Soil Chemistry Group, Institute of Biogeochemistry and Pollutant Dynamics, Department of Environmental Systems Science, ETH Zurich, Universitätstrasse 16, CHN, CH-8092 Zürich, Switzerland.
Soil Chemistry Group, Institute of Biogeochemistry and Pollutant Dynamics, Department of Environmental Systems Science, ETH Zurich, Universitätstrasse 16, CHN, CH-8092 Zürich, Switzerland.
Sci Total Environ. 2022 Apr 10;816:151567. doi: 10.1016/j.scitotenv.2021.151567. Epub 2021 Nov 8.
In freshwater wetlands, redox interfaces characterized by circumneutral pH, steep gradients in O, and a continual supply of Fe(II) form ecological niches favorable to microaerophilic iron(II) oxidizing bacteria (FeOB) and the formation of flocs; associations of (a)biotic mineral phases, microorganisms, and (microbially-derived) organic matter. On the volcanic island of Iceland, wetlands are replenished with Fe-rich surface-, ground- and springwater. Combined with extensive drainage of lowland wetlands, which forms artificial redox gradients, accumulations of bright orange (a)biotically-derived Fe-rich flocs are common features of Icelandic wetlands. These loosely consolidated flocs are easily mobilized, and, considering the proximity of Iceland's lowland wetlands to the coast, are likely to contribute to the suspended sediment load transported to coastal waters. To date, however, little is known regarding (Fe) mineral and elemental composition of the flocs. In this study, flocs from wetlands (n = 16) across Iceland were analyzed using X-ray diffraction and spectroscopic techniques (X-ray absorption and Fe Mössbauer) combined with chemical extractions and (electron) microscopy to comprehensively characterize floc mineral, elemental, and structural composition. All flocs were rich in Fe (229-414 mg/g), and floc Fe minerals comprised primarily ferrihydrite and nano-crystalline lepidocrocite, with a single floc sample containing nano-crystalline goethite. Floc mineralogy also included Fe in clay minerals and appreciable poorly-crystalline aluminosilicates, most likely allophane and/or imogolite. Microscopy images revealed that floc (bio)organics largely comprised mineral encrusted microbially-derived components (i.e. sheaths, stalks, and EPS) indicative of common FeOB Leptothrix spp. and Gallionella spp. Trace element contents in the flocs were in the low μg/g range, however nearly all trace elements were extracted with hydroxylamine hydrochloride. This finding suggests that the (a)biotic reductive dissolution of floc Fe minerals, plausibly driven by exposure to the varied geochemical conditions of coastal waters following floc mobilization, could lead to the release of associated trace elements. Thus, the flocs should be considered vectors for transport of Fe, organic carbon, and trace elements from Icelandic wetlands to coastal waters.
在淡水湿地中，具有中性 pH 值、氧气梯度陡峭和连续供应二价铁 (Fe(II)) 的氧化还原界面形成了有利于微好氧铁(II)氧化细菌 (FeOB) 和絮体形成的生态小生境；生物和非生物矿物相、微生物和（微生物衍生的）有机质的组合。在冰岛这个火山岛上，湿地中富含铁的表层水、地下水和泉水得到了补充。与低地湿地的广泛排水相结合，形成了人工氧化还原梯度，明亮的橙色生物衍生富铁絮体的积累是冰岛湿地的共同特征。这些松散的絮体很容易被移动，而且考虑到冰岛低地湿地靠近海岸，它们很可能会增加输送到沿海水域的悬浮泥沙负荷。然而，迄今为止，人们对絮体的（铁）矿物和元素组成知之甚少。在这项研究中，对来自冰岛各地湿地的絮体（n=16）使用 X 射线衍射和光谱技术（X 射线吸收和 Fe Mössbauer）以及化学提取和（电子）显微镜进行了分析，以全面表征絮体的矿物、元素和结构组成。所有絮体都富含铁（229-414mg/g），絮体的铁矿物主要由水铁矿和纳米晶纤铁矿组成，单个絮体样品中含有纳米晶针铁矿。絮体矿物学还包括粘土矿物中的铁和相当数量的非晶态铝硅酸盐，最可能是埃洛石和/或硅线石。显微镜图像显示，絮体（生物）有机物主要由矿物包裹的微生物衍生成分（即鞘、茎和 EPS）组成，表明常见的 FeOB 亮发菌属和泉发菌属。絮体中的微量元素含量在低μg/g 范围内，但几乎所有微量元素都用羟胺盐酸盐提取。这一发现表明，絮体中铁矿物的（生物）还原性溶解，很可能是由于絮体移动后暴露于沿海水域多变的地球化学条件所致，可能导致相关微量元素的释放。因此，这些絮体应该被视为从冰岛湿地向沿海水域输送铁、有机碳和微量元素的载体。
