Enhanced-solubilization of multicomponent dense immiscible liquid in homogeneous porous media
Complex multi-component nonaqueous phase liquid (NAPL) mixtures can significantly limit the effectiveness of groundwater remediation. The use of enhanced-flushing technologies has emerged as a promising technique for the remediation of sites contaminated with NAPL. A series of one-dimensional column experiments was conducted to quantify the effectiveness of four flushing agents (also called "solubilizing agents"; the terms are synonymous in this study) for the removal of a uniformly distributed multi-component NAPL source within a homogeneous porous medium. The columns were established with NAPL saturations (Sn) targeted between 10- 20%, consisting of an equal 1:1:1 mole mixture of tetrachloroethene (PCE), trichloroethene (TCE), and cis-1,2-dichloroethene (DCE). The solubilization agents investigated included 5 wt.% solutions of two complexing sugars: hydroxypropyl-beta-cyclodextrin (HPCD) and methyl-beta-cyclodextrin (MCD), a 5 wt.% solution of a surfactant: sodium dodecyl sulfate (SDS), and a 50 wt.% solution of ethanol (EtOH). For comparison purposes, a water flush (pump-and-treat) experiment was also conducted as a control, representing the system by which no enhanced- solubilization occurred. In general, initial effluent NAPL concentrations were successfully predicted using Raoult's Law, with minimal deviation (error) between the observed and predicted concentrations. The experimental data were used to test the efficiency of each flushing agent by analyzing the normalized contaminant mass recovery, the mass ratio of contaminant removed to reagent used, and the mole ratio of contaminant removed to reagent used. Mass flux reduction analysis was also used to test the efficiency of the various flushing agents. The results of the analyses showed that EtOH was the most efficient flushing agent when analyzed based on a normalized mass recovery basis. SDS exhibited the most efficient removal when the experiments were analyzed by mass-contaminant removed to mass reagent used. Based on the moles-contaminant removed to moles reagent used ratio efficiency evaluation, MCD was the most effective flushing agent. The results of these experiments indicate that the addition of a chemical flushing agent greatly reduces the time needed to remove each NAPL component, compared to flushing with water alone (i.e., pump and treat). PCE showed the greatest relative solubility enhancement for all enhanced flushing agents tested. MCD and SDS were most effective flushing agents for the three-component NAPL system, removing all three components (DCE/TCE/PCE) with the least number of pore volumes. The HPCD flush showed no appreciable solubility enhancement for DCE and TCE; however, PCE did show a 10-fold enhancement in solubility for this experiment. In terms of mass flux reduction/mass removal (MFR/MR) evaluation, DCE (the highest solubility NAPL component) exhibited the greatest nonideal (inefficient) response during both the water and HPCD flushing experiments. Mass flux reduction behavior was less ideal for the MCD flush compared to the other enhanced-flushing agents. EtOH showed superior performance when evaluated based on a normalized contaminant mass recovery basis, but showed relatively poor performance based on a mass ratio or mole ratio removal evaluation. SDS surpassed all other flushing agents based on a contaminant mass- recovery to reagent used basis, whereas MCD exceeded the removal efficiencies of the other enhanced flushing agents. In general, technical grade HPCD and MCD are more expensive than SDS and EtOH, and therefore may be less desirable choices for use at greater scales; however, their toxicities to microbial communities in the subsurface and potential concerns to human health and the environment are negligible compared to SDS and EtOH. Results from this study indicate that several criteria should be used to evaluate the removal effectiveness of flushing agents for multi-component NAPL systems. Due to the variety of remediation scenarios that could be encountered in the field, these findings will be helpful in developing more efficient and effective remediation strategies and for enabling more accurate prediction of resulting NAPL concentrations to meet the needs of various contaminated sites impacted by chlorinated compounds such as PCE, TCE, and DCE.