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.
Highlights
Nowadays, persistent organic compounds (POPs)are considered chemical compounds of great environmental concern [1]
MO, USA). para-chlorobenzoic acid (p-CBA) was obtained from the same company, and it was used as a model compound at an initial concentration of 4 μM
The first part of this study focuses on the effect of thermal treatment on the structure and major physicochemical characteristics of the examined minerals/potential catalysts for the heterogeneous catalytic oxidation process
Summary
Persistent organic compounds (POPs)are considered chemical compounds of great environmental concern [1]. Several classes of them occur in aquatic ecosystems [2] but with rather low concentrations, ranging from ng/L–μg/L (they are termed as micropollutants). They are very stable compounds and cannot be degraded or removed by the application of conventional biological wastewater treatment technologies [1]. Their quantities are still quite low, they can constantly accumulate in fragile ecosystems, and there is a high probability of side effects in living organisms throughout the food chain [2]. The treated wastewaters may contain several compounds of emerging concern, released into water bodies and eventually ending up in marine environments [3]
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