UNITED STATES ENVIRONMENTAL PROTECTION AGENCY
WSG72
Date Signed: March 7, 1994
MEMORANDUM
SUBJECT: Clarification on Chlorination BAT for Cyanide
FROM: William R. Diamond, Acting Director
Drinking Water Standards Division, OGWDW
TO: Drinking Water Branch Chiefs, Region I-X
I am writing to inform you that one of the best available technologies (BATs) for cyanide is not defined specifically enough
in the rule language for the Phase V regulation. Chlorine is listed as one of the three BATs for cyanide in § 141.62(c) and § 142.62(b).
Chlorinating water containing cyanide at or above the MCL without any regard to pH can lead to a buildup of an equally toxic
compound called cyanogen chloride. The proper terminology for this BAT is alkaline chlorination as was discussed in the preamble to
both the proposed and final Phase V regulations. The rule language in §141.62(c) and §142.62(b) will be amended to state that the
BAT is alkaline chlorination (pH >= 8.5) in the Phase VIb regulation.
In the meantime, please use the attached Public Water System (PWS) Warning developed by Region VIII when cyanide
levels exceed the MCL and chlorination is selected as the BAT. The PWS Warning contains important information regarding: 1)
the potential public health threat that can occur if chlorine is applied at too low of a pH; 2) reasons why alkaline chlorination is the
BAT instead of just chlorination; 3) chemistry and minimum pH requirements of alkaline chlorination; and 4) guidelines on the
application of alkaline chlorination.
Please forward this information to your State Drinking Water Program Directors as soon as possible so that they can share
this information with public water systems when the MCL for cyanide is exceeded. EPA will be submitting this PWS Warning to the
Journal of the American Water Works Association for their consideration for publication and more widespread distribution. If you
have any questions about the PWS Warning, please call Jeff Kempic at (202) 260-9567, Bob Clement at (303) 293-1259 or Dr.
Robert Benson at (303) 293-1694.
Attachments
cc: Robert J. Blanco, 4604
Alan A. Stevens, TSD
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PUBLIC WATER SYSTEM WARNING
APPLIES ONLY TO PWSs THAT USE CHLORINE TO OXIDIZE CYANIDE IN DRINKING WATER THAT
EXCEEDS THE 0. 2 MG/L MAXIMUM CONTAMINANT LEVEL (MCL).
The Environmental Protection Agency (EPA) is publishing this warning to alert public water systems
(PWSs) of the potential health hazard that results when chlorine (C12) is used to oxidize cyanide (CN) in
drinking water that exceeds the 0.2 mg/1 MCL. When chlorine is used to oxidize water contaminated with
cyanide, a disinfection by-product (DBF) called cyanogen chloride (CNC1) is formed. EPA has no direct data on
the toxicity of cyanogen chloride in drinking water. Due to this lack of toxicological data, EPA currently has no
plans to regulate cyanogen chloride. However, chronic exposure to cyanogen chloride may be as harmful as
chronic exposure to cyanide at and above the MCL. Additionally, cyanogen chloride is volatile and is
extremely irritating to the respiratory tract at and above the MCL.
Chlorine quickly and effectively oxidizes cyanide and cyanogen chloride in a process called
alkaline chlorination. Alkaline chlorination oxidizes cyanide to harmless bicarbonate (HCO3) and nitrogen gas
(N2) by using excess chlorine at pH values greater than at least 8.5. The higher the pH the faster the reaction
proceeds. Alkaline chlorination is prescribed over simple chlorination because of the need to destroy cyanide
and cyanogen chloride as quickly as possible. EPA will change the regulations specifying the best available
technology (BAT) for removing cyanide from chlorination to alkaline chlorination.
Because cyanogen chloride is not regulated and not routinely analyzed for, it is important that cyanogen
chloride be oxidized at the water treatment plant prior to the first customer. Research on this subject has not
been performed at levels representative of the MCL for drinking water. The available research has been
conducted at low levels (less than 0. 1 mg/1) and at high levels (10-50 mg/1) of cyanogen chloride. Research at
both the high and low levels shows the same important observation: that the higher the pH, the faster the
cyanogen chloride is oxidized to bicarbonate and nitrogen gas. Alkaline chlorination's ability to oxidize cyanide
has made it a popular method to destroy cyanide and avoid the buildup of cyanogen chloride in the metal
plating industry. This same technology can be used in the drinking water treatment industry to destroy cyanide
when levels exceed the MCL. The process of alkaline chlorination proceeds through two oxidative steps.
The first-stage of alkaline chlorination converts cyanide to cyanate (CNO). The following chemical
equation can be used to calculate chemical dosages for chlorine and hydroxides in the first-stage:
EPA PWS cyanide warning
EPA contacts: Robert W. Benson (303-293-1694), Robert T. Clement (303-293-1259), Jeffrey B. Kempic (202-260-9567)
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Cl, + 2OH+ CN- CNO + 2C1+ HO
However, the reaction is not that simple. The oxidation of cyanide to cyanate progresses through
the intermediate compound cyanogen chloride according to the following reactions:
CN+ OCI+ H2O -» CNC1 + 2OH
CNC1 + 2OH -» CNO + Cl + H2O
The oxidation of cyanide to cyanogen chloride occurs instantaneously at all pH values. The oxidation of
cyanogen chloride to cyanate occurs rapidly at pH values of 8.5 and above. The hypochlorite ion (OCL) is the
active chlorine species in the first stage. Research shows that within one hour the cyanogen chloride
concentration has been significantly reduced "'. Research conducted at the higher levels of cyanogen chloride
show that at pH values less than 7.1 oxidation proceeds very slowly'2 (Figure 1 & 2). Research conducted at the
lower levels of cyanogen chloride show that oxidation occurs at this very slow rate at pH 5.8 (Figure 3).
Therefore, to avoid a buildup of cyanogen chloride, the first-stage of alkaline chlorination should be conducted
at pH 8.5 or above. This is accomplished by the addition of hydroxide ions to increase the pH.
Lower temperature waters appear to slow the reaction down. Temperatures less than 20 degrees
Celsius increase the reaction time4 for the hydrolysis of cyanogen chloride (Figure 4). Therefore, even higher
pH values or longer contact times may be necessary to compensate for slower reaction times in low
temperature waters.
The second-stage of alkaline chlorination converts cyanate to bicarbonate and nitrogen gas and proceeds
rapidly in the presence of excess chlorine. The following chemical equation can be used to
N. S. Chamberlin and H. B. Snyder, Jr., Technology of Treating Plating Wastes, 1955, p. 277
2 G. E. Eden et al., Destruction of Cyanide in Waste Water by Chlorination, J. Soc. Chem. Ind., 69: 244-
249, (August 1950).
Yuefeng Xie and David A. Reckhow, Stability of Cyanogen Chloride in the Presence of Sulfite and
Chlorine, AWWA Proceedings of 1992 WQTC, Toronto, November 15-19, 1993.
4
G. E. Eden and A. B. Wheatland, Effect of Temperature and the Presence of Hypochlorite on the Rate
of Hydrolysis of Cyanogen Chloride in Alkaline Solution, J. Soc. Chem. Ind., 69: 166-169, (June 1950).
EPA PWS cyanide warning
EPA contacts: Robert W. Benson (303-293-1694), Robert T. Clement (303-293-1259), Jeffrey B. Kempic (202-260-
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calculate chemical dosages for chlorine and hydroxides in the second-stage:
3C12 + 2CNO+ 6OH -» 2HCO3+ N2 + 6C1+ 2H2O
Hypochlorous acid (HOCI) is the active chlorine species that destroys cyanate. Even a small
percentage of hypochlorous acid will oxidize cyanate. For example, at pH 8.5 (about 10% hypochlorous acid),
cyanate is almost completely oxidized in 10 minutes (Figure 5). Oxidation occurs even more quickly at lower
pH values.
In conclusion, the speed at which cyanogen chloride is oxidized is controlled by the concentration of the
hypochlorite ion. The hypochlorite ion concentration increases as the pH increases and the entire reaction is
dependent upon excess chlorine being available. The best way to know if the cyanogen chloride is oxidized
below the MCL for cyanide is laboratory testing. However, EPA does not require this testing for the following
reasons: 1) Labs available to perform this testing are few; 2) There is not a laboratory certification process for
cyanogen chloride; 3) The holding times are very short making it difficult to assess the accuracy of the levels of
cyanogen chloride in drinking water. Therefore, in lieu of testing, the following guidelines are provided to
ensure that cyanide is completely oxidized to bicarbonate and nitrogen gas without the buildup of cyanogen
chloride and that the water will be safe to drink prior to the first customer:
1. Maintain a pH of 8.5 or greater.
2. Maintain high free chlorine residual, but not greater than 4.0 mg/1.
3. Choose the percent removal needed and ensure that the contaminated water remains in contact with
excess free chlorine, prior to the first customer, according to the following Table [adapted from the kinetics
reactions developed by research conducted by Dr. David A. Reckhow and Yuefeng Xie, University of
Massachusetts, Amherst].
Percent Removal
90
95
99
99.9
Ct (min-mg/1)
68
89
136
204
4. Compensate for the temperature effects with longer contact time or higher pH values.
EPA expects that the number of PWSs with cyanide greater than the MCL in their raw water to be
low; however, those that do may have significant levels. For example, contamination from one
EPA PWS cyanide warning
EPA contacts: Robert W. Benson (303-293-1694), Robert T. Clement (303-293-1259), Jeffrey B. Kempic (202-260-9567)
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cyanide heap leaching operation has reported cyanide levels as high as 2.7 mg/1 in the receiving stream. PWSs
with very high levels of cyanide in their raw water should consult with the state and experts in this field to
ensure that above guidelines are adequate.
EPA PWS cyanide warning
EPA contacts: Robert W. Benson (303-293-1694), Robert T. Clement (303-293-1259), Jeffrey B. Kempic (202-260-9567)
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o
FIGURE 1
J£ 20 Z5 • 30 35 4Q 4-5 SO 55 6
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LJ
a:.
LJ
£.-:
cc
O
_J
X
a
Q
0
FIGURE 2
1O 15 20 25 30
RETENTION IN MINUTES
Effect of pH on hydrolysis of c^iinogen chloride to
in presence of n ehlsramine residual.1
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100
a 3o
0
o
o
O
10
0
K pH 5.B
O -pH 7,0'
V pH 8.2 '
V pH 9,0
« pH 10.0
O pH 10,7:
20 , 40 60 86
Time (minute)
100
FIGTJRE 3 Degradation of CNCi in the Presence of Cuioriite
(1
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•o-os
FIGURE 4
Effect of temperature on the rate of hydrolysis of
cyanogen chloride at a pH vaJac of approximately
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CL
BO rnSnS
tit hM* 9,9
40 mini, dl pH*9G5
mini, of pH * 9,2
— IO num. or pH "S,4
3O 4O -BO
RETENTION (ti f/INUTES
FIGURE 5 Time required for cyanate destruction,1
10
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11
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