Clopyralid degradation using solar-photocatalytic/ozone process with olive stone activated carbon

Abstract

Clopyralid degradation in ultrapure water was investigated by homogeneous ozonation (O3), catalytic ozonation with Olive Stone Activated Carbon (O3/OSAC), under simulated sunlight radiation by photolytic ozonation (O3/Daylight) and photocatalytic ozonation (O3/OSAC/Daylight). The OSAC was characterized by several analytical techniques, such as Nitrogen adsorption desorption at 77 K, Scanning Electron Microscopy (SEM-EDX), determination of pHpzc and X-ray Photoelectron Spectroscopy (XPS). This herbicide exhibited a rapid oxidation rate in the simultaneous presence of ozone, activated carbon and simulated sunlight. The combined process led to a complete removal of clopyralid in less than 30 min. Solar-photocatalytic ozonation was the most efficient process both in terms of clopyralid elimination rate and mineralization. Clopyralid degradation mainly occurs through hydroxyl radical in those systems involving ozone. For comparison purposes, the different processes can be modelled by simple first-order kinetics in clopyralid concentration (rate constants khomo = 0.0159 min−1, homogeneous ozonation; kO3/Daylight = 0.0410 min−1, kO3/OSAC = 0.0705 min−1, kO3/OSAC/Daylight = 0.1433 min−1). 90% of Total Organic Carbon (TOC) removal after 3 h was reached in photocatalytic ozonation (O3/OSAC/Daylight) and photolytic ozonation (O3/Daylight) compared to about 70% using O3/OSAC. Single ozonation just led to 20% TOC reduction under similar operating conditions. Furthermore, small oxygenated organic and inorganic by-products were analyzed by ionic chromatography. As a rule of thumb, TOC removal led to the formation of low weight carboxylic acids, taking place the release of chlorides and nitrates. Finally, toxicity evolution during O3, O3/OSAC, O3/Daylight and O3/OSAC/Daylight were monitored considering phytotoxicity trials of Lactuca Sativa germination. Phytotoxicity assays indicate almost total detoxification after the photocatalytic ozonation treatment.