Biochar Nanoparticles and Their Interaction with Engineered Cerium Oxide Nanoparticles.

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 soil constituents and pyrolyzed biomass waste (biochar) has been added to soil to improve soil fertility and assisting in soil remediation.  Cerium oxide (ceria) is an oxidation catalyst widely used commercially. In this study, we describe the behavior of biochar nanoparticles (biochar-NPs) alone and in the presence of ceria nanoparticles (CeO₂-NPs) using electron microscopy and time-resolved dynamic light scattering.  Biochar-NPs were produced by filtration of an aqueous suspension of a pulverized pecan shell biochar produced at 500 ^oC.  Transmission electron microscopic (TEM) images indicate different types of particles and modes of aggregation. Biochar-NPs are stable towards homoaggregation in water at neutral pH; CeO₂-NPs are stable towards homoaggregation at pH from 2 to 11, except in the neutral range. At pH 5.3, CeO₂-NPs are positively charged and biochar-NPs are negatively charged.  To examine the hetero-aggregation kinetics at pH 5.3, the concentrations of CeO₂-NP and biochar-NP were adjusted so that the scattered light intensities of biochar-NPs were negligible compared to those of CeO₂-NPs.  Biochar-NPs had no effect on the colloidal stability of CeO₂-NPs at low biochar-NP/CeO₂-NP ratios.  However, as the biochar-NP/CeO₂-NP ratio increased, the initial growth rate of CeO₂-NPs hydrodynamic diameter increased accordingly due to heteroaggregation, reaching a maximum value, and then declining eventually to zero at high biochar-NP/CeO₂-NP ratios.  The reversibility of heteroaggregation was tested at the maximum ratio by abruptly increasing the pH from 5.3 to pH 10, where both CeO₂-NPs and biochar-NPs were negatively charged.  Although hetero-aggregation ceased when the pH increased, no disaggregation was observed, despite the high electrostatic repulsion between the particles, indicating poor reversibility.  The underlying cause(s) or irreversibility will be discussed.