Plasma-catalytic methane pyrolysis for hydrogen production and synthesis of CNTs-nZVI nanocomposite for water remediation

The development of efficient and sustainable materials for water remediation is essential to address the growing contamination of aquatic environments by potentially toxic metal(loid)s, while simultaneously reducing the environmental impact of material synthesis. In this work, carbon nanotubes-nano zerovalent iron (CNTs-nZVI) composites were produced via methane Plasma-Catalytic pyrolysis, a single-step, electrified, and solvent-free process that simultaneously converts methane into syngas and carbon nanomaterials. Produced syngas contained 79.6 ±3.7 vol.% hydrogen, 19.9 ±3.9 vol.% carbon monoxide, and 0.5 ±0.2 vol.% methane, which confirms more than 99% conversion. The CNTs-nZVI composite structure, consisting of single-walled CNTs and core-shell, iron-carbon nanoparticles with a metallic iron core, was confirmed by comprehensive microstructural, chemical, and textural characterization using TEM, STEM, EDS, BET, XRD, STA, XPS, and Raman spectroscopy. The remediation performance of the CNTs-nZVI composite was evaluated with nine metal(loid)s in water. The as-synthesized material showed rapid and highly selective removal of Pb(II) exceeding 96% within 24 hours. Thermal activation at 450 °C significantly enhanced long-term adsorption, achieving removal efficiencies of approximately 55% for Cr(VI) and Cu(II) and ~30% Cd(II) and As(V) after 48 days. Thermal activation also completely eliminates the volatile organic compounds present in as-synthesized CNTs-nZVI, as confirmed by GC-MS analysis. These results demonstrate the potential of the plasma-catalytic pyrolysis process to valorize methane and iron oxide into hydrogen-rich syngas and CNTs-nZVI composites, which can be used as sustainable water-treatment materials.