pH-dependent surface charging of metal oxides

Aqueous dispersions of metal oxide particles are of great practical interest due to their wide-ranging industrial applications and everyday uses under the conditions required. The latter involves the broad variability of pH and dissolved materials among them electrolytes which have definite role in charge neutralization in the vicinity of particle surface. In this work, the pH-dependent surface charging of metal oxides due to the specific adsorption of H+/OH- in the presence of indifferent and specific ions, and other simultaneous processes at solid/water interface such as hydration of surface, dissociation of surface sites, dissolution of solid matrix, then hydrolysis of dissolved ions are discussed. The governing role of solid particles will be showed by means of characteristic examples of aluminum, iron, titanium and zinc oxides, which have significant differences in chemical properties considering e.g., their pH-dependent dissolution and the speciation of dissolved ions

Surface charge characterization of metal oxides by potentiometric acid–base titration, revisited theory and experiment

The increased interest in architecting nanocomposites and in clarifying their impact on health and environment necessitates a standardized method of surface characterization, because the behaviour of nanoparticles is governed largely by their surface free energy and interfacial chemistry. Potentiometric acid–base titration has been used for decades to determine surface charge density of oxides, tremendously useful in understanding and modelling oxide/electrolyte interfaces. The method has solid theoretical and experimental foundation. Yet, published values of the point of zero charge (PZC) of chemically identical oxides scatter widely: the deviation of seven units of pH is the rule, rather than an exception. The PZC must reflect the inherent charging behaviour, linked exclusively to the protonation, and determined by the chemistry of the oxides. Assignment of PZC (just like assignment of equilibrium constant of any chemical reaction) requires strict conditions and consistent protocol. We present here such a protocol, and some examples of application. The protocol leads through the basic steps: pH buffer calibration, proton concentration calibration in the background electrolytes, and titrations of oxides. Built-in tests help to eliminate artificial side effects simultaneously from the measurement of pH and from the oxide/electrolyte interfacial equilibria. Although these may seem self-evident, literature survey proves the need to go back to basics in order to obtain precise, reliable and comparable pH-dependent charging of oxides. It is most general that the oxides contain some unrecognized surface impurities and have a strongly pH-dependent dissolution. These distort the results of titrations; their effect increases with decreasing particle size and can become dominant for nanoparticles. The protocol presented here contains the necessary feedback loops to easily recognize and exclude these side effects

SURFACE CHARACTERIZATION OF A POLYACRYLONITRILE BASED ACTIVATED CARBON AND THE EFFECT OF pH ON ITS ADSORPTION FROM AQUEOUS PHENOL AND 2,3,4-TRICHLOROPHENOL SOLUTION

Granular highly nanoporous activated carbon was prepared from polyacrylonitrile (PAN). The BET surface area was found to be 544 m2/g with V\rm tot = 0.278 cm3/g, and V\rm micro= 0.266 cm3/g. Aqueous treatment reduced the surface area to 364 m2/g. This carbon contains O and N related surface functional groups. Surface groups containing these hetero atoms are responsible for the acid/base character of this carbon in aqueous solutions. The microporous pore network produces a wide hysteresis loop, observed when the granular carbon was studied by continuous titration. This loop, which was found to diminish, but not to disappear completely when the particle size was reduced, is attributed to irreversible hydrolysis of surface esters and/or lactones. The surface concentration of the functional groups titrated by the equilibrium Boehm method is 449.2 \mu equiv/g, of which 112.9 \mu equiv/g have acidic and 336.3 \mu equiv/g have basic character. Within the acidic species three subgroups were distinguished corresponding to carboxylic, lactonic and phenolic surface groups. PAN based carbon was investigated as a means of removing weak aromatic acids, phenol and 2,3,4-trichlorophenol from aqueous solutions. Both the adsorption capacity and the overall interaction parameter, K (both derived from a fit to the Langmuir equation) depend on the adsorbed species and on the pH. The former is a consequence of the different water solubilities of the solute molecules, while the latter stems from the pH sensitivity of both the surface functional groups and these weak acids. The effect of pH is more marked for 2,3,4-trichlorophenol

Preface

Special issue of Periodica Polytechnica Chemical Engineering is devoted to papers presented in the 10th jubilee Colloid Chemistry Conference organized in Budapest in 2012

Felületkémia és szelektivitás nagyfelületű szeneken = Chemical structure and performance of high surface area carbons

A hamumentes polimer alapú szeneken vizsgáltuk a felületkémia és a szorpciós tulajdonságok közti kapcsolatot. Morfológiailag igen hasonló, vízmentes és vizes közegben jellemzett oxigén, nitrogén és hidrogén heteroatomot tartalmazó szeneken mind gőz-, mind hígoldat adszorpciós folyamatokban igazoltuk a felület kémiai tulajdonságainak meghatározó jellegét. A felületi tulajdonságok nem csak az egyensúlyt, de a folyamatok sebességét is befolyásolják. Új függvényt vezettünk be a póruskitöltődés nyomonkövetésére és ennek használhatóságát mind poláros, mind apoláros gőzök esetén bemutattuk. Vizsgáltuk további, elsősorban fém heteroatomok beviteli lehetőségeit polimer alapú szénmátrix kialakításának különböző stádiumaiban. Megállapítottuk, hogy lignocellulóz alapú szenek esetében a Na és Si atomok bevitele (vízüveges módosítás) gyakorlatilag nem befolyásolja a vizsgált szorpciós tulajdonságokat, de kedvezően hat a mechanikai tulajdonságokra. Amennyiben a fémionok hozzáadása a prekurzor polimer szintézise során ill. a pórusos szerkezet rögzítése (szárítás) előtt történik, a mátrix szerkezete, a fajlagos felület és a pórusszerkezet is jelentősen módosul. Utólagos kezelés esetén a porozitáshoz kapcsolható változásokat elsősorban a pórusok eltömődése okozza. | The relationship between surface chemistry and sorption performance was studied on ash-free polymer-based carbons. In carbons that were morphologically similar and contained oxygen, nitrogen and hydrogen hetero atoms, the role of the surface chemistry was demonstrated in sorption processes from both the vapour and the liquid phase. The surface properties influence not only the adsorption equilibrium but play a role in the kinetics as well. A new function was introduced to follow the pore filling. Its applicability was shown for polar and nonpolar vapours. The possibility of introducing further, mainly metal, heteroatoms was studied at various stages of the carbon synthesis. In the case of lignocellulosic matrices we found that introducing Si and Na atoms (in the form of waterglass) results in improvement of mechanical strength rather than of the adsorption properties. When metal ions were added to the polymer precursor during the polymerization step or before the drying process, both the morphology and the porosity of the matrix can be severely influenced. Post-carbonization treatment, however, leads to obstruction of the pore entrance and thus a reduction of the accessible porosity

Antioxidant Effect of Humic Substances from Hungarian Leonardite

Humic substances are natural substances that are continuously formed from the decay of plant residues. These materials have a very diverse range of properties, making them versatile. According to many new studies, these humic substances also exhibit antioxidant propensities. The aim of this paper was to shed light on whether humic substances really have antioxidant properties