Solubility controls on aluminum in drinking water at relatively low and high pH

Potential control of soluble aluminum in drinking water by formation of solids other than Al(OH) 3 was examined. At pHs below 6.0, Al +3 solids containing sulfate, silica or potassium are thermodynamically favored versus amorphous Al(OH) 3; however, in this work no evidence could be obtained that solids other than Al(OH) 3 would form in practice. At pHs above 9, aluminum and magnesium were discovered to form complex solid phases of approximate composition AlMg 2(OH) 7, AlMg 2SiO 2(OH) 7 or Al(SiO 2) 2(OH) 3 dependent on circumstance. Formation of these solids provide a mechanistic explanation for enhancements to precipitative softening obtained in practice by dosing Al +3 salts; that is, improved flocculation/settling and removal of silica from water that interferes with calcium precipitation. The solids also maintain residual aluminum below regulatory guidelines at high pH >9.5.