MgAl-layered double hydroxides/sodium alginate beads for nitrate adsorption from groundwater and potential use as a slow-release fertilizer

The excessive occurrence of nitrate in water can be a threat to the environment and human health. Yet, nitrate is an essential nutrient for plant growth. In this study, MgAl-layered double hydroxides/sodium alginate (MgAl-LDHs/SA) beads, synthesized via the crosslinking of MgAl-LDHs and sodium alginate, were employed for nitrate adsorption in both synthetic solution and groundwater, and then tested for slow nitrate release. The effects of pH on nitrate adsorption were insignificant within a wide range (pH 4–10, Langmuir maximum adsorption capacity ∼20–22 mg NO3−/g), while the presence of co-existing anions may negatively affect nitrate removal (HCO3− ≫ Cl− ≈ SO42−). The spent beads slowly swelled for 47 days at pH 7 (to ∼800%) in water before gradually dissolving. In soil slurries, the beads also enhanced the water retention capacity of the soil, implying their ability to retain water for plant growth. Nitrate release from the spent beads in water was slow (still in progress after 40 days) and in soil slurries was low (cumulative desorption <30%), due to the strong complexes formed between LDHs and nitrate molecules. However, the release behavior can be manipulated by changing the physical form of the beads (e.g., grinding; duration in water shortened to ∼5 days, desorption in soil doubled). Results of this study show the MgAl-LDHs/SA beads have the potential of being adsorbents for pump-and-treat water purification and then slow release fertilizers (SRFs) for better nutrient management for crop growth.