The extensive use of titanium dioxide nanoparticles (nano-TiO2) in many consumer products has raised concerns about possible risks to the environment The magnitude of the threat may depend on whether nano-TiO2 remains dispersed in the environment, or forms much larger-sized aggregates or clusters. Currently, limited information is available on the issue. In this context, the purpose of the present article is to report initial measurements of the morphology and rate of formation of nano-TiO2 aggregates in aqueous suspensions as a function of ionic strength and of the nature of the electrolyte in a moderately acid to circumneutral pH range typical of soil and surface water conditions. Dynamic light scattering results show that 4-5 nm titanium dioxide particles readily form stable aggregates with an average diameter of 50-60 nm at pH approximately 4.5 in a NaCl suspension adjusted to an ionic strength of 0.0045 M. Holding the pH constant but increasing the ionic strength to 0.0165 M, leads to the formation of micron-sized aggregates within 15 min. At all other pH values tested (5.8-8.2), micron-sized aggregates form in less than 5 min (minimum detection time), even at low ionic strength (0.0084-0.0099 M with NaCl). In contrast, micron-sized aggregates form within 5 min in an aqueous suspension of CaCl2 at an ionic strength of 0.0128 M and pH of 4.8, which is significantly faster than observed for NaCI suspensions with similar ionic strength and pH. This result indicates that divalent cations may enhance aggregation of nano-TiO2 in soils and surface waters. Optical micrographs show branching aggregates of sizes ranging from the 1 microm optical limit of the microscope to tens of micrometers in diameter.

Nanosized silver sulfide (α-Ag(2)S) particles were identified in the final stage sewage sludge materials of a full-scale municipal wastewater treatment plant using analytical high-resolution transmission electron microscopy. The Ag(2)S nanocrystals are in the size range of 5-20 nm with ellipsoidal shape, and they form very small, loosely packed aggregates. Some of the Ag(2)S nanoparticles (NPs) have excess S on the surface of the sulfide minerals under S-rich environments, resulting in a ratio of Ag to S close to 1. Considering the current extensive production of Ag NPs and their widespread use in consumer products, it is likely that they are entering wastewater streams and the treatment facilities that process this water. This study suggests that in a reduced, S-rich environment, such as the sedimentation processes during wastewater treatment, nanosized silver sulfides are being formed. This field-scale study provides for the first time nanoparticle-level information of the Ag(2)S present in sewage sludge products, and further suggests the role of wastewater treatment processes on transformation of Ag nanoparticles and ionic Ag potentially released from them.

The production of the neurotoxic methylmercury in the environment is partly controlled by the bioavailability of inorganic divalent mercury (Hg(II)) to anaerobic bacteria that methylate Hg(II). In sediment porewater, Hg(II) associates with sulfides and natural organic matter to form chemical species that include organic-coated mercury sulfide nanoparticles as reaction intermediates of heterogeneous mineral precipitation. Here, we exposed two strains of sulfate-reducing bacteria to three forms of inorganic mercury: dissolved Hg and sulfide, nanoparticulate HgS, and microparticulate HgS. The bacteria cultures exposed to HgS nanoparticles methylated mercury at a rate slower than cultures exposed to dissolved forms of mercury. However, net methylmercury production in cultures exposed to nanoparticles was 6 times greater than in cultures treated with microscale particles, even when normalized to specific surface area. Furthermore, the methylation potential of HgS nanoparticles decreased with storage time of the nanoparticles in their original stock solution. In bacteria cultures amended with nano-HgS from a 16 h-old nanoparticle stock, 6-10% of total mercury was converted to methylmercury after one day. In contrast, 2-4% was methylated in cultures amended with nano-HgS that was aged for 3 days or 1 week. The methylation of mercury derived from nanoparticles (in contrast to the larger particles) would not be predicted by equilibrium speciation of mercury in the aqueous phase (<0.2 μm) and was possibly caused by the disordered structure of nanoparticles that facilitated release of chemically labile mercury species immediately adjacent to cell surfaces. Our results add new dimensions to the mechanistic understanding of mercury methylation potential by demonstrating that bioavailability is related to the geochemical intermediates of rate-limited mercury sulfide precipitation reactions. These findings could help explain observations that the "aging" of mercury in sediments reduces its methylation potential and provide a basis for assessing and remediating methylmercury hotspots in the environment.

A large fraction of engineered nanomaterials in consumer and commercial products will reach natural ecosystems. To date, research on the biological impacts of environmental nanomaterial exposures has largely focused on high-concentration exposures in mechanistic lab studies with single strains of model organisms. These results are difficult to extrapolate to ecosystems, where exposures will likely be at low-concentrations and which are inhabited by a diversity of organisms. Here we show adverse responses of plants and microorganisms in a replicated long-term terrestrial mesocosm field experiment following a single low dose of silver nanoparticles (0.14 mg Ag kg−1 soil) applied via a likely route of exposure, sewage biosolid application. While total aboveground plant biomass did not differ between treatments receiving biosolids, one plant species, Microstegium vimeneum, had 32 % less biomass in the Slurry+AgNP treatment relative to the Slurry only treatment. Microorganisms were also affected by AgNP treatment, which gave a significantly different community composition of bacteria in the Slurry+AgNPs as opposed to the Slurry treatment one day after addition as analyzed by T-RFLP analysis of 16S-rRNA genes. After eight days, N2O flux was 4.5 fold higher in the Slurry+AgNPs treatment than the Slurry treatment. After fifty days, community composition and N2O flux of the Slurry+AgNPs treatment converged with the Slurry. However, the soil microbial extracellular enzymes leucine amino peptidase and phosphatase had 52 and 27% lower activities, respectively, while microbial biomass was 35% lower than the Slurry. We also show that the magnitude of these responses was in all cases as large as or larger than the positive control, AgNO3, added at 4-fold the Ag concentration of the silver nanoparticles.

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The production of methylmercury (MeHg) by anaerobic microorganisms depends in part on the speciation and bioavailability of inorganic mercury to these organisms. Our previous work with pure cultures of methylating bacteria has demonstrated that the methylation potential of mercury decreased during the aging of mercuric sulfides (from dissolved to nanoparticulate and microcrystalline HgS). The objective of this study was to understand the relationship between mercury sulfide speciation and methylation potential in experiments that more closely simulate the complexity of sediment settings. The study involved sediment slurry microcosms that represented a spectrum of salinities in an estuary and were each amended with different forms of mercuric sulfides: dissolved Hg and sulfide, nanoparticulate HgS (3-4 nm in diameter), and microparticulate HgS (>500 nm). The results indicated that net MeHg production was influenced by both the activity of sulfate-reducing microorganisms (roughly represented by the rate of sulfate loss) and the bioavailability of mercury. In the presence of abundant sulfate and carbon sources (supporting relatively high microbial activity), net MeHg production in the slurries amended with dissolved Hg was greater than in slurries amended with nano-HgS, similar to previous experiments with pure bacterial cultures. In microcosms with minimal microbial activity (indicated by low rates of sulfate loss), the addition of either dissolved Hg or nano-HgS resulted in similar amounts of net MeHg production. For all slurries receiving micro-HgS, MeHg production did not exceed abiotic controls. In slurries amended with dissolved and nano-HgS, mercury was mainly partitioned to bulk-scale mineral particles and colloids, indicating that Hg bioavailability was not simply related to dissolved Hg concentration or speciation. Overall, the results suggest that models for mercury methylation potential in the environment will need to balance the relative contributions of mercury speciation and activity of methylating microorganisms.

The development of antibiotics revolutionized human health, providing a simple cure for once dreaded diseases such as tuberculosis. However, widespread production, use, and mis-use of antibiotics have contributed to the next-generation concern for global public health: the emergence of multiple drug-resistant (MDR) infectious organisms (a.k.a. “superbugs”). Recently, nanotechnology, specifically the use of nanomaterials (NMs) with antimicrobial activity, has been presented as a new defense against MDR infectious organisms. We discuss the potential for NMs to either circumvent microbial resistance or induce its development in light of our current state of knowledge, finding that this question points to a need for fundamental research targeting the molecular mechanisms causing antimicrobial activity in NMs. In the context of current microbial nanotoxicology studies, particularly reductionist laboratory studies, we offer suggestions and considerations for future research, using an illustrative example from our work with silver nanoparticles.

Titanium dioxide (TiO(2)) is the most extensively used engineered nanoparticle to date, yet its fate in the soil environment has been investigated only rarely and is poorly understood. In the present study, we conducted two field-scale investigations to better describe TiO(2) nano- and larger particles in their most likely route of entry into the environment, i.e., the application of biosolids to soils. We particularly concentrated on the particles in the nano-size regime due to their novel and commercially useful properties. First, we analyzed three sewage sludge products from the US EPA TNSSS sampling inventory for the occurrence, qualitative abundance, and nature of TiO(2) nano- and larger particles by using analytical scanning electron microscopy and analytical (scanning) transmission electron microscopy. Nano- and larger particles of TiO(2) were repeatedly identified across the sewage sludge types tested, providing strong evidence of their likely concentration in sewage sludge products. The TiO(2) particles identified were as small as 40 nm, and as large as 300 nm, having faceted shapes with the rutile crystal structure, and they typically formed small, loosely packed aggregates. Second, we examined surface soils in mesocosms that had been amended with Ag nanoparticle-spiked biosolids for the occurrence of TiO(2) particles. An aggregate of TiO(2) nanoparticles with the rutile structure was again identified, but this time TiO(2) nanoparticles were found to contain Ag on their surfaces. This suggests that TiO(2) nanoparticles from biosolids can interact with toxic trace metals that would then enter the environment as a soil amendment. Therefore, the long-term behavior of TiO(2) nano- and larger particles in sewage sludge materials as well as their impacts in the soil environment need to be carefully considered.