Chitosan synthesized locally with a degree of deacetylation 71% and chitosan with a degree of deacetylation 68% from Sigma Aldrich were used to investigate adsorption of Cu<sup>2+</sup> ion in aqueous solution. The results obtained from equilibrium isotherm adsorption studies of Cu<sup>2+</sup> ion were an-alyzed in five adsorption models namely: Langmuir, Freundlich, Temkin, Elovich and Dubin- Ra-dushkevich. The isotherms equation was indicated to be well fitted to Langmuir, Freundlich, Temkin and Elovich under the concentration range studied. The kinetic parameters were evaluated utilizing the pseudo-first-order and pseudo-second-order equations, and the adsorption kinetics followed the mechanism of the pseudo-second-order equation for all systems studied, evidencing chemical sorption as the rate-limiting step of adsorption mechanism and not involving a mass transfer in solution. The FTIR studies revealed that the greater sorption of heavy metal was attributed to the large number of primary amine groups available on the surfaces of the chitosan and the abundant carboxyl groups on chitosan.
The presence of heavy metal ions in the environment has received extensive attention due to their increasing release to the atmosphere. Chitin is the most abundant natural fiber next to cellulose and is similar to cellulose in many respects. The most abundant source of chitin is the shell of crab and shrimp. Chitosan was discovered in 1859 by Professor C. Rouget. Chitosan contains 2-acetamido-2-deoxy-
Amino groups of chitosan NH2 are responsible for complex formation, in which nitrogen is a donor of electron pairs, although hydroxyl groups―OH can also participate in sorption. The mechanism of combining these reactive groups with ions of heavy metals is much differentiated and can depend on the ion type, pH and also the main components of the solution. Great influence on sorptivity has the deacetylation degree of chitosan: the higher it is, the more efficient is the sorption process. The deacetylation degree usually does not exceed 95%, because production of chitosan with a higher deacetylation degree is very costly.
A wide range of various treatment techniques such as ion exchange, biodegradation, oxidation, solvent extraction and adsorption have been reported to be used for removal of heavy metal ions from industrial effluents. However, adsorption has been widely accepted as one of the most effective pollutant removal processes, with low cost, ease in handling, low consumption of reagents, as well as scope for recovery of value-added components through desorption and regeneration of adsorbent [
The linear form of the isotherms are more frequently used for practical reasons, as they offer the means to determine constants and other parameters describing the adsorption kinetics from experimental. The mathematical correlation is usually depicted by graphs expressing the amount adsorbed on the solid-phase against the residual concentration in solution [
The Langmuir model assumes that the maximum adsorption takes place in a monolayer of the adsorbate molecules on the adsorbent surface and that all adsorption sites have equivalent energy and negligible interaction between adsorbed molecules [
The Langmuir isotherm model used for monolayer adsorption can be represented the following equation.
The Langmuir isotherm was evaluated using the model
where
The plot of
where
The Freundlich adsorption isotherm, however, is an empirical model and can be used in the case of a heterogeneous surface energy system [
where
where the factor
The Elovich isotherm assumes that the adsorption sites multiply exponentially with adsorption, indicating multilayer adsorption. The Elovich equation is express as follows [
where
The Dubinin-Radushkevich isotherm equation, which is more generally used to distinguish between physical and chemical adsorption, is given by the following equation
where
where
where E is the mean adsorption energy (kJ/mol), and K is the Dubinin-Radushkevich constant.
This isotherm contains a factor that explicitly taking into the account of adsorbent-adsorbate interactions. By ignoring the extremely low and large value of concentrations, the model assumes that heat of adsorption (function of temperature) of all molecules in the layer would decrease linearly rather than logarithmic with coverage [
In order to evaluate the mechanism of adsorption and the potential steps controlling the rate of adsorption, the basic characteristics of a good adsorbent considering adsorption kinetics under constant temperature and optimum solution pH, were determined, by using various initial metal ion concentrations. The kinetics of adsorption was determined by analyzing adsorptive uptake of heavy metals from the prepared copper (II) ion at different time intervals. The pseudo-first-order and pseudo-second-order kinetic models were applied to the experimental data to predict the adsorption kinetics onto chitosan. The linearity of each model when plotted indicates whether the model suitably described the adsorption process or not. The pseudo-first-order equation is generally expressed as shown in equation below [
where
The pseudo-second-order equation model is given below is
where
Chitosan with a degree of deacetylation about approximately 70% was obtained from Sigma Aldrich Germany, Sodium hydroxide (NaOH), Hydrochloric acid (HCl) and copper (II) suplhate pentahydrate all reagent used were of analytical grade. Deionized and distill water was used to prepared all reagents. Crab shells were obtained from Lagos (Nigeria) Lagoon waters (osa) and was used to synthesized the local chitosan with degree of deacétylation of 71 %.
Isolation of chitosan from crab shell wastes involves four traditional steps deproteinization (DP), demineralization (DM), decolorization (DC), and deacetylation (DA). The wet crab is washed and dried follow by grinding and sieving to reduced the surface area, the grinded crab is then stored in a plastic bottle. The crab shell was place in a solution of 3.5% NaOH
Batch adsorption studies were carried out at room temperature using a conical flasks with 50 mL of the working Cu(II) ion solution of different concentrations and stirrers at 300 rpm in view of the results from previous work [
The experiments were performed in duplicates and the amount of Cu(II) ions adsorbed in milligram per gram was determined by using the following mass balance equation
In which,
Copper (II) sulphate pentahydrate (CuSO4∙5H2O) analar grade is used as the source for copper stock solution. The solution is prepared with di ionized water the copper (II) stock solution (1000 mg/L) was made by dissolving 3.916 g of 99% CuSO4∙5H2O in one liter di ionized water. Samples of different concentrations of copper (II) are prepared from this stock solution.
The concentrations of metal ions were measured using an atomic absorption spectrophotometer (Varian AA 240). All reported copper concentrations are the mean value of three replicates.
FTIR analysis was used to characterize the surface chemistry of chitosan before adsorption of copper. FTIR analysis was used to identify the type of functional groups on the adsorbent surface peaks at 3500 - 3100 cm−1 related to N-H and C-N valent vibrations and at 3400 cm−1 related to symmetrical valent vibration of free NH2 and OH group for
The degree of deacetylation (DD) of the chitosan was calculated using the baseline by Domszy and Roberts (1985). The computation equation for the baseline is given below:
The SEM micrograph of chitosan powder of four different magnifications. The morphology has globules of crystalline structure with smooth surface. As it was prepared as fine ground powder, the morphology shows that the specimen is in particle structures. In a study [
Electron Dispersive Spectroscopy (EDS) is a test to examine the presence of elements through amplitude of wavelength for the x-ray emitted after the electron was hit by the electron beam. For the emission of x-ray, the atoms must contain minimum of K-shell and L-shell where the electron is allow to dislodge from shell to shell. Therefore, hydrogen being the only elements in the periodic table with only K shell is not detectable with EDS [
Atomic number | Element symbol | Element name | Confidence level | Concentration percentage | Certainty percentage | Error percentage |
---|---|---|---|---|---|---|
7 | N | Nitrogen | 100 | 40.3 | 97.5 | 2.5 |
6 | C | Carbon | 100 | 31.6 | 99.3 | 0.7 |
8 | O | Oxygen | 100 | 28.1 | 95.4 | 4.6 |
Atomic number | Element symbol | Element name | Confidence level | Concentration percentage | Certainty percentage | Error percentage |
---|---|---|---|---|---|---|
8 | O | Oxygen | 100 | 39.6 | 98.4 | 1.6 |
11 | Na | Sodium | 100 | 32 | 98.8 | 1.2 |
6 | C | Carbon | 100 | 14.9 | 99.1 | 0.9 |
7 | N | Nitrogen | 100 | 13.5 | 97.8 | 2.2 |
The effect of concentration at room temperature at PH = 6, and dosage 1 g on both locally develop and commercial chitosan.
The graph of
The graph of
The contact time represents the time necessary for the adsorption process to reach equilibrium. For each sample the adsorption capacity was studied as a function of time. The contact time was varied from 30 to 90 minutes. The effect was studied at room temperature while stirring at 300 rpm using a constant adsorbent dose of 1 g. The adsorption reached equilibrium at short contact time for both commercial and locally develop chitosan, which was due to the availability of active sites on the adsorbent surface. As the active sites were occupied, adsorption slowed down and finally an equilibrium stage was reached.
Langmuir isotherm model of copper (II) onto commercial and locally developed chitosan
The linearized form of Equation (2) (Langmuir isotherm) was plot in
The linearized form of Equation (2) (Langmuir isotherm) was plot in
Elovich Isotherm for copper (II) onto commercial and local developed chitosan (
The linearized form of the Elovich isotherm Equation (7) was used to plot
The elovich constant terms are summarized in
The linearized form of the Elovich isotherm Equation (7) was used to plot
Temkin Isotherm
The linearized form of Temkin isotherm model Equation (12) is plotted for commercial chitosan in
The linearized form of Temkin isotherm model Equation (12) is plotted for locally developed chitosan in
Freundlich Isotherm
The linearized form of Freundlich isotherm Equation (5) is plotted in
The linearized form of Freundlich isotherm Equation (5) is plotted in
Dubinin-Radushkevich Isotherm for locally developed chitosan
Dubinin-Radushkevich model constants of
Equilibrium isotherm are essential for describing the mechanism of adsorption for The equilibrium data of Cu2+ ion were subjected to five different adsorption isotherm models: Langmuir, Elovich, Temkin, Freundlich and Dubinin-Radushkevich are summarized in
Runs | Time (mins) | Co (mg/L) | Ce (mg/L) | % Removal |
---|---|---|---|---|
1 | 30 | 30 | 0.15 | 99.5 |
2 | 45 | 30 | 0.06 | 99.8 |
3 | 60 | 30 | 0.1 | 96.67 |
4 | 75 | 30 | 0.07 | 99.77 |
5 | 90 | 30 | 0.08 | 99.73 |
Runs | Time (mins) | Co (mg/L) | Ce (mg/L) | % Removal |
---|---|---|---|---|
1 | 30 | 30 | 0.53 | 98.23 |
2 | 45 | 30 | 0.55 | 98.17 |
3 | 60 | 30 | 0.61 | 97.97 |
4 | 75 | 30 | 0.58 | 98.07 |
5 | 90 | 30 | 0.55 | 98.17 |
Adsorbent | Adsorbent constants | |||
---|---|---|---|---|
a (mg/g) | b (L/mg) | RL | R2 | |
C Chitosan | 1.49 | 3728.56 | 0 | 0.9999 |
L Chitosan | 1.4435 | 92.491 | 0.000361 | 0.9999 |
Adsorbent | Adsorbent constants | ||
---|---|---|---|
Qm (mg/g) | KE (L/mg) | R2 | |
C Chitosan | 0.00502 | 0 | 0.9915 |
L Chitosan | 0.0301 | 0.0000 | 0.9006 |
Adsorbent | AT (L/g) | bT | B | R2 |
---|---|---|---|---|
C Chitosan | 4.41 | 500519.6 | 0.00495 | 0.983 |
L Chitosan | 4.297 | 99580 | 0.02488 | 0.81 |
1/n | n | KF (mg/g) | R2 | |
---|---|---|---|---|
C Chitosan | 0.003 | 333.33 | 1.48 | 0.985 |
L Chitosan | 0.008 | 125 | 1.45 | 0.998 |
Adsorbent | Kad (mol2/kJ2) | E (kJ/mol) | R2 |
---|---|---|---|
Local Chitosan | 2.290 × 10−9 | 0.59 | 0.9901 |
Pseudo-first-order (Lagergren model) for adsorption of Cu(II) onto chitosan (
Pseudo-second-order reaction model for adsorption of Cu2+ ion on chitosan (
The data on the dependence of adsorption capacity on time were used for kinetic analysis. Figures 18-21 show the tests of pseudo-first-order rate equation (Lagergren Model), and pseudo-second-order rate equation (Ho Model) respectively. The results of the fitted data, in
Coefficient of empirical kinetic models for local and commercial chitosan from crab shell
Pseudo-first-order reaction model | Pseudo-second-order reaction model | |||||||
---|---|---|---|---|---|---|---|---|
Adsorbent | Metal | qecal. | qeexp | k1 | qecal. | qeexp | R2 | k2 |
C-C | Cu2+ | 1.096 | 1.4965 | 0.104 | 1.4993 | 1.4965 | 0.999 | 10.35 |
L-C | Cu2+ | 0.1042 | 1.4735 | 0.085 | 1.4728 | 1.4735 | 0.999 | 66.02 |
Chitosan was produced from crab shell to remove Cu(II) from aqueous solution. Pseudo-second-order kinetics model was the better model to describe the adsorption behavior of copper ion. The adsorption data for Cu(II) were well fitted to Langmuir, Freundlich and Elovich isotherm model.
b Langmuir isotherm constant (L/mg)
m amount of chitosan membrane (g)
n number of experimental data
q adsorption amount (mg/g)
a maximum adsorption capacity (Langmuir isotherm constant) (mg/g)
t time (min)
V volume of the solution (L)
K is the Dubinin-Radushkevich constant (kJ2/mol)