Diverse performances for Pb(II) adsorption by in situ formed Fe(III) oxyhydroxide derived from ferrate(VI) reduction and ferrous oxidation

Two in situ formed Fe(III) oxyhydroxides (FeOx) originated from ferrate reduction (designated FeOx-Fe-VI) and ferrous oxidation by H2O2 (designated FeOx-Fe-II) were compared in the aspects of morphology, hydrolyzed species, surface binding mechanism of lead. The theoretical maximum adsorption capacity calculated from the Langmuir model toward Pb(II) was 929.54 and 810.37 mg/g Fe by FeOx-Fe-VI and FeOx-Fe-II, respectively. At pH 6 and the same Fe/Pb ratio, the kinetic rate of Pb removal by the FeOx-Fe-VI process was 8 times faster. FTIR, SEM, and Ferron assay suggest FeOx-Fe-VI was associated with a lesser polymerization degree and contained more reactive hydroxyl-Fe polymers than those in the FeOx-Fe-II sample. SAXS verified that the particles possessed a smaller, more homogeneous, and open structure when Fe was hydrolyzed by ferrate reduction than ferrous oxidation. XPS coupled with fractal analysis suggests the different sorption capacities of Pb(II) can be ascribed to their distinct growth patterns. Fast cluster agglomeration during FeOx-Fe-II fabrication decreased the exposure of effective adsorption sites. In comparison, the incompact assemblies of FeOx-Fe-VI clusters facilitated Pb(II) ions to access the interstices of octahedral FeO6 units and formed an edge-sharing complex. This work provides new insight into mechanisms of particle fabrication and heavy metal removal of Fe(III) formed in situ.