Reusable polyamine-functionalized polyacrylonitrile adsorbent for dual cationic/anionic dye removal

Water pollution by synthetic dyes poses severe environmental and health hazards due to their high toxicity, chemical stability, and treatment resistance. An efficient poly-aminated polyacrylonitrile (PA–PAN) microscale adsorbent was synthesized and evaluated for the removal of cationic neutral red (NR) and anionic methyl orange (MO) dyes from aqueous solutions. The PAN matrix was prepared via radical-initiated precipitation polymerization of acrylonitrile and polyamine-functionalized with diethylenetriamine to yield PA–PAN. Multiple analytical techniques, including FTIR, SEM-EDX, XRD, CHN, and TGA, confirmed surface functionalization and structural, textural, and morphological features. XRD confirmed a transition from semi-crystalline to amorphous structure, and SEM revealed 100–150 nm PA–PAN nanoparticles with abundant porosity, offering accessible active sites for dye adsorption. The PA-PAN exhibited high capacities of 212.42 mg/g for MO and 127.82 mg/g for NR at initial pH 3.0 and 6.0–7.0, respectively, at 298 K, attributed to structural differences between the dyes. The adsorption followed the Langmuir isotherm, indicating predominantly monolayer coverage on a heterogeneous surface with multiple interaction mechanisms. Adsorption kinetics followed the pseudo-second-order model (R2 > 0.999), with MO achieving a half-life of ∼4.5 min and NR ∼8.2 min, indicating rapid surface binding and diffusion-controlled adsorption. Thermodynamic analysis confirmed spontaneous and exothermic behavior. Furthermore, PA-PAN retained strong performance over five regeneration cycles using 0.1 M NaOH for MO and 0.1 M HCl for NR, highlighting its robustness and applicability for wastewater treatment.