Sorption enhancement of TBBPA from water by fly ash-supported nanostructured γ-MnO2

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JIEC-1973; No. of Pages 10 Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx

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Sorption enhancement of TBBPA from water by fly ash-supported nanostructured g-MnO2 Yun Zhang a,*, Lingyun Jing b, Xinghua He a, Yanfeng Li a, Xin Ma a a

College of Chemistry and Chemical Engineering, Institute of Biochemical Engineering & Environmental Technology, Lanzhou University, Lanzhou 730000, PR China b College of Resources and Environmental Science, Lanzhou University, Lanzhou 730000, PR China

A R T I C L E I N F O

Article history: Received 26 October 2013 Received in revised form 11 March 2014 Accepted 17 March 2014 Available online xxx Keywords: TBBPA Fly ash Nanostructured MnO2 Adsorption mechanism

A B S T R A C T

Tetrabromobisphenol A (TBBPA), an emerging contaminant, has been detected frequently in the environmental media. This study reports the adsorption kinetics, equilibrium and thermodynamics of TBBPA on fly ash modified by nanostructured MnO2 (FA@nM). The resulting nano-adsorbent was characterized by means of the Fourier transform infrared spectra (FT-IR), XRD and scanning electron microscope (SEM). The adsorption results showed that the equilibrium adsorption capacity has been significantly improved by increasing the initial TBBPA concentration and contact time. While a large reduction of TBBPA uptake was observed in alkaline conditions and at high temperatures and ionic strength. The equilibrium between TBBPA and FA@nM was achieved in approximately 40 min with removal of 98% of the TBBPA. The sorption kinetics were well described by a pseudo-first-order rate model, while both Langmuir and Freundlich models described the sorption isotherms well at 298 K. Thermodynamic parameters suggested that the adsorption of TBBPA is exothermic and spontaneous at the temperatures studied. As fly ash is a low-cost material that is often available on-site, it offers an interesting alternative to high-cost advanced wastewater treatment systems for removing such organic pollutants. ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

1. Introduction Tetrabromobisphenol A (TBBPA), known as an emerging contaminant, is one of the most widely used brominated flame retardant (BFR) in commercial and industrial applications for the purpose of fire prevention. TBBPA has been recognized as a cytotoxicant, immunotoxicant and thyroid hormone agonist with the potential to disrupt estrogen signaling [1]. The high usage and limited water solubility may lead to persistence in the environment and possibly bioaccumulation. Potential environmental concerns have led to increasing regulation and restriction on their production and use. TBBPA and its dimethylated derivative have been detected in various environmental matrices such as soil, air, sediments, sewage effluent, fishes and human tissues [2–4]. It is reported that dissolved TBBPA in wastewater is expected to be quite high compared to other BFRs and even the very low

* Corresponding author. Tel.: +86 931 8912528; fax: +86 931 8912113. E-mail address: [email protected] (Y. Zhang).

concentrations could cause endocrine disruption [5]. Thus, it is of significance to remove TBBPA in the contaminated environment. Current studies of TBBPA removal in wastewater focus mainly on biotransformation, photochemical degradation, catalytic decomposition and adsorption [6–9]. Sorption processes are of particular significance because that combination of TBBPA with adsorbents will decrease their transportability in surface water and mobility in aquifers so as to impact strongly on the fate of TBBPA in environment [10]. Fasfous et al. investigated the sorption of TBBPA on multiwalled carbon nanotubes (MWCNTs). 96% removal of TBBPA was observed after 60 min and reduced dramatically in alkaline conditions and at high temperatures [11]. Potvin et al. compared the TBBPA removal by conventional activated sludge and membrane bioreactors and found that the removal mechanisms included adsorption and biological degradation [12]. Ji et al. prepared adsorbents of multiwall carbon nanotubes/iron oxides to remove TBBPA. It was observed the adsorbents could be separated easily and regenerated [13]. However, to our best knowledge, no studies were conducted to investigate the interaction of TBBPA with fly ash modified by manganese dioxide.

http://dx.doi.org/10.1016/j.jiec.2014.03.027 1226-086X/ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

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Fly ash, as a coal combustion product from thermal power plants, is abundantly available and thus cheap in Chinese market. It contains various metal oxides, i.e. SiO2, Al2O3, Fe2O3, CaO, etc. among which several oxides have been reported to possess a strong binding with organic pollutants [14]. The superior properties of high porosity, large surface area, ultralight weight and chemically inert nature make it a strong candidate as nonconventional and low-cost adsorbents. Unexpectedly no adequate attention has been paid to adsorption of TBBPA on fly ash despite the potential advantage. MnO2 is a naturally occurring soil and sediment component that has been shown to oxidize organic compounds containing phenolic or aniline moieties and adsorb metal ions [15,16]. Consequently, it seems reasonable to propose that the modification of fly ash by nanostructured MnO2 (FA@nM) may strongly complex with the TBBPA molecules based on previous studies [14,15]. While, so far no relevant test for the above mentioned hypotheses have been studied. This study focused on the adsorption removal of TBBPA by fly ash modified with nanostructured MnO2 from water. The primary objective was to evaluate adsorption properties of the synthesized composite, FA@nM, as a function of TBBPA concentrations, pH, contact time and ionic strength. The sorption mechanisms of TBBPA onto FA@nM will be investigated from points of thermodynamics and kinetics. The impacts of solution chemistry conditions on adsorption were evaluated and the equilibrium adsorption data were applied to Langmuir, Freundlich, Temkin and Dubinin– Radushkevich (D–R) isotherm models as well which will lead to development of more efficient adsorbents and prediction of the fate, transport and risk of TBBPA in the environment.

2. Experimental 2.1. Materials and analytical The fly ash samples were obtained from a local municipal power plant in Lanzhou, Gansu province, PRC. Before use, they were treated with 30% aqueous ethanol solution at 40 8C for 24 h to remove organic matter. Then the resulting product was filtered, washed with deionized water, and dried in an oven at 100 8C over night. Next, the sample was powdered, ground and sieved to the desired particle size. Finally, the product was stored in a vacuum desiccator. TBBPA (4,40 -isopropylidenebis(2,6-dibromophenol)) with a water solubility of 4.16 mg L1 at 25 8C and log Kow of 4.50 was purchased from Aldrich chemicals (USA, purity >97%). The chemical structure of TBBPA is shown in Fig. 1. Ultrapure water of resistivity 18.2 MV cm was obtained direct from a Milli-Q Plus water purification system (Millipore Corporation). Nitric acid and sodium hydroxide were used to adjust the pH. All other chemicals and reagents used were of analytical reagent (AR) grade obtained from Retell Fine Chemical Co., Ltd. (Tianjin, PRC).

Br

Br

HO

OH

Br

Br

TBBPA MW 543.9; Log Kow 9.7; pKa 7.5/8.5 Fig. 1. Structure and some physical properties of TBBPA.

Zeta potential measurements were conducted using a JS94H microelectrophoresis instrument (Powereach Instruments, Shanghai, China). The FA@nM sample was suspended in 0.01 mol L1 KCl solution, and the aqueous suspension was equilibrated with TBBPA (0.1 mmol L1) at different pH for 16 h. A minimum 10 readings were recorded and the mean value was reported. TBBPA concentrations were quantified using a Unico UV/vis-721 spectrophotometer (Shanghai Unico Co. Ltd., Shanghai, China) by monitoring the absorbance at 209 nm. Quartz glass cells (Hellma) of 10 mm path length were used. Linear calibration curves (absorbance versus concentration) were used to determine the concentration of TBBPA. The detection limit of TBBPA concentration by UV spectroscopy was 0.001 mmol L1 much lower than the lowest TBBPA concentration in this study. The adsorption capacity, Qt (mg g1) and the percentage removal were calculated using the following Eqs. (1) and (2), respectively Qt ¼

ðC i  C e ÞV=1000 W

% removal ¼

Ci  Co  100 Ci

(1)

(2)

where Qt is the adsorption capacity in mg g1 at time t, Ci, Co and Ce are the initial, outlet and equilibrium concentration in mg L1, V is the volume of solution in mL and W is the total amount of adsorbents in g. 2.2. Preparation and characterization of fly ash modified by nanorods of g-MnO2 (FA@nM) In this study, nanorods of g-MnO2 were prepared from MnSO4 using (NH4)2S2O8 as an oxidizing agent. The scheme figure is shown in Fig. 2. First, 10.8172 g of MnSO4H2O and 14.6048 g of (NH4)2S2O8 were dissolved in 70.0 mL of deionized water. 4.0000 g of fly ash were then blended and heated at 120 8C for 2 h. A darkbrown precipitate was separated, washed, and dried at 70 8C. FT-IR spectra of dried FA@nM and TBBPA loaded FA@nM were recorded using a Bruker Vertex 70 FT-IR spectrometer with ATR device (4000–400 cm1). The morphology of FA@nM was examined using a scanning electron microscope (SEM) XL20, Philips. X-Ray diffraction analysis (XRD, Cu Ka radiation, l = 1.54 A˚, 50 kV, 150 mA, Rigaku RU200B) was used for the structure characterization of g-MnO2 and FA@nM. Patterns were recorded over the 2u range 5–808 with a step size of 0.028 and a count time of 3.0 s. The surface area of the synthesized composite was determined by the BET (Micromeritics ASAP 2010 apparatus) method. 2.3. Adsorption experiments 200 mg L1 of TBBPA stock solution was prepared as follows: 20.0 mg TBBPA powder (purity >97%) was dissolved in methanol and then transferred into a 100 mL volumetric flask. The stock solution was diluted with ultrapure water to obtain standard solutions with concentration ranging from 1 to 150 mg L1. Sevenpoint calibration curves were used to perform the quantification of the analyte. Measurements were performed in three replicates and the average of these replicates were reported. Batch equilibrium experiments were carried out in triplicates on a temperature controlled shaker incubator at 120 rpm. In the pH studies, 0.1 g of FA@nM and 100 mL TBBPA solution (50 mg L1) with a range of pH values from 2.0 to 11.0 which was adjusted by the dropwise addition of NaOH/HNO3 as required were transferred into a conical flask and shaken at 298 K for 2 h. Adsorbent dosage experiments were conducted by shaking 100 mL TBBPA solutions (50 mg L1) with different masses of FA@nM (0.05–0.6 g) at 298 K for 2 h. For comparison, the adsorption experiments at the dosage of

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Fig. 2. The scheme for the preparation of FA@nM.

1 g L1 were also conducted using fly ash and nanostructured MnO2 as adsorbents. In kinetic experiments and to study the effects of contact time on FA@nM adsorption, 0.1 g adsorbent was added to a 250 mL solution of TBBPA (50 mg L1) and shaken at 298 K. Samples of 0.1 mL were taken at predetermined time intervals for the analysis of the lead ions in the solution. For determination of equilibrium adsorption isotherm and to study the effect of initial TBBPA concentration on FA@nM adsorption, 0.1 g of FA@nM and 100 mL of various concentrations (1–150 mg L1) of TBBPA solutions were shaken for 2 h. The ionic strength of the solutions was adjusted to the range of 0.01–0.2 M by adding an appropriate amount of sodium nitrate salt, then 0.1 g adsorbent and 100 mL TBBPA solution (50 mg L1) with diverse ionic strength were shaken for 2 h at 298 K. 3. Results and discussion 3.1. Characteristic of the adsorbents FA@nM The reaction involved in the synthesis of g-MnO2 is listed below: MnSO4 þ ðNH4 Þ2 S2 O8 þ 2H2 O ¼ g-MnO2 þ ðNH4 Þ2 SO4 þ 2H2 SO4

In order to differentiate the structure of g-MnO2 component in the composites, FT-IR analysis of g-MnO2, fly ash, FA@nM were performed and the corresponding spectra are shown in Fig. 3. It can be seen from Fig. 3 that the spectrum of FA@nM shows a peak at 3490 cm1 indicating the presence of silanol (Si–OH). The band at 980 and 1100 cm1 corresponds to Si–O symmetric stretching vibrations from fly ash. The peak observed between 1200 and 850 cm1 can be attributed to TO4 (where T = Si, Al) asymmetric stretching vibrations. The band observed between 780 and 795 cm1 corresponds to symmetric vibration of Si–O–Si. The band at 460–470 cm1 indicates the presence of Si-H group it also assigned as T-O bending mode [17,18]. Importantly, two peaks located at 540 and 718 cm1 occurred in the spectra of FA@nM can be ascribed to the Mn–O and Mn–O–Mn vibrations indicating that fly ash was modified by g-MnO2 as expected. A scanning electron microscope was used for obtaining microscopic images of FA@nM. The shape of the FA@nM and the morphologies of g-MnO2 can be seen clearly from Fig. 4. For example, Fig. 4(a) and (c) shows FA@nM and TBBPA loaded FA@nM, respectively. It can be seen that the fly ash were homogeneously

Fig. 3. FT-IR spectra of MnO2, fly ash and FA@nM.

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Fig. 4. SEM micrographs of FA@nM and TBBPA loaded FA@nM.

decorated with nanorods g-MnO2 structures (Fig. 4(a) and (b)), which was consistent with the FT-IR observations. There were significant differences in the morphologies between FA@nM and TBBPA loaded FA@nM (Fig. 4(c) and (d)). After adsorption of TBBPA, the surface of FA@nM (Fig. 4(d)) tends to be more smooth compared with its original form (Fig. 4(b)) indicating the surface binding of TBBPA with FA@nM. Fig. 5 shows the X-ray power diffraction diagrams for the prepared nanorods g-MnO2 and the composited FA@nM. No significant structural changes were observed during composition process, and it could be thought that g-MnO2 was modified on the fly ash successfully. The specific area of the FA@nM determined by the N2-BET adsorption isotherm (677 m2 g1), was significant higher than that of pristine fly ash (384 m2 g1). It could be expected a large adsorption capacity of FA@nM for the target pollutants. 3.2. Batch sorption studies 3.2.1. Effect of solution pH The effect of pH on TBBPA removal was investigated in the pH ranges of 2.0–11.0 at 298 K for 2 h as shown in Fig. 6(a). From the corresponding data, an increase in pH corresponds to an increase in adsorption, reaching the maximum adsorption capacity at pH 7.1. When the solution pH varied from 2.0 to 7.1 the adsorption of TBBPA increased from 10.36 to 43.12 mg g1. This could be explained by that pH governs the speciation distribution of TBBPA. For example, undissociated TBBPA dominates at pH < 7.5, the monoanion is the primary form between pH 7.5 and 8.5, while

dibasic TBBPA in the form of negatively charged phenoxy ion is prevalent at pH > 8.5. On the other hand, the variation of the pH affects the surface properties of FA@nM. As shown in Fig. 6(b), the zeta potentials of FA@nM were measured at different pH in the range of 2.0 to 10.0. It can be seen that the point of zero charge (pHpzc) of FA@nM was 6.8. Hence, the FA@nM has positive zeta potentials at pH < 6.8 indicating that it is positively charged. On the contrary, it is negatively charged with pH > 6.8. Thus, with the pH increasing from 2.0 to about 6.8, lower that the pHpzc, TBBPA is primarily in the neutral form and the adsorption capacities of TBBPA increase gradually due to a reduction of the proton as competing ions in solution and increasing solubility of TBBPA. The strong adsorption of TBBPA on FA@nM might be attributed to physical adsorption and reaction with manganese oxides [19]. At pH > 6.8, the increase of solution pH over the pKa of the ionizable solute leads to increasing ionization and solubility which might promote the adsorption process. However, the electrostatic repulsions between the negatively charged surface of the FA@nM and the TBBPA anions could cause the steep decreasing of adsorption capacities of TBBPA on FA@nM. Here the optimum pH for TBBPA adsorbed by the FA@nM was determined to be 7.1 and all the following adsorption experiments were conducted at this most favorable pH value. 3.2.2. Effect of adsorbent dosage The adsorbent dosage of 0.02, 0.05, 0.10, 0.15, 0.20 and 0.25 g were used in 100 mL solutions of TBBPA (40 mg L1) under pH 7.0 to test their effects on TBBPA adsorption. As shown in Fig. 7(a), the FA@nM has a high level of performance in terms of removal of

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JIEC-1973; No. of Pages 10 Y. Zhang et al. / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx

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Fig. 5. X-ray power diffraction diagrams for MnO2 and FA@nM; (a) the prepared MnO2 nanorods and (b) the composite material, FA@nM.

(a) 40

Adsorption capacity (mg/g)

50

30 20 10 0

2

4

6

8

(a)

40

80.0 60.0

30 40.0 20 20.0

10 10 0.0

pH 30

0.5

1.0

1.5

2.0

2.5

0.0 3.0

Adsorbent dosage (g/L) (b)

15

30

Adsorption capacity (mg/g)

Zeta potential (mV)

100.0

Adsorption removal (%)

Adsorption capacity (mg/g)

50

0

–15

–30 2

4

6

8

10

pH Fig. 6. Effect of initial pH on the adsorption of TBBPA on FA@nM (a) and zeta potentials of FA@nM.

(b) 20

10

0 0.0

0.1

0.2

0.3

Concentration of NaCl (mol/L) Fig. 7. Adsorption of TBBPA by FA@nM as a function of adsorbent dosage and ion strength.

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1.6

30

log (Qe-Qt)

Adsorption capacity (mg/g)

40

20 10

0

30

60

0.8 0.4

(a) 0

1.2

(b)

90

120

0.0

0

10

time (min) 1.5

30

40

40

30

Qt (mg/g)

1.2

t/Qt

20

time (min)

20

0.9

10

(c)

0.6 0

10

20

30

time (min)

(d) 40

0 0.0

1.0

2.0

3.0

4.0

ln t

Fig. 8. The effect of contact time on the adsorption of TBBPA on FA@nM (a) and the fitting of various kinetic equations (b) pseudo first-order model; (c) pseudo-second-order model; d, Elovich model).

TBBPA. When resin dosage was above 0.15 g, the adsorption effective was up to 90% because increasing the adsorbent doses provides a greater surface area or adsorption sites [20]. However, reverse trend was observed with adsorbed TBBPA by unit amount of FA@nM. The adsorption capacity decreased from 39.27 mg g1 at FA@nM dose of 0.1 g L1 to 15.71 mg g1 at FA@nM dose of 2.5 g L1. Once the interaction of TBBPA–FA@nM reached equilibrium, the addition of extra adsorbent was probably left unutilized or unsaturated. These unutilized mass of FA@nM were accounted however during the calculation of removal capacity, leading to reduction of adsorbed amount [21]. On the other hand, the adsorption capacity at FA@nM dose of 0.1 g L1 (39.27 mg g1) was considerably improved compared with pristine fly ash (4.31 mg g1) and nanostructured MnO2 (7.08 mg g1). This may be attributed to the better dispersion, increased surface area and more activated sites obtained after modification of nanostructured MnO2 on fly ash. For the isotherm and kinetic batch experiments, 1 g L1 for FA@nM was chosen as appropriate adsorbent doses. 3.2.3. Effect of ionic strength The ionic strengths of 0.01, 0.05, 0.10, 0.20 and 0.25 mol L1 were used to test their effects on TBBPA adsorbed by FA@nM. In Fig. 7(b), it clearly shows that the adsorption amount of TBBPA onto FA@nM decreased to some extent with increases in the concentration of electrolyte. For instance, when the concentration of NaCl varied from 0.01 to 0.2 mol L1, the adsorption of TBBPA dropped from 23.85 to 6.21 mg g1. This phenomenon could be attributed to the influence of the ionic strength on the activity coefficients of TBBPA, which limit their transfer to the adsorbent surfaces. It is concluded that the adsorption of TBBPA by FA@nM mainly proceeds through outer-sphere adsorption. 3.3. Kinetics of TBBPA adsorption The effect of contact time on the adsorption capacity of FA@nM was investigated in the time ranges of 5 min–2 h under pH of 7.0 at

298 K. The adsorption data for TBBPA uptake versus contact time for a fixed adsorbent amount are shown in the Fig. 8(a). It was observed that the maximum adsorption capacity of FA@nM for TBBPA was 39.15 mg g1 at around 40 min at 100 mL solutions. According to these data, initial adsorption of TBBPA was rapid on FA@nM. The adsorption sites on the FA@nM were quickly covered by TBBPA and the adsorption rate became dependent on the rate at which TBBPA were transported from the bulk liquid phase to the actual adsorption sites. Thus the contact time of 2 h was used in the following sections to ensure the adsorption equilibrium of TBBPA onto FA@nM. The dynamics of the adsorption process in terms of the order and the rate constant can be evaluated using the kinetic adsorption data. The kinetic parameters as important information are used in designing and modeling of the adsorption operation to predict the adsorption rate. The kinetics of removal of TBBPA is clearly explained in the literature using pseudo first-order, second-order and Elovich kinetic models to examine the rate controlling mechanism of the adsorption process such as chemical reaction, diffusion control and mass transfer. Lagergren showed that the rate of adsorption of solute on the adsorbent is based on the adsorption capacity and followed a pseudo first-order equation [22] which is often used for estimating kad considered as mass transfer coefficient in the design calculations. The linear form of the pseudo first-order equation is described by Eq. (3): lnðQ e  Q t Þ ¼ lnQ e  K 1 t

(3)

where Qe and Qt are the amounts of TBBPA adsorbed (mg g1) at equilibrium time and at any instant of time, t, respectively, and K1 (min1) is the rate constant of the pseudo first-order sorption. The plot of log (Qe  Qt) versus t gives a straight line as shown in Fig. 8(b) for the pseudo first-order adsorption kinetics of removal of TBBPA using the FA@nM. The values of first order rate constants K1 and Qe at 298 K, are calculated and listed in Table 1. The

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JIEC-1973; No. of Pages 10 Y. Zhang et al. / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx Table 1 Comparison of pseudo-first-order, pseudo-second-order and Elovich kinetic model for TBBPA adsorption by FA@nM. Kinetic model

Parameters

Pseudo-first-order Pseudo-second-order Elovich model

K1 (min1) 0.0596 K2 (g mg1 min1) 5.23718E  05

Qe1,cal (mg g1) 39.81 Qe2,cal (mg g1) 156.25

a

b

3.4945

0.0630

R2 0.9960 R2 0.7176 R2 0.9900

coefficient of determination (R2) is found to be 0.9960 and Qe calculated from the equation is 39.81 mg g1 which is very close to the true value of Qe obtained from experiments for 100 mg L1 of initial TBBPA concentrations (39.15 mg g1). Thus, it shows the applicability of pseudo first-order kinetic model for the removal of TBBPA using FA@nM. Ho developed a pseudo-second-order kinetic expression for the sorption system of divalent metal ions using sphagnum moss peat [23]. This model has since been widely applied to a number of sorption systems described in the following form: dQ t ¼ K 2 ðQ e  QtÞ2 dt

(4)

where K2 (g mg1 min1) is the second-order rate constant. From the boundary conditions, t = 0 to t and Qt = 0 to Qt, the integrated form of the equation becomes Eq. (5): 1 1 ¼ þ K 2t Qe  Qt Qe

(5)

Eq. (5) can be written in a linear form, as given by Eq. (6): t t 1 ¼ þ Qt Qe h

(6)

where h = K2Qe2 that can be regarded as the initial sorption rate as t approaches 0. The application of the second-order kinetics by plotting t/Qt versus t as shown in Fig. 8(c) yielded the second-order rate constant, K2, estimated equilibrium capacity Qe, and the coefficient of determination (R2) for the initial concentration of 100 mg L1. As can be seen from Table 1, the calculated Qe value shows a bad agreement with the experimental value and the obtained values for coefficient of determination (R2) are less than 0.7176 which indicates that the second-order kinetic model does not describe well the removal of TBBPA by FA@nM as adsorbents. Elovich equation is a rate equation based on the adsorption capacity describing adsorption on highly heterogeneous adsorbents which is commonly expressed as Eq. (7) [24]: dQ t ¼ aexpðbQ t Þ dt

importance for designing fixed-bed column as the primary design parameters of fixed-bed adsorption column such as the breakthrough time and shape of the breakthrough curve are dependent on rate of adsorption. With the use of adsorption rate kinetic constants, the mass transfer coefficient and equilibrium capacity of adsorbent at different fluid phase concentration can be obtained. Amount of TBBPA adsorbed in solid surface is estimated using the kinetic equation which is required to estimate the fluid phase concentration in fixed-bed column operation. Overall, these observations suggest that TBBPA sorption by FA@nM followed the first-order reaction indicating that the process controlling the rate may be a one-step sorption. The correlation coefficient of above 0.9900 at pH 7.0 also confirms the applicability of Elovich equation suggesting the chemical reaction involved in TBBPA adsorption on FA@nM. 3.4. Adsorption equilibrium isotherms and effect of initial TBBPA concentration Initial TBBPA concentration was adjusted in the ranges of 1– 150 mg L1 for adsorption on the FA@nM under pH of 7.0 at 298 K for 2 h as shown in Fig. 9. The increasing initial TBBPA concentration resulted in an increase in the TBBPA on FA@nM. The removal percentage of TBBPA by the beads increased rapidly with increase in the TBBPA concentration in the range of 1– 40 mg L1, while increased slowly as the TBBPA concentration was higher than 40 mg L1. At the low concentration, TBBPA in the solution would interact with the binding sites and thus facilitated more than 90% adsorption for the adsorbent. With increasing TBBPA concentration, there is an increase in the amount of TBBPA adsorbed due to increasing driving force of TBBPA towards the active sites on the adsorbents. When TBBPA concentration was 40 mg L1, amount of TBBPA absorbed by FA@nM was 39.18 mg g1. At higher concentrations, more TBBPA was left un-adsorbed in solution due to the saturation of binding sites. However, there is a decrease in the active sites on the sorbents as more TBBPA are adsorbed. As can be seen from Fig. 8, the adsorption capacity on FA@nM in 50 mg L1 TBBPA solution reached the maximum while the effective (72%) is much lower than that in 40 mg L1 (97.9%) and it shows few adsorption (<40%) when the concentration of TBBPA was below 10 mg L1. This indicates that FA@nM was effective for being used to treat polluted solution with TBBPA of concentrations in ranges of 10–40 mg L1. Adsorption isotherms are important to describe the adsorption mechanism for the interaction of TBBPA on the adsorbent surface for the design of an adsorption process. In the present study, as the

(7)

where a (mg g1 min1) is the initial adsorption rate and b (g mg1) is the desorption constant related to the extent of the surface coverage and activation energy for chemisorption. Eq. (7) is simplified by assuming ab  t and by applying the boundary conditions Qt = 0 at t = 0 and Qt = Qt at t = t, as given by Eq. (8): Qt ¼

1

b

lnðabÞ þ

1

b

lnt

(8)

Fig. 8(d) represents the application of linear form of Elovich kinetic equation which is a plot between Qt and ln t. The Elovich kinetic constants a and b are obtained from the intercept and the slope, respectively. The coefficient of determination (R2) obtained is 0.9907 which is found to be more than the value calculated using pseudo second-order kinetic model as shown in Table 1. To get the adsorption rate of TBBPA onto the solid surface of FA@nM is of great

7

Fig. 9. Adsorption of TBBPA by FA@nM as a function of initial concentration.

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Fig. 10. Equilibrium isotherm for TBBPA adsorption on FA@nM (a) Langmuir; (b) Freundlich; (c) Temkin; (d) D–R isotherm).

adsorbent developed is new, it is essentially required to test the equilibrium data obtained for TBBPA removal using FA@nM with different isotherm models. 3.4.1. Langmuir isotherm Langmuir model has been widely applied to many metal ions sorption process. The basic assumption of the Langmuir theory is that uptake of metal ions occurs on a homogenous surface by monolayer adsorption without any interaction between adsorbed ions [25]. The model takes the following linear form: Ce 1 Ce ¼ þ Q e bQ m Q m

(9)

where Qm is the quantity of adsorbate required to form a single monolayer on unit mass of adsorbent (mg g1) and Qe is the amount adsorbed on unit mass of the adsorbent (mg g1) when the equilibrium concentration is Ce (mg L1) and b (L mg1) is Langmuir constant that is related to the apparent energy of adsorption. A further analysis of the Langmuir equation can be made on the basis of a dimensionless equilibrium parameter, RL [26], also known as the separation factor, given by Eq. (10): RL ¼

1 1 þ bC 0

(10)

The value of RL lies between 0 and 1 for a favorable adsorption, while RL > 1 represents an unfavorable adsorption, and RL = 1 represents the linear adsorption, while the adsorption operation is irreversible if RL = 0. The isotherm data has been linearized using Eq. (10) and is plotted between Ce/Qe versus Ce which is shown in Fig. 10(a). The Langmuir constant Qm, a measure of the monolayer adsorption capacity of FA@nM, is obtained as 40.16 mg g1 which was close to the experimental data (39.15 mg g1) of adsorption capacity that indicated a good adsorption on the FA@nM and the Qm of FA@nM was higher than some Qm values of other sorbents reported by literatures (Table 2). Another constant denoting adsorption energy, b, is found to be 2.3714 L mg1. The high value of related coefficient (R2 = 0.9999) obtained indicates a good agreement between the experimental values and isotherm parameters and also confirms the monolayer adsorption of TBBPA onto FA@nM surface. The dimensionless parameter, RL, a measure of adsorption favorability is found in the range of 0.0028–0.2966 (0 > RL < 1) when C0 is varying from 1 to 150 mg L1. The results confirm the favorable adsorption process for TBBPA removal using FA@nM especially when initial TBBPA concentration is lower than 50 mg L1. 3.4.2. Freundlich isotherm The Freundlich isotherm theory says that the ratio of the amount of solute adsorbed onto a given mass of sorbent to the

Table 2 Comparison of TBBPA adsorption capacity of FA@nM with other reported adsorbents. Adsorbent

Graphene oxide MWCNTs DPA-MIPs MWCNTs/Fe3O4 GX soil FA@nM

Operating conditions

Qm

pH

T (K)

C0 (mg L

6.0 7.0 NA 6.8 5.8 7.0

298 303 303 308 203 298

1 20 1200 10 1.5 40

1

)

Ref. 1

(mg g

115.77 64.4 45 6.4 0.27 39.15

) Zhang et al. [33] Fasfous et al. [11] Yin et al. [34] Ji et al. [13] Sun et al. [35] In this study

NA: not available.

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JIEC-1973; No. of Pages 10 Y. Zhang et al. / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx Table 3 Constants and correlation coefficients obtained by linear regression for the Langmuir, Freundlich, Redlich–Peterson, and Temkin Isotherm models of TBBPA adsorption onto FA@nM. Isotherm model Langmuir Isotherm Freundlich Isotherm Temkin model D–R model

KL (L mg ) 2.371 KF (mg g1)/(mg L1) 53.99 KT (L mg1) 3.053 K (mol2 kJ2) 2.346E  07

1

Adsorbent

Qm (mg g ) 40.16 1/n 1.894 bT (kJ mol1) 64.09 E (kJ mol1) 1459

1 lnC e n

R 0.9999 R2 0.9931 R2 0.9499 R2 0.9797

(11)

where Kf (mg11/n L1/n g1) and n (g L1) are the Freundlich constants characteristics of the system, indicating the relative adsorption capacity of the adsorbent related to the bonding energy and the adsorption intensity, respectively. The Freundlich constants Kf and n are obtained by plotting the graph between log Qe versus log Ce as shown in Fig. 10 (b). The values of Kf and n are 53.99 and 0.5278, respectively. It is found that the coefficient of determination obtained from the Freundlich constants for FA@nM is 0.9931, which is lower than that for Langmuir isotherm model as given in Table 3. 3.4.3. Tempkin isotherm Tempkin isotherm equation [28] assumes that the heat of adsorption of all the molecules in the layer decreases linearly with the coverage of molecules due to the adsorbate–adsorbate repulsions and the adsorption of adsorbate is uniformly distributed and that the fall in the heat of adsorption is linear rather than logarithmic. The linearized Tempkin equation is given by Eq. (12) Q e ¼ BT lnAT þ BT lnC e

(12)

where BT = (RT)/bT, T is the absolute temperature in K and R is the universal gas constant (8.314 J mol1 K1). The constant bT is related to the heat of adsorption, AT is the equilibrium binding constant (L min1) corresponding to the maximum binding energy. Fig. 10 (c) shows a plot of Qe versus lnCe at a constant temperature of 298 K. To evaluate the adsorption potentials of the adsorbent for adsorbates the Tempkin isotherm constants AT and bT are calculated: 3.0532 and 64.09 L min1, respectively. The obtained AT values indicated a good potential for TBBPA. The coefficient of determination (R2) for Tempkin isotherm model is 0.9491 which confirms the worse fit of equilibrium data as compared with the Langmuir and Freundlich model. 3.4.4. Dubinin–Radushkevich (D–R) isotherm Dubinin and Radushkevich [29] have proposed another isotherm which is not based on the assumption of homogeneous surface or constant adsorption potential, but is applied to estimate the mean free energy of adsorption (E). D–R equation is represented in a linear form by Eq. (13): lnQ e ¼ lnQ m  K e2

DG˚ (kJ mol1) 298 K

2

concentration of the solute in the solution is not constant at different concentrations. The heat of adsorption decreases in magnitude with increasing the extent of adsorption [27]. The linear Freundlich isotherm is commonly expressed as follows: lnQ e ¼ lnK f þ

Table 4 Thermodynamic parameters for the adsorption of TBBPA onto FA@nM.

Parameters 1

(13)

where K (mol2 kJ2) is a constant related to mean adsorption energy; and e is the Polanyi potential, which can be calculated from

9

FA@nM

308 K

DH˚ (kJ mol1) DS˚ (J mol K1) R2 318 K

9.501 9.246 8.992 17.08

25.44

0.9749

Eq. (14). 

e ¼ RTln 1 þ

1 Ce



(14)

The sorption energy can also be worked out using the following relationship: 1 E ¼ pffiffiffiffiffiffiffiffiffiffi 2K

(15)

Fig. 10(d) shows the plot between lnQe and e2 at 298 K. The constants, Qm and K obtained for Dubinin–Radushkevich isotherm model are 88.21 mg g1 and 2.346  107 mol2 kJ2, respectively. The value of Qm is higher than that obtained from Langmuir isotherm model and the experimental Qm. The value of coefficient of determination (R 2= 0.9797) indicates that applicability of fitting the equilibrium data with Dubinin–Radushkevich isotherm model. Overall, the experimental data are found to be fitted well with the Langmuir isotherm model which confirms that the sorption process is monolayer favorable sorption because of large surface area of FA@nM. It is also fitted with Freundlich and Dubinin– Radushkevich isotherm model. The result suggests that the adsorption process of TBBPA onto FA@nM includes both physisorption and chemisorption and the monolayer adsorption is prevalent. 3.5. Adsorption thermodynamics and effect of temperature Thermodynamics parameters can de determined using the equilibrium constant, Kd (Qe/Ce) which depends on temperature. The change in enthalpy (DH8) and entropy (DS8) associated to the adsorption process were calculated by using following equation [30]: lnK d ¼

DS R



DH  RT

(16)

where R (8.3145 J mol1K1) is the ideal gas constant, and T (K) is the temperature. The Gibbs free energy, DG8, of specific adsorption is calculated from the equation:

DG ¼ DH  T DS

(17)

DH8, DS8 and DG8 parameters can be calculated according to Eqs. (16) and (17). The DH8 values between 20.9 and 418.4 kJ mol1 are usually considered as the comparable values for the chemical adsorption process [31]. The thermodynamics of the adsorption of TBBPA onto FA@nM were carried out at 298–318 K at optimum pH value of 7.0 and adsorbent dosage level of 1.0 g L1. The contact time for adsorption was maintained at 2 h. The results were given in Table 4. From Table 4 it is clear that negative value of DH8, 17.09 kJ mol1, suggests the exothermic nature of the adsorption, The negative values of DS8, 25.44 J mol1 K1 showed that TBBPA in bulk phase was in a much more chaotic distribution compared to the relatively ordered surface of adsorbent during the adsorption process. The negative values of DG8 indicate the spontaneous nature of the adsorption process. In addition, amount of TBBPA absorbed at three different temperatures showed an decrease trend with increase in temperature accompanied by increase in DG8 suggested

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JIEC-1973; No. of Pages 10 10

Y. Zhang et al. / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx

that adsorption of TBBPA onto FA@nM was less favorabale at higher temperature. The heat of adsorption value between 0 and 20 kJ mol1 indicates the physical adsorption process [32]. In this study, the values of DG8 are in the range of 8.992 to 9.501, indicating the sorption is mainly physical in nature. 4. Conclusions The high potential of FA@nM for removal of TBBPA from aqueous solution was demonstrated in this study. The sorption capacity increased with an increase in initial TBBPA concentration and contact time, but decreased with an increase in pH (pH > 7.0) and temperature. It also showed a significant effect of ionic strength on the adsorption process. The equilibrium between TBBPA and FA@nM was approximately achieved in 40 min with removal of 98% of TBBPA. The Langmuir model exhibited a slightly better fit to the sorption data than the Freundlich model. The sorption kinetics was found to follow pseudo-first-order model expression. The negative value of DG8 (8.992 to 9.501) indicates the feasibility and spontaneity of the sorption process. The negative value of DH8 suggests the exothermic nature of the sorption. Acknowledgments The authors gratefully acknowledge financial supports from the National Natural Science Foundation of China (No. 21304040) and Chinese Postdoctoral Funds (2013M532090). References [1] S. Strack, T. Detzel, M. Wahl, B. Kuch, H.F. Krug, Chemosphere 67 (2007) 405. [2] A. Mehran, A. Pedro, S. Andreas, B. Ake, Environ. Int. 29 (2003) 683. [3] Y. Li, Q. Zhou, Y. Wang, X. Xie, Chemosphere 82 (2011) 204.

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