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Keywords:

  • Ni-Al LDH;
  • adsorption;
  • methyl orange dye;
  • isotherm

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. ACKNOWLEDGMENTS
  8. LITERATURE CITED

In the present work, Ni-Al layered double hydroxide (Ni-Al LDH) was synthesized by coprecipitation method from their nitrate salts and utilized as an adsorbent for the removal of methyl orange (MO) dye from its aqueous solution. The synthesized Ni-Al LDH was characterized using thermogravimetric analysis (TGA), X-ray diffraction (XRD), and N2 adsorption–desorption analysis. Batch adsorption isotherm experiments were conducted with methyl orange dye at three different temperatures (30, 40, and 50°C). Adsorption isotherm data were fitted with Langmuir, Freundlich, and Redlich–Peterson models. It was found that the Langmuir and Redlich–Peterson isotherm models best described the adsorption of MO on calcined Ni-Al LDH. The experimental results revealed that the increase in temperature increases the adsorption capacity of MO on calcined Ni-Al LDH adsorbent. The maximum adsorption capacity was found to be 5.7 × 10−4 mol g−1 at 50°C. The influence of pH on the adsorption of MO dye indicated that the adsorbent has good structural stability in the studied pH range. Thermodynamic studies authenticated that the adsorption of MO dye on calcined Ni-Al LDH was spontaneous, endothermic and entropy driven process. The results indicate that the calcined Ni-Al LDH can be employed as an adsorbent for removal of dye from aqueous solution. © 2013 American Institute of Chemical Engineers Environ Prog, 33: 154–159, 2014


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. ACKNOWLEDGMENTS
  8. LITERATURE CITED

Treatment of dye wastewater from aqueous system has been a technological challenge for many decades [1]. The dye effluents generated from various industries such as textile, plastic, pulp and paper, among others, can cause harmful effects on animals, human beings and aquatic lives [2]. Owing to stringent environment regulations, these dye stuff effluents must be treated to reduce their impact on the environment. Numerous treatment methods (physical, chemical, and biological) have been adopted for the treatment of dye wastewater [3]. Although every method has its own advantages and disadvantages, adsorption has been recognized to be one of the most promising and cost effective processes for treating dye wastewater. Among the various adsorbents, activated carbon has been found to be an effective and versatile adsorbent for the treatment of dye wastewater. However, the high price and difficulties involved in the regeneration of activated carbon forced the researchers to search for efficient and economic alternative adsorbents [3, 4].

Recently, Layered double hydroxides (LDH), also known as hydrotalcites, have been widely used as adsorbent for various applications due to their unique anion exchange capability. In addition, the spent adsorbent can be easily regenerated and its adsorption capacities are comparable with the fresh LDH adsorbents [5-7]. In general, LDH consists of positively charged metal hydroxide sheets with anions located between the layers to compensate the positive layer charges. The composition of LDH is generally represented by the following equation [7-10]:

  • display math(1)

where M2+ represents divalent cations ( inline image), M3+ represents trivalent cations ( inline image), inline image, represents inorganic or organic anions ( inline image), m is the number of interlayer water and inline image is the layer charge density of LDH.

Many adsorption experiments have been conducted on LDH adsorbents for the adsorption of organic chemicals (dye molecules, pesticides, and drugs) as well as adsorption of gases [7-10]. On the other hand, very few investigations have been focused on the adsorption of MO dye on LDH adsorbents [11-13]. To our best knowledge, adsorption of MO dye on calcined Ni-Al LDH adsorbent has not been investigated before.

In the present work, Ni-Al layered double hydroxide (Ni-Al LDH) is synthesized by coprecipitation method and its adsorbent potential is examined for the removal of methyl orange (MO) dye from its aqueous solution. The structural and thermal properties of the synthesized Ni-Al LDH are characterized by thermogravimetric (TG), X-ray diffraction (XRD), and nitrogen adsorption-desorption analysis. Adsorption of MO onto Ni-Al LDH is experimented by batch adsorption technique. Experiments are carried out at three different temperatures (30, 40, and 50°C) and the data are fitted with Langmuir, Freundlich, and Redlich–Peterson models to obtain the corresponding isotherm constants.

MATERIALS AND METHODS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. ACKNOWLEDGMENTS
  8. LITERATURE CITED

Materials

Nickel nitrate hexahydrate (Ni(NO3)2.6H2O), aluminum nitrate nonahydrate (Al(NO3)3.9H2O), sodium nitrate (NaNO3), methyl orange dye (C.I. 13025, C14H14N3NaO3S) and sodium hydroxide (NaOH) were procured from Merck India, Mumbai, India. All the chemicals were used as received. Water used in this work was obtained from the Millipore System.

Preparation of Ni-Al LDH

A facile coprecipitation route was employed for the synthesis of Ni-Al LDH. First, an aqueous solution containing Ni(NO3)2.6H2O, Al(NO3)3.9H2O and NaNO3 was prepared by taking the mole ratio of the nitrate salts as 2:1:2, respectively. With above prepared solution, 0.2M NaOH solution was added drop wise with vigorous stirring until the pH reaches to 10 ± 0.2. After which, the mixture was stirred at room temperature for 16 h. Finally, the obtained precipitate was filtered and washed thoroughly with Millipore water until the pH of the filtrate was neutral. Subsequently the material was dried in a vacuum oven and ground with a mortar and pestle to get particles smaller than 160 µm. Then, the above synthesized, pristine Ni-Al LDH sample was calcined at 500°C for 5 h in a muffle furnace (air atmosphere) with a heating rate of 2°C min−1 to use in adsorption study.

Characterization of Ni-Al LDH

Thermo gravimetric analysis (TGA) of the as-synthesized Ni-Al LDH was carried out in the Mettler Toledo thermo gravimetric analyzer (TGA/SDTA 851® model) under N2 atmosphere between 25 and 800°C with a heating rate of 10°C min−1. The X-ray diffraction (XRD) patterns of the Ni-Al LDHs were recorded using Bruker AXS instrument equipped with Cu Kα (λ = 1.5406 Å) radiation operating at 40 kV and 40 mA between 2θ ranging between 5 and 50° with a scan speed of 0.05° s−1. Nitrogen adsorption/desorption isotherm of the calcined Ni-Al LDH were measured at −196°C using Beckmen-Coulter surface area analyzer (SATM 3100 model). Before N2 adsorption/desorption analysis, the sample was degassed at 200°C for 2 h and the surface area was calculated using a multipoint Brunauer-Emmett-Teller (BET) model.

Adsorption Experiments

Calcined Ni-Al LDH was used as an adsorbent for the adsorption of MO dye. Batch equilibrium technique was followed for the adsorption isotherm studies. First, MO dye was dried at 100°C for 2 h to remove the moisture. A stock solution with concentration of 6.11 × 10−3 mol dm−3 (2000 ppm) was prepared and the experimental solutions of the desired concentration were obtained by successive dilutions of the stock solution. For adsorption isotherm experiments, 50 mL of MO dye solution of known initial concentrations ranging between 6.11 × 10−5 and 3.05 × 10−3 mol dm−3 (20 and 1000 ppm) were shaken with 0.05 g of adsorbent (calcined Ni-Al LDH) in an incubator shaker (Labtech®, Korea) at 150 rpm and at natural pH of the dye solution. The solution and solid phase were separated by centrifugation at 8000 rpm for 30 min in a centrifuge (Sigma Laborzentrifugen Gmbh, Model 4k15C). About 10 mL of the supernatant was collected without disturbing the centrifuged solution and the concentration of dye was determined by UV–vis spectrophotometer at wavelength of 472 nm. The amount of CV dye adsorbed at equilibrium was calculated by the following equation:

  • display math(2)

where qe is the amount of MO dye adsorbed at equilibrium (mol g−1), V is the volume of the solution (dm3), m is the mass of the adsorbent (g), C0 and Ce are the initial and equilibrium concentrations of the MO dye (mol dm−3), respectively. All data reported in this work were the averages of two or three experiments with relative error of about 5%.

RESULTS AND DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. ACKNOWLEDGMENTS
  8. LITERATURE CITED

Characterization of Ni-Al LDH Adsorbent

Thermogravimetric analysis of as-synthesized Ni-Al LDH is shown in Figure 1. Two regions of weight losses are observed during calcination of as-synthesized Ni-Al LDH. The first region of weight loss in the temperature range lower than 230°C is attributed to the removal of physisorbed and interlayer water molecules. The second region of weight loss in the temperature ranges between 230 and 500°C is ascribed to the release of the hydroxyl group from brucite layers and NO2 from the interlayer anions [14]. The DTG curve also suggests that the complete decomposition of as-synthesized LDH occurs at 345°C. There is no weight loss observed above 500°C. It implies that there is no phase change after 500°C and hence the calcination temperature is fixed at 500°C for the calcined Ni-Al LDH adsorbent.

image

Figure 1. Thermogravimetric analysis of Ni-Al LDH. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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The XRD pattern of Ni-Al LDH (as-synthesized and calcined) is shown in Figure 2. As-synthesized Ni-Al LDH displays a single phase corresponding to the layered double hydroxide with sharp asymmetric reflections at 003, 006, and 009 planes and broad asymmetric reflections at 015 and 018 planes. The position of the basal peak of Ni-Al LDH indicates the distance between two adjacent metal hydroxide sheets (d003). The d-spacing is calculated using Bragg's equation:

  • display math(3)

where λ is the X-ray wavelength (1.5418 Å). The calculated d-spacing value (from 2θ value of 11.42°) is 0.77 nm, which corresponds to the thickness of the brucite layers as well as the size of the anion and number of the water molecules existing in the interlayer [14]. During calcination, the as-synthesized Ni-Al LDH is converted to more crystalline phase (see Figure 2). The disappearance of the planes 003 and 006 suggests that the complete degradation of as-synthesized LDH and the formation of oxides of Ni and Al.

image

Figure 2. XRD pattern of Ni-Al LDHs. (a) As-synthesized Ni-Al LDH (b) Calcined Ni-Al LDH. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Nitrogen adsorption/desorption analysis of the calcined Ni-Al LDH measured at −196°C is depicted in Figure 3. The shape of the isotherm is type II according to IUPAC classification, which indicates the existence of spaces between particles of nanometric size that forms the intraparticle porosity. The narrow hysteresis loop of desorption corresponding to type H3 reveals that the pores are cylindrical in nature [15]. BET surface area and pore size of the calcined Ni-Al LDH estimated from the N2 adsorption-desorption analysis is found to be 205.76 m2 g−1 and 0.4139 mL g−1, respectively. This high surface area as compared to other LDHs suggests that the synthesized calcined Ni-Al LDH would be a suitable candidate for adsorption.

image

Figure 3. N2 adsorption/desorption isotherm of calcined Ni-Al LDH. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Adsorption Isotherms

Generally, adsorption isotherm with a specific adsorbate is carried out to estimate the adsorption characteristics. The adsorption isotherm indicates how the adsorbate molecules are distributed between the liquid phase and the solid phase when the adsorption process reaches an equilibrium state. The adsorption isotherms of MO dye on calcined Ni-Al LDH are measured at three different temperatures. The results are fitted with three important isotherm models such as Langmuir, Freundlich, and Redlich–Peterson to identify a suitable model to be used for design purposes. The models are fitted with nonlinear expression without linearization because linearization leads to an error (high or low correlation coefficient) value of correlation coefficients (R2), depending on in which form it is linearized. Consequently, it is difficult to predict the actual adsorption mechanism. Thus the nonlinear method is the best way to get the actual isotherm parameters [16].

The Langmuir adsorption is based on the assumption of monolayer adsorption on a structurally homogeneous adsorbent, where all the sorption sites are identical and energetically equivalent. The homogeneous Langmuir adsorption isotherm is represented by the following equation [17]:

  • display math(4)

where qe is the adsorbed amount of the dye at equilibrium (mol g−1), Ce is the equilibrium concentration of the dye in solution (mol dm−3), Qmax is the maximum adsorption capacity (mol g−1) and KL is the constant related to the free energy of adsorption (dm3 mol−1).

Freundlich adsorption isotherm is one of the most widely used empirical equations [18], which is derived by assuming a heterogeneous surface with a nonuniform distribution of heat of adsorption over the surface. This isotherm is suitable for a highly heterogeneous surface. The Freundlich isotherm is represented by the following equation:

  • display math(5)

where KF is the Freundlich isotherm constant (mol/g(dm3/mol)1/n), which is an indicative of the extent of adsorption (i.e., adsorption capacity) and 1/n is the adsorption intensity (dimensionless). The exponent 1/n is usually <1.0 because sites with the highest binding energies are utilized first, followed by weaker sites, and so on.

The three parameter Redlich–Peterson isotherm model [19] combines the features of both Freundlich and Langmuir isotherm equations. It approaches the Freundlich model at high concentrations and Langmuir model at low concentration. Redlich-Peterson isotherm can be described by the following equation:

  • display math(6)

where KRP and α are the Redlich–Peterson constants. “g” is the exponent which lies between 0 and 1 and can characterize the isotherm as if g = 1, the Langmuir will be the preferable isotherm, while if g = 0, the Freundlich will be the preferable isotherm. This model can describe the adsorption process over a wide range of concentrations.

All the three isotherm models are fitted with the experimental data as shown in Figures 4a–4d. Typical parameter values obtained by non-linear curve fitting are listed in Table 1. The adsorption of MO dyes on calcined Ni-Al LDH increases with an increase in the temperature (see Figures 4a–4c and Table 1). This displays that the interaction energy between the dye molecules and LDH is very strong with increase in the temperature. In addition, the increase in the temperature enhances the diffusion of the dye molecules into the internal and external pores of the adsorbents. This affects the adsorption equilibrium and consequently affects the total adsorption capacity. The maximum adsorption capacity of the calcined LDH is found to be 5.7 × 10−4 mol g−1 at 50°C. The correlation coefficients (R2) of both Langmuir and Redlich–Peterson models are greater than the Freundlich model at all temperatures representing monolayer coverage of Ni-Al LDH adsorbent with MO dye molecules. The R2 values of the three models decrease in the order of: Redlich–Peterson > Langmuir > Freundlich. This indicates that the equilibrium data are better fitted by the three-parameter model rather than the two-parameter models. However, the “g” values of the Redlich–Peterson model are close to unity suggesting that the calcined Ni-Al LDH contains more homogeneous surfaces, which might play a vital role in the adsorption of MO dye. Based on the above, it can be concluded that both Langmuir and Redlich–Peterson model could better describe the adsorption of MO on calcined LDH. The observed dye uptake by calcined Ni-Al LDH (obtained from the Langmuir parameters) at various temperatures are very close to the stoichiometric uptakes. The increase in the value of b (b = KLQmax) with increasing temperature hints the endothermic nature of the process. The decreased values of KL with increasing temperature signify the low energy requirement for MO dye adsorption.

Table 1. Adsorption isotherm parameters of methyl orange dye on calcined Ni-Al LDH adsorbent at different temperatures.
ParametersTemperature (°C)
304050
Langmuir model
Qmax (mol g−1)2.9 × 10−44.8 × 10−45.7 × 10−4
KL (dm3 mol−1)3.475 × 1032.273 × 1031.959 × 103
R20.9930.9880.966
Freundlich model
KF (mol g−1 (dm3 mol−1)1/n)1.88 × 10−35.04 × 10−35.61 × 10−3
1/n0.3200.4130.395
R20.9370.9650.959
Redlich–Peterson model
KRP (mol g−1)0.9490.9481.796
α ((dm3 mol−1)g)3.818 × 1031.883 × 1031.370 × 103
g0.9940.9900.858
R20.9940.9880.967
image

Figure 4. Adsorption isotherms of MO dye on calcined Ni-Al LDH. (a) Langmuir model (b) Freundlich model (c) Redlich-Peterson model (d) Comparison of adsorption models with experimental data. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Effect of pH

The effect of pH on the adsorption capacity of MO dye on calcined Ni-Al LDH is carried out at different pH ranging between 2 and 11 and the results are shown in Figure 5. It is seen that the percentage of adsorption decreases with increasing pH of the solution from 2 to 11. This result is consistent with the adsorption of MO on Zn-Al LDH [13]. In general, the surface of the calcined Ni-Al LDH contains large number of binding sites and positively charged at low pH due to the protonation reaction on the surface [13]. This increases the electrostatic attraction between the negatively charged MO dye and LDH, which results in increased adsorption at lower pH. Correspondingly the electrostatic attraction decreases with an increase in the pH resulting in the decreased percentage of adsorption. Furthermore, the increased percentage of adsorption at pH = 2 displays that the calcined Ni-Al LDH have good structural stability, i.e., the structure of the layered materials with hydroxide sheets is not destroyed at lower pH [13]. Hence, the prepared calcined Ni-Al LDH can be adopted for adsorption of dyes even for strong acidic conditions.

image

Figure 5. Effect of pH for the adsorption of MO dye on calcined Ni-Al LDH (Concentration = 3.05 × 10−3 mol dm−3; Adsorbent dosage = 0.05 g; Shaker speed = 150 rpm). [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

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Thermodynamic Parameters

Thermodynamic parameters of MO adsorption on Ni-Al LDH are determined, since both energy and entropy are the key factors to be considered in any process design. The Gibbs free energy change for the adsorption process is expressed by the following equation:

  • display math(7)

where ΔG0 is the change in free energy (kJ mol−1) and R is the gas constant (8.314 J mol−1 K−1). The change in enthalpy and entropy are related to the Gibbs free energy and are calculated based on the adsorption isotherms by the following equations:

  • display math(8)
  • display math(9)

where ΔH0 is the change in enthalpy (kJ mol−1), ΔS0 is the change in entropy (kJ mol−1 K−1), T is the absolute temperature (K), K1 and K2 are the Langmuir constants at T1 = 30°C and T2 = 50°C, respectively.

The thermodynamic parameters (ΔG0, ΔH0, and ΔS0) for the adsorption of MO on Ni-Al LDH are presented in Table 2. The obtained negative values of ΔG0 at different temperatures indicate that the adsorption of MO is thermodynamically feasible and spontaneous in nature. The positive value of ΔH0 confirms the endothermic nature of dye adsorption. This explains the increase of MO dye adsorption efficiency as the temperature increased. In general, ΔH0 value between 5 and 40 kJ mol−1 corresponds to physisorption mechanism and in the range between 40 and 800 kJ mol−1 corresponds to chemisorption mechanism [20]. In this work, the ΔH0 value is found to be 17.27 kJ mol−1 indicating that the adsorption seems to be physisorption. The higher positive value of ΔS0 characterizes the increased randomness at the solid-solution interface during the adsorption of MO on calcined Ni-Al LDH. The driving force for adsorption is entropy effect; the entropy contribution is greater than the free energy of adsorption.

Table 2. Adsorption parameters for the adsorption of methyl orange dye on Ni-Al LDH.
ΔG0 (kJ mol−1)ΔH0 (kJ mol−1)ΔS0 (J mol−1 K−1)
30°C40°C50°C
−20.54−19.72−20.7617.27120.24

Practical Implications

The calcined Ni-Al LDH adsorbent investigated in this work has demonstrated its effectiveness for the removal of MO dye, which highlights an optimistic outlook for the practical application of the adsorbent. The maximum dye sorption capacity of the Ni-Al LDH (5.70 × 10−4 mol g−1 at 50°C) is comparable with other type of LDH adsorbents (2.64 × 10−4 mol g−1 at 25°C for orange II dye on Mg4Al NO3 LDH; 2.33× 10−4 mol g−1 at 25°C for orange G dye on Mg-Fe LDH; 5.46 × 10−4 mol g−1 at 35°C for methyl orange on Zn-Al LDH) [10, 11, 13]. For real applications, at the end of the adsorption process, when the working capacity of an adsorbent is exhausted, the dye loaded adsorbent materials must be regenerated and reused allowing recovery of the dye. Adsorbents are generally regenerated with acid or alkali solution. To optimize the regeneration step, more works are essential. It is noteworthy to mention that the dye adsorbed LDH material may be used for other organics removal such as naphthalene, phenol which is quite attractive and aids the overall economics of the water treatment system [21]. Furthermore, to assess the application of this adsorbent at industrial scale and test its ability to handle shock-loads in practical situations, laboratory column and subsequent pilot scale studies are required. The performance of the adsorbent to remove dyes from real wastewater also needs to be examined during the pilot study. Finally, for application at the industrial level, a detailed cost-benefit-analysis and comparison of different processes with adsorption using this adsorbent must be worked out.

CONCLUSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. ACKNOWLEDGMENTS
  8. LITERATURE CITED

The adsorption potential of the calcined Ni-Al LDH adsorbent for methyl orange dye has been successfully investigated by batch adsorption experiments. The prepared Ni-Al LDH has been characterized with TGA, XRD, and BET surface area analysis. TGA and XRD analysis reveal that the calcination of Ni-Al LDH forms the oxides of nickel and alumina at 500°C. Higher surface area (205.76 m2 g−1) is obtained for the calcined Ni-Al LDH. Adsorption isotherm experiments discloses that the adsorption capacity of the adsorbent increases with an increase in the temperature. The maximum adsorption capacity of the Ni-Al LDH is found to be 5.7 × 10−4 mol g−1 at 50°C. The adsorption data are very well fitted with Langmuir and Redlich–Peterson model suggesting that the monolayer adsorption might be a dominant mechanism for adsorption. The adsorption of MO dye on calcined LDH is strongly dependent on pH of the solution. Thermodynamic parameters indicate that the adsorption is spontaneous and endothermic under the experimental conditions. This investigation suggests that the calcined Ni-Al LDH could be used as an alternate adsorbent for the treatment of MO dyes from aqueous solution.

ACKNOWLEDGMENTS

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. ACKNOWLEDGMENTS
  8. LITERATURE CITED

The authors are thankful to Center for Nanotechnology and Department of Chemistry, IIT Guwahati, for providing the instrumental facilities.

LITERATURE CITED

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. MATERIALS AND METHODS
  5. RESULTS AND DISCUSSION
  6. CONCLUSION
  7. ACKNOWLEDGMENTS
  8. LITERATURE CITED
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