An in-situ investigation on the speciation and relationship of Zn and Cd in contaminated sediments: Bulk EXAFS, and micro synchrotron X-ray fluorescence

Abstract

The determination of the speciation and binding types of metals is critical to assessing mobility and bioavailability in soils and sediments. The current study determined the speciation of Zn and Cd in contaminated sediments in the Baiyin district using X-ray absorption fine structure (XAFS) and micro-X-ray fluorescence (μ-XRF). The results revealed that the speciation of Zn in sediments was quite complicated and related to the mineralogical composition and physiochemistry in different locations, with speciation including Zn2+, Zn–Al layered double hydroxide (LDH), Zn primary minerals (sphalerite and franklinite), Zn-Calcite, and Zn-Ferrihydrite. In comparison to Zn, the speciation of Cd in sediments was found to be simpler and less influenced by the geochemical properties of the samples. The μ-XRF analysis indicated a strong correlation between the distribution of Cd and hot spots of Ca and Zn. The XAFS analysis confirmed that the dominant form of Cd was adsorbed on calcite and illite, suggesting a high level of bioavailability, mobility, and potential environmental risk. The results also showed that Zn and Cd coexisted and had a synergistic effect in the calcite and clay minerals of contaminated sediments. This study is the first report on the in-situ speciation of heavy metals in alkaline and calcareous sediment of north China, and the use of spectroscopy techniques allowed for the reliable identification of the speciation of Zn and Cd.