Department of Ecosystem Science and Management, University of Wyoming, Laramie, Wyoming 82071, United States.
X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Lemont, Illinois 60439, United States.
Environ Sci Technol. 2021 Mar 2;55(5):3419-3429. doi: 10.1021/acs.est.0c06310. Epub 2021 Feb 18.
Layered manganese (Mn) oxides, such as birnessite, can reductively transform into other phases and thereby affect the environmental behavior of Mn oxides. Solution chemistry strongly influences the transformation, but the effects of oxyanions remain unknown. We determined the products and rates of Mn(II)-driven reductive transformation of δ-MnO, a nanoparticulate hexagonal birnessite, in the presence of phosphate or silicate at pH 6-8 and a wide range of Mn(II)/MnO molar ratios. Without the oxyanions, δ-MnO transforms into triclinic birnessite (T-bir) and 4 × 4 tunneled Mn oxide (TMO) at low Mn(II)/MnO ratios (0.09 and 0.13) and into δ-MnOOH and MnO with minor poorly crystallized α- and γ-MnOOH at high Mn(II)/MnO ratios (0.5 and 1). The presence of phosphate or silicate substantially decreases the rate and extent of the above transformation, probably due to adsorption of the oxyanions on layer edges or the formation of Mn(II,III)-oxyanion ternary complexes on vacancies of δ-MnO, adversely interfering with electron transfer, Mn(III) distribution, and structural rearrangements. The oxyanions also reduce the crystallinity and particle sizes of the transformation products, ascribed to adsorption of the oxyanions on the products, preventing their further particle growth. This study enriches our understanding of the solution chemistry control on redox-driven transformation of Mn oxides.
层状锰（Mn）氧化物，如钠锰矿，可以被还原转化为其他相，从而影响 Mn 氧化物的环境行为。溶液化学对转化有强烈的影响，但氧阴离子的影响尚不清楚。我们在 pH 值为 6-8 且 Mn(II)/MnO 摩尔比范围很宽的条件下，确定了在磷酸盐或硅酸盐存在下，Mn(II)驱动的δ-MnO（一种纳米颗粒六方钠锰矿）还原转化的产物和速率。在没有氧阴离子的情况下，δ-MnO 在低 Mn(II)/MnO 比（0.09 和 0.13）下转化为三方钠锰矿（T-bir）和 4×4 隧道型 Mn 氧化物（TMO），在高 Mn(II)/MnO 比（0.5 和 1）下转化为 δ-MnOOH 和 MnO，并伴有少量结晶不良的 α-和 γ-MnOOH。磷酸盐或硅酸盐的存在会大大降低上述转化的速率和程度，这可能是由于氧阴离子在层边缘的吸附，或在 δ-MnO 的空位上形成 Mn(II,III)-氧阴离子三元配合物，从而不利于电子转移、Mn(III)的分布和结构重排。氧阴离子还会降低转化产物的结晶度和颗粒大小，这归因于氧阴离子在产物上的吸附，阻止了它们进一步的颗粒生长。这项研究丰富了我们对溶液化学控制 Mn 氧化物氧化还原驱动转化的理解。
