Performance and Evaluation of Slow-Release Fertilizer Encapsulated by Waterless Synthesized GO Sheets

Abstract

Slow-release fertilizer was developed by encapsulating NPK compound pellets with graphene oxide (GO) sheets employing a waterless synthesis technique. As-prepared GO sheets were characterized by XRD, Raman, XPS, FTIR, SEM, and EDS. The XRD patterns of the GO sheets indicate that the peak for the GO is observed at 2θ = 9.3°, and the peak (002) for graphite vanished. Moreover, a higher intensity ratio of the Raman ID/IG of the GO sheets than that of pristine graphite confirms the oxidation of the graphite. The FTIR and XPS analyses provided information on electronic structure, chemical structure, and oxygen-bonding neighbors. The SEM images indicated the GO sheet, whereby its morphology resembles a thin curtain or corrugated shape. The EDS spectrum of coated GO-F pellets revealed the distribution of C, O, N, P, and K elements in the synthesized materials. Afterwards, GO shell formation on fertilizer pellets greatly improved the slow-release characteristics of fertilizer, thus providing plants with their requisite nutrients and reducing environmental pollution.

1. Introduction

Crop production is impacted by extreme weather conditions, causing chemical fertilizers to be used in excessive and disproportionate quantities to raise crop yields. However, this practice has no benefits in terms of ensuring crop yield and nutritional quality. It carries the risk of environmental pollution and soil acidification, which also poses a threat to food security [1]. According to a Ministry of Agriculture and Rural Affairs of the People’s Republic of China survey in 2017, China’s fertilizer use rate of 328.5 kg/ha is much higher than the world average (120 kg/ha) [2,3]. Hence, reducing the use of chemical fertilizers and maintaining high-yielding crops are long-term goals of the Chinese government. In addition, another problem associated with traditional fertilizers is their fast dissolution in soil compared to their rate in plant absorption. This leads to fertilizer loss and contamination of the surrounding environment. Consequently, to enhance the delivery efficiency of nutrients to crops and minimize the harmful effects on the environment with the goal to make agricultural production more efficient, resilient, and sustainable, it is crucial to accelerate the development of new materials and innovative technologies [4].

In order to improve the efficiency of fertilizer use and reduce environmental issues caused by its application, slow-release fertilizer (SRF) has been developed. This type of high-efficiency fertilizer reduces nutrient loss and enhances utilization efficiency through the slow release of nutrient elements through diffusion, dissolution, or other mechanisms [5]. SRF can be categorized into coated and non-coated types according to their release mechanisms. Coated SRF is a substrate with a coating that physically separates the nutrient from the soil [6], allowing for gradual diffusion or release after the coating breaks. On the other hand, non-coated SRF achieves its effect by reducing solubility in water through different combinations of fertilizer and slow-release material. In particular, coated fertilizer significantly improves slow-release effectiveness compared to chemical fertilizer. In water, the release time of coated SRF is 20–30 times longer than that of uncoated fertilizer [7]. Compared with non-coated fertilizer, coated fertilizer can enhance nitrogen utilization efficiency and reduce nitrogen loss. In the present technology, coated SRF uses synthetic polymer coatings [8], such as polyolefin and polyvinylidene chloride. But these coatings are difficult to degrade, leading to accumulation over time, which usually necessitates the use of either organic solvents or toxic polymerization initiators, or requires time-consuming and complex procedures, which not only increase the costs of production but also lead to health issues [9]. Furthermore, the majority of SH-based synthetic polymers, including poly (acrylic acid), poly (acrylamide), and copolymer, are non-biodegradable and contribute to environmental pollution [10]. Therefore, research into the development of coated SRFs, green fertilizers that can regulate the nutrient release rate, meet the nutrient requirements of a variety of crops at different growth stages through a single application, have a straightforward manufacturing process, and are environmentally benign, represents a significant area of interest within the field of new fertilizers.

Graphene oxide (GO) is a unique material with excellent chemical properties, adsorption capacity, and biocompatibility. It has a 2D structure, large surface area (2600 m²/g) [11], cost-effective production, and the ability to interact directly with different biomolecules and nanoparticles. Moreover, its structure is enriched by various functional groups, including epoxy-functional (–CH (O) CH–), hydroxyl (–OH), carboxyl (–COOH), and ester (–COO–) groups [12,13], making it an ideal substrate for functionalization processes. GO finds applications in several fields. In optics, the optical properties of GO are governed by the functionalization of surfaces and play a pivotal role in tailoring their optical properties [14]. In catalysis, GO’s inherent functional groups enable it to accelerate chemical reactions, which benefits sectors such as energy production and environmental applications [15]. In biomedical applications, the functionalization of GO’s carboxyl groups also demonstrates GO’s promise in creating enzyme electrodes that can be used in biomedicine and clinical diagnostics [12]. In the agriculture field, the application of GO-based nanomaterials was shown to improve the efficiency of agrochemicals through targeted delivery and slow-release action and to promote plant growth and development in the soil environment [16]. Safikhan et al. (2017) [17] showed GO NPs in agricultural crop improvement. Soil-based graphene oxide treatment significantly enhanced the physicochemical qualities of the soil. Lalwani et al. (2014) [18] showed that lignin peroxidase (ligninolytic enzyme released from white-rot fungi) can effectively degrade GO in the soil, and then turn it into a completely harmless product, thus avoiding secondary pollution of the environment. In Silybum marianum, a study showed that graphene oxide use increased chlorophyll content and RWC and improved the growth potential and yield of plants [4]. These studies confirmed the potential application of GO as a slow-release fertilizer; however, the preparation methods of GO employed as either the KClO₃-based Brodie [19] and Staudenmaier [20] method or the KMnO₄-based Hummers [21] method have been the three primary routes for synthesizing graphite oxide, which all depend on the homogeneous reactions of graphite with mixed strong oxidants [22]. While the Hummers method has advantages over the technique of Brodie and Staudenmaier, there are some practical issues that must be resolved, such as long reaction time, security risks, difficult quality control, and a large amount of acid liquid waste [23]. Consequently, the cost of synthesis and processing was still too high as a coating material on the surface of fertilizer, limiting its application in agriculture.

Therefore, we aim to find a green, safe, and economical way to prepare GO for employment as a coating material. In this paper, a novel waterless synthesis method is presented for the GO sheet as a slow-release fertilizer coating material. The slow-release performance and evaluation of the prepared GO-F were analyzed and can also provide a theoretical basis for the further development of GO-Fs with high efficiency, a simple process, and reduced environmental pollution.

2. Materials and Methods

2.1. Materials

Fine flakes of graphite (99% carbon basis, 600 mesh particle size (≥90%)) were purchased from Gongyi Baichuan Environmental Protection Engineering Co., Ltd. (Henan, China), and fully dried in an oven at 80 °C for 24 h before use. Concentrated sulfuric acid (98 wt.%, H₂SO₄), phosphoric acid (85 wt.%, H₃PO₄), potassium permanganate (KMnO₄), hydrogen peroxide (30 wt.%, H₂O₂), and ethanol (99 wt.%, C₂H₆O) were from purchased Guangzhou Chemical Reagent Factory (Guangzhou, China), Damao Chemical Reagent Factory (Tianjin, China), Yien Chemical Technology Co., Ltd. (Tianjin, China), and Fengchuan Chemical Reagent Co., Ltd. (Shanghai, China), respectively. All the chemicals are analytical grade, and their solutions were prepared using deionized water (18.2 MΩ) from Dongguan Chaojing Water Treatment Technology Co., Ltd., Dongguan, China.

2.2. Synthesis of GO Material

The waterless synthesis of GO was achieved by modification of the improved Hummer’s method and preparing the sample of GO. The advantage of this method is the fact that the oxidation process does not require the addition of additional deionized water. After the reaction was completed, the GO product was immediately separated from the acid solution. The concentrated H₂SO₄ is recovered for future use. A total of 2 g of graphite powder was added into a mixture of concentrated H₂SO₄ and H₃PO₄ with a volume ratio of 9:1 (81:9 mL) [24], then put into a 500 mL flask for mechanical stirring (300 rpm) in an ice bath, followed by the gradual addition of KMnO₄ (13 g) to keep the temperature of the suspension below 20 °C. The reaction mixture was then heated to 50 °C and mixed by mechanical stirring for 12 h. Thereafter, the reaction mixture was cooled down to room temperature, also to recover the sulfuric acid for further use. One subsequent addition of H₂O₂ (3 mL) stopped the oxidation process and the color of the reaction mixture changed to bright yellow, indicating a high oxidation level of graphite. After this process, centrifugation was performed to separate the precipitate from the liquid, and the deionized water was added until a neutral pH was reached. Following this process, centrifugation (4500 rpm) and repeated washing with deionized water and ethanol until the supernatant became neutralized to prevent any inorganic ion sedimentation was conducted. The obtained GO solution was freeze-dried for two days to obtain the GO sheet material. The process of GO sheet material synthesization is illustrated in Figure 1.

2.3. Preparation of GO-Coated Fertilizer Particles

After freeze-drying for two days to obtain the GO sheet, gently remove the GO sheet with fine-pointed tweezers, and pay attention to the application of force during the removal process to maintain the fluffy state of the GO samples. Then, grab composite fertilizer particles with tweezers and apply mechanical force to coat the GO sheet on the surface of the fertilizer to obtain GO-coated fertilizer particles (GO-F), as shown in Figure 2. Finally, dry the GO-F particles and store them in sealed containers for future use.

2.4. Characterization

The materials’ structures were measured using X-ray diffraction (XRD, Rigaku Ultima IV, Rigaku, Tokyo, Japan). Raman spectra of graphene oxide with different oxidation degrees were measured at 20 °C with a LabRAM HR Evolution spectrometer (Horiba JY, France SAS) using a laser wavelength of 532 nm in the range from 1000 to 3500 nm cm⁻¹. X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Scientific, East Grinstead, UK) analyzes elemental composition and bonding state information. Physicochemical properties of employing Fourier-transform infrared (FT-IR) spectra were recorded on an Avatar 360 Nicolet instrument (Thermo Fisher Scientific, Shanghai, China) by measuring the samples. The field emission scanning electron microscope (FE-SEM, Zeiss Sigma 300, Carl Zeiss, Germany) operating at 30 kV was used to characterize the area, size, and morphology. An energy-dispersive X-ray spectroscopy (EDS, Bruker Quantax XFlash SDD 6/30, Berlin, Germany) at 15 kV was used to identify and quantify the elements (C, O, N, P, and K) present in the GO-F.

2.5. Nutrient Release Properties of GO-F

The nutrient release behavior of GO-F in water at 25 °C was measured in accordance with the Chinese National Standard of slow-release fertilizer protocol (GB 23348-2009). In total, 1 g of GO-F and F samples was sealed in a 100-mesh nylon bag and added to 50 mL of distilled water, put in a 150 mL flask, and incubated at 25 °C in the incubator. After 1, 1.5, 2, 3, 4, 5, 6, 7, 8, 9, and 10 h, respectively, the samples were filtered, and a certain amount of solution was taken out to measure the N, P, and K content. Subsequently, 10 mL of deionized water was added to each flask to maintain the desired initial concentration. All samples were tested in triplicate and the average value was taken as the N, P, and K concentration of each sample. The nutrient release longevity of GO-F and F was defined as the time at which the cumulative nutrient release reached 100%. Finally, the concentration of N, P, and K was measured by a nutrient rapid analyzer, and the contents of N, P, and K were further calculated.

3. Results and Discussion

3.1. Characterization of GO Sheet and GO-F

3.1.1. Structures of GO Sheet and Graphite

The XRD patterns of GO sheets and graphite powder were analyzed by scanning the sample through a range of 2θ angles from 5° to 40° with Cu-Kα radiation (λ = 1.5406 Å) to analyze the interlayer spacing. Figure 3 shows that the graphite flakes showed a characteristic sharp peak at 2θ = 26° (d = 0.34 nm), referring to graphite as the presence of a hexagonally ordered structure in the graphitic materials [25]. Following the oxidation of graphite, the peak for the GO sheet is observed at 2θ = 9.3° (d = 1.0.6 nm), attributed to the forming of epoxy (–CH (O) CH–), hydroxyl (–OH), and carboxyl (–COOH) groups due to oxidation [12,13,26,27], resulting in a wider interlayer distance of the GO sheet. Additionally, the observed peak (002) for graphite completely vanished in the case of the GO sheet, consistent with the existing literature [28]. The initial results of XRD demonstrated the successful synthesis of the GO sheets.

3.1.2. Raman Analysis of GO Sheet and Graphite

Raman spectroscopy provides essential information for characterizing the crystal structure of graphene derivatives and determining the presence of disorder or defects in the graphitic material resulting from the synthesis route. Figure 4 displays the Raman spectra of the GO sheet and graphite upon its transformation from graphite to graphene oxide, requiring an oxidation process to structurally change, during the chemical process, graphite into graphene oxide. The pristine graphite material shows a sharp G band, attributed to the in-plane stretching motion between sp² carbon atoms. The D band is recognized to be a disordered band originating in structural defects, edge effects, and dangling sp² carbon bonds that break the symmetry. For pristine graphite, the D and G peak positions were centered at 1292.95 and 1485.92 cm⁻¹, while the GO sheet peaks were at 1350.70 and 1573.24 cm⁻¹, respectively. The G band of the GO sheet broadens and up-shifts in comparison to what is observed in the pristine graphite (strong G band at 1573.24 cm⁻¹ due to first-order scattering of the E₂g mode) [29]. A possible explanation of this behavior is the presence of isolated double bonds which resonate at higher frequencies. Furthermore, the D band at 1350.70 cm⁻¹ becomes prominent, indicating that the oxidation process influences the size of the in-plane sp² domains [30]. In addition, the D band represents the disorder or defect band that shows defects caused by breaks in the infinite-symmetry carbon honeycomb-shaped lattice [31]. The shift of the G band to a higher wavenumber upon graphite oxidation suggests amorphization of the graphite and the presence of a significant amount of sp³ carbon [32].

The intensity ratio of ID/IG provides direct evidence of the degree of functionalization. In this study, the ID/IG ratio of the graphite and GO sheet was found to be 0.19 and 0.863, respectively. This result confirmed that the intensity ratio of ID/IG of GO was higher than that of pristine graphite, which confirms the oxidation of graphite, referring to the D band, which increased during oxidation since oxygen functional groups were introduced into the graphitic chain [33], where lattice defects could be a type of sp³ defect (epoxides) caused [34].

3.1.3. FTIR and XPS Analysis of GO Sheet

The FTIR spectrum can identify functional groups and determine molecular structures. FTIR analysis evidence for the presence of various peaks corresponding to various oxygen-containing functional groups confirms the formation of the GO sheet, as shown in Figure 5. In the spectrum of the GO sheet, a broad peak appeared at 3433 cm⁻¹ in the high-frequency area, attributed to the existence of hydroxyl groups and surface-adsorbed water [34]. The peak at 2985 cm⁻¹ represents the asymmetric and symmetric stretching vibrations of C–H bonds [35]. The peak at 1631 cm⁻¹ is attributed to the stretching vibration of the carbonyl group (C=O) from the ketones [36]. The peak at 1401 cm⁻¹ is attributed to the stretching vibrations of (C–OH) side groups, and the one at 1051 cm⁻¹ is attributed to the C–O (alkoxy) stretching of the epoxy or peroxide group [35,37].

To further investigate the obtained GO sheet composite after the presentation of oxygen-containing functional groups, XPS analysis was conducted, as shown in Figure 6. For the GO sheet, as illustrated in Figure 6a, the XPS C 1s spectrum of the GO sheet was deconvoluted into four component peaks centered at 284.8 eV, 286.8 eV, 287.9 eV, and 288.5 eV, respectively. These peaks correspond to sp² carbon (284.8 eV) [38,39,40], and/or sp³ hybridized carbon corresponds to the following oxygen-containing groups [41]: C–O–C epoxy group (286.8 eV) [39,40], C=O carbonyl group (287.9 eV) [40,42], and O–C=O carboxyl group (288.5 eV) [43,44], respectively. The high-resolution O 1s XPS spectrum of the GO flake contains three fitted peaks at 531.9 eV, 530.7 eV, and 529 eV (see Figure 6b). The peaks at 531.9 eV and 530.7 eV indicate the presence of surface oxygen (C=O and O–C=O) [44,45], while the peak at 529 eV corresponds to O-M lattice oxygen [46], possibly resulting from the reduction of KMnO₄ to MnO₂ by oxidation. The presence of oxygen-containing functional groups confirms the successful oxidation of the graphite into the GO sheet.

3.1.4. SEM-EDS Analysis of GO Sheet and GO-F Particle

The analysis of the GO sheet and GO-F were observed by SEM at a magnification of 10.00 K, as shown in Figure 7a–c, respectively. As observed in Figure 7a, the waterless synthesized GO sheet shows that when graphite is oxidized and exfoliated into GO sheets, there is evidence of GO sheets whereby their morphology resembles a thin curtain or corrugated shape, which was the result of deformation upon the exfoliation and restacking process [47,48]. The synthesized GO membrane formed from GO sheets coated in each fertilizer particle (GO-F) is observed in Figure 7b to have formed a complete shell around the fertilizer core. A more detailed examination of the cross-sectional GO shell reveals a thickness of approximately 50–60 µm (Figure 7b). The high-magnification SEM imaging of the surface of the GO-F demonstrates that it has a dense GO shell without any apparent apertures outside (Figure 7c). Figure 7d presents the results of the EDS analysis, which confirms the chemical composition of the GO-F particles. The analysis revealed that the particles are composed of C (40.74%), O (42.92%), N (5.63%), P (9.58%), and K (1.13%) elements, respectively.

3.2. Slow-Release Behavior of GO-F Particle in Water

To assess the nutrient release of the GO-F and F pellets (i.e., in a granular-form commercial granular fertilizer), experiments were performed to determine their release rates of NPK nutrients in water dissolution. As shown in Figure 8a–c, more than 81.51%, 85.64%, and 81.55% of the N, P, and K in the uncoated F pellets were released within 4, 2, and 4 h (Figure 8a, Figure 8b, and Figure 8c), respectively. The results indicated that the release rate of NPK nutrients was arranged in different stages since the GO sheets coating the fertilizer pellets (i.e., GO-F) were used as a coating to delay the whole release process. In the initial stage, spanning from 0 to 4 h, the release rate was relatively slow compared with the uncoated F pellets’ stage. At this stage, only approximately 43.41%, 57.07%, and 55.1% of the NPK ions were released from the GO-F into the water. The burst release of NPK ions takes place in the stage from 7 to 8 h, after which about 92.96%, 95.81%, and 94.45% of the NPK ions were released from the fertilizers, in agreement with the results of Zhang et al. [49]. However, after 6 h, the coated GO-F pellets cracked on the sheet, which led to the penetration of water into the NPK compound fertilizer core through these tiny cracks, inducing the rapid release of NPK ions. The data also reveal that the release of NPK ions from the GO-F to the water reaches its equilibrium after about 10 h, indicating that the sheet has a slow-release ability, which can meet the nutrient requirement of plants during this time. Meanwhile, as shown in Figure 8d, uncoated F pellets immediately structurally collapsed with water; conversely, the coated GO-F pellets remained in as good a shape as they began when placed underwater, without structural collapse. This demonstrates that the GO sheet-coated fertilizer (i.e., GO-F) can provide a promising coating technique for the slow-release characteristic.

4. Conclusions

This study demonstrates that the GO-F coating technique offers a promising approach to achieving slow-release behavior. The GO-F particle was developed by encapsulating NPK compound pellets with GO sheets, employing a waterless synthesis technique. The findings indicated that uncoated F pellets exhibited a higher release rate, with 81.51%, 85.64%, and 81.55% of their NPK ions released within 4, 2, and 4 h, respectively. In contrast, the release of NPK ions from the coated GO-F pellets was significantly lower, with only approximately 43%, 57%, and 55% released at this stage. These findings demonstrate that the GO sheet, when employed as a coating material, exhibits excellent slow-release behavior of the encapsulated GO-F particle. With the unique characteristics of GO, this newly developed method could be used for developing fertilizers that have slow-release behavior, thereby providing plants with their requisite nutrients, enhancing productivity, and reducing environmental pollution.