Synchrotron‐based X‐Ray Approaches for Examining Toxic Trace Metal(loid)s in Soil–Plant Systems

Abstract

Elevated levels of trace metal(loid)s reduce plant growth, both in soils contaminated by industrial activities and in acid agricultural soils. Although the adverse effects of trace metal(loid)s have long been recognized, there remains much unknown both about their behavior in soils, their toxicity to plants, and the mechanisms that plants use to tolerate elevated concentrations. Synchrotronbased approaches are being utilized increasingly in soil-plant systems to examine toxic metal(loid)s. In the present review, brief consideration is given to the theory of synchrotron radiation. Thereafter, we review the use of synchrotron-based approaches for the examination of various trace metal(loid)s in soil-plant systems, including aluminum, chromium, manganese, cobalt, nickel, copper, zinc, arsenic, selenium, and cadmium. Within the context of this review, X-ray absorption spectroscopy (XAS) and X-ray fluorescence microscopy (m-XRF) are of particular interest. These techniques can provide in situ analyses of the distribution and speciation of metal(loid)s in soil-plant systems. The information presented here serves not only to understand the behavior of trace metals in soil-plant systems, but also to provide examples of the potential applications of synchrotron radiation that can be used to advantage in other studies.Synchrotron-based X-Ray Approaches for Examining Toxic Trace Metal(loid)s in Soil-Plant Systems Peter M Kopittke, Peng Wang,* Enzo Lombi, Erica Donner T race metals and metalloids, hereafter referred to as metal(loid)s, are natural components of the geosphere, hydrosphere, biosphere, and atmosphere, but their presence at elevated concentrations can result in toxicity. The importance of elevated trace metals in the environment has long been recognized (e.g., Veitch, 1904). Elevated levels of a range of trace metal(loid)s occur in sites contaminated by mining, industry, and transport. Similarly, acid soils, in which soluble aluminum (Al) and manganese (Mn) are increased, comprise ~3.95 billion ha of the global ice-free land or ~40% of the world's arable land (von Uexküll and Mutert, 1995;Eswaran et al., 1997). These degraded lands may be improved by liming of acid agricultural soils or by burying and landfilling of polluted soil, but the cost of remediation is prohibitive in many cases. As a result, there is a continued interest in understanding the toxic effects of metal(loid)s in the soilplant continuum.Investigation of metal(loid) toxicity requires a multidisciplinary and multitechnique approach. Although many complementary techniques are suitable for the examination of metal(loid)s in soil-plant systems, it is not the intention of this review to compare this multitude of approaches. Rather, the focus of the present review is to consider synchrotron-based approaches, these being some of the few techniques that allow for in situ measurements of metal(loid) distribution and speciation with minimal sample preparation. Of particular interest here is synchrotron-based X-ray fluorescence microscopy (m-XRF...

High‐Energy Resolution Fluorescence Detected X‐Ray Absorption Spectroscopy: A Powerful New Structural Tool in Environmental Biogeochemistry Sciences

“…levels to the atomic level. In this context, synchrotron radiation techniques are very powerful, particularly to study the distribution (micro X‐ray fluorescence imaging) and the chemical speciation (X‐ray absorption spectroscopy, XAS) of trace metals or metalloids (Sarret et al, 2013; Kopittke et al, 2017). X‐ray absorption spectroscopy probes the speciation of a target chemical element (i.e., its valence, composition, and structure of the coordination shells) and is widely used for this aim (Lombi et al, 2011).…”

Small‐scale distribution of copper in a Technosol horizon studied by nano‐scale secondary ion mass spectrometry

“…The composition of class 13 pointed in particular to Cu being adsorbed on Fe oxides, but the area represented by this class was very small. Except for class 9, the mentioned Cu‐enriched classes were not enriched in C. Thus, direct interactions of Cu ions with organic matter might be less essential in this technogenic material than in natural soil, where Cu directly binds to functional groups of soil organic matter, especially carboxyl groups . This lack of direct interaction may be affected by the composition of organic matter, dominated by aromatic C, but poor in carboxylic C (6%), as pointed out before.…”

“…Frequently, methods operating at the micrometer scale such as electron‐microprobe analysis (EMPA) or scanning‐electron microscopy (SEM) with energy‐dispersive X‐ray spectroscopy (EDX) have been applied, although individual particles may be present, which are tens to hundreds of nanometers in size. Identifying these particles requires methods operating at the appropriate spatial scale, e.g., transmission‐electron microscopy on samples prepared with a focused ion beam or synchrotron radiation‐based X‐ray spectromicroscopy . Spatially resolved data (maps) obtained by X‐ray spectromicroscopy have been evaluated by image difference, correlation analysis, and principal component analysis‐cluster analysis .…”

Emergent Properties of Microbial Activity in Heterogeneous Soil Microenvironments: Different Research Approaches Are Slowly Converging, Yet Major Challenges Remain

“…The first two of these instruments, in scanner mode, can in principle produce elemental maps like that of Figure 6A . Starting in the mid- to late 1990s, synchrotron facilities around the world began offering soil scientists the opportunity to run various types of analyses, including X-ray absorption near-edge structure (XANES) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopies, which rapidly became popular because of the very useful information it is able to provide on the molecular environment of atoms, and therefore on element speciation (Prietzel et al, 2003; Schumacher et al, 2005; Solomon et al, 2005, 2012; Kinyangi et al, 2006; Christl and Kretzschmar, 2007; Wan et al, 2007; Strawn and Baker, 2009; Hesterberg et al, 2011; Milne et al, 2011; Jassogne et al, 2012; Kopittke et al, 2017). In most cases, the target of interest in this type of analysis is extremely minute in extent, a most a few μm 2 , but occasionally researchers have attempted to map properties over a slightly larger area, among other things to try to assess the heterogeneity of the composition of OM in soils ( Figure 6B ).…”