Alkali modified hydrochar of grape pomace as a perspective adsorbent of Pb2+ from aqueous solution

Journal of Environmental Management 182 (2016) 292e300

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Research article

Alkali modified hydrochar of grape pomace as a perspective adsorbent of Pb2þ from aqueous solution Jelena T. Petrovi
c a, *, Mirjana D. Stojanovi
c a, Jelena V. Milojkovi
c a, Marija S. Petrovi
c a,  stari
c b, Marija L. Mihajlovi
ca Tatjana D. So c a, Mila D. Lausevi
a b

Institute for Technology of Nuclear and Other Mineral Raw Materials, 86 Franchet d’Esperey St., 11000 Belgrade, Serbia Faculty of Technology and Metallurgy, University of Belgrade, 4 Karnegijeva St., 11000 Belgrade, Serbia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 2 April 2016 Received in revised form 15 July 2016 Accepted 25 July 2016

Hydrochar produced via hydrothermal carbonization of grape pomace was considered as novel sorbent of Pb2þ from aqueous solution. In order to enhance the adsorption capacity, hydrochar was chemically modified using 2 M KOH solution. Both materials were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy and X-ray diffraction technique. Batch experiments were performed to examine the effect of sorbent dosage, pH and contact time. Obtained results showed that the KOH treatment increased the sorption capacity of hydrochar from 27.8 mg g1 up to 137 mg g1 at pH 5. Adsorption of lead on either of the materials was achieved through ion-exchange mechanism, chemisorption and Pb2þ-p interaction. The Sips isotherm model gave the best fit with the experimental data obtained for Pb2þ sorption using activated hydrochar. The adsorption kinetic followed a pseudo second-order model. Thermodynamic parameters implied that the Pb2þ binding for hydrochar surface was spontaneous and exothermic process. Findings from this work suggest that the hydrothermal carbonization is a promising route for production of efficient Pb 2þ sorbents for wastewater treatment. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Grape pomace hydrochar Lead removal KOH activation Adsorption isotherms Mechanism

1. Introduction Over the years, increasing anthropogenic activity and spillage of industrial waters into watercourses have become a worldwide environmental problem. Conventional methods for water purification, such as coagulation/flocculation, chemical oxidation, membrane filtration, adsorption on activated carbon and electrochemical treatment, are oftentimes expensive, polluting, time consuming and mostly ineffective at low concentrations of contaminants (Elaigwu et al., 2014; Iftikhar et al., 2009; Regmi et al., 2012; Kilic et al., 2013). Therefore, there is a growing interest of utilization of novel, low-cost and efficient technologies for wastewater treatment (Harman et al., 2007; Elaigwu et al., 2014). Due to its cumulative effects, heavy metals in waters are potential threat to the environment, humans and other living organisms (Regmi et al., 2012). One of the most prevalent, toxic metals in industrial effluents is lead (Pb2þ) (Milojkovi
c et al., 2014; Mohan et al., 2015). It is well known that Pb2þ may cause mental, brain

* Corresponding author. E-mail address: [email protected] (J.T. Petrovi
c). http://dx.doi.org/10.1016/j.jenvman.2016.07.081 0301-4797/© 2016 Elsevier Ltd. All rights reserved.

and liver damage, and also can be assimilated and concentrated in animal tissues (Guyo et al., 2015; Wang et al., 2015a, 2015b). The adsorption of Pb2þ using waste biomass has been confirmed as a simple, inexpensive and efficient method for treatment of polluted waters (Milojkovi
c et al., 2014; Wang et al., 2015a; Lu et al., 2012; Guyo et al., 2015). Comparatively, development of new methods for thermochemical conversion of waste biomass into valuable products is on the increase. Biochars derived from lignocellulosic residuals via dry pyrolysis and hydrothermal carbonization (HTC) has been broadly examined for different environmental applications (Sun et al., 2015; Mohan et al., 2015; Xue et al., 2012). The latter conversion procedure offers significant advantages relative to dry pyrolysis, such as mild reaction conditions and high conversion efficiency of wet biomass load (Tan et al., 2015). Produced hydrochars, although with lower porosity and carbon content in comparison to pyrolized biochars are very abundant with the reactive oxygen functional groups (OFGs) (e.g., hydroxyl and carboxylic) (Sun et al., 2015). Therefore, hydrochars may represent alternative low-cost and efficient sorbents of heavy metals from polluted waters (Xue et al., 2012; Regmi et al., 2012; Sun et al., 2015). Structure and adsorption capacity of hydrochar depends primarily on the HTC temperature,

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reaction time and feedstock type (Parshetti et al., 2013; Dai et al., 2014). In order to further enhance heavy metal sorption ability of hydrochars, several studies have investigated different methods of tailoring the hydrochar's surface structure (Jin et al., 2016; Regmi et al., 2012). Thus, activation of hydrochars obtained from peanut hull and Cymbopogon schoenanthus L. Spreng, using hydrogen peroxide (H2O2), have significantly improved hydrochar's sorption capacities for Pb2þ and Cu2þ, respectively (Xue et al., 2012; Zuo et al., 2016). Xue et al. (2012) have reported that the Pb2þ removal from aqueous solution using H2O2-activated hydrochar was enhanced more than 20 times in comparison to untreated material, while Zuo et al. (2016) have reported an increase of Cu2þ removal for 30%. Also, cold alkali modification of hydrochar using potassium hydroxide solution (KOH) has been proposed as a relatively simple procedure to obtain more efficient hydrochar-based heavy metal sorbents (Regmi et al., 2012, Sun et al., 2015). Regmi et al. (2012) have investigated the removal of Cu2þ and Cd2þ from the solution using untreated and KOH modified switchgrass hydrochar, while Sun et al. (2015) have examined the efficiency of KOH-activated hydrochars from sawdust, wheat straw and corn stalk as alternative adsorbents of Cd2þ and heavy metals from aqueous multimetal system. Both authors have reported that alkali modification enhances the content of OFGs on hydrochar surface, which subsequently improved metal adsorption capacity of activated hydrochars (Regmi et al., 2012; Sun et al., 2015). Grape is one of the most common fruit crops in Mediterranean Europe with annual production of about 29 million tones, of which more than 70% is consumed for the production of wine. During winemaking, significant amount of pomace is generated (20e25% from feedstock). Common disposal of this wet biomass on landfill sites may cause serious environmental problems like uncontrolled decomposition, moldering, bad odor spreading and leakage of waste fluids. Therefore appropriate reuse of pomace would contribute to global environmental protection and sustainable development of wine industry. In this study, for the first time suitability of grape pomace hydrochar (GP-HC) as an adsorbent for Pb2þ removal from aqueous solution was investigated. Additionally, inspired by studies of Regmi et al. (2012) and Sun et al. (2015), GP-HC was activated with 2 M KOH. The effect of pH, adsorbent dosage, concentration of the heavy metal, temperature and contact time on the adsorption process of a single component system for both materials was studied, in order to understand its kinetics, thermodynamics and mechanisms of Pb2þ removal. 2. Materials and methods 2.1. Chemicals All the chemicals and reagents used in the present study were of analytical grade. The primary standard solution of Pb2þ (2000 mg L1) was prepared by dissolving the weighed amount of Pb(NO3)2  3H2O in distilled water. Solutions of various Pb2þ concentrations used in experiments were prepared by diluting the primary stock solution with distilled water. 2.2. HTC hydrochar preparation Preparation of used HC has been described in detail in our previous study (Petrovi
c et al., 2016a). Briefly, about 250 g of the air dried and homogenized biomass (particle size <0.5 mm) was mixed with distilled water at 1:5 ratio and carbonized in 2000 mL autoclave (model 10253, Deutsch&Neumman), at 220  C 1 h. After a

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reaction period and cooling of the reactor to room temperature, solid and liquid products were separated by filtration and collected. The separated HC was then rinsed several times with distilled water and dried at 105  C in the oven. Alkali activation was carried out by stirring 5 g of the obtained HC with 500 mL of 2 M KOH solution for 1 h at room temperature (25 ± 0.5  C). Afterwards, the obtained activated hydrochar (HCact) was filtered, rinsed with distilled water and adjusted to the neutral pH with 0.1 M HNO3/KOH solution. During the final step, the HCact was filtered again and dried overnight in an oven at 105  C. 2.3. Characterization of hydrochar The point of zero charge (pHPZC) of HC and HCact surface was determined based on the method proposed by Milonjic et al. (Milonjic et al., 1975). More precisely, 0.1 g of HC and HCact were shaken with 50 mL of KNO3 solution (0.001 M and 0.01 M) for 24 h at 250 rpm. The initial pH values (pHi) of KNO3 were adjusted from 2.0 to 12.0 using 0.01 M HNO3 and/or 0.01 M KOH. After 24 h the suspension was filtered and the final pH was measured (pHf). The change in pHf vs pHi were plotted for the determination of the pHPZC. Scanning electron microscopy (SEM) was performed using a JSM-6610 (JEOL Inc., USA) in order to analyze the surface structure of HC and HCact before and after Pb2þ sorption. All samples were coated with gold, placed on the adhesive carbon disc and recorded. Spectroscopic analyses were conducted with Thermo Scientific Nicolet iS50 FT-IR spectrometer. The KBr pastilles with 0.8 mg sample and 80 mg KBr have been recorded in transmission mode to identify the chemical functional groups present on the original and Pb-loaded HCact. The spectra were obtained in the range of 4000e400 cm1. X-ray diffraction analyses were used to determine the phase composition of HCact before and after Pb2þ sorption. Samples were analyzed on the X-ray diffractometer (“PHILIPS”, Model PW-1710), with curved graphite monochromator and a scintillation counter. The intensity of diffracted X-ray CuKa (l ¼ 1.54178) were measured on the room temperature at intervals of 0.02 2q and time of 1 s, and in the range of 4 e65 2q. X-ray tube was loaded with a voltage of 40 kV and current of 30 mA, while the slots to guide primary and diffracted beam were 1 and 0.1 mm. 2.4. Batch adsorption studies In order to investigate the effect of pH, mass balance, contact time and Pb2þ initial concentration batch experiments were performed. The effect of solution pH on the adsorption of Pb2þ onto HC and HCact was conducted in the pH range from 2.0 to 7.0. Adsorbent dose of 0.5 g L1 was added to 100 mL volumetric flasks containing 50 mL of standard Pb2þ solution (100 mg L1). All flasks were put on top of a Heidolph Unimax1010 orbital shaker, and shaken at room temperature (25 ± 0.5  C) for 60 min at 250 rpm. The adsorbents doses of 0.4e4.0 g L1 were agitated with 50 mL of Pb2þ solution (100 mg L1) at pH 5.0 for 60 min in order to investigate the mass balance. Batch kinetic experiments were carried out at different contact times (5e120 min) contacting 0.5 g L1 of HC or HCact and 50 mL of Pb2þ solution (100 mg L1) at pH 5.0. For isotherm studies, 50 mL of lead solutions of different concentrations (40e180 mg L1) were shaken with 0.5 g L1 HCact for 60 min (250 rpm) at different temperatures (298, 308 and 318 K). To address the contribution of possible ion-exchange mechanism during Pb2þ sorption in obtained filtrates, except Pb2þ, the released cations (Ca2þ, Mg2þ, Kþ and Naþ) were also measured. The content of Ca2þ, Mg2þ, Kþ and Naþ released from HCact mixed only

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with distilled water at pH 5.0, served as a control to eliminate the potential influence of van der Waals forces and Hþ attachment under acidic conditions. The content all above mentioned cations in the resultant filtrates were measured using Atomic Absorption Spectrophotometer (AAS) (Analytic Jena Spekol 300). The pH value of all Pb2þ solutions was maintained constant at pH around 5.0 using 0.1 M KOH and/or 0.1 M HNO3 solutions. All experiments were performed in triplicate and average values of obtained results are shown. The amounts of metal ion adsorbed on hydrochars in mg g1 at equilibrium qeq were calculated using following equation:


qeq ¼

C0  Ceq m

 V

(1)

where V, is the volume of the Pb2þ solution (L), m is the amount of adsorbent (HC or HCact) (g); C0 and Ceq are the initial and equilibrium concentrations of the Pb2þ ions (mg L1) respectively. To investigate adsorption kinetics and isotherm models, linear and non-linear fitting method were employed using Origin 9.0 software.

3. Results and discussion 3.1. Characterization of adsorbent SEM images and EDX diagrams of HC and HCact are presented in Fig. 1(a, c), to illustrate potential structural differences caused by KOH treatment. Additionally, surface morphology and EDX graphs of both pristine and activated hydrochars loaded with Pb2þ have been presented in Fig. 1(b, d). SEM images of HC and HCact (Fig. 1(a, c)) revealed porous structures, with disorganized structural cracks and channels, of both materials. From these figures is quite obvious that the surface of HCact was rougher compared to the HC, presumably because the activation with KOH caused an increase of surface cracks due to the removal of impurities from the partially blocked pores (Trakal et al., 2014). These cracks and pores facilitate the diffusion of Pb2þ ions to the interior of HCact by providing a larger contact surface i.e. more binding sites for Pb2þ, in comparison to the HC. Absence of Pb2þ aggregates on the surface of both HC and HCact indicates that the precipitation on surface did not occur. The EDX spectrum for HC and HCact showed the presence of O, Fe, Mg, Al, Si, K, Ca and Na peaks. Furthermore, the successful Pb2þ-

Fig. 1. SEM images and corresponding EDX spectra of (a) HC, (b) HC after Pb2þ adsorption, (c) HCact and (d) HCact after Pb2þ adsorption.

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loading was confirmed by Pb peaks present on EDX graphs. The HCact exhibited a higher Pb2þ peak than HC, which may be an indication of its enhanced adsorption upon alkali activation. Besides, significant reductions of K peak in Pb-loaded HCact, was followed by the decreasing of Ca peak in EDX (Fig 1(d)). This may indicate the presence of ion-exchange mechanism during Pb2þ binding to the HCact. The XRD patterns of the HC and HCact are showed and compared on Fig. 2(a) and (b). The crystal cellulose structure in HCs was confirmed by distinct sharp peak at 2q ¼ 22.7, which is assigned to the (002) interlayer reflection (Fig. 2(a)) (Parshetti et al., 2013). This suggests that cellulose was not degraded completely after HTC at 220  C. However, the peak of cellulose has weakened in the HCact as a result of partial hydrolysis of cellulose due to KOH treatment (Fig. 2(b)). Conversely, increase of crystallinity from 34.0% in the HC to 41.7% in the HCact was appreciable. Enhancement of crystallinity index after alkali treatment may be induced by a partial removal of the amorphous constituents of the fibers (Sghaier et al., 2012). Similarly, Sghaier et al. (2012) observed an increase of crystallinity due to the alkali treatment of Agave americana L. fiber. Furthermore, HC and HCact have showed reduction of crystalline index, as well as the intensity of peak at 2q ¼ 22.7 after Pb2þ sorption, probably due to participation of crystalline components in Pb2þ removal (Fig. 2a, b). FTIR spectra of HC and HCact before and after Pb2þ sorption are shown in Fig. 2(c, d). The FTIR spectra of the HC exhibited characteristic aromatic peaks, as well as presence of OFGs that have been reported in our previous work (Petrovi
c et al., 2016a). After alkali treatment the peaks at 3385 cm1, 1617, 1058 and 1033 cm1 attributed to OeH vibrations of hydroxyl groups and CeO stretching vibration of primary and/or alkyl substituted ethers became more apparent (Fig. 2(d)). Sun et al. (2015) previously reported

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similar observations for KOH modified hydrochars. This suggests that HCact have more OFGs than HC, i.e. more electron donating sites for Pb2þ, and hence an increased Pb2þ adsorption ability. Also, the peak at 1617 cm1 attributed to aromatic C]C bond in HCact was more prominent (Fernandez et al., 2015). Aforementioned groups play a major role during interaction between hydrochar surface and Pb2þ (Hamid et al., 2014). On the contrary, bands at 1445 (CeH deformation in the lignin and carbohydrates), 1369 (phenolic eOH), 1280 (eOCH3 from lignin), 1161 (eCeOeCe from cellulose), 782 cm1 (aromatic CH3) showed decreased intensity, while 1701 (C]O from cellulose and lignin), 1513 (C]C from aromatic ring) and 1318 cm1 (CeO stretching band from syringyl groups) completely disappeared. Interaction of metal ions with the OFGs (eCOOH and eOH) on HC surface led to the reduction of these bands intensities after Pb2þ sorption and its shift to a lower wavelengths (Fig. 2(c), (d)). This is a sign of one of the proposed Pb2þ binding mechanisms, complexation with OFGs followed by Hþ release. Reduction of solution pH (from pH of 5.0 to 4.34) during adsorption confirmed these allegations. This result is in accordance with findings previously reported by Wang et al. (2015a). Additionally, weakening and shifting of aromatic C]C (1617 cm1) and CeH (782 cm1) bands after Pb2þ sorption suggests a second possible mechanism, Pb2þ-p electron interaction (Wang et al., 2015a). Namely, heterocyclic compounds may easily bind Pb2þ through the formation of coordination bonds between the d-electron of metal and C]C (p electron) bond in the unsaturated and aromatic systems (Wang et al., 2015a).

3.2. The effect of pH and pHpzc The effect of solution pH was investigated in the range of 2.0e7.0, and the results are shown in Fig. 3(a). It was observed that

Fig. 2. X-ray diffractograms of (a) HC and (b) HCact and FT-IR spectra of (c) HC and (d) HCact before and after Pb2þ sorption.

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an increase of pH value resulted in an increase of Pb2þ removal, and reached a maximum at a pH 5.0. At lower pH values removal of Pb2þ was hindered as a result of competition between Pb2þ and Hþ ions for active sites on the HCact surface (Mohan et al., 2015; Singh et al., 2008; Wang et al., 2015b). On the other hand, increased pH led to the reduction of Hþ concentration in the solution, thus the functional groups of hydrochar surface (mostly OFGs) become deprotonated and more applicable for the binding of positively charged metal ions (Wang et al., 2015b). Above the pH of 6.0, a slight decrease in sorption of Pb2þ was observed, probably due to striving of Pb2þ ions to hydrolyze and precipitate rather than to being

adsorbed. In order to avoid the formation of metal hydroxides precipitates, all equilibrium and kinetic studies were carried out at an initial pH of 5.0. Additional insight into the pH influence on the removal of metals provides pHPZC of the sorbent. At pH < pHPZC the surface charge of the sorbent is positive which facilitate removal of anions (Wang et al., 2015b; Petrovi
c et al., 2016b). Conversely, at pH > pHPZC, binding of cations is favored. The pHPZC of HC has been 4.5 while the highest adsorption was achieved at pH 5 which is in agreement with the concept of pHPZC. On the contrary, the pHPZC of HCact was 6.0 (Fig. S1). As the maximum of the adsorption occurs at pH < pHPZC (Fig. 3(a)), it could be concluded that repulsive electrostatic forces have shown meager influence, and that other mechanisms of metal binding are responsible for the removal of Pb2þ using HCact (Milojkovi
c et al., 2014; Petrovi
c et al., 2016b). 3.3. Effect of adsorbent dosage The effects of HC and HCact doses on Pb2þ sorption are shown in Fig. 3(b). The amount of Pb2þ adsorbed by the different doses (0.4e4.0 g L1) of HC and HCact was reduced from 28.0 to 12.2 mg g1 and 137.0 to 47.8 mg g1, respectively (Fig. 3(b)). In both cases, the dose of 0.5 g L1 gave the highest qeq values. Similar trends have been reported earlier for Pb2þ removal using different adsorbents (Bulut and Baysal, 2006; OuYang et al., 2014; Petrovi
c et al., 2016b; Singh et al., 2008). Petrovi
c et al. (2016b) observed decrease of corn silk capacity from 67 to 11.1 mg g1 as the dose increase from 1 to 16 g L1, while OuYang et al. (2014) reported decrease of capacity from 36.42 to 8.51 mg g1 with a rise of partially hydrolyzed bamboo dose from 1 to 5 g L1. 3.4. Effect of contact time The effects of contact time on Pb2þ removal from aqueous solution by both hydrochars were performed in time period between 5 and 120 min. As Fig. 3(c) shows, sorption of metal mostly occurred during the initial 60 min in the HC and 45 min in the HCact. The rapid removal was presumably caused by an abundance of active sites on the surface of both hydrochars (Guyo et al., 2015). Lead sorption capacity, qeq, of HC and HCact, were 27.8 mg g1 and 137 mg g1, respectively, suggesting that the KOH treatment significantly enhanced the ability of HC to remove solution Pb2þ. Similar results of the efficiency of alkali activated hydrochars have been reported by Regmi et al. (2012) and Sun et al. (2015) for Cu2þ and Cd2þ, respectively. 3.5. Kinetics studies Three kinetic models were applied to the obtained experimental data of Pb2þ removal using HC and HCact within 120 min: the Lagergren pseudo-first order (Lagergren, 1898), pseudo-second order (Ho and McKay, 1999) and Weber-Morris intra-particle diffusion model (Weber and Morris, 1963). The linear Lagergren pseudo-first-order rate could be expressed by equation:

 .   .  1=qt ¼ k1 qeq ð1 þ tÞ þ 1 qeq

(2)

The pseudo-second-order rate is presented as:

  .   t=qt ¼ 1=k2 q2eq þ 1 qeq t

Fig. 3. Effect of process parameters on Pb adsorbent dosage and (c) contact time.

2þ

adsorption onto HC and HCact: (a) pH, (b)

(3)

where qeq and qt are the amount of Pb2þ ion adsorbed per HCact (mg g1) at equilibrium and time t calculated from kinetic models. The k1 (min1), and k2 (g mg1 min1) are the pseudo-first-order,

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and pseudo-second-order rate constant respectively. While the intra-particle diffusion model is given as equation:

q ¼ Kid t 0:5 þ C

(4)

where q (mg g1) is amount adsorbed in time t (min), Kid represents the intra-particle diffusion rate constant (mg g1 min1/2) and C is the intercept. The Kid can be calculated from the slope of the linear plot of q versus t1/2. The results of pseudo-second kinetic model are displayed in Fig. 4, while calculated coefficients and sorption capacities of the applied kinetic models are summarized in Table 1. The correlation coefficient (R2) for both hydrochars were closest to unit (R2 ¼ 0.999) for the pseudo-second-order model. Moreover, the equilibrium adsorption capacities determined by pseudo-second model were in correlation with experimental results (qeq,exp). Therefore, it could be concluded that sorption of Pb2þ on the HC and HCact followed pseudo-second order model. This model implies chemical adsorption as the rate controlling step (Ho and McKay, 1999) and involves the chemical interaction between Pb2þ ions and polar functional groups at the surface on the HC and HCact (Ho et al., 2001). This is in agreement with results of FTIR analysis of the Pb-loaded HCact. Also, the pseudo-second order has been described

297

previously for sorption kinetic of Pb2þ onto Phytolacca americana L (Wang et al., 2015b). and Prosopis africana shell hydrochar (Elaigwu et al., 2014). Weber-Morris intra-particle diffusion model for Pb2þ removal onto HC and HCact did not pass modeling procedure using Origin 9.0, suggesting that intra-particle diffusion is not the only ratecontrolling step. As can be seen from Fig. S2, two different linear zones are observed. The first zone indicates that adsorption of metal ions occurred on the adsorbents external surface, while the second linearity suggesting diffusion and intra-particle adsorption of Pb2þ ions on the active sites (Liu and Zhang, 2009; Wang et al., 2015b). Thus, it can be concluded that the removal of Pb2þ with HC and HCact occurs in two simultaneous stages. Similar results have been observed by Liu and Zhang (2009) for Pb2þ removal using pinewood and rice husk hydrochars. 3.6. Sorption isotherm models Three isotherm models, Langmuir, Freudlich and Sips were applied for analyzing the Pb2þ adsorption process on HCact surface in accordance to the obtained experimental data. Sorption experiments were conducted at contact time of 60 min to ensure equilibrium. The Langmuir isotherm model is based on the assumption of a monolayer adsorption process onto a homogeneous surface, with constant adsorption energy and without transmigration of adsorbate in the plane of the surface. This isotherm model has been represented by the following equation (Langmur, 1918):

qe ¼

qm KL Ce 1 þ KL Ce

(5)

where the Ce is the equilibrium concentration (mg L1), qm is the maximum amounts of Pb2þ ion adsorbed on the hydrochars (mg g1) and KL is the Langmuir constant (L mg1), related to the affinity of the binding sites. The Freundlich isotherm, based on the assumption of chemisorption onto heterogeneous surface with different sites energies, can be expressed in its non-linear form (Freundlich, 1906): 1=n

qe ¼ KF Ce

Fig. 4. Pseudo-second order kinetic adsorption curves of HC and HCact (pH ¼ 5.0, T ¼ 25  C, t ¼ 60 min, adsorbent dosage 0.5 g L1).

Table 1 Kinetic parameters for Pb2þ adsorption onto HC and HCact. Adsorbent

1

qeq, exp (mg g ) Pseudo-first-order model qeq, cal (mg g1) k1 (min1) R2 Pseudo-second-order model qeq, cal (mg g1) k2 (g mg1 min1) R2 Weber-Morris diffusion model Kid1 (mg g1 min1/2) C1 (mg g1) R2 Kid2 (mg g1 min1/2) C2 (mg g1) R2

HC

HCact

27.8

137

25.6 10.0 0.899

136 1.24 0.901

33.3 0.001 0.985

138 0.004 0.999

2.74 1.92 0.996 0.16 28.88 0.401

5.31 97.94 0.960 0.27 138.95 0.837

(6)

where qe, KF, Ce, 1/n represents the amount of the adsorbate per unit weight of adsorbent (mg g1), a constant relating to adsorption capacity (Freundlich constants), equilibrium Pb2þ ion concentration in the solution (mg L1) and adsorption intensity, respectively. The Sips model, also known as the Langmuir-Freundlich model, is useful to anticipate heterogeneous adsorption as well as to exceed the limitation of the increasing concentration that occurs in Freundlich isotherm model (Sips, 1948). At the low sorbate concentration this isotherm is reduced to the Freundlich isotherm, while at high concentration of sorbate, Sips model assume form of the Langmuir isotherm. The Sips model is commonly expressed by the equation:

qe ¼ qm þ

KS Cens 1 þ KS Cens

(7)

where qm represents the maximum adsorption capacity (mg g1), KS the equilibrium constant (L mg1) and nS is the model exponent and vary between 0 and 1 (Singh et al., 2008). The obtained isotherm parameters are summarized in Table 2. The correlation coefficient R2 showed that the Sips isotherm gave the best fit with the experiment (Table 2). Alatalo et al. (2013) also observed the best fit of the Sips model in relation to other models

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Table 2 Parameters and determination coefficients of the isotherm models for Pb2þ adsorbed onto HCact. Langmuir isotherm model

HCact

Freundlich isotherm model

Sips isotherm model

qm (mg g1)

KL (L mg1)

R2

KF (mg g1) (L mg1)1/n

1/n

R2

qm (mg g1)

KS (L mg1)

R2

140.8

0.7700

0.9861

103.9

0.0700

0.8131

139.0

0.5600

0.9985

DG ¼ RT ln Kd

(8)

DG ¼ DH  T DS

(9)

ln Kd ¼ DH =RT þ DS =R

(10)

where the Kd is the distribution coefficient for adsorption (qe/Ce), R is the universal gas constant (8.314  103 kJ moL1 K1), T is the temperature (K). The DH and DS values were obtained from the slope and the intercept of the plot 1/T versus ln Kd. The values of thermodynamic parameters, determined for the three test temperatures are listed in Table 3. The negative values of DG imply that the adsorption of Pb2þ onto HCact was spontaneous and favorable process (Anastopoulos et al., 2013). The DH is also negative, revealing the exothermic nature of the process. The positive values of DS suggest increasing randomness in the solid/solution system interface during the adsorption process (Milojkovi
c et al., 2014). Fig. 5. Non-linear fits of Sips isotherm model for the Pb2þ adsorption by HCact at 298, 307 and 318 K (pH ¼ 5.0, t ¼ 60 min, adsorbent dosage 0.5 g L1).

for defining the adsorption of Pb2þ onto anaerobically digested sludge and pulp and paper industry sludge hydrochars. Besides, maximum of sorption capacity of the Sips model (qm) for the HCact (of 139 mg g1) is in excellent agreement with the experimental one. Additionally, HCact showed better adsorption capacity for Pb2þ than activated carbon (26.94 mg g1) (Singh et al., 2008), peanut shell (52.80 mg g1) (Wang et al., 2015a) and sludge-derived biochars (30.88 mg g1) (Lu et al., 2012), Prosopis africana shell (45.30 mg g1) (Elaigwu et al., 2014), pine wood (4.25 mg g1), rice husk (2.4 mg g1) hydrochars (Liu and Zhang, 2009), as well as H2O2-modified peanut hull (22.82 mg g1) hydrochar (Xue et al., 2012). According to these findings it can be emphasized that GPHCact may be used as a perspective adsorbent of Pb2þ from aqueous solution. The Sips isotherm model for the Pb2þ adsorption on the HCact at different temperatures is presented in Fig. 5. 3.7. Thermodynamic studies The thermodynamic parameters, the changes of Gibbs free energy (DG ), enthalpy (DH ) and entropy (DS ) determined by studying the sorption process of Pb2þ by HCact at three different temperatures (298, 308 and 318 K) were calculated according to the following equations:

3.8. Pb2þ adsorption mechanisms Previous studies have shown that the sorption of heavy metals was directly correlated with the amount OFGs on hydrochar's surface (Xue et al., 2012; Liu and Zhang, 2009). This is confirmed by FTIR analysis where two possible mechanisms were introduced (i) surface complexation between OFGs and Pb2þ and (ii) Pb2þ- p interaction with electrons of C]C. Additionally, the FTIR results revealed the possible presence of ionexchange between Pb2þ and cations containing functional groups (ReOeMþ or ReCOOeMþ) (Lu et al., 2012). In order to confirm ion-exchange mechanism, amount of Kþ, Naþ, Mg2þ, Ca2þ in filtrates after sorption of Pb2þ were measured at different initial Pb2þ concentration (Fig. 6). While Pb2þ occupies binding sites during sorption, these cations should be released into filtrates. The obtained results indicated that the ion-exchange mechanism might partly contribute to the removal of Pb2þ by HCact. The most significant cation involved in ion-exchange was Kþ followed by Ca2þ, Mg2þ and Naþ in a descending manner (Fig. 6). Generally, total amount of the exchangeable cations released from HCact was lower than total amount of Pb2þ removed from solution. Hence, it may be concluded that ionexchange mechanism, followed by chemisorption and surface complexation, are major binding mechanisms of Pb2þ onto activated hydrochar of grape pomace. This is in accordance with previously proposed mechanisms of Pb2þ binding for peanut

Table 3 Thermodynamic parameters. Temperature (K)

298 308 318

Parameters

DG (kJ mol1)

DH (kJ mol1)

DS (J mol1 K1)

qeq (mg g1)

17.91 18.44 18.97

2.070

53.14

139.0 139.5 127.0

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Fig. 6. A comparative view of the adsorbed Pb2þ and the released cations during ion exchange at different Pb2þ initial concentrations.

shell biochar (Wang et al., 2015a), sludge-derived biochar (Lu et al., 2012) and peanut hull hydrochar surface (Xue et al., 2012). 4. Conclusion The ability of HC and HCact to remove lead from aqueous solutions was studied in batch mode examining the influence of solution pH value, adsorbent dose, and contact time. It was found that the increase of Pb2þ removal using KOH-activated GP-HC rose almost fivefold in comparison to the untreated GP-HC. The obtained Pb2þ sorption capacity of HCact (137 mg g1) was much higher than that of commercial activated carbon (26.94 mg g1) and many other similar materials. The results of XRD, SEM and FTIR analysis of both, HC and HCact, showed structural changes which might affect the increase of the Pb2þ sorption following KOH treatment. Furthermore, FTIR spectra of samples before and after sorption seem to correlate well with the proposed mechanisms of metal binding for OFGs present on the HC surface. The sorption isotherms of Pb2þ onto HCact were well described by the Sips model, while kinetic of Pb2þ removal followed the pseudo secondorder kinetic model. Thermodynamic studies reveled that Pb2þ sorption on alkali activated hydrochar is spontaneous and exothermic process. Our results suggest that the GP can be a promising precursor for production of low-cost hydrochars for lead removal from wastewaters. Acknowledgment The authors are grateful to the Serbian Ministry of Education, Science and Technological Development for the financial support of this investigation included in the project TR 31003. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jenvman.2016.07.081. References €kil€ €a €, M., 2013. Alatalo, S.M., Repo, E., Ma a, E., Salonen, J., Vakkilainen, E., Sillanpa Adsorption behavior of hydrothermally treated municipal sludge & pulp and paper industry sludge. Bioresour. Technol. 147, 71e76. Anastopoulos, I., Massas, I., Ehaliotis, C., 2013. Composting improves biosorption of Pb2þ and Ni2þ by renewable lignocellulosic materials. Characteristics and mechanisms involved. Chem. Eng. J. 231, 245e254. Bulut, Y., Baysal, Z., 2006. Removal of Pb(II) from wastewater using wheat bran. J. Environ. Manag. 78, 107e113.

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