A method of scavenging oil from an oil-water mixture includes providing a plurality of functionalized fly ash particles having functionalized surfaces including reactive groups or reactive materials having hydrophobic groups covalently bound to the reactive groups or reactive materials. The oil-water mixture is contacted with the plurality of functionalized fly ash particles. The plurality of functionalized fly ash particles absorb oil from the oil-water mixture to form oil-laden fly ash particles. The oil-laden fly ash particles can be fed into a combustion process to generate heat from oil absorbed thereon, or absorbed oil from the oil-laden fly ash particles can be separated using a desorption process, and the oil recovered after separating.