Optimisation of soil washing method for removal of petroleum hydrocarbons from contaminated

Development of a BBD, and statistical analysis

The TPHs removal results were assessed statistically to create a response surface model and identify the best conditions for SWM (Table S2). Based on the results, the minimum, maximum and mean TPHs removal in SWM values were 63.5, 94.5 and 76.7%, respectively. According to fitting the model, quadratic model suggested (Table 3).

Table 3 A summarized view of the model’s performance.

By using the ANOVA, the results of TPHs (Table S3) showed to be highly dependable and had a very low probability value for the quadratic regression model, demonstrating that it could accurately explain the codes within the actual data and predicted values. This model was incredibly valid and proficient at predicting responses, as indicated by the correlation coefficients (R2, R2adj, and R2 predict). High correlation is present between R2, R2adj, and R2 predict, indicating that the model can predict responses19,20. The F-value and p-value in the ANOVA analysis are reliable statistical indicators for determining the data deviation factors. These indexes led to the selection of a substantial statistical model with a high F-value and a low p-value (≤ 0.05). Fisher’s F-test showed that a quadratic model would be the most suitable to fit the relationship between the predicted and experimental values of TPHs removal. The F-value of TPHs has been determined to be 1064.5 and the p-values (0.0001) provide clear evidence of an effective fit between the experimental and the expected response values. An Adequate Precision ratio (117.1) indicates an acceptable model21. Based on the ANOVA results, it was found that due to the physical nature of SWM, there is no significant relationship between the variables, so the mathematical relationship is simplified. The equation for the percentage of TPHs removal in SWM was expressed in terms of coded factors:

$$ {\text{TPHs removal }}\left( \% \right) \, = {74}/{75 } + { 5}/0{4} \times {\text{A }} + { 5}/0{2} \times {\text{B }} + { 7}/{4} \times {\text{C }} + { 4}/{9} \times {\text{D }} + { 7}/{7} \times {\text{E }} + \, 0/{8} \times {\text{BD }} + \, 0/0{9} \times {\text{A}}^{2} \, + \, 0/{6} \times {\text{B}}^{2} \, + { 2}/{3} \times {\text{C}}^{2} \, + \, 0/{5} \times {\text{D}}^{2} \, + { 2}/{2} \times {\text{E}}^{2} $$


The impact of parameters on the removal of TPHs from SWM

Through the use of response surface plots or contour plots generated by RSM, it is possible to illustrate the interaction between variables and to pinpoint the most advantageous values for the variables.

The 3D graph in (Fig. 2) displays the TPHs removal efficiency based on the five influence washing parameters in SWM. Each of the tested parameters, including washing solution pH, liquid/soil ratio, surfactant concentration, washing cycles, and retention time, had a substantial influence on TPHs removal efficiency.

Figure 2

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

Washing solution pH effect

The efficiency of SWM in TPHs removal is significantly influenced by the washing solution pH. The results of the effect of pH (4–8) on the efficiency of TPHs removal were surveyed and showed in (Fig. 2a). As the pH of the solution increased, the efficacy of the removal of TPHs increased significantly, with the highest efficacy at pH levels slightly basic and close to 8. The negative charges of organic and inorganic components of the soil colloid surfaces are increased by the increasing of the pH of the soil solution. This proliferation of negative charge will bring about the dispersion of soil particles and the desorption of organic pollutants from the colloid surface to the washing solution, owing to the repulsion between the negatively charged heads of organic matter and soil colloids. The solubility of organic matter increases with an increase in soil solution pH, since the main organic matter constituents (humic acid and fulvic acid) are soluble in alkaline solutions. When organic material is dissolved into the soil solution, the petroleum hydrocarbon molecules that are bound tightly are also released or made available for dissolution by surfactants by lowering surface tension10. According to a previous study conducted on diesel-contaminated soils, an increase in pH or alkalinity was reported, extracting more diesel than when in an acidic phase, and it was found that oils are more soluble under alkaline conditions than acidic22. In Liu and coworkers’ study, remediation of diesel-contaminated soil at alkaline pH in the presence of tween 80 was better than other pH23. In addition, similar outcomes of the effect of pH were reported in Jiang and coworkers’ study24.

Liquid/soil ratio effect

The effectiveness of SWM in the removal of TPHs was heavily affected by the liquid to soil ratio. The results of the effect of this parameter (20:1–60:1) on the efficiency of TPHs removal were surveyed and showed in (Fig. 2a). When the liquid to soil ratio enhanced, the elimination of TPHs significantly improved, with the highest effectiveness nearly accomplished at elevated levels. As anticipated, the application of a substantial amount of surfactant solution facilitates the TPHs removal. By having high liquid to soil ratio, greater TPHs concentrations are created between the two phases, thus intensifying the transfer of contaminants from the solid to the aqueous solution. Despite this, the cost of the procedure rises exponentially with the growth of the liquid to soil ratio, thus making it of utmost importance to identify the ideal value for this parameter25. Evaluation of the prior studies uncovered that the liquid to solid ratio was altered depending on the concentration and type of pollutants, the kind of surfactant, and the conditions of the soil washing process. In the study conducted by Lin and coworkers, the optimum ratio of liquid to soil was found to be 60:126. Also, in Wang and coworkers study 60:1 ratio reported as a prepare ratio in the presence of EDTA27.

Surfactant concentration effect

The effectiveness of SWM in the removal of TPHs was heavily affected by the surfactant concentration. The results of the effect of this parameter (5–10 mg kg−1) on the efficiency of TPHs removal were surveyed and showed in (Fig. 2b). Surfactants are compounds that reduce the surface tension between two liquids or between a liquid and a solid, allowing for better mixing and dispersion of the solution. The concentration of surfactant used in soil washing can have a significant effect on the efficiency of the process. At low concentrations, surfactants may not be effective in removing contaminants from soil. This is because the surface tension of the water is not reduced enough to allow for effective mixing with the contaminants. As the concentration of surfactant increases, more contaminants are solubilized and removed from the soil28,29. However, at high concentrations, surfactants can become less effective due to factors such as foam formation or precipitation of the surfactant. Foam formation can occur when too much surfactant is added to the solution, leading to reduced contact between the solution and soil particles. Precipitation can occur when too much surfactant is added, causing it to bind with other compounds in the solution and become insoluble. Therefore, it is important to optimize the concentration of surfactant used in soil washing processes to ensure maximum efficiency while avoiding negative effects such as foam formation or precipitation. This optimization process may involve testing different concentrations of surfactant on contaminated soil samples and monitoring their effectiveness in removing contaminants29. Prior studies have found various levels of surfactants as the most favorable concentration, which is influenced by the kind and amount of pollutants in the soil, the type of surfactant and the retention time of the process30,31.

Washing cycles effect

The efficacy of SWM in the removal of TPHs was profoundly affected by the washing cycles. An examination of the impact of this parameter (1–3 time) on the effectiveness of TPHs removal was conducted and depicted in (Fig. 2c). Generally, the more times the soil washing is performed, the more successful it will be. Every washing cycle eliminates a certain amount of pollutants from the surface, and multiple washings can contribute to the removal of more pollutants. However, there are also some potential drawbacks to multiple washings. For example, excessive washing can lead to the loss of valuable nutrients and organic matter from the soil. Repeated washing may not be practical or cost-effective in all situations. Overall, the optimal washing cycles for soil remediation will depend on a variety of factors, including the type and concentration of contaminants present in the soil, as well as site-specific conditions, such as soil texture and moisture content. A thorough site assessment and careful planning are essential for achieving effective and sustainable results with soil washing7,32. In Rui and coworkers’ study, 3 time washing was reported as an optimum multi-step washing to remediation of pollutants33. Also, in Piccolo and coworkers’ study, the highest performance to heavy metals removal was got in 3 times washing34.

Retention time effect

The potency of SWM in the removal of TPHs was deeply affected by the retention time. A study of the influence of the 30–60 min parameter on TPHs removal efficiency was performed and presented in (Fig. 2d). Soil washing requires a certain length of contact between the soil and washing solution, which is known as the retention time. The effect of retention time on soil washing depends on various factors, such as the type of contaminants present in the soil, the type of washing solution used, and the soil properties. Increasing retention time can improve the efficiency of soil washing by allowing more time for the contaminants to dissolve or desorb from the soil particles and be removed by the washing solution. However, there is a limit to how much retention time can be increased before diminishing returns are observed. Different pollutants may have different optimal retention times for effective removal. Increasing retention time may also increase the amount of water required for soil washing, which can lead to higher costs and potential environmental effects, such as increased water usage and discharge. Overall, while increasing retention time can improve soil washing efficiency sometimes, it is important to consider other factors such as cost-effectiveness and environmental effects when determining optimal retention times for specific soil remediation projects28,35,36. In Bianco and coworkers’37 and Kang and coworkers38 studies, 60 min reported as an optimum retention time to PAHs removal from soil.

Predicted optimum condition

Finally, we used Design-Expert software to estimate the optimal value of experiment variables for TPHs removal by SWM. pH of solution 7.8, liquid to solid ratio 50, reaction time 52 min, surfactant concentration 7.9 mg kg−1 and washing cycles 3 times were an optimum condition of SWM. In this situation, 98.8% of TPHs removed from soil. According to the prediction of the software, more than 99.9% of TPHs can be removed in the mentioned conditions. This difference can be due to measurement errors, device accuracy, the presence of interfering agents, etc. The reaction time had highest effect on SWM (7.7), then the surfactant concentration (7.4), the pH of solution (5.04), liquid to solid ratio (50.2) were effective respectively, and washing cycles (4.6) had lowest impact on SWM.

Supplementary investigationReaction kinetics

The TPHs removal from the soil in the optimal condition of the SWM was showed by first- and second-order model used were expressed by the [Eqs. (4) and (5)]. The results of kinetics study at optimum condition were presented in (Table 4).

Table 4 First- and second-order coefficient.

The first-order model gave a slightly better fit to the experimental results than the second-order model, based on the square regression coefficient R2. The removal of TPHs from the soil was regulated by the first-order kinetic model. Similarly, Li and coworkers39 study, and Khalladi and coworkers14 studies, first-order kinetic reported as a best model of fitting.

Effect of temperature, mixing speed, air flow and sonication on SWM performance

The effect of reactor temperature on TPHs removal performance in SWM at 20, 25, 30, 35, and 40 ºC investigated. The results illustrated in (Fig. 3). Temperature can have a significant impact on the efficiency and effectiveness of soil washing processes. Higher temperatures can increase the rate of chemical reactions and solubility of contaminants in the soil, leading to faster and more thorough removal of pollutants33. However, excessively high temperatures can also cause problems such as increased vaporization of water and chemicals, which can lead to loss of materials and reduced efficiency. Additionally, high temperatures may also cause changes in the physical properties of the soil, such as increased viscosity or decreased permeability, which can hinder the washing process40. Therefore, it is important to control and optimize the temperature carefully during soil washing processes to ensure maximum effectiveness while minimizing any negative effects. The analysis of (Fig. 3) shows that, within the tested temperature range, the removal performance is not significantly affected, though a minor variation is present. The temperature dependent processes control the removal, desorption, and dissolution of elements, whereas the separation of incrustation or soil-trapped TPHs is mainly determined by mechanical conditions. Consequently, this test has revealed that other parameters in TPHs could have a greater impact than temperature. In Peng and coworkers’ study, similar results were reported25.

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).

The effect of mixing speed on TPHs removal performance in SWM at 0, 50, 100, 150, and 300 rpm investigated. The results illustrated in (Fig. 4). The process involves mixing the contaminated soil with water and other chemicals to extract the contaminants. Therefore, the mixing speed plays a crucial role in the soil washing process performance. The effect of mixing speed on soil washing process can be summarized as follows35:


Extraction efficiency: The extraction efficiency of contaminants from soil increases with increasing mixing speed. This is because higher mixing speeds create more turbulence, which enhances the contact between the soil particles and the…

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