Underground waters resist forced depollution

May 27, 2011 By Emmanuel Barraud
Chemical industry generated many pollutions. Credit: Flickr / pheochromocytoma / Creative Commons

An experiment conducted at the EPFL Environmental Biotechnology Laboratory has demonstrated the role played by micro-organisms in the degradation – without oxygen – of a very common pollutant: vinyl chloride.

The traces of human industrial activity persist deep inside the ground. And sometimes for a long time! In Switzerland alone, the Federal Office for the Environment records no less than 50,000 polluted sites, including 4000 that have been declared “contaminated” and require clean-up operations. This is a process that the Confederation is aiming to complete by 2025.

But some pollutants, such as chlorinated hydrocarbons (CHCs), prove to be particularly insidious. A study recently conducted at EPFL has just shown that a process that aims to “boost” the bacteria in the ground by injecting nutrients in order to accelerate the depollution of groundwater aquifers risked instead extending the lifespan of a particularly carcinogenic substance, vinyl chloride (or chloroethene).

This substance is derived from chlorinated solvents (such as perchlorethylene or trichlorethylene) that are still used today in a wide range of industrial procedures, from degreasing metal parts to the chemical cleaning of clothes. “These solvent types have been developed because they resist oxidation and are less dangerous to use than oil by-products because they do not burn”, explains Christof Holliger, Director of the EPFL Environmental Biotechnology Laboratory. “The problem is that they become much more harmful when they are not completely de-chlorinated”.

This is a side effect that went unnoticed until the 1980s. Previously there was no incentive for industry to exercise any particular caution in processing its chlorinated waste, which generally ended up in the sewage system or in the ground.

What happens to the poison?

It’s when the conversion of CHCs occurs without oxygen – in an anaerobic environment – that the situation can be most serious. Bacteria then remove the chlorines from the solvents absorbed by the ground, which then converts them, all being well, into ethylene, a non-harmful substance. However, the de-chlorination is often incomplete, leading to a build-up of vinyl chloride.

This carcinogenic substance makes the water, which it may contaminate, unfit for consumption. It’s then crucial to know precisely how it develops once it becomes enclosed in an aquifer.

This is the objective of the study undertaken by the EPFL Environmental Biotechnology Laboratory in partnership with the University of Neuchâtel. The scientists built some lab “columns” from which they extracted the air to replicate the conditions of a groundwater reserve. Then they circulated inside it a solution containing vinyl chloride and mineral salts, in one column enriched with acetate intended to “feed” the bacteria. Their results are published in the current issue of the Journal of Environmental Quality.

Do not feed the bacteria...

After several weeks, the quantities of vinyl chloride in the two columns began to diminish, and then disappeared completely after four months. Ethylene, a harmless by-product, was detected only in the column that contained acetate. But the concentration proved to be too low for all the vinyl chloride to have been reduced to this substance. “Our tests provide a good indication that this reduction process is not the only one involved in the of vinyl chloride. Another phenomenon is therefore at work, possibly an anaerobic oxidation towards another substance -- we do not yet know which”, Christof Holliger adds.

Concerning the difference observed between the two columns: “It shows that the processes said to be part of ‘improved natural reduction’, designed to ‘feed’ the bacteria so that they carry out a more effective ethylene reduction, in fact risk impairing the oxidation, leading ultimately to an increase in the duration of the presence of vinyl chloride”, he concludes.

This study will oblige engineers and government officials to revise the models they have been using until now to calculate the timescale for a de-contamination, by including from now on the anaerobic oxidation at work. It should also encourage them to take more care in using procedures designed to accelerate it, which may prove counterproductive. More time will be required than was anticipated to correct the errors of the past.

Explore further: US exports help Germany increase coal, pollution

Provided by Ecole Polytechnique Federale de Lausanne

5 /5 (3 votes)
add to favorites email to friend print save as pdf

Related Stories

Wintertime air chemistry to be studied

Mar 02, 2011

NOAA scientists and their colleagues from Boulder, Colo., and across the country have gathered in Erie, Colo., for a month-long study of the chemistry of the wintertime atmosphere, which they hope will shed ...

Groundwater threat to rivers worse than suspected

Nov 02, 2010

Excessive groundwater development represents a greater threat to nearby rivers and streams during dry periods (low flows) than previously thought, according to research released today by CSIRO.

Modern society made up of all types

Nov 04, 2010

Modern society has an intense interest in classifying people into ‘types’, according to a University of Melbourne Cultural Historian, leading to potentially catastrophic life-changing outcomes for those typed – ...

When robots learn from our mistakes

May 26, 2011

(PhysOrg.com) -- Robots typically acquire new capacities by imitation. Now, EPFL scientists are doing the inverse -- developing machines that can learn more rapidly and outperform humans by starting from failed ...

Recommended for you

Pacific summit to urge action on climate change

13 minutes ago

Pacific island leaders will renew calls for meaningful action on climate change at a regional summit opening in Palau on Tuesday, amid fears rising seas will swamp their low-lying nations.

US plans widespread seismic testing of sea floor

Jul 26, 2014

(AP)—The U.S. government is planning to use sound blasting to conduct research on the ocean floor along most of the East Coast, using technology similar to that which led to a court battle by environmentalists in New Jersey.

Fire ecology manipulation by California native cultures

Jul 26, 2014

Before the colonial era, 100,000s of people lived on the land now called California, and many of their cultures manipulated fire to control the availability of plants they used for food, fuel, tools, and ritual. Contemporary ...

User comments : 3

Adjust slider to filter visible comments by rank

Display comments: newest first

that_guy
3 / 5 (1) May 27, 2011
There's no better way for them to describe this than "anaerobic oxidation"?

The reaction with oxygen without oxygen.

Do they mean:
oxidation without air?
Chemical decomposition without oxygen?
biological oxidation?

Did anyone once point out that the term they were using was an oxymoron
hush1
4 / 5 (2) May 27, 2011
lol

"What we tried, did not work."
You need Pysorg and 678 words to say this.

Yes to your last question. You self addressed it.
antialias
5 / 5 (3) May 28, 2011
Did anyone once point out that the term they were using was an oxymoron

Anaerobic oxidation is just an oxidation process where the oxidizyng agents do not stem out of the air (e.g. if oxygen is that agent then it was first bound up in another molecule and needed to be liberated/transferred to the one which is the 'oxidated')

Oxidation does not necessarily mean that oxygen is involved. A chemical reaction that results in an increase of the oxidation number (which mostly means loss of electrons) is called an oxidation. Besides Oxygen such elemnts as Fluorine, Bromine and Clorine can also used as oxidysers.

Head over to wikipedia and type in "anaerobic oxidation" (and for further reading: "oxidation")