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Ind. Eng. Chem. Res. 2010, 49, 2388–2400

Cu(II), Zn(II), Ni(II), and Cd(II) Complexes with HEDP Removal from Industrial Effluents on Different Ion Exchangers Dorota Kołodyn´ska* Department of Inorganic Chemistry, Faculty of Chemistry, Maria Curie-Sklodowska UniVersity, Maria Curie-Sklodowska Sq. 2, 20-031 Lublin, Poland

The aim of this research is to investigate sorption characteristic of polyacrylate anion exchangers and chelating ion exchangers for the removal of Cu(II), Zn(II), Ni(II), and Cd(II) complexes with HEDP (1-hydroxyethylene1,1-diphosphonic acid) from aqueous solutions. Optimum sorption conditions were determined as a function of phase contact time (1-180 min), pH (5-13), ion exchanger dosage (0.1-1.0 g), temperature (293-333 K), and chloride ions concentration (0.01-0.5 mol/L of NaCl). The Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R) models were applied to describe the adsorption isotherm of Cu(II), Zn(II), Ni(II), and Cd(II) complexes with HEDP on Amberlite IRA 458 and Amberlite IRA 958 as well as Purolite S-920 and Purolite S-930. In the case of M(II)-HEDP complexes the adsorption capacities of Amberlite 458 were found to be 1.96 meq/g for Cu(II), 3.94 meq/g for Zn(II), 2.98 meq/g for Ni(II), and 4.25 meq/g for Cd(II). The metal ions were desorbed using 1 M HCl. The ion exchange capacity of ion exchangers applied decreased 8% in the recovery of Cu(II)-HEDP after 10 times of the sorption-desorption processess. Experimental data were also tested in terms of sorption kinetics using the pseudo-first-order and pseudosecond-order kinetic models. The results showed that the sorption processes of Cu(II), Zn(II), Ni(II), and Cd(II) complexes with HEDP on Amberlite IRA 458 and Amberlite IRA 958 as well as Purolite S-920 and Purolite S-930 followed well the pseudo-second-order kinetics. Introduction Phosphonates are a group of chelating agents that are used in high quantities in oil production in the textile industry to stabilize peroxide based bleaching agents, in industrial and household detergent formulations, and in nuclear medicine as bone-seeking carriers for radionuclides.1-4 In detergent applications four phosphonates, HEDP (1-hydroxyethylene-1,1-diphosphonic acid), ATMP (aminotrimethylenephosphonic acid), EDTMP (ethylenediaminetetra-(methylenephosphonic acid)), and DTPMP (diethylenetriaminepentamethylenephosphonic acid), are used.5 The chemical structures of these phosphonates were presented in the paper by Popov et al.6 HEDP is an efficient sequestrant of Ca(II), and in detergent applications it is used to inhibit growth of CaCO3 crystals on the fabric. It is also a very strong inhibitor of mineral precipitation and growth of CaCO3 and CaSO4, which is a significant problem in commercial water treatment processes (cooling waters and desalination systems). Another important application of HEDP is protection against corrosion. HEDP can dissolve the oxidized materials on metal surfaces and form deposits of insoluble complexes with Fe(III) to prevent the metals from corrosion. However, it was considered that because complexes of HEDP with Zn(II) are soluble in the solution, the anodic process of zinc corrosion was accelerated in the presence of this chelating agent (synergistic inhibition effect has been observed).7-9 The increase in the phosphonates use in Europe is a result of the scrutiny imposed by regulatory agencies on other complexing agents such as DTPA (diethylenetriaminepentaacetic acid) and EDTA (ethylenediaminetetraacetic acid) because of their potential of remobilization of heavy metals. The increase in the use of phosphonates is also related to voluntary bans of polyphosphates in cleaning applications introduced because of * To whom correspondence should be addressed. Tel.: +48 (81) 5375736. Fax: +48 (81) 5333348. E-mail: [email protected].

euthrophication and the necessity to improve the ecological conditions in natural waters.10,11 Phosphonates adsorb very strongly onto almost all mineral surfaces. This behavior distinguishes them from the corresponding aminocarboxylates, which exhibit much weaker interactions with mineral surfaces, especially near neutral pH. Some of the investigated adsorbents for phosphonates are calcite, clays, aluminum oxides, iron oxides, zinc oxide, hydroxoapatite, and barite.12-17 For all those compounds very strong adsorption is observed in the pH range of natural waters. On the other hand, in the case of the presence of metal ions such as Fe(III), Zn(II), or Cu(II), strong adsorption of the uncomplexed phosphonates, which resulted in a dissociation of the complexes on metal ions and the phosphonates, was discovered. Therefore, in the presented paper the method of simultaneous removal of heavy metal ions and phosphonates was proposed. The commonly used phosphonic acid HEDP was used as a complexing agent for heavy metal ions removal by the ion exchange method. Among typical methods of heavy metal ions removal from solutions such as precipitation, coagulation and flocculation, and electrolytic methods, evaporation and adsorption ion exchange receives considerable interest due to high efficiency and low operational costs. However, the removal of the complexed heavy metal ions is a problem affecting most metal related fields of industry. Purification of the waters and wastewaters containing heavy metal complexes requires separation of heavy metal ions from the complexing agents. One possible strategy is first, precipitation on the complexing agents (aminopolycarboxylic acids such as EDTA, NTA (nitrilotriacetic acid), DTPA or phosphonic acids such as HEDP, ATMP) by adding strong acids. Second, precipitation of heavy metal ions from such solutions as hydroxides, sulfides, or carbonates should be performed. The solubility of the heavy metal contaminant and the required cleanup standards impose chemical precipitant. Commonly coagulants and flocculants are additionally used to increase particle size through aggregation. Another strategy assumes

10.1021/ie9014414  2010 American Chemical Society Published on Web 01/26/2010

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a

Table 1. Data of Ion Exchangers Applied properties

Amberlite IRA 458

Amberlite IRA 958

matrix functional groups particle size [mm] BET surface area [m2/g] average pore diameter [nm] moisture content [%] total capacity [meq/mL] shipping weight [g/L] swelling [%] operating pH range thermal stability [K]

A-DVB quaternary ammonium 0.60-0.90 2.03 3.6 57-64 1.25 720 20 Cl-fOH- form 0-14 373

A-DVB quaternary ammonium 0.67-0.85 2.03 6.46 66-72 0.80 720 20 Cl-fOH- form 0-14 373

a

Amberlite IRA 67

Purolite S-920

Purolite S-930

A-DVB tertiary amine 0.55-0.75 4.05 7.1 56-64 1.60 718 20 OH-fCl- form 0-7 343

PS-DVB thiouronium 0.30-1.20 26.05 9.13 48-54 1.60 700-735 5 H+fHg2+ form 1-13 353

PS-DVB iminodiacetate 0.30-1.20 20.61 5.49 55-65 1.10 710-745 25 H+fNa+ form 2-11 343

Abbreviations: PS-DVB, styrene divinylbenzyne copolymer; A-DVB, acrylic divinylbenzyne copolymer.

removal of metal ions from solutions by their reduction or electrodeposition but these processes are incomplete and additional purification is required after removal of a basic portion. Therefore, more effective solution is application of anion exchangers or in the case of weaker complexing agents then EDTA chelating ion exchangers. Simultaneous removal of heavy metal ions and anionic ligands in the form of negative complexes using anion exchangers is characterized by some advantages. Among them, low operating cost efficiency, little energy consumption, using cheap regenerant chemicals, waste minimization, instrumentation and automation, operating life expectancy of the anion exchange system are important factors. There is, however, a number of limitations such as, for example, increase of costs of the process for treating concentrated solutions which must be taken into careful account in the design stages.18-20 The polyacrylate anion exchangers used in the presented paper are characterized not only by good sorption capacity for metal ions but they are also resistant to organic impurities. For, example Amberlite IRA 958 found application in sugar refining plants for sorption of large organic molecules. It also sorbes significant amounts of [Cu(CN)3]2- and [Cu(CN)4]3- compared to [Ni(CN)4]2-.21-23 In the paper by Riveros,23 it was found that Amberlite IRA 458 has a weak affinity for [Au(CN)2]- but strong affinity for [Fe(CN)6]4- and [Zn(CN)4]2-. The order of affinities for metallocyanide Zn(II) > Fe(II) > Cu(II) > Ni(II) > Au(I) is due to its high hydrophilic nature along with its high ionic density. For sorption of [Pb(edta)]2- weakly basic Amberlite IRA 68 (now denoted as Amberlite IRA 67) is the most appropriate. It is worth stressing that Amberlite IRA 458, Amberlite IRA 958, and Amberlite IRA 68 have larger preference for [Pb(edta)]2- and [Cd(edta)]2- than polystyrene resins such as Amberlite IRA 400 and Amberlite IRA 900.24 As for chelating ion exchangers Purolite S-930 can be used for removal of Cd(II).25 In the paper by Seggiani et al.,26 it was found that Lewatit TP 207, Purolite S-930 and Amberlite IRC748 applied for the recovery of Ni(II) from the sulfate solutions reveal the sorption capacity equal to 1.32 mmol/g. Purolite S-930 and Amberlite IRC-748 have also high affinity for copper.27 In the presented paper the ion exchange process was studied by means of column experiments as well as batch investigations. The effect of the solution pH, ion exchanger dosage, phase contact time, temperature, and the chloride ions concentration on the sorption of the Cu(II), Zn(II), Ni(II), and Cd(II) complexes with HEDP on Amberlite IRA 458, Amberlite IRA 67, and Amberlite IRA 958 as well as Purolite S-920 and Purolite S-930 was determined. The adsorption isotherm models were applied to describe the sorption process, and probable

mechanism was explained. The attempts were also made to study kinetics of sorption process. Experimental Section Anion Exchangers and Chemicals. The following ion exchangers have been used: Amberlite IRA 458 (the strongly basic, gel anion exchanger), Amberlite IRA 958 (the strongly basic, macroporous anion exchanger), and Amberlite IRA 67 (the weakly basic, gel anion exchanger). These resins were supplied by Rohm and Haas Co. (Chauny, France). Purolite S-920 (the macroporous, chelating ion exchanger with the thiourea functional groups) and Purolite S-930 (the macroporous chelating ion exchanger with the iminodiacetate functional groups) supplied by Purolite International, Ltd. (Llantrisant, United Kingdom) were also used. Before the experiments, the resins were washed with hydrochloric acid (0.1 M) and sodium hydroxide (0.1 M) to remove impurities from their synthesis. After pretreatment they were washed with deionized water. Their properties are listed in Table 1. All the solutions of Cu(II), Zn(II), Ni(II), and Cd(II) complexes with HEDP were prepared with deionized water and analytical grade reagents by the mixing of appropriate metal chlorides or nitrates with the HEDP solution. The pH was adjusted to 7.5 and 11.5. HEDP (CAS no. 2809-21-4) was obtained from POCh (Gliwice, Poland). The other chemicals used were of analytical grade. The Dynamic Method. The ion exchangers were packed in a column with the inner diameter of 1 cm and the length of 25 cm. The prepared solutions of complexed metal ions with HEDP (initial concentration 1 × 10-3 M, M(II)-HEDP ) 1:1) were passed continuously downward through the resin beds (bed volume 10 mL), keeping the flow rate at 0.6 mL/min. The mass (Dg) and volume (Dv) distribution coefficients as well as the working (Cw) and total (Cr) ion exchange capacities of M(II) were calculated from the determined breakthrough curves according to eqs 1-3:28 Dg )

j - (V0 + Vi) V m

(1)

Dv )

j - (V0 + Vi) V Vj

(2)

Vec0 Vj

(3)

Cw )

The total ion exchange capacities (Cr) were calculated by integration along the curve using the mathematical option of the graphical program.

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The Static Method. The appropriate aqueous phase (50 mL) and ion exchanger (0.5 g) were put into a conical flask and shaken mechanically using the laboratory shaker Elpin type 357, (Elpin-Plus, Poland) for 1-180 min. After the pH of solutions was stabilized and equilibrated, the ion exchanger was filtered. The experiments were conducted in the two parallel series. The reproducibility of the measurements was within 5%. The concentrations of analyzed metals at the equilibrium (qe) and at the specific time (qt) at room temperature were calculated according to eqs 4 and 5:29 qe )

(c0 - ce)V m

(4)

qt )

(c0 - ct)V m

(5)

The Adsorption Studies. Adsorption isotherm studies were carried out using the batch equilibrium technique. The initial concentrations of the studied solutions were prepared in the range 1.0 × 10-3 to 1.5 × 10-2 M. A 0.5 g portion of the ion exchanger sample and 50 mL solution of Cu(II), Zn(II), Ni(II), and Cd(II) complexes with HEDP were placed in 100 mL flasks for 3 h at room temperature until equilibrium was reached. The concentration of M(II) ions in the filtrate was determined using the AAS method. In the case of the assumption that ion exchanger is an adsorbent possessing specific charge many isotherm equations, such as Langmuir, Freundlich, Temkin, and DubininRadushkevich (D-R), can be used for modeling the adsorption equilibrium. The use of these models for the study of the sorption behavior of different sorbents has been consistently discussed by many researchers.30-36 The Langmuir equation may be written as q0KLce qe ) 1 + KLce

(6)

The Freundlich model can be as follows: qe ) KFce1/n

(7)

The Temkin adsorption isotherm is expressed as qe )

( )

RT ln(KTce) bT

(8)

The D-R equation was given by qe 2 ) e-βε Xm

(9)

To investigate the effect of pH, 50 mL of 1.0 × 10-3 M complexed with the HEDP metal solutions of Cu(II), Zn(II), Ni(II), and Cd(II) and 0.5 g of Amberlite IRA 458, Amberlite IRA 958, Amberlite IRA 67 as well as Purolite S-920 and Purolite S-930 were used. Experiments were performed in the pH range 5-13. The solutions were shaken for 3 h at 180 rpm at room temperature. The appropriate ion exchanger dosage was determined for the Cu(II) complexes with HEDP using Amberlite IRA 458. Different 0.1-1.0 g anion exchanger samples and 50 mL solution of Cu(II) complexes were placed in 100 mL flasks for 3 h at 180 rpm at room temperature until equilibrium was reached, as previously described.

As for the temperature, the investigations were conducted for 0.5 g of the ion exchanger sample and 50 mL solution of Cu(II), Zn(II), Ni(II), and Cd(II) complexes with HEDP in the temperature range 293-313 K. The Selectivity Coefficients Studies. Ion exchange resins, when placed in a solution, reach an equilibrium state between ions in solution and those on the resin. To determine the selectivity coefficients, the 0.5 g ion exchanger sample and 50 mL solution of Cu(II) and Zn(II) complexes with HEDP in the binary system were mixed in 100 mL flasks until equilibrium was reached. In all experiments, the ratio between the concentrations in solution of two exchangeable (X) complexes was kept close to the unity. From the concentration of the abovementioned complexes in the resin phase the selectivity coefficients were calculated as follows: KAB )

¯ ][B] [A [B¯][A]

(10)

The selectivity coefficients were determined in the two parallel series. The reproducibility of the measurements was within 5%. The Kinetic Studies. To examine the controlling mechanism of sorption processes such as mass transfer and chemical reaction, several kinetic models were used to test the experimental data. Kinetic experiments were made by using 50 mL solution of M(II) complexes with HEDP with the concentration 1.0 × 10-3 M (M(II)-HEDP ) 1:1). The samples were taken at different time intervals (1-180 min). There was performed a kinetic analysis of adsorption by the use of the pseudo-firstorder and pseudo-second-order equations:37,38 log(qe - q) ) log(qe) t t 1 ) + q qe k2qe2

k1t 2.303

(11)

(12)

Additionally, the intraparticle diffusion model, which refers to the theory proposed by Weber and Morris was also tested. Apparatus. The pH values were measured with the Radiometer pH meter PHM 84 (Copenhagen, Denmark) with glass REF 451 and calomel pHG 201-8 electrodes. The concentrations of heavy metal ions were measured with the AAS spectrometer ContrAA, (Analytic Jena, Jena, Germany). Surface morphology of the ion exchangers was studied by the scanning electron microscope LEO1430 VP (Carl Zeiss, Jena, Germany) with the EDX detector (Ro¨ntec GmbH, Berlin, Germany). The specific surface area, pore volume, and pore diameter of the studied ion exchangers were measured using ASAP 2405 (Micromeritics Instrument Co., Norcross, USA) and AUTOSORB-1 CMS (Quantachrome Instruments, Boynton Beach, USA). FT-IR spectra were obtained using spectrometer ALPHA including the Platinum ATR sampling module with a single bounce diamond crystal and a DTGS detector (Bruker Optik GmbH, Ettlingen, Germany). Results and Discussion To establish the affinity series of the Cu(II), Zn(II), Ni(II), and Cd(II) complexes with HEDP in the M(II):HEDP ) 1:1 system, at pH 11.5 on Amberlite IRA 458 and Amberlite IRA 958 the breakthrough curves were determined (Figures 1a,b). As follows from the spatial diagram,39,40 at pH values from 3 to 6 HEDP occurs as H2hedp2- and with the studied metal ions forms neutral complexes of [M(H2hedp)] type. At pH from

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6 to 11 metal ions should occur as [M(Hhedp)] complexes, whereas at pH above 11 they should occur as bivalent complexes of the [M(hedp)]2- type. Therefore for the studied anion exchangers Amberlite IRA 458 and Amberlite IRA 958 with the quaternary ammonium functional groups, at studied pH, the anion exchange reactions can be written as 2R-N+(CH3)3Cl- + [M(hedp)]2- a [R-N+(CH3)3]2[M(hedp)]2- + 2Cl- (13) where R is the anion exchanger skeleton (A-DVB). The analogous mechanism was described for the sorption of the arsenate H2AsO4- and HAsO42- ions as well as cyanide complexes of the [Cu(CN)3]2-, [Cu(CN)4]3-, and [Ni(CN)4]2types on polyacrylate anion exchangers.41,42 For Amberlite IRA 458 and Amberlite IRA 958 the following advantages over the conventional polystyrene anion exchangers such as Dowex 1 × 8, Lewatit MonoPlus M 500, or Amberlite IRA 400 should be listed: the ability to reposition their long spacer arm to provide two or three quaternary ammonium exchange sites, the hydrophilic character of the polyacrylic matrix, and the hydrogen bonding possibilities provided by the carbonyl group. In the case of the weakly basic anion exchanger Amberlite IRA 67 with the tertiary amine functional groups, at the neutral and basic pH the following reactions should take place: pH 7.0 R-+NH(CH3)2Cl- + [M(Hhedp)]- a [R-+NH(CH3)2][M(Hhedp)]- + Cl-

(14)

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pH 11.5 2R-+NH(CH3)2Cl- + [M(hedp)]2- a [R-+NH(CH3)2]2[M(hedp)]2- + 2Cl-

(15)

Taking the above statements into account for Amberlite IRA 67, the breakthrough curves were also determined (Figure 1c,d). The breakthrough curves are not of typical S shape. The plateau on the plots suggests that at pH 7.0 sorption of different types of complexes takes place. According to the spatial diagram39,40 reported in literature in the system at pH 7.0 there is about 47% of [M(Hhedp)]- and [M(hedp)]2- complexes each and these complexes probably undergo sorption in the resin phase. However, at pH 11.5 due to poor dissociation of the tertiary amine functional group of Amberlite IRA 67, the sorption of binuclear complexes of [Cu(hedp)]2- is very low. Under these conditions OH- ions are more preferable. As follows from the mass (Dg) and volume (Dv) distribution coefficients calculated for the above-mentioned complexes (Figure 2a,b), the anion exchangers taking into account their applicability for removal of the Cu(II), Zn(II), Ni(II), and Cd(II) complexes with HEDP can be arranged as follows: Cu(II)-HEDP ) 1:1

IRA 958 > IRA 458 > IRA 67

Zn(II)-HEDP ) 1:1

IRA 67 > IRA 458 > IRA 958

Cd(II)-HEDP ) 1:1

IRA 67 > IRA 958 > IRA 458

Ni(II)-HEDP ) 1:1

IRA 958 > IRA 458 > IRA 67

For Amberlite IRA 458 and Amberlite IRA 958 the calculated values of the working (Cw) and total (Cr) sorption capacities

Figure 1. The breakthrough curves of Cu(II), Zn(II), Ni(II), and Cd(II) complexes with HEDP on Amberlite IRA 458 (a), Amberlite IRA 958 (b), Amberlite IRA 67 at pH 7.0 (c), and Amberlite IRA 67 at pH 11.5 (d) (c0 ) 1 × 10-3 M, bed volume ) 10 mL, flow rate at 0.6 mL/min).

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equilibrium adsorption capacities calculated (q) depending on the pseudo-second-order model much closer to the experimental data (qe,exp) also proved the suitability of pseudo-second-order model. The quaternary ammonium functional groups of Amberlite IRA 458 and Amberlite IRA 958 were quick to interact with M(II)-HEDP complexes and the sorption rate became dependent on the rate at which the complexes were transported from the liquid phase to the functional groups. The sorption order can be as follows: Cd(II) > Cu(II) > Zn(II) > Ni(II). However, for chelating ion exchangers with the thiourea (Purolite S-920) and iminodiacetate (Purolite S-930) functional groups the selectivity effect of these resins toward heavy metal ions is caused by the presence of sulfur and nitrogen atoms in the functional groups, respectively. Therefore, for Purolite S-920 the following reaction can be written:43,44

Figure 2. (a,b) Comparison of the mass (Dg) and volume (Dv) distribution coefficients of Cu(II), Zn(II), Ni(II), and Cd(II) complexes with HEDP on Amberlite IRA 458, Amberlite IRA 958, and Amberlite IRA 67.

are as follows: 1.88 meq/mL, 1.48 meq/mL and 2.14 meq/mL, 1.98 meq/mL (for Cu(II)); 0.70 meq/mL, 0.39 meq/mL and 0.91 meq/mL, 0.61 meq/mL (for Zn(II)); 0.98 meq/mL, 0.88 meq/ mL and 1.46 meq/mL, 1.87 meq/mL (for Ni(II)); 0.07 meq/ mL, 0.08 meq/mL and 0.12 meq/mL, 0.16 meq/mL (for Cd(II)), respectively. The obtained results were confirmed by the comparison of the sorption morhphology of the studied ion exchangers (Figures 3a-d). The rejected FT-IR spectra of Amberlite IRA 458 and Amberlite IRA 958 as well as Purolite S-920 and Purolite S-930 are presented in Figure 4a-d. The Influence of Phase Contact Time, pH, Ion Exchanger Dosage, Metal Concentration, and Temperature. Optimum sorption conditions were determined as a function of phase contact time, pH, ion exchanger dosage, temperature and chloride ions concentration. The effect of the phase contact time on the sorption capacity of Amberlite IRA 458, Amberlite IRA 958, and Amberlite IRA 67 as well as Purolite S-920 and Purolite S-930 was investigated in the time range from 1 to 180 min (Figure 5a-e). For all studied ion exchangers the experiments were carried out at pH 11.5. It was observed that the initial sorption of Cu(II), Zn(II), Ni(II), and Cd(II) complexes with HEDP was rapid on strongly basic polyacrylate anion exchangers. The equilibrium was reached in less than 30 min. Thus, the contact time of 120 min was used in the following sections. However, for Amberlite IRA 67 the sorption was very low. To investigate the sorption rate, the kinetic data obtained from batch experiments have been analyzed using the pseudo-firstand pseudo-second-order equations proposed by Largergen and Ho.37,38 The good linearity and the values of correlation coefficients (R2) indicate that HEDP complexes sorption can be approximated with the pseudo-second-order equation. The exemplary values for the Cu(II)-HEDP equilibrium sorption, the values of the rate constants and the correlation coefficients are presented in Table 2. As can be seen from Table 2, the pseudo-second-order model provided better correlation coefficients than the pseudo-first-order model for all the ion exchangers studied, suggesting the pseudo-second-order model was more suitable to describe the sorption kinetics of Amberlite IRA 458, Amberlite IRA 958, Purolite S-920, and Purolite S-930 for M(II)-HEDP complexes. At the same time, the fact of

whereas in the case of Purolite S-930 the decomposition of the M(II)-HEDP complexes in the resin phase and the sorption of metal ions should be assumed according to the reactions: [M(hedp)]2- f M2+ + hedp4-

(17)

As Purolite S-930 possesses nitrogen atoms with a free electron pair, it can act as a Lewis base and form coordination bonds with heavy metal ions (Lewis acids). Comparing the sorption capacities of polyacrylate anion exchangers with the values for the chelating ion exchanger Purolite S-920 (Figures 5a-d), it should be stated that the polyacrylate anion exchangers Amberlite IRA 458 and Amberlite IRA 958 are more effective than Purolite S-920. The aqueous solution pH is an important operational parameter in the sorption process because of its effect on the solubility of the metal ions or complexes, concentration of the counterions on the functional groups of the ion exchanger and the degree of their ionization. The effect of pH on the Cu(II), Zn(II), Ni(II), and Cd(II) complexes with HEDP sorption was investigated in the pH ranges from 5 to 13 at room temperature for 120 min. Figure 6a presents the results obtained for the Cu(II)-HEDP complexes on Amberlite IRA 458, Amberlite IRA 958, Amberlite IRA 67 as well as Purolite S-920 and Purolite S-930, whereas Figure 6b presents the comparison of the results obtained for Cu(II) Zn(II), Ni(II), and Cd(II) sorption on Amberlite IRA 67. It can be observed that the highest sorption of M(II) complexes with HEDP on the polyacrylate anion exchangers (except for Amberlite IRA 67) and the chelating ion exchangers was obtained at pH > 8. With the pH increase from 5 to 13, the Cu(II)-HEDP sorption increased from 3.15 to 5.62 mg/g, while the highest sorption on the weakly basic anion exchanger Amberlite IRA 67 was obtained at pH < 6, when

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Figure 3. (a-d) SEM scans of Amberlite IRA 958 (a, b) and Purolite S-930 (c, d) after the sorption of Cu(II) and Cd(II) complexes with HEDP.

Figure 4. (a-d) FT-IR spectra of Amberlite IRA 458 (a), Amberlite IRA 958 (b), Purolite S-920 (c), and Purolite S-930 (d) before and after the sorption of Cu(II) complexes with HEDP, respectively.

pH increased from 5 to 13, the amount of M(II)-HEDP complexes sorbed decreased even to 0.25 mg/g. In general, sorption of M(II)-HEDP complexes by Amberlite IRA 67 was affected more than that on the strongly basic anion exchangers Amberlite IRA 458 and Amberlite IRA 958. For Amberlite IRA 67 the variations in amount of M(II)-HEDP complexes sorbed with increasing of pH could be explained on the basis of the nature of weakly basic anion exchangers

under different pH conditions. Several authors have investigated the selective sorption of heavy metal ions by weakly basic anion exchangers.45-47 It was observed that this type of anion exchangers reveals a strong preference for heavy metal salts under neutral conditions and for strong acids at low pH values. At neutral pH nitrogen atoms of these groups are not protonated. Therefore, they have a free electron pair and can act as Lewis bases with metal ions (Lewis acids).

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Figure 5. (a-e) The effect of the phase contact time on the sorption of Cu(II), Zn(II), Ni(II), and Cd(II) complexes with HEDP on Amberlite IRA 458 (a), Amberlite IRA 958 (b), Amberlite IRA 67 (c), Purolite S-920 (d), Purolite S-930 (e) (c0 ) 1 × 10-3 M, pH ) 11.5, anion exchanger dose ) 10 g/L, agitation speed ) 180 rpm).

Additionally, equivalent amounts of anions have to occur to maintain electroneutrality in the liquid and resin phases. As a total, the process can be presented as the sorption of heavy metal salts: R-N(CH3)2 + M

2+

-

-

+ 2Cl a R-N(CH3)2(M , 2Cl ) (19) 2+

In neutral or slightly alkaline solutions due to the poor dissociation of the free base form, the sorption of strong acids is almost completely suppressed.40 With decreasing pH, the

sorption of acids increases and the metal salts are desorbed. Consequently, regeneration of anion exchanger can be achieved by applying a strong acid: R-N(CH3)2(M2+,2Cl-) + HCl a R-+NH(CH3)2Cl- + MCl2 (20) Contrary to the sorption of metal salts the uptake of acids develops stoichiometrically. In the acidic form, the anion exchanger cannot be used for adsorption of heavy metal salts. In a second step of the entire regeneration it has, therefore, to

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Table 2. Kinetic Parameters for the Sorption of Cu(II) Complexes with HEDP on Amberlite IRA 458, Amberlite IRA 958, Purolite S-920, and Purolite S-930 type

ion exchanger

qe,exp [mg/g]

q [mg/g]

k1 [1/min]

R2

PS-I

IRA 458 IRA 958 S-920 S-930

5.31 5.35 5.12 5.09

2.88 4.32 5.20 5.05

0.143 0.189 0.041 0.033

0.8203 0.9447 0.9962 0.9971

type

ion exchanger

qe,exp [mg/g]

q [mg/g]

k2 [1/min]

h [mg/g min]

R2

PS-II

IRA 458 IRA 958 S-920 S-930

5.31 5.35 5.12 5.09

5.44 5.39 5.03 5.10

0.057 0.098 0.004 0.033

1.697 2.861 0.094 0.126

0.9997 0.9996 1.0000 0.9996

be neutralized and reconverted to the free base form, for example, by means of sodium hydroxide:47 R-+NH(CH3)2Cl- + NaOH a R-N(CH3)2 + NaCl + H2O (21) Due to the formation of water molecules again, an extreme tendency toward the free base form occurs: R-N(CH3)2 · H2O a R-+NH(CH3)2OH-

(22)

In the presented paper the same tendency was observed. The effect of the dosage of ion exchanger in the range 0.1-1.0 g on the sorption of Cu(II) complexes with HEDP (1 × 10-3 M) was also examined. The results are given in Figure 7. It is evident that the sorption percent rapidly increases for the anion exchange dose from 0.1-0.5 g and then is almost

constant. Therefore the minimum value of the anion exchanger (0.5 g) was chosen for further experiments. In the presented paper the studies of the sorption of Cu(II), Zn(II), Ni(II), and Cd(II) complexes with HEDP on Amberlite IRA 458, Amberlite IRA 958, and Amberlite IRA 67, as well as Purolite S-920 and Purolite S-930 were also carried out at 293, 303, 313, 323, and 333 K. The examplary data for the sorption of Cu(II) complexes with HEDP on Amberlite IRA 458, Amberlite IRA 958, and Amberlite IRA 67 as well as Purolite S-920 and Purolite S-930 as a function of temperature are presented in Figure 8. It was found that the sorption capacity of Cu(II), Zn(II), Ni(II) on Amberlite IRA 458 decreased from 5.63 to 5.46 mg/g, 3.43 to 2.99 mg/g, and 4.27 to 4.13 mg/g with temperature, respectively. But for Cd(II), the adsorption capacity decreased from 9.47 to 9.82 mg/g with increase in temperature. For Amberlite IRA 958 and Amberlite IRA 67, as well as Purolite S-920 and Purolite S-930, the obtained results were similar. On the basis of the obtained results, the thermodynamic parameters for the sorption of the M(II)-HEDP complexes such as Gibbs free energy (∆Go), enthalpy (∆Ho) and entropy (∆So) were determined using the following equations:48 log

qe ∆So ∆Ho + )ce 2.303RT 2.303R ∆Go ) ∆Ho - T∆So

(23)

(24)

The values of ∆Ho and ∆So were obtained from the slope and intercept of the plots of log qe/ce versus 1/T. In Table 3 the calculated values for Cu(II)-HEDP complexes were collected. The values of ∆Go were more negative with the increasing temperature, which indicates that the sorption process is more

Figure 6. (a,b) The effect of pH on the sorption of Cu(II) complexes with HEDP on Amberlite IRA 458, Amberlite IRA 958, Amberlite IRA 67, Purolite S-920, and Purolite S-930 (a) as well as of Cu(II), Zn(II), Ni(II), and Cd(II) complexes with HEDP on Amberlite IRA 67 (b) (c0 ) 1 × 10-3 M, t ) 120 min, anion exchanger dose ) 10 g/L, agitation speed ) 180 rpm).

Figure 7. The effect of the anion exchanger Amberlite IRA 458 dosage on the percentage of Cu(II) complexes with HEDP (c0 ) 1 × 10-3 M, t ) 120 min, pH ) 9.0, ionic strength ) 1.0, agitation speed ) 180 rpm).

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Figure 8. The effect of temperature on the sorption of Cu(II) complexes with HEDP on Amberlite IRA 458, Amberlite IRA 958, Amberlite IRA 67, Purolite S-920, and Purolite S-930 (c0 ) 1 × 10-3 M, t ) 180 min, agitation speed ) 180 rpm). Table 3. Thermodynamic Parameters for the Sorption of Cu(II)-HEDP Complexes on Amberlite IRA 458, Amberlite IRA 958, Amberlite IRA 67, Purolite S-920, and Purolite S-930 ion exchanger Amberlite IRA 458

Amberlite IRA 958

Amberlite IRA 67

Purolite S-920

Purolite S-930

T [K] ∆Go [kJ/mol] ∆Ho [kJ/mol] ∆So [J/mol K] 293 303 313 323 333 293 303 313 323 333 293 303 313 323 333 293 303 313 323 333 293 303 313 323 333

-2.68 -2.97 -3.25 -3.54 -3.83 -2.61 -2.93 -3.23 -3,57 -3.89 -0.50 -0.57 -0.63 -0.70 -0.76 -2.61 -2.91 -3.24 -3.55 -3.88 -2.44 -2.86 -3.27 -3.68 -4.10

5.78

28.87

6.76

32.00

2.67

6.51

9.83

41.84

9.99

41.33

favorable at high temperature, whereas the positive values of ∆So indicate that there is an increase in the degree of the randomness during the adsorption process. The ∆Ho values were found to be 5.78 kJ/mol for Ambelite IRA 458, 6.76 kJ/mol for Amberlite IRA 958, and 2.67 kJ/mol for Amberlite IRA 67, as well as 9.83 kJ/mol for Purolite S-920 and 9.99 kJ/mol for Purolite S-930, which reveals the endothermic character of the sorption process. The highest value of change of the enthalpy of sorption for Purolite S-930 can confirm formation of strong coordinate bonds between the Cu(II) ions and their functional groups. The chelate effect concerns also a large positive change in entropy on formation of a chelate complex, which means that the change of Gibbs free energy with the temperature will be negative so the equilibrium constant will become larger as temperature increases.49 In the paper by Mosayebi et al.50 it was found that HEDP added to an aqueous corrosive media loses the properties of

Figure 9. The effect of NaCl concentration on the sorption of Cu(II) complexes with HEDP on Amberlite IRA 458 (c0 ) 1 × 10-3 M, t ) 180 min, agitation speed ) 180 rpm).

corrosion inhibitor in the presence of more than 1200 ppm of chloride ions and temperatures greater than or equal to 315 K, and it cannot be used to control the corrosion in a cooling water system. Therefore the influence of chloride concentration on the sorption of Cu(II) complexes with HEDP on Amberlite IRA 458 was also examined. The increase of chloride ions concentration from 0.01 to 0.5 M of NaCl significantly reduces the amount of sorbed complexes only for the following 0.2, 0.3, and 0.5 M concentrations of NaCl (accordingly about 79%, 98%, and 99%) (Figure 9). The results indicate that sorption of the Cu(II) complexes with HEDP proceeds according to the anion exchange mechanism. The analogous results were found in the case of As(V) or Cr(VI) sorption (the present Cl- ions competes against As(V) or Cr(VI) for the sorption sites).51 The Adsorption Studies. Adsorption isotherm data describe how adsorbates interact with adsorbents and are critical in optimizing the use of adsorbents. Thus, the correlation of equilibrium data by either theoretical or empirical equations is essential for the practical design and operation of the adsorption system. The adsorption isotherms for Cu(II), Zn(II), Ni(II), and Cd(II) complexes with HEDP were determined with different concentrations of solution from 1.0 × 10-3 to 1.5 × 10-2 M under the sorption condition already selected. The experimental data were fitted to the Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R) isotherms and the isotherm constants in eqs 6-9 and the correlation coefficients (R2) are given in Tables 4-7. The Langmuir model is based on the assumption that adsorption can only occur at a fixed number of definite localized sites. The constants q0 and KL are the charactersistics of the Langmuir equation and can be determined from its linearized form by plotting ce/qe versus ce. The Freundlich constant 1/n and KF were evaluated by plotting log qe against log ce using eq 7. The linear regression analysis gives the slope (1/n) and intercept (KF). The constants KF and 1/n indicate the relative sorption capacity and intensity, respectively. However, the Temkin isotherm takes into account the adsorbing metal-ion exchanger interactions. The isotherm constants bT and KT can be determined by ploting qe versus ln ce. According to the Dubinin-Radushkevich model sorption is related to the porous structure of the ion exchanger. The slope of the plot ln qe versus ε gives β and the ordinate intercept yields the sorption capacity Xm. The

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Table 4. The Isotherm Constants for the Sorption of Cu(II) Complexes with HEDP on Amberlite IRA 458, Amberlite IRA 958, Purolite S-920, and Purolite S-930

Langmuir isotherm

Freundlich isotherm

Temkin isotherm

Dubinin-Radushkevich isotherm

ion exchanger

qe,exp [meq/g]

q0 [meq/g]

KL [L/meq]

R2

IRA 458 IRA 958 S-920 S-930

2.16 2.23 1.66 1.64

1.96 2.18 1.87 1.84

1.20 4.44 0.38 0.54

0.9647 0.9954 0.9750 0.9891

ion exchanger

qe,exp [meq/g]

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

n[-]

R2

IRA 458 IRA 958 S-920 S-930

2.16 2.23 1.66 1.64

1.17 1.01 0.82 0.93

8.48 5.28 2.49 2.47

0.8716 0.7075 0.9944 0.9791

ion exchanger

qe,exp [meq/g]

KT [L/g]

bT [J/mol]

R2

IRA 458 IRA 958 S-920 S-930

2.16 2.23 1.66 1.64

144.81 134.44 2.08 2.04

524.71 493.46 298.38 283.00

0.9082 0.9086 0.9017 0.9333

ion exchanger

qe,exp [meq/g]

Xm [meq/g]

β [mol2/J2]

R2

IRA 458 IRA 958 S-920 S-930

2.16 2.23 1.66 1.64

2.26 2.22 1.45 1.74

2.00 × 10-9 2.01 × 10-9 4.02 × 10-9 4.00 × 10-9

0.9646 0.7314 0.9877 0.9898

Table 5. Estimated Isotherm Constants for the Sorption of Zn(II) Complexes with HEDP on Amberlite IRA 458, Amberlite IRA 958, Purolite S-920, and Purolite S-930

Langmuir isotherm

Freundlich isotherm

Temkin isotherm

Dubinin-Radushkevich isotherm

ion exchanger

qe,exp [meq/g]

q0 [meq/g]

KL [L/meq]

R2

IRA 458 IRA 958 S-920 S-930

3.72 3.86 3.80 3.85

3.92 3.73 3.92 3.68

3.03 0.03 3.23 0.49

0.9311 0.9074 0.8726 0.8744

ion exchanger

qe,exp [meq/g]

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

n[-]

R2

IRA 458 IRA 958 S-920 S-930

3.72 3.86 3.80 3.85

2.28 2.17 2.32 2.01

3.25 5.44 3.19 2.53

0.8057 0.9267 0.7978 0.9141

ion exchanger

qe,exp [meq/g]

KT [L/g]

bT [J/mol]

R2

IRA 458 IRA 958 S-920 S-930

3.72 3.86 3.80 3.85

2.87 1.97 1.34 1.59

71.36 78.24 62.79 77.81

0.9904 0.9441 0.9536 0.8411

ion exchanger

qe,exp [meq/g]

Xm [meq/g]

β [mol2/J2]

R2

IRA 458

3.72

2.26

1.00 × 10-4

0.8058

IRA 958 S-920 S-930

3.86 3.80 3.85

1.91 2.20 2.98

2.00 × 10-9 2.00 × 10-9 3.00 × 10-9

0.7558 0.7549 0.9079

adsorption capacity, q0, which is a measure of the maximum sorption capacity corresponding to the complete monolayer coverage showed values with good agreement with the experimental capacities qe.exp. For Amberlite 458 these values are as follows: 1.96 meq/g for Cu(II), 3.92 meq/g for Zn(II), 2.99 meq/g for Ni(II) and 4.24 meq/g for Cd(II) complexes with HEDP, respectively. The adsorption coefficient, KL, related to the apparent energy of adsorption on the ion exchanger, was greater in the case of Cd(II) and Ni(II) complexes than for Zn(II) and Cu(II); values equal 14.22 L/meq, 7.10 L/meq, 3.03 L/meq, and 1.20 L/meq, respectively. Furthermore, the values of Temkin adsorption potential constant for the anion exchanger Amberlite IRA 458 (KT) are 144.81 L/g for Cu(II), 2.87 L/g for Zn(II), 11.88 L/g for Ni(II) and 6.45 L/g for sorption of Cd(II) complexes with HEDP, respectively. The values of Temkin constant (bT) that related to the heat of sorption are estimated to be 524.7, 71.4,

214.1, and 64.2 J/mol, respectively. These low values indicate a weak interaction between the above-mentioned complexes and the surface of ion exchangers supporting an ion-exchange mechanism of the process. The comparison of the sorption capacities of studied ion exchangers calculated from the modeled isotherm data reveals that only in the case of the Langmuir model are there no significant differences between experimental and modeled values of the sorption capacities. This indicates that different isotherm models are not appropriate in describing the use of the polyacrylate anion exchangers Amberlite IRA 458 and Amberlite IRA 958 as well as and the chelating ion exchangers Purolite S-920 and Purolite S-930 for the simultaneous removal of Cu(II), Zn(II), Ni(II), and Cd(II) and HEDP in the form of binuclear complexes from wastewaters. This statement was also confirmed on the basis of the correlation coefficient of determination (R2),

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Table 6. Estimated Isotherm Constants for the Sorption of Ni(II) Complexes with HEDP on Amberlite IRA 458, Amberlite IRA 958 and Purolite S-920 and Purolite S-930

Langmuir isotherm

Freundlich isotherm

Temkin isotherm

Dubinin-Radushkevich isotherm

ion exchanger

qe,exp [meq/g]

q0 [meq/g]

KL [L/meq]

R2

IRA 458 IRA 958 S-920 S-930

2.96 3.43 1.56 2.05

2.99 3.47 1.51 2.04

7.10 1.28 10.56 1.14

0.9006 0.9993 0.9974 0.9705

ion exchanger

qe,exp [meq/g]

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

n[-]

R2

IRA 458 IRA 958 S-920 S-930

2.96 3.43 1.56 2.05

1.62 1.96 1.32 1.33

3.92 3.77 9.24 6.76

0.8452 0.8317 0.7812 0.8341

ion exchanger

qe,exp [meq/g]

KT [L/g]

bT [J/mol]

R2

IRA 458 IRA 958 S-920 S-930

2.96 3.43 1.56 2.05

11.88 17.38 21.17 19.45

214.08 183.46 191.38 134.07

0.8733 0.9497 0.9725 0.9266

ion exchanger

qe,exp [meq/g]

Xm [meq/g]

β [mol2/J2]

R2

IRA 458

2.96

3.16

7.00 × 10-6

0.9333

IRA 958 S-920 S-930

3.43 1.56 2.05

3.47 1.55 2.48

2.05 × 10-9 1.06 × 10-9 1.10 × 10-9

0.8777 0.7867 0.8187

Table 7. Estimated Isotherm Constants for the Sorption of Cd(II) Complexes with HEDP on Amberlite IRA 458, Amberlite IRA 958, Purolite S-920, and Purolite S-930

Langmuir isotherm

Freundlich isotherm

Temkin isotherm

Dubinin-Radushkevich isotherm

ion exchanger

qe,exp [meq/g]

q0 [meq/g]

KL [L/meq]

R2

IRA 458 IRA 958 S-920 S-930

3.92 4.74 3.87 4.92

4.24 4.94 4.04 4.44

14.22 10.56 8.65 6.63

0.9829 0.9964 0.9946 0.9687

ion exchanger

qe,exp [meq/g]

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

n[-]

R2

IRA 458 IRA 958 S-920 S-930

3.92 4.74 3.87 4.92

5.84 5.32 3.69 3.38

2.05 2.45 2.08 1.97

0.7306 0.7557 0.8586 0.9250

ion exchanger

qe,exp [meq/g]

KT [L/g]

bT [J/mol]

R2

IRA 458 IRA 958 S-920 S-930

3.92 4.74 3.87 4.92

6.45 7.56 4.07 3.40

64.15 65.15 69.95 59.17

0.9413 0.9766 0.9759 0.9269

ion exchanger

qe,exp [meq/g]

Xm [meq/g]

β [mol2/J2]

R2

IRA 458 IRA 958 S-920 S-930

3.92 4.74 3.87 4.92

3.26 2.66 3.57 4.61

2.02 × 10-4 4.00 × 10-9 4.03 × 10-9 4.01 × 10-9

0.7306 0.7345 0.8842 0.9456

where also the Langmuir isotherm model mostly provides a better fit of our experimental data with the other isotherm models. The Selectivity Coefficients Studies. Knowing the appropriate initial concentrations of the M(II)-HEDP complexes and the concentrations obtained in the resin phase, the determination of the selectivity coefficients based on eq 10 was possible. The variations of the selectivity coefficients for Cu(II) and Zn(II) complexes with HEDP in the binary system on Amberlite IRA 485, Amberlite IRA 958, Amberlite IRA 67, Purolite S-920, and Purolite S-930 are presented in Figure 10. In all experiments, the ratio between the concentrations in solution of two exchangeable (X) complexes was kept close to the unity. The obtained results show that the selectivities for the polyacrylate anion exchangers Amberlite IRA 458, Amberlite IRA 958, and Amberlite IRA 67 for these complexes are lower (with the selectivity coefficients around

1 for Amberlite IRA 67 and Amberlite IRA 458) than for the chelating ion exchangers Purolite S-920 and Purolite S-930. However, the evaluation of the results is complicated by the fact that the heavy metal-HEDP solution is very complex with different compositions of complexes at different pH as well as M(II) and HEDP concentrations including possibility of the sorbed complexes decomposition in the resin phase especially for chelating ion exchangers. Desorption. For the desorption of sorbed M(II) complexes with HEDP from Amberlite IRA 458, Amberlite IRA 958, and Purolite S-920, as well as M(II) ions from Purolite S-930, 1 M HCl was used. Approximately from 92 to 99% of the sorbed species were desorbed from the above-mentioned ion exchangers. It was observed that the sorption for cycles 2-10 was slightly lower than that for cycle 1. After 10 cycles, the total decrease in sorption efficiency for Amberlite IRA 458 was 9.6%, for Amberlite IRA 958 was 7.5%, for Purolite

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HEDP to Purolite S-920. In the case of Purolite S-930 the sorption of M(II) ions was observed. For the polyacrylate anion exchangers Amberlite IRA 458 and Amberlite IRA 958, the affinity series are as follows: Cd(II) > Zn(II) > Ni(II) > Cu(II), whereas for the Purolite S-920 and Purolite S-930 they were found to be Cd(II) > Zn(II) > Cu(II) > Ni(II) and Cd(II) > Zn(II) > Ni(II) > Cu(II), respectively. All ion exchangers can be regenerated very efficiently using 1 M HCl and can be used in multiple cycles of operation without considerable loss in capacity.

Figure 10. Variations of the selectivity coefficients for Amberlite IRA 485, Amberlite IRA 958, Amberlite IRA 67, Purolite S-920, and Purolite S-930 (c0 ) 1 × 10-3 M, M(II)-HEDP ) 1:1, t ) 180 min, agitation speed ) 180 rpm).

Figure 11. The sorption-desorption effeciency of Cu(II) in the presence of HEDP on Amberlite IRA 458 (sorption-desorption cycles no. 1-10, desorption agent ) 1 M HCl, c0 ) 1 × 10-3 M, t ) 180 min, agitation speed ) 180 rpm, the ion exchanger-desorption agent ratio ) 1:100).

S-920 was 6.3%, and for Purolite S-930 was 11.7%, which shows that they have good potential to sorb M(II) ions in the presence of HEDP. The examplary data for the Cu(II) sorption-desorption process in the presence of HEDP on Amberlite IRA 458 are presented in Figure 11. Conclusions The major findings are summarized as follows: M(II)-HEDP complexes are effectively sorbed (>99%) at the time of 20 min at optimum pH 11.5 for the strongly basic anion exchangers Amberlite IRA 458 and Amberlite IRA 958 as well as the chelating ion exchanger Purolite S-920 and Purolite S-930, except for the weakly basic anion exchanger Amberlite IRA 67 where the optimal pH was established to be 7.0. The M(II)-HEDP complexes sorption kinetics is accoriding to the pseudo-second-order equation and its efficiency was reduced by the presence of interfering chloride ion. The equilibrium adsorption data were fitted to the four isotherm modelssthe Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich ones. It was found that the Langmuir model correlates the experimental data most effecitvely. Comparing the polyacrylate anion exchangers to the selective chelating ion exchangers, it was found that Amberlite IRA 458 and Amberlite 958 offer almost equal experimental capacity for the sorption of Cu(II), Zn(II), Ni(II), and Cd(II) complexes with

Nomenclature j ) (A) ) concentrations of the A(II)-HEDP complex or A(II) (A ion in the resin phase or in the solution respectively j ) (B) ) concentrations of the B(II)-HEDP complex or (B B(II) ion in the resin phase or in the solution respectively β ) constant related to the sorption energy (mol2/J2) bT ) Temkin constant related to the heat of sorption (J/mol) c0 ) initial concentration of a solution of M(II) (mg/L) ce ) concentration of M(II) in the aqueous phase at equilibrium (mg/L) or (meq/L) ct ) concentration of M(II) in the aqueous phase at time t (mg/L) Cw ) working ion exchange capacity (meq/mL) Cr ) total ion exchange capacity (meq/mL) Dg ) mass distribution coefficient Dv ) volume distribution coefficient ε ) Polanyi potential given by ε ) RT ln (1 +1/ce) ∆G° ) Gibbs free energy (kJ/mol) ∆H° ) enthalpy (kJ/mol) h ) initial sorption rate (mg/g min) KBA ) selectivity coefficient KL ) Langmuir constant related to the free energy of sorption (L/meq) KF ) Freundlich adsorption capacity (meq/g(meq/L)1/n) 1/n ) Freundlich constant related to the surface heterogenity KT ) Temkin isotherm constant (L/g) k1, k2 ) equilibrium rate constant (1/min) m ) mass of the ion exchanger (g) qe ) amount of M(II) sorbed at equilibrium (mg/g) or (meq/g) q0 ) Langmuir monolayer sorption capacity [meq/g) q ) amount of metal complexes sorbed at time t (mg/g) R ) gas constant (J/mol K) ∆S° ) entropy (J/mol K) T ) temperature (K) j ) volume of effluent at c ) c0/2 (determined graphically) V (mL) V ) volume of the solution (L) V0 ) dead volume in the column (liquid volume in the column between the bottom edge of anion exchanger bed and the outlet) (mL) Ve ) effluent volume to the break point (L) Vi ) void (interparticle) anion exchanger bed volume which amounts to ca. 0.4 of the bed volume (mL) Vj ) bed volume (mL) Xm ) D-R sorption capacity (meq/g) Literature Cited (1) Nowack, B. Aminopolyphosphonate removal during wastewater treatment. Water Res. 2002, 36, 4636–4642. (2) Nowack, B. Environmental chemistry of phosphonates. Water Res. 2003, 37, 2533–2546.

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ReceiVed for reView September 14, 2009 ReVised manuscript receiVed December 29, 2009 Accepted December 30, 2009 IE9014414