Point Of Zero Net Proton Charge Research Articles

In Ukrainian

The paper discusses the results of saponite research from the Tashkiv deposit of Ukraine. X-ray structural analysis proved the necessity of preliminary cleaning of saponites from mineral impurities. The study of the morphology, nanoprofile and topography of the surface of saponite by the methods of SEM-microscopy and Atomic Force Microscopy revealed that the mineral is represented by aggregates of nano- and microparticles of a pyramidal shape. Its characteristic feature is the heterogeneity of isomorphic substitutions of ions in the tetrahedral and octahedral sheets of the structural elementary package. According to X-ray fluorescence analysis, saponite contains a significant amount of Fe3+, which isomorphically replaces magnesium Mg2+ and, accordingly, is located mastly in the octahedral sheet of the structural package with a charge from +0.37 to +0.35. The number and mechanism of isomorphic substitutions determine the presence of a total negative charge of the crystal lattice (from –0.38 to –0.3), the value of which ensures intensive interaction with water molecules of the interpacket space with the formation of surface OH groups. Accordingly, both acidic and alkaline Lewis and Brønsted centers are present on the surface with a predominance of acidic ones, so the acidity function is 5.82, and the point of zero proton charge is pH = 5.5. During dispersion in water, a part of the alkaline centers of the side surface are transformed into Brønsted acid centers as a result of their protonation, which causes an increase in the pH of the dispersion medium to pH = 8–8.6. Accordingly, the isoionic state is reached at pH = 7.5. The difference in pH values characterizing the isoionic state of the surface and the point of zero net proton charge (PZNPC) indicates the presence of weak acid-alkaline centers on the surface. The study of the adsorption of acid-alkaline dyes showed the adsorption of alkaline (pK = 1¸3) and acid (pK = 7¸14) dyes on saponites. The latter is significantly reduced due to the preliminary hydration of the solid surface - mainly the lateral edges of the particles. Acidic dyes are not adsorbed from a dispersion medium with pH < 5.5 (PZNPC), and basic dyes are adsorbed at pH > 5,5( PZNPC).

Clay particle charging in apolar media

Abstract Advancements in electrophoretic displays (e.g., the Amazon Kindle®) and new high-quality printing technologies (e.g., HP Indigo®) have been made possible by the ability to control and understand particle charging in apolar media. While previous work has investigated the acid-base charging mechanisms of oxides and similar particles which have only one charging mechanism in water, the purpose of this study was to determine if and how clay particles, which have two known aqueous charging mechanisms, charge in apolar media. Characterization of kaolinite and montmorillonite surface charging properties and mechanisms were conducted in apolar media utilizing different surfactants. It was found that clay particle charging behavior as a function of surfactant concentration mimicked the expected trend of previously studied oxides, which can charge only via an acid-base mechanism. Furthermore, by comparing the point of zero charge of the edge groups (edge PZC) or the point of zero net proton charge (PZNPC) of the clay particle to the empirical effective pH (pHEff) of the surfactant, the polarity of the particle charge can be predicted. In addition to particle charging, it was shown that the surfactants also improve the stability against aggregation and settling of the particles.

Adsorption of Antimonate by Kaolinite

Core Ideas Sb(V) adsorption by kaolinite is pH and ionic strength dependent, with adsorption decreasing with increasing pH and ionic strength. Sb(V) adsorption is hysteretic in acidic systems, but reversible in alkaline. Adsorbed Sb(V) generates a downward shift in the isoelectric point and an upward shift in the point of zero net proton charge. Phosphate decreases Sb(V) adsorption, but the influence of sulfate is minimal. The macroscopic findings indicate that Sb(V) is retained by innersphere in acidic and outersphere in neutral to alkaline systems. The environmental fate and behavior of antimony (Sb) is controlled by adsorption processes at the solid–solution interface of surface‐reactive solids. One such solid is kaolinite, which bears the aluminol (≡AlOH) surface functional group. The objectives of this study are to characterize antimonate [Sb(V) in the Sb(OH)6– species] adsorption by kaolinite as a function of pH, ionic environment, and competing ligands (PO4 and SO4); to describe the reversibility of the adsorption and the influence of adsorbed Sb(V) on the surficial properties of kaolinite; and to develop predictive surface complexation models. The results of this study show adsorbed Sb(V) is at a maximum in strongly acidic suspensions, and retention decreases with increasing pH and ionic strength. Adsorption is hysteretic in pH < 5 suspensions, becoming reversible with increasing pH. The point of zero net proton charge (PZNPC) increases from 4.5 in KNO3 to 5.6 in KSb(OH)6. Correspondingly, the isoelectric point decreases from 4.5 in KNO3 to <3 in KSb(OH)6. The macroscopic findings indicate that Sb(V) is retained by a combination of inner and outersphere mechanisms, with the former dominating in acidic suspensions. The inclusion of SO4 had little impact on the retention of Sb(V); however, PO4 reduced Sb(V) retention. The inclusion of both inner‐ and outersphere complexes in the triple layer surface complexation model (TLM) provided an adequate description of the Sb(V) adsorption envelopes. Further, the TLM successfully predicted ligand retention in competitive systems without re‐optimization.

A NOVEL METHOD FOR THE DETERMINATION OF THE POINT OF ZERO NET PROTON CHARGE OF COLLOIDAL KAOLINITE IN AQUEOUS SOLUTIONS

In this work, a novel method to determine the point of zero net proton charge (PZNPC) of colloidal kaolinite in aqueous solutions was presented through the measurement of zeta potential of the particles in various concentrations of NaCl solution and the mathematic regression of the zeta potential versus pH curves. This method is based on the observation that there are two common intersection points (CIP) in the two curves of zeta potential versus pH of colloidal kaolinite with a low and a high NaCl concentration. The observation might be attributed to that NaCl would compress the electric double layer of kaolinite particles and affect the efficiency of the protonation/deprotonation of the HD-faces. The experimental result showed that the PZNPC of kaolinite is 5.37.

Influence of Morphology and Crystallinity on Surface Reactivity of Nanosized Anatase TiO2 Studied by Adsorption Techniques. 2. Solid–Liquid Interface

The aim of this study was to provide new insight into the evaluation of the effect of the crystallinity, size, and morphology of TiO2-anatase nanoparticles on their acid–base properties at the solid–liquid interface. This was achieved through monitoring the evolution in the surface charge density with the solution acidity and point of zero net proton charge (PZNPC) for a set of anatase nanoparticles with a mean size in the 5–20 nm range and various shapes. Different anatase nanoparticles were obtained by a sol–gel synthesis approach using different precursors, pH conditions, and various inorganic or organic additives. The measured PZNPC values were found to vary more than one pH unit depending on the degree of crystallinity and presence of differently exposed surfaces. To discriminate slight differences in the surface reactivity at the solid–water interface, high-resolution titration curves of surface charge for each anatase sample were recorded, and a fine analysis by the titration derivative isotherm summation (TDIS) method and proton affinity distributions (PADs) was performed. They provided accurate data on the strength (pK position) of various local domains of proton adsorption with their relative surface contributions in relation with the particle morphology. The increase in the proportion of sites that are more present on the {101} faces or that of medium acid sites largely present on the {100} faces shifted the point of zero net proton charge to more acidic pH values for particles possessing a majority of such surfaces. In addition to these experiments, the relative acidity of the surface sites on the different surfaces of anatase was evaluated with the multisite complexation model (i.e., MUSIC model) applied to the theoretically optimized surfaces, and a comparison was drawn.

Aggregation of ferrihydrite nanoparticles in aqueous systems

Crystal growth by non-classical mechanisms, such as oriented aggregation, frequently involves an aggregation step. The aggregation of nanoparticles is sensitive to solution variables like ionic strength and pH, as well as the presence and concentration of other chemical species. Aggregation is a critical first step during the early stages of oriented aggregation. Time-resolved, cryogenic transmission electron microscopy was employed to characterize the degree of aggregation, the reversibility of aggregation, and the influence of additives on aggregation in aqueous suspensions. In this work, freshly synthesized ferrihydrite nanoparticles in aqueous suspension were employed as the model system. These nanoparticles are largely aggregated even with a solution pH several pH units away from the point of zero net proton charge (PZNPC) and a very low ionic strength (<10−4 M). Reversibility of aggregation was observed to be time-dependent. Finally, chemical additives dramatically change the evolution of aggregation state with strongly coordinating ligands strongly suppressing aggregation, even after aging at an elevated temperature.

Investigation of radionuclide 63Ni(II) sorption on ZSM-5 zeolite

The sorption of 63Ni(II) from aqueous solution using ZSM-5 zeolite was investigated by batch technique under ambient conditions. ZSM-5 zeolite was characterized by point of zero net proton charge (PZNPC) titration. The sorption was investigated as a function of shaking time, pH, ionic strength, foreign ions, humic acid (HA), fulvic acid (FA) and temperature. The results indicate that the sorption of 63Ni(II) on ZSM-5 zeolite is strongly dependent on pH. The sorption is dependent on ionic strength at low pH, but independent of ionic strength at high pH values. The presence of HA/FA enhances 63Ni(II) sorption at low pH values, whereas reduces 63Ni(II) sorption at high pH values. The sorption isotherms are simulated by Langmuir model very well. The thermodynamic parameters (i.e., ∆H0, ∆S0 and ∆G0) for the sorption of 63Ni(II) are determined from the temperature dependent sorption isotherms at 293.15, 313.15 and 333.15 K, respectively, and the results indicate that the sorption process of 63Ni(II) on ZSM-5 zeolite is spontaneous and endothermic.

Surface Complexation of Carboxylate Adheres Cryptosporidium parvum Öocysts to the Hematite−Water Interface

The interaction of viable Cryptosporidium parvum oocysts at the hematite (alpha-Fe2O3)-water interface was examined over a wide range in solution chemistry using in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. Spectra for hematite-sorbed oocysts showed distinct changes in carboxylate group vibrations relative to spectra obtained in the absence of hematite, indicative of direct chemical bonding between carboxylate groups and Fe metal centers of the hematite surface. The data also indicate that complexation modes vary with solution chemistry. InNaCl solution, oocysts are bound to hematite via monodentate and binuclear bidentate complexes. The former predominates at low pH, whereas the latter becomes increasingly prevalent with increasing pH. In a CaCl2 solution, only binuclear bidentate complexes are observed. When solution pH is above the point of zero net proton charge (PZNPC) of hematite, oocyst surface carboxylate groups are bound to the mineral surface via outer-sphere complexes in both electrolyte solutions.

Montmorillonite surface properties and sorption characteristics for heavy metal removal from aqueous solutions

Surface properties of montmorillonite (MMT) and its adsorption characteristics for heavy metals have been investigated with nickel and copper as sorbate from aqueous solutions. Employing the potentiometric and mass titration techniques in batch experimental methods, the point of zero charge (PZC) and point of zero net proton charge (PZNPC) of MMT edges at different ionic strengths present pH PZC and pH PZNPC to be 3.4 ± 0.2. A crossing point was observed for the proton adsorption vs. pH curves at different ionic strengths of KCl electrolyte and in investigating MMT remediation potentialities as sorbent for heavy metals polluted waters, the effects of heavy metal concentration, pH, MMT dosage, reaction time and temperature for Cu 2+ and Ni 2+ uptake were studied. The sorption of metal ions by MMT was pH dependent and the adsorption kinetics revealed sorption rate could be well fitted by the pseudo-second-order rate model. The data according to mass transfer and intraparticle diffusion models confirmed diffusion of solutes inside the clay particles as the rate-controlling step and more important for the adsorption rate than the external mass transfer. Adsorption isotherms showed that the uptake of Cu 2+ and Ni 2+ could be described by the Langmuir model and from calculations on thermodynamic parameters, the positive Δ G° values at different temperatures suggest that the sorption of both metal ions were non-spontaneous. Change in enthalpy (Δ H°) for Ni 2+ and Cu 2+ were 28.9 and 13.27 kJ/mol K respectively, hence an endothermic diffusion process, as ion uptake increased with increase in temperature. Values of Δ S° indicate low randomness at the solid/solution interface during the uptake of both Cu 2+ and Ni 2+ by MMT. Montmorillonite has a considerable potential for the removal of heavy metal cationic species from aqueous solution and wastewater.

Photocatalytic reduction of Cr(VI) to Cr(III) in solution containing ZnO or ZSM-5 zeolite using oxalate as model organic compound in environment

Photocatalytic reduction of Cr(VI) to Cr(III) in aqueous solution containing ZnO or ZSM-5 zeolite under ambient condition was studied by using oxalate as model organic compound in the natural environment. ZSM-5 zeolite was characterized by X-ray diffraction (XRD), and point of zero net proton charge (PZNPC) titration. The effect of illumination time, mass content of catalyst (m/V), Cr(VI) initial concentrations, pH, ionic strength, and oxalate concentrations on the photocatalytic reduction of Cr(VI) was determined. The results indicate that the PZNPC of ZSM-5 zeolite is at pH 3.6 ± 0.1. At C[Cr(VI) (initial)] = 2.00 × 10 −4 mol/L, pH 7.5 ± 0.1 and after illumination time of 24 h, the reduction of Cr(VI) were 1.1 × 10 −5 mol/L (no ZSM-5 zeolite, 4.0 × 10 −3 mol/L oxalate) and 1.0 × 10 −5 mol/L (0.4 g/L ZSM-5 zeolite, no oxalate), respectively; whereas the reduction of Cr(VI) achieved 1.0 × 10 −4 mol/L in the presence of 0.4 g/L ZSM-5 zeolite and 4.0 × 10 −3 mol/L oxalate. The removal of Cr(VI) from solution is dependent on pH value. The results are important for the application of zeolites in the treatment of Cr(VI) polluted solution in the natural environment.

Biosorption of Cu(II) from aqueous solution by Fucus serratus: Surface characterization and sorption mechanisms

In this work, the brown alga Fucus serratus ( FS) used as a low cost sorbent has been studied for the biosorption of copper(II) ions in batch reactors. Firstly, the characterization of the surface functional groups was performed with two methods: a qualitatively analysis with the study of FT-IR spectrum and a quantitatively determination with potentiometric titrations. From this latter, a total proton exchange capacity of 3.15 mmol g −1 was extrapolated from the FS previously protonated. This value was similar to the total acidity of 3.56 mmol g −1 deduced from the Gran method. Using the single extrapolation method, three kinds of acidic functional groups with three intrinsic p K a were determined at 3.5, 8.2 and 9.6. The point of zero net proton charge (PZNPC) was found close to pH 6.3. Secondly, the biosorption of copper ions was studied. The equilibrium time was about 350 min and the adsorption equilibrium data were well described by the Langmuir’s equation. The maximum adsorption capacity has been extrapolated to 1.60 mmol g −1. The release of calcium and magnesium ions was also measured in relation to the copper biosorption. Finally, the efficiency of this biosorbent in natural tap water for the removal of copper was also investigated. All these observations indicate that the copper biosorption on FS is mainly based on ion exchange mechanism and this biomass could be then a suitable sorbent for the removal of heavy metals from wastewaters.

Determination of Point of Zero Charge of Tunisian Kaolinites by Potentiometric and Mass Titration Methods

AbstractThis paper deals with determining points of zero charge of natural and Na+‐saturated mineral kaolinites using two methods: (1) acid‐base potentiometric titration was employed to obtain the adsorption of H+ and OH− on amphoteric surfaces in solutions of varying ionic strengths in order to determinate graphically the point of zero net proton charge (PZNPC) defined equally as point of zero salt effect (PZSE); (2) mass titration curve at different electrolyte concentrations in order to estimate PZNPCs by interpolation and to compare with those determined by potentiometric titrations. The two methods involved points of zero charge approximately similar for the two kaolinites between 6.5‐7.8, comparable to those reported previously and were in the range expected for these clay minerals. The comparison of potentiometric surface titration curves obtained at 25 °C and those published in the literature reveals significant discrepancies both in the shape and in the pH of PZNPCs values.

Fullerol cluster formation in aqueous solutions: Implications for environmental release

Here we report on the characteristics of fullerol in aqueous systems and examine those conditions that affect the physical state of fullerol in water. When dispersed in water fullerol forms polydisperse suspensions characterized by both small ( ∼ 100 nm ) and large associations ( > 500 nm ). These clusters are charged with a point of zero net proton charge (PZNPC) of approximately pH 3. Though the size of fullerol clusters may be manipulated through changes in solution chemistry, principally pH, cluster formation cannot be entirely prevented through these means alone. The fullerol cluster structure is amorphous as revealed by X-ray diffraction analysis, which is in contrast to clusters of C 60 formed through dissolution in toluene and then introduced into water through sonication (SONnC 60). The SONnC 60 clusters are crystalline with a structure similar to that of unreacted C 60 crystals.

Some Interface Electrochemical Properties of Kaolinite

AbstractThe interface electrochemical properties of clay were theoretically analyzed to obtain some relationships among point of zero net charge (PZNC), point of zero net proton charge (PZNPC), intrinsic surface reaction equilibrium constants (K in 1‐pK model, Kinta1 and Kinta2 in 2‐pK model, *KintNa and *Kint in inert electrolyte chemical binding model) and structural negative charge density (σst) of clay, and some interface electrochemical parameters of kaolinite were measured. The following main conclusions were obtained. For clay possessing structural negative charges, the PZNC independent of electrolyte concentration (c) should exist just as amphoteric solid without structural charges such as oxides or hydroxides. A common intersection point (CIP) should appear among the potentiometric (or acid‐base) titration curves at different c and the pH at the CIP should be pHPZNC. A CIP among potentiometric titration curves at different c for kaolinite was observed, and the value of pHPZNC of kaolinite was 2.16. The values of pHPZNPC were decreased with increasing c, which arises from the presence of structural negative charges of kaolinite. In addition, it was observed that a good linear relationship existed between pHPZNPC and lg c. According to the values of pHPZNC and σst measured, the intrinsic surface reaction equilibrium constants, pK and pKinta1 and pKinta2 of 1‐pK and 2‐pK models could be directly calculated for clay, and the values of pK, pKinta1 and pKinta2 for kaolinite were 2.93, 1.90 and 3.97, respectively. These experimental values of pKinta1 and pKinta2 for kaolinite are obviously lower than those optimized with fitting programs in literatures, which maybe arises from the introduction of a type of permanent negatively charged sites in the models of literatures. An interesting result obtained in this study is that the inert electrolyte chemical binding does not exist for kaolinite, which also arises from the presence of structural negative charges.

Acid-base chemistry of montmorillonitic and beidellitic-montmorillonitic smectite

Suspension of a Tunisian purified smectite and American montmorillonite are studied by acid-base potentiometric and mass titrations. These experimental methods are used to determine the point of zero net proton charge (PZNPC). A very good agreement is observed between the two kinds of experiments. The two Namontmorillonites, studied at different ionic strengths, present proton adsorption vs. pH curves with a common crossing point. The PZNPC of the edge sites are 8.02 for Tunisian purified smectite and 8.11 for pure American montmorillonite. By analyzing the proton adsorption or desorption (H+ vs. pH) curves, one may assume the presence of four active sites at the surface. The montmorillonite surface undergoes two successive protonations and two successive deprotonations. Below pH A1OH groups) occur simultaneously. For pH > PZNPC and in alkaline pH range, deprotonation of surface hydroxyl groups exposed at the edge sites (>SiOH, and >A1OH at high pH) of montmorillonite platelets causes an overall negative charge.

Surface charge heterogeneity of kaolinite in aqueous suspension in comparison with montmorillonite

An analogous study to 2:1 type montmorillonite [Tombácz, E., Szekeres, M., 2004. Colloidal behavior of aqueous montmorillonite suspensions: the specific role of pH in the presence of indifferent electrolytes. Appl. Clay Sci. 27, 75–94.] was performed on 1:1 type kaolinite obtained from Zettlitz kaolin. Clay minerals are built up from silica tetrahedral (T) and alumina octahedral (O) layers. These lamellar particles have patch-wise surface heterogeneity, since different sites are localized on definite parts of particle surface. pH-dependent charges develop on the surface hydroxyls mainly at edges besides the permanent negative charges on silica basal plane due to isomorphic substitutions. Electric double layers (edl) with either constant charge density on T faces (silica basal planes) or constant potential at constant pH on edges and O faces (hydroxyl-terminated planes) form on patches. The local electrostatic field is determined by the crystal structure of clay particles, and influenced by the pH and dissolved electrolytes. The acid–base titration of Na-kaolinite suspensions showed analogous feature to montmorillonite. The initial pH of suspensions and the net proton surface excess vs. pH functions shifted to the lower pH with increasing ionic strength indicating the presence of permanent charges in both cases, but these shifts were smaller for kaolinite in accordance with its much lower layer charge density. The pH-dependent charge formation was similar, positive charges in the protonation reaction of (Si–O) Al–OH sites formed only at pHs below ∼ 6–6.5, considered as point of zero net proton charge (PZNPC) of kaolinite particles. So, oppositely charged surface parts on both clay particles are only below this pH, therefore patch-wise charge heterogeneity exists under acidic conditions. Electrophoretic mobility measurements, however, showed negative values for both clays over the whole range of pH showing the dominance of permanent charges, and only certain decrease in absolute values, much larger for kaolinite was observed with decreasing pH below pH ∼ 6. The charge heterogeneity was supported by the pH-dependent properties of dilute and dense clay suspensions with different NaCl concentrations. Huge aggregates were able to form only below pH ∼ 7 in kaolinite suspensions. Coagulation kinetics measurements at different pHs provided undisputable proofs for heterocoagulation of kaolinite particles. Similarly to montmorillonite, heterocoagulation at pH ∼ 4 occurs only above a threshold electrolyte concentration, which was much smaller, only ∼ 1 mmol l − 1 NaCl for kaolinite, than that for montmorillonite due to the substantial difference in particle geometry. The electrolyte tolerance of both clay suspensions increased with increasing pH, pH ∼ 6–6.5 range was sensitive, and even a sudden change occurred above pH ∼ 6 in kaolinite. There was practically no difference in the critical coagulation concentration of kaolinite and montmorillonite (c.c.c.∼ 100 mmol l − 1 NaCl) measured in alkaline region, where homocoagulation of negatively charged lamellae takes place. Rheological measurements showed shear thinning flow character and small thixotropy of suspensions at and above pH ∼ 6.7 proving the existence of repulsive interaction between uniformly charged particles in 0.01 M NaCl for both clays. The appearance of antithixotropy, the sudden increase in yield values, and also the formation of viscoelastic systems only at and below pH ∼ 6 verify the network formation due to attraction between oppositely charged parts of kaolinite particles. Under similar conditions the montmorillonite gels were thixotropic with significant elastic response.

Intrinsic surface reaction equilibrium constants of structurally charged amphoteric hydrotalcite-like compounds

The relative equations among intrinsic surface reaction equilibrium constants ( K in 1-p K model, K a1 int and K a2 int in 2-p K model, and ∗K Na int and ∗K Cl int in inert electrolyte chemical binding model), points of zero charge (PZC), and structural charge density ( σ st) for amphoteric solids with structural charge were established to investigate the effects of σ st on intrinsic equilibrium constants and PZC. The intrinsic equilibrium constants of HTlc with general formulas [(Zn,Mg) 1− x Al x (OH) 2](Cl,OH) x and [Mg 1− x (Fe,Al) x (OH) 2](Cl,OH) x were evaluated. The following main conclusions were obtained. For amphoteric solids with structural charge, a point of zero net charge (PZNC) independent of electrolyte concentration ( c) exists. A common intersection point (CIP) should appear among the acid–base titration curves at different c, and the pH at the CIP is pH PZNC. The p K, p K a1 int, and p K a2 int may be expressed as a function of pH PZNC and σ st, and these intrinsic equilibrium constants can be directly calculated from pH PZNC and σ st. The inert electrolyte chemical binding does not exist for amphoteric surfaces with structural charge. PZNC is not equal to the point of zero net proton charge (PZNPC) when σ st≠0. pH PZNC > pH PZNPC when σ st>0; pH PZNC < pH PZNPC when σ st<0; and pH PZNC = pH PZNPC only when σ st=0. With increasing c, the difference between pH PZNC and pH PZNPC decreases; i.e., pH PZNPC moves forward to pH PZNC with increasing c. For the HTlc samples studied, with increasing x, the pH PZNC and the p K a1 int and p K a2 int decrease, and the p K increases. These results can be explained on the basis of the affinity of metal cations for H + or OH − and the electrostatic interaction between the charging surface and H + or OH −.

Points of Zero Charge and Potentiometric Titrations

Based on the various categories of surface charge and the definitions of points of zero charge proposed by Sposito (1984, 1998), the determination of the adsorbed proton charge (σH) and the point of zero net charge (PZNPC) of insoluble oxides and layer silicates in aqueous solution was examined in detail using potentiometric titration experiments. The confusion, various interpretations and difficulties associated with points of zero charge, including the point of zero net proton charge (PZNPC), the point of zero net charge (PZNC) and the point of zero charge (PZC), were discussed and clarified. The importance of comparing potentiometric titration curves for pure insoluble oxides in the presence and absence of specific adsorption and of layer silicates with a permanent negative charge was emphasized. The dependency of expressions of points of zero charge on the model assumed and the weakness of the method of potentiometric titrations widely used for the determination of points of zero charge were discussed.

Removal of phosphates from aqueous solutions by using bauxite II: the activation study

AbstractIn this study, acid treatment and heat treatment methods have been investigated in order to enhance the phosphate adsorption capacity of bauxite. For this purpose, a series of bauxites treated with 0.1–1.0 M HCl and another one heated at various temperatures between 200 and 1000 °C were subjected to standardized orthophosphate adsorption tests. Besides determining chemical and mineralogical compositions, TGA and DTA were performed and point of zero net proton charge (PZNPC), surface area, mean particle size and porosity were measured for selected samples. The results have shown that the acid treatment of bauxite has a negative effect on the phosphate adsorption capability. On the other hand, it has been observed that the phosphate adsorption capacity of bauxite could be increased by heating. The optimum heating temperature was determined as 600 °C. Bauxite heated at 600 °C exhibited about an eight‐fold increase in surface area compared with raw bauxite. The results showed that the activation of bauxite occurred via dehydration of boehmite and diaspore, being the hydrated mineral phases. Maximum adsorption efficiencies for ortho‐, tripoly‐ and glycerophosphates were achieved in the slightly acidic pH range. It was found that the adsorption capacities of thermally‐activated bauxite for all phosphate species investigated were higher than that of raw bauxite. But the increase in adsorption efficiency is not proportional to the increase in specific surface area. It was found that the relative adsorptivity of phosphate species investigated is in the order of orthophosphate &gt; tripoly(phosphate) &gt; glycerophosphate. In addition, it was found that the desorption trends of these phosphate species were similar to the results obtained for raw bauxite. Copyright © 2003 Society of Chemical Industry

Proton Adsorption and Electrokinetics of an Argentinean Montmorillonite

Suspensions of an argentinean Na+-exchanged montmorillonite were studied by acid–base potentiometric titrations, mass titrations and electrophoresis. Potentiometric titrations were employed to obtain proton adsorption vs pH curves at different NaCl concentrations whereas mass titrations were used to determine the dependence of the point of zero net proton charge (PZNPC) with the ionic strength. The studied Na+montmorillonite presented proton adsorption vs pH curves typical of materials carrying mainly structural negative charges and few hydroxyl surface groups: the PZNPC decreased with the ionic strength, and the different titration curves ran almost parallel to each other. No crossing point could be observed for several titration curves performed at different NaCl concentrations. The EM was also typical of particles that carry mainly negative charges: there was only a slight decrease in the negative electrophoretic mobility when the pH was decreased from 11 to 3. The kinetics of proton adsorption–desorption processes was fast in alkaline media and slower at pH lower than ca. 8.5. This behavior appears to be a consequence of edge-to-face interactions and suggests that the edges become positively charged at pH <8.5. A model of the 2:1 type clay water interface is proposed to fit proton adsorption and electrophoretic data of the studied sample and those previously published for an illite. The model assumes the presence of structural negative charges in the clay particle whereas variable charge sites and cation exchange sites are assumed to be located at the particle surface. The model predicts that H+adsorption at high pH takes place mainly on variable charge sites. They become positively charged at pHs lower than the PZNPC, which is in agreement with kinetics measurements. H+adsorption is only important at pH < 6 and it reaches values close to the cation exchange capacity of the sample at pHs 2–3. Although this proton adsorption is high at low pHs, it takes place mainly by a H+–Na+exchange mechanism and does not modify appreciably the net negative charge of the particle. This nearly constant charge induces a negative surface potential that is responsible for negative zeta potential of the particles. The negative surface potential also modifies the normal behavior of the variable charge sites so that the PZNPC is predicted to change by changing the electrolyte concentration. The model also predicts that the affinity of surface sites for cations is higher in illites than in montmorillonites. This is probably due to the fact that isomorphic substitutions take place beneath the plane of basal oxigens in illites, modifying their electronic population and increasing their cation binding capabilities.