Dissolution kinetics of pyroaurite-type layered double hydroxide doped with Zn: Perspectives for pH controlled micronutrient release

Abstract Layered double hydroxides (LDH) have a high potential for use as a source of micronutrients for plants since their dissolution rates may be controlled within the pH range observed in rhizospheres of plants in need of nutrients. Here the Zn 2 + release from a pyroaurite-type LDH host at different pH was investigated. Zinc doped Mg–Fe(III) LDH intercalated with nitrate (PY(Zn) NO3 ) was synthesized by co-precipitation. For reference, both the nitrate and carbonate forms without Zn doping (PY NO3 and PY CO3 ) were also synthesized. The LDH with a Mg:Fe(III) molar ratio of 2 were found to be pure. LDH dissolution was monitored at a constant pH (4–8) to study the release of framework metal cations, dissolution stoichiometry and release kinetics. The LDH dissolved incongruently, releasing Mg 2 + and Zn 2 + , and leaving an Fe(III)-enriched residue. The stoichiometry of metal release at a particular pH remained the same throughout the dissolution. Release kinetics of Zn 2 + and Mg 2 + from PY LDH was ‘first order’ with respect to the metal cations in the LDH, providing rate constants in the range of 0.16 × 10 − 3 to 10.1 × 10 − 3  min − 1 , with the highest rate constants at low pH. Metal ion (M) release had a fractional order with respect to [H + ], resulting in overall release kinetics according to: rate = k H  × [M][H + ] n with n in the range 0.25–0.43. The type of interlayer anion did not affect the overall release kinetics. Lower amounts of Zn 2 + were released with respect to Mg 2 + as pH increased, indicating that Zn 2 + was increasingly retained in the residual Fe(III)-enriched phase. The present study demonstrates not only the pH-controlled release of micronutrient metal cations doped into the LDH framework but also points to resorption as critical parameters for optimizing the release profile. Furthermore, the present study indicates how the incorporation of the desired metal cation within the structure of the LDH may be adapted for other micronutrient/s such as Mn or Cu with the intention of using them as fertilizer.