Enhanced Trace Tl Removal with Ferrate through the Addition of Mn(II): Effect and Mechanism

Thallium (Tl) is highly toxic and mobile compared with the other priority pollutant metals, and trace Tl pollution is difficult to control. Some mineral deposits in South China contain high levels of Tl, and Tl related pollution incidents have increasingly occurred in recent years. We previously found that ferrate [K₂FeO₄, Fe(VI)] could oxidize Tl(I) into Tl(III) and remove the resulting Tl(III). However, the Tl-bearing ferric nanoparticles may penetrate the filtration tank and impact water quality. Herein, with the addition of a low dosage of Mn(II) (50 µg/L), the Tl removal efficiency surged from 42% to 84% [Fe(VI): 0.5 mg/L, Tl: 1 µg/L, pH 7.0]. Time for the formation of 4 µm of floc shortened from 75 min in the Fe(VI) group to 30 min in the Fe(VI) + Mn(II) group. In the reaction process, Mn(II) was oxidized into MnO₂, part of Tl(I) was oxidized into Tl(OH)₃, and Fe(VI) was reduced into ferric (hydr)oxides. Negatively charged MnO₂ (zeta potential: -40.6 mV at pH 7.0) interacted with positively charged ferric (hydr)oxides (zeta potential: 0.8 mV at pH 7.0) through electrostatic attraction with the formation of ferric (hydr)oxides/MnO₂ floc. Tl(I) and Tl(OH)₃ were adsorbed by the negatively charged ferric (hydr)oxides/MnO₂ floc and subsequently removed through solid-liquid separation. By adjusting the molar ratio of Fe(VI)/Mn(II), effective removal of trace Tl was achieved in authentic polluted waters.