Influence of dissolved organic matter on photogenerated reactive oxygen species and metal-oxide nanoparticle toxicity

Abstract

The effect of humic acid (HA) or fulvic acid (FA) on reactive oxygen species (ROS) generation by six metal-oxide nanoparticles (NPs) and their toxicities toward Escherichia coli was investigated under UV irradiation. Dissolved organic matter (DOM) decreased OH generation by TiO2, ZnO, and Fe2O3, with FA inhibiting OH generation more than HA. The generated OH in NPs/DOM mixtures was higher than the measured concentrations because DOM consumes OH faster than its molecular probe. None of NPs/FA mixtures produced O2−. The generated O2− concentrations in NPs/HA mixtures (except Fe2O3/HA) were higher than the sum of O2− concentrations that produced as NPs and HA were presented by themselves. Synergistic O2− generation in NPs/HA mixtures resulted from O2 reduction by electron transferred from photoionized HA to NPs. DOM increased 1O2 generation by TiO2, CuO, CeO2, and SiO2, and FA promoted 1O2 generation more than HA. Enhanced 1O2 generation resulted from DOM sensitization of NPs. HA did not increase 1O2 generation by ZnO and Fe2O3 primarily because released ions quenched 1O2 precursor (3HA*). Linear correlation was developed between total ROS concentrations generated by NPs/DOM mixtures and bacterial survival rates (R2 ≥ 0.80). The results implied the necessity of considering DOM when investigating the photoreactivity of NPs.