Inhibition of nitrification and carbon dioxide evolution as rapid tools for ecotoxicological assessment of drilling fluids

Abstract

A comparison of the sensitivity and rapidity of the inhibition of nitrification and respiration processes with mortality, as indices of bacterial toxicity assessment was performed. The toxicants employed were four drilling fluids: oil-based Paradril, a synthetic based IMCO-O, water–based IMCO-W and Gel/KCL/Polymer. The static test for acute toxicity assessment was employed for a 48h period. The oxidation of Ammonia to Nitrites by Nitrosomonas, and the oxidation Nitrites to Nitrates by Nitrobacter were the indices for the nitrification process. Respiration was monitored by changes in the carbon (IV) oxide evolution by Escherichia coli. Mortality was monitored using changes in the aerobic colony count of viable cells. The 8h median lethal concentrations (LC50) of drilling fluids to all three-test organisms were significantly higher than the corresponding 8h median effective concentrations (EC50). The 8h EC50 of Paradril, IMCO–O, IMCO–W and Gel/KCL/Polymer to Nitrosomonas were 0, 84.4, 690.1 and 0 mg/L and for Nitrobacter were 0, 0, 134.2 and 0 respectively. The 8h EC50 of these toxicants to E. coli were 471.4, 0, 192.9 and 75.3 mg/L respectively. The 8h LC50 of these toxicants to Nitrosomonas were 1190.7, 964.7, 4673.2 and 844.9mg/L respectively. The 8h LC50 of these toxicants to Nitrobacter were 1410.2 0, 639.0 and 2065.9mg/L and for E. coli 414.8, 1374.1, 971.8 and 1900.8mg/L respectively. Results of 8h EC50 for all test organisms revealed differences in the sensitivities of the organisms to the toxicants while 8h LC50 results of toxicants for test organisms were not as definite. However 36h LC50 results exhibited definite differences in the sensitivities of the organisms to the toxicants. Results showed that inhibition of nitrite oxidation by Nitrobacter was more sensitive than inhibition of nitrite formation by Nitrosomonas and carbon VI oxide evolution by E coli. Inhibition of nitrite formation and CO2 evolution displayed similar levels of sensitivities at 95% confidence levels. These results indicate that monitoring inhibition of metabolic processes rather than mortality was a more rapid and sensitive tool for ecotoxicological evaluation of chemicals employed in the petroleum industry in the Niger Delta.

Keywords: Effective concentration, lethal concentration, Nitrification, Carbon IV Oxide evolution.

AJAZEB Vol. 6 2004: pp. 21-25