Sedimentation volume silica

The colloidal stability of silica Suspensions in the present work was assessed by sediment volumes and from the optical coagulation rate constant. In the first method, 50 mg of silica was dispersed in 5 cm3 polymer solution (concentration 10-2 g cm 3) in a narrow tube and the sediment height found at equilibrium. Coagulation rates of the same systems were found by plotting reciprocal optical densities (500nm, 1cm cell) against time. When unstable dispersions were handled, the coagulation was followed in... 

The sediment volume of silica in CCl solutions of poly (methyl methacrylate) was approximately 9 cc g-l but variable results were found in solutions of polystyrene, depending on the molecular wt. of the polymer. Lower M.W. samples are poor stabilizers and the dispersions are so unstable that optical coagulation rates could not be measured with confidence. Figure 5 shows the general trend in CCl. All polymers, whatever their composition, are superior to the pure solvent. 

Figure 5. Sediment volume of silica 186 in CCl solutions of polymers of various styrene contents. Squares, block polymers circles, random copolymers or homopolymers.

In the absence of polymer the sediment volume of silica depends on the non-solvent fraction of the medium as shown in Figure 6. The sediment volume assessment of steric stabilization behavior of the copolymers is illustrated in Figures 7a to 7c. At low styrene contents, both the random and block copolymers show a steady increase in sediment volume as the non-solvent content is raised up to the phase separation value. With polystyrene and random copolymers of high styrene content, the sediment volume stays largely constant with alteration in the non-solvent fraction until the theta-point is approached and then continues to become larger as the limit of solubility is reached. In Figure 7b only the data points of RC 86 are shown, RC 94 giving almost identical values. 

Colloid stability assessments of silica dispersions in CCI4 by sediment volume and coagulation rate are in general concordance and confirm the pattern previously reported by Barron and Howard... 

Figure 6. Sediment volume of silica 227 in polymer-free CCl /C Hj mixtures.

Figure 7. Sediment volumes of silica 186 in solutions of polymers at various non-solvent contents. Plateau adsorption, (a) curve 1, RC 08 2, RC 86 3, PS III. (b) curve 1, BC 10 ...

Poly(methyl methacrylate) provides a level of stabilization even though the solution in CCl is below the 0-temperature. All the copolymers, both random and block, are better stabilizers than PMM, the methacrylate units acting as anchors, with stabilizing sequences of styrene loops, of block copolymers, or mixed loops and tails, of random copolymers, at better than 0-conditions. Higher M.W. polystyrenes give silica dispersions too unstable to measure by our optical method the sediment volumes are between those of poly(methyl methacrylate) solutions and pure solvent. 

The addition of water to immiscible nonpolar liquids increases the sedimentation volume of the solid of the.particles containing polar spots,.because the water helps form bridges. On the other hand, if partly hydrophobic particles are dispersed in water, the sedimentation volume is high because of association between the hydro-phobic spots on particles. This is reduced when a polar solvent such as dioxane is added, which renders all of the surface water-wettable by adsorption of the polar organic material at the hydrophobic areas. These factors are important to consider when silica is used as a thickening agent. 

The sediment sample is allowed to dry in open air and then sieved. To 20g of the sample 20% distilled water is added for deactivation purposes and the excess water is then bound to active silica (Siloxid), so that a powdery consistency is obtained. The insecticides studied are extracted with petroleum ether (b.p. 30-60°C) in a Soxhlet apparatus. The extract is concentrated using the vacuum rotary evaporator and the coextractants are separated on a Celite oleum column. The petroleum ether eluate is then concentrated to a volume of 1ml and used for gas chromatography under the following conditions [10, 31-33],... 

Adsorption column preparation and loading. In order to obtain satisfactory results, the tube must be uniformly packed with the adsorbent uneven distribution may lead to the formation of cracks and channels and to considerable distortion of adsorption band shapes. If there is any doubt concerning the uniformity of particle size of the adsorbent powder it should be sifted before use to remove the larger particles fines are removed from the adsorbent using a sedimentation procedure immediately prior to column packing. In this the alumina or silica gel adsorbent is stirred into between five to ten times its volume of the selected solvent or solvent system, allowed to settle for five minutes and the supernatant liquor decanted off the procedure is repeated until the supernatant liquid is clear. 

U.S. EPA has listed 16 PAH as priority pollutants in wastewaters and 24 PAH in the category of soils, sediments, hazardous wastes, and groundwaters. Some common PAH compounds including the ones listed by U.S. EPA as priority pollutants are presented in Table 2.13.1. All these analytes, as well as any other compound that has a polyaromatic ring, may be analyzed by similar methods. The analytical steps include extraction of the sample with methylene chloride or an appropriate solvent, concentration of the solvent extract into a small volume, cleanup of the extract using silica gel (for dirty samples), and determination of PAH by HPLC, GC, or GC/MS. The HPLC method is superior to packed column GC analysis which suffers from a coelution problem. 

Artificial sediment formulated from constituents such as silica sand and peat moss according to standardized recipes, intended to match the physical characteristics (e.g., grain size, TOC) of the site under investigation. Volume 2(10). 

Soil, sediment (2,3,7,8-TCDD) Soxhlet extraction of sample volume reduction clean-up on basic silica/acidic, silica/alumina, elution with CH2CI2 in hexane analysis clean-up on silver nitrate silica or 2,3,7,8-TCDD-specific alumina, elution with CH2CI2 in hexane analysis repeating of clean-up or extraction if needed HRGC/LRMS (SIM) HRGC/MS/MS (SIM) 1 ng/g <1 ng/g 40-90 57-102 Simon et al. 1989... 

Thus from the thermodynamic data it follows that if acid or neutral silica-bearing solutions arrived in the depositional basin, the action of the presumed geochemical barriers (gradients of pH, Eh, concentration) could not operate in isothermal conditions of chemogenic deposition of silica, to form cherty or cherty-iron sediments. Only a change in concentration due to evaporation of substantial volumes of water in closed basins could have led to deposition of silica. An easy calculation shows that to deposit chert bands 0.3-0.5 cm thick in that way, a 100-m water layer has to be evaporated. Thus the formation of thick piles of Precambrian iron formations would have required the evaporation of a fantastic amount of water from restricted... 

In another vein, double layers play a role In the salt-sieving phenomenon, mentioned In the Introduction to Volume I, and already known to Aristotle. When seawater percolates over a compact sediment of slllcate-like particles, under some conditions the effluent Is potable. Basically the phenomenon is attributable to the negative adsorption of (in this case) anions, leading to the Donnan expulsion of electrolyte, see sec. 3.5b. Over-demand may lead to salt penetration the screening of the double layers around the silica particles (reduction of x ) makes the pores between them effectively wider. For this problem technical solutions had to be found. 

Fig. 14 Normalized Dcoii/Bo of PDMS coated silica suspension with = 0.3 in a symmetric mixture of toluene and heptane (solid circles) along with hard sphere suspension (open squares) at similar volume fraction. The hydrodynamic interactions expressed in H(q) for the two systems (solid squares for the hard sphere suspension) are shown in the inset [101]. This system is crystallized by sedimentation as seen in the photograph...

The within- and between-bottle homogeneity was verified and the variances were compared with the method variance (assessed on the basis of replicate analyses of an extract). A 500 mg portion of the sediment was extracted in a Soxhlet apparatus with 120 mL of a mixture of n-hexane and acetone 90/10 (v/v) for 6 h. The extract was concentrated and taken into a volume of 1 mL iso-octane. The concentrated extract was eluted over 2 g deactivated silica (with 15% water) with 15 mL of petroleum ether. The eluate was concentrated to a volume of 1 mL and treated with 1 mL of a solution containing 34 g L tetrabutylammonium sulphate, and sodium sulphite and isopropanol for the removal of sulphur. The concentrated eluate was cleaned over a column packed with 1.5 g deactivated silica (with 5% water). An internal standard solution (CB 198) was added to the clean extract. The mixture was separated on an Ultra-2 column (length 50 m, internal diameter 0.2 mm, film thickness 0.33 mm. He carrier gas, 65 min gradient from 90 to 275°C) with electron capture detection. No significant difference in the variance of the between- and the within-bottle series and the analytical method itself Therefore, it was concluded that the material is homogeneous for CBs. 

Figure 1. Change in volume on settling (small circles, left scale), expressed as a change in capacitance, as a function of time for a 35% suspension of 5-pm silica particles (Minusil). Also shown, the height of the sediment (large circles, right scale). (From Braun et al., 1987).

If wet soil at the land surface is thought of as simply a very concentrated suspension of sediment in water, the sediment concentration would be on the order of 1,600,000 mg/liter. If the rate of silica release into solution were related to total solid surface exposed per unit volume of water and thus proportional to the sediment concentration, it would take 3-4 minutes contact time to increase the silica concentration to 12 mg/ liter based upon the data for soil A (Figure 15). 

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