Adsorption of phenol on microwave-assisted activated

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Oct 22, 2018 - Journal of Molecular Liquids 274 (2019) 309–314. ⁎ Corresponding author. E-mail address: [email protected] (L. Sellaoui).
Journal of Molecular Liquids 274 (2019) 309–314

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Adsorption of phenol on microwave-assisted activated carbons: Modelling and interpretation Lotfi Sellaoui a,⁎, Mouna Kehili b, Eder Claudio Lima c, Pascal S. Thue c, Adrián Bonilla-Petriciolet d, Abdelmottaleb Ben Lamine a, Guilherme L. Dotto e, Alessandro Erto f a

Laboratory of Quantum and Statistical Physics, LR18ES18, Monastir University, Faculty of Sciences of Monastir, Tunisia Laboratory of Environmental Bioprocesses, Centre of Biotechnology of Sfax, University of Sfax, PO, Box 1177, 3018 Sfax, Tunisia Institute of Chemistry, Federal University of Rio Grande do Sul (UFRGS), Av. Bento Gonçalves 9500, P.O. Box 15003, 91501-970 Porto Alegre, RS, Brazil d Instituto Tecnológico de Aguascalientes, Aguascalientes, 20256, Mexico e Chemical Engineering Department, Federal University of Santa Maria (UFSM), 1000, Santa Maria, RS, Brazil f Department of Chemical Engineering, Materials and Industrial Production, University of di Napoli Federico II, P.le Tecchio, 80, 80125 Napoli, Italy b c

a r t i c l e

i n f o

Article history: Received 10 July 2018 Received in revised form 25 August 2018 Accepted 20 October 2018 Available online 22 October 2018 Keywords: Phenol Activated carbon Statistical physics model Microwave preparation

a b s t r a c t A theoretical study of phenol adsorption on microwave-assisted activated carbons has been performed using statistical physics models. These models have been selected based on the adsorption isotherms profiles and applied to obtain a detailed explanation of the phenol adsorption mechanism and to understand the adsorbent performance. In particular, the phenol adsorption was analyzed using a model that assumed two layers with different adsorption energies. Modelling results, based on the number of phenol molecules adsorbed per site, suggested that two relative positions of phenol adsorbed on activated carbon surface could occur. Besides, at high temperatures, an aggregation phenomenon of phenol molecules was also hypothesized and interpreted. This adsorption process was associated to two adsorption energies that characterized the interactions between phenol – adsorbent and phenol – phenol molecules. The analysis of adsorption energies indicated that all interactions were weak resulting in a physisorption process. © 2018 Elsevier B.V. All rights reserved.

1. Introduction The pollution of groundwater and natural water is a worldwide issue and different technologies have been proposed to face it. Among these, adsorption has received wide attention and many applications have been successfully reported in the literature, e.g. the in-situ permeable adsorptive barriers for groundwater remediation [1–4]. One of the main reasons for the wide applications of adsorption is its exceptional versatility, which allows the use of this technology for the treatment of fluids containing one or several pollutants [3–9]. It can be applied for the separation of an extensive set of pollutants with different properties and in different operative conditions [8–13]. In particular, adsorption of organic molecules is one of the most frequent applications for environmental protection purposes [13,14]. In the plethora of organic compounds, phenol is a relevant pollutant regarding its high toxicity that can cause adverse effects to both humans and biota [15]. Previous studies have reported the experimental findings for the phenol adsorption on several activated carbons [16–19]. Experimental data obtained in these studies have been modelled using ⁎ Corresponding author. E-mail address: sellaouilotfi@yahoo.fr (L. Sellaoui).

https://doi.org/10.1016/j.molliq.2018.10.098 0167-7322/© 2018 Elsevier B.V. All rights reserved.

traditional adsorption models, which can be theoretical, semiempirical and empirical [17–22]. In general, the analysis of adsorption data with reliable models can represent a duplex advantage: to provide a fundamental tool for the design and operation of adsorption devices and to attribute further insights into the adsorption mechanism for process and economical optimisation [21–24]. In particular, the phenol adsorption isotherms obtained with different adsorbents have been studied and discussed mainly employing Langmuir, Freundlich and Liu models [25]. Although these models are popular in adsorption modelling, the interpretations of the adsorption properties using the parameters included in their analytical formulation are limited. In fact, these models cannot be used to understand, from a statistical physics perspective, the adsorption dynamics and the parameters that impact it. To this aim, the use of adsorption models derived from statistical physics [26–28] can provide useful information, which can help in the elucidation of the role of the main process parameters. The application of these models has received limited attention in the last years and, particularly, for the adsorption of organic molecules such as phenols. Indeed, the chemical structure of the phenols includes two main chemical groups, i.e. aromatic ring and –OH group, which can be both active in the adsorption on activated carbon. Moreover, the effect of characteristic parameters (e.g. pH) on the chemical properties of these

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180

180

ADS1 150

150 T= 25 °C T= 30 °C T= 35 °C T= 40 °C T= 45 °C T= 50 °C

90 60

Q (mg/g)

Q (mg/g)

120

120

T= 25 °C T= 30 °C T= 35 °C T= 40 °C T= 45 °C T= 50 °C

90 60

30 0

ADS2

30

0

100

200

300

400

0 0

500

100

200

300

400

500

C (mg/L)

C (mg/L) 120

FeZnCW-1.0

ADS4

120

100

100

80

80

Q (mg/g)

Q (mg/g)

140

T= 25 °C T= 30 °C T= 35 °C T= 40 °C T= 45 °C T= 50 °C

60 40 20

100

200

300

400

T= 25 °C T= 30 °C T= 35 °C T= 40 °C T= 45 °C T= 50 °C

40

20

0 0

60

500

C (mg/L)

0 0

100

200

300

400

500

600

C (mg/L)

Fig. 1. Phenol adsorption isotherms at 25–50 °C on activated carbons prepared with microwave technology.

structures is known to affect the adsorption performance of an adsorbent. In this scenario, it can be beneficial to have a modelling tool that can give specific information on the adsorption geometry and, consequently, on the effect of the main process parameters. In this work, grand canonical ensemble-based analytical models were selected to model the phenol adsorption on activated carbons obtained with microwave technology. They included mono- and multilayer models with one and two energies. These models assumed that the adsorption of phenol ends up after a given number of adsorbed layers were formed. The understanding and elucidation of the phenol adsorption mechanism using activated carbons, which were obtained from microwave synthesis, is reported in this paper via the investigation of the parameters of the best fitting adsorption model. Theoretical interpretations were correlated with some physico-chemical properties of the activated carbons to provide a better explanation of the phenol adsorption mechanism. Hence, new insights of the mathematical modelling of phenol adsorption on activated carbons were reported with the aim of providing a valuable information to support adsorption operations. 2. Experimental investigation Isotherms of phenol adsorption on adsorbents prepared from Sapelli sawdust and microwave technology were used in this

study [25]. The adsorbent preparation was performed using a mixture of 100.0 g of Sapelli sawdust, 20.0 g of lime, 80 g of zinc chloride and about 50.0 mL of water [25]. A homogeneous paste was obtained after mixing, and it was subsequently dried at 85–90 °C. Microwave technology was used for the adsorbent preparation using 30.0 g of the mass, which was inserted in a quartz vertical tube. Pyrolysis of the precursor was performed in a quartz tube that was heated for 320 s at 100% of potency under N2 atmosphere (150 mL min−1). A cooling down step of the adsorbent sample was performed for 5 min at the inert atmosphere (i.e., 60 mL min−1 of N 2 ) and room temperature. The adsorbents were obtained with two inorganic/organic ratios 1:1 and 1:1.5. Surface chemistry of these adsorbents was further modified using iron (III) chloride, and new adsorbent samples were obtained. This activating agent was introduced in the mixture of the biomass, lime and zinc chloride following the same synthesis protocol for the pyrolysis with microwaves. In particular, the mixture of inorganics was: 20% lime + 40% zinc chloride + 40% iron(III) chloride. Two proportions of organic (i.e., inorganic ratios of 1:1 and 1.5:1.0) were used to obtain the adsorbents. All these carbonized samples were chemically activated using an HCl solution with a concentration of 6 mol L −1 as previously described [18–20]. These activated carbons were labeled as ADS1, ADS2, ADS3 and ADS4.

Table 1 Mathematical expressions of the selected models. n is the number of phenol molecules per adsorbent site, NM is the density of receptor site, and c1 and c2 are the concentrations at halfsaturation for the first and second layers, respectively. Model

MMOE Q¼

DLMTE

nN M 1þð

c1=2 c Þ

n

Q ¼ nN m

MMS ðcc Þn þ2ðcc Þ2n 1

2

1þðcc Þn þðcc Þ2n 1

2

Q = nNM [F1(c) + F2(c) + F3(c) + F4(c)]/[G(c)]: the expression of this model is reported in reference [29]

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311

Table 2 Parameters of DLMTE model for the phenol adsorption with activated carbons ADS1 and ADS2. T (°C)

ADS1

ADS2

25

30

35

40

45

50

25

30

35

40

45

50

n

0.5

0.83

1.12

1.44

2.1

2.24

0.58

0.93

1.23

1.54

2.19

2.58

Qsat (mg/g) E1 (kJ/mol) E2 (kJ/mol)

189 −11 −5

174 −13 −7

169 −15 −8

165 −19 −9

161 −22 −11

160 −24 −13

204 −14 −7

202 −16 −9

196 −18 −10

189 −23 −13

179 −26 −15

175 −30 −16

3. Theoretical study: description of adsorption isotherms and selection of statistical physics models In general, the form of adsorption isotherms reflects interesting physicochemical information of the investigated adsorption system, and it can be used to select the best model for simulation and data interpretation. Fig. 1 reports the experimental phenol adsorption isotherms for the four adsorbents at six temperatures. Based on Fig. 1, all the phenol adsorption isotherms trend to the saturation condition at high adsorbate concentration. Moreover, as expected for the adsorption of an organic adsorbate on activated carbons, adsorption isotherms have the typical exothermic trends as, for all tested adsorbents, phenol adsorption uptake decreases when the temperature increases. This isotherm behaviour suggests that different models can be used for the fitting and description of the experimental results, assuming that phenol adsorption on all adsorbents occurred by the formation of a fixed number of adsorbate layers [29]. Therefore, different statistical physics-based adsorption models were selected to understand the phenol adsorption on the activated carbons obtained by microwave technology. Explicitly, the first model, namely a monolayer model with one energy (MMOE), assumed that the adsorption of phenol molecules implied the formation of only one layer [29]. This model also considers that there is a single adsorption energy, which is related to the bonding of the phenol molecules to the surface of activated carbon assuming that it can accept n phenol molecules [29]. The second selected model was based on the formation of two adsorbate layers during the adsorption. This model implied two double layers with different adsorption energies and was labeled as DLMTE. This model considered that the phenol adsorption was characterized by two energies ε1 and ε2 describing the interactions adsorbate - adsorbent (i.e., phenol - activated carbon, first layer) and adsorbate - adsorbate (i.e., phenol – phenol, second layer) [29]. Finally, a model based on the presence of multilayers with saturation (MMS) was used for phenol adsorption analysis. In this model, a saturation of adsorption sites occurs by multilayer interactions. In particular, it assumed that a variable number (N2) of phenol molecule layers were formed, being this number limited [29]. The first adsorbed layer of phenol molecules is characterized by ε1 energy, while all the other layers have ε2 energy describing the adsorbateadsorbate interactions [29]. Note that MMOE and DLMTE models are based on the formation of a fixed number of layers (i.e., one and two layers, respectively), while the number of layers for

adsorption may vary in MMS model depending on the system under analysis. Table 1 contains the equations of tested adsorption models. 4. Fitting of phenol adsorption isotherms The three models were applied to correlate all the phenol adsorption isotherms obtained with the four activated carbons at different temperatures. Models parameters were estimated with the LevenbergMarquardt method. This is a standard technique for solving nonlinear least squares problems, and it can be applied to select the best model and its corresponding parameter values. Statistical analysis of model performance was done with R2 and RMSE (root mean square error) where the best adsorption model was selected. Indeed, R2 values varied from 0.981 to 0.988, 0.994 to 0.997 and 0.983 to 0.985 for MMOE, DLMTE and MMS, respectively. On the other hand, RMSE values ranged from 2.11 to 4.57, 2.10 to 3.52 and 3.27 to 5.13 for MMOE, DLMTE and MMS, respectively. These results showed that DLMTE outperformed the other statistical physics models and, consequently, it was chosen to study the phenol adsorption on the investigated activated carbons. Parameters of DLMTE model are reported in Tables 2 and 3 for all activated carbons. 5. Adsorption isotherms interpretation via the statistical physics model parameters 5.1. Adsorption capacity at saturation: First parameter The adsorption capacity at saturation can be calculated from DLMTE model using the expression Qsat = 2nNm and results are reported in Tables 2 and 3. Saturation adsorption capacities ranged from 82 to 204 mg/g for all the activated carbons. Comparably, the phenol adsorption capacities of the tested activated carbons at different temperatures followed the trend: ADS2 N ADS1 N ADS3 N ADS4. This result suggests that the activated carbon ADS2, which was obtained with an organic/inorganic ratio of 1:1.5, ZnCl2 and lime, was the best for phenol adsorption. This chemical treatment improved the adsorbent properties for the phenol adsorption because new functional groups were formed on the adsorbent surface favouring the interaction with phenol molecules. On the contrary, activated carbon ADS4 adsorbed the lowest phenol amount. Several studies [13,16,19] showed that the adsorbent characterization plays an important role to explain the removal performance of a certain adsorbate. In particular, results apparently indicated that the chemical reactions of inorganics reagents (i.e., FeCl3, ZnCl2, lime)

Table 3 Parameters of DLMTE model for the phenol adsorption on the activated carbons ADS3 and ADS4. T (°C)

ADS3

ADS4

25

30

35

40

45

50

25

30

35

40

45

50

n

0.42

0.48

0.65

0.88

0.92

1.2

0.33

0.55

0.82

0.91

0.98

1.12

Qsat (mg/g) E1 (kJ/mol) E2 (kJ/mol)

150 −7 −4

143 −9 −6

134 −11 −8

129 −13 −9

122 −15 −11

120 −17 −12

112 −5 −3

100 −6 −5

94 −8 −7

88 −10 −9

85 −11 −10

82 −13 −11

220

Number of phenol molecules per site

L. Sellaoui et al. / Journal of Molecular Liquids 274 (2019) 309–314

Adsorption capacity at saturation (mg/g)

312

ADS1 ADS2 ADS3 ADS4

200 180 160 140 120 100 80 25

30

35

40

45

2,8 ADS1 ADS2 ADS3 ADS4

2,4 2,0 1,6 1,2 0,8 0,4 25

50

30

35

40

45

50

Temperature (°C)

Temperature (°C) Fig. 2. Adsorption capacities at saturation for phenol adsorption on activated carbons obtained with microwave technology.

Fig. 4. Number of phenol molecules per site for the activated carbons obtained with microwave treatment.

during the preparation of adsorbent ADS4 can block its micropores, resulting in a reduction in the total pore volume and surface area that concur to produce adsorbents with low adsorption capacities. Indeed, from characterization data [24], ADS4 showed the lowest total pore volume (0.34443 cm3/g) [25] and BET surface area (647.05 m2/g) [25] in comparison with other tested adsorbents. It is likely that these textural parameters affected the removal of phenol. Fig. 2 reports the phenol adsorption capacities as a function of temperature for all adsorbents. It is clear that ADS3 and ADS4 showed a very similar trend in the slope of the curve, with a gradual decrease in the adsorption capacity as function of the different temperatures. Differently, the adsorbents ADS1 and ADS2 have a less uniform behavior, and the loss of adsorption capacity mainly occurs at 40 °C for ADS2 and at 30 °C for ADS1, confirming the best performance of ADS2 at usual operating temperatures (25–30 °C).

process. Results reported in Tables 2 and 3 showed that the n values for these activated carbons varied from 0.33 at 25 °C to 2.58 at 50 °C suggesting that the phenol molecule changed its adsorption position from a parallel to a vertical position on the tested activated carbons. This adsorption geometry behaviour was more evident for adsorbents ADS1 and ADS2 regarding the data of Table 2. The transition of phenol adsorption position was less significant for the activated carbons ADS3 and ADS4 when compared with ADS1 and ADS2. The parallel position of phenol molecules adsorption on these activated carbons was more dominant. Chemically, this fact could be probably related to the physicochemical properties of these adsorbents. Indeed, if the activated carbon adsorbent is highly functionalized but has a low surface area, the interaction of phenol with adsorbent surface could become difficult. Hence, the phenol molecules need a multianchorage to establish sufficient interactions to be adsorbed. On the other side, it can be hypothesized that the different series of adsorbents have common interactions with different parts of the phenol structure. The physico-chemical properties of adsorbents ADS3 and ADS4 determine the occurrence of interactions between these activated carbons and the aromatic ring of phenol molecule, likely by Van der Waals interactions between carbon atoms (from adsorbent and adsorbate sides). On the contrary, for the adsorbents ADS1 and ADS2, an interaction with the\\OH groups can also be hypothesized because there are functional groups with an electron donor characteristic. To provide a visual scheme of the different occurring interactions, Fig. 3 shows the parallel and vertical positions of phenol molecules adsorbed on the tested activated carbons. Fig. 4 reports the trend of the values of n for all the activated carbons. It can be deduced that an aggregation phenomenon at high temperature

5.2. Number of phenol molecules adsorbed per site: Second parameter Modelling results were used to estimate the number of phenol molecules fixed per activated carbon receptor site during the adsorption process. DLMTE model with two energies offers a direct way to explain the geometry of the phenol molecules during the adsorption. The adsorption geometry description relied on the comparison of the values of this parameter. Referring to previous works [24,26,29], two interesting cases can be inferred from this parameter. The phenol molecules interact with several adsorbent functional groups when n b 1 implying that the adsorption is performed with a parallel geometry. On the contrary, if n ≥ 1, a vertical adsorption geometry can be hypothesized in this second case. In these two scenarios, adsorption can be classified as a multi-anchorage (n b 1) or multi-molecular (n N 1)

PM RS AC (a)

(b)

Fig. 3. A schematic explanation of (a) parallel and (b) vertical positions of phenol molecules adsorbed on activated carbon (PM: phenol molecule, RS: receptor site, AC: activated carbon).

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occurred during the phenol adsorption process, but it differed for each activated carbon. For example, the maximum values of n were 1.20 and 1.12 for ADS4 and ADS3 reflecting that phenol molecule could be in the form of monomer. This finding was contrary to the results obtained for adsorbents ADS1 and ADS2 where the phenol molecule could be in the form of a dimer. This aggregation could be favored by the different adsorption energies involved, as well as by the different types of interactions, as described above. Effectively, the phenol molecules aggregation by dispersion forces or Van der Waals carbon carbon interactions are facilitated when the –OH groups are linked to the carbon surface. In fact, in this case, the lateral interactions (repulsive) between phenol molecules are low because the asymmetric groups that promote them are involved in the anchorage to the carbon surface. Evidently, all this is valid for the first layer which seems to determine the behaviour of the whole phenomenon. 5.3. Characterization of the phenol adsorption via the analysis of the adsorption energy DLMTE model allows the characterization of phenol adsorption by the estimation of two adsorption energies which provides important information regarding the interaction of phenol - adsorbent surface and phenol - phenol molecules using the two following equations: E1 ¼ RTlnðcs =c1 Þ

ð1Þ

E2 ¼ RTlnðcs =c2 Þ

ð2Þ

where cs is the adsorbate solubility. Adsorption energy values calculated with this model are given in Tables 2 and 3. The energies of phenol – adsorbent interactions (first layer) of all activated carbons were higher than the energies calculated for the phenol – phenol interactions (second layer). This trend is reasonable and standard because the activated carbon surface – phenol interaction was related to the first energy, while the phenol - phenol interaction corresponded to the second energy. As commonly known, the induced dipole in the molecular structure of phenols is responsible for the formation of a second layer, and it is interesting to observe that the energy of the second layer is still high enough to assure a significant contribution. Finally, it can be affirmed that the adsorption of phenol molecules can be classified as physisorption because all adsorption energies were weak. 6. Conclusion This study reports the application of statistical physics model, which was based on the formation of double adsorbate layers with different adsorption energies, to theoretically analyse the phenol adsorption on different activated carbons prepared by microwave treatment. Results showed that two different adsorption positions of phenol molecules on activated carbons could be observed for the two series, depending on the adsorbents properties. For the adsorbents obtained without FeCl3, a transition from parallel to vertical position was observed by an increase of the temperature, possibly by \\OH group adsorption on specific functional groups of the carbons. This fact could explain the higher adsorption capacity also observed at high temperature, ascribable to the lower magnitude of lateral interactions between phenol molecules. Differently, a parallel position was retrieved for the adsorbents prepared with FeCl3. This adsorption behavior could be associated to the absence of effective functional groups for phenol binding. Therefore, these adsorbents showed the lower adsorption capacity together with the lowest adsorbent porosity. The modelling results suggested also that an aggregation phenomenon could occur at high temperature which could be favored by the different adsorption energies involved, as well as by the different types of interactions.

313

Finally, the phenol removal using the microwave-based activated carbons was based on physisorption process, in which a non-negligible contribution is provided by the second adsorption layer too. Acknowledgements All financial and technical support from CNPq (Brazil) is acknowledged. Authors also thank the Chemaxon software: Marvin Sketch (academic research license, Version 18.13.0) that was used for the calculation of physical-chemical properties of phenol. References [1] A. Erto, I. Bortone, A. Di Nardo, M. Di Natale, D. Musmarra, Permeable adsorptive barrier (PAB) for the remediation of groundwater simultaneously contaminated by some chlorinated organic compounds, J. Environ. Manag. 140 (2014) 111–119. [2] F. Santonastaso, I. Bortone, S. Chianese, A. Erto, A. Di Nardo, M. Di Natale, D. Musmarra, Application of a discontinuous permeable adsorptive barrier for aquifer remediation. A comparison with a continuous adsorptive barrier, Desalin. Water Treat. 57 (48–49) (2016) 23372–23381. 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