photorelease of TDCu for the 24 h irradiation of site ER 2 was 27.7 nM (114 nmol/ g dry sed.).
The relationship may provide insight into the role Cu-binding ligands play in the process of
photochemical redissolution.
There are two possible explanations of the trend. One is that photoproduction of CDOM
from the sediment is eventually balanced by photobleaching by photoproduced radicals such as
superoxide as seen in the leveling off of absorbance (Figure 11). A secondary explanation may
be that Cu photoprotects sedimentary organic matter from photorelease. Goldstone and Voelker
(2000) demonstrated that in natural waters, CDOM photosensitizes the production of the radical
anion superoxide. Superoxide radicals then continue to react with CDOM causing it to
photobleach and thus change optical properties of the water. Unpublished data from R.F.
Whitehead demonstrated that higher dissolved Cu concentrations help to protect CDOM from
photobleaching effects. This was determined by spiking in Cu (II) in natural water from the Cape
Fear River, NC. With each additional spike of Cu (II) (20 to 200 nM) a decrease in the net loss of
absorbance was observed implying that organically complexed Cu may be a sink for radicals
controlling CDOM photobleaching such as superoxide. Photoproduced TDCu from sediment
resuspensions may affect the optical properties of water by supplying Cu to the water column
(net photoproduction of Cu correlated with time, Figures 9b and 10b). This unrecognized supply
of Cu could be photoprotecting sedimentary organic matter within the resuspensions. This is an
important process because photoproduced Cu may change the photochemical cycling of CDOM
in estuarine and coastal locations during and after sediment resuspension events.
15
IMPLICATIONS
Photorelease of TDCu from irradiated resuspended sediments appears to be a common
occurrence in sediments from diverse biogeochemical settings. For 10 of 13 sites, significant
TDCu photorelease was demonstrated, and for 11 of 13 sites, DOC was photoproduced at
environmentally relevant TSS conditions (138-420 mg/L). Greater TDCu was produced at lower
TSS, suggesting that shading or competitive photoreactions may occur at higher TSS. The direct
correlation found between leachable Cu and net photorelease of TDCu is important when
considering anthropogenically altered sediments from marinas and industrialized estuaries.
Highly impacted sites such as ER and PC demonstrated greater net TDCu photorelease. Net
DOC and TDCu photorelease were significantly correlated for timed exposures of sediments at
individual sites. Long-term irradiations (12 and 24 h) yielded greater photolytic release, with a
marked increase at 4-6 h which may correspond to the progressive photodissolution of organic
layers on the irradiated particles. Photoproduced TDCu persists in the dissolved phase for at least
24 h after irradiation stops perhaps due to complexation by strong organic ligands which
prevents rapid particle adsorption.
Since Cu is an established micronutrient and micropollutant in aquatic environments, this
previously unrecognized source of dissolved Cu may have potential biological impacts. Shank et
al. (2004) established that bioavailability and toxicity of Cu are dominated by organic
complexation. Biogeochemical cycling of Cu may be altered by increases in photoproduced
TDCu altering free ion concentrations and potentially affecting the photodegradation of strong
Cu ligands (Shank et al. 2006). These factors may influence compositions of phytoplankton
communities (Croot et al. 2000) and possibly surpass toxicity ranges for species like
16
Synechococcus which experienced a 60% decrease in gowth at [Cu2+] = 10-8.7 M (Miao et al.
2005).
Photochemical release of TDCu can be compared to three other measured fluxes to the
ocean, benthic, wet deposition, and riverine. Fluxes for 6 h irradiated resuspended sediments
were calculated using averages of three sites that demonstrated both the highest (38 nmol/g res.
sed. or 6.33 nmol/g res. sed. h) and lowest (2 nmol/g res. sed. 0.33 nmol/g res.sed h) net
photoreleased Cu concentrations. Rates were applied to a modest TSS concentration of 100 g/m3
and were considered as occurring in the upper 1 m of the water column. These calculations yield
fluxes of 33 nmol/m2/h and 633 nmol/m2/h. Riverine flux was calculated using values reported in
Libes (2009). Average river discharge was 3.74 x 1016 L/y with Cu input of 23 x 10-9 mol/L and
if dispersed over the continental shelf area of 52 x 106 km2 reported by Stacey (1992), the flux
would be 1.9 nmol/m2/h. Average benthic fluxes for various sediments is typically 12.5
nmol/m2/h either into or out of the sediments (Rivera-Duarte 1995). Kieber et al. (2004) reported
that the Cu rainwater flux to the ocean would be 0.91 nmol/m2/h using the average Cu
concentrations from wet deposition over 10 years. Although photolytic fluxes from sediment are
episodic, they are comparable to benthic, rainwater, and riverine fluxes and are a previously
unrecognized source of TDCu to estuarine ecosystems.
17
Table 1. Site abbreviation, TSS, net photorelease of Cu, % organic carbon, leachable Cu, and leachable Fe
Site
TSS (mg/L) Cu net photorelease (nM) % organic carbon Leachable Cu (µmol/g) Leachable Fe (µmol/g)
PG
0.41 ± 0.04 -0.5 ± 0.6
1.93 ± 0.51
0.07 ± 0.01
163 ± 74
BC 1
0.19 ± 0.01 3.9 ± 1.9
3.93 ± 0.43
0.39 ± 0.01
96.6 ± 17
PC
0.15 ± 0.01 4.1 ± 1.6
2.17 ± 0.08
0.85 ± 0.13
82.4 ± 13.4
HC
0.18 ± 0.00 0.7 ± 0.7
3.70 ± 0.78
0.23 ± 0.12
25.1 ± 0.7
CB/S
0.46 ± 0.01 1.9 ± 1.1
1.81 ± 0.12
0.16 ± 0.02
80.1 ± 36
CB/NE 0.42 ± 0.01 0.1 ± 0.4
2.03 ± 0.27
0.30 ±0.23
51.8 ± 2.6
WP
0.43 ± 0.01 3.4 ± 1.5
3.62 ± 0.35
0.16 ± 0.02
88.6 ± 38
GC
0.20 ± 0.01 4.7 ± 2.1
7.26 ± 0.87
0.35 ± 0.02
30.2 ± 2.3
ER 1
0.40 ± 0.01 3.2 ± 1.0
1.76 ± 0.20
0.42 ± 0.02
45.5 ± 1.2
BC 2
0.14 ± 0.01 5.0 ± 0.5
7.73 ± 0.53
0.95 ± 0.12
331 ± 20
JC
0.22 ± 0.05 1.1 ± 0.1
5.49 ± 1.16
0.07 ± 0.00
9.21 ± 2.2
LA
0.18 ± 0.10 5.5 ± 0.9
1.58 ± 0.09
0.20 ± 0.01
149 ± 5.4
ER 2
0.26 ± 0.03 12 ± 1.0
2.74 ± 1.01
1.62 ± 0.31
217 ± 30
PG = Prince George Creek, NC; BC 1 & 2 = Bradley Creek, NC; PC = Paradise Creek, NC; HC = Harvey’s Creek, NC; CB/S =
Carolina Beach, Seahorse, NC; CB/NE = Carolina Beach, North End; WP = West Point, VA; GC = Goodby’s Creek, FL; ER 1 & 2 =
Elizabeth River, VA; JC = Jacob’s Creek, NC; LA = Mississippi River Delta, LA
18
Table 2. Site abbreviation, TSS, irradiation time, net photorelease of Cu, % organic carbon, leachable Cu, and leachable Fe
Site TSS (mg/L) Time (h) Cu net photorelease (nM) % organic carbon Leachable Cu (µmol/g) Leachable Fe (µmol/g)
JC
0.15 ± 0.05 12
28 ± 3.2
5.49 ± 1.2
0.09 ± 0.03
1.00 ± 0.5
BC 2 0.23 ± 0.02 12
10 ± 2.8
6.27 ± 0.56
0.73 ± 0.09
38.2 ± 4.8
ER 2 0.24 ± 0.09 12
2.3 ± 0.3
NA
1.31 ± 0.51
161 ± 58
ER 2 0.23 ± 0.02 24
16 ± 1.7
2.38 ± 1.3
1.59 ± 0.13
131 ± 42
BC 2 = Bradley Creek, NC; ER 2 = Elizabeth River, VA; JC = Jacob’s Creek, NC
19
FIGURE LEGENDS
Figure 1. Map of study sites. Base image from Google EarthTM.
Figure 2. Average total dissolved Cu concentrations (nM) for light and dark flasks for each
sampled site. Sites are arranged in increasing leachable Cu concentrations from left to right.
Error bars represent ± standard deviations for triplicate analyses.
Figure 3. Average DOC concentrations (µM) for light and dark flasks for each sampled site.
Error bars represent ± standard deviation for triplicate analyses.
Figure 4. Effect of photoreleased Cu (nmol Cu/g dry sed.) on the leachable Cu concentration
(nmol Cu/g dry sed.) of selected sites; R2 = 0.6662, y = 0.0282x + 3.5629, p < .05, n = 11. Error
bars represent ± standard deviation for triplicate analyses
Figure 5. (a.) Effect of varying TSS on BC sediment of light and dark treatments; R2 = 0.984, y =
3.3312x + 192.16. Error bars represent ± standard deviations of triplicate analyses. (b.) Effect of
net photorelease Cu (nM) from varied TSS irradiations of BC on DOC (µM).
Figure 6. Effect of readsorption of Cu (nM) onto sediment particles after irradiation of site BC 2.
Figure 7. Effect of Cu (nM) and DOC for 12 h irradiation of Jacob’s creek sediment. (a) TdCu
(nM) of light and dark irradiations at time points. (b) Effect of time (min) on the net photorelease
20
of Cu (nM) (R2 = 0.876, p < 0.10, y = 0.0184x + 1.256). (c) DOC (µM) of light and dark
irradiations at time points. (d) Effect of net photorelease of Cu (nM) on the net photorelease of
DOC (µM) (R2 = 0.7484, p < 0.05, y = 3.793x + 0.3214). Error bars represent data from a
different resuspension of the same sediment.
Figure 8. Effect of Cu (nM) and DOC for 12 h irradiation of Bradley creek 2 sediment. (a) TdCu
(nM) of light and dark irradiations at time points. (b) Effect of time (min) on the net photorelease
of Cu (nM (R2 = 0.9706, p < 0.002, y = 0.147x + 0.3598). (c) DOC (µM) of light and dark
irradiations at time points. (d) Effect of net photorelease of Cu (nM) on the net photorelease of
DOC (µM) (R2 = 0.7791, p < 0.10, y = 6.321x + 9.231). Error bars represent data from a
different resuspension of the same sediment.
Figure 9. Effect of Cu (nM) and DOC for 12 h irradiation of Elizabeth River 2 sediment. (a)
TdCu (nM) of light and dark irradiations at time points. (b) Effect of time (min) on the net
photorelease of Cu (nM) (R2 = 0.7460, p < 0.10, y = 0.0162x + 0.9243). (c) DOC (µM) of light
and dark irradiations at time points. (d) Effect of net photorelease of Cu (nM) on the net
photorelease of DOC (µM) (R2 = 0.7460, p < 0.05, y = 0.3926x + 2.1897). Error bars represent
data from a different resuspension of the same sediment.
Figure 10. Effect of Cu (nM) and DOC for 24 h irradiation of Elizabeth River 2 sediment. (a)
TdCu (nM) of light and dark irradiations at time points. (b) Effect of time (min) on the net
photorelease of Cu (nM) (R2 = 0.9740, p < 0.01, y = 1.164x + 1.815). (c) DOC (µM) of light and
21
dark irradiations at time points. (d) Effect of net photorelease of Cu (nM) on the net photorelease
of DOC (µM).
Figure 11. Absorbance at A300 vs. net photorelease of TDCu (R2 = 0.9607, y = 1.03ln(x) – 1.03.
Error bars represent ± standard deviations from triplicate analyses.
22
Fig. #1
23
Fig. #2
24
Fig. #3
25
Fig. #4
26
Fig. #5
27
Fig. #6
28
Fig. #7
29
Fig. #8
30
Fig. #9
31
Fig. #10
32
Fig. #11
33
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