Development of an integrated and eco-friendly approach for producing nanomagnetite-based coconut coir biosorbents: elucidation of the adsorption mechanism of Rhodamine B and methylene blue

The present study focused on optimizing the biosorbent production through chemical modification of natural coconut fibers (NCFs) using a biorefinery process combined with nanoparticle impregnation and its adsorptive performance for removing Rhodamine B (Rh-B) and methylene blue. NCF was chemically modified via the formosolv process using formic acid (85% wt.) and 2% v/v HCl as catalyst at 100 °C for 1 h under atmospheric pressure, yielding formosolv coconut fiber (FCF). Nanomagnetic composites (NCF/Fe₃O₄ and FCF/Fe₃O₄) were produced through wet co-precipitation by mixing FeSO₄·7H₂O and FeCl₃·6H₂O salts (1:2 mol/mol) in an alkaline medium at different concentrations. This process generated four biosorbents, such as NCF, FCF, NCF/Fe₃O₄, and FCF/Fe₃O₄, which were characterized by chemical composition, XRD, FTIR, and magnetic properties. (1) The biorefinery process caused a chemical modification in the fibers, increasing the cellulose content and the crystallinity indexes. (2) The formosolv process enhanced the iron incorporation. (3) FCF exhibited the highest Rh-B removal efficiencies (REs), approximately 98% for FCF and 74% for FCF/Fe₃O₄. (4) For MB, RE values were 98% for FCF and 93% for FCF/Fe₃O₄. (5) These biosorbents were produced via a simple, fast, and cost-effective methodology and showed great potential for adsorption purposes. (6) Isotherms revealed distinct behaviors for the dyes, indicating different interactions with adsorbents. (7) Rh-B fitted better to the Freundlich model, suggesting a physisorption mechanism, while MB showed better fitting to the Langmuir model, suggesting the occurrence of chemisorption. (8) Thermodynamic parameters indicated that the adsorption of both dyes was endothermic and spontaneous for all biosorbents. (10) Biorefinery-processed fibers exhibited higher enthalpy values, indicating stronger interaction with MB and Rh-B. (11) Regeneration studies suggested that a higher NaOH concentration enhanced the desorption efficiency. Overall, the proposed modification routes have demonstrated the capability to produce eco-friendly magnetic biosorbents with high adsorption potential through a simple, fast, and low-cost approach.