Point Of Zero Charge Values Research Articles

Arsenic sorption by carbonate-rich aquifer material, a control on arsenic mobility at Zimapán, México.

Arsenic retention by carbonate-rich aquifer material at Zimapán, México, was studied to gain insight into the processes controlling arsenic mobility in natural systems. Batch experiments showed that retention of soluble As (V) on carbonate-rich aquifer material was 35.3-90% in the pH range of 7-9 found in Zimapán natural water. Chemical and mineralogical compositions, point of zero charge (PZC), and experimental arsenic retention at various pH of three composite samples of limestone (M1, M2, M3) showed that sorption (adsorption and coprecipitation) may be one of the main processes controlling arsenic mobility in the Zimapán As-polluted aquifer. The PZC values approximately corresponded to the PZC reported for the main minerals present in each sample: hydrous ferric oxides (M1), calcite (M2), and the range from hydrous ferric oxides and calcite (M3). The chemical and mineralogical compositions of each sample explain the obtained PZC values. Experimental and modeled arsenic retention at various pH values on sample M1 corresponded to reported arsenic adsorption onto hydrous ferric oxides. Coprecipitacion of complex Ca arsenates or arsenic adsorption onto calcite or clay minerals could be the main processes of arsenic retention on samples M2 and M3. Groundwater flow through the granular, carbonate-rich, shallow aquifer may decrease the water As content as a result of these interactions. A remediation method based on the promotion of polluted water flow into the shallow aquifer could be developed from these results.

Surface characterization of plasma sprayed oxide materials: estimation of surface acidity using mass titration.

The aim of this study was to characterize the influence of plasma spraying on the point of zero charge (PZC) of Al2O3-, Cr2O3- and TiO2-based materials. PZC is one of the most important parameter, which describes the acidity of oxide material in aqueous environments. PZC values of several plasma sprayed oxides were determined using mass titration method. Studies were performed for initial spray powders and plasma sprayed coating materials. In addition, mass titration experiments were performed for water-washed and nonwashed samples. It was found that mass titration is a suitable method to estimate the surface acidity of relatively coarse sample powders. It was found for most of the studied materials that the limiting pH values (assumed to be close PZC) were close to those reported in literature for the PZC values of traditionally manufactured oxide materials. On the other hand, mass titration curves of some oxide samples showed unexpected deviation in curve shapes and limiting pH. These deviations were probably due to selective dissolution of sample contaminations or sample material.

Modelling soil surface charge density using mineral composition

The proposed mathematical model consists of a first approach to estimate the soil composition from an experimental titration curve and the quantitative analysis of the soil mineral constituents. Mineral mixtures made using different percentages of kaolinite, illite and quartz were used to simulate a natural porous media and evaluate the proposed mathematical model assuming no interaction among the solid components of mixtures. Three soil samples from Argentine provinces collected from surface horizons (0–15 cm depth, horizon A 1) and typified as Typic Haplocryoll, Rhodudult and Typic Hapludoll were titrated by successive additions of small volumes of 0.1 M HCl and/or KOH at ionic strength of 10 −3, 10 −2 and 10 −1 M of KCl. Soils are strongly reflective of the individual point of zero charge (PZC) of the soil components. Iron oxides have high PZC values while silica, clays and soil organic matter have low PZC. In these soils, the difference between isoelectric point (IEP) and PZC was used to evaluate the existence of specific adsorption and the soils charge behaviour was fitted using the developed mathematical model. A theoretical titration curve was calculated using the charge density and area fraction of each soil or mineral mixtures components. The experimental and theoretical titration curves present a regression coefficient, R 2, higher than 0.95 and a significant p-level p=0.00 for both, soils and mineral mixtures. This model could also be used to interpret the contaminants mobilization in the natural environments, which is dependent of the surface charges and on the mineralogical components of the soils as well as of the chemical interactions among contaminants and soil constituents.

Electrokinetic Behavior of El Maghara Coal: Effect of Some Inorganic Ions.

In this article, a number of preliminary concepts relating to the effects of some inorganic additives on electrokinetic behavior of El Maghara coal surface have been introduced. EL Maghara coal generally exhibits a point of zero charge (PZC) at a certain pH value. Removing the siliceous minerals from coal by deashing process would increase its PZC value. Solution chemistry models were used to elucidate the mechanism of action of some inorganic ions. Electrokinetic potential measurement is considered as one of the tools could be used to understand the nature of the interface and the mechanism of adsorption of solute species at the interface. Zeta potential values usually are considered as important information that determines the stability of colloidal dispersion.

Experimental Studies and Theoretical Interpretation of the Calorimetric Effects Accompanying Ion Adsorption at Oxide/Electrolyte Interfaces: Application of Flow Adsorption Calorimetry

Application of flow-adsorption calorimetry is demonstrated to measure the enthalpic effects accompanying adsorption of simple ions at oxide/electrolyte interfaces. A new kind of experiment is presented, in which calorimetric effects are recorded, accompanying the change of the concentration of the inert electrolyte at constant pH equal to the point of zero charge (PZC). It is shown that carrying out calorimetric experiments in that way decreases by one half the number of parameters that must be determined by best fitting the experimental data by theoretical expressions. The theoretical expressions applied here are essentially those developed in our previous publications, though certain theoretical aspects are discussed here more rigorously. The adsorption of NaCl on an anatase sample was selected for study. A special high-purity anatase sample was prepared, having a high surface area equal to 61.7 m2/g and a PZC value close to that of pH of distilled water. A striking difference was found between the enthalpies of proton adsorption in the absence and in the presence of Na+, Cl- ions.

Alteration of Imogolite, Allophane, and Acidic Soil Clays by Chemical Extractants

AbstractPretreatment of soils to enable physical separation and purification of minerals may alter their structural or chemical composition and preclude meaningful surface charge characterization. A natural imogolite and allophane, a synthetic imogolite (Al/Si = 2.01) and allophane (Al/Si = 1.64), and clays from the Bs and BC horizons of a Spodosol containing imogolite were subjected to standard H2O2 treatments before and after extraction with buffered dithionite‐citrate‐bicarbonate (DCB) to determine if effects on their surface charge characteristics and bulk composition were reversible. The H2O2 slightly shifted the point of zero charge (PZC) determined by electrophoretic mobility (EM) of natural imogolite and allophane toward more positive values, probably by the partial oxidation of retained organics, whereas it had no effect on the PZC values of soil clays. Five DCB extractions removed the most Al and Si from the synthetic allophane and the least Al from the synthetic imogolite. More Si than Al was removed from the synthetic imogolite by five DCB extractions. Citrate retained on allophane and soil clays after DCB extraction was probably partially transformed to oxalate by subsequent H2O2 treatment. The EM in synthetic allophane could not be restored by oxidation of retained citrate following DCB treatment. Synthetic imogolite, after five DCB extractions, had only positive mobility up to pH 11.8. The DCB extraction, commonly used to remove Fe oxide from imogolite and allophane samples isolated from soils, irreversibly altered mineral surfaces and modified surface charge characteristics.

Changes in surface characteristics of silicon nitride prepared for extrusion

The preparation step for extrusion (with binders) of Si 3N 4 powder, as well as the extrusion process itself, modify the original properties of the powder so that its recovery and subsequent rework are not possible. X-ray photoelectron spectroscopy (XPS), X-ray diffraction, infra-red spectroscopy, potentiometric titrations and zeta potential determinations have been used to identify the surface modifications in Si 3N 4 due to addition of binders, and to examine powders after elimination of the binders by extraction with organic solvents and heat treatment. The original Si 3N 4 with binders showed a shift in zeta potential and point of zero charge (PZC) to lower pH, indicating the adsorption of organic compounds on the Si 3N 4 surface at this step. The appearance of one O ls peak at 532.3 eV in the XPS spectrum showed the presence of Si 2N 2O. After extraction with organic solvents the PZC value was close to that of the Si 3N 4 + binders (pH = 3.2), indicating incomplete removal of organics; this was confirmed by XPS determination (C ls intensity data) and chemical analysis. The presence of Si 2N 2O was not modified as shown by XPS and the value of the isoelectric point. Heat treatment removes the organics completely but produces an oxidation of Si 3N 4 to SiO 2 as evidenced by a shift in the binding energy of the O ls photopeak to 534.0 eV.

Cation Exchange Capacity and Condition of Zero Charge of Hydroxy-Al Montmorillonite

AbstractThe effect of interlayering montmorillonite with different amounts of hydroxy aluminum cation on the cation exchange capacity (CEC) and the point of zero charge (PZC) of the clay was studied. The CEC decreased as the Al content of the clay increased. Both the CEC and the loss in the CEC were linearly dependent on the Al content, thus indicating the predominance of a single polynuclear interlayered species. Its composition is [Al6(OH)16.5]1.5+. Acid base titration of the interlayered montmorillonites produced values for the PZC in the range 4.5-5.9, increasing with the Al content of the materials. The values, however, did not represent real conditions of zero charge and some samples were still negatively charged at the determined PZC. The PZC values obtained by acid-base titration are thought to be adversely affected by secondary reactions involving the interlayered material with a concomitant release of soluble Al in amounts dependent on the pH and ionic strength.

Electrochemistry of the zircon—water interface

The electrokinetic and surface charge behaviours of a very pure natural zircon sample and a commercial grade zircon have been studied as a function of pH and KNO 3 concentration. The isoelectric point (IEP) and point of zero charge (PZC) of the natural zircon were determined to fall at a pH of 5.7 ± 0.1 and 5.9 ± 0.1 respectively. For the commercial grade zircon the IEP and PZC values were 5.5 ± 0.1 and 6.1 ± 0.1 respectively. A triple layer site-binding model was used to describe the electrical properties of the zircon—water interface. The surface charge and the zeta potential have been simultaneously calculated using this model and are in good agreement with the experimental data. The best-fit parameters pertaining to this model have been extracted by using both graphical extrapolation and computational methods. Infrared and X-ray photoelectron spectroscopy both indicated that kaolinite was present as an impurity in the commercial grade zircon.

Temperature effects on the point of zero charge and isoelectric point of a red soil rich in kaolinite and iron minerals

The effects of temperature (373–1373 K) on the point of zero charge (PZC) and isoelectric point (IEP) of a red soil rich in kaolinite and iron minerals were studied. PZC values of the soil treated at 373 and 573 K indicated the presence of iron oxide. The soil calcined between 773 and 1173 K shows a PZC almost coincident with the respective values of kaolinite. At 1373 K, the PZC of the soil is nearer to the value of iron oxide. In the entire temperature range studied the PZC values were lower than the IEP values. An approach of PZC and IEP values was observed after a partial removal of iron oxide by the dithionite-citrate-bicarbonate (DCB) method. The analyses of the PZC and IEP values, of electron probe micro analysis (EPMA) data and of specific surface areas evidence a specific adsorption of iron oxide on kaolinite. Finally, the dissolution sequence of iron and aluminium contained in soil was determined using hydrochloric acid.

An experimental correlation between points of zero charge and X-ray photoelectron spectroscopy chemical shifts of oxides

Measurements are reported of X-ray photoelectron spectroscopy (XPS) investigations, performed under conditions of ultrahigh vacuum, of several metal oxides either produced commercially (high purity) or prepared in the laboratory. The same oxides were also subjected to electrochemical measurements in aqueous solutions for the determination of the point of zero charge (PZC) values and/or the isoelectric points (IEPs). The PZC (IEP) values appear to be linearly correlated with the sum of the metal and oxygen XPS chemical shifts (DM + DO). The higher (more alkaline) the PZC (IEP), the lower the XPS (DM + DO) term and vice versa. The weighting of the different components in the accuracy of the correlation is discussed. It is shown how a cross comparison between experimental XPS and electrochemical data obtained for the same samples can be used as a diagnostic criterion for the occurrence of secondary phenomena at the solid/liquid boundary.

Effect of HNO 3 treatment on the surface acidity of activated carbons

A homologous series of activated carbon samples was prepared from a low ash source carbon by controlled oxidation using nitric acid. The physicochemical properties (specifically, BET surface area, ash content, acid/base neutralization, and ammonia adsorption) were determined. Each property value changed systematically with increasing oxidation treatment; the more oxidized sample showed higher surface acidities without significant changes in BET area or ash content. The point of zero charge (PZC) of each sample was determined by mass and acid/base potentiometric titration. The PZC values also changed systematically with the extent of oxidation; the more oxidized the carbon, the lower its PZC. It is concluded that the PZC can be used as a convenient index for correlating changes in surface acidity of carbons.