Abstract: Iron-bearing clay minerals are predominant in soils and sediments, and they undergo oxidation-reduction cycles as a result of natural processes such as wetting/drying cycles and plant root respiration. However, the kinetics and mechanisms of multiple redox cycles of iron in clay-rich sediments and consequences of such cycling on sediment properties are poorly understood. The objective of this study was to understand how multiple redox cycles of Fe in clay-rich sediment affect the rate and extent of Fe bioreduction and the physicochemical properties of the sediment. A natural sediment sample containing Fe-bearing montmorillonite from Hanford, Washington, USA was size-fractionated [2.0 to 0.5 μm (Hanford-C) and 0.5 to 0.02 μm (Hanford-F)] and redox-cycled for four times. Bioreduction was achieved utilizing Geobacter sulfurreducens and re-oxidization was performed with sparged air. Time-course change of total Fe (II) was monitored to measure the rate and extent of Fe (III) bioreduction. Redox-cycled sediments were characterized to determine the physicochemical changes. Both the initial rate and extent of bioreduction fluctuated across the four redox cycles, but they ultimately decreased from 5.3 μmol g−1 h−1 and 22.9% to nearly zero by the fourth cycle. These fluctuation patterns were likely due to a combined effect of reductive dissolution of small/poorly crystalline clay particles (by 3–5%) and clay mineral structural changes, as evidenced by redox induced changes of aqueous chemistry, surface area, cation exchange capacity, mineralogy, and Mossbauer parameters. Once these small/poorly crystalline clay particles were dissolved, structural Fe in residual larger and more crystalline clay particles was largely reversible across additional redox cycles, as revealed by Mossbauer spectroscopy through the first three cycles.

中文翻译：

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美国华盛顿州汉福德遗址地下沉积物微生物介导的铁氧化还原循环

摘要： 含铁粘土矿物在土壤和沉积物中占主导地位，它们在干湿循环和植物根系呼吸等自然过程中经历氧化还原循环。然而，人们对富含粘土沉积物中铁的​​多次氧化还原循环的动力学和机制以及这种循环对沉积物性质的影响知之甚少。本研究的目的是了解富粘土沉积物中 Fe 的多次氧化还原循环如何影响 Fe 生物还原的速率和程度以及沉积物的物理化学性质。对来自美国华盛顿州汉福德的含铁蒙脱石的天然沉积物样品进行粒度分级 [2.0 至 0.5 μm (Hanford-C) 和 0.5 至 0.02 μm (Hanford-F)]，并进行四次氧化还原循环。利用硫还原地杆菌实现生物还原，并用鼓泡空气进行再氧化。监测总 Fe (II) 的时程变化以测量 Fe (III) 生物还原的速率和程度。表征氧化还原循环沉积物以确定物理化学变化。生物还原的初始速率和程度在四个氧化还原循环中都有波动，但最终在第四个循环中从 5.3 μmol g-1 h-1 和 22.9% 降至接近零。这些波动模式可能是由于小/结晶性差的粘土颗粒的还原溶解（3-5%）和粘土矿物结构变化的综合影响，氧化还原引起的水化学、表面积、阳离子交换容量、矿物学和 Mossbauer 参数。