# Treatment of Water Contaminated with Diesel Using Carbon Nanotubes
## 3 Results
### Adsorption/Desorption Cycles  
Three consecutive cycles of adsorption and desorption were performed to test the possibility of regeneration of the carbon nanotubes and to evaluate a possible recovery of the adsorbed gas oil.  
The % adsorption was calculated in the same way as reported in the previous paragraph (3.1), while the percentage of desorption was calculated with the following expression (Vol\({}_{\text{-final diesel desorbed}}\)/Vol\({}_{\text{initial diesel adsorbed}}\)) \(\times\) 100, where Vol\({}_{\text{-final diesel desorbed}}\) represents the volume of diesel obtained after the desorption process while Vol\({}_{\text{-initial diesel adsorbed}}\) represents the volume of diesel initially adsorbed.  
Figure 6: FT-IR spectra of pre-adsorption (**a**) and post-adsorption carbon nanotubes for systems with 0.3 g (**b**) and 1.0 g (**c**) carbon nanotubes.  
In particular, the system consisting of 10 mL of diesel, 40 mL of water and 0.5 g of carbon nanotubes was taken as a reference, as it was simpler in terms of experimental operation and equally effective for evaluating the desorption and adsorption process.  
The data shown in Figure 7, first of all, show that the % of adsorption decreases as the number of cycles increases; therefore, there is a decrease in the quantities of diesel fuel adsorbed as the cycles progress. In particular, there is a decrease in the % adsorption of about 5% in the second cycle and of about 8% in the third cycle. All this can be motivated by a small quantity of diesel, which cannot desorb during the desorption phase and which is retained inside the nanotubes.  
Most likely, this small amount of diesel that is retained is the one that is most inside the carbon nanotubes and that cannot be easily desorbed and would need drastic extraction methods. Furthermore, it is also possible to attribute this slight decrease in the % of adsorption to a small loss of the carbon nanotubes following the operations carried out during the cycles. However, it should be emphasized that this decrease in % adsorption remains low and is, therefore, acceptable.  
The % of diesel desorbed is always lower than the quantity adsorbed, with values ranging between 9 and 15%; in this case, the previous hypothesis relating to a small quantity of diesel, which is withheld, is confirmed. The acetone extraction method probably does not allow for total desorption. It is possible and desirable that alternative methods could lead to total desorption.  
As is known, the choice of solvent is mainly guided by considerations, such as high solvent power towards the components to be transferred, easy separability and availability at low costs. It is likely that other solvents may have higher solvent extraction power for diesel, but in this experimental research, we chose a solvent that would allow for an adequate compromise between solvent power and availability at low costs.  
Finally, there is a decrease in the percentage of desorption as the number of cycles increases.  
In general, it is difficult to identify an adsorption and desorption threshold for nanotubes as these are certainly conditioned by the nature of the species being adsorbed as well as by times and operating conditions. Compared to our case study, however, it is possible to outline the following operating capacities of the nanotubes used: with 0.7 g of CNTs, at room temperature, after only 2.5 min, all 10 mL of diesel is adsorbed. As far as desorption is concerned, the first desorption cycle has only a 5% loss of recovered diesel.  
Figure 7: Percentage of adsorbent and desorbed diesel as a function of the number of cycles in adsorbance/desorbance.