Jesús C. Echeverría

RETENTION MECHANISMS OF Cd ON ILLITE

The adsorption of metals by clay minerals is a complex process involving different mechanisms, and is controlled by different variables which can interact. The aim of this work was to study the retention mechanisms of Cd on illite. We obtained Cd adsorption isotherms at constant pH, adsorption edges as a function of pH, adsorption isotherms at 5, 25 and 45°C (pH = 7), and response surfaces of the simultaneous effect of pH, initial concentration, ionic strength, and temperature on the retention of Cd on illite. Below pH 6, adsorption of Cd on illite is via ion exchange with H3O+ and Na+ ions which saturate the exchange sites, the exchange with Na+ being the main mechanism between pH 4.5 and 6.0. For pH values >6, the effect of ionic strength on the amount of Cd2+ retained decreased with pH, being negligible at pH 8; the proton stoichiometry was greater than for pH values <6 and an increase in the temperature favored the retention of Cd. These facts are compatible with a more specific process involving hydrolyzed species, in which Cd can associate with illite as an inner sphere complex.

A fiber-optic sensor to detect volatile organic compounds based on a porous silica xerogel film

Fiber-optic sensors are increasingly used for the determination of volatile organic compounds (VOCs) in air matrices. This paper provides experimental results on the sensitivity of a fiber-optic sensor that uses a film of a porous silica xerogel as the sensing element. This film was synthesized by the sol-gel process and affixed to the end of the optical fiber by the dip-coating technique. This intrinsic sensor works in reflection mode, and the transduction takes place in the light that travels through the core of the fiber. The VOCs included in this research cover a wide range of compounds with different functional groups and polarities. The highest sensitivity was for 2-propanol (13.1±1.4 M(-1) nm(-1)), followed by toluene (11.4±1.4 M(-1) nm(-1)), and 1-butylamine (9.5±0.4 M(-1) nm(-1)). Acetone and cyclohexane had the lowest sensitivity of all studied VOCs. Limits of detection varied between 9.1×10(-5) M for 1-butylamine and 1.6×10(-3) M for ethanol. Silanol groups on the xerogel surface act as weak acids and interact strongly with molecules that contain OH groups like alcohols, π-electrons like toluene, or a lone pair of electrons like toluene. Stronger interaction of methanol and ethanol with the silanol groups on the film led to some irreversible adsorption of these analytes at room temperature.

Fiber optic sensors based on hybrid phenyl-silica xerogel films to detect n-hexane: determination of the isosteric enthalpy of adsorption

We investigated the response of three fiber optic sensing elements prepared at pH 10 from phenyltriethoxysilane (PhTEOS) and tetraethylsilane (TEOS) mixtures with 30, 40, and 50% PhTEOS in the silicon precursor mixture. The sensing elements are referred to as Ph30, Ph40 and Ph50, respectively. The films were synthesized by the sol–gel method and affixed to the end of optical fibers by the dip-coating technique. Fourier transform infrared spectroscopy, N2 adsorption–desorption at 77 K and X-ray diffraction analysis were used to characterize the xerogels. At a given pressure of n-hexane, the response of each sensing element decreased with temperature, indicating an exothermic process that confirmed the role of adsorption in the overall performance of the sensing elements. The isosteric adsorption enthalpies were obtained from the calibration curves at different temperatures. The magnitude of the isosteric enthalpy of n-hexane increased with the relative response and reached a plateau that stabilized at approximately −31 kJ mol−1 for Ph40 and Ph50 and at approximately −37 kJ mol−1 for Ph30. This indicates that the adsorbate–adsorbent interaction was dominant at lower relative pressure and condensation of the adsorbate on the mesopores was dominant at higher relative pressure.

Combining DLS, XRD, SEM-EDAX and EXAFS in the study of Zn(II) retention on a palygorskitic clay

Abstract Clay materials play a key role in determining the retention capacity of a soil, and are widely used in waste treatments. One of the most commonly used clays is palygorskite. The aim of this research is to determine the chemical species formed by Zn when retained in a palygorskitic clay material. Adsorption isotherm analysis is useful in studying the retention process, because it provides a macroscopic view of the retention phenomena. Complementary techniques are needed in order to study the different retention processes. Sorption isotherms of Zn on palygorskitic clay were carried out; the supernatant was analysed by means of dynamic light scattering (DLS) and the residues by using X-ray diffraction (XRD), scanning electron microscopy-energy dispersive angle X-ray (SEM- EDAX)analysis and extended X-ray absorption fine structure (EXAFS). Isotherm analysis shows that the global retention process could be due to the sum of two separate processes, adsorption and surface precipitation via solid-solution. This is supported by DLS, which shows that z potential increases as the Zn(II) is retained onto clay surfaces but remains constant during the precipitation process. The XRD pattern corresponding to the Zn-clay system showed weak new peaks, probably from zincite. The microanalysis by X-ray fluorescence of several spots selected for their different electronic densities indicated that the retained Zn was randomly distributed across the clay surface. Analysis by EXAFS supports the hypothesis of retention via adsorption and solid-solution surface precipitation.