The stability of engineered nanoparticles (NPs) in soil is a subject of intense interest due to the potential hazardous effects of NPs in the environment. Pyrogenic biomass materials from wildfires (chars) are ubiquitous constituents of soil, and pyrolyzed biomass wastes (biochars) are attracting interest for improving soil fertility in agriculture and assisting in soil remediation. In this study, heteroaggregation of cerium oxide nanoparticles (CeO2-NPs) and biochar nanoparticles (biochar-NPs) was investigated in DI water at pH 5.6 using time-resolved dynamic light scattering. Biochar-NPs of fairly uniform size (30-220 nm) were produced by filtration of an aqueous suspension of a pulverized pecan shell biochar produced at 500 oC. The zeta potentials of CeO2- and biochar-NPs were 45 and -23 mV, respectively, at pH 5.6 and both NPs were stable to homoaggregation. To examine the influence of biochar-NPs on the colloidal stability of CeO2-NPs, the concentrations of CeO2-NP and biochar-NP suspensions were adjusted so that the scattered light intensities of biochar-NPs were less than 8% of that of CeO2-NPs. At low biochar-NP/CeO2-NP ratios, biochar-NPs had no effect on the hydrodynamic diameter of CeO2-NPs. However, as the biochar-NP/CeO2-NP ratio increased, the initial growth rate of CeO2-NPs increased accordingly and reached a maximum value of ~30 nm/min due to heteroaggregation at the �optimum� biochar-NP/CeO2-NP ratio. With further increase in the biochar-NP/CeO2-NP ratio above the optimum value, the growth rate of CeO2-NPs decreased eventually to zero at high biochar-NP/CeO2-NP ratios. The reversibility of heteroaggregation was tested at the optimum biochar-NP/CeO2-NP ratio by increasing pH from 5.6 to 11, where the zeta potentials of CeO2-NPs and biochar-NPs declined to -20 and -42 mV, respectively. Although the heteroaggregation ceased when pH increased, no disaggregation was observed at pH 11, despite the high electrostatic repulsion presumably existing between CeO2-NPs and biochar-NPs, indicating poor reversibility. The underlying cause(s) will be discussed.

Interactions between cerium oxide nanoparticles and biochar nanoparticles

248th National Meeting (2014)

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