Removal of persistent heavy metals using marine biomass and engineered biopolymer under challenging seawater conditions

• Marine crab biomass and chitosan tested for heavy metal removal in saline water • Box–Behnken design optimized adsorption under realistic seawater conditions • Chitosan showed higher capacity; biomass effective for Pb²⁺ and Cu²⁺ uptake • Kinetic and isotherm models reveal monolayer, site-specific adsorption behavior • Valorization of marine waste supports sustainable water treatment solutions This study compares the performance of a marine-derived biomass and commercial chitosan for the removal of multiple heavy metals from saline wastewater under simulated seawater conditions. Both materials were characterized by FTIR, SEM, TGA, and EDS to establish structure–function relationships. A Box–Behnken design was applied to optimize key operational parameters (contact time, dose, salinity, sulfate concentration), achieving an optimal setup at 95 min, 5.5 g/L sorbent dose, ∼20 g/L salinity, and 1.5 g/L sulfate. Adsorption performance was tested using single and mixed metal solutions. The commercial chitosan shows high metal removal and high selectivity. This is attributed to its high presence of free amine groups. On the other hand, the blue crab biomass also demonstrated notable effectiveness, especially for Pb²⁺. Kinetic fittings indicated pseudo-first order (PFO) and pseudo-second order (PSO) models provided the best fits, with chemisorption and external surface interactions as dominant mechanisms. Isotherm modeling suggests the Langmuir and Dubinin–Radushkevich as well-fitted models. This suggests monolayer adsorption with site-specific interactions. Fixed-bed column breakthrough curves could confirm the higher capacity and delayed saturation of chitosan compared to biomass. These results show that marine biomass valorization and the chitosan-based biopolymers as promising, sustainable strategies for treating complex saline effluents.