Modelling long-term hydraulic conductivity behaviour of zero valent iron column tests for permeable reactive barrier design

Abstract

In this paper, a numerical model to simulate the hydraulic conductivity reduction observed during long-term laboratory column tests is proposed. The column tests are carried out to study dissolved heavy metals removal by using granular zero valent iron (ZVI). The proposed model is also used to analyse the main causes of hydraulic conductivity reduction observed during laboratory column tests. Expansive iron corrosion, precipitation of reaction products, and gas formation are the processes considered in the proposed model. Numerical simulation results show that to reproduce hydraulic behaviour of the experimental systems, the change of pores geometry due to expansive iron corrosion and precipitation of reaction products, which determines a possible stoppage of gas bubbles, should be considered. Furthermore, model results show that only a small percentage of the iron available is corroded during the tests (from 0.4% to 1.9%). According to the model, the average diameter of gas bubbles that better fit the experimental results varies between 0.16 and 0.19 mm. While assuming gas absence (or its possible escape), higher values of iron corrosion rate should be considered to fit the experimental results.

Predicting the Hydraulic Conductivity of Metallic Iron Filters: Modeling Gone Astray

“…The approach conventionally used in the Fe 0 water filtration literature [16,19,[27][28][29][30] is not derived from Equation (10) and related simplifications. Moreover, the presentation above reveals that chemical reactions yielding contaminants reductive transformation in Fe 0 /H 2 O systems (Equations (3) and (4)) are included in just one of seven processes making up the dynamics of oxide scale formation and transformation.…”

“…Conventionally, numerical models are used to predict the long-term behavior of Fe 0 filters (see [19] and the references cited therein). Data for such models should be derived from well-designed laboratory column experiments performed under operational conditions (e.g., experimental duration, flow velocity, temperature and water chemistry) mimicking as closely as possible those expected in the field.…”

“…However, such studies currently provide at best information for understanding the early service life of Fe 0 filters (some months). In fact, experiments lasting for more than one year are very scarce [19,62,63], while it is not established that k iron determined under such conditions would describe the behavior of Fe 0 filters for 10 or 15 years. Moreover, according to Alamilla et al [51], it is even not likely that this will be the case because of the well-documented non-linearity of the kinetics of iron corrosion [64] …”

“…A reduction reaction implying electrons transfer from Fe 0 to the contaminants (e.g., RCl = chlorinated hydrocarbon, cf. Equation (1)) can be at best a side reaction under environmental conditions [13][14][15][16][17][18][19]. …”

“…Alternative strategies have been discussed in [16,19,[27][28][29][30]. These alternative strategies are based on Equation (1) and mentioned herein for the sake of completeness.…”