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 (FeO) originated from ferrate reduction (designated FeO-Fe) and ferrous oxidation by HO (designated FeO-Fe) 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 FeO-Fe and FeO-Fe, respectively. At pH 6 and the same Fe/Pb ratio, the kinetic rate of Pb removal by the FeO-Fe process was 8 times faster. FTIR, SEM, and Ferron assay suggest FeO-Fe was associated with a lesser polymerization degree and contained more reactive hydroxyl-Fe polymers than those in the FeO-Fe 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 FeO-Fe fabrication decreased the exposure of effective adsorption sites. In comparison, the incompact assemblies of FeO-Fe clusters facilitated Pb(II) ions to access the interstices of octahedral FeO 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.