Activated Carbon Process For The Treatment Of Cadmium (2) Containing Wastewaters

United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-83-061  Oct. 1983
Project Summary
Activated  Carbon  Process for
the  Treatment  of Cadmium(ll)-
Containing  Wastewaters
C. P. Huang
  This study deals with the removal of
cadmium(ll) from two synthetic cadmium
plating wastewaters, namely cadmium
(ll)-tetrafluoroborate and cadmium(ll)-
cyanide solutions, by activated carbon
adsorption process. Among a total of
17 different  types of commercial
activated carbon tested, it was found
that the acidic activated carbons,
namely Nuchar SA and Nuchar SN ex-
hibited the greatest cadmium(ll) re-
moval capacity.
  In batch mode experiments, the
effects of pH, carbon to cadmium(ll)
ratio, temperature, particle size, ionic
strength and mixing rate on cadmium(ll)
removal were  studied.  A suspension-
polymerization technique was used to
prepare activated carbon beads from
powdered carbon. The cadmium(ll)
adsorption characteristics of  the  ac-
tivated carbon beads were studied and
compared with the original  powdered
carbon.  Chemical regeneration with
strong  acids  of the used  activated
carbon was also investigated along with
wet thermal-chemical process.
  Continuous flow column packed with
activated  carbon beads was used to
treat the cadmium(ll)-tetrafluoroborate
wastewater with successive adsorption-
regeneration runs.
  The costs of activated carbon adsorp-
tion systems, i.e., continuously mixed
and packed column, were also estimated
and compared with several other treat-
ment processes including  insoluble
sulfide precipitation, alkaline neutral-
ization precipitation and ion exchange
processes.
  This Project Summary was developed
by EPA's Industrial Environmental
Research  Laboratory, Cincinnati,  OH,
to announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
  It has been estimated that more than
45% of all cadmium is used in electro-
plating, and that 10% of total industrial
cadmium use ends up in  wastewater
streams. The removal of cadmium(ll)from
wastewater stream, therefore, is of great
importance, especially in the protection
of human health. Although precipitation
is the method most commonly used, it has
some  definite drawbacks. The main
disadvantage is the separation of chem-
ical precipitates. Furthermore, the pres-
ence of organic and inorganic complex
forming agents in wastewater always
renders precipitation and other processes,
such as ion exchange,  ineffective. The
major  objective of  this research project
was to investigate the treatability of
activated carbon adsoprtion process for
cadmium(ll)-containing wastewater.
  Major factors affecting the extent of
cadmium(ll) removal by activated carbon
adsorption process, such as pH, particle
size, mixing rate, carbon types, carbon-
to-cadmium ratio, ionic strength, and
temperature were thoroughly studied.
Both batch and column reactors were
used in this study.  Finally, an effort was
made  to estimate costs  of  activated
carbon adsorption  processes;  i.e., com-
pletely mixed tank with carbon separation
and column packed with activated carbon
beads.  A similar economical analysis was
made  for other treatment processes;
namely, alkaline-neutralization precipi-

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tation, insoluble sulfide precipitation and
ion exchange.

Conclusions
Among a total of 17 different types of
commercial activated carbons tested, the
acidic activated carbons such as Nuchar
SA and Nuchar SN appear to be most
effective adsorbents for cadmium(ll)
removal. Figure 1 exemplifies the effect
of carbon type on cadmium(ll) removal.
An acidic activated carbon, generally, has
a low pH of zero point of change, pH ZPc-
The extent of cadmium(ll) removal
depends significantly on the pH value of
the wastewater. For cadmium(ll)-tetra-
fluoroborate wastewater, the amount of
cadmiurn(ll) removal increases with pH,
reaches a plateau then remains constant.
Chemical precipitation apparently takes
place at high pH value, i.e., >9. For
cadmium(ll)-cyanide wastewater the
extent of cadmium(ll) removal peaks at pH
6. While cadmium(ll) removal by chemical
precipitation mechanism is inhibited by
the presence of cyanide, it is noted that
the presence of cyanide also depresses
cadmium(ll) removal due to formation of
cadmiurn(ll)-cyanide complexes which
are not as adsorbable as the uncomplexed
cadmium(ll) ions.
Figure 2 shows the general adsorption
characteristics of cadmium(ll) from syn-
thetic cadmium(ll)-tetrafluoroborate waste-
water as affected by pH and various
cadmium concentrations; Figure 3 dem-
onstrates the adsorption behavior of
cadmium(ll) from another synthetic
wastewater, cadmium(ll)-cyanide.
Other factors such as carbon-to-
cadmium ratio, ionic strength, and
temperature can also have an effect on
cadmium(ll) removal. Less than a critical
carbon-to-cadmium ratio, increasing
temperature, and ionic strength all
depress cadmium(ll) removal at various
significant levels.
The rate of cadmium(ll) removal by
activated carbon adsorption reaction is
very fast, reaching equilibrium adsorption
in a few minutes of reaction time. Varying
parameters such as pH, carbon dose,
ionic strength and temperature do not
affect the rate of cadmium(ll) removal.
Carbon type, i.e., powdered, granular or
"beaded", appears to be the most
influencing variable; powdered activated
carbons attain adsorption equilibrium
more rapidly than the other types of
activated carbons.
Since protons compete favorably with
cadmium(ll) ions, particularly at low pH
region, the cadmium(ll)-laden activated
carbon can be readily regenerated by
strong acids such as sulfuric, hydrochloric
o
I
1
1001- • Nuchar S-N
• Nuchar S-A (base washed)
^DarcoS-51
V Darco HOC
8Q\- O Filtrasorb 100
A Filtrasorb 200
O Filtrasorb 300
O Filtrasorb 400
L V Darc° HD 300°
® Nuchar WV-W
© Blank
40
20
100 ml suspension
10'* M Cd(BF
-------
250
Cd-BF4 Solution
NucharS-N(2g/l)
Room Temperature
0.1 MNaCIO4
2 hr. reaction
time
10
250
200
O 5 x 70~5 M
glxW4M
2 x W4 M
-© 3x 10'4M
150
100
50
Cd
Cd-BF4 Solution
. NucharS-A(2g/l)
Room Temperature
0.1 M NaCIO4
2 hr. reaction
time
10
Figure 2. The cadmiumfll) removal characteristics of Nuchar SN(top) and Nuchar SA fbottom}
from synthetic cadmium(ll)-tetrafluoroborate wastewater.
specifically, the optimal operational
conditions and control parameters.
Further work should also be focused on
the separation of powdered activated
carbon from water phase and the effect of
separation operation such as coagulation
on the cadmium(ll) retaining capacity of
the activated carbon.
-------
725
too
50
25
I
Original
Cd
Concen-
tration
1.0g/INucharS-N
0.1 M NaCIOt
Room Temperature
Reaction Time = 24 hours
5x10'sM
8x10~*M
1 xW4M
2 x W-* M
10'* M
40 60 80
Flow Rate gpm
100
Figure 4.
10
Estimated total costs of vari-
ous cadmiumfll) treatment
processes.
125
100 -
J.Og/INucharS-A
0.1 M /VaC/O4
Reaction Time = 24 hours
Room Temperature
Original Cd Concentration
10
Figure 3. The cadmiumfll) removal characteristics ofNuchar SN(top) andNuchar SA fbottom)
from synthetic cadmiumfll)-cyanide wastewater.
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C. P. Huang is with University of Delaware, Newark, DE 19711.
Mary Stinson is the EPA Project Officer (see below).
The complete report, entitled "Activated Carbon Process for the Treatment of
Cadmiums-Containing Wastewaters," (Order No. PB 83-247 155; Cost:
$13.00, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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