Many phenolic compounds are related to the group of toxic substances, the occurrence of which in wastewater has a negative impact on the environment. In addition to extraction, biochemical, and membrane methods for the isolation of hydroxybenzenes from aqueous media, sorption methods using active carbon, nonionic polymers, and ion-exchange materials are also actively used. The purpose of this work was to establish the features of the sorption of phenol, resorcinol and phloroglucinol from aqueous solutions with FIBAN fibers. Sorption was studied under static conditions at a temperature of 298 K. An assessment of the amount of absorbed sorbates by fibers of various nature indicates that FIBAN A-1 fiber exhibits better capacity characteristics relative to phenolic compounds. This fact is due to the nature of the matrix (styrene-divinylbenzene) and the high basicity of the functional groups of this sorbent, which increase the number of sorption centers for the uptake of hydroxybenzenes. The isotherms of the sorption of phenol, resorcinol and phloroglucin in the concentration range 0.5-10 mmol/dm3 indicate the affinity of FIBAN A-1 to these sorbates. In the entire range of concentrations studied, the highest recovery rates are characteristic of trihydroxybenzene. The paper provides a formal analysis of the obtained equilibrium curves based on the choice of the sorption equation that most closely describes the experimental dependences. The apparent constants of sorption equilibrium are calculated, the energy characteristics of uptake process are estimated.

In this brief review, we consider various characterizations of “monomeric” reversed phases for elucidating the interactions governing adsorbate retention in liquid chromatography. Conventional methods related to the assessment of retention capacity and hydrophobicity (specifically methylene selectivity) using single mobile phase compositions are discussed with a focus on dispersion interactions, along with their inherent strengths and limitations. An alternative approach involving separation maps through relative retention analysis is proposed. It is noted that, in real reversed-phase adsorbents, the density of the attached alkyl chains is typically one half of that of solid n-alkanes. In this case, adsorbate molecules to penetrate into the attached phase, and the process depends on the molecular shape. Consequently, conventional “monomeric” reversed phases exhibit specific selectivity towards substances with specific structures. The review also notes that current analytical methods often do not pay sufficient attention to the difference between the substance retention mechanisms, absorption and adsorption, because the predominant parameters of these mechanisms are quite different. Moreover, in the two most widely used very interesting and informative methods, linear solvation energy relationships (LSERs) and the hydrophobic-subtraction model, this characteristic has not received due attention. Taking into account that the method does not distinguish adsorbates retained by different mechanisms, absorptive versus adsorptive, to the obtained significant discrepancies between the calculated and experimental data do not seem extraordinary. The interpretation of the results of an LSER analysis is also complicated by uncertainties in the contributions of partial properties of adsorbates in both mobile and stationary phases to the total solvation energy, as only their difference is typically calculated. Nonetheless, a comparison of different columns in identical mobile phases can yield informative insights. A drawback of the second approach is the necessity of using multiple columns with substantial qualitative differences in the adsorbate retention among them. Furthermore, a possibility of the decomposition of all interactions into distinct types seems questionable, because the method does not involve any orthogonal (independent of the applied calculation method) properties.

n this work we studied the flavonoid composition of tangerine skins using 10 kinds of tangerines from various producers. The tangerine skins were peeled from fruit purchased from the market in Belgorod and dried at room temperature outside direct sunlight. The compositions of “green” ethanol-based extraction solutions and the “weighed portion of plant material: volume of extraction solution” ratios were chosen for extraction to ensure a high yield of flavonoids. For separation, reverse-phase HPLC on the Kromasil 100-5C18 stationary phase was used in environmentally friendly mobile phase compositions that contained ethanol as an organic modifier. In gradient elution mode all mobile phases contained 0.2 vol. % orthophosphoric acid. To assign extract components, we used samples of a number of substances and parameters of electronic absorption spectra in comparison with published data. It was found that the chromatographic profiles of the extracts of the studied samples were noticeably different. A number of the least lipophilic compounds, derivatives of cinnamic acids, were found in all extracts. After that, we discovered two flavanone glycosides, the main of which was hesperidin (hesperitin-7-rutinoside) with a content of is 26.0-39.9 mg per 1 g of dried raw material, and naringin (naringenin-7-neohesperidiside) in significantly smaller quantities, from 0 to 1.15 mg per 1 g. But of greatest interest were polymethoxylated flavones, among which the main component was nobiletin (5,6,7,8,3′,4′-hexamethoxyflavone) with a content from 0.03 to almost 11 mg/g in dried peel and tangeretin (5,6,7,8,4′-pentamethoxyflavone), found in slightly smaller quantities, 0.1-5.4 mg/g. It was established that nobiletin and tangeretin were mostly accumulated in the skins of small tangerines. On the whole, tangerine skin is a great source of highly biologically active flavonoids, so its processing is an important task for the food industry.

В работе установлено, что в плодах красного винограда и в продуктах его переработки основными фенольными кислотами являются не хлорогеновая (продукт ацилирования хинной кислоты кофейной), как указывается в ряде публикаций, а кафтаровая (продукт ацилирования винной кислоты кофейной) и коутаровая (продукт ацилирования винной кислоты п-кумаровой). Для хроматографирования исследованных образцов была выбрана стационарная фаза Symmetry C18 и подвижные фазы системы «ацетонитрил – 10 об.% муравьиной кислоты – вода», удобные при определении антоцианов винограда. Показано, что кафтаровая кислота во всех исследованных составах подвижной фазы полностью отделяется от коутаровой и от трех изомерных монокофеоилхинных кислот (3-кофеоилхинной, 4-кофеоилхинной и 5-кофеоилхинной), т.е. идентификация по временам удерживания этих кислот возможна. Однако для хроматографирования следует выбирать составы подвижной фазы, при которых нет соэлюирования какой-либо из этих кислот с обычно присутствующим в экстрактах, соках и винах дельфинидин-3-глюкозидом. Полное разделение указанных компонентов возможно как при относительно низком содержании ацетонитрила (менее 6 об.%) в подвижной фазе, так и при относительно высоком (не менее 8.5 об.%). Это позволило предложить вариант градиентного элюирования для разделения всех интересующих соединений. Градиентный режим требуется для элюирования из колонки всех антоцианов и сопутствующих экстрактивных веществ. При этом, предложенные условия разделения оказались благоприятными для детектирования полидатина (одного из изомерных глюкозидов ресвератрола). Это позволило опровергнуть еще одно распространенное заблуждение, согласно которому ресвератрол (а не его глюкозид) является одним из важнейших биологически активных веществ в винограде. В предложенных условиях значительно сильнее удерживаемый ресвератрол соэлюируется с одним из ацилированных антоцианов, но анализ площадей соответствующих пиков свидетельствует о том, что именно полидатин выступает доминирующей формой присутствия ресвератрола в винограде.

In this work, a study was made of the sorption of boric acid from aqueous solutions by fibers of the FIBAN brand (Institute of Physical Organic Chemistry of the National Academy of Sciences of Belarus, Belarus), which have functional amino groups in their structure. The possibility of using FIBAN A-1, A-5, and A-6 fibrous materials for the extraction of boron compounds from aqueous solutions has been established. According to the sorption kinetic curves, a high rate of sorption of boric acid by high- and low-basic anion exchangers was noted. Also, some equilibrium characteristics of analyte sorption by the studied sorbents are estimated in the work. A complex multivariant mechanism of sorption of boric acid by FIBAN A-1 and A-5 fibers was revealed, which includes both chemisorption uptake of H 3 BO 3 by amino groups of sorbents in the range of low concentrations and the possibility of formation and fixation of polyborates in the phase of the ion exchanger in the range of high concentrations (superequivalent sorption).

A method for assessing the antioxidant properties of anthocyanins was proposed, using a reaction with an aqueous solution of potassium permanganate followed by chromatographic determination of the concentration of various anthocyanins in complex mixtures in comparison with the original solution. The main feature of the method is the exclusion of the occurrence of a chain of successive oxidation reactions of each initial anthocyanin, which is actually observed when using traditional methods for determining antioxidant properties. For this purpose, a lack of oxidant was used, which should be consumed mainly in the first stages for each antioxidant, under the assumption that the activity of the initial (least oxidized) anthocyanin is higher than that of the products formed from it. Only such a scheme allows comparing antioxidant activity (as a time-dependent parameter instead of the usually determined antioxidant capacity) depending on the structure of the molecule. Based on a study of the oxidation of 3-glucosides with potassium permanganate of five different main natural anthocyanidins (in extracts of grape fruits and leaves of Cercis canadensis), a dependence of antioxidant activity was established, which increased in the series: Pn3Glu

The alterations in current-voltage and transport characteristics of highly basic and strongly acidic ion-exchange membranes, during the electrodialysis of solutions containing a heterocyclic amino acid and a strong electrolyte, were studied. An increase in the catalytic activity of the water splitting process at the surface of heterogeneous MK-40 and MA-41 membranes upon prolonged contact with proline and tryptophan solutions was found. A significant effect of electroconvection on the components mass transfer through the cation-exchange membrane in the intensive current mode of electrodialysis was revealed for the solution containing a heterocyclic amino acid along with mineral salt (NaCl). This led to a reduction in the length of the “plateau” of the membrane’s current-voltage characteristics, in comparison with the characteristics for an individual sodium chloride solution with the same concentration. The changes in the characteristics of the studied ion-exchange membranes caused by contact with solutions containing heterocyclic amino acids during electrodialysis were reversible when applying electrochemical regeneration (cleaning in place) using the overlimiting current mode, corresponding to the region of facilitated transport for these ampholytes.

— Excessive contents of boron and its compounds can exhibit toxic effects despite their necessity for functioning living organisms. An urgent task is to remove them in various ways, including the use of sorption technologies. Boron-selective sorbents with functional OH groups and granular anion exchangers with functional amino groups are capable of extracting boron compounds. This paper is devoted to studying boric acid sorption under static conditions on FIBAN A-5 fibrous sorbent, which shows a high affinity for boron and can be used as an alternative to anion-exchange resins produced in the form of granules. The features of the kinetics and equilibrium of the sorption by the studied fiber brought in contact with boric acid solutions have been studied. It has been found that the time required to reach the equilibrium state decreases with the intensification of stirring and an increase in the concentration gradient in a solution. Sorption isotherms are of a stepwise character due to the existence of boron in different forms at concentrations above 0.025 M. The influence of temperature, pH of a solution, and ionic form of the sorbent on the capacity of FIBAN A-5 fiber has been determined.

The features of organic fouling have been revealed for highly basic anion exchange membranes during prolonged electrodialysis in solutions containing the aromatic amino acid tyrosine. With increased operation time when using MA-41 heterogeneous membranes in tyrosine solution, an increase in hydrophobicity and roughness characteristics of the material surface is detected. A reduction in tyrosine flux through the membrane occurs which is caused by its pores plugging and deposition of the amino acid at the membrane surface induced by tyrosine adsorption and local supersaturation of the solution in the membrane phase. The long-term contact of the anion exchange membrane with a solution of tyrosine leads to some structural changes in the anion exchange material. An accumulation of the studied amino acid with phenolic fragment and tyrosine oxidation products (DOPA, DOPA-quinone) is found and confirmed by IR- and UV-spectroscopy techniques. The organic fouling is accompanied by an increase in density and a decrease in moisture content of the studied membrane. A comparative analysis of the chemical and electrochemical cleaning results for fouled samples of the MA-41 membrane demonstrates a partial restoration of the material transport characteristics using electrochemical cleaning in the intensive current mode of electrodialysis. The best efficiency of regeneration is reached when carrying out chemical cleaning with a solution of hydrochloric acid, providing almost complete restoration of the membrane characteristics.

В работе проведено исследование сорбции борной кислоты из водных растворов волокнами марки ФИБАН ("Институт физико-органической химии Национальной академии наук Беларуси", Беларусь), имеющими в своем строении функциональные аминогруппы. Установлена возможность использования волокнистых материалов ФИБАН А-1, А-5 и А-6 для извлечения соединений бора из водных растворов. Согласно кинетическим кривым сорбции отмечена высокая скорость сорбции борной кислоты высоко- и низкоосновными анионообменниками. Также в работе оценены некоторые равновесные характеристики сорбции аналита исследуемыми сорбентами. Выявлен сложный многовариантный механизм поглощения борной кислоты волокнами ФИБАН А-1 и А-5, который включает как хемосорбционное поглощение H 3 BO 3 аминогруппами сорбентов в области низких концентраций, так и возможность образования и закрепления в фазе ионообменника полиборатов в области высоких концентраций (сверхэквивалентная сорбция).

In the present study, the possibility of using a strong-base fibrous material FIBAN A-1 (Institute of Physical Organic Chemistry, National Academy of Sciences of Belarus, Belarus) as a sorbent to remove tryptophan from aqueous solutions is investigated. The capacity characteristics of the anion exchangers in the form of granules and fibers containing quaternary ammonium bases as functional groups with respect to the chosen amino acid are analyzed. The influence of the pH of the medium on the sorption of amino indolylpropanoic acid onto the A-1 fiber is determined. IR spectroscopy confirms the fact of the fixation of amino acids from solutions with pH ~5.0–5.5 due to the charge exchange of the zwitterion by the OH groups of the sorbent. The specific features of both the equilibrium and kinetics of tryptophan sorption from solutions with a pH close to the isoelectric point are studied. Based on the analysis of the isotherms of sorption of the amino acid at different temperatures, a monomolecular mechanism of the sorptive sorption process and its endothermic character are established. The kinetic characteristics of tryptophan sorption indicate a significant contribution of external diffusion to the velocity of the investigated process.

The article presents a study of the behaviour of the MA-41 anion-exchange membrane and MB-2 bipolar membrane during the electrodialysis of a solution containing tyrosine and sucrose. It establishes changes in current-voltage, transport, and structural characteristics of ion-exchange membranes. The study of the evolution of membrane characteristics during a prolonged contact with solutions containing an aromatic amino acid and disaccharide is aimed at providing a deeper understanding of and finding solutions to the problem of organic fouling of membranes, which complicates the electromembrane separation of components of the solution during microbiological synthesis of amino acids. It was found that the fluxes of tyrosine and sucrose through the MA-41 membrane measured after its operation during 50-hour electrodialysis reach higher values than during the first hours of operation after the system reaches a steady state. However, it was noted that when the membrane continues to be used, the flux of components through the MA-41 membrane decreases. What is more, this change is pronounced with a high current density. This decrease in mass transport, an increased voltage drop on the MB-2 and MA-41 membranes, and lower values for the effective ОН- ion transport number for the MA-41 membrane are associated with the phenomenon of organic fouling confirmed by revealed structural changes in the ion-exchange material, which become significant after a prolonged contact (more than 60 hours) with a mixed solution of tyrosine and sucrose. These changes are associated with the accumulation of an amino acid and its oxidation product, 3,4-dihydroxyphenylalanine, in the membrane phase, as well as with a decrease in the content of sucrose absorbed by the membrane.

Adsorption isotherms are obtained for L-histidine on carbon nanotubes using aqueous solutions at temperatures of 25, 35, 45, 55, 65, and 80°C. The isotherms are interpreted by using the cluster adsorption model and analyzing the dependence of the strength of the bond nanotube–amino acid on temperature. Equations of the adsorption isotherms are obtained for all temperatures. The calculated results indicate that L-histidine is fixed on a nanotube’s surface in the form of monomers and different clusters. The equilibrium characteristics of adsorption are found to change as the temperature rises. Values obtained for the coefficients of equilibrium are used to calculate apparent enthalpy Δ H * of L-histidine adsorption by nanotubes, according to the van’t Hoff equation.

The characteristic features of the mass transport of the components through the ion-exchange membranes during conventional electrodialysis of a ternary aromatic amino acid–disaccharide–mineral salt solution with inert spacers are studied. The mutual influence of the components of the system during the transport through MA-41 and MK-40 heterogeneous membranes is revealed. It is shown that the fluxes of phenylalanine through the membranes reach lower values at a higher concentration of sucrose in the feed solution. Here, lower values of the degree of desalination are observed when compared to the lower concentration of the carbohydrate in the mixture. It is found that most losses of sucrose during desalination are due to its mass transport through the cation-exchange membrane, while phenylalanine, through anion-exchange. The application of an electrodialysis scheme with bipolar and anion-exchange membranes at the next stage provides effective separation of the aromatic amino acid and disaccharide from the preliminary demineralized solution due to the conjugated transport of phenylalanine through the anion-exchange membrane with the hydroxyl ions generated at the inner interface of the bipolar membrane.

This work provides an overview of the processes of fouling, the deposition of substances on the surface or in the pores of membranes, leading to deterioration of their performance. Degradation and fouling phenomena in various membrane materials, as well as the mechanisms of these processes, are considered. It is shown that, despite the difference in the chemical composition, morphology of membranes, ion exchangers, and the phenomena in which they are used, the phenomena leading to clogging of their surface and pores are largely similar. Among the main substances that contaminate membranes are organic molecules, polyelectrolytes, crystals of inorganic substances formed from ions contained in a solution, as well as colloidal particles, and biological organisms. The binding strength of foulants essentially depends on their nature and on the chemical composition of the membranes. At the same time, many fouling phenomena have their own characteristics. For example, in the processes of electricity generation in fuel cells or hydrogen production in electrolyzers, the formation of oxides or metal particles is observed in the membrane pores due to the electrolysis processes. The consequences of the processes of fouling and methods of their control are also considered. It should be noted that cleaning of membranes is still the main method of preventing fouling. At the same time, in recent years, research has been intensively developed in the field of inhibition of corrosion processes, as well as the creation of integrated approaches that integrate various processing processes, including both membrane and other technologies.

This article’s main focus is to highlight significant aspects of amino acid solution demineralization. The main part of the amino acid production method requires the provision of downstream treatment solutions for the process of desalination. Electrodialysis (ED) and electrodeionization (EDI) are prospective technologies for such treatment. The article presents a brief review of the first studies and current research on electromembrane desalination of amino acid solutions as well as the analysis of some electrochemical features for the mineral salt–amino acid system (model solution) in an ED process based on the experimental results. The influence of various factors on the desalination of neutral amino acid-containing solutions and on target product losses in this process is estimated. The behavior of aliphatic (alanine) and aromatic (phenylalanine) amino acids in the electromembrane system is considered in mixed solutions with inorganic electrolytes. The influence of various mineral cations (Na+, K+ and NH4+) and anions (NO3−, SO42−, Cl−) on the features of the transport and current–voltage characteristics of ion-exchange membranes in the electrodialysis of phenylalanine- and alanine-containing solutions is considered. A comparative analysis of the desalination parameters of AA solutions in electrodialysis with the following pairs of heterogeneous MA-41/MK-40, MA-40/MK-40 and homogeneous AMT/CMT membranes is carried out. The minimum amount of amino acid loss along with rather high values of the degree of desalination are revealed in electrodialysis with polypropylene spacers in comparison with EDI, ED with a copolymer of styrene and divinylbenzene as spacer, as well as ED with a smooth deionization channel. At the same time, EDI is the most promising method to reach the highest desalination degree in the considered range of mineral salt content.