Influence of Surface Functional Groups on Deposition and Release of TiO₂ Nanoparticles

A clear understanding of the factors governing the deposition and release behaviors of engineered nanoparticles (NPs), such as TiO₂ NPs, is necessary for predicting their transport and fate in both natural and engineered aquatic environments. In this study, impacts of specific chemistries on TiO₂ NP deposition, as a function of TiO₂ NP concentration and ionic strength/valence, were investigated using self-assembled monolayers (SAMs) with five different ending chemical functionalities (-CH₃, -OH, -COOH, -NH₂, and -CONH₂). The fastest deposition and maximum deposition mass were observed on -NH₂, followed by -COOH, -CONH₂, -CH₃, and -OH, showing that contact angle and zeta potential of surfaces were not good indicators for predicting the deposition. Specific interactions, for instance, between -COOH and -CONH₂ and TiO₂, significantly affected their deposition. Deposition rate increased linearly with TiO₂ NP concentration; however, specific deposition rate was dependent on the type of SAMs. The increase of monovalent (Na⁺) and divalent (Ca²⁺) led to different changes in deposition rates for the SAMs due to different functionalities. Results also showed that favorable SAM (e.g., -NH₂) had lowered release of NPs compared to unfavorable surface (e.g., -OH). The obtained deposition and release behaviors will support more accurate prediction of the environmental fate of nanoparticles.