Fenton-Like Oxidation of Polycyclic Aromatic Hydrocarbons in Soils Using Electrokinetics

An integrated electrochemical oxidation process that utilizes electrokinetics (EK) to deliver the oxidant (5–10% hydrogen peroxide, H2O2) and chelant 40 mM of ethylenediaminetetraacetic acid (EDTA) or diethylenetriaminepentaacetic acid (DTPA)] or iron chelate (1.4 mM Fe-EDTA or Fe-DTPA) to oxidize polycyclic aromatic hydrocarbons (PAHs) in soils was investigated. Batch and bench-scale EK experiments were conducted using: (a) kaolin, a low permeability clayey soil, spiked with phenanthrene at 500 mg/kg, and (b) former manufactured gas plant (MGP) soil, a high buffering silty soil, contaminated by a variety of PAHs (1493 mg/kg). Batch experiments showed that chelant solutions dissolve native iron minerals to form soluble Fe-chelates that remain available even at higher pH conditions of soil for the Fenton-like oxidation of the PAHs. In EK experiments, a 5–10% H2O2 solution was delivered from the anode and a chelant solution or iron-chelate was delivered from the cathode. Preflushing of soil with 5% ethanol and ferrous sulfate (1.4 mM) prior to oxidant delivery was also investigated. An electric potential of 2 VDC/cm was applied in all tests to induce electroosmotic flow for 5–8 days for kaolin and 25 days for the MGP field soil. In the absence of any chelating agent, phenanthrene oxidation was catalyzed by native iron present in kaolin soil, and 49.8–82.3% of phenanthrene was oxidized by increasing H2O2 concentration from 5–10%. At 5% H2O2 concentration, phenanthrene oxidation was not increased by using 40 mM EDTA, 40 mM DTPA or 1.4 mM Fe-DTPA, but it increased to 70% using 1.4 mM Fe-EDTA. Maximum phenanthrene oxidation (90.5%) was observed by 5% ethanol preflushing and then treating with 5% H2O2 at the anode and 1.4 mM Fe-EDTA at the cathode. However, preflushing with 1.4 mM ferrous sulfate did not improve phenanthrene oxidation. The results with the MGP field soil indicated that delivery of 5% H2O2 alone resulted in oxidation of 39.8% of total PAHs (especially 2- and 3-ring PAHs). The use of EDTA and Fe-EDTA did not increase PAHs oxidation in this soil. Overall, the results reveal that an optimized in situ combined technology of EK and Fenton-like process has the potential to oxidize PAHs in low permeability and/or high buffering soils.