Importance of the Free Zn Species in Batch Nitrification Systems

The inhibitory effect of Zn on nitrifying bacteria was examined in the presence and absence of ethylenediaminetetraacetic acid (EDTA). EDTA addition reduced nitrification inhibition due to the formation of strong complexes and the reduction in free Zn species. Furthermore, formation of strong complexes prevented Zn from being adsorbed by bacterial surfaces. Zn was measured by voltammetric techniques, which enable the differentiation between strong ZnEDTA complexes and labile Zn. Inhibition of nitrification depended on free, labile and biosorbed fractions of Zn and did not correlate with the total concentration. Thus, both the Free Ion Activity Model (FIAM) and the Biotic Ligand Model (BLM) were applicable in expressing the influence of Zn on nitrification inhibition.