REMOVAL OF MERCURY IONS FROM MIXED AQUEOUS METAL SOLUTIONS BY NATURAL AND MODIFIED ZEOLITIC MINERALS T. GEBREMEDHIN-HAILE1,2, M. T. OLGUíN1∗ and M. SOLACHE-RíOS1 1 Instituto Nacional de Investigaciones Nucleares, Departamento de Química, A.P. 18-1027 Col. Escandón, Delegación Miguel Hidalgo, México, D.F., México; 2 Facultad de Química, Universidad Autónoma del Estado de México, Paseo Colón y Paseo Tollocan s/n, Toluca, Estado de México (∗ author for correspondence, e-mail: [email protected], Fax: 53 29 73 01)

(Received 17 October 2002; accepted 9 April 2003)

Abstract. Research works on the removal of mercury from water by zeolitic mineral show that small quantities of this element are sorbed. In this work the mercury sorption from aqueous solutions in the presence and absence of Cu(II), Ni(II) and Zn(II) onto a Mexican zeolitic mineral unmodified and modified with cysteamine hydrochloride or cystamine dihydrochloride was investigated in acidic pH. The zeolitic minerals were characterized by thermogravimetric analysis, scanning electron microscopy, X-ray diffraction and FTIR. The sorption kinetics behavior and the retention isotherms for mercury were determined in the natural and treated zeolitic mineral samples. It was found that the amounts of sulfur on the modified zeolitic minerals were 0.375 (cysteamine hydrochloride) and 0.475 (cystamine dihydrochloride) mmol g−1 , which were not saturated to their total capacities of adsorption for the maximum concentration used (0.310 mM). Under the experimental conditions, the retention of mercury was the highest for the zeolitic minerals treated with the organic compounds, with adsorption capacities ranging from 0.0107 to 0.0509 mmol Hg g−1 . The retention was not affected by the presence of others heavy metals studied in this work as expected. Keywords: adsorption, cysteamine HCl, cystamine HCl, mercury, modified zeolites, natural zeolites

1. Introduction Zeolites are hydrated aluminosilicates of the alkaline and alkaline earth metals. According to the international mineralogical association, a zeolite mineral is a crystalline substance with a structure characterized by a framework of linked tetrahedra, consisting of four O atoms surrounding a cation (SiO4 and AlO4 ), arranged so that each oxygen is sheared between two tetrahedra. The framework has a net negative charge of one at the site of each aluminium atom, because the aluminium has one less positive charge than the silicon, and is balanced by the exchangeable cation (named extra-framework cations). The zeolite framework contains opened cavities in the form of channels and cages. The H2 O molecules and the extraframework cations usually occupied these cavities. The channels are large enough to allow the passage of guest species. The zeolites cation exchange behavior (Breck, 1974) depends on (a) the nature of the cation species, the cation size, both anhydrous and hydrated cations, and cation charge, (b) the temperature, (c) the concentration of the cation species in Water, Air, and Soil Pollution 148: 179–200, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.

180

T. GEBREMEDHIN-HAILE ET AL.

solution, (d) the anion species associated with the cation in solution, (e) the solvent (most of the exchange has been carried out in aqueous solutions, although some work has been done in organic solvents), and (f) the structural characteristics of the zeolite framework (electrical and stereological properties of the exchange sites as well as on the size and geometry of the pores, the cavities and the intramineral microchannels). Crystalline aluminosilicates, e.g., natural zeolites, are gaining great importance in treating liquid wastes, because they have high ionic exchange capacities and are highly selective for many metals (Tsitsivilli et al., 1992). The zeolites have similar surface chemistry to the clay minerals; however, natural zeolites can occur as millimeter (or greater) sized particles and are free of shrink – swell behavior. As a result, zeolites exhibit superior hydraulic properties and different electrostatic characteristics, which contribute for their superior ion exchange properties (Sullivan et al., 1997; Bowman et al., 2001). There are in nature, more than 40 different species of zeolitic minerals, however only clinoptilolite, mordenite, ferrierite and erionite are found abundantly; therefore they are considered economically accessible (Zamzow et al., 1990). Among the most common and abundant zeolitic minerals the clinoptilolite is the most studied. It has been used as environmental remedy to remove heavy metals by an ion exchange mechanism. This mineral is found associated with pyrite, halite, mordenite, heulandite and phillipsite, which are known to increase the adsorption properties of the minerals (Curkovic et al., 1997). Zamzow and Murphy (1992) have shown the removal of heavy metals like Cu, Cd, Zn etc., by using different zeolitic minerals where phillipsite proved to be the most efficient material, while the mordenites had the lowest uptakes. They found as well that sodium was the most effective exchangeable ion to exchange heavy metals. It has been experimentally shown that chabazite, clinoptilolite and mordenite natural zeolites with a high Si/Al ratio, selectively exchange large cations such as + 2+ and Sr2+ (Roque-Malherbe, 2001). Cs+ , Rb+ , K+ , NH+ 4 , Na , Ba Most of the research works done on zeolitic minerals show that the pretreatment with NaCl enhances their ion exchange ability (Zamzow et al., 1990; Curkovic et al., 1997; Pavon-Silva et al., 2000). Mexican clinoptilolite exchanged with sodium was found to be selective to Zn(II) from a solution containing Cd(II) and Ni(II) with a selective order: Zn(II) > Cd(II) > Ni(II). Curkovic et al. (1997) were able to remove Pb and Cd from water using a Croatian clinoptilolite treated with NaCl. They found retention efficiencies of 90 and 70%, respectively. Mercury is removed from wastewater by different methods such as chemical precipitation, conventional coagulation, lime softening, activated carbon adsorption, reverse osmosis and ion exchange. The last one is desirable because of it’s relative simplicity (Blanchard, 1984). A significant number of researchers have worked with zeolitic minerals and have determined selective sequences of these minerals for a range of metals, and

REMOVAL OF MERCURY BY ZEOLITIC MINERALS

181

they all agree that clinoptilolite shows a very low affinity to mercury. This mineral is more selective to toxic metals like Pb than Ca and Na but not to Hg. In natural waters HgCl2 , the most common mercuric salt, does not dissociate to a great extent. Many researchers have been looking for alternatives to increase the removal efficiency of metals from water by zeolitic minerals. A complete removal of Pb, Hg and Cd was achieved with simulated wastewater containing 50 mg L−1 of the metals, using polyacrylamide grafted hydrous tin(IV) oxide gel having carboxylate functional groups (Shubha et al., 2001). Other authors have used rice husk ash to remove mercury from water where the maximum adsorption was achieved at pH 6 and with a contact time of 3 hr (Tiwari et al., 1995). Khalid et al. (1999) have also used the same material to remove mercury from nitric acid solutions. They were able to remove 10.2 g of Hg by using 1 Kg of rice husk ash. Since heavy metals in general and mercury in particular show very high affinity to sulfur, some papers (Chiarle et al., 2000; Nooney et al., 2001; Celis et al., 2000; Mercier and Pinnavaia, 1998; Lagadic et al., 2001; Ritche et al., 2001) have been published concerning materials with sulfur in their structure or functionalized with organic compounds containing sulfur in order to increase the removal efficiency of heavy metals like Hg, Ag, Pb, Zn and Cd. Chiarle et al. (2000) used Doulite GT-73 which has a thiol functional group to remove mercury from an aqueous acidic solutions. Nooney et al. (2001) have used mesoporous materials functionalized with 3-mercaptopropyltrimethoxysilane ligands to remove mercury and silver; they found that the adsorption was directly proportional to the amount of sulfur in the material. Furthermore the material was more selective to mercury than to silver. Celis et al. (2000) have used clay minerals functionalized with thiol functional group and the functionalized materials were not selective to sodium and calcium. Functionalized phyllosilicates were also found to have a very high adsorption capacity for mercury, the removal achieved by these materials was up to 603 mg g−1 of mercury. In addition, these materials showed equal affinity to Pb and Cd. Materials formed by the grafting of a thiol to mesoporous silica molecular sieves and a thiol-functionalized magnesium phyllosilicate have been used as well in the removal processes (Mercier and Pinnavaia, 1998; Lagadic et al., 2001). Ritche et al. (2001) have shown the role of polycysteine and other polyamino acid functionalized microfiltration membranes on the capture of Hg, Pb, Cd and Cr. Furthermore, these materials could be regenerated. In general many materials have been studied for the removal of mercury from water; Lagadic et al. (2001) have done a good review of the materials capable of removing low concentrations of toxic metal ions from contaminated water. So far there is not any report on zeolitic minerals modified with organic compounds of sulfur in order to increase the efficiency of these materials to remove mercury from water. Thus the objective of the present work was to determine the properties of a zeolitic mineral and modify it with organic compounds of sulfur for the removal of mercury from homoionic and mixed heavy metal solutions.

182

T. GEBREMEDHIN-HAILE ET AL.

2. Materials and Methods 2.1. Z EOLITIC MINERAL AND REAGENTS The Mexican zeolitic mineral, clinoptilolite-heulandite from Taxco, Guerrero; was supplied by Lumogral Company. The diameter of the particles used to carry out the experimentation was 2.2 ± 0.34 mm, it was determined by an electron microscopy. The height and width of various particles in the sample were measured and the area of each particle was calculated, although they had irregular forms. Finally, in order to standardize the measurements, the particles were considered as spheres and their diameters were calculated. The composition and characteristics of the zeolitic mineral have been reported elsewhere (Rivera-Garza, 2000). Merck supplied cysteamine hydrochloride and cystamine dihydrochloride (for synthesis), all other reagents used in the experiments were of commercial analytic grade and the experiments were performed in duplicate to verify reproducibility. 2.2. M ODIFICATIONS OF THE ZEOLITIC MINERAL Mercury removal studies were carried out using a natural and three modified zeolitic mineral samples. ZNa Zeolitic mineral treated with 0.1 M NaCl solutions at 90 ◦ C over a period of 36 hr, washed with deionized water until no presence of chloride ions was indicated in the washing solution, using a silver nitrate solution. The sample was then dried at 80 ◦ C for 1 hr. ZNaS The Na-treated zeolitic mineral was put in reflux with a 0.05 M cysteamine hydrochloride (S) solution at 90 ◦ C over a period of 36 hr, washed with deionized water until no presence of chloride ions was indicated in the washing solution, using a silver nitrate solution. The sample was then dried at 80 ◦ C for 1 hr. ZNaSS The Na-treated zeolitic mineral was put in reflux with a 0.05 M cystamine dihydrochloride (SS) solution at 90 ◦ C over a period of 36 hr, washed with deionized water until no presence of chloride ions was indicated in the washing solution using a silver nitrate solution. The sample was then dried at 80 ◦ C for 1 hr. For comparison purposes, the natural zeolitic mineral (Z) was also used in the experimentation process.

REMOVAL OF MERCURY BY ZEOLITIC MINERALS

183

2.3. C HARACTERIZATION OF ZEOLITIC MINERALS The thermo gravimetric analysis was carried out with a TGA 51 TA thermo gravimetric analyzer, which was operated in a nitrogen atmosphere and with a heating rate of 10 ◦ C min−1 from 20 to 800 ◦ C. X-ray diffraction (XRD) analysis was performed on the zeolitic minerals (treated and untreated) to confirm the crystal structure and the identity of the components of the zeolitic mineral. Powder diffractograms of the sample were obtained with a Siemens D500 diffractometer coupled to a copper-anode X-ray tube. Conventional diffractograms were used to identify the compounds present in the zeolitic mineral. For scanning electron microscopy observation, the samples were mounted directly on the sample holders and covered with graphite. Finally the images were observed at 10 and 20 KeV by a Phillips XL30 electron microscope. The chemical composition of the zeolitic samples was determined by an EDS system coupled to the electron microscope. The FTIR spectra of each sample were obtained at room temperature by means of the Nicolet magna IRT M 550 spectrometer from 400 to 4000 cm−1 . For this purpose, pellets with the sample and KBr (ratio 1:100) were prepared. The BET surface areas were determined by standard multipoint techniques adsorbing nitrogen. A Micromeritics Gemini 2360 instrument was used. The samples were heated at 60 ◦ C during 2 hr before specific surface areas measurements. 2.4. M ERCURY UPTAKE CURVES A constant amount of zeolitic mineral samples (0.150 g) were mixed with 15 mL of aqueous mercury nitrate [Hg(NO3 )2 ] solution (0.031 mM) and were shaken for 10, 20 and 30 min, 1, 3, 5, 20, 24 and 72 hr. A few drops of nitric acid were added to the Hg(NO3 )2 solution in order to prevent the formation of oxides by maintaining the pH around 3. After shaking, the solid phases were separated by centrifugation. The supernatant solutions were analyzed for their mercury content using a GBC 932 plus atomic absorption spectrometer, at a wavelength of 253.7 nm. At the equilibrium, the pH changed slightly to values higher than 3. In the crystalline zeolites, ion exchange kinetic is controlled by diffusion of ions within the aluminosilicate structure. It has been shown that, for spherical particles, the extent of exchange follows (at its initial steps) the relationship (Breck, 1974):
Di t , (1) Qt /Q∞ = 6/r π where t (s) is the contact time, Qt and Q∞ (mg g−1 ) are the exchanged amounts of time t and at equilibrium t = ∞, respectively, and r (cm) represents the radius of the exchanged particles. In this work r was assumed to be 0.22 cm. This value corresponded to the mean particle size observed by electron microscopy. Qt /Q∞ was plotted against t 1/2 . From the slope of the curve, Di (apparent diffusion coefficient) was then estimated.

184

T. GEBREMEDHIN-HAILE ET AL.

2.5. I SOTHERMS For the isotherm experiments 0.031, 0.062, 0.093, 0.124, 0.155 and 0.310 mM Hg(NO3 )2 stock solutions were prepared in distilled water. Zeolitic mineral samples (0.150 g) were put in contact with 15 mL of the mercury nitrate solutions (pH 3). The mixtures were shaken for 24 hr and the phases were separated by centrifugation. The mercury concentrations in the liquid phases were determined as mentioned above. Mercury adsorption isotherms were fit to the Langmuir equation (Celis et al., 2000; Li and Bowman, 1997): Ce /Cs = Ce /Cm + 1/Cm L

(2)

where Cs is the amount of mercury adsorbed at the equilibrium concentration (Ce ), Cm is the maximum adsorption capacity of the adsorbent, and L is the Langmuir constant, which is related to the free energy of adsorption. Cm and L can be calculated from the linear plot of Ce /Cs vs. Ce . 2.6. M ERCURY REMOVAL FROM MIXED METAL SOLUTIONS The mercury removal behavior from aqueous solutions was studied in the presence of copper, nickel and zinc. For this purpose, stock solutions of the each nitrate salt were prepared (0.106, 0.109 and 0.105 mM copper, nickel or zinc, respectively). The mixtures used in the experiments were: Hg-Cu, Hg-Ni, Hg-Zn, Hg-Cu-Ni, Hg-Cu-Zn, Hg-Ni-Zn, and Hg-Cu-Zn-Ni. These solutions at pH 3 (15 mL) were put in contact with 0.150 g of each zeolitic mineral (Z, ZNa, ZNaS and ZNaSS) and shaken for 24 hr. The zeolitic minerals were separated by centrifugation. The concentrations of Hg, Ni and Zn in the remaining solutions were determined using a GBC 932 plus atomic absorption spectrometer, wavelength of 253.7, 341.5, and of 213.9 nm, respectively. For Cu determinations a Perkin Elmer atomic absorption spectrometer at wavelength 325 nm was used. Atomic absorption standards of copper, nickel, zinc and mercury for the metals analysis were obtained from Merck.

3. Results and Discussion 3.1. C HARACTERIZATION The thermogravimetric analysis showed a thermal degradation characteristic for this kind of materials (Z and ZNa) (Tsitsivilli et al., 1992). From the thermogravimetric analysis for both cysteamine hydrochloride and cystamine dihydrochloride, an important weight loss (almost a 100%) was observed for these compounds from 200 to 300 ◦ C (Figures 1a and b, respectively). The zeolitic minerals treated with

REMOVAL OF MERCURY BY ZEOLITIC MINERALS

185

Figure 1. TGA of (a) cysteamine hydrochloride, (b) cystamine dihydrochloride.

NaCl and modified with the organic compounds (ZNaS and ZNaSS) showed a weight loss at about 350 ◦ C (Figures 2a and b, respectively), this weight loss at the temperature mentioned above was not observed for Z and ZNa. According to these results, it could be suggested that the zeolitic minerals had retained the organic compounds. In general, the clinoptilolite tuffs contain 70–90% of clinoptilolite in association with authigenic species, such as montmorillonite, celadonite, chlorite, calcite, low cristobalite and sometimes, mordenite, as well as high-temperature resistant minerals such as quartz, plagioclase, biotite and potassium feldspar. The principal components of the Mexican mineral used in this work were reported elsewhere (Rivera-Garza et al., 2000).

186

T. GEBREMEDHIN-HAILE ET AL.

The XRD patterns of the Z, ZNa, ZNaS and ZNaSS are shown in Figures 3a, b, c, and d, respectively. The Z sample shows the principal diffraction peaks corresponding to clinoptilolite (reference card, JCPDS 25-1349D) and impurities like quartz (Figure 3a). When the zeolitic mineral was treated with a NaCl solution, some differences in the XRD pattern were observed with respect to Z. Mainly, new diffraction peaks at 29 and 42◦ were observed (Figure 3b). These changes can be attributed to the presence of Na+ into the zeolite network. When the zeolitic mineral (ZNa) was treated with cysteamine hydrochloride or cystamine dihydrochloride, notable changes were not observed (Figures 3c and d). Mumpton and Ormsby (1976) reported the characterization of zeolites in sedimentary rocks from different regions. The crystals of these materials have characteristic monoclinic symmetry of blades and laths, some of which are similar to the coffin shape of megascopic heulandite. The Mexican clinoptilolite-heulandite crystals are morphologically similar to that observed by these authors as it can be seen in Figure 4. No morphological changes were observed in the ZNa in comparison to the Z sample. However, some surface modifications as different crystals were observed in the ZNaS and ZNaSS samples (Figures 5a and b), which could be attributed to the modification. Furthermore from the elemental analysis (EDS), the presence of sulfur on the modified minerals was verified. In preliminary studies (Haile et al., 2001), it was verified that the organic compounds used in this work, are retained preferentially in the zeolitic mineral treated with NaCl. The elemental chemical composition of the zeolitic mineral and those treated with the organic compounds are shown in Table I. When the zeolitic mineral was treated with NaCl solution (ZNa), the amount of Na+ increased considerably in the zeolitic mineral, and K+ and Ca2+ diminished, but not in the same proportion because of selectivity for the materials and their concentrations. 0.470 meqK g−1 and 0.080 meqCa g−1 were exchanged by Na+ . 0.610 meqNa g−1 was exchanged by other ions present in the natural zeolitic mineral. An excess of NaCl in the zeolitic mineral after the treatment was not possible because chloride ions were not detected by the AgNO3 test in the washing solutions, furthermore chlorine was not observed by the EDS analysis. Calcium and potassium diminished in this sample and they did not change when treated with cysteamine hydrochloride or cystamine dihydrochloride, but the quantity of sodium diminished. It has been reported that sodium is the most effective exchangeable ion (Zamzow and Murphy, 1992). Therefore, it can be suggested that the retention of the organic compounds by the zeolitic mineral could have taken place by ion exchange with sodium. The content of cysteamine ion in the ZNaS was 0.375 meq g−1 , in this case the sodium plays a more important role in the ion exchange process than K+ and Ca2+ because of the ion selectivity, Table II; this behavior was also observed for the ZNaSS, and the amount of cystamine ions retained by the zeolitic mineral was of 0.475 meq g−1 .

REMOVAL OF MERCURY BY ZEOLITIC MINERALS

187

Figure 2. TGA of (a) ZNaS, (b) ZNaSS.

Pabalan and Bertetti (2001) have reported that the selectivity for potassium is higher than for sodium in clinoptilolite, therefore, it is more difficult to remove K+ than Na+ in this zeolitic mineral. Flanigen and Khatami (1971) have considered two groups of vibration frequencies in all zeolites: internal vibrations of T-O (considered insensitive to structure) and vibrations of external linkages between tetrahedral, due to topology and the mode of arrangement of the structure.

188

T. GEBREMEDHIN-HAILE ET AL.

Figure 3. Diffractograms of the different zeolitic minerals: (a) Z, (b) ZNa, (c) ZNaS and (d) ZNaSS.

Figure 4. SEM image of Z.

REMOVAL OF MERCURY BY ZEOLITIC MINERALS

Figure 5. SEM images of: (a) ZNaS, (b) ZNaSS.

189

190

T. GEBREMEDHIN-HAILE ET AL.

TABLE I Chemical composition of the different zeolitic mineral samples Element

Z

ZNa

ZNaS

ZNaSS

(Wt. %) O Na Mg Al Si S K Ca Fe

45.96 ± 0.42 0 1.74 ± 0.22 7.38 ± 0.06 35.91 ± 0.21 0 5.04 ± 0.01 1.35 ± 0.02 2.64 ± 0.41

43.33 ± 1.13 2.66 ± 0.37 1.84 ± 0.63 7.50 ± 0.62 36.65 ± 4.12 0 3.22 ± 2.26 1.18 ± 0.74 3.64 ± 2.90

47.65 ± 0.25 1.56 ± 0.34 1.53 ± 0.04 8.62 ± 0.62 33.92 ± 1.71 1.20 ± 0.13 3.48 ± 1.44 1.01 ± 0.86 1.05 ± 0.50

42.08 ± 0.58 1.49 ± 0.11 1.93 ± 0.28 7.10 ± 0.78 38.01 ± 1.27 1.52 ± 0.02 3.02 ± 0.20 1.27 ± 0.33 3.60 ± 1.41

TABLE II Amounts of sodium, potassium, calcium, cysteamine and cystamine ions in the different zeolitic mineral samples Zeolitic

Ion (meq g−1 )

mineral

Na+

K+

Ca2+

Cysteamine

Cystamine

Z ZNa ZNaS ZNaSS

–a 1.16 0.68 0.65

1.29 0.82 0.89 0.77

0.67 0.59 0.50 0.63

0 0 0.37 0

0 0 0 0.47

a Na was not detected.

Assignation of vibrations bands proposed for the Mexican zeolite mineral studied in this work have been reported elsewhere (Rivera-Garza et al., 2000). No notable changes were observed in the vibrations bands in the FTIR spectra of the ZNa (Figure 6a) compared with the vibration bands of Z. When the sodium zeolitic mineral was treated with organic compounds (cysteamine hydrochloride and cystamine dihydrochloride), changes in the range from 400 to 2000 cm−1 vibration bands were observed, as can be seen in Figures 6b and c, respectively). The presence of cysteamine or cystamine ions could be confirmed by the observation of a band assigned to C-H vibration in the 1250–1500 cm−1 region and a stretching band attributed to the C-S vibration in the 680–780 cm−1 region (Ritche et al., 2001). Although, the major band for the thiol groups (ZNaS), which usually

REMOVAL OF MERCURY BY ZEOLITIC MINERALS

191

Figure 6. FTIR Spectra: (a) ZNa, (b) ZNaS and (c) ZNaSS.

appears around 2555 cm−1 (Lagadic et al., 2001) was not observed, probably due to the small quantity of sulfur in this material. The BET surface areas found for the zeolitic minerals were about the same and the mean value was 16.7 ± 0.8 m2 g−1 , which is expected for natural zeolites and the modifications did not affect this parameter. 3.2. M ERCURY UPTAKE AND DIFFUSION COEFFICIENTS Hg(II) uptake in the different materials (Z, ZNa, ZNaS and ZNaSS) was monitored for 72 hr. The mercury sorption by the zeolitic minerals attains its equilibrium after approximately 10 hr (Figure 7a and b). The equilibrium uptake values were 0.00421 ± 0.00001 mmol Hg/gZ, 0.00467 ± 0.00004 mmol Hg/gZNa, 0.00473 ± 0.00003 mmol Hg/gZNaS and 0.00476 ± 0.00001 mmol Hg/gZNaSS at room temperature at initial mercury solution concentration of 0.031 mM. No desorption of mercury was observed up to 72 hr of contact time in all the cases studied in the present work. According to these results, the contact time chosen for the rest of the experiments was 24 hr.

192

T. GEBREMEDHIN-HAILE ET AL.

Figure 7. Mercury uptake curves: (a) ZNa and (b) ZNaS.

Diffusion of Hg(II) into the ZNa is about 2 times faster than in Z, ZNaS and ZNaSS, as can be observed in Table III. There are other components in Z (impurities of the natural material) that can affect the pass of Hg(II) through the zeolitic network, because in this case, the zeolitic mineral was not previously treated or it is more difficult for Hg(II) to be exchanged by the exchangeable cations from the natural zeolitic mineral (for example Ca2+ ) due to the mobility into the network of the zeolite. It is important to note that the diffusion of Hg(II) into the ZNaS and ZNaSS in comparison with ZNa, is affected by the presence of cysteamine or cystamine ions on the surface of the zeolitic minerals which can obstruct the porous of the zeolitic mineral (Table III). Olguín et al. (1994), determined the diffusion coefficient values for UO2+ 2 in erionite and Y zeolite. They concluded that the diffusion coefficient depended on the structure of each zeolite. Olguín et al. (1996), found that the diffusion of strontium through the cavities of the erionite is influenced by the concentration of

REMOVAL OF MERCURY BY ZEOLITIC MINERALS

193

TABLE III Diffusion coefficients for the different zeolitic mineral samples Zeolitic mineral

Di (cm2 s−1 )

Z ZNa ZNaS ZNaSS

1.22836E-07 ± 0.98438E-07 2.72603E-07 ± 0.35951E-07 1.63214E-07 ± 0.33330E-07 1.71868E-07 ± 0.17135E-07

this ion and the pH of the solution. According to Table III, it can be deduced that the cations present in the zeolitic mineral, the impurities and the surface chemical modifications play an important role on the diffusion behavior of mercury. It can also be suggested that the predominant mercury species (Puigdomenech; Ritchie et al., 2001) in the initial and equilibrium conditions are, Hg2+ , HgOH+ and Hg(OH)2 (Figure 8). These species could also affect the mercury diffusion, probably due to processes such as Hg(OH)2 precipitation on the zeolitic mineral surface at experimental pH values (3.0–3.5). 3.3. I SOTHERMS Figures 9a–d show the isotherms obtained for the mercury retention by the zeolitic mineral samples. The retention of mercury was the same for the four materials (Z, ZNa, ZNaS and ZNaSS) at the beginning of the isotherms (0.031 mM). The highest mercury retention was found for the modified zeolitic minerals being in contact with the highest initial concentration solution (0.310 mM), and the retention behavior was as follows: Z < ZNa < ZNaS ≈ ZNaSS. The retention of mercury is higher for the sodium zeolitic mineral than for the natural material, this behavior was expected since the ionic exchange in zeolites is more effective when the natural materials are treated with sodium ions (Zamzow et al., 1990; Curkovic et al., 1997; Pavon-Silva et al., 2000) and mercury retention by the Z and ZNa samples is attributed to cation exchange. The zeolitic mineral upon modification showed an increase in Hg(II) sorption. The increase in the Hg(II) sorption on both zeolitic minerals upon modification is attributed to the metal-chelating S functionality, which provides the zeolitic mineral surface with a remarkable affinity for Hg(II). Lagadic et al. (2001) found that two thiol groups are involved in the binding of the Hg(II) ions, suggesting that the Hg(II) ions are complexed in a bidentate manner. In other studies, some adsorbents were found to bind Hg(II) ions quantitatively to each complexation site in the material. It is still unclear why the Hg(II) ions can complex in two

194

T. GEBREMEDHIN-HAILE ET AL.

Figure 8. Species-distribution diagram obtained by simulation (1 × 10−5 M mercury nitrate solution).

Figure 9. Isotherms: Z (
), ZNa (), ZNaS () and ZNaSS (
).

REMOVAL OF MERCURY BY ZEOLITIC MINERALS

195

TABLE IV Langmuir coefficients, Cm and L, for Hg2+ adsorption on zeolitic minerals Zeolitic mineral sample

Cm (mmol g−1 )

L (L mmol−1 )

r

Z ZNa ZNaS ZNaSS

0.01072 ± 0.00064 0.03957 ± 0.00037 0.05099 ± 0.00003 0.05045 ± 0.00158

39.48265 ± 14.32485 32.06127 ± 4.10933 38.32861 ± 2.40409 47.86329 ± 5.26779

0.97831 ± 0.01344 0.99442 ± 0.00526 0.98916 ± 0.00496 0.93593 ± 0.02568

different coordination modes with the thiol groups, and further investigations of the structural features of the Hg(II) complexing process are necessary. The mercury uptake did not increase considerably from ZNaS to ZNaSS probably because the maximum mercury concentration used in this work was not enough to observe this effect or because the Hg(II) interaction with –SH groups for the case of ZNaS is more favorable than a Hg(II) interaction with –SS– groups for the case of ZNaSS, probably due to the steric impediment. It has been observed that the presence of the thiol groups in other materials improves its loading capacities for heavy metals (Mercier and Pinnavaia, 1998), since the reaction between the Hg(II) ions and the thiol groups is the most thermodynamically favorable (Lagadic et al., 2001) and it has been shown that metal sorption is directly related to the amount of thiol groups available in the sorbent (Mercier and Pinnavaia, 1998). The results of the present work show that in addition to the –SH group, –SS– also plays an important role in the removal of mercury from aqueous solutions. The mercury(II) adsorption isotherms had an affinity Langmuir character for all zeolitic minerals (Z, ZNa, ZNaS and ZNaSS). The mercury adsorption data were, therefore, described by the Langmuir equation with regression coefficients r > 0.93 (Table IV). Data in Table IV shows, that the Langmuir maximum adsorption capacities (Cm ) of zeolitic minerals for Hg(II) are greatly enhanced upon modifications. The adsorption capacities (Cm ) of the zeolitic minerals increased from 0.01070 to 0.05099 mmol Hg g−1 . Although, considering the standard deviations, the affinities (L) seem to be similar for all cases. The maximum adsorption capacities for ZNaS (0.05099 ± 0.00003 mmol Hg g−1 ) and ZNaSS (0.05045 ± 0.00158 mmol Hg g−1 ) are similar and represent a 13.6 and a 10.6% of the total –SH and –SS– groups present in the materials, respectively. 3.4. M ERCURY REMOVAL FROM MIXED METAL SOLUTIONS Table V shows the retention of mercury by the zeolitic mineral samples (Z, ZNa, ZNaS and ZNaSS), in mercury homoionic and Hg, Zn, Cu and Ni mixed metal

196

T. GEBREMEDHIN-HAILE ET AL.

solutions. The selectivity of the metal ions for a given modification depends on many factors, including the metal acidity, the size, the hydration, etc. The ionic radii of the ions studied in this work were: 1.16 Å for Hg(II), 0.87 Å for Cu(II), 0.83 Å for Ni(II) and 0.88 Å for Zn(II) (considering in all the cases a coordination number of 6) (Huheey, 1978). In the Z sample, it was observed that the highest retention of mercury corresponded to the homoionic solution; this behavior was expected since there is not any other metal to interfere for the exchange sites in the zeolitic mineral (M1, M2, M3 or M4 sites of the clinoptilolite) (Tsitsivilli et al., 1992). When mercury and other metals are present in the solutions, the mercury retention decreases and the lowest retention (19%, taking the sorption of mercury by Z in a homoionic solution as a reference point) is observed when three metals (Hg-Ni-Zn) are present in the solutions, because of the competition of these metals for the exchange sites into the zeolite network. The mercury sorption by ZNa increased 33% in relation to Z in a single component system. The lowest mercury sorption by the ZNa (35%, considering the mercury sorption by ZNa in a homoionic solution as a reference point) was obtained when the metals Cu, Zn, Ni and Hg were present. Mercury retentions by ZNa, in homoionic or in mixed metal solutions were always higher than those found for Z. The highest sorption of mercury was found for the ZNaSS (42 and 7% more than the sorption obtained for Z and ZNa, respectively, in homoionic solution) and then for the ZNaS (36 and 3% more than the sorption obtained for Z and ZNa, respectively, in homoionic solution) as it can be observed in Table V. The retention of mercury by ZNaSS and ZNaS was not considerably affected by the presence of the other metals in solutions. This behavior can be explained by the affinity of the Hg(II) for the sulfur groups of the zeolitic mineral treated with cystamine dihydrochloride (–SS–) or cysteamine hydrochloride (–SH). It has been reported that cysteine, which contains, a thiol group, has much different stabilities for the same metals. This is because the thiol is a soft group and hence, it interacts very strongly with soft metal ions like Hg(II) and no interferences could be expected from ions such as Ca2+ , Mg2+ and Na+ (hard metals), which are known to have little affinity for thiol groups (Huheey, 1978). It is important to note that even for the organically modified materials most of the ion exchange capacity of the zeolites is compensated by alkaline and earth alkaline ions. Therefore metal binding by mechanisms other than coordination with thiol/disulfide groups, i.e. ion exchange or surface precipitation, can not be generally excluded. Celis et al. (2000) investigated the competition between Hg(II) and non-specifically adsorbed cations, such as Na+ and Ca2+ , for the thiol moiety. Their results showed very little difference between the adsorption of Hg in the presence and absence of Na+ or Ca2+ background electrolytes (Celis et al., 2000). In agreement with the previous work where the heavy metal selectivity was not affected by the presence of electrolytes normally associated with groundwater and waste streams, our results seem to confirm the high affinity of mercury not only to the

197

REMOVAL OF MERCURY BY ZEOLITIC MINERALS

TABLE V Removal of mercury by the different zeolitic minerals in Hg homoionic and Hg, Zn, Cu and Ni mixed metal solutions (mmol of Hg g−1 of zeolitic minerals) Systems

Z

ZNa

ZNaS

ZNaSS

Hg CuHg ZnHg NiHg CuNiHg ZnNiHg CuZnHg CuZnNiHg

0.00681 ± 0.00010 0.00393 ± 0.00013 0.00492 ± 0.00014 0.00357 ± 0.00012 0.00200 ± 0.00018 0.00122 ± 0.00096 0.00246 ± 0.00033 0.00150 ± 0.00012

0.00905 ± 0.00001 0.00755 ± 0.00001 0.00861 ± 0.00004 0.00821 ± 0.00004 0.00671 ± 0.00005 0.00641 ± 0.00006 0.00626 ± 0.00001 0.00584 ± 0.00011

0.00930 ± 0.00001 0.00925 ± 0.00005 0.00902 ± 0.00002 0.00893 ± 0.00004 0.00911 ± 0.00004 0.00870 ± 0.00001 0.00936 ± 0.00007 0.00913 ± 0.00017

0.00972 ± 0.00001 0.00960 ± 0.00002 0.00981 ± 0.00001 0.00984 ± 0.00002 0.00964 ± 0.00005 0.00987 ± 0.00005 0.00990 ± 0.00009 0.00988 ± 0.00009

thiol moiety but also to the –SS– groups. Other cations like Cu2+ , Ni2+ or Zn2+ do not present a significant interference on the removal efficiencies of the modified zeolitic minerals. Copper from mixed solutions was analyzed after the sorption equilibrium was reached, and the results are shown in Table VI. Z did not retain any copper and in most of the systems its retention by the other materials was low. A retention of 28% of the initial concentration of copper by ZNa was found when this element was present with mercury. The copper sorption diminished from a 28 to an 18% of its initial concentration when this element was present with Hg and Ni, or Hg and Zn. For the most complex system (HgZnNiCu), the copper sorption by ZNa was the lowest (12.7% of its initial concentration). This is obviously attributed to the high competition of the ions Hg(II), Zn(II), Ni(II) and Cu(II) for the ionic exchange sites of the ZNa network. When the zeolitic mineral was modified with the sulfur compounds (ZNaS or ZNaSS), the copper sorption clearly diminished. This result shows the effect of the cysteamine or cystamine ions retained on the surface of the zeolite that prevents the diffusion of copper ions into the zeolite network. Although, according to Huheey (1978) the Cu(II) ion is classified as a borderline acid. Table VI shows the retention of nickel by the different materials. As it can be observed, ZNaSS retained a little amount of nickel, a 15% of its initial concentration, when this element was put together with Hg and about an 8% of its initial concentration for both mixed metal solutions HgZnNi and HgCuNi. This behavior was not observed with copper as mentioned above, although the Ni(II) ion is classified as hard acid (Huheey, 1978). For the case of zinc, no sorption was observed by Z and there was a retention of a 28% of its initial concentration by ZNa when this element was present with

198

T. GEBREMEDHIN-HAILE ET AL.

TABLE VI Copper, nickel and zinc concentration in solution after 24 hr of contact with the different zeolitic mineral samples in mixed metal solutions (meq 100 mL−1 ) Mixed metal solution

Z

ZNa

ZNaS

ZNaSS

0.0600 ± 0.0002 0.0610 ± 0.0002 0.0610 ± 0.0002 0.0620 ± 0.0002

0.0430 ± 0.0001 0.0510 ± 0.0002 0.0520 ± 0.0002 0.0550 ± 0.0002

0.0590 ± 0.0002 0.0620 ± 0.0002 0.0570 ± 0.0002 0.0620

0.0610 ± 0.0002 0.0620 ± 0.0002 0.0620 0.0630

0.0670 ± 0.0003 0.0660 ± 0.0003 0.0670 ± 0.0001 0.0670

0.0650 ± 0.0001 0.0690 ± 0.0006 0.0690 ± 0.0002 0.0680 ± 0.0004

0.0700 ± 0.0011 0.0690 ± 0.0002 0.0690 ± 0.0001 0.0660 ± 0.0002

0.0580 ± 0.0005 0.0610 ± 0.0001 0.0630 ± 0.0006 0.0680 ± 0.0008

0.0610 ± 0.0010 0.0600 ± 0.0010 0.0590 ± 0.0009 0.0610 ± 0.0120

0.0440 0.0520 ± 0.0004 0.0480 0.0550

0.0570 ± 0.010 0.0570 ± 0.010 0.0580 ± 0.0008 0.0550 ± 0.0008

0.0550 ± 0.0002 0.0530 ± 0.0003 0.0520 0.0530 ± 0.0040

Copper HgCu HgNiCu HgZnCu HgZnNiCu Nickel HgNi HgZnNi HgCuNi HgZnNiCu Zinc HgZn HgNiZn HgCuZn HgZnNiCu

Cu, Ni and Zn initial concentrations: 0.0630, 0.0681 and 0.0612 meq 100 mL−1 , respectively.

mercury, and the zinc sorption diminished from a 28 to a 21% when this element was present together with Hg and Cu, and from 28 to 15% when this element was present with Hg and Ni. The zinc sorption by ZNa was the lowest (10% of its initial concentration) for the most complex system (HgZnNiCu). When the zeolitic mineral was modified with the organic compounds (ZNaS and ZNaSS), a slight sorption of zinc was observed (a 10 and a 14% of its initial concentration, respectively), as can be observed in the Table VI. This behavior is similar to that observed for nickel, although the Zn(II) is classified as a borderline acid. Lagadic et al. (2001) have studied the adsorption of Hg in the presence of Pb and Cd by a thiol-functionalized magnesium phyllosilicate clay and they found it to be highly effective for the adsorption of these ions, in our case, the zeolites showed more selectivity for mercury than for Cu, Ni and Zn.

REMOVAL OF MERCURY BY ZEOLITIC MINERALS

199

Acknowledgements We acknowledge financial support from CONACyT project 35322-E and we thank L. Carapia and the technicians of the Chemistry Department (ININ) for technical support.

References Blanchard, G., Maunaye, M. and Martin, G.: 1984, ‘Removal of heavy metals from waters by means of natural zeolites’, Water Res. 18, 1501–1507. Bowman, R. S., Li, Z., Roy, S. J., Burt, T., Johnson, T. L. and Johnson, R. L.: 2001, in Smith and Burns (eds), Physicochemical Groundwater Remediation, Kluwer Academic/Plenum Publishers, Chapter 8. Breck, W.: 1974, Zeolite Molecular Sieves, Wiley-Interscience, New York. Celis, R., Hermosin, M. C. and Cornejo, J.: 2000, ‘Heavy metal adsorption by functionalized clays’, Environ. Sci. Technol. 34, 4593–4599. Chiarle, S., Ratto, M. and Rovatti, M.: 2000, ‘Mercury removal from water by ion exchange resins adsorption’, Water Res. 34, 2971–2978. Curkovic, L., Cerjan-Stefanovic, S. and Filipan, T.: 1997, ‘Metal ion exchange by natural and modified zeolites’, Water Res. 31, 1379–1382. Flanigen, E. M. and Khatami H.: 1971, Molecular Sieve Zeolites, American Chemical Society, 1, Washington. Haile, T., Olguín, M. T. and Solache-Ríos, M.: 2001, ‘Zeolita Natural Mexicana para la Remoción de Mercurio del Agua’ Memorias del 2◦ Congreso Mexicano de Zeolitas Naturales, Ed. UAMAzcapozalco, pp. 153–156. Huheey, J. E.: 1978, Inorganic Chemistry: Principles and Structures of Reactivity, Happer & Row. Khalid, N., Ahmad, S., Naseer, S. K and Ahmed, J.: 1999, ‘Removal of mercury from aqueous solutions by adsorption to rice husks’, Separat. Sci. Technol. 34, 3139–3153. Lagadic, I. L., Mitchell, M. K. and Payne, B. D.: 2001, ‘Highly effective adsorption of heavy metal ions by a thiol-functionalized magnesium phyllosilicate clay’, Environ. Sci. Technol. 35, 984– 990. Li, Z. and Bowman, R. S.: 1997, ‘Counterion effects on the sorption of cationic surfactant and chromate on natural clinoptilolite’, Environ. Sci. Technol. 31, 2407–2412. Mercier, L. and Pinnavaia, T. J.: 1998, ‘Heavy metal ion adsorbents formed by grafting of a thiol functionality to mesoporous silica molecular sieves: Factor affecting Hg(II) uptake’, Environ. Sci. Technol. 32, 2749–1754. Mumpton, F. A. and Ormsby, W. C.: 1976, ‘Morphology of zeolites in sedimentary rocks by scanning electron microscopy’, Clay and Clay Minerals 24, 1–23. Nooney, R. I., Kalyanaraman, M., Kennedy, G. and Maginn, E. J.: 2001, ‘Heavy metal remediation using functionalized mesoporous silicas with controlled macrostructure’, Langmuir 17, 528–533. Olguín, M. T., Solache, M., Asomoza, M., Acosta, D., Bosch, P. and Bulbulian, S.: 1994, ‘UO2+ 2 sorption in natural Mexican erionite and Y zeolite’, Separat. Sci. Technol. 29, 2161–2178. Olguín, M. T., García-Sosa, I. and Solache-Ríos M.: 1996, ‘Sorption of strontium by Mexican erionite’, J. Radioanalyt. Nucl. Chem. Articles 204, 415–422. Pabalan, R. T. and Bertetti, F. P.: 2001, in D. L. Bish and D. W. Ming (eds), Reviews in Mineralogy and Geochemistry, Mineralogical Society of America, Chapter 14. Pavon-Silva, T. B., Campos, E. and Olguín, M. T.: 2000, ‘Remoción de níquel, cadmio y zinc del agua, utilizando clinoptilolita heulandita’, Ciencia Ergo Sum 7, 251–258.

200

T. GEBREMEDHIN-HAILE ET AL.

Puigdomenech, I.: Program MEDUSA (Make Equilibrium Diagrams Using Sophisticated Algorithms), http://www.inorg.Kth.se/Ignasi/Index.html. Ritchie, S. M. C., Kissick, K. E., Bachas, L. G., Sikdar, S. K., Parikh, C. and Bhattacharyya, D.: 2001, ‘Polycysteine and other polyamino acid functionalized microfiltration membranes for heavy metal capture’, Environ. Sci. Technol. 35, 3252–3258. Rivera-Garza, M., Olguín, M. T., García-Sosa, I., Alcántara, D. and Rodríguez-Fuente: 2000, ‘Silver supported on natural Mexican zeolite as an antibacterial material’, Microporous and Mesoporous Materials 39, 431–444. Roque-Malherbe R.: 2001, in H. S. Nalwa (ed.), Handbook of Surfaces and Interfaces of Materials, Academic Press, Chapter 12, pp. 495–522. Shubha, K. P., Raji, C. and Anirudhan, T. S.: 2001, ‘Immobilization of heavy metals from aqueous solutions using polyacrylamide grafted hydrous Tin(IV) oxide gel having carboxylate functional groups’, Water Res. 35, 300–310. Sullivan, E. J., Hunter, D. B. and Bowman, R. S.: 1997, ‘Topological and thermal properties of surfactant-modified clinoptilolite studied by tapping-mode atomic force microscopy and highresolution thermogravimetric analysis’, Clays and Clay Minerals 45, 42–53. Tiwari, D. P., Sing., D. K. and Saksena, D. N.: 1995, ‘Hg(II) adsorption from aqueous solutions using rice-husk ash’, J. Environ. Engineer. 121, 479–481. Tsitsivilli, G. V., Andronikashvili, T. G., Kirov, G.N., Felizova, L. D.: 1992, Natural Zeolites, Ellis Horwood, U.K. Zamzow, M. J., Eichbaum, B. R., Sandgren, K. R. and Shanks, D. E.: 1990, ‘Removal of heavy metals and other cations from wastewater using zeolites’, Separat. Sci. Technol. 25, 1555–1569. Zamzow, M. J. and Murphy, J. E.: 1992, ‘Removal of metal cations from water using zeolites’, Separat. Sci. Technol. 27, 1967–1984.