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

Bachelorarbeit von Nina Siebers

Abstract:

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. 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, 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 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. 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.

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. 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. Most methods like complexation and coagulation 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. 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. 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. 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 used activated carbon developed from fertiliser waste to remove Hg(II), Cr(IV), Pb(II) and Cu(II). Furthermore, Sreedhar and Anirudhan, 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. Another advantage is that clay minerals are capable of removing many metal ions. A number of adsorption studies have been performed using montmorillonite and bentonite. Green-Ruiz 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.

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. Furthermore, the modification with ligands containing metal-chelating groups like -SH and -NH2 was also studied. 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. Manohar 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.

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.

Table of Contents:

	Index of figures	VII
	Index of tables	IX
	Index of schemes	X
	Index of equations	XI
	ABSTRACT	XII
1.	INTRODUCTION	1
1.1	General information	1
1.1.1	MERCURY AND RESULTING ENVIRONMENTAL PROBLEMS	1
1.1.2	THE REMOVAL OF MERCURY BY ADSORPTION USING CLAY MINERALS	1
1.2	Theoretical Background	4
1.2.1	STRUCTURE AND PROPERTIES OF BENTONITE	4
1.2.2	CHELATING AGENTS	6
1.2.3	CHARACTERISATION TECHNIQUES	7
1.2.3.1	Nuclear magnetic resonance spectroscopy (NMR)	7
1.2.3.2	X-ray diffraction (XRD)	7
1.2.3.3	Fourier transform infrared spectroscopy (FT-IR)	8
1.2.3.4	Thermogravimetric analysis (TGA)	8
2.	AIMS	9
3.	EXPERIMENTAL	10
3.1	Material	10
3.1.1	REAGENTS AND CHEMICALS	10
3.1.2	INSTRUMENTS AND GLASSWARE	11
3.2	Methods	14
3.2.1	SYNTHESIS	14
3.2.1.1	Purification of bentonite	14
3.2.1.2	Synthesis of AEPE	14
3.2.1.3	Preparation of functionalised bentonite	15
3.2.2	CHARACTERISATION	17
3.2.2.1	Characterisation of AEPE with NMR	17
3.2.2.2	Characterisation of the clay products from purification and modification	17
3.2.3	ADSORPTION OF HG(II) FROM AQUEOUS SOLUTION	18
3.2.3.1	Effect of pH	18
3.2.3.2	Effect of contact time	18
3.2.3.3	Adsorption isotherm	18
3.2.3.4	Effect of ionic strength	19
3.2.3.5	Interfering ions	19
3.2.3.6	Effect of adsorbent dose and kinetics	19
4.	RESULTS AND DISCUSSION	20
4.1	Characterisation	20
4.1.1	1H AND 13C NMR SPECTRA OF AEPE	20
4.1.2	XRD PATTERN	21
4.1.2.1	XRD pattern of raw and purified bentonite	21
4.1.2.2	Comparison of XRD pattern of product from every step of modification	22
4.1.3	FT-IR SPECTRA	24
4.1.3.1	Characterisation of purified bentonite with FT-IR	24
4.1.3.2	Comparison of FT-IR spectra of products from modification	25
4.1.4	TGA	27
4.1.5	BET	29
4.2	Adsorption of Hg(II) from aqueous solution	31
4.2.1	EFFECT OF PH	32
4.2.2	EFFECT OF CONTACT TIME	34
4.2.3	ADSORPTION ISOTHERM	36
4.2.4	EFFECT OF IONIC STRENGTH	40
4.2.5	INTERFERING IONS	41
4.2.5.1	Cations	41
4.2.5.2	Anions	42
4.2.5.3	Heavy metal ions	44
4.2.6	EFFECT OF ADSORBENT DOSE AND KINETICS	45
4.2.6.1	Effect of adsorbent dose	45
4.2.6.2	Kinetics	46
5.	CONCLUSION AND OUTLOOK	49
6.	REFERENCES	50
7.	APPENDIX	57

Text Sample:

Chapter 4.2.5, Interfering ions:

Cations: The adsorption of Hg(II) on AEPE-bentonite was tested under optimised conditions in the presence of Na+, K+ and Mg2+ and the results are listed in Table 10 (see Table 10. Effect of cations on extraction of Hg(II) at a concentration of 0.1 mM by AEPE-bentonite).

As can be seen in Table 10, Na+ and K+, monovalent alkali cations, only showed a slight effect on Hg(II) adsorption at high concentration. The trend of a decrease in the adsorption efficiency is in agreement with the trend observed in increasing the ionic strength. The decrease of adsorption efficiency for Hg(II) ions with increasing concentrations of NaNO3 and KNO3 can again be explained by the decreasing complex formation constant with increasing ionic strength due to lower activity of the analyte. The results for Na+ and K+ are in agreement with previous work where Celis found that Na+ slightly affected Hg(II) adsorption by sepiolite grafted with 3-mercaptopropyltromethoxysilane. On the other hand, the presence of 1:2 electrolytes, in this case Mg(NO3)2, affected strongly the adsorption of Hg(II), even in low concentration of Mg(NO3)2. In effect, the ionic strength of the solutions containing Mg2+ is higher than those containing Na+ and K+ ions in the same concentration, namely 0.3 M and 3.0 M for a concentration of 0.1 and 1.0 M Mg(NO3)2, respectively. For this study, it seems that the adsorption efficiency of AEPE-bentonite is dependent to the ionic strength and independent of the type of electrolyte used in this study. Moreover, these results confirm that with the condition used in this study, the adsorption of Hg(II) on the adsorbent occurred via complexation with the ligand AEPE, rather than the unspecific ion exchange.

Anions: The effect of the presence of anions at high concentrations on the Hg(II) adsorption of AEPE-bentonite is shown in Table 11. Compared to the results of the presence of 1.0 M NaNO3, the presence of NaCl, Na2SO4 and NaH2PO4 in the same concentration affected the adsorption efficiency in a higher extent. In this case, the effect that is responsible for the decreasing efficiency can possibly be attributed to both ionic strength and complex formation between the anions and Hg(II). In Table 12 the equilibrium constants for Hg(II) complexes are listed (see Table 11. Effect of anions on extraction of Hg(II) at a concentration of 0.1 mM by AEPE-bentonite and Table 12. Aqueous speciation reactions and complex formation constants of Hg(II) ions).

Besides PO43- ions, the presence of Cl- ions in a high concentration strongly affect Hg(II) adsorption by AEPE-bentonite. The adsorption efficiency decreased nearly 23% when the initial concentration was 1.0 M NaCl compared to an initial concentration of 0.1 M NaCl. This effect can again be explained by the decrease of the complex formation constant of Hg(II) ions and AEPE with increasing ionic strength. The adsorption efficiency nearly decreased 15% for an initial concentration of 1.0 M NaCl compared to the blank (NaNO3) in the same concentration. The reason for this effect could also be attributed to the complex formation between Hg(II) and Cl- ions. At a NaCl concentration of 0.1 and 1.0 M, most of the Hg(II) ions are Hg-Cl complexes. The a values for the single species are calculated, and the dominant species for an aqueous system containing 0.1 mM Hg(II) and 0.1 M NaCl are HgCl2 (39%), HgCl3- (30%) and HgCl42- (31%). However, the adsorption efficiency for Hg(II) was still high (94%) in this condition and it seems that all these species can also form complex with AEPE on bentonite. On the other hand, when the concentration of NaCl increased to 1.0 M, the adsorption efficiency decreased sharply. The dominant species in solution is HgCl42-, which possibly undergoes the effect of a high ionic strength. Moreover, it seems that the increase in ionic strength affects HgCl42- species differently from other Hg(II) species (i.e. Hg2+, Hg(OH)+, Hg(OH)2) found in solution containing NaNO3 and Na2SO4. As a result, the complexation of HgCl42- with the ligand is less favourable, leading to a decrease in adsorption efficiency.

The equilibrium constants of NO3- and SO42- complexed with Hg(II) and of the complex HgClOH are very small so that no significant complexation with Hg(II) ions occurs and the dominant species in the system is Hg(OH)2. A decrease in adsorption efficiency in these cases is probably a result of the increasing ionic strength of solution when using NaNO3 and Na2SO4 in a higher concentration. Another reason for the low impact of NO3- on the adsorption efficiency could be that NO3- undergoes non-specific adsorption by oxide surfaces rather than complexation with Hg(II) ions (Sposito, 1984), thus they are not likely to influence the specific complex formation of Hg(II) with the chelating agent AEPE.

Striking is the effect of NaH2PO4 on the adsorption capacity of Hg(II) ions. At an initial concentration of 0.1 M, the adsorption efficiency decreased to 57% and only 17% of Hg(II) could be removed out of the aqueous solution at an initial concentration of 1.0 M NaH2PO4. To avoid precipitation, the initial pH of all solutions was set to a value of 3.0. It was observed that the pH of the metal solutions containing NaH2PO4 was 3.1 during the adsorption experiment while the pH of the solution increased to » 6.0-7.0 for the solutions containing other salts. The reason for the effect of NaH2PO4 is not clear yet. First, NaH2PO4 has an ionic strength of 0.35 M and 3.5 M for a concentration of 0.1 M and 1 M, respectively. This high ionic strength could be one reason for the high decrease in adsorption efficiency even at low NaH2PO4 concentration. Nevertheless, Na2SO4 also exhibits a very close ionic strength to that of NaH2PO4 and such ionic strength did not affect the efficiency in a high degree as found in the case of NaH2PO4. Thus, another possible reason is that some of Hg(II) form complexes with PO43- to give Hg(OH)2H2PO4- (log K=14.04). This complex, if formed, may have low affinity to the ligand AEPE, resulting in low adsorption efficiency. Moreover, the buffer effect of the used compound may also affect the efficiency. As shown in previous experiments, pH 3 is not the optimum pH for Hg(II) adsorption.

Heavy metal ions: The effect of the presence of heavy metals other than Hg(II) ions on the adsorption is summarized in Table 13 (see Table 13. Effect of heavy metal ions on extraction of Hg(II) at a concentration of 0.1 mM by AEPE-bentonite.

As can be seen in Table 13, the extraction of Hg(II) by AEPE-bentonite was not affected by the presence of Fe(III). On the other hand, the presence of Ni(II) and especially Pb(II) at high concentrations resulted in a slight decrease in the extraction efficiency for Hg(II). These results show that AEPE-bentonite has a high selectivity towards Hg(II) ions. The slight decrease of the adsorption efficiency in the presence of high concentrations of Pb(II) ions is explained by Pearson rule. Pb(II) belongs to the group of the ‘borderline’ cations, which have an intermediate character and posses affinity for both, hard and soft ligands. As a result, it can complex with the N and S sites of the chelating ligand AEPE. Nevertheless, the affinity of AEPE towards the soft cation Hg(II) is much higher.