Nano-titanium dioxide (TiO ) is the most widely used form of nanoparticles in commercial industry and comes in 2 main configurations: rutile and anatase. Rutile TiO is used in ultraviolet (UV) screening applications, whereas anatase TiO crystals have a surface defect that makes them photoreactive. There are numerous reports in the literature of photo-induced toxicity to aquatic organisms following coexposure to anatase nano-TiO and UV. All natural freshwater contains varying amounts of natural organic matter (NOM), which can drive UV attenuation and quench reactive oxygen species (ROS) in aquatic ecosystems. The present research examined how NOM alters the photo-induced toxicity of anatase nano-TiO . Daphnia magna neonates were coexposed to NOM and photoexcited anatase nano-TiO for 48 h. Natural organic matter concentrations as low as 4 mg/L reduced anatase nano-TiO toxicity by nearly 100%. These concentrations of NOM attenuated UV by <10% in the exposure system. However, ROS production measured using a fluorescence assay was significantly reduced in a NOM concentration--dependent manner. Taken together, these data suggest that NOM reduces anatase nano-TiO toxicity via an ROS quenching mechanism and not by attenuation of UV. Environ Toxicol Chem 2017;36:1661-1666. © 2016 SETAC.

Increased use of titanium dioxide (TiO2) nanoparticles (NPs) in domestic and industrial applications has increased the risk for adverse environmental outcomes based on an elevated likelihood of organism exposure. Anatase TiO2 NP exposure to ultraviolet A (UV‐A) radiation in aquatic environments generates radical oxygen species (ROS), which may ultimately be responsible for increased organism toxicity. We have identified and measured the 2 most relevant ROS species, hydroxyl and superoxide radicals, and described that ROS can be modeled using the highly reactive hydroxyl radical to provide an upper bound for toxicity. The TiO2 NPs were co‐exposed to increasing natural organic matter (NOM) amounts (measured as concentration of dissolved organic carbon [DOC]) and simulated‐sunlight UV‐A intensities. Radical production rate was determined using fluorescence spectroscopy and was positively correlated with increases in TiO2 concentration and UV‐A intensity, and negatively correlated with increased DOC concentration. Daphnia magna toxicity was also found to decrease with NOM addition, which is attributed to the decreased radical production rate with increased DOC concentrations. We demonstrate that the rate of ROS production from simulated‐sunlight–irradiated TiO2 NPs can be quantified using relatively simple fluorescent techniques. We show that toxicity to TiO2 NP varies greatly with conditions, and that concentration alone is a poor predictor of toxicity. Describing toxicity/hydroxyl radical measurement may be a more accurate way to describe overall risk. We provide a framework for a simple model to describe toxicity/hydroxyl radical. These conclusions demonstrate the importance of considering exposure conditions as a means of risk management during TiO2 NP toxicity testing, waste management, and regulatory decisions. Environ Toxicol Chem 2021;40:1322–1334. © 2021 SETAC

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