Predicting the Precipitation of Mineral Phases in Permeable Reactive Barriers

Abstract

The working lifetime of permeable reactive barriers (PRBs) using Fe 0 as the reactive media is limited by precipitation of secondary minerals, due to reaction of groundwater with Fe 0 . Since PRBs are emplaced at sites with widely differing groundwater chemistry, the suite of minerals that precipitate, as well as the rate of their formation, can vary widely. Using plausible phases obtained from field PRBs, the study shows that chemical equilibrium modeling can correctly predict the amounts of precipitates formed, based on the thermodynamic properties of Fe 0 and groundwater constituents. These predictions were compared to the results from the solid phase analysis from a field column experiment and from a field-installed PRB at Y-12 Plant, Oak Ridge, TN. Using the column chemical data molar distributions of the precipitates along the flow path were modeled. The maximum precipitation at the Fe 0 -sand interface at the influent end was predicted, where pore water showed high saturation index (SI) with respect to calcite and iron (oxyhydr)oxide. In the absence of flow information, the field sampling data were used to construct an SI-pH diagram, from which the extent of reaction with Fe 0 , the potential for precipitate buildup, and relative residence time for the pore water were identified. Kinetic and heterogeneous flow effects were also discussed. To illustrate the application of chemical equilibrium modeling to the design and planning phase of PRBs, groundwater data from four PRB sites were analyzed. The analysis shows that up to 0.63 cm 3 /L solid could form in pore water using an average Fe 0 dissolution rate, leading to severe clogging of Fe 0 medium over a 10-yr period of operation.

Performance of a PRB for the Remediation of Acidic Groundwater in Acid Sulfate Soil Terrain

“…Several researchers (Karvonen 2004;Komnitsas et al 2007;Li and Benson 2005;Liang et al 2003;McMahon et al 1999;Puls et al 1999a;Sarr 2001;Vogan et al 1999;Yabusaki 2001) have reported that armouring on the surface of reactive materials and chemical clogging of pores, by precipitated compounds during chemical reactions inside the PRB, decrease the long-term performance parameters such as reactivity, porosity and permeability. Significant concerns relating to precipitation of iron and aluminium oxides and hydroxides also exist in alkaline PRBs with acidic groundwater in ASS terrain because their solubilities are pH-dependent.…”

Calcite precipitation dominates the electrical signatures of zero valent iron columns under simulated field conditions

“…Although encouraging treatment efficiency has been documented (Gu et al, 1998;Morrison et al, 2001;Su and Puls, 2003), reactions between ZVI and groundwater constituents causing ZVI corrosion and subsequent secondary mineral precipitation contribute to long term performance reduction of ZVI PRBs (Liang et al, 2003;Slater and Binley, 2003;Wilkin et al, 2003;Jin Suk et al, 2009). Carbonate minerals (often precipitated concurrent with iron oxides (Mackenzie et al, 1999;Phillips et al, 2000;Liang et al, 2003)), are known to significantly impact ZVI reactivity and hydraulic efficiency in systems with frequently encountered high carbonate groundwater conditions through surface passivation and pore clogging (Agrawal and Tratnyek, 1996;Phillips et al, 2000;Liang et al, 2003;Slater and Binley, 2003;Wilkin et al, 2003;Jeen et al, 2006;Jeen et al, 2007;Jin Suk et al, 2009). …”

“…iron oxides or calcite) on the electrical responses. Yet PRB precipitate mineralogy is normally complex, being dependent on the groundwater chemistry at the site, in many cases including both iron oxides and calcite (Furukawa et al, 2002;Morrison et al, 2002;Liang et al, 2003;Phillips et al, 2003;Liang et al, 2005 and electronic (iron metal) conductors causing decreases in electrical conduction and polarization and, presumably, in surface reactivity.…”

Permeable Reactive Barrier (PRB) Technology: An Innovative Solution for the Remediation of Acidic Groundwater from Acid Sulphate Soil (ASS) Terrain

“…Indraratna and Vafai (1997)). However, there are limited studies on the long-term performance of PRBs, especially with respect to chemical and/or biological clogging (Liang et al, 2003). Some researchers (e.g.…”