Analysis of Iron, Scandium, Samarium, and Zinc in Commercial Fertilizers and the Chemistry behind the Stability of These Metals in the Fertilizers

Abstract

Fertilizers are the indispensable materials for farming and one of the major components of the current world economy. It is essential to understand the chemical structures of fertilizers to provide best quality products to the consumers. In this study, chemical structures of some frequently used commercial fertilizers (compost, DAP, and TSP) and their phosphate-metal interaction chemistry were studied employing both analytical and theoretical methods. Three types of fertilizer samples from the mid-southern part of Bangladesh were collected to quantify the content of two micronutrient metals (iron and zinc) and two non-essential metals (scandium and samarium). Neutron activation analysis (NAA) coupled with γ-ray spectrometry was employed to analyze the content of the metals where three standard reference materials, namely IAEA-SL-1 (Lake Sediment), IAEA-Soil-7, and NIST Coal Fly Ash 1633b, were used. Concentration of Fe (2964 - 24,485) mg/kg, Sc (3.50 - 11.80) mg/kg, Sm (2.19 - 26.69) mg/kg, and Zn (243 - 4426) mg/kg were determined in the fertilizer samples. Extremely high concentrations of Fe and Zn were quantified in some of the compost and phosphate fertilizers in comparison with other studies of different countries. Quantum mechanical calculations were performed to understand the molecular level interactions of Fe and Zn with triple super phosphate (TSP) and diammonium phosphate (DAP) fertilizers by employing DFT-B3LYP/SDD level theory. Results showed that both Fe and Zn have high affinity with the phosphate fertilizers, but Fe compound showed stronger binding affinity than the Zn compounds, which supported the experimental results. Another interesting finding was that the compounds of Fe and Zn attached to the oxygen of the phosphate group of the fertilizers by covalent-like bonding. HOMO-LUMO gaps of the Fe-DAP/TSP complexes were observed significantly lower than the Zn-DAP/TSP, which also demonstrated that Fe compound could have higher affinity to attach with the phosphate group of DAP and TSP fertilizers.