Department of Natural Resources and Environmental Sciences, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States.
CAS Key Laboratory of Mineralogy and Metallogeny/Guangdong Provincial Key Laboratory of Mineral Physics and Materials, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, PR China.
Environ Sci Technol. 2020 Nov 3;54(21):13701-13708. doi: 10.1021/acs.est.0c04043. Epub 2020 Oct 22.
An elevated activity of (bi)carbonate in soils and sediments (pCO, ∼2%) above current atmospheric CO (∼0.04%) could influence the iron cycling in mineral-water interfacial chemistry. However, the impact of (bi)carbonate on mineral transformation is unclear. Here, a model short range-ordered iron oxyhydroxide, two-line ferrihydrite, was used to evaluate the impact of (bi)carbonate on mineral transformation at near-neutral pH using experimental geochemistry, X-ray diffraction, X-ray absorption spectroscopy, transmission electron microscopy, and Fourier transform infrared spectroscopy. Results showed that (bi)carbonate promoted the transformation of ferrihydrite to hematite and retarded the goethite formation. As pCO increased from 408 to 20,000 ppmv at 40 °C, the transformation efficiency of ferrihydrite increased from 53 to 95%, and the formation of hematite increased from 13 to 76%. During the formation of hematite, a terminal ligand on a Fe(III)O octahedral monomer such as a hydroxyl or water was displaced to form Fe(III)O octahedral dimers and/or polymers. Because the Fe-O bond of ≡(Fe-O)-CO is much weaker than that of ≡Fe-O-H, the -OCO group can be more easily replaced by two terminal -OH groups; the dehydration/rearrangement between Fe(III)O octahedral monomers was enhanced under high pCO. Results suggest that high carbonate activity is an important geochemical parameter controlling the occurrence of hematite in oxic environments and, in turn, iron cycling in the critical zone.
土壤和沉积物中（bi）碳酸盐的活性升高（pCO2，约为 2%），高于当前大气 CO2（约为 0.04%），可能会影响矿物-水界面化学中的铁循环。然而，（bi）碳酸盐对矿物转化的影响尚不清楚。在这里，使用实验地球化学、X 射线衍射、X 射线吸收光谱、透射电子显微镜和傅里叶变换红外光谱，使用模型短程有序铁氢氧化物——两线水铁矿，来评估近中性 pH 下（bi）碳酸盐对矿物转化的影响。结果表明，（bi）碳酸盐促进了水铁矿向赤铁矿的转化，并阻碍了针铁矿的形成。随着 pCO2 从 40°C 时的 408 ppmv 增加到 20,000 ppmv，水铁矿的转化效率从 53%增加到 95%，赤铁矿的形成从 13%增加到 76%。在赤铁矿形成过程中，Fe(III)O 八面体单体上的末端配体（如羟基或水）被取代，形成 Fe(III)O 八面体二聚体和/或聚合物。由于≡(Fe-O)-CO 中的 Fe-O 键比 ≡Fe-O-H 中的弱得多，-OCO 基团更容易被两个末端-OH 基团取代；在高 pCO2 下，Fe(III)O 八面体单体之间的脱水/重排得到增强。结果表明，高碳酸盐活性是控制氧化环境中赤铁矿出现的一个重要地球化学参数，并进而控制了关键带中的铁循环。
