8 research outputs found
Competitive Adsorption of Cd(II), Cr(VI), and Pb(II) onto Nanomaghemite: A Spectroscopic and Modeling Approach
A combined modeling
and spectroscopic approach is used to describe
CdĀ(II), CrĀ(VI), and PbĀ(II) adsorption onto nanomaghemite and nanomaghemite
coated quartz. A pseudo-second order kinetic model fitted the adsorption
data well. The sorption capacity of nanomaghemite was evaluated using
a Langmuir isotherm model, and a diffuse double layer surface complexation
model (DLM) was developed to describe metal adsorption. Adsorption
mechanisms were assessed using X-ray photoelectron spectroscopy and
X-ray absorption spectroscopy. PbĀ(II) adsorption occurs mainly via
formation of inner-sphere complexes, whereas CrĀ(VI) likely adsorbs
mainly as outer-sphere complexes and CdĀ(II) as a mixture of inner-
and outer-sphere complexes. The simple DLM describes well the pH-dependence
of single adsorption edges. However, it fails to adequately capture
metal adsorption behavior over broad ranges of ionic strength or metal-loading
on the sorbents. For systems with equimolar concentrations of PbĀ(II),
CdĀ(II), and CrĀ(VI). PbĀ(II) adsorption was reasonably well predicted
by the DLM, but predictions were poorer for CrĀ(VI) and CdĀ(II). This
study demonstrates that a simple DLM can describe well the adsorption
of the studied metals in mixed sorbateāsorbent systems, but
only under narrow ranges of ionic strength or metal loading. The results
also highlight the sorption potential of nanomaghemite for metals
in complex systems
Sublethal and Reproductive Effects of Acute and Chronic Exposure to Flowback and Produced Water from Hydraulic Fracturing on the Water Flea <i>Daphnia magna</i>
Hydraulic fracturing is an industrial
process allowing for the
extraction of gas or oil. To fracture the rocks, a proprietary mix
of chemicals is injected under high pressure, which later returns
to the surface as flowback and produced water (FPW). FPW is a complex
chemical mixture consisting of trace metals, organic compounds, and
often, high levels of salts. FPW toxicity to the model freshwater
crustacean <i>Daphnia magna</i> was characterized utilizing
acute (48 h median lethal concentrations; LC<sub>50</sub>) and chronic
(21 day) exposures. A decrease in reproduction was observed, with
a mean value of 18.5 neonates produced per replicate over a 21 day
chronic exposure to 0.04% FPW, which was a significant decrease from
the average of 64 neonates produced in the controls. The time to first
brood was delayed in the highest FPW (0.04%) treatment. Neonates exhibited
an LC<sub>50</sub> of 0.19% of full-strength FPW, making them more
sensitive than adults, which displayed an LC<sub>50</sub> value of
0.75%. Quantitative PCR highlighted significant changes in expression
of genes encoding xenobiotic metabolism (<i>cyp4</i>) and
moulting (<i>cut</i>). This study is the first to characterize
chronic FPW toxicity and will help with the development of environmental
monitoring and risk assessment of FPW spills
Thermodynamic Analysis of Nickel(II) and Zinc(II) Adsorption to Biochar
While numerous studies have investigated
metal uptake from solution
by biochar, few of these have developed a mechanistic understanding
of the adsorption reactions that occur at the biochar surface. In
this study, we explore a combined modeling and spectroscopic approach
for the first time to describe the molecular level adsorption of NiĀ(II)
and ZnĀ(II) to five types of biochar. Following thorough characterization,
potentiometric titrations were carried out to measure the proton (H<sup>+</sup>) reactivity of each biochar, and the data was used to develop
protonation models. Surface complexation modeling (SCM) supported
by synchrotron-based extended X-ray absorption fine structure (EXAFS)
was then used to gain insights into the molecular scale metalābiochar
surface reactions. The SCM approach was combined with isothermal titration
calorimetry (ITC) data to determine the thermodynamic driving forces
of metal adsorption. Our results show that the reactivity of biochar
toward NiĀ(II) and ZnĀ(II) directly relates to the site densities of
biochar. EXAFS along with FT-IR analyses, suggest that NiĀ(II) and
ZnĀ(II) adsorption occurred primarily through proton-active carboxyl
(āCOOH) and hydroxyl (āOH) functional groups on the
biochar surface. SCM-ITC analyses revealed that the enthalpies of
protonation are exothermic and NiĀ(II) and ZnĀ(II) complexes with biochar
surface are slightly exothermic to slightly endothermic. The results
obtained from these combined approaches contribute to the better understanding
of molecular scale metal adsorption onto the biochar surface, and
will facilitate the further development of thermodynamics-based, predictive
approaches to biochar removal of metals from contaminated water
Linking Topographical Ring Features to Geochemical and Geophysical Anomalies
Circular features in forests seen from air have been studied for several decades at different locations around the world. Forest rings, as they are called in Canadaās boreal forests, express several geochemical (pH, carbonate content) and geophysical (surface potential) anomalies on their 20ā30 m wide ring edges. Although it has been proposed that microbial processes may cause these anomalies, the exact mechanisms of ring formation are still unknown. We focused on the Thorn North forest ring in Ontario, Canada to correlate the surface potential anomaly to soil gas concentrations. Field measurements showed that the surface potential drop at the ring edge center is framed by peaks in CO2 production, which is linked to O2 depletion and methane generation. Carbon isotope signatures were found to drop to lighter values (down to ā20ā°), suggesting increased respiration. Higher concentrations of uronic acids bound to extracellular polymeric substances were found, indicating that the surface potential anomaly is linked to respiration. 16S rRNA gene sequencing of shallow soil did not indicate a dominant microbial group on the edges; instead, principal component analysis showed that the microbial composition was controlled by the substrate (clayey vs. sandy soil), therefore future studies should focus on deeper ground layers.</p
Rare Earth Element Adsorption to Clay Minerals: Mechanistic Insights and Implications for Recovery from Secondary Sources
The energy transition will have significant mineral demands
and
there is growing interest in recovering critical metals, including
rare earth elements (REE), from secondary sources in aqueous and sedimentary
environments. However, the role of clays in REE transport and deposition
in these settings remains understudied. This work investigated REE
adsorption to the clay minerals illite and kaolinite through pH adsorption
experiments and extended X-ray absorption fine structure (EXAFS).
Clay type, pH, and ionic strength (IS) affected adsorption, with decreased
adsorption under acidic pH and elevated IS. Illite had a higher adsorption
capacity than kaolinite; however, >95% adsorption was achieved
at
pH ā¼7.5 regardless of IS or clay. These results were used to
develop a surface complexation model with the derived binding constants
used to predict REE speciation in the presence of competing sorbents.
This demonstrated that clays become increasingly important as pH increases,
and EXAFS modeling showed that REE can exist as both inner- and outer-sphere
complexes. Together, this indicated that clays can be an important
control on the transport and enrichment of REE in sedimentary systems.
These findings can be applied to identify settings to target for resource
extraction or to predict REE transport and fate as a contaminant
Reducing Conditions Influence U(IV) Accumulation in Sediments during <i>In Situ</i> Bioremediation
This study presents field experiments conducted in a
contaminated
aquifer in Rifle, CO, to determine the speciation and accumulation
of uranium in sediments during in situ bioreduction.
We applied synchrotron-based X-ray spectroscopy and imaging techniques
as well as aqueous chemistry measurements to identify changes in U
speciation in water and sediment in the first days follwing electron
donor amendment. Limited changes in U solid speciation were observed
throughout the duration of this study, and non-crystalline U(IV) was
identified in all samples obtained. However, U accumulation rates
strongly increased during in situ bioreduction, when
the dominant microbial regime transitioned from iron- to sulfate-reducing
conditions. Results suggest that uranium is enzymatically reduced
during Fe reduction, as expected. Mineral grain coatings newly formed
during sulfate reduction act as reduction hotspots, where numerous
reductants can act as electron donors [Fe(II), S(II), and microbial
extracellular polymeric substances] that bind and reduce U. The results
have implications for identifying how changes in the dominant reducing
mechanism, such as Fe versus sulfate reduction, affect trace metal
speciation and accumulation. The outcomes from this study provide
additional insights into uranium accumulation mechanisms in sediments
that could be useful for the refinement of quantitative models describing
redox processes and contaminant dynamics in floodplain aquifers
Persisting Effects in <i>Daphnia magna</i> Following an Acute Exposure to Flowback and Produced Waters from the Montney Formation
Hydraulic fracturing extracts oil and gas through the
injection
of water and proppants into subterranean formations. These injected
fluids mix with the host rock formation and return to the surface
as a complex wastewater containing salts, metals, and organic compounds,
termed flowback and produced water (FPW). Previous research indicates
that FPW is toxic to Daphnia magna (D. magna), impairing reproduction, molting, and maturation time; however,
recovery from FPW has not been extensively studied. Species unable
to recover have drastic impacts on populations on the ecological scale;
thus, this study sought to understand if recovery from an acute 48
h FPW exposure was possible in the freshwater invertebrate, D. magna by using a combination of physiological
and molecular analyses. FPW (0.75%) reduced reproduction by 30% and
survivorship to 32% compared to controls. System-level quantitative
proteomic analyses demonstrate extensive perturbation of metabolism
and protein transport in both 0.25 and 0.75% FPW treatments after
a 48 h FPW exposure. Collectively, our data indicate that D. magna are unable to recover from acute 48 h exposures
to ā„0.25% FPW, as evidence of toxicity persists for at least
19 days post-exposure. This study highlights the importance of considering
persisting effects following FPW remediation when modeling potential
spill scenarios
Field- and Lab-Based Potentiometric Titrations of Microbial Mats from the Fairmont Hot Spring, Canada
<p>Potentiometric titrations are an effective tool to constrain the protonation constants and site concentrations for microbial surface ligands. Protonation models developed from these experiments are often coupled with data from metal adsorption experiments to calculate microbial ligand-metal binding constants. Ultimately, the resulting surface complexation models can be used to predict metal immobilization behavior across diverse chemical conditions. However, most protonation and metal-ligand thermodynamic constants have been generated in laboratory experiments that use cultured microbes which may differ in their chemical reactivity from environmental samples. In this study, we investigate the use of <i>in situ</i> field potentiometric titrations of microbial mats at a carbonate hot spring located at Fairmont Hot Springs, British Columbia, with the aim to study microbial reactivities in a natural field system. We found that authigenic carbonate minerals complicated the potentiometric titration process due to a ācarbonate spikeā introduced by the contribution of inorganic carbonate mineral dissolution and subsequent carbonate speciation changes during the transition from low to high pH. This inhibits the determination of microbial surface ligand variety and concentrations. Our preliminary study also highlights the need for developing novel probes to quantify <i>in situ</i> microbial mat reactivity in future field investigations.</p