Effects of inorganic anions on Fenton oxidation of organic species in landfill leachate.

Fenton oxidation typically achieves only a moderate removal efficiency of chemical oxidation demand (COD) in treatment of a landfill leachate, incomparable to some other chemical oxidation processes (e.g. electrochemical oxidation). A traditional viewpoint is that the remaining highly oxidized organic species become recalcitrant to hydroxyl radical (OH·)-induced oxidation at a higher degree of Fenton oxidation. Although this may, at least partially, be true, the effects of inorganic anions, which are abundant in leachate, on Fenton oxidation have been largely ignored. The objective of this study is to assess the roles of three inorganic anions (nitrate, sulfate, and chloride) originally present in or externally added to leachate during Fenton oxidation of refractory organic species in leachate. Nitrate (NO₃⁻) did not significantly affect the COD removal rate. In contrast, sulfate (SO₂⁴⁻) and chloride (Cl⁻), particularly the latter, inhibited Fenton oxidation. Their inhibiting effect may be ascribed to: (1) scavenging of OH·; and (2) competition with H₂O₂ to form Fe(III) complexes, thus preventing the Fe(II) regeneration from Fe(III) reduction. At 2.125H₂O₂:initial COD(COD(0)) = 9.0, [H₂O₂]:[Fe²⁺] = 3, and pH = 3.0, 96, 62, 49, and 37% of COD(0) (1,254 mg L(-1)) were removed in the presence of 0, 1270, 5000, and 10, 000 mg L⁻¹ Cl⁻, respectively.