Silica Polymerization from Supersaturated Dilute Aqueous Solutions in the Presence of Alkaline Earth Salts

Abstract

The early stages of silica polymerization in aqueous solution proceed according to a mechanism based on three steps: nucleation, particle growth, and agglomeration of the particles. Application of time-resolved static and dynamic light scattering as a powerful in situ technique in combination with spectrophotometric analysis of the monomer consumption based on the molybdenum blue method was carried out to further investigate this 3-step process. Experiments were carried out at four different initial silicic acid contents covering a range between 350 and 750 ppm in the presence of either 10 mM NaCl or 5 mM of a mixture of CaCl and MgCl. The process in all cases was initiated with a drop of pH to 7. Addition of the salts made possible an analysis of the impact of an electrolyte on the process. Independent of the presence or absence of salt, particle growth in step two proceeded as a monomer-addition process without being interfered significantly by Ostwald-ripening. The growing particles were compact with a homogeneous density. The size of the particles approached final values between 5 and 20 nm with the actual value increasing with decreasing initial silicic acid content. Above a certain concentration of initial silica content, which depends on the level of added salt, particle-particle interactions caused agglomeration. The presence of electrolyte shifted this level from ∼2000 ppm to a range between 500 and 750 ppm. The resulting agglomerates had a fractal dimension of 2. Independent of the conditions, particle growth could be described with a simple nucleation and growth model.

X-ray Absorption Spectroscopy Reveals Mechanisms of Calcium and Silicon Fouling on Reverse Osmosis Membranes Used in Wastewater Reclamation

“…More Si/Ca clusters with R 2 < 0.5 were observed on the lead membrane, suggesting that siloxane could be prevalent, consistent with findings at the full-scale OCWD system . Conversely, the larger number of Si/Ca clusters with R 2 > 0.5 on the tail membrane is consistent with large, highly concentrated Si particles, which we interpret to be silica and whose deposition we speculate is enhanced by Ca 2+ . ,, …”

“…However, we expect the concentration of siloxane in the fouling layer to be lower than silica because previous researchers have observed siloxanes (Si–C) in wastewater effluent at concentrations of 20 μg/L, which is three orders of magnitude lower than the Si concentration measured for the feedwater used in the pilot system investigated here (20 mg/L). We thus interpret the large (10’s–100’s of μm in dimension) Si-rich particles on the tail membrane observed via XRF to be aggregated silica particles, , whereas lower-concentration Si observed in the background layer may comprise silica and/or siloxane.…”

“…Ca mineral standards (calcite, CaCO 3 ; gypsum, CaSO 4 ·2H 2 O; aragonite, CaCO 3 ; brushite, CaHPO 4 ·2H 2 O; monetite, CaHPO4; apatite, Ca 10 (PO 4 ) 6 (OH) 2 ; anorthite, CaAl 2 Si 2 O 8 ; and Ca chloride dihydrate, CaCl 2 ·2H 2 O) were purchased (see Table S2 for vendors and purity). Amorphous Ca carbonate (ACC) and Ca oxalate were synthesized according to existing protocols in the literature. − Synthesis details and chemical information are provided in the SI (Section III). Minerals were prepared for measurement by grinding 2 mg of Ca mineral with 20 mg of boron nitride using a mortar and pestle.…”

“…Conversely, if a reference material cannot be reconstructed from the sample spectra principal components, then it is absent from the mixture of species that comprise the sample. 37 Standards were prepared for XANES analysis as described below. Reference spectra selected via TTA were then used in LCF of sample spectra.…”

dynamics of non-dense sodium silicate - water system studied by molecular dynamics

“…If we look into the experimental studies on silica polycondensation, we see that the researchers implemented a spectrum of methods, including silico-molybdate method [3]- [6], 29 Si Nuclear Magnetic Resonance [7]- [11], Dynamic Light Scattering [5], [6], [11], Static Light Scattering [5], [6], [12] Small-Angle X-ray Scattering [7], [9]- [11], Fourier-Transform Infrared Spectroscopy [8] and Transmission Electron Microscopy [13]. In the majority of the cases, they employed a combination of two (or more) experimental techniques to ensure their mutual verification and access to complementing data.…”