Sorption of Cd(II), Pb(II), Cu(II), and Zn(II ... - ACS Publications

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Ind. Eng. Chem. Res. 2010, 49, 4700–4709

Sorption of Cd(II), Pb(II), Cu(II), and Zn(II) Complexes with Nitrilotris(Methylenephosphonic) Acid on Polystyrene Anion Exchangers Marcin Siek, Dorota Kołodyn´ska, and Zbigniew Hubicki* Department of Inorganic Chemistry, Faculty of Chemistry, Maria Curie-Skłodowska UniVersity, Maria Curie-Skłodowska Sq. 2, 20-031 Lublin, Poland

The performance of the polystyrene anion exchangers Lewatit MonoPlus M 500, Lewatit MonoPlus M 600, Lewatit MonoPlus MP 500, Lewatit MonoPlus MP 64, and Lewatit MP 62 as well as Amberlyst A 21 was investigated for their further application in purification of wastewaters containing nitrilotris(methylenephosphonic acid) (NTMP) and heavy metal ions. The comparison involved evaluation of the sorption/desorption behavior of NTMP complexes of Cd(II), Pb(II), Cu(II), and Zn(II) on the resins with different basicity of functional groups. The batch method, in which a fixed amount of anion exchanger was contacted with the sample solution for increasing periods of time and the amount of metal ion remaining in the solution measured as a function of the elapsed time, was used to obtain information on the kinetics of species formed in the presence of added complexing agent. The effects of pH, metal(II) ion and ligand concentration, and their molar ratio on the sorption as well as type of functional groups of the anion exchangers used were determined. The sorption data were fitted to various models to obtain certain constants related to the sorption phenomena. The effect of the regeneration solution was also investigated. Introduction Phosphonate complexing agents have a great range of use in industry and households. Phosphorus structural analogues of EDTA, NTA, or DTPA are used in boilers, cooling water, and detergents. They inhibit scale formation and corrosion. The world consumption of phosphonates was 56 000 tons in 1998.1 In recent years phosphonates have still been under investigation as cheap materials for metal protectionsespecially carbon steel.2-4 In the textile industry, they stabilize peroxide-based bleaching agents and are bone-seeking carriers for radionuclides in nuclear medicine. Application of phosphonates in medicine is a result of their low toxicity. They also have a very low bioconcentration factor, but their radionuclide complexes have very high affinity for bones.5 Aminophosphonates are interesting as ligands for platinum, and those complexes are antitumor drugs.6 On the other hand, the heavy metals complexes with phosphonic acids complexes are dangerous for the environment and as a result they are interesting research topics.8 The stability constants of the metal complexes with phosphonic acids increase with the increasing number of phosphonic groups (Table 1).7 One of the best known examples of aminophosphonates described in the literature is nitrilotris(methylenephosphonic acid) NTMP (also called ATMP in other publications). NTMP H6ntmp, (H6L), is a well-known compound with interesting acid-base and complexing properties. Its chemical structure is presented below: The composition of NTMP anions depends on the pH and the ionic medium in solution. The successive five steps of deprotonation lead finally to [HN(CH2-PO3)3]5- species, and this process takes place only at very high pH values. For the six possible equilibrium states, L6- + H+ a HL5- (1); HL5+ H+ a H2L4- (2); H2L4- + H+ a H3L3- (3); H3L3- + H+ a H4L2- (4); H4L2- + H+ a H5L- (5); H5L- + H+ a H6L (6), where L ) N(CH2PO3)3, the following protonation constants * To whom correspondence should be addressed. Tel.: +48 (81) 5375511. Fax: +48 (81) 533 33 48. E-mail: zbigniew.hubicki@ poczta.umcs.lublin.pl.

were found: log K1 ) 12.1; log K2 ) 7.30; log K3 ) 5.86; log K4 ) 4.64; log K5 ) 1.5; log K6 ) 0.3, respectively.7,9 The speciation distribution of NTMP depending on the pH value is presented in Figure 1. Compare the values of the first and the second protonation constants (log K1 and log K2), it was stated that the first protonation of NTMP occurs on the nitrilo nitrogen. Insignificant differences between log K2 and log K3 and between Table 1. Comparison of the Stability Constants (log K) of Metal Complexes with AMPH, IDHP, HEDP, NTMP, and EDTPH M(II) Cd(II) Pb(II) Cu(II) Zn(II) Co(II) Ni(II) Ca(II) Mg(II)

M+L ML + H M + HL M+L ML + H M + HL M+L ML + H M + HL M+L ML + H M + HL M+L ML + H M + HL M+L ML + H M + HL M+L ML + H M + HL M+L ML + H M + HL

AMPH

IDHP

HEDP

NTMP

EDTPH

5.1 2.0

6.0 4.5

7.15 6.4

6.8 6.2 8.1 2.6 5.0 1.7 4.5 1.6 5.3 1.6 1.67 1.06 2.0 1.3

10.1021/ie901748k  2010 American Chemical Society Published on Web 04/26/2010

11.8 4.6 6.7

7.5

6.34 10.7

7.6 17.8

6.5 4.7

5.1

6.2 8.2

8.3 14.1

6.3 3.3

6.3 7.5

8.3 12.4

5.9 3.5

8.2 7.1

8.9 12.3

6.4 9.4 2.4 9.2 2.7

3.3 7.3 3.8

8.85 4.0 9.42 4.3

9.44 5.75 10.0 5.4

Ind. Eng. Chem. Res., Vol. 49, No. 10, 2010

Figure 1. Distribution diagram for NTMP (the species generated after successive deprotonation of phosphonic groups).

log K3 and log K4 indicate a small effect of protonation of one phosphate in the case of further protonation. Because log K5 is smaller than log K4, the further protonation on the phosphonate group already protonated (PO3H-) is very unfavorable compared with that on PO32-.10 In the case of coordination of metal ion, the basicity of O- of the phosphonate of NTMP decreases. While the first protonation of a complex occurs on the phosphonates O- uncoordinated to the metal ion, the first protonation constant of the complex ML (KMHL) should be smaller than that of the phosphonate O- of the free ligand, where the second protonation of NTMP (log K2) corresponds to the first protonation of phosphonates ion of the free ligand. Nevertheless, the first protonation constants of complexes (log KMHL ) 8.8-9.7) are larger than the second ones of the free ligand (log K2 ) 7.15). This fact suggests that, in the first protonation process in ML, the proton attacks nitrilo nitrogen of NTMP (structure II) rupturing the M-N bond of the complex [M(ntmp)]4- (structure I).

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ions catalyzed degradation of NTMP yields to two diphosphonic acid breakdown products: iminodimethylenephosphonic acid and formyl iminodimethylenephosphonic acid.24 Lesueur et al.25 check out NTMP degradation by UV light. However, the literature does not report about the application of synthetic ion exchangers for removal of the complexes of Cu(II), Zn(II), Pb(II), and Cd(II) with NTMP. Moreover, the M(II):NTMP ) 1:2 system was chosen in the studies. The systems of this type have not been investigated so far (there is no paper on the adsorption of M(II):NTMP ) 1:2 complexes). Therefore, from the cognitive point of view, it is essential to compare them with the extensively described M(II) and NTMP complexes of the molar ratio M(II):NTMP ) 1:1. NTMP does not find household applications. However, its industrial consumption grows up rapidly.24 NTMP has been reported for example to protect aluminum from corrosion.26-31 The purpose of the presented studies is to focus attention on the sorption of heavy metal ions i.e. Cd(II), Pb(II) Cu(II), and Zn(II) in the presence of nitrilotris-(methylenephosphonic acid) (NTMP) on strongly, medium, and basic anion exchangers. Synthetic ion exchangers are known for selective adsorption and removal of metal ions from waters and wastewaters. The anion exchange method proved to be effective for simultaneous removal of heavy metal ions and chelating ligands (in the form of anionic complexes). Therefore, the study was carried out to evaluate the sorption capacity of Lewatit MonoPlus M 500, Lewatit MonoPlus M 600, Lewatit MonoPlus MP 500, Lewatit MonoPlus MP 64, and Lewatit MP 62 as well as Amberlyst A 21 and its affinity for M(II)-NTMP complexes. The optimal pH values, pseudo-first- or pseudo-second-order of ion exchange process, and maximal sorption capacities for a single complex system and mixtures of complexes systems were determined. As a result, the correlations between types of functional groups and sorption process parameters were obtained. Desorption and interferention studies were also carried out. Experimental Section

As follows from the literature data, NTMP can form different complexes with metal ions.10-14 When heavy metal ions form complexes with phosphonate chelating agents, they have high solubility in a wide range of pH values. As a result, soils rich in phosphonates can transfer Pb(II), Cd(II), Cu(II), Zn(II), and other ions, even in strongly basic solutions and should be purified from toxic metal complexes. NTMP can be removed during wastewaters treatment by adsorption processes. In the paper by Nowack,15 it was also found that for wastewaters from dyeing and bleaching processes the removal efficiency of NTMP was at least 93%. It is well-known that phosphonates of NTMP type readily sorb on sewage sludge, clays, hydroxyapatite, and aluminum oxides. Recent publications by Nowack and Stone,16-18 as well as Kan et al.19 presented adsorption of NTMP complexes onto goethite and the adsorption mechanism of NTMP onto calcite. Luca et al.20 applied NTMP in the ion exchange process for removal of U(VI) salts on the polyacrylonitrile beads, and Zenobi et al.21 studied adsorption of different phosphonic acids onto boehmite. Nowack and Stone22,23 described also oxidative degradation of NTMP by Mn(II) ions and molecular oxygen. The Mn(II)

Resins and Chemicals. The polystyrene anion exchangers with different basicity of functional groups i.e. Lewatit MonoPlus M 500, Lewatit MonoPlus M 600, Lewatit MonoPlus MP 500, Lewatit MonoPlus MP 64, and Lewatit MP 62 were supplied by Lanxess and Amberlyst A 21 was supplied by Rohm and Haas. For the experiments, the resins were first pretreated with 1 M HCl and 1 M NaOH to remove impurities from their synthesis. After rinsing with demineralized water the anion exchangers were applied in the experiments. Short characteristics of these resins as given by the manufacturers are listed in Table 2. Nitrilotris(methylenephosphonic acid) (NTMP) was supplied by Sigma-Aldrich. All other reagents used were of analytical grade. The solution of M(II) complexes with NTMP were prepared from the concentrated stock solutions of Cd(II) and Pb(II) nitrates as well as Cu(II) and Zn(II) chlorides and the appropriate amount of NTMP for M(II)-NTMP ) 1:1 and M(II)-NTMP ) 1:2 (concentrations given in the further part of the paper are those of metal ions). pH of the obtained complexes was adjusted in the range 3-12. Static Method. The equilibrium isotherms and kinetics of Cd(II), Pb(II), Cu(II), and Zn(II) complexes with NTMP on the above-mentioned anion exchangers were investigated by batch operation as a function of pH, phase contact time, metal(II) ion and ligand concentration, and their molar ratio on the sorption. In all experiments, 100 mL Erlenmeyer flasks containing 0.2 g of the desired anion exchanger and 20 mL of M(II)-NTMP

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Table 2. Characteristics of Anion Exchangers Used in the Investigations typical properties matrixa

anion exchanger Lewatit MonoPlus Lewatit MonoPlus Lewatit MonoPlus Lewatit MonoPlus Lewatit MP 62 Amberlyst A 21 a

M 500 M 600 MP 500 MP 64

S-DVB S-DVB S-DVB S-DVB S-DVB S-DVB

(g) (g) (m) (m) (m) (m)

active groups

delivery form

bead size [mm]

total capacity [eq/L]

-N+(CH3)3 -N+(CH3)2C2H4OH -N+(CH3)3 -N+(CH3)3/-N(CH3)2 -N(CH3)2 -N(CH3)2

chloride chloride chloride chloride/free base free base free base

0.62 ( 0.05 0.62 ( 0.05 0.63 ( 0.05 0.59 ( 0.05 0.47 ( 0.06 0.49 - 0.69

1.3 1.25 1.1 1.3 1.7 1.25

S-DVB: polystyrene crosslinked with divinylbenzene. g: gel (microporous) structure. m: macroporous structure.

solutions were shaken in a 180 rpm shaker for 1-120 min at room temperature (25 ( 1 °C). Then the anion exchangers were separated by filtration except kinetic study, where the anion exchangers were separated after the determined time. The speed of shaking was chosen in the preliminary studies (not presented in the paper), where it was shown that further increase in precipitation rate does not cause the increase of the sorption effectiveness. Additionally, the time of equilibrium is very short, and it was not necessary to use higher speed of shaking. In order to determine the final M(II) concentration in solution, the atomic absorption spectrometry method (AAS) was used. The concentrations of analyzed metal ions at the equilibrium, qe (mg/ g), and at the time t, qt (mg/g), were obtained according to qe )

(c0 - ce)V m

(7)

qt )

(c0 - ct)V m

(8)

where 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). The experiments were repeated three times, and the mean values were used in the analysis of data. Standard deviation and analytical errors were calculated and the maximum error was found to be (5%. The effect of initial pH on the removal of Cd(II), Pb(II), Cu(II), and Zn(II) complexes with NTMP from aqueous solution was studied in the range of 3.0-12.0. pH was adjusted using 0.1 M HCl or 0.1 M NaOH solutions. For these experiments, 20 mL of appropriate solution was placed in flasks and 0.2 g of the anion exchanger was added. The solutions were shaken at 180 rpm for 120 min. (25 ( 1 °C). The influence of the initial concentrations of M(II) complexes with NTMP was investigated in the M(II):NTMP ) 1:1 and M(II):NTMP ) 1:2 systems at the optimal pH. A 20 mL portion of appropriate solutions at different concentrations in the range 1-7 × 10-3 M were added into the flask with 0.2 g of the anion exchanger. The concentration range given is sufficient for obtaining a plateau on the isotherm diagrams. These concentrations are also predicted based on the solubility the heavy metal complexes with NTMP and in agreement with their concentration ranges of occurring for example in metal coating processes. The solutions were shaken at 180 rpm for 120 min. (25 ( 1 °C). Kinetic Studies. As far as the kinetic studies go, 100 mL Erlenmeyer flasks containing 0.2 g of the desired anion exchanger and 20 mL of M(II)-NTMP solutions (in the M(II): NTMP ) 1:2 system) were shaken in a 180 rpm shaker for 1-120 min at room temperature (25 ( 1 °C). After equilibrium, accomplishment the anion exchangers were separated by filtration. In order to determine the final M(II) concentration in

solution, the AAS method was used. The kinetic parameters for sorption on the above-mentioned anion exchangers were determined using the pseudo-first-order as well as the pseudosecond-order models, which have linear forms as follows: log(q1 - qt) ) log(q1) -

k 1t 2.303

t t 1 ) + qt q2 k2q22

(9) (10)

where q1 and q2 are the amount of metal complexes sorbed at equilibrium (mg/g) for the pseudo-first-order and pseudo-secondorder kinetic models, qt is the amount of metal complexes sorbed at time t (mg/g), k1 and k2 are the equilibrium rate constants (1/min) for the pseudo-first-order and pseudo-second-order kinetic models. Adsorption Studies. Adsorption of metal complexes, qe (mg/ g), can be related to the equilibrium concentration of these complexes ce (mg/L), by both the Langmuir isotherm model qe ) q0 -

qe bce

(11)

and the Freundlich isotherm model log qe ) log KF +

1 log ce n

(12)

where KF and 1/n is the Freundlich constant related to the sorption capacity and q0 and b are the Langmuir constants. The Langmuir constants represent the maximum sorption capacity for the solid phase loading and the energy constant related to the heat of sorption, respectively. The Freundlich constant bF (also denoted as 1/n) is related to the sorption intensity. The intercept and the slope of the linear plot of log qe vs log ce provide the values of KF and 1/n, respectively. The constants q0 and b can be also evaluated from the intercept and the slope of the linear plot of ce/qe vs ce, respectively. Desorption Studies. The desorption of Cd(II), Pb(II), Cu(II), and Zn(II) complexes with NTMP on Lewatit MonoPlus M 500 was investigated by mixing 0.2 g of the sorbed by NTMP complexes anion exchanger with 20 mL of 1 M HCl, H2SO4, HNO3, NaCl, and HEDP solutions. The solutions were shaken at 180 rpm for 120 min (25 ( 1 °C). Then 1 mL samples were withdrawn in order to determine the concentrations of the analyzed metal ions by the AAS method. Interference Agents. The sorption of Cd(II), Pb(II), Cu(II), and Zn(II) complexes with NTMP on Lewatit MonoPlus M 600, Lewatit MonoPlus M 500, and Lewatit MP 500 was investigated by mixing 0.2 g of anion exchanger with 20 mL of mixture 4 × 10-3 M NaCl, NaNO3, Na2SO4, CaCl2, and thiourea solutions containing 4 × 10-3 M M(II)-NTMP complexes. The solutions were shaken at 180 rpm for 120 min (25 ( 1 °C). Then 1 mL


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Figure 2. Comparison of the sorption capacity (qe) for the Cd(II):NTMP ) 1:2 complexes on the polystyrene anion exchangers Lewatit MonoPlus M 500, Lewatit MonoPlus M 600, Lewatit MonoPlus MP 500, Lewatit MonoPlus MP 64, Lewatit MP 62, and Amberlyst A 21 depending on the pHe value of the sorption (0.2 g of anion exchanger, 20 mL, c0 ) 4 × 10-3 M, t ) 2 h).

samples were withdrawn in order to determine the concentrations of the analyzed metal ions by the AAS method. Apparatus. The ELPINE type 357, Elpin-Plus shaker was employed to mix the resin and the liquid phase. All pH measurements were made with a Radiometer pH meter (Model PHM 82) using a set of glass REF 451 and calomel pHG201-8 electrodes. An atomic absorption spectrometer ContrAA, Analytic Jena, was used for quantitative determination of the metal ions(II) concentration. Results and Discussion Ion Exchangers. Types of functional groups being active centers of the ion exchangers are given in Table 2. This table includes also the maximal capacities of the ion exchangers Lewatit MonoPlus M 500, Lewatit MonoPlus M 600, and Lewatit MonoPlus MP 500 as well as Lewatit MonoPlus MP 64, Lewatit MP 62, and Amberlyst A21. Of the chosen ion exchangers, Lewatit MonoPlus M 500 and Lewatit MonoPlus MP 500 (anion exchangers type I) and Lewatit MonoPlus M 600 (anion exchanger type II) differ in basicity of functional groups. Type I contains the trimethylamine group, and type II, the dimethylhydroxyethylamine group. Type I is more strongly basic than type II, but more difficult to regenerate. Type II has a higher thermal stability but is more sensitive to oxidants. In the case of the macroporous anion exchanger Lewatit MonoPlus MP 500, it is characterized by the smallest sorption capacity (about 1.1 equiv/L). Metal-Ligand Ratio. Formation of NTMP complexes with metal ions largely depends on the metal-ligand molar ratio, pH, and ionic medium in the solution. For example, in the case of the Cu(II):NTMP ) 1:1 system for the respective concentrations, 0.5, 1.0, 1.5, and 2.0 × 10-3 M accordingly 68.8%, 76.6%, 74.0% and 74.0% of Cu(II) ions are bound in the form of complexes. Due to the use of a double excess of NTMP, the amount of Cu(II) ions bound increased to over 90%. In the case of a double excess of Cu(II) ions, their complexing amounted to 36.6%. Calculations of Cu(II) ions complexing extent at pH ) 6 and an equimolar Cu(II)-NTMP ratio indicate that nearly all Cu(II) ions in solution should be bound in the form of complexes, with about 30% of it in the form of [Cu(ntmp)]4and about 70% in the form of [Cu(Hntmp)]3-. 31 For heavy metal complex solutions of Cd(II), Pb(II), Cu(II), and Zn(II) with NTMP prepared by us, it was found that the

Pb(II) complexes obtained in the M(II):NTMP ) 1:1 system and the initial concentration 1 × 10-3 M are insoluble in the pH range from 2.5 to 12.5. For the Cd(II) complexes with NTMP in the same ratio, it is possible to prepare 1 × 10-3 M solution in a full pH range, but at higher concentration (above 2 × 10-3 M) these complexes start to be insoluble. Cd(II) and Pb(II) complexes with NTMP obtained in the ratio 1:2 are soluble in a wide range of pH below the concentration level of 1 × 10-2 M. For Cu(II) and Zn(II) complexes in the same molar ratio, they were found to be soluble in the whole range of concentration and pH. Taking the above into account, the experiments were carried out in the M(II): NTMP ) 1:2 system in the range 1-7 × 10-3 M. The concentration increase leads to the decreasing sorption capacity. Additionally, as shown in the paper by Kozachkova et al.32 for high concentrations of NTMP the polynuclear complexes are formed with participation of coordinated phosphonic groups. They are not bound directly to metal ion but form intermolecular hydrogen bonds, cations of the bases added to the system, and solvent molecules. Effect of pH on Sorption Values. pH is an important controlling parameter in all the adsorption processes. NTMP is an acid with six acidic hydrogen atoms. When one metal ion is complexed by two NTMP molecules, pH value has great influence on charge and building of complexes. pH is an important controlling parameter in all the adsorption processes. NTMP is an acid with six acidic hydrogen atoms. When one metal ion is complexed by two NTMP molecules, pH value has great influence on charge and building of complexes. In the coordination zone of the M(II)-NTMP complex, phosphonic groups and hydrophobic organic parts originating from the ligand can be distinguished. The charge of formed complexes depends on pH. In the case of neutral complexes, the complex ion-water dipole interaction disappears, and only the dipole-dipole interaction remains. The complex of this type is characterized by greater hydrophobicity compared to the negative complexes M(II)-NTMP. Taking the above into account in the next stage, the influence of pH on the sorption effectiveness of Cd(II), Pb(II), Cu(II), and Zn(II) complexes with NTMP was investigated in the range from 3 to 12. Figures 2-5 present the comparison of the sorption capacity depending on the equilibrium pH (pHe) of Lewatit MonoPlus M 500, Lewatit MonoPlus M 600,

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Figure 3. Comparison of the sorption capacity (qe) for the Pb(II):NTMP ) 1:2 complexes on the polystyrene anion exchangers Lewatit MonoPlus M 500, Lewatit MonoPlus M 600, Lewatit MonoPlus MP 500, Lewatit MonoPlus MP 64, Lewatit MP 62, and Amberlyst A 21 depending on the pHe value of the sorption (0.2 g of anion exchanger, 20 mL, c0 ) 4 × 10-3 M, t ) 2 h).

Figure 4. Comparison of the sorption capacity (qe) for the Cu(II):NTMP ) 1:2 complexes on the polystyrene anion exchangers Lewatit MonoPlus M 500, Lewatit MonoPlus M 600, Lewatit MonoPlus MP 500, Lewatit MonoPlus MP 64, Lewatit MP 62, and Amberlyst A 21 depending on the pHe value of the sorption (0.2 g of anion exchanger, 20 mL, c0 ) 4 × 10-3 M, t ) 2 h).

Figure 5. Comparison of the sorption capacity (qe) for the Zn(II):NTMP ) 1:2 complexes on the polystyrene anion exchangers Lewatit MonoPlus M 500, Lewatit MonoPlus M 600, Lewatit MonoPlus MP 500, Lewatit MonoPlus MP 64, Lewatit MP 62, and Amberlyst A 21 depending on the pHe value of the sorption (0.2 g of anion exchanger, 20 mL, c0 ) 4 × 10-3 M, t ) 2 h).

Lewatit MonoPlus MP 500, Lewatit MonoPlus MP 64, and Lewatit MP 62 as well as Amberlyst A 21 in the M(II):NTMP

) 1:2 system. Figure 6 presents the exemplary results obtained in the M(II):NTMP ) 1:1 system for the Cd(II)


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Figure 6. Comparison of the sorption capacity (qe) for the Cd(II):NTMP ) 1:1 complexes on the polystyrene anion exchangers Lewatit MonoPlus M 500, Lewatit MonoPlus M 600, Lewatit MonoPlus MP 500, Lewatit MonoPlus MP 64, and Lewatit MP 62 depending on the pH value of the sorption (0.2 g of anion exchanger, 20 mL, c0 ) 1 × 10-3 M, t ) 2 h).

complexes. Depending on the initial pH, it was stated that in the M(II):NTMP ) 1:2 system for the studied Cd(II), Pb(II), Cu(II), and Zn(II) complexes, the sorption capacities of strongly basic anion exchangers increase with the increasing pH value from 3 to 6 and above these values they remain almost constant. For Lewatit MonoPlus MP 64 and Lewatit MP 62 as well as Amberlyst A-21, these values gradually decrease, which is consistent with the behavior of medium and weakly basic anion exchangers. They achieve maximal sorption capacities in the pH range 1-7. Taking into account the values of sorption capacities depending on equilibrium pH, they were found to be maximal at pH range 5-7 for strongly basic anion exchangers. In the Cd(II):NTMP ) 1:2 system, maximal sorption values were obtained at equilibrium pH 4 (Figure 2). Maximal sorption capacities for the Cd(II) and NTMP complexes for Lewatit MonoPlus M 500, Lewatit MonoPlus M 600, and Lewatit MonoPlus MP 500 are 42.29, 43.59, and 41.29 mg/ g, respectively. For the Pb(II) complexes, the values are 49.22, 49.30, and 46.31 mg/g, respectively. In the case of Cu(II) and Zn(II), these values are smaller: 20.36, 20.62, and 20.47 mg/g as well as 21.34, 21.43, and 29.97 mg/g. For medium and weakly basic anion exchangers, a drastic drop of sorption effectiveness in the range of equilibrium pH 7-9 was found. Therefore, these anion exchangers were not taken into consideration in further studies. In the Cd(II):NTMP ) 1:1 system, such relation does not occur (Figure 6). As mentioned earlier for the M(II)-NTMP complexes in the molar ratio 1:1 and 1:2, complexes of [M(ntmp)]4- and [M(Hntmp)]3- types are formed. They compete in the sorption process whereby those of [M(ntmp)]4- type are preferentially sorbed by the functional groups of the anion exchangers. As follows from Figures 2-5, solution pH changes toward more basic in the sorption process. For the medium and weakly basic anion exchangers, this change may amount to an even 4 units (for pH of the initial solution of M(II):NTMP ) 1:2 complexes equal 3). This indicates that OH- ions are released into the solution, and the molecules of the complex occupy their sites in the ion exchanger structure. For the strongly basic anion exchangers, the exchange of Cl- ions into the anion M(II)NTMP complexes is observed. For the strongly basic anion exchangers, Lewatit MonoPlus M 500 and Lewatit MonoPlus M 600 as well

as Lewatit MonoPlus MP 500, there may be proposed the following sorption mechanism: 4R-N+(CH3)3Cl- + [M(ntmp)]4- a [R-N+(CH3)3]4[M(ntmp)]4- + 4Cl- (13) 3R-N+(CH3)3Cl- + [M(Hntmp)]3- a [R-N+(CH3)3]3[M(Hntmp)]3- + 3Cl- (14) 4R-N+(CH3)2C2H4OHCl- + [M(ntmp)]4- a [R-N+(CH3)2C2H4OH]4[M(ntmp)]4- + 4Cl- (15) 3R-N+(CH3)2C2H4OHCl- + [M(Hntmp)]3- a [R-N+(CH3)2C2H4OH]3[M(Hntmp)]3- + 3Cl- (16) where R is the anion exchange skeleton. In the case of the medium and weakly basic anion exchangers Lewatit MonoPlus MP 64, Lewatit MP 62, and Amberlyst A21, this can be as follows: 4R-N+(CH3)3 /N(CH3)2OH- + [M(ntmp)]4- a [R-N(CH3)3 /N(CH3)2]4[M(ntmp)]4- + 4OH- (17) 3R-N+(CH3)3 /N(CH3)2OH- + [M(Hntmp)]3- a [R-N(CH3)3 /N(CH3)2]3[M(Hntmp)]3- + 3OH- (18) 4R-N+(CH3)2OH- + [M(ntmp)]4- a [R-N + (CH3)2]4[M(ntmp)]4- + 4OH- (19) 3R-N+(CH3)2OH- + [M(Hntmp)]3- a [R-N+(CH3)2]3[M(Hntmp)]3-+ 3OH- (20) Kinetic Studies. In order to analyze the sorption processes of Cd(II), Pb(II), Cu(II), and Zn(II) complexes with NTMP on Lewatit MonoPlus M 500, Lewatit MonoPlus M 600, and Lewatit MonoPlus MP 500, two kinetic models were used, including the pseudo-first- and pseudo-second-order models. Figure 7 present the effect of the phase contact time on the sorption of the studied complexes under the optimal sorption conditions that is the M(II)-NTMP molar ratio 1:2 and pH 6-7. For the Cd(II):NTMP ) 1:2 and Pb(II):NTMP ) 1:2 complexes, the maximal sorption capacities were found at pH ) 7. These

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Figure 7. Comparison of the sorption capacity (q) for the M(II):NTMP ) 1:2 complexes (M(II) ) Cd(II), Pb(II), Cu(II), Zn(II)) on the polystyrene anion exchangers Lewatit MonoPlus M 500, Lewatit MonoPlus M 600, and Lewatit MonoPlus MP 500 depending on the time of the sorption (0.2 g of anion exchanger, 20 mL, c0Cd ) 3 × 10-3 M, c0Pb ) 2 × 10-3 M, c0Cu ) 1 × 10-3 M, c0Zn ) 3 × 10-3 M, pH ) 7 for Cd(II) and Pb(II), pH ) 6 for Cu(II) and Zn(II)).

Figure 8. Comparison of the sorption capacity (q) for the Cd(II):NTMP ) 1:1 complexes on the polystyrene anion exchangers Lewatit MonoPlus M 500, Lewatit MonoPlus M 600, Lewatit MonoPlus MP 500, Lewatit MonoPlus MP 64, and Lewatit MP 62 depending on the time value of the sorption (0.2 g of anion exchanger, 20 mL, c0 ) 1 × 10-3 M, pH ) 12).

sorption values were slightly higher than at pH ) 6. However, for the Cu(II):NTMP ) 1:2 and Zn(II):NTMP ) 1:2 complexes, the maximal sorption capacities were found at pH ) 6. For comparison, Figure 8 presents the results obtained for the Cd(II): NTMP ) 1:1 complexes. The effect of the phase contact time on the sorption process was studied in the time range from 1 to 120 min at pH 6.0-7.0. It was observed that an increase in contact time corresponds to an increase in sorption. The sorption increased rapidly during the first 10 min, then it was moderate up to 30 min, and thereafter, the sorption remained constant which may be explained by the saturation of the available adsorption sites present in the anion exchanger phase of Lewatit MonoPlus M 500, Lewatit MonoPlus M 600, and Lewatit MonoPlus MP 500. According to the obtained results, it was observed that the time necessary to reach the equilibrium was 30 min. As follows, the sorption process is well-described by using the pseudo-second-order model (R2 > 0.98), but the pseudofirst-order model was not suitable (R2 < 0.47) (Table 3). The

results from both models were compared by statistical F-test. The results from F-test clearly show that the pseudo-first-order equation did not overlap data points. The same deduction came from good correspondence between q2 and qe,exp. A pseudosecond-order reaction designates that the speed of ion exchange is proportional with the concentration of metal complexes and number of active places in the ion exchanger surface. Adsorption Studies. Sorption processes proved to be effective for the removal of pollutants from wastewaters. The characteristics of adsorption behavior are generally inferred in the terms of both adsorption kinetics and equilibrium isotherms. They are also important tools to understand the adsorption mechanism, viz. the theoretical evaluation and interpretation of thermodynamic parameters. Moreover, the most appropriate method in designing the sorption systems and in assessing the performance of the sorption systems is to have an idea on sorption isotherms. The Langmuir and Freundlich isotherms are the most commonly used to describe the adsorption characteristics of the adsorbent used in water and wastewater treatment.


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Table 3. Pseudo-First- and Pseudo-Second-Order Kinetic Parameters for the Sorption of Cd(II), Pb(II), Cu(II), and Zn(II) Complexes with NTMP on Lewatit MonoPlus M 500, Lewatit MonoPlus M 600, and Lewatit MonoPlus MP 500 pseudo-first-order

pseudo-second-order

M(II):NTMP ) 1:2

q1 [mg/g]

k1 [1/min]

R

Cd(II) Pb(II) Cu(II) Zn(II)

9.92 15.59 1.55 1.76

0.132 0.082 0.108 0.030

0.9488 0.9842 0.7110 0.8720


5.54 5.73 1.64 2.37

0.103 0.066 0.106 0.084

0.6235 0.5789 0.7544 0.8314


16.74 8.71 1.86 2.88

0.107 0.045 0.141 0.045

0.9987 0.4703 0.6470 0.5639

2

k2 [1/min]

q2 [mg/g]

h [mg/(g min)]

R2

qexp

F-test

34.364 27.701 2.607 24.213

0.9999 0.9999 0.9984 0.9999

30.62 45.13 6.20 15.81

237.5 14.0 5.7 1491.6

30.488 44.248 1.765 23.364

1.0000 0.9999 0.9859 1.0000

30.48 44.58 6.25 16.11

994.3 7273.5 0.19 1674.6

17.668 27.701 5.187 12.210

0.9999 0.9999 0.9998 1.0000

30.00 43.61 6.08 16.73

28.1 4269.8 3.4 308.0

Lewatit MonoPlus M 500 30.76 44.05 6.31 15.84

0.036 0.014 0.065 0.096

Lewatit MonoPlus M 600 30.67 44.84 6.07 16.20

0.032 0.022 0.048 0.089

Lewatit MonoPlus MP 500 30.39 44.05 6.13 16.86

0.019 0.014 0.138 0.043

Table 4. Parameters of the Langmuir and Freundlich Isotherms for the Sorption of Cd(II), Pb(II), Cu(II), and Zn(II) Complexes with NTMP in the Single Component System on Lewatit MonoPlus M 500, Lewatit MonoPlus M 600, and Lewatit MonoPlus MP 500a Langmuir constants M(II):NTMP ) 1:2

qe,exp [mg/g]

q0 [mg/g]


45.22 52.17 18.96 26.67

46.85 52.20 18.40 28.16


44.15 53.18 19.50 27.46


45.62 45.38 16.32 26.02

a

F-test R2

Freundlich constants

[V1/V2]

KF [mg/g(L/mg)1/n]

n [-]

R2

Lewatit MonoPlus M 500 0.087 0.9991 0.968 0.9996 5.373 0.9991 0.217 0.9835

1.45 37.43 16.91 179.95

16.04 40.67 12.13 12.40

5.19 24.56 11.45 6.64

0.7227 0.7408 0.7369 0.9050

46.38 53.13 19.66 28.03


3.21 58.75 1.29 24.26

15.36 42.10 13.07 13.65

5.03 24.27 11.74 7.77

0.6704 0.6667 0.7565 0.9295

44.90 45.64 16.79 26.05

Lewatit MonoPlus MP 500 0.081 0.9969 17.89 0.128 0.9972 375.95 0.165 0.9861 175.91 0.094 0.9958 414.30

16.57 40.63 11.38 12.24

5.60 49.04 10.08 7.95

0.8991 0.3207 0.5785 0.9779

b [L/mg]

The critical value of F-test is 4.28 at the significance level R ) 0.05.

The Langmuir model is probably the best known and most widely applied sorption isotherm. The Langmuir constants qo and b are related to the adsorption capacity and the energy of adsorption, respectively. The Freundlich model is the empirical model applied to nonideal sorption on heterogeneous surfaces as well as multilayer sorption. The fit of data to Freundlich isotherm indicates the heterogeneity of the sorbent surface. The magnitude of the exponent 1/n 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. The adsorption data of Cd(II), Pb(II), Cu(II), and Zn(II) complexes with NTMP in the 1:2 system on Lewatit MonoPlus M 500, Lewatit MonoPlus M 600, and Lewatit MonoPlus MP 500 were analyzed in terms of both Langmuir and Freundlich equations. Table 4 shows the constants of the Langmuir and Freundlich equations obtained for the anion exchangers under consideration in the single metal systems. It is evident that the Langmuir isotherm equation better describes the adsorption processes of Cd(II), Pb(II), Cu(II) and Zn(II) complexes with NTMP from aqueous solution compared to the Freundlich equation. The correlation coefficients (R2) used to describe the matching between the experimental data and the theoretical model were greater than 0.99. The maximum sorption capacities were obtained for the Pb(II) and Cd(II) complexes with NTMP on Lewatit MonoPlus M 500 and Lewatit MonoPlus M 600. The order of the sorbed complexes on these anion exchangers in an equilib-

rium state was as follows: Pb(II) > Cd(II) > Zn(II) > Cu(II). For Lewatit MonoPlus MP 500, the analogous affinity series can be shown as follows: Pb(II) ) Cd(II) > Zn(II) > Cu(II). The results obtained in the quaternary metal system including Cd(II)-Pb(II)Cu(II)-Zn(II) complexes with NTMP are presented in Table 5. It was found that due to the competition reaction the maximum adsorption capacity for the Cu(II) and Zn(II) complexes with NTMP both in the binary and quartenary systems are comparable. A result of complexes similarity is low selectivity of anion exchangers but differences cannot be discussed without IR or NMR data for complex solutions. Interference Agents. Though, in the presented paper, the studies of wastewater containing NTMP acid were not carried out, some research was made applying the systems showing to some extent actual conditions concerning solution composition, pH, etc. For example, in the paper by Tang et al.,33 there was proposed the following composition 1-50 g/L aminophosphonate and 1-10 g/L fluoride ions as a mixture for coating magnesium alloys by chromate free bath. After the coating process, the wastewater contains, among others, aminophosphonate complexes of various metals like magnesium and vanadium. Other publications describe similar mixtures for coating zinc, alluminium, or steel alloy. In Table 6, the results concerning the effect of the NaCl, NaNO3, Na2SO4, and CaCl2 as well as thiourea commonly applied in galvanic covering mixtures are presented. As follows from the

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Table 5. Parameters of the Langmuir and Freundlich Isotherms for the Sorption of Cd(II), Pb(II), Cu(II), and Zn(II) Complexes with NTMP in the Quaternary Component System (Cd(II)-Pb(II)-Cu(II)-Zn(II)) on Lewatit MonoPlus M 500, Lewatit MonoPlus M 600, and Lewatit MonoPlus MP 500a Langmuir constants M(II):NTMP ) 1:2

qe.exp [mg/g]

q0 [mg/g]


13.77 12.94 5.64 5.11

14.45 12.87 5.62 5.20


13.56 14.56 6.10 5.37


13.78 12.37 5.90 4.79

a

F-test R2

Freundlich constants

[V1/V2]

KF [mg/g(L/mg)1/n]

n [-]

R2


3.60 64.04 43.29 16.98

4.56 12.28 4.75 3.78

3.84 53.42 23.22 11.79

0.7291 0.6199 0.9739 0.9577

13.99 14.85 6.28 5.51


4.70 125.04 73.45 81.76

5.48 12.89 5.04 4.05

4.76 40.38 20.07 12.23

0.5303 0.8502 0.9864 0.9581

14.46 12.25 6.17 4.81

Lewatit MonoPlus MP 500 0.238 0.9972 0.675 0.9994 0.886 0.9977 2.944 0.9998

5.05 80.80 33.96 24.79

5.06 11.77 4.82 3.63

4.19 68.21 21.47 13.19

0.7702 0.6674 0.9383 0.9396

b [L/mg]

The critical value of F-test is 4.28 at the significance level R ) 0.05.

Table 6. Values of the Sorption (%) of Cd(II), Pb(II), Cu(II), and Zn(II) Complexes with NTMP in the Single Component System on Lewatit MonoPlus M 600, Lewatit MonoPlus M 500, and Lewatit MP 500 with Various Interfering Substances

Table 7. Values of the Desorption (%) of Cd(II), Pb(II), Cu(II), and Zn(II) Complexes with NTMP in the Single Component System Desorbed from Lewatit MonoPlus M 500, Lewatit MonoPlus M 600, and Lewatit MonoPlus MP 500

M(II):NTMP ) 1:2

M(II):NTMP ) 1:2 1 M HCl 1 M H2SO4 1 M HNO3 1 M NaCl 1 M HEDP

NaCl

NaNO3

Na2SO4

CaCl2

S ) C(NH2)2


92.34 77.94 81.67 83.72

Lewatit 85.88 65.63 78.31 70.31

MonoPlus M 600 84.47 67.62 69.22 56.28 71.38 76.71 54.51 76.93 76.54 59.04 78.46

84.27 71.02 78.54 80.38


93.58 79.24 77.47 80.31

Lewatit 82.26 61.31 70.76 67.43

MonoPlus M 500 81.26 64.61 58.80 48.38 65.63 71.21 46.73 74.65 80.38 51.85 71.27

80.46 68.86 73.27 77.50


91.09 71.45 74.68 74.24

Lewatit MP 79.26 76.45 50.89 55.20 62.97 62.52 63.60 72.23

500 54.78 37.60 37.35 47.53

54.49 66.64 66.95

Lewatit MonoPlus M 500 Cd(II) Pb(II) Cu(II) Zn(II)

15.42 76.80 90.12 46.11

83.98 15.80 80.31 96.38

80.78 85.58 95.21 94.87

53.57 84.41 92.66 92.60

29.02 45.78 23.62 36.66

61.57 88.50 87.89 88.82

21.29 51.05 28.20 35.90

50.63 74.46 81.37 80.00

87.98 69.19 76.12 91.76

Lewatit MonoPlus M 600

76.05 64.19 74.42 67.65

obtained data, a significant drop in sorption effectiveness of M(II)-NTMP complexes with Cd(II), Pb(II), Cu(II), and Zn(II) is observed only in the case of sodium sulfate(VI) addition. Thiourea has the smallest effect on the sorption on the anion exchangers of M(II)-NTMP complexes. Desorption Studies. In order to investigate the sorption mechanism and regeneration methods of applied anion exchangers, the desorption experiments were conducted (Table 7). The results of the desorption experiment show that 1 M HNO3 could effectively desorb the M(II) complexes with NTMP. Similar results were obtained in the case of 1 M NaCl. However, it was not very effective in washing away the Cd(II)-NTMP complexes. A 1 M portion of H2SO4 in the case of macroporous Lewatit MonoPlus MP 500 did not washed away the Pb(II)NTMP complexes well due to probable precipitation of lead sulfate in the anion exchanger phase. However 1 M HCl was not very effectiwe for all complexes. Good desorption results were obtained using 1 M HEDP for strongly basic anion exchanger Lewatit MonoPlus MP 500. Conclusions The obtained results indicate that, in the presence of NTMP, the effectiveness of sorption of the studied heavy metal ions


22.09 72.70 88.63 50.27


9.82 57.49 87.75 33.42

93.58 15.80 91.56 91.85

94.92 87.33 88.99 92.98

Lewatit MonoPlus MP 500 91.18 16.32 84.00 93.94

90.92 70.95 88.12 96.98

depends on the kind of complex form in the anion exchanger phase and the type of the anion exchanger used. Strongly basic anion exchangers have much higher sorption at neutral pH, contrary to weakly basic anion exchangers, which have good sorption only in strong acidic solution. The type of complexes undergoing sorption depends on the M(II)-NTMP molar ratio and the process of sorption is very fast in the M(II):NTMP ) 1:2 system. Equilibrium is attained in less than 30 min with over 98% of adsorption. Equilibrium of adsorption is better described the Langmuir model than the Freundlich model. The best kinetic correlation under the experimental conditions investigated was provided by the pseudo-second-order kinetic model. Literature Cited (1) Nowack, B. Enviromental chemistry of phosphonates. Water Res. 2003, 37, 2533–2546. (2) Hang, T. T. X.; True, T. A.; Nam, T. H.; Oanh, V. K.; Jorcin, J.-B.; Pe´be`re, N. Corrosion protection of carbon steel by an epoxy resin containing organically modified clay. Surf. Coat. Technol. 2007, 201, 7408–7415. (3) Azim, S. S.; Sathiyanarayanan, S.; Venkatachari, G. Anticorrosive properties of PANI-ATMP polymer containing organic coating. Prog. Org. Coat. 2006, 56, 154–158.

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ReceiVed for reView November 4, 2009 ReVised manuscript receiVed March 31, 2010 Accepted April 11, 2010 IE901748K

Sorption of Cd(II), Pb(II), Cu(II), and Zn(II ... - ACS Publications

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