Reaction Condition Effects on Nickel Sorption Mechanisms in Illite-Water Suspensions

Abstract

Nickel sorption in illite suspensions was studied as a function of pH (4.5–8.0), reaction time (3 h, 24 h, and 1 wk) and ionic strength (I = 0.1 and 0.003 M) using x-ray absorption spectroscopy (XAS) to characterize the Ni sorption complexes formed. The formation of Ni-Al layered double hydroxide (LDH) phases was observed at pH values >6.25, with increasing formation of these phases with time, and a faster formation rate with increasing pH. Comparison among samples with the same Ni loading, but different reaction times and pH values, showed larger second neighbor scattering for the samples reacted at higher pH, which had a faster Ni sorption rate. Most likely this was due to a greater importance of Ni-Al LDH precipitation relative to other (mononuclear) Ni sorption mechanisms at higher pH, and/or a higher Al content in the Ni-Al LDH phase formed at lower pH (slower Ni sorption rate). Lowering the ionic strength resulted in increased Ni sorption for the entire pH range studied. Our extended x-ray absorption fine structure spectroscopy (EXAFS) data indicated that this was due to significant outer-sphere Ni sorption occurring at the planar sites at low I, leading to a reduced importance of Ni-Al LDH formation in the overall Ni sorption process at pH > 6.50.