Engineered Nano-Adsorbents for Concurrent Heavy Metal Detoxification and Feedstock Upgrading of Waste Cooking Oils in Delta State, Nigeria

The study designed and tested multifunctional nano-adsorbent systems that sequester heavy metals while enhancing the quality of waste cooking oil (WCO) for biodiesel production in the Warri-Effurun industrial zone of Delta State, Nigeria. The waste oil from ten commercial centres contained high levels of Pb, Cd, Ni, and Cr, all of which exceeded the WHO/FAO limits, indicating the need to purify them using advanced purification methods. Two designed adsorbents (thiol-functionalized magnetic biochar (Fe3O4 -Biochar-SH) and amine-functionalized graphene oxide (GO -NH2)) were produced and characterized by SEM/TEM, FTIR, XRD, BET analysis, and zeta potential. Spontaneous and endothermic processes were confirmed by batch adsorption experiments showing rapid pseudo-second-order kinetics (R2 ≥ 0.989), Langmuir monolayer behaviour, and favourable thermodynamic behaviour (ΔG ≥ 0, ΔH ≥ 0). Biochar-Fe3O4-SH performed best, with Pb removal of 97.5% and over 87% for metals. Purification considerably enhanced the quality of FFA; peroxide value and moisture were reduced to 1.14 meq/kg and 0.12, respectively. Biodiesel derived from purified WCO demonstrated a better FAME composition (96.8%), improved viscosity and density, improved oxidative stability (6.1h), and full compliance with ASTM/EN standards. Regeneration experiments showed 86.7% capacity retention over five cycles. In general, using Fe3O4-Biochar-SH as an adsorbent, this study demonstrates that it is scalable and reusable and can accomplish both WCO detoxification and biofuel feedstock upgrading.