Enhanced spent oil bioremediation in contaminated soils using biostimulants: kinetics, half-life, and efficacy...

The persistence of petroleum hydrocarbons in contaminated soils remains a significant environmental concern, necessitating effective and sustainable remediation strategies. This study evaluated the potential of selected ecofriendly organic biostimulants: egusi peel (Ymp), sawdust (Sd), and cassava peel (Csp), applied individually and in combination, to enhance the biodegradation of spent oil in contaminated soil over a 180-day period under controlled microcosm conditions. Biodegradation efficiency was assessed by monitoring residual oil concentrations at regular intervals. Kinetic models (zero-, first-, second-, and third-order) were applied to describe degradation patterns and estimate rate constants. All amended treatments showed improved hydrocarbon reduction compared to the untreated control. The combination of Ymp + Sd exhibited the highest performance, with an average degradation rate of 0.269%/day and 48.42% removal, while Ymp alone also showed notable efficiency. Kinetic analysis indicated variable degradation behavior across treatments while statistical differences (p < 0.001), among treatments were observed. Complementary correlation and principal component analyses further elucidated relationships between biostimulants properties and performance. The findings demonstrated that ecofriendly agro-waste biostimulants enhanced hydrocarbon biodegradation and highlighted promising biostimulant formulations; however, further studies incorporating microbial characterization and field validation are required.