The photo-enhanced dissolution of semiconducting birnessite by organic reductants prevalently occurs in the euphotic zones of natural aquatic systems. However, the underlying photoelectrons transfer mechanisms and kinetic factors are not fully understood. In this study, the anoxic photoreduction of δ-MnO₂ (a phase analogue for natural birnessite) by citrate under simulated solar irradiation at pH 4.0, 5.5 and 7.0 was investigated, and the structural evolution of δ-MnO₂ during the reaction was examined. The photoreduction of δ-MnO₂ was observed at all three pHs. ~86.0% and ~32.7% Mn²⁺ was released into the solution at pH 4.0 and 5.5, respectively; while negligible amount of dissolved Mn²⁺ was produced at pH 7.0. Mn average oxidation state (AOS) in both the bulk and surface of δ-MnO₂ lowered significantly with decreasing pH (3.39/2.56, 3.57/3.07 and 3.78/3.19 in bulk/surface at pH 4.0, 5.5 and 7.0, respectively). The increase in the photoreduction rate and extent of δ-MnO₂ at lower pHs indicated the photoelectron transfer was pH-dependent. From the amount of reduced Mn(III/II) in structure and Mn²⁺ in solution, the average photoelectron transfer amount per mol Mn was calculated as 1.81, 0.95 and 0.22 at pH 4.0, 5.5 and 7.0, respectively. The retention of Mn(II) (~13% in content) in the structure of reacted δ-MnO₂ was only observed at pH 4.0 rather than higher pHs. The photoreduction of Mn(IV) involved two successive steps of single-electron transfer to produce Mn(III) and further Mn(II), and the above results suggested the second-step photoelectron transfer was more effective under acidic conditions. The potential difference between the conduction band (CB) and MnO₂/Mn²⁺ half-reaction was enlarged as pH decreased, which conduced to the optimal effectiveness of photoelectron transfer and thorough reduction of Mn(IV/III) under lower pH conditions. The pH-dependent Mn photoreduction occurred throughout the structure of δ-MnO₂, whereas for the dark reaction, the influence of pH on the reduction extent only took effect on the surface. These observations implied the dark-reaction electron transfer predominantly occurred at the surface sites. By contrast, according to the semiconductor energy band theory, the excited CB photoelectrons were shared by all atoms in the lattice of δ-MnO₂, and each Mn atom had equal chance to be reduced by photoelectrons. Furthermore, the accumulation of photo-reduced and large-size Mn(III/II) in the layer caused the spatial expansion of the in-plane crystallization, and could possibly alter the layer symmetry or even promote the long-term structural transformation towards tunneled Mn oxides. This work provides insights into the photocatalytic geochemical performance of the naturally-abundant birnessite, which is influenced by varied pH conditions on Earth's surface.

半导体水钠锰矿被有机还原剂光增强溶解的现象普遍发生在天然水生系统的富营养区。然而，尚未完全理解潜在的光电子转移机理和动力学因素。在这项研究中，缺氧光还原δ-的MnO ₂（相位类似物对天然水钠锰矿）通过模拟太阳照射下柠檬酸盐在pH 4.0，5.5和7.0进行了研究，并且δ-的MnO的结构演进₂检查在反应过程中。δ-MnO的光还原₂在所有三个pH值进行了观察。在pH 4.0和5.5时，〜86.0％和〜32.7％的Mn ²⁺释放到溶液中；而溶解的Mn ²⁺量可忽略不计在pH 7.0下产生。在块状和δ-的MnO表面两者的Mn平均氧化态（AOS）₂显著随pH值降低（3.39 / 2.56，3.57 / 3.07，并在pH为4.0，分别为5.5和7.0，在本体/表面3.78 / 3.19）。在光还原率和δ-MnO的程度的增加₂在较低的pH指示的光电子转移是pH依赖性。从结构中还原的Mn（III / II）和溶液中的Mn ^2+的量，在pH 4.0、5.5和7.0下，每摩尔Mn的平均光电子转移量分别计算为1.81、0.95和0.22。锰（II）（以含量〜13％）的中的结构反应δ-MnO的保持₂仅在pH 4.0而不是更高的pH下观察到。Mn（IV）的光还原涉及两个连续的单电子转移步骤，从而生成Mn（III）和进一步的Mn（II），以上结果表明第二步光电子转移在酸性条件下更为有效。随着pH的降低，导带（CB）与MnO ₂ / Mn ²⁺半反应之间的电势差增大，这导致了在较低pH条件下光电子转移的最佳有效性以及Mn（IV / III）的彻底还原。该pH依赖性的Mn光还原发生，导致的结构δ-的MnO ₂，而对于黑暗反应，pH值对还原程度的影响仅影响表面。这些观察结果表明暗反应电子转移主要发生在表面部位。与此相反，根据半导体能带理论，被激发的光电子的CB通过在晶格中的所有原子共享δ-的MnO ₂，并且每个Mn原子都有相等的机会被光电子还原。此外，层中光还原和大尺寸Mn（III / II）的积累引起面内结晶的空间扩展，并可能改变层的对称性，甚至可能促进向隧道结构的长期结构转变锰氧化物。这项工作为自然丰富的水钠锰矿的光催化地球化学性能提供了见识，这受到地球表面不同pH条件的影响。