ACTIVATED CARBONS FILLED WITH ZnS AS ADSORBENTS
Kurt A. Kraus, Chemist
Frederick Nelson, Chemist
Chemistry Division
Oak Ridge National Laboratory
Oak Ridge, Tennessee 37830
J. S. Gilbert, Graduate Student
Department of Environmental Engineering
University of Tennessee
Knoxville, Tennessee 37916
INTRODUCTION
Many years ago we showed in our laboratory that highly insoluble transition metal
sulfides can be used for removal of metals from solution provided the starting sulfide is
more soluble than the one produced from the exchanging ions of the feed [1-3].   The
exchange reaction characterizing this process (illustrated for ZnS)
ZnS + (2/i) M*z -»• M2/ZS + Zn* (D
apparently occurs in the solid phase without the sulfide first going into solution.
Exchange reactions of this type have long been known to occur with natural minerals,
but with these they seem to be limited to the surface of the particles.   If the adsorbers
are prepared as microcrystalline or amorphous materials by precipitation and drying,
these exchange reactions often are not limited to the surface of the particles and in many
cases extend through the bulk of the materials.
When bulk exchange occurs, large capacities or uptakes can be achieved.   Indeed, as
shown in Table I [2], uptakes can at times exceed 10 mol/liter of bed.   For this reason, *"•
as well as others [4], have proposed the use of these sulfides for processing of waste
streams where removal of (toxic) transition elements is desired.   To our knowledge, this
technique has not been used extensively.   While we are not clear regarding the reasons
for the relative unpopularity of these interesting materials for processing, we suspect that
it may in part result from the fact that they are not easily prepared as granules of sufficient size for column operation and that they sometimes tend to disperse (or peptize).
Table I. Uptake of Metals by Sulfides (at 50% Breakthrough) [2]
Sulfide
Metal
Mol/kg
Moll Bed
% Theor.
CdS
Ag(I)
13.5
12.1
98
CdS
Cu(H)
4.8
4.3
69
CdS
Hg(II)
4.4
4.0
64
78
CdS
Au(IlI)
3.6
3.2
ZnS
Agfl)
14.5
20.3
71
65
5
ZnS
Cu(II)
6.6
9.3
ZnS
Hg(II)
0.5
0.8
Two years ago we reported at this conference [5] on the use of activated carbon as a
carrier for inorganic adsorbents which, by themselves, are difficult to prepare in forms
suitable for column operation.   We illustrated this application of activated carbon by
1046