Equilibrium and kinetic modeling of adsorption of urea nitrogen onto chitosan coated dialdehyde cellulose

Abstract

The adsorption of urea nitrogen onto chitosan coated dialdehyde cellulose (CDAC) under urease catalysis was studied in a batch system. The equilibrium isotherm of urea nitrogen adsorption onto CDACs with different degree of oxidation (DO) and the kinetics of adsorption with respect to the DO of CDAC (41, 60 and 74%), the initial urea nitrogen concentration (544, 635 and 785 mg L −1), temperature (37, 42 and 47 °C) and CDAC/urease weight ratio (100:3, 100:2 and 100:1) were investigated. Langmuir and Freundlich adsorption models were applied to describe the experimental isotherm and isotherm constants. Equilibrium data fitted very well to the Langmuir model in the entire saturation concentration range (152–756 mg L −1). The maximum monolayer adsorption capacities obtained from the Langmuir model are 50.0, 60.6 and 69.2 mg g −1 for the CDACs with DO 41, 60 and 74%, respectively, at 37 °C and CDAC/urease weight ratio 50:1. Pseudo first-order and second-order kinetic models were used to describe the kinetic data and the rate constants were evaluated. The experimental data fitted well to the second-order kinetic model, which indicates that chemical adsorption is the rate-limiting step, instead of mass transfer. The DO of CDAC, initial urea nitrogen concentration, temperature and CDAC/urease weight ratio affected the adsorption capacity significantly. The activation energy is 11.64 kJ mol −1 for the adsorption of the urea nitrogen onto the CDAC under urease catalysation at DO 74% of CDAC, initial urea nitrogen concentration 635 mg L −1 and CDAC/urease weight ratio 50:1.