The interactions between unpurified manufactured nanoparticles (NPs; iron oxide NPs, ∼7 nm) and standard Suwannee River humic acid (SRHA) were investigated under a range of environmentally relevant conditions. At low pH, approximately 35% of the total iron was in the dissolved phase (<1 kDa), present from the initial synthesis, whereas at pH more than 4, this concentration was negligible because of the formation of new particles via hydrolysis. Dynamic light scattering results indicated that extensive aggregation of NPs began at approximately pH 5 to 6 and reached a maximum at approximately pH 8.5, whereas with added SRHA, aggregation was shifted to lower pH values of 4 to 5 and was affected by SRHA concentration. Aggregation could be explained mainly by charge neutralization. Further, more detailed investigations by flow field-flow fractionation and transmission-electron microscopy were performed under a more restricted set of conditions (pH 2–6) to examine the aggregation process. Results indicated the formation of SRHA surface coating on iron oxide NPs of approximately 1 nm and the increase in thickness of this coating with the increase of SRHA concentration. Iron oxide NPs were shown to form increasingly large aggregates with increases in both pH (from 2 to 6) and SRHA concentration (from 0 to 25 mg/L). The structure and aggregation mechanism of these aggregates were found to be both pH and SRHA concentration dependent, with open, porous aggregates in the absence of SRHA and compact aggregates in the presence of SRHA.

Despite increasing application of silver nanoparticles (NPs) in industry and consumer products, there is still little known about their potential toxicity, particularly to organisms in aquatic environments. To investigate the fate and effects of silver NPs in fish, rainbow trout (Oncorhynchus mykiss) were exposed via the water to commercial silver particles of three nominal sizes: 10 nm (N₁₀), 35 nm (N₃₅), and 600–1600 nm (N_Bulk), and to silver nitrate for 10 days. Uptake into the gills, liver, and kidneys was quantified by inductively coupled plasma-optical emission spectrometry, and levels of lipid peroxidation in gills, liver, and blood were determined by measurements of thiobarbituric acid reactive substances. Expression of a suite of genes, namely cyp1a2, cyp3a45, hsp70a, gpx, and g6pd, known to be involved in a range of toxicological response to xenobiotics was analyzed in the gills and liver using real-time PCR. Uptake of silver particles from the water into the tissues of exposed fish was low but nevertheless occurred for current estimated environmental exposures. Of the silver particles tested, N₁₀ were found to be the most highly concentrated within gill tissues and N₁₀ and N_Bulk were the most highly concentrated in liver. There were no effects on lipid peroxidation in any of the tissues analyzed for any of the silver particles tested, and this is likely due to the low uptake rates. However, exposure to N₁₀ particles was found to induce expression of cyp1a2 in the gills, suggesting a possible increase in oxidative metabolism in this tissue.