Adsorption of phenols on reduced-charge montmorillonites modified by bispyridinium dibromides: Mechanism, kinetics and thermodynamics studies

Colloids and Surfaces A: Physicochem. Eng. Aspects 482 (2015) 222–230

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Adsorption of phenols on reduced-charge montmorillonites modified by bispyridinium dibromides: Mechanism, kinetics and thermodynamics studies Zhongxin Luo, Manglai Gao ∗ , Senfeng Yang, Qiang Yang State Key Laboratory of Heavy Oil Processing, College of Science, China University of Petroleum, Beijing 102249, PR China

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Organo-RCMs were prepared using HMBP to modify RCMs through ion exchange. • The uptake of phenol onto HMBPRCMs was positively related to clay layer charge. • The uptake of phenols onto HMBPMt decreased in the order: phenol > PCP > PMP > PNP. • The ␲-␲ polar interaction played a dominated role in the adsorption of phenols.

a r t i c l e

i n f o

Article history: Received 13 March 2015 Received in revised form 16 May 2015 Accepted 20 May 2015 Available online 22 May 2015 Keywords: Hexamethylene bis-pyridinium dibromides Layer charge Adsorption Phenols Adsorption mechanism

a b s t r a c t A series of reduced-charge montmorillonites (RCMs) were modified by hexamethylene bispyridinium dibromides (HMBP), then used to remove phenols from aqueous solution. The effects of concentration of HMBP (C), clay layer charge, contact time (t), temperature (T) and pH were investigated using a batch technique. The results implied that the clay layer charge had significant influence on phenol adsorption and the optimum conditions were as follows: C of 1.0CEC, 120 min, 25 ◦ C and pH of 6.0. The adsorption mechanism of phenol on the HMBP-Mt was studied by comparing the adsorption characters of substituted phenols, the results indicated that the uptake of phenols onto HMBP-Mt decreased in the order: phenol > p-chlorophenol > p-methylphenol > p-nitrophenol. This indicated that the – polar interaction existing between the pyridine ring and benzene ring in phenols played a dominated role in the adsorption of phenols. The adsorption kinetics demonstrated that the adsorption of phenol onto HMBP-Mt followed the pseudo-second-order model. The adsorption isotherms at the temperatures of 25, 40 and 55 ◦ C were determined and modeled using four different models. The best-fitted adsorption isotherm models were found to be in the order: Langmiur ≈ Redlich-Peterson > Temkin > Freundlich. The thermodynamic study of adsorption process showed that the adsorption of phenol with HMBP-Mt was carried out spontaneously, and the process was randomly increasing and exothermic in nature. © 2015 Elsevier B.V. All rights reserved.

∗ Corresponding author. Tel.: +86 10 89733680; fax: +86 10 6972790. E-mail address: [email protected] (M. Gao). http://dx.doi.org/10.1016/j.colsurfa.2015.05.014 0927-7757/© 2015 Elsevier B.V. All rights reserved.

Z. Luo et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 482 (2015) 222–230

1. Introduction Phenol and its derivates are multipurpose raw materials which are used extensively in the areas of paint, coal conversion, pharmaceutical, paper, wood, rubber, polymeric resin, petroleum and petrochemical industries [1]. The depollution of phenol-containing wastewater is considered as one of the top priority since phenols are not only harmful to organisms but also causes bad taste and odor even at low concentrations [2]. Various processes such as biological treatment [3,4], chemical treatment [5], catalytic oxidation [6], ion exchange [7], solvent extraction [8], membrane separation [9] and adsorption [10–12], have been developed to treat the phenol-containing effluents. Among these technologies, adsorption is proved to be one of the most versatile and widely used methods for purification and separation in wastewater treatment, since it is convenient and effective [13]. Activated carbons are the most widely used adsorbents due to their high adsorption abilities for organic pollutants [14–17]. However, the poor mechanical strength, the difficult regeneration and the high disposal cost make it less economically viable as an adsorbent [18–20]. Undoubtedly, low-cost and easily available adsorbents offer a lot of promising benefits for the removal of organic pollutants from wastewater [21]. In recent years, clay minerals have attracted significant attention as adsorbents for the removal of toxic metals and organic pollutants from aqueous solutions due to their low cost and high efficiency [22]. Montmorillonite (Mt), a low-cost and abundant clay, is highly valued for its adsorptive properties because of its favorable physical and chemical characteristics [23]. The adsorption properties of Mt can be extremely improved by the modification with organic modifier. Previous researches [24–27] have shown that when the exchanged quaternary ammonium cation has one or more long-chain alkyl functional groups, the mechanism of sorption has been attributed to solute partitioning between water and the organic phase created by the conglomeration of the flexible alkyl chains, which is characterized by linear, noncompetitive isotherms and aqueous solubility dependence of the sorption process. Conversely, when the exchanged quaternary ammonium cation has benzyl, phenyl, and/or short-chain alkyl groups, the mechanism of sorption has been attributed to a physical adsorption process, which is characterized by competitive, nonlinear isotherms with no clear solubility dependence. The properties of the resultant organo-montmorillonite are highly dependent on the molecular structure of the organic modifier and the layer charge of the parent clay [28]. Huang and Zhu [29] found that the sorption capacity of 2,4-dichlorophenol onto cetyltrimethylammonium bromide (CTMA)-bentonite increased with increasing bentonite layer

223

charge, while the reverse was observed with tetramethylammonium bromide (TMA)-bentonite. This result can be explained by the fact that CTMA aggregates and siloxane surfaces provided the major sorption sites on CTMA-bentonite and TMA-bentonite, respectively. The adsorption capacity of organoclays is not only related to the type and size of organic modifier but also associated with the structure of pollutant molecules [28]. Previous investigation showed that Benzyltrimethylammonium bromide (BTMA) modified bentonite displays a high affinity for phenol, possibly because phenol molecules interact favorably with the benzene ring in BTMA ion through increased ␲-␲ type interactions [30]. Gu et al. [31] found that the ␲-␲ polar interaction existing between the aromatic rings of aniline and pyridine ring plays a key role in the adsorption. Accordingly, it is possible to improve its adsorption capacity towards specific organic compounds present in the wastewater by introducing some special functional groups into the chemical structure of the organic modifier. In this paper, a series of reduced-charge montmorillonites (RCMs) modified by Hexamethylene bis-pyridinium dibromides (HMBP) were applied in the removal of phenol from aqueous solution. The effects of the concentration of HMBP, clay layer charge, contact time, temperature and pH on phenol adsorption were investigated. The comparison of adsorption characters of phenols with various substituted group on HMBP-Mt was carried out to investigate the adsorption mechanism of phenol. The equilibrium data fitted with four isotherm equations (Langmuir, Freundlich, Temkin, and Redlich–Peterson) and the adsorption kinetics was examined using the pseudo-first-order and pseudo-second-order equations. The thermodynamics parameters (G◦ , H◦ , and S◦ ) were also calculated. The results obtained from this work would not only enrich the species of adsorbents for the treatment of phenolcontaining wastewater, but also provide the theoretical basis for its further research. 2. Materials and methods 2.1. Materials The original montmorillonite (Mt), obtained from Zhejiang Institute of Geology and Mineral Resources, China, had a cationexchange capacity (CEC) of 0.99 mmol g−1 . The reduced-charge montmorillonites (RCMs) were prepared by heating a sample of the Li+ -saturated Mt at 100, 120, 150, 170, and 200 ◦ C, respectively, in a muffle furnace for 24 h [32]. The RCMs thus obtained were denoted as Mt-Tm, where m was the heating tempera-

Fig. 1. The chemical structures and molecule dimensions of HMBP and phenols.

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Table 1 The physical properties of the phenols in this study. Phenols

Formula

Molecular weight (g mol-1 )

log(Kow )a

Solubility (g L-1 )

pKa

Phenol PCP PMP PNP

C6 H5 OH C6 H4 ClOH C6 H4 CH3 OH C6 H4 NO2 OH

94.1 128.6 108.1 139.1

1.46 2.39 2.93 1.91

93 27 20 1.7

9.89 9.37 10.26 7.15

a

Kow is the partition coefficient of n-octanol/water.

ture. The CEC values of RCMs gradually decreased with increasing preparation temperature, which were 0.80, 0.68, 0.56, 0.38 and 0.18 mmol g−1 , respectively. Hexamethylene bis-pyridinium dibromide (HMBP) was prepared according to the method reported by Musilek et al. [33] and characterized in previous study [32]. Phenol, p-chlorophenol (PCP), p-methylphenol (PMP) and p-nitrophenol (PNP) used as adsorbates were of analytical grade supplied by Tianjin Guangfu Fine Chemical Research Institute, China. The chemical structures and molecule dimensions of HMBP and phenols are depicted in Fig. 1. The physical properties of the phenols [34,35] in this study are presented in Table 1. 2.2. Preparation of organoclays The preparation of organoclays was performed by the following procedure: 2.0 g of Mt or RCMs was first dispersed in deionized water, into which a pre-dissolved stoichiometric amount (0.25–2.0CEC) of HMBP was slowly added. The reaction mixtures were shaken in water bath oscillator for 3 h at 60 ◦ C to allow equilibrium to be attained. After centrifugation, the products were washed free of bromide anions (tested by AgNO3 ), dried at 80 ◦ C for 24 h and then pulverized to pass through a 200 mesh sieve. The organoclays thus obtained were assigned as HMBP-Mt-Tm, where m was the heating temperature (◦ C) for the preparation of the RCMs. 2.3. Characterization methods The XRD patterns were recorded using a X-ray diffractometer (Shimadzu XRD–6000 powder diffractometer) operating at 40 kV and 40 mA with Cu K␣ radiation ( = 0.15406 nm). The 2 ranging from 1.5◦ to 10◦ for the clay samples at the scanning rate of 1◦ min−1 was recorded. The determination of Zeta potentials: 0.1 g of HMBP-Mt was dispersed in the deionized water (25 mL), then the solution pH was adjusted by the solution of HCl or NaOH, Zeta potentials of HMBP-Mt at different pH values (2, 4, 6, 8, 10 and 12) were determined using Zetasizer Nano ZS at 25 ◦ C. 2.4. Adsorption experiments Adsorption experiments were conducted by batch mode in stoppered conical flasks. 0.100 g of adsorbent was dispersed in a 50 mL conical flask where 25 mL solution of phenol or substituted phenols with different initial concentrations of 0.5–3.0 mmol L−1 was added. The pH was adjusted by adding a small amount of dilute HCl and NaOH solution using a CT-6023 pH meter. All experiments were performed at a shaking speed of 200 rpm for a certain time at a given temperature to ensure the equilibrium of the adsorption process. The concentrations of phenol, p-chlorophenol (PCP), pmethylphenol (PMP) and p-nitrophenol (PNP) in aqueous solutions were determined by using a Shimadzu UV-2550 spectrophotometer at 270, 280, 277 and 317 nm, respectively. The adsorption capacities and removal efficiency (R%) of phenol and substituted phenols on adsorbent were calculated using the following equations: qe =

C0 − Ce V m

(1)

R% =

C0 − Ce × 100% C0

(2)

where qe (mmol g−1 ) is the amount of phenols adsorbed onto the adsorbents; C0 and Ce (mmol L−1 ) are the initial and equilibrium liquid-phase concentrations of phenols, respectively, V (L) is the volume of the solution, and m (g) is the weight of the adsorbents used. The kinetic studies were performed following a similar procedure at 25 ◦ C, solution initial pH 6, the initial concentration of phenol was set as 0.5 mmol L−1 , and the samples were separated at predetermined time intervals (2, 5, 10, 15, 20, 25, 30, 60, 90, 120, 180, 240 min). The uptake of phenol at time t, qt (mmol g−1 ), was calculated by the following equation: qt =

C0 − Ct V m

(3)

where Ct is the concentration of the adsorbate (mmol L−1 ) in solution at time t. Adsorption isotherms were obtained by batch technique at 25, 40 and 55 ◦ C in a 50 mL conical flask (with sealed cape), respectively. 25 mL of phenol solution with varying initial concentrations (0.5–3.0 mmol L−1 ) were mixed with 0.100 g of HMBP-Mt at pH 6. The dispersions were shaken for 120 min at the stirring speed of 200 rpm, then centrifuged and calculated. All the experiments were run in triplicate and average values were reported. Standard deviations were found to be within ±3%. Control experiments demonstrated that desorption of HMBP from adsorbents and the loss of phenols were negligible. 3. Results and discussion 3.1. Effect of the concentration of HMBP and clay layer charge Fig. 2(a) depicts the removal efficiency (R%) of phenol and d001 basal spacing of HMBP-Mt as the concentration of HMBP increases from 0.0 to 2.0CEC. It is apparent that the phenol removal efficiency (R%) increases sharply at low concentration of modifier and then gradually plateau with the concentration further increasing. The removal efficiency (R%) of phenol on Mt (0.0CEC) is about 20%, while the removal efficiency of phenol on HMBP-Mt increases to 70% with the amount of HMBP increasing to 1.0CEC. Thus, the uptake of phenol is positively related to the amount of HMBP. The d001 values show an increase in the basal spacing from 1.22 nm to 1.39 nm for HMBP-Mt with the concentration of HMBP increasing. The basal spacing reaches a plateau of 1.38 nm when the initial concentration of HMBP is 0.8CEC, and then it only increases slightly with continuous increasing of the amount of HMBP. The removal efficiency (R%) of phenol is proportional to the basal spacing of HMBP-Mt. This can be explained in terms of several factors. First, with the increase of the concentration of HMBP, the interlayer spacing of HMBP-Mt is enlarged, which contributes to the intercalation of phenol. Second, the clay surface is converted from hydrophilicity to hydrophobicity after HMBP modification [36], which is helpful for adsorbing phenol. Moreover, the interaction existing between the aromatic rings of phenol and pyridine ring of HMBP may also play a key role in the adsorption. From the above analysis, 1.0CEC HMBP can be consid-

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Fig. 3. The effect of contact time and reaction temperature on the uptake of phenol (C0 = 0.5 mmol L-1 , pH 6.0).

Fig. 2. The effects of concentration of HMBP (a) and clay layer charge (b) on the uptake of phenol and the d001 basal spacing of corresponding organoclays (C0 = 0.5 mmol L-1 , pH 6, T = 25 ◦ C, t = 120 min).

ered to be a sufficient amount to ensure ion exchange completely, and this amount is chosen to prepare a series of HMBP-RCMs. The effect of clay layer charge on the removal efficiency (R%) of phenol and the d001 basal spacing of HMBP-RCMs obtained from the XRD results are shown in Fig. 2(b). It can be seen that the uptake of phenol by HMBP-RCMs depends strongly on the clay layer charge. The removal efficiency of phenol decreases as layer charge decreases, i.e., the removal efficiency is positively related to the clay layer charge. The reason for this phenomenon is that the intercalated HMBP cations rather than the exposure of the siloxane surfaces are the main adsorption sites. The adsorption amount of HMBP to high-charged clay is larger than that to low-charged clay because of their different CEC values [32], and then the adsorption capacity of organoclays with relatively high layer charge can be improved. Likewise, the d001 values decreases with clay layer charge decreasing, resulting in the low uptake of phenol. This is because the larger interlayer space within organoclays provides a higher potential for the uptake of more phenol [37]. HMBP-Mt has the highest removal efficiency of phenol, thus HMBP-Mt is chosen in the following studies. 3.2. Effect of contact time and temperature The absorption capacities (qt , mmol g−1 ) of phenol by HMBP-Mt versus contact time at different temperatures (25, 40 and 55 ◦ C) are presented in Fig. 3. It is obvious that a significant and fast adsorption of phenol occurs in the first 30 min. Thereafter, the adsorption rate decreases and the adsorption equilibrium reaches in about 120 min. It can be explained that the adsorption sites are abundantly avail-

Fig. 4. The effect of initial solution pH on the uptake of phenol (C0 = 0.5 mmol L-1 , T = 25 ◦ C, t = 120 min).

able for adsorption during the initial stage, and after a lapse of time, the remaining vacant surface sites are difficult to be occupied [38]. Generally, the adsorption capacity increases with time and reaches a constant value where no more phenol is removed from the solution. At this equilibrium point, the phenol amount being adsorbed onto the HMBP-Mt is in a state of dynamic equilibrium with the phenol amount desorbed from the adsorbent [1]. The equilibrium adsorption of phenol onto HMBP-Mt is observed at 120 min which is fixed as the equilibrium contact time. From Fig. 3, it can be also seen that the uptake of phenol decreases with increasing the temperature from 25 to 55 ◦ C, which may be due to decreased surface activity with an increase in temperature. The result suggests that the adsorption between phenol and HMBP-Mt is an exothermic process. Therefore, 25 ◦ C is selected as an optimum reaction temperature. 3.3. Effects of pH The effect of pH on phenol adsorption capacity and the zeta potential of HMBP-Mt are investigated in the pH range of 2.0–12.0, and the results are listed in Fig. 4. It is obvious that the uptake of phenol highly depends on the initial solution pH, which affects the surface properties of the adsorbent and ionization of the phenol. The adsorption capacity of phenol increases with pH increasing from 2 to 6 and then starts decreasing with increasing pH. The zeta potentials for HMBP-Mt are all inferior to zero and found to be decreasing with an increase of pH, which indicates the negatively

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charged surface of HMBP-Mt under all experimental pH values. A low pH leads to an increase in H+ ion concentration in the system, and the cation exchange between H+ and HMBP2+ , resulting in a decline in hydrophobicity of HBMP-Mt, which does not favor the adsorption of phenol. When pH of solution goes beyond the pKa of phenol (9.89 in Table 1), phenol chiefly exist as negative phenolate ion (C6 H5 O− ) [19], which would repel the negative charge on the surface of HMBP-Mt, so the adsorption capacity decreases. With the increase of pH, phenol is dissociated to a higher degree [34] and the HMBP-Mt surface is charged more negatively, leading to increasing electrostatic repulsion force between them. As a consequence, the uptake of phenol decreases sharply after pH 10.0. Based on the above experimental results, pH 6 is selected as an optimum pH value. 3.4. Adsorption mechanism In general, several possible mechanism including hydrophobic interaction, electrostatic interaction and special molecular interaction involves in the adsorption of phenolic compounds [39]. HMBP molecule has a hexyl chain linking two pyridine rings and the XRD results (Fig. 2) indicate that the HMBP molecule can only lie in the monolayer with the heteroaromatic ring parallel or at an angle to the silicate planes and the two cation heads linked to the siloxane surface in the clay interlayer [32]. Accordingly, HMBP could not form effective organic phase in the interlayer of clay for contaminants partition [40]. This implies that the hydrophobic interaction between the phenol and adsorbent plays a minor role in the adsorption process. In addition, the electrostatic attraction force between phenol molecules and the negatively charged surface of HMBPMt (according to the zeta potential results) is limited when pH 6, which contributes little adsorption capacity of on HMBP-Mt. Therefore, the special molecular interaction especially – interactions may play a major role in phenol adsorption. Xu and Zhu [40] reported that the maximum adsorption amount of phenol on the hexamethonium bromide (HM) modified clay was about 13 mg g−1 , which was far less than that of phenol adsorbed by the HMBP-Mt (30.6 mg g−1 in this study), this resulted from that the existence of pyridine rings in HMBP could enhance the adsorptive ability through – interactions existing between phenols and HMBP, which are consistent with other studies dealing with adsorption of phenols on organoclays [31,41]. To further explore the – interaction between phenol and HMBP-Mt, several para-substituted phenols (PCP, PNP and PMP) are chosen as adsorbates. The experiments are performed with the initial phenols concentrations in the range of 0.5–3.0 mmol L−1 ,

Fig. 5. Equilibrium adsorption capacities of phenols onto HMBP-Mt (pH 6, T = 25 ◦ C, t = 120 min).

and the results are shown in Fig. 5. As shown in Fig. 5, the substituted phenols demonstrate lower adsorption capacities than phenol independent of initial phenols concentrations, which follows the order of phenol > PCP > PMP > PNP. It is proposed that the introduction of substituent groups would alter the – interactions between phenols and HMBP [34]. The introduction of electronwithdrawing groups can reduce the – interactions between the pyridine ring and phenols by decreasing the ␲ electron density of aromatic ring. And it is quite plausible that a decrease in – interactions between phenols and pyridine ring of HMBP can reduce the adsorptive ability and vice versa. As both chloro- and nitro- are electron-withdrawing groups, the weaker – interactions lead to lower adsorption capacities for PCP and PNP than for phenol. The nitro group has stronger electron-withdrawing ability than the chloro group, hence the – interaction are weaker for PNP than for PCP. In the case of PMP, the – interaction would be enhanced due to the weak electron-donating effect of methyl group. However, the introduction of methyl group would cause the PMP to have more difficulties in moving within the interlayer of HMBP-Mt because of its non-planar structure (see Fig. 1) and small interlayer space of HMBP-Mt. As a consequence, the steric effect results in reduced affinity between PMP and HMBP-Mt. The combined contribution of the steric effect and electron-donating effect generates PMP lower adsorption onto HMBP-Mt than phenol. On the basis of the above analysis and the molecule size of HMBP and phenols

Fig. 6. Schematic illustration of the interaction between HMBP and phenol or PMP in the interlayer of HMBP-Mt.

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Table 2 Langmuir, Freundlich, Redlich–Peterson and Temkin isotherm model constants and correlation coefficients for adsorption of phenol onto HMBP-Mt (according to Eqs. (7)–(11) and Fig. 8). Model

Parameters

-1

Langmuir

Freundlich

Redlich–Peterson

Fig. 7. Pseudo-second-order plots of phenol adsorption onto HMBP-Mt at 25, 40 and 55 ◦ C.

(Fig. 1), the interaction between HMBP and phenols in the interlayer of HMBP-Mt can be schematically illustrated in Fig. 6 (phenol and PMP are chosen as representatives). Fig. 6 clearly shows that phenol enter into the interlayer of HMBP-Mt easier than PMP and the – interaction between the pyridine ring and substituted phenols is positively related to the ␲ electron density of benzene ring. Thus, it is reasonable to infer that the – polar interaction existing between the pyridine ring and benzene ring in phenols plays an important role in this study, as suggested by other studies dealing with adsorption of phenols on organoclays [30,41]. 3.5. Adsorption kinetics The adsorption kinetics describes the rate of adsorbate uptake on adsorbent and it controls the equilibrium time. The kinetic parameters are helpful for predicting adsorption rate, which gives important information for designing and modeling the processes. The pseudo-first-order kinetic model and pseudo-second-order kinetic model are used to elucidate a possible mechanism involved in the adsorption. The integral form of the pseudo-first-order model is expressed in linear form using the following equation [42]: ln(qe − qt ) = ln qe − k1 t

(4)

The pseudo-second-order kinetic model is based on the assumption of chemisorption of the adsorbate on the adsorbents [43]. This model is given as [44,45]: 1 1 t = + t qt q2 k2 q22

(5)

where k1 (min−1 ) and k2 (g mmol−1 min−1 ) are rate constants, qe and q2 (mmol g−1 ) are the maximum adsorption capacity for the pseudo-first-order and pseudo-second-order adsorption, respectively. t (min) is the contact time, k1 and qe are calculated from the slope and the intercept of ln(qe –qt ) against t, respectively (figure not shown). k2 as well as q2 can be determined from the intercept and slope of t/qt versus t in Fig. 7, respectively. The corresponding kinetic parameters are tabulated in Table 2. The pseudo-second-order rate constants are used to calculate the initial sorption rate given by [46,47]: It can be seen that the experimental values (qe,exp ) show good agreement with the calculated ones (q2,cal ) at different temperature. Meanwhile, R2 for the pseudo-first-order kinetic model are between 0.8015 and 0.8766, whereas R2 are all above 0.999 for the pseudo-second-order kinetic model fitting. The initial sorption rate, h, is found to decrease with temperature increasing, and the pos-

Temperature (K)

Temkin

qmax (mmol g ) KL (L mmol-1 ) R2 RL RSS KF (mmol/g (L/mmol)1/n ) n R2 RSS KRP (L g-1 ) B (L mmol-1 )g g R2 RSS A T (L g-1 ) B (J mol-1 ) R2 RSS

298

313

328

0.455 1.467 0.996 0.185 2.349 0.260 2.013 0.978 13.36 0.737 1.742 0.917 0.995 1.996 13.918 0.102 0.995 3.269

0.454 1.096 0.999 0.233 0.574 0.228 1.863 0.984 8.496 0.526 1.221 0.944 0.999 0.499 10.952 0.100 0.991 4.851

0.447 0.860 0.997 0.279 2.161 0.198 1.750 0.977 14.07 0.350 0.677 0.989 0.996 1.786 8.642 0.097 0.989 2.693

sible reason might be that too high temperature would slow down the adsorption process [46]. Hence, the adsorption of phenol onto HMBP-Mt is not suitable for the pseudo-first-order reaction, while the pseudo-second-order kinetics describes quite well the uptake of phenol onto HMBP-Mt. 3.6. Adsorption equilibrium isotherm Adsorption isotherms are very important for the optimization of the adsorption system. In this study, four adsorption isotherms, namely the Langmuir, Freundlich, Redlich–Peterson (R–P) and Temkin isotherms in their non-linear forms are applied to the equilibrium data of phenol adsorption on HMBP-Mt. The Langmuir model supposes that the adsorption takes place at a specific surface with the single coating layer on the surface [48–50] and is expressed as [51]: qmax KL Ce qe = 1+K (7) C L e

where qmax (mmol g−1 ) is the theoretical monolayer capacity, KL (L mmol−1 ) is Langmuir constant related to the affinity of the binding sites. The essential characteristics of Langmuir isotherm can be expressed by a dimensionless constant, RL , known as the separation factor or equilibrium parameter, is defined by the following formula [34]:

RL =

1 1 + KL C0

(8)

where C0 (mmol L−1 ) is the highest initial phenol concentration, the value of RL indicates the type of isotherm to be irreversible adsorption (RL = 0), favorable adsorption (0 < RL < 1), unfavorable adsorption (RL > 1) and linear adsorption (RL = 1). Freundlich model is an empirical equation based on sorption on a heterogeneous surfaces or surfaces supporting sites of varied affinities [46]. The isotherm is expressed as the following formula: 1/n qe = KF Ce (9)where KF (mmol/g (L/mmol)1/n ) and n are Freundlich constants related to sorption capacity and sorption intensity of the adsorbent. qe represents the quantity of phenol adsorbed onto HMBP-Mt for a unit equilibrium concentration. A value for n above one indicates a normal Langmuir isotherm while n below one is indicative of cooperative adsorption [52]. The Redlich–Peterson (R–P) equation [33] is widely used as a compromise between Langmuir and Freundlich systems. This model has three parameters and incorporates the advantageous

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significance of both models. R–P model can be represented as follows: K C qe = RF eg (10)where KRF (L g−1 ) and B (L mmol−1 )g ) are 1+BCe

Redlich–Peterson isotherm constants whereas g is the exponent which lies between 0 and 1. R–P equation transforms to Henry’s equation when g = 0, whereas it transforms to Langmuir equation when g = 1. The Temkin isotherm model contains a factor that explicitly takes into account the adsorbent–adsorbate interactions. The heat of adsorption of all the molecules in the layer would decrease linearly with coverage due to adsorbent–adsorbate interactions. The adsorption is characterized by a uniform distribution of binding energies, up to some maximum binding energy. The Temkin isotherm is shown in Eq. (11) [46,53]: qe = Bln (AT Ce )(11)where B = RT/bT (J mol−1 ) is the Temkin constant related to heat of sorption. A (L g−1 ) is the equilibrium binding constant corresponding to the maximum binding energy. R (8.314 J mol−1 K−1 ) is the universal gas constant and T (K) is the absolute solution temperature. The optimum isotherm out of the four above mentioned is determined by non-linear regression analysis, using ORIGIN 8.5 software. The Langmuir, Freundlich, Redlich–Peterson and Temkin adsorption isotherms of phenol onto HMBP-Mt at different temperature values (25, 40 and 55 ◦ C) are shown in Fig. 8. The corresponding parameters of the four adsorption isotherms are given in Table 3. Due to the highest R2 values (R2 > 0.995) of Langmuir and Redlich–Peterson models listed in Table 3, they are the most suitable equations to describe the adsorption equilibrium of phenol onto HMBP-Mt at 25–55 ◦ C. As can be seen from Fig. 8, the qe values predicted from the Langmuir model agree well with the experimental values. The applicability of the model suggests monolayer

Table 3 Models parameters obtained in adsorption of phenol onto HMBP-Mt. Kinetic model

Pseudo-first-order

Pseudo-second-order

Parameter

qe,exp (mmol g-1 ) k1 (min-1 ) qe.cal (mmol g-1 ) R2 RSS k2 (g mmol-1 min-1 ) q2.cal (mmol g-1 ) h (mmol (g min)-1 ) R2 RSS (×103 )

Temperature (K) 298

313

328

0.0884 0.0182 0.0199 0.8766 3.023 3.496 0.0897 0.0281 0.9999 1.083

0.0795 0.0055 0.0251 0.7658 0.594 3.888 0.0806 0.0252 0.9998 1.985

0.0722 0.0036 0.0319 0.8015 0.212 3.628 0.0734 0.0195 0.9996 4.809

coverage of phenol at the outer surface of the HMBP-Mt. All the RL values are between 0 and 1, indicating that the adsorption of phenol on the HMBP-Mt is favorable at the conditions being studied. As the reactant temperature increases from 25 to 55 ◦ C, the qmax value of phenol adsorption shows some decrease. Additionally, the RL values increase from 0.185 to 0.279, this indicates that adsorption is more favorable at lower temperature. The suitability of the Langmuir isotherm to fit the data is confirmed by the exponent value of R–P model, the g values are close to 1, which means that the isotherms conform to Langmuir model better than Freundlich model. Freundlich equation is in general not as good as Langmuir or Redlich–Peterson equation. This phenomenon is possibly derived from its assumption of heterogeneous adsorbent surface, whereas the surface of HMBP-Mt is relatively uniform. The KF value shows a decrease tendency with the rise of temperature, and that the n values are large than one reveals the favorable adsorption.

Fig. 8. Adsorption isotherm plots for phenol onto HMBP-Mt at 25, 40 and 55 ◦ C: (a) Langmuir, (b) Freundlich, (c) Redlich–Peterson, and (d) Temkin isotherm model.

Z. Luo et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 482 (2015) 222–230 Table 4 Thermodynamic parameters for the adsorption of phenol onto HMBP-Mt.

the selection of high efficient adsorbent according to the special interaction between functional groups.

Temperature (K)

G◦ (kJ mol-1 )

H◦ (kJ mol-1 )

S◦ (J mol-1 K-1 )

298 313 328

−18.06 −18.21 −18.43

−14.47

11.97

References

The determination coefficients of Temkin isotherm equations (R2 = 0.989–0.995) are relatively high thus can also represent the experimental data well. 3.7. Adsorption thermodynamics The thermodynamic parameters such as Gibbs free energy change G◦ , standard enthalpy H◦ and standard entropy S◦ are also studied to better evaluate the feasibility of the adsorption process. Experiments are performed at three different temperatures (298, 313 and 328 K). G◦ , H◦ and S◦ can be calculated by the following equations [34]: G◦ = −RT ln KL ln KL =

S o H o − R RT

229

(12) (13)

where KL (L mol−1 ) is from Langmuir equation, R is the ideal gas constant (8.314 J mol−1 K−1 ) and T is the temperature in Kelvin. In the application of Eq. (13), the values of lnK are plotted against 1/T, the H◦ and S◦ values are calculated from the slope and intercept of the plot (figure not shown, R2 = 0.991, RSS = 1.9168 × 10−4 ). Thermodynamic parameters are given in Table 4. The negative G◦ values indicate the feasibility and spontaneity of the adsorption process. The values of G◦ are close to each other indicating that such spontaneity is independent on the temperature where the adsorption occurs [1]. Generally, the G◦ value is in the range of 0 to −20 kJ mol−1 and −80 to −400 kJ mol−1 for physical and chemical adsorptions, respectively [54]. In this study, the G◦ values are close to −18 kJ mol−1 , indicating that the adsorptions are mainly physical in nature. The negative values of H◦ values are demonstrating the exothermic nature of the adsorption, which is in agreement with the experimental observation. The magnitude of the H◦ value lies in the range of 2.1–20.9 and 80–200 kJ mol−1 for physical and chemical adsorptions, respectively [55]. In this study, the H◦ value is −14.47 kJ mol−1 , indicating the reinforcement of physical adsorption. The positive S◦ for phenol adsorption on HMBP-Mt suggests increasing in randomness or disorder at the solid-liquid interface during the adsorption process [56]. 4. Conclusions The prepared organo-RCMs were characterized and investigated as adsorbents for the removal of phenols from aqueous solutions. The key influence factors were investigated in detail, the experimental results showed that the optimal conditions for the uptake of phenol on HMBP-Mt were as follows: 1.0CEC of the concentration of HMBP, contact time of 120 min, temperature of 298 K, and pH of 6.0. The results indicated that the uptake of phenol onto HMBP-Mt was spontaneous and exothermic process. The adsorption isotherm models fitted the equilibrium data in the order: Langmuir ≈ Redlich-Peterson > Temkin > Freundlich isotherms. The maximum adsorption capacity was obtained as 0.455 mmol g−1 . The adsorption kinetics was found to follow the pseudo-secondorder model. The uptake of phenols onto HMBP-Mt decreased in the order: phenol > PCP > PMP > PNP. The – polar interaction existing between the pyridine ring and benzene ring in phenols was the main adsorption driving force, which provided a new idea for

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