Download NOTE Size fractionation of nanoparticulate metal sulfides in oxic water of N

Geochemical Journal, Vol. 50, pp. 281 to 286, 2016
doi:10.2343/geochemj.2.0408
NOTE
Size fractionation of nanoparticulate metal sulfides in oxic water of
Lake Teganuma, Japan
NORIKO NAKAYAMA,1* TAKAYUKI TOKIEDA,2 ASAMI S UZUKI,1 TAEJIN KIM,1
TOSHITAKA GAMO1 and HAJIME OBATA1
1
Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan
2
Meteorological College, 7-4-81 Asahi, Kashiwa, Chiba 277-0852, Japan
(Received September 14, 2015; Accepted November 18, 2015)
Concentrations of acid volatile sulfides (AVS) as metal sulfides in three different size fractions (<30 nm, 30–200 nm,
>200 nm) at Lake Teganuma were measured using a nano-filtration followed by a purge-trap gas chromatography with a
flame photoionization detector (GC-FPD). Fresh water samples were collected at four sites in the lake and sequentially
filtered with 30 nm and 200 nm pore size filters on site immediately after the sampling. The concentrations of unfiltered
AVS (AVStotal) ranged from 0.6 to 1.4 nmol/kg, among which the highest concentration was found in a lotus colony site.
Except for the lotus colony site, the relative AVS abundances in the three size-fractions were quite similar. It was found
that >80% of AVStotal existed in the <30 nm size fraction, while only 10~20% in 30–200 nm and >200 nm size fractions.
In the lotus colony site, on the other hand, <30 nm fraction contributed only ~5% but the 30–200 nm size fraction exhibited most dominant contribution (~80%), although the AVStotal concentration in the lotus colony site was similar to those
in other sites. Present observation shows that metal sulfides exist in fresh water environment and mainly reside in <30 nm
size fraction, but even larger metal sulfide nanoparticles with the size of 30–200 nm can be formed, which seem to be
formed from <30 nm size fraction through a relatively rapid process.
Keywords: metal sulfides, nanoparticle, size fractionation, acid volatile sulfides, Lake Teganuma
In oxic aquatic environments, sulfides had been considered to be promptly oxidized and/or precipitated (German et al., 1990; Feely et al., 1994), dissolved sulfides
were thus believed to be significantly low concentrations
and have negligible impact on biogeochemical system in
the natural environment. However, recent studies revealed
that sulfides exist in micro to pico-molar concentrations
even in oxic aquatic environment and stable metal sulfides
are formed (Cutter et al., 1999; Nakayama et al., 2015).
It is also suggested that some of the metal sulfides may
exist in a nanoparticulate form, rather than in a dissolved
form, which even more stabilizes the metal sulfides and
enables the metal sulfides being long-range transported
in oxic water column since nanoparticles have smaller
settling rate (Sander and Koschinsky, 2011; Yücel et al.,
2011; Nakayama et al., 2015). The nanoparticles should
eventually undergo oxidative dissolution and supply trace
metals in the area distant from their sources, which could
be an important long-range transportation process of the
trace metals in aquatic biogeochemical cycles. It is therefore quite important to understand physiochemical properties of the metal sulfide, such as particle size, stability,
and lifetime.
INTRODUCTION
Trace metals such as Fe, Cu, and Zn are now wellrecognized as essential micronutrients and their amounts
in waters can critically affect biological production in
aquatic system (Bruland et al., 1991; Hunter et al., 1997),
although it is still under intensive debate which chemical
forms are dominant in aquatic waters and how they are
taken into biological cell system from surrounding aquatic
waters (Gerringa et al., 2000; Worms et al., 2006). It is
therefore quite important to clarify the chemical species
of the trace metals and their biogeochemical cycles in
aquatic environments. Previous studies indicated that
these trace metals interact with organic ligands and form
stable organic-metal complexes in seawater (Bruland and
Lohan, 2004). Recent studies, however, suggested that
some of the trace metals bind with trace sulfide and form
metal sulfides, which might stabilize the trace metals even
in natural oxic aquatic system (Rickard and Luther, 2006).
*Corresponding author (e-mail: [email protected])
Copyright © 2016 by The Geochemical Society of Japan.
281
Fig. 1. Map of Lake Teganuma and sampling sites (site 1–4).
Traditionally, particulate and dissolved forms have
been defined by an operational point of view: whether it
passes through 0.2 (or 0.45) mm pore size filters or not.
One limitation in this commonly used definition is that
no further information about the size fractionation below
0.2 (or 0.45) mm could be obtained and one cannot distinguish whether they exist in a fully dissolved form or
nanoparticulate form. Nishioka et al. (2013) showed that
the hydrothermal Fe distributed over 3000 km distance
in the deep ocean around the Central Indian Ridge segment and a large portion of the dissolved Fe was in a soluble fraction (<1000 kDa) rather than in a colloidal fraction (1000 kDa-0.2 mm). They concluded that the physicochemical forms of dissolved Fe in seawater differ in
different Fe sources, which is a key factor for controlling
residence time and large scale distribution of the dissolved
Fe. For metal sulfides, only few studies suggested the
existence of the metal sulfide-bearing nanoparticles (<200
nm in size) and the potential extended lifetime of metal
sulfide in aquatic environment due to their potential stability and lower sinking rate than those in lager particles
(Rozan et al., 1999; Yücel et al., 2011; Nakayama et al.,
2015). At present, no information is available on the
chemical and physical properties of the metal sulfide
nanoparticles in the particle size below 200 nm. In this
study, we report the size-fractionated concentrations of
acid volatile sulfides (AVS) (<30 nm, 30–200 nm, >200
282 N. Nakayama et al.
nm) as metal sulfides in the fresh water samples collected
at Lake Teganuma. We show that the presence of metal
sulfide nanoparticles with diameter of 30–200 nm and
their dynamic size variation in the oxic aquatic system
for the first time.
SAMPLING LOCATION AND SAMPLING METHODS
Lake Teganuma is located in the suburbs of metropolitan Tokyo area, Japan and has a surface area of 4 km2
with an average water depth of 0.86 m with a maximum
depth of 3.8 m (Fig. 1). Lake Teganuma has been under a
hypertrophic condition for more than four decades due to
large-scale land reclamation and rapid urbanization in
surrounding areas, and known as one of the worst polluted lakes in Japan. Because of the highly hypertrophic
condition, Lake Teganuma exhibited several unique
biogeochemical features. It was found that sulfate was
abundant (500–3600 m mol/kg) and high densities of
sulfate-reducing bacteria (SRB) were present in surface
sediments in a littoral reed and an offshore sites throughout the year (Fukui and Takii, 1990), which is indicative
that the sulfate reduction actively occurs in the surface
sediments. Recently, Tokieda et al. reported significantly
high concentrations of dissolved CH4 and CO 2 in waters
over the lake and much higher concentration in a lotus
site throughout the year (submitted to Limnology and
Oceanography, 2015) and indicated an existence of
anaerobic environment in the sediment of the lake. They
suggested that enhanced biological production of organic
matter in the water column and its subsequent sedimentation and decomposition in the sediment lead an enhanced
O2 consumption and formation of anaerobic environment
in the sediment which promoted productions of CH4 and
CO2 in the sediment and contributed maintaining the observed higher CH4 and CO2 concentrations in the water
column.
All surface fresh water samples were collected during
morning time on April 16, 2015 at the four sites of the
lake. The four sites are shown in Fig. 1; near the estuary
of Ohori river (site 1), which brings its river water into
the lake for reducing the hypertrophic condition, near the
lotus site located in southern part of the lake (site 3) and
its opposite shore (site 2), and near the Tega river in the
eastern part of the lake (site 4), through which lake water
outflows. All water samples were collected from the shore
using an acid cleaned polypropylene beaker and transferred into acid cleaned polyethylene cubic containers.
In all sampling sites, surface temperature and dissolved
oxygen were measured using a multi-parameter portable
sensor (Handy Polaris, Oxy Guard International A/S,
Farum, Denmark).
WATER FILTRATION
Immediately after the water sampling, fresh water samples were filtered through a 0.2 mm-pore size capsule filter (AcroPakTM 200 Capsules with Supor Membrane, Pall
Co., San Diego, CA) using a peristaltic pump (Millipore
Model XX 80 000 00, Merck Millipore Corp., Darmstadt,
Germany) connected by a Tygon tubing. Filtrates were
collected in acid-cleaned glass vials and sealed with
Teflon-lined septum and aluminum crimp cap. Residual
filtrates were further filtered with custom-made 30 nmpore size polyethylene hollow-fiber filters (Sterapore,
Mitsubishi Rayon Co., Ltd., Tokyo, Japan) and subsequent
filtrates were collected in glass vials. Prior to the filtration, all filters used in the present study were cleaned by
the procedure employed by Nishioka et al. (2013) and
Kim et al. (2015). To examine adsorption and contamination during the filtration process, each filtrate was refiltrated with the same pore size capsule filters and
analyzed as shown in the filtration sequence (Fig. 2).
GAS CHROMATOGRAPHIC H2S D ETERMINATION
The AVS concentrations were quantified by converting sulfides to H2S by adding 1.5 mL of 1 M phosphoric
acid to the filtrate samples before the determination of
H2S with a gas chromatographic method as employed in
the previous studies (Radford-Knoery and Cutter, 1993).
Fig. 2. Sequence of nano-filterations to obtain AVS (acid volatile sulfide) in the <30 nm and <200 nm size fraction using 30
nm and 200 nm pore size filters. AVS concentrations in all filtrates were further quantified by GC-FPD. Additional refiltrations (dotted line) were also conducted to check the effect
of the adsorption and contamination during filtration processes.
Some modifications were made to the original method to
improve detection sensitivity (Nakayama et al., 2015).
H2S in the filtrates were extracted with a purge and trap
method. Water samples (50 mL) were transferred into a
gas stripping vessel and purged with He (>99.9999%)
carrier gas stream (100 mL/min). Extracted H2S gas and
other dissolved gases were passed through a gas trap column cooled with liquid nitrogen and trapped/concentrated
in the trap column. The water vapor was trapped using a
borosilicate glass U-tube immersed in ethanol held at
around –50∞C before other extracted gases were trapped
onto the column. After the gas stripping/trapping for 25
minutes, the trap column was removed from the liquid
nitrogen bath and immersed immediately into a hot water
bath (~95∞C) to release trapped gases. The released gases
were then injected into a gas chromatography system
equipped with a flame photometric detection (GC-FPD,
GC-2010, Shimadzu, Kyoto, Japan). We confirmed that
the effect of H2S adsorption in the dehydration unit and
related carry-over were small through a laboratory test
experiment, which ensured accurate H2S quantification
in the present study. The FPD detector was employed to
maximize detection selectivity and sensitivity for sulfurcontaining compounds. A wide-bore capillary column of
30 m ¥ 0.53 mm inner diameter ¥ 40 mm (HP-PLOT/Q +
PT, Agilent Technology, Santa Clara, California) was used
at a constant temperature of 110∞C (Nakayama et al.,
2015). The entire gas-stripping and GC system was constructed with Sulfinert®-treated tubing to prevent adsorption of sticky sulfur compounds onto the inner walls of
the system. The calibration of entire H2S detection sys-
Size fractionation of metal sulfide nanoparticules in Teganuma 283
Fig. 3. (a) Size-resolved AVS concentrations in four different size fractions (<30 nm: yellow, 30–200 nm: blue, >200 nm: green,
and unfiltered (total): black in the site 1–4). (b) Relative abundance of AVSs in different size fractions to the total AVS in the site
1–4. <30 nm: yellow, 30–200 nm: blue, >200 nm: green.
tem including gas stripping process was carried out by
applying the same analysis for freshly prepared standard
solutions of sodium sulfide nonahydrate (Wako Pure
Chemical Industries, Osaka, Japan, Ltd. Cat. No. 19703362) (Cutter and Oatts, 1987). To precisely determine
H2S, we ran six different standard solutions after measurement of one sample whose concentration was very different. We chose the six concentrations so that the sample concentration was in the middle of the standard concentrations, for example, 30, 50, 70, 80, 100, 120 pmol/
kg of standard solutions for the sample whose concentration was about 70 pmol/kg. On the basis of the standard
deviation of blanks, detection limit (2s) of <5 pmol for
H2S was obtained. The precision was estimated about 5%
as a relative standard deviation from the multiple measurements of 0.5 nmol/kg standard solution (n = 10). We
confirmed that re-filtered filtrate yielded only less than
~5% of deviation from the original filtrates through triplicate AVS measurements of each filtrates, which indicated negligible adsorption or contamination during each
filtration steps (Table 1). AVS total , AVS <30nm , and
AVS<200nm were determined by the analytical procedure
mentioned above. The AVS concentrations in two size
fractions (AVS30–200nm, AVS>200nm) were then derived as
AVS<200nm – AVS<30nm and AVStotal – AVS<200nm, respectively.
RESULTS AND DISCUSSION
The size fractionated AVS concentrations (AVS<30nm,
AVS 30–200nm , AVS >200nm , and AVS total) in all sites are
shown in Fig. 3a. The AVS concentrations of unfiltered
water samples (AVStotal) ranged from 0.6 to 1.4 nmol/kg
284 N. Nakayama et al.
with the highest observed in the lotus colony site (site 3).
In open ocean, the AVStotal concentration was reported to
be ~0.1 to 0.2 nmol/L in mixed layer, which decreased
with the depth and reached nearly constant concentrations
(0.03–0.05 nmol/L) at pycnocline (Radford-Knoery and
Cutter, 1994). In a submarine hydrothermal area, on the
other hand, it was reported as 0.35 nmol/kg throughout
the water column (Nakayama et al., 2015). The AVStotal
concentrations observed in the present study are higher
than those reported in the mixed layer of the open ocean
and over the hydrothermal area by a factor of 3–14 and
2–4, respectively, while they are much higher than those
in the pycnocline by a factor of 12–50. In Lake Teganuma,
lager input of nutrients into the lake and subsequent high
sedimentation of organic matter were reported in previous studies (Matsunashi et al., 2002; Deepak, 2007). It
was also suggested that sulfate reduction in the surface
sediments of the lake was occurred throughout the year
(Fukui and Takii, 1990). Since sufficient dissolved oxygen was available in the lake water in all sampling sites
(144 to 434 mmo/l, Table 1), it is considered that an active production of H2S in the anaerobic sediment contributes the observed high AVStotal concentrations in the
lake.
The relative abundances of AVS in the three size fractions were similar in the site 1, 2, and 4, but those were
significantly different in the lotus colony site (site 3) as
shown in Fig. 3b. In the site 1, 2, and 4, 80–90% of the
AVStotal existed in the smallest size fraction (d < 30 nm)
and 10–20% of the AVStotal was in the size fraction larger
than 30 nm. The relative fraction of the AVS<30nm was
fairly constant in the site 1, 2, and 4 (82–89%), while
that for AVS30–200nm was variable among the three sites
Table 1. Variations in triplicate AVS measurements of the filtrates for the size fraction <30 nm and <200 nm. A comparison
of filtered and re-filtered samples to estimate the effect of adsorption and contamination during the filtration process is also
shown. Dissolved oxygen (DO) concentration and water temperature during water sampling were also provided. AVS concentrations are given in nmol/kg.
Filtration
Stera-pore (30 nm)
Acropack (200 nm)
Site 1 (Temp. = 16.8∞C, DO = 193 mmol/l)
1st
0.64
0.73
0.72
0.70
0.68
0.74
0.72
0.71
Ave.
0.75
0.62
0.65
0.67
0.74
0.73
0.72
0.73
Ave.
STDV
STDV(%)
0.68
0.02
2.7
0.72
0.01
1.5
Ave.
2nd
Site 2 (Temp. = 17.2∞C, DO = 372 mmol/l)
1st
0.48
0.51
0.52
Ave.
0.50
2nd
0.57
0.64
0.52
0.58
Ave.
0.47
0.53
0.58
0.53
0.56
0.59
0.59
0.58
Ave.
STDV
STDV(%)
0.51
0.02
3.2
0.58
0.001
0.25
Site 3 (Temp. = 16.8∞C, DO = 144 mmol/l)
1st
0.10
0.04
0.07
0.07
1.24
1.05
1.12
1.14
Ave.
0.09
0.05
0.09
0.08
1.25
1.12
1.12
1.17
Ave.
STDV
STDV(%)
0.074
0.004
5.5
1.15
0.02
1.9
Ave.
2nd
Site 4 (Temp. = 16.2∞C, DO = 434 mmol/l)
1st
1.05
0.85
0.72
0.87
0.99
0.93
0.84
0.92
Ave.
0.75
0.86
0.90
0.83
0.86
0.85
0.99
0.90
Ave.
STDV
STDV(%)
0.85
0.03
3.1
0.91
0.01
1.5
Ave.
2nd
Value is AVS nmol/kg.
(2–13%). The relative fractions of AVS>200nm in the three
sites were relatively constant (6–11%). The observed
AVS <30nm in all sampling sites except for the site 3
strongly suggests that metal sulfides mainly exist in d <
30 nm and they have a lifetime longer enough to spread
over the lake even in an oxic water environment.
The size-fractionation of AVS at site 3 was an exceptional in the present study. The most abundant size-fraction in the site was d = 30–200 nm, which occupied about
78% of the AVStotal in the site, while this size fraction in
other sites took up only 2–13% of the AVS total. The
AVS30–200nm in the site 3 was 1.1 nmol/kg, which is 14–
62 time higher than those in other sites although the
AVStotal in the site 3 was slightly higher than those in
other sites by a factor of 1.4–2.2, suggesting that most of
the increased AVS30–200nm fraction was arisen from the
decreased AVS<30nm fraction and potential existence of
unique nanoparticle growth processes in the site 3.
Tokieda reported that the dissolved CH4 concentration at
the lotus site remained several mmol during March to May
from their year-round measurement (Tokieda et al., submitted to Limnology and Oceanography, 2015), which was
by three orders of magnitudes higher than those in the
ordinary water environments and indicative of strong reducing condition in the site. Since the significant increase
was observed only in the site 3 (lotus site), where strong
reducing environment in the sediment was suggested
(present study was conducted in April), the potential metal
sulfide growth processes in the site 3 may be related to
the highly reducing environment in the site, although further study is required to find out the formation mechanism. Free sulfides (H2S, HS– and S2–) produced by sulfate
reducing bacteria can form metal complexes with trace
metals, for example, Fe2+, which might further promote
coagulation of metal sulfides to nanoparticles (d = 30–
200 nm), and/or attachment of metal sulfides to preexisting nanoparticles/microorganisms (d = 30–200 nm). Microbial sulfur-rich fragments found within the cell of certain types of bacteria (e.g., Berg et al., 2014) may be also
responsible for the increase, although further study is required to find out the detailed process for increasing
nanoparticles. It should be also pointed out that we found
quite different AVS size distribution in only site 3 despite the rather short distances between each observation
sites (less than 3 km), though a slight increase of the
AVS30–200nm in site 2 (nearest to the site 3) was observed.
The observed local change in the AVS size distribution
suggests that the lifetime of the AVS30–200nm may be short
compared to that of AVS<30nm in the present study area.
CONCLUSION
The present study finds several important aspects of
metal sulfide nanoparticles. Firstly, metal sulfide exist
even in an oxic fresh water of Lake Teganuma, secondly,
Size fractionation of metal sulfide nanoparticules in Teganuma 285
they generally reside in the size range of <30 nm but they
can be in nanoparticulate form with the size of 20–300
nm, and thirdly, the ultrafine metal sulfides (d < 30 nm)
may grow up to larger size nanoparticles (d = 30–200 nm)
through a relatively rapid process. In aquatic sciences,
particulate substances, which passes through 0.2 (or 0.45)
mm pore size of filters, have been treated as a “dissolved”
fraction, however, present results clearly show that such
simple size fractionation is not sufficient for the characterization of metal sulfide nanoparticles. Finer size-classification in the dissolved fraction is necessary to shed
light on the dynamics of ultrafine nanoparticles in aquatic
environment. Although the potential rapid growth to the
metal sulfide nanoparticles (d = 30–200 nm) and its possible connection to a strong reducing sedimentary environment was suggested, their formation mechanism and
morphology of such nanoparticles are not yet understood.
Dynamic nanoparticulate growth process of metal sulfide
as found in this study should be quite important in the
view of their biogeochemical cycle, and further detailed
studies on chemical morphology of nanoparticles, size
distributions of metal sulfides, concentrations of related
trace metals, and interaction between physicochemical
processes in water and sediment are required.
Acknowledgments—We thank Mayuko Kasai for her kind assistance in sample collection. NN thanks Dr. Hiroshi Furutani
for valuable discussion at various stages during this study. We
also thank three anonymous reviewers for their valuable comments which improved the manuscript. This study was partly
supported by a Grant-in-aid for Scientific Research (C) (No.
15K00516) from the Japan Society for the Promotion of Science.
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