Oxyhydroxides values

Adsorption of Mo to Mn oxyhydroxides produces an isotopic fractionation that appears to follow that of a closed-system equilibrium model as a function of the fraction of Mo adsorbed (Fig. 8). Barling and Anbar (2004) observed that the 5 TVlo values for aqueous Mo (largely the [MoOJ species) were linearly correlated with the fraction (/) of Mo adsorbed (Fig. 8), following the form of Equation (14) above. The 5 Mo-f relations are best explained by a MOaq-Mn oxyhydroxide fractionation of +1.8%o for Mo/ Mo, and this was confirmed through isotopic analysis of three solution-solid pairs (Fig. 8). The data clearly do not lie... 

Next we explore using the 5 Fe value of the ferric oxide/oxyhydroxide precipitate as a proxy for 5pe(ni)aq, which allows Equation (21) to be used to calculate the Ape(ni)-Fe(n) fractionation from the measured 5 Fe values for the ferric precipitate and Fe(II)aq. This approach is valid when the molar proportion of Fe(III)3q is very small. However, if there is a significant Fe isotope fractionation between Fe(III)3q and ferric hydroxide precipitate, this must be taken into account. As discussed in the previous chapter (Chapter 10A Beard and Johnson 2004), at low... 

Figure 17. Predicted Fe isotope variations produced by redox cycling of Fe due to APIO and DIR by bacteria, as envisioned from Fig. 16. The initial 6 Fe of the pool, as well as that of the influx of Fe(II), [JF n).Exi] is assumed to be zero. Solid lines show the 6 Fe values for ferric oxide/oxyhydroxide deposits as a function of solidification of the pool as an iron deposit is formed, and dashed lines show the 6 Fe values for Fe(II),. Depending upon the relative fluxes of external Fe(II), [Tpem-Ext], return of Fe(II) to the pool by DIR [Tpean Bio]. and precipitation of ferric oxide/oxyhydroxides by APIO [JF nnppd, a wide range of 6 Fe values can be produced.

Using coprecipitation methods with a suitable mixture of solutions described above, the resulting LDH materials are often poorly crystallized and exhibit compositional fluctuations due mainly to the difference in the values of the pH at which the precipitation of M(II)(OH)2 and M(III)(OH)3 hydroxides occurs. Consequently, the chemical formula of the final material may not reflect the composition of the solution prior to the precipitation as noted in Chapter 1. Controlling the amount of anion incorporated under such conditions is very difficult. A "chimie douce method has been proposed by Delmas et al. in an effort to overcome this problem [181,182]. The process is illustrated schematically in Fig. 8. Since the synthesis starts from a highly crystalline layered y-oxyhydroxide precursor, it was suggested that this favored the formation of very crystalline LDHs with controllable M(1I)/M(III)... 

For removing low levels of priority metal pollutants from wastewater, using ferric chloride has been shown to be an effective and economical method [41]. The ferric salt forms iron oxyhydroxide, an amorphous precipitate in the wastewater. Pollutants are adsorbed onto and trapped within this precipitate, which is then settled out, leaving a clear effluent. The equipment is identical to that for metal hydroxide precipitation. Trace elements such as arsenic, selenium, chromium, cadmium, and lead can be removed by this method at varying pH values. Alternative methods of metals removal include ion exchange, oxidation or reduction, reverse osmosis, and activated carbon. 

An indication of the degree of exothermicity of sulphide oxidation reactions can be gained by comparing the enthalpy of formation (A//f), that is, a measure of the energy locked up in each chemical species, relative to native elements. The difference in enthalpies of formation of all reactants and all products defines the enthalpy (heat released or absorbed) of the reaction. Thermodynamic data on sulphide minerals, such as pyrite, are notoriously varied and disputed, and the values in Table 4 must be treated with caution. Nevertheless, depending on whether one defines the reaction as ending in an aqueous solution (equation 5), an intermediate secondary sulphate (e.g., melanterite - equation 6) or in complete oxidation to an oxyhydroxide (equation 7), the calculated reaction enthalpy (AH°) released is of the order of at least 1000 kJ/mol. 

In very acidic solutions (pH < 2.4-3) with ionic strengths below 0.1 M and at 25 °C and 1 bar pressure, scorodite has a pK of about 25.83 0.07. The pK of amorphous Fe(III) arsenate is approximately 23.0 0.3 under the same conditions (Langmuir, Mahoney and Rowson, 2006). At higher pH values, scorodite dissolves incongruently, which means that at least one of its dissolution products precipitates as a solid. The incongruent dissolution of scorodite in water leads to the formation of Fe(III) (oxy)(hydr)oxide precipitates that is, Le(III) (hydrous) oxides, (hydrous) hydroxides and (hydrous) oxyhydroxides (Chapter 3). During the formation and precipitation of the iron(III) (oxy)(hydr)oxides, As(V) probably coprecipitates with them (Chapter 3 also see Section 2.7.6.3). The dissolution rate of scorodite at 22 °C in pH 2-6 water is slow, around 10—9 —10—10 mol m-2 s-1, which explains its presence in many mining wastes (Harvey et al., 2006). 

Figure 8.10 Sorption constants for Cu, Zn and Cd on natural oxyhydroxides as a function of pH obtained from field measurements. The points were obtained in the Carnon River, UK (V, Johnson, 1986) in 40 sites in Canadian lakes representing a variety of geological settings, lake pH values, and trace element concentrations in the sediments and in the overlying waters (O, Tessier, 1992) and in streams affected by acid mine drainage (A, Chapman et at., 1983). Log= Fe-M / Fe-ox [Mz+] (adapted from Tessier, 1992).

A reaction sequence analogous to that in Eq. 4.40 can also be developed for the specific adsorption of bivalent metal cations (e.g., Cu2+, Mn2 or Pb2+) by metal oxyhydroxides.21 In this application the abstract scenario in the first row of Table 4.3 is realized with A = =Al-OH, B = M2+, C = =Al-OH - - M2+, D = = Al-OM+, and E = H where M is the metal complexed by an OH group on the surface of an aluminum oxyhydroxide. Analysis of pressure-pulse relaxation kinetics data leads to a calculation of the second-order rate coefficient kf, under the assumption that the first step in the sequence in Eq. 4.40 is rate determining. Like k(l, the rate coefficient for the dissolution of a metal-containing solid (Section 3.1 cf. Fig. 3.4), measured values of k, correlate positively in a log log plot with kw,. , the rate coefficient for water exchange on the metal... 

Rare earth hydroxide nanocrystals are commonly synthesized via the precipitation of to form gel-like R(OH)3 in basic aqueous solutions, which is quite straightforward with appropriate pH values of 6-8 for Y and La-Lu. Sc would precipitate even in an acidic solution. The crystallized rare earth hydroxide is then obtained after annealing or aging the gel-like precipitation with mother liquor. With elevated temperature, the dried rare earth hydroxides could be dehydrated into oxyhydroxide and oxide in steps. 

For heavy lanthanides like Yb and Sc, the hydrothermal treatment may lead to the formation of oxyhydroxides ROOH. The use of alkali hydroxide or ammonia has strong effect on the phase and morphology of ScOOH (Zhang et al., 2005c). With NaOH as precipitator, pH values in 10-14, or with 2.5 mol L NaOH, a-ScOOH nanorods are obtained, similar to that of YbOOH obtained by Wang and Li (2002). However, when higher... 

---