Mineralogy of permeable reactive barriers for the attenuation of subsurface contaminants

Permeable reactive barriers (PRBs) are a relatively recent development of a passive system to remediate subsurface waters containing organic or inorganic contaminants. Groundwater flow under a natural gradient passes through a permeable curtain of treatment medium that either precipitates the contaminants as relatively insoluble compounds or transforms the contaminants into environmentally acceptable or benign species. The most widely adopted treatment medium is submillimetrie zero-valent iron. a substance that is highly reactive, environmentally acceptable. and is readily available as a in anti factured product derived from the recycling of scrap iron and steel. Organic compost wastes have also been used to ameliorate inorganic contaminants, and two case studies of the utilization of composts to reduce sulfate and precipitate metals are presented, primarily from a mineralogical perspective. In cores of the reacted treatment media. the most abundant secondary product formed in situ is Fe oxyhydroxide, but a variety of precipitates has been identified. For example, secondary pyrite. greigite, and native nickel are present at a site at which replacement of organic material by sulfides is common. At all industrial site. secondary pyrite. covellite. chalcopyrite, and bornite have formed in the treatment medium. and whereas replacement of organic material by Fe oxyhydroxides is widespread, replacement by Sulfides is rare. The secondary Sulfides and metals are volumetrically small and are unlikely to impede the permeability of the treatment medium. bill the formation of Fe oxyhydroxides and secondary carbonates in the presence of zero-valent iron requires further monitoring to determine whether the secondary precipitates and the consumption of Fe-0 will appreciably lessen the effectiveness of such PRBs over the long term. Current indications are that PRBs are both an environmentally effective and I cost-effective technique of remediation.