Quantitative zinc speciation in soil with XAFS spectroscopy: Evaluation of iterative transformation factor analysis

We employed a combination of selective sequential extractions and bulk XAFS spectroscopy, and extracted spectral XAFS components and their concentrations by iterative transformation factor analysis (ITFA), in order to determine the Zn speciation in a smelter-contaminated, acidic soil. We compared the speciation by ITFA with one we performed earlier using principal component analysis and linear combination fit. ITFA identified 4 different species, two different franklinite-type phases (Zn-Fe spinels), sphalerite (Zn sulfide) and Zn oxalate, the latter forming as a precipitate during chemical extraction with oxalate solution. The second spinel-type phase could be extracted with the help of ITFA, although no appropriate reference sample was available. Spinel 1 and 2 have Zn-O distances of 1.96 and 1.99 Å, and Zn-Fe distances of 3.53 and 3.48 Å, respectively. The results from ITFA gave much better fits of experimental spectra and are better in line with elemental mapping and XAFS microspectroscopy. The major advantage of investigating XAFS data with ITFA is the possibility to derive all species even when part of the references are not available.