Influence of Physicochemical Factors on Biodiesel Retention in Clayey Residual Soil

Soil can naturally attenuate contamination, depending on its interaction with contaminants. Soils can delay the spread of the contaminant plume and even reduce the bioavailability of the contaminant because of their contaminant retention capacity. The retention capacity may be influenced by several physicochemical factors of both the soil and the contaminant. The aim of this study was to assess the retention capacity of a clayey, basaltic residual soil to an organic contaminant (soybean biodiesel) under different variable conditions, such as moisture content, void ratio, and presence or absence of biostimulation nutrients, specifically nitrogen (N), phosphorus (P), and potassium (K). Experiments were conducted using 100-g samples of dry soil under different void ratios (1.14, 1.24, and 1.34) and different moisture contents (15, 25, and 35%). Two series of experiments were conducted: one in the absence of nutrients and another in the presence of nutrients. The soil samples were initially sterilized, and they were spiked with biodiesel at a concentration of $40\mathrm{g}/\mathrm{kg}$ of dry soil (4%) and homogenized. The extent of retention of the contaminant in the soil was assessed immediately and after 15 and 30 days of contamination. Residual contaminant content in the soil was determined based on the difference between the initial contaminant mass and the extracted contaminant mass using ultrasound technique. Results showed that the moisture content had a profound impact on contaminant retention—as the moisture content increased, contaminant retention decreased. The presence of nutrients also reduced the retention capacity of the soil under all tested conditions in this study. Generally, the amount of residual contaminant in the soil increased with the contact time. Overall, biostimulation nutrients and high moisture content reduced the contaminant retention, creating conditions favorable for contaminant migration, which increase environmental risk associated with the contamination.