ENHANCED WATER RETENTION AND NUTRIENT DELIVERY USING MINERAL FERTILIZER-ENRICHED HYDROGELS

УДК 66.01

ABSTRACT

This article compares the physicochemical properties of hydrogels enriched with various mineral fertilisers with those of traditional hydrogels. The maximum degree and rate of swelling of hydrogels enriched with mineral fertilisers and microelements were determined and compared with traditional hydrogels, revealing their advantages and disadvantages.

АННОТАЦИЯ

В данной статье сравниваются физико-химические свойства гидрогелей, обогащенных различными минеральными удобрениями, с традиционными гидрогелями. Определены максимальная степень и скорость набухания гидрогелей, обогащенных минеральными удобрениями и микроэлементами, и проведено сравнение с традиционными гидрогелями, выявлены их преимущества и недостатки.

Keywords: hydrogel, mineral fertiliser, microelements, ammophos, urea, degree of swelling, swelling rate.

Ключевые слова: гидрогель, минеральное удобрение, микроэлементы, аммофос, карбамид, степень набухания, скорость набухания.

Introduction. Currently, as a result of rapid growth in agricultural production, there is an increasing global demand for technologies that conserve natural water resources and polymer hydrogel products. Water shortages are leading to a decline in crop yields. To solve these problems, it is important to industrially produce and apply environmentally friendly, inexpensive agricultural hydrogels that can absorb and retain large amounts of water and then easily release it to plants through the root system during droughts.

Hydrogels are three-dimensional (3D) structural networks of interconnected hydrophilic polymer matrices capable of retaining large amounts of water (by definition > 10%) and exhibiting useful properties such as softness, hardness, biocompatibility, extensibility, and deformability. The interconnection between hydrophilic functions ensures their structural integrity and prevents their instantaneous dissolution in an aqueous environment. Their ability to retain and store large amounts of water or biological solutions, combined with their unique softness and adaptability, resembles natural soft tissues and therefore makes them promising materials for mimicking the properties of these tissues. Hydrogels can be manufactured as thin films or, depending on requirements, moulded into any shape, length, size or various configurations. The high water absorption of hydrogels is due to the presence of hydrophilic functional groups such as -OH, -COOH, -CONH-, -NH₂, SO₃H and others. For many advanced applications, these hydrogel networks are created using various polymers. To create a hybrid structure, components of natural or synthetic origin are used to support the hydrogel structure, protect it or give it new functions. Most polymers used to prepare hydrogels are non-toxic and are considered suitable for many biomedical applications, from skin dressings to implants, and they are successfully used in all these areas.

Cross-linked hydrogels possess rheological properties similar to those of solids, with very high viscosity (105 Pa·s) and high elasticity (shear stress of 2000 Pa). As hydrogels are insoluble in water, they serve as research objects for studying swollen polymer networks, and research is also being conducted into their practical applications.

Research methodology. Two types of hydrogel were synthesised: a hydrogel enriched with ammophos and ammonium polyphosphate, and a hydrogel enriched with NPK fertilisers (this has been described in detail in our previous work [5]). The synthesised hydrogels were dried in an oven at 50 °C until a constant mass was reached, then soaked. Every 20–25 minutes the hydrogels were weighed on a scale and the results were entered into a table.

Table 1.

Degree of swelling of traditional and synthesised hydrogels

Table 1 shows that the maximum swelling degree of locally produced hydrogel is currently 150 g/g, while that of hydrogel enriched with NPK fertilisers reaches 195 g/g, and for hydrogel enriched with ammonium polyphosphate and ammophos, it is 245 g/g.

Results and discussion

If Table 1 is presented in graphical form, the difference in the degree of swelling of hydrogels can be clearly seen.

Figure 1. Swelling degree (g/g) of conventional and fertilizer-enriched hydrogels as a function of time

The degree of swelling of traditional hydrogels and hydrogels enriched with fertilisers was studied over time, and the results are shown in Figure 1. As can be seen from the graph, the degree of swelling in all hydrogel samples increases rapidly in the initial stages and then gradually approaches equilibrium.

Traditional hydrogel demonstrated the lowest swelling capacity compared to other samples. Its swelling capacity gradually increased over time, reaching an equilibrium value of approximately 150 g/g after about 225 minutes. This indicates that the ability of the unmodified polymer network to absorb water is limited.

In comparison, the hydrogel enriched with ammonium polyphosphate and ammophos (APP+AM) demonstrated a significantly higher degree of swelling throughout the entire time interval. The degree of swelling of this sample increased rapidly, reaching approximately 245 g/g at equilibrium, which significantly exceeds the values for traditional hydrogel. Such a high swelling capacity is explained by the hydrophilicity of the phosphate and ammonium groups in the hydrogel composition, which increase the affinity of the polymer network to water, leading to an increase in osmotic pressure inside the hydrogel. The hydrogel enriched with NPK fertilisers exhibited intermediate swelling behaviour. Its equilibrium swelling degree was approximately 195 g/g, which is higher than that of traditional hydrogel but lower than that of hydrogel enriched with APP+AM. The increase in the degree of swelling compared to traditional hydrogel is due to the formation of additional hydrophilic centres as a result of the introduction of nutrient salts into the polymer matrix. At the same time, the lower degree of swelling compared to the APP+AM system can be explained by the limitation of excessive expansion of the polymer network due to strong interionic interactions or partial screening of hydrophilic groups. It was also noted that the swelling process in all hydrogel samples reached equilibrium after approximately 225-250 minutes. This indicates that fertiliser enrichment mainly affects the quantitative value of the degree of swelling without significantly changing the overall characteristics of the swelling kinetics. Overall, the results confirm that enriching hydrogels with fertilisers significantly increases their water retention capacity, with APP+AM-modified hydrogel being particularly promising for agricultural practice. The high degree of swelling of synthesised hydrogels enriched with ammophos and ammonium polyphosphate, as well as hydrogels enriched with NPK fertilisers, compared to traditional hydrogels can be explained as follows: The main water absorption capacity of hydrogels is based on two properties: the first is water retention through hydrogen bonds, and the second is long-term water retention due to the spaces between three-dimensional networks. When the hydrogel is enriched with mineral fertilisers, the degree of swelling increases sharply compared to traditional hydrogels due to the enhancement of both of the above properties. Ammophos, ammonium polyphosphate and NPK fertilisers used in hydrogel modification are rich in hydrophilic groups. In addition, fertilisers used to enrich the hydrogel, located between the three-dimensional branches of the hydrogel, increase the porosity between them, which leads to a sharp increase in the degree of swelling.

The advantages of hydrogels enriched with synthesised mineral fertilisers compared to traditional hydrogels are as follows:

• Synthesised hydrogels have a significantly higher water absorption capacity.
• It has been established that synthesised hydrogels are capable of retaining absorbed water in their composition for 48-62 days, whereas local hydrogels obtained by traditional methods retain water for 36-48 days.
• Synthetic hydrogels, along with long-term retention of absorbed water, provide plants with convenient absorption of mineral fertilisers in a water-soluble state.
• Synthesised hydrogels containing mineral fertilisers not only save water but also reduce excessive consumption of mineral fertilisers, which brings significant economic benefits.

Hydrogels enriched with synthetic mineral fertilisers, along with the above advantages, also have a number of disadvantages.

• Too rapid water absorption can lead to a decrease in groundwater levels due to rapid swelling of the hydrogel during crop irrigation or precipitation.
• In some cases, a high degree of swelling causes a certain portion of hydrogels to accumulate in one place due to improper agricultural treatment of the sowing areas, which, as a result of excessive moisture, causes root rot and plant death.

Conclusion. In this study, the physicochemical properties of hydrogels enriched with various mineral fertilisers and microelements were compared with those of traditional hydrogels. The results showed the following:

The hydrogel enriched with ammonium polyphosphate and ammophos (APF+AM) had the highest degree of swelling, reaching a maximum value of 245 g/g. This confirms the ability of this hydrogel to retain water for a long time and have a high capacity for liquid absorption. The hydrogel enriched with NPK fertilisers showed a significant improvement in swelling degree compared to traditional hydrogel, reaching a maximum swelling degree of 195 g/g. However, its lower value compared to hydrogel enriched with APP+AM is explained by differences in the number of hydrophilic groups and the structure of the network. The maximum swelling degree of traditional hydrogel was 150 g/g, which indicates its limited water retention capacity. In all hydrogel samples, the swelling process reaches equilibrium after 225-250 minutes, i.e. the process of enrichment with fertilisers does not significantly affect the rate of swelling kinetics, but only increases the maximum degree of swelling. Hydrogels enriched with mineral fertilisers have the ability not only to retain water for a long time, but also to deliver mineral fertilisers in a form that is convenient for plants. At the same time, their high degree of swelling provides economic benefits and saves water resources. The disadvantages identified include the fact that rapid swelling of hydrogels in some cases can lead to a decrease in the groundwater level, as well as to the rotting of plant roots due to improper agricultural practices. Overall, hydrogels enriched with APP+AM and NPK demonstrated significant advantages over traditional hydrogels and can be used in agricultural practice as promising materials that provide high water retention capacity, effective plant irrigation, and mineral fertiliser savings.

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