Acid dye removal: comparison of surfactant-modified mesoporous FSM-16 with activated carbon derived from rice husk.

Cetyltrimethylammonium bromide (CTAB)-modified mesoporous molecular sieve FSM-16, prepared by a hydrothermal process (373 K, 3 days), was tested as an adsorbent for acid dye (acid yellow, AY, and acid blue, AB) removal in comparison with as-prepared FSM-16 and activated carbon (AC) derived from rice husk (50 vol% H3PO4, 773 K, 2.5 h). The adsorption isotherms, sorption kinetics, and pH effects upon acid dyes sorption on the adsorbents were thoroughly investigated. The structures of different adsorbents were characterized by XRD, FTIR spectroscopy, N2 adsorption measurements, and thermogravimetric (TG) analysis. It was found that the ultimate capacity of the adsorbents varied in the order FSM-16 > modified FSM-16 > AC and followed first-order rate kinetics. The adsorption isotherm of acid dyes on FSM-16 is of type IV, according to the IUPAC classification, drastically different from that of CTAB/FSM-16, which showed a type I isotherm. The latter sample had better adsorption performance at low concentration of acid dyes than the former. As compared to activated carbon of microporous character, the CTAB/FSM-16 sample achieved higher performance at low concentrations. This was due to the successful narrowing of the pore opening of FSM-16 using CTAB with maintenance of a considerable portion of the pore volume. Powder XRD and N2 adsorption studies of the CTAB/FSM-16 material indicated that the textural properties of the support were preserved during the hydrothermal synthesis and that the channels remained accessible, despite a significant reduction in surface area (ca. 26%). TG studies, on the other hand, confirmed that the modified material presented a higher hydrophobicity than that of the CTAB-free FSM-16 sample.