BacterioFiles 313 - Colonies Correct Chloride Corrosion

Ancient Roman nails
By Takkk - Own work,CC BY-SA 3.0

This episode: Bacteria could help treat corrosion to preserve ancient iron artifacts!

Thanks to Drs. Pilar Junier and Edith Joseph for their contributions!

Download Episode (13.4 MB, 14.7 minutes)

Show notes:
Journal Paper:
Comensoli L, Maillard J, Albini M, Sandoz F, Junier P, Joseph E. 2017. Use of Bacteria To Stabilize Archaeological Iron. Appl Environ Microbiol 83:e03478-16.

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Post questions or comments here or email to bacteriofiles@gmail.com. Thanks for listening!

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Episode outline:

• Background: Imagine future human digging through landfills
• What would he find? Probably a lot of plastic
• Maybe your old electronic device you threw away
• Wouldn’t look quite the same though
• US history museums have many old tools
• Like iron cannons, big black and lumpy
• Always wonder what they looked like new
• But iron artifacts can be found much older than that
• Despite being tough, iron susceptible to chemical corrosion
• Oxygen creates rust, various iron oxides
• If chlorine in environment, acidic ferric chloride can form and eat away
• Several ways currently to try to recover artifacts
• Chemical treatment: alkaline sulfide
• Lets chlorine ions come out and reduces corrosion
• But slow and tricky to measure progress
• Electrolyte reduction – redox treatment, removes corrosion
• Harsh and significant loss of mass
• Also produces hydrogen, explosive
• Plasma treatment doesn’t do much about chlorine, just creates cracks in corrosion layer
• Would be nice to have better ways
• What’s new: Now, Lucrezia Comensoli, Julien Maillard, Monica Albini, Frederic Sandoz, Pilar Junier, and Edith Joseph, with Wafa Kooli, publishing in Applied and Environmental Microbiology, have developed an interesting new treatment strategy, using mineral-eating bacteria!
• Microbes considered in other types of conservation/restoration of artifacts
• Breaking down nitrate/sulfate crusts on stone, self-healing concrete by mineralization
• Removing organic matter from frescoes
• Forming protective layer on copper
• So why not iron?
• Many microbes manipulate iron for energy or as nutrient
• Iron is mostly in insoluble form in presence of oxygen
• Siderophores solubilize and bring inside cells
• Some bacteria can access solid iron too; must be transferring electrons out of cells
• Here's Dr. Junier describing their thinking regarding microbes: 2
• Methods: Chose to study Desulfitobacterium hafniense
• Anaerobic, very versatile in electron acceptors it can use
• Nitrate, sulfite, metals, humic acids, halogenated organic compounds
• Esp interesting cos of reducing iron
• Grew D. hafniense with soluble iron in presence of chlorine ions, in form of NaCl (salt)
• Saw some iron reduced in control with no cells; probably reducing agent added to remove O
• But with cells, even more iron reduced beyond control amount
• Actually about all added was reduced
• color of solution changed from orange (rust) to black (reduced iron)
• So chloride doesn’t interfere
• Good for soluble iron, but what about solid?
• Put bacteria with mineral called akaganeite, synthetic, mix of Fe, O, Cl
• Worst kind of corrosion found on artifacts
• Here, control had not much improvement
• But with cells, lots of iron reduced
• Synthetic akaganeite is too pure; artifacts have other elements mixed in
• Next tested rusty pieces of steel
• Took these pieces and let sit outdoors for a while until corroded
• Red-color like rust, Fe+O, with atoms of silicon, carbon, aluminum, and calcium
• No-cells control turned black yellow (probably cos of the reducing agent)
• D. hafniense, at least one of two strains tested, turned it grey
• Mostly covered with crystals mostly Fe, O, and C, and some P, S, Mg, Ca
• Chemical testing showed abiotic stuff was called mackinawite (Fe S) and some C
• Cells’ crystals were mostly vivianite, Iron phosphate. And elemental sulfur and some others
• Nice and stable iron compound
• Then final test, actual corroded archaeological objects!
• Roman nails, prepared for treatment by sandblasting like normal
• Dr. Joseph tells an amusing story about acquiring these nails: 6
• Started with brown-red color with orange spots, iron and oxygen
• No-cell controls had no change; corrosion too established/stable
• But with bacteria, nails turned dark-grey, could see clusters of crystals
• Magnetite and vivianite, dark grey and blue colors
• Covered whole surface after 3-7 days
• Summary: Iron-reducing bacteria in this study converted damaging rust and corrosion on iron objects into more stable and desirable forms
• Applications and implications: Could be very useful for archaeology
• As Edith said, already demand for process in museums and such
• Though process needs tweaking: magnetite color is ok but blue vivianite not as acceptable
• This process could be more desirable than chemical methods, as Dr. Junier explains: 5
• Though I'm not sure how results compare exactly; certainly takes a lot less time
• But it could also be useful for other purposes, not just archaeology. Here's Dr. Joseph: 4
• What do I think: Very cool to use microbes for this purpose
• They are powerful chemists though, so makes sense
• Often microbes are problematic for what they do to metals instead of helpful
• Nice to see them helpful instead
• There are some tricky limitations to applying this to larger artifacts, though. Here's Pilar again: 7
• Edith and Pilar have some interesting ideas of applying and developing this work in the future: 8,9

Transcript statements:
2
So the general reason we go into bacteria or fungi, depends on the topic, is because we believe they are the best chemists on the planet, and then you can, by understanding their metabolism, apply that metabolism for particular problem or concern, like corrosion in this case.

4
So of course, the main issue of this project is how we can preserve the heritage, and this is of course important for the next generation. But we can also see some application in other domain and field of society. For example, iron is also used for pipeline and different many architectural part, and for this it can also be preserved.

5
now we have turned to trying to find alternatives to many treatments that are done currently more in chemical or industrial way, and we are trying to use microbes to replace these in what is so-called green chemistry, and that's at the end how can this type of research can build back into society.

6
The first time when I remember when I come to museum asking if I can have some object to make some treatment with some bacteria and fungi, they just told me, "Are you crazy? We don't want any bacteria here!" So it was quite, now they are quite demanding, so it's funny to see how they change their mind.

7
And also more in general, for us when we develop something in the laboratory, obviously our constraints are relatively small, because we know what we want to achieve. But it is easy to put a Roman nail inside a culture bottle, but it's not the same if you try to do this with a cannon or with any type of artifact that is larger than one or two centimeters.

8
So in this case, as Pilar say, we mainly develop actually in the lab, and what we would like to do is pursue this research, but in this case we've an in situ treatment and in particular involves some conservator ... that can give us their feedback and how they can be used as a protocol in the museum.

9
I think what is really interesting is this whole general idea has built basically Edith's and from the beginning she was very into bringing the final practitioner right into research so they can tell us what are the limitations, what are the conditions, if they understand the protocols, and that will lead us to real kits ready to use in the future.