Exploring the existing state of petroleum based on the adsorption capacity of carbon tetrachloride in petroleum-contaminated soil

Purpose The existing state of petroleum in soil not only affected the migration of other substances but also changed soil ecology. The purpose of this paper was to investigate the distribution coefficient ( K d ) of carbon tetrachloride (CT) in soils with different contents of petroleum and organic matter and to judge the existing state of petroleum in soil by the interaction of organic matter and petroleum with CT. Methods Three different kinds of soil were used to prepare petroleum-contaminated soils with different concentration gradients and to age them. The adsorption capacity ( Q ) of CT in the different soil–water systems was determined by headspace-gas chromatography, and the adsorption isotherms were drawn. Results K d demonstrated a rapid enhancement effect in relatively low and high petroleum content. K d demonstrated the slow enhancement effect in a relatively moderate petroleum content (the ratio of petroleum content to organic matter content is slightly greater than 5%), which can be observed in petroleum-contaminated soils of high mineral content. The slow enhancement of K d was due to the formation of a separate petroleum phase (SPP) on the mineral surface by adsorption of petroleum, and SPP thin film can interact with minerals to weaken the adsorption of CT. The adsorption coefficient ( K SPP ) of CT in SPP increased linearly with the increase of SPP film thickness ( R 2 > 0.998). The increasing rate of K SPP depended on the specific surface area and mineral content of the soil. Conclusion When the ratio of soil petroleum content to organic matter content was greater than 5%, the SOM reached the state of petroleum saturation. The extra petroleum coated mineral surface can kill soil microbes and reduce soil microbial community diversity. The petroleum saturation point of SOM provides a basis for risk assessment of petroleum-contaminated soils and the protection of soil biodiversity.