The mineral characteristics of Fe-bearing nanoparticles in hot springs
According to the HRTEM images, the Fe-bearing nanoparticles mainly range in size from 50 to 200 nm. The morphology of the Fe-bearing nanoparticles in the hot spring varies and includes triangular, axiolitic, and irregular shapes. Among these, the majority of Fe-bearing nanoparticles exhibit biomimetic morphologies, resembling cells or microorganisms. Furthermore, these Fe-bearing nanoparticles with biomimetic morphologies often aggregate, suggesting that they may undergo a growth process [15].
The selected area electron diffraction patterns indicate that the Fe-bearing nanoparticles found in hot springs can exhibit crystal form, amorphous form, and a transition from amorphous to crystalline forms. This transition from amorphous to crystalline forms suggests the growth process of these nanoparticles in hot springs.
The chemical composition of Fe-bearing nanoparticles primarily consists of Fe and O, but there are also various trace elements present in these nanoparticles, including Si, S, Ca, Zn, Cr, Ni, and Mo. This finding suggests that Fe-bearing nanoparticles have the ability to transport elements over significant distances in hot springs.
Origin of Fe-bearing nanoparticles in hot spring
Nanomaterials have gained significant attention in recent years due to their potential applications in technology and medicine [16]. Consequently, the production of engineered nanoparticles has experienced a significant surge, with their usage extending to various industries, consumer goods, and environmental remediation practices [17]. For instance, engineered Fe-oxide nanoparticles hold promise for in-situ remediation of polluted soils [18]. As a result, the majority of nanoparticles present in the environment are synthetic or human-made [19, 20]. These synthetic nanoparticles typically exhibit regular shapes, such as spheres or cubes, and possess a pure composition.
However, a fraction of the global pool of environmental Fe minerals is constantly present as nanoparticles. In contrast, solid bulk minerals are exposed to various processes such as shearing, straining, weathering, and dissolutive reactions, which also result in the formation of nanoparticles [14]. Therefore, Fe-bearing nanoparticles can also be a natural occurrence. Indeed, natural Fe-bearing nanoparticles have been discovered in the air, soil, icebergs, glaciers, and water in various environmental settings [21–23]. Additionally, these natural nanoparticles typically have irregular shapes and complex elemental compositions, distinguishing them from engineered nanoparticles [24–26].
In this study, it was observed that most Fe-bearing nanoparticles and nanoparticle aggregations in hot springs have irregular shapes. The composition of these nanoparticles and aggregations is also found to be complex. Based on these findings, we propose that the Fe-bearing nanoparticles in hot springs have a natural origin. Specifically, some of the Fe-bearing nanoparticles with biomimetic morphologies, such as cell-like or microorganism-like shapes, may be formed as a result of microbial activity. Similar morphologies have been observed in other aquatic environments as well [23]. On the other hand, Fe-bearing nanoparticles without biomimetic morphologies may be formed through weathering of minerals or as growth nuclei in super-saturated fluids. Additionally, since nanoparticle aggregations are commonly found in hot springs, the aggregation process (i.e. ripening) may be the main mechanism responsible for the growth of Fe-bearing nanoparticles in this ecosystem.
Potentially bioavailable properties of Fe-bearing nanoparticles
Dissolved Fe in the form of Fe-bearing nanoparticles exhibits different behavior compared to aqueous inorganic species, as their reactivity can change over time [27, 28]. Iron nanoparticles are considered to be intermediate in terms of their physical and chemical properties between soluble and particulate forms, and their small size enhances their solubility and reactivity by several orders of magnitude. This suggests that nanoparticles may play a significant role in acting as a reservoir of iron that can be transformed into a form that is available for biological processes. Furthermore, studies conducted on cultures have shown that freshly-prepared, poorly-ordered colloids containing Fe can provide the necessary Fe for cell growth, whereas more crystalline phases are not bioavailable [29, 30]. The solubility and reactivity of Fe nanophase are crucial factors in the biogeochemical cycling of Fe in various aquatic environments, including seawater systems and acid mine drainage environments [21]. Hot springs, characterized by a diverse range of chemical compositions and surface temperatures that can range from ambient to 100°C, and pH levels varying from less than 2 to 10, also exhibit a wide range of total dissolved Fe concentrations spanning over five orders of magnitude, from less than 1 µg L− 1 to over 100 mg L− 1 [2, 4–9, 31]. Therefore, due to their high temperatures and diverse chemical compositions, hot springs are expected to provide favorable conditions for the proliferation of biological activity associated with Fe-bearing nanoparticles.
In this study, the morphological structures of Fe-bearing nanoparticles resemble those commonly found in terrestrial bacteria, and some nanoparticles contain biophile elements such as S and N. One of the most noteworthy characteristics of Fe-bearing nanoparticles in hot springs is their bioavailability, as indicated by their resemblance to bacteria [32, 33]. Furthermore, the size and number of these nanoparticles generally increase in culture, suggesting that they might have an impact on the circulation and availability of elements in the environment [34, 35].
Environmental indicator of iron oxide nanoparticles
The physical and chemical characteristics of water, including temperature, pH, ion concentration, oxygen supersaturation, organic matter content, and redox potential (Eh), can vary. These conditions fluctuate across different locations and depths, impacting microbial activities and the formation of biotic iron oxides. Additionally, iron oxide nanostructures are commonly found in various ecosystems, making them useful for monitoring environmental conditions due to their measurable physical and chemical properties [13].
In this study, various iron oxide nanoparticles, such as goethite and hematite, are found in the hot spring. Goethite is commonly observed in aquatic environments with both acidic and alkaline pH levels [13]. In specific environments that are rich in silicate, nanogoethite has been reported as the prevailing precipitated phase. However, in most cases, it is transformed from other less stable oxyhydroxide nanoparticles (e.g. ferrihydrite and lepidocrocite) in water.
The morphology of goethite nanoparticles is predominantly acicular or needle-like, primarily because the (010) surface possesses high surface energy and undergoes more rapid coarsening [36]. However, in this study, the goethite nanoparticles largely exist as aggregates, exhibiting a nearly pseudohexagonal morphology. This morphology is indicative of the alkaline conditions present, as well as the presence of silicates [37], which aligns with the chemical characteristics of the hot springs studied [38, 39].
The morphology of hematite exhibits significant variation and is highly sensitive to environmental conditions, particularly when compared to goethite [36]. In natural settings, hematite nanoparticles are predominantly formed through the dehydration of oxyhydroxides, such as goethite, or through a combination of aggregation and internal rearrangements, as observed in ferrihydrite. The typical shapes of hematite crystals, whether on the nano or macro scale, include rounded, rhombohedral, and platy forms. The coarsely crystalline morphology of hematite often exposes the (001) basal plane, which is believed to be associated with high-temperature processes exceeding 100 ℃ [40]. In this study, the hematite nanoparticles primarily exhibited irregular hexagonal morphologies, similar to nanoparticle aggregation, particularly under neutral pH conditions and at low temperatures [41]. Additionally, the nanoparticles were predominantly coarsely crystalline, with the (-120) and (003) crystal planes observed, suggesting that they cannot be formed under high-temperature conditions. These findings align with the results obtained for goethite nanoparticles and are in accordance with the chemical characteristics of hot springs.
Above all, our results demonstrate that the morphology of iron oxide nanoparticles is closely related to the pH and temperature of the hot spring. Furthermore, these particles can be utilized as indicators to assess environmental conditions.