In situ chemical oxidation (ISCO) is generally considered to be an effective remedial option for chlorinated solvent sites, and is widely used by the DoD and remediation practitioners. However, the success of ISCO and in situ technologies in general can be limited at low permeability (low-K) and/or high heterogeneity sites, leaving only expensive options such as thermal treatment. The electrokinetically delivered, thermally-activated persulfate (EK-TAP) technology can potentially overcome the respective limitations of ISCO and thermal approaches, by improving persulfate delivery and activation in low-K and heterogeneous materials, while avoiding the high energy and vapor treatment costs associated with traditional thermal remedies. This demonstration project was conducted at Naval Air Station Jacksonville, Florida to assess and validate the performance of an electrokinetic technique to promote uniform and effective distribution of persulfate in low-K and heterogeneous subsurface materials, for the purposes of improving site remediation at low-K sites.
The EK-TAP technology consists of two main components: i) delivery of persulfate through low-K and heterogeneous soils using direct current (DC); followed by ii) heat activation of the persulfate, by raising the temperature of the soil and pore water by electrical resistance heating (ERH) using alternating current (AC).
A phased testing approach was planned for the demonstration, but unfortunately, due to budget constraints, only the technical objectives associated with the first phase of testing (i.e., Phase 1 dipole test to distribute persulfate within the clay unit at the Site) were assessed. These objectives were:
Each of the technical objectives were achieved, and the demonstration showed that EK can achieve relatively uniform transport of persulfate in low-K materials, which is a critical and distinct advantage of the EK technology over other conventional advective flow-based approaches. EK-enhanced delivery is a safe and relatively more controllable approach compared to high-pressure/fracturing injection and thermal approaches, and the EK technology also represents a remedial alternative with excellent environmental performance.
The duration of the Phase 1 dipole test ran for several months longer than anticipated due to a disruption in the supply of the potassium bicarbonate pH buffer, which impacted system uptime. However, once the supply of this pH buffer was restored, system uptime recovered and was maintained through the end of the test.
Based on the information and experience obtained from this demonstration, there are three main cost drivers to consider when evaluating implementation costs in future projects, including: (i) footprint, depth interval, and volume of target treatment zone and contaminant mass; (ii) presence and location of above-ground and subsurface utilities; and (iii) site geochemistry, particularly pH and iron. A cost comparison was developed and showed that EK-TAP can be potentially more cost favorable than electrical resistive heating, and that the EK-TAP approach is slightly more cost favorable than direct-injection in situ chemical oxidation (ISCO) and fracturing enhanced zero-valent iron direct injection. Thus, at sites where low-K material and/or a high-degree of heterogeneity likely preclude the consideration for direct injection, EK-TAP provides a cost-effective solution for implementing ISCO using persulfate.
When considering the use of EK-TAP at a site, additional attention may be required concerning electrical safety, elevated concentrations of iron in the treatment zone, corrosion of wetted metallic components, potential regulatory limitations for pH control amendments, cathodic protection measures when implementing the technology near “sensitive” utilities, and informing local and facility departments about the proposed remedy.