Synergistic Actions of Biostimulation and Bioaugmentation in Microbial Biodegradation of Total Petroleum Hydrocarbons (TPH) Using Keratinaceous Materials

The total petroleum hydrocarbons (TPH) in used engine oil-polluted soil constrains its natural bioattenuation. Synergistic actions of biostimulation and bioaugmentation in microbial biodegradation mediated by the addition of keratinaceous materials resulted in the biodegradation of TPH. In this study, keratinous materials served as the source of organic nutrients to improve the soil’s physicochemical properties and promote the proliferation of keratinolytic and hydrocarbonoclastic bacteria. After the biodegradation activity, the TPH declined from 48,865 mg kg⁻¹ to a threshold lower than 10,000 mg kg⁻¹ in 180 days except in the control groups. The TPH biodegradation efficiencies ranged between 70.5–91.5% in the treatment groups S1, S2 and S3 while natural bioattenuation in control groups S4 and S5 ranged between 18.5–22.5%. Biodegradation kinetics modelling indicated decay rates (0.0075 to 0.0137 per day) and half-life (50.38 to 91.29 days) in the treatment groups, while the control group recorded decay rates (0.0011 to 0.0014 per day) and half-life (487.29 to 616.74) days. From the culture-independent analysis to identify the keratinolytic and hydrocarbonoclastic bacteria in the soil, the changes in bacterial community structures bacterial dynamics, diversity, community structure and phylogenesis were determined. The keratinolytic and hydrocarbonoclastic bacteria identified were made up of Proteobacteria (52%), Actinobacteriota (22%), Firmicutes (4%), Planctomycetota (2%), Acidobacteriota (2%) and Chloroflexi (1%). The AlkB gene copies in the treatment groups increased from 6.25 × 10⁵ to 1.25 × 10⁷ copies ng DNA⁻¹, while the control groups decreased from 6.20 × 10⁵ copies ng DNA⁻¹ to 6.05 × 10⁴ copies ng DNA⁻¹ in 180 days.