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Microwave vacuum drying of marine sediment determination of moisture content, Kinetyka suszenia - artykuły

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ELSEVIER
Analytica Chimica Acta 342 (1997) 247-252
ANALYTICA
CHIMICA
ACTA
Microwave vacuum drying of marine sediment: determination of
moisture content, metals and total carbon
P.A. Tanner*, L.S. Leong
Department
of
Biology and Chemistry, City University
of
Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong
Received 9 September 1996; received in revised form 15
November 1996: accepted 22 November 1996
Abstract
No
significant difference was found in the moisture content of marine sediment determined by using traditional oven drying
at 105°C and microwave-drying under vacuum. Subsequent determinations of the major and trace metals, Al, Ca, Fe, Cr, Cu,
Mn, Ni, Pb, and Zn, gave similar results after employing the different drying procedures. The total carbon contents were also
investigated and again no significant difference was found. Vacuum microwave drying requires only 10 min of time compared
to more than 8 h for oven-drying. Contamination of samples is also minimized when microwave digestion is adopted in the
same system for the subsequent analysis of metal contents.
Keywords: Drying; Vacuum; Microwave; Sediment; Metals; Total carbon
1. Introduction
assessment of the effects of different methods including
oven-, air- and freeze-drying upon the obtained moist-
ure and metal content was carried out previously, and
we found that oven- and freeze-drying gave similar
values for moisture and metal contents of sediment,
but that air drying gave significantly different results
[3] under the ambient laboratory conditions.
In the past few years, there has been a growing
interest in the use of microwave radiation in analytical
and environmental chemistry. Zlotorzynski [4]
reviewed the latest advances in the application of
microwave energy to analytical chemistry including
digestion, extraction, chemical reaction, pre-concen-
tration, and desorption of samples. With respect to soil
and sediment studies, the adoption of microwave oven
for sample digestion prior to the analysis of metal
contents is well documented [5-81. However we are
unaware of the use of the microwave vacuum techni-
The analyses of heavy metals and total carbon
contents in marine sediment are widely used to assess
long-term anthropogenic inputs to the marine environ-
ment. In any given sediment study program, the
storage of large batch of bulk samples can cause
congestion of laboratory space. The usual practice
involves drying a small portion of the collected sam-
ples to remove moisture and then weighing and storing
them until the subsequent analyses. Although standard
procedures are available for the instrumental analysis
[l] and digestion [2] of sediment samples, less atten-
tion has been paid to their pre-treatment, especially the
drying procedures. In view of this, a comparative
*Corresponding
author. E-mail: bhtan@cityu.edu.hk;
fax: +852
2788 7406.
0003-2670/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved.
PII
SOOO3-2670(96)00582-X
248
PA. Tanner; L.S. Leong/Anctlvtica Chimica Actu 342 (1997) 247-252
que for marine sediment drying, although it has been
ment: Aberdeen Typhoon Shelter and Shuen Wan. At
applied to the drying of foodstuffs [9], wood [IO], and
Shuen Wan, the major intrusive igneous rock forma-
cotton [ll]. Under reduced pressure the sediment
tion is granodiorite, whereas sedimentary and water-
water is lost at lower temperatures so that the decon-
laid volcaniclastic
rocks occur near the former
position or loss of volatile organic matter occurs less
sampling site. Aberdeen Typhoon Shelter sediment,
and local hot spots are avoided. Furthermore, sample
collected by a Ponar grab, was deeply situated beneath
oxidation is minimized. This paper documents a
seawater (>lO m), highly anoxic, finely particle-sized,
comparison of traditional oven drying at 105°C with
and heavily polluted by industrial and domestic sew-
microwave drying of sediment under vacuum prior to
age discharges. Samples were collected by using a
the analyses of metals using inductively coupled
plastic scoop from the shallow coastal region in Shuen
plasma atomic emission spectrometry (ICP-AES)
Wan, where the sediment was oxic, subjected to tidal
and flame atomic absorption spectrometry (FAAS),
flushing and currents, coarsely particle-sized, and less
as well as total carbon content using a CHN analyzer.
prone to direct pollution sources.
2.3.
Reagents
2.
Experimental
2.1.
Apparatus
Reagent water was prepared firstly by ion-exchange
and then double distillation with a Cyclon Fistreem
system. AnalaR concentrated nitric acid (69%) for
digestion was purchased from BDH. Standard metal
stock solutions (1000 ppm) for calibration purpose
were purchased from High-Purity Standards. Standard
cystine powder (Sigma) was used for single-point
calibration of the CHN analyzer.
Traditional drying was conducted at 105°C at the
mid-upper shelf of a She1 Lab oven (range: 50°C to
2OO”C), with ceramic crucibles used as sample
holders. No microwave vacuum drying equipment
was available at City University of Hong Kong
(CityU) so that this operation was performed by Mile-
stone S.r.1. in Italy. The unit employed was a Milestone
LAVIS-1000 multiMOIST system with micro-proces-
sor-controlled power up to 1000 W. All sediment
samples were digested at CityU in CEM Teflon-lined
digestion vessels using a CEM MDS-2000 microwave
digester. Perkin-Elmer Plasma 1000 ICP-AES and
Shimadzu AA-6501s FAAS instruments were
employed for the determination of major and minor
metals. A Leco CHN-900 carbon, hydrogen and nitro-
gen determinator model 600-800-300 was employed
for the determination of total carbon content in the
samples. All glassware were immersed in a mixture of
laboratory grade detergent and bleach for 16 h, then
rinsed thoroughly with tap water and immersed in an
acid bath (1
:
2
:
9 concentrated nitric
:
hydrochloric
acid
:
water) for 16 h, then triple-rinsed with deio-
nized water, and finally rinsed with deionized double-
distilled water and dried in air before use.
2.4.
Procedures
2.4.1. Determination of moisture content
In the laboratory, the silt-clay fraction (<63 urn)
was wet-sieved from the bulk sediment with seawater
collected in situ, in order to normalize the grain size
effects in analyses. The sieved silt-clay slurry was
collected in a beaker and mixed thoroughly with a
glass rod. Approximately 40 cm3 of this slurry was
then transferred to each of the ten 50 cm3 Nelgene
tubes and spinned down at 2000 rpm for 10 min with a
bench top centrifuge. The supematant seawater was
decanted and the residue was homogenized. Sub-
samples of two 5 g portions were collected from each
tube, weighed to three decimal places (mass X), and
then subjected to traditional oven drying and micro-
wave drying under vacuum, respectively. For the
latter, samples of up to ca. 8.0 g, weighed in
50 cm3 quartz tubes or polypropylene vials, were dried
under 650 mbar vacuum for up to 15 min. The routine
analytical microwave drying method for subsequent
analyses utilized a one-step programme with a time of
10 min using 500 W power. At the end of the drying
2.2.
Sample collection
Bulk sediment samples were collected from two
different locations in the Hong Kong marine environ-
l?A.
Tanner, LX Leong/Analytica Chimicn
Actu
342 (1997) 247-252
249
procedure, the sample mass was E Percent moisture
contents were determined as 100(X-Y)/X. All the
dried and weighed samples were then ground to
fine powder in a mortar and pestle, and stored in a
desiccator containing
silica gel before chemical
concentrated nitric acid in a CEM lined digestion
vessel and batches of 5 samples plus a blank (without
sediment) were digested in a CEM MDS-2000 micro-
wave digester. The digested solutions were then
cooled in a fumehood, filtered through Whatman
541 paper, and the washings were made up to
25 cm3 (in 20% nitric acid solution). ICP-AES was
used for the determination of Al, Ca, Cr, Cu, Fe, Mn,
Ni and Zn, while FASS was used for the determination
of Pb in the digested sample solutions. Selection of the
appropriate analytical lines for each metal in ICP-AES
analyses.
2.4.2.
Determination of metal contents
Metals in sediment were leached according to the
USEPA Method SW846-3051 [2]. A dried and finely
ground sample of 0.500 g was mixed with 10 cm3
Table 1
Instrumental descriptions for CEM MDS-2000 microwave digester, Perkin-Elmer Plasma 1000 ICP-AES, Shimadzu AA-6501s FAAS, and
Leco CHN-900 analyzer
Instrumental descriptions
Microwave digester
1. Specifications
Magnetron frequency: 2455 MHz
Nominal power: 630250 W
Power: 100% of nominal value
Pressure: 85 psi
Time: 20 min
Time after pressure: 10 min
Temperature: 175°C
Number of vessels: 6 (maximum)
Gas type: air-acetylene
Background correction: deuterium lamp
Flow: 2.0 1 min-’
Burner height: 7 mm
Slit: 0.5 nm
Pb: 2 17.0 nm
RF
2. Method 3051
FAAS
1. Settings
ICP-AES
2. Analyte line
1. Settings
power:
Argon plasma flow: 15.0
1
min-’
Auxiliary flow: 1.O 1 min-’
Nebulizer uptake: 1.0 1 min..’
Pump rate: 1.0 cm3 min-’
Viewing height: 15 mm
Viewing window: 0.1 nm
Background correction: auto
Read delay time: 25 s
Al: 308.215 nm
Ca: 315.887 nm
Cr: 357.869 nm
Cu: 324.754 nm
Fe: 259.940 nm
Mn: 257.610 nm
Ni: 341.476 nm
Zn: 213.856 nm
1038 W
2. Analyte lines
CHN analyzer
Operating conditions
Oxidation furnace temperature: 950°C
Reduction furnace temperature: 650°C
Helium flow: 100 cm3 min-’
Helium pressure: 40 psi
System pressure: 3 psi
250
PA. Tarmel; L.S. L.eong/Analytica Chimica Acta 342 (1997) 247-252
analysis was facilitated by using the wavelength char-
acterization tables and graphics system of the spectro-
meter [12], and several different wavelengths were
used for each metal in trial analyses. The final instru-
mental conditions of the microwave digester, ICP-
AES, and FAAS are summarized in Table 1. Dilution
of sample solutions was carried out when necessary.
Instrumental calibration was performed by 3-point
external calibration. Calibrator solutions of each metal
were prepared from 1000 ppm stock solutions imme-
diately before analysis, diluted to various concentra-
tions with deionized double-distilled water, and
matrix-matched with the same amount of acid as that
in the sample solutions. The accuracy and precision of
the instrument and calibration curve were evaluated
and checked with replicate analysis of control samples
at the beginning and subsequently at every tenth or
twentieth sample analyzed. Analysis of blank samples
was conducted to check the purity of reagents and
cleanliness of apparatus. The method recoveries for
metals using the microwave digestion method were
determined for several certified reference materials,
oven dried at 105°C: (a) BCSS-1, coastal marine
sediment; (b) PACS- 1, harbour marine sediment (both
purchased from National Research Council Canada);
and (c) Buffalo River sediment (from NIST). The %
recoveries (listed in the order (a), (b), (c); and where
ND indicates not determined), were for Al 30,27, ND;
Ca 70,38,73; Cr 41,54,53; Cu 92,86,77; Fe 80,68,
71; Mn 91,56,83; Ni 87,85, ND; Zn 97,63,77; Pb 74,
74, ND.
micro-processor) installed with Microsoft Excel and
SigmaStat. Significant differences of the determined
physico-chemical parameters between the two drying
methods were compared using Student’s t-test at
cc=O.O5 according to Zar [13].
3.
Results and discussion
3.1.
Moisture content
A preliminary experiment to investigate the time
required for complete drying using the vacuum micro-
wave system was conducted with 5-replicate sediment
samples collected from Aberdeen Typhoon Shelter.
The samples were weighed 7 times during the
vacuum-drying process of 15 min. The weights of
samples were expressed relative to 100% at the initial
time. Polynomial regression of the mean weight per-
cent of the samples,
W,
against time,
t
min, gave the
equation:
W =
99.9 -
15.5t + 1.38t2 +
0.0409t3,
(IV = 5; R* =
1.00)
(1)
Thus the difference in weight percent from
t=lO min to t=l
1
min was found to be 0.06%. The
standard deviation of the determined weight percent
for the 5 sediment samples, s, was found to decrease
with time according to the regression:
s=2.06 -
0.267t +
0.00917?, (N = 6;
R* =
0.936).
(2)
After a drying time of 10 min the range in
moisture content was less than 1% of the mean value
(N=5). This precision was considered satisfactory,
and since from (1) the loss in mass after 10 min
was negligible, the drying operation was set at
this time.
The results obtained for moisture contents of
sediment samples using oven drying and vacuum
microwave drying are given in Table 2. No significant
difference was found between the moisture contents
so determined (Student’s t-test, a=0.05). The
precisions of the two drying methods for the
determination of moisture content, as indicated
by the standard
2.4.3.
Determination of total carbon content
A CHN analyzer was employed for the determina-
tion of total carbon content in sediment. Dried and
finely ground sediment samples of 2.000f0.200 mg
(nominal weight for the CHN analyzer) was weighed
using a Leco 650 micro-balance and encapsulated in a
tin capsule. Standard cystine (29.99% carbon content),
weighed in the same manner as the samples, was used
for calibrating the CHN analyzer. Table 1 also sum-
marizes the instrumental conditions for the CHN
analyzer.
2.5.
Calculations and statistics
Calculations were performed using an IBM-com-
patible personal computer (80486DX2- 100 MHz
deviations,
were found to
be comparable (Table 2).
RA. Tame< LX L.eong/Analytica Chimica Acta 342 (1997) 247-252
251
Table 2
Moisture content (%), metals @g g-l), and total carbon (%) in marine sediment samples using oven and vacuum microwave drying methods
Aberdeen typhoon shelter
Shuen Wan
Oven drying
Microwave drying
Oven drying
Microwave drying
Moisture content
Pb
Zn
Mn
CU
Ni
Cr
Fe
Al
Ca
Total carbon
59fl (2.4)
95f3 (3.5)
428f14 (3.2)
383flO (2.5)
327f5 (1.5)
29+2 (5.2)
5513 (4.6)
32 8OOf8OO (2.5)
40OOOf7000 (18)
79OOf200 (2.1)
2.6fO.l (3.8)
58.0f0.5 (0.9)
93+5 (4.8)
430f13 (2.9)
382f12 (3.1)
331fll (3.3)
29f2 (6.3)
54+3 (6.3)
32 OOO+lOOO (4.0)
37 000~7000 (20)
76OOf300 a (4.2)
2.5fO.l (4.0)
56.8f0.8 (1.4)
128f5 (4.1)
177f4 (2.3)
651f13 (2.0)
430+23 (5.3)
12.3f0.7 (5.7)
17.4zt0.8 (4.6)
27 9OO~t800 (2.8)
22 OOOf8000 (35)
16OOOf200 (1.3)
2.3fO.l (4.3)
56.3f0.8 (1.4)
130f5 (3.6)
173f5 (2.8)
626zt23 (3.7)
427+17 (4.0)
13f2 (13)
19.5zt0.8 (4.1)
27 OOOf 1000 (3.9)
29000f6000 a (19)
15400f400 a (2.5)
2.3f0.2 (8.7)
Values expressed as mean&SD (N=lO), with % coefficients of variation in parentheses.
a Indicates significant difference between drying methods (Student’s t-test, a=O.O5).
3.2. Metal contents
wave-drying. Again, no significant difference was
found between the results so determined (Student’s
t-test, c~=O.05). The carbon content is some 13 times
smaller than that of the CHN calibration standard, and
in view of this the agreement is good.
The amounts of Al, Ca, Cr, Cu, Fe, Mn, Ni, Pb, and
Zn determined in the two sets of sediment samples
using oven- and microwave-drying prior to digestion
and instrumental analysis are given in Table 2. A wide
range of analyte concentrations - from Al in the range
of 40273 pg g-’ to that of Ni 12.3 pg g-’ - were
chosen in order to test the trends in metals obtained for
the different drying methods. Except for Al in Shuen
Wan sediment, and Ca in sediment from both sites, no
significant difference was found between the deter-
mined metal contents when using the two drying
methods (Student’s t-test, c~=O.05). The differences
are not associated with the drying process, but with the
non-homogeneous distribution of shell material and
aluminosilicates in the sediment samples. This was
particularly notable from the visual inspection of the
Shuen Wan coastal sediment. The standard deviations
of the ten-replicate determinations were small in
nearly all cases (Table 2). Aluminium was again an
exception. The coefficients of variation (CV) in the
determined chemical parameters for the two drying
methods ranged between 1.3% and 8.7% (except for
Ni from Shuen Wan microwave-dried: 13%, and for all
determinations of Al: 18% to 35%).
4.
Conclusions
Under vacuum, the boiling point of water is lowered
and thus evaporation takes place at a lower tempera-
ture. The advantages of using microwave drying of
sediment under vacuum conditions include a milder
thermal mode of drying, rapid removal of vapours
during the shorter analysis time, and the minimization
of degradation or oxidation of samples. Traditional
techniques like oven- or air-drying are time consum-
ing (>8 h to some days) and microwave vacuum
drying of sediment provides a faster turn-around
time on a higher number of samples. In the present
study, the agreement of the determined moisture
content for the two different drying methods
employed is good. Considerable error is to be expected
from the uncertainty in the water content of the “wet
sediment”. In the subsequent analyses of metals and
carbon in sediment samples, good agreement has also
been found when using the two different sample
drying procedures. In this work we have employed
two different microwave systems in the analytical
process. The sample contamination may be further
minimized when the same microwave system and
3.3.
Total carbon content
Table 2 also presents the results for total carbon
content of the two samples using oven- and micro-
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