Biochar composites for methylene blue removal by adsorption

Applied Clay Science 168 (2019) 11–20

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Applied Clay Science journal homepage: www.elsevier.com/locate/clay

Research Paper

MgAl-LDH/Biochar composites for methylene blue removal by adsorption a,⁎

a

b

c

a

d

e

L. Meili , P.V. Lins , C.L.P.S. Zanta , J.I. Soletti , L.M.O. Ribeiro , C.B. Dornelas , T.L. Silva , M.G.A. Vieirae

T

a

Laboratory of Processes, Center of Technology, Federal University of Alagoas, Brazil Applied Eletrochemistry Laboratory, Chemistry and Biotechnology Institute, Federal University of Alagoas, Brazil Laboratory of Separation Systems and Process Optimization, Center of Technology, Federal University of Alagoas, Brazil d TECNANO, School of Nursing and Pharmacy, Federal University of Alagoas, Brazil e Department of Development of Processes and Products, School of Chemical Engineering, State University of Campinas, Brazil b c

ARTICLE INFO Keywords: Dyes/pigments Clays Layered double hydroxides Biochar Synthesis

Abstract: In this work, LDH-biochar composites were synthesized in different molar ratios of Mg:Al (2:1, 3:1 and 4:1) using co-precipitation method. The composites were applied to remove an organic dye from aqueous solutions by adsorption. The composites and the pure bovine bone biochars were characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) measurements, thermogravimetric analysis (TG/DTG), dispersive energy spectroscopy (DES) and scanning electron microscopy (SEM). The methylene blue dye adsorption experiments were conducted in a finite bath. The results indicate that pH 12 is more suitable for dye adsorption process, with a removal > 95% for all composites. The adsorption kinetic was best described by the pseudo-second order model, reaching the equilibrium in approximately 20 min. The Redlich-Peterson model fit the adsorption equilibrium isotherms satisfactorily. It was obtained a maximum adsorption capacity of 406.47 mg·g−1 at 40 °C. Negative values of ΔG indicate the spontaneity of the adsorption process. The positive value of ΔH (30.72 kJ·mol−1) indicates the physical nature of the adsorption and the positive value of ΔS (0.1863 kJ·mol−1) indicates that there was a change in the structure of the adsorbent and increased randomness during the fixing of the dye.

1. Introduction Layered Double Hydroxide (LDH), also known as hydrotalcite-type compound, is an anionic clay, with structure similar of brucite [Mg (OH)2] consisting of stacked of positive layers separated by interlamellar region constituted of anions and water. Its general formula is +3 +x −n [M+2 Ax/n·mH2O, where M+2 is a divalent metal, M+3 a 1−xMx (OH)2] trivalent metal and A−n, an anion n valent; usually where the ratio between M+2/M+3 is 0.1 ≤ x ≤ 0.5 molecules (Aramendía et al., 1999; Guo et al., 2018; Menezes et al., 2014; Vial et al., 2008). LDH are materials capable of incorporating negatively charged species in their interlamellar region to neutralize the positive charges of the lamellae through the ion exchange mechanism. Also, LDH have high adsorption capacity, low costs of raw materials, easy way to prepare and are positively charged through electrostatic interactions, and hydrogen bonds in their wide surface area (Guo et al., 2018; Kuznetsova et al., 2017; Menezes et al., 2014). However, they are not mechanically resistant to be used continuously or to undergo regeneration, because they can be sprayed or exfoliated. Therefore, its application is more effective when

⁎

supported by larger recalcitrant particles, with interesting environmental potentials, such as the activated bio-carbon (Nishimura et al., 2010; Takehira, 2017; Wang et al., 2016). Activated biochar has high adsorptive capacity, surface density of functional groups, highly condensed structure, high surface area and, volume of pores(Kim et al., 2016). Its characteristics depend on the raw material and the production condition. Recently, these materials have been used as supports for nanoparticles in order to reduce their agglomeration and increase the surface area (Chen et al., 2015; Wang et al., 2016, 2015a; Xue et al., 2015; Yao et al., 2014; Zhang et al., 2013). In literature, some works reported LDH nanoparticles supported in biochar which have been used to remove contaminants from water solutions by adsorption. Zhang et al. (Zhang et al., 2013) functionalized activated cotton-wood biochar with MgAl-LDH nanoparticles via spontaneous high-mount method and the resulting material was used for the removal of phosphate obtaining a maximum adsorption capacity of 410 mg.g−1. Wang et al. (2015b) precipitated NiMn-LDH nanoparticles in pristine biochar and compared the material obtained with

Corresponding author. E-mail address: [email protected] (L. Meili).

https://doi.org/10.1016/j.clay.2018.10.012 Received 9 May 2018; Received in revised form 5 October 2018; Accepted 22 October 2018 0169-1317/ © 2018 Elsevier B.V. All rights reserved.

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Ni/Mn oxide-modified pinewood feedstock in the removal of arsenic. The NiMn-LDH/biochar composite was more efficient removing the contaminant at low concentrations with 98% efficiency, with great capacity for chemical regeneration with NaOH. Tan et al. (2016) obtained adsorbents from the pyrolysis of MgAl-LDH pre-coated ramie stem (Boehmeria nivea L.) for the removal of violet crystal dye. It was obtained a high dye removal indicating their effectiveness as an adsorbent agent. Wan et al. (2016) functionalized bamboo biochars with different amounts of MgeAl and MgeFe LDH's for the phosphate removal from aqueous solutions. The Mg-Al/biochar composite had better performance than Mg-Fe/biochar. Brazil is one of the largest producers and exporters of beef in the world. In Brazil, the average number of cattle slaughtered, from 2014 to the second half of 2018, is 15,620,248 beef cattle/semester (IBGE, 2018). Because of increased demand for meat, the global meat production is projected to be 15% higher in 2027, relative to current period. The developing countries, including Brazil, are projected to account for the vast majority of the total increase (FAO, 2018a). The competitive price of Brazilian meat products is related to the availability of feed supplies at competitive prices and improved pastures in extensive livestock due to good weather, resulting in increased carcass weight with low costs (FAO, 2018a, 2018b). The large number of slaughter in Brazil supplies a large amount of raw material for products from bovine bone, among them bone-biochar. Due to the high availability of this resource that, in addition, is abundant and renewable, researches that develop products from this resource, like LDH-Biochar, are important for the progress of industry in the country. Taking into account the current research in this area and the importance of the development of new high performance materials for the contaminants removal from water, the present work aimed to evaluate the adsorption of methylene blue dye by MgAl-LDH/biochar composites produced with different Mg:Al molar ratios.

entire procedure was performed changing the Mg:Al molar ratio to obtain materials with different compositions. The composites were also obtained in the molar proportions of 3:1 and 4:1 (Mg:Al). 2.3. Materials characterization The produced materials and the pure biochar were characterized by the following techniques: Nitrogen adsorption by the Brunauer, Emmet and Teller (BET) and Barret, Joyner and Halenda (BJH) methods to determine the surface area and pore diameter of the materials, Fourier Transform Infrared Spectroscopy (FT-IR) X-ray diffraction (XRD) to evaluate the crystalline structure of the material, Thermogravimetric Analysis (TG / DTG) to investigate the degradation temperature of the materials, Dispersive Energy Spectroscopy (DES) for determination of the elements presence and by Scanning Electron Microscopy (SEM) for morphological analysis. Nitrogen adsorption isotherms were obtained from an ASAP 2020 (Accelerated Surface Area and Porosimetry System) equipment from Micromeritics. This equipment has two independent vacuum systems: one for sample preparation and one for analysis. This equipment allows one sample to be treated and another to be analyzed simultaneously. Thus, the BET analysis was divided into two stages: the first step called the sample treatment (degassing) and the subsequent step referring to the analysis of the sample. Sample degassing is the process to which the sample is cleaned before analysis is performed, then heated and placed under vacuum. This step is important because most solid materials absorb moisture and other contaminants when exposed to the atmosphere and being analyzed without proper treatment can provide unreliable data and equipment damage. The degassing step consisted in subjecting the sample to 350 °C under vacuum below 10−3 mmHg for 9 h. After this step, the samples were analyzed in order to obtain important data, such as: surface area, pore diameter and volume. Using Attenuated Total Reflectance (ATR) as an accessory. The pure sample was placed directly on the accessory crystal and then pressed. Air was used in the background and 50 scans were taken in a spectral range of 400 to 4000 cm−1. The XRD used was the Shimadzu DRX-6000 brand equipment, while the thermal analysis was performed on a Shimadzu DTG 60H thermobalance, with a heating rate of 10 °C.min−1 from room temperature to 900 °C under a dynamic air atmosphere with a flow rate of 50 mL.min−1. Samples were packed in 70 μL platinum crucibles and approximately 14 mg in mass. SEM/EDS results were obtained from a Scanning Electron Microscope with X-ray Dispersive Energy Detector (LEO Electron Microscopy, SEM: Leo 440i, EDS: 6070), Voltage: 0.3 to 30 kV, Current: 1 pA to 1 μA, Detectors: SE and EDS. The images were captured with a magnification of 500 X. For gold metal coating of composites, it was used a Sputter Coater EMITECH (K450) with the thickness of Au layer estimated at 200 A°.

2. Materials and methods 2.1. Materials For the synthesis of MgAl-LDH/biochar the following analytical reagents grade were used: magnesium chloride hexahydrate (MgCl2.6H2O), aluminum chloride hexahydrate (AlCl2.6H2O) and sodium hydroxide (NaOH) (all Synth/Brazil). Bovine bone biochar (Bonechar/Brazil), it will be called “biochar” from now on, had an average diameter of 0.45 mm. The adsorbate solutions were produced in various concentrations using the methylene blue dye (C16H18N3SCl.3H2O) (Synth/Brazil). 2.2. Preparation of compositions of MgAl (LDH)/Biochar MgAl-LDH/biochar composite were synthesized by co-precipitation method as described by Zhang et al. (2013). To produce the composite with proportion of 2 mol of Mg and 1 mol of Al (2:1), in a beaker were weighed 3.62 g of aluminum chloride and 6.09 g of magnesium chloride and then 20 mL of deionized water was added. The mixture was under constant stirring for 30 min until complete dissolution of the chemicals. After dissolution, this mixture was transferred to another beaker containing 1 g of the biochar, under constant stirring. A sodium hydroxide solution (3 M) was dripped with a burette until the solution reached pH 10. The above procedure should last for 2 h, after that, the solution was stirred for a further 2 h. After this time, the solution was transferred to a centrifuge (Petrotest, Petrocen 6-15H) where it remained for 5 min at 3000 rpm/min to obtain the MgAl-LDH/biochar composite. This last procedure was repeated six times and in each of them, the solid phase (composite) was washed with deionized water. After washing, the material was placed in Petri dishes and dried in an oven (Fanem, Orion 515) for 16 h at 60 °C. When leaving the oven the material was macerated and sifted until reaching an average diameter < 0.50 mm. The

2.4. Adsorption studies The adsorption of methylene blue dye by adsorption were carried out in finite bath in a Shaker Incubator (SOLAB - SL 222). The pH effect, the equilibrium time through the kinetics experiments and the influence of temperature and concentration on the adsorption isotherm were evaluated. 2.4.1. pH influence The influence of pH on the dye adsorption was investigated adjusting the pH of the solutions for the values: 2; 5.5; 7; 9.5 and 12. A volume of 50 ml of each methylene blue solutions (100 ppm) with pH adjusted were placed in Erlenmeyers flasks, in which 0.1 g of the composites where added. The dye solutions with adsorbents were taken into the incubator and shaken at 140 rpm for 24 h at 30 °C. After this time, the samples were filtered using a filter paper and the absorbance was determined in a UV–visible spectrophotometer (Hach DR2800) at wavelength 665 nm. The adsorption capacity of the sorbent material 12

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(qt) in mg.g−1 was determined by Eq. 1, where C0, Ce and Ct are the concentrations in mg.L−1 of the solution at the beginning, at equilibrium and at time t, respectively. W is the mass in g of adsorbent used and V is the volume of the solution in liters. The quantity adsorbed at equilibrium (qe) was calculated by Eq. 2,16 while the removal efficiency in percentage was calculated based on Eq. 3.

qt = qe =

(C0

Ct )·V W

(1)

( C0

Ce )·V W

(2)

%R =

C0

Ct C0

·100

2.5. Regeneration experiments Adsorption regeneration cycles were performed to investigate the durability of the adsorbent after long use. In the regeneration study, methanol (CH3OH) and sodium chloride (NaCl) were used as eluents. Adsorption of the dye from the composite was performed by mixing 0.1 g of the adsorbent in 50 mL of the solution (100 ppm) in an Erlenmeyer flask followed by stirring at 140 rpm for 2 h. After 2 h, the solution was filtered and the liquid phase had its absorbance read and the solid part was dried in an oven at 60 °C for 2 h. The dry material was added in 20 mL of the methanol stirred for 2 h at 140 rpm. After this period of time, the sample was again filtered, washed and dried at 60 °C for 2 h. The dried material was used in another adsorption test, which was repeated six time. This experiment was performed in the same manner for 0.5 M sodium chloride (Machado et al., 2011; Pavan et al., 2008; Prola et al., 2013).

(3)

2.4.2. Kinects study The adsorption kinetics of the dye in the composite samples were conducted mixing 0.1 g of adsorbent in 50 mL of dye aqueous solution at pH 12, 30 °C, pH 12,0 and 100 ppm of initial concentration. The mixture was placed in the incubator at 140 rpm for different times: 5, 10, 15, 30, 60, 120 and 240 min. The samples were filtered to ensure that no solid was present and the absorbance read. The kinetic data of for dye adsorption were tested with pseudo first order (PFO) (Lagergren, 1898) and pseudo second order (PSO) (Ho and McKay, 2000, 1999, 1998) models, expressed in Eqs. 4 and 5.

q t = q e (1

qt =

e

3. Results and discussion 3.1. Materials characterization Fig. 1 shows the diffractograms of the biochar and the composites. It was observed that there is little crystallinity present in the coals, and this crystallinity is due to the ashes, the main residue of the degradation of carbonaceous material. The conditions of pyrolysis operation, such as temperature, residence time, as well as the biomass origin, affect the composition of the coal. The crystalline phases found in bovine biochar and their compositions were as follows: quartz (SiO2) in the range of 20-40o; (K2O, Al2O3 and SiO2) between 30-40o, dolomite (CaO, MgO and CO2) and kaolinite (Al2O3, SiO2 and H2O) between 40-60o. The presence of MgAl-LDH on the surface of the biochar was confirmed by the presence of characteristic peaks between 5 and 150; since these peaks were absent in pure biochar (Huang et al., 2017; Lonappan et al., 2018; Viana, 2013). Fourier Transform Infrared Spectroscopy (FT-IR) was able to identify the main absorption bands of the samples, as shown in Fig. 2. The bands verified at 3400 and 1640 cm−1 are characteristic of OH stretching vibrations (ʋOH) and elongation and bending vibrations HOH (δH2O flexion) of the hydroxyl groups of the composites and/or the adsorbed and/or interlamellar water of the sorbent. The bands located around 1100 cm −1 and 500–800 cm −1 are vibrations related to the Al-OH and M-O, where M can be Mg or Al stretching, characterizing

(4)

k1·t )

k2·t·qe2 (5)

(1 + k2·t· qe )

where k1 and k2 are first and second order adsorption kinetics (min−1 and g.mg −1.h−1), respectivel, qt and qe are the adsorbed adsorbent (mg.g−1) in time, respectively. 2.4.2.1. Equilibrium study. The adsorption isotherms were determined in a manner similar to the kinetics. 0.1 g of the composite were mixed in 50 mL of dye solutions (pH 12) at concentrations ranging from 5 to 500 ppm at temperatures of 30, 40, 50 and 60 °C with constant speed of 140 rpm for 2 h. After that time, the samples were filtered (porosity 26–44 μm), and the absorbance was determined via a spectrophotometer (Paz et al., 2013). The Langmuir (Langmuir, 1918), Freundlich (Freundlich, 1906) and Redlich-Peterson (Redlich and Peterson, 1959) models, represented in eqs. 6, 7 and 8 respectively, were used to describe the data of the dye adsorption isotherms.

qe =

Ce. Q. k1 1 + (Ce. k1 )

qe = kf . Ce1/ nf qe =

Biochar/MgAl 4:1

(6) (7)

Biochar/MgAl 3:1

Ce. krp b

(8)

(1 + arp. Ce rp ) −1

where Q is the maximum adsorption capacity (mg.g ), kl is the Langmuir constant (L.mg−1), kf is the Freundlich constant (mg.g−1) (mg.L−1) -1/nf, 1/nf is the heterogeneity factor, krp (L.mg−1), arp (L.mg−1)brp and brp are Redlich-Peterson constants. The quality of the kinetic and isothermic fits were evaluated by average relative error (ARE), presented in Eq. 9.

ARE =

100 n

n i=1

qi, model qi, exp qi, exp

Biochar/MgAl 2:1 Biochar LDH

(9)

10

where qi,model is each value of q provided by the adjustment, qi,exp is each value of q measured experimentally and n is the number of experimental points.

20

30

40

50

60

70

80

90

2Θ (degrees) Fig. 1. X-ray diffraction of pure bio‑carbon and composites produced with LDH. 13

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Fig. 2. Infrared spectra of biochar and composites produced with LDH.

the bonds between cations and oxygen, respectively. Peaks around 450 cm−1 are characteristic of Mg-OH bonds (Adebajo et al., 2003; Frost and Kloprogge, 1999). The two peaks observed next 1633–3385 cm−1 in composites spectra were associated with sorbed water molecules or OeH stretching vibrations and H-O-H stretching and bending vibrations of the hydroxyl groups in the biochar/MgAl (Li et al., 2016; Zhang et al., 2013). The presence of these absorption bands demonstrates the efficiency in the production of the composite biochar/ LDH, which is responsible for the improvement of the adsorption capacity of the adsorbent. The results of the textural properties determined from the N2 adsorption measurements are presented in Table 1. In this table, the

values of surface area (SBET), pore volume (Vt) and pore diameter (DBJH) are listed. It is possible to observe that, with the increasing of the Mg:Al molar ratio, the surface area increases and, the volume and diameter of the pores decreased. This is due to the fact that the nanoparticles of LDH are supported in the pores of the biochar, decreasing the pore size, thus increasing the surface area (Song et al., 2005; Vallet-Regí et al., 2004). It was possible to construct the adsorption isotherms, which reveal details about the characteristics of the material, Fig. 3. It can be observed that in all cases, the isotherms are type IV, characteristics of mesoporous materials, according to the classification BDDT (Braunauer, Deming and Teller), and exhibit hysteresis type H3 for most materials and H4 for biochar/LDH 4:1, which can be said that these adsorbents do not have well-defined mesoporous structures, according to IUPAC classification (IUPAC, 1985). These characteristics are related to materials with pores of constant cross-section (cylindrical or hexagonal, for example). The inflections around P/P0 between 0.5 and 0.8 confirm this structural feature of pores. The results obtained by the textural analysis were corroborated by the images obtained from the SEM (Fig. 4). It is possible to verify the good distribution of the LDH particles in the surface of the biochar, which causes an increase in the surface area, and the occupation of the pores, thus reducing the volume of the pores, confirming that the composite formation occurred. Furthermore, from the DES analysis it was confirmed that with increasing of Mg:Al

Table 1 Results of the parameters evaluated by the nitrogen adsorption assay (BET method) for bovine bone biochar and for LDH-produced composites.

Biochar Biochar/MgAl 2:1 Biochar /MgAl 3:1 Biochar /MgAl 4:1

SBET (m2/g)

Vt (cm3/g)

DBJH (nm)

94.3867 46.4306 59.6286 151.2591

0.235371 0.1167 0.0963 0.2003

8.3212 8.0059 4.7371 3.5135

SBET: area by BET method; Vt: total pore volume; DBJH: pore diameter in desorption. 14

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Fig. 3. Adsorption/desorption curves of biochar and composites produced with LDH.

molar ratio, 2:1, 3:1 and 4:1, there was an increase in the amount of Mg (11.31%, 16.38% and 22.91%, respectively) and the amount of aluminum remained practically constant (9.06%, 9.19% and 8.02%, respectively). Analyzing the thermogravimetric (TG) and differential thermal analysis (DTG) curves, shown in Fig. 5, it was possible to identify the

composite decomposition bands. In general, LDH containing chloride as an interlamellar anion have three stages of decomposition. The first mass loss occurs up to approximately 220 °C and is associated with the decomposition of the interlamellar and adsorbed water molecules in the compound. The other stapes between 300 and 500 °C, are related to structural dehydroxylation and the consequent loss of the lamellar

Fig. 4. SEM (500×): (a) 2:1 MgAl- LDH / biochar; (b) 3:1 MgAl-LDH/biochar; (c) 4:1 MgAl- LDH/biochar. 15

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Fig. 5. Thermal analysis of the bio‑carbon and composites produced with LDH.

structure, accompanied by the formation of HCl (Choudary et al., 2002; Islam and Patel, 2009; Kameda et al., 2008; Silion et al., 2010).

observe that there is a rapid growth in the adsorption with the increase of the pH. According to Zhao (2011) the high adsorption increase with respect to the increase in pH can be attributed to the surface charge and the availability of binding sites presented on the surface of the composites, a characteristic inherited from the LDH (Zhao et al., 2011).

3.2. Adsorption studies 3.2.1. pH influence Fig. 6a and b show the pH effect used in the adsorption tests. It is possible to observe that for all Mg:Al molar ratios, > 95% of the dye were adsorbed on the surface of the composite when using pH 12. The high removal capacity at high pH values is associated with surface charge and the availability of bonding sites presented on the surface of composites, a characteristic inherited from LDH. The pH interferes with the degree of dissociation of the dye and at pHs lower than 4.0 the dissolution of LDH occurs (Ambrogi et al., 2009). In addition, there is a rapid increase in adsorption with increasing pH, a fact that may be related to the neutralization of the negative charges on the surface from LDH of the material due to the action of the cationic dye when occupying the active sites. In Fig. 6a and b it can be observed that at pH below 4.0 the dye removal in percentages above 40% for composites is still present. In this case, probably the dye adsorption occurs mostly on the remaining biochar surface, due to the dissolution of the supported LDH (De Sá et al., 2013; Li et al., 2009; Mittal et al., 2010; Tan et al., 2016; Zhao et al., 2011). It is possible to

3.2.2. Kinect and equilibrium studies The adsorption kinetics is one of the most important factors in the evaluation of the efficiency of an adsorbent. Fig. 7 presents the experimental data and fit of PFO and PSO models. It can be observed that the composites can adsorb a greater amount of dye when compared to the pure biochar, fact related to the great capacity of ionic exchange acquired by the material when the LDH is incorporated to the biochar. This behavior can corroborate the results presented in the textural analysis where the composites have greater surface areas than biochar. Another fact that should be taken into account when analyzing the figure is that the methylene blue adsorption performed by the composites enters in equilibrium faster when compared to the pure biochar. The equilibrium time was reached for the composites at around 20 min and for the pure biochar at 60 min. These results demonstrate that the synthesized composites present promising potential for the adsorption removal of dyes. Table 2 shows the kinetic parameters obtained from the fit of the 16

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Fig. 6. a) Absorption capacity of methylene blue by the composites produced as a function of the pH of the dye, b) Removal (%) of the methylene blue by the composites produced as a function of the pH of the dye. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

PFO and PSO models to the experimental data. It was observed that for the biochar the PFO model presented better adjustment when compared to the PSO model, with close values of qe experimental and q calculated, and, values of ARE (%) much smaller. However, for the composites, the PSO model was the one that obtained the best results in terms of the capacity of experimental adsorption compared to the calculated one, as well as, R2 and ARE (%). Although the models used do not describe the adsorption mechanism, both assume that the driving force that causes adsorption is the difference between the mean concentration of the solid phase and the equilibrium concentration. In this way, the adsorption rate would be proportional to the driving force for the PFO model and proportional to the square of the driving force for the PSO model (Chang and Juang, 2004; Yang and Al-Duri, 2005). Thus, the results of the adjustments of the models corroborate with that observed experimentally, where the adsorption performed by the composites was

Table 2 Kinetic parameters of samples of pure biocarbons and composites produced. Model

Biochar

MgAl

2:1 MgAl/ Biochar

3:1 MgAl/ Biochar

4:1 MgAl/ Biochar

qeexperimental

44.231

46,450

65.600

67.352

68.244

43.330 0.094 0.987 6.376

46,347 16,347 0,999 0,351

65.448 0.1874 0.999 1.183

65.983 0.610 0.996 2.187

67.697 0.323 0.998 1.893

47.195 0.003 0.999 1.590

46,536 0,201 0,999 0,227

66.970 0.009 0.999 1.538

67.135 0.036 0.998 1.517

68.590 0.026 0.998 1.780

PPO q1 (mg g−1) k1 (min−1) R2 ARE(%) PSO q2 (mg g−1) k2 (min−1) R2 ARE(%)

Fig. 7. Experimental data of the biochar and composites produced with LDH adjusted to the kinetic models of pseudo-first order and pseudo-second order. 17

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Fig. 8. Methylene blue adsorption isotherms to the 2:1 MgAl / Biochar sample, adjusted by the Langmuir, Freundlich and Redlich-Peterson models. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

much faster than in the pure biochar. From the kinetic tests, it was possible to evaluate that the materials impregnated in the ratio of 3:1 and 4:1 did not show removal efficiency much higher than 2:1. Thus, and due to LDH production savings, in the remaining tests only the 2:1 MgAl-LDH/biochar composite was applied. The adsorption isotherms of methylene blue in the 2:1 MgAl-LDH/ biochar are shown in Fig. 8. The Langmuir, Freundlich and RedlichPeterson isotherms were used to describe the adsorption behavior of the methylene blue dye. According to the R2 values, the Redlich-Peterson model presented slightly better values compared to Langmuir model (with very close statistical values), besides having the smallest relative mean errors, presented in Table 3. Redlich-Peterson model combine elements from Langmuir and Freundlich models. Hence, the value of parameter β obtained by Redlich-Peterson model was close to unity (mainly at 40 °C) indicating that the isotherm had the behavior more suitable with Langmuir than Freundlich isotherm (Albadarin and Mangwandi, 2015; Redlich and Peterson, 1959). A good fit was also obtained by Langmuir model indicating a monolayer adsorption with uniform energy distribution in the active sites. In addition, the model

considers that there are no lateral interactions between adsorbate molecules. It is considered, therefore, that after the site is occupied by the molecule, no further type of interaction occurs at the site (Akhtar et al., 2006; Elmoubarki et al., 2017). The values of q obtained from the Langmuir model were 326.85 mg.g−1, 406.47 mg.g−1 and 398.76 mg.g−1 for 30, 40 and 50 °C, respectively. The adsorption capacity increases with increasing temperature, this behavior confirms that the adsorption process is naturally endothermic (Mittal et al., 2015a, 2015b). The values obtained are higher than many reported in the literature: 61.3 mg.g−1 (pork bones biochar treated with 1.0 mmol.g−1 of H2SO4) (IriarteVelasco et al., 2016), 128 mg.g−1 (MgeAleLDH/Carbon quantum dots composite) (Zhang et al., 2014), 36.4 mg.g−1 (Syagrus coronata fiber) (Silva et al., 2017), 280.0 mg.g−1 (SBA-15)(Dong et al., 2011), 294.12 mg.g−1 (rattan sawdust) (Hameed et al., 2007). The results of the regeneration tests for methanol and sodium chloride are shown in Fig. 9. It was observed that after 6 cycles the capacity of removal of the composite reduced from values between 65

Table 3 Isothermal parameters of the composite 2:1 MgAl/Biochar. adjusted by the Langmuir models. Freundlich and Redlich-Peterson. Modelo Langmuir Q KL RL R2 ARE (%) Freundlich XF KF R2 ARE (%) Redlich-Peterson KR AR β R2 ARE (%)

30 °C

40 °C

50 °C

326.85 0.0319 0.5109 0.964 21.03

406.47 0.0303 0.5238 0.981 10.37

398.76 0.0672 0.3315 0.931 16.37

3.2337 52.084 0.974 13.66

2.7813 51.173 0.964 10.54

3.6533 86.724 0.875 20.113

25.601 0.2677 0.7916 0.969 17.64

12.769 0.0351 0.9801 0.981 10.41

16.757 0.0073 1.3186 0.943 18.37

Fig. 9. Desorption cycles of the 2:1 MgAl / Biochar composite using NaCl and methanol and as desorbent agents. 18

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Acknowledgment

Table 4 Thermodynamic Parameters. ΔG° (kJ.mol−1) 303.15 K

313.15 K

323.15 K

−26.12

−26.85

−29.85

∆H° (kJ.mol−1)

∆S° (kJ.mol−1)

30.72

0.1863

The authors thank to National Council for Scientific and Technological Development (CNPq/Brazil), Coordination for the Improvement of Higher Education Personnel (CAPES/Brazil) and Foundation for Research Support of the State of Alagoas (FAPEAL/ Brazil). The authors also thank Prof. Bin Gao (Department of Agricultural and Biological Engineering, University of Florida) for the theoretical and technical support, that was very important for the development of the work.

and 70 mg.g−1 to values between 40 and 45 mg.g−1. The last values are close to those obtained for pure biochar. LDH undergoing regeneration processes have difficulties in maintaining their structure due to the ease in which they are exfoliated (Nishimura et al., 2010; Takehira, 2017; Wang et al., 2016). Supporting the LDH in biochar is an alternative to improve its application as an adsorbent agent while maintaining its special ion exchange characteristics. Despite the reduction in the adsorption capacity of the composite, probably due to the LDH disintegration, the presence of the biochar favored the permanence of the adsorptive capacity. The thermodynamic parameters, such as the standard Gibbs free energy change (ΔG), the standard enthalpy change (ΔH) and the standard entropy change (ΔS) were evaluated from the adsorption isotherms. The thermodynamic parameters are important for a better understanding of the adsorption phenomena. To obtain these parameters, it is necessary to determine the thermodynamic equilibrium constant (Ke) from the best fit to the isotherm data. Eqs. 9 and 11 were used to calculate ΔG, ΔH and ΔS. The slope and intercept coefficients of the ΔG (kJ.mol−1) versus temperature (K) curve provide the values of ΔH and ΔS, respectively. Table 4 summarizes the activation energy values and thermodynamic parameters (ΔG, ΔH and ΔS) (Dotto et al., 2013; Milonjić, 2007).

G° =

R T ln K e

G°

H°

=

T

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(10)

S°

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The negative values of ΔG indicate the spontaneity of the process, making it more favorable with the increase in temperature. The value of ΔH was positive indicating that the ion exchange during the adsorption was of physical nature (weak attraction forces) and endothermic. The positive value of ΔS indicates an increase in the degrees of freedom for the adsorption adsorption. Moreover, this value may indicate that there was a change in the structure of the adsorbent and increased randomness during the fixation of the methylene blue in the active sites present in the composite (Chen et al., 2009; Fu et al., 2015; Ma et al., 2012; Silva et al., 2017). 4. Conclusion In this study, it was possible to produce satisfactorily MgAl-LDH/ biochar composites, guaranteed by the characterization analyzes, and use them efficiently for the removal of methylene blue present in aqueous solution. A material with a maximum adsorption capacity at 40 °C of 406.47 mg.g−1 was obtained and with rapid action on dye removal, it reaches equilibrium in up to 20 min, with removal of up to 95% at pH 12, indicating the feasibility of the material in dye adsorption process under basic solution conditions. The kinetic model of pseudo-first order was the best fit for the results for the biochar and the pseudo-second order best fit the results obtained for the composites. The Redlich-Peterson and Langmuir equilibrium models provided good fit the equilibrium isotherms, demonstrating that the adsorption mechanism occurs with the distribution of adsorbate on the surface in monolayers with uniform energy distribution in the active sites. The regeneration tests showed that after 6 cycles the removal capacity was reduced to values close to those obtained for the pure biochar. The thermodynamic analysis indicates that the methylene blue adsorption in MgAl-LDH/biochar is spontaneous, endothermic and that there was a change in the structure of the material. 19

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