Chain length and acidity of carboxylic acids influencing adsorption/desorption mechanism and kinetics over anion exchange membrane

Formic and acetic acid pKa values increase under nanoconfinement

Organic acids are prevalent in the environment and their acidity and the corresponding dissociation constants can change under varying environmental conditions. The impact of nanoconfinement (when acids are confined within nanometer-scale domains) on physicochemical properties of chemical species is poorly understood and is an emerging field of study. By combining infrared and Raman spectroscopies with molecular dynamics (MD) simulations, we quantified the effect of nanoconfinement in silica nanopores on one of the fundamental chemical reactions-the dissociation of organic acids. The pKa of formic and acetic acids confined within cylindrical silica nanopores with 4 nm diameters were measured. MD models were constructed to calculate the shifts in the pKa values of acetic acid nanoconfined within 1, 2, 3, and 4 nm silica slit pores. Both experiments and MD models indicate a decrease in the apparent acid dissociation constants (i.e., increase in the pKa values) when organic acids are nanoconfined. Therefore, nanoconfinement stabilizes the protonated species. We attribute this observation to (1) a decrease in the average dielectric response of nanoconfined aqueous solutions where charge screening may be decreased; or (2) an increase in proton concentration inside nanopores, which would shift the equilibrium towards the protonated form. Overall, the results of this study provide the first quantification of the pKa values for nanoconfined formic and acetic acids and pave the way for a unifying theory predicting the impact of nanoconfinement on acid-base chemistry.

Phosphates Transfer in Pristine and Modified CJMA-2 Membrane during Electrodialysis Processing of NaxH(3-x)PO4 Solutions with pH from 4.5 to 9.9

Phosphate recovery from different second streams using electrodialysis (ED) is a promising step to a nutrients circular economy. However, the relatively low ED performance hinders the widespread adoption of this environmentally sound method. The formation of "bonded species" between phosphates and the weakly basic fixed groups (primary and secondary amines) of the anion exchange membrane can be the cause of decrease in current efficiency and increase in energy consumption. ED processing of Na_xH_(3-x)PO₄ alkaline solutions and the use of intense current modes promote the formation of a bipolar junction from negatively charged bound species and positively charged fixed groups. This phenomenon causes a change in the shape of current-voltage curves, increase in resistance, and an enhancement in proton generation during long-term operation of anion-exchange membrane with weakly basic fixed groups. Shielding of primary and secondary amines with a modifier containing quaternary ammonium bases significantly improves ED performance in the recovery of phosphates from Na_xH_(3-x)PO₄ solution with pH 4.5. Indeed, in the limiting and underlimiting current modes, 40% of phosphates are recovered 1.3 times faster, and energy consumption is reduced by 1.9 times in the case of the modified membrane compared to the pristine one. Studies were performed using a new commercial anion exchange membrane CJMA-2.

Adsorption of amphetamine on deep eutectic solvents functionalized graphene oxide/metal-organic framework nanocomposite: Elucidation of hydrogen bonding and DFT studies

The efficient and selective removal of amphetamine (AMP) from water bodies is significant for environmental remediation. In this study, a novel strategy for screening deep eutectic solvent (DES) functional monomers was proposed based on density functional theory (DFT) calculations. Using magnetic GO/ZIF-67 (ZMG) as substrates, three DES-functionalized adsorbents (ZMG-BA, ZMG-FA, and ZMG-PA) were successfully synthesized. The isothermal results showed that the DES-functionalized materials introduced more adsorption sites and mainly contributed to the formation of hydrogen bonds. The order of the maximum adsorption capacity (Q_m) was as follows: ZMG-BA (732.110 μg⋅g^-1) > ZMG-FA (636.518 μg⋅g^-1) > ZMG-PA (564.618 μg⋅g^-1) > ZMG (489.913 μg⋅g^-1). The adsorption rate of AMP on ZMG-BA was the highest (98.1%) at pH 11, which could be explained by the less protonation of -NH₂ from AMP being more favorable for forming hydrogen bonds with the -COOH of ZMG-BA. The strongest affinity of the -COOH of ZMG-BA for AMP was reflected in the most hydrogen bonds and the shortest bond length. The hydrogen bonding adsorption mechanism was fully explained by experimental characterization (FT-IR, XPS) and DFT calculations. Frontier Molecular Orbital (FMO) calculations showed that ZMG-BA had the lowest HOMO-LUMO energy gap (E_gap), the highest chemical activity and the best adsorption capability. The experimental results agreed with the results of theoretical calculations, proving the validity of the functional monomer screening method. This research offered fresh suggestions for the functionalized modification of carbon nanomaterials to achieve effective and selective adsorption for psychoactive substances.

Influence of Electroconvection on Chronopotentiograms of an Anion-Exchange Membrane in Solutions of Weak Polybasic Acid Salts

Visualization of electroconvective (EC) vortices at the undulated surface of an AMX anion-exchange membrane (Astom, Osaka, Japan) was carried out in parallel with the measurement of chronopotentiograms. Weak polybasic acid salts, including 0.02 M solutions of tartaric (NaHT), phosphoric (NaH₂PO₄), and citric (NaH₂Cit) acids salts, and NaCl were investigated. It was shown that, for a given current density normalized to the theoretical limiting current calculated by the Leveque equation (i/i_lim^theor), EC vortex zone thickness, d_EC, decreases in the order NaCl > NaHT > NaH₂PO₄ > NaH₂Cit. This order is inverse to the increase in the intensity of proton generation in the membrane systems under study. The higher the intensity of proton generation, the lower the electroconvection. This is due to the fact that protons released into the depleted solution reduce the space charge density, which is the driver of EC. In all studied systems, a region in chronopotentiograms between the rapid growth of the potential drop and the attainment of its stationary values corresponds to the appearance of EC vortex clusters. The amplitude of the potential drop oscillations in the chronopotentiograms is proportional to the size of the observed vortex clusters.

Chloroquine and hydroxychloroquine inhibitors for COVID-19 sialic acid cellular receptor: Structure, hirshfeld atomic charge analysis and solvent effect

COVID-19, the pandemic disease recently discovered in Wuhan (China), severely spread and affected both social and economic activity all over the world. Attempts to find an effective vaccine are challenging, time-consuming though interminable. Hence, re-proposing effective drugs is reliable and effective alternative. Taking into account the genome similarity of COVID-19 with SARS-CoV, drugs with safety profiles could be fast solution. Clinical trials encouraged the use of Chloroquine and Hydroxychloroquine for COVID-19 inhibition. One of the possible inhibition pathways is the competitive binding with the angiotension-converting enzyme-2 (ACE-2), in particular with the cellular Sialic acid (Neu5Ac). Here, we investigate the possible binding mechanism of ClQ and ClQOH with sialic acid both in the gas phase and in water using density functional theory (DFT). We investigated the binding of the neutral, monoprotonated and diprotonated ClQs and ClQOHs to sialic acid to simulate the pH effect on the cellular receptor binding. DFT results reveals that monoprotonated ClQ⁺ and ClQOH⁺, which account for more than 66% in the solution, possess high reactivity and binding towards sialic acid. The Neu5Ac-ClQ and the analogues Neu5Ac-ClQOH adducts were stabilized in water than in the gas phase. The molecular complexes stabilize by strong hydrogen bonding and π - π stacking forces. In addition, proton-transfer in Neu5Ac-ClQOH⁺ provides more stabilizing power and cellular recognition binding forces. These results shed light on possible recognition mechanism and help future breakthroughs for COVID-19 inhibitors.

Application of seed residues from Anadenanthera macrocarpa and Cedrela fissilis as alternative adsorbents for remarkable removal of methylene blue dye in aqueous solutions

Two novel ecological and low-cost adsorbents were prepared from seed residues of the tree species Anadenanthera macrocarpa and Cedrela fissilis for the removal of methylene blue dye in water. The materials were comminuted and characterized by different techniques. The particles of samples have a rough surface with cavities. The optimum dosage and pH for both materials were 1 g L^-1 and pH 8. The pseudo-second-order model was the most suitable for describing the adsorption kinetics for both systems. The Anadenanthera macrocarpa presented a maximum experimental capacity of 228 mg g^-1, while the Cedrela fissilis, a similar capacity of 230 mg g^-1 at 328 K. The Tóth model was proper for describing the equilibrium curves for both systems. The thermodynamic indicators show that the adsorption process is spontaneous and endothermic for both materials. The application of materials for the simulated effluent treatment showed 74 and 78% of color removal using Anadenanthera macrocarpa and Cedrela fissilis samples, respectively. Overall, seed residues of Anadenanthera macrocarpa and Cedrela fissilis could be potentially applied for adsorptive removal of colored contaminants in wastewater.