One-pot synthesis of siliceous ferrihydrite - coated halloysite nanorods in alkaline medium: Structure, properties and cadmium adsorption performance

The effects of formation modes of ferrihydrite-low molecular weight organic matter composites on the adsorption of Cd(II)

The interactions between iron oxides and organic matter (OM) play vital roles in the geochemical cycle of cadmium (Cd). However, the effects of the formation modes of ferrihydrite (Fh)-low molecular weight OM (e.g., fulvic acid (FA)) composites on Cd(II) adsorption remain poorly understood. The immobilization mechanisms of Cd(II) on Fe-OM composites formed by adsorption and coprecipitation at varying C/Fe molar ratios were investigated by means of adsorption batch experiments, two-dimensional correlation spectroscopy, and surface complexation models (SCMs). The composites formed by adsorption or coprecipitation exhibited a crystal structure similar to that of Fh. Ligand exchange and hydrogen bonding were identified as the primary mechanisms between components in adsorption composites and coprecipitates, respectively. Compared to coprecipitates, the adsorption composites showed a higher adsorption capacity and formed ternary complexes (Fh-FA-Cd). In coprecipitates, Cd(II) primarily interacted with the carboxyl and hydroxyl groups of FA and the hydroxyl groups of Fh. With increasing C/Fe molar ratios, the FA functional group (R-COOH) in adsorption composites responded more quickly to Cd(II). However, the order of functional group reactions in coprecipitates was unaffected by C/Fe molar ratios, which is due to the irregular distribution of C and Fe elements. SCM calculation results indicated that Cd(II) distribution on Fh in adsorption composites was higher than that in coprecipitates. The molar ratios of C/Fe and Cd(II) concentrations influenced the distribution of Cd(II) on the composites, with the highest proportion of Cd(II) on Fh reaching about 70%. These findings contribute to understanding Cd behavior in environments with periodically fluctuating redox conditions.

Experimental and advanced equilibrium studies on the enhanced adsorption of phosphate, cadmium, and safranin dye pollutants using methoxy exfoliated glauconite

Natural glauconite, as a mixed-layered clay mineral, was subjected to exfoliation processes, producing silicate monolayers or individual sheets that were further modified with methanol into methoxy exfoliated glauconite (Mth/EXG). The structure was assessed as an enhanced adsorbent for three types of common water contaminants, including phosphate (PO4 3-), safranin-O dye (SFR), and cadmium metal ions (Cd2+). The Mth/EXG structure achieved promising adsorption capacities at the saturation points equal to 269.9 mg/g for PO4 3-, 312 mg/g for SFR, and 234.5 mg/g for Cd2+ which are significantly better than the reported values for several studied adsorbents of higher costs and complex production procedures. The adsorption processes and the predicted regulated mechanisms in terms of the adsorbate/adsorbent interface were illustrated based on the steric and energetic findings that correspond to the applied monolayer equilibrium model of one energy site. The structure displays active site densities of 82.5 mg/g (PO4 3-), 136.3 mg/g (SFR), and 83.4 mg/g (Cd2+), which illustrate the high uptake performance of SFR. Also, the steric parameters reflected the suitability of each existing site to be filled with 4 ions of PO4 3-, SFR, and Cd2+. The adsorption energy (less than 40 kJ/mol) in conjunction with free adsorption energy from D-R model (8-16 kJ/mol) and steric parameters validate the dominant impact of the multi-ionic physical mechanisms (hydrogen bonding and van der Waals forces), in addition to the assistant impact of some weak chemical processes that might be assigned to the formed inner-sphere complex. Also, these reactions all occurred spontaneously with exothermic behaviors according to the thermodynamic functions. Additionally, the structure exhibit significant affinity for the studied pollutants even in the existing of completive chemical including anions, cations and organic molecules.

Investigation of adsorption potential of acid violet 90 dye with chitosan/halloysite/boron nitride composite materials

In this study, composite adsorbents consisting of a mixture of chitosan (CTS), boron nitride (h-BN) and halloysite (HNT) were used for the adsorption of Acid Violet 90 (AV90) dye in a batch system. Adsorbents CTS, CTS/HNT, CTS/h-BN and CTS/h-BN/HNT beads were prepared by simple dropping method and dried in a freeze dryer. The beads were characterized by FT-IR, SEM and zeta potential analysis. The effects of pH (2-8) and dye concentration (50-250 mg/L) on AV90 adsorption properties of beads were investigated. In addition, Langmuir, Freunlich, Temkin and Henry adsorption isotherm models were used to examine the dye adsorption mechanism. It was observed that the Langmuir and Freundlich adsorption isotherm models were in good agreement with the experimental data. In the dye concentration range studied, the qm values of CTS, CTS/h-BN1, CTS/h-BN3, CTS/HNT/h-BN1, CTS/HNT/h-BN3, CTS/HNT obtained from the Langmuir isotherm model was 27.62, 17.80, 10.11, 8.71, 32.57, 19.96 mg/g, respectively. Pseudo-first order, pseudo-second order and intra-particle diffusion kinetic models were used to examine the adsorption kinetics of adsorbents. As a result, it is thought that the use of this study in the field of dye adsorption can be an innovative and important study.

The pH dependent surface charging and points of zero charge. X. Update

Surfaces are often characterized by their points of zero charge (PZC) and isoelectric points (IEP). Different authors use these terms for different quantities, which may be equal to the actual PZC under certain conditions. Several popular methods lead to results which are inappropriately termed PZC. This present review is limited to zero-points obtained in the presence of inert electrolytes (halides, nitrates, and perchlorates of the 1st group metals). IEP are reported for all kinds of materials. PZC of metal oxides obtained as common intersection points of potentiometric curves for 3 or more ionic strengths (or by means of equivalent methods) are also reported, while the apparent PZC obtained by mass titration, pH-drift method, etc. are deliberately neglected. The results published in the recent publications and older results overlooked in the previous compilations by the same author are reported. The PZC/IEP are accompanied by information on the temperature and on the nature and concentration of supporting electrolyte (if available). The references to previous reviews by the same author allow to compare the newest results with the PZC/IEP of similar materials from the older literature.

Morphology and Selected Properties of Modified Potato Thermoplastic Starch

Potato thermoplastic starch (TPS) containing 1 wt.% of pure halloysite (HNT), glycerol-modified halloysite (G-HNT) or polyester plasticizer-modified halloysite (PP-HNT) was prepared by melt-extrusion. Halloysites were characterized by FTIR, SEM, TGA, and DSC. Interactions between TPS and halloysites were studied by FTIR, SEM, and DMTA. The Vicat softening temperature, tensile, and flexural properties were also determined. FTIR proved the interactions between halloysite and the organic compound as well as between starch, plasticizers and halloysites. Pure HNT had the best thermal stability, but PP-HNT showed better thermal stability than G-HNT. The addition of HNT and G-HNT improved the TPS’s thermal stability, as evidenced by significantly higher T5%. Modified TPS showed higher a Vicat softening point, suggesting better hot water resistance. Halloysite improved TPS stiffness due to higher storage modulus. However, TPS/PP-HNT had the lowest stiffness, and TPS/HNT the highest. Halloysite increased Tα and lowered Tβ due to its simultaneous reinforcing and plasticizing effect. TPS/HNT showed an additional β-relaxation peak, suggesting the formation of a new crystalline phase. The mechanical properties of TPS were also improved in the presence of both pure and modified halloysites.