Dose- and Time-Dependent Attenuation of Total Petroleum Hydrocarbons, Polycyclic Aromatic Hydrocarbons and BTEX in Spent-Engine-Oil-Contaminated Soil Amended with African Breadfruit (Treculia africana) Compost

Spent engine oil contaminates soil with persistent and toxic hydrocarbon compounds. This study investigated the effectiveness of African breadfruit (Treculia africana) compost as a biostimulant for remediating spent-engine-oil-contaminated sandy-loam soil. Each reactor contained 4.0 kg of soil and 500 g of spent engine oil, amended with 0, 0.5, 1.0, or 1.5 kg of compost (control, T1, T2, and T3, respectively), with three replicates per treatment. Total petroleum hydrocarbons (TPH), polycyclic aromatic hydrocarbons (PAHs), and benzene, toluene, ethylbenzene, and xylenes (BTEX) were monitored on days 0, 14, 28, 42, and 56. Data were analysed using two-way factorial ANOVA on log10-transformed concentrations, partial eta-squared effect sizes, Tukey’s honestly significant difference tests, and apparent first-order attenuation models. Compost dosage, incubation duration, and their interaction significantly influenced all contaminant groups (p < 0.001). By day 56, T3 achieved the highest attenuation of TPH (88.10%), PAHs (87.13%), and BTEX (90.98%), compared with 70.71%, 72.37%, and 73.77%, respectively, in the control. T3 also exhibited the highest apparent first-order rate constants and shortest half-lives across all contaminant groups. However, residual concentrations remained above the protocol’s target values, indicating incomplete remediation. African breadfruit compost demonstrates potential as a locally available biostimulant, although extended incubation, contaminant mass-balance assessments, microbial mechanism investigations, and field validation are necessary to establish its practical applicability.