Mixed cation surface precipitates

Model M9 Mono- and multinuclear complexes and precipitation. The best model yet achieved comprises reactions 203, 204, 221, 902, and precipitation. Simulations of uptake data and Nco are shown in Figures 5A,B, 5C, and 6. Model M9 fits both uptake and Nco data well. The fitting exercise reinforces our inferences and the conclusion offered by Hayes and Katz (10), namely that uptake data alone are inadequate to derive a robust sorption model. Spectroscopic information is needed to support selection of reactions. Future models must account for the mixed-cation hydroxide precipitates observed by Scheidegger et al. (36), Towle et al. (22), and Thompson et al (35), but we found it unnecessary to invoke a co-precipitate or variable activity precipitate. Obviously, M9 is not a unique solution, A continuum of multimeric species seems more likely than a single species. More data are needed to test for protonation/deprotonation of all proposed surface complexes and precipitates, and sorption by the precipitates themselves. 

Ni Sorption on Clay Minerals A Case Study. Initial research with Co/clay mineral systems demonstrated the formation of nucleation products using XAFS spectroscopy, but the stmcture was not strictly identified and was referred to as a Co hydroxide-like stmcture (11,12). Thus, the exact mechanism for surface precipitate formation remained unknown. Recent research in our laboratory and elsewhere suggests that during sorption of Ni and Co metal ions, dissolution of the clay mineral or aluminum oxide surface can lead to precipitation of mixed Ni/Al and Co/Al hydroxide phases at the mineral/water interface (14,16,17,67,71). This process could act as a significant sink for metals in soils. The following discussion focuses on some of the recent research of our group on the formation kinetics of mixed cation hydroxide phases, using a combination of macroscopic and molecular approaches (14-17). 

These above studies suggest that three phenomena occur at the mineral/liquid interface to cause formation of mixed-cation hydroxide phases (1) non-specific and/or specific adsorption (2) dissolution of A1 where the dissolution rate is most probably dependent on the surface morphology and impurities present and (3) precipitation of a mixed Ni/Al phase. The last step is rapid and proceeds until the cation concentrations correspond to the solubility product of the hydrotalcite-like phase. Dissolution of A1 appears to be the rate-limiting step 16,17). 

Towle et al. [91] interpret their EXAFS and TEM results of the C0/AI2O3 (Linde A alumina powder) system as a surface precipitation of Co(OH)2 with many vacancies, while Scheidegger et al. [92] report, also on the basis of EXAFS, the formation of mixed cation phases. Finally, Brown et al. [93] show in situ EXAFS and X-ray absorption measurements with the result that Co (and Pb) adsorb at pH 6.8 on 7-AI9O3 as inner-sphere complexes without forming three-dimensional precipitates or diffusing into the oxide, but multinuclear species are formed on the oxide. 

Hydroxylated groups, which arc the cause of the surface charge (see Chapter 6), may al.so act as coordination sites for di.s.solved cations, or may be substituted by anions (surface coordination). They may also act as nucleation Sites for a solid phase in the case of surface precipitation. Therefore, several possible mechanisms should be considered in the overall phenomena of adsorption, and most of them may be described in terms of the classical concepts of coordination chemistry. However, adsorption exhibits some specific characteristics because one of the partners is a solid with structural and physical properties that may play an important role. Although adsorption often involves only the surface of the particles, if the solid exhibits appreciable ionic mobility (in the case of an ion-exchange material), or electronic mobility (for a mixed-valence material), acid-base surface reactions may trigger various phenomena in the core of the particles, reactions which in turn may cause ionic and/or electron transfers through the. solid-.solution interface. 

LDAO/SDS Interaction. Mixing of cationic and anionic surfactant solutions results In the formation of a mixed species that Is more surface active than the Individual species. The enhanced synergistic effect has been explained (2,3) by showing that a close-packed adsorption of electroneutral R R takes place (R" " and R represent the long chain cation and anion respectively). In the case of Ci2 and C14-DAO, a 1 1 LDAO/SDS molar ratio produces a minimum In surface tension and Is accompanied by an Increase In pH In the bulk solution the association seems to be of the type R R", and the absence of visible precipitate may be attributed to the solubilization of the R R" complex In the solution. In the region where LDAO Is In excess, the structure Is probably [cationic (LDAOH ) anionic (SDS)] nonlonlc (LDAO), while [cationic (LDAOH anionic (SDS)] anionic (SDS) Is formed when SDS Is In excess. Equal molar concentration results In cationic (LDAOH ) anionic (SDS) complex which should favor precipitation. However, at pH >9, there Is no Indication of precipitation (even when the total solute concentration Is 0.35 M). When the pH Is below 9, then precipitation will take place. 

Smectite-type materials were synthesized with a hydrothermal method [5]. The aqueous solution of sodium silicate (Si02 / NajO= 3.22) and sodium hydroxide was mixed with the aqueous solution of metal chloride to precipitate Si-M (M divalent metal cation, Si M = 8 6) hydroxides. The precipitation pH of Si-M hydroxide was controlled by changing the molar ratio of sodium hydroxide to sodium silicate. After separating and washing of Si-M hydroxide, slurries were prepared from Si-M hydroxide and water. The Si-M slurries were treated hydrothermally in an autoclave at 473 K under autogaseous water vapor pressure for 2 h. The resultant samples were dried at 353 K then we obtained smectite samples. The smectite-type materials are denoted by the divalent species in octahedral sheets and BET surface area, e.g., Ni-481 for the Ni2+ substituted smectite-type material with a surface area of 481 m2g. ... 

Solidification with cement generally is accomplished with a Portland cement and other additives. The quantity of cement can be varied according to the amount of moisture in the waste. Heavy metal cations in the waste form insoluble carbonates and hydroxides at the high pH of the mixture. The surface of the hardened mass can be coated with asphalt or other material to reduce leaching of hazardous components. If the waste is mixed with anhydrous cement and water there is the possibility of ions incorporation in the cement structure during the hydrolysis process. Heavy metal ions could bind with the cement by the process of chemisorption, precipitation, surface adsorption,... 

However, the nature, crystallinity (Kinniburg and Jackson, 1976, 1981 McKenzie, 1980), crystal size, and surface charge of metal oxides and mixed metal oxides (e.g., Fe-Al oxides Violante et al., 2003) also play an important role in the sorption selectivity of trace elements in cationic form. McBride (1982) compared the sorption behavior of different Al precipitation products of different crystallinity. The Cu sorption capacity followed tlie order noncrystalline Al-hydroxide > poorly crystalline boehmite > gibbsite. Iron and Mn oxides are... 

The influence of the cations and anions has been discussed separately with the solution properties and reactions in the main focus. It has, however, been known over 100 years that anions play a crucial role for the stabilization and coagulation of colloids. More recently, the contribution of anions on the stabilization of particles, biocolloids, and bubbles has received renewed attention. - In these papers, it has been pointed out that there exists a collaborative interaction between cations and anions upon adsorption of one of the complexes from solution. At high concentrations this effect renders the simple indifferent ions specific and selective to each other. It is also seen as a dependency on the acid-base pair chosen for the regulation of the pH. This effect certainly needs to be added as an extension to (correction of) the DLVO theory. However, as shown in this paper, it is just as probable that the anion and cation collaborate during the adsorption and formation of gels and precipitates at the surface. The presence of such mixed phases has been confirmed experimentally, e.g., during the formation of hydroxoapatite in silica gel layers. ... 

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. 

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