The exchange of metal and protons with natural particles requires long
equilibration time due to their porous and heterogeneous structure. A
pparent exchange measured in the hulk solution results from binding to
the available sites as well as from diffusion inside the particle, We
reduced diffusion to a predictable component of the system by fixing
the time interval between successive titrant additions and ensuring th
at, the same pH increment occurs each time. We assume that local equil
ibrium prevails and the apparent delays in equilibration are entirely
caused by diffusion. Under these assumptions, it is possible to explai
n apparent proton binding to a divinylbenzene macroporous sphere as di
ffusion into a Donnan domain where mobile ions are kept inside the par
ticle by the opposite charge of strong sulfonate sites. Similar analys
is of Hamilton Harbour sediments shows that one-third to one-half of t
he binding capacity resembles diffusion into a Donnan domain. The rema
ining binding occurs on ampholyte sites such as iron oxide or silicate
surfaces.