3-Mercaptopropyltrimethoxysilane Modified Diatomite: Preparation and Application for Voltammetric Determination of Lead (II) and Cadmium (II)

In this study, functionalized diatomite was prepared by grafting of 3-mercaptopropyltrimethoxysilane (MPTMS) to diatomite (MPTMS-diatomite).Thediatomitewith thermal treatment from 100 to 700Cwas functionalized byMPTMSunder dry and humid conditions. The obtained MPTMS-diatomite was characterized by X-ray diffraction (XRD), thermal gravity-differential scanning calorimeter (TG-DSC), and Fourier transformation infrared (FT-IR).The results showed that an increase in treatment temperature seems to reduce the loading of MPTMS onto diatomite. The humidity of diatomite was favorable for the grafting of functional groups on the surface. The possible mechanisms of MPTMS loading to diatomite (MPTMS-diatomite) were also proposed. The performance of a carbon paste electrode (CPE) modified withMPTMS-diatomite in the simultaneous determination of Cd(II) and Pb(II) ions was addressed.


Introduction
Heavy metals (Hg(II), Pb(II), Cd(II), Cu(II), Co(II), Ni(II), Zn(II), etc.), which are commonly found in wastewaters discharged from chemical manufacturing, mining, extractive metallurgy, nuclear, and other industries, cause danger to humans and aquatic animals and plants and therefore must be controlled urgently [1].The detection of heavy metal ions in water samples has received much attention over the past few years.The commonly used methods for metal analysis are atomic absorption spectrometry (AAS) and electrothermal atomic absorption spectrometry (ETAAS), inductively coupled plasma-mass spectrometry (ICP-MS), inductively coupled plasma atomic emission spectrometry (ICP-AES), and inductively coupled plasma optical emission spectrometry (ICP-OES) [2][3][4].These techniques can assist quantitative analyses of the total amount of toxic metals in water samples.However, disadvantages such as instrument costs and complex operation procedures may limit the use of these techniques.
Anodic stripping voltammetry (ASV) is a useful electrochemical method for detecting trace metals because of its wide linear dynamic range and low detection limit which results from a preconcentration step performed directly in the voltammetric cell.Several porous materials have been utilized as electrode modifiers with the aim of preconcentrating the analyte at the electrode surface via a selective reaction due to special function groups grafted to electrode surface [5][6][7][8][9][10].
Diatomite, which consists of a wide variety of sized diatoms in a structure containing up to 80-90% pores, is a pale-coloured, soft, lightweight sedimentary rock composed principally of silica fossils of aquatic unicellular alga [11].Diatomite has received considerable attention for its unique combination of physical and chemical properties including large porosity, fine particle size, and chemical inertness [12].Silanol groups often occur on the diatomite surface especially on the natural diatomite surface.The density of the silanol groups can be controlled by hydrating diatomite.When these silanol groups combine with organosilane through a series of chemical reaction, the functional groups at the terminal end of the organosilanes can be anchored to the surface of the diatomite to form surface activated products.The diatomite in combination with surface coated functional groups offers a potential materials for adsorption [13,14] and modifying electrodes to determine the dye [13].Recently, the usage of silicabased materials (especially organically modified silicates) for modifying glassy carbon electrodes (GCEs) applied to the voltammetric detection of metal ions such as Hg(II), Cd(II), and Pb(II) has been extensively reported [6,7].However the use of organosilane modified diatomite as an electrode modifier less pronounced so far [15].To the best of our knowledge, the use of a 3-mercaptopropyltrimethoxysilane (MPTMS) which grafted diatomite for modifying electrode in connection with the electrochemical analysis of Pb (II) and Cd (II) has not yet been reported.In this paper, the preparation of MPTMS modified diatomite was demonstrated.The effects of thermal treatment of diatomite and the humidity of diatomite on MPTMS functionalization and the possible proposed mechanisms of MPTMS grafting diatomite surface were discussed.The MPTMS-diatomite which was used to modify electrode for simultaneous determination of Pb(II) and Cd(II) in aqueous solution by differential pulse anodic stripping voltammetry method (DP-ASV) was also studied.

Materials. Diatomite sample was collected from Phu
Yen province, Vietnam, after removing organic stuff by repeated sedimentation and desiccation at 100 ∘ C. The 3mercaptopropyltrimethoxysilane (MPTMS, >99%, Merck) and chloroform (Merck) were used as functional reagent and solvent, respectively.The chemical composition of diatomite analyzed by EDS was listed in Table 1.
All working solutions of reagents were prepared from chemicals for analysis (Merck, Germany).Acetate buffer solutions (ABS) of pH = 4,5 were prepared from CH 3 COOH and CH 3 COONa (Merck).pH of the ABS was adjusted by adding 1.0 M nitric acid or 1.0 M sodium hydroxide solutions into the ABS and then checking by pH meter.Working solution of Cd(II) and Pb(II) was prepared daily from stock solution (1000 ppm, Merck, Germany).Nafion (C 7 HF 13 O 5 S⋅C 2 F 4 , Merck) was used as an adhesive and electric conductivity material.

The Functionalization of Diatomite with MPTMS (MPTMS-Diatomite
).The diatomite was calcinated at 100, 300, 500, and 700 ∘ C for 3 hours.The incorporation of MPTMS into calcinated diatomite was conducted under dried and humid conditions.For the dry condition, 2 grams of calcinated diatomite was mixed with 2 g of MPTMS and 36 mL of chloroform in a 250 mL flask covered with watch glass.After mixing the slurry for 4 days at ambient temperature by a magnetic stirrer, it was washed with 10 mL of chloroform and filtered through a Buchner funnel.The separated solid was dried at 70 ∘ C for 24 hours.The functionalized diatomite is denoted as 100D, 300D, 500D, and 700D.The number implies temperature at which diatomite was heat-treated prior to functionalization.For the humid condition, the calcinated diatomite was hydrated for 2 hours in a desiccator containing saturated NaCl solution at the bottom before functionalization.Two grams of hydrated diatomite was mixed with 2 g of MPTMS for two hours.The solid was filtered and the extra MPTMS was extracted with chloroform in a Soxhlet until the solution in flask was clear.The resulting samples are denoted as 100H, 300H, 500H, and 700H.With the aim of studying the applicability of the material MPTMS-diatomite, the use of MPTSE-diatomite as an electrode modifier was performed.

Preparation of Working Electrode-Modified Electrode.
The preparation procedure was carried out as [16].In brief, glassy carbon electrode (GCE) was first polished with 0.05 m Al 2 O 3 slurries on a polishing cloth and then rinsed ultrasonically with 2 M HNO 3 , absolute ethanol, and doubledistilled water.Subsequently, the GCE was electrochemically cleaned by cyclic potential scan between −1.5 V and +1.5 V in 0.5 M ABS pH 4.5 with the scan rate of 0.10 V s −1 .After that, the electrode was rinsed with double-distilled water.Nafion-MPTMS-diatomite composite was prepared by dispersing 5.0 mg MPTMS-diatomite powder into 1.0 mL Nafion (0.1 wt%) and sonicated.The GCE was coated with 5 L of the composite and dried at room temperature (∼25 ∘ C) (denoted as MPTMS-diatomite/GCE).

Voltammetric
Procedure.Solution under study (final volume of 10 mL) containing 0.1 M ABS (pH 4.5) and 100 ppb Cd(II) and 200 ppb Pb(II) was transferred into the electrochemical cell with the 3 electrodes.Then, voltammetric measurement was conducted as follows: (i) For surveying voltammetric characteristics of Cd (II) and Pb(II) on the MPTMS-diatomite/GCE: Cd(II) and Pb(II) was accumulated on the surface of the modified electrode at a potential of −1.2 V ( acc ) for an accumulated time of 60 s ( acc ).During this stage, the electrode was rotated at a constant rate of 1000 rpm.
After that, the electrode rotation was off.Then, cyclic voltammograms were recorded from −1.2 V to +0.2 V (forward potential scan) and subsequently from +0.2 V to −1.2 V (reverse potential scan) at a scan rate of 0.1 V s −1 .
(ii) For differential pulse anodic stripping voltammetric (DP-ASV) determination of Cd(II) and Pb(II) on electrode MPTMS-diatomite/GCE: during the DP-ASV procedure, the Cd(II) and Pb(II) were accumulated on the surface of the rotating modified electrode as done for the above procedure.After that, the electrode rotation was off for 10 s and then DP-ASV voltammograms were recorded from −1.2 V to +0.2 V at a scan rate of 0.02 V s −1 and suitable differential voltammetry technique parameters.During the potential anodic scan, Cd(0) and Pb(0) on the electrode surface were oxidized to Cd(II) and Pb(II).DP-ASV voltammograms of blank solution (without Cd(II) and Pb(II) and prepared from double-distilled water) were similarly recorded before each measurement.The Cd(II) and Pb(II) quantitation was done by standard addition method.

Apparatuses.
The phases were monitored by powder X-ray diffraction (XRD), recorded on 8D Advance Bucker, Germany with CuK  radiation in the range of from 20 ∘ to 60 ∘ with a scan step size of 0.01 ∘ and a scan step time of 0.04 s.The amounts of MPTMS incorporated into diatomite were evaluated by TG-DSC SETARAM under ambient atmosphere.Morphology of diatomite was observed by scanning electron microscopy (SEM.Jeole-3432) and transmission electron microscope (TEM) (JEOL JEM-2100F).Fourier transform infrared (FT-IR) spectra of the typical samples (1 wt% sample + 99 wt% KBr) were collected on Shimadzu FT-IR 8010M; the wavenumber range was 500-4000 cm −1 .A CPA-HH5 Computerized Polarographic Analyzer (produced by Laboratory of Computer Application to Chemistry, Institute of Chemistry, Vietnam Academy of Science and Technology) was used for voltammetry experiments.All measurements were done in the cell with three electrodes: a GCE with a diameter of 2.8 ± 0.1 mm used for formatting the modified electrode as working electrode, Ag/AgCl/3 M KCl as reference electrode, and a platinum wire as auxiliary electrode.All measurements were carried out at room temperature.

Preparation of MPTMS Modified Diatomite (MPTMS-Diatomite)
. Figure 1 shows XRD patterns of diatomite calcinated at 100 ∘ C, 300 ∘ C, 500 ∘ C, and 700 ∘ C. The raw diatomite (dried at 100 ∘ C) consisted of mainly amorphous structure.At 300 ∘ C, the characterized peak of quartz was observed indicating that the silica in amorphous form was crystallized to form quartz crystallites.The diffraction intensity of quartz phase increases with an increase in calcinated temperature.
Figure 2 shows SEM and TEM observation.Morphology of raw diatomite shows a cylindrical structure with a ca.length of 10-20 m and an internal pore diameter of about 2-3 m.There are nearly regular arrays of submicron pore in a diameter of ca. 100 nm in the wall.
In order to investigate the surface characteristics of raw and calcined diatomite, FT-IR analyses were carried out in the range 400-4000 cm −1 .The main absorption bands for raw diatomite, as shown in Figure 3, were observed at 466, 528, 690, 798, 914, 1022, 1153, 3622, and 3695 cm −1 .The absorption peaks around 466 and 528 cm −1 represent to the Si-O-Si bending vibration [17].The 690 and 798 cm −1 bands reflect SiO-H vibration while the 910 cm −1 band corresponds to the Si-O stretching of silanol group.The bands at 1153 and 1022 cm −1 are strong due to stretching vibration of the siloxane (-Si-O-Si-) groups [18].The band at 1639 cm −1 corresponds to H-O-H bending vibration of water.The bands at 3695 and 3622 are attributed to the internal silanol due to strongly interacting vicinal OH [19].The surface is also predominantly covered by physically adsorbed water in the pore and water bounded to the surface hydroxyl groups via H-bonds [19,20].By removing adsorbed water, upon heating the diatomite at 700 ∘ C, the OH groups on diatomite surface may be removed.The disappearance of the OH groups may have a significant effect on MPTMS grafting to diatomite, which is dependent on not only the pore structure of diatomite but also the surface functional groups.
Figure 4 shows DSC and TG diagrams of diatomite functionalized by MPTMS at dried conditions.The DSC and TG curves were similar for all samples.The exothermic peak, together with no any ignition loss at around 300 ∘ C, should be attributed to the crystallization of amorphous silica to quartz.The result also agrees with the fact that the crystallization of quartz was detected at 300 ∘ C by XRD analysis.However, even for sample 300D-700D which was calcinated at 300 ∘ C for 3 hours before functionalization, this peak is also observed.This can be explained by a reconstruction to form amorphous silica during silane treatment.The exothermic peaks and ignition losses at around 520 ∘ C should be assigned to the decomposition and oxidation of MPTMS incorporated into diatomite.The hydrolysis of MPTMS into silanols occurs in water.The silane incorporated into diatomite is due to the condensation of silanols of diatomite and silane.In this work, it is suggested that the silane is strongly bonded to diatomite surface as silane is not removed by washing chloroform, dispersed in water, or heated at 100 ∘ C. Then the amount of MPTMS incorporated into diatomite is proportional to the amount of ignition loss in range 100 ∘ C to 700 ∘ C calculated by TG.Therefore, the ignition loss (100-700 ∘ C) was assumed as MPTMS loading to diatomite.The amount of MPTMS seems to increase slightly from 5.6% to 6.3% for D100 and D300 and decrease remarkably with a further increase in thermal treatment temperature (see Table 2).
Figure 5 shows TG-DSC diagrams of functionalized diatomite under humid conditions.The curves were similar to all samples.The thermal behaviors were also similar to the case of samples under dried conditions except for ignition loss together exothermic peaks at around 300 ∘ C observed for most cases.From Table 2, the amount of MPTMS incorporated into diatomite seems to be higher in the samples in dried conditions and the diatomite thermal treated at 100 ∘ C-300 ∘ C is most favorable for MPTMS functionalization.
There are two types of silanols, isolated and H-bonded silanols on diatomite surface [20].At room temperature, both types of silanols are H-bonded with water.With an increase in temperature, dehydration occurs.At first, the desorption of water and the exposure of more and more isolated silanols were favorable for the silanols of silane adsorbing onto diatomite surfaces.This accounts for the fact that the high amount of MPTMS was incorporated into diatomite calcinated in the range of 100-300 ∘ C. The calcinated diatomite exposed to water-saturated medium could create more silanols, which consequently adsorb MPTMS more strongly than the diatomite under dried conditions.As the temperature increases to more than 300 ∘ C, the silanols begin to condense to form siloxane bridges that are not favorable for coupling reactions.In the case of hydrated diatomite, two decompositions of silane incorporated into diatomite are observed at around 320 ∘ C and 520 ∘ C in TG-DSC curves instead of only 520 ∘ C in the case of the dried diatomite.Based on the results reported by Johansson et al. [21] about adsorption of silane coupling agents onto kaolin surfaces, we suggest two possible mechanisms by which MPTMS react with diatomite surface.The first proposed mechanism involves four steps: (1) MPTMS was converted to the reactive silanol formed by hydrolysis (2) Condensation of the organosilane to oligomers is presented:  (3) Formation of hydrogen bonds between the oligomers and the OH groups on the diatomite surface is presented: (4) Finally, a covalent linkage is formed under drying The second proposed mechanism involves only two steps: (1) MPTMS is converted to the reactive silanol form by hydrolysis (R: (2) The silanol groups react directly with hydroxyl groups on the diatomite surface It is assumed that silane incorporated in diatomite by the first mechanism is more stable than one by the second mechanism.It means that it will be decomposed at higher temperature with the silane incorporated by the latter.For silane treatment with dried diatomite, the only ignition loss and exothermic peak at around 520 ∘ C could be assigned to the decomposition of silane grafting through the first mechanism.When diatomite is hydrated before silane treatment, the incorporation of MPTMS might occur through both mechanisms.Then, ignition loss and exothermic peak at around 300 ∘ C in TG-DSC for 100-700H might be assigned to decomposition of silanes in which MPTMS is grafted through the second mechanism.
The effect of hydrated times on the loading of MPTMS was also investigated.The diatomite calcined at 100 ∘ C for 3 hours was hydrated in a desiccators containing saturated NaCl solution for 0, 1, 2, 3, 6, 8, and 10 h.The functionalization was conducted as in the case of humid conditions.The amount of MPTMS incorporated into diatomite estimated from TG analysis was listed in Table 3.
As can be seen, the amount of MPTMS peaked around 3 hours and then decreased as the time of hydrate was prolonged.Those results provide further evidence that water moisture favors the coupling reaction of silanols of silane and diatomite.However, the hydrated time prolonged means that there was too much water in the channel of diatomite, and the silane molecules will be significantly hydrated and condense each other instead of being bound to the wall and be removed under chloroform reflux condition; that is, the loading amount will decrease.Figure 7 shows TEM and SEM observation of MPTMS modified diatomite.The inclusions were highly dispersed on the diatomite surface (Figure 6(a)).However, pores in diatomite surface around 40-50 nm were also observed by TEM (Figure 6(b)).This means that the MPTMS-diatomite is combining highly porous structure with the binding properties of the thiol groups, leading to a high number of reactive sites that are readily accessible for the target analyte (Pb(II) and Cd(II)).

Voltammetric Characteristics of Cd(II) and Pb(II) on MPTMS-Diatomite/GCE and Limit of Detection (LOD).
Figure 7 shows anodic voltammetric curves using electrodes modified by MPTMS-diatomite containing different MPTMS amounts.The percentage number in parenthesis presents the ignition loss which is assigned to as MPTMS amount.Cd(II) and Pb(II) ion showed anodic peaks at approximate −0.806 and −0.584 V in 0.1 M ABS (pH 4.5), respectively.For bare GCE and Nafion/GCE the broad oxidation peaks were observed.The intensity of anodic peak increases with an increase in MPTMS amount from 6.59% to 9.88%.A well-defined oxidation peak and high current response were observed using the MPTMS (9.88%)-diatomite electrode with highest MPTMS amount.Then, the MPTMS-diatomite with the highest amount of incorporated MPTMS was selected for modifying electrodes.
K 3 [Fe(CN) 6 ]/K 4 [Fe(CN) 6 ] was selected as a probe to evaluate the performance of the prepared electrodes by cyclic voltammetry.Figure 8 presents the electrochemical responses of bare GCE and MPTMS-diatomite/GCE in 1.0 mM K 3 Fe(CN) 6 /K 4 Fe(CN) 6 (10 −3 M and 0.1 M KCl) solution.It is clear that the cathodic-anodic curves using electrode modified by MPTMS-diatomite were broad compared to that of bare GCE, indicating that the electron-transfer kinetics of Fe(CN) 6 3− /Fe(CN) 6 4− are gradually prevented, which is probably related to the higher ohmic drop value in the electrode process [22].
The redox behavior of Cd(II) and Pb(II) was investigated by cyclic voltammetry in a potential range from −1.2 to +0.2 V as shown in Figure 9.The Cd(II) and Pb(II)  anodic and cathodic peaks were observed for the diatomite-MPTMS/GCE; the peak separation of anodic and cathodic potentials for Cd(II) and Pb(II) were 0.226 V and 0.139 V, respectively.Therefore, the electrochemical process of Cd(II) and Pb(II) on the electrode surfaces was quasi-reversible.
The analytical signals for electrode surface were stable for at least 5 measuring cycles with the relative standard deviation (RSD) from 2.75 to 3.5%.These results indicated very good reproducibility for Cd(II) and Pb(II) detection with the developed electrode.The anodic peak current of Cd(II) and Pb(II) ( P ) was higher than the cathodic peak current, ca.1.5-fold and 16.5-fold, respectively.It means that the anodic curves can be used for analytical signals.
The thiol group binds Cd(II) and Pb(II) to surface complexes because of its high affinity to metal ions.The metals were accumulated in electrode due to reduction reaction and then dissolved in solution through oxidation reaction.The electrochemical reaction occurred as follows [23,24]: (1) Electrochemical process of Pb(II) and Cd(II) on electrodes modified by MPTMS-diatomite was illustrated by Figure 10.
Figure 11 shows the DP-ASV of Cd(II) and Pb(II) with different concentrations.An obvious anodic peaks were observed at around −0.82 and −0.61 V.These observations can be attributed to the oxidation species of Cd(II) and Pb(II), respectively, which have been deposited into the surface of MPTMS-diatomite/GCE during reduction processes.The intensity of anodic peak increased with an increase in Cd(II) and Pb(II) concentration from 20 to 300 ppb.A calibration plot of the anodic current response versus Cd(II) concentration and Pb(II) is presented in the insets of Figure 11.The results show that the current peak response was linear to the Cd(II) concentration with a R 2 of 0.999 in the range of 20-300 ppb and the Pb(II) concentration with R 2 of 0.994 in the range of 20-150 ppb.The limit of detection (LOD) was calculated on the basis of (3.3 S  /b) criteria (S  represents the standard deviation of the intercept while b represents the slope of the calibration curve defined for the LOD concentration range (20-300 ppb)).The LOD for Cd(II) and Pb(II) calculated was 15.9 ppb and 6.9 ppb, respectively.
Table 4 shows a comparison of MPTMS-diatomite-GCE developed with other GCEs modified with other nanoparticles for simultaneous determination of Pb(II) and Cd(II).As seen in Table 4, the GCE modified with MPTMS-diatomite developed in the present work shows good LOD compared with other similar materials/GCE reported previously in the literature [21][22][23][24].This indicates that MPTMS-diatomite is a potential material for electrode modifiers.

Conclusions
The effects of functionalization conditions on the loading of MPTMS in diatomite were investigated.Diatomite from Phu Yen consists of mainly amorphous structure.The crystallization to form quartz occurs at more than 300 ∘ C. The diatomite with thermal treatment in the range of 100-300 ∘ C is favorable for functionalization.The humidity of diatomite also affects the functionalization level significantly.The MPTMS loading around 9.8% peaked as diatomite was hydrated for 3 hours.
We have demonstrated that MPTMS-diatomite is useful to prepare chemically modified glassy carbon electrodes.The electrode modified by MPTMS-diatomite exhibited potential for the use of the simultaneous determination of cadmium and lead by DP-ASV.The stripping peak currents of the two metal ions had linear relationships with the concentrations in the range of 20 to 300 ppb (i (A) = −0.0202+ 0.0036 C (ppb), R 2 = 0.9997) for Cd(II) and 20 to 150 ppb (i (A) = −0.1179+ 0.0069 C (ppb), R 2 = 0.9943) for Pb(II).The LOD for Cd(II) and Pb(II) calculated was 15.9 ppb and 6.9 ppb.

Figure 1 :
Figure 1: XRD patterns of diatomite calcined at different temperatures.

Table 2 :
The amount of MPTES incorporated into diatomite estimated by TG.

Table 3 :
The total amount of ignition loss of MPTMS-diatomite in the range 100 ∘ C−700 ∘ C.