The Effect of Thermal Treatment on the Physicochemical Properties of Minerals Applied to Heterogeneous Catalytic Ozonation

Abstract

In order to enhance the efficiency of heterogeneous catalytic ozonation, the effect of thermal treatment on three commonly used and inexpensive minerals, i.e., zeolite, talc and kaolin (clay), which present different physicochemical properties as potential catalysts, has been examined for the removal of para-chlorobenzoic acid (p-CBA). p-CBA is considered a typical micro-pollutant, usually serving as an indicator (model compound) to evaluate the production of hydroxyl radicals in ozonation systems. The catalytic activity of selected solid catalysts was studied for different pH values (6, 7 and 8) and different temperatures (15 °C, 25 °C and 35 °C). The mechanism of radicals’ production was also verified by the addition of tert-butyl alcohol (TBA). The respective thermal behavior study showed that the point of zero charge (PZC) of these minerals increased with the increase of applied treatment temperature, as it removed crystalline water and hydroxyls, thus improving their hydrophobicity. Circa-neutral surface charge and the presence of hydrophobicity were found to favor the affinity of ozone with solid/catalytic surfaces and the subsequent production of hydroxyl radicals. Therefore, zeolite and talc, presenting PZC 7.2 and 6.5 respectively, showed higher catalytic activity after thermal treatment, while kaolin with PZC equal to 3.1 showed zero to moderate catalytic efficiency. The degradation level of p-CBA by oxidation was favored at 25 °C, while the pH value exerted positive effects when it was increased up to 8.

Comparative study on heterogeneous and homogeneous catalytic ozonation efficiency in micropollutants' removal

“…All solutions were prepared in distilled water. Based upon preliminary experiments (Psaltou et al 2020a(Psaltou et al , 2020b, calcite (a widely found mineral) was selected as solid catalyst for the heterogeneous catalytic ozonation experiments. CoCl 2 •6H 2 O (PRS Panreac, Spain) and FeSO 4 •7H 2 O (Chem-lab, Belgium) were used as sources for the dissolved metal cations, applied as homogeneous catalysts.…”

“…Furthermore, the ozone molecules are more stable in the aqueous solutions of lower temperature; therefore, in this case the ozone decomposition reactions and the respective production of radicals are slower (Psaltou et al 2020a). Thus, the removal efficiency of p-CBA was decreased by reducing the solution temperature (Figure 1(b)).…”

“…Calcite is a low-cost, raw material with high commercial availability and it can be used without further modification. In a previous research (Psaltou et al 2020a(Psaltou et al , 2020b, it was pointed out that it can be used as a catalyst with high removal efficiency, especially for the case of p-CBA, which cannot be removed by its oxidative reaction with ozone molecules. Therefore, it is considered as an interesting alternative for application in heterogeneous catalytic ozonation systems, although its activity in different experimental conditions or against other micropollutants has not been yet clearly evaluated.…”

Catalytic Membrane Ozonation

“…The reduction of pH/temperature or the addition of carbonate alkalinity are more effective ways to control the BrO 3 − production; however, it is neither cost effective, nor practical considering full-scale applications [34,36]. Furthermore, the control of these parameters may present also negative effects regarding the micropollutants removal, e.g., by slowing down the production of hydroxyl radicals [39]. Therefore, a key challenge is to develop a process that can effectively degrades micropollutants and simultaneously minimizes the formation of BrO 3 − [36].…”

“…Ozone escape to the gas phase at the 3rd min of ozonation process and its typical half-life at various temperatures[39,40].…”