(374a) Advanced Oxidation Processes for the Destruction of Chlorinated Pesticides: Free Radical Mechanisms, Pathways, Products and Reaction Products

Advanced oxidation processes including ozonation, direct ultraviolet (UV) photolysis, ozonation, hydrogen peroxide/ultraviolet radiation/ (HP/UV) oxidation, and hydrogen peroxide/ozone (HP/ozone) oxidation were studied for the destruction of the chlorinated pesticides alachlor and heptachlor. Reaction mechanisms and pathways were proposed for these oxidation processes, and the product formations were studied. The oxidation mechanisms, as the formation of intermediates and products for the above AOPs were studied. More importantly, the adverse effects of free-radical scavengers including natural organic matter (NOM) such as humic acid, and the carbonate species on the reaction kinetics were studied.

Direct UV photolysis and HP/UV oxidation of alachlor were studied for different UV intensities resulting in 99% reduction of alachlor. Maximum destruction of alachlor was observed for the HP/UV oxidation. The pseudo-first order rate constants were 10-15 times higher for HP/UV oxidation than for direct UV photolysis at similar UV intensities, highlighting the role of hydrogen peroxide in free-radical kinetics. Influence of pH and free-radical scavengers such as bicarbonate and carbonate ions, as well as NOM were evaluated for UV photolysis and HP/UV oxidation. Limited studies were conducted for ozonation of alachlor in OFW under different pH values, having highest oxidation efficiency at pH 7 or above, since the process was facilitated by the hydroxyl radical as the primary oxidant.

Ozonation of heptachlor was studied under different conditions such as pH, initial pesticide concentration, and influence of free radical scavengers such as carbonate species and NOM. The results showed that direct ozonation facilitated decomposition of the pesticide, while the hydroxyl radical was not effective as primary oxidant. The influence of hydrogen peroxide and NOM on the free radical reaction kinetics were evaluated. The decline in heptachlor decomposition rate was exponentially proportional to NOM concentration. Ozonation studies also showed that the decomposition rates for 1-hydroxyclordene, the hydrolytic metabolite of heptachlor, were three times those for heptachor. The heptachlor oxidation products for ozone and HP/ozone oxidation processes were formaldehyde, acetaldehyde, glyoxal, glyoxylic acid and heptachlor epoxide, although these processes manifested subtle variations in reaction mechanisms. The concentrations of heptachlor and heptachlor epoxide detected in the product products were much below their regulated MCLs of 0.4 micro-grams/liter and 2 micro-grams/liter, respectively. The study further demonstrated that ozonation was more effective than HP/ozone oxidation in the destruction of heptachlor.

The investigations showed that the AOPs under investigation were very effective in destruction of alachlor and heptachlor, meeting their MCL requirements at short reaction times and low oxidant dosages. The proposed AOP reaction mechanisms could adequately explain the product formations. The ddition of hydrogen peroxide to UV radiation accelerated the decomposition of alachlor due to faster generation of hydroxyl radicals. Free-radical scavengers including carbonate species and NOM adversely affected the oxidation efficiencies.