Aggregation of TiO₂ and Ag nanoparticles in soil solution – Effects of primary nanoparticle size and dissolved organic matter characteristics

Highlights

• NP size effect on aggregation differs more in the presence of DOM than in the absence.

• Ag/TiO₂ NP formed larger aggregates with hydrophilic DOM than with hydrophobic DOM.

• The effect of DOM on NP colloidal stability may not be considered as uniform.

Abstract

The colloidal stability of nanoparticles NP in soil solution is important to assess their potential effects on ecosystems. The aim of this work was to elucidate the interactions between initial particle size d_i, particle number concentration (N₀) as well as the characteristics of dissolved organic matter (DOM) for stabilizing Ag NP and TiO₂ NP.

In batch experiments using time-resolved dynamic light scattering (DLS), we investigated the aggregation of TiO₂ NP (79 nm, 164 nm) and citrate-stabilised Ag NP (73 nm, 180 nm) in Ca²⁺ solution (2 mM) and two soil solutions, one extracted from a farmland and one from a floodplain soil (each containing 2 mM Ca²⁺).

Our results demonstrate that the initial particle size and the particle number concentration affected aggregation more strongly in the presence of DOM than without DOM. The composition of DOM also affected aggregate size: NP formed larger aggregates in the presence of hydrophilic DOM than in the presence of hydrophobic DOM. Hydrophilic DOM showed a larger charge density than hydrophobic DOM. If Ca²⁺ is present, it may bridge DOM molecules, which may lead to greater NP destabilization. The results demonstrate that DOM interaction with NP may not only vary for different DOM characteristics (i.e. charge density) but may also be influenced by the presence of multivalent cations and different NP material; thus the effect of DOM on NP colloidal stability is not uniform.