Insights into sequential chemical extraction procedures from quantitative XRD: a study of trace metal partitioning in sediments related to frog malformities

Sequential chemical extraction (SCE) and quantitative X-ray diffraction (QXRD) were used in combination to assess trace metal speciation and availability in sediment from two separate marshes in western Vermont, USA, one with high (45%) and one with low (<5%) northern leopard frog (Rana pipiens) malformity rates. The types of malformities observed in the field are very similar to malformities produced in laboratory studies where frogs are exposed to waters and sediments with elevated levels of trace metals such as Co, Cu, Ni, and Zn. recoverable metals, as determined by US EPA method 200.2 (HCl–HNO3), are statistically higher in the malformity-affected marsh than in the marsh with low malformity rates, suggesting possible causality. Analysis of trace metal speciation by SCE and QXRD, however, implies that the most common reservoir of Co, Cr, Cu, Ni, and Zn in the sediment is trioctahedral clay minerals, including chlorite, high charge smectite and vermiculite. The only trace metal that is not associated with trioctahedral clay is Pb, which appears to be contained in poorly crystalline or amorphous oxyhydroxides. The proportions of trace metals in the exchangeable fraction are variable, with values of ≤3% of total Cr and Pb, 2–17% of total Cu and Ni, and up to 24% of total Co and Zn. Total metals concentrations may be slightly elevated relative to background values, and the partitioning of Co, Cu, Ni, and Zn into the exchangeable fraction implies a mechanism for interaction with frogs. Furthermore, partitioning of Pb into poorly crystalline or amorphous oxyhydroxides suggests that it might be mobilized when water levels rise and cause reduction and dissolution of oxyhydroxides in submerged sediment. The difference in speciation between Pb on the one hand, and Co, Cr, Cu, Ni, and Zn on the other, implies that only Pb has an anthropogenic source. Mn speciation differs from all other metals in that it is subequally partitioned into exchange sites, hydroxides and trioctahedral clay. Fe is predominantly partitioned into trioctahedral clay, with lesser amounts in hydroxide form. ndicates that expandable trioctahedral clay dissolves progressively throughout the SCE procedure, including during extractions intended to (1) oxidize sulfides and organic matter, and (2) reduce oxides and hydroxides, indicating that SCE procedures are not entirely phase selective. It also underscores the need for quantitative mineralogical analysis in conjunction with SCE, and has important implications for the interpretation of metal speciation by SCE alone and without corroborating QXRD analyses.