Comparison of the Efficiency of Different Plants to Remove Total Petroleum Hydrocarbons from Oilfield Soils

Crude oil is a complex combination of many hydrocarbon and non-hydrocarbon compounds, including heavy metals, which affect the physical and chemical properties of the soil, cause the soil particles to stick and connect and then cause the soil to become stiff and impenetrable. Contamination of soil with petroleum hydrocarbons is a significant environmental problem, which has received remarkable attention in recent decades. Petroleum hydrocarbons are resistant and hazardous pollutants. Some petroleum hydrocarbons such as benzene are mutagenic and carcinogenic materials for humans. There are many physical and chemical methods to remediate oil-contaminated soils. Phytoremediation is a relatively new technology for refining contaminated soils in which resistant plants are used to remove or reduce the concentration of inorganic, radioactive, and organic pollutants, especially petroleum compounds, from the environment.

Sufficient amounts of about 50 kg of soil contaminated with petroleum hydrocarbons were collected from regions (0-30 cm soil depth) adjacent to the oil wells west of Kermanshah province. Uncontaminated soil samples were also taken from sites at the lowest distance to the contaminated sites. The aim of this study was to compare the efficiency of different plants to remove total petroleum hydrocarbons from oilfield soils. In this study, after determining the total amount of petroleum hydrocarbons, the contaminated and uncontaminated soils were mixed in 4 treatments with different weight ratios (0, 10, 25, and 35%). This experiment was established as completely randomized design with 3 replications for 6 different plants (Barley, Grass, Alfalfa, Hemp, Camelina, and Vicia ervilia). One treatment without plant was considered to remove soil matrix effects on petroleum hydrocarbon concentrations. Plants were harvested at the end of their growing season (90-120 days). Soils and plant samples from the experimental pots were analyzed for their important properties (including some physiological characteristics of the plants, as well as the percentage of reduced petroleum hydrocarbons in the soils). The gravimetric method was used to determine the concentration of petroleum hydrocarbons in the soil. After measuring the properties of the soil and plant, the normality of the data was checked by the Anderson–Darling test, and the homogeneity of the variance of the treatments was checked by using Levene's test. Analysis of data variance was done using ANOVA and average data comparison was done using LSD test at 5 and 1 percent probability levels (SAS 9.4 and SPSS 26).

In general, the growth of most plants showed a decreasing trend in proportion to the increase in soil pollution levels. However, the growth decline rates of different plants were not similar. Camelina was very sensitive to oil pollution and the plant could not tolerate pollution even at 10% level. After camelina, alfalfa was highly sensitive to oil pollution. The highest dry weight of the aerial parts of the hemp plant in the soil without oil contamination was observed at the rate of 111.22 grams in the pot. The leaf area of all studied plants in contaminated soils decreased compared to the control treatment (without contamination) so with the increase in the percentage of contamination, the leaf area of the plants was significantly reduced. The highest amount of leaf surface was observed in unpolluted soil and in the hemp plant. Except for the Camelina plant, which was completely destroyed at different levels of pollution, the rest of the plants showed a noticeable decrease in growth. The total petroleum hydrocarbons in soil were measured again 120 days after the start of cultivation, and its difference with the total amount of petroleum hydrocarbons at the beginning of cultivation was determined as the reduction of petroleum hydrocarbons and reported as a percentage. According to the mean comparison results, the percentage of reduced petroleum hydrocarbons was not significantly different among cultivated and non-cultivated treatments, although, it was significantly affected by soil pollution levels. Since all the studied soils contained natural bacteria and were not sterilized, the eliminated part of petroleum hydrocarbons is probably decomposed and removed by native bacteria in the soils. Therefore, the strengthening of native bacteria in these soils may increase the decomposition and degradation of petroleum hydrocarbons.

The results of this research show that the presence of petroleum hydrocarbons in the soil caused a decrease in growth and other physiological characteristics in all studied plants. Although the Camelina was able to germinate in soils contaminated with petroleum hydrocarbons, the presence of these pollutants in the soil prevented the optimum growth of the plant, so its use in subsequent studies of phytoremediation of oil-contaminated soils, was not recommended. The results showed that there is no statistically significant difference between cultivated and non-cultivated treatments at different pollution levels, and the reduction of the total petroleum hydrocarbons in the soil was probably done by native microorganisms in the soil. It is recommended to take into consideration the efficiency of the plant species used, the type of polluting hydrocarbons, and the duration of contamination in future research to obtain better results.