Hydroxidic phases

Many of the procedures used for technical analysis of aluminum hydroxides are readily available from the major producers of aluminum hydroxides. Phase Composition. Weight loss on ignition (110°—1200°C) can differentiate between pure (34.5% Al(OH)2) ttihydroxides and oxide—hydroxides (15% Al(OH)2). However, distinction between individual ttihydroxides and oxide —hydroxides is not possible and the method is not useful when several phases are present together. X-ray powder diffraction is the most useful method for identifying and roughly quantifying the phase composition of hydroxide products. 

Clay may promote hydrolysis of the metal at low pFI, but also inhibit hydrolysis at high pH (McBride, 1991). At a higher pH, clay prevents complete hydrolysis of the metal due to the affinity of the charged polymeric metal ions for the silicate surface. This keeps the metal from becoming a separate hydroxide phase. 

In setting up a reaction path, we find there is no entry in the thermo.dat database for Cr(OH)3(s). To write the kinetic reaction, we can use the mineral Cr203 as a proxy, since it is the dehydrated form of the hydroxide phase. This substitution alters the reaction s free energy yield, but forward progress is favored so strongly that the reaction rate predicted is not affected. If this were not the case, we would need to add to the database a mineral Cr(OH)3 (s) of appropriate stability. 

As pH rises, the metal content of drainage water tends to decrease. Some metals precipitate directly from solution to form oxide, hydroxide, and oxy-hydroxide phases. Iron and aluminum are notable is this regard. They initially form colloidal and suspended phases known as hydrous ferric oxide (hfo, FeOOH n O) and hydrous aluminum oxide (HAO, AlOOH nH.2O), both of which are highly soluble under acidic conditions but nearly insoluble at near-neutral pH. 

Acidification of the aqueous potassium hydroxide phase with 6A7 hydrochloric acid gives -toluenesulfonamide. After being dried at 85° (50 mm.) the sample weighs 110-120 g. (86-94%) and melts at 132-134°. 

Sequential extraction showed that, for both reduced and oxidized tailings, most of the total Fe (75-85%) is in the residual phase with most of the remainder in the iron-hydroxide phase. Less than 1% Fe is in the mobile adsorbed-exchangeable-carbonate and water soluble phases in both reduced and oxidized tailings. 

Copper was shown to be much more mobile with about 28% total Cu being water soluble or easily extractable 40-50% in iron hydroxide phases and only about 20-30% in the residual. 

Although the elemental analysis indicates an enrichment in Al, no crystalline Al phase was observed in any of the sediment samples. Neither XRD nor electron diffraction showed a separate Al hydroxide phase. 

Oxide/hydroxide minerals of Mn(III,IV), Fe(III), Co(III), and Pb(IV) are thermodynamically stable in oxygenated solutions at neutral pH, but are reduced to divalent metal ions under anoxic conditions in the presence of reducing agents. Changes in oxidation state dramatically alter their solubility. Reduction of Fe(III) to Fe(II), for example, increases iron solubility with respect to oxide/ hydroxide phases by as much as eight orders of magnitude (1). 

In fact, the polymer is quite stable with respect to precipitation. Once isolated it can be kept in aqueous solution indefinitely (37). This stability is presumably kinetic in origin. Since all evidence points to a different internal structure for the polymer from all crystalline ferric oxide or hydroxide phases, the reorganization required for precipitation would be expected to have a high activation energy. Addition of base to pol5maer solutions does produce an immediate precipitate, presumably by cross-linking the polymer particles. In hydrolyzed ferric nitrate solutions with less than 2.5 base equivalent per mole of iron the eventual precipitates observed are probably formed directly from low molecular weight components. The low rate of dissociation would then be another factor in polymer stability. 

It should be borne in mind that the mechanism may change in the course of the deposition. As the metal is depleted from solution, the complexmetal ratio will increase and may pass the point where no solid hydroxide phase is present in the solution. In this case, the ion-by-ion process will occur (initially in parallel with the hydroxide mechanism, later maybe exclusively) if the conditions are suitable. 

In Ref. 8, crystals ca. 5 mn in size were deposited from a nitrilotriacetate (NTA)-complexed bath (no ammonia) at 40°C (a lower temperature than most CdS depositions). The composition of the bath was such that Cd(OH)2 was present as a colloidal phase (cluster mechanism-see Chap. 3). Under conditions where no hydroxide phase was present and the reaction proceeded via an ion-byion mechanism, much larger crystals (>70 mn) and a red-shifted spectrum were found. See Section 10.2.2 for more detail on the dependence of crystal size on the deposition mechanism. 

An alternative cleanup procedure is the partition of the raw extract, which often contains considerable amounts of lipid material, between an organic and an aqueous sodium hydroxide phase. With this partitioning scheme, the analytes are further fractionated into estrogens and nonestrogens. The presence of phenolic groups in the molecules of estrogens such as diethylstilbestrol and zeranol ensures their complete extraction from organic phases such as chloroform or tert.-butyl methyl ether into the aqueous sodium hydroxide phase (435, 438, 447). Further purification could be accomplished by neutralization of the sodium hydroxide solution and back-extraction of the contained diethylstilbestrol into diethyl ether (435), or adjustment of the pH of the sodium hydroxide solution to 10.6-10.8 and back-extraction of the contained zeranol into a chloroform phase (447). 

Oxidation with air in the presence of electrolytes. Various iron oxide or iron oxide hydroxide phases are formed depending on the electrolyte used Possible electrolytes include iron(II) chloride solutions [2.18], ammonium chloride [2.19], or ammonium carbonate-carbonic acid [2.20]... 

Precipitation Processes. In principle, all iron oxide hydroxide phases can be prepared from aqueous solutions of iron salts (see Table 23). However, precipitation with alkali produces neutral salts (e.g., Na2S04, NaCl) as byproducts which enter the wastewater. 

The use of mixtures of sodium hydroxide and benzyltrimethylammonium chloride or tetrabutylammonium bromide failed to enhance the DPGE alkylation of HEC by the in situ formation of the corresponding quaternary ammonium hydroxide phase transfer catalyst. These quaternary ammonium halides are too soluble in aqueous /-butyl alcohol and are preferentially extracted into the organic phase. Mixtures of benzyltrimethylammonium hydroxide and sodium acetate were also ineffective in enhancing the DPGE alkylation of HEC for the same reason, namely preferential solubility of benzyltrimethylammonium acetate in the organic phase. 

Table IV shows the predicted and measured concentration of Cu, Pb, Cd, and Zn in the GSL North Arm brine. The measured values are compared with the predicted solubilities calculated for the basic carbonate, carbonate, and hydroxide phases. The lead sulfate form was also included. The measured and calculated values in most cases appear with coefficients of variation representing the six brines used to provide the average values reported.

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