Amendment of Crude-Oil-Contaminated Soil with a Pig Waste-Cassava Peel Biochar Blend: Response Surface Optimisation

Abstract

Crude-oil contamination of soil remains a major environmental concern in petroleum-producing regions, creating a need for effective and locally available amendment materials for reducing petroleum hydrocarbon concentrations. This study evaluated the effect of a pig waste–cassava peel biochar blend on total petroleum hydrocarbon (TPH) reduction in crude-oil-contaminated soil and optimised the treatment conditions using response surface methodology (RSM). Soil was artificially contaminated with crude oil at a concentration of 15 g/kg and amended with different proportions of pig waste and cassava peel biochar. A central composite design comprising 13 experimental runs was employed to investigate the effects of treatment duration (2–7 weeks) and cassava peel biochar composition on TPH reduction. The experimental data were fitted to a quadratic response surface model and analysed using analysis of variance. The model was statistically significant (p < 0.0001), with an R² of 0.9741, an adjusted R² of 0.9556 and a predicted R² of 0.8838. Cassava peel biochar composition significantly influenced TPH reduction (p = 0.0001), while the interaction between treatment duration and biochar composition was also significant (p = 0.0002). The quadratic effect of treatment duration was significant (p = 0.0156), despite the corresponding linear term being non-significant (p = 0.8289), indicating a nonlinear response to treatment duration. The optimisation model identified a treatment duration of 7 weeks and a blend containing 10% cassava peel biochar and 90% pig waste as the reported optimum condition, with a predicted response of 4572.76 [unit to be verified] and a desirability of 0.843. The findings demonstrate the potential of combining pig waste with cassava peel biochar as an amendment for the treatment of crude-oil-contaminated soil and provide an optimised treatment condition for further validation.