Honeycomb-like porous activated carbon for efficient copper (II) adsorption synthesized from natural source: Kinetic study and equilibrium isotherm analysis

Journal of Environmental Chemical Engineering 7 (2019) 103236

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Honeycomb-like porous activated carbon for efficient copper (II) adsorption synthesized from natural source: Kinetic study and equilibrium isotherm analysis Somen Mondal, Subrata Kumar Majumder

T

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Department of Chemical Engineering, Indian Institute of Technology Guwahati, Guwahati, Assam 781039, India

ARTICLE INFO

ABSTRACT

Keywords: Activated carbon Adsorption kinetics Adsorption isotherms Characterization

The preparation of cost-effective potentially efficient adsorbents of metal pollutants by activated carbons from the natural sources is the challenging research to the scientific community. In the present study, the characteristic features of highly potential activated carbon synthesized from various natural sources are reported. Among the synthesized activated carbons, the best one by its surface area is applied for the analysis of its kinetics and isotherms for copper (II) adsorption. Four different seeds and three leaves were used as the precursors in a chemical activation (three-step) process composed of the impregnation, carbonization and subsequent activation to produce the activated carbons. As per present investigation, Indian gooseberry seeds mixed with their shells showed the highest BET surface area of 1360 m2/g. The maximum adsorptivity of the activated carbon was found to be 66.79 mg/g at 60 °C. The adsorption followed pseudo-second-order and Elovich model kinetics with the Langmuir and Dubinin–Radushkevich (D–R) adsorption isotherms and was spontaneous and endothermic in nature. This AC could be used as a suitable cost-effective copper (II) adsorbent in water purification processes. Moreover, the ACs could be reused after regeneration with high removal efficiency.

1. Introduction

[11], hazelnut shell [12], cassava peel [13], date stones [14,15], grape bagasse [16], pecan shell [17], chestnut shell and grape seeds [18], hazelnut husks [19], peanut shells [20], ceiba pentandra [21], apricot stone [22], rubber wood sawdust [23] and pinewood sawdust [24] were utilized as the copper adsorbent so far (Table 1). The present work aims to find out an effective precursor for the preparation of AC, to be utilized as a useful and value-added copper (II) adsorbent. The four seeds such as, Indian gooseberry (Phyllanthus emblica) seeds with its shells, harde whole (Terminalia chebula) seeds, jackfruit (Artocarpus heterophyllus) seeds, betel (Areca catechu) nut and three leaves like boiled tejpatta (Cinnamomum tamala), Indian rubber (Ficus elastica) leaves and extracted mint leaves (Mentha leaves) were used to make the ACs by chemical activation, which was not used earlier (as per available literature) for the copper (II) adsorption processes and these three leaves were not utilized previously for the ACs preparation. The best AC according to the BET surface area and pore volume (gooseberry seeds and their shells AC, GB10) was characterized (by BET-N2 adsorption, X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), Fourier-transform infrared spectroscopy (FT-IR), Raman spectroscopy, thermo gravimetric analysis (TGA) and energy-dispersive X-ray spectroscopy (EDS)) and used to remove the

Copper is a harmful water contaminant after a specific limit, released from the metallurgical and electrical industries [1,2]. There are several methods for the dismissal of copper from the contaminated water. Adsorption is one of the most efficient processes among them. Recently, making of new cost-effective and efficient adsorbent for the dismissal of various harmful contaminants from the water resources is a challenge to the researchers [3]. Activated carbons (ACs) are extensively used as an efficient and effective adsorbent for the water purification. It has high surface area and high volume of pores with a well-developed internal structure. The physical and chemical properties of the ACs are depended mainly on the characteristics of the precursor materials [4]. The researchers from several decades have been tried to find out the appropriate precursor materials for the synthesis of the ACs for its utilization as an adsorbent of pollutants. In brief, the gooseberry seeds [5], harde wholes [6] and harde whole seeds [7], jackfruit seeds [8] and betel nuts [9,10] were used as the precursor materials for the production of the ACs. The ACs derived from the precursors prescribed herewith, their BET (Brunauer–Emmett–Teller) surface area and utilization are tabulated in Table S1. The ACs prepared from sewage sludge

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Corresponding author. E-mail address: [email protected] (S.K. Majumder).

https://doi.org/10.1016/j.jece.2019.103236 Received 3 April 2019; Received in revised form 1 June 2019; Accepted 23 June 2019 Available online 04 July 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.

Journal of Environmental Chemical Engineering 7 (2019) 103236

S. Mondal and S.K. Majumder

Nomenclature ae b B B′ be C CA Ce E ΔG0 ΔH0 KC k1 k2 kf

kp

initial adsorption rate of Elovich equation (mg/g min) Langmuir's equilibrium constant (l/mg) Temkin adsorption isotherm constant (related to the heat of adsorption) Dubinin–Radushkevich (D–R) model constant (mol2/kJ2) extent of surface coverage for chemisorption in Elovich equation (g/mg) intercept of intra-particle diffusion model solid phase equilibrium concentration (mg/l) solution concentration at equilibrium (mg/l or mol/g) mean free energy of adsorption (kJ/mol) Gibbs free energy change (kJ/mol) standard enthalpy change (kJ/mol) equilibrium constant of adsorption pseudo-first-order rate constant (1/min) pseudo-second-order rate constant (g/mg min) Freundlich's constant (related to adsorption capacity, mg1−1/nl1/n/g)

kt n N qcal qe qexp Qm qs qt R ΔS0 T t ε

rate constant of the intra-particle diffusion model (mg/ g min1/2) Temkin adsorption isotherm constant (equilibrium binding energy, l/mg) Freundlich's constant (related to adsorption intensity) number of data points equilibrium values obtained from the isotherm model (mg/g) adsorption capacity at equilibrium (mg/g or mol/g) equilibrium values obtained from the experiment (mg/g) maximum adsorption capacity (mg/g) Dubinin–Radushkevich (D–R) model constant or adsorption capacity (mol/g) adsorption capacity at time t (mg/g) universal gas constant (kJ/mol K) standard entropy change (kJ/mol K) temperature (K) time (min) Polanyi potential

Table 1 Activated carbons (ACs) used for the copper (II) removal. Adsorbents

Activating agents

pH

Qm (mg/g)

Time (min)

Fitted isotherm model

Fitted kinetic model

Reference

Sewage sludge-based AC Hazelnut shell AC Cassava peel AC Date stones AC

H3PO4 and ZnCl2 H2SO4 ZnCl2 H3PO4

5.0 6.0 5.0 5.0

1440 120 11520 480

Langmuir Langmuir Langmuir Langmuir

Pseudo-second-order Pseudo-second-order – Pseudo-second-order

[11] [12] [13] [14,15]

Grape bagasse AC Pecan shells AC Chestnut shell and Grape seeds AC Hazelnut husks AC

H3PO4 H3PO4 ZnCl2 ZnCl2

5.0 3.6 5.0 5.7

7.73 and 10.56 58.27 56.17 31.25 and 18.68 from Cu2+, Ni2+, Zn2+ mixture 43.47 95 98.04 and 32.15 6.65

180 4320 480 60

Pseudo-second-order – – –

[16] [17] [18] [19]

Peanut shells AC Ceiba pentandra hulls AC Apricot stone AC Rubber wood sawdust AC Pinewood sawdust AC Gooseberry seeds with its shells AC (GB10)

Steam Steam H2SO4 H3PO4 H3PO4 H3PO4

2.0–6.0 6.0 6.0 6.0 2–6 9.56

58.27 20.80 24.21 5.72 24.65 66.79

1440 80 2880 240 60 360

Langmuir, D–R Langmuir Freundlich Langmuir, Freundlich Langmuir Langmuir – Langmuir – Langmuir, D–R

– Pseudo-second-order – Pseudo-second-order Pseudo-second-order Pseudo-second-order, Elovich

[20] [21] [22] [23] [24] Present study

metal ions from the sulfate solutions by adsorption. The parametric effects of the initial pH, contact time and the temperature on the adsorption capacity were investigated. The adsorption isotherms, kinetics and the thermodynamics of the copper (II) adsorption onto the AC (GB10) were analyzed based on the experimental results. This study will be useful to the investigators to produce an efficient and cost-effective AC to get the remedy of the metal contaminants with the complete understanding of the metal ion adsorption kinetics and isotherms.

Indian Rubber leaves and Mint leaves. The sources were designated as GB, HW, JF, BN, BT, IR and ML respectively. The dried sample is first impregnated with the activating agent (50% diluted (volume/volume) H3PO4 (85%) solution at a ratio of 1:4 (precursor:H3PO4 solution, weight ratio) for 16–18 h at room temperature (30 °C)). The sample (impregnated) is then transferred to the crucible (made by silica) and pyrolyzed at 500 °C (heating rate was 10 °C/min) for 1 h in a furnace (Make: Ikon Instruments, Model: IKO-011). The pyrolysis was done in an air atmosphere. After pyrolysis, the ACs were specified as GB10W, HW10W, JF10W, BN10W, BT10W, IR10W and ML10W. In the final step, the pyrolyzed samples were activated again in the pure H3PO4 (85%) at 95 °C and were designated as GB10, HW10, JF10, BN10, BT10, IR10, and ML10. The prepared ACs were then washed several times with the milli-Q water to remove the unwanted loosely bound materials and extra acidity. Finally, the ACs were dried in an oven for 24 h and conserved for further experimentations. The AC samples were crashed and sieved before use to utilize a particular size range in the adsorption study. The screening sieves were used to measure the particle size, which ranges from 150 to 250 μm with an average of 200 μm.

2. Materials and methods 2.1. Preparation of activated carbons (ACs) ACs were synthesized from the various natural sources ensuring chemical activation process (three-step). The AC sources were collected from the local market and IIT Guwahati campus, cleaned by washing with the milli-Q water (produced by the Millipore Water Production Unit, Model: Elix-3, Make: Millipore), peeled out in case of fruit and then cuts into pieces and finally dried in an oven at 95 °C for 5 days. The sources were Indian gooseberry seeds combined with their shells, harde whole seeds, jackfruit seeds, Betel (Areca catechu) nut, boiled Tejpatta, 2

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2.2. Characterization of activated carbons (ACs)

488 nm monochromatic light from a Raman system (Make: Horiba, Model: LabRam).

2.2.1. Specific surface area and porosity The ACs surface area and porosity were examined by the N2 adsorption (at 77.3 K within the P/P0 range of 0.01–1.0) using the surface area analyzer (BET, Make: Micromeritics, Model: Tristar II). The ACs were heated at 110 °C for 24 h in an oven and the degassing process was continued for 3 h under vacuum at 150 °C prior to the measurements.

2.2.5. Surface morphology and compositional analysis The surface morphology and structure of GB10 (the best AC according to surface area) prepared from the Indian gooseberry seeds with their shells were examined with the FESEM images (Make: Zeiss, Model: Sigma). The components of GB10 prior and after the adsorption were examined by EDS.

2.2.2. Fourier-transform infrared spectroscopy (FT-IR) analysis FT-IR (Model: IRAffinity-1, Make: Shimadzu, Japan) was used to determine the functional groups present on the surface of the ACs by the infrared spectra transmission. KBr and 0.5% of AC samples were ground finely to form the pellets and used for the measurements. Before the tests, the KBr and the samples were dried for 24 h at a temperature of 110 °C. Each spectrogram was the average of 30 scans (wavelengths of the infrared spectra ranges from 400 to 4000 cm−1).

2.3. Adsorption studies The adsorption studies were done on the prepared AC with the laboratory grade copper sulfate solutions of various concentrations (40.08, 50.06, 60.10, 70.08, 80.04, 92.12 and 100 mg/l). The copper concentrations in the contaminated solutions before and after the experimentations were estimated by calibrating the Atomic Absorption Spectrophotometer (AAS) (Model: Spectra AA 220FS, Make: Varian) by using copper sulfate solution. The Copper (II) adsorptive removal capacity onto the prepared activated carbon was determined by the batch mode experiments. The influences of contact time and initial pH on the adsorption capacity of the AC (GB10) were investigated.

2.2.3. X-ray diffraction (XRD) analysis The scientific study of the crystal structures and the physical properties of the minerals of the AC samples were obtained using XRD (Model No.: D8 Advance, Make: Bruker, Netherlands) with Cu-Kα radiation of wavelength 1.5406 Å (40 kV, 40 mA) over the 2θ range 10–60° at a rate of 3°/min.

2.3.1. Impact of the initial pHs The consequences of the pHs on the adsorptive removal capacities were examined by varying pH using 0.04 g of ACs (at room temperature 30 °C the sample pH was 4.12). ACs were added to 50 ml of copper sulfate solutions (copper concentration 100 mg/l) with the initial pHs of

2.2.4. Raman spectroscopy analysis Raman spectroscopy is a useful technique to identify the molecules. The nature of the produced carbon was recognized by scattering a

Fig. 1. (a) N2 adsorption–desorption isotherms of the ACs from the seeds, (b) N2 adsorption–desorption isotherms of the ACs from the leaves, (c) BJH pore size distribution curves of the ACs from the seeds and (d) BJH pore size distribution curves of the ACs from the leaves. 3

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increase at P/P0 < 0.1, specifies the presence of the sufficient amount of micropores whereas a distinct hysteresis at the higher relative pressure of P/P0 = 0.4–1.0, suggests the presence of a huge number of mesopores [28]. The BET and Langmuir surface area of the prepared samples are shown in Table 2. The N2 adsorption-desorption isotherm results were satisfied by the BJH pore size distribution data obtained from Fig. 1(c) and (d). Among the three leaves, the highest pore size distribution range was obtained for BT10 (2.61–4.72 nm, Fig. 1(c)), whereas among the four seeds the highest pore size distribution was obtained for GB10 (2.84–7.28 nm, Fig. 1(d)), which were reflected in their BET surface area values (Table 2). It indicates that the AC prepared from the Indian gooseberry seeds along with their shells using H3PO4 (heating rate of 10 °C/min) acquired the highest BET surface area (1360 m2/g) with the highest mesopore volume (0.1333 cm3/g, difference between the total pore volume and micropore volume) and the both properties were enhanced after the second activation (Table 2). The best AC according to the highest BET surface area created amongst the prepared ACs was GB10, which was used for the copper (II) adsorption study in the present case.

3.04, 5.24 and 9.56. The 100 mg/l CuSO4 solution forms copper hydroxide [Cu(OH)2] precipitate at a pH range of 5.5–8.5, after that pH the precipitate dissolves and forms a blue color solution of soluble copper ammine complex [25]. The sample pHs were determined by a tabletop pH meter (Make: Equiptronics, Model: EQ610). The pHs were maintained by the addition of hydrochloric acid (HCl, 0.1 N) and ammonium hydroxide (NH4OH, 5%) solutions to the initial solution. 2.3.2. Impact of the temperature The copper (II) adsorption studies were performed at varying temperatures (30 °C, 40 °C, 50 °C and 60 °C) to study the impact of temperature on the adsorption process at a particular pH of 9.56 (at that pH value, the maximum adsorption capacity is obtained) for 5 h equilibrium time. 2.3.3. Kinetics and the equilibrium adsorption studies Experiments were performed in batch mode at room temperature with varying copper (II) concentrations (40.08, 50.06, 60.10, 70.08, 80.04, 92.12 and 100 mg/l) with the addition of 0.04 g of ACs into 50 ml contaminated solutions with the continuous agitation using the magnetic stirrers until the equilibrium reached. The tests were performed for different time intervals. The persisting copper (II) concentrations were measured by AAS to calculate the adsorption capacities of the ACs.

3.1.2. Fourier-transform infrared spectroscopy (FT-IR) analysis The functional groups present in the ACs were examined by the FTIR as shown in Fig. 2(a). As the precursor materials are different, the FT-IR spectrum of ACs showed peaks at different wavelengths. All the prepared AC samples showed peaks at the wavelength of 1525–1550 cm−1, which corresponds to the NeO stretching of the nitro compound. Peaks observed at 3200–3550 cm−1 for all the ACs can be allocated to the presence of strong OeH stretching of alcohol or the NeH stretching of the amine. HW10 and JF10 showed peaks at 3270–3310 cm−1, which can be allocated to the strong OeH stretching of the carboxylic acid group. The active C]O stretching of aldehyde or α,β-unsaturated ester or aliphatic ketone at the wavelength of 1705–1740 cm−1 was obtained for the ACs GB10, HW10, BT10, IR10, and ML10. The active NeH stretching of amine salt was observed for the samples GB10, HW10, IR10 and ML10 at the wavelength of 2800–3000 cm−1. The medium CeH stretching of alkane at 2840–3000 cm−1 (HW10), medium CeH stretching of aldehyde at 2800–2850 cm−1 (GB10, IR10 and ML10) and 2695–2830 cm−1 (BN10, BT10, IR10 and ML10), strong C-H bending of 1,2,4-trisubstituted at 860–900 cm−1 (BN10) and 1,2,3-trisubstituted at 680–720 cm−1 (BT10) were also identified. The peaks at 1266–1342 cm−1 (HW10, BN10, and BT10) and 1290–1372 cm−1 (HW10) indicated the presence of strong NeO bond stretching of aromatic amines and nitro compounds. The evidence of strong CeO stretching of ester (HW10), alkyl or aryl ether (GB10, JF10, BN10, and IR10), vinyl ether or primary alcohol (JF10, BN10, and ML10) were confirmed by the presence of

3. Results and discussion 3.1. Characterization of the prepared activated carbons (ACs) 3.1.1. Specific surface area and porosity The specific surface area and porosity of the ACs derived from the various natural sources after pyrolysis and after the secondary activation was examined with the help of N2 adsorption–desorption isotherms (Fig. 1). The adsorption isotherms help to estimate the preliminary statistics of the adsorptive removal mechanism and the porous nature of the prepared samples. The ACs prepared from the seeds (GB10, GB10W, HW10, HW10W, BN10, BN10W, JF10, and JF10W) revealed a type-I isotherm and H4 type of hysteresis (Fig. 1(a)). At relatively higher pressures, the invariant essence of the isotherms indicates the highly microporous materials with a limited pore size distribution [26] which can increase the adsorptive removal capacity at low relative pressures by increasing the energy of interaction between the solid faces and the gas-molecules [27]. ACs obtained from the leaves (BT10, BT10W, IR10, IR10W, ML10, and ML10W) showed the similar types of adsorption–desorption isotherms (type-IV) and the H3 type of hysteresis loops specifying the porous structure of the ACs (Fig. 1(b)). A sudden

Table 2 The BET and Langmuir surface area, pore distribution and average micropore diameters of the ACs. Activated carbons (H3PO4 activation at 500 °C at a rate of 10 °C/min)

Pore volume (cm3/g) Total pore volume

Micropore volume

GB10W GB10 HW10W HW10 JF10W JF10 BN10W BN10 BT10W BT10 IR10W IR10 ML10W ML10

0.2949 0.4252 0.3623 0.4467 0.3026 0.3163 0.2259 0.2976 0.2649 0.3601 0.0819 0.1691 0.1108 0.1853

0.2737 0.2919 0.3805 0.4389 0.1142 0.1283 0.1226 0.1636 0.0891 0.1572 0.0465 0.1139 0.0448 0.1042

4

BET average pore diameter (nm)

BET surface area (m2/g)

Langmuir surface area (m2/g)

1.4355 1.2505 1.5463 1.4127 1.1094 1.1191 1.1572 1.2028 1.1119 1.1684 1.1765 1.2722 1.0997 1.1881

821.99 ± 14.34 1359.98 ± 19.50 937.08 ± 18.46 1264.82 ± 23.89 1091.10 ± 9.37 1130.74 ± 10.47 780.99 ± 9.12 989.54 ± 11.43 953.07 ± 7.43 1232.79 ± 11.46 278.49 ± 2.99 531.64 ± 6.58 402.92 ± 3.54 624.04 ± 6.84

1232.80 ± 2.97 2135.05 ± 12.74 1389.59 ± 1.41 1873.01 ± 2.69 1966.33 ± 36.02 1995.23 ± 32.28 1280.39 ± 12.24 1653.96 ± 18.83 1728.95 ± 33.13 2181.90 ± 36.46 490.44 ± 8.41 895.87 ± 11.55 729.11 ± 13.97 1072.93 ± 15.23

Journal of Environmental Chemical Engineering 7 (2019) 103236

S. Mondal and S.K. Majumder

Fig. 2. Characterization of the ACs (a) FTIR spectrograms, (b) XRD diffractograms, and (c) Raman spectrograms.

peaks at 1250–1310 cm−1, 1200–1275 cm−1 and 1020–1070 cm−1. The peaks at 1020–1070 cm−1 or 1040–1050 cm−1 also corresponds to the presence of strong S]O bond stretching of sulfoxide and COeOeCO anhydride bond stretching. The medium C-N bond stretching of the amine compound can be represented by the wavelength of 1020–1250 cm−1 (IR10). The peaks found at 905–915 cm−1(GB10 and JF10) and 665–730 cm−1 (BT10) are due to the strong C]C bending of the mono-substituted or 1,2,3-tri-substituted alkenes. Peaks at 684–706 cm−1 correspond to the C]C bending of alkene (BT10) [29–31,54]. All the ACs prepared by H3PO4 activation, bands observed at 980 cm−1 and 1050 cm−1 which correspond to PeOeP and PeOeC bond stretching. Presence of phosphate functional groups on the surface of the ACs facilitates the metal ions adsorption in case of H3PO4 activation. The FTIR spectrogram is shown in Fig. 2(a) demonstrated the presence of the functional groups in the ACs which mainly controlled by the constituents present in the precursor for the same activating agent and process parameters.

diffractograms showed the formation of broad diffused peaks at around 2θ = 21–28° and 2θ = 42–48° for all the samples. It continues with the reflections from (0 0 2) plane and (1 0 0) plane respectively of graphitic carbon [32,33]. The presence of broad, diffused peaks shows the low crystallinity and amorphous nature of the tested samples as opposed to sharp peaks observed in highly crystalline substances. It occurs due to the relatively low pyrolysis temperature compared to that of graphite (> 2700 °C) [32]. The flatter peaks for (1 0 0) indicate honeycomb-like structure for sp2 hybridized carbon [34]. 3.1.4. Raman spectroscopy analysis The Raman spectroscopy is an important tool to draw the inference of the existence of defects or disorders. The Raman spectrograms showed that the ACs exhibited three significant peaks at 1306–1361 cm−1 (D-band), 1541–1578 cm−1 (G-band) and −1 2756–2842 cm (2D-band) (Fig. 2(c)). The ID/IG of 0.76–1.05 indicated the formation of the disordered carbons. The broad 2D-band in the presence of G-band is the proof of the existence of sp2 carbonized graphitic materials. IG/I2D ratios greater than one (IG/I2D = 1.39–4.52) are the proof of the formation of multilayer in the prepared samples. The best sample (GB10) was multilayered, amorphous and sp2-

3.1.3. X-ray diffraction (XRD) analysis The powder X-ray diffractograms of the prepared AC samples were collected between 2θ values of 10–60° as shown in Fig. 2(b). The 5

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S. Mondal and S.K. Majumder

hybridized graphitic carbon (ID/IG = 0.99 and IG/I2D = 3.34) as illustrated in Fig. 2(c).

was experimented with the varying pH values (3.04, 5.24 and 9.56) taking 100 mg/l copper (II) solutions. It was observed that with the increase in pH values from 3.04 to 9.56, an increment of the adsorptive removal capacities occurred from 18.73 mg/g to 44.85 mg/g (Fig. 6(a)). The solution pHs effect the solution chemistry along with the binding sites of the adsorbent. The adsorption rate decreased due to the decrease in the availability of the vacant sites with respect to time. In acidic solution, the Cu2+ ions must compete with the H+ ions for its adsorption onto the bonding sites. The smaller H+ ions were having, the higher mobility towards the adsorption sites reduces the Cu2+ adsorption in the acidic solution making the bonding sites positively charged. With the increase of pH values, the OH− ions increased and H+ ions decreased. Some functional groups like bonded PO43−, OH− ions can also make the surface of the AC negatively charged, which can increase the adsorptive removal of copper (II) in the primary range [11,42]. The copper amine complex (Cu(NH3)42+) formed at higher pH value (pH = 9.56), is also positively charged which facilitates its adsorption on the negatively charged AC surfaces, but only their velocities towards the adsorbent differ. Due to the bulkiness of Cu[NH3]42+ ions over the Cu2+ ions, the reaction kinetics become slower at the pH 9.56. As a result, the higher concentration gradient retained for a longer time resulting in higher adsorption capacities at the higher pH values. The activated carbons derived from the various natural sources by the researchers and their utilization as the copper (II) adsorbents at different pH values with their fitted kinetics and isotherms were tabulated in Table 1. The AC (GB10) obtained in the present experiment acquired the highest copper (II) adsorption capacity so far at 60 °C temperature.

3.1.5. Thermo-gravimetric (TG) and difference thermo-gravimetric (DTG) analyses The TGA and DTG curves of GB10 evidently stated the existence of three temperature zones during activation (Fig. 3). The initial weight loss of 4.09% at a temperature of 190 °C corresponds to the dehydration process [35,36]. The moisture contents obtained in the TGA analysis are mainly the bound moisture as the materials were heated at 100 °C for 24 h before performing the analysis. In the second step, the weight loss of 9.87% at a temperature of 610 °C is due to the decay of the volatile constituents [37]. The cellulose, hemicellulose, and lignin present in the precursor probably decomposed in the range of 190–610 °C [38]. Above the temperature 610 °C, the decomposition of the AC starts [39]. The total weight loss of the AC was 32.63% in the thermal degradation process. The DTG curves demonstrated the endothermic nature of the process (Fig. 3). The AC is not completely degraded at the temperature of 1080 °C which corresponds to the high thermal stability of the prepared sample in the inert nitrogen atmosphere (Fig. 3) [40,41]. 3.1.6. Surface morphology and composition analyses The FE-SEM photograph of GB1OW (Fig. 4(a) and (b)), the pyrolyzed AC without subsequent activation represented the existence of porous and non-uniform surface (honeycomb-like structure) due to the consequence of H3PO4. Pyrolysis at 500 °C in the presence of H3PO4, developed the pores with the removal of volatiles by the degradation of the cellulose, hemicelluloses and lignin compound from the precursor. During the subsequent activation with pure 85% H3PO4, the concentrated acid corroded the pores, the mesopore volume increased and the micropore volume decreased. Hence, the secondary AC surfaces contain the well-developed pores (Fig. 4(c) and (d)). Fig. 4(e) and (f) demonstrates the FE-SEM photograph of GB10 next to the adsorptive removal of copper. The AC sample was regenerated by 0.1 M HCl at room temperature (30 °C) with the continuous stirring for 2 h (Fig. 4(g) and (h)). EDS mapping of the GB10 next to the adsorption and desorption processes (Fig. 4(i) and (j)), indicated that some copper (II) retained on the surface of AC after the regeneration process which was represented by green dots. The compositions of the ACs (GB10W and GB10) were examined by EDS analyzer and shown in Fig. 5(a) and (b). GB10 showed the presence of higher and lower amount of copper in the samples when tested after the adsorption and desorption processes (Fig. 5(c) and (d)). Elemental analyses of the AC (GB10) are shown in Table 3. The fixed carbon percentage increases from 63.07 (GB10W) to 70.42 (GB10) after the subsequent activation which is also demonstrated by the EDS spectrum (Fig. 5(a) and (b)). The adsorption capacity of GB10, shown in Fig. 5(c), holds the maximum value of 73.00 mg/g. The true amount which depends on the operating variables was given in Section 3.2. The yield% of GB10W (before washing) was 87.34%, decreased to 46.56% in GB10 (after washing and drying). The second activation after pyrolysis improves the carbon content in the produced activated carbons (Fig. 5(a) and (b)). The recovery of previously adsorbed copper (II) from the AC by acidic solution left some little amount of copper (II) on it (5%) which was demonstrated by Fig. 5(d). More recovery is possible by increasing the temperature or acid concentration or regeneration time. The acid solution also causes some damages at the honey-comb structure of the AC (Fig. 4(g) and (h)).

3.2.2. Impact of the heat on adsorption Temperature plays a vital role in the adsorption processes. In general, with the increase of warmth after a certain limit the adsorption decreases. But, in the case of chemical adsorption, the rise in temperature increases the adsorption since the heat energy can chemically change the adsorbent and its adsorption sites and activity [43]. The heat energy may enhance the interaction of the adsorbate molecules with the active sites of the adsorbent. The better adsorption at higher temperature might also indicate the endothermic nature of the process. The present study demonstrated the effect of temperatures (30–60 °C) on the Cu (II) adsorption processes as shown in Fig. 6(b). Within temperature 30–40 °C, it is observed that the highest steepness of the straight line than the others. It indicated that the highest rate of adsorption was obtained at this temperature range. The increase of adsorption with the rise in temperature proved that the chemical adsorption was prominent there [43].

3.2. Adsorption studies

Fig. 3. Thermal (TG and DTG) analysis of the best AC (heating rate, 10 °C/min; protective N2 gas flow rate, 40 ml/min and normal N2 gas flow rate, 20 ml/ min).

3.2.1. Impact of the initial pH of the solution The impact of pHs on the adsorptive removal capacities of the AC 6

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Fig. 4. Surface morphology (a, b) FE-SEM image of GB10W, (c, d) FE-SEM image of GB10, (e, f) FE-SEM image of GB10 after copper (II) adsorption, (g, h) FE-SEM image of GB10 after copper (II) desorption and (i, j) EDS mapping of GB10 after copper (II) adsorption and desorption.

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Fig. 5. EDS spectrum of (a) GB10W, (b) GB10, (c) GB10 after the copper (II) adsorption and (d) GB10 after the copper (II) desorption.

were obtained from qt versus t1/2 plot (Fig. 7(c)). The non zero value of intercept C as shown in Table 4, reveals the fact that the adsorption process follows a complex mechanism not only the intra-particle diffusion. Elovich model is used to inspect the existence of chemisorption processes on the heterogeneous surfaces. The linearized Elovich model is represented as [46]:

Table 3 The elemental analysis of the AC (GB10). Elemental analysis (wt%) GB10W

GB10

C H O N S P

75.70 2.63 15.70 3.30 0.40 3.40

C H O N S P

79.30 1.66 16.70 2.19 0.10 3.60

qt =

qt ) = log qe

k1 t 2.303

ARE (%) =

(1)

(2)

where the pseudo-second-order rate constant (g/mg min) is denoted by k2. The k2 and qe values can be estimated from the t/qt versus t plot (Fig. 7(b)). Estimation of the kinetic model parameters (pseudo-secondorder) is disclosed in the supplementary material (Table S2). The intraparticle diffusion model is expressed as [14,45]:

where kp is the rate constant (mg/g min

N 1

qcal

qexp qexp

(5)

3.2.4. Adsorption isotherms Adsorption isotherms are very important to understand how the adsorbate ions or molecules interact with the binding sites of the adsorbent surfaces. Langmuir, Freundlich, Temkin and Dubinin–Radushkevich (D–R) models were studied in the present context. Monolayer adsorptive removal processes generally described by the Langmuir adsorption model is represented by [46]:

(3)

qt = kp t 1/2 + C

100 N

where N represents the number of data points. The equilibrium values calculated from the experiment and the models are designated as qexp and qcal (mg/g) respectively. The pseudo-second-order model and the Elovich model both showed the integrity of fit the experimental data as per the correlation coefficients of 0.998 and 0.997 respectively with the ARE% of 5.87 and 0.61. The experimental data that satisfied the Elovich model with the ARE% of only 0.61 signifies the chemical adsorption similar to the earlier study reported by the investigator [12].

where qe and qt are the amount of copper (II) adsorbed (mg/g) on the AC at equilibrium and at time t respectively, and k1 is the rate constant of the pseudo-first-order kinetic model and was obtained from the slope of log(qe − qt) versus t (min) plot (Fig. 7(a)). The pseudo-second-order kinetic model is expressed as [45]:

t 1 t = + qt qe k2 qe2

(4)

where the initial adsorption rate (mg/g min) and the extent of surface coverage by the chemical adsorption (g/mg) are represented by ae and be which were calculated from the qt versus ln t plot (Fig. 7(d)). A model calculation of Elovich model is shown in Table S3. The average relative errors (ARE%) were determined by Eq. (5), to find out the goodness of fit of the experimental and model data.

3.2.3. Adsorption kinetics The kinetics of the copper (II) adsorption process were enunciated with the help of pseudo-first-order, pseudo-second-order, intra-particle diffusion, and Elovich models. A linearized pseudo-first-order equation can be represented as [11,44]:

log(qe

ln(ae be) 1 + ln t be be

1/2

) and C is the intercept which 8

Journal of Environmental Chemical Engineering 7 (2019) 103236

S. Mondal and S.K. Majumder

qe = B ln kt + B ln Ce

(8)

where the Temkin adsorption isotherm constants kt (equilibrium binding energy, l/mg),B (associated to the heat of adsorption) were calculated from the intercept and slope of qe versus ln Ce plot respectively (Fig. 8(c)). Dubinin–Radushkevich (D–R) isotherm is widely used to examine the adsorption processes on the heterogeneous surfaces where the adsorption potentials are variable. The D–R model is represented by [49]:

ln qe = ln qs

2

B

(9)

where ε represents the Polanyi potential, is defined by

= RT ln 1 +

1 Ce

(10)

In Eq. (9), the adsorptive removal capacities (mol/g) and the equilibrium copper (II) concentrations (mol/g) are denoted by the terms qs and qe respectively. qs and B′ (mol2/kJ2), the model constants are obtained from the ln qe versus ε2 plot (Fig. 8(d)). Ce is the equilibrium concentration of copper (II) in solution (mol/l), R represents the gas constant (kJ/mol K), and T (K) represents the temperature (Eq. (10)). E, mean free energy required for adsorptive removal is obtained by [50]:

1

E=

The chemical adsorption process occurs when the value of E lies between 8 and 16 kJ/mol, and the value bellow 8 kJ/mol supported the physisorption process [16,51]. In the current context, the E value 20.41 kJ/mol at a pH of 9.56, corresponds to the particle diffusion dominated chemical adsorption process. Model calculation of D–R isotherm is shown in Table S5. The isotherm constants are arranged in tabular form in Table 5. The experimental data were satisfied well by the Langmuir isotherm (correlation coefficient 0.993 and ARE% 3.77). The Langmuir adsorption capacity (monolayer) was found to be 47.62 mg/g, close to the experimental data 44.85 mg/g. The value of n (Freundlich isotherm), greater than 1 (n = 8.05) proved the favorable adsorption of Cu2+ onto the heterogeneous surfaces of the prepared AC. It is seen that with increasing temperature the adsorptive removal capacity increases as shown in Fig. 6(b), indicated the endothermic nature of the adsorption process which can also be explained by the value of B′. The adsorption of Cu2+ onto the AC satisfies both Langmuir and Dubinin–Radushkevich adsorption isotherms. The Langmuir isotherm explained the monolayer adsorption of the Cu2+ ions on the homogeneous sites whereas the Dubinin–Radushkevich (D–R) isotherm furnishes the overall heterogeneous faces of the solid. Hence, the AC surfaces consist of heterogeneous sites, similar concerning the adsorption phenomena [52]. The Cu2+ adsorption capacities of the ACs prepared from the various sources are compared in Table 1. The AC prepared from the gooseberry seeds with their shells (GB10) can be used as the promising Cu2+ adsorbent at 60 °C temperature with the adsorption capacity of 66.79 mg/g.

Fig. 6. Parametric effects on the copper (II) adsorption (a) pH and time (Cu2+ concentration, 92.12 mg/l; volume of Cu2+ solution, 50 ml; AC dose, 0.04 g; pH, 3.04–9.56; temperature, 303 K and contact time, 30–360 min), and (b) temperature (Cu2+ concentration, 92.12 mg/l; volume of Cu2+ solution, 50 ml; AC dose, 0.04 g; pH, 9.56; temperature, 303–333 K and contact time, 360 min).

Ce 1 C = + e qe bQm Qm

(6)

The solution concentrations (mg/l) and the adsorptive copper removal capacities (mg/g) at equilibrium are denoted by the terms Ce and qe respectively. b (l/mg) is the adsorption equilibrium constant, and Qm represents the maximum adsorptivity (mg/g). The model coefficients, estimated from the plot of Ce/qe versus Ce (Fig. 8(a)). A sample calculation of Langmuir isotherm model is shown in the supplementary material (Table S4). The Freundlich isotherm (used for the heterogeneous systems), is represented as [47]:

log qe = log kf +

1 log Ce n

(11)

2B

3.2.5. Thermodynamic parameters The Gibbs free energy change (ΔG0), standard enthalpy change (ΔH0) and standard entropy change (ΔS0) were investigated to postulate the adsorption mechanism. The thermodynamic parameters can be estimated by the following equations [53].

(7)

where kf and n represent the adsorption capacity and adsorption intensity, can be calculated from the log qe versus log Ce plot (Fig. 8(b)). The Temkin model represents the molecular interactions, associated with the following assumptions: (a) with the increase of surface coverage the heat of adsorption decreases linearly and (b) the binding energy of the adsorption sites are uniformly distributed. The Temkin adsorption isotherm is expressed as [48]:

KC =

CA Ce

G0 = ln K C = 9

(12)

RT ln K C S0 R

H0 RT

(13) (14)

Journal of Environmental Chemical Engineering 7 (2019) 103236

S. Mondal and S.K. Majumder

Fig. 7. Copper (II) adsorption kinetics: (a) pseudo-first-order model, (b) pseudo-second-order model, (c) intra-particle diffusion model and (d) Elovich model (Cu2+ concentration, 92.12 mg/l; volume of Cu2+ solution, 50 ml; AC dose, 0.04 g; pH, 9.56; temperature, 303 K and contact time, 30–360 min).

solution concentration, time and temperature. The recovered AC was used again for the adsorption with 0.05 g carbon stirred in 50 ml CuSO4 solution (92.12 mg/l) of pH 9.56 for 6 h and observed that the adsorption capacity was decreased by only 1.55%. Hence the prepared adsorbent AC can be reused successfully after desorption or regeneration process.

Table 4 Kinetic parameters for the copper (II) adsorption onto the AC (Cu2+ concentration, 92.12 mg/l; volume of Cu2+ solution, 50 ml; AC dose, 0.04 g; pH, 9.56; temperature, 303 K and contact time, 30–360 min). Kinetic models

Kinetic parameters

R2

ARE%

Pseudo-first-order Pseudo-second-order Intra-particle diffusion Elovich

k1 = 0.0147 k2 = 0.0008 kp = 1.879 ae = 7.80

0.978 0.998 0.972 0.997

50.29 5.87 2.13 0.61

qe = 27.34 qe = 48.54 C = 18.41 be = 0.12

4. Conclusions In the present study, the best surface area (1360 m2/g with the total pore volume 0.4252 cm3/g) AC was prepared from the gooseberry seeds with their shells. Batch adsorption studies of the copper (II) onto the AC, performed at 60 °C and a pH value of 9.56 resulted in the maximum copper (II) adsorption capacity of 66.79 mg/g. The experimental data were better described by the pseudo-second-order and the Elovich models with the correlation coefficients of R2 = 0.998 and 0.997, and with the ARE% of 5.87 and 0.61 respectively, which suggested the chemical adsorption. The chemical adsorption along with the physical adsorption onto the prepared activated carbon was well satisfied by the Langmuir and Dubinin–Radushkevich (D–R) adsorption models. The chemically preferred, spontaneous and endothermic type of adsorption onto the prepared AC was demonstrated by the thermodynamic parameters ΔG0, ΔH0 and ΔS0.The regeneration data showed that the AC could be reused with the loss of only 1.55% adsorption capacity. The results stated that the AC synthesized from the gooseberry seeds with their shells may be used significantly as a cost-effective, efficient and reusable copper (II) adsorbent from the contaminated basic solutions.

KC, CA, and Ce are the equilibrium constant, solid phase equilibrium concentration (mg/l) and liquid phase equilibrium concentration (mg/ l) respectively. The values of ΔH0 and ΔS0 were calculated from the ln KC versus 1/T plot (Van’t Hoff plot) (Fig. 9). The thermodynamic parameters for the copper (II) adsorption onto the AC (GB10) are tabulated in Table 6. The negative values of ΔG0 demonstrated the spontaneous nature of the process which was favored by the temperature. The positive values of ΔH0 and ΔS0 provide the shreds of evidence of the endothermic adsorption process and the increase of randomness with increasing the temperature as reported earlier for the adsorption of copper (II) onto the ACs [11,14]. The value of ΔH0 less than 40 corresponds mainly to the multilayered physical adsorption process. 3.2.6. Desorption or regeneration studies Desorption or regeneration study is important to recover the adsorbed metals and for the reuse of the adsorbent. After the adsorption for 6 h, the AC was washed and dried. 0.04 g of AC was then stirred for 2 h in 50 ml 0.1 M HCl solution at room temperature (30 °C), and the resultant solution was analyzed by AAS to obtain the copper (II) concentration. It was observed that experimentally 98.44% of the adsorbed metals were recovered in 2 h (93.15% as shown in Fig. 5(c) and (d)). The recovery percentage may be increased with the increase of HCl

Conflict of interests None declared.

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Journal of Environmental Chemical Engineering 7 (2019) 103236

S. Mondal and S.K. Majumder

Fig. 8. Adsorption isotherms: (a) Langmuir, (b) Freundlich, (c) Temkin and (d) Dubinin–Radushkevich (D–R) (Cu2+ concentration, 40.08, 50.06, 60.10, 70.08, 80.04 and 92.12 mg/l; volume of Cu2+ solution, 50 ml; AC dose, 0.04 g; pH, 9.56; temperature, 303 K and contact time, 360 min). Table 5 Adsorption constants for the adsorption of Cu2+onto the AC (Cu2+ concentration, 40.08, 50.06, 60.10, 70.08, 80.04 and 92.12 mg/l; volume of Cu2+ solution, 50 ml; AC dose, 0.04 g; pH, 9.56; temperature, 303 K and contact time, 360 min). Isotherms

Temperature (K)

Constants

Langmuir Freundlich Temkin D–R

303 303 303 303

Qm = 47.62 kf = 26.33 B = 4.98 qs = 0.0010

2

b = 0.197 n = 8.05 Kt = 108.11 B′ = 0.0019

R

ARE%

0.993 0.871 0.850 0.847

3.77 0.59 2.34 2.52

Table 6 Thermodynamic parameters for the copper (II) adsorption onto the AC (GB10) (Cu2+ concentration, 92.12 mg/l; volume of Cu2+ solution, 50 ml; AC dose, 0.04 g; pH, 9.56; temperature, 303–333 K and contact time, 360 min). Temperature, T (K)

ΔG0 (kJ/mol)

ΔH0 (kJ/mol)

ΔS0 (kJ/mol K)

303 313 323 333

−0.0308 −0.2964 −0.4711 −0.8937

10.32

0.035

Acknowledgements The authors are grateful to the analytical laboratory, Department of Chemical Engineering and Central Instrument Facility (CIF), Indian Institute of Technology Guwahati for the assistance and support to perform the necessary analyses. The authors would like to thank Mr. Saptarshi Sengupta, Jadavpur University for his initial help during the ACs preparation. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jece.2019.103236. References [1] E.R. Parker, Metallurgical problems arising from the use of copper in electric equipment, J. Electr. Eng. 62 (12) (1943) 531–534. [2] S.G. Temple, Recent developments in properties and protection of copper for electrical uses, Metall. Rev. 11 (1) (1966) 47–60. [3] L. Aljerf, High-efficiency extraction of bromocresol purple dye and heavy metals as chromium from industrial effluent by adsorption onto a modified surface of zeolite: kinetics and equilibrium study, J. Environ. Manage. 225 (2018) 120–132. [4] H. Liu, J. Zhang, C. Zhang, N. Bao, C. Cheng, Activated carbons with well-developed

Fig. 9. Van’t Hoff plot of copper (II) adsorption equilibrium constant (Cu2+ concentration, 92.12 mg/l; volume of Cu2+ solution, 50 ml; AC dose, 0.04 g; pH, 9.56; temperature, 303–333 K and contact time, 360 min).

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