'Emulsion locks' for the containment of hydrocarbons during surfactant flushing

Enhanced air sparging for groundwater remediation using alginate gel-based removable hydraulic barriers

The objective of this study was to investigate the effect of a removable physical barrier on the air sparging performance using a lab-scale aquifer model was investigated. The barrier was installed in water-saturated porous media, prior to the air sparging, by injecting calcium chloride aqueous solution into the aquifer with pre-applied alginate solution. Changes in the air flow direction and air flux at the media surface during air sparging were evaluated. With a hydrogel barrier set at the center of the media, the airflow detoured the barrier resulting in a bimodal air flux distribution at the media surface. While employing two gel-formed barriers positioned away from the media's center, the airflow concentrated specifically on the gap between the barriers. The hydrogel was successfully removed using a sodium bicarbonate solution (1.0 mol/L). Using the hydrogel barrier, the performance of air sparging was significantly enhanced for removing contaminants [tetrachloroethene (PCE) and n-hexane mixture] due to increased air flux; 9.8% of PCE applied (7.8 g) was removed during 120 min air sprging for the gel barrier system whereas no PCE was removed for the control. Alginate gel did not show significant sorption capacity for PCE. It was stable in the contaminant up to 68 days with reasonable loss of its mass. Findings of this study present a promising option for air sparging process specifically targeting the contaminant source zone in the aquifer.

Temporary hydraulic barriers using organic gel for enhanced aquifer remediation during groundwater flushing: Bench-scale experiments

This study presents the use of organic gel-forming material for the construction of hydraulic barriers in aquifer, which can be easily removed after use. Experiments on the performance of the temporary hydraulic barrier during NAPL removal (aquifer flushing) were also conducted. An aqueous solution of sodium alginate was injected into the horizontally oriented, 2-dimensional flow chamber packed with sand, followed by gelation using a calcium solution. The alginate gel formed in the porous media produced a circular shape barrier (24 cm diameter, 1.3 cm thickness) that was successfully removed using sodium bicarbonate solution (1.0 M) in 72 h, whereas the gel was stable for 7 days during simulated groundwater flushing at the same flow rate as the sodium bicarbonate solution. When circular hydraulic barriers (12 cm diameter each, 14 cm apart) were set on either side of the NAPL (n-hexane and PCE mixture)-contaminated zone, the increased water flux during water flushing resulted in significantly increased PCE removal by almost 108%. When a surfactant solution (sodium dodecyl sulfate, 0.037%) was applied, the influenced groundwater flow controlled by hydraulic barriers on the NAPL removal was amplified by 196% removal.

Zein-Bonded Graphene and Biosurfactants Enable the Electrokinetic Clean-Up of Hydrocarbons

Nonaqueous phase liquids (NAPL, e.g., hydrocarbons and chlorinated compounds) are common groundwater pollutants. Electrokinetic remediation of NAPLs uses electric fields to draw them toward electrodes and remove them from groundwater. The treatment requires NAPL mobility. Emulsification increases mobility, but at a risk for downstream receptors. We propose using alkaline aqueous solutions of zein and graphene nanoparticles (GNP) to form conductive materials, which could also act as barriers to control NAPL migration. Alkaline zein-GNP solutions can be injected in the polluted soil and solidified by neutralizing the pH (e.g., with glacial acetic acid, GAA). Shear rheology experiments showed that zein-GNP composites were cohesive, and voltammetry showed that GNP increased electrical conductivity of zein-based materials by 3.5 times. Gas chromatography-mass spectroscopy (GC-MS) demonstrated that the electrokinetic treatment of model sandy aquifers yielded >60% and ∼47% removal of emulsified toluene in freshwater and in salt solutions, respectively (with 30 min treatment using a 10 V differential voltage between a zein-GNP and an aluminum electrode. NaCl was used as model salt contaminant. The conductivity of surfactant solutions was lower in saline water than in freshwater, explaining differences in toluene removal. Toluene-water emulsions were stabilized using the natural surfactants lecithin and saponin. These surfactants acted synergistically in stabilizing emulsions in either freshwater or salt solutions. Lecithin and saponin likely interacted at toluene-water interfaces, as indicated by the morphology, interfacial tension and compressional rigidity of toluene-water interfaces with both components (relative to interfaces of either lecithin or saponin alone). The compressional behavior of interfacial films was well-described by the Marczak model. Electrokinetic treatment of saturated model sandy aquifers also decreased the turbidity of emulsions of water and either tricholoroethylene (TCE, by ∼41%) or diesel (by ∼75%), in the presence of a bacterial biosurfactant. This decrease was used as semiquantitative indicator of NAPL removal from water.

Selective solvent filters for non-aqueous phase liquid separation from water

Injectable filters permeable to water but impermeable to non-polar solvents were developed to contain non-aqueous phase liquids (NAPL) in contaminated aquifers, hence protecting downstream receptors during NAPL remediation. Filters were produced by injecting aqueous solutions of 0.01% chitosan, hydroxyethylcellulose and quaternized hydroxyethylcellulose into sand columns, followed by rinsing with water. Polymer sorption onto silica was verified using a quartz-crystal microbalance with dissipation monitoring. Fluorescence and gas chromatography mass spectroscopy showed low ppm range concentrations of non-polar solvents (e.g., hexane and toluene) in water eluted from the filters (in the absence of emulsifiers). The contact angles between polymer-coated surfaces and hexane or toluene were > 90°, indicating surface oleophobicity. Organic, polar solvents (e.g. tetrahydrofuran and tetrachloroethylene, TCE) were not separated from water. The contact angles between polymer-coated surfaces and TCE was also > 90°. However, the contact area with polymer coated surfaces was greater for TCE than non-polar solvents, suggesting higher affinity between TCE and the surfaces. Emulsifiers can be used to facilitate NAPL extraction from aquifers. Emulsion separation efficiency depended on the emulsifier used. Emulsions were not separated with classical surfactants (e.g. Tween 20 and oleic acid) or alkaline zein solutions. Partial emulsion separation was achieved with humic acids and zein particles.