Effects of molecular weight-fractionated natural organic matter on the phytoavailability of silver nanoparticles

Abstract

Ubiquitous natural organic matter (NOM) controls the environmental behavior, bioavailability, and toxicity of silver nanoparticles (AgNPs). However, NOM is heterogeneous and little is known about the reaction between its specific components and AgNPs. We investigated the effect of molecular weight (MW)-fractionated NOM (M_f-NOM) and its chemical composition on the stability, dissolution, and bioavailability of polyvinylpyrrolidone-coated AgNPs in their interactions with rice (Oryza sativa L.) seedlings. The amendment of Suwannee River NOM modified the size and surface charge of the AgNPs as well as their released Ag ion concentrations, thereby decreasing total Ag uptake rates. M_f-NOM addition also inhibited AgNP availability, but the extent of inhibition differed among NOM fractions. This effect was likely associated with the dissolution, not the aggregation of AgNPs. Moreover, neither the aromaticity nor the fluorescence intensity at the humic-like peaks of M_f-NOM explained its effects on AgNP bioavailability. The thiol group of cysteine, rather than the hydroxyl group of its analogue (serine) or the carboxylic groups of malic and citric acids, was more effective in reducing AgNP bioavailability. Hence, in AgNP risk assessments, in addition to the types and sources of NOM, the heterogeneity of the obtained NOM samples should be considered.