The production and use of nanoparticles leads to the emission of manufactured or engineered nanoparticles into the environment. Those particles undergo many possible reactions and interactions in the environment they are exposed to. These reactions and the resulting behavior and fate of nanoparticles in the environment have been studied for decades through naturally occurring nanoparticulate (1–100 nm) and colloidal (1–1000 nm) substances. The knowledge gained from these investigations is nowhere near sufficiently complete to create a detailed model of the behavior and fate of engineered nanoparticles in the environment, but is a valuable starting point for the risk assessment of these novel materials. It is the aim of this Review to critically compare naturally observed processes with those found for engineered systems to identify the “nanospecific” properties of manufactured particles and describe critical knowledge gaps relevant for the risk assessment of manufactured nanomaterials in the environment.

Riverine transport of iron (Fe) and arsenic (As) is affected by their associations with natural organic matter (NOM) and suspended iron (oxy)hydroxides. Speciation has a strong influence on element transport from the headwaters to the ocean because NOM may be transported over longer distances compared to iron (oxy)hydroxides. We show that Fe speciation changes along the flow path of a boreal watercourse, as water moves from NOM-rich, acidic first-order streams with pH as low as 3.9 to less acidic higher-order systems (up to pH 6.4). Analysis by Flow Field-Flow Fractionation and chemical equilibrium modeling revealed that Fe from wetland-dominated headwaters was mainly exported as Fe-NOM complexes; in catchments with a stream order >1 and with higher pH, Fe was present in Fe-NOM complexes and precipitated as nanoparticulate iron(oxy)hydroxides which aggregated as the pH increased, with their size eventually exceeding the membrane filters cutoff (0.2 μm). The measured NOM-bound Fe decreased with increasing pH, from 0.38 to 0.16 mmol Fe·g(NOM)(-1). The high concentrations of NOM-bound Fe emphasize the importance of boreal catchments to Fe export to the oceans. Concentrations of As in the <0.2 μm fraction but larger than what is usually considered "truly dissolved" (<1000 g·mol(-1)), decreased from 75% to 26% with increasing pH. The As in this size range was mainly associated with NOM but at pH >4.5 became associated with iron(oxy)hydroxides, and its transport thus became more coupled to that of the iron(oxy)hydroxides downstream in the circumneutral streams.

Detecting and quantifying engineered nanoparticles (ENPs) in complex environmental matrices requires the distinction between natural nanoparticles (NNPs) and ENPs.

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The speciation of iron(III) in oxic seawater is dominated by its hydrolysis and sedimentation of insoluble iron(III)-oxyhydroxide. As a consequence, many oceanic areas have very low iron levels in surface seawater which leads to iron deficiency since phytoplankton require iron as a micronutrient in order to grow. Fortunately, iron solubility is not truly as low as Fe(III) solubility measurements in inorganic seawater would suggest, since oceanic waters contain organic molecules which tend to bind the iron and keep it in solution. Various iron-binding organic ligands which combine to stabilize dissolved iron have been detected and thoroughly investigated in recent years. However, the role of iron-binding ligands from terrestrial sources remains poorly constrained. Blackwater rivers supply large amounts of natural organic material (NOM) to the ocean. This NOM (which consists mainly of vascular plant-derived humic substances) is able to greatly enhance iron bioavailability in estuaries and coastal regions, however, breakdown processes lead to a rapid decrease of river-derived NOM concentrations with increasing distance from land. It has therefore been argued that the influence of river-derived NOM on iron biogeochemistry in offshore seawater does not seem to be significant. Here we used a standard method based on 59Fe as a radiotracer to study the solubility of Fe(III)-oxyhydroxide in seawater in the presence of riverine NOM. We aimed to address the question how effective is freshwater NOM as an iron chelator under open ocean conditions where the concentration of land-derived organic material is about 3 orders of magnitude smaller than in coastal regions, and does this iron chelating ability vary between NOM from different sources and between different size fractions of the river-borne NOM. Our results show that the investigated NOM fractions were able to substantially enhance Fe(III)-oxyhydroxide solubility in seawater at concentrations of the NOM ≥ 5 μg L− 1. Terrigenous NOM concentrations ≥ 5 μg L− 1 are in no way unusual in open ocean surface waters especially of the Arctic and the North Atlantic Oceans. River-derived humic substances could therefore play a greater role as iron carriers in the ocean than previously thought.

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