In the present work we present experimental and computational results on the efficacy of two simple polymers, polyethylene glycol (PEG) and polyvinyl alcohol (PVA) on silica scale inhibition. The aim of this study is to compare the experimental and computational approaches, in order to explain why PEG is an effective silica inhibitor, while PVA is not. Based on the computational studies, PVA prefers to form interactions between the polymeric chains and not between the polymeric chains and the silicic acid molecules. In contrast, PEG does induce stabilizing interactions between the polymeric chains and the silicic acid molecules. These computational predictions are confirmed by experimental scale inhibition studies, which prove indeed that PEG is an efficient silica scale inhibitor, whereas PVA is not.
Inorganic scaling and fouling are undesirable processes occurring in process waters that are supersaturated with respect to scaling cations and anions.1 Mineral scaling is the result of nucleation and crystal growth phenomena that follow predictable pathways. This is because the mineral scales are composed of well-defined crystalline salts, the most common being calcium carbonate, calcium sulfate dehydrate (gypsum), metal sulfides, depending on the particular water chemistry and operational parameters (eg. temperature).2 Among inorganic foulants, colloidal (amorphous) silica is a unique case for the following reasons: (a) it is not a crystalline phase because it is not composed of discrete cations and anions, (b) it is essentially an inorganic polymer constructed of Si-O-Si bonds, (c) it has unique surface chemistry due to its surface silanol (-Si-OH) groups, (d) "traditional" scale inhibitors (eg. phosphonates or anionic polymers) are not effective in its control. Although colloidal silica can be a useful material for a plethora of applications,3 it is still an undesirable issue in water treatment.
Silica scale formation is an intricately complex process.4 Existing data reveal that silica solubility is essentially independent of pH in the range of 6 to 8. Silica exhibits "normal" solubility characteristics, its solubility increasing proportionally to temperature. Silica formation is actually a polymerization event, not a cation-anion association based on electrostatic attraction. When silicic acid polymerizes, a plethora of structural motifs are created, including rings of various sizes, cross-linked polymeric chains of different molecular weights, oligomeric structures, etc. The resulting "silica scale" is a complex and amorphous solid (colloidal silica).
