Sodium alginate–Cupressus macrocarpa hydrogel beads as an entropy-driven inner-sphere chemisorbent for cadmium(II) across the trace-to-industrial concentration window

Mancilla H.B.; Silva C.E.; Avila S.C.; Ureta L.V.; Marcelo H.E.B.; Roca Y.A.B.I.; Jindal M.K.; Rasheed Z. · Separation and Purification Technology (2026) · DOI: 10.1016/j.seppur.2026.139535

Resumen

Cadmium discharged from electroplating, battery-recycling, and pigment industries reaches concentrations of 6–200 mg L−1 necessitate highly efficient, sustainable sorbents. This study reports a green hydrogel adsorbent prepared by ionotropic crosslinking of sodium alginate with Cupressus macrocarpa leaf powder in CaCl2, and uses Response Surface Methodology (RSM) coupled to a Central Composite Design (CCD) to optimise Cd(II) removal. The optimiser identified an optimum at pH 7.99, t = 59.81 min, C₀ = 40.65 mg L−1 and dose = 99.68 mg, giving R = 98.94%. Four independent lines of evidence converge on inner-sphere chemisorption: (i) pseudo-second-order kinetics, (ii) thermodynamic analysis returning ΔG° = −21.2 and − 23.6 kJ mol−1 at 291 and 303 K, ΔH° = +39.2 kJ mol−1, ΔS° = +207 J mol−1 K−1, the signature of partial desolvation of Cd2+ on binding; (iii) Cd uptake of 104 μmol m−2 based on the measured BET surface area of 2.83 m2 g−1; and (iv) an XRD lattice expansion of the cellulose (200) reflection (Δd = +0.09 Å), without crystalline Cd-phase formation. This work validates that biopolymer hydrogels outperform high-surface-area carbons by utilizing internal functional group density, providing a scale-up-ready solution for the entire industrial-to-trace concentration window.

Instituciones: Universidad Nacional del Centro del Perú