Selective capture of thallium(I) ion from aqueous solutions by amorphous hydrous manganese dioxide

• HMO exhibits great sorption selectivity towards Tl(I) in the presence of competing Ca(II) at much higher levels.
• The adsorbed Tl(I) onto HMO was oxidized to insoluble Tl(III) in low pH range.
• The specific inner-sphere complexes predominated the Tl(I) adsorption process.
• The exhausted HMO particles could be efficiently regenerated by NaOH–NaOCl binary solution.

Thallium, a highly toxic pollutant, has greater toxicity to mammals than common heavy metals such as lead, cadmium, mercury and efficient techniques for its removal are penurious. In the present study, amorphous hydrous manganese dioxide (HMO), an excellent sorbent for heavy metals retention, was synthesized by using oxidation and chemical precipitation method and its selective adsorption behavior toward Tl(I) was evaluated. The uptake of Tl(I) onto HMO is pH-dependent and dropped significantly with the pH rise from 2 to 4, and then followed by rapid promotion with further increased pH to 7. XPS spectrum analysis confirmed that more than half of the adsorbed Tl(I) was oxidized to insoluble Tl(III) in low pH range (2.02), while little adsorbed Tl(I) was oxidized around neutral pH range (5.83). Compared to a macroporous cation exchanger D-001 and an metal-specific chelating resin Amberlite IRC-748, HMO exhibited stronger adsorption selectivity toward Tl(I) in the presence of high concentrations of competing Ca(II). In addition, the sorption of Tl(I) onto HMO is fast and the kinetic data could be reasonably fitted by the pseudo-first-order model. The exhausted HMO particles could be efficiently regenerated by NaOH–NaOCl binary solution of alkalinity at 7.8% and active chlorine at 5.0%. All results presented strongly indicated that amorphous HMO is a sorbent of great potential for selective Tl(I) removal from contaminated waters.