Colour removal from model solutions by coagulation--surface charge and floc characterisation aspects.

Chemical coagulation applied for colour removal from dye bearing solutions has been investigated from the point of view of surface charge progression. Two commercially used dyes, i.e. CI Acid Blue 113 and CI Disperse Blue 26 have been tested, employing three common coagulants: alum, aluminium chloride and ferric chloride. Coagulant type and dose level and pH of the dye solution have been studied as process parameters affecting surface charge and degree of colour removal after filtration of coagulated dye. It has been found, that both dyes could be almost completely removed with the tested coagulants, when supplied however at different dosages. From one side, the correlation between surface charge and colour removal suggests that destabilization of colour colloids occurs as a result of charge neutralization followed by removal by filtration. From other side, the lack of re-stabilization and continued high colour removal even at increased coagulant dosages implies enmeshment of destabilized dye colloids into the hydroxy flocs/precipitates of metal coagulant by "sweep coagulation". Finally, flocs from selected sludge samples have been examined for particle size and shape by image analysis and their key characteristics summarised.