Fabrication of granular activated carbons derived from spent coffee grounds by entrapment in calcium alginate beads for adsorption of acid orange 7 and methylene blue

Accepted Manuscript Fabrication of granular activated carbons derived from spent coffee grounds by entrapment in calcium alginate beads for adsorption of acid orange 7 and methylene blue Kyung-Won Jung, Brian Hyun Choi, Min-Jin Hwang, Tae-Un Jeong, Kyu-Hong Ahn PII: DOI: Reference:

S0960-8524(16)31077-X http://dx.doi.org/10.1016/j.biortech.2016.07.098 BITE 16860

To appear in:

Bioresource Technology

Received Date: Revised Date: Accepted Date:

23 June 2016 21 July 2016 22 July 2016

Please cite this article as: Jung, K-W., Choi, B.H., Hwang, M-J., Jeong, T-U., Ahn, K-H., Fabrication of granular activated carbons derived from spent coffee grounds by entrapment in calcium alginate beads for adsorption of acid orange 7 and methylene blue, Bioresource Technology (2016), doi: http://dx.doi.org/10.1016/j.biortech.2016.07.098

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1

Fabrication of granular activated carbons derived from spent coffee

2

grounds by entrapment in calcium alginate beads for adsorption of

3

acid orange 7 and methylene blue

4

Kyung-Won Junga, Brian Hyun Choia, Min-Jin Hwangb, Tae-Un Jeonga, Kyu-Hong

5

Ahna,*

6

a

7

Technology, 5 Hwarang-ro 14-gil, Seongbuk-gu, Seoul 02792, South Korea

8

b

9

Munsan, Jinju, Gyeongsangnam-do 52834, South Korea

Center for Water Resources Cycle Research, Korea Institute of Science and

Future Environmental Research Center, Korea Institute of Toxicology, 17 Jeigok-gil,

10

*

11

958-5832; fax: +82-2-958-6854)

12

Abstract

Corresponding author: Kyu-Hong Ahn (E-mail address: [email protected], Tel: +82-2-

13

Biomass-based granular activated carbon was successfully prepared by entrapping

14

activated carbon powder derived from spent coffee grounds into calcium-alginate beads

15

(SCG-GAC) for the removal of acid orange 7 (AO7) and methylene blue (MB) from

16

aqueous media. The dye adsorption process is highly pH-dependent and essentially

17

independent of ionic effects. The adsorption kinetics was satisfactorily described by the

18

pore diffusion model, which revealed that pore diffusion was the rate-limiting step

19

during the adsorption process. The equilibrium isotherm and isosteric heat of adsorption

20

indicate that SCG-GAC possesses an energetically heterogeneous surface and operates

21

via endothermic process in nature. The maximum adsorption capacities of SCG-GAC

22

for AO7 (pH 3.0) and MB (pH 11.0) adsorption were found to be 665.9 and 986.8 mg/g

23

at 30 oC, respectively. Lastly, regeneration tests further confirmed that SCG-GAC has

1

promising potential in its reusability, showing removal efficiency of more than 80%

2

even after seven consecutive cycles.

3 4

Keywords

5

Activated carbon; spent coffee grounds; calcium-alginate beads; adsorption; acid orange

6

7; methylene blue

7 8 9

1. Introduction The presence of dyestuff in effluents from multiple industries such as food, paper,

10

carpets, rubbers, cosmetics, and textiles is a major concern because it can directly

11

damage the entire ecosystems due to its resistance to biodegradation and high stability

12

towards light and/or oxidizing agents (Doğan et al., 2009). It is estimated that roughly

13

7×10 5 tons of dyestuff are produced annually and approximately 15% of the produced

14

dyes is released into the environment without proper treatment (Rafatullah et al., 2010).

15

The presence of minuscule concentrations of dyestuff in wastewater (~1 mg/L) is easily

16

visible and undesirable, as the color of wastewater is recognized as the primary factor

17

when determining the degree of contaminant (Lee et al., 2006). Moreover, the major

18

concern of widespread dyes in water systems is that they are toxic to aquatic organisms,

19

as well as carcinogenic and mutagenic to humans (Ma et al., 2015). Hence, the

20

development of industrially applicable and efficient treatments for dye containing

21

wastewater is imperative before disposal to aquatic ecosystems.

22

Among numerous approaches towards dye removal from wastewater (i.e., physical,

23

chemical, biological, advanced oxidation processes, electrochemical), activated carbon-

24

based adsorption processes has been found to be superior than other methods because of

25

its attractive features such as high specific surface area, surface reactivity, efficiency,

1

and convenience in removing targets from aqueous solution (Li et al., 2016; Sun et al.,

2

2015). The major drawback of the conventional activated carbon-based adsorption

3

process is the use of non-renewable and relatively expensive raw materials (i.e., wood

4

and coal), and therefore, it is still considered to be a costly process with restricted

5

applicability. To overcome such drawbacks and to attain better economic viability,

6

adsorption processes using non-conventional low-cost activated carbon derived from

7

renewable and inexpensive materials such as naturally abundant biomass, agricultural,

8

and industrial wastes have been examined (Li et al., 2016; Rafatullah et al., 2010; Sun et

9

al., 2015).

10

Coffee is one of the most abundant agricultural products, as well as one of the most

11

widely consumed beverages in the world. Approximately six million tons of solid

12

residue known as spent coffee grounds (SCG)/exhausted coffee residue is produced

13

during the brewing process (Franca et al., 2009). In addition to further conversion of

14

SCG into valuable products such as enzymes, organic acids, fuels, ethanol, dyes,

15

compost, and antioxidant phenolic compounds (Murthy and Naidu, 2012), preparation

16

of SCG-based activated carbon as an effective adsorbent for the removal of pollutants

17

(e.g., heavy metals, dyes, and organic contaminants) from aqueous solutions also has

18

been proposed and scrutinized (Franca et al., 2009; Ma and Ouyang, 2013; Reffas et al.,

19

2010; Safarik et al., 2012). From these studies, SCG-based activated carbon has been

20

considered as an effective and promising adsorbent for removing pollutants from

21

aqueous solution. However, in some cases, its use is still considered undesirable due to

22

its difficulty in recovering or separating the powdered SCG-based activated carbon from

23

the mixture during the post-adsorption stage, which can substantially obstruct its

24

beneficial utility on an industrial scale.

25 26

To bypass this critical disadvantage, alginate (alginic acid), a naturally occurring polysaccharide extracted from brown seaweed, has recently received considerable

1

attention as researchers have sought to immobilize powdered activated carbon by cross-

2

linking the powder to the matrix. One of the most significant properties of alginate

3

solution is the ease to form hydrogels (egg-box structure) by selective ionic interactions

4

with divalent metals such as Ca (Ai et al., 2011). Taking this factor into consideration,

5

the objective of this work is to investigate a convenient and economical method for

6

dyestuff removal from an aqueous solution by adsorption using SCG-based granular

7

activated carbon (SCG-GAC), which is prepared by entrapping powdered SCG-based

8

activated carbon in porous calcium-alginate beads. In this study, a mono-azo anionic

9

dye-acid orange 7 (AO7) and cationic dye-methylene blue (MB) were chosen as model

10

pollutants because of their versatility and wide application in multiple industries such as

11

medical processing, paper colorization, cotton dying, and so on. In addition to

12

monitoring the physicochemical properties of SCG-GAC, its adsorption properties

13

including effects of additives, kinetics and equilibrium isotherms were evaluated at

14

different temperatures.

2+

15 16

2. Materials and methods

17

2.1. Chemicals and materials

18

Raw-SCG, a by-product collected from coffees houses, was used as a precursor.

19

Raw-SCG was washed with distilled deionized water (Milli Q plus, Merck Millipore

20

Co., Germany) to remove impurities such as dust and water soluble substances from the

21

surface. After drying at 105 oC for 24 h, the fraction passing through a 425 µm sieve

22

(mesh No. 40) was collected prior to its activation. Anionic AO7 (C16H11N2NaO4S,

23

MW= 350.32 g/mol) was supplied by Tokyo Chemical Industry (Japan). Cationic MB

24

(C16H18CIN3S·3H2O, MW= 373.90 g/mol) was purchased from Samchun Pure

25

Chemical Co., Ltd. (South Korea). Each stock solution was prepared by dissolving 2.0 g

1

of AO7 and MB in 1.0 L of deionized distilled water, respectively, without further

2

purification and was diluted to appropriate concentrations. The measurement of AO7

3

and MB concentrations was performed using a UV Spectrophotometer (DS5000, HACH

4

Company, Netherlands). The maximum wavelength (λmax) for AO7 and MB was found

5

to be 485 and 663 nm, respectively. After the adsorption, the respective concentrations

6

were calculated using the calibration curves in the range of 0-20 mg/L (Abs485 at 20

7

mg/L= 1.465) for AO7 and 0-10 mg/L (Abs663 at 10 mg/L= 2.012) for MB. The removal

8

efficiencies were expressed as percent ratio of removed dye concentrations to their

9

initial concentrations.

10 11 12

2.2. Preparation of SCG-based granular activated carbon (SCG-GAC) First, in the carbonization step, the dried raw-SCG was heated in a horizontal electric

13

furnace ((TF-80-6, Woori Science Instrument Co., South Korea) with a quartz tubular

14

reactor up to 450 oC with a heating rate of 7 oC/min. After dwelling for 2 h under a

15

nitrogen atmosphere (flow rate= 25 mL/min), the product was cooled to room

16

temperature under a nitrogen atmosphere. Then, KOH was impregnated into the

17

carbonized product in 5:1 ratio w/w (g KOH/g carbonized-SCG) for 1 h, and the

18

mixture was oven-dried at 105 oC until it was completely dried. Thereafter, activation of

19

the impregnated SCG was carried out via pyrolysis. The impregnated-SCG was placed

20

in a horizontal electric furnace at 700 oC for 1 h with a ramp rate of 7 oC/min under

21

nitrogen protection (flow rate= 25 mL/min). After cooling to room temperature, the

22

samples were transferred to a bottle containing 200 mL of 0.1 M HCl and stirred for 1 h.,

23

followed by filtering and washing repeatedly with distilled deionized water until the

24

filtrate pH became neutral (pH 6.5-7.0). The samples were dried at 105 oC for 24 h, and

25

then stored in sealed bottles before their use. As shown in Fig. S1 (supplementary data),

1

the percentage removals of AO7 and MB at 200 mg/L of the initial concentration were

2

99.9 and 100%, respectively, after 24 h of contact time using powdered SCG-based

3

activated carbon, because the prepared activated carbon has more suitable textural

4

properties for AO7 and MB adsorption than those of raw-SCG and carbonized-SCG, as

5

provided in the supplementary data (Fig. S2 and Table S1). The N2

6

adsorption/desorption isotherms was determined as a typical type I isotherm according

7

to the international union of pure and applied chemistry (IUPAC) classification and a

8

remarkable uptake at low relative pressure (Fig. S3) further confirmed that the

9

micropores were predominant in the prepared activated carbon. Thus, the experimental

10

data suggest that the activated carbon was successfully prepared and could be used as a

11

highly effective adsorbent to remove AO7 and MB from aqueous solutions.

12

SCG-GAC was prepared from the powdered SCG-based activated carbon according

13

to the procedure described by the Jung research group (Jung et al., 2016). In detail,

14

2.0% (w/v) sodium alginate solution was prepared by mixing sodium alginate in 500

15

mL of distilled deionized water with stirring for 1 h at room temperature. Subsequently,

16

2.0% (w/v) of prepared powdered SCG-based activated carbon was added to the

17

alginate solution, and the mixture was then stirred for 10 h. After the mixture became

18

homogeneous, it was dropped through a burette into 2.0% (w/v) calcium chloride to

19

form granules under gentle stirring. The excess unbounded calcium chloride was

20

removed by washing several times with distilled deionized water, and the product was

21

then oven-dried at 45 oC for 24 h. From here on, the prepared SCG-based granular

22

activated carbon is denoted as SCG-GAC.

23 24

2.3. Characterization of SCG-GAC

1

The morphology and the microstructure of the prepared SCG-GAC were investigated

2

by scanning electron microscopy (SEM, S-4200, Hitachi Co., Japan). Fourier transform

3

infrared (FTIR) spectra were recorded using a FTIR spectrometer in the 4000-600 cm-1

4

range (NICOLET iS10, Thermo Scientific, USA) to identify existing functionalities

5

within the prepared material. The porosity of SCG-GAC were characterized by N2

6

vapor adsorption/desorption studies at 77K using a nano porosity system (NP-XQ,

7

Mirae Scientific Instruments, South Korea). Based on the results of N2-mediated

8

adsorption/desorption isotherms, surface area, pore diameter, and total pore volume of

9

the product were measured using the Brunauer-Emmett-Teller (BET), the Barrett-

10

Joyner-Halenda (BJH), and the Horvath-Kawazoe methods. In addition, the pore size

11

distribution was estimated by the BJH method from the acquired desorption branches.

12

To determine the point of zero charge (pHpzc), the solution pH was adjusted in a range of

13

2.0-10.0 (with intervals of “1.0”) with 0.1 M HCl and NaOH using a LabQuest2

14

portable meter (LQ2-LE, Vernier, USA); the SCG-GAC was crushed into powder and

15

added to the reaction vessels. After shaking for 1 h at 30 oC for homogeneity, zeta

16

potential was measured using a zeta potential analyzer (Zetasizer Nano ZS, Malvern

17

Instrument. Ltd., UK).

18 19 20

2.4. Batch adsorption experiments The influence of solution pH on the adsorption capacities of AO7 and MB were

21

monitored to determine optimal conditions for further investigations. The effect of

22

background salt (ionic strength) on the adsorption performance also evaluated. Based on

23

the results of preceding tests, to evaluate the adsorption properties of SCG-GAC for

24

AO7 and MB and elucidate the main adsorption mechanisms, the adsorption kinetics

25

and equilibrium isotherms were carried out at three different temperatures (10, 20, and

1

30 oC). Lastly, to assess the reusability of saturated SCG-GAC, regeneration tests were

2

carried out. The detailed experimental conditions for each test are described below. All

3

experiments were carried out in duplicate and the experimental errors were found to be

4

within ±1% of the presented average values.

5 6 7

2.4.1. Influences of solution pH and ionic strength The influence of solution pH on the adsorption capacity of the adsorbent was

8

evaluated by mixing 0.1 g of SCG-GAC with 50 ml of stock solutions containing 200

9

mg/L of AO7 or MB in the reaction vessel. The pH of the solutions were adjusted to

10

3.0-11.0 (with intervals of “2.0”) using 0.1 M HCl and NaOH; the reaction vessels were

11

then homogeneously shaken in an orbit shaking incubator at 200 rpm with controlled

12

temperature at 20 oC (WIS02011, WiseCube®, Germany). After 24 h of contact time, the

13

concentrations of the dyes within the solutions were analyzed immediately. In the same

14

manner, the effect of ionic strength on the adsorption performance was also evaluated

15

with varying salt concentration (0.5, 1.0, 2.0, and 3.0 g/L) because salts are often found

16

in dye containing wastewater, particularly NaCl as it is the most common salt existent in

17

the wastewater (Zille et al., 2003). The pH of the solutions was fixed at 3.0 and 11.0 for

18

AO7 and MB, respectively, while other experimental procedures and conditions were

19

unaltered. The removal efficiencies and the amount of AO7 and MB adsorbed onto

20

SCG-GAC were calculated using the following equations:

Removal rate (%) =

Qe =

(Ci - C f )

V ( Ci − C e ) M

Ci

× 100

(1)

(2)

1

where Ci and Cf are the AO7 and MB concentration (mg/L) of the initial and final

2

solution, respectively. Qe is the adsorbed amount of AO7 and MB per unit weight of

3

adsorbent (mg/g) at a given time, M is the mass of the adsorbent (g), V is the volume of

4

dye solutions (L), and Ci and Ce (mg/L) are the initial and equilibrium concentrations of

5

the target compounds, respectively.

6 7 8 9

2.4.2. Adsorption kinetics The adsorption kinetics were investigated to evaluate the adsorption properties of SCG-GAC by mixing 0.5 g of SCG-GAC in a reaction vessel containing 250 mL of

10

2000 mg/L AO7 or MB. Based on the preceding experiment, the solution pH was

11

adjusted to pH 3 for AO7, and pH 11 for MB using 0.1 M of HCl and NaOH. The

12

solution temperature was controlled at 10, 20, and 30 oC. Aliquots were taken

13

periodically at designated time intervals; the amounts of AO7 and MB adsorbed onto

14

SCG-GAC were then calculated as described earlier. Using the obtained experimental

15

data, the pore diffusion model was employed to fully identify the diffusion mechanisms

16

of dye molecules onto SCG-GAC. The pore diffusion model equations and numerical

17

method for solving the equations (initial and boundary conditions) are given as the

18

following, respectively (Jung et al., 2016): Pore diffusion model:

εp

∂C p

∂t

+ ρ

p

∂q = ε ∂t

p

∂C p  1 ∂  2 r D p r 2 ∂ r  ∂ r 

(3)

Initial condition:

C p (r, t = 0) = 0

(4)

Boundary conditions: At the center of the particle:

At the surface of the particle:

∂C p ∂r

(5)

=0 r=0

ε p Dp

∂C p ∂r

= k f ( C − Cs )

(6)

r =Rp

1

where εp is the porosity of the particle, ρp is the particle density of the adsorbent (kg/L),

2

rp is the radial coordinate of the particle (m), Dp is the internal pore diffusivity (m2/s),

3

Cp is the solution concentration in pores (mg/L), q is the adsorbate concentration in

4

particle phase (mg/g), t is time (s), kf is the external-film transfer coefficient (m/s), and

5

C and Cs are the concentrations in the bulk aqueous phase and the external surface

6

(mg/L), respectively. The particle porosity, density, and radius of the prepared SCG-

7

GAC were measured as 0.65, 0.281 kg/L, and 2.0316 mm, respectively.

8 9

2.4.3. Adsorption equilibrium isotherms

10

Adsorption equilibrium isotherm studies were carried out by subjecting 0.1 g of

11

SCG-GAC in 50 mL of AO7 and MB solutions with different concentrations, ranging

12

from 50 to 5000 mg/L at three different temperatures (10, 20, and 30 oC). Contact time

13

of 24 h was chosen based on the results of kinetics studies; other experimental

14

procedures were set constant as before. The experimental results were analyzed using

15

three adsorption isotherm models, Langmuir, Freundlich, and Sips models, which are

16

expressed as the following, respectively (McKay et al., 2014):

Qe =

Q m K LC e 1 + K LC e

(7)

Qe = K F C e1 / n

(8)

Q m K S C e1 / n Qe = 1 + K S C e1 / n

(9)

1

where Qe is the adsorbed amounts of AO7 and MB per unit weight of adsorbent (mg/g)

2

at an equilibrium concentration of adsorbate in bulk solution (Ce, mg/L). KL, KF, and KS

3

are the Langmuir, Freundlich, and Sips constants, respectively. Qm denotes the

4

maximum adsorption capacity and n is the heterogeneity factor.

5

Using the obtained results from equilibrium isotherms, the isosteric heat of

6

adsorption and its variation with surface loading were determined by the Clausius-

7

Clapeyron equation, expressed as follows (Jung et al., 2016):

Qst  ∂ ln Ce  = RT 2  ∂T  N

(10)

8

where Qst is the isosteric heat of adsorption (kJ/mol), R is the gas constant (8.3145 J/mol

9

K), Ce is the equilibrium concentration (mol/L), T is temperature (K), and N is the moles

10

adsorbed.

11 12

2.4.4. Evaluation of experimental data

13

In order to evaluate the applicability of equilibiurm isotherm models for fitting the

14

experimental data, the coefficient of determination (R2) and error functions including

15

the residual root mean square error (RMSE), the sum of the squares of the errors (SSE),

16

and the chi-square test (χ2) were employed, expressed as the following, respectively:

n

∑

1− R2 =

(Q e.n − Q m .n ) 2

n =1

(11)

n

∑

(Q e.n − Q m .n )

2

n =1

RMSE =

1 n (Q e . n − Q m . n ) 2 ∑ n − 1 n =1

n

SSE = ∑ ( Qm ,n − Qe ,n )

2

(12)

(13)

n =1

2

χ =

n

∑ n =1

(Q e . n − Q m . n ) 2 Q e.n

(14)

1

where Qe and Qm are the adsorbed amount per unit weight of adsorbent (mg/g) at

2

equilibrium and the calculated value at equilibrium from the model, respectively. n is

3

the number of observation.

4 5 6

2.4.5. Regeneration test To assess the recyclability potential of the used SCG-GAC, regeneration tests were

7

repeated for seven cycles. In this test, after the equilibrium conditions were reached,

8

each saturated SCG-GAC by AO7 and MB was taken immediately from the reaction

9

vessels; the samples were then put into 100 mL of methanol. Subsequently, the mixtures

10

were shaken homogeneously at 200 rpm for 24 h with controlled temperature at 20 oC.

11

After washing with distilled deionized water and thoroughly drying at 45 oC for 24 h,

12

the adsorption tests were carried out at initial dye concentrations of 200 mg/L following

13

the same procedures as those used for the equilibrium isotherm experiments. The

14

removal efficiencies and the adsorbed amount of AO7 and MB onto SCG-GAC were

15

then calculated by using Eqs. (1) and (2), respectively.

1 2

3. Results and discussion

3

3.1. General characteristics of SCG-GAC

4

Fig. S4 (see supplementary data) depicts the general characteristics of the prepared

5

SCG-GAC, including morphological, textural, and surface chemical properties. As

6

shown in Fig. S4(A), SCG-GAC is identified as a black sphere with an average

7

diameter of 2.03±0.14 mm, an indicative of successful immobilization of the powdered

8

SCG-based activated carbon into calcium-alginate matrix. The SEM images in Fig.

9

S4(B), taken at a magnitude of ×40 (inset) and ×100, delineate that SCG-GAC bears an

10

egg-like structure with wrinkled and rough surface. Such heterogeneous surface may be

11

beneficial for the uptake of dye molecules in an aqueous solution (Ai et al., 2011).

12

N2 adsorption/desorption isotherms at 77 K for the prepared SCG-GAC were

13

analyzed and core textural properties, including BET surface area, total pore volume,

14

and an average pore size, are presented in Fig. S4(C). It can be seen that the isotherm is

15

classified as a typical type I isotherm, indicating a typical microporous surface

16

according to the IUPAC classification. A remarkable uptake of the N2 vapor at low

17

relative pressure (P/P0) is due to the micropore filling effect. The determined BET

18

surface area and the total pore volume for SCG-GAC were 704.23 m2/g and 0.2926

19

cm3/g, respectively. The pore size distribution estimated by the BJH method from the

20

desorption branch (Fig. S4(C)-inset) further confirmed that the average pore size of

21

SCG-GAC was 2.1966 nm. Even though the estimated average pore size exceeded the

22

micropore range (< 2.0 nm), the prepared SCG-GAC can be considered a microporous

23

material because it had a peak at 1.7754 nm and this is very close to the upper limit of

24

the micropore range (Ma et al., 2015). The overall structural analysis suggest that the

25

prepared SCG-GAC possesses suitable properties as an adsorbent for removing AO7

1

and MB, from an aqueous solution because the molecular dimensions of AO7 and MB

2

in water are 0.544 × 1.003 × 1.567 nm and 0.400 × 0.793 × 1.634 nm, respectively

3

(Zhao et al., 2013).

4

The FTIR spectra of both naked and exhausted SCG-GAC were obtained from 4000

5

to 600 cm-1, shown in Fig. S4(D). The peak observed at 3339.92 cm-1 is caused by

6

residual water or hydrogen-bonded O−H stretching vibrations existent in the alginate

7

matrix. The peaks at 3000-2800 cm-1 (2924.54 and 2853.66 cm-1) are axial and

8

equatorial sp3 C-H stretching vibrations (Bedin et al., 2016) from the alginate. The

9

peaks in the wave number range of 1600-1300 cm-1 (1599.61 and 1421.68 cm-1)

10

correspond to aromatic C=C stretching vibrations and COO- stretching vibrations (Liou,

11

2010). The peak observed at 1028.08 cm-1 represents C-O stretching vibrations and the

12

existence of multiple shoulders adjacent to the main peak is attributed via the positional

13

differences among these functional groups (OH and COO-) within the alginate matrix

14

(Papageorgiou et al., 2010). The peak observed in 800-600 cm-1 region can be attributed

15

to different types (aromatic and non-aromatic) of C=C bending vibrations (Unur, 2013).

16

After the adsorption of the substrates, the major peaks identified from the naked

17

adsorbent were slightly shifted due to the adsorbent’s interaction with adsorbate

18

molecules. However, the values fall within the range of the assigned functional groups,

19

indicating that they are involved in the interaction with the adsorbate, but no chemical

20

alterations regarding the functional groups occurred. In addition, the shift of the bands

21

and their decrease of corresponding intensities observed in 800-500 cm-1 region can be

22

attributed to the presence of π- π interactions between dye molecules and aromatic

23

regions of SCG-GAC (Bedin et al., 2016).

24 25

3.2. Influences of solution pH and ionic strength

1

The solution pH is one of the most influential factors controlling the adsorption

2

process due to its close relations with the surface charge (protonated or deprotonated) of

3

the adsorbent surface and the degree of ionization degree of dye molecules (Elmoubarki

4

et al., 2015; Franca et al., 2009). In this study, the influence of solution pH on the

5

adsorption of AO7 and MB was analyzed by varying the solution pH from 3.0 to 11.0,

6

and the results are shown in Fig. 1(A). The adsorption capacities were strongly

7

dependent on the solution pH and diametric trends were observed between AO7 and

8

MB, where the highest adsorption performance was found to be 97.4% (73.5 mg/g) at

9

pH 3.0 for AO7 and 99.6% (75.2 mg/g) at pH 11.0 for MB. Similar observations were

10

reported by several groups and demonstrated that such phenomena occur due to the

11

possession of strong ionic characters by AO7 and MB under acidic and alkaline

12

conditions, respectively (Ai et al., 2011; Benhouria et al., 2015; Doğan et al., 2009;

13

Elmoubarki et al., 2015; Nourmoradi et al., 2015; Reddy et al., 2013; Uddin et al., 2009).

14

In detail, as depicted in Fig. 1(A), the adsorbed amount of AO7 dwindled with

15

increasing solution pH from 3.0 to 11.0, which may be due to the following reasons:

16

First, the formation of negatively charged electrical double layer near the surface of the

17

SCG-GAC may induce an electrostatic repulsion between dissociated AO7 (pKa1: 11.4,

18

pKa2: 1.0) and the negatively charged layer near the surface of SCG-GAC. Another

19

possible explanation may be due to the increased degree of tautomerization within the

20

dye molecule with an increase of pH condition. The proton in the hydroxyl group can

21

go under intramolecular transfer to one of the nitrogen atoms in the azo group as shown

22

in Fig. S5 (see supplementary data). Because the predominant species of AO7 in water

23

is the hydrazine tautomer at relatively higher pH condition (Alosmanov, 2016), the ketal

24

group in the hydrazine tautomer may undermine the adsorption capacity due to its lower

25

degree of interaction with the adsorbent compared to the azo tautomer due to the lack of

1

hydrogen bonding at the oxygen atom of the substrate. The transferred hydrogen may be

2

less prone to hydrogen bonding as it is located in the inner core of the dye molecule.

3

Lastly, the point of zero charge (pHpzc) for this study was 7.6 (Fig. S6), and as a result,

4

the surface charge of SCG-GAC will be negatively charged at pH > pHpzc suppressing

5

the adsorption capacity of AO7 (60.5% and 45.8 mg/g at pH 11).

6

In contrast, it was found that the adsorption capacity of MB remained almost

7

constant, but it significantly increased from 7.0 to 11.0 (pH > pHpzc). This is due to the

8

stronger electrostatic attraction between the positively charged MB (pKa= 3.8) and

9

more negatively charged surface of SCG-GAC when the solution pH was 11.0, and

10

thereby the adsorption capacity of MB was higher than that at pH 7.0 (95.8% and 70.8

11

mg/g) and 9.0 (98.9% and 74.8 mg/g). At lower solution pH values (pH < pHpzc),

12

protonation at the surface of the adsorbent takes place, which causes electrostatic

13

repulsion with the MB molecules. Additionally, high concentration of H+ will protonate

14

the carboxylate groups within the alginate matrix, as well as form a positively charged

15

double layer at the surface, repelling the MB from approaching the active sites, and

16

ultimately plummeting the adsorption capacity. These results confirmed that the

17

adsorption of the prepared SCG-GAC for AO7 and MB were pH dependent; therefore,

18

the remaining tests (ionic strength, kinetic and equilibrium isotherm) were investigated

19

at constant pH conditions of 3.0 and 11.0 for AO7 and MB, respectively.

20

Followed by the study on the influence of pH, the influence of ionic strength on the

21

adsorption capacities of AO7 and MB onto SCG-GAC also was investigated by varying

22

the NaCl concentration from 0.5 to 3.0 g/L because the adsorption process in some

23

cases is markedly affected by the existence of electrolytes due to their interference on

24

the electrostatic interaction between the adsorbent and absorbent, drastically impacting

25

the adsorption process (Doğan et al., 2009; Özdemir et al., 2006). However, contrary to

1

expectations, superficial change of the adsorption capacities for AO7 and MB were

2

observed when increasing the ionic strength even to a NaCl concentration of 3.0 g/L

3

(Fig. 1(B)). Similar results were reported for the adsorption of dyes such as AO7, MB,

4

and reactive red 24 using zinc oxide coated activated carbon and paper mill sewage

5

sludge derived activated carbon, respectively (Li et al., 2011; Nourmoradi et al., 2015).

6

These results suggest that the binding affinity between SCG-GAC and dye molecules is

7

not significantly affected by background salt concentrations; thus, the adsorption of

8

AO7 and MB predominantly takes place at the internal-layer of SCG-GAC (ionic bonds,

9

strongly bonding) rather than the external-layer (electrostatic or Van der Waals

10

interactions, weakly bonding) because the latter could be more prone to the influence

11

from ionic intervention than the former (Zhu et al., 2015).

12 13 14

3.3. Adsorption kinetics Figs. S7(A) and (B) (see supplementary data) delineate that the adsorption kinetics

15

appear to involve three phases over the course of reaction, insinuating that the

16

adsorption is comprised of multiple steps. Generally, adsorption process can be

17

categorized into three consecutive diffusion steps, followed by an equilibrium step: (1)

18

bulk diffusion: transport of the adsorbate toward the external surface of the adsorbent;

19

(2) film diffusion or boundary layer diffusion: diffusion of the adsorbate to the external

20

surface of the adsorbent via the boundary layer; and (3) pore diffusion or intraparticle

21

diffusion: diffusion of the adsorbate within the pores and interior surface of the

22

adsorbent except for a small amount of adsorption (Sheha and El-Khouly, 2013). As

23

previously mentioned, the pore size of SCG-GAC is larger than the molecular sizes of

24

AO7 and MB. Thus, it can be conjectured that pore diffusion is the primary mechanism

25

for the investigated adsorption process. Despite the prevalence of reaction-based models

1

such as pseudo-first and pseudo-second models in kinetic analysis, these models are

2

insufficient to utterly interpret and elucidate the diffusion mechanisms involved in

3

adsorption processes. To overcome such limitations, a numerical simulation model, pore

4

diffusion model (Eq. (3)), was used to describe the adsorption kinetics of AO7 and MB

5

onto SCG-GAC and determine the external-film mass transfer (kf) and the pore

6

diffusion coefficients (Dp) at three different temperatures.

7

Figs. 2(A) and (B) represent the concentration decay curves of AO7 and MB,

8

respectively, along with their corresponding theoretical predictions for SCG-GAC at

9

three different temperatures. The results reveal that the employed pore diffusion model

10

demonstrates an excellent agreement with the experimental data, suggesting that the

11

model accurately describes the dynamics of the adsorption. The calculated parameters in

12

terms of the external-film transfer coefficient and the pore diffusion coefficient are

13

listed in Table 1 with the coefficient of determination values (R2). The obtained values

14

propose that the pore diffusion is the main rate-limiting step during the adsorption of

15

AO7 and MB. As seen, the external-film transfer coefficients for AO7 and MB vary

16

with respect to temperature and were found to be in the range of 2.228 × 10-6 to 3.101 ×

17

10 -6 m/sec and 2.280 × 10 -7 to 4.291 × 10-6 m/sec, respectively. The pore diffusion

18

coefficients were found to be in a range of 6.12 × 10 -10 to 1.36 × 10 -9 m/sec for AO7 and

19

7.823 × 10 -10 to 2.443 × 10-9 m/sec for MB, with a high R2 values around 0.99, which

20

reflects high accuracy.

21

The simulation for the concentration gradients of particle phase AO7 and MB

22

demonstrated that the pore diffusion rate of dye molecules as a function of contact time

23

gradually increased with increasing temperature, as shown in Figs. S8 and S9 (see

24

supplementary data). These phenomena may be caused by not only the increased

25

mobility and kinetic energy of the adsorbate with lowered boundary layer thickness, but

1

also decreased intraparticle diffusion resistance for adsorbing dye molecules with

2

increasing temperature. In addition, the adsorbates may penetrate deeper and faster at

3

higher temperatures via swelling of the internal structure (Baccar et al., 2013). These

4

findings confirmed that the adsorption mechanism is an endothermic process.

5

Comparing AO7 and MB, the relatively lower external-film transfer and pore diffusivity

6

of AO7 are likely due to its heavier molecular weight (350.32 g/mol for AO7 vs. 319.85

7

g/mol for MB) and larger dimensions compared to those of MB.

8 9 10

3.4. Adsorption equilibrium isotherms Figs. 3(A) and (B) show the plots of equilibrium concentration (Ce) versus the

11

adsorption capacities of the AO7 and MB at different temperatures (10-30 oC),

12

respectively. The plotted curves indicated that SCG-GAC has high binding affinity

13

towards the substrates. As seen, the adsorption capacities of AO7 and MB onto SCG-

14

GAC gradually increased with increasing temperature, and reached from 460.5 mg/g at

15

10 oC to 634.6 mg/g at 30 oC for AO7 and 465.5 mg/g at 10 oC to 710.2 mg/g at 30 oC

16

for MB. These findings are consistent with the obtained kinetic results, suggesting the

17

enhancement of adsorbent-adsorbate interaction via simultaneous increase of both

18

kinetic energies of the two matters and the reactivity at the surface of the adsorbent

19

(Chowdhry et al., 2011). To quantitatively evaluate the respective adsorption capacities

20

of SCG-GAC for AO7 and MB and to unearth it adsorption behaviors, three isotherm

21

models, Langmuir, Freundlich, and Sips models, were employed and the obtained fitting

22

curves are shown in Figs. 3(A) and (B). Also, the determined isotherm model constants

23

along with R2, RMSE, SSE, and χ2 for the three isotherm models, are summarized in

24

Table 2. Both AO7 and MB exhibited the largest R2 values and the lowest error

25

functions values were estimated by the Sips isotherm model. Generally, a larger R2 and

1

smaller error functions values reflect greater coherence between the model and

2

experimental results. Out of the employed models, the Sips model is the most accurate

3

representation of the experimetnal adsorption of the dye molecules. Therefore, the

4

adsorption of AO7 and MB onto SCG-GAC possibly took place on an energetically

5

heterogeneous surface in a multi-step fashin (Jung et al., 2016).

6

In the Sips isotherm model, KS and 1/n represent the affinity of the binding sites and

7

adsorption intensity (heterogeneity factor), respectively. A favorable adsorption is

8

predicted when KS is large and 1/n is small. As shown in Table 2, KS values for AO7

9

and MB increased from 0.0007 to 0.0134 and from 0.0114 to 0.0387, while the 1/n

10

values decreased from 1.1328 to 0.8012 and from 0.7035 to 0.4851, respectively,

11

indicating that at higher temperature, adsorptoin of the substrates are favored. As also

12

illustrated in Table 2, all values of 1/n in both AO7 and MB are less than 1 (0 < 1/n < 1),

13

indicating favorable adsorption. In addition, the lowest 1/n values at 30 oC reveal that

14

the adsorbate-adsorbent interactions might be strong (Ai et al., 2011; Hameed et al.,

15

2007). The adsorption mechanism is believed to be endothermic because the

16

interactions between the adsrobent and the adsrobate are enhanced at higher

17

temperatures. The maximum adsorption capacities of AO7 and MB obtained from the

18

Sips model gradually increased from 500.5 to 665.9 mg/g and 553.2 to 986.8 mg/g,

19

respectively. These values are superior to those of many other adsorbents and even

20

comparable to powdered adsorbents reported in the literature (Table 3).

21 22 23

3.5. Isosteric heat of adsorption Owing to its usefulness for evaluating the strength of interaction between the

24

adsorbate molecules and adsorbent lattice atoms, which is viable as a measure of the

25

energetic heterogeneity of a solid surface, the isosteric heat of adsorption and its

1

variation with surface loading has been generally adopted (Himeno et al., 2005). Thus,

2

in order to identify the underlying factors in the increasing trend in adsorption capacities

3

for AO7 and MB, the isosteric heat of AO7 and MB adsorptions were evaluated by the

4

Clausius-Clapeyron equation (Eq. 10) as functions of the adsorbed amount

5

(0.00068669-9.0789 mol/kg for AO7 and 0.0000057825-14.0364 mol/kg for MB) at

6

different temperatures. As depicted in Figs. 4(A) and (B), isosteric heat of adsorption

7

(Qst) varies with an increase in adsorbed amount, indicating that SCG-GAC has an

8

energetically heterogeneous surfaces. It is also observed that the isosteric heat of

9

adsorption values decreased from 130.5050 to 58.2439 kJ/mol for AO7 and 174.5457 to

10

57.0930 kJ/mol for MB at lower surface loading, whereas a substantial increase was

11

detected as the loading was increased, ranging from 58.2442 to 206.0887 kJ/mol for

12

AO7 and 57.0947 to 281.3331 kJ/mol for MB, respectively. From these observations, it

13

can be concluded that surface energetic heterogeneity and/or adsorbate-adsorbate

14

interaction initially took place at low surface coverage when adsorbing AO7 and MB

15

from aqueous solutions, while the interactions between adsorbate-adsorbent become

16

dominant as the loading increased, resulting in high isosteric heat of adsorption values

17

(Himeno et al., 2005).

18 19 20

3.6. Regeneration test Since reusability of a spent adsorbent is one of the most crucial factors when

21

assessing the cost-effectiveness of the overall processes, the regeneration of exhausted

22

SCG-GAC was carried out. As shown in Fig. 5, the adsorption performance of SCG-

23

GAC gradually decreased with increasing regeneration cycles as a result of incomplete

24

and/or partial recovery of available active sites after the regeneration process.

25

Nevertheless, it is clear that the removal efficiencies or the adsorption capacity were

1

maintained at certain levels (70.2% and 51.6 mg/g for AO7 and 75.4% and 57.1 mg/g

2

for MB) even after seven cycles. These values are comparable to those of various fresh

3

adsorbents (Table 3), demonstrating that SCG-GAC offers sustainability for treatment

4

of dye-containing wastewater. Generally, owing to their simplicity and effectiveness,

5

continuous flow column-type adsorption processes have been employed to ensure

6

practical applicability of adsorbents in the real field. From this point of view, research

7

on the adsorption of AO7 and MB onto SCG-GAC under a continuous flow fixed-bed

8

column condition with a detailed investigation of various operating parameters (solution

9

pH, bed height, flow rate, initial dye concentration) and their prediction for column

10

breakthrough is currently underway.

11 12 13

4. Conclusion This study describes the preparation of SCG-GAC and its application to AO7 and

14

MB adsorption from an aqueous solution. The results indicated that adsorption was

15

highly dependent on the solution pH with respect to the binding affinity between SCG-

16

GAC and dye molecules, while it was independent of ionic effects. Furthermore, the

17

acquired experimental results of dye adsorption suggest that the surface of SCG-GAC is

18

energetically heterogeneous and the adsorption process is governed by an endothermic

19

mechanism. Finally, regeneration results clearly indicated that SCG-GAC is a highly

20

potent adsorbent that can simultaneously meet the requirements of environmental and

21

economic benefits.

22 23

Acknowledgement

24

This work was supported by grants from the National Research Council of Science and

25

Technology (Project No. Asia-02-002).

1 2

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3

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1 2

Fig. 1. Effects of solution pH (A) and ionic strength (B) on the adsorption of AO7 and

3

MB onto SCG-GAC. Initial AO7 and MB concentrations, 200 mg/L; solution pH range,

4

3.0-11.0 (ionic strength: pH 3.0 and 11.0 for AO7 and MB, respectively); NaCl

5

concentration range, 0.5-3.0 g/L; temperature, 20 oC; SCG-GAC dosage, 0.1 g; working

6

volume, 50 mL; contact time, 24 h.

Fig. 2. Concentration decay curves of AO7 (A) and MB (B) on the SCG-GAC at different temperatures. Initial dyes concentrations, 2000 mg/L, solution pHs, 3.0 (AO7) and 11.0 (MB); temperature range, 10-30 oC; SCG-GAC dose, 0.5 g; working volume, 250 mL; reaction time, 24 h.

Fig. 3. Adsorption equilibrium isotherms for AO7 (A) and MB (B). Initial dyes concentration range, 50-5000 mg/L; solution pHs, 3.0 (AO7) and 11.0 (MB); temperature range, 10-30 oC; SCG-GAC dosage, 0.1 g; working volume, 50 ml; contact time, 24 h.

Fig. 4. Isosteric heat of adsorption for AO7 (A) and MB (B).

1 2

Fig. 5. Regeneration tests for the AO7 and MB adsorptions onto SCG-GAC. Initial dyes

3

concentration, 200 mg/L; adsorption pHs, 3.0 (AO7) and 11.0 (MB); temperature, 20 oC;

4

adsorbents dose, 0.1 g; working volume, 50 mL; contact time, 24 h; desorption

5

conditions, 200 mL of methanol at 20 oC for 24 h.

6 7

1

Table 1 Kinetic parameters for AO7 and MB adsorptions onto SCG-GAC. Temperature (oC) Parameters

AO7

MB

Units 10

20

30

kf

m/sec

2.228 × 10 -6

2.381 × 10 -6

3.101 × 10 -6

Dp

m2/sec

6.124 × 10-10

9.432 × 10-10

1.356 × 10 -9

R2

-

0.991

0.992

0.991

kf

m/sec

2.280 × 10 -7

2.414 × 10 -6

4.291 × 10 -6

Dp

m2/sec

7.823 × 10-10

1.243 × 10 -9

2.443 × 10 -9

R2

-

0.989

0.990

0.988

Table 2 Adsorption isotherm parameters for AO7 and MB adsorptions onto SCG-GAC. AO7 Temperature (oC)

Parameters

Langmuir

Freundlich

Sips

MB

10

20

30

10

20

30

Qm

525.9763

573.7681

622.7023

471.5177

556.184

667.8028

KL

0.0014

0.0024

0.0058

0.0029

0.0047

0.0062

R2

0.9968

0.9962

0.9938

0.9861

0.984

0.9685

RMSE

1.093E+01

1.295E+01

1.953E+01

2.056E+01

2.721E+01

4.645E+01

SSE

1.133E+02

1.682E+02

3.814E+02

4.226E+02

7.401E+02

2.158E+03

χ2

9.832E+02

1.668E+03

3.723E+03

3.272E+03

5.783E+03

1.830E+04

KF

19.3872

32.6082

63.075

34.0668

57.417

79.3159

1/n

0.3784

0.3370

0.2785

0.3091

0.2737

0.2603

R2

0.9282

0.9354

0.9396

0.9571

0.9585

0.9801

RMSE

5.005E+01

5.368E+01

6.104E+01

3.613E+01

4.384E+01

3.691E+01

SSE

2.505E+03

3.325E+03

3.725E+03

1.306E+03

1.921E+03

1.362E+03

χ2

2.157E+04

2.488E+04

3.229E+04

1.451E+04

2.064E+04

1.509E+04

Qm

500.4862

588.8762

665.8587

553.196

653.1532

986.8108

KS

0.0007

0.0033

0.0134

0.0114

0.0197

0.0387

1/n

1.1328

0.9362

0.8012

0.7035

0.6653

0.4851

R2

0.9979

0.9966

0.9968

0.9972

0.9951

0.994

RMSE

8.581E+00

1.239E+01

1.395E+01

9.300E+00

1.508E+01

2.027E+01

SSE

7.335E+01

1.598E+02

1.946E+02

8.646E+01

5.204E+02

4.106E+02

χ2

8.012E+02

1.495E+03

1.894E+03

1.517E+03

2.506E+03

4.257E+03

1

Table 3 Comparison of maximum dye adsorption capacity obtained in this study with

2

previous data. Adsorbent

Adsorption capacity (mg/g)

Dye

References

SCG-GAC

665.9

This study

Spent brewery grains

30.5

Silva et al., 2004

Surfactant-modified zeolite

0.63

Banana peel-activated carbon

333

Activated carbon coated with ZnO

66.22

SCG-GAC

986.8

This study

Surfactant-modified zeolite

15.68

Jin et al., 2008

Tea waste

85.16

Activated carbon derived from coffee

AO7

Ma et al., 2015 Nourmoradi et al., 2015

Uddin et al., 2009 Reffas et al.,

181.8

grounds Activated carbon/cobalt

33.58

ferrite/alginate composite beads Banana peel-activated carbon

1,263

Calcium alginate-bentonite-activated

994.06

carbon composite beads Activated carbon coated with ZnO

66.66

Activated carbon prepared from

704.2

sucrose spherical carbon 3 4

36

Jin et al., 2008

2010 MB

Ai et al., 2011 Ma et al., 2015 Benhouria et al., 2015 Nourmoradi et al., 2015 Bedin et al., 2016

1

Graphical Abstract

2 3

37

1

- We studied adsorption of acid orange 7 and methylene blue from aqueous solutions.

2

- Spent coffee grounds has been converted into granular activated carbon (SCG-GAC).

3

- Pore diffusion and Sips models fit the adsorption data of the SCG-GAC.

4

- Maximum adsorption capacities were 665.9 mg/g for AO7 and 986.8 mg/g for MB.

5

- Regeneration tests show the saturated SCG-GAC has excellent reusability potential.

6 7 8

38