The riboflavin—dihydroriboflavin system in acidic medium on mercury: Part I. A chronocoulometric investigation

The riboflavin (RF)—dihydroriboflavin (DRF) system in 0.01 M HClO₄ + 0.09 M NaClO₄ has been studied on mercury by the single-step chronocoulometric technique. At ?0.040 V/SCE, where RF is still electro-inactive, this substance is adsorbed according to a Langmuir isotherm with an adsorption coefficient K_O = 5.2 × 10⁶ 1 mol^?1, giving rise to a single adsorbed monolayer. At potentials along the plateau of the RF polarographic adsorption prewave, as well as at more negative potentials, DRF is adsorbed with formation of two overlapping monolayers. This behaviour denotes strong attractive vertical interactions between overlapping adsorbed DRF molecules. The progressive shift in the chronocoulometric Q vs. E curve for electro-oxidation of adsorbed DRF towards more positive potentials with an increase in the surface concentration of DRF confirms the strength of these vertical interactions. The simultaneous presence, with formation of a charge-transfer complex, of adsorbed RF and DRF molecules along the plateau of the polarographic RF prewave, as postulated by Tedoradze and co-workers [21,22] is excluded.     

Characteristics of granular boehmite and its ability to adsorb phosphate from aqueous solution

In this study, we investigated the surface properties of granulated boehmite with vinyl acetate (G-BE20) and measured the amount of phosphate it adsorbed and the effect of contact time and solution pH on the adsorption process. The specific surface area (144.9?m²/g) and the number of surface hydroxyl groups (0.88?mmol/g) of G-BE20 were smaller than those of virgin boehmite (BE), which gave a specific surface area and number of surface hydroxyl groups of 297.0?m²/g and 1.08?mmol/g, respectively. The amount of phosphate adsorbed increased with the temperature. The isotherm model of Langmuir was used to fit experimental adsorption equilibrium data for phosphate adsorption onto G-BE20. The calculated thermodynamic parameters show the spontaneous and endothermic nature of the adsorption process. The equilibrium adsorption onto G-BE20 was reached within 16?h and the amount of phosphate adsorbed was 8.4?mg/g. The kinetic mechanism of phosphate uptake was evaluated with two different models: the Largergren pseudo first- and pseudo second-order models. The data obtained showed a better fit to the pseudo second-order model (0.991) than to the pseudo first-order model (0.967), as indicated by the r values. The rate constants for the adsorption of phosphate onto G-BE20 were calculated as 0.481?1/h and 0.029?g/mg?h. The adsorption of phosphate onto G-BE20 was the maximum in the pH range 3.0-4.0.     

Improved adsorption and desorption behavior of Cd on thiol-modified bentonite grafted with cysteamine hydrochloride

Clay adsorbents are considered an inexpensive and readily available solution for removing heavy metals, including cadmium, from the environment to reduce pollution. In this study, thiol-modified bentonite (SH-Bent) was prepared by grafting cysteamine hydrochloride onto natural bentonite (Bent). The effects of pH, equilibrium contact time, and temperature on the adsorption–desorption behavior of Cd²⁺ were studied, and adsorption isotherm models were applied to examine the adsorption behavior of SH-bent. SH-Bent demonstrated better performance and stability for Cd²⁺ adsorption than Bent. SH-Bent exhibited an enhanced adsorption capacity for Cd²⁺ at equilibrium of 49.3 mg/g at pH 6, 120 min, and 303 K, which was 42-fold higher than that of Bent under the same conditions. An investigation of the desorption behavior of Cd²⁺ adsorbed on Bent and SH-Bent in simulated acid rain revealed that SH-Bent has high stability, with a desorption rate of 5.73% at pH 4.5, 60 min, and 303 K, which was much lower than that demonstrated by Bent under the same conditions (45.68%). The Langmuir equation was the best-fitted adsorption isotherm model, closely followed by the Freundlich, Tempkin, and Dubinin–Radushkevich models. A significant difference in diffusion was observed between the two types of clay according to the intraparticle diffusion model. The adsorption–desorption processes of SH-Bent and Bent fit the pseudo-second-order model best among the five kinetic models examined. The information provided in this study can be used to apply thiol-modified clay for wastewater treatment or for the removal of cadmium from soil.

Graphical abstract

     

Admittance measurements of adsorption properties of 2,2′-bipyridine complexes: Part I. Cobalt(II) and nickel(II) complexes in aqueous solutions

The complex [Co bipy₃]²⁺ in aqueous perchlorate solution yields on the d.c. polarogramthree adsorption type pre-waves and a main reduction wave at ?1.2 V (SCE) corresponding to the reduction to monovalent complex. From admittance measurements and other results it can be concluded that two most positive pre-waves at ?0.2 V and ?0.5 V (SCE) are of non-faradaic nature being related to the adsorption of the complex with specifically adsorbed anions of the supporting electrolyte with the formation of a compact adsorbed layer of much lower interfacial capacity. The most negative pre-wave at ?0.9 V just preceding the main wave corresponds to the reduction of the adsorbed complex. The main wave at ?1.2 V is distorted by the adsorption of reaction products. Very similar and also strong adsorption-coupled interaction was found with divalent nickel trisbipyridine complex.     

Enhanced adsorption of roxarsone onto humic acid modified goethite from aqueous solution

The adsorption of roxarsone (ROX) on the humic acid modified goethite (HA-α-FeOOH) was evaluated for several potential environmental factors. Results showed that 1) ROX had a higher adsorption capacity onto HA-α-FeOOH than unmodified α-FeOOH; 2) the adsorption of ROX increased with a decrease in pH; 3) the high ionic strength significantly inhibited the adsorption capacity of HA-α-FeOOH; and 4) a higher temperature yielded greater adsorption, since the process for ROX to be adsorbed by HA-α-FeOOH was a spontaneous endothermic reaction. The maximum adsorption capacity of ROX was found to be 80.71?mg?·?g^?1, when the temperature was 308?K. Meanwhile, the inhibitory effects of ionic strength and PO₄^3? on the adsorption of ROX onto HA-α-FeOOH were enhanced with an increase in concentration. In addition, the adsorption equilibrium data obeyed Langmuir isotherm model and the kinetic data were well described by the pseudo-second-order kinetic model. From the infrared spectra of HA-α-FeOOH, it could be deduced that the ROX adsorption onto HA-α-FeOOH was achieved via the ion exchange between the arsenic acid and the carboxyl group on adsorbent, as well as the formation of As-O-Fe bond between Fe-O and arsenic acid ions.     

Adsorption of light green and brilliant yellow anionic dyes using amino functionalized magnetic silica (Fe3O4@SiO2@NH2) nanocomposite

Abstract

Fe₃O₄@SiO₂@NH₂ nanocomposite was prepared for highly effective adsorption of two anionic dyes one of which is triarylmethane dye (light green, LG) and the other is azo dye (brilliant yellow, BY). The characterization results demonstrated that superparamagnetic Fe₃O₄ nanoparticles were covered with silica and functionalized with amino groups successfully without losing magnetic character. The effects of adsorbent dosage, contact time, pH, temperature, and dye molecular structure on the adsorption were investigated. Acidic pH was better for both LG and BY, on the other hand, alkaline pH was favorable to some extent for LG in comparison with BY due to the contribution of stacking effect in addition to electrostatic attraction. Kinetic data demonstrated that the driving force for adsorption process could be explained by pseudo-second order mechanism in both systems. The equilibrium data were more compatible with Langmuir isotherm than those of Freundlich isotherm and the maximum adsorption capacities of Fe₃O₄@SiO₂@NH₂ calculated from Langmuir isotherm model for LG and BY at 30?°C and natural pH of the solution were 40.2 and 35.5?mg g^?1. Thermodynamic calculations related to temperature dependence demonstrated that the adsorption process was spontaneous and exothermic.     

Removal of thorium from aqueous solution by ordered mesoporous carbon CMK-3

The ability of ordered mesoporous carbon CMK-3 has been explored for the removal and recovery of thorium from aqueous solutions. The textural properties of CMK-3 were characterized using small-angle X-ray diffraction and N₂ adsorption–desorption, and the BET specific surface area, pore volume and the pore size were 1143.7 m²/g, 1.10 cm³/g and 3.4 nm. The influences of different experimental parameters such as solution pH, initial concentration, contact time, ionic strength and temperature on adsorption were investigated. The CMK-3 showed the highest thorium sorption capacity at initial pH of 3.0 and contact time of 175 min. Adsorption kinetics was better described by the pseudo-second-order model and adsorption process could be well defined by the Langmuir isotherm. The thermodynamic parameters, △G°(298 K), △H° and △S° were determined to be -0.74 kJ·mol^?1, 9.17 kJ·mol^?1 and 33.24 J·mol^?1·K^?1, respectively, which demonstrated the sorption process of CMK-3 towards Th(IV) was feasible, spontaneous and endothermic in nature. The adsorbed CMK-3 could be effectively regenerated by 0.02 mol/L HCl solution for the removal and recovery of Th(IV).     

Adsorption of di-2-pyridyl ketone salicyloylhydrazone on silica gel: characteristics and isotherms

The adsorption of DPKSH onto silica gel was investigated, at 25±1 °C and pH 1, 4.7 and 12. For the same DPKSH concentration interval, the minimum required time of contact for adsorption maximum at pH 4.7 was smaller than at pH 1 and the maximum amount of DPKSH adsorbed per gram of silica at pH 1 is smaller than at pH 4.7. At pH 12 the DPKSH adsorption onto silica gel was not significant. The adsorption data followed Langmuir, Freundlich and Dubinin-Radushkevich (D-R) isotherms. The maximum amount of solute adsorbed (m_ads^max) and the adsorption constant, K_L, were derived from Langmuir isotherm. The Freundlich constants 1/n and K_F related, respectively, to the energetic heterogeneity of adsorption sites and an empirical constant were evaluated. The mean sorption free energy (E) of DPKSH adsorption onto silica gel was calculated from D-R isotherm indicating a physical adsorption mode. Finally, conductimetric titrations showed the silica particle basicity and acidity as 0.002 and 0.3 mmol g⁻¹, respectively.     

Influence of solution pH on stability of aluminum oxide suspension in presence of polyacrylic acid

The effect of solution pH and molecular weight of polyacrylic acid (PAA) on its adsorption as well as on stabilization-floculation properties of the colloidal Al₂O₃ and electrolyte solution systems was studied. The measurements showed that at pH = 6, the presence of the polymer of molecular weight 2?000 and 240?000 does not change stability of Al₂O₃ suspension. However at pH =3 and 9 the effect of polyacrylic acid is significant. At pH = 3 it creates destabilization of the suspension while at pH = 9 PAA it improves significantly the stability of Al₂O₃. It was shown that the increase in solution pH affects conformation of adsorbed macromolecules which causes the decrease in PAA adsorbed amount and thickness of polymer adsorption layer. By comparing the values of diffusion layer and surface charges, main effects responsible for the decrease in surface charge and ζ potential of the solid in the presence of the polymer as well as suspension stability were determined.     

Sorption study of uranium from aqueous solution on ordered mesoporous carbon CMK-3

The ability of ordered mesoporous carbon CMK-3 has been explored for the removal and recovery of uraium from aqueous solutions. The textural properties of CMK-3 were characterized using small-angle X-ray diffraction and N₂ adsorption–desorption, and the BET specific surface area, pore volume and the pore size were 1143.7 m²/g, 1.10 cm³/g and 3.4 nm. The influences of different experimental parameters such as solution pH, initial concentration, contact time, ionic strength and temperature on adsorption were investigated. The CMK-3 showed the highest uranium sorption capacity at initial pH of 6.0 and contact time of 35 min. Adsorption kinetics was better described by the pseudo-second-order model and adsorption process could be well defined by the Langmuir and Freundlich isotherm. The thermodynamic parameters, ?G°(298 K), ?H° and ?S° were determined to be ?7.7, 21.5 k J mol^?1 and 98.2 J mol^?1 K^?1, respectively, which demonstrated the sorption process of CMK-3 towards U(VI) was feasible, spontaneous and endothermic in nature. The adsorbed CMK-3 could be effectively regenerated by 0.05 mol/L HCl solution for the removal and recovery of U(VI). Complete removal (99.9 %) of U(VI) from 1.0 L industry wastewater containing 15.0 mg U(VI) ions was possible with 2.0 g CMK-3.     

Adsorption and emission enhancement behavior of 4,4′-bipyridine on dispersed montmorillonite nano-sheets under aqueous conditions

The adsorption and photochemical behavior of 4,4′-bipyridine (BPy) on an exfoliated single montmorillonite nano-sheet (MtNS) was investigated in water. Under the appropriate conditions, BPy was adsorbed on the MtNS as a monomer state. BPy exhibited a relatively large red shift by 30?nm in the absorption spectrum, and showed significant emission enhancement in the fluorescence spectrum, contrary to the non-fluorescent property of BPy in water without MtNS. From the investigation using protonated BPy (BPyH₂²⁺) under acidic conditions, it was turned out that BPy was adsorbed as BPyH₂²⁺ on MtNS even under neutral conditions. From the analysis based on the Langmuir adsorption isotherm, the saturated adsorption amount of BPy on MtNS was calculated to be 3.2?×?10^?2 molecule nm^?2.     

Effect of halides and alkali cations on the Zn(Hg)/Zn(II) electrode reactions

The title subject has been studied by galvanostatic single-pulse, chronopotentiometric and equilibrium measurements on the Zn(Hg)/Zn(II) electrode in x M KI+(1?x) M KCl (x from 0 to 1), 1 M KBr and 1 M MeCl (Me=Li, Na, K and Cs) solutions of pH 3 at 25°C. Quantitative information about the effect of specifically adsorbed halides on the rates of the Zn(II)/Zn(I) and the Zn(I)/Zn(Hg) steps is obtained separately (for the latter step mainly at potentials near ?1.0 V(SCE)), and the latter step seems to be more influenced than the former by the adsorption. An attempt is made to correlate the adsorption effect on the rate of the Zn(II)/Zn(I) step to double-layer parameters according to recent models for such effects. The extra current observed at potentials where the halides are adsorbed, seems to vary with the surface activity of the specifically adsorbed ion. The lack of any observed kinetic effect of Cs⁺, which is specifically adsorbed at these potentials, is possibly due to the Cs⁺ specific adsorption enhancing the Cl^? specific adsorption and vice versa, so that the decelerating and accelerating effects by these ions may cancel each other.     

Removal and recovery of uranium from aqueous solution by tea waste

Experiments on the removal and recovery of U(VI) from aqueous solution by tea waste were conducted. The adsorbent was characterized by scanning electron microscope and energy dispersive spectrometer before and after the adsorption treatment. The removal of U(VI) amounts to 86.80?% at optimum pH 6. The adsorption process reaches its equilibrium in 12?h at 308?K, and the kinetic characteristic can be described by the pseudo-second-order kinetic equation. The amount of adsorption increases from 22.92 to 142.21?mg?g^?1 with the decrease of tea waste dosage from 100 to 10?mg for solution with an initial uranium concentration of 50?mg?L^?1. Desorption for the four strippants is higher than 80?%. The equilibrium data are more agreeable with Freundlich isotherm than Langmuir isotherm.     

Determination of Copper by Adsorptive Stripping Voltammetry in the Presence of Calcein Blue

An electrochemical adsorptive stripping approach is presented for the trace measurement of copper in some real samples. The method is based on the reduction of Cu²⁺ at pH 5.5 calcein blue (CB) containing solution at ?250 mV (vs. Ag/AgCl), adsorption of Cu? CB complex on hanging mercury drop electrode (HMDE) and the voltammetric determination by further reduction to Cu⁺ at HMDE. Experimental optimum conditions were determined in the fundamental studies. At the experimental optimum conditions the adsorbed complex of Cu²⁺ and calcein blue gives a well defined cathodic stripping peak current at ?0.135 V, which has been used for the determination of copper in the concentration range of 0.02 to 15 ng/mL with accumulation time of 90 s. The relative standard deviation (RSD) for the determination of 0.5 and 6.0 ng mL^?1 were 2.60 and 1.94% respectively. (n=10). The method has been applied to the analysis of copper in analytical reagent grade salts and tap water, mineral water and drug samples with satisfactory results.     

Phenol removal from aqueous solution by carbon xerogel

Carbon xerogel (CX) was used for phenol adsorption from aqueous solution. CX was synthesized by sol?Cgel polycondensation of resorcinol with formaldehyde using sodium carbonate (Na₂CO₃) as catalyst. Then, it was dried by convective drying technique and pyrolyzed under inert atmosphere. Phenol adsorption kinetics was very fast, what was attributed to the presence of open pore structure. The kinetic studies showed that the adsorption process could be fitted to a pseudo-second-order model and the particle diffusion process is the rate-limiting step of the adsorption. The phenol removal was maximum and unaffected by pH changes when the initial pH of the phenol solution was in the range of 3?C8. The optimum adsorbent dose obtained for phenol adsorption onto CX was 0.075?g/50?cm³ solution. The Langmuir model described the adsorption process better than the Freundlich isotherm model and the monolayer adsorption capacity is 32?mg?g^?1. Among the desorbing solutions used in this study, the most efficient desorbent was EtOH (100?%) which released about 87?% of phenol bound with the CX.     

The inhibitory effect of bipyridine and its cobalt complex on the corrosion behaviour of carbon steel in acidic medium

The bipyridine (bipy) and its cobalt complex (Co-bipy) were tested as corrosion inhibitors for N80 carbon steel in 0.1 M H₂SO₄ solution by electrochemical polarization and electrochemical impedance spectroscopy (EIS) method. Scanning electron microscopy (SEM) techniques were used to characterize the mild steel surface. The test results showed that the complex and ligand are mixed-type inhibitors and the compounds are adsorbed on the steel surface according to Temkin adsorption isotherm. The inhibition efficiency of the inhibitors follows the trend Co-bipy > bipy. The adsorption of the inhibitors can be classifies as physical adsorption.     

Adsorption kinetic,thermodynamic and desorption studies of phosphate onto hydrous niobium oxide prepared by reverse microemulsion method

A type of Nb₂O₅⋅3H₂O was synthesized and its phosphate removal potential was investigated in this study. The kinetic study, adsorption isotherm, pH effect, thermodynamic study and desorption were examined in batch experiments. The kinetic process was described by a pseudo-second-order rate model very well. The phosphate adsorption tended to increase with a decrease of pH. The adsorption data fitted well to the Langmuir model with which the maximum P adsorption capacity was estimated to be 18.36 mg-P g⁻¹. The peak appearing at 1050 cm⁻¹ in IR spectra after adsorption was attributed to the bending vibration of adsorbed phosphate. The positive values of both ΔH° and ΔS° suggest an endothermic reaction and increase in randomness at the solid-liquid interface during the adsorption. ΔG° values obtained were negative indicating a spontaneous adsorption process. A phosphate desorbability of approximately 68% was observed with water at pH 12, which indicated a relatively strong bonding between the adsorbed phosphate and the sorptive sites on the surface of the adsorbent. The immobilization of phosphate probably occurs by the mechanisms of ion exchange and physicochemical attraction. Due to its high adsorption capacity, this type of hydrous niobium oxide has the potential for application to control phosphorus pollution.     

Adsorption of thorium from aqueous solution by HDTMA+-pillared bentonite

The ability of hexadecyltrimethylammonium cation pillared bentonite (HDTMA⁺-bentonite) has been explored for the removal and recovery of thorium from aqueous solutions. The adsorbent was characterized using small-angle X-ray diffraction, high resolution transmission electron microscopy and Fourier transform infrared spectroscopy. The influences of different experimental parameters such as solution pH, initial thorium concentration, contact time and temperature on adsorption were investigated. The HDTMA⁺-bentonite showed the highest thorium sorption capacity at initial pH of 3.5 and contact time of 60?min. Adsorption kinetics was better described by the pseudo-second-order model and adsorption process could be well defined by the Langmuir isotherm. The thermodynamic parameters, ?G° (298?K), ?H° and ?S° were determined to be ?31.78, ?23.71?kJ/mol and 27.10?J/mol?K, respectively, which demonstrated the sorption process of HDTMA⁺-bentonite towards Th(VI) was feasible, spontaneous and exothermic in nature. The adsorption on HDTMA⁺-bentonite was more favor than Na-bentonite, in addition the saturated monolayer sorption capacity increased from 17.88 to 31.20?mg/g at 298?K after HDTMA⁺ pillaring. The adsorbed HDTMA⁺-bentonite could be effectively regenerated by 0.1?mol/L HCl solution for the removal and recovery of Th(VI). Complete removal (99.9?%) of Th(VI) from 1.0?L industry wastewater containing 16.8?mg Th(VI) ions was possible with 7.0?g HDTMA⁺-bentonite.     

Estimating of surface activity of Cl− and Br− ions in the (In-Ga)/[N-methylformamide + mc KCl + (1 − m)c KClO4] and (In-Ga)/[N-methylformamide + mc KBr + (1 − m)c KClO4] systems by different methods

Differential capacitance curves in the (In-Ga)/[N-methylformamide + mc KCl + (1 ? m)c KClO₄] and (In-Ga)/[N-methylformamide + mc KBr + (1 ? m)c KClO₄] systems are measured using an ac bridge for the following molar portions m of the surface-active anion: 0, 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, and 1. The Cl^? and Br^? anions specific adsorption in the systems can be described quantitatively by the Frumkin isotherm. The principal parameters of Cl^? and Br^? anions adsorption at the (In-Ga)/N-methylformamide interface are determined by different methods. Unlike Ga/N-methylformamide interface, where the adsorption energy increased in the sequence I^? ≈ Br^? < Cl^?, at the (In-Ga)/N-methylformamide interface it increased in the reverse sequence: Cl^? < Br^? < I^?. The adsorption parameters at the charge density q = 0, obtained by three different methods, are close to each other. However, the parameters α₁ and α₂, which characterize the charge effect on the adsorption energy, when determined by the analyzing of dependences of adsorption potential drop E _ads on ln(mc), differ from those determined by two other methods. The error may be caused by the assuming that the adsorption potential drop is proportional to the coverage of dense layer with the specifically adsorbed ions.     

William B. Guenther,Chemical Equilibrium. A Practical Introduction for the Physical and Life Sciences,Plenum Press,New York-London (1975), 248 pp., price U.S. $ 23.40

The adsorption and related interfacial behavior of uracil at a mercury electrode/electrolyte solution interface has been studied by differential capacitance and maximum bubble pressure methods in 0.5 M NaF plus 0.01 M Na₂HPO₄ buffer pH 8.0. At concentrations below 24 mM uracil is adsorbed in a flat orientation on the electrode surface and occupies an area of 63 Ų. At higher concentrations and at potentials close to ?0.5 V the adsorbed uracil undergoes a reorientation and adopts a perpendicular stance on the electrode surface where it occupies an area of 39 Ų. In this perpendicular stance uracil undergoes a strong intermolecular stacking interaction with its neighbors similar to that observed between adjacent pyrimidines in nucleic acids.