Nondestructive speciation of solid environmental samples: fate of heavy metal elements in estuarine sediments

Abstract

In this dissertation, nondestructive chemical speciation methods for solid environmental samples using Mössbauer spectroscopy and X-ray absorption fine structure (XAFS) were developed. Further, the behavior of heavy-metal elements in sediments collected from the Tama River estuary in Tokyo, Japan, was investigated by these methods. Prior to speciation, 33 elements in the estuarine sediments were determined by activation analyses. The iron speciation in the sediments was conducted by using Mössbauer spectroscopy in order to clarify the vertical distribution of the iron species. Although XAFS can be measured for many elements, unlike Mössbauer spectroscopy, the XAFS spectra of different species often severely overlap in the whole spectral region. For resolving the XAFS spectra of mixtures and to quantify the species therein, partial least-squares (PLS) and artificial neural networks (ANN) were employed. The overlapping spectra were successfully deconvoluted, and the relative abundances of the thus-determined iron species were consistent with the Mössbauer results. Since the availability of the multivariate calibration of XAFS was confirmed for Fe, it was extended to the speciation of Cr, Mn, and Zn. The Cr and Zn increased with depth in the forms of hydroxide and sulfide, respectively, while there was no significant variation in the chemical state of Mn.