Sediments silica

In weathering situations, saturation of fluids with SiC relative to any species of pure silica is probably only rarely achieved. In continental and shallow sea deposits, silica is precipitated in some initially amorphous form, opaline or chert when lithified or extracted by living organisms. Authigenically formed silicates are probably not in equilibrium with quartz when they are formed. As compaction increases in sediments, silica concentrations in solution are again above those of quartz saturation (15 ppm) and again it must be assumed that the diagenetic minerals formed are not in equilibrium with a silica polymorph except where amorphous silica is present. It is possible that burial depths of one or two kilometers are necessary to effectively stabilize that quartz form. It must be anticipated that the minerals formed under conditions of silica saturation near the earth s surface will be a minority of the examples found in natural rock systems. 

Coagulation involves the addition of chemicals to alter the physical state of dissolved and suspended solids. This facilitates their removal by sedimentation and filtration. The most common primary coagulants are alum ferric sulfate and ferric chloride. Additional chemicals that may be added to enhance coagulation include activate silica, a complex silicate made from sodium silicate, and charged organic molecules called polyelectrolytes, which include large-molecular-weight polyacrylamides, dimethyl-diallylammonium chloride, polyamines, and starch. 

Zebiihr et al. (29) developed an automated system for determining PAHs, PCBs and PCDD/Fs by using an aminopropyl silica column coupled to a porous graphitic carbon column. This method gives five fractions, i.e. aliphatic and monoaromatic hydrocarbons, polycyclic aromatic hydrocarbons, PCBs with two or more ortho-chlorines, mono-ort/io PCBs, and non-ortho PCBs and PCDD/Fs. This method employed five switching valves and was successfully used with extracts of sediments, biological samples and electrostatic filter precipitates. 

Sediment type %CaCO % clastic and clayey material % amorphous silica % pelagic sed. Composition... 

One of the most complex separation schemes utilizes flash liquid chromatography and PLC to obtain petropophyrins both from geochemical samples or those synthesized and used subsequently as standards [110]. Ocampo and Repeta [111] described the scheme of petroporphyrins isolation in which at the first step the sediment extract is fractionated into ten fractions on silica gel using dichlo-romethane (fractions 1 to 4), a mixture of dichloromethane-acetone with increasing acetone concentrations (for fractions 5 to 9), and, at last, dichlo-romethane methanol (4 1) (fraction 10). Next, the fifth fraction was separated on silica PLC plates using dichloromethane-acetone (97.5 2.5 v v v) as a developer. Two purple bands (with Rj 0.53 and 0.50) were recovered from silica and purified further on a silica gel column with dichloromethane-acetone (97.5 2.5, v v v) as an eluent. The emiched fraction was then separated by PLC with the same solvent mixture, and the purple bands containing two bacteriopheophytin allomers were recovered with acetone. 

Ferruginous chert in which abundant silica occurs formed below the seafloor by the mixing of ferruginous sediments and hydrothermal components (Kalogeropourous and Scott, 1983). 

Several types of mineralizations, massive-type stockwork mineralization and a sulfide-bearing sediment layer were described (Halbach et al., 1989). The outer portion of the massive sulfide sample (late-stage) is composed of barite, realgar, orpiment, amorphous silica and hydrous Fe-Mn oxides, small amounts of sphalerite, galena and pyrite. The central portion consists mainly of sphalerite, pyrite and galena with small... 

Osthols E (1995) Thorium sorption on amorphous silica. Geochim Cosmochim Acta 59(7) 1235-1249 Palmer MR, Edmond JM (1993) Uranium in river water. Geochim Cosmochim Acta 57 4947-4955 Paytan A, Moore WS, Kastner M (1996) Sedimentation rate as determined by Tla activity in marine barite. Geochim Cosmochim Acta 60(22) 4313-4320... 

DeMaster DJ, Kuehl SA, Nittrouer CA (1986) Effects of suspended sediments on geochemical processes near the mouth of the Amazon river - examination of biological silica uptake and the fate of particle-reactive elements. Cont Shelf Res 6 107-125... 

Mineral Oil Hydraulic Fluids. Methods are available for analysis of the hydrocarbon components of mineral oil hydraulic fluids (predominantly straight and branched chain alkanes) in environmental samples. Some of these methods are summarized in Table 6-3. In general, water and sediment samples are extracted with a suitable solvent in a Soxhlet extractor (for solid samples) or in separatory funnel or shake flask (for liquid samples) (Bates et al. 1984 Peterman et al. 1980). The extract is cleaned up on silica gel or Florisil columns using a nonpolar solvent to elute the nonpolar alkanes. Analysis is usually performed by GC/MS (Bates et al. 1984 Kawamura and Kaplan 1983 Peterman et al. 1980). Method performance has not been reported, although 82% recovery of aliphatic hydrocarbons was reported for rainwater (Kawamura and Kaplan 1983). 

Sediment Soxhlet extraction liquid-liquid partition cleanup fractionation on silica gel columns GC/MS Not specified Not specified Bates et al. 1984... 

LAS and major metabolites (SPC) PBDEs, PCDDs, PCDFs sewage sludge Coastal sediments Soxhlet (toluene) and clean-up with silica gel (QqQ) GC-EI-MS [Pg.34]

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. 

Therefore, a systematic examination of the influences of pH and electrolyte concentration on the state of flocculation of both silica types seemed promising. For a quantitative examination of the state of flocculation a Laserphoto Sedimentometer was used to follow the turbidity and the sedimentation of the suspension as a function of time. This method was described in detail in our earlier work (1,10). 

The pyrogenic flame hydrolyzed silica Aerosil 200, a commercial product from Degussa, was used as a dispersion in doubly distilled water (1). The precipitated silica was prepared by hydrolysis of orthosilicic acid tetraethylester in ammoniacal solution according to the method of Stober, Fink and Bohn (11). The prepared suspension was purified by repeated centrifugation, separation from solvent and redispersion of the sediment in fresh water. Finally, the water was evaporated and the wet silica dried at 150°C for about half an hour. 

The pH value and the electrolyte concentration show decisive influences on the flocculation and the sedimentation behaviour of precipitated silica. Precipitated silica could not be flocculated at high pH values without the addition of an electrolyte. At pH> 3.5 no change in the state of the suspension was observed, e.g. the sedimentogramms of the suspensions with polymer addition up to saturation (0 1) have the same shapes as that of the sedi-mentogram of the polymer free suspension. At pH 2.3 and even at a very low polymer concentration (c(40,000/PEG) > 3 10-5 mg/ml,... 

In the flocculated state for all suspensions, immediately after mixing, no transmission of light could be detected through the cuvette used and the sedimentation behaviour was different. Thus, instead of the parameters used for the pyrogenic silica systems we used the following characteristics for the precipitated silica mixtures to describe the flocculation and sedimentation behaviour ... 

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