The study of toluene adsorption by humin extracted from a forest soil

Background and Objective: Soil plays an important role in the environment by controlling the fate and availability of organic compounds due to the ability to absorb these substances. The absorption and desorption of these substances in the soil is basically controlled by soil organic matter. Therefore, to clean up organic chemicals, it is important to study the absorption behavior of these substances by soil organic matter. Soil organic matter has a complex structure and composition that depends on the source of organic matter. Soil organic matter plays an important role in environmental processes, the transfer of natural and unnatural pollutants and their fate in the soil environment. Several studies have concluded that adsorbent properties can have a significant correlation with the adsorption of hydrophobic organic materials. For example, the adsorption of hydrophobic organic substances to soil organic substances is inversely proportional to the polarity of these substances. It has been reported that the absorption of chloroaliphatic chemicals decreases with the increase of oxygenated functional groups in the organic matter. The normal absorption coefficient of organic carbon (Koc) of naphthalene was inversely proportional to the polarity of organic matter ((O+N)/C) of different soils and sediments. Volatile organic compounds (VOCs) are organic chemical compounds that have a high vapor pressure under normal conditions, which causes these substances to quickly enter the atmosphere. Toluene is volatile organic compounds found in petroleum products such as diesel. These materials are well known for soil and surface water pollution. Toluene has been shown to cause various risks to human health such as carcinogenicity. Leakage of these materials during transportation and storage can pollute groundwater, land, soil, and atmosphere. The American Environmental Protection Agency has determined the maximum amount of toluene to be 1000 micrograms per liter. Volatile organic compounds are one of the main sources of polluting the environment, among which the compounds mentioned above are one of the most effective polluting substances due to their high consumption in industry and agriculture. Also, these materials are abundant in petroleum products such as gasoline and diesel. Humin has a high capacity to absorb petroleum hydrocarbons and can play the role of an absorbent for these substances in water and soil environments. These hydrocarbons are produced in large quantities in the industry. These substances pose harmful risks to the health of humans and living beings. The purpose of this research is to investigate the role of humin extracted from soil in absorbing toluene as a part of petroleum hydrocarbons. In order to better understand the movement process of these materials in the soil and to evaluate the effect of humin as a part of soil organic matter on volatile petroleum hydrocarbons, it is necessary to investigate the behavior and extent of absorption of these materials.
Methodology: For extraction of humin, a soil with a high percentage of organic carbon was needed, and for this purpose, after examining the soils of northern Iran, the desired soil was sampled from the research forest of the University of Tehran with the financial classification of Sol and was air-dried for the next steps. After these steps, it was passed through a two mm sieve and stored in the laboratory. The chemical properties of soil and organic matter were measured, and the results are shown in Table 1. First, the soil sample was shaken for 48 hours with hydrochloric acid at a ratio of 1:10 on a reciprocating shaker and then centrifuged. Then the sample was shaken with distilled water on a shaker and centrifuged again. After completing the first step, the soil sample was mixed with a ratio of 1:10 with 0.1 normality NaOH for 24 hours shaken and then centrifuged, the supernatant solution was discarded and finally the above process until the supernatant solution of the sample turned pale yellow. This continued to be repeated 17 times. To investigate the interaction between toluene and humin, first, 0.1 g of humin was weighed and poured into a 120 ml vial. Then 30 ml of background solution was poured on the sample. Amounts of 0.2, 0.35, 0.5, 0.75, 1, 1.5 and 5 microliters were injected with a Hamilton syringe and closed with a rubber cap and sealed by an aluminum cover. In order to reach the equilibrium state, the samples were shaken at the equilibrium time obtained in the previous step on the shaking device with 150 revolutions and at a temperature of 25 ± 2 centigrade inside the incubator, and finally after reaching the equilibrium time, samples were taken by a gas syringe and the absorbed values was read.
Findings: Absorption of volatile hydrophobic substances is due to hydrophobic and van der Waals forces, and the reason for the initial increase is due to the empty surfaces of the absorbent, but later on, with the reduction of absorbent levels, the effect of the surface agent decreased and most of the hydrophobic forces were the agent of absorption. For this reason, the partition coefficient and absorption percentage decreased at once and then remained almost constant.
Conclusion: Fitting the absorption data with the surface absorption equations showed that the humin-toluene system had a good correlation with the Freundlich equation, and this correlation was well seen in the absorption of toluene. The absorption of toluene on humin was probably due to the roughness of the surfaces and little penetration in the structure of humin. In general, the absorption of hydrocarbon substances, especially volatile substances, on humic substances is a complex phenomenon. The absorption of toluene on humic material is only the absorption of a small part of hydrocarbon materials on a part of humic materials, and more research should be done on the absorption and desorption of these hydrocarbon materials on humic materials.