# Speciation of the Removed Pollutants in Bioremediation of Hydrocarbon-Contaminated Soil
## 4 Discussion  
Looking at Figures 3 and 4, it is worth noting the behavior of clusters C16-C19 and C20-C24, which show values lower than those of the other clusters for both mesocosms and both assessments (average on the total and bottom). This result could be due to two contemporary and contrary effects:  
1. The removal of alkanes belonging to these cluster;
2. The generation of these alkanes due to the degradation of other hydrocarbons. As expected, the biostimulated mesocosms efficiently removed diesel oil from polluted soil, by 66% and 72%, respectively, with a slightly higher value for the inoculated system (BS + IN), whereas the NA test had almost no removal (around 2%). These findings are in line with the data achieved in microcosms where the tests of diesel oil biodegradation were carried out with the same soil and similar operative conditions [9]. [PERSON] et al. [16] studied the removal of aliphatic and polycyclic aromatic hydrocarbons (PAHs) in microcosms of 500 g of polluted soil for 60 days. They used two consortia, namely, one with mixed identified microorganisms (C1) and one with identified hydrocarbon-degrading strains (C2). They also tested a control microcosm where only  
Figure 4: Removal efficiency by clusters of BS + IN mesocosm. Figure 3: Removal efficiency by clusters of BS mesocosm. natural attenuation was applied and one with biostimulation. The results for the removal of aliphatic compounds were as follows:  
* 70% in the microcosm C1;
* 87% in the microcosm C2;
* 1% in the natural attenuation;
* 35% with biostimulation. Correspondingly, for the PAH removal, the results were as follows:  
* 65% in the microcosm C1;
* 85% in the microcosm C2;
* 0% in the natural attenuation;
* 24% with biostimulation. Comparing these data to ours, good agreement of the findings achieved in the tests with mixed microorganisms can be observed, i.e., 66% (BS) and 70% (C1). Regarding aromatics and PAHs, the comparison is less sharp since our result refers to aromatics (PAHs are part of aromatics): in the mesocosms, we achieved the removal of about 80% for both mesocosms, while [PERSON] et al. removed 85% of their PAHs in the microcosm C2. With a gross comparison, the lower values achieved for the mesocosms can be also ascribed to the different management during the runs, especially the absence of aeration, whereas in the microcosms, the soil was periodically aerated. Provided that the consortia were not analyzed by strains in our mesocosms, altogether, the findings for removal efficiency are satisfactory, especially considering the scale of the systems (8.4 kg of soil). The speciation of hydrocarbons, carried out as the average value on the total amount of soil and the bottom, allowed for a more detailed picture of the removal efficiency given by clusters of compounds. The results reflect the integral ones. However, the value for aromatics needs a comment: among the clusters, it is always the highest for both mesocosms. [PERSON] et al. [17] studied the effects of temperature on the biodegradation of PAHs in microcosms (5 g of soil) for 60 days. They found that the optimal temperature was in the range of 20-28 \({}^{\circ}\)C. They also showed that at higher temperatures, as drawbacks, co-metabolism starts and the main PAH degraders deactivate, with a decrease in PAH removal. This seems in line with the results achieved in our tests, which were carried out at temperatures over 20 \({}^{\circ}\)C during the first 80 days of the runs, as monitored by the geophysical probe. This can support the high values obtained in both mesocosms, including the removal at the bottom of each mesocosm. In other words, also at the bottom of the mesocosms, the conditions for the process were more suitable at the start of the runs than towards the end when the depletion of nutrients and oxygen could have occurred. [PERSON] et al. [18] studied the removal of petroleum from sludge-polluted soil by different consortia and with different strategies of inoculation in biopsies of about 2500 kg.