Selective removal of thallium from water by MnO₂-doped magnetic beads: Performance and mechanism study

Aqueous thallium has posed an increasing threat to environment as human's intensified activities in mining, refining, process and discharge. Remediation on thallium pollution has been of up-most importance to water treatment. In present work, MnO₂ and magnetic Fe₃O₄ have been implanted to sodium alginate (SA) in presence of carboxyl methyl cellulose (CMC), and the resultant beads consisted of SA/CMC/MnO₂/Fe₃O₄ were characterized. The materials were applied to treatment of Tl-contaminated water as adsorbent in lab. The removal results revealed that the adsorption capacity reached 38.8 mg (Tl)·g (beads)^-1 and almost 100 % removal efficiency was achieved. The residual Tl was below 0.1 μg·L^-1, meeting the discharge standard regulated in China. The kinetic adsorption was better described as a pseudo-second-order and three-step intra-particle diffusion model. Freundlich isotherm was well fitted the experimental data. The absorbent shown an excellent competitive specificity (K_Tl/M: ∼10⁴!) over common hazardous ions Cu²⁺, Cd²⁺, Co²⁺, Pb²⁺ and Cr³⁺, as well as naturally abundant K⁺ and Na⁺ (K_Tl/M: 10-10²) in mimic environmental conditions. Regeneration and reusability of the absorbent was also verified by five absorption-desorpotion cycles. XPS results revealed that a redox reaction between Mn⁴⁺ with Tl⁺, and an ion exchange of H⁺ (^-O-Fe) and Tl⁺ were assumed to be main process for the specific capturing. This study provided an efficient SA/CMC/MnO₂/Fe₃O₄ composite beads that could be a promising adsorbent for Tl-polluted water treatment.