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

Cerium dioxide nanoparticles (CeO 2 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 CeO 2 NPs in aqueous environments. However, the effects of light intensities and HA fractions on the interaction mechanism between CeO 2 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 CeO 2 NPs and HA. The absolute content of reactive oxygen species (ROS) and quantum yield (Φ) of 3 HA* were inhibited when HA (10 mg of C L −1 ) interacts with CeO 2 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 Φ of 3 HA* due to surface adsorption between HA and CeO 2 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 CeO 2 NPs. These findings are essential for us to improve the understanding of the impacts of HA on CeO 2 NPs under different conditions in natural waters.