Physics
Scientific paper
Nov 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011gecoa..75.6584t&link_type=abstract
Geochimica et Cosmochimica Acta, Volume 75, Issue 21, p. 6584-6599.
Physics
Scientific paper
Various sorption phenomena, such as aging, hysteresis and irreversible sorption, can cause differences between contaminant (ad)sorption and desorption behavior and lead to apparent sorption 'asymmetry'. We evaluate the relevance of these characteristics for neptunium(V) (Np(V)) sorption/desorption on goethite using a 34-day flow-cell experiment and kinetic modeling. Based on experimental results, the Np(V) desorption rate is much slower than the (ad)sorption rate, and appears to decrease over the course of the experiment. The best model fit with a minimum number of fitting parameters was achieved with a multi-reaction model including (1) an equilibrium Freundlich site (site 1), (2) a kinetically-controlled, consecutive, first-order site (site 2), and (3) a parameter ψ, which characterizes the desorption rate on site 2 based on a concept related to transition state theory (TST). This approach allows us to link differences in adsorption and desorption kinetics to changes in overall reaction pathways, without assuming different adsorption and desorption affinities (hysteresis) or irreversible sorption behavior a priori. Using modeling as a heuristic tool, we determined that aging processes are relevant. However, hysteresis and irreversible sorption behavior can be neglected within the time-frame (desorption over 32 days) and chemical solution conditions evaluated in the flow-cell experiment. In this system, desorption reactions are very slow, but they are not irreversible. Hence, our data do not justify an assumption of irreversible Np(V) sorption to goethite in transport models, which effectively limits the relevance of colloid-facilitated Np(V) transport to near-field environments. However, slow Np(V) desorption behavior may also lead to a continuous contaminant source term when metals are sorbed to bulk mineral phases. Additional long-term experiments are recommended to definitely rule out irreversible Np(V) sorption behavior at very low surface loadings and environmentally-relevant time-scales.
Kersting Annie B.
Powell Brian A.
Tinnacher Ruth M.
Zavarin Mavrik
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