Effect of soil fulvic acid on nickel(II) sorption and bonding at the aqueous-boehmite (γ-AIOOH) interface

The influence of soil-derived fulvic acid (SFA) on Ni^II sorption and speciation in aqueous boehmite (γ-AIOOH) suspensions was evaluated using a combination of sorption experiments and Ni K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy measurements. Cosorption of SFA at the aqueous-boehmite interface modifies both the extent of Ni^II sorption as well as the local structure of the sorbing Ni^II ions. In SFA-free suspensions, Ni^II sorbs by forming inner-sphere bidentate mononuclear complexes with surface aluminol groups. Addition of SFA increases Ni^II sorption at pH conditions below the sorption edge observed in SFA-free suspensions and diminishes Ni^II sorption at pH above the SFA-free sorption edge. When SFA is co-sorbed to boehmite, Ni^II sorbs by forming both ligand-bridging ternary surface complexes (Ni ^II-SFA-boehmite) as well as surface complexes in which Ni ^II remains directly bonded to aluminol groups, that is, binary Ni^II-boehmite or metal-bridging ternary surface complexes (SFA-Ni^II-boehmite). The relative contribution of the individual sorption complexes depends heavily on geochemical conditions; the concentration of ligand-bridging complexes increases with increasing SFA sorption and decreasing pH. The local structure of sorbed Ni^II does not change with increasing reaction time even though the extent of sorption continues to increase. This supports a slow uptake mechanism where surface or intraparticle diffusion processes are rate-limiting. This work demonstrates that the association of humic constituents with soil minerals can significantly modify the mechanisms controlling trace metal sorption and transport in heterogeneous aquatic environments.