Zinc release from Zn-Mg-Fe(III)-LDH intercalated with nitrate, phosphate and carbonate: The effects of low molecular weight organic acids

Abstract

There is growing interest in using layered double hydroxides (LDH) as controlled release systems of essential nutrients for plants. The Zn-Mg-Fe(III)-LDH intercalated with nitrate (LDH-NO3−), phosphate (LDH-HPO42−) and carbonate (LDH-CO32−) were synthesized with the aim of investigation the effect of oxalic acid (OX), tartaric acid (TAR) and citric acid (CIT) (three common low molecular organic acids (LMWOAs) available in soil solution) to release structural Zn. In this regard, effects of time, initial pH and background electrolyte on the release of structural Zn from LDH in the presence of LMWOAs were performed based on the batch technique. According to the results, although the Zn-complexing abilities of organic acids were pH-dependent, the overall extent of Zn release was sensitive to the concentration of organic acid as well as to the type of available cation in the electrolyte solution and intercalated anion. Among the LMWOAs, CIT was the most effective ligand in releasing Zn, followed by OX and TAR. Also, our observation suggested that the intercalation of carbonate strongly suppressed the release of Zn in the absence and presence of organic acids, especially in CaCl2 electrolyte. The effect of organic acids on the kinetics of Zn release (KCl solution; initial pH = 6.0) were investigated and several kinetic models were tested to describe the time-dependent Zn release. Results showed that Elovich and pseudo-second order equations were the best-fit equations among five kinetic models used. The presence of the LMWOAs generally increased initial release rate (h) and the extent of Zn released at equilibrium (qe) from LDH; however, the enhancement effect of CIT was pronounced in LDH-NO3− and LDH-HPO42−.