Surfactant-Enhanced Remediation of Subsurface ... - ACS Publications

Chapter 20

Surfactant-Enhanced Remediation of Subsurface Contamination

Downloaded by NORTH CAROLINA STATE UNIV on May 16, 2013 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0594.ch020

Review of Emerging Technologies and Panel Recommendations Candida C. West Robert S. Kerr Environmental Research Laboratory, U.S. Environmental Protection Agency, Ada, OK 74820

These are exciting and extremely challenging times for those of us who are professionals in thefieldof subsurface remediation. We have been charged to bring to practice innovative technologies that have shown substantial promise for improving the way we currently practice reclamation of ground water to regulatory acceptability. Much of the current pressure for the development of innovative technologies stems from the widespread recognition that, with the exception of the case of dissolved plumes in homogeneous media, we are largely unable to clean up sites to regulatory contaminant levels using current technology. Many innovative technologies are currently at various stages of development including those that use the addition of chemical amendments to the extractionfluidfor the purpose of chemically altering the way contaminants partition from aquifer solids and pore spaces into the mobilizing fluid. Surfactants are one class of chemical additives that may be used successfully to enhance the remediation process, particularly pump-and-treat technology as it is currently practiced. Surfactant enhanced subsurface remediation has been identified as one of the technologies worthy of serious evaluation. Over the last few years, the growth in the number of government, academic, and industry research laboratories conducting research on some aspect of surfactant-based remediation has been short of incredible. There is tremendous momentum behind this effort which has been primarily focussed on developing the solid science base that is going to be required to bring these technologies tofruition.The energy of that momentum was evident by the number and quality of the presentations made at the two-day session of the "207th American Chemical Society Meeting" in San Diego. Our Goal - A Public Mandate The current list of contaminated sites that have been successfully remediated is woefully short. Those sites that have been successfully remediated are generally those where the contaminants are present in dissolved form and the geology is relatively

This chapter not subject to U.S. copyright Published 1995 American Chemical Society

In Surfactant-Enhanced Subsurface Remediation; Sabatini, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


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homogeneous. Unfortunately, this is generally not the case at the majority of sites; and the complexity of the contaminant matrix and the geology have significantly hampered remedial action. In response to this realization, there is increased pressure by the public and the regulatory community to improve our performance in subsurface remediation in a timely and scientifically defensible manner. As part of the effort to address remedial needs, we are faced with the difficult task of recognizing and developing the best course of action to be taken to develop surfactant-based remediation into viable and valid remedial tools. Based on current needs for innovative remedial technologies, it appears that the greatest contribution surfactant-based remedial technologies can make is in the area of remediating nonaqueous phase liquids, particularly dense nonaqueous phase liquids for which there are currently no remedial tools available. To address this, it is necessary to identify the issues that must be resolved to develop this remedial tool in a logical and accomplishable order. To this end, an invited panel was assembled at the conclusion of the symposium for the purpose of providing a forumfromwhich key issues crucial to the development of this technology could be identified and discussed. The panel represented several segments of the community having a commitment in surfactant-based remediation. The panel members were: Dr. Linda Abriola, Department of Environmental Engineering at the University of Michigan, and the principal investigator for several research projects in surfactant-enhanced aquifer remediation; Mr. James Greenshields of ICI Surfactants, a major manufacturer and distributor of surfactants; Dr. Abdul Abdul, a researcher with General Motors who has conducted laboratory andfieldresearch evaluating the use of surfactants for enhancing removal of contaminants; Dr. Jeffrey Harwell, Institute for Applied Surfactant Research, University of Oklahoma, who has an extensive background in surfactant use in industrial and environmental applications, including enhanced oil recovery; and the author, as a representative researcher for the U.S. Environmental Protection Agency. The audience was equally broad in its representation of these segments of the provider and user community. What Course Do We Take? Panel Discussion The Need for an Interdisciplinary Approach. It became clear through the symposium discussions that the most challenging aspect of surfactant-based technology development will be the necessity for creating a forum through which open collaboration and communication between expertsfrommany disciplines can be achieved. There is a fundamental distinction between research that is conducted solely for the simple extension of fundamental knowledge and that which is conducted for the purpose of producing afinishedtechnological product ready for market use. This effort will require the active participation of experts covering all aspects of the surfactant utilization process: microbiologists for the determination of surfactant biodegradability and specific metabolic pathways; toxicologists evaluation of the acceptability of injecting a chemical amendment into the subsurface environment given possible receptors; chemists and geologists for evaluation of the compatibility of surfactant solutions given specific water chemistry and geochemistry; hydrologists for designing injection/extraction systems that will provide adequate delivery of the


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surfactant to the contaminant and proper hydrologie recovery of the injection solution; and engineers for process development of appropriate recovery processes for surfactant reuse and contaminant treatment. The primary stakeholder in this business is the general public and it is our duty to work towards acceptable technologies that will aid in cleaning up our nation's ground water and not make a bad situation worse. We will be required to share ideas and results developed at the laboratory bench, at small-scale field demonstrations, and full-scalefieldprojects. Where Are We Now? When research in the area of surfactant-based remediation was begun, the logical question asked was whether or not using surfactants could enhance contaminant removal or destruction sufficiently to warrant further investigation. It would be accurate, I believe, to say that bench-scale laboratory experiments have shown that the enhanced removal of residual phase contaminants from ground water via solubilization or mobilization in surfactant solutions warrants further evaluation and development. If bench-scale experimental results were directly applicable to field situations, there could be realized one to several orders of magnitude reduction in the flush volume required to remove a given mass of contaminant using surfactantamended pump-and-treat remediation. The potential to use surfactants to enhance bioremediation is perhaps less clear, due to the lack of a fundamental understanding of the mechanism by which surfactants cause microbial toxicity or aid in microbial utilization of contaminants. The questions now posed are directed towards the development of fundamental data required to move this technology to small-scalefielddemonstrations. Can we adequately delineate the area of contamination to be remediated and deliver the surfactant solution to it effectively? If so, what percentage of the surfactant flush can we recapture? Given some escape rate, what will be the surfactant's fate and how far will it travel before it is reduced to safe concentrations given processes such as sorption and/or biodégradation? What is a "safe" concentration? What are the biodégradation products and what are their toxicity to the same set of receptors? Can the recovered surfactant be processed for reuse and can the contaminant be recovered from the surfactant solution for treatment? How might the surfactant affect other intrinsic or active remediation processes? These were the range of questions that were addressed by the panel and audience. Analogies to EOR. There are those who would say that we should use the example set in enhanced oil recovery (EOR) as evidence that the use of surfactants to remediate aquifers should not be pursued. Surfactant flooding was developed by the oil industry as a way to improve tertiary recovery of oil deposits. Both the oil companies and the surfactant manufacturers put substantial research resources into the development of this technology which for various reasons never came to fruition. However, the motivations and the stakeholders are as different in these two applications as are the environments in which they are used. Enhanced oil recovery involved injection of massive volumes of surfactant solution into hostile environments of high temperature and pressure over areas typically orders of magnitude greater than proposed for ground-water remediation. Additionally, the economical feasibility was driven by the


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price of oil, which at its peak was approximately $50/barrel, whereas the recovery of a single barrel of a contaminant such as tetrachloroethylene may well run thousands to tens of thousands of dollars. Given these significant differences in application and return, the two scenarios are not directly comparable. It is, however, important to point out that environmental researchers have examined many of the theories developed for EOR application for the prediction of surfactant behavior and its usefulness in this field. An example of this is the relationship between the bond and capillary numbers for the prediction of the onset of mobilization, as conducted by Dr. Linda Abriola and her coworkers. Surfactant Receptors, Fate, and Toxicity. There is legitimate concern over injecting surfactants into ground water and possibly trading one contaminant for another. To allay this concern, it will be necessary to develop an extensive database on the toxicity of surfactants and their breakdown products to possible receptors. Many surfactants are toxic to aquatic organisms at relatively low concentrations (on the order of low parts per million) and should perhaps not be considered in situations where the injection solutions could reach and impact any surface waters such as swamps, rivers, streams, or lakes. This will require calculated estimates of rates of loss through sorption and degradation, time of travel, and rates of dilution. It is clear that surfactants will either need to be completely non-toxic to the receiving system or be known to biodegrade within a reasonable period of time. However, an acceptable rate of biodégradation will be completely dependent on the specific use of the surfactant. The surfactant solution must be stable for a sufficient period of time required to do the job for which it was intended, but not so recalcitrant as to represent a long-term contaminant in itself. Currently, most degradation data on surfactants are for aerobic systems. There are very little anaerobic data. If surfactants are to be considered for use as part of a treatment train concept, for instance as part of a biologically mediated process, the effect of the surfactant on the microbial population of the system needs to be determined. It was pointed out that the need for an extensive database on biodégradation rates, products, and toxicity, combined with the need to have a thorough understanding of surfactant phase-behavior and chemical compatibility with various water and mineral geochemistries, may necessitate focussing on a narrow selection of surfactants that could be studied intensely and recommended for use. This would reduce the research costs associated with the development of this database. It seems clear, however, that regulatory guidelines for surfactant acceptability must be developed before the research community can focus on a select group of surfactants and essentially put all its eggs in a few baskets. Nonaqueous Phase Liquids and Contaminant Delineation. Adequate contaminant delineation and characterization has been identified as crucial to successfully instituting surfactant-enhanced remediation. This is particularly true for dense, nonaqueous phase liquid (DNAPL) contamination for which surfactant remediation has been identified as a key innovative technology. DNAPLs are particularly difficult to detect either directly via soil coring due to their elusive nature (i.e. sinking deep into aquifers) or by



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inference based on dissolved aqueous-phase concentrations. Geophysical techniques for source delineation are being developed, but may be relatively expensive and may not be available in the near future. Another delineation technique using partitioning tracers appears to be a promising technique and may be more rapidly developed for use than geophysical techniques. Again, unless we can improve our ability to locate and delineate residual and free-phase liquids, surfactants cannot be properly delivered for the purpose of removal of the DNAPL. Innovative surfactant-based remediation technologies cover a wide range of contaminant types, both chemical and physical. Many address the enhancement of the remediation of dissolved plumes either by increasing the bioavailability of the contaminant or immobilizing the contaminant making it available for subsequent abiotic or biotic in situ treatment. Remediation of dissolved plumes using modified or enhanced pump-and-treat remediation in relatively homogeneous media probably has a high likelihood of success, but represents a small fraction of the sites currently mandated for clean-up. One of the most serious and difficult situations for which there is no practical remedial solution, even under uniform, homogeneous aquifer conditions, is that of remediation of nonaqueous phase liquids (NAPLs). The limitation for remediation of NAPLs comesfromthe large volumes of material introduced into the environment relative to its aqueous solubility, posing the limiting factor for pump-andtreat remediation. The vast majority of Superfund and RCRA sites are contaminated with NAPLs, and it is projected that tens to thousands of years would be required to remove the volume of NAPL based on calculations of the mass removed per pore volume at contaminant saturation. The problem is exacerbated by the fact that saturation is rarely, if ever, achieved making the time required to remove these materials even greater. It is the remediation of NAPLs for which surfactants can make its greatest contribution, either through the process of solubilization or mobilization. Field Demonstrations. As has been the case in the past, many researchers expressed the need for field sites which could qualify for use as research demonstrations for surfactant-based remediation such as the field site in Borden, Canada. It was recognized that the United States seems to be moving in this direction and that the DoD has provided funding for the development of several field research sites. It was recommended that researchers submit proposals which would provide collaborative, holistic small-scale demonstration approaches encompassing innovative techniques for site characterization, contaminant source delineation, injection/extraction well geometries, and treatment systems for surfactant reuse and contaminant removal and treatment. As part of this collaboration, a stepwise test protocol for small-scale field demonstrations needs to be developed and modified asfieldand laboratory data are collected and analyzed. These test sites would also provide an opportunity to study surfactant interactions at more complex sites than have been typically studied. Research intensive smallscale pilot demonstrations would then provide the information required to develop fullscale site remediation.


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Closing Remarks One of the major barriers to the development of innovative technologies is the risk of failure, or worse, the risk of exacerbating the situation. Thisriskbarrier is evident by the user community's reluctance to approve installation and operation costs for an unproven technology while still being held liable for the installation of a "standard" technology if the innovative technology should fail. Nor do regulatory agencies feel compelled to share in the costs of supporting these new technologies. The panel discussion was closed with the suggestion to form an expert panel consisting of representatives of all of the disciplines and stakeholders previously mentioned for the purpose of evaluating and promoting surfactant-based remediation technologies. It would be the panel's responsibility to offer their expert services for evaluation of proposed surfactant-aided remediation projects. The availability of this service would help prevent irresponsible use of surfactant-based remediation technologies and promote communication between scientists, users, and the regulatory community. It was speculated that the formation of a cooperative research and development agreement (CRADA), spearheaded by a regulatory agency such as the EPA, might be an appropriate vehicle through which to form this panel. In this way, the panel could facilitate possible opportunities for both smallscale and scaled-up field tests which could provide the kind of scientifically defensible data required to demonstrate the economic feasibility and appropriate application of surfactant-based aquifer remediation. The opportunities for research in and application of surfactant-based remediation are growing. In order for our efforts to be fruitful it will be necessary to continue communication through symposia such as this one and through the creation of expert panels as discussed previously. It was generally agreed upon by the panel discussion participants that there is a need to meet at least every other year and to continue to hold panel discussions for the purpose of interchange of ideas. The author would like to thank all of those who participated in the panel discussions. It is hoped this chapter is an accurate reflection of your ideas and suggestions and may be in some way useful to your endeavors in surfactant-enhanced remediation. RECEIVED

December 13,1994


Surfactant-Enhanced Remediation of Subsurface ... - ACS Publications

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