Upcycling PVC and styrofoam waste into adsorptive PVC/SPS composite membranes for efficient Pb(II) and Cr(III) removal

Plastic waste accumulation and heavy-metal contamination of water are two pressing environmental problems. In this work, polyvinyl chloride (PVC) and Styrofoam waste were upcycled into three types of adsorptive composite membranes (ACMs) — PVC, PVC/polystyrene (PVC/PS), and PVC/sulfonated polystyrene (PVC/SPS) — through a simple solution-casting technique and applied for the removal of Pb²⁺ and Cr³⁺ from contaminated water. The fabricated ACMs were characterized by FTIR, XPS, FE-SEM, TGA/DSC, and XRD, and their density, porosity, water uptake, and ion-exchange capacity were evaluated. Adsorption was monitored by atomic absorption spectrometry as a function of contact time, initial metal concentration, temperature, and pH. The adsorptive PVC/SPS membrane achieved the highest removal efficiency, reaching 94.0±0.67% for Pb²⁺ and 91.5±1.30% for Cr³⁺, with sorption capacities following the order PVC < PVC/PS < PVC/SPS. Pore-size analysis revealed progressive narrowing after adsorption (PVC: 93.8 → 73.2 nm; PVC/PS: 65.3 → 54.6 nm; PVC/SPS: 40.8 → 21.9 nm), consistent with metal-ion deposition on the membrane surface. The non-linear fitting of Langmuir and Freundlich isotherm models suggested predominately homogeneous adsorption behavior of PVC, while heterogeneous for modified PVC/PS and PVC/SPS membranes. The data fitted best with pseudo-second-order kinetics indicating chemisorption on membrane surface, while thermodynamic analysis (ΔG° < 0, ΔH° > 0, and ΔS° > 0) confirmed a spontaneous, endothermic process. The ACMs also exhibited high selectivity against competing cations and retained more than 90% of their initial activity over five regeneration cycles. These findings demonstrate that PVC/SPS composite membranes offer an economical route for upcycling plastic waste into efficient adsorbents for polluted water treatment.