Coprecipitation mechanisms

For both adsorption and coprecipitation mechanisms, the expected correlation between hydrolysis and adsorption (or coprecipitation) is seen in Figs. 3-5. Cr(III), which hydrolyzes at the lowest pH of the three metal ions, was also removed by adsorption and coprecipitation at the lowest pH. Conversely, Ni(II), which hydrolyzes at the highest pH, was removed by adsorption and coprecipitation at the highest pH. 

The enhancement seen at low pH is very unusual and is difficult to attribute to simple coprecipitation mechanisms operating in tandem with adsorption. Ni(II) could not coprecipitate with Cr(IIl) because the HCO was preformed. Any effects due to Cr(lll) remaining in solution or being stripped from the surface to form a coprecipitate with Ni(II) would be seen in the data for Ni(II) adsorption onto HCO [in the absence of Zn(II)], and there would be no enhancement in the presence of Zn(II). If Ni(II) were to coprecipitate with Zn(II), then one would expect to observe enhanced removal of Ni(II) in the presence of Zn(II) onto HFO, which was not the case. 

The removal of Zn(II) by adsorption onto HCO in ammoniacal solutions is illustrated in Fig. 20. The data around pH 6-7 do not follow the characteristic sigmoidal curve, with the percent removal decreasing sharply followed by an immediate sharp increase. The data are modeled as two distinct curves (see Sec. VI) to highlight the probable cause of this apparent deviation from normal adsorption behavior. Curve 1 represents the removal assuming that a coprecipitation mechanism is operating. Curve 2 represents the removal assuming that an adsorption mechanism is operating. 

The removal of Zn(II) by coprecipitation with HFO in ammoniacal solutions is illustrated in Fig. 21, and its removal by coprecipitation with HCO in ammoniacal solution, in Fig. 22. In the case of HFO. the pattern is similar to that observed for adsorption. Coprecipitation, in this case, results in only a slight increase in removal. In the case of HCO, however, the pattern is quite different, with removal considerably enhanced at all pH values. These results indicate that the coprecipitation mechanism is very effective and can overcome the effect of a complexing ligand to promote heavy metal removal over a wide range of pH values and conditions. 

In this work the formation of nickel, cobalt or zinc aluminum hydrotalcite-type coprecipitates upon impregnation of y-alumina at near neutral pH and ambient temperature was confirmed by EXAFS and X-ray dif action. The role of the metal ion concentration in solution on the composition of the supported coprecipitate was studied as well as the influence of the specific surface area of the Y-alumina. The deposition of Co(II), Ni(II) and Zn(II) ions onto a commercial almnina was first investigated. Since the coprecipitation mechanism is likely to be affected both by the impurity level and the thermal pretreatment of the carriers before impregnation, supports of high purity were prepared by hydrolysis of aluminum alkoxides and were submitted to identical pretratments immediately before impregnation. The deposition of Ni(ll) onto these supports was then examined. 

Occlusions, which are a second type of coprecipitated impurity, occur when physically adsorbed interfering ions become trapped within the growing precipitate. Occlusions form in two ways. The most common mechanism occurs when physically adsorbed ions are surrounded by additional precipitate before they can be desorbed or displaced (see Figure 8.4a). In this case the precipitate s mass is always greater than expected. Occlusions also form when rapid precipitation traps a pocket of solution within the growing precipitate (Figure 8.4b). Since the trapped solution contains dissolved solids, the precipitate s mass normally increases. The mass of the precipitate may be less than expected, however, if the occluded material consists primarily of the analyte in a lower-molecular-weight form from that of the precipitate. 

Coprecipitation is a partitioning process whereby toxic heavy metals precipitate from the aqueous phase even if the equilibrium solubility has not been exceeded. This process occurs when heavy metals are incorporated into the structure of silicon, aluminum, and iron oxides when these latter compounds precipitate out of solution. Iron hydroxide collects more toxic heavy metals (chromium, nickel, arsenic, selenium, cadmium, and thorium) during precipitation than aluminum hydroxide.38 Coprecipitation is considered to effectively remove trace amounts of lead and chromium from solution in injected wastes at New Johnsonville, Tennessee.39 Coprecipitation with carbonate minerals may be an important mechanism for dealing with cobalt, lead, zinc, and cadmium. 

The coefficient actually measures multiple processes (reversible and irreversible adsorption, precipitation, and coprecipitation). Consequently, it is a purely empirical number with no theoretical basis on which to predict adsorption under differing environmental conditions or to give information on the types of bonding mechanisms involved. 

Figure 15.4(A) shows the effect of the R = Zn2+/Al3+ ratio, which determines the charge density of the LDH layer, on the Freundlich adsorption isotherms. K values are far higher than those measured for smectite or other inorganic matrices. The increase in Kf with the charge density (Kf= 215, 228, 325mg/g, respectively, for R = 4, 3 and 2) is supported by a mechanism of adsorption based on an anion exchange reaction. The desorption isotherms confirm that urease is chemically adsorbed by the LDH surface. The aggregation of the LDH platelets can affect noticeably their adsorption capacity for enzymes and the preparation of LDH adsorbant appears to be a determinant step for the immobilization efficiency. [ZnRAl]-urease hybrid LDH was also prepared by coprecipitation with R = 2, 3 and 4 and Q= urease/ZnRAl from 1 /3 up to 2.5. For Q < 1.0,100 % of the urease is retained by the LDH matrix whatever the R value while for higher Q values an increase in the enzyme/LDH weight ratio leads to a decrease in the percentage of the immobilized amount.

The mechanism of coprecipitation reUes upon the condensation of hexa-aquo complexes in solution in order to build brucite-like layers having a uniform distribution of both metallic cations and solvated interlamellar anions [7]. Two methods of coprecipitation have been commonly used precipitation at low supersaturation and precipitation at high supersaturation. 

The simplest technique may be coprecipitation. In this method, a reagent is added to the stock solution that is destabilized and precipitated. Better mixing at a microscopic level is then achieved without mechanical grinding and mixing. Insoluble carboxylates such as citrates, oxalates and carbonates or hydroxides are the most suitable reagents. 

Analysis of graft copolymer fractions shows that they always contain PVC and BD-AN copolymer in sufficient quantity to eliminate the hypothesis of mechanical coprecipitation (for a mixture we find ca. 95% homopolymer). The amount of PVC in the first fraction is higher (ca. 70-80%) than in the last. 

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