T H E importance of alum coagula-electrolytes commonly found in water, tion in removing colloidal impuri-have been the subject of many investities from water is well known. . Workers have found though much valuable experimental that good flocculation with both alum work has been done on the problems and ferric sulfate occurs within a relainvolved and although the techniques tively narrow pH range. Unfortufor coagulation with alum are well ad-nately, these results have also been vanced, the mechanisms have never considered true for the coagulation of been completely elucidated. Coagula-clay suspensions. This has led to the tion with alum, as other coagulations, belief that there is but a narrow range requires that a colloidal suspension be of coagulant dosage within which good made unstable. The two factors re-flocculation will result, and that the lated to the stability of colloidal sus-coagulant dosage for all waters must pensions are the zeta potential (some-be held within these limits. times incorrectly referred to as the Various theories have been proposed "charge") and hydration. When co-to explain the role of alum in the coagulation of a colloidal suspension oc-agulation of turbid waters. In 1948, curs, the degree of hydration or the Black (13) attempted to summarize zeta potential, or both, are insufficient existing theory as to the mechanism to maintain suspension stability. As of coagulation. He first postulated the isoelectric point is approached, the that organic color and turbidity were zeta potential and hydration decrease negatively charged colloidal particles, and coagulation may occur.or, more accurately, colloids with Most of the laboratory research that negative zeta potential. According to has been done in the past in connec-him, coagulation takes place in three tion with water clarification has been phases: in coagulation with aluminum or ferric 1. As the coagulant dissolves and the salts. The formation and properties trivalent aluminum or ferric ions become of alum and ferric floes. and the effects available, they neutralize the negative of varying the initial pH with sodium charge on the particles of color or turhydroxide or calcium hydroxide in the bidity, and ... their neutralizing power presence of different concentrations of is very great. This, in the opinion of 1467
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