Mineral sorbents for ammonium recycling from industry to agriculture

In tropical environments, nutrient-poor soils are commonly found, leading to high fertilizers application rates to support agricultural activities. In contrast, anthropogenic activities generate large amounts of effluents containing nitrogen. In this study, two minerals (natural zeolite and vermiculite) were tested to remove NH₄⁺ from an industrial effluent with high pH and contents in Na⁺ and K⁺. Afterwards, they were tested as an alternative slow-release fertilizer in the soil. To verify the best conditions to adsorb NH₄⁺, batch tests were conducted using synthetic solutions and an industrial effluent. In general, the efficiency of both minerals in removing NH₄⁺ was high (85% for zeolite and almost 70% for vermiculite) as well as the ability to decrease the industrial effluent pH. In this process, more NH₄⁺ and K⁺ ions were removed in comparison with Na⁺, which remained in solution. These minerals were tested as slow-release fertilizers by leaching with distilled water (both minerals releasing 2 mg L^-1 NH₄⁺) and with an acid solution (releasing 10 mg L^-1 NH₄⁺ from zeolite and 50 mg L^-1 NH₄⁺ from vermiculite-corresponding only to 12% of total NH₄⁺ retained by zeolite and 29% by vermiculite). During the test of soil incubation with zeolite-NH₄⁺, the NH₄⁺ ions of the exchangeable sites were retained for a longer period, minimizing their loss by leaching and biological nitrification. Consequently, soil acidification was prevented. Therefore, both minerals showed high efficiency in removing NH₄⁺ from solution which can then be slowly released as a nutrient in the soil.