Chapter 11. Breakdown of organic contaminants in soil by manganese oxides: a short review
by K.L. Johnson, C.M. McCann and C.E. Clarke
Manganese oxides are natural, ubiquitous and important components of the soil environment. Much scientific research has focused upon their interactions with inorganic contaminants in soil. Yet there is a large body of literature that shows clearly that Mn oxides are powerful oxidants of numerous organic compounds. It is widely suspected that Mn oxides are inherently involved in the soils’ natural defence mechanism against such contaminants. With the number of sites being classified as organically contaminated on the increase, and a surge in emerging organic contaminants, exogenous Mn oxide addition may provide a potential remediation option. This chapter reviews the reactions between synthetic and natural Mn oxides with three key classes of organic contaminants: phenols, N-containing compounds and polycyclic aromatic hydrocarbons (PAHs). Historically, the mechanistic understanding of reactions between organics and Mn oxides centred on the oxidative transformation of representative components of natural organic matter (NOM) via the well documented process of reductive dissolution. However, the fate of organic contaminant transformations in soils by Mn oxides is dependent upon the processes of sorption, oxidation and polymerization. There is a strong interdependence between Mn oxides, NOM and organic contaminants. NOM is the main sink for organics and Mn oxides can transform phenolic moieties, a prominent component of NOM. Factors affecting the interactions between Mn oxides, NOM and contaminant/contaminant-NOM complexes are discussed. Overall, transformation of organic contaminants by Mn oxides follows a pathway which involves complex formation, electron transfer and radical formation followed either by polymerization to create larger more aromatic molecules or further oxidation resulting in smaller molecules. Manganese oxides, have demonstrated the capacity to react with some of the most recalcitrant organic pollutants, such as PCBs and PAHs, yet the mechanisms by which these reactions occur is still unclear. Mn oxides theoretically show excellent potential as soil amendments. Further research on environmentally relevant systems rather than clean chemical model systems are required in conjunction with field remediation studies.
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