Acid water containing toxic levels (>0.1 ppm) of aluminum is treated to dissolve therein both silicate ions in an amount to create a solution containing at least 40, preferably at least 100, micromoles per liter of silicon and a silicon:aluminum ratio (based on metals) of at least about 6:1 and preferably 10:1 in the water to form a non-toxic complex with aluminum and alkali or alkali metal ions to increase the pH of the water. The preferred treatment agent is a hydraulic calcium silicate, in the form of cement clinker, optionally silicate-enriched with a soluble silicate.