Photochemical acetochlor degradation induced by hydroxyl radical in Fe-amended wetland waters: Impact of pH and dissolved organic matter.

Iron (Fe) plays a critical role in the formation of hydroxyl radical (OH) which may participate in the indirect photodegradation of aquatic contaminants. While Fe photochemistry has been extensively studied, the efficacy of iron amendments for contaminant attenuation in sunlit natural waters has not been well researched. We studied the efficacy of this approach by monitoring OH induced acetochlor (AC) degradation and determining OH production rates with terephthalate (TPA) as a probe. Surface wetland waters as well as model fulvic acid (FA) solutions were amended with Fe(III) salt at different concentrations at pH values of 2.7, 5, and 7.6. We observed no significant enhancement in the AC degradation rate at circumneutral pH. At pH 5, AC degradation increased by more than 50% with an Fe addition up to an [Fe]T  ≈ 6 μM and plateaued at high [Fe]T . At the highly acidic pH of acid mine drainage (AMD) waters, AC degradation was enhanced by two-orders-of magnitude with increasing [Fe]T and no plateau was observed under the conditions tested ([Fe]T  ≤ 500 μM). While the Fe induced relative difference in OH production rates determined using TPA was useful in elucidating the reaction mechanism for different dissolved organic matter types at different pH values, the absolute value of OH production rates over-predicted the transformation of AC suggesting the existence of unknown side reactions and/or alternative reactive intermediates.