Kinetics, thermodynamics, equilibrium, surface modelling, and atomic absorption analysis of selective Cu(ii) removal from aqueous solutions and rivers water using silica-2-(pyridin-2-ylmethoxy)ethan-1-ol hybrid material

The removal of heavy metals is attracting considerable attention due to their undesirable effects on the environment. In this investigation, a new adsorbent based on silica functionalized with pyridin-2-ylmethanol (SiPy) was successfully synthesized to yield to a hybrid material. FTIR, SEM, TGA, and specific surface area analysis were used to characterize the structure and morphology of the SiPy hybrid material. Various heavy metal ions such as Cu(ii), Zn(ii), Cd(ii), and Pb(ii) were selected to examine the adsorption efficiency of the newly prepared adsorbent, optimized at varying solution pH, contact time, concentration, and temperature. The adsorbent SiPy displayed good adsorption capacity of 90.25, 75.38, 55.23, and 35.12 mg g−1 for Cu(ii), Zn(ii), Cd(ii), and Pb(ii), respectively, at 25 min and pH = 6. The adsorption behaviors of metal ions onto the SiPy adsorbent fitted well with the pseudo-second-order kinetic mode and the isotherm was better described by the Langmuir isotherm. The thermodynamic studies disclose spontaneous and endothermic adsorption process. Furthermore, the SiPy adsorbent retained good selectivity and regeneration properties after five adsorption–desorption cycles of Cu(ii). A computational investigation of the adsorption mechanism indicates that the N-pyridine, O-hydroxyl, and ether O-atoms play a predominant role during the capture of Cu(ii), Zn(ii), Cd(ii), and Pb(ii). This study proposes the SiPy adsorbent as an attractive material for the selective removal of Cu(ii) from real river water and real industrial wastewater.


Introduction
With intensive industrial activity and urbanization, aquatic environment contamination by heavy metal ions has become one of the major environmental pollution sources of groundwater and surface waters 1,2 since heavy metal ions are not biodegradable, and tend to accumulate and remain in living organisms for long periods, causing various disorders such as skin and liver diseases. 3,4 Copper, zinc, cadmium, and lead are the most hazardous metals listed on the US Environmental Protection Agency's list of priority pollutants. However, their use cannot be avoided because of their necessity in tanneries, batteries, paints, coatings, dyeing, textile mill, and other growing industries. 5 These industries release heavy metal ions into the environment and polluted water is used for irrigation and consumption. 6 It has become essential to search for new methodologies to extract them from environmental aqueous solutions.
In recent years, various techniques and technologies were used for the removal of heavy metal ions such as ion exchange, 7 reverse osmosis, 8 chemical precipitation, 9 membrane ltration, 10 photocatalysis methods, 11,12 ltration, 13 electrochemical technology, 14 solvent extraction, 15 and occulation. 16 However, these techniques are limited in their practical use due to the generation of hazardous by-products. 17 Adsorption is however considered a promising and effective purication technique for heavy metal ions due to its simplicity, low cost, and high efficiency. 18 Conventionally, adsorbents such as carbon nanotubes, 19 zeolites, 20 chitosan, 21 and cellulose 22 have gained attention due to their promising potential in environmental domains. Mesoporous silica, however, offers a number of assets such as a large specic surface area, high thermal and mechanical stabilities, as well as regular porosity and exibility toward surface chemical modication. 23,24 In this respect, mesoporous silica gel can be easily functionalized to signicantly increase the sorption of heavy metals ions from wastewater. [25][26][27][28][29][30][31][32] The efficiency and selectivity of these adsorbents partly rely on the donor atoms such as oxygen, nitrogen, and sulfur onto the surface of the material that are responsible for forming a complex with heavy metals. [33][34][35][36][37] In this context, pyridine has attracted attention because of its structure and well-known coordination chemistry. 38,39 Pyridine is thus an excellent graing group for functionalizing silica and can be widely applied in environmental cleanup via the chelating reaction. [40][41][42] In this work, a silica gel impregnated with a ligand-based functionalized pyridine was synthesized and characterized for the extraction of heavy metal from aqueous solutions. On the other hand, density functional theory (DFT), noncovalent interaction (NCI), quantum theory of atoms in molecules (QTAIM) approaches, and the localized orbital locator (LOL) were employed to gain a better understanding of the metal ion's adsorption mechanism and selectivity of the ligand structure. 43

Materials and methods
All solvents and chemicals (Sigma-Aldrich, purity 99.5%) were of analytical grade and used without further purication. Silica gel (Sigma-Aldrich) with particle size in the range of 70-230 mesh, surface area of 470-530 m 2 g À1 , pore diameter of 52-73Å, and pore volume of 0.7-0.85 cm 3 g À1 was activated before use by heating at 120 C for 24 h. The silylating agent 3-glycidoxypropyltrimethoxysilane was used without purication. All metal ion quantities were determined by atomic absorption spectroscopy (AAS) using a Spectra Varian A.A. 400 spectrophotometer. The pH value was controlled using a pH 2006, J. P. Selecta; elemental analyses were performed by the Microanalysis Centre Service (CNRS). FT-IR spectra were obtained using a PerkinElmer System 2000. SEM images were obtained on a FEI-Quanta 200. Mass loss determinations were performed in a 90 : 10 O 2(g) /N 2 (g) atmosphere on a PerkinElmer Diamond TG/ DTA at a heating rate of 10 C min À1 . The specic surface area of modied silica was determined using the BET equation. The nitrogen adsorption-desorption was obtained by means of a Thermoquest Sorptomatic 1990 analyzer aer the material had been purged in a stream of dry nitrogen.

Preparation of 3-glycidoxypropyl-functionalized silica (SiEp)
The surface modication of mesoporous silica material by 3glycidoxypropyltrimethoxysilane (SiEp) was carried out according to our published method. 44 Briey, 10 g of activated silica was dissolved in toluene (40 mL), in which were added triethylamine (240 mL). Then, 3-glycidoxypropyltrimethoxysilane (13.3 mL) was added to the reaction and stirred at solvent reux temperature for a period of 24 h under nitrogen atmosphere. The nal product was ltered and extracted via Soxhlet extraction with ethanol and dichloromethane (1 : 1) for 12 h. The crude product was vacuum dried at 70 C over 24 h.

Fabrication of pyridine-substituted silica (SiPy)
Aer converting the hydroxyl-pyridine (L 1 ) (Scheme 1) to its alcoholate derivative (0.8 mmol) using Na/THF, it was added to SiEp (1.00 g) in DMF (30 mL). The mixture was stirred and reuxed for 24 h to give a brown solid (SiPy), which was puried using a similar procedure used for SiEp.

Batch adsorption experiments
The effect of concentration ([Mn(II)] ¼ 10 to 300 mg L À1 ), time (5 to 35 min), pH (1 to 7), temperature (25 to 45 C), and selectivity (quaternary metal systems) parameters on the adsorption of Cu(II), Zn(II), Cd(II), and Pb(II) were studied in the batch method. Briey, 10 mg of the SiPy adsorbent was dispersed to 10 mL of aqueous solutions. The pH values were adjusted with diluted HCl and NaOH solutions, the mixture was shaken at 25 C for 25 min. Aer extraction, the metal ion concentrations were monitored by AAS and the adsorption capacity was calculated using eqn (1). 45 Here, q e (mg g À1 ) denotes the adsorption capacity, C 0 (mg L À1 ) and C e (mg L À1 ) are the initial and equilibrium concentrations respectively, V (mL) refers to the volume of the solution, and W (mg) refers to the mass of the adsorbent.

Linker synthesis
The synthesis of the new material SiPy is presented in Scheme 1. The rst step of the synthesis consisted of mixing activated silica gel with 3-glycidoxypropyltrimethoxysilane to form the material (SiEp). The second step consisted of the synthesis of the target ligand L 1 , which was converted to its pyridine alcoholate using metallic sodium in THF, which was then immobilized on the SiEp surface in DMF at reux. 45 3.2. Characterization of the material 3.2.1. Elemental analysis. The result of the elemental analysis, determined for SiPy, showed high percentages for carbon (5.97%) and nitrogen (1.08%), which are not present on the starting silica. This result indicates that the organic matter (pyridine unit) has been immobilized on the silica network, thus supporting a successful functionalization. This immobilization of the organic matter on the inorganic silica network was also conrmed by the analyses below.
3.2.2. FTIR characterization. The specic peaks observed at 3440 cm À1 , 1087 cm À1 , and 798 cm À1 related to the silica backbone were identied. 54 The SiPy material was analyzed using FTIR spectra and compared with pure silica SiG and SiEp (Fig. 1). Aer functionalization, the SiEp spectrum exhibits a new peak at 2890 cm À1 , which originates from the C-H stretching bond. The spectrum of the nal material (SiPy) showed a new peak at 1462 cm À1 , which was attributed to the stretching vibration of C]N. This spectrum thus shows that the material was successfully modied by the pyridine derivative.
3.2.3. Scanning electron micrographs (SEM). As we can see from SEM imaging (Fig. 2), the surface morphology of the nonfunctionalized and functionalized silica particles are clearly different. Indeed, an irregular and smooth surface was observed for silica particles (SiG), whereas a rough and porous morphology was observed for the hybrid material SiPy, where Scheme 1 Synthetic route to prepare the hybrid material SiPy. the agglomeration of molecules was much elevated, thus successfully conrming the modication of the silica surface.
3.2.4. Surface properties. While a porous structure was observed for SiPy, it was of interest to determine its eventual mesoporous character by nitrogen adsorption/desorption (Fig. 3). Such a character was conrmed by a type IV isotherm, and a visible hysteresis loop, indicating a H2-type prole based on theIUPAC classication. 55 The surface area for free silica SiG is 305.21 m 2 g À1 with a pore volume of 0.770 cm 3 g À1 . The BET surface area of SiEpand SiPy decreased to 277.08 and 261.33 m 2 g À1 , respectively, due to pore blocking by the chemical bonding of free silica with organic functional groups. It means that part of the pores were blocked by organic groups, which led to a decrease in the size of the pores. The total pore volume of free silica was 0.770 cm 3 g À1 and it decreased to 0.68 and 0.66 cm 3 g À1 for SiEp and SiPy, respectively.
3.2.5. Thermogravimetric analysis (TGA). The thermal stability of SiG, SiEp, and SiPy was examined by TGA analysis (Fig. 4). SiG showed a weight loss of less than 3.15% from 25 to 110 C due to the evaporation of water; the second step of 5.85% from 110-800 C, resulting from the condensation of the silanol groups. 56,57 SiEp also exhibited two decomposition stages at 1.34% from 25-110 C and 10.8% from 200 to 800 C due to the desorption of water and the degradation of organic moieties graed to silica. Therefore, the two distinct weight loss steps of SiPy represent a loss of 4.13% in the 25-100 C range and 12.4% in the interval of 270-800 C, which were attributed to the vaporization of water and degradation of organic compounds.

Effect of contact time and adsorption mechanism.
The study of the impact of the contact time is very important for the investigation of metal ions removal. For this purpose, the time of adsorption of Cu(II), Cd(II), Cd(II), and Pb(II) was varied from 5 to 35 min. From Fig. 6, the adsorption capacity increased rapidly during the rst 15 min and then reached equilibrium; this indicates the high dispersibility of the organic group receptor in the aqueous phase, and likely stems from the rapid interaction between the individual metal ions and the active sites of the adsorbent.
The potential mechanisms controlling the sorption of Cu(II), Zn(II), Cd(II), and Pb(II) by SiPy were studied using rst, secondorder, and intra-particle diffusion models. The linear form of the three models can be calculated as eqn (2)       Pseudo-rst-order model: where q e (mg g À1 ) and q t (mg g À1 ) are the amounts of adsorbate at equilibrium and at time t (s), respectively, and K 1 (min À1 ) is the rate constant of the rst-order adsorption.
Pseudo-second-order model: where K 2 (g (mg min À1 ) À1 ) is the second order rate constant of adsorption.
Intra-particle diffusion model: where k pi (mg g À1 min À1/2 ) is the intra-particle diffusion rate constant, and constant C i indicates the thickness of the boundary layer. The Cu(II), Zn(II), Cd(II), and Pb(II) experimental adsorption rate data were tted to the three kinetic models. All the kinetic data of adsorption onto SiPy, calculated from the related linear tting curves, are shown in Fig. S1 and S2. † The corresponding parameters by the pseudo-second-order kinetic model are listed in Fig. 6 and Table 1. The values R estimated from the pseudosecond-order kinetic model were signicantly higher than those obtained from the pseudo-rst-order kinetic model. The results suggested a better tting of the pseudo-second-order, assuming that the adsorption was a chemisorption process.
Then, it is important to further study the intra-particle diffusion model, which has been widely applied to provide the rate-controlling steps affecting the adsorption mechanism. 60,61 Fig. S3a-d † shows the plot of q e vs. t 1/2 accompanied by two distinct stages with different line slopes. Lines do not pass through the origin, indicating that the mechanism is complex and that intra-particle diffusion is not the only step controlling the adsorption process but that two steps occurred during the adsorption process. In this case, both external and internal diffusion can be involved in the adsorption mechanism. The rst step corresponded to the instantaneous adsorption or external surface adsorption and the second to the progressive adsorption or intra-particle diffusion stage. Table 2 shows that the diffusion rate constants decreased following the order k p1 > k p2 . The high slopes of the rst step indicate that the removal rate of Cu(II), Zn(II), Cd(II), and Pb(II) ions is higher at the beginning of the process due to the availability of a large number of active sites on the adsorbent surface. The lower slopes of the second parts are due to the decrease in the concentration gradient, which makes the diffusion of Cu(II), Zn(II), Cd(II), and Pb(II) ions into the mesopores of the adsorbent slower, thus leading to a low removal rate. As the data showed, intra-particle diffusion was the real rate-controlling process of the adsorption process.
3.3.3. Inuence of initial concentration and adsorption isotherms. The impact of varying concentration of metal ions has been evaluated under optimal conditions. Fig. 7 indicates that the adsorption efficiency of all the metal ions increased with the increase in the initial concentration of Cu(II), Zn(II), Cd(II), and Pb(II). These very high adsorption capacities can be justied by the available adsorption sites on the surface of the adsorbents, which are easily occupied by the metal ions. But the number of active sites on the SiPy surface is limited when they are fully occupied. Thus, saturated adsorption would be achieved at that time. 62 The equilibrium adsorption of the metal ions on SiPy was in the order Cu(II) > Zn(II) > Cd(II) > Pb(II) (Fig. 7).
The adsorption mechanism of metal ions and the SiPy adsorbent was investigated by Langmuir, Freundlich, Dubinin-Radushkevich (D-R), and Temkin isotherm models, 63,64 which are recalled below: Langmuir model: C e /q e ¼ C e /q m + 1/qK L where q e (mg g À1 ) and q (mg g À1 ) represent the equilibrium and the saturated adsorption capacity, respectively. C e (mg g À1 ) is the concentration of metal ions in solution at equilibrium, q m (mg g À1 ) is the theoretical saturation adsorption capacity, and K L (L mg À1 ) is the equilibrium Langmuir constant. Freundlich model: Where K F is the Freundlich constant and n is the value used to suggest the heterogeneity of the interface. Dubinin-Radushkevich (D-R) model: ln(q e ) ¼ ln(q m ) À b3 2 (7) where ß (mol J À1 ) 2 is a constant related to the mean free energy E of adsorption (E ¼ (2b) À0.5 ), and 3 (J mol À1 ) is the Polanyi potential related to the equilibrium concentration (3 ¼ RT ln(1 + 1/C e )). Temkin model: where b t (J mol À1 ) and A t (L mg À1 ) are the Temkin isotherm constant, T denes the Kelvin temperature (K), and R (8.314 J mol À1 K À1 ) denes the universal gas constant. The plots of the linear tting curves of the Langmuir, Freundlich, D-R, and Temkin isotherm mod-els for the adsorption of Cu(II) Zn(II), Cd(II), and Pb(II) onto the surface of SiPy are presented in Fig. S4-S7. † The experimental data and the tted curves for the single-metal system are shown in Fig. 7 with the corresponding parameters listed in Table 3.
It is obvious that the R 2 values are higher for the Langmuir model, thus conrming that the adsorption process belongs to monolayer adsorption. The maximum adsorption capacity is found for Cu(II) with 95.60 mg g À1 , a high value that shows that the SiPy adsorbent could be used potentially for the adsorption of Cu(II) from industrial wastewater and river waters.
All thermodynamic parameters are listed in Table 4. The negative values of DG suggest that the adsorption process of SiPy for Cu(II), Zn(II), Cd(II), and Pb(II) is thermodynamically favorable and spontaneous. Meanwhile, the positive value of DH indicates that the adsorption process has an endothermic nature. On the other hand, the obtained negative values of DS points out an increase in the randomness and disorder at the surface of the adsorbent Fig. 8. 3.3.5. Adsorption selectivity for Cu(II). The adsorption selectivity of SiPy was examined using aqueous solutions containing Cu(II), Zn(II), Cd(II), and Pb(II) with optimum concentration of each metal ion (180.66 10 À3 , 140.10 10 À3 , 99.70 10 À3 , and 95 63 10 À3 g L À1 for Cu(II), Zn(II), Cd(II), and Pb(II), respectively). According to Fig. 9, SiPy shows excellent adsorption selectivity for Cu(II) ions. Furthermore, given this selectivity, our adsorbent could be tested for its applicability in the removal of Cu(II) from river waters, which is discussed in the next section.
3.3.6. Desorption and recycling. As adsorbents, regeneration is of signicant importance for any application that guarantees economic perspectives and durability of operation in practice. The adsorbent was subjected to ve cycles of Cu(II) adsorption using HCl (2 mol L À1 ) as the eluent. Aer ve regeneration cycles, the adsorption efficiency of the adsorbent for Cu(II) can still reach 93.81% (Table 5). This result conrms the stability of the prepared SiPyand establishes the feasibility of the high regenerative capacity of the adsorbent.
3.3.7. Application in real water treatment. In order to test its feasibility in a real application, an adsorption experiment was carried out on real water samples originating from (i) Ghis river (located next to Al Hoceima) where: pH ¼ 7.7, total dissolved solids (TDS) ¼ 1297 mg L À1 and conductivity s ¼ 1733 mS cm À1 ; (ii) Touissit-Boubekker river (in the Jerada-Oujda region) where: pH ¼ 7.1, TDS ¼ 2031 mg L À1 and s ¼ 2301 mS cm À1 . The adsorption efficiency of SiPy (10 mg) was investigated under optimal conditions by the batch method using 10 mL of the river water. The percentage removal efficiency of Cu(II) ion was found to be high, up to 92% and 94% from Ghis and Touissit-Boubekker rivers, respectively, for SiPy (Table 6). Thus, SiPy is not only an economic adsorbent but it is also potentially feasible to be transformed to a high value-added product, i.e., a material for larger industrial application for extracting Cu(II) from wastewater.
3.3.8. Comparison with similar adsorbents. Table 7 shows the comparison of the adsorption capacity of SiPy for Cu(II) with other literature adsorbents. It is clear that SiPy has a higher adsorption efficiency for Cu(II) ion (90.25 mg g À1 ).

Adsorption mechanism
3.4.1. FTIR technique. The organic fraction of the surface of the material plays an important role in the removal of metal ions. The FTIR spectra of SiPy before and aer the adsorption of Cu(II) indicate that the absorption peak at 1462 cm À1 belonging to the N]C group disappeared aer the adsorption of Cu(II)  (Fig. 10). 77 The results demonstrate that the N]C group exhibits marked binding of Cu(II).

Theoretical investigations.
Theoretical investigation was undertaken to get insights into the mechanism of adsorption of metal ions with SiPy. The following theoretical methods were used: DFT, QTAIM, LOL, NCI, and dual descriptor. 78 Weak interactions were found between oxygen and hydrogen atoms (O 11 /H 29 ), (O 11 /H 25 ), (O 22 /H 13 ) as well as a repulsive interaction within the pyridine ring (Fig. 11). Since nitrogen N 27 and O 24 do not interact, we can deduce that these atoms may be involved in coordination with metal ions to give the SiPy-metal hybrid. Oxygen O 11 only makes weak interaction with the hydrogen atoms so it can be still be bound to the metals.
Dual descriptors are a useful function used to reveal the reactive sites. Fig. 12 shows that N 27 , O 11 , O 24 are available for coordination with metal ions. Actually, NCI and dual descriptors conrm that these three are responsible for forming the SiPy-metal hybrid. An optimization by the DFT(B3LYP) method afforded all possible structures of (Cu(II)-SiPy, Zn(II)-SiPy, Cd(II)-SiPy, and Pb(II)-SiPy (Fig. 13)). The bond lengths between the metal and the neighboring atoms as well as the coordination type is also given. The optimized (M(II)-SiPy) structures form stable complexes but these complexes do not have the same stability, given the bond lengths and the type of coordination (Fig. 13). For instance, the bond lengths of Cu(II)-SiPy (Cu 33 -O 12 ¼ 1.95Å) are smaller compared to those for Zn(II)-SiPy (Zn 33 -O 12 ¼ 2.08Å) and even smaller for Cd(II)-SiPy Cd-O 12 ¼ 2.31Å) and Pb(II)-SiPy (Pb 33 -O 12 ¼ 2.47Å).
In order to better understand metal coordination, we have used the NBO (natural bond orbital) 79  The values of E (2) are useful to indicate the binding strength, which is found to be higher for the Cu(II) complex with a charge transfer LP(O 12 ) / LP*(Cu(II)) of 16.63 kcal mol À1 compared to LP(O12) / LP*(Zn(II)) ¼ 16.11 kcal mol À1 and LP(O12) / LP*(Zn(II)) ¼ 6.03 kcal mol À1 for Zn(II) ( Table 8). For the Pb(II)-SiPy complex, a lower E (2) value was found, indicating a low interaction strength of Cu(II)-O10. Thus, the computational investigation completely replicates the experimental selectivity (Fig. 9). 80 All the interatomic areas have positive V 2 r cp values, and a low value of the electron density, r cp z 0 (Table 9), which shows that the interatomic surface cps has identied noncovalent bonds. Thus, the involved bond in coordination is either ionic or of van der Waals type. However, the value of the electron density of the Cu(II) complex is r cp ¼ 0.08 (Cu 33 -O 10 ), which is higher compared to the ones found for the Zn(II) complex with r cp ¼0.05 (Zn 33 -O 10 ), as well as for the Cd(II) complex with r cp ¼0.02 (Cd-O 10 ) and the Pb(II) complex. Thus, the stability of the four complexes follows the order Cu(II) > Zn(II) > Cd(II) > Pb(II).
The structures of the complexes shown in Fig. 14 show the BCP point for the (M(II)33-N10, M(II)33-O12, M(II)33-O17, and M(II)33-O19), which conrms the stability of the complexes. In the contour map, the blue color represents a low electron density region, whereas the red color shows a high electron density. The purple color means the absence of electron density (valence electrons). Such a color is not found for the Cu(II) complex, whereas a small area is detected for the Zn(II) complex. However, a larger area is found for the Cd(II) complex, and the Pd(II) complex has a much larger purple area. Thus, the stability of our four complexes is conrmed as follows: Cu(II) > Zn(II) > Cd(II) > Pb(II).

Conclusion
In conclusion, a new simple and low-cost method for the highly efficient cleanup of heavy metal ions in water has been presented, using silica organically modied with pyridine, which has been synthesized and fully characterized. The maximum sorption capacity aer reaching the equilibrium (30 min) was 90.25, 75.38, 55.23, and 35.12 mg g À1 for Cu(II), Zn(II), Cd(II), and Pb(II) respectively, with an optimal sorption pH of 7. The adsorption kinetics process is dominated by the pseudosecond-order model, which indicates a homogeneous character. The adsorption properties indicated monolayer adsorption, chemisorption binding mechanism, and an endothermic and spontaneous process. The SiPy adsorbent was able to maintain 93.81% of Cu adsorption capacity even aer ve cycles of adsorption and desorption experiments. A computational study suggested that the removal of the metal ions is the synergistic effect of C]N, R-O-R, and R-OH groups functionalized onto the SiPy material. The C]N group plays a major function during the complexation of metal ions. Furthermore, the computational investigation conrmed the affinity of our hybrid material for Cu(II). The results suggested that the SiPy adsorbent displays great advantages of high adsorption capacity toward Cu(II), rapid response, high selectivity, and good reusability, which makes it as a good candidate for wastewater treatment applications.

Conflicts of interest
The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to inuence the work reported in this paper.