A recent study showed that the release from paints is ~3 to 27 μg/cm2 ·day, or in other terms, around 0.2 to 1.8% of the Cu NPs present in the paint were released in 180 days . Cu concentrations in water near the painted material can range from 40 to 630 mg/L; Cu can be dissolved, complexed with organics, or in nano- and micron-scale particles . While the objective of Cu in these paints is to maintain the surface free of specific target organisms, the above mentioned concentrations may have a negative impact on non-target organisms. However, depending on the exchange of the water surrounding the painted surfaces, the concentrations may decrease by 1–4 orders of magnitude within a few tens of cm from the surface. World demand for fungicides is over 660,000 tons/yr , of which only a fraction are copper based. In California alone, 7300 metric tons of copper pesticides are sold annually. In Cu-based pesticides, organic copper compounds represent 56% by weight, copper sulfate 34%, cupric oxide 4%, and cuprous oxide 6% . At present there is no separate tracking of copper nanopesticides. Given the benefit of slower release of Cu2+ from copper nanopesticides, compared to copper sulfate, NPs are likely to increase their share of the market. Manufacturer recommendations for the use of Cu2 nanopesticide indicate that it can be used for a wide range of crops, including vegetables ,fruits as well as trees . The Cu2 nanopesticides are typically applied at initiation of new growth and repeated at 2–4 week intervals to control a wide range of fungal and bacterial diseases. They may be applied at rates of 0.05 to 0.8 g/m2 per event . This corresponds to around 10–50 mg per plant, depending on application amount and planting density. Uptake by crop plants exposed to Cu NPs varies depending on plant species, Cu NP, mode of application, and growth media . For example, for lettuces , alfalfa and cilantro exposed via soil,vertical grow racks uptake of nCu, nCuO, and the two nCu2 nanopesticides resulted in accumulation of Cu mostly in the roots, with little translocation to the stems, and almost none to leaves . Cu NPs may accumulate in the outer parts of root tissues , as shown in micro-XRF studies .

However, cucumbers exposed to nCu via the soil translocated it readily to upper plant tissues, including stems, leaves and fruits . When the application of Cu NPs is foliar, a much larger fraction of the Cu taken up by the plant remains in leaves or fruits, although some plants do exhibit translocation from upper tissues to roots by phloem. For example, lettuces exposed to nCu2-b, following the recommended application amount, accumulated around 1350–2010 mg Cu/ kg dry weight after 30 days of foliar exposure. The accumulated Cu was mostly inside the tissues, since the lettuces were washed thoroughly . While most of the copper was sequestered in the leaves, a small fraction was translocated to root tissues through phloem loading . A fraction was in the form of nano- and micro-particulates, although not necessarily as Cu2. Considering typical US daily lettuce consumption, the Cu content in the leaves would represent an additional 2.2–3.3 mg Cu/person-day, which is within recommended intake guidelines of 0.7–10 mg Cu/person-day . However, a diet rich in fruits and vegetables protected with Cu nanopesticides, in addition to other sources of Cu, may result in elevated Cu intake for some individuals.Once released into the environment, NPs immediately begin to undergo a number of transformations . Homoand hetero-aggregation, coating with natural organic matter, sedimentation, dissolution, oxidation in oxic environments, reduction or sulfidation in anoxic waters all initiate from the moment the dry ENM powder is placed in an aqueous medium. However, different processes dominate at various stages, depending on ENM composition and environmental parameters. Aggregation of Cu NPs in natural waters depends on ENM speciation, aqueous media characteristics, specifically ionic strength , natural organic matter concentration, and pH . Given their high density , Cu NPs settle out rapidly once they reach micron scale. nCu, with an isoelectric point of pH 2.1, rapidly forms highly poly disperse micron-scale aggregates in simple salt solutions and natural waters , both in the absence and presence of organisms . Rapid aggregation of nCu leads to fast sedimentation; for instance, only 20% of initial nCu mass was detected after 6 h in 10 mM NaCl at pH 7 . However, in the presence of zebra fish and moderately hard freshwater, ~40% of the initial mass of nCu remained suspended after 48 h, indicating a fraction of the nanoparticles may have been stabilized by NOM released by the fish.

In contrast, nCuO is relatively stable in freshwater, with a critical coagulation concentration of 40 mM NaCl at pH 7 . However, stability of nCuO is strongly influenced by salinity and pH . Rapid aggregation and sedimentation of nCuO occurs at high IS due to complete screening of electrostatic charges on particle surfaces . Sedimentation of nCuO is also pH dependent; it reached values of 64%, 40%, and 39% at pH 4, 7, and 11, respectively in 10 mM NaCl . Particle size also influences sedimentation. Sedimentation of 50 nm CuO in seawater was faster, compared with10 nm CuO , indicating that smaller particles have longer resident time in suspension. Adsorption of ions in natural waters may strongly influence nanoparticle stability. nCuO is stabilized by the presence of phosphate ions , which reverse NP surface charge polarity at concentrations as low as 0.1 mg PO4 3−/L. NOM, surfactants, and polymers stabilize Cu NPs via electrostatic and/or steric influences . The critical coagulation concentration of nCuO increased from 40 to 75 mM NaCl in the presence of 0.25 mg/L Suwanee River NOM . Similarly, the aggregation and sedimentation of Cu NPs were suppressed by extracellular polymeric substances from a marine phytoplankton and activated sludge . When 10 mg/L nCuO was suspended in 10 mM NaCl for seven days, suspended Cu increased from 0.10 mg/L in the absence to 0.32 mg/L in the presence of SRNOM, and to 0.48 mg/L in the presence of algal EPS . Bovine serum albumin stabilized nCuO more than alginate and activated sludge EPS due to a stronger steric repulsive energy . Due to the abundance of polymeric stabilizer and very negative surface charge, nCu2-b was much more stable than nCu and nCuO in freshwater and up to 100 mM NaCl, sedimenting very slowly . Homoaggregation is important very early on, or when there are few natural colloids present. Once the Cu NPs reach natural waters, the concentration of suspended particles will be 4–6 orders of magnitude greater than that of the NPs. Under these conditions, heteroaggregation is likely to overwhelmingly dominate the fate of the Cu NPs . Dissolution of Cu NPs in natural waters over a 90-day period generally correlates with ENM aggregation and oxidation state, pH, and NOM, although in saline waters the formation of insoluble complexes also drives dissolution.

Highly aggregated Cu NPs have a reduced surface area, which decreases the dissolution rate. In simple salt solutions, dissolution rate was nCu2- b > nCu ≫ nCuO, and in all cases, the dissolution rate decreased as pH increased from 4 to 11 . Dissolution of Cu NPs was enhanced at high IS in the presence of NOM due to additional complexation. For example,vertical hydroponics dissolution of Cu2-b after 90 days was 7.0%, 10.9%, and 17.4% at 1, 10, and 100 mM NaCl, respectively . nCu underwent rapid dissolution followed by complex formation in waters with moderate to high salinity, likely as a result of the ENM being in a non-oxidized state . EPS and other NOM can coat the Cu ENM surfaces, in some cases reducing the initial dissolution rate . However, the released Cu2+ may be bound by negatively charged functional groups in NOM, driving dissolution. Dissolution of Cu2-b after 90 days increased at 1, 10, and 100 mM NaCl to 12.7%, 13.5%, and 20.7% when 5 mg of carbon/L from phytoplankton EPS was present; and to 8.4%, 13.2%, and 18.8% with 5 mg/L SRNOM . The overall rates of dissolution depend on aggregation, NOM coating, and other ions present . The dissolution of nCuO is very slow: on the order of weeks in freshwater, and in seawater dissolution is ≤1% after months, over a wide range of initial nanoparticle concentrations . However, these studies were performed under almost saturated conditions, where the maximum solubility of CuO may be reached. Inunsaturated and somewhat idealized conditions, the dissolution of nCuO may occur in a matter of hours to days . Surface water renewal and immobilization of NPs on a substrate can lead to accelerated dissolution, even for these relatively insoluble NPs. The transformation of Cu in the environment is controlled by the chemistry of both the particles and the environment . Cu is commonly partitioned into aqueous , solid , and biological media .As a result, inorganic Cu ions in natural waters exist mostly as complexes of carbonate, hydroxide, and NOM . The fraction of free Cu ions decreases by increasing IS and pH in the presence of EPS . In fact, free Cu ions were non-detectable at pH 11 . Moreover, under low redox conditions and high S2−, CuS will form relatively insoluble compounds, even at the nanoscale.

Bio-accumulation of Cu and/or Cu NPs has been observed in many studies, from cell membranes in single-cell organisms , to aquatic filter feeders that pack and excrete Cu in pseudofeces , in fish , marine invertebrates , and in terrestrial plants . While it is likely that Cu is internalized as Cu2+ or in organic complexes, in some cases Cu NPs are ingested or taken up from soil into the organisms, where they likely dissolve. In terrestrial plants, translocation of Cu was observed from roots to above ground tissues when the exposure was via soil or hydroponic media, and from leaves to stems and roots when the exposure was foliar. In many cases bio-accumulation factors are 2 to 4 orders of magnitude. The accumulated knowledge on NP fate and transport, as reflected in the nanoFate model , indicates that Cu-based NPs would enter the environment mostly via treated effluent from wastewater treatment plants , biosolids from WWTPs applied to agriculture, and Cu-based nanopesticides. Assuming a continuous input of Cu NPs, and the dissolution and transformation of the NPs once released, the concentrations of dissolved Cu2+ in the freshwater would increase by < 0.1 μg/L relative to background, and the concentrations of small aggregates of Cu NPs in the water column would be < 1 ng/L . Most of the Cu would be accumulated in the sediment beds of freshwater and marine environments. Agricultural soils receiving WWTP bio-solids would accumulate Cu at concentrations ranging from 1 to 10 μg/kg above the background Cu concentrations . Future studies should address questions regarding the effect of Cu NP coatings, the nature of the released Cu species from paints and coatings, and the potential accumulation of Cu and Cu NPs in agricultural applications where application rates will likely be continuous and at higher concentrations than other releases.Although Cu is an essential element for many biological processes, doses of Cu above the required level can be toxic to many organisms. As such, the concentration and bio-availability of Cu in natural environments is very important when considering ecosystem health . The bio-availability of Cu depends on speciation and environmental factors including pH, redox potential, water, soil and sediment type, water hardness, and organic content . In comparison to terrestrial organisms, Cu tends to be quite toxic to aquatic biota, whose sensitivity to Cu and Cu NPs depends on factors such as surface-area-to-volume ratio, respiratory rates, and, for fish, flow rate over gill surfaces, among others . Cu is more bioavailable in aquatic than in terrestrial systems, where it can be bound in minerals. In addition to the known toxic effect of exposure to non-nano Cu, there is the potential for additional nano-related toxicity resulting from exposure to Cu NPs in the environment . The following studies present a hierarchical assessment of Cu NP toxicity.