An effective strategy to synthesize water-soluble iron heterocomplexes containing Dubb humic acid chelating agent as efficient micronutrients for iron-deficient soils of high pH levels

Abstract

Humic acid (HA), the most known and popular water-soluble organic matter extracted from a variety of sources, has many applications in agriculture. Moreover, Fe-HA complexes are water-insoluble at high pH levels and thus they cannot be applied for iron-delivery to alkaline Fe-deficient soils. To solve this issue and to increase solubility and stability of Fe-HA complexes in alkaline soils (iron delivery to plants), herein, Dubb humic acid (DHA) extracted from lignite source was used and characterized by various techniques including X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), Brunauer-Emmett-Teller (BET) surface area, UV–Visible (UV–Vis) absorption and photoluminescence (PL) spectra, diffuse reflectance spectra (DRS), Fourier-transform infrared (FT-IR) spectra, gel permeation chromatography (GPC), and laser scattering particle size distribution analysis. To achieve iron complexes which were suitable for application in iron-deficient soils, firstly, some heterocomplexes containing non-DHA primary complexing agents were designed and synthesized using ammonium citrate (AC), ammonium sulfate (AS), and fulvic acid (FA) ligands. These complexes were water-soluble and could be easily used to synthesize final iron heterocomplexes with DHA coordinating agent, which were water-soluble in neutral and alkaline conditions. The desired heterocomplex structures with general formula of (A)n-(Metal)x-(B)m (A = DHA, Metal = Fe, B = AC, AS or FA) were synthesized through optimizing pH, DHA percentage, and initial iron ion concentration. Structural characterizations of the synthesized heterocomplexes were conducted by above-mentioned techniques. Elemental analysis of these heterocomplexes (particularly the AS containing sample) showed very much higher iron contents compared with commercial iron containing chelates. These results established that iron heterocomplexes designed in this study had an extraordinary potential for acting as valuable inorganic micronutrients for iron-deficient soils in alkaline conditions.