CHUCK NEWELL: Our last lecture described how we're finally
starting to recognize that these abiotic body degradation reactions should
be part of the discussion about natural attenuation
and contaminated groundwater sites.
There's been a lot of research in this area,
a lot of information to sort through in terms of understanding
how these abiotic reactions work.
So a lot going on in this area.
DAVE ADAMSON: Yeah, and it could be a little challenging if what you're
used to is attenuation by biological pathways, the abiotic reactions,
as we saw last time.
And they're definitely little bit different.
CHUCK NEWELL: And there's also the fact that these abiotic processes,
although, their natural reactions, they rely on several different players
to proceed.
And if there's something missing at the site,
then you're not going to see this type of attenuation
from these abiotic reactions.
DAVE ADAMSON: I think this slide that we showed during our last lecture
sort of highlights the different players that are involved in this.
We're talking about biogeochemical reaction.
So moving from left to right on here, we've
got the biological component, the geological component,
and then sort of a chemical reaction that we're dealing with.
So the key point, though, in all these reactions,
remember, that we've got a mineral usually involved
and that minerals in sort of reduced form.
So you need to have the conditions that support those sorts of reactions
to happen.
CHUCK NEWELL: And it's different from this whole valent iron process, right?
DAVE ADAMSON: Exactly.
So one of the questions that we want to introduce here
is, which minerals are we actually talking about here?
I think the one that most people are familiar with then
are these group of minerals called iron sulfide.
So we're talking about iron sulfide, simple FES, or mackinawite, or pyrite,
in these cases.
And we've got a picture here then of iron sulfide.
This is a scanning electron microscope image
of that from Michelle Scherer at University of Iowa's lab.
CHUCK NEWELL: Neat.
And sort of how big is that particle?
Do you have any sense of the scale?
DAVE ADAMSON: Well, I'd say it's definitely
falls in the teeny-weeny category.
But just looking at it, I think we're talking, you know,
the individual crystals on the order of a few
microns up to 20 microns, something like that.
CHUCK NEWELL: Great, great.
DAVE ADAMSON: So those are the iron sulfides,
but there's a few other ones that are definitely of interest and definitely
been established as reactive minerals.
So we're also talking about iron oxides and iron
hydroxides, things like magnetite and hematite, as well as mixed iron
hydroxides.
So most people seeing these referred to as green rust--
which can be a lot of different types of compounds
that fall under that category-- as well as iron carbonates, like siderite.
And then there's these phyillosilicate clays, biotite, vermiculite,
those sorts of things.
CHUCK NEWELL: And so this is really from the world of soil science.
Those soil scientists know about all this stuff.
This can be unfamiliar to some of us, coming
from the ground waterfield, or chemical engineering,
or environmental engineering, some great names there.
Just a quick one, one of the metal, under iron hydroxides, I see magnetite.
I see hematite.
What's the one that starts with L?
How do you pronounce that one?
DAVE ADAMSON: I would throw that back to you.
I'm not really sure.
CHUCK NEWELL: More research is needed on that particular one.
DAVE ADAMSON: More research is needed.
So those are the minerals that are important,
but what geochemical conditions then are we
talking about that need to be present in order for these reactions to occur?
So remember, since they're being mediated by these mineral species,
we need to have those mineral species present.
So we need to have iron rich minerals or some sort of source of iron
within the ground water.
We need to have natural sources of sulfate
in order to have sulfide be formed.
We also then need to have generally anaerobic conditions.
The biological reductions here are occurring
through the actions of iron reduction and sulfate reductions.
So these occur in an anaerobic conditions.
And then we need to have organic carbon present.
We need to have enough organic carbon to supply those bugs in order for them
to do those reduction reactions.
CHUCK NEWELL: Now the anaerobic conditions can they be natural,
or they can then be sort of man-made?
DAVE ADAMSON: They can be in both cases.
So you could have a man-made condition that supports a natural reaction.
CHUCK NEWELL: Great.
DAVE ADAMSON: And then we ask ourselves, well,
which minerals are important for which contaminants?
And so we're going to go through a couple
slides here, where we talk about the reactivity of iron sulfides, magnetite,
and green rust-- which are sort of three are the most important minerals
out there-- and which contaminants then can be degraded by these minerals.
So if you see, chlorinated solvents, we've
got a yes in each one of those categories.
And these are probably the most widely studied, definitely reactive
with a wide range of minerals.
Pesticides, definitely evidence for iron sulfides, but not the other ones.
Munitions, magnetite, yes, maybe iron sulfides.
And then metals, definitely there's redox sensitive metals.
So iron sulfides can mediate reactions in those.
