Aminoalkylated Merrifield Resins Reticulated by Tris-(2-chloroethyl) Phosphate for Cadmium, Copper, and Iron (II) Extraction
We aimed to synthesize novel substituted polymers bearing functional groups to chelate heavy metals during depollution applications. Three polyamine functionalized Merrifield resins were prepared via ethylenediamine (EDA), diethylenetriamine (DETA), and triethylenetetramine (TETA) modifications named, respectively, MR-EDA, MR-DETA, and MR-TETA. The aminoalkylated polymers were subsequently reticulated by tris-(2-chloroethyl) phosphate (TCEP) to obtain new polymeric resins called, respectively, MR-EDA-TCEP, MR-DETA-TCEP, and MR-TETA-TCEP. The obtained resins were characterized via attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), elemental analysis (EA), and thermogravimetric (TGA), thermodynamic (DTA), and differential thermogravimetric (DTG) analysis. The synthesized resins were then assayed to evaluate their efficiency to extract metallic ions such as Cd2+, Cu2+, and Fe2+ from aqueous solutions.
Recently, the solid phase extraction (SPE) has been shown to be an excellent separation technique due to immiscibility of resins with aqueous phase, low rate of physical degradation, minimum release of toxic organic solvents, and recycling options [1, 2]. In SPE, the organic extractants are either sorbed or anchored to an inert polymeric support [3, 4]. In this context, grafted resins have been used in separation of hazardous metallic ions in industrial effluents [5, 6]. In fact, many polymers were reported to contain functional chelating groups; (i) morin (2′,3,4′,5,7-pentahydroxyflavone) which was covalently attached to Merrifield’s resin and used in metal sorption and recognition , (ii) chelating ion exchange which was prepared by functionalizing Merrifield resin with 2,2′-pyridylimidazole and used to selectively adsorb and separate nickel from other base metal ions in synthetic sulfate solutions , and (iii) polymer-supported triazoles were used to extract metals such as Cd, Fe, Mg, Ni, and Co from aqueous solutions . Indeed, it has been reported that adsorption properties and selectivity for metallic ions depend on the molecular structure of adsorption materials, particularly, polyamine functional groups, and nitrogen atoms [10–13]. Besides, other resins modified by the organophosphorus reagents such as polyethylene imine methylene phosphonic acid have also been performed to remove copper ions from aqueous media . In fact, there are many research articles describing the synthesis procedure of chloromethylated polystyrene or Merrifield resin attached with polyamine groups and employed them as adsorbent for heavy metals removal from aqueous solutions [15–18].
In the present paper we attempt to investigate modified Merrifield resins functionalized by polyamines and phosphorus derivatives to adsorb metallic ions in an aqueous phase.
Merrifield resin (MR reticulated by 2% divinylbenzene, 200–400 mesh, 2.1 mmol Clg−1, Fluka), ethylenediamine (EDA), diethylenetriamine (DETA), triethylenetetramine (TETA), and Tris-(2-chloroethyl) phosphate (TCEP) were purchased from Sigma-Aldrich. Tetrahydrofuran (THF) and dimethylformamide (DMF) were purchased from Sigma-Aldrich. Absolute ethanol, triethylamine (TEA), and diethyl ether were purchased from Prolabo. Iodide potassium (KI), CdCl2H2O, FeCl24H2O, and CuCl26H2O were purchased from Fluka.
2.2. Instrumental Analyses
Infrared analysis was carried out by using the attenuated total reflectance technique (ATR-FTIR), with a Nicolet FTIR 200 spectrophotometer (Thermo Scientific). Elemental analysis of C, H, and N was performed by Perkin Elmer analyzer CHN Series II 2400. Elemental analysis of P was performed by ICP-OES analysis on a HORIBA Scientific. DTA, TGA, and DTG analyses were carried out with a Mettler Toledo TGA/DSC 1 Star System (power 400 Watts). The amount of remaining metal ions in solution was evaluated by flame atomic absorption spectroscopy (FAAS) analysis on a Perkin-Elmer PinAAcle 900 T. The detection limits for Cd2+, Cu2+, and Fe2+ are 0.8, 1.5, and 5 mgL−1, respectively, and the corresponding wavelengths for the studied metals are 228.8, 324.75, and 248.33 nm, respectively.
2.3. Extraction Procedure of Metal Cations
The synthesized polymers (0.1 g) were incubated with 20 mL of aqueous solution of metal ion at 25°C for 24 h. Aqueous monometallic solutions of CdCl2H2O, FeCl24H2O, and CuCl26H2O were prepared at a concentration of 2.10−4 molL−1 in relation with each metal ion in distilled water (pH = 6-7). The suspension was filtrated on filter paper (previously washed several times with distilled water until disappearance of conductivity in washing water). The amount of remaining metal ions was evaluated by FAAS analysis of the filtrate. The extraction percentage (%) was calculated using the following equation:where and are the concentrations of the metal ion in initial and final solutions, respectively.
2.4. Synthesis of Merrifield Resin with Ethylenediamine Group (MR-EDA)
MR-EDA was prepared according to the literature  with some modification. The synthesis procedure was performed according to Figure 1. First, commercially available Merrifield resin was suspended in DMF for 10 h followed by the addition of EDA. The amine is used in excess to act as the base and increase the rate of reaction. Then, the reaction mixture was stirred at room temperature for 1.5 h and then for 24 h at 70–80°C. After completing the reaction, the resin was washed several times with deionized water and then with absolute ethanol. After, the obtained resin was dried under vacuum at 50°C for 48 h.
2.5. Synthesis of Merrifield Resin with Diethylenetriamine Group (MR-DETA)
The reaction involved MR, DMF, and excess of DETA. The reaction conditions of the product were similar to that of MR-EDA (see Figure 1).
2.6. Synthesis of Merrifield Resin with Triethylenetetramine Group (MR-TETA)
The reaction involved MR, DMF, and excess TETA. The reaction conditions of the product were similar to that of MR-EDA (see Figure 1).
2.7. Synthesis of MR-EDA Reticulated by Tris-(2-chloroethyl) Phosphate (MR-EDA-TCEP)
The synthesis procedure was performed according to Figure 2. MR-EDA was suspended in THF followed by the addition of certain amount of TCEP, a small amount of KI used as catalysts. Then, a certain amount of TEA used as base was added to the flask. The reaction mixture was heated at 70°C for 72 h. The cooled mixture was filtered and washed with distilled water and with diethyl ether. After, the obtained resin was dried in a vacuum at 50°C for 48 h.
2.8. Synthesis of MR-DETA Reticulated by Tris-(2-chloroethyl) Phosphate (MR-DETA-TCEP)
The reaction medium contained MR-DETA, THF, KI, TCEP, and TEA. The reaction conditions of the product were similar to that of MR-EDA-TCEP (see Figure 2).
2.9. Synthesis of MR-TETA Reticulated by Tris-(2-chloroethyl) Phosphate (MR-TETA-TCEP)
The reaction medium contained MR-TETA, THF, KI, TCEP, and TEA. The reaction conditions of the product were similar to that of MR-EDA-TCEP (see Figure 2).
3. Results and Discussion
3.1.1. ATR-FTIR Analysis
The structures of synthetic chelating resins MR-EDA, MR-DETA, and MR-TETA can be confirmed by comparing the ATR-FTIR spectra of Merrifield resin before and after reaction with polyamines as shown in Figure 3. The spectra showed the disappearance of the CH2-Cl band which appears at 1265 cm−1 from the Merrifield resin  and the appearance of new bands at 3320 cm−1 and a broad peak at 1670 cm−1, which could be attributed to the stretching vibration and deformation vibration of N-H, respectively, accounting for NH/NH2 introduced groups. These findings agree with the literature . The proposed synthesis route to the prepared resins is presented schematically in Figure 1.
On the other hand the ATR-FTIR spectra of the resins MR-EDA-TECP, MR-DETA-TECP, and MR-TETA-TCEP (Figure 4) showed the appearance of new bands of symmetrical and asymmetrical stretching vibration characteristics of P-O-C observed at, respectively, 1030 and 1076 cm−1. Indeed, new peak appeared at 1221 cm−1 and could be attributed to the stretching vibration of P=O. These findings confirmed the functionalization of the resins by the tris-(2-chloroethyl) phosphate group. The proposed synthesis route to the newly prepared resins is presented schematically in Figure 2.
3.1.2. Elemental Analysis
Table 1 presented the N percentage in chelating resins determined by elemental analyses. Obtained results showed that the content of nitrogen in MR-EDA, MR-DETA, and MR-TETA were 4.24, 5.03, and 6.80%, respectively. These findings suggest that the resin structure, particularly polyamine chain length and nitrogen atoms composition, influences the N content. Besides, elemental analysis showed also that the P content in MR-EDA-TCEP, MR-DETA-TCEP, and MR-TETA-TCEP resins was 1.47, 1.44, and 1.52%, respectively, which confirm the grafting of TCEP groups on the aminoalkylated resins. In fact, the polyamine groups (EDA, DETA, and TETA) loading in MR-EDA, MR-DETA, and MR-TETA are 9.07, 12.24, and 17.69% by weight, respectively. In addition, TCEP group loading in MR-EDA-TCEP, MR-DETA-TCEP, and MR-TETA-TCEP is 27.64, 27.08, and 28.58% by weight, respectively.
3.1.3. TGA, DTA, and DTG Analyses
The characteristic TGA, DTA, and DTG curves of MR, MR-EDA, MR-DETA, MR-TETA, MR-EDA-TECP, MR-DETA-TECP, and MR-TETA-TECP were presented in Figures 5, 6, and 7, showing the complexity of the thermal decomposition for chemically modified resins. The thermal stability of MR resin was divided into three regions: 190–325°C, 325–425°C, and above 600°C. In fact, the exothermic peak which appears nearly to 320°C was detected, in TGA, by a weight loss up to 28.8% in the temperature range 190–325°C that can explain the elimination of chloromethylated group CH2Cl . In addition, the exothermic phenomena that corresponded to a weight loss up to 23% in the temperature range between 325°C and 425°C may suggest the thermal degradation of the cross-linked structure of MR resin and the polymer seems to be decomposed completely at 600°C . The DTA curves (Figure 6) of MR resin showed three exothermic peaks observed at 305, 330, and 509°C. This data may suggest that the decomposition of the resin took place in steps. The DTA curves of MR-EDA, MR-DETA, MR-TETA, MR-EDA-TCEP, MR-DETA-TCEP, and MR-TETA-TCEP showed that the glass transition temperature () was, respectively, 184, 163, 204, 196, 215, and 200°C. Besides, all resins present two exothermic peaks that could indicate their decomposition. The substitution at the benzene ring of polystyrene determined three degradation stages with different weight losses depending on the chemical structure of the substituent (Table 2). Hence, we have considered the weight losses (), up to 250°C as insignificant, representing the solvent and water retained in the sample. From Figure 7 and Table 2 one can see that the thermal stability of the studied compounds decreases in the following order: MR-EDA > MR-TETA > MR-DETA > MR-EDA-TCEP > MR-TETA-TCEP > MR-DETA-TCEP = MR.
3.2. Metal Ions Extraction by the Synthesized Polymers
Table 3 showed that the order of adsorption capacity of the chelating resins against Cd2+, Cu2+, and Fe2+ ions was, respectively, MR-TETA > MR-DETA > MR-EDA, which agree with the other investigation . This increase in adsorption percentage could be explained by the higher content of nitrogen atoms in functional groups and thus imposed more coordination with metallic ions.
The orders of adsorption capacity for Cd2+ are as follows: MR-DETA-TCEP > MR-EDA-TCEP > MR-TETA-TCEP, MR-EDA-TCEP > MR-EDA, MR-DETA-TCEP > MR-DETA, and MR-TETA > MR-TETA-TCEP. Besides, the orders of adsorption capacity for Cu2+ are as follows: MR-TETA-TCEP > MR-DETA-TCEP > MR-EDA-TCEP and MR-EDA-TCEP > MR-EDA and MR-DETA-TCEP > MR-DETA and MR-TETA-TECP > MR-TETA. In addition, the orders of adsorption capacity for Fe2+ are as follows: MR-EDA-TCEP > MR-DETA-TCEP > MR-TETA-TCEP and MR-EDA-TCEP > MR-EDA and MR-DETA > MR-DETA-TCEP and MR-TETA > MR-TETA-TCEP. Hence, it appears that MR-TETA-TECP and MR-TETA resins exhibited good adsorption selectivity for Fe(II) and Cu(II), respectively.
This high adsorption selectivity could be explained by the high affinity of Fe(II) and Cu(II) ions to the polyamine and organophosphate groups in studied resins. On the basis of hard-soft acid-base (HSAB) theory, Fe(II) and Cu(II) were classified as intermediate ions. Therefore, they have affinities to two types of soft ligands which contain nitrogen atoms of polyamine groups and hard ones which contain oxygen atoms of organophosphate groups. The deference of adsorption capacity of metallic ions such as Cd2+, Cu2+, and Fe2+ by the three synthesized polymers MR-EDA-TCEP, MR-DETA-TCEP, and MR-TETA-TCEP can be explained essentially by the compatibility factor between the cations size and the ligands cavity size in polymer matrix, which was probably influenced by the cross-linking degree. The length of amine chain and the cross-linking by the TCEP is directly proportional. However, the active sites are less readily available and efficiency of the resin is reduced. In fact, for Fe2+ which has a less size than Cu2+, the adsorption percentage increases in the sense: MR-EDA-TCEP, MR-DETA-TCEP, and MR-TETA-TCEP. But, for Cu2+, which has a higher size than Fe2+, the same phenomena decreases in the same sense. In the case of Cd2+, which has a higher size than the two cations Cu2+ and Fe2+, the adsorption percentage is better in MR-DETA-TCEP than in the two resins MR-EDA-TECP and MR-TETA-TCEP. In fact, in MR-DETA-TCEP, the Cd2+ was simply incorporated in the complex sites compared to the two other resins. It can be explained by the high compatibility between the Cd2+ cations size and the ligands cavity size in MR-DETA-TCEP polymer matrix.
In the present paper, three chelating resins were synthesized via chemical grafting of polyamine groups on Merrifield resin, leading to resin polymers named MR-EDA, MR-DETA, and MR-TETA. In fact, these chelating resins are reticulated by tris-(2-chloroethyl) phosphate (TCEP) to access to new polymers, called MR-EDA-TECP, MR-DETA-TECP, and MR-TETA-TECP, respectively. These synthesized polymers were characterized by ATR-FTIR, EA, TGA, DTA, and DTG analysis and were tested for their ability to extract metallic ions from aqueous solutions, particularly for Cu2+, Fe2+, and Cd2+. The obtained results show that MR-DETA-TECP exhibited good efficiency towards Cd2+ (73%) while MR-TETA-TCEP and MR-TETA were efficient against Cu2+ (95%) and Fe2+ (96%), respectively. However, further approaches are still required in order to highlight and understand the involvement of polymer modifications and their mechanisms for metallic extractions, as well as their application in environmental conditions.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to thank the Tunisian Ministry (Research Unit: 05/UR/12-05) which financed this project.
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