Inhibition of the activated sludge-associated enzyme phosphatase by transition metal oxyanions

Abstract

Organic esters of phosphoric acid and other organophosphorous compounds are enzymatically hydrolyzed during wastewater treatment by microbial phosphoesterases, especially by phosphomonoesterase (phosphatase). For physiological reasons, the enzyme is inhibited by its main inorganic reaction product, ortho-phosphate. It is known that oxyanions of transition metals, resembling the molecular topology of ortho-phosphate, e.g. vanadate and tungstate, are more potent inhibitors for microbial alkaline phosphatase than phosphate. To proof this effect for activated sludge, a multitude of samples from a communal wastewater treatment plant was exposed at pH values from 7.00 to 8.50 to tungstate, vanadate, and molybdate. Inhibition effects were determined by a sensitive fluorimetric microplate assay and characteristic parameters (IC50 and IC20 concentrations) were deduced from modelled dose-response functions. Mean inhibitor concentrations (in brackets: ranges) causing 50% inactivation (IC50) at pH 7.50 were 2.5 (1.3–4.1) μM tungstate, 2.9 (1.6–5.5) μM vanadate, and 41.4 (33.6–56.7) μM molybdate. Vanadate and tungstate concentrations between 0.6 and 0.7 μM provoked a 20% (IC20) inhibition. The inhibition efficiency of tungstate and molybdate decreased with increasing pH, whereas vanadate reacted pH independently. These results underline the necessity to consider enzyme inhibition assessing the limitations and potentials of biological wastewater treatment processes. HIGHLIGHTS Sensitive fluorimetric microplate assay applied to analyze phosphatase inhibition.; Vanadate and tungstate are strong inhibitors of sludge-associated alkaline phosphatase.; Molybdate has lower inhibition potency than the other oxyanions.; Inhibition efficiency of tungstate and molybdate decreased with increasing pH.; Enzyme inhibition needs more attention in the performance assessment of wastewater treatment.;

Keywords

• activated sludge
• alkaline phosphatase
• enzyme inhibition
• tungstate
• vanadate
• wastewater treatment