Characterizing nickel adsorption-desorption on kaolinite

The pH dependent (∼2-11) adsorption-desorption behavior of nickel (Ni 2+ ) on kaolinite was characterized in two experiments in deionized (DI; I = 0.005 M) water and a higher ionic strength coal mine drainage water ( I = ∼0.1 M). In the adsorption (using NaOH) phases of all experiments, ∼10 mg/L Ni 2+ spikes were completely removed by pH ∼8.8-9.7. In the two (1 d and 3 d long) adsorption experiments conducted in DI water, 29-36% of the spiked Ni 2+ was adsorbed at the onset of testing (pH ∼2.3) consistent with adsorption to permanent structurally charged sites (outer sphere complexation). The sorption edges became sharper at pH ∼6.7-6.8 (the inflection point) where ∼45-53% of the spiked Ni 2+ was adsorbed consistent with the activation of variable charged edge sites (inner sphere complexation). At the end of the desorption (using HCl) phase of the DI water experiments (pH∼2.0-2.2), ∼31-55% of the adsorbed Ni 2+ has desorbed. Lower concentrations of the desorbed Ni 2+ (∼31%) in the 3 d vs the 1 d experiments in DI water (∼55%) at pH ∼2.0-2.2 suggest increasing reaction/residence time increases Ni 2+ stability on kaolinite surfaces. In contrast to the DI water experiments, a decrease in adsorption was observed during the adsorption phase of the mine drainage water experiment. For example, in the desorption phase, most (∼95%) of the adsorbed Ni 2+ was desorbed at pH ∼2.1. EXAFS spectra suggest Ni 2+ adsorption to both basal mineral surfaces and variable edge sites. The first coordination shell spectra (∼1.65 Å) correspond to a Ni–O scattering path, likely to basal mineral surfaces, whereby the Ni atoms are in an octahedral coordination environment irrespective of pH. The second coordination shell spectra (∼2.7 Å) correspond to Ni–Al/Si/Ni scattering paths reflecting Ni 2+ adsorption on mineral edged as a NiO 6 octahedra bond which increase in intensity with increasing pH. A single Ni–Ni scattering path was also observed at pHs greater than ∼7.2, possibly indicating the formation of NiO 6 octahedra clusters on the kaolinite surface.