Aluminum Hydr oxides

Aluminum has only one stable oxidation state (+3) within the electrochemical window of water, but it forms numerous relatively stable compounds with oxygen and hydrogen, which differ in their degree of hydration and in their crystallographic structure. The nominal degree of hydration indicated by a chemical name/formula reported in the literature does not necessarily reflect the actual degree of hydration. 

PZCs/IEPs of aluminum (hydr)oxides are presented in Tables 3.1 through 3.263. PZCs/IEPs of aluminum oxide and of gibbsite are compiled in [773]. Acid-base properties of the Keggin A,3 polymer are discussed in [774], 

PZCs/IEPs of aluminum oxides (nominally AEO,) are presented in Tables 3.1 through 3.181. Previous compilations of PZCs/IEPs of aluminum oxides were published in [54,775-778]. A previous compilation of lEPs of aluminum oxides was published in [519] (powders and single crystals). 

PZCs/IEPs of aluminum oxide from different commercial sources are presented in Tables 3.1 through 3.148. 

1 y-Alumina from Akzo Properties BET specific surface area 208 mVg [779], 265 m7g [780], 270 in /g (product code DRUM 1696) [781]. 

Aluminum is an abundant element that is present as a major element in all soils. However, it appears mainly in aluminosilicate minerals (Chapter 8), gibbsite being almost the only aluminum mineral appearing in an important extent in soils (Huang et al. 2002). There are also poorly crystalline aluminum hydroxide or oxohydroxide colloids that are highly reactive, which can strongly adsorb either nutrient or pollutant ionic species. 

The aluminum speciation and precipitation equilibria play an important role both in its state in the soil solution (and soil acidity) and in the presence of aluminum minerals in soil environments hence, we will devote some discussion to this point. Al(III) in aqueous solution undergo the following hydrolysis reactions involving aqueous mononuclear species  

Amethyst Shades of violet from small amounts of Fe(lll) 

Rose quartz Rose-red or pink color due to small amounts of 

Onyx Another microcrystalline banded mineral, having 

---

The calculation illustrates that SO4 is bound strongly to aluminum (hydr)oxide surfaces under acid or slightly acid conditions. At pH above 7 adsorption of SO occurs only to a very small extent. 

Assuming a correlation between surface complexation and aqueous hydrolysis exists, the trend in strengths of surfaces complexes for An in different oxidation states onto a given mineral would be in the order An4+ > AnC>2+ > An3+ > AnOj. Several authors have provided evidence for linear relations between the first hydrolysis constant of metals and the intrinsic constant associated to the formation of surface species of metals as S-OMamorphous silica (Schindler Stumm 1987), hydrous ferric oxides (Dzombak Morel 1990), aluminum (hydr-)oxides and kaolinite (Del Nero et al. 1997, 1999a). 

These questions can be answered directly with nanocluster solutes. Some can be easily synthesized and are metastable in solution for experimentally long periods of time. They have structural features in common with aluminum (hydr)oxide minerals so that ideas about reaction pathways at mineral surfaces can be examined in detail. Most importantly, they are sufficiently small that computer-based models can be used to understand the reactions. 

Hiemstra et al. [44] suggested a complicated electrostatic model, whieh was combined with the 1-pK concept with inert electrolyte binding (reactions (5.23), (5.44) and (5.45)). This model was used to explain the asymmetry in charging curves of aluminum (hydr)oxides. Different penetration of the surface by anions and cations of inert electrolyte was modeled by three different electrostatic planes. The counterions are adsorbed mostly in the 2 plane (which corresponds to the /9 plane), but cations of inert electrolyte are bound by one type of subsurface sites in the surface plane. In contrast, anions bound by these sites split their charge 50-50 between the surface plane and the 1 plane (placed between the surface and the 2 plane. Two capacitances (between the surface and the 1 plane on the one hand, and between the 1 and 2 planes on the other) were adjustable parameters of the electrostatic model. 

The charging curves of aluminum (hydr)oxides reported in [112] merge at pH 6-8 (no clear CIP), and they behave nearly as expected outside that pH range. The charging curves of goethite in [537] showed a CIP in the presence of NaNOj and NaCI, but in the presence of Li salts there was no clear CIP. Other sets of charging curves without a clear CIP are reported in [160,588-590]. 

Yang, X. et al.. Surface acid-base properties and hydration/dehydration mechanisms of aluminum (hydr)oxides, J. Colloid Interf. Sci., 308. 395, 2007. 

Yang, X.F. et al., Adsorption of phosphate at the aluminum (hydr)oxides-water interface Role of surface acid-base properties. Colloids Surf. A, 297, 84, 2007. 

Hiemstra, T., Yong, H., and van Riemsdijk, W.H., Interfacial charging phenomena of aluminum (hydr)oxides, Langmuir, 15, 5942,1999. 

Persson, P. Laid, E. Ohman, L.-O. "Vibradon spectroscopy study of phenylphosphonate at the water-aluminum (hydr)oxide interface", J. Colloid Interface Sci. 1997,190, 341-349. 

The main mechanism of ligand adsorption is ligand exchange the surface hydroxyl is exchanged by another ligand. This surface complex formation is also competitive OH ions and other ligands compete for the Lewis acid of the central ion of the hydrous oxide (e.g., the Al(iii) or the Fe(III) in aluminum or ferric (hydr)oxides). The extent of surface complex formation (adsorption) is, as with metal ions, strongly... 

In Table 5.4 the contributions of the individual weathering reactions were assigned and combined in such a way as to yield the concentrations of Ca2+, Mg2+, Na+, K+, and H+ measured in these lakes the amounts of silicic acid and aluminum hydroxide produced and the hydrogen ions consumed were calculated stoichiometrically from the quantity of minerals assumed to have reacted. Corrections must be made for biological processes, such as ammonium assimilation and nitrification and the uptake of silicic acid by diatoms. Some of the H4Si04 was apparently lost by adsorption on aluminum hydroxide and Fe(III)(hydr)oxides, but the extent of these reactions was difficult to assess. 

Aluminum (oxy)(hydr)oxides (amorphous) — 9.4 (NaCI electrolyte) Goldberg and Johnston (2001)... 

Additional information on adsorption mechanisms and models is in Stollenwerk (2003), 93-99 and Prasad (1994). Foster (2003) also discusses in considerable detail how As(III) and As(V) may adsorb and coordinate on the surfaces of various iron, aluminum, and manganese (oxy)(hydr)oxides. In adsorption studies, relevant laboratory parameters include arsenic and adsorbent concentrations, adsorbent chemistry and surface area, surface site densities, and the equilibrium constants of the relevant reactions (Stollenwerk, 2003), 95. Once laboratory data are available, MINTEQA2 (Allison, Brown and Novo-Gradac, 1991), PHREEQC (Parkhurst and Appelo, 1999), and other geochemical computer programs may be used to derive the adsorption models. 

Adsorption of arsenic Iron, aluminum, and manganese (oxy)(hydr)oxides widely occur as sorbents and coatings on other solid materials in nature. They are often important in adsorbing arsenic from water ((Stollenwerk, 2003), 73 Chapter 3). Below the ZPCs of the (oxy)(hydr)oxides, the presence of... 

Like sediments, colloids are often important in sorbing and transporting arsenic in soils (Sadiq, 1997 Waychunas, Kim and Banheld, 2005). Colloids may consist of clay minerals, organic matter, calcium carbonate, and various aluminum, manganese, and iron (oxy)(hydr)oxides (Sadiq, 1997). Important iron (oxy)(hydr)oxides include goethite, akaganeite (/J-FeO(OH)), hematite, ferrihydrites, and schwertman-... 

Aluminum (oxy)(hydr)oxide coprecipitation followed by filtration ArsenX"/ sorbent Bauxsol sorbent... 

Corundum is aluminum oxide, q -A1203, which has a hexagonal crystalline structure that is analogous to hematite. However, water treatment systems most often use activated alumina, which is typically produced by thermally dehydrating aluminum (oxy)(hydr)oxides to form amorphous, cubic (y), and/or other polymorphs of corundum (Clifford and Ghurye, 2002, 220 Hlavay and Poly k, 2005 Mohan and Pittman, 2007). When compared with corundum, amorphous alumina tends to have higher surface areas, greater numbers of sorption sites, and better sorption properties. 

Depending on the types of hydrotalcites and the chemistry of the arsenic-contaminated waters, As(V) sorption capacities can be very good, perhaps more than 500 mg As(V) g-1 of hydrotalcite (Lazaridis etal., 2002, 321-322), or relatively poor (4.545 mg As(V)/uncalcinated hydrotalcite and 5.609 mg As(V)/calcinated hydrotalcite (Yang et al., 2005, 6809 Table 7.2). Even with excellent As(V) sorption abilities, hydrotalcites are rather expensive compared to aluminum (oxy)(hydr)oxides (Douhova et al.,... 

Although both Fe(III) sulfates and chlorides are effective in coprecipitating As(V) from water, the sulfates may produce less turbidity and corrosion (Han et al., 2003 Floch and Hideg, 2004, 76). As(V) coprecipitation with iron (oxy)(hydr)oxides may be further improved by filtering out the precipitates with membranes or sand. Han et al. (2003) were able to reduce arsenic concentrations to below 2pgL-1 with Fe(III) doses of 6 mgL-1 followed by membrane filtration (Table 7.1). Using Fe(III) sulfate coprecipitation followed by sand filtration, Yuan et al. (2003) found that 0.25 mM of Fe(III) could inexpensively remove about 98 % of 1 mgL-1 of As(V) from household water supplies. Dosages of aluminum sulfate (0.25 mM as Al(III)) achieved about 95 % arsenic removal. 

---