Influence of Ca2+ and Suwannee River Humic Acid on aggregation of silicon nanoparticles in aqueous media

Silicon nanoparticles (NPs) have potential applications in many fields including microelectronics, biomedical imaging, and most recently energetics. Even though silicon NPs are thought to be harmless, their full impact on the environment and human health needs further investigation due to their potential increased use and recent toxicity data. Various techniques were used to characterize silicon NPs that are being considered for use in energetics. These techniques included dynamic light scattering (DLS), electron microscopy (EM), X-ray diffraction (XRD) and atomic force microscopy (AFM). Experiments were also conducted on the early stage aggregation kinetics of silicon NPs in the presence of Ca2+ and Suwannee River Humic Acid (SRHA). The addition of SRHA in the presence of Ca2+ resulted in increased attachment efficiencies and decreased critical coagulation concentration (from 0.4 to 0.1 M). The enhanced aggregation was attributed to bridging generated by SRHA aggregates as evidenced by selected area electron diffraction (SAED) and energy dispersive spectroscopy (EDS). SAED verified the bridging to be amorphous phase comprised of humic substances rather than artifacts of silicon crystallites. Element distribution analyses were also used in the delineation of the silicon NP aggregation mechanisms in the absence and presence of SRHA.