Influence of organic acid and base solution chemistry on interfacial and transport properties of mixed wastes in the subsurface.

Abstract

Groundwater contamination by organic compounds is widespread. Many such compounds exhibit sufficiently low solubility in water that they exist and migrate in the subsurface as a separate liquid phase. Current cleanup technologies are inefficient at treating nonaqueous phase liquid (NAPL)-contaminated sites. As such, there is a need for more appropriate remediation methods, stimulating a demand for an expanded knowledge of the physical, chemical, and transport properties controlling NAPL behavior in the subsurface. This study sought to delineate the relationship between the solution chemistry and transport properties of multiphase wastes containing surface-active solutes, such as organic acids and bases. Controlling pH and surface-active solute concentration for air-water, o-xylene-water, and tetrachloroethylene-water systems was shown to effect changes in interfacial tension, wettability, and ultimately, the drainage capillary pressure-saturation relationship in quartz sands. The value of pH relative to the pK_a of the organic acid or base emerged as a primary parameter determining the net surface activity, and ultimately, the capillary pressure. In systems containing an organic acid, lowering the pH through the pK_a induced formation of the protonated neutral acid, which was highly surface-active at the air-water interface, but partitioned readily into an adjacent organic liquid phase, where its surface activity was minimal. The capillary pressure-saturation relationships for the air-water and organic-liquid water systems therefore showed opposing trends with respect to pH. Raising pH increased capillary pressure in air-water-quartz systems, but decreased capillary pressure in organic liquid-water-quartz systems proportionally to the corresponding interfacial tension for each system. In systems containing an organic base, lowering the pH induced formation of protonated monomer and dimer forms of the base, which not only reduced surface tension at the air-water interface, but also sorbed to the quartz-water interface, resulting in reductions in the hydrophilicity of the surface as measured by the contact angle. Replacing air with an organic liquid as the nonaqueous phase induced partitioning of dodecylamine, where, similar to the case with octanoic acid, its surface activity was minimal. Capillary pressure measurements in these systems directly reflected the reductions in both interfacial tension and contact angle brought about by changes in the solution chemistry.