Electrodialytic removal of tungsten and arsenic from secondary mine resources - Deep eutectic solvents enhancement.

Tungsten is a critical raw material for European and U.S. economies. Tungsten mine residues, usually considered an environmental burden due to e.g. arsenic content, are also secondary tungsten resources. The electrodialytic (ED) process and deep eutectic solvents (DES) have been successfully and independently applied for the extraction of metals from different complex environmental matrices. In this study a proof of concept demonstrates that coupling DES in a two-compartment ED set-up enhances the removal and separation of arsenic and tungsten from Panasqueira mine secondary resources. Choline chloride with malonic acid (1:2), and choline chloride with oxalic acid (1:1) were the DES that in batch extracted the average maximum contents of arsenic (16%) and tungsten (9%) from the residues. However, when ED was operated at a current intensity of 100 mA for 4 days, the extraction yields increased 22% for arsenic and 11% for tungsten, comparing to the tests with no current. From the total arsenic and tungsten extracted, 82% and 77% respectively were successfully removed from the matrix compartment, as they electromigrated to the anolyte compartment, from where these elements can be further separated. This achievement potentiates circular economy, as the final treated residue could be incorporated in construction materials production, mitigating current environmental problems in both mining and construction sectors.

Effect of pressure on the catalytic activity of subtilisin Carlsberg suspended in compressed gases.

We studied the effect of pressure up to 300 bar on the catalytic efficiency of subtilisin Carlsberg suspended in compressed propane, near-critical ethane, near-critical carbon dioxide and tert-amyl alcohol, at constant temperature and fixed enzyme hydration. Increasing pressure lowered the catalytic efficiency of the enzyme in all the solvents, resulting in positive activation volumes, delta V#. The delta V# values in compressed propane and in tert-amyl alcohol were similar and larger in magnitude than the value reported in the literature for the same reaction in an aqueous buffer, although within the range of typical delta V# values in aqueous media. In the near-critical fluids, the delta V# were much larger, e.g., an increase in pressure of only 200 bar causing a sixfold decrease in the catalytic efficiency of subtilisin in carbon dioxide. These data should reflect the proximity of ethane and carbon dioxide to the critical point, and the resulting condensation of solvent molecules about the solutes, yielding negative solute partial molar volumes.

Supercritical fluids are superior media for catalysis by cross-linked enzyme microcrystals of subtilisin Carlsberg.

We report on the performance of cross-linked enzyme microcrystals (CLECs) of subtilisin Carlsberg in supercritical fluids (SC-fluids). The catalytic activity of CLECs in SC-ethane was found to be 2- to 10-fold greater than in hexane under the same conditions, using CLECs dried by propanol washing. Air-dried CLECs and lyophilized powders showed much lower activities, reflecting the same hydration hysteresis effects as in organic solvents. Reaction rates were much lower in SC-CO(2), especially at higher water activity, probably as a result of acid-base effects of carbonic acid on the enzyme.

Understanding the ion jelly conductivity mechanism.

The properties of the light flexible device, ion jelly, which combines gelatin with an ionic liquid (IL) were recently reported being promising to develop safe and highly conductive electrolytes. This article aims for the understanding of the ion jelly conductive mechanism using dielectric relaxation spectroscopy (DRS) in the frequency range 10(-1)-10(6) Hz; the study was complemented with differential scanning calorimetry (DSC) and pulsed field gradient nuclear magnetic resonance (PFG NMR) spectroscopy. The room temperature ionic liquid 1-butyl-3-methylimmidazolium dicyanamide (BMIMDCA) used as received (1.9% w/w water content) and with 6.6% (w/w) of water content and two ion jellies with two different ratios BMIMDCA/gelatin/water % (w/w), IJ1 (41.1/46.7/12.2) and IJ3 (67.8/25.6/6.6), have been characterized. A glass transition was detected by DSC for all materials allowing for classifying them as glass formers. For the ionic liquid, it was observed that the glass transition temperature decreases with the increase of water content. While in subsequent calorimetric runs crystallization was observed for BMIMDCA with negligible water content, no crystallization was detected for any of the ion jelly materials upon themal cycling. To the dielectric spectra of all tested materials, both dipolar relaxation and conductivity contribute; at the lowest frequencies, electrode and interfacial polarization highly dominate. Conductivity, which manifests much more intensity relative to dipolar reorientations, strongly evidences subdiffusive ion dynamics at high frequencies. From dielectric measures, transport properties as mobility and diffusion coefficients were extracted. Data treatment was carried out in order to deconvolute the average diffusion coefficients estimated from dielectric data in its individual contributions of cations (D(+)) and anions (D(-)). The D(+) values thus obtained for IJ3, the ion jelly with the highest IL/gelatin ratio, cover a large temperature range up to room temperature and revealed excellent agreement with direct measurements from PFG NMR, obeying to the same VFT equation. For BMIMDCA(6.6%water), which has the same water amount as IJ3, the diffusion coefficients were only estimated from DRS measurements over a limited temperature range; however, a single VFT equation describes both DRS and PFG NMR data. Moreover, it was found that the diffusion coefficients and mobility are similar for the ionic liquid and IJ3, which points to a role of both water and gelatin weakening the contact ion pair, facilitating the translational motion of ions and promoting its dissociation; nevertheless, it is conceivable the existence of a critical composition of gelatin that leads to those properties. The VFT temperature dependence observed for the conductivity was found to be determined by a similar dependence of the mobility. Both conductivity and segmental motion revealed to be correlated as inferred by the relatively low values of the decoupling indexes. The obtained results show that ion jelly could be in fact a very promising material to design novel electrolytes for different electrochemical devices, having a performance close to the IL but presenting an additional stability regarding electrical measurements and resistance against crystallization relative to the bulk ionic liquid.

Lipase catalyzed esterification of glycidol in organic solvents.

We studied the resolution of racemic glycidol through esterification with butyric acid catalyzed by porcine pancreatic lipase in organic media. A screening of seven solvents (log P values between 0.49 and 3.0, P being the n-octanol-water partition coefficient of the solvent) showed that neither log P nor the logarithm of the molar solubility of water in the solvent provides good correlations between enantioselectivity and the properties of the organic media. Chloroform was one of the best solvents as regards the enantiomeric purity (e. p.) of the ester produced. In this solvent, the optimum temperature for the reaction was determined to be 35 degrees C. The enzyme exhibited maximum activity at a water content of 13 +/- 2% (w/w). The enantiomeric purity obtained was 83 +/- 2% of (S)-glycidyl butyrate and did not depend on the alcohol concentration or the enzyme water content for values of these parameters up to 200 mM and 25% (w/w), respectively. The reaction was found to follow a BiBi mechanism.

Lipase catalyzed esterification of glycidol in nonaqueous solvents: solvent effects on enzymatic activity.

We studied the effect of organic solvents on the kinetics of porcine pancreatic lipase (pp) for the resolution of racemic glycidol through esterification with butyric acid. We quantified ppl hydration by measuring water sorption isotherms for the enzyme in the solvents/mixtures tested. The determination of initial rates as a function of enzyme hydration revealed that the enzyme exhibits maximum apparent activity in the solvents/mixtures at the same water content (9% to 11% w/w) within the associated experimental error. The maximum initial rates are different in all the media and correlate well with the logarithm of the molar solubility of water in the media, higher initial rates being observed in the solvents/mixtures with lower water solubilities. The data for the mixtures indicate that ppl apparent activity responds to bulk property of the solvent. Measurements of enzyme particle sizes in five of the solvents, as function of enzyme hydration, revealed that mean particle sizes increased with enzyme hydration in all the solvents, differences between solvents being more pronounced at enzyme hydration levels close to 10%. At this hydration level, solvents having a higher water content lead to lower reaction rates; these are the solvents where the mean enzyme particle sizes are greater. Calculation of the observable modulus indicates there are no internal diffusion limitations. The observed correlation between changes in initial rates and changes in external surface area of the enzyme particles suggests that interfacial activation of ppl is only effective at the external surface of the particles. Data obtained for the mixtures indicate that ppl enantioselectivity depends on specific solvent-enzyme interactions. We make reference to ppl hydration and activity in supercritical carbon dioxide.

Solvent effects on the catalytic activity of subtilisin suspended in compressed gases.

We studied a model transesterification reaction catalyzed by subtilisin Carlsberg suspended in carbon dioxide, propane, and mixtures of these solvents under pressure. To account for solvent effects due to differences in water partitioning between the enzyme and the bulk solvents, we measured water sorption isotherms for the enzyme in each solvent. We measured catalytic activity as a function of enzyme hydration and obtained bell-shaped curves with maxima at the same enzyme hydration (12%) in all the solvents. However, the activity maxima were different in all media, being much higher in propane than in either CO(2) or the mixtures with 50 and 10% CO(2). Considerations based on the solvation ability of the solvents did not offer an explanation for the differences in catalytic activity observed. Our results suggest that CO(2) has a direct adverse effect on the catalytic activity of subtilisin. (c) 1996 John Wiley & Sons, Inc.

Solvent effects on the catalytic activity of subtilisin suspended in organic solvents.

We studied a model transesterification reaction catalyzed by subtilisin Carlsberg suspended in toluene, n-hexane, diisopropyl ether, and mixtures of these solvents. To account for solvent effects due to differences in water partitioning between the enzyme and the bulk solvents, we measured water sorption isotherms for the enzyme in each solvent. We measured catalytic activity as a function of enzyme hydration and obtained bell-shaped curves with maxima at the same enzyme hydration in all the solvents. However, the activity maxima were different in all the media, being the lowest in toluene. Differences in the partitioning of substrates and product between the bulk solvent phase and the enzyme active site were accounted for but could not explain the lower catalytic activity observed in toluene. The fact that toluene is very similar to one of the substrates suggested the possibility of competitive inhibition by this solvent. We derived a model allowing for differences in solvation of the substrates, by using thermodynamic activities instead of concentrations, as well as for competitive inhibition by toluene. The model fit the experimental data well, confirming that toluene had a direct adverse effect on the catalytic activity of the enzyme.