Ind. Eng. Chem. Res. 2008, 47, 6221–6227
6221
Polyaspartic Acid As a New Complexing Agent in Removal of Heavy Metal Ions on Polystyrene Anion Exchangers Dorota Kołodyn´ska, Zbigniew Hubicki,* and Marzena Ge¸ca Department of Inorganic Chemistry, Faculty of Chemistry, Maria Curie-Sklodowska UniVersity, Maria Curie-Sklodowska Sq. 2, 20-031 Lublin, Poland
In the presented paper, the sorption of copper(II) and zinc(II) in the presence of polyaspartic acid sodium salt on polystyrene ion exchangers from aqueous solutions was studied. This polyaspartic acid sodium salt is an active, water soluble polyaminocarboxylate with the multifunctional property profile. It is a complexing agent of a new generation as it undergoes biodegradation thus being an alternative for the reagents of the EDTA or NTA type. On the basis of the research, the applicability of gel and macroporous polystyrene anion exchangers with different functional active groups, Lewatit MonoPlus M 500, Lewatit MonoPlus M 600, and Lewatit MonoPlus MP 500 as well as Lewatit MonoPlus MP 64 and Lewatit MP 62, was determined by the dynamic technique. Batch experiments were also carried out to determine the factors affecting sorption and kinetics of the sorption process. Introduction In all stages of industrial production from gaining raw material to the final product, there are wastewaters formed. They often contain such substances as heavy metal ions which cause irreversible changes in living organisms. Wastewaters containing heavy metal ions are most frequently purified using chemical (neutralization, reduction, and/or oxidation precipitation), physicochemical (sorption, extraction, ion exchange), or electrochemical methods. The choice of method depends on the kind of wastewater, composition, form, and concentration of removed components as well as the required state of purification.1 The wastewaters containing heavy metal ions as well as complexing agents constitute a particular group of wastewaters.2–12 Many complexing agents can be classified as environmentally relevant, since they are poorly microbiologically degradable and exhibit excellent water solubility.13,14 Because of the capability of binding and masking metal ions, synthetic chelating agents are widely applied in many industries but they have also some undesired features such as the remobilization of radionuclides and toxic heavy metals from sediments and soils. They are also only partially removed during drinking water treatment by filtration and biodegradation steps; therefore, wastewaters containing complexing agents need special treatment methods which depend mainly on the complexing agent, metal ions, and their concentrations. For diluted solutions, the ion exchange,15,16 binding-ultrafiltration,17 and adsorption into activated carbons,18,19 inorganic materials,20 and functional polymers21,22 have been used. These techniques generate concentrates from which the metals must be removed or recovered prior to disposal. The alternative may be breaking the metal chelates into free metal and free chelating agent followed by separation of the metal in an insoluble form, and then, the chelating agent should be discharged with the treated effluent. As the complexed metal ions are not destroyed, using the above-mentioned methods the recovery of metal process should be aided by electrowinning and/or electrorefining. In this case, the solution is either searching for an effective and economical method of simultaneous removal of heavy metal ions and complexing agents or * To whom correspondence should be addressed. Tel.: +48 (81) 5375511. Fax: +48 (81) 533 33 48. E-mail address: hubicki@hermes. umcs.lublin.pl.
eliminating complexing agents unfriendly for the environment and replacing them with new agents of comparable physicochemical properties. Taking into account a growing number of such detergents and fertilizers containing complexing agents of a new generation, studies of this type compounds are not only topical but indispensable from the point of view of environmental protection. One alternative to the conventional chelating agents might be polyaspartates, a group of acidic polyamides such as Baypure DS 100 produced by Bayer (Germany). This is a new dispersing agent for use in a wide range of applications, for example to soften water in washing machines and dishwashers as well as to improve dispersion of pigment particles in the production of paper, paints, and other products. Baypure DS 100 displays a dispersing effect comparable to the best dispersing agents in its class. It is also suitable for the production of latex-based coating compounds. It has advantages over conventional dispersing agents in terms of performance and biodegradability.23–27 The chemical structure of polyaspartic acid is shown in Figure 1. More details concerning its chemical properties are presented in our earlier paper.28 Elaboration of methods of simultaneous removal of heavy metal ions and complexing agents from wastewaters is equally important to searching for new ones. Thus the aim of this study was to explain the influence of chemical conditions on the ion exchange capacity and on the kinetics of copper and zinc uptake by anion exchangers in the presence of Baypure DS 100 (in the paper denoted as DS). Copper(II) and zinc(II) were selected as they are widely used in various industries and have negative effects on the environment. In the investigations, the polystyrene anion exchangers with different basicity of functional groups
Figure 1. Chemical structure of polyaspartic acid.
10.1021/ie800472y CCC: $40.75 2008 American Chemical Society Published on Web 07/19/2008
6222 Ind. Eng. Chem. Res., Vol. 47, No. 16, 2008 Table 1. Characteristics of Ion Exchangers Used in the Investigations typical properties ion exchanger Lewatit Lewatit Lewatit Lewatit Lewatit Lewatit a
MonoPlus MonoPlus MonoPlus MonoPlus MP 62 SP 112
M 500 M 600 MP 500 MP 64
matrixb,c
active groups
bead size [mm]
total capacity [equiv/dm3]
thermal stability [K]
PS-DVBa (g) PS-DVB (g) PS-DVB (m) PS-DVB (m) PS-DVB (m) PS-DVB (m)
sN+(CH3)3 (type 1) sN+(CH3)2C2H4OH (type 2) sN(CH3)2 (type 1) sN+(CH3)3/sN(R)2 sN(R)2 sSO3H
0.62 ( 0.05 0.62 ( 0.05 0.63 ( 0.05 0.59 ( 0.05 0.47 ( 0.06 0.62 ( 0.04
1.30 1.25 1.10 1.30 1.70 1.30
333 333 313 313 313 393
PS-DVB: polystyrene crosslinked with divinylbenzene. b g ) gel structure. c m ) macroporous structure.
(Lewatit MonoPlus M 500, Lewatit MonoPlus M 600, Lewatit MonoPlus MP 500 (strongly basic), Lewatit MonoPlus MP 64 (medium basic), and Lewatit MP 62 (weakly basic)) were used. The two most common methods were usedsthe column and the batch techniques. In order to measure affinity of the Cu(II)-DS and Zn(II)-DS complexes, the breakthrough curves were determined by the dynamic method. Analogous studies were carried out for the strongly acidic cation exchanger Lewatit SP 112. Batch experiments were also carried out in order to determine the recovery factors (%R) as well as the concentration of analyzed complexes at equilibrium (qe) and at the specific time (qt) under different metal-ligand ratios, metal concentrations, phase contact times, and pH values. Some details important for the choice of optimal conditions for the desired degree of exchange were also discussed. Experimental Details Anion Exchangers and Their Characteristics. Five different anion exchangers and one cation exchanger were produced by Bayer (Germany). Prior to the use, they were converted to the sodium, hydrogen (in the case of cation exchanger) or chloride, and hydrate amine/chloride or hydrate amine (in the case of anion exchangers) forms. Anion exchangers were also used in the acetate form. Important physical and chemical properties of these ion exchangers are presented in Table 1. Solutions. Aqueous Cu(II) and Zn(II) stock solutions were prepared from copper(II) or zinc(II) chlorides (POCh, Poland) and the required amount of Baypure DS 100 (Bayer, Germany). The behavior of copper(II) and zinc(II) was investigated at pH values without adjustment. For individual complexes, the pH values were the following: (7.0) Cu(II)-DS ) 1:1, (8.0) Cu(II)-DS ) 1:2, (8.3) Zn(II)-DS ) 1:1, (9.0) Zn(II)-DS ) 1:2. All other reagents produced by POCh (Poland) were of analytical grade. Dynamic MethodsColumn Procedure. The breakthrough curves of Cu(II)-DS and Zn(II)-DS were determined using 20 mL of the swollen ion exchanger in the appropriate form. The frontal analysis process was carried out in glass columns of a diameter 1.0 cm filled with the ion exchanger. The prepared solutions were passed continuously downward through the resin beds keeping the flow rate at 0.8 mL/cm2 · min. The effluent was collected in 50 mL fractions (to the breakthrough point) and then in 250-500 mL fractions in which the metal(II) content was determined. The sorption parameters were calculated from the determined breakthrough curves according to the procedure presented earlier. 29 Static MethodsBatch Procedure. In the batch experiments, an aliquot of dry resin (0.5 g) and 50 mL of the aqueous solution were placed in the 100 mL glass stoppered flask and shaken at 180 rpm in different time intervals. The measurements were made at the constant temperature 298 K. The preliminary experiments showed that sorption is fast and the removal rate is negligible after 2 h. Therefore, a contact time of 2 h was
used for batch tests. After equilibrium, the concentrations of metal ions in the aqueous phase were analyzed by AAS. The experiment was conducted in the two parallel series (the reproducibility of the measurements was within 5%). The concentrations of chloride ions were not determined. The recovery factor was calculated from the equation: %R )
c0 - ct × 100% c0
(1)
The resin phase concentrations of metal ions at the equilibrium, qe (mg/g), and at the specific time, qt (mg/g), were obtained according to the following: (c0 - ce)V (2) m (c0 - ct)V (3) qt ) m Where, c0 is the initial concentration of M(II) in the aqueous phase (mg/L), ce is the concentration of M(II) in the aqueous phase at equilibrium (mg/L), ct is the concentration of M(II) in the aqueous phase at time t (mg/L), V is the volume of the solution (L), and m is the mass of the ion exchanger (g). FT-IR/PAS Studies. The FT-IR/PAS spectra of the ion exchangers before and after the sorption processes were recorded in order to control the sorption process. Apparatus. A laboratory shaker ELPHINE type 357 produced in Poland was used to shake the anion exchangers and liquid phases. An atomic absorption spectrometer (Varian Spectr AA-880) was used for quantitative determination of the concentrations of Cu(II) and Zn(II). The pH was measured with a Radiometer pH meter (Model PHM 82). FT-IR/PAS scans were made by Bio-Rad Excalibur 3000MX spectrometer equipped with the photoacoustic detector MTEC 300. qe )
Results and Discussion FT-IR/PAS Studies Results. The FT-IR/PAS data of all the anion exchangers showed characteristic peaks related to the vibration of O-H at 3600 cm-1 as well as peaks connected with the matrix of the applied anion exchangers (polystyrene cross-linked by divinylbenzene) as well as functional groups (quaternary ammonium groups as well as tertiary and secondary amines). After the sorption of Cu(II) ions in the presence of DS in the obtained spectra occur two strong bands related to the asymmetrical and symmetrical stretching bonds of the carboxylic group in the range 1590-1650 cm-1. As follows from the literature data for heavy metal complexes with aminopolycarboxylic acids, the position of the peak at 1607 cm-1 (assigned to the vibration of carbonyl group) is connected with the nature of M-O bond.30 For Cu(II)-DS complexes
Ind. Eng. Chem. Res., Vol. 47, No. 16, 2008 6223
Figure 2. FT-IR/PA spectra of Lewatit MonoPlus M 500 and Lewatit MonoPlus MP 500 before and after sorption of Cu(II) complexes with DS.
sorbed on Lewatit MonoPlus M 500 and Lewaiti MonoPlus MP 500, the maxima of the peaks are at 1604 and 1607 cm-1, respectively which is indicated by the ionic character of the M-O bond. The exemplary reordered FT-IR/PAS spectra for Lewatit MonoPlus M 500 and Lewatit MonoPlus MP 500 are presented in Figure 2. Column Studies Results. In order to determine optimal conditions of sorption of anionic complexes of copper(II) and zinc(II) ions with DS, the breakthrough curves in the 1 mM M(II)-1 mM DS and 1 mM M(II)-2 mM DS systems on the polystyrene anion exchangers of different functional groups types were determined. Figure 3 (plots of concentration c/c0 (dimensionless) vs the volume of effluent (mL)) show the effect of the molar ratio of copper(II) or zinc(II) concentration to DS on sorption effectiveness on Lewatit MonoPlus M 500, Lewatit MonoPlus M 600, and Lewatit MonoPlus MP 500. For Lewatit MonoPlus MP 64 and Lewatit MP 62, the obtained breakthrough curves are not presented. Figure 4 presents the analogous breakthrough curves obtained for the strongly acidic cation exchange resin Lewatit SP 112. It was indicated that for Lewatit SP 112 in the H+ form complex decomposition does not occur both for the complexing degree Cu(II)-DS ) 1:1 and 1:2 in the ion exchanger phase. One can suppose that then only a small number of Cu(II) ions, which were not complexed, undergo sorption, and it can be expressed as follows: nRSO3- - H+ + Mn+ a nRSO3- - Mn+ + nH+ -
(4)
Where, (SO3 ) is a functional group attached to the cation exchanger, Mn+ is a metal cation, and n is a coefficient of the reaction component, depending on the oxidation state of metal ion. As follows form the obtained results the copper(II) complexes with DS in general exhibit higher affinity compared to the zinc(II) complexes of this type. It is in agreement with the values of the stability constants of copper(II) and zinc(II) complexes with DS - log KCu(II)-DS ) 4.8, log KZn(II)-DS ) 2.2, respectively. 24 It is also evident that in the 1 mM M(II)-1 mM DS system different types of anionic complexes are sorbed on the strongly basic anion exchangers (Lewatit MonoPlus M 500, Lewatit MonoPlus M 600, and Lewatit MonoPlus MP 500). In the intial stage, c/c0 reached the constant value (for bed effluent volume V equal to about 1500 mL) and then gradually increased until
Figure 3. Breakthrough curves of Cu(II) and Zn(II) complexes with DS in the M(II)-DS ) 1:1 and M(II)-DS ) 1:2 systems on Lewatit MonoPlus M 500 (a), Lewatit MonoPus M 600 (b), and Lewatit MonoPLus MP 500 (c) in the chloride and acetate (Ac) forms.
the total exhaustion of the bed (c/c0 ) 1) took place. At the higher molar ratio M(II)-DS ) 1:2, one type of complex is sorbed for all studied anion exchangers because in this case typical “S” shaped curves were obtained. Bearing in mind the statement that the anion exchanger prefers the cation which forms with the anionic ligand, the stronger negative complex or the complex with the greater average ligand number it is evident that the effect of complex formation is outweighed by other factors presumably by the steric effect because for the 1:2 molar ratio of metal ion to ligand obtained complexes have less affinity for the anion exchanger compared to the complexes of a 1:1 ratio. It is also noticeable that for the medium (Lewatit MonoPlus MP 64) and weak (Lewatit MP 62) anion exchangers the exchange zone formation time is much shorter than for the strongly basic anion exchangers mentioned earlier. The efficiency of sorption is lower due to the fact that these anion exchangers with the amine functional groups ionize only under
6224 Ind. Eng. Chem. Res., Vol. 47, No. 16, 2008 Table 3. Comparison of Porosity, BET Surface Area, and Pore Diameter of Studied Ion Exchangers31 ion exchanger
porosity
BET surface area [m2/g]
average pore diameter [nm]
M 500 MP 500 MP 64 MP 62 SP 112
naa 35-40 40-45 25-30 45-50
naa 45-50 40-50 45-50 75-85
naa (60b) 40 (100b) 50 30 30
a na ) data not available. b Measurements made by means of AUTOSORB-1 CMS, Quantachrome by authors.
Figure 4. Breakthrough curves of Cu(II) complexes with DS in the M(II)-DS ) 1:1 and 1:2 systems on Lewatit SP 112. Table 2. Weight (Dg) and Volume (Dv) Distribution Coefficients As Well As the Total (Cr) Ion Exchange Capacities (mg/mL) for Cu(II) and Zn(II) Complexes with DS on Anion Exchangers Calculated Based on the Column Studies M(II)-DS system
Dg
Dv
Cr
248.27 215.37 158.14 145.97
82.49 71.56 52.54 48.50
5.1 3.3 3.2 3.0
203.43 194.10 143.32 121.99
71.39 68.12 50.30 42.81
4.2 3.7 3.2 2.8
302.42 53.86 177.83 164.06
75.61 13.46 44.46 41.02
4.6 2.2 2.8 2.5
131.71 134.28 70.65 78.37
30.68 31.28 16.46 18.26
2.1 2.0 1.1 1.2
107.93 88.62 34.91 80.78
30.98 25.44 10.02 23.19
2.0 1.8 0.8 1.7
Lewatit MonoPlus M 500 Cu(II)-DS ) 1:1 Zn(II)-DS ) 1:1 Cu(II)-DS ) 1:2 Zn(II)-DS ) 1:2 Lewatit MonoPlus M 600 Cu(II)-DS ) 1:1 Zn(II)-DS ) 1:1 Cu(II)-DS ) 1:2 Zn(II)-DS ) 1:2 Lewatit MonoPlus MP 500 Cu(II)-DS ) 1:1 Zn(II)-DS ) 1:1 Cu(II)-DS ) 1:2 Zn(II)-DS ) 1:2 Lewatit MonoPlus MP 64 Cu(II)-DS ) 1:1 Zn(II)-DS ) 1:1 Cu(II)-DS ) 1:2 Zn(II)-DS ) 1:2 Lewatit MP 62 Cu(II)-DS ) 1:1 Zn(II)-DS ) 1:1 Cu(II)-DS ) 1:2 Zn(II)-DS ) 1:2
acidic conditions, and under alkaline conditions, exchangers exist as free bases. The obtained results were also confirmed by the calculated distribution coefficients as well as the sorption capacities of anion exchangers. These values obtained from the column studies are presented in Table 2. Taking into account the kind of functional groups, it was found that Lewatit MonoPlus M 500sthe strongly basic, gel anion exchanger of type 1 with -N+(CH3)3 functional groupssis more effective for sorption of Cu(II) and Zn(II) complexes than Lewatit MonoPlus M 600sthe anion exchanger of type 2 with -N+(CH3)2C2H4OH functional groups. When the kind of structure is taken into consideration, the macroporous anion exchanger Lewatit MonoPlus MP 500 is more effective than the gel anion exchanger of this type Lewatit MonoPlus M 500. Compared to the gel structure anion exchangers, macroporous anion exchangers have a special matrix with the additional system of larger pore channels which allow for large molecules or ions not only to enter the spongelike matrix but also to be
eluted during the regeneration procedure. According to the evidence31 and obtained results of porosity and surface properties of the studied anion exchangers (Table 3), it was established that the size of sorbed complexes is the factor influencing sorption effectiveness. Though it is difficult to estimate the extent of polyaspartic acid polymerization which depends on the way the synthesis is carried out,23 one can suppose that in the case of Lewatit MonoPlus M 500 too large molecules of complexes are excluded from the ion exchange phase to some extent due to the sieve effect. It was also found that Lewatit MonoPlus MP 500 and M 500 in the chloride form are the most effective in the sorption of copper(II) complexes with DS. In the case of the acetate form of this anion exchanger, different types of complexes are sorbed. In this part of the investigation, the effect of regenerating agent was also studied. The exhausted anion exchangers were regenerated by 1 M NaCl (Figure 5). The total regeneration of the resin bed (desorption efficiency 96%-98%) by sodium chloride is quick and complete as follows from the mass balance. Ten cycles of exhaustion-regeneration of the tested anion exchanger Lewatit MonoPlus MP 500 were carried out without significant change of its capacity. Effect of pH. The pH of the solution has also a significant impact on the removal of heavy metal ions, especially in the presence of other components of solution. In the case of Baypure DS 100, this effect was investigated for all studied anion exchangers in the commercial form. Figure 6 shows the percentage of removal of copper(II) as a function of pH (range 2-9). Heavy metals are known to exist as free ions in a strongly acidic medium. Under these conditions, they are not exchanged into anion exchangers. With an increase of pH, formation of soluble aqua-complexes and hydroxides of heavy metal ions is observed. But in the presence of DS 100 in the system under acidic conditions, only their insignificant influence on metal sorption was shown. At low pH levels, the major functional groups of DS 100, i.e. carboxyl and amines, responsible for
Figure 5. Elution profile of Cu(II) and Zn(II) complexes with DS in the M(II)-DS ) 1:1 and 1:2 systems on Lewatit MonoPlus MP 500.
Ind. Eng. Chem. Res., Vol. 47, No. 16, 2008 6225
Figure 6. Recovery factor (%R) of Cu(II) complexes with DS in the Cu(II)-DS ) 1:1 system on anion exchangers depending on the pH value.
the metal binding are undissociated. This affected weak interactions between DS 100 and metal ions (proton competition). With the increasing pH value, the removal of heavy metal ions increases in the presence of DS 100. This is caused by the binding of metal ions with DS 100 molecules due to the complexation reactions (water molecules as ligands of the inner sphere are substituted by carboxyl groups of DS 100) as a result of functional groups’ dissociation; therefore, the following expression can be written as follows: M2+ + HL a [ML]+ + H+ 2+
M
(5)
+
+ 2HL a [ML2] + 2H -
+
2+
+ 3HL a [ML3] +2H
2+
+ 4HL a [ML4] +2H
M M
2-
+
(6) (7) (8)
Where, M is for metal ion and HL is for the neutral form of polyaspartic acid. The decrease of heavy metal concentrations in the raffinate with the increasing pH value from 2 to 9 is caused by the increasing quantity of metal ions needed for formation of complexes. Possibly with higher pH values more carboxylic groups of DS l 00 are dissociated to form polyanionic species. It was also stated that in the case of strongly basic anion exchangers the highest sorption percentages were obtained for pH > 7. For the medium basic macroporous anion exchanger Lewatit MonoPlus MP 64, the recovery factors are high in the range of pH from 2 to 6 and then gradually decrease. Such behavior of medium and weakly basic ion exchangers is in agreement with the literature data as the ion exchangers of this type achieve complete ion exchange capacity in acidic solutions (they are fully ionized at pH values less than 7.0).32,33 However, the OH- group sorption is predominant in the basic medium. Effect of the Phase Contact Time. The contact time between the anion exchanger and the solution is an important factor when the effectiveness of metal ions removal is investigated. The data obtained for the heavy metal ion sorption in the presence of DS in the 1 mM M(II)-1 mM DS and 1 mM M(II)-2 mM DS systems on Lewatit MonoPlus MP 500 are presented in Figure 7. For Cu(II) and Zn(II) in the single metal ion systems, the time to reach an equilibrium state was about 2 h. For Lewatit MonoPlus M 500, the amount of Cu(II) complexes removed and the removal efficiency were about 6.26 mg/g and 98%, respectively whereas for the Zn(II), they were 5.36 mg/g and 82% (c0 ) 1 mM). Generally, for all studied anion exchangers, the removal of Cu(II) was higher than that of Zn(II). However, for strongly basic polystyrene anion exchangers, at equilibrium, the removal of Cu(II) and Zn(II) was more differentiated than for the weakly basic anion exchanger Lewatit MP 62 (3.17 mg/g and 50% and for Cu(II) and 2.81 mg/g and 43% for Zn(II), respectively). Sorption Isotherms. Sorption equilibrium is usually described by an isotherm equation whose parameters express the
Figure 7. Recovery factors (%R) of Cu(II) and Zn(II) complexes with DS in the M(II)-DS ) 1:1 and 1:2 systems on Lewatit MonoPlus MP 500 depending on the phase contact time.
surface properties and affinity of the ion exchanger. Equilibrium models have been developed to describe the sorption isotherm relationships (equations often used are developed by Langmuir, Freundlich, and Dubinin-Radushkevich, etc.). For the solid-liquid systems the linear form of these isotherms can be expressed as follows: Langmuir isotherm model: ce ce 1 + ) qe qob qo
(9)
Freundlich isotherm model: log qe ) bF log ce + log KF
(10)
The plots of ce/qe versus ce and log qe versus log ce give a straight lines. The values q0 and b as well as KF and bF (bF )1/n) were calculated from the slopes and intercepts of the linear forms of these plots. The constant q0 is the equilibrium constant and the b gives the inverse of the theoretical monolayer capacity. The fit of data to the Freundlich isotherm indicates the heterogeneity of the sorbent surface. The magnitude of the exponent bF gives an indication of the adequacy and capacity of the adsorbent/adsorbate system. In most cases, an exponent between 1 and 10 shows beneficial adsorption, but values above 1 indicate favorable adsorption. The isotherm constants obtained from the linearized plots of the Langmuir and Freundlich equations and the values of correlation coefficients (r2) for the copper(II) and zinc(II) complexes with DS on Lewatit MonoPlus MP 500 are presented in Table 4, as an example. Adsorption Kinetics. The prediction of batch sorption kinetics is necessary for design of optimum industrial sorption systems. Several kinetic expressions based on the sorbent concentration such as first-order and pseudo-second-order relationships elaborated by Lagergren, Ritchie, Weber, Morris,
6226 Ind. Eng. Chem. Res., Vol. 47, No. 16, 2008 Table 4. Langmuir and Freundlich Constants for Cu(II) and Zn(II) Complexes with DS in the M(II)-DS ) 1:1 System on Lewatit MonoPlus MP 500 Langmuir constants M(II)-DS ) 1:1
Freundlich constants 2
qo [mg/g]
b [L/mg]
r
17.15 13.97
0.09 0.03
0.982 0.946
Cu(II) Zn(II)
KF [mg/g]
n [-]
r2
1.92 1.05
1.72 1.06
0.841 0.785
Table 5. Kinetic Parameters and Standard Deviations for Sorption of Cu(II) and Zn(II) Complexes with DS in the M(II)-DS ) 1:1 System on Lewatit Anion Exchangers first-order anion exchanger M 500 M 600 MP 500 MP 64 MP 62
pseudo-second-order
intraparticle diffusion
system
k1
q1
r2
k2
q2
r2
ki
r2
Cu(II)-DS ) 1:1 Zn(II)-DS ) 1:1 Cu(II)-DS ) 1:1 Zn(II)-DS ) 1:1 Cu(II)-DS ) 1:1 Zn(II)-DS ) 1:1 Cu(II)-DS ) 1:1 Zn(II)-DS ) 1:1 Cu(II)-DS ) 1:1 Zn(II)-DS ) 1:1
0.12 0.02 0.07 0.04 0.04 0.06 0.05 0.03 0.06 0.02
3.10 2.96 1.92 1.35 5.89 5.66 3.20 2.78 1.25 1.09
0.994 0.959 0.822 0.916 0.973 0.982 0.921 0.985 0.893 0.988
0.11 0.06 0.13 0.15 0.14 0.01 0.15 0.03 0.03 0.01
6.25 5.89 5.67 5.39 6.14 5.78 5.47 4.48 2.57 2.54
0.999 0.994 1.000 0.999 0.999 0.999 0.999 0.997 0.972 0.985
0.26 0.31 0.21 0.40 0.34 0.60 0.32 0.44 0.15 0.25
0.674 0.953 0.579 0.567 0.847 0.941 0.842 0.911 0.891 0.940
Ho, and McKay are also known.34,35 The linear forms of the kinetic rate equations can be rewritten as follows: first-order: log(q1 - qt) ) log(q1) -
k1 t 2.303
(11)
pseudo-second-order: t 1 1 ) + t qt k q 2 q2
(12)
2 2
Where, q1, q2 are the amounts of heavy metal complexes with DS sorbed at equilibrium (mg/g), qt is the amount of heavy metal complexes with DS sorbed at time t (mg/g), k1 is the equilibrium rate constant of the first order sorption (1/min), and k2 is the equilibrium rate constant of the pseudosecond order sorption (g/mg · min). As follows from literature data, except for Ho’s pseudosecond-order model, no other models represent the experimental kinetic data for the entire sorption period well for most of the systems. The parameters of adsorption kinetics of copper complexes with DS onto the Lewatit anion exchange groups assessed above describe the rate of their uptake (Table 5). The experimental results show the rapid initial sorption rate followed by a slower stage. Initially, the functional groups are accessible for the metal complexes and hence a higher rate of adsorption is observed. The obtained data for this anion exchanger also indicate that the correlation coefficients for the linear plots of the pseudo-second-order equation are much better than those obtained for the first-order and intraparticle diffusion equations. 4. Conclusions 1. Due to common exploitation of complexing nonbiodegradable (EDTA, DTPA) as well as more biodegradable agents such as NTA, EDDS, IDS, and DS, it is essential to search for the effective and economical methods for removal of anion complexes of heavy metals with these agents from waters and wastewaters. The research results indicate possible application of the polystyrene anion exchangers such as Lewatit MonoPlus MP 500, Lewatit MonoPlus M 500, and Lewatit MonoPlus M 600 in technologies of heavy metals recovery, especially when they contain copper(II) from wastewater streams.
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ReceiVed for reView March 25, 2008 ReVised manuscript receiVed May 25, 2008 Accepted May 27, 2008 IE800472Y
