Insights into the role of the phosphate group in vastly improved adsorption of thorium and uranium using a triptycene-based knitted aryl polymer network

Abstract

We demonstrated the efficient and selective removal of Th(IV) and U(VI) from aqueous solutions using a triptycene-based knitted aryl polymer (KAP). The adsorption efficiency is governed by multiple parameters including pH (2–10), initial ion concentrations (25–800 ppm), adsorbent characteristics, and the presence of competing ions (Cu²⁺, Sr²⁺, Co²⁺, K⁺, Na⁺, Zn²⁺ and Cs⁺) in solution. The specially designed KAP (TP_DPP) consisted of nanopores (surface area of 480 m² g⁻¹) with PO groups that can potentially interact with Th and U, resulting in 760 mg g⁻¹ and 644 mg g⁻¹ adsorption capacities, respectively. Maximum adsorption was achieved at pH 6 for both Th(IV) and U(VI), indicating a balance between favourable ion speciation and deprotonated, highly interactive phosphate functional groups on the TP_DPP framework. TP_DPP also registered a distribution coefficient (K_d) of 10⁶ mL g⁻¹ for Th and 10⁵ for U. γ-Irradiated TP_DPP (50, 100, 200, and 300 kGy) retained excellent adsorption capacities for both Th(IV) and U(VI). Chemical stability was tested by treating TP_DPP with acidic and basic solutions before adsorption experiments. The reduction of Th uptake varied from 1.2% in the presence of Sr²⁺ to a maximum of 4.2% in the presence of Co²⁺. The effect of interfering ions on the reduction of U uptake varied from 0.6% in the presence of Na to a maximum of 13% in the presence of Cu²⁺. This work highlights TP_DPP as a superior adsorbent for Th and U removal, offering a sustainable and effective solution for nuclear waste management and groundwater remediation.