Arsenic speciation in multiple metal environments: II. Micro-spectroscopic investigation of a CCA contaminated soil

The speciation of arsenic (As) in a copper-chromated-arsenate (CCA) contaminated soil was investigated using micro-focused X-ray fluorescence (μXRFμXRF) and micro-focused X-ray absorption fine structure (μXAFSμXAFS) spectroscopies to determine if and how the co-contaminating metal cations (Cu, Zn, Cr) influenced the speciation of As. 15μXRF images were collected on 30-μm polished thin sections and powder-on-tape samples from which Pearson correlation coefficients (ρ)(ρ) between As and various metal species were determined based on the fluorescence intensity of each element in each image pixel. 29 μXAFSμXAFS and two bulk-XAFS spectra were collected from depths of 0–20 cm (LM-A) and 20–40 cm (LM-B) to determine the chemical speciation of As in the soil by target analyses of principal components with circa   52 reference spectra and linear least-square combination fitting of individual experimental spectra with a refined reference phase list (32) of likely As species. Arsenic and metal cations (Cr, Mn, Fe, Cu, Zn) accumulated in distinct, isolated areas often not larger than 50×50μm in which the Pearson correlation between the elements was strongly positive (ρ>0.75)(ρ>0.75). The correlation of As to Zn and Cr decreased from >0.9 to <0.8 and increased to Cu from ∼0.6 to >0.8 with depth. Arsenic occurred predominantly in the +5 oxidation state. Abstract factor analysis and linear least square combination fit analysis suggested that As occurred as a continuum of fully and poorly-ordered copper-arsenate precipitates with additional components being characterized by surface adsorption complexes on goethite and gibbsite in the presence and absence of Zn. Precipitates other than copper-based ones, e.g., scorodite, adamite and ojuelaite were also identified. The significant co-localization and chemical speciation of As with Cu suggest that the speciation of As in a contaminated soils is not solely controlled by surface adsorption reactions, but significantly influenced by the co-contaminating metal cation fraction. Future studies into As contaminated soil therefore need to focus on identifying the speciation of As and the co-localizing metal cations.

Graphical abstractArsenic speciation in soils is partly controlled by co-sorbing metal cations. Here we show how As(V) co-precipitated with Cu and Zn to form poorly ordered precipitates.Figure optionsDownload full-size imageDownload high-quality image (83 K)Download as PowerPoint slide