Adsorption of Inorganic Materials
on Activated Carbon
FREDERICK NELSON, Chemist
HAROLD O. PHILLIPS, Chemist
KURT A. KRAUS, Chemist
Oak Ridge National Laboratory
Oak Ridge, Tennessee 37830
INTRODUCTION
Adsorption of inorganic ions and electrolytes by activated carbon has been of interest
for well over half a century (1,2, 3). Certain aspects of the work which we describe are thus
by no means new, though our point of view may shed additional light on the complex
phenomena which seem to be occurring. We shall divide our discussion into several areas
and illustrate these with recent experiments carried out in our laboratory.
Activated carbons have long been known to exhibit cation exchange properties at high
pH and to some extent in neutral solutions; carboxylic acid groups, phenolic groups,
lactones, etc., are presumably involved (4,5,6). We shall illustrate this area with a few alkali
metal separations. These also illustrate, not unexpectedly, that selectivities do not follow a
simple atomic weight progression rule and that they are dependent on the pH of the
solutions.
A second set of adsorption phenomena deals with uptake of electrolytes (anions and
cations) from neutral and slightly acidic solutions. Here it is convenient to distinguish
between cases where the cation is a metal and where it is the hydrogen ion. We shall
illustrate the former mode of adsorption by some results with Cu(II)-chloride solutions. In
this case the adsorption equilibria are difficult to describe by conventional ion exchange
relationships.
When the cation is the hydrogen ion, one deals with adsorption of acids. Explanations
here are not without controversy. Some like to distinguish different mechanisms for acid
adsorption from dilute and from concentrated solutions (7). As we shall discuss in another
paper which specifically deals with adsorption of acids by activated carbons and with the
activity coefficient relationships of acids in the carbon phase, we do not believe that
different mechanisms occur. However, there is no doubt that a significant amount of the
adsorbed mineral acids is difficult to remove by washing with water; further, the adsorbed
anion can be replaced by other anions as expected for an anion exchanger. Structures
containing carbonium ions have been suggested as permanently attached anion exchange
sites in the carbon (6, 8). While we are not convinced that this is the case, at least with the
preparations with which we have worked, rapid exchangeability of the anions is well
established.
The activated carbons are reminiscent of weak base anion exchangers of the organic
type, and more particularly, of the hydrous oxide type, where acid adsorption also occurs
and, at constant pH, the anions are exchangeable (9). As with many hydrous oxides, the
relative amounts of cation (other than hydrogen ion) adsorption and anion adsorption
depends on the pH of the surrounding medium. We shall briefly describe a technique based
on radioactive tracers (the "double tracer technique") by which the relative extent of anion
and cation uptake can be established. We believe that this technique is generally useful for
elucidating the complex adsorption phenomena typical of activated carbons.
A very interesting group of adsorption reactions occurs at high ionic strength (10,11).
Here certain negatively charged complex ions seem to be strongly adsorbed in a manner
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