Ammonium removal from water by natural and modified zeolite: kinetic, equilibrium, and thermodynamic studies

Zeolite is commonly used to remove ammonium from water. To improve its adsorption performance, natural zeolite (N-Z) was modified by NaCl immersion and calcination. Batch experiments were used to investigate the adsorption performance of N-Z, immersion zeolite (M-Z-1), and immersion-calcination zeolite (M-Z-2). Kinetic studies showed that the adsorption rate followed the sequence M-Z-2>M-Z-1>>N-Z. The pseudo-second-order model fitted the N-Z, M-Z-1, and M-Z-2 data well. The k(s) values of the two types of modified zeolite were considerably larger than that of N-Z; the k(s) of M-Z-2 was slightly larger than that of M-Z-1. These results indicate that the adsorption rate of M-Z-2 is only slightly faster than that of M-Z-1, which agrees with the experimental results. Equilibrium studies showed that the adsorption capacities of M-Z-1 and M-Z-2 were 1.4-1.5 times higher than that of N-Z. Adsorption isotherms fitted the Freundlich model well. The k(F) and 1/n values of the modified zeolites were similar, which indicates that M-Z-2 does not exhibit significant improvements compared with M-Z-1. Thermodynamic studies showed that the G degrees, H degrees, and S degrees of all zeolite samples were negative, which indicates that adsorption was spontaneous, exothermic, and orderly. The G degrees values of both types of modified zeolites were similar and more negative than that of N-Z. This similarity suggests that NaCl modification effectively improves the adsorption potential of zeolite.