Metal accumulation and tolerance mechanisms in Potamogeton crispus under single-metal exposure to Cd, Zn, Ni and Li: Implications for phytoremediation of metal-contaminated waters

• Multi-metal accumulation hierarchy in P. crispus: Zn > Ni > Cd > Li • Low metals induced hormesis; high levels caused phytotoxicity • Multi-scale analysis (SEM/FTIR/TGA) revealed metal-binding mechanisms • Key functional groups (–OH, –COOH, –NH) immobilized metals via complexes • P. crispus shows high potential and limits for aquatic phytoremediation This study evaluated the submerged macrophyte Potamogeton crispus for its ability to accumulate cadmium (Cd), zinc (Zn), nickel (Ni), and lithium (Li) in metal-contaminated waters and its suitability for phytoremediation. A 70-day static mesocosm experiment was performed in which four independent single-metal exposure series (Cd, Zn, Ni, and Li) were established, each with four concentration levels and one metal-free control; water and plant samples were regularly collected. Metal concentrations were quantified by flame atomic absorption spectrometry, and scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS), Fourier-transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA) were used to probe microstructural changes, functional-group interactions, and thermal stability under metal stress. Potamogeton crispus showed strong accumulation capacity for all four metals, with concentration-dependent uptake; maximum accumulation occurred for Zn (up to 885.38 mg·kg⁻¹), followed by Ni, Cd, and Li. Low-concentration exposure stimulated plant growth, suggesting hormesis, whereas high concentrations suppressed growth and induced visible toxicity, indicating a tolerance threshold. SEM-EDS showed surface roughening, particle aggregation, and metal deposits, consistent with cell-wall-based immobilization and structural damage at high concentrations. FTIR spectra indicated that hydroxyl, carboxyl, and amide groups are key sites for metal binding, as reflected by changes in band intensity and peak shifts. TGA profiles showed that metal exposure modified thermal decomposition and increased residual mass, consistent with partial stabilization of organic matter through metal–organic associations. Overall, this multi-scale analysis demonstrates the robust accumulation capacity and moderate tolerance of P. crispus to Cd, Zn, Ni, and Li under single-metal exposure, provides baseline data for evaluating its use in remediating waters polluted by mixtures of traditional and emerging metals, and offers mechanistic insights to inform future multi-metal experiments.