Jason K. Kirby

Fate and Risks of Nanomaterials in Aquatic and Terrestrial Environments

Over the last decade, nanoparticles have been used more frequently in industrial applications and in consumer and medical products, and these applications of nanoparticles will likely continue to increase. Concerns about the environmental fate and effects of these materials have stimulated studies to predict environmental concentrations in air, water, and soils and to determine threshold concentrations for their ecotoxicological effects on aquatic or terrestrial biota. Nanoparticles can be added to soils directly in fertilizers orplant protection products or indirectly through application to land or wastewater treatment products such as sludges or biosolids. Nanoparticles may enter aquatic systems directly through industrial discharges or from disposal of wastewater treatment effluents or indirectly through surface runoff from soils. Researchers have used laboratory experiments to begin to understand the effects of nanoparticles on waters and soils, and this Account reviews that research and the translation of those results to natural conditions. In the environment, nanoparticles can undergo a number of potential transformations that depend on the properties both of the nanoparticle and of the receiving medium. These transformations largely involve chemical and physical processes, but they can involve biodegradation of surface coatings used to stabilize many nanomaterial formulations. The toxicity of nanomaterials to algae involves adsorption to cell surfaces and disruption to membrane transport. Higher organisms can directly ingest nanoparticles, and within the food web, both aquatic and terrestrial organisms can accumulate nanoparticles. The dissolution of nanoparticles may release potentially toxic components into the environment. Aggregation with other nanoparticles (homoaggregation) or with natural mineral and organic colloids (heteroaggregation) will dramatically change their fate and potential toxicity in the environment. Soluble natural organic matter may interact with nanoparticles to change surface charge and mobility and affect the interactions of those nanoparticles with biota. Ultimately, aquatic nanomaterials accumulate in bottom sediments, facilitated in natural systems by heteroaggregation. Homoaggregates of nanoparticles sediment more slowly. Nanomaterials from urban, medical, and industrial sources may undergo significant transformations during wastewater treatment processes. For example, sulfidation of silver nanoparticles in wastewater treatment systems converts most of the nanoparticles to silver sulfides (Ag₂S). Aggregation of the nanomaterials with other mineral and organic components of the wastewater often results in most of the nanomaterial being associated with other solids rather than remaining as dispersed nanosized suspensions. Risk assessments for nanomaterial releases to the environment are still in their infancy, and reliable measurements of nanomaterials at environmental concentrations remain challenging. Predicted environmental concentrations based on current usage are low but are expected to increase as use increases. At this early stage, comparisons of estimated exposure data with known toxicity data indicate that the predicted environmental concentrations are orders of magnitude below those known to have environmental effects on biota. As more toxicity data are generated under environmentally-relevant conditions, risk assessments for nanomaterials will improve to produce accurate assessments that assure environmental safety.

Solubility and batch retention of CeO2 nanoparticles in soils.

There is a paucity of information on the environmental fate of cerium oxide nanoparticles (CeO2 NPs) for terrestrial systems that may be exposed to CeO2 NPs by the application of biosolids derived from wastewater treatment systems. Using ultrafiltration (UF), dissolution, and nonequilibrium retention (Kr) values of citrate-coated (8 nm diameter) CeO2 NPs and partitioning (Kd) values of dissolved Ce(III) and Ce(IV) were obtained in suspensions of 16 soils with a diversity of physicochemical properties. Dissolution of CeO2 NPs studied in solutions was only significant at pH 4 and was less than 3.1%, whereas no dissolved Ce was detected in soils spiked with CeO2 NPs. Kr values of CeO2 NP were low (median Kr=9.6 L kg(-1)) relative to Kd values of dissolved CeIII and CeIV (median Kd=3763 and 1808 L kg(-1), respectively), suggesting low CeO2 NP retention in soils. Surface adsorption of phosphate to CeO2 NP caused a negative zeta potential, which may explain the negative correlation of log Kr values with dissolved phosphate concentrations and the significant reduction of Kr values upon addition of phosphate to soils. The positive correlation of Kr values with clay content suggested heterocoagulation of CeO2 NPs with natural colloids in soils. Co-addition of CeO2 NPs with biosolids, on the other hand, did not affect retention.

Transport of silver nanoparticles in saturated columns of natural soils.

With industrialization and urbanization soils are increasingly exposed to engineered nanoparticles (ENP), yet knowledge regarding the transport of ENP in natural soils is lacking, a process that was examined further in the current study. Saturated columns of 11 natural soils with varying physical and chemical properties were spiked with two pore volumes of a solution containing 1.7 mg Ag L(-1) as polyvinylpyrrolidone (PVP)-coated silver nanoparticles (AgNP) (40 nm actual diameter) and eluted at a constant flow rate of 1 ml min(-1). Breakthrough of Ag was analyzed using filtration theory and a HYDRUS-1D transport model that incorporated two-site kinetic attachment-detachment. Separate kinetic batch studies suggested fast heteroaggregation between negatively charged AgNP and positively charged sites on the common soil colloids maghemite or montmorillonite. The concentration of such sites in the soil correlates positively with the oxalate-extractable aluminum concentration of the soils, a measure that correlated positively with collision efficiency. This correlation thus suggested favorable deposition of AgNP and/or enhanced straining following heteroaggregation of AgNP with mobile soils colloids. Occurrence of heteroaggregation was supported by the batch studies, enhanced size-exclusion in the soil with the highest porosity, and reversible attachment-detachment predicted from HYDRUS modeling, whereas straining and favorable deposition were suggested by irreversible attachment. Our study suggests that under similar experimental conditions, PVP-coated AgNP would rapidly interact with natural colloids in soils significantly reducing their mobility and hence potential risk from off-site transport.

Toxicity, biotransformation, and mode of action of arsenic in two freshwater microalgae (Chlorella sp. and Monoraphidium arcuatum)

The toxicity of As(V) and As(III) to two axenic tropical freshwater microalgae, Chlorella sp. and Monoraphidium arcuatum, was determined using 72-h growth rate-inhibition bioassays. Both organisms were tolerant to As(III) (72-h concentration to cause 50% inhibition of growth rate [IC50], of 25 and 15 mg As[III]/L, respectively). Chlorella sp. also was tolerant to As(V) with no effect on growth rate over 72 h at concentrations up to 0.8 mg/L (72-h IC50 of 25 mg As[V]/L). Monoraphidium arcuatum was more sensitive to As(V) (72-h IC50 of 0.25 mg As[V]/L). An increase in phosphate in the growth medium (0.15-1.5 mg PO4(3-)/L) decreased toxicity, i.e., the 72-h IC50 value for M. arcuatum increased from 0.25 mg As(V)/L to 4.5 mg As(V)/L, while extracellular As and intracellular As decreased, indicating competition between arsenate and phosphate for cellular uptake. Both microalgae reduced As(V) to As(III) in the cell, with further biological transformation to methylated species (monomethyl arsonic acid and dimethyl arsinic acid) and phosphate arsenoriboside. Less than 0.01% of added As(V) was incorporated into algal cells, suggesting that bioaccumulation and subsequent methylation was not the primary mode of detoxification. When exposed to As(V), both species reduced As(V) to As(III); however, only M. arcuatum excreted As(III) into solution. Intracellular arsenic reduction may be coupled to thiol oxidation in both species. Arsenic toxicity most likely was due to arsenite accumulation in the cell, when the ability to excrete and/or methylate arsenite was overwhelmed at high arsenic concentrations. Arsenite may bind to intracellular thiols, such as glutathione, potentially disrupting the ratio of reduced to oxidized glutathione and, consequently, inhibiting cell division.

A method for determination of retention of silver and cerium oxide manufactured nanoparticles in soils

Environmental context.Soils are the environmental compartment likely to be exposed most to manufactured nanoparticles, but there is no method available at present to assess their retention, which determines potential mobility and bioavailability. Optimisation and application of a method to determine retention values for silver (Ag) and cerium oxide (CeO2) manufactured nanoparticles in soils found in many cases that they differed from the partitioning of their bulk and soluble counterparts. Wider application of this method can assist in comparing the risk of many different manufactured nanoparticles with other contaminants in soil systems and model their relationship to soil properties. Abstract.Methods to study the retention of manufactured nanoparticles (MNP) are lacking for soils that are likely to be increasingly exposed to MNP. In this study we present, for the first time, a method to determine retention values (Kr) of Ag and CeO2 MNP, that can be ranked among solid–liquid partitioning (Kd) values of bulk (micrometre-sized) forms, soluble salts and other possible contaminants of soils. After method optimisation, suspensions containing 1.24 mg kg–1 Ag as Ag MNP and 1.30 mg kg–1 Ce as CeO2 MNP were added to five soils. More than 7% of Ag MNP occurred as soluble AgI after 24 h and the range of Kr values of Ag MNP (77–2165 L kg–1) and CeO2 MNP (1.1–2828 L kg–1) contrasted with Kd values of soluble AgI, CeIII and CeIV salts and bulk Ag and CeO2 powders in different soils.

Dissolution kinetics of macronutrient fertilizers coated with manufactured zinc oxide nanoparticles.

The solubility of Zn in Zn fertilizers plays an important role in the agronomic effectiveness of the fertilizer. On the basis of thermodynamics, zinc oxide (ZnO) nanoparticles (NPs) should dissolve faster and to a greater extent than bulk ZnO particles (equivalent spherical diameter >100 nm). These novel solubility features of ZnO NPs might be exploited to improve the efficiency of Zn fertilizers. In this study, we compared the Zn solubility and dissolution kinetics of ZnO nanoparticles and bulk ZnO particles coated onto two selected granular macronutrient fertilizers, urea and monoammonium phosphate (MAP). The main Zn species on coated MAP and urea granules were zinc ammonium phosphate and ZnO, respectively. Coated MAP granules showed greater Zn solubility and faster dissolution rates in sand columns compared to coated urea granules, which may be related to pH differences in the solution surrounding the fertilizer granules. The kinetics of Zn dissolution was not affected by the size of the ZnO particles applied for coating of either fertilizer type, possibly because solubility was controlled by formation of the same compounds irrespective of the size of the original ZnO particles used for coating.

Copper speciation and isotopic fractionation in plants: uptake and translocation mechanisms

The fractionation of stable copper (Cu) isotopes during uptake into plant roots and translocation to shoots can provide information on Cu acquisition mechanisms. Isotope fractionation ((65) Cu/(63) Cu) and intact tissue speciation techniques (X-ray absorption spectroscopy, XAS) were used to examine the uptake, translocation and speciation of Cu in strategy I (tomato-Solanum lycopersicum) and strategy II (oat-Avena sativa) plant species. Plants were grown in controlled solution cultures, under varied iron (Fe) conditions, to test whether the stimulation of Fe-acquiring mechanisms can affect Cu uptake in plants. Isotopically light Cu was preferentially incorporated into tomatoes (Δ(65) Cu(whole plant-solution ) = c. -1‰), whereas oats showed minimal isotopic fractionation, with no effect of Fe supply in either species. The heavier isotope was preferentially translocated to shoots in tomato, whereas oat plants showed no significant fractionation during translocation. The majority of Cu in the roots and leaves of both species existed as sulfur-coordinated Cu(I) species resembling glutathione/cysteine-rich proteins. The presence of isotopically light Cu in tomatoes is attributed to a reductive uptake mechanism, and the isotopic shifts within various tissues are attributed to redox cycling during translocation. The lack of isotopic discrimination in oat plants suggests that Cu uptake and translocation are not redox selective.

Arsenic Species Determination in Biological Tissues by HPLC–ICP–MS and HPLC–HG–ICP–MS

The use of high-pressure liquid chromatography coupled directly or by a hydride generation system to an inductively coupled plasma mass spectrometer for the unambiguous measurement of 13 arsenic species in marine biological extracts is described. The use of two chromatography systems; a Supelcosil LC-SCX cation-exchange column eluted with a 20 mM pyridine mobile phase adjusted to pH 2.2 and 2.6 with formic acid, with a flow rate of 1.5 mL min−1 at 40°C, and a Hamilton PRP-X100 anion-exchange column eluted with 20 mM NH4H2PO4 buffer at pH 5.6, with a flow rate of 1.5 mL min−1 at 40°C, was required to separate and quantify cation and anion arsenic species. Under these conditions, arsenous acid could not be separated from other arsenic species and required the use of an additional hydride generation step. Arsenic species concentrations in a locally available Tasmanian kelp (Durvillea potatorum), a certified reference material (DORM-2), and a range of commercially available macroalgae supplements and sushi seaweeds have been measured and are provided for use as in-house quality control samples to assess the effectiveness of sample preparation, extraction, and measurement techniques.

Zn isotope evidence for immediate resumption of primary productivity after snowball Earth

The Ediacaran period began with the deglaciation of the ca. 635 Ma Marinoan snowball Earth and the deposition of cap dolostones on continental shelves worldwide during post-glacial sea-level rise. These carbonates sharply overlie glacial sediments deposited at low paleolatitudes and preserve negative carbon isotope excursions. The snowball Earth hypothesis invokes an almost complete cessation of primary productivity in the surface ocean. Because assimilatory uptake of Zn appears to fractionate its isotopes, Zn isotope ratios measured in carbonate precipitated in the surface ocean should track fluctuations in primary productivity. Here we report the first Zn isotopic data, together with carbon and oxygen isotopic profiles from a Neoproterozoic cap dolostone, the Nuccaleena Formation in the Flinders Ranges, South Australia. We interpret the Zn isotopic data in terms of a two-stage evolution of the deglacial ocean. Slightly ^(66)Zn-enriched values at the base of the cap dolostone indicate immediate resumption of the biological pump upon melting of the surface ocean, but this signal was diluted by intense surface runoff that drove δ^(66)Zn (^(66)Zn/^(64)Zn, versus the JMC Lyon reference) values down to the composition of continentally derived Zn. A subsequent rise in δ^(66)Zn records a vigorous increase in primary production and export from a nutrient-laden surface ocean.

Measurement of Trace Elements in Marine Environmental Samples using Solution ICPMS. Current and Future Applications

The strengths and weaknesses of using inductively coupled plasma mass spectrometer (ICPMS) measurements of samples in solution for marine environmental analyses using real world examples is discussed. ICPMS can detect nanogram per litre concentrations of trace elements but suffers from polyatomic interferences generated from the sample matrix. Most of the routine trace elements measured in marine biological tissue and sediment digests, with the notable exceptions of iron, chromium, vanadium, and selenium, are not subject to severe interferences. Low recoveries of trace elements from sediments are due to the inability of extraction acids to remove trace elements such as chromium and nickel from sediment matrices. The use of ICPMS offers the advantage that elements such as phosphorus, which previously required elaborate digestion procedures and a colorimetric determination, can be rapidly determined using nitric acid digestion alone. The use of flow injection coupled with ICPMS allows on-line preconcentration of trace metals and metalloids using chelation by ion-exchange resins or hydride generation and trapping as well as separation from matrix elements. Thus, the routine determination of trace elements and inorganic and methylated arsenic, antimony, mercury, and germanium species in open-ocean waters is possible. The coupling of HPLC and GC to ICPMS allows the measurement of metal and metalloid species in biological and sediment extracts. However, extraction of unaltered species from matrices presents a challenge. Many of the species found in the environmental samples are not known and analytical standards are not available. The concurrent use of HPLC-MS is needed to confirm these species.