Kinetics and Mechanisms of Silica Scale Formation on Organic Macromolecule-Coated Surfaces
Open AccessSilica scaling is a major issue in membrane-based desalination. However, the mechanism of silica scaling on organic foulant coated surfaces is not fully understood. Recent studies reported differentiated roles of surface charge in silica scaling. Some studies reported that positively charged surfaces are more favorable for silica scaling while others found increased silica nucleation in the presence of carboxyl groups. In addition, the possible co-existence of bulk nucleation and heterogeneous nucleation complicates the understanding of silica scaling. The objective of this study is to elucidate the mechanisms of silica scaling on organic foulant covered surfaces. We use four types of representative macromolecules, lysozyme (LYZ), bovine serum albumin (BSA), humic acid (HA), and alginic acid (Alg) as model organic foulants for surface coating. We investigated the kinetics of silica scaling on organic foulant covered surfaces by quantifying the deposited silica using a quartz crystal microbalance with dissipation monitoring (QCM-D). We employed two conditions, low oversaturation with negligible bulk nucleation and high oversaturation with fast bulk nucleation, to investigate the kinetics of surface-induced heterogeneous nucleation of silicic acid monomers and deposition of silica aggregates, respectively. For surface-induced heterogeneous nucleation of silicic acid monomers, the deposited silica mass increases in the order LYZ < BSA < HA < Alg. For the deposition of silica aggregates, the mass increases in the order LYZ < HA < Alg < BSA. To elucidate the mechanisms, we analyzed the interfacial free energy and hydration energy using contact angle measurements, and also measured the interaction forces between foulant macromolecules and silica surface using an atomic force microscope.
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