Cationic Acid Research Articles

Enhanced pozzolanic reactivity in hydrogen-form zeolites as supplementary cementitious materials

Pozzolans rich in silica and alumina react with lime to form cementing compounds and are incorporated into portland cement as supplementary cementitious materials (SCMs). However, pozzolanic reactions progress slower than portland cement hydration, limiting their use in modern construction due to insufficient early-age strength. Hence, alternative SCMs that enable faster pozzolanic reactions are necessary including synthetic zeolites, which have high surface areas and compositional purity that indicate the possibility of rapid pozzolanic reactivity. Synthetic zeolites with varying cation composition (Na-zeolite, H-zeolite), SiO2/Al2O3 ratio, and framework type were evaluated for pozzolanic reactivity via Ca(OH)2 consumption using ion exchange and in-situ X-ray diffraction experiments. Na-zeolites exhibited limited exchange reactions with KOH and Ca(OH)2 due to the occupancy of acid sites by Na+ and hydroxyl groups. Meanwhile, H-zeolites readily adsorbed K+ and Ca2+ from a hydroxide solution by exchanging cations with H+ at Brønsted acid sites or cation adsorption at vacant acid sites. By adsorbing cations, the H-zeolite reduced the pH and increased Ca2+ solubility to promote pozzolanic reactions in a system where Ca(OH)2 dissolution/diffusion was a rate limiting factor. High H-zeolite reactivity resulted in 0.8 g of Ca(OH)2 consumed per 1 g of zeolites after 16 h of reaction versus 0.4 g of Ca(OH)2 consumed per 1 g of Na-zeolite. The H-zeolite modulated the pore fluid alkalinity and created a low-density amorphous silicate phase via mechanisms analogous to two-step C-S-H nucleation experiments. Controlling these reaction mechanisms is key to developing next generation pozzolanic cementitious systems with comparable hydration rates to portland cement.

Revealing the Mechanism of Ketonization of Acetic Acid on HBEA Zeolite via Metadynamics Simulations

Ketonization of biomass‐derived carboxylic acids offers a promising approach to remove oxygen and upgrade into valuable chemicals. However, the mechanism on Brønsted acid site (BAS) confined in micropores of zeolite remains elusive, due to the difficulty to observe the reaction intermediates experimentally and to locate the transition state via static density functional theory (DFT) calculations. Herein, ketonization of acetic acid on HBEA at 673 K was studied by metadynamics simulations based on DFT. The first reaction step was the concerted protonation and dehydroxylation of acetic acid, resulting in an electrophilic acylium cation and H2O (ΔG‡ = 1.53 eV). The subsequent likely steps, including C‐C coupling through the nucleophilic attack of acetic acid (path I), ketene (path II), and 1,1‐dihydroxyethene and (path III) toward acylium cation were tracked and compared. The high barriers for formation of more nucleophilic intermediates of ketene and 1,1‐dihydroxyethene made the overall path II and III less favorable than the direct coupling between acetic acid and acylium cation (path I, ΔG‡ = 2.15 eV). These results indicate that acylium cation is a key intermediate, and the C‐C coupling between acetic acid and acylium cation via nucleophilic attach is the most favorable path on BAS confined in zeolite.

Electrochemical CO2 Reduction in Acidic Electrolytes: Spectroscopic Evidence for Local pH Gradients.

Inspired by recent advances in electrochemical CO2 reduction (CO2R) under acidic conditions, herein we leverage in situ spectroscopy to inform the optimization of CO2R at low pH. Using attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) and fluorescent confocal laser scanning microscopy, we investigate the role that alkali cations (M+) play on electrochemical CO2R. This study hence provides important information related to the local electrode surface pH under bulk acidic conditions for CO2R, both in the presence and absence of an organic film layer, at variable [M+]. We show that in an acidic electrolyte, an appropriate current density can enable CO2R in the absence of metal cations. In situ local pH measurements suggest the local [H+] must be sufficiently depleted to promote H2O reduction as the competing reaction with CO2R. Incrementally incorporating [K+] leads to increases in the local pH that promotes CO2R but only at proton consumption rates sufficient to drive the pH up dramatically. Stark tuning measurements and analysis of surface water structure reveal no change in the electric field with [M+] and a desorption of interfacial water, indicating that improved CO2R performance is driven by suppression of H+ mass transport and modification of the interfacial solvation structure. In situ pH measurements confirm increasing local pH, and therefore decreased local [CO2], with [M+], motivating alternate means of modulating proton transport. We show that an organic film formed via in situ electrodeposition of an organic additive provides a means to achieve selective CO2R (FECO2R ∼ 65%) over hydrogen evolution reaction in the presence of strong acid (pH 1) and low cation concentrations (≤0.1 M) at both low and high current densities.

Integrated deep eutectic solvent extraction and resin adsorption for recovering polyphenols from citrus reticulata Blanco peels: Process optimization, compositional analysis, and activity determination

The recovery of polyphenols from Citrus reticulata Blanco peels was studied based on integrated deep eutectic solvent (DES) extraction and resin adsorption. The DES prepared with choline chloride and ethylene glycol (molar ratio 1:4) and hydrophobic interaction/weakly acidic cation exchange mixed-mode adsorption resin HD-1 were used as the extractant and adsorbent respectively. The operating parameters of integrated process were optimized using completely randomized design and response surface methodology (RSM). The extraction kinetics was studied and the composition of extract was analyzed by UPLC-MS. Finally, the antioxidant activity of extract and its inhibitory ability on α-glucosidase and α-amylase were determined. Under the optimal operating conditions (liquid–solid ratio of 20:1, extraction temperature of 43 °C, extraction time of 4.8 h, resin amount of 6.6 g), the polyphenols yield of integrated process was increased by 2.15-fold and the time was shortened by 52.48 % compared with that of traditional sequential process. The extraction kinetic curves of both integrated and sequential processes conformed to pseudo-first-order kinetic model, and the extraction speed of integrated process was significantly higher than that of sequential process. The main polyphenols in extract were neohesperidoside, poncirin, narirutin, and hesperidin, etc. The extract exhibited a strong antioxidant capacity with DPPH free radical scavenging rate of extract reached 79.54 % of that of vitamin C. The inhibition ratios of extract against α-glucosidase and α-amylase were 97.81 % and 427 % of that of acarbose respectively. The results indicated that the extract has a potential application for prevention and control of type 2 diabetes mellitus.

The Effective Separation of Gallium, Vanadium, and Aluminum from a Simulated Bayer Solution by Resin Exchange.

The effective recovery of gallium from wastewater discharge in the Bayer process is promising for the long-term development of gallium resources. The adsorption and desorption behavior of gallium (Ga), vanadium (V), and aluminum (Al) ions on a strong acidic styrene cation exchange resin (JK resin) from a simulated Bayer solution was systematically investigated by static experiments. The results showed that the optimum conditions for separating Ga from V and Al were at low temperatures and short contact times, with 78.30%, 15.16%, and 6.63% of the adsorption efficiency at 25 °C and 60 min, respectively, for Ga, V, and Al. The adsorption kinetics of Ga3+ conformed to the pseudo-second order model, and the static saturation adsorption capacity was 18.25 mg/g. The Langmuir model fitted the adsorption isotherm of gallium well, and the maximum adsorption capacity was 1.11 mg/g at 25 °C. FT-IR spectroscopy and XPS showed that the mechanism of the Ga3+ adsorption was only related to the interaction of the oxygen atoms of the amide oxime group (C=NOH). The separation of Ga, V, and Al can be achieved by desorbing 98% of Al with low concentrations of ammonia and 90% of Ga with low concentrations of hydrochloric acid. The results indicate that JK resin is an efficient adsorbent for separating gallium and vanadium in alkaline solutions.

Structure-driven development of a biomimetic rare earth artificial metalloprotein

The 2011 discovery of the first rare earth-dependent enzyme in methylotrophic Methylobacterium extorquens AM1 prompted intensive research toward understanding the unique chemistry at play in these systems. This enzyme, an alcohol dehydrogenase (ADH), features a La3+ ion closely associated with redox-active coenzyme pyrroloquinoline quinone (PQQ) and is structurally homologous to the Ca2+-dependent ADH from the same organism. AM1 also produces a periplasmic PQQ-binding protein, PqqT, which we have now structurally characterized to 1.46-Å resolution by X-ray diffraction. This crystal structure reveals a Lys residue hydrogen-bonded to PQQ at the site analogously occupied by a Lewis acidic cation in ADH. Accordingly, we prepared K142A- and K142D-PqqT variants to assess the relevance of this site toward metal binding. Isothermal titration calorimetry experiments and titrations monitored by UV-Vis absorption and emission spectroscopies support that K142D-PqqT binds tightly (Kd = 0.6 ± 0.2 μM) to La3+ in the presence of bound PQQ and produces spectral signatures consistent with those of ADH enzymes. These spectral signatures are not observed for WT- or K142A-variants or upon addition of Ca2+ to PQQ ⸦ K142D-PqqT. Addition of benzyl alcohol to La3+-bound PQQ ⸦ K142D-PqqT (but not Ca2+-bound PQQ ⸦ K142D-PqqT, or La3+-bound PQQ ⸦ WT-PqqT) produces spectroscopic changes associated with PQQ reduction, and chemical trapping experiments reveal the production of benzaldehyde, supporting ADH activity. By creating a metal binding site that mimics native ADH enzymes, we present a rare earth-dependent artificial metalloenzyme primed for future mechanistic, biocatalytic, and biosensing applications.

CO2-to-methanol electroconversion on a molecular cobalt catalyst facilitated by acidic cations

CO2-to-methanol electroconversion on a molecular cobalt catalyst facilitated by acidic cations

Effects of electron beam irradiation on the postharvest quality characteristics of winter jujube (Zizyphus jujuba mill. cv. Dalidongzao)

Winter jujube was irradiated with 0.5 kGy electron beam (EB) and stored at 0 °C for 60 days. The quality of winter jujube was analyzed every 10 days to explore the effect of EB on antioxidant activity of winter jujube. Results indicated that 0.5 kGy EB reduced the decay rate of winter jujube (25.95% the 60th day), maintained the hardness and cell membrane integrity, delayed the changes of total soluble solids (TSS) and ascorbic acid (AA) contents, moreover maintained high contents of total phenols (TPC) and flavonoids. Furthermore, the activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT) and ascorbate peroxidase (APX) increased, and the free radical scavenging ability of 2,2-diphenyl-1-picrylhydrazyl (DPPH) radicals and 2,2-azino-bis-3-ethylbenzothiazoline-6-sulfonic acid (ABTS) cation radicals and the ferric ion reducing antioxidant power (FRAP) increased. Research demonstrated the potential of EB in the preservation of winter jujube and provided important information for the application of irradiation technology.

A novel multifunction of wearable ionic conductive hydrogel sensor for promoting infected wound healing

Conductive hydrogels have attracted widespread attention in applications of artificial biological tissues and ionic skin. Herein, a highly ionically conductive and biocompatibility hydrogels that integrate sensing, low swelling, and antimicrobial are prepared by the composition of chitosan (CS) chains, tannic acid (TA), polyacrylic acid (PAA), and metal cations. The hydrogels with high conductivity had tensile strain repeatable adhesion, wide strain detection, biocompatibility, and motion sensing capabilities. Particularly, in the aspect of hydrogel strain sensing based on ion conduction, it has high nonliearity (GF=2.05) with a large strain (20 %-1400 %), and excellent cycle stability. In addition, owing to the low-swelling effect and dipole-dipole interaction between the existence of free catechol groups of the zwitterionic group and TA in the zwitterionic sulfobetaine methacrylate (SBMA), the hydrogels display repeatable and controllable adhesiveness. Meanwhile, the covalent cross-linking between AA and SBMA synergistically enhances the mechanical properties of the hydrogels (169 kPa, 1474 %). Furthermore, as demonstrated in the L929 mouse wound infection model, the results indicated that the prepared hydrogel group had antimicrobial activity and reduced inflammation, thereby accelerating wound healing. The biocompatible and mutli-functional conductive hydrogels provide a new method for the design of novel type ionic skin device.

Fluorination/Dearomatization of C6F5 Groups: An FLP Route to an Electrophilic Borane and Non-Coordinating Anions.

B(C6F5)3 and the corresponding anion [B(C6F5)4]- are ubiquitous in main group and transition metal chemistry. Known derivatives are generally limited to the incorporation of electron donating substituents. Herein we describe electrophilic fluorination and dearomatization of such species using XeF2 in the presence of BF3 or Lewis acidic cations. In this fashion the anions [HB(C6F5)3]-, [B(C6F5)4]- and [(C6F5)3BC≡NB(C6F5)3]-, are converted to [FB(C6F7)3]-, [B(C6F7)4]-, and [(C6F7)3BC≡NB(C6F7)3]-, respectively. Similarly, the borane adducts (L)B(C6F7)3 (L=MeCN, OPEt3) are produced. These rare examples of electrophilic attack of electron deficient rings proceed as [XeF][BF4] acts as a frustrated Lewis pair effecting fluorination and dearomatization of C6F5 rings.

Rare earth elements recovery and mechanisms from coal fly ash by column leaching using citric acid

Rare earth elements (REEs) play an irreplaceable role in supporting the advancement of the global economy and technology, but their limited supply drives researchers to devise effective strategies for the recovery of REEs from alternative sources, including coal fly ash (CFA). In this work, a new column leaching method using citric acid as the lixiviant was proposed to enhance the recovery of REEs from CFA at room temperature. The mechanism of column leaching was investigated based on sample properties, leaching results, and the adsorption characteristics and complexation of citric acid. REEs in the CFA were found to be hosted into the amorphous aluminosilicate matrix in the form of oxides and fluorides using SEM-EDS. Compared to the maximum recovery of total REEs (TREEs, < 30 %) in the conventional agitation leaching tests, the recovery of TREEs in the column leaching tests increased significantly to 64.8 %, while the leaching rate of impurities (Al, Ti, and Fe) was less than 13.7 %. Results of the Zeta potential measurements showed that the citric acid and rare earth cations were adsorbed on the surface of CFA particles. These adsorption phenomena may make REEs bind to the CFA surface, thereby resulting in low leaching recoveries of REEs. Rare earth-citrate complexes in the lixivium were isolated by antisolvent precipitation, demonstrating that rare earth cations can be complexed by citrate. During the column leaching process, the leached rare earth ions were complexed with citrate and then carried away with the flowing liquid phase, which improved the recovery of REEs.

Fabrication of Dicarboxylic-Acid- and Silica-Substituted Octacalcium Phosphate Blocks with Stronger Mechanical Strength

Octacalcium phosphate (OCP) is an attractive base material to combine into components developed for medical purposes, especially those used in bone replacement procedures, not only because of its excellent biocompatibility but also because of its ability to intercalate with multiple types of molecular layers such as silica, dicarboxylic acid, and various cations. On the other hand, there are no examples of simultaneous substituting for several different compounds on OCPs. Therefore, in this study, the physical and mechanical strength (DTS: diametral tensile strength) of OCPs substituted with both silica and dicarboxylic acids (thiomalate: SH-malate) were evaluated. By optimizing the amount of SH-malate, we were able to prepare a block consisting of OCPs with both silica and SH-malate supported in the interlayer. The composition of the OCP-based compound comprising this block was Ca8Na1.07H6.33(PO4)4.44(SiO4)1.32(SH-malate)2.40·nH2O. Interestingly, the low mechanical strength, a drawback of silica-substituted OCP blocks, could be improved by dicarboxylic acid substituting. The dicarboxylic acid addition increased the mechanical strength of silica-substituted OCP blocks, and the acid successfully incorporated into the interlayer, even with the presence of silica. These results are expected to advance the creation of better silica-substituted OCPs and improved bone replacement materials.

Inherited Structure Properties of Larch Arabinogalactan Affected via the TEMPO/NaBr/NaOCl Oxidative System.

Arabinogalactan (AG), extracted from larch wood, is a β-1,3-galactan backbone and β-1,6-galactan side chains with attached α-1-arabinofuranosyl and β-1-arabinopyranosyl residues. Although the structural characteristics of arabinogalactan II type have already been studied, its functionalization using 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) oxidation remains a promising avenue. In this study, the oxidation of AG, a neutral polysaccharide, was carried out using the TEMPO/NaBr/NaOCl system, resulting in polyuronides with improved functional properties. The oxidation of AG was controlled by analyzing portions of the reaction mixture using spectrophotometric and titration methods. To determine the effect of the TEMPO/NaBr/NaOCl system, air-dried samples of native and oxidized AG were studied by Fourier-transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopy, as well as by gel permeation chromatography. Compounds that model free (1,1-diphenyl-2-picrylhydrazyl (DPPH)) and hydroxyl radicals (iron(II) sulfate, hydrogen peroxide, and salicylic acid) were used to study the antioxidant properties. It was found that, in oxidized forms of AG, the content of carboxyl groups increases by 0.61 mmol compared to native AG. The transformation of oxidized AG into the H+ form using a strong acid cation exchanger leads to an increase in the number of active carboxyl groups to 0.76 mmol. Using FTIR spectroscopy, characteristic absorption bands (1742, 1639, and 1403 cm-1) were established, indicating the occurrence of oxidative processes with a subsequent reduction in the carboxyl group. The functionality of AG was also confirmed by gel permeation chromatography (GPC), which is reflected in an increase in molecular weights (up to 15,700 g/mol). A study of the antioxidant properties of the oxidized and protonated forms of AG show that the obtained antioxidant activity (AOA) values are generally characteristic of polyuronic acids. Therefore, the TEMPO oxidation of AG and other neutral polysaccharides can be considered a promising approach for obtaining compounds with the necessary controlled characteristics.

Positively Charged Amino Acid-Modulated Interfacial Chemistry and Deposition Textures for Highly Reversible Zinc Anodes.

Interfacial active water molecule-induced parasitic reactions and stochastic Zn2+ transport-caused dendrite issue significantly impede the implementation of aqueous Zn-ion batteries. Herein, three positively charged amino acids, namely arginine, histidine, and lysine, were utilized as adsorption-type electrolyte additives to enhance the stability and reversibility of Zn anodes. Combined theoretical and experimental analyses verified that these amino acid cations can synergistically modulate the interfacial microenvironment and promote orientational Zn deposition. The adsorbed amino acid cations reconfigured the interfacial electric double layer structure, forming SO42-- and H2O-poor interfaces, thereby retarding hydrogen evolution and corrosion side reactions. Simultaneously, the preferential adsorption of the amino acid cations at specific facets induced crystallographic orientational Zn deposition along unterminated facets. Three deposition architectures, namely planar texture, subvertical alignment, and vertical erection, were obtained, all effectively inhibiting dendrite formation. Consequently, symmetric cells with the three amino acid cations exhibited high stripping/plating reversibility of over 2000 cycles at 5 mA cm-2. Moreover, MnO2-based full cells exhibited markedly improved stabilities compared with their additive-free counterparts.

Eco-friendly closed-loop recycling of nickel, cobalt, manganese, and lithium from spent ternary lithium-ion battery cathodes

Recycling technology is essential for managing waste and addressing environmental issues related to scrapping power lithium batteries. A closed-loop recycling technique was proposed in this work for maximizing usage of lithium (Li), manganese (Mn), cobalt (Co), and nickel (Ni) resources in spent ternary lithium battery (SNCMB) cathodes. A green and sustainable leaching process utilizes citric acid (CA) and hydrogen peroxide (HP) to efficiently extract Ni, Co, Mn, and Li from SNCMB cathodes. Maximizing leaching rates for Ni (96.69 %), Co (95.38 %), Mn (97.64 %), and Li (98.72 %) under optimal conditions (80 ℃, 4 mol/L CA, 20 g/L solid to liquid (S/L) ratio, 2 vol% HP, 70 min). The Avrami equation models leaching kinetics, requiring a temperature of 80 ℃ to accelerate the process. Hydrogen-type strong acidic cation exchange resin (HR) effectively adsorbs Ni, Co, Mn, and Li ions from leachates through ion exchange under optimal conditions, reducing costs and eliminating the need for separating metals with similar chemical properties. This multi-stage adsorption recycling process renders leachates both economical and eco-friendly. The metal-enriched HR composite (HRC) from the ion-exchange process prepares ternary lithium battery (NCMB) cathodes to maximize metal usage. Microwave heating achieves excellent structure and electrochemical performance of regenerated ternary lithium battery (RNCM) cathodes, with 153.81 mAh/g discharge capacity at 0.1C rate and 90.92 % capacity retention after 50 cycles at 900 ℃ for 4 h. This innovative, efficient, and eco-friendly technique allows for closed-loop recycling of Ni, Co, Mn, and Li from SCNMB cathodes, providing new avenues for lithium battery recycling.

Highly Selective Electrochemical Baeyer-Villiger Oxidation through Oxygen Atom Transfer from Water.

The Baeyer-Villiger oxidation of ketones is a crucial oxygen atom transfer (OAT) process used for ester production. Traditionally, Baeyer-Villiger oxidation is accomplished by thermally oxidizing the OAT from stoichiometric peroxides, which are often difficult to handle. Electrochemical methods hold promise for breaking the limitation of using water as the oxygen atom source. Nevertheless, existing demonstrations of electrochemical Baeyer-Villiger oxidation face the challenges of low selectivity. We report in this study a strategy to overcome this challenge. By employing a well-known water oxidation catalyst, Fe2O3, we achieved nearly perfect selectivity for the electrochemical Baeyer-Villiger oxidation of cyclohexanone. Mechanistic studies suggest that it is essential to produce surface hydroperoxo intermediates (M-OOH, where M represents a metal center) that promote the nucleophilic attack on ketone substrates. By confining the reactions to the catalyst surfaces, competing reactions (e.g., dehydrogenation, carboxylic acid cation rearrangements, and hydroxylation) are greatly limited, thereby offering high selectivity. The surface-initiated nature of the reaction is confirmed by kinetic studies and spectroelectrochemical characterizations. This discovery adds nucleophilic oxidation to the toolbox of electrochemical organic synthesis.

Amino Acid Ionic Liquid Coated Magnetic Core Fe&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt;@SiO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; Nanoparticles Coupled with UV Spectrophotometry for the Separation /Analysis Congo Red

Congo red has strong tinting strength, low price that it was used in meat dyeing. Amino acid ionic liquids (AAILs) containing amino acid cations or anions are synthesized from natural amino acids, which has the characteristics of low toxicity. Nowadays, Fe&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt;@SiO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;@AAIL composites has seemly not been reported for the separation/analysis of congo red. In this work, Fe&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt;@SiO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;@AAIL nanoparticles were used as magnetic solid phase adsorbents in combination with UV spectrophotometry for separation of congo red.

Trivalent Cations Slow Electron Transfer to Macrocyclic Heterobimetallic Complexes.

Incorporation of secondary redox-inactive cations into heterobimetallic complexes is an attractive strategy for modulation of metal-centered redox chemistry, but quantification of the consequences of incorporating strongly Lewis acidic trivalent cations has received little attention. Here, a family of seven heterobimetallic complexes that pair a redox-active nickel center with La3+, Y3+, Lu3+, Sr2+, Ca2+, K+, and Na+ (in the form of their triflate salts) have been prepared on a heteroditopic ligand platform to understand how chemical behavior varies across the comprehensive series. Structural data from X-ray diffraction analysis demonstrate that the positions adopted by the secondary cations in the crown-ether-like site of the ligand relative to nickel are dependent primarily on the secondary cations' ionic radii and that the triflate counteranions are bound to the cations in all cases. Electrochemical data, in concert with electron paramagnetic resonance studies, show that nickel(II)/nickel(I) redox is modulated by the secondary metals; the heterogeneous electron-transfer rate is diminished for the derivatives incorporating trivalent metals, an effect that is dependent on steric crowding about the nickel metal center and that was quantified here with a topographical free-volume analysis. As related analyses carried out here on previously reported systems bear out similar relationships, we conclude that the placement and identity of both the secondary metal cations and their associated counteranions can afford unique changes in the (electro)chemical behavior of heterobimetallic species.

Effect of Fractionation Columns on the Elution of Rare Earth Elements Recovered from Acid Mine Drainage

Rare earth elements (REE) can be found in expressive contents in different secondary sources, such as acid mine drainage (AMD). This work evaluated separation of light and heavy rare earth elements (REE) from an acid mine drainage (AMD) generated in a former uranium mine in Brazil by using ion exchange. This AMD presents pH 3.50, total REE content of 97 mg L−1 and 1.3 g L−1 of sulfate and was used in the REE loading experiments. Loading experiments were carried out in columns using a commercial strong acid cation (SAC) exchange resin. Elution was performed with 0.01 mol L−1 NH4EDTA in systems with one, two and three columns. Regarding the loading step, the resin presented a total loading capacity of 0.58 mmol g−1. The resin proved to be more selective for light REE with adsorption efficiency of 78% and 48% for heavy REE. Regarding elution, high efficiencies between 90 and 100% were achieved for REE. The final REE solution is approximately 10 times more concentrated in the liquor related to the acid mine water. Better fractionation results were achieved for the system with three columns. Although the complete separation of the REE into pure elements was not possible, two distinct fractions of heavy and light REE could be obtained, and La was completely separated from the other REE. In order to improve fractionation and separate the REE into individual ones, the concentrated fractions can proceed to subsequent ion exchange systems.

Effect of the type of ion-exchange resin on Mn&lt;sup&gt;2+&lt;/sup&gt; adsorption in the presence of competing cations

Present in soils, ground and surface waters, manganese is among the most common metals in Earth crust. It is also an essential trace element to the functioning of several enzymes in the human body. However, exposure to high manganese concentrations can also be harmful to humans with psychiatric and motor effects and therefore, manganese concentrations in drinking water and also industrial effluents are regulated. In the current work, the adsorption of Ca&lt;sup&gt;2+&lt;/sup&gt;, Mg&lt;sup&gt;2+&lt;/sup&gt; and Mn&lt;sup&gt;2+&lt;/sup&gt; on three different ion-exchange resins: (i) aminophosphonic acid - chelating (Purolite S950), (ii) polyacrylic weak acid cation (Purolite C104E) and (iii) polystyrene strong acid cation (Purolite C100) was investigated. The results revealed that Purolite S950 had the highest Mn2+ uptake (37.9 mg/mL-resin or 0.69 mmol/mL-resin) as compared to Ca&lt;sup&gt;2+&lt;/sup&gt; (3.2 mg/mL-resin or 0.08 mmol/mL-resin) and Mg&lt;sup&gt;2+&lt;/sup&gt; (~0 mg/mL-resin) and was selected for further kinetics and equilibrium studies. The results indicated Purolite S950 as particularly suited to be applied in the treatment of neutral mine waters with high Mg/Mn ratios. Additionally, Purolite S950 showed a small affinity for Ca&lt;sup&gt;2+&lt;/sup&gt; and therefore an efficient Mn&lt;sup&gt;2+&lt;/sup&gt; removal will depend on the Ca/Mn ratio of the mine water under treatment. According to the kinetic analysis, manganese sorption on Purolite S950 was described by the pseudo-second order model (r&lt;sup&gt;2&lt;/sup&gt; &gt; 0.98) with an activation energy of 10.40 kJ/mol and thus pore-difussion was the rate controlling step of the process. In terms of equilibrium studies, manganese sorption on Purolite S950 followed the Langmuir model with maximum loadings of up to 41.5 mg/mL-resin. The thermodynamic modelling indicated an exothermic process (-85.0 kJ/mol, as standard enthalpy) with a standard entropy of -274 J/mol.K, which was ascribed to the release of two adsorbed H&lt;sup&gt;+&lt;/sup&gt; ions for each Mn&lt;sup&gt;2+&lt;/sup&gt; ion taken up from solution