Environment-friendly synthesis of Feldspar-KH₂PO₄ complexes by mechanochemical reaction

Introduction

As global food consumption continues to rise in the coming years. Global food demand will increase approximately 60% by 2050 compared to 2005, according to the United Nations’ Food and Agriculture Organization (1). Crop production has to be increased in agriculture to fulfill global food demand. One of the most efficient ways to spur plant growth is fertilizer, which can play a significant role in increasing crop productivity (2). Environmental pollution is caused by fertilizer use, which results in health problems. Several factors contribute to environmental pollution, including the high solubility of nutrients and their leaching followed by soil mobility (3).

Plants can only use a small number of chemical fertilizers because they are highly soluble in water. Several processes affect the rest of them when they are applied to a field, such as adsorption, degradation, runoff, and leaching. In other words, soluble chemical fertilizers are quite ineffective. Because fertilizers are derived from inorganic anions such as nitrate and phosphate, excessive application of fertilizers can cause contamination of surface water and soil (4).

There have been several types of slow-release fertilizers made from different materials, including various minerals (5–8). Slow-release mineral fertilizers have gained a lot of attention for their environment-friendly properties and ability to maintain soil fertility. Minerals are usually charged to protect nutrients from rapid degradation as well as having a crystal structure (8–13).

The mineral feldspar is the most common one found in rocks. A group of minerals that are known collectively as feldspar is a mineral with the chemical formula x Al (Al, Si)₃O₈, where x might be calcium (Ca), sodium (Na), or potassium (K). Because of their high alumina and alkali contents, feldspars are predominantly utilized in industrial applications (14). Feldspar is used to describe a wide range of materials. We use feldspar in a substantial number of everyday products, including drinking glasses, windshields, fiberglass for insulation, bathroom floor tiles, shower basins, and even the dishes on our tables. Feldspar is ubiquitous. An infinite number of tetrahedral SiO₂ and AlO₄ networks comprise the crystal structure of feldspars (14).

According to Zhang et al. (15), the minerals that contain at least some albite or anorthite are called plagioclase feldspars. In contrast, those that include alkali feldspars are known as orthoclase feldspars (16). Due to feldspars’ industrial application, the latter category is especially interesting. Depending on the type of alkali they contain, feldspars can also be divided into sodium, potassium, and mixed types (15). Thermal gravimetric analysis (TGA), Fourier transforms Infrared (FT-IR), and x-ray diffraction (XRD) were used to explain the unique properties of Feldspar (17).

There is a wide range of applications for feldspars, from fluxing agents for ceramics to fillers for paint, rubber, and adhesive industry purposes (14, 18). Therefore, this research aims to optimize control/slow-release fertilizers (C/SRFs) by combining feldspar minerals with KH₂PO₄ fertilizers using a dry mechanochemical approach. We aim to highlight the significance of adjusting the milling time and rotation speed in the synthesis of feldspar slow-release fertilizers by studying their mechanochemical properties.

Experimental sections

Mechanochemical processes and materials

High-purity feldspar from the Yutum Granite Suite (KAlSi3O8) (part of the Aqaba complex) (14, 19) and Pan- reac, PRS undergoes mechanical reactions with potassium dihydrogen phosphate (KH₂PO₄). Ball mill, planetary (Pulverisette-7, Fritsch, Germany), was used for milling under a choice of time and rotation speed, using 6.0°g (3:1) feldspar/fertilizer and 7 steel balls of 15°mm diameter. In each experiment, alternating 10°min of milling with 5°min of rest was done to prevent excessive heat from accumulating during milling. Two discussed parameters were studied in Table 1.

Characterization

X-ray diffraction measurements were performed on the Feldspar-KH₂PO₄ samples before and after milling. Radiation was Cu K has a range of 2° between 5 and 90°. The produced samples were examined using infrared spectrometers as the NEXUS and the EPS-870. At a resolution of 2°cm^–1, the sensors were scanned from 4,000 to 500°cm^–1 an advanced sensor (not KBr). Using a NETZSCH STAT-409 PC thermal gravimetric analyzer in an N₂ atmosphere, the analysis was performed in 10^°C/min increments from room temperature to 1,000^°C.

A 100-ml glass beaker was used to conduct the leaching tests with KH₂PO₄ ground in distilled water. The parameters were the following: 1.0°g of ground sample, 20°ml of distilled water, and 24°h of incubation. Vacuum filtration with 0.45°m pore size filter paper was used after leaching to separate solids from liquids. The parameters of the liquid ion chromatography were determined by the concentration of the K⁺, NH₄⁺, and PO³₄^– ions (nutrients) in the filtrate along with determining the total nitrogen released from urea by the Kjeldahl Nitrogen Determination instrument (IC, Thermo Scientific, column series Dionex, CS-5000 + DP, Germany).

Results and discussion

Hae-Jin Jung, Md Abdul Malek, JiYeon Ryu, BoWha Kim, Young-Chul Song, HyeKyeong Kim, and Chul-Un Ro.

There are no studies available providing data on slow- release fertilizer systems synthesized from feldspar and fertilizers. More research is needed on the effectiveness of different parameters in fertilization. During amorphous material developed and reacted with salts during the production of feldspar, changing the solubility profile of compounds and generating new interactions. It has been established that feldspar has a neutral, layered alkaline structure that does not interact with other substances, including the study’s target phosphate salts.

Feldspar-KH₂PO₄ System

We investigated the influence of the milling speed and the milling time in the high-energy ball mill. To achieve the most suitable conditions for utilizing the slow-release properties of nutrients, a series of experiments were conducted with feldspar and selected fertilizers.

Mechanochemical synthesis of Feldspar-KH₂PO₄ as SRF - effect of milling speed

In this study, the phosphate salts of interest were studied as amorphous feldspars produced by the formation of polymorphous aggregates. The emergence of amorphous materials changed the solubility profiles of the salts and enhanced interactions between the products that were obtained. As shown in Figure 1, amorphization of the Feldspar-KH₂PO₄ (3:1 weight ratio) sample mixture is influenced by mill rotational speed. The mill rotational speed ranged between 100 and 600°rpm for all experimental runs, with a fixed milling time of 120°min. When mill rotation speeds between 100 and 400°rpm were lowered, characteristic patterns related to KH₂PO₄ were still visible in the milled products. The higher mill speeds yielded a complete amorphous reduction of the samples, which suggests the amorphous structure of feldspar incorporates KH₂PO₄.

Each of the experimental runs was run for 120°min with mill rotation speeds ranging from 100 to 600°rpm for the FT-IR spectra to be collected. The characteristic spectrums of KH₂PO₄ at 1,281°cm^–1 remained in the milled products at lower mill rotational speeds, such as 100−400°rpm, as shown in Figure 2. As a result, the clear band at 1,281°cm^–1 on the KH₂PO₄ spectrum was eliminated from sample mixtures that were milled at speeds of 400°rpm, which suggests successful intercalation of KH₂PO₄ in feldspar, distinctive due to the overlap of Al-OH, Si- O, and P-O vibrations, wide bands were seen in the area approximately 1,000°cm^–1 for all samples (20–22).

Moreover, the shoulder band of feldspar at (1,100°cm^–1) vanished when milling over 400°rpm, which indicates a better mechanochemical reaction, occurs at high milling speeds than at low speeds. The thermal gravimetric analysis (TGA) patterns of Feldspar-KH₂PO₄ samples that were milled for 2°h at different milling speeds were shown in Figure 3.

Figure 3 shows that feldspar did not present significant mass loss during TGA analysis, with a mass loss of 0.5%. This small loss shall be related to the evaporation of impurities like carbonates and the free water elimination of feldspar, which is a good match for the predicted value of 0.3% (10), demonstrating the high purity of the feldspar that was employed. The phosphate salts generally disintegrate in three steps, starting endothermic peaks at 230, 270, and 360°C for potassium dihydrogen phosphate (Figure 3). For samples milled at 100 and 200°rpm, clear mass loss obtained at 230°C illustrated that these low speeds of rotation were not enough to incorporate KH₂PO₄ into feldspar, Nevertheless, no mass loss characteristics of the raw materials were observed in the milled samples of potassium dihydrogen phosphate milled at 400 and 600°rpm, which indicates that they are destroyed during grinding; this is in agreement with the observations of other instrumental techniques showing that amorphous phases were formed by combining the reagents.

Figure 4 depicts the 24-h release curve of K⁺ and PO³₄^– nutrients from Feldspar-KH₂PO₄ sample mixes milled at varied rotating rates can be determined. As shown by the results from samples prepared at 100 and 200°rpm, the release of both K⁺ and PO₄^3– reached around 1,003 and 983°ppm, respectively. These results indicate a greater mill rotational speed is required for complete amorphization of the starting materials to incorporate KH₂PO₄ into the amorphous feldspar structure. As the mill speed was increased from 400 to 600°rpm, the release of kaolin and phosphate nutrients into the solution decreased markedly. This was achieved by reaching 974°ppm at 400°rpm and sharply decreasing to 484°ppm at the sample. This suggests 600°rpm, suggesting the addition of KH₂PO₄ to feldspar in amorphous form. Specifically, metals such as Al and Si, especially those that are present in Al-Si-O networks, retard the diffusion movement of these elements (23). In water, both K⁺ and PO³₄^– dissolve as a result of KH₂PO₄, and these chains are attached to Al-Si-O molecules.

A correlative study with Kaolinite-KH₂PO₄ (5) is investigated to make sense and clarify the effect of milling speed at a fixed wt ratio and milling time to the release of K⁺ from different complexes when dispersed into distilled water for 24°h; the results are displayed together in Table 2.

Figure 5 clarifies the correlation process between Kaolinite-KH₂PO₄ (5) and Feldspar-KH₂PO₄ complexes results in Table 2, examining the effectiveness of these complexes as SRF by milling at 600°rpm for various time intervals and comparing the nitrogen release results to determine which one is the most suitable as SRF.

Figure 5 and Table 2 demonstrate that it is possible to reduce disintegration of K + nutrients by increasing milling speed, which is in agreement with IR, TGA, and XRD results (5); however, results refer that Kaolinite-KH₂PO₄ is more suitable than feldspar as a nutrient carrier and best for use and application as SRF.

Feldspar-KH₂ PO₄ as SRF: Effect of milling time

Amorphization of the Feldspar-KH₂PO₄ sample mixture was studied using the XRD patterns shown in Figure 6. Throughout all experimental runs, milling speed was fixed at 600°rpm, and milling intervals ranged from 60 to 180°min. As can be seen in Figure 6, most of the characteristics associated with KH₂PO₄ had been removed from milled products at a lower milling time of about 60°min. Furthermore, using a 120°min milling time, KH₂PO₄ was reduced to an amorphous state and the featured peaks disappeared, supporting the idea that KH₂PO₄ was incorporated into the amorphous structures of the feldspar.

As can be seen in Figure 7, the thermal dehydroxylation of the feldspar caused a mass loss of 0.3%. In the milled samples, potassium dihydrogen phosphate, the mass loss characteristics were not observed, which could indicate their destruction in the process of grinding. This was confirmed by other instrumental methods that showed that amorphous phases originated from the reagent mixture. Additionally, Figure 7 shows that 3°h of milling resulted in just one mass loss event; however, samples milled for 60 and 120°min showed two additional mass losses, suggesting that milling duration affects mechanochemical synthesis.

Figure 8 illustrates how nutrients are released from samples milled to 600°rpm as a function of milling time after 24°h of leaching in distilled water. Both K⁺ and PO₄^3– were detected at over 927°ppm in sample mixtures that were prepared after 60°min of milling time. This indicates sufficient milling time was required to cause complete amorphization of the starting materials to allow KH₂PO₄ to be incorporated into the amorphous feldspar structure.

During milling for 180°min, both K⁺ and PO₄^3– nutrients were released substantially less, reaching around 708 and 653.8°ppm and reaching more than 780°ppm for samples obtained from milling at 120°min. This indicates that KH₂PO₄ had sufficient time at 120°min to incorporate into the structure of feldspar. Milling for long periods may cause the compound to escape slightly from feldspar.

Conclusion

A novel Feldspar-KH₂PO₄ mixture loaded with KH₂PO₄ demonstrated remarkable mobility in solution because of the nutrient release characteristics of KH₂PO₄, which confirmed that, in addition to being simple and economical, the mechanochemical route also proved to be effective for synthesizing Feldspar-KH₂PO₄ complex samples that act as intelligent fertilizers. By optimizing the ratio of feldspar to fertilizer (3:1 wt ratio) and milling speed at 600°rpm for 2°h, this study illustrated the ability to open a novel approach to the preparation of SRF and higher possibilities for the preparation of low-cost precursor biomass material. The nutrient results indicated that KH₂PO₄ had a sufficient time and milling speed to incorporate KH₂PO₄ into the structure of feldspar.

Acknowledgments

The University of Jordan and Tafila Technical University are greatly acknowledged for their support of this research.

References