Insights into the effect of humic acid on Ni(II) sorption mechanism on illite: Batch, XPS and EXAFS investigations

Abstract

In this study, the influence of humic acid (HA) on Ni(II) sorption mechanism at illite/water interface was investigated using batch, EXAFS, and XPS. The results showed that HA can apparently inhibit the sorption of Ni(II) on illite, especially at neutral to weak alkaline conditions under a short aging time. Under low pH conditions, ion exchange (IE) and HA-bridging complexes were the main sorption mechanisms. Under neutral to weak alkaline conditions, HA-bridging complexes and surface precipitates (such as α-Ni(OH)2) gradually became predominant for Ni(II) sorption on illite at a short term. The inhibition of HA for Ni(II) sorption was deduced that: (i) the competition of soluble HA-Ni complexes, and (ii) HA-coated illite surface restrained the formation of surface-induced precipitates. With the increasing aging time, the inhibiting effect of HA became weak with respect to the surface precipitates such as Ni-Al LDH and Ni(OH)2 forming gradually. This might be caused by the growth of the available Al3+ and the surface loading of Ni(II). Moreover, β-Ni(OH)2 was the dominant species for Ni(II) sorption on illite under strong alkaline conditions. These findings in this study are providing deep insights into the understanding and prediction of Ni(II) environmental behaviors in soils enriched with natural organic matters.