Comparative Effects of Bone Char, Spent-Palm-bunch, and NPK Biodegradation Treatment on Total Hydrocarbon Content (THC) of Crude-oil Polluted Soil

This study comparatively evaluated the efficacy of two organic amendments: bone char and spent palm bunch (SPB), against conventional NPK (20:10:10) fertilizer and natural attenuation in reducing total hydrocarbon content (THC) in crude oil-polluted soil over a 10-week incubation period, across application rates ranging from 0.5 to 14.0 kg. After contamination with THC from an initial concentration of 38 mg/kg in unpolluted soil to 23,560 mg/kg, soil acidification and a decrease in nitrogen and phosphorus availability were detected. All amendments produced statistically significant THC reductions relative to the unamended control (p < 0.05). Bone char achieved the fastest degradation kinetics, with a first-order rate constant of 0.021 day⁻¹, a half-life of 33.01 days, and biostimulation efficiency of 79.37% at 6.5 kg application, outperforming both NPK (half-life ≈ 57.76 days) and SPB (half-life ≈ 99.02 days). The highest degradation efficiency was observed at intermediate application rates, which did not increase linearly with the dosage, indicating an optimal nutrient threshold. Total THC degradation was below 25% in all treatments within the study period. These results identify bone char as a highly effective, low-cost alternative to inorganic fertilization for accelerating hydrocarbon bioremediation in tropical, petroleum-impacted soils, with implications for scalable, field-level restoration practice.