United States
Environmental Protection
Agency
Water Engineering
Research Laboratory
Cincinnati, OH 45268
Research and Development
EPA/600/S2-88/031 July 1988
Project Summary
Destruction of Cyanide in
Wastewaters: Review and
Evaluation
6. Chris Weathington
Twelve technologies were applied
to the destruction of cyanides in
wastewaters by reviewing available
literature and by conducting
discussions with industry and
government agencies. The 12
technologies reviewed were alkaline
chlorination, natural degradation,
oxidation of cyanide by sulfur dioxide
and air, ozonation alone and in
conjunction with ultraviolet (UV)
irradiation, precipitation by addition
of heavy metals, ion exchange,
activated carbon, reverse osmosis,
electrodialysis, the Kastone* process,
electrolytic hydrolysis, and titanium
oxide/UV irradiation.
The scope of the problem of
cyanide in wastewater or leachate
and the effectiveness of 12 different
technologies which have been
applied to the destruction of cyanide
were assessed by review of available
literature and discussions with in-
dustry and government authorities.
Brief summaries of the problem and
the current technologies are con-
tained in this report. In general, the
primary concern with cyanide
effluent is its propensity to form
complexes which are difficult to
remove and can later break down to
highly toxic forms.
During the course of this study, a
new approach to the destruction of
cyanide and cyanide complexes was
developed. This process involves the
reduction of cyanide, iron cyanides,
"Mention of trade names or commercial products
does not constitute endorsement or
recommendation for use
and thiocyanates by irradiation with
ultraviolet light in the presence of a
catalyst, titanium oxide. The UV/TIO2
process parameters were inves-
tigated by the Hittmann Ebasco
laboratory using a series of test
mixtures in distilled water, and, after
the optimal conditions were
established, four composite indus-
trial waste samples were tested to
examine the effects of different
matrices on the process. Recom-
mendations were made for further
evaluation of the chemistry and
development of the T1O2/UV process
technology for field testing.
Copper chloride was found to
increase the rate of cyanide con-
version in simple and complex
cyanide. Temperature was found to
have little significant effect, while pH
in the range 9.0 to 13.0 had no effect
on the conversion. The most effective
wavelength for irradiation was in the
range of 340-360 nm. Titanium oxide
at 200 ppm and copper chloride at
300 ppm were the optimal concen-
trations of catalysts. Precipitates of
metal hydroxides and chromates
were found to inhibit the reaction as
the result of what appears to be a
preferential absorption of the UV
radiation. Under optimal laboratory
conditions a solution containing ap-
proximately 60 ppm of iron cyanide is
converted to cyanate in 1 hr.
Thiocyanate does not effect the
conversion of iron cyanides at con-
centrations of 600 ppm and is itself
converted to sulfate and cyanate.
This Project Summary was
developed by ERA'S Water Engineering
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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
More than 350,000 tons of cyanide
compounds are produced each year in
the United States. Much of this
production is used in metal elec-
troplating, hardening of steel, paint
manufacturing, photographic processing,
and mining and ore dressing operations.
Ferricyanides and ferrocyanides are
found in waste streams from all these
processes. The most common waste
treatment processes are alkaline chlor-
ination, ozonation, and electrolytic
decomposition, but none of these
processes are effective in destroying
complex cyanides, particularly ferro-
cyanides and ferricyanides. Although
these compounds are considered
nontoxic in their natural states, sunlight
irradiation of solutions containing them
results in conversion to free cyanides.
This study assesses 12 different
technologies that have been applied to
the destruction of cyanides by reviewing
available literature and by conducting
discussions with industry and govern-
ment agencies. The twelve cyanide
destruction technologies reviewed were:
• Alkaline chlorination
• Natural degradation
• Oxidation of cyanide by sulfur dioxide
and air
• Ozonation alone and in conjunction
with ultraviolet (UV) irradiation
• Precipitation by addition of heavy
metals
• Ion exchange
• Activated carbon
• Reverse osmosis
• Electrodialysis
• Kastone process
• Electrolytic hydrolysis
• Titanium oxide/UV irradiation
Though a wide variety of techniques
have been discussed for detoxifying
cyanide, the only process currently found
effective and tested on a pilot scale for
the treatment of complex cyanides is
ozonation in conjunction with UV
irradiation. Cyanide concentrations
increased in laboratory studies using
(1) titanium oxide as a photo catalyst in a
solution containing simple cyanides and
(2) irradiation with UV light. This report
presents the results of a bench-scale
investigation for evaluating the
effectiveness of the titanium oxide/UV
process for the treatment of wastewaters
containing simple cyanide, complex iron
cyanides, and thiocyanate. A series of
test mixtures containing ferrocyanide,
ferricyanide, simple cyanide, and
thiocyanate were used to determine
optimum concentrations, and several
industrial waste samples were then
treated to examine process effectiveness
on complex matrices.
Test Procedure and Results
A photochemical reactor was used for
UV treatment of various surrogate
cyanide solutions and industrial effluents.
A 450-watt, high-pressure quartz
mercury vapor lamp was used to provide
UV energy. This high-pressure lamp
was chosen because of its high energy
output and wide spectral band. To
enhance the incident radiation of the
solution, the outside of the reactor vessel
was covered with aluminum foil. Eleven
different one-liter mixtures containing
ferrocyanide, ferricyanide, thiocyanate,
and potassium cyanide of analytical
reagent grade were used for ex-
perimental evaluation.
Dry compressed air was sparged
through the solution at 250 mL/min, and
the solution was stirred for 10 min. The
UV lamp was then turned on, and
titanium oxide (anatase form, 80 to 100
mesh, anhydrous), copper chloride or
other reagents were added, depending
on the test being performed. Air sparging
and stirring were continued throughout
the irradiation. After treatment, the
solution was decanted and filtered to
remove the residue of titanium oxide,
ferric oxide, and copper hydroxide. The
supernatant was analyzed for the pa-
rameters of interest. All procedures used
either EPA standard methods or methods
described in Standard Methods for the
Evaluation of Water and Wastewater.
The following key test results were
observed:
• Cyanide conversion performance
improved with air sparging.
• Concentrations of the catalysts
(titanium oxide and copper chloride)
were varied to determine the optimal
concentrations. Optimal concentra-
tions of 200 mg/L titanium oxide and
50 mg/L copper chloride at pH 13
destroyed 99.9 to 100 percent of the
total cyanide available in 100 mg/L of
the ferrocyanide and ferricyanide
mixtures respectively. However, all
subsequent tests used excess of
copper chloride.
• Tests conducted on a mixture of 100
mg/L ferrocyanide and 100 mg/L
ferricyanide (93.6 mg/L total cyanid^
with 300 mg/L copper chloride am
200 mg/L of titanium (as catalysts) fc
a 2-hr irradiation showed m
detectable cyanide (the detection lim
was 0.1 mg/L).
Varying the pH between 9 and 1
resulted in no significant differences i
the cyanide destruction at a titaniur
oxide concentration of 200 mg/L and
copper chloride concentration of 30
mg/L.
Off-gases from the experiments di
not contain hydrogen cyanide.
Temperature effects were tested <
19°, 26°, and 358C. In all cases,
mixture of 100 mg/L ferrocyanide an
100 mg/L ferricyanide was reduced I
no detectable cyanide after 2 h
Higher temperatures appeared I
improve cyanide conversion at higru
concentrations of ferricyanide an
ferrocyanide.
A test mixture at pH 13 containin
100.9 mg/L ferrocyanide (42.6 mg,
cyanide) was spiked with 1000.2 mg
potassium thiocyanate (597.8 mg,
thiocyanate) and irradiated for 2 h
Under optimal operating conditions
200 mg/L titanium oxide and 50 mg
copper chloride, 100 percent of ti
total cyanide and 47.3 percent
thiocyanate were removed.
Optimal conditions for the destructK
of thiocyanate were investigate
Additions of copper chloride had ;
inhibitory effect above 50 mg/L. Tl
optimal titanium oxide concentratii
and pH were 1000 mg/L and 1
respectively. Test results for optimi
UV wavelength for thiocyana
destruction were inconclusive as th
indicated that a wide band of ener
was needed for the thiocyana
conversion.
Wastewater samples from tl
following processes were tested
determine the applicability of t
titanium oxide/UV process:
Gold processing effluent
Cadmium electroplating bath waste
Composite electroplating waste
Black and white photoprocessi
reducer
Encouraging results were obtain
after high dilution and filtration, a
after pH adjustment in the case
gold processing effluent. Simila
encouraging results were obtair
with high dilution for cadmium plat
wastes and composite electroplat
waste. The black and white pi
toprocessing reducer tests showed
cyanide destruction after 1 hr
irradiation.
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Conclusions
A number of technologies are
currently being used for the removal of
cyanide compounds from effluents. The
most widely accepted and used are
alkaline chlorination, ozonation, ozonation
with irradiation, electrolytic hydrolysis,
hydrogen peroxide oxidation, and
precipitation processes. Of these pro-
cesses, alkaline chlorination is the most
frequently used for removal of the free
cyanides; it does not, however, remove
iron cyanides. Electrolytic hydrolysis and
ozonation with UV do remove iron-
complex cyanides and free cyanide, but
these treatment techniques have only
recently been used for waste treatment
at a few electroplating or photo-
processing plants. These processes
incur high capital and operational costs,
depending on the type of waste stream
treated and concentration of cyanide.
The titanium oxide/UV technique
seems to have distinct advantages.
Some of these include the lack of need
for temperature or pH control, the ability
to use nontoxic and inexpensive chem-
icals, and the destruction of thiocyanates,
and complex and free cyanides.
Bench-scale tests showed that
complex cyanides and simple-cya-
nide-contaming effluents can be des-
troyed by using UV irradiation with
titanium oxide and copper chloride as
catalysts. The process requires only two
reagents and no pH or temperature
control for the conversion of iron
cyanides and simple cyanides. Effluents
that contain significant concentrations of * *
thiocyanate can also be treated by this
process without affecting the conversion
of complex or simple cyanides.
Prior treatment of some waste
streams is necessary to remove
chromates and metal hydroxides No
other pretreatment appears to be
required for the four industrial waste
effluents tested. The rate of conversion
of complex cyanide to cyanate appears
to be limited by the amount of radiant
energy absorbed by the system.
The full report was submitted in
fulfillment of Contract Nos. 68-03-3037
and 68-03-3197 to Hittman Ebasco
Associates, Inc., and PEI Associates,
Inc., under the sponsorship of the U.S.
Environmental Protection Agency.
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B. Chris Weathington is with Hittman Ebasco Associates, Inc., Columbia, MD
21046.
S. Jackson Hubbard is the EPA Project Officer (see below).
The complete report, entitled "Destruction of Cyanide in Wastewaters: Review
and Evaluation," (Order No. PB 88-213 046/AS; Cost: $14.95, 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
Official Business
Penalty for Private Use $300
EPA/600/S2-88/031
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