Enhancement of Solubilization and Sorption Behaviors of Polycyclic Aromatic Hydrocarbons Through Involvement of Gemini Surfactants in Soil-Water Systems

Abstract

Contamination of soil, sediment and water by polycyclic aromatic hydrocarbons (PAHs) has been recognized as a major, widespread, environmental waste concern. As a consequence, surfactant enhanced remediation (SER) has emerged as a promising technology for the removal of toxic PAHs. Gemini surfactants as a new generation of surfactants have structures and properties that are unique to the world of surfactants, such as greater efficiency in reducing surface tension and unusual aggregation morphologies in comparison with conventional surfactants. They have generated a growing interest owing to their superior performance in soil and water remediation applications. In this research, systematic studies have been physicochemically investigated in detail via tensiometric, conductometric and solubilization techniques to get insight into the micellar, interfacial and enhanced solubilization aspects of selected single and equimolar bi and ternary Gemini/Gemini and Gemini/conventional surfactant systems. A variety of mutual interaction parameters associated with the micellarization and solubilization process have been correlated through several theoretical treatments to understand the synergism and antagonism in solubilization capabilities of multi-component surfactant systems. Nonideality has been found due to a change in the microenvironment of the surfactant solution when various surfactants are mixed. The results have shown the solubilization power depends on the micellar and interfacial properties of the surfactant and their association with the hydrophilic/hydrophobic properties of solutes. Based on the solubilization analysis, the binding of select symmetric and dissymmetric Gemini surfactants with soil particles and their equilibrium distribution between solid and aqueous phases were evaluated. The adsorption isotherm is plotted and modeled to measure the surface coverage by surfactant molecules under a given condition to study the adsorption mechanism and, hence, determine the interfacial properties of modified solids. The sorption capacity of modified soils and natural soils and the overall partitioning of representative polycyclic aromatic hydrocarbons (PAHs) in a soil-water-surfactant system with a soil-sorbed Gemini surfactant and Gemini micelles are compared and related coefficients are developed. Major contributions coming from this research include: 1) A set of methodologies to evaluate and assist in the design of multi-component Gemini surfactant systems to enhance the solubility of polycyclic aromatic hydrocarbons (PAHs); 2) A clearer view of the effect of the structure of surfactants including the hydrophilic head group, hydrophobic chain length and spacer length, and the chemical nature of the solute on the micellar, interfacial and solubilization properties of surfactant systems; 3) Optimum Gemini surfactant mixtures for potential engineering application; 4) An explanation of the contaminant distribution pattern produced by the behavior of Gemini surfactant adsorption onto soils; 5) An investigation into the partitioning behavior of PAHs in a soil-water-Gemini surfactant and a development of the relative coefficients to produce an in-depth understanding of the mechanisms for surfactant enhanced remediation technology. The outputs of this study will be useful to understand and predict the solubilization and adsorption properties of Gemini surfactant systems and provide proof for exploring new surfactant systems for practical engineering applications.