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An Integrated Approach to Standard Methods, Materials, and Databases for the Measurements Used To Develop Surface Complexation Models Thomas A. Duster* Applied Chemicals and Materials Division, Materials Measurement Laboratory, National Institute of Standards and Technology, 325 Broadway, Boulder, Colorado 80305, United States inconsistencies by compiling and reanalyzing raw sorption data sets from the literature using a single self-consistent model. This approach has been applied to cation/anion-surface interactions for only a small number of mineralse.g.,2,3 and its success requires consensus on the use of a single SCM formulation within a community where strong “model camps” exist. To support the characterization and remediation objectives for a diverse range of contaminated lands, there is a significant need for a more flexible database approach that can rapidly extend to additional compounds and environmental surfaces. Collecting the relevant surface complexation data will require a community effort and, in my view, a new analytical paradigm for sorption measurements. Despite the transformative work of Dzombak and Morel, there still exist no mechanisms to calibrate distinct sorption measurements or identify and quantify individual sources of experimental or analytical uncertainties. For example, if I search the literature and find three equilibrium constants that describe the same surface complexation reaction but vary by two orders-of-magnitude, how do I determine if this discrepancy resulted from differences in model assumptions, mineral sourcing, or simply the choice of orption reactions between groundwater contaminants and experimental container? And, perhaps more fitting for this environmental surfaces (e.g., minerals, colloids, bacteria) discussion, what metrics should I use during the critical review influence contaminant fate/transport, bioavailability, and process to justify excluding individual value(s) from the ecotoxicity. In complex geochemical systems, predictively database? There are few, if any, ways to definitively address modeling the partitioning of contaminants among the sorbed these questions without thinking beyond database development and aqueous phases depends on the availability and quality of to an integrated and comprehensive surface complexation data inputs, including reactive surface site concentrations for standards program that also includes reference materials and material constituents and thermodynamic equilibrium constants methods. for surface complexation reactions. Hence, collecting and An illustration of how a community standards framework archiving these parameters in a coherent database would could be implemented is depicted in Figure 1. At the heart of significantly improve the efficacy of contaminated land the framework is an international collaborative that partakes in characterizations and remediation efforts. round-robin surface complexation studies using a single lot of However, the development of such a database is hampered target material and uniform experimental methods. Decisions by substantive variations in the surface complexation models regarding the types of materials and sorbates to test would be (SCMs) used to calculate the relevant parameters. SCMs often made by a steering committee. The round-robin construct differ in their treatment of surface heterogeneities, reaction minimizes the experimental burden on participating laboratostoichiometries, and/or surface electrostatics,1 and may use ries (with each assigned only a very small fraction of the total conflicting values for the stability constants of aqueous titration/sorption studies for a target material) while still complexes needed to constrain the sorbent−sorbate equiliallowing for intra- and interlaboratory replicates to evaluate and brium constants. The titration and batch sorption data required quantify specific sources of uncertainty. With commitments as SCM inputs may also be collected using disparate from a sufficient number of laboratories and full-time methodologies. These factors lead to a lack of data consistency organization by a project coordinator, a large suite of sorption and capability between studies, and as a result, site concentrations and equilibrium constants for surface reactions cannot simply be amassed from the published literature. The Received: May 27, 2016 Published: July 11, 2016 database approach of Dzombak and Morel2 addresses SCM
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This article not subject to U.S. Copyright. Published 2016 by the American Chemical Society
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DOI: 10.1021/acs.est.6b02669 Environ. Sci. Technol. 2016, 50, 7274−7275
Environmental Science & Technology
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classically coupled sorption/SCM study construction of Stumm and others. But we must remember that professional hydrogeologists and environmental engineers depend on surface complexation parameters every day to protect groundwaters from contamination and optimize remediation processes. They have the tools to effectively complete these tasks (e.g., geochemical modeling programs), but do they have sufficient high-quality sorption data sets to parametrize the associated predictive models? I suggest that the answer to this question is currently “no”, but implementation of the standards framework proposed above could rapidly improve this situation.
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AUTHOR INFORMATION
Corresponding Author
*Phone: (303) 497-3486; e-mail:
[email protected]. Notes
Figure 1. An integrated standards framework for surface complexation measurements that facilitates information and resource sharing, collective decision making, and coordinated research between an international collaborative of laboratories. This framework would be repeated for each material of interest to the collaborative.
The author declares no competing financial interest.
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ACKNOWLEDGMENTS T.D. appreciates valuable insights from participants of the Standard Methods, Materials, and Databases for Surface Complexation Measurements workshop at the Spring 2016 ACS meeting. The workshop was supported by the ACS Geochemistry Division. Contribution of the National Institute of Standards and Technology, an agency of the United States government; not subject to copyright in the United States.
data could be collected for several environmental materials each year. Raw titration curves and sorption edges from the roundrobin effort would be archived and form the foundation for a sorption database. Storing the feature common to all SCMs the raw datawould enable analysis of these data sets using any existing SCM by coordination within the collaborative or by individuals in need of specific parameters. The flexibility afforded by this archival approach essentially acknowledges that having the ability to use different SCM formulations (e.g., nonelectrostatic models vs triple-layer models) may be advantageous in specific applications. In addition, a database populated with internally consistent sorption data for a variety of environmental materials could serve as a vital resource for those wishing to test novel SCMs. While procuring a material that is truly archetypal of the material population it represents may be impossible, the alternative is our current paradigmsourcing/fabricating materials using an unsystematic approach that varies by laboratory. As with the use of uniform experimental methods, simplifying investigations to a single lot for each material type removes uncontrolled sources of variation from these studies. Testing the influence of material sourcing and/or fabrication techniques could be the topic of complementary studies. In addition, procuring and testing a sufficiently large material lot has the added benefit of allowing for its certification and distribution as a reference/research material, thereby creating a catalog of control standards to calibrate interlaboratory measurements and/or deploy in experiments that benefit from the availability of well-characterized materials (e.g., reactive transport studies). Nearly 40 years ago, the natural organic matter4 and clay5 communities underwent an identical shift toward identifying and characterizing reference materials (e.g., Suwanee River fulvic acid and KGa-1 kaolinite, respectively) to represent the endless varieties of these materials present in the environment. Greater standardization of test materials in the surface complexation community would likely echo these unmitigated success stories. Still, there will be those who wonder why such effort should be expended to collect rather mundane sorption data sets. After all, in the contemporary literature, newer, more high-tech spectroscopic approaches have become favored over the
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REFERENCES
(1) Lützenkirchen, J. Preface. In Surface Complexation Modeling; Lützenkirchen, J., Ed. Elsevier: Oxford, U.K., 2006. (2) Dzombak, D. A.; Morel, F. M. M. Surface Complexation Modeling: Hydrous Ferric Oxide; Wiley: New York, NY, 1990. (3) Karamalidis, A. K.; Dzombak, D. A. Surface Complexation Modeling: Gibbsite; Wiley: New York, NY, 2010. (4) MacCarthy, P.; Malcolm, R. L. The need to establish a reference collection of humic substances. In Trace Organic Analysis: A New Frontier in Analytical Chemistry, Chesler, S. N., Hertz, H. S., Eds.; U.S. National Bureau of Standards Special Publication No. 519: Gaithersburg, MD, 1979. (5) Van Olphen, H.; Fripiat, J. J. Data Handbook for Clay Minerals and Other Non-Metallic Minerals; Pergamon Press: Oxford, England, 1979. Cited by : Costanzo, P. A.; Guggenheim, S. Baseline studies of the Clay Minerals Society source clays: Preface. Clays Clay Miner. 2001, 49 (5), 371.10.1346/CCMN.2001.0490501
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DOI: 10.1021/acs.est.6b02669 Environ. Sci. Technol. 2016, 50, 7274−7275
