Sorption of heavy metals on clay minerals (or oxides) affects their migration, transformation, and bioavailability in the Earth surface environment (Li et al. 2012). Sorption mechanisms for heavy metals include outer-sphere complexation, inner-sphere complexation, surface-induced precipitation, diffusion, and isomorphic substitution with the mineral structure (Sparks 2005). In the natural environment, Co(II) and Ni(II) adsorption may significantly affect their partitioning behavior and fate. Therefore, it is of great importance to gain a molecular-scale understanding of the competitive sorption behavior and underlying mechanisms of Co(II) and Ni(II) at environmental interfaces. This study reports competitive sorption of Co(II) and Ni(II) on γ-Al2O3 at pH 6.0 and 7.5 through macroscopic batch sorption experiments conducted with advanced extended X-ray absorption fine structure (EXAFS) spectroscopy.

In the macroscopic sorption experiments, competitive adsorption between Co(II) and Ni(II) was observed at pH 6.0. The sorption amount of Co(II) at 48 h in a binary-solute system was about 18.6 μmol g−1, significantly lower than the 66.5 μmol g−1 observed in the single-solute system at pH 6.0 (Fig. 1). Previous studies (Thompson et al. 1999; Voegelin and Kretzschmar 2005) have suggested inner-sphere complexation is the driving force for Co(II) and Ni(II) sorption on γ-Al2O3 in an acidic environment. Ni(II)—with electronegativity of 1.91—is expected to be a stronger competitor for the similar sites on γ-Al2O3 than Co(II), with electronegativity of 1.88 (Li and Xue 2006).

Fig. 1
figure 1

Sorption amount of Co and Ni in single-solute or binary-solute systems at the end of reaction from 0.1-M NaNO3 solution with 0.8-mM initial concentration. Sorption of Co at pH 6.0 (a) and pH 7.5 (c); sorption of Ni at pH 6.0 (b) and pH 7.5 (d)

Full size image

However, no competition effect was observed at pH 7.5, as evidenced by the negligible difference between the sorption amounts of Co(II) and Ni(II) in single- and binary-solute systems. To further interpret the competitive sorption behavior of Co(II) and Ni(II), EXAFS analysis was performed in the Shanghai Synchrotron Radiation Facility (SSRF) and the Beijing Synchrotron Radiation Facility (BSRF). The k 3-weighted EXAFS spectra were collected for the sorption samples prepared at pH 7.5 and for several model compounds (Fig. 2). EXAFS spectra for the Ni single sorption sample showed a weak splitting at 3.7 Å−1, a shoulder at 5.2 Å−1, and a distinctive beat pattern at ~8.0 Å−1, all of which correspond to the characteristics for a layered double hydroxide (LDH) model compound (Aimoz et al. 2012). Fourier transform of the EXAFS data resolved two main peaks, corresponding to the nearest shell of oxygen atoms (first shell) and metal–metal (Me–Me) backscattering pairs (second shell). Shell-by-shell fits yielded a Ni–O distance of 2.06 Å, with coordination number (CN) of 6 for the first shell. The second shell was fitted with 3.9 ± 0.4 Ni and 1.4 ± 0.6 Al at the same RMe–Me distance of 3.05–3.07 Å. Very similar fitting results were obtained for Co EXAFS analysis (Table 1). The fitting data are in agreement with previous research (Thompson et al. 1999; Voegelin and Kretzschmar 2005) and suggest the formation of LDH surface precipitates under alkaline conditions. Distinct from surface complexation reaction, surface precipitation of LDH does not rely on surface sorption sites, therefore resulting in rather weak competitive sorption of Co(II) or Ni(II) in binary-solute systems at pH 7.5. However, further study is needed to ascertain whether Ni and Co co-precipitated in one LDH phase or formed through separate LDH phases.

Fig. 2
figure 2

The k-edge EXAFS results of Co and Ni sorption samples and model compounds prepared in single- and binary-solute systems at pH 7.5. The k3-weighted EXAFS spectra of Co (a) and Ni (c) and their Fourier transforms without phase-shift corrected of Co (b) and Ni (d). Colored lines represent experimental data and dark gray lines the spectral fits

Full size image
Table 1 EXAFS Fitting results for model compounds and sorption samples (S 20  = 0.90)
Full size table

To conclude, the macroscopic and spectroscopic findings together suggest that the competitive behavior and the corresponding microstructures of Co(II) and Ni(II) on γ-Al2O3 are dominantly controlled by solution pH. This provides a crucial scientific basis to understand the potential risk of Co(II) and Ni(II) in the aqueous environment. Specifically, in this work, the sorption amount of Co(II) was significantly reduced by the simultaneous presence of Ni(II) at pH 6.0, implying that the competition of Ni(II) increases environmental contamination risk of Co(II) in an acidic environment, such as acid mine drainage or tailings disposal systems.