Indeed, if you compare the interaction lifetimes for how long an ion remains bound, only a handful of Fe 3+ ions are mobile at any time (Fig. ). While the median time a Na + ion remains bound to a carboxyl group is on the order of 10 ns, the median time for a Fe 3+ ion to remain bound to the same carboxyl group can effectively be as long as our simulations are. This is only seen for Fe 3+ interacting with the carboxyl groups attached to the hemicellulose, as hydroxyl or acetyl group dwell times (Figs. and) do not show a similar pattern.