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, andMR-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 Cd, Cu, and Fe from aqueous solutions.


Introduction
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 [7], (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 [8], and (iii) polymer-supported triazoles were used to extract metals such as Cd, Fe, Mg, Ni, and Co from aqueous solutions [9].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][11][12][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 [14].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][16][17][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.

Experimental
where   and   are the concentrations of the metal ion in initial and final solutions, respectively.

Synthesis of Merrifield Resin with Ethylenediamine Group (MR-EDA)
. MR-EDA was prepared according to the literature [16] 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.

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).

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).

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.
International Journal of Polymer Science

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  CH 2 -Cl band which appears at 1265 cm −1 from the Merrifield resin [15] 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/NH 2 introduced groups.These findings agree with the literature [16].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.  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.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 CH 2 Cl [19].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 [20].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.2).Hence, we have considered the weight losses ( < 10%), 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.

Metal Ions Extraction by the Synthesized Polymers.
Table 3 showed that the order of adsorption capacity of the chelating resins against Cd 2+ , Cu 2+ , and Fe 2+ ions was, respectively, MR-TETA > MR-DETA > MR-EDA, which agree with the other investigation [16].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 Cd

Sample
Weight loss (, %) and temperature range (Δ, ∘ C) Step I Step  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 Cd 2+ , Cu 2+ , and Fe 2+ 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 Fe 2+ which has a less size than Cu 2+ , the adsorption percentage increases in the sense: MR-EDA-TCEP, MR-DETA-TCEP, and MR-TETA-TCEP.But, for Cu 2+ , which has a higher size than Fe 2+ , the same phenomena decreases in the same sense.In the case of Cd 2+ , which has a higher size than the two cations Cu 2+ and Fe 2+ , 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 Cd 2+ was simply incorporated in the complex sites compared to the two other resins.It can be explained by the high compatibility between the Cd 2+ cations size and the ligands cavity size in MR-DETA-TCEP polymer matrix.
3.1.2.Elemental Analysis.Table 1 presented the N percentage in chelating resins determined by elemental analyses.

Table 2 :
TGA data for Merrifield resin and its substituted derivatives.

Table 3 :
Extraction percentage results of metal ions by the synthesized polymers.
Note: each result is the mean from three determinations ± standard deviation.