A Kinetic, Thermodynamic, and Mechanistic Approach toward

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A Kinetic, Thermodynamic, and Mechanistic Approach toward Adsorption of Methylene Blue over Water-Washed Manganese Nodule Leached Residues K. M. Parida,* Swagatika Sahu, K. H. Reddy, and P. C. Sahoo Colloids and Materials Chemistry Department, Institute of Minerals & Materials Technology, Bhubaneswar-751013, Orissa, India

The present study aims to investigate and develop a cheap adsorption method for color removal from wastewater using water-washed manganese nodule leached residues (WMNLR) as adsorbent. The method was employed for the removal of methylene blue (MB), and the influence of various factors such as adsorbent dose, adsorbate concentration, electrolytes (NaCl, Na2HPO4, Na2SO4, NaNO3, and Na2CO3), temperature, and pH was studied. The equilibrium of adsorption was modeled by using the Langmuir and Freundlich isotherm models; the kinetic parameters and intraparticle diffusion were also then determined for the MB-WMNLR system. Kinetic data were well described by the pseudo second-order model. The thermodynamics of the MB-WMNLR system indicates spontaneous and endothermic nature of the process. The results in this study indicated that WMNLR was an attractive candidate for removing MB (98%), which could be reused up to four times without significant loss in its adsorption capacity. 1. Introduction Industries such as dye manufacturing, tanneries, cosmetics, coffee pulping, pharmaceuticals, food processing, electroplating, and distilleries release colored and toxic effluents to water bodies rendering them murky, dirty, and unusable for further use. Among these industries, the textile industry lines first in treatment of dyes for coloration of fiber. Many dyes are hard to degrade due to their complex structure and xenobiotic properties. Several studies have been undertaken on the toxicity of dyes and their impact on ecosystems, which show certain dyes’ degradation. Consequently, there is considerable need to treat these effluents prior to their discharge into receiving water.1,2 Environmental regulations in most countries have made it mandatory to decolorize dye wastewater prior to discharge.3 The removal of toxic pollutants from wastewater using various techniques like precipitation, ion exchange, chemical oxidation, and adsorption is an important and widely studied research area.4-10 Over the years, a number of works have used different waste and low cost materials11,12 such as waste rice husk,13 wheat husk,14 neem leaf powder,15 powdered activated sludge, perlite,16 bamboo dust, coconut shell, groundnut shell, rice husk and straw, duck weed,17 sewage sludge,18 sawdust carbon19 and gram husk,20 coal bottom ash,21,22 bagasse fly ash,23 deoiled soya,24-26 carbon slurry,27,28 red mud,29 and industrial wastes30,31 as adsorbents for removal of various dyes from wastewaters. MB is a thiazine (cationic) dye, which is most commonly used for coloring paper, temporary hair colorant, dyeing cottons, wools, and so on. Although MB is not considered to be a very toxic dye, it can reveal very harmful effects such as difficulties in breathing, vomiting, diarrhea, and nausea in humans.32 Manganese nodule leached residue (MNLR) is one of the efficient waste materials, which was produced from manganese nodule processing plant, possessing a high surface area, and which can also act as an effective adsorbent for removal of copper, nickel, chromate, phosphate, and sulfate33-37 from synthetic solutions and industrial wastewater. MNLR was washed with water to remove sulfate, and the other impurity * To whom correspondence should be addressed. Fax: 916742581637. E-mail: [email protected].

present in the surface is designated as water-washed manganese nodule leached residue (WMNLR). The leached residue mainly contains the oxides in the form of MnO2, Fe2O3, Al2O3, and SiO2 having variable oxidation states, which is an effective oxidizing agent and has high adsorption capacity.38-43 In our previous study, we have reported the adsorption of selenite, chromium, nickel, and phosphate using MNLR as adsorbent.36,38-40 The present study was designed to explore the feasibility of using WMNLR as an effective adsorbent for removal of MB from contaminated water bodies. Effects of various parameters such as pH, concentration of methylene blue, adsorbent dose, and temperature were studied to optimize the process parameters. 2. Experimental Section 2.1. Preparation of WMNLR. Manganese nodule leached residue (MNLR) was obtained during the pilot plant testing of NH3-SO2 leaching of Indian Ocean manganese nodules at Regional Research Laboratory, Bhubaneswar, India. To obtain a narrow size of fine powders, MNLR was crushed, sieved, and the size fraction (50-75 µm) was collected for further study. Next, the sample was processed for removal of (NH4)2SO4 impurities. In a typical run, about 50 g of MNLR in 500 mL of water was stirred at room temperature for 4 h, filtered, washed thoroughly in distilled water, air-dried for 2 days, and kept in a desiccator for further use. The water-washed manganese nodule leached residue is henceforth abbreviated as WMNLR. 2.2. Characterization Techniques. FTIR spectra of the samples were recorded in a Varian FTIR spectrophotometer (FTS-800) in the range of 400-4000 cm-1 taking KBr as the reference. Surface morphology was studied by scanning electron microscopy (Hitachi S-3400N), equipped with EDX analyzer. The SEM analysis was done by gold sputtering process. 2.3. Methylene Blue Adsorption Studies. Adsorption of MB was conducted in dark condition using a batch technique in a constant temperature water bath at (25 ( 1.0) °C. An accurately weighed quantity of WMNLR (0.6 g) was added to 1 L of stirred MB solution taken in a three-necked round-bottom flask equipped with a magnetic stirrer.44 The MB concentration was kept in the range 10-100 mg/L and pH ranging from 3.0 to

10.1021/ie101866a  2011 American Chemical Society Published on Web 12/10/2010

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Figure 1. SEM and EDX spectra of WMNLR (A) before MB adsorption and (B) after MB adsorption.

10.2. The pH of the suspension was not controlled as the reaction proceeds because addition of inorganic ionic compounds for pH adjustment34 may disturb the adsorption process. At specific time intervals, aliquots of 10 mL suspension were filtered through a 0.22 µ syringe end filter and used for determination of dye concentration using a Varian Carry IE UV-vis spectrophotometer (model EL 96043181) fitted with a Carry 100 software using 10 mm matched quartz cells at characteristic wavelengths (MB, λmax ) 660 nm) at initial suspension pH ) 3.0. Residual content of the MB is calculated by the following formula: residual content (n)% )

Figure 2. FTIR spectra of WMNLR (A) before MB adsorption and (B) after MB adsorption.

Ct × 100 Co

in which Ct is MB concentration at time “t”, and Co is the initial concentration of the MB under study. The effects of pH, temperature, MB concentration, loading of adsorbent, and different electrolytic salt on adsorption were studied. Regeneration study was performed by separating the adsorbent (MB loaded WMNLR) through filtration, subsequently washing with distilled water several times, and finally calcinating at 300 °C. 3. Results and Discussion 3.1. Characterization. 3.1.1. Surface Morphology Studies. SEM morphograph and EDS study (Figure 1) of WMNLR before and after MB adsorption were investigated. Figure 1A shows a semicircular or elongated shape, indicating the pure leached residue shape, a good platform for MB adsorption. Yet in Figure 1B, the leached residue particles are in the aggregated form, indicating that surface was covered with adsorbed MB molecule. The appearance of additional elemental peaks for C, S, and N (structural elements of MB) with Mn, Fe, Si, Al, Cu, Co, Ni, Pb, and Zn in EDS of WMNLR after MB adsorption clearly indicates the distribution of MB on WMNLR surface. 3.1.2. FTIR Results. The FTIR spectra of WMNLR before and after adsorption are presented in Figure 2A and B, respectively. The peak observed at 2363 cm-1 may be due to MnOOH.36 The peak corresponding to 1636 cm-1 is due to the H-O-H bending of adsorbed water. The peak that appeared at 1030 cm-1 may be attributed to the Si-O or Si-O-Al of silicate minerals in both Figure 2A and B. Yet in Figure 2B, sharp and elongated absorption bands at 1550 and 1350 cm-1

Figure 3. Effect of initial pH of suspension on adsorption of MB at initial concentration of MB 30 mg/L and WMNLR loading 0.6 g/L.

may be assigned to N-O stretching, indicating that MB was adsorbed on the surface of WMNLR. 3.2. Adsorption Studies. The contribution of different parameters on MB adsorption is as follows. 3.2.1. Effect of pH. To determine the optimum pH conditions for the adsorption of MB over WMNLR, the effect of pH was observed over the entire pH range of 3-10. As shown in Figure 3, the adsorption of MB increased with a decrease in pH. Higher adsorption of MB at lower pH is probably due to an increase in reduction potential of Mn (hydro) oxide/Mn2+ couple. The factors that affect the MB adsorption are mainly: (1) surface charge of WMNLR and (2) reduction potential of Mn (hydro) oxide/Mn2+ couple. At any PH below the point of zero charge of WMNLR (pHpzc ) 4.5),36 the surface of the metal oxide becomes positively charged due to protonation of hydroxyl group, which does not favor cationic dye (MB) adsorption; however, it improves the reduction potential of Mn (hydro) oxide/Mn2+ couple and thus enhances the MB adsorption. On the other hand, at higher pH, the surface charge density decreases, which favors the MB adsorption and reduces the reduction potential of Mn

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Figure 4. Adsorption of MB dye as a function of concentrations of NaCl: particle concentration ) 0.6 g/L; temperature ) 25 ( 1 °C; [MB] ) 30 mg/L; pH ) 3.0.

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competing with MB adsorption. At higher NaCl concentration, there was hardly any enhancement of MB adsorption. In presence of high NaCl concentration, the thickness of the diffuse double layer can be compressed,45 which facilitates the close approach of MB molecule to WMNLR, leading to remarkable increase of attractive forces and sorption of MB molecule on particle surface. Besides that, the ionization of MB was suppressed with the increase in NaCl concentration due to the common ion effect, which forces the exclusion of MB molecules from the solution phase to the particle surface, as a consequence of hydrophobic effect. The effect of other anions on the adsorption of MB by WMNLR is illustrated in Figure 5. A series of experiments were performed in which different electrolytes, for example, NaCl (0.02 mol/L), Na2HPO4 (0.02 mol/L), Na2SO4 (0.02 mol/L), NaNO3 (0.02 mol/L), and Na2CO3 (0.02 mol/L), were added to MB solution at initial pH 3.0. The presence of carbonate and phosphate inhibited the adsorption of MB, whereas presence of nitrate, chloride, and sulfate ion accelerated the rate of adsorption. Among all the anions, carbonate inhibited the adsorption process to a maximum extent. In contrast, the presence of sulfate ion led to an increase in adsorption of MB on WMNLR. Because of the highest mobility of sulfate, it can compress the electrical double layer between WMNLR and water interface and makes the catalyst surface available for dye molecules to approach each other more closely, leading to enhancement in the rate of adsorption. The inhibition of MB adsorption by inorganic salts at acidic pH is in the following order: 22CO23 > HPO4 > NO3 > Cl > SO4

Figure 5. Adsorption of MB dye in the presence of different electrolytes, NaCl, NaNO3, Na2CO3, Na2SO4, and Na2HPO4, of 0.02 mol/L each.

(hydro) oxide/Mn2+ couple. A little change in MB adsorption at higher pH is probably due to compensation of these two factors. 3.2.2. Effect of Electrolytes. Figure 4 illustrates the influence of sodium chloride on the adsorption of MB. NaCl concentration was varied at various levels such as 0.02, 0.04, 0.1, and 0.2 mol/L. It can be seen from the figure that at lower concentration (0-0.04 mol/L), the adsorption decreases with increase in NaCl concentration. This could be due to addition of inorganic ions affecting electrical double layer of WMNLR in water and

3.2.3. Effect of Adsorbent and Adsorbate Concentration. The variation of the uptake of the dye with varying amount of WMNLR and MB concentration is illustrated in Figure 6A and B, respectively. As expected, the amount of MB adsorption increases with increased adsorbent (WMNLR) concentration; however, the adsorption capacity (mg/g) decreases with the increasing amount of WMNLR loading, indicating that adsorption is dependent upon the availability of binding sites. The adsorption capacity increases from 16.6 to 87.6 mg/g as the MB concentration increases from 10 to 100 mg/L; however, the percentage of adsorption decreases with increased adsorbate concentration. Increase in adsorption with increase in loading capacity of adsorbent is due to availability of greater number of active sites on the surface, whereas decrease in MB adsorption with increase in adsorbate concentration is due to lack of enough active sites for accommodation of adsorbate molecules. 3.2.4. Kinetics Study. The kinetic study explains the rate of adsorption of MB on WMNLR. The kinetics of MB on

Figure 6. Effect of adsorbent dose and adsorbate concentration for the adsorption of MB on WMNLR.

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Table 1. Adsorption Kinetics of MB on WMNLR first-order kinetics pH

qe(exp) (mg/g)

qe(cal) (mg/g)

R

3 4.5 6 8.5 10

48.105 38.18 29.95 27.80 27.92

26.99 10.38 14.64 07.24 09.00

0.9164 0.9582 0.9927 0.9519 0.8103

2

Table 2. Langmuir and Freundlich Isotherms of MB Adsorption on WMNLR

second-order kinetics

Langmuir

k2 (mg/(g min))

qe(cal) (mg/g)

R

0.019 0.025 0.032 0.035 0.036

51.28 38.18 29.25 27.80 27.92

0.9961 0.9998 0.9998 0.9997 0.9911

2

WMNLR is calculated using pseudo first-order and pseudo second-order mechanisms at different pH values. The comparison was done between the experimental data, calculated data, and the regression correlation coefficient (R2). The pseudo firstorder model can be explained from the Lagergren rate equation. It may be represented as46 ln(qe - qt) ) ln qe - k1t

(1)

where qe is the amount of MB adsorbed per unit mass of adsorbent at equilibrium (mg/g), qt is the amount of MB adsorbed at time t (mg/g), and k1 is the equilibrium rate constant of pseudo first-order (min-1). The plot ln(qe - qt) versus time (figure not given) gave fairly straight lines with an R2 value above 0.91 for the pH range 3-8.5. However, the calculated qe value (from intercept of this plot ln qe) does not agree with the experimental results. As intercept does not equal the equilibrium uptake of MB, the reaction is not likely to be first-order, even if this plot has high correlation coefficient with the experimental data.47 The kinetics of adsorption can be represented by a pseudo second-order model,48 which leads to the equation:

()

t 1 1 ) + t 2 qt q k2qe e

(2)

where k2 is the pseudo second-order rate constant (g mg-1 min-1). All data regarding the pseudo second-order are shown in Table 1. In the pseudo second-order kinetics, the calculated qe values are nearly the same as experimental values, and the regression coefficient was found to be 0.99, which also confirms that adsorption phenomena follow second-order kinetics. Similar results were also reported on adsorption of MB onto hazelnut shell44 or adsorption of Cr by WMNLR.39 3.2.5. Effect of Temperature and Adsorption Isotherm Studies. The percentage of MB adsorption increases with increase in temperature from 25 to 45 °C (shown in Supporting Information Figure S1) due to the increase in active surface centers available for adsorption. The equilibrium data were correlated by both Langmuir and Freundlich equations. The isotherm can be well described by the linear form of the Langmuir equation:49 Ce Ce 1 + ) qe Qob Qo

(3)

where Ce (mg/L) is the equilibrium concentration of the adsorbate, and Qo and b are Langmuir constants related to maximum adsorption capacity and energy of adsorption. Regression coefficient values (R2) indicated the best fitting of the data to the Langmuir equation (Table 2). Qo and b were determined from the slopes and intercepts of the straight-line plot between Ce/qe versus Ce (Figure S2) and are given in Table 2. The increase in the value of Qo with the increase in temperature indicates an increase in the adsorption capacity of

temp(K) 298 308 318

Freundlich 2

Qo (mg/g)

b (L/mg)

R

104.17 107.53 125.00

0.181 0.310 0.500

0.986 0.988 0.998

Kf (mg/g)

1/n

R2

35.64 47.75 50.47

0.248 0.204 0.221

0.974 0.999 0.992

Table 3. Thermodynamic Parameter of MB Adsorption on WMNLR thermodynamic parameters temp(°C) 25 35 45

∆G° (kJ/mol)

∆H° (kJ/mol)

∆S° (kJ/(mol K))

-7.274 -8.975 -10.930

47.38

0.1830 0.1829 0.1833

the adsorbent for MB and the endothermic nature of this process. The value of Langmuir equilibrium constant (Ka) indicates the ability of adsorbent toward MB, which increases with increase in temperature. This suggests an increase in affinity of MB toward WMNLR surface with the rise in temperature. The equilibrium data at different temperatures were also fitted to the Freundlich adsorption equation,50 which is given as follows: log qe ) log Kf +

1 log Ce n

(4)

where Kf and n are Freundlich constants. The value of n gives an indication of favorability and Kf (mg/ g) the capacity of the adsorbent. Linear plots of log Ce and log qe (Figure S3) show that the adsorption of MB on WMNLR follows the Freundlich model. The value of 1/n between 0.1 and 1 represents good adsorption of MB on WMNLR (Table 2). 3.2.6. Thermodynamic Parameters. Thermodynamic parameters such as free energy change (∆G°), enthalpy change (∆H°), and entropy change (∆S°) for the adsorption of MB on WMNLR were calculated by the application of the following equations:51 ∆Go ) -RT ln Ka

(5)

∆Go ) ∆Ho - T∆So

(6)

Values of these parameters are compiled in Table 3. The vant’s Hoff plot (Figure S4) is plotted between the ln Ka versus 1/T to determine the enthalpy change (∆H°). The values of ∆G° and ∆S° are determined from eqs 5 and 6, respectively. A positive value of ∆H° indicates an endothermic nature of the adsorption process, while positive values of ∆S° suggest increased randomness at the solid-solution interface. The negative value of (∆G°) confirms the spontaneous nature of adsorption. 3.2.7. Mechanism of Adsorption. The adsorption of MB on WMNLR was found to be rapid at the initial period of contact time, became slow, and then stagnated with the increase in contact time. The mechanism for the removal of dye may be assumed to involve the following four steps:52 migration of dye from bulk of the solution to the surface of the adsorbent, diffusion of dye through the boundary layer to the surface of the adsorbent, adsorption of dye at an active site on the surface of WMNLR, and intraparticle diffusion of dye into the interior pores of the WMNLR.

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Figure 7. Intraparticle diffusion kinetics for adsorption of MB onto WMNLR: initial concentration 30 mg L-1, WMNLR dose 0.6 g, pH 3.0, at 25 °C.

The boundary layer resistance will be affected by the rate of adsorption and increase in contact time, which will reduce the resistance and increase the mobility of MB during adsorption. The uptake of dye at the active sites of WMNLR can mainly be governed by either liquid phase mass transfer rate or intraparticle mass transfer rate. 3.2.7.1. Mass Transfer Coefficient. Mass transfer coefficient, βL (ms-1) of MB at the WMNLR solution interface, was determined by using eq 7:53

(

ln

) (

)

mKa 1 + mKa Ct 1 ) ln βLSst Co 1 + mKa 1 + mKa mKa

(7) -1

where Ka is the Langmuir equilibrium constant (L g ), m is the mass of adsorbent (g), and Ss is the surface area of adsorbent (m2 g-1). This model is not valid for the system because it failed to produce a linear graphical relation between ln((Ct)/(Co) (1)/(1 + mKa)) versus t. 3.2.7.2. Intraparticle Diffusion Model. The adsorbate species are most probably transported from the bulk of the solution into the solid phase through intraparticle diffusion/transport process, which is often the rate-limiting step in many adsorption processes. The possibility of intraparticle diffusion was explored by using the intraparticle diffusion model:54 qt ) Kdif√t + C

(8)

where C is the intercept and Kdif is the intraparticle diffusion rate constant (mg min-1/2 g-1). The values of qt were found to be linearly correlated with values of t1/2. The intraparticle diffusion plot has been given in Figure 7. The values of intercept (shown in Supporting Information Table S1) give an idea about the boundary layer thickness; that is, the larger intercept means the greater is the boundary layer effect.55 The applicability of the intraparticle diffusion model indicates that it is the ratedetermining step. 3.2.8. Regeneration Study. Finally, the effects of regeneration cycles on the adsorption activity were also investigated. In this way, the MB loaded WMNLR can be subjected to several consecutive adsorption cycles. After four cycles, the adsorption capacity decreases only by 8% (Figure S5). 4. Conclusion The potential of WMNLR as adsorbents for removal of MB from contaminated water bodies has been examined, and the following conclusions are drawn.

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(1) WMNLR may be an alternate to costly adsorbents for the removal of MB from wastewater, which can satisfactorily be regenerated by calcining at 300 °C. It has good adsorption capacity, which is comparable with the other low-cost adsorbent reported for the removal of MB (Table S2). (2) Adsorption of MB is optimum in lower pH, that is, below the pHPZC of WMNLR. Up to 98% adsorption can be achieved with 0.6 g of WMNLR/lit. (3) This study also showed that the equilibrium of adsorption of the MB on WMNLR system is suitably described by the Langmuir and Freundlich isotherms. Pseudo second-order and intraparticle diffusion models can be used to predict the adsorption kinetics. (4) The negative value of free energy change indicated the spontaneous nature of sorption and confirmed affinity of WMNLR for the basic dye (MB). (5) The presence of electrolyte can apparently influence the adsorption capacity of WMNLR. Anions like CO3-, PO42-, and Cl- inhibit adsorption, whereas SO42- and NO3- accelerate the adsorption process of MB. Acknowledgment We are extremely thankful to Prof. B. K. Mishra, Director, IMMT, Bhubaneswar 751013, Orissa, India, for his constant encouragement and permission to publish this paper. Special thanks to DOD, Government of India, for financial support. Supporting Information Available: Effect of temperature (Figure S1), Langmuir isotherm plot (Figure S2), Freundlich isotherm plot (Figure S3), van’t Hoff plot (Figure S4), regeneration activity of WMNLR (Figure S5), intraparticle diffusion parameter (Table S1), comparison of adsorption capacities of various adsorbents (Table S2), and literature cited 56-60 related to Table S2. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Pagga, U.; Taeger, T. Development of a method for adsorption of dyestuffs on activated sludge. Water Res. 1994, 28, 1051. (2) Reife, A. Othmer Encyclopedia of Chemical Technology; John Wiley & Sons, Inc.: New York, 1993. (3) Hawkes, F. R.; Lourenco, N. D.; Pinheiro, H. M.; Delee, W. Colour in textile effluents - sources, measurement, discharge consents and simulation: a review. J. Chem. Technol. Biotechnol. 1999, 74, 1009. (4) Stephenson, R. J.; Sheldon, J. B. Coagulation and precipitation of a mechanical pulping effluent-I. Removal of carbon, colour and turbidity. Water Res. 1996, 30, 781. (5) Chiou, M. S.; Chuang, G. S. Competitive adsorption of dye metanil yellow and RB15 in acid solutions on chemically cross-linked chitosan beads. Chemosphere 2006, 62, 731. (6) Salem, I. A.; El-maazawi, M. Kinetics and mechanism of color removal of methylene blue with hydrogen peroxide catalyzed by some supported alumina surfaces. Chemosphere 2000, 41, 1173. (7) Gupta, V. K.; Rastogi, A. Biosorption of lead from aqueous solutions by green algae Spirogyra species: Equilibrium and adsorption kinetics. J. Hazard. Mater. 2008, 152, 407. (8) Gupta, V. K.; Jain, R.; Varshney, S.; Saini, V. K. Removal of Reactofix Navy Blue 2 GFN from aqueous solutions using adsorption techniques. J. Colloid Interface Sci. 2007, 307, 326. (9) Gupta, V. K.; Jain, R.; Varshney, S. Electrochemical removal of hazardous Dye Reactofix Red 3 BFN from industrial effluents. J. Colloid Interface Sci. 2007, 312, 292. (10) Ali, I.; Gupta, V. K. Advances in water treatment by adsorption technology. Nat. Protoc. 2007, 1, 2661. (11) Gupta, V. K.; Suhas. Application of low cost adsorbents for dye removal-A review. J. EnViron. Manage. 2009, 90, 2313. (12) Gupta, V. K.; Carrott, P. J. M.; Ribeiro Carrott, M. M. L.; Suhas. Low cost adsorbents: Growing approach to wastewater treatment - A review. Crit. ReV. EnViron. Sci. Technol. 2009, 39, 783.

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ReceiVed for reView September 7, 2010 ReVised manuscript receiVed November 16, 2010 Accepted November 22, 2010 IE101866A