Impact of Acid Dopants (HCl vs. H₂SO₄) on the Structure, Performance, and Reusability of Bio-Based Luffa/Polyaniline...

Sustainable and low-cost Luffa cylindrica/polyaniline (LC/PAN) composite adsorbents were developed for the removal of methylene blue (MB) from aqueous solutions. The effect of acid dopants (hydrochloric acid and sulfuric acid) on the structural properties and adsorption performance of the composites was systematically investigated. LC/PAN materials were synthesized via a dual-acid doping approach and characterized using FT-IR, Raman, SEM, XRD, TGA, and BET techniques. Batch adsorption experiments were conducted to examine the influence of adsorbent dosage, initial dye concentration, pH, and temperature, while adsorption behavior was analyzed using kinetic, isotherm, and thermodynamic models. Although the HCl-doped LC/PAN exhibited a higher BET surface area (20.8 m²/g), the H₂SO₄-doped composite showed superior MB adsorption capacity (Q_max ≈  10 mg/g), indicating that surface chemistry plays a more critical role than total porosity. The adsorption process was rapid, followed pseudo-second-order kinetics and the Langmuir isotherm, and was spontaneous and exothermic. These results demonstrate that acid dopant selection is a key factor in tailoring LC/PAN biocomposites for efficient and sustainable dye adsorption.