Sol-Gel Synthesis of CaFe₂O₄ Spinel Nanoparticles Using Marine Algae-Derived Calcium Oxide for Efficient Adsorption

In this study, spinel CaFe₂O₄ nanoparticles were fabricated for the first time via the sol-gel method using biogenic calcium oxide (CaO) derived from the marine alga Corallina elongata as a sustainable, available, and low-cost precursor. The synthesized nanoparticles were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and point of zero charge (pHpzc) analysis. Batch adsorption experiments were performed to evaluate Congo red (CR) dye removal by the fabricated spinel nanoparticles as an efficient adsorbent. The effects of pH, contact time, adsorbent dosage, and initial dye concentration were systematically optimized. Kinetic data followed the pseudo-second-order model, while equilibrium data fitted the Langmuir isotherm, confirming monolayer chemisorption on a homogeneous adsorbent surface. Thermodynamic analysis demonstrated a spontaneous, endothermic adsorption mechanism. A maximum adsorption capacity of 84.5 mg g^-1 and 94.7% CR removal were achieved within 50 min under optimized conditions, establishing marine biomass-derived CaFe₂O₄ as a promising, sustainable, low-cost adsorbent for CR-contaminated wastewater remediation.