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Adsorption of mercury(II) on dithizone-immobilized natural zeolite
Accepted Manuscript Title: Adsorption of Mecury(II) on Dithizone-Immobilized Natural Zeolite Author: Mudasir Mudasir Karelius Karelius Nurul Hidayat Aprilita Endang Tri Wahyuni PII: DOI: Reference:
S2213-3437(16)30095-1 http://dx.doi.org/doi:10.1016/j.jece.2016.03.016 JECE 1018
To appear in: Received date: Revised date: Accepted date:
6-1-2016 29-2-2016 8-3-2016
Please cite this article as: Mudasir Mudasir, Karelius Karelius, Nurul Hidayat Aprilita, Endang Tri Wahyuni, Adsorption of Mecury(II) on DithizoneImmobilized Natural Zeolite, Journal of Environmental Chemical Engineering http://dx.doi.org/10.1016/j.jece.2016.03.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Publication manuscript Adsorption of Mecury(II) on Dithizone-Immobilized Natural Zeolite
Mudasir Mudasir*[email protected], [email protected], Karelius Karelius, Nurul Hidayat Aprilita and Endang Tri Wahyuni Chemistry Department, Faculty of Mathematics and Natural Sciences, Gadjah Mada University, Sekip Utara, P.O. Box Bls. 21, Yogyakarta 55281, Indonesia, * Corresponding author: Tel.: +62 274 545188; Fax: +62 274 513339
Abstract Adsorption of Hg(II) ions onto selective adsorbent of dithizone-immobilized natural zeolite (DIZ) from Wonosari, Yogyakarta, Indonesia has been investigated in batch mode. Some parameters influencing immobilization of dithizone and adsorption of Hg(II) were optimized including effect of temperature and reaction time on the effectiveness of dithizone immobilization as well as effect of pH, contact time and initial concentration of Hg(II) ion on the efficiency of the Hg(II) adsorption. Preliminary application of the adsorbent in the removal of Hg(II) ion in river water spiked with Hg(II) was also studied. The FT-IR and XRD analytical results show that the surface of natural zeolite can be modified by immobilization of selective organic ligand towards Hg(II) ions. The optimum conditions for the adsorption of Hg(II) is achieved at pH 5 and 90 minutes of contact time. Kinetics and adsorption isotherm studies suggest that the capacity, affinity and selectivity of the DIZ in adsorbing hazardous metal ions such as Hg(II) is significantly improved compared to those of non-immobilized activated natural zeolite (AZ). The Hg((II) adsorption capacity for AZ and DIZ at optimum condition is 8.0 and 13.1 µmol/g,
respectively and the both adsorption follows first-order reaction. It has also been demonstrated that the DIZ adsorbent may be applied in the removal of Hg(II) ion in the river water spiked with Hg(II) ion up to 99,36 % from the initial concentration of 8 mg L-1 in two serial batch adsorptions.
Keywords: mercury; selective adsorbent; natural zeolite; dithizone; Immobilization
1. Introduction Many toxic heavy metal ions have been discharged into the environment as industrial wastes, causing serious soil and water pollution [1]. Mercury (Hg), lead (Pb), copper (Cu), iron (Fe), and chromium (Cr) are common metals that tend to accumulate in organisms, causing numerous diseases and disorders [2]. Hg(II) has especially received increasing attention as a serious pollutant due to its toxicity and bio-accumulative properties. In aquatic system, mercury can be converted by bacteria to methyl mercury, which can be magnified hundreds of thousands of times through aquatic food chain, posing a potential risk to humans and wildlife in the aquatic that consume fish. The major resources of mercury pollution in the aquatic are industries including electrical and medical equipment (batteries and thermometers), paint and color industries, catalyst, fungicides or pesticides. In addition, coal combustion, geothermal gas and waste incineration emit large quantities of mercury into the atmosphere [3]. Numerous processes exist for removing dissolved heavy metals, including ion exchange, precipitation, phytoextraction, ultrafiltration, reverse osmosis, and electrodialysis [4]. However, these methods generally require high cost and technology, thus making it not suitable for developing countries like Indonesia. Therefore, the use of alternative low-cost materials as potential sorbents for the removal of heavy metals has been increasing recently [5-6].
Zeolites are naturally occurring hydrated aluminosilicate minerals. They belong to the class of minerals known as “tectosilicates”. Most common natural zeolites are formed by alteration of glass-rich volcanic rocks (tuff) with fresh water in playa lakes or by seawater [7]. The structures of zeolites consist of three-dimensional frameworks of SiO4 and AlO4 tetrahedra. The aluminum ion is small enough to occupy the position in the center of the tetrahedron of four oxygen atoms, and the isomorphous replacement of Si4+ by Al3+ produces a negative charge in the lattice. The net negative charge is balanced by the exchangeable cation (sodium, potassium, or calcium). These cations are exchangeable with certain cations in solutions such as lead, cadmium, zinc, and manganese [8,9]. The fact that zeolite exchangeable ions are relatively innocuous (sodium, calcium, and potassium ions) makes them particularly suitable for removing undesirable heavy metal ions from industrial effluent waters. However, like many other natural adsorbants, the adsorption of zeolite towards metal ions is not selective, especially when alkaline and alkaline-earth metal ions are also available in the solution in high level of concentrations. Therefore, modification of the surface of natural adsorbants including zeolite is frequently done using a specific and sensitive ligand for heavy metal ions to enhance the capacity and selectivity of the adsorbants [10-11]. For example, some host materials has been used for immobilization of sugarcane bagasse waste biomass for biosorption of chromium [12] and Mangifera indica waste biomass for biosorption of Pb(II) [13]. Moreover, pre-treated biomaterials such as rosa bourbonia phyto-biomass for removal of Pb (II) and Cu (II) [14], groundnut (Arachis hypogaea) shell for divalent ions [15] as well as lemon grass (Cymbopogon citratus) for Pb(II), Cd(II) and Zn(II) ions [16] also has been reported as promising adsorbants of heavy metal ions. Meanwhile, Asasian, et al has reported elimanation of mercury by adsorption onto activated carbon prepared from the biomass material [17].
Dithizone (diphenylthiocarbazone) is a suitable ligand for such purposes because it contains several N-donor atoms, -NH as well as –SH groups which is excellent for analytical and spectroscopic applications. This compound is also very specific for chelating heavy metal ions such as Pb(II), Cd(II), Cu(II) and Hg(II) [18, 19]. Therefore, it is quite reasonable to use this chelating compound as modifiying agent in the surface of various adsorbent materials such as alumina [20], polymer poly (EGDMA-HEMA) microbeads [10] and silica gel [11, 21-22]. The immobilization of dithizone onto the surface of the above polymer as well as silica gel has been reported to be successfully used for the removal and selective pre-concentration of heavy metals. The aim of this study is to prepare a mercury-selective adsorbent by immobilization of dithizone onto the surface of zeolite and to study its adsorption characteristics towards mercury(II) ion. The modified adsorbent is intended to be used for the adsorption of mercury pollutant in industrial liquid waste as well as the supporting material in the solidphase extraction process for pre-concentration of mercury ion. Dithizone has been selected as the organic ligand in this study because this ligand is very selective for Hg, Cd and Pb. In this study, we use natural zeolite from Wonosari, Yogyakarta, Indonesia as a supporting material for the immobilization of dithizone because it is easily obtained in Indonesia and its price is cheaper than the synthetic polymer or silica gel.
2. Materials and Methods 2.1 Materials Mercury(II) chloride salts (HgCl2) and dithizone (1,5-diphenylthiocarbazone) were purchased from Merck, Germany. Natural zeolite was obtained from Wonosari, Yogyakarta, Indonesia. Zeolite activation and dithizone immobilization were prepared and characterized according to previously reported procedure [23, 24]. Organic solvents were
of reagent grade and used as received. For all solutions, double distilled water was used and the buffer solutions were prepared by mixing citric acid and sodium citrate for pH < 7 and sodium dihydrogen phosphate (NaH2PO4.H2O) and sodium hydrogen phosphate (Na2HPO4.12H2O) for pH: 7-9 and the pH-value of the resulting solution was checked by pH-meter. 2.2 Instrumentations The pH measurements were conducted by a TOA pH meter model HM5B calibrated against two standard buffer solutions of pH 4.0 and 9.2. Infrared spectra of natural zeolite and dithizone-immobilized zeolite were measured from KBr pellets by a Shimadzu FT-IR/8201 PC spectrophotometer. Mercury(II) determination was performed by a GBC 932 AA atomic absorption spectrophotometer equipped with GBC HG-3000 vapour generation accessory at Laboratory of Analytical Chemistry, Chemistry Department, Gadjah Mada University, Yogyakarta, Indonesia. X-ray diffraction (XRD) spectra of natural zeolite and dithizone-loaded natural zeolite were recorded on Phillips model PW 3710 BASED XRD spectrophotometer (Shimadzu 6000X, radiation source: Cu, K-lambda 1.5402 nm). 2.3 Dithizone Immobilization on Zeolite The following procedure which is analog to the previously reported procedure [2224] was applied for immobilization of dithizone on the surface of zeolite. Zeolite (4.0 g) was added to 80 ml toluene and mixed with 1.0311 g dithizone in a 500 mL flask. The mixture was heated and stirred for 4h and the temperature of the mixture was varied at 50, 70, 90 and 110 oC. The product was filtered and washed consecutively with toluene, ethanol and water several times until the filtrate showed no characteristic color of dithizone. The dithizone-immobilized zeolite (DIZ) was then dried in an oven at 70 oC for 6 h and filtered through 200-mesh filter. The obtained DIZ was then characterized and used for
adsorption study. 2.4 Adsoption Study 2.4.1 Effect of pH on the Hg(II) adsorption
The experiment was done by batch system. The adsorbent (20 mg, 200 mesh) was interacted with 10 mL of Hg(II) ion (5 mg/L) solution and the pH of the solution was varied from 3.0 to 9.0 using buffer solution. The mixture was stirred using horizontal shaker for 60 minutes. The mixtures was then filtered and the concentration of Hg(II) ion in the filtrate was determined by cold-vapor (CV) AAS. For a control solution, the same concentration of Hg(II) solution at corresponding evaluated pH was treated in the similar procedure but without the addition of adsorbent. 2.4.2 Effect of contact time on the Hg(II) adsorption This experiment was carried out to evaluate kinetic aspects of Hg(II) ion adsorption on the DIZ adsorbent. A similar procedure to that described in the previous section (effect of pH) was applied but the stirring/contact time was varied from 5 to 180 minutes. The obtained data were then evaluated by modified Langmuir-Hinshelwood equation [16]:
C ln( 0
CA
CA
)
= k1 ( t
CA
)+K
(1)
Where Co is initial concentration of metal ion and CA is concentration of metal ion at equilibrium. Plot of ln(C0/CA)/CA versus t/CA gives the slope and Y-intercept equal to firstorder adsorption rate (k1) and adsorption equilibrium constants (K), respectively. 2.4.3 Effect of initial concentration of metal ions on the adsorption capacity The effect of initial concentration of Hg(II) ions on the adsorption capacity of DIZ adsorbent was examined by conducting adsorption experiment in various concentration of Hg(II) ion. The similar procedure to pH effect study was applied but the concentration of
Hg(II) was varied in the range of 0 – 10 mg/L. The obtained data were evaluated by Langmuir isotherm adsorption model (Eq. 2) to determine adsorption capacity (b) and equilibrium constant (K). C 1 1 = + C m bK b
(2)
Where C is concentration of Hg(II) ion at equilibrium and m is mole of Hg(II) ion adsorbed by 1.0 g of adsorbent. The value of K was then used to calculate the adsorption energy (E) based on the equation of standard Gibbs-Hemholtz relation (Eq. 3). E = -∆Go = RT ln K
(3)
Where R is gas constant (8,314 J/mol K) and T is absolute temperature (K). 2.4.4 Removal of Hg(II) from aqueous solutions Fifteen milliliter of synthetic sample solution (river water) containing 8 mg/L Hg(II) was interacted with 0.02 g AZ or DIZ adsorbents. The mixture was stirred for 90 minutes and the adsorbent was filtered using Whatman 42 filter. The filtrates were collected and subjected to cold vapor atomic absorption spectrophotometric analysis (CVAAS) for the determination of Hg(II) ion concentration. For the second adsorption, the same procedure as first adsorption was applied to 10 mL of filtrate obtained from the first adsorption and the filtrates were again subjected for CV AAS for the determination of Hg(II) concentration remaining in the filtrate after second adsorption.
3. Results and Discussion 3.1 Dithizone Immobilization and its Characterization 3.1.1 Dithizone immobilization on zeolite Dithizone immobilization on zeolite has been done by mixing dithizone dissolved in toluene and natural zeolite that has been activated with hydrofluoric acid (HF) for 4 hours [22-24]. The mixture was stirred and heated at various temperatures of 50, 70, 90
and 110 oC. The results obtained were then filtered and washed subsquently with toluene, ethanol and double-distilled water until no dithizone trace was detected in the filtrate. The obtained dithizone-zeolite was dried in an oven at a temperature of 70 °C for 6 hours. After drying, the dithizone-zeolite was sieved to ensure the homogeneity of particle size. The adsorbants obtained from the above process was a reddish brown solid hereinafter called dithizone-immobilized zeolite (DIZ). In this experiment, the adsorbent was first washed with toluene to remove the remaining free (non-immobilized) dithizone on the surface of zeolite. The further washing of the adsorbent using ethanol and water is aimed to remove semi-polar and polar impurities, respectively. The obtained adsorbant was again dried in the oven to remove the remnants of solvents that still exist on the surface of DIZ adsorbent. The immobilization of dithizone in this study is basically coating the solid surface of natural zeolite with selective organic compound with the hope that the active groups available on the organic ligand can give additional serve as an active site of adsorbent. Organic ligand, such as dithizone, has been known to have several active groups such as -SH and -NH in its structure, therefore it is expected to enhance the interaction facilities between Hg(II) and the surface of the adsorbent via covalent interactions. There are several factors that affect the immobilization of dithizone on the surface of zeolite including temperature, contact time and the amount of dithizone used in immobilization. In this study we have specifically investigated the effect of various heating temperature (50, 70, 90 and 110 oC) on the effectiveness of dithizone immobilization on the surface of natural zeolite. The adsorbents obtained from different heating temperature of mixing were characterized by FT-IR spectroscopy and the results are shown in Fig. 1. Comparison of the infrared spectra of AZ (Fig. 1A) and those of DIZ (Figure 1B) reveals that immobilization process conducted at temperature of 50 and 70 °C gives the
adsorbent with higher content of dithizone than that carried out at temperature of 90 and 110 oC. This conclusion can be derived by directly observing the higher vibration intensity of dithizone immobilized at temperature of 50 and 70 °C as compared to that obtained at temperatures of 90 and 110 oC. The effectiveness of immobilization at lower temperature may be attributed to the stability of dithizone towards oxidation at lower temperatures. As a result, the immobilization carried out at 50 and 70 °C undergoes more effectively. At higher temperature e.g. 90 or 110 °C, and with the existence of atmospheric air, dithizone can be easily damaged by oxidation, therefore it becomes more difficult to immobilize dithizone on the surface of the natural zeolite.
The DIZ adsorbents obtained at various temperatures were also subjected to X-ray diffraction (XRD) analysis. For the purpose of comparison, the XRD patterns of free dithizone and DIZ prepared at temperature of 50, 70 oC are presented in Fig. 2, while the important data of the XRD spectra are collected in Table 1. It is clearly observed from the table that the peaks characteristic for dithizone appear more intense on DIZ prepared at temperature of 50 °C than those done at 70 oC. Therefore, for the next experiment, we have selected temperature of 50 °C as immobilization temperature of dithizone. Another reasons for the selection of this temperature is that heating at lower temperatures is more easily achieved and maintained than the higher one. Moreover, heating at lower temperature also reduces the risk of the oxidation process of dithizone.
3.1.2 Characterization of dithizone-immobilized zeolite The effect of dithizone immobilization on the surface of zeolite may be observed from the changes in the wavenumber of infrared spectra of functional groups by comparing the infrared spectra of AZ, DIZ and free dithizone (see Figure 1B, 1C and 1D). It is clearly
shown in Fig. 1D that the infrared spectra of free dithizone has very sharp absorption bands at wavenumber of 1496.7 cm-1 and 1438.8 cm-1, indicating the presence of the double bond C=C stretching of the aromatic phenyl group, whereas absorption band at wavenumber of 748.3 cm-1 and 678.9 cm-1 represents the vibration of aromatic C-H bending, which is normally observed in 900-690 cm-1. Absorption band at wavenumber of 2912.3 cm-1 is attributed to the presence of benzene substitution. Absorption band at 3552.6 cm-1 indicates the existence of N-H stretching vibration, while the sharp absorption band at 1589.2 cm-1 belongs to the N-H bending vibration. Weak absorption band in the region around 2500 cm-1 indicates the presence of –S-H group, while the weak peak at 2200 cm-1 region is due to the presence of the C=N. Absorption band at wave number 1350-1000 cm-1 is the C-N stretching vibration. Upon immobilization of dithizone onto zeolite surface as presented in Fig. 1C, the FT-IR spectra significantly change (Fig. 1B), which is characterized by the shift of absorption and the existence of new absorption bands. These phenomena suggest the successful immobilization of dithizone onto the zeolite surface. Absorption band at wavenumber of 1488.9 cm-1 and 1461.9 cm-1 indicates the presence of C=C double bonds of the aromatic phenyl group which is supported by the appearance of absorption band at wavenumber of 3012 cm-1, showing the stretching vibration of aromatic C=C. The absorption at 2854.5 cm-1 shows the stretching of aliphatic C-H vibration, and absorption at wavenumber of 763.8 cm-1 and 686.6 cm-1 indicate the presence of aromatic C-H bending vibration. Absorption band at wavenumber of 1311.5 cm-1 shows the stretching of the C-N vibration, while that at wavenumber of 3448.5 cm-1 indicates the presence of N-H stretching vibration. Weak absorption band at wave number 2345.3 cm-1 is likely due to the presence of the S-H stretching vibration. All of the IR bands mentioned above indicate the presence of dithizone in the zeolite material. Meanwhile, absorption which is
characteristics for zeolites still appears as a strong absorption at 1049.2 cm-1, indicating the presence of internal asymmetric stretch of the primary building blocks of zeolite, e.g. Si-O and Si-O-Si. Absorption band at wavenumber of 1639.4 cm-1 shows the O-H bending vibration of silanol group. Also, absorption band appears at 470.6 cm-1 and 428.2 cm-1 suggesting the availability of bending vibration of Si-O-Si [25].
Additional characterization of the obtained adsorbents has been performed using XRD analysis. In this case, the d space and 2θ angle values which are characteristic for both zeolites and dithizone are examined and compared to determine whether the dithizone has been successfully immobilized on surface of the zeolite. Based on the results of XRD analysis of AZ (Figure not shown), DIZ (Fig. 2A) and free dithizone (Fig. 2C), it is easily noticed that several peaks which are characteristic for both zeolites and dithizone can be found in DIZ, indicating that dithizone has successfully been immobilized on the surface of the zeolite. The detailed XRD peaks of DIZ are listed in Table 2. This finding is consistent with the results of IR spectroscopic analysis and supported the conclusion that immobilization of dithizone has been successfully done as discussed in the previous section. 3.2 Adsorption study of Hg(II) ions In this study, the ability of DIZ to adsorb Hg(II) has been systematically evaluated by examining several factors that affect the ability of DIZ in adsorbing Hg(II). The parameters studied include the effect of pH, adsorption kinetics and adsorption isotherms. Evaluation of the effect of pH on the Hg(II) adsorption of the adsorbent has been conducted with the purpose to determine the optimum pH of adsorption. The adsorption kinetics was studied on the basis of adsorption rate constant data evaluated by using the
Langmuir-Heinshelwood equation and Santosa models [26]. Furthermore, evaluation of adsorption isotherms has also been carried out to obtain the adsorption capacity, the equilibrium constant as well as the adsorption energy at constant temperature. This evaluation has been determined by the Langmuir-Heinselwood equation. The adsorption energy data obtained from this evaluation is then used to classify the interaction mechanism operated in the adsorption processes between adsorbate of Hg(II) and the adsorbent of DIZ, e.g. physical or chemical interaction and the results are compared with those done using AZ. Figure 3 gives the scheme of possible interaction mechanism between Hg(II) and the active sites of DIZ adsorbant.
3.2.1 Effect of pH on the Hg(II) adsorption Study of the effect of pH on the adsorption of Hg(II) has been conducted by interacting 5 mg/L of Hg(II) solution with DIZ adsorbent at different pH of 3, 4, 5, 6, 7, 8 and 9. For the purpose of comparison, adsorption of Hg(II) on AZ is also conducted. After interaction, the remaining metal ions in the solution was determined by cold vapor AAS method to calculate the adsorbed Hg(II). In this study, the control solution, i.e. the same concentration of Hg(II) solution at various pH but without the addition of the adsorbent was also used for the purpose of error correction that may arise from the precipitation of Hg(II) ions at certain pH or from metal loss caused by the adsorption of metal ions on the surface walls of the container during the process of interaction.
The pH of the solution plays an important role in the adsorption process, because the effectiveness of the adsorption is determined by the chemical form (speciation) of Hg(II) ions in solution and active groups on the surface of the adsorbent. These speciations is basically pH dependent. Fig. 4 gives the results of the effect of pH on the amount of adsorbed Hg(II) on the adsorbents.
Figure 4(A) and (B) clearly show that there is a significance difference in the amount of the adsorbed Hg(II) on each adsorbent at the same pH. In general, it can be concluded that the ability of dithizone-immobilized zeolite in adsorbing Hg(II) is higher than that of zeolite itself. The activated zeolite without immobilization of dithizone is able to adsorp Hg(II) but the adsorbed Hg(II) is relatively small. This may be due to the role of pores and active sites available in zeolite, such as aluminol, silanols and siloxanes. In contrast, we clearly observe that the ability of zeolite-immobilized dithizone to adsorb Hg(II) are relatively high, doubled than that of activated zeolite. This high ability of adsorption is probably due to the involvement of not only the pores and active site of zeolite, but also the additional active groups from immobilized dithizone on the surface of the zeolite such as -SH and -NH. This provides much more active groups that are ready to interact with metal ions, e.g. Hg(II). The twice increase in the Hg(II) adsorption ability of dithizone-immobilized zeolite may also be explained with respect to Hard and Soft Acid-Base (HSAB) principle. In dithizone-immobilized zeolite adsorbent, there are -SH and –NH active groups which are classified as soft bases according to the HSAB theory. This functional groups prefer to interact with soft acid such as Hg(II) ions. Therefore, it is quite easy to understand that Hg(II) ion will favorably bind to the active groups of dithizone, leading to much more Hg(II)ion is adsorbed by dithizone-immobilized zeolite as compared to that adsorbed by activated zeolite. In Fig. 4, it appears in general that Hg(II) ion adsorbed by the two adsorbents increase with increase of the pH of metal ion solution and after reaching the maximum value, the amount of adsorbed Hg(II) starts to decrease. This trend may be explained as follow, at low pH, the concentration of H+ ion in the solution is high, resulting in the active groups on the surface of the adsorbent be protonated and produce a partially positive
charge on the functional groups/active site of adsorbent. This condition is not favorable for the occurence of the interaction between the positively charged Hg(II) and the protonated active groups on the surface of adsorbent, due to electrostatic repulsion. Furthermore, the high concentration of H+ in solution at low pH also induces strong competition between H3O+ ions and the Hg(II) ion in solution to bind to the active group on the surface of the adsorbent. Therefore it is understandable that the higher acidity of the solution (lower pH) leading to the lower adsorption of Hg(II) ions on the adsorbent surfaces. The amount of Hg(II) adsorbed by the two types of adsorbent tends to increase with the increase of pH of the solution. In this study, we found that the optimum pH for the adsorption of Hg(II) on zeolite is at pH 7, which is close to the reported pH of 8 for the adsorption of Hg(II) onto activated carbon prepared from the biomass material [17]. At pH = 7, it is expected that the protonated H+ on the surface of the adsorbent is released, so that the active groups on the surface of the adsorbent is becoming negatively charged. On the other hand, Hg(II) ion in the solution is still in the form of positive ion, leading to very supportive condition for the interaction between Hg(II) and the active groups on the surface of the adsorbent. In case of DIZ, the optimum pH of adsorption is reached at pH 5. The lower optimum pH for DIZ compared to AZ is probably due to the effect of dithizone immobilization. The same optimum pH of 4-6 has been reported by Yu, et al [21] for the adsorption of Hg(II) on dithizone-immobilized silica gel, while optimum pH of 7 has also been reported for the adsorption of Hg(II) onto surface of dithizone-alumina [20]. At pH 5, the amount of H+ ions available in solution is not much enough to be able to protonate the active group of the adsorbent, so that the equilibrium reaction between metal ions (M2+) and dithizone ligand (H2DZ) to form metal-dithizone complex, M(HDZ)n favorably goes the right side, i.e. the formation of metal dithizonate complexes:
M2+
+
H2O + nH2DZ
M(HDZ)n +
H3O+
However, further increasing of pH results in the decrease of the amount of adsorbed Hg(II). This may be attributed to the deposition of Hg(II) as Hg(OH)2 (Ksp Hg(OH)2 = 3.13x10-26). Also, the functional groups on the adsorbent will undergo de-protonation at higher pH, resulting in a partially negative charged active site. Similarly, the Hg(II) ion at higher pH also tend to form negatively charged complex in the excess of OH- ions. Therefore, electrostatic repulsion of negatively charged species occurs in the solution, as the result, the metal ions are becoming very difficult to interact with the active groups of the adsorbent, leading to the decline in the number of adsorbed metal ions.
3.2.2 Isotherm adsorption Isothermal adsorption models are often used to determine the adsorption capacity and it shows a correlation between the activity of the adsorbent and the adsorbed amount of a substance at constant temperature is the Langmuir adsorption isotherm. This model assumes that the adsorbent surface is homogeneous and the magnitude of the adsorption energy is equivalent to each active adsorption sites[27]. Each of the active sites of the adsorbent will adsorb one molecule of adsorbate. When the adsorption sites has not been saturated yet with adsorbate molecules, the increase in the concentration of the adsorbate will always be accompanied by an increase in the amount of metal ions adsorbed. Conversely, if the adsorption sites have been saturated with the adsorbate molecule, the increase in adsorbate concentration will not increase the amount of metal ions adsorbed [28]. In this study, the adsorption isotherms of Hg (II) on the two types of adsorbent has been studied using the following procedure, a solution of Hg(II) with various
concentrations of 0.5, 1, 2, 4, 6, 8 and 10 mg/L was interacted with the adsorbent at optimum pH and interaction time of corresponding adsorbent and the results are presented in Fig. 5. The figure shows that the two adsorbents give the same pattern of Hg(II) adsorption, i.e. the greater the initial concentration of metal ions, the more metal ions are adsorbed by the adsorbent. The amount of adsorbed Hg(II) ion is then constant at a certain concentration of Hg(II). This situation is happened because the metal ions in solution occupy active sites of the adsorbent, the greater the concentration of metal ions in the solution, the more active sites of the adsorbent are occupied by metal ions, if all the active sites on the surface of the adsorbent has been occupied by Hg(II) ions, no more Hg(II) ion can be adsorbed by the adsorbent, and therefore the amount of adsorbed Hg(II) remains constant. This condition indicates that the equilibrium between the active sites on the adsorbent and Hg(II) ion has been reached, hence the addition of Hg(II) ion will not result in any significant change in the amount of Hg(II) ion adsorbed by the adsorbent.
For adsorption using AZ (Fig. 5B), it is observed that the Hg(II) adsorption with initial concentration in the range of 0.5 to 2 µg/ml has been undergone slowly, whereas at concentrations above 2 mg/L, adsorption occur rapidly and achieve optimum conditions at concentration of 4 mg/L. The slow adsorption may occur because of the active groups available in AZ are silanols, aluminol and siloxane which is classified as hard base according HSAB theory. At low concentration, Hg(II) ion is difficult to interact with active groups on the surface of the adsorbent due to the low selectivity and affinity of the adsorbent towards soft-acid of Hg(II) ion. At high concentration, the amount of Hg(II) ion in the solution is quite large, therefore, it increases the possibility of interaction between the Hg(II) ion and the active groups on the surface of the adsorbent, resulting in faster adsorption process of the adsorbent and Hg(II) ion. The same condition is also applied to
DIZ adsorbent (Fig. 5A), however in this case the adsorption of Hg(II) on the adsorbent involves not only the active group of the zeolite, but also the active groups of ditizone, which have great affinity and selectivity to ions Hg (II). Therefore, the adsorption using DIZ adsorbent achieves the optimum condition at higher initial concentration than that of activated zeolite, e.g. at 6 mg/L, indicating that DIZ has more active sites with greater affinity and selectivity that can be occupied by Hg(II) ion. Adsorption thermodynamic parameters including adsorption capacity, adsorption equilibrium constant and adsorption energy can be determined by applying the Langmuir adsorption isotherm models to the obtained data, according to the following equation: C 1 C = + m bK b By plotting C/m against C, the value of K (equilibrium constant) and b (adsorption capacity) can be calculated from the slope and the intercept the graph. From the K value, the adsorption energy can then be calculated using the following standard relation: Eads = ∆G 0 = RT ln K where R is the general gas constant (8.314 J / mol K) and T is the temperature (Kelvin). The results of the calculation of Langmuir isothermal adsorption parameters are presented in Table 3. Based on the data summarized in Table 3, it is shown that the adsorption capacity of Hg(II) for DIZ adsorbent (13.1 x 10-6 mol g-1) is comparable the to reported value of 20 x 10-6 mol g-1 for dithizoze-silica gel at pH=6 [20]. The little bit lower result in this study is probably due to the lower amount of dithizone that can be immobilized onto the surface of zeolite compared to that of silica gel. However, it is clearly observed that the adsorption capacity and adsorption equilibrium constant of Hg(II) for DIZ adsorbent is still higher than those of AZ. This suggests that the immobilization of dithizone on zeolite has increased the adsorption capacity of zeolite due to the increase in the number of new active
sites from immobilized dithizone. The functional groups of dithizone has been known to have high affinity and selectivity to Hg(II) ion. The above explanation is also supported by finding that the value of the equilibrium constant of the reaction is quite large, meaning that the complex formation reaction favorably goes to the right side, e.g. the formation of the products. Hence, much more Hg(II) ion is binding to the active groups of dithizone immobilized on the zeolite. Adsorption process can occur by means of physical interaction (physisorption), chemical interaction (chemisorption) or mixture of them. Adamson [29] categorized adsorption as physisorption if its adsorption energy is less than 20.92 kJ/mol and considered as chemisorption if its adsorption energy is more than 20.92 kJ/mol. Based on this classification, we may conclude that the adsorption of Hg(II) on both AZ and DIZ adsorbents is classified as chemisorption because the their adsorption energy is higher than 20.92 kJ/mol, i.e. 36.09 kJ/mol and 39.55 kJ/mol, respectively. The possible chemical interactions operated in the adsorption process are electrostatic interaction between Hg(II) ion and silanol, or aluminol groups of zeolite (in AZ and DIZ adsorbents), as well as coordination covalent bonds between Hg(II) ion and functional groups of immobilized dithizone on the surface of the zeolite (in DIZ only).
3.2.3 Kinetics of adsorption Adsorption kinetics was studied by determining the adsorption rate constant and adsorption equilibrium constants. The data were obtained by interacting 5 mg/L of Hg(II) solution with the two types of adsorbent at different contact time of 5, 15, 30, 60, 90, 120 and 180 minutes and at corresponding optimum pH of adsorption. Effect of interaction time on the amount of adsorbed Hg(II) ion is presented in Fig. 6. Adsorption kinetics study is aimed to determine the value of adsorption rate constant which is able to give information on the rate of adsorption process. The
information about the time required to achieve adsorption equilibrium is very useful to determine the contact time required to achieve optimum result of adsorption. Generally, when the adsorption equilibrium has been reached, the increase in interaction time will not significantly change the amount of metal ions adsorbed by the adsorbent. Fig. 6 shows clearly that the amount of Hg(II) adsorbed by DIZ is always larger than that adsorbed by AZ. This finding again supports the explanation that the dithizone immobilized on the surface of the zeolite has improved both selectivity and affinity/capacity of the adsorbent towards Hg(II) ion due to the existence of additional active groups from dithizone, i.e. -SH and –NH. These two active groups have preferency to interact with Hg(II) ion, leading to more Hg(II) ion is adsorbed by DIZ as compared to that adsorbed by AZ.
Based on Fig. 6, it can also be observed that in general the amount of metal ions adsorbed by the adsorbent increases with the increase in interaction time between adsorbents and Hg(II) ion. Aftrer reaching optimum value of time, the adsorbed Hg(II) remains relatively constant. The general trend of Hg(II) adsorption on AZ and DIZ is relatively the same. However, in case of DIZ, it is observed that at the first 5 minutes of contact time the amount of Hg(II) adsorbed on the adsorbent increases significantly and reaches a constant value after 90th minute of contact time. On the other side, the adsorption of Hg(II) by AZ occurs slowly at the beginning of adsorption, and reaches a constant value after 60th minute of interaction. The constant value of adsorption indicates that the equilibrium has been achieved between Hg(II) ions in solution and the active groups on the surface of the adsorbent. The difference in value of optimum contact time between the two types of adsorbent is presumably due to the longer time needed for the complexation between dithizone on the surface of zeolite and Hg(II) ion. Since there is
various active groups available on DIZ, the interaction involves not only the active groups of the zeolite, but also the active groups of dithizone, e.g. via chelate formation. Therefore it can easily be understood that the time required for DIZ to reach equilibrium is much longer compared to that needed by AZ. The data obtained from this experiment was then evaluated to determine the adsorption kinetic parameters, e.g. the rate of adsorption (k) and adsorption equilibrium constants (K), using the following Langmuir-Heishelwood adsorption kinetics model: C ln 0 C k1t A + K = C0 − C A C0 − C A C ln 0 C t A In plot of versus , the values of K and k1 are respectively given by C0 − C A C0 − C A the Y-intercept and the slope of the plot. Results of k and K calculation are presented in Table 4. In order to obtain the best model of the adsorption kinetics, the kinetic parameters which have been evaluated by Langmuir-Heinshelwood equation are compared with those obtained by the kinetic model proposed by Santosa [26], as follows: ln
C0
C A
CA
ln Plot of
C0
C A
CA
=
k1 t + K CA
versus
t gives K and k1 values respectively as the Y-intercept and CA
slope of the plot. Detailed calculation results are presented in Tabel 5. Comparing Table 4 and 5, it is clearly observed that the kinetic parameters evaluated by Santosa model [26] gives better results from the point of view of linearity (rvalue). The data presented in Table 4 and 5 also suggest that the value of the adsorption rate constant (k) for DIZ is larger than that of AZ. This indicates that the immobilization of
dithizone on the surface of zeolite increases the rate of adsorption. In the adsorption of Hg(II) by AZ, the metal ion interact with active groups of siloxane, silanol and aluminol groups which according to HSAB (Hard Soft Acid-Base) principle are classified as the hard base, hence it would be more difficult for soft acid species like Hg(II) to interact with. This reason explains the lower rate constants observed for the Hg(II) adsorption on AZ. On the other hand, the higher value of adsorption rate constant for DIZ is probably due to the diversity of active groups available on the surface of adsorbent as a result of dithizone immobilization on the surface of the zeolite. The active groups of -SH and –NH are classified as soft bases according to HSAB theory which have great affinity and selectivity towards soft acid such as Hg(II). As a result, a lot of Hg(II) has been able to be adsorbed by DIZ in a relatively short time. The fact that the value of the equilibrium constant (K) for DIZ is larger compared to that of AZ also supports this explanation. This finding indicates that the adsorption capacity of DIZ towards Hg(II) is larger than that of AZ, resulting in more Hg (II) ion is able to be adsorbed by the DIZ adsorbant.
3.3 Removal of mercury(II) from river water spiked with Hg(II) Application of adsorbent for the removal of Hg(II) ion in the river water spiked with Hg(II) has been studied by diluting a solution containing certain concentration of Hg(II) ion with filtered river water in a 100 mL of volumetric flask so that the final concentration of Hg(II) in the solution is 8 mg/L. The river water used for the study was taken from Progo river, Bantul, Yogyakarta Province, Indonesia at two points of sampling, before and after the bridge. The detailed composition of the river water is not specifically determined in this study, but it is expected to contain some heavy metal ions other than Hg(II) as well as domestic waste because the river is flowing across the city of Yogyakarta and home industrial and domestic wastewaters are normallly thrown directly to the rives.
The AZ and DIZ adsorbents were then interacted with the two types of Hg(II)-spiked river water samples at optimum conditions of Hg(II) adsorption. The mixture solution was then filtered and analyzed using CV-AAS to determine the content of Hg(II) remaining in the filtrate. To determine the effectiveness of the adsorbent in the adsorption of Hg(II) in the samples, the filtrate from the first interaction was interacted again with the two adsorbents in the same way as for the first interaction. Detailed results of experiment is presented in Fig. 7. It can be seen from this figure that approximately 51.73% of Hg(II) ion can be removed from the solution at first adsorption by AZ and up to 80.89% by using DIZ. For the second adsorption, up to 97.98% and 99.36% of Hg(II) has been able to be removed from the solution using AZ and DIZ adsorbents, respectively. Based on these results, again we see that the DIZ adsorbent has greater effectiveness in the removal of Hg(II) ion from the river water samples compared to AZ adsorbent. This better performance of the modified adsorbant can be achieved because of the influence of the higher affinity and selectivity of functional groups of dithizone that has been immobilized on the surface of the zeolite towards Hg(II) ion.
4. Conclusion Dithizone-immobilized zeolite (DIZ) adsorbent has been prepared by mixing natural zeolite and dithizone in the toluene medium at 50 oC for 4h to yield brown-reddish adorbent with high affinity and selectivity to Hg(II) ion. The optimum conditions for the adsorption of Hg(II) ion using this adsorbent is achieved at pH 5 and 90 minutes of contact time. The kinetic study suggests that the rate of adsorption of Hg(II) on DIZ is higher than that of activated zeolite (AZ). Adsorption isotherm experiment reveals that the adsorption Hg(II) on DIZ is best described by Langmuir model with adsorption capacity, equilibrium constant and energy higher than that of AZ. This higher performence of DIZ has been found to be the effect of the functional groups of dithizone immobilized on the surface of
the zeolite. This modified adsorbent has been successfully applied in the removal of Hg(II) ion in the river water spiked with Hg(II) ion with satisfied results, suggesting that the modified adsorbant may be used in the future for removal of Hg(II) in industrial waste or used as promising material for solid phase extraction and preconcentration of Hg(II) . Acknowledgement This work is partially supported by Directorate General of Higher Education (DGHE), Ministry of Education and Culture, The Republic of Indonesia through National Strategic Research Grant (Penelitian Stranas) fiscal year 2014 contract no. LPPMUGM/1041/LIT/2014 dated May 5, 2014.
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[16] A. Babarinde, K. Ogundipe, K. T. Sangosanya, B. D. Akintola, A. E. Hassan, Comparative study on the biosorption of Pb(II), Cd(II) and Zn(II) using Lemon grass (Cymbopogon citratus): Kinetics, isotherms and thermodynamics, Chem. Inter. 2 (2016) 89-102. [17] N. Asasian, T. Kaghazchi, M. Soleimani, Elimination of mercury by adsorption onto activated carbon prepared from the biomass material, J. Ind. Eng. Chem. 18 (2012) 283-289. [18] Z. Marczenko, Separation and Spectrophotometric Determination of Elements, Chichester, West Sussex, UK.: Ellis Horwood Ltd., Chap. 3, 1986, pp. 88-94. [19] L. C. Thomas, G. J. Chamberlin, Colorimetric chemical analytical methods, Salisbury, England: The Tintometer, Ltd, 1980. [20] M. E. Mahmoud, M. M. Osman, O. F. Hafez, A. H. Hegazi, E. Elmelegy, Removal and preconcentration of lead(II) and other heavy metals from water by alumina adsorbents developed by surface-adsorbed dithizone, Desalination 251 (2010) 123130. [21] H-M. Yu, H. Song, M-L. Chen, Dithizone immobilized silica gel on-line preconcentration of trace copper with detection by flame atomic absorption spectrometry, Talanta 85 (2011) 625-630. [22] M. Mudasir, D. Maryanti, G. Pramiyanti, R. Roto, Preconcentration study of Pb(II) and Cd(II) from aqueous solution using silica gel loaded with dithizone, J. Ion Exchange 18 (2007) 516-517. [23] M. Mudasir, K. Wijaya, S. Suseno, P.D. Ola, A. Suseno, “Proceeding of 2nd International Conference for Young Chemist (ICYC-2006): Environmental Chemistry, 24-27 May 2006, USM, Penang-Malysia, pp. 1-9.
[24] M. Mudasir, D. Siswanta, P. D. Ola, Adsorption characteristics of Pb(II) and Cd(II) ions on dithizone-loaded natural zeolite, J. Ion Exchange 18 (2007) 418-323. [25] R. M. Silverstein, B. C. Bassler and T. C. Morrill, Spectrometric Identification of Organic Compounds, 4th edition, Menlo Park, California, 1963. [26] S.J. Santosa, D. Siswanta, A. Kurniawan, W.H. Rahmanto, Hybrid of chitin and humic acid as high performance sorbent for Ni(II), Surf. Sci. 601 (2007) 5155–5161. [27] E. Guibal, J.M. Tobin, Chitosan Sorbent for Platinum Sorption from Diliute Solution, Ind. Eng. Chem. Res. 38 (1998) 4011-4023. [28] J. Oscik, Adsorption, Ellis Harwood Limited, England, 1982. [29] A.W. Adamson, Physical Chemistry of Surfaces, 5th edition, John Wiley and Sons Inc, New York, 1990.
Fig. 1 Infrared spectra of dithizone-immobilized natural zeolite (DIZ) at various temperature of reflux, 50 °C (A), 70 °C (B), 90 °C (C) and 110 °C (D).
Fig. 2 XRD patterns of dithizone-immobilized zeolite at various temperature of reflux, 50 o C (A), 70 oC (B) and free dithizone (C). Fig. 1; Mudasir, et al (JECE, 2016)
Fig. 2; Mudasir, et al (JECE, 2016) Fig. 3; Mudasir, et al (JECE, 2016) Fig. 3 Possible interaction mechanism between Hg(II) and the active sites of dithizoneimmobilized zeolite (DIZ) adsorbant. Fig. 4; Mudasir, et al (JECE, 2016)
Fig. 4 Effect of pH on the adsorption of Hg(II) on dithizone-Immobilized zeolite (A) and activated zeolite (B).
Fig. 5 Effect of initial concentration of Hg (II) on the amount of Hg(II) adsorbed by dithizone-immobilized zeolite (A) and activated zeolite (B).
Fig. 6 Effect of interaction time on the amount of adsorbed Hg(II) ion on dithizoneimmobilized zeolite (A) and activated zeolite (B).
Fig. 7 The effectiveness of activated zeolite (AZ) and zeolite-immobilized ditizon (ZID) in the removal of Hg(II) in the river water samples taken from the west bridge (1) and the eastern bridge (2) of river Progo, Bantul DIY spiked with Hg(II). Before adsorption (blue), after first adsorption (red) and after second adsoption (green). Table 1. Interpretation of 2θ and d-space values as well as the relative intensity (I/Io) of XRD peaks for dithizone-immobilized zeolite (DIZ) obtained at heating temperature of 50 and 70 °C Materials 2θ d I/Io I (degree) (spacing) 9.5400 9.2633 26 116
DIZ prepared at 50 oC
DIZ prepared at 70 oC
Free dithizone
13.5250 15.3756 19.7064
6.5416 5.7579 4.5014
26 52 48
114 230 215
2.7899 9.3200 13.5416 15.3765 19.7370
2.7899 9.4815 6.5334 5.7579 4.4945
11 3 29 13 40
49 12 115 50 160
32.0696 10.8532 13.4141 14.8671
2.7887 8.1452 6.5954 5.9539
9 16 20 100
36 340 420 2078
19.0712 32.3946
4.6499 2.7615
40 10
834 215
Table 2. Interpretation of 2θ, d-space values as well as the relative intensity of the XRD peaks for dithizone-immobilized zeolite (DIZ) 2θ(degree) d I/Io Interpretation (spacing) ▓ 9.5400 9.8481 13.5250 15.3756 19.7064 22.3818 25.7457 26.4010 27.8320 28.2876 32.0546
9.2633 8.9742 6.5416 5.7579 4.5014 3.9690 3.4575 3.3732 3.2030 3.1524 2.7899
26 41 26 52 48 65 100 41 49 31 11
dithizone zeolite dithizone dithizone dithizone zeolite zeolite zeolite zeolite zeolite dithizone
Table 3. Langmuir isothermal adsorption parameters on Hg(II) on activated zeolite adsorbent (AZ) and dithizone-immobilized zeolite (ZID). Langmuir isothermal adsorption parameters Adsorbent
Adsorption
Equilibrium
adsorption
capacity (b) (mol g-1)
constant (K) (L mol-1)
energy (kJ mol-1)
Plot Linearity (r)
8.00 x 10-6 13.1 x 10-6
AZ ZID
1.92 x 106 7.71 x 106
36.09 39.55
0.9993 0.9998
Table 4. The kinetic parameters of Langmuir-Heishelwood adsorption for of Hg(II) on activated zeolite (AZ) and dithizone-immobilized zeolite (DIZ). Langmuir-Heishelwood kinetic parameters Adsorbent 1st order ▓ Equilibrium Plot linearity adsorption (r) rate constants constants (mol 1 (min-1) L) 1.00 x 10-7 4.50 x 10-3
AZ DIZ
2.76 x 102 4.77 x 104
0.8109 0.9692
Table 5. Parameters of adsorption kinetics calculated by model of Santosa [26] for Hg(II) ion on activated zeolite (AZ) and dithizone-immobilized zeolite (DIZ). Parameters of adsorption kinetics according to Santosa model [26] Adsorbent 1st order Equilibrium Plot linearity adsorption (r) rate constants constants (mol 1 (min-1) L) -3 AZ 8.10 x 10 2.76 x 103 0.9479 DIZ 13.0 x 10-3 4.77 x 105 0.9720
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S2213-3437(16)30095-1 http://dx.doi.org/doi:10.1016/j.jece.2016.03.016 JECE 1018
To appear in: Received date: Revised date: Accepted date:
6-1-2016 29-2-2016 8-3-2016
Please cite this article as: Mudasir Mudasir, Karelius Karelius, Nurul Hidayat Aprilita, Endang Tri Wahyuni, Adsorption of Mecury(II) on DithizoneImmobilized Natural Zeolite, Journal of Environmental Chemical Engineering http://dx.doi.org/10.1016/j.jece.2016.03.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Publication manuscript Adsorption of Mecury(II) on Dithizone-Immobilized Natural Zeolite
Mudasir Mudasir*[email protected], [email protected], Karelius Karelius, Nurul Hidayat Aprilita and Endang Tri Wahyuni Chemistry Department, Faculty of Mathematics and Natural Sciences, Gadjah Mada University, Sekip Utara, P.O. Box Bls. 21, Yogyakarta 55281, Indonesia, * Corresponding author: Tel.: +62 274 545188; Fax: +62 274 513339
Abstract Adsorption of Hg(II) ions onto selective adsorbent of dithizone-immobilized natural zeolite (DIZ) from Wonosari, Yogyakarta, Indonesia has been investigated in batch mode. Some parameters influencing immobilization of dithizone and adsorption of Hg(II) were optimized including effect of temperature and reaction time on the effectiveness of dithizone immobilization as well as effect of pH, contact time and initial concentration of Hg(II) ion on the efficiency of the Hg(II) adsorption. Preliminary application of the adsorbent in the removal of Hg(II) ion in river water spiked with Hg(II) was also studied. The FT-IR and XRD analytical results show that the surface of natural zeolite can be modified by immobilization of selective organic ligand towards Hg(II) ions. The optimum conditions for the adsorption of Hg(II) is achieved at pH 5 and 90 minutes of contact time. Kinetics and adsorption isotherm studies suggest that the capacity, affinity and selectivity of the DIZ in adsorbing hazardous metal ions such as Hg(II) is significantly improved compared to those of non-immobilized activated natural zeolite (AZ). The Hg((II) adsorption capacity for AZ and DIZ at optimum condition is 8.0 and 13.1 µmol/g,
respectively and the both adsorption follows first-order reaction. It has also been demonstrated that the DIZ adsorbent may be applied in the removal of Hg(II) ion in the river water spiked with Hg(II) ion up to 99,36 % from the initial concentration of 8 mg L-1 in two serial batch adsorptions.
Keywords: mercury; selective adsorbent; natural zeolite; dithizone; Immobilization
1. Introduction Many toxic heavy metal ions have been discharged into the environment as industrial wastes, causing serious soil and water pollution [1]. Mercury (Hg), lead (Pb), copper (Cu), iron (Fe), and chromium (Cr) are common metals that tend to accumulate in organisms, causing numerous diseases and disorders [2]. Hg(II) has especially received increasing attention as a serious pollutant due to its toxicity and bio-accumulative properties. In aquatic system, mercury can be converted by bacteria to methyl mercury, which can be magnified hundreds of thousands of times through aquatic food chain, posing a potential risk to humans and wildlife in the aquatic that consume fish. The major resources of mercury pollution in the aquatic are industries including electrical and medical equipment (batteries and thermometers), paint and color industries, catalyst, fungicides or pesticides. In addition, coal combustion, geothermal gas and waste incineration emit large quantities of mercury into the atmosphere [3]. Numerous processes exist for removing dissolved heavy metals, including ion exchange, precipitation, phytoextraction, ultrafiltration, reverse osmosis, and electrodialysis [4]. However, these methods generally require high cost and technology, thus making it not suitable for developing countries like Indonesia. Therefore, the use of alternative low-cost materials as potential sorbents for the removal of heavy metals has been increasing recently [5-6].
Zeolites are naturally occurring hydrated aluminosilicate minerals. They belong to the class of minerals known as “tectosilicates”. Most common natural zeolites are formed by alteration of glass-rich volcanic rocks (tuff) with fresh water in playa lakes or by seawater [7]. The structures of zeolites consist of three-dimensional frameworks of SiO4 and AlO4 tetrahedra. The aluminum ion is small enough to occupy the position in the center of the tetrahedron of four oxygen atoms, and the isomorphous replacement of Si4+ by Al3+ produces a negative charge in the lattice. The net negative charge is balanced by the exchangeable cation (sodium, potassium, or calcium). These cations are exchangeable with certain cations in solutions such as lead, cadmium, zinc, and manganese [8,9]. The fact that zeolite exchangeable ions are relatively innocuous (sodium, calcium, and potassium ions) makes them particularly suitable for removing undesirable heavy metal ions from industrial effluent waters. However, like many other natural adsorbants, the adsorption of zeolite towards metal ions is not selective, especially when alkaline and alkaline-earth metal ions are also available in the solution in high level of concentrations. Therefore, modification of the surface of natural adsorbants including zeolite is frequently done using a specific and sensitive ligand for heavy metal ions to enhance the capacity and selectivity of the adsorbants [10-11]. For example, some host materials has been used for immobilization of sugarcane bagasse waste biomass for biosorption of chromium [12] and Mangifera indica waste biomass for biosorption of Pb(II) [13]. Moreover, pre-treated biomaterials such as rosa bourbonia phyto-biomass for removal of Pb (II) and Cu (II) [14], groundnut (Arachis hypogaea) shell for divalent ions [15] as well as lemon grass (Cymbopogon citratus) for Pb(II), Cd(II) and Zn(II) ions [16] also has been reported as promising adsorbants of heavy metal ions. Meanwhile, Asasian, et al has reported elimanation of mercury by adsorption onto activated carbon prepared from the biomass material [17].
Dithizone (diphenylthiocarbazone) is a suitable ligand for such purposes because it contains several N-donor atoms, -NH as well as –SH groups which is excellent for analytical and spectroscopic applications. This compound is also very specific for chelating heavy metal ions such as Pb(II), Cd(II), Cu(II) and Hg(II) [18, 19]. Therefore, it is quite reasonable to use this chelating compound as modifiying agent in the surface of various adsorbent materials such as alumina [20], polymer poly (EGDMA-HEMA) microbeads [10] and silica gel [11, 21-22]. The immobilization of dithizone onto the surface of the above polymer as well as silica gel has been reported to be successfully used for the removal and selective pre-concentration of heavy metals. The aim of this study is to prepare a mercury-selective adsorbent by immobilization of dithizone onto the surface of zeolite and to study its adsorption characteristics towards mercury(II) ion. The modified adsorbent is intended to be used for the adsorption of mercury pollutant in industrial liquid waste as well as the supporting material in the solidphase extraction process for pre-concentration of mercury ion. Dithizone has been selected as the organic ligand in this study because this ligand is very selective for Hg, Cd and Pb. In this study, we use natural zeolite from Wonosari, Yogyakarta, Indonesia as a supporting material for the immobilization of dithizone because it is easily obtained in Indonesia and its price is cheaper than the synthetic polymer or silica gel.
2. Materials and Methods 2.1 Materials Mercury(II) chloride salts (HgCl2) and dithizone (1,5-diphenylthiocarbazone) were purchased from Merck, Germany. Natural zeolite was obtained from Wonosari, Yogyakarta, Indonesia. Zeolite activation and dithizone immobilization were prepared and characterized according to previously reported procedure [23, 24]. Organic solvents were
of reagent grade and used as received. For all solutions, double distilled water was used and the buffer solutions were prepared by mixing citric acid and sodium citrate for pH < 7 and sodium dihydrogen phosphate (NaH2PO4.H2O) and sodium hydrogen phosphate (Na2HPO4.12H2O) for pH: 7-9 and the pH-value of the resulting solution was checked by pH-meter. 2.2 Instrumentations The pH measurements were conducted by a TOA pH meter model HM5B calibrated against two standard buffer solutions of pH 4.0 and 9.2. Infrared spectra of natural zeolite and dithizone-immobilized zeolite were measured from KBr pellets by a Shimadzu FT-IR/8201 PC spectrophotometer. Mercury(II) determination was performed by a GBC 932 AA atomic absorption spectrophotometer equipped with GBC HG-3000 vapour generation accessory at Laboratory of Analytical Chemistry, Chemistry Department, Gadjah Mada University, Yogyakarta, Indonesia. X-ray diffraction (XRD) spectra of natural zeolite and dithizone-loaded natural zeolite were recorded on Phillips model PW 3710 BASED XRD spectrophotometer (Shimadzu 6000X, radiation source: Cu, K-lambda 1.5402 nm). 2.3 Dithizone Immobilization on Zeolite The following procedure which is analog to the previously reported procedure [2224] was applied for immobilization of dithizone on the surface of zeolite. Zeolite (4.0 g) was added to 80 ml toluene and mixed with 1.0311 g dithizone in a 500 mL flask. The mixture was heated and stirred for 4h and the temperature of the mixture was varied at 50, 70, 90 and 110 oC. The product was filtered and washed consecutively with toluene, ethanol and water several times until the filtrate showed no characteristic color of dithizone. The dithizone-immobilized zeolite (DIZ) was then dried in an oven at 70 oC for 6 h and filtered through 200-mesh filter. The obtained DIZ was then characterized and used for
adsorption study. 2.4 Adsoption Study 2.4.1 Effect of pH on the Hg(II) adsorption
The experiment was done by batch system. The adsorbent (20 mg, 200 mesh) was interacted with 10 mL of Hg(II) ion (5 mg/L) solution and the pH of the solution was varied from 3.0 to 9.0 using buffer solution. The mixture was stirred using horizontal shaker for 60 minutes. The mixtures was then filtered and the concentration of Hg(II) ion in the filtrate was determined by cold-vapor (CV) AAS. For a control solution, the same concentration of Hg(II) solution at corresponding evaluated pH was treated in the similar procedure but without the addition of adsorbent. 2.4.2 Effect of contact time on the Hg(II) adsorption This experiment was carried out to evaluate kinetic aspects of Hg(II) ion adsorption on the DIZ adsorbent. A similar procedure to that described in the previous section (effect of pH) was applied but the stirring/contact time was varied from 5 to 180 minutes. The obtained data were then evaluated by modified Langmuir-Hinshelwood equation [16]:
C ln( 0
CA
CA
)
= k1 ( t
CA
)+K
(1)
Where Co is initial concentration of metal ion and CA is concentration of metal ion at equilibrium. Plot of ln(C0/CA)/CA versus t/CA gives the slope and Y-intercept equal to firstorder adsorption rate (k1) and adsorption equilibrium constants (K), respectively. 2.4.3 Effect of initial concentration of metal ions on the adsorption capacity The effect of initial concentration of Hg(II) ions on the adsorption capacity of DIZ adsorbent was examined by conducting adsorption experiment in various concentration of Hg(II) ion. The similar procedure to pH effect study was applied but the concentration of
Hg(II) was varied in the range of 0 – 10 mg/L. The obtained data were evaluated by Langmuir isotherm adsorption model (Eq. 2) to determine adsorption capacity (b) and equilibrium constant (K). C 1 1 = + C m bK b
(2)
Where C is concentration of Hg(II) ion at equilibrium and m is mole of Hg(II) ion adsorbed by 1.0 g of adsorbent. The value of K was then used to calculate the adsorption energy (E) based on the equation of standard Gibbs-Hemholtz relation (Eq. 3). E = -∆Go = RT ln K
(3)
Where R is gas constant (8,314 J/mol K) and T is absolute temperature (K). 2.4.4 Removal of Hg(II) from aqueous solutions Fifteen milliliter of synthetic sample solution (river water) containing 8 mg/L Hg(II) was interacted with 0.02 g AZ or DIZ adsorbents. The mixture was stirred for 90 minutes and the adsorbent was filtered using Whatman 42 filter. The filtrates were collected and subjected to cold vapor atomic absorption spectrophotometric analysis (CVAAS) for the determination of Hg(II) ion concentration. For the second adsorption, the same procedure as first adsorption was applied to 10 mL of filtrate obtained from the first adsorption and the filtrates were again subjected for CV AAS for the determination of Hg(II) concentration remaining in the filtrate after second adsorption.
3. Results and Discussion 3.1 Dithizone Immobilization and its Characterization 3.1.1 Dithizone immobilization on zeolite Dithizone immobilization on zeolite has been done by mixing dithizone dissolved in toluene and natural zeolite that has been activated with hydrofluoric acid (HF) for 4 hours [22-24]. The mixture was stirred and heated at various temperatures of 50, 70, 90
and 110 oC. The results obtained were then filtered and washed subsquently with toluene, ethanol and double-distilled water until no dithizone trace was detected in the filtrate. The obtained dithizone-zeolite was dried in an oven at a temperature of 70 °C for 6 hours. After drying, the dithizone-zeolite was sieved to ensure the homogeneity of particle size. The adsorbants obtained from the above process was a reddish brown solid hereinafter called dithizone-immobilized zeolite (DIZ). In this experiment, the adsorbent was first washed with toluene to remove the remaining free (non-immobilized) dithizone on the surface of zeolite. The further washing of the adsorbent using ethanol and water is aimed to remove semi-polar and polar impurities, respectively. The obtained adsorbant was again dried in the oven to remove the remnants of solvents that still exist on the surface of DIZ adsorbent. The immobilization of dithizone in this study is basically coating the solid surface of natural zeolite with selective organic compound with the hope that the active groups available on the organic ligand can give additional serve as an active site of adsorbent. Organic ligand, such as dithizone, has been known to have several active groups such as -SH and -NH in its structure, therefore it is expected to enhance the interaction facilities between Hg(II) and the surface of the adsorbent via covalent interactions. There are several factors that affect the immobilization of dithizone on the surface of zeolite including temperature, contact time and the amount of dithizone used in immobilization. In this study we have specifically investigated the effect of various heating temperature (50, 70, 90 and 110 oC) on the effectiveness of dithizone immobilization on the surface of natural zeolite. The adsorbents obtained from different heating temperature of mixing were characterized by FT-IR spectroscopy and the results are shown in Fig. 1. Comparison of the infrared spectra of AZ (Fig. 1A) and those of DIZ (Figure 1B) reveals that immobilization process conducted at temperature of 50 and 70 °C gives the
adsorbent with higher content of dithizone than that carried out at temperature of 90 and 110 oC. This conclusion can be derived by directly observing the higher vibration intensity of dithizone immobilized at temperature of 50 and 70 °C as compared to that obtained at temperatures of 90 and 110 oC. The effectiveness of immobilization at lower temperature may be attributed to the stability of dithizone towards oxidation at lower temperatures. As a result, the immobilization carried out at 50 and 70 °C undergoes more effectively. At higher temperature e.g. 90 or 110 °C, and with the existence of atmospheric air, dithizone can be easily damaged by oxidation, therefore it becomes more difficult to immobilize dithizone on the surface of the natural zeolite.
The DIZ adsorbents obtained at various temperatures were also subjected to X-ray diffraction (XRD) analysis. For the purpose of comparison, the XRD patterns of free dithizone and DIZ prepared at temperature of 50, 70 oC are presented in Fig. 2, while the important data of the XRD spectra are collected in Table 1. It is clearly observed from the table that the peaks characteristic for dithizone appear more intense on DIZ prepared at temperature of 50 °C than those done at 70 oC. Therefore, for the next experiment, we have selected temperature of 50 °C as immobilization temperature of dithizone. Another reasons for the selection of this temperature is that heating at lower temperatures is more easily achieved and maintained than the higher one. Moreover, heating at lower temperature also reduces the risk of the oxidation process of dithizone.
3.1.2 Characterization of dithizone-immobilized zeolite The effect of dithizone immobilization on the surface of zeolite may be observed from the changes in the wavenumber of infrared spectra of functional groups by comparing the infrared spectra of AZ, DIZ and free dithizone (see Figure 1B, 1C and 1D). It is clearly
shown in Fig. 1D that the infrared spectra of free dithizone has very sharp absorption bands at wavenumber of 1496.7 cm-1 and 1438.8 cm-1, indicating the presence of the double bond C=C stretching of the aromatic phenyl group, whereas absorption band at wavenumber of 748.3 cm-1 and 678.9 cm-1 represents the vibration of aromatic C-H bending, which is normally observed in 900-690 cm-1. Absorption band at wavenumber of 2912.3 cm-1 is attributed to the presence of benzene substitution. Absorption band at 3552.6 cm-1 indicates the existence of N-H stretching vibration, while the sharp absorption band at 1589.2 cm-1 belongs to the N-H bending vibration. Weak absorption band in the region around 2500 cm-1 indicates the presence of –S-H group, while the weak peak at 2200 cm-1 region is due to the presence of the C=N. Absorption band at wave number 1350-1000 cm-1 is the C-N stretching vibration. Upon immobilization of dithizone onto zeolite surface as presented in Fig. 1C, the FT-IR spectra significantly change (Fig. 1B), which is characterized by the shift of absorption and the existence of new absorption bands. These phenomena suggest the successful immobilization of dithizone onto the zeolite surface. Absorption band at wavenumber of 1488.9 cm-1 and 1461.9 cm-1 indicates the presence of C=C double bonds of the aromatic phenyl group which is supported by the appearance of absorption band at wavenumber of 3012 cm-1, showing the stretching vibration of aromatic C=C. The absorption at 2854.5 cm-1 shows the stretching of aliphatic C-H vibration, and absorption at wavenumber of 763.8 cm-1 and 686.6 cm-1 indicate the presence of aromatic C-H bending vibration. Absorption band at wavenumber of 1311.5 cm-1 shows the stretching of the C-N vibration, while that at wavenumber of 3448.5 cm-1 indicates the presence of N-H stretching vibration. Weak absorption band at wave number 2345.3 cm-1 is likely due to the presence of the S-H stretching vibration. All of the IR bands mentioned above indicate the presence of dithizone in the zeolite material. Meanwhile, absorption which is
characteristics for zeolites still appears as a strong absorption at 1049.2 cm-1, indicating the presence of internal asymmetric stretch of the primary building blocks of zeolite, e.g. Si-O and Si-O-Si. Absorption band at wavenumber of 1639.4 cm-1 shows the O-H bending vibration of silanol group. Also, absorption band appears at 470.6 cm-1 and 428.2 cm-1 suggesting the availability of bending vibration of Si-O-Si [25].
Additional characterization of the obtained adsorbents has been performed using XRD analysis. In this case, the d space and 2θ angle values which are characteristic for both zeolites and dithizone are examined and compared to determine whether the dithizone has been successfully immobilized on surface of the zeolite. Based on the results of XRD analysis of AZ (Figure not shown), DIZ (Fig. 2A) and free dithizone (Fig. 2C), it is easily noticed that several peaks which are characteristic for both zeolites and dithizone can be found in DIZ, indicating that dithizone has successfully been immobilized on the surface of the zeolite. The detailed XRD peaks of DIZ are listed in Table 2. This finding is consistent with the results of IR spectroscopic analysis and supported the conclusion that immobilization of dithizone has been successfully done as discussed in the previous section. 3.2 Adsorption study of Hg(II) ions In this study, the ability of DIZ to adsorb Hg(II) has been systematically evaluated by examining several factors that affect the ability of DIZ in adsorbing Hg(II). The parameters studied include the effect of pH, adsorption kinetics and adsorption isotherms. Evaluation of the effect of pH on the Hg(II) adsorption of the adsorbent has been conducted with the purpose to determine the optimum pH of adsorption. The adsorption kinetics was studied on the basis of adsorption rate constant data evaluated by using the
Langmuir-Heinshelwood equation and Santosa models [26]. Furthermore, evaluation of adsorption isotherms has also been carried out to obtain the adsorption capacity, the equilibrium constant as well as the adsorption energy at constant temperature. This evaluation has been determined by the Langmuir-Heinselwood equation. The adsorption energy data obtained from this evaluation is then used to classify the interaction mechanism operated in the adsorption processes between adsorbate of Hg(II) and the adsorbent of DIZ, e.g. physical or chemical interaction and the results are compared with those done using AZ. Figure 3 gives the scheme of possible interaction mechanism between Hg(II) and the active sites of DIZ adsorbant.
3.2.1 Effect of pH on the Hg(II) adsorption Study of the effect of pH on the adsorption of Hg(II) has been conducted by interacting 5 mg/L of Hg(II) solution with DIZ adsorbent at different pH of 3, 4, 5, 6, 7, 8 and 9. For the purpose of comparison, adsorption of Hg(II) on AZ is also conducted. After interaction, the remaining metal ions in the solution was determined by cold vapor AAS method to calculate the adsorbed Hg(II). In this study, the control solution, i.e. the same concentration of Hg(II) solution at various pH but without the addition of the adsorbent was also used for the purpose of error correction that may arise from the precipitation of Hg(II) ions at certain pH or from metal loss caused by the adsorption of metal ions on the surface walls of the container during the process of interaction.
The pH of the solution plays an important role in the adsorption process, because the effectiveness of the adsorption is determined by the chemical form (speciation) of Hg(II) ions in solution and active groups on the surface of the adsorbent. These speciations is basically pH dependent. Fig. 4 gives the results of the effect of pH on the amount of adsorbed Hg(II) on the adsorbents.
Figure 4(A) and (B) clearly show that there is a significance difference in the amount of the adsorbed Hg(II) on each adsorbent at the same pH. In general, it can be concluded that the ability of dithizone-immobilized zeolite in adsorbing Hg(II) is higher than that of zeolite itself. The activated zeolite without immobilization of dithizone is able to adsorp Hg(II) but the adsorbed Hg(II) is relatively small. This may be due to the role of pores and active sites available in zeolite, such as aluminol, silanols and siloxanes. In contrast, we clearly observe that the ability of zeolite-immobilized dithizone to adsorb Hg(II) are relatively high, doubled than that of activated zeolite. This high ability of adsorption is probably due to the involvement of not only the pores and active site of zeolite, but also the additional active groups from immobilized dithizone on the surface of the zeolite such as -SH and -NH. This provides much more active groups that are ready to interact with metal ions, e.g. Hg(II). The twice increase in the Hg(II) adsorption ability of dithizone-immobilized zeolite may also be explained with respect to Hard and Soft Acid-Base (HSAB) principle. In dithizone-immobilized zeolite adsorbent, there are -SH and –NH active groups which are classified as soft bases according to the HSAB theory. This functional groups prefer to interact with soft acid such as Hg(II) ions. Therefore, it is quite easy to understand that Hg(II) ion will favorably bind to the active groups of dithizone, leading to much more Hg(II)ion is adsorbed by dithizone-immobilized zeolite as compared to that adsorbed by activated zeolite. In Fig. 4, it appears in general that Hg(II) ion adsorbed by the two adsorbents increase with increase of the pH of metal ion solution and after reaching the maximum value, the amount of adsorbed Hg(II) starts to decrease. This trend may be explained as follow, at low pH, the concentration of H+ ion in the solution is high, resulting in the active groups on the surface of the adsorbent be protonated and produce a partially positive
charge on the functional groups/active site of adsorbent. This condition is not favorable for the occurence of the interaction between the positively charged Hg(II) and the protonated active groups on the surface of adsorbent, due to electrostatic repulsion. Furthermore, the high concentration of H+ in solution at low pH also induces strong competition between H3O+ ions and the Hg(II) ion in solution to bind to the active group on the surface of the adsorbent. Therefore it is understandable that the higher acidity of the solution (lower pH) leading to the lower adsorption of Hg(II) ions on the adsorbent surfaces. The amount of Hg(II) adsorbed by the two types of adsorbent tends to increase with the increase of pH of the solution. In this study, we found that the optimum pH for the adsorption of Hg(II) on zeolite is at pH 7, which is close to the reported pH of 8 for the adsorption of Hg(II) onto activated carbon prepared from the biomass material [17]. At pH = 7, it is expected that the protonated H+ on the surface of the adsorbent is released, so that the active groups on the surface of the adsorbent is becoming negatively charged. On the other hand, Hg(II) ion in the solution is still in the form of positive ion, leading to very supportive condition for the interaction between Hg(II) and the active groups on the surface of the adsorbent. In case of DIZ, the optimum pH of adsorption is reached at pH 5. The lower optimum pH for DIZ compared to AZ is probably due to the effect of dithizone immobilization. The same optimum pH of 4-6 has been reported by Yu, et al [21] for the adsorption of Hg(II) on dithizone-immobilized silica gel, while optimum pH of 7 has also been reported for the adsorption of Hg(II) onto surface of dithizone-alumina [20]. At pH 5, the amount of H+ ions available in solution is not much enough to be able to protonate the active group of the adsorbent, so that the equilibrium reaction between metal ions (M2+) and dithizone ligand (H2DZ) to form metal-dithizone complex, M(HDZ)n favorably goes the right side, i.e. the formation of metal dithizonate complexes:
M2+
+
H2O + nH2DZ
M(HDZ)n +
H3O+
However, further increasing of pH results in the decrease of the amount of adsorbed Hg(II). This may be attributed to the deposition of Hg(II) as Hg(OH)2 (Ksp Hg(OH)2 = 3.13x10-26). Also, the functional groups on the adsorbent will undergo de-protonation at higher pH, resulting in a partially negative charged active site. Similarly, the Hg(II) ion at higher pH also tend to form negatively charged complex in the excess of OH- ions. Therefore, electrostatic repulsion of negatively charged species occurs in the solution, as the result, the metal ions are becoming very difficult to interact with the active groups of the adsorbent, leading to the decline in the number of adsorbed metal ions.
3.2.2 Isotherm adsorption Isothermal adsorption models are often used to determine the adsorption capacity and it shows a correlation between the activity of the adsorbent and the adsorbed amount of a substance at constant temperature is the Langmuir adsorption isotherm. This model assumes that the adsorbent surface is homogeneous and the magnitude of the adsorption energy is equivalent to each active adsorption sites[27]. Each of the active sites of the adsorbent will adsorb one molecule of adsorbate. When the adsorption sites has not been saturated yet with adsorbate molecules, the increase in the concentration of the adsorbate will always be accompanied by an increase in the amount of metal ions adsorbed. Conversely, if the adsorption sites have been saturated with the adsorbate molecule, the increase in adsorbate concentration will not increase the amount of metal ions adsorbed [28]. In this study, the adsorption isotherms of Hg (II) on the two types of adsorbent has been studied using the following procedure, a solution of Hg(II) with various
concentrations of 0.5, 1, 2, 4, 6, 8 and 10 mg/L was interacted with the adsorbent at optimum pH and interaction time of corresponding adsorbent and the results are presented in Fig. 5. The figure shows that the two adsorbents give the same pattern of Hg(II) adsorption, i.e. the greater the initial concentration of metal ions, the more metal ions are adsorbed by the adsorbent. The amount of adsorbed Hg(II) ion is then constant at a certain concentration of Hg(II). This situation is happened because the metal ions in solution occupy active sites of the adsorbent, the greater the concentration of metal ions in the solution, the more active sites of the adsorbent are occupied by metal ions, if all the active sites on the surface of the adsorbent has been occupied by Hg(II) ions, no more Hg(II) ion can be adsorbed by the adsorbent, and therefore the amount of adsorbed Hg(II) remains constant. This condition indicates that the equilibrium between the active sites on the adsorbent and Hg(II) ion has been reached, hence the addition of Hg(II) ion will not result in any significant change in the amount of Hg(II) ion adsorbed by the adsorbent.
For adsorption using AZ (Fig. 5B), it is observed that the Hg(II) adsorption with initial concentration in the range of 0.5 to 2 µg/ml has been undergone slowly, whereas at concentrations above 2 mg/L, adsorption occur rapidly and achieve optimum conditions at concentration of 4 mg/L. The slow adsorption may occur because of the active groups available in AZ are silanols, aluminol and siloxane which is classified as hard base according HSAB theory. At low concentration, Hg(II) ion is difficult to interact with active groups on the surface of the adsorbent due to the low selectivity and affinity of the adsorbent towards soft-acid of Hg(II) ion. At high concentration, the amount of Hg(II) ion in the solution is quite large, therefore, it increases the possibility of interaction between the Hg(II) ion and the active groups on the surface of the adsorbent, resulting in faster adsorption process of the adsorbent and Hg(II) ion. The same condition is also applied to
DIZ adsorbent (Fig. 5A), however in this case the adsorption of Hg(II) on the adsorbent involves not only the active group of the zeolite, but also the active groups of ditizone, which have great affinity and selectivity to ions Hg (II). Therefore, the adsorption using DIZ adsorbent achieves the optimum condition at higher initial concentration than that of activated zeolite, e.g. at 6 mg/L, indicating that DIZ has more active sites with greater affinity and selectivity that can be occupied by Hg(II) ion. Adsorption thermodynamic parameters including adsorption capacity, adsorption equilibrium constant and adsorption energy can be determined by applying the Langmuir adsorption isotherm models to the obtained data, according to the following equation: C 1 C = + m bK b By plotting C/m against C, the value of K (equilibrium constant) and b (adsorption capacity) can be calculated from the slope and the intercept the graph. From the K value, the adsorption energy can then be calculated using the following standard relation: Eads = ∆G 0 = RT ln K where R is the general gas constant (8.314 J / mol K) and T is the temperature (Kelvin). The results of the calculation of Langmuir isothermal adsorption parameters are presented in Table 3. Based on the data summarized in Table 3, it is shown that the adsorption capacity of Hg(II) for DIZ adsorbent (13.1 x 10-6 mol g-1) is comparable the to reported value of 20 x 10-6 mol g-1 for dithizoze-silica gel at pH=6 [20]. The little bit lower result in this study is probably due to the lower amount of dithizone that can be immobilized onto the surface of zeolite compared to that of silica gel. However, it is clearly observed that the adsorption capacity and adsorption equilibrium constant of Hg(II) for DIZ adsorbent is still higher than those of AZ. This suggests that the immobilization of dithizone on zeolite has increased the adsorption capacity of zeolite due to the increase in the number of new active
sites from immobilized dithizone. The functional groups of dithizone has been known to have high affinity and selectivity to Hg(II) ion. The above explanation is also supported by finding that the value of the equilibrium constant of the reaction is quite large, meaning that the complex formation reaction favorably goes to the right side, e.g. the formation of the products. Hence, much more Hg(II) ion is binding to the active groups of dithizone immobilized on the zeolite. Adsorption process can occur by means of physical interaction (physisorption), chemical interaction (chemisorption) or mixture of them. Adamson [29] categorized adsorption as physisorption if its adsorption energy is less than 20.92 kJ/mol and considered as chemisorption if its adsorption energy is more than 20.92 kJ/mol. Based on this classification, we may conclude that the adsorption of Hg(II) on both AZ and DIZ adsorbents is classified as chemisorption because the their adsorption energy is higher than 20.92 kJ/mol, i.e. 36.09 kJ/mol and 39.55 kJ/mol, respectively. The possible chemical interactions operated in the adsorption process are electrostatic interaction between Hg(II) ion and silanol, or aluminol groups of zeolite (in AZ and DIZ adsorbents), as well as coordination covalent bonds between Hg(II) ion and functional groups of immobilized dithizone on the surface of the zeolite (in DIZ only).
3.2.3 Kinetics of adsorption Adsorption kinetics was studied by determining the adsorption rate constant and adsorption equilibrium constants. The data were obtained by interacting 5 mg/L of Hg(II) solution with the two types of adsorbent at different contact time of 5, 15, 30, 60, 90, 120 and 180 minutes and at corresponding optimum pH of adsorption. Effect of interaction time on the amount of adsorbed Hg(II) ion is presented in Fig. 6. Adsorption kinetics study is aimed to determine the value of adsorption rate constant which is able to give information on the rate of adsorption process. The
information about the time required to achieve adsorption equilibrium is very useful to determine the contact time required to achieve optimum result of adsorption. Generally, when the adsorption equilibrium has been reached, the increase in interaction time will not significantly change the amount of metal ions adsorbed by the adsorbent. Fig. 6 shows clearly that the amount of Hg(II) adsorbed by DIZ is always larger than that adsorbed by AZ. This finding again supports the explanation that the dithizone immobilized on the surface of the zeolite has improved both selectivity and affinity/capacity of the adsorbent towards Hg(II) ion due to the existence of additional active groups from dithizone, i.e. -SH and –NH. These two active groups have preferency to interact with Hg(II) ion, leading to more Hg(II) ion is adsorbed by DIZ as compared to that adsorbed by AZ.
Based on Fig. 6, it can also be observed that in general the amount of metal ions adsorbed by the adsorbent increases with the increase in interaction time between adsorbents and Hg(II) ion. Aftrer reaching optimum value of time, the adsorbed Hg(II) remains relatively constant. The general trend of Hg(II) adsorption on AZ and DIZ is relatively the same. However, in case of DIZ, it is observed that at the first 5 minutes of contact time the amount of Hg(II) adsorbed on the adsorbent increases significantly and reaches a constant value after 90th minute of contact time. On the other side, the adsorption of Hg(II) by AZ occurs slowly at the beginning of adsorption, and reaches a constant value after 60th minute of interaction. The constant value of adsorption indicates that the equilibrium has been achieved between Hg(II) ions in solution and the active groups on the surface of the adsorbent. The difference in value of optimum contact time between the two types of adsorbent is presumably due to the longer time needed for the complexation between dithizone on the surface of zeolite and Hg(II) ion. Since there is
various active groups available on DIZ, the interaction involves not only the active groups of the zeolite, but also the active groups of dithizone, e.g. via chelate formation. Therefore it can easily be understood that the time required for DIZ to reach equilibrium is much longer compared to that needed by AZ. The data obtained from this experiment was then evaluated to determine the adsorption kinetic parameters, e.g. the rate of adsorption (k) and adsorption equilibrium constants (K), using the following Langmuir-Heishelwood adsorption kinetics model: C ln 0 C k1t A + K = C0 − C A C0 − C A C ln 0 C t A In plot of versus , the values of K and k1 are respectively given by C0 − C A C0 − C A the Y-intercept and the slope of the plot. Results of k and K calculation are presented in Table 4. In order to obtain the best model of the adsorption kinetics, the kinetic parameters which have been evaluated by Langmuir-Heinshelwood equation are compared with those obtained by the kinetic model proposed by Santosa [26], as follows: ln
C0
C A
CA
ln Plot of
C0
C A
CA
=
k1 t + K CA
versus
t gives K and k1 values respectively as the Y-intercept and CA
slope of the plot. Detailed calculation results are presented in Tabel 5. Comparing Table 4 and 5, it is clearly observed that the kinetic parameters evaluated by Santosa model [26] gives better results from the point of view of linearity (rvalue). The data presented in Table 4 and 5 also suggest that the value of the adsorption rate constant (k) for DIZ is larger than that of AZ. This indicates that the immobilization of
dithizone on the surface of zeolite increases the rate of adsorption. In the adsorption of Hg(II) by AZ, the metal ion interact with active groups of siloxane, silanol and aluminol groups which according to HSAB (Hard Soft Acid-Base) principle are classified as the hard base, hence it would be more difficult for soft acid species like Hg(II) to interact with. This reason explains the lower rate constants observed for the Hg(II) adsorption on AZ. On the other hand, the higher value of adsorption rate constant for DIZ is probably due to the diversity of active groups available on the surface of adsorbent as a result of dithizone immobilization on the surface of the zeolite. The active groups of -SH and –NH are classified as soft bases according to HSAB theory which have great affinity and selectivity towards soft acid such as Hg(II). As a result, a lot of Hg(II) has been able to be adsorbed by DIZ in a relatively short time. The fact that the value of the equilibrium constant (K) for DIZ is larger compared to that of AZ also supports this explanation. This finding indicates that the adsorption capacity of DIZ towards Hg(II) is larger than that of AZ, resulting in more Hg (II) ion is able to be adsorbed by the DIZ adsorbant.
3.3 Removal of mercury(II) from river water spiked with Hg(II) Application of adsorbent for the removal of Hg(II) ion in the river water spiked with Hg(II) has been studied by diluting a solution containing certain concentration of Hg(II) ion with filtered river water in a 100 mL of volumetric flask so that the final concentration of Hg(II) in the solution is 8 mg/L. The river water used for the study was taken from Progo river, Bantul, Yogyakarta Province, Indonesia at two points of sampling, before and after the bridge. The detailed composition of the river water is not specifically determined in this study, but it is expected to contain some heavy metal ions other than Hg(II) as well as domestic waste because the river is flowing across the city of Yogyakarta and home industrial and domestic wastewaters are normallly thrown directly to the rives.
The AZ and DIZ adsorbents were then interacted with the two types of Hg(II)-spiked river water samples at optimum conditions of Hg(II) adsorption. The mixture solution was then filtered and analyzed using CV-AAS to determine the content of Hg(II) remaining in the filtrate. To determine the effectiveness of the adsorbent in the adsorption of Hg(II) in the samples, the filtrate from the first interaction was interacted again with the two adsorbents in the same way as for the first interaction. Detailed results of experiment is presented in Fig. 7. It can be seen from this figure that approximately 51.73% of Hg(II) ion can be removed from the solution at first adsorption by AZ and up to 80.89% by using DIZ. For the second adsorption, up to 97.98% and 99.36% of Hg(II) has been able to be removed from the solution using AZ and DIZ adsorbents, respectively. Based on these results, again we see that the DIZ adsorbent has greater effectiveness in the removal of Hg(II) ion from the river water samples compared to AZ adsorbent. This better performance of the modified adsorbant can be achieved because of the influence of the higher affinity and selectivity of functional groups of dithizone that has been immobilized on the surface of the zeolite towards Hg(II) ion.
4. Conclusion Dithizone-immobilized zeolite (DIZ) adsorbent has been prepared by mixing natural zeolite and dithizone in the toluene medium at 50 oC for 4h to yield brown-reddish adorbent with high affinity and selectivity to Hg(II) ion. The optimum conditions for the adsorption of Hg(II) ion using this adsorbent is achieved at pH 5 and 90 minutes of contact time. The kinetic study suggests that the rate of adsorption of Hg(II) on DIZ is higher than that of activated zeolite (AZ). Adsorption isotherm experiment reveals that the adsorption Hg(II) on DIZ is best described by Langmuir model with adsorption capacity, equilibrium constant and energy higher than that of AZ. This higher performence of DIZ has been found to be the effect of the functional groups of dithizone immobilized on the surface of
the zeolite. This modified adsorbent has been successfully applied in the removal of Hg(II) ion in the river water spiked with Hg(II) ion with satisfied results, suggesting that the modified adsorbant may be used in the future for removal of Hg(II) in industrial waste or used as promising material for solid phase extraction and preconcentration of Hg(II) . Acknowledgement This work is partially supported by Directorate General of Higher Education (DGHE), Ministry of Education and Culture, The Republic of Indonesia through National Strategic Research Grant (Penelitian Stranas) fiscal year 2014 contract no. LPPMUGM/1041/LIT/2014 dated May 5, 2014.
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Fig. 1 Infrared spectra of dithizone-immobilized natural zeolite (DIZ) at various temperature of reflux, 50 °C (A), 70 °C (B), 90 °C (C) and 110 °C (D).
Fig. 2 XRD patterns of dithizone-immobilized zeolite at various temperature of reflux, 50 o C (A), 70 oC (B) and free dithizone (C). Fig. 1; Mudasir, et al (JECE, 2016)
Fig. 2; Mudasir, et al (JECE, 2016) Fig. 3; Mudasir, et al (JECE, 2016) Fig. 3 Possible interaction mechanism between Hg(II) and the active sites of dithizoneimmobilized zeolite (DIZ) adsorbant. Fig. 4; Mudasir, et al (JECE, 2016)
Fig. 4 Effect of pH on the adsorption of Hg(II) on dithizone-Immobilized zeolite (A) and activated zeolite (B).
Fig. 5 Effect of initial concentration of Hg (II) on the amount of Hg(II) adsorbed by dithizone-immobilized zeolite (A) and activated zeolite (B).
Fig. 6 Effect of interaction time on the amount of adsorbed Hg(II) ion on dithizoneimmobilized zeolite (A) and activated zeolite (B).
Fig. 7 The effectiveness of activated zeolite (AZ) and zeolite-immobilized ditizon (ZID) in the removal of Hg(II) in the river water samples taken from the west bridge (1) and the eastern bridge (2) of river Progo, Bantul DIY spiked with Hg(II). Before adsorption (blue), after first adsorption (red) and after second adsoption (green). Table 1. Interpretation of 2θ and d-space values as well as the relative intensity (I/Io) of XRD peaks for dithizone-immobilized zeolite (DIZ) obtained at heating temperature of 50 and 70 °C Materials 2θ d I/Io I (degree) (spacing) 9.5400 9.2633 26 116
DIZ prepared at 50 oC
DIZ prepared at 70 oC
Free dithizone
13.5250 15.3756 19.7064
6.5416 5.7579 4.5014
26 52 48
114 230 215
2.7899 9.3200 13.5416 15.3765 19.7370
2.7899 9.4815 6.5334 5.7579 4.4945
11 3 29 13 40
49 12 115 50 160
32.0696 10.8532 13.4141 14.8671
2.7887 8.1452 6.5954 5.9539
9 16 20 100
36 340 420 2078
19.0712 32.3946
4.6499 2.7615
40 10
834 215
Table 2. Interpretation of 2θ, d-space values as well as the relative intensity of the XRD peaks for dithizone-immobilized zeolite (DIZ) 2θ(degree) d I/Io Interpretation (spacing) ▓ 9.5400 9.8481 13.5250 15.3756 19.7064 22.3818 25.7457 26.4010 27.8320 28.2876 32.0546
9.2633 8.9742 6.5416 5.7579 4.5014 3.9690 3.4575 3.3732 3.2030 3.1524 2.7899
26 41 26 52 48 65 100 41 49 31 11
dithizone zeolite dithizone dithizone dithizone zeolite zeolite zeolite zeolite zeolite dithizone
Table 3. Langmuir isothermal adsorption parameters on Hg(II) on activated zeolite adsorbent (AZ) and dithizone-immobilized zeolite (ZID). Langmuir isothermal adsorption parameters Adsorbent
Adsorption
Equilibrium
adsorption
capacity (b) (mol g-1)
constant (K) (L mol-1)
energy (kJ mol-1)
Plot Linearity (r)
8.00 x 10-6 13.1 x 10-6
AZ ZID
1.92 x 106 7.71 x 106
36.09 39.55
0.9993 0.9998
Table 4. The kinetic parameters of Langmuir-Heishelwood adsorption for of Hg(II) on activated zeolite (AZ) and dithizone-immobilized zeolite (DIZ). Langmuir-Heishelwood kinetic parameters Adsorbent 1st order ▓ Equilibrium Plot linearity adsorption (r) rate constants constants (mol 1 (min-1) L) 1.00 x 10-7 4.50 x 10-3
AZ DIZ
2.76 x 102 4.77 x 104
0.8109 0.9692
Table 5. Parameters of adsorption kinetics calculated by model of Santosa [26] for Hg(II) ion on activated zeolite (AZ) and dithizone-immobilized zeolite (DIZ). Parameters of adsorption kinetics according to Santosa model [26] Adsorbent 1st order Equilibrium Plot linearity adsorption (r) rate constants constants (mol 1 (min-1) L) -3 AZ 8.10 x 10 2.76 x 103 0.9479 DIZ 13.0 x 10-3 4.77 x 105 0.9720
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