Advanced Biopolymer-Based Zn-Co-Fe/Ca Nanocomposites for High-Performance Dye Removal From Aqueous Media

Due to the toxicity, persistence, and high visibility of synthetic dyes, the discharge of dye-contaminated effluents from textile and allied industries represents a major environmental concern. This study reports the development of a sustainable, high-performance biopolymer-based nanocomposite incorporating zinc, cobalt, iron, and calcium (Zn-Co-Fe/Ca) to overcome the limitations of conventional adsorbents. The primary objective was to evaluate the adsorption efficiency and mechanistic behaviour of the developed adsorbent for methylene blue (MB) removal from aqueous media. The biopolymer-based Zn-Co-Fe/Ca nanocomposite, hereafter referred to as Biopolymer-based Zn-Co-Fe/Ca Magnetic Nanocomposite (BPMNC), was synthesised via chemical co-precipitation followed by ionic crosslinking within a calcium alginate matrix. Comprehensive physicochemical characterisation using scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) surface area analysis, thermogravimetric analysis (TGA), and point of zero charge (pHpzc) confirmed effective metal immobilisation, mesoporous surface characteristics, and strong dye–surface interactions. Batch adsorption experiments were conducted in triplicate (n = 3) by varying pH (3–10), adsorbent dosage (0.5–2.5 g), contact time (10–90 min), initial MB concentration (4–20 mg L–1), and temperature (30–70 °C). Adsorption kinetics and equilibrium behaviour were analysed using pseudo-first-order and pseudo-second-order models, along with Langmuir and Freundlich isotherms. Thermodynamic parameters (ΔG0, ΔH0, and ΔS0) were determined using Van’t Hoff analysis. At pH 6, an adsorbent dose of 1.0 g and an initial dye concentration of 12 mg L–1 with a contact time of 60 min resulted in a maximum MB removal efficiency of approximately 97%. The pseudo-second-order model 1 provided the best fit to the experimental data, with high correlation coefficients (R² = 0.9614–0.9808). Among the investigated concentrations, only the 20 mg L⁻¹ system exhibited an R² value exceeding 0.98, while the remaining concentrations showed R² values between 0.96 and 0.98, indicating that the adsorption kinetics were generally well described by the pseudo-second-order model. The Langmuir isotherm (R2 = 0.99) suggested monolayer adsorption on a homogeneous surface. SEM and FTIR analyses further confirmed significant dye–surface interactions, and thermodynamic results indicated a spontaneous and endothermic adsorption process. Consequently, the biopolymer-based adsorbent demonstrated high and stable MB removal at near-neutral pH due to synergistic multi-metal interactions and surface functionalities, highlighting its potential as a sustainable adsorbent for dye-laden wastewater treatment.