土壤铁/锰氧化物的协同共生及其对铬形态的影响

Iron oxides and manganese oxides are widely distributed in soils and sediments, and their synergistic coexistence is more likely to occur. They both could affect the speicies, toxicity, and migration ability of elemental chromium. In hypergene environments, adsorbed Mn(II) participates in the transformation of ferric oxide crystal structures, and the catalytic oxidation of Mn(II) to manganese oxides with the higher valence states of manganese usually occurred in oxygen atmosphere. Manganese oxides accelerate the formation of ferric oxides due to the oxidation of Fe(II). As reported in our previous work, Mn(II) ions located in the crystal structures and adsorbed onto crystal surface possibly affect the transformation process of the crystal structures of ferric oxides, however, the exact mechanism remains unclear. The catalytic oxidation mechanism and kinetics of Mn(II) to manganese oxides in the presence of ferric oxides will also be different due to the diversity of the crystal structures of ferric oxides. The chemical compositions and proportions of ferric oxides and manganese oxides would be changed during the reaction process, which will subsequently influence the chromium species by adsorption and oxidation. The above problems need to be further addressed. In this project, the influences of Mn(II) sites, including the crystal surface and lattice, on the transformation process of ferric oxides will be systematically studied. In the open natural environment, the catalytic mechanism and kinetics of ferric oxides with various crystal structures will be investigated during the oxidation process of Mn(II) by oxygen in air. The intermediate products will be analyzed and their corresponding influence on chromium species will be studied in detail. During the above adsorption, catalytic oxidation, and transformation process, the effect of clay minerals, organic matters, pH, temperature, and concentration will also be concerned. The proposed research is very likely to reveal the synergistic coexistence mechanism as well as reaction kinetics of iron oxides and manganese oxides in hypergene environments. Such work will also provide the new theoretic foundation on the formation and evolution of the mixed minerals of iron oxides and manganese oxides in soils and sediments. This research can also offer guidance for the remediation of chromium-polluted soils.

土壤铁/锰氧化物分布广泛，能协同共生，影响有毒元素铬的形态、毒性与迁移能力。表生环境中，吸附态Mn(II)参与铁氧化物晶体结构转变，还可被其催化氧化生成高价态锰氧化物，锰氧化物又促使氧化铁生成。目前，Mn(II)在铁氧化物晶体结构转变过程中的作用机制尚不明确，晶体结构不同的氧化铁催化氧化Mn(II)的机理与动力学也会有所差异，铁/锰氧化物协同生成反应不同阶段产物组分对Cr形态的影响如何未见报道。本项目拟探明表面吸附态和晶格内Mn(II)对铁氧化物晶体结构转变过程的影响，分析有氧体系中Mn(II)在不同结构铁氧化物表面催化氧化机理与动力学，明确铁/锰氧化物共生反应不同阶段对Cr(III)吸附氧化与催化氧化的机制，探讨粘土矿物、有机质、pH、温度和浓度等对上述反应的影响。研究结果有望揭示土壤铁/锰氧化物协同共生机理，为阐明其形成演化机制提供新的依据，为铁/锰氧化物缓解土壤铬污染提供理论指导。

天然铁/锰氧化物分布广泛，多相互促进彼此生成并共存于表生环境，影响有毒元素铬环境化学行为与效应。然而，锰氧化物促进表面铁氧化物生成及晶体结构不同的氧化铁催化氧化Mn(II)的机理尚不清楚，铁/锰氧化物协同生成反应不同阶段产物组成对Cr形态影响如何也未见报道。本项目研究了铁/锰氧化物相互促进生成的转化过程，阐明了有氧体系中Mn(II)/Fe(II)催化氧化生成铁锰氧化物机理，探明了模拟共生的铁/锰氧化物及天然铁锰结核吸附氧化Cr(III)的行为，考察了粘土矿物、太阳光辐射及环境因素的影响。结果表明，偏酸体系中黑锰矿易歧化生成层状结构氧化锰前驱体，并进一步转化为隧道结构氧化锰矿物；溶解氧促进了水锰矿表面吸附态Mn2+ads直接氧化，并沿着晶面外延式生长而使晶体增大；太阳光激发NO3−催化氧化Mn(II)/Fe(II)生成了常见铁锰氧化物，如施氏矿物和水钠锰矿等，且太阳光辐射可诱导水铁矿更多向纤铁矿转化。封闭体系中水锰矿氧化Fe2+aq产物主要为针铁矿和纤铁矿，且氧化速率随着Fe2+aq浓度和pH升高而加快；开放有氧体系中，空气（氧气）引入能显著提高Fe2+aq氧化速率和针铁矿结晶度；Fe2+aq和Mn2+aq相互催化加速彼此氧化生成并形成铁锰氧化物共存体。光化学生成的施氏矿物与蒙脱石形成复合物后，其吸附固定Cr(VI)能力增强。水锰矿-针铁矿复合物不仅能吸附Cr(VI)，表面吸附态Fe(II)将部分Cr(VI)还原成Cr(III)并吸附于矿物表面。天然铁锰结核能有效氧化溶解态Cr(III)为Cr(VI)，新生成Cr(VI)多吸附在铁锰结核表面；溶解氧参与Cr(III)氧化反应，Cr(III)氧化速率随溶解氧浓度上升而增大，而铁锰结核有催化加速氧化作用。研究结果揭示了土壤铁/锰氧化物协同共生机理，可为阐明其形成演化机制提供新的依据，还可为铁/锰氧化物缓解土壤铬污染提供理论指导。