Iron plaque on aquatic plant roots are ubiquitous and sequester metals in wetland soils; however, the mechanisms of metal sequestration are unresolved. Thus, characterizing the Fe plaque and associated metals will aid in understanding and predicting metal cycling in wetland ecosystems. Accordingly, microscopic and spectroscopic techniques were utilized to identify the spatial distributions, associations, and chemical environments of Fe, Mn, Pb, and Zn on the roots of a common, indigenous wetland plant (Phalaris arundinacea). Iron forms a continuous precipitate on the root surface, which is composed dominantly of ferrihydrite (ca. 63%) with lesser amounts of goethite (32%) and minor levels of siderite (5%). Although Pb is juxtaposed with Fe on the root surface, it is complexed to organic functional groups, consistent with those of bacterial biofilms. In contrast, Mn and Zn exist as discrete, isolated mixed-metal carbonate (rhodochrosite/hydrozincite) nodules on the root surface. Accordingly, the soil-root interface appears to be a complex biochemical environment, containing both reduced and oxidized mineral species, as well as bacterial-induced organic-metal complexes. As such, hydrated iron oxides, bacterial biofilms, and metal carbonates will influence the availability and mobility of metals within the rhizosphere of aquatic plants.
This paper shows that synchrotron-based fluorescence and absorption-edge computed microtomographies (CMT) are well-suited for determining the compartmentalization and concentration of metals in hyperaccumulating plant tissues. Fluorescence CMT of intact leaf, stem, and root samples revealed that Ni concentrated in stem and leaf dermal tissues and, together with Mn, in distinct regions associated with the Ca-rich trichomes on the leaf surface of the nickel hyperaccumulator Alyssum murale "Kotodesh". Metal enrichment was also observed within the vascular system of the finer roots, stem, and leaves but absent from the coarser root, which had a well-correlated metal coating. Absorption-edge CMT showed the three-dimensional distribution of the highest metal concentrations and verified that epidermal localization and Ni and Mn co-localization at the trichome base are phenomena that occurred throughout the entire leaf and may contribute significantly to metal detoxification and storage. Ni was also observed in the leaf tips, possibly resulting from release of excess Ni with guttation fluids. These results are consistent with a transport model where Ni is removed from the soil by the finer roots, carried to the leaves through the stem xylem, and distributed throughout the leaf by the veins to the dermal tissues, trichome bases, and in some cases the leaf tips.
Arsenic is a contaminant in the groundwater of Holocene aquifers in Bangladesh, where Ϸ57 million people drink water with arsenic levels exceeding the limits set by the World Health Organization. Although arsenic is native to the sediments, the means by which it is released to groundwater remains unresolved. Contrary to the current paradigm, ferric (hydr)oxides appear to dominate the partitioning of arsenic in the near surface but have a limited impact at aquifer depths where wells extract groundwater with high arsenic concentrations. We present a sequence of evidence that, taken together, suggest that arsenic may be released in the near surface and then transported to depth. We establish that (i) the only portion of the sediment profile with conditions destabilizing to arsenic in our analysis is in the surface or near-surface environment; (ii) a consistent input of arsenic via sediment deposition exists; (iii) retardation of arsenic transport is limited in the aquifers; and (iv) groundwater recharge occurs at a rate sufficient to necessitate continued input of arsenic to maintain observed concentrations. Our analyses thus lead to the premise that arsenic is liberated in surface and near-surface sediments through cyclic redox conditions and is subsequently transported to well depth. Influx of sediment and redox cycling provide a long-term source of arsenic that when liberated in the near surface is only weakly partitioned onto sediments deeper in the profile and is transported through aquifers by groundwater recharge.redox ͉ Ferric (hydr)oxides R esolving the processes responsible for high concentrations of dissolved arsenic is essential for addressing the human health calamity within Bangladesh and West Bengal, India (1), where we are witnessing the largest mass poisoning in history. Furthermore, deciphering the processes and conditions responsible for arsenic partitioning to the aqueous phase within the Ganges-Brahmaputra Delta may also help diminish arsenicinduced hazards within deltas throughout subtropical and tropical regions of Asia. Although important work has been done to this end, numerous observations conflict with the prevailing theory that reductive dissolution of iron (hydr)oxides at well depth (i.e., 30-50 m) results in the high concentration of arsenic within drinking water (2-6). Although iron (hydr)oxides have been detected in oxidized upper sediments (7), as well as in orange Pleistocene sediments (8, 9), they do not appear widespread in the gray Holocene aquifer at depths of (and below) the highest arsenic concentrations (6,8,9). Moreover, proxies for active bacterial metabolism, namely redox potential (refs. 4,5, and 10 and www.bgs.ac.uk͞arsenic͞Bangladesh), concentration of dissolved electron acceptors (e.g., sulfate, Fig. 1) and their products (e.g., methane), and molecular hydrogen (4), are all inconsistent with ongoing ferric-iron reduction at well depths of 30 to 40 m. Finally, solid-phase arsenic concentrations in the aquifer sediments are relatively low (typically Ͻ3 mg͞kg) compared ...
The formation of an Fe(III) precipitate (plaque) on the surface of aquatic plant roots may provide a means of attenuation and external exclusion of metals. Presently, the mechanisms of metal(loid) sequestration at the root surface are unresolved. Accordingly, we investigated the mechanisms of Fe and As attenuation and association on the roots of two common aquatic plant species, Phalaris arundinacea (reed canarygrass) and Typha latifolia (cattail) using X-ray absorption spectroscopy and X-ray fluorescence microtomography. Iron plaque of both P. arundinacea and T. latifolia consist predominantly of hydrated iron oxides (ferrihydrite) with lesser amounts of goethite and minor levels of siderite. Typha latifolia, however, differs from P. arundinacea by having a significant contribution from lepidocrocite as well as a greater proportion of crystalline minerals. Coexistence of goethite and lepidocrocite suggests the presence of chemically diverse microenvironments at the root surface. Arsenic exists as a combination of two sorbed As species, being comprised predominantly of arsenate- (approximately 82%) with lesser amounts (approximately 18%) of As(III)-iron (hydr)oxide complexes. Furthermore, both spatial and temporal correlations between As and Fe on the root surfaces were observed. While the iron (hydr)oxide deposits form a continuous surficial rind around the root, As exists in isolated regions on the exterior and interior of the root. Root surface-associated As generally corresponds to regions of enhanced Fe levels and may therefore occur as a direct consequence of Fe phase heterogeneity and preferential As sorption reactions.
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