Determination of the Henry's law constant for dimethyl sulfide in seawater

Chemosphere. Vol. 35, No. 3. pp. 535-544, 1997

Pergamon PII: S0045-6535(97)00118-5

© 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0045-6535197 $17.00+0.00

DETERMINATION OF THE HENRY'S LAW CONSTANT FOR DIMETHYL SULFIDE IN SEAWATER

P.K.Wong 1"2and Y.H. Wang t'3

~Centre for Environmental Studies and 2Department of Biology, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong, and 3Department of Urban and Environmental Sciences, Peking University, Beijing, 100871, China (Received in Germany 16 October 1996; accepted 3 February 1997)

Abstract

This study describes a modified method to measure the Henry's law constant (Hc) of dimethyl sulfide (DMS) across air-water interface. A fixed amount of DMS was added into a 50 ml gas-tight glass syringe with air-water (seawater or distilled water) and equilibrated twice with different volumes of air but identical volume of water. The Hc of DMS in water could be determined by measuring the concentrations of DMS in the air phases of two equilibrium by gas chromtography. The He of DMS in seawater and distilled water determined by the modified method at 18°C were 0.069 and 0.056, respectively, and the relative standard deviation was less than 5%. The modified method was used to determine the effects of temperature on Hc of DMS in seawater and concentration of NaCl on He of DMS in freshwater.

@ 1997 Elsevier Science Ltd

Keywords: Henry's law constant, Dimethyl sulfide, Seawater

To whom correspondence may be addressed

535

536 Introduction

Dimethyl sulfide (DMS) is a volatile compound produced by the decomposition of dimethyl sulfoniopropionate (DMSP) which is synthesized in the surface water of ocean by marine phytoplankton [1]. Some of the DMS thus produced is transported to the atmosphere and becomes the primary precursor of sulfuric acid aerosols in the remote marine environment. Thus DMS poses an important influence on the earth's radiation balance and climate [2]. These findings attract the study on the transport of DMS from the sea to the atmosphere. The study of exchange of DMS between air and seawater is often considered as a two-layer system. Liss and Sluter [3] suggested a method to calculate the transport rate of DMS from the sea to the atmosphere based on the Fick's law of molecular diffusion, and a Henry's law constant (He) has been used to evaluate the flux of DMS from the sea to the atmosphere. However different values of Hc of DMS between air and seawater were reported [4, 5] and made these Henry's law constants of DMS are not reliable. In addition, values of Hc for other volatile solutes of environmental concern are often not available. Mackay and Shiu [6] presented a review of common methods for measuring Hcs of various volatile solutes by (1) use of vapor pressure and solubility data, and (2) direct measuring air and aqueous concentration in system at equilibration.

The first method suffers from the lack of reliable

solubility and vapor pressure data for the target solute. The second method is difficult to be carded out with accuracy at the low concentrations of the target solute in the environment. Dacey et al. [5] used a very complicate glass sparger sytsem to measure the Hc of DMS. Recently, a new method to measure the Hcs of solutes has been presented [7]. But It has not been used in the determination of the Hc of DMS.

The method obtained the Hc of target solute by a ratios of the headspace

concentrations of the solute measuring from a pair of sealed bottles possessing differing liquid volumes. The present study describes a modification of the above method. This modified method offers a simple operation with significant improvement in precision to measure the Hc of DMS in seawater. It is based on the twice equilibrium with different gas volumes in a single gas tight glass syringe rather than using a pair of sealed bottles possessing differing liquid volumes.

Theory

When a fixed amount (M with unit of moles) of a volatile solute is added to a known volume (Va) of air in a gas-tight container with a known volume (Vw) of water, the volatile solute will be

537 partitioned in the air and water phases of the container at equilibrium according to the following equation: M - CwVw+ C,V,

(1)

where Cw= concentration of volatile solute in water (rag/l) C, = concentration of volatile solute in air (i.e. headspace of the conatiner) (rag/l) V~ = volume of water in the container Va = volume of air (i.e. beadspace) in the container

according to the previous study [8] that the Hc of DMS in seawater, especially at low concentration, can be expressed as: Hc = C,/Cw,

thus,

M = C,(V,,]I"I,) + C,V, = c . (VdHo + v . )

(2)

In the present study, a fixed amount of volatile solute, namely DMS, will be added into a gas-tight glass syringe with the same volumes of water and air (i.e. V,1 = Vw~), DMS in total moles of M~ will be partitioned in the air and water phases of the syringe in the first equilibrium according to the equation (3). The concentration of DMS in the air phase (i.e. C,,) will be determined by sampling a small volume of air from the beadspace of the syringe and analyzed by gas chromatogrpahy. Then the air volume of the syringe will be increased by pulling down the piston of the syringe while the volume of water in the syringe is unchanged (i.e. Va > Vw2), and the DMS in total moles ofM 2 will be partition into the air and water phases of the syringe in the second equilibrium according to the equation (4). The concentration of DMS in the air phase (C~) in the second equilibrium will be determined by gas chromatography as those mentioned in the first equilibrium.

M, = C,,[(V,,]I-Ic) + V~]

(3)

M 2 = C,2[(V,~cI,) + V~]

(4)

Since the volume of air sampled from the headspace (i.e. air phase) for the OC analysis in first equilibrium is very small as comparing to the total volume of the air phase of the syringe, the total

538 moles of DMS in first and second equilibrium are assumed to be equal (i.e. M~ = M2). From equations (3) and (4)

C., [(V,/Ho) + V..] = C.2[(V,:-Io)+ V.2] Then

(c.,) (Vw/Hc) + (C.,) (V.,) = (C,2) (V~-Ic) + (C,~) (V.,) (Cat) (V,frlc) - (C a) (VJHc) = (C a ) (Va) - (C,,) (V,,) Hc = [(Ca,) (V.)- (C.~) (V.)] / [(C.,) (V.9- (C,,) (V.,)] Hc = [V. (C., - C.9] / [(C,9 (V,9 - (C.,) (V,,)]

(5)

If equation (5) is divided by Vw. He = (Ca,- Ca2) / [(C,2) (V,2)/Vw - (C,,) (V,t)/Vw] since R~ = VJVw and R 2 = Va/V, are the gas liquid phase volume ratio,

He = (C,,- C,z)/(R,C,2" RIC,I)

(6)

Since the concentration of the solute (C) is proportional to the peak area (A) determined by gas chromatography (i.e. C = fA), from eqution (6), the He can be determined by the following equation.

He = (A,- A2)/(R:A2- R,A,)

(7)

Materials and Methods

Gas chromatographic analysis of dimethyl sulfide Concentration of DMS in the air phase of a gas-tight glass syringe was measured by the method described by Wang and Jiao [9]. In brief, 100 ~tl of air sample from the headspace of the syringe was injected into a Shimazu GC-9A gas chromatograph equipped with a Shumazu C-R2A integrator. A 1.6 m x 3.2 mm stainless steel GC column packed with 10% SE-30 on 60-80 mesh chromasorb (Shimazu) was used. The temperature for the Injector and detector was 150°C. Carrier gas was nitrogen and the flow rate was 50 ml/min. FID (flame ionization detector, Shimazu) was

539 employed with hydrogen and air flows of 40 and 400 ml/min, respectively.

The column

temperature was 800C and the retention time o f a DMS standard was 0.57 min.

Determination of Henry's law constant of dimethyl sulfide

In a 50 ml gas tight glass syringe, 10 ml of seawater (or distilled water) and 10 ml of DMS-free air were added (Vs = Vw). Then 1.0 ~tl DMS (Riedel-de Hahn AG) was added at the concentration of 84.5 mg/L and the end of the syringe was sealed by a telfon cap. After the syringe was incubated in a water bath at 180C for 10 min, it was then vigorously shaken at the water bath for I min to establish the first equilibrium of DMS between gas and water phases. One hundred microliter of headspace gas of the syringe was sampled from the telfon cap-sealed end of the syringe and injected to GC to measure the concentration of DMS. Peak area (A~) of DMS (retention time = 0.57 min) was recorded. Then the piston of the gas-tight glass syringe was slowly pulled down and 30 ml of DMS-free air was introduced into the gas-tight glass syringe by a needle inserted into syringe from the telfon cap-sealed end (i.e. total headspace volume became 40 ml). The syringe was incubated in the water for 10 min, and then vigorously shaken in the water bath for 1 min to establish the second equilibrium of DMS between air and water phases. One hundred microliter of headspace gas of the syringe was sampled from the telfon cap sealed end of the syringe and injected to the GC, and peak area (A2) of DMS was recorded. The Henry's law constant (Hc) of DMS in seawater (or distilled water) was calculated according to the equation (7).

Effects of temperature and concentration of NaCl on the Henry's law constant of DMS

Effect of various temperatures (18, 25, 35 and 440C) on the Henry's law constant (Hc) of DMS in seawater was determined by the procedures described above except the first and second equilibrium were conducted at various temperature in a water bath. Effect of concentration of on the He of DMS in distilled water was determined at 18°C by the procedures described above except the various concentrations of NaCI NaC1 (0, 2, 10, 20, 24 and 32%, w:v) in distilled water were used.

540 Results and Discussion

The modified method was evaluated by the determination of the Henry's law constants (Hcs) of DMS in the seawater and distilled water (i.e. freshwater).

In order to study the effect of

concentration of NaCI on the Hc of DMS in water, the Hc of DMS in distilled water was also determined in the present study by the modified method. Table 1 shows that the precision of the modified method to determine the He of DMS in six seawater and six distilled water samples at 18°C was very high. The RSD of the analysis by the modified method was less than 5%. The Hc of DMS in seawater was 0.069 which was lower than that reported by Loveiock et al. [4]. In their study, the method of measurement was same as that described by Dacey et al. [5]. In order to further evaluate the applicability of the modified method, effects of temperature and concentration of NaCI (i.e. salinity) of the Hc of DMS were determined by the modified method.

Table 1.The precision for Henry'slaw constant(Hc)ofDMSinthe seawater and di~illed waterat 18°C

No.

Al

A2

Hc

AI

A2

Hc

(~tV)

(p.V)

(seawater)

(laV)

(~tV)

(distilled water)

1

16468

13969

0.072

20125

17214

0.059

2

13780

11540

0.069

17593

15144

0.057

3

9360

7773

0.073

13694

11894

0.053

4

7985

6679

0.069

11699

10040

0.058

5

4871

4109

0.066

9853

8406

0.061

6

3978

3328

0.069

7179

6121

0.061

Mean

0.069

0.058

SDa

0.0025

0.003

RSD b(%)

3.6

5.1

aSD = standard deviation bRSD = relative standard deviation

541

Effect of temperature on the Henry's law constant of DMS

Mean, standard deviation (SD) and relative standard deviation (RSD) values of the Henry's law constant (Hc) of DMS in seawater are listed in Table 2. The dependence of He of DMS in seawater on temperature has been well described by the vantHoff equation for temperatures effect on an equilibrium constant. Result from the linear regression of log Hc against I/T is shown in Fig. 1. The r value of the regression fitted the vantHoff equation well. The least squares regression equation was log He = 1736/T(K) + 4.806, the r value was 0.9997 for DMS in seawater.

Table 2. The effect of temperature on Henry's law constant (He) of DMS in seawater at 18°C

Hc No. of Temp (*C)

samples

Mean

SD'

RSD b (%)

18

3

0.069

0.003

4.34

25

3

0.094

0.0088

9.36

35

3

0.149

0.01

6.71

44

3

0.211

0.0078

3.69

"SD = standard deviation bRSD = relative standard deviation

Effect of concentration of NaCl on the Henry's law constant of DMS

The effect of concentration of NaCI (i.e. salinity) on the Henry's law constant (He) of DMS in distilled water are shown in Table 3 and Fig. 2. A strong linear relationship between log Hc of DMS and concentration of NaCl was obtained in the present study (Fig. 3).

542

-0.6 DgT) -0.7

-0.8

_t

-0.9

-1.0

-1.1

-1.2

i 3.2

3.1

i 3.3

T 3.4

3.5

1/T (K, x l O 4)

Fig. 1. The effect of temperature on Henry's law constant (Hc) of DMS in seawater.

Table 3. The effect of concentration of NaCl on Henry's law constant (He) of D M S distilledwater at 18°C

NaCl (%)

0

2 10

20

24

Hc

0.061

0.089

0.146

0.178

0.058

32 0.263

in

543 0.4

0.3

0,2

0.1

0.0 i

i

i

i

0

10

20

30

40

N | C I C o n c h . (%)

Fig. 2. The effect of concentration of NaCI on Henry's law constant (He) of DMS in distilled water at 18°C.

-0.4

-0.0

-0.8

-1.0

-1.2

-1 4

f

r

i

i

0

10

20

30

40

NaCI C o n c . (%)

Fig. 3. The linear relationship between log Henry's law constant (He) of DMS and concentration of NaCl in distilled water at 18°C.

544 Conclusion

The modified method provides a simple and precise measurement of the Henry's law constant (Hc) of DMS in seawater and freshwater, especially at low concentration of DMS in the target environments, The Hc of DMS in seawater was strongly temperature dependent, The Hc of DMS showed a linear relationship between log Hc and concentration of NaCl, and The modified method will be used to measure the Henry's law constants (Hc) of other volatile solutes in freshwater and seawater.

Acknowledgement

Y.H. Wang was supported by a research fellowship of the Centre for Environmental Studies, The Chinese University of Hong Kong.

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