Effect of Soil Fulvic Acid on Nickel(II) Sorption and Bonding at the Aqueous-Boehmite (γ-AlOOH) Interface

The influence of soil-derived fulvic acid (SFA) on NiII sorption and speciation in aqueous boehmite (γ-AlOOH) suspensions was evaluated using a combination of sorption experiments and Ni K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy measurements. Co-sorption of SFA at the aqueous-boehmite interface modifies both the extent of NiII sorption as well as the local structure of the sorbing NiII ions. In SFA-free suspensions, NiII sorbs by forming inner-sphere bidentate mononuclear complexes with surface aluminol groups. Addition of SFA increases NiII sorption at pH conditions below the sorption edge observed in SFA-free suspensions and diminishes NiII sorption at pH above the SFA-free sorption edge. When SFA is co-sorbed to boehmite, NiII sorbs by forming both ligand-bridging ternary surface complexes (NiII−SFA−boehmite) as well as surface complexes in which NiII remains directly bonded to aluminol groups, that is, binary NiII−boehmite or metal-bridging ternary surface complexes (SFA−NiII-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 NiII 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.