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 CO₂ reduction reactions. Oleic acid (OA) stabilized Cu₂O and CuO NPs promote the MeOH oxidation reaction with 88% and 99.97% selective HCOH formation, respectively. Alternatively, CO₂ 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 CO₂ into CO with a ∼1.7-fold increase in CO/H₂ production ratios compared to bulk Cu oxides. The OA stabilized Cu oxide NPs also show 7.6 and 9.1-fold increases in CO/H₂ 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 CO₂ Sorbents: A New Class 4 Category

Hybrid Class 1/Class 2 supported amine CO₂ 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 ²⁹Si CP-MAS and 2-D FSLG ¹H–¹³C CP HETCOR NMR spectroscopies to probe the overall molecular interactions of aminosilane/silica, polyamine [poly­(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 H₂O and TGA steam treatments (H₂O stability) and to oxidative degradation (thermal stability). High percentages of CO₂ capture retained (PCR) and organic content retained (OCR) values after H₂O testing of <i>N</i>-(3-(trimethoxysilyl)­propyl)­ethylenediamine (TMPED)/PEI and (3-aminopropyl)­trimethoxysilane (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–NH₂···NH₂–TMPED and PEI–NH₂···HO–Si/O–Si–O (TMPED, T²) 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/SiO₂ 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 bis­(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 SiO₂ 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 −NH₂ (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 La³⁺, Nd³⁺, Eu³⁺, Dy³⁺, and Yb³⁺ (Ln³⁺) 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 −NH₂ plus −NH) recovered, overall, the highest percentage of Ln³⁺ 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 Ln³⁺ 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 bis­(trifluoromethanesulfonyl)­imide [OMPhTz]­[Tf₂N], 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>₁) 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