AbstractBACKGROUNDThis research investigated the solidification/stabilization process (S/S) efficiency of contaminated soil by total petroleum hydrocarbons (TPH) in the waste of a used oil treatment plant using quick lime (QL) as a binder and modified clay (MC) as an additive. Several objectives were considered, including reducing the mobility and leaching of TPH, investigating the effect of MC on the QL hydration process in the presence of organic matter, the possibility of S/S waste reuse, and estimating the S/S cost for the optimum mixture.RESULTSThe unconfined compressive strength (UCS) increased about 45% by increasing 25% of QL without MC, but these S/S wastes did not satisfy the waste reuse standards. Additionally, adding any ratio of MC to samples containing QL increased UCS significantly. The UCS of all samples at the age of 28 days with different QL and MC proportions obtained more than 350 kPa (the minimum UCS required for S/S waste disposal in the landfill). Moreover, the toxicity characteristic leaching procedure (TCLP) test revealed that TPH leaching of contaminated soil was 3.8 mg L−1 but adding only 15% of QL could reduce TPH leaching to less than 1 mg L−1, which is the maximum allowable TPH leaching.CONCLUSIONThe combination of QL and MC decreased TPH leaching, significantly. The scanning electron microscopy analysis also showed that QL hydration was improved by increasing the amount of MC. Finally, a sample with 35% QL and 15% MC was selected as an optimum sample due to its cost of $80.5/ton. © 2021 Society of Chemical Industry (SCI).