Noncrystalline Al hydroxide

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... 

Adsorption of As on selected sorbents (ferrihydrite, noncrystalline Al-hydroxide, and Andisol) was determined at pH values ranging from 4.0 to 8.0. Suitable amounts of As were added to an Andisol sample at pH 4.5 as a mixture with LMMOAs (OX, MAL, or citrate) or phosphate in order to have a maximum surface coverage of As (as determined by adsorption isotherms) and an initial orgaitic ligand or phosphate/As molar ratios ranging from 0 to 5. 

Other experiments were carried out on the adsorption of As in the absence or presence of MAL on different sorbents (goethite, ferrihydrite, noncrystalline Al hydroxide, and Andisol). Suitable amounts of As (90-100% of surface coverage of As for each sorbent) were added to each sorbent in the presence of increasing concentrations of MAL to have initial MAL/As molar ratios ranging from 0 to 5. After 24 h of reaction, the final suspensions were centrifuged at 10,000 g for 20 ntin and filtered through a 0.22- o,m filter. 

Fig. 9. Amounts (%) of arsenate (As) adsorbed onto goethite, noncrystalline Al-hydroxide, and Andisol at pH 4.0 in the presence of increasing concentrations of malate (MAL) (150, 150, 416 mmol As was initially added per kg of goethite, noncrystalline Al-hydroxide, and Andisol, respectively).

Noncrystalline aluminosilicates (allophanes), oxides, and hydroxides of Fe, Al, and Mn, and even the edges of layer silicate clays to a lesser extent, provide surface sites for the chemisorption of transition and heavy metals. All of these minerals present a similar type of adsorptive site to the soil solution a valence-unsatisfied OH or H2O ligand bound to a metal ion (usually Fe ", AF, or Mn ). For example, on iron oxides, a trace metal, M, may bind according to the reaction... 

As with metal cations, anion chemisorption occurs on soil minerals that possess surface hydroxyl groups. The most important minerals in this regard are noncrystalline aluminosilicates (allophanes) oxides and hydroxides of Fe, Al, and Mn and layer silicate clays (edge sites only). It is the H2O or valence-unsatisfied OH ligands bound to surface metal ions (usually Fe, Al, or Mn) that are the sites of chemisorption. In general terms, the surface reaction can be written... 

The solid phase of soils can consist of many Al-bearing materials, including organic matter oxides and hydroxides of Al noncrystalline aluminosilicates layer silicate clays and various primary minerals. Any model of Al solubility that is based on the dissolution reaction of only one of these materials is likely to be too simple. Nevertheless, some of these models will now be considered in turn. 

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