Download Chemical control of biocorrosion - European Federation of Corrosion

Chemical control of biocorrosion
Christine Gaylarde
UFRGS
Control of Biocorrosion
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Good housekeeping
Physical cleaning
Chemical treatment – biocide use
Alternative treatments – physical
methods, biological control
Industrial biocides
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What are biocides?
How are they used?
How do they work?
Why do they not always work?
How do we test biocides? –in the
laboratory, - in the field.
• What are the environmental problems
associated with biocide use?
An ideal biocide
• Kills all target organisms
• Has no effect on other organisms
• Does not affect the material to be protected nor the
external environment
• Does not cause allergies, irritation or other
diseases in humans or livestock
• Is efficient under different conditions
• Is cheap (cost/benefit ratio) and stable
• Is biodegradable
• Does not exist
Biocide groups
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Oxidizing agents
Protein denaturants and enzyme poisons
Surfactive agents
Inhibitors of sterol systhesis
Inhibitors of photosynthesis
Oxidizing agents
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Chromate
Halogens – chlorine, bromine
Organohalogens – chloramines, bromamines
Ozone
• Wide activity
• Generally inexpensive
Disadvantages
• Corrosive
• Inactivated by organics
• Reaction with organic material can produce
carcinogens (esp. Cl)
• pH range limited
Bromine activity with pH
Protein denaturants and
enzyme poisons
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Bisthiocyanates
Heavy metals
Aldehydes – formaldehyde, glutaraldehyde
Formaldehyde-releasing agents
Isothiazolones e.g. BIT (benzoisothiazolone)
Disadvantages
• Many ecotoxic and persistent
• Often inactivated by hydrogen sulfide
• Isothiazolones irritant and relatively
expensive
Surfactive agents
• Quaternary ammonium compounds
• Quaternary phosphonium compounds
• Dispersant activity. May be environmentally
friendly.
Disadvantages
• High concentrations required
• Foaming may be a problem
• Microorganisms readily develop resistance
Inibitors of sterol synthesis
• Triazoles
• Imidazoles (e.g., carbendazin)
• Pyridine derivatives
• Fungicidal (block ergosterol synthesis) and,
to some extent, algicidal
Disadvantages
• Ineffective against most bacteria
(some activity by inhibition of C-55
isoprenoid alcohol)
Inhibitors of photosynthesis
• Herbicides (algicides)
– Urea derivatives, e.g., Diuron
Often incorporated into paints to prevent
biofouling. Theoretically inactive against fungi
and bacteria.
Fungal biofilms on paint with and
without carbendazin or Diuron
How are biocides used?
Remember that prevention is
better than treatment. Good
housekeeping is the first choice
for control of biocorrosion.
Other options
• Correct system design – easy cleaning, no
dead spaces, access points for sampling
• Use of corrosion resistant materials, with
low biofilm forming potential
• Use of protective coatings
• Cathodic protection
Before biocide application:
• Pinpoint the source of contamination
• Ensure that biocide will gain access to the
appropriate sites
• Know your system – ensure that biocide will not
be inactivated by physico-chemical properties
• CLEAN THE SITE AS THOROUGHLY AS
POSSIBLE
Cleaning sludge from
the bottom of a diesel
storage tank
Biocide application
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Continuous treatment (low concentrations)
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Risk of resistance
Expensive
Shock, or slug, treatment
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Highly effective biocide necessary
Monitoring essential
Incorporation into coatings
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May protect against “holidays”
Leaching rate important
Biocide application
• Mixtures of products
• Alternating products
– Ensure no negative interaction
– Low concentrations may be possible when
mixtures used (“Hurdles concept”)
Why do biocides fail?
1. The microbial populations are
resistant
• Microorganisms with high lipid content in
the cell envelope generally more resistant.
• Many environmental isolates have been
shown to be naturally resistant to
isothiazolones and quats.
2. The microbial population
becomes resistant
• Genetic change.Gene transfer (usually on
plasmids). Has been shown for heavy metals,
some quats.
• Adaptive change. Continuous biocide treatment
selects out resistant members of the population.
Biocide treatment of a secondary
oil production system
Sanders, 1988
3. The biocide is inactivated
• Biological. 2-bromo-2-nitropropane-1,3diol inactivated by a resistant Fusarium
solani allows bacteria to grow.
• Chemical. High levels of hydrogen sulfide
inhibit heavy metal, isothiazolone action
4. The biocide removes bacterial
competitors, allowing growth of
other corrosive organisms
• E.g. A bactericide may allow algal growth.
Algae can release glycolic acid, which may
react with corrosion inhbitors.
5. The biocide is less active in
biofilms
• Biofilm model
• ESEM of biofilm
Why are sessile organisms
resistant?
1. Cellular mechanisms
• Altered cell metabolism
• Altered reproduction rate
• Consortia effects:
– Breakdown of biocide by one or more species
– Synergistic interactions causing biocide
inactivation
Why are sessile organisms
resistant?
2. Biofilm mechanisms
• Reduced activity of biocide within biofilm
caused by:
– Adsorption onto inorganics
– Chelation by biofilm polymers
– Ionic binding to biofilm matrix
• Lack of penetration into/through the biofilm
Lack of penetration
• Diffusion of biocide into biofilm
• Assuming that time taken to reach half its
external concentration is 100 times the cell
half-life:
Biocide with a concentration
exponent of 6 (e.g. a phenolic)
When biocide concentration increases 2x, cell death rate increases 26
Biocide with a concentration
exponent of 0.7 (e.g. a heavy metal)
When biocide concentration increases 2x, cell death rate increases 20.7
Distribution of biocides in
biofilms
• 1. Biotic. Uptake into cells.
• 2. Abiotic.
– Solution in interstitial water
– Ionic binding to polymers
– Van der Waals’ binding (important for
surfactants)
– Chelation by polymer
In a film of 100%
polysaccharide, for 10mM Cu,
17% Cu taken up is chelated,
leaving up to 83% available to
act against the cells.
Testing biocide efficacy in the
laboratory
Diffusion in agar
2 biocides, one as a gradient in the agar, the other on the strip
Disadvantages
• Biocide can only be tested against easily
grown bacteria and only under conditions
suitable for their growth
• Cidal activity is not tested
• Result depends on ability of biocide to
diffuse through agar (may be a good
indication of biofilm penetration, however)
Minimum Inhibitory
Concentration
Varying dilutions of biocide in growth medium inoculated with
relevant microorganism
Incubation at suitable growth
temperature
MIC = 7.5 to 10 ppm
Subculture from last
positive and all negative tubes into
growth medium without biocide
Disadvantages of MIC
• Only medium and conditions suitable for
growth are used
• Mixtures of microorganisms cannot be used
because of possible interactions
• The LETHAL effect of the biocide is not
rigorously tested
Time/kill curve
Suspension of cells in appropriate medium
Incubate at appropriate temperature with varying
(concentrations of) biocides
Samples taken at various time intervals
Measure viable cells by any suitable method
Viable bacteria in highly contaminated
fuel/water system treated with an Isothiazolone biocide
Activity measurements
Activity measurements
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FDA (fluorescein diacetate) hydrolysis
ATP measurement
Enzyme assays (e.g., hydrogenase)
CO2 release, with or without radioactive
label
• Radioactivity measurements with other
marked compounds
35S2–
produced by SRB incubated
with biocide
Testing biocide activity in
biofilms
The Robbins Device
Formaldehyde-releasing agent against Pseudomonas
attached to metal surface 200ppm 3h contact time
Epifluorescence
microscopy using
acridine orange
Untreated
Biocide treated
Formaldehyde-releasing agent 200ppm/3h
Slope = 0.573
Quat 200ppm/3h
Slope = 0.062
Quat 200ppm/6h
Slope = 0.088
Summary – slopes for 2 biocides
at 200ppm
Biocide/
treatment
time
DHEM/3h
Quat/6h
Quat/3h
Slope
0.573
0.088
0.062
Biocides and the environment
Directive 98/8/EC of the European
Parliament and of the Council
• concerning the placing of biocidal products
on the market
• 16 February, 1998
Biocidal products:
• are aimed at the control of organisms
harmful to human or animal health
• are aimed at the control of organisms that
cause damage to natural or manufctured
products
• can pose risks to humans, animals and the
environment due to their intrinsic properties
and associated use patterns.
Some definitions
• Biocidal products
– Active substances and preparations containing them, intended
to destroy, render harmless, prevent the action of, or otherwise
exert control on any harmful organism by chemical or
biological means.
• Active substance
– A substance or microorganism having general or specific
action on or against harmful organisms.
• Harmful organism
– Any organism which has an unwanted presence or detrimental
effect for humans, their activities, or the products they use or
produce, or for animals or the environment.
Biocides should:
• be sufficiently effective
• have no unacceptable effect on the target
organisms (e.g., resistance)
• cause no unnecessary suffering and pain to
vertebrates
• have no unacceptable effect on the
environment and on human or animal health
Some suggested ecotoxicological
studies
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Acute toxicity to fish
Acute toxicity to Daphnia
Growth inhibition test on algae
Inhibition of microbial activity
Bioconcentration in the environment
Degradation
– biotic
– abiotic (pH, light)
Fate and behaviour in the
environment (soil and water)
• Rate and route of degradation – processes
involved, metabolites and degradation
products
• Absorption and desorption
• Mobility
• Extent and nature of bound residues
– Residues: substances which remain as a result
of biocide use, including metabolites and
degradation products
Case studies
1. The system
• A heat exchanger using well water, pH8,
low-chromate zinc and phosphonate as
corrosion inhibitors and an organic sulfur
compound as biocide when required
The problem
• Increased corrosion rate
• Total aerobic bacterial count 5 x 104 to 1.5 x
106/ml
• Increased SRB counts
The response
• Biocide added at 50mg/L
three times per week
The result
• Bacterial counts remained the same.
• After 10 months, heat exchange efficiency
was reduced, causing losses in the steel
production line.
• The oil-based cooling system was found to
be corroded and was replaced.
• One month later, biofouling was detected
and biocide addition was increased to every
day.
• Three months later, heat exchanger tubes
were found to be blocked with corrosion
products.
• Iron-oxidising bacteria were detected in
tubercles.
The next response
• Various additives (biocides and corrosion
inhibitors) were tried.
• High corrosion rates and microbial
populations continued.
The panic response
• A detailed study of the system
• Laboratory testing of potential
biocides
Results
• Source of the problem was a change in the
water source (recycling) and an oil leak into
the water.
• The biocide was incompatible with the
chromate corrosion inhibitor.
• Microorganisms detected in the system
included Gallionella, Sphaeotilus,
Thiobacillus, Desulfovibrio
Laboratory test results
• Using water from the system itself, the
following biocide regime was determined:
• Weekly treatment with a mixture of
– Sodium dimethyldithiocarbamate
– Methylene bisthiocyanate
– 2-(thiocyanomethylthio)benzothiazole
2. The system
• A cooling tower in a syrup manufacturing
plant
The problem
• Reduced efficiency due to slime formation
The response
• Biocide use –
15ppm of 20% 2,2-dibromo-3nitroilopropionamide (DBNPA) weekly
The result
• Tower efficiency restored after one month
• Treatment continued.
• After 14 months, slime reformed.
The response
• Laboratory tests to assess sensitivity
of microbial population to the DBNPA
concentration used (ATP assay).
Result
• Population still sensitive!
The next response
• Investigation of the history of the system.
• No recent changes detected, apart from the
installation of an automatic biocide delivery
pump.
The final answer
Return to manual biocide addition!
How to choose a biocide
The wrong way
Identify the existence of a microbial problem
Contact a biocide provider
Buy the cheapest biocide suggested for this system
Apply product at recommended concentration (or less)
Wait for improvement
The right way
Collect physico-chemical data
Identify compatible biocides
Calculate the probable cost (including environmental cost)
Identify problem-causing organisms
Obtain data on sensitivity to chosen biocides
Apply chosen product – at correct concentration
Monitor result