Interactions between cerium dioxide nanoparticles and humic acid: Influence of light intensities and molecular weight fractions

Cerium dioxide nanoparticles (CeO2 NPs) are ubiquitous in the water environment due to the extensive commercial applications. The complexity of heterogeneous humic acid (HA) plays a significant role in affecting the physicochemical properties of CeO2 NPs in aqueous environments. However, the effects of light intensities and HA fractions on the interaction mechanism between CeO2 NPs and HA are poorly understood. Here, we provided the evidence that both light intensities (>3 E L-1 s(-1)) and molecular weights (>10 kDa) can effectively affect the interactions between CeO2 NPs and HA. The absolute content of reactive oxygen species (ROS) and quantum yield (Phi) of (3)HA* were inhibited when HA (10 mg of C L-1) interacts with CeO2 NPs. However, they were positively correlated with the increasing irradiation time and simulated sunlight intensities. High molecular weights of HA fraction (>100 kDa) restrained the ROS generation and Phi of (3)HA* due to surface adsorption between HA and CeO2 NPs blocking reactive sites, competitive absorption for simulated sunlight. Fourier transform infrared and three-dimensional excitation-emission matrix fluorescence spectroscopy confirmed that the carboxylic groups of HA have high complexation capacity with CeO2 NPs. These findings are essential for us to improve the understanding of the impacts of HA on CeO2 NPs under different conditions in natural waters.