Silica scaling of reverse osmosis membranes preconditioned by natural organic matter

Reverse osmosis (RO) membranes were preconditioned in this study with humic acid, sodium alginate, or bovine serum albumin, and subsequently examined for silica scaling using the water matrix representative of concentrated brackish groundwater. The results suggested that water matrix combined with organic foulants affected silica scaling. High ambient pH favored the moderate silica ionization and thus the silica homogeneous polymerization to potentially form low molecular weight silica oligomers. The resulting scaling layer was dense and highly impermeable. Under the high Ca proportion at a given hardness, membrane scaling was enhanced through the Ca-induced silica scaling and the formation of intermolecular bridges between adjacent silica species. In contrast, high Mg hardness may facilitate the sustainable growth of silica oligomers to form the ringed high molecular weight oligomers by reducing the required energy for chain deformation. The deposition of these oligomers caused a loose scaling layer with reduced hydraulic resistance to water permeation. During the scaling tests under similar water matrix, the membranes slightly fouled by organics suffered severe flux decline due to an available space provided by the pre-existing organic fouling layer for subsequent silica scaling. [Display omitted] •Oligomers were identified as the dominant silica species under various water matrix.•High ambient pH and Ca hardness resulted in the formation of the dense silica scaling layer with a low water permeation.•Preconditioning of RO membranes prominently enhanced silica scaling.•Silica scaling rendered the entire fouling layer hydraulic irreversible.