Abstract
Solid iron corrosion products (FeCPs), continuously generated from iron corrosion in Fe0-based permeable reactive barriers (PRB) at pH > 4.5, can lead to significant porosity loss and possibility of system’s failure. To avoid such failure and to estimate the long-term performance of PRBs, reliable models are required. In this study, a mathematical model is presented to describe the porosity change of a hypothetical Fe0-based PRB through-flowed by deionized water. The porosity loss is solely caused by iron corrosion process. The new model is based on Faraday’s Law and considers the iron surface passivation. Experimental results from literature were used to calibrate the parameters of the model. The derived iron corrosion rates (2.60 mmol/(kg day), 2.07 mmol/(kg day) and 1.77 mmol/(kg day)) are significantly larger than the corrosion rate used in previous modeling studies (0.4 mmol/(kg day)). This suggests that the previous models have underestimated the impact of in-situ generated FeCPs on the porosity loss. The model results show that the assumptions for the iron corrosion rates on basis of a first-order dependency on iron surface area are only valid when no iron surface passivation is considered. The simulations demonstrate that volume-expansion by Fe0 corrosion products alone can cause a great extent of porosity loss and suggests careful evaluation of the iron corrosion process in individual Fe0-based PRB.
Highlights
Solid iron corrosion products (FeCPs), continuously generated from iron corrosion in Fe0-based permeable reactive barriers (PRB) at pH > 4.5, can lead to significant porosity loss and possibility of system’s failure
The performance of F e0-based PRBs are generally satisfactory, questions remain on the long-term effectiveness of PRBs, which are expected to operate for d ecades[18,19,20]
It is assumed that only the volumetric expansive corrosion of iron contributes to the porosity loss of the system
Summary
Since this study considers the condition that iron corrodes in DI water, the porosity change data for 100% F e0 column reacted with DI water were used to calibrate the parameters of the model These parameters were used to simulate the porosity loss in the systems with different iron mixing ratios. The simulation with higher coefficient of passivation (n) shows a more rapid porosity loss in the beginning phase but only slight porosity change after long-term corrosion This indicates that the rate of diffusion process decreases with the increase of the thickness of the generated corrosion products. According to simulation results, when the porosity value of the simulation with constant corrosion rate (in mm/year) reaches 0, there is still 0.09 m 3/m3 Fe0 volume fraction left in the system It means the PRB system loses its capability to remove contaminants before the iron is completely consumed. Porosity and Fe0 volume fraction versus time assuming goethite as reaction product
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