3.1 Volatilization characteristics of toluene and water
The curves of samples #1, #3, and #5 in Fig. 3 represent volatilization process of toluene, water and total mass loss by volatilization in clay, fine sand and corase sand, respectively. Volatilization rate is defined as the mass loss of toluene or water per unit area and unit time by volatiliaztion. According to Fig. 3, it can be seen that the volatilization process of toluene can be divided into two stage: stage Ⅰ and stage Ⅱ. At stage Ⅰ, volatilization rate of toluene linearly decreases with time. It shows a low volatilization rate at stage Ⅱ. As can be seen from Fig. 3, the volatilization process of toluene only have stage Ⅰ in coarse and fine sand. The volatilization process of toluene in clay has stage Ⅰ and stage Ⅱ. At stage Ⅰ, the average volatilization rate of clay, fine sand and coarse sand are 667, 786 and 762 mg/(min⋅m2), respectively. Volatilization rate of toluene in clay is significantly lower than that in sand. It can be explained by the fact that sand has higher air permeability, and sand has well volatilization channel. In addition, the clay particles are smaller in size and have larger specific surface area, causing higher adsorption capacity(Song et al. 2018). At Stage Ⅱ, the duration time of volatilization process for coarse sand and fine sand are shorter obviously, which can be ignored. Compared with clay, adsorption capacity of sand is pretty weak. Most of the toluene in sand has volatilized into air at stage Ⅰ. So generally the volatilization process of stage Ⅱ in sand will not occur. For clay, there obviously exists a low-speed volatilization process at stage Ⅱ. The main reason for this phenomenon is that the clay has strong adsorption capacity for toluene, which weakens the volatile capacity of toluene. Under the same initial concentration, the proportion of adsorption phase(solid phase) toluene in clay is higher than that in sand, resulting in lower volatilization rate of toluene.
Figure 4 (a), (b) and (c) are the variation results of the mass proportion of volatilization of toluene and evaporation of water with time in clay, fine sand and coarse sand, respectively. It is found that the mass loss proportion of toluene by volatilization is much higher than that of water in early stage Ⅰ. This is attributed to the fact that the vapor pressure of toluene is higher than that of water. Vapor pressures of toluene and water are 22.1 mmHg and 17.5 mmHg at 20 ℃, respectively(Hauxwell et al. 1968). With the increase of vapor pressure, there exists greater pressure difference at liquid-gas interface. So the toluene molecules are more easily to escape from liquid into pore space of soil, resulting in the improvement of volatilization process of toluene.
3.2 Effect of toluene on water
The curves of samples #2, #4, and #6 in Fig. 3 represent volatilization process of water without toluene in clay, fine sand and corase sand, respectively. As we can see from Fig. 3(a), volatilization rate of water for sample #2 with time decrease slowly. It is noted, however, evaporation rate of water from samples(#1) presents rising curve process, when the volatilization rate of toluene decreases significantly. It is indicated that the volatilization of toluene can inhibit evaporation of water. A possible explanation is that polar water is more easily sorbed on clay than weakly polar toluene molecules(Tekrony &Ahlert 2001). Researchers(Smith et al. 1990, Unger et al. 1996) studied the adsorption of VOCs vapor to vadose zone soils from 0% water content to saturation. It was found that water effectively competed with VOCs for mineral surfaces and therefore suppressed VOCs adsorption considerably. In other words, the adsorption of toluene molecules are weaker than water molecules, resulting that the toluene molecules may mainly exist outside of water film. Therefore, higher concentration of toluene inhibits the evaporation of water.
3.3 Effect of water content on volatilization of toluene
Figure 5 shows the mass loss of toluene due to volatilization over time in clay and fine sand under different water content, as well as the variation of volatilization rate with increase of water content in clay. As observed from the volatilization curve of toluene, the influence of water content on volatilization of toluene in clay can not be neglected. Researchers(Donaldson et al. 1992, Tekrony &Ahlert 2001, Wilson et al. 1994) found that there might exist competitive adsorption relationship between water molecules and toluene molecules for the binding sites on soil particle surface(Fig. 7). That is, conpared with toluene molecules, water molecules present stronger polar. So it has stronger competitive adsorption capacity on the surface of clay particles(He et al. 2008, Poulsen et al. 1998). In other words, water molecules occupy part of the binding sites of toluene molecules(Fig. 7). As is shown in Fig. 7, more toluene molecules are desorbed from the soil binding sites and the volatilization process of toulene is promoted. Increases in water content after a dry period can cause VOCs fluxes that are many times higher than the average background level, which is called "wet-dog" effect(Petersen et al. 1996, Poulsen et al. 1998). This so-called "wet-dog" effect, where VOCs are released to the soil air during an increase of water content, has been reported by Petersen(1996) and Poulsen(1998). When the water content of soil increases, VOCs molecules are released to the pore spaces of soil with potentially large fluxes of VOCs to the atmosphere as a result(Poulsen et al. 1998). This is justified by the decrease of adsorption in the solid phase, related to reduce contact between the vapor phase contaminant and the soil matrix due to the presence of water(Alvim-Ferraz et al. 2006, Petersen et al. 1996). These results show that the increase of water content decreases the clay partilcles adsorption capacity to VOCs molecular, enhancing the volatilization of toluene. Sterrett (1989) obtained similar phenomenons by studying soil vacuum extraction after rains at field sites with very dry soil. In Fig. 5(a), compared with 5% water content, the toluene volatilization rate at 10% and 15% increased 13% and 22%, respectively. This also proves the existence of such kind of competitive adsorption.
However, the effect of water content in clay on promoting volatilization only exists at the lower water content stage. The increase of clay water content will also reduce clay permeability(Niu et al. 2012, Qin et al. 2010). Figure 5 presents that the volatilization rate of toluene gradually increases with the water content before 15%, and then decreases significantly. It can be explained that water molecules will occupy the pore space of clay, and high water content will reduce its air permeability. In Fig. 7, the vo latilization channels of toluene from soil are blocked when water content is high, resulting in the reduction of volatilization of toluene. Figure 6 shows the elapsed time at toluene mass loss reaching 90% in samples with different water content. In Fig. 6, it took 150, 136, 107 and 210 minutes for the samples with 5%, 10%, 15% and 20% water content to reach the 90% removal rate, respectively. It shows that there exists an optimal water content of 15% under which the maxium volatilization rate can be reached. This conclusion can also be confirmed in Fig. 5 (a). When the water content reached 25%, the removal rate of toluene by volatilization reached 67%, and the removal rate of toluene by volatilization did not increase with the elapsed time. This means that the volatilization channels are severely blocked, and toluene molecules are trapped in clay.
Since adsorption of toluene and water on sand is weak, competitive adsorption can be neglected. The increase of water content will block the volatilization channels in sand to some degree. However, compared with clay, the effect of water content on volatilization process of toluene in sand is relatively small. As can be seen from Fig. 5, the change of water content in sand has less influence on mass loss of toluene by volatilization. In addition, the permeability of sand is much higher than that of clay (Islam et al. 2021, Taheri et al. 2018). For the two samples with water content of 1% and 10%, the time periods to reach 90% toluene removal rate are 63min and 81min, respectively (Fig. 6). The increase of water content will block the volatilization channels in sand. However, compared with clay, the effect of water content on volatilization process of toluene in sand is relatively small.
In summary, there are two main mechanisms for the effect of water content on volatilization process of toluene in clay. The first is the mechanism of competitive adsorption between toluene molecules and water molecules in clay. Water molecules have a competitive advantages than toluene molecules adsorbed on the soil surface. The higher water content is, the stronger volatilization process of toluene will be. The second is that the increase of water content decreases soil permeability and thus blocks the volatilization channels of toluene in clay. When water content is low, the mechanism of molecular competitive adsorption plays a dominant role, and the water content has little effect on the volatilization channels of toluene. But for the samples with different water content, the blocking mechanism of volatilization channels under the influence of water content plays an increasingly important role with the increase of water content. That is, volatilization rate of toluene decreases with the increasing of water content. As shown in Fig. 5(a) and Fig. 6, there exists an optimal water content of 15% under which the maxium volatilization rate can be reached. As discussed above, water content has an important effect on the volatilization process of toluene in clay. However, such kind of effect in sand is relatively weak.