United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-84-023 Mar. 1984
Project Summary
Sulfide Precipitation of Heavy
Metals: Effect of Complexing
Agents
D. Bhattacharyya and Y. Ku
Sulfide precipitation behavior of
heavy metal ions in the presence of
chelating agents was evaluated in
terms of type of metal ion, chelating
agent type and concentration, pH,
sulfide dosage and reaction time.
Theoretical solubilities of metal sulf ides
were calculated for the various metal-
sulfide-chelating agent systems and
compared with the experimental values.
Experiments with Zn and Cd showed
that Ksp's of ZnS and CdS of fresh
precipitates were one or two log cycles
higher than the aged precipitate Ksp
values. With metal sulfides of higher
solubilities (such as, zinc and nickel),
high concentrations (greater than 10
mg/l) of strong chelating agent (such as
EDTA) substantially reduced the extent
of sulfide precipitation.
For sparingly soluble metal sulfides
(such as, copper and cadmium), the
effect of the chelating agent was
insignificant; residual concentration of
less 1mg/l was obtained even with the
strong chelating agent at a pH 4. The
presence of a high concentration of
citrate caused partial dissolution of
copper sulfide. NiS precipitation was
effective only for short reaction times
(less than 5 minutes) or in the absence
of oxygen. CdS precipitation was
effective even in the presence of a
strong chelating agent, but the removals
of cadmium were interfered with in the
Ni-containing-systems. Continuous
experiments were also conducted to
evaluate the effectiveness of the settling
operation and to establish selective
metal sulfide separation.
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
Process wastewaters containing heavy
metals result from metal finishing and
production operations and certain inor-
ganic chemical manufacturing operations.
Depending on their origin, these process
wastewaters may also contain chelating
agents such as EDTA and citrate. The
effective removal of heavy metals or their
selective recovery, even at high pH, is
often not feasible with conventional
hydroxide precipitation, the method most
often used for treating industrial waste-
waters.
In hydroxide precipitation of heavy
metals, the metals are removed by
adjusting the pH of the wastewater with
lime or caustic soda until the metals
exhibit minimum solubilities. The metal
hydroxides can be removed by flocculation
and sedimentation. Some common limit-
ations of the hydroxide process are:
- Hydroxide precipitates tend to reso-
lubilize if the solution pH is increased
or decreased from their minimum
solubility values; maximum removal
efficiency may not be achieved
unless the pH is controlled within a
narrow range.
- The theoretical minimum solubility
for different metal hydroxides occurs
at different pH values; hence, the
removal of metals by hydroxide
precipitation of mixed metal wastes
may not be effective.
Sulfide precipitation is an effective
alternative to hydroxide precipitation for
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removing various heavy metal ions from
industrial wastewaters. An earlier bench-
scale study evaluated the effectiveness of
sulfide precipitation using Na2S. That
investigation showed that high metal
removal efficiencies can be achieved.
Although several literature citations
mention the effect of chelating agents on
sulfide precipitation, no quantitative
research of this effect has been reported
under various operating conditions for
metal sulfide precipitations. Since metal
chelates are highly dissociated at low pH
values, effective metal removal by sulfide
precipitation is possible even at pH 3-4.
Selective precipitation of heavy metal
sulfides can be obtained by the addition of
proper chelating agents.
The overall objective of the present
investigation was to establish the extent
of precipitation of selected heavy metals
with sodium sulfide in the presence of
several commonly used chelating agents.
Extensive bench-scale experiments were
conducted with several synthetic systems.
Dissociation of Complexing
Agents
In the absence of metal ions, complex-
ing agents may form complexes with a
proton, H+. The stepwise acid dissociation
constants for polyprotinated ligands can
be expressed as follows:
HnL H + Hn-lL (1)
Hn-!L H + Hn-2L (2)
HL H + L (3)
in which "L" stands for ligand.
The corresponding stepwise dissociation
constants for several complexing agents
are shown Table 1. Using these stepwise
dissociation constants, a set of simulta-
neous equations can be solved to obtain
the distribution of various species in the
solution under different experimental
conditions. A typical species distribution
of EDTA is computed and shown in Figure
1 as a function of pH.
The equilibrium concentration of all
species present in a metal-sulfide-
chelating agent system can be calculated
by solving a series of simultaneous
equations comprised of equations for
materials balance, solubility product of
metal sulfide, expressions of species, pH,
total metal concentration, total sulfide
concentration and total complexing agent
concentration. A computer program was
designed to solve these equations and
calculate the equilibrium concentration
of all species. The equations and the
Table 1. Acid Dissociation Constants of Chelating Agents
Dissociation Constant (at 25C)
Complexing
Agents
Citric acid
Cyanide
DCTA
EDTA
Gluconic acid
HEDTA
NTA
Oxalate
Tartrate
pK,
3.00
9.20
2.51
2.07
2.30
2.40
1.89
1.40
2.90
pKz
4.40
---
3.60
2.75
4.28
5.40
2.49
3.80
4.10
pK3
6.10
...
6.20
6.24
9.67
10.00
9.73
...
—
pK*
16.00
...
11.78
10.34
...
-_.
...
.__
—
100
Figure 1. Distribution in various species of EDTA in aqueous solution as a function of pH.
computer program are presented in the
final report.
Metal-Sulfide-Chelating Agent
Equilibrium Calculations
Using the calculation procedure re-
ferred to above, the theoretical values
of residual metal concentrations, CM,
were calculated for various zinc-sulfide-
chelating agent systems at a sulfide
dosage of 1.0x and 8.9x10"3M chelating
agent concentration and plotted as a
function of pH. Zinc residual concentra-
tions are always higher in the presence of
chelating agents, indicating that the
formation of zinc chelates reduces the
extent of ZnS precipitation. For example,
at pH 8, zinc residual concentration is
about 7 mg/l in the presence of the
strong chelating agent EDTA, whereas
residual concentration is about 10 :> mg/1
in the absence of any chelating agents.
The final report shows the residual zirc
concentrations in the presence of other
chelating agents of various degrees of
metal chelate stability. Weak chelating
agents such as tartrate and gluconic acid
had little influence on the residual con-
centration. The interference of these
chelating agents, in terms of reduction
of sulfide precipitation, increases in order
of: tartrate < gluconic acid < citrate <
NTA (nitroloacetic acid) < HEDTA (hydro-
xyethyl-ethyine-diaimetriacetic acid) <
EDTA.
The pH effect on the residual zinc
concentration with Ksp values is shown in
the final report. At low pH, removal of zinc
is not complete because the protonation
of sulfide tends to form HzS. and part of
the HzS may be lost into the air, thus
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decreasing the extent of ZnS precipitation.
In alkaline conditions, the metal forms
stable chelates because of the deprotona-
tion of the chelating-agent. For pH 10,
formation of metal hydroxide chelates
becomes another competing reaction in
the presence of some weak chelating
agents and increases residual concentra-
tion. But with strong chelating agents, no
significant residual change occurs even
at high pH because of the formation of
highly stable chelates.
The effect of soluble sulfide concentra-
tion (Si) on sulfide precipitation is also
shown in the final report. At a higher ST,
the residual concentration is always
lowered. The ST value increases as the
sulfide dosage exceeds 1.0x. A higher
dosage of sulfide always lowers the
residual metal concentration due to com-
mon ion effect.
Equilibrium calculations for Cu, Cd and
Ni sulfide precipitation were made in the
presence of various chelating agents.
Because of the high reactivity of Cu with
sulfide (Ksp = 1.2x10~35), the formation
of CuS is more favorable over the formation
of copper chelates. The effect of the
formation of metal-EDTA chelates on
various metal sulfide precipitation reac-
tions was also plotted. Metal sulfides
with small solubility products (Ksp), ouch
as Cu and Cd, always have low residual
concentrations and are less influenced by
the presence of chelating agents.
Comparison with Hydroxide
Precipitation
In the final report residual metal
concentrations after hydroxide precipita-
tion with EDTA are presented for several
heavy metals. The results are considerably
higher than those for sulfide precipitation,
especially at low pH values. For example,
at pH 6, zinc does not form a hydroxide
precipitate but the residual is about 16
mg/l after sulfide precipitation. The
difference is even greater for zinc sulfide.
In this study, EDTA, citrate, tartrate and
gluconic acid were used as chelating
agents. Among these chelating agents,
EDTA has the highest complex formation
equilibrium constants, which indicate
that EDTA may form more stable chelates
with the zinc ion than other chelating
agents used in this study. For weak
chelating agents such as citrate and
tartrate, the sulfide precipitation is
quite complete, indicating that the for-
mation of zinc chelates with citrate or
tartrate is much less favorable of zinc
sulfide precipitation.
According to Le Chatelier's principle of
equilibrium, the residual metal concen-
tration increases with an increase in the
amount of chelating agent present in
initial feed solution. This was verified for
the zinc—EDTA system. Experiments
were conducted at pH 8.0 and in the
presence of various amounts of EDTA for
the zinc-EDTA system. The residual zinc
concentration increased with an increas-
ing amount of EDTA present in the
solution. A high concentration of EDTA
caused severe interference in the precipi-
tation of the zinc sulfide. For example, 72
mg/l of zinc residual did not precipitate
with sulfide in the presence of 500 mg/l
of EDTA. Assumptions can be made that
about 70% of the EDTA formed 1:1
chelates with the zinc at EDTA concentra-
tions less than 500 mg/l and EDTA/Zn
ratios less than 1.0. In other words, about
12.5 mg/l of soluble zinc were present
with 100 mg/l of EDTA. This assumption
agreed with the fact that almost no
precipitation occurred when the initial
zinc concentration was 10 mg/l and the
EDTA concentration was 100 mg/l (3.4x
10~4M). Some experiments were carried
out for the zinc-citrate system. The
presence of citrate had a negligible
influence on the precipitation of zinc
sulfide. This is attributed to the weak
chelation ability of citrate. The sulfide ion
competed more successfully than citrate
for the coordinative position on the zinc
ion.
Effect of pH Value
The species distribution of different
forms of metal chelates present in the
solution varies at different pH values. In
order to evaluate the effect of pH on the
removal of metal by sulfide precipitation
in the presence of chelating agents, a
series of experi ments was ca rried out at a
1.05x sulfide dosage and various pH
values in the presence of 10 or 100 mg/l
of EDTA. The residual zinc concentrations
at various pH values appear in the final
report. Removal of zinc by sulfide
precipitation was relatively incomplete at
pH 3 because of the competition of the
sulfide ion for H+. On the other hand,
protonated EDTA species, H
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batch experiments. Sulfide dosage fluc-
tuation may have been caused by some-
what higher residuals.
Multi-Metal System
A mixture of Cu2+, Cd2+, Zn2+, and Ni2+
ions was used in this study with and
without EDTA. Results were plotted
against the residual metal concentrations
for batch experiments at the same
operational conditions. Relatively low
residual metal concentrations were
obtained for copper and zinc. However,
removals were not complete for nickel
and cadmium ions because of the
interference between these metal ions.
Without the Ni ion, a low residual
cadmium concentration was achieved for
a mixture of Cu2+, Cd2+, and Zn2+ ions by
sulfide treatment.
Another important observation in this
research pertained to the sedimentation
of metal sulfide precipitates in the sett-
ling column. Sedimentation is one of the
most used unit operations in wastewater
treatment. In this study, a 90-cm long
glass column with a 7-cm diameter was
used as a sedimentation column. At a
feed solution flowrate of 100 cmVmin
the detention time in the sedimentation
column was 35 minutes. The surface
loading rate was 2.6 cmVfcm2) (min). In
the final report, metal sulfide precipitate
removals are shown as a function of the
initial concentration of EDTA. The ZnS
precipitate was removed quited effect-
ively. The experimental data show that
about 20-30% of Cu, 60-80% of Cd, and
60-100% of Ni precipitate were removed
by sedimentation. The presence of EDTA
improved the sedimentation of copper
and nickel sulfides.
D. Bhattacharyya and Y. Ku are with the University of Kentucky, Lexington, KY
40506-0046.
Alfred B. Craig, Jr., is the EPA Project Officer (see below).
The complete report, entitled "Sulfide Precipitation of Heavy Metals: Effect of
Complex!ng Agents," (Order No. PB 84-141 514; Cost: $14.50, 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
•fr U.S. GOVERNMENT PRINTING OFFICE, 1984 — 759-015/7636
United States
Environmental Protection
Agency
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Cincinnati OH 45268
Official Business
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