오염 토양 세척 공정에서의 계면활성제 재사용을 위한 연구

Abstract

Remediation of soil contaminated with hazardous hydrophobic organic compounds (HOCs) such as polycyclic aromatic hydrocarbons (PAHs) is a major environmental concern due to the toxic and carcinogenic properties of these compounds. PAHs are released into the environment as a result of combustion of fossil fuels or by accidental discharge. Due to their hydrophobicity, PAHs have low water solubility and are strongly sorbed to soil and sediment. Therefore, biodegradation of PAHs is very slow, resulting in their persistence in environments for long periods of time. A potential technology for rapid removal of PAHs sorbed to soils is soil washing with a surfactant solution. While the use of surfactants significantly enhances the performance of soil washing, operation costs are significantly increased as surfactant dosages are increased. Even though many approaches to reduce surfactant dosage have been introduced such as physical separation or chemical degradation of contaminants from surfactant solution, some of these have many drawbacks due to high-energy requirements, incomplete separation, or formation of potentially hazardous intermediates. Therefore, it is necessary to develop a surfactant recovery technology that is more simple, economic, and effective. In this study, selective adsorption of a HOC using activated carbon as a means of recovering surfactant after a soil washing process was investigated. In order to evaluate feasibility of selective adsorption process, we examined in various experimental conditions. And also we developed mathematical partitioning model for soil washing and selective adsorption to elucidate each system. As a model system, phenanthrene (PHE) was selected as a representative HOC and five different of nonionic surfactants (Triton X-100 (TX100), Tween 40 (TW40), Tween 80 (TW80), Brij 30 (B30), Brij 35 (B35)) and one of anionic surfactant (sodium dodecyl sulfate: SDS) were used. Four types of activated carbons that differed in size (Darco 100 (D100), Darco 20-40 (D20), 12-20 (D12) and 4-12 (D4) mesh sizes) were used in adsorption experiments. The efficiency of the selective adsorption process was represented as selectivity parameter. A selectivity value larger than 1 indicates that more contaminants relative to surfactant are adsorbed onto the activated carbon and that surfactant recovery is theoretically possible. In various surfactant solutions, the selectivity was much higher than 1, but surfactant recovery and soil washing efficiency were significantly differed by surfactant types. This indicated that to select proper surfactant, we should consider the soil washing efficiencies, surfactant losses, PHE removal and surfactant recovery efficiencies. In the surfactant mixtures as nonionic-anionic surfactant (TX100-SDS system), the sorbed amount of TX100 onto activated carbon was reduced by addition of SDS. Moreover the selectivity was also much higher than 1, thus activated carbon adsorption can be a good method for surfactant recovery in nonionic-anionic surfactant mixtures as well as in pure nonionic surfactant solutions. The model system (PHE/TX100) for soil washing and selective adsorption was analyzed using a mathematical partitioning model and compared to intrinsic sorption of PHE without the effect of sorbed surfactant. PHE was sorbed onto activated carbons or soils in a greater amount than an estimated value by intrinsic sorption, even though surfactant molecules covered most of activated carbon and soil surfaces. The enhanced sorption of PHE was attributed to the function of sorbed surfactant as an effective sorbent in soil washing and surfactant recovery process both. As a result, the molar solubilization ratio for sorbed surfactant (MSRs) was higher than that for micellar surfactant in bulk solution (MSR).In various soil washing scenarios, activated carbon adsorption process was evaluated using mathematical model. The simulation results suggest that only a small amount of activated carbon is necessary to effectively remove contaminants from surfactant solutions. And selective adsorption process using highly adsorptive activated carbon (D100) was more efficient than without selective adsorption process or using lower adsorptive activated carbon (D20). The adsorption technology using activated carbon is simple, fast, and inexpensive, and thus may be a reasonable alternative to recovery surfactant in soil washing process.