Aluminum, based on its low solubility, is typically not found in any significant concentrations in well or surface waters. Aluminum, when present in an RO feed water, is typically colloidal in nature (not ionic) and is the result of alum carryover by an on-site or municipal clarifier or lime-softener. Alum (aluminum sulfate) is a popular coagulant that is effective in the absorption and precipitation of naturally occurring, negatively charged colloidal material (e.g. clay and silt) from surface waters. Alum, when introduced into water, disassociates into trivalent aluminum and sulfate. The hydrated aluminum ion reacts with the water to form a number of complex hydrated aluminum hydroxides, which then polymerize and starts absorbing the negatively charged colloids in water. Fouling by aluminum-based colloid carryover can occur, with alert levels for the RO designer ranging from 0.1 to 1.0 ppm aluminum in the feed water.
Aluminum chemistry is complicated by the fact that it is amphoteric. Aluminum at low pH’s can exist as a positively charged trivalent cation or as an aluminum hydroxide compound. Aluminum at high pH’s can exist as a negatively charged anionic compound. Typically, the range of least solubility for aluminum compounds is in the pH range of 5.5 to 7.5.