RESEARCH ARTICLE Aging of TiO2 Nanoparticles Transiently Increases Their Toxicity to the Pelagic

During their aquatic life cycle, nanoparticles are subject to environmentally driven surface modifications (e.g. agglomeration or coating) associated with aging. Although the ecotoxi-cological potential of nanoparticles might be affected by these processes, only limited infor-mation about the potential impact of aging is available. In this context, the present study investigated acute (96 h) and chronic (21 d) implications of systematically aged titanium di-oxide nanoparticles (nTiO2; ~90 nm) on the standard test species Daphnia magna by follow-ing the respective test guidelines. The nTiO2 were aged for 0, 1, 3 and 6 d in media with varying ionic strengths (Milli-Q water: approx. 0.00 mmol/L and ASTM: 9.25 mmol/L) in the presence or absence of natural organic matter (NOM). Irrespective of the other parameters, aging in Milli-Q did not change the acute toxicity relative to an unaged control. In contrast, 6 d aged nTiO2 in ASTM without NOM caused a fourfold decreased acute toxicity. Relative to the 0 d aged particles, nTiO2 aged for 1 and 3 d in ASTM with NOM, which is the most en-vironmentally-relevant setup used here, significantly increased acute toxicity (by approxi-mately 30%), while a toxicity reduction (60%) was observed for 6 d aged nTiO2