Effect of chemical structure on the microbial nitrification inhibition and copper corrosion inhibition properties of azole compounds

Azoles are a class of nitrogen-containing heterocyclic compounds that are widely used as metal corrosion inhibitors in different industrial processes and consumer products. These compounds are severely inhibitory towards microorganisms responsible for nitrification and can impair the efficiency of biological nitrogen removal in wastewater treatment plants. This study assessed the effect of chemical structure on the toxicity of simple azole compounds towards microbial nitrification with the aim of identifying substitution patterns leading to decreased inhibition. Nitrification inhibition by the widely used azoles, pyrazole and 1,2,4-triazole, and several of their derivatives with dimethyl-, diethyl-, carbonyl-, amino substituent groups was studied. The results indicated that, while pyrazole and 1,2,4-triazole were severely inhibitory towards nitrification, addition of dimethyl- and diethyl-substituents greatly lowered the toxicity response of these compounds. The fifty percent inhibition concentrations (IC50) of pyrazole and 1,2,4-triazole were very low, 0.07 and 0.11 mg L⁻¹, respectively, indicating a high level of inhibition. In contrast, nitrification inhibition by the dimethyl- and diethyl-substituted azoles was at least 30–700 times lower compared to the unsubstituted parent compounds, and in the best case, 3,5-diethyltriazole was found to be completely non-toxic. Preliminary evaluation of the corrosion inhibition efficiency of the alkyl-substituted azole compounds under alkaline conditions was conducted electrochemically. All the azoles tested were able to inhibit copper corrosion to a significant extent. For the pyrazoles, the corrosion inhibition performance improved with the addition of dimethyl- and diethyl-substituents. The inhibition efficiency by pyrazole, 3,5-dimethylpyrazole and 3,5-diethylpyrazole was 71.5, 77.6 and 85.0%, respectively, with respect to the control with no inhibitor. In the case of triazoles, the corrosion inhibition efficiency was slightly lower for the alkyl-substituted triazoles (77.7% for 3,5-dimethyl-1,2,4-triazole and 82.9% for 3,5-diethyl-1,2,4-triazole) than for the unsubstituted 1,2,4-triazole (89.6%). An important implication of these results is the possibility of replacing conventional azole compounds by their non-toxic substituted counterparts in industrial applications. This would prevent inhibition of microbial nitrification without compromising the efficacy of the azoles as inhibitors of copper corrosion.