# Treatment of Water Contaminated with Diesel Using Carbon Nanotubes
## 3 Results  
### Adsorption Kinetics  
The following Figure 2a-d show the adsorption kinetics for systems characterized by 40 mL of distilled water, 10 mL of diesel and 0.3, 0.5, 0.7 and 1.0 g of carbon nanotubes, respectively. The % of adsorption was calculated as \([\text{Vol}_{\text{\tiny{Diesel init. }}}-\text{Vol}_{\text{\tiny{Diesel finat. }}})/\text{Vol}_{\text{\tiny{Diesel init. }}}]\times 100\), where the initial volume of diesel represents the amount of diesel before the adsorption process, and the final volume represents the amount of diesel still present in the system after the adsorption process. Figure 2: Percentage of diesel adsorbed as a function of time and of different quantities of carbon nanotubes (**a**) 0.3; (**b**) 0.5; (**c**) 0.7; (**d**) 1.0 (g). Using 0.3 g of non-purified carbon nanotubes (Figure 2a), it can immediately be seen that the adsorption rate is very high, so much so that we can state that the phenomenon is kinetically immediate. However, a percentage of diesel adsorption equal to 100% is not observed. In fact, the maximum value is around 80%. The evaluation of the percentage of adsorption remains almost constant, even for long times. By increasing the amount of nanotubes to 0.5 g (Figure 2b), the results obtained are very similar to the previous case in the first 15 min. At longer times, the behavior is different; in fact, the percentage value of adsorption stands at about 90% after 30 min to reach about 100% of adsorbed diesel fuel after 60 min. Thus, 0.3 g of nanotubes is insufficient for almost total adsorption of the nanotubes, which, in this case, reaches saturation after about 20 min. Further, 0.5 g of nanotubes, being a greater quantity than the previous ones, does not reach saturation and is able to totally adsorb, although with longer times compared to the subsequent systems, which have higher quantities of nanotubes. By increasing the quantities of carbon nanotubes to 0.7 and 1.0 g, respectively (Figure 2c,d), the usual immediate \"triggering\" of the adsorption phenomenon is highlighted in just 2.5 min and the achievement of the maximum adsorbed quantity of diesel fuel. These data lead us to think that 0.7 g of CNTs can already be considered as a suitable quantity to obtain the full adsorption of diesel fuel in water. Using 1 g of CNTs, on the other hand, the system has an excess aliquot of carbon nanotubes not needed by the process. Figure 3 reports the adsorption capacity of carbon nanotubes. The adsorption capacity was calculated as the ratio of the variation in diesel volumes with respect to the grams of nanotubes used (qe = Vol.\({}_{\text{Diesel init. }}-\text{Vol.\({}_{\text{Diesel final}}\cdot)}\)/g CNTs). As the system was stirred, the carbon nanotubes transferred mainly into the diesel phase, and as the amount of carbon nanotubes in the system increased, the CNT agglomeration phenomena were increasingly visible to the naked eye. This agglomeration phenomenon has already been reported in the literature [75]. All this leads to the incomplete availability of the adsorption sites and, consequently, a decrease in the adsorption capacity of the carbon nanotubes. An excess of carbon nanotubes, therefore, does not lead to an increase in adsorption capacity. The maximum adsorption capacity is reached in almost all cases within the first 15 min and then stabilizes. In  
Figure 3: Adsorption capacity (qe = mL \({}_{\text{Diesel}}\)ads/gCNTs) in the different systems consisting of 40 mL water + 10 mL diesel and different quantities of carbon nanotubes: 0.3; 0.5; 0.7; 1.0 g. The results obtained show that the qe values decrease as the amount of carbon nanotubes used in the adsorption systems increases. This is probably due to the agglomeration phenomenon of carbon nanotubes, which becomes increasingly significant as their quantities increase. particular, for the system with the lowest content of carbon nanotubes (0.3 g), the highest adsorption capacity value recorded is equal to 27 and is reached after about 15 min.