Synchrotron micro-XRF and micro-XANES characterisation of metal entrapment in anthropogenic tufa

Anthropogenic tufa deposits forming in hyperalkaline environments near legacy industrial waste sites can act as passive sinks for toxic metals offering potential for nature-based remediation. To unravel the distribution and mechanisms of metal incorporation in tufa we investigated two tufa samples from a stretch of slag-contaminated stream near Consett (N.E. England) using synchrotron-micro-X-ray fluorescence (μ-SXRF) and micro-X-ray absorption near edge structure (μ-XANES). Analyses revealed that metal content were significantly higher in the upstream sample compared to the downstream sample for As, Fe, Mn, Pb and Zn. Element maps revealed metals distributed heterogeneously as diffuse enrichments, linear features and discrete concentrations between calcite growths. Prominent bands of Cu, Fe, Mn, Ti and Zn were also observed between laminates. Statistical analysis of whole images and transects along the direction of tufa growth in the upstream sample gave notable correlations ( p < 0.005) between: Mn and Zn ( R = 0.66); Fe and Zn ( R = 0.64); Fe and Mn ( R = 0.55); and Cu and Fe ( R = 0.61). Linear combination fitting (LCF) of μ-XANES spectra indicated that Pb micro-sized particles might contain phases of galena (PbS), Pb(II)oxides, cerussite (PbCO₃) and/or crocoite (CrPbO 4 ), while sorbed phases of Pb were not favoured in the LCF fit. These LCF results raise questions about transport and transformation of Pb from the weathered slag and the stability of Pb trapped within tufa. The highly alkaline stream conditions (pH 10.3–11.3) favour metal co-precipitation with CaCO 3 whilst incorporating atmospheric carbon. Overall, these findings highlight the potential of tufa as a natural attenuation material for trapping metals in contaminated post-industrial landscapes. Further study is required to help understand how these tufa metal capturing processes could be harnessed.