Groundwater Chemistry Has a Greater Influence on the Mobility of Nanoparticles Used for Remediation than the Chemical Heterogeneity of Aquifer Media

Abstract

The application of nanoscale zerovalent iron (nano-ZVI) particles for groundwater remediation has spurred research into the influence of the collector heterogeneity on the nano-ZVI mobility. The chemical heterogeneity of surfaces within aquifer media affects their surface charge distribution and their affinity for nano-ZVI. The groundwater chemistry affects the properties of both aquifer surfaces and the nano-ZVI particles. Commercial poly­(acrylic acid)-coated nano-ZVI (PAA–nano-ZVI) particles were tested in column experiments using two solution chemistries and silica collectors with different degrees of chemical heterogeneity, achieved by ferrihydrite coating. A porous media filtration model was used to determine the attachment efficiency of PAA–nano-ZVI particles, and the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory was used to describe the interactions between PAA–nano-ZVI particles and the aquifer “collectors”. The mobility of PAA–nano-ZVI particles suspended in ultrapure water depended on the extent of ferrihydrite coating on the collector surfaces. The mobility of PAA–nano-ZVI particles under environmentally relevant conditions was independent of the collector chemical heterogeneity. The size of PAA–nano-ZVI aggregates doubled, inducing gravitational sedimentation and possibly straining as mechanisms of particle deposition. There was no repulsive energy barrier between particles and collectors, and the DLVO theory was unable to explain the observed particle attachment. Our results suggest that the groundwater chemistry has a greater influence on the mobility of PAA–nano-ZVI particles than the collector chemical heterogeneity. A better understanding of polymer adsorption to nanoparticles and its conformation under natural groundwater conditions is needed to further elucidate nanoparticle–collector interactions.

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