Electrokinetic generation of reactive iron-rich barriers in wet sediments: implications for contaminated land management

Abstract

Here we describe preliminary research into the in situ electrokinetic generation of continuous iron-rich precipitates to act as sub-surface barriers for the containment of contaminated sites. This is achieved using sacrificial iron electrodes emplaced either side of a soil/sediment mass to introduce iron into the system, and their dissolution and re-precipitation under the influence of an applied (DC) electric field. Continuous vertical and horizontal iron-rich bands (up to 2 cm thick) have been generated over a timescale of 300–500 h, at voltages of <5 V with an electrode separation of between 15 and 30 cm. The thickness of the iron-rich band increases as the applied voltage is increased. Geotechnical tests in sand indicate that the iron-rich band produced is practically impervious (coefficient of permeability of 10−9 ms⁻¹ or less), and has significant mechanical strength (unconfined compressive strength of 10.8 N mm⁻²). By monitoring the current, the integrity of the iron-rich band may be assessed, and by continued application of current, the barrier may ‘self heal’. The iron-rich barrier is composed of amorphous iron, goethite, lepidocrocite, maghemite and native iron

Electrokinetic remediation of plutonium-contaminated nuclear site wastes: Results from a pilot-scale on-site trial

“…Element remobilisation/redistribution was most apparent in this soil (Fig. 3a), with ITRAX data indicating: (a) supply of Fe to the anodic compartment of the cell (via electrode dissolution), with subsequent precipitation of Fe (as Fe oxides/oxyhdroxides [14,15]) across the anodic zone on experimental completion/cessation of applied voltage [14]; (b) mobilisation of Mn in the anodic compartment, followed by migration towards the cathode and precipitation at the interface between the acid and alkaline zones (where a black precipitate was visually observed); and (c) mobilisation of Ca and Sr from the anodic compartment, with subsequent migration towards the cathode and precipitation (most likely as carbonates) on encountering alkaline conditions in the cathodic compartment. These trends were confirmed by (a) repeat scanning of differently orientated soil sections via ITRAX, and (b) WD-XRF analysis (data not shown) on destructively sampled material.…”

“…1). This Fe-rich "wall" acts as a chemical and physical barrier to contaminant migration [14,15]. The contaminants which are remobilised by the acid or alkaline conditions generated by the technique are, depending on their physicochemical behaviour, either precipitated at (or around) the pH/Eh "jump" in the cell, sorbed onto the Fe (and Mn) mineral phases precipitated in the Fe-rich barrier, forced to migrate towards the appropriately charged electrode, or washed from the cell by electro-osmotic flow ( Fig.…”

Lab-scale feasibility tests for sediment treatment using different physico-chemical techniques

“…To provide a rough estimation for the order of magnitude of treatment costs, it can be considered that energy consumption during electrokinetics is heavily affected by the sediment salt content and can hardly be estimated lower than 500 kWh m −3 , but values up to about 2,000 kWh m −3 cannot be excluded ; the overall cost for electrokinetics has been roughly estimated in the range 150-1,150 USD m −3 (USEPA 2007), but the value is also strictly related to the local cost of energy. In order to reduce costs, low-tech and low-energy electrokinetic applications were investigated Faulkner et al 2005). As for persulfate-based chemical oxidation, which resulted in the highest PAH removal, using the oxidant/sediment and the activator/oxidant ratios adopted in the present study and assuming chemical prices as reported by Block and Cutler (2005), a cost of chemicals of about 800 USD m −3 for persulfate and 500 USD m −3 for ferrous sulfate would be estimated; assuming also a specific heat of sediment of 1 kJ kg −1 K −1 (DeLapp and LeBoeuf 2004) and 20% heat losses, the energy consumption for heating the sediment to 60°C would be estimated to be about 13 kWh m −3 , while it can be roughly evaluated that the mixing apparatus would consume about 110 kWh m −3 .…”

Review: Technical and Policy Challenges in Deep Vadose Zone Remediation of Metals and Radionuclides

“…Electrokinetic methods have been used to indurate subsurface materials for geotechnical applications and to develop low permeability ironrich bands. 137,138 Permeability control using these types of subsurface barriers have the potential to provide more targeted sequestration, lower maintenance, and greater longevity than surface barriers. However, evaluating their performance needs to consider their effectiveness as a function of overall permeability reduction, areal extent of the barrier, and integrity of the barrier, all of which can be difficult to evaluate in the subsurface.…”