IMPACT OF ARSENICALS ON NITRIFICATION 

 IN AQUEOUS SYSTEMS 



HARVEY W. HOLM and MARILYN F. COX 



Environmental Research Laboratory, Environmental Protection Agency, 



College Station Road, Athens, Georgia 



ABSTRACT 



The impact of both arsenate and cacodylic acid (at arsenic concentrations of 0, 

 0.1, 1, 10, 100, and 1000 mg/liter) on mixed populations of nitrifiers in model 

 aqueous systems containing ammonia was determined by measuring levels of 

 ammonia and nitrite over a 24-day incubation period. Ai'senate decreased the 

 rate of oxidation of ammonia by Nitrosomonas only at high concentrations (100 

 and 1000 mg/liter) of arsenic; low levels of arsenate (0.1, 1, and 10 mg As/liter) 

 had no effect on the oxidation rate, in comparison with arsenic-free controls. 

 The oxidation of nitrite to nitrate by Nitrobacter was affected by all 

 concentrations of arsenic added as arsenate; low concentrations (0.1, 1, and 10 

 mg/liter) delayed the oxidation of nitrite and high concentrations (100 and 1000 

 mg/liter) inhibited the process. The only impact of cacodylic acid on 

 nitrification occurred at 1000 mg As/liter. The oxidation of ammonia by 

 Nitrosomonas was delayed by the arsenical, but the Nitrobacter population was 

 not affected. Although cacodylic acid is not toxic to the nitrification process, its 

 degradation product, arsenate, can inhibit nitrification if it is in an available 

 form. This inhibition of the Nitrobacter population may promote the accumula- 

 tion of nitrite in the environment. 



An understanding of the impact of pollutants on nutrient cycling in 

 aquatic ecosystems is important to decision makers in industry and 

 to surveillance and enforcement personnel in local, state, and federal 

 government. These individuals need to know (l)how a pollutant 

 affects an environment, (2) what the consequence of an altered 

 environment might be, and (3) how the environment influences the 

 fate of a pollutant. 



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