J. Cent. South Univ. Technol. (2009) 16: 0569-0574

DOI: 10.1007/s11771-009-0095-2

Adsorption of Pb²⁺ on macroporous weak acid adsorbent resin from aqueous solutions: Batch and column studies

XIONG Chun-hua(熊春华), FENG Yu-jie(冯宇杰), YAO Cai-ping(姚彩萍)

(Department of Applied Chemistry, Zhejiang Gongshang University, Hangzhou 310012, China)

Abstract: The adsorption properties of a novel macroporous weak acid resin (D152) for Pb²⁺ were investigated with chemical methods. The optimal adsorption condition of D152 resin for Pb²⁺ is at pH 6.00 in HAc-NaAc medium. The statically saturated adsorption capacity is 527 mg/g at 298 K. Pb²⁺ adsorbed on D152 resin can be eluted with 0.05 mol/L HCl quantitatively. The adsorption rate constants determined under various temperatures are k_{288 K}=2.22×10^-5 s^-1, k_{298 K}=2.51×10^-5 s^-1, and k_{308 K}=     2.95×10^-5 s^-1, respectively. The apparent activation energy, E_a is 10.5 kJ/mol, and the adsorption parameters of thermodynamics are ?H^Θ=13.3 kJ/mol, ?S^Θ=119 J/(mol?K), and ?G^Θ_{298 K} =-22.2 kJ/mol, respectively. The adsorption behavior of D152 resin for Pb²⁺ follows Langmuir model.

Key words: Pb²⁺; macroporous weak acid resin; adsorption; thermodynamics

1 Introduction

The contamination of water recourses by heavy metal is a serious worldwide environmental problem. Lead is one of the most important pollutants and is of particular interest because of its toxicity and widespread presence in the environment [1]. The removal of heavy metal ions from contaminated water can be achieved by several processes, such as chemical precipitation [2], solvent extraction [3], membrane separation [4] and electrochemical reduction [5]. Most of these processes may be ineffective, extremely expensive, or generate secondary pollution. In recent years, the adsorption process has received much attention and becomes one of the more popular methods for the removal of heavy metals from the wastewater because of its competitive and effective process for the above purpose. Though numerous adsorbents have been reported for the removal of Pb²⁺, such as activated carbon [6], chitosan [7] and clay [8], additional alternative materials for this purpose are still needed. An effective adsorbing material should consist of a stable and insoluble matrix. And polymeric materials have been demonstrated to be such kind of material [9-11]. Many studies have shown the functionalization of a polymeric matrix with different chelating functionalities for metal ions removal or separation [12-15]. As one of these materials, macroporous weak acid resin (D152) is an excellent absorbent because of its strong mechanical stability to high linear flow rates, and high chemical resistibility to harsh conditions. D152 resin contains the functional group of —COOH, which possesses not only protons that can exchange with cations, but also oxygen atoms that can coordinate directly with metal ions. Therefore, the adsorption ability of D152 resin for enriching metal ion may be very strong. In addition, D152 resin is also inexpensive and easy to regenerate.

Until now, little work has been published on the application of D152 resin for the removal of Pb²⁺. The present work was undertaken to study the adsorption of Pb²⁺ on macroporous D152 resin in aqueous solutions in the form of batch and column processes. The effects of various adsorption conditions, such as pH, temperature and shaking speed were also investigated.

2 Experimental

2.1 Materials and instrument

Macroporous D152 resin was provided by Nankai University, China. The properties are shown in Table 1. Standard solution of Pb²⁺ was prepared by dissolving Pb(NO₃)₂·6H₂O (analytical reagent grade) and concentrated nitric acid in purified water. The buffer solution with pH 3.50-6.50 was prepared from HAc-NaAc solution. Other chemicals used were of analytical reagent grade. Pb²⁺ was determined with Shimadzu UV-2550 UV-VIS spectrophotometer. Mettler toledo delta 320 pH meter was used for measuring pH. The sample was shaken with a DSHZ-300A temperature constant shaking machine.

Table 1 General description and properties of absorbent

2.2 Experimental method

2.2.1 Batch adsorption experiment

A desired amount of treated resin was weighed and added into a conical flask, in which a known volume of buffer solution with pH 5.4 was added. After 24 h, a required amount of standard solution of Pb²⁺ was added. The flask was shaken in a DSHZ-300A temperature constant shaking machine at the constant temperature. Aliquot samples were taken from the flask at appropriate time intervals for analysis until adsorption equilibrium reached. The adsorption capacity Q and distribution coefficient D were calculated with the following formulas, respectively:

Q=(C₀-C_e)V/m                               (1)

D=Q/C_e                                                         (2)

where C₀ and C_e are the initial and equilibrium concentrations of Pb²⁺ in solution, mg/mL; V is the total volume of solution, mL; and m is the mass of the resin, g.

2.2.2 Measurement of Pb²⁺

The concentration of unadsorbed Pb²⁺ in the sorption medium was determined by spectrophotometry. A solution containing Pb²⁺ less than 100 μg was accurately added into a 25 mL colorimetric tube, and then 2 mL of 0.1% xylenol orange solution and 10 mL methenamine-HNO₃ buffer solution (pH 5.4) were added. After the addition of redistilled water to the mark of colorimetric tube, the absorbency was determined in a   vessel (1 cm) on a Shimadzu UV-2550 UV-VISIBLE spectrophotometer at the wavelength of 560 nm and was compared with that of the blank test.

2.2.3 Elution test

A desired amount of resin was added into a mixed solution composed of buffer solution (pH 6.00) and a known amount of standard solution of Pb²⁺. After the equilibrium reached, the concentration of Pb²⁺ in the aqueous phase was determined, and then the adsorption capacity of the resin for Pb²⁺ was obtained. The resin separated from aqueous phase was washed three times with buffer solution (pH 6.00). The resin that adsorbed Pb²⁺ was shaken with eluant. After reaching equilibrium, the concentration of Pb²⁺ in aqueous phase was determined and then the percentage of elution was obtained.

3 Results and discussion

3.1 Influence of pH on distribution coefficient (D)

The pH of the aqueous solution is an important parameter that controls the sorption process. The adsorption behavior of Pb²⁺ on the D152 resin in HAc-NaAc medium was investigated in the pH range of 3.50-6.50. The results are shown in Fig.1, indicating that the distribution coefficient is the highest at pH 6.00 and is decreased by either raising or lowering pH [16]. Therefore, the following experiments were performed at pH 6.00 in the HAc-NaAc system.

Fig.1 Influence of pH on distribution coefficient of Pb²⁺ adsorption by D152 resin (resin 15.0 mg, T=298 K, C₀=0.333 mg/mL, r=100 r/min)

3.2 Isotherm adsorption curve

The Langmuir model is perhaps the best known isotherm to describe adsorption from a liquid solution [17]. The model assumes uniform energies of adsorption on the surface and no interaction between adsorbed molecules. The model also assumes that adsorption is limited to complete surface coverage by a monomolecular layer and can be represented by the following equation:

Q=Q⁰bC_e/(1+bC_e)                             (3)

where  C_e is the equilibrium concentration of metal ion, Q is the adsorbing capacity in equilibrium state, Q⁰ is the saturated sorbing capacity, and b is the Langmuir constant. It is usually rewritten in the following form:

C_e/Q=C_e/Q⁰+1/(Q⁰b)                           (4)

It is equally used to analyze batch equilibrium data by plotting C_e/Q vs C_e, which yields a linear plot if the data conform to the Langmuir isotherm. Five parts of 15.0 mg resin were added into a mixed solution composed of buffer solution (pH 6.00) individually. After 24 h, standard solutions of Pb²⁺ at initial concentrations of 0.267, 0.300, 0.333, 0.366 and 0.400 mg/mL were added respectively. When the adsorption equilibrium arrived, the experimental data were recorded.

Plotting of C_e/Q vs C_e gives a straight line. The correlation coefficient of straight line R²=0.997 6 was achieved. The values of Q⁰ and b determined from the figure are found to be 527 mg/g and 379.5 mL/mg, respectively. The results shown in Fig.2 indicate that the Langmuir-type adsorption isotherm is suitable for equilibrium studies.

Fig.2 Langmuir plot for adsorption of Pb²⁺ by D152 resin (resin 15.0 mg, r=100 r/min, pH 6.00)

A comparison of the maximum capacity of D152 resin with that of some other adsorbents reported in literatures is given in Table 2. Differences of metal uptake are due to the properties of each adsorbent such as structure, functional groups and surface area.

3.3 Determination of adsorption rate constant at different temperatures and apparent activation energies

The experiments were carried out by the above-mentioned method at 288, 298 and 308 K, respectively, and a series of data about the adsorption amounts and time were obtained (Fig.3). According to Ref.[26], the adsorption rate constant k can be calculated from the following two equations, respectively:

-ln(1-F)=kt                                 (5)

F=Q_t/Q_∞                                                        (6)

where  Q_t and Q_∞ are the adsorption amounts at time t and at equilibrium state, respectively. A straight line was

Table 2 Comparison of the maximum adsorption capacity capacities of Pb²⁺ by various adsorbents

Fig.3 Adsorption rate curves of Pb²⁺ adsorption by D152 resin (C₀(Pb²⁺)=0.333 mg/mL, resin 30.0 mg, pH 6.00)

obtained by plotting -ln(1-F) vs t (Fig.4). The adsorption rate constant of D152 resin for Pb²⁺ can be found from the slope of the straight line, which are   k_{288 K}=2.22×10^-5 s^-1, k_{298 K}=2.51×10^-5 s^-1, and k_{308 K}= 2.95×10^-5 s^-1, respectively. The linear relationship of -ln(1-F)—t indicates that the liquid film diffusion is the predominating step of the adsorption process.

According to the Arrhenius formula:

lgk=-E_a/(2.30RT)+lg A                        (7)

the slope of straight line, which was attained by plotting lg k vs 1/T (Fig.5) and calculated by linear fitting, yields the apparent activation energy of E_a=10.5 kJ/mol. It can

Fig.4 Determination of adsorption rate constant of Pb²⁺ adsorption by D152 resin (C₀(Pb²⁺)=0.333 mg/mL, resin  30.0 mg, pH 6.00)

Fig.5 Linear plot of lg k vs 1/T for adsorption of Pb²⁺ by D152 resin (C₀(Pb²⁺)=0.333 mg/mL, resin 30.0 mg, pH 6.00)

be seen from the rate constant that the adsorption speed accelerates when the temperature increases within the scope of experimental temperature.

3.4 Influence of adsorption temperature on distribution coefficient and determination of thermodynamic parameters

Under the experimental condition shown in Fig.6, the distribution coefficient of the resin for Pb²⁺ at the temperature ranging from 288 to 308 K was measured. A straight line was obtained by plotting lg D against 1/T with a correlation coefficient of 0.996 0. The result obviously indicates that it is favorable for the adsorption with the adsorption temperature going up. This implies that the adsorption process is an endothermic process. So the adsorption reaction is a chemical adsorption. According to the following equation:

lg D=-?H^Θ/(2.30RT)+?S^Θ/(2.30R)               (8)

?H=13.3 kJ/mol can be computed from the slope of the line, and then ?S=119 J/(mol·K) can be obtained from the intercept of the line. According to

?G^Θ=?H^Θ-T?S^Θ                                         (9)

?G^Θ_{288 K}=-21.0 kJ/mo1, ?G^Θ_{298 K}=-22.2 kJ/mo1, ?G^Θ_{308 K}= -23.3 kJ/mo1 are obtained. These reveal that the adsorption reaction is a spontaneous one under the experimental condition [27].

Fig.6 Influence of temperature on distribution coefficient of Pb²⁺ adsorption by D152 resin (C₀(Pb²⁺)=0.333 mg/mL, resin 15.0 mg, pH 6.00)

3.5 Effect of shaking speed on adsorption rate

The adsorption rate curves at different shaking speeds were studied in the range from 50 to 150 r/min. Three portions of 30.0 mg of resin were weighed accurately and put into conical flasks individually. The experiment was carried out by using the above mentioned method. The data are shown in Fig.7. The result shows that it is advantageous for the adsorption rate when the shaking speed rises, but the influence of shaking speed on adsorption capacity is not very obvious.

Fig.7 Influence of shaking speed on Pb²⁺ adsorption rate by D152 resin (C₀(Pb²⁺)=0.333 mg/mL, resin 30.0 mg, pH 6.00, T=298 K)

3.6 Elution

The elution test was carried out according to the above mentioned method. 30.0 mL eluant was added. The results listed in Table 3 show the elution percentages are different when the concentration of HCl is changed and 0.05 mol/L HCl is the best.

Table 3 Results of elution test of Pb²⁺ (resin 15.0 mg, pH 6.00, T=298 K)

3.7 Dynamic adsorption and desorption

3.7.1 Dynamic adsorption curve

Dynamic adsorption was conducted using a glass column packed with the newly prepared D152 resin at room temperature [28]. 300 mg of resin was accurately weighed. After the column was conditioned, a solution with a concentration of 0.200 mg/mL Pb²⁺ was continuously fed into the column at a constant flow rate of 0.25 mL/min. The effluents from the column were analyzed quantitatively according to the above mentioned method. Breakthrough data were acquired by plotting the volume of solution that was passed through the column vs the concentration of Pb²⁺ in the effluent. When the concentration of Pb²⁺ emerging from the bottom of the column (C₁, mg/mL) is equal to that entering the top of the resin bed (C₂, mg/mL), the experiment was terminated. Then a plot of C₁/C₂ vs the volume of effluent gives a typical breakthrough curve in Fig.8.

Fig.8 Dynamic adsorption curve for Pb²⁺ adsorption by D152 resin at C₂=0.200 mg/mL and flow rate of 0.25 mL/min

3.7.2 Dynamic desorption curve

The dynamic desorption curve of D152 resin for Pb²⁺ was obtained based on the volume of desorption solution and the Pb²⁺ concentration in the desorption solution (Fig.8). With respect to the stripping of Pb²⁺ from D152 resin, 0.05 mol/L HCl was employed and a flow rate of 0.15 mL/min for elution of Pb²⁺ from the column was used. The results are shown in Fig.9.

Fig.9 Dynamic desorption curve for Pb²⁺ adsorption by D152 resin at flow rate of 0.15 mL/min

4 Conclusions

(1) A novel macroporous weak acid resin (D152) is employed as a new adsorbent material for uptaking of Pb²⁺ in aqueous solutions. The adsorption characteristics are examined at different pH values, temperatures, shaking speeds and initial metal ion concentrations, in the form of batch and column processes.

(2) The maximum adsorption capacity of D152 resin for Pb²⁺ is evaluated to be 527 mg/g at 298 K in the HAc-NaAc system. And it is found that 0.05 mol/L HCl solution provides effectiveness of the desorption of Pb²⁺ from D152 resin.

(3) Isotherm studies show that the adsorption process of D152 resin for Pb²⁺ obeys the Langmuir model. This suggests that the adsorption of Pb²⁺ by D152 resin is monolayer-type and agrees with the observation that the metal ion adsorption from an aqueous solution usually forms a layer on the adsorbent surface.

(4) Thermodynamic parameters including standard enthalpy (?H^Θ), standard entropy (?S^Θ) and standard free energy (?G^Θ) indicate that the adsorption of Pb²⁺ on D152 resin is a spontaneous reaction and is endothermic in nature.

(5) Breakthrough curves are obtained from column experiments. The experimental results demonstrate that D152 resin can be used for the removal of Pb²⁺ from aqueous solution.

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(Edited by CHEN Wei-ping)

Foundation item: Project(2008F70059) supported by the Scientific and Technological Research Planning of Zhejiang Province, China

Received date: 2008-11-03; Accepted date: 2009-01-06

Corresponding author: XIONG Chun-hua, Professor; Tel: +86-571-88932083; E-mail: xiongch@163.com