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김종성, 이종협

서울대학교 화학공학과

Modeling the Removal of Heavy Metal Ions from Aqueous Solution using Ceramic Beads Impregnated with Chelating Agents

Jong Sung Kim and Jongheop Yi

Dept. of Chemical Engineering, Seoul National University

INTRODUCTION

The removal of hazardous heavy metal ions from the aqueous streams is a significant industrial waste problems. The use of ion exchange chelation molecules to selectively extract metal ions of waste streams is currently receiving substantial attention [1]. This interest stems from an awareness of advances made in the development of commercial extractants for metal separation in the hydrometallurgical industries.

Inorganic supports have been used as the stationary phases in chromatographic metal separations. However, this technique has mainly been used as analytical purpose. In recent years there have been attempts to immobilize various chelating agents on ceramic materials by chemical surface modification methods [2].

The objective of this study is to model the metal removal process of the impregnated ceramics.

EXPERIMENTAL

□-alumina beads (□1.5mm, Grace) and silica beads (□0.65mm, Fuji Silysia) were used as supports. 2-hydroxy-5-nonylaceto-phenone oxime in Kerosen (LIX 84, Henkel), □-aminopropyltriethoxy silane (Aldrich) and glutaraldehyde (25wt% in water) were used without further purification.

Silanization and silanization-bridging method were used for attachment of LIX 84 at □-alumina surface [3].

MODELING

The reaction for exchange of metal ions may be given by [4] :

[eqn: rm M^2+~ +~ 2 {bar HR}~~ □ ~~{bar MR_2} ~+~ 2H^+]                                   (1)

Here,[eqn: rm {bar HR}] and[eqn: rm {barMR_2}] represent the chelating agents and metal-chelating agent complex respectively, and overbar denotes species in the solid phase. The reaction of eq (1) is a reversible reaction wherein a cation and hydrogen ions are exchanged. Thus, the degree of extraction of metal ions depends on the pH of the aqueous phase. The expression for the ion exchange reaction, R, can be written as

[eqn: rm R~ =~ k_+~ C_M~ q_HR^2~ ~~~ -~~ k_-~ q_{M R_2}~ C_H^2~~]                               (2)

where CM and CH denotes concentrations in the solution, and qHR and[eqn: rm q_MR_2]  are concentrations in the beads. In this reaction, equilibrium constant, Ke ,can be defined as follows,

[eqn: rm K_e~ =~ {q^*~ C_H^2} over {C^*~ q _HR ^*2 }~]                                                    (3)

where * represents the equilibrium state

New conditional equilibrium constant, K, can be defined as

[eqn: rm K_~ = ~K_e over {c_H^2} ~ = ~q^*~ over {C^*~ q _HR ^*2 }=~ q*~C_H^*2 over {C^*~ (Q-q_HR^*2 ) }~]                          (4)

In eq (4), Q is the maximum capacity of the bead.

Introducing dimensionless variables, material balance in the bulk solution can be written as follows.

[eqn: rm - dX over {d tau} = N_M (X - X^* )]	(5)

where    [eqn: rm N_M ~=~ Bi~{4□R^3 N} over V]

X = C/C0

[eqn: rm tau~ =~{t ~D_eff} over R^2]                                                (6)

Bi = KfmR/Dm,

In the above expressions, Kfm is the film mass transfer coefficient, R is the radius of the bead, Dm is the molar diffusivity of metal ions, N is the number of the bead, and V is the volume of the metal solution.

Material balance inside the beads can be written as follows.

[eqn: rm{ partialX over partialtau}~=~ ({ partial^2 X} over partialzeta^2 + {2 over zeta} partialX over partialzeta )]/A                                      (7)

where :    □ = [eqn: rm r over R]

[eqn: rm A~ =~ epsilon_p ~+~{2□_p~ KQ^2} over { SQRT {4KQC_0 X + 1 }~ (2KQC_0 X + 1+ SQRT {4KQC_0 X + 1 } ) }]

Where,[eqn: rm epsilon_p] is the porosity of the beads

The initial conditions and boundary conditions are given by

[eqn: rm X_b =1]	At □= 0 :	(8)

[eqn: rm X = 0]	At □= 0 :                                      (9)

[eqn: rm {partial X}over{partialzeta} =0]	at [eqn: rm zeta = 0]                         (10)

[eqn: rm Bi ( X - X^* ) = {partial X} over{partial it zeta}rightvert_{itzeta=1}]        at [eqn: rm zeta = 1]	(11)

where Xb = C/Cb

Eq (5) ∼ (11) were solved by using IVPAG package included in IMSL package.  By finite difference method, PDE of eq (7) was transformed into a set of ordinary differential equations.

PARAMETER SENSITIVITY AND PARAMETER ESTIMATION

Fig. 1 shows the calculated concentration changes for the bead radius of 0.25mm, 0.5mm and 1.8mm. Fig. 2 shows the concentration profiles inside the beads calculated with varying time.

Effective diffusion coefficient, Deff, is generally defined as follows,

[eqn: rm D_eff~ =~{D_m epsilon_p over tau_p}]                                                    (12)

where[eqn: rm tau_p] is the tortuosity. The value of Deff can be evaluated by matching the experimental data to the data calculated from the solution of a model equation which describes the ion exchange process.

REFERENCES

[1]	Warshawsky, A. In : J. A. Marinsky, and Y. Marcus (Editors), Ion Exchange and Solvent Extraction, Marcel Dekker, New York, vol. 8 (1981), pp. 229-281.

[2]	Bradshaw, J, S., Krakowiak, K. E. and Izatt, R. M., Aza-crown Macrocycles, John Wiley and Sons, New York (1993).

[3]	Yi, J. and Tavlarides L. L., Ph. D. Thesis, Syracuse Univ. (1992).

[4]	Loureiro, J. M., Costa, C. A. and Rodrigues, A. E., Chem. Eng. Sci., 43(5),  1115-1123 (1988).