A novel route for preparation of chemically activated carbon from pistachio wood for highly efficient Pb(II) sorption

Journal of Environmental Management 236 (2019) 34–44

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Research article

A novel route for preparation of chemically activated carbon from pistachio wood for highly efficient Pb(II) sorption

T

Seyed-Ali Sajjadia, Alireza Meknatia, Eder Claudio Limab,∗, Guilherme L. Dottoc, Didilia Ileana Mendoza-Castillod, Ioannis Anastopoulose,∗∗, Fadi Alakhrasf, Emmanuel I. Unuabonahg, Pardeep Singhh, Ahmad Hosseini-Bandegharaeii,j,∗∗∗ a

Environment Health Engineering Department & Social Determinants of Health Research Centre, Gonabad University of Medical Sciences, Gonabad, Iran Institute of Chemistry, Federal University of Rio Grande do Sul (UFRGS), Av. Bento Gonc¸ alves 9500, Postal Box 15003, 91501-970 Porto Alegre, RS, Brazil c Environmental Processes Laboratory, Chemical Engineering Department, Federal University of Santa Maria–UFSM, 1000, Roraima Avenue, 97105-900 Santa Maria, Brazil d Instituto Tecnológico de Aguascalientes, Av. López Mateos 1801, Aguascalientes, 20256, Aguascalientes, Mexico e Laboratory of Biochemical Engineering & Environmental Technology (LBEET), Department of Chemical Engineering, University of Patras, 26504 Patras, Greece f Department of Chemistry, College of Science, Imam Abdulrahman Bin Faisal University, P.O. Box 1982, Dammam 31441, Saudi Arabia g Environmental and Chemical Processes Research Laboratory, Department of Chemical Sciences, Redeemer's University, PMB 230, Ede, Osun State, Nigeria h School of Chemistry, Faculty of Basic Sciences, Shoolini University, Solan (Himachal Pradesh)-173212, India i Department of Environmental Health Engineering, Faculty of Health, Sabzevar University of Medical Sciences, Sabzevar, Iran j Department of Engineering, Kashmar Branch, Islamic Azad University, PO Box 161, Kashmar, Iran b

ARTICLE INFO

ABSTRACT

Keywords: Adsorption Pb(II) ion Two-stage activation Activated carbon NH4NO3 NaOH

Pistachio wood-derived activated carbon prepared by a two-stage process (PWAC-2), conducting two consecutive chemical activation processes with NH4NO3 and NaOH, respectively. The results showed that explosive characteristic of NH4NO3 can primarily be employed to produce a char, with a large surface area and a highlyordered pore structure, which can be subjected to a second activation process with NaOH to prepare a more suitable activated carbon, with a highly porous structure and useful functional groups, for removal of lead ions from aqueous media. An L25 Taguchi experimental design was used by varying impregnation ratio, activation time and temperature in both pre- and post-activation stages, and the results showed that, in both stages, a small activating agent/precursor and a proportional low activation time suffice for preparation of an advantageous activated carbon for Pb(II) adsorption. A comprehensive study was performed on the equilibrium, kinetic and thermodynamic aspects of Pb(II) adsorption by the new activated carbon. The results exhibited that, having had a high lead adsorption capacity (190.2 mg g−1), a high adsorption rapidness, and thermodynamic favorability, PWAC-2 is a beneficial alternative for utilization in full-scale plants of lead removal from waters and wastewaters.

1. Introduction

increased intracranial pressure, headache, nephropathy, and dysfunction of the nervous system are some lead poisoning symptoms in adults (Villa et al., 2014). In children, lead poisoning can lead to delayed intellectual development (Dietrich et al., 1993). This element is naturally present in the environment and is speeded via some industrial activities of human. Different industries, particularly those involving in the production of battery, electro-plating, mining, paper and pulp, metal processing, lead smelting, metal-finishing, etc., produce lead-laden

Lead is regarded as one of the most harmful heavy metals owing to the widespread presence in the environment and its chemotoxic effects on different forms of life (Abdel-Halim et al., 2003). Lead has no biological function in the human body and is harmful, even at low concentrations, since can disturb the metabolism of some important nutrients, like calcium (Mahaffey, 1977). Arthralgia, abdominal pains,

Corresponding author. Corresponding author. ∗∗∗ Corresponding author. Department of Engineering, Kashmar Branch, Islamic Azad University, PO Box 161, Kashmar, Iran. E-mail addresses: [email protected] (E.C. Lima), [email protected] (I. Anastopoulos), [email protected], [email protected] (A. Hosseini-Bandegharaei). ∗

∗∗

https://doi.org/10.1016/j.jenvman.2019.01.087 Received 13 September 2018; Received in revised form 31 December 2018; Accepted 24 January 2019 0301-4797/ © 2019 Elsevier Ltd. All rights reserved.

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wastewaters (Hosseini-Bandegharaei et al., 2014). A variety of methods, such as ion-exchange (Tavakoli et al., 2017), membrane processes (Gürel et al., 2005), chemical precipitation (Matlock et al., 2002), adsorption (Fu and Wang, 2011), electrochemical processes (Liu et al., 2013), etc., have been applied to remove lead ion from water and wastewater. Amongst these technologies, adsorption is considered to be one of the most profitable technologies (Giannakoudakis et al., 2018; Guo et al., 2017; Sardella et al., 2015; Wahid et al., 2017), since it is economically feasible and efficient in the removal of some types of pollutants. Among the adsorbents used for water and wastewater treatment, usage of activated carbon as adsorbent have gained favor in the last decades, owing to its availability, adequate chemical and physical stability, high surface area, porous structure, and high adsorption capacity. A wide variety of carbonaceous materials can be used as the precursor for the production of commercial activated carbons, like natural coal, Petroleum residues, lignite, peat, wood, and various agricultural by-products. Since commercial activated carbons are often expensive (Baccar et al., 2009), mainly due to extensive use of expensive precursors (e.g. coal), a large-scale effort has been conducted to use agricultural wastes for preparing cost-effective and advantageous activated carbon samples (Alslaibi et al., 2013; Nowrouzi et al., 2017; Singh et al., 2013). On the other hand, in addition to the cost-effectiveness of many agricultural by-products or even being practically free at the processing sites, converting such waste materials into activated carbon which are then applied for the pollution control purposes is very interesting from the view point of environment and sustainability concept. Hence, in the last decades, many activated carbons have produced from different agricultural waste sources, like waste leaves, peels, pruned woods, nut shells, etc., and have applied to the removal of heavy metals, including lead, from waters and wastewaters. For example, Brudey et al. (2016) prepared three chemically activated carbons from lignocellulosic precursors, using sulfuric acid, and utilized them for lead adsorption. Demiral et al. (2014) used the chestnut shell for preparation of phosphoric acid-activated carbon to remove lead ions from water media. Nadeem et al. (2006) reported the removal of Pb(II) ions by adsorption on the steam activated carbon prepared from M. oleifera pod. Li et al. (2010) used zinc chloride as an activating agent for preparation of activated carbon from Enteromorpha prolifera (EP) and optimized the conditions for its usage in the removal of Pb(II) from water. Many activated carbons utilized for different purposes are prepared by a two-stage process in which the primary carbonization is followed by an extra activation or modification stage. In such activated carbons, the purpose of the first step usually is to make the carbon content richer and to develop an initial porosity and functionality, while the second activation (or modification) step helps in changing various chemical and/or physical properties of the precursor, like porosity and functionality, to improve its performance for various potential applications. For example, Goel et al. (2005) treated commercial granular activated carbon with alkali sulphide to enrich its sulfur functional groups and use it in removing lead ions from aqueous solutions. Zubrik et al. (2017) employed a two-stage preparation method by activation of the firstly pyrolyzed biomass with KOH to prepare a more effective activated carbon for Cd(II) ion adsorption from water media. Wu et al. (2017) utilized Shengli lignite for obtaining a series of porous carbons (PCs) via hydrothermal carbonization and chemical activation with KOH for application as electric double layer capacitors. However, production of activated carbon via a two-stage chemical activation process, using two different chemical agents, has been rarely used by the researchers. A previous group study has shown that, due to its explosive characteristics at high temperatures, ammonium nitrate (NH4NO3) can be employed as a novel activating reagent for production of a surfaceengineered activated carbon with regular-shaped pores and high surface area, using pistachio wood wastes (PWAC) (Sajjadi et al., 2018). In the present work, to improve the porosity and increasing the surface

functional groups, pistachio wood wastes were subjected to a two-stage activation process using NH4NO3 and NaOH, respectively, as chemical activating agents, to produce a new activated carbon (PWAC-2) for Pb (II) ion removal from water bodies. The influence of physic-chemical parameters on the Pb(II) adsorption by PWA-2 was investigated comprehensively, using batch-mode adsorption experiment. Also, the thermodynamic parameters were evaluated, and equilibrium and kinetic data were fitted to different models. 2. Experimental 2.1. Chemicals and reagents Analytical grade chemicals and reagents were purchased from Merck (Darmstadt, Germany) and used, unless otherwise stated. N2 gas (99.9995%) was supplied by Azaroxide Co., Iran. Ultrapure water (Milli-Q Millipore, 18.2 MΩ cm−1) was used in all experiments. All lead solutions used in experiments were synthesized from nitrate salt by dissolving in acidic medium. The laboratory glassware and polyethylene bottles were kept in nitric acid solution (10%, v/v) overnight, rinsed thoroughly with ultrapure water and dried before use. An appropriate amount of Pb(NO3)2 was dissolved in a weakly acidic medium to prepare 100 mL of Pb(II) stock solution (1000 mg L−1). Working solutions of Pb(II) (100 mL) with various concentrations (5.0–25.0 mg L−1) were prepared by diluting the stock solution. Nitric acid solution (0.1 mol L−1) was prepared from concentrated nitric acid (65% m/m) by diluting with ultrapure water. Sodium hydroxide solution (0.1 mol L−1) for pH adjustment was prepared by dissolving 2.01 g of NaOH in enough water and diluted to 500 mL of ultrapure water. 2.2. Preparation of pistachio wood-based activated carbon (PWAC-2) samples Biomass obtained from the pistachio wood (PW) wastes of gardens of Gonabad, Iran, was rinsed with tap water for removing any impurity and adhering dirt, dried at 80 °C for 48 h, grounded and sieved to obtain wood particles sizes between 0.5 and 2 mm. Twenty-five adsorbents were obtained in two stages under an N2 flow of 150 cm3 min−1, using sequential activation with NH4NO3 and NaOH, respectively. Table S1 shows the Taguchi experimental design utilized to identify the most appropriate activating conditions of PW samples where the NH4NO3/ PW impregnation ratio (i.e., 0–8 wt %), NH4NO3 activation time (i.e., 1–3 h), NH4NO3 activation temperature (i.e., 400–900 °C), NaOH impregnation ratio (i.e., 10–50%), NaOH activation time (i.e., 1–3 h) and NaOH activation temperature (i.e., 400–900 °C) were chosen as the operating variables to ameliorate the lead adsorption properties of PW based adsorbents. After the NH4NO3 activation step, the adsorbents obtained were thoroughly washed with 0.1 M HCl, rinsed with doubly distilled water several times and dried at 105 °C for 48 h. Samples obtained were further mixed at 150 rpm with NaOH solutions during 24 h, at the different impregnation ratios reported in Table S1. Impregnated samples were oven-dried for 48 h and activated according to the experimental conditions reported in Table S1. All adsorbents were: thoroughly washed with 0.1 M HCl solution two times to remove the inorganic salts and ash, rinsed with distilled water until neutralization, dried at 105 °C for 24 h and stored in airtight plastics for further adsorption tests. Results obtained from orthogonal array L25 were the basis to determine the most appropriate preparation conditions of PW based adsorbents for lead adsorption from aqueous media. The Pb(II) adsorption capacity of prepared adsorbents was the response variable of this experimental design. Using batch reactors, lead adsorption experiments were conducted with the produced adsorbent samples at pH 6, ambient temperature, shaking speed of 160 rpm for 2 h, adsorbent dosage of 0.2 g L−1, and initial lead concentration of 25 mg L−1 using an adsorbent mean particle size of ∼0.45 mm. An ANOVA analysis of the 35

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signal-to-noise ratio was used to identify the best activation conditions from Taguchi experimental design, using the statistical software Minitab 15.

The adsorption equilibrium studies at different temperatures were performed using the experimental data obtained from the studies on the effect of initial lead concentration, in which the equilibrium adsorption capacity, qe (mg g−1), was computed using the below relation:

2.3. Apparatus

qe =

A vertical stainless steel reactor inserted into an electrical furnace was used for conducting pyrolysis processes, using a porcelain crucible covered with a lid, under N2 gas (99.9995% purity) atmosphere at a flow rate of 150 cm3 min−1. Scanning electron microscopic (SEM) micrographs were taken by a VEGA//TESCAN instrument after gold-palladium sputter coating of PWAC-2 by a Model SC 7620 sputter coater, at an accelerating voltage of 15 kV. EDX technique was used for determining elemental composition, using a VEGA//TESCAN instrument for analysis. A nitrogen gas adsorption analyzer (BELSORP-mini II instrument) was used for calculation of BET (Brunauer–Emmett–Teller) specific surface area, pore size, and pore volume. An FTIR Nicolet spectrometer (AVATAR 370-FTIR Thermo Nicolet) was applied to detect surface functional groups of the activated carbon (AC; PWAC-2) after their pelleting using KBr discs. A Shimadzu atomic absorption spectrometer (AA-7000F) with an air-acetylene flame was used for Pb (II) determination in aqueous solutions after suitable dilution, according to the users' manual provided by the manufacturer and using lead hollow cathode lamp of radiation wavelength of 283.3 nm as the source. All measurements of pH values in the solutions were performed by a PHS-3BW Model pH-meter (Bel, Italy) equipped with a combined glass–calomel electrode. The mixtures of activated carbon and adsorbate solution were separated using a Sartorius membrane filter (0.45 μm pore size).

Ct ) C0

× 100

×V

(2)

where Ce (mg L ) is the remaining lead concentration at the equilibrium situation. V (L) is the volume of lead solution phase, and W (g) is the mass of AC. The kinetics studies of each Pb(II)/AC adsorption system was performed by employing the experimental results obtained from the studies on the effect of time, in which the uptake of lead at any time, qt (mg g−1), was determined by Eq. (3):

qt =

(C 0

Ct ) W

×V

where Ct (mg L fied time ‘t’.

−1

(3)

) is the remaining lead concentration at a pre-speci-

2.5. Modeling of adsorption equilibrium and kinetic data A nonlinear form of several two- and three-parameter models were utilized to find the best kinetic and equilibrium models obeyed by the adsorption process. In order to evaluate the goodness-of-fit of each model with experimental data, determination coefficient (R2), normalised standard deviation (Δq (%)), root mean square error (RMSE), and chi-square (χ2) were computed, using the expressions reported in Section 1S (HosseiniBandegharaei et al., 2016b; Tran et al., 2017c). Additionally, the validity of kinetic studies was judged by using Homogenous particle diffusion model (HPDM), and the following relation was employed for calculating the fractional attainment of equilibrium F(t) at any specified time ‘t’:

The performances of the activated carbon samples for the adsorption of Pb(II) ion was comprehensively tested under different batch experimental conditions, using an adsorbent mean particle size of ∼0.45 mm. All experiments were conducted in triplicate, in which RSD % values were lesser than 3.9%, and the obtained average values were used for drawing the plots and for all calculations. In each experiment, lead solution (100 mL) with known initial concentration and pH was added into a 250-mL conical flask containing a weighed amount of activated carbon. The conical flasks were shaken at optimum agitation speed (150 rpm; see Fig. S1) for the required time at a determined temperature, using a temperature set reciprocating shaker. The dependence of PWAC-2 efficiency on pH (2–10) was investigated at a lead concentration of 12.5 mg L−1 using 0.1 M NaOH or HNO3 solution for adjusting the adsorbate solution to the predetermined values. Also, the efficiencies dependency of two other carbons, commercial activated carbon (CAC) and activated carbon obtained from one-stage activation process (PWAC) (Sajjadi et al., 2018), were obtained at the same conditions and the obtained results were compared with those of PWAC-2. The effect of initial lead concentration (5–25 mg L−1) was assessed at optimum pH and ambient temperature. For investigation of the effect of temperature and thermodynamic studies, adsorption studies were conducted at different temperatures (288–328 K), for various initial concentrations (7.5–22.5 mg L−1). The effect of contact time was tested for different initial concentrations (7.5, 12.5, 17.5 and 22.5 mg L−1) at the optimum conditions. After conducting each adsorption experiment, the mixture was filtered, and the remaining concentration of the lead solution was measured by the flame atomic absorption spectrometry (FAAS) method. The removal efficiency of each AC sample towards lead (RE; %) at any time was determined by Eq. (1):

(C0

C e) W

−1

2.4. Batch adsorption efficiencies

RE % =

(C0

Ft =

C0 C0

Ct Ce

(4)

3. Results and discussion 3.1. Preparation studies Fig. S2 shows the effect of different parameters (i.e. impregnation ratio, activation temperature, and activation time, in both pre- and post-activation processes) on the adsorption capacities of produced PWAC-2 samples. From the results exhibited in this figure, it can be found that the best conditions for NH4NO3/PW impregnation ratio, first stage activation time, first stage activation temperature, NaOH/PWAC impregnation ratio, second stage activation time, and second stage activation temperature respectively were 4.0 wt%, 1.0 h, 800 °C, 30% wt.%, 1.0 h, and 800 °C, which were used to produce PWAC-2 for performing further studies. Also, based on the obtained results, the effect of tested variables follows the following trend: Pre-stage impregnation ratio > Pro-stage impregnation ratio > Pro-stage temperature > Pre-stage temperature > Pre-stage activation time > Pro-stage activation time. 3.1.1. Calculation of PWAC-2 yield Carbonization temperature, type of precursor, carbonization time, and activating agent influence the yield of produced activated carbon with different degrees. Therefore, PWAC-2 yield was computed using the following relation:

(1)

PWAC

−1

where C0 and Ct (mg L ) are, respectively, the Pb(II) concentration at the initial and pre-specified time ‘t’.

2 yeild (%) =

WPWAC WPW

2

× 100

(5)

where WPWAC-2 and WPW (g) are the weight of activated carbon 36

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produced and the dry pistachio wood used for the production of activated carbon. The yield of the final product (PWAC-2) was determined to be 33.0%. Using dry agricultural precursors for production of activated carbon, releasing of a large amount of volatiles at high temperatures is the primary reason for the loss of weight in the carbonization process. Also, some activation agents may play a significant role in the loss of weight through different mechanisms, like catalyzing of oxidation reactions by the strong bases such as KOH or NaOH. Amongst the two chemical reagents (i.e. NH4NO3 and NaOH) used for activation in this work, NaOH can play a more significant role in the decreasing of the yield of activated carbon. Proposing several reactions to explain the interaction of anthracite with NaOH, Lillo-Ródenas and co-workers tried to provide some insights into the chemical activation mechanism of carbon in the presence of NaOH as activation agent and indicated that the following reaction is thermodynamically possible and occurs during the process (Lillo-Ródenas et al., 2003):

C+ 6NaOH

2Na + 2Na2CO3 + 3H2

(6)

3.2. Characterization studies 3.2.1. BET analysis The BET approach using nitrogen adsorption/desorption isotherm is the most practical procedure for evaluating the surface area and getting insight into the size of the pores existing in the adsorbent (Tran, 2017). Therefore the BET approach was used for determination of textural properties of PWAC-2, including BET surface area (SBET; 1884 m2 g−1), total pore volume (Vtotal; 0.994 cm3 g−1), monolayer volume (Vm; 293.8 cm3 g−1), and mean pore diameter (2.11 nm). Using NH4NO3 alone and through a one-stage activation process, Hosseini-Bandegharaei et al. (Sajjadi et al., 2018) synthesized a pistachio-wood derived activated carbon (PWAC) by applying impregnation ratio (NH4NO3/ PW; wt.%) of 5%, pyrolysis temperature of 800 °C, and a pyrolysis time of 2 h. They reported that the SBET, Vtotal, Vm, and the mean pore diameter of PWAC were respectively 1448 m2 g−1, 0.901 cm3 g−1, 265.8 cm3 g−1, and 2.49 nm. Notably, mean pore diameter and monolayer volume in PWAC-2 are lower and higher than those of PWAC, respectively, which reveals that pre-activation stage with NaOH has a significant effect on the developing of new micropores in the structure of activated carbon. Such enhancement in the micropore volume due to using a strong base in the second pyrolysis has been reported by Zubrik et al. who used KOH as activation agent to improve the properties of a firstly pyrolyzed char for Cd(II) adsorption (Zubrik et al., 2017).

Fig. 1. SEM micrographs of PWAC-2.

of the shapes of channels in PWAC-2, which have been developed in the first activation stage. Therefore, having had parallel-shaped channels and as an engineered porous activated carbon, PWAC-2 can confer better accessibility of inner surfaces and diffusion of contaminants into its interior pores is easier, compared to commercial activated carbons.

3.2.2. Study of surface morphology In some granular porous sorbents, the existence of too many microsize transitional pores results in both inaccessibility of the inner surfaces (Rahmani-Sani et al., 2017) and slowness of adsorption rate (Alahabadi et al., 2017), due to the constraints occurring for the diffusion process. Therefore, the surface morphology of adsorbent is one of the principal properties which may have a significant influence on the suitability of an adsorption system in removal of a particular contaminant from aqueous phases. To observe the surface morphology, SEM micrographs of PWAC-2 were taken and shown in Fig. 1. The previous study showed that, due to explosion at high temperatures, using https://www.sciencedirect.com/topics/earth-and-planetarysciences/ammonium-nitratesNH4NO3 as an activation agent leads to preparation of a valuable AC with well-developed and regular-shaped pores (Sajjadi et al., 2018). Like PWAC which was prepared by one stage activation with NH4NO3 alone at NH4NO3/PW of 5 wt% (Sajjadi et al., 2018), and unlike commercial activated carbons, SEM micrographs of the PWAC-2 show that this new activated carbon also possesses highly regular-shaped and well-developed channels which can facilitate the accessibility of lead ions to the interior surface of activated carbon granules. This result means that undergoing a further activation process with NaOH does not make a significant change in the regularity

3.2.3. EDX analysis Fig. 2(a) shows the result of elemental analysis of PWAC-2. In comparison to PWAC (Sajjadi et al., 2018), the carbon (81.45%) and oxygen (16.45%) contents of PWAC-2 are lower and higher than those of PWAC, which were 91.24 and 5.40%, respectively. The higher oxygen content of PWAC-2 is relevant to the second stage of activation with NaOH, indicating more oxygen-containing (hydroxyl, lactonic and carboxylic functional) groups which can lead to better attraction of mineral pollutants, like heavy metals ions, from aqueous solutions. 3.2.4. FTIR study FTIR spectra is a useful technique for qualitative identification of functional groups of sorbent. As exhibited in Fig. 2(b), it was found that PWAC-2 spectrum has a peak at 3412 cm−1 which is characteristic of eOH stretching vibration in phenol groups, carboxylic groups, or sorbed water. The observed shoulder at 3316 cm−1 can be attributed to vibrational stretching eNH2 group. The relatively intense band around 1670 cm−1 and the weak band at 1786 cm−1 are assigned to the stretching vibration of C]O bond in lactonic and carboxylic groups. 37

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3.0; 93.7, 75.4, and 63.0% at pH 6.0; and 68.3, 62.2, and 43.9% at pH 9.0. At the low pHs, there is a high concentration of H+ ion which competes with the adsorption of Pb(II) ion (Milojković et al., 2019) and, along with the repulsive force between Pb(II) ions and the positively charged surface of AC (Mazaheri et al., 2015), causes low removal percentages for all activated carbon. The positive charge of AC surface eases at pH > 6.0, and finally changes to negative charge at higher pHs, which is expected to cause an increase in removal percentages beyond pH 6.0. However, the changes in the speciation of Pb(II) ions at the high pHs, and its precipitation as Pb(OH)2 (Li et al., 2002), cause the removal percentages of lead to be diminished. Several mechanisms may happen in the adsorption process of Pb(II) ion onto the surface of activated carbon such as ion-exchange (Song et al., 2018), chelating ion-exchange (Yin et al., 2007), and π-cation donor–acceptor electron interaction (Ding et al., 2016). Due to the higher ability of PWAC-2 for Pb(II) adsorption, further investigations were focused on this activated carbon, and pH 6.0 was elected as the optimum value to be used in other studies. 3.3.2. Effect of ionic strength and other heavy metal ions To evaluate the ability of PWAC-2 to sorb Pb(II) ion from water bodies at different ionic strengths, several adsorption experiments were performed at various concentrations of NaCl. The data were summarised in Fig. 4(a) in which increasing the ionic strength has led to diminishing of lead removal. As Wang et al. mentioned (Wang et al., 2010), the decreases in the removal percentage can be ascribed to the increasing of Na+ concentration and its competition with lead ions for the adsorption sites of PWAC-2 surface. Several tests also were conducted to investigate the influence of other heavy metal ions on the removal percentages of PWAC-2. Fig. 4(b) shows that, for all activated carbons, raising the concentration of another toxic metal ion decrease the removal percentages of lead ion. These results show that other metal ions compete for the adsorption sites, especially when they are present at high concentrations. However, the result shows that PWAC-2 is of a good performance and, even when the initial concentration of foreign metal ion is about 10 times greater than that of Pb(II) ion, it is capable of removing more than 50% of lead from the solution media.

Fig. 2. EDX analysis results (a) and FTIR spectrum (b) of PWAC-2.

The peak occurring at 1148 cm−1 is a confirmation of the presence of CeO groups of AC structure, in phenols, alcohols, acids, ethers and esters (Puziy et al., 2002). Also, the characteristic C]C double bond in the aromatic rings at 1300 cm−1 can be observed as a shoulder. In addition, the weak broaden peak at approximately 812 cm−1 belongs to out-of-plane bending vibration of aromatic CeH bond.

3.3.3. Effect of activated carbon dose The removal efficiency of Pb(II) ion versus various PWAC-2 dosages (0.05–0.25 g L−1) was evaluated for the newly produced activated carbon, and the results have been shown in Fig. 3(b). As can be seen from the figure, initially the increase in AC dosage caused a sharp increase in the removal performance, but beyond a specific value, the increase in removal percentage is not very high and, almost, reaches a constant value. This trend can be explained by this reality that as the AC dose increases the number of adsorption sites increases and thus more Pb(II) ions can be attached to these sites. It can also be observed that, by increasing PWAC-2 dose from 0.05 to 0.25 g L−1, the removal percentages increase from 67.3 to 99.0%, showing the economic beneficiary of PWAC-2 for Pb(II) adsorption from aqueous solutions.

3.2.5. The value of pHPZC The pH of point of zero charge (pHPZC), at which the net (external and internal) surface charge of a sorbent is zero, was measured by the “drift method”, due to its simplicity and versatility (Tran et al., 2017a). The results showed that the pHpzc of PWAC-2 was 8.1. 3.3. Studies on lead adsorption 3.3.1. Effect of pH Usually, the pH of aqueous phase has an essential role in the removal of a pollutant by an adsorbent, because of its effects on the speciation of solute and surface chemistry of the adsorbent. The pHdependency of adsorption of Pb(II) ion onto PWAC-2 was compared with those onto PWAC (Sajjadi et al., 2018) and commercial activated carbon (CAC). To subtract the precipitated amounts from the whole removed amounts at high pHs, the precipitated amounts of lead ions were obtained by repeating the experiments without adding any adsorbent, at the final pHs of those conducted in the presence of activated carbon. Fig. 3(a) exhibits the influence of pH on the removal percentage of Pb(II) ion for all these activated carbons. The results show that the removal of percentages of lead increase as pH increases and, after achieving the maximum amounts at approximately initial pH 6.0, they start to decrease with further increase of pH. As can be observed, PWAC-2, PWAC, and CAC respectively showed the percentage removals of approximately 70.4, 43.4, and 52.1% at pH

3.3.4. Effect of initial lead concentration The result obtained from the study of the effect of initial Pb(II) concentration on the removal efficiency has been reported in Fig. 3(c) in which it can be seen that there was a decrease in removal efficiencies at the higher initial concentrations. As can be seen from Fig. 3(c), increasing the initial concentration from 5.5 to 25.0 mg L−1 cause the removal percentage to be decreased from 96.7 to 70.9%. Although the affinity between the PWAC-2 surface and Pb(II) ion is high, the adsorption sites existing on any surface is limited, and this the primary cause of decreasing removal efficiency at higher concentrations. 3.3.5. Effect of contact time . The study of the effect of contact time (Fig. 3(d)) showed that, for 38

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Fig. 3. (a) Influence of pH on the removal of Pb(II) ion by PWAC-2, PWAC, and CAC (initial concentration 12.5 mg L−1, AC dosage 0.1 g L−1, contact time 60 min, and temperature 25 °C), (b) influence of adsorbent dose on the removal of Pb(II) ion by the studied PWAC-2 (initial concentration 12.5 mg L−1, contact time 60 min, initial pH 6.0, and temperature 25 °C), (c) influence of initial concentration on the removal of Pb(II) ion by PWAC-2 (AC dosage 0.1 g L−1, contact time 60 min, initial pH 6.0, and temperature 25 °C), and (d) influence of contact time on the removal of Pb(II) ion by PWAC-2 (initial concentration 7.5, 12.5, 17.5, and 22.5 mg L−1, AC dosage 0.1 g L−1, initial pH 6.0, and temperature 25 °C).

all initial concentrations, increasing contact time leads to increasing the removal efficiencies, until the attainment of equilibrium. The figure reveals that, for all the initial concentrations studied, initially the removal efficiencies increase very sharply with the increase in contact times. However, at the higher contact times, the increase in the removal percentage is not very high and, almost, reaches a constant value. Fig. 3(d) also shows that almost a contact time of > 30 min is necessary for equilibrium to be attained. These results imply that the lead adsorption onto PWAC-2 accomplishes with a good rapidness, which can be attributed to the rapid accessibility of lead ions to the adsorption sites on the surface of this activated carbon.

different kinetic models it was employed the correlation coefficient (R2), error functions, and theoretical adsorption capacities obtained from these plots, and the results were reported in Table 1. The results revealed that pseudo-second-order and Ritchie model fit better the experimental results than the other models, indicating that the interaction between lead ions and adsorption sites is the rate-controlling step of the whole process of Pb(II) adsorption onto the surface of PWAC2. To have a better judgment on the obtained kinetic results, and also for validating the possible effect of film diffusion and/or pore diffusion on the adsorption of Pb(II) ion into the granules of PWAC-2, the data obtained from the experiments were fitted with the equations of homogeneous particle diffusion model (HPDM). This kinetic model describes the counter diffusion of two species in a quasi-homogeneous media, and usually is presented with equations (7) and (8): If the mass-transfer controlling stage is pore diffusion of ions into the AC granules, the following equation will be linear (Cortina et al., 1998; Hosseini-Bandegharaei et al., 2011):

3.3.6. Kinetic studies The kinetic studies are fundamental in adsorption science and technology for the reason that such studies have a significant role in designing the full-scale removal plants. To find rate-controlling step and to explore the mechanism involved in Pb(II) ion adsorption, several kinetic models were considered as described in Section 3S. The capability of the kinetic models to explain the lead adsorption onto the studied activated carbons was examined by nonlinear plots of qt vs. t, and the results are summarised in Fig. 5(a). To evaluate the

ln(1

Ft2 ) = kPD t

(7)

where t is the adsorption time (s), Ft is fraction of equilibrium 39

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Fig. 4. (a) Influence of ionic strength on the removal of Pb(II) ion by PWAC-2, and (b) influence of the presence of the heavy metal ions on the removal of Pb (II) ion by PWAC-2 (initial concentration 12.5 mg L−1, initial pH 6.0, AC dosage 0.1 g L−1, contact time 60 min, and temperature 25 °C).

attainment at the time t, and kPD is rate constant for pore diffusion. If the diffusion of Pb(II) ion across the external film surrounding the AC granules controls the process of mass-transfer, the following equation will be linear (Cortina et al., 1998):

ln(1

Ft ) = kFD t

(8)

Fig. 5. Plots of different kinetic models (a) and multi-linear fit of experimental data to Weber-Morris kinetic model (b) for sorption of Pb(II) onto PWAC-2 at pH 6.0, different initial concentrations (7.5–22.5 mg L−1), AC dosage 0.1 g L−1, and temperature 25 °C.

where kFD is rate constant for film diffusion. Fig. 3(S) shows the linear kinetic plots of HPDM model for different Pb(II) concentrations, in which the low correlation coefficients and, especially, the faraway intercepts from zero exhibit that neither pore diffusion nor film diffusion is the rate-controlling steps for the entire adsorption process of Pb(II) into PWAC-2 granules. Usually, Weber-Morris kinetic model is used to find if the intraparticle diffusion affects on the adsorption kinetic of a solute onto the surface of the solid support. The following equation can present this kinetic model.

qt = kip t + I

(9)

where kid is the intra-particle diffusion rate constant (mg g−1 min−0.5), and I is a constant whose values is proportional to the boundary layer. Weber-Morris model was fitted with the experimental data and Fig. 5(b) shows that whole plots of qt versus t0.5 do not pass through the 40

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Table 1 Parameters and the values of error functions of different kinetic models for sorption of lead by PWAC-2 at different initial concentrations and 298 K. Model; C0 (mg/L)

Calculated Parameters and the values of error functions

Pseudo-first order 7.50 12.5 17.5 22.5 Pseudo-second order 7.50 12.5 17.5 22.5 Ritchie 7.50 12.5 17.5 22.50 Elovich 7.5 12.5 17.5 22.5

k1 (min−1) 1.866.10−1 2.852.10−1 4.401.10−1 6.051.10−1 k2 (g mg−1 min−1) 2.578.10−3 2.979.10−3 4.028.10−3 5.152.10−3 kR 2.190.10−1 3.852.10−1 6.445.10−1 9.193.10−1 α (mg g−1 min−1) 3.251.101 1.274.102 4.880.102 1.555.103

qe,cal (mg g−1) 71.28 114.1 145.9 165.7 qe,cal (mg g−1) 84.93 129.3 160.0 178.4 qe,cal (mg g−1) 84.93 129.3 160.0 178.43 β (g mg) 5.127.10−2 4.046.10−2 3.986.10−2 4.198.10−2

R2 0.9859 0.9925 0.9930 0.9942 R2 0.9969 0.9990 1.000 1.000 R2 0.9969 0.9992 0.9996 0.9997 R2 0.9827 0.9918 0.9964 0.9975

χ2 2.603 5.724 7.674 6.623 χ2 0.255 0.485 0.442 0.314 χ2 0.255 0.485 0.442 0.314 χ2 1.539 3.265 2.997 2.909

Δq (%) 8.280 8.258 7.391 6.055 Δq (%) 2.169 2.596 1.974 1.313 Δq (%) 2.168 2.596 1.974 1.313 Δq (%) 6.217 6.033 4.747 4.442

RMSE 2.034 4.139 6.701 7.652 RMSE 0.955 1.346 1.538 1.701 RMSE 0.955 1.346 1.538 1.701 RMSE 2.253 4.330 4.778 5.008

ARE (%) 0.6856 0.6820 0.5463 0.3666 ARE (%) 0.04703 0.06740 0.03898 0.01723 ARE (%) 0.04702 0.06738 0.03898 0.01723 ARE (%) 0.3866 0.3640 0.2253 0.1973

origin point and, therefore, their intercepts are not close (or equal) to zero. However, the Weber-Morris plots for all initial concentrations display a good multi-linearity, signifying that (i) the rate-controlling step is not intra-particle diffusion, but it is involved in the adsorption process and (ii) the adsorption process is controlled by two or more simultaneous mechanisms. The results obtained from multi-linearity of Weber-Morris model (Table 2S) showed that PWAC-2 has higher values of kid,1 and kid,2 at higher initial concentrations, which can be ascribed to higher rapidness in the mass transfer of Pb(II) ion into the PWAC-2 structure. Fig. 5(b) also shows that, unlike at initial concentrations of 7.5 and 12.5 mg L−1, lead adsorption into the PWAC at initial concentrations of 17.5 and 22.5 mg L−1 has a third stage with a small intraparticle diffusion rate constant (kip,3) value which is related to the diffusion of Pb(II) ion into the fine pores existing in the most interior surface of PWAC-2. 3.3.7. Adsorption equilibrium Equilibrium study for an adsorption systems is a tool to describe the interaction of sorbate and sorbent at the equilibrium condition, which is very helpful in optimization of sorbent usage and designing full-scale plants for treatment of polluted waters and wastewaters. Therefore, to find the isotherm model best obeyed by the experimental results and to study the characteristics of Pb(II)/PWAC-2 interaction, the isotherm models described in Section 4S were employed. The isotherm equations were fitted with the adsorption data at various temperatures, and Fig. 6 and Figs. 4S–7S shows the obtained results. Also, R2 values, along with the statistical error indices, were summarised for all isotherms in Table 2 and Tables 3S–6S. The results show that, amongst the two-parameter models, the best data fit belongs to Langmuir model, indicating the occurrence of adsorption by a homogenous monolayer process without any internal interactions between the sorbed Pb(II) ions on the PWAC-2 surface. On the other hand, the studied three-parameter isotherms also show a good fit with the experimental results of Pb(II) adsorption at various temperatures, based on the high R2 values and the low values obtained for statistical indices (Table 2 and Tables 3S–6S). As reported in these tables, the theoretical maximum adsorption capacity (qmax) of new activated carbon obtained from all three-parameter models at various temperatures is close to the value obtained from Langmuir model (190.2 mg L−1). Also, all the exponents of three-parameter models are close to unity, signifying the adsorption of Pb(II) onto PWAC-2 is a homogeneous process, and the Langmuir model best describes the process.

Fig. 6. Plots of different isotherm models for sorption of Pb(II) onto PWAC-2 at pH 6.0, AC dosage 0.1 g L−1, and temperature 25 °C.

3.3.8. Comparison of the PWAC-2 with other activated carbons Table 3 lists the adsorption capacity of various activated carbons used for lead adsorption from aqueous solutions, along with activation agents used, activation agent/precursor weight ratios, pHZPC values, and SBET magnitudes. The results show that the adsorption capacity of PWAC-2 is higher than that of other activated carbons (ACs). Also, although the activation agent/precursor weight ratios utilized in this study are lower than the magnitudes used for the preparation of other ACs, the BET surface area of PWAC-2 is larger than that of other ones listed in Table 3. Regarding to these results, along with the other properties like rapid adsorption kinetic and the regular pores, it seems that PWAC-2 has a high potential to be used for Pb(II) adsorption from aqueous media. (Singh et al., 2008) (Wang et al., 2010) (Issabayeva et al., 2006) (Demiral et al., 2014) (Song et al., 2010) (Boudrahem et al., 2009) (Acharya et al., 2009) (Aroua et al., 2008) (Imamoglu and Tekir, 2008) (Goel et al., 2005) (Li et al., 2010) (Sekar et al., 2004) (Nadeem et al., 2006). 3.3.9. Thermodynamic studies Thermodynamic assessments are an indispensable and eminent component in adsorption studies since they indicate both the type of 41

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Table 2 Parameters of different isotherm models and the values of error functions computed for the removal of lead by PWAC-2 at 298 K. Isotherm

Calculated parameters and values of error functions

Langmuir

qmax (mg g−1) 190.2 KF (mg1−(1/n) L1/n g−1) 113.7 KT-P (L g−1) 28.03 E (kj mol −1) 9.685.10−1 KR-P (L mg−1) 367.459 qmax (mg g−1) 188.9 qmax (mg g−1) 188.3

Freundlich Tempkin Dubinin–Radushkevich Redlich-Peterson Sips Toth

nR-P 1.010 nS 0.9774 nT 0.9590

b (L mg−1) 1.983 n 3.733 B 35.60 q0 (mol g−1) 161.2 aR-P 1.897 KS (L g−1) 2.013 KT (mg L−1) 1.913

R2 0.9993 R2 0.9122 R2 0.9730 R2 0.8851 R2 0.9993 R2 0.9993 R2 0.9994

χ2 0.138 χ2 15.12 χ2 3.985 χ2 51.289 χ2 0.133 χ2 0.1447 χ2 0.1398

Δq (%) 1.480 Δq (%) 18.71 Δq (%) 7.503 Δq (%) 23.143 Δq (%) 1.535 Δq (%) 1.635 Δq (%) 1.610

RMSE 1.186 RMSE 13.07 RMSE 7.245 RMSE 14.952 RMSE 1.116 RMSE 1.130 RMSE 1.112

ARE (%) 0.02191 ARE (%) 3.499 ARE (%) 0.5630 ARE (%) 5.356 ARE (%) 0.02357 ARE (%) 0.02672 ARE (%) 0.02591

Table 3 List of sorption capacity of various activated carbons used for lead, along with activation agents used, activation agent/precursor weight ratios, pHZPC values, and SBET magnitudes. Precursor

Activating agent

Activation agent/precursor weight ratio

pHzpc

SBET (m2 g-1)

Maximum sorption capacity (mg g−1)

Ref.

Pistachio wood wastes

Stage 1: NH4NO3 Stage 2: NaOH sulfuric acid phosphoric acid NI (commercial) phosphoric acid NI (commercial) zinc chloride zinc chloride NI (commercial) zinc chloride

Stage 1: 1/20 Stage 1: < 1/3 NIa 2.4/1 NI 1/3 NI 3/4 1/1 NI 1/2

8.1

1884

190.2

This work

– – 1.43 ft 10.1 – – 1.9 –

– – 957.04 1611 1245 890b 1322 941 1092

134.22 98.39 95.2 138.9 17.193 71.429 43.85 82.42 13.05

NI (commercial) zinc chloride sulfuric acid Steem

NI 1/1 1.8/1 –

5.3 – – 8.17

1000 1688 265.96 725.0

21.88 146.85 26.50 –

Singh et al. (2008) Wang et al. (2010) Issabayeva et al. (2006) Demiral et al. (2014) Song et al. (2010) Boudrahem et al. (2009) Acharya et al. (2009) Aroua et al. (2008) Imamoglu and Tekir (2008) Goel et al. (2005) Li et al. (2010) Sekar et al. (2004) Nadeem et al. (2006)

Tamarind wood Polygonum orientale Linn Palm shell Chestnut shell Coconut-shell Coffee residue Tamarind wood Palm shell Hazelnut husks Coconut shell Enteromorpha prolifera (EP) Coconut shell pods of Moringa oleifera (M. oleifera) a b

Not Identified. Determined at activation agent/precursor weight ratio of 1/1.

where ΔH° is the enthalpy change of the process (kJ.mol−1), ΔS° is the entropy change of the process (kJ.mol−1), T is the temperature of system (K), and R is the universal gas constant (8.314 J mol−1.K−1). Also, ΔG° (the Gibbs energy change of the process; kJ.mol−1) of the process can be computed from the following relation:

Table 4 Computed thermodynamic parameters for Pb(II) sorption onto PWAC-2 at different temperatures. Temprature (K)

ΔG° (kJ mol−1)

ΔH° (kJ mol−1)

ΔS° (J mol−1 K−1)

288 298 308 318 328

−29.911 −32.018 −34.125 −36.232 −38.338

30.761

210.668

G° = H °

H° 1 × + R T

S° R

(11)

The slope and intercept of the plot of ln bM against 1/T (Fig. 8S) were used to calculate ΔH° and ΔS° during the process of Pb(II) adsorption on the for range above temperature (288–328 K), and ΔG° value at any studied temperature was computed from Eq. (11). The values of the thermodynamic parameters have been reported in Table 4 in which the negative values of ΔG° shows that Pb(II) adsorption onto new activated carbon is of a spontaneous, feasible, and favorable. Furthermore, the results revealed that an increase in temperature brings about a higher degree of spontaneity for the adsorption process, regarding to more negative values of ΔG° at higher temperatures. The positive value of ΔH° signifies that the Pb(II) removal is an endothermic nature. Similar results have been reported by Sekar et al. (2004), Acharya et al. (2009), and Li et al. (2010) who have studied various types of activated carbons for adsorption of Pb(II) ion from aqueous media. Since the ΔH° for a physisorption process has a value between 2.1 and 20.9 kJ/mol (Alahabadi et al., 2017; Tran et al., 2016), the ΔH° value for PWAC-2 (30.761 kJ/mol) indicates that, probably, a blend of physiochemical and physical interactions, like chelating complexations,

mechanism involved in the process and the nature of the process. Use of isotherm studies is the most accurate basis for calculating thermodynamic parameters (Rahmani-Sani et al., 2017; Tran et al., 2017b). Since the adsorption of Pb(II) ion onto PWAC-2 obey from Langmuir isotherm model, the thermodynamic equilibrium constant (dimensionless) is numerically equal to the Langmuir equilibrium constant in L.mol−1 (bM) can be used as a thermodynamic equilibrium constant (Azizian et al., 2018; Lima et al., 2019; Liu, 2009). Therefore, isothermal studies were repeated for five different temperatures (288, 298, 308, 318, and 328 K) and the obtained bM values were utilized for evaluating the thermodynamic parameters by the well-known van't Hoff equation in the following form:

ln (bM ) =

T S°

(10) 42

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ion-exchange, and π-cation donor–acceptor electron, are involved in the adsorption of Pb(II) ion onto this activated carbon. Also, the positive value of ΔS° indicates the more randomly organization lead ion at the solid/solution interface, which is attributable to the liberation of water of hydration during the adsorption process (HosseiniBandegharaei et al., 2016a; Khamirchi et al., 2018).

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4. Conclusion A good-quality activated carbon was produced from pistachio wood wastes by two-stage chemical activation using NH4NO3 and NaOH, respectively, as chemical activation agents. An L25 Taguchi experimental design was used by varying impregnation ratio, activation time and temperature in both pre- and post-activation stages. The influence of tested variables exhibited the following trend: Pre-stage impregnation ratio > Pro-stage impregnation ratio > Pro-stage temperature > Pre-stage temperature > Pre-stage activation time > Pro-stage activation time. The activated carbon sample produced at the best conditions (PWAC-2) was of mesoporous character and had a surface area of 190.2 m2g-1. Studying the influence of pH, the adsorption performance of PWAC-2 was compared to those of both commercial activated carbon (CAC) and activated carbon obtained from one-stage activation process (PWAC), and results revealed the superiority of PWAC-2 for lead adsorption at different pHs. Also, although the activation agent/precursor weight ratios utilized in this study are lower than the magnitudes used for the preparation of other ACs for Pb (II) adsorption, the adsorption capacity of PWAC-2 was higher than those of all the activated carbon applied for this purpose. Other advantages of PWAC-2, like rapid adsorption, high adsorption capacity, and the favorability of adsorption process, were derived from kinetic, equilibrium, and thermodynamic studies, respectively. Overall, PWAC2 is a very beneficial activated carbon for using in the treatment plants for removal of lead from water media. Acknowledgment Gonabad University of Medical Sciences, Gonabad, Iran, has offered the financial support of this work. We also thanks CNPq (Brazil) for financial support. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jenvman.2019.01.087. References Abdel-Halim, S., Shehata, A., El-Shahat, M., 2003. Removal of lead ions from industrial waste water by different types of natural materials. Water Res. 37, 1678–1683. Acharya, J., Sahu, J., Mohanty, C., Meikap, B., 2009. Removal of lead (II) from wastewater by activated carbon developed from Tamarind wood by zinc chloride activation. Chem. Eng. J. 149, 249–262. Alahabadi, A., Hosseini-Bandegharaei, A., Moussavi, G., Amin, B., Rastegar, A., KarimiSani, H., Fattahi, M., Miri, M., 2017. Comparing adsorption properties of NH4Clmodified activated carbon towards chlortetracycline antibiotic with those of commercial activated carbon. J. Mol. Liq. 232, 367–381. Alslaibi, T.M., Abustan, I., Ahmad, M.A., Foul, A.A., 2013. A review: production of activated carbon from agricultural byproducts via conventional and microwave heating. J. Chem. Technol. Biotechnol. 88, 1183–1190. Aroua, M.K., Leong, S.P.P., Teo, L.Y., Yin, C.Y., Daud, W.M.A.W., 2008. Real-time determination of kinetics of adsorption of lead(II) onto palm shell-based activated carbon using ion selective electrode. Bioresour. Technol. 99, 5786–5792. Azizian, S., Eris, S., Wilson, L.D., 2018. Re-evaluation of the century-old Langmuir isotherm for modeling adsorption phenomena in solution. Chem. Phys. 513, 99–104. Baccar, R., Bouzid, J., Feki, M., Montiel, A., 2009. Preparation of activated carbon from Tunisian olive-waste cakes and its application for adsorption of heavy metal ions. J. Hazard. Mater. 162, 1522–1529. Boudrahem, F., Aissani-Benissad, F., Aït-Amar, H., 2009. Batch sorption dynamics and equilibrium for the removal of lead ions from aqueous phase using activated carbon developed from coffee residue activated with zinc chloride. J. Environ. Manag. 90, 3031–3039.

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