11 research outputs found
Bacterial Respiration of Arsenate and Its Significance in the Environment
Although arsenic is a trace element in terms of its natural abundance, it nonetheless
has a common presence within the earth's crust. Because it is classified as a
group VB element in the periodic table, it shares many chemical and biochemical
properties in common with its neighbors phosphorus and nitrogen. Indeed, in the
case of this element's most oxidized (+5) oxidation state, arsenate [HAsO_4^(2-) or
As (V)], its toxicity is based on its action as an analog of phosphate. Hence,
arsenate ions uncouple the oxidative phosphorylation normally associated with
the enzyme glyceraldehyde 3-phosphate dehydrogenase, thereby preventing the
formation ofphosphoglyceroyl phosphate, a key high-energy intermediate in glycolysis.
To guard against this, a number of bacteria possess a detoxifying arsenate
reductase pathway (the arsC system) whereby cytoplasmic enzymes remove internal
pools of arsenate by achieving its reduction to arsenite [H_2AsO_3- or As
(III)]. However, because the arsenite product binds with internal sulfhydryl
groups that render it even more toxic than the original arsenate, efficient arsenite
efflux from the cell is also required and is achieved by an active ion ''pumping'' system (1). The details of this bacterial arsenic detoxification phenomenon have
been well established in the literature, and Chapter 10 in this volume provided
a thorough review. Here, we discuss bacterial respiration of arsenate and its significance
in the environment. As a biological phenomenon, respiratory growth
on arsenate is quite remarkable, given the toxicity of the element. Moreover, the
consequences of microbial arsenate respiration may, at times, have a significant
impact on environmental chemistry
Selective Electrocatalytic Activity of Ligand Stabilized Copper Oxide Nanoparticles
Ligand stabilization can influence the surface chemistry of Cu oxide nanoparticles (NPs) and provide unique product distributions for electrocatalytic methanol (MeOH) oxidation and CO{sub 2} reduction reactions. Oleic acid (OA) stabilized Cu{sub 2}O and CuO NPs promote the MeOH oxidation reaction with 88% and 99.97% selective HCOH formation, respectively. Alternatively, CO{sub 2} is the only reaction product detected for bulk Cu oxides and Cu oxide NPs with no ligands or weakly interacting ligands. We also demonstrate that OA stabilized Cu oxide NPs can reduce CO{sub 2} into CO with a {approx}1.7-fold increase in CO/H{sub 2} production ratios compared to bulk Cu oxides. The OA stabilized Cu oxide NPs also show 7.6 and 9.1-fold increases in CO/H{sub 2} production ratios compared to weakly stabilized and non-stabilized Cu oxide NPs, respectively. Our data illustrates that the presence and type of surface ligand can substantially influence the catalytic product selectivity of Cu oxide NPs
Selective Electrocatalytic Activity of Ligand Stabilized Copper Oxide Nanoparticles
Ligand stabilization can influence the surface chemistry of Cu oxide nanoparticles (NPs) and provide unique product distributions for electrocatalytic methanol (MeOH) oxidation and CO<sub>2</sub> reduction reactions. Oleic acid (OA) stabilized Cu<sub>2</sub>O and CuO NPs promote the MeOH oxidation reaction with 88% and 99.97% selective HCOH formation, respectively. Alternatively, CO<sub>2</sub> is the only reaction product detected for bulk Cu oxides and Cu oxide NPs with no ligands or weakly interacting ligands. We also demonstrate that OA stabilized Cu oxide NPs can reduce CO<sub>2</sub> into CO with a ∼1.7-fold increase in CO/H<sub>2</sub> production ratios compared to bulk Cu oxides. The OA stabilized Cu oxide NPs also show 7.6 and 9.1-fold increases in CO/H<sub>2</sub> production ratios compared to weakly stabilized and nonstabilized Cu oxide NPs, respectively. Our data illustrates that the presence and type of surface ligand can substantially influence the catalytic product selectivity of Cu oxide NPs
Spectroscopic Investigation of the Mechanisms Responsible for the Superior Stability of Hybrid Class 1/Class 2 CO<sub>2</sub> Sorbents: A New Class 4 Category
Hybrid
Class 1/Class 2 supported amine CO<sub>2</sub> sorbents demonstrate
superior performance under practical steam conditions, yet their amine
immobilization and stabilization mechanisms are unclear. Uncovering
the interactions responsible for the sorbents’ robust features
is critical for further improvements and can facilitate practical
applications. We employ solid state <sup>29</sup>Si CP-MAS and 2-D
FSLG <sup>1</sup>H–<sup>13</sup>C CP HETCOR NMR spectroscopies
to probe the overall molecular interactions of aminosilane/silica,
polyamine [polyÂ(ethylenimine), PEI]/silica, and hybrid aminosilane/PEI/silica
sorbents. A unique, sequential impregnation sorbent preparation method
is executed in a diffuse reflectance infrared Fourier transform spectroscopy
(DRIFTS) setup to decouple amine binding mechanisms at the amine–silica
interface from those within bulk amine layers. These mechanisms are
correlated with each sorbents’ resistance to accelerated liquid
H<sub>2</sub>O and TGA steam treatments (H<sub>2</sub>O stability)
and to oxidative degradation (thermal stability). High percentages
of CO<sub>2</sub> capture retained (PCR) and organic content retained
(OCR) values after H<sub>2</sub>O testing of <i>N</i>-(3-(trimethoxysilyl)Âpropyl)Âethylenediamine
(TMPED)/PEI and (3-aminopropyl)Âtrimethoxysilane (APTMS)/PEI hybrid
sorbents are associated with a synergistic stabilizing effect of the
amine species observed during oxidative degradation (thermal gravimetric
analysis-differential scanning calorimetry, TGA-DSC). Solid state
NMR spectroscopy reveals that the synergistic effect of the TMPED/PEI
mixture is manifested by the formation of hydrogen-bonded PEI–NH<sub>2</sub>···NH<sub>2</sub>–TMPED and PEI–NH<sub>2</sub>···HO–Si/O–Si–O (TMPED,
T<sup>2</sup>) linkages within the sorbent. DRIFTS further determines
that PEI enhances the grafting of TMPED to silica and that PEI is
dispersed among a stable network of polymerized TMPED in the bulk,
utilizing H-bonded linkages. These findings provide the scientific
basis for establishing a Class 4 category for aminosilane/polyamine/silica
hybrid sorbents
Novel Polyethylenimine–Acrylamide/SiO<sub>2</sub> Hybrid Hydrogel Sorbent for Rare-Earth-Element Recycling from Aqueous Sources
Recycling
rare-earth elements (REEs) becomes increasingly important
because of their supply vulnerability and increasing demands in industry,
agriculture, and national security. Hybrid hydrogel sorbents are outstanding,
because of their high stability and selectivity. Organic–inorganic
hybrid hydrogels were synthesized by thermopolymerization of acrylamide
onto PEI polymer chains with <i>N</i>,<i>N</i>′-methylene bisÂ(acrylamide) as a cross-linker. The grafted
network was evidenced by diffuse reflectance infrared Fourier transform
spectroscopy (DRIFTS) and X-ray photoelectron spectroscopy (XPS).
The porous structure was observed by scanning electron microscopy
(SEM). The degree of cross-linking, the degree of PEI grafting, and
the SiO<sub>2</sub> concentration were studied to optimize the adsorption
of REEs. The pH value of the medium greatly affected REE adsorption
capacity, where the almost-neutral conditions gave the strongest bonding
of REEs to active sites. Moreover, kinetic studies showed that the
rate-determining step of the adsorption process was chemical sorption,
and that REE diffusion within micropores was the control step for,
specifically, intraparticle diffusion. The adsorbents showed excellent
selectivity and recyclability for REEs through five adsorption–desorption
cycles in contact with synthetic acid mine drainage solution. A high
separation toward REEs over fouling metals was achieved by using a
citrate-based buffer eluent solution. This hybrid hydrogel shows promise
for the recycling of REEs from aqueous solutions
Recovering Rare Earth Elements from Aqueous Solution with Porous Amine–Epoxy Networks
Recovering
aqueous rare earth elements (REEs) from domestic water sources is
one key strategy to diminish the U.S.’s foreign reliance of
these precious commodities. Herein, we synthesized an array of porous,
amine–epoxy monolith and particle REE recovery sorbents from
different polyamine, namely tetraethylenepentamine, and diepoxide
(E2), triepoxide (E3), and tetra-epoxide (E4) monomer combinations
via a polymer-induced phase separation (PIPS) method. The polyamines
provided −NH<sub>2</sub> (primary amine) plus −NH (secondary
amine) REE adsorption sites, which were partially reacted with C–O–C
(epoxide) groups at different amine/epoxide ratios to precipitate
porous materials that exhibited a wide range of apparent porosities
and REE recoveries/affinities. Specifically, polymer particles (ground
monoliths) were tested for their recovery of La<sup>3+</sup>, Nd<sup>3+</sup>, Eu<sup>3+</sup>, Dy<sup>3+</sup>, and Yb<sup>3+</sup> (Ln<sup>3+</sup>) species from ppm-level, model REE solutions (pH ≈
2.4, 5.5, and 6.4) and a ppb-level, simulated acid mine drainage (AMD)
solution (pH ≈ 2.6). Screening the sorbents revealed that E3/TEPA-88
(88% theoretical reaction of −NH<sub>2</sub> plus −NH)
recovered, overall, the highest percentage of Ln<sup>3+</sup> species
of all particles from model 100 ppm- and 500 ppm-concentrated REE
solutions. Water swelling (monoliths) and ex situ, diffuse reflectance
infrared Fourier transform spectroscopy (DRIFTS) (ground monoliths/particles)
data revealed the high REE uptake by the optimized particles was facilitated
by effective distribution of amine and hydroxyl groups within a porous,
phase-separated polymer network. In situ DRIFTS results clarified
that phase separation, in part, resulted from polymerization of the
TEPA-E3 (<i>N</i>-<i>N</i>-diglycidyl-4-glycidyloxyaniline)
species in the porogen via C–N bond formation, especially at
higher temperatures. Most importantly, the E3/TEPA-88 material cyclically
recovered >93% of ppb-level Ln<sup>3+</sup> species from AMD solution
in a recovery–strip–recovery scheme, highlighting the
efficacy of these materials for practical applications
Nuclear Spin Relaxation and Molecular Interactions of a Novel Triazolium-Based Ionic Liquid
Nuclear
spin relaxation, small-angle X-ray scattering (SAXS), and
electrospray ionization mass spectrometry (ESI-MS) techniques are
used to determine supramolecular arrangement of 3-methyl-1-octyl-4-phenyl-1H-triazol-1,2,3-ium
bisÂ(trifluoromethanesulfonyl)Âimide [OMPhTz]Â[Tf<sub>2</sub>N], an example
of a triazolium-based ionic liquid. The results obtained showed first-order
thermodynamic dependence for nuclear spin relaxation of the anion.
First-order relaxation dependence is interpreted as through-bond dipolar
relaxation. Greater than first-order dependence was found in the aliphatic
protons, aromatic carbons (including nearest neighbors), and carbons
at the end of the aliphatic tail. Greater than first order thermodynamic
dependence of spin relaxation rates is interpreted as relaxation resulting
from at least one mechanism additional to through-bond dipolar relaxation.
In rigid portions of the cation, an additional spin relaxation mechanism
is attributed to anisotropic effects, while greater than first order
thermodynamic dependence of the octyl side chain’s spin relaxation
rates is attributed to cation–cation interactions. Little interaction
between the anion and the cation was observed by spin relaxation studies
or by ESI-MS. No extended supramolecular structure was observed in
this study, which was further supported by MS and SAXS. nuclear Overhauser
enhancement (NOE) factors are used in conjunction with spin–lattice
relaxation time (<i>T</i><sub>1</sub>) measurements to calculate
rotational correlation times for C–H bonds (the time it takes
for the vector represented by the bond between the two atoms to rotate
by one radian). The rotational correlation times are used to represent
segmental reorientation dynamics of the cation. A combination of techniques
is used to determine the segmental interactions and dynamics of this
example of a triazolium-based ionic liquid