3 research outputs found
Separation and quantification of quantum dots and dissolved metal cations by size exclusion chromatography–ICP-MS
<div><p>ABSTRACT</p><p>The prevalence of engineered metallic nanoparticles within electronic products has evoked a need to assess their occurrence and fate within environmental systems upon potential release of these nanoparticles. Quantum dots (QDs) are mixed-metal nanocrystals with the smallest of particle sizes (2–10 nm) that readily leach heavy metal cations in water, potentially creating a co-occurrence of nanoparticulate and dissolved metal pollutants. In this report, we develop a size exclusion chromatography–inductively coupled plasma–mass spectrometry method (SEC-ICP-MS) for the rapid separation and quantification of ~5-nm-sized CdSe/ZnS QDs and dissolved Cd<sup>2+</sup> and Zn<sup>2+</sup> cations in water. The SEC-ICP-MS method provided a wide chromatographic separation of CdSe/ZnS QDs and dissolved Cd<sup>2+</sup> and Zn<sup>2+</sup> cations only when using the smallest SEC column pore size available and an eluent composition that prevented loss of metals to column polymer surfaces by using a surfactant to ensure elution of QDs (ammonium lauryl sulfate) and a complexing ligand to ensure elution of metal cations (ethylenediaminetetraacetate). Detection limits were between 0.2 and 2 µg L<sup>–</sup><sup>1</sup> for Cd<sup>2+</sup> and Zn<sup>2+</sup> among dissolved cation and QD phases, and ranges of linearity covered two to three orders of magnitude. Gold nanoparticles of sizes 5, 10, 20 and 50 nm were also effectively separated from dissolved Au<sup>3+</sup> cations, illustrating the method applicability to a wide range of nanoparticle sizes and compositions. QD and dissolved metal concentrations measured by SEC-ICP-MS were comparable to those measured using the more conventional method of centrifuge ultrafiltration on split samples for dissolved and total metals. The applicability of the SEC-ICP-MS method to environmental systems was verified by measuring QDs and dissolved metals added to samples of natural waters. The method was also applied to monitoring CdSe/ZnS dissolution kinetics in an urban river water. The SEC-ICP-MS developed here may offer improved automation for characterising heterogeneous suspensions containing >1 µg L<sup>–</sup><sup>1</sup> heavy metals.</p></div
Oxygen Isotope Indicators of Selenate Reaction with Fe(II) and Fe(III) Hydroxides
Selenate (SeO<sub>4</sub><sup>2‑</sup>) reduction to elemental
selenium is an important Se immobilization process in subsurface environments
that could be mediated by FeÂ(II)-rich minerals or selenate-respiring
microorganisms. We report the kinetic isotope effects for <sup>18</sup>O within selenate during abiotic reactions with iron-bearing hydroxides
within laboratory experiments. Selenate was reduced to Se(0) by a
green rust (chloride interlayer type) and ferrous hydroxide, the two
known environmentally relevant mineral reductants for selenate. Reaction
kinetics are described by a rapid, low-fractionating uptake step caused
by diffusive exchange between selenate and chloride followed by a
slower, high-fractionating reduction step caused by electron transfer
from structural FeÂ(II). The dual-phase kinetics cannot be described
with the traditional Rayleigh fractionation model; however, well after
the initial uptake step, the extent of selenate reaction is well correlated
with δ<sup>18</sup>O values in accordance with the Rayleigh
model. Selenate-<sup>18</sup>O enrichment (ε<sub>O</sub>) was
nearly identical for reaction with chloride green rust (22.7 ±
2.2‰) and ferrous hydroxide (22.1 ± 1.1‰) which
suggests a common reduction mechanism by structural FeÂ(II). The minor
enrichment due to anion exchange alone (1.4 ± 0.2‰) was
confirmed using iowaite, a nonredox active MgÂ(II)–FeÂ(III) layered
double hydroxide. Our ε<sub>O</sub> results may contribute to
Se isotope forensics to identify selenate reduction within field sites
and to possibly distinguish between abiotic and biotic reduction processes
Susceptibility of Goethite to Fe<sup>2+</sup>-Catalyzed Recrystallization over Time
Recent
work has shown that iron oxides, such as goethite and hematite,
may recrystallize in the presence of aqueous Fe<sup>2+</sup> under
anoxic conditions. This process, referred to as Fe<sup>2+</sup>-catalyzed
recrystallization, can influence water quality by causing the incorporation/release
of environmental contaminants and biological nutrients. Accounting
for the effects of Fe<sup>2+</sup>-catalyzed recrystallization on
water quality requires knowing the time scale over which recrystallization
occurs. Here, we tested the hypothesis that nanoparticulate goethite
becomes less susceptible to Fe<sup>2+</sup>-catalyzed recrystallization
over time. We set up two batches of reactors in which <sup>55</sup>Fe<sup>2+</sup> tracer was added at two different time points and
tracked the <sup>55</sup>Fe partitioning in the aqueous and goethite
phases over 60 days. Less <sup>55</sup>Fe uptake occurred between
30 and 60 days than between 0 and 30 days, suggesting goethite recrystallization
slowed with time. Fitting the data with a box model indicated that
17% of the goethite recrystallized after 30 days of reaction, and
an additional 2% recrystallized between 30 and 60 days. The decreasing
susceptibility of goethite to recrystallize as it reacted with aqueous
Fe<sup>2+</sup> suggested that recrystallization is likely only an
important process over short time scales