Highly selective copper ion imprinted clay / polymer nanocomposites designed by visible light radical photopolymerization

There is an urgent demand worldwide for the development of highly selective adsorbents and sensors of heavy metal ions and other organic pollutants. Within these environmental and public health frameworks, we are combining the salient features of clays and chelatant polymers to design selective metal ion adsorbents. Towards this end, the ion imprinting approach has been used to develop a novel nanohybrid material for the selective separation of Cu ions in aqueous solution. The Cu-imprinted polymer/ montmorillonite nanocomposite (IIP/Mt) and nonimprinted polymer/montmorillonite nanocomposite (NIP/Mt) were prepared by radical photopolymerization process in the visible light. Ion imprinting was indeed important as the recognition of copper ions by IIP/Mt was significantly superior to that of NIP/Mt that is the nanocomposite synthesized in the same way but in the absence of Cu ions. The adsorption process as batch study was investigated under the experimental condition affecting same parameters such as contact time, concentration of ions metals and pH. The adsorption capacity of Cu ions is maximized at pH 5. Removal of Cu ion achieved equilibrium within 15 minutes; the results obtained were found to be fitted by the pseudo-second order kinetics model. The equilibrium process was well described by the Langmuir isothermal model and the maximum adsorption capacity was found to be 23.6 mg/g.


Introduction
There is a global concern of heavy metal pollution of the environment with a high risk for these elements to enter the food chain via natural and anthropogenic routes [1].Heavy metal ions pollutants affect humans after ingestion of water and food [2,3].For example, children are particularly vulnerable to the neurotoxic effects of lead, and exposure, even at relatively low levels, may cause serious, and in some cases irreversible, neurological damage [4].
For these reasons, heavy metal pollution is regarded as one of the most challenging global problems and requires combined actions such as detections at minute levels and removal at a separation/filtration stage using high capacity selective adsorbents.The first issue of the problem can be addressed by develop highly sensitive and selective sensors for a YES/NO response of the sensors to indicate whether or not a sample is contaminated by heavy metals; ideally the output response is processed to return a quantitative value of the metal ion concentration in the target sample [5,6].In addition to sensitive devices, the second important issue is to develop high capacity adsorbents to remove or to pre-concentrate heavy metals in order to have a complete picture of the pollution by metal ions from the surface physicochemical point of view.Towards this end, clays can be regarded as natural inorganic "sponges" to retain heavy metal ions [7,8].However, aluminosilicates are negatively charged and retention of heavy metal ions could be achieved through cation exchange reactions.In this sense, clays are not selective [8] and organo-modification permits to impart to clays functional groups for the specific retention of metal ions.For example, NH2, SH, C=C, calixarene, crown-ethers and cryptand, dendrimer among others have been attached to clays via ammonium salts, diazonium salts, silane coupling agents or other strategies [9, 10,11,12,13,14,15,16,17].In a more elaborated picture, clay/polymer nanocomposites can also be envisaged if the polymer possesses pendant functional groups able to complex metal ions [18], yet selectivity remains challenging.Some of us reported various synthetic approaches for the syntheses of claypolymer nanocomposites including free radical, cationic and ring-opening poymerizations and click chemistry [19,20,21,22,23].By incorporating suitably selected functional groups onto clay surface a wide variety of polymerization systems can successfully be applied in an in situ manner.
To push the frontiers of selectivity further, a smart strategy consists of making clay/ion imprinted polymer nanocomposites [24,25], therefore combining the best of the two worlds: high capacity mineral adsorbent [26] and highly selective organic material c.a. ion imprinted polymers [27,28,29,30,31].We have previously demonstrated that such materials can be designed by UV-triggered radical photopolymerization of functional and crosslinker in the presence of organo-clay [32].In a unique approach, we have efficiently organo-modified clay using the versatile aryl diazonium instead of ammonium salts or silane coupling agents bearing photoinitiator.
Yet, the original design of nanohybrids we have reported recently requires UV light initiation.
Instead, herein we simplify the process by triggering radical photopolymerization under visible light using diazonium-modified clay as a macro-initiator and camphorquinone (CQ) as photosensitizer.CQ is indeed a very efficient photosensitizer for generating ultrathin polymer coatings by simulated sunlight-induced surface-confined radical photopolymerization [33].To the very best of our knowledge, this strategy has never been reported previously, it is highly interesting simply because the clay/IIP nanocomposite can be designed using natural day or simulated day light or white light.
Herein, montmorillonite was first cation exchanged with a diazonium compound bearing dimethyl amino groups to provide a macro-photoinitiator, and visible light radical photopolymerization was conducted in the presence of the modified clay, the monomers (4vinylpyridine, (VP) methacrylic acid (MA), CQ and the heavy metal ions.Thorough rinsing of the final product, permitted to remove the metal ions and to create the artificial receptor sites for the selective recognition of copper.

Chemicals and Reagents
All chemical reagents and materials were purchased from Aldrich and used without further purification.Milli-Q Plus water purification system was employed to prepare deionized water.

Instrumentation and characterization
X'Pert PRO (PANalytical) instrument fitted with a Co Kα X-ray source (1.789Ǻ) was employed to record the diffraction patterns of the samples.
Infrared spectra were recorded using a Nicolet Magna-IR 550 spectrometer in the attenuated total reflection (ATR) mode, in the 4000-450 cm − 1 range.AK Alpha instrument (Thermo) fitted with a monochromated Al Kα X-ray source (hν= 1486.6 eV, spot size = 400 µm) was used for XPSmeasurements.Aflood gun was employed for static charge compensation.The analyzer was operated at 80 and 200 eV pass energy for the narrow regions and survey spectra, respectively.Elemental atomic concentrations were computed using the integrated peak areas and the corresponding sensitivity factors provided by the manufacturer.
The determination of metal ion concentration was performed with Avanta/GBC flame atomic absorption spectrophotometer equipped with a hollow cathode lamp and deuterium background corrector at respective wavelength resonance line using an air-acetylene flame.

Diazonium modification of montmorillonite
To an aqueous suspension of clay (100 mg dispersed in 10 ml of de-ionized water) we added dropwise an aqueous solution of diazonium salt (500 mg in 10 ml of deionized water).The intercalated clay was washed thoroughly with deionized water and dried at 60°C overnight.

Synthesis of Cu 2+ _IIP/Mt nanocomposite
The IIP/Mt nanocomposite was prepared in two major sequential steps.

Metal ion adsorption
Adsorption of Cu 2+ on the IIP/Mt nanocomposite and NIP/Mt nanocomposite was examinated using batch experiments.To prepare adsorption isotherms, a series of samples containing 4mg of an appropriate IIP/Mt and NIP/Mt as equilibrated with 10mL solution containing various concentrations (5-20 mg/l) of Cu 2+ .The solution was shaken at room temperature for 2h.After adsorption, the mixture was isolated by centrifugation, then filtered through a 0.45 µm membrane filter and examined with atomic absorption spectroscopy (AAS).The adsorption amount (qe; mg/g) was calculated using: Where 0 and   are initial and equilibrium concentration (mg/L), m is the sorbent mass (g), and V is the solution volume (L).
Kinetic studies were conducted to determine the adsorption rate of Cu 2+ from water samples as follows: 4mg of each sorbent were dispersed in 10mL solution containing the same initial Cu 2+ concentration of 20 mg/L.Each mixture was continuously batch oscillated at room temperature for 5,10,15,30,60,90 and 120 min.After each time period, solutions were filtered and analyzed by AAS to determine the concentration of Cu 2+ in the supernatant.
To investigate the effect of pH, 4 mg IIP/Mt and NIP/Mt were added to 20mL sample solutions containing 20 mg/L of Cu(II) ion at pH range 2.0-9.0,respectively, pH values were adjusted by 1mol/L HNO3 and 1 mol/L NaOH.

Selectivity experiments
To estimate the selectivity of the ion-imprinted polymer/Mt nanocomposite and their corresponding non-imprinted polymer/Mt nanocomposites materials for copper adsorption, 5mg of the sorbent was added into 10ml of 20mg/L binary solution of Cu 2+ /Zn 2+ , Cu 2+ /Pb 2+ , and Cu 2+ /Fe 3+ for 1h.The mixture was magnetically stirred at 300 rpm, then left to decant for 15 min, centrifuged, and filtered.Finally, the concentration of copper and the interfering ions was measured by AAS.
The distribution coefficients, Kd (ml/g), of Cu 2+ , Zn 2+ , Pb 2+ , and Fe 3+ were calculated using equation: The selectivity coefficients for the binding of a copper ion in the presence of other competing ions in binary systems were calculated according to the following equations: Where M n+ (n=2 or 3) represents Cu 2+ competing ions mentioned above.The coefficients give an indication as to how selective the nanocomposite is for Cu 2+ ions in the presence of other competing species in solution.
The relative selective coefficient K', which represents the enhanced effect of imprinting on selectivity and adsorption affinity for the template onto the ion imprinted polymer/montmorillonite nanocomposite.The K' of the IIP/Mt nanocomposite against the NIP/Mt nanocomposite as determined using equation where KIIP/Mt and KNIP/Mt are the selectivity coefficients of the IIP/Mt and NIP/Mt, respectively.

Desorption and regeneration
To estimate the reuse ability of IIP/Mt nanocomposite, synthesized adsorbant were contacted with Cu 2+ solution for adsorption processus.After adsorption, the IIP/Mt were placed in the desorption medium (1M, HNO3 solution) and stirred for 120 minutes at room temperature.This procedure was repeated for many times until Cu 2+ could not be detected in aqueous phase.Then, the adsorbent was washed thoroughly with double distilled water to a neutral pH to determine reusability of IIP/Mt nanocomposite, the desorption-adsorption procedure was repeated eight times using the same sorbent.

Synthesis strategy and mechanistic aspects
The synthesis strategy of IIP/Mt nanocomposites was carried out in two main steps starting by a simple diazonium cation exchange reaction with Na + (Scheme 1).
Scheme 1. Schematic representation of the synthesis procedure of Cu 2+ _IIP/ clay nanocomposite.NB: Under irradiation conditions the azo compound undergoes cis-trans isomerization.In fact, this behavior would be more beneficial since it would lead to a better intercalation.
The cation exchange reaction led to the intercalation of the diazonium cations into the clay.
Heating at 60 °C was achieved in order to obtain stable aluminosilicate-aryl bond by dediazonation of the interfacial diazoether [32].For the photopolymerization step, we have deliberately selected CQ as the photosensitizer for two reasons.First, it absorbs the light in the This issue is particularly important for our case as chain growth occurs only on the clay surface.
The overall, mechanism is presented in Scheme 2.
Scheme 2. Mechanisn of visible light photopolymerization on the clay surface by using camhorquinone.The structure of the attached azoic initiator is simplified for the sake of clarity.
The mixture was irradiated using a Ker-Vis blue photoreactor equipped with six lamps (Philips TL-D 18 W) emitting light nominally at 400-500 nm at room temperature for 4h.For (Mt_DZ), the d001 peak position is shifted to lower values (Figure 1b) compared to the untreated clay (Figure 1a).The inter-lamellar distance of soda montmorillonite (Mt), modified montmorillonite (Mt_DZ) and montmorillonite/ionic imprinted polymer nanocomposite (IIP/Mt) were estimated from the peak (001) using the Bragg equation n =  d sin After the cation exchange reaction between the Na + cations and the diazonium salt, the interlayer distance was increased from 1.34 nm to 1.66 nm, which proves the intercalation of diazonium salt between the montmorillonite layers.After photopolymerization the distance d001 moves to the lower value of 2 (d001=1.8nm), which means that after the in-situ radical photopolymerization the polymer chain is intercalated between the clay layers [37].It is to note that Figure 1(c) showing the X-ray diffraction of the IIP/Mt nanocomposite, suggests that this adsorbent is characterized by (00l) (1< l <4) reflection which testifies for regular distribution of the polymer within the clay nanosheets.The positions (001), ( 002) and (003) peaks at d=1.87, 1.02 and 0.52 nm, respectively, indicate an intercalated crystalline structure of the IIP in the interlayers.Figure 2 shows the FTIR spectra of Mt, Mt_DZ, IIP/Mt and NIP/Mt.The structure of the purified montmorillonite is characterized by the bands located in the 1000-1200cm -1 range assigned to Si-O-Si.The peaks at 3635 and 918cm -1 are in line with the dominant presence of dioctahedral smectite with (Al, Al-OH) [38].The absorption band at 1625 cm -1 corresponds to OH from the water molecules absorbed by the clay.After clay modification the FTIR analysis confirmed the presence of the diazonium in the clay interspacings.FTIR data of modified clay with diazonium prove  2 + band at 2210cm -1 .Moreover, after clay modification we observe the appearance of a band at 1621cm -1 assigned to the pyridine ring, and an intense band at 1447cm -1 which ascribed to the stretching vibration of C-N of 4-vinylpyidine.One can also note the presence of a new signal near 1718cm -1 corresponding to the carbonyl of the methacrylic acid.These FTIR observations are in line with the coexistence of an organic polymer fraction in the final composite material.3b the disappearance of the Na1s peak compared to the pristine clay (Figure 3a).The intercalation leads also to an increase in the C1s/Si2p intensity ratio and appearance of a tiny N1s peak (~400 eV) from the intercalated diazonium.The synthesis of the IIP/Mt_Cu results in a significance change in the survey region (Figure 3c) which exhibits sharp C1s and O1s peaks from the polymer and the existence of a Cu2p doublet.The high resolution Cu2p from IIP/Mt_Cu is shown in insert.Nitrogen is also slightly more visible as vinylpyridine was used here as a comonomer.After extraction, the Cu2p vanishes (Figure 3d).

Effect of pH
The pH of the solution has a significant influence on the adsorption process as it is well-known to affect the protonation of surface functional groups.Figure 4 displays the adsorption capacity of Cu 2+ by the IIP/Mt support; the maximum is noted at pH 5. Above this value, adsorption starts to decrease due to precipitation of copper [39].For low pH values the adsorption of copper ions is also low this could be due to the protonation of functional sites and consequently a competition between protons from the acidic medium and Cu 2+ ions towards the artificial receptor sites of copper.In acidic medium, adsorption increases with pH as reported for clays

Effect of contact time and a kinetic study
The effect of contact time on the adsorption of Cu 2+ , Zn 2+ , Pb 2+ and Fe 3+ on IIP/Mt and NIP/Mt nanocomposites is displayed in Figure 5.The adsorption of Cu 2+ ions is rapid; the adsorption capacity increases with time and reaches a maximum during the first 15 minutes, then increases to saturation.The extent of adsorption of Cu 2+ is much higher than that of Zn 2+ , Pb 2+ and Fe 3+ ions (Figure 5a).This rapid equilibrium could be explained by the high affinity of complexation and geometry between Cu 2+ ions and the cavities of the nanocomposite structure.It is known that the removal of the template from the polymer network creates smart cavities that allow us to know the shape and chemical functionality of the Template Cu 2+ [41].In contrast, for the NIP/Mt adsorbent, there is no significantly distinct adsorption is noted for copper.As a matter of fact, one can note in Figure 5b that the maximum adsorption is noted for zinc and not copper.
From Figures 5a-b, clearly it is essential to fabricate a copper ion imprinted polymer in order to achieve selective removal of this template ion.To study the adsorption process, pseudo-first order and pseudo-second order models were used to control the check adsorption kinetics of IIP/Mt and NIP/Mt.
Where qt (mg/g) and qe (mg/g) are the adsorption capacities of metal ions on IIP/Mt and NIP/Mt at time t and equilibrium, respectively; k1 (L/min) and k2 (g/mg min) are the first order and second order adsorption constants, respectively.The kinetic modeling for the adsorption of metal ions on IIP/Mt nanocomposites is shown in Figure 6 and the adsorption kinetics constants and the correlation coefficient values R² are summarized in Table 1.The values of the correlation coefficient R² of the pseudo-second-order model are significantly higher than those of the other models and closer to unity which accounts for pseudo-second order mechanism of the adsorption process on the IIP/Mt nanocomposite.It follows that chemisorption is the driving force for the adsorption process on the actual IIP/Mt nanocomposite [44].
Table1.Kinetic parameters of the pseudo-first-order and the pseudo-second -order rate equations for ions metals adsorption on IIP/Mt nanocomposites.

Adsorption isotherms
Figure 7 shows the effects of aqueous phase Cu 2+ concentrations on equilibrium adsorption capacity on IIP/Mt.The adsorption capacity increases with increasing concentration in the aqueous phase.Adsorption of the metal ion saturates at 23.562 mg/g (qmax).In this study the adsorption isotherm is applied to study the interactions between metal ions and the active sites of our adsorbent (IIP/Mt).The adsorption isotherm is expressed for an adsorbent-adsorbate pair as a function of concentration.The adsorption isotherm of Cu 2 + ions was checked whether it fits the Langmuir [45 ] or Freundlich [46] isotherm models (see Figure 8) as described by equations ( 7) and ( 8), respectively.The Langmuir isotherm theory assumes that adsorption is single-layer and takes place at homogeneous sites specific to the adsorbent and the Freundlich isotherm assumes that adsorption is multi-layer and that the surface of the adsorbent is heterogeneous. .
Where qe (mg/g) is the adsorbed amount at equilibrium, Ce is the equilibrium concentration of the metal ions (mg/L), KL (L/mg), KF( 1−1/ / 1/ ) are the Langmuir equilibrium and the Freundlich constants, respectively; qm the maximum adsorption capacity (mg/g), n is the heterogeneity factor, and 1/n value is related to the sorption intensity.Table 2 lists all isotherm parameters.The data show that Langmuir model (Figure 8a) is more suitable to describe the adsorption reaction of Cu 2+ on the IIP/Mt with the experimental data with higher R² values.Therefore, these results imply that the adsorption process of IIP/Mt is surface monolayer adsorption and the adsorption sites are homogeneous [47,48].

Regeneration of IIP/Mt
In order to evaluate the reusability and the renewability of the synthesized sorbent, several adsorption-desorption cycles were carried out on the same sample of IIP/Mt nanocomposite d at the same Cu 2 + concentration of 20 mg/l.The desorption of the Cu 2 + ions adsorbed from the IIP/Mt nanocomposites was carried out using a 2M HNO3 solution.Several cycles of adsorption-desorption were applied to the IIP/Mt adsorbent.The results displayed in Figure 9 permit to claim stability of the IIP/Mt nanocomposite adsorbent which could be regenerated without significant loss of performance.Indeed, up to 8 adsorption/desorption cycles were applied to the robust adsorbent designed so far which did not show any sign of decrease in performances. .

4-Conclusion
Montmorillonite-based ion imprinted polymer nanocomposite was prepared by radical photopolymerization under visible light exposure.The pre-polymerization complex was prepared by mixing Cu(NO3)2 and functional monomers of 4-VP and MAA mixed in DMF/H2O.Then, this solution was mixed with the cross-linker (EDGMA), photoinitiator (camphorquinone, CQ) and modified montmorillonite by diazonium (Mt_DZ).We show that diazonium salt is a good intercalant for clay and for triggering in situ radical photopolymerization.In this study, Cu 2+ -imprinted polymer/ montmorillonite nanocomposite was successfully prepared and applied for the pre-concentration and determination of Cu 2+ ions in aqueous solutions.The experimental data followed the Langmuir isothermal model and pseudo-second order kinetics.In adition, these artificial sites shaped to accommodate Cu 2+ could uptake copper at high extent compared to the competing metal ions Zn 2+ , Pb 2+ and Fe 3+ , but also much more than non-imprinted polymer/montmorillonite nanocomposite.The selectivity with other ions confined that IIP/Mt showed high specific Cu 2+ ions.
This work demonstrates for the very first time the efficiency of diazonium salt to initiate visible light radical photopolymerization within the interlayer spacings of layered aluminosilicates.
The method could be applied to other nanomaterials which could serve as nanoscale platforms to be coated by ion or molecularly imprinted polymers.

Figure 3 displays
Figure 3 displays XP spectra of Mt, Mt_DZ and the nanocomposites before and after extraction of copper.The survey regions are displayed in figures 3a-d; and the main peaks are Al2p (~74

Figures
Figures 3e and 3f display the high resolution C1s and N1s peaks of the IIP.Interestingly, the C1s shows features due to C-N (~286 eV) and the functional group COOH (~peak component at 289 eV).As far as the peak component at 292 eV is concerned, it is ascribed to aromatic species from the vinylpyridine repeat units and the diazonium-derived aryl groups attached to the clay sheets.The N1s feature from the IIP (Figure3f) shows free and quaternized nitrogen species.The latter could be due to protonation by the COOH group of the methacrylic acid or by the OH groups from clay.
visible region of the electromagnetic radiation.Although there are many visible light acting photoinitiating systems[34,35,36], CQ is the safest photoinitiator and therefore, widely used in dental applications.

Table 3 )
Table2.Adsorption equilibrium constants for Langmuir and Freundlich isotherms.To evaluate the selective properties of the IIP/Mt and NIP/Mt nanocomposites, adsorption of Cu 2+ was conducted in the presence of competitive ions in binary systems.In our work, the binary solute solutions, including Cu 2+ /Zn 2+ , Cu 2+ /Pb 2+ and Cu 2+ /Fe 3+ were studied to explore the selectivity to template Cu 2+ at room temperature.The distribution coefficients (Kd), the selectivity coefficients (K) and the relative selectivity coefficients (K ') were calculated (see .It is noted that the distribution coefficient value of Cu 2 + is greater than that of the other metal ions.Therefore, high selectivity coefficients were determined for IIP/Mt and NIP/Mt nanocomposites: 10.308, 6.515 and 8.524 for Cu 2 + /Zn 2 + , Cu 2 + /Pb 2 + and Cu 2 + /Fe 3 + , respectively.The selectivity for Cu 2 + is due to the ion imprints shaped in the polymer network of the nanocomposite.The geometry, charge and size of the prints account for the selective recognition of Cu 2+ ions over the other cationic competitors.
Author Contributions:Conflicts of Interest: "The authors declare no conflict of interest."Preprints(www.