Journal du génie chimique et de la technologie des procédés
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Effect of pH Solution on the Optical Properties of Cationic Dyes in Dye/Maghnia Montmorillonite Suspensions

Elaziouti A, N. Laouedj & Bekka Ahmed

The interaction of three cationic dyes (Methylene blue, MB; Neutral red, NR; and Malachite green oxalates, MG) with montmorillonite clay from Maghnia deposit in aqueous suspensions was investigated using visible absorption spectroscopy. The stability of the optical properties of dyes in aqueous solutions and in dye/clay suspension was assessed in terms of the absorption bands maxima as a function of pH. The optimal pH for favorable adsorption of the dyes, i.e. ≥ 90% has been achieved in aqueous solutions at 6 for NR and at 7 for both MB and MG respectively. The adsorption of MB leads to the methacromatic effect. MB agglomerates occurred predominantly in the clay with high charge density, even with low charge density. The redistribution of MB molecules over the surface of clay at higher acidic medium causing a reduction of the dye aggregation. However, the interactions of NR and MG with clay differ in certain aspects from those of MB. The adsorption of NR is accompanied by a red shift of the main band at low pH, whereas, the conjugate form of NR, remains stable at neutral and higher pH. A blue shift of the main band is observed at pH 6, which is consistent with externally and internally adsorbed dye cations. For MG, a slight stability of the main bands in clay suspension at acidic and neutral pH, while, the band attributed to MG cations shifs to longer wavelengths at pH 9. To explain the relationship between the layer charge density and the adsorption, so- colled model 2-pKa of surface complexation (SCM) was developed.