Occurrence of heavy metals and polycyclic aromatic hydrocarbons in typical used mineral oil from China: implications for risk management

Heavy metals and polycyclic aromatic hydrocarbons (PAHs) are the important contaminants in used mineral oil (UMO) and likely pose environmental risks in the processes of disposal and regeneration. However, little information on the concentrations and associated with the risk of heavy metals and PAHs in various types of UMO from different industries is available. In this study, the occurrence of heavy metals and PAHs in the UMOs from typical industries in China was investigated. The median concentrations of 12 metals were 0.1–28 mg/kg. PAHs were mainly composed of 2- and 3-ring compounds and the median concentrations of 16 PAHs were 0.03–7.6 mg/kg in different UMO. The cluster analysis showed that samples with high heavy metal and PAH concentrations mainly originated from machine manufacturing and vehicle industries. The random forest model suggested that the species and phase of UMO were the main influencing factors associated with heavy metal concentrations, and the relative importance was 71.1–19.9%, 100–30.7%, 42.3–20.7%, 38.0–11.8%, and 100–24.1% for Cu, Zn, Ni, Cr, and Pb models, respectively, while industry was the main influencing factor associated with PAHs in UMO, and the relative importance was 81.5–51.5%, 27.7–16.5%, 83.4–25.3%, 94.8–21.4%, and 53.3–16.1% for 2- to 6-ring models, respectively.