Cleaning up the large number of groundwater contamination sites is a significant and complex environmental challenge. The environmental industry is continuously looking for remediation methods that are both effective and cost-efficient. Over the past 10 years there have been amazing, important developments in our understanding of key attenuation processes and technologies for evaluating natural attenuation processes, and a changing institutional perspective on when and where Monitored Natural Attenuation (MNA) may be applied. Despite these advances, restoring groundwater contaminated by anthropogenic sources to allow for unrestricted use continues to be a challenge. Because of a complex mix of physical, chemical, and biological constraints associated with active in-situ cleanup technologies, there has been a long standing focus on understanding natural processes that attenuate groundwater contaminant plumes. We will build upon basic environmental science and environmental engineering principles to discover how to best implement MNA as a viable treatment for groundwater contamination plumes. Additionally we will delve into the history behind and make predictions about the new directions for this technology. Any professional working in the environmental remediation industry will benefit from this in-depth study of MNA. We will use lectures, readings, and computational exercises to enhance our understanding and implementation of MNA. At the completion of this course, students will have updated understanding and practical tools that can be applied to all possible MNA sites.
Other Abiotic Reactions: Hydrolysis
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來自 Rice University 的課程
Natural Attenuation of Groundwater Contaminants: New Paradigms, Technologies, and Applications
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Abiotic Degradation Principles
In this series of lectures, we will focus on the principles of abiotic degradation and discuss how these processes support monitored natural attenuation. You will be learning about the key reactions and minerals that are involved in abiotic degradation. We will also be covering what methods are used to determine if abiotic degradation is relevant at a particular site, and the rates at which these reactions may be occurring
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Pedro AlvarezGeorge R. Brown Professor of Civil and Environmental Engineering
Department of Civil and Environmental Engineering
Charles NewellVice President
GSI Environmental Inc.
David AdamsonSenior Environmental Engineer
GSI Environmental Inc.
Well Dave, I know you said at the end of the last lecture that we're going to
revisit a different type of a biotic degradation process and we skipped over,
and what is that?
>> Well let me ask you and see if you can remember,
what reaction we're talking about here.
>> Okay, I think this is hydrolysis, is that right?
>> Right!
>> Okay, [LAUGH] well, I did sort of try to brush up on some of the material this
morning before I showed up, drinking my coffee.
But anyway, I remember back in the early days of groundwater cleanups,
when I first got into this business in the late 1980's, 1990's.
We thought many of these contaminants, which really were the solvents,
were pretty recalcitrant.
But even then, we recognized that hydrolysis was an important reaction for
a couple of these organics, a couple of these chlorinated solvents.
>> Yeah, and hydrolysis is a pretty basic chemical reaction.
So we knew that it caused transformation of a few of these man-made
compounds long before we started to clean them up out of groundwater.
>> So how was hydrolysis different than other abiotic reactions and
biotic reactions for that matter?
>> Well let's take a look at this again then on this first slide,
we sort of take a look at a compound like 1,1,1-TCA.
>> Very common and
degreasers and things of that nature so- >> Yeah.
>> A lot of that, lot of plumes out there >> And the biotic pathway for
this, it needs specific geochemical conditions to happen,
you need to have the right microbes present.
You need and not have a lot of competition or
inhibition from other things that might be out there.
Abiotic by reactive minerals,
we've spent the last few lectures talking about that, in terms of what's needed.
You need to have specific geochemical conditions as well as the right minerals
and then again, that microbial contribution as well.
But what we're talking about today then, is basically hydrolysis,
another abiotic reaction, so what's needed for that?
>> Nothing and isn't it this idea that it's just water molecules bang
into that contaminant.
And just because of the structure of the water molecule and
the structure of the contaminant, it will break apart,
it will fly apart with enough of these collisions.
>> Yeah. >> Is that one way to think about it?
>> That's one way to think about it.
So, again, really nothing that we need to have this reaction proceed.
It's got distinct byproducts that we can sort of look for and
a portion of those byproducts are innocuous.
So sort of a very favorable reaction if it's relevant for these compounds.
>> Great. >> So let's take this reaction again,
take a look at it.
We're talking about nucleophilic substitution in this case.
So there's our water molecule, and we're going to have that react with 1,1,1-TCA.
So on this case, we're replacing one of the chlorides in there with this hydroxide
iron in the intermediate in this case is this 2.2-Dichloroethanol.
And after that we then get acetic acid after a few more steps.
>> So you could make vinegar with this reaction?
>> If you had enough maybe you could,
most times you're talking about pretty low yields in these cases.
So maybe not enough to really turn your aquifer acidic in these cases.
>> I think I should stay away from the vinegar business, take it from 1,1,1-TCA?
>> Yeah. >> Okay.
>> Look at this in a little bit more detail.
Hydrolysis we're talking about an SN2 reaction in these cases so
some of the steps are showing again, here.
But what I'd like to point out then is there's this carbocation
intermediate that's formed.
So that's the second one here on that case.
So eventually, if you go through that hydrolysis reaction you're going to get
acetic acid out of that.
That intermediate is also subject to dehydrohalogenation reaction,
an E1 reaction.
So in this case, out of that you get an alternative end product,
you're looking at 1,1 DCE.
1,1-Dichloroethene, which is shown there.
>> A key sort of daughter product of these things.
>> Mm-hm. >> But both those reactions,
the dehydrohalogenation and
the hydrolysis is sort of lumped under this hydrolysis umbrella, is that right?
>> Yeah, exactly.
And so looking at those again, we can kind of take a look at those products.
And what we see when we do these in sort of a controlled situation, is
you see a product yield where 80% of that initial 1,1,1-TCA shows up as acetic acid.
>> A non-toxic compound.
>> A non-toxic compound.
20% of it ends up as 1,1-DCE.
>> And why is that, why do you see this difference?
>> Well it's pretty simple.
In this case it's kinetics is the reason that faster, the top reaction there
is basically 5X faster so you see a higher yield of that acetic acid.
>> So you got the 1,1,1-trichlorethane, the water molecule hits it, and then it
just turns out that more often than not it's going to form this acetic acid.
But one out of every four or
one out of every five is going to then turn into this 1,1-dichlorethane.
>> Yeah.
So what are the really the key contaminants in terms of
being relevant for hydrolysis?
Well there's quite a few of them listed under here.
You see a lot of ethane typed compounds and also chlorinated methane,
carbon tetrachloride.
So all of these undergo hydrolysis reactions.
And the various products that you might expect to see with those are there.
And these might not be the end products.
They might be subject to further degradation.
But they're sort of these things that you might see if you were measuring for them.
And they occur as sort of a result of this hydrolysis type reaction.
>> Okay, well how about some other contaminants that I care about?
How about the Petroleum hydrocarbons, how about TCE, how about these other guys?
>> Well, I know you care a lot about a lot of these things, and unfortunately I hate
to break it to you, we can't really rely on hydrolysis for these types of things.
So again, yeah, we're talking about a lot of the chlorinated ethanes,
the things that might form from PCE or TCE.
1,1-DCE itself is not subject to further hydrolysis.
Chloroform, a lot of the petroleum hydrocarbons, and again
sort of the list of emergent containment, probably not of relevant for hydrolysis.
>> So chloroethanes, you've got thumbs up for some of those important ones but
then it's this desert maybe for these other compounds.
>> Yeah.
Looking at rates that might be associated with these reactions,
we can rely on a great paper that came out in water research in 2011,
where they compiled a lot of these things and again,
we're talking primarily about a fairly limited number of compounds.
So they were looking at trichloroethane as well as chloroethane.
They did a great job of putting sort of what they saw for sort of rate constants.
They even converted them into half life for those half life people.
So they listed them here in some of the products, and
temperatures that are associated with those.
>> So thumbs up for me.
It has a lot of information.
Some of those papers from 1959,
but where are some of the ranges that we're seeing in that table?
>> Yeah, so TCA in this case, we're looking at half-life
somewhere around the order of one to 10 years in this case.
So pretty reasonable half-life in this case.
The one thing that you might want to be aware of, though,
in this case is that TCA is actually relatively biodegradable.
So, if you want to do a comparison between this abiotic hydrolysis rate and
biodegradation, maybe about an order magnitude or
even two orders of magnitude which you see in difference of this rates.
The biodegradation is generally faster in these cases.
>> But the biodegradation only occurs under that certain sites or
some sites where there's no biodegradation, right?
>> Yeah. And then a lot of these cases one
to ten years in terms of half life might be relevant for
what your needs are in terms of the viability of natural attenuation.
So, not to dismiss that the hydrolysis rates are actually relevant here.
>> So that's cool, but now let's talk about temperature.
How does it affect it?
>> Well, these are chemical reactions.
So if you have a higher temperature, you're going to proceed at a faster rate.
And this follows the Arrhenius equation, and just take a look at this.
If you see your term there with the T, the temperature term.
If you increase that,
you're going to increase the k associated with that reaction.
>> And I'm a sort of a fan of history of science and
I like reading up about some of this stuff.
And Arrhenius Equation is a key thing that we use a lot in our business thinking
about how temperature might affect these rates.
Turns out this guy is a Swedish chemist around the turn,
around the 1900 time period.
He was one of the first guys besides this equation,
thinking about how enzymes worked, how reactions worked,
identified the potential for global warming from the burning of coal.
But he's been on it in a 1900 time frame was that he's in Sweden and
global warming would be a great thing.
>> No, well we're in a little different place now, but good to know.
>> But we still use this formula for the future.
>> Yeah, and in this case, just remember if the temperature increases,
k is also going to increase.
So, one way to look at this sort of temperate dependence of these rates is to
plot a bunch of data.
So, this is a pretty standard [INAUDIBLE] plot in this nice paper,
looking at hydrolysis of TCA.
And so in this plot here, we've got the natural log of the rate coefficient
the hydrolysis rate coefficient associated with these various temperatures.
So as you're moving front the bottom right to the upper left,
you're seeing an increase in temperature associated with an increase in rate.
>> Okay, now this is one of my least favorite types of way to
express temperature.
This is one over temperature in degrees kelvin.
What does this mean in real life in terms of
how much temperature change will increase the rates or change the rates?
Well, one way to look at it is sort of, you might see a cold weather site,
a cold run water might be something like 10 degrees Celsius and
a hot maybe in a warmer climate you might see something like 25 degree Celsius.
The rate increase associated with that sort of the very matter temperature
changes is 16 time increased in the hydrolysis rate coefficient.
So pretty significant.
>> That's a lot and that's the increase in the rate of insinuation.
I think some folks are actually gone to TCA sites and try to heat them
up just a little bit to get it to 25 degrees increase that hydrolysis rate.
>> Mm-hm.
>> Mm-hm.
So is this process relevant at you site, at person's site?
>> Well let's take a look at some of the.
Some of the key factors here.
Basically if you're at a TCA site you see it disappearing.
You look for 1,1-DCE and acetic acid as products.
And you can use the ratio of those things maybe even to help you
estimate the age of the release.
Particularly at sites where the conditions might not be favorable for biodegradation.
>> A little tough to do in some cases, a lot of other factors in there that
whole dating thing can be tough but there are papers about how to try to do it.
>> Yeah, and it's relying on that it's a predictable reaction,
that you may be able to actually know what those rate coefficients are with some
>> Some degree of uncertainty.
>> It's a lot of unknowns in that dating stuff.
>> Yeah.
Rates are significant, and in a lot of cases from natural attenuation,
because we're talking about releases that may have occurred decades ago, so
if you half-lives in the one to ten year range, you might have seen
quite a bit of degradation of related, just the hydrolysis.
And then again, these types of reactions are must less dependent on
the geo-chemical conditions or other things that might be present.
One thing that we like to highlight is the positive impact of a warmer
ground water temperature.
>> Okay well, let's wrap up.
>> So hydrolysis again, a key abiotic degradation pathway.
Doesn't rely on the presence of other minerals, microbes or
even geochemical conditions to a large part.
And these hydrolysis rates are pretty predictable.
If you know the chemical and you know the temperature, you can get
these rates from the literature, a lot of good, sort of basic information about it.
>> Yeah, and you can look for
these very specific products of these reactions in order to sort of do some
forensic information about whether the process is relevant at your site.
>> But it's only limited to a couple of these key chemicals where that water will
bang into that chemical and it will >> It will fall apart,
1,1,1-trichloroethane is sort of the big guy in this world.
