Continuous monooxygenase-mediated biodegradation of phenol derivatives in wastewater: Optimization of flow conditions

Abstract

Nitrophenols and halogenated phenols are major pollutants found in a pharmaceutical production, and have been included in US-EPA’s priority pollutant list. Biological treatment is increasingly attractive due to its low energy consumption and use of non-toxic reagents. In this work, we developed a system for degrading the phenol derivatives via HadA cells expressing a flavin-dependent monooxygenase HadA enzyme. The HadA cells were immobilized into alginate beads (so-called HadA@alginate), which could be set up into a fixed bed column for a continuous operation. Quinone products, generated from this enzymatic conversion, could be consumed by cells, highlighting the self-sustaining nature of this system. Flow compositions (e.g., carbon, nitrogen sources, calcium ions) and conditions (e.g., air feeding) were comprehensively optimized for a robust operation (e.g., no swelling of beads, prolonged cell activity). Upon optimal conditions, 0.05 mM solutions of 4-nitrophenols (4-NP), 2,4-dinitrophenols (2,4-DNP), and 4-chlorophenols (4-CP) could be completely degraded at recycle ratios (i.e., recycle-to-purge ratio) of 3.3, 0.5, and 7.6 respectively. We achieved the specific degradation rates of 3.24 × 10−4 mg4-NP/mgcell/h, 2.45 × 10−3 mg2,4-DNP/mgcell/h, and 3.43 × 10−3 mg4-CP/mgcell/h. We also demonstrated the use of technology to degrade 4-NP in a real wastewater collected from a pharmaceutical plant.