Biogenic palladium nanoparticles reclaimed by a novel strain Citrobacter braakii Z1 accelerating RDX biodegradation in the munition effluent

Abstract

The microbial degradation of high-energy explosives has emerged as a cost-effective environmental remediation strategy, but is hindered by low degradation efficiency due to the slow electron transfer. Biogenic Pd⁰ nanoparticles (bio-Pd⁰) demonstrate dual functionality, maintaining catalytic activity for N-NO₂ bond reduction while substantially enhancing electron transfer efficiency. This study introduces an innovative single-cell system that synergistically combines chemical catalysis with biological enhancement through in situ synthesis of nanoparticles. Comprehensive characterization through electron microscopy, spectroscopic analysis, and electrochemical measurements revealed that bio-Pd⁰ synthesized by Citrobacter braakii Z1 (C. braakii Z1) were preferentially localized in the periplasmic space and extracellular matrix, forming a stable hybrid named bio-Pd⁰@C. braakii Z1. This process relied on the bio-reduction mediated by biohydrogen and extracellular electron transfer processes via c-type cytochromes (c-Cyts) and flavins. Notably, RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine) degradation efficiency increased from 11 % with microbes alone to nearly 100 % with bio-Pd⁰. Although the observed RDX degradation in inactivated bio-Pd⁰@C. braakii Z1 and the detected hydrogenolysis intermediates indicated the participation of chemical catalysis, the negative correlation between RDX degradation rate with the Pd:biomass ratios suggested the biology process dominated RDX degradation. Meanwhile, the higher output current in bio-Pd⁰@C. braakii Z1 and the extracellular electron consumption by RDX indicated the bio-Pd⁰-mediated extracellular biodegradation was the primary driver for improved RDX removal efficiency. Toxicity evaluation confirmed that the in-situ combination of Pd-catalyzed chemical destruction and biological enhancement in a single cell is a feasible and environmentally friendly strategy for RDX removal with a reduced ecological risk.