Diffusion is only one way mass actually moves — and in most real equipment, it's not even the way that's doing most of the work.
Say "mass transfer" to most engineering students and Fick's Law is the first (and often only) thing that comes to mind. That's a problem, because in almost every piece of real absorption, distillation, or aeration equipment, the bulk fluid is turbulently mixed and species get carried around mostly by bulk fluid motion — convection — which is typically orders of magnitude faster than molecular diffusion. Diffusion still matters. It just doesn't matter everywhere. It matters in one thin place: right at the interface, where turbulence can't reach.