sciences, there appears to be no generalizable quantitative information available on
these transport pathways. Active transport across membranes always competes with
passive (diffusive) transport. Hence it appears likely that active transport will
generally be more important for ionic species than for neutral ones because neutral
species have a much higher diffusive membrane permeability than ionic species. The
most likely effect of active transport is as a barrier against the uptake of ionized
chemicals. Efflux transporters imbedded in the apical membrane of epithelial cell
layers can effectively reduce the uptake of substrate chemicals. This has been
demonstrated in Fischer et al. (2013). Increased uptake of ionized chemicals by
influx transporters is rather unlikely to occur because these transporters would have
to be located in all membranes of the epithelial cell layers and the energy demand for
such a transport cascade would be high.
7 Conclusions
Much more experimental data for ionic sorption in well-defined environmentally
relevant scenarios are needed. “Well defined” means that not only the pH is
controlled in such experiments but also the availability of competing ions and/or
corresponding ions for ion-pair formation is reported and possible concentration
dependencies are investigated. In general, we are lacking experimental data for
zwitterions (glyphosate as a prominent example) and polyvalent ions. We are also
lacking systematically determined sorption data for all kinds of organic ions to
α-glycoprotein, a transport protein in blood that is known to be important for cations.
Existing sorption data for albumin have been measured for fatty-acid free albumin
which might not be representative for the albumin in vivo. Hence, additional data for
fatty-acid-loaded albumin would be of interest. Sorption experiments to soil have so
far focused on amine-cations and pharmaceuticals and need to be diversified.
Whenever new experimental data become available, they should be used to evaluate/recalibrate existing predictive models and possibly develop new models. In any
case, it is clear that IOCs will demand sorption models that are much more complex
than anything that may have been useful for neutral organic chemicals in the past.
While empirical log K ow models have brought us a long way for neutral chemicals, a
good mechanistic concept will be required for any IOC sorption model with a
satisfying predictive power. Further investigation into the parallels with the sorption
of metal ions may be helpful.
8 Summary
This review first gives an overview on the principles that govern ionic sorption in
environmental systems which are more complex than the simple partition processes
of neutral chemicals. Our current knowledge on various topics such as
56
L. Henneberger and K.-U. Goss
these transport pathways. Active transport across membranes always competes with
passive (diffusive) transport. Hence it appears likely that active transport will
generally be more important for ionic species than for neutral ones because neutral
species have a much higher diffusive membrane permeability than ionic species. The
most likely effect of active transport is as a barrier against the uptake of ionized
chemicals. Efflux transporters imbedded in the apical membrane of epithelial cell
layers can effectively reduce the uptake of substrate chemicals. This has been
demonstrated in Fischer et al. (2013). Increased uptake of ionized chemicals by
influx transporters is rather unlikely to occur because these transporters would have
to be located in all membranes of the epithelial cell layers and the energy demand for
such a transport cascade would be high.
7 Conclusions
Much more experimental data for ionic sorption in well-defined environmentally
relevant scenarios are needed. “Well defined” means that not only the pH is
controlled in such experiments but also the availability of competing ions and/or
corresponding ions for ion-pair formation is reported and possible concentration
dependencies are investigated. In general, we are lacking experimental data for
zwitterions (glyphosate as a prominent example) and polyvalent ions. We are also
lacking systematically determined sorption data for all kinds of organic ions to
α-glycoprotein, a transport protein in blood that is known to be important for cations.
Existing sorption data for albumin have been measured for fatty-acid free albumin
which might not be representative for the albumin in vivo. Hence, additional data for
fatty-acid-loaded albumin would be of interest. Sorption experiments to soil have so
far focused on amine-cations and pharmaceuticals and need to be diversified.
Whenever new experimental data become available, they should be used to evaluate/recalibrate existing predictive models and possibly develop new models. In any
case, it is clear that IOCs will demand sorption models that are much more complex
than anything that may have been useful for neutral organic chemicals in the past.
While empirical log K ow models have brought us a long way for neutral chemicals, a
good mechanistic concept will be required for any IOC sorption model with a
satisfying predictive power. Further investigation into the parallels with the sorption
of metal ions may be helpful.
8 Summary
This review first gives an overview on the principles that govern ionic sorption in
environmental systems which are more complex than the simple partition processes
of neutral chemicals. Our current knowledge on various topics such as
56
L. Henneberger and K.-U. Goss