Direct Orange 34 dye fixation by modified kaolin

The present study investigated the adsorption behaviour of Direct Orange 34, a highly toxic dye used in textile industries in Tunisia, on modified kaolinite-rich clays. A kaolin from the Sidi Bader (SDB) area was activated with hydrochloric acid to create the activated clay referred to hearafter as SDBa, or treated with FeSO4•7H2O to obtain its Fe-saturated form, Fe-SDB. The adsorbents were characterized by X-ray diffraction, X-ray fluorescence, transmission electron microscopy, BET surface area and zeta-potential measurements. The equilibrium adsorption data were analysed using the Langmuir and Freundlich isotherms. The estimated adsorption capacities (qm) for the dye were improved in the Fe-loaded samples. The good fit (R² = 0.99) with a pseudo-second order expression suggests that the adsorption process could be effective following a chemisorption mechanism. At acidic pH, the optimum dye-retention rate was achieved for SDB (83%) after 60 min. The uptake decreased at neutral pH and increased again in alkaline media. This behaviour might be explained by the formation of covalent bonds between the OH radicals on the external surface and the negatively charged dye molecules. On the other hand, Fe impregnation increased the zeta potential of kaolinite, leading to a greater adsorption capacity compared to its natural and acid-activated counterparts. In addition, the adsorption rate increased when increasing the suspension temperature from 283 to 313 K. The modified kaolinite-rich materials showed satisfactory affinity for adsorbing this reactive dye.