Additive-Driven Dissolution Enhancement of Colloidal Silica. 1. Basic Principles and Relevance to Water Treatment

The effect of various chemical additives (small molecules and polymers) on the dissolution of two kinds of colloidal silica (Aerosil 200 and laboratory-synthesized, SSD) is systematically studied at pH 10. The silica scale dissolvers tested are 5-carboxybenzotriazole ( CBZT ), amino-tris(methylene phosphonic acid) ( AMP ), a phosphino-polycarboxylic acid ( PPCA ), diethylenetriamine pentacarboxylic acid ( DETPA ), a proprietary polymer ( Genesol 40 ), poly(acrylic acid) ( PAA ), ethylenediamine-tetrakis(methylenephosphonic acid) ( EDTMP ), phosphonobutane-1,2,4-tricarboxylic acid ( PBTC ), sodium metaborate, and N -phosphonomethylimino-diacetic acid ( PMIDA ). Of the polymeric additives only Genesol 40 shows some dissolution activity, dissolving ∼280 ppm silica at 10 000 ppm dosage after 72 h. PBTC and DETPA are the best-performing additives of all those tested. PBTC is effective even at the 2500 ppm dosage, as it solubilizes ∼290 ppm silica after 72 h. Its efficiency is dosage-dependent. DETPA is also an effective silica dissolver. Its behavior is similar to that of PBTC . Its best dosage is 7500 ppm, which yields dissolution of 322 ppm silica (after 24 h), 340 ppm (after 48 h), and 333 ppm (after 72 h). SSD silica is a very recalcitrant deposit showing resistance to dissolution even by the most effective additives, PBTC and DETPA .