Electokinetic and adsorption properties of different titanium dioxides at the solid/solution interface

Six samples of titanium dioxide of different phase compositions and specific surface areas have been characterized by XRD, Raman-and FTIR spectroscopy, adsorption of nitrogen, electrophoresis. Adsorption of Zn(II) ions at the TiO₂/NaCl aqueous solution interface as well as the effect of adsorption on the structure of electrical double layer have been studied. The influence of ionic strength, pH and presence of ions on the adsorption of Zn(II) ions at the TiO₂/NaCl solution interface have also been investigated. The zeta potential, surface charge density, parameters of adsorption edge pH_50% and ΔpH_10–90% for different concentrations of basic electrolyte have been determined. Studied unpurified samples showed lower values of isoelectric point pH_iep compared with literature data due to the presence of anion impurities. The antibate dependence between pH_iep values and particle size has been established. Adsorption of Zn(II) ions using monophase samples is completed at a lower pH than for the biphase TiO₂. Appearance of the point CR3 is associated with the charge turnover from positive to negative at high values of pH and formation of Zn(OH)₂.      

Influence of the structure of boundary layers and the nature of counterions on the position of the isoelectric point of silica surfaces

Dependences of electrokinetic potentials of different silica materials (nano-and ultraporous glasses, a quartz glass plane-parallel capillary, and monodisperse spherical particles of silicon oxide) on the pH of solutions containing single-, double-, and triple-charged cations have been compared. It has been shown that the degree of hydration of a single-charged cation and the structure of an interface substantially affect the position of the isoelectric point (IEP). The most hydrated Na⁺ ions have virtually no effect on the position of the IEP up to their concentration of 0.1 M irrespective of the thickness of an ion-permeable layer at the surface of a solid phase. A reduction in the radius of a hydrated cation (K⁺, Cs⁺) enables its penetration into an ion-permeable layer and, as a consequence, causes the IEP to shift toward larger pH values depending on the parameters of this layer. Two IEPs are observed in LaCl₃ solutions: one at a pH value close to pH_IEP in NaCl solutions and another at a higher pH value corresponding to the charge reversal of the Stern layer.     

Specific adsorption of cesium on silica-titania

The amphoteric nature of hydrous silica-titania gel and its sorption behaviour towards cesium were studied. In NaNO₃ solution, the point of zero change (pH_pzc) of the gel was found to be 4.22 by pH-titration. The fraction of protonated, deprotonated and neutral surface hydroxyl groups as a function of pH have been computed. Sorption of cesium increased with the increase of pH, reached maximum at a pH of 7 followed by a plateau. Significant uptake of cesium was observed even when the pH was less than pH_pzc. Nearly 70% sorption was observed at pH_pzc. The free energy of specific adsorption was found to be –18.7 kJ·mol^–1.     

Studies of the Influence of Polyelectrolyte Adsorption on Some Properties of the Electrical Double Layer of ZrO2- Electrolyte Solution Interface

An influence of the molecular weight of the polymer and inorganic contaminations of zirconia on the adsorption and electrokinetic properties on ZrO₂-electrolyte solution interface was studied. Two polymers were used; polyacrylic acid (PAA) and polyacrylamide (PAM). On the basis of the obtained dependencies, main factors responsible for observed changes of zeta potential and surface charge of washed and contaminated ZrO₂ were determined. It was showed, that the change of ionic structure in the Stem layer depends on the number and arrangements of-COOH groups in PAA and PAM macromolecules. These groups are responsible for the conformation of polymer chains near the surface and have direct influence on the amount of the adsorbed polyelectrolyte. The inorganic ions, present on the surface of the oxide, blocking some part of active sites, making them inaccessible for adsorbing by carboxylic groups polymer chains. That makes the adsorption on the contaminated oxide lower than on the washed one. From the comparison of the determined values of the diffuse layer charge and surface charge, the main factor influencing the zeta potential changes at different pH, molecular weight and polymer concentration was determined. Also was demonstrated, that the contaminations of the solid are reason for considerable shift of pH_pzc in relation to pH_icpof the zirconia. Thickness of the adsorption layers and free energies of the adsorption of polyacrylic acid and polyacrylamide on the surface of ZrO₂ were calculated.     

Residual changes in the surface charge of TiO2 (anatase) suspensions as a result of exposure to a 44 MHz radiofrequency electric field

Potentiometric titrations of TiO₂ (anatase) suspensions in solutions of 10^–2, 10^–3, and 10^–4 M NaCl prior to treatment with a radiofrequency (RF) electric field gave values of pH_pzc and pH_iep of 5.75±0.1 and 5.85±0.1, respectively, within the range of literature values. Surface charge plots versus pH for untreated samples gave curves with a common intersection point, indicating that NaCl acts as an indifferent electrolyte. Identical suspensions of anatase were then exposed for 30 min to an RF electric field with a peak-to-peak, no-load amplitude of 34V and a frequency of 44 MHz. Portions of the treated suspension were titrated after different time periods following removal of the field (2, 15, 30, and 45 min). At 2 min, the pH_pzc had shifted to 6.50±0.1, and at 15, 30 and 45 min, the curves for the three NaCl concentrations were found to lack a common intersection point, an effect which became more pronounced with time. It is proposed that RF treatment results in the formation of a porous gel layer of hydrous titanium dioxide on the surface of the anatase particles that subsequently undergoes a slow ion-exchange reaction involving ions of the supporting electrolyte.     

Influence of pH of the BSA solutions on velocity of the rising bubbles and stability of the thin liquid films and foams

Influence of pH of the BSA solutions on velocity of the rising bubbles, stability of foams, and properties of single foam and wetting films was studied. It was found that the solution pH affected significantly the BSA surface activity and properties of the protein adsorption layer under dynamic and static conditions. At pH close to the isoelectric point (pH_IEP=4.8) the BSA showed the highest surface activity. The equilibrium microscopic foam films of thicknesses of 64–80 nm, depending on the BSA concentration, were obtained at pH=5.8. Under dynamic conditions the bubble rising velocity was reduced in a highest degree and the foam formed were most stable at the solutions pH-5.8 and 4.8. Lowering the bubble velocity shows that the BSA adsorption layer was formed, which retarded fluidity of the bubble surface. When the solution pH was significantly lower (pH=3.9) or much higher (pH=10) than the pH_IEP then the BSA practically had no influence on the bubble velocity and the foam stability was drastically reduced. Moreover, the pH variations affected also the time of the three-phase contact (TPC) formation on mica surface covered by the BSA adsorption layers. These pH dependent changes in the BSA surface activity indicate that the BSA linear conformers, existing at pH far away from the pH_IEP, have much higher affinity to aqueous phase resulting from higher net electrical charge present over the extended BSA molecule conformers.     

The ELECTRICAL INTERFACIAL LAYER at theTiO2 (ANATASE)/ELECTROLYTE INTERFACE - ADSORPTION of Zn(II) and Cd(II) IONS

Mechanism of adsorption of Zn(II) and Cd(II) ions at the TiO₂ (anatase)/electrolyte interface has been studied by different experimental techniques (potentiometric titration, microelectrophoresis and adsorption measurements of zinc and cadmium species). It was found that the point of zero charge (pzc) of anatase (pH =5.8) was shifted to the lower pH values with increasing concentrations of Zn(II) or Cd(Il) ions. The surface charge of anatase in the presence of Zn(II) and Cd(II) for pH > pH_pzc was higher than that observed for original sample in NaClO₄ solutions only. Due to low coverage of anatase surface with Zn(II) or Cd(II) species almost no shift of the isoelectric point (iep) or charge reversal were observed. Adsorption density vs. pH plots for both Zn(Il) or Cd(II) showed, typical for multivalent ions, presence of “adsorption edge.”     

Kinetic study of barrierless coagulation of sols in electrolyte solutions

The flow ultramicroscopy method is employed to investigate the coagulation kinetics of hydrosols of α-Fe₂O₃, α-Fe00H, γ-Al₂O₃, and Cr₂O₃ with electrolytes (NaCl and KCl) under conditions corresponding to the absence of an ion-electrostatic barrier. It is found that, at pH values corresponding to pH_IEP of particles, as well as at those differing from pH_IEP, but at sufficiently high ionic strengths of coagulating electrolytes, the observed rate of coagulation is close to or above that calculated in terms of the Smoluchowski theory of fast coagulation. The data obtained are analyzed and possible reasons for the observed effect are considered with regard to the fractal structure of aggregates, the polydispersity of initial particles, and the concepts of additional interparticle attractive forces arising at high electrolyte concentrations.     

Cr(VI)在Mg-Al型类水滑石上的吸附-脱附性研究I. 吸附性

研究了Cr(VI)在带结构正电荷的Mg-Al型类水滑石(HTlc)上的吸附性能, 考察了pH、无机电解质添加剂NaCl, NaNO₃, Na₂SO₄和Na₃PO₄及有机添加剂EDTA和柠檬酸等因素的影响, 并结合红外光谱和XRD实验结果探讨了吸附机理. 研究表明, Mg-Al型HTlc对Cr(VI)有很强的吸附能力, 其吸附动力学和吸附等温线分别符合准二级速率方程和Langmuir方程, 饱和吸附量达105 mg/g, 有望成为一种优良的含Cr(VI)污水处理剂和Cr(VI)污染土壤修复剂. 初始pH增大, 吸附量降低. 无机电解质和有机添加剂均能明显抑制Cr(VI)在HTlc上的吸附, 其抑制吸附作用的强弱顺序分别为Na₃PO₄≥Na₂SO₄≥NaCl>>NaNO₃和柠檬酸＞EDTA. Cr(VI)在HTlc上的吸附可分为层间的离子交换吸附和外表面的吸附, 其中外表面的吸附层在微观上又可分为因化学键合作用而形成的内络合层和因静电作用而形成的外络合层.     

Effect of the Surface Structure of Poly(styrene-co-acrolein) Microspheres and Its Modification by Protein on Electrosurface Properties

The electrophoretic mobility of poly(styrene-co-acrolein) microspheres was studied as a function of storage time. It was shown that pH_IEP2.0 is retained but the abnormal dependence of electrophoretic mobility on NaCl concentration is replaced by classical dependence. When comparing chemisorption of bovine serum albumin (BSA) on the microsphere surface for various latex samples, the differences in the isotherm patterns was revealed; moreover, the prevalence of surface concentration of carboxyl groups over that of aldehyde groups resulted in a decrease in adsorption. After the modification of the microspheres by protein, the values of pH_IEPfall within the range of 3.5–5.0 and their dependence on the amount of surface-bound protein passes the minimum. The results obtained are discussed in terms of the different arrangement patterns of protein molecules on the microsphere surface and the changes of BSA macromolecule conformations under the effect of a dispersion medium and as a result of chemical interaction with the polymer surface.     

Sorption of alkali cations at the zirconium oxide/ aqueous electrolyte interface

Sorption of the ions, Li⁺, Na⁺ and K⁺ on hydrous zirconia has been studied at 298 K and in the concentration and pH ranges 0.1–1.0 mol dm⁻³ and 4.0–10.0, respectively. It has been found that sorption sequence of the studied ions depends on both ion concentration and the pH value of the solution. The intrinsic ionization and complexation constants of the above ions with surface sites of hydrous zirconia are reported. The point of zero charge of hydrous zirconia has been determined as pH_pzc = 4.0.     

Determination of pH value of isoelectric solution and identification of passive iron surface phases using immersion method

Using the method of phase modeling, the pH values of solutions corresponding to the uncharged surface of passive iron and ferric oxide γ-Fe₂O₃ (pH₀) are compared. According to the theory of connected places, the charge of metal oxide surface is determined by the adsorption or desorption of hydrogen ions leading to a change in the potential drop at the oxide/solution interface. Preliminarily passivated iron electrode was washed with twice-distilled water and placed into 0.5 M NaNO₃ solution with various pH values; the variation in the potential (ΔE) with time was studied. The pH₀ value for passive electrode under the open-circuit conditions was determined by the dependence of ΔE on the pH value (pH₀ 6.2 ± 0.1). The pH₀ value was close to that for γ-Fe₂O₃ (pH₀ 6.2), which was determined by the method of potentiometrical titration of oxide suspension in the nitrate solution. The introduction of surface-active ions Ba²⁺ and Cl^? changes the charge of passive iron surface: Ba²⁺ ions increase the electrode potential, while Cl^? ions decrease it. Comparing the pH₀ values for passive electrode and metal oxides, one can identify the composition of passive electrode surface.     

Electrosurface Properties of Hydr(oxides) and Oxide Nanostructures in 1 : 1 Electrolyte Solutions: 1. Adsorption Characteristics of Boehmite, Goethite, and Silicon Dioxide

The adsorption of potential-determining ions on aluminum and iron hydroxides and silicon dioxide was measured in KCl and NaCl background solutions. It was shown that dependences of the adsorption on pH and the concentration of a background 1 : 1 electrolyte (NaCl) have the shape characteristic of oxide surfaces; the of values pH_PZCwere determined. Results of adsorption measurements were used for the determination of constants of surface reactions and adsorption potentials of ions in terms of the 2-pK model.     

Insight from adsorption properties of Xylidyl Blue embedded hydrogel for effective removal of uranyl: Experimental and theoretical approaches

Applications of a hybrid material consisting of polyacrylamide (PAA) and Xylidyl Blue (XB) for the removal of uranyl ions from aqueous solutions has been investigated with all details. Adsorption experiments were performed at batch mode and constant temperature. Experimental parameters affecting adsorption process such as pH, initial uranyl concentration, time and temperature were studied on the removal of the uranyl ions. The isotherms assays were carried out with synthetic solutions and adsorption data were evaluated by using Langmuir, Freundlich and Dubinin–Radushkevich (D–R) isotherm models. Morphological and chemical characterizations of new synthesized material were investigated by UV-VIS-NIR spectroscopy and SEM/EDX techniques and pH_pzc experiments. The results of the kinetic experiments are consistent with pseudo-second-order models and intra-particle diffusion models with a slightly better fit to the latter. Equilibrium was achieved within 3 h. The value of rate constant for adsorption process was calculated as 1.055 mol⁻¹ kg min⁻¹ at 318 K. The calculated thermodynamic parameters (ΔG°, ΔH° and ΔS°) indicated that the adsorption of uranyl ions onto XB@PAA was feasible, spontaneous and endothermic nature under the studied temperature. The developed material has also a potential as a sensor because its color turn from pink to red by adsorption of uranyl ions.     

Evaluation of phosphate removal from water by calcined-LDH synthesized from the dolomite

In the present study, the characteristics of the adsorption of phosphate on calcined layered double hydroxides (LDHs) from aqueous solution were evaluated under laboratory conditions. The anionic clays were Mg₃pH₁₀-LDH synthesized by co-precipitation and D ₁pH₁₀-LDH synthesized from the dolomite. The equilibrium isotherm showed that the uptake of phosphate ion by Mg₃pH₁₀(500)-LDH was in agreement with the Langmuir and Freundlich equations; the Langmuir model gave a better fit to the experimental data than the Freundlich one. The equilibrium isotherm showed, however, that the adsorption of the phosphate on D₁pH₁₀(500)-LDH was accompanied with precipitation reac tion of the phosphate salts. A mechanism proposed for the removal of phosphate ion has been confirmed by X-ray diffraction and FT-IR spectroscopy.     

Adsorptive removal of malachite green and Rhodamine B dyes on Fe3O4/activated carbon composite

This study demonstrates the adsorption experiments of toxic dyes malachite green (MG) and Rhodamine B (RB) on Fe₃O₄-loaded activated carbon (AC). AC, which is known to be a high-capacity adsorbent, was aimed to be easily separated from aqueous media by loading it with Fe₃O₄. Fe₃O₄-loaded AC was prepared by the coprecipitation method and named magnetic activated carbon (M-AC), and the produced M-AC was characterized by x-ray diffraction (XRD), thermogravimetric analysis (TGA), and pH_pzc analyses. MG and RB adsorption by the M-AC was performed separately by batch technique and the effects of adsorbent amount, solution pH, and initial dye concentration on the adsorption were explored. Maximum removal efficiencies were found to be 96.11% for MG and 98.54% for RB, and the Langmuir isotherm model was the most fitted isotherm model for the adsorption. The kinetic and thermodynamic studies showed that the adsorption proceeded via the pseudo-second-order kinetic model and endothermic in-nature for both dyes.     

Surface charge and adsorption of Na+ and Cl− ions at the zirconium dioxide/water interface

The surface charge and adsorption densities of Na⁺ and Cl^– ions at the zirconium dioxide/electrolyte interface have been determined as a function of pH for 0.1, 0.01 and 0.001 M solutions of NaCl. Using potentiometric titration of the surface hydroxy groups, it was found that the point of zero charge occurred at pH 4.3±0.15. The results are discussed in terms of site binding model of the electric double layer. The ionization and complexation constants have also been determined.     

Adsorption of Cl<Superscript>−</Superscript> ions on γ-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript> oxide from nitrate solutions

The adsorption of chloride ions on γ-Fe₂O₃ oxide (maggemite) from nitrate solution is studied using the method of potentiometric titration and an ion-selective electrode. The specific character of adsorption is determined. It is shown that the maggemite surface coverage with Cl^? ions increases with increasing concentration of ions in the solution, decreasing pH value, and increasing potential. The adsorbability of ions changes drastically in the pH range about pH₀ (γ-Fe₂O₃)6.2. It is found that the adsorption of chloride ions from neutral nitrate solution exponentially increases in the potential range from 0.1 to 1.0 V. The type of adsorption isotherm and the adsorption parameters are determined. It is found that, in the absence of external polarization, the concentration dependences of adsorption of Cl^? ions are complex-shaped, and their initial portions are described by the Langmuir isotherm. Further increase of adsorption is explained by the penetration of Cl^? ions inwards the oxide.     

Electrokinetic properties of aluminum oxide nanodispersions in NaCl solutions

Using a Zetasizer Nano analyzer, electrophoretic mobility and particle sizes are studied as depending on the pH (3–11) and concentration (10^?3–10^?1 M) of NaCl background solutions for two samples of aluminum oxide, namely, γ-oxide of the Degussa brand and a sample synthesized by aluminum chloride hydrolysis. The values of the electrokinetic ζ potential are calculated through the Smoluchowski equation and with allowance for the effect of the polarization of electrical double layer on the electrophoretic mobility within the framework of the Overbeek-Boes-Wiersema model (ζ_W). It is established that, in the region of the isoelectric point (IEP), at (pH-pH_IEP) = ±1.5, the |ζ_W|-pH dependences are linear and their slopes are almost equal for both aluminum oxide samples. The difference between the positions of IEPs for γ-oxide and synthesized oxide sample may be related to different degrees of their surface hydration.     

Zein film as a novel natural biopolymer membrane in electrochemical detections

Permselective modifier films are very important in preparing highly sensitive electrochemical sensors. In this work, for the first time, the behavior of gold and glassy carbon electrodes coated with biocompatible zein film as a permselective membrane for the electrochemical detection of various compounds has been investigated. For this purpose, several electroactive cationic (methylene blue, brilliant green, and thionine) and anionic (potassium ferricyanide, alizarin red S, and riboflavin-5’-phosphate) compounds have been used as model. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) showed that zein membranes prepared from casting solution containing 1% zein in ethanol/water have porous structures with high nanometric roughness. The capacitance values of electrical double layers of electrodes modified with zein film were very high for hydrophilic ions in comparison with hydrophobic ions. Point of zero charge pH (pH_pzc) of zein membrane was 4.8. The results of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) as well as pH_pzc study indicated that zein permselective membrane acts as ion exchanger film for selected cationic compounds with fast electrochemical kinetics responses in aqueous solution (pH=7). This behavior was confirmed by circulating solutions containing model compounds from homemade continuous cell equipped with polyamide membranes modified with zein film.

Graphical Abstract