NITRATE RELEASE FROM LAYERED DOUBLE HYDROXIDES AS POTENTIAL SLOW-RELEASE FERTILIZERS

Layered double hydroxides (LDHs) are potential anionic intercalators. To obtain nitrate slow-release fertilizers, LDHs with the general formula [M1-x2+Mx3+(OH)(2)](x-)(NO3)(x center dot)yH(2)O (where M2+ = Mg2+ and M3+ = Al3+ and/or Fe3+) were synthesized by the method of co-precipitation at constant alkaline pH. The X-ray diffraction (XRD) measurements showed that crystallinity increases with the pH value and with the M2+/M3+ molar ratio. The FTIR measurements showed characteristic bands of free nitrate in the interlayer, as well as minor carbonate ion contamination. The compositions and concentrations of intercalated nitrate ions were confirmed by TGA/DTA measurements. The nitrate release curves showed two patterns of complementary release - fast initial release (A) and slow gradual release (D). The latter is related to the planar orientation of the intercalated nitrate ion. The synthesized materials investigated in this study proved to be highly promising as matrices for nitrate slow-release fertilizers.