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Removal of heavy metal ions by using dithiocarbamated-sporopollenin
Separation and Purification Technology 52 (2007) 461–469
Removal of heavy metal ions by using dithiocarbamated-sporopollenin ¨ u, Mustafa Ersoz ∗ Nuri Unl¨ Department of Chemistry, Faculty of Arts and Sciences, University of Selcuk, Konya, Turkey Received 14 February 2006; received in revised form 25 May 2006; accepted 25 May 2006
Abstract In this study, the sorption conditions of Cu(II), Pb(II) and Cd(II) metal ions onto dithiocarbamated-spororpollenin (DTC-S) have been investigated. The different variables affecting the sorption capacity such as pH of the solution, sorption time, initial metal ion concentration and temperature have been investigated. Experimental data were exploited for kinetic and thermodynamic evaluations related to the sorption processes. Sorption isotherms correlated well with the Langmuir type sorption isotherm and sorption capacities were found to be 0.2734, 0.4572 and 0.0631 mmol g−1 for Cu(II), Pb(II) and Cd(II) metal ions, respectively. Sorption processes for three target heavy metal ions were found to follow pseudo-second order type sorption kinetics. Intraparticle diffusion was found to take part in sorption processes but it could not be accepted as the primary rate determining step. On the evaluation of the results obtained for the mean free energies of sorption (E) and enthalpy of sorption (H◦ ) it was observed that the resin mainly shows the characteristics of a chelating exchanger. Thermodynamic parameters, H◦ , S◦ and G◦ were also calculated from graphical interpretation of the experimental data. Standard heats of sorption (H◦ ) were found to be endothermic and S◦ values were calculated to be positive for the sorption of Cu(II), Pb(II) and Cd(II) ions onto the adsorbent. Negative G◦ values indicated that sorption process for these three metal ions onto DTC-Sp is spontaneous. © 2006 Elsevier B.V. All rights reserved. Keywords: Heavy metals; Dithiocarbamate; Sporopollenin; Langmuir isotherm; Freundlich isotherm; D–R isotherm; Kinetics; Thermodynamics
1. Introduction Heavy metal ions are the agents causing various diseases by binding vital cellular components of living organisms like structural proteins, enzymes, and nucleic acids. They are also present in aquatic media either as ions or compound forms and unfortunately are not biodegredable [1]. Cadmium, tin, lead, iron, copper, mercury, nickel, zinc and chromium are well known heavy metal pollutants. Waste effluents arisen from many industrial facilities like mining operations, textile industries, metal plating cause heavy metal pollution which is one of the major environmental problem that has to be controlled or solved [2]. Since their undesired effects on human physiology and ecological systems, heavy metals should be removed from the waste streams prior to their discharge. There are some well known heavy metal removal processes from waste effluents which include chemical precipitation, membrane filtration, ion exchange, sorption on carbon
∗
Corresponding author. E-mail address: [email protected] (M. Ersoz).
1383-5866/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2006.05.026
and coprecipitation. But these methods sometimes do not provide satisfactory removal rate to meet the pollution control limits or subjected to be expensive [3]. Therefore unconventional low-cost alternative technologies or sorbents are needed for wastewater treatment. Natural materials which are available from industrial waste products or agricultural operations can be used as potential inexpensive sorbents [4]. In literature various types of low-cost adsorbents have been cited including chitin/chitosan, lignin, bark/tanen rich materials, dead biomass, etc. [4]. The choice of an effective resin is an important concept and one of the rules of selection is based on the concept of hard and soft acids and bases [5]. Because sulphur-containing groups are namely soft bases and therefore it is reasonable to use sulphur containing functional group materials for removal of soft acids like Cu, Pb, Cd, etc. [6]. Dithiocarbamates are sulphur bearing functional groups and therefore dithiocarbamated resins are thought to be suitable for removal of metals of those soft acids. In the literature different types of dithiocarbamate functional groups were added to different polymeric supports for heavy metal removal and/or separation purposes, and sometimes achieving more satisfying subsequent instrumental analyses. Ima et al. [7], prepared
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tosylcellulose which was then reacted with amines and later converted to the dithiocarbamates. Murthy and Ryan [8] attached different types of amines onto microcrystslline cellulose support and prepared some different dithiocarbamated celluse derivatives. These resins were used for preconcentration of some metal ions from sea and tap water samples. Espinola et al. [9] prepared immobilized ditihiocarbamate covalently bonded to silica gel, by reacting a 3-propylethylenediamine group with carbon disulphide. The resin was used to extract some heavy metal ions from ethanol. Lezzi and Cobianco [10] introduced two sulphur bearing functional groups – dithiocarbamate and thiourea – into poly(styrene-g-ethylene glycols) by reacting the amino groups with CS2 and methylisothiocyanate. Denizli et al. [11] incorparated dithiocarbamate functional group to the monosize polystrene microsphers and exploited for the removal of mercury ions from aqueous solutions. Venkatesan et al. [12] studied the extraction of cobalt from aqueous solutions using dithiocarbamate grafted silica gel. They prepared the resin by reacting ␥-aminopropyltriethoxysilane with alkaline carbon disulphide. Roy et al. [13], synthesized a polydithiocarbamate resin supported on macroreticular styrene–divinylbenzene copolymer for the removal of some trace and heavy metal ions. These dithiocarbamate derivated chelating resins were prepared generally in two steps which are addition of amino groups into the polymeric support and conversion of the amino groups to the dithiocarbamates by reacting with carbon disulphide. Sporopollenin is a natural biopolymer which occurs in the outer membranes of moss and fern spores and most pollen grains [14]. It has been shown that spore and pollen membranes have two layers; the inner one is known as intine, and the outer one containing a material, which is called sporopollenin, is known as exine [15]. Sporopollenin is highly resistant to chemicals, is stable, has constant chemical structure and exihibits a good sability after even prolonged exposure to mineral acid and alkalis [16]. Infra-red and 13 C NMR spectroscopic studies on sporopollenin derived from pteridophyta and spermatophyta have shown that sporopollenin has aliphatic, aromatic, hydroxyl, carbonyl/carboxyl and ether functions in various portions in its polymeric structure [17]. Sporopollenin with its polymeric structure is suitable for adding some functional groups on it and would be a candidate low-cost adsorbent for removal of heavy metals from aquatic media. Some modified forms of the sporopollenin cited in the literature has been used as anion, cation or ligand exchangers [18–21]. Pehlivan et al. [22] prepared bis diaminoethyl glyoximated (bDAEG) and carboxylated (DAEC) sporopollenin resins studied the sorption behaviour of some heavy metal ions as a function of pH. Pehlivan et al. [23] studied the effects of pH and temperature on the sorption of some metal ions and they explained the uptake mechanism of the metal ions with the coordination to the donor nitrogen and oxygen atoms of glyoxime and of carboxylate functional groups present on the resin. But in literature there are not any study on the dithiocarbamate functionalized sporopollenin. In this study dithiocarbamated-sporopollenin was prepared as a novel adsorbent and utilized for removal of some heavy metal ions. For this purpose sorption conditions of Pb(II), Cu(II) and
Cd(II) ions have been investigated and also the nature of the sorption process with respect to its kinetics and thermodynamic aspects were evaluated. 2. Materials and methods 2.1. Materials The resin used was Lycopodium clavatum spores (sporopollenin) with 20 m particle size obtained from Fluka Chemicals. Sporopollenin obtained from L. clavatum spores were used as the supporting resin material for the preparation of dithiocarbamated-sporopollenin resin. All the other reagents used were of analytical reagent grade (Merck). Metal salts of CuCl2 ·2H2 O, Pb(NO3 )2 , CdCl2 ·H2 O were used to prepare metal ion solutions. The solutions (1000 mg L−1 ) were prepared by dissolving appropriate amounts of metal salts in doubly distilled water. The working solutions were prepared by diluting the stock solutions to appropriate volumes. Heavy metals are often present in industrial wastewaters together with some complex forming organic compounds and their separation from complexing compounds is very complicated due to the high stability constants of these complexes. Therefore, acetic acid–sodium acetate medium was chosen as the representative of complexing agents. For this purpose sorption studies were conducted in 0.1 M of acetic acid–acetate medium. This medium was also chosen to maintain an approximate equal ionic strength for the working solutions. 2.2. Preparation of the dithiocarbamated-sporopollenin resin Dithiocarbamated derivative of sporopollenin was synthesized in two steps. First, ethylenediamine (DAE) groups were introduced into the sporopollenin structure and then the amine groups were converted to dithiocarbamates by reacting with carbon disulfide. 2.2.1. Preparation of diaminoethyl-sporopollenin Diaminoethyl-sporopollenin (DAE-S) was prepared as described in the literature [22]. According to this procedure a suspansion of sporopollenin from L. clavatum in dry toluene (150 mL) containing 1,2-diaminoethane (50 mL) was mixed and refluxed for 9 h. This procedure was also applied in some works for the preparation of DAE-S and carboxylated diaminoethylsporopollenin (CDAE-sporopollenin) resins [18–21]. The reaction was as follows: S + NH2 –CH2 –CH2 –NH2 → S–NH–CH2 –CH2 –NH2 (DAE-sporopollenin) where S represents sporopollenin. 2.2.2. Preparation of dithiocarbamated-sporopollenin The amine groups of DAE-S were converted into their dithiocarbamates according to the procedure given in the literature [7].
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A mixture of previously obtained DAE-S (5 g), 20 mL of carbon disulfide, 36 mL of aqueous 28% ammonia and 200 mL of methanol was stirred at room temperature for 7 days. Upon completion of the reaction, dithiocarbamated-sporopollenin (DTCS) was filtered, washed throughly with doubly distilled water and methanol and dried. The end product was stored in a stoppered bottle in an ammonia gas atmosphere to prevent acid catalised dithiocarbamate break down. DTC-S formation reaction was:
capacities should be investigated. Thus, the effect of pH on sorption capacities were examined by varying the initial pHs of the solutions. The initial concentrations of metal ions were taken as 10, 20, 30 mg L−1 for Cu(II), Pb(II) and Cd(II), respectively. The effect of pH on the specific sorption of heavy metal ions on chelation by dithiocarbamated-sporopollenin has shown in Fig. 1. As it can be seen from Fig. 1, sorption capacities
2.3. Sorption experiments
increased with increasing pH up to an optimum pH. The resin exhibited low affinity towards Cd(II) and Pb(II) at pHs lower than 4. But for Cu(II), an adsorption maximum was obsorved at pH 4. Lower sorption capacities at lower pH values can be attributed to the competitive sorption of H3 O+ ions and metal ions for the same active sorption sites. This behaviour may be further explained with the dithiocabamate functional group introcuced to the sporopollenin. These functional groups must have taken part in metal uptake process by complexation which is pH dependent, and the nature of the active sites and sorbate must have been changed with pH [3,25]. After an optimum pH value sorption capacity started to decrease with increasing the pH [as in the case for Cu(II) and Pb(II)]. The actate complexation/competition with dithiocarbamate functionalities thought to be important after this optimum pH. The sorption capacities reached a maximum at pH values 4.0, 5.5 and 7.0 for the metal ions Cu(II), Pb(II) and Cd(II), respectively. For all subsequent experiments, these optimum pH values were used.
All the sorption equilibrium experiments were conducted batchwise; the sorption equilibrium was attained by shaking 0.05 g of sporopollenin in 30 mL of aquous solutions containing metal ions which were shaken at 100 rpm for a predetermined time period. Sorption experiments were carried out in an incubator at controlled temperature. After the predetermined sorption time, solution was filtered and the metal ion concentrations were measured. Initial and equilibrium metal ion concentrations in the aquous solutions were detemined by using a UNICAM 930 model flame atomic absorption spectrometer equiped with deuterium lamp background correction, Hallow Cathode Lamp and air acetylene burner. Initial pH of the solutions was adjusted to desired pH by adding hydrochloric acid or sodium hydroxide solutions to the medium to maintain a constant pH. A JENWAY 3010 model pH meter was used to adjust a desired pH value. The pH of the solution was buffered only between the pH values of 4–5.5. Temperature experiments were carried out between 20 and 65 ◦ C at optimum pH values for each metal ions. Experiments were repeated three times in each case. The amount of sorbed metal ion was calculated from the change in the metal concentration in the aqueous solution before and after equilibrium and the weight of the dry sporopollenin. The amount of metal ion adsorbed by sporopollenin was calculated as: (C0 − C)V (1) W where q is the amount of metal ions adsorbed onto unit amount of the resin (mmol g−1 ). C0 and C are the concentrations of metal ions in the initial and equilibrium concentrations of the metal ions in aqueous phase (mmol L−1 ), V is the volume of the aqueous phase (L) and W is the dry weight of the resin (g). q=
3.2. The effect of contact time and sorption dynamics The effect of contact time on metal ion uptake capacities has shown in Fig. 2. As seen from the figure, there is a rapid uptake kinetics and sorption equilibria would be accepted to be attained within the first 30 min. But unless otherwise stated, 1 h of equilibration time was chosen as the optimum contact
3. Results and discussion 3.1. Effect of pH It is a well known phenomenon that sorption of heavy metal ions by resins is dependent on pH because of the pH dependencies of the complexation reactions or physisorption processes at the adsorption surface. Since sporopollenin has some functional groups on its natural stucture [24] and also dithiocarbamate funtionality on its dithiocarbamated form, effect of pH on sorption
Fig. 1. The effect of pH on the sorption of Cu(II), Pb(II) and Cd(II) metal ions. [() Pb(II)] C0 = 20 mg L−1 ; [() Cu(II)] C0 = 10 mg L−1 ; [() Cd(II)] C0 = 30 mg L−1 ; t: 120 min; m(DTC-S): 0.05 g; medium: 0.1 M acetic acid–acetate; temperature: 20 ◦ C.
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Fig. 2. Sorption rates of heavy metal ions onto DTC-S. [() Pb(II)] C0 = 30 mg L−1 , pH 5.5; [() Cu(II)] C0 = 8 mg L−1 , pH 4.0; [() Cd(II)] C0 = 15 mg L−1 , pH 7.0; m(DTC-S): 0.05 g; medium: 0.1 M acetic acid–acetate; temperature: 20 ◦ C.
time to ensure that equilibrium conditions were achived. When kinetic considerations are taken into account, the presence of functional groups on a chelating polymer is usually evaluated together with the accessibility to these functional groups without sterical hinderence, which is greatly determined by polymeric matrics. And, the sorbents with the best charateristics are defined as those having hydrophilic macroporous structures. In our study the observed rapid uptake kinetics can be attributed to the hydophilic porous structure of the sporopollenin itself [26] and easily available dithiocarbamate functional groups on DTC-S. There are a lot of parameters effecting the sorption rates like structural properties of the sorbent, metal ion properties, initial concentration of metal ions, pH, temperature, chelate formation rate or presence of competing ions. Therefore, it is difficult to make a comparison between the sorbents. But, when compared with the results stated in the literature where time required to attain equilibrium ranged from 30 min to 7 h, the sorption of Cu(II), Pb(II) and Cd(II) metal ions onto DTC-S is seemed to be suitable from kinetic considerations. It is known that sorption kinetics are dependent or controlled by different kind of mechanisms like mass transfer, diffusion control, chemical reactions, and particle diffusion. In order to clarify the kinetic characteristics of the sorption three well known kinetic models were applied to evaluate experimental data. For this purpose Lagergren’s pseudo-first order kinetic model, pseudo-second order kinetic model and intra particle diffusion model were used.
Fig. 3. Lagergren’s pseudo-first order plots for heavy metal ions on DTC-S. [() Pb(II)] C0 = 30 mg L−1 , pH 5.5; [() Cu(II)] C0 = 8 mg L−1 , pH 4.0; [() Cd(II)] C0 = 15 mg L−1 , pH 7.0; m(DTC-S): 0.05 g; medium: 0.1 M acetic acid–acetate; temperature: 20 ◦ C.
3.2.1. First-order kinetics The linearized form of the first-order rate equation by Lagergren and Svenska [27] is given as: kads t (2) log(qe − qt ) = log qe − 2.303 where qe and qt are the amounts of the metal ions adsorbed (mg g−1 ) at equilibrium and at time t (min), respectively, and kads is the sorption rate constant (min−1 ). The plots of log(qe − qt ) versus t gives a straight line and the rate constants (kads ) and theoratical equilibrium sorption capacities, qe (theoric), can be calculated from the slopes and intercepts. In Fig. 3 Lagergren’s plots and constants related to these plots are given in Table 1. Straight lines obtained from Lagergren’s plots suggest the applicability of the pseudo first-order kinetic model to fit the experimental data. But it is also required that theoreticaly calculated equilibrium sorption capacities, qe (theor.), should be in accordance with the experimental sorption capacity, qe (exp.), values. As can be seen from Table 1, linear correlation coefficients of the plots, except for Pb, are not good. And, qe (theor.) and qe (exp.), values are not in agreement and with each other. So, it could suggested that the adorption of Cu(II), Pb(II) and Cd(II) metal ions onto DTC-S is not a first-order reaction. 3.2.2. Second-order kinetics The experimental data were also applied to the pseudo-second order kinetic model which is given with the equation below: 1 1 t t (3) = + qt k2 qe2 qe
Table 1 First-order, second-order and intraparticle diffusion rate constants Metal
Cu Pb Cd
First-order rate constants (min−1 )
qe (exp.) (mg g−1 )
kads
2.587 18.30 3.81
0.0318 0.0882 0.0262
Second-order rate constants (g mg−1
qe (theor.) (mg g−1 )
R2
k2
0.866 14.86 0.566
0.9791 0.9902 0.9548
0.090 0.014 0.209
min−1 )
Intraparticle diffusion rate constants
qe (theor.) (mg g−1 )
R2
kid (g g−1 min−1/2 )
R2
2.62 18.98 3.82
0.9975 0.9996 0.9997
83.20 1141.20 103.40
0.9961 0.9249 0.9888
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Fig. 4. Pseudo second-order plots for heavy metal ions on sporopollenin. [() Pb(II)] C0 = 30 mg L−1 , pH 5.5; [() Cu(II)] C0 = 8 mg L−1 , pH 4.0; [() Cd(II)] C0 = 15 mg L−1 , pH 7.0; m(DTC-S): 0.05 g; medium: 0.1 M acetic acid–acetate; temperature: 20 ◦ C.
where k2 (g mg−1 min−1 ) is the rate constant of pseudo secondorder sorption reaction. Second-order kinetic is said to be applicable if the plot of t/qt versus t shows linearity. Fig. 4 shows the plots obtained from the graphical interpretation of the data for the second-order kinetic model. The constants related to these plots are given in Table 1. As it can be seen from the results given in Table 1, correlation coefficients are higher compared to the results obtained from the first-order kinetics. And, theoretical and experimental qe values are in a good accordance with each other. So, it is possible to suggest that the sorption of Cu(II), Pb(II) and Cd(II) metal ions onto DTC-S followed a second-order type reaction kinetics. It is known that ordinary exchange processes are rapid and controlled mainly by diffusion, whereas those in a chelating exchanger are slower and controlled either by particle diffusion or by a second order chemical reaction [6]. Moreover, it is a general understanding that pseudo-second order kinetics provide best fits to the experimental data for the sorption systems where chemisorption seems significant in the rate controlling step [28]. DTC-S behaves as a chelating exchanger because of the dithiocarbamate functionality present on it and sorption to the resin would take place via valency forces between the metal ions and the sorbent. Therefore, second-order chemical reaction kinetics would be expected to be followed in the sorption processes and for our resin it is possible to state that chemisorption is the rate determining step in the sorption process. 3.2.3. Intraparticle diffusion In aqueous solutions, when sorptions to the porous sorbents are considered, mesopores may act as micropores because of the potentially formation of water layers on the pore walls [29]. And also it is known that, both pore and surface diffusions may play role during the sorption processes to the macroporous structures [30]. Since because sporopollenin has a porous structure, the effect of intraparticle diffusion to the sorption process should also be taken into accaunt.
Fig. 5. Intraparticle diffusion plots for heavy metal ions on DTC-S.
Intraparticle diffusion model is expressed with the equation given by Weber and Morris [31]: qt = kid t 1/2
(4)
where qt is the amount of metal ions adsorbed at time t (g g−1 ), kid is the intraparticle diffusion rate constant (g g−1 min−1/2 ). Plots of qt versus t1/2 are shown in Fig. 5a and b. In such graphs, initial curved/steep sloped portions are attributed to the bulk diffusion or exterior sorption rate which is very high. Only the subsequent linear portions can be attributed to intraparticle diffusion. And, plateau portions represent the equilibrium. In sorption processes intraparticle diffusion can be accepted as a rate determining step when if the plots of qt versus t1/2 show linearity. As can be seen from Fig. 5, because of the non-linear distribution of the plots and deviation of the curves from the origin, intraparicle diffusion cannot be accepted as the only rate determining step for the sorption of the heavy metal ions studied on DTC-S. But on the comparison of the kid values calculated from the linear portions of the plots (Table 1), it is possible to suggest that intraparticle diffusion, which may play an important role as a rate determining step, is said to be more effective for Pb(II) ions. The kid value for Pb(II) is higher than those for Cu(II) and Cd(II). This can be attributed to the ralatively small radius of the hydrated Pb(II) ions.
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3.3.2. Freundlich isotherms Freundlich equation is derived to model the multilayer sorption and for the sorption on heterogeneous surfaces. Linearised form of the Freundlich equation [33] is given by the following equation: log qe = log KF +
Fig. 6. Effect of initial concentration on heavy metal sorption on DTC-S. [() Pb(II)] pH 5.5; [() Cu(II)] pH 4.0; [() Cd(II)] pH 7.0; m(DTC-S): 0.05 g; t: 60 min; medium: 0.1 M acetic acid–acetate; temperature: 20 ◦ C.
3.3. Sorption capacity Fig. 6 shows the effect initial metal ion concentrations of Cu(II), Pb(II) and Cd(II) metal ions on the equilibrium sorption capacities. It is clear from Fig. 6, that the sorption capacities were incresed with increasing the initial concentrations and reached a plateau which represents the maximum sorption capacity of resin. This behaviour can be explained with the high driving force for the mass transfer. The analysis of the sorption isotherms is important for design purposes. Therefore, experimental data were analyzed with three well known sorption isotherm models of Langmuir, Freundlich and D–R to develop an equation which accurately represents the results. 3.3.1. Langmuir isotherms Langmuir sorption isotherm models the monolayer covarage of the sorption surfaces and assumes that sorption occurs on a structrurally homogeneous adsorbent and all the sorption sites are energetically identical. Linearised form of the Langmuir equation [32] is given by the following equation: 1 1 1 = 0+ qe Q bQ0 Ce
1 log Ce n
(6)
where qe is the equilibrium solute concentration on adsorbent (mmol g−1 ), Ce the equilibrium concentration of the solute (mmol L−1 ), KF the Freundlich constant (mmol g−1 ) which indicates the sorption capacity and represents the strength of the adsorptive bond and n is the heterogenity factor which represents the bond distribution. According to this equation the plot of the log qe versus log Ce gives a straight line and KF and n values can be calculated from the intercept and slope of this straight line, respectively. The constants related to Langmuir and Freundlich equations are given in Table 2. On the comparison of the R2 values, we can conclude that sorption isotherm data obtained for the metal ions studied on DTC-S can be described better by Langmuir equation. This result predicts the homogenity of the sorption sites on DTCS. In our previous work [24] sorption isotherm data obtained for unmodified sporopollenin were found to fit Freundlich type equation better. Therefore, it is possible to suggest that after the modification process, dithiocarbamated-sporopollenin (DTC-S) became structrurally homogeneous and all the sorption sites on the adsorbent are energetically identical. The Langmuir constant Q0 indicates the sorption capacity of the sorbent. As it can be seen from Table 2, Q0 values were found to be 0.2734, 0.4572 and 0.0631 mmol g−1 for Cu(II), Pb(II) and Cd(II), respectively. These sorption capacities are much higher (especially for Cu and Pb) than the unmodified form of the sporopollenin itself for which sorption capacities were given as 0.0195, 0,0411 and 0.0146 mmol g−1 , respectively [24]. As can be seen from Table 3, these sorption capacities are comparable values when compared to some other dithiocarbamate functionalized resins given in the literature.
(5)
where qe is the amount of solute adsorbed on the surface of the adsorbent (mmol g−1 ), Ce the equilibrium ion concentration in the solution (mmol L−1 ), Q0 the maximum surface density at monolayer covarage and b is the Langmuir sorption constant (L mmol−1 ). The plots of 1/qe versus 1/Ce give a straight line and the values of Q0 and b can be calculated from the intercept and slope of the plots, respectively.
3.3.3. D–R isotherms D–R isotherm describes sorption on a single type of uniform pores. In this respect the D–R isotherm is an anologue of Langmuir type but it is more general because it does not assume a homogeneous surface or constant sorption potential [36]. The D–R isotherm is given with the following equation, Q = Qm exp(−kε2 )
(7)
Table 2 Langmuir, Freundlich and D–R isotherm constants Metal
Cu Pb Cd
Langmuir isotherm parameters
Freundlich isotherm parameters
D–R isotherm parameters
Q0 (mmol g−1 )
b (L mmol−1 )
R2
KF (mmol g−1 )
n
R2
Qm (mmol g−1 )
k (mol2 kJ−2 )
E (kJ mol−1 )
R2
0.2734 0.4572 0.0631
2.032 1.268 15.44
1.00 0.9976 0.9951
0.153 0.248 0.0597
1.96 1.70 5.63
0.9572 0.9949 0.9225
0.809 1.668 0.102
−0.0055 −0.0064 −0.0019
9.53 8.84 16.22
0.9795 0.9992 0.949
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Table 3 Sorption capacities of some dithiocarbamate functionalized resins Resin supporting material
Cellulose (different derivatives) Poly(styrene-g-ethylene glycols) Polystyrene microspheresa Styrene–divinylbenzene copolymer Poly(acrylaminophosphonic) fibers Activated carbon Sporopollenin (our study) a
Sorption capacities (mg g−1 )
Reference
Cu
Pb
Cd
7.3–15 1.85 3.76 35.0 2.03 38.0 17.4
6.9–9.5 1.95 6.17 23.0 5.72 – 94.7
– 1.69 1.92 – 0.094 – 7.09
[7] [10] [11] [13] [34] [35] –
Sorption capacities in the presence of the heavy metal ions.
and linearized form of the equation is given as, ln Q = ln Qm − kε2
(8)
where ε (Polanyi Potential) is [RT ln(1 + (1/Ce ))], Q is the amount of solute adsorbed per unit weight of adsorbent (mol g−1 ), k is a constant related to the sorption energy (mol2 kJ−2 ), Qm is the sorption capacity (mol g−1 ). The values of Qm and k were calculated from the intercept and slope of the ln Q versus ε2 plots and presented in Table 2. The mean free energy of sorption (E) was calculated from the k values using the equation E = (−2k)−0.5
icent values, (KD ) as a function of temperature (Fig. 7). It was obsorved that the distribution coefficent values, (KD ), increased with increase in temperature and that shows the endothermic nature of the sorption. Thermodynamic parameters like free energy change (G◦ ), enthalpy change (H◦ ), and entropy change (S◦ ) were estimated using the following equations. The Gibb’s free energy change of the process is related to the distribution coefficient (KD ), by the equation below: ◦
G = −RT ln KD ◦
◦
G = H − T S ◦
(9)
Langmuir and Freundlich isotherms do not give any idea about sorption mechanisms. But, the magnitude of E is a useful mean for estimating the type of sorption process. If this value is between 8 and 16 kJ mol−1 , sorption process can be explained by ion exchange [30]. In this study, for Pb(II) and Cu(II) ions E values were calculated to be between the values of ion exchange. But, for Cd(II) the calculated E value was found to be above the value of 16 kJ mol−1 that indicates the chelating behaviour of DTC-S. Therefore, it is possible to say that sorption mechanism of heavy metal ions (Cu(II), Pb(II) and Cd(II)) on DTC-sporopollenin can be explained with an ion exchange process. But chelating effect of the ditihiocarbamate functional groups on DTC-S is also thought to take part in the sorption process and chemisorption was found to be the rate determining step. According to the E values it is also possible to make a comparison between the comlexation strengths of the metal ions with dithiocarbamate functionality. E values for the metal ions studied were found to be in order of Cd(II) > Cu(II) > Pb(II) and this order may be correlated well with the complexation strengths. It is well known that b values calculated from the Langmuir isotherms are constants related to the affinity between the adsorbent and sorbate. A large value of b implies strong binding. The b values (given in Table 2) are also supporting and in accordance with the proposed complexation strenghts given above. 3.4. Thermodynamics of the sorption The effect of temperature on the sorption of heavy metal ions onto DTC-S is given as the plots of the distribution coeff-
log KD =
(10) ◦
(11) ◦
H S − 2.303R 2.303RT
(12)
where KD is the distribution coefficient (cm3 g−1 ), and R is gas constant (kJ mol−1 K−1 ). According to the last equation given above the values of H◦ and S◦ can be calculated from the slopes H◦ /2.303R and intercepts S◦ /2.303R of log KD versus 1/T plots. The calculated values of thermodynamic parameters were given in Table 4. As it can be seenfrom Table 4, positive H◦ values represents the endothermic nature of the sorption process. The heat of sorption values between 5.0 and 100 kcal mol−1 (20.9–418.4 kJ mol−1 ), which are heats of chemical reactions, are frequently assumed as the comparable values for the chemical sorption processes. H◦ values were found as 23.61, 25.67 and 21.24 kJ mol−1 for Cu(II), Pb(II) and Cd(II), respectively. According to the calculated E values, previously
Fig. 7. log KD –1/T graphs for the sorption of Cu(II), Pb(II) and Cd(II) onto DTC-S. [() Cu(II)] C0 = 75 mg L−1 , pH 4.0; [() Pb(II)] C0 = 10 mg L−1 , pH 5.5; [() Cd(II)] C0 = 10 mg L−1 , pH 7.0; m(DTC-S): 0.05 g; t: 60 min; medium: 0.1 M acetic acid–acetate.
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Table 4 Thermodynamic parameters for the sorption of Cu(II), Pb(II) and Cd(II) on DTC-S Metal
Cu
Pb
Cd
C0 (mg L−1 )
75
10
10
H◦ (kJ mol−1 )
23.61
25.67
21.24
S◦ (J K−1 mol−1 )
T (K)
G◦ (kJ mol−1 )
R2
119.15
298 303 313 323 338
−11.30 −12.49 −13.68 −14.87 −16.66
0.9922
146.49
303 313 323 338
−18.72 −20.19 −21.65 −23.85
0.9962
119.07
298 303 313 323 338
−14.25 −14.84 −16.03 −17.22 −19.00
0.9795
we described the adorption mechanism of the heavy metal ions [Cu(II), Pb(II) and Cd(II)] onto the sorbent as an ion exchange process. But, it is clear from the H◦ values beyond the heats of chemisorption that dithiocarbamate functional groups also contribute to the sorption process with their chelating effects. These results are also in good agreement with the results and comments done on the calculated E values and support the possibility of the chemisorption that is the rate determining step in the sorption process. Negative values of G◦ indicates the spontaneous nature of the reaction. The reaction is favored and getting easier at higher temperatures. S◦ values were found to be positive due to the exchange of the metal ions with more mobile ions present on the exchanger, which would cause increse in the entropy during the adsoption process. In the case of physisorption, which may also contribute to the total sorption process, water molecules released from the hydrated metal ions or from the sorption surface may also cause an increase in the entropy.
(2)
(3)
(4) (5)
characteristics of the sorption sites on sporopollenin. Sorption capacities order was found as Pb(II) > Cu(II) Cd(II). Sorption of the Cu(II), Pb(II) and Cd(II) ions onto DTCS can be explained with an ion exchange process. But it is evident from the values of mean free energy of sorption (E) and enthalpy of sorption (H◦ ) that dithiocarbamate functional groups are also taking part in the sorption process. The sorption process for the heavy metal ions studied on DTC-S, was found to be endothermic, spontaneous and can be explained with pseudo second-oder type kinetic model. Intraparticle diffusion effect was obseved to be more effective for Cu(II) and Cd(II) metal ions but, from kinetic considerations intraparticle diffusion cannot be accepted as the rate determinig step. DTC-S mainly showed characteristics of a chelating exchanger. DTC-S showed higher sorption capacities when compared to the unmodified form of the sporopollenin.
3.5. Regenerability In regeneration studies 0.2 and 2 M of concentrations of HCl and HNO3 were used. But only about 10% of the adsorbed metals were recovered. Similiar behaviour were also observed in some works done on some sulfur bearing chelating resins. Lezzi and Cobianco [10] prepared chelating resins by introducing dithiocarbamate and methyl thiourea groups on poly(styrene-gethylene glycol) polymer. They studied the adsorption of some heavy metal ions [Hg(II), Cu(II), Pb(II) and Cd(II)] and dithiocabamate resin was reported to be regenerated by complete mineralization of the metal complexes. In a similiar work by Lezzi et al. [37], only 25% of the Hg(II) ions were reported to be eluted from the thiol chealating resins by treatment with a 6 N HCl solution. 4. Conclusions (1) Sorption on DTC-S can be expressed better with Langmuir type sorption isotherms which shows the homogeneous
References [1] G.T. Spiro, W.M. Stigliani, Chemistry of the Environment, Prentice Hall, New Jersey, 1996. [2] S. Rengaraj, Y. Kim, C.K. Joo, J. Yi, J. Colloid Interf. Sci. 273 (2004) 14. [3] V.C. Taty-Costodes, H. Fauduet, C. Porte, A. Delacroix, J. Hazard. Mater. B 105 (2003) 121. [4] E.S. Bailey, T.J. Olin, R.M. Bricka, D.D. Adrian, Water Res. 33 (1999) 2469. [5] R.G. Pearson, J. Am. Chem. Soc. 85 (1963) 3533. [6] G. Kantipuly, S. Katragadda, A. Chow, H.D. Gesser, Talanta 37 (1990) 491. [7] S. Ima, M. Muroi, A. Hamaguchi, Anal. Chim. Acta 113 (1980) 139. [8] R.S.S. Murthy, D.E. Ryan, Anal. Chim. Acta 140 (1982) 139. [9] J.G.P. Espinola, J.M.P. Freitas, S.F. Oliveira, C. Airoldi, Colloid Surf. A: Physicochem. Eng. Aspects 87 (1994) 33. [10] A. Lezzi, S. Cobianco, J. Appl. Polym. Sci. 54 (1994) 889. [11] A. Denizli, K. Kesenci, Y. Arıca, E. Pis¸kin, React. Funct. Polym. 44 (2000) 235. [12] K.A. Venkatesan, T.G. Srinivasan, P.R.V. Rao, Colloid Surf. A: Physicochem. Eng. Aspects 180 (2001) 277. [13] P.K. Roy, A.S. Rawat, P.K. Rai, Talanta 59 (2003) 239. [14] F. Zetzche, H. Vicari, Helv. Chim. Acta 14 (1931) 58.
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Removal of heavy metal ions by using dithiocarbamated-sporopollenin ¨ u, Mustafa Ersoz ∗ Nuri Unl¨ Department of Chemistry, Faculty of Arts and Sciences, University of Selcuk, Konya, Turkey Received 14 February 2006; received in revised form 25 May 2006; accepted 25 May 2006
Abstract In this study, the sorption conditions of Cu(II), Pb(II) and Cd(II) metal ions onto dithiocarbamated-spororpollenin (DTC-S) have been investigated. The different variables affecting the sorption capacity such as pH of the solution, sorption time, initial metal ion concentration and temperature have been investigated. Experimental data were exploited for kinetic and thermodynamic evaluations related to the sorption processes. Sorption isotherms correlated well with the Langmuir type sorption isotherm and sorption capacities were found to be 0.2734, 0.4572 and 0.0631 mmol g−1 for Cu(II), Pb(II) and Cd(II) metal ions, respectively. Sorption processes for three target heavy metal ions were found to follow pseudo-second order type sorption kinetics. Intraparticle diffusion was found to take part in sorption processes but it could not be accepted as the primary rate determining step. On the evaluation of the results obtained for the mean free energies of sorption (E) and enthalpy of sorption (H◦ ) it was observed that the resin mainly shows the characteristics of a chelating exchanger. Thermodynamic parameters, H◦ , S◦ and G◦ were also calculated from graphical interpretation of the experimental data. Standard heats of sorption (H◦ ) were found to be endothermic and S◦ values were calculated to be positive for the sorption of Cu(II), Pb(II) and Cd(II) ions onto the adsorbent. Negative G◦ values indicated that sorption process for these three metal ions onto DTC-Sp is spontaneous. © 2006 Elsevier B.V. All rights reserved. Keywords: Heavy metals; Dithiocarbamate; Sporopollenin; Langmuir isotherm; Freundlich isotherm; D–R isotherm; Kinetics; Thermodynamics
1. Introduction Heavy metal ions are the agents causing various diseases by binding vital cellular components of living organisms like structural proteins, enzymes, and nucleic acids. They are also present in aquatic media either as ions or compound forms and unfortunately are not biodegredable [1]. Cadmium, tin, lead, iron, copper, mercury, nickel, zinc and chromium are well known heavy metal pollutants. Waste effluents arisen from many industrial facilities like mining operations, textile industries, metal plating cause heavy metal pollution which is one of the major environmental problem that has to be controlled or solved [2]. Since their undesired effects on human physiology and ecological systems, heavy metals should be removed from the waste streams prior to their discharge. There are some well known heavy metal removal processes from waste effluents which include chemical precipitation, membrane filtration, ion exchange, sorption on carbon
∗
Corresponding author. E-mail address: [email protected] (M. Ersoz).
1383-5866/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.seppur.2006.05.026
and coprecipitation. But these methods sometimes do not provide satisfactory removal rate to meet the pollution control limits or subjected to be expensive [3]. Therefore unconventional low-cost alternative technologies or sorbents are needed for wastewater treatment. Natural materials which are available from industrial waste products or agricultural operations can be used as potential inexpensive sorbents [4]. In literature various types of low-cost adsorbents have been cited including chitin/chitosan, lignin, bark/tanen rich materials, dead biomass, etc. [4]. The choice of an effective resin is an important concept and one of the rules of selection is based on the concept of hard and soft acids and bases [5]. Because sulphur-containing groups are namely soft bases and therefore it is reasonable to use sulphur containing functional group materials for removal of soft acids like Cu, Pb, Cd, etc. [6]. Dithiocarbamates are sulphur bearing functional groups and therefore dithiocarbamated resins are thought to be suitable for removal of metals of those soft acids. In the literature different types of dithiocarbamate functional groups were added to different polymeric supports for heavy metal removal and/or separation purposes, and sometimes achieving more satisfying subsequent instrumental analyses. Ima et al. [7], prepared
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tosylcellulose which was then reacted with amines and later converted to the dithiocarbamates. Murthy and Ryan [8] attached different types of amines onto microcrystslline cellulose support and prepared some different dithiocarbamated celluse derivatives. These resins were used for preconcentration of some metal ions from sea and tap water samples. Espinola et al. [9] prepared immobilized ditihiocarbamate covalently bonded to silica gel, by reacting a 3-propylethylenediamine group with carbon disulphide. The resin was used to extract some heavy metal ions from ethanol. Lezzi and Cobianco [10] introduced two sulphur bearing functional groups – dithiocarbamate and thiourea – into poly(styrene-g-ethylene glycols) by reacting the amino groups with CS2 and methylisothiocyanate. Denizli et al. [11] incorparated dithiocarbamate functional group to the monosize polystrene microsphers and exploited for the removal of mercury ions from aqueous solutions. Venkatesan et al. [12] studied the extraction of cobalt from aqueous solutions using dithiocarbamate grafted silica gel. They prepared the resin by reacting ␥-aminopropyltriethoxysilane with alkaline carbon disulphide. Roy et al. [13], synthesized a polydithiocarbamate resin supported on macroreticular styrene–divinylbenzene copolymer for the removal of some trace and heavy metal ions. These dithiocarbamate derivated chelating resins were prepared generally in two steps which are addition of amino groups into the polymeric support and conversion of the amino groups to the dithiocarbamates by reacting with carbon disulphide. Sporopollenin is a natural biopolymer which occurs in the outer membranes of moss and fern spores and most pollen grains [14]. It has been shown that spore and pollen membranes have two layers; the inner one is known as intine, and the outer one containing a material, which is called sporopollenin, is known as exine [15]. Sporopollenin is highly resistant to chemicals, is stable, has constant chemical structure and exihibits a good sability after even prolonged exposure to mineral acid and alkalis [16]. Infra-red and 13 C NMR spectroscopic studies on sporopollenin derived from pteridophyta and spermatophyta have shown that sporopollenin has aliphatic, aromatic, hydroxyl, carbonyl/carboxyl and ether functions in various portions in its polymeric structure [17]. Sporopollenin with its polymeric structure is suitable for adding some functional groups on it and would be a candidate low-cost adsorbent for removal of heavy metals from aquatic media. Some modified forms of the sporopollenin cited in the literature has been used as anion, cation or ligand exchangers [18–21]. Pehlivan et al. [22] prepared bis diaminoethyl glyoximated (bDAEG) and carboxylated (DAEC) sporopollenin resins studied the sorption behaviour of some heavy metal ions as a function of pH. Pehlivan et al. [23] studied the effects of pH and temperature on the sorption of some metal ions and they explained the uptake mechanism of the metal ions with the coordination to the donor nitrogen and oxygen atoms of glyoxime and of carboxylate functional groups present on the resin. But in literature there are not any study on the dithiocarbamate functionalized sporopollenin. In this study dithiocarbamated-sporopollenin was prepared as a novel adsorbent and utilized for removal of some heavy metal ions. For this purpose sorption conditions of Pb(II), Cu(II) and
Cd(II) ions have been investigated and also the nature of the sorption process with respect to its kinetics and thermodynamic aspects were evaluated. 2. Materials and methods 2.1. Materials The resin used was Lycopodium clavatum spores (sporopollenin) with 20 m particle size obtained from Fluka Chemicals. Sporopollenin obtained from L. clavatum spores were used as the supporting resin material for the preparation of dithiocarbamated-sporopollenin resin. All the other reagents used were of analytical reagent grade (Merck). Metal salts of CuCl2 ·2H2 O, Pb(NO3 )2 , CdCl2 ·H2 O were used to prepare metal ion solutions. The solutions (1000 mg L−1 ) were prepared by dissolving appropriate amounts of metal salts in doubly distilled water. The working solutions were prepared by diluting the stock solutions to appropriate volumes. Heavy metals are often present in industrial wastewaters together with some complex forming organic compounds and their separation from complexing compounds is very complicated due to the high stability constants of these complexes. Therefore, acetic acid–sodium acetate medium was chosen as the representative of complexing agents. For this purpose sorption studies were conducted in 0.1 M of acetic acid–acetate medium. This medium was also chosen to maintain an approximate equal ionic strength for the working solutions. 2.2. Preparation of the dithiocarbamated-sporopollenin resin Dithiocarbamated derivative of sporopollenin was synthesized in two steps. First, ethylenediamine (DAE) groups were introduced into the sporopollenin structure and then the amine groups were converted to dithiocarbamates by reacting with carbon disulfide. 2.2.1. Preparation of diaminoethyl-sporopollenin Diaminoethyl-sporopollenin (DAE-S) was prepared as described in the literature [22]. According to this procedure a suspansion of sporopollenin from L. clavatum in dry toluene (150 mL) containing 1,2-diaminoethane (50 mL) was mixed and refluxed for 9 h. This procedure was also applied in some works for the preparation of DAE-S and carboxylated diaminoethylsporopollenin (CDAE-sporopollenin) resins [18–21]. The reaction was as follows: S + NH2 –CH2 –CH2 –NH2 → S–NH–CH2 –CH2 –NH2 (DAE-sporopollenin) where S represents sporopollenin. 2.2.2. Preparation of dithiocarbamated-sporopollenin The amine groups of DAE-S were converted into their dithiocarbamates according to the procedure given in the literature [7].
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A mixture of previously obtained DAE-S (5 g), 20 mL of carbon disulfide, 36 mL of aqueous 28% ammonia and 200 mL of methanol was stirred at room temperature for 7 days. Upon completion of the reaction, dithiocarbamated-sporopollenin (DTCS) was filtered, washed throughly with doubly distilled water and methanol and dried. The end product was stored in a stoppered bottle in an ammonia gas atmosphere to prevent acid catalised dithiocarbamate break down. DTC-S formation reaction was:
capacities should be investigated. Thus, the effect of pH on sorption capacities were examined by varying the initial pHs of the solutions. The initial concentrations of metal ions were taken as 10, 20, 30 mg L−1 for Cu(II), Pb(II) and Cd(II), respectively. The effect of pH on the specific sorption of heavy metal ions on chelation by dithiocarbamated-sporopollenin has shown in Fig. 1. As it can be seen from Fig. 1, sorption capacities
2.3. Sorption experiments
increased with increasing pH up to an optimum pH. The resin exhibited low affinity towards Cd(II) and Pb(II) at pHs lower than 4. But for Cu(II), an adsorption maximum was obsorved at pH 4. Lower sorption capacities at lower pH values can be attributed to the competitive sorption of H3 O+ ions and metal ions for the same active sorption sites. This behaviour may be further explained with the dithiocabamate functional group introcuced to the sporopollenin. These functional groups must have taken part in metal uptake process by complexation which is pH dependent, and the nature of the active sites and sorbate must have been changed with pH [3,25]. After an optimum pH value sorption capacity started to decrease with increasing the pH [as in the case for Cu(II) and Pb(II)]. The actate complexation/competition with dithiocarbamate functionalities thought to be important after this optimum pH. The sorption capacities reached a maximum at pH values 4.0, 5.5 and 7.0 for the metal ions Cu(II), Pb(II) and Cd(II), respectively. For all subsequent experiments, these optimum pH values were used.
All the sorption equilibrium experiments were conducted batchwise; the sorption equilibrium was attained by shaking 0.05 g of sporopollenin in 30 mL of aquous solutions containing metal ions which were shaken at 100 rpm for a predetermined time period. Sorption experiments were carried out in an incubator at controlled temperature. After the predetermined sorption time, solution was filtered and the metal ion concentrations were measured. Initial and equilibrium metal ion concentrations in the aquous solutions were detemined by using a UNICAM 930 model flame atomic absorption spectrometer equiped with deuterium lamp background correction, Hallow Cathode Lamp and air acetylene burner. Initial pH of the solutions was adjusted to desired pH by adding hydrochloric acid or sodium hydroxide solutions to the medium to maintain a constant pH. A JENWAY 3010 model pH meter was used to adjust a desired pH value. The pH of the solution was buffered only between the pH values of 4–5.5. Temperature experiments were carried out between 20 and 65 ◦ C at optimum pH values for each metal ions. Experiments were repeated three times in each case. The amount of sorbed metal ion was calculated from the change in the metal concentration in the aqueous solution before and after equilibrium and the weight of the dry sporopollenin. The amount of metal ion adsorbed by sporopollenin was calculated as: (C0 − C)V (1) W where q is the amount of metal ions adsorbed onto unit amount of the resin (mmol g−1 ). C0 and C are the concentrations of metal ions in the initial and equilibrium concentrations of the metal ions in aqueous phase (mmol L−1 ), V is the volume of the aqueous phase (L) and W is the dry weight of the resin (g). q=
3.2. The effect of contact time and sorption dynamics The effect of contact time on metal ion uptake capacities has shown in Fig. 2. As seen from the figure, there is a rapid uptake kinetics and sorption equilibria would be accepted to be attained within the first 30 min. But unless otherwise stated, 1 h of equilibration time was chosen as the optimum contact
3. Results and discussion 3.1. Effect of pH It is a well known phenomenon that sorption of heavy metal ions by resins is dependent on pH because of the pH dependencies of the complexation reactions or physisorption processes at the adsorption surface. Since sporopollenin has some functional groups on its natural stucture [24] and also dithiocarbamate funtionality on its dithiocarbamated form, effect of pH on sorption
Fig. 1. The effect of pH on the sorption of Cu(II), Pb(II) and Cd(II) metal ions. [() Pb(II)] C0 = 20 mg L−1 ; [() Cu(II)] C0 = 10 mg L−1 ; [() Cd(II)] C0 = 30 mg L−1 ; t: 120 min; m(DTC-S): 0.05 g; medium: 0.1 M acetic acid–acetate; temperature: 20 ◦ C.
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Fig. 2. Sorption rates of heavy metal ions onto DTC-S. [() Pb(II)] C0 = 30 mg L−1 , pH 5.5; [() Cu(II)] C0 = 8 mg L−1 , pH 4.0; [() Cd(II)] C0 = 15 mg L−1 , pH 7.0; m(DTC-S): 0.05 g; medium: 0.1 M acetic acid–acetate; temperature: 20 ◦ C.
time to ensure that equilibrium conditions were achived. When kinetic considerations are taken into account, the presence of functional groups on a chelating polymer is usually evaluated together with the accessibility to these functional groups without sterical hinderence, which is greatly determined by polymeric matrics. And, the sorbents with the best charateristics are defined as those having hydrophilic macroporous structures. In our study the observed rapid uptake kinetics can be attributed to the hydophilic porous structure of the sporopollenin itself [26] and easily available dithiocarbamate functional groups on DTC-S. There are a lot of parameters effecting the sorption rates like structural properties of the sorbent, metal ion properties, initial concentration of metal ions, pH, temperature, chelate formation rate or presence of competing ions. Therefore, it is difficult to make a comparison between the sorbents. But, when compared with the results stated in the literature where time required to attain equilibrium ranged from 30 min to 7 h, the sorption of Cu(II), Pb(II) and Cd(II) metal ions onto DTC-S is seemed to be suitable from kinetic considerations. It is known that sorption kinetics are dependent or controlled by different kind of mechanisms like mass transfer, diffusion control, chemical reactions, and particle diffusion. In order to clarify the kinetic characteristics of the sorption three well known kinetic models were applied to evaluate experimental data. For this purpose Lagergren’s pseudo-first order kinetic model, pseudo-second order kinetic model and intra particle diffusion model were used.
Fig. 3. Lagergren’s pseudo-first order plots for heavy metal ions on DTC-S. [() Pb(II)] C0 = 30 mg L−1 , pH 5.5; [() Cu(II)] C0 = 8 mg L−1 , pH 4.0; [() Cd(II)] C0 = 15 mg L−1 , pH 7.0; m(DTC-S): 0.05 g; medium: 0.1 M acetic acid–acetate; temperature: 20 ◦ C.
3.2.1. First-order kinetics The linearized form of the first-order rate equation by Lagergren and Svenska [27] is given as: kads t (2) log(qe − qt ) = log qe − 2.303 where qe and qt are the amounts of the metal ions adsorbed (mg g−1 ) at equilibrium and at time t (min), respectively, and kads is the sorption rate constant (min−1 ). The plots of log(qe − qt ) versus t gives a straight line and the rate constants (kads ) and theoratical equilibrium sorption capacities, qe (theoric), can be calculated from the slopes and intercepts. In Fig. 3 Lagergren’s plots and constants related to these plots are given in Table 1. Straight lines obtained from Lagergren’s plots suggest the applicability of the pseudo first-order kinetic model to fit the experimental data. But it is also required that theoreticaly calculated equilibrium sorption capacities, qe (theor.), should be in accordance with the experimental sorption capacity, qe (exp.), values. As can be seen from Table 1, linear correlation coefficients of the plots, except for Pb, are not good. And, qe (theor.) and qe (exp.), values are not in agreement and with each other. So, it could suggested that the adorption of Cu(II), Pb(II) and Cd(II) metal ions onto DTC-S is not a first-order reaction. 3.2.2. Second-order kinetics The experimental data were also applied to the pseudo-second order kinetic model which is given with the equation below: 1 1 t t (3) = + qt k2 qe2 qe
Table 1 First-order, second-order and intraparticle diffusion rate constants Metal
Cu Pb Cd
First-order rate constants (min−1 )
qe (exp.) (mg g−1 )
kads
2.587 18.30 3.81
0.0318 0.0882 0.0262
Second-order rate constants (g mg−1
qe (theor.) (mg g−1 )
R2
k2
0.866 14.86 0.566
0.9791 0.9902 0.9548
0.090 0.014 0.209
min−1 )
Intraparticle diffusion rate constants
qe (theor.) (mg g−1 )
R2
kid (g g−1 min−1/2 )
R2
2.62 18.98 3.82
0.9975 0.9996 0.9997
83.20 1141.20 103.40
0.9961 0.9249 0.9888
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Fig. 4. Pseudo second-order plots for heavy metal ions on sporopollenin. [() Pb(II)] C0 = 30 mg L−1 , pH 5.5; [() Cu(II)] C0 = 8 mg L−1 , pH 4.0; [() Cd(II)] C0 = 15 mg L−1 , pH 7.0; m(DTC-S): 0.05 g; medium: 0.1 M acetic acid–acetate; temperature: 20 ◦ C.
where k2 (g mg−1 min−1 ) is the rate constant of pseudo secondorder sorption reaction. Second-order kinetic is said to be applicable if the plot of t/qt versus t shows linearity. Fig. 4 shows the plots obtained from the graphical interpretation of the data for the second-order kinetic model. The constants related to these plots are given in Table 1. As it can be seen from the results given in Table 1, correlation coefficients are higher compared to the results obtained from the first-order kinetics. And, theoretical and experimental qe values are in a good accordance with each other. So, it is possible to suggest that the sorption of Cu(II), Pb(II) and Cd(II) metal ions onto DTC-S followed a second-order type reaction kinetics. It is known that ordinary exchange processes are rapid and controlled mainly by diffusion, whereas those in a chelating exchanger are slower and controlled either by particle diffusion or by a second order chemical reaction [6]. Moreover, it is a general understanding that pseudo-second order kinetics provide best fits to the experimental data for the sorption systems where chemisorption seems significant in the rate controlling step [28]. DTC-S behaves as a chelating exchanger because of the dithiocarbamate functionality present on it and sorption to the resin would take place via valency forces between the metal ions and the sorbent. Therefore, second-order chemical reaction kinetics would be expected to be followed in the sorption processes and for our resin it is possible to state that chemisorption is the rate determining step in the sorption process. 3.2.3. Intraparticle diffusion In aqueous solutions, when sorptions to the porous sorbents are considered, mesopores may act as micropores because of the potentially formation of water layers on the pore walls [29]. And also it is known that, both pore and surface diffusions may play role during the sorption processes to the macroporous structures [30]. Since because sporopollenin has a porous structure, the effect of intraparticle diffusion to the sorption process should also be taken into accaunt.
Fig. 5. Intraparticle diffusion plots for heavy metal ions on DTC-S.
Intraparticle diffusion model is expressed with the equation given by Weber and Morris [31]: qt = kid t 1/2
(4)
where qt is the amount of metal ions adsorbed at time t (g g−1 ), kid is the intraparticle diffusion rate constant (g g−1 min−1/2 ). Plots of qt versus t1/2 are shown in Fig. 5a and b. In such graphs, initial curved/steep sloped portions are attributed to the bulk diffusion or exterior sorption rate which is very high. Only the subsequent linear portions can be attributed to intraparticle diffusion. And, plateau portions represent the equilibrium. In sorption processes intraparticle diffusion can be accepted as a rate determining step when if the plots of qt versus t1/2 show linearity. As can be seen from Fig. 5, because of the non-linear distribution of the plots and deviation of the curves from the origin, intraparicle diffusion cannot be accepted as the only rate determining step for the sorption of the heavy metal ions studied on DTC-S. But on the comparison of the kid values calculated from the linear portions of the plots (Table 1), it is possible to suggest that intraparticle diffusion, which may play an important role as a rate determining step, is said to be more effective for Pb(II) ions. The kid value for Pb(II) is higher than those for Cu(II) and Cd(II). This can be attributed to the ralatively small radius of the hydrated Pb(II) ions.
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3.3.2. Freundlich isotherms Freundlich equation is derived to model the multilayer sorption and for the sorption on heterogeneous surfaces. Linearised form of the Freundlich equation [33] is given by the following equation: log qe = log KF +
Fig. 6. Effect of initial concentration on heavy metal sorption on DTC-S. [() Pb(II)] pH 5.5; [() Cu(II)] pH 4.0; [() Cd(II)] pH 7.0; m(DTC-S): 0.05 g; t: 60 min; medium: 0.1 M acetic acid–acetate; temperature: 20 ◦ C.
3.3. Sorption capacity Fig. 6 shows the effect initial metal ion concentrations of Cu(II), Pb(II) and Cd(II) metal ions on the equilibrium sorption capacities. It is clear from Fig. 6, that the sorption capacities were incresed with increasing the initial concentrations and reached a plateau which represents the maximum sorption capacity of resin. This behaviour can be explained with the high driving force for the mass transfer. The analysis of the sorption isotherms is important for design purposes. Therefore, experimental data were analyzed with three well known sorption isotherm models of Langmuir, Freundlich and D–R to develop an equation which accurately represents the results. 3.3.1. Langmuir isotherms Langmuir sorption isotherm models the monolayer covarage of the sorption surfaces and assumes that sorption occurs on a structrurally homogeneous adsorbent and all the sorption sites are energetically identical. Linearised form of the Langmuir equation [32] is given by the following equation: 1 1 1 = 0+ qe Q bQ0 Ce
1 log Ce n
(6)
where qe is the equilibrium solute concentration on adsorbent (mmol g−1 ), Ce the equilibrium concentration of the solute (mmol L−1 ), KF the Freundlich constant (mmol g−1 ) which indicates the sorption capacity and represents the strength of the adsorptive bond and n is the heterogenity factor which represents the bond distribution. According to this equation the plot of the log qe versus log Ce gives a straight line and KF and n values can be calculated from the intercept and slope of this straight line, respectively. The constants related to Langmuir and Freundlich equations are given in Table 2. On the comparison of the R2 values, we can conclude that sorption isotherm data obtained for the metal ions studied on DTC-S can be described better by Langmuir equation. This result predicts the homogenity of the sorption sites on DTCS. In our previous work [24] sorption isotherm data obtained for unmodified sporopollenin were found to fit Freundlich type equation better. Therefore, it is possible to suggest that after the modification process, dithiocarbamated-sporopollenin (DTC-S) became structrurally homogeneous and all the sorption sites on the adsorbent are energetically identical. The Langmuir constant Q0 indicates the sorption capacity of the sorbent. As it can be seen from Table 2, Q0 values were found to be 0.2734, 0.4572 and 0.0631 mmol g−1 for Cu(II), Pb(II) and Cd(II), respectively. These sorption capacities are much higher (especially for Cu and Pb) than the unmodified form of the sporopollenin itself for which sorption capacities were given as 0.0195, 0,0411 and 0.0146 mmol g−1 , respectively [24]. As can be seen from Table 3, these sorption capacities are comparable values when compared to some other dithiocarbamate functionalized resins given in the literature.
(5)
where qe is the amount of solute adsorbed on the surface of the adsorbent (mmol g−1 ), Ce the equilibrium ion concentration in the solution (mmol L−1 ), Q0 the maximum surface density at monolayer covarage and b is the Langmuir sorption constant (L mmol−1 ). The plots of 1/qe versus 1/Ce give a straight line and the values of Q0 and b can be calculated from the intercept and slope of the plots, respectively.
3.3.3. D–R isotherms D–R isotherm describes sorption on a single type of uniform pores. In this respect the D–R isotherm is an anologue of Langmuir type but it is more general because it does not assume a homogeneous surface or constant sorption potential [36]. The D–R isotherm is given with the following equation, Q = Qm exp(−kε2 )
(7)
Table 2 Langmuir, Freundlich and D–R isotherm constants Metal
Cu Pb Cd
Langmuir isotherm parameters
Freundlich isotherm parameters
D–R isotherm parameters
Q0 (mmol g−1 )
b (L mmol−1 )
R2
KF (mmol g−1 )
n
R2
Qm (mmol g−1 )
k (mol2 kJ−2 )
E (kJ mol−1 )
R2
0.2734 0.4572 0.0631
2.032 1.268 15.44
1.00 0.9976 0.9951
0.153 0.248 0.0597
1.96 1.70 5.63
0.9572 0.9949 0.9225
0.809 1.668 0.102
−0.0055 −0.0064 −0.0019
9.53 8.84 16.22
0.9795 0.9992 0.949
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Table 3 Sorption capacities of some dithiocarbamate functionalized resins Resin supporting material
Cellulose (different derivatives) Poly(styrene-g-ethylene glycols) Polystyrene microspheresa Styrene–divinylbenzene copolymer Poly(acrylaminophosphonic) fibers Activated carbon Sporopollenin (our study) a
Sorption capacities (mg g−1 )
Reference
Cu
Pb
Cd
7.3–15 1.85 3.76 35.0 2.03 38.0 17.4
6.9–9.5 1.95 6.17 23.0 5.72 – 94.7
– 1.69 1.92 – 0.094 – 7.09
[7] [10] [11] [13] [34] [35] –
Sorption capacities in the presence of the heavy metal ions.
and linearized form of the equation is given as, ln Q = ln Qm − kε2
(8)
where ε (Polanyi Potential) is [RT ln(1 + (1/Ce ))], Q is the amount of solute adsorbed per unit weight of adsorbent (mol g−1 ), k is a constant related to the sorption energy (mol2 kJ−2 ), Qm is the sorption capacity (mol g−1 ). The values of Qm and k were calculated from the intercept and slope of the ln Q versus ε2 plots and presented in Table 2. The mean free energy of sorption (E) was calculated from the k values using the equation E = (−2k)−0.5
icent values, (KD ) as a function of temperature (Fig. 7). It was obsorved that the distribution coefficent values, (KD ), increased with increase in temperature and that shows the endothermic nature of the sorption. Thermodynamic parameters like free energy change (G◦ ), enthalpy change (H◦ ), and entropy change (S◦ ) were estimated using the following equations. The Gibb’s free energy change of the process is related to the distribution coefficient (KD ), by the equation below: ◦
G = −RT ln KD ◦
◦
G = H − T S ◦
(9)
Langmuir and Freundlich isotherms do not give any idea about sorption mechanisms. But, the magnitude of E is a useful mean for estimating the type of sorption process. If this value is between 8 and 16 kJ mol−1 , sorption process can be explained by ion exchange [30]. In this study, for Pb(II) and Cu(II) ions E values were calculated to be between the values of ion exchange. But, for Cd(II) the calculated E value was found to be above the value of 16 kJ mol−1 that indicates the chelating behaviour of DTC-S. Therefore, it is possible to say that sorption mechanism of heavy metal ions (Cu(II), Pb(II) and Cd(II)) on DTC-sporopollenin can be explained with an ion exchange process. But chelating effect of the ditihiocarbamate functional groups on DTC-S is also thought to take part in the sorption process and chemisorption was found to be the rate determining step. According to the E values it is also possible to make a comparison between the comlexation strengths of the metal ions with dithiocarbamate functionality. E values for the metal ions studied were found to be in order of Cd(II) > Cu(II) > Pb(II) and this order may be correlated well with the complexation strengths. It is well known that b values calculated from the Langmuir isotherms are constants related to the affinity between the adsorbent and sorbate. A large value of b implies strong binding. The b values (given in Table 2) are also supporting and in accordance with the proposed complexation strenghts given above. 3.4. Thermodynamics of the sorption The effect of temperature on the sorption of heavy metal ions onto DTC-S is given as the plots of the distribution coeff-
log KD =
(10) ◦
(11) ◦
H S − 2.303R 2.303RT
(12)
where KD is the distribution coefficient (cm3 g−1 ), and R is gas constant (kJ mol−1 K−1 ). According to the last equation given above the values of H◦ and S◦ can be calculated from the slopes H◦ /2.303R and intercepts S◦ /2.303R of log KD versus 1/T plots. The calculated values of thermodynamic parameters were given in Table 4. As it can be seenfrom Table 4, positive H◦ values represents the endothermic nature of the sorption process. The heat of sorption values between 5.0 and 100 kcal mol−1 (20.9–418.4 kJ mol−1 ), which are heats of chemical reactions, are frequently assumed as the comparable values for the chemical sorption processes. H◦ values were found as 23.61, 25.67 and 21.24 kJ mol−1 for Cu(II), Pb(II) and Cd(II), respectively. According to the calculated E values, previously
Fig. 7. log KD –1/T graphs for the sorption of Cu(II), Pb(II) and Cd(II) onto DTC-S. [() Cu(II)] C0 = 75 mg L−1 , pH 4.0; [() Pb(II)] C0 = 10 mg L−1 , pH 5.5; [() Cd(II)] C0 = 10 mg L−1 , pH 7.0; m(DTC-S): 0.05 g; t: 60 min; medium: 0.1 M acetic acid–acetate.
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Table 4 Thermodynamic parameters for the sorption of Cu(II), Pb(II) and Cd(II) on DTC-S Metal
Cu
Pb
Cd
C0 (mg L−1 )
75
10
10
H◦ (kJ mol−1 )
23.61
25.67
21.24
S◦ (J K−1 mol−1 )
T (K)
G◦ (kJ mol−1 )
R2
119.15
298 303 313 323 338
−11.30 −12.49 −13.68 −14.87 −16.66
0.9922
146.49
303 313 323 338
−18.72 −20.19 −21.65 −23.85
0.9962
119.07
298 303 313 323 338
−14.25 −14.84 −16.03 −17.22 −19.00
0.9795
we described the adorption mechanism of the heavy metal ions [Cu(II), Pb(II) and Cd(II)] onto the sorbent as an ion exchange process. But, it is clear from the H◦ values beyond the heats of chemisorption that dithiocarbamate functional groups also contribute to the sorption process with their chelating effects. These results are also in good agreement with the results and comments done on the calculated E values and support the possibility of the chemisorption that is the rate determining step in the sorption process. Negative values of G◦ indicates the spontaneous nature of the reaction. The reaction is favored and getting easier at higher temperatures. S◦ values were found to be positive due to the exchange of the metal ions with more mobile ions present on the exchanger, which would cause increse in the entropy during the adsoption process. In the case of physisorption, which may also contribute to the total sorption process, water molecules released from the hydrated metal ions or from the sorption surface may also cause an increase in the entropy.
(2)
(3)
(4) (5)
characteristics of the sorption sites on sporopollenin. Sorption capacities order was found as Pb(II) > Cu(II) Cd(II). Sorption of the Cu(II), Pb(II) and Cd(II) ions onto DTCS can be explained with an ion exchange process. But it is evident from the values of mean free energy of sorption (E) and enthalpy of sorption (H◦ ) that dithiocarbamate functional groups are also taking part in the sorption process. The sorption process for the heavy metal ions studied on DTC-S, was found to be endothermic, spontaneous and can be explained with pseudo second-oder type kinetic model. Intraparticle diffusion effect was obseved to be more effective for Cu(II) and Cd(II) metal ions but, from kinetic considerations intraparticle diffusion cannot be accepted as the rate determinig step. DTC-S mainly showed characteristics of a chelating exchanger. DTC-S showed higher sorption capacities when compared to the unmodified form of the sporopollenin.
3.5. Regenerability In regeneration studies 0.2 and 2 M of concentrations of HCl and HNO3 were used. But only about 10% of the adsorbed metals were recovered. Similiar behaviour were also observed in some works done on some sulfur bearing chelating resins. Lezzi and Cobianco [10] prepared chelating resins by introducing dithiocarbamate and methyl thiourea groups on poly(styrene-gethylene glycol) polymer. They studied the adsorption of some heavy metal ions [Hg(II), Cu(II), Pb(II) and Cd(II)] and dithiocabamate resin was reported to be regenerated by complete mineralization of the metal complexes. In a similiar work by Lezzi et al. [37], only 25% of the Hg(II) ions were reported to be eluted from the thiol chealating resins by treatment with a 6 N HCl solution. 4. Conclusions (1) Sorption on DTC-S can be expressed better with Langmuir type sorption isotherms which shows the homogeneous
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