Exploring the synergistic effects of goethite intercalated coal in the presence of humic acids for enhanced growth of Sinapis alba

Abstract Background Iron oxide mineral–humic complexes serve as a reservoir of bioavailable Fe for plants, releasing metal ligands and providing Fe–humic complexes directly usable by plant Fe-uptake mechanisms. In this study, we synthesized and characterized goethite α-FeOOH (G) nanoparticles (NPs) intercalated in coal (GC) to estimate the bioactivity effect of humic acids (HA). The synthesized GC NPs were characterized by X-ray diffraction, scanning electron microscopy (SEM), Mössbauer spectroscopy, N2 adsorption–desorption Brunauer–Emmett–Teller (BET) specific surface area, zeta potential, hydrodynamic particle diameter, iron ions release, and a phytoassay method of root elongation using the higher plant Sinapis alba. Results X-ray diffraction revealed that G was the primary phase in both GC and GC–HA complexes. Mössbauer spectroscopy analysis identified a goethite-doped Fe2+-in the GC samples. The intercalation of G into the coal matrix increased the specific surface area of GC, enhancing its HA sorption capacity. In addition, GC–HA demonstrated superior plant growth stimulation compared to HA and GC alone, indicating its role in colloidal stability. In contrast to GC, GC–HA exhibited a more consistent and time-dependent release of Fe3+ and Fe2+. This sustained Fe release from GC–HA, coupled with the formation of Fe3+ and more bioavailable (soluble) Fe2+ humic complexes is a promising result in terms of iron nanofertilizers production. Conclusions The use of goethite nanoparticles intercalated within a coal matrix and subsequently complexed with HA contributes to prolonged phytoactivity by employing slowly released nutrient additives within the coal mesoporous matrix. Graphical Abstract