Successful Remediaiton Case for GW with Chromium IV

[NRCatBNA]

Is there any successful case of remediation project for groundwater contamination with metals primary with chromium (Cr VI)?

Thanks for all feed back!
nschuang@itri.org.tw

 

[kenvlach]

Yes.

Both ’pump and treat’ and in situ methods are in use at Superfund and other sites.
’Pump and treat’ methods utilize the same procedures as the metal finishing industry uses for Cr⁺⁶ wastewaters: Ion exchange, chemical reduction, and electrochemical reduction (e.g., electrocoagulation), plus precipitation of Cr⁺³ hydroxide. Not sure whether electrocoagulation is actually in use at any major site, but have seen it demonstrated, and it has the advantage of simultaneously treating other metals and organics.

In situ methods can only be used if it is considered sufficient to convert the Cr⁺⁶ to the less hazardous Cr⁺³, which has a solubility < 1 ppm at pH 6.5-8.5.

http://www.finishing.com/Library/metalpH.html

Either a PBR Permeable Reactive Barrier or injection of a sodium hydrosulfite (dithionite) solution can be used. See ‘Metals and Inorganics’ at

http://www.rtdf.org/public/permbarr/prbsumms/

The Hanford listing describes the use of rows of dithionite injection wells to create a chemical barrier.
I haven’t reviewed all of the listings, but one PRB site in Denmark utilizing Fe⁰ as the chemical reducing agent is being re-worked due to exhaustion of the Fe⁰ in areas of higher than expected Cr⁺⁶ concentrations. Some PBR designs allow for reactant replenishment, and of course it is not a problem when using a virtual PBR (series of injections wells, as at Hanford).

 

[ripsnub]

I was involved in a large scale pump and treat remediation system at an EPA Superfund site remediating hex chrome (Cr⁺⁶) contaminated groundwater. Process used initial influent pH adjustment (6.5 down to 5.0) with sulfuric acid followed by bag filtration and finally ion exchange. P&T has been very successful at this site, with removal efficiency near 99.9%. Hex chrome average influent concentrations were between 50 and 100 mg/l at the beginning of the project and now are generally below 0.5 mg/l. System operating flow rates have ranged from 80 to 150 gpm. We also conducted pilot scale testing using insitu reduction with some success. Regenesis HRC and HRC-X is also shown to be an affective approach to insitu chromium treatment. Regenesis will soon be releasing a new product specifically for metals remediation as they are in the pilot test stage. As mentioned in the response above, zero valent iron (Fe⁰) type passive reactive barriers are also known technologies for chromium remediation. Some agencies may not allow the types of insitu chemical reduction because the source is still in the ground, albeit in the reduced Cr⁺³ form, agencies typcially stating that it could be resoluablized if groundwater conditions changed dramatically, although highly unlikely.

good luck.