Various toxic chemicals are discharging to the environment due to rapid industrialization and polluting soil, water, and air causing numerous diseases including life-threatening cancer. Among these ...pollutants, Cr(VI) or hexavalent chromium is one of the most carcinogenic and toxic contaminants hostile to human health and other living things. Therefore, along with other contaminants, the removal of Cr(VI) efficiently is very crucial to keep our environment neat and clean. On the other hand, silica has a lot of room to modify its surfaces as it is available with various sizes, shapes, pore sizes, surface areas etc. and the surface silanol groups are susceptible to design and prepare adsorbents for Cr(VI). This review emphases on the progress in the development of different types of silica-based adsorbents by modifying the surfaces of silica and their application for the removal of Cr(VI) from wastewater. Toxicity of Cr(VI), different silica surface modification processes, and removal techniques are also highlighted. The adsorption capacities of the surface-modified silica materials with other parameters are discussed extensively to understand how to select the best condition, silica and modifiers to achieve optimum removal performance. The adsorption mechanisms of various adsorbents are also discussed. Finally, future prospects are summarized and some suggestions are given to enhance the adsorption capacities of the surface-modified silica materials.
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•Surface-modified silica-based materials are promising materials for the removal of Cr(VI) from wastewater.•Amine functionalized mesoporous silica showed higher adsorption capacities.•Post-grafting method showed better performance than co-condensation/pre-synthesis method.•Silica surface-modification has plenty of room to apply for efficient hexavalent chromium removal processes.
Ionic liquids (ILs), a class of unique substances composed purely by cation and anions, are renowned for their fascinating physical and chemical properties, such as negligible volatility, high ...dissolution power, high thermal stability, tunable structure and miscibility. They are enjoying ever-growing applications in a great diversity of disciplines.
IL-modified silica, transforming the merits of ILs into chromatographic advantages, has endowed the development of high-performance liquid chromatography (HPLC) stationary phase with considerable vitality. In the last decade, IL-functionalized silica stationary phases have evolved into a series of branches to accommodate to different HPLC modes. An up-to-date overview of IL-immobilized stationary phases is presented in this review, and divided into five parts according to application mode, i.e., ion-exchange, normal-phase, reversed-phase, hydrophilic interaction and chiral recognition. Specific attention is channeled to synthetic strategies, chromatographic behavior and separation performance of IL-functionalized silica stationary phases.
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•Ionic liquids (ILs) are amazing ligands for HPLC stationary phases.•IL-functionalized materials are classified according to HPLC modes.•We illustrate strategies for preparation of IL-functionalized stationary phases.•We describe characterizations and LC evaluations of IL-based stationary phases.•We put forward trends and perspectives on IL-based stationary phases.
•New adsorbents were prepared by combining chitosan and poly(1-vinylimidazole) using a facile technique.•High adsorption capacity was observed with the new adsorbent.•Synergistic effect was realized ...from chitosan and poly(1-vinylimidazole).•Interaction mechanism was also evaluated.
Chitosan and poly(1-vinylimidazole) are both potential adsorbents to remove Cr(VI). Here, we designed the preparation of new adsorbents by combining chitosan and poly(1-vinylimidazole) to get the synergistic effect for the removal of hexavalent chromium. Trimethoxysilyl group terminated poly(1-vinylimidazole)-modified-chitosan composite was successfully synthesized by one-step free radical polymerization based on the grafting backbone of chitosan and vinylimidazole. The resulting adsorbents were used for the removal of Cr(VI) ions from the aqueous solution. The modified chitosan composite was characterized by ATR, FTIR, BET isotherm studies, elemental analysis, TGA, DSC, FE-SEM, and EDX. ATR. FTIR results confirmed the presence of the imidazole group in modified chitosan. The adsorption results were described by the Langmuir isotherm model with a maximum adsorption capacity of 196.1 mg/g for modified chitosan, however, the chitosan yielded 151.5 mg/g. It has been observed that the adsorption of chromium fitted better with the pseudo-second-order kinetics. The modified chitosan composite exhibited greater adsorption capacity than chitosan for hexavalent chromium and has potential application for Cr(VI) removal from aqueous solution containing other common ions with regeneration ability. This novel approach of modifying chitosan with telomerized poly(1-vinyl imidazole) offers potential application in wastewater treatment of different industries releasing Cr (VI).
•Three EPG-containing pentafuorophenyl (PFP) stationary phases were synthesized.•The phases were evaluated by the Tanaka and Neue tests in RPLC and Tanaka test in HILIC.•One of the phases showed very ...high selectivity in RPLC and another in HILIC.•The phases were used successfully for the analysis of the real samples of chili pepper extract.•This work will open a new door for the analysis of complex samples both in 1D and 2D LC.
Pentafluorophenyl (PFP) stationary phase is one of the most important phases after the C18 phase in terms of its applications. Three embedded polar groups (EPG)-containing stationary phases were newly synthesized to act the EPGs as additional interaction sites. The silica surface was initially modified with (3-aminopropyl)trimethoxysilane (APS). The APS-modified silicas were coupled with 2,3,4,5,6-pentafluorobenzoic acid, 2,3,4,5,6-pentafluorophenylacetic acid, and 2,3,4,5,6-pentafluoro-anilino(oxo)acetic acid to obtain Sil-PFP-BA, Sil-PFP-AA, and Sil-PFP-AN phases, respectively. The new phases were characterized by elemental analysis, ATR-FTIR, scanning electron microscopy (SEM), and thermogravimetric analysis (TGA). The phases were evaluated with the Tanaka and Neue tests in reversed-phase liquid chromatography (RPLC). In addition, they were characterized as hydrophilic phases by the Tanaka test protocol used in hydrophilic interaction chromatography (HILIC) separation mode. The Sil-PFP-AA phase showed the highest molecular shape selectivity in RPLC, while Sil-PFP-AN achieved the highest separability in HILIC compared to the commercial PFP reference column. The Sil-PFP-AA phase was successfully applied for the analysis of capsaicinoids from real samples of fresh chili peppers (Capsicum spp.) in RPLC and the Sil-PFP-AN phase for vitamin C (ascorbic acid) in HILIC.
Reversed-phase liquid chromatography (RPLC) is the most popular separation mode in high-performance liquid chromatography (HPLC). The design and synthesis of new separation materials to meet growing ...demands has always been a major concern to enhance the selectivity of stationary phases. Therefore, variety of new organic stationary phases for columns has been a key factor in the development of HPLC as one of the major separation tool. This review summarized the development trend of stationary phases and their characterizations with improved shape selectivity especially for the separation of shape-constrained isomers. The design of several novel stationary phases, based on the molecular-shape recognition concept, is also briefly reviewed along with the applications. Up-to-date information about the newly developed phases and the driving forces for the high shape selectivity will also be focused in this review.
•Advancement of new shape-selective stationary phases is described.•Design and preparation strategies of various stationary phases are discussed.•All of the developed highly shape-selective stationary phases were summarized.•Various interaction mechanisms are also described.
Novel stationary phases have been emerging recently. A β-alanine-derived embedded urea and amide group-containing C18 phase (Sil-Ala-C18) was prepared for the first time. The media were packed into a ...150 × 2.1 mm HPLC column, and the newly designed column was evaluated with the Tanaka and Neue test protocols in reversed-phase liquid chromatography (RPLC) separation mode. Moreover, it was characterized by the Tanaka test protocol in hydrophilic interaction chromatography (HILIC) separation mode. The new phase was characterized by elemental analysis, attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and solid-state 13C cross-polarization magic angle spinning (CP/MAS) NMR spectroscopy at variable temperatures. The chromatographic evaluation involved very good separation of nonpolar shape-constrained isomers, polar and basic compounds in RPLC, and highly polar compounds in HILIC compared to the commercial reference columns. The Sil-Ala-C18 phase was able to separate the challenging β- and γ-isomers of tocopherol. The phase was also successfully applied for the separation of the isomers of tocopherol (vitamin E) and capsaicinoids from real samples of chili peppers (Capsicum spp.) in RPLC and ascorbic acid (vitamin C) in HILIC.
Column purchasing cost is an important issue for an analyst to analyze complex sample matrices. Here, we report the development of an amino acid (β-alanine)-derived stationary phase (Sil-Ala-C12) ...with strategic and effective interaction sites (amide and urea as embedded polar groups with C12 alkyl chain) able to separate various kinds of analytes. Owing to the balanced hydrophobicity and hydrophilicity of the phase, it showed exceptional separation abilities in both reversed-phase high-performance liquid chromatography (RP-HPLC) as a hydrophobic phase and hydrophilic interaction chromatography (HILIC) as a hydrophilic phase. Remarkably, the baseline separation was achieved for the challenging β- and γ-isomers of tocopherol. Usually, three columns such as pentafluorophenyl or C30, C18, and sulfobetaine HILIC are required for the analysis of vitamin E, capsaicinoids, and vitamin C in chili peppers (Capsicum spp.), respectively. However, only Sil-Ala-C12 was able to separate these analytes. A single column can serve 3-4 purposes, which suggests that Sil-Ala-C12 had the potential to reduce column purchasing costs.
•A new alternating copolymer-based stationary phase prepared using a facile technique.•High molecular shape selectivity was observed with the new phase.•Tocopherol isomers were baseline ...separated.•Interaction mechanism was also evaluated.
The synthesis of a new alternating copolymer-grafted silica phase is described for the separation of shape-constrained isomers of polycyclic aromatic hydrocarbons (PAHs) and tocopherols in reversed-phase high-performance liquid chromatography (RP-HPLC). Telomerization of the monomers (octadecyl acrylate and N-methylmaleimide) was carried out with a silane coupling agent; 3-mercaptopropyltrimethoxysilane (MPS), and the telomer (T) was grafted onto porous silica surface to prepare the alternating copolymer-grafted silica phase (Sil-alt-T). The new hybrid material was characterized by elemental analyses, diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, and solid-state 13C and 29Si cross-polarization magic-angle spinning (CP/MAS) NMR spectroscopy. The results of 13C CP/MAS NMR demonstrated that the alkyl chains of the grafted polymers in Sil-alt-T remained disordered, amorphous, and mobile represented by gauche conformational form. Separation abilities and molecular-shape selectivities of the prepared organic phase were evaluated by the separation of PAHs isomers and Standard Reference Material 869b, Column Selectivity Test Mixture for Liquid Chromatography, respectively and compared with commercially available octadecylsilylated silica (ODS) and C30 columns as well as previously reported alternating copolymer-based column. The effectiveness of this phase is also demonstrated by the separation of tocopherol isomers. Oriented functional groups along the polymer main chains and cavity formations are investigated to be the driving force for better separation with multiple-interactions with the solutes. One of the advantages of the Sil-alt-T phase to that of the previously reported phase is the synthesis of the telomer first and then immobilized onto silica surface. In this case, the telomer was characterized easily with simple spectroscopic techniques and the molecular mass and polydispersity index of the telomer were determined by size exclusion chromatography (SEC) before grafting onto silica surface. Moreover, both of the monomers were commercially available. Therefore, the technique of preparation was very facile and better separation was achieved with the Sil-alt-T phase compared to the ODS, C30 and other previously reported alternating copolymer-based columns.
For the first time, we synthesized multiple embedded polar groups (EPGs) containing linear C18 organic phases. The new materials were characterized by elemental analysis, IR spectroscopy, 1H NMR, ...diffuse reflectance infrared Fourier transform (DRIFT), solid-state 13C cross-polarization magic angle spinning (CP/MAS) NMR, suspended-state 1H NMR, and differential scanning calorimetry (DSC). 29Si CP/MAS NMR was carried out to investigate the degree of cross-linking of the silane and silane functionality of the modified silica. Solid-state 13C CP/MAS NMR and suspended-state 1H NMR spectroscopy indicated a higher alkyl chain order for the phase containing four EPGs than for the phase with three EPGs. To correlate the NMR results with temperature-dependent chromatographic studies, standard reference materials (SRM 869b and SRM 1647e), a column selectivity test mixture for liquid chromatography was employed. A single EPG containing the C18 phase was also prepared in a similar manner to be used as a reference column especially for the separation of basic and polar compounds in reversed-phase liquid chromatography (RPLC) and hydrophilic interaction liquid chromatography (HILIC), respectively. Detailed chromatographic characterization of the new phases was performed in terms of their surface coverage, hydrophobic selectivity, shape selectivity, hydrogen bonding capacity, and ion-exchange capacity at pH 2.7 and 7.6 for RPLC as well as their hydrophilicity, the selectivity for hydrophilic–hydrophobic substituents, the selectivity for the region and configurational differences in hydrophilic substituents, the evaluation of electrostatic interactions, and the evaluation of the acidic–basic nature for HILIC-mode separation. Furthermore, peak shapes for the basic analytes propranolol and amitriptyline were studied as a function of the number of EPGs on the C18 phases in the RPLC. The chromatographic performance of multiple EPGs containing C18 HILIC phases is illustrated by the separation of sulfa drugs, β-blockers, xanthines, nucleic acid bases, nucleosides, and water-soluble vitamins. Both of the phases showed the best performance for the separation of shape-constrained isomers, nonpolar, polar, and basic compounds in RPLC- and HILIC-mode separation of sulfa drugs, and other polar and basic analytes compared to the conventional alkyl phases with and without embedded polar groups and HILIC phases. Surprisingly, one phase would be able to serve the performance of three different types of phases with very high selectivity, and we named this phase the “smart phase”. Versatile applications with a single column will reduce the column purchasing cost for the analyst as well as achieve high separation, which is challenging with the commercially available columns.
•Chitosan-based nanofibers are promising materials for the treatment of wastewater.•Chitosan can be functionalized with various functionalities and therefore can adsorb different pollutants.•In ...nanofibers surface area increase leading to increase adsorption efficiency.•Crosslinking process gives the chitosan-based nanofiber much strength, which is favorable for an adsorbent.•Chitosan-based nanofibers could be transformed into various composites, membranes, films etc. facilitates for wastewater treatment.
Now-a-days water pollution is one of the most concerned problem as it is increasing due to industrialization and human activities. The applications of electrospun nanofibers are increasing day by day due to their special properties such as huge surface-to-volume ratio, possibilities of surface modification and conversion to small mesh mat or membrane etc. Chitosan-based electrospun nanofibers have more avenues due to their additional benefits of non-toxicity, biocompatibility, biodegradability and cost-effectiveness. Although chitosan-based nanofibers have been using in various biomedical fields and reviewed, applications as adsorbents for the removal of heavy metals, dyes and other pollutants are relatively new and not reviewed yet. In this regard, the present review explores various types of chitosan-based nanofibers prepared with pure chitosan, mixed with other materials, surface-modified chitosan, magnetic chitosan etc., their adsorption performance and applications for the removal of heavy metals, dyes, and miscellaneous pollutants.
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