Residual Cr3+ on the biodegradation of partially hydrolyzed polyacrylamide after polymer gel formation by Bacillus licheniformis SP01

The residual Partially Hydrolyzed Polyacrylamide (HPAM) and the crosslinker (Cr³⁺) after polymer gel formation caused blockages and decreased recovery. Microorganisms can degrade HPAM to address the issues, but Cr³⁺ inhibits the process and consequently reduces oil recovery. This study investigates an underexplored area—the biodegradation of HPAM in the presence of Cr³⁺. HPAM and crude oil were innovatively used as the sole nitrogen(N) and carbon(C) sources, respectively, to isolate the highest-growing strain from Daqing Oilfield production water, identified as Bacillus licheniformis SP01. Fourier transform infrared spectroscopy, high-performance liquid chromatography and scanning electron microscopy analyses revealed Cr³⁺-induced structural and morphological changes in HPAM after biodegradation. B. licheniformis SP01 exhibited a maximum HPAM degradation rate of 39.66 %, which declined to 32.41 % (a 7.25 % decrease) upon exposure to Cr³⁺. This was accompanied by a 43.9 % drop in biomass and declines in amidase and urease activity by 35.2 % and 19.5 %, respectively, implying that Cr³⁺ impairs microbial growth and enzymatic activities, thereby reducing HPAM biodegradation. Mass balance and stoichiometry investigations of C and N in HPAM indicate that Cr³⁺ alters both the biodegradation pathway and final degradation products. Furthermore, this study explores the impact of reservoir conditions on biodegradation with residual Cr³⁺ present and predict the optimal degradation rate of 37.10 %. These results highlight the inhibitory effects of Cr³⁺ on HPAM biodegradation mechanism and propose a bioremediation strategy to mitigate reservoir plugging, improving oil recovery after polymer gel formation.