Elimination of trace organic contaminants during enhanced wastewater treatment with horseradish peroxidase/hydrogen peroxide (HRP/H2O2) catalytic process

Abstract

The potential of horseradish peroxidase combined with hydrogen peroxidase (HRP/H2O2) catalytic process was assessed as an enhanced wastewater treatment technology to eliminate trace organic contaminants (micropollutants). Kinetic experiments with 17α-ethinylestradiol (EE2) as a phenolic micropollutant in synthetic buffered solutions showed that the apparent first-order rate constant of EE2 transformation (kEE2) increased linearly with increasing HRP concentration but was independent of the concentrations of H2O2 and EE2. The observed kinetic behaviors of EE2 could be well explained by the known chemistry of the HRP/H2O2 system considering the reaction of Compound II with EE2 as the rate-limiting step in the catalytic cycle of HRP under the condition of excess H2O2 over HRP. Using this HRP/H2O2 kinetic model, the second-order rate constants of the reaction of Compound II (k3) with a few selected phenolic micropollutants including EE2 and phenol and aniline could be determined and compared with those from the literature. Good correlations were found between the k3 of various phenolic or anilinic compounds (from this study and the literature) versus Hammett sigma constants, which can be used for predicting the elimination efficiency of phenolic or anilinic micropollutants. Experiments conducted using lake water and wastewater effluent matrices showed that significant elimination of phenolic micropollutants (i.e., % elimination levels of 20%–100%) could be achieved under the following treatment condition: 1.4–5.6nM (12–48U/L) of HRP, 20μM of H2O2, and 1h of reaction time. Elimination of the other tested micropollutants without phenolic moieties was less than 5%. The elimination levels of phenolic micropollutants (e.g., EE2) in real water matrices spiked at 2μM could be well predicted by the HRP/H2O2 kinetic model with the corresponding k3 value. However, relatively lower elimination levels were observed when the phenolic micropollutants were spiked at 1μg/L, indicating some reduction in the performance of the HRP/H2O2 process for eliminating phenolic micropollutants present at environmentally relevant concentrations (e.g., sub μg/L).