*3.5. Oil*/*Solvent Absorption Capacity of SCA and Its Recycleability*

Owing to their low density, high porosity, and surface hydrophobicity, the silane-treated cellulose aerogels may be an ideal candidate for the selective absorption of oils and organic solvents from water. To examine the oil/solvent absorption behavior of MEMO silane-coated cellulose aerogel, several oils and organic solvents, such as toluene and gasoline were used.

Figure 9 shows the first minutes of the waste motor oil sorption process. The material absorbed the motor oil easily while floating in water, indicating high capacity absorption of the aerogel. After 5 min, there was no trace of waste motor oil on the water, showing that the sorption was completed successfully. Figure 10 shows the sorption kinetics of the oils and solvents on the silane-coated cellulose aerogel. The absorption rates were quite high at the very first stage and saturation was achieved after 75 h for all types of oils and solvents.

**Figure 9.** Waste motor oil absorption test of the silanized cellulose aerogel with MEMO silane.

**Figure 10.** Absorption kinetics of oils and organic solvents on the silane-coated cellulose aerogel.

The experiment showed the linear relationship for both ln (*Qm*/(*Qm*−*Qt*)) and *t*/*Qt* versus absorption time, *t*, for two representative adsorbates: vacuum oil and waste motor oil as shown in Figure 11. These results mean that the adsorption kinetics of the SCA followed the pseudo-first-order and pseudo-first-order equation quite similarly.

Equations (3) and (4) were used to calculate the absorption rate constants *k*1, *k*2, and correlation coefficient *R*<sup>2</sup> from Figure 11 as seen in Table 3.

The results in Table 3 indicated that the *R*<sup>2</sup> value of the pseudo second-order model of vacuum oil is higher than that of the pseudo first-order model. While the R2 value of the pseudo second-order model of waste motor oil is lower than that of the pseudo first-order model. These results mean that the pseudo second order model can predict better the oil absorption behavior for vacuum oil and the pseudo first order model is better for waste motor oil in this work. The absorption processing of vacuum oil is faster than the absorption of waster motor oil because the *k*<sup>1</sup> and *k*<sup>2</sup> values of vacuum oil is higher than those of waste motor oil.

**Figure 11.** Pseudo-first-order and pseudo-second-order absorption linear fitting of the vacuum oil and waste motor oil onto SCA.

**Table 3.** Summary of the maximum oil absorption capacities and the absorption rate constants of the SCA using the pseudo-first-order and pseudo-second-order models.


Figure 12 shows the maximum absorption capacities of the oils and organic solvents on the silane-coated cellulose aerogel. The results showed that the absorbent had sorption capacity ranging from 631 ± 15.9% to 1081 ± 20.1% by weight gain. The high oil/solvent absorption capability of the silane-coated cellulose aerogel can be attributed to its highly porous structure and hydrophobic silane coating.

**Figure 12.** Absorption capacities of silane-coated cellulose aerogel for various oils and organic solvents as indicated by weight gain.

Because the weight gain of aerogel is related to the density of the respective oils and organic solvents, it can be normalized by dividing the oils and organic weight gain by the density of each respective oil and organic solvent. The results are reported in Figure 13. As shown in Figure 13, the highest absorption capacity was found for toluene and gasoline probably because these organic solvents possess the lowest viscosity. A lower viscosity would facilitate the penetration of solvent into the porous network of the aerogel more easily, leading to a higher oil/solvent absorption capacity.

**Figure 13.** Absorption capacities normalized by the density of the respective oil or organic solvent.

Furthermore, for the recyclability test, the used absorbent was directly squeezed by hand and reused to absorb the oil and organic solvent. The absorption capacities of SCA for ten cycles are shown in Figure 14.

**Figure 14.** The cyclic adsorption capacity of sample SCA for gasoline and toluene.

After ten cycles, the adsorption capacity of the SCA for representative gasoline decreased from 916% to 862%. For toluene, the adsorption capacity decreased from 1081% to 989 % after ten cycles.
