Metals In Nodules Research Articles

Potential Ecological Risk Assessment of Heavy Metals in the Fe-Mn Nodules in the Karst Area of Guangxi, Southwest China.

We investigated the concentration of heavy metals in the Fe-Mn nodules in soils derived from the carbonate rocks of typical karst areas in Guangxi, Southwest China. Compared with the soil background values in Guangxi, heavy metals are substantially enriched in the following order: Cd (268.13) > Pb (39.46) > Cr (11.80) > Zn (8.43) > Ni (6.16) > Cu (3.65). CaCl2 extraction results indicate that heavy metals in the nodules are substantially stable, while the proportions of the metals released to the surrounding environment are extremely low (≤ 0.003%). Moreover, pH-static leaching experiments indicate that heavy metals can hardly be dissolved into the surrounding soil environment under natural conditions (pH 6-8). However, once the soil environment is acidified, heavy metals incorporated in the nodules will gradually release into the surrounding environment, thereby causing potential ecological risks.

“Zero-Waste”: A Sustainable Approach on Pyrometallurgical Processing of Manganese Nodule Slags

A continuously growing demand for valuable non-ferrous metals and therefore an increase in their prices at the metal exchanges makes it necessary and profitable to investigate alternative metal resources. Polymetallic deep-sea nodules contain cobalt, copper, manganese, molybdenum and nickel, and are highly abundant on the sea floor. Developing a metallurgical process to recover the metal content from manganese nodules can close the predicted supply gap of critical metals like cobalt. This paper investigated a potential extraction process for valuable metals from manganese nodules supplied by the German Federal Institute for Geosciences and Natural Resources. The samples originated from the German license area of the Clarion-Clipperton Zone in the Pacific Ocean. Due to a low concentration of valuable metals in nodules, a pyrometallurgical enrichment step was carried out to separate cobalt, copper, molybdenum and nickel in a metallic phase. The manganese was discarded in the slag and recovered in a second smelting step as ferromanganese. To aid the experiments, FactSageTM was used for thermodynamic modeling of the smelting steps. To increase metal yields and to alter the composition of the metal alloys, different fluxes were investigated. The final slag after two reduction steps were heavy-metal free and a utilization as a mineral product was desired to ensure a zero-waste process.

Micrometer-scale internal structure and element distribution of Fe-Mn nodules in Quaternary red earth of Eastern China

Fe-Mn nodules are the common feature of tropical and subtropical soils and contain abundant information of pedogenic processes, palaeoenvironmental changes, and element geochemistry. The main aim of the present study was to determine the internal structure and spatial distribution of elements in the Fe-Mn nodules to better understand the 3D internal structure, enrichment, and dynamics of heavy metals in nodules and to provide more aspects to explore the possible heavy metal sequestration and pedoenvironmental implications of Fe-Mn nodules in soils. The studied Typic Plinthudult was developed on Quaternary red earths in Eastern China. The Fe-Mn nodules in the Bs horizon of soil were separated and classified into four size fractions (5–8, 3–5, 2–3, and 1–2 mm). The 3D microstructure of Fe-Mn nodules was examined by means of synchrotron radiation-based X-ray microcomputed tomography (SR-mCT), and the spatial distribution of Fe and Mn in nodules was studied by scanning electron microscopy (SEM) equipped with an energy-dispersive X-ray spectrometer (EDS). The association of heavy metals with Fe and Mn oxides in nodules was described by selective chemical dissolution, SEM/EDS, and principal component analysis. The SR-mCT images indicated that the 5–8, 3–5, and 2–3 mm nodules exhibited well-defined ring structures, while the 1–2 mm nodule exhibited homogeneous fabric. The internal microstructures of nodules could be divided into four parts: Fe-rich ring, Mn-rich ring, Fe and Mn overlapped ring, and the gap between ring structures. The Fe-Mn nodules were significantly enriched in Mn, Pb, Ni, Cu, and Zn relative to the soil matrix. In particular, the concentrations of Mn and Pb in the nodules were 150 and 90 times greater than those in the soil matrix, respectively. A clear partitioning of heavy metals between Mn and Fe oxide phases was observed in the nodules, indicating that Pb was mainly present in Fe oxides, while Ni, Cu, and Zn were mainly associated with Mn oxide phases. The SR-mCT and SEM-EDS revealed the detailed internal microstructure of the Fe-Mn nodules and geochemical dynamics of heavy metals in the soil system. The Fe-Mn nodules have very high scavenging ability in sequestrating toxic heavy metals in soils, such as Pb and Ni. The microstructure and spatial distribution of Fe and Mn in nodules reflected the cycle of alternating drying and wetting conditions and served as an important basis for inferring the pedogenic processes and pedoenvironmental conditions.

Influence of zinc on cadmium-induced toxicity in nodules of pigeonpea (Cajanus cajan L. Millsp.) inoculated with arbuscular mycorrhizal (AM) fungi

Arbuscular mycorrhizal (AM) fungi are known to alleviate heavy-metal stress in plants. The intent of the present work was to analyze accumulation of heavy metals (Cd and Zn) in nodules of two Cajanus cajan (L.) Millsp. genotypes and their subsequent impact on nitrogen fixation, oxidative stress, and non-protein thiols (glutathione and phytochelatins) with and without AM fungus Glomus mosseae. Accumulation of Cd and Zn in nodules resulted in sharp reduction in nodule number, nodule dry mass as well as nitrogen fixation (leghemoglobin and nitrogenase (N2ase)), although Cd had more pronounced effects than Zn. Cd-induced lipid peroxidation, H2O2 accumulation, and electrolyte leakage were largely reversed by Zn supplementation. Zn application significantly altered the negative effects of Cd on the synthesis of non-protein thiols, suggesting antagonistic behaviour of Zn. Higher concentration of Zn was more effective in lessening the negative effects of Cd than its lower concentration. Remarkable genotypic variation was found, with more severe effects of both the metals in P792 than Sel 85N. Glomus mosseae attenuated the phytotoxic effects of metals in nodules by decreasing metal uptake, oxidative stress, and by enhancing defense system ultimately leading to better nitrogen-fixing potential of pigeonpea nodules.

The composition of Fe-Mn nodules as determined by synchrotron X-ray analysis (Review of publications)

In recent years, the Fe-Mn nodules of different genesis have been intensely studied using synchrotron X-ray microscopy, which permits analyzing undisturbed samples. Three types of analysis are performed at once in this case, i.e., microfluorescence and microdiffraction analyses and a detailed analysis of the fine structure of the absorption spectra. This permits researchers to determine the dispersal of elements, to identify the minerals-carriers in soils, and to reveal the mechanism of heavy metal fixation on a molecular scale using the same experimental device. It is found that heavy metals in nodules are subdivided into siderophilic (As and Cr) and manganophilic (Pb, Co, Cu, and Cd) elements. Zinc is fixed predominantly by phyllosilicates. Nickel is fixed by Mn-oxides in some nodules and by Fe-hydroxides in others. Nodules with a complicated structure are formed in many soils. Their stability is ensured by the relatively stable goethite contained in the ferruginated envelope of the nodules, whereas unstable minerals (feroxyhyte, vernadite, and birnessite) are concentrated in the ferromanganic nuclei of the nodules.

Cerium anomaly variations in ferromanganese nodules and crusts from the Indian Ocean

Fifty analyses of rare earth elements as well as mineralogical studies have been carried out on a suite of manganese nodules and crusts from the Central Indian Basin and the Western Indian Ocean. The aim was to identify the processes controlling the REE patterns of the phases hosting the REE in the manganese nodules, with an emphasis on an understanding of the Ce anomaly. This has involved separating the encrusting layers and nuclei physically as well as Fe-Mn oxides from the aluminosilicate phase chemically (using a 2 M HCl leach) prior to analysis. The presence of nodule nuclei seems to have little influence (mostly <5% to a maximum of 30%) on the overall magnitude of the Ce anomalies in these nodules. The ratios of concentrations of elements in the acid leachates and the corresponding bulk values yield flat REE patterns indicating that the aluminosilicate phase contributes very little to the Ce anomalies. Interelement relations indicate that the Ce anomalies are largely controlled by the amorphous mineral phase FeOOH. xH 2O. The relationship of Fe, Ce anomaly and δ-MnO 2 further suggests that Ce is chemisorbed onto iron oxyhydroxides which are epitaxially intergrown with δ-MnO 2. The regional distribution of the Ce anomaly values appears to depend on many of the factors responsible for the uptake of other minor metals in nodules and crusts.

Rapid growth of a deep-sea manganese nodule

Recent studies on marine ferromanganese nodules have suggested a relationship between their growth rate and their chemical composition1–3. The suggestion is based on the hypothesis that the metals in nodules have two sources of supply: bottom seawater (hydrogenous) and sediment pore water (diagenetic)4–7. The diagenetic source calls for upward diffusion of Mn mobilized in a deeper, low Eh zone of the sediment column and its subsequent precipitation in an oxidized surface zone8. Addition of the remobilized Mn to surface nodules would augment their accretion rate. It would also render the oxide layers to contain higher Mn/Fe ratios than layers resulting from hydrogenous precipitation. We report here a radiometrically determined accretion rate of 168±24 mm Myr−1 for a deep-sea manganese nodule from the Peru Basin in the Pacific. This rate is about two orders of magnitude higher than that frequently found in nodules of deep-sea origin, and emphasizes the importance of sediment diagenetic processes on the growth of manganese concretions.

New constraint on the maintenance of Mn nodules at the sediment surface

Investigations into the association of manganese nodules with pelagic sediment in many areas of the deep ocean have mainly considered either (1) the source of metals in nodules, or (2) the occurrence of nodules predominantly at the sediment surface. The second problem is duscussed here. The mechanisms previously proposed have failed to consider that nodules retain their orientation at the sediment surface for several hundred-thousand years, during which time several tens of centimetres of sediment are fluxed down beneath them. Although we have no conclusive evidence, we consider the most plausible explanation for the surface occurrence of nodules to be bioturbation by infauna.

Compositional variation in Pacific Ocean ferromanganese nodules and its relationship to sediment accumulation rates

Recent work on the geochemistry of deep- and shallow-water manganese nodules, and the post-depositional migration of manganese and related metals in marine sediments, has emphasized the role of diagenesis in the formation of manganese nodules and concretions. These observations have led to a reappraisal of the contrasting compositions and modes of formation of open oceanic and shallow marine nodules. The Mn Fe ratio in Pacific Sea floor nodules is highest along the eastern margin and lowest in the southwest central region. The Fe Co and Mn Ni ratios are also highest in the eastern Pacific but lowest in the north and south central Pacific. The enrichment of Mn in marginal nodules is caused by the diagenetic migration of Mn within the Recent sediments and its precipitation at the sediment surface. The Mn Fe ratio is therefore a useful index of diagenetic derivation of metals in nodules. The depletion of minor metals in marginal nodules, in spite of an adequate source, must similarly be controlled by diagenetic processes within the sediments. Since diagenesis is largely governed by the amount and type of organic matter in sediments and this depends on the rate of sediment accumulation, diagenesis is most marked in marginal, rapidly accumulating sediments and minimal in open oceanic sediments. The regional variations in nodule composition therefore reflect the broad pattern of sedimentation in the Pacific, an explanation which is at variance with the current view that volcanism controls the composition and the formation of manganese nodules.