Mutually reinforcing removal of uranium(VI) and nitrate enabled by pH-buffered nitrogen-doped carbon nanotube-calcium...

The coexistence of uranium(VI) (U(VI)) and nitrate (NO₃^-) in uranium mining wastewater poses significant environmental risks, as NO₃^- enhances U(VI) mobility in groundwater. Herein, nitrogen-doped carbon nanotubes (BCNTs) derived from corn stover were hybridized with calcium phosphate (CaP) sourced from mussel shells to prepare BCNTs-CaP-1 and BCNTs-CaP-2. Structural and compositional characterizations confirmed successful CaP immobilization, well-maintained tubular morphology, and abundant pyrrolic-N active sites. Owing to the reversible transformation of H₂PO₄^-, BCNTs-CaP-1 possesses excellent pH buffering capacity, with ultrahigh U(VI) adsorption capacities reaching 453.3-519.0 mg/g across a broad pH range of 3.0-7.0. This property effectively resolves the difficulties in controlling solution pH conditions caused by pH-dependent U(VI) speciation. In binary U(VI)-NO₃^- systems, the U(VI) adsorption capacity is well maintained at 445.7-498.7 mg/g. Notably, the synergistic effect induced by U(VI) strengthens electrostatic attraction, which raises the NO₃^- adsorption capacity from 202.7-336.3 mg/g (single-component system) to 255.3-399.7 mg/g (binary U(VI)-NO₃^- system). The composite exhibits excellent reusability over seven consecutive adsorption-desorption cycles. Mechanistic analysis reveals that U(VI) is mostly immobilized via phosphate precipitation accompanied by reduction to U(IV), while NO₃^- is adsorbed through electrostatic attraction and part of the NO₃^- is further reduced to NO₂^-. The BCNTs-CaP-1 composite effectively improves the unsatisfactory removal efficiency of traditional adsorbents for cation-anion mixtures. This study offers a sustainable technology to prepare multifunctional adsorbents from agricultural waste, supporting the treatment of U(VI) and NO₃^- co-contaminated wastewater.