활성탄소섬유에 의한 폐액내에 있는 피콜린산-킬레이트

금속이온의 흡착

이상문, 정종헌*, 오원진*, 이보성, 유승곤

충남대학교 공과대학 화학공학과

*한국원자력 연구소 제염·해체연구실

The Adsorption of Picolinic acid-Chelated Metal

Ions in Liquid Waste on Activated Carbon Fibers

S.M. Lee, C.H. Jung*, W.Z. Oh*, B.S. Rhee, S.K. Ryu

Dept. of Chemical Engineering, Chungnam National University,

*Decontamination & Decommissioning Dept.,

Korea Atomic Energy Research Institute

INTRODUCTION

Adsorption process is one of the promising techniques for the removal of metal ions in liquid waste  resulting from the chemical decontamination of nuclear facilities. Especially, it is effective as an alternative method for the precipitation process in the treatment of liquid waste containing organic chelated agent such as EDTA, picolinic acid, etc.

In this study, it was investigated on the adsorption behaviors of activated carbon fibers(ACFs) for the picolinate complexes with various metal ions (such as Cu2+,Co2+,Ni2+), which has a unique characteristics of greater adsorption rate and capacity for organic and inorganic molecules or ions than commercially available active carbons(ACs)[1].

EXPERIMENTAL

Two ACFs were prepared by activation of isotropic pitch-based carbon fibers with steam diluted in nitrogen. Textural characteristic of ACFs was determined from nitrogen adsorption isotherm of ACFs at 77K. Pore size distributions of ACFs were as analyzed by H-K Method[2]. Single component aqueous solution was prepared by dissolving CuCl2·2H2O, CoCl2·6H2O, and NiCl2·6H2O to distilled water at various concentration respectively. And also the effect of picolinic acid was investigated for above metals.

Metal content was determined by flame Atomic Absorption Spectrophotometry(Perkin-Elmer 1100B). If necessary, samples were diluted with distilled water prior to metal analysis. All samples were filtered through 0.45□m membrane filter prior to analysis. NaOH and HCl were used for pH adjustment. An Metrohm combination pH electrode(Model# 602) was used to measure pH. A temperature of 25℃ was used for all experiments in the batch mode.

RESULT & DISCUSSION

Pore size distributions by H-K method shown in Fig. 1 and the textural characteristics of ACFs in Table 1. indicate that the volumes of all of the narrow micropores and the supermicropores increase on activation, and the average pore size is about 10Å. A-20  has greater adsorption capacity  for all metal  ions than A-7 (Fig. 2). This is evidently related to the larger surface area and pore volume, considering of  the similar chemical properties of two ACFs.

The adsorption isotherms of Cu2+, Co2+, and Ni2+, on the variation of the molar ratio of metal to picolinate ligand are presented in Figs 3∼5. The adsorption isotherms of Cu2+, Co2+, Ni2+ on this ACF might be included in type L(2) according to Giles′classification [3].

In the absence of picolinic acid,  the amount of adsorbed metal increases in the order: Ni<Co<Cu. This order is determined by many factors such as ionic potential, chemical properties, ionic radius, and hydrolysis. Having in mind the hydration number of 6 and similar ionic potential values of the metal ions studied (Cu2+: 2.78, Co2+: 2.78, Ni2+: 2.89), and hydrated radii of the metals: Cu2+- 0.419 nm, Co2+-0.423 nm, Ni2+-0.404 nm[4], the adsorption order is expected to be as follows: Co<Cu<Ni. The results might be due to the differences of their polarizing power.

The adsorption isotherms (Figs.3∼5) were observed to be strong functions of the ratio of metal ion and picolinic acid concentrations. Adsorption of picolinate-metal chelates on ACFs was found  to be more favorable than that of free metal ion. At the precipitation-free experimental conditions a dominent species of the metals in solution is supposed to exist as the positively charged  free metal ions, while picolinic acid-chelated metal species is supposed to exist as the  neutrally or less positively-charged type. Thus, most of the metal removed was in the form of M-Pic chelates(M: metal, Pic:Picolinate), and the free metal species remains in solution. Therefore, the greater the picolinic acid concentration, the greater the fraction of metal ions present in the chelated form which preferes to be adsorbed by ACF for Pic/M≤1.

The adsorption isotherms of Cu2+, Co2+, Ni2+ on the activated carbon fiber(ACF) at temperature of 298K and 318K are shown in Fig. 6.  It is noteworthy that for a given equilibrium concentration, the amount  of Cu2+and Co2+ adsorbed at 298K is higher than those at 318K, which suggest that these adsorption processes are exothermic, while  the amount of Ni2+ adsorbed on ACF at 298K is lower than at 318K, indicating this endothermic process.

REFERENCE

[1]  S.S Barton, Carbon, 25, 77 (1987)

[2] Linda S. Cheng, Ralph T. Yang, Chemical Engineering              Science, 49, 2599 (1994)

[3] C. H. Giles, T. H. MacEwan, S. N. Nakhwa, and D. Smith,          J. Chem. Soc. 3973 (1960)

[4]  E. R. Nightmighle, J.Phys. Chem., 63, 1381 (1959).

Table 1. Textural Characteristics of ACFs

	SBET (m2/g)	total pore volume (cc/g)	micropore volume (cc/g)	Ave. pore radius (Å)
A - 7	683	0.28	0.26	9.06
A -20	1850	1.07	0.74	9.84