Environ. Sci. Technol. zyxwvut 1992, 26, 2298-2300 COMMUNICATIONS Effect of Two Electron Acceptors on Atrazine Mineralization Rates in Soil Dhlleepan R. Nalr” and Jerald L. Schnoor zyxwvuts Hazardous Substances Research Laboratory, 120 Engineering Research Facility, Department of Civil and Environmental Engineering, University of zyxwvutsrqp Iowa, Iowa 52242 Introduction Atrazine zyxwvutsr [2-chloro-4-(ethylamino)-6-(isopropylamino)- s-triazine] and its metabolites, deethylatrazine and de- isopropylatrazine, are the most frequently detected pes- ticides in surface waters of the midwestern United States zyxwvu (1) and in Iowa groundwater (2). Atrazine is also one of the most heavily used pesticides for corn in the corn belt states (3). At the present time, Iowa and other states are considering increased labeling restrictions on atrazine or outright bans due to persistence of the compound in surface water and groundwater in excess of its maximum contaminant level allowable of 3 ppb. In the upper soil profile, atrazine is biotransformed aerobically to deethylatrazine and deisopropylatrazine, but in deeper soil and groundwater, the role of an alternate electron acceptor for biotransformationbecomes important as oxygen gets depleted. Nitrate is usually the alternate electron acceptor under agricultural fields in the corn belt states. Atrazine and other s-triazine biodegradation re- search in different media and environmental conditions have been extensively reviewed (4-6). Jessee et al. (7) found that a facultative anaerobicbacterium could degrade about half of 75 mg/L atrazine in 1 week under anaerobic conditions. Goswami and Green (8)) however, did not detect mineralization of atrazine in anaerobic systems, but mineralization of atrazine in aerobic soils has been widely reported in the research literature (9). Behki and Khan zyxwvut (10) isolated three species of Pseudomonas from agricul- tural soil with a 14-year history of atrazine treatment and found that these species could utilize atrazine as a sole source of carbon. Dealkylation of both the side chains was performed with preferential utilization of the isopropyl side chain. Cook and Hutter (11) isolated three strains of Pseudomonas and two strains of Klebsiella pneumoniae that were able to use s-triazines as sole and limiting sources of nitrogen for growth. Giardina et al. (12) isolated a soil bacterium, a Norcardia strain from a soil enrichment culture, that could utilize atrazine as the sole source of carbon and nitrogen to form dealkylated metabolites and deaminated metabolites. They found that dealkylation preceded deamination in this species and that the final product formed was 2-chloro-4-amino-s-triazine. However, no studies have been reported in the literature on atrazine mineralization under denitrifying conditions in soil systems. This research, therefore, compared the mineralization rates of 14C-labeled atrazine in agricultural soil under oxygenated conditions and denitrifying condi- tions, to ascertain whether movement of atrazine to anoxic zones in the soil profiie will affect biotransformation rates and subsequently its persistence. Experimental Procedures Soil used for the experiments was taken from the top 150 cm of the ground surface, from an experimental plot just adjacent to Lily Lake, Amana, IA. The soil is classified as of the Nodeway-Ely series and is of silt-loam texture (organic matter 2.2%, pH 6.3, and CEC 23.5 mequiv/100 g of soil). The soil was air-dried, pulverized, and passed through a 2-mm sieve. The soil was then homogenized, and 500-g portions of the whole soil were added to reactors. Three types of reactors were used for the batch studies. Wide-mouth 1000- and 500-mL Erlenmeyer Pyrex flasks were used for aerobic and anoxic bioreactor studies, re- spectively, while 50-mL serum bottles were used for mi- crocosm reactor studies. For the microcosm reactors, 30 g of soil was added to each and sealed with gas-tight rubber caps. For all the reactors simulating anoxic conditions, the fresh soil was added directly to the reactors without any preparation. The bioreactors were then sealed with neoprene stoppers with two glass tubing inserts and were sealed air-tight with paraffin wax. Controls were prepared using glass beads and sterilized soil as media. The soil for the sterile controls was first autoclaved in trays for 1 h at 250 OF and 14 psi. The autoclaved soil was then trans- ferred to the flasks, stoppered, and autoclaved again for 15 min at 250 OF and 14 psi. All reactors were prepared in replicates. Uniform 14C ring labeled atrazine (specific activity 19.4 pCi/mg) or 14C isopropyl side chain labeled atrazine (specific activity 4.9 pCi/mg), obtained from Ciba-Geigy (Greensboro, NC) was added to the soil at a concentration of 0.37 ppm (pg/g of soil). Chemical purity was 95% and >99%, and radiochemical purity was 98.9% and 98.4%, for the ring labeled and isopropyl side chain labeled atrazine, respectively. Deionized water was added (deox- ygenated for anoxic soils) after atrazine application to bring the soil to 60% field capacity (water-holdingcapacity) for aerobic soils and 100% field capacity for all anoxic soils. Nitrate at a soil water concentration of 10 mg/L NO< N (0.16 mM), was added to the anoxic reactors only, to sim- ulate anoxic conditions with nitrate as the electron ac- ceptor. The anoxic bioreactors were supplemented with 10 mg/L NO3- N (0.16 mM) on days 30,67, and 109, and the anoxic microcosm reactors on days 36, 68, and 92 to make up for nitrate depletion. The headspaces of the reactors were filled with Nz/Ozor N2 gas mixture for ae- robic and anoxic incubations, respectively. The bioreactors were individually wrapped with aluminum foil to shield off light, and the microcosm reactors were placed in boxes shielded from light. The temperature in the laboratory was in the range of 20-25 “C. At 10-14-day intervals, the air in the reactors was re- moved by a vacuum pump from one outlet and fresh gas was passed into the reactors from the other. The gases evacuated were trapped in C02 traps consisting of glass tubes and 0.2 N NaOH (prepared with C0,-free deionized 2298 Envlron. Scl. Technol., Vol. 26, No. 11, 1992 0013-936X/92/0926-2298$03.00/0 0 1992 American Chemical Soclety