Anaerobic Tailings Sampling and Preparation

The IKMHSS tailings samples were collected by excavating a pit to about 1 m, samples were collected and composited across the pit face for discrete depth intervals on the basis of morphological transitions (color, consistency, etc. details in Hayes et al., 2014 (link)). A core extending to 2 m depth was extracted adjacent to the excavated pit to acquire deep tailings and recover the presumed un-oxidized, originally deposited material. Samples were sealed in double bagged low O₂ diffusion bags and transported on dry ice (-78°C). Samples were sub-sectioned under anaerobic atmosphere (H₂:N₂ = 5%:95%) in a vinyl glove box (Coy Laboratory Products, MI) to obtain three representative splits from each depth increment and limit post sampling oxidation.
Splits were analyzed as in Hayes et.al. (2014) (link) for (i) moisture content and particle size; (ii) sieved (<2 mm), lyophilized at -80°C and 130 mbar prior to chemical analysis; (iii) kept field moist, frozen, and in darkness prior to sieving and grinding in preparation for XRD and XAS analysis; or (iv) prepared as thin sections following anaerobic drying at room temperature and vacuum imbedding in metal free epoxy (EPO-TEK 301–2FL; Epoxy Technologies, Inc.). Additional details regarding sample preparation procedures are available in the SM.
Reference materials (for As, Pb, and Zn) were collected from mineral source distributors, whereas others, e.g. Zn and As sorbed jarosite, schwertmannite, and As(III) and As(V) sorbed 6-line ferrihydrite, were synthesized in accordance with published methods and equilibrated with 0.1 mM As, Pb, or Zn for 24 hours centrifuged and washed (Bigham et al., 1990 ; Cornell and Schwertmann, 2003 ; Gao et al., 2013 (link); Regenspurg and Peiffer, 2005 ) (see Table S1 for details). The adsorbed metal(loid) mass (in g kg^-1), calculated on the basis of loss from solution, was 14 (As^III, ferrihydrite), 14 (As^v, ferrihydrite), 9.6 (Pb, hematite) and 3.3 (Zn, ferrihydrite). All reagents used were ACS grade or better. The identities of all references were confirmed by XRD.