Coagulation mechanism of lignin with Al₂ (SO₄) ₃ as a coagulant was investigated. Aluminium ion combines to acidic groups of lignin, such as sulfonic acid and carboxylic acid groups, and then aluminium polynuclear complexes and insoluble aluminium hydroxide are formed by pH adjustment in a state including lignin combined with aluminium ions. Floc thus formed becomes larger by binding each together, and readily precipitates.
ζ-Potential of floc measured by microelectrophoresis is generally considered to be the indicator for setting the best coagulation pH. However, in the case of lignin as pollutant, it is not easy to find the best coagulation pH by ζ-potential measurement, because its value changes remarkably with both coagulation pH and relative concentration of lignin. Lignin floc formed by coagulation with excessive amount of Al₂ (SO₄) ₃ shows the minimum value of ζ-potential at the best coagulation pH. On the other hand, ζ-potential of lignin floc formed with a small amount of Al₂ (SO₄) ₃ by the usual way increases simply with the increase of pH.