Silicas adsorption

Supply of MU water for a medium-pressure (450 psig) WT boiler, from a surface water source with very variable suspended solids and hardness (sugar refinery, South Africa). The process used is a. carbonate removal using hot-lime precipitation softening coupled with silica adsorption by magnesia addition b. clarification in anthracite filters and c. cation ion-exchange for the balance of hardness removal. 

Complete resolution and quantitation by HPLC of mixtures of monohydroxy fatty acids containing positional isomers of various chain lengths is a technical problem, because their separation occurs according to both chain length and the position of the hydroxyl group on both reverse-phase and silica adsorption HPLC columns. 

The surface of the silica may be dynamically coated with transition metals, and the selectivities observed can be attributed to the complexes between the metal ions and the analyte species [56], The use of silver-impregnated silica (adsorption of salts of transition metals on the silica surface) has been used for the analysis of saturated and unsaturated fatty acid methyl esters (FAME) and triacylglycerols (TAG) [57], The retention of the unsaturated FAME and TAG can be attributed to the stability of the complex that is formed between the K electrons of the carbon-carbon double bonds and the silver ions. The predominant interaction for saturated analytes is with the polar silanol groups. The secondary interactions are those of the silver ions with the unpaired electrons of the carbonyl oxygens of the analytes. The amount of silver adsorbed onto the silica and the pH (employment of acidic or basic modifiers) have been determined to have an effect on the retention and resolution of certain acidic and basic compounds and fatty acids [58]. 

The aromatics content of aviation turbine fuel is included in the aviation turbine fuel specification (ASTM D-1655). Another test method for aromatics content (ASTM D-5186) involves the injection of a small aliquot of the fuel sample onto a packed silica adsorption column and elution with supercritical carbon dioxide as the mobile phase. Mono- and polynuclear aromatics in the sample are separated from nonaromatics and detected with a flame ionization detector. The chromatographic areas corresponding to the mono- and polynuclear and nonaromatic components are determined, and the mass percent content of each of these groups is calculated by area normalization. The results obtained by this method are at least statistically more precise than those obtained by other test methods (ASTM D-1319, ASTM D-2425). 

Clean-up Silica adsorption EPA Method 3630 or GPC EPA Method 3640 None... 

The dense silica (DS) process involves the exposure of titania particles to aqueous silica solutions of increasing silica concentration. The process is examined in this chapter by relating silica adsorption on titania surfaces to solution pH and concentration and to the various monomeric, multimeric, and polymeric species present in aqueous solutions of silica. Microelectrophoresis and gas adsorption studies reveal that adsorption of monomeric silica occurs via hydrated cation sites that constitute only approximately 40% of titania surfaces. These anchoring sites provide a base for complete surface coverage and buildup of silica multilayers (coatings), a buildup that occurs when the silica concentration is increased sufficiently at the chosen pH (around 10 in the DS process) to induce polymerization. 

The isoelectric point was apparently unaffected at low silica coverages—below ca. 0.5 wt %. This fact indicates that silica adsorbs preferentially onto surface sites in a manner that does not affect average surface charge-pH behavior. A similar suggestion was made by Kononov et al. (35) in describing silica adsorption onto fluorite. 

Iler DS coatings have been a major success in the pigment world, and the Iler process may well be of value in the world of useful photocatalysis. Studies of silica adsorption on titania, via uptake and electrophoresis measurements and gas adsorption characterization procedures, have enabled an explanation of the nature of specific interactions between aqueous silica and titania. Binding is proposed to occur preferentially via hydrated cation sites on titania, and the occurrence of such binding is concluded to provide the basis for the subsequent surface polymerization necessary for the buildup of coherent multilayer silica. 

Silica adsorbs on gamma-AI2O3 in a broad pH range (5, 6), which might be explained by assuming that the silica species in solution have a similarly broad range of pKa values (between 6 and 10). The silica adsorption does not exhibit the sharp maximum value normally seen for weak acids at the pH value equal to the acid s pKa value. Silica adsorption was reduced by the presence of divalent metal ions, possibly as a result of reduced silicate species activity. 

Fig. 8-5. Anomalous As values for adsorption on silica. Adsorption from various solvents on 16% HjO-SiOj (small pore). Reprinted from Journalcf Chromatography...

Silica Adsorption Amino acids 1. BuOH/AcOH/HjO (4 1 1) 2. PhOH/HjO (3 1) Ninhydrin or densitometer Two-way development... 

Clinoptilolite-H palygorskite-Hcalcite K-feldspar-1-dolomite-1-quartz Clinoptilolite-H sepiolite-Hcalcite K-feldspar-h dolomite-1-quartz Amorphous hydroxides (mainly Fe)- - H4Si04-H Mg " -> Fe-montmorillonite Nontronite -1- Mg + + reduced sulfur->saponite + FeSa Dissolved silica adsorption by clay minerals Amorphous aluminosilicate reconstitution ... 

P-05 17-P-12 18-P-12 23-P-17 28-P-IO Mesoporous silica adsorbent, VOC removal 18-P-12 Mesoporous silicas, adsorption 17-P-12... 

DNA Extraction by Silica Adsorption The silica DNA extraction method is an efficient method able to recover up to 90% of the sample DNA. The extracted DNA has various sizes ranging from 100 bp DNA fragments and up to high-molecular-weight DNA. 

The adsorption maximum at pH 9-10 has been seen by others [26] as reflecting the first of silicic acid (pA l = 9.45 at 25°C), although it has been argued by different workers that this maximum in turn was an indication that SiO(OH)J adsorbed alone [29] or that Si(OH)4 and SiO(OH)J adsorbed concurrently [26]. Early modeling studies [26,28] related silica adsorption to exchange with substrate surface hydroxyl groups, although clearly this situation would not be the case for adsorption on fluoride minerals. Davis and Leckie... 

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