Preparation and characterization of sulphonated bio-adsorbent from waste hawthorn kernel for dye (MB) removal

Journal of Molecular Liquids 287 (2019) 110988

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Preparation and characterization of sulphonated bio-adsorbent from waste hawthorn kernel for dye (MB) removal Yasin Akköz a, Ramazan Coşkun b,⁎, Ali Delibaş b a b

Institute of Science, Yozgat Bozok University, 66900 Yozgat, Turkey Department of Chemistry, Faculty of Science and Arts, Yozgat Bozok University, 66900 Yozgat, Turkey

a r t i c l e

i n f o

Article history: Received 13 April 2019 Received in revised form 9 May 2019 Accepted 16 May 2019 Available online 21 May 2019 Keywords: Adsorption Activation Sulphonated bio-adsorbent Methylene blue

a b s t r a c t In this study, hawthorn kernel (HK), obtained from agricultural waste, which is eco-friendly and low cost material was selected as bio-adsorbent to remove of methylene blue (MB) from aqueous solutions. Since hawthorn kernel has low adsorption capacity, a new hawthorn-based bio-adsorbent having sulphonic acid groups (SHK) was prepared by activation using sulfuric acid with a very simple method. The optimal activation conditions were determined as 6 h, 85 °C, in 18 M sulfuric acid, with a sample/acid ratio of 1/5 (mass/volume). To determine the basic properties such as chemical structure, porosity and surface properties of the HK and SHK, they were characterized by Boehm Titration, BET, FTIR, SEM-EDX, XRD and pHpzc methods. Also, the thermal behavior of both adsorbents was examined by TGA. It was observed that activation has improved the thermal stability and also caused more amorphous structure. Optimum adsorption conditions were determined considering the amount of adsorbent, solution pH, MB initial concentration, and contact time and temperature parameters. Activation significantly changed the surface properties of the HK such as total acidity, functionality, porosity and surface pH. For MB adsorption on both adsorbents, pseudo-second-order kinetic model was found to be appropriate to explain kinetic behavior and equilibrium data fitted better to the Langmuir isotherm model. The maximum MB adsorption capacities of HK and SHK were calculated as 49.5 and 151.5 mg/g, respectively. It was determined from the calculated thermodynamic parameters that the adsorption of MB onto HK is spontaneous at low concentrations and high temperatures, but the adsorption of MB onto SHK is spontaneous at both high concentrations and low temperatures. After reusability studies which were carried out with 100 mg/L MB for five times, the SHK lost about 1% of its removal efficiency. As a result it was observed that SHK could be used as an efficient adsorbent for MB removal from aqueous solutions. © 2019 Published by Elsevier B.V.

1. Introduction With increasing industrialization and population, the discharge of water polluting materials is also increasing. These pollutants also include dyes originating from various industries such as textiles, leather, paper, cosmetics, and generally having intense color and high toxicity [1]. For example methylene blue (MB) is a typical toxic dye. If the amount of methylene blue, one of the most commonly used dyes, is above a certain amount (N7 mg/kg) in the water, it causes health problems such as high blood pressure, abdominal pain, nausea and mental disorder [2]. Therefore, the removal of these contaminants from the aqueous medium is an essential necessity. Before discharge of wastewater including dyes to the water resources, dyes they should be removed from aqueous solution by using any method and this should also be economical and effective. In recent years, various methods such as ⁎ Corresponding author at: Department of Chemistry, Faculty of Science and Arts, Yozgat Bozok University, 66900 Yozgat, Turkey. E-mail address: [email protected] (R. Coşkun).

https://doi.org/10.1016/j.molliq.2019.110988 0167-7322/© 2019 Published by Elsevier B.V.

adsorption, coagulation, flocculation, ion exchange, ozonation, membrane filtration, anaerobic treatment, and photo degradation have been used for this purpose [3]. Among these methods, adsorption is preferred as the most economical and efficient method of treatment to remove the dyes [4]. In the adsorption method, the choice of the material (adsorbent), which performs the holding process, is very important. Activated carbon is the most popular adsorbent but its use is limited due to its high cost and some disadvantages [5,6]. This has led to search for new, efficient and economical adsorbents that can be alternative to active carbon to remove pollutants, especially dyes, from aqueous media. Some research studies have shown that biological materials can be utilized as bio-adsorbent for removing dyes [7,8]. Especially agricultural by-products, such as nutshells [5], wheat shells [9], rice husk [10], garlic peel [11], raw beech sawdust [12], coir pith [13], peanut hull [14], tea waste [15], neem leaf powder [16], cotton waste [17], spent coffee grounds [18], banana peel [19], orange peel [20], coffee husks [21], white pine sawdust [22] and onion membrane [23], papaya seed [24], olive and apricot stones [25,26] are included in many studies due to their low cost and environmental friendliness. However, most of

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these low-cost adsorbents generally have low sorption potentials [27]. Increasing surface area and functionality of the adsorbents lead to increase in the adsorption capacity of these adsorbents [28]. Therefore, these wastes are generally being activated with physical and chemical activations by using various agents [29–31]. The use of acidic solutions in modification forms carboxylic groups on bio-adsorbent surface, in other words, it provides more oxygenated functional groups and develops micropores that give rise to an increase in the surface area too [4,32,33]. As a result all these increase the adsorption of the positively charged pollutants on the adsorbent that is activated with acidic solutions. H2SO4, HNO3, H3PO4, H2O2, (NH4)2S2O8 compounds are often used as agents in acid activation [34]. Hawthorn (Crataegus monogyna) of which belongs to Rosaceae family and is common in northern hemisphere, is grown abundantly with very different types in Turkey and it is consumed as a fruit [35,36]. Hawthorn kernel that is an agricultural waste has not taken place in the scientific literature as adsorbent for dye removal yet. The aim of this study has been as follow; (1) to prepare and characterize activated bioadsorbent (SHK) from hawthorn kernel (HK) by acid activation; (2) to optimize the methylene blue adsorption onto HK and SHK according to the amount of adsorbent, solution pH, initial concentration, contact time and temperature parameters; (3) to evaluate with regard to kinetic and isotherm parameters to explain the mechanism of MB adsorption onto HK and SHK.

The above procedure was performed to optimize the activation temperature (25, 45, 65, 85 and 105 °C), acid concentration (4.55, 9.10, 13.65 and 18.20 M) and sample/acid ratio (mass/volume) (1/1; 1/3; 1/ 5; 1/7 and 1/10). The optimal activation conditions were determined as 6 h, 85 °C, and 18 M H2SO4 with sample/acid ratio of 1/5. Preparation steps of bio-adsorbent were given in Fig. 1. Bio-adsorbent prepared at optimal activation conditions was named as SHK in the studies. 2.4. Adsorption studies and analysis Adsorption studies were carried out in 150 ml Erlenmeyers. 50 mL of MB solution, prepared from stock solution at the desired concentration, was put into the Erlenmeyer then adjusted to the required pH by using a digital pH-meter (WTW 32101 model) with 0.1 M HCl or 0.1 M NaOH. Later, 0.1 g adsorbent was added to erlenmeyer it was placed into a shaker at room temperature and shaked at certain times at agitation rate of 200 rpm. After that samples were centrifuged at 5000 rpm for 5 min; the absorbance of MB was measured by the UV–Vis spectrophotometer (Schimadzu UV-1208) at wavelength 665 nm (λmax.). The amount of MB adsorbed onto the HK and SHK at time t (qt) and dye removal percentage (DR%) were calculated using Eqs. (2) and (3) respectively.

2. Materials and methods DRð%Þ ¼

2.1. Chemicals Cationic dye, methylene blue (MB) (CI = 52,015; chemical formula: C16H18ClN3S; molecular weight = 319.86 g/mol) and all other used chemicals were Merck and were used without any purification. Stock solution of the MB (1000 mg/L) was prepared by dissolving required amount of MB in ultra-pure deionized water. 2.2. Bio-adsorbent

ðC o −C tÞ V W

ð2Þ

ðC o −C t Þ  100 Co

ð3Þ

qt ðmg=g Þ ¼

where, C0 (mg/L) is the initial dye concentration, Ct (mg/L) is concentration of dye at time t, V is the volume of solution (L) and W is the weight of bio-adsorbent (g). The effects of adsorbent amount (0.025–0.3 g), pH (3−11), initial dye concentration (25–400 mg/L), time (0–360 min) and temperature (25-60 °C) onto adsorption were investigated.

Hawthorn fruits were collected from hawthorn trees in the campus of Yozgat Bozok University in September, Turkey. The seeds of the hawthorn fruits were removed and they were washed with distilled water to remove residues and then were dried in the daylight. After that the dried seeds were milled, then were passed through sieve with 65 meshes and stored in an airtight plastic container for using analyses and adsorption experiments. This bio-adsorbent was named as HK in the studies. 2.3. Activation of the HK with H2SO4 and optimization of conditions To prepare bio-adsorbent including sulphonic acid group from the HK, it was activated with sulfuric acid (H2SO4). To optimize activation time, 2.0 g of washed and dried hawthorn seeds were added to each of the nine different reaction balloons of 100 ml. After adding 10 ml of concentrated H2SO4 (18.20 M) into the each was stirred for different times (0.5, 1, 2, 3, 4, 5, 6, 7 and 8 h) at 85 °C under reflux (continuous stirring). After the treatment, the cooled samples were poured into 50 mL of distilled water and then filtered. Filtered the activated samples were thoroughly washed with 0.1 M NaHCO3 until the pH of the wash water was close to 7. And then they were dried at 55 °C. Yields for each activation steps were determined gravimetrically using given equation below. Yieldð%Þ ¼

Wa  100 Wo

ð1Þ

where Wo and Wa represent initial weight and activated weight of the hawthorn seed.

Fig. 1. Preparation steps of the SHK bio-adsorbent.

Y. Akköz et al. / Journal of Molecular Liquids 287 (2019) 110988

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Table 1 Results of BET and Boehm Titration of bio-adsorbents. Sample

Surface area (BET) × 104 (m2/g)

Pore volume (cm3/g)

Pore size (nm)

Carboxylic (mmol/g)

Lactonic (mmol/g)

Phenolic (mmol/g)

Acidic (mmol/g)

Basic (mmol/g)

223 1251

2.18 34.07

39.1 37.7

0.530 1.350

0.270 0.510

0.070 0.240

0.870 2.100

0.600 0.470

HK SHK

Fig. 2. The infrared spectrums of the HK, SHK and DL-SHK.

2.5. Characterization Fourier transform infrared (ATR-TIR), Perkin Elmer Spectrum 400, was used to determine functional groups present on the HK and SHK over the wave number range of 400–4000 cm−1. The surface morphology of the HK and SHK was examined by Field Emission-Environmental Scanning Electron Microscope-Energy Distribution Spectrometry (FEESEM-EDS), (FEIQuanta 450 FEG). The surface area, pore volume and average pore diameter of the samples were determined by Micromeritics Gemini VII analyzer. Thermal stability of the adsorbents was carried out by using a Setaram Labsys TG- DTA/DSC thermobalance in N2 atmosphere. The samples were exposed with isothermal heating process at 70 °C for 20 min. to remove the moisture and then cooled to 30 °C. TGA were performed with 10 °Cmin−1 heating rate from 30 °C to 800 °C. XRD diffraction analysis were performed with PANalytical – Empyrean using Cu Kα radiation (λ = 0.1542 nm).

the hemicellulose structure. From the results, the optimal activation conditions, high dye removal was obtained, were determined as 6 h, 85 °C, 18 M sulfuric acid and sample/acid ratio of 1:5 (mass/volume).

3. Results and discussion 3.1. Determination of optimum activation conditions In order to determine the appropriate activation conditions, the yield of the product obtained as a result of each process, the dye removal percentage and surface properties of the activated sample were evaluated together (results were given in supplementary file). When all the data were analyzed (see supplementary file), it was seen that the total pore volume and also the activation efficiency of the SHK prepared under the optimum activation conditions was the highest. This could be explained by the fact that the volatile components are removed from the structure and the cellulose is degraded by acid activation of

Fig. 3. pHpzc graph of the HK and SHK.

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Fig. 4. SEM image of (a) HK, (b) SHK and (cx2000, dx50000) DL-SHK.

3.2. Characterization BET and Boehm Titration analysis results of unactivated (HK) and activated (SHK) were given in Table 1. After activation, as seen in Table 1 the amounts of the active groups especially carboxylic groups which are more effective in adsorption [28,37] were increased at considerable ratios (2.4 folds). Also, from the total acid amount results, it can be said that the surface of the SHK is mainly acidic (as seen the pHpzc results and final pH values in the Table 6). It is also observed that BET surface area and the pore volume of the SHK are also increased and has meso pores which are effective in the adsorption [38]. In particular, a 15.6 folds increase in total pore volume is considered as a result of the removal of volatile substances, nitrogenous

compounds by acidic hydrolysis and degradation of the cellulose and hemicellulose in the HK structure [39,40]. Although there is not a major increase in the BET surface area after activation, the great increase in the amount of adsorption could be attributed to the functional groups, especially sulfonyl groups, provided by activation. As a result, these properties improved has clearly contributed to the adsorption of the SHK. In the FT-IR spectra (Fig. 2.) of the HK, observed broad band between 3600 and 3000 cm−1 indicates the O\\H stretching vibration of the hydroxyl groups belonging to phenols, carboxylic groups or it could be related to the stretching vibration of –NH groups [41,42]. Also, the bands at 2924 cm−1 and 2855 cm−1 are step from aliphatic C\\H stretching. The bands at 1735 cm−1 and 1235 cm−1 are characteristic for the

Fig. 5. EDX results for HK and SHK obtained optimized condition.

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Fig. 6. TGA/DTA thermograms of HK and SHK obtained optimized condition.

C_O and C\\O vibration respectively [43]. Moreover, the bands at 1645, 1510 cm−1and 1456 cm−1 came from the C_C vibration in olefinic structures or –CONH (due to nitrogen content of the HK in EDX spectra) and aromatic ring [38]. And also band at 1032 cm−1 came from the \\C\\O\\C\\ vibration [44]. The FTIR result of the HK indicated that its surface major groups are \\OH, \\COOH, C_C, \\N (\\CONH) and \\C\\O\\C\\. After activation with sulfuric acid, the changes in the chemical structure of HK are evident in the FTIR spectra. In the FT-IR spectra of the SHK new bands were observed that came from sulphonic acid at 1150 cm−1 and 863 cm−1 belonging to O_S_O and S\\O vibration [45,46]. Also after activation by H2SO4, the observed band in the spectrum of HK between 3500 and 3000 cm−1, expanded to a range of 3500–2000 cm−1 because of the OH stretching vibrations in the -SO3H group [46] and the band at 1645 cm−1, observed in the HK spectra belonging C_C band in the olefinic structure, was disappeared. This indicates that the olefinic structure is effective in sulfonation. These results indicate that

Fig. 7. XRD pattern of HK and SHK obtained optimized condition.

the HK is modified with H2SO4, in other words, it is possible to perform of its sulphonation. In the FT-IR spectra of DL-SHK (dye loaded SHK), after dye loading, the bands at 1380, 1318 and 1217 cm−1 are assigned to the C\\N\\S, C_N stretching and tertiary amine groups in MB molecule, respectively. Also the intensity of the band at 1594 cm−1 increased due to aromatic groups in the MB molecule. In addition, the bands between 1200 and 1100 and 1000–750 cm−1 are belongs to S_O and S\\O sulfonate groups [45–47] are another evidence for interaction between the MB and sulfonyl groups. Therefore, these results indicate that MB is

Fig. 8. Effect of the adsorbent dosage on MB adsorption onto HK and SHK (Ci_100 mg/L; pH = 10; T = 25 °C; t = 6 h; V = 50 ml).

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adsorbed onto the SHK and it is seen that sulfonyl groups are particularly effective in the adsorption. The results in Fig. 3 show pHpzc values of the HK and the SHK. The surface charge of the HK and the SHK were calculated as 7.20 and 3.98 respectively. It means that the surface charge of the HK and the SHK are zero at this pHs respectively. At pH value N7.20 the surface charge is negative for the HK but the surface charge is negative for the SHK at pH values N3.98. Hence, SHK can be used to remove cationic pollutants such as cationic dyes and heavy metals in a wide pH range. SEM images of HK, SHK and also dye loaded- activated howthorn kernel (DL-SHK) are shown in Fig. 4. When the SEM images are examined, it is seen that the original HK has a surface without pores structure (Fig. 4a). But after activation, the surface structure of HK is disrupted and different sizes pores are formed as can be seen from the SEM image of the activated SHK (Fig.4b). It could be said that activation with H2SO4 caused a significant pore structure formed contrary to studied in literature [48]. After dye loading, it is obviously seen that the pores in the SHK surface were filled by the dye molecules and also surface of SHK was covered with MB (Fig. 4c–d). A detailed EDX microanalysis was performed for both HK and SHK and the results were depicted in Fig.5. As seen the results, beside the major elements such as carbon, nitrogen and oxygen, some minor elements such as sodium, magnesium, aluminum and phosphorus have been determined in HK structure. Nitrogen, oxygen, sodium, magnesium, aluminum and phosphorus were removed from the structure of the HK by H2SO4 activation and its carbon composition was increased. While the weight percentage of carbon in unactivated HK is about 49.56, this value, under optimum activation conditions, was increased to 66.81 by H2SO4 activation, because of the loss of volatile matter and the decomposition of cellulose, hemicelluloses and lignin. Moreover after activation, a peak belonging to sulfur atom was observed in the EDX analyze which also supports suggested FT-IR result of the SHK. This is evidence that HK is sulfonated by H2SO4 activation. After activation, the absence of N in the structure of SHK may be attributed to the removal of the amine and its derivatives from the structure as a result of the acidic hydrolysis of HK. As a result, H2SO4 activation under optimum conditions and washing with bicarbonate solution has increased the number of pores significantly which are important parameter for adsorption process. Thermal stability analysis results of the HK and SHK were depicted in Fig. 6. As seen the figure, the thermal decomposition of the HK and SHK followed in different steps. Although the TGA curve of HK showed a small mass loss (b0.5%) at a temperature below 200 °C due to the removal of volatiles and evaporation of the adsorbed water, the degradation of HK took place basically in two steps. Large mass loss (about 48% of the initial one) observed in the first step between 200 and 381 °C may be attributed to the degradation of cellulose, hemicelluloses and lignin [49]. The second mass loss (about 14% of the initial one) was observed at temperatures between 381 and 675 °C attributed to the degradation of carbonaceous materials [50] and the residue is 38% at 675 °C. As can be seen from TGA thermogram of the SHK, although there is a mass loss about 4% until 190 °C due to evaporation of the bound water, almost whole degradation of SHK occurred in single-step between 190 and 750 °C. It was observed that total mass loss is about 46% and the residue is about 54% at 750 °C. The small decrease in thermal stability of SHK could be attributed to the introduction of more easily decomposable carboxyl and sulfonyl groups formed with acid activation than cellulose [51]. But, when the residue amounts of HK and SHK were compared, it could be said that SHK has more thermal stability, necessary to utilise at high temperature applications, than HK [52]. However, the improved properties of the SHK, especially the porous structure which was formed due to the removal of volatile matters and the decomposition of significant compounds of the HK (cellulose, hemicelluloses and lignin) increased the applicability of SHK for MB adsorption. XRD patterns for HK and SHK were given in Fig. 7. As seen the figure, while more sharpener diffraction peaks at 2θ 35° and 23° was observed

Fig. 9. Effect pH on MB adsorption onto HK and SHK (Ci = 100 mg/L; T = 25 °C; t = 6 h; V = 50 ml; m = 2 gL−1 (inside: digital image of HK and SHK after adsorption)).

for HK but broad peaks which indicating the amorphous carbon structure were observed for SHK [53,54]. Moreover, decreasing of the signal intensity show that HK was converted to the amourphous structure (SHK), important for the adsorption process, by acid activation.

Fig. 10. Effect of time on MB adsorption onto HK and SHK (Ci = 100 mg/L; pH = 10; T = 25 °C; V = 50 ml; m = 2 gL−1).

Y. Akköz et al. / Journal of Molecular Liquids 287 (2019) 110988

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Fig. 11. The adsorption kinetic graphs of the MB onto HK and SHK.

Table 2 Results of kinetic parameters. Pseudo first order Sample HK SHK

Pseudo second order

qe,exp (mg∙g−1)

qe,cal (mg∙g−1)

k1 min−1

R2

27.34 49.99

13.13 25.18

0.0128 0.0142

0.9563 0.9630

Intraparticle diffusion model

qe,cal (mg∙g−1)

k2x103 (g/ mg min−1)

28.25 51.28

2.44 1.57

3.3. Adsorption studies 3.3.1. Effect of the adsorbent dosage The effect of adsorbent dosage on dye removal for both HK and of the SHK was studied as using varying amounts of the adsorbent (0.5–6 g/L) at 100 mg/L constant MB concentration. The results were illustrated in Fig. 8. It can be clearly seen from the Fig. 5 that both adsorbents show similar adsorption behavior except for dye removal percentage. Dye removal percent of both adsorbents increased with increase in mass of adsorbent up to a certain amount and after that almost remains constant. This could be attributed to increase of the number adsorption centers with increasing adsorbent dosage [55]. The highest dye removal is % 80 for 5 g HK/L but of the SHK is nearly %100 for 1.0 g SHK/L. These differences between the dye removal percentages could be explained with increasing functionality per adsorbent amount (see Table 1) after activation. In other words after activation, SHK has large number of functional groups and a larger active surface area, leading to an increase in adsorption capacity.

R2

C

0.9990 0.9994

11.27 23.97

kint (mg/ g min−1) 1.0422 1.6645

R2 0.7816 0.8132

While MB removal by HK is 54.68% at 6 h for 100 mg/L MB solution, of the SHK is 99.96% under the same conditions. It could be attributed to pore volume increasing with activation and the pores formed by activation. Since MB molecules diffuse easily into the pores of the SHK, further adsorption is observed [58]. To determine the kinetics and mechanism, Lagergren, pseudosecond-order and intra-particle diffusion models were used in this study. The linear equations of these models were given below respectively. ln ðqe −qt Þ ¼ lnqe −k1 t

ð4Þ

t 1 t ¼ þ qt k2 q2e qe

ð5Þ

qt ¼ A þ kint t 0:5

ð6Þ

3.3.2. Effect of initial pH value The effect of pH on adsorption of MB onto HK and SHK was examined in the range of 3.0–11.0 and the results were shown in Fig. 9. As seen from the figure the MB removal performance of HK was under the quantitative value at all the studied pHs for 100 mg/L MB, whereas that of the SHK over the quantitative value that it was excellent. This high dye removal of the SHK can be attributed to easily ionizable functional groups occurred after activation and also the negative surface of the SHK. The surface of the SHK is negatively charged when the pH of the medium is over that of pHpzc (pHpzc = 3.98) and this negative charge increase as the pH of the medium increases. This could also attract more MB and high removal of the MB occurs. That is why SHK can be used as a good adsorbent with high removal efficiency for cationic dyes such as MB in a wide pH range. Similar results for the adsorption of methylene blue onto different adsorbents have been reported in the literature [55–57]. 3.3.3. Effect of contact time The effect of contact time on the MB removal with HK and SHK was illustrated in Fig. 10. As seen clearly from the figure, the MB removal of both the HK and the SHK rapidly increase in the first 1 h due to initially abundant active groups on the surface; then slows down because of steric effect of the previously adsorbed MB molecules. And finally both adsorbent reached equilibrium due to saturation of the active centers.

Fig. 12. Effect of concentration on MB adsorption onto HK and SHK (pH = 10; t = 6 h; T = 25 °C; V = 50 ml; m = 2 gL−1).

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Fig. 13. The adsorption isotherm graphs of the MB onto HK and SHK.

Table 3 Isotherm parameters for MB adsorption onto CS, SCS and PCS. Freundlich

Langmuir

Temkin

2

Sample

KF (L/mg)

n

R

HK SHK

3.72 104.18

2.10 11.17

0.9408 0.5696

KL (L/mg)

Qm (mg/g)

0.037 5.50

43.7 151.5

where, Qt and Qe (mgg−1) are the adsorption amount of the MB onto the HK or SHK at time t and equilibrium, k1 (min−1), k2 (gmg−1 min−1) and kid (mg/ g min−1/2) are the rate constant for the pseudo-first-order, the pseudo second-order and the intraparticle diffusion models respectively. Using the data in Fig. 10, the kinetic curves for these kinetic models, ln(Qe-Qt) vs. t for first order, t/Qt vs. t for second order and Qt vs. t1/2 for intra-particle diffusion, were drawn (Fig. 11). For both HK and SHK, the kinetic constants calculated and regression coefficients (R2) for each kinetic model were given in Table 2. Pseudo second order kinetic model is better suited due to higher correlation coefficients (R2) and close values of the qe, exp and qe, cal for Table 4 Comparison of the MB maximum capacities of the HK, SHK and those of different bio-adsorbents reported in the literature. Adsorbent

Act. temp./time (°C/h)

Ads. time (h)

Ci (mg/L)

Removal effc. (%)

Qmax. Ref. (mg/g)

CS SCS PCS SWC PWC AC (commercial) SDC SD GAC (commer.) RSAC RTS SMTS RMS SMMS HK

– 150/8 150/8 120/24 120/24 –

3 3 5 2 2 2

50–450 50–450 50–450 50–500 50–500 50–500

97–85 ͂ 100 100 100–29.4 100–51.8 ͂ 100

147 555.56 222.22 39.68 88.49 –

[57] [57] [57] [61] [61] [61]

150/24 50/4 – 110/24 – −/24 – −/24 –

3 3 3 0.04 2 1 1 0.5 6

50–500 50–500 50–500 20–60 25–125 25–125 50–250 50–250 25–500

92–67 87.1–50 ͂100 99.5–97.5 80–50 100–90 99–92 100–97 67.6–23

51.4 46.1 – 62.50 16.611 34.483 25.36 58.08 49.50

SHK

85/6

6

25–500

100–60.2

151.52

[62] [62] [62] [63] [64] [64] [65] [65] This study This study

R2

AT (L/mg)

B

R2

0.9964 1.000

0.261 1,55 × 104

10.58 10.81

0.9776 0.5904

both adsorbents HK and SHK. This means that the rate limiting step is chemisorption for MB adsorption onto the both adsorbents [59]. 3.3.4. Effect of initial MB concentration To investigate concentration effect of MB onto the HK and SHK adsorption, the adsorption capacity (Q) and dye removal (DR%) of both the HK and the SHK for MB were studied in a series of different initial concentrations (25–500 mg/L) of MB solution. The results were illustrated in Fig. 12. It could be seen that with the increasing of MB concentration, the adsorption capacity of both adsorbents increased until it reached a saturation value (40.1 mg/ g for HK, 149.8 mg/ g for SHK) and then has remained unaltered with increased concentration. While removal efficiency of the HK decreased with increasing MB concentration, the SHK preserved its high removal efficiency until 300 mg/L and then decreased with further concentration increase. High adsorption capacity and high removal efficiency of the SHK can be attributed to the increasing functional groups and the charge characteristics of the HK surface to change from positive to negative with activation. To clarify the interaction of MB with the HK and SHK, the equilibrium adsorption data were examined according to the Langmuir, Freundlich and Temkin models. While the Langmuir model assuming that adsorption occurred on homogeneous surface, the Freundlich and Temkin models assuming that adsorption taken place on heterogeneous surface [55]. Linearized equations of these models were given in the Eqs. (7), (8) and (9) respectively. Ce 1 Ce ¼ þ qe Q m K L Q m ln qe ¼ ln K F þ

ð7Þ 1 lnC e n

qe ¼ BlnAT þ BlnC e

ð8Þ

ð9Þ

Y. Akköz et al. / Journal of Molecular Liquids 287 (2019) 110988

9

Fig. 14. Effect of temperature on MB adsorption onto HK and SHK (t = 6 h; V = 50 ml; pH = 10; m = 2 gL−1).

In the equations, Ce (mg/g) and qe (g/mg) are the amount of dye remained in the solution and adsorbed onto the bio-adsorbent at equilibrium. Qm (mg/g) is the saturated adsorption capacity and KL (L/mg) is the Langmuir isotherm constant that is related to the energy of adsorption. Kf and n constants, which are related to the adsorption capacity and adsorption intensity, are the Freundlich isotherm constants. AT (L/ mg) is Temkin constant and B is Temkin parameter that is related to the heat of adsorption. The isotherm curves, Ce/qe vs Ce, lnqe vs lnCe and qe vs lnCe for Langmuir, Freundlich and Temkin models respectively, were drawn and shown in Fig. 13. Calculated isotherm constants from the drawn isotherm curves for the HK and SHK were given in Table 3. As seen from the table, Langmuir equation fits well to the adsorption data for HK (with R2 of 0,9964). But when we consider the n value of Freundlich equation (which is N1) and correlation coefficient (0.9776) of Temkin equation, they are also seemed to be favorable. This condition can be evaluated as the supporting evidence to the presence of heterogeneous adsorption sites with more or less identical adsorption energies on the adsorbent HK. On the comparison of the Freundlich and Temkin models before (HK) and after activation (SHK) it is clearly seen that heterogeneous adsorption sites on the adsorbent HK are successfully converted to the homogeneous uniform identical sites on the adsorbent SHK. Therefore, adsorption data fitted best for the adsorbent SHK to the Langmuir equation with a high correlation coefficient of 1.000. All these results point out that adsorption of MB to the HK and SHK is monolayer and also the adsorption centers of them have identical energy according to the Langmuir model. Also, calculated values of KL (L mg-1) that is a constant related to the free energy of MB adsorption for the HK and SHK are 0.037 and 5.50 respectively. This high KL value of SHK indicates that SHK is more favorable than HK for MB adsorption [60]. Moreover, the Langmuir monolayer capacity values of the HK and SHK, Qm, were calculated as 49.5 mg/ g, 151.5 mg/ g respectively (Table 3). Activation with H2SO4 enhanced Langmuir capacity of the HK by ~306.1% due to obtaining a large number of additional adsorption sites (Table 1). As a result, activation of HK with sulfuric acid has positive influence on the capacity. In other words functional groups with higher adsorption efficiency have been attached to the surface by H2SO4 activation. Also, it is observed

that the adsorption capacity and dye removal efficiency of SHK for MB have high capacity and high dye efficiency when it compared to some adsorbents given in the literature except for SCS and PCS (Table 4). Considering the steps of preparing the activated carbon and its high cost, it could be said that SHK will be a suitable adsorbent for MB removal due to process simplicity and cheap cost. [60–65] 3.3.5. Effect of temperature The effect of temperature on adsorption capacity of HK and SHK were studied at 25 °C, 35 °C, 45 °C and 60 °C for different initial concentration of the MB and results were illustrated in Fig. 14. The adsorption capacity of both HK and SHK increased with increasing temperature in all concentrations studied, except 100 and 200 mg/L for SHK. But the adsorption capacity of SHK has not been affected by increasing the temperature for 100 and 200 mg/L MB due to high adsorption capacity (Fig.13). This indicates that adsorption process of MB onto the HK and SHK is endothermic. Rising temperature increases the surface activity of the HK and SHK as well as the mobility of the MB molecules and rate of intraparticle diffusion of MB and that resulted in increase in adsorption capacities of the HK and SHK [66,67]. The numerical values of the thermodynamic parameters are very important for applicability of adsorption process. Hence, Gibbs free energy change (ΔG°), Enthalpy change (ΔH°) and Entropy change (ΔS°) for adsorption of MB onto HK and SHK were calculated using the following equations and the calculated values were given in Table 5 [68]. ΔGo ¼ ΔH o −TxΔSo Kd ¼

qe Ce

ln K d ¼

ΔSo ΔHo − R RT

where Kd is equilibrium constant for adsorption, qe is the amount of the adsorbate adsorbed at equilibrium (mg/g), Ce is the equilibrium

Table 5 Thermodynamic parameters for MB adsorption onto HK and SHK. Conc. (ppm)

100 200 300 400

HK

SHK

ΔH (kJ/mol)

ΔS (kJ/molK)

28.85 21.81 22.23 19.30

0.092 0.061 0.060 0.049

ΔG (kJ/mol) 298 K

308 K

318 K

333 K

1.434 3.632 4.350 4.698

0.514 3.022 3.750 4.208

−0.406 2.412 3.150 3.718

−1.786 1.497 2.250 2.983

ΔH (kJ/mol)

ΔS (kJ/molK)

– – 177.7 70.6

– – 0.598 0.238

ΔG (kJ/mol) 298 K

308 K

318 K

333 K

– – −0.504 −0.324

– – −6.484 −2.704

– – −12.464 −5.084

– – −21.434 −8.654

10

Y. Akköz et al. / Journal of Molecular Liquids 287 (2019) 110988 Table 6 pH change during adsorption studies in different pHs. pHinitial

pHfinal

2.32 3.89 6.05 7.97 10.00

2.14 2.53 2.56 2.54 2.57

concentration of adsorbate in the solution (mg/L), R is ideal gas constant (8.314 J/mol K) and T is the temperature (K). As seen from the table, the Gibbs free energy change value, (ΔG°), in the adsorption of MB to HK is negative for only 100 mg/L at 45 °C and 60 °C but this values both 300 mg/L and also 400 mg/L for SHK is negative for all studied temperatures. Also, the spontaneity of the endothermic interactions of MB with SHK was influenced by increase in the temperature [68]. It shows that before activation the interaction between HK and MB is weak at low temperatures, but after activation the interaction between SHK and MB is effective at low temperatures as a result of the contribution of functional groups with high adsorption efficiency. In briefly, these result show that activation make the surface of HK more energetic. The enthalpy values (ΔH°) of the adsorption of MB in both HK and SHK are positive. This indicates that the adsorption nature of MB is endothermic for both the adsorbents [69]. Also, the enthalpy values for both adsorbents decreased with increasing concentration. It indicates that the interaction between adsorbed molecules is weak, this means to compatible with the Langmuir model. Entropy change values for the HK and the SHK were found positive for all concentration studied in the temperature range of 298 K–333 K. It shows that the randomness increased at adsorbate/adsorbent interface in both the adsorbents [70]. 4. Adsorption mechanism of MB onto the SHK To clarify the adsorption mechanism of MB onto the SHK, initial and final pH of medium were measured before and after adsorption and given in Table 6. As seen from the table, final pH value of adsorption medium decreased in of all the studied pH. This might be attributed to the formation of ion exchange between SHK acidic groups (\\COOH, \\SO3H and \\OH) and methylene blue [63], where H+ would be released during the interaction, hence the medium pH decrease. As a result it could be said that adsorption mechanism of the MB adsorption onto the SHK is predominantly ion exchange mechanism besides the π-π and electrostatic interaction between MB and SHK. Suggested mechanism was given as schematic in Fig. 15. 5. Reusability of the SHK for MB adsorption It is important to be reusable of the adsorbents in terms of economic cost. Fig. 16 shows the reusability performance of SHK for 100 mg/L MB. When the figure is examined, it is seen that the dye removal efficiency of the SHK is 99.87% for the MB at the first usage. But this high value

Fig. 16. Reusability of the SHK (Ci = 100 mg/L; T = 25 °C; t = 6 h; V = 50 ml; pH = 10; m = 2 gL−1).

for the MB decreased to 98.81% at the end of the 5th usage of the SHK. After five times usage, a reduction of about 1% in dye removal efficiency indicates that the SHK has effective reusability for dyes such as MB. Considering the difficulty of regeneration and the high cost of activated carbon, the SHK could be used as an adsorbent to remove cationic dyes from aqueous solutions. 6. Conclusions In this study, preparation an efficient adsorbent that could be used to remove of cationic contaminants, especially MB, from aqueous solution was performed at low cost without needing for high temperature carbonization. It was seen that activation was effective important to obtain porous adsorbent and changed the surface chemical structure of the HK that was confirmed with FTIR, SEM-EDX, XRD, BET and pHpzc methods. Moreover acid activation enhanced the thermal stability of the HK. While HK carried out only 80% removal of 100 mg/L MB with high adsorbent amount (5 g/L), SHK was able to remove almost 100% with much lower adsorbent amount (2 g/L). Adsorption kinetic results and equilibrium data showed that the adsorption of MB onto both HK and SHK better fit to the pseudo-second-order kinetic model and more suitable for the Langmuir Isotherm model. Furthermore, activation was also significantly changed the adsorption capacities (calculated as 49.5 and 151.5 mg/g for the HK and SHK respectively). Calculated enthalpy values showed that MB adsorption onto both adsorbent was endothermic. From the entropy values, it was determined that the adsorption of MB onto HK was spontaneous in low dye concentration and at high temperatures but adsorption onto SHK was spontaneous at both low temperatures and high concentrations. As results, SHK prepared with

Fig. 15. Suggested mechanism for MB adsorption onto SHK.

Y. Akköz et al. / Journal of Molecular Liquids 287 (2019) 110988

simple method could be used as an efficient adsorbent with its superiority properties such as using in wide pH range, reusability and high adsorption capacity for MB removal. Declaration of Competing Interest There are no conflicts to declare. Acknowledgements The authors gratefully thank the financial support provided by the Scientific Research Projects Unit of Yozgat Bozok University (Grant no. 6602c-FEF/17-118). The authors would like to thank Assoc. Prof. Dr. Nuri ÜNLÜ for the critical reading of the paper and his valuable contribution. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.molliq.2019.110988. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]

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