Adsorption of dyes from aqueous solutions by microwave modified bamboo charcoal

Adsorption of dyes from aqueous solutions by microwave modified bamboo charcoal

Chemical Engineering Journal 195–196 (2012) 339–346 Contents lists available at SciVerse ScienceDirect Chemical Engineering Journal journal homepage...

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Chemical Engineering Journal 195–196 (2012) 339–346

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej

Adsorption of dyes from aqueous solutions by microwave modified bamboo charcoal Peng Liao a,b, Zainab Malik Ismael a, Wenbiao Zhang c, Songhu Yuan a,⇑, Man Tong a,b, Kun Wang a, Jianguo Bao a a b c

State Key Lab of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, PR China Environmental Research Institute, Huazhong University of Science and Technology, Wuhan, 430074, PR China Zhejiang A & F University, Lin’an 311300, 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

" A novel cost-effective biosorbent

was investigated for dyes adsorption. " Microwave modified bamboo charcoal could be used as a potential adsorbent for dye removal. " The mechanism regarding bamboo charcoal adsorption of dyes was explored.

a r t i c l e

i n f o

Article history: Received 2 March 2012 Received in revised form 25 April 2012 Accepted 25 April 2012 Available online 5 May 2012 Keywords: Bamboo charcoal Microwave Adsorption Dyes Wastewater

a b s t r a c t The performance and mechanism of bamboo charcoal (BC) and microwave modified BC (BC-MW) for the adsorption of methylene blue (MB) and acid orange 7 (AO7) from aqueous solutions were investigated. MB and AO7 adsorption was stronger on BC-MW than on BC due to the greater pore diameter, higher hydrophobicity as well as more surface charge. Kinetic results showed that the surface and intraparticle diffusion were the rate-controlling steps for the adsorption. All adsorption isotherms were highly nonlinear and fitted well by Freundlich and Dubinin–Radushkevich models. Hydrophobic, p–p and electrostatic interactions are mainly responsible for the adsorption of dyes, while the surface area and pore volume of BCs in conjunction with H-bond interaction had minute contribution. The adsorption was pH-dependent with more adsorption at pH below 5 for MB and below 3 for AO7, respectively. Higher ionic strength led to higher adsorption capacity. The thermodynamic analysis showed that the adsorption was spontaneous and endothermic. The distributions of the adsorption site energy on BCs were calculated to be heterogeneous. Furthermore, the fixed-bed columns packed with BC and BC-MW resulted in an excellent adsorption of both dyes. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Environmental contamination by dyes has gained widespread public attention as a pervasive problem [1–3]. The removal of dyes from wastewaters is an important environmental issue. There are several strategies for the treatment of dye wastewater, such as advanced oxidation processes (AOPs) [4–6], biological treatment [7],

⇑ Corresponding author. Tel./fax: +86 27 67848629. E-mail address: [email protected] (S. Yuan). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.04.092

ion exchange [8] and adsorption [9]. Adsorption has been proven to be an effective process for dye removal due to its low cost, high adsorption capacity and environmental friendliness [9,10]. Porous materials, particularly activated carbon (AC), have been commonly used as adsorbents for dye removal from wastewaters [9]. Unfortunately, AC has some shortcomings such as slow desorption kinetics and difficulties in regeneration, arising primarily from the closed, irregular-shaped, and highly porous micropores with wide pore size distributions [11]. Additionally, AC is mainly made from the non-renewable source of coal, which implies decreasing application in the future [12,13]. Therefore, it is of great importance to

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develop cost-effective and environmentally friendly adsorbents for dye removal. Numerous novel adsorbents, such as natural minerals (e.g., clays, zeolites and other siliceous materials, etc.), waste materials (e.g., by-product from agriculture and industry, etc.) and bioadsorbents [14], have been developed for the treatment of dye effluents. Among these materials, bioadsorbents attract much interest because they can be produced from a variety of natural materials (i.e. chitosan, peat, biomass, etc.). In recent years, a new-style bioresource, bamboo charcoal (BC), has been recognized as a promising adsorbent because it is cost-effective, renewable and environmentally friendly [13,15]. BC can be produced from the widespread fast-growing speed and short growth period moso bamboo plants in China. The bamboo and bamboo residues can be transformed to BC at a high temperature under nitrogen atmosphere, which is a mature technology used in China. Currently, the area of bamboo forest is 4.84 million hm2 with an average increase of 126,000 hm2 per year in China, according to the statistics of the Sixth National Forest Resources Survey. Additionally, the cost of BC is much less compared with AC in China [16]. BC, as a newly emerged bioresource with porous structure and large surface area, has attracted extensive attention for their potential use in wastewater treatment. Several researchers revealed that BC could effectively adsorb many contaminants, such as antibiotics, dibenzothiophene, nitrate-nitrogen, N-vinylpyrrolidone and so on, in wastewaters [13,17–20]. Nonetheless, to our knowledge, the adsorption of dyes by BC has been seldom investigated, and the mechanism for the adsorption has not been adequately interpreted. In order to increase the adsorption capacity of carbon-based adsorbents for dyes, attention has been paid on the structure and surface modification. Because most dyes have large-size and charges [21], a large pore size, high surface charge as well as hydrophobic characteristic are theoretically required for the adsorbents. Microwave (MW) is a part of electromagnetic spectrum occurring in the frequency range of 300 MHz to 300 GHz [5], with the properties of molecular-level heating, which leads to homogeneous and quick thermal reactions [22,23]. The advantages of MW radiation over other types of radiation, such as plasma irradiation, for the modification or regeneration of adsorbent are in terms of energy savings and better performance with ulterior adsorption capacity as well as the rate of adsorption [24,25]. Modification of adsorbents by MW radiation results in an increase in the average pore size and hydrophobicity [26]. Moreover, BC is a good MW absorber receiving MW energy directly through dipole rotation and ionic conduction [26]. Thus, it is possible to increase the adsorption of dyes on BC modified by MW radiation (BC-MW). In the present study, the adsorption of dyes onto BC and BC-MW were investigated. Two commonly used dyes with opposite charge, methylene blue (MB) and acid orange 7 (AO7), were examined. The main objectives are (1) to identify the key process controlling the rate of dye adsorption by BCs, (2) to provide direct evidence regarding the relative importance of multiple mechanisms for the adsorption of dyes, and (3) to evaluate the performance for dye removal from wastewaters.

2.2. BC production and modification The BC was prepared according to the following procedure. Moso bamboo was firstly heated to evaporate water at 100– 150 °C for 2–3 days, followed by pre-carbonization at 150–270 °C for 2–3 days. Then, it was carbonized at 270–450 °C for 1–2 days, calcined between 450 and 800 °C for 0.5–1 day, and finally kept at 800 °C for a short time. All the processes were carried out under nitrogen atmosphere. The obtained massive BC was ground and sieved through a 100mesh screen. After washed with DI water, the BC particles were dried at 105 °C. One portion of BC particles were modified by a household 2450MHz MW oven (MM72lAAU, China). The schematic diagram of the experimental apparatus is shown in Fig. S1 in the Supplementary materials (SM). A quartz crucible was installed on the bottom of MW oven, where the sample was exposed to MW irradiation. BC-MW was obtained by the radiation of BC at 550 W for 5 min, which was proved as an effective means of modification. 2.3. Batch experiments The adsorption was carried out in a 30-mL serum bottle. For the adsorption kinetics, 20 mL of aqueous solution containing 50 mg/L MB and AO7, respectively, were mixed with 0.05 g of adsorbents. Preliminary experiments showed that adsorption on this dosage gave a straightforward comparison of adsorption data. The solution contained 5 mM CaCl2 as background electrolyte and 200 mg/L of NaN3 as biocides. The bottles were sealed and placed in a shaking table at 150 rpm in dark (293 ± 1 K). At regular time intervals, the bottles were taken out for dye analysis. With respect to adsorption isotherms, the equilibrium time was set at 48 h with different initial MB and AO7 concentrations at 5–100 mg/L and different temperatures at 293, 303, and 313 K. The initial pH was set at 7.0 and the variation was less than 0.2 during the whole process. Control experiments without BCs resulted in less than 5% loss of dyes. To investigate the effect of pH and ionic strength, the adsorption at fixed dye concentration of 50 mg/L were conducted at different pHs (1–12) and ionic strengths (0–1 M) at 293 K. The pH was adjusted by 1 M HNO3 solution and 1 M NaOH, and the ionic strength was provided by the addition of NaNO3. All the experiments were conducted in duplicate. 2.4. Column experiments Dynamic adsorption experiments were carried out in a laboratory-scale column (Fig. S2). A glass column of 50 cm length and 20 mm internal diameter was filled with 12 g of BC and BC-MW, respectively. Experiments were carried out with an influent solution of 200 mg/L MB and 100 mg/L AO7, respetively, at 293 K and pH of 7.0. An upward flow was controlled at constant rate of 6.7 mL/min using a peristaltic pump. Samples of the effluent were collected and analyzed for dye concentration.

2.5. Analysis and characterization 2. Materials and methods 2.1. Chemicals MB (>99%) and AO7 (>99%) were purchased from Mecicine group chemical reagent Co. Ltd. and Shanghai cystal pure reagent Co. Ltd., respectively (see Table S1 in the Supplementary materials (SM) for their physicochemical properties). DI water (18.0 mX cm) obtained from a Millipore Milli-Q system was used. All the other reagents were above analytical grade.

After filtration through 0.45 lm membrane, the concentrations of MB and AO7 in the filtrate were measured at 663 and 484 nm, respectively, on a Cary 50 ultraviolet visible (UV) spectrophotometer (Varian, USA). For the characterization of BC and BC-MW, elemental analysis was performed using a Vario Micro cube elementar analyzer (Elementar Company, Germany). The Brunauer– Emmett–Teller (BET) surface area, pore volume and pore size distribution were determined using a Micromeritics ASAP 2020 nitrogen adsorption apparatus. The surface binding and elemental

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aromatic structure. BC-MW has more surface negative charge compared with BC in the pH range of 4–9 (Fig. S5). The DRIFT spectra (Fig. S6) show that BC has absorption bands at 3000–3400, 1710, 1580, 1441 and 1258 cm 1. Strong peak at 1580 cm 1 is attributed to CAC stretching vibration of polyaromatic [email protected], which proves the presence of polyaromatic structure in BC. The strong peak at 1710 cm 1 and broad peak at 1258 cm 1 are due to [email protected] stretching in the carboxylic acid group and CAO stretching vibration in phenol, respectively [13]. The CAH stretching mode (1441 cm 1) is not significant. The broad peaks at 3000–3400 cm 1 for AOH may be resulted from the water adsorbed on BC or from the carboxylic acid group [28]. MW radiation remarkably reduces the surface functional groups because of the destruction of oxygen-containing functional groups at high temperatures [29]. The XPS results (Fig. S7 and Table S2) demonstrate that BC primarily consists of graphite sheets and functional groups, i.e., CAC, CAO, [email protected], COOA, p–p and [email protected] A substantial increase in graphite and p–p groups after MW regeneration suggests the higher hydrophobicity for BC-MW relative to BC.

speciation was analyzed by X-ray photoelectron spectroscopy (XPS) using VG Multilab 2000 Electron Energy Spectrometer (Thermo Electron Corporation), with the surface excitation at 1486.6 eV by an Al Ka X-ray source. The survey and high-resolution spectra of C1s was collected, calibrated with the binding energy of C1s at 284.6 eV, which was deconvolved by an XPS peak 4.1 software. The surface functional groups were qualitatively measured with diffuse reflectance infrared Fourier transform spectroscopy (DRIFT, VERTEX 70, Bruker Corporation). Sample discs were prepared by mixing 5 mg of the samples with 95 mg of KBr in an agate mortar and scanned in a range from 450 to 4000 cm 1. The surface morphology was scanned by environmental scanning electron microscopy (ESEM, FEI Quanta 200). The zeta potentials (f) versus pH were measured using a Micro electrophoresis apparatus (JS94H, Shanghai Morning in Digital Technology Equipment Co., Ltd.). 2.6. Data analysis Regarding the adsorption kinetics, pseudo first- and second order equations as well as the diffusion-based Weber–Morris model were used to identify the key process controlling the adsorption rate. For adsorption isotherms, Langmuir model (LM), Freundlich model (FM), modified Freundlich model (M-FM) and Dubinin–Radushkevich model (D–R) were employed. Van’t Hoff equations were used to calculate the thermodynamic parameters, and a methodology developed by Carter et al. [27] was used for site adsorption energy distribution analysis. With respect to the fixed-bed column adsorption, Thomas model was used for data fitting. Detailed information regarding these models and equations are presented in SM.

3.2. Adsorption kinetics The adsorption kinetics of dyes by BC and BC-MW are presented in Fig. 1a. The kinetic data were better fitted by the pseudo second order model than by the pseudo first order model (Table 2). The pseudo second order model indicates that chemisorption dominated in the adsorption process [30]. The difference in the adsorbed concentration of adsorbate at equilibrium (qe) and at time t (qt) is the key driving force for the adsorption, and the adsorption capacity is proportional to the number of active adsorption sites occupied on the adsorbent [30–32]. There are three steps involved in pseudo second order kinetic model: (i) the dye molecules diffuse from liquid phase to liquid–solid interface; (ii) the dye molecules move from liquid–solid interface to solid surfaces; and (iii) the dye molecules diffuse into the particle pores [30,33]. Herein, the diffusion of dye molecules from aqueous phase to the liquid–BCs interface was much faster than the surface and intraparticle diffusion processes because the adsorption was performed under rigorous shaking conditions. To reveal the relative contribution of surface and intraparticle diffusion to the kinetic process, the kinetic adsorption data were further fitted with the Weber–Morris model (Fig. 1b and Table 2). It is concluded that the intraparticle diffusion is the sole ratecontrolling step if the regression of qe versus t1/2 is linear and the plot passes through the origin [1]. Our fitting results shows that the regression was linearly, but the plot did not pass through the origin (C – 0). Therefore, the adsorption kinetics of dyes on BCs was regulated by both surface and intraparticle diffusion processes. The surface-diffusion mechanism is likely attributed to the electrostatic attraction. MB and AO7 have positive and negative charge, respectively. The surface of BCs was negatively charged at pH 7

3. Results and discussion 3.1. Characterization of BC and BC-MW The N2 adsorption/desorption isotherms of BC and BC-MW (Fig. S3a) show that both adsorption curves are of type IV according to the IUPAC classification, which characterizes the mesoporous structure. The pore size distribution (Fig. S3b) reveals the predominant existence of mesopores (2–50 nm). This pore structure is consistent with those reported [12,15]. The specific surface area, pore volume and surface elemental composition of the BCs are summarized in Table 1. It is apparent that BC-MW has greater external surface area and average pore diameter and, lower micropore area than virgin BC, and the BET surface areas and pore volumes are approximate. The ESEM images indicate that BC-MW has a larger pore diameter than BC (Fig. S4). Results of elemental analysis suggest that the main elements of BC are carbon, oxygen, hydrogen and nitrogen. It is noteworthy that MW treatment dramatically reduced O/C ratio, polarity index (O + N)/C, and H/C ratio of BC, indicating a high hydrophobicity and the presence of

Table 1 Main characteristics of BC and BC-MW. Sorbents

BC BC-MW a b c d e f g h

Elemental compositiona (%)

Sexterc

Smicrod

Davere

Pore volume (cm3/g)

C

H

O

N

(O+N)/C

O/C

H/C

(m2/g)

(m2/g)

(m2/g)

(nm)

Vmicrof

Vmeg

Vth

81.38 85.97

2.45 0.92

9.46 6.31

0.45 0.54

0.12 0.08

0.12 0.07

0.03 0.01

255.76 255.07

24.2 58.2

231.5 196.9

2.3 3.22

0.107 0.091

0.113 0.129

0.22 0.22

SBET

Determined by Vario Micro cube elementar analyzer. SBET : BET surface area calculated in the relative pressure region P/P0 = 0.05–0.35. Sexter: External surface area determined by t-plot methods. Smicro: Micropore area determined by t-plot methods. Daver: Average pore diameter obtained from BJH equation using N2 isotherms. Vmicro: Micropore volume determined by t-plot methods. Vmes: Mesopore volume, calculated by Vt–Vmicro. Vt: Total pore volume determined at P/P0 = 0.97.

b

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Fig. 1. (a) Adsorption kinetics of MB and AO7 by BC and BC-MW. Symbols stand for the experimental data and lines stand for the pseudo-second-order fitting, (b) Weber–Morris model plots of MB and AO7 adsorption on BC and BC-MW. Error bars represent standard deviations. The reaction conditions were at 50 mg/L initial dyes concentration, 0.05 g BCs, 293 K and pH 7.0.

(Fig. S5). As a consequence, the adsorption rate constants (k2) of MB were higher than those of AO7. Moreover, the thickness of the boundary layer (C) of MB were remarkably larger than those of AO7 by BCs (Table 2), implying that surface-diffusion played a more important role for MB relative to AO7. The intraparticle diffusion mechanism is pore-size dependent [34]. The k2 values for BCMW were higher than BC because BC-MW had larger pore diameter than BC. The dye molecules may fit into the interstitial area when the pore diameter is large, and may further increase the effective adsorption sites when the pore diameter is sufficiently large [34]. In addition to the electrostatic attraction and pore-size dependent mechanisms, there are other mechanisms contributing to the surface-diffusion and intraparticle diffusion, which will be discussed later. 3.3. Adsorption isotherms and mechanisms Fig. 2 demonstrates the adsorption isotherms of the two dyes on BC and BC-MW, and Table 3 lists the corresponding modeling

results. Both FM and D–R models fitted the adsorption isotherms data better than LM (Table 3 and Table S3), implying the heterogeneous adsorption sites on both adsorbents. MB exhibited much higher adsorption nonlinearity, as indicated by the larger n values, than AO7 implying a more heterogeneous distribution of the adsorption sites. Yang et al. [35,36] addressed that the adsorption of many organic chemicals on carbon nanutubes (CNTs) can be described by Polanyi–Manes model (PMM), and PMM was applicable for both pore filling and surface adsorption. D–R isotherm is based on the Polanyi–Manes adsorption theory [37]. In this study, both surface adsorption and micropore filling were responsible for the adsorption of dyes on BC and BC-MW as both FM and D–R model fitted the experimental data well. This is consistent with the kinetic adsorption results. The mean sorption energy (E) derived from the D–R isotherm model can be used to distinguish chemical and physical adsorption. Table 3 shows that the E values were 13.61–15.43 kJ/mol for MB and 12.7–15.43 kJ/mol for AO7, which were in the range of 8–16 kJ/mol for chemisorption [33]. This also agrees with the kinetic results. FM-based thermodynamics and energy distribution analysis are available in literature and thus the comparison with the literature data could be readily made [27]. Therefore, the following discussion will be mainly focused on the results obtained from FM fitting. Single point adsorption coefficients (log K) were calculated at Ce = 10 mg/L according to the fitted FM. For both MB and AO7, the log K for BC-MW was approximately 1–1.2 times larger than for BC (Table 3). Because BC and BC-MW had similar BET surface areas and pore volumes, the difference in log K was probably due to the difference in surface functionality [38]. On the other hand, the log K values for MB were about 1.1–1.4 orders of magnitude higher than those for AO7 on BC and BC-MW. This may be resulted from their different physical-chemical properties. Concerning the adsorption of dyes on BCs, several possible mechanisms should be considered: (i) hydrophobic interactions between dyes and BCs; (ii) p–p electron-donor–acceptor (EDA) interactions; (iii) H-bond between the dye groups and the O-containing functional groups of BCs; and (iv) electrostatic attraction between dyes and BCs. Hydrophobic interaction can be evaluated by the hydrophobic parameter of organic chemicals, log KOW (octanol-water distribution coefficient) [39]. The log K of MB and AO7 for BCs were consistent with their relative log KOW (5.85 for MB and 1.25 for AO7). This suggests that hydrophobic interactions somewhat regulated the adsorption of MB and AO7 by BCs. The hydrophobicity of BC-MW was higher than BC as indicated by the less surface functional groups and the lower oxygen as well as higher graphite contents. MB had more alkyl and aromatic moieties than AO7, so MB can interact with the graphite of BC-MW via hydrophobic interactions resulting in more MB adsorption than AO7. A recent study also suggested the adsorption of chloramphenicol by NaOH modified BC via hydrophobic interaction [13]. In order to single out other mechanisms for the adsorption besides hydrophobic interactions, solubility-normalized coefficients (KF-Cs) were obtained (Table 3). The KF-Cs values of MB were still much higher than those of AO7. This

Table 2 Coefficients for the pseudo-first-order and pseudo-second-order adsorption kinetic models as well as Weber–Morris model.a Adsorbate

a

Adsorbent

qe,exp (mg/g)

Pseudo-first-order

Pseudo-second-order

Weber–Morris model

qe,cal (mg/g)

k1 (1/h)

R2

qe,cal (mg/g)

k2 (g/mg/h)

R2

C

kip (mg/g/h1/2)

R2

MB

BC BC-MW

14.99 17.32

2.61 4.28

0.078 0.086

0.875 0.866

14.97 17.51

0.112 0.120

0.998 0.998

12.60 13.00

0.355 0.69

0.852 0.951

AO7

BC BC-MW

4.91 9.28

3.97 3.02

0.072 0.058

0.971 0.926

5.64 9.43

0.026 0.074

0.967 0.998

0.49 5.67

0.738 0.599

0.942 0.949

The reaction conditions were at 50 mg/L initial dyes concentration, 0.05 g BCs, 293 K and pH 7.0.

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Fig. 2. Adsorption isotherms of MB and AO7 on BC and BC-MW at different temperatures. Symbols stand for experimental data and lines stand for the FM fitting. Error bars represent standard deviations.

Table 3 Fitting results of FM and D–R model to the adsorption isotherm data. Adsorbate

MB

Adsorbent

BC

BC-MW

AO7

BC

BC-MW

Temperature (K)

Freundlich model (FM)

Dubinin–Radushkevich model (D–R)

KF

n

log K

R2

KF-Cs

qm (mol/g)

b (mol2/kJ2)

E (kJ/mol)

R2

293 303 313 293 303 313

4.48 5.70 6.39 10.55 15.34 19.33

0.47 0.40 0.38 0.28 0.30 0.22

3.12 3.16 3.19 3.30 3.49 3.51

0.978 0.978 0.955 0.947 0.907 0.860

698.15 701.78 388.71 217.79 399.14 202.31

1.3 1.5 1.7 2.0 2.6 3.4

exp( exp( exp( exp( exp( exp(

4) 4) 4) 4) 4) 4)

2.6 2.7 2.6 2.1 2.2 2.1

exp( exp( exp( exp( exp( exp(

3) 3) 3) 3) 3) 3)

13.87 13.61 13.87 15.43 15.07 15.43

0.935 0.962 0.932 0.897 0.982 0.856

293 303 313 293 303 313

1.55 2.09 2.15 3.66 3.86 4.22

0.33 0.26 0.27 0.25 0.24 0.22

2.52 2.58 2.60 2.81 2.83 2.85

0.928 0.978 0.963 0.890 0.898 0.920

74.36 50.11 46.58 65.08 62.29 56.47

3.9 4.2 4.6 7.2 8.6 8.8

exp( exp( exp( exp( exp( exp(

4) 4) 4) 4) 4) 5)

2.6 2.2 2.5 2.1 2.7 3.1

exp( exp( exp( exp( exp( exp(

3) 3) 3) 3) 3) 3)

13.86 15.07 14.14 15.43 13.61 12.70

0.943 0.984 0.983 0.921 0.944 0.942

clearly suggests the contribution of other mechanisms to the adsorption of dyes by BC. Similar results were also obtained by Pan and Wang et al. [38,39] for the adsorption of organic chemicals on CNTs. p–p interaction is another important mechanism governing the adsorption of both p-acceptors and p-donors to graphene surfaces of carbon-based adsorbents [36,38–42]. BCs contained abundant graphene structure. Both dyes were rich in aromatic moieties. Thus, MB and AO7 can both interact with the electron-rich sites at the graphite surfaces of BCs via p–p interactions. As BC-MW have higher fraction of graphite structure than BC, larger adsorption of dyes on BC-MW than BC was reached. Furthermore, MB has more functional groups than AO7, so it can interact with BC more strongly than AO7. Zhu et al. [43] argued that p–p EDA interaction resulted in the stronger adsorption of 2-methylphenol than nonpolar aromatics on wood charcoal. Chen et al. [40,41] also proposed that the strong adsorptive interaction between CNTs and nitroaromatics was due to p–p EDA interaction.

Besides hydrophobic and p–p interaction, H-bond also influences chemical adsorption [32,36,38–43]. Since the oxygen content of BC-MW was much lower compared with BC, BC-MW should have adsorbed lesser dyes than BC if H-bond interaction played a significant role. The much higher adsorption of dyes on BC-MW than on BC ruled out the contribution of H-bond. This result is different from those reported [2,13], where H-bond contributed to the adsorption to some extend. 3.4. Effect of pH and ionic strength on the adsorption The pH of the aqueous solution is an important factor controlling dye adsorption [2,3,44–46]. As showed in Fig. 3a, pH played an important role for both MB and AO7 adsorption. Adsorption coefficients of MB rapidly increased from pH 1 to 5, decreased from pH 5 to 7, and became approximately constant from pH 7 to 12. The adsorption of AO7 has the similar trend with MB except the maximum adsorption at pH 3. When the adsorption occurs at pH

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negatively charged. At pH 5, MB and BC surface are oppositely charged, thus the adsorption is enhanced. Moreover, stronger pHdependent adsorption is observed on BC-MW than on BC at pH 5 because the surface of BC-MW is more negatively charged compared with BC. As a result, electrostatic interaction also contributes to the adsorption of dyes on BCs. Fig. 3b shows the effect of ionic strength on the adsorption of dyes on BCs. The log K values for both MB and AO7 were increased by less than 0.25 unit with the increase in NaNO3 concentration from 0 to 1 M at pH 7. This implies that ionic strength had minute influence on the adsorption of dyes on BCs. At pH 7, the anionic form of AO7 is dominant in solution and the net surface charge on the BCs is negative. The increase in ionic strength can enhance the uptake of ionic compounds such as AO7 because of the screening effect of the surface charge and the depressed electrostatic repulsion by the added salt [47,48]. In contrast, the screening effect was less important for MB adsorption because log K values were increased with minimal electrostatic attractive interaction. In the presence of high ionic concentration, the thickness of the diffuse double layer was compressed [46,49], which facilitated the approach of MB molecule to BCs. Compared with BCs, an increasing ionic strength greatly depressed the adsorption of ionic compounds on CNTs because the compounds were ‘‘pushed out’’ at high ionic concentration [50].

3.5. Adsorption thermodynamics and site energy distribution

Fig. 3. Effect of (a) pH and (b) ionic strength on log K for single-point adsorption of dyes on BC and BC-MW. Error bars represent standard deviations.

less than 3, the adsorption is increased due to the electrostatic attraction between the negatively charged deprotonated AO7 and positively charged BC surface. At pH between 3 and 9, the adsorption of AO7 on BCs is depressed because the BC surface is

The adsorption thermodynamics is critical for the interpretation of adsorption mechanisms [51]. Thus, the thermodynamic parameters, such as Gibbs energy (DG0), enthalpy (DH0), and entropy (DS0), for the adsorption of dyes on BCs were calculated. The negative values of DG0 in the temperature range of 293–313 K (Fig. 4a and b) suggest the spontaneous nature for the adsorption of MB and AO7 on BCs. Moreover, Fig. 4a and b show that DG0 became more negative as temperature increased at the same adsorbate loading, indicating that the driving force for the adsorption of both

Fig. 4. Standard Gibbs free energy change (DG0) for the adsorption of (a) MB and (b) AO7 at different adsorbate loadings and temperatures, (c) adsorption site energy of dyes on different adsorbate loadings at 313 K and (d) adsorption site energy distribution curves of dyes on BC and BC-MW at 313 K.

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dyes increased with increasing temperature. For a given adsorbents at a certain temperature, DG0 became more negative when the adsorbed MB and AO7 increased, which is in consistent with a stronger driving force. The affinity of the solute to the adsorbent surface was related to the position of the energy distribution mean on the energy axis [27]. The higher value of mean energy is in accordance with the higher adsorption affinity. As shown in Fig. S8, both the average adsorption energy (E) and site energy distribution F(E) for BCs follow the order of 313 K > 303 K > 293 K. This further proved that the adsorption of MB and AO7 were elevated at higher temperatures. As illustrated in Fig. 4c, BC-MW exhibited higher adsorption site energies than BC for both MB and AO7. For a given adsorbent at the same adsorbate loading above 5 mg/g, higher values of E were obtained for MB than for AO7, implying that BCs have more adsorption sites for MB than for AO7. This also agreed with the order of site energy distribution curves (Fig. 4d). Fig. 4c also presents that E decreased dramatically for both adsorbents with increasing MB and AO7 loadings. MB and AO7 molecules occupied the high energy adsorption sites first at low concentrations and then spread to the low energy adsorption sites at high concentrations. The similar result was also reported by Wang et al. [51]. The DH0 values for the adsorption were positive at the three temperatures (Fig. S9a and b) indicating a endothermic adsorption of MB and AO7 on BCs. The decrease of DH0 with increasing MB and AO7 loadings suggests that the adsorption process became less endothermic. In addition, the positive values of DS0 at three temperatures reflect the high affinity of MB and AO7 to BCs and the increase in randomness at the solid-solution interface (Fig. S9c and d). 3.6. Fixed bed column adsorption To further evaluate performance of the adsorption, the fixedbed column adsorption of MB and AO7 by BC and BC-MW were conducted. The breakthrough curves are illustrated in Fig. 5 and

the results are tabulated in Table 4. It can be seen that Thomas model fitted all the experimental data well. This suggests that the rate driving force obeys second order reversible reaction kinetics, which was in agreement with the aforementioned batch kinetic results. The maximum adsorption capacities of MB on BC and BCMW attained 26.46 and 35.34 mg/g, respectively, and for AO7 reached 7.41 and 10.45 mg/g, respectively (Table S3). Although the retention time for the adsorption in column is less than in batch mode, approximate adsorption capacity was reached, which is different from the reported lower capacity in column than in batch by other adsorbents [52]. This suggests that BCs can be regarded as a potential adsorbent for the adsorption of dyes in wastewater. 3.7. Compared with other carbon-based adsorbents The adsorption capacities for the removal of MB on conventional carbon-based adsorbents, including AC, CNTs, charcoal and coal, are summarized in Table S4. It can be seen that BC has lower adsorption affinity to MB compared with other carbonaceous materials. However, the practical application of these materials on a large scale is limited by source materials, i.e., non-renewable source of coal, and cost of regeneration and potential toxicity (ie., CNTs) [12,13,53]. Besides, traditional carbon-based adsorbents predominantly consist of micropores and wide pore size distributions. Adsorption of bulky organic chemicals may be impeded by the size-exclusion effect [11]. BC has relative less graphite structure but more O-containing groups and porous structure than the carbon-based adsorbents reported [13,15,42,51]. These features make BC a promising adsorbent for removing bulky organics such as dyes. Another notable advantage of BC is its lower cost ($421– 571 per ton [15]) in comparison with commercial carbon-based materials in China ($1200–2000 per ton). Additionally, our undergoing work shows BC can be effectively regenerated by MW radiation without loss of adsorption capacity (data no shown). Therefore, this new biosorbent is expected to have applicability in the removal of dyes and even other organic pollutants from wastewater. 4. Conclusions

Fig. 5. Breakthrough curves for adsorption of MB and AO7 by BC and BC-MW. Symbols stand for experimental data and line stand for Thomas model fitting. The reaction conditions were at 200 mg/L MB and 100 mg/L AO7 initial concentration, 12 g BCs, constant rate 6.7 mL/min, 293 K and pH 7.0.

The adsorptive removals of dyes on the renewable bioresource of BC and MW modified BC were investigated. The experimental results confirmed that MW modification can increase the adsorption of MB and AO7 on BC. Greater pore diameter and higher hydrophobicity as well as more surface charges were reached by the modification. The adsorption of MB and AO7 on BCs were mainly regulated by surface and intraparticle diffusion. Hydrophobic, p–p and electrostatic interactions were mainly responsible for the adsorption of dyes, while the surface area and pore volume of BCs and H-bond interaction had minute contribution. The thermodynamic and site energy analysis indicated that adsorption of dyes on BCs was spontaneous, and BCs had heterogeneous adsorption sites for dye adsorption. The fixed-bed column experiments

Table 4 Comparison of adsorption capacities obtained in batch and column experiments. Adsorbate

Adsorbents

Inlet concentration (mg/L)

Batch capacity (mg/g)

Column capacity Qe-exp (mg/g)

Qe-cal (mg/g)

kTh (mL/min/mg)

Total removal (%)

R2

MB

BC BC-MW

200 200

26.46 35.34

29.26 31.41

32.94 34.22

0.116 0.127

68.20 73.22

0.995 0.998

AO7

BC BC-MW

100 100

7.41 10.45

5.45 7.76

6.61 10.35

0.174 0.270

33.03 47.01

0.997 0.998

346

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