Volatilization from Subsurface Aqueous Solutions

Water evaporation and contaminant volatilization from subsurface aqueous solutions are two companion processes that affect contaminant partitioning between the liquid and gaseous phases. Temperature-induced evaporation may affect the concentration of the natural constituents of the subsurface water and thus affect contaminant dissolution in this water. 

Water evaporation occurs when the vapor pressure of the water at the surface, which is temperature dependent, is greater than the water pressure in the subsurface, which is dependent on relative humidity and temperature. The isothermal evaporation process is described by Stumm and Morgan (1996) via a reaction progress model, in which the effects of the initial reaction path are based on the concept of partial equilibrium. Stumm and Morgan (1996) describe partial equilibrium as a state in which a system is in equilibrium with respect to one reaction but out of equilibrium with respect to others. As an example, Stumm and Morgan (1996) indicate (Fig. 7.1) that water with a negative residual alkalinity (i.e.. 

2[Ca ] [HCOj ]) tends to increase its calcinm hardness as a result of water evaporation into the atmosphere, and conseqnently its alkalinity and pH decrease. Eventually, after excessive evaporation, the water may reach saturation, becoming a Na , SO or Cl brine. 

From Fig. 7.1, it may be observed that residual alkalinity initially increases with water evaporation, concomitant with a pH increase, and that negative residnal alkalinity and pH decrease even if alkalinity is due to any kind of anions. Because evaporation leads to a change in the quality of water, the water properties as a solvent for organic contaminants are also changed. This fact should be considered when dealing with contaminant partitioning between phases. 

The molecnlar weight of water vapor (MW= 18) is less than that of air (MW= 29). As snch, the diffnsion of water vapor into the surrounding atmosphere, which consists of a mixtnre of water and air, leads to a buoyant force with upward macroscopic movement. The natural evaporation phenomenon is not only the effect of heat transfer but also a buoyancy-induced motion. The system is at steady state when the vapor pressure of the water at the surface is less than that in the air above, and the resulting condensation is governed by the slow process of molecular diffusion and lamilar flow. 

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Volatilization of contaminants from subsurface aqueous solutions into the subsurface gas phase or the (above ground) atmosphere is controlled by the vapor pressnre. Componnds with high vapor pressure tend to accnmnlate in the gas phase, which may be considered a kind of componnd solnbility in the atmosphere. Partitioning between the liquid and the gas phases is described by Henry s law and is expressed as... 

Piwoni, M.D. and Banerjee, P. Sorption of volatile organic solvents from aqueous solution onto subsurface solids. J. Contam. Hydro ., 4(2) 163-179, 1989. 

Abiotic transformation of contaminants in subsurface natural waters result mainly from hydrolysis or redox reactions and, to lesser extent, from photolysis reactions. Complexation with natnral or anthropogenic ligands, as well as differential volatilization of organic compounds from multicomponent hquids or mixing with toxic electrolyte aqueous solutions, may also lead to changes in contaminant properties and their environmental effects. Before presenting an overview of the reactions involved in contaminant transformations, we discuss the main chemical and environmental factors that control these processes. 

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