Refinement of the density-modified displacement method for efficient treatment of tetrachloroethene source zones

Emulsion-based recovery of a multicomponent petroleum hydrocarbon NAPL using nonionic surfactant formulations

Surfactants can aid subsurface remediation through three primary mechanisms - solubilization, mobilization and/or emulsification. Among these mechanisms, emulsification in porous media is generally not well studied or well understood; particularly in the context of treating sources containing multicomponent NAPL. The objective of this research was to elucidate the processes responsible for recovery of a multicomponent hydrocarbon NAPL when surfactant solutions are introduced within a porous medium to promote the formation of kinetically-stable oil-in-water emulsions. Emulsifier formulations considered here were selected to offer similar performance characteristics while relying on different families of non-ionic surfactants - nonylphenol ethoxylates or alcohol ethoxylates - for emulsification. The families of surfactants have particular environment relevance, as alcohol ethoxylates are often used where replacement of nonylphenol content is necessary. Results from batch and column studies suggest performance of the two formulations was similar. With both, a synergistic combination of emulsification and mobilization led to recovery of a synthetic gasoline NAPL. The relative contribution of solubilization to the recovery was found to be minor. Moreover, the physical processes associated with emulsification and mobilization acted to limit the amount of preferential recovery (or fractionation) of the multicomponent NAPL.

Colloidal biliquid aphron demulsification using polyaluminum chloride and density modification of DNAPLs: optimal conditions and common ion effect

Dense non-aqueous phase liquids (DNAPLs) entrapped and pooled in aquifers serve as a long term source of groundwater contamination because of their low solubility and high density. Density modification displacement (DMD) with colloidal biliquid aphrons (CBLAs) is a promising approach to prevent DNAPL downward migration during surfactant-based remediation processes. CBLA demulsification and quick release of internal light organic matter is the key to density modification of DNAPLs. In this work, it is reported for the first time that polyaluminum chloride (PAC) could destabilize CBLAs efficiently. The optimum conditions for demulsification of CBLAs by PAC were obtained; the effects of several specific ions in groundwater on demulsification of CBLAs by PAC were investigated. The results indicated that the CBLA was completely demulsified by PAC within 10 minutes and released light organic matter. It recorded the highest demulsification efficiency when the addition ratio (VPAC/VCBLA) was 2 : 1, concentration of PAC was 0.7 g L-1 and the PVR of CBLAs was 8. Most cations (sodium, magnesium and calcium ions) had inhibitory effects on demulsification of CBLAs by PAC with increasing ion concentration, but iron ions had no effect on it. Sulfate anions showed a stronger inhibitory effect on demulsification of CBLAs by PAC compared to chloride ions. With PAC as the demulsifier, CBLAs could be demulsified efficiently, irreversibly modifying the density of trichloroethylene in 5 minutes.

Surfactant-enhanced aquifer remediation: Mechanisms, influences, limitations and the countermeasures

In recent years, surfactant-enhanced aquifer remediation (SEAR) has attracted increasing interest duo to the high efficiency of removing non-aqueous phase liquids (NAPLs) from aquifer. A thorough understanding of SEAR is necessary for its successful implementation in field remediation. This paper reviewed the SEAR technology in a comprehensive way based on the recent research advances. Firstly, an overview of the basic processes and mechanisms underlying the technology was presented. Secondly, applications of SEAR and the factors that influence the performance were summarized. Thirdly, the key limitations of SEAR, which are downward migration of dense-NAPLs, secondary pollution of surfactants, adsorptive, precipitative and partitioning loss of surfactants, and heterogeneity of the aquifer, were reviewed. Finally, the recent advances in modifying SEAR to overcome the limitations were discussed in detail. The review will promote our understanding of SEAR technology and provide some useful information to improve the performance of SEAR in applications.

Sub-CMC solubilization of dodecane by rhamnolipid in saturated porous media

Experiments were conducted with a two-dimensional flow cell to examine the effect of monorhamnolipid surfactant at sub-CMC concentrations on solubilization of dodecane in porous media under dynamic flow conditions. Quartz sand was used as the porous medium and artificial groundwater was used as the background solution. The effectiveness of the monorhamnolipid was compared to that of SDBS, Triton X-100, and ethanol. The results demonstrated the enhancement of dodecane solubility by monorhamnolipid surfactant at concentrations lower than CMC. The concentrations (50–210 μM) are sufficiently low that they do not cause mobilization of the dodecane. Retention of rhamnolipid in the porous medium and detection of nano-size aggregates in the effluent show that the solubilization is based on a sub-CMC aggregate-formation mechanism, which is significantly stronger than the solubilization caused by the co-solvent effect. The rhamnolipid biosurfactant is more efficient for the solubilization compared to the synthetic surfactants. These results indicate a strategy of employing low concentrations of rhamnolipid for surfactant-enhanced aquifer remediation (SEAR), which may overcome the drawbacks of using surfactants at hyper-CMC concentrations.

Partition behavior of surfactants, butanol, and salt during application of density-modified displacement of dense non-aqueous phase liquids

Density-modified displacement (DMD) is a recent approach for removal of trapped dense NAPL (DNAPL). In this study, butanol and surfactant are contacted with the DNAPL to both reduce the density as well as release the trapped DNAPL (perchloroethylene: PCE). The objective of the study was to determine the distribution of each component (e.g., butanol, surfactant, water, PCE) between the original aqueous and PCE phases during the application of DMD. The results indicated that the presence of the surfactant increased the amount of n-butanol required to make the NAPL phase reach its desired density. In addition, water and anionic surfactant were found to partition along with the BuOH into the PCE phase. The water also found partitioned to reverse micelles in the modified phase. Addition of salt was seen to increase partitioning of surfactant to BuOH containing PCE phase. Subsequently, a large amount of water was solubilized into reverse micelles which lead to significantly increase in volume of the PCE phase. This work thus demonstrates the role of each component and the implications for the operation design of an aquifer treatment using the DMD technique.

Behavior of DNAPL mixture of organometallic and chlorinated solvent in the presence of surfactants and alcohols as density modifying agents

This work evaluates the behavior of surfactant and alcohols in combination with a mixture of tributyltinchloride (TBT) and tetrachloroethylene (PCE) with the goal of modifying the mixed oil from being a dense non-aqueous phase liquid (DNAPL) to a light non-aqueous phase liquid (LNAPL). Phase behavior of the mixed oil was studied under various combinations of surfactant, alcohol, and salinity. Phase density conversion was examined using pseudo-ternary phase diagrams constructed between the mixed oil, surfactant solution (4 wt%), and two types of alcohols (n-butyl alcohol (BuOH) and tert-butyl alcohol (TBA)). Aqueous phase solubilization and oil phase density modification were studied at varying alcohol to surfactant (A/S) ratios. The results showed that the optimum surfactant system was sodium dihexylsulfosuccinate (SDHS) and hexadecyl diphenyloxidedisulfonate (C16DPDS) (3.6 wt% and 0.4 wt%, respectively) with salt (NaCl) of 3 wt%. From pseudo-ternary phase diagrams, BuOH was found to produce a larger LNAPL region than TBA. From solubilization studies, the surfactant system plus either TBA or BuOH caused PCE preferential solubilization and this preference was more pronounced at higher total surfactant concentration in the system with TBA addition. In terms of density modification, BuOH produced lower oil density than TBA at high A/S ratio. This phase behavior knowledge can be used to optimize site remediation of organometallic DNAPLs.

Enhanced remedial amendment delivery to subsurface using shear thinning fluid and aqueous foam

A major issue with in situ subsurface remediation is the ability to achieve an even spatial distribution of remedial amendments to the contamination zones in an aquifer or vadose zone. Amendment delivery to the aquifer using shear thinning fluid and to the vadose zone using aqueous foam has the potential to enhance the distribution. 2-D saturated flow cell experiments were conducted to evaluate the enhanced fluid sweeping over heterogeneous system, improved contaminant removal, and extended amendment presence in low-permeability zones achieved by shear thinning fluid delivery. Unsaturated column and flow cell experiments were conducted to investigate the improvement on contaminant mobilization mitigation, amendment distribution, and lateral delivery implemented by foam delivery. It was demonstrated that the shear thinning fluid injection enhanced the fluid sweeping and increased the delivery of remedial amendment into low-perm zones. The presence of amendment distributed by the shear thinning fluid in the low-permeability zones was increased. Foam delivery was shown to mitigate the mobilization of highly mobile contaminant from sediments. It also achieved more uniform amendment distribution in a heterogeneous unsaturated system, and demonstrated remarkable increasing in lateral distribution of the injected liquid compared to direct liquid injection.

Encapsulation of nZVI particles using a Gum Arabic stabilized oil-in-water emulsion

Stabilization of reactive iron particles against aggregation and sedimentation is a critical engineering aspect for successful application of nZVI (nanoscale zero valent iron) within the contaminated subsurface environment. In this work we explore the stability and reactivity of a new encapsulation approach that relies upon Gum Arabic to stabilize high quantities of nZVI (∼ 12 g/L) in the dispersed phase of a soybean oil-in-water emulsion. The emulsion is kinetically stable due to substantial repulsive barriers to droplet-droplet induced deformation and subsequent coalescence. Sedimentation time scales were found to be on the order of hours (τ=4.77 ± 0.02 h). Thus, the use of Gum Arabic represents an advance in stabilizing nZVI-in-oil-in-water emulsions. nZVI within the emulsion was shown to be reactive with both TCE degradation and H(2) production observed. Degradation rates were observed to be on the same order of magnitude as those reported for less stable, aqueous suspensions of nZVI. TCE consumption within the emulsion was described with an equivalent aqueous phase rate coefficient of ∼ 5 × 10(-4)L(aq)/m(2)h.

Iron-mediated trichloroethene reduction within nonaqueous phase liquid

Aqueous slurries or suspensions containing reactive iron nanoparticles are increasingly suggested as a potential means for remediating chlorinated solvent nonaqueous phase liquid (NAPL) source zones. Aqueous-based treatment approaches, however, may be limited by contaminant dissolution from the NAPL and the subsequent contaminant transport to the reactive nanoparticles. Reactions occurring within (or at the interface) of the NAPL may alleviate these potential limitations, but this approach has received scant attention due to concerns associated with the reactivity of iron within nonaqueous phases. Results presented herein suggest that iron nanoparticles are reactive with TCE-NAPL and exhibit dechlorination rates proportional to the concentration of (soluble) water present within the NAPL. Reactivity was assessed over a 12-day period for five water contents ranging from 0.31 M to 4.3M, with n-butanol used to enhance water solubility in the NAPL. Rates of dechlorination were generally slower than those reported for aqueous-phase dechlorination, but were not observed to slow over the course of the 12-day period. The lack of observed deactivation may indicate the potential that highly efficient (with respect to utilization of available electrons) dechlorination reactions can be engineered to occur within nonaqueous liquids. These results suggest a need for subsequent investigations which focus on understanding the mechanisms of the reactions occurring within NAPL, as well as those assessing the utility of controlling both the iron and water content within a NAPL source zone.

Enhanced remedial amendment delivery through fluid viscosity modifications: experiments and numerical simulations

Low-permeability zones are typically bypassed when remedial fluids are injected into subsurface heterogeneous aquifer systems. Therefore, contaminants in the bypassed areas may not be contacted by the amendments in the remedial fluid, which may significantly prolong remediation operations. Laboratory experiments and numerical studies have been conducted to investigate the use of a shear-thinning polymer (Xanthan gum) to improve access to low-permeability zones in heterogeneous systems. The chemicals sodium mono-phosphate and the surfactant MA-80 were used as the remedial amendments. The impact of polymer concentration, fluid injection rate, and permeability contrast in the heterogeneous systems has been studied in a series of eleven two-dimensional flow cell experiments. The Subsurface Transport over Multiple Phases (STOMP) simulator was modified to include polymer-induced shear-thinning effects. The experimental and simulation results clearly show that using the polymer leads to an enhanced delivery of remedial amendments to lower-permeability zones and an increased sweeping efficiency. An added benefit of using the polymer is the stabilization of the displacing front when density differences exist between displaced and displacing fluids. The modified STOMP simulator was able to predict the experimental observed fluid displacing behavior well and might be used to predict subsurface remediation performance when a shear-thinning fluid is used to remediate a heterogeneous system at larger scales.

Surfactant-enhanced remediation of organic contaminated soil and water

Surfactant based remediation technologies for organic contaminated soil and water (groundwater or surface water) is of increasing importance recently. Surfactants are used to dramatically expedite the process, which in turn, may reduce the treatment time of a site compared to use of water alone. In fact, among the various available remediation technologies for organic contaminated sites, surfactant based process is one of the most innovative technologies. To enhance the application of surfactant based technologies for remediation of organic contaminated sites, it is very important to have a better understanding of the mechanisms involved in this process. This paper will provide an overview of the recent developments in the area of surfactant enhanced soil and groundwater remediation processes, focusing on (i) surfactant adsorption on soil, (ii) micellar solubilization of organic hydrocarbons, (iii) supersolubilization, (iv) density modified displacement, (v) degradation of organic hydrocarbon in presence surfactants, (vi) partitioning of surfactants onto soil and liquid organic phase, (vii) partitioning of contaminants onto soil, and (viii) removal of organics from soil in presence of surfactants. Surfactant adsorption on soil and/or sediment is an important step in this process as it results in surfactant loss reduced the availability of the surfactants for solubilization. At the same time, adsorbed surfactants will retained in the soil matrix, and may create other environmental problem. The biosurfactants are become promising in this application due to their environmentally friendly nature, nontoxic, low adsorption on to soil, and good solubilization efficiency. Effects of different parameters like the effect of electrolyte, pH, soil mineral and organic content, soil composition etc. on surfactant adsorption are discussed here. Micellar solubilization is also an important step for removal of organic contaminants from the soil matrix, especially for low aqueous solubility organic contaminants. Influences of different parameters such as single and mixed surfactant system, hydrophilic and hydrophobic chain length, HLB value, temperature, electrolyte, surfactant type that are very important in micellar solubilization are reviewed here. Microemulsion systems show higher capacity of organic hydrocarbons solubilization than the normal micellar system. In the case of biodegradation of organic hydrocarbons, the rate is very slow due to low water solubility and dissolution rate but the presence of surfactants may increase the bioavailability of hydrophobic compounds by solubilization and hence increases the degradation rate. In some cases the presence of it also reduces the rate. In addition to fundamental studies, some laboratory and field studies on removal of organics from contaminated soil are also reviewed to show the applicability of this technology.

Experimental and numerical validation of the total trapping number for prediction of DNAPL mobilization

The total trapping number (N(T)), quantifying the balance of gravitational, viscous, and capillaryforces acting on an entrapped dense nonaqueous phase liquid (DNAPL) droplet was originally developed as a criterion to predict the onset and extent of residual DNAPL mobilization in porous media. The ability of this approach to predict mobilization behavior, however, has not been rigorously validated in multidimensional systems. In this work, experimental observations of residual tetrachloroethene (PCE) mobilization in rectangular columns are compared to predictions obtained using a multiphase compositional finite-element simulator that was modified to incorporate the dependence of entrapped residual,flow, and transport parameters on the total trapping number. Consistent with calculated NT values (1.21 x 10(-3)-1.10 x 10(-2)), residual PCE-DNAPL was mobilized immediately upon contact with a low-interfacial tension (IFT) surfactant solution and rapidly migrated downward to form a bank of mobile DNAPL. The numerical model accurately captured the onset and extent of PCE-DNAPL mobilization, the angle and migration of the DNAPL bank, the swept path of the surfactant solution, and cumulative PCE recovery. These findings demonstrate the utility of the total trapping number for prediction of DNAPL mobilization behavior during low-IFT flushing.