XPS studies (Fig. 3B-III) has shown only CuI speciation present on Cp
surface under similar pulp conditions (Eh ~−100 mV and pH ~9).
NEXAFS analysiswas able to showCuII oxidation products, as absorption
cross-section (transition probability) for CuII has been estimated to be
approximately 25 times that of CuI because of the availability of d-like
unfilled states in the nominally d9 system compared with the d10
configuration [49]. The absorption peak energy of 931.3 eV reported
for the CuII–oxygen species (Fig. 6cii) in the spectra was the same as
that observed by Goh et al. [17] for copper metal, exposed to air for
several days and for CuO powder [46]. It is also close to the value of
931.2 eV reported for tenorite (CuO) by van der Laan et al. [48]. Jiang
et al. [47] reported CuII oxidation peak at BE 930.0 eV for Cu thin film
after in-situ oxidation at nominally 0.2 V. Therefore the Cu L3-edge
absorption energies determined in the present investigation for the Cp
sample at reductive conditions are directly comparable with those
found previously [46–48]. The formation of CuII–oxygen species may result
from exposure to air or desorption of xanthate from the mineral
surface. Both xanthate and perxanthate were observed in solution
using UV–vis, Fig. 2. The decomposition of CuEX in solution results in
the formation of perxanthate and the precipitation CuII oxides and
hydroxides which adsorb to the mineral particle surfaces [24].
The Fe L2,3-edge NEXAFS spectra of Eh modified Cp sample are
shown in Fig. 6di and dii. The Fe L2,3-edge NEXAFS spectra were
acquired from the same 12 ROI from where Cu L-edge spectra were
collected. Fe L3-edge absorption spectra showed two leading peaks,
one at 707.4 eV and a slightly more intense peak at 709.0 eV. The peak
at 707.4 eV is attributed to bulk FeIII in Cp and the peak at 709.0 eV is attributed to iron oxidation products (iron oxides and hydroxides).
Buffering the pulp pH to ~9 stabilises iron oxidation products on Cp surface
and leads to an increase in surface oxidation products at reductive
pulp potentials. In addition, the Cp surface, unprotected by adsorbed
collector, may have oxidised between removal from the aqueous
phase and introduction into the PEEM/XPS. A few spectra (E, C and L
in Fig. 6di and dii) showed the dominant peak at 707.4 eV, indicating
even at reductive pulp conditions, some of the Cp particles remained
relatively less oxidised.