The high-resolution XPS spectra of Cr2p core region measured
on the surface of S-3-650 sample before and after adsorption of Cr
(VI) from solutions with initial Cr(VI) concentration of 105 mg/L
and initial pH values of 2.0, 2.7 and 3.5 are shown in Fig. 4. As can be seen, after adsorption two Cr2p peaks are detected on the
surface of the adsorbent, which can be attributed to the binding
energies of Cr2p3/2 (576.8 eV) and Cr2p1/2 (586.3 eV) orbital’s,
respectively, all corresponding to the signal of Cr(III) [34]. This
implies that the chromium on the surface layer of the adsorbent is
in the form of Cr(III). In other words, despite of the acidity of the
initial solutions, the removal of Cr(VI) by the SSH activated carbon
all involves adsorption and reduction processes. Such a coupled
removal process of Cr(VI) has also been observed on other
carbonaous and biomaterial adsorbents, which involved oxidation
of adsorbent surface and reduction of Cr(VI) to Cr(III) at the same
time through electron transfer [7,35].
The results revealed that the adsorption isotherms obtained at a
fixed initial pH value may not really reflect the highest adsorption
capacity of the adsorbents. Therefore, to fully explore the
adsorption ability of the SSH activated carbons, the predetermined
optimum pH values corresponding to the solutions with different
initial Cr(VI) concentrations were used for the subsequent
adsorption isotherm studies