Bentonite Functionalised with 2-(3-(2-aminoethylthio)propylthio)ethanamine (AEPE) for the Removal of Hg(II) from Wastewaters

Index

Index of figures

Index of tables

Index of schemes

Index of equations

Abstract

1 Introduction
1.1 General information
1.1.1 Mercury and resulting environmental problems
1.1.2 The removal of mercury by adsorption using clay minerals
1.2 Theoretical Background
1.2.1 Structure and properties of bentonite
1.2.2 Chelating agents
1.2.3 Characterisation techniques
1.2.3.1 Nuclear magnetic resonance spectroscopy (NMR)
1.2.3.2 X-ray diffraction (XRD)
1.2.3.3 Fourier transform infrared spectroscopy (FT-IR)
1.2.3.4 Thermogravimetric analysis (TGA)

2 Aims

3 Experimental
3.1 Material
3.1.1 Reagents and chemicals
3.1.2 Instruments and glassware
3.2 Methods
3.2.1 Synthesis
3.2.1.1 Purification of bentonite
3.2.1.2 Synthesis of AEPE
3.2.1.3 Preparation of functionalised bentonite
3.2.2 Characterisation
3.2.2.1 Characterisation of AEPE with NMR
3.2.2.2 Characterisation of the clay products from purification and modification
3.2.3 Adsorption of Hg(II) from aqueous solution
3.2.3.1 Effect of pH
3.2.3.2 Effect of contact time
3.2.3.3 Adsorption isotherm
3.2.3.4 Effect of ionic strength
3.2.3.5 Interfering ions
3.2.3.6 Effect of adsorbent dose and kinetics

4 Results and Discussion
4.1 Characterisation
4.1.1 ^[1] H and ^[13] C NMR spectra of AEPE
4.1.2 XRD pattern
4.1.2.1 XRD pattern of raw and purified bentonite
4.1.2.2 Comparison of XRD pattern of product from every step of modification
4.1.3 FT-IR spectra
4.1.3.1 Characterisation of purified bentonite with FT-IR
4.1.3.2 Comparison of FT-IR spectra of products from modification
4.1.4 TGA
4.1.5 BET
4.2 Adsorption of Hg(II) from aqueous solution
4.2.1 Effect of pH
4.2.2 Effect of contact time
4.2.3 Adsorption isotherm
4.2.4 Effect of ionic strength
4.2.5 Interfering ions
4.2.5.1 Cations
4.2.5.2 Anions
4.2.5.3 Heavy metal ions
4.2.6 Effect of adsorbent dose and kinetics
4.2.6.1 Effect of adsorbent dose
4.2.6.2 Kinetics

5 Conclusion and Outlook

6 References

7 Appendix

Abbreviations

Abbildung in dieser Leseprobe nicht enthalten

Index of figures

Figure 1. Schematic representation of the crystal structure of montmorillonite

Figure 2. Illustration of the sheet structure of a 2:1 montmorillonite (after Wypych and Satyanarayana, 2004)

Figure 3. XRD-pattern of (a) raw bentonite and (b) purified bentonite; * = quartz; I = impurity

Figure 4. XRD-pattern of (a) purified bentonite, (b) bentonite-NH2, (c) bentonite-NH-Br and (d) AEPE-bentonite

Figure 5. FT-IR spectrum (transmission, KBr pellet) of purified bentonite

Figure 6. FT-IR spectrum (Transmission, KBr-pellet) of (a) the purified bentonite, (b) bentonite-NH2, (c) bentonite-NH-Br and (d) AEPE-bentonite

Figure 7. TGA and DTGA curves of (a) purified bentonite, (b) bentonite-NH2, (c) bentonite-NH-Br and (d) AEPE-bentonite

Figure 8. BET plot for purified bentonite and AEPE-bentonite

Figure 9. Adsorption capacity of the AEPE-bentonite (grey bar) obtained from single batch compared to the adsorption capacity of purified bentonite (black bar)

Figure 10. Effect of pH on the adsorption of HG(II) ions by AEPE-bentonite and purified bentonite; initial Hg(II) concentration: 20 mg/L; adsorbent does: 0.01 g; contact time: 60 min

Figure 11. The effect of contact time on the adsorption of Hg(II) by AEPE-bentonite and purified bentonite; initial Hg(II) concentration: 20 mg/L; adsorbent dose: 0.01 g; pH:

Figure 12. The effect of contact time and initial concentration on the adsorption of Hg(II) onto AEPE-bentonite; adsorbent dose: 0.01 g, pH:

Figure 13. Equilibrium adsorption isotherm for the removal of Hg(II) from aqueous solution at a temperature of 24.5 °C

Figure 14. Langmuir adsorption isotherm for the removal of Hg(II) from aqueous solution at a temperature of 24.5 °C

Figure 15. Freundlich adsorption isotherm for the removal of Hg(II) from aqueous solution at a temperature of 24.5 °C

Figure 16. Effect of ionic strength on Hg(II) adsorption by AEPE-bentonite; initial Hg(II) concentration: 20 mg/L; adsorbent dose: 0.01 g; pH: 4.0; contact time: 20 min

Figure 17. Effect of different adsorbent dose on adsorption efficiency of Hg(II) onto AEPE-bentonite as a function of contact time; initial concentration of Hg(II): 100 mg/L, pH:

Figure 18. Pseudo-first order kinetics of Hg(II) ions uptake by AEPE-bentonite for different adsorbent dose

Figure 19. Pseudo-second order kinetics of Hg(II) ions uptake by AEPE-bentonite for different adsorbent dose

Figure 20. ^[13] C-NMR spectrum of AEPE

Figure 21. ^[1] H-NMR spectrum of AEPE

Figure 22. FT-IR spectrum (Transmission, KBr pellet) of the purified bentonite

Figure 23. FT-IR spectrum (Transmission, KBr pellet) of bentonite-NH

Figure 24. FT-IR spectrum (Transmission, KBr pellet) of bentonite-NH-BR

Figure 25. FT-IR spectrum (Transmission, KBr pellet) of AEPE-bentonite

Index of tables

Table 1. Chemical list

Table 2. Volumes and concentrations of the stock solution used to prepare the standard solutions for the calibration curve of Hg(II); standards were diluted with 1% HNO3 to 50 mL

Table 3. Pipettes and glassware used in this study

Table 4. X-ray data of raw and purified bentonite: literature values and measured values in this study

Table 5. X-ray data for the d001 value of the products of modification

Table 6. Characteristic wavenumber of stretching vibrations in FT-IR spectrum of purified bentonite: Literature and observed values

Table 7. Characteristic wavenumber of bending vibrations in FT-IR spectrum of purified bentonite: Literature and observed values

Table 8. Characteristic parameters obtained from Langmuir and Freundlich plot, where R^[2] is the correlation coefficient

Table 9. Value of separation parameter, RL, based on the Langmuir isotherm equation obtained

Table 10. Effect of cations on extraction of Hg(II) at a concentration of 0.1 mM by AEPE-bentonite

Table 11. Effect of anions on extraction of Hg(II) at a concentration of 0.1 mM by AEPE-bentonite

Table 12. Aqueous speciation reactions and complex formation constants of Hg(II) ions (Sarkar et al, 1999)

Table 13. Effect of heavy metal ions on extraction of Hg(II) at a concentration of 0.1 mM by AEPE-bentonite

Table 14. Characteristic parameters obtained from pseudo-first and pseudo-second order plot, where R^[2] is the correlation coefficient

Index of schemes

Scheme 1. Synthesis of 2-(3-(2-aminoethylthio)propylthio)ethanamine (AEPE)

Scheme 2. Reaction of the purified bentonite with 3-aminopropyltriethoxysilane to yield bentonite-NH

Scheme 3. Reaction of bentonite-NH2 with ethyl-2-bromopropionate to give bentonite-NH-Br

Scheme 4. Reaction of bentonite-NH-Br with AEPE to yield AEPE-bentonite

Scheme 5. Preparation of functionalised bentonite with AEPE

Scheme 6. pH dependent reaction of functional surface sites of bentonite (Toth, 2002)

Index of equations

Equation 1. Braggsche equation

Equation 2. Brunauer, Emmet and Teller equation

Equation 3. Specific surface area

Equation 4. Adsorption capacity

Equation 5. Langmuir equation

Equation 6. Freundlich equation

Equation 7. Separation parameter

Equation 8. Pseudo-first order equation

Equation 9. Pseudo-second order equation

Equation 10. Adsorption capacity at equilibrium and rate constant of pseudo-second order sorption as function of adsorbent does

Equation 11. Rate law for pseudo-second order kinetics

Abstract

In this study, natural bentonite clay (Cernic International Co., LTD) was first purified and then functionalised with the chelating ligand 2-(3-(2-aminoethylthio)propylthio)ethanamine (AEPE) to improve the adsorption capacity and selectivity towards Hg(II) ions. The surface modification was characterised with the help of powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), BET isotherm to determine the specific surface area while the thermal stability of the samples was studied using thermogravimetric analysis (TGA). FT-IR and TGA demonstrated the presence of the chelating ligand on the modified clay. XRD pattern indicated that the chelating agent AEPE was only grafted onto the surface of the purified bentonite, whereas the interlayer distance did not change. N2 specific surface area measurement also indicated the coverage of AEPE onto the surface of purified bentonite.

Adsorption of Hg(II) ions from aqueous solutions as a function of pH, contact time, initial concentration, ionic strength, interfering ions and adsorbent dose was studied. The adsorption process followed a pseudo-second order kinetics and monolayer adsorption. The adsorption of Hg(II) ions increased with increasing pH and reached a plateau value in the pH range of 4.0-8.0. The removal of Hg(II) was found to be higher than 99% at an initial concentration of 20 mg/L using adsorbent dose of 0.01 g. The presence of NaNO₃ as background electrolytes at concentration ranging from 0.01 to 2.0 M decreased the adsorption of Hg(II) ions. Furthermore, the adsorption capacity increased with increasing adsorbent dose. Sorption data analysis was carried out using Langmuir isotherm for the uptake of Hg(II) ions at a concentration range of 20-400 mg/L. The adsorption process was found to be favourable as the separation parameter is less than unity (R_L<1). The maximal adsorption capacity was found to be 48.69 mg/g obtained from Langmuir equation.

1 Introduction

1.1 General information

1.1.1 Mercury and resulting environmental problems

Mercury is a natural occurring liquid heavy metal, which refers to high-density metallic elements such as cobalt, copper, iron etc. By the decomposition of minerals in rocks, in the ground and by water and wind erosion, mercury is released to the nature (Siegel et al., 1975). Moreover, human activities have raised the natural concentrations decisively because of large application of mercury compounds in the industry, e.g. in the chlorine-alkali manufacturing industries for the production of chlorine and sodium hydroxide by means of mercury cathodes (Hylander and Meili, 2003), the paint and battery manufacturing industries and oil refinery. As a result, mercury is found in an increasing amount in the sewage of these branches of industries. This can lead to severe environmental problems if mercury is introduced into natural water sources without proper treatment.

Mercury compounds cause serious health damages. All mercury compounds are toxic for human (WHO, 2000) and in particular organic mercury compounds. The major effects of mercury poisoning manifest as neurological and renal disturbances as it can easily fit the blood-brain barrier and affect the foetal brain. High concentrations of mercury cause impairment of pulmonary function and kidney, chest pain and dyspnousea (Manohar et al., 2002). Mercury can also be accumulated in the food chain, e.g. in fish, and will be taken up with consumption, leading to poisoning. Therefore, the removal of mercury in water and wastewaters is important and necessary.

1.1.2 The removal of mercury by adsorption using clay minerals

Many applications are already set up for the removal of mercury out of wastewater as for example precipitation, coagulation and flocculation, solvent extraction, complexation, adsorption, filtration, membrane processes and activated carbon adsorption (Koene and Janssen, 2001; Manohar et al., 2002). However, most of these techniques have some disadvantages, which make the application sometimes problematic. To remove heavy metals by precipitation as hydroxide or sulphide compounds, the dosage necessary for sulphide precipitation is difficult to determine and the process requires a specific pH range to operate.

For coagulation and flocculation, the process is to combine colloidal particles into larger aggregates that can also adsorb dissolved organic and inorganic contaminants. The removal is facilitated by subsequent sedimentation and filtration processes. The most commonly used coagulants are aluminium or iron(III)-based salts (e.g., aluminium sulphate and ferric sulphate). However, the efficiency of this process is limited to a certain pH range depending on the salt used. Moreover, the coagulant containing metals must be filtered, resulting in additional costs (Stumm and Morgan, 1962). Most methods like complexation and coagulation (Bose et al., 2002) were discussed and evaluated regarding to benefit and disprofit.

Adsorption is another promising approach for the removal of metals out of wastewater. It is one of the most important physicochemical process that occurs at the solid-liquid and solid-gas interfaces. It has become an alternative method for removal, recovery and recycling of toxic heavy metals from wastewater (Chang et al., 2002). Different conventional and non-conventional adsorbents have been studied for removal of various metal ions, e.g. activated carbon.

The adsorption on activated carbon occurs when molecules or ions adhere to the internal wall of pores in carbon particles produced by thermal activation. The major disadvantage of the adsorption process using activated carbon is that the cost is comparatively high, depending on the characteristics of the influent wastewater (U.S. EPA, 2000). Hence the search and development for cheaper methods and materials has drawn a lot of interest. The material should be inexpensive, local available, technical feasible and good for engineering application. Srivastava et al. (1989) found that the waste slurry generated in fertiliser plants exhibited good adsorption efficiency for Hg(II), Cr(IV) and Pb(II), but a poor efficiency for Cd(II), Ni(II), Co(II) and Zn(II). Gupta et al. (1998) used activated carbon developed from fertiliser waste to remove Hg(II), Cr(IV), Pb(II) and Cu(II). Furthermore, Sreedhar and Anirudhan (2000) examined the adsorption properties of coconut husk grafted with polymerised acrylamide. The adsorption properties towards Hg(II) were studied and it was shown that the material exhibited a very high adsorption capacity for the metal. Nevertheless, the recovery of the sorbed metals and the regeneration of the adsorbent were doubtful.

Compared to the different adsorptive materials mentioned above, clays have unique features which are high specific surface area and low cost because of their occurrence in most soil and sediment environments (Bailey et al., 1999). Another advantage is that clay minerals are capable of removing many metal ions (Garcia et al., 1999). A number of adsorption studies have been performed using montmorillonite (Lin and Juang, 2002; Green-Ruiz, 2005) and bentonite (Borisover et al., 2001; Naseem and Tahir, 2001). Green-Ruiz (2005) found that one gram of natural montmorillonite is capable of removing 96.8-98.8% of Hg(II) in solutions having concentration range of 0.25 to 1.00 mg/L, while the removal efficiency decreased to 79.9% when the initial concentration of Hg(II) increased up to 10 mg/L. However, inherent limitations of these materials for the use as adsorbent of heavy metals are their low loading capacity, relatively small metal ion binding constants and low selectivity to the type of metal (Mercier and Detellier, 1995; Mercier and Pinnavasia, 1998).

To circumvent these limitations, the modification of materials is carried out. The amount of metal cation uptake by clays increased after various physical and chemical treatments. These treatments lead to structural modification (chemical composition, changes in interlayer distance, etc.), textural modification (surface area, porosity, etc.) and also change in the acidic properties of the clay (Wypych and Satyanarayana, 2004). Furthermore, the modification with ligands containing metal-chelating groups like -SH and -NH2 was also studied (Mercier and Detellier, 1995). These chelating agents can be grafted onto the surface of the clay material, via chemical reaction between the reactive groups of the layer and that of the reactant molecule to establish covalent bonds, which ensure chemical, structural and thermal stability for the compound (Wypych and Satyanarayana, 2004). Manohar et al. (2002) studied the adsorption capacity of clay functionalised with 2-mercaptobenzimidazole and the removal of Hg(II) was found to be higher than 99% when the initial concentration was 50 mg/L. Moreover, the adsorption capacity of the functionalised clay was, compared to that of untreated clay, much higher. Therefore, the modification seems to be a promising approach for the removal of Hg(II) ions out of wastewater.

To study the adsorption properties of functionalised clay, certain parameters were investigated e.g. metal concentration, pH, contact time, temperature, ionic strength, particle size of adsorbent and adsorbent dose (Manohar et al., 2002; Erdmoglu et al., 2004; Green-Ruiz, 2005).

In this study, bentonite was modified with 2-(3-(2-aminoethylthio)propylthio)ethanamine (AEPE) to improve the removal efficiency for mercury. AEPE is a chelating ligand which can complex very well with soft metal ions due to its electron donors, i.e. nitrogen and sulphur atoms. The main objective of this work is to prepare the modified bentonite and to investigate the potential of AEPE-modified bentonite as adsorbent for removing Hg(II) ions from water. The effect of pH, contact time, initial concentration, ionic strength, interfering ions and adsorbent dose was studied in order to optimise the removal process. The adsorption isotherm study was also performed.

1.2 Theoretical Background

1.2.1 Structure and properties of bentonite

Abbildung in dieser Leseprobe nicht enthalten

Figure 1. Schematic representation of the crystal structure of montmorillonite (after Yan et al., 1996).

Bentonite is a clay material generated frequently from the alteration of volcanic ash, consisting predominantly of smectite minerals, usually montmorillonite. Other smectite group minerals include hectorite, saponite, beidelite and nontronite. The main component of bentonite is montmorillonite, which is a hydrophilic mineral. The specific surface area (SSA) of montmorillonite ranges from 524 m^[2] /g up to 725 m^[2] /g for pure clay mineral calculated from crystallographic data. (Ersahin et al., 2006; Theng, 1979). Montmorillonite is an aluminiumhydrosilicate, which belongs to the group of the phyllosilicates (sheet structure-silicates) and has a nominal chemical formula of [(Al_1.67Mg_0.33)Si₄O10(OH)2]Na_0.33.nH₂O (Derek et al., 2001). The phyllosilicates contain bi-dimensional tetrahedral or octahedral sheets. The tetrahedral cations are normally Si^[4] +. The octahedral cations normally are Al^[3+] (Figure 1). Each layered sheet is about 10 Å thick with literature lateral dimensions of 0.1 mm to more than 1 mm. Montmorillonite is the chief agent in the group of the three-layered silicates, which are also called smectite. Bentonite can be found along with accompanying minerals like quartz, feldspar and mica. The presence of these minerals has an impact on the industrial value of a deposit, reducing or increasing its value depending on the application. Montmorillonite is by far the most common clay of the smectite group. The layered structure of montmorillonite consists of a dioctahedral 2:1 layer with charge balancing and exchangeable cations located between the layers (Brindley, 1961).

The ratio 2:1 means that it has two tetrahedral sheets, where four oxygen atoms arranged tetrahedral surround a silicon atom, which is called silicate sheet ((Si₂O₅)^[2-] ), sandwiching a central octahedral sheet that consists of six oxygen atoms arranged octahedral surround an aluminium atom, which is also called gibbsite layer, (Al₂(OH)₄) (Moore and Reynoids, 1989; Wypych and Satyanarayana, 2004).

Abbildung in dieser Leseprobe nicht enthalten

Figure 2. Illustration of the sheet structure of a 2:1 montmorillonite (after Wypych and Satyanarayana, 2004).

The substitution of aluminium for silicon in the tetrahedral sheet creates a charge imbalance. The charge imbalance of montmorillonite is about 0.66 per unit cell (Brindley and Brown, 1980), which means that the cations are not fixed between the layers. This net positive charge deficiency is balanced by exchangeable cations (e.g. [Abbildung in dieser Leseprobe nicht enthalten]) adsorbed between the unit layers on the edges (Murray, 2000). The layers contain ions that are very loosely bound to one another and easily exchangeable with ions in bulk solution such as toxic heavy metals. They are called exchangeable ions. These cations are also the components of interest. The property of clays to exchange one cation with another is called cation exchange capacity (CEC), which is the total amount of cations adsorbed on the clay, expressed in miliequivalents (meq) per hundred grams of dry clay (Hunter, 1982). Up to 20% of the CEC is due to broken edges, which are undercoordinated metal ions ([Abbildung in dieser Leseprobe nicht enthalten]) on the broken edges on the surface of the clay. They react with water and form surface hydroxyl groups in order to complete their coordination sphere (Johnston, 1996). Montmorillonite has a CEC value of 81-124 meq/100 g (Thomas et al., 1999). These cations play an important role because they are responsible for the neutralisation of the layer charge. They also have large chemical leverage because they govern many reactions in which clay minerals take part. The exchangeability of cations in the clay by cations of a particular aqueous solution is determined by (Wypych and Satyanarayana, 2004):

- The nature of the clay mineral
- The nature of the cation (hydration energy, size, valency)
- The concentration of the electrolyte and the pH of the exchange solution
- The population of exchange sites on the clay

These properties, namely the ability to bind and exchange water, cations and organic complexes or protons due to their negative charge of the tetrahedral layer, make bentonite useful for modification and also for wastewater purification.

1.2.2 Chelating agents

Every metal is able to complex with a ligand, which must be either an anion or a polar molecule. Besides, the ligand is always the electron donor and the metal atom or ion the electron acceptor. The bond between the ligand and the metal atom is a coordinative bond. For chelation to occur, two conditions have to be fulfilled. First, the ligand must contain at least one donor atom, which is able to establish a bond with the metal atom. The second condition is that the location of the functional groups in molecule of ligand should be suitable for the formation of complex, in particular a ring including the metal atom.

The chelating agents can be characterised according to the donor atoms present in the molecule. If there is only one pair of electrons involved in s-bonding with the metal, like ammonia or chloride, this ligand is called unidentate (literally ’one-toothed’). Molecules, which own two or more coordinative places, are called bidentate and multidentate.

Elements, which can act as a donor for metal chelation are members of the V and VI group of the periodic table, as for example: nitrogen, phosphorus, oxygen and sulphur. The groups that undergo coordinative bonds are either basic or acidic groups. The basic groups contain an atom with a lone pair of electrons that can coordinate with a metal atom/ion or a proton. Examples for this are -NH2 (amino), -NH (imino), C=O (carbonyl), -OH (alcohol) or -S- (thioether) groups. The acidic groups lose a proton and then establish a bond to the metal. Examples for this are -COOH (carboxylic), -SO₃H (sulphonic) or -SH (thioenolic and thiophenolic) groups (Bell, 1977).

This provides a basis for consideration of the properties of ligands that are of major significance in chelation. The nature of the functional group will give an idea of the selectivity of the ligand towards trace elements.

In practice, inorganic cations (metal ions) may be divided in 3 groups (Dwyer and Mellor, 1964):

Group I- ‘hard’ cations: These cations preferentially react via electrostatic interactions (to gain in entropy caused by changes in orientation of hydration water molecules). This group includes alkaline and alkaline earth metals [Abbildung in dieser Leseprobe nicht enthalten] that form rather weak outer-sphere complexes with only hard oxygen containing ligands.

Group II- ‘borderline’ cations: These cations have an intermediate character. The metals in this group are: [Abbildung in dieser Leseprobe nicht enthalten]. They possess affinity for both hard and soft ligands.

Group III- ‘soft’ cations: These ions tend to form covalent bonds. The metals in this group are: [Abbildung in dieser Leseprobe nicht enthalten].

There is always a special order for the affinity of metal ions towards donor ligands. For soft metals, the following affinity order of donor atoms to bind the metals is observed: O < N < S. A reversed order is observed for hard cations. For bidentate ligands, affinity for a soft metal increases with overall softness of the donor atom:(O, O) < (O, N) < (N, N) < (N, S). The order is reversed for hard metals. In general, for ligands essentially involved group I and group II metals contain O sites, and N and S sites for metals of group II and group III (Mark, 2004).

1.2.3 Characterisation techniques

There are several techniques used to characterise functionalised clay materials. Most of them are spectroscopic techniques.

1.2.3.1 Nuclear magnetic resonance spectroscopy (NMR)

Nuclear magnetic resonance spectroscopy provides the information of nuclei, such as ^[1] H or ^[13] C. This technique is highly sensitive to small variations in chemical structure. Liquids can directly be analysed by dissolving in proper a solvent. In case of insoluble compounds such as clay minerals, solid-state nuclear magnetic resonance is used to characterize these samples (Fribolin, 1992).

1.2.3.2 X-ray diffraction (XRD)

Powder X-ray Diffraction (XRD) is one of the primary techniques to characterise materials. Powder XRD can provide information about crystalline structure (or lack thereof) in a sample.

The XRD is based on the diffraction of X-rays in the reticule layer. This layer is formed by a regular sterical arrangement of the atoms. Only because of the reticule the X-rays can be diffracted and interfere constructive, so that a detector can detect them. The coherence between wavelength, incident radiation, angel of incidence and the space of the reticule is described by the “Braggschen equation” (Moore and Reynoids, 1989):

Abbildung in dieser Leseprobe nicht enthalten

1.2.3.3 Fourier transform infrared spectroscopy (FT-IR)

Infrared spectroscopy technique is one of the most commonly used spectroscopic techniques that engage with the infrared region of the electromagnetic spectrum. It is widely used for analysis both organic and inorganic samples of different types, such as gases, liquids and solids by using various sampling accessories.

Each functional group has its own absorption characteristic frequencies of IR radiation. Thus, the spectra of IR spectroscopy analysis provide the presence of chemical functional groups in the matrix. This gives information about molecular structure and chemical nature of the sample.

The characteristic frequencies of functional groups can be divided into different regions, such as: 4000-2500 cm^[-1] as single bond stretching vibration, 2500-2000 cm^[-1] as triple bond, 2000-15000 cm^[-1] as double bond region and the fingerprint region at 1500-500 cm-^[1], which is unique for every molecule.

In Fourier transform infrared (FT-IR) spectroscopy all frequencies are examined simultaneously (Kellner et al., 2004).

1.2.3.4 Thermogravimetric analysis (TGA)

Thermogravimetry is a method that measures the weight change in materials as a function of temperature and/or time in air or nitrogen atmosphere. This technique usually is used to determine the thermal stability and composition of both organic and inorganic material (Kellner et al., 2004). Its application provides information about moisture and volatile contents, the composition, the thermal stability and oxidative stability of the material.

2 Aims

The aims of this research are to synthesise the chelating ligand AEPE and graft it onto the surface of the clay material bentonite to enhance the adsorption capacity and selectivity towards Hg(II) ions. Then, the optimal conditions, such as pH, contact time, initial concentration, ionic strength, interfering ions and adsorbent dose shall be determined. Moreover, the adsorption kinetics and isotherms shall be identified.

3 Experimental

3.1 Material

3.1.1 Reagents and chemicals

Chemicals used in this study were listed in Table 1.

Table1. Chemical list.

Abbildung in dieser Leseprobe nicht enthalten

Organic solvents used were of analytical grade and used without further purification, except for acetonitrile that was dried with calcium hydride.

All metal solutions for adsorption experiments were prepared with deionised water and the ionic strength was controlled with 0.01 M NaNO3. For this, 2 mL of Hg(II) standard solution with a concentration of 1000 mg/L were pipetted into a volumetric flask of 100 mL and 1 mL of 1 M NaNO₃ was added. The solution was diluted to 100 mL, therefore, the concentration of Hg(II) and NaNO₃ was 20 mg/L and 0.01 M, respectively. The pH of the solutions was adjusted to a desired value using either NaOH or HNO3 solutions.

The standard solutions for the calibration curve were prepared as followed:

Table2. Volumes and concentrations of the stock solution used to prepare the standard solutions for the calibration curve of Hg(II); standards were diluted with 1% HNO3 to 50 mL.

Abbildung in dieser Leseprobe nicht enthalten

The SnCl₂ solution and the acid carrier for Hg(II) reduction for cold vapour atomic absorption spectroscopy analysis were prepared as followed:

SnCl2 solution: 100 g of SnCl2 were dissolved in 200 mL concentrated HCl and diluted with water to a volume of 1000 mL. After 2 h the SnCl2 solution was filtered using a membrane filter.

Acid carrier: the mixture of 20 mL HNO3 and 40 mL of H2SO4 were diluted with water to a volume of 1000 mL.

3.1.2 Instruments and glassware

Centrifuge

A Centaur 2, Sanyo centrifuge was used to separate clay from solution in the clay purification and metal adsorption study.

Cold vapour atomic absorption spectrometer

Cold vapour atomic absorption spectrometer model Perkin Elmer Analyst 300 coupled with FIAS 400 system was used to measure the mercury concentration.

Exsiccator

An exsiccator from Schott Duran was utilised to dry the samples.

Fourier transform infrared spectroscopy

Fourier transform infrared spectrometer (FT-IR) model Nicolet FT-IR Impact 410 was used for the characterisation of the product from every step of purification and modification of bentonite. Infrared spectra were recorded from 400 to 4000 cm-^[1] in transmittance mode by KBr pellet technique.

Nuclear magnetic resonance spectrometer

Nuclear magnetic resonance spectrometer (NMR) model Varian Mercury+400 was used for the characterisation of the chelating ligand AEPE. NMR spectra were recorded in CDCl3.

Oven

The purified clay was dried by a Memmert UM-500 oven at 110 °C for 1 day.

pH meter

A pH meter model pH 211 (Hanna instruments) was used for pH measurement.

Rotary evaporator

Solvents used during synthesis were removed by a Rotary Evaporator model EYELA N-100.

Stirrer/Hot plate

Stirrer/Hot Plate model CORNING was used in this study.

Surface area analyser

The surface area was determined with a Quantachrome, Autosorb-1 with N2 as sample gas at -195.8°C.

Thermogravimetric analysis

Thermogravimetric measurements were performed using Simultaneous Thermal Analyser (STA) model 409 (Netzsch) at a heating rate of 10 °C/min under nitrogen atmosphere.

Vacuum pump

In this study a vacuum pump model V-700 (Büchi) was used.

Water bath

A water bath obtained from Bosstech Thailand model Thermal TMP/1 was used to control the temperature during adsorption experiments constant.

X-Ray diffraction

X-ray pattern was recorded on a Rigaku 1200 + series X-ray diffractometer, equipped with a monochromator in a 2 q range of 0-30° using a CuKa source accelerated at 40 kV and 30 mA.

The used pipettes and glassware are listed in Table 3.

Table3. Pipettes and glassware used in this study.

Abbildung in dieser Leseprobe nicht enthalten

3.2 Methods

3.2.1 Synthesis

To enhance the adsorption capacity of the clay material bentonite, it is necessary to remove any accompanying material like quartz. For this, a purification step is essential.

To improve the adsorption capacity towards Hg(II) ions, a special ligand named AEPE has to be synthesised and grafted onto the surface of the purified bentonite within three steps of modification.

3.2.1.1 Purification of bentonite

Bentonite was purified by fractionated sedimentation. 30 g of bentonite were dispersed into 1000 mL deionised water and then the mixture was stirred for 3 h at room temperature. The colloid bentonite was collected and separated from quartz sediments by centrifugation at 4000 rpm for 8 min. The obtained clay was dried at 100°C.

Altogether it was preformed 6 batches, so that a total amount of 180 g bentonite was purified. The purified bentonite was characterised with XRD, FT-IR, TGA and nitrogen adsorption/desorption isothermal analysis to determine the surface area.

3.2.1.2 Synthesis of AEPE

The synthesis of AEPE is schematically shown in Scheme 1.

Abbildung in dieser Leseprobe nicht enthalten

Scheme1. Synthesis of 2-(3-(2-aminoethylthio)propylthio)ethanamine (AEPE).

About 3 g of sodium metal was transferred into a beaker containing 60 mL of ethanol. The solution was kept at 10-20 °C. Then 6.9 g of cysteamine hydrochloride was added to the solution and stirred for 15 min. 3 mL 1,3-dibromopropane was further added and the mixture was stirred for 4 h at 40 °C under N2 atmosphere. The solvent was removed by rotary evaporation. A sodium hydroxide solution (15 g NaOH in 45 mL water) was added to the obtained residue and the mixture was left stand overnight. The ligand was extracted with 20 mL dichloromethane and the organic phase was washed with water (2x40 mL) and dried over anhydrous sodium sulphate. After filtration, dichloromethane was removed by rotary evaporator. The obtained product was visible in the form of an oily, yellow liquid. A total of nine batches were performed and approximately 25 g AEPE were synthesised. The chelating agent AEPE was characterised by ^[1] H-NMR and ^[13] C-NMR.

3.2.1.3 Preparation of functionalised bentonite

The method for the modification of bentonite with the ligand AEPE can be divided into three steps. The first step of modification is schematically shown in Scheme 2.

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Scheme2. Reaction of the purified bentonite with 3-aminopropyltriethoxysilane to yield bentonite-NH2.

5 g of the purified bentonite were dispersed into 125 mL acetonitrile. The mixture was stirred for 24 h at room temperature under N2 atmosphere. Then 10 mL of (3-Aminopropyl)-triethoxysilane was added and the mixture was stirred for 24 h at 90 °C under reflux condition. The clay was filtered off with a suction filter and filter paper with a pore size of 0.45 mm and washed with ethanol (3x125 mL) and dichloromethane (4x125 mL) and dried in an exsiccator. The product was named bentonite-NH2. The bentonite-NH2 was characterised by FT-IR, XRD and TGA.

The second step of the modification of the purified bentonite with AEPE is schematically shown in Scheme 3.

Abbildung in dieser Leseprobe nicht enthalten

Scheme3. Reaction of bentonite-NH2 with ethyl-2-bromopropionate to give bentonite-NH-Br.

For the second step, the clay from step I, bentonite-NH2, was dispersed in 125 mL of dry acetonitrile and stirred for 24 h at room temperature under N2 atmosphere. Then, 6 mL of ethyl-2-bromopropionate were added and the mixture was stirred for 24 h at 90 °C under reflux condition. The solid was further filtered off, washed like that mentioned in the first step and dried in an exsiccator. The product was named bentonite-NH-Br and characterised by FT-IR, XRD and TGA.

The third step of the modification of the purified bentonite with AEPE is schematically shown in Scheme 4.

Abbildung in dieser Leseprobe nicht enthalten

Scheme4. Reaction of bentonite-NH-Br with AEPE to yield AEPE-bentonite.

The product from step II, bentonite-NH-Br, was dispersed in 125 mL of dry acetonitrile and stirred for 24 h at room temperature under N2 atmosphere. Then, 2.55 g AEPE were added and the mixture was stirred at 90 °C under reflux condition. Filtration and washing step were carried out as mentioned above.

The product AEPE-bentonite was characterised by FT-IR, XRD, TGA and nitrogen adsorption/desorption isothermal analysis to determine the surface area. Altogether 4 batches were preformed whereas 2 batches had the double amount and a total amount of approximately 30 g of bentonite was modified.

The overall reaction of the modification is presented in Scheme 5.

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Scheme5. Preparation of functionalised bentonite with AEPE.

3.2.2 Characterisation

To control the progress and the success of the purification and the modification of the clay material and also the synthesis of the ligand AEPE, it is necessary to identify the product of each step of purification and modification and compare the results of characterisation to one another.

The synthesised ligand was characterised by ^[1] H-NMR and ^[13] C-NMR.

3.2.2.1 Characterisation of AEPE with NMR

The chelating agent AEPE was characterised by ^[1] H-NMR and ^[13] C-NMR. The ligand was dissolved in deuterated chloroform (CDCl₃).

3.2.2.2 Characterisation of the clay products from purification and modification

All clay products obtained from purification and modification of bentonite were characterised by FT-IR (KBr pellet technique), XRD (flat surface technique) and TGA. Additionally an XRD pattern of raw bentonite was also recorded.

The purified bentonite and the final modified bentonite were additionally characterised for surface area with BET isotherm.

For FT-IR, a KBr (potassium bromide) pellet technique was used. The KBr pellet was prepared by grinding the solid sample with solid (KBr) and applying great pressure to the dry mixture. The sample was mixed with KBr in a ratio of 1 : 100 (Sample : KBr).

The powder X-ray diffraction must be done with finely ground samples. For that, the sample was ground in a mortar until it becomes a fine-grained sample. The sample was put in the middle of the glass slide and pressed flat with another glass slide. Several repeats involving pressing and cleaning off powder around the well were necessary. It was important that the top of the sample be coplanar with the top of the glass slide holder. Then the XRD pattern was recorded.

The BET isotherms for the determination of the surface area of the purified and AEPE-bentonite were recorded with N₂ as sample gas at -195.8°C.

3.2.3 Adsorption of Hg(II) from aqueous solution

A batch method was employed to study the adsorption of Hg(II) ion from aqueous solution onto functionalised and purified bentonite. 0.01 g of adsorbent were added to 5 mL of Hg(II) solution. The initial concentration of mercury was 20 mg/L. The ionic strength of the solutions was controlled by 0.01 M NaNO₃. The mixture was stirred for 60 min and the solid was separated by centrifugation. The solutions were analysed using a cold vapour atomic adsorption spectrometer to determine the residual Hg(II) concentration. The initial pH of Hg(II) solution was 4.0. The pH of the solutions was monitored before and after adsorption. All adsorption experiments were carried out in triplicate.

3.2.3.1 Effect of pH

The effect of pH on the adsorption of Hg(II) on modified and unmodified bentonite was examined in the pH range of 1.0-8.0. The pH of the metal solutions was adjusted with HNO3 and NaOH solutions before completing the dilution with water and the concentration of Hg(II) was still 20 mg/L.

3.2.3.2 Effect of contact time

The effect of contact time on the adsorption of Hg(II) on modified and purified bentonite was examined at pH 4.0. The adsorption experiments were carried out using contact time ranging from 1 to 60 min.

Furthermore, the effect of the initial concentration on equilibrium time for AEPE-bentonite was investigated using initial concentration of 20 and 100 mg/L, and contact time ranging from 1-60 min and 5-60 min, respectively.

3.2.3.3 Adsorption isotherm

The adsorption isotherm study was carried out using an initial concentration ranged from 20 to 400 mg/L and the initial pH was 4.0. The temperature was kept constant at 24.6 °C during the adsorption experiments and the contact time was 20 min. The isotherm was only investigated for modified bentonite, because the results from previous experiments indicated that no significant adsorption occurs onto the unmodified bentonite.

3.2.3.4 Effect of ionic strength

The effect of ionic strength on the adsorption of Hg(II) on modified bentonite was examined using NaNO₃ in a concentration range of 0.01 to 2 M. The effect was examined using an initial pH of 4.0 and a contact time of 20 min.

3.2.3.5 Interfering ions

The effect of interfering ions on the adsorption of Hg(II) on AEPE-bentonite was investigated using different cations, anions and heavy metal ions. The effects of the cations [Abbildung in dieser Leseprobe nicht enthalten] were studied using nitrate salts and anions [Abbildung in dieser Leseprobe nicht enthalten] using sodium salts with two different concentrations (0.1 and 1 M). The effect of the presence of heavy metal ions other than Hg(II) ions on extraction efficiency was studied using Pb(II), Fe(III) and Ni(II) ions. They were added separately to the solution containing Hg(II) ions (0.1 mM) in two different concentrations (0.1 and 1 mM). To avoid precipitation as hydroxide of the metal ions, the initial pH was adjusted to a value of 3.0 and a contact time of 20 min was used.

3.2.3.6 Effect of adsorbent dose and kinetics

To study the effect of adsorbent dose and kinetics of adsorption of Hg(II) onto AEPE-bentonite, the adsorbent dose was varied using 0.01, 0.05 and 0.1 g of adsorbent and the adsorption experiment was performed at different contact time ranging from 5-60 min. The initial concentration of Hg(II) was 100 mg/L and the pH was set to a value of 4.0.

4 Results and Discussion

4.1 Characterisation

4.1.1 [1]H and [13] C NMR spectra of AEPE

AEPE was synthesised from the reaction between cysteamine hydrochloride and 1,3-dibromopropane via nucleophilic substitution reaction. Synthesis of AEPE was shown in Scheme 1. The method was developed from that proposed by Choudhury et al. (1991). In this work, ethoxide abstracted a proton of the -SH group to generate the nucleophile that could react with 1,3-dibromopropane, which have two bromide atoms as leaving groups. The synthesis of AEPE was done at the mole ratio of cysteamine hydrochloride : 1,3-dibromopropane of 2:1. The product was obtained as yellow oil. The yield of AEPE was about 79 %.

The ^[1] H-NMR spectrum of AEPE was recorded in CDCl3. The spectrum showed 3 multiplets in aliphatic proton region, according to symmetrical structure of the chelating ligand. The received data are as follows: d (ppm) 1.77 [Abbildung in dieser Leseprobe nicht enthalten], 2,53 [Abbildung in dieser Leseprobe nicht enthalten] and 2.78 [Abbildung in dieser Leseprobe nicht enthalten]. The ^[1] H-NMR spectrum of AEPE is presented in Figure 21.

The ^[13] C-NMR spectrum was obtained with chemical shifts as followed: d (ppm) 29.40 [Abbildung in dieser Leseprobe nicht enthalten], 30,46 (2C, s, SCH2CH2NH2), 36.18 ([Abbildung in dieser Leseprobe nicht enthalten] and 41.02 [Abbildung in dieser Leseprobe nicht enthalten]. The ^[13] C-NMR spectrum of AEPE is presented in Figure 20.

The obtained spectra proofed the success of the synthesis of the chelating ligand AEPE.

4.1.2 XRD pattern

4.1.2.1 XRD pattern of raw and purified bentonite

With the help of X-ray diffraction the layer distance between the montmorillonite crystals can be measured. The distance is defined by the (001)-reflection, which can be annualised into the d-value. The XRD pattern and data of raw and purified bentonite are presented in Figure 3 and Table 4, respectively.

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Figure3. XRD-pattern of (a) raw bentonite and (b) purified bentonite; * = quartz; I = impurity.

The XRD pattern of the raw bentonite shows the characteristic peaks of bentonite at 7°, 19° and 30° 2 q. A difference in XRD pattern of raw and purified bentonite was observed. The typical peaks, which belong to quartz are clearly visible in the pattern of the natural bentonite at 21°-26° 2 q, while they are absent in the pattern of the purified bentonite. This is a proof for the success of the purification.

The d003 peak of purified bentonite slightly shifts from that observed in raw bentonite, but it is also one of the characteristic peaks for bentonite. The XRD patterns of raw and purified bentonite show that quartz and other impurities, except the impurity at 29.4° 2 q, were removed by centrifugation technique. Yang et al. (2006) also used the centrifugation technique to remove impurities out of natural montmorillonite.

Table4. X-ray data of raw and purified bentonite: literature values and measured values in this study.

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4.1.2.2 Comparison of XRD pattern of product from every step of modification

As illustrated in Figure 4 and listed in Table 5, the XRD patterns for all bentonite samples are quite similar, with only the reflection at 28.4° 2 q showing some loss of definition.

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Figure4. XRD-pattern of (a) purified bentonite, (b) bentonite-NH2, (c) bentonite-NH-Br and (d) AEPE-bentonite.

A comparison of all XRD patterns indicates that the original structure was preserved after modification. No significant difference was observed in the basal spacing of the four samples (Figure 4). Normally, the intercalation of molecules in the interlayer region of clay mineral causes significant change in the interlayer distance. The results reveal that the ligand was only fixed solely on the outer surface of the clay and not into the interlayer region.

Shanmugharaj et al. (2006) studied the effect of different solvent media on the modification of montmorillonite with 3-aminopropyltriethoxysilane and found that the interaction between the trifunctional silane and clay took place by two different processes: (i) the adsorption of silane onto the broken edges of the clay surface and (ii) the intercalation of the silane between the clay layers. These two processes depend on the surface energy of the solvent used as dispersing medium.

Burgentzle et al. (2004) reported the influence of solvents on swelling capacity of pure sodium montmorillonite clay and found out that the interlayer distance of clay materials depends on both polar and dispersive components of the solvent surface energy. If the solvent has a surface energy lower or equal with that of the clay material, it can wet the clay material and thereby make it possible for the silane to react with the reactive surface sites of the clay. On the other hand, if the surface energy of the solvent is higher than the surface energy of the clay material, wetting becomes harder and the reaction between the silane and the surface hydroxyl sites becomes fewer.

When the silane is diffused in between the clay layers, grafting of the silane onto the interlayer space becomes possible (Shanmugharaj et al., 2006). Distilled water, tetrahydrofuran, toluene and ethylene glycol were used by Shanmugharaj et al. (2006) as solvents. It was found that modified clay obtained by using ethylene glycol as solvent showed the lowest amount of intercalated silane, while the modified clay obtained by using water as solvent showed the highest amount of intercalated silane and the spacing distance also increased. By the results observed in this study, it is possible that the surface energy of acetonitrile (26.6 mJ/m^[2] ) (Kang et al., 2003), which was used as solvent in this study, is not high enough to disperse the single layers of the clay (surface energy of montmorillonite is 44 mJ/m^[2] ) (Shanmugharaj et al., 2006) and the intercalation of the ligand AEPE seems to be less likely to occur. Therefore, there was no difference in the XRD pattern and d spacing value of the modified and unmodified bentonite.

Table5. X-ray data for the d001 value of the products of modification

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4.1.3 FT-IR spectra

4.1.3.1 Characterisation of purified bentonite with FT-IR

In a FT-IR spectrum of smectites one can observe tetrahedral framework vibrations of Si-O-Si and Si-O-Al and stretching- and bending-vibrations of hydroxide groups. The FT-IR spectrum of purified bentonite recorded in this study is presented in Figure 5.

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Figure5. FT-IR spectrum (transmission, KBr pellet) of purified bentonite.

In Table 6 and Table 7, the characteristic wavenumbers of bentonite reported in the literature are listed and classified along with the values observed in this study.

Table6. Characteristic wavenumber of stretching vibrations in FT-IR spectrum of purified bentonite: Literature and observed values.

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Table7. Characteristic wavenumber of bending vibrations in FT-IR spectrum of purified bentonite: Literature and observed values.

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The information from the recorded FT-IR spectrum of the purified bentonite is in agreement with the data reported previously. Moreover, the results also indicate that the purification was successful because the quartz typical peaks at 800 cm-^[1] (vSi-O-Si) and a board peak at 1150-1090 cm-^[1] are missing.

4.1.3.2 Comparison of FT-IR spectra of products from modification

To control the success of the modification of purified bentonite with the ligand AEPE, it is necessary to record FT-IR spectra of the product of each modification step and compare them with the spectra of the product of the previous step. All FT-IR spectra of the products from the single step of modification are presented in the same diagram to see the development of modification (Figure 6).

In this study, the chelating agent was grafted onto the surface of bentonite and chemical bonding took place. The grafting of the molecules of the coupling agent onto the surface of bentonite will result in a change the FT-IR spectra. Therefore, FT-IR spectra commonly provide information of the surface modification (Madejova, 2003). Comparing the spectrum of the purified bentonite to that of the products from each step of modification, it can be seen that the characteristic structural peak of bentonite are the main peaks present in the spectra. Moreover, there is an additional band situated at 2920 cm-^[1] occurred in the spectrum of bentonite-NH2 (Figure 6 and Figure 23) that belongs to an aliphatic C-H stretching vibration. These results fit to the structure of this intermediate.

Shanmugharaj et al. (2006) also observed a new occurring band after modifying montmorillonite with 3-aminopropyltriethoxysilane at 2930 cm-^[1] with a small hump at 2850 cm-^[1], which belong to the -CH asymmetric and symmetric stretching of -CH2 groups, respectively and confirmed the successful modification with 3-aminopropyltriethoxysilane. They also found a new occurring peak at 700 cm-^[1] (Figure 23) that corresponded to the -CH out-of-plane deformation and further supported the successful modification of purified bentonite with 3-aminopropyltriethoxysilane. In addition, a band at 1036 cm-^[1] (vSi-O stretching vibration) has increased its intensity (Figure 23), which indicates the interaction of the silane with the layer surface.

When compare the spectrum of the product obtained from the first and second step of modification, no additional occurring bands can be observed. On the other hand, the spectrum of the final products shows a new and small band at 3252 cm-^[1] which corresponds to a NH2 stretching vibration of the ligand AEPE. The presence of this band confirmed the success of the grafting of AEPE onto the surface of bentonite.

Abbildung in dieser Leseprobe nicht enthalten

Figure6. FT-IR spectrum (Transmission, KBr-pellet) of (a) the purified bentonite, (b) bentonite-NH₂, (c) bentonite-NH-Br and (d) AEPE-bentonite.

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