Suggestion: Add selected chemicals A to the water. Select chemicals A to add that will attach fairly strongly to the chemicals B want to remove from the water. Now the chemicals B don't want in the pure water have joined (bonded) with added chemicals A to form larger molecules C that have a harder time than chemicals B flowing through the pores in the membrane. So, can use membranes with larger pores that the pure water can more easily flow through and still block relatively large chemicals C.

Anyone try any such thing so far?