Aluminum hydroxide complex species

In several of the studies of aqueous chemistry of aluminum that have been made since about 1950, polynuclear complexing mechanisms have been proposed to identify and describe the dissolved aluminum hydroxide complex species (3, JO, 11). The formulae proposed have generally been based on stoichiometric considerations and pH measurements assuming the polynuclear species were ionic, and that equilibrium was attained. The complex ions reported by Hsu and Bates (8) were single six-mem-bered rings Ale (OH) 12 or multiples of this unit. Johansson (JO) identified a structural unit containing 13 aluminum and 40 oxygen atoms with various numbers of protons in crystalline basic aluminum sulfate. Because this solid formed readily, the same structural unit of aluminum was proposed as a solute species. Most of the proposed formulae for polynuclear complexes, however, have not been derived from structural considerations. 

The conclusion that polynuclear hydroxide complex ions do not constitute a significant part of the solute species of aluminum at equilibrium in the solutions studied here is in accord with the conclusions of Frink and Sawhney (4). The microcrystalline gibbsite itself is metastable in the sense that as crystals grow in size with longer aging time their solubility decreases. Although at equilibrium metastable species can be ignored, it can be so diflBcult to attain an equilibrium condition that the concept has little practical usefulness. 

The thermodynamic data for fluoride complexes of aluminum, from published literature (12), were recalculated to zero ionic strength by means of the Debye-Hiickel equation. These, with stability data from our experiments with aluminum hydroxide species, provide a basis for deciding which complexes will be predominant when pH and dissolved fluoride concentrations are known. Conclusions drawn concerning the hydroxide species show that the polymeric aggregates of Al(OH)3 should be considered colloidal and they are not included in this calculation as equilibrium solute species. Hydroxide complexes of significance are the monomer A10H- and the anion Al(OH)4". Fluoride complex species constitute the series AlFn " where n ranges from one to six. 

The lines showing proportion of free to total aluminum should meet in a smooth curve rather than at a 90° angle in the region where both fluoride and hydroxide species are important. A simple procedure for calculating the positions of such a curve has been described by the writer (5) for systems where only two complex species need to be considered at a time. 

A1(00CCH3)3) a white solid soluble in water. It is usually obtained as the dibasic salt, basic aluminum ethanoate, A1(0H)(CH3C00)2. It is prepared by dissolving aluminum hydroxide in ethanoic acid and is used extensively as a mordant in dyeing and as a size for paper and cardboard products. The solution is hydrolyzed and contains various complex aluminum-hydroxyl species and colloidal aluminum hydroxide. 

The earth s surface contains rocks and minerals, as well as clays and soils. The minerals may take many forms, including primary aluminosilicate minerals and other complex species that makeup the hard surface, or crust, of the planet. Aluminosilicates are made from aluminum, silicon, and oxygen and account for approximately 82% of the earth s crust by weight. Because of their structure, the soil surface chemistry is dominated by the reactions that occur on oxide and hydroxide-rich surfaces. These surfaces are extremely hydrophilic, layers of adsorbed water molecules surrounded by varying volumes of bulk water. 

Delhi soils by studying its speciation in the soil profile and to assess if there was any spatial variability. Soils representing the Aravali Ridge and the alluvial floodplains of river Yamuna were collected as a single, undisturbed core up to a depth of lm and the profile differentiated into four layers- 0-17 cm, 17-37 cm, 37-57 cm, and 57-86 cm. Pseudo total Aluminum and Iron in the soils were speciated into the operationally defined species (weakly exchangeable, organic matter complexes, amorphous oxides and hydroxides, and crystalline or free oxides) by widely recommended selective extraction procedures. Both A1 and Fe in these soils are bound predominantly as Fe oxides and silicates and have only very low percentages in the easily mobilizable pools. 

Aluminum binds to nucleoside phosphates mainly through the basic terminal phosphate groups. Nucleosides mono-, di-, and triphosphates demonstrate similar phosphate basicity. Aqueous solutions of Al3+ and nucleoside phosphates have a tendency to form ternary complexes with hydroxide in a pH-dependent manner. In addition, there is a possibility of Al3+-bridged complexes being formed. Fig. 3 shows the species distribution for the A13+-ATP system. At physiological pH the merged hydroxo mono complexes predominate [9, 18]. 

Neutral and Polymeric Aluminum and Iron. The association constants and enthalpies of aluminum and iron hydroxides have been evaluated by comparing the critically selected data of Baes and Mesmer (51) with that of R. M. Siebert and C. L. Christ (personal communication, 1976). Differences between the two data sets are negligible and the final selection was from Baes and Mesmer (51) because data on more complexes are found there. Important new species added to tjjie model are the polynuclear complexes Fe2(0H)2 and Fes(OH). Some controversy has arisen over the existence of Fe(0H) and A1(0H)3. Baes and Mesmer (51) have indicated that although the formation constant of A1(0H)3 is only known from one measurement (52) and has a large uncertainty, it is real, with a log K < -15.0 for the reaction... 

Once they have reached higher pH, reducing conditions of the intestinal tract (Davis et al, 1992), sulhdes should be more stable, and may actually precipitate if reduced sulfur is present. Other solids, such as hydroxides or hydroxy-sulfates of aluminum, and possibly iron, may also precipitate. The increased pH should also lead to the increased sorption onto particulates of various metals and metalloids such as lead and copper (Smith, 1999). However, in vitro tests (Ruby et al, 1993) indicate that the increased complexing with unprotonated organic acids and enzymes helps offset the pH-driven precipitation and sorption of the base metals that were dominantly chloride-complexed in the stomach fluids. Arsenic and other oxyanionic species are likely to be sorbed as the stomach acids are neutralized, but may be partially desorbed once higher pH values are reached in the intestine (Ruby et al, 1996). 

Chemical leach tests on the bulk settled dust samples showed that the dusts are quite chemically reactive. Leach solutions have high alkali-nities, due to the rapid partial dissolution of calcium hydroxide from concrete particles. Indoor dust samples produced higher pH levels (11.8-12.4) and alkalinities (—600 mg CaCOa) than outdoor dusts (pH 8.2-10.4 alkalinity —30mgL CaCOa), indicating that outdoor dust samples had reacted with rainfall or other water prior to collection. Thurston et al (2002) found that the leachate pH of the dusts decreased with decreasing particle size. Some metals or metalloids in the dusts (aluminum, chromium, antimony, molybdenum, barium, copper, zinc, cobalt, nickel) are readily leached by deionized water many of these form oxyanion species or carbonate complexes that are most mobile at the alkaline pH s generated by the leachates. 

A central issue in the attempt to establish a reliable database is the requirement of critically evaluated thermodynamic data for several key species. One such pivotal element is aluminum, which has an extensive literature of solubility and thermochemical data from which to choose, for each of the aqueous species or complexes. The aluminum species are fundamental to the calculation of solubility and reaction state with respect to many silicates and aluminum oxides and hydroxides and are principal components in numerous surface chemical reactions in the environment. Two key chapters in this volume address this fundamental problem Apps and Neil give a critical evaluation of the data for the aluminum system and Hem and Roberson present the kinetic mechanisms for hydrolysis of aluminum species. 

Lead enters surface water from atmospheric fallout, run-off, or wastewater. Little lead is transferred from natural minerals or leached from soil. Pb ", the stable ionic species of lead, forms complexes of low solubility with major anions in the natural environment such as the hydroxide, carbonate, sulfide, and sulfate ions, which limit solubility. Organolead complexes are formed with humic materials, which maintain lead in a bound form even at low pH. Lead is effectively removed from the water column to the sediment by adsorption to organic matter and clay minerals, precipitation as insoluble salt (the carbonate, sulfate, or sulfide) and reaction with hydrous iron, aluminum, and manganese oxides. Lead does not appear to bioconcentrate significantly in fish but does in some shellfish such as mussels. When released to the atmosphere, lead will generally occur as particulate matter and will be subject to gravitational settling. Transformation to oxides and carbonates may also occur. 

A Ithough it is the most abundant of the metallic elements in the outer crust of the earth, aluminum usually occurs in natural waters in concentrations below 100 micrograms per liter. High concentrations occur rarely and usually are associated with water having a low pH. The chemical properties of aluminum which control its behavior in water have been studied extensively. This paper is based on current research by the U.S. Geological Survey and on published literature. The principal topics considered here are the processes by which aluminum combines with hydroxide ions to form complexes and polymers, the influence of these processes on solubility of aluminum and the forms of dissolved species to be expected in natural water, and the relative importance of fluoride and sulfate complexes of aluminum. The experimental work is briefly summarized here. Details are published elsewhere (6). 

Two sulfate complexes of aluminum A1S04 and Al( 804)2" also are reported (I). The same type of computation as that described for fluoride-hydroxide systems was used to prepare Figure 12, showing the predominant species to be expected at various pH s and sulfate activities. 

---