Delineating the role of surface grafting density of organic coatings on the colloidal stability, transport, and sorbent behavior of engineered nanoparticles

Abstract

Aqueous stability and sorption affinity (towards target environmental contaminants) of engineered nanoparticles, composed of inorganic nanoparticles and surface stabilizers, underpin their environmental behavior and application potential for a variety of proposed technologies. However, fundamentally delineating the role of surface coatings (in terms of surface coating density) remains outstanding for a number of particle systems. To address this critical issue, we describe colloidal stability, transport, and sorption behavior of engineered manganese oxide nanoparticles as a function of specific surface grafting density. We observed higher grafting density results in higher colloidal stability due to higher steric repulsion forces. Additionally, humic acid (HA) significantly improved the stability of NPs with lower grafting density, while showing negligible effects in higher grafting density. Deposition behavior did not correlate with grafting density, regardless of organic coating types. In the presence of HA, deposition behavior of negatively charged NPs was not altered, while deposition of positively charged NPs was dependent on HA presence. Higher grafting density of CTAB also enhanced chromate sorption capacity due to the increasing number of favorable functional groups. Taken together, this work clearly demonstrates the critical need to fully understand NP surface coating dynamics as they relate to fundamental material behavior and performance.