Ammonium nitrate occlusion vs. nitrate ion exchange in natural zeolites

Several salts, such as NH₄NO₃ and KNO₃, can be occluded in natural zeolites through molten salt treatments, which increase the capacity of zeolites to store nutrients in addition to exchangeable cations. Zeolites occluded with NH₄NO₃ are potential candidates as slow-release fertilizers. This study focuses on determining the rates of NH₄NO₃ uptake by natural zeolites from molten salt and on evaluating kinetics of ion release from NH₄NO₃-occluded zeolites. The kinetics of ion release from NH₄NO₃-occluded zeolites were compared with those of NH₄⁺-saturated zeolites. NH₄NO₃-occluded zeolites were prepared by treatment in molten NH₄NO₃ at 185°C for different time periods, or at 250°C for 4 h. Uptake of molten NH₄NO₃ by erionite apparently reached equilibrium after 8 h of treatment at 185°C. The total N uptake by NH₄NO₃-occluded zeolites was not related to cation-exchange capacity but dependent on the structural type of the zeolite. Approximately a twofold increase in total N loading capacities of zeolites was achieved through salt occlusion compared with NH₄⁺ exchange from solution. For example, phillipsite exhibited a N loading capacity of 76 g N kg^-1 when treated with molten NH₄NO₃, but showed a capacity of only 46 g N kg^-1 through NH₄⁺ exchange. A simulated soil solution was used to examine the kinetics of ion release from both NH₄NO₃-occluded and NH₄⁺-saturated zeolites. Occluded NH₄⁺ and NO₃^- were released slowly and steadily as in the case of exchanged NH₄⁺ alone from zeolites during 30-d treatment in a simulated soil solution. It is evident from this study that salt-occluded zeolites are potential candidates as slow-release fertilizers.