Adsorption Behaviour of Cu ( II ) Ions from Aqueous Solution on Chitosan

The chemical contamination of water with a wide range of toxic substances, in particular heavy metals, is a serious environmental problem due to their potential toxicity to humans. Therefore, there is a need to develop technologies that can remove toxic pollutants found in wastewaters. Adsorption is one of the more popular methods for the removal of pollutants from the wastewater. Chitosan is a derivative from N-deacetylation of chitin – a naturally occurring polysaccharide from crustacean and fungal biomass and it has been found to be capable of chemically or physically adsorbing various heavy metal ions, including lead, vanadium, platinum, silver, cadmium, chromium and copper, from wastewaters. In this study, a batch adsorption system was applied to study the adsorption of Cu (II) ions from aqueous solution by chitosan. The adsorption capacities and rates of Cu (II) ions onto chitosan were evaluated. Experiments were carried out as function of time, adsorbent mass and concentration of Cu (II) ions. Langmuir and Freundlich adsorption models were applied to describe the isotherms. Equilibrium data agreed very well with the Langmuir model. The kinetic experimental data correlated well with the second-order kinetic model, indicating that the chemical sorption was the rate-limiting step.

The chemical contamination of water with a wide range of toxic substances, in particular heavy metals, is a serious environmental problem due to their potential toxicity to humans.Therefore, there is a need to develop technologies that can remove toxic pollutants found in wastewaters.Adsorption is one of the more popular methods for the removal of pollutants from the wastewater.Chitosan is a derivative from N-deacetylation of chitin -a naturally occurring polysaccharide from crustacean and fungal biomass -and it has been found to be capable of chemically or physically adsorbing various heavy metal ions, including lead, vanadium, platinum, silver, cadmium, chromium and copper, from wastewaters.
In this study, a batch adsorption system was applied to study the adsorption of Cu (II) ions from aqueous solution by chitosan.The adsorption capacities and rates of Cu (II) ions onto chitosan were evaluated.Experiments were carried out as function of time, adsorbent mass and concentration of Cu (II) ions.Langmuir and Freundlich adsorption models were applied to describe the isotherms.Equilibrium data agreed very well with the Langmuir model.The kinetic experimental data correlated well with the second-order kinetic model, indicating that the chemical sorption was the rate-limiting step.
Water pollution due to toxic metals is a serious environmental and public problem [1][2][3][4][5].Moreover, faced with more and more stringent regulations, water pollution has also become a major source of concern and a priority for most industrial sector.Heavy metals ions are often found in the environment as a result of their wide industrial uses.They are toxic to the organisms.For this reason, heavy metal removal from wastewaters is a crucial issue of concern to health.Several methods have been used for the removal of heavy metals such as: sorption, ion exchange, filtration, coagulation, precipitation, membrane separation, reduction, solvent extraction, and reverse osmosis [6][7][8][9][10][11].
From these methods, adsorption is considered to be an effective and economical method for removal of pollutants from wastewater [12].The adsorption capacity of several low-cost adsorbents, such as biopolymes, zeolites, clay, or certain waste products from industrial operations (fly ash, coal, and oxides) has been investigated.The biopolymers can be obtained from renewable sources and selectively adsorb several metallic ions [13].
Among these biopolymers, chitosan has proved to be an extremely promising material because of its excellent metal-binding capacities and low cost as compared with other adsorption materials [14][15][16][17][18][19][20].Chitosan is a linear polysaccharide based on a glucosamine unit (fig.1), and is obtained through chitin deacetylation, which is a major component of crustacean shells and one of the most abundant biopolymers in nature.
The amine groups on chitosan are much more reactive than the acetamide groups on chitin The free electron doublet of nitrogen on amine groups is responsabile for the sorption of metal cations.The protonation of amine groups in acidic solutions is responsible for the electrostatic attraction of metal anions.
In this work, the sorption properties of samples of chitosan with respect to copper ions was investigated in static conditions.

Experimental part
Samples of chitosan flakes highly viscous purchased from Sigma Aldrich Chemie GmbH was used in this study.Practical grade chitosan from crab shells has a minimum of 85% percent deacetylation and a viscosity > 200cps (1% in 1% acetic acid); and the total impurities are ≤ 1%.
Stock solutions of 6373 mg/L copper were prepared by dissolving the corresponding salt (CuCl 2 ) (Merck a.r.grade) in distilled water.Aqueous solutions of Cu(II) used in this study were prepared by diluting the stock solutions.
Adsorption experiments were carried out by batch method by using a Heidolph RZR 2041 stirrer.The quantity of chitosan was varied between 0.1-2.5 g, and the speed rotation was 150 rpm (rotation per minute).The solution volume used was 100 mL, and the copper concentration in solution was in the range of 166.5 -690 mg/L.
Metalic ion concentrations in initial solution, and in solution after the adsorption on the chitosan were determined by atomic adsorption spectrometry by using a type AAS 1N Carl Zeiss Jena atomic adsorption spectrophotometer.The next parameters: wavelenght λ= 324.75 nm, gap = 4, photomultiplication = 1, lamp current = 6 mA, work range 1 -100 A (absorbance units), time constancy = 0.5 s were used in copper concentration in solution determination.

Results and discussion
Kinetic studies of chitosan were performed as batch tests, with the sample removed from the shaker between 1 min and 840 min.These tests were performed in order

Effect of contact time
The chitosan samples (0.1000±0.0001 g, and 0.2000±0.0001)was weighed into a glass vial with a screw top.A total of 100 mL of the cooper 360 mg/L was added to each vial via a volumetric dropper, and the vials were capped.The initial pH was 4.3, and the temperature was 22 °C.Each sample was then filtered, and tested for metal ion concentration.
The adsorption capacity of an adsorbent, Q, is calculated by means of equilibrium studies and then summarized using the equilibrium equations of Langmuir and Freundlich.Equation (1) establishes the mass balance of process at equilibrium condition: (1) where: Q = adsorbent capacity, mg/g; C i = the initial metal ion concentration, mg/L; C f = the metal ion concentration remained in solution at different times, mg/L; V = solution volume, L; m = adsorbent mass (g).
The quantity of heavy metal uptake by chitosan was calculated at different times, and experimental data are presented in figure 2.
filtration.The same solution was shaked with a quantity of chitosan by 0.2 g in order to establish the influence of chitosan quantity of adsorption capacity.The results obtained in function of time are presented in figure 3.
Results (fig.3) indicate that adsorption capacity increased with an increase in contact time before equilibrium is reached.Other parameters such as dose of adsorbent, pH of solution and agitation speed was kept optimum, while temperature was kept at 22°C.It can be seen that in this case (a solution with 690 mg/L Cu(II)) the equilibrium was reached for a 0.1 g chitosan after 5 h, and for a 0.2 g chitosan after 8 h.This result is important, as equilibrium time is one of the important parameters for an economical wastewater treatment system.

Adsorption isotherm
The most simple adsorption isotherm is based on the assumption that every adsorption center is equivalent and the capacity of the particles to bond some components is independent of how many adiacent centers are occupied or not with sorbat.Dates obtained from copper retaining on chitosan are correlative with Langmuir and Freundlich models function of the next equations: (2) or (3) where: K f and 1/n are adsorption isotherm parameters (capacity and intensity); K L and a are Langmuir model parameters.These models are made in linear form by logarithmation.
From this figure it can be seen that equilibrium adsorption of copper onto chitosan is reached in approximately 5 h in case of using a 0.1 g quantity of chitosan, and after 7.30 in case of using a 0.2 g chitosan quantity.
A 100 mL of a solution containing 690 mg/L Cu(II) was treated with 0.1 g chitosan by shaking with a 150 rpm speed rotation for 1-780 min before phase separation through (5)

Table 1 LANGMUIR AND FREUNDLICH PARAMETERS FOR COPPER REMOVAL
For the determination of adsorption isotherms synthetic solutions with 690 mg/L, 360 mg/L, 166.5 mg/L and 40 mg/ L copper concentrations were used.The solution volume used was 100 mL, the chitosan quantity used was 0.2 g, the contact time was 720 min, and the temperature was 22°C.Results obtained are presented in table 1(fig.4 and  5).
From figures 4 and 5 it can be seen that in case of Cu(II) adsorption on chitosan, the correlation parameter between experimental and the date obtained by Freundlich linearized isotherm is R = 0.9131, and for Langmuir isotherm is R = 0.9603.This fact leads to the conclusion that Cu(II) adsorption on chitosan may be the best described by Langmuir isotherm:

Kinetics of adsorption
In order to investigate the controlling mechanisms of adsorption processes such as mass transfer and chemical reaction, the first-order, second-order and intraparticle diffusion equations were used to test the experimental data.The first-order kinetic model of Chiou and Li [21] and Annadurai and Krishnan [22] is given as: (7) where q e and q t are the amounts of Cu(II) adsorbed on adsorbent (mg/g) at equilibrium and at time t, respectively and k 1 is the rate constant of first-order adsorption (min -1 ).Straight line plots of log(q e -q t ) against t were used to determine the rate constant, k 1 and correlation coefficient, R, values for Cu(II) under different concentration range conditions (fig.6).
In this case we obtained for k 1 the value 8.87 .10 -3 , and for R the value 0.9456.
The second-order equation [23] may be expressed as: where k 2 is the rate constant of second-order adsorption (g/mg .min).Straight-line plots t/q t against t were tested to obtain rate parameters and the results suggested the applicability of this kinetic model to fit the experimental data.The rate constant of second-order adsorption obtained from the experimental data presented in figure 7 is k 2 = 3.76 .10 -4 (g/mg .min), and the correlation coefficient R is 0.9953.
The intraparticle diffusion rate [21] can be described as: (9) where k i is intraparticle diffusion rate (mg .g -1.min -0.5 ).The k i is the slope of straight-line portions of the plot of q t against t 0.5 .
The value of k i determined from experimental data is 4.53 (mg .g -1.min -0.5 ), and correlation coefficient R is 0.9074.
Based on the correlation coefficients, the adsorption of Cu(II) on chitosan is best described by the second-order equation (R=0.9953).The second-order kinetic model assumes that the rate-limiting step may be chemical adsorption [24].In many cases, the second-order equation

Effect of the chitosan mass
The increase of the chitosan mass from 0.1 g to 2.5 g dramatically increases the retention of Cu(II) from 24.66% up to 91.21% (fig.9).
The removal efficiency (E) of adsorbent on Cu(II) was defined as: (10) where C 0 and C 1 are the initial and equilibrium concentration of Cu(II) solution (mg/L), respectively.

Conclusions
Studies developed by using Cu(II) as ion models for removal of heavy metals from aqueous solutions by means adsorption onto chitosan have evidenced that a quantity of 2.5 g chitosan and 7 h shaking could be enough for the strong reduction of polluted wastes from 690 mg/L to 60.65 mg/L with a removal efficiency by 91.21%.
The amount of Cu(II) ions adsorbed was influenced by parameters such as the initial Cu(II) concentration, contact time, adsorbent quantity.
The maximum adsorption capacity (170.16mg Cu(II)/g chitosan) obtained in case of the solution with 690 mg/L Cu(II) is comparable with adsorption capacity of chitosan reported in other studies [2].
Equilibrium data agreed very well with the Langmuir model.The adsorption process could be best described by the second-order equation.This suggests that the ratelimiting step may be the chemical adsorption (chemiosorption) not the mass transport.
Results also showed that this adsorbent can be a good candidate for adsorption of not only copper ions but also other heavy metal ions in wastewater stream.

Fig. 2 .Fig. 3 .
Fig. 2. Adsorption isotherm of Cu(II) onto chitosan at pH 4.3 and 22°C as function of time by using 0.1 g chitosan and 0.2 g chitosan samples and a solution of 166.5 mg/L Cu(II)

Fig. 9 .
Fig. 9. Effect of chitosan quantity on the adsorption of Cu(II) on chitosan.Cu(II) concentration was 690 mg/L; agitation speed 150 rpm; contact time 7 h ; temperature 22°C

Adsorption Behaviour of Cu(II) Ions from Aqueous Solution on Chitosan CARMEN DELEANU 1 , CLAUDIA MARIA SIMONESCU 1 *, IONEL CONSTANTINESCU 1
University Politehnica of Bucharest, Faculty of Applied Chemistry and Materials Science, Department of Inorganic Technology and Environmental Protection, 17 Polizu Str., 011061, Bucharest, Romania