Fast, Efficient and Environmentally Friendly Extraction of Cu(II) and Zn(II) by Hybrid Silicas Impregnated with Acidic Organic Extractants

Laboratoire de Chimie des Matériaux, (BP 1524 El M’naouer Oran-Algerie ) Oran, Algeria Laboratoire de Reconnaissance et Procédés de Séparation Moléculaire IPHC (UMR 7178 du CNRS), University of Strasbourg, 25 rue Becquerel, F-67087 Strasbourg Cedex 2, France Lebanese International University, Beirut, P.O. Box 146404 Mazraa, Lebanon *Corresponding author: Dr. Hafida MILOUDI, Tel: +213 6 61 72 66 45 or +33 3 68 85 27 01; Fax: +213 41 51 44 28 Email: mmiloudi@yahoo.com, miloudi.hafida@univ-oran.dz ABSTRACT


INTRODUCTION
Several Industries such as mining, metallurgical and chemical factories massively contribute to the ongoing contamination of the environment. Furthermore, the mismanagement of sewage sludge contaminates soil with heavy metals such as cobalt, copper, zinc, lead, cadmium, and mercury. Naturally acidic rainwater can subsequently lixiviate these metals resulting in their disposal in the sea. Therefore, the necessity to remove these metals is two-fold: 1. To stop this source of pollution 2. To recover these precious metals in order to satisfy the global demand.
Consequently, the development of efficient and environmentally friendly systems for the extraction of metal cations, in synthetic solutions or industrial effluents, became a necessity. These performing systems are based on the synthesis of novel selective ligands on one hand, and on the other hand the elaboration of new porous organo-mineral hybrid materials, capable of trapping the metal cations. In this scope, one of the most used methods is the solid-liquid extraction , chosen in this work. Using this technique, various porous inorganic and organic (clays, zeolites, resins) solids have been widely used in the recovery of metal cations. However, the described extraction capacity in the literature is very limited [1][2][3][4][5][6][7]. In order to increase this latter, the materials used can be functionalized with organic extractants [1,2,[8][9][10][11][12][13][14].
In this work, we have synthesized mesostructured silicas according to the protocol described by Firouzi et al. [15], and subsequently modified by Boos et al. [16]. These silicas were then calcined and impregnated with three acidic organic chelators, an acylisoxazolone 3-phenyl-4-benzoyl-5-isoxazolone (HPBI), an acylpyrazolone 1-phenyl-3-methyl-4-stearoyl-5-pyrazolone (HPMSP) and an organophosphoric acid di-(2-ethylhexyl)-phosphoric acid (DEHPA) [13]. The prepared silicas were used in the solid-liquid extraction of Cu (II) and Zn (II) in a sulfate medium. The various factors affecting the extraction were studied. The extraction equilibrium for the two cations was reached within 15 minutes, with very high extraction capacity and separations were carried out successfully.  The synthesis, impregnation and characterization of silicas used in this work were detailed in a previous work [13].

Metal extraction procedure
The extraction of the metal M(II) was carried out in polypropylene tubes thermoregulated at 25.0 + 0.2 °C. In a typical experiment, 0.1 g of solid was mechanically stirred in 10 ml of aqueous phase for 1 h, a sufficient time to reach the equilibrium. Then, the two phases were separated with a high-speed centrifuge (10000 rpm for 10 min), the equilibrium pH of the liquid phase (pH eq ) was measured using a combined glass electrode connected to a Digilab 517 pH-meter (Crison, Barcelona, Spain) and the metal content in the aqueous phase was determined by ICP/AES ( JY138 ultratrace, Horiba Jobin Yvon, UK). Aqueous solutions of the following composition were prepared: [(H + , Na + ) SO 4 2-] = 0.33 M, [M(II)] = 100 ppm (~ 1.6 x 10 -3 M for Cu(II) and ~ 1.5 x 10 -3 M for Zn(II)), with different initial pH's (pH i ) between 1.0 and 5.8.
The extraction percentage %R of a metal M in a solid-liquid extraction, can be determined from its analytical concentrations in the aqueous phase before and after extraction (Eq. (1)): where R is the extraction percentage.
[M m+ ] 0 is the initial concentration of the metal (mmol/l).
[M m+ ] aq is the equilibrium concentration of the metal in the aqueous phase (mmol/l).
Another ratio can also be defined: (2) S = number of moles of HL s in the impregnated solid. Table 1 presents the concentrations of the ligands in mmol per 1 g of functionalized solid. The capacity of an extracting solid, expressed in m eq / g, is defined by the maximum amount of metal ion that can be immobilized per unit of solid mass (solid). It depends essentially on the number of functional groups accessible on the surface of the material, on the specific surface area, and the dilution of the functional groups in the solid matrix and the stoichiometry of the complex between the metal and the ligand. The Cu (II) capacity of impregnated silicas was determined in 0.33M sulfate medium by placing the same solid mass in contact with increasing amounts of metal ion, to allow the saturation of the solid. This was done by increasing the metal ion concentration in the solution without changing the volume ratio of the phases.
As for the stripping of metals and for each experimental point, an extraction of the metal was performed at a given equilibrium pH. The liquid and solid phases were then separated. During the extraction, we added to each solid phase a volume of nitric acid (in general HNO 3 , 1M) equal to the volume of the aqueous phase used. The pH of the nitric acid solution was less than that of the initial extraction pH determined in the previous study. The system was stirred for one hour. The stripped metal present in the aqueous phase by then was titrated.
The effect of the stripping on the silica structure was investigated through an analysis of the solids by X-ray diffraction XRD.

RESULTS
Our work revolves around the study of the extracting properties of silica supports impregnated by HPBI, HPMSP and / or DEHPA in order to confirm the importance of their use in the extraction of transition metals cations in sulfuric aqueous medium and their eventual separation. The preparation and characterization of the different silicas have been described in a previous work [10,13]. We have investigated the kinetics and extraction capacity of the used supports towards the metal to be extracted. We also stu died the pH of the aqueous medium and the concentration of ligands in the silica matrix as the main parameters influencing the outcome of the extraction.
We conducted Cu (II) and Zn (II) extraction tests on blanks (non-functionalized calcined silicas) [13], and no extraction was observed. This prompted us to incorporate chelating ligands in the silica for the cations extraction following their complexation within the silica matrix.
For each extraction system M 2+ /(impregnated silica), we conducted a series of extractions for which the initial pH were the same. However, the contact time was ranging from 5 to 60 min. The initial pH and the pH at equilibrium are summarized in Table 2: Zn-MCM-CI-DEHPA 6.19 3.09 ± 0.03 Figure 1 represents the extraction percentage as a function of time. It is important to note that the equilibrium was reached in less than 15 min in all three cases. With the aim of determining the kinetic order of the extraction process, a pseudo-first order expressed in equations (3, 4) and a pseudosecond order expressed in equations (5, 6) were studied. For a pseudo-first order reaction, the rate law is expressed as: Where k 1 : the rate constant (min -1 ), q e : quantity of the metal ion extracted at equilibrium (mmol/g), q t : quantity of the metal ion extracted at time t.
After equation 3 is integrated between 0 and t for the time and between 0 and q t for the quantity of the metal ion extracted, we obtain: For a pseudo-second order reaction, the rate law is expressed as: where k 2 is the rate constant (g.mmol -1 .min -1 ) After equation 5 is integrated, the following integrated law is obtained: can be rearranged as follows: We determined the rate constants and correlation coefficients, by plotting Ln (q e -q t ) as a function of time t for pseudo-first order and t/q t versus time for pseudo-second order. The results are summarized in Table 3     After comparing the correlation coefficients for the three different cases, we noted that the experimental points were consistent with the pseudo-second order model. Therefore, based on this model, we recalculated taking into account all the experimental points (Table  4). b q e,exp : quantity of the metal ion extracted at equilibrium and experimentally determined.
The correlation coefficients determined as well as the amounts of metal extracted at equilibrium were in good agreement with those determined experimentally.
Based on the theoretical study of Azizian [17,18], the pseudo-second order model is best suited when the initial metal concentration is low. Indeed, in our case, we worked with a large excess of ligand (S/M ≥ 3.43) and the kinetic order observed was certainly in agreement with the theory. However, it is to note that this result does not exclude in any case, second order possible chemisorption [19,20] where the adsorption of metal cations taking place by complexation of this latter by ligands present in the solid phase.
In Table 5 we have summarized the data obtained by other research groups who worked with with the same ligands used in this study, on different functionalized solids. We also included the results of teams who used the liquid-liquid extraction technique [21,26].  [26] 120 min [26] It is of note that the equilibration time was reached more quickly in the case of the extraction of Zn (II) by DEHPA incorporated in the silica. The longest equilibration time corresponded to the extraction of zinc and copper by the resin impregnated by DEHPA. This was probably due to the textural properties of the two types of solids (Silica vs Resin). In general, for an extracting solid with a given chemical composition, the extraction kinetics of a metal ion depend on the solid's particle size and its average pore diameter. Decreasing the particles' size and increasing the average pore diameter help in achieving equilibrium more rapidly (Table 6)   Although the pore diameter of the resin is greater than twice that of silica, which promotes rapid kinetics, this is largely reduced by the other textural properties. The grain size of the mesostructured silicas is much smaller than that of the resin and their specific surface area is four times larger than that of the resin. This leads to a higher accessibility of the silica active sites to metal cations compared to the resin's.
In the remainder of the study, the contact time will be set to 60 minutes which will be sufficient to reach equilibrium.

Influence of the pH
The domain of extraction pH of Cu (II) by calcined impregnated silicas with different ligands was determined in a previous work [13].
The extraction was carried out at more acidic pH compared to those of the liquid-liquid extraction of copper with the same ligands [28] in the different cases, with an extracted amount of 0.16 mmol / g of impregnated silica.

Extraction capacity of MCM-CI towards Copper
The capacity was determined by measuring the amount of metal ion present in solution before (n 0 ) and after saturation (n): In order to determine the Cu (II) extraction capacity of MCM-CI-HPBI and MCM-CI-HPMSP silicas, we have chosen initial pH's that were able to lead to maximum extraction. The initial pH's (pH 0 ) and equilibrium pH's are represented in Table 7.

MCM-CI-HPBI MCM-CI-HPMSP
pH 0 1.88 ± 0,05 3.48 ± 0.02 pH 1.81 ± 0,02 3.01 ± 0.02 Figure 4 shows the experimental results; In both cases, we observe two plateaus, the first one appears for Cu(II) initial concentrations of less than 600 ppm, and the second appears when the Cu(II) initial concentration is greater than 2000 ppm.  The capacity value obtained in the case of MCM-CI-HPBI was in agreement with the values generally obtained by related materials [5]. However, those obtained by MCM-CI-HPMSP were substantially larger.

Zn(II) extraction by silicas
We have studied the extraction of zinc by non-functionalized silicas and silicas impregnated with different ligands.

Influence of the pH
The pH influence on Zn(II) extraction pH by the different impregnated silicas was studied in a previous work [13]. The best extraction was obtained with the silica impregnated by DEHPA. The extraction pH's were similar to those of the liquid-liquid extraction of Zn (II) by the same ligands [29].

Influence of ligand concentration
We have studied the effect of DEHPA concentration on the extraction of Zinc. The results are shown in Figure 5. For 0.43 mmol/g of Zn(II), the MCM-CI-DEHPA was saturated with an extraction rate of approximately 66%. Whereas, for 0.42 mmol/g of Zn (II), the MCM-CI-HPBI was saturated with a 45% extraction rate. This difference may be due to the higher availability of DEHPA compared to HPBI.

Extraction capacity of MCM-CI-DEHPA towards zinc
We investigated the zinc extraction capacity of MCM-CI-DEHPA at pH 0 = 6.20 ± 0.08 and pH eq= 3.14 ± 0.05. The results are presented in Figure 6.     In order to test the above hypotheses, we performed the extraction of copper and zinc, from the same solution containing 100 ppm of each metal, by different systems, at two different pH's. The results are summarized in Table 8.   Table 8, we can conclude that it is possible to selectively separate copper and zinc by using the systems we synthesized and elaborated. It should be noted that there was no further extraction of zinc by MCM-CI-HPBI nor by MCM-CI-HPMSP in the presence of copper. Furthermore, no extraction of copper by MCM-CI-DEHPA in the presence of zinc was observed.

Copper/zinc selectivity of the calcined impregnated silicas
The S/M t ((Number of moles of HL / Total number of moles of metal (zinc + copper)) were 1.77; 1.74 and 1.84 respectively, under the conditions of these experiments. However, in the conditions of figures 7, 8 and 9, the S/Cu or S/Zn were practically doubled. It seems that the two metals compete for binding to the active sites, thereby disadvantaging the extraction of the less reactive (towards these sites) metal and even cancelling it.

Stripping of metallic cations
In order to recover the extracted metal cations and ensure the best performance of our functionalized silicas and their ability to be reused, we studied the stripping of copper and zinc by the silicas that have showed high extraction capacities towards these cations.
We presented the extraction-stripping by plotting the following experimental curves: n (M ex ) = f(pH eq ) for extraction n (M str ) = f(pH eq ) for stripping pH eq : extraction pH at equilibirum.  Figure 10 shows the evolution of the number of moles of copper extracted by MCM-CI-HPMSP and stripped as a function of pH. The stripping was carried out using a solution of 1M solution of nitric acid.  out immediately after the phase separation following the extraction, by adding the acid solution to the silica still impregnated by the extraction. This was a source of error as the extraction is low at these pH's, the stripping percentage can surpass 100 %. However, this error becomes negligible for extractions with % R beyond 50%. Furthermore, XRD diffractograms showed that the structure was not affected by the stripping process.

Cu(II) stripping from MCM-CI-HPBI
Copper extraction by silicas impregnated with HPBI was high starting with pH= 0; therefore, the stripping must be carried out with very acidic solutions such as 6M HNO 3 . We tried to recover the copper extracted by silicas impregnated with HPBI, playing on the stripping time. Experimental extraction-stripping curves are shown in Figure 11. The maximum amount of recovered copper was approximately 35% after 24 hours. On the other hand, The XRD diffractograms, showed that the silica structure was completely destroyed by acid attack during the stripping; This may explain the low recovery of copper from the MCM-CI-HPBI.

Zn(II) stripping from MCM-CI-DEHPA
The extraction/stripping of zinc was realized with a 1M nitric acid solution. The results are presented in Figure 12. Zinc can be recovered at more than 95% out of the quantity extracted by silica impregnated with DEHPA, without any change in the structure. In the various cases of extraction capacity curves (Figures 4 and 6), we obtained the same shape presenting two plateaus. In Table 9, we have reported, for each system, the concentration of extracted metal at each plateau and the R L/M ratio which is defined as: R L/M = amount of ligand (mmol/) / Amount of extracted metal (mmol/g). Unlike liquid-liquid extraction, the amount of ligand in the solid-liquid extraction was limited and distributed over a large area. However, the formation of complexes ML + ,Xtype, whose R L/M ratio was 1, could be favorable at high metal concentrations [5]. The charge of the complex would indeed be compensated by the anions of the aqueous phase (eg. HSO 4 -).

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For Cu-MCM-CI-HPMSP systems, the R L/M ratio was 1 at the 2 nd plateau, which corresponded to the formation of the complex CuL + X -. This ratio was 2 at the 1 st plateau and corresponded to the formation of CuL 2 . This was in agreement with the liquid-liquid extraction by HPMSP where the concentration of the latter was largely in excess with regard to copper, which promoted the formation of CuL 2 . Thus, we could conclude that all HPMSP ligands were accessible, reactive and arranged to form a square planar complex with copper.
Concerning the Cu-MCM-CI-HPBI system, at high concentration of copper, a R L/M ratio of 1.4 (2 nd plateau) was obtained. This corresponded to the formation of CuL + ,Xcomplex with the existence of some inaccessible sites (eg. crystallized HPBI). When the copper concentration was low, the ratio was 3.4 (1 st plateau). This ratio corresponded to the formation of CuL 2 , with the presence of unreachable sites. Some of these sites are reactive, but could be hidden or poorly placed to coordinate a single copper, thus, we came to complex copper by these sites under CuL + form at high concentrations of copper. This was consistent with the plateau obtained at 0.42 mmol/g for copper extraction by MCM-CI-HPBI when the R L/M = 6, which implies that some molecules were inaccessible to copper [10].
In the liquid-liquid extraction of Zn by DEHPA, different complex species could be formed such as for example ZnL 2 , ZnL 2 (HL), and ZnL 2 (HL) 2 . A high Zn/DEHPA ratio in organic phase promotes the formation of polymeric complexes of type (ZnL 2 ) n with: 2 ≤ n ≤ 3,5 [25,29]. In case DEHPA was immobilized on a mesostructured silica such as MCM-CI-DEHPA, it would be very difficult to accurately determine the stoichiometry of the complex formed during the extraction of zinc. The R L/M ratio was 2.1 at the 2 nd plateau which may correspond to the formation of ZnL 2 complex. This ratio was 2.6 at the 1 st plateau and could relate to the formation of ZnL 2 (HL) which was a very probable complex to form in the case of zinc [30]. The formation of ZnL + ,Xwas unlikely to happen. These results were consistent with the plateau of the zinc extraction by the MCM-CI DEHPA with a rate of 66% of Zn, when the concentration of DEHPA was 0.43 mmol/g. The ratio obtained was 4.26 ( Figure 5) most likely corresponding to the formation of ZnL 2 (HL) 2 .
The different silicas used in this work had almost the same ligand load and similar specific surface areas, despite this, the capacity to extract zinc was lower (0.27 mmol/g) than that for copper. This could be explained by the difference of stoichiometries of the formed complexes for copper and zinc.

CONCLUSION
In the first instance, we tested the extraction of Cu (II) and Zn (II) by the various impregnated silicas as a function of the pH of the aqueous phase. The different silicas extracted copper and / or zinc at very acidic pH's.
We then thoroughly studied the extraction process by investigating the extraction kinetics. The reactions were very fast for the different cases, and they were of pseudo-second order type. Extraction capacities were also considered. Up to 0.54 mmol/g and 0.27 mmol/g of Cu (II) and Zn (II) were respectively extracted by the impregnated silicas. We have highlighted the extraction of two complex types; ML + and ML 2 depending on the initial concentration and the nature of the ligand.
The striping of Cu (II) and Zn (II) by the silicas impregnated with HPMSP and DEHPA, could easily be completed by a 1 M nitric acid solution without the structure of the silicas being damaged. In the case of the Cu (II) stripping by MCM-CI-HPBI, as extraction occurred starting at pH of -0.19, the process was only effective in more acidic media (6M HNO 3 ). Under these experimental conditions, the structure collapsed.
Further tests for the separation of Cu (II) and Zn (II) by the impregnated silica were conducted as a function of the pH. The silicas impregnated HPBI and HPMSP were selective for copper while those impregnated by DEHPA were selective for zinc.