Study on the efficient selective oil absorption and recovery of hydrophobic chitosan aerogels

Chitosan (CS) aerogels were prepared by an ice templating method and their surfaces were then coated with reduced graphene oxide to form modified biodegradable CS (MCS) aerogels. The MCS aerogels retained the porous CS structure after modification. Based on the characteristics of high porosity and hydrophobicity, the MCS aerogels exhibited a highly selective absorption ability of oils from water. The oil absorption capacity of MCS aerogels could achieve 92.63 g g−1 for chloroform. In addition, the compressive strength of the MCS aerogels increases after modification, so they are recyclable. Therefore, the MCS aerogels could be a kind of green, biodegradable, efficient, and cost-effective absorbent for spilled oils.


Introduction
The exploitation and transportation of petroleum have created many environmental problems including water pollution resulting from oil spillage and chemical leakage [1].Oil spills pose a threat to aquatic ecosystems and the whole environment in general.Traditionally, spilled oils could be cleaned through various methods, such as mechanical collection, bioremediation, and adsorption [2].Among these methods, adsorption is viewed as one of the most effective strategies because they are affordable to use and readily available.Aerogels have been reported to be very efficient oil absorbents because of their lightweight and high porosity [3].Recently, aerogels based on carbon nanotubes and graphene have been used as oil absorbents and have shown high oil absorption capacities [4].However, these materials are expensive, which limits their practical application.
Given the environmentally friendly requirement and low cost, the development of polysaccharidebased aerogels has attracted extensive interest from researchers.Chitosan (CS), as one kind of biodegradable and biocompatible polymer, is natural, environmentally friendly, and non-toxic [5].Chitosan and its derivatives have also been used as absorbents for many types of waterborne pollutants including heavy metals, phenolic dyes, and organics [6].As an absorbent, chitosan possesses the advantages involving low cost, abundant resources, and low possibility of secondary pollution.However, chitosan is hydrophilic.Therefore, chemical modification is necessary to make it oleophilic before it can be used for oil absorption.
Absorbents for organic liquids based on CS aerogels were prepared by a versatile approach.The CS aerogels were modified with rGO to obtain rGO-modified CS aerogels (MCS aerogels).By modification with rGO, the compressive strength of the aerogels was improved.More importantly, rGO is oleophilic [7].By modification with rGO, hydrophobic surface characteristics of CS aerogels can be achieved.The combination of hydrophobic surface, low density, and high strength makes the cost-effective and biodegradable MCS aerogels have potential in oil spill clean-up.

Preparation of MCS aerogels
A homogeneous CS solution was prepared by mixing a 4.0% acetic acid solution of 50.0 mL and chitosan of 0.4 g with vigorous stirring.After the dissolution of chitosan, 5.0% glutaraldehyde solution of 0.5 mL was mixed with the CS solution under stirring as the crosslinking agent at room temperature.The mixtures stood for about 6 h, and then they were frozen unidirectionally and used with liquid nitrogen.The frozen mixtures were freeze-dried for 48 h to obtain CS aerogels under less than 15 Pa (Alphal-2, Christ, German).The CS aerogels were immersed in the rGO suspension and then sonicated for 60 min.The aerogels were removed from the solution washed with copious amounts of ethanol and then freeze-dried for 24 h to obtain MCS aerogels.

Characterization
Oil absorption capacity, hydrophobicity, and other physical and chemical properties of the CS and MCS aerogels were measured [8].

Characteristics of CS and MCS aerogels
Chitosan aerogels were fabricated by unidirectional freeze-drying.The prepared CS aerogels were then immersed in the rGO suspension to allow the rGO nanosheets to diffuse into the aerogels and to adsorb onto the surface of the porous CS structure.After coating with rGO, the color of the aerogels changed from yellow to black, as shown in Figure 1 (a).
A scanning electron microscope (SEM) was used to characterize the pore textures of the CS and MCS aerogels, and the results are shown in Figure 1 (b-g).Figure 1 (b, c, e, and f) shows the crosssections and vertical sections of the CS (left) and MCS aerogels (right).They show almost the same pore texture, which indicates that the rGO coating did not affect the pore texture of the CS aerogels.However, the morphology of the pore walls in the MCS aerogel is completely different from that of the CS aerogels.The pore walls of the MCS aerogels are coarse and full of plates (Figure 1 (g)), while the pore walls of the CS aerogels are smooth (Figure 1 (d)).These plates are the rGO which form a thin coating on the CS.These results indicate that rGO coating is uniformly deposited on the pore walls of the CS aerogels.

(b) (a)
The above analyses show that rGO is coated onto the surfaces of the pore's walls of the CS aerogels to form MCS aerogels.The rGO coating would affect the physical properties of aerogels, such as density (ρ), porosity (ϕ), and mechanical strength (σ).These physical properties are important properties of absorbents.As can be seen from Table 1, the compressive strength of the aerogel increased with the formation of rGO coating but the porosity of the aerogel did not change obviously.As a result, the high porosity (98.72% vol%) and low density (0.0205 g cm −3 ) would provide the MCS aerogel a potential high absorption capacity for spilled oils, and the superior compressive strength would help to achieve recyclability of the MCS aerogel in spilled oils clean-up.Figure 2 (a) shows the thermogravimetric analysis (TGA) of the rGO, CS, and MCS aerogels.CS and MCS aerogels show similar weight loss trends from room temperature to 800℃.The total weight residual rate of the MCS aerogels is slightly higher than that of the CS aerogels, indicating that rGO has a positive effect on the thermal stability of MCS aerogels.The better thermal stability would enable MCS aerogels to be applied to absorb spilled oil in harsh conditions.
Porous chitosan readily adsorbs moisture because of many hydroxyl groups in CS macromolecules.
The moisture absorption of the CS aerogels and MCS aerogels at 55% RH was tested.As shown in Figure 2 (b), the moisture absorption (1.25%) of the MCS aerogels was significantly lower than that (8.68%) of the CS aerogels.So, the rGO coatings improved the resistance of the MCS aerogels to water.

Absorption capacity of CS and MCS aerogels for organic liquids
The hydrophobicity of the aerogels was characterized via water contact angle measurement.CS aerogel showed a water contact angle of about 47° (Figure 3 (a)), while the MCS aerogel showed a contact angle of about 123° (Figure 3 (b)).This indicates that the MCS aerogel is hydrophobic.Figure 3 also shows a CS aerogel (c-f) and an MCS aerogel (g-j) placed on a corn-oil (dyed with Sudan III)-water (dyed with Methyl blue) mixture.When the CS aerogel was placed on corn oil (Figure 3 (c)), it absorbed the corn oil from the mixture in only a few minutes (Figure 3 (d and e)).However, as shown in Figure 3 (f), the CS aerogel also absorbed the water, thereby reducing the separation selectivity and efficiency.
Figure 3 (g-j) shows that the MCS aerogel only absorbed the corn oil.Then, the MCS aerogel filled with oil floated on the water (Figure 3 (i)), thus facilitating the easy collection of oil.Selective absorption is a key characteristic of ideal sorbents.In addition, MCS aerogel showed a quicker absorption rate than CS aerogel, and it usually took less than one minute to complete the absorption.The rGO coating made MCS aerogel a selective absorbent for oil from an oil-water mixture.Therefore, the MCS aerogels should have the potential to effectively collect spilled oil in the Marine.Figure 4 (b) shows the Q values of CS and MCS aerogels for several organic liquids, including chloroform (CF), dimethyl formamide (DMF), methylbenzene (MB), tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), corn oil, diesel oil, lubricating oil, mineral oil, olive oil, and vacuum pump oil.All values of absorption capacity were over 60 g g -1 and a maximum was reached for CS with chloroform (94.28 g g -1 ).The CS and MCS aerogels exhibited similar oil absorption capacities.For a given substance, the Q values for MCS are lower than those for CS.For example, for corn oil, the Q value of MCS is about 1.47% lower than that of CS.The slight decrease in the Q value is because the rGO coating increases the density of the MCS aerogel.The absorption capacity of MCS aerogels is high although it has a slight decrease compared with CS aerogels [9].
The order of the Q values for the organic liquids was as follows: MB < THF < dimethyl DMF < diesel oil < lubricate oil < vacuum pump oil < mineral oil < olive oil < corn oil < DMSO < CF.As shown in Table 2, the order of the Q values corresponds to the density of the liquid.As reported, the absorption capacity is related to the density of the organic liquid [10].When the porosity is fixed, the volume filled with organic liquids (void volume) is fixed too, and the void volume of an absorbent is dependent on the porosity.As a result, the Q value of the same MCS aerogel for different organic liquids is proportional to the density of an organic liquid.For both the CS and MCS aerogels, the absorption rates of all the tested organic liquids were rapid, but the rates of the CS aerogels were slower than those for the MCS aerogels.The sorption kinetics of corn and diesel oil by the CS and MCS aerogels are shown in Figure 5 (a-d).It can be seen from the figure that there is a functional relationship between the adsorption capacity Q t of the oil and the adsorption time.The sorption capacities increased with sorption time until they reached saturation, and the sorption kinetics could be described by the second-order model.The fitting parameters for the two oils are listed in Table 3. "Half-life" (t 1/2 ), which is defined as the time needed to reach one-half of the saturated adsorption capacity, could be used to assess the absorption rate of the aerogels.A smaller t 1/2 suggests a faster absorption.As shown in Table 3, the t 1/2 values for the MCS aerogels are smaller than those of the CS aerogels, which indicates that the MCS aerogels have faster absorption rates.So, the rGO coating made the aerogels more oleophilic.In addition, absorption rates were affected by the viscosity of oils.As shown in Table 3, the t 1/2 values were much smaller for the low-viscosity oil (diesel oil).This is because oils with a low viscosity can more easily penetrate the porous structure of the aerogels.

Recyclability of CS and MCS aerogels
Recyclability and recoverability are of key importance for practical oil cleanup applications, especially in the practical applications.The oil absorbed by the CS and MCS aerogels was collected by centrifugation, which is more eco-friendly than combustion or heat treatment.The recycle behavior of the CS and MCS aerogels for diesel oil sorption for six cycles is shown in Figure 6 (a).For the CS aerogel, the sorption capacity for diesel oil decreased by about 33% after six cycles.For the MCS aerogel, the saturation adsorption capacity can still reach 94% of the initial value in the second cycle.This is because the absorbed diesel oil could not be completely removed by centrifugation.For subsequent cycles, the MCS aerogel had saturation sorption capacities of about 93% of that in the first cycle.The difference between the CS and MCS aerogels in recyclability is due to the rGO coating.As shown in Figure 6 (b and c), after six cycles, CS aerogels were deformed, while no significant deformation occurred in MCS aerogels.This is mainly because rGO coating can effectively enhance the strength of MCS aerogels.Those results suggested that MCS aerogels have both high adsorption properties and desirable recoverability.

Conclusion
The MCS aerogels were prepared in a facile way including unidirectional freeze-drying and dip coating.The MCS aerogels had a higher compressive strength than the CS aerogels.The MCS aerogels also exhibited high selectivity when used as an absorbent for collecting oil on a water surface.Compared to other previously reported oil absorbents, MCS aerogels are more cost-effective because they only require a facile and environmental preparation process.Importantly, the absorption capacity of the MCS aerogel is high and it only decreases by about 7% after six cycles of oil absorption.This demonstrates that the MCS aerogels have excellent recyclability.The MCS aerogels possess the desirable absorbent properties of low density, superior absorption capacity, high selectivity, excellent recyclability, and low cost.Therefore, the MCS aerogel should be potential in the collecting of spilled oils.

Figure 2 .
Figure 2. (a) TGA curves of the rGO, CS, and MCS aerogels; (b) Moisture absorption of the CS and MCS aerogels at 55% RH.

Figure 3 .
Figure 3. (a) Photos of a water contact angle of a CS aerogel; (b) MCS aerogel; (g-j) Absorption of corn oil (dyed with Sudan III) on water (dyed with methyl blue) by CS (c-f) and MCS aerogels.

Figure 4 (
Figure4(a) illustrates the absorption capacity Q of the different CS aerogels.The Q values of the aerogels decreased as the CS concentration increased.However, it is not a linear relationship, which may be because the CS aerogels with low CS concentrations collapse and shrink more than those with high CS concentrations, indicating an obvious effect of CS concentrations on the absorption capacity.Figure4(b) shows the Q values of CS and MCS aerogels for several organic liquids, including chloroform (CF), dimethyl formamide (DMF), methylbenzene (MB), tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), corn oil, diesel oil, lubricating oil, mineral oil, olive oil, and vacuum pump oil.All values of absorption capacity were over 60 g g -1 and a maximum was reached for CS with chloroform (94.28 g g -1 ).The CS and MCS aerogels exhibited similar oil absorption capacities.For a given substance, the Q values for MCS are lower than those for CS.For example, for corn oil, the Q value of MCS is about 1.47% lower than that of CS.The slight decrease in the Q value is because the rGO coating increases the density of the MCS aerogel.The absorption capacity of MCS aerogels is high although it has a slight decrease compared with CS aerogels[9].The order of the Q values for the organic liquids was as follows: MB < THF < dimethyl DMF < diesel oil < lubricate oil < vacuum pump oil < mineral oil < olive oil < corn oil < DMSO < CF.As shown in Table2, the order of the Q values corresponds to the density of the liquid.As reported, the absorption capacity is related to the density of the organic liquid[10].When the porosity is fixed, the volume filled with organic liquids (void volume) is fixed too, and the void volume of an absorbent is dependent on the porosity.As a result, the Q value of the same MCS aerogel for different organic liquids is proportional to the density of an organic liquid.

Figure 4 .
Figure 4. (a) The Q value of the CS aerogels; (b) Absorption capacities of the CS and MCS aerogels for various organic liquids.

Figure 5 .
Figure 5. (a) Sorption kinetics of diesel oil by the CS aerogel; (b) The MCS aerogel; (c) Sorption kinetic of corn oil by the CS aerogel; (d) MCS aerogel.

Figure 6 .
Figure 6.(a) Recyclability of the CS and the MCS aerogels; (b1) and (c1) Photos of the CS and the MCS aerogels after the first cycle; (b2) and (c2) After six cycles.

Table 1 .
Physical properties of the porous CS and MCS aerogels.

Table 2 .
Absorption properties of MCS aerogels for organic liquids with different densities.

Table 3 .
Fitting parameters for the CS and MCS aerogel sorption kinetics.