Recovery of grape waste for the preparation of adsorbents for water treatment: Mercury removal

Journal Pre-proof Recovery of grape waste for the preparation of adsorbents for water treatment: Mercury removal ˜ N.M. Zu´ niga-Muro (Writing - original draft) (Conceptualization) (Methodology) (Investigation), A. Bonilla-Petriciolet (Supervision) (Project administration) (Writing - review and editing), D.I. Mendoza-Castillo (Investigation) (Writing - review and editing), H.E. ´ Reynel-Avila (Investigation) (Writing - review and editing), C.J. Duran-Valle (Investigation) (Writing - review and editing), H. Ghalla (Software) (Investigation) (Writing - review and editing), L. SellaouiSoftware, Investigation) (Writing review and editing)

PII:

S2213-3437(20)30086-5

DOI:

https://doi.org/10.1016/j.jece.2020.103738

Reference:

JECE 103738

To appear in:

Journal of Environmental Chemical Engineering

Received Date:

14 November 2019

Revised Date:

30 January 2020

Accepted Date:

1 February 2020

˜ Please cite this article as: Zu´ niga-Muro NM, Bonilla-Petriciolet A, Mendoza-Castillo DI, ´ Reynel-Avila HE, Duran-Valle CJ, Ghalla H, Sellaoui L, Recovery of grape waste for the preparation of adsorbents for water treatment: Mercury removal, Journal of Environmental Chemical Engineering (2020), doi: https://doi.org/10.1016/j.jece.2020.103738

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RECOVERY OF GRAPE WASTE FOR THE PREPARATION OF ADSORBENTS FOR WATER TREATMENT: MERCURY REMOVAL 1

N.M. Zúñiga-Muro , A. Bonilla-Petriciolet 1*, D.I. Mendoza-Castillo 1,2, H.E. Reynel-Ávila 1,2, C.J. Duran-Valle 3, H. Ghalla 4, L. Sellaoui 4 1

Instituto Tecnológico de Aguascalientes, Aguascalientes, México, 20256 2

CONACYT, Cátedras Jóvenes Investigadores, 03940, México 3

Universidad de Extremadura, Badajoz, 06006, España Monastir University, Monastir, 5000, Tunisia

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Highlights      

Recovery of grape bagasse biomass to prepare adsorbents was proposed New adsorbents for mercury removal were synthetized from grape bagasse Pyrolysis of grape bagasse was analyzed to improve mercury adsorption Mercury – grape bagasse char interactions were analyzed via DFT Oxygenated and silicon-based functional groups were involved in mercury adsorption Two-site Langmuir model correlated the mercury adsorption isotherms

ABSTRACT. This study has focused on the recovery of grape bagasse biomass to prepare new adsorbents for

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the removal of mercury from aqueous solutions.

Keywords: Grape bagasse waste, Pyrolysis, Char, Water treatment, Adsorption, Mercury.

Corresponding author: 524499105002, [email protected]

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1. INTRODUCTION. Industrial activities generate significant amounts of waste and by-products that generally cause serious environmental problems. Recovery operations use these discarded materials as a feedstock to create valuable products, which is an alternative approach to reduce the waste generation and the extraction of natural resources [1]. In this context, the grapes are one of the most harvested fruits with an estimated world production of 75 million tons/year [2]. Statistical data indicate that more than 50 % of the grapes is used in the wine and juice industries and that approximately 7 % of the initial processed mass is grape bagasse (GB) [2-4]. GB consists of seeds and skins, its main chemical components are carbon (46.6 %), oxygen (45.5 %), hydrogen (6.3 %) and nitrogen (1.7 %) and is generally used as animal food, incinerated or just discarded [3,5-8]. Based on these

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facts, previous studies have concluded that this biomass could be used as raw material to obtain a variety of value-added products, including new materials for environmental applications (e.g.,

for preparing carbon-based materials) [3,5,7,8,13,14]. Therefore, the preparation of GB-based adsorbents is an attractive alternative to develop low cost and ecofriendly water treatment processes. It is important to remark that wastewater generation and the need for water treatment

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have promoted the search for alternative technologies and the application of novel materials with outstanding properties [9-12].

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In the industrial production of adsorbents (e.g., activated carbons, chars) for water treatment, a significant amount of the used precursors is related to lignocellulosic materials [15-17]. These adsorbents are obtained by pyrolysis and their physicochemical properties depend on the

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characteristics of the precursor and the preparation conditions [18,19]. Specifically, the pyrolysis of GB produces a gas phase formed mainly of CO, CO2, H2 and CH4; a liquid containing methanol, acetone, furfuryl alcohol and phenol; and a solid phase that can be classified as a char [3]. This

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product distribution (i.e., the amount of solid, liquid and gas) depends on the experimental conditions applied in the pyrolysis [3,20]. The char generated in the GB pyrolysis is a porous carbon material, which can been employed as an adsorbent for heavy metals, dyes and other

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chemical compounds present in groundwater and wastewaters [8,21]. The use of GB-based chars as adsorbents in water treatment requires a tailored surface chemistry, which is the most important

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property of the adsorbent compared to the textural parameters (e.g., surface area and pore distribution) [13,18,22]. The surface chemical properties of GB-based adsorbents can be modified via chemical and physical treatment methods to obtain a specific performance for the removal of the target pollutants from water. The possible modification method used to improve the adsorbent properties should be selected considering both technical and economic factors. According to the literature, the biomass of GB has been used in its natural form for the removal of heavy metals such as Cd(II) and Pb(II) in aqueous solutions showing adsorption capacities of 53.8 and 42.3 mg/g, respectively [5,7]. GB biomass has been also utilized to produce activated carbons for the adsorption of Cu(II) and dyes (basic blue 9 and acid yellow 36) with adsorption 3

capacities from 43 to 417 mg/g [8,14]. These results have shown that the GB-based adsorbents can be effectively used in water purification. But, to the best of author’s knowledge, this biomass has not been utilized to remove mercury from aqueous solutions. Mercury can be found in water resources due to anthropogenic and geogenic factors, such as the geological composition of aquifers or improper management of polluted industrial effluents [23]. This heavy metal is considered a hazardous chemical for human beings and ecosystems due to its toxicological profile and it has also been categorized as a priority pollutant in the context of water treatment [24-27]. Mercury adsorption is a challenge and adsorbents with specific surface functionalities are required to obtain a competitive removal performance [19,28]. The aim of this study was to analyze the preparation of GB-based chars via pyrolysis and its

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application in the mercury removal from aqueous solutions as an alternative for the recovery of this residual biomass. A full experimental factorial design was defined to analyze the effects of the pyrolysis conditions of the GB biomass on the physicochemical properties and the mercury

adsorption capacity of the corresponding chars. The best adsorbent was selected and the mercury

adsorption thermodynamics was studied. Density functional theory (DFT) calculations were used

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to characterize the adsorbent-adsorbate interatomic interactions and to understand the mercury

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adsorption mechanism.

2. METHODOLOGY.

2.1 Recovery of the precursor and its usage in the preparation of chars for mercury adsorption

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GB was collected from a local wine factory, washed with deionized water, dried, crushed and sieved to obtain a particle size of 0.84 - 1.0 mm. This precursor was physicochemically characterized at room temperature using FTIR spectroscopy and XRD analysis.

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The GB biomass was pyrolyzed to obtain the adsorbents in a Carbolite Eurotherm CTF 12165/550 tubular furnace using a quartz tube with loads of 20 g. A full factorial experimental design was established for the analysis of the impact of the pyrolysis conditions (i.e., pyrolysis

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temperature and dwell time) on the mercury adsorption properties of the GB chars. In this experimental design, the adsorbents were prepared with pyrolysis temperatures of 600, 800 and

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1000 °C and dwell times of 1, 2 and 3 h. Table 1 shows the corresponding experimental design utilized in the preparation of the chars. A heating rate of 10 °C/min and a N2 flow of 200 mL/min were used during the pyrolysis. The adsorbent samples obtained from the experimental design were weighted and washed with deionized water under constant stirring at 120 rpm until to obtain a constant pH in the washing solution. Then, the adsorbents were dried at 110 °C for 24 h and the particles with size of 0.50 - 0.84 mm were selected and used in the mercury adsorption tests.

2.2 Mercury adsorption studies

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All mercury adsorption studies were carried out by triplicate using batch adsorbers under constant stirring of 120 rpm. In particular, 0.02 g of the adsorbents and 10 mL of the mercury solutions (which were prepared from reactive grade HgCl2) were used in all experiments. The mercury adsorption performance of the adsorbents obtained with the experimental design was analyzed at 30 °C, pH 1.5 and 24 h, where the pH of the mercury solutions was adjusted using HNO3. Mercury concentration was quantified using an atomic absorption spectrophotometer (Ice 3000 Thermo Scientific) with a linear calibration curve (i.e., 10 - 300 mg/L). The response variable of the experimental design was the mercury adsorption capacity (q, mg/g), which was calculated using the adsorbate mass balance for the batch adsorber 𝑞 = 𝑉(

𝐶0 −𝐶𝑓 𝑚

)

(1)

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where V is the volume of mercury solution (L), C0 and Cf are the initial and final mercury

concentrations (mg/L) in the aqueous solutions utilized in the adsorption experiments and m is the adsorbent mass (g).

The results of the adsorption capacity of the experimental design (Table 1) were used to

identify the best preparation conditions for GB-based char. The best adsorbent was utilized in

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additional experiments to determine kinetic and equilibrium thermodynamic parameters of the mercury removal. Adsorption kinetic studies were performed at pH 4 and 30 °C using mercury

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solutions with initial concentrations of 100 and 200 mg/L. Mercury adsorption kinetic profiles were quantified at operating times from 5 to 1440 min. The same adsorbent dosage was also

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utilized in both kinetic and equilibrium adsorption studies. Pseudo second order model was applied for kinetic data modeling and for the calculation of the adsorption rate constants via a nonlinear data regression. Mercury adsorption isotherms were measured at different conditions of pH (1.5, 3 and 4) and temperature (30 and 40 ºC) with adsorbate concentrations from 10 to 900

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mg/L and an equilibrium time of 24 h. These experimental isotherms were fitted by a Langmuirtype adsorption model. Enthalpy change for the mercury removal using the best GB-based char

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was calculated using the distribution coefficient approach and Van’t Hoff’s plot [29].

2.3 Physicochemical characterization of the GB-based chars

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Physicochemical characteristics of GB-based adsorbents were studied via N2 adsorption-

desorption isotherms, elemental analysis, X-ray fluorescence (XRF), Boehm titration, Fourier Transform Infrared (FTIR) and X-ray diffraction (XRD) techniques. N2 adsorption-desorption isotherm studies were carried out at 77 K with an equipment Quadrasorb Evo de Quantachrome Instruments, where the samples were previously degassed for 24 h at 120 ºC. Elemental analysis (C, H, O and N) was performed with an equipment LECO CHNS 932, while a Bruker S8 Tiger analyzer was utilized in XRF.

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Boehm-based method titrations were performed with 0.25 g of the adsorbents, which were kept in contact with 25 mL of solutions of NaOH (0.1 M), Na2CO3 (0.05 M) and NaHCO3 (0.1 M) to estimate the amount of acidic groups in the adsorbent surface. These tests were carried out at 30 °C for 48 h. Subsequently, these aqueous solutions were titrated with HCl (0.1 M) until reaching the neutralization that was verified with a pH-meter. The amount of acidic sites was estimated under the assumptions that NaOH neutralized carboxylic, phenolic and lactonic groups, Na2CO3 was utilized to neutralize the carboxylic and lactonic functionalities, while NaHCO3 neutralized only the carboxylic functional groups [30]. Surface chemistry analysis was carried out with FTIR spectra of the GB-based chars. These spectra were obtained with a Nicolet iS10 Thermo Scientific spectrometer equipped with a DTGS KBr detector using KBr pellets where

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FTIR analysis was done in the range of 4000 - 600 cm-1. XRD patterns of selected samples of adsorbents were recorded in a PANalytical Empyrean X-ray diffractometer equipped with a

PIXcel1D detector and the Bragg-Brentano geometry, on a range of 5° ≤ 2θ ≤ 100° with CuKα

nickel filtered radiation (λ = 1.5406 Å, 45 kV, 30 mA). HighScore Plus software and the PDF 2

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database were employed for the analysis and interpretation of diffraction patterns.

2.4 Theoretical analysis of the mercury adsorption mechanism via DFT calculations

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DFT calculations were performed to understand the mercury adsorption mechanism using GB chars. These calculations were done at the ground state using Gaussian 09 program and GaussView as a visualization package [31,32]. A simplified molecular structure for the GB-based

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adsorbent surface (M) was proposed employing the results of physicochemical characterization, see Figure 1. The molecular structures of M and the complex M  Hg2+ were optimized in the context of DFT method using the B3LYP exchange correlation hybrid functional. For all atoms,

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except Hg, the polarized 6-311G(d,p) basis set was utilized. ECP LanL2DZ basis set was employed for the heavy atom Hg. Harmonic vibrational modes were computed at the same levels of theory and the absence of any imaginary frequency confirmed that the optimized structures

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were stable.

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3. RESULTS AND DISCUSSION. 3.1 Analysis of preparation conditions of grape bagasse-based adsorbents for mercury

removal and their physicochemical properties The yields for the preparation of the GB-based chars via pyrolysis are reported in Table 1.

These yields ranged from 33.7 to 38.2 % where the highest value corresponded to the adsorbent prepared at 600 °C for 1 h. Demiral and Ayan [3] obtained similar results in the GB pyrolysis at the same temperature, reporting a char yield of 36.8 %. In general, the adsorbent yield decreased slightly with pyrolysis conditions (i.e., temperature and dwell time), where the lowest char yield (33.7 %) was obtained for the biomass pyrolyzed at 1000 °C and 3 h. Note that the GB pyrolysis 6

showed char yields higher than those reported for other adsorbents produced from lignocellulosic biomasses, for example, corncob, almond shell and olive stone [33,34]. This result could be attributed to the lignin content of GB (~ 42 %), which has been recognized as the most stable compound of these lignocellulosic materials [8,33,34]. On the other hand, the results of mercury adsorption for the chars prepared via the full factorial experimental design are also reported in Table 1. The maximum mercury adsorption capacity was 5.3 mg/g for the adsorbent obtained at 600 °C and 2 h, while the char prepared at the pyrolysis conditions of 1000 °C and 3 h showed the lowest adsorption capacity of 0.5 mg/g. Mercury adsorption capacity decreased with the pyrolysis conditions utilized to obtain the chars and this trend agreed with other studies concerning to the preparation of chars from lignocellulosic

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biomasses [35]. This result can be associated to the thermal degradation of surface functional groups that could be involved in the mercury adsorption [35,36]. Note that it was expected that increments on the pyrolysis temperature and dwell time decreased the number of surface functionalities of GB-based chars.

N2 adsorption-desorption isotherms of selected GB chars are reported in Figure 2, while their

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textural parameters are shown in Table 2. According to the IUPAC classification, these adsorbents exhibited a type III isotherm, which corresponds to materials with low porosity that can be

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characterized by a weak adsorbate-adsorbent interaction [37]. Specific surface area of char samples calculated with Brunauer-Emmett-Teller (SBET) model ranged from 5.7 m2/g (adsorbent obtained at 1000 °C for 2 h) to 21.9 m2/g (adsorbent obtained at 600 °C and 1 h). Some studies

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have reported SBET of 2.0 - 280.0 m2/g for different lignocellulosic-based materials and the GBbased chars can be considered as adsorbents with low surface area [13,18,38,39]. Density Functional Theory was also utilized to estimate the specific surface areas (SDFT), see Table 2. The

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highest value of SDFT was 12.8 m2/g and corresponded to the adsorbent with the maximum mercury adsorption capacity (i.e., 5.3 mg/g). Overall, the specific surface area decreased with pyrolysis temperature. Similar results have been reported by other researchers in the preparation

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of adsorbents from lignocellulosic biomasses [40-42]. For example, Paethanom et al. [42] reported that the specific surface areas of rice husk chars varied from 141 m2/g (pyrolysis at 600

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°C) to 117 and 46 m2/g (pyrolysis at 800 and 1000 °C). These results can be attributed to the adsorbent structural ordering and to the fact that a high pyrolysis temperature can develop the porous structure in the char, which can reach the point where the walls of the pores become so thin that they can collapse, thus reducing the available char surface area [40,42]. Total pore volume (VTotal) of char samples ranged from 0.010 to 0.028 cm3/g and they also exhibited a low microporosity (i.e., 0.001 - 0.005 cm3/g). It is convenient to remark that the GB char with the highest mercury adsorption capacity showed the best textural parameters. Specific surface area and total pore volume of this char decreased after the mercury adsorption (i.e., 9.49 m2/g and 0.0225 cm3/g, respectively). These results indicated that textural properties of the GB chars 7

affected their mercury adsorption performance, which was consistent with the results reported by other authors [42,43]. Table 3 shows the results of elemental composition and XRF analysis of the GB adsorbents. In general, the percentage of C of these chars increased from 63.6 to 74.9 wt% with both pyrolysis temperature and dwell time, while the content of H, N and O decreased. These results can be explained taking into account that the changes in the pyrolysis conditions caused the loss of surface functional groups, the release of volatile matter and the loss of H, N and O atoms attached to C due to the degradation of the structural core. At the same time, the increment of pyrolysis conditions (i.e., temperature and/or dwell time) resulted in a higher content of fixed carbon, thus implying that the chars became carbonaceous-based materials especially at the highest pyrolysis

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temperature [42,44,45]. Results also showed that the inorganic phase of the adsorbents was composed of several elements (e.g., Al, Ca, Si, P, K, Fe), where Si was the most abundant. The highest quantity of Si (i.e.,  3.58 wt%) was found in the adsorbent with the highest mercury

adsorption capacity, which was prepared at the pyrolysis conditions of 600 °C and 2 h. This result

suggested the participation of Si moieties in the mercury adsorption mechanism. The best

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adsorbent was further analyzed after the adsorption experiments and mercury was identified in its structure ( 1.11 wt%).

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In terms of the char surface chemistry, Boehm titration results showed that the acidic groups in the char surface decreased with respect to the conditions used in the pyrolysis of GB, which ranged from 0.11 to 0.27 mmol/g, see Table 1. Herein, it is important to highlight that the acidic

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functional groups have been considered as the main functionalities involved in the heavy metal adsorption on chars obtained from lignocellulosic feedstocks [22]. Therefore, this trend on the loss of functional groups can be partially associated with the mercury adsorption properties of the

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GB chars prepared via the experimental design. However, the best GB adsorbent did not show the highest amount of acidic functional groups. These findings indicated that the adsorption performance of this char was the result of its surface chemistry (in terms of acidic functional

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groups and Si moieties) and its textural parameters. Surface chemistry of the best adsorbent demonstrated the predominant presence of phenolic groups (0.15 mmol/g) in comparison to

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carboxylic and lactonic functionalities (0.02 and 0.03 mmol/g, respectively). With illustrative purposes, Figure 3 displays the FTIR spectra of the GB precursor, the best

char for mercury adsorption and the adsorbent obtained at the maximum pyrolysis temperature and dwell time (i.e., 1000 °C and 3 h). Spectrum of the precursor showed absorption bands that corresponded to O-H of alcohols, phenols and carboxylic acids (3600 - 3000 cm-1), C-H from aliphatic and aromatic structures (2920 - 2853, 1420 cm-1), C=C of aromatic rings from oxygenated groups as lactones (1630 cm-1) and C=O from aldehydes and ketones (1127 cm-1) [5,8,14]. The lignin in the precursor structure was also confirmed by the strong band of C-O at 1051 cm-1, while the Si-OH absorption band was identified at 787 cm-1 [5,46]. FTIR spectra of 8

the GB biomass and the char samples were similar. The spectra of the chars showed the reduction of different absorption bands depending of the pyrolysis conditions used in their preparation, thus confirming the loss of surface functional groups. FTIR spectrum of the best adsorbent after mercury removal is also given in Figure 3. A loss of intensity in the absorption bands associated to O-H, C=C, C=O and Si-OH groups was observed indicating that these functionalities could be interacting with the metallic adsorbate. XRD patterns of the GB biomass, the best adsorbent and the char obtained at 1000 °C and 3 h are shown in Figure 4. These diffractograms indicated that the crystal structure of these samples contained SiO2. However, it was also confirmed that the content of SiO2 on the adsorbent surface was partially lost due to the pyrolysis conditions where these results agreed with XRF analyses

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and other studies reported in literature [5]. Two broad peaks at 22.9 and 44.3 ° 2 were identified in the XRD patterns of char samples indicating the presence of a graphene-like structure, whose

intensity increased with the pyrolysis conditions due to they promoted a structural ordering in the adsorbent [47,48]. Figure 4 also displays the XRD pattern of the best adsorbent loaded with

mercury where it was possible to identify that the signals associated to SiO2 shifted drastically.

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This result was an evidence of the participation of Si moieties in the mercury adsorption process.

In the literature, some studies have reported the surface chemistry modification of adsorbents

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using silicon in order to improve the mercury adsorption capacity, where the affinity of Si to this adsorbate has been pointed out [49]. In summary, the best GB char for mercury removal was obtained via pyrolysis at 600 °C and 2 h using N2 flow of 200 mL/min and a heating rate of 10

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°C/min. This adsorbent was selected for further adsorption studies including the theoretical understanding of the mercury removal mechanism via DFT calculations.

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3.2 Experimental mercury adsorption performance of the best grape bagasse char. Kinetic and thermodynamic mercury adsorption studies were performed using the GB char with the highest adsorption capacity. Results of adsorption kinetics at pH 4 and 30 °C are reported

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in Figure 5. The adsorption equilibrium was reached with  20 h at tested experimental conditions and the pseudo second order model was used to estimate the adsorption kinetic parameters, see

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Table 4. Mercury adsorption rate for GB char was 5.47E-03 g/mg min at 100 mg/L and 1.10E-03 g/mg min at 200 mg/L where the calculated equilibrium adsorption capacities were 6.86 and 13.73 mg/g, respectively; while the experimental adsorption capacities were 7.8 and 14.3 mg/g for the same initial concentrations. This kinetic model showed a R2 > 0.964, see Table 4. Mercury adsorption isotherms at different pH are given in Figure 6a-c. These results showed that the maximum adsorption capacity of the GB char increased with pH varying from 20.8 to 32.3 mg/g. This adsorption behavior can be attributed to the fact that the increment in solution pH caused the deprotonation of the different surface functional groups of GB char. Therefore, the amount of available adsorption sites for mercury binding increased under these experimental 9

conditions. Note that there was also a reduction of repulsion forces between the adsorbent surface and mercury ions and the competition of protons for the binding acidic sites also decreased [19,50]. Adsorption isotherms at pH 4, 30 and 40 °C (see Figures 6c and 6d) indicated that the mercury removal with this GB char was an endothermic process. Mercury removal increased from 32.3 to 45.9 mg/g at tested temperatures. It is convenient to remark that the temperature of the solution showed a greater impact on the mercury adsorption than the increment of pH. Estimated enthalpy for mercury adsorption on GB char was 24.48 kJ/mol, which could be associated to an adsorption process involving both physical and chemical interactions [51]. An endothermic nature of mercury adsorption has been reported by different authors [39,52,53]. Mercury adsorption capacities of the best GB char were higher than those reported for several adsorbents e.g.: multi-

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walled carbon nanotubes (10.9 mg/g), Brazilian pepper biochars (24.2 mg/g) and bone charcoal (21.0 mg/g) [38,54,55].

Results of the adsorbent surface characterization showed that the GB char comprised oxygenated and silicon functionalities, which could be responsible of mercury binding.

Considering this adsorbent surface heterogeneity, the isotherm data were fitted to a two-site

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Langmuir model [56]. This isotherm model was developed based on the traditional Langmuir equation to handle the presence of different adsorption sites on the adsorbent surface [56]. The

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two-site Langmuir equation has been successfully applied to represent the equilibrium adsorption of different metals (not mercury) on biomasses, bone char, activated carbon and goethite [56-59]. 𝑞′ =

𝑏1 𝑞1 𝐶𝑒 1+𝑏1 𝐶𝑒

+

𝑏2 𝑞2 𝐶𝑒 1+𝑏2 𝐶𝑒

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The equation of this two-adsorption site isotherm is given by

(2)

where q’ is the total adsorption capacity of the adsorbent in mg/g, q1 and q2 are the maximum

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adsorption capacities for the two different adsorption sites (i.e., oxygenated and silicon functionalities for GB char) also given in mg/g, b1 and b2 are the adsorption equilibrium constants in L/mg and Ce is the mercury equilibrium concentration in the aqueous solution given in mg/L,

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respectively.

The two-site Langmuir model correlated satisfactorily the adsorption data with determination coefficients (R2) > 0.99 where the parameters of this model are given in Table 5. The equilibrium

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constant b2 was higher than b1 being an indicator of the heterogeneous adsorbent surface with at least two different mercury-adsorption site affinities. Note that the mercury adsorption sites with high equilibrium constants presented high binding energies [58]. The value of the two-site Langmuir parameters increased with pH and temperature because these operating variables promoted the adsorption process as already discussed. Overall, the active sites 1 (i.e., oxygenated functional groups) presented a major contribution to the mercury removal in comparison to active sites 2 (i.e., Si moieties), see Figure 6.

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3.3 DFT calculations and its relationship with the mercury adsorption mechanism Results from char characterization were utilized to analyze the role of surface functionalities on the mercury adsorption via DFT calculations. Figure 7 illustrates the two oxygenated functional groups (carboxylic acid and lactone functional groups) and Si-based functionality that were selected for this theoretical analysis with DFT calculations. Table 6 reports the results of DFT calculations for the proposed atomic interactions between GB char surface functionalities and mercury ions. Calculated binding energies ranged from -1239 to -1233 kJ/mol and atomic bond distances varied from 2.813 to 3.653 Å. DFT results indicated that the interaction between carboxylic acid functionalities and mercury ions was the strongest, followed by the interaction of Si moieties and lactone functional groups. These results confirmed that the oxygenated functional

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groups could be the main active sites involved in the mercury adsorption from aqueous solution using the GB-based chars. DFT calculations were also consistent with the characterization results

of the surface chemistry of the chars, where the best adsorbent contained the highest amount of

Si moieties, which contributed to the mercury removal. These simulations also supported the assignation of active sites 1 (oxygenated functional groups) and 2 (silicon moieties) in the two-

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site Langmuir isotherm model. Consequently, the adsorption of mercury from aqueous solution

with GB-based chars was associated to the presence of both, oxygenated and silicon

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functionalities on the adsorbent surface.

4. CONCLUSIONS

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This study has discussed and analyzed the preparation of alternative adsorbents for mercury removal via the pyrolysis of grape bagasse biomass. Grape bagasse chars were synthetized using different pyrolysis conditions, which were improved to maximize the mercury adsorption from

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aqueous solution. Mercury adsorption capacities of grape bagasse adsorbents were determined by their heterogeneous surfaces that contained phenolic, carboxylic, lactonic and silicon functionalities. Mercury adsorption with the best grape bagasse char was endothermic with a

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maximum adsorption capacity up to 45.9 mg/g. DFT calculations confirmed that carboxylic functional groups and Si moieties were the main active sites for mercury adsorption using grape

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bagasse chars. Results of this study contribute to the recovery of the grape bagasse biomass to obtain value-added materials for water pollution control and environmental protection.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflict of interest.

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Credit author statement N.M. Zúñiga-Muro: Writing – Original draft, Conceptualization, Methodology, Investigation A. Bonilla-Petriciolet: Supervision, Project administration, Writing – Review and Editing D.I. Mendoza-Castillo: Investigation, Writing – Review and Editing H.E. Reynel-Ávila: Investigation, Writing – Review and Editing C.J. Duran-Valle: Investigation, Writing – Review and Editing H. Ghalla: Software, Investigation, Writing – Review and Editing

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L. Sellaoui: Software, Investigation, Writing – Review and Editing

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16

Figure captions Figure 1. Proposed surface structure of the grape bagasse-based char (M) for the analysis of mercury adsorption via DFT. a) Optimized structure calculated with the B3LYP/6-311G(d,p) theory level. b) ESP map of M with isovalue = 0.0004 where ESP value ranged from -133.5 kJ/mol (red) to +133.5 kJ/mol (blue). Figure 2. N2 adsorption-desorption isotherms at 77 K for the chars obtained from the pyrolysis of grape bagasse biomass. Figure 3. FTIR spectra of grape bagasse biomass and selected char samples. Figure 4. X-ray diffraction patterns of grape bagasse biomass and selected char samples. Figure 5. Adsorption kinetics for the mercury removal from aqueous solution using the best

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grape bagasse-based char. Experimental conditions: pH 4 and 30 °C. Figure 6. Adsorption isotherms for the mercury removal from aqueous solution using the best grape bagasse-based char. Experimental conditions: a) pH 1.5 and 30 °C, b) pH 3.0 and 30 °C, c) pH 4.0 and 30 °C, d) pH 4 and 40 °C.

Figure 7. Surface functionalities of grape bagasse-based char analyzed in the mercury

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Lactone functional group, c) Silicon functional group.

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adsorption via DFT calculations. Type of adsorption site: a) Carboxylic acid functional group, b)

17

Figure 1.

b)

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18

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19

1000 °C, 3 h

Transmittance, %

600 °C, 2 h

Precursor Si-OH C-H C=C O-H 3800

3000

2200 Wavenumber, cm-1

C=O C-O

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C-H

1400

600

After mercury adsorption

-p

Before mercury adsorption

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na

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Figure 3.

20

Graphene-like structure

SiO2

900 0

1000 °C, 3 h

Intensity, a.u.

700 0

500 0

300 0

100 0

600 °C, 2 h

-1000

-3000

30

10

90

70

50

2θ,

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Precursor

110

After mercury adsorption

Before mercury adsorption

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na

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Figure 4.

21

Mercury adsorption capacity, mg/g

Pseudo second order model 200 mg/L 100 mg/L

16

14.3 mg/g

12 7.8 mg/g

8

4

0 200

400

600

800 1000 Time, min

1400

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Figure 5.

1200

22

1600

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0

a) two-site Langmuir model Adsorption capacity of oxygenated groups Adsorption capacity of Si moieties

60

40 20.8 mg/g

20

0 0

200

400

600

800

1000

b) two-site Langmuir model Adsorption capacity of oxygenated groups Adsorption capacity of Si moieties

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60

25.0 mg/g

0

na

20

400

600

800

1000

two-site Langmuir model Adsorption capacity of oxygenated groups Adsorption capacity of Si moieties

60 40

200

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0

-p

20

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c)

Mercury adsorption capacity, mg/g

40

32.3 mg/g

0

0

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d)

400

600

800

1000

two-site Langmuir model Adsorption capacity of oxygenated groups Adsorption capacity of Si moieties

60

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200

45.9 mg/g

40 20 0 0

200

400

600

800

1000

Mercury equilibrium concentration, mg/L Figure 6. 23

a)

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b)

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c)

Figure 7.

24

Table captions Table 1. Experimental design utilized for the preparation of grape bagasse-based chars for the mercury adsorption from water. Table 2. Textural parameters of the grape bagasse-based chars obtained from pyrolysis. Table 3. Elemental composition and XRF analysis of the grape bagasse-based chars obtained from pyrolysis. Table 4. Kinetic adsorption parameters for the mercury removal from aqueous solution using the best grape bagasse-based char. Experimental conditions: pH 4 and 30 °C. Table 5. Results of the two-site Langmuir model for the mercury adsorption isotherms using the best grape bagasse-based char.

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Table 6. Results of DFT calculations for the atomic interactions of the proposed adsorbent

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surface structure (M) and mercury ion.

25

Table 1. Pyrolysis conditions

Char performance

Temperature, ºC

Dwell time, h

Yield, %

q, mg/g

Total acidity, mmol/g

1

600

1

38.2

4.2

0.27

2

600

2

37.7

5.3

0.20

3

600

3

37.5

4.9

0.24

4

800

1

36.0

2.8

0.21

5

800

2

35.5

2.1

0.17

6

800

3

35.3

1.8

0.13

7

1000

1

34.0

1.3

0.18

8

1000

2

33.8

0.7

0.18

9

1000

3

33.7

0.5

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Sample

26

0.12

Table 2. Textural parameters Sample

SBET, m /g

SDFT, m2/g

VTotal, cm3/g

VMicropore, cm3/g

1

21.91

9.94

0.0252

0.003

2

15.50

12.84

0.0281

0.005

3

9.48

5.20

0.0145

0.001

4

15.64

5.40

0.0147

0.001

5

-

3.29

0.0105

0.001

6

6.83

4.45

0.0136

0.004

7

6.30

4.56

0.0133

0.002

8

5.73

4.36

0.013

0.002

9

12.57

5.40

0.0147

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Symbol “-“ indicates a poor adjustment of BET model.

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2

27

0.001

Table 3. Elemental analysis, wt% Sample

C

H

N

O*

XRF, wt% Si

Al

Ca

P

K

Fe

Others

69.40 2.46 3.13 19.58 2.69 0.81 0.66 0.47 0.30 0.24

0.26

2

63.60 3.99 3.76 21.88 3.58 1.00 0.71 0.56 0.30 0.28

0.34

3

71.10 3.24 3.00 18.24 2.16 0.67 0.60 0.37 0.20 0.17

0.24

4

67.10 1.91 3.04 21.74 3.33 0.86 0.63 0.56 0.35 0.20

0.28

5

69.80 1.99 2.27 20.88 2.67 0.68 0.63 0.37 0.23 0.22

0.26

6

71.10 2.11 2.28 19.51 2.51 0.69 0.61 0.42 0.28 0.23

0.26

7

72.90 1.68 1.80 18.44 2.54 0.74 0.68 0.46 0.28 0.23

0.24

8

72.90 1.49 1.27 18.81 2.80 0.81 0.68 0.42 0.31 0.23

0.28

9

74.90 1.39 1.59 17.10 2.52 0.72 0.60 0.43 0.28 0.23

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1

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* This value was calculated by difference.

28

0.24

Table 4. Mercury initial concentration, mg/L Adsorption kinetic model

100

200

R2

0.964

0.982

k, g/mg min

5.47E-03

1.10E-03

qte, mg/g

6.86

13.73

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Pseudo second order

Parameter

29

Table 5. Adsorption conditions

Parameter

T, °C

q1, mg/g

b1, L/mg

q2, mg/g

b2, L/mg

q’, mg/g

R2

1.5

30

26.92

8.08E-04

17.92

3.00E-03

20.9

0.994

3.0

30

26.21

8.94E-04

19.70

3.30E-03

25.89

0.997

4.0

30

30.23

1.02E-03

24.70

3.98E-03

33.03

0.992

4.0

40

45.10

1.06E-03

32.78

4.23E-03

46.76

0.993

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pH

30

Table 6. Char surface functionality

Estimated atomic distance for the

Energy, kJ/mol

interaction M Hg, Å 3.447

-1239

Lactone

3.653

-1233

Silicon moiety

2.813

-1237

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Carboxylic acid

31