Synthesis of carbon dots and carbon dot loaded alginate beads using macroalgae Stypopodium schimperi as a precursor and heavy metal phycoremediation

Abstract The study aimed to synthesize Stypopodium schimperi -derived carbon dots (SSCDs) and SSCD-loaded alginate beads (A-SSCDs) via a hydrothermal synthesis method and to investigate their adsorption capacity toward Hg(II), Sn(IV), Zn(II), Cr(VI), and Pb(II). The selective behavior of SSCDs and A-SSCDs against the determined heavy metals was determined using fluorescence quenching and ICP-OES (Inductively Coupled Plasma-Optical Emission Spectrometry) respectively. Characterization of synthesized SSCDs, A-SSCDs and adsorption studies were carried out by field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), Fourier transform infrared (FTIR), UV–Vis spectroscopy, fluorescence spectroscopy and thermogravimetric analysis (TGA). HR-TEM revealed that the carbon dots were spherical with diameters of approximately 10–15 nm. A-SSCD beads exhibited a broad amorphous halo (2θ≈20–30°), typical of disordered carbon structures, which provides a high density of binding sites. Carbon dot incorporation improved the thermal resistance of the alginate beads, indicating stronger structural integrity. Both SSCDs and A-SSCDs demonstrated the highest selectivity toward Cr(VI). A-SSCD beads achieved a maximum experimental adsorption capacity of 132.14 mg g⁻ 1 at an initial Cr(VI) concentration of 250 mg L⁻ 1 and a theoretical Langmuir monolayer capacity of 181.82 mg g⁻ 1 (R 2 = 0.986). The use of the invasive brown alga S. schimperi as a carbon source offers a sustainable and cost-effective route for water purification, converting ecologically problematic biomass into a value-added adsorbent. The synthesized carbon dot–alginate composite provides a green, low-cost, and efficient material for Cr(VI) removal from aqueous environments, combining biomass valorization with advanced nanomaterial design. Graphical Abstract