Phosphate removal from solution by composite of MCM-41 silica with rice husk: Kinetic and equilibrium studies

Composite of MCM-41 silica with rice husk was synthesized under hydrothermal conditions using cetyltrimethylammonium bromide (CTAB) as an organic template, aqueous ammonia solution (NH4OH) and rice husk. Rice husk served as not only a silica source but also as a substrate for the deposition of MCM-41. The synthetic hybrid composites were characterized by powder X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM) and tested as sorbents for phosphate from aqueous solution. Kinetic data and equilibrium uptake isotherms were measured. The effects of different experimental parameters such as contact time, initial phosphate concentration, solution pH, adsorbent mass, and the presence of competitive ions on phosphate uptake were investigated. The phosphate uptake kinetics were found to be fast and equilibrium was achieved after 30 min. The phosphate uptake was found to be highly pH dependent. Studies on the effects of competing ions, without keeping the initial pH constant, indicated that phosphate uptake and K-d values decreased in the presence of CO32- and NO32-, but SO42- ions showed little or no effect. With keeping the initial pH constant at 6, the presence of these competing ions had no clear effect on the uptake of phosphate. The phosphate uptake by composite of rice husk with MCM-41 could be described well by the Langmuir isotherm equation. Adsorption kinetic data correlated well with pseudo-second-order model, which suggested that the uptake process might be chemical sorption. (C) 2015 Elsevier Inc. All rights reserved.