Hydroxide-Source-Dependent Polymorphism and Phase Stability of Cobalt(II) Hydroxides in Diffusion-Driven Systems.

Hydroxides of cobalt(II) exist predominantly in two polymorphic forms, namely, the blue-green α-form [α-Co(OH)2] and reddish ß-form [ß-Co(OH)2]. These hydroxides have a layered structure with interlayer galleries of around 7 and 4 Å, respectively, for α- and ß-Co(OH)2. In most of the previous studies, both the polymorphs were synthesized separately, and a few of them showed that the α-form gets converted to a thermodynamically more stable ß-form via physical processes. In the present work, we have optimized the conditions for the simultaneous synthesis of both polymorphs under identical conditions in the same reactor using the 1D reaction-diffusion framework by employing different outer electrolytes. We found that the polymorph chemistry of Co(OH)2 depends on the source and concentration of OH- rather than other reaction conditions or later physical transformation. The products are characterized to confirm their morphology, structure, and chemical environment. We observed that the use of NaOH and NH4OH as the OH- precursor leads to α-Co(OH)2 only; however, with NaOH, a continuous precipitate is formed, and with NH4OH, periodic precipitation is formed. On the other hand, with hydrazine (HYZ) as the OH- source, Liesegang bands of α-Co(OH)2 and ß-Co(OH)2 as granules are formed throughout the diffusion reactor. Another intriguing observation on the HYZ system is that at its high concentration, the bands of α-Co(OH)2 get converted to ß-Co(OH)2. We articulate the reasons and mechanism of those observations.

Cu(II)-Metallized Three-Layered Cu-8HQ Complex with Hierarchical Crystallite Morphologies Synthesized via Reaction-Diffusion.

Self-organization of regular band patterns of the precipitate via a reaction-diffusion (RD) framework is called Liesegang phenomenon. This non-equilibrium system is emerging as an efficient method for synthesizing materials with unique morphologies that may have desired properties. The formation of continuous precipitation inside a band with poor control over the shape and size of sparingly soluble salts has been well documented. However, only a few reports on forming organic-inorganic bonds are available. In the present work, we demonstrate the formation of 2D frameworks of bis-(8-hydroxyquinoline) copper(II) in the agar gel via RD. The macroscopic particles were dumbbell-shaped, with aspect ratios ranging from 2.7 (inner bands) to 0.7 (outer bands). The particles were composed of ribbon-shaped crystallites at the microscopic level, each with three layers of parallelogram prismatic monoclinic sheets stacked over one another, which could easily be exfoliated. The powder X-ray diffraction patterns at low angles and the surface areas of the crystallites indicated the formation of metal-organic frameworks. It was observed that the sizes of the particles could be tuned by controlling the extent of diffusion using reactant concentrations. Since such heterostructures have energy storage capacity, the cyclic voltammograms of the unexfoliated and exfoliated materials showed that they fall in the pseudocapacitor category with potential application as the electrode material. The frameworks were further characterized by techniques such as optical and electron microscopy, X-ray diffraction, IR spectroscopy, and UV-visible spectrophotometry.

Precise Evaluation of Spatial Characteristics of Periodically Precipitating Systems via Measurement of RGB (Red, Green, and Blue) Values of Pattern Images.

In the present study, a method is described for precise determination of spatial characteristics of Liesegang bands formed by employing a classical 1D setup using a web-based free resource (https://www.ginifab.com/feeds/pms/color_picker_from_image.php). The method involves the compartmentalization of the information on each pixel into R (red), G (green), or B (blue) values from the pattern images obtained using a simple digital camera. The values can further be converted to absorbance values by using the system blank. Each trough (or peak) in the graph of RGB values (or absorbance values) corresponds to a band in the pattern. The method is employed to determine the spacing and width of the periodically precipitating AgCl, AgBr, and Co(OH)2 in an agar gel. It is observed that AgCl shows revert banding, and AgBr shows revert banding at the top of the tube and then diverges to regular banding at the bottom of the tube, whereas the Co(OH)2 patterns explicitly show regular banding under given experimental conditions. It is also observed that minute instabilities, such as the formation of secondary bands, can also be visualized by the present method.

Cr(VI)-Mediated Homogeneous Fenton Oxidation for Decolorization of Methylene Blue Dye: Sludge Free and Pertinent to a Wide pH Range.

Fe(II)-mediated Fenton process is commonly employed for oxidative degradation of recalcitrant pollutants in wastewater. However, the method suffers from limitations like narrow working pH range and iron sludge formation. The present work deals with the degradation of Methylene Blue (MB) dye using Fenton-like oxidation by replacing Fe(II) with Cr(VI), which eliminates the limitations of classical Fenton oxidation. The Fenton-like oxidation of MB is brought about by HO• radicals generated by the disproportionation of chromium-coordinated peroxo complexes. It was observed that the working pH range for the Cr(VI)-mediated Fenton oxidation was 3-10, and no sludge formation takes place up to four cycles as the oxidation remains in the pure solution phase. The complete mineralization of dye was confirmed by observing the decay of MB peaks by a spectrophotometer and cyclic voltammetry. The reaction parameters like pH of the solution, temperature, degradation time, concentrations of H2O2, Cr(VI), and MB were studied for optimal performance of the Cr(VI) as the catalyst. Kinetic studies revealed that the Cr(VI)-mediated Fenton reaction follows pseudo-first-order reaction kinetics and depends on the concentration of HO• radicals. The proposed Cr(VI)-mediated Fenton oxidation in the present work is best suited for the degradation of organic dyes by adding H2O2 as a precursor in chromate-contaminated wastewaters.

Valorization of Uncharred Dry Leaves of Ficus benjamina towards Cr (VI) removal from Water: Efficacy Influencing Factors and mechanism.

The potential of uncharred biomaterial derived from dry leaves of Ficusbenjamina (Family: Moraceae,local name: Weeping Fig) plant to remove Cr(VI) from aqueous samples was investigated. In the present work, treatment of dilute acids was used for activating the adsorption centres on the biomass instead of cumbersome charring process. The plant material was characterized using FT-IR, FE-SEM and EDX. Various influencing factors such as pH of equilibrating solution, contact time, Cr (VI) concentrations, adsorbent dose and temperature were optimized to obtain maximum sorption efficacy. The interactions among the biomaterial and Cr (VI) in water were studied by fitting the sorption data in four different adsorption isotherms. The data fitting and experimental evidences indicated formation of monolayer of Cr(VI) over the biomass surface. The process followed pseudo-second order kinetics and was thermodynamically spontaneous under laboratory conditions and reached equilibrium in 24 hours. Maximum adsorption capacity of 56.82 mg/g was obtained at the pH 2 when the concentration before adsorption was 200 mg L-1 of Cr(VI) with 24 hours of equilibration time and 2.50 g L-1 of dose of biomaterial at room temperature. The sorption efficiency was found to be better than many charred bio-based materials.

Revert Banding in One-Dimensional Periodic Precipitation of the (AgNO3 + KBr) System in Agar Gel.

A periodically precipitating system wherein interband distance successively decreases is known as revert Liesegang banding. The phenomenon is rare, and the underlying mechanism is implicit. In the present paper, the Liesegang system comprising of AgNO3 and KBr as the outer and inner electrolyte pair showing revert banding in agar gel by employing a 1D experimental setup is studied under varying concentrations of participating species. Revert banding was observed under all the experimental conditions. The concentrations of inner and outer electrolytes were found to play a major role in reverting since they build the ionic strength inside Liesegang tubes. We hypothesize that the band reverting is the interplay of van der Waals and electrical double-layer interactions, and hence classical DLVO (Derjaguin-Landau-Verwey-Overbeek) theory can be applied to interpret reverting. We propose that revert deposition of precipitates is the outcome of flocculation and peptization of sols, which is the manifestation of balancing attractive and repulsive interactions acting on colloidal particles responsible for band formation.

Evaluation of the antioxidant activity of wheatgrass (Triticum aestivum L.) as a function of growth under different conditions.

The antioxidant activity of wheatgrass, which is consumed as a dietary supplement, was estimated at different levels. The methods employed include FRAP (ferric reducing antioxidant power), ABTS (2,2'-azobis-3-ethylbenzthiazoline-6-sulfonic acid) and DPPH (1,1'-diphenyl-2-picrylhydrazyl) assays. Aqueous and ethanol extracts of wheatgrass grown under different conditions over a period of 6, 7, 8, 10 and 15 days were used. Lipid peroxidation and oxygen radical absorbance capacity (ORAC) were determined and utilized to check the potency of a few selected extracts. Different conditions used for growth were (1) tap water, (2) tap water with nutrients, (3) soil and tap water, and (4) soil with nutrients. For comparison, a commercially available wheatgrass tablet was analysed. To explain the reasons behind the observed differences, the total phenolic and flavonoid contents of the extracts were measured. These contents increased with growth under all the conditions. The ethanol extracts were found to have a higher phenolic and flavonoid content than the aqueous extracts. The highest FRAP values occurred on day 15 of growth under condition 4, the values being 0.463 and 0.573 mmol of ascorbic acid and Trolox equivalents/100 g fresh wheatgrass for aqueous and ethanol extracts, respectively. In the aqueous extracts no specific trend was observed with the DPPH assay for the different conditions nor for the growth period. In the case of ethanol extracts, however, it increased with the growth period and the wheatgrass grown in condition 4 was found to be the most effective. These extracts were also found to inhibit significantly ascorbate-Fe2+ induced lipid peroxidation in rat liver mitochondria. The ORAC values of aqueous and ethanol extracts of day 10 with condition 4 were found to be 39.9 and 48.2, respectively, being higher than those reported for many natural extracts or vegetables.