Combined Effect of Pulverized Costus afer Stem and Cow dung Amendment on Contaminants of Concern in Crude Oil–Contaminated Soil

Abstract

This study investigated the combined effect of pulverised Costus afer stem and cow dung amendments on contaminants of concern (heavy metals, PAHs, and TPHs) in crude oil-contaminated soil. Non-polluted soil samples were obtained from farmland along Mini Orlu Road, near Ada George, Port Harcourt, Rivers State, Nigeria, and divided into groups. Twenty kilograms of soil was contaminated with 1,000 mL of crude oil. Fresh whole stems of Costus afer were pulverised using an electric blender, measured, and introduced into the polluted soil samples, while cow dung was thoroughly mixed with the polluted soil to provide nutrients and hydrocarbon-degrading microorganisms. The samples were incubated for 60 days before laboratory analysis. Heavy-metal results indicated that the control sample (Group 1) had a relatively higher cadmium concentration (0.22 mg/kg) than the other groups, whereas the polluted untreated soil (Group 2) contained detectable mercury, lead, arsenic, and chromium. Treatments in Groups 3–9 generally reduced or stabilised heavy-metal concentrations, with no marked increase in the amended soils. PAH concentrations differed among treatments. Group 2 showed elevated phenanthrene and acenaphthene concentrations, whereas Group 5 (polluted soil amended with 200 g cow dung) produced the greatest reduction, with several PAHs approaching zero. Combined treatments (Groups 8 and 9) produced mixed outcomes. Group 2 also recorded the highest TPH concentrations across the C9–C14 fractions. In Group 5, TPH concentrations decreased substantially, including a reduction in C9 to 0.26 µg/L. Group 4 (100 g cow dung) showed a smaller reduction, while treatments containing Costus afer and combined amendments produced inconsistent responses.