Absorption of Sulfur Dioxide in Water with High Salinity

IRINA PINCOVSCHI1, CRISTINA MODROGAN2* 1University Politehnica of Bucharest, Faculty of Hydraulic Machinery and Environmental Engineering, Department of Hydraulics, 1-7 Polizu Str., 011061, Bucharest, Romania 2 University Politehnica of Bucharest, Faculty of Applied Chemistry and Materials Science, Department Analytical Chemistry and Environmental Engineering, 1-7 Polizu Str., 011061, Bucharest, Romania

The importance of studied problem consists in the fact that the majority of planet water is of high salinity varying between 12 g/L (Black Sea) and 35 g/L (Atlantic Ocean). It results that the pollution of such waters with SO 2 is quite different, varying in large limits [1].
Sulfur dioxide is removed from flue gas by absorption processes because of its toxicity. In fossil-fired power plants at the coast, alkaline seawater is often used as a scrubber agent to chemisorb the acid SO 2 . In arid regions frequently energy-intensive seawater desalination plants which produce fresh water and brine as a waste product are connected with the power plants. The brine is able to substitute seawater in flue gas desulfurization. However, for the design of such a process, systematic investigations of the influence of ions in the brine on SO 2 absorption are lacking. Hence, a reliable process modeling and prediction of the brine's absorption capacity are not possible. Several working groups have modeled the solubility of SO 2 in sea water on the basis of available models. Abdulsattar [3] has modeled the SO 2 solubility in seawater in a temperature range from 10 to 25 °C. The activity coefficients of the ions were calculated from the Bromley model using an extended Debye-Hückel term [4], and the activity coefficients of the molecularly dissolved components were determined on basis of the coefficients of Rabe and Harris [5] as well as Garrels and Christ [6]. Al-Enezi [7] investigated the solubility of SO 2 in sea water solutions at salinities from 0 to 65 g/kg, temperatures between 10 °C and 40 °C and constant SO 2 partial pressure of 22.4 Pa. The applied model is based on the approach of Abdulsattar [3] and includes an extended Debye-Huckel term to describe the activity coefficients. As a result, a quadratic equation of the SO 2 solubility was fitted as a function of temperature and salinity. However, the equation allows no extrapolations to other SO 2 partial pressures and is therefore limited in their applicability [8][9][10][11][12][13][14][15][16][17].
Sulfur dioxide affects the environment in different ways, like direct absorption in water, acid rains and health damages [2,[18][19]. The concentration of SO 2 in residual gases emitted by thermal power plants is about 0.15-0.25%. The SO 2 concentration can to attain bigger values, like in metalurgical ones (1-2 %).
In contrast to the work cited above, in this work the absorption of SO 2 in fundamental binary systems (water + 1 salt) present in seawater was experimentally * email: c_modrogan@yahoo.com investigated and modeled first to determine reliable model parameters for the main components of the electrolyte systems seawater.

Theoretical approach
The SO 2 absorption in water is a phisicochemical process developped as follows: (1) In the equation (1)  where: p SO2 (e) represents the equilibrium pressure of sulfur dioxide, H -Henry constant. The equation (5) can be transformed in a more explicit form: (6) where S represents the sum of chemisorbed species: (7) and K 1 ' is the ratio between chemically and physically absorbed species: (8) Equation (2) highlights the equilibrium concentration of physically absorbed sulfur dioxide: (9) Combining equations (5) and (9) one obtain: The concentration of chemisorbed species [H 2 SO 3 ] can be expressed, combining the relations (3), (4) and (7): (11) By substituting the equation (11) in (10) we obtain the equation (6). The equilibrium constants K 2 =1.7x10 -2 and K 3 =6.2 x 10 -8 from equation (6) have the significance of ionization constants, indicating the H 2 SO 3 strength [11]. The low values of ionization constants K 2 and K 3 are indicating the weak character of sulfurous acid (H 2 SO 3 ). Both constants can be used in calculating the proton concentration after the first and the second ionization step, according to equations: where: [H + ] 1 represents the proton concentration after the first step of ionization; [H + ] 2 -the proton concentration after the second step of ionization; [C] -the total sulfur dioxide concentration in water.
Considering the equation (6) one can see the correlation between SO 2 equilibrium pressure, the solution pH and temperature (K 2 and K 3 are functions of temperature). This dependence can be put in evidence experimentally.

Experimental part
The absorbtion of SO 2 in water was experimented in a device presented in figure 1 [16]. It contains a SO 2 measurement vessel (1) having the volume V, an absorption vessel (2) whose volume is v, a mercury manometer (3) and a water manometer (4). The mercury manometer is used for high SO 2 concentrations and the water manometer for low SO 2 concentrations. The order of operations is the following: by slowly opening the tap (8) a vacuum (∆h 1 ) is made in the vessel (1). Then closing the tap (8) and opening the tap (5) pure SO 2 is introduced, establishing the initial pressure. Closing the tap (5) and opening the tap (9) the communication between the vessel (1) and (2) is established, permitting the SO 2 absorption in the absorbent contained in the vessel (2) till the equilibrium is attained. The SO 2 absorption determines the creation of a vacuum (∆h 2 ). Knowing ∆h 1 and ∆h 2 values and the amount of absorbent (g), a point on equilibrium diagram can be represented.
The absorbed SO 2 volume is ν abs The non-absorbed SO 2 volume ν r is The equilibrium SO 2 pressure ∆h e is: Knowing the amount of absorbent g = 3g H 2 O, the SO 2 concentration C SO2 can be calculated (in g SO 2 /1000 g H 2 O) as follows:

Results and discussions
The experimental method permits to determine the following equluilibrium curves: 1 From the figure 2 one can see that the SO 2 cnceentration in liquide phase is decreasing when the salinity S is increasing. This dependence results also from the figure 2. The knowing of these dependences permits to establish tha SO 2 pollution in different conditions.
The results obtained are presented in figure 3 as a equilibrium diagramme between the solubility of water S [g/L] and SO 2 concentration C SO2 [mol/L]. The variation of marine solubility from 10 g/L to 40 g/L determines the variation of SO 2 concentration in water from 2.6 mol/L to about 3.2 mol/L. In consequence, the variation of water salinity in these limits can detemine a quite big difference of water pollution.

Conclusions
The study reveals a big influence of salinity of water on SO 2 concentration in water, showing that even at big salinity (40 g/L) the concentration is big enough to create the conditions for natural waters acidification. The SO 2 concentration determined by SO 2 absorption from polluting gases can attain 26.5 g/1000 g water. This concentration is quite low for water acidification because of weak character of H 2 SO 3 resulting from SO 2 absorption. Unfortunately in natural waters there are favorable conditions for SO 2 oxidation, conducting finally to H 2 SO 4 formation. Only 0.5 H 2 SO 4 g/1000g H 2 O can determine the value of water pH about 3, damaging the fauna and flora. In order to put in evidence the influence of SO 2 partial pressure and temperature on SO 2 concentration in water, equilibrium diagrams have been determined. These diagrams can be also used to design SO 2 absorption devices.