Impossible except with dialysis and osmosis membranes or by usong adsorbptipn and exchange (so not based on size), size of a water molecule is roughly 0.0003 mi

Impossible except with dialysis and osmosis membranes or by usong adsorbptipn and exchange (so not based on size), size of a water molecule is roughly 0.0003 micron some pollutants are just 5-10 times that... Of course things are a bit more complicated by the shape of the molecules.