Ammonia monooxygenase-mediated cometabolic biotransformation of volatile 4-chlorophenol by nitrifying counter-diffused biofilms: A combined MD simulation, DFT calculation and experimental study

Ammonia monooxygenase (AMO)-mediated organic pollutants cometabolism has been widely observed during biological nitrogen removal. However, its molecular evidence remains unclear, hindering its applicability. Furthermore, conventional nitrification systems encounter significant challenges such as air pollution and the loss of ammonia-oxidizing bacteria, when dealing with wastewater containing volatile organic pollutants. This study developed a nitrifying membrane-aerated biofilm reactor (MABR) to enhance the biodegradation of volatile 4-chlorophenol (4-CP). Results showed that 4-CP was primarily removed via Nitrosomonas nitrosa-mediated cometabolism (42.41±4.53%) in the presence of NH4+-N, supported by the increased nicotinamide adenine dinucleotide (NADH) and adenosine triphosphate (ATP) content, AMO activity and the related genes abundance. Hydroquinone, detected for the first time and produced via oxidative dechlorination, as well as 4-chlorocatechol was primary transformation products of 4-CP. AMO structural model was constructed for the first time using homology modeling. Molecular dynamics simulation suggested that the ortho-carbon in 4-CP was more susceptible to metabolism than ipso-carbon. Density functional theory calculation revealed that 4-CP was metabolized by AMO via H-abstraction-OH-rebound reaction, with a significantly higher rebound barrier (16.37 kcal·mol-1) at the ipso-carbon as compared to the ortho-carbon (6.7 kcal·mol-1). This study fills the knowledge gap on the molecular mechanism of AMO-mediated organic pollutants cometabolism, providing practical and theoretical foundations for enhanced volatile organic pollutants removal using nitrifying MABR.