Two divalent cation-based coagulants, magnesium chloride and manganese chloride, were used to treat synthetic textile wastewaters containing the azo-dye pigment Levafix Brilliant Blue EBRA. The jar-tests were performed in the presence or absence of auxiliary dyeing chemicals. They proved that (i) both divalent cation-based coagulants were effective in the treatment of those alkaline effluents, (ii) better performances in terms of color removal, residual turbidity, and settled volume, were achieved with manganese chloride, and (iii) the presence of dyeing auxiliaries significantly increases the required coagulant demand for treating the textile effluent. The dye removal mechanisms were investigated by combining observations of freeze-dried sediments with transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy and selected area electron diffraction, Fourier transform infrared spectroscopy, adsorption experiments, and aggregates size measurements with a laser sizer under cyclic shear conditions. The results show that brucite (Mg(OH)(2)) particles are formed when applying MgCl(2) to the textile wastewaters, whereas a mixture of feitknechite (β-MnOOH) and hausmannite (Mn(3)O(4)) is obtained when using MnCl(2). More poorly crystallized particles are formed in presence of auxiliary dyeing chemicals. The adsorption experiments suggested that the azo-dye pigment adsorbs onto the surface of precipitating phases, whereas the aggregation dynamics indicated that a charge-neutralization mechanism underlies the formation of aggregates. The dye removal is then consistent with a precipitation/adsorption mechanism.
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