Biosorption of Cadmium (II) from Aqueous Solutions by Prunus Avium Leaves

A new biosorbent from Prunus avium (sweet cherry) leaves was used to remove cadmium(II) from aqueous solutions. The biosorption of cadmium(II) was found to be dependent on solution pH, initial metal ion concentrations, biosorbent dose, and contact time. The experimental equilibrium b iosorption data were analyzed by two widely used two-parameters, Langmuir and Freundlich isotherm models. The Langmuir model gave a better fit than the Freundlich model. The kinetic studies indicated that the biosorption process of the cadmium ions followed well pseudo-second-order model. It was concluded that Prunus avium leaves powder can be used as an effective, low cost, and environmentally friendly biosorbent for the removal of Cd(II) ions from aqueous solution.


Introduction
The major sources of Cd(II) release into the environment b y waste streams are electrop latin g , s meltin g , alloy man u factu ring , p ig men ts, p lastic, b attery , min ing and refining processes. Cad miu m is one of the to xic heavy metals, which is regarded as an element of h igh to xicity. Different methods have been used on cadmiu m content reduction fro m water and industrial waste such as chemicalprecip itati on, ion exchange, membrane filtrat ion, electro lytic methods, reverse osmosis, solvent extract ion, and activated carbon adsorption [1]. These conventional techniques can reduce cadmiu m ions, but they do not appear to be highly effective due to the limitations in the pH range as well as the high material and operational costs. Different biosorbentmaterial s have been investigated for removal of cad miu m ions fro m aqueous solut ions such as p lan t materials like wheat straw [2], ag ricu ltu ral waste b io mass [3], rice h usk [4], orange peel [5], nut husk [6], sugar beet pulp [7], orange waste [8], coconut shell [9], juniper bark and wood [10] and wheat stem [11].In recent years, considerable attention has been focused on the removal o f cad miu m io ns fro m aqueous solution using adsorbents derived from lo w-cost t ree leav es su ch as loqu at leaves [12], sidiu mgu a ja va eaves [13], maize leaf [14], u lmus leav es [15], Sco ly mus hispanicus. [16], Ulmus carpinifolia and Fraxinus excelsior tree leaves [17], In the present work, We have studied the potential of cadmiu m(II) ions biosorption on Prunus avium (sweet cherry) co ming fro m P. avium tree leaves. Results fro m this study can be used to assess the utility of P. avium leaves powder for cad miu m(II) ions removal fro m water and industrial wastewaters.

Ads or bent
The raw Prunus avium (sweet cherry) leaves were collect ed fro m a local p lantation. The leaves were thoroughly rinsed with water to remove dust and soluble materials. Then it was allo wed to dry at room temperature. The dried leaves was grounded to a fine powder in a grinding mill (Retsch RM 100) and sieved to get size fraction < 44 µm, and then dried in an oven at 60 o C for 24 h.

Materials
All the chemicals used were of analytical reagent (AR) grade. Stock solutions of 1000 mg/ L of cad miu m(II) ionswere prepared fro m nitrates of cadmiu m which was purchased from Fluka A G using double distilled water. Desired test solutions of cadmiu m ions were prepared using appropriate subsequent dilutions of the stock solution. The range of concentrations of cadmiu m ions prepared from standard solution varies between 10 and 100 mg/ L. Befo re mixing the adsorbent, the pH of each test solution was adjusted to the required value with 0.1 M NaOH or 0.1 M HCl.

Anal ysis
The concentrations of Cd(II) ions in the solutions before and after equilibriu m were determined by AAS6300 Ato mic absorption spectrometer (Shimad zu, Japan). The pH of the solution was measured with a WTW pH meter using a combined glass electrode. Fourier Transform Infrared Spectroscopy, FTIR (IR Prestige-21, Shimad zu, Japan) was used to identify the different chemical functional groups present in the P. avium leaves powder. FTIR analyses also used to determine the functional groups which are responsible for the cad miu m b inding with P. avium leaves powder. The analysis was carried out uing KBr and the spectral range varying fro m 4000 to 400 cm −1 .

Biosorpti onExperi ments
Batch biosorption experiments were conducted by mixing biosorbent with cad miu m ion solutions with desired concent ration in 250 mL glass flask. The glass flasks were stoppered during the equilibration period and placed on a temperature controlled shaker at a speed 120 r/ min. The effect of pH on the equilib riu m biosorption of Cd(II) was investigated by mixing, The amount of biosorption was calculated based on the difference between the init ial (C o , mg/ L) and final concentration (C e , mg/ L) in every flask, as follows: where q e is the metal uptake capacity (mg/g), V the volu me of the metal solution in the flask (L) and M is the dry mass of biosorbent (g). Percent removal (% R) o f cad miu m ions was calculated fro m the following equation:

FT-IR Analysis
To investigate the functional groups of P. avium and cadmiu m loaded on P. avium, a FT-IR study was carried out and the spectra are shown in Figures. 1. and 2. The P. avium leaves display a number of absorption peaks, reflecting their complex nature. A strong and broad peak at 3356cm −1 results due to the stretching of the N-H bond of amino groups and indicative of bonded hydroxyl group of alcohols and phenols. A change in peak position to 3375 cm -1 in the spectrum of the cadmiu m loaded P. avium, Fig.2, indicates the binding of cadmiu m with amino and/or hydroxyl groups. The absorption peaks at 2939 cm −1 and 2893cm −1 could be assigned to -CH stretching vibrations of -CH 3 and -CH 2 functional groups. The shoulder peak in pure P. avium leaves powder at 1701 cm -1 is shifted to lowe frequency 1696 cm -1 due to the involvement of carbonyl -CO group fro m carboxy lic acids in the biosorption process of cadmiu m ions with P. avium leaves powder. The peak at 1635cm −1 indicates the fingerprint reg ion of CO, C-O and O-H groups, which exist as functional groups of P. Avium and shift ing of this peak to 1631cm −1 , indicated involvement of these groups in cad miu m b inding. The band at 1377 cm -1 corresponding of C-O stretching was shifted to 1404 cm -1 . The region between 1284 and 1000 cm -1 is the fingerprint region, OH, and C-H bending vibration and C-O stretching vibration absorption bands. The intense band at 1029 cm -1 can be assigned to the C-O of alcohols and carboxylic acids. The shift of the peak fro m 1029 to 1010 cm −1 also suggests the involvement of C-O group in binding Cd(II). The shifts in the absorption peaks generally observed indicate the existence of a cadmiu m b inding process taking place on the surface of the P. avium leaves powder.

Effect of PH
The pH has been identified as one of the most important parameter that is effect ive on metal sorption. It is directly related with co mpetition ability of hydrogen ions with metal ions to active sites on the biosorbent surface. The effect of pH on the biosorption of Cd(II) ions onto P.avium leaves powder was studied at pH 1.0-8.0. The maximu m biosorpti on was observed at pH 6.5 for Cd(II). Therefore, the remain ing all biosorption experiments were carried out at this pH values. The biosorption mechanisms on the P.avium leaves powder surface reflect the nature of the physicochemical interaction of the solution. At highly acidic p H (pH < 1.0), the overall surface charge on the active sites became positive and metal cations and protons complete for bind ing sites on cell wall, which results in lower uptake of metal. The biosorbent surface was mo re negatively charged as the pH solution increased from 1.0 to 6.0. The functional groups of the P. avium leaves powder were more deprotonated and thus available for the metal ions. Decrease in b iosorption yield at higher pH (p H > 6) is not only related the formation of soluble hydroxylated complexes of the metal ions (cad miu m ions in the form of Cd(OH) 2 ) but also to the ionized nature of the cell wall surface of the bark powder under the studied pH.

Effect of Contact Ti me
The rate of biosorption is important for designing batch biosorption experiments. Therefore, the effect of contact time on the biosorption of d Cd(II) was investigated. The biosorption yield of Cd(II) increased considerably until the contact time reached 120 min. Further increase in contact time did not enhance the biosorption, so, the optimu m conta ct time was selected as 120 min for further experiments

Effect of Adsorbent Dose on Biosorption
The biosorbent dosage is an important parameter because this determines the capacity of a b iosorbent for a given initial concentration. The biosorption efficiency for Cd(II) ions as a function of b iosorbent dosage was investigated. The percentage of the metal biosorption steeply increases with the biosorbent loading up to 0.5 g/0.1 L. This result can be explained by the fact that the biosorption sites remain unsaturated during the biosorption reaction whereas the number of sites available for biosorption site increases by increasing the biosorbent dose. The maximu m biosorptio n 94.44% for Cd(II) was attained at biosorbent dosage, 0.5 g/0.1 L. Therefore, the optimu m b iosorbent dosage was taken as 0.5 g/0.1 L for further experiments.

Biosorpti on Isotherms
An adsorption isotherm descris the fraction of sorbate mo lecules that are part itioned between liquid and solid phases at equilibriu m. Two isotherm models were tested:

Freundlich isotherm
The Freundlich isotherm model is the well known earliest relationship describing the adsorption process. This model applies to adsorption on heterogeneous surfaces with the interaction between adsorbed molecules and the application of the Freundlich equation also suggests that sorption energy exponentially decreases on completion of the sorption centers of an adsorbent. This isotherm is an emp irical equation and can be employed to describe heterogeneous systems and is expressed as follo ws in linear form [18]: where K F is the Freundlich constant related to the bonding energy. 1/n is the heterogeneity factor and n (g/L) is a measure of the deviation from linearity of adsorption.Freundlich equilibriu m constants were determined fro m the plot of log q e versus log C e , Figure 3 on the basis of the linear of Freundlich equation (3).The n value indicates the degree of non-linearity between solution concentration and adsorption as follows: if n=1, then adsorption is linear; if n<1, then adsorption is a chemical process; if n>1, then adsorption is a physical process. The n value in Freundlich equation was found to be 1.15 fo r P. avium , Table 1. Since n lie between 1 and 10, this indicate the physical biosorption of cadmiu m (II) onto P. avium. The values of regression coefficients R 2 are regarded as a measure of goodness of fit of the experimental data to the isotherm models.

Lang mu ir isotherm
The Lang muir isotherm assumes monolayer adsorption on a uniform surface with a fin ite nu mber o f adsorption sites [19]. Once a site is filled, no further sorption can take place at that site. As such the surface will eventually reach a saturation point where the maximu m adsorption of the surface will be achieved. The linear form of the Lang muir isotherm model is described as: where K L is the Lang muir constant related to the energy of adsorption and qmax is the maximu m adsorption capacity (mg/g) . Values of Lang muir parameters q max and K L were calculated fro m the slope and intercept of the linear p lot of C e /qe versus C e as shown in Figure 4. Values of qmax, K L and regression coefficient R 2 are listed in Table 1. These values for P. avium biosorbent indicated that Langmuir model describes the biosorption phenomena favarouble.
The essential characteristics of the Lang muir isotherm parameters can be used to predict the affinity between the sorbate and sorbent using separation factor or dimensionless equilibriu m parameter, R L e xpressed as in the following equation:

Biosorpti on Ki netics
Parameters fro m t wo kinetic models, pseudo first-order and pseudo second-order were fit to experimental data to examine the biosorption kinetics of cadmiu m (II) uptake by P. avium leaves powder.

Pseudo First-Order Kinetics
The pseudo-first order equation of Lagergren [20] is generally exp ressed as follows: Where qe and qt are the sorption capacities at equilibriu m and at time t , respectively (mg/g) and K 1 is the rate constant of pseudo-first order sorption, (1/ min). A fter integration and applying boundary conditions, q t = 0 to q t = q t at t = 0 to t = t ; the integrated form of equation (6) beco mes: The equation applicable to experimental results generally differs fro m a true first order equation in two ways: the parameter k 1 (qe -q t ) does not represent the number of available sites; and the parameter log q e is an adjustable parameter which is often not found equal to the intercept of a plot of log (q e -q t ) against t, whereas in a true first order sorption reaction log qe should be equal to the intercept of log(q e -q t ) against t. In order to fit equation (7) to e xperim ental data, the equilibriu m sorption capacity, q e must be known. In many cases is unknown and as chemisorption tends to become un measurably slow, the amount sorbed is still significantly s maller than the equilibriu m amount. In most cases in the literature, the pseudo-first order equation of Lagergren does not fit well fo r the whole range of contact time and is generally applicab le over the initial 20 to 60 minutes of the sorption process. Furthermore, one has to find some means of ext rapolating the experimental data to t = ∞, on treating qe as an adjustable parameter to be determined by trial and error. For this reason, it is therefore necessary to use trial and error to obtain the equilibriu m sorption capacity, in order to analyze the pseudo-first order model kinetics.
The pseudo first order rate constant can be obtained fro m the slope of plot between log (q e -q t ) against time, t. Figure 5 shows the Lagergren pseudo-first order kinetic plot fo r the adsorption of cadmiu m ions onto P. avium leaves powder. The calcu lated values and their co rresponding linear regress ion correlation coefficient values are shown in Table 2. The linear regression correlat ion coefficient value found 0.9506,which shows that this model cannot be applied to predict the adsorption kinetic model. The pseudo second-order rate expression, which has been applied for analyzing chemisorption kinetics rate [20] is expressed as: 2 2 ) ( t e t q q K dt dq − = (8) Where q e and q t are the sorption capacity at equilibriu m and at time t, respectively (mg/g) and k is the rate constant of pseudo-second order sorption, (g/mg min). For the boundary conditions to q t = 0 to q t = q t at t = 0 to t = t; ; the integrated form of equation (8) Where t is the contact time (min ), q e (mg/g) and q t (mg/g) are the amount of the solute adsorbed at equilib riu m and at any time, t. If pseudo-second order kinetics is applicable, the plot of t/q t versus t of equation (9) should give a linear relationship, fro m which q e and K 2 can be determined fro m the slope and intercept of the plot, Fig. 6. The pseudo-second o rder rate constant K 2 , the calcu lated q e value and the corre sponding linear regression correlat ion coefficient value are given in Table 2. At all init ial metal concentrations, the linear regression correlation coefficient R 2 values were h igher. The higher values confirm that the adsorption data are well represented by pseudo-second order kinetics. t (min) Figure 6. Pseudo-second order kinetic model for Cd (II) biosorption onto P. avium leaves powder at 303 K

Conclusions
The potential of P. avium leaves powder for the removal of Cd(II) ions fro m aqueous solutions and wastewater was dependent on biosorption process such as pH, init ial metal ions concentration, biosorbent dose, and contact time,. The Lang mu ir and Freundlich b iosorption isotherms were demo nstrated to provide best correlation for the biosorption of Cd(II) ions onto P. avium leaves powder. The kinetic results provided the best correlation of the experimental data of biosorption of cadmiu m ions onto P. avium leaves powder by pseudo second-order equation. it can be concluded that since the P. avium leaves are an easily, locally availab le, lo w-cost adsorbent and has a considerable high biosorption capacity , it may be treated as an alternative adsorbent for treat ment of wastewater containing cadmiu m (II) ions.