Mechanism for the destruction of carbon tetrachloride and chloroform DNAPLs by modified Fentonʹs reagent

The destruction of a carbon tetrachloride DNAPL and a chloroform DNAPL was investigated in reactions containing 0.5 mL of DNAPL and a solution of modified Fentonʹs reagent (2 M H2O2 and 5 mM iron(III)-chelate). Carbon tetrachloride and chloroform masses were followed in the DNAPLs, the aqueous phases, and the off gasses. In addition, the rate of DNAPL destruction was compared to the rate of gas-purge dissolution. Carbon tetrachloride DNAPLs were rapidly destroyed by modified Fentonʹs reagent at 6.5 times the rate of gas purge dissolution, with 74% of the DNAPL destroyed within 24 h. Use of reactions in which a single reactive oxygen species (hydroxyl radical, hydroperoxide anion, or superoxide radical anion) was generated showed that superoxide is the reactive species in modified Fentonʹs reagent responsible for carbon tetrachloride DNAPL destruction. Chloroform DNAPLs were also destroyed by modified Fentonʹs reagent, but at a rate slower than the rate of gas purge dissolution. Reactions generating a single reactive oxygen species demonstrated that chloroform destruction was the result of both superoxide and hydroxyl radical activity. Such a mechanism of chloroform DNAPL destruction is in agreement with the slow but relatively equal reactivity of chloroform with both superoxide and hydroxyl radical. The results of this research demonstrate that modified Fentonʹs reagent can rapidly and effectively destroy DNAPLs of contaminants characterized by minimal reactivity with hydroxyl radical, and should receive more consideration as a DNAPL cleanup technology.