Comparison of Different Extraction Methods

Each extraction method has its own characteristics. In order to compare the advantages and disadvantages of different pretreatment methods, the best conditions of each method were selected and the extraction effect of the same pollutant was compared. The soil samples were all 10g, and the extraction solvent was acetone ∶ n-hexane = 1∶ 1 (solvent dosage: 50mL for shaking extraction, 50mL for ultrasonic extraction and 50 ml for soxhlet extraction 150mL). The results are shown in Table 2.4.

Table 2. 4 Extraction effect of three pretreatment methods (unit: ng)

Figure 2.7 can show the measurement results more intuitively.

As can be seen from Figure 2.7, the order of overall extraction effect of these three pretreatment methods is Soxhlet extraction 16h, ultrasonic extraction for 20min (repeated for 3 times) and oscillating extraction for 30min (repeated for 3 times). Soxhlet extraction method has obvious advantages in extracting polycyclic aromatic hydrocarbons from soil, but it is slightly inferior to organochlorine pesticides, and the latter two methods are slightly better than ultrasonic extraction method.

This is because the adsorption and desorption of pollutants in soil components have a rapid reversible stage, followed by a slow irreversible stage. In the rapid stage, pollutants are adsorbed in an unstable form, so they are easily desorbed. In the slow stage, pollutants enter the micropores of soil components (caused by diffusion) and are adsorbed in a relatively stable form, which is difficult to desorb. Soxhlet extraction can ensure the purity of the extraction solvent in the filter paper sleeve and improve the desorption rate in the rapid stage, and because of the long extraction time, it is possible for the extraction solvent to enter the soil micropores, so the extraction is more sufficient; Although ultrasonic extraction and oscillating extraction can not make the solvent extracted in the container durable and pure, the concentration of pollutants in the extraction solvent is obviously reduced step by step by selecting multiple extractions, which provides favorable conditions for rapid desorption. At the same time, because ultrasonic extraction is more direct and effective, the soil particles vibrate violently, and the extraction solvent can easily enter the soil micropores, which can also achieve relatively good results. Because of the simple energy supply mode, oscillation extraction has poor extraction effect in slow stage and poor overall effect.

Fig. 2.7 Comparison of Extraction Results of Three Pretreatment Methods