leaching hexavalent chromium from concrete debris

[sam74]

I thought maybe someone might have some suggestions or ideas. I'm involved in the demolition of a 60 year old building with a 32 acre footprint. It had two stories. It came down rather quickly (less than 4 months)and there is plenty of debris on the first floor pad. Lots of steel and concrete from the second floor and concrete encased beams.

The landfill where we are shipping this is not set up to process waste water runoff and apparently hexavalent chromium is collecting in their detention ponds as stormwater runoff is leaching it from our concrete debris. So they have stopped us from shipping any more concrete. We had stormwater outfalls at the site tested and they are showing elevated levels of hexavalent chromium and we could be looking at a NPDES violation for polluting waterways.

We are researching ways to treat this onsite so we can resume landfill shipments. I found this article on the internet

http://news.sciencemag.org/sciencenow/2007/03/29-03.html

Anyone have any ideas. We are thinking of creating cells by berming the concrete three or four feet high to create moats that we could fill with water and an additive to reduce the hexavalent chromium to trivalent chromium. What other additives could we use?

 

[bimr]

The method that you are considering will not leach the chromium from the debris; it will precipitate the chromium into an insoluble form. The chromium will still be in the debris, just in a different form.

I would not expect the sugar concept to work in real life. It will be very difficult to mix the materials into a concrete debris pile and the concrete itself. The method has been proposed for anaerobic soil treatment.

You might consider segregating the pieces of concrete that have been exposed to chromium and treating this exposed debris separately. It would seem that the chromium contamination would be from spillage on the floor or possibly from a tank wall.

You may be better off spraying the debris with iron sulfide and crushing the concrete. Iron sulfide will also precipitate the chromium, making the chromium less likely to leach. You may be able to pass the TLCP test because of the high pH of the concrete in the debris.

 

[moltenmetal]

This all depends on whether you're concerned only about the leachate, or whether the measured chromium content of the concrete afterward is of concern.

From a toxicological standpoint, only the Cr+6 is a problem, but regulatory bodies often don't make this distinction. They will often still be concerned about the remaining Cr content, even if it is all Cr+3 and no longer leaches.

Reducing Cr+6 (which is an oxidant) to Cr+3 can be done with any number of reducing reagents. Making Cr go away obviously gets HARDER (i.e. next to impossible) if your reduce it first.

 

[sam74]

The Cr(VI) occurs naturally in concrete according to my research. We just exposed a great deal at one time. The great majority has been demolished to a state that looks similar to soil.

We are concerned with the leachate on site due to possible NPDES violations.

I believe at the land fill they are pumping the pond water through steel wool and have placed some in the pond itself I'm guessing to reduce to Cr(III). But they are not set up to handle waste water and have currently refused anymore shipments of concrete from us.

So we are also concerned with reducing the Cr(VI) to Cr(III) on site so we may begin shipments again.

Can you propose any methods of on-site mixing of reagents with our concrete debris in a dry or wet form?

 

[bimr]

The first step is to have a TCLP prepared. That will tell you wether you have a problem or not.

However, ferrous sulfate is commonly used to reduce hexavalent chromium to trivalent chromium. You would lhave to use liquid ferrous sulfate in order to react with the chromiun.

You probably will need to do some lab and pilot tests.

http://onlinelibrary.wiley.com/doi/10.1111/j.1600-0536.1993.tb03403.x/abstract

http://www.mpa.uni-stuttgart.de/publikationen/otto_graf_journal/ogj_1996/beitrag_laskowski.pdf

http://www.rmc-foundation.org/images/Hex Chrom Executive Summary.pdf

http://pubs.acs.org/doi/abs/10.1021/es9402967

 

[ekolog16]

I've used calcium polysulfide before for hexavalent chromium reduction in water. The reduction works very well by mixing it at about 5% solution. Dealing with the hydrogen sulfide generated made things difficult however. You can consider using ferrous sulfate as bimr talks about or a safer reducing agent. Definitely conduct batch tests before you purchase your reducing agents. The book: Chromium (VI) Handbook (2005) by Guertin, J., Jacobs, J., and Avakian, C. from CRC Press might be helpful.

Are the quantities of chromium (VI) contaminated concrete too high to use a hazardous waste landfill?

The EPA has some information about a Superfund site with similar issues.

http://yosemite.epa.gov/r10/cleanup...4bb0780587882565280056f5d1!OpenDocument#facts

They used a haz waste landfill for the debris but mixed the onsite soil with a reducing agent and then solidified with cement.

Are you sure all of the concrete contains high Cr(VI) concentrations? I agree with bimr on the source of the Cr(VI) in the concrete. Elevated concentrations are usually where chromic acid was used, such as plating rooms. You might be able to save some money if you can segregate the clean from the contaminated.

 

[sam74]

There has been talks of using sodium bisulfite injection contraptions in three catchbasins where all/most of site catchbasins eventually drain to. Initail estimated cost of about $25000 each installed.

We are using a hazardous waste landfill but they were not set up to treat contaminated runoff. Although they were forced to set something up to treat what they captured recently.

I'm fairly sure that it is just leaching from the concrete debris because it is in a soil like state and there is so much of it exposed to the elements. I suppose there are other possible explanations that I am unaware of. The building was a former gasous diffusion urainium processing plant that has all equipment removed and was for the most part completely decontaminated.

 

[moltenmetal]

I sincerely doubt that you'd see enough Cr+6 to be of a regulatory concern, leaching out of crushed concrete that was not contaminated with Cr+6 in the first place.

You can TRY to completely decontaminate something to your heart's content, but a water-soluble species like that is going to end up where it chooses to go.

 

[StoneCold]

Maybe you should make it back into large concrete blocks and sent that to the haz waste landfill?

Regards
Stonecold

 

[chicopee]

A little bit late to ask a question but I'll try anyway. What were the previous uses of that 60 year old building?

 

[sam74]

From four posts above

The building was a former gaseous diffusion urainium processing plant that has all equipment removed and was for the most part completely decontaminated.

 

[moltenmetal]

You're putting far too much faith in this term "completely decontaminated". If Cr+6 was used, and spilled, no surficial decontamination activity would be effective at removing it all. Your demolition of the concrete in a sense IS an effective decontamination process for this material.

The question is, can you merely convert it back to Cr+3 and leave a non leachate toxic material behind, or will the remaining insoluble Cr STILL leave the material in violation of the regs? If the former, reducing agents properly applied may do the job for you. If the latter, hazardous waste landfilling is probably the only option you have available to you, unless there's a risk assessment approach that can be used to get around the regulatory requirement on Cr.