Removal of Congo Red, hexavalent chromium and divalent copper by electrospun PAN/PEI/polylysine nanofibrous membranes: Insights into adsorption mechanisms

Water pollution caused by organic dyes and heavy metals severely threatens ecological security and human health. Herein, a novel polyacrylonitrile/ polyethyleneimine/ε-polylysine (PAN@PEI@ε-PL) electrospun nanofibrous membrane (NF) was fabricated for synchronous removal of Congo Red (CR), divalent copper (Cu(II)), and hexavalent chromium Cr(VI) from aqueous solutions. Brunauer-Emmett-Teller (BET) analysis confirmed that the membrane possessed a typical mesoporous structure with a specific surface area of 0.2283 m 2 /g, average pore diameter of 28.82 nm, and total pore volume of 0.001645 cm 3 /g, providing abundant adsorption sites and fast mass transfer channels. Under optimal conditions (optimal pH: CR = 6, Cu(II) = 5, Cr(VI) = 3; adsorption equilibrium within 120 min; 35 °C), the maximum Langmuir adsorption capacities of PAN@PEI@ε-PL NF for CR, Cu(II), and Cr(VI) reached 1311.6 mg/g, 376.1 mg/g and 490.8 mg/g, respectively. Adsorption kinetics and isotherms well fitted the pseudo-second-order model and Langmuir model, indicating monolayer chemical adsorption. Thermodynamic studies revealed the adsorption process was spontaneous and endothermic. The adsorption mechanism was synergistically driven by electrostatic attraction, hydrogen bonding, Cr(VI) reduction (65.68% Cr(VI) converted to Cr(III)), and coordination interaction. After 5 consecutive adsorption-desorption cycles, the removal efficiencies still remained 77.8% (CR), 56.5% (Cu(II)), and 67.4% (Cr(VI)). Benefiting from high specific surface area, rich amino/amide active sites, favorable mesoporous structure, and excellent stability, PAN@PEI@ε-PL NF shows great potential in simultaneous purification of dye-heavy metal composite wastewater.