Chemical Modification of Cellulose Using a Green Route by Reactive Extrusion with Citric and Succinic Acids

Cellulose is a natural, unbranched, and fibrous homopolymer that is a major component in several agroindustrial residues. The aim of this study was to extract cellulose from oat hulls and then to modify it using a green route to obtain esterified cellulose through reaction with organic acids employing the reactive extrusion process, which is a process that presents some advantages, including low effluent generation, short reaction times, and it is scalable for large scale use. Citric (CA) and succinic (SA) acids were employed as esterifying agents in different concentrations (0, 5, 12.5, and 20%). Modified cellulose samples were characterized by their degree of substitution (DS), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (DRX), scanning electron microscopy (SEM), wettability, oil and water absorption capacities, and thermal stability. DS of modified samples ranged from 2.28 to 3.00, and FTIR results showed that the esterification occurred in all samples for both acids by observation of important bands at 1720 and 1737 cm−1 for samples modified with CA and SA, respectively. All modified samples presented increased hydrophobicity. The modification did not have an influence on the morphological structure or crystallinity pattern of all samples. This study proved to be possible to modify cellulose using a simple and ecofriendly process based on reactive extrusion with organic acids.


Introduction
Cellulose is one of the most abundant natural, renewable, and biodegradable polymers; it is an unbranched and fibrous homopolymer that can be obtained from plants or synthesized by bacteria. The cellulose chain consists of β-D-glucose units repetedly joined by β (1-4) glycosidic bonds, with three hydroxyl groups per monosaccharide unit [1][2][3], which makes cellulose an excellent platform for chemical modifications [4]. So, cellulose can be modified to be used in the cosmetic, pharmaceutic, and food industries and in agricultural systems, among others [5][6][7].
Agroindustrial residues can be considered interesting sources for cellulose extraction, and in the last few years, an increased interest in the obtainment of cellulose from these materials using different approaches was seen, which can be considered a promising alternative for the production of sustainable products at affordable prices to reduce the dependency on petroleum-based products [8]. The use of agroindustrial residues to obtain new products is inserted into the concept of biorefineries, meeting the vision of a sustainable economy using biological resources, maximizing benefits and profits through strategies to add value to the plant biomass chain [9,10].
Modifications in the cellulose structure are largely studied to enhance its properties. Among all the possible modification types, esterification is one of the most reported Polysaccharides 2022, 3 293 approaches [13,14]. The esterified cellulose transfers from a hydrophilic to hydrophobic nature due to the introduction of hydrophobic aliphatic branches [15].
In the last few years, a great interest in green routes to obtain new materials from renewable sources, including modified cellulose, was seen. According to Liyanage et al. [16], the use of green technologies for the production of versatile materials can reduce carbon footprint. Organic acids such as citric or succinic acids gained attention as esterifying and crosslinking agents for carbohydrate polymers in many aspects, they are safe, inexpensive, UV resistant, and biocompatible multifunctional monomers that are listed in the generally regarded as safe (GRAS) category by the US Food and Drug Administration (FDA) [3, [13][14][15][16][17][18][19][20].
Citric acid and other polycarboxylic acids are described as efficient esterification and crosslinking agents for polysaccharides, such as cellulose. The reaction of cellulose with polycarboxylic acids occurs due to the attachment of the carboxylic group from acid via esterification with a cellulosic hydroxyl group, and a further reaction via esterification with another cellulosic hydroxyl group can produce a crosslink between cellulose chains [3,4,14,19,21]. Although the catalytic mechanism of cellulose esterification with organic acids is not completely elucidated, esterification catalyzed by α-hydroxy acids is proposed to proceed via a nucleophilic attack of the acylant by the hydroxyl groups of the α-hydroxy acid, followed by a second nucleophilic attack of the intermediate formed by the hydroxyl groups of cellulose [1,22].
According to Gil-Giraldo et al. [4], the reactive extrusion process is an efficient method for modifying cellulose, and it is considered an ecofriendly technological solution since the extruder is used as a reactor where chemical reactions such as esterification and crosslinking can be performed. Reactive extrusion combines the thermomechanical energy to promote the reaction between cellulose and organic acids in a single process without using other reagents. Additionally, reactive extrusion is a continuous process that has commercial viability, and it is easy to adapt to large industrial scales, offering short reaction times (2-3 min) [23,24].
The use of reactive extrusion to obtain esterified cellulose through reaction with organic acids is not fully exploited in the literature. The aim of this study was to extract cellulose from oat hulls and then to modify it using a green route to obtain esterified cellulose through a reaction with organic acids (citric and succinic acids) employing the reactive extrusion process.

Materials
The oat hull was acquired from a local oat processing industry (SL Alimentos, Mauá da Serra, PR, Brazil). The citric acid (CA) and succinic acid (SA) of analytical grades were purchased from Synthlab (Synthlab, Diadema, Brazil), like all other reagents and solvents.

Extraction of the Cellulose from Oat Hulls
The extraction of the cellulose was performed following the process described by Marim et al. [25]. Oat hulls (10 g) were treated with 250 mL of peracetic acid (50% acetic acid, 38% hydrogen peroxide, and 12% distilled water). The suspensions were maintained on a mechanical stirrer with a controlled temperature of 60 • C for 24 h. After this procedure, the samples were filtered, washed with distilled water to reach pH 5-6, and dried at 35 • C for 24 h in an air circulation oven.
Cellulose and hemicellulose contents were determined by the Van Soest method [26]. Lignin contents were determined by the TAPPIT222 om-88 method [27].

Modification of Cellulose by Reactive Extrusion
The citric acid (CA) and succinic acid (SA) were used in different concentrations (0, 5.0, 12.5, and 20.0% g acid/100 g cellulose) as esterifying agents. CA or SA in different concentrations were dissolved in distilled water and mixed with the cellulose (100 g), resulting in a final moisture content of 32% (g water/100 g cellulose). Each mixture was slowly added to sealed plastic bags and equilibrated for 1 h before extrusion. A control sample was extruded without any reagent other than water, resulting in the same final moisture content of 32%. The reactive extrusion parameters were based on a previous study by Gil-Giraldo et al. [4]. All samples were extruded in a single screw extruder (AX Plastics, Diadema, SP, Brazil) with a screw diameter of 1.6 cm and a screw length/diameter ratio (L/D) of 40, with four heating zones and a matrix of 0.8 cm in diameter. The temperature in all zones was 100 • C, and the screw speed was 60 rpm. The modified cellulose was collected, placed in an oven, dried to constant weight at 45 • C, and grounded and sieved through an 80-mesh sieve. The samples were washed three times with absolute ethanol to remove the unreacted CA or SA, and finally, the washed samples were air-dried at 45 • C before being characterized. Cellulose samples prepared by reaction with CA were labeled CA5%, CA12.5%, and CA20% throughout the study, while samples prepared with SA were labeled SA5%, SA12.5%, and SA20% throughout the study.

Determination of the Degree of Substitution (DS)
The DS of the modified cellulose was determined using the method of Karnitiz et al. [28]. The carboxylic acid concentration (C COOH ) per gram of the modified cellulose was determined by retro titration using hydrochloride acid and sodium hydroxide. The DS was calculated according to the equation as follows: DS = [(C NaOH · VNaOH ) − (4·C HCl ·V HCl )]/Mm, where: C NaOH is the sodium hydroxide solution concentration (mmol/L), C HCl is the hydrochloric acid solution concentration (mmol/L), V NaOH is the sodium hydroxide volume (L), V HCL is the hydrochloric acid volume spent on the titration of sodium hydroxide unreacted (L), and Mm is the cellulose mass used (g).

Fourier Transform-Infrared Spectroscopy (FTIR)
The pulverized and dried samples were mixed with potassium bromide and compressed into tablets. The FTIR analyses were carried out with a Shimadzu FTIR-8300 (Kyoto, Japan), which has a spectral resolution of 4 cm −1 and a spectral range of 4000-500 cm −1 .

X-ray Diffraction (XRD)
The crystallinity of each sample was investigated using XRD. The samples were finely powdered using an analytical mill (IKA basic 23), and the analysis was performed using a PANalytical X'Pert PRO MPD diffractometer (Almelo, The Netherlands) with copper Kα radiation (λ = 1.5418

Modification of Cellulose by Reactive Extrusion
The citric acid (CA) and succinic acid (SA) were used in different concentrations (0, 5.0, 12.5, and 20.0% g acid/100 g cellulose) as esterifying agents. CA or SA in different concentrations were dissolved in distilled water and mixed with the cellulose (100 g), resulting in a final moisture content of 32% (g water/100 g cellulose). Each mixture was slowly added to sealed plastic bags and equilibrated for 1 h before extrusion. A control sample was extruded without any reagent other than water, resulting in the same final moisture content of 32%. The reactive extrusion parameters were based on a previous study by Gil-Giraldo et al. [4]. All samples were extruded in a single screw extruder (AX Plastics, Diadema, SP, Brazil) with a screw diameter of 1.6 cm and a screw length/diameter ratio (L/D) of 40, with four heating zones and a matrix of 0.8 cm in diameter. The temperature in all zones was 100 °C, and the screw speed was 60 rpm. The modified cellulose was collected, placed in an oven, dried to constant weight at 45 °C, and grounded and sieved through an 80-mesh sieve. The samples were washed three times with absolute ethanol to remove the unreacted CA or SA, and finally, the washed samples were airdried at 45 °C before being characterized. Cellulose samples prepared by reaction with CA were labeled CA5%, CA12.5%, and CA20% throughout the study, while samples prepared with SA were labeled SA5%, SA12.5%, and SA20% throughout the study.

Determination of the Degree of Substitution (DS)
The DS of the modified cellulose was determined using the method of Karnitiz et al. [28]. The carboxylic acid concentration (CCOOH) per gram of the modified cellulose was determined by retro titration using hydrochloride acid and sodium hydroxide. The DS was calculated according to the equation as follows: DS = [(CNaOH·VNaOH) − (4·CHCl·V HCl)]/Mm, where: CNaOH is the sodium hydroxide solution concentration (mmol/L), CHCl is the hydrochloric acid solution concentration (mmol/L), VNaOH is the sodium hydroxide volume (L), VHCL is the hydrochloric acid volume spent on the titration of sodium hydroxide unreacted (L), and Mm is the cellulose mass used (g).

Fourier Transform-Infrared Spectroscopy (FTIR)
The pulverized and dried samples were mixed with potassium bromide and compressed into tablets. The FTIR analyses were carried out with a Shimadzu FTIR-8300 (Kyoto, Japan), which has a spectral resolution of 4 cm −1 and a spectral range of 4000-500 cm −1 .

X-ray Diffraction (XRD)
The crystallinity of each sample was investigated using XRD. The samples were finely powdered using an analytical mill (IKA basic 23), and the analysis was performed using a PANalytical X'Pert PRO MPD diffractometer (Almelo, The Netherlands) with copper Kα radiation (λ = 1.5418 Ǻ) under the operational conditions of 40 kV and 30 mA. All the assays were performed with a ramp rate of 1°/min. The relative crystallinity index (CI) was calculated as follows: .100, where I002 is the intensity at 2θ = 22° and Iam is the amorphous intensity at 2θ = 18°.

Scanning Electron Microscopy (SEM)
SEM analyses were performed on the FEI Quanta 200 equipment (Hillsboro, OR, USA). Samples were incubated in an air circulation oven (Marconi MA 035, Piracicaba, SP, Brazil) at 60 °C for 3 h and then kept in desiccators containing anhydrous calcium chloride for 7 d. The dried samples were assembled for viewing on bronze stumps using double-sided tape. Afterward, the samples were covered with a thin layer of gold (40-50 nm), and an accelerated voltage of 20 kV was used for all samples.

Differential Scanning Calorimetry (DSC)
) under the operational conditions of 40 kV and 30 mA. All the assays were performed with a ramp rate of 1 • /min. The relative crystallinity index (CI) was calculated as follows: CI (%) = [(I 002 − I am ) /(I 002 )].100, where I 002 is the intensity at 2θ = 22 • and I am is the amorphous intensity at 2θ = 18 • .

Scanning Electron Microscopy (SEM)
SEM analyses were performed on the FEI Quanta 200 equipment (Hillsboro, OR, USA). Samples were incubated in an air circulation oven (Marconi MA 035, Piracicaba, SP, Brazil) at 60 • C for 3 h and then kept in desiccators containing anhydrous calcium chloride for 7 d. The dried samples were assembled for viewing on bronze stumps using double-sided tape. Afterward, the samples were covered with a thin layer of gold (40-50 nm), and an accelerated voltage of 20 kV was used for all samples.

Differential Scanning Calorimetry (DSC)
DSC analyses were performed on a Shimadzu DSC 60 (Kyoto, Japan) calorimeter. Approximately 3.0 mg of each sample were placed in platinum containers and heated from 30 to 450 • C at a heating rate of 5 • C/min in a helium atmosphere.

Thermogravimetric Analysis (TGA)
TGA of the samples was performed using the Shimadzu TGA-50 (Kyoto, Japan) equipment. The scans were performed from room temperature up to 600 • C with a heating rate of 20 • C/min under a nitrogen flow of 20 mL/min.

Wettability
Samples were mixed into two different systems with two immiscible solvents with different density values: water (d: 1.000 g/cm −3 ) and dichloromethane (d: 1.335 g/cm −3 ) and water (d: 1.000 g/cm −3 ) and chloroform (d: 1.490 g/cm −3 ), allowing us to observe the affinity of each sample for each solvent [29].

Water Absorption Capacity (WAC) and Oil Absorption Capacity (OAC)
WAC and OAC were determined following the methodology described by Gil-Giraldo et al. [4]. Approximately 0.5 g of each sample (M0) and 15 mL of water (or soybean oil) (M1) were added to a centrifuge tube. Then, the samples were kept in a water bath for 30 min and centrifuged for 30 min at 9000 rpm (Hettich Centrifuge, Universal model 320R, Darmstadt, Germany). The non-adsorbed water (or soybean oil) (M2) was removed, and the water (or soybean oil) absorbed by the samples was estimated as the difference between M1 and M2. WAC was calculated as WAC (g/g) = (M1 − M2)/M0, and OAC was calculated as OAC (g/g) = (M1 − M2)/M0.

Statistical Analysis
Analyses of variance (ANOVA) and Tukey's mean comparison test (p ≤ 0.05) were performed with the R program.

Results
Oat hull bleaching was performed with peracetic acid to obtain cellulose. Raw oat hull presented 26% cellulose, 30% hemicellulose, and 21% lignin, and these results are in agreement with other authors [4,11,30]. After bleaching with peracetic acid, the obtained sample presented a composition of 78% cellulose, 8% hemicellulose, and 3% lignin, and this sample was labeled as cellulose. The sample was employed in this study to obtain the modified samples by reactive extrusion with organic acids.

Degree of Substitution (DS)
Esterification implies the substitution of hydroxyl groups of cellulose by less polar ester groups, and the DS is indicative of the substitution level. DS can be defined as the average number of substituted hydroxyl groups per glucose unit, and it has a maximum value of 3 [1,22,31]. DS results of cellulose samples modified by reactive extrusion are shown in Table 1. It is possible to observe that the DS values increased with the increase of CA and SA concentration and that DS values were not affected by the type of acid employed. Ratanakamnuan et al. [31] reported similar DS values to those obtained in this study, their values ranged from 2.41 and 2.69 for cotton cellulose esterified with several fatty acids (C4 to C12) under microwave heating, and they stressed that shorter reaction times are required to obtain samples with higher DS when the microwave power increases. Hoang et al. [7] and Ji et al. [15] reported that in the esterification of cellulose with citric acid, the increased concentration of the acid enhances the interaction between carboxylic groups of citric acid and hydroxyl groups of cellulose.
Xin et al. [20] presented DS results ranging from 0.337 to 1.191 in cellulose succinates under a catalyze-free condition. Qin et al. [18] reported that cellulose succinates obtained by reaction with SA performed in a high-energy stirring ball mill presented higher degrees of substitution when subjected to higher SA concentrations.

Fourier-Transform Infrared (FTIR) Spectroscopy
The FTIR spectra of the cellulose, control sample and modified cellulose with CA and SA are shown in Figure 1. It is possible to see that all samples that were subjected to reactive extrusion with CA and SA presented a pronounced band located between 1720 and 1737 cm −1 , which can be associated with the C=O stretching of carbonyl in the ester bonds, being an indication that esterification using CA and SA occurred. These results are similar to those presented by other authors [4,6,7,14,15,17,18,20,22], who also used CA or SA as esterifying agents for cellulose; these authors reported that bands between 1720 to 1750 cm −1 can be used to show the success of cellulose esterification. Hoang et al. [7] and Ji et al. [15] reported that in the esterification of cellulose w citric acid, the increased concentration of the acid enhances the interaction betw carboxylic groups of citric acid and hydroxyl groups of cellulose.
Xin et al. [20] presented DS results ranging from 0.337 to 1.191 in cellulose succin under a catalyze-free condition. Qin et al. [18] reported that cellulose succinates obtai by reaction with SA performed in a high-energy stirring ball mill presented higher deg of substitution when subjected to higher SA concentrations.

Fourier-Transform Infrared (FTIR) Spectroscopy
The FTIR spectra of the cellulose, control sample and modified cellulose with CA SA are shown in Figure 1. It is possible to see that all samples that were subjected reactive extrusion with CA and SA presented a pronounced band located between 1 and 1737 cm −1 , which can be associated with the C=O stretching of carbonyl in the e bonds, being an indication that esterification using CA and SA occurred. These results similar to those presented by other authors [4,6,7,14,15,17,18,20,22], who also used CA SA as esterifying agents for cellulose; these authors reported that bands between 172 1750 cm −1 can be used to show the success of cellulose esterification. For the cellulose and control sample, this band appeared with lower intensity (Figure 1). Considering that cellulose was extracted from oat hulls, and it remained with 8% of hemicellulose, this band in the cellulose and control samples (extruded without organic acids) possibly corresponds to the acetyl or uronic groups of the hemicelluloses [25]. Gil-Giraldo et al. [4] reported that this band could also be explained by the thermomechanical treatment applied to cellulose during extrusion, being attributed to the C=O group from the opened terminal glucopyranose rings.
As observed in Figure 1, all samples presented a broad band near 3500 cm −1 that can be assigned to O-H stretching groups. The band at 2900 cm −1 appeared due to C-H stretching [4,6]. All the bands between 1057 and 1162 cm −1 correspond to C-O and C-O-C stretching vibration in cellulose [4,11,25].

X-ray Diffraction (XRD)
The X-ray diffractograms and relative crystallinity indexes (IC) of the cellulose, control sample, and modified cellulose with CA and SA are shown in Figure 2. It is possible to see that all samples presented characteristic peaks of cellulose type I, at 2θ = 16, 22, and 34 • [4,11,18,31]. New peaks at 2θ = 26 and 32 • were observed only for the SA20% sample, and these peaks are reported by other authors as typical peaks of SA [32], which possibly remained in the sample after modification.
hemicellulose, this band in the cellulose and control samples (extruded without organic acids) possibly corresponds to the acetyl or uronic groups of the hemicelluloses [25]. Gil-Giraldo et al. [4] reported that this band could also be explained by the thermomechanical treatment applied to cellulose during extrusion, being attributed to the C=O group from the opened terminal glucopyranose rings.
As observed in Figure 1, all samples presented a broad band near 3500 cm −1 that can be assigned to O-H stretching groups. The band at 2900 cm −1 appeared due to C-H stretching [4,6]. All the bands between 1057 and 1162 cm −1 correspond to C-O and C-O-C stretching vibration in cellulose [4,11,25].

X-ray Diffraction (XRD)
The X-ray diffractograms and relative crystallinity indexes (IC) of the cellulose, control sample, and modified cellulose with CA and SA are shown in Figure 2. It is possible to see that all samples presented characteristic peaks of cellulose type I, at 2θ = 16, 22, and 34° [4,11,18,31]. New peaks at 2θ = 26 and 32° were observed only for the SA20% sample, and these peaks are reported by other authors as typical peaks of SA [32], which possibly remained in the sample after modification. The extrusion process with CA and SA employed in this study did not affect the polymorph type of cellulose compared to the cellulose sample. Other authors that subjected cellulose extracted from oat hulls to modification by reactive extrusion also reported that this process did not affect the inherent crystalline structure of the extruded samples [4,11]. Ji et al. [15] reported that esterification of nanocellulose with CA has no influence on the position of peaks and no changes on the crystalline allomorph of the cellulose. Gil-Giraldo et al. [4] also reported that esterification with CA possibly occurred mainly at the cellulose surface, without affecting the inner crystalline structure. According to Qin et al. [18], chemical reagents are difficult to enter into the crystalline region of cellulose because of their stable structure.
The crystallinity index (CI) of cellulose was 42%, which is typical of a semicrystalline material, and a very close value (43%) was reported by Gil-Giraldo et al. [4] for cellulose from oat hull. The CI for all extruded samples (with and without reagent) slightly decreased, and the CI values ranged from 37 to 39% for samples modified with CA ( Figure 2) and for 36 to 37% for samples modified with SA (Figure 2). During the reactive extrusion process, cellulose fibers were exposed to high temperatures and high shear forces; however, these processing conditions did not affect the crystallinity pattern of the modified samples. These results agreed with other authors that used reactive extrusion to esterify cellulose [4]. Qin et al. [18] reported a decrease in CI of cellulose esterified through a reaction with SA in a high-energy stirring ball mill. They attributed this decrease to the disruption of the intramolecular and intermolecular hydrogen bonds in cellulose, which effectively can destroy its crystalline structure. Figure 3 show the SEM images of the oat hull, cellulose, and cellulose samples modified with citric acid 20% (CA20% sample) and succinic acid 20% (SA20% sample). It is possible to observe that raw oat hull presented a compact and uniform structure, with the fibers bundled with hemicellulose and lignin components, which are typical for lignocellulosic materials [4,6,25,33]. The extraction process with peracetic acid removed the external layer composed mainly of hemicellulose and lignin; as a result, the cellulose sample showed a different morphology, with long and individualized fibers [4,30,34].  Figure 4 present the DSC curves of the cellulose, control sample, and modified cellulose with CA and SA. All samples showed an endothermic peak near 50 °C, which can be associated with water loss [4,21]. It is possible to observe that all samples showed Both CA and SA samples presented long and individualized fibers, showing no difference from the cellulose sample, and these results were consistent with XRD results, which showed that reactive extrusion with the organic acids did not affect the crystallinity pattern of cellulose. Gil-Giraldo et al. [4] subjected cellulose to reactive extrusion with CA employing a higher acid concentration (40%) and also reported that the surface morphology of the cellulosic fibers did not change. He et al. [14] also reported that the morphology of nanocellulose was not significantly affected by esterification, suggesting that only the surface of the cellulose was modified during reaction with citric acid. Cui et al. [17] reported a slight increase in roughness on the cellulose fiber surface after esterification with CA. Figure 4 present the DSC curves of the cellulose, control sample, and modified cellulose with CA and SA. All samples showed an endothermic peak near 50 • C, which can be associated with water loss [4,21]. It is possible to observe that all samples showed an exothermic peak near 350 • C, which can be attributed to cellulose depolymerization. This event was also reported by other authors [4,35], and it can be observed that the esterification of cellulose by reactive extrusion did not affect the thermal stability observed from DSC curves.

Thermogravimetric Analysis (TGA)
TGA and DTGA curves from the cellulose, control samples, and cellulose samples modified with CA and SA are shown in Figures 5 and 6, respectively. All samples presented a small first degradation stage at 50-100 °C (Figures 5a and 6a), which can be attributed to the loss of water from the samples [4,5,36].

Thermogravimetric Analysis (TGA)
TGA and DTGA curves from the cellulose, control samples, and cellulose samples modified with CA and SA are shown in Figures 5 and 6, respectively. All samples presented a small first degradation stage at 50-100 • C (Figures 5a and 6a), which can be attributed to the loss of water from the samples [4,5,36].
All samples started to decompose near 250 • C, and it can be observed that for unmodified cellulose, the temperature that corresponds to the maximum weight loss rate was 367 • C, and for all modified samples, this value decreased. Samples modified with CA had the maximum weight loss rate ranging from 360 to 364 • C, and for samples modified with SA these values ranged from 349 to 362 • C (Figures 5b and 6b), which is an indication of a decrease in thermal stability of the modified samples. Ji et al. [15] also reported that the thermal stability of nanocellulose decreased after esterification with CA. All samples started to decompose near 250 °C, and it can be observed that for unmodified cellulose, the temperature that corresponds to the maximum weight loss rate was 367 °C, and for all modified samples, this value decreased. Samples modified with CA had the maximum weight loss rate ranging from 360 to 364 °C, and for samples modified with SA these values ranged from 349 to 362 °C (Figures 5b and 6b), which is an indication of a decrease in thermal stability of the modified samples. Ji et al. [15] also reported that the thermal stability of nanocellulose decreased after esterification with CA.   Figures 7 and 8 show the wettability results in the two tested systems, water/dichloromethane and water/chloroform, respectively. The wettability property is directly influenced by the polar and non-polar groups present on the molecule's surface [4,37,38].

Wettability
As can be seen in Figures 7 and 8, the cellulose and control samples showed an affinity for water (polar solvent), while all the modified samples showed an affinity for dichloromethane and chloroform, which are non-polar solvents, indicating that the hydrophobic character of cellulose changed by chemical modification through reaction with CA and SA. According to Hubbe et al. [37], the substitution of polar hydroxyl groups in the cellulose surface for less polar groups can affect the wettability results. He et al. [14] reported increases in cellulose hydrophobicity upon esterification with CA. Gil-Giraldo et al. [4] also observed an increased affinity by non-polar solvents when cellulose was esterified by reactive extrusion with CA. According to Adewuyi and Pereira [39], the poor hydrophobicity of cellulose limits its application, and this limitation can be overcome by the surface modification of cellulose by replacing its hydroxyl groups, which can be an effective strategy to improve its hydrophobicity.  Figures 7 and 8 show the wettability results in the two tested systems, water/dichloromethane and water/chloroform, respectively. The wettability property is directly influenced by the polar and non-polar groups present on the molecule's surface [4,37,38].

Wettability
As can be seen in Figures 7 and 8, the cellulose and control samples showed an affinity for water (polar solvent), while all the modified samples showed an affinity for dichloromethane and chloroform, which are non-polar solvents, indicating that the hydrophobic character of cellulose changed by chemical modification through reaction with CA and SA. According to Hubbe et al. [37], the substitution of polar hydroxyl groups in the cellulose surface for less polar groups can affect the wettability results. He et al. [14] reported increases in cellulose hydrophobicity upon esterification with CA. Gil-Giraldo et al. [4] also observed an increased affinity by non-polar solvents when cellulose was esterified by reactive extrusion with CA. According to Adewuyi and Pereira [39], the poor hydrophobicity of cellulose limits its application, and this limitation can be overcome by the surface modification of cellulose by replacing its hydroxyl groups, which can be an effective strategy to improve its hydrophobicity.

Water Absorption Capacity (WAC) and Oil Absorption Capacity (OAC)
WAC and OAC results are shown in Table 2. For the modified cellulose, WA ranged from 6.46 to 7.49 (g/g). When compared to cellulose (9.27 g/g) and the sample (8.55 g/g), all the modified samples present significantly lower WAC value 2). It is important to observe that the type of acid or the acid's concentration significantly affect the WAC values of the modified samples.

Water Absorption Capacity (WAC) and Oil Absorption Capacity (OAC)
WAC and OAC results are shown in Table 2. For the modified cellulose, WA ranged from 6.46 to 7.49 (g/g). When compared to cellulose (9.27 g/g) and the sample (8.55 g/g), all the modified samples present significantly lower WAC value 2). It is important to observe that the type of acid or the acid's concentration significantly affect the WAC values of the modified samples. 6.46 f ± 0.01 7.26 bc ± 1.3 Figure 8. Dispersion of cellulose, control sample, and modified cellulose with CA and SA in a water/chloroform system.

Water Absorption Capacity (WAC) and Oil Absorption Capacity (OAC)
WAC and OAC results are shown in Table 2. For the modified cellulose, WAC values ranged from 6.46 to 7.49 (g/g). When compared to cellulose (9.27 g/g) and the control sample (8.55 g/g), all the modified samples present significantly lower WAC values ( Table 2). It is important to observe that the type of acid or the acid's concentration did not significantly affect the WAC values of the modified samples. From the OAC results (Table 2), it is possible to see that the cellulose sample presented the lowest OAC value (1.80 g/g), while the modified samples CA 12.5%, CA 20%, and SA 20% showed the highest values. It can also be observed that the increase of acid concentrations resulted in samples with higher OAC. This increase in hydrophobic capacity of the modified samples can be attributed to the decrease of free hydroxyl groups on the cellulose surface by reaction with CA and SA. Adewuyi and Pereira [39], in their study with modified cellulose through reaction with suberic acid, also reported the increase in hydrophobicity of modified samples, which was observed by the decrease in WAC and increase in OAC values. Gil-Giraldo et al. [4], in their study with modified cellulose with CA, also described lower WAC and higher OAC values for cellulose modified with CA.

Conclusions
All the results showed that it was possible to obtain cellulose samples with increased hydrophobicity after employing a reactive extrusion process to perform the esterification of cellulose with two organic acids, citric and succinic acid. FTIR confirmed the modification by the appearance of new bands at 1720 and 1737 cm −1 for samples modified with CA and SA, respectively. Regardless of the type of acid used, all modified samples showed higher affinity for non-polar solvents and increased oil absorption capacity.
When compared to the conventional processes employed to obtain esterified cellulose, reactive extrusion can be considered an ecofriendly and efficient alternative for cellulose modification, with some advantages such as low effluent generation, short reaction times, lower investment costs, the simplicity of operation due to its continuous nature, and additionally, it is scalable for large scale use.