Compatibility of chelators with permanganate for chelator-enhanced permanganate-based ISCO remediation of chlorinated ethenes DNAPLs

Abstract Permanganate-based ISCO (in situ chemical oxidation) has been commonly utilized to remediate chlorinated ethenes DNAPLs (dense non-aqueous phase liquids), but the treatment is seriously hindered by the formation of MnO2 rinds that encapsulate DNAPLs. We report herein a proactive approach to the MnO2 problem by chelating and stabilizing the Mn(III) species formed in situ, the precursors of MnO2. To identify suitable chelators for chelator-enhanced permanganate-based ISCO, chemical compatibility of sixteen representative commercial organic and inorganic chelators with permanganate was first investigated. Organic chelators generally induced rapid consumption of permanganate and concurrent severe MnO2 precipitation, when mixed with permanganate. In contrast, polyphosphate-based inorganic chelators showed high compatibility due to the redox-inert nature of the chelators. Among all chelators tested, only pyrophosphoric acid (PPA) could effectively stabilize the Mn(III) species and mitigate the generation and precipitation of MnO2 when permanganate was exposed to trichloroethene (TCE, a model compound of chlorinated ethenes) liquid. Further studies show that mitigation of MnO2 precipitation by PPA significantly improved permanganate oxidation of TCE DNAPL in the water/DNAPL biphasic system and in the sand column. Overall, the study indicates the potential of chelator-enhanced permanganate-based ISCO for the remediation of chlorinated ethenes DNAPLs.