Recovery of Glucose from Aqueous Solutions by Adsorption on Green Carbon

M. Mini Margaret

1. Introduction

Lignocellulosic wastes have been proposed as large renewable resources for chemicals and sugars. Lignocellulosic biomass has generated increasing interest in the bioconversion of this feedstock into liquid fuels and chemical products [1–5]. Cellulose and hemicellulose in agro-wastes are hydrolysed into sugar monomers that can be converted into liquid fuels [6–10]. Ionic liquids (ILs) are useful for dissolving biomass and separating lignin from hemicellulose and cellulose [11–14]. The high solubility of glucose in these aqueous ionic liquids makes separation challenging, and formidable problems are likely to be encountered in practice [15–18]. The ultimate goal of biorefineries is to develop processes that could convert biomass efficiently into fuels, power, heat, and value-added products. One of the most studied concepts is the so-called sugar platform, in which biopolymers (cellulose and hemicellulose) are hydrolysed into monomers (sugars). Fermentable sugars may then be converted biochemically into various products. Hydrolysis is carried out enzymatically with cellulases and hemicellulases or using an acid, most often dilute sulphuric acid. Lignocellulosic biomass contains various sugar monomers such as xylose, mannose, glucose, fructose, and galactose, but also arabinose and rhamnose, which are released on hydrolysis [19]. Adsorption equilibrium data have been published for glucose, fructose, sucrose, arabinose, xylose, and some oligosaccharides on strong acid cation (SAC) exchange resins in K⁺, Na⁺, Ca²⁺, or Fe²⁺ forms [20–23]. In this work, we consider the adsorption of glucose as a possible method for separation.

2. Materials and Methods

Glucose (Loba Chemie Pvt. Ltd.) was used to study the adsorption process. Anthrone (Loba Chemie Pvt. Ltd.) and concentrated sulphuric acid (Merck, 98%) were used for the colour reaction of sugars. Prosopis juliflora bark was collected from local areas for the preparation of green carbon, which was used as the adsorbent for the adsorption process. Hydrochloric acid (Merck) and sodium hydroxide (Loba Chemie) were used to adjust the pH of the glucose solution.

2.1. Glucose solution preparation

A stock solution of 1000 mg/L was prepared by dissolving 1 g of glucose separately in distilled water in a 100 mL standard measuring flask. Working solutions of the desired concentrations were prepared by successive dilution of the stock solution. The concentration of glucose was analysed using a UV–visible spectrometer (PerkinElmer Lambda 25).

2.2. Green carbon preparation

Green carbon means carbon prepared from cellulose-based material by a thermal method without using any chemicals. A high-temperature reactor was used for the preparation of green carbon. Prosopis juliflora bark was cut into chips and sun-dried. The dried P. juliflora chips were packed and supported on either side by asbestos wool in a vertical-type high-temperature reactor. This reactor was kept inside the tubular furnace. The furnace temperature was controlled by a digital temperature controller. The reactor temperature was increased to 200 °C and maintained at that temperature for 3 hours in the absence of air, and the reactor was continuously evacuated during the carbonisation reaction to remove volatile organics, hydrogen and moisture. On further increasing the temperature to the range of 250–350 °C, the P. juliflora chips became green carbon.

2.3. Determination of glucose concentration by the anthrone method

Carbohydrates are dehydrated by concentrated H₂SO₄ to form furfural and its derivatives, which condense with anthrone to form a blue-green complex with an absorption maximum at 578 nm. The concentration of glucose was measured spectrophotometrically by the anthrone method. Briefly, 2 mL of chilled 75% H₂SO₄ and 4 mL of chilled anthrone solution were progressively added to 10 mL boiling tubes, and then 1 mL of glucose solution was added to the tubes separately. The tubes were placed in a boiling water bath for 15 minutes and cooled, and the optical density was measured at 578 nm against a blank and converted to concentration from the calibration curve.

3. Results and Discussion

3.1. Characterisation of the glucose solution

The maximum absorption of the glucose solution was determined spectrophotometrically using anthrone reagent. The blue-green complex formed was characterised with a UV–visible spectrometer. The maximum absorption of the glucose–anthrone complex is at 578 nm, as shown in Fig. 1.

UV–visible absorbance spectrum from 400 to 700 nm showing a single absorption peak of the glucose–anthrone complex near 560–580 nm. — Fig. 1. UV–visible spectrum of the glucose–anthrone complex

3.2. Characterisation of the adsorbent

The adsorbent (Prosopis juliflora green carbon, PJGC) was characterised by FT-IR spectroscopy and BET surface area analysis to find out the nature of the active sites, structure, pore size and pore diameter.

3.3. FT-IR spectrum of green carbon

The FT-IR spectrum of green carbon is shown in Fig. 2. The O–H stretching observed at 3430 cm⁻¹ corresponds to the OH groups present in the glucose units of cellulose in P. juliflora chips. The peak at 1040 cm⁻¹ corresponds to C–O–C stretching in the cyclic form present in the glucose unit. The peak at 1117 cm⁻¹ corresponds to C–O–C stretching between two glucose units in cellulose. The peaks at 1014 and 1117 cm⁻¹ are less intense in green carbon. This may be due to the cracking of glucose units in the cellulose polymers. Moreover, the peak at 1625 cm⁻¹ is more intense in green carbon, which may be due to the oxidation of alcoholic groups to carbonyl groups in the glucose units of cellulose. Thus, the characterisation confirms the loss of OH groups and a smaller number of ether linkages, which proves the formation of green carbon.

FT-IR transmittance spectrum (%T versus wavenumber from 4000 to 400 cm⁻¹) of the green carbon, with strong absorption bands near 3430 and 1625 cm⁻¹. — Fig. 2. FT-IR spectrum of green carbon

3.4. BET surface area

The BET surface area of PJGC was measured by the nitrogen adsorption isotherm method. The BET surface area was found to be 2.7219 m²/g. The pore size of green carbon is in the range of 12–15 Å. The density of green carbon is 0.4857 and the particle size of green carbon is 0.667 mm.

3.5. Adsorption isotherms

The adsorption isotherms of glucose on PJGC at different concentrations were studied, and it was observed that the data are well matched with both the Langmuir and Freundlich adsorption isotherms (Figs. 3 and 4). The Langmuir equation is represented as

\frac{C _{e}}{Q _{e}} = \frac{1}{Q _{max} K _{L}} + \frac{C _{e}}{Q _{max}}

(1)

where Q_e is the equilibrium saccharide concentration on the adsorbent (mg/g), C_e is the equilibrium saccharide concentration in solution (mg/L), Q_max is the monolayer capacity of the adsorbent (mg/g) and K_L is the Langmuir constant (L/g).

Linear Langmuir plot of Ce/Qe (g/L) against Ce (mg/L) for glucose adsorption on PJGC, with five data points closely following a fitted straight line. — Fig. 3. Langmuir isotherm for the adsorption of glucose on PJGC

Table 1. Langmuir adsorption isotherm parameters for the adsorption of glucose on PJGC

Saccharide	Q_max	K_L	R²
Glucose	4.0159	0.4874	0.9947

The Freundlich equation is represented as

ln Q_{e} = ln K_{F} + \frac{1}{n} ln C_{e}

(2)

where Q_e is the equilibrium saccharide concentration on the adsorbent (mg/g), C_e is the equilibrium saccharide concentration in solution (mg/L), K_F is the Freundlich constant (L/g) and n (dimensionless) is the heterogeneity factor.

Linear Freundlich plot of ln Qe against ln Ce for glucose adsorption on PJGC, with five data points lying close to a fitted straight line. — Fig. 4. Freundlich isotherm for the adsorption of glucose on PJGC

Table 2. Freundlich adsorption isotherm parameters for the adsorption of glucose on PJGC

Saccharide	n	K_F	R²
Glucose	2.6543	1.3873	0.99657

The various parameters obtained from the Langmuir and Freundlich adsorption isotherms are given in Tables 1 and 2. The R² values for both adsorption models of glucose on PJGC are close to 1, indicating that the adsorption of glucose followed the Langmuir and Freundlich adsorption isotherms. A value of n greater than one corresponds to multilayer adsorption of glucose.

3.6. Desorption studies

The desorption of glucose is shown in Fig. 5. Desorption studies of glucose were carried out in the presence of water and ethanol at room temperature. Mild conditions are satisfactory for reaching complete glucose recovery after adsorption. The high desorption of sugars from PJGC indicated that the sugars were weakly adsorbed (physisorption) on PJGC. The high desorption is promising for the separation of sugars by adsorption. PJGC is the best adsorbent for the recovery of sugars from aqueous solutions.

Line plot of percentage of desorption (92–100%) against volume of solvent (mL) for water and ethanol, with ethanol reaching 100% desorption at a lower volume than water. — Fig. 5. Desorption of glucose from PJGC in water and ethanol (time: 10 minutes; temperature: 30 °C)

4. Conclusion

Prosopis juliflora green carbon was used as an adsorbent for the adsorption of glucose. It was characterised by FT-IR spectroscopy and BET surface area analysis. The percentage of adsorption decreased with an increase in the concentration of glucose. This is attributed to the saturation of the active sites and surface area of the adsorbent. From the adsorption studies, adsorption isotherms were calculated. The adsorption isotherm data are well matched with the Langmuir and Freundlich adsorption isotherm models. Adsorption isotherm studies were used to find out the maximum adsorption, the number of maximum layers and the adsorption capacity of each layer. A desorption study was carried out using solvents such as water and ethanol to find out the binding stability of glucose on the adsorbent. Glucose was completely desorbed in both water and ethanol. This high desorption of sugars from PJGC indicated that the sugars were physisorbed on PJGC. The experimental methods carried out in the present study can be suggested to fractionate glucose from lignocellulosic wastes in food industries.

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Recovery of Glucose from Aqueous Solutions by Adsorption on Green Carbon

Abstract

Keywords