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|Title: ||The role of humic non-exchangeable binding in the promotion of metal ion|
|Authors: ||Bryan, Nick D.|
Jones, Dominic L.M.
Keepax, Rose E.
Farrelly, Dean H.
Abrahamsen, Liam G.
Evans, Nicholas D.M.
|Issue Date: ||2007|
|Publisher: ||© Royal Society of Chemistry|
|Citation: ||BRYAN, N.D. ... et al, 2007. The role of humic non-exchangeable binding in the promotion of metal ion. Journal of Environmental Monitoring, 9 (4), pp. 329-347|
|Abstract: ||Metal ions form strong complexes with humic substances. When the metal ion is first complexed
by humic material, it is bound in an ‘exchangeable’ mode. The metal ion in this fraction is
strongly bound, however, if the metal–humic complex encounters a stronger binding site on a
surface, then the metal ion may dissociate from the humic substance and be immobilised.
However, over time, exchangeably-bound metal may transfer to a ‘non-exchangeable’ mode.
Transfer into this mode and dissociation from it are slow, regardless of the strength of the
competing sink, and so immobilisation may be hindered. A series of coupled chemical transport
calculations has been performed to investigate the likely effects of non-exchangeable binding upon
the transport of metal ions in the environment. The calculations show that metal in the nonexchangeable
mode will have a significantly higher mobility than that in the exchangeable mode.
The critical factor is the ratio of the non-exchangeable first-order dissociation rate constant and
the residence time in the groundwater column, metal ion mobility increasing with decreasing rate
constant. A second series of calculations has investigated the effect of the sorption to surfaces of
humic/metal complexes on the transport of the non-exchangeably bound metal. It was found that
such sorption may reduce mobility, depending upon the humic fraction to which the metal ion is
bound. For the more weakly sorbing humic fractions, under ambient conditions (humic
concentration etc.) the non-exchangeable fraction may still transport significantly. However, for
the more strongly sorbed fractions, the non-exchangeable fraction has little effect upon mobility.
In addition to direct retardation, sorption also increases the residence time of the nonexchangeable
fraction, giving more time for dissociation and immobilisation. The nonexchangeable
dissociation reaction, and the sorption reaction have been classified in terms of two
Damkohler numbers, which can be used to determine the importance of chemical kinetics during
transport calculations. These numbers have been used to develop a set of rules that determine
when full chemical kinetic calculations are required for a reliable prediction, and when
equilibrium may be assumed, or when the reactions are sufficiently slow that they may be ignored
|Description: ||This article was published in the journal, Journal of Environmental Monitoring [Royal Society of Chemistry] and definitive version can be found at: http://www.rsc.org/Publishing/Journals/em/index.asp|
|Appears in Collections:||Published Articles (Chemistry)|
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