Evolution of Physicochemical Structure of Waste Cotton Fiber (Hydrochar) During Hydrothermal Carbonation

2.1 Experimental

First, the waste cotton fabric was bleached and washed several times with tap water and dried in an oven at 80°C for 12 h. The cotton fabric was then cut into pieces of 0.25 cm². Cotton fabric pieces (12 g), glucose, and MCC were added to deionized water (600 mL) taken in a beaker and stirred at 400 rpm for 30 min at room temperature to dissolve the reactants or disperse them evenly in water. Then, this was transferred to a high-pressure autoclave of 1,000- mL capacity and sealed. The reaction was allowed to continue at 200–280°C for a certain period of time and then cooled to room temperature. The reaction product was withdrawn, and the solid product was separated by centrifugation. After separation, the product in the liquid phase was retained for testing, and the solid product was washed several times with ethanol until the filtrate was clear and was then finally washed several times with distilled water. Finally, the solid product was dried in a dry box at 120°C for 4 h and tested.

2.2 Characterization methods

Field-emission scanning electron microscopy (SEM) was employed to examine the morphologies of different samples. The structures of samples were examined by X-ray photoelectron spectroscopy (XPS). The surface characteristics of the samples were studied by Fourier transform infrared (FTIR) spectroscopy. Carbon and hydrogen contents in the samples were determined using Vario Micro cube elemental analyzer. The surface compositions of the samples were determined by X-ray photoelectron spectrometry. The liquid products were analyzed by high-performance liquid chromatography (HPLC). Refraction differential detector (RID) and SH1011 column were used for the analysis.

3.1 Analysis of decomposition products of the three materials in subcritical water

HPLC analysis showed the main hydrolysis product of cotton and MCC to be glucose. Figure 1a shows the yields of glucose from hydrolysis of cotton fiber at different temperatures. It was evident that high temperature was beneficial for hydrolysis of cotton. An important observation was that the yield of glucose reached the maximum of 4.5% at 240°C after 2.5 h, whereas its original level did not exceed 0.05%. In fact, it decreased continuously to a much lower value and reached 0% at 260°C after 1.5 h. At 28°C, no glucose was detected in the liquid products. Figure 1b shows the yields of glucose obtained on the hydrolysis of MCC at different temperatures. The highest yield was 7.87% at 220°C after 1.5 h, whereas hydrolysis at 200°C for 2.5 h gave 4.6% yield. When the temperature was increased to 260°C, no glucose was detected in the product.

Figure 1

The glucose yield curves: (a) WCF, (b) MCC hydrolysis. MCC, microcrystalline cellulose; WCF, waste cotton fiber.

In subcritical water, diffusion coefficient and dynamic viscosity of water increased with increase in ion product constant. It provided a contrary circumstance for hydrothermal hydrolysis and conversion of cellulose. Water not only acted as a reaction medium but also as a reactant to accelerate the decomposition of cellulose, producing glucose as the main product [9]. In general, the trends in hydrolysis of cotton and cellulose to produce glucose were similar. However, glucose easily underwent decomposition in subcritical water, which reduced the yield as a whole. Results showed that the rate of decomposition of glucose was faster than the rate of hydrolysis of cellulose [10]. The degradation pathways of both cotton and MCC included cellulose–glucose–glucose. It is noteworthy that, at very high temperature, some insoluble matter formed from cellulose covered the glucose, which hindered the hydrolysis of cellulose to oligosaccharides. In the case of cotton fibers, this encapsulation effect was more apparent. This is due to the fact that cotton is coated with a layer of wax, pectin, and proteins. The wax layer is a hydrophobic barrier made up of lipids which covers the cotton fiber and serves in keeping the surface of the cellulose clean and water proof. Wax, pectin, and protein had relatively stable structures, which reduced the area of contact between cellulosic cotton and water and decreased the rate of hydrolysis of glucose at low temperatures [11, 12].

To evaluate the stability of glucose under hydrothermal conditions, the rate of hydrolysis of glucose was determined under the same hydrothermal conditions. Results showed that glucose was very unstable under hydrothermal conditions and the decomposition rate increased rapidly with increasing temperature. As shown in Figure 2, 29.7% of glucose was hydrolyzed at 180°C after 1 h, while only 49.9% remained after 1 h at 200°C. As the reaction progressed, the decomposition of glucose and its residual amount decreased. Above 240°C, glucose decomposed completely in 1 h, and the residue was close to 0%.

Figure 2

The residual curve of glucose hydrothermal decomposition at different temperatures.

3.2 Elemental analysis of hydrothermal carbonization products

Figure 3 shows the elemental analysis of solid products, namely WCF, MCC, and glucose, at different temperatures (8 h). It was evident that carbon contents of the products increased with increase in water temperature. The carbon contents of products of WCF and MCC after hydrothermal carbonization showed a steep rise, due to rapid carbonization after the required temperature was reached. The carbonization temperature of MCC was 220–~240°C, while that of WCF was 240–~260°C. This illustrates that the carbonization temperature of cellulose was lower than that of WCF. The C content of carbonized product of cotton fiber was 75.46% at 280°C. However, carbonization products of hydrothermal treatment of glucose had higher carbon content at the same temperature. At 240°C, the C content of carbonized products of cotton fiber was 78.52%. The C content of glucose also showed a step-like graph between160°C and~ 220°C. This was because when the temperature of water was below 180°C; there were many products of intermolecular dehydration of glucose polymers produced during the hydrothermal process. Above 200°C, the degree of hydrolysis of glucose molecules significantly increased and the products were mainly formed by the polymerization of small molecules, causing an apparent increase in the extent of carbonization. The C contents of products showed a decreasing trend at 260°C due to the decomposition of products at high temperature. Moreover, the C contents of the products of the three materials were far lower than those of ordinary activated carbon (about 90%) [13, 14]. However, the O contents were more, which showed that there were an abundant number of oxygen groups on the surfaces of the hydrothermally carbonized products.

Figure 3

The elemental analysis of carbonized products of three substances at different temperatures.

3.3 Analysis of the crystal structures of hydrothermal carbonization products

Figure 4 shows that cellulose and cotton fiber have the same crystal structure. With the increase in temperature, both the carbonized products showed transformation from crystalline to amorphous forms. The characteristic peaks of cellulose-I at 220°C and 240°C were absent, which suggested that the gradual increase in water temperature could destroy the crystal structure of cotton fiber and cellulose. Compared to MCC, complete carbonization of cotton fiber occurred at higher temperature. Both the carbonized products had similar large crystal structures, which appeared at 2θ = 22.7° (crystal surface index is 002). This was the diffraction peak of aromatic carbon-layered structure, which illustrated a highly disordered structure of the products, and the degree of graphitization was very low. Reasons for the high carbonization temperature of cotton fibers were the strong hydrogen bonds and stable glycosidic linkages, which made the macromolecular cellulose of cotton fibers very stable [15]. When the water temperature was below 250°C, hydrolysis of cellulose occurred mainly at the surface, producing cellobiose and glucose.

Figure 4

XRD spectra of (a) WCF, (b) MCF, and glucose at different temperatures. MCF, microcrystalline cellulose fiber; WCF, waste cotton fiber; XRD, X-ray diffraction.

3.4 Morphological and structural analysis of hydrothermal carbonization products

Figure 5 shows the SEM images of carbonized products of cotton fiber, MCC, and glucose (8 h). In the case of cotton fiber, a fibrous structure could still be observed in products when the reaction temperature was below 240°C. With the increase in temperature, the fibrous structure disappeared, showing irregular granular particles adhered to each other. At 280°C, there were some spherical products, but the overall morphology of the products did not change. Hydrochar morphologies of MCC and cotton fiber were different. The products in the case of MCC at 200°C were not only quantitative but also spherical. Furthermore, as the temperature of water increased, the spherical degree increased gradually. Most of the products were spherical in shape with uniform distribution (average 1.8 μm) at 260°C. In the case of glucose, the products were mainly spherical and showed some adhesion at 180°C. Further, with the increase in temperature, CMSs appeared consistently, and their quality was better than those obtained from MCC. CMSs had uniform size and good dispersibility at 240°C (average 0.5 μm). Combining the results of X-ray diffraction (XRD) analyses of carbonized products such as cotton and cellulose hydrolysis, glucose was found to be an important intermediate product in the transformation of cotton fibers to carbonaceous microspheres. Only some of the molecules of the cotton fiber could be hydrolyzed to glucose, whereas the hydrolysis rate of MCC was much higher under hydrothermal conditions. Based on this, it could be speculated that cotton fiber did not react in the order of “cellulose → glucose → glucose decomposition products.” However, cellulose macromolecules in cotton fibers broke irregularly and generated polysaccharides of certain molecular weights. These polysaccharides underwent dehydration reaction, making the products irregularly granular.

Figure 5

SEM images of (a) WCF, (b) MCC, and (c) glucose carbonized products at different temperatures. MCC, microcrystalline cellulose; SEM, scanning electron microscopy; WCF, waste cotton fiber.

3.5 Chemical nature of hydrothermal carbonization products

To further investigate the process of hydrothermal carbonization of cotton fiber, the structures of products and glucose were analyzed. Figure 6 shows the infrared (IR) spectra of the three samples of carbonized products and glucose. As shown in the figure, all the three samples contained the same functional groups, which were similar to glucose [16]. Peaks at 3,441 and 1,023 cm⁻¹ were assigned to –OH stretching vibrations, whereas peak at 2,923 cm⁻¹ was attributed to C–H stretching vibrations. Peaks at 1,299 and 1,208 cm⁻¹ could be assigned to C–O–C stretching vibrations [17]. The three samples showed some new characteristic peaks. These include peaks of benzene ring at 1,509 cm⁻¹ and C=C at 1,614 cm⁻¹. However, these characteristic peaks were the strongest in the case of glucose, while they were the weakest for hydrothermal products of cotton fibers [18]. In the case of hydrothermal carbonization products of cotton fibers, peaks for –C=O vibrations and –COOH symmetric stretching vibrations were stronger than the corresponding peaks in glucose and MCC samples, which appeared at 2,850 and 1,398 cm⁻¹. FTIR results showed that cotton fiber, MCC, and glucose reacted during the hydrothermal degradation. SEM analysis showed that under hydrothermal conditions carbonization pathways and chemical reactions of all the three substances were overlapped. In other words, some macromolecules of cotton fibers and MCC hydrolyzed to glucose monomers, which in turn underwent dehydration and fragmentation to form hydrothermally carbonized products [19]. Besides, IR spectral analysis suggested that aromatization process occurred after glucose decomposition. Therefore, characteristic peaks of benzene ring in the hydrothermal products of glucose were stronger and the degree of carbonization was higher. However, inter- and intramolecular dehydration of cotton fibers in the carbonization process was stronger, which resulted in a large number of the –C=O and –COOR groups.

Figure 6

FTIR patterns of glucose and carbonized products of glucose at 240°C (G), MCC at 260°C (M), and WCF at 280°C (W). FTIR, Fourier transform infrared; MCC, microcrystalline cellulose; WCF, waste cotton fiber.

The oxygen-containing functional groups in hydrothermal products of cotton fibers and glucose were investigated by XPS. As shown in Figure 7, both the products had similar type of multiple peaks. Figure 7a and b shows four signals attributed, respectively, to the aliphatic/aromatic carbon (R–C₆H₅) (284.5 eV), phenol, ethanol, and ether groups (–C–O) (285.2 eV), carbonyl groups (–C=O) (287.3 eV), carboxylic groups (–COOH), and esters and lactones (289.2 eV). Moreover, Figure 7b and d shows the three multiple peaks in the XPS spectrum of O1s at 531.7, 532.5, and 533.5 eV, which are typical of the carbonyl group (–C=O), hydroxyl group (C–OH), and ester group (C–O–C), respectively. Calculating the areas of multi-peaks of C1s and O1s, it could be concluded that the C/O mass ratios on the surfaces of hydrochars were 5.79 and 5.85 [20]. This indicated that the carbonization degree of glucose was higher than that of cotton fiber. The peak area of R–C₆H₅ in carbonized products of glucose was larger, whereas the peak areas of –C=O and – COOH were larger in hydrothermally carbonized products of cotton fibers. This suggested that carbonization processes of cotton fibers and glucose were different. During hydrolysis, the complex chemical structure of cotton fiber made it difficult to be hydrolyzed completely, but the molecular chains were broken. Inter- and intramolecular dehydration reactions in large molecules occurred under hydrothermal conditions, which increased the number of oxygen-containing functional groups in the products and reduced the carbonization degree. Glucose molecules underwent fragmentation and were carbonized to microspheres by intermolecular dehydration and aromatic condensation. This analysis also verified the abovementioned speculation from SEM image analysis.

Figure 7

C1s (a) and O1s (b) of carbonized products of WCF at 280°C, C1s (c) and O1s (d) of carbonized products of glucose at 240°C. WCF, waste cotton fiber.

3.6 The formation and mechanism of hydrothermal carbonization products from cotton fiber

Through previous researches [21, 22] and experimental analysis, it was obvious that there were two pathways in the carbonization of cotton fibers (Figure 8). First, the hydronium ions generated from water attacked the glycosidic linkages of cotton cellulosic macromolecules, resulting in the hydrolysis of cotton. However, due to the structural stability of cotton fibers and limited number of hydronium ions, only some part of cotton formed monosaccharides, whereas the other parts of the chain formed oligomers [23, 24]. Monosaccharides and oligomers followed different pathways: (1) glucose was split into four-carbon sugars, namely phloroglucinol, furan, organic acids, aldehydes, etc. [25, 26]. These compounds formed a soluble polymer after intermolecular dehydration and aromatization. Some of the hydroxyl groups of monomers underwent dehydration to form C=O, while other molecules formed C=C through keto–enol tautomeric or intramolecular dehydration. Subsequently, aromatic clusters were produced by the condensation reaction of aromatic molecules. When the concentration of aromatic clusters in the aqueous solution reached the critical supersaturation point, a burst nucleation occurred; this grew outwards by diffusion toward the surface of the chemical species present in the solution. (2) Oligomers formed during hydrolysis of cotton could not undergo further hydrolysis. The glucose broke under high temperature and pressure of hydrothermal environment and underwent intra-/intermolecular dehydration and fragmentation reactions, which led to an increase in carbon content. The main products were organic compounds, which were insoluble in water. After the reaction, the morphologies of hydrothermal carbonization products were irregular and the particles were bound.

Figure 8

The mechanism diagram of WCF hydrothermal carbonization paths. (a) WCF carbonization path for CMSs (i). (b) WCF carbonization path for particulate matter (ii).

CMSs, carbon microsphere; WCF, waste cotton fiber.