Anthraquinone removal by a metal-organic framework/polyvinyl alcohol cryogel-immobilized laccase: Effect and mechanism exploration

Anthraquinones (AQs) are hard-degraded substances that can cause significant harm to the environment. Many treatment methods are under consideration, among which biodegradation by laccase is competitive. AQs can be degraded by laccase, but the mechanism is indistinct, and the free laccase lacks stability and reusability. In this study, an eco-friendly metal–organic framework (MOF) composite, a MOF/polyvinyl alcohol (PVA) cryogel, is synthesized to construct a MOF/PVA immobilized laccase, i.e., MOF/PVA/Lac. Among all the MOF/PVA/Lac, the MIL-68(Al)/PVA/Lac (MIL-68(Al) is one of MOFs) showed excellent enzymatic properties where the pH stability, thermal stability, and operational stability were enhanced. The MIL-68(Al)/PVA/Lac achieved a high removal efficiency (95.86%) of alizarin green within 12 h and maintained more than 60% of its activity after six reaction cycles. Using molecular docking, mass spectrometry (MS), and two-dimensional nuclear magnetic resonance (2D-NMR), the final product of the degradation process, p-toluidine, was identified. The affinity sites of laccase for alizarin green were determined, and the reaction mechanism, C-α oxidation, was confirmed.