Master of Science (MSc)
Biological and Chemical Sciences
Faculty of Science
Surface chemistry is the study of the chemical and physical phenomena that transpire at interfaces such as liquid-solid, solid-gas, liquid-gas, and liquid-liquid. To study the reactions occurred at the surfaces, surface sensitive techniques come to play. Adsorption of arsenical compounds (liquid adsorbate), particularly arsenate (iAs) and dimethylarsinic acid (DMA), to the surface of iron bearing materials (solid adsorbent) that frequently are found in geosorbents such as hematite (Fe2O3) has been studied in this thesis. Arsenical compounds can pollute the environment through natural sources and anthropogenic activities, and ultimately contaminate water. Absorbing these pollutants to a material as an absorber is one way of filtering drinking water. One of the efficient absorbents for arsenicals are hematite nanoparticles. Hematite is an iron-oxide and a soil component that has a great affinity for the arsenical compounds.
The present study was performed under pH 7 at temperature ranges of 5-35℃ for iAs and 5-50℃ for DMA. DMA has only one hydrogen and at pKa 7 it is dissociated and becomes negatively charged. Arsenate has three hydrogens, and at pKa 7 its two hydrogens are dissociated, and it becomes negatively charged. At around pH 7, the hematite nanoparticles (Fe2O3) are positively charged. As a result, electrostatic attraction occurs between these negatively charged arsenicals to the surface of the positively charged hematite. Effectiveness of this attraction varies by factors such as temperature, concentration, time, and pH. Temperature was found to be the main factor, specifically in the case of DMA. Temperature eased the formation of a type of strong binding structure called bidentate inner sphere in which covalent binding is involved. Due to the two methyl groups in DMA, bidentate inner sphere cannot be easily formed on the surface. Therefore, DMA cannot be absorbed strongly, and it primarily forms weak van der Waals monodentate inner sphere or outer sphere structures on the surface. DMA needs energy to defeat this strain in the transition state. The energy needed is called barrier energy or activation energy, and is provided by increasing the temperature. In comparison, based on previous studies, arsenate forms predominantly bidentate inner sphere at room temperature. As a result, it may not need high temperature to incapacitate the transition state boundaries. Arsenate has been studied alongside DMA for estimating its activation energy in this study. The activation energy was calculated for both iAs and DMA under identical conditions to compare the results. Four levels of concentration were examined for iAs and DMA in 0.01 M of NaCl solution.
The surface sensitive technique used in this study was attenuated total internal reflectance Fourier transform infrared spectroscopy (ATR-FTIR) under environmentally relevant conditions to study the in situ surface interactions of the arsenicals at the molecular level. Initial observed rates (robs) of the first 2 minutes of the adsorption reaction were extracted and upon that pseudo-first order rate constants (kads) were quantified at different temperatures and concentrations. Eventually, in order to calculate the activation energy, Arrhenius equation was employed.
The significance of this study is that it would be beneficial for cleaning drinking water from arsenical compounds by interpreting the obtained kinetic parameters and modeling the suitable system.
soldoozy, sara, "TEMPERATURE-DEPENDENT KINETIC STUDIES OF ARSENICALS ADSORPTION FROM SOLUTION TO HEMATITE NANOPARTICLES" (2019). Theses and Dissertations (Comprehensive). 2204.