PCE dechlorination

[elarocque]

Remediation Problem 4.1

Given: PCE --> TCE --> DCE isomers

Find: What is not a significant pathway for degradation of perchloroethene in a soil-groundwater system?

ANS Choices:

(A) biological oxidation

(B) chemical reduction

© deamination

(D) hydrolysis

Solution:

i know the correct answer is C, given PCE does not possess NH_2 (amine functional group); however...

from studying LeGrega (Chap 4 - Fate...):

wouldn't a better answer choice be biological reduction, not bio. oxidation (anaerobic dechlorination is the pathway LeGrega discusses on pg. 206)? while most organic substances degrade under biological oxidation, anaerobic mode is reported by LeGrega to be FAR more efficient than aerobic

also - anyone know explain why deamination is even an answer choice?-am i correct to consider it to be way out of place?

Thanks,

-Eric

 

[Wyatt72]

If I am reading your post right then I agree with you. In paractice as well as LaGrega I believe you are correct. Anyone else have any thoughts?

 

[EnvEngineer]

It a pretty bogus question give the answere choices, there are three things that fall out of standard practice which mess you up. First PCE is fully oxidized so it cannot be biologically oxidized with any effenciency, oxidation systems for treatement of PCE depend on co-metabilization which is not considered biological oxidation.. The second issue is chemical reduction, this makes little sence because althought valid in theory I do not know of any chemicals which can chemically reduce PCE, reductive dechlorination is a biological process not a chemical.

Lastly I dont see how hydrolysis can be worked into treatment at.

Seems like the answere is taken as the most non-correct rather than a correct answere.

 

[Dleg]

I don't do remediation work so I can't speak from experience like these guys, but I looked at my notes from this problem, and my take was just what you came up with, and what EnvEngineer said above: it was just simply a "which is the most non-correct answer' question. Don't sweat it - if you did the research to be questioning what the correct solution should be, then you're in good shape for the exam.

 

[elarocque]

Thanks for the help everyone - the countdown is on no doubt

 

[Guest]

Oohhh .. you asked about something that I know about !!

Check out this article ---> http://en.wikipedia.org/wiki/Dehalococcoides

Note that the bacteria Dehalococcoides ethenogenes utilizes O2 as an energy source hence the biological oxidation.

From Todd H. Weidemier, generally recognized degradation mechanisms for chlorinated solvent species are as follows;

PCE = Reductive dechlorination

TCE = Reductive decholorination, cometabolism

DCE = Reductive decholorination, direct biological oxidation

First PCE is fully oxidized so it cannot be biologically oxidized with any effenciency, oxidation systems for treatement of PCE depend on co-metabilization which is not considered biological oxidation.

Perhaps, it would be more correct to say that PCE is fully halogenated as all hydrogen groups have been substituted with a chloride atom.

The second issue is chemical reduction, this makes little sence because althought valid in theory I do not know of any chemicals which can chemically reduce PCE, reductive dechlorination is a biological process not a chemical.

Please note the reaction sequence above from Todd H. Weidemeier and further explanation provided in the EPA Technical Guidance, Technical Protocol for Evaluation of Natural Attenuation

Lastly I dont see how hydrolysis can be worked into treatment at

Hydrolysis is a very inefficient pathway but still recognized as an abiotic pathway for reducing chlorinated solvents; an example of acknowledgement can be found in the following peer-reviewed paper http://www.adventusgroup.com/pdfs/ISCR_pathways_paper.pdf

When I reviewed hydrolysis, my recollection was the half-life was something on the order of the 50-yr to 100-yr time frame for chlorinated species.

I don't do remediation work so I can't speak from experience like these guys, but I looked at my notes from this problem, and my take was just what you came up with, and what EnvEngineer said above: it was just simply a "which is the most non-correct answer' question. Don't sweat it - if you did the research to be questioning what the correct solution should be, then you're in good shape for the exam.

Dleg is spot-on! You found the answer that didn't fit based on your basis of knowledge - that's your only worry!

Thanks for the help everyone - the countdown is on no doubt

Awesome!!

Feel free to post more questions - especially with respect to environmental remedation!

Good luck!

JR