Removal of Heavy Metal Ions Using Modified Celluloses Prepared from Pineapple Leaf Fiber.

Since large amounts of pineapple leaves are abandoned after harvest in agricultural areas, the possibility of developing value-added products from them is of interest. In this work, cellulose fiber was extracted from pineapple leaves and modified with ethylenediaminetetraacetic acid (EDTA) and carboxymethyl (CM) groups to produce Cell-EDTA and Cell-CM, respectively, which were then used as heavy metal ion adsorbents. A solution of either lead ion (Pb2+) or cadmium ion (Cd2+) was used as wastewater for the purpose of studying adsorption efficiencies. The adsorption efficiencies of Cell-EDTA and Cell-CM were significantly higher than those of the unmodified cellulose in the pH range 1-7. Maximum adsorptions toward Pb2+ and Cd2+ were, for Cell-EDTA, 41.2 and 33.2 mg g-1, respectively, and, for Cell-CM, 63.4 and 23.0 mg g-1, respectively. The adsorption behaviors of Cell-CM for Pb2+ and Cd2+ fitted well with a pseudo-first-order model, but those of Cell-EDTA for Pb2+ and Cd2+ fitted well with a pseudo-second-order model. All of the adsorption behaviors could be described using the Langmuir adsorption isotherm. Desorption studies of Pb2+ and Cd2+ on both adsorbents using 1 M HCl suggested that regenerability of Cell-EDTA was, for both adsorbates, better than that of Cell-CM. Moreover, adsorption measurements in a mixture of Pb2+ and Cd2+ at various ratios showed that for both adsorbents the adsorption of Pb2+ was higher than that of Cd2+, while the adsorption selectivity for Pb2+ of Cell-CM was greater than that of Cell-EDTA. This study showed that the modified cellulosic adsorbents made from pineapple leaves were able to efficiently adsorb metal ions.

Introduction

The use of biomass residues as feedstock for sustainable energy and material innovation development has become a challenging field of research. Intensively cultivated in Thailand and an important crop in terms of revenue, pineapple contributes a potential biomass feedstock. Very large amounts of pineapple leaf (PAL) are generated and subsequently abandoned after the fruit harvest. The leaves are not suitable for fuel production and cattle feed; hence, it is of great interest to find a use for PAL as a resource for the production of products with added value.

In general, dried PAL consists of α-cellulose (74.5–87.2%), hemi-cellulose (12.3–20.4%), lignin (3.46–8.7%), and other components (water, wax, oil, fat, pectin, tannin), with quantities depending on the type of pineapple and the location of the harvest area. Cellulose fiber is part of the cell wall of PAL consisting of d-glucose (C6H12O6) polymers involving a β 1-4 glycosidic bond, and it may be in crystalline and/or amorphous forms. Due to the unit linkage of d-glucose needed to form polymer chains and the hydrogen bonding within the molecule, cellulose fiber is naturally strong and tough. Normally, cellulose fiber extracted from PAL can be obtained using an acid solution (HCl or H2SO4) or alkaline solution (NaOH, known as a soda process) in the first step. After that, a bleaching process (NaClO or NaClO2) is carried out to eliminate residual lignin, and subsequently, the purified cellulose fiber is produced.[1−3] Cellulose fiber commonly contains a hydroxyl group, which can be reacted with numerous reagents to provide different, useful derivatives; these are used in various applications, including as reinforcement agent composites in materials,[4−7] biomedical engineering,[8,9] and adsorbents.[10−12]

Heavy metal contamination in the environment is a serious concern worldwide. The ions of those metals are highly toxic to the environment, as well as to animals and humans, in the event that the ions contaminate water resources.[10,13−15] Therefore, having effective technology to treat these metal ions is vital. Adsorption using an adsorbent is one of the most effective methods to reduce the number of metal ions in water due to the fact that the process is relatively simple, low in cost, and highly efficient. More importantly, advantages increase when the adsorbent can be regenerated[16,17] and disposed after use.[17,18]

Several papers have reported on the modification of cellulose extracted from different natural resources—such as wood sawdust,[19] corncob,[20] orange peel,[21,22] and sugarcane bagasse[23−25]—for use as metal ion adsorbents. Due to the low adsorption efficiency of the hydroxyl group of the extracted cellulose, this group must be modified before it can be used in practice. Examples of functional groups bonded to extracted cellulose to increase its adsorption efficiency include carboxyl,[26] amidoxime,[27,28] amide,[29] ethylenediaminetetraacetic acid (EDTA),[18,25] sulfur as an anionic ligand,[30] and triethylenetetramine.[31,32] Common metal ions that are studied include Pb, Cd, Cu, Zn, and Ni.[16,18,30,33−36] It was found that the adsorption efficiencies of these modified celluloses were clearly higher than those of the unmodified cellulose.

The elimination of Pb2+ and Cd2+ from wastewater or natural water has been of particular interest in many studies,[37,38] because these two heavy metal ions can have severe adverse effects on human health and the environment, as stated earlier. Indeed, a number of studies have reported on the use of different modified cellulosic adsorbents for the adsorption of Pb2+ and Cd2+. Gurgel et al. used mercerized sugarcane bagasse with succinic anhydride for removal of Pb2+ and Cd2+.[24] The authors demonstrated that the adsorption capacity of modified mercerized sugarcane bagasse exhibited an increase for both Pb2+ (83 mg g–1) and Cd2+ (44 mg g–1) relative to modified nonmercerized sugarcane bagasse; this result was due to the increase in the number of introduced carboxylic acid groups. Furthermore, another research group modified mercerized sugarcane bagasse with ethylenediaminetetraacetic dianhydride (EDTAD), achieving the highest adsorption capacities of 333 and 149 mg g–1 for Pb2+ and Cd2+, respectively. EDTAD was also used by d’Halluin et al. to modify cellulose fiber paper.[18] Their modified cellulosic adsorbent was very robust and had high hydrophilicity, disposability, and low toxicity. The adsorption efficiencies obtained were 227 and 102 mg g–1 for Pb2+ and Cd2+, respectively.

Considering the high adsorption efficiency, wide availability, high disposability and efficiency, and inexpensive methodology associated with modified cellulosic adsorbents prepared from modifications of cellulose with either carboxylic acid or EDTA group, the current study, therefore, used PAL as a new source of cellulose for developing modified cellulosic adsorbents for removal of Pb2+ and Cd2+. These modified celluloses were used to study the adsorption of Pb2+ and/or Cd2+ in both single and binary metal ion systems. The regenerability of the adsorbents was also investigated using a solution of 1 M HCl. Moreover, adsorption isotherms and adsorption kinetic models (pseudo-first-order and pseudo-second-order models) were used to investigate the behavior of the adsorbent–adsorbate system.

Results and Discussion

PAL Composition

Compositions of the PAL determined using the TAPPI standard method are presented in Figure . It was found that the total amounts of extractives (fat, wax, oil, tannin, pectin) in ethanol + benzene, ethanol, and hot water were about 40%. Moreover, the amount of α-cellulose (35.4 ± 5.4%) was greater than that of hemi-cellulose (16.6 ± 0.4%). This high amount of α-cellulose relative to that of hemi-cellulose can be advantageous when using PAL as a natural source for turning it into adsorbents. The remaining constituents consisted of lignin and ash (about 16% in total). Moreover, although the amount of α-cellulose seemed to be too low for economic utilization, the use of α-cellulose in this work can still be considered to demonstrate and develop a good methodology of adsorption studies for academic and industrial purposes.

Figure 1

Compositions of PAL determined using the TAPPI standard test method. Note that holo-cellulose (44.60%) is the combination of α-cellulose and hemi-cellulose.

Modifications of the Extracted Cellulose

The extracted cellulose products before and after modification with the EDTA or carboxymethyl (CM) group were characterized using Fourier transform infrared (FT-IR), as shown in Figure . For the extracted cellulose, the observed broad peak around 3600–3200 cm–1 was assigned to the −OH stretching vibration. The band at 2890 cm–1, meanwhile, belonged to the stretching vibrational mode of C–H, while the bands at 1428, 1364, 1320, 1025, and 896 cm–1 were ascribed to the stretching and bending vibrations of the −CH2, −CH, −OH, and C–O bonds in the cellulose backbone.[39] For Cell-EDTA, a new and strong adsorption peak at a wavelength of 1588 cm–1 appeared, which was an indication of the carboxyl group (O=C–O– stretching).[40,41] In addition, there was a stronger adsorption peak at around 1400–1300 cm–1, which was an indication of the C–N stretching vibration of the EDTA group.[41] This suggested that the EDTA group had been successfully substituted onto the cellulose backbone during the modification. In the spectra of Cell-CM, two new observed peaks at wavelengths of 1727 and 1639 cm–1 appeared; the former band was assigned to the C=O stretching of the carboxyl group,[42] while the latter was assigned to the adsorption of H2O onto Cell-CM.[43,44] The bands around 1450–1300 cm–1, ascribed to the vibrations of C–O and −OH of the carboxyl group, overlapped with the bands of the −CH2, −CH, −OH, and C–O bonds in the cellulose backbone. Nonetheless, this suggested that the −OH groups of the extracted cellulose could be successfully substituted with the carboxymethyl group.

Figure 2

FT-IR spectra of extracted cellulose, Cell-EDTA, and Cell-CM.

In addition, the physical morphologies and the elemental compositions of the extracted cellulose, Cell-CM, and Cell-EDTA were examined using scanning electron microscopy (SEM) with energy-dispersive spectrometry (EDS); see Figure . All three samples had a long fiber structure with an average diameter of 5–8 μm. The EDS analysis showed that the surface of each sample of cellulose mainly consisted of the cellulose backbone (C and O). The EDS spectra of the surfaces of Cell-EDTA and Cell-CM revealed the existence of N atoms on the former and Na atoms on the latter. Both the FT-IR and EDS results verified that there had been successful modifications of the extracted cellulose with the desired functional groups, EDTA and carboxymethyl.

Figure 3

SEM images with elemental compositions of extracted cellulose, Cell-EDTA, and Cell-CM.

Adsorption Studies of Extracted Cellulose, Cell-EDTA, and Cell-CM with Pb2+ and Cd2+

EDS analyses after adsorption of Pb2+ and Cd2+ are presented in Figure . A peak for Pb or Cd appeared on the spectra of the extracted cellulose, Cell-EDTA, and Cell-CM after performing the metal ion adsorption, confirming that Pb2+ or Cd2+ was adsorbed on the surface of the samples. Moreover, the results demonstrated that the adsorption mechanism of Cell-CM was ion exchange between the Na+ of Cell-CM and Pb2+ or Cd2+ from the solution,[45] as indicated by the disappearance of Na+ atoms (Figure , Cell-CM/Pb and Cell-CM/Cd). For Cell-EDTA, the adsorption mechanism was covalent coordination, which happens due to the ability of the EDTA group to adsorb metal ions by chemisorption.[18]

Figure 4

Elemental compositions after adsorption of Pb or Cd using extracted cellulose, Cell-EDTA, and Cell-CM. Conditions for adsorption: concentration of Pb2+ or Cd2+ = 100 mg L–1, pH = 6, at room temperature for 90 min.

To further characterize the samples after adsorption with Pb2+ or Cd2+, they were analyzed by FT-IR, and the spectra were compared with those before adsorption, as presented in Figure . For the extracted cellulose (Figure a), all of the bands of the samples after adsorption were very similar to those before adsorption, which might be due to the very low adsorption of the metal ions by the extracted cellulose. For Cell-EDTA (Figure b) and Cell-CM (Figure c), it was clearly observed that the band indicating the O=C–O– stretching of the EDTA group (1588 cm–1) and the band indicating the C=O stretching of the carboxyl group (1727 cm–1) almost disappeared, which suggested that the EDTA or carboxymethyl group participated in the coordination.[46] The characteristic absorption bands of Pb–O and Cd–O are normally observed at around 700–400 cm–1,[47,48] but they overlapped with the absorption bands of the adsorbents; thus, identifying them was difficult.

Figure 5

FT-IR spectra of (a) extracted cellulose, (b) Cell-EDTA, and (c) Cell-CM before and after the adsorption of Pb2+ or Cd2+. Conditions for adsorption: concentration of Pb2+ or Cd2+ = 100 mg L–1, pH = 6, at room temperature for 90 min.

Effect of pH on the Adsorption Efficiency of Cellulose, Cell-EDTA, and Cell-CM

The effect of the pH of the metal ion solutions on the adsorption efficiency of the modified celluloses was studied, and a comparison was made with the case of the extracted cellulose (Figure ). The pH values for the study were varied in the range of 1–7. It can be clearly seen that the optimal adsorption efficiencies of the extracted cellulose, Cell-EDTA, and Cell-CM fell in the pH range 5–7. Specifically, the maximum adsorption efficiencies of Cell-EDTA were about 40 mg g–1 for Pb2+ and 33 mg g–1 for Cd2+, while the maximum adsorption efficiencies of Cell-CM were about 63 mg g–1 for Pb2+ and 23 mg g–1 for Cd2+. Meanwhile, at approximately 7 mg g–1 for Pb2+ and 4 mg g–1 for Cd2+, the maximum adsorption efficiencies of the extracted cellulose were much lower than those of Cell-EDTA and Cell-CM. For pH values from 1 to 3, the adsorption efficiencies of all three samples decreased with reducing pH values, owing to the change in the number of protons in the solution. When the number of protons increased (by lowering the pH), the metal ions had to compete with protonation of the carboxyl and carboxylate groups, and this increased the positive surface charge of the adsorbent.[18,27,49] At higher pH values, varying pH had a minimal effect on efficiency (and thus a pH of 6 was preferred in later tests). Note that the adsorption measurement could not be made with metal ion solutions above pH 7 due to the precipitation of the hydroxide forms of the metals.

Figure 6

Adsorption efficiencies (qe) of extracted cellulose, Cell-EDTA, and Cell-CM for (a) Pb2+ and (b) Cd2+ in the pH range of 1–7. Conditions for adsorption: concentration of Pb2+ or Cd2+ = 100 mg L–1, at room temperature for 90 min.

Although these two adsorbents cannot, in general, be compared because the adsorption mechanisms of the adsorbent–adsorbate system are different, this work will still discuss the specific adsorption mechanism of each system and compare, in relative terms, the adsorption efficiency as of a matter of interest.

It was interesting to see that the adsorption efficiencies of Cell-CM for Pb2+ were considerably higher than those of Cell-CM for Cd2+, while the adsorption efficiencies of Cell-EDTA for Pb2+ were only slightly higher than those of Cell-EDTA for Cd2+. These differences were due to the influence of the electronegativity of each metal on the formation of a chemical bond with each adsorbent, which can be explained as follows. (i) The adsorption on Cell-CM occurs by an ionic bond; thus, a higher electronegativity of a metal results in a better exchange with a sodium ion, thereby forming a better ionic bond. In other words, Pb2+ has a much higher adsorption efficiency relative to Cd2+ since the electronegativity of Pb (2.33) is much greater than that of Cd (1.69). (ii) The adsorption on Cell-EDTA occurs by covalent coordinate bonding (a shared pair of electrons); thus, a metal that has a higher electronegativity (Pb) is able to share electrons with the EDTA group better than a metal that has a lower electronegativity (Cd). Furthermore, in comparisons using the same adsorbate, the adsorption efficiency of Cell-CM for Pb2+ was higher than that of Cell-EDTA for Pb2+, whereas the adsorption efficiency of Cell-CM for Cd2+ was lower than that of Cell-EDTA for Cd2+. This may have been due to the fact that the influence of the adsorption by ionic bonding was dominant when the adsorbate (Pb) had a very high electronegativity. Nonetheless, adsorption by the covalent coordinate bonding became dominant when the adsorbate (Cd) had a low electronegativity. The adsorption efficiencies of the extracted cellulose were invariably much lower than those of Cell-CM and Cell-EDTA, owing to the low adsorption efficiency of the hydroxyl group.

Effect of Contact Time on Metal Ion Adsorption of Cellulose, Cell-EDTA, and Cell-CM

The time required for adsorption of metal ions is an important factor requiring evaluation in a study of the adsorption process. Figure shows plots of the amounts of Pb2+ or Cd2+ adsorbed onto the extracted cellulose, Cell-EDTA, and Cell-CM within 160 min; from these results, it is clear that the adsorption efficiencies of the two modified celluloses were significantly greater than that of the unmodified cellulose. It should be noted that the experiments in this section were carried out at pH 6, with this value being chosen based on the optimal range of values for pH in Section . The maximum adsorption efficiencies of the extracted cellulose for Pb2+ and Cd2+ were the lowest among the samples, at 5.6 and 4.1 mg g–1, respectively. For Cell-EDTA, the maximum adsorption efficiencies for Pb2+ and Cd2+ were 41.2 and 33.2 mg g–1, respectively. Meanwhile, the adsorption efficiency values of Cell-CM for Pb2+ and Cd2+ were 63.4 and 23.0 mg g–1, respectively. In other words, both Cell-EDTA and Cell-CM gave higher adsorption efficiency for Pb2+ than that for Cd2+. This is certainly because the electronegativity of Pb (2.33) is greater than that of Cd (1.69), so the formation of bonds between Pb2+ and the EDTA or CM group is more efficient. In Figure , it can also be seen that for both Pb2+ and Cd2+, Cell-CM reached its maximum adsorption efficiency much faster than Cell-EDTA (within about 13 min); in contrast, the adsorption efficiency of Cell-EDTA gradually increased with time and, for both Pb2+ and Cd2+, reached equilibrium within about 90 min. These results can perhaps be explained by the formation of ionic bonds being faster than the formation of covalent coordinate bonds. In other words, the covalent coordinate bonding involves a bond breakage and a bond formation that can slow down the entire reaction, while the ionic bond formation involves only an exchange of ions.

Figure 7

Effect of contact time on metal ion adsorption of (a) Pb2+ and (b) Cd2+ onto extracted cellulose, Cell-EDTA, and Cell-CM. Conditions for adsorption: concentration of Pb2+ or Cd2+ = 100 mg L–1 at pH 6 and room temperature.

It can also be seen in Figure a,b that the adsorption efficiency of Cell-CM as regards Pb2+ was significantly higher than that as regards Cd2+ (63.4 mg g–1 for Pb2+ and 23.0 mg g–1 for Cd2+), while the adsorption efficiency of Cell-EDTA as regards Pb2+ and Cd2+ was slightly different (41.2 mg g–1 for Pb2+ and 33.2 mg g–1 for Cd2+). The former result can be explained by the fact that because the adsorption of the metal ions onto Cell-CM occurs by ionic bonding, the electronegativity of each metal becomes the dominant cause of adsorption. That is, because the electronegativity of Pb (2.33) is greater than that of Cd (1.69), the adsorption efficiency of Cell-CM as regards Pb2+ is much higher than that regarding Cd2+. However, in the case of Cell-EDTA, adsorption of the metal ions occurs by covalent bonding; the latter observation (regarding Cell-EDTA) can be explained on the basis of calculating the energy required to form a chemical bond between the adsorbent and the adsorbate (heat of formation). As no data have been collected for the values of the heat formation of Cell-EDTA-Pb and Cell-EDTA-Cd, the values for the heat formation of PdO (−51.72 kcal mol–1) and CdO (−62.35 kcal mol–1)[50] might be used to explain this behavior instead; such an approach suggests that the adsorption efficiency of Cell-EDTA for Pb2+ is indeed greater than that of Cell-EDTA for Cd2+. Moreover, the adsorption efficiencies of Cell-EDTA that would be estimated based on the heat of formation with these two metal ions did not appear to be significantly different, which is in good agreement with the observations regarding actual efficiencies.

Another interesting result observed in both Figure a,b was that the adsorption efficiencies of Cell-CM for Pb2+ were much greater than those of Cell-EDTA for Pb2+ (Figure a). In contrast, the adsorption efficiencies of Cell-CM as regards Cd2+ were, in the equilibrium state, slightly lower than those of Cell-EDTA as regards Cd2+ (Figure b). For the case where the metal ion or adsorbate possesses high electronegativity (Figure a), effects of bond formation occurring by ionic bonding, which requires less energy, could be dominant. Consequently, the adsorption of Pb2+ on Cell-CM is more effective than that on Cell-EDTA. Meanwhile, for the case where the adsorbate possesses low electronegativity (Figure b), a complicated system, which cannot be easily described, becomes dominant: many factors have an effect on the adsorption efficiency, such as the number of active sites per adsorbate, the specific interaction between the adsorbate and each adsorbent, and the diffusion of the adsorbate in each adsorbent medium.[51]

Kinetic Models for Metal Ion Adsorption of Cell-EDTA and Cell-CM

Data from the previous sections can now be used to predict the kinetic behavior of adsorption. In general, the linear forms of a pseudo-first-order model and a pseudo-second-order model can be expressed using eqs and 2, respectively[52]where q1 and q2 are the number of adsorbed metal ions (mg g–1) at equilibrium, q is the number of adsorbed metal ions (mg g–1) at time t (min), and k1 and k2 are the rate constants.

The average experimental values of q at time t of Pb2+ and Cd2+ onto Cell-EDTA and Cell-CM from Figure were re-plotted with nonlinear fitting curves using the expressions of pseudo-first- and pseudo-second-order models, as shown in Figure . The kinetic parameters (q1, q2, k1, and k2) and the statistical information (coefficient of determination (R2), adjusted determination coefficient (Radj2), sum of squared errors (SSE),[53] and Akaike’s information criterion (AIC)[54]) were also collected, as shown in Table . The formulas of SSE and AIC are shown in eqs and 4, respectivelywhere N is the number of data points. q and q are the number of adsorbed metal ions at the data point i; they are obtained from the calculation regarding the nonlinear fitting and the experiment, respectively. Np is the number of parameters in the model.

Figure 8

Plots of nonlinear pseudo-first-order and nonlinear pseudo-second-order kinetic models for the adsorption of Pb2+ and Cd2+ onto (a) Cell-EDTA and (b) Cell-CM.

Table 1

Values of Parameters in the Pseudo-First- and Pseudo-Second-Order Models for Cell-EDTA and Cell-CM

	adsorbate
adsorbent, model	Pb2+	Cd2+
Cell-EDTA, pseudo-first order
q1 (mg g–1)	40.26	32.69
k1 (min–1)	0.06	0.05
R2	0.9930	0.9886
Radj2	0.9924	0.9870
SSE (%)	15.56	11.46
AIC	13.12	15.174
Cell-CM, pseudo-first order
q1 (mg g–1)	63.22	22.91
k1 (min–1)	0.52	0.39
R2	0.9982	0.9982
Radj2	0.9981	0.9980
SSE (%)	6.55	1.15
AIC	1.01	–16.10
Cell-EDTA, pseudo-second order
q2 (mg g–1)	45.90	37.43
k2 (min–1)	7950.35	3499.21
R2	0.9956	0.9967
Radj2	0.9520	0.9962
SSE (%)	9.84	3.26
AIC	6.70	3.87
Cell-CM, pseudo-second order
q2 (mg g–1)	64.92	24.08
k2 (min–1)	409 188.77	9162.05
R2	0.9934	0.9820
Radj2	0.9929	0.9802
SSE (%)	24.44	11.19
AIC	19.43	11.16

The important information obtained from Figure and Table are as follows. (i) The values of q1 and q2 represent the maximum adsorption efficiency of each adsorbent–adsorbate system under the performed conditions. All values of q2 were slightly greater than those of q1. (ii) The values of R2 and Radj2 cannot be used to determine with certainty which model is the best linear fit with the experimental data.[55] The values of SSE and AIC have to be considered for making the final decision. According to SSE and AIC conventions, the model that has a SSE value closer to zero and a smaller AIC value is the best fit with experimental data. It can be seen from the results that for the adsorption of Pb2+ and Cd2+ onto Cell-EDTA both the SSE and AIC values of the pseudo-first-order model were significantly greater than those of the pseudo-second-order model, indicating that the pseudo-second-order model fitted better with the experimental data. (iii) For adsorption of Pb2+ and Cd2+ onto Cell-CM, both the SSE and AIC values of the pseudo-first-order model was meaningfully smaller than those of the pseudo-second-order model. This suggests that the pseudo-first-order model fits better with the experimental data here. The fact that Cell-EDTA and Cell-CM follow different models may be a result of either the difference in adsorbents or the concentration of the adsorbate.[56]

Isotherm of Metal Ion Adsorption of Cell-EDTA and Cell-CM

The adsorption behaviors of Cell-EDTA and Cell-CM were investigated by plotting data fit to both the Langmuir isotherm and the Freundlich isotherm, as presented in Figure . In general, the Langmuir isotherm is used to describe the adsorption behavior of a chemisorption monolayer at the surface of an adsorbent[57] and can be expressed using eq (52)where qe is the number of adsorbed metal ions at equilibrium (mg g–1), qm is the maximum adsorption capacity of the adsorbent (mg g–1), Ce is the concentration of metal ions in solution at equilibrium (mg L–1), and kL (L mg–1) is the Langmuir constant.

Figure 9

Equilibrium curves for the adsorption of Pb2+ and Cd2+ onto (a) Cell-EDTA and (b) Cell-CM. Conditions for adsorption: concentration of Pb2+ or Cd2+ in the range of 30–160 mg L–1 at pH 6 and room temperature.

The Freundlich isotherm, meanwhile, is used to describe the adsorption behavior on heterogeneous adsorbent surfaces, assuming that multilayer adsorption takes place on a heterogeneous surface. Equation gives the linearized Freundlich isotherm model[52,58]where qe (mg g–1) is the adsorption capacity at equilibrium, Ce is the concentration of metal in solution at equilibrium (mg L–1), and kF and 1/nF are the empirical Freundlich constants associated with the capacity and intensity of adsorption, respectively.

A full list of the adsorption parameters and the statistical values (SSE and AIC) associated with the Langmuir and Freundlich isotherms can be found in Table . The SSE and AIC values were used to determine which adsorption model had a better goodness fit with the experimental data. These values were established based on nonlinear fitting analysis. It can be seen that for both Cell-EDTA and Cell-CM and for both metal ions the R2 and Radj2 values associated with the Langmuir isotherms are greater than those associated with the Freundlich isotherms and the SSE and AIC values associated with the Langmuir isotherms are significantly smaller than those associated with the Freundlich isotherms. These data clearly indicate that the Langmuir isotherm is more effective at describing the adsorption behaviors of both Cell-EDTA and Cell-CM. This confirms that the adsorption mechanism for both Cell-EDTA and Cell-CM was chemisorption,[18,59] meaning that a monolayer of each adsorbate formed on the surface of the adsorbents.

Table 2

Values of Parameters in the Langmuir and Freundlich Isotherms for Cell-EDTA and Cell-CM

	adsorbate
adsorbent, model	Pb2+	Cd2+
Cell-EDTA, Langmuir
qm (mg g–1)	63.92	48.02
kL (L mg–1)	0.01	0.02
R2	0.9976	0.9909
Radj2	0.9972	0.9898
SSE	4.41	14.13
AIC	8.84	9.17
Cell-EDTA, Freundlich
kF (mg g–1)(mg L–1)−1/n	2.80	2.90
1/nF	0.55	0.51
R2	0.9902	0.9597
Radj2	0.9885	0.9546
SSE	17.99	62.80
AIC	20.08	30.94
Cell-CM, Langmuir
qm (mg g–1)	155.06	37.26
kL (L mg–1)	0.01	0.01
R2	0.9775	0.9746
Radj2	0.9747	0.9714
SSE	210.04	20.36
AIC	43.02	19.68
Cell-CM, Freundlich
kF (mg g–1)(mg L–1)−1/n	3.5619	1.46
1/nF	0.67	0.55
R2	0.9591	0.9618
Radj2	0.9540	0.9571
SSE	382.24	30.55
AIC	49.00	23.74

Desorption Studies of Cell-EDTA and Cell-CM

The ability to desorb metal ions from an adsorbent is an important factor in determining the lifetime of the adsorbent. Thus, regenerability of Cell-EDTA and Cell-CM was investigated. In general, various desorbing or regenerating agents—such as acids (HCl, HNO3, and H2SO4), bases (NaOH), and chloride salts (NaCl, KCl, and CaCl2)—can be used for regeneration of adsorbents. In this study, 1 M HCl was selected to study the regenerability of these two adsorbents since it has a high desorption efficiency and is widely used.[60] First, each adsorbent was used to adsorb 100 mg L–1 of Pb2+ or Cd2+ solution at pH 6. Second, each adsorbed adsorbent was regenerated using 1 M HCl solution. Finally, the adsorbent was washed with DI water until a neutral pH was reached. These steps were repeated for five cycles. The regenerability of each adsorbent was determined using eq where i is the cycle number of the experiment. The results are plotted in Figure and show that the regenerability values for Cell-EDTA were in the range of 96–94% for Pb2+ and 98–95% for Cd2+—both of which were higher than those for Cell-CM, which were in the range of 43–41% for Cd2+ and 62–44% for Pb2+. The adsorption efficiencies of Cell-EDTA remained high at 40.5 mg g–1 for Pb2+ and 31.8 mg g–1 for Cd2+ for the fifth cycle (versus 41.2 mg g–1 for Pb2+ and 33.2 mg g–1 for Cd2+ for the first cycle), while the adsorption efficiencies of Cell-CM dropped significantly to 43.5 mg g–1 of Pb2+ and 15.6 mg g–1 of Cd2+ by the fifth cycle (starting from 63.4 mg g–1 for Pb2+ and 23.0 mg g–1 for Cd2+ for the first cycle). This indicated that the desorption efficiency of each cycle for Cell-EDTA or the regenerability of Cell-EDTA was significantly better than that of Cell-CM. This is due to the fact that Cell-EDTA captures each metal ion using covalent bonding, which is weaker than the ionic bonding used with Cell-CM, and thus the desorption process between Cell-EDTA and the adsorbates occurs more easily. Another point that was observed when comparing the regenerabilities of the two adsorbates using the same adsorbent was that the values of Cd2+ were slightly higher than those of Pb2+. This could have been because the Pb–adsorbent bond strength was greater than the Cd–adsorbent bond strength.

Figure 10

Regenerability of Cell-EDTA and Cell-CM for five cycles: (a) Cd2+ and (b) Pb2+ solutions. Conditions for adsorption: concentration of Pb2+ or Cd2+ = 100 mg L–1, pH = 6, at room temperature for 30 min. Conditions for desorption: 50 mL of 1 M HCl, at room temperature for 30 min.

Adsorption Studies of Cell-EDTA and Cell-CM in a Mixture of Pb2+ and Cd2+

Cell-EDTA and Cell-CM were studied using a binary adsorption system, as shown in Figures and 12, respectively. These experiments were employed to understand the adsorption behavior of the adsorbates in the system and determine the adsorption ability of the adsorbates. The solutions used for the study were a mixture of Pb2+ and Cd2+ at various total metal ion concentrations of Pb2+/Cd2+ (see Table S1). It was found that both adsorbents adsorbed Pb2+ more efficiently than Cd2+. For both adsorbents, this behavior can be explained with reference to the covalent index,[61] given in eq for Cell-EDTA, and using the effect of ionic bond formation for Cell-CM.where En is the electronegativity value and r is the atomic radius in nanometers. The (En, r) values of Pb and Cd are (2.33, 0.180 nm) and (1.69, 0.158 nm), respectively. Thus, the covalent indices of Pb2+ and Cd2+ were determined to be 0.977 and 0.451 nm, respectively, indicating that the ability of Pb2+ to bond with both adsorbents was greater than that of Cd2+. This study suggests that in an adsorption system, where various metal ions are present and one of these two adsorbents is used, the order of the adsorption ability from the greatest to the lowest in the adsorption system will be the same as the order of the covalent index value of each metal from the greatest to the smallest.

Figure 11

Adsorption study of Cell-EDTA for a solution mixture of Pb2+ and Cd2+: Contour plots of (a) Pb2+ adsorbed and (b) Cd2+ adsorbed versus Pb2+ + Cd2+ concentration. Conditions for adsorption: pH = 6, at room temperature for 90 min.

Figure 12

Adsorption study of Cell-CM for a solution mixture of Pb2+ and Cd2+: Contour plots of (a) Pb2+ adsorbed and (b) Cd2+ adsorbed versus Pb2+ + Cd2+ concentration. Conditions for adsorption: pH = 6, at room temperature for 90 min.

For Cell-CM, the ability of the adsorbent for metal ion adsorption depends on the ability to form ionic bonding between the carboxyl group and each metal ion. This ability is also governed by the En value of each metal. The results of the adsorption abilities of Pb2+ and Cd2+ by Cell-CM presented in Figure indicate that the qm value of Pb2+ is much greater than that of Cd2+, which is in good agreement with their respective En values. Moreover, the adsorption ability of Pb2+ onto Cell-CM (Figure a) is greater than that of Pb2+ onto Cell-EDTA (Figure a). This is probably because ionic bond formation takes place much more quickly than covalent bonding. In other words, Cd2+ was able to compete with Pb2+ for only a relatively short time for adsorption onto Cell-CM, unlike in the case of Cell-EDTA, where the time to reach equilibrium was much longer, meaning that Cd2+ ions had a greater opportunity to compete with Pb2+ ions; hence, the difference in selectivity between Pb2+ and Cd2+ was smaller for Cell-EDTA than for Cell-CM.

Conclusions

Using a soda or alkaline process, α-cellulose fiber was extracted from PAL following which it was modified with either EDTA or carboxymethyl groups to produce Cell-EDTA or Cell-CM, respectively. These two modified celluloses were successfully prepared and used as adsorbents for adsorption studies of heavy metal ions (Pb2+ and Cd2+) in single and binary adsorption systems. It was found that Cell-EDTA had maximum adsorption for Pb2+ and Cd2+ at 41.2 and 33.2 mg g–1, respectively, while Cell-CM gave maximum values of 63.4 and 23.0 mg g–1. All of these values were much greater than those of the unmodified cellulose. Comparing the adsorption values (found in this work) with other adsorbents reported in the literature (see Table S1), it can be seen that both of the modified celluloses are excellent adsorbents that are efficient for the removal of Pb2+ and Cd2+. However, these two adsorbents possess lower adsorption efficiencies when compared to some other adsorbents that have the same functional group, potentially due to the differences in the number of active sites per weight of the adsorbents. In the study of adsorption time, it was found that for both Pb2+ and Cd2+ solutions the time required to reach adsorption equilibrium was much longer for Cell-EDTA than for Cell-CM. This is due to the facts that the adsorption process occurs via covalent coordinate bonding for Cell-EDTA and that this is naturally slower than the ionic bonding associated with Cell-CM. However, compared to Cell-CM, Cell-EDTA was much more effective in terms of regenerability using 1 M of HCl solution. In the kinetic model study, a pseudo-first-order model described well the adsorption behaviors of Pb2+ and Cd2+ onto Cell-CM. By contrast, a pseudo-second-order model fitted better the adsorption behaviors of Pb2+ and Cd2+ onto Cell-EDTA. Additionally, the adsorption behavior for each adsorbate onto each adsorbent was well described using the Langmuir isotherm, since the adsorption behaviors of both adsorbents involved monolayer chemisorption. In the study of a binary adsorption system, the amounts of Pb2+ adsorbed onto both Cell-EDTA and Cell-CM were greater than those of Cd2+, and Cell-CM was more selective as regards Pb2+ adsorption than Cell-EDTA. This study showed that cellulose fiber can be extracted from pineapple leaves and can be modified into heavy metal ion adsorbents.

Experimental Section

Determination of PAL Composition

The composition of the extracted cellulose fiber—ethanol–benzene extractives, ethanol extractives, hot water extractives, holo-cellulose, hemi-cellulose, α-cellulose, lignin, and ash—were analyzed using the TAPPI standard test methods (see the Supporting Information for details).

Extraction of Cellulose from PAL

The leaves of pineapple (Smooth Cayenne) were collected from a pineapple farm in Bangsaphan, Prachuap Khiri Khan province, Thailand. After being dried overnight in an air oven at 70 °C, the dried PALs were blended using a mechanical blending machine. PAL fiber was then extracted from the blended sample (50 g) using 500 mL of 10% sodium hydroxide (NaOH, 98%, Carlo Erbra) at 100 °C for 1 h. The extracted fiber was filtered and rinsed using tap water until the pH was neutral. The obtained fiber was then bleached using 6 g of sodium chlorite (NaClO2, 80%, Ajax FineChem), 2 mL of glacial acetic acid (CH3COOH, 98.0%, QReC), and 640 mL of DI water at 60 °C for 30 min. After that, it was filtered and rinsed using tap water until the pH was neutral before being dried overnight at 70 °C in an air oven to produce the final extracted cellulose fiber.

Modifications of Extracted Cellulose Fiber

The extracted cellulose fiber normally has a low adsorption efficiency due to its hydroxyl group status. The hydroxyl group can be modified into another functional group that has a higher adsorption efficiency. In this study, EDTA and carboxymethyl groups were investigated as follows.

EDTA Group

The extracted cellulose was modified using EDTA (denoted as Cell-EDTA) as follows.[18] The extracted cellulose (1 g) was pretreated with 50 mL of 10% NaOH solution for 24 h. Then, the pretreated cellulose was immersed in 100 mL of dimethyl sulfoxide (C2H6OS, 99.5%, Riedel-de Haen) at 60 °C and treated with 3 g of EDTA dianhydride (C10H12N2O6, Sigma-Aldrich), followed by 5 mL of pyridine (C5H5N, 99+%, Alfa Aezar), all while being stirred under N2 gas. After continuous stirring at 60 °C for 20 h, the modified cellulose was filtered and washed with 100 mL of DI water, 100 mL of ethanol (C2H5OH, 99.9%, QReC), and 100 mL of acetone (C3H6O, 99.5%, QReC), respectively. After that, the sample was dried overnight at 65 °C in an air oven to produce the final product (Cell-EDTA).

Carboxymethyl Group

Carboxymethyl cellulose (denoted as Cell-CM) was prepared as follows. The extracted cellulose (3.0 g) was treated with a solution of NaOH (10%, w/v) mixed with isopropyl alcohol (99.7%, Qrec) using a ratio of extracted cellulose/10% NaOH solution/isopropyl alcohol of 1:4:20 (w/v/v). The mixture was stirred using a hotplate stirrer (Scilogex MS7-H550-S) for 1 h at room temperature. After that, 3.6 g of sodium monochlorite (Ajax FineChem) was added into the mixture, and then the temperature was increased to 55 °C and left for 3 h. Next, the solution was neutralized using 90% acetic acid (98.0%, QReC), followed by filtering and washing with an ethanol solution (70% v/v). Finally, the modified cellulose was kept in an open container for drying overnight at room temperature before using it as an adsorbent.

Characterization of Extracted Cellulose and Modified Celluloses

The physical, morphological, and chemical compositions of the samples before and after modification were observed using a scanning electron microscope equipped with an energy-dispersive X-ray spectrometer (SEM-EDS, model: JEOL JSM7600F). The samples were coated with platinum before imaging.

Functional groups of the samples were examined using a Fourier transform infrared (FT-IR) spectrometer (model: Bruker Optics, α-E), using the attenuated total reflection (ATR) mode.

Adsorption Studies

Different concentrations of Pb2+ (Pb(NO3)2, 99.5%, Qrec) and Cd2+ (Cd(NO3)2·4H2O, 98%, HiMedia) in DI water were prepared and used to investigate the adsorption efficiency of the modified celluloses in the pH range of 1–7. The adsorption kinetics models were determined using each sample (1.0 g) to adsorb 100 mg L–1 of lead or cadmium ions for different times (1, 3, 5, 9, 12, 15, 20, 50, 90, 120, and 150 min). An adsorption kinetics model was formulated using the information from the experimental data. In addition, the adsorption isotherms were investigated using each sample (1.0 g) to adsorb lead or cadmium ions at different concentrations in the range of 30–160 mg L–1. Furthermore, the adsorption efficiency of modified cellulose in a binary system of adsorption was investigated using various lead-to-cadmium ratios and various total concentrations of lead and cadmium ion solutions (more details can be found in Table S2). An atomic absorption spectrometer (AAS, PerkinElmer, HGA-800) was used to analyze the residual metal ion concentration after adsorption. Finally, experiments for studying the recyclability of the samples were carried out using a solution of 1 M HCl to observe the regenerative ability of each sample.