Aluminum
Aluminum is one of the most abundant elements on the face of the Earth.
Aluminum occurs in many rocks and ores, but never as a pure metal in nature. Although the metal itself is insoluble, many of its salts are readily soluble. Other aluminum salts, however, are quite insoluble and consequently, aluminum is not likely to occur in surface waters because it precipitates and settles or is absorbed as aluminum hydroxide or aluminum carbonate, or other compounds.
These are the reasons it rarely occurs in natural waters in concentrations greater than a few tenths of a mg/L. The exceptions are mostly waters of very low pH.
Because aluminum is so abundant and widely distributed, most natural waters have ample opportunity to dissolve it. The low concentrations common to water at near neutral pH must therefore be a result of the chemistry of the element.
Aluminum occurs in many silicate rock minerals such as feldspars, feldspathoids, micas and many amphiboles. Aluminum hydroxide in the form of gibsite is a fairly common mineral. Aluminum also is present in zeolites.
Most common of all sedimentary aluminum bearing minerals are the clays. The clay minerals have a layer structure in which aluminum is octahedrally coordinated with six oxide or hydroxide ions to form one type of a layer, and silicon tetrahedrally coordinated with oxygen to form a second type of layer.
These layers alternate in various ways, forming the various clay structures. Clays are present in most natural water environments.
The cation Al+3 predominates in many solutions where the pH is less than 4.0. If the pH is raised slightly to a pH of 4.5 to 6.5, a process of polimarization occurs which results in units of various sizes with the structural pattern of gibsite.
The solubility of aluminum is strongly influenced by complexing. This implies that the chemical factors that control aluminum concentration are difficult to define because the reactions by which aluminum is dissolved or precipitated cannot be treated reliably by equilibrium methods. Only a rather small amount of information is available regarding actual aluminum concentrations in water. Occasional reported concentrations of 1.0 mg/L or more in water with near neutral pH and no unusual concentrations of complexing ions probably represents particulate material. `
Because of its low solubility, aluminum is generally a minor constituent in domestic and irrigation waters and is therefore of little importance. Very little information exists on the toxicity levels of aluminum towards fish and other aquatic life.
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