Optimisation of soil washing method for removal of petroleum hydrocarbons from contaminated soil around oil storage tanks using response surface methodology

Abstract

Total petroleum hydrocarbons (TPHs), which are often found in soil, water, sediments, and air. These compounds are a type of pollutant that can have a serious negative impact on living things and human health. Soil washing method is a remediation technique used to remove contaminants from the soil. This process involves the use of water or other solvents to extract contaminants from the soil, followed by separation and disposal of the contaminated solution. This research engineered the effectiveness of soil washing method to remove TPHs from a genuine, sullied soil sample. After analyzing the physical and chemical properties of the soil, the Box-Benken Design (BBD) technique was used to optimize the variables that influence the process's effectiveness. A quadratic model was suggested based on the BBD design, correlation coefficients, and other factors. The minimum, maximum and mean removal of TPHs during the stages of the study were 63.5, 94.5 and 76.7%, respectively. The correlation between the variables was strong, as shown by the analysis of variance (ANOVA), F-value (1064.5) and P-value (0.0001), and the proposed model was highly significant. The most effective soil washing method (SWM) was obtained with pH 7.8, liquid to solid ratio 50:1, reaction time 52 min, surfactant concentration 7.9 mg kg^-1, and three washings. A removal rate of 98.8% was accomplished for TPHs from the soil in this context. The kinetic results indicate that the kinetic of TPHs removal follows the first-order kinetics (R² = 0.96). There was not a major difference in the process's efficiency based on temperature. The removal efficiency heightened from 0 to 150 rpm and then remained steady. Introducing air flow increased the rate of removal, and the combination of ultrasonic waves with the reaction environment increased the process efficiency and decreased the time for the process and the amount of times it needed to be washed. An analysis of the washed soil both physically and chemically revealed a substantial decrease in the concentration of other elements.

Figure 1

Diagram of the SWM.

Figure 2

3D plots of significant interaction terms (washing solution pH, liquid/soil ratio, surfactant concentration, washing cycles, and retention time).

Figure 3

The effect of temperature on TPHs removal in SWM (pH 7.8, L/S ratio = 50:1, surfactant concentration = 7.9 mg L⁻¹, washing cycles = 3 times, and retention time = 52 min).

Figure 4

The effect of mixing speed on TPHs removal in SWM (pH 7.8, L/S ratio = 50:1, surfactant concentration = 7.9 mg L⁻¹, washing cycles = 3 times, and retention time = 52 min).

Figure 5

The effect of airflow rate on TPHs removal in SWM (pH 7.8, L/S ratio = 50:1, surfactant concentration = 7.9 mg L⁻¹, washing cycles = 3 times, and retention time = 52 min).

Figure 6

The effect of airflow rate on TPHs removal in SWM (pH 7.8, L/S ratio = 50:1, surfactant concentration = 7.9 mg L⁻¹, washing cycles = 3 times, and retention time = 52 min).

Figure 7

Changes of trace and major elements during soil washing.