Mineralogy of Permeable Reactive Barriers for the Attenuation of Subsurface Contaminants

Abstract

Permeable reactive barriers (PRBs) are a relatively recent development of a passive system to remediate subsurface waters containing organic or inorganic contaminants. Groundwater fl ow 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 submillimetric zero-valent iron, a substance that is highly reactive, environmentally acceptable, and is readily available as a manufactured 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 identifi ed. For example, secondary pyrite, greigite, and native nickel are present at a site at which replacement of organic material by sulfi des is common. At an 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 sulfi des is rare. The secondary sulfi des and metals are volumetrically small and are unlikely to impede the permeability of the treatment medium, but 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 a cost-effective technique of remediation.

A CRITICAL REVIEW ON THE PROCESS OF CONTAMINANT REMOVAL IN FE0–H2O SYSTEMS

“…The advent of readily available and low-cost elemental iron materials (Fe 0 materials) in water treatment (mostly groundwater remediation and wastewater treatment) has resulted in the use of materials from various origins [1][2][3][4][5]. Different classes of organic and inorganic compounds have been successfully removed from the aqueous phase in the presence of Fe 0 materials [1,[6][7][8][9][10][11][12][13][14][15].…”

“…Different classes of organic and inorganic compounds have been successfully removed from the aqueous phase in the presence of Fe 0 materials [1,[6][7][8][9][10][11][12][13][14][15]. The earliest detailed surveys involving contaminant removal by Fe 0 materials in laboratory batch systems raised speculation that the decontamination occurs through a reduction reaction at the surface of the metal [16][17][18][19][20].…”

Elemental Iron (Fe0) for Better Drinking Water in Rural Areas of Developing Countries

“…In cores of the reacted treatment media, the most abundant secondary product formed in situ is Fe oxyhydroxide (iron corrosion products -iron hydroxides and oxides), but a variety of precipitates has been identified. For example, secondary pyrite, greigite, cove llite, chalcopyrite, and bornite have formed in the treatment medium (Jambor et al 2005, Mackenzie et al 1999. The secondary sulfides are volumetrically small and are unlikely to impede the permeability of the treatment medium, but the formation of Fe oxyhydroxides and secondary carbonates in the presence of Fe 0 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.…”

“…The secondary sulfides are volumetrically small and are unlikely to impede the permeability of the treatment medium, but the formation of Fe oxyhydroxides and secondary carbonates in the presence of Fe 0 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 a cost-effective technique of remediation (Henderson and Demond 2007, Jambor et al 2005, Laine and Cheng 2007. A trend persists in the scientific literature terming iron PRBs as a reduction technology (Kim et al 2008, Laine andCheng 2007) although contaminant reduction has not been traceably demonstrated.…”

“…The long term feasibility of Fe 0 reactive barriers in the cleanup of contaminated groundwaters has been demonstrated in laboratory column studies and confirmed by field installations. Column studies and fields installations indicate that Fe 0 maintains its reactivity over long periods of time (Jambor et al 2005). Many forms of iron have been proposed for water treatment.…”