Mobilization of LNAPL by using polymer-based alcohol emulsion in contaminated aquifers

Abstract

Environmental pollution, notably soil pollution, is one of the urgent problems at present. Light refined petroleum products (gasoline, diesel, engine oil, etc.) which represent light non-aqueous phase liquids (LNAPL) are among the most dangerous environmental pollutants. Penetration of LNAPL into the soil or groundwater makes it unsuitable for plant, animal, and microorganism life. Therefore, the search for effective ways to dispose of hydrocarbon pollution is of great importance. One of the most widely known LNAPL remediation methods is Pump-and-treat (PT), which is considered inefficient due to long operation, and low recovery efficiency (around 60%). Injecting non-Newtonian fluids such as polymers presents a significant industrial application interest for in situ remediation of contaminated soil, especially for aquifers with significantly high permeability and strong heterogeneity. Nevertheless, there is some limitation to using high-viscosity fluids. For instance, the injection of a high-viscosity polymer solution during soil remediation can harm the environment by increasing the injection pressure, leading to aquifer erosion or soil uplift. Hence, developing eco-friendly, sufficiently viscous, and biodegradable fluid is highly in demand. The main goal of this study is to improve the removal of LNAPL (here diesel fuel) with a new polymer based formulation that promotes its dissolution or mobilization. The use of viscous polymer solutions can contribute to stable oil displacement in heterogeneous porous media and simultaneously carry active matters (surfactants, co-solvents) to hard-to-reach low-permeability media. To embody the goal, the remediation of diesel was tested in one-dimensional 30 cm long silica-based sand-pack columns. Biopolymer “xanthan gum” was used due to its rheological and biodegradability properties. Primary flushing by xanthan gum with a concentration of 0.1% w/w, promoted stable displacement in saturated porous media and allowed the recovery of up to 80% of diesel fuel. Further, the polymer solution was formulated with the addition of alcohol solvents to improve sweeping performance. Therefore, fifteen types of biodegradable homologous alcohols (from ethanol to n-octanol) were applied for batch experiments to formulate polymer-based alcohol mixtures. These homologous alcohols are known to exhibit different partitioning behavior upon contact with diesel fuel. Environmentally friendly surfactant sodium dodecyl sulfate (SDS) was used to stabilize a mixture of alcohol and polymer solution. To obtain a stable formulation, batch experiments were carried out at various surfactant concentrations and various volume fractions of polymer solution, alcohol, and diesel fuel. Batch experiments were carried out in small polypropylene vials to test the mechanisms of alcohol partitioning in the presence of diesel fuel. According to the results of batch experiments, ethanol, 1- propanol, and 2-propanol exhibit a mechanism of diesel dissolution into an aqueous phase, which leads to a decrease in the volume of diesel fuel. Homologous alcohols from n-butanol to n-octanol promoted partitioning into the organic phase. As a result, swelling of the volume of diesel fuel was observed, which is typical for the mobilization mechanism. However, only 1-pentanol, 1-hexanol, 1-heptanol, 1- octanol, and 2-octanol among mobilization alcohols showed stable emulsions when mixed with a xanthan-SDS solution. Further, the rheological behaviors of xanthan-SDS-alcohol mixtures were characterized through a rheometer. The preliminary results showed that they exhibit non-Newtonian shear-thinning behavior. Rheological experiments are still ongoing to select the best formulation regarding viscosity properties. Thus, the final polymer-based alcohol emulsion will be used for the secondary flushing (after polymer flushing) in a 1D column to remove residual diesel.

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