Relative importance of humic and fulvic acid on ROS generation, dissolution, and toxicity of sulfide nanoparticles

In this study, the effect of natural organic matter (NOM) composition (humic acid (HA) or fulvic acid (FA)) on dissolution, reactive oxygen species (ROS) generation, and toxicity of sulfide nanoparticles (NPs) was investigated under UV irradiation. NOM acted as a UV filter or antioxidant, decreasing ROS (O-2(center dot-), center dot OH, and O-1(2)) generation by WS2 and MoS2 NPs. The higher light-absorbing fractions of HA in NP/HA mixtures and the faster reaction rate of HA with ROS resulted in higher inhibition effect of HA than FA on O-2 center dot(-) and center dot OH generation by WS2 and MoS2 NPs. Both HA and FA completely inhibited O-1(2) generation by WS2 and MoS2 NPs. NOM could transfer electrons to CdS and promote its O-2 center dot(-) generation. No measurable amount of center dot OH was generated by CdS with or without NOM. FA decreased O-1(2) generation by CdS more significantly than HA due to the higher reaction rate between FA and O-1(2). HA showed a higher inhibition effect on the dissolution rate of CdS and WS2 NPs than FA. Both HA and FA played minor roles in the toxicity of CdS toward Escherichia coli but decreased the toxicity of MoS2 and WS2 due to the reduced ROS generation and/or dissolution concentrations. (C) 2017 Elsevier Ltd. All rights reserved.