Removal capacity and adsorption mechanism of heat-treated palygorskite clay for methylene blue

In this work, palygorskite clay was activated by heat treatment method and its capacity for the removal of a typical cationic dye, methylene blue, was studied. The activated samples were characterized by XRD and BET techniques. The physical quantities including the cation exchange capacity (CEC) of the activated samples, the variation of pH during the adsorption process and the concentration of metal ions in the supernatant were also measured. The results show that the adsorption capacity of the heat-treated sample increases with temperature and reaches the maximum at 700 °C, and then decreases with further increasing temperature. In general, with the increasing calcined temperature, the variation trend of the adsorption capacity of activated samples for dye is accordant with that of CEC. The results of the metal ions leached from the adsorbent demonstrate the leaching amount of divalent ions (Mg2+, Ca2+) in all samples are significantly higher than that of univalent (Na+, K+) and trivalent ions (Fe3+, Al3+). In addition, the changing tendency of leaching amount of divalent ions with the increasing temperature is consistent with that of adsorption capacity of the heat-treated samples whether the dye solution is acidic, neutral or alkaline. The leaching of divalent ions should be the main factor affecting the adsorption capacity of heat-activated palygorskite for methylene blue.

► Heat treatment method can improve adsorption capacity of palygorskite for dye.
► Change of CEC basically accorded with that of adsorption capacity.
► Cation exchange is a main mechanism in the adsorption process.
► A decisive influence of the leaching of divalent ions on adsorption.