United States
Environmental Protection
Agency
Water Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-86/051 July 1986
&EPA Project Summary
Sulfide Precipitation of Nickel
and Other Heavy Metals from
Single- and Multi-Metal
Systems
D. Bhattacharyya and L. F. Chen
Precipitation behavior of heavy metals
(Ni, Co, Cd, Cu, and Zn) was studied ex-
tensively in single- and multi-metal sys-
tems. Kinetic studies showed that NiS ox-
idation (as a function of pH, oxygen, and
reaction time) caused the dissolution of
NiS. CoS precipitation behavior was similar
to NiS. Effective NiS (or CoS) precipitation
would require high pH, short detention
times, or closed reactors (under N2 at-
mosphere). In the multi-metal systems
without nickel, CdS and ZnS precipitation
were the same as in their single-metal
systems; however, the precipitations were
affected in nickel-containing systems be-
cause of NiS oxidation. Results of CuS
precipitation from multi-metal systems
(with or without nickel) were substantial-
ly better than the single-metal case.
The use of ultrafiltration proved that the
particle size of CdS was influenced by pH.
A higher pH (greater than 8) favored the
formation of colloidal precipitates. The set-
tling characteristics of precipitate were im-
proved significantly by the addition of
coagulants (anionic polymer or calcium).
The effectiveness of the polymer was con-
trolled by polymer type, pH, and polymer
concentration. Freshly prepared CaS slur-
ries were quite successful in supplying
both sulfide source and coagulant.
This Project Summary was developed
by EPA's Water Engineering Research Lab-
oratory, 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 in-
formation at back).
Introduction
Scope of Study
Extensive studies of sulfide precipitation
were conducted to remove heavy metals
(such as Co, Cd, Cu, Ni, and Zn) from syn-
thetic wastewaters. Kinetic studies of
nickel and cobalt were conducted in dif-
ferent reactor systems (closed or open).
In addition, the interactions between dif-
ferent metals were also investigated for
various mixed-metal solution sytems. In
the presence of EDTA (ethylenediamine
tetraacetic acid), a strong chelating agent,
the effectiveness of selective precipitation
was evaluated by varying the EDTA con-
centration and the pH. Selective coagula-
tion studies were also conducted with col-
loidal metal sulfide precipitates by adding
synthetic polymers. Some metal precipita-
tions were conducted with calcium sulfide
slurries (freshly prepared or commercially
available) to provide a simultaneous sulfide
source and coagulant in one step.
Background
Sulfide precipitation is a well-known
means of removing heavy metals from
solutions because of the low solubilities
of metal sulfides. A number of investiga-
tions have proved that sulfide precipitation
processes remove heavy metals from in-
dustrial wastewaters more effectively than
the conventional hydroxide precipitation
processes. Industrial wastewaters usually
contain a mixture of heavy metals, but
most of the fundamental studies of sulfide
precipitation were conducted using single-
metal solutions. The interactions between
different metals may significantly affect
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the metal precipitation behavior. Most of
the heavy metals (such as Cu, Cd, Zn, and
Hg) can be effectively precipitated at short
contact times; but for metals like nickel
and cobalt, the precipitation kinetics is
more complex because of the post dissolu-
tion of nickel sulfide and cobalt sulfide.
At a pH <10.0, dissolution of nickel sul-
fide occurs with time in open reactor sys-
tems (in the presence of oxygen). No dis-
solution has been observed in closed reac-
tor systems. Several articles in the litera-
ture have reported the observation of this
phenomenon. The knowledge of the mech-
anisms of nickel sulfide reactions would
be beneficial in terms of reactor design for
multi-metal systems. Nickel ions have
been reported to possess strong catalytic
effects on sulfide oxidation. Mixed-metal
solution systems containing nickel are of
particular interest because the presence
of nickel may result in stronger and more
complicated metal interactions than would
the presence of other metals.
Cobalt resembles nickel in many aspects
and is a valuable strategic metal. Great in-
terest has been shown in its separation
and recovery. Experimental data are need-
ed to examine the effectiveness of cobalt
removal by sulfide precipitation and to
compare its precipitation behavior with
that of nickel.
In the presence of chelating agents, se-
lective separation of heavy metals may be
possible because of the difference in sta-
bility among the various metal-chelate
complexes.EDTA (ethylenediamine tetra-
acetic acid), a rather strong chelating
agent, could be used in mixed-metal solu-
tion systems to evaluate the effectiveness
of this technique on metal separation with
sulfide precipitation.
Metal sulfide precipitation tends to form
colloidal precipitates that cause some sep-
aration problems in either settling or filtra-
tion processes. To meet increasingly strin-
gent effluent standards for heavy metals,
several different approaches were consid-
ered to solve these problems. The addition
of coagulants such as polymer can aggre-
gate colloidal precipitates after sulfide
precipitation. This research suggests that
adding coagulants may be an effective
method to help separate the precipitates
from solution. The addition of calcium sul-
fide slurry instead of sodium sulfide solu-
tion, to supply both a sulfide source and
a coagulant [Ca(ll)] at the same time, may
also be an effective technique.
Procedures
In this study, most of the experiments
were conducted with the addition of
sodium sulfide solution to metal solutions.
In some experiments, calcium sulfide slur-
ry was used as a sulfide source. Two cat-
egories of metal solution systems, single-
metal and multi-metal, were used in either
an open or a closed reactor (Figure 1) at
a controlled pH value. Polymers used as
coagulants were added after sulfide pre-
cipitation in the sedimentation studies.
Results and Discussion
The extent of metal sulfide precipitation
is expected to be a function of pH, reac-
tion time, initial metal concentration, sul-
fide dosage, and the presence of chelating
agents and other interfering ions. With
some metals such as nickel and cobalt, the
precipitation is dependent on the reactor
system (closed or open).
Several different types of sulfide com-
pounds (e.g., Na2S, NaHS, H2S(g), and
CaS) can be used in metal precipitation.
In this study, most of the experiments
were conducted with sodium sulfide solu-
tion. Calcium sulfide was used in some
specific experiments. Generally, exper-
iments were conducted in an open reac-
tor system with a high mixing speed.
The separation results are reported in
terms of residual metal concentration,
pH Electrode
Gas
which refers to the amount of soluble
(filtered through 0.45-/^m filter), unreacted
metal in solution. In some cases where
colloidal particles are formed, soluble
metal concentrations can be compared
with filtration through an ultrafiltration
membrane. In all discussions and figures,
pH refers to the reaction pH and is con-
trolled by a pH control device.
Sulfide Precipitation of Solutions
Containing Multi-Metal Ions
Additional studies were carried out in
solutions containing more that one metal
ion to examine the interactions between
different metal ons and to observe the
possible differences in individual metal
sulfide precipitations. Previous studies
have indicated that the presence of nickel
ions would affect the sulfide precipitation
behavior of cadmium. Thus most of the
multi-metal solution systems being stud-
ied contained nickel.
Selective Metal Precipitation in the
Presence of EDTA
Water streams that contain heavy metals
may also contain some chelating agents
such as EDTA, citrate, oxalate, etc. Exten
Na2S Injection
Acid or Alkali Injection
••— Auto
Titrimeter
-*- N2 Outlet
(V)->- Sampling
Reactor
Magnetic Stirrer
Figure 1. Schematic diagram of reactor system.
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sive studies have evaluated the effects of
some chelating agents on metal sulfide
precipitation when the molar ratio of che-
late to metal is low. EDTA had the stron-
gest effect on reducing the extent of sul-
fide precipitation of metals such as cad-
mium, copper, nickel, and zinc. By varying
the apparent stability constants (KML) of
the metal-EDTA complex, it is possible to
achieve selective precipitation by adding a
proper amount of EDTA and varying the pH.
Flocculation and Sedimentation of
Metal Sulfide Precipitates
Metals are known to be effectively
removed by sulfide precipitation because
of the low solubility product (Ksp) of metal
sulfides. However, the formation of fine
particle precipitates in sulfide precipitation
processes causes some separation dif-
ficulties. Hence synthetic polymers have
been considered as flocculants to aggre-
gate the fine particles. Also, some chem-
icals such as Al(lll) form small, polymeric,
hydroxo-metal complexes in solutions.
These complexes act as coagulants by ab-
sorbing the colloidal particles and neut-
ralizing the charge on the particles.
Conclusions
The important conclusions of this study
are as follows:
1. Metal interactions had significant ef-
fects on the precipitation behavior of
cadmium, zinc, or nickel mainly be-
cause the nickel ion had catalytic ef-
fects on nickel sulfide oxidation.
2. Selective precipitation can be achiev-
ed effectively in the presence of the
proper amount of EDTA and the ad-
justment of pH.
3. High pH (pH >8) favored the forma-
tion of some colloidal cadmium sul-
fide precipitate.
4. The settling characteristcs of pre-
cipitates could be significantly im-
proved with the addition of anionic
polymer at a lower pH or the use of
freshly prepared calcium sulfide
slurry.
5. In the presence of EDTA, a high
degree of metal removal with sulfide
could be achieved with the addition
of calcium.
6. According to the kinetic studies of
nickel and cobalt sulfide reaction, the
proposed mechanism may be possi-
ble for cobalt and nickel reactions;
that is, in the presence of oxygen,
dissolution was a consecutive reac-
tion that occurred immediately after
sulfide precipitation. The dissolution
rate was a function of pH, oxygen,
and initial metal concentration. At a
lower pH range, the dissolution rate
increased as the pH decreased.
Recommendations
Recommended topics for further studies
based on this research are as follows:
1. More kinetic studies for cobalt and
nickel reaction with sulfide to obtain
the possible reaction rate expression.
2. Identification of soluble sulfur
species formed during NiS and CoS
dissolution.
3. Further studies of the catalytic ef-
fects of nickel and copper on metal
dissolution.
4. A design for continuous separation,
floccuiation, settling, and filtration on
the basis of the data obtained in this
study.
5. A search for some inhibitors to pre-
vent the dissolution of nickel sulfide
and cobalt sulfide in the presence of
oxygen.
The full report was submitted in fulfill-
ment of Contract No. CR-807760 by the
University of Kentucky under the sponsor-
ship of the U.S. Environmental Protection
Agency.
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D. Bhattacharyya and L. F. Chen are with University of Kentucky, Department of
Chemical Engineering, Lexington, KY 40506-0046,
Thomas J. Powers is the EPA Project Officer (see below).
The complete report, entitled "Sulfide Precipitation of Nickel and Other Heavy
Metals from Single- and Multi-Metal Systems," (Order No. PB 86-190972/AS;
Cost: $16.95, 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:
Water Engineering 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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