IMPACT OF ARSENICALS ON NITRIFICATION 291 



Economic factors prevent the use of individual field studies to 

 answer these questions for every pollutant. Instead, smaller 

 laboratory-associated studies must be completed and the data 

 ultimately extrapolated to a field situation. To meet this end, we 

 have focused much of our laboratory's research on the fate and 

 impact of pollutants in different environmental systems. Our 

 environmental system studies largely involve mixed microbial popula- 

 tions that are important in the nutrient cycling process. 



For this study, we investigated the impact of two arsenicals, 

 cacodylic acid (hydroxydimethylarsine oxide) and sodium arsenate, 

 on the nitrification process in aqueous systems. Cacodylic acid, 

 which is representative of the organic arsenicals, is used in the 

 cotton-growing states as a defoliant (Versar, Inc., 1976). Sodium 

 arsenate was chosen because it is a common metabolic product of the 

 organic arsenicals (Woolson and Kearney, 1973). 



Oxidation of nitrogen from a reduced state, such as ammonia, to 

 a more oxidized state, such as nitrite and nitrate, is called 

 nitrification (Hardy and Holsten, 1972). These nitrogen transforma- 

 tions are usually mediated by two types of chemosynthetic, 

 autotrophic bacteria. Ammonia oxidizers, typified by Nitrosomonas, 

 get energy for growth by oxidizing ammonia to nitrite; nitrite 

 oxidizers of the Nitrobacter type complete the process by oxidizing 

 nitrite to nitrate. 



The objectives of this research were to develop simple systems to 

 study the impact of pollutants on nitrification and to determine 

 whether arsenicals, represented by sodium arsenate and cacodylic 

 acid, signific£intly alter nitrification rates. These objectives were met 

 by the following steps: (1) Mixed populations of nitrifiers were 

 cultured in an aqueous medium; (2) the reproducibility and reli- 

 ability of the system was determined; and (3) the impact of 

 arsenicals on nitrification was determined by measuring the oxida- 

 tion of ammonia and nitrite. 



MATERIALS AIMD METHODS 



Growth Medium 



Nitrification rates were determined in a chemically defined 

 medium containing micronutrients and macronutrients. In a final 

 volume of 1000 ml, the filter-sterilized micronutrient stock con- 

 tained EDTA-Na2, 500 mg; FeS04 • 7H2O, 200 mg; ZnS04 • 7H2O, 

 10 mg; MnCU • 4H2 O, 3 mg; H3 BO3 , 30 mg; C0CI2 * 6H2 O, 20 mg; 

 CUCI2 • 2H2O, 1 mg; NiCl2 • 6H2O, 2 mg; and Na2Mo04 • 2H2O, 

 3 mg. Each liter of the chemically defined medium contained NH4CI, 



