Influence of pH and Ionic Impurities on the Adsorption of Poly(acrylic) Dispersant onto a Zinc Oxide Surface

We have studied the properties of aqueous suspensions of ZnO powders with different purities. Our results suggest that powder purity determines the amount of dispersant necessary to form a stable aqueous suspension as well as the maximum adsorption capacity of the dispersant: the higher the positive surface charge of the as-received ZnO powders, the higher the amount of dispersant adsorbed onto the metal oxide surface. The surface charge of the ZnO particles in suspension is affected by the concentration of zinc as well as sulfate ions, which are the major impurities in the supernatant. The pH of the aqueous ZnO suspensions increases with increasing concentration of poly(acrylic) dispersant until the maximum adsorption capacity is attained. Further additions of dispersant do not increase the pH because of a buffer formation with impurity ions.     

Surface modification of SiC powders by hydrolysed aluminium coating

SiC powders are surface modified to behave like alumina in aqueous suspensions by coating the powders with in situ generated hydrolyzed aluminium from dilute aqueous aluminum nitrate solutions in a pH range of 3–4.5 using hexa-methylene-tetramine as the base generator at ambient temperature. By examining the zeta potential and rheological properties of aqueous suspensions of the powder with different Al-coverage, it was observed that the coated powder begins to show alumina-like surface properties at an Al-coverage of 0.1 mg/m², in contrast to 0.5 mg/m² that was the minimum value reported earlier for observing the effect. This is explained by proposing a coating mechanism which proceeds through adsorption of a layer of cationic hydrolysed aluminum molecules, such as [Al(OH)(H₂O)₅]²⁺ and [Al₂(OH)₂(H₂O)₈]⁴⁺, during which the particles attain alumina-like surface charge properties. The modified powder retained alumina-like surface characteristics when stored under moist conditions and in acidic suspensions. The low value of Al-coverage and the resulting low increase (<10%) of powder surface area are advantageous for preparation of concentrated SiC and SiC–Al₂O₃ composite suspensions in acidic aqueous medium with rheological properties similar to that of their counterparts containing alumina only.     

Protective mechanisms occurring on zinc coated steel cut-edges in immersion conditions

Electrochemical processes occurring on the cut-edge of a galvanized steel immersed in NaCl solutions were studied using numerical simulations, and in situ current and pH profiles measured over the cut-edge. These results clearly demonstrate that only the steel surface remote from the zinc coating is cathodically active, oxygen reduction being strongly inhibited in the vicinity of zinc. This trend was confirmed by local polarization curves recorded on these distinct areas. Ex-situ AES and SEM analysis and cathodic polarization curves in solutions containing Zn²⁺ ions led to conclude that this cathodic inhibition was related to the fast nucleation of a dense Zn(OH)₂ film on the steel surface. After a long term exposure, a new galvanic coupling takes place between the Zn(OH)₂ covered area, showing an anodic activity, and the remaining steel surface covered by bulky white zinc corrosion products.     

The Adsorbing Colloid Flotation of Lead(II) and Zinc(II) by Hydroxides

Abstract

The effects of pH, ionic strength, and specific ions on the adsorbing colloid flotation of lead(II) and zinc(II) with AI(OH)₃ and Fe(OH)₃ with sodium lauryl sulfate (NLS) as collector are reported. Floc foam flotation of lead with Fe(OH)₃ and gelatin, with MnO₂ and NLS, and with CaCO₃ and NLS or hexadecyltrimethylammonium bromide was studied. Lead is efficiently removed with Fe(OH)₃ and NLS; zinc with Al(OH)₃ and NLS. A theoretical study was made of the effect of the charges of ions present as inert electrolyte; separations decrease in efficiency with increasing charge at constant ionic strength.     

Synthesis and characterization of nano-sized calcium zincate powder and its application to Ni–Zn batteries

Nano-sized calcium zincate powders used as active materials for a secondary Zn electrode were prepared by a chemical co-precipitation method. The properties were studied by thermal gravimetric analysis (TGA), micro-Raman spectroscopy and nitrogen adsorption–desorption experiments. The secondary Zn electrodes using chemical co-precipitation calcium zincate powders (CP-ZnCa) and ball-milled calcium zincate powders (BM-ZnCa), were examined and compared. The electrochemical performance of the secondary Zn electrodes was systematically investigated by cyclic voltammetry and galvanostatic charge/discharge measurements. It was demonstrated that the electrochemical properties of the secondary Zn-pasted electrode using CP-ZnCa powders were greatly improved, as compared with conventional secondary ZnO electrodes. The results indicated that secondary Ni-Zn batteries using CP-ZnCa powders exhibited a better charge/discharge property and a longer life-cycle performance, compared with those based on ball-milled ZnO + Ca(OH)₂ (BM-ZnCa) powders.     

Electrostatic charge dependence on surface hydroxylation for different Al2O3 powders

The effect of surface chemistry on the electrostatic charge in α-Al₂O₃ powders was studied. Net charge and tribo-charge of dry powders were measured by the Faraday double cup procedure. High electrostatic charge values were observed in powders with high surface hydroxylation by IR spectroscopy. The nature of hydroxyl groups at the surface determines the polarity of the electrostatic charge. Mixture of particles having intrinsic electrostatic positive and negative charges follows the lever rule. Dry mixtures of differently charged particles favour the dispersion of smaller particles on the surface of the largest ones. An effective random dispersion was achieved by a low-energy process that allowed preserving the morphology of the supporting and supported particles.     

Burning of a Cotton Fabric Impregnated by Synthetic Zinc Carbonate Hydroxide as a Flame Retardant

Zinc carbonate hydroxide [ZnCO₃·Zn(OH)₂] synthesized by means of the multiple-bath method was deposited onto a cotton fabric, and its uniformity was ensured by means of squeeze rolls. Prolonged burning was observed on treated specimens: 200.5 sec for 20.20% of ZnCO₃·Zn(OH)₂ added to 100 g of a dry fabric increased to 337.5 sec for a 45.39% addition. The ashes of the treated specimens were subjected to X-ray diffraction analysis, and the result was compared with data for zinc and zinc-oxide powders. The existence of zinc oxide was detected in the ashes, but no traces of the metallic zinc were discovered. Therefore, it can plausibly be assumed that a reduction-oxidation reaction occurs during the smoldering process.__________Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 4, pp. 73–77, July–August, 2005.     

The separation of europium from a middle rare earth concentrate by combined chemical reduction,precipitation and solvent-extraction methods

The chemical reduction of pure europium(III) chloride solutions was investigated using reagents comprising reactive metals (Zn and Mg), metal amalgams (Zn-Hg, Na-Hg and Eu-Hg), metal hydride (NaBH₄) and nitrogenous reductants (N₂H₄ and NH₂OH). Using 100% excess of reducing agent and of ammonium sulphate, efficient precipitation of europium(II) sulphate was obtained with the metal amalgams (99·7–99·9%) and with zinc metal (99·8%), whereas only partial precipitation was obtained with magnesium metal (69%), and no precipitation was observed with the other reagents. Application of the method to synthetic rare earth chloride solutions containing europium 7·5, neodymium 5, samarium 35 and gadolinium 20 g dm⁻³ gave efficient precipitation of europium(II) sulphate with zinc and europium amalgams, but no selective precipitation with sodium amalgam. Reduction of an authentic middle rare earth chloride solution with zinc amalgam gave 97·5% recovery of europium(II) sulphate containing (as a percentage of the total rare earths) europium 92, samarium 3·5, neodymium 2, cerium 1, praseodymium 0·6 and gadolinium 0·5%. Conversion of the europium(II) sulphate to europium(II) chloride, followed by re-precipitation of the sulphate increased the europium content only to 96·5%, whereas replacement of the re-precipitation by solvent extraction of the trivalent rare earth impurities into solutions of commercial organophosphorus or carboxylic acids in xylene increased the europium content to > 99·98%. The zinc ions introduced into the middle rare earth mother liquor during the reduction procedure can be removed by solvent extraction into a commercial phosphine oxide (Cyanex 925), without loss of rare earth values.     

Synthesis of spherical metal oxide particles using homogeneous precipitation of aqueous solutions of metal sulfates with urea

Spherical micron sized porous particles of goethite FeO(OH), boehmite AlO(OH), anatase TiO₂, Zn(OH)₂ and binary mixtures of these oxides have been synthesized by homogeneous precipitation from aqueous solution containing urea in the presence of corresponding metal sulfates. Metal (hydrous) oxide particles obtained show spherical morphology and consist of agglomerated randomly oriented nanocrystallites. Synthesized metal oxide hydroxides were characterized using Brunauer-Emmett-Teller (BET) surface area and Barrett-Joiner-Halenda porosity (BJH), X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy-dispersive X-ray microanalysis (EDX).     

Cyclic voltammetric study of zinc and zinc oxide electrodes in 5.3 M KOH

The behaviour of zinc and zinc oxide in 5.3 M KOH in the presence of alkaline earth oxides, SnO, Ni(OH)₂ and Co(OH)₂ was examined by cyclic voltammetry. The influence of the alkaline earth oxides was compared with additives of established effects (Bi₂O₃, LiOH, Na₂CO₃ and CdO). The alkaline earth oxide each exhibits a distinct behaviour towards zincate. Whereas, a single process of interaction with zincate was shown by CaO; two modes of reaction were obtained with SrO and BaO. Solid solution formation was noticed with BeO and MgO. The other additives forming solid solution with ZnO were CdO, SnO. The ionic sizes of Ni(OH)₂ and Co(OH)₂ allow solid solution formation with Zn(OH)₂. Both Bi₂O₃ and Na₂CO₃ enter into complexation with zincate. LiOH forms two distinct zincates, of which one is an oxo zincate leaching the `hydroxyl' functionality. Cyclic voltammetry revealed the deposition of the oxide/hydroxide additives as metal prior to the onset of zinc deposition and the potential range for this additive metal deposition is almost the same for different additives (SnO, CdO, Ni(OH)₂). The beneficial action of these additives to zinc alkaline cells is associated with a substrate effect. The implication of this electrocatalytic deposition of metals on a zinc oxide electrode is also discussed.     

Ultrasmall Metal Oxide Particles: Preparation,Photophysical Characterization,and Photocatalytic Properties

This paper gives an overview of the author's activities in the research of extremely small metal oxide particles in recent years. In particular, the synthesis of transparent colloidal solutions of extremely small zinc oxide, titanium dioxide, hematite, and titanium/iron mixed oxide particles (2 nm < d 20 nm) in water, ethanol, and 2-propanol is described. Quantum (Q)-size effects are observed during particle growth and at me final stages of synthesis. A simple molecular orbital (MO) picture is presented for the qualitative interpretation of these effects, while quantitative calculations have been carried out using a quantum mechanical model developed by Brus. The photophysical properties of the particles have also been investigated extensively. Fluorescence spectra of the ZnO sols suggest that adsorbed electron relays are necessary to shuttle electrons from the conduction band to lower-lying traps. Excess negative charge on the particles, resulting from either deprotonated surface hydroxyl groups or from photogenerated or externally injected charge carriers, causes a blue-shift in the electronic absorption spectrum, which is explained by electrostatic and MO models. The zero point of charge (pH_zpc) of the aqueous colloidal suspensions has been determined by several independent methods. While zinc oxide, titanium dioxide, and titanium/iron mixed oxide particles exhibit considerable photocatalytic activity (as illustrated for the reduction of molecular oxygen and the oxidation of various halogenated carboxyl acids), hematite particles are only found to oxidize S(IV) under bandgap illumination to a reasonable extent (ϕ < 0.3). A mechanism involving surface-bound molecules and free radical intermediates is presented to explain these differences in reactivity.     

Electrokinetics and stability of a cellulose acetate phthalate latex

An experimental investigation is described on the surface electric characterization of a commercially available latex, Aquateric, composed of cellulose acetate phthalate polymer particles, and used in enteric-controlled drug release. Since the surface charge of dispersed systems is an essential parameter governing most of their behavior, it is of fundamental importance to characterize how that quantity changes in the different environments in which the colloids could be used. The experimental method used in this work is electrophoresis; we report measurements of electrophoretic mobility of the latex as a function of pH and ionic concentration in the dispersion medium. It is shown that the zeta potential of the polymer particles is negative for the whole pH range studied and increases with pH as the dissociation of surface acetic acid groups proceeds. A plateau value is found for pH > 5, corresponding to complete dissociation of available ionizable sites. The values of the electrophoretic mobility (μ_e) and the zeta potential (ζ) of Aquateric are also analyzed as a function of the concentration of 1-1 (NaCl) and 2-1 (CaCl₂) concentration. The anomalous surface conductance (associated to the mobility of counterions adsorbed in the inner part of the electric double layer of the particles) manifests in a maximum in the |μ_e|-NaCl concentration plot for 10⁻³M concentration. No such behavior is observed in the presence of CaCl₂ solutions, where only a decrease of the mobility with ionic strength is observed. The effect of AlCl₃ concentration on the mobility is also considered; it is found that at pH 2 aluminum ions adsorb on the particles and render them positively charged. When the pH of the suspensions is not maintained constant, the hydrolysis of aluminum gives rise to a less efficient control of the charge of the particles and no positive mobilities are observed. Electrophoretic mobility measurements as a function of pH at constant AlCl₃ concentration show an abrupt change of μ_e from negative to positive, interpreted as due to surface precipitation of Al(OH)₃. When the pH is further increased, a second charge reversal is found, corresponding to the isoelectric point (pH of zero zeta potential) of Al(OH)₃. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65: 2721–2726, 1997     

Effect of ZnO loading to activated carbon on Pb(II) adsorption from aqueous solution

The effect of zinc oxide loading to granular activated carbon on Pb(II) adsorption from aqueous solution was studied in comparison with zinc oxide particles and oxidized activated carbon. Cu(II), Cd(II) and nitrobenzene were used as reference adsorbates to investigate the adsorption. The BET surface area and point of zero charge (pH_PZC) in the aqueous solution were measured for the adsorbents. The adsorption isotherms were examined to characterize the adsorption of heavy metals and organic molecules. The heavy metal adsorption was improved by both the zinc oxide loading and the oxidation of activated carbon. In contrast, the adsorption of nitrobenzene was considerably reduced by the oxidation, and slightly decreased by the zinc oxide loading. The zinc oxide loading to the activated carbon was found to be effectively used for the Pb(II) adsorption whereas only a part of surface functional groups was used for the zinc oxide particles and the oxidized activated carbon. From the experimental results, the surface functional groups responsible for the Pb(II) adsorption on the zinc oxide loaded activated carbon were considered to be hydroxyl groups that formed on the oxide, while those on the oxidized activated carbon were considered to be carboxylic groups.     

Zinc electrowinning with 2-butyne-1,4-diol

The beneficial effect of 2-butyne-1,4-diol on zinc current efficiency in the presence of nickel impurity has been examined. Several techniques including HPLC, absorption spectrophotometry and constant current deposition experiments have shown that a trimer of 2-butyne-1,4-diol is responsible for the removal of Ni ions from the electrolyte, thus increasing the current efficiency of the zinc electrowinning process from sulphate solutions (60 g/L Zn + 200 g/L H₂SO₄) in the presence of Ni ions.     

Effects of a small addition of Mn on the corrosion behaviour of Zn in a mixed solution

The effects of a small addition of Mn (0.4 wt%) on the corrosion behaviour of pure Zn (99.995 wt%) in a mixed solution (0.1 M NaCl + 0.1 M Na₂SO₄ + 0.01 M NaHCO₃, pH 8.4) were investigated by electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and X-ray photoelectron spectroscopy (XPS). The electrochemical impedances of both Zn and Zn–0.4Mn have been successfully fitted with a suitable EIS equivalent circuit model. Fitted impedance results revealed that 0.4 wt% Mn improved both the pore resistance and charge transfer resistance of Zn in the mixed solution. As a result, both anodic and cathodic reaction rates were reduced. X-ray photoelectron spectroscopy (XPS) analysis showed that the corrosion films formed in the mixed solution consisted of zinc oxide (ZnO), zinc hydroxide (Zn(OH)₂) and zinc hydrozincite (Zn₅(CO₃)₂(OH)₆). The role of small addition of Mn is that it promotes the precipitation of hydrozincite in the pores of corrosion film. An “alleviation of local acidification” mechanism is proposed to explain the investigated results.     

Preparation of ZnO/Eu mixed films by electrochemical precipitation

The electrochemical preparation of europium doped zinc oxide and europium oxide/hydroxide as thin films is investigated. First, a thermodynamic study of the Eu-Cl-H₂O system has been carried out at 25 and 70 °C in order to predict the electrochemical behaviour of Eu(III) dissolved in aqueous solution containing chloride ions. A comparison of the Eu-Cl-H₂O and Zn-Cl-H₂O systems indicates the possible coprecipitation of ZnO and Eu(OH)₃ from deposition solutions containing well-adjusted Eu(III)/Zn(II) concentrations ratio. The thermodynamic predictions have been confirmed experimentally by the electrochemical co-deposition of ZnO/Eu thin films on conducting electrode substrates at −1.4 V versus MSE. The presence of europium in the film is detected for Eu(III)/Zn(II) concentration ratio at (0.6 mM/5 mM) which is lower than the predicted value. Increasing Eu(III) concentration leads to the rapid appearance of two phases: dispersed zinc oxide nanorods and, at the bottom of the rods, a covering layer containing Eu(OH)₃ and zinc. The density of ZnO rods decreases and the rod size increases with increasing Eu(III) concentration in the bath. Above 1 mM EuCl₃, a dramatic fall in the current density is observed with the formation of a less conducting ZnO/Eu mixed deposit.     

Suppression of the memory effect observed in alkaline secondary batteries under partial charge-discharge conditions

Under partial charge-discharge cycling conditions at a state of charge (SoC) of 50-70%, a greater effect on the suppression of the memory effect, which occurs in rechargeable alkaline batteries containing a Ni-electrode, was observed using conductive materials such as nano-sized Co(OH)₂ and CoO powders in the Ni-electrode. The nano-sized Co(OH)₂ powder, prepared by the reverse micelle method, consisted of approximately 15-60 nm spherical-shaped particles. The Ni-Cd cells containing approximately 15- or 20-nm-sized Co(OH)₂ powder and 18 nm CoO powder in the Ni-electrode did not show any memory effect after a 150-cycle partial charge-discharge cycling test, and γ-NiOOH, which is mainly the origin of the memory effect, was not detected in the Ni-electrode, whereas the Ni-Cd cell containing the same content of 500 nm Co(OH)₂ powder and 18 nm CoO in the Ni-electrode showed the memory effect. The nano-sized Co(OH)₂ powder could be more homogeneously dispersed in the Ni-electrode when compared to the 500 nm powders and might develop a superior electro-conductive network between the Ni(OH)₂ particles.     

Zinc dissolution in ammonium chloride electrolytes

The anodic dissolution of zinc RDE in NH₄Cl and NH₄Cl + ZnCl₂ electrolytes at pH 5.5 was studied. XPS and SEM data indicate that the zinc electrode is covered by a porous film composed of a mixture of metallic zinc and zinc hydroxide. The thickness of the film and the zinc hydroxide content is much higher in zinc-containing electrolytes than in NH₄Cl and, although the thickness and Zn(OH)₂ amount decrease with anodic polarization, the electrode is never free from oxidized compounds. Electrochemical results indicate that at low overpotentials the rate of zinc dissolution is determined by the removal of the oxidized blocking particles, while at high overpotentials Zn dissolution takes place through the porous layer.     

Degradation mechanisms of nickel oxide electrodes in zinc/nickel oxide cells with low-zinc-solubility electrolytes

Nickel oxide electrodes that suffered capacity degradation during extended cycling in zinc/nickel oxide cells were examined by a variety of chemical and physical techniques. Nickel hydroxyzincates, which have been speculated to cause such capacity degradation, were also examined. Powder X-ray diffraction experiments indicated that the intersheet distance between layers of turbostratic nickel hydroxide increased when zinc was incorporated. Photoelectron spectra (XPS) showed that this material is probably a mixture of NiOH)₂ and ZnO or Zn(OH)₂. Raman spectroscopy data also supported this conclusion. XPS indicated that the form of zinc in degraded nickel oxide electrodes is probably ZnO or Zn(OH)₂. Significant increases in resistivity were found in cycled nickel oxide electrodes, and optical microscopy provided visible evidence of mechanical damage during cycling. These results suggest that the observed capacity degradation was largely mechanical in nature, and not due to the formation of nickel-zinc double hydroxides, as had been reported by others. Cell-cycling experiments indicated that the mechanical degradation is largely irreversible.     

Effect of cations and anions on properties of zinc oxide particles synthesized in supercritical water

Hydrothermal synthesis of zinc oxide fine particles from zinc salt (Zn(CH₃COO)₂, ZnSO₄, Zn(NO₃)₂) and alkali metal hydroxide (LiOH, KOH) aqueous solution was carried out with a Ti alloy batch reactor in supercritical water. Particle size synthesized in LiOH solution was relatively smaller than that in KOH. Emission spectra of the particle produced from ZnSO₄ and LiOH aqueous solution shows the highest intensity among these systems. Hydrothermal synthesis of zinc oxide fine particles from Zn(NO₃)₂ and LiOH solution was also carried out with a flow-through apparatus for continuous production and rapid heating of the starting solution to supercritical states. Nanoparticles having an average particle diameter of 16 nm was produced at 659 K and 30 MPa.