Analytical methods used to characterize the solid-phase speciation of metal(loid)s

Abstract

The ecosystem and human health risks associated with metal(loid)s in soils, sediments and mine wastes are strongly influenced by their solid-phase speciation. This presentation will review a range of methods commonly used by geoscientists to measure the various chemical (e.g. oxidation state) and physical (e.g. morphology, particle size) forms of an element which together make up the total concentration of that element in a sample. Traditional macroscopic techniques for determining solid-phase speciation include methods such as sequential chemical extractions, which can be used for indirectly assessing the partitioning of metals in solid materials, and X-ray diffraction (XRD), which can be used to identify crystalline phases. Microscopic methods range from optical techniques (e.g. transmitted and reflected light microscopy) to microbeam methods that are used to determine near-surface compositions (e.g. electron microprobe, laser-ablation ICP-MS, proton-induced X-ray emission (PIXE)). Over the last two decades, many environmental investigations have employed synchrotronbased microscopic methods that can be used to determine the in situ speciation of metal(loid)s in solid materials. With careful sample collection and preparation, techniques such as X-ray absorption fine structure spectroscopy (XAFS) can provide information on metal(loid) oxidation states and coordination environments that are essential for assessing the environmental risks associated with these elements. Recent studies demonstrate that determination of the total concentrations of metal(loid)s in soils, sediments and mine wastes does not give sufficient information on the environmental availability of these elements, or their potential risks to human health. In the future, ecological and human health risk assessments should incorporate information on the solid-phase speciation of metal(loid)s to ensure that realistic management guidelines are established.