Modelling the porosity loss in granular iron based permeable reactive barriers

Abstract

The quality of groundwater resources globally has been under serious threat due to their exposure to a broad spectrum of anthropogenic pollutants. Permeable reactive barriers (PRBs) are an innovative technology being used for in-situ remediation of polluted groundwater for the past three decades. Metallic iron (Fe0) has been presented as the most efficient reactive medium for PRBs, and Fe0-PRBs can eliminate a large variety of both inorganic and organic compounds from aqueous solutions. Although the performance of installed Fe0-PRBs (using granular Fe0) has been generally satisfactory, there is still uncertainty on how to properly estimate their service life. The long-term porosity loss of a Fe0-PRB is a key factor to determine its service life or its long-term effectiveness. To date, efforts to characterize the long-term porosity loss of Fe0-PRBs have paid little attention to the inherent porosity loss due to the volumetric expansive nature of iron corrosion. The present work is the first attempt to root the estimation of the service life of Fe0-PRBs on the inherent characteristics of Fe0 and its corrosion products. This study presents a review of the Fe0-PRBs literature that reports the porosity loss based on field reports, laboratory column tests, and numerical model studies. Data on reported porosity loss, their estimation methods, and the corresponding geochemical conditions are summarized and analysed. A new mathematical model based on Faraday’s Law is established to describe the porosity change caused by iron corrosion products (FeCPs) in a hypothetical Fe0-based PRB through-flowed by deionized water. Moreover, a three-dimensional (3-D) numerical groundwater flow and transport model of a Fe0-PRB was developed to assess how porosity heterogeneity of the barrier medium may affect groundwater flow over time and influence the long-term effectiveness. A 3-D high resolution aquifer outcrop analogue was utilized to implement aquifer heterogeneity. Contaminant plume migration and groundwater residence time were investigated to evaluate the treatment performance of the PRB. The literature review reveals that the current estimation methods for porosity loss of Fe0-PRBs, which are based on core sample studies and stoichiometric calculations, may significantly underestimate the effect of iron corrosion products. In addition, the Darcy flux has the strongest positive correlation with the long-term porosity loss. The heterogeneity within the aquifer and the barrier should be well studied. Iron corrosion rates derived from the Faraday’s law based mathematical model are up to 7 times larger than the corrosion rate used in previous modeling studies. This suggests that the previous models have underestimated the impact of in-situ generated FeCPs on the porosity loss. The model simulations demonstrate that volume-expansion by Fe0 corrosion products alone can cause to a great extent porosity loss and emphasizes the need for a careful evaluation of the iron corrosion process in individual Fe0-based PRB. The findings of the 3-D model simulation demonstrate that the heterogeneity of porosity reduction of the barrier medium is an important factor in estimating the long-term performance of a continuous-wall Fe0-PRB. Ignoring the porosity heterogeneity of the barrier medium leads to an underestimation of the by-passing flow by 30%-41% in a ten-year simulation, and of contaminant plume spread over time. The overall results of this work provide an important contribution and give practical implications for the future design of Fe0-PRBs. This study developed a new modeling approach to describe the effect of generated iron corrosion products on long-term porosity loss of the PRB system, and a comprehensive 3-D model to simulate the groundwater flow and to assess the long-term effectiveness of the Fe0-PRB. The thesis highlights the potential impact of volume-expansion by Fe0 corrosion products, and the porosity heterogeneity of the barrier medium on the longevity estimation of Fe0-PRBs.