Performance of a small-scale subsurface flow constructed wetland for thallium-containing wastewater

Thallium (Tl) is an extremely toxic heavy metal with high mobility, posing severe threats to aquatic environments. Although constructed wetlands are widely used for heavy metal wastewater treatment, their performance in removing Tl remains largely unexplored. This study demonstrates that a lab-scale subsurface flow CW achieved over 96% Tl removal under varied hydraulic conditions. Mass balance analysis over 137 days showed that 97.6% of the influent Tl was removed, with the remainder (2.4%) discharged in the effluent. Of the influent Tl, 52.8% was retained in the ceramsite, 44.4% in the soil, and 0.4% in the plants. The synergistic removal mechanisms involved: initial immobilization of Tl via pH-dependent adsorption; a distinct rhizospheric barrier effect in Phragmites australis, which restricted Tl largely to the roots with limited upward translocation; and significant microbial community restructuring under Tl stress, which may have contributed to the transformation of mobile Tl into stable solid phases. XPS analysis suggested an increased proportion of Tl(Ⅲ) to 30.92% in soil and 43.62% in substrate after treatment. This study provides the first systematic evidence for the multi-mechanistic synergy enabling efficient Tl removal in subsurface flow CWs, offering a theoretical and practical basis for engineering applications. • The constructed wetland achieved high and stable Tl removal (>96%) across varied hydraulic conditions. • Mass balance analysis revealed substrate adsorption dominated Tl sequestration (99.6%), while plant uptake was negligible. • Thallium stress alters the microbial community structure, thereby facilitating the accumulation of heavy metals in constructed wetlands. • The operation of the constructed wetland facilitated the transformation of thallium from a mobile ionic state into a stabilized solid phase.