Sol-gel silica doped with 3-(2-naphthoyl)-1,1-dibutylselenourea, an efficient precursor for removal of Pb(II) and Zn(II) from water samples

ABSTRACT 3-(2-naphthoyl)-1,1-dibutylselenourea was prepared and Sol-gel silica was doped with it. The selenourea organic derivative was characterised in solid state by single-crystal X-ray diffraction, and doped sol-gel materials were characterised by FT-IR and SEM. The doping results were verified from their characteristic prominent bands v(cm−1) = 1753 and 1199 predominantly assigned to C = O and C = Se, respectively. The processed sol-gel was used as adsorbent to investigate its efficiency for removal of M(II) ions from aqueous solution using batch techniques (M = Pb and Zn). The porosity study reveals that the doped sol-gel contains pores which are responsible for the adsorption of metal ions, besides pores present in the material, the ligand possesses active sites (-NH, = O, = Se) readily available for coordination with metal ions, and the material thus acts as efficient chelating agent. Adsorption kinetics, isotherm, effect of equilibration time, initial concentration of adsorbate, and pH on the metal extraction were studied and conditions were optimised. Metal remediation capacity of the hybrid material i.e. sol-gel is pH dependent, and maximum removal was obtained at pH 5 (Pb) and pH 6 (Zn) with in 35 and 15 minutes, respectively. The prepared adsorbent shows rapid equilibrium and enough stability at elevated temperature in the given medium. Desorption of metal ions was carried out in 0.1 M HNO3 solution, and thereafter, sol-gel silica adsorbent was successfully regenerated and reused.


Introduction
Heavy metals are toxic and their presence in small amount contaminate water easily.Metals are metalloid having relatively high density, persistent in the environmental segments and are toxic at low concentration [1,2].The concentration of such metals is increased to a considerable level due industrialisation and urbanisation [3].For sensing and remediation of toxic ions in various environmental segments, several efforts are underway to control their adverse effects in a timely manner [4].Lead (Pb(II)) is considered as one of the most serious heavy metals and has been actively associated with various human activities for the last several centuries.The increasing concentration of Lead in water may have different effects i.e hypochromic anaemia with basophilic stippling of the erythrocytes [5].Environmental toxicants, particularly heavy metals including Pb(II), are potentially preventable exposures that may explain population variation in cardiovascular disease rates.Exposure to the metal under discussion has also been reported to be a source of incidence coronary heart disease, stroke and peripheral arterial disease [6].High lead levels in children are the main source of diseases like decreased play activity, low intelligence quotient and poor school performance [7].Besides these disorders in human, donana mining catastrophe in 1998 was caused by acid pyrite sludge which was rich in Zn and Pb [8].Excess concentration of zinc in the body becomes toxic and cause various ailments.Major industrial sources of Zinc are electroplating and ore processing, and drainage from both active and inactive mining operations [9].Furthermore, Zinc is an important component of brass, bronze, die casting metal, other alloys, rubber and paints [10].Zinc is one of essential trace elements, and the human body has effective mechanisms, both on body and cellular levels, to maintain homoeostasis over a broad exposure range; however, it is reported that the excess of Zinc in body may cause deficiency of copper [11,12] as has been reported in several mammalian species [13,14].
Different methods are used to eliminate excess concentration of toxic elements from the environment.Some of these remediation techniques are time consuming and cannot be applied under some conditions [15].Chemists are trying to use cost effective, efficient adsorbent materials with preferably high selectivity for removal of toxic metals ions from various environmental samples.Activated carbon [16], metal oxides [17], minerals, ion-exchange and chelating resins are some of the adsorbents, which have been used for removal of unwanted metal ions [18].Thiourea derivatives are efficient coordinating reagents to a number of metal ions and have been reported as metal ions sensors [19][20][21].Selenourea derivatives are also accessible in laboratory and have a wide ranged applications including their chelates formation with soft transition metal ions [22][23][24][25].
Traditional sorbents suffer from some limitations such as low binding strength, poor selectivity, and low resistance towards chemicals, heat and radiations.Silica gel has been one of the most widely used solid substrates [26] because it does not swell, has good mechanical strength and can sustain a wide range of high temperature, the groups according to the requirements, can be introduced on silica gel.Surface modifications of silica gel is an easy and commonly applicable approach which is attained as a result of covalent interaction between the two materials [27,28].Recently, highly effective silicabased sorbents have been prepared by synthesising mesoporous silica followed by covalent binding of a surface monolayer of ligand [29].To avoid the complexity and high cost associated with the covalent binding method, a simple method such as sol-gel method is an efficient method, which leads to solid sorbents, in which the complexing or chelating reagents are doped in porous substrates [30].Literature contains many reports related to hybrid silica materials [31][32][33], but we did not find studies related to removal of toxic metal ions with the help of selenourea doped material.This article deals with the mesoporous composite adsorbent based on Sol-gel silica doped with selenourea, which has been actively used for removal of selected metal ions (Zn and Pb) from aqueous medium.The surface and active hydroxyl sites of mesoporous silica establish strong H-bonding and dispersive interactions with 3-(2-naphthoyl)-1,1-dibutylselenourea, affording formation of stable mesoporous composite adsorbent with doped ligand i.e. 3-(2-naphthoyl)-1,1-dibutylselenourea.The composite adsorbent is reversible after several chemical treatments and is reusable for several cycles with negligible loss of efficiency, has the potentials to serve effectively in on-site analysis as well as in removal of Zn and Pb ions.

Chemicals and reagents
The standard Schlenk techniques were used to carry out all preparative works under dry nitrogen atmosphere.Chemicals used in this study were mostly obtained from Sigma-Aldrich and were used without further purification.Double distilled water with pH 6.95 ± 0.05 and density 0.998 g/cm 3 was used throughout.

Instrumentation
Infrared spectra were obtained on a Specac single reflectance ATR instrument (4000-400 cm −1 ).X-ray powder diffraction patterns were obtained using a Bruker D8 AXE diffractometer (Cu-Kα).The samples were scanned between 20 and 80 degrees in a step size of 0.05 with a count rate of 9 sec.Thin films were carbon coated using Edwards E-306A coating system before carrying out SEM analysis (performed using a Philips XL 30FEG instrument).Metal analysis was carried out by the inductively coupled plasma-Mass spectrometry, Model HORIZON, FISSION.
Single-crystal X-ray crystallographic measurements were carried out using graphite monochromator Mo/K α radiation on a Bruker APEXII diffractometer.The structures were solved by direct methods and refined by full-matrix least-squares on F 2 .All calculations and refinements were carried out using the SIR97 [34,35], SHELXL97 [36], WinGX [37] and PLATON [38].All non-hydrogen atoms were refined with anisotropic atomic displacement parameters.Hydrogen atoms were placed in calculated positions and were assigned isotropic atoms.

Preparation of the selenourea
According to literature procedure [39], solution of KSeCN (0.01 mmol, 1.53 g) in acetone (25 mL) was prepared, and to this solution, 2-naphthoyl chloride (0.01 mmol) was slowly added.The reaction mixture was stirred for ca.30 min.To the same reaction mixture, dibutyl amine (0.01 mmol) was added and stirring was continued for further 30 min.The reaction mixture was poured into 0.1 M HCl (ca.200 mL), and the resulting precipitates were separated by filtration, washed with copious amounts of water and dried in open air.Crystals of the resultant selenourea compound were obtained in ethanol.

Preparation of doped gel
Sol-gel was doped with 3-(2-naphthoyl)-1,1-dibutylselenoureaby mixing same amount of tetraethoxysilane and deionised water (40 mL each).To this solution, ethanolic solution of 3-(2-naphthoyl)-1,1-dibutylselenourea in the presence of NH 4 F (0.01 M, 0.37 g) was added.The mixture was kept undisturbed to stand for 3 days for gel formation.The gel was kept in oven at ca. 40°C, so that constant weight was achieved.After drying, it was crushed and strained.The strained gel was shacked with distilled water to get rid of free 3-(2-naphthoyl)-1,1-dibutylselenourea.Before use, the sorbent was soaked in buffer solution to allow the leachable reagent to filter out.The xerogel was also prepared without organic reagent in accordance with the reported procedure [29].

Sorption process
Sorption studies of Zn and Pb metal ions were carried out with doped sol-gel separately.Samples collected from various places were filtered with pretreated filter paper.Water samples were spiked with metals under study.To water sample, appropriate volume of stock solution of metal salt was added, and its pH was maintained using HNO 3 .An amount of 25 cm 3 of spiked water sample solutions was stirred for 50 min, at room temperature, with 0.2-0.5 g of doped xerogel.Pretreated filter paper was used to filter sample solution.The optimised samples were analysed for determination of metal ions using ICP-MS.
For sorption isotherm trial, the batch technique was used at room temperature (23 ± 1°C).Solutions of different concentrations containing Zn and Pb were prepared (at different pH) for adsorption studies.A volume of 10 mL of metal solution was taken in six ampoules, and 0.05 g of the doped sol-gel was added to each sample.For better sorption results, the pH and time was adjusted by shaking the mixture in a mechanical shaker.The concentration of adsorbed metal was determined by Atomic Absorption Spectrometer.The following equation (1) was used to calculate the percent removal of the respective metal from the given aqueous solution: Where, Ci = initial, Cf = final concentration of metal ion in solution.
Similarly, the adsorbed metal by doped sol-gel was calculated using the following equation ( 2): Where, Q e = Equilibrium sorption capacity, Ci = initial metal concentration, C f = Equilibrium metal concentration, V = volume of solution containing sorbate and W = mass of sorbent.
The molecular structure of NDS along with structurally important bond lengths and angles is shown in Figure 1, while the data pertinent to crystal structure refinements and determination is given in Table 1.Structurally analogous compounds to NDS have been reported in literature, which provide enough grounds for comparison of structural aspects [39].These compounds are efficient ligands with a variety of metal ions in a bidentate fashion through Se and O sites.[40,41].The caption of Figure 1 reflects selected structural parameters of NDS.The N1-C11 and N1-C12 bonds are shorter i.e. 1.399 and 1.403 Å, whereas the average C-N single  The intermolecular hydrogen bond of 2.493 Å between O1 and H17A is found, which is relatively stronger and stabilises the supramolecular structure of the compound.All bond angles are in their respective limits and geometry around C12 is trigonal planar; the oxygen and selenium atoms are slightly twisted, which makes the compound as a bidentate ligand with an appropriate metal ion and can afford six membered stable metallacycle.The structural features of NDS are well comparable with simple thiourea derivatives [42,43].

FT-IR findings of the xerogel and doped sol-gel
The FTIR spectra of undoped (a), ligand (b) and silica gel with doped ligand (c) are given in Figure 2. It is clear from spectrum that silica gel gives two bands v(cm −1 ) = 957) and 1067, which can be assigned to -C-C-bending movements, while the ligand shows two prominent bands v(cm-1) = 1753 and 1199, which are characteristic peaks correspond to C = O and C = Se, respectively [44,45].The spectra measure after doping the ligand onto the surface of silica gel (c), wherein the characteristic peaks of silica gel and selenourea appear, confirming the doped sol-gel.Once the gel gets dried, the interactions between selenourea and the silica gel become more stronger, which pushes the organic derivative into the free available space of the gel, and the hybrid structure is thus stabilised.

Thermal analysis of un-doped gel and doped gel
The TG analyses were carried out for sol-gel, and it was observed that the material is stable at relatively high temperature, which can be confirmed by the fact that the percent loss in total mass is negligible.The addition of selenourea to the silica gel increase the decomposition behaviour of the material.The TG/DTA profiles of the blank and doped gels clearly show variation with increased temperature.It was noted that the first loss in weight of blank sol-gel and doped sol-gel happens below 100°C; this is because of loss of moisture contents being adsorbed on the surface.However, extra weight loss was observed between 200 and 300°C in doped sol-gel.The organic part from the silica network showed decomposition at nearly 250°C that was responsible for the exothermic peak in DTA profiles of doped sol-gel.Whereas the loss of adsorbed water from silica surface was responsible for the endothermic peak below 100°C.The figures given below represent the DTA curves of sol-gel, and it displays that thermal decomposition of doped NDS take place in one step.

Porosity studies of undoped sol-gel, doped sol-gel and doped sol-gel after sorption of metal
The morphology of undoped and doped sol-gel are explained by SEM images listed, as in Figure 3(a-d).The porosity of the materials can clearly be seen in these images in addition to that the physiochemical properties of the same material are significantly changed by metalligand complexation.The SEM images, 5a and 5b, were taken at 100 and 2.0 μm for sol-gel (un-doped) that show suitably enhanced porosity for doping of the ligand.Porosity appears after doping the ligand onto the surface of the silica gel is shown in image taken at 1 μm (Figure 3(c) and (d).Images 3c and 3d taken at 1 μm present better comparison between the materials before and after metal-ligand complexation, respectively.There images give a straightforward difference in porosity silica gel after complexation with the ligand.White clusters covering small holes is enough justification of metal-ligand interactions.

Effect of pH on sorption study of metal ions
The results displayed in Figure 4 show the percent sorption of the respective metal ions in pH 1-10.Concentration, temperature and the amount of adsorbent were kept constant during the process.It was noted that with the increase in pH the sorption of the metals increased.This increase was observed up to pH 4. When the medium is basic or strongly acidic, the sorbent retained less number of metal ions; this may be at low pH the H + ions compete the metals ions, while high pH will activate silanol groups, which leads to  precipitate the metal ions as hydroxides.Comparing the metal ions at pH 5 for Pb(II), pH 6 for Zn(II) was chosen the optimal conditions for removal.While comparing the blank sol-gel and doped sol-gel, the intake of metal ions for doped sol-gel is relatively high.

Effect of equilibrium time on removal of metals ions
The effect of time on sorption process was examined for NDSSG (Figure S1), while the other parameters were kept constant.From the experiment, it is revealed that the rate of sorption is very high for Zn, and it reaches to the maximum in 15 min, while for Pb(II), it takes relatively longer time 35 min, so comparatively Zn occupy rapidly the active sites on NDSSG.After that an equilibrium is achieved, and no further increase is seen.So, the optimal contact time for maximum equilibrium sorption was found to be 40 min.

Effect of mass of sorbent on removal of metals ions
The efficiency of the sorbent is measured by its amount, so it is the important parameter of the process.To optimise the efficient amount of the sorbent, 0.01-0.09g of NDSSG was treated with 10 mg/L concentration of metal ions.The results are given in Figure S2; it can be seen that increasing the amount of sorbent from 0.025 to 0.1 g, the sorption of metal ions increase; however, reaching at 0.05 g the rate of sorption becomes constant.So, 0.08 g was selected the optimum quantity for absolute sorption.

Sorption studies
Sorption of Pb(II) and Zn(II) on NDSSG in mg/g was studied, while the concentration was changed from 15 mg/L to 65 mg/L under optimised conditions to get maximum removal of the metals ions (Figure 5).It was noted that sorption process is rapid at low concentration but become slow when the concentration is high.The equilibrium reaches to 60 mg/L for Zn(II), while for Pb(II), it is 70 mg/L.These results reveal that NDSSG is good chelating agent towards Pb(II), so the maximum removal for lead was 4.9 mg/g, while for Zinc it was 4.5 mg/g.The sorption mechanism was further explained by Langmuir and Freundlich isotherm (for more details see Table 2).

Sorption kinetics
The factors affecting the sorption processes and the mechanism are well explained by chemical kinetics.The physiochemical properties of sorbent are important along with other conditions such as temperature and pressure, which determine the nature of sorption process.The rate constant is an important information to evaluate the mechanism of sorption process.During the process of batch sorption, the adsorbate molecules diffuse into the interior of the porous adsorbent.The kinetic model for sorption of Pb(II) and Zn(II) was monitored by pseudo-first-order and second-order on doped sol-gel material.

Pseudo-first-order model
The pseudo-first-order equation is given as: Where q t (mg/g) is the amount of sorbate Pb(II) and Zn(II) adsorbed in time t, q e (mg/g) the sorption capacity at equilibrium, k r (min −1 ) the rate constant for pseudo-first-order model and t (min) is the time.By integrating equation at initial concentrations qt = 0 at t = 0 and qt = qt at t = t, the equation becomes; Sorption rate constant (k f ) and sorption capacity (q e ) for the sorption of Pb(II) and Zn(II) ions by doped sol-gel were calculated from the slope and intercept of the plot of log (q e -q t ) against t (Figure S3).The values of both rate constant (k f ) and sorption capacity (q e ) indicate the efficiency of sorbent for trapping of metals from aqueous solution.These values show that both metals don't follow pseudo first-order kinetics.

Pseudo-second-order model
The pseudo-second-order model can be represented by the following equation,  Where, k s (g/mg min) denotes rate constant of pseudo-second-order model.Integration of above equation gives new equation after applying boundary conditions, q t = 0 when t = 0 and q t = q t for t = t: Moreover, the initial sorption constant h (mg/g min), can be defined as The initial sorption rate h, the equilibrium sorption capacity (q e ) and the pseudo-order rate constant k s were obtained from the slope and intercept of the plot 't/q t ' against 't' for doped sol-gel sorbent (Figure S4).The values and graphs show the sorption is following the Pseudo-second-order kinetics for both Pb(II) and Zn(II) ions.

Sorption isotherms
Amount of adsorbed metal per unit mass of sorbent at constant temperature is related to the concentration of same metal in the equilibrium solution.Sorption isotherm is the term used to explain this relationship.This data is often expressed graphically by using equations, Freundlich and Langmuir isotherms.In this study, we applied the sorption data in the equations of two isotherms.The following equations are used to represent the Langmuir and Freundlich isotherms

Langmuir isotherm
In Figure S5, plot of C e /Q e versus C e gives straight line, indicating the applicability of the Langmuir isotherm for the system under consideration.It is clear from the obtained straight line that Pb(II) sorption is followed by Langmuir isotherm and Zin(II) removal data could not be fitted in Langmuir isotherm.The Pb(II) with slope Q 0 = 5.35 mg/g), intercept (i.e.K L = 0.06) and 0.99 value of correlation coefficient R. The values calculated from the slope and intercept of isotherm straight line equation are used to compare and correlate with sorption properties of doped sol-gel matrix.On comparison, it was found that Q 0 is greater than Q e that means sorption of Pb(II) on doped sol-gel follows monolayer sorption pattern, which further implicates that adsorbent surface is not fully covered.

Freundlich isotherm
Freundlich isotherm appears not to be equally effective for showing the data adjustment on both metals, as shown in Figure S6.The values calculated from the slope and intercept of isotherm straight line equation are used to compare and correlate with sorption properties of doped sol-gel matrix.On comparison, it was found that Q 0 is greater than Q e that means sorption of Zinc metal on organically doped sol-gel follows Freundlich sorption isotherm pattern, which further implicates that adsorbent surface is not fully covered.

Recovery and desorption of metal ions
The recovery of the sorbent material was carried out using solution of 0.1 M HNO 3 .The dry NDSSG was mixed with 20 mL of 0.1 M HNO 3 , and the mixture was stirred for 40 min.The amount of metal was examined by using ICP.When it was confirmed that no amount of the adsorbed metal is remaining on the sorbent material, the NDSSG was washed with deionised water.The material was further treated with NaHCO 3 solution to neutralise the remaining amount of acid, and the final material was dried at 50°C.The process resulted almost 100% elution of metal ions; the sorbent was used three times following the identical procedure.The residue of filtration obtained after sorption was treated with HNO 3 solution of specific concentration for the recovery of metal desorption.Desorption of the metal from 25 cm 3 sample solutions on the doped xerogel was carried out by heating the residue with HNO 3 solution for 10 min.The solution was filtered, and the filtrate was diluted to 25 cm 3 by dilution with nitric acid solution.The solution was screened for desorption of metal contents with the help of ICP-MS.

Conclusion
The sorption of Pb(II) and Zn(II) ions from aqueous medium using sol-gel doped with 3-(2-naphthoyl)-1,1-dibutylselenourea was successfully carried out.The sorption of both metal ions is greatly dependent on pH, adsorbate, initial concentration of metal ion and metal-adsorbent contact time.The functional groups of the prepared ligand, -NH, C = O and C = Se, play very important role in trapping metal through efficient complexation.Kinetic and adsorption isotherms were applied to discuss rate of adsorption, adsorption equilibrium and the potency of the adsorbent.The following main features were explored for the prepared adsorbent material.
The prepared adsorbent material possesses very interesting features, like the equilibrium is quickly achieved, it is stable at high temperature and is easily recyclable.These features determine the significance of the doped sol-gel for removing metal ions from the aqueous medium.This approach is promising to resolve the challenging issue of effective remediation and removal of heavy metals from environmental samples.

Figure 4 .
Figure 4. Sorption of metal ions on the surface of NDSSG as a function of pH (contact time = 60 min, solution volume = 20 mL, NDSSG dose = 50 mg, metal concentration = 10 mg/L).

Figure 5 .
Figure 5. Sorption of metal ions by changing the concentration of metal ions under optimised parameters.

Table 1 .
Data related to crystal structure determination and refinement of NDS.bond length fall in the range of ca.1.472 Å.The solid-state data indicate that lone pairs of nitrogen atoms get delocalised, and some double bond characters are produced between N and C11 and C12.All other structural features of the compound are within the expected range.

Table 2 .
Sorption kinetics of Lead and Zinc.