Zinc coordination to multiple ligand atoms in organic-rich surface soils

We report on the solid-phase speciation of naturally occurring Zn in metalliferous organic-matter-rich surface soils. Synchrotron-based studies were used to probe elemental distribution and associations in soil particles (μ-XRF) together with the mineralogy (μ-XRD) and Zn bonding environment (Zn-μ-XANES) at the micrometer-scale level. The average bonding environment of Zn was also probed for bulk soils using XANES. We found the distribution of elements within soil particles to be heterogeneous; however, some elements are consistently co-located. While conventional XRD analyses of whole soils did not identify any Zn mineral phase, synchrotron-based-μ-XRD analyses indicated that sphalerite (ZnS) is present in a particle from a wetland soil (soil labeled G3). Linear combination fit (LCF) analyses of XANES spectra collected for bulk soils (Zn-XANES) and μm-regions (Zn-μ-XANES) within soil particles suggest Zn bonds to oxygen-, nitrogen-, and sulfur-functional groups in these sulfur-, nitrogen-, and zinc-rich organic surface soils. The XANES spectra of all bulk soils and of all μm-regions except for the wetland soil (G3), where ZnS was the most significant constituent, were best fitted by the Zn-arginine reference compound and therefore seems to indicate Zn bonding to nitrogen. Thus, these results provide compelling evidence of the formation of highly covalent Zn-organic bonds in the organic-rich surface soils that were studied. This may explain in part why metal partition coefficients (K_d) are generally higher in organic soils, and why the toxic thresholds for total metal concentrations are higher in organic than in mineral soils.