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Straining Credibility: A General Comment Regarding Common Arguments Used to Infer Straining As the Mechanism of Colloid Retention in Porous Media William P. Johnson,* Huilian Ma, and Eddy Pazmino Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112, United States
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n the past decade there has been a proliferation of papers in colloid filtration science that have concluded straining to be the dominant mechanism of retention of colloids in porous media under unfavorable deposition conditions (conditions in which significant colloid collector repulsion exists). Although straining is likely an important potential retention mechanism under particular circumstances (e.g., large colloid collector size ratios), inference of straining is made in many cases for relatively small colloid collector ratios on the basis of three arguments: (1) attachment was not expected under unfavorable conditions; (2) trends in collector efficiency (η) with size did not match the trend expected from colloid filtration theory (CFT); and (3) incorporation of a “straining” coefficient into the numerical simulations of transport produced a better fit to the results. The purpose of this comment is to demonstrate that these three arguments do not, in fact, eliminate alternative mechanisms. It should be noted that CFT, as used herein, refers to existing correlation equations used to predict colloid retention under favorable conditions, and does not refer to any modifications to CFT that will in the future, hopefully, allow prediction of colloid retention under unfavorable conditions. r 2011 American Chemical Society
Argument 1 is based on the assumption that the granular material being examined does not display sufficient surface charge heterogeneity or roughness to cause local reduction or elimination of colloid collector repulsion. However, the assumption is never quantitatively examined since it is not readily possible to measure nanoscale surface heterogeneity, and since there are no readily available expressions to estimate the impacts of measurable roughness on repulsion. Usually the argument is made that acid base treatments remove heterogeneity; however, this is a weak argument since heterogeneity may arise from, for example, impurities or substitutions within mineral matrices that are exposed on the surface even after the removal of surface coatings. Hence, it is not possible to rule out the possibility of retention via local elimination of repulsion via surface charge heterogeneity or roughness, and there are many experimental observations of colloid retention on acid base-treated open collector surfaces that suggest the presence of surface charge heterogeneity and/or roughness (e.g., refs 1 and 2). The problem with argument 2 is the underlying unspoken assumption that CFT can predict trends in η versus colloid size for experiments conducted under unfavorable conditions. CFT addresses favorable conditions (absent significant colloid collector repulsion), but it cannot address unfavorable conditions. Incorporation of significant energy barriers into the numerical models underlying CFT yields zero attachment, yet in experiments conducted under these unfavorable conditions, one commonly observes retention of colloids on the open surfaces of the collectors (e.g., refs 1 and 2). We therefore cannot expect CFT to accurately predict a trend in η versus colloid size under unfavorable conditions. The actual trend will almost certainly not be equivalent to that under favorable conditions, since we know that deposition under unfavorable conditions is related to the heights of the energy barriers, the depths of secondary energy minima, and these features will yield different forces/torques driving attachment and detachment for different colloid sizes. This effect causes the so-called deposition efficiency (R) to vary with changes in fluid velocity.3 Even if we did have a theory to tell us what the trend in η should be as a function of colloid size under unfavorable conditions, we do not presently have good experimental control regarding the surface properties of the colloids/nanoparticles as a function of size, since our principal means of characterizing the surface (zeta potential) is too coarse to represent the influence of discrete attractive heterodomains on
Published: March 29, 2011 3831
dx.doi.org/10.1021/es200868e | Environ. Sci. Technol. 2011, 45, 3831–3832
Environmental Science & Technology the surfaces, which can cause colloid collector interaction to become locally attractive. The upshot is that the mismatch between observation and CFT predictions for trends in η versus colloid size only proves that we do not have a complete theory for prediction of colloid retention under unfavorable conditions; it certainly does not specifically suggest straining as a mechanism of retention. Argument 3 results from misunderstanding the difference between kinetic versus mechanistic modeling. Researchers observe that the retention of colloids under unfavorable conditions is disproportionately loaded toward the influent end of the porous medium column relative to expectations from CFT (spatially constant deposition rate coefficient). Improved simulations (that fit both effluent and retained concentrations) are obtained when a depth-dependent deposition rate coefficient is used in the advection dispersion attachment simulations. The improved fit to effluent and retained concentrations obtained from addition of a depth-dependent rate coefficient to the model shows only that the deposition was depth dependent; it does not prove any particular mechanism leading to the observed depth dependence. Calling the depth-dependent parameter a “straining” parameter does not confer upon it a mechanistic tie to straining. The depth-dependent parameter may in fact represent mechanisms other than straining. It has been shown previously, for example, that heterogeneity in attachment efficiencies among the colloid (nanoparticle) population also leads to depth-dependence of the deposition rate coefficient (e.g., ref 4). In the first argument it is presumed that no surface charge heterogeneity or roughness exists on the granular media. In the second argument, CFT is presumed to predict retention by all mechanisms other than straining under unfavorable conditions, leading to a false dichotomy between CFT and straining. In the third argument, the nonmechanistic rate coefficient is presumed to have a mechanistic tie to straining. Stated in this plain fashion, the weakness of the three arguments used to infer straining becomes obvious. Inference of straining as the major mechanism of deposition in column transport experiments on the basis of the above three arguments is increasingly common, but is far from definitive, and must therefore be considered highly tentative. Notably, distinction of predominance of straining in pore throats too small to pass versus wedging in grain to grain contacts, versus retention on the open surface of the collector is easily made via direct observation. In the absence of such a distinction, alternative mechanisms such as surface charge heterogeneity and roughness should be considered equally viable to straining under unfavorable conditions.
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(3) Johnson, W. P.; Tong, M.; Li, X. On colloid retention in saturated porous media in the presence of energy barriers: The failure of R, and opportunities to predict η. Water Resour. Res. 2007, 43, W12S1310.1029/2006WR005770. (4) Li, X.; Scheibe, T. D.; Johnson, W. P. Apparent Decreases in Colloid Removal Rate Coefficients with Distance of Transport under Unfavorable Deposition Conditions: A General Phenomenon. Environ. Sci. Technol. 2004, 38 (21), 5616–5625.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ REFERENCES (1) Johnson, W. P.; Pazmino, E.; Ma, H. Direct Observations of Colloid Retention in Granular Media in the Presence of Energy Barriers and Pitfalls of Inferring Mechanisms from Indirect Observations. Water Res. 2010, 44 (4), 1158–116910.1016/j.watres.2009.12.014. (2) Johnson, W. P.; Tong, M. Simulated and Experimental Influence of Hetero-Domain Size on Colloid Deposition Efficiencies on Overall Like-Charged Surfaces. Environ. Sci. Technol. 2006, 40 (16), 5015–502110.1021/es060450c. 3832
dx.doi.org/10.1021/es200868e |Environ. Sci. Technol. 2011, 45, 3831–3832
