Anionic Clays as Potential Slow-Release Fertilizers: Nitrate Ion Exchange

Several nitrate containing anionic clays were synthesized at different temperatures and the kinetics of NO₃^- release were determined to test their suitability as slow-release N fertilizers. A sample (Mg:Al = 2:1) synthesized at 60°C with smaller particle size released 75, 86 and 100% of its NO₃^- in 1, 3 and 7 days, respectively when equilibrated with a simulated soil solution. On the other hand, the 175°C/2 hrs sample with larger particle size released 65, 77 and 84% of its nitrate in 1, 3 and 7 days, respectively. Another anionic clay (synthesized at 175°C/24 hrs) of higher charge density (Mg:Al = 2:1) containing NO ₃^- was equilibrated with a 0.012 N NaCl or Na ₂CO₃ to test the role of different anions in releasing the NO₃^- anion from the interlayers. The results showed that Cl^- released more NO₃^- than did CO ₃^2- from this anionic clay after all the treatment times probably as a result of the CO₃^2- anion blocking the release of NO₃^- from the interior of the crystals. When a lower charge density (Mg:Al = 3:1) sample (synthesized at 175°C/48 hrs) was equilibrated with 0.02N solution of anions the release of nitrate was as follows: Cl^- < F^- < SO₄⁼ ≤ CO₃^2-. These results suggest that the divalent SO ₄^- and CO₃^2- anions are more effective in the release of NO₃^- from this lower charge density anionic clay. Time-resolved structural analysis of NO₃ ^- exchange with CO₃^2- in the above anionic clay using synchrotron x-ray diffraction showed that ion exchange is rapid because of small crystal size and lower charge density. Thus the release of NO₃^- from anionic clays is an interplay among the type of anions present in soil solution, their concentration, pH of soil solution, the charge density and crystal size of anionic clay etc.