1. Occurrence and Significance
Aluminum (Al) is the second element in Group IIIA of the periodic table. It has an atomic number of 13, an atomic weight of 26.98, and a valence of 3. The average abundance in the earth’s crust is 8.1%; in soils it is 0.9 to 6.5%; in streams it is 400 μg/L; in U.S. drinking waters it is 54 μg/L, and in groundwater it is <0.1 μg/L. Aluminum occurs in the earth’s crust in combination with silicon and oxygen to form feldspars, micas, and clay minerals. The most important minerals are bauxite and corundum, which is used as an abrasive. Aluminum and its alloys are used for heat exchangers, aircraft parts, building materials, containers, etc. Aluminum potassium sulfate (alum) is used in water-treatment processes to flocculate suspended particles, but it may leave a residue of aluminum in the finished water.
Aluminum’s occurrence in natural waters is controlled by pH and by very finely suspended mineral particles. The cation Al3+ predominates at pH less than 4. Above neutral pH, the predominant dissolved form is Al(OH)4−. Aluminum is nonessential for plants and animals. Concentrations exceeding 1.5 mg/L constitute a toxicity hazard in the marine environment, and levels below 200 μg/L present a minimal risk. The United Nations Food and Agriculture Organization’s recommended maximum level for irrigation waters is 5 mg/L. The possibility of a link between elevated aluminum levels in brain tissues and Alzheimer’s disease has been raised. The U.S. EPA secondary drinking water regulations list an optimal secondary maximum contaminant level (SMCL) of 0.05 mg/L and maximum SMCL of 0.2 mg/L.
2. Selection of Method
The atomic absorption spectrometric methods (Sections 3111 D and E, and 3113 B) and the inductively coupled plasma methods (Sections 3120 and 3125) are free from such common interferences as fluoride and phosphate, and are preferred. The Eriochrome cyanine R colorimetric method (3500-A1 B) provides a means for estimating aluminum with simpler instrumentation.