Hemp shives conformally-coated by ALD ZnO for a sustainable application in wastewater remediation

A new hybrid nanocomposite for wastewater treatment was developed using hemp shiv fibers coated with ZnO nanolayers. Macro hemp shives were selected for their adsorption capacity and sustainability, while ZnO provided photocatalytic activity. Low-temperature atomic layer deposition (ALD) anchored ZnO nanolayers with controlled thicknesses (30, 70, 90 nm) onto hydroxyl groups on the hemp shives' surface. The effect of the ZnO thickness on adsorption and photocatalytic performance toward water organic pollutants was systematically investigated. Characterization included spectroscopic ellipsometry, scanning electron microscopy (SEM), transmission electron microscopy (TEM) with electron energy loss spectroscopy (EELS) and selected area electron diffraction (SAED), X-ray diffraction (XRD), thermogravimetric analysis (TGA), N2 adsorption-desorption analysis, and Fourier transform infrared spectroscopy (FTIR). Methylene blue (MB) adsorption kinetics showed ZnO coatings partially shield the intrinsic adsorption aptitude of pure porous fibers' surface, with a shielding increasing with the ZnO thickness. Photocatalytic activity under UV irradiation was evaluated using three emerging water pollutants: MB dye, sodium dodecyl sulfate (SDS), and phenol. Hemp shives coated with a 30 nm-thick ZnO layer exhibited the highest degradation efficiencies for all the tested pollutants (about 70% after 4 h of light irradiation). Recycling tests confirmed material stability, with preserved ZnO coating and crystal structure after repeated use. Scavenger experiments indicated both radicals and photo-generated holes contribute to photocatalytic mechanism. The synergistic combination of adsorption and photocatalysis makes ZnO-coated hemp shives a promising and sustainable material for advanced wastewater treatment.