Total silica concentration

Typically, the total silica concentration in most surface waters is 5 to 15 ppm Si02, but this may be much higher (notably in arid areas), rising to 50 to 70 ppm Si02 or higher. 

Influence of Total Silica Concentration on Aggregation Kinetics, Dilution of the silicate solution would be expected to give rise to a crossover in the type of aggregation behavior of the primary silicate particles. For relatively concentrated systems ([Si02] 5 wt %), a... 

Figure 5. Fractal dimensionality of aqueous silica aggregates differing in total silica concentration obtained from SAXS spectra. The spectra were recorded after 5 days of reaction (pH 4.0 at 25 °C) in sealed polyethylene bottles. SAXS spectra were recorded at the Synchrotron Radiation Source of Daresbury Laboratories, United Kingdom, on beam line 8.2.

Figures 7, 8, and 9 describe changes in aqueous silica concentration, as the pH is lowered from ca. 13 to ca. 6, at various total silica concentrations (up to and including 10 2 mol/dm3). Runs were performed in the absence and presence of titania. This series of experiments was performed at 25 °C (34), but nevertheless provides a framework for the understanding of DS deposition.

Two substances that are frequently of concern in ion exchange demineralization are silica and organics. The organics are frequently present in natural waters as aromatic polycarboxylic acid derivatives known as humic and fulvic acids. Silica may be the limiting factor in the efficiency of the anionic resins, and (particularly in boiler feedwater applications) the lower the concentration before ion exchange demineralization, the better. Reverse osmosis will frequently produce 90% or greater reductions in total silica concentrations. However, performance should be tested on the specific water to be treated since trie results can be variable and the reason for differences between waters is not yet understood. 

Influence oe Total Silica Concentration ON Aggregation Kinetics... 

The rate of aggregation was shown in general to be related to the total silica concentration as follows ... 

It is unlikely that small colloidal particles such as the 48-mer can be nucleated unless the total silica concentration exceeds 300 ppm, since the equilibrium solubility of the 48-mer (equivalent SiOj diameter of 1.52 nm) is about 290 ppm (see also Figure 3.56). 

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