Challenges in using allylthiourea and chlorate as specific nitrification inhibitors

Abstract

Allylthiourea (ATU) and chlorate (ClO3−) are often used to selectively inhibit nitritation and nitratation. In this work we identified challenges with use of these compounds in inhibitory assays with filter material from a biological rapid sand filter for groundwater treatment. Inhibition was investigated in continuous-flow lab-scale columns, packed with filter material from a full-scale filter and supplied with NH4+ or NO2−. ATU concentrations of 0.1–0.5 mM interfered with the indophenol blue method for NH4+ quantification leading to underestimation of the measured NH4+ concentration. Interference was stronger at higher ATU levels and resulted in no NH4+ detection at 0.5 mM ATU. ClO3− at typical concentrations for inhibition assays (1–10 mM) inhibited nitratation by less than 6%, while nitritation was instead inhibited by 91% when NH4+ was supplied. On the other hand, nitratation was inhibited by 67–71% at 10–20 mM ClO3− when NO2− was supplied, suggesting significant nitratation inhibition at higher NO2− concentrations. No chlorite (ClO2−) was detected in the effluent, and thus we could not confirm that nitritation inhibition was caused by ClO3− reduction to ClO2−. In conclusion, ATU and ClO3− should be used with caution in inhibition assays, because analytical interference and poor selectivity for the targeted process may affect the experimental outcome and compromise result interpretation.