Polyethylene glycol outer layer regulates heterogeneous nucleation to alleviate Cr(VI) stress and promote biomineralization and denitrification performance of strain LX-6: Performance and mechanism.

Photovoltaic wastewater often contains hexavalent chromium (Cr(VI)), fluoride (F-), and poorly biodegradable polyethylene glycol (PEG), and this complex pollution poses a severe challenge to biological denitrification systems. This study focused on strain LX-6 (Acinetobacter sp.), which possesses both heterotrophic nitrification-aerobic denitrification (HNAD) and microbial-induced calcium precipitation (MICP) capabilities, to systematically investigate the inhibitory effects of Cr(VI) on denitrification and mineralization and to elucidate the mechanism by which PEG alleviates Cr(VI) stress through heterogeneous nucleation regulation. Cr(VI) significantly inhibited biomass, electron transport system activity (ETSA), HNAD enzyme activity, and mineralization efficiency in a concentration-dependent manner. PEG coordinated with calcium via its ether bonds and hydroxyl groups, providing additional heterogeneous nucleation sites and acting synergistically with extracellular polymeric substances to construct a composite nucleation interface, thereby promoting the rapid formation of a dense mineralized shell on the bacterial surface during the early stage of the reaction. This shell acted as a diffusion barrier and effectively mitigated the direct cytotoxicity of Cr(VI). Compared with the Cr(VI)-only treatment, PEG increased adenosine-5'-triphosphate by 16.21% and ETSA by 92.86%, decreased malondialdehyde by 47.52%, and significantly enhanced antioxidant enzyme activity. The electrochemical impedance characteristics showed significant improvement, with the limiting current recovering from 0.287 mA to 0.562 mA. Cr(VI) was ultimately bioreduced and immobilized as Cr(III) precipitates, whereas PEG is primarily removed by adsorption and coprecipitation. This study provides new insights into the mechanisms and technical approaches for the synergistic advanced treatment of photovoltaic composite wastewater containing Cr(VI), F-, and recalcitrant PEG.