Microplastics and Nanoplastics in Aquatic Environments: Aggregation, Deposition, and Enhanced Contaminant Transport.

Plastic litter is widely acknowledged as a global environmental threat, and poor management and disposal lead to increasing levels in the environment. Of recent concern is the degradation of plastics from macro- to micro- and even to nanosized particles smaller than 100 nm in size. At the nanoscale, plastics are difficult to detect and can be transported in air, soil, and water compartments. While the impact of plastic debris on marine and fresh waters and organisms has been studied, the loads, transformations, transport, and fate of plastics in terrestrial and subsurface environments are largely overlooked. In this Critical Review, we first present estimated loads of plastics in different environmental compartments. We also provide a critical review of the current knowledge vis-à-vis nanoplastic (NP) and microplastic (MP) aggregation, deposition, and contaminant cotransport in the environment. Important factors that affect aggregation and deposition in natural subsurface environments are identified and critically analyzed. Factors affecting contaminant sorption onto plastic debris are discussed, and we show how polyethylene generally exhibits a greater sorption capacity than other plastic types. Finally, we highlight key knowledge gaps that need to be addressed to improve our ability to predict the risks associated with these ubiquitous contaminants in the environment by understanding their mobility, aggregation behavior and their potential to enhance the transport of other pollutants.

Influence of Aqueous Inorganic Anions on the Reactivity of Nanoparticles in TiO2 Photocatalysis.

The influence of inorganic anions on the photoreactivity and aggregation of titanium dioxide nanoparticles (NPs) was assessed by dosing carbonate, chloride, nitrate, phosphate, and sulfate as potassium salts at multiple concentrations. NP stability was monitored in terms of aggregate morphology and electrophoretic mobility (EPM). Aggregate size and fractal dimension were measured over time by laser diffraction, and the isoelectric point (IEP) as a function of anion and concentration was obtained by measuring EPM versus pH. Phosphate, carbonate, and to a lesser extent, sulfate decreased the IEP of TiO2 and stabilized NP suspensions owing to specific surface interactions, whereas this was not observed for nitrate and chloride. TiO2 NPs were exposed to UV-A radiation, and the photoreactivity was assessed by monitoring the production of reactive species over time both at the NP surface (photogenerated holes) and in the bulk solution (hydroxyl radicals) by observing their reactions with the selective probe compounds iodide and terephthalic acid, respectively. The generation of photogenerated holes and hydroxyl radicals was influenced by each inorganic anion to varying degrees. Carbonate and phosphate inhibited the oxidation of iodide, and this interaction was successfully described by a Langmuir-Hinshelwood mechanism and related to the characteristics of TiO2 aggregates. Chloride and nitrate do not specifically interact with TiO2, and sulfate creates relatively weak interactions with the TiO2 surface such that no decrease in photogenerated hole reactivity was observed. A decrease in hydroxyl radical generation was observed for all inorganic anions. Quenching rate constants for the reaction of hydroxyl radicals with each inorganic anion do not provide a comprehensive explanation for the magnitude of this decrease, which arises from the interplay of several physicochemical phenomena. This work shows that the reactivity of NPs will be strongly influenced by the makeup of the waters they are released into. The impact of anion species on hydroxyl radical inhibition was as follows: carbonate > chloride > phosphate > nitrate > sulfate.

Impact of aggregate size and structure on the photocatalytic properties of TiO2 and ZnO nanoparticles.

Aggregation of photocatalytic semiconductors was determined to reduce the generation of free hydroxyl radicals in aqueous suspensions in a fashion dependent on aggregate size and structure. Static light scattering measurements were used to follow temporal changes in the fractal dimension of aggregating TiO(2) and ZnO nanoparticles. At length scales comparable to nanoparticle size, the structure of aggregated TiO(2) nanoparticles was independent of particle stability and the associated aggregation rate, consistent with the fused nature of TiO(2) primary particles in the initial suspension. In contrast, ZnO aggregates were characterized by smaller fractal dimensions when ionic strength, and the resulting aggregation rate, were increased. The photocatalytic activity of ZnO and TiO(2) in generating free hydroxyl radicals varied with aggregate structure and size, consistent with theory that predicts reduced reactivity as aggregates become larger and more dense.

Chlorpyrifos degradation via photoreactive TiO2 nanoparticles: Assessing the impact of a multi-component degradation scenario.

High concentrations of pesticides enter surface waters following agricultural application, raising environmental and human health concerns. The use of photoreactive nanoparticles has shown promise for contaminant degradation and surface water remediation. However, it remains uncertain how the complexity of natural waters will impact the photodegradation process. Here, we investigate the photoreactivity of titanium dioxide nanoparticles, the capability to degrade the pesticide chlorpyrifos, and the effect of and impact on bacteria during the photodegradation process. Loss of chlorpyrifos in solution resulted solely from photocatalytic oxidation, with 80% degradation observed after 24 h in our reactor, either in the presence or absence of bacteria. Degradation of chlorpyrifos to chlorpyrifos oxon and 3,5,6-trichloro-2-pyridinol was observed via LC/MS-MS and effectively modeled for the given reactor conditions. Bacterial inactivation occurred over 60 min and was not impacted by the presence of chlorpyrifos. The relative affinity of bacteria and chlorpyrifos for the nanoparticle surface decreased the amount of Reactive Oxygen Species (ROS) detected in the bulk by up to 94%, suggesting that ROS measurements in simplified systems may overestimate the reactivity of photoreactive nanoparticles in complex environments.

Heterogeneities in fullerene nanoparticle aggregates affecting reactivity, bioactivity, and transport.

Properties of nanomaterial suspensions are typically summarized by average values for the purposes of characterizing these materials and interpreting experimental results. We show in this work that the heterogeneity in aqueous suspensions of fullerene C(60) aggregates (nC(60)) must be taken into account for the purposes of predicting nanomaterial transport, exposure, and biological activity. The production of reactive oxygen species (ROS), microbial inactivation, and the mobility of the aggregates of the nC(60) in a silicate porous medium all increased as suspensions were fractionated to enrich with smaller aggregates by progressive membrane filtration. These size-dependent differences are attributed to an increasing degree of hydroxylation of nC(60) aggregates with decreasing size. As the quantity and influence of these more reactive fractions may increase with time, experiments evaluating fullerene transport and toxicity end points must take into account the evolution and heterogeneity of fullerene suspensions.