Structural Incorporation of Manganese into Goethite and Its Enhancement of Pb(II) Adsorption

Natural goethite (α-FeOOH) commonly accommodates various metal elements by substituting for Fe, which greatly alters the surface reactivity of goethite. This study discloses the enhancement of Mn-substitution for the Pb²⁺adsorption capacity of goethite. The incorporated Mn in the synthesized goethite presents as Mn­(III) and causes a slight decrease in the a and c of the unit cell parameters and an observable increase in the b direction due to the Jahn–Teller effect of the Mn­(III)­O₆ octahedra. With the Mn content increasing, the particle size decreases gradually, and the surface clearly becomes roughened. The Pb²⁺ adsorption capacity of goethite is observably enhanced by Mn substitution due to the modified surface complexes. And the increased surface-area-normalized adsorption capacity for Mn-substituted goethite indicated that the enhancement of Pb adsorption is not only attributed to the increase of surface area but also to the change of binding complexes. Extended X-ray absorption fine structure (EXAFS) analysis indicates that the binding structures of Pb²⁺ on goethite presents as edge-sharing complexes with a regular R_Pb–Fe = 3.31 Å. In the case of Mn-goethite, Pb²⁺ is also bound with the Mn surface site on the edge-sharing complex with a larger R_Pb–Mn = 3.47 Å. The mechanism for enhancing Pb²⁺ adsorption on Mn-goethite can be interpreted as the preferred Pb²⁺ binding on the Mn site of Mn-goethite surface. In a summary, the Mn-goethite has great potential for material development in environmental remediation.