Effects of humic acid on the adsorption and fixation of ammonium and potassium ions on montmorillonite

The presence of organic coatings in soils may considerably modify the ability of minerals to immobilize nutrient cations. Based on the preparation of montmorillonite-humic acid complexes (MH), we examined the changes in major properties of montmorillonite (Mt) before and after humic acid (HA) coating, and evaluated the influence of these differences on the behavior of ammonium (NH4+) and potassium (K+) ions on Mt by batch experiments. HA coating glued clay particles together, and significantly decreased the cation exchange capacity and specific surface area. An HA coating significantly increased the retention capacity of Mt for NH4+ and K+, and enhanced the selectivity for NH4+ over K+. Kinetic studies showed a more rapid adsorption rate and slower fixation rate for both cations, especially for NH4+, after HA coating. An HA coating changed mainly the surface properties of Mt and consequently increased the retention competitiveness of NH4+ on Mt.