Coagulant transformation - the way from solution till flocs

摘要

Hydrolysis of trivalent metal coagulants (aluminium and ferric salts) was studied and evaluated. It was demonstrated that hydrolysis and the transformation of water soluble aluminium- and ferric-hydroxide polymers (complexes) into weakly water soluble sols are very quick processes. The sol particles have positive electric charge, and they are able to adsorb onto the surface of suspended solids and change their negative electric charge. Sol formation is a very complicated process. Although water soluble hydroxide complexes are positively charged, they are able to bond with each other. This fact is a serious contradiction to Coulomb's Law. The hydroxide groups of positively charged aluminium- and ferric-hydroxides form hydrogen bonds with water molecules. Relatively thick water layer covers the hydroxide complexes, reducing the repulsive forces existing among them. Among the metal hydroxide chains there are water molecules, connecting with each other and the hydroxide oligomers by hydrogen bond. These associates into positively charged, weakly water soluble sols. A smaller fraction of sols adsorb onto the surface of suspended solid particles, changing their electric charge and facilitate their aggregation. The larger fraction of positively charged sols aggregate with each other, because the repulsive forces are reduced by water molecule layers, situated between sols.