Dissolution and Aggregation of Metal Oxide Nanoparticles in Root Exudates and Soil Leachate: Implications for Nanoagrochemical Application

Knowledge of dissolution, aggregation, and stability of nanoagrochemicals in root exudates (RE) and soil leachate will contribute to improving delivery mechanisms, transport in plants, and bioavailability. We characterized aggregation, stability, and dissolution of four nanoparticles (NPs) in soybean RE and soil leachate: nano-CeO, nano-MnO, nano-Cu(OH), and nano-MoO. Aggregation differed considerably in different media. In RE, nano-Cu(OH), and nano-MoOincreased their aggregate size for 5 days; their mean sizes increased from 518 ± 43 nm to 938 ± 32 nm, and from 372 ± 14 nm to 690 ± 65 nm, respectively. Conversely, nano-CeOand nano-MnOdisaggregated in RE with time, decreasing from 289 ± 5 nm to 129 ± 10 nm, and from 761 ± 58 nm to 143 ± 18 nm, respectively. Organic acids in RE and soil leachate can be adsorbed onto particle surfaces, influencing aggregation. Charge of the four NPs was negative in contact with RE and soil leachate, due to organic matter present in RE and soil leachate. Dissolution in RE after 6 days was 38%, 1.2%, 0.5%, and <0.1% of the elemental content of MoO, Cu(OH), MnO, and CeONPs. Thus, the bioavailability and efficiency of delivery of the NPs or their active ingredients will be substantially modified soon after they are in contact with RE or soil leachate.