Removal of Pb(II) ions from aqueous solutions by

Journal of the Taiwan Institute of Chemical Engineers 58 (2016) 264–273

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Removal of Pb(II) ions from aqueous solutions by sulphuric acid-treated palm tree leaves Ahmed M. Soliman a,b,∗, Hanan M. Elwy c, Thies Thiemann a, Yasamin Majedi d, Felix T. Labata e, Nathir A.F. Al-Rawashdeh a,f a

Department of Chemistry, Faculty of Science, United Arab Emirates University, P.O. Box 15551, Al Ain, UAE Hot Laboratory Center, Atomic Energy Authority, Cairo, P.O. Box 13759, Egypt National Organization of Drug Control and Research, 6 Abu Hazem St., Giza, Egypt d Department of Chemical and Petroleum, Faculty of Engineering, United Arab Emirates University, Al Ain, UAE e Department of Arid Agriculture, Faculty of Food and Agriculture, United Arab Emirates University, Al Ain, UAE f Department of Chemistry, Jordan University of Science and Technology, Irbid, Jordan b c

a r t i c l e

i n f o

Article history: Received 5 January 2015 Revised 17 May 2015 Accepted 24 May 2015 Available online 20 June 2015 Keywords: Adsorption Activated carbon Lead Palm tree leaves

a b s t r a c t The adsorption of Pb(II) ions on chemically activated carbon derived from palm tree leaves was investigated. The activated carbon (AC) was characterized by Infrared absorption spectroscopy (FTIR), Brunauer–Emmett– Teller (BET) method and Scanning Electron Microscopic (SEM) analysis. The surface area (SBET ), pore volume, and pore diameter of AC were determined by adsorption isotherms for N2 . The effect of pH, equilibrated contact time, initial concentration of Pb(II), adsorbent dosage, and temperature were investigated. The maximum uptake of lead ion was obtained at pH 5.5 ± 0.3. The adsorption equilibrium was established after 180 min. The adsorption of lead ions by AC increased with its initial concentration in the medium, and reached a maximum at an approx. concentration of 100 mg Pb L−1 . Pseudo-first order and pseudo-second order kinetic models were applied to study the kinetics of the adsorption process. The pseudo-second order kinetic model provided the best correlation (R2 > 0.9981) with the experimental data. It was found that the adsorption of Pb(II) on AC correlated better with the Langmuir than with the Freundlich isotherm equation in the concentration range studied. The study indicates that date palm derived AC could be used as an efficient adsorbent for the removal of lead ions from aqueous solutions. A maximum removal efficiency of 98.6% Pb(II) was obtained at a 4 g L−1 solid-to-liquid ratio and an initial heavy metal concentration of 50 mg L−1 . © 2015 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction Sources of water pollution of all kinds (industrial and agricultural wastes, sewage, etc.) present a grave concern for public health. Toxic components, such as heavy metals, are of special concern because of their potential for long-term accumulation in soils and sediments. Unlike other pollutants, heavy metals are not biodegradable and can accumulate in the environment and food chain, creating significant health risks. High levels of exposure to heavy metals have been known to cause cancer, organ damage, joint diseases, and in extreme cases, death [1–3]. As one of the toxic metals, lead is one of the major environmental pollutants present in drinking water and in air [3]. The main source of environmental lead pollution stems from the long use of leaded gasoline, where higher concentrations of lead ∗ Corresponding author at: Department of Chemistry, Faculty of Science, United Arab Emirates University, P.O. Box 15551, Al Ain, UAE. Tel.: 00971-50-330-4356. E-mail address: [email protected], [email protected] (A.M. Soliman).

can still be found in soil and plant matter along the roadside, even in countries, where leaded gasoline has been replaced many years ago [4]. The lead can eventually migrate in the soil and enter watercourses [5]. Industrial effluents from metal plating and finishing, acid battery manufacturing, printing, lead mining, ceramics, and glass industries also contain lead [2,3]. Consequently, various technologies have been developed to remove lead from wastewaters. Well-known traditional technologies include membrane filtration and reverse osmosis [6,7], chemical precipitation [8], ion exchange [7,9], flotation and electrochemical methods [7,10], coagulation-flocculation [7], and adsorption [11,12]. All of these technologies have advantages and limitations. They are either low-cost but not very effective as they cannot remove trace levels of metal ions, or effective in that they can lower lead concentration to parts per million levels, but costly. Metal ion adsorption is quite promising due to its high efficiency, easy handling, the availability of different adsorbents and its cost effectiveness. Recent research has shown that it is possible to use agricultural wastes or byproducts like spent grain and peanut shells as an

http://dx.doi.org/10.1016/j.jtice.2015.05.035 1876-1070/© 2015 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

A.M. Soliman et al. / Journal of the Taiwan Institute of Chemical Engineers 58 (2016) 264–273

absorbent for heavy metal removal [13,14]. The sorption properties of these materials are due to the presence of functional groups such as hydroxyl, carboxyl and amino groups, which have a high affinity for metal ions [15]. This technology has great potential as many agricultural by-products are easily available and can be obtained at low cost. However, the adsorption capacity of agricultural by-products is generally low compared to ion exchange resins. Adsorption utilizing activated carbons appear to be the most widely used industrial adsorption method for the removal of pollutants in the industry. Several studies have reported on the use of this method for the adsorption of metal ions from wastewaters. These activated carbons have uniquely powerful adsorption properties, and it is possible to readily modify their surface groups [16]. The surface groups can be produced or modified by oxidation with air oxygen or with liquid oxidants (such as acids) before or during the carbonization process [16]. The majority of recent adsorption studies for the removal of lead ions from aqueous solutions were carried out with activated carbon produced from different low cost sources. The most common materials used for the preparation of activated carbon are wood, coal, lignin, petroleum coke, and polymers [17]. Recently, the preparation of activated carbons from agricultural wastes or byproducts has been given serious attention due to the growing interest in low-cost activated carbons from renewable biomass, especially for applications concerning the treatment of wastewater [18–22]. These agricultural wastes impact negatively on the environment because of an indiscriminate disposal of such wastes. Hence, producing activated carbon from these wastes is an alternative method of waste reduction and reuse. Conventional technologies are not able to provide low-cost and effective removal of heavy metals from wastewater. Adsorption processes using agricultural by-products can potentially provide a low cost solution, but the adsorption capacity of these materials needs to be improved. Therefore, there is a need for a technology that provides an adsorption process using agricultural wastes or by-products with high adsorption capacity. One such recently recognized agricultural waste-product that can be converted to activated carbon is the date palm leaf [23–25]. The Arabian peninsula as an arid region has limited sources of thus far non-used agricultural wastes. One such non-used agricultural waste are date palm leaves. Date palms belong to the family Palmae, of genus Phoenix, species dactylifera. Mainly, they are grown in the Gulf States and on the Mediterranean coast. Phoenix dactylifera has been cultivated since prehistoric times in most of the Gulf, SouthAsian and African countries. There are currently over 44 million date palms in the United Arab Emirates (UAE) that can be grouped into 199 different varieties, of which 8.5 million are in the Al Ain region. Together, they produce ∼1 million tons of palm leaves annually, as agricultural waste. The common practice to get rid of these wastes is by burning, which causes a potential pollution impact on the environment. The date palm leaves (Phoenix dactylifera) as plant material possess cellulose (33.5%), hemicellulose (26%) and lignin (27%), as main components [9,26]. Concentrated sulphuric acid (H2 SO4 ) is a powerful dehydrating agent, which has been frequently used as cleaning and de-ashing agent of activated carbon precursors [27]. Thus, the use of H2 SO4 for activating purposes may be advantageous as far as process costs (H2 SO4 is a low-cost chemical) and the chemical composition of activated carbon are concerned. Also, it may result in the preparation of activated carbon with different porous structures [19]. The primary aim of the present study is to investigate the adsorption properties of the activated carbon from sulphuric acid-treated date palm leaves, and its potential use to remove lead (II) ions from aqueous solutions. In this work, the effect of several operating parameters, such as contact time, initial concentration, particle size, adsorbent dose and pH were investigated. The adsorption equilibrium was analyzed using Langmuir and Freundlich models. Thermo-

265

dynamic and kinetic parameters were determined for adsorption of lead ions to explain the feasibility of the process. The results of the present study may provide useful data for future scale up using date palm leaves as a low-cost adsorbent for the removal of lead ions from aqueous solutions. 2. Materials and methods 2.1. Adsorbent Naturally collected date palm leaves from the United Arab Emirates University Campus at Maqam, Al Ain, Abu Dhabi, UAE were washed with deionized water, air-dried and cut into small pieces. The dried small pieces were soaked in 25% (w/w) H2 SO4 at room temperature (rt) for 24 h. The treated leaves were washed carefully with distilled water and dried at rt. Then, the leaf mass was transferred to an oven for carbonisation at different temperatures for 24 h. The activated carbon (AC) was allowed to cool to rt and repeatedly washed with deionized water, then with 1% aq. NaHCO3 until the pH of the supernatants remained constant at around pH 6.0. Then, the obtained activated carbon was dried at 105 ± 1 °C for 12 h to remove residual moisture, ground and sieved by standard steel meshs to select particles between 300 μm and 425 μm in size. The particles were sealed in polyethylene containers for further use. 2.2. Reagents All chemicals used were of analytical reagent grade. Deionized water (4 μS) was used in all experiments. A 25% (w/w) H2 SO4 solution was prepared by diluting appropriate amounts of conc. H2 SO4 with water. The stock solution of 1000 mg L−1 Pb(II) was prepared by dissolving appropriate amounts of Pb(NO3 )2 in 1 L of water and the concentrations of Pb(II) used in this study were obtained by dilution of the stock solution. The pHs of the solutions were adjusted to the required value by using 0.1 mol L−1 NaOH and/or 0.1 mol L−1 HCl solutions. 2.3. Methods Unless otherwise stated, the batch adsorption experiments were performed with 50 mL of solution in a capped plastic flask, shaken at 200 rpm, using a mechanical shaker equipped with a thermostatic water bath at 25 ± 1 °C, for a specified period of time. Samples were then filtered, and the Pb(II) concentration was evaluated by ethylenediaminetetraacetic acid (EDTA) titration. In a control experiment, to validate the titrimetric Pb-analysis, ICP-OES measurements were used for the quantification of the Pb-content. 2.3.1. Effect of the adsorbent dose Activated carbon doses of 0.1–50 g L−1 were added to the Pb(II) solution and the resulting suspensions were shaken for 48 h, at 25 ± 1 °C. Then, the samples were filtered, and the Pb(II) concentration in the filtrate was determined. 2.3.2. Effect of the contact time An activated carbon dose of 5 g L−1 was added to the Pb(II) solutions (100 mg Pb(II) L−1 ) and agitated for different contact times, ranging from 5 to 300 min at 25 ± 1 °C. At prescribed time intervals, the solutions were filtered, and the Pb(II) concentration in the filtrate determined. 2.3.3. Effect of pH The effect of pH on the amount of Pb(II) adsorbed onto activated carbon was investigated over the pH range of 1–7, thereby avoiding the precipitation of Pb(OH)2 under basic conditions [28]. The pH was adjusted using NaOH and/or HCl solutions. An activated carbon dose

266

A.M. Soliman et al. / Journal of the Taiwan Institute of Chemical Engineers 58 (2016) 264–273 Table 1 The weight loss of the samples upon activation at different temperatures. Sample

Activation temperature (°C)

Mass of AC before heating (g)

Mass of AC after heating

Loss percentage (%)

1 2 3 4 5

250 300 350 400 450

3.026 3.066 3.866 4.566 4.406

2.256 1.696 1.795 2.114 1.215

25.4 44.7 53.6 53.7 72.4

of 5 g L−1 was added to a series of flasks with Pb(II) containing solutions (100 ppm) at different pH values. The resulting suspensions were agitated for 3 h on a shaker at 25 ± 1 °C, prior to removal, filtration and analysis of Pb(II).

where Ci is the initial concentration of Pb(II) (mg L−1 ) and Ct is the concentration at time t (mg L−1 ).

2.3.4. Sorption studies Equilibrium experiments were performed on solutions with an initial Pb(II) concentration varying from 10 to 1000 ppm, where the solutions were gently shaken with 150 mg of activated carbon from palm leaves at temperatures of 29, 39, and 56 °C. After 3 h, the samples were filtered, and the filtrate was analyzed. Also, for the sorption experiment using a 100 ppm Pb(II) solution shaken with a carbon dose of 5 g L−1 , samples were taken at different time intervals. The results obtained from these experiments were utilized to evaluate the kinetic and thermodynamic parameters (G, H, and S), for the adsorption process using well-known equations, as shown in the results and discussion section. In a control experiment, a 50 ppm Pb(II) solution (125 mL) was treated at 25 °C with activated carbon (0.5 g = 4 g L−1 ), and samples were taken after 10 min., 20 min., 30 min., 1 h, and 1 h 30 min and 2 h, where the Pb content was analyzed by ICP-OES. In order to evaluate the possibility of the release of metal ions from activated leaves, a blank was run with the adsorbent in deionized water only. Only trace amounts (at negligible ppb levels) of metals were found in the blank.

3.1. Preparation of the activated carbon

2.4. Instrumentation The pH of the solutions was measured using a portable Orion420+A waterproof digital pH/CON meter. FTIR measurements were carried out using a Thermo Nicolet Nexus 870 FTIR instrument in the region of 400–4000 cm−1 . The samples were prepared as KBr pellets, comprised of a 1 mg sample of dried carbon material and 100 mg ground KBr. The surface area of the activated date palm derived carbon material, carbonized at 250 °C, was measured by nitrogen physisorption technique using Autosorb1 Quantachrome instrumentation. Before each measurement, the sample was degassed at 200 °C for at least 2 h. Also, the nitrogen adsorption/desorption isotherm was measured. The surface structure and morphology of the activated carbon were characterized using a scanning electron microscope (JEOL Jsm-5600 SEM) coupled with an Energy Dispersive X-ray Spectrometer (EDXS) to carry out elemental analyses. ICP-OES measurements were carried out with a Varian 710-ES instrument. 2.5. Data evaluation The equilibrium adsorption capacity (qe , mg g−1 ) of Pb(II) adsorbed on treated palm leaves was calculated, using the following:

qe =

(Ci − Ce )V w

(1)

where Ci and Ce are the initial and equilibrium concentrations of Pb(II) (mg L−1 ), respectively, V is the volume of the Pb(II) solution (L), and w is the weight (g) of the treated palm leaf adsorbent. The Pb(II) removal efficiency (R in %) is given by

R=

(Ci − Ct ) Ci

× 100%

(2)

3. Results and discussion

Initially, dried, H2 SO4 treated date palm leaves were activated for 24 h at different temperatures, at 250, 300, 350, 400 and 450 °C, respectively. The weight loss was measured (Table 1) and infrared spectra were taken for the individual carbons activated at different temperatures. Fig. 1 shows two representative infrared spectra, one for a carbon sample activated at 250 °C and the other for a carbon sample activated at 450 °C. The infrared of the sample activated at 250 °C shows a complex pattern, pointing to a complexity of functionalities on the surface and potentially within the material. Indicative for a carbonyl or most likely a carboxyl group is the absorption band at 1720 cm−1 , which would signify these functionalities to be present on the surface of the material. Peaks at 2919 and 2850 cm−1 suggest C–H, denoting that alkyl functions or an alkyl backbone is still present in the material. The infrared of the sample activated at 450 °C is devoid of the formerly discussed bands. Peaks at 673, 613, 595 cm−1 may indicate that aromatic units are still present within the material. This observation is in accord with the significant loss of carbon in the sample activated at 450 °C as compared to the sample activated at 250 °C. The carbon loss would most likely mean a loss of all surface carboxyl groups at 450 °C. The absorption band at 1721 cm−1 is less pronounced in carbon samples activated at 300 and 350 °C and is already totally absent with the carbon activated at 400 °C. Carbon activated at 500 °C is very light in color, indicating the samples to consist mostly of ash. The acidity/alkalinity of the carbon samples was measured upon adding 0.1 g of sample to deionized water (25 mL) and leaving at rest for 24 h. The acidity/alkalinity of the carbon samples was found to depend on the activation temperature, where with the alkalinity increases with an increase of activation temperature (pH 2.78 at 200 °C; pH 5.21 at 350 °C; and pH 8.00 at 500 °C). This would make sense as the functional groups created by the oxidative sulfuric treatment, as there are carboxylic acid, sulfonic acid and analogous groups, would be extruded at the higher temperatures. At the same time it may be that the inorganic components would also partially calcinate so that any carbonates would transform to the more basic oxides. Upon evaluation of the carbon samples, which took into account the carbon burn-off percentage and the presence of functional groups, the authors used date palm leaf derived carbon activated at 250 °C for the further studies described below. 3.1. Activated carbon characterization The FTIR spectra of the date palm leaf derived activated carbon and the Pb(II)-loaded AC are shown in Fig. 2. These spectra show a broad stretching vibrational band of hydroxyl (–OH) at about 3400 cm−1 , vibrational bands of aliphatic C–H at 2926 cm−1 , stretching vibrational band of carboxyl group (C=O) in carboxylic acid or quinone type structure at 1715 cm−1 , stretching vibrational bands of C=O (in –COO− ) or C=C centering at 1621 cm−1 , bending vibrational band of C–H at 1442 cm−1 [29]. Vibrational bands at 1105, 800, and


267

Fig. 1. FT-IR spectra of carbon activated at 250 °C (left) and at 450 °C (right).

20.0 4000

3500

3000

473.68

800

1105.26

25.0

2926.32

B 30.0

1115.79

2926.32 2852.63

3431.58

35.0

3421.05

%Transmittance

40.0

1715.79 1715.79 1621.05 1631.58 1442.10 1442.10

A

800 620

45.0

473.68

50.0

2500

2000

1500

1000

500

-1

Wavenumber (cm ) Fig. 2. (A) FT-IR spectra of lead(II)-loaded activated carbon and (B) activated carbon at 250 °C.

473 cm−1 are the normal vibration modes of the SO4 2− tetrahedral configuration [30]. Since the bands at 3400, 2926, and 1715 cm−1 are ascribable to ν (O–H), ν (C–H) and ν (C=O) vibrations, this suggests that H2 SO4 causes an oxidation of the date palm leaves. In contrast, the analysis of the Pb(II)-loaded FTIR spectrum shows some characteristic bands that can be assigned to the involvement of main functional groups in Pb(II) loading. When comparing Fig. 2A to 2B, one can see that binding of Pb(II) to the surface of the activated carbon produces a change in positions and intensities of some peaks. After Pb(II) loading, the stretching band at 3421 cm−1 (of –OH group) is shifted to 3431 cm−1 and becomes less broad. The stretching band at 1621 cm−1 (of C=O in the –COO− group) shifted to 1631 cm−1 . The vibrational band at 1105 cm−1 (indicative of the SO4 2− tetrahedral configuration) is shifted to 1115 cm−1 , and the peak intensity decreases and becomes broader. The shift in wavenumber corresponds to a change in energy of the functional group, suggesting the involvement of carboxyl, hydroxyl, and especially of the sulphate groups in the binding of Pb(II) and the formation of a complex [20].

The surface morphology was investigated by SEM and is shown in Fig. 3. It can be observed that the activated carbon particles (Fig. 3A and B) have a rough surface. At the magnification used, macropores can be seen that range from 1 – 12 μm in diameter. Magnification of a macropore shows it partially filled with carbon detritus and most likely some inorganic particles originating from the activation. In the upper wall of the magnified macropore the embedding of some mesopores can be discerned. This inclusion of mesopores and potentially also micropores is in accord with the mean pore diameter of 52 A˚ , found by BET measurements of the sample. In the SEM images of the lead(II)-loaded material (Fig. 3C and D), new formations of small particles and agglomerations were observed. The SEM images show that at least some of the lead adsorption has occurred at the surface of the AC. EDXS analyses of the activated carbon before and after Pb(II) adsorption has been carried out and it is shown in the supporting material (Figs. 1 and 2). The EDXS analysis of the carbon after the adsorption experiment shows the presence of different lead species on the carbon. Also, the EDXS analyses show sulphur content, which is incorporated during the activation with H2 SO4 acid. The adsorption isotherms for N2 at –196 °C were determined for the activated carbon. The surface area (SBET ), pore volume, and pore diameter of AC were found to be 64.12 m2 g−1 , 8.35 × 10−2 cm3 g−1 , ˚ respectively. This activated carbon contains micropores, and 52.1 A, a high percentage of micropores, mesopores and few macropores (62.25% micropore, 36.5% mesopores, and 1.25% macropores). Our study shows a higher surface area than that found for activated carbons prepared via the phosphoric acid treatment of date palm leaflets [20]. 3.2. Effect of adsorbent dose The extent of Pb(II) removal versus the adsorbent (AC) dosage in the range of 0.1–50 g L−1 is presented in Fig. 4, where the initial concentration of the Pb(II) solution was 100 ppm. From Fig. 4, one can observe that the removal efficiency of Pb(II) increases from 19.2% to 92.6% with the increase of the AC dosage from 0.1 to 5.0 g L−1 , while a further increase in AC dosage has little added effect on the removal efficiency of Pb(II). The removal efficiency of Pb(II) by AC after 50 min. achieves a peak value (92.6%) with an adsorbent dosage of 5.0 g L−1 , then levels out with a further increase of the dosage to 50 g L−1 . Also, Fig. 4 shows that the values of the adsorption capacity (qe ) decreases from 102 to 9.1 mg g−1 as the AC dosage is increased from 0.1 to 50 g L−1 . This might be due to more adsorption sites remaining unoccupied at a higher dosage of AC [20,31].

268


Fig. 3. SEM micrographs of unloaded (A and B) and lead(II)-loaded AC (C and D) (A: unloaded AC ×600, B: unloaded AC ×8000; C: loaded-lead(II) AC ×5500, and D: loaded-lead(II) AC ×8000).

110.0

100

80.0

80 60.0 60 40.0

e

40

-1

20.0

20

Removal efficiency (R in %)

120

Adsorption capacity (q , mg g )

Removal efficiency (R in %)

100.0

100.0

90.0

80.0

0

0.0 0

10

20

30

40

50

60

-1

Dose of AC (g L ) Fig. 4. Effect of dosage of activated carbon on Pb(II) adsorption at 25 ± 1 °C after 50 min, using an initial lead concentration of 100 ppm.

70.0 0

50

100

150

200

250

300

350

Time (min) Fig. 5. Effect of contact time on Pb(II) adsorption: AC dosage = 5 g L−1 , C0 = 100 ppm, at 25 ± 1 °C.

3.3. Effect of contact time Fig. 5 shows the effect of contact time on the removal efficiency of Pb(II) by AC, using 0.5 g L–1 AC with a 100 ppm Pb(II) solution. In Fig. 5, an extremely fast initial adsorption process during the first 30 min can be noted, followed by a longer period of much slower uptake. In this case, the optimal adsorption time that shows the maximum removal efficiency has been found to be 180 min with a Pb(II) removal efficiency of 98%. A second experiment was performed, where 0.4 g L–1 AC was used with a 50 ppm Pb(II) solution, with the remaining Pb(II) concentration being measured by ICP-OES. After 60 min., this led to 95.3% Pb(II) removal, after 90 min. to 96.9% Pb(II) removal, after 120 min. to 97.1% Pb(II) removal. Maximally, 98.6% Pb(II)

removal could be achieved, with a residual Pb(II) concentration of 0.68(5) ppm, which is very near to or below the recommended upper limit of Pb(II) of 0.4 ppm to 2.0 ppm for irrigation water in a number of developed countries [32,33] and lower than the former FAO recommended upper limit of Pb(II) of 5 mg L–1 for agricultural irrigation with treated wastewater [34–37], a value that is still often used as a benchmark for scientific analytical work. 3.4. Effect of pH on Pb(II) ions removal by AC The pH of a solution can have an effect on both the surface carbon of the adsorbent and the species of metal ion present in the solution.


180

269

100.0

-1

Adsorption capacity (q , mg g )

140

e

-1

Adsorption Capacity (q , mg g )

b 160 95.0

e

120

a c

100 80 60

90.0

85.0

80.0

40 20

75.0

0

1

2

3

4

5

6

7

8

0

pH

400

600

800

1000

-1

Initial concentration of Pb(II) (mg L )

Fig. 6. Effect of pH on the adsorption of Pb(II) ions by AC. AC dosage = 5 g L−1 , initial concentration of Pb(II) = 100 ppm, contact time: 3 h, at 25 ± 1 °C.

The hydrolysis and precipitation of metal ions in aqueous solutions is pH dependent. Therefore, the pH of the aqueous solution is one of the most important parameters affecting adsorption capacity. In the present study, the effect of pH values on the adsorption capacity of Pb(II) ions by sulfuric acid-treated palm leaves (AC) was investigated as shown in Fig. 6. To avoid the precipitation of Pb(OH)2 under basic conditions [28], the initial pH of the solutions was varied from pH 1 to pH 7 with an increment of 1.0 pH units. Fig. 6 shows that the adsorption capacity of AC is significantly enhanced with increasing pH up to pH 5.5, before starting to decrease at higher pH values. Since the optimum pH for adsorption of Pb(II) by AC was found to be 5.5, this pH was used for all further experiments in this study. The pH affects the adsorbate surface charge and the degree of ionization of the adsorbent speciation, consequently affecting the adsorption process of Pb(II) from water [20]. Under acidic conditions, the adsorbate surface will be completely covered with H+ ions and the Pb(II) ions cannot compete with them for adsorption sites. However, with increasing pH, the competition from the protons decreases and the positively charged Pb(II) ions can be adsorbed at the negatively charged sites on the adsorbent [38]. Under these circumstances, the adsorbing cations may be species such as Pb2+ , Pb(OH)+ , Pb(OH)2 , Pb(OH)3 − , and Pb(OH)4 2- [39]. At pH lower than pH 8, Pb(II) ions are the dominant species. Pb(OH)2 is present at pH higher than 8 [40]. Adsorption occurs at a comparatively low pH (∼6.0) for Pb(II) ions [41]. The general hydrolysis reaction of Pb(II) ions is given by

Pb2+ + 2nH2 O ⇔ Pb(OH )n 2−n + nH3 O+

200

(3)

where n depends on the pH value. To understand the mechanism of adsorption in terms of electrostatic attraction between the AC surface and the lead ions, the point of zero charge (pHzpc ) was determined (Fig. 3, supplementary materials). The pHpzc was determined to be at pH 3.5. At a pH lower than pH 3.5, the carbon surface is positively charged, thus the adsorption capacity is low. At a pH higher than pHpzc , the surface is negatively charged and thus the adsorption capacity increases reaching a maximum at pH 5.5, due to the electrostatic interaction with the positive lead species. 3.5. Effect of initial Pb(II) concentration and temperature on adsorption The effect of the initial concentration of Pb(II) ions in the range of 10 to 1000 ppm on the adsorption at different temperatures is shown

Fig. 7. Effect of initial concentration on the adsorption of Pb(II) ions by AC. Contact time is 3 h and pH = 5.5. At temperature: (a) 56 ± 1 °C, (b) 39 ± 1 °C, and (c) 29 ± 1 °C.

in Fig. 7. Fig. 7 shows that the adsorption equilibrium capacity of AC is significantly enhanced with an increase of the initial concentration of Pb(II) ions. Then, it reaches a plateau value at an approx. initial Pb(II) concentration of 100 ppm. Above this value, the adsorption equilibrium capacity does not significantly change with the initial concentration of Pb(II) ions. Furthermore, the results shown in Fig. 7 demonstrate that the adsorption capacity of Pb(II) ions slightly increases with an increase in temperature, indicating that the adsorption of Pb(II) ions onto AC is weakly endothermic in nature. 3.6. Adsorption isotherms To design adsorption systems, usually it is very crucial to analyze the equilibrium data to be able to construct adsorption isotherms. These isotherms provide information on the capacity of adsorbent, which is a needed parameter for an adsorption system. In the present study, the experimental adsorption data reported for the adsorption of Pb(II) by AC has been analyzed using the two most common adsorption models, which are the Langmuir and Freundlich adsorption isotherms. 3.6.1. Langmuir isotherm The Langmuir isotherm assumes a monolayer adsorption on a surface with a finite number of identical adsorption sites, and is used to determine the maximum adsorption capacity of an adsorbent, which is a very important parameter for an adsorption system [42]. The nonlinear Langmuir equation can be expressed as

qe =

qmax KCe 1 + KCe

(4)

The linear form of Langmuir equation can be rewritten as

Ce 1 Ce = + qe Kqmax qmax

(5)

where Ce (mg L−1 ) is the equilibrium concentration of the adsorbate in solution, qe (mg g−1 ) is the amount adsorbed per unit mass of adsorbent at equilibrium, qmax (mg g−1 ) is the Langmuir adsorption constant, representing the maximum adsorption capacity, K (L mg−1 ) is the Langmuir adsorption constant, which is related to the affinity of binding sites and hence the adsorption bonding energy. The practical limiting adsorption capacity, when the surface of the adsorbent is

270

A.M. Soliman et al. / Journal of the Taiwan Institute of Chemical Engineers 58 (2016) 264–273 Table 2 Langmuir and Freundlich constants for adsorption of Pb(II) ions on activated carbon. Temperature (°C)

Langmuir constants qmax

29 39 56

(mg g

−1

)

61.15 79.52 88.61

Freundlich constants

K (L mg−1 )

R2

Kf

n

R2

0.0113 0.0161 0.0168

0.99812 0.99606 0.99242

2.374 6.886 7.542

1.720 2.460 2.280

0.97045 0.99113 0.99337

RL

0.470 0.383 0.373

4

Data 34 4.5

a 3.5

a 4.0

b

-1

C /q (g L )

3

3.5

lnq

e

c

e

e

c

b

2.5

2

3.0

1.5

1

2.5

0.5 20

40

60

80

100

120

140

2.0 2.5

-1

C (mg L )

3

3.5

4

4.5

5

lnC

e

e

Fig. 8. The linearized Langmuir adsorption isotherms for Pb(II) adsorption on AC. AC dosage = 5 g L−1 , initial concentration of Pb(II) = 100 ppm, contact time: 3 h, pH = 5.5, Contact time: 3 h, at (a) 29, (b) 39, and (c) 56 ± 1 °C. Linear regression analysis equations: (a) y = 1.4414 + 0.016354x, R2 = 0.99812; (b) y = 0.77906 + 0.012575x, R2 = 0.99606; (c) y = 0.67185 + 0.011286x, R2 = 0.99242.

completely covered with adsorbate, is represented by qmax . Based on Eq. 5, a plot of Ce /qe versus Ce should be a straight line with a slope of 1/qmax and an intercept at 1/Kqmax . The experimental adsorption data reported for the adsorption of Pb(II) by AC have been fitted to the linear form of the Langmuir equation (Eq. 5), and the plot is depicted in Fig. 8. The Langmuir adsorption constants (qmax and K) and their correlation regression coefficients (R2 ), calculated from the plots in Fig. 8, are listed in Table 2. The regression correlation coefficients (R2 ), presented in Table 2, indicate that the adsorption equilibrium data for Pb(II) removal by AC fit well to the Langmuir model. The R2 values are higher than 0.992 for all studied temperatures. As seen from Table 2, the values of qmax and K increase with increasing temperature, indicating that the Pb(II) ions are favorably adsorbed by AC at higher temperatures and that the adsorption of Pb(II) ions by AC is endothermic. The essential quality of the Langmuir isotherm can be measured by calculating a dimensionless constant referred to as the separation factor, or equilibrium parameter (RL ) using

RL =

1 1 + KCo

(6)

where Co (mg L−1 ) is the initial concentration of adsorbate and K (L mg−1 ) is the Langmuir constant described above in Eqs. 4 and 5. The equilibrium parameter (RL ) has the following possibilities: 0 < RL < 1 for favorable adsorption, RL > 1 for unfavorable adsorption, RL = 1 for linear adsorption, RL = 0 for irreversible adsorption [43]. The values of RL have been calculated using Eq. 6 and are summarized in Table 2. As seen from Table 2, the values of RL are lying between 0 and 1, indicating a favorable adsorption.

Fig. 9. The linearized Freundlich adsorption isotherms for Pb(II) adsorption on AC. AC dosage = 5 g L−1 , initial concentration of Pb(II) = 100 ppm, contact time: 3 h, pH = 5.5, contact time: 3 h, at (a) 29, (b) 39, and (c) 56 ± 1 °C. Linear regression analysis equations: (a) y = 2.0205 + 0.43857x, R2 = 0.99337; (b) y = 1.9295 + 0.40647x, R2 = 0.99113; (c) y = 0.8646 + 0.58131x, R2 = 0.97045.

3.6.2. Freundlich isotherm The multi-side adsorption isotherm for heterogeneous surfaces is fully described by the empirical Freundlich adsorption isotherm, which is expressed by

qe = K f Ce1/n

(7)

where Kf and n are the Freundlich constants related to the adsorption capacity and adsorption intensity, respectively. The linear form of Eq. 6 is given as follows:

ln qe = ln K f +

1 ln Ce n

(8)

The experimental adsorption data was fitted to the linear form of the Freundlich isotherm (Eq. 8) for Pb(II) ions adsorbed on AC and is shown in Fig. 9. The Freundlich adsorption constants (Kf and n) and their correlation regression coefficients (R2 ) calculated from the plots in Fig. 9 are listed in Table 2. The correlation coefficients determined for the Freundlich model are relatively lower as compared to the Langmuir model indicating that the data are not found in as good agreement as for the Langmuir model. The values of constant Kf , which is a measure of adsorption capacity, are increasing with increasing temperature, confirming that the adsorption of Pb(II) ions by AC is endothermic. Also, it was observed from the linear form of the Freundlich model that the values of constant n increased, but not regularly, with an increasing temperature of the solution. It is worth to mention that the values of n, at all studied temperatures, are higher than 1.7 (1.7–2.5) representing a favorable adsorption [44]. It has been reported that in the Freundlich model the most common

A.M. Soliman et al. / Journal of the Taiwan Institute of Chemical Engineers 58 (2016) 264–273 Table 3 Thermodynamic parameters for the adsorption of Pb(II) on AC at various temperatures. Kc

G (kJ mol−1 )

H (kJ mol−1 )

S (kJ mol− 1 K−1 )

302 312 329

45.83 48.06 55.62

−9.64 −10.16 −11.05

6.06

0.052

4.5

4.25

value for n is greater than one (n > 1). This may be due to a distribution of surface sites or any factor that causes a decrease in adsorbent– adsorbate interaction with increasing surface density [45]. Generally, the values of n in the range of 1–10 represent a good adsorption [44]. The correlation regression coefficients (R2 ) that are presented in Table 2 show that the adsorption process is better described by the Langmuir model than by the Freundlich model, with a value of RL lying between 0 and 1. The Langmuir fit is consistent with a strong monolayer adsorption at specific sites on the surface of the AC [39]. These findings are similar to other studies, which also have shown that Pb(II) adsorption fits a Langmuir model [18,20–22,46,47]. Moreover, the qmax and the Kf values increase with an increase in temperature, as would be expected for sorption that is endothermic in nature. In conclusion, sulphuric acid treated date palm leaves (AC) have a good affinity for Pb(II) ions and their adsorption capacity increases with increasing temperature of the solution. 3.7. Thermodynamic adsorption parameters Determination of thermodynamic adsorption parameters has an important role to evaluate the heat change and spontaneity of the adsorption process. From the temperature effect, the thermodynamic parameters have been calculated for the adsorption system in the present study. The effect of temperature on the adsorption of Pb(II) ions by AC was studied at 302, 312 and 329 K to determine the thermodynamic equilibrium constant (Kc ) for the adsorption process. Kc was obtained from the linear Langmuir model plots and was determined by plotting ln(Ce /qe ) versus Ce and extrapolating to zero Ce [48,49]. The obtained value of Kc at different temperatures is reported in Table 3. The thermodynamic adsorption parameters, namely, standard enthalpy change (Ho ), standard entropy change (So ), and standard Gibbs free energy change (Go ) were calculated from the values of Kc at different temperatures. The variation of the thermodynamic equilibrium constant (Kc ) with temperature can be expressed by van’t Hoff equation as follows:

ln Kc =

S o R

−

H o RT

(9)

where R is the ideal gas constant (8.314 J mol−1 K−1 ), and T is the absolute temperature. The change in entropy (So ) and the change in enthalpy (Ho ) were determined from the plot of ln Kc versus (1/T). The linear plot derived from the experimental data of this study is shown in Fig. 10. The values of Ho and So were determined from the slope and the intercept, respectively, of the linear plot in Fig. 10, and are summarized in Table 3. It can be seen from Table 3, that the positive value of H° confirms that the process is endothermic. This might be due to an increase in chemical interaction between adsorbate ions and surface functional groups of the adsorbent or due to the increase of the intraparticle diffusion rate of adsorbate ions into the pores of the adsorbent at higher temperature, as diffusion is an endothermic process [18]. The positive value of entropy indicates an increase in randomness at the solid-solution interface during the adsorption, and might be due to the changes taking place in structure of the adsorbent and dispersal of water solvate molecules from Pb(II) ions during the adsorption process [18]. The increase in entropy and the absorption of

lnK

c

T (K)

271

4

3.75

3.5 0.003

0.0031

0.0032

0.0033

0.0034

-1

1/T (K ) Fig. 10. Plot of the Langmuir constant (ln K) versus temperature (1/T). Linear regression analysis equation: y = 6.2252 – 728.34x, R2 = 0.98719.

heat (endothermic process) for adsorption surface reactions have been reported in several previous studies using different types of activated carbon [18,21,50–52]. The standard Gibbs free energy change (Go ) is related to the change in entropy (So ) and change in enthalpy (Ho ) by

G o = H o − T S o

(10)

By using Eq. 10, the standard Gibbs free energy change (Go ) at different temperatures was calculated and summarized in Table 3. The negative values of Go indicate that the adsorption process is spontaneous with high preference of Pb(II) ions for AC, which is usually the case for many reported adsorption systems in solution [18,21,50–52]. 3.8. Kinetics adsorption study In order to investigate the mechanism of adsorption process, which is very important for the designing of an adsorption reactor, the adsorption kinetics was determined. There are various adsorption kinetics models that have been used to describe the dependence of the adsorption process on time. The most common models are expressed by the Lagergren equation for a pseudo-first order kinetic model based on solid capacity [53], and by the Ho equation for a pseudo-second order kinetic model based on solid phase sorption [54]. The general equation for nth order kinetic model is given by [18,47]:

dqt = kads (qe − qt )n dt

(11)

where qe (mg g−1 ) is the amount of adsorbate adsorbed on the surface of the adsorbent at equilibrium, qt (mg g−1 ) the amount of adsorbate at any contact time t, and kads is the rate constant of the adsorption reaction, where its unit is depending upon the order of the reaction. Ho’s pseudo-second order kinetic model equation was derived by integrating Eq. (11) for n equals 2 with the boundary conditions t = 0 to t, and qt = 0 to qt with the final equation given by [54]:

t 1 1 = + t qt qe kads q2e

(12)

In the present study, the experimental data for adsorption of Pb(II) ions on AC was found to appropriately fit the pseudo-second order

272


adsorption of Pb(II) by tree fern [53], peat [54] and spent grain [56] were 87, 29.8, and 12.33 kJ mol−1 , respectively. It is worth mentioning that the reported value of Ea for adsorption of Pb(II) ions on date palm leaflets activated by phosphoric acid was found to be 10.2 kJ mol−1 (for dry carbons) and 11.2 kJ mol−1 (for moistened carbons) [21]. These findings were presented as evidence for chemical adsorption as the rate-controlling step. Therefore, the calculated value of activation energy for the adsorption of Pb(II) ions by CA suggests that also here the adsorption rate-controlling step is most likely chemical adsorption. The reported value of Ea for physical adsorption is usually less than 4 kJ mol−1 , since in physical adsorption the system reaches the equilibrium rapidly and representsweak adsorption forces [56]. The chemical adsorption involves stronger forces and is more specific than physical adsorption. Thus, Ea for chemical adsorption commonly lies between 8 and 84 kJ mol−1 .

2 a b

c

-1

t/q (min g mg )

1.5

t

1

0.5

4. Conclusions 0 0

20

40

60

80

100

120

140

t (min) Fig. 11. Plot of pseudo second order kinetics model for the adsorption of Pb(II) ions on AC at different temperatures: 29 °C (a), 39 °C (b), and 56 °C (c). AC dosage = 5 g L−1 , initial concentration of Pb(II) = 100 ppm, contact time: 3 h, pH = 5.5, contact time: 3 h. Linear regression analysis equations: (a) y = 0.2047 + 0.013839x, R2 = 0.99333; (b)y = 0.078876 + 0.012905x, R2 = 0.99782; (c) y = 0.034668 + 0.010016x, R2 = 0.99886. Table 4 Second order kinetic model parameters for Pb(II) ions adsorption on AC. T (°C)

qe (mg g−1 )

Kads .104 (g min−1 mg−1 )

R2

29 39 56

72.26 77.49 99.84

9.36 21.20 28.90

0.99333 0.99782 0.99886

model expressed by Eq. 12. Poor correlation coefficients were obtained for a pseudo-first order kinetic model (R2 much less than that for second order, plot not shown). The plot of the pseudo second order kinetics model for the adsorption of Pb(II) ions on AC at various temperatures is shown in Fig. 11, and the kinetic adsorption parameters are listed in Table 4. A second-order kinetic reflects that the rate of adsorption of Pb(II) ions on the AC surface depends on both the adsorbent and adsorbates [55]. From the data in Table 4, it can be observed that the adsorption rate constant (kads ) increases with increasing temperature. The same trend can be observed for the value of qe . These results show that the adsorption of Pb(II) ions on AC is faster at higher temperature. The dependence of the rate constant on temperature is well known and can be described by the Arrhenius equation, the linear form of that equation being as follows:

ln kads = ln A −

Ea 1 R T

(13)

where kads is the adsorption rate constant, A is a temperature independent factor (called collision frequency factor) having the same unit as kads , Ea (J mol−1 ) is the activation energy of the metal adsorption, R is the ideal gas constant (8.314 J mol−1 K−1 ), and T (K) is the solution temperature. To calculate the activation energy (Ea ) for the adsorption of Pb(II) ions by AC, ln(kads ) was plotted against 1/T using the listed values of kads in Table 4 for the pseudo second order model. When using the experimental data for kads at different temperatures as listed in Table 4, the regression equation for the linear plot (not shown) was found to be y = 6.2542 – 3949.3x (R2 = 0.93). Thereby, the calculated activation energy for the adsorption process was found to be 32.83 kJ mol−1 . It has been reported that the activation energy for the

In the present study, adsorption of Pb(II) ions on activated carbon, prepared by immersion of date palm leaves in sulphuric acid with subsequent carbonization at 250 °C for 24 h, was investigated as a function of adsorbent dose, contact equilibration time, pH, initial lead(II) ions concentration, and temperature of the solution. The results of this study summarize the different operating conditions that would be required for the efficient removal of Pb(II) ions from aqueous solution by activated carbon (AC). Based on the results of this study, we can conclude the following: i. The removal yield of Pb(II) ions by AC increased with increasing contact time and reached the equilibrium state within 3 h. The highest value of removal efficiency (R%) was found to be at AC dosage of 5 g L−1 (R% = 92.6%) for a 100 ppm Pb(II) solution. With an AC dosage of 4 g L−1 a max. R% of 98.6% could be achieved for a 50 ppm Pb(II) solution. ii. The adsorption capacity of the adsorbent towards Pb(II) ions is highly dependent on the pH of the solution, with a maximum removal of Pb(II) found at pH 5.5±0.3. iii. The results of adsorption isotherms showed that the adsorption process is better described by the Langmuir model than by the Freundlich model, with a value of RL , lying between 0 and 1, indicating a favorable adsorption. It was observed that the isotherm constant increased with increasing temperature. The values of qmax increased from 61.15 to 88.61 mg g−1 , when the solution temperature increased from 29 to 56 °C, which shows the adsorption process to be endothermic. iv. The positive value of the enthalpy change, Ho (6.06 kJ mol−1 ), confirms that the adsorption process is endothermic. The positive value of entropy change, So (0.052 kJ mol−1 K−1 ), indicates an increase in randomness at the solid-solution interface during the adsorption. The negative values of the Gibbs free energy change (G°) at different temperatures reflect the feasibility and spontaneous nature of the adsorption process of Pb(II) ions on AC. v. The kinetic analysis of the study shows that the adsorption of Pb(II) ions on AC could be described well with the secondorder kinetic model. The rate constants increased with increasing temperature, which indicates that the adsorption of Pb(II) ions by AC is faster at higher temperatures. vi. The activation energy for the adsorption process was found to be 32.83 kJ mol−1 . These findings were presented as evidence for chemical adsorption as the rate-controlling step. vii. The surface area (SBET ), pore volume, and pore diameter of AC ˚ respecwas 64.12 m2 g−1 , 8.35 × 10−2 cm3 g−1 , and 52.1 A, tively. The present study shows higher surface areas than that found for activated carbons prepared via phosphoric acid treatment of date palm leaflets.


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