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Journal of Environmental Management 182 (2016) 292e300

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Journal of Environmental Management

journal homepage: www.elsevier .com/locate/ jenvman

Research article

Alkali modified hydrochar of grape pomace as a perspective adsorbentof Pb2þ from aqueous solution

Jelena T. Petrovi�c a, *, Mirjana D. Stojanovi�c a, Jelena V. Milojkovi�c a, Marija S. Petrovi�c a,Tatjana D. �So�stari�c a, Mila D. Lau�sevi�c b, Marija L. Mihajlovi�c a

a Institute for Technology of Nuclear and Other Mineral Raw Materials, 86 Franchet d’Esperey St., 11000 Belgrade, Serbiab 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

Article history:Received 2 April 2016Received in revised form15 July 2016Accepted 25 July 2016Available online 3 August 2016

Keywords:Grape pomace hydrocharLead removalKOH activationAdsorption isothermsMechanism

* Corresponding author.E-mail address: j.petrovic@itnms.ac.rs (J.T. Petrovi

http://dx.doi.org/10.1016/j.jenvman.2016.07.0810301-4797/© 2016 Elsevier Ltd. All rights reserved.

a b s t r a c t

Hydrochar produced via hydrothermal carbonization of grape pomace was considered as novel sorbentof Pb2þ from aqueous solution. In order to enhance the adsorption capacity, hydrochar was chemicallymodified using 2 M KOH solution. Both materials were characterized by Fourier transform infraredspectroscopy, scanning electron microscopy and X-ray diffraction technique. Batch experiments wereperformed to examine the effect of sorbent dosage, pH and contact time. Obtained results showed thatthe KOH treatment increased the sorption capacity of hydrochar from 27.8 mg g�1 up to 137 mg g�1 at pH5. 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 experimentaldata obtained for Pb2þ sorption using activated hydrochar. The adsorption kinetic followed a pseudosecond-order model. Thermodynamic parameters implied that the Pb2þ binding for hydrochar surfacewas spontaneous and exothermic process. Findings from this work suggest that the hydrothermalcarbonization is a promising route for production of efficient Pb 2þ sorbents for wastewater treatment.

© 2016 Elsevier Ltd. All rights reserved.

1. Introduction

Over the years, increasing anthropogenic activity and spillage ofindustrial waters into watercourses have become a worldwideenvironmental problem. Conventional methods for water purifi-cation, such as coagulation/flocculation, chemical oxidation,membrane filtration, adsorption on activated carbon and electro-chemical treatment, are oftentimes expensive, polluting, timeconsuming and mostly ineffective at low concentrations of con-taminants (Elaigwu et al., 2014; Iftikhar et al., 2009; Regmi et al.,2012; Kilic et al., 2013). Therefore, there is a growing interest ofutilization of novel, low-cost and efficient technologies for waste-water treatment (Harman et al., 2007; Elaigwu et al., 2014).

Due to its cumulative effects, heavy metals in waters are po-tential threat to the environment, humans and other living organ-isms (Regmi et al., 2012). One of the most prevalent, toxic metals inindustrial effluents is lead (Pb2þ) (Milojkovi�c et al., 2014; Mohanet al., 2015). It is well known that Pb2þ may cause mental, brain

�c).

and liver damage, and also can be assimilated and concentrated inanimal tissues (Guyo et al., 2015; Wang et al., 2015a, 2015b). Theadsorption of Pb2þ using waste biomass has been confirmed as asimple, inexpensive and efficient method for treatment of pollutedwaters (Milojkovi�c et al., 2014; Wang et al., 2015a; Lu et al., 2012;Guyo et al., 2015).

Comparatively, development of new methods for thermo-chemical conversion of waste biomass into valuable products is onthe increase. Biochars derived from lignocellulosic residuals via drypyrolysis and hydrothermal carbonization (HTC) has been broadlyexamined for different environmental applications (Sun et al., 2015;Mohan et al., 2015; Xue et al., 2012). The latter conversion proce-dure offers significant advantages relative to dry pyrolysis, such asmild reaction conditions and high conversion efficiency of wetbiomass load (Tan et al., 2015). Produced hydrochars, althoughwithlower porosity and carbon content in comparison to pyrolizedbiochars are very abundant with the reactive oxygen functionalgroups (OFGs) (e.g., hydroxyl and carboxylic) (Sun et al., 2015).Therefore, hydrochars may represent alternative low-cost andefficient sorbents of heavy metals from polluted waters (Xue et al.,2012; Regmi et al., 2012; Sun et al., 2015). Structure and adsorptioncapacity of hydrochar depends primarily on the HTC temperature,

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J.T. Petrovi�c et al. / Journal of Environmental Management 182 (2016) 292e300 293

reaction time and feedstock type (Parshetti et al., 2013; Dai et al.,2014).

In order to further enhance heavy metal sorption ability ofhydrochars, several studies have investigated different methods oftailoring the hydrochar's surface structure (Jin et al., 2016; Regmiet al., 2012). Thus, activation of hydrochars obtained from peanuthull and Cymbopogon schoenanthus L. Spreng, using hydrogenperoxide (H2O2), have significantly improved hydrochar's sorptioncapacities for Pb2þ and Cu2þ, respectively (Xue et al., 2012; Zuoet al., 2016). Xue et al. (2012) have reported that the Pb2þ

removal from aqueous solution using H2O2-activated hydrocharwas enhanced more than 20 times in comparison to untreatedmaterial, 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 simpleprocedure to obtain more efficient hydrochar-based heavy metalsorbents (Regmi et al., 2012, Sun et al., 2015). Regmi et al. (2012)have investigated the removal of Cu2þ and Cd2þ from the solutionusing untreated and KOH modified switchgrass hydrochar, whileSun et al. (2015) have examined the efficiency of KOH-activatedhydrochars from sawdust, wheat straw and corn stalk as alterna-tive adsorbents of Cd2þ and heavy metals from aqueous multi-metal system. Both authors have reported that alkali modificationenhances the content of OFGs on hydrochar surface, which subse-quently improved metal adsorption capacity of activated hydro-chars (Regmi et al., 2012; Sun et al., 2015).

Grape is one of the most common fruit crops in MediterraneanEurope with annual production of about 29 million tones, of whichmore than 70% is consumed for the production of wine. Duringwinemaking, significant amount of pomace is generated (20e25%from feedstock). Common disposal of this wet biomass on landfillsites may cause serious environmental problems like uncontrolleddecomposition, moldering, bad odor spreading and leakage ofwaste fluids. Therefore appropriate reuse of pomace wouldcontribute to global environmental protection and sustainabledevelopment of wine industry.

In this study, for the first time suitability of grape pomacehydrochar (GP-HC) as an adsorbent for Pb2þ removal from aqueoussolution was investigated. Additionally, inspired by studies ofRegmi et al. (2012) and Sun et al. (2015), GP-HC was activated with2 M KOH. The effect of pH, adsorbent dosage, concentration of theheavy metal, temperature and contact time on the adsorptionprocess of a single component system for both materials wasstudied, in order to understand its kinetics, thermodynamics andmechanisms of Pb2þ removal.

2. Materials and methods

2.1. Chemicals

All the chemicals and reagents used in the present study were ofanalytical grade. The primary standard solution of Pb2þ

(2000 mg L�1) was prepared by dissolving the weighed amount ofPb(NO3)2 � 3H2O in distilled water. Solutions of various Pb2þconcentrations used in experiments were prepared by diluting theprimary stock solution with distilled water.

2.2. HTC hydrochar preparation

Preparation of used HC has been described in detail in ourprevious study (Petrovi�c et al., 2016a). Briefly, about 250 g of the airdried and homogenized biomass (particle size

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with distilled water at pH 5.0, served as a control to eliminate thepotential influence of van der Waals forces and Hþ attachmentunder acidic conditions.

The content all above mentioned cations in the resultant fil-trates were measured using Atomic Absorption Spectrophotometer(AAS) (Analytic Jena Spekol 300). The pH value of all Pb2þ solutionswas maintained constant at pH around 5.0 using 0.1 M KOH and/or0.1 M HNO3 solutions. All experiments were performed in triplicateand average values of obtained results are shown.

The amounts of metal ion adsorbed on hydrochars in mg g�1 atequilibrium 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 ofadsorbent (HC or HCact) (g); C0 and Ceq are the initial and equilib-rium concentrations of the Pb2þ ions (mg L�1) respectively.

To investigate adsorption kinetics and isotherm models, linearand non-linear fitting method were employed using Origin 9.0software.

Fig. 1. SEM images and corresponding EDX spectra of (a) HC, (b) HC af

3. Results and discussion

3.1. Characterization of adsorbent

SEM images and EDX diagrams of HC and HCact are presented inFig. 1(a, c), to illustrate potential structural differences caused byKOH treatment. Additionally, surface morphology and EDX graphsof both pristine and activated hydrochars loaded with Pb2þ havebeen presented in Fig. 1(b, d).

SEM images of HC and HCact (Fig. 1(a, c)) revealed porousstructures, with disorganized structural cracks and channels, ofboth materials. From these figures is quite obvious that the surfaceof HCact was rougher compared to the HC, presumably because theactivationwith KOH caused an increase of surface cracks due to theremoval of impurities from the partially blocked pores (Trakal et al.,2014). These cracks and pores facilitate the diffusion of Pb2þ ions tothe interior of HCact by providing a larger contact surface i.e. morebinding sites for Pb2þ, in comparison to the HC. Absence of Pb2þ

aggregates on the surface of both HC and HCact indicates that theprecipitation 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-

ter Pb2þ adsorption, (c) HCact and (d) HCact after Pb2þ adsorption.

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loading was confirmed by Pb peaks present on EDX graphs. TheHCact exhibited a higher Pb2þ peak than HC, which may be anindication of its enhanced adsorption upon alkali activation. Be-sides, significant reductions of K peak in Pb-loaded HCact, was fol-lowed by the decreasing of Ca peak in EDX (Fig 1(d)). This mayindicate 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 wasconfirmed by distinct sharp peak at 2q¼ 22.7�, which is assigned tothe (002) interlayer reflection (Fig. 2(a)) (Parshetti et al., 2013). Thissuggests that cellulose was not degraded completely after HTC at220 �C. However, the peak of cellulose has weakened in the HCact asa result of partial hydrolysis of cellulose due to KOH treatment(Fig. 2(b)). Conversely, increase of crystallinity from 34.0% in the HCto 41.7% in the HCact was appreciable. Enhancement of crystallinityindex after alkali treatment may be induced by a partial removal ofthe amorphous constituents of the fibers (Sghaier et al., 2012).Similarly, Sghaier et al. (2012) observed an increase of crystallinitydue to the alkali treatment of Agave americana L. fiber. Furthermore,HC and HCact have showed reduction of crystalline index, as well asthe intensity of peak at 2q¼ 22.7� after Pb2þ sorption, probably dueto participation of crystalline components in Pb2þ removal (Fig. 2a,b).

FTIR spectra of HC and HCact before and after Pb2þ sorption areshown in Fig. 2(c, d). The FTIR spectra of the HC exhibited charac-teristic aromatic peaks, as well as presence of OFGs that have beenreported in our previous work (Petrovi�c et al., 2016a). After alkalitreatment the peaks at 3385 cm�1, 1617, 1058 and 1033 cm�1

attributed to OeH vibrations of hydroxyl groups and CeO stretch-ing vibration of primary and/or alkyl substituted ethers becamemore apparent (Fig. 2(d)). Sun et al. (2015) previously reported

Fig. 2. X-ray diffractograms of (a) HC and (b) HCact and FT-IR spe

similar observations for KOH modified hydrochars. This suggeststhat HCact have more OFGs than HC, i.e. more electron donatingsites for Pb2þ, and hence an increased Pb2þ adsorption ability. Also,the peak at 1617 cm�1 attributed to aromatic C]C bond in HCactwas more prominent (Fernandez et al., 2015). Aforementionedgroups play a major role during interaction between hydrocharsurface and Pb2þ (Hamid et al., 2014). On the contrary, bands at1445 (CeH deformation in the lignin and carbohydrates), 1369(phenolic eOH), 1280 (eOCH3 from lignin), 1161 (eCeOeCe fromcellulose), 782 cm�1 (aromatic CH3) showed decreased intensity,while 1701 (C]O from cellulose and lignin), 1513 (C]C from aro-matic ring) and 1318 cm�1 (CeO stretching band from syringylgroups) completely disappeared. Interaction of metal ions with theOFGs (eCOOH andeOH) on HC surface led to the reduction of thesebands intensities after Pb2þ sorption and its shift to a lowerwavelengths (Fig. 2(c), (d)). This is a sign of one of the proposedPb2þ bindingmechanisms, complexationwith OFGs followed by Hþ

release. Reduction of solution pH (from pH of 5.0 to 4.34) duringadsorption confirmed these allegations. This result is in accordancewith findings previously reported by Wang et al. (2015a). Addi-tionally, weakening and shifting of aromatic C]C (1617 cm�1) andCeH (782 cm�1) bands after Pb2þ sorption suggests a secondpossible 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-elec-tron of metal and C]C (p electron) bond in the unsaturated andaromatic systems (Wang et al., 2015a).

3.2. The effect of pH and pHpzc

The effect of solution pH was investigated in the range of2.0e7.0, and the results are shown in Fig. 3(a). It was observed that

ctra of (c) HC and (d) HCact before and after Pb2þ sorption.

J.T. Petrovi�c et al. / Journal of Environmental Management 182 (2016) 292e300296

an increase of pH value resulted in an increase of Pb2þ removal, andreached amaximum at a pH 5.0. At lower pH values removal of Pb2þ

was hindered as a result of competition between Pb2þ and Hþ ionsfor 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 tothe reduction of Hþ concentration in the solution, thus the func-tional groups of hydrochar surface (mostly OFGs) become depro-tonated and more applicable for the binding of positively chargedmetal ions (Wang et al., 2015b). Above the pH of 6.0, a slightdecrease in sorption of Pb2þ was observed, probably due to strivingof Pb2þ ions to hydrolyze and precipitate rather than to being

Fig. 3. Effect of process parameters on Pb2þ adsorption onto HC and HCact: (a) pH, (b)adsorbent dosage and (c) contact time.

adsorbed. In order to avoid the formation of metal hydroxidesprecipitates, all equilibrium and kinetic studies were carried out atan initial pH of 5.0. Additional insight into the pH influence on theremoval of metals provides pHPZC of the sorbent. At pH < pHPZC thesurface charge of the sorbent is positive which facilitate removal ofanions (Wang et al., 2015b; Petrovi�c et al., 2016b). Conversely, atpH > pHPZC, binding of cations is favored. The pHPZC of HC has been4.5 while the highest adsorption was achieved at pH 5 which is inagreement with the concept of pHPZC. On the contrary, the pHPZC ofHCact was 6.0 (Fig. S1). As the maximum of the adsorption occurs atpH < pHPZC (Fig. 3(a)), it could be concluded that repulsive elec-trostatic forces have shown meager influence, and that othermechanisms of metal binding are responsible for the removal ofPb2þ 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 inFig. 3(b). The amount of Pb2þ adsorbed by the different doses(0.4e4.0 g L�1) of HC and HCact was reduced from 28.0 to12.2mg g�1 and 137.0 to 47.8mg g�1, respectively (Fig. 3(b)). In bothcases, the dose of 0.5 g L�1 gave the highest qeq values. Similartrends have been reported earlier for Pb2þ removal using differentadsorbents (Bulut and Baysal, 2006; OuYang et al., 2014; Petrovi�cet al., 2016b; Singh et al., 2008). Petrovi�c et al. (2016b) observeddecrease of corn silk capacity from 67 to 11.1 mg g�1 as the doseincrease from 1 to 16 g L�1, while OuYang et al. (2014) reporteddecrease of capacity from 36.42 to 8.51 mg g�1 with a rise ofpartially hydrolyzed bamboo dose from 1 to 5 g L�1.

3.4. Effect of contact time

The effects of contact time on Pb2þ removal from aqueous so-lution by both hydrochars were performed in time period between5 and 120 min. As Fig. 3(c) shows, sorption of metal mostlyoccurred during the initial 60min in the HC and 45min in the HCact.The rapid removal was presumably caused by an abundance ofactive sites on the surface of both hydrochars (Guyo et al., 2015).Lead sorption capacity, qeq, of HC and HCact, were 27.8 mg g�1 and137 mg g�1, respectively, suggesting that the KOH treatmentsignificantly enhanced the ability of HC to remove solution Pb2þ.Similar results of the efficiency of alkali activated hydrochars havebeen 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 experimentaldata of Pb2þ removal using HC and HCact within 120 min: theLagergren pseudo-first order (Lagergren, 1898), pseudo-secondorder (Ho and McKay, 1999) and Weber-Morris intra-particlediffusion model (Weber and Morris, 1963).

The linear Lagergren pseudo-first-order rate could be expressedby equation:

1=qt ¼�k1

.qeq

�ð1þ tÞ þ

�1.qeq

�(2)

The pseudo-second-order rate is presented as:

t=qt ¼�1=k2q

2eq

�þ�1.qeq

�t (3)

where qeq and qt are the amount of Pb2þ ion adsorbed per HCact(mg g�1) at equilibrium and time t calculated from kinetic models.The k1 (min�1), and k2 (g mg�1 min�1) are the pseudo-first-order,

J.T. Petrovi�c et al. / Journal of Environmental Management 182 (2016) 292e300 297

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

q ¼ Kidt0:5 þ C (4)

where q (mg g�1) is amount adsorbed in time t (min), Kid representsthe intra-particle diffusion rate constant (mg g�1 min�1/2) and C isthe intercept. The Kid can be calculated from the slope of the linearplot of q versus t1/2.

The results of pseudo-second kinetic model are displayed inFig. 4, while calculated coefficients and sorption capacities of theapplied kinetic models are summarized in Table 1. The correlationcoefficient (R2) for both hydrochars were closest to unit(R2 ¼ 0.999) for the pseudo-second-order model. Moreover, theequilibrium adsorption capacities determined by pseudo-secondmodel were in correlation with experimental results (qeq,exp).Therefore, it could be concluded that sorption of Pb2þ on the HCand HCact followed pseudo-second ordermodel. This model implieschemical adsorption as the rate controlling step (Ho and McKay,1999) and involves the chemical interaction between Pb2þ ionsand polar functional groups at the surface on the HC and HCact (Hoet al., 2001). This is in agreement with results of FTIR analysis of thePb-loaded HCact. Also, the pseudo-second order has been described

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

Adsorbent

HC HCact

qeq, exp (mg g�1) 27.8 137Pseudo-first-order modelqeq, cal (mg g�1) 25.6 136k1 (min�1) 10.0 1.24R2 0.899 0.901Pseudo-second-order modelqeq, cal (mg g�1) 33.3 138k2 (g mg�1 min�1) 0.001 0.004R2 0.985 0.999Weber-Morris diffusion modelKid1 (mg g�1 min�1/2) 2.74 5.31C1 (mg g�1) 1.92 97.94R2 0.996 0.960Kid2 (mg g�1 min�1/2) �0.16 �0.27C2 (mg g�1) 28.88 138.95R2 0.401 0.837

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 L�1).

previously for sorption kinetic of Pb2þ onto Phytolacca americana L(Wang et al., 2015b). and Prosopis africana shell hydrochar (Elaigwuet al., 2014).

Weber-Morris intra-particle diffusion model for Pb2þ removalonto HC and HCact did not pass modeling procedure using Origin9.0, suggesting that intra-particle diffusion is not the only rate-controlling step. As can be seen from Fig. S2, two different linearzones are observed. The first zone indicates that adsorption ofmetal ions occurred on the adsorbents external surface, while thesecond linearity suggesting diffusion and intra-particle adsorptionof Pb2þ ions on the active sites (Liu and Zhang, 2009; Wang et al.,2015b). Thus, it can be concluded that the removal of Pb2þ withHC and HCact occurs in two simultaneous stages. Similar resultshave been observed by Liu and Zhang (2009) for Pb2þ removalusing pinewood and rice husk hydrochars.

3.6. Sorption isotherm models

Three isotherm models, Langmuir, Freudlich and Sips wereapplied for analyzing the Pb2þ adsorption process on HCact surfacein accordance to the obtained experimental data. Sorption experi-ments were conducted at contact time of 60 min to ensureequilibrium.

The Langmuir isotherm model is based on the assumption of amonolayer adsorption process onto a homogeneous surface, withconstant adsorption energy and without transmigration of adsor-bate in the plane of the surface. This isotherm model has beenrepresented by the following equation (Langmur, 1918):

qe ¼ qmKLCe1þ KLCe(5)

where the Ce is the equilibrium concentration (mg L�1), qm is themaximum amounts of Pb2þ ion adsorbed on the hydrochars(mg g�1) and KL is the Langmuir constant (L mg�1), related to theaffinity of the binding sites.

The Freundlich isotherm, based on the assumption of chemi-sorption onto heterogeneous surface with different sites energies,can be expressed in its non-linear form (Freundlich, 1906):

qe ¼ KFC1=ne (6)

where qe, KF, Ce, 1/n represents the amount of the adsorbate per unitweight of adsorbent (mg g�1), a constant relating to adsorptioncapacity (Freundlich constants), equilibrium Pb2þ ion concentra-tion in the solution (mg L�1) and adsorption intensity, respectively.

The Sips model, also known as the Langmuir-Freundlich model,is useful to anticipate heterogeneous adsorption as well as toexceed the limitation of the increasing concentration that occurs inFreundlich isotherm model (Sips, 1948). At the low sorbate con-centration this isotherm is reduced to the Freundlich isotherm,while at high concentration of sorbate, Sips model assume form ofthe Langmuir isotherm. The Sips model is commonly expressed bythe equation:

qe ¼ qm þ KSCnse

1þ KSCnse(7)

where qm represents the maximum adsorption capacity (mg g�1),KS the equilibrium constant (L mg�1) and nS is the model exponentand 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 gavethe best fit with the experiment (Table 2). Alatalo et al. (2013) alsoobserved the best fit of the Sips model in relation to other models

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

Langmuir isotherm model Freundlich isotherm model Sips isotherm model

qm (mg g�1) KL (L mg�1) R2 KF (mg g�1) (L mg�1)1/n 1/n R2 qm (mg g�1) KS (L mg�1) R2

HCact 140.8 0.7700 0.9861 103.9 0.0700 0.8131 139.0 0.5600 0.9985

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 L�1).

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for defining the adsorption of Pb2þ onto anaerobically digestedsludge and pulp and paper industry sludge hydrochars. Besides,maximum of sorption capacity of the Sips model (qm) for the HCact(of 139 mg g�1) is in excellent agreement with the experimentalone. Additionally, HCact showed better adsorption capacity for Pb2þ

than activated carbon (26.94 mg g�1) (Singh et al., 2008), peanutshell (52.80 mg g�1) (Wang et al., 2015a) and sludge-derived bio-chars (30.88 mg g�1) (Lu et al., 2012), Prosopis africana shell(45.30 mg g�1) (Elaigwu et al., 2014), pine wood (4.25 mg g�1), ricehusk (2.4 mg g�1) hydrochars (Liu and Zhang, 2009), as well asH2O2-modified peanut hull (22.82 mg g�1) hydrochar (Xue et al.,2012). According to these findings it can be emphasized that GP-HCact may be used as a perspective adsorbent of Pb2þ from aqueoussolution. The Sips isotherm model for the Pb2þ adsorption on theHCact at different temperatures is presented in Fig. 5.

3.7. Thermodynamic studies

The thermodynamic parameters, the changes of Gibbs free en-ergy (DG�), enthalpy (DH�) and entropy (DS�) determined bystudying the sorption process of Pb2þ by HCact at three differenttemperatures (298, 308 and 318 K) were calculated according to thefollowing equations:

Table 3Thermodynamic parameters.

Temperature (K) Parameters

DG� (kJ mol�1) DH� (kJ m

298 �17.91 �2.070308 �18.44318 �18.97

DG� ¼ �RT ln Kd (8)

DG� ¼ DH� � TDS� (9)

ln Kd ¼ �DH�=RT þ DS�=R (10)

where the Kd is the distribution coefficient for adsorption (qe/Ce), Ris the universal gas constant (8.314 � 10�3 kJ moL�1 K�1), T is thetemperature (K). The DH� and DS� values were obtained from theslope and the intercept of the plot 1/T versus ln Kd. The values ofthermodynamic parameters, determined for the three test tem-peratures are listed in Table 3.

The negative values of DG� imply that the adsorption of Pb2þ

onto HCact was spontaneous and favorable process (Anastopouloset al., 2013). The DH� is also negative, revealing the exothermicnature of the process. The positive values of DS� suggest increasingrandomness in the solid/solution system interface during theadsorption process (Milojkovi�c et al., 2014).

3.8. Pb2þ adsorption mechanisms

Previous studies have shown that the sorption of heavymetals was directly correlated with the amount OFGs onhydrochar's surface (Xue et al., 2012; Liu and Zhang, 2009). Thisis confirmed by FTIR analysis where two possible mechanismswere introduced (i) surface complexation between OFGs andPb2þ and (ii) Pb2þ- p interaction with electrons of C]C. Addi-tionally, the FTIR results revealed the possible presence of ion-exchange between Pb2þ and cations containing functionalgroups (ReOeMþ or ReCOOeMþ) (Lu et al., 2012). In order toconfirm ion-exchange mechanism, amount of Kþ, Naþ, Mg2þ,Ca2þ in filtrates after sorption of Pb2þ were measured at differentinitial Pb2þ concentration (Fig. 6). While Pb2þ occupies bindingsites during sorption, these cations should be released into fil-trates. The obtained results indicated that the ion-exchangemechanism might partly contribute to the removal of Pb2þ byHCact. The most significant cation involved in ion-exchange wasKþ followed by Ca2þ, Mg2þ and Naþ in a descending manner(Fig. 6). Generally, total amount of the exchangeable cationsreleased from HCact was lower than total amount of Pb2þ

removed from solution. Hence, it may be concluded that ion-exchange mechanism, followed by chemisorption and surfacecomplexation, are major binding mechanisms of Pb2þ onto acti-vated hydrochar of grape pomace. This is in accordance withpreviously proposed mechanisms of Pb2þ binding for peanut

ol�1) DS� (J mol�1 K�1) qeq (mg g�1)

53.14 139.0139.5127.0

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

J.T. Petrovi�c et al. / Journal of Environmental Management 182 (2016) 292e300 299

shell biochar (Wang et al., 2015a), sludge-derived biochar (Luet al., 2012) and peanut hull hydrochar surface (Xue et al., 2012).

4. Conclusion

The ability of HC and HCact to remove lead from aqueous solu-tions was studied in batch mode examining the influence of solu-tion pH value, adsorbent dose, and contact time. It was found thatthe increase of Pb2þ removal using KOH-activated GP-HC rosealmost fivefold in comparison to the untreated GP-HC. The ob-tained Pb2þ sorption capacity of HCact (137 mg g�1) was muchhigher than that of commercial activated carbon (26.94 mg g�1)and many other similar materials. The results of XRD, SEM and FTIRanalysis of both, HC and HCact, showed structural changes whichmight affect the increase of the Pb2þ sorption following KOHtreatment. Furthermore, FTIR spectra of samples before and aftersorption seem to correlate well with the proposed mechanisms ofmetal binding for OFGs present on the HC surface. The sorptionisotherms of Pb2þ onto HCact were well described by the Sipsmodel, while kinetic of Pb2þ removal followed the pseudo second-order kinetic model. Thermodynamic studies reveled that Pb2þ

sorption on alkali activated hydrochar is spontaneous andexothermic process. Our results suggest that the GP can be apromising precursor for production of low-cost hydrochars for leadremoval from wastewaters.

Acknowledgment

The authors are grateful to the Serbian Ministry of Education,Science and Technological Development for the financial support ofthis 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.

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Alkali modified hydrochar of grape pomace as a perspective adsorbent of Pb2+ from aqueous solution1. Introduction2. Materials and methods2.1. Chemicals2.2. HTC hydrochar preparation2.3. Characterization of hydrochar2.4. Batch adsorption studies

3. Results and discussion3.1. Characterization of adsorbent3.2. The effect of pH and pHpzc3.3. Effect of adsorbent dosage3.4. Effect of contact time3.5. Kinetics studies3.6. Sorption isotherm models3.7. Thermodynamic studies3.8. Pb2+ adsorption mechanisms

4. ConclusionAcknowledgmentAppendix A. Supplementary dataReferences