Response Surface Optimization of Hydrogen Peroxide Oxidation for Cyanide Remediation of Cassava-Effluent-Impacted Soils in Southern Nigeria

Cassava-processing wastewater can introduce cyanogenic contaminants into soils receiving untreated discharges. This study evaluated laboratory-scale hydrogen peroxide (H₂O₂) oxidation of cyanide-contaminated soils from three cassava-processing-impacted locations in Ozoro and its environs, Delta State, Nigeria, and optimized the process using response surface methodology. Baseline soil pH, moisture content, electrical conductivity, organic matter and cyanide were determined. Cyanide was analysed by an acid-distillation/colorimetric procedure using copper chloride, sodium hydroxide trapping, chloramine-T and pyridine-barbituric acid, with spectrophotometric measurement at 578 nm. A randomized three-factor, three-level Box–Behnken design comprising 17 runs was executed in Design-Expert 13.0.1.0. The factors were H₂O₂ load (0.5–1.5%), water content (20–100%) and reaction time (24–96 h), while cyanide reduction was modelled separately for Leo, Oroledo and Erovie soils. Baseline pH ranged from 6.32 to 6.81, organic matter from 15.00 to 48.54%, and the cyanide concentrations were 853, 614 and 1041 mg CN⁻ kg⁻¹ dry soil, respectively. All the fitted quadratic equations were significant (p < 0.0001) and the R² values obtained were 0.9911, 0.9934 and 0.9922 for Leo, Oroledo and Erovie respectively. The optimum found in the experimental domain by numerical optimization was 1.0% H₂O₂, 60% water content and 60 h, which resulted in the observed cyanide reduction of 73.44%, 80.85% and 87.14%, respectively. The H₂O₂ quadratic coefficients were all negative, suggesting significant curvature and that further increases in oxidants beyond the centre of the plot did not consistently increase the removal. The results are encouraging for laboratory application, and site-specific results, limitations of available analytical and operational data suggest that field application needs to be independently validated, site-specific oxidant-demand assessment, and full soil-quality characterization.