A novel chemically modified cellulose (DTD) adsorbent bearing pendent methyl benzalaniline chelating group was synthesized. This new adsorbent was used for the removal of Cu2+ and Pb2+ heavy metal ions from aqueous solution. The chemical and structural characteristics of the adsorbent were determined using FT-IR, 13C CP-MAS NMR, SEM, EDX and TGA analysis. The adsorption parameters, such as pH, adsorbent dose, contact time, initial metal ion concentration and temperature were optimized. Adsorption kinetic parameters were fitted into pseudo-first-order and pseudo-second-order models. The kinetic data fitted well to the pseudo-second-order kinetic model. The adsorption isotherms such as Freundlich and Langmuir isotherms have been investigated. Thermodynamic parameters have also been evaluated. The negative values of △ G0 and △ H0 reveal that the adsorption system is spontaneous and exothermic in nature. The modified cellulose was challenged with microorganisms as a function of contact time. The biocidal results showed that the chemically modified cellulose has bactericidal effect against the bacterial species.
Heavy metal pollution has become a serious problem with the rapid increase of global industrial activities. Industrial uses of metals and other domestic processes have introduced substantial amounts of potentially toxic heavy metals into the atmosphere and into the aquatic and terrestrial environments. The contamination of the aquatic systems with toxic heavy metal ions is a problem of global concern. Among the heavy metals, lead causes encephalopathy, cognitive impairment, behavioral disturbances, kidney damage, anemia and toxicity to the reproductive system [
In recent years, increasing costs and environmental considerations associated with the use of commercial adsorbents, have led to a significant body of research work aimed at developing new low-cost adsorbents derived from renewable resources. In this context, the advantages of using cellulose as the basis for new adsorbent design lie primarily in its high abundance, low cost and the relative ease with which it can be modified chemically [
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 waste water. The adsorption process offers flexibility in design and operation and in many cases produces treated effluent suitable for re-use, free of color and odor. In addition, because adsorption is sometimes reversible, the regeneration of the adsorbent with resultant economy of operation may be possible [
With the growing of public health awareness of disease transmissions and cross-infection caused by the microorganisms, the use of antimicrobial materials has been increased in many applications. The continuous search for potential antimicrobial agents has lead to identification of antimicrobial biomaterials that are based on polymers or their composites. In recent years, antibacterial textile fibers have gained an increasing attention because they offer several interesting properties. It could be either bactericidal (to kill bacteria) or bacteriostatic (to prevent the bacterial proliferation) and in the two cases it protects the human body [
In this work, we report the chemical modification of cellulose using sodium metaperiodate (NaIO4) oxidation followed by condensation with p-toluidine. The modified cellulose contains methyl formylimino groups which can both act as chelating group with metal ions and possess antimicrobial activities.
Cellulose (Loba), p-toluidine (Alfa Aesar), sodium metaperiodate (Sigma-Aldrich) was used as received. Copper and lead salts were procured from Sigma-Aldrich chemicals. All other chemicals and solvents used were either of analytical grades or purified according to standard procedures.
Metal salts CuSO4∙5H2O, Pb(NO3)2 were used for preparing stock solutions. Stock solutions of 1000 mg/L of standardized Cu2+, Pb2+ ions were prepared by dissolving the exact amount of the salts in double distilled water. The stock solutions were diluted to the required experimental concentration for the batch adsorption experiments.
The oxidation reaction using sodium metaperiodate was carried out onto cellulose before the coupling process. Sodium metaperiodate oxidation is a highly specific reaction that cleaves the bond between C2-C3 of the glucosidic ring and converts into the 2,3-dialdehydic groups, following the mechanism of Malaprade reaction, without significant side reactions [
The heavy metal ion concentration of the solutions before and after equilibrium was determined by Atomic Absorption Spectrometer AA6300 (Shimadzu, Japan). The pH of solution was measured using a Hanna pH meter using glass electrode. FT-IR analysis was carried out using Shimadzu Spectrophotometer with KBr pellets. The SEM images of the DTD and metal loaded DTD were analyzed using a Leo Gemini1530 scanning electron microscope. Thermo gravimetric analysis (TGA) was recorded using a Perkin-Elmer analyzer in static air at a heating rate of 10˚C/min. Solid-state 13C CP-MAS NMR spectra were performed at 100.52 MHz on a Bruker AMX-200 spectrometer.
Batch adsorption experiments were carried out by shaking the flasks using a horizontal bench shaker (Orbitek- Teqip-ACT/EQ/454) at 200 rpm. The experimental data obtained in batch studies were used to calculate the percentage removal of heavy metal ions by using mass balance equation.
where C0 and Ce are initial and equilibrium final concentrations (mg/L) of the metal solutions respectively.
The effect of pH on the adsorption of DTD was carried out in the pH range of 2.0 to 10.0 at 30˚C. The samples were then shaken in a horizontal bench shaker at 200 rpm at a different solution pH for 60 min and then filtered through Whatman 42 filter paper. The filtrate was analyzed using AASC.
Batch adsorption experiments were carried out at different adsorbent dosages of DTD from 5 to 25 mg at a pH of 6.0, by keeping the contact time and temperature constant.
Batch adsorption experiments were carried out by varying contact time of 20 - 120 min by keeping all other parameters constant.
Initial metal ion concentrations were investigated in the range 50 to 300 mg/L at a pH 6.0.
The adsorbed metal amount qe (mg/g) was determined by using the following mass balance relationship:
where V is the volume of the solution (L); and m is the adsorbent mass (g).
The amount of metal adsorbed at time (t), qt (mg/g), was calculated using the following equation:
Ct―the concentration of metal solution at any time t (mg/L).
The antimicrobial activities of modified cellulose DTD against Escherichia coli, Staphylococcus aureus and Enterococcus faecalis were examined using the agar well diffusion assay method. Diluted bacterial cultures were spread on sterile Mueller-Hinton agar plates, after which modified cellulose (50 µl) were placed on impregnated discs with 6 mm diameter for testing. The plates were incubated for 24 h at 37˚C under aerobic conditions and the diameter of the inhibition zones of each disc were measured and recorded [
Synthesis of chemically modified cellulose containing pendent methoxy benzalanilene group in the polymer chain is presented in
The FTIR spectra of native cellulose (RA) and DTD are presented in
hence the C-C ring stretching frequency is not observed in the modified cellulose. A new peak at 1624 cm−1 is clearely due to the -N=CH- stretching frequency and the C-N stretching frequency appeared at 1517 cm−1 which supports the formation of methyl benzalaniline pendent groups in the chemically modified cellulose. To further establish the structure of chemically modified cellulose, solid state 13C-NMR spectroscopy was performed on both native and modified cellulose. 13C CP-MAS NMR spectra of natural cellulose and DTD are given in
TG traces of cellulose and DTD are shown in
Scanning electron micrographs of DTD and metal loaded DTD are shown in Figures 6(a)-(c). Modified cellulose (DTD) surface is more irregular, rough and has open porous structure. The presence of pores in DTD suggests the possibility of the metal ions to be trapped and adsorbed onto the surface. These cavities are large enough to allow the metal ions to penetrate into the surface, and interact therein with the surface chelating groups. Surface morphology of the metal ion adsorbed DTD shows layers of metal ions on to porus surface. The
particle size was measured by particle size analyser and was found to be 448.3 nm. The value of the average particle size of the adsorbent provides more surface area for the removal of Cu2+ and Pb2+ ions from the aqueous media.
The pH of the solution affects the adsorptive process through protonation and deprotonation of functional groups of the active sites of the adsorbent surface. Initial pH values were varied from 2 - 10 and the corresponding % adsorption is given in
Adsorbent dose was varied from 5 mg - 25 mg and the percent removal is shown in
tion of the coordinating sites in the cellulose chain.
The initial concentration of metal ions is an important factor for effective adsorption. The percentage removal of metal ions Cu(II), Pb(II) at different metal ion concentration (10 - 30 mg/L) were performed by keeping all other parameters constant. An adsorption isotherm can be used to characterize interaction of the adsorbates with adsorbents and optimizing the use of adsorbents. Adsorption isotherms are the basic requirements for designing any adsorption system. The distribution of metal ions between the liquid phase and the adsorbent is a measure of the position of equilibrium in the adsorption process and can be expressed by a series of isotherm models. The non-linear forms of the Langmuir [
dlich adsorption isotherms based on experimental observations are given in
Langmuir isotherm models the monolayer coverage of adsorption surface. This model assumes that the maximum adsorption occurs at specific structurally homogeneous adsorption sites within the adsorbent and intermolecular forces decreases rapidly with the distance from adsorption surface. The non-linear equation of Langmuir isotherm model is expressed as;
where Ce―the equilibrium concentration of the metal ions in the solution (mg/L), qe―the adsorbed value of the metal ion at equilibrium concentration (mg/g),
Langmuir binding constant which is related to the energy of adsorption. The data obtained qe, KL and correlation coefficient (R2) values are shown in
where b―the Langmuir adsorption equilibrium constant and Co―the initial metal ion concentration. The values of RL lie between 0.0081 and 0.0069, indicating suitability of the chemically modified cellulose as adsorbent for Cu(II) and Pb(II) from aqueous solution.
The Freundlich isotherm model is related to multilayer adsorption, heterogeneous surface and interaction between adsorbed molecules. The nonlinear form of the Freundlich equation is given by
where
The adsorption capacity of the present DTD adsorbent has been compared with other adsorbents reported for copper and lead.
Isotherm model | Parameters | Cu2+ ion | Pb2+ ion |
---|---|---|---|
Langmuir | K (L/mg) qm (mg/g) R2 | 0.0737 157.3 0.9686 | 0.0962 153.5 0.9763 |
Freudlich | Kf (mg/g) n R2 | 38.05 3.6 0.8032 | 41.1 3.74 0.9675 |
When compared to other cellulose based adsorbents, the present chemically modified adsorbent is a highly efficient one for the removal of Pb2+ and Cu2+ ions from aqueous solution.
Adsorption kinetics provides valuable information about the controlling mechanism of the adsorption process, rate of the adsorbate uptake and optimum operating conditions for the full-scale batch process. Adsorption kinetic models, such as the pseudo-first-order [
The effect of adsorption time on the removal of Pb2+ and Cu2+ by modified chelating DTD is presented in
For better understanding of the kinetic mechanism which governs the whole process, the experimental data obtained were fitted with the well known kinetic pseudo-first-order and pseudo-second-order models according to the following equations:
where ki is the pseudo-first-order rate constant (min−1) of adsorption and qe and qt (mg/g) are the amounts of
metal ion adsorbed at equilibrium and time t (min) respectively. The linear form of pseudo-second-order equation can be written as
where k2 is the pseudo-second order rate constant of adsorption (g/(mg∙min)).
All the kinetic parameters of the removal process are summarized in
The thermodynamic studies were conducted at various temperatures (300 - 335 K). It provides information on energetic changes that are associated with adsorption and adsorption process is spontaneous or not. The thermodynamic parameters for the adsorption including the Gibbs free energy change
where
A most important problem is the recyclability of adsorbent and multi-usability. The regenerated adsorbent was used up to five adsorption-desorption cycles with Cu(II) and Pb(II) ions and the results are given in
Kinetic Model | Parameters | Pb2+ ion | Cu2+ ion |
---|---|---|---|
Pseudo-first order | kad (min−1) qe, cal (mg/g) R2 | 0.059 62.08 0.836 | 0.023 26.12 0.958 |
Pseudo-second order | qe, cal (mg/g) k (g∙mg−1∙min−1) h (mg∙g−1∙min−1) qe, exp (mg/g) R2 | 52.63 2.23 × 10−3 6.17 48.5 0.997 | 52.63 1.47 × 10−3 4.08 49.2 0.996 |
Parameters | Temperature (K) | Cu2+ ion | Pb2+ ion |
---|---|---|---|
∆H (KJ∙mol−1) | 82.7 | 118 | |
∆S (J∙mol−1) | 249.4 | 356.8 | |
∆G (KJ∙mol−1) | 303 | 0.92119 | 1.389526 |
313 | 0.450489 | 0.559536 | |
323 | 0.232738 | 0.325101 | |
333 | 0.003064 | 0.065355 |
The acid solutions of 0.1 M of H2SO4, HCl and CH3COOH were used as eluents. On comparison 0.1 M solutions of H2SO4 and HCl has equal efficiency for the regeneration of Cu(II) and Pb(II) metal ions than 0.1 M solution of CH3COOH. This is due to the weak acidic nature of CH3COOH. Even after five cycles of adsorption- desorption, the efficiency of DTD did not exhibit a significant decrease. The polymeric resins with azomethine group are acid resistant and the adsorbents had a good potential for reuse. The results shows adsorption-desorp- tion process is a reversible process which specifies the formation of coordinate bond between the chelating groups and the metals.
The modified cellulose DTD was tested by standard disc diffusion method with E. coli, E. faecalis and S. aures and the results are presented in
Compound | Zone of inhibition (mm) | ||
---|---|---|---|
Gram negative bacteria | Gram positive bacteria | ||
E. coli | E. faecalis | S. aureus | |
Untreated cellulose (RA) | --- | --- | --- |
Modified cellulose (DTD) | 8.0 | 14.2 | 12.2 |
selected microorganisms while the modified cellulose DTD showed activity against the same microorganisms. The significant antimicrobial activity of the modified cellulose is due to the presence of methyl benzalaniline pendent groups in the cellulose chain. These new modified cellulosic materials have good antimicrobial properties and can be used in many medicinal applications.
Removal of Pb2+, Cu2+ and antimicrobial activities was carried out using novel cellulose adsorbent bearing pendent methyl benzalaniline groups (DTD). Adsorbent capacity of the chemically modified cellulose towards Cu2+ and Pb2+ is 157.3 and 153.5 mg/g respectively. The adsorbent is an active one over a wide range of pH values. The new modified cellulose also shows promising antibacterial activity. The adsorption kinetic studies revealed that the adsorption process fits with the pseudo-second-order model while the adsorption isotherm studies confirmed that the experimental results follow the Langmuir model. Thermodynamic studies showed that the adsorption process is feasible and exothermic in nature.