Process for separating a substance from a liquid medium containing the substance in dissolved form

A process for separating solute molecules or ions with a molecular weight of less than 5000 from a liquid solution thereof, comprising the steps of:

• (a) introducing into said solution at least one macromolecular substance selected from the group consisting of a polyvinylalcohol, polyacrylamide, polyvinylpyrrolidone, polyacrylates, dextrans, dextrins, albumin, pectic acid, polyacrylic acid, strongly cationic polyelectrolyte of the polyamine type and polyamides dissolved or dispersed in the form of a colloid and capable of fixing the said solute molecules or ions to form macromolecular associations of molecules, said macromolecular substance forming a homogeneous liquid phase with said liquid solution before and after fixing the said solute molecules whereby said macromolecular association of molecules remain dissolved or dispersed in said solution;
• (b) allowing at least a substantial fraction of said solute molecules or ions to be fixed by the molecules of said macromolecular substance;
• and
• (c) passing the homogeneous liquid medium thus obtained through a semipermeable ultrafiltration membrane with a porosity of from 50 to 100,000 Angstroms and capable of retaining the macromolecular association of molecules formed between said macromolecular substance and said solute molecules or ions while allowing the molecules of the solvent of said liquid solution to pass through said membrane.