Ammonium transport in zeolite and zero-valent iron columns: Experiments and tempered fractional modeling

Ammonia nitrogen contamination in groundwater poses risks of eutrophication and drinking-water quality deterioration. Understanding ammonium transport in reactive porous media is critical for predicting its subsurface fate. This study utilized batch adsorption experiments, column transport tests, and a tempered one-sided density (TOSD) transport model to investigate the transport behavior of ammonium in two reactive media-zeolite and zero-valent iron (ZVI)-which serve as model systems for in-situ groundwater remediation. The results of the batch adsorption experiments for zeolite and ZVI were consistent with the pseudo-second-order and pseudo-first-order kinetic adsorption models respectively. Both adsorption isotherms conformed the Freundlich model, and the results indicated greater ammonium uptake by zeolite than by ZVI. The column breakthrough curves exhibited anomalous transport behavior, and the TOSD model (a fractional-order modeling framework) accurately described the transport experimental data, proving the applicability of the TOSD method in describing the anomalous transport of ammonium ions. The model fitting parameters suggested that adsorption and rate-limited mass exchange may be the plausible factors influencing ammonium retention. This study provides experimental evidence and modeling tools for ammonium ion transport in reactive media, which is significance for mitigating groundwater pollution.