Influence of pH, Iron Source, and Fe/Ligand Ratio on Iron Speciation in Lignosulfonate Complexes Studied Using Mössbauer Spectroscopy. Implications on Their Fertilizer Properties

Iron chlorosis is a very common nutritional disorder in plants that can be treated using iron fertilizers. Synthetic chelates have been used to correct this problem, but nowadays environmental concerns have enforced the search for new, more environmentally friendly ligands, such as lignosulfonates. In this paper, Fe coordination environment and speciation in lignosulfonate (LS) complexes prepared under different experimental conditions were studied by 57Fe Mössbauer spectroscopy in relation to the Fe-complexing capacities, chemical characteristics of the different products, and efficiency to provide iron in agronomic conditions. It has been observed that the complex formation between iron and lignosulfonates involves different coordination sites. When Fe2+ is used to prepare the iron–LS product, complexes form weak adducts and are sensitive to oxidation, especially at neutral or alkaline pH. However, when Fe3+ is used to form the complexes, both Fe2+ and Fe3+ are found. Reductive sugars, normally present in lignosulfonates, favor a relatively high content of Fe2+ even in those complexes prepared using Fe3+. The formation of amorphous ferrihydrite is also possible. With respect to the agronomical relevance of the Fe2+/Fe3+ speciation provided by the Mössbauer spectra, it seems that the strong Fe3+–LS complexes are preferred when they are applied to the leaf, whereas root uptake in hydroponics could be more related with the presence of weak bonding sites.