Biosorption of Cu (II), Pb (II), and Cr (III) ions in single and combined systems using Spirulina platensis Immobilised on magnetic banana peels: Kinetic, isotherm and reusability analyses

• A novel magnetic bio-composite (SP/MBP) was successfully developed by immobilizing Spirulina platensis onto magnetic banana peels, exhibiting enhanced surface functionality, improved structural roughness, and effective magnetic recoverability for multiple wastewater treatment cycles. • Remarkable single-metal removal efficiencies were attained, with SP/MBP (0.8:1) eliminating up to 97.67% Cu (II), 95.21% Pb (II), and 93.89% Cr (III), underscoring its robust affinity and efficacy as a sustainable biosorbent for heavy-metal-contaminated effluents. • Competitive adsorption studies demonstrated distinct metal-binding hierarchies, with Cu (II) exhibiting the greatest suppression in multi-metal systems, while Pb (II) and Cr (III) retained higher adsorption affinities—offering essential insights into sorption behaviour in actual industrial wastewater matrices. • Kinetic modelling validated pseudo-first-order mechanisms (R² > 0.98) regulating metal uptake, while equilibrium data exhibited exceptional alignment with both Langmuir and Freundlich isotherms (R² > 0.99), with maximum capacities of 896.28 mg/g (Pb), 139.53 mg/g (Cu), and 438.62 mg/g (Cr). • The Extended Langmuir and Extended Freundlich isotherms effectively characterized binary systems, affirming surface heterogeneity and competitive interactions, therefore substantiating the appropriateness of SP/MBP for intricate, multi-solute wastewater contexts. • Regeneration experiments demonstrated partial recyclability of the biosorbent, with performance decreasing over four cycles, yet still indicating promising feasibility for cost‑effective, magnetically assisted recovery in continuous treatment processes. This research investigates the biosorption of Cu (II), Pb (II), and Cr (III) from aqueous solutions using Spirulina platensis immobilized on magnetic banana peels (SP/MBP). The composite biosorbent was synthesized by co-precipitation and characterized using SEM-EDX and FTIR studies, which verified effective immobilization and the presence of functional groups responsible for metal binding. Batch investigations evaluated the impacts of immobilization ratio, metal competition, contact time (10-120 minutes), and initial concentration (50-250 mg/L). A mass ratio of 0.8:1 achieved the greatest removal efficiency in individual systems: 97.67% for Cu (II), 95.21% for Pb (II), and 93.89% for Cr (III). Competitive analyses indicated that Cu (II) was significantly impacted in multi-metal circumstances, with its removal efficiency declining to 82.57% in the Pb–Cu system, 72.41% in the Cr–Cu system, and 60.38% in the ternary Pb–Cu–Cr system. Kinetic modelling showed that the pseudo-first-order model most accurately described the adsorption behaviour (R² > 0.98), indicating the predominance of physisorption. Isotherm modelling in individual systems shown robust alignment with both Langmuir and Freundlich models (R² > 0.99), with maximal adsorption capacities of 896.28 mg/g (Pb), 139.53 mg/g (Cu), and 438.62 mg/g (Cr). The results from the binary system were well characterized by the extended Langmuir and Freundlich models, validating competitive interactions. Regeneration experiments indicated a significant decrease in performance throughout four cycles. SP/MBP is an efficient, cost-effective, magnetically separable biosorbent ideal for extracting heavy metals from intricate wastewater, demonstrating significant promise for real-world environmental applications.