Nitrification inhibition by heavy metals and chelating agents

Date of Completion

January 2002


Engineering, Environmental




Nitrification is generally considered the rate determining step in biological nutrient removal. Although nitrification inhibition by heavy metal and chelating agent has been addressed in the literature, few researchers have quantified the link between inhibition and metal speciation. In this work, the extent of inhibition was calculated from the kinetics of ammonium oxidation and nitrite oxidation. The inhibition correlated well with free metal cation, [Ni 2+] or [Cd2+], but not the total metal concentration. ^ Batch experiments were conducted to test whether the kinetics of metal partitioning affect nitrification inhibition. Intracellular Zn, Ni, and Cd concentrations continued to increase with time beyond 24 hours after metal addition, whereas intracellular Cu attained equilibrium after 4 hours. An intraparticle diffusion model adequately fit the slow Zn, Ni, and Cd internalization kinetics. The inhibition was not a function of the sorbed metal fraction, rather a correlation with the intracellular Zn, Ni, and Cd concentration was observed. Further, the inhibitory mechanism of Cu was very different from Zn, Ni, and Cd and may involve rapid loss of membrane integrity. ^ Shock load experiments were conducted in continuous flow bioreactors to compare the metal partitioning and inhibition between batch and continuous metal addition. A mathematical model incorporating metal partitioning adequately fit the metal dynamics in the continuous flow reactors. However, at the same free metal cation concentrations, the inhibition in continuous flow reactor was much higher than in the short-term batch assays, most likely due to the slow kinetics of Zn, Ni and Cd internalization and the effect of continued metal exposure in the continuous flow reactor. ^ The order of nitrification inhibition by chelating agents in the batch assays was: EDA >> EDTA >> DTPA. Inhibition by EDTA was completely relieved by adding appropriate amount of complexing partners (e.g., Ca2+, Mg2+, and Fe3+) whereas inhibition by EDA was not. Inhibition by both EDTA and DTPA, but not EDA, correlated with the depletion of cellular Ca2+. Further, inhibition by EDA was paired with substantial leakage of cellular K+ and disruption of plasma membrane integrity inferred from LIVE/DEAD® Baclight™ bacterial viability assays. Therefore, EDA inhibits via a different operative mechanism than EDTA and DTPA. ^