Selectivity and regeneration performance of natural and formaldehyde-crosslinked Fucus vesiculosus for the single and competitive adsorption of Cr3+, Cd2+, Co2+, and Ni2+

The persistent release of toxic transition metals associated with industrial activities and landfill leachates represents a major concern for water quality, highlighting the need for cost-effective and efficient materials for metal removal from both single and multicomponent aqueous systems. This study assessed the adsorption performance of natural and formaldehyde-crosslinked Fucus vesiculosus toward Cr 3+ , Cd 2+ , Co 2+ , and Ni 2+ in single- and quaternary-component systems. The structural and chemical characteristics of the resulting biosorbents were examined using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM–EDS) and Fourier transform infrared spectroscopy (FTIR). The results demonstrated that formaldehyde crosslinking enhanced the structural stability of the algal biomass while preserving the functional groups involved in metal binding, thereby improving its adsorption performance. The optimal pH was 4 for Cr 3+ and Cd 2+ , and 6 for Ni 2+ and Co 2+ . Adsorption equilibrium was reached within 120 min for Cd 2+ , Ni 2+ , and Co 2+ , whereas Cr 3+ required 360 min. The adsorption kinetics of Cr 3+ , Cd 2+ , and Ni 2+ were best described by the pseudo-second-order model, while Co 2+ followed the pseudo-first-order model. The Langmuir model provided the best fit for Cr 3+ , Cd 2+ , and Ni 2+ , whereas the Freundlich model better described Co 2+ adsorption. Formaldehyde crosslinking increased the maximum Langmuir adsorption capacity for Cr 3+ , Cd 2+ , and Co 2+ , whereas Ni 2+ exhibited comparable adsorption capacities for both biosorbents. Under competitive conditions, Cr 3+ exhibited the highest selectivity, markedly suppressing the adsorption of the divalent metal ions. Five consecutive adsorption-desorption cycles demonstrated improved adsorption stability of the crosslinked biosorbent, particularly for Cd 2+ and Ni 2+ , while a preliminary laboratory-scale economic assessment indicated that its higher preparation cost may be partially offset by its superior performance during repeated operation.