Experimental evidence of power-law trapping-time distributions in porous media

We present experimental results of solute transport in porous samples made of packings of activated carbon porous grains. Exchange experiments, where the tagged solution initially saturating the medium is replaced with the sa me solution without tracer, are accurately described by macroscopic transpo rt equations. On the other hand, in desorption experiments, where the tagge d solution is replaced by water, the solute concentration exhibits a power- law decay for long rimes, which requires a more detailed, mesoscopic descri ption. We reproduce this behavior within a continuous-time random-walk appr oach, where the waiting time distribution is related to the desorption isot herm. Results are compatible with a power-law trapping time distribution wi th divergent first moment, characteristic of anomalous (sub)diffusion. [S10 63-651X(99)14411-8].