|MadSci Network: Chemistry|
The treatment of water with alum salts is effective at removing colloidal material. Most colloids have a negative surface charge. Aluminium ions adsorb strongly onto these surfaces, and help the colloidal material to coagulate and settle out. Aluminium ions are very insoluble in water at normal pH values. Unless the water is very acidic (pH 4 or less), the aluminium ions from the alum will slowly form an aluminium hydroxide precipitate, which will also settle out. I do not think there is a problem with residual aluminium, provided the water is left to settle for some time, and filtered. It is fairly easy to test water for the presence of dissolved aluminium. We have such an experiment in the first year of our environmental chemistry laboratory. We have had difficulty in finding waters that even give a detectible result. I boiled some water containing lemon juice in an aluminium billy. It gave a measurable result a day later. But when we did the same experiment the following year, the residue of that water had a visible aluminium hydroxide cloud, and gave a zero test result. If ever there were a problem with significant amounts of dissolved aluminium in water, the very acidic water involved could easily be neutralized with a small amount of lime or magnesia (this would make the water 'hard' as a side effect, but not make it toxic or unpalatable). Any aluminium in solution would be quite effectively removed. It would be worth checking with the local water supply authorities. They sometimes do use alum, and they certainly monitor the water supply regularly for dissolved aluminium, and would have safe and effective remedies to remove it if it were found.
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