Hello,
If I am not mistaken the adsorption/desorption of micro-pollutans only acts between the dissolved pollutant and the suspended sediment, so no adsorption/desorption with the bed sediment.
The law is pretty simple, it first gets the difference between the concentrations and equilibrium:
Kd C - Cs
being Kd the distribution coefficient, C the dissolved pollutant's concentration and Cs the fraction of suspended matter that is polluted.
So if there is too much dissolved pollutant KdC-Cs is > 0 so the pollutant is adsorbed by sediments. On the other hand if there is not enough dissolved polluntant then KdC-Cs is < 0 so there is desoprtion.
Then this difference is multiplied by the speed of desoprtion to get the balance.
I suggest you check the Theory Guide (v8p1/documentation/waqtel/theory_guide) for more info.
I am just an user of micropol, so it is better if a moderator can answer this question but in the meantime I hope I helped.
Santiago