The dissolution rate of the bulk glass over a six-day test was found to average 0.6 micrograms/mm2/day, with the rate peaking at 1.05 micrograms/mm2/day after 1 hour and ending the six-day test at 0.2 micrograms /mm2/day. microgram/mm2/day, releasing 60 ppm ions into the solution. A fertilizer is an addition to a plant's soil to provide important nutrients to a plant that may lack them. In addition to these nutrients that the plant needs to survive, there are fewer required nutrients that are still important for a plant's growth and sustainability.
While the concept of glass microsphere fertilizers is not necessarily a new concept, there is an ongoing desire to find new and improved ways to deliver plant nutrients into the system. Potassium polyphosphate glass fertilizers are common models for glass fertilizers because they consist of a majority of the two major macronutrients, while also containing at least one micronutrient as a glass modifier in lower concentrations compared to the rest of the glass network.7. While it is possible to get the nitrogen into the glass, it remains to be determined how to control the release rate of the nitrogen.
One of the main problems of the use and overuse of fertilizer is its contribution to the nitrogen cycle. When the extra H is removed and only NH3 remains, the molecule evaporates into the air.12 So while it is good to get the nitrogen into the glass, it is important to be able to control the release rate. If this cannot be controlled, most of the nitrogen supplied will not be used.
A diagram detailing the nitrogen cycle and how nitrogen is lost from soil can be seen in Figure 1 below.
Iron-Phosphate Glass Fertilizers
Methods and Procedures
Batching and Melting
Microsphere Production
Microspheres were imaged using a Leica M165 FC optical microscope and processed using the Leica Application Suite (LAS). The diameter of the particles was found using LAS and the average particle diameter was found from these measurements. These values were recorded and an average diameter was found by taking the sum of the diameters and dividing by 20.
A dissolution test is performed on the bulk glass to determine the chemical durability and release rate of the glass. The glass was cut into small rectangular pieces with a diamond saw and then polished to make them free of surface contaminants. The dimensions of the pieces were measured using a caliper and the surface area was calculated using the equation 𝑆𝐴 = 2𝐿𝑊 + 2𝐿𝐻 + 2𝐻𝑊.
The pieces were weighed individually and the ratio between the mass and surface area of the pieces was found. Then, each part was placed in a container filled with 35 mL of DI water and allowed to stand for a predetermined time (1 hour, 1 day, 3 days, 6 days). After the specified time, the pieces were removed from the water and dried in a drying oven at about 125℃ for ~30 minutes and weighed again.
From the observed change in mass, the mass/surface ratio was recalculated with.
Scanning Electron Microscopy
The concentration of phosphorus, in the form of PO4- and iron, in the form of Fe3+ ions in solution was found using UV Vis spectrophotometry. A ThermoSpectronic Genesys 20 spectrophotometer analyzing at 400 nm was used to collect phosphate ion data.
X-Ray Diffraction
UV Vis Spectroscopy
Differential Scanning Calorimetry
Results and Discussion
Microspheres
This value was used to calculate the average surface area of the microspheres using the equation 𝑆𝐴 = 4𝜋𝑟2, which was found to be 0.25 mm2. To ensure that these conditions are kept constant during the test, it was determined that 0.00106 g of microspheres should be added to 0.00729 ml of water. Scaling up to usable amounts add 1.06 g of microspheres to 7.29 mL of water to ensure that the ratio remains the same.
Dissolution
This is consistent with a diffusion process, as the concentration of these ions on the surface decreases with time, and further reactions require ions from the deeper layers of the glass to migrate to the surface through the process of ion diffusion. It takes longer for the ions to diffuse from the depth of the glass according to the equation 𝑑 = (2𝐷𝑡)0.5, where D is the diffusion rate given by Stuart Anderson's relation and t is time. D is affected by two factors, the "gate" of the P2O5 rings in the glass and the bond energy of the unbonded oxygens in the glass.
These two factors determine the rate at which the ions diffuse out of the glass and into the solution.18. The equation outlines the scenario where if 5 Fe atoms e.g. removed from the glass in one second, the next 5 will dissolve in 4 seconds, and the next five in 9 seconds, and so on. This is illustrated in Figure 5, where the arrows represent the movement of the diffusing ions and the thick yellow line represents the location of diffusing atoms in the given time.
The results of this test showed that the release rate of glass litter could be controlled well enough to understand the rate at which these glasses dissolve and show promise for their use in the fertilization process. As seen in Table 3, when the pH of the water was tested at neutral pH, the pH of the water after dissolution was acidic, with a pH of 5. The hexaaquairon(III) complex is formed when the Fe3+ ion is surrounded by six water atoms.
The electron pair that is on the oxygen is pulled towards the Fe ion, which pulls the electrons on the O-H bond closer to the oxygen sand, creating a greater positive charge on the hydrogen. This equation describes how the act of attracting the oxygen electrons to the Fe ion creates an apparent increase in the hydrogen content in the solution. This process can be repeated to remove more hydrogen and increase the hydrogen content.19 This process is described in Figure 6.
Scanning Electron Microscopy
EDS data also showed that the concentration of calcium and potassium dropped to almost zero levels in the process of forming the microspheres. The beads were mainly made of phosphorus and oxygen, with iron lightly dispersed throughout the microsphere. It is assumed that the calcium and potassium ions were lost during the microsphere formation process due to evaporation.
This happens less easily for phosphorus, as it is the glass network former, or for iron, due to its thermal stability 23 .
UV VIS Spectrophotometry
Conclusion
Phosphorus was seen in concentrations of 81ppm and iron in concentrations of 7ppm. According to literature, phosphorus is required at approximately 40-60 ppm, while iron is required at approximately 100 ppm. Both of these values are off the recorded values, indicating that the chosen composition will not work for glass fertilizer application.
In addition, the surface of the glass lacks calcium and potassium, which means that it would take longer for these to reach the plant itself. Also, the oxidation state of the iron in the glass is currently useless for plant growth. In future tests, melting in a reduced atmosphere should be performed to generate Fe2+ ion compared to Fe3+ ion.
Likewise, a new preparation method, such as the sol gel method, may need to be used to create microspheres containing the two elements that evaporated from the microspheres during the oxygen propane flame method. However, despite all these shortcomings from this experiment, there is still promise for creating a glass fertilizer suitable for growing wine grape plants.
