Cattails and Bog Metals

2020 May 17

(Most of this posting is about decomposing plants and mud. But, if you go all the way to the end, there is a butterfly picture as a reward)

Just down the hill from our house, there is a spot where the hill flattens out and catches water, making a bog. Cattails grow there. Right at the moment (May 13, 2020), the cattails have not yet re-sprouted, and so there are flat expanses of decomposing plant matter saturated with water.


The road we live on cuts through this bog, and the water oozes from the decaying vegetation into the ditch, which forms the headwaters of the little creek that runs beside the road all winter. At the beginning of the ditch, where the water has just flowed out of the ground, the color just looks kind of generally like you would expect mud to look.


But, as the water continues flowing past the bog, it quickly turns a strong red color.

Looking closer, we see there are quantities of red-brown sediments, along with a sheen of oil on the water.


And digging down a bit into the sediment, we see that under the very fine light-colored material, there is a darker red mud that contains granules about the size of sand.


Neither the red mud nor the oil are the result of any sort of human pollution, they are actually natural products of the bog. The oil is the result of anaerobic decomposition of the bog plants, which liberates fatty acids and hydrocarbons in the fermentation process. I believe this is an example of the sort of process that eventually leads to the formation of oil shale and petroleum deposits, given time.

The part I personally am most interested in, though, is the red sediments.

That mixture of red muds and sands is a type of iron ore commonly called “Bog Iron”. Bog iron consists of hydrated iron oxides, and if the deposition conditions are good it can be practically pure, with little or no contamination by silicate sands or gravels. For a long time, this sort of thing was very popular for iron ores, as a blacksmith could simply go to a nearby swamp, scoop it up, and take it back to the bloomery and dry it out for smelting into iron. The deposits tend to be fairly small, though, so while a blacksmith could easily get the few tons per year that he might need, these days they have been largely eclipsed by the much bigger igneous deposits and the massive Banded Iron Formations that currently provide most of the world’s supply of iron.

The way that bog iron forms is kind of interesting. What happens is this: First, the plant matter in the bog dies, falls over, and starts to rot. This is initially rotted by aerobic organisms, that need oxygen to grow. But, since the bog is flooded with water and oxygen does not actually dissolve in water very well, the oxygen quickly gets depleted. At this point, the anaerobic fermentative organisms take over. These break down the complex molecules into simpler ones, and in the process extract energy for their growth. This is similar to what yeast do when fermenting sugars during beer and wine production, they split the sugars into carbon dioxide and ethyl alcohol to produce energy in the absence of oxygen. The fermenting organisms can carry out a lot of different breakdown operations, and eventually they reduce much of the dead plant matter into simple molecules like acetic acid, citric acid, oxalic acid, ethyl alcohol, and various other water-soluble species.

At this point, those simpler organic molecules still contain a significant amount of food value, but any bacteria that want to extract energy from them have a problem. See, the respiration process that living things use to extract energy from organics is basically electrochemical. The bacteria can induce the food molecules to release electrons, and the bacteria can extract energy from these electrons as they flow through. But the problem is, in order to get the energy, the bacteria then need to pass off the electrons to an electron acceptor. For aerobic organisms (like us) that are exposed to air, they can use oxygen for this, because oxygen is an excellent electron acceptor. But, we already said that the oxygen was consumed from the bog water early on, so that’s no good. The fermentative organisms are able to break down complex organic molecules in such a way that one fragment of the molecule is the electron donor, and the other is the electron acceptor. But that only works if the molecule is big enough to break into smaller bits.

But down in the bottom of the bog, the oxygen is gone and the molecules are too simple to ferment, so what can the anaerobic bacteria down there use for an electron acceptor? Well, what happens here is that, below the layer of rotting muck, we get to the mineral subsoil. This subsoil often contains minerals that incorporate iron oxides and manganese oxides. And, it turns out that both iron and manganese can act as electron acceptors. The iron atoms in, say, hematite (Fe2O3) are in the +3 oxidation state, which means that they are short three electrons. Similarly, the manganese is usually present as MnO2, and the manganese is in the +4 oxidation state and is short four electrons. If bacteria use them as electron acceptors, they can convert the Fe[3+] to Fe[2+], and the Mn[4+] to Mn[2+]. And the thing is, while Fe[3+] and Mn[4+] are both highly insoluble in water, Fe[2+] and Mn[2+] are both very water-soluble.

So what happens is, the iron and manganese under the muck layer get converted to their soluble forms by the bacteria. They then dissolve in the water, which slowly flows out of the bog. But then, when the water oozes out of the ground and into a stream or ditch where it is exposed to oxygen, the iron and manganese can re-react with oxygen in the air. This converts them back into their insoluble forms, and they precipitate as bog iron and bog manganese.

I actually have a professional interest in this process, because I think it could be adapted into a metal-extraction process that would be considerably more energy-efficient and less environmentally damaging than current practices for mining iron and managnese. If, instead of just waiting for small amounts of bog iron to happen naturally, we constructed artificial wetlands on top of ore bodies, we could design them to force all the organic-rich water to flow through places where there was iron and manganese to dissolve. The dissolved metals could then be pumped up to the surface, precipitated from the water, and the water could be put back into the wetland to dissolve more metal. I’m envisioning something like this:


And during this entire process, instead of a massive hole in the ground that would have to be dealt with later, we would have an innocuous wetland that would be congenial to wildlife. Basically, this would be a lot more like farming metals than mining metals. And given that iron and manganese are both pretty ubiquitous (iron is the the fourth most common element in the Earth’s crust after oxygen, silicon, and aluminum; manganese is the 12th most common, so it’s not exactly scarce either), this could be done in many places and would basically make both metals essentially unlimited in supply.

So, anyway, I’m experimenting with some “test wells” in our bog (PVC pipes that go down about 2 feet below the bottom of the muck layer) to see if I can enhance the iron (and maybe manganese) recovery from the swamp water. We’ll see how it goes.


And, finally, here is the butterfly I promised. It is a Mourning Cloak that was sunning itself on the end of the handlebars of Sandy’s bicycle on April 25, 2020:


Mourning Cloaks spend the winter as adult butterflies hiding under the bark of trees, so they are usually the first butterflies that we see in the spring.

3 Responses
  1. Barbara and Philip Philip Quenzi permalink
    May 17, 2020

    We are enjoying the new path your blog is taking.

  2. Steve Plumb permalink
    May 21, 2020

    I heartily agree. A little geology, chemistry, and a butterfly makes for a good read.

  3. May 28, 2020

    This is fascinating, but I haven’t had the time yet to properly consume it. I’ve got it bookmarked, though.

Comments are closed.