NOTES
THE REACTIVITY
AND
COMMENT
OF DISSOLVED SILICON IN SOME NATURAL
ABSTRACT
Several samples of natural waters, from the
deep North Atlantic
Ocean and four rivers
in southern England, have been examined for
the presence of dissolved silicon unreactive
under the conditions
generally
used for its
absorptiometric
determination;
none was detected. Polymeric silicon added to the samples
depolymerized
completely
within a few days,
most rapidly in seawater. When filtered river
water was frozen and subsequently
thawed,
the silicon was largely present in an unreactive form; the unreactive
fraction
could be
removed by membrane filtration.
The silicon
became entirely reactive on standing at about
2% for several hours. Unreactive
forms of
dissolved silicon were not formed when natural waters of salinity
>27%, were similarly
treated.
Dissoelved silicon in natural waters is now
measured almost entirely by absorptiometric methods involving reaction of the
silicon with molybdate ions to form silicomolybdic acid. Higher polymers of silicic
acid probably
do not react completely
during the reaction times used (Alexander
1953), so there has been uncertainty concerning the extent to which analyses of reactive dissolved silicon represent the total
dissolved silicon concentration in various
natural waters. The presence of significant
concentrations of unreactive dissolved silicon could considerably affect conclusions
on the geochemical behavior of silicon in
The form of dissolved
the hydrosphere.
silicon in natural waters has been a subject
of controversy; presumably in seawater and
most freshwaters the dissolved silicon occurs dominantly in reactive form and little,
if any, is highly polymerized
(Krauskolpf
1956; Sill&r 1961; Armstrong 1965). However, there seem to have been no comparisons of reactive and total dissolved silicon
based on the methods currently in use.
We have examined samples of natural
waters from several sources to see whether
they contain any unreactive dissolved silicon and have also measured .the rate of
depolymerization
of polymeric silicic acid
WATERS
added to some of these waters. We have
also investigated the important finding of
Kobayashi (1967) that if river water is
frozen before analysis the freshly melted
water may contain unreactive silicon.
We are grateful to Dr. F. Culkin for
collecting the deep seawater. One of us
(TML) wishes to thank the Natural Environment Research Council for the award
of an Advanced Course Studentship.
METHODS
Samples were collected with plastic sampling apparatus, filtered as soon as possible through Oxoid membrane filters (pore
diam, 0.5-1.0 p), and stored in polyethylene bottles.
Reactive dissolved silicon was de,termined by the method of Mullin and Riley
( 1955), which usually gave a coefficient
of variation of less than 0.5%; salt error
corrections were made using factors from
mixtures of seawater and river water. The
method determines only monomeric and
low molecular weight polymeric silicic acid
units (Alexander 1953). Here, we use the
terms reactive and unreactive with reference to the procedure of Mullin and Riley.
Total dissolved silicon was determined
by the same procedure, after depolymerization of any large silicic acid polymers
by the me,thod of Morrison and Wilson
( 1969). The sample is made 0.01 M with
respect to sodium hydroxide and digested
on a boiling water bath for 10 min. The
effectiveness of this procedure was checked
with a standard solution of silicon that
had been allowed to polymerize until about
60% was unreactive; recoveries were in the
range of 99.8-100.2%. When applied to
river water, the procedure gave a coefficient of variation of less than 1%. Recoveries of silicon are low for waters
containing cations, such as magnesium, in
sufficient amounts to precipitate under the
conditions used. For saline waters, therefore, the cations were removed before the
473
474
NOTES
AND
COMMENT
TABLE 1. Typical results fey reactive and totai
dissolved silicon from replicate analyses of samp.ks
from three rivers in Hampshire,
England
River
Test
Itchen
Beaulieu
Reactive
dissolved
( ,Wliter
4,728
4,827
5,533
Si
)
Total dissolved Si
(/.&liter)
4,734
4,824
5,532
alkaline digestion on a column (22 cm x
1 cm3) of Zeo-Karb 225 cation exchange
resin. The eluate from the addition to the
column of 25 ml of seawater and 65 ml
of distilled water washings was collected,
neutralized with dilute potassium hydroxide solution, diluted to 106 ml, and aliquots
taken for analysis. The recovery of dissolved silicon in this procedure was greater
than 99%.
-L
120
I_1‘0
Tlme(hr)
FIG. 1. Changes in reactivity with time for silicon added in a partially polymerized
form to three
water samples kept at about 25C. n -m
Seawater:
initial
reactive
silicon concentration
45
hg/liter;
5,500 ,ug/liter silicon added.
A -A
River water:
initial reactive silicon concentration
4,651
pg/liter;
1,721 p.g/liter
silicon
added.
0-O
Distilled
water:
5,500 pg/liter
silicon
added.
RESULTS
Reactizjity of silicon in some
natural waters
Nine samples collected in summer from
three rivers in Hampshire, England, were
analyzed in replicate for reactive and total
dissolved silicon (Table 1). In no case
was there a significant difference between
the concentrations of reactive and total dissolved silicon (i.e., no unreactive silicon
was detected ) .
To see whether any unreactive silicon
was produced during regeneration of dissolved silicon from phytoplankton, we collected a sample of water from the River
Thames during a diatom bloom. The water,
initially low in dissolved reactive silicon,
was kept unfiltered in the dark. Periodically, subsamples were drawn from it, filtered, and analyzed for both reactive and
total dissolved silicon. Over a period of
53 days the reactive dissolved silicon concentration increased from 42 to 510 pg/liter, but at no time was a detectable amount
of unreactive dissolved silicon produced.
Seawater collected from a depth of 2,000
m in the North Atlantic Ocean ( 36”N,
10” W) was also analyzed for reactive and
No detectable
total dissolved
silicon.
amount of unreactive silicon was found as
shown by the following results:
Reactive dissolved silicon before cation
removal = 838 pg/liter.
Reactive dissolved silicon after cation removal = 835 pug/liter.
Total dissolved silicon after cation removal = 838 pug/liter.
Depolymerization
of unreactive
silicon
A solution containing polymeric silicon
was prepared by adding acid to a standard
silicon solution until the pH was 8.2 and
then allowing it to stand for 1 month.
About 60% of the silicon was unreactive;
the total silicon concentration
was 172
mg/liter;
no detectable amount was removed by membrane filtration.
This solution was added to samples of distilled water
and filtered samples of water from the
River Itchen and the English Channel.
Subsamples were withdrawn
at intervals
and analyzed for reactive silicon (Fig.
1). The polymeric silicon always depolymerized after it had been added to the
water sample, but the rate varied, being
fastest in seawater and slowest in distilled
water. For example, 5,500 pug/liter of polymeric silicon was completely depolymerized after 18 hr in seawater, whereas
distilled water, to which a similar amount
of polymeric silicon had been added, still
NOTES
AND
contained 3.1% unreactive
silicon after
12 days.
Depolymerization
in distilled water proceeded by a first order reaction; the process was presumably related to the dilution
of the polymeric silicon. The much more
rapid depolymerization
in seawater appeared to follow zeroth order kinetics
initially, suggesting that it may have proceeded by a surface-limited
reaction. It
would therefore be unwarranted
to regard the observed rates of depolymerization as applicable to environmental situations without further work.
The effect of freezing on the reactivity
of dissolved silicon
Filtered river and saline water samples
were frozen at -20C for not less than 2
days, then melted, and analyzed for reactive silicon. River water samples, containing 4,000-5,000 pg Si/liter, just after
melting consistently had reactive silicon
concentrations only lO-20% of those meaA considerable
sured before freezing.
amount of particulate matter was visible
in the freshly melted water. When the
samples were kept at about 25C after melting, the reactive silicon increased rapidly,
usually re~turning to its prefreezing level
after about 3 hr.
For any particular river water sample
the value for total dissolved silicon determined immediately after melting, without
removal of any particles present, agreed
with the reactive silicon concentration before freezing. If the particles were removed by filtration
at any stage in the
period soon after melting, no unreactive
silicon was detectable in the filtrate and
so there was no increase in reactive silicon
with time.
These findings suggest that, on thawing,
a large proportion of the silicon originally
present in the reactive form occurs as polymeric silicon associated with particles large
enough to be retained by a membrane
filter of pore diameter 0.51.0 p. After
melting, the polymers gradually depolymerize by a first order reaction to give
silicic acid units small enough to be determined by the method of Mullin and Riley
475
COMMENT
(1955).
We began measurements only
when the sample had completely melted.
Extrapolation of the kinetic data suggested
that the silicon in the initial melt water
was completely unreactive. In river water
a small amount of particulate
material
remained after depolymerization
was complete, showing that the original particles
were not entirely
siliceous.
A similar
freezing effect was observed with distilled
water containing about 7,000 ,ug Si/liter,
but in this case the rate of depolymerization was about 10 times slower; although
visible particles were not apparent, the
deactivated silicon could be removed by
filtration.
In contrast, no freezing effect was found
with .two filtered samples of seawater containing 40 and 840 pg Si/liter respectively.
Much less particulate material was visible
in the melt water from saline samples. At
intermediate salinities (the proportion of reactive silicon deactivated by freezing was
found to decrease with increase in salinity;
samples with salinity >27%0 showed no effect. If the salinity of a river wa.ter sample
was increased to 35%0with sodium chloride,
then the water still exhibited an appreciable freezing effect, suggesting that the
absence of a freezing effect in seawater is
not due to sodium chloride alone.
CONCLUSIONS
We found no detectable amount of unreactive silicon in any of the natural water
samples analyzed. Polymeric silicon added
to the water samples was unstable and
depolymerized
completely within
a few
days. These findings support the view that
all the dissolved silicon in seawater and
most freshwaters is reactive in the analytical procedures generally used and that no
appreciable amount exists in the form of
large polymers.
The results of the experiments in which
water samples were frozen extend the findings of Kobayashi ( 1967). Samples having
a salinity greater than 27%0 can be analyzed for reactive silicon immediately after
melting without significant error due to
deactivation.
However, samples of lower
476
salinity
NOTES AND COMMENT
should
be
melted
and
then
left
overnight, to ensure complete depolymerization, before analysis for reactive silicon.
J. D. BURTON
T. M. LEATHERLAND
Department of Oceanography,
The University,
Southampton, SO9 SNH, England.
P. s. LISS
School of Environmental
Sciences,
University of East Angliu,
Norwich, NOR 88C, England.
REFERENCES
ALEXANDER, G. B. 1953. The reaction of low
molecular
weight silicic acid with molybdic
acid.
J. Amer. Chem. Sot. 75: 5655-5657.
ON THE PRESENCE OF 0.1-0.5-p
ARMSTRONG, F. A. J. 1965. Silicon, p. 409-432.
In J. P. Riley and G. Skirrow [eds.], Chemical
oceanography,
v. 1. Academic.
KOBAYASHI, J. 1967. Silica in fresh water and
estuaries, p. 41-55.
In H. L. Golterman
and
R. S. Clymo [eds.], Chemical environment
in
the aquatic habitat.
North-Holland.
KRAUSKOPF, K. B. 1956. Dissolution
and precipitation of silica at low temperatures.
Geochim. Cosmochim. Acta 10: l-26.
MORRISON, I. R., AND A. L. WILSON.
1969. The
absorptiometric
determination
of silicon in
water. Part VI. Determination
of polymeric
silicic acid.
Analyst 88: 54-61.
MULLIN, J. B., AND J. P. RILEY.
1955. The
calorimetric
determination
of silicate
with
special reference to sea and natural waters.
Anal. Chim. Acta 12: 162-176.
SILLkN, L. G. 1961. The physical chemistry of
sea water, p. 549-581.
In M. Sears [ed.],
Oceanography.
Publ. Amer. Ass. Advan. Sci.
67.
DISSOLVED ORGANIC MATTER
ABSTRACT
“Dissolved”
organic
matter
(DOM)
in
seawater was fractionated
by filtration
with
filters of different
pore size.
Samples below 600 m in one station of the
East China Sea contained
0.1-0.5-p
DOM.
In a sample from 1,573 m, DOM between
0.1 and 0.5 p amounted to 24% of the total.
In other stations, a large part of the DOM
was composed of fractions smaller than 0.1 ,u.
The nature of 0.1-0.5-p DOM is discussed.
Several methods of filtration have been
used to estimate #the quantity and size distribution of particulate material in seawater (Armstrong
1958; Jeffrey and Hood
1958; Saijo 1964; Saijo and Takesue 1965;
Mullin 1965).
Organic matter that has passed through
a membrane filter of 0.45-p pore size is
generally called “dissolved.” However, various fractions <0.45 ,x can be present in
dissolved organic matter. Jeffrey and Hood
(1958) filtered solutions of 14C-labeled organic materials in seawater through 1.5-p,
0.45-p, and lo-rnp filters; about 90% of
total dissolved organic matter was in the
range <lO rnp.
IN SEAWATER
This work is a preliminary
report on
the presence of 0.1-0.5-p dissolved organic
matter in seawater.
Samples were collected during the Hakuho-Maru cruise in the East China Sea
( May 1968) with Nansen water samplers.
Each 100 ml of sample was filtered through
47-mm-diam Millipore
HA filters (porosity, 0.45 p) under vacuum of about 10
mm of Hg. The same raw water was also
filtered through Membrane MF-14 filters
(porosity, 0.10 p). All filters were soaked
and washed in distilled water before use
until free of organic carbon and then
washed by seawater to be measured.
The organic carbon in each filtrate was
determined by the method of Menzel and
Vaccaro ( 1964). In every case, 2 or 3
replicate samples were run. The standard
deviation between duplicates at the OS2.0 mg C/liter level was within ~0.05 mg
C/liter.
The concentration of dissolved organic
carbon (DOC) was around 1.0-1.2 mg C/
liter at the surface layer, decreasing to 0.50.8 mg C/liter in the deep water (Table 1).