Adsorption of hexavalent chromium onto Bamboo Charcoal grafted by Cu2+-N-aminopropylsilane complexes: Optimization, kinetic, and isotherm studies

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JIEC 3146 1–12 Journal of Industrial and Engineering Chemistry xxx (2016) xxx–xxx

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Adsorption of hexavalent chromium onto Bamboo Charcoal grafted by Cu2+-N-aminopropylsilane complexes: Optimization, kinetic, and isotherm studies Wu a,*, Zhu Ming b, Shengxin Yang b, Yiang Fan b, Peng Fang b, Haitao Sha b, Ligen Cha b

Q1 Yunhai

a

Key Laboratory of Integrated Regulation and Resources Development of Shallow Lakes, Ministry of Education, Hohai University, Xikang Road #1, Nanjing, 210098, China b College of Environment, Hohai University, Xikang Road #1, Nanjing, 210098, China

A R T I C L E I N F O

Article history: Received 7 June 2016 Received in revised form 27 September 2016 Accepted 23 October 2016 Available online xxx Keywords: Bamboo charcoal Cr(VI) Response surface methodology Adsorption

A B S T R A C T

The adsorption mechanism of Cr(VI) uptake onto Bamboo Charcoal grafted by Cu2+-N-aminopropylsilane complexes (BC/Cu-N) was investigated. The properties of BC/Cu-N were characterized using XRD, FTIR, SEM, EDS, potentiometric acid–base titration and electrochemical analysis. Results indicated that the framework integrity of BC was kept after modification and the quantity of functional groups on BC/Cu-N surface has changed. There existed functional groups on the BC/Cu-N which could consume OH and the pHpzc was found to be 6.20. Moreover, electrochemical analysis showed that new electron transfer pathway was imported. Parameters such as pH, initial Cr(VI) concentration, adsorbent dosage and temperature were optimized using RSM. Analysis of variance of the quadratic model for Cr(VI) was suitable to predict the adsorption of Cr(VI) (F value = 134.23 and P value < 0.001) with a high correlation (R2 = 0.9921). The results showed that the initial Cr(VI) concentration and the adsorption capacity of Cr(VI) were positively related. Adsorption data of Cr(VI) were better fitted by Sips, Temkin and D-R models with the maximum adsorption capacity of 17.9383 mg/g. In addition, the pseudo second-order kinetic model was found to be more suitable for the adsorption of Cr(VI). ß 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

10 11

Introduction

12 13 14 15 16 17 18 19 20 21 22 23

Attention has been drawn to the wastewater containing heavy metals which have been harmful to environment and human health even at a very low concentration [1]. Among the various heavy metal ions, chromium is known to be a typical heavy metal pollutant and greatly impairs ecosystems [2]. The toxicity of chromium relates to valence states. Hexavalent chromium (Cr(VI)) is generally more toxic and can be easily absorbed by human body and accumulated in the body. What’s more, long-term exposure to Cr(VI) is susceptible to cause lung cancer [3]. Comparing with various conventional treatment methods, adsorption, one of the most effective and attractive method, applied in the removal of heavy metal ion from wastewater. It

* Corresponding author. Tel.: +86 25 83786697; fax: +86 25 83786697. E-mail address: [email protected] (Y. Wu).

presents several advantages associated with no chemical sludge and a high removal efficiency to remove metal ions. Hence, a lot of attention has been focused towards the exploitation of cheaper and effective adsorbents. Bamboo charcoal (BC) used as a potential adsorbent has a porous structure and a huge specific surface area. BC is already applied to removal of toxic pollutants from waters, such as heavy metal ions [4,5], dyes [6] and so on. Wang et al. studied the adsorption of cadmium (II) by BC. The result demonstrated the highest monolayer adsorption capacity of BC was obtained 12.08 mg/g, providing theoretical basis for developing BC as adsorbent [7]. In spite of extensive research, adsorption capacity of BC shows slightly insufficient and adsorption performance also has limitation [8]. In order to raise the utilization value of BC, an innovative processing system was developed using various chemicals and temperatures [9]. Adsorption capacity and the feasible removal rate of modified BC will enlarge during the process of adsorption of pollutants. Therefore, modified BC have been widely studied and applied to pollution purification industry

http://dx.doi.org/10.1016/j.jiec.2016.10.034 1226-086X/ß 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

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JIEC 3146 1–12 Y. Wu et al. / Journal of Industrial and Engineering Chemistry xxx (2016) xxx–xxx

2

The electrochemical properties were investigated under a three-electrode cell configuration at room temperature. The working electrode was BC/Cu-N. Platinum wire was used as counter electrodes and the Ag/AgCl electrode was applied as the reference electrode. The electrolyte was deoxygenated with pure nitrogen for about 30 min prior to experiments. The cyclic voltammogram (CV) measurement of the electrodes was carried out with an electrochemical workstation (CHI660D, Chenhua Instruments, China).

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Adsorption experiment

112

BC was cut into clumps, and boiled in distilled water for 30 min. After cooling to room temperature, the BC was dried at 105 8C overnight. Finally, it was sieved (<60 mesh) for later use. 2.08 g CuSO45H2O was mixed with 3.0 g 3-Aminopropyltrimethoxysilane (the molar mass proportion of CuSO45H2O and 3-Aminopropyltrimethoxysilane is 1:2) in 50 mL of n-hexane and stirred for 30 min with magnetic stirring apparatus in a round-bottom flask. This was followed by the addition of 3.0 g BC powder. The mixture was stirred for 17 h. The solid was filtered, washed with deionized water, further rinsed with ultrasonic treatment and finally dried at 102 8C for 24 h. The obtained adsorbent was called as BC modified by Cu2+-N-aminopropylsilane complexes (BC/Cu-N).

A comparison removal efficiency of two adsorbents were studied as follows: 0.1 g of BC, BC/Cu-N were added into 50 mL of Cr(VI) solution, respectively. The pH was adjusted to 6.0 by adding negligible volume of 0.1 M HCl or NaOH solution. And it was followed by water bath shaking with 140 rpm for 180 min at 35  1 8C. Meanwhile, in order to measure the effect of the ionic strength to absorption experiment, the ionic strength was altered by 0, 0.01 and 0.1 M NaCl, respectively. The batch experiment designed by RSM was achieved in 250 mL of Erlenmeyer flask. The pH, initial Cr(VI) concentration, adsorbent dosage (BC/Cu-N) and temperature were designed as independent variables and adsorption capacity was designed as response variable. Table 1 shows the independent variables and coded levels in the experimental design. Experimental design and results of the central composite design are demonstrated in Table 2. For adsorption isotherm experiments, six kinds of different concentrations (2, 4, 6, 8, 10 and 12 mg/L) of Cr(VI) were implemented with different temperature (30, 40 and 50 8C) shaking with 140 rpm for 180 min. Kinetics experiments was carried out when the concentrations of Cr(VI) were 4, 8 and 12 mg/ L vibrating with 140 rpm for different times (10, 30, 60, 90, 120, 150, 180 and 240 min) at 40 8C. The pH of Cr(VI) solution was adjusted to 6.0 and the addition amount of BC/Cu-N was 0.04 g. All drugs were purchased from Sinopharm Chemical Reagent Co. Ltd, China. These reagents were used without further purification. Deionized water was used throughout. After the reaction, the supernate was collected by filtration. The Cr(VI) concentration was measured by UV-spectrophotometer (Ruili Analytical Instrument Corporation Ltd., Beijing, China). All data was the average of twice test results.

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81

Physico-chemical characterization

Data analysis

143

82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102

XRD patterns were measured with an X-ray diffractometer (ARL Corporation, Switzerland) using a Cu target and a nickel medium. It operated at 40 kV and 100 mA in the 2u angle of 5– 808. FTIR spectra of the samples were recorded (Bruker corporation, German) and the spectra were at scope of 400  4000 cm1. The morphology changes were observed by SEM (S4800, Hitachi Ltd., Japan). Meanwhile, elementary composition of the surface of adsorbent was analyzed by EDS (S4800, Hitachi Ltd., Japan). The potentiometric acid–base titration was performed using a computer controlled titration system (AutoCAT9000 automatic titrator, Sash America). The experiment was carried out by agitating 0.05 g BC/Cu-N with 50 mL of 0.01 M NaCl background electrolyte. Initial pH was adjusted to 3.0 by 0.01 M HCl. In order to eliminate disturbances of CO2, nitrogen was bubbled into the titration system. The whole titration process was conducting at 25 8C. The ready-prepared suspension was stirred for 2 h, then slowly titrated to pH 10.5 with 0.05 M NaOH titrant at a fixed increment (0.01 mL). The interval time of titration is 10 min [16]. Blank experiment was operated without adsorbent repeatedly.

Calculation formula of removal rate and adsorption capacity

144 145

1. Removal rate of Cr(VI) is expressed as Eq. (1):

147 148

42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66

[9–11]. Iron-Modified Bamboo Charcoal using as adsorbent for adsorption of arsenic was testified [12]. The addition of iron increased the adsorption capacity of target contaminant. The ability of bamboo charcoal based, iron-containing adsorbent for Cr(VI) removal also proved this inference [13]. Besides, N-(2Aminoethyl) aminopropylsilane coordinated with Cu2+ grafted by MCM-41 and SBA-15 was studied [14]. It illustrated that N-(2Aminoethyl) aminopropylsilane coordinated with Cu2+ enhanced adsorption capacity and Langmuir coefficient. Until now, almost no one focused on the feasibility of Cr(VI) uptake onto Bamboo Charcoal grafted by Cu2+-N-aminopropylsilane complexes (BC/Cu-N). In the present work, the goal of research is to explore the adsorption mechanism of BC/Cu-N for removal of Cr(VI) from aqueous solution. The physicochemical characteristic of BC/Cu-N is examined with X-ray diffraction (XRD), Fourier transform infrared spectra (FTIR), scanning electron microscopy (SEM), Energy Dispersive Spectroscopy (EDS), potentiometric acid– base titration and electrochemical determination. The operating parameters of pH, initial Cr(VI) concentration, adsorbent dosage and temperature were optimized by response surface methodology (RSM). In order to explore the adsorption mechanism and generalize the use of adsorption materials, it is indispensable to confirm adsorption isotherm model [15]. Moreover, various kinetic models are also employed to study the adsorption mechanism of Cr(VI) on BC/Cu-N.

67

Materials and methods

68

Grafting of Cu2+-N-aminopropylsilane complexes on BC

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C 0 C e Removal ¼ 100% C0

(1)

where C0 and Ce are the initial and final concentrations of Cr(VI) (mg/L), respectively.

Table 1 The independent variables and coded levels in the experimental design. Variable

Coded levels

pH Initial Cr(VI) concentration (mg/L) Adsorbent dosage (g) Temperature (8C)

A B C D

2

1

0

1

2

2 2 0.03 30

4 4 0.04 35

6 6 0.05 40

8 8 0.06 45

10 10 0.07 50

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JIEC 3146 1–12 Y. Wu et al. / Journal of Industrial and Engineering Chemistry xxx (2016) xxx–xxx Table 2 Experimental design and results of the central composite design. Running order

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

152 153

Adsorbent capacity qe (mg/g)

A

B

C

D

Cr

1 0 1 0 0 1 1 1 1 1 0 2 0 2 1 1 0 0 1 0 0 1 0 1 0 1 0 1 1 1

1 0 1 0 0 1 1 1 1 1 0 0 0 0 1 1 0 0 1 0 0 1 0 1 2 1 2 1 1 1

1 0 1 0 0 1 1 1 1 1 0 0 2 0 1 1 0 0 1 0 2 1 0 1 0 1 0 1 1 1

1 2 1 2 0 1 1 1 1 1 0 0 0 0 1 1 0 0 1 0 0 1 0 1 0 1 0 1 1 1

8.3875 5.4300 4.3750 5.5000 4.9900 4.1125 6.2000 6.2417 4.1125 8.8625 5.5800 5.3600 3.9786 4.7300 9.4000 3.0083 5.5600 5.5000 6.1500 5.5500 8.6000 6.3250 5.4800 3.0333 9.31 4.3875 1.5800 2.9583 9.1500 2.9500

ðC 0 C e ÞV M

Because of the interaction between adsorbent and adsorbate, the reduction of adsorbent surface adsorption energy is linear related to surface coverage. Temkin model is calculated using Eq. (5): qe ¼

qe ¼ qDR expðbe2 Þ 

e ¼ RTln 1 þ

where V is volume of the solution (mL), M is the mass of dry adsorbent (g).

Isotherm modeling In this study, the Langmuir [17], the Freundlich [18], the Dubinin-Radushkevich [19], the Temkin [20] and the Sips [21] models were employed to fit the experimental data. Langmuir model is used to describe the situation where distribution of adsorption sites is uniform, given as Eq. (3). And adsorption process is monolayer adsorption that occupied adsorption sites cannot be adsorbed any more. qe ¼

166 165 167 168 169 170

qm K L C e 1 þ K LCe

1=n

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(3)

where KL is Langmuir constants related to adsorption capacity (L/ mg), qm is the maximum adsorption capacity (mg/g). Freundlich model is empirical formula which describes the process that involves heterogeneous adsorption. It is represented by Eq. (4): qe ¼ K F Ce

1 Ce



1 E ¼ pffiffiffiffiffiffiffi 2b

(2)

(4)

where KF is the Freundlich constant which can forecast adsorption capacity of per unit mass of adsorbent under the equilibrium concentration (mg/g), n indicates the strength, nature and the distribution of heterogeneity site in the adsorption process. The stronger the numerical values, the greater otherness in the adsorption processes.

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(5)

Eq. (6) can be expressed in another form:

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193 192 191 (7) (8)

(9)

where qDR is maximal adsorption capacity (mg/g), b is D-R constant related to free energy of absorption (mol2/kJ2), E represents mean free energy (kJ/mol). Usually, E value is used to speculate the type of adsorption reaction. It means that the adsorption process is physical properties when E value is less than 8 kJ/mol. Ion exchange has happened in the adsorption process when E value lies between 8 kJ/mol and 16 kJ/mol. The adsorption is dominated by chemisorption if E value is greater than 8 kJ/mol [23,24]. Sips model is calculated using Eq. (10):

195 194

197 196

199 198 200 201 202 203 204 205 206 207 208

1=ns

qe ¼ 157 158 159 160 161 162 163 164

RT lnaT C e bT

where bT is Temkin constant related to adsorption heat (kJ/mol) and aT is balanced binding energy constant related to maximal binding energy (L/mg). Dubinin-Radushkevich (D-R) model is mainly used to estimate the adsorption energy and distinguish the physical and chemical properties of adsorption process. It can describe the adsorption on both homogeneous and heterogeneous surfaces [22]. It is represented by Eq. (6): (   2 ) 1 (6) qe ¼ qDR exp b RTln 1 þ Ce

2. Adsorption capacity qe is expressed as Eq. (2): qe ¼

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Coded value

3

qm as Ce

1=ns

1 þ as Ce

;

(10)

where qm is adsorption energy of monomolecular layer (mg/g) and as is Sips model constant related to adsorption energy. The heterogeneous adsorption has happened when 1/ns is equal to zero. The otherness of distribution of adsorption point is large. It means distribution of adsorption point is relatively homogeneous when 1/ns is closed to zero. Sips model equates with Langmuir model when 1/ns is 1. In order to explore adsorption mechanism and adsorption rate of BC/Cu-N onto Cr(VI), pseudo first-order kinetic model [25], pseudo second-order kinetic model [26], Spahn and Schlunder model [27] and intraparticle diffusion model [28] are used for simulating the dynamics of adsorption process.

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Kinetics modeling Pseudo first-order kinetic model is often used to fit equilibrium phase, which can be written as Eq. (11):

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qt ¼ qe ð110k1 t=2:303 Þ

(11)

where qt is the amount of Cr(VI) adsorbed (mg/g) at time t (min) and k1 is rate constant of pseudo first-order kinetic model (min1). The assumptions of pseudo second-order kinetic model are that the adsorption process involves in chemisorption, electron sharing or ion exchange between adsorbent and adsorbate. The pseudo

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232

second-order kinetic model can be written as follows: t 1 t ¼ þ qt k2 q2e qe

(12)

h ¼ k2 ðqeÞ2

(13)

234 233

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where k2 is pseudo second-order rate constant (g/(mg min)), and h is initial reaction rate (mg/(g min)). Spahn and Schlunder model are mainly used to analyze external diffusion process when the adsorbent across the film to the surface of adsorbent. It is represented by Eq. (14): lnC t ¼ lnC 0 kext t

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(14)

where kext is external diffusion constant, and Ct is the Cr(VI) concentration (mg/L) at time t (min). If the adsorption process is controlled by external diffusion, there is a good linear relation between ln Ct and t. The intraparticle diffusion model is often used to describe ion diffusion in the adsorbent micropore [29]. qt ¼ kp;i t 0:5 þ C

(15)

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where kp,i is the rate constants of intraparticle diffusion in different stage. In this model, adsorption process involves in intraparticle diffusion when it demonstrates a linear relationship between qt and t0.5. The intrapaticle diffusion rate is dominant if the line passes through origin. If not, external diffusion and intraparticle diffusion collectively control the adsorption rate [30].

255

Results and discussion

256

Characteristic and structure of BC/Cu-N

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XRD patterns of the BC and BC/Cu-N are presented in Fig. 1(a). The peaks at 22.88 and 42.828 for BC are assigned to 002, 100 crystal plane, indicating the amorphous nature of the BC and low degree of graphitization [9]. The peak position at 22.88 did not shift after grafting even if the intensity of the high-index diffractions decreased slightly. It proves that grafting did not destroy the structure of BC but led to the increase of structural disorder to some degree. Besides, the absence of slight peak at 33.168, 378 and 59.88 may due to loading of Cu2+-N-aminopropylsilane complexes. FTIR spectrum of BC and BC/Cu-N are shown in Fig. 1(b). And characteristic peak and corresponding functional group are demonstrated in Table 3. As shown in Fig. 1(b), three bands of

BC and BC/Cu-N at 2361 cm1 and 2363 cm1, 1699 cm1 and 1636 cm1, 1065 cm1 and 1111 cm1, which are assigned to the vibrations of the C=N, stretching vibrations of the C=O, stretching vibrations of the C-O, respectively. The corresponding change of intensity and shift of peaks means the change of amount of functional group. The characteristic peak of BC at 1558 cm1 assigned to the stretching vibrations of the C=C is not observed at the spectra of BC/Cu-N. This fact is indicative of destruction of C=C after grafting Cu2+-N-aminopropylsilane complexes. Moreover, a new peak at 3462 cm1 attributed to the stretching vibrations of the O-H in the spectrum of BC/Cu-N may belong to hydroxyl of water from grafting process. This characteristic peak at 1400 cm1 has been observed in FTIR spectra of BC/Cu-N and it can be explained by the introduction of ammonium salts during the reaction between 3-Aminopropyltrimethoxysilane and CuSO45H2O [14]. In Figs. 2 and 3, SEM images and corresponding EDS of BC and BC/Cu-N are illustrated. It is clear that the surface of BC is smooth and has small hole. The major components of BC are C and O displayed in corresponding EDS. In the Fig. 2(b), the surface of BC/ Cu-N is relatively coarse and has a large number of holes that conductive to the adsorption reaction. In addition, EDS shows the major components of BC/Cu-N are C, O and Cu. The distribution of Cu imply that Cu2+-N-aminopropylsilane complexes could be successfully grafted on BC. The back-titration curves of BC/Cu-N and blank experiment are demonstrated in Fig. 4. The back-titration process of BC/Cu-N consumed more NaOH solution than blank experiment and obvious buffer was found in back-titration curves of BC/Cu-N. It can be explained that addition OH reacts with H+ of solution and the substance on the surface of BC/Cu-N while addition OH just react with H+ in the blank experiment. The amount of titration of acid, alkali and pH in titration point were recorded. The pH in titration point, total concentration of H+ after experiment, and concentration of active sites of BC/Cu-N were obtained. The gran plot (Eqs. (16) and (17)) are shown as [31]: On the acidic side : Ga ¼ ðV 0 þ V a þ V b Þ10pH 100

270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305

(16) 307 306

On the alkaline side : Ga ¼ ðV 0 þ V a þ V b Þ10ð13:8pHÞ 100

(17)

where V0 represents the initial volume of the suspension. Va and Vb are the total volume of acid solution and the volume of NaOH used in titration at each point, respectively. The point of intersection of gran plot and X axis are Veb1 and Veb2 corresponding to Veb10 and Veb20 of blank experiment, respectively. The concentration of active sites was obtained by calculating adsorption capacity of H+ at the

Fig. 1. (a) XRD patterns of the BC and BC/Cu-N (It equipped with a Cu target and a nickel medium, operating at 40 kV and 100 mA in the 2u angle of 5–80˚); (b) FTIR spectra of the BC and BC/Cu-N (The spectra were at scope of 400  4000 cm1).

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Table 3 The characteristic peak and corresponding functional group of BC/Cu-N. Material types

Perssad

BC (cm1) BC/Cu-N (cm1)

315

Hs ¼

C5 5O

C5 5C

Ammonium salts

C–O

2361 2363

1699 1636

1558 –

– 1400

1065 1111

0

ðV eb2 V eb1 ÞC b ðVeb2 Veb1 ÞC b V0

ðV b V eb1 ÞC b V0 þ Vb

electron transfer abilities and speeds under different Cr(VI) concentration [34].

350 351

Comparison removal efficiency

352

As shown as Fig. 7, BC without modification hardly wipes off Cr(VI) while removal efficiency of BC/Cu-N reaches 69.89%. It illustrates that the adsorption capacity of BC can be raised by grafting Cu2+-N-aminopropylsilane complexes. The adsorption capacity of 3-aminopropylsilane grafted onto various mesoporous silicas have been demonstrated [35]. Meanwhile, the coordination of Cu2+ with organic groups enhances the adsorption capacity and equilibrium constant of adsorption [36].

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Fitting of the process model and statistical analysis

361

Table 2 displays the experimental results of orthogonal tests. In order to optimize experimental results, RSM obtained an optimal model by fitting experimental conditions and results. In this test, a second-order polynomial was achieved which can be utilized to describe the experimental variable: the independent variables are pH (A), initial metal concentration (B), adsorbent (C), and temperature (D). The dependent variable is the response (qe). The second-order polynomial model equation of Cr(VI) adsorption is as follows as Eq. (20):

362 363 364 365 366 367 368 369 370

(18)

Gran plot of BC/Cu-N and blank experiment during the backtitration process is demonstrated in Fig. 5(a). During the titration, OH involves three main reaction during the back-titration process, including consuming the excess H+ added from the acid titration (before Veb1), contributing to pH value of system (after Veb2), and reacting with solid surface of BC/Cu-N (between Veb1 and Veb2) [31]. The consumption of proton total concentration in every titration point was calculated [32], as followed as Eq. (19): TOTH ¼

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C5 5N

– 3462

surface of BC/Cu-N [32], as shown as Eq. (18): 0

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O–H

(19)

where Cb is the concentration of NaOH, Veb1 is the volume of NaOH used in titration at Gran point to zero at acidic side. The titration curve of TOTH vs. pH plot is shown in Fig. 5(b). When TOTH value equals to zero, the zero point of charge is the beginning of formation of deprotonation stage from protonation stage. Hence, the pHpzc value of BC/Cu-N is calculated to be 6.20. This research adopts CV to explore the electrochemical activity of BC and BC/Cu-N, as shown as Fig. 6(a). There is an apparent sharp of BC at 0.3 V while it is found that BC/Cu-N has two apparent peak at 0.6 and 0.1 V, which indicates that the reaction path of BC/Cu-N and FeCl3 electrolyte in solution is different from that of BC and FeCl3 in electrolyte solution. It can be explained that different electron transfer ways led to different reaction paths [33]. The CV of BC/Cu-N in the Cr(VI) solution under different pH is presented in Fig. 6(b). It means that the intensity of current peaks is disparate under different pH, which can be attributed to the difference of existence form of Cr(VI) when the pH of solution is different. The rate of electron exchange is also varied may due to different Cr(VI) concentration solution. CV of BC/Cu-N under different concentrations of Cr(VI) is shown in Fig. 6(c). It can be found that Cr(VI) concentration affects redox peaks of CV of BC/CuN. The current intensity increases with the increase in solution concentration, which manifests that BC/Cu-N has different

qeðCrÞ ¼ 5:440:004965A þ 1:97B1:05C þ 0:022D 0:080AB þ 0:078AC0:055AD0:37BC0:042BD þ0:002344CD0:093A2 þ 0:007469B2 þ 0:22C 2 þ0:012D2 (20) The results of ANOVA table for response surface quadratic model for Cr(VI) adsorption is presented in Table 4. The regression was statistically significant when the F-value was found to be 134.23 with low P-values (P-value < 0.001). The larger the F-value and the smaller P-values, the better statistical significance of the second-order polynomial model. In addition, the determination coefficient of model (R2 = 0.9632) and adjusted determination

Fig. 2. SEM image of BC (a) and BC/Cu-N (b).

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Fig. 3. EDS mapping of BC (a) and BC/Cu-N (b).

380 381 382 383 384 385 386 387 388 389 390 391

coefficient (Adj R2 = 0.9847) were in essential agreement and both close to 1.0. It indicates that the second-order polynomial has better fitting and can be used for predicting experimental results. Usually, adequate precision value is used to detect signal to noise ratio [37]. The model does not reliable until signal to noise ratio value is greater than 4 [38]. Adequate precision of 43.428 for Cr(VI) indicates an adequate signal. Hence, this response surface model can be used to describe the adsorption of Cr(VI). Fig. S1, Supplementary data shows the contrast of actual and predicted removal capacity. Actual value and predicted removal capacity demonstrated a good agreement which indicates the second-order polynomial is credible.

392

Effect of process variables

393 394 395 396 397 398 399 400

Three dimensional (3D) response surface plots employed four variable (A, B, C, D), as shown in Fig. 8(a–f). The effect of two factors on the relationship of factors and response are examined by holding the other factors at central level. The significant analysis result shows that the initial metal concentration (B) (F value = 1414.50, P value < 0.0001) and adsorbent (C) (F value = 401.37, P value < 0.001) have great influence upon adsorption capacity. However, the pH (A) (F value = 9.001E-3, P

Fig. 4. Back-titration curves of BC/Cu-N.

value < 1.0) and Temperature (D) (F value = 0.17, P value < 0.1) exert relatively little impact on adsorption capacity.

401 402

Effect of Cr(VI) concentration The mutual effects of Cr(VI) concentration with pH, adsorbent dosage, and temperature are visualized in Fig. 8(a–c). As increase of Cr(VI) concentration from 4 mg/L to 8 mg/L, adsorption capacity increases. The B linear coefficient is +1.97 from Eq. (20) which indicates the adsorption capacity and Cr(VI) concentration have a positive correlation. A certain amount of adsorbent has quantitative adsorption sites. The low adsorption capacity results from adsorption sites can’t combine with enough Cr(VI) when Cr(VI) concentration is low. As the Cr(VI) concentration increased, concentration gradient on the contact surface between solution and adsorbent also increased. Hence, the force overcoming the various mass transfer resistance of Cr(VI) from the aqueous to the solid phase increases, resulting in higher collision between Cr(VI) and BC/Cu-N surface increasing the uptake of Cr(VI) [39]. When adsorbent dosage was 0.04 g and the initial concentration was in the range of 4  8 mg/L, adsorption capacity increased quickly with the increase in metal ions. At that time, adsorption sites of adsorbent did not become saturation. However, when adsorbent dosage was 0.05 g and the initial concentration was in the range of 4  8 mg/L, the increased speed of adsorption capacity is lower than the former. It can be deduced that adsorbent particles easy to condense into pieces when adsorbent dosage is larger. Reduced the contact area of adsorbent and metal ions leads to the reduction of adsorption capacity. Thus, as the adsorbent dosage is low, the increase speed of adsorption capacity along with the increase in metal ion concentration was higher than the increase speed when the adsorbent dosage was high [40].

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Effect of adsorbent dosage 3D surface graphs presented the effect of adsorption capacity and adsorbent dosage in Fig. 8(c–e), revealing adsorption capacity reduced with the increase of adsorbent dosage. C linear coefficient is 1.05 (Eq. (20)) which indicates this is a negative correlation between the adsorption capacity and adsorbent dosage. With a certain Cr(VI) concentration, the reaction reached a stage. At that time, the concentration of residual metal ions decreases and the pressure of concentration gradient from residual metal ions also correspondingly decreases. The driving force supplied by pressure of the concentration gradient does not enough to overcome

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Fig. 5. Gran plot of BC/Cu-N (a) and corresponding TOTH-pH plot (b).

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transfer resistance of the solution. Hence, adsorption capacity does not increase with the increase of adsorption dosage [41]. In addition, the possibility of aggregation adsorbent particles also increases with the increase of adsorbent dosage which leads to decrease in effective adsorption sites. That is reason that adsorption capacity has been reduced.

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Effect of pH and ion strength pH is a key control variable during adsorption process related to the type and ionic state of functional groups present on the

adsorbent and Cr(VI) in the solution. Fig. 8(a, e, and f) illustrates the interaction effects of the pH with metal ion concentration, adsorption dosage and temperature on the adsorption capacity of BC/Cu-N. Moreover, Fig. 9(a) demonstrates the influence of pH (2  9) on the adsorption capacity under three different ionic strengths. It was found that the pH change of Cr(VI) has little effect on adsorption capacity when the pH value is in the range of 4  8. Besides, A linear coefficient is 4.965E  3 (Eq. (20)) which indicates this is uncorrelated between the adsorption capacity and pH (4  8).

Fig. 6. (a) CV of BC and BC/Cu-N (Electrolyte solution is 0.001 mol/L FeCl3 and 0.01 mol/L Na2SO4 solution; Scanning rate is 0.05 V/s); (b) CV of BC/Cu-N under different pH (The concentration of Cr(VI) is 8 mg/L; Electrolyte solution is 0.01 mol/L Na2SO4 solution; Scanning rate is 0.05 V/s); (c) CV of BC/Cu-N under different concentrations of Cr(VI) (The electrolyte solution is 0.01 mol/L Na2SO4 solution; Scanning rate is 0.05 V/s).

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Table 4 ANOVA table for response surface quadratic model.

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Source

Sum of squares

df

Mean square

F-value

Prob. > F

Model A B C D AB AC AD BC BD CD A2 B2 C2 D2 Residual Lack of fit Pure error Cor total

123.53 5.917E-004 92.98 26.38 0.011 0.10 0.097 0.049 2.14 0.029 8.789E -005 0.23 1.530E-003 1.31 4.264E-003 0.99 0.73 0.25 124.52

14 1 1 1 1 1 1 1 1 1 1 1 1 1 1 15 10 5 29

8.82 5.917E-004 92.98 26.38 0.011 0.10 0.097 0.049 2.14 0.029 8.789E -005 0.23 1.530E-003 1.31 4.264E-003 0.066 0.073 0.051

134.23 9.001E-003 1414.50 401.37 0.17 1.58 1.48 0.75 32.49 0.44 1.337E-003 3.57 0.023 19.93 0.065

<0.0001 0.9257 <0.0001 <0.0001 0.6833 0.2285 0.2432 0.4005 <0.0001 0.5179 0.9713 0.0782 0.8808 0.0005 0.8024

The similar discipline was demonstrated in Fig. 9(a) under three different ionic strengths. Adsorption capacity without Cl ions has no obvious change, when pH value is in the range of 4  7 which coincides with the conclusion from the response surface model. Fig. 9(a) shows adsorption capacity without Cl ions rises to 9.11 mg/g from 8.15 mg/g when pH increases from 2 to 4. The Cr(VI) in the solution exists in the form of oxy anions under different pH was demonstrated Fig. 9(b) (MINTEQ [42] and Excel software). It can be seen that the dominant form of Cr(VI) at low pH is HCrO4. Increasing the pH will shift the concentration of HCrO4 to other forms, such as CrO42 [43,44]. HCrO4 is the major form of Cr(VI) when the pH value is in the range of 2  5. It is found that zero potential of BC/Cu-N is about 6.20 from acid–base titration curve (Fig. 5(b)). Since pH is less than 6.20 (pHpzc), the surface of adsorbent is protonated and carries a positive electric charge. Therefore, electrostatic attraction between protonated adsorbent and HCrO4 promotes the adsorption of Cr(VI). Adsorption capacity reduces from 9.20 mg/g to 8.91 mg/g when pH increases from 7.0 to 9.0. Since pH is greater than 6.20 (pHpzc), the surface of adsorbent is deprotonated and carries a negative charge. CrO42 is the major form of Cr(VI) when pH is greater than 8.0. Adsorbent with a negative charge and CrO42 have electrostatic repulsion. So electrostatic repulsion restrains the adsorption of Cr(VI). Adsorption capacity doesn’t change significantly when pH increases from

Fig. 7. Removal efficiency of BC and BC/Cu-N.

1.44

0.3595

4.0 to 7.0. When pH is in the range of 4.0  7.0, CrO42 form gradually replaces of HCrO4 form. But the surface of the adsorbent is almost the protonation state (6.20 > pH > 4.0). Hence, electrostatic attraction between CrO42 and HCrO4 holds the adsorption of Cr(VI). When pH is in the range of 6.20  7.0, ion exchanges between adsorbent and Cr(VI) may maintain the adsorption of Cr(VI) as well. The influence of ionic strength on the adsorption process is usually used for indirectly distinguishing whether adsorption process involves internal complexing mechanism or external complexing mechanism. It involves internal complexing mechanism when the influence of ion strength on adsorption process is low, or increase of the ionic strength promotes adsorption process. Otherwise, it involves external complexing mechanism when increase of the ionic strength restrains adsorption process [45]. As shown as Fig. 9(a), adsorption capacity of Cr(VI) is altered under three kinds of ion strengths (0, 0.01 and 0.1 M), and adsorption capacity reduces with increase of ion strength. Therefore, it can be inferred that the adsorption process of BC/ Cu-N on Cr(VI) solution involves external complexing mechanism. The phenomenon can be explained. (1) Under high ionic strength, the thickness of diffused double layer on the periphery of adsorbent particles is compressed, and repulsive force between particles is reduced, increasing the aggregation likelihood of adsorbent particles. Aggregation of adsorbent particle reduces the number of effective adsorption sites of the adsorbent surface. (2) When the adsorption process involves the external complexing mechanism, ion exchange competition will happens between initiate electrolyte ions and Cr(VI), which have an inhibitory effect on the adsorption process, and the adsorption capacity decreases with the increase in the ionic strength [46].

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Effect of temperature The combined effects of temperature with metal ion concentration, adsorption dosage and pH on the adsorption capacity are shown in Fig. 8(b, d and f), respectively. As can be seen from Fig. 8(b–f), the adsorption capacity has slightly increased with increase in temperature, which indicates the adsorption is an exothermic reaction. On one hand, the rise of temperature results to weak expansion of BC/Cu-N. Adsorbent pore size and pore volume is also enlargement. On the other hand, the increase of diffusion rate of Cr(VI) in the solution makes Cr(VI) ion can pass faster through the external boundary layer and into the internal pores of BC/Cu-N [20,47,48].

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Fig. 8. Response surface plots for the combined effect of (a) metal ions concentration and pH (adsorption dose is 0.05 g and temperature is 40˚ C), (b) initial metal ions concentration and temperature (pH is 6.0 and adsorption dose is 0.05 g), (c) initial metal ions concentration and adsorbent dosage (pH is 6.0 and temperature is 40˚ C), (d) adsorbent dosage and temperature (initial metal ions concentration is 6.0 mg/L and pH is 6.0), (e) adsorbent dosage and pH (initial metal ions concentration is 6.0 mg/L and temperature is 40˚ C), (f) temperature and pH (temperature is 40˚ C and pH is 6.0) on adsorption capacity of Cr(VI).

530

Adsorption isotherms

531 532 533 534 535 536 537 538 539 540

Fitted curves of the isotherm models are shown in Fig. S2(a), Supplementary data (Langmuir), Fig. S2(b), Supplementary data (Freundlich), Fig. S2(c), Supplementary data (D-R), Fig. S2(d), Supplementary data (Sips) and Fig. S2(e), Supplementary data (Temkin), respectively. Meanwhile, corresponding isotherm model parameters are revealed in Table S1, Supplementary data. It is found that fitting results of Langmuir and Freundlich model are poor because the R2 are less than 0.90. By contrast, Sips employed to describe the heterogeneous systems fits well and all of 1/ns are less than 1.0, which manifests that distribution of

adsorption sites of BC/Cu-N is relatively homogeneous. In addition, the greater R2 value means fitting degree of Tempkin is also great. Indirect interaction between BC/Cu-N and Cr(VI) makes adsorption heat on the surface of the adsorbent reduces linearly with the increase of the surface coverage of adsorbent. Temkin model constant (bT) is 28.1595, 152.0891 and 48.1597 J/ mol at 30 8C, 40 8C and 50 8C, correspondingly. Low bT value shows that applied force between BC/Cu-N and Cr(VI) ion is weak in the process of adsorption. The E value of D-R model is 1.8866, 1.9166 and 2.0805 (<8.0 kJ/mol) at 30 8C, 40 8C and 50 8C, correspondingly, which can deduce the physical adsorption process.

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Fig. 9. (a) The influence of pH on the adsorption capacity under different ion strengths (adsorbent dosage is 0.04 g and temperature is 40˚ C; (b) Diagram for Cr(VI) chemical species in aqueous solution under different pH.

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Kinetic model

554 555

The fitting effect of experiment data with pseudo first-order kinetic model and pseudo second-order kinetic model are

illustrated in Fig. S3, Supplementary data. Model parameters and correlation coefficient are shown in Table S2, Supplementary data. The comparison is done among the experimental data in Fig. S3, Supplementary data, fitting degree of kinetic curve, and the

Fig. 10. (a) The XRD of BC/Cu-N before and after adsorbing Cr(VI); (b) Attenuated total reflection infrared spectrogram of BC before and after adsorption; (c) The SEM of BC/CuN after adsorbing Cr(VI); (d) The EDS of BC/Cu-N after adsorbing Cr(VI).

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regression coefficient (R2). It can be found that pseudo secondorder described the experiment data better for a higher R2 value. It can be concluded that electron sharing or ion exchange during the adsorption process may promotes adsorption reaction. Table S2, Supplementary data reveals the adsorption capacity of Cr(VI) rises from 4.3631 mg/g to 13.8762 mg/g when Cr(VI) concentration ups from 4 mg/L to 8 mg/L. Adsorption capacity increased with the increase of metal ions concentration. In addition, initial reaction rate h rises from 4.4653 mg/g min to 18.5177 mg/g min with the increase of initial Cr(VI) concentration. The diffusion rate of adsorbent in the solution increases with the increase of initial concentration, resulting to rise in initial reaction rate to a certain extent. Although the pseudo second-order kinetic model can analyze kinetic data of adsorption of Cr(VI) onto BC/Cu-N very well, it cannot be a good choice to reflect the adsorption mechanism of mass transfer in the adsorption process. In order to explain the mass transfer in the adsorption process, Spahn and Schlunder model and intraparticle diffusion model are adopted to analyze kinetic data. Spahn and Schlunder model as applied to adsorption of Cr(VI) on BC/Cu-N is shown in Fig. S4(a), Supplementary data. The fitting parameters are shown in Table S3, Supplementary data. Fig. S4(a), Supplementary data indicated that the adsorption process of Cr(VI) onto BC/Cu-N shows a good linear relationship in the first 60 min in three different concentrations (ln Ct and t) which can infer external diffusion is the major adsorption process in the first 60 min. Meanwhile, Table S3, Supplementary data kext increases with the increase of initial Cr(VI) concentration. It can conclude that the increase of initial Cr(VI) concentration can rise the diffusion rate of Cr(VI) from the solution to the surface of BC/Cu-N. The process of intraparticle diffusion is illustrated in Fig. S4(b), Supplementary data. Intraparticle diffusion model parameters and correlation coefficient are presented in Table S3, Supplementary data. As can be seen from Fig. S4(b), Supplementary data, intraparticle diffusion was not unique rate control of the adsorption process. Adsorption process of Cr(VI) onto BC/Cu-N divided into three stages: The first part was external diffusion represented an extremely fast uptake (the needed time in this stage too short to be shown in the figure). Cr(VI) complexed with the active sites on the adsorbent surface. Intraparticle diffusion was followed when the active adsorption sites on the adsorbent surface were saturated. Cr(VI) further combined with redundant adsorption site on the surface of BC/Cu-N and diffused into the micropore of adsorbent along with the increase of mass transfer resistance. Hence, the adsorption was slowed. It can be learned that the Cr(VI) concentration was 4 mg/L, 8 mg/L and 12 mg/L when diffusion rate was 0.0764, 0.1666 and 0.2322, respectively. The increase of Cr(VI) concentration can increase corresponding concentration gradient pressure. At that time, the diffusion rate increases. Thus, it promotes the adsorption process. Finally stage was an equilibrium phase. Adsorption process achieved dynamic balance after the external or internal adsorption sites were all saturated so the rate of adsorption fall markedly. As shown as Table S3, Supplementary data, the diffusion rate kp.2 were already low.

616

Adsorption mechanism

617 618 619 620 621 622 623

XRD diffraction pattern before and after adsorbing Cr(VI) onto BC/Cu-N are revealed in Fig. 10(a). As can be seen from the Fig. 10(a), (002) crystal plane diffraction peak still exists at 22.88 in BC/Cu-N after the adsorption of Cr(VI). The position of the diffraction peak doesn’t shift and the intensity reduces slightly. It’s easy to see that the structure of BC/Cu-N after the adsorption of Cr(VI) doesn’t destroy.

11

FTIR of BC before and after adsorption are illustrated in Fig. 10(b). The location of the functional group characteristic peak doesn’t shift and the strength of correlative characteristic peaks decreases. It suggests that the corresponding functional groups involve in the adsorption process so that the amount of functional groups reduces. Hence, light transmittance increases [49]. The SEM and EDS of BC/Cu-N adsorbed Cr(VI) are shown in Fig. 10(c and d). Contrasting Figs. 2, 3, 10(c and d), it can be seen that the surface of BC/Cu-N has some scrap material after adsorbing Cr(VI). In addition, BC/Cu-N after adsorbing Cr(VI) has Cr from EDS. Hence, it can conclude that a part of Cr(VI) was adsorbed in the surface of BC/Cu-N. The morphologic change of BC/ Cu-N may result from adsorption of Cr(VI).

624 625 626 627 628 629 630 631 632 633 634 635 636

Conclusions

637

The BC/Cu-N was proven to be an economical and potential adsorbent for the removal of Cr(VI) from aqueous solution. Parameters were optimized for the adsorption of Cr(VI) using RSM. The initial Cr(VI) concentration was influential on the adsorption capacity of Cr(VI), while the increase of temperature slightly increased the adsorption capacity of Cr(VI). Moreover, pH (4  7) exerted little impact on Cr(VI) adsorption capacity, and the adsorbent dosage was negatively related to adsorption capacity. Analysis of variance of the quadratic model for Cr(VI) was suitable to predict the adsorption of Cr(VI) with high correlation. Sips, Temkin and D-R models were better models to describe the adsorption behavior of Cr(VI) onto BC/Cu-N with a maximum adsorption capacity of 17.9383 mg/g. Furthermore, the kinetic data signified that the adsorption of Cr(VI) ions onto BC/Cu-N followed well the pseudo second-order kinetic model. The characterization indicated that Cr(VI) was successfully adsorbed on the BC/Cu-N. Therefore, the BC/Cu-N can be study as a potential adsorbent.

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Acknowledgement

655

This work was supported by A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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Appendix A. Supplementary data

659

Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jiec.2016.10.034.

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