Effect of oxidant dosage on integrated electrochemical remediation of contaminant mixtures in soils

Abstract

Many sites are contaminated with contaminant mixtures, commonly heavy metals and polycyclic aromatic hydrocarbons (PAHs), which pose a great challenge for remediation. The objective of this research was to investigate coupled Fenton-like oxidation and electrokinetic remediation of low permeability soils contaminated with both heavy metals and PAHs. This remediation process aims at simultaneous oxidation of organic contaminants and removal of heavy metals. Fenton's reagent, consisting of hydrogen peroxide (H₂O₂) and native iron catalyst, is utilized for chemical oxidation. Laboratory batch and electrokinetic experiments were performed on kaolin (a low permeability soil) spiked with nickel and phenanthrene each at a concentration of 500 mg/kg of dry soil to represent typical heavy metal and PAH contaminants found at contaminated sites. Experiments were conducted using H₂O₂ solution in 5%, 10%, 20% and 30% concentrations and also using deionized (DI) water as control. For electrokinetic experiments, a voltage gradient of 1 VDC/cm was applied and H₂O₂ solution was introduced at the anode for a total duration of four weeks. Batch tests showed that phenanthrene oxidation increases from 76% to 87% when the H₂O₂ concentration increases from 5% to 30%. The electrokinetic experiments showed substantial electroosmotic flow in all the tests. Approximately one pore volume of flow was generated in the DI baseline test and about 1.2-1.6 pore volumes were generated in case of H₂O₂ tests. Phenanthrene was partially oxidized in the H₂O₂ tests and its removal from the soil was insignificant. Oxidation of phenanthrene increased with increasing concentration of H₂O₂; a maximum of 56% oxidation was observed with 30% H₂O₂. Nickel migrated from anode to cathode. This migration was more pronounced in the H₂O₂ tests as compared to the DI baseline test. Nickel precipitated in all the tests near the cathode due to high pH conditions. These results emphasize that the optimization of H₂O₂/catalyst concentration and voltage gradient as well as the control of soil pH are required to increase the removal of nickel and the oxidation of phenanthrene.

Keywords:

• Electrokinetic remediation
• chemical oxidation
• soil
• clays
• mixed contamination
• heavy metals
• organic contaminants

Notes

*All the tests were performed in duplicates (total number of tests = 10).