Migration of ammonium nitrogen in ion-absorbed rare earth soils during and post in situ mining: a column study and numerical simulation analysis

Abstract

● Column experiments with an inclined slope were applied to simulate NH₄–N transport.

● The transport of NH₄–N was simulated via HYDRUS-2D.

● The chemical non-equilibrium model well described the transport process.

● The lateral flow led to the preferential loss of surface NH₄–N.

● Flow rate and rainfall intensity affected the adsorption and leaching of NH₄–N.

Ion-absorbed rare earth mines, leached in situ, retain a large amount of ammonium nitrogen (NH₄–N) that continuously releases into the surrounding environments. However, quantitative descriptions and predictions of the transport of NH₄–N across mining area with hill slopes are not fully established. Here, laboratory column experiments were designed with an inclined slope (a sand box) to examine the spatial temporal transport of NH₄–N in soils collected from the ionic rare earth elements (REE) mining area. An HYDRUS-2D model simulation of the experimental data over time showed that soils had a strong adsorption capacity toward NH₄–N. Chemical non-equilibrium model (CNEM) could well simulate the transport of NH₄–N through the soil-packed columns. The simulation of the transport-adsorption processes at three flow rates of leaching agents revealed that low flow rate enabled a longer residence time and an increased NH₄-N adsorption, but reduced the extraction efficiency for REE. During the subsequent rainwater washing process, the presence of slope resulted in the leaching of NH₄–N on the surface of the slope, while the leaching of NH₄–N deep inside the column was inhibited. Furthermore, the high-intensity rainfall significantly increased the leaching, highlighting the importance of considering the impact of extreme weather conditions during the leaching process. Overall, our study advances the understanding of the transport of NH₄–N in mining area with hills, the impact of flow rates of leaching agents and precipitation intensities, and presents as a feasible modeling method to evaluate the environmental risks of NH₄–N pollution during and post REE in situ mining activities.

Graphical abstract

Keywords

Ion-absorbed rare earth / Ammonium nitrogen transport / HYDRUS-2D / Numerical simulation

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