Chapter 7 Frontiers in assessing the role of chemical speciation and natural attenuation on the Bioavailability of Contaminants in the Terrestrial Environment

Abstract

This chapter has sought to provide the reader a series of case studies that exemplify cutting edge techniques and methodologies for assessing the role of chemical speciation and natural attenuation on the bioavailability of contaminants in the terrestrial environment. We provide here a summary of the findings of the techniques and case studies presented to reinforce the strength of the techniques described. Isotopic dilution techniques represent a flexible and powerful tool to assess changes in metal exchangeability as a result of a number of processes such as natural attenuation, rhizosphere reactions, remediation processes, etc. However, it must be remembered that this information is operationally defined mainly as a function of the equilibration time. Nonetheless, a combination of isotopic dilution techniques together with spectroscopic methods, such as those described in this chapter, represent an exciting possibility to obtain quantitative and mechanistic information regarding important processes controlling contaminant's bioavailability in soil. The strength of synchrotron-based spectroscopic techniques has been described. For instance through the application of XANES it has been demonstrated that Cr is present in ambient particulate matter in a wide variety of forms that probably represent an equally broad set of sources. The majority of the Cr analysed within the aerosol particulate case study presented in Section 7.3 was present in relatively unreactive phases such as Cr(III) oxy(hydro)oxides, Cr(0) and Cr-carbides, but Cr(VI) is also present in some ambient particles. This finding was consistent with complementary laboratory ageing experiments, which suggest that ambient particulate Cr(VI) has a lifetime of days. Analysis of Peruvian mummy teeth by using synchrotron micro-XRF mapping has provided an insight into the environmental exposure of individuals to trace metals that would be difficult to obtain by other means. It is apparent from this study that the trace metal concentration in the cementum ring system differs from that of the rest of the tooth. Metals such as Br and Pb appear in the cementum of teeth from different individuals, indicative of different lifetime exposure. In addition the two distinct Zn bands suggest an endogenous origin for the Zn. It is clear that micro-focused XAFS is an ideal tool for the analysis of ambient particulate matter. The specific advantages of micro-XAFS include spatial resolution of a few microns, element specific analysis, oxidation state determination, elemental correlations and detailed structural information, even in non-crystalline samples. These are all important factors that must be determined for a more complete understanding of particulate matter composition and its impacts on health. A better understanding of the complex chemical composition of ambient particulate matter may lead to improvements in the effectiveness of particulate matter regulations. Soil samples affected by a smelter were analysed by XAFS spectroscopy to determine the speciation and local coordination environment of As in the samples. The level of understanding achieved through the determination of local coordination and oxidation state provided by these studies would be extremely difficult to acquire by other means. This study stemmed from the need to assess the stability of As carriers in soil for the development of a conceptual model for human health and environmental risks associated with the soil containing up to 270 ppm As. The idea of remobilisation of trace metals due to microbial activity is supported by XAFS measurements along with the spatial associations provided by the SEM. This spectroscopic evidence suggests that trace element mobility can be directly associated with dissimilatory metal reducing bacteria. By combining both electron microscopy and synchrotron-based techniques with other available selective extraction procedures one can investigate the partitioning of trace metals in lacustrine sediment and similarly iron rich lateritic soil profiles. In particular, the ability to estimate concentrations of metals associated with carbonate Fe (particularly siderite and ankerite) and magnetite Fe materials in weathered sediment profiles contributes to our increased understanding with respect to bioavailability. The application of the spectroscopic procedures to laterite environments provides insight into the dominant modes of occurrence of different trace metals associated with Fe and Mn minerals in weathered laterites and how they will behave in the presence of biologically mediated redox environments. © 2008 Elsevier B.V. All rights reserved.