Isotherm and kinetic study of Malachite green and Congo red removal using Spirulina platensis/CMC/ZnO as a green reusable bioadsorbent

Abstract

A novel reusable bioadsorbent, Spirulina platensis/CMC/ZnO, was synthesized using Spirulina platensis algae, carboxymethyl cellulose (CMC), and zinc oxide nanoparticles via a co-precipitation method. The material was thoroughly characterized by FT-IR, EDS, elemental mapping, XRD, TGA, and FE-SEM techniques. This bioadsorbent was employed to remove Malachite Green (MG) and Congo Red (CR) dyes from binary mixtures, showing significantly enhanced efficiency compared to individual components. Optimization using Box–Behnken Design (BBD) involved 27 experiments, achieving maximum removal efficiencies of 99.4 % for MG and 99.8 % for CR, with optimal conditions requiring 0.009 g of adsorbent and a 40-minute contact time. Isotherm studies indicated that both dyes followed the Langmuir model in single-dye systems, and the extended Langmuir model was applicable to binary systems. Maximum adsorption capacities in single solutions were 104 mg/g for MG and 80 mg/g for CR, slightly decreasing to 102 mg/g and 72 mg/g in binary systems. Kinetic analysis revealed that the adsorption process followed a pseudo-second-order model. The bioadsorbent demonstrated high reproducibility (%RSD < 3.5 %), good reusability for at least six cycles, and strong thermal and salt stability, making it a promising candidate for dye removal in harsh environmental conditions.