Zirconium-loaded alginate/biochar hydrogel beads modified by low-pressure cold plasma for selective phosphate removal from aqueous solution

Phosphate contamination is a leading driver of eutrophication in freshwater ecosystems, yet conventional adsorption materials often fail to deliver the selectivity and capacity required for practical remediation. Zirconium-functionalized alginate-coffee husk biochar hydrogel beads (Zr-SA/BC) offer a promising platform for phosphate capture, yet their performance is frequently constrained by restricted accessibility of Zr coordination sites within the swollen polymer matrix. To address this accessibility bottleneck, low-pressure cold plasma (LPCP) was applied as a post-treatment surface engineering strategy at discharge powers of 10, 15, and 20 W. Moderate plasma activation at 10 W (P10) yielded the highest swelling ratio, enhancing phosphate diffusion into the hydrogel matrix and accessibility to Zr coordination sites, resulting in a maximum Langmuir adsorption capacity of 72.97 mg g −1 exceeding that of most previously reported Zr-based bioadsorbents. Kinetic data conformed to the pseudo-second-order model (R 2 = 0.949–0.999), indicating chemisorption-dominated uptake. Thermodynamic analysis indicated spontaneous (ΔG° = −4.88 to −9.25 kJ mol −1 ), endothermic (ΔH° = +22.62 kJ mol −1 ), and entropy-driven adsorption. Combined evidence from FTIR spectral shifts, EDS elemental mapping, pH-dependent behavior, competitive-ion selectivity, and thermodynamic trends is consistent with surface-complexation-assisted phosphate uptake involving ligand exchange at Zr-OH sites as the predominant adsorption mechanism. Optimal removal occurred under mildly acidic to near-neutral conditions (pH 4–6), and P10 demonstrated high selectivity against competing anions (Cl - , NO 3 - , SO 4 2- , HCO 3 - ) with stable performance over four reuse cycles. Notably, performance declined at higher plasma powers (15 and 20 W), suggesting that excessive plasma exposure may induce surface restructuring or reduce the accessibility of active sites. These findings reveal a critical LPCP activation window and establish plasma power as a tunable variable for balancing surface reactivity and structural integrity in metal-functionalized hydrogel adsorbents.