Optimized solid composite bacterial agents for biodegradation of benzo[a]pyrene contaminated soils: Effects, microbial dynamic changes and mechanisms

Solid composite bacterial agents (SCBA) present an innovative bioremediation strategy for the in situ degradation of benzo[a]pyrene (BaP), a recalcitrant compound that poses significant challenges to remediation in contaminated soils. In this study, an SCBA for BaP degradation was prepared using vacuum freeze-drying technology. Response surface methodology was employed to optimize the protective agent ratio, resulting in a composite formulation comprising 14 % skimmed milk powder, 8 % glycerol, and 8 % sucrose. This formulation achieved a freeze-drying survival rate of (89.65 %±2.04)% and an effective viable bacterial count of (1.50 ± 1.07)× 10 ¹ ¹ CFU/g in the SCBA. After 60 days of BaP-contaminated soil remediation, the BaP degradation rates in the activated bacterial liquid combined with co-metabolized substrate (CMS) (JYC) and the SCBA combined with CMS (JFC) groups were 57.23 % and 45.33 %, respectively, reflecting increases of 41.35 % and 29.45 % compared to the control (CK) group. Incorporating SCBA and CMS significantly enhanced the microbial BaP remediation efficiency and improved soil organic matter, alkaline nitrogen content, as well as fluorescein diacetate, catalase, and dehydrogenase activity. Furthermore, this approach enriched the population of beneficial bacteria involved in BaP degradation, fostering a more efficient and stable microbial network. Enhanced signaling between microorganisms facilitated faster material exchange and closer inter-bacterial contact. The CMS provided essential nutrients, enabling microorganisms to adapt more rapidly to their environment and reducing microbial remediation.