Diffusion and binding of protons in sediments

The exchange of metal and protons with natural particles requires long equilibration time due to their porous and heterogeneous structure. Apparent 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 that, the same pH increment occurs each time. We assume that local equilibrium prevails and the apparent delays in equilibration are entirely caused by diffusion. Under these assumptions, it is possible to explain apparent proton binding to a divinylbenzene macroporous sphere as diffusion into a Donnan domain where mobile ions are kept inside the particle by the opposite charge of strong sulfonate sites. Similar analysis of Hamilton Harbour sediments shows that one-third to one-half of the binding capacity resembles diffusion into a Donnan domain. The remaining binding occurs on ampholyte sites such as iron oxide or silicate surfaces.