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

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 How to cite this paper: Rahman, Md.S., Hossain, S.M., Rahman, M.T. and Kabir, M. (2019) Analysis of Iron, Scandium, Samarium, and Zinc in Commercial Fertilizers and the Chemistry behind the Stability of These Metals in the Fertilizers. Journal of Agricultural Chemistry and Environment, 8, 155-171. https://doi.org/10.4236/jacen.2019.83013 Received: July 1, 2019 Accepted: August 13, 2019 Published: August 16, 2019 Copyright © 2019 by author(s) and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/ Open Access Md. S. Rahman et al. DOI: 10.4236/jacen.2019.83013 156 Journal of Agricultural Chemistry and Environment 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.


Introduction
Fertilizers are indispensable materials, providing essential nutrients to soil to maximize food production. Fertilizers are becoming one of the essential factors of the world economy [1]. So, it is easily comprehensible that the need for fertilizer is increasing tremendously. World demand for phosphate fertilizers was 41,700,000 tons in 2013, but it is expected to become 46,600,000 tons by 2018 [1]. Among the phosphate fertilizers, DAP and TSP are the most consumed fertilizers [2], because both can supply high content of phosphorous, where DAP can also provide high amount of nitrogen [3]. Manufacturers sometimes mix high amounts of phosphate ores and recycled by-products to the fertilizers to meet the nutritional needs of soil [4] [5]. In this way, excessive amount of trace metals could be ingested into the fertilizers. The common forms of those metals in the fertilizers are oxides and sulphates [6] [7]. Since some of these metallic compounds show great affinity to the phosphate groups, after the application of the phosphate fertilizers, those compounds could retain in the topsoil for a longer period of time [8]. Moreover, metals could also stay in soil and water for an extended period of time by changing their oxidation state and worsening the soil environment [9]. Every stakeholder should maintain the quality of the fertilizers, starting from manufacturing process to packaging and must state quality control results on their packages according to the suggestion and trend reported by international regulatory bodies such as FAO, USGS, or USDA [1] [10] [11] [12]. In this study, excessive amount of micronutrient metals, e.g. Fe and Zn detected in some of the fertilizer samples. Therefore, this study tried to find out the reasons behind the high concentration of the metals in the phosphate fertilizer samples by employing density functional theory calculations.
It is important to know because excessive exposure of the essential nutrient metals could cause severe environmental and health hazards. Elevated level of Fe can cause "Bronzing" of the rice leaves, which reduces the rice-yield; it can even cause complete crop failure [13]. Zinc is also an essential trace element since it has the antagonistic capacity against copper and cadmium toxicity [14] [15], but application of large doses of zinc over extended periods of time by diverse sources such as fertilizers, pesticides, and manure could cause Zn induced iron  [16].
Scandium and samarium metals do not get absorbed by the plants to a measurable extent, so these metals should not have any significant role in agricultural soil and the human diet, but Rim et al. [17] reported that samarium could be slightly toxic in its soluble form.
Quantification of metal contents in diversified types of samples can be accomplished by various methods such as Inductively Coupled Plasma-Atomic Emission Spectrometry (ICPAES) [18], Continuum Source Graphite Furnace Atomic Absorption Spectrophotometry (CS-GFAAS) [19], Wavelength Dispersive X-Ray Fluorescence Spectrometry (WD-XRF) [20], Proton-Induced X-ray Emission (PIXE) [21], and Neutron Activation Analysis (NAA) [8]. Each method has its own advantages and disadvantages, but NAA method was used in this study because it needs no chemical treatment, non-destructive, matrix independent only based on the (n, γ) nuclear reaction, and IAEA regarded it as a "Reference Method" [22]. The only difficulty to run this method could be its overall cost.
Density functional theory (DFT) is one of the most effective ways to study different chemical, material, and biological system [23]. To comprehensively understand the structural changes, binding energy changes, and other modifications occurred by strong interaction of metallic compounds with fertilizers, DFT calculations can play a successful role [8].
In this study, we investigated Fe, Sc, Sm, and Zn contents in frequently used commercial fertilizers from the mid-southern region of Bangladesh employed by neutron activation analysis (NAA). In addition, quantum mechanical calculations revealed the structural characteristics of the fertilizers, TSP and DAP, and the compounds interacting with them. The structural changes occurred in fertilizers due to the interaction of Fe and Zn compounds, and the reasons behind the compounds high affinity to the fertilizers were explored.

Sample Collection
The detail method of sample collection was explained in our earlier study [8].
Concisely, total ten phosphate (TSP and DAP) and compost fertilizer samples were collected to observe the level of essential (Fe and Zn) and non-essential metals (Sc and Sm) from the mid-southern part of Bangladesh namely; Alfadanga and Shaltha in Faridpur, Agargaon, Mirpur-2 (Kingshook Nursery), Savar (Gerua Bazar) in Dhaka, and Mohammadpur in Magura [8]. Sample identification numbers were assigned as, C11-L, C13L, C14-L, T22-L, T32-L, T42-L, T52-L, T62-L, D24-L, and D54-L where C, T, and D means compost, TSP, and DAP, respectively. Coordination data of the sample collection points are presented in Table S1.

Sample Preparation for INAA Analysis
Sample preparation and correction of the interference were also described in the Journal of Agricultural Chemistry and Environment previous study [8]. In brief, collected samples were taken into an electric oven to dry about 65˚C until having constant weight. About 60 mg of dried, homogeneous, and powdered fertilizer samples were heat sealed in a small polyethylene bag. Three standard reference materials (SRMs) were used where IAEA-Soil-7 was used as a standard, and IAEA-SL-1, NIST-1633b (Coal Fly Ash) were used as the control samples.

Quality Control and Detection Limit
Ratio of the measured concentrations of the studied metals in control samples (NIST-1633b Coal Fly Ash and IAEA-SL-1) to their certified concentrations gave a strong quality control result for this experiment ( Figure S1(a) and Figure   S1(b)). Deviations were found within 5% for most of the metals in both cases except Sm in SL-1 had 12% deviation, and the deviation calculated for Zn in NIST 1633b was 28%. Overall, the QC results provided reliability of the calculated results. A three-σ criterion [8] was employed to calculate the detection limit of studied metals (Table S2).

Computational Method
Gaussian 09 software package [24] was used to optimize the structures of DAP and TSP and their complexes with FeSO 4 , ZnO, and ZnSO 4 at gas phase. Vibrational frequencies were calculated with the density functional theory (DFT) employing (BLYP) correlation functional [25]. All calculations were conducted by SDD basis set, which can produce reliable results for the interaction between metallic compounds and phosphate fertilizers [8] [26]. After computing, several thermochemical properties such as change of electronic energies, enthalpies, Gibbs free energies, HOMO-LUMO gaps, dipole moments, hardness and softness of the fertilizers, and the fertilizer-metal complexes were investigated.   Table 1). The ranges of iron concentration in soil samples of Punjab (India) reported 2800.0 -5700.0 mg/kg (Table S3). Therefore, the repeated use of extremely high iron-enriched fertilizers could be turned into beneficiary evil.  The average Zn concentration in the phosphate fertilizers in European market was reported 166 mg/kg [27] where average Zn content in superphosphate fertilizer of Egypt market was 107.80 mg/kg [28] and Zn content in phosphate fertilizers of Chile market was 41.3 to 600.0 mg/kg [29]. So, the concentrations of Zn in sample C11-L and D54-L were found about 20 times and 27 times higher than the European market, respectively.

Iron and Zinc Content in Compost, TSP, and DAP
Zinc is unevenly distributed in soil and its concentration ranges between 73.0 to 320.0 mg/kg in Punjab (India) [30]. Kabata-Pendias and Pendias [31]

Scandium and Samarium Content in Compost, TSP, and DAP
The concentrations of scandium in compost, TSP, and DAP are 3.496 -7.092 mg/kg, 5.735 -11.796 mg/kg, and 6.965 -8.423 mg/kg, respectively (Figure 1(b)).       Frontier molecular orbitals (MO) i.e., highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), and their energy gaps are essential factors to measure the chemical reactivity, extent of affinity and kinetic stability of a complex compound [8]. Larger HOMO-LUMO gap of the complexes means high kinetic stability but low chemical reactivity. In that case, an electron requires high energy to be promoted from HOMO to a relatively high-energy level LUMO. The HOMO and LUMO energy gaps of all metal compounds, TSP, and TSP-metal complexes are summarized in Table 4 and    (Table 4). These results confirmed that Fe-TSP complex is more reactive and stable than Zn-TSP complexes.  Table 5).    (Table 5 and Figure 4). So, both Fe and Zn metals have a strong affinity to the oxygen of PO 4 group. HOMO-LUMO gap, dipole moment change, and hardness and softness of the complexes (Table 4) could help to understand the interactions more comprehensively.

Interaction and Binding of Fe and Zn with DAP
Electronic energy, enthalpy, and Gibbs free energy of metal-fertilizer complexes are summarized in Table 3 Table 4 and Figure 5. It was observed that Fe has a significant effect on the frontier molecular orbital energies. Compared to DAP, the HOMO and LUMO energy gaps of the Fe-DAP and Zn-DAP complexes are significantly decreased where the HOMO-LUMO gap of Fe-DAP was least. The HOMO and LUMO energy gap of the DAP, DAP-FeSO 4 , DAP-ZnO, and The results suggest that Fe-complex is chemically more reactive to DAP than Zn-complexes. Dipole moments of these complexes are 5.0713, 5.1759, 8.1366, and 7.3715 Debye, respectively. Trend of the dipole moment is also proving the earlier assumption. Moreover, Fe-DAP complex structure is softer than the Zn-DAP complexes (Table 4).
Therefore, quantum mechanical calculation confirmed that both Fe-DAP/TSP and Zn-DAP/TSP complexes are thermodynamically stable, which supports experimental results. It was also observed that Fe-DAP/TSP complexes are more reactive and stable than Zn-DAP/TSP complexes because Fe-DAP complex has lower HOMO-LUMO gap and Fe-PO 4 bond distance is smaller than the Zn-PO 4 .

Conclusion
Concentrations of essential metals, Fe and Zn, in some of the fertilizer samples were found to be surprisingly high. Density functional theory revealed that Fe and Zn have strong affinity with the PO 4 group present in DAP and TSP. It was proved because both Fe and Zn with the oxygen of the PO 4 group formed covalent like bonding, and the complexes were found thermodynamically stable.
HOMO-LUMO gap indicated that Fe compound was more prone to attach with the PO 4 group of the fertilizers due to lower HOMO-LUMO gap than Zn-fertilizer complexes. Therefore, the combined experimental and theoretical studies revealed that excess Fe and Zn could be stayed with fertilizers in the soil over a long period, gradually be bioaccumulated by the application of either excess phosphate fertilizers or excess metal ingested fertilizers, and eventually could go into food web. Journal of Agricultural Chemistry and Environment