Green synthesis of nanoscale zero-valent iron impregnated walnut shell biochar as efficient adsorbent for metal(loid)s purification: Performance and mechanism insight

The escalation of metal(loid) contamination in global water poses severe risks to ecological environment safety. Meeting the ‘pollution control with waste’ strategy, we utilized eco-friendly waste tea leaf extract and walnut shell to first fabricate a greenly synthesized nanoscale zero-valent iron impregnated walnut shell biochar (G-nZVI/WSB) for the simultaneous sequestration of cadmium (Cd), lead (Pb), and arsenic (As) from aqueous systems. Since the interactions among these metal(loid)s during adsorption on G-nZVI/WSB and their corresponding mechanisms remain unclear, we conducted a series of batch experiments coupled with characterization techniques. The optimal pyrolysis temperature and Fe/C ratio for G-nZVI/WSB were determined to be 650 °C and 20 %, respectively. Under optimal conditions (contact time = 24 h, pH = 6), G-nZVI/WSB exhibited theoretical maximum adsorption capacities for Cd(II), Pb(II), and As(III) of 52.96, 135.1, and 8.414 mg g⁻¹, respectively, which were significantly higher than those of WSB carrier. In single and combined systems, the monolayer and chemical adsorption of metal(loid)s were predominant, with their competitive affinity for active sites following the order of Pb(II) > Cd(II) > As(III). Characterization analyses using FTIR, SEM, XRD, and XPS confirmed the specific adsorption mechanisms of G-nZVI/WSB for these metal(loid)s, including cation-π interactions, coprecipitation, oxidation, and coordinated complexation. Notably, G-nZVI/WSB exhibited excellent adsorption stability over five cycles of adsorption-desorption, which achieved the efficient purification of multi-contaminated actual wastewaters to meet the standards for wastewater discharge or drinking water. These findings demonstrate the significant potential of G-nZVI/WSB as an eco-friendly and promising nanocomposite for treating metal(loid)-contaminated wastewater.