Polymeric surface modifi cation/functionalization is a promising approach to increase the mobility of reactive nanoparticles in the subsurface and may allow
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for effi cient emplacement of particles for in situ remediation. Signifi cant progress has been made in this area. However, there are many remaining challenges and ongoing research and development opportunities to improve the eff ectiveness of this technology. First, eff ective subsurface remediation requires the ability to emplace reactive nanomaterials in the contaminated zone.
Although several fi eld demonstrations and applications of reactive nanoparticles for in situ remediation have been conducted, the controlled or designed transport and emplacement of nanoparticles in the subsurface was not achieved because of the absence of fundamental, quantitative understanding of the transport of concentrated polyelectrolyte modifi ed nanoparticles in the subsurface. Transport and deposition of nanoparticles in saturated porous media has typically been quantitatively modeled using a conventional clean bed fi ltration model based on the advection–dispersion equation for colloid transport [ 31 ]. However, the constraints on this model (e.g., the assumption of irreversible deposition, no aggregation, and perfect sink collectors) make it inappropriate for modeling the injection of high particle concentration dispersions used in remediation. An alternative approach to predicting the transport of polymer modifi ed nanoparticles at high concentration is needed.
This approach must take into account the eff ect of the adsorbed layer properties, geochemical–physical conditions, hydrodynamics, and particle concentration in order to predict the transport and emplacement of polymeric modifi ed nanoparticles using various surface modifi ers in diff erent geochemical–physical and hydrodynamic conditions.
Second, the nanomaterials used for remediation are often more expensive than their bulk counterparts; hence, methods to improve the selectivity and effi ciency of injected materials are still needed. This can be achieved through synthesis of reactive materials that specifi cally adsorb heavy metals [ 46 ], or have reactive sites specifi c for the target contaminants. Surface modifi ers that can serve a dual role, that is, improve transport and improve selectivity, are needed. For example, surface modifi ers that inhibit reactivity until the nanoparticles are emplaced in the contaminant source zone could also increase the eff ectiveness of nanoparticle remediation strategies by maintaining high reactivity until they are in a location where the contaminant concentrations are the highest. Since the mobility of reactive nanomaterials in the subsurface depends on the type of surface modifi cation used, appropriate surface modifi cation can allow one to select the transport distance, minimize the potential for “run away” particles, and decrease the potential for negative eff ects of nanoparticles on the environment and human health.
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269
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© 2009 William Andrew Inc.
for Water Purifi cation: Nanoscale Eff ects on Catalytic Activity, Selectivity, and Sustainability
Timothy J. Strathmann, 1, 3 , Charles J. Werth, 1, 3 , and John R. Shapley 2, 3 1
Department of Civil and Environmental Engineering, 2 Department of Chemistry, and 3
Center of Advanced Materials for the Purification of Water with Systems, University of Illinois at Urbana−Champaign, Urbana, IL, USA
19.1 Introduction 270