Transport of Hg(II) through bulk liquid membrane containing calix[4]arene thioalkyl derivative as a carrier

Desalination 262 (2010) 215–220

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Transport of Hg(II) through bulk liquid membrane containing calix[4]arene thioalkyl derivative as a carrier Fozia T. Minhas, Shahabuddin Memon ⁎, M.I. Bhanger National Center of Excellence in Analytical Chemistry, University of Sindh, Jamshoro 76080, Pakistan

a r t i c l e

i n f o

Article history: Received 23 February 2010 Received in revised form 3 June 2010 Accepted 4 June 2010 Available online 29 June 2010 Keywords: Hg(II) First order reactions Kinetic laws Bulk liquid membrane Carrier

a b s t r a c t In this study effectiveness of 5,11,17,23 tetrakis[(propylthio)methyl]-25,26,27,28-tetrahydroxycalix[4]arene (1) towards Hg(II) across bulk liquid membrane has been examined. The effect of the fundamental parameters influencing the transport process, e.g. carrier concentration, type of solvent used, effect of temperature and time dependency of the process have been investigated. The mass transfer of Hg(II) was analyzed on the basis of kinetic max max ) laws of two consecutive irreversible first order reactions. Thus, the kinetic parameters (k1, k2, Rmax m , tmax, Jd , Ja for the transport of Hg(II) were studied. The activation energy values for the extraction and re-extraction were found as 19.59 and 27.57 kJ mol− 1, respectively. These values demonstrate that the process is diffusionally controlled by Hg(II). Observations show that the membrane entrance and exit rate constants (k1, k2) increase with increasing stirring speed, carrier concentration and temperature. The effect of solvent on k1 and k2 was found to be in the order of CH2Cl2 N CHCl3 N CCl4. The effectiveness and reproducibility of Hg(II) transport has proved that 1 is an efficient carrier of Hg(II) in BLM process. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Remediation of hazardous pollutants from environment is a technological challenge for scientists today. Every day thousands of pollutants are introduced into air and water ecosystem due to anthropogenic activities. Mercury is one of the notorious toxicant widespread in atmosphere which is highly toxic, carcinogenic and nonbiodegradable. The WHO sets mercury allowed level in drinking water at 0.006 mg L− 1 (for inorganic mercury) [1]. Emissions from the coal smoke, agricultural wastes, mining, combustion are the main sources of mercury pollution in the environment by human activities [2]. It is also released to the atmosphere by various natural phenomena, including volcanic eruptions, forest fires and the weathering of geological formations [3,4]. Moreover, it is used in fever and rectal thermometers, barometers, electrical switches, blood pressure cuffs, thermostats, fluorescent light bulbs, paints, art supplies, fungicides and light ballasts. In certain cultures mercury is utilized in medicine as well as in religious ritual [5]. There are numerous studies establishing mercury toxicity as occupational health hazard for dental staff [6], chloralkali factory workers [7] and gold miners [8–11]. The emitted mercury both natural and anthropogenic in air after traveling long distances eventually passes into rivers, lakes and oceans [12]. In aquatic environments, inorganic mercury is microbiologically transformed into lipophilic organic compound, methylmercury. This transformation makes mercury more

⁎ Corresponding author. Tel.: +92 22 2772065; fax: +92 22 2771560. E-mail address: [email protected] (S. Memon). 0011-9164/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2010.06.014

prone to biomagnification in food chains. Consequently, marine food is the principal reason of mercury in non-occupationally exposed populations. Mercury occurs principally in three different chemical forms: elemental mercury Hg(0), mercury salts Hg(II) and organic mercury. The toxicity of mercury depends strongly on its redox state. Mercury causes hearing loss, brain damage, speech difficulty, impaired vision, autism, chromosome breakage and dysfunction of liver and central nervous system [2,13–15]. The first occurrence of widespread mercury poisoning in humans, now called Minamata disease, was reported in Minamata, Japan [16]. Besides this, toxicity of mercury compounds has been observed in fish, birds and even mammals [17]. Basically Hg(II) blocks essential functional groups in biomolecules and also displaces essential metal ions from them. It is known as one of the strongest thiolbinding agents. Intracellular mercury therefore attaches itself to thiol residues of proteins particularly glutathione and cysteine resulting in inactivation of sulfur and blocks related enzymes, cofactors and hormones [18]. The other functional groups besides SH for which mercury has high affinity include, –CONH2, –NH2, –COOH and –PO4 [19]. The development of new and reliable methods for the removal of mercury from environmental and biological sites/materials is of particular significance. Conventional methods of mercury treatment are precipitation, coagulation, electrodialysis, reverse osmosis, adsorption, solvent extraction, chemical oxidation and reduction and ion exchange. However, they are ineffective at metal concentrations less than 100 mg L− 1 [13]. The Liquid membrane (LM) technology has been effectively used to treat aqueous streams contaminated with heavy metal ions like copper, zinc, cadmium, nickel, mercury and lead [20–26].

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Also, these methods reduce the solvent inventory requirements and also allows the use of expensive and highly selective extractants, which otherwise would be uneconomic in solvent extractions [27]. Among LM technologies bulk liquid membranes (BLM) are suitable for screening novel carrier-mediated transport systems on a laboratory scale. In general, the mobile carrier contained in the liquid membrane governs the efficiency and selectivity of the liquid membrane transport and therefore a lot of novel carriers have been explored to create selective separation systems [28]. In the last decade, macrocyclic ligands have attracted much attention as novel host compounds for ionic species [29]. In this study, we have focused on the calixarenes, whose cavity can recognize the size of a guest molecule and the framework is available for the arrangement of the functional groups to create a specific affinity for a target guest molecule [30–32]. To date, various calixarene derivatives have been synthesized and utilized as carriers for liquid membrane transport [33–37]. The selective Hg(II) extraction efficiency of calix[4] arene thioalkyl derivative (1) shown in Fig. 1 has been evaluated previously by our group [38]. The present study examines the effectiveness of 1 as a carrier and explores the effect of solvent, stirring rate, carrier concentration, and temperature on the efficiency of transport process. 2. Experimental 2.1. Materials Analytical reagent-grade chemicals were used for the preparation of all the solutions. All aqueous solutions were prepared with deionized water passed through a Milli-Q system (ELGA Model CLASSIC UVF, UK). The carrier 1 used in the study was synthesized according to literature methods [38]. 2.2. Apparatus The absorbance was recorded on a Spectronic® 20 Genesys ™ (CAT 4001/4, USA). A Gallenkamp magnetic stirrer model APP SS610/5, UK and Teflon-coated magnetic bars were used for kinetic study. Cotransport experiments were conducted using a thermostated (Grants Instruments, model W14, Cambridge, England) apparatus at 298 K and transport experiments were carried out in a U-type cell (Fig 2). 2.3. Kinetic procedure An organic solution (15 mL) containing the carrier was placed at the bottom of the cell and two portions of aqueous solutions (10 mL) were carefully added on top of them. Both surface areas were 2.5 cm2. The organic phase was stirred at 90 rpm magnetically. The initial compositions of the phases consist of the donor phase, which was an aqueous mercury picrate (2.5 × 10− 5 M) [39], membrane phase, which was made up by dissolving carrier 1 (1.0× 10− 3 M) in organic solvent and acceptor phase that is double distilled water. Samples were taken from both water phases (acceptor and donor phases) at regular time intervals

Fig. 1. Calix[4]arene thioalkyl derivative (1).

Fig. 2. Bulk liquid membrane apparatus (U-type cell) used for the transport of Hg(II).

and the mercury picrate concentration was analyzed by spectrophotometric method. Each experimental result reported is the arithmetic mean of two independent measurements. 3. Results and discussion A bulk liquid membrane (BLM) generally transfer component of interest between two aqueous phases i.e. donor phase and acceptor phase. In the present work, transport of Hg(II) as a function of carrier concentration, temperature, type of solvent and stirring rate through BLM was studied in detail. The kinetic behavior for the variation of Hg (II) picrate concentration with time was directly measured in both the donor (Cd) and acceptor (Ca) phases, respectively using UV–Vis spectrometer at regular time intervals of 20 min for a total of 660 min. The reduced dimensionless concentrations were used for experimental purposes; Rd =

Cd C C R = m Ra = a Cdo m Cdo Cdo

ð1Þ

where Cdo is the initial Hg(II) concentration in the donor phase, while Cd, Cm, and Ca represent the Hg(II) concentration in donor, membrane and acceptor phases, respectively. The material balance with respect to the reduced concentrations can be expressed as; Rd + Rm + Ra = 1: From this expression, the kinetic behavior of the consecutive irreversible first order reactions can be described as follows; k1

k2

Cd → Cm → Ca

ð2Þ

where k1 and k2 are the apparent membrane entrance and exit rate constants, respectively. Both the values of k1 and k2 are related with each other, there are three possibilities (1) if values of k1 and k2 are comparable, i.e. k1 ≈ k2, it means rate of transport at both donor– membrane and membrane–acceptor interfaces are equal; (2) k1 ≫ k2 shows that rate at donor–membrane interface is high as compared to membrane–acceptor interface, in this case membrane carries high metal content; (3) In k1 ≪ k2 there is reverse of above stated condition, membrane here carries a little metal quantity. Hence, the system in this study worked on third condition. The kinetic scheme for consecutive reaction systems can be described by the following rate equations [25,39–42]. dRd = −k1 Rd ≡Jd dt

ð3Þ

dRm = k1 Rd −k2 Rm dt

ð4Þ

dRa = k2 Rm = Ja dt

ð5Þ

F.T. Minhas et al. / Desalination 262 (2010) 215–220

Fig. 3. Time dependence of Rd, Rm and Ra for the transport of Hg(II).Membrane phase: 1 × 10− 3 M carrier in chloroform, stirring rate 90 rpm at 298 K.

where J is the flux, when k1 ≠ k2; and integrating the above differential equations give; ð6Þ

Rd = expð−k1 tÞ Rm =

k1 ½expð−k1 tÞ− expð−k2 tÞ k2 −k1

Ra = 1−

k1 ½k expð−k1 tÞ−k1 expð−k2 tÞ k2 −k1 2

max

Rm

 −k = ðk −k Þ 2 1 2 k1 k2

=

−

ð10Þ

by combining Eqs. (9) and (10), the following relationship can be obtained [43].

k2 =

  1 ln Rmax

ð11Þ

m

tmax

Numerical analysis by non-linear curve fitting permits the rate constants to be determined, the value of k1 is directly obtained by iteration from Eq. (6). This value is introduced as a constant value in Eqs. (7) and (8). An initial value of k2 is obtained from Eq. (11) that was introduced in Eqs. (7) and (8) and iterated. By considering the first order time differentiation Eqs. (6–8) at tmax, one obtains the following equations: dRd dt

j

max

= −k1

 −k = ðk −k Þ 1 1 2 k1 max ≡ Jd k2

j

ð8Þ

ð9Þ

 1 k ln 1 k1 −k2 k2

dRa dt

dRm dt

 tmax =

Fig. 4. Stirring rate dependence of k1, k2 for the transport of Hg(II) at 1 × 10− 3 M carrier concentration in CHCl3 at 298 K.

ð7Þ

while the maximum values of Rm and tmax (when dRm/dt = 0) can be written as;

ð12Þ

Table 1 Kinetic parameters of Hg(II) at different stirring rates (solvent: CHCl3; T = 298 K).

217

max

j

= k2

 −k = ðk −k Þ 2 1 2 k1 max ≡ Ja k2

ð13Þ

ð14Þ

=0 max

j

dRd dRa = dt dt

j

ð15Þ

It was noted that at t = tmax, the system may be in steady state because the concentration of Hg(II) in the membrane does not change with time (Eq. 14). As a result of this the exit and entrance fluxes are equal and have opposite signs (Eq. 15). The activation energy value is obtained from the Arrhenius equation using the k1 and k2 values at different temperatures. lnðkÞ = lnðAÞ−

Ea RT

ð16Þ

It can be seen that Rd versus t yields a decreasing monoexponential curve whereas the time variation of both Rm and Ra is bi-exponential. The actual numerical analysis was carried out by nonlinear curve fitting. The variation of Rd, Rm and Ra with time through BLM is shown in Fig. 3. 3.1. Effect of stirring speed on Hg(II) transport It is well known that hydrodynamic conditions play an important role in mass transfer from one phase to another phase at the interface between two liquid phases. In order to examine the effect of hydrodynamics on the Hg(II) transfer, experiments were carried out with different stirring rates, 30, 60, and 90 rpm at 298 K, while the carrier (1) concentration in CHCl3 was 1 × 10− 3 M. A plot of k against stirring speed is given in (Table 1 and Fig. 4) indicates that the stirring speed affects the transport rates of Hg(II) through the liquid membrane. The Table 2 Effect of carrier 1 concentration on the kinetic parameters for transport of Hg(II) ions (solvent: CHCl3, 298 K and 90 rpm).

Stirring rate (rpm)

k1 ×104 (min− 1)

k2 ×103 (min− 1)

max Rm

tmax (min)

×104 Jmax d (min− 1)

×104 Jmax a (min− 1)

Ion transported (%)

Conc.

30 60 90

5.00 8.00 18.0

2.71 4.39 5.20

0.127 0.122 0.124

760 480 400

− 3.41 − 5.47 − 10.3

3.41 5.47 10.3

55.2 62.2 69.7

1 × 10− 5 6.00 1 × 10− 4 14.0 1 × 10− 3 18.0

max k1 × 104 k2 × 103 Rm (min− 1) (min− 1)

2.50 4.12 5.20

tmax × 104 Jmax × 104 Ion Jmax d a (min) (min− 1) (min− 1) transported (%)

0.201 640 0.108 540 0.124 400

− 3.82 − 8.03 − 10.3

3.82 8.03 10.3

50.2 60.0 69.7

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F.T. Minhas et al. / Desalination 262 (2010) 215–220

Fig. 6. Time variation v/s Ra for the transport of Hg(II) in different solvents at 1 × 10− 3 M carrier concentration, stirring rate 90 rpm at 298 K.

1×10− 3 M in CHCl3 at 298 K and 90 rpm stirring speed. The results are shown in Table 2 and Fig. 5(a, and b). The experiments demonstrate that the flux of Hg(II) increases with an increase in the carrier (1) concentration in the membrane. It is quite obvious from Eqs. (7)–(8) that the flux is dependent on the carrier concentration. In a lower carrier concentration, the interface between the donor phase and membrane is not saturated by the carrier. Therefore, the flux increases with an increase of carrier concentration. In addition, a blank experiment was performed in which the membrane contained no carrier. No detectable movement of Hg(II) through the liquid membrane was found in the blank experiment. It has been found that the transport of Hg(II) through liquid membrane is fulfilled by the carrier (1).

3.3. Effect of solvent on Hg(II) transport Fig. 5. Time dependence of Ra (a) and Rd (b), for the transport of Hg(II) at different carrier concentration in CHCl3, stirring rate 90 rpm at 298 K.

flux values, k1, k2 increase, while tmax decreases with an increase of the stirring speed. However, it is noticed that a stronger agitation promotes the interfacial interaction at the donor–membrane and membrane– acceptor interfaces; by decreasing the thickness of the diffusion layer eventually increases the kinetic rate of chemical reaction on the interface [44]. But at extremely higher stirring speeds the interface could be deformed (surfaces increased by bump formation) and drops of the acceptor phase may be transferred mechanically to the donor phase [45]. In this study, it has been observed that 69.7% of Hg(II) was transferred to the receiving phase at 90 rpm.

3.2. Effect of carrier concentration on Hg(II) transport In the BLM transport, carrier plays the role of a phase transfer catalyst. It forms an extractable complex diffusing in the membrane and then releases the analyte into the acceptor phase. In order to investigate the effect of carrier concentration on the process, a series of experiments were performed by varying amount of the carrier (1) in the range 1×10− 5 to Table 3 Kinetic parameters of Hg(II) in different solvents. (T = 298 K; stirring speed is 90 rpm). Solvent

k1 × 104 (min− 1)

k2 × 103 (min− 1)

max Rm

tmax (min)

× 104 Jmax d (min− 1)

× 104 Jmax a (min− 1)

Ion transported (%)

CH2Cl2 CHCl3 CCl4

21.0 18.0 3.00

6.40 5.20 2.02

0.22 0.12 0.16

240 400 920

− 12.2 − 10.3 − 2.15

12.2 10.3 2.15

74.4 69.7 47.5

The effect of organic solvent in membrane on the transport efficiency of 1 for Hg(II) was evaluated by using CHCl3, CH2Cl2 and CCl4. The results are shown in Table 3 and Fig. 6. The membrane entrance (k1) and exit (k2) rate constants as well as the flux (Jmax) values decrease in order as CH2Cl2 (74.4%) N CHCl3 (69.7%) N CCl4 (47.5%). This is consistent with the literature reported [44]. Considering the polarity of the examined solvents (Table 4), one can attribute that the viscosity and polarity are essential factors for solvent selection for BLM phenomena.

Table 4 Physicochemical characteristics of solvents [44]. Solvent

ε0

nD

μ

η

Vm

CH2Cl2 CHCl3 CCl4

9.08 4.81 2.24

1.42 1.45 1.47

1.96 1.35 0

0.44 0.58 0.97

64.2 96.5 96.5

ε0 dielectric constant (20 °C). nD refractive index (20 °C). μ dipole moment (D). η viscosity(cP). Vm molar volume (M− 1).

Table 5 The kinetic parameters of Hg(II) transport using carrier 1 at different temperatures (solvent, CHCl3; stirring rate, 90 rpm). Temp. (K)

k1 × 104 (min− 1)

k2 × 103 (min− 1)

max Rm

tmax (min)

× 104 Jmax d (min− 1)

× 104 Jmax a (min− 1)

298 303 308 313

18 22 23 27

5.20 7.40 8.25 9.04

0.124 0.060 0.071 0.095

400 380 320 260

− 10.3 − 13.2 − 14.0 − 16.1

10.3 13.2 14.0 16.1

F.T. Minhas et al. / Desalination 262 (2010) 215–220

219

the diffusion steps are rate determining at donor–membrane or membrane–acceptor interfaces. The rate of transport in such systems can be increased by decreasing film thickness, or by increasing the mobility of the diffusible species. The overall transport kinetics in many other liquid membrane processes is governed by chemical reaction kinetics, especially at the membrane–acceptor interface. Chemical reactions are sufficiently slow in comparison with diffusion controlled processes. The Ea values in diffusion controlled processes are quite low (b84 kJ mol− 1). On the other hand, the Ea values of processes controlled by chemical reactions are high due to a strong effect of temperature on the actual speed constants. Thus, activation energy (Ea) is used as an indicator whether transport through liquid membrane processes is diffusion controlled or chemically controlled by reactions. The activation energy values are 19.59 and 27.57 kJ mol− 1 for extraction and reextraction, respectively. Thus, the obtained values of Ea specify that the transport of Hg(II) is diffusion controlled process [46]. Fig. 7. Arrhenius plots for the transport of Hg(II) in bulk liquid membrane.

3.5. Suggested mechanism The liquid membrane operated here is shown schematically in Fig. 8. The Hg(II) is transported from the donor phase to the acceptor phase via a liquid membrane. Movement of the Hg(II) through the BLM is accomplished by the presence of the carrier 1. After complexation of the mercury picrate ion pair with carrier on the left side of the membrane, the complex diffuses down its concentration gradient. On the right side of the membrane, carrier ion pair complex is decomplexed and mercury picrate ion pair is released into the acceptor phase. The free carrier diffuses back across the liquid membrane and the cycle starts again. The net result is the transport of Hg(II)from the aqueous source phase to the aqueous receiving phase across the bulk of the organic phase [44].

3.6. Comparative study

Fig. 8. Mechanism of the ion pair mediated transport through bulk liquid membrane.

A comparison of kinetic parameters of calix[4]arene thioalkyl derivative have been made with other Hg(II) selective calix[4]arene derivatives (solvent: CHCl3, 298 K) as given in Table 6. It has been found that the results obtained in this study are comparatively better than most of the studies reported in the literature [25,39,41,42,47,48].

4. Conclusions 3.4. Effect of temperature The effect of temperature on the transport of the Hg(II) through the liquid membrane containing 1 in CHCl3 was examined at 293, 303, 308, and 313 K, respectively. The experimental results are collected in Table 5 and Fig. 7. It is quite obvious that k1 and k2 increase with an increase in the temperature. Table 5 shows also that tmax decreases with an increase of temperature. The results suggest that the transport of the Hg(II) could be described by the kinetics laws of two consecutive irreversible first order reactions (as shown above) in the present case. An Arrhenius type plot is followed perfectly in Fig. 7. For many liquid membrane systems,

A kinetic study of the transport of Hg(II) using calix[4]arene thioalkyl derivative (1) as a new and effective metal carrier was described in this paper. The results show that Hg(II) can be effectively transported through a liquid membrane containing 1, and that the transport depends on the ion transported and other parameters, i.e. the stirring speed, carrier concentration, solvent type and the temperature. The apparent rate constants, k1 and k2 of interfacial transport of extraction and re-extraction are determined on the basis of a scheme implying two consecutive irreversible first order reactions. This method offers important advantages such as simplicity and economy.

Table 6 Comparison of kinetic parameters of calix[4]arene thioalkyl derivative with other Hg(II) selective calix[4]arene derivative (solvent: CHCl3, 298 K). Carrier

Stirring rate (rpm)

k1 × 103 (min− 1)

k2 × 103 (min− 1)

max Rm

tmax (min)

Jmax × 103 (min− 1) d

Jmax × 103 (min− 1) a

Reference

Bis-calix[4]arene nitrile derivative Calix[4]arene dinitrile derivative Calix[4]arene tetranitrile derivative Calix[4]arene ketone derivative Ester derivative of bis-calix[4]arene Calix[4]arene biphenyl derivative Calix[4]arene dinitrile-oligomer Calix[4]arene thioalkyl derivative

500 500 500 500 500 300 300 90

3.8 2.5 1.6 2.16 3.03 9.69 15.9 1.80

3.72 12.5 12.4 1.59 16.8 4.38 18.2 5.20

0.372 0.13 0.10 0.42 0.124 0.52 0.34 0.124

266.1 161.1 189.6 537.5 124.5 149.53 58.69 400

− 1.38 − 1.62 − 1.18 − 6.05 − 2.08 − 2.28 − 6.26 − 1.03

1.38 1.62 1.18 6.05 2.08 2.28 6.26 1.03

[39] [41] [41] [42] [47] [48] [25] Current study

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