Environ. Sci. Technol. 1991, 25, 481-488
Christakos, G. On the Problem of Permissible Covariances and Variogram Models. Water Resour. Res. 1984, 20, 251-265. Isaaks, E. H.; Srivastava, R. M. A n Zntroduction t o Geostatistics; Oxford University Press: New York, 1989. Rossi, M. Impact of Spatial Clustering on Geostatistical Analysis. M.Sc. Thesis, Dept. of Applied Earth Sciences, Stanford University, Stanford, CA, 1988. Parker, H.; Srivastava, R. M. Relative Variograms and Robust Spatial Continuity Measures; Third International Congress of' Geostatistics Avignon, France, 1988; Vol. 2.
(14) Isaaks, E. H.; Srivastava, R. M. Spatial Continuity Measures for Probabilistic and Deterministic Geostatistics. Math. Geol. 1988, 20, 313-341. (15) Broecker, W. S. Chemical Oceanography; HBJ Inc.: New York, 1974.
Received for review November 9, 1989. Revised manuscript received May 2 1 , 1990. Accepted October 23, 1990. Financial support was provided to D.P. by a grant from the McGee Fund, School of Earth Sciences, Stanford University.
Metal( II)Ion Binding onto Chelating Exchangers with Nitrogen Donor Atoms: Some New Observations and Related Implications Arup K. Sengupta," Yuewei Zhu, and Diane Hauze
Fritz Engineering Laboratory, Environmental Engineering Program, Lehigh University, Bethlehem, Pennsylvania 18015 Chelating exchangers with only nitrogen donor atoms exhibit some unusual properties in relation to metal ion uptake from the aqueous phase. Specifically, for these chelating exchangers, (a) the metal ion uptake increases with an increase in competing Ca2+and Na+ ion concentrations; (b) metal removal capacities remain almost unaffected at pH as low as 1.0; (c) regeneration or desorption of metal ions with ammonia is very efficient while acid regeneration is ineffective; and (d) both metal cations and other anions can be removed simultaneously from the aqueous phase. The foregoing behaviors are not observed at all for other commonly used chelating exchangers with carboxylate, iminodiacetate, or aminophosphonate functionalities. A molar-exchange model, where the individual nitrogen donor atoms bind metal ions independently on molar basis, can explain the anomalous characteristics of these specialty chelating exchangers. Because of their unusual properties, these chelating exchangers with only nitrogen donor atoms offer new application opportunities for heavy-metals removal unattainable through other exchangers. ~~
Introduction Chelating Exchangers: General Background and Metal Ion Selectivity. Chelating exchangers are, in general, coordinating copolymers with covalently bound side chains, which contain one or multiple donor atoms (Lewis bases) that can form coordinate bonds with most of the toxic metal ions (Lewis acids). Due to coordination-type interactions, all such chelating exchangers offer extremely high selectivity toward commonly encountered toxic M(I1) cations, namely, Cu2+,Pb2+,Ni2+, Cd2+,and Zn2+,over competing alkaline (Na+,K+) and alkaline-earth (Ca2+,Mg2+)metal cations. Depending on the number of donor atoms present in a pendant ligand of the polymer, the repeating functional groups are often referred to as mono-, bi-, or polydentate. In the recent past, a great deal of interest has been observed in relation to the applicability of these chelating polymers for removal, separation, and purification of metal ions from heavy-metal-contaminated water, wastewater, and solid wastes (2-6). State of the art reviews on synthesis and properties of a great number of chelating polymers including the recently developed ones have been provided by Hudson (7) and Warshawsky (8). Table I provides functionalities and other salient information for five different chelating exchangers used in this study, of which the first three (iminodiacetate, car0013-936X/91/0925-0481$02.50/0
boxylate, aminophosphonate) are quite popular and traditionally used in many industrial applications (3,5).The last two chelating exchangers (Dow Chemical, Midland, MI) have only nitrogen donor atoms and are weakly basic; on protonation, they develop fixed positive charges. For convenience, the exchanger with three N atoms and the one with two N atoms per functional group in Table I will be referred to as DOW 3N and DOW 2N, respectively. Figure 1provides experimentally determined Cu(II)/Ca and Ni(II)/Ca separation factors at pH 4.0 for three different chelating exchangers, and the high selectivities of the metal ions can be readily noted ( 4 ) . Separation factor is a dimensionless measure of relative selectivity between two competing ions and, in this case, equal to the ratio of distribution coefficient of the metal(I1) concentration between the exchanger phase and the aqueous phase to that of calcium ion and is given as follows: separation factor
[m]
aMICa =
[RM][ ~ a 2 + 1 [M2+][RCa]
(1)
where and [M2+] correspond to exchanger- and aqueous-phase concentrations of M(II), respectively. It is true that Coulombic and hydrophobic interactions are present in such chelating exchange processes, but their roles are relatively insignificant compared to Lewis acidbase interaction in contributing toward such high metal ion selectivities. Another generic similarity among all chelating exchangers lies in the fact that these functionalities are weak acid or weak base, and therefore, all of them exhibit high affinity toward hydrogen ions. Differences in metal ion selectivities among different chelating exchangers in Figure 1 are closely related to the metalligand stability constants of different functionalities; this aspect is, however, outside the central objective of this article. Metal Ion Binding onto Chelating Polymers and Premises of the Study. In a chelating polymer, the functional groups with the donor atoms are rigidly (covalently) bound to repeating monomers (like styrene) which, again, are fixed as part of a three-dimensional network cross-linked through divinylbenzene. As a result, the ligands in the polymer phase experience considerable strains to orient themselves spatially around the receptor metal ions. This strain may not allow the individual functionalities in the polymer phase to reproduce their aqueousphase metal-ligand configurations. Extensive experimental results are available for more widely used chelating
0 1991 American Chemical Society
Environ. Sci. Technol., Vol. 25, No. 3, 1991 481
Table 1. Background Information on Some Chelating Polymers
functionalitv
/cnxcoar
a!!
donor atoms per functional group
matrix
acid-base characteristic
manufacturer and trade name
one N and two 0
polystyrene
weak base and weak acid Rohm and Haas, IRC-718
one 0
polymethacrylate weak acid
one N and two
polystyrene
weak base and weak acid Rohm and Haas, ES-467
three N
polystyrene
weak base
Dow Chemical, DOW 3N or XFS 4195
two N
polystyrene
weak base
Dow Chemical, DOW 2N or XFS 43084
\cn,coer
cog-
n
o
I n @-.N;,;c". :o-
0
9;
@
..
Rohm and Haas, DP-1; Biorad Inc., Biorex 70
p > - i - C H . Q
..
@
(N)CH,~ycH,cnoH-cn, I
.. R denotes the repeating unit in the polymer matrix. Manufacturer's literature provides additional information.
,
n
(",%
Imim&mlate:
-c.,"1(.,PO.
& cw,cw *R
