Regrowth of Broken Hydroxide Flocs: Effect of Added Fluoride

Hydrous oxides of Al(III) and Fe(III) play a large part in environmental processes and in the action of coagulants used in water and wastewater treatment. Aggregates (flocs) of hydroxide precipitates can be rather weak and are easily broken by applied shear. It is usually found that broken flocs do not fully regrow under low-shear conditions, and this could be a serious disadvantage in practical applications. The irreversible nature of floc breakage suggests that some form of specific, chemical interaction between precipitate particles must be at least partly responsible. On the basis of experiments reported here and elsewhere, we propose that hydroxyl bridges between particles play a part. When these are broken, there is a reduction in the number of "active" surface groups that are able to form new bridges. When small amounts of fluoride are added during breakage of Al flocs, there can be significant improvement in floc regrowth, although this depends on a number of factors, especially pH. With Fe flocs, fluoride has no noticeable effect. These results can be explained by the formation of soluble Al-F complexes and some dissolution of the Al(OH)3 precipitate. This creates a new surface with more "active" groups that can form new hydroxyl bridges.