Adsorbent gibbsite

The term alumina hydrates or hydrated aluminas is used in industry and commerce to designate aluminum hydroxides. These compounds are tme hydroxides and do not contain water of hydration. Several forms are known a general classification is shown in Figure 1. The most weU-defined crystalline forms ate the trihydroxides, Al(OH) gibbsite [14762-49-3], bayerite [20257-20-9], and nordstrandite [13840-05-6], In addition, two aluminum oxide—hydroxides, AIO(OH), boelimite [1318-23-6] and diaspote [14457-84-2], have been clearly defined. The existence of several other forms of aluminum hydroxides have been claimed. However, there is controversy as to whether they ate truly new phases or stmctures having distorted lattices containing adsorbed or intedameUar water and impurities. 

When the gibbsite is dehydrated a structural collapse occurs with a large increase in surface area. The boehmite sample has a nominal surface area of 325 m /g. The infrared spectrum of the boehmite shows distinct structure in the OH stretching region, with two peaks located at 3090 and 3320 cm". There are three features at 1648, 1516 and 1392 cm" that are due to adsorbed water and carbonate, which are removed upon heating the boehmite to 350 0 in hydrogen. 

Boron is adsorbed on Fe/Al oxides (goethite and gibbsite) via an inner-sphere mechanism with shifts in zero point of charge (Goldberg et al., 1993). Boron adsorption on Fe/Al oxides increases from pH 3 to 6,... 

Ladeira, A.C.Q., Ciminelli, V.S.T., Duarte, H.A., et al. (2001) Mechanism of anion retention from EXAFS and density functional calculations arsenic (V) adsorbed on gibbsite. Geochimica et Cosmochimica Acta, 65(8), 1211-17. 

IR studies indicate that As(V) is chemisorbed to HAO, as indicated by changes in the vibrations assigned to the As(V)(0,0H) tetrahedron (Table 5). Myneni et al. (1998) found similarities in the position of v(As-O) peaks and v(As-OAl) peaks in a poorly crystalline Al arsenate and in samples of As(V) adsorbed on gibbsite (but many peaks were obscured, as noted above). Goldberg and Johnston (2001) concluded that As(V) chemisorbs on am-HAO after observing that (1) the positions of the v(As-O) modes of sorbed As(V) were indifferent to pH, (2) the Raman spectra of As(V) in... 

The degree of crystallinity of A1 oxides and hydroxides also is important for As adsorption. For comparable experimental conditions, x-ray amorphous Al(OH)3 adsorbed more As(V) per gram than crystalline Y-AI2O3 and gibbsite (Anderson et al., 1976 Anderson and Malotky, 1979 Ghosh andTeoh, 1985 Kingston et al., 1971). 

In an average upper-crustal granodiorite, it is mainly feldspars that weather to form clay minerals (eqns. 4.13 4.14). Since feldspars are framework silicates, the formation of clay minerals (sheet silicates) must involve an intermediate step. This step is not at all well understood although it has been proposed that fulvic acids, from the decay of organic matter in soil, may react with aluminium to form a soluble aluminium-fulvic acid complex, with aluminium in six-fold coordination. This gibbsitic unit may then have Si04 tetrahedra adsorbed on to it to form clay mineral structures. 

For gibbsite type surfaces the characteristic protonation reaction is given by Eq. (43) and the surface charge by Eq. (62). In the presence of a simple 1-1 electrolyte of s.a. ions that weakly adsorb at the Stern plane Eqs. (62) and (55) to (59) can be combined to give... 

ELECTRON SPIN RESONANCE SPECTROSCOPY Electron spin resonance (ESR) is a technique that can also be used on aqueous samples and has been used to study the adsorption of copper, manganese, and chromium on aluminum oxides and hydroxides. Copper(II) was found to adsorb specifically on amorphous alumina and microcrystalline gibbsite forming at least one Cu-O-Al bond (McBride, 1982 McBride et al., 1984). Manganese(II) adsorbed on amorphous aluminum hydroxide was present as a hydrated outer-sphere surface complex (Micera et al., 1986). Electron spin resonance combined with electron spin-echo experiments revealed that chromium(III) was adsorbed as an outer-sphere surface complex on hydrous alumina that gradually converted to an inner-sphere surface complex over 14 days of reaction time (Karthein et al., 1991). 

Phosphate ions, like F", adsorb strongly on aluminous mineral surfaces at low pH, and have been shown to accelerate gibbsite dissolution at pH <3 (Bloom and Erich, 1987). The rate of gibbsite dissolution in 1.0 X 10" M phosphate at pH 3.0 is more than 30 times the rate in 0.1 M KNO3 (Fig. 7-2). The data of Bloom and Erich (1987) show an apparent 0.88-order dependence on solution phosphate. Their data suggest that dissolution is not a simple function of the quantity of PO4 adsorbed. The reaction is also pH independent. 

Aluminum hydroxides. Gibbsite, Al(OH)3, the most abundant aluminum hydroxide in soils (Jackson, 1964 Hsu, 1977), is especially abundant in old soils affected by intensive weathering under tropical climates. Under these circumstances, the desilication of the minerals leaves aluminum hydroxide as a residue. Aluminum hydroxides and aluminum polymers occurring in cryptocrystaUine or non-crystalline states display variable charges and become positively charged under acid conditions, adsorbing anions such as SO4", NO3, Cl , and PO (Hsu, 1977). Table 7-5a shows major A1 minerals. 

Figure 10. Electron micrograph of microcrystalline gibbsite from solution F with negative gold particles adsorbed...

Adsorption of the major anions takes place at low pH values of the soil. Under these conditions, the anions chloride and nitrate are electrostatically and indifferently attracted to positive charges of some soil colloids. Sulfate is more attracted to surfaces, whilst phosphates are specifically and strongly adsorbed. Oxides and hydroxides of Fe and Al, as well as allophane and kaolin-ite, favor anion adsorption. Hydrous oxides such as goethite (a-FeOOH) or gibbsite (y-A1(0H)3) generate positive charges below their zero point of charge Allophane,... 

Spectral characterization of 5-SSA in presence of aluminium ions or adsorbed onto gibbsite... 

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