Pollution characteristics of polycyclic aromatic hydrocarbons in common used mineral oils and their transformation during oil regeneration

Abstract

The pollution characteristic of polycyclic aromatic hydrocarbons (PAHs) in common used mineral oils, semi-refined oils, refined oils and solid wastes produced during the used mineral oil regeneration process was analyzed. The results showed that total PAHs content in six common used mineral oils was as follows: used engine oil > used quenching oil > used casting oil > used hydraulic oil > used antirust oil > used industrial lubricating oil. Furthermore, this order was dependent on the source of PAHs and oil working temperatures. Additionally, total PAHs content in regenerated oils was as follows: semi-refined oil > refined oil > crude oil, which was related to the catalytic cracking process of crude oil and adsorption refining process of semi-refined oil. The ranking of total PAHs content in regenerated wastes varied depending on the regeneration technology used as follows: waste adsorption sand > acid sludge > waste clay > precipitation sludge > cracked residue. In all types of used mineral oils and regenerated wastes, the maximum and minimum proportions of the total PAHs content were composed of 2–3 ring-PAHs and 5–6 ring-PAHs, respectively. The majority of PAHs in the used mineral oils entered into regenerated wastes during regeneration process, while a small number remained in the regenerated oil.

Introduction

With the widespread use of coal and petroleum in industrial production and transportation, now mineral oil has become an important fossil fuel worldwide. Mineral oil extracted from oil, coal and oil shale cannot be used continually because external factors alter its original physical and chemical properties, resulting in generation of waste mineral oil (Pratt et al., 1999). There are a variety of toxic materials in mineral oil including heavy metals, benzene series and polycyclic aromatic hydrocarbons (PAHs). This was closely related to the usage and production process of used oils (Magiera et al., 2003). Accordingly, used mineral oil has the potential to cause serious harm to the ecological environment and human health if it is discharged directly into the environment. According to the statistics, a barrel of used mineral oil (200 L) dumped into lake or sea can cause about 3.5 km² area of the water pollution (Qiao et al., 2015). If poured into the soil, it can make contaminated site grow nothing even for 3 to 5 years (Qiao et al., 2015). Because of the obvious toxicity and flammability, used mineral oil has been listed as a class 8 hazardous waste (HW08) according to the National Hazardous Wastes Catalogue in China (Wang et al., 2013).

However, used mineral oil has high regeneration value through appropriate regeneration technology. Recycled used mineral oil can be used though either energy regeneration or materials regeneration (Zenon et al., 2010). The energy regeneration mainly includes direct combustion and regeneration into fuel oil. Pollutants in the used mineral oil can cause serious secondary pollution to the environment when used mineral oil was burned directly without any treatment. Regeneration into fuel oil is a major renewable way for used mineral oil at present through pyrolysis and catalytic pyrolysis methods. However a lot of secondary pollutants such as oily wastewater, acid slag, waste clay produced in the course of regeneration should be disposed safely. The materials regeneration of used mineral oils refers to the regeneration into the high quality base oil through depth refining process such as short-range distillation, flocculation, hydrofining, and reduced pressure distillation (Bartz, 1998).

Developed countries have formulated a series of strict and detailed laws, rules, regulations and policies for used mineral oil management from collection to disposal, which promotes environmental management and reuse of used mineral oils. In China, there are still many problems associated with used mineral oil management. Compared with the total quantity, the actual amount of used mineral oil brought into hazardous waste management is relatively less (recovery of 6%), and the regeneration utilization is relatively low. What is more, there has been a large amount of environmental pollution events of used mineral oils. Further research into the pollution characteristics of used mineral oil is needed to improve the value of recycled oils and strengthen the environmental management of used mineral oils.

PAHs are considered to be persistent contaminants that may be toxic to living creatures. Because of their mutagenic, carcinogenic and teratogenic properties, 16 PAHs have been classified as priority substances by the United States Environmental Protection Agency (USEPA) and the European Union (EU) (Lindgren et al., 2014). PAHs have been found in crude oil and the recycled oils. In order to further understand the pollution characteristics of PAHs in used mineral oils and regenerated products, this study was conducted to characterize the residual level, composition and transportation of PAHs in used mineral oil, regenerated oil products and regenerated wastes using the City of Chongqing, China as the study area. The results presented herein will be useful for environmental risk assessment and pollution control management of used mineral oil.