Freezing, cationic concentration

The effect of certain factors on the recovery eflBciency of various cations from water by freezing was studied. These included the mixing rate, initial pH, specific conductance or total dissolved solids, cation concentration and initial and finaj volume. 

Cationic quaternary ammonium compounds such as distearyldimethylammonium-chloride (DSDMAC) used as a softener and as an antistatic, form hydrated particles in a dispersed phase having a similar structure to that of the multilayered liposomes or vesicles of phospholipids 77,79). This liposome-like structure could be made visible by electron microscopy using the freeze-fracture replica technique as shown by Okumura et al. 79). The concentric circles observed should be bimolecular lamellar layers with the sandwiched parts being the entrapped water. In addition, the longest spacings of the small angle X-ray diffraction pattern can be attributed to the inter-lamellar distances. These liposome structures are formed by the hydrated detergent not only in the gel state but also at relatively low concentrations. 

In an electrolyte solution, each formula unit contributes two or more ions. Sodium chloride, for instance, dissolves to give Na+ and Cl- ions, and both kinds of ions contribute to the depression of the freezing point. The cations and anions contribute nearly independently in very dilute solutions, so the total solute molality is twice the molality in terms of NaCl formula units. In more concentrated solutions, the ions do not move independently. For instance, some stick together to form ion pairs and other aggregates of small numbers of ions. The effect of the solute on the freezing point in these solutions is very difficult to predict. We can write... 

It was concluded syllogistically that hemolysis after freeze-thawing will be induced because (1) concentrated sodium chloride removes membrane phospholipid and cholesterol and (2) enhances the permeability of the cells to cations. The leaky cell becomes rich with cations and undergoes osmotic lysis when the suspending medium is diluted by thawing. 

Figure 7 Scatterplots of (Ca -h Mg ) and HCOa" versus S04 for groundwaters in the proglacial zone of Finsterwalderbreen, a polythermal-based valley glacier on Svalbard. The groundwaters were sampled from three wells, and show high concentrations of the divalent cations and S04 due to the dissolution of secondary sulfate salts and freeze concentration effects. By contrast, HCOs concentrations are depressed because of the high concentrations of Ca, so that there is an inverse association between HC03 and S04. Full details can be found in Cooper et al. (2002) (reproduced by permission of Elsevier from J. Hydrol.

A scheme devised by Thurman and Malcolm (1981) uses XAD-8 to concentrate and isolate aquatic humic substances. According to this scheme, the sample is first filtered through 0.45 /am silver-membrane filters and acidified. After concentration on XAD-8 resin, the humic acid fraction is precipitated at pH 1. Both humic acid and fulvic acid fractions are hydrogen saturated by passing the sample through a cation-exchange resin in the H-form. These fractions are then freeze-dried to yield low-ash samples of aquatic humic and fulvic acids. This extraction scheme is outlined in Table 7. These authors successfully isolated humic substances from a number of surface and groundwaters. Even samples with DOC values of 0.7 mg C/L could be proc-... 

Hydrogen saturate fulvic acid fraction by immediately passing O.IN NaOH eluate through cation-exchange resin in H-form. Continue cation-exchange process until final concentration of Na+ is less than 0.1 part per million. Freeze-dry humic acid and fulvic acid fractions. 

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