The influence of competing sorbents on the dynamics and mechanisms of metal reactions in natural systems: A multi-scale approach
There is a gap in our current understanding of metal sorption mechanisms in multi-sorbent systems, where several competing mechanisms are available for metal uptake. In this study, molecular scale spectroscopic and microscopic techniques were combined with macroscopic sorption and desorption studies to elucidate the intrinsic metal sorption mechanisms to soils and common reactive soil constituents. The research findings demonstrated that the presence of ubiquitous coatings such as humic acid and goethite on clay mineral surfaces affect metal sorption, but do not change the intrinsic metal sorption mechanisms to the underlying clay mineral. In the absence of a humic acid coating, stable Ni-Al layered double hydroxide (LDH) surface precipitates were formed on kaolinite. A less stable nickel hydroxide formed in the presence of a 5-wt% organic coating, whereas in the presence of a 1-wt% organic coating, a more stable Ni-Al layered double hydroxide (LDH) formed. Similarly, the mechanisms governing Zn uptake in a goethite coated kaolinite system were studied as a function of pH. At pH 5, Zn was mainly bound to kaolinite edge sites. At pH 7, the dominant sorption mechanism was found to change from the initial formation of inner-sphere complexes with the goethite coating to the formation of a Zn-Al LDH surface precipitate at the kaolinite surface with increasing reaction time. In a study to the Zn speciation in a non-treated smelter soil and the aluminosilicate- and compost-treated soil revealed no significant differences in speciation. Amorphous, Zn containing surface precipitates made up 60% of the total Zn fraction in the smelter contaminated soils. Desorption studies however indicated that Zn-containing surface precipitates are only stable at near neutral pH and therefore the formation of metal containing surface precipitates did not lead to a permanent immobilization of the metal. Synchrotron based X-ray microscopy, combined with C1s-near edge X-ray absorption fine structure (NEXAFS) spectroscopy, was applied to study the interactions humic acids with metals in situ. Changes in the position of the C 1s(C=O) → 1π*C=O electronic transitions in the NEXAFS spectra could well be used to qualitatively describe metal interactions with specific functional groups of the biopolymers.