Preparation and characterization of mesoporous activated carbons from waste watermelon rind by using the chemical activation method with zinc chloride

2.1 Materials

Watermelon rind used in this study was obtained from a local market in Edirne, Turkey. Watermelon rind was washed with double distilled water and placed in a blender with electromechanical controls to break into small pieces. The watermelon rinds broken into small pieces were left under the sun for two days on the summer days to evaporate their water. Then, dried watermelon rinds (dWRs) were milled to be smaller than 50 mesh, and stored in brown glass bottles. ZnCl₂ and HCl were obtained from Merck. Sodium hydroxide, sodium carbonate, and sodium bicarbonate were obtained from Sigma–Aldrich.

2.2 Preparation of activated carbon

Five grams of dWR were inserted into each of five flasks, and ZnCl₂ solutions whose impregnation ratios (ZnCl₂/dWR) were 0.5/1, 1/1, 2/1, 3/1 ve 4/1 (w/w) were added to these flasks in order to activate the carbons. The mixtures in the flasks were refluxed below their boiling points for an hour and then dried in petri dishes in an incubator for 24 h at 105 °C. After drying, the carbonizations of the mixtures were completed by baking in a furnace (nüve, MF 120) at different temperatures, i.e., 400, 500, 600, 700 and 800 °C for 30, 60, 90 and 120 min. separately. The obtained watermelon activated carbons (WACs) were treated with 0.1 N HCl solutions, and then washed with double distilled water until neutral pH was attained. After an additional drying step in the incubator at 105 °C, the WACs were ground and stored for future use.

2.3 Characterization techniques

Elemental contents (C, H, N, S) of dWR and WAC at 600 and 700 °C were obtained using an elemental analyzer (Leco TrueSpec Micro). Oxygen content was calculated from the difference. The surface areas and pore volumes were carried out by measuring N₂ adsorption–desorption isotherms at liquid nitrogen temperature (−196 °C) on an automatic apparatus (Micromeritics TriStar II 3020) after degassing the carbon samples sequentially at 105 °C for 30 min and at 300 °C for 120 min. under N₂ atmosphere to remove volatile contaminations. The surface areas of WACs with different impregnation ratios, carbonization temperatures, and carbonization times were calculated by applying the Brunauer–Emmett–Teller (BET) model (Lowell and Shields, 1991) to the isotherm data of the adsorption branch in the relative pressure range p/p₀ < 0.21. The total pore volumes were obtained at a relative pressure of about 0.98, and micropore surface areas and micropore volumes were estimated using t-plot method (Lowell and Shields, 1991). The mesopore areas and the mesopore volumes were calculated by subtracting the micropore values from the total values.

FTIR spectroscopy (Perkin Elmer, FT-IR/FIR Spectrometer Frontier) was used to determine the functional groups present in dWR and WAC. The point of zero charge was measured to obtain information of the surface charge on WAC via a batch equilibrium procedure (Milonjić et al., 1975). This measurement was carried out as follows: 50 mL of KNO₃ solution (0.1 M) was placed in an Erlenmeyer flask including 0.1 g of WAC. The initial pH was adjusted between 2 and 12 by addition of NaOH or HCl (0.1 N). After a contact time of 48 h under magnetic agitation at 150 rpm, the ultimate pH was determined and plotted versus the initial pH.

The surface functional groups containing oxygen were determined using Boehm titration (Boehm, 1966, 1994). WACs of 0.5 g were separately placed in 50 mL of the following solutions of 0.05 N: sodium hydroxide, sodium carbonate, sodium bicarbonate, and hydrochloric acid. The vials were sealed and agitated for 24 h and then filtered. 10.00 mL aliquots were taken. NaHCO₃ and NaOH samples were acidified with the addition of 20.00 mL standardized 0.05 N HCl, while for Na₂CO₃, 30.00 mL of 0.05 N HCl was added to neutralize the diprotic base completely and then allowed a strong acid-strong base titration. The excess base or acid was titrated with standardized 0.05 N HCl or NaOH, respectively. The number of acidic sites was found under following assumptions. NaOH neutralizes carboxylic, lactonic, and phenolic groups. Na₂CO₃ neutralizes carboxylic and lactonic groups. NaHCO₃ neutralizes only carboxylic groups. Also, the number of basic sites was calculated from the amount of hydrochloric acid that reacted with WAC. Furthermore, dWR and WAC were analyzed using a scanning electron microscopy (SEM; FEI-QUANTA FEG 250) to observe the variations on the surface morphologies.

3.1 Elemental analyses

Elemental compositions of dWR and WACs carbonized at 400–800 °C are given in Table 2. The carbon content of WAC substantially increased compared to dWR after activation process. Hydrogen and oxygen contents showed the opposite change trend as expected. Because of dWR decomposition in the activation and carbonization processes, volatile compounds, such as hydrogen and oxygen left the carbonaceous product. Therefore, the carbonaceous product became rich in carbon.

3.2 Influence of chemical impregnation ratio and temperature on the surface area

The surface areas of WACs prepared at different temperatures and different impregnation ratios are shown in Fig. 1. As the carbonization temperature increases from 400 to 700 °C, the surface area of WAC increases, but increasing temperature from 700 to 800 °C causes to decrease the surface area of WAC dramatically. It is observed that the surface areas of WACs are below 800 m²/g when the carbonization processes are completed at 400, 500, and 800 °C. The surface areas of WACs carbonized at 600 and 700 °C are much larger than the others. One of the other parameters affecting the surface areas is impregnation ratio shown in Fig 1. For all carbonization temperatures, as the impregnation ratio increases from 0.5 to 2, the surface area of WAC increases, but increasing impregnation ratio from 2 to 4 leads to decrease the surface area of WAC. The surface area is a maximum value (1156 m²/g) at the temperature of 700 °C with ZnCl₂ activation in which impregnation ratio is 2/1, but the surface areas (1121 and 1098 m²/g respectively) of WAC carbonized at 600 °C with impregnation ratio of 2/1 and 3/1 are quite close to each other. The difference of these surface areas obtained at 600 and 700 °C is about 50 m²/g. Therefore, WACs carbonized at 600 and 700 °C were analyzed in depth.

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Figure 1 Influences of carbonization temperature and impregnation ratio on the surface area of prepared WACs.

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3.3 Influence of chemical impregnation ratio and temperature on the pore volume

The influences of impregnation ratio on the micropore and mesopore volumes of prepared activated carbon at 600 and 700 °C are shown in Fig. 2a and b respectively. These figures indicate that the mesopores are well developed in the activated carbons prepared in this study. The effect of ZnCl₂ on the formations of micropore and mesopore can be understood with two competing mechanisms – micropore formation and pore widening (Rodriguez-Reinoso and Molina-Sabio, 1992). At low ZnCl₂-to-dWR, micropore formation is dominating mechanism and pore widening becomes important as the ratio increases at 600 °C. Therefore, mesopore volume increases until the impregnation ratio of 3/1. Then, the total pore volume decreases at the impregnation ratio of 4/1. For the total pore volume, similar trend is observed in Fig. 2b indicating the carbonization temperature of 700 °C. Total pore volume increases until the impregnation ratio of 3/1, and then, decreases at the impregnation ratio of 4/1. Furthermore, it is obvious that increasing carbonization temperature from 600 to 700 °C does not change the total pore volume (micropore volume + mesopore volume) substantially, but it causes the micropore volume to decrease. In Fig. 2b, the maximal value of the total pore volume of WAC prepared with impregnation ratio of 3/1 at 700 °C is found to be 0.95 cm³/g, and the total volume of WAC with impregnation ratio of 2/1 at 700 °C is 0.91 cm³/g. The difference between these pore volumes, i.e., 0.95 and 0.91 cm³/g may be negligible, but the maximal value of the total pore volume of WAC prepared impregnation ratio of 3/1 at 600 °C is 1.41 cm³/g. Although the surface areas of WACs carbonized at 600 and 700 °C with impregnation ratio of 2/1 and 3/1 were determined to be close to each other, the pore volume of WAC prepared impregnation ratio of 3/1 at 600 °C was calculated as a high value which was more than the others, with the difference of about 50%.

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Figure 2 Influences of impregnation ratio on the micropore and mesopore volumes of prepared WACs at (a) 600 and (b) 700 °C.

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Table 3 represents the average pore diameters (4V/A by BET) of WACs produced with the different impregnation ratios at 400–800 °C. As the impregnation ratio was raised for all WACs produced at 400–800 °C, the average pore diameter increased until the impregnation ratio of 3/1, but the average pore diameter decreased when the impregnation ratio becomes 4/1. The maximal value of the average pore diameter was determined to be 5.12 nm for WAC with the impregnation ratio of 3/1 at 600 °C.

3.4 Nitrogen adsorption–desorption isotherms

The physisorption isotherms have been classified into six different types by the International Union of Pure and Applied Chemistry (IUPAC) (Sing, 1985). As shown in Figs. 3 and 4, N₂ adsorption–desorption isotherms of WACs prepared with different impregnation ratios at 600 and 700 °C fit the mixture of type I and type IV with hysteresis loops. Hysteresis loops indicate that WACs have mesoporous, and WACs with narrower hysteresis loops have less mesoporous. The hysteresis loop of WAC prepared with impregnation ratio of 3/1 at 600 °C is the largest compared to the others. It means that WAC prepared with impregnation ratio of 3/1 at 600 °C has the highest mesoporous, and the shape of isotherm of WAC prepared with impregnation ratio of 3/1 at 600 °C resembles type IV. These findings were consistent with the pore volume of WAC prepared impregnation ratio of 3/1 at 600 °C, as shown in Fig. 2a.

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Figure 3 N₂ adsorption–desorption isotherms of WACs prepared for different impregnation ratios (ZnCl₂/dWR) at 600 °C.

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Figure 4 N₂ adsorption–desorption isotherms of WACs prepared for different impregnation ratios (ZnCl₂/dWR) at 700 °C.

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3.5 FTIR spectra of dWR and WAC

FTIR spectra of WAC prepared at different conditions were determined to be similar. For this reason, only dWR and WAC spectra were compared in Fig. 5. At the spectrum of dWR, the broad absorption band between 3200 and 3400 cm⁻¹ shows the existences of OH groups and the vibration of N—H stretching (Dağdelen et al., 2014). The bands located at 2923 and 2854 cm⁻¹ indicate C—H vibrations in methylene and methyl groups (Sun and Webley, 2010). The band at 1734 cm⁻¹ indicates carbonyl C⚌O groups. The band located at 1634 cm⁻¹ corresponds to olefinic C⚌C stretching (Yu and Luo, 2014). The bands at around 1415 and 1365 cm⁻¹ are due to the C—H in-plane bending vibrations in methyl and methylene groups (Yang and Qiu, 2010). The bands located at 1236 cm⁻¹ and an intense band at approximately 1027 cm⁻¹ belong to C—O stretching vibrations in alcohols, phenols, or ether or ester groups (Hesas et al., 2013). WAC spectrum has less adsorption bands than dWR spectrum. This indicates that various functional groups present in dWR spectrum disappeared after the carbonization and activation steps. Similar to dWR spectrum, WAC spectrum exhibited the bands between 1260 and 1050 cm⁻¹ are attributed to C—O stretching in carboxyl acids, alcohols, phenols and esters (Martins et al., 2015).

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Figure 5 FTIR images of dWR and WAC.

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3.6 Point of zero charge and Boehm titration

The value at which pH (ultimate) is equal to pH (initial) was accepted to be pH_ZPC, and the charge surface is neutral at this pH value. The pH_ZPC of an adsorbent is a fundamental characteristic to comprehend interfacial properties. Moreover, the pH_ZPC of an adsorbent helps to deduce which ionic species can be adsorbed by the activated carbon at desired pH. The value of pH_ZPC of WACs produced at 600 and 700 °C was determined to be 5.45. The surface of WAC is positively charged at the pHs below 5.45, while it is negatively charged at pHs above 5.45. Thus, it is predicted that the adsorption of ionic species with positively charged by WAC will be proper at pHs above 5.45. On the other hand, the adsorption of ionic species with negatively charged by WAC will be appropriate at pHs below 5.45 because of electrostatic interactions.

As shown in Table 4, increasing carbonization temperature (at the impregnation ratios of 3/1 and 2/1 for WACs carbonized at 600 and 700 °C, respectively) reduces the amount of acidic functional groups and increases the basic surface groups of WAC. The increase in carbonization temperature will make some functional groups decompose to form CO and CO₂. This is caused by the instability of acidic groups at high temperatures (Boehm, 1994). On the contrary, basic groups are increased when the temperature is increased. These groups can be formed during the cooling of WAC after the carbonization process. This cooling process makes possible to be the fixation of oxygen in the active sites (Boehm, 1994). Similar result was also obtained by Prahas et al. for phosphoric acid activated jackfruit peel waste (Prahas et al., 2008). Furthermore, the result of point of zero charge (pH_pzc = 5.45) is consistent with Boehm titration results, which explains the dominance of acidic groups on the surface of WACs.

3.7 SEM observation of dWR and WAC

The SEM images of dWR at the operating condition with 2000× magnification and WACs at the operating condition with 1000× magnification are presented in Fig. 6(a–c). SEM image of dWR in Fig. 6a shows that dWR has smooth surface without distinctive pores. In Fig. 6b, SEM image of activated carbons prepared by using ZnCl₂ chemical activation with impregnation ratio of 3/1 at 600 °C illustrates an irregular and heterogeneous surface morphology with a well-developed porous structure in various sizes. Moreover, in Fig. 6c, SEM image of activated carbons prepared by using ZnCl₂ chemical activation with impregnation ratio of 2/1 at 700 °C is similar to Fig. 6b, but there are fewer holes in Fig. 6c than in Fig. 6b. This result supports that WAC prepared by using ZnCl₂ chemical activation with impregnation ratio of 3/1 at 600 °C has the highest pore volume compared to the other WACs, see Fig. 2.

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Figure 6 SEM images of (a) dWR and WACs carbonized at (b) 600 and (c) 700 °C.

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