Treatment of contaminated soil to petroleum hydrocarbons with Vetiver phytoremediation

Abstract
Industrialization can be one of the main causes of environmental pollution increasing, especially petroleum hydrocarbons in the water, air, and soil. Polluted soils in the evaporation ponds are one of the primary sources of pollution in the oil refineries. Phytoremediation is the use of plants to remove contaminants from contaminated soils, to accumulate them in the roots and to remove them from the stems and leaves, or to decompose these compounds within the plant. Choosing the right plant, especially in soils with low or moderate pollution, plays an important role in the process of phytoremediation. Vetiver is a suitable plant for phytoremediation. In this project, first, soil samples were collected and the survival and resistance of vetiver were investigated. After that, executive works started in the Dalan area, including planting plants in the area, installing a reservoir, installing a drip irrigation system, sampling contaminated and non-contaminated soils in the area, and sampling the soil adjacent to the plant roots and measuring the amount of Petroleum hydrocarbons. The vetiver plant can grow and survive in oil-polluted soils. After a year of phytoremediation effect, the Vetiver plant reduced the amount of total petroleum hydrocarbons and aromatic hydrocarbons by about six times and nine times, respectively. Further phytoremediation treatment with vetiver root (soil rhizosphere near the root) could further reduce the amount of total petroleum hydrocarbons and aromatic hydrocarbons.

Introduction
Phytoremediation is a cost-effective plant-based approach of remediation that takes advantage of the ability of plants to concentrate elements and compounds from the environment and to metabolize various molecules in their tissues. It refers to the natural ability of certain plants called hyperaccumulators to bioaccumulate, degrade, or render harmless contaminants in soils and water. Bioremediation is very much involved in the degradation, eradication, restriction, or reclamation varied chemical and physical hazardous substances from the nearby with the action of all-inclusive microorganisms. The oil refinery is one of the top 10 sources of environmental pollution. In the last few years, petroleum hydrocarbons pollution, due to its toxicity to microorganisms as well as humans, has become one of the most serious global concerns. Among petroleum hydrocarbon pollutants, 16 polycyclic aromatic hydrocarbons (PAHs) are known as important hazardous pollutants due to their stability in the environment. The use of plants to remove contaminants from contaminated soils, their accumulation in roots, and their removal from stems and leaves is called phytoremediation. The term phytoremediation was first introduced in 1991 to describe methods of reducing the environmental pollution. Phytoremediation is an inexpensive in-situ method that has no destructive effects on the environment. Phytoremediation is a cost-effective technology that should be considered an environmental-friendly method for remediation of contaminated areas. Vetiver plant (according to the new scientific name classification called Chryspogon zizanioides L.) is suitable for phytoremediation due to its special properties. Vetiver is called “khas” in Iran. Vetiver is widely distributed in Africa, India, Myanmar, Sri Lanka, and Southeast Asia. This plant is a genus of weeds and has no rhizomes. It has a voluminous root system with delicate structures that can grow quickly so that sometimes its root depth reaches 3 to 4 meters in the first year of growth. Chrysopogon zizanioides, commonly known as vetiver is a perennial bunchgrass of the family Poaceae, native to India. Vetiver grows to 150 centimeters (5 ft.) high and form clumps as wide. Under favorable conditions, the erect culms can reach 3m in height. The stems are tall and the leaves are long, thin, and rather rigid. The flowers are brownish-purple. Unlike most grasses, which form horizontally spreading, mat-like root systems, vetiver roots grow downward, 2 meters (7 ft.) to 4 meters (13 ft.) in depth. The vetiver bunch grass has a gregarious habit and grows in tufts. Shoots growing from the underground crown make the plant frost and wildfire resistant, and allow it to survive heavy grazing pressure. The leaves can become up to 300 centimeters (10 ft.) long and 8 millimeters (0.3 in) wide. The panicles are 15 centimeters (6 in) to 30 centimeters (12 in) long and have whorled, 25 millimeters (1 in) to 50 millimeters (2 in) long branches. Vetiver grass is a phytoremediator plant but is also grown for many other purposes. The plant helps to stabilize soil and protects it against erosion, but it can also protect fields against pests and weeds. Vetiver has favorable qualities for animal feed. From the roots, oil is extracted and used for cosmetics, aromatherapy, herbal skincare and ayurvedic soap. Due to its fibrous properties, the plant can also be used for handicrafts, ropes and more.

Methodology
Three thousent vetiver seedlings were prepared from its production center in Khuzestan province, Ramhormoz city. After replanting in seedling pots, kept with care for two months. The intended land for the phytoremediation project was 23,000 square meters, which had been used for years as an evaporation pond for the effluent of the Farashband refinery without the use of any plastic, or cement, or geomembrane liners. In the first step, two of the three main ponds were selected. After that, the wastewater was removed from the evaporation ponds and the soil around the ponds was poured into them to fill the ponds. Then the land was leveled and a quarter of it was selected for the project. Holes were dug to plant the plants with a hole machine. The physical and chemical properties of the wastewater and the contaminated soil were determined by the laboratory of Farashband Gas Refinery Company. Total petroleum hydrocarbons (TPHs) were measured by Parham Gostar Shiraz Laboratory and cyclic aromatic hydrocarbons were measured by gas chromatography-mass spectrometry (GC-MS) of the central laboratory of Shiraz University.

Conclusion
Vetiver plant can grow and survive in a variety of oil pollution conditions, such as contaminated soil in refineries. Results of this study showed that after one year of phytoremediation, vetiver reduced the amount of total petroleum hydrocarbons and Polycyclic aromatic hydrocarbons by about six and nine times, respectively. In addition, Results showed that further phytoremediation treatment with vetiver root (soil rhizosphere near the root) could further reduce the amount of total petroleum and polycyclic aromatic hydrocarbons.