Effective removal of trace thallium from surface water by nanosized manganese dioxide enhanced quartz sand filtration

Thallium (Tl) has drawn wide concern due to its high toxicity even at extremely low concentrations, as well as its tendency for significant accumulation in the human body and other organisms. The need to develop effective strategies for trace Tl removal from drinking water is urgent. In this study, the removal of trace Tl (0.5 μg L^-1) by conventional quartz sand filtration enhanced by nanosized manganese dioxide (nMnO₂) has been investigated using typical surface water obtained from northeast China. The results indicate that nMnO₂ enhanced quartz sand filtration could remove trace Tl(I) and Tl(III) efficiently through the adsorption of Tl onto nMnO₂ added to a water matrix and onto nMnO₂ attached on quartz sand surfaces. Tl(III)-HA complexes might be responsible for higher residual Tl(III) in the effluent compared to residual Tl(I). Competitive Ca²⁺ cations inhibit Tl removal to a certain extent because the Ca²⁺ ions will occupy the Tl adsorption site on nMnO₂. Moreover, high concentrations of HA (10 mgTOC L^-1), which notably complexes with and dissolves nMnO₂ (more than 78%), resulted in higher residual Tl(I) and Tl(III). Tl(III)-HA complexes might also enhance Tl(III) penetration to a certain extent. Additionally, a higher pH level could enhance the removal of trace Tl from surface water. Finally, a slight increase of residual Tl was observed after backwash, followed by the reduction of the Tl concentration in the effluent to a "steady" state again. The knowledge obtained here may provide a potential strategy for drinking water treatment plants threatened by trace Tl.