Measurement and modelling of the intra-particle diffusion and b-term in reversed-phase liquid chromatography

Acid precipitation-hydrothermal synthesis of needle-like hydroxyapatite for protein adsorption from waste phosphogypsum

In order to promote the high-value utilization of waste phosphogypsum (PG), hydroxyapatite was directly synthesized from PG by acid precipitation-hydrothermal method (PGHAP), which was used for the adsorption of bovine serum albumin (BSA) and lysozyme (LYS). The synthesized PGHAP was characterized by XRD, SEM, FTIR and BET, and the effects of various factors on protein adsorption capacity were studied. The results showed that PGHAP exhibits a clear needle-like morphology, high crystallinity, and an average size of about 200 nm. The pH had the greatest effect on the adsorption of protein, and the highest adsorption capacity was obtained at pH 4.0. In addition, the adsorption mechanism of protein on PGHAP was explored by adsorption kinetics and adsorption isotherm. The adsorption of protein on PGHAP conforms to the Intra-particle diffusion model kinetic model, the maximum adsorption capacity of protein on PGHAP can reach 31 mg/g, which is comparable to other adsorbents in this field. In addition, the adsorption behaviour of PGHAP on protein is more appropriately described by Langmuir isotherm model, which indicates that the binding site with uniform energy on the surface of PGHAP realizes the monolayer adsorption of protein. The main adsorption mechanisms are ion exchange, co-precipitation, complexation reaction and so on. Therefore, the needle-like PGHAP synthesized from waste PG is a protein adsorbent with industrial application potential.

Organic-solvent ditch overlap in reversed-phase liquid chromatography: A molecular dynamics simulation study in cylindrical 6-12 nm-diameter pores

Mass transport through the mesopore space of a reversed-phase liquid chromatography (RPLC) column depends on the properties of the chromatographic interface, particularly on the extent of the organic-solvent ditch that favors the analyte surface diffusivity. Through molecular dynamics simulations in cylindrical RPLC mesopore models with pore diameters between 6 and 12 nm we systematically trace the evolution of organic-solvent ditch overlap due to spatial confinement in the mesopore space of RPLC columns for small-molecule separations. Each pore model of a silica-based, endcapped, C18-stationary phase is equilibrated with two mobile phases of comparable elution strength, namely 70/30 (v/v) water/acetonitrile and 60/40 (v/v) water/methanol, to consider the influence of the mobile-phase composition on the onset of organic-solvent ditch overlap. The simulations show that, as the pore diameter decreases from 9 to 6 nm, the bonded-phase density extends and compacts towards the pore center, which leads to increased accumulation of organic-solvent excess and thus enhanced organic-solvent diffusivity in the ditch. Because the acetonitrile ditch is more pronounced than the methanol ditch, acetonitrile ditch overlap sets in at less severe spatial confinement than methanol ditch overlap. The pore-averaged methanol and acetonitrile diffusivities are considerably raised by ditch overlap in the 6 nm-diameter pore, but also benefit from the ditch (without overlap) in the 7 to 12 nm-diameter pores, whereby local and pore-averaged effects are generally larger for acetonitrile than methanol.

Comprehensive analysis of the effective and intra-particle diffusion of weakly retained compounds in silica hydrophilic interaction liquid chromatography columns

A detailed analysis of intra-particle volumes and layer thicknesses and their effect on the diffusion of solutes in hydrophilic interaction liquid chromatography (HILIC) was made. Pycnometric measurements and the retention volume of deuterated mobile phase constituents (water and acetonitrile) were used to estimate the void volume inside the column, including not only the volume of the mobile phase but also part of the enriched water solvent acting as the stationary phase in HILIC. The mobile phase (hold-up) volume accessible to non-retained components was estimated using a homologous series approach. The joint analysis of the different approaches indicated the formation of enriched water layers on the hydrophobic silica mesopore walls with a thickness varying significantly with mobile phase composition. The maximal thickness of the enriched water layers, which corresponded to the minimum void volume accessible to unretained solutes, marked a transition in the retention behavior of the studied analytes. Discrepancies between deuterated solvent measurements and pycnometry were explained by the existence of an irreplaceable water layer adsorbed on the silica surface. Regarding the diffusion behavior in HILIC, peak parking experiments were used to interpret the influence of the acetonitrile content on the effective diffusion coefficient Deff. A systematic decrease in Deff and molecular diffusion Dm was observed with decreasing acetonitrile concentration, primarily attributed to variations in mobile phase viscosity. Notably, Deff/Dm remained nearly unaffected by variations in mobile phase composition. Finally, the effective medium theory was used to make a comprehensive analysis of Dpart/Dm to study the contribution to band broadening when the solute resides in the mesopores. The obtained data unveiled a curvature with a minimum corresponding to conditions of maximum water-layer thickness and retention. For the weakly retained compounds (k' < 0.5) the Dpart/Dm-values were found to be relatively high (order of 0.35-0.5), which directly reflects the high γsDs/Dm-values that were observed (order 0.35-7).

Detailed analysis of the effective and intra-particle diffusion coefficient of proteins at elevated pressure in columns packed with wide-pore core-shell particles

To determine the efficiency that can be obtained in a packed-bed liquid-chromatography column for a particular analyte, a correct determination of the molecular and effective diffusion coefficients (Dm and Deff) of the analyte is required. The latter is usually obtained via peak parking experiments wherein the flow is stopped. As a result, the column pressure rapidly dissipates and the measurement is essentially conducted at ambient pressure. This is problematic for analytes whose retention depends on pressure, such as proteins and potentially other large (dipolar) molecules. In that case, a conventional peak parking experiment is expected to lead to large errors in Deff. To obtain a better estimate ofDeff, the present study reports on the use of a set-up enabling peak parking measurements under pressurized conditions. This approach allowed us to report, for the first time, Deff for proteins at elevated pressure under retained conditions. First, Deff was determined at a (average) pressure of about 105 bar for a set of proteins with varying size, namely: bradykinin, insulin, lysozyme, β-lactoglobulin, and carbonic anhydrase in a column packed with 400 Å core-shell particles. The obtained data were then compared to those of several small analytes: acetophenone, propiophenone, benzophenone, valerophenone, and hexanophenone. A clear trend between Deff and analyte size was observed. The set-up was then used to determine Deff of bradykinin and lysozyme at variable (average) pressures ranging from 28 bar to 430 bar. These experiments showed a decrease in intra-particle and surface diffusion with pressure, which was larger for lysozyme than bradykinin. The data show that pressurized peak parking experiments are vital to correctly determine Deff when the analyte retention varies significantly with pressure.

Olive stone as an eco-friendly bio-adsorbent for elimination of methylene blue dye from industrial wastewater

Adsorbents synthesized by activation and nanoparticle surface modifications are expensive and might pose health and ecological risks. Therefore, the interest in raw waste biomass materials as adsorbents is growing. In batch studies, an inexpensive and effective adsorbent is developed from raw olive stone (OS) to remove methylene blue (MB) from an aqueous solution. The OS adsorbent is characterized using scanning electron microscopy (SEM), Fourier Transform Infra-Red (FTIR), and Brunauer-Emmett-Teller (BET) surface area. Four isotherms are used to fit equilibrium adsorption data, and four kinetic models are used to simulate kinetic adsorption behavior. The obtained BET surface area is 0.9 m2 g-1, and the SEM analysis reveals significant pores in the OS sample that might facilitate the uptake of heavy compounds. The Langmuir and Temkin isotherm models best represent the adsorbtion of MB on the OS, with a maximum monolayer adsorption capacity of 44.5 mg g-1. The best dye color removal efficiency by the OS is 93.65% from an aqueous solution of 20 ppm at the OS doses of 0.2 g for 90 min contact time. The OS adsorbent serves in five successive adsorption cycles after a simple filtration-washing-drying process, maintaining MB removal efficiency of 91, 85, 80, and 78% in cycles 2, 3, 4, and 5, respectively. The pseudo second-order model is the best model to represent the adsorption process dynamics. Indeed, the pseudo second-order and the Elovich models are the most appropriate kinetic models, according to the correlation coefficient (R2) values (1.0 and 0.935, respectively) derived from the four kinetic models. The parameters of the surface adsorption are also predicted based on the mass transfer models of intra-particle diffusion and Bangham and Burt. According to the thermodynamic analysis, dye adsorption by the OS is endothermic and spontaneous. As a result, the OS material offers an efficient adsorbent for MB removal from wastewater that is less expensive, more ecologically friendly, and economically viable.

Mobile-Phase Contributions to Organic-Solvent Excess Adsorption and Surface Diffusion in Reversed-Phase Liquid Chromatography

Fast transport of retained analytes in reversed-phase liquid chromatography occurs through surface diffusion in the organic-solvent (OS)-enriched interfacial "ditch" region between the hydrophobic stationary phase and the water (W)-OS mobile phase. Through molecular dynamics simulations that recover the OS excess adsorption isotherms of a typical C₁₈-stationary phase for methanol and acetonitrile, we explore the relation between OS properties, OS excess adsorption, and surface diffusion. The emerging molecular-level picture attributes the mobile-phase contribution to surface diffusion to the hydrogen-bond capability and the eluting power of the OS. The higher affinity of methanol for the formation of W-OS hydrogen bonds at the soft, hydrophobic surface presented by the bonded-phase (C₁₈) chains reduces the OS excess and the related viscosity drop in the ditch. The lower eluting power of methanol, however, translates to increased bonded-phase contacts for analytes, which can increase their mobility gain from surface diffusion above the gain observed with acetonitrile.

Essentials of LC Troubleshooting, VI: How Wide Should Those Peaks Be?

In this latest installment of our series on recurring challenges in liquid chromatography (LC), we turn our attention to peak widths that are wider than expected. Estimating the expected peak widths can help determine when a column and LC system are not working quite right, and to decide when troubleshooting should be initiated.

An approach to high throughput measurement of accurate retention data in liquid chromatography

Efforts to model and simulate various aspects of liquid chromatography (LC) separations (e.g., retention, selectivity, peak capacity, injection breakthrough) depend on experimental retention measurements to use as the basis for the models and simulations. Often these modeling and simulation efforts are limited by datasets that are too small because of the cost (time and money) associated with making the measurements. Other groups have demonstrated improvements in throughput of LC separations by focusing on "overhead" associated with the instrument itself - for example, between-analysis software processing time, and autosampler motions. In this paper we explore the possibility of using columns with small volumes (i.e., 5 mm x 2.1 mm i.d.) compared to conventional columns (e.g., 100 mm x 2.1 mm i.d.) that are typically used for retention measurements. We find that isocratic retention factors calculated for columns with these dimensions are different by about 20%; we attribute this difference - which we interpret as an error in measurements based on data from the 5 mm column - to extra-column volume associated with inlet and outlet frits. Since retention factor is a thermodynamic property of the mobile/stationary phase system under study, it should be independent of the dimensions of the column that is used for the measurement. We propose using ratios of retention factors (i.e., selectivities) to translate retention measurements between columns of different dimensions, so that measurements made using small columns can be used to make predictions for separations that involve conventional columns. We find that this approach reduces the difference in retention factors (5 mm compared to 100 mm columns) from an average of 18% to an average absolute difference of 1.7% (all errors less than 8%). This approach will significantly increase the rate at which high quality retention data can be collected to thousands of measurements per instrument per day, which in turn will likely have a profound impact on the quality of models and simulations that can be developed for many aspects of LC separations.

High-efficiency adsorption of Cd(II) and Co(II) by ethylenediaminetetraacetic dianhydride-modified orange peel as a novel synthesized adsorbent

The treatment of heavy metal (HM) wastewater is a critical and considerable challenge. Fruit peel-based HM adsorption is a promising way for the water pollution control and the reuse of agricultural waste. In this study, a novel adsorbent based on orange peel was synthesized for the first time by introducing abundant -COO groups with ethylenediaminetetraacetic dianhydride (EDTAD) to eliminate Cd(II) and Co(II) of sewage solution. The synthesized adsorbent displayed excellent adsorption capacity of 51.020 and 40.486 mg/g for Cd(II) and Co(II), respectively, and the adsorption equilibrium was achieved within 5 min, following the Langmuir isotherm model and the pseudo-second-order model. Surface characterization of adsorbents by scanning electron microscopy-energy dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy confirmed that ion exchange, complexation, and physical adsorption could occur during the adsorption process. The rapid and highly efficient adsorption performance suggests EDTAD-modified synthesized orange peel possesses great potential for HM removal from sewage systems.

Essentials of LC Troubleshooting, Part II: Misbehaving Retention Times

Several liquid chromatography (LC) troubleshooting topics never get old because there are some problems that persist in the practice of LC, even as instrument technology improves over time. There are many ways for things to go wrong in an LC system that ultimately manifest as deviations from expected retention times. Developing a short list of the likely causes of these deviations can help streamline our troubleshooting experience when retention-related problems occur.

Accuracy of retention model parameters obtained from retention data in liquid chromatography

In liquid chromatography (LC), it is often very useful to have an accurate model of the retention factor, k, over a wide range of isocratic elution conditions. In principle, the parameters of a retention model can be obtained by fitting either isocratic or gradient retention factor data. However, in spite of many of our own attempts to accurately predict isocratic k values using retention models trained with gradient retention data, this has not worked in our hands. In the present study we have used synthetic isocratic and gradient retention data for small molecules under reversed-phase LC conditions. This allows us to discover challenges associated with predicting isocratic k's without the confounding influences of experimental issues that are difficult to model or eliminate. The results indicate that it is not currently possible to consistently predict isocratic retention factors for small molecules with accuracies better than 10%, even when using synthetic gradient retention data. Two distinct challenges in fitting gradient retention data were identified: 1) a lack of 'uniqueness' in the parameters; and 2) an inability to find the global optimum fit in a complex fitting landscape. Working with experimental data where measurement noise is unavoidable will only make the accuracy worse. This article is protected by copyright. All rights reserved.

A Molten-Salt Pyrolysis Synthesis Strategy toward Sulfur-Functionalized Carbon for Elemental Mercury Removal from Coal-Combustion Flue Gas

The emission of mercury from coal combustion has caused consequential hazards to the ecosystem. The key challenge to abating the mercury emission is to explore highly efficient adsorbents. Herein, sulfur-functionalized carbon (S-C) was synthesized by using a molten-salt pyrolysis strategy and employed for the removal of elemental mercury from coal-combustion flue gas. An ideal pore structure, which was favorable for the internal diffusion of the Hg0 molecule in carbon, was obtained by using a SiO2 hard template and adjusting the HF etching time. The as-prepared S-C with an HF etching time of 10 h possessed a saturation Hg0 adsorption capacity of 89.90 mg·g−1, far exceeding that of the commercial sulfur-loaded activated carbons (S/C). The S-C can be applied at a wide temperature range of 25–125 °C, far exceeding that of commercial S/C. The influence of flue gas components, such as SO2, NO, and H2O, on the Hg0 adsorption performance of S-C was insignificant, indicating a good applicability in real-world applications. The mechanism of the Hg0 removal by S-C was proposed, i.e., the reduced components, including sulfur thiophene, sulfoxide, and C-S, displayed a high affinity toward Hg0, which could guarantee the stable immobilization of Hg0 as HgS in the adsorbent. Thus, the molten-salt pyrolysis strategy has a broad prospect in the application of one-pot carbonization and functionalization sulfur-containing organic precursors as efficient adsorbents for Hg0.

Detailed numerical analysis of the effect of radial column heterogeneities on peak parking experiments with slowly diffusing analytes

The origin of the peak skewness that can be observed when applying the deconvolution method to isolate the diffusion process from the flow processes for peak parking experiments conducted under conditions of slow radial equilibration and strong trans-column velocity gradients was investigated. Numerical simulations were carried out for a variety of trans-column velocity profiles and a broad range of experimental conditions and system parameters were investigated. Results show that, under the aforementioned conditions, the traditionally employed variance subtraction method displays a consistent error which follows the dynamics of the diffusive relaxation during both the peak parking and the flow steps. It is also found that, under the same conditions, the peak deconvolution method is bound to produce deconvoluted "parking-only" peaks that are strongly asymmetric, despite the perfectly symmetric nature of the pure diffusion process marking this parking step. It is shown that this asymmetry is acquired during the flow step following the parking stop. During this step, parked and non-parked peaks are deformed in different ways, despite being subjected to the same trans-column velocity profile. This different deformation cannot be filtered away with the deconvolution or the variance subtraction method, hence introducing an error. Solutions to alleviate the peak skewness and the variance error consist of parking the peak close to the inlet or the outlet or exiting the parked peak through the column inlet (flow reversal method). Under the considered conditions, these approaches could reduce the error on the measured effective diffusion coefficient up to 87%. Carrying out the variance subtraction or the deconvolution process with a peak that has also been parked for a substantially long parking time instead of using a "no-parking" peak as is customary done, is another option to counter the effect.

Highly effective remediation of high-arsenic wastewater using red mud through formation of AlAsO4@silicate precipitate

High-arsenic wastewater derived from the metallurgical industry of nonferrous minerals is one of the most dangerous arsenic (As) sources that usually follow the emission of massive hazardous arsenic-bearing wastes. Considering the properties of red mud (RM), we propose an alternative and environmentally friendly method for the efficient remediation of high-arsenic wastewater using RM through formation of AlAsO₄@silicate precipitate, aiming at ''zero-emission of hazardous solid waste''. The results show nearly 100% of arsenic could be stepwisely removed from high-arsenic wastewater and reduce the arsenic concentration from 6100 mg/L to 40 μg/L using RM at room temperature. The highest arsenic removal capacity of RM reaches 101.5 mg/g at a RM-to-wastewater ratio of 40 g/L due to the superior arsenic adsorption and the co-precipitation of arsenate and Al³⁺ to form insoluble aluminum arsenate. The silicate shell of arsenic-loaded RM created at an alkaline condition acts as an arsenic stabilizer, resulting in a leached arsenic concentration of 1.2 mg/L in TCLP tests. RM acts as a highly effective arsenic remover and stabilizer for the disposal of high-arsenic wastewater. It shows great potential for the remediation of wastewater containing heavy metals with varying concentrations to produce clean water available for industrial purpose.