White Paper
Storage-stable Aqueous Solutions of Chlorine Dioxide
Tom McWhorter
November 14, 2014
Abstract
Chlorine dioxide is a powerful, selective biocide. Aqueous solutions of chlorine
dioxide have many applications but typically have had limited shelf life and must
therefore be produced on-site, as needed, from stable chemical reagents. CDG
has developed a ready-to-use, concentrated (3,000 ppm) chlorine dioxide
solution that is storage stable for extended periods (at least nine months), at
room temperature.
Storage stable chlorine dioxide solutions afford advantage over traditional
approaches to making chlorine dioxide solutions on site:
•
Storage stable solutions are of precisely-known concentration, in contrast to
on-site solutions, which are of variable concentration. Knowing the strength of
the solution enables precise dosing.
•
Storage stable solutions are pure; they contain negligible chlorine, chlorite
ion, chlorate ion, chloride ion, chlorous acid or perchlorate ion. For example,
CDG Solution 3000 contains <10 ppm chlorine in 3000 ppm chlorine dioxide.
This means that when the solution is used at a typical 0.6 ppm chlorine
dioxide, the chlorine concentration of the product is < 2 parts per billion
(PPB). This is in contrast to on-site-made solutions, which are of variable
purity, and may contain any or all of the aforementioned contaminants,
sometimes in high concentrations. Solution purity enables enhanced control
of disinfection by-products, and helps to minimize corrosion.
•
Storage stable solutions eliminate on-site mixing of chemicals, evolution of
chlorine dioxide fumes into the workplace and the start-up, maintenance, and
calibration associated with on-site generators.
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Storage stable chlorine dioxide solutions offer advantage for virtually all existing
small-scale applications of chlorine dioxide, and facilitate chlorine dioxide use for
new applications that were previously impractical.
Introduction
Chlorine dioxide (ClO2) is a strong but highly selective oxidizer. It has been used
in aqueous solution for many decades for the disinfection of drinking water and
for other water treatment applications. One of the chief benefits of chlorine
dioxide is that it does not react with organic materials to form chlorinated
hydrocarbons, such as trihalomethanes, the consumption of which has been
linked to cancer and other health problems. Chlorine dioxide reacts with various
contaminants to produce chlorite (ClO2-) and chloride (Cl-) ion. In the
environment, chlorite ion reacts more slowly with various reducing agents to
produce more of the generally innocuous chloride ion. Thus –although some care
must be taken to limit environmental release of chlorite ions, the long term impact
of chlorine dioxide is to produce the mostly harmless chloride ions.
Chlorine dioxide solutions are used as hard-surface sterilants, and as a
disinfecting wash for poultry and many types of fruits and vegetables. Since
chlorine dioxide is a dissolved gas in water, it penetrates organic materials such
as polysaccharide bio-film and semi-permeable reverse osmosis membranes to
kill microorganisms within the organic film and on the untreated side of the film.
Several firms offer products called “Stabilized Chlorine Dioxide”, “Chlorine
Dioxide Solution” or similar names. These materials are not chlorine dioxide;
rather, they are dissolved (sometimes buffered) sodium chlorite (NaClO2) which,
when mixed with an acid “activator”, produce chlorine dioxide solutions.
Opportunities abound for errors in mixing and –even when reagents are mixed
properly– the resulting solution may contain high levels of salt, acid, chlorine,
chlorous acid and other impurities; once mixed, the resultant chlorine dioxide
solutions have a short shelf life.
Applications
In general, CDG Solution 3000 can be used for any application served by on-sitegenerated chlorine dioxide and more. CDG Solution 3000 is generally preferred
where its advantages in terms of purity, precise control, safety, portability, and
convenience are most highly valued.
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The following list of applications is meant to be suggestive only and is not
comprehensive. For more information on these or other applications, contact
CDG at 484-821-0780 or visit the website at www.cdgenvironmental.com.
Municipal Drinking Water – CDG Solution 3000TM can be used to supply any of
the well-known applications for chlorine dioxide in drinking water. These include:
• Disinfection without producing halogenated hydrocarbons like THMs or
HAAs.
• Removal of metals like iron or manganese.
• Oxidation of taste and odor compounds like hydrogen sulfide and organic
compounds.
• Oxidation of phenols without production of chlorphenols.
• Control of bio-fouling in filters and other devices.
• Control of Giardia and Cryptosporidia.
Reverse Osmosis and ultrafiltration- control of bio-fouling- CDG Solution 3000TM
can be fed continuously at chlorine dioxide doses <1ppm upstream of microfilters in RO/UF plants. In this use, chlorine dioxide may extend the useful life of
the microfilters and will often totally eliminate the need to shut down membrane
modules for off-site cleaning. Chlorine dioxide in this mode will also kill
microorganisms inside the membranes and in the permeate water. Chlorine-free
chlorine dioxide at this dose has does not damage membranes. Chlorine dioxide
can also be used at higher doses for off-site cleaning of membranes.
Water Filter control of bio-fouling- CDG Solution 3000TM can be used to control or
eliminate bio-fouling of water filters, often greatly extending the service life of
filter cartridges, increasing flow and reducing backpressure.
Water well Remediation - CDG Solution 3000TM can be used by water well
service companies to control and eliminate bacteria that block water flow and
produce color and odor in the wells and aquifers.
Cooling towers- control of algae, legionella and bio-fouling – CDG Solution
3000™ liquid concentrate is being used as an effective biocide in cooling towers
to eliminate bio-fouling that can reduce cooling efficiency.
One of the most efficient biocides for microorganism control in cooling towers is
chlorine dioxide. This is especially true since chromate treatment has been
phased out and towers are being operated in the alkaline pH range. In the
alkaline range chlorine is not an effective biocide because of the poor
dissociation to HOCl.
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Disinfection - Chlorine dioxide has been shown to be an effective agent in killing
a number of harmful organisms; CDG SOLUTION 3000™ liquid concentrate can
be used for food and non-food contact hard surface disinfection in food
processing plants, research laboratories, and other settings.
Agriculture - CDG SOLUTION 3000™ liquid concentrate is being used to purify
animal drinking water and greenhouse or hydroponic water. It can be used in a
number of other agricultural applications to remove bio contamination in farm
water. It is also useful in the treatment of agricultural storage facilities, animal
transport vehicles & confinement facilities.
Heat exchangers – control of bio-fouling- CDG Solution 3000TM is effective at
removing and controlling bio-film that insulates heat exchange surfaces and
reduces efficiency. Ultra-pure chlorine dioxide from CDG Solution 3000TM
coupled with pH control does not cause corrosion on stainless steel heat
exchange surfaces.
Hospitals and Nursing Homes – control of Legionella in water systemsHospitals, nursing homes, and other institutions with recirculating hot water
systems often have major build-up of bio-film on the insides of water pipes.
Chlorine and other disinfectants are not able to penetrate the polysaccharide biofilm and kill bacteria that grow in the film. Biofilm accumulates until pieces slough
off and are carried with water to the points of use. There, biofilm and the bacteria
that it contains are aerosolized in showers and other bathroom fixtures where
legionella and other pathogens can infect the population, which often contains a
high proporation of immunocompromised individuals. CDG Solution 3000TM can
be employed either continuously or during system shut-down to remove biofilm
from water pipes and kill pathogens.
Food Processing- Chlorine dioxide can be used for process plant disinfection and
the removal of bacteria from water used in food processing. CDG SOLUTION
3000™ liquid concentrate has proven effective in the disinfection of poultry wash
water and the disinfection and treatment of poultry facilities. CDG Solution
3000TM is also effective for pathogen control in washing vegetables.
Mold Remediation – CDG Solution 3000TM can be used as a wash, an aerosol,,
or as a source of chlorine dioxide gas to kill mold and mold spores in buildings.
Odor Control – Chlorine dioxide from CDG Solution 3000 TM can be used to
eliminate odors from waste treatment or environmental odors from sources such
as tobacco smoke or building fires.
Paper manufacturing- CDG Solution 3000TM is effective at controlling bacteria,
mold and slime growth in white water tanks and other problem areas in papermaking.
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Production of chlorine dioxide solutions
Chlorine dioxide is produced in a variety of ways. Most of the production
processes suitable for small-scale use (less than a few thousand pounds per
day) are based on reaction of sodium chlorite with chlorine and/or various acids
in aqueous solution. Many of these processes are based on the reaction:
2NaClO2 + Cl2 => 2ClO2 + 2NaCl
(Reaction 1)
CDGE’s Gas:Solid process uses this chemistry by passing a dilute chlorine gas
stream over a bed of solid sodium chlorite.
Other processes are based on a reaction of an acid with dissolved sodium
chlorite to produce chlorous acid, which dissociates over time to form chlorine
dioxide.
NaClO2 + HCl => HClO2 + NaCl
(Reaction 2)
Followed by:
5HClO2 => 4ClO2+ HCl + 2 H2O
(Reaction 3)
Many different acids are used in variations of this reaction,
The rate of conversion of HClO2 to ClO2 varies depending on pH and other
factors. Nearly complete conversion often requires hours.
In yet another common process, chlorine dioxide is made by reacting sodium
chlorite solution with sodium hypochlorite (bleach) and acid”
2NaClO2 + NaOCl + HCl => 2ClO2 + 2NaCl + NaOH
(Reaction 4)
Since NaOCl is notoriously unstable, frequent adjustment is required to maintain
optimum production in this reaction,
All of these solution-production technologiesare characterized by production of
chlorine dioxide solutions which also contain all of the other products and byproducts of the reaction plus any un-reacted feedstock reagents. Typical
contaminants in these products include elemental chlorine, chlorous acid, sodium
chlorite, sodium chlorate, hydrochloric acid and sodium chloride; other organic
by-products also may be present, if an organic acid (e.g., lactic acid or citric acid)
is used.
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Use of this powerful oxidizer and disinfectant has been limited because chlorine
dioxide typically has exhibited limited stability in aqueous solution. For this
reason, most chlorine dioxide applications currently require generation of the
chemical near the point of use.
The usable concentration of chlorine dioxide solution is limited for safety reasons.
Chlorine dioxide gas can decompose spontaneously and exothermically at high
concentrations. OSHA lists the safe limit as 10% (76 mm partial pressure) in air
at atmospheric pressure. Other expert sources identify the limit as 16% (120 mm
partial pressure) or even higher. Even at partial pressure of 150 mm and higher,
a spontaneous decomposition is quite mild and characterized as a “puff”. As the
gas phase concentration increases, the decomposition becomes more forceful.
At 30% concentration in air (partial pressures of about 228 mm) or higher,
explosions can be highly energetic. The presence of water vapor elevates the
concentration at which decomposition occurs.
Even when the concentration of a solution is stable—as with CDG Storage
Stable Chlorine Dioxide Solution-- shipping must be with care. Figure 1 shows
the vapor pressure of chlorine dioxide gas above aqueous solutions of the gas as
a function of temperature and concentration.
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As is the case with aqueous solutions of most gases, the solubility of chlorine
dioxide decreases as temperature increases – i.e. for a given concentration of
dissolved gas, the partial pressure of the gas above the solution at equilibrium is
a positive function of temperature.
Aqueous solutions with concentrations of chlorine dioxide below 3,000 ppm are
safe for shipment, so long as they are protected from very-high temperatures.
The gas in the headspace above these liquids might, at most, reach 115 mm Hg
if the temperature of the liquid reached 50oC (122oF). Containers of solution 3000
have been tested at temperatures up to 75 oC (167 oF) with no gas phase
reaction.
Storage-Stable Chlorine Dioxide Solutions
CDG has developed a novel means for producing storage-stable aqueous
solutions of chlorine dioxide, which maintain their integrity for extended periods,
even at room temperature. CDG Solution 3000 can be stored at room
temperature for up to nine months while maintaining more than 90% of their
original 3000 ppm concentration. These solutions contain 3000 ppm chlorine
dioxide and less than 10 ppm elemental chlorine. Therefore, if the chlorine
dioxide is applied at 1 ppm, the chlorine in the applied liquid will be less than 3.3
ppb.
Solution 3000 should be stored at temperatures between 32 oF and 110 oF
Although solution 3000 is shipped in containers that are opaque to UV light,
storage of the containers and use of the product in areas shaded from direct
sunlight or fluorescent light is an advisable precaution.
CDG offers specially adapted Colder Fittings for most container sizes. These
fittings enable changing from one container to another without release of any gas
or solution.
Conclusion
Storage stable chlorine dioxide solutions afford advantages over traditional
approaches to making chlorine dioxide solutions on site. Storage stable solutions
are of precisely-known concentration. Knowing the strength of the solution
enables precision dosing. Storage stable solutions are pure; they contain no
chlorine, which has been shown to attack many metal and polymeric substrates..
Solution purity enables enhanced control of disinfection by-products, and helps to
minimize corrosion potential. Storage stable solutions eliminate on-site mixing of
chemicals, evolution of chlorine dioxide fumes into the workplace, and the startup, maintenance, and calibration associated with on-site generators.
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Storage stable chlorine dioxide solutions have utility for virtually all existing smallscale applications of chlorine dioxide, and facilitate chlorine dioxide use for new
applications which were previously impractical.
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