Nanoparticle affinity for natural soils: a functional assay for determining particle attachment efficiency in complex systems

Few standardized methods and reference systems have been established for evaluating the behavior of engineered nanomaterials in complex media like natural soils. In this study, a working method for a functional assay to determine the affinity of nanoparticles (NPs) for soil surfaces by batch test is provided along with a case study demonstrating the utility of the method using a proposed reference soil (LUFA 2.2) and gold nanoparticles (AuNPs) with four different surface treatments (citrate [Cit], gum arabic [GA], polyvinylpyrrolidone [PVP], and branched polyethylenimine [bPEI]). Particle attachment efficiencies (α) of AuNPs with LUFA 2.2 soil spanned four orders of magnitude, with α_bPEI-Au > α_PVP-Au > α_Cit-Au ≫ α_GA-Au, suggesting that GA-Au will be significantly more mobile in this soil than the other AuNPs. These results run counter to a widely held assumption that acquired macromolecular coatings in natural systems will likely mask engineered coatings and dictate NP mobility. Moreover, the trend in attachment efficiency could not be predicted based on differences in intrinsic or extrinsic NP properties alone, which supports the need for development and validation of well-controlled functional assays performed in complex media.