Zeolite-supported nanoscale zero-valent iron: New findings on simultaneous adsorption of Cd(II), Pb(II), and As(III) in aqueous solution and soil

Highlights: • The intrinsic limitation of zeolite for AsO₂⁻ adsorption was overcome. • Removal efficiencies for Cd²⁺, Pb²⁺, and AsO₂⁻ were rarely high. • The competitiveness ranking of the pollutants was Pb(II) > As(III) > Cd(II). • The removal of the pollutants were stable within a wide pH range. • XRD and XPS confirmed the precipitation and reduction of Cd(II) and Pb(II). - Abstract: Nanoscale zero-valent iron (NZVI) has a high adsorption capacity for heavy metals, but it forms aggregates easily. In this study, zeolite-supported nanoscale zero-valent iron (Z-NZVI) was synthesized from a simplified liquid-phase reduction of iron(III) salts which simultaneously adsorbed As(III), Cd(II) and Pb(II) from aqueous solution and soil. Scanning electron micrographs showed that aggregation was eliminated and the NZVI evenly dispersed onto the surface of zeolite. FTIR spectra reveal that NZVI was protected from oxidization on the surface of Z-NZVI. XRD and XPS patterns confirmed the formation of Cd(OH)₂, Pb⁰, and FeAsO₄ in Z-NZVI after adsorption. The experimental maximum adsorption capacity of Z-NZVI was 11.52 mg As(III)/g, 48.63 mg Cd(II)/g, and 85.37 mg Pb(II)/g at pH 6, respectively, much higher than that of zeolite. Batch experiments indicate that various adsorption mechanisms including electrostatic adsorption, ionic exchange, oxidation, reduction, co-precipitation, and complexation coexisted with the selected heavy metals. Due to the formation of multiphase compounds on the Z-NZVI, the synergy and competition among heavy metals were concurrent. Most arsenic, cadmium and lead in the soil samples were immobilized after mixing with 30 g/kg Z-NZVI. These results suggest that Z-NZVI has great potential for treating water and soil multi-contaminated with heavy metals.