Adsorption Study of Pb(II), Cd(II), Cu(II) and Cr(III) Ions in Aqueous Medium using C-4-Hydroxy-3 Methoxyphenylcalix[4]Resorcinarene Dodecaacetate

Introduction

Heavy metal contamination in water is a serious problem, particularly in the health and environmental aspects. It occurs since many industries discharge their waste without any proper treatment. Most of the heavy metal are toxic and may be bio-accumulated in human body. In addition, heavy metal may cause serious health hazards¹. In this context, WHO regulated the standard quality of metal ion in the drinking water. For instance, the regular limits for Pb(II), Cd(II), Cu(II) and Cr(III) should not exceed 0.01, 0.003, 2.0 and 0.05 mg/L, respectively². To eliminate the amount of metal ions in the aquatic environment, a proper and effective wastewater treatment is highly required. Various techniques have  been developed such as adsorption, ion-exchange, coagulation and precipitation. Among them, adsorption offers several advantageous such as simple operation, good efficiency and the possibility to recycle the adsorbent. It is also cheap and can be applied at low concentration of metal ions³. A number of natural and synthetic adsorbents have been applied in the removal of heavy metal ions such as zeolite⁴, humic acid⁵, modified chitosan^6-8, biosorbent⁹ and calixarene^10-11. Calixarene is a cyclic oligomer of phenol which posses unique structure and shape. The presence of cavity and the unlimited functional group modification has led this macrocycle to be mainly applied in host-guest chemistry¹². This study will more focus on the cyclic oligomer of resorcinol namely calix[4]resorcinarene. The previous reports showed that calix[4]resorcinarene was significantly efficient in heavy metal adsorption^13-17. The design of calix[4]resorcinarene-based-adsorbent was carried out by installing numbers of chelating atoms into its framework. The potential functional groups which display good affinity towards the metal ions include amine, alcohol, ether, phosphine, carbonyl and carboxylic acid derivatives^{8, 18-21}. In the connection with our study, we have synthesized C-4-hydroxy-3-methoxyphenylcalix[4]resorcinarene dodecaacetate containing acetoxy and alkoxy moieties. Beside the simple adsorbent preparation, it was expected that the increase of chelating atoms may be beneficial to the heavy metal sorption.  Therefore, the adsorption performance of the synthesized calix[4]resorcinarene in the removal Pb(II), Cd(II), Cu(II) and Cr(III) ions will be investigated.

Experimental Part

Materials and Instrumentation

Adsorbent of C-4-hydroxy-3-methoxyphenylcalix[4]resorcinarene dodecaacetate was prepared according to the previous study (Figure 1). The adsorbent was sieved to give the particle size of 100 mesh. The stock solutions of Pb(NO₃)₂, Cu(NO₃)₂, Cd(NO₃)₂ and Cr(NO₃)₃ (1,000 mg/L in HNO₃ 0.5 M) were purchased from E. Merck. The stock solution was diluted with deionized water to give the desired concentration. The concentration of the metal ions was determined by atomic absorption spectrometer (AAS, Perkin Elmer 3110 USA) and was corrected with the blank solution. Solutions of HCl (0.1 M) and NaOH (0.1 M) were utilized to adjust the pH of the solution. The pH was determined by  pH meter (Orion model 370).

Figure 1:  The Structure of C-4-hydroxy-3-methoxyphenylcalix [4]resorcinarene dodecaacetate   Click here to View figure

 

Adsorption Study

The adsorption was carried out in batch system. As much as 0.1 g of the adsorbent was introduced into the 10 mL of metal solution in certain concentration and followed with the shaking at temperature ambient for certain period of time. The filtration of the mixture gave the filtrate which was analyzed by means of AAS spectrometer to determine the metal concentration. Additionally, the concentration of metals in blank solution was also measured. The adsorption capacity q (mmol/g) for such experiment was calculate based on Eq. 1.

where C_o was the initial concentration of metal ion, C_f was the metal ion concentration after certain period of time. The volume of metal solution and the mass of the adsorbent were defined as V and W, respectively.

In this study, several adsorption parameters were evaluated including pH, mass of adsorbent and interaction time. The kinetic study was also performed by using three models. The first-order model was proposed by Santosa²² which was based on the concentration of the adsorbate in the aqueous phase  and can be described by the eq. 2.

where t was adsorption time (minute), k was first order adsorption rate constant (minute^-1) and K was equilibrium constant (mol/L). In addition, the next two models (Lagergren and Ho) were based on the concentration of the adsorbate in the solid phase. The Lagergen’s pseudo-first order and the Ho’s pseudo-second order were generally expressed by eq. 3 and 4^23-24, respectively.

where q_e and q_t were the adsorption capacity (mg/g) at the equilibrium and contact time t, while k₁ and k₂ were the rate constants of the pseudo-first order (mg.g^-1.min^-1) and the pseudo-second order (g.mg^-1.min^-1), respectively.

Results and Discussions

We have performed adsorption study of some selected heavy metal cations (Pb(II), Cd(II), Cu(II) and Cr(III)) by using C-4-hydroxy-3-methoxyphenylcalix[4]resorcinarene dodecaacetate as adsorbent. The study was carried out under different pH, adsorption dose and contact time in order to determine the maximum adsorption capacity. In addition, the kinetic study was also conducted to determine the kinetic mechanism of the adsorption of the heavy metals.

Effect of pH

pH is one of the important parameter in the adsorption study since it may affect the nature of both adsorbent surface and the metal ion²⁵. In this study, the adsorption was carried out in the range pH of 2-6. As displayed on Figure 2, the adsorption profiles for all metal ions were dependant to the pH value and displayed similar pattern. In acidic condition, the adsorption capacity was relatively low. Increasing the pH enhanced the capacity until the optimum pH and the higher pH led to the decrease of adsorption capacity.

Figure 2: The Adsorption profiles of all metal ions   Click here to View figure

 

These results can be explained by considering electronic interaction between the adsorbent and the speciation of metal ions^26-28 in different pH. In term of metal speciation, both Pb and Cd are mainly existed as Pb²⁺ and Cd²⁺ under pH 6. On the other hand, the major species of Cr³⁺ and Cu²⁺ can be found under pH 2 and 4, respectively. Increasing the pH value allowed to these metal ion to coordinate with ligand exists on the system to form the metal complex which posses lower positive charge (M^(n-x)+).

In low pH, all the metal ions were in the form of free metal ion (Mⁿ⁺). However, the proton was also present in the medium and was the potential Lewis acid to interact with the adsorbent. Therefore, the competition between the proton and the metal ion may decrease the adsorption capacity. The increase of adsorption capacity toward all the metal ions were observed starting from pH 3 until the optimum pH values were reached. The rationalization of these results was probably due to the decrease of the proton concentration. Therefore, the adsorbent will preferably interact with the metal ion. While the optimum pH of Pb(II) and Cu(II) was 4, the maximum adsorption capacity for Cd(II) and Cr(III) was reached at pH 5. Performing the study above the optimum pH diminished the adsorption capacity due to the increasing of the hydroxyl ion concentration. This allowed the formation of lower charge metal complex and, therefore, decrease the interaction between metal and adsorbent.

Effect of Adsorbent Dose

The effect of the adsorption dosage on the removal of the metal ions was investigated by varying the adsorbent mass ranged from 0.0175 to 0.15 g (Figure 3). The study was carried out at the initial metal concentration of 10 ppm at temperature ambient and under the optimum pH. It was expected that increasing the dose may supply more the adsorption sites and may, therefore, enhance the capacity. However, we observed the decrease on the adsorption capacity for all the metal ions with the increasing of the adsorption dose. The reason may be owing to the aggregation of the adsorbent due to the high concentration of adsorbent. As the consequence, the available adsorption sites were decreased²⁹.

Figure 3: The effect of the adsorption dosage on the removal of the metal ions   Click here to View figure

 

Effect of Contact Time

The effect of contact time to the removal of metal ion using calix[4]resorcinarene adsorbent was presented in Figure 4 and 5. The adsorption of Cu(II), Cd(II) and Cr(III) dramatically increased at 3 min. After 3 min of interaction time, the adsorption capacity of Cd(II) was relatively constant, while that of Cu(II) and Cr(III) tended to decrease. The decrease on the capacity was probably due to the weak interaction between the adsorbent and the metal ion. On the other hand, the rate of Pb(II) adsorption was relatively low comparing to the others. The rapid increase of the adsorption capacity occurred at the first 30 min. The capacity gradually increased and reached the equilibrium at 360 min.

Figure 4 : The effect of contact time to the removal of Cr(III), Cd(II), and Cu(II)   Click here to View figure

 

Figure 5: The effect of contact time to the removal of Pb(II)   Click here to View figure

 

Kinetic Study

In order to investigate the mechanism of metal sorption on calix[4]resorcinarene adsorbent, three kinetic equation models were utilized. They were first order of Santosa, pseudo first-order of Lagergren and pseudo-second order of Ho. Unfortunately, the adsorption kinetic of Cu(II), Cd(II) and Cr(III) ions could not be studied due to the very rapid sorption at the initial stage and the decrease of capacity as the function of time.

Figure 6 : The three kinetic models (a) Santosa’s first order (b) Lagergren’s pseudo-first order (c) Ho’s pseudo-second order   Click here to View figure

 

Table 1: The three kinetic profiles of the adsorption of Pb(II) onto the calix[4]resorcinarene

Kinetic Parameter		Pb(II)
qe (mg/g)		0.4454
Santosa’s first order	k (1/min)	0.0004
	r2	0.8823
Lagergren’s pseudo-first order	k1 (1/min)	0.0012
	qe (mg/g)	0.2775
	r2	0.8185
Ho’s pseudo-second order	k2 (g/mg.min)	0.0865
	qe (mg/g)	0.3936
	r2	0.9995

 

The kinetic profiles of the adsorption of Pb(II) onto the calix[4]resorcinarene by using the three kinetic models were depicted in (Figure 6a-c and Table 1). By comparing the value of the linear regression value (R²) of each model, it was found that Ho’s pseudo-second order (R² = 0.9995) had the highest linearity comparing to Santosa’s first order (R² = 0.8823) and Lagergren’s pseudo-first order (R² = 0.8185). It indicated that that adsorption process could be better described by the second order of Ho.

According to the Ho’s equation, the adsorption capacity at the equilibrium can be determined from the slope of the plot of t/q_t vs t. The capacity was 0.3936 mg/g and was more closer to the experimental adsorption capacity. Moreover, the calculated rate constant (k₂) was found to be 0.0865 g/mg.min.

Conclusion

Adsorption study of several heavy metal ions of Pb(II), Cd(II), Cu(II) and Cr(III) onto C-4-hydroxy-3-methoxyphenylcalix[4]resorcinarene dodecaacetate has been successfully carried out. The results showed that the adsorbent was effective to remove Pb(II) and Cu(II) at pH 4 as well as Cd(II) and Cr(III) at pH 5. It was noticed that the increase of the adsorbent dose led to the decrease of adsorption capacity. It has been also found that the adsorption rate for Pb(II) was relatively lower than the that for the others. Moreover, the adsorption of Pb(II) followed the pseudo second order model of Ho. From this study, C-4-hydroxy-3-methoxyphenylcalix[4]resorcinarene dodecaacetate was expected to become an alternative adsorbent in the removal of heavy metals.            

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