Removal of nitrate by synthetic organosilicas and organoclay: Kinetic and isotherm studies

In this work, octadecyltrimethylammonium (ODTMA) chloride was used to prepare MCM-41 at room temperature while cetyltrimethylammonium (CTMA) bromide was used to synthesize MCM-48 and layered organosilica material under hydrothermal conditions. These synthetic organosilica materials, in addition to a commercially available organoclay, Cloisite®10A, were characterized by powder X-ray diffraction (XRD) and tested with the occluded surfactant as sorbents for nitrate. The Langmuir and Freundlich models were applied with determination of the equilibrium isotherms and the isotherm constants. Adsorption kinetics data were tested using pseudo-first-order, pseudo-second-order and intraparticle diffusion models. The results showed that the highest nitrate uptake capacities were achieved with Cloisite®10A (0.359 ± 0.003 meq/g) followed by a layered organosilica (0.287 ± 0.008 meq/g). MCM-48and MCM-41 silica adsorbed 0.096 ± 0.002 and 0.157 ± 0.005 meq/g of nitrate, respectively. Kinetics studies showed that the adsorption followed pseudo-second-order kinetic model. Layered organosilica gave a good fit to the Langmuir model among all the studied samples with correlation coefficient (R²) close to unity (0.99).