Ultrafast Removal of Cationic Dye Using Agrowaste-Derived

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Ultrafast removal of cationic dye using agrowaste derived mesoporous adsorbent Raju Dutta, Tallam V. Nagarjuna, Sachin Mandavgane, and Jayant D. Ekhe Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 04 Nov 2014 Downloaded from http://pubs.acs.org on November 5, 2014

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Industrial & Engineering Chemistry Research

Ultrafast removal of cationic dye using agro-waste derived mesoporous adsorbent Raju Dutta †, Tallam V. Nagarjuna ‡, Sachin A. Mandavgane ‡,*, Jayant D. Ekhe †,* †

Department of Chemistry, Visvesvaraya National Institute of Technology, Nagpur-440010, India

‡

Department of Chemical Engineering, Visvesvaraya National Institute of Technology, Nagpur-440010, India

Corresponding authors: E-mail address: [email protected] (Jayant D. Ekhe) †,* Tel.: +91 712 2801603, Fax: 91-712-2223969 E-mail address: [email protected] (Sachin A. Mandavgane) ‡,* Tel.: +91 712 2801563, Fax: 91-712-2223969

1 ACS Paragon Plus Environment


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Abstract: High purity amorphous silica extracted from rice husk ash (RHA) was utilized for synthesising a low cost, novel mesoporous adsorbent (RHS-MCM-41) for removal of cationic dye brilliant green (BG) from water. Transforming silica to mesoporous aluminosilicate structure, resulted ≈ 9 fold enhancement of surface area generating negatively charged surface active sites. Adsorbent was characterized with Small Angle XRD, SEM, EDS, Nitrogen adsorption-desorption analysis and FTIR spectroscopy. Equilibrium state was achieved very fast (within 15 min) following pseudo-second-order kinetics (ks=315x10-4 g/mg min) with a very high adsorption capacity of 250 mg/g. Variation in adsorbent surface functionality revealed that presence of negative charge centres favour BG adsorption. Elovich model fitting the system explicated involvement of chemisorption process. Intra-particle diffusion models elucidate two step diffusion and saturation stage. Equilibrium batch adsorption data are best explained with LangmuirFreundlich isotherm models. Keywords: RHA, mesoporous, adsorption, brilliant green, kinetics


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1. Introductions Environmental pollution especially of aquatic body is posing huge threats to living beings.1 Having strong persistent colour and high COD (Chemical Oxygen Demand) loading, dye containing waters are aesthetically and environmentally unacceptable.2 Various types of industries like textile, pulp and paper, paint, lather use different dyes and pigments for their final product preparation.3 Over 7x105 tons of dyes and pigments comprising of more than 10,000 different molecules are produced worldwide annually. Almost 15% of the total dyestuff is lost during the dyeing process in different textile industries and come out with their effluents.4 Cationic dyes cause more toxicity than anionic.3,5 Brilliant Green (BG) is a cationic dye extensively used in textile and paper industries. In paper industries, about 0.8-1.0 kg of BG is used per ton of paper produced.6 BG causes irritation to the lungs and affects the gastrointestinal tract.7 Several studies have reported investigating the efficiency of different wastewater treatment technologies viz. chemical coagulation, flocculation, microbial degradation, photocatalytic degradation, advanced oxidation processes, membrane separation, nanofltration and adsorption. Owing to its high efficiency adsorption is the most economic and extensively employed process. Interest is growing on usability of low cost and widely available adsorbents.8,9 Variety of natural and synthetic materials e.g. kaolin,3,1 saklikent mud,7 rice husk ash,6 Neem Leaf Powder,10 bottom ash and deoiled soya11,

bagasse

fly

ash

(BFA),12

Super

adsorbent

polymer,13

poly

(N-

isopropylacrylamide-co-itaconic acid) hydrogels,14 Acron based novel adsorbent,15 have been applied for aqueous phase adsorptive removal of BG.



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Rice husk, accounts for almost one-fifth of global annual gross rice production (545 million metric tons)16 when burnt in boiler produces rice husk ash (RHA) a solid waste that contains ≈84% silica.17 Thus harvesting silica from RHA is a sustainable way for development of novel adsorbent and catalysts. Zeolite molecular sieves are aluminosilicate materials can be used as potential adsorbents due to their large surface area, uniform pore size with high degree symmetry providing a suitable environment for size exclusive separation and adsorption. In view of all above citations particularly looking at the proved applicability of adsorption technique and need for faster, effective removal of BG, it is found interesting to modify abundantly available RHA into much more effective adsorbent for the removal of BG from aqueous medium. In the present study we have synthesized RHS-MCM-41, a mesoporous adsorbent with large surface area using high purity silica extracted from RHA. Particularly it is worth investigating when scanty references are available and those too utilize expensive precursors of silica.18,19 Kinetics, thermodynamic and isotherm studies were performed to understand the mechanism and nature of adsorption process. 2. Material and methods 2.1. Materials The chemicals used in present work were of analytical grade and used without any further purification. BG dye (hydrogen sulphate) (C.I.: 42040, Molar mass: 482.64 g/mol, Chemical Formula: C27H34N2O4S, Solubility: 100 g/L) was purchased from Merck, India. Chemical structure of BG is illustrated in Figure 1. The maximum absorption in visible wavelength (λmax) is observed to be 624 nm. Sodium hydroxide, Nitric acid, 4 ACS Paragon Plus Environment

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Sulfuric

acid,

Al2(SO4)3.16H2O

were

procured

from

Merck,

India.

Cetyltrimehylammonium bromide (CTAB) was purchased from Sigma-Aldrich while RHA was supplied by Yash Agro Industries Ltd. Nagpur, India. 2.2. Extraction of silica (RHA-Si) from RHA RHA-Silica (RHA-Si) was extracted by alkaline digestion method. The RHA obtained was of greyish black colour. The residual carbon and other heavy metals in RHA were removed by calcination at 700 °C for 3 h under air atmosphere followed by 2 h acid digestion with 2 M HNO3. 10 g of pre-treated RHA was digested with 50 ml of 6 N NaOH solution for 1 h with constant stirring. The extract was collected via filtration through Whatman 41 filter paper. The residue was washed twice with 100 ml portion of boiling water. Precipitated RHA-Si was obtained on neutralizing the NaOH extract with 1 N H2SO4.20,21 After an 18 h of aging the precipitated silica was filtered, thoroughly washed with distilled water, dried overnight at 100 °C. Finally about 8.5 g silica was obtained from 10 g of RHA. Subsequently this RHA-Si obtained in batches was collectively used for adsorbent synthesis. 2.3. Synthesis of adsorbent (RHS-MCM-41) Hydrothermal synthesis of RHS-MCM-41 molecular sieve with Si/Al ratio 30 was carried out adopting the reported method with minor modifications.22,23 The molar composition of the final gel mixture was SiO2: 0.2 CTAB: x Al2O3: 0.89 H2SO4: 120 H2O (x depends on Si/Al ratio). 10 g of RHA-Si and 12.6 g NaOH was dissolved in 50 ml deionized water with constant stirring at 80-90 °C. The silica solution thus obtained was allowed to cool to room temperature. A required amount of Al2(SO4)3.16H2O dissolved in 20 ml deionized water was added slowly to the above silica solution under vigorous 5 ACS Paragon Plus Environment


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stirring and simultaneously solution pH was brought down to 10.5 with dilute H2SO4. During this process a thick gel appeared. After stirring the gel vigorously for 30 min, an aqueous solution of template, cetyltrimethylammonium bromide (CTAB) was added to it followed by stirring for another 30 min. After the whole process was over, resultant gel mixture was transferred to a Teflon lined stainless steel autoclave, sealed and kept in hot air oven at 145 °C for 48 h. After the aging period (48 h) reaction mixture was cooled to room temperature, filtered and washed with EtOH followed by deionized water several times and dried in hot air oven for 24 h at 100 °C. Finally the dried material was calcined at 550 °C for 1 h under nitrogen and then for 6 h under air atmosphere with a heating rate of 2 °C/min. The calcined material was used as adsorbent (RHS-MCM41). 2.4. Adsorption Experiments Batch adsorption studies were carried out in stoppered 100 ml Erlenmeyer flasks. A BG stock solution of 1000 mg/L was prepared by dissolving 1 g of dye in deionized water and other required concentrations were prepared by subsequent dilution. The standard batch adsorption procedure was followed using water bath shaker equipped with thermostat. In each flask a constant volume (50 ml) of dye solution was stirred with a predefined amount of adsorbent. Each time after completion of the experiment the solutions were filtered and dye concentration of filtrate was measured by spectrophotometric method using UV-1800 (Shimadzu Japan). Experiments were performed in triplicates and average values are reported. 2.5. Instruments used for adsorbent characterization XRD powder diffraction pattern of calcined RHS-MCM-41was obtained from Bruker AXS-D8 advance, using CuKα radiation (λ =1.5406 Å) and Si(Li) linear position6 ACS Paragon Plus Environment

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sensitive detector. The diffractogram was recorded in 2θ range 1.5-10° with a step size 0.02° and count time 6 s at each point. Infrared spectra of adsorbent before and after BG adsorption were recorded on IR Affinity-1 (Shimadzu) by diffuse reflectance spectroscopy (DRS) technique. FTIR spectra were recorded separately for pure dye and adsorbent before and after dye adsorption in the range 4000 to 400 cm-1. Surface area, pore volume and pore diameter of RHS-MCM-41 was measured using nitrogen adsorption-desorption method at 77 K in ASAP-2010 Porosimeter (Micrometrics). Prior to analysis the sample was degassed at 623 K at 10-5 Torr for overnight. Surface structure and morphology was observed with Scanning Electron Microscope (JOEL JSM-7600F). Elemental composition of adsorbent was determined with Energy Dispersive Spectroscopic (EDS) unit attached with SEM. 3. Results and discussions 3.1. Characterization of adsorbent (RHS-MCM-41) FTIR spectra of three samples calcined RHS-MCM-41, after BG adsorption and pure dye (BG) are presented in Figure 2 (a), (b) and (c) respectively. The intense peaks at 1072 cm-1 and 800 cm-1 correspond to the asymmetric and symmetric Si-O stretching vibrations. The peaks originated due to silicate framework vibrations, are observed in between 1200 and 500 cm-1. Absorption band around 1228 cm-1 and 460 cm-1 was due to asymmetric stretching of Si-O-Si bridge and bending of surface Si-O groups. The peaks at 1188 cm-1 and 1344 cm-1 and originated from aromatic C-N stretching and C-H deformation of dye molecule and did not result any significant shift after adsorption. The peak at 1415 cm-1 was from O-H bending of adsorbed water molecules. The absorption



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band at 1576 cm-1 which observed small shift to 1581cm-1 after adsorption was due to aromatic C=C ring stretch conjugated with C=N in the quinoid structure of BG. Another peak at 1387 cm-1 which originated from terminal –CH3 bending shifted to 1391 cm-1 after adsorption. The adsorption of BG on RHS-MCM-41 are well explained by observed features the three spectra.24-27 Small angle powder XRD diffractogram of RHS-MCM-41 is presented in Figure 3. The diffraction peaks from (100), (110) and (200) were observed and indexed to hexagonal lattice geometry of typical MCM-41 material. The observed d100 and d110 spacing are 44.9 Å and 33.1 Å respectively. Hexagonal unit cell parameter (a0) was calculated using the formula a0 = 2d100/√3 found to be 51.84 Å The calcined mesoporous material possesses well defined pore structure formed via condensation of silanol groups. The XRD pattern is well in parity with literature reports thus confirming the formation of desired material.22,23,28 Nitrogen adsorption-desorption study showed that BET surface area of RHA-Si and RHS-MCM-41 was 95 m2/g and 840 m2/g respectively. Average pore diameter and total pore volume of the later was 3.8 nm and 1.54 cc/g. The BET adsorption isotherm with the pore size distribution plot of RHS-MCM-41 is presented in Figure 4.The Figure 5(a) & (b) present the SEM micrograph of RHA-Si and RHS-MCM-41respectively. It is evident that highly porous morphology is imparted in RHS-MCM-41 compared to the precursor RHA-Si. The micrograph also reveals highly agglomerated structure of RHSMCM-41 which may be attributed to the enhanced surface energy. 3.2 Adsorption Studies


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3.2.1. Effect of contact time and initial dye concentration (C0) The effect of contact time on BG adsorption onto RHS-MCM-41 was studied with differing initial concentrations (C0) from 100 to 1000 mg/L visualising the results in Figure 6. It is evident that rapid dye uptake occurred within first 2 min followed by gradual falling down of rate leading to system equilibration in ≈15 min; thereby 15 min considered to be equilibration time for further studies. Initially the higher dye concentration gradient across the boundary layer and number of available sites on sorbent may be majorly contributing to the high adsorption rate which in turn reduced with increasing contact time.6,29 This reduction of rate may be attributed to firstly the aggregation of dye molecules filling up the mesopores thereby increasing mass transfer resistance to BG molecules and secondly of the active site saturation on adsorbent surface. With increasing C0 initial rate of adsorption slows down which may be due to increased competition among dye molecules. Equilibration time required in present study is extremely less compared to reported studies supporting the fastest kinetics observed with BG/RHS-MCM-41 system. 3.2.2. Effect of adsorbent dosage Number of available sorption sites, a direct consequence of sorbent dosage influences sorption capacity. It is evident from Figure 7 that increasing dosage result sharp increase in percent removal which dies down gradually forming a plateau. The initial rapid BG removal may be due to greater number of available sorption sites. With dosage 2, 3 and 4 g/L dye removal obtained was 88%, 91% and 92% respectively with 500 mg/L dye solution. Incremental dye removal became very low with further increase



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in dosage. Possible reason may be setting up of equilibrium between solution phase and solid phase dye concentration. Thus dose of 3 g/L considered to be optimum. 3.2.3. Effect of initial pH Several important factors like stability and structure of dye molecules, dissociation of functional groups on adsorbent are predominantly controlled by solution pH.6 Hence initial pH of dye solution (pH0) plays a very important role in controlling particularly adsorption efficiency. The natural pH of 500 mg/L BG solution was found to be 3.15. Below this value BG solution started decolourizing automatically and at higher value, the solution becomes unstable resulting turbidity and finally precipitation. Similar observation was also reported by many researchers.10,30,31 Thus natural pH (3.15) was considered ideal for carrying out adsorption studies. 3.2.4. Effect of Temperature Experiments in current study were carried out at three different temperatures viz. 303, 313 and 323 K to study temperature effect of adsorption. Variations of sorption capacity (qe) with equilibrium dye concentration (Ce) at different temperatures are presented in Figure 8 indicating increased sorption capacity (qe) with temperature. Physical adsorption is an exothermic process thereby increase in temperature would result decreased sorption capacity. In case of diffusion controlled sorption process (intra-particle

transfer-pore

diffusion),

adsorption

capacity

may

increase

with

temperature because of enhanced mobility of adsorbate (BG) molecules. 6 Besides the process involving chemisorption may also favour higher adsorption at elevated temperature.7,32


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3.3. Adsorption kinetics Kinetic study can explain the rate and detail mechanism of adsorption process. In current study, the kinetics of BG adsorption onto RHS-MCM-41 was investigated using Lagergren pseudo-first-order model, pseudo-second-order model, Elovich model and intra-particle diffusion model. Pseudo-first-order kinetic model was unable to explain the adsorption process satisfactorily. Corresponding values of parameters are listed in Table (S1).in Supporting Information. 3.3.1. Pseudo-second-order kinetic model Adsorption processes with chemisorption being the dominated mechanism are believed to follow pseudo-second-order kinetic model.33,34 It is expressed in linear and non-linear form as 2

kq t qt = s e 1 + ks qet

(1)

t 1 t = + 2 qt ks qe qe

(2)

Where, ks is the pseudo-second-order rate constant (g/mg.min). Values of qe and ks can be easily calculated using the linear plot (Figure 9) of t/qt vs t in Eq. 2. From Table 1 it can be observed that values qe (calculated) are well in parity with qe (experimental), resulting the coefficient of determination (R2) near to unity. Thus, it is evident that adsorption mechanism of BG onto RHS-MCM-41 can better be explained with the pseudo-second-order kinetic model. 3.3.2. Elovich Model 11 ACS Paragon Plus Environment


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Linear form of Elovich model 35,36 is presented as qt =

1

β

ln t +

ln αβ

(3)

β

Or as

qt = a ln t + b Where, a =

1

β

and b =

(4)

ln αβ

β

qt is the adsorption capacity at time t (mg/g), α is the initial adsorption rate (mg/(g .min)) and β is related to the activation energy for chemisorption during any particular experiment. The values of a and b can be determined from the slope and intercept of linear plot presented in Figure 10 between qt and ln t in Eq. (4) are summarized in Table 2. From the figure and values of coefficient of determination (R2) in Table 2 it may be observed that Elovich model efficiently explains the mechanism for chemisorption of BG stacked on RHS-MCM-41. 3.3.3. Intra-particle diffusion It is assumed that, the mechanism of intra-particle diffusion involves four steps: bulk diffusion, film diffusion, pore-diffusion and finally adsorption of molecules on active sites. Adsorption through intra-particle-diffusion pathway is a diffusive mass transfer process where adsorption capacity may be expressed in terms of square root of time. The mathematical expression for intra-particle diffusion process in a simplified form is qt = kid t

1 2

(5)

+I

Where qt is the amount of dye uptake (mg/g) at time t, kid is the intra-particle diffusion rate constant (mg/g.min1/2) and I is the intercept indicative of boundary layer thickness. 12 ACS Paragon Plus Environment

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The higher the value of I higher is boundary layer effect. The plots of qt against t1/2 for different initial concentrations (C0) are presented in Figure (S1). Each plot consists of three segments, indicating film diffusion, intra-particle diffusion leading to final saturation state .37,38 Table (S2) contains values of kid and I for different initial concentrations. The plot not passing through origin indicating that intra-particle diffusion is not the only rate limiting step. 3.4. Adsorption equilibrium Equilibrium adsorption isotherm has a fundamental importance to design an adsorption system. It relates the distribution of adsorbate molecules between solid and solution phase.39 In present study five isotherm models such as Langmuir, Freundlich, Temkin

(two

parameters)

and

Langmuir-Freundlich,

Redlich-Peterson

(three

parameters) have been applied on equilibrium experimental data. Langmuir model assumes that finite number of binding site are homogeneously distributed over adsorbent surface showing same affinity towards adsorbed molecules for monolayer adsorption and there is no interaction between the adsorbed molecules themselves. 40 The model can be mathematically presented as

qe =

qm K L Ce 1 + K L Ce

(6)

Where, Ce is equilibrium solution phase concentration of BG (mg/L), qe and qm are equilibrium and maximum adsorption capacity (mg/g) and K L is Langmuir adsorption constant (L/mg). The linear form is

Ce Ce 1 = + qe qm K L qm

(7)



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Freundlich adsorption isotherm model hypothesize that, interaction between the adsorbed molecules not restricted to monolayer formation and with increase in bulk phase concentration, adsorbed phase concentration also increase thereby resulting in exponential decrease of sorption energy.40 Mathematical form of Freundlich isotherm model is 1

(8)

qe = K F C e n

Where, K F is Freundlich constant ((mg/g) (L/mg)1/n) and (1/n) is heterogeneity factor. The magnitude of (1/n) giving the favorability of adsorption process. Values of n>1 favours the adsorption condition.41 Rearranging the Eq. (8) we get the linear form as Eq. (9)

1 ln qe = ln K F +   ln Ce n

(9)

Temkin isotherm model explicitly considers the interaction between the adsorbate and adsorbent. It is assumed that (i) the heat of adsorption of all the molecules in the layer decreases linearly with coverage due to adsorbate-adsorbent interaction (ii) the adsorption is characterized by uniform distribution of binding energies, up to some maximum binding energy.39,42 The model can be expressed by the following equation

qe = Where, B1 =

RT (ln KT Ce ) or q e = B 1 ln K T + B 1 ln C e b

(10)

RT is related to the heat of adsorption and KT is the equilibrium binding b

constant corresponding to the maximum binding energy.


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The Redlich-Peterson (R-P) and Langmuir-Freundlich (L-F) isotherms can be applied to homogeneous as well as heterogeneous systems. The mathematical form of R-P and L-F model is presented in Eq. (11) and (12) respectively.

qe =

K RCe β 1 + aR Ce

(11)

and β

qe = qmax

Ce β K + Ce

(12)

The different parameters of Langmuir, Freundlich, Temkin, R-P and L-F isotherm at 303, 313 and 323 K temperature and coefficients of determination (R2) calculated from experimental data are summarized in Table (S 3). The nonlinear plots of Langmuir, Freundlich and Temkin models are presented in Figure 11 and that of R-P and L-F models in Figure 12. Comparing R2 among the two parameter isotherms it is observed that Langmuir and Temkin models explain the process better than Freundlich whereas among three parameter models L-F suits best. Overall L-F is the best to explain the system as the basic assumption of Langmuir adsorption model is adsorption takes place at specific homogeneous sites within the adsorbent whereas Freundlich theory assumes that adsorption involves heterogeneous type of interactions. 3.5. Error analysis Model equations employed for explaining the isotherm data are empirical. The quality of any isotherm model to fit an experimental data is typically based on the magnitude of coefficient of determination (R2) for that regression. In other way the model equation with R2 value near to unity are expected to fit the system better. But inherent bias in 15 ACS Paragon Plus Environment


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linearisation favours the non-linear regression to determine different isotherm parameter set. Different error functions can satisfactorily judge the best fit isotherm model for the studied system. Six different non-linear error functions (Table S 4)

6,43

were used in the

study. The values of different error functions are summarized in Table 3. Studying the Table it is well observed that Langmuir-Freundlich isotherm is the best fit for adsorption of BG on RHS-MCM-41. 3.6. Adsorption thermodynamics The Gibbs free energy change (∆G0) can be correlated to the adsorption equilibrium constant through the classic Van’t Hoff equation: (13)

∆G 0 = − RT ln K d

Again, Gibbs free energy change at constant temperature can be expressed in terms of enthalpy and entropy change by Eqn. (14). (14)

∆G 0 = ∆H 0 − T ∆S 0

Thus combining the two equations (13) and (14), we get ln K d =

− ∆G 0 ∆S 0 ∆H 0 1 = − RT R R T

(15)

Here, T is the absolute temperature (°K) and R is universal gas constant (J/(mol K)) and the other terms having their corresponding units. Kd is distribution coefficient. The value of enthalpy (∆H0) and entropy (∆S0) change can be determined from the slope and intercept of the plot of ln K d versus (1/T) respectively. ∆G0 can be obtained by substituting the value of ∆H0 and ∆S0 in Eq. (17). All these values are listed in Table 4. The negative values of ∆G0 indicate spontaneity of process. Positive values of ∆H0 indicate endothermic nature of overall process which is contradictory to routine


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expectation. Hence it can be easily envisaged that there might be more than one factor contributing to net enthalpy change. Sorption from solution is governed not only by sorbent-sorbate interaction but also by sorbent-solvent and sorbate-solvent interactions. Adsorption of BG molecules onto RHS-MCM-41 is associated with i) breaking of solvation sheath around the BG molecules ii) desorption of previously adsorbed water molecules from adsorbent surface and iii) finally adsorption of BG molecules onto RHSMCM-41 surface.6,44 The overall change in enthalpy is integration of individual enthalpy changes. In overall process endothermic steps dominated over the exothermic resulting in positive value of ∆H0. The observation may also be explained in other way. The adsorbent surface charge in contact with water is delocalized through solvation. Dye adsorption needs desorption of H2O molecules followed adsorption of cationic dye molecules. Due to larger dimension, BG molecules accommodated on a given surface is less in number than H2O. Hence for neutralizing the surface charge greater number of positive ions is needed. This requires more number of desolvated free cations. So collectively, energy requirement for desorption of H2O molecules and breakage of stable solvation sheaths of cations suppresses the exothermic enthalpy of adsorption. 3.7. Adsorption mechanism Adsorption proceeds via rapid binding of dye molecules to sorbent surface followed by slow intra-particle diffusion through pores. Comparative study of kinetic models suggests chemisorption may be a major contributing phenomenon

45,46

which is

also supported by endothermic nature of overall process. Intra-particle diffusion contributes to the mass transfer process through macro and mesopores. The effect of



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structure and surface charges on adsorption have also been studied and result is presented in Figure 13 where it is clear that on formation of mesoporous molecular sieve structure (RHS-MCM-41), amorphous RHA-Si showed a huge hike in adsorption from 36% to 92%. Again surface chemical structures of RHS-MCM-41 played crucial role in overall process. The corresponding schematics of surface chemical structures before and after H-exchanging are shown in Figure 14(a) and (b) respectively. It is evident that maximum silanol groups and aluminium centres are present as anionic form over the aluminosilicate surface before H-exchange and thereby Na+ ions playing the charge balancing role. After H-exchange these are converted to -OH groups generating brönsted acidic sites with reduced negative charge density. Thus while approaching; the cationic BG molecules experience greater attraction towards surface of RHS-MCM-41 (Na-form) compared to H-from. Consequently the former showed higher removal (92%) than the later (67%). The possible type of interaction between BG molecule and adsorbent surface may be presented by Figure 14(c). Hence it may be suggested that cationic BG molecule can show an electrostatic interaction with the sorbent surface. 3.8. Comparative efficiency of different adsorbents for BG removal Till date several adsorbent materials were studied for aqueous phase BG removal. Some of them are summarized in Table 5. It emerges out that present study dealt with a wide range of dye concentration which is rarely covered in other reported studies. The adsorbent used in present study showed excellent BG removal capacity within very short time span. Ultrafast adsorption kinetics along with high removal capacity stands for the novelty of RHS-MCM-41 as adsorbent. Activated carbon, a commercially used adsorbent lacks for its high cost. Generation of novel activated


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carbon followed by utilization in BG removal was also reported in some previous studies. Acron based activated carbon studied in 10 to 50 mg/L concentration range, took 30 min to reach equilibrium.15 In another study comparative performance of jute stick derived charcoal, physically activated carbon (ACS) and chemically activated carbon (ACC) in BG removal was reported. All three cases the equilibration time required was more than 10 h. Even though surface area of ACC (2304 m2/g) is thrice than that of RHS-MCM-41 (demonstrating equivalent kinetic adsorption capacity as ACC) amount of BG adsorbed per unit surface area is half (0.124 mg/m2) than present material (0.276 mg/m2).47 DP/MWCNTs composite was reported to give a very high adsorption capacity of 476.19 mg/g but the kinetics of dye adsorption was quite slow as compared to present study.

30

Thus it is conspicuous that for aqueous phase BG

removal, time and amount of adsorbent required in present work is very less compared to almost all reported studies. 4. Conclusion The present study showed RHS-MCM-41 to be a promising adsorbent for BG removal over a wide range of concentrations at its natural solution pH. Pseudo-secondorder model best explained process kinetics with a very high value (315x10-4 g/mg min) of kinetic constant (ks) that indicates very faster adsorption. Intra-particle diffusion involved two stage diffusion and finally saturation. Equilibrium isotherm study revealed that L-F is the best among the models applied, subsequently inferring that surface binding sites on RHS-MCM-41 are homogeneously distributed having equal type of interaction with dye. But adsorption is not limited to monolayer due to strong interaction. Maximum monolayer adsorption capacity was found to be 232 mg/g at 303K which



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increased to 250 mg/g at 323K. Higher adsorption capacity at elevated temperature indicates overall endothermic dye uptake. The negative value of ∆G0 implies spontaneity and thermodynamic feasibility of adsorption process that increased with temperature. It may be proposed from sorbent structure and functionality study that BG uptake on RHS-MCM-41 is predominantly favoured by the electrostatic interaction between positively charged dye molecules and negative charge centres on adsorbent. It can be concluded finally that adsorbent synthesized in present study gives highest BG removal in shortest period. Supporting Information Tables including parameters for Pseudo-first-order kinetics, Intra-particle diffusion study, Equilibrium adsorption parameters, Type of error functions used for error analysis and figure representing Intra-particle diffusion. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgements The authors express their sincere gratitude to Department of Science and Technology, India for funding (SR/FTP/ETA-110/2010) the research. Authors are thankful to Dr B D Kulkarni, Distinguished Scientist CSIR India, for his valuable suggestions and encouragement. Sophisticated characterization facilities provided by SAIF, IIT Bombay; SAIF, Kochi and JNARDDC, Nagpur are acknowledged.


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Nandi, B.K.; Goswami, A.; Purkait, M.K. Adsorption characteristics of brilliant green dye on kaolin, J. Hazard. Mater. 2009, 161, 387-395.

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Figure 1: Chemical Structure of Brilliant Green


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Figure 2: FT-IR Spectra of (a) calcined RHS-MCM-41, (b) Dye adsorbed RHS-MCM-41 and (c) pure BG



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Figure 3: XRD of RHS-MCM-41


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Figure 4: Nitrogen adsorption-desorption isotherm of RHS-MCM-41and inset pore size distribution



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Figure 5: SEM image of (a) RHA-Si (b) RHS-MCM-41


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Figure 6: Effect of contact time and initial concentration on BG removal (30°C)



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Figure 7: Effect of adsorbent dose on BG adsorption (C0= 500mg/L, 30°C)


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Figure 8: Effect of operating temperature on adsorption capacity of BG onto RHS-MCM41(C0= 500mg/L)



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Figure 9: Pseudo-second order adsorption kinetics of BG onto RHS-MCM-41 at different C0 (30°C)


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Figure 10: Elovich plot for BG adsorption on RHS-MCM-41 at different initial concentrations (30°C)



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Figure 11: Two parameter non-linear isotherm (30°C)


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Figure 12: Three parameter non-linear plot isotherms Langmuir-Freundlich and RedlichPeterson (30°C)



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Figure 13: Effect of sorbent structure and functionality on percentage BG removal (C0= 500mg/L, 30°C)


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Figure 14: Schematics of surface functionalities in RHS-MCM-41 (a) Na-form, (b) Hform, and (c) Interaction with dye. 41 ACS Paragon Plus Environment


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Table 1 Pseudo-second-order kinetic parameters C0 (mg/L) 100 500 700 1000

qe, exp (mg/g) 33.20 154.70 210.64 286.96

qe, calc (mg/g) 34.3922 156.2500 212.7659 285.7143

ks (g/(mg min)) 0.06692 0.03151 0.01105 0.00879

R2 0.999961 0.999959 0.999617 0.999555

Table 2 Parameters of Elovich plot C0 (mg/L) 100 500 700 1000

a (mg/(g min)) 1.2821 6.0808 11.2098 14.0437

R2 0.939 0.979 0.985 0.983

b (mg/g) 29.583 137.458 173.760 240.870

Table 3 Different error values of equilibrium models (non-linear regression) Error functions (303K) Langmuir Freundlich Temkin Langmuir-Freundlich Redlich-Peterson

R2 0.944789 0.979484 0.982215 0.992425 0.979534

SSE 3318 879.47 730.16 311.06 877.51

ARE 18.605 8.8136 7.1988 5.0146 8.8021

EABS 160.21 75.847 72.393 49.751 75.765

χ2 42.935 12.3031 7.10625 3.05392 12.2687

SRE 23.791 14.083 11.778 7.9278 14.068

Table 4 Thermodynamic parameters C0 (mg/L) 100 500 700 1000

∆H0 (kJ/mol) 62.0391 39.8507 22.8219 12.7370

∆S0 (J/(mol K)) 245.9946 150.8908 89.2508 48.1048

∆G0 (kJ/mol) 303K -12.497 -5.869 -4.221 -1.825


313K -14.957 -7.378 -5.113 -2.3057

323K -17.417 -8.887 -6.006 -2.786

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Table 5 Comparative of different adsorption studies on BG Adsorbent

C0 (mg/L)

(Langmuir capacity) qm

Ref

(mg/g) 5-40

65.42

[3]

50-200

21.60

[6]

Saklikent mud

1-20

1.18

[7]

Neem leaf powder

10-50

133.7

[10]

Bagasse fly ash

50-200

116.28

[12]

Acron based AC

10-50

1.73

[15]

50- 1000

476.19

[30]

50-200

58.48

[31]

1000

52, 150, 286

[47]

50-200

166.67

[45]

500

235.3

[48]

20-100

125

[49]

100-1000

232.6

Present study

Kaolin RHA

DP/MWCNTs Treated Saw dust Charcoal, ACS, ACC from Jute stick Cactus fruit peel Residue acid mixture Red clay RHS-MCM-41



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99x68mm (300 x 300 DPI)

ACS Paragon Plus Environment

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Ultrafast Removal of Cationic Dye Using Agrowaste-Derived

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