Influence of pH on the Reductive Transformation of Birnessite by Aqueous Mn(II) Joshua P. Lefkowitz, † Ashaki A. Rouff, ‡ and Evert J. Elzinga †, * † Department of Earth & Environmental Sciences, Rutgers University, Newark, New Jersey 07102, United States ‡ School of Earth and Environmental Sciences, Queens College, Queens, New York 11367, United States * S Supporting Information ABSTRACT: We investigated the effect of pH (5.5-8.5) on the mineralogical transformation of hexagonal birnessite induced by reaction with aqueous Mn(II) (50-2200 μM), using batch sorption experiments, X-ray diffraction analyses, X-ray absorption and infrared spectroscopic measurements. Samples reacted at pH < 7.0 exhibited disrupted stacking of birnessite sheets, but no mineralogical transformation products were observed. At pH 7.0 and 7.5, reaction with Mn(II) under anoxic conditions caused reductive transformation of birnessite into manganite (γ-MnOOH), whereas at pH 8.0 and 8.5, conversion into hausmannite (Mn 3 O 4 ) occurred. Feitknechtite (β- MnOOH) is a major transformation product at low Mn(II) inputs at pH 7.0-8.5, and represents a metastable reaction intermediate that is converted into manganite and possibly hausmannite during further reaction with Mn(II). Thermodynamic calculations suggest that conversion into hausmannite at alkaline pH reflects a kinetic effect where rapid hausmannite precipitation prevents formation of thermodynamically more favorable manganite. In oxic systems, feitknechtite formation due to surface catalyzed oxidation of Mn(II) by O 2 increases Mn(II) removal relative to anoxic systems at pH ≥ 7. The results of this study suggest that aqueous Mn(II) is an important control on the mineralogy and reactivity of natural Mn-oxides, particularly in aqueous geochemical environments with neutral to alkaline pH values. ■ INTRODUCTION Hexagonal birnessite type minerals have garnered much interest due to their natural ubiquity, unique structural and reactive properties, and for their potential impact on the fate and transport of a diverse range of chemical species in the environment. 1-5 These phyllomanganates dominate Mn mineralogy in a variety of geochemical environments with reported occurrences in soils, marine Mn-Fe nodules, and desert varnishes. 1 Hexagonal birnessites are the main product derived from the biologically catalyzed oxidation of Mn- (II), 2,4,6-8 and are structurally characterized by mineral sheets with hexagonal layer symmetry and a significant proportion of reactive anionic vacancy sites. 9 They are further noted for signi ficant Mn(IV) content, with reported average Mn oxidation states of 3.7-4.0, 2,8,9 as well as for large specific surface area, 10 high redox potential, 11 and low point of zero charge. 12 Their physical and chemical characteristics provide these minerals with high reactivity with respect to both sorption and redox reactions, explaining their critical role in determining the speciation and distribution of trace element and pollutant species in the environment. 2,13-19 Recent work has shown that hexagonal birnessites are subject to structural and mineralogical changes during reaction with aqueous Mn(II), which suggests that the dissolved Mn(II) concentration represents an important control on the structure and reactivity of Mn-oxides in aqueous geochemical environ- ments. Early work on interactions between aqueous Mn(II) and hexagonal birnessite focused on the adsorption of the aqueous metal by the mineral substrate. 20-22 Tu et al. 23 and Mandernack et al. 24 demonstrated that under oxic conditions, reaction of Mn(II) with hexagonal birnessite yielded a variety of Mn-oxide mineral products dependent on pH, with formation of nsutite (γ-Mn(IV,III)(O,OH) 2 ) and ramsdellite (Mn(IV)- O 2 ) observed at pH 2.4, cryptomelane (K 1.3-1.5 Mn(IV, III) 8 O 16 ) and groutite (α-Mn(III)OOH) at pH 4.0 and 6.0, respectively, and feitknechtite (β-Mn(III)OOH) and manganite (γ-Mn(III)OOH) at pH > 7. Bargar et al. 25 observed the formation of feitknechtite during reaction of aqueous Mn(II) with biogenic hexagonal birnessite in oxic systems at circum- neutral pH, and attributed this to electron exchange between adsorbed Mn(II) and structural Mn(IV) yielding Mn(III). A recent study by Elzinga 26 demonstrated that reaction of aqueous Mn(II) with hexagonal birnessite under anoxic conditions at pH 7.5 leads to bulk transformation of the mineral into manganite through a reductive transformation Received: May 10, 2013 Revised: July 18, 2013 Accepted: July 22, 2013 Published: July 22, 2013 Article pubs.acs.org/est © 2013 American Chemical Society 10364 dx.doi.org/10.1021/es402108d | Environ. Sci. Technol. 2013, 47, 10364-10371