Chemically modified cellulose bearing metal binding sites like Schiff base and carboxylic acid groups was synthesized and characterized through Fourier transform infrared and solid state 13C–nuclear magnetic resonance (NMR) analysis. The chemically modified cellulose (Cell-PA) adsorbent was examined for its metal ion uptake ability for Cu(II) and Pb(II) ions from aqueous solution. Kinetic and isotherm studies were carried out under optimum conditions. Pseudo-second-order kinetics and Langmuir isotherm fit well with the experimental data. Thermodynamic studies were also performed along with adsorption regeneration performance studies. The adsorbent (Cell-PA) shows high potential for the removal of Cu(II) and Pb(II) metal ions, and it shows antibacterial activity towards selected microorganisms.

INTRODUCTION

The problem of coping with the presence of heavy metal ions has become a top priority in water treatment (Gadd & White 1993). Heavy metal pollution is a serious problem that adversely affects public health. Unlike organic pollutants, heavy metals are non-biodegradable in the environment and can accumulate in living tissues, particularly in human bodies causing significant physiological disorders such as damage to the central nervous system and blood composition, production of energy and irreversible damage of vital organs (Wahi et al. 2009).

Various treatment technologies for the removal of heavy metals from wastewaters include chemical precipitation, ion exchange, coagulation–flocculation, membrane separation, electrochemical treatment and adsorption. Amongst all the treatment processes mentioned, adsorption using sorbents is one of the most popular and effective processes for the removal of heavy metals from wastewater. The adsorption process offers flexibility in design and operation and in many cases produces treated effluent suitable for re-use, free of colour and odour. In addition, because adsorption is sometimes reversible, the regeneration of the adsorbent with resultant economy of operation may be possible (Kelleher 2001).

Nowadays, among the various solid adsorbents, chelating resins are widely used in the removal of heavy metals due to their high adsorption capacities and selectivity (Sun & Wang 2006; Kumar et al. 2007). We have reported a series of synthetic polymeric resins bearing Schiff base chelating groups which can effectively remove heavy metal ions from aqueous solution (Ravikumar et al. 2011a, 2011b, 2012, 2013). Schiff bases that are built of nitrogen and oxygen donor atoms are well known for their very good selectivity towards complexation of transition metals ions, and low affinity to alkali and alkaline earth metal ions. Moreover, many Schiff bases contain additional donor groups, which make them very good candidates for metal ion complexation. Even though such polymeric adsorbents are effective, cost of production is high and disposal of exhausted adsorbents causes serious environmental problems. Therefore the use of low cost, green adsorbents has attracted intense research in recent years.

Cellulose and its derivatives play an important role as green adsorbents for the removal of heavy metal ions. Cellulose is the most abundant, renewable natural source of organic materials. Unmodified cellulose has a low heavy metal adsorption capacity as well as variable physical stability. Therefore, chemical modification of cellulose can be carried out to achieve adequate structural durability and efficient adsorption capacity for heavy metal ions (Kamel et al. 2006). Modified cellulose with chelating groups such as carboxyl (Marchetti et al. 2000), amino (Maekawa & Koshijima 1990) and amidoxime (Saliba et al. 2005) have been used as potential adsorbents for the heavy metal ions such as Cu2+, Pb2+, Ni2+, Cd2+ and Cr3+. Various cellulose derivatives are produced by chemical modifications. The periodate oxidation of cellulose performed under mild aqueous conditions causes specific cleavage of the C2-C3 bond of the glucopyranose ring, which results in the formation of two aldehyde groups per unit (Nevell & Whistler 1963; Galbraikh et al. 1971; Kim et al. 2000; Kim & Kuga 2001).

Because of mildness of the reaction conditions, one can easily control the amount of introduced aldehyde groups. The reaction between aldehyde and primary amines is a facile procedure allowing introduction of various chelating substitutions. Using this method recently, we have reported chemically modified cellulose bearing benzalaniline chelating groups for the removal of heavy metal ions from aqueous solution (Ravikumar & Saravanan 2015). A typical metal binding ligand may be defined as any ion or molecule possessing a pair of non-bonding electrons. Carboxyl groups have two lone pairs of electrons on the oxygen. Two carboxyl groups are required to form a chelate with a divalent metal. In this regard, a spheroidal cellulose adsorbent was synthesized through a grafting reaction using acrylonitrile and subsequent saponification using sodium hydroxide giving the cellulose carboxyl groups on its surface. This modified cellulose adsorbent was used for the removal of Cu(II) ions from aqueous solutions by forming a bidentate arrangement between the Cu(II) and the carboxyl groups on the adsorbent (Liu et al. 2002).

The prime objective of the present work was to synthesize chemically modified cellulose having Schiff base and carboxylic acid chelating groups at the C2-C3 bond of the glucopyranose ring and to evaluate the new adsorbent towards uptake of heavy metal ion in aqueous solution. In recent years, antibacterial textile fibres have gained increasing attention because they offer several interesting properties. They could be either bactericidal (to kill bacteria) or bacteriostatic (to prevent the bacterial proliferation) and in the two cases they protect the human body (Bourgeois 2000). Hence an attempt has been made in the current work to evaluate the antibacterial activity of the modified cellulose.

MATERIALS AND METHODS

Materials

Cellulose (Loba), p-aminobenzoic acid (Alfa Aesar) and sodium metaperiodate (Sigma-Aldrich) were used as received. Copper and lead salts were procured from Sigma-Aldrich. The solvents such as DMSO and ethanol were purified according to standard procedures (Vogal 1989).

Synthesis of chemically modified cellulose

In the synthesis of chemically modified cellulose (Cell-PA) with the aim of creating aldehyde groups on the C2-C3 bond of the glucosidic ring on cellulose, the oxidative reaction with sodium metaperiodate was carried out before the Schiff base forming reaction. Periodate-oxidized celluloses were prepared by oxidizing 10 g of cellulose powder suspended in water (1 L) with sodium metaperiodate at ambient temperature in the dark. The amount of sodium metaperiodate (NaIO4) used was 1.3 times as much as the theoretical amount (13 g) for cellulose powder. The concentration of NaIO4 was kept at 0.4 M and stirred in the dark for 4 h, so as to have approximately 30 carbonyl groups per 100 glucoside units. At the end of the oxidation the solid formed was filtered and washed with double distilled water to neutral conditions (pH = 7.0) and dried.

The dialdehyde cellulose thus obtained (0.5 g) was suspended in 100 mL water, and a few drops of concentrated HCl were added as catalyst for the Schiff base forming reaction. p-Aminobenzoic acid (1.5 g) dissolved in water was then added to the dialdehyde cellulose suspension and stirred at 70 °C for 5 h. The brown-coloured chemically modified cellulose was washed with diluted HCl, hot water and ethanol and filtered. It was then dried under vacuum and used for adsorption studies.

Preparation of metal ion solution

Stock solutions of Cu(II) and Pb(II) ions were prepared by dissolving the required quantity of copper sulphate and lead nitrate salts in 1,000 mL of double distilled water to have a concentration of 1,000 mg/L of Cu(II) and Pb(II) ions. The standard solutions of metal ions were prepared from the stock solution by diluting to different concentration using double distilled water. The pH of the working solution was adjusted to 6.0 by using 0.1 M HCl or 0.1 M NaOH.

Batch adsorption experiments

In the present study, batch adsorption experiments were carried out to examine the kinetics, mechanism, adsorption isotherms and thermodynamics for the maximum removal of heavy metal ions at optimum conditions. The various operating parameters of pH, adsorbent dose, initial metal ions concentration, contact time and temperature were studied to find out the optimum conditions for the maximum removal of Cu(II) and Pb(II) ions. One hundred millilitres of Cu(II) and Pb(II) ions solutions with a concentration range of 10–50 mg/L were placed in 100 mL conical flasks. A measured quantity of adsorbent was added to the solution. The conical flasks were kept in a temperature controlled incubation shaker and then shaken at a constant speed of 200 rpm with different temperatures of 303 K to 343 K. After the equilibrium time, the adsorbent and supernatant was separated by centrifugation. The supernatant was analyzed for the residual Cu(II) and Pb(II) ion concentration by atomic absorption spectrometry (AAS; Shimadzu-AA6300, Japan with detection limit 0.006 mg/L). All the experimental studies were done in duplicate.

The percentage removal of Cu(II) and Pb(II) ions can be calculated by using the following equation:
formula
1
where C0 and Ce are initial and equilibrium final concentrations (mg/L) of the metal solutions respectively.
The amount of metal ions adsorbed onto Cell-PA at equilibrium time (qe, mg/g) was determined by using the following mass balance relationship:
formula
2
where V is the volume of the solution (L); and m is the adsorbent Cell-PA mass (g).

A non-linear approach was applied for the most widely used adsorption isotherm models of Langmuir (Langmuir 1918) and Freundlich (Freundlich 1906), which were compared for the present adsorption system using MATLAB 7.1 software. The adsorption isotherm parameters, correlation coefficient (R2) values and error values were estimated by using this software directly by fitting the adsorption equilibrium data to the different adsorption isotherm models.

The amount of metal adsorbed onto Cell-PA at time t, qt (mg/g), was calculated using the following equation:
formula
3
where Ct is the concentration of metal solution at any time t (mg/L). The different adsorption kinetic models of pseudo-first-order (Lagergren 1898), pseudo-second-order (Ho & McKay 1999) and Weber–Morris intra-particle diffusion (Weber & Morris 1963) were compared for the adsorption experiment of Cu(II) and Pb(II) ion adsorption onto Cell-PA.

Batch desorption experiments

The desorption of Cu(II) and Pb(II) ions was carried out by leaching spent Cell-PA with varying concentration of HCl solution. A measured amount of spent Cell-PA (0.2 g) was placed in Erlenmeyer conical flasks with varying concentration of HCl solution in a temperature-controlled incubation shaker at a rotating speed of 200 rpm and at 30 °C. The concentration of desorbed Cu(II) and Pb(II) ions in the solution was measured by using AAS.

Analytical method

The surface morphologies of the adsorbent (Cell-PA) and metal-loaded adsorbent were analyzed using a Leo Gemini1530 microscope. The concentration of heavy metal ions in the solution before and after equilibrium was determined using AAS (Shimadzu-AA6300, Japan). Fourier transform infrared (FTIR) analysis was used to identify the different chemical functional groups present in the adsorbent. Solid-state 13C-NMR spectra were obtained, to determine the functional groups that are responsible for the metal binding with the adsorbent, at 100.62 MHz on a Bruker AMX-200 spectrometer. The pH of solution was measured using a Hanna pH meter.

Antimicrobial activity test

The antimicrobial spectra of Cell-PA bearing an active azomethine functional group were determined against selective microorganisms, i.e. Enterococcus faecalis, Escherichia coli, and Staphylococcus aureus, by using the agar well diffusion method. Bacterial cultures were spread on sterile Mueller-Hinton agar plates, after which modified cellulose (50 μL) was placed on impregnated discs with 6 mm diameter for testing. The plates were incubated with the tested bacteria at 37 °C under aerobic conditions. After incubation for 24 h, the modified cellulose produced inhibition zones on each disc, which were measured and recorded (El-Khoulya et al. 2011).

RESULTS AND DISCUSSION

Synthesis of chemically modified cellulose bearing Schiff base and carboxylic acid group

Synthesis of chemically modified cellulose through oxidation and condensation reactions with NaIO4 and p-aminobenzoic acid is explained in Figure 1. With cellulose, when oxidation condition becomes stronger the number of carbonyl groups per 100 units of glucoside quickly increases. In the present study NaIO4 concentration was kept at 0.4 M and stirred for 4 h to have 30(±2) carbonyl groups per 100 glucoside units (Princi et al. 2006). The FTIR spectra of purchased cellulose and Cell-PA are presented in Figure 2. The IR spectrum of the cellulose showed characteristic cellulose peaks. Absorbance at 3,348 cm−1 (-OH stretching), 2,903 cm−1 (C-H stretching), 1,664 cm−1 (C-C ring stretching and -OH in plane bending), 1,430 cm−1 (-CH2 bending), 1,371 cm−1 (-CH bending) and 1,058 cm−1 (C-O-C stretching) are in good agreement with the reported values (El-Khoulya et al. 2011). In the Cell-PA spectrum (Figure 2), due to the presence of pendant carboxylic acid groups the -OH stretching frequency is observed as a broad band around at 3,369 cm−1. The imine -CH stretching frequency appears at 2,922 cm−1. The carbonyl stretching frequency of the acid groups shows a strong band at 1,690 cm−1. The azomethine (-CH = N-) stretching frequency appears at 1,605 cm−1. The appearance of the above-mentioned new peaks in the IR spectra indicates chemical modification has been successfully carried out. To further establish the structure of chemically modified cellulose, solid state 13C-NMR spectroscopy was performed on both natural and modified cellulose. Solid state 13C-NMR spectra of natural cellulose and Cell-PA are given in Figure 3. The signals of the five carbons of the pyranose ring of native cellulose appeared between δ = 62.4 and 102.4 ppm and the carbon of -CH2-OH signal appeared at δ = 44.2 ppm. In the chemically modified cellulose due to the cleavage of pyranose ring and formation of Schiff base, the 13C-NMR spectrum shows considerable changes. In the Cell-PA the five carbons show signals between δ = 64.7 and 104.6 ppm. The carbon of -CH2-OH group is shifted to δ = 32.2 ppm. The two broad peaks at δ = 170.1 and δ = 131.5 ppm in the modified cellulose confirms the formation of Schiff base along with pendant -COOH groups. The -CH = N- carbon and the -COOH carbon appeared as a broad peak at δ = 170.1 ppm. The aromatic carbons of the benzene rings showed a broad peak at δ = 131.5 ppm. These values are typical of a cellulose reported in literature. The above observations confirm the formation of chemically modified cellulose.
Figure 1

Synthesis of Cell-PA.

Figure 1

Synthesis of Cell-PA.

Figure 2

FTIR spectrum of (a) cellulose, (b) Cell-PA, (c) Cu-loaded Cell-Pa and (d) Pb-loaded Cell-Pa.

Figure 2

FTIR spectrum of (a) cellulose, (b) Cell-PA, (c) Cu-loaded Cell-Pa and (d) Pb-loaded Cell-Pa.

Figure 3

Solid state 13C-NMR spectra of (a) cellulose, (b) Cell-PA.

Figure 3

Solid state 13C-NMR spectra of (a) cellulose, (b) Cell-PA.

Metal ion uptake studies

Effect of initial pH value

The solution pH is an important parameter in the adsorption process. Adsorption of Cu(II) and Pb(II) ions is strongly dependent upon pH. The effect of pH variations on the adsorption of Cu(II) and Pb(II) was studied in the pH range of 2.0 to 8.0. As seen in Figure 4, adsorption of metal ions onto Cell-PA increased with increase in pH, reached a maximum at pH = 6.0 and thereafter decreased with increase in solution pH. At lower pH, the protonation of active sites, imine groups on the adsorbent surface, takes place resulting in the electrostatic repulsions between the metal cations and the protonated groups, preventing the adsorption of the metal ions and hence resulting in lower metal ion uptake. At higher pH, free imine groups are available on the adsorbent for ion adsorption. Moreover at higher pH, the free carboxylic acid groups are converted into carboxylate ions and available for metal ion adsorption. The optimum pH for the maximum removal of Cu(II) and Pb(II) ions was found to be 6.0.
Figure 4

Influence of initial pH value.

Figure 4

Influence of initial pH value.

Effect of adsorbent dosage

The effect of adsorbent dose on the removal of Cu(II) and Pb(II) ions by Cell-PA was investigated to measure the optimum dose for the maximum removal of Cu(II) and Pb(II) ions and the results are shown in Figure 5.
Figure 5

Effect of adsorbent dose on percentage removal.

Figure 5

Effect of adsorbent dose on percentage removal.

The figure shows that the removal of Cu(II) and Pb(II) ions was increased with increase in Cell-PA dose. This is because the number of available active sites of Cell-PA increased with increase in Cell-PA dose, which results in the increase in removal of Cu(II) and Pb(II) ions. However, unsaturation of active sites of Cell-PA during the adsorption process contributed to the decrease in the equilibrium adsorption capacity with increase in the Cell-PA dose. In the present case an optimum adsorbent dose of 30 mg was chosen for all other experiments.

Effect of contact time

The effect of contact time on the removal of Cu(II) and Pb(II) ions by Cell-PA was studied to identify the equilibrium time for the maximum removal of Cu(II) and Pb(II) ions and the results are shown in Figure 6. Adsorption was observed to increase with the contact time then reached almost a plateau after 60 min. The efficiency of the absorbent was evaluated from pseudo-first-order, pseudo-second-order and intra-particle diffusion kinetic models to understand the mechanism of adsorption on Cell-PA. The rate constant of adsorption was determined from the pseudo-first-order equation, which is generally expressed as
formula
4
where qe and qt are the adsorption capacities (mg/g) at equilibrium and time t, respectively, and k1 is the rate constant for pseudo-first-order adsorption. From the plots of ln (qeqt) versus t, the values of k1 and qe were determined. The linear form of the pseudo-second-order kinetic model is expressed as
formula
5
where k2 is the rate constant for second-order adsorption (g/(mg·min) and is determined from the linear plot of t/qt versus t.
Figure 6

Effect of contact time on percentage removal.

Figure 6

Effect of contact time on percentage removal.

The Weber and Morris intra-particle diffusion model was plotted in order to verify the diffusion mechanism, expressed as
formula
6
where C is the intercept and k is the intra-particle diffusion rate constant, (mg/g, min1/2), which can be calculated from the slope of the linear plot of Q versus t1/2. The various parameters calculated from the plots of the three kinetic models are presented in Table 1.
Table 1

Kinetic parameters of removal of Cu(II) and Pb(II) onto Cell-PA

Kinetic modelParametersCu(II)Pb(II)
Pseudo-first-order k1 (min−10.0829 0.0691 
qe,cal (mg/g) 98.18 64.86 
R2 0.829 0.841 
Pseudo-second-order qe,cal (mg/g) 55.56 58.82 
k2 (g/(mg·min)) 8.181 × 10−4 0.1138 × 10−4 
qe,exp (mg/g) 44.3 49.3 
R2 0.983 0.994 
Intra-particle diffusion model k (mg/(g.min1/2)) 3.933 3.708 
C 8.776 16.09 
R2 0.926 0.911 
Kinetic modelParametersCu(II)Pb(II)
Pseudo-first-order k1 (min−10.0829 0.0691 
qe,cal (mg/g) 98.18 64.86 
R2 0.829 0.841 
Pseudo-second-order qe,cal (mg/g) 55.56 58.82 
k2 (g/(mg·min)) 8.181 × 10−4 0.1138 × 10−4 
qe,exp (mg/g) 44.3 49.3 
R2 0.983 0.994 
Intra-particle diffusion model k (mg/(g.min1/2)) 3.933 3.708 
C 8.776 16.09 
R2 0.926 0.911 

The pseudo-second-order kinetic model best fitted the experimental data with correlation coefficient (R2 = 0.994) value close to 1 as compared to pseudo-first-order kinetics and intra-particle diffusion model. From the kinetic results it is observed that the adsorption of Cu(II) and Pb(II) ions onto Cell-PA follows the pseudo-second-order kinetic model.

Effect of metal ion concentration

The rate of removal of Cu(II) and Pb(II) ions by the Cell-PA is a function of initial Cu(II) and Pb(II) ion concentration, which makes initial ion concentration an important parameter for the effective adsorption process. The effect of initial metal ion concentration on the adsorption efficiency of Cu(II) and Pb(II) ions removal by Cell-PA was investigated and the results are shown in Figure 7. It is clear that, as the initial metal ion concentrations were increased from 25 to 200 mg/L, the percentage of adsorption was gradually decreased. This may be due to the saturation of active adsorption sites on the modified cellulose.
Figure 7

Effect of metal ion concentration.

Figure 7

Effect of metal ion concentration.

The experimental data on the effect of initial Cu(II) and Pb(II) ions concentration of modified cellulose of the test medium were applied to the adsorption isotherm models of Langmuir and Freundlich.

The non-linear equation of the Langmuir isotherm model is expressed as
formula
7
where Ce is the equilibrium concentration of the metal ions in the solution (mg/L), qe is the amount of metal adsorbed (mg/g), is the maximum adsorption capacity (mg/g) and KL is the Langmuir constant which is related to the energy of adsorption. The essential feature of the Langmuir model can be expressed in terms of the dimensionless constant separation factor RL (Hall et al. 1966) given by the following equation:
formula
8
where b is the Langmuir adsorption equilibrium constant (L/mg) and C0 is the initial metal ion concentration (mg/L). It has been established that for favorable adsorption 0 < RL < 1, for unfavorable RL> 1, for linear RL = 1 and the adsorption process is irreversible RL = 0.
The non-linear form of the Freundlich equation is given by
formula
9
where is the Freundlich constant ((mg/g) (L/mg)1/n) related to bonding energy: 1/n is the heterogeneity factor and n is a measure of the deviation from linearity of the adsorption process (g/L).
The various parameters calculated from the Langmuir and Freundlich isotherm plots are given in Figure 8 and Table 2. The values were found to be higher for the Langmuir isotherm (0.986 for Cu(II) and 0.972 for Pb(II)) when compared to the Freundlich isotherm (0.968 and 0.918 for Cu(II) and Pb(II) respectively). In addition the values of q calculated from the Langmuir isotherm are quite close to the experimental values for Cu(II) and Pb(II) ions. The values of RL are 0.063 and 0.078 for Cu(II) and Pb(II) ions. The RL value lies between 0 and 1 indicating a favorable adsorption process.
Figure 8

Non-linear adsorption Isotherms for adsorption of Cu(II) (a) and Pb(II) (b) onto Cell-PA.

Figure 8

Non-linear adsorption Isotherms for adsorption of Cu(II) (a) and Pb(II) (b) onto Cell-PA.

Table 2

Isotherm parameters for the adsorption of Cu(II) and Pb(II) onto Cell-PA

Isotherm modelParametersCu(II)Pb(II)
Langmuir KL (L/mg) 0.0922 0.3535 
qm (mg/g) 85.43 87.00 
RL 0.063 0.078 
R2 0.9862 0.9717 
Freundlich Kf (mg/g) 20.94 34.51 
n 3.429 4.642 
R2 0.968 0.9186 
Isotherm modelParametersCu(II)Pb(II)
Langmuir KL (L/mg) 0.0922 0.3535 
qm (mg/g) 85.43 87.00 
RL 0.063 0.078 
R2 0.9862 0.9717 
Freundlich Kf (mg/g) 20.94 34.51 
n 3.429 4.642 
R2 0.968 0.9186 

Effect of temperature

Metal ion adsorption was evaluated with 100 mg/L of Cu(II) and Pb(II) ion concentration separately with Cell-PA adsorbent ranging from 30 °C to 60 °C. The thermodynamic parameters for the adsorption, the Gibbs free energy change , the enthalpy change , and the entropy change , were calculated using the following equations:
formula
10
formula
11
formula
12
where KC is the equilibrium constant, Ce is the equilibrium metal ion concentration in solution (mg/L), CAe is the amount of metal ion adsorbed on the adsorbent per litre of solution at equilibrium (mg/L), R is the gas constant 8.314 J/(mol.K) and T is the absolute temperature (K) respectively.
The thermodynamic parameters , and were evaluated from the slope and intercept of the van 't Hoff plot (Figure 9) and the values are represented in Table 3.
Table 3

Thermodynamic parameters for the adsorption of Cu(II) and Pb(II) onto Cell-PA

Metal ionsTemperature (K)− ΔG (kJ mol−1)ΔS (kJ mol−1)− ΔH (kJ mol−1)
Cu(II) 303 −2.30 0.0136 6.435 
313 −2.17 
323 −2.03 
333 −1.90 
343 −1.76 
Pb(II) 303 −2.97 0.0130 6.910 
313 −2.84 
323 −2.71 
333 −2.58 
343 −2.45 
Metal ionsTemperature (K)− ΔG (kJ mol−1)ΔS (kJ mol−1)− ΔH (kJ mol−1)
Cu(II) 303 −2.30 0.0136 6.435 
313 −2.17 
323 −2.03 
333 −1.90 
343 −1.76 
Pb(II) 303 −2.97 0.0130 6.910 
313 −2.84 
323 −2.71 
333 −2.58 
343 −2.45 
Figure 9

Van 't Hoff plot for thermodynamic study.

Figure 9

Van 't Hoff plot for thermodynamic study.

Negative means a chemically exothermic process and hence the chelating mechanism may generally dominate over the ion exchange mechanism. With increase in temperature from 30 °C to 60 °C, the negative values of indicate that the Cu(II) and Pb(II) ions–Cell-PA adsorption system was rapid and thermodynamically spontaneous. Exothermic adsorption suggests the chelating of metal ions with the Schiff base moieties present in the cellulose is the principal mechanism of adsorption.

Evidence of adsorption of Cu(II) and Pb(II) ions onto the chelating sites of modified cellulose can be inferred from FTIR spectra of Cu(II)- and Pb(II)-loaded adsorbent (Figure 2). When compared to the modified cellulose, —OH stretching frequency of —COOH group shifted from 3,369 cm−1 to 3,347 cm−1 in the metal-loaded cellulose. The stretching frequency of the carbonyl group of acid also shifted from 1,690 cm−1 to 1,685 cm−1. Similarly the shape and intensity of -N = CH- stretching frequency suggest that the —COOH groups and -CH = N- bonds are the main adsorption sites for Cu(II) and Pb(II) adsorption. This fact is further verified through the scanning electron microscopy (SEM) images of Cu(II)- and Pb(II)-loaded modified cellulose (Figure 10(b) and 10(c)). The metal-adsorbed cellulose has different surface morphologies when compared to the modified cellulose (Figure 10(a)).
Figure 10

SEM morphology of (a) Cell-PA, (b) Cu-loaded Cell-PA, and (c) Pb-loaded Cell-PA.

Figure 10

SEM morphology of (a) Cell-PA, (b) Cu-loaded Cell-PA, and (c) Pb-loaded Cell-PA.

Furthermore, the energy-dispersive X-ray (EDX) spectra of Cu(II)- and Pb(II)-adsorbed modified cellulose shows the presence of peaks corresponding to Cu(II) and Pb(II) in the spectrum (Figure 11(a)11(c)), supporting the adsorption of Cu(II) and Pb(II) ions onto modified cellulose (Cell-PA). The Schiff base synthesis is confirmed by the presence of a high N content of 15.44 wt% in the EDX of Cell-PA. Based on the above observations a molecular model of adsorption of Cu(II) and Pb(II) metal ions onto the modified cellulose is given in Figure 12.
Figure 11

EDX spectra of (a) Cell-PA, (b) Cu-loaded Cell-PA, and (c) Pb-loaded Cell-PA.

Figure 11

EDX spectra of (a) Cell-PA, (b) Cu-loaded Cell-PA, and (c) Pb-loaded Cell-PA.

Figure 12

Molecular model of Cell-PA-metal system.

Figure 12

Molecular model of Cell-PA-metal system.

Regeneration

Regeneration of any exhausted sorbent is an important factor in the adsorption process for improving the process economics. Regeneration allows for the repeated use of the sorbent material and decreasing costs. The chemically modified cellulose adsorbate was treated with 0.1 M HCl solutions for an hour, and the filtrate was evaluated to determine the metal recovery by the AAS. The adsorbent was washed with water to remove the acid present on the adsorbent surface and it was used in further cycles as an adsorbent. After the fifth adsorption–desorption cycle, the adsorption capacities of Cu(II) and Pb(II) were found to be about 80% and 74% respectively of the fresh adsorbent and the results are given in Figure 13. The slight decrease in the efficiency of the adsorbent after five cycles may be attributed to the poisoning of the chelating sites.
Figure 13

Adsorption–desorption cycles.

Figure 13

Adsorption–desorption cycles.

Comparison of adsorption capacities with other adsorbents

A comparison of the maximum adsorption capacities with some recent results obtained using different types of adsorbent is presented in Table 4. The maximum adsorption capacities of the present cellulose bearing Schiff base and carboxylic acid chelating adsorbent is a highly efficient one for the removal of Cu(II) and Pb(II) ions compared with other materials. It shows very good complexation properties towards Cu(II) and Pb(II) and has potential as a commercial adsorbent.

Table 4

Comparison of adsorption capacity of various adsorbents for Cu(II) and Pb(II) ion adsorptions

 Adsorption capacity (mg/g)
Adsorbent usedCu2+ ionPb2+ ionReferences
Wood pulp–acrylic acid adsorbent – 6.0 Abdel-Aal et al. (2006)  
Cellulosic pulp adsorbent 49.6 – Bao-Xiu et al. (2006)  
Tetraethyleneamine grafted cellulose 30.0 – Kubota & Suzuki (1995a)  
Hydroxylamine grafted cellulose (amidoxime) 51.0 – Kubota & Shigehisa (1995b)  
Cellulose–glycidyl methacrylate 60.0 – Navarro et al. (1999)  
Acrylamide grafted cellulose 18.0 – Raji & Anirudhan (1998)  
Hyperbranched aliphatic polyester grafted cellulose 9.8 – Liu (2007)  
Vinyl pyrrolidone grafted cellulose – 32.3 Aly et al. (2005)  
Poly(hydroethyl acrylate)-grafted cross-linked poly(vinyl chloride) 55 – Liu et al. (2006)  
Grafted silica 16.5 – Chiron et al. (2003)  
Silica gel functionalized with ditopic zwitterionic Schiff base 41.31 – Wang et al. (2014)  
6-Deoxy-6-mercaptocellulose 22.0 28.0 Aoki et al. (1999)  
Citric acid modified cellulose 24.0 64.3 Low et al. (2004)  
Acrylic acid grafted cellulose 17.2 55.9 Guclu et al. (2003)  
Amidoximated bacterial cellulose 46.2 52.6 Chen et al. (2009)  
Succinylatedmercericed cellulose 56.8 147.1 Vinicius et al. (2009)  
Chemically modified cellulose bearing Schiff base (Cell-PA) 87.00 85.43 Present work 
 Adsorption capacity (mg/g)
Adsorbent usedCu2+ ionPb2+ ionReferences
Wood pulp–acrylic acid adsorbent – 6.0 Abdel-Aal et al. (2006)  
Cellulosic pulp adsorbent 49.6 – Bao-Xiu et al. (2006)  
Tetraethyleneamine grafted cellulose 30.0 – Kubota & Suzuki (1995a)  
Hydroxylamine grafted cellulose (amidoxime) 51.0 – Kubota & Shigehisa (1995b)  
Cellulose–glycidyl methacrylate 60.0 – Navarro et al. (1999)  
Acrylamide grafted cellulose 18.0 – Raji & Anirudhan (1998)  
Hyperbranched aliphatic polyester grafted cellulose 9.8 – Liu (2007)  
Vinyl pyrrolidone grafted cellulose – 32.3 Aly et al. (2005)  
Poly(hydroethyl acrylate)-grafted cross-linked poly(vinyl chloride) 55 – Liu et al. (2006)  
Grafted silica 16.5 – Chiron et al. (2003)  
Silica gel functionalized with ditopic zwitterionic Schiff base 41.31 – Wang et al. (2014)  
6-Deoxy-6-mercaptocellulose 22.0 28.0 Aoki et al. (1999)  
Citric acid modified cellulose 24.0 64.3 Low et al. (2004)  
Acrylic acid grafted cellulose 17.2 55.9 Guclu et al. (2003)  
Amidoximated bacterial cellulose 46.2 52.6 Chen et al. (2009)  
Succinylatedmercericed cellulose 56.8 147.1 Vinicius et al. (2009)  
Chemically modified cellulose bearing Schiff base (Cell-PA) 87.00 85.43 Present work 

Antimicrobial activity

The biocidal activity of Cell-PA was examined against S.aureus, E. coli, and E. faecalis by standard disc diffusion method. From Table 5, the results show that the untreated cellulose was not active, while the Cell-PA showed activity against the above-mentioned microorganisms. The significant antimicrobial activity of Cell-PA is due to the presence of pendant azomethine and carboxylic acid groups in the cellulose chain. The new chemically modified cellulose exhibited good antimicrobial properties and can be used in different medicinal applications.

Table 5

Antimicrobial activity of Cell-PA

 Zone of inhibition (mm)
MicroorganismsUntreated celluloseCell-PA
Gram-positive bacteria E. faecalis – 11.0 
S. aureus – 9.0 
Gram-negative bacteria E. coli – 10.0 
 Zone of inhibition (mm)
MicroorganismsUntreated celluloseCell-PA
Gram-positive bacteria E. faecalis – 11.0 
S. aureus – 9.0 
Gram-negative bacteria E. coli – 10.0 

CONCLUSION

Cellulose with chemical modifications used in this study is an efficient adsorbent for the removal of Cu(II) and Pb(II) ions from aqueous solution. The adsorption capacities of Cu(II) and Pb(II) on Cell-PA depended strongly upon the pH of the solution. The adsorption process is spontaneous and of exothermic nature . The values suggest that adsorption by Cell-PA is presumably due to chemisorption. The active sites in cellulose, which bind with metal ions, are Schiff base and carboxylic acid groups as evidenced from FT-IR studies. The adsorption isotherm studies confirmed that the experimental results follow the Langmuir model with adsorption capacity of Cell-PA towards Cu(II) and Pb(II) being 87 and 85 mg/g respectively. The adsorption kinetics was found to follow the pseudo-second-order kinetic model. The recyclability of adsorbent was carried out successfully for the retrieval of adsorbent for further wastewater treatment. Also, the new modified cellulose was toxic against bacterial species. The Schiff base and carboxylic acid groups present in the cellulose backbone are responsible for the higher adsorption efficiency, recyclability and antibacterial activities. The low cost, renewable nature and the efficiency of the present green adsorbent suggest that it may serve as an efficient adsorbent for the remediation of heavy metals from aquatic environment.

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