Adsorption/desorption tests of Zn and the constant pressure microfiltration were conducted using bentonite slurry in order to clarify the adsorption/desorption properties of heavy metal for clayey soil as well as the change in soil property due to cation-exchange reaction caused by the addition of ions. The average specific filtration resistance of bentonite cake calculated from the result of microfiltration experiment decreased with the increase of Zn
2+ uptake. It was shown that Zn
2+ adsorbed onto bentonite was efficiently desorbed by the addition of Na
+, K
+, NH
+4 and Ca
2+. The marked decrease in the average specific filtration resistance was observed in cases where Zn
2+ was desorbed by the addition of ions, and the average specific filtration resistance became almost constant being independent of the amount of Zn
2+ preadsorbed onto bentonite expect the case of Na
+. Thus, it was shown that the permeability of bentonite cake is largely influenced by the cation-exchange reaction.
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