Precipitation multivalent cation

Keeping in mind the above studies of multivalent cations (Fe, Cr and Mn) in aqueous medium, some experiments involving redox or complexometric reactions of these metal ions have been carried out, using ultrasound (20 kHz) and its effect on the precipitation, oxidation, reduction and decomposition of complex have been evaluated. An Ultrasonic Processor model P2 with a titanium tip of diameter 12 mm and 250 watts power was used. In the subsequent sections details of some of the interesting experiments, carried out in aqueous solutions of salts of Fe, Cr and Mn in their different oxidation states, have been discussed. 

The adsorption densities ( r ) on minerals (C< CMC) of the salt type are in some cases higher because of precipitation of the ionic surfactant with multivalent cations in the bulk phase. Measurements were carried out to determine the fraction f the precipitated surfactant by divalent cations Ca and Ba " leading to a decrease in its equilibirum concentration. They showed a shift of the adsorption maximum towards lower values of r, even after a correction of the adsorption density due to the precipitation. On the other hand, a direct co-adsorption of the precipitated surfactant on a mineral surface cannot be excluded. 

The flow enhancement effect is completely suppressed in the presence of trivalent cations. Even at the preparation stage for aluminium sulfate (Al2(SOi)3), the polymer is precipitated at a concentration of c = 5 ppm the flakes thus formed can be clearly seen, suspended in solution. Consequently, no drag-reduction effect can be expected and the experimental solution behaves as a Newtonian fluid (Fig. 31). Therefore, it is likely that multivalent cations from complexes or ionotropic gels and that by exceeding a critical value they may precipitate out of solution. 

Micellar solutions of anionic amphiphiles are usually not stable with respect to the addition of di- or multivalent cations since a precipitation occurs (hard water). In exceptional cases, where precipitation does not occur, the question arises as to how the uni-, di- and multivalent ions compete for binding to the micelles. Due to the high value of I l close to the micellar surface a counterion of high charge will be strongly favored and there is a discrimination between the different types of ions299. For example for SDS close to the CMC, -e(r[)/kT — 7 and if the presence of small amounts of calcium ions does not affect [Pg.74]

Purity of the effluent to be treated organic pollution and multivalent cations must be avoided (1) EM and BPM swell in the presence of aromatic compounds and (2) metaUic hydroxides precipitate in the membranes when the divalent cation concentrations reach 1 ppm. 

Total multivalent cations content in the treated fluid was in the range less than 1-5 ppm. Indeed, the multivalent cations may precipitate by association with OH ions in the stack. This precipitation may occur in the CEM during the ionic transfer from the acid to the base compartment, thus destroying this membrane. It is to be noted that the BPM is not affected by this problem. 

Both As(V) and As(III) may precipitate within soils and sediments, but the constituents inducing precipitation vary dramatically. Arsenate, similar to phosphate, tends to precipitate with hard multivalent cations such as aluminum and ferric iron under acidic conditions and calcium and magnesium under alkaline conditions arsenate may also replace 804 or, in particular, PO4 in minerals due to similar size and charge characteristics (Smedley and Kinniburgh, 2002). Various heavy metals (e.g., Mn, Cd, Pb) and alkali earth metals (Ca, Mg, Ba) also have... 

Multivalent metals can be separated from the alkali metals by solvent extraction or precipitation methods. Cation exchangers and electrolysis with a mercury cathode are useful in separating the metals of other groups from the alkali metals. 

The primary reaction of alkali with reservoir water is to reduce the activity of multivalent cations such as calcium and magnesium in oilfield brines. Upon contact of the alkali with these ions, precipitates of calcium and magnesium hydroxide, carbonate, or silicate may form, depending on pH, ion concentrations, temperature, and so on. If properly located, these precipitates can cause diversion of flow within the reservoir, leading to better contact of the injected fluid with the less-permeable and/or less-flooded flow channels. This then may contribute to improved recovery. Also, this reduction of reservoir brine cation activity will lead to more surfactant activity, resulting in lower IFT values (Mayer et al., 1983). 

Abstract Sulfur sols are colloidal solutions of elemental sulfur or of sulfur-rich compounds. The particles in these solutions have diameters of O.l-l.O iim and consist either of Ss molecules (hydrophobic sulfur sols) or of chain-like sulfur compounds with hydrophilic end groups like sulfonate or functionalized organic groups (hydrophilic sulfur sols). Both types of sols are stabilized by the negative charge of the particles which results in mutual repulsion. Therefore, cations and especially multivalent cations are able to precipitate the sol particles. While hydrophilic sulfur sols can be prepared with sulfur concentrations of up to 600 g hydrophobic sols are much more dilute (<0.1 g 1 ). Sols of these types occur both in industrial desulfurization plants where sulfide is oxidized to elemental sulfur as well as in cultures of certain oxidizing sulfur bacteria. 

In the course of attempts to determine adsorption isotherms of anionic surfactants on selected clays two other phenomena requiring separate investigation were noted, namely, salting-out of surfactants by NaCl, and surfactant precipitation as calcium or magnesium salts by multivalent cations displaced from clays. 

Precipitation of anionic surfactants by multivalent cations is well known and it has been studied intensively (10, 11, 12,13). Powney and Addison (10) found that the addition of small amounts of CaCl to dilute sodium dodecyl sulfate solutions caused precipitation of the calcium salt. The addition of small amounts of n-hexanol was reported to postpone precipitation to higher CaCl concentrations, and increasing the surfactant concentration to a value greater than the CMC was found by Pearson and Lawrence (13) to prevent precipitation of the calcium salt of dodecyl sulfate owing to fixation of calcium ions by the micelles. 

Where surfactants were used in these experiments they were present in relatively large concentration. Hill and Lake, for example, injected a solution containing 0.046 meq/ml of a surfactant mixture having an average equivalent weight of 410 thus, the surfactant concentration was about 1.9 wt.%. Since this concentration is far above the CMC for the surfactants involved, multivalent cations may be bound by the micelles with the result that calcium sulfonate precipitation does not occur. 

To evaluate the importance of multivalent cation precipitation of surfactants by multivalent cations present in the formation brine or resulting from cation exchange and dissolution of minerals such as limestone, dolomite, etc., a very limited study of the calcium tolerance of selected alkylbenzene sulfonates was undertaken. 

Salting-out of alkylbenzene sulfonates from relatively dilute solutions by NaCl has been found to be substantial if the alkyl chain contains more than about 12 carbons. Similarly, the multivalent cation tolerance of alkylbenzene sulfonates in dilute solutions has been found to be small and strongly dependent on equivalent weight. In general, if the equivalent weight exceeds 350 (alkyl chain more than 12 carbons), the calcium tolerance appears to be extremely small. The addition of short-chain alcohols appears to be of limited benefit in preventing either salt-ing-out by NaCl or multivalent cation precipitation. 

Gelation and precipitation occurs with multivalent cations and with some polycations. However, solubilization was observed with proteins (soy protein, caseinate) in the isoelectric range. At low pH, CMC may form cross-links through lactonization between carboxylic acid and free hydroxyl groups. ... 

A different effect occurs with the use of polycarboxy-lates in combination with zeolites. Small amounts of polycarboxylates or phosphonates can retard the precipitation of sparingly soluble calcium salts such as CaCOs (the threshold effect ). As they behave as anionic polyelectrolytes, they bind cations (counterion condensation), and multivalent cations are strongly preferred. Whereas the pure calcium salt of the polymer is almost insoluble in water, mixed Ca/Na salts are soluble, i.e. only overstoichiometric amounts of calcium ions can cause precipitation. Polycarboxylates are also able to disperse many solids in aqueous solutions. Both dispersion and the threshold effect result from the adsorption of the polymer on to the surfaces of soil and CaCOs particles, respectively. 

Polyacrylic acids (PAA) are good at both scale inhibition and dispersion, and are more effective than SHMP. Polyacrylic acids with high molecular weight distribution show the best dispersion abiHty at the cost of scale inhibition ability. However, precipitation may occur with cationic polyelectrolytes or multivalent cations such as aluminium or iron, resulting in fouling the membranes. Blend inhibitors are a combination of low (2000—... 

Alginates have interesting ion-exchange properties most monovalent counterions (except Ag ) form soluble alginate salts, whereas divalent and multivalent cations (except Mg " ) form gels or precipitates. The affinity was found to follow the order [60-63] ... 

Partially hydrolysed polyacrylamides, however, exhibit a high viscosity level due to coil expansion, but succumb to aging and are not stable to multivalent cations (precipitation). 

It is to be noted that precipitation and redissolution of the above type can lead to an abstraction (abstraction of surfactant from solution) maximum and, if precipitation is not totally isolated from adsorption, it can consequently lead to an apparent adsorption maximum. In the case of clays, a number of monovalent and multivalent cations such as those of Na, Ca, Al, etc. can be expected to be present in a supernatant of it and to produce precipitations. 

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