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. 2020 Jan 9;5(2):1127–1133. doi: 10.1021/acsomega.9b03310

Spout Fluidized Bed Assisted Preparation of Poly(tannic acid)-Coated Urea Fertilizer

Yu Wang †, Heling Guo †, Xiaolin Wang †, Zhiyuan Ma ‡, Xie Li †, Rui Li †, Qian Li †, Rongjie Wang †, Xin Jia †,*
PMCID: PMC6977027  PMID: 31984269

Abstract

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Slow-release fertilizers (SRFs) have been widely used to reduce environment pollution derived from excessive nutrients. Coated fertilizers have been designed and prepared using various materials. However, development of new green coating materials and simple process is still a huge challenge. In this study, tannic acid (TA), a natural polyphenol, was used to prepare poly(tannic acid) (PTA)-coated fertilizers with urea prills as the core, and the technology of the coating process in a spout fluidized bed was developed. PTA coating could be formed rapidly by the fast oxidation of TA by an oxidation solution containing CuSO4 and H2O2. The coated urea release behavior was systematically studied in water and soil. In both water and soil, the release rate of nitrogen from coated urea is much slower than that from raw urea. Raw urea was completely dissolved within 30 min, while 27% of urea was released from coated urea. The pot experiments indicated that coated urea has a positive effect on the plant growth as well. Our results provide an effective method to prepare environment-friendly SRFs, indicating a promising application in sustainable agriculture.

Introduction

Rapid population growth has made food shortages severe, and global food demand is expected to double by 2050.1 Fertilizers have been used as crucial agrochemicals to supply plant nutrients, which play a pivotal role in maintaining food production sufficiently for the continuously growing population of the world. It has been reported that ∼70% of the nutrients of plant growth come from fertilizers.2 However, overuse of fertilizers has caused many serious environmental problems that have been a widespread concern, such as water, air, and soil pollution.3−5 For example, urea is widely used as a nitrogenous fertilizer. It is highly soluble in water, but the rapid and excessive release of nitrogen is harmful to plants. It may be converted to ammonia by an enzyme or microorganism, leading to easier volatilization and leaching to the natural environment. Therefore, a slow-release fertilizer (SRF) has been designed and developed to mitigate fertilizer pollution and to avoid plants from suffering damage from fast nutrient release.6−8

Compared with traditional fertilizers, SRFs have many advantages such as improving fertilizer utilization efficiency,9,10 reducing the frequency of fertilization,11 and alleviating the potential impact on the environment.12−14 Coated fertilizers are among the most developed SRFs. Generally, they are prepared by coating leakproof materials such as inorganic, synthetic, or natural polymers on a nutrient core.10 The nondegradability and nonrenewability of synthetic polymers are always accompanied with environmental pollution and additional manufacturing costs, which have limited the widespread application of SRFs. Biocompounds such as cellulose,15 starch,16 chitosan,17 and some superabsorbents10,18−20 have been widely used in SRFs due to their renewability and biodegradability.21 However, the coating materials with green, natural, and low-cost properties still have a big room for improvement. More recently, inspired by mussel adhesion, dopamine and catecholamines have been used for surface modification and functionalization of versatile materials. In our previous study, polydopamine (PDA) is a potential candidate to make coated SRFs,22−25 but it is expensive and does not meet the industrial utilization purpose and sustainable development. Tannic acid (TA), a type of commercial polyphenol, is usually extracted from plants, such as Tara pods, gallnuts, and Sicilian Sumac leaves. It has a similar adhesion property and forms a poly(tannic acid) (PTA) coating, which could be a substitute for PDA as a coating material of SRFs.

Various methods have been applied to prepare coated fertilizers, such as rotary drum, rotary turntable, and fluidized bed.26 The rotary drum and rotary turntable technologies have been widely used, but they have shortcomings. They require a large quantity of coating materials, generate local humidity, form holes, and collapse in the coating, leading to a rough coating with low repeatability and poor fertilizer efficiency.27 The fluidized bed can form a uniform coating by using melt or liquid coating materials, which can be controlled by adjusting coating conditions including coating cycles, temperature, and spraying rate.28,29 However, there are no reports on PTA-coated fertilizers made by a spout fluidized bed, which provides an economic and efficient method for the preparation of promising coated SRFs.

In this study, the oxidation rates of TA by various oxidation solutions were studied by UV–vis spectroscopy. PTA coating was formed on four different substrates by spraying technology to investigate the forming mechanism of PTA coating, which was analyzed by FT-IR, static water contact angles, and EDX. The morphology of the PTA coating was investigated by SEM and AFM. Urea, the most widely used nitrogen fertilizer, was used as a starting material to prepare the coated fertilizer by a single spout fluidized bed using optimized PTA formation conditions. Compared with industrial urea prills, the coated urea release rate was slower in water and soil. The release behavior of coated urea was evaluated by the pot experiment. Such results provide a promising methodology to prepare coated fertilizers and shine light on the development of new SRFs.

Materials and Methods

Materials

Urea prills (⌀ 1.18–3.35 mm, 46.4% N) were purchased from Xinlianxin Energy and Chemical Co., Ltd. (Xinjiang, China). Tannic acid (TA, 99%) was supplied by Sigma-Aldrich (Shanghai, China). Anhydrous cupric sulfate (CuSO4) was purchased from Shengao Chemical Reagent Co., Ltd. (Tianjin, China). Hydrogen peroxide (H2O2) was purchased from Yongsheng Fine Chemical Co., Ltd. (Tianjin, China). Sodium periodate (NaIO4) was purchased from Kelong Chemical Reagent Factory (Chengdu, China). Polyvinyl alcohol (PVA) was purchased from Shanghai Titan Technology Co., Ltd. (Shanghai, China). Glass, polyurethane (PU), polycarbonate (PC), poly(tetrafluoroethylene) (PTFE), and silicon rubber (Si-rubber) were received from Shenzhen Jinhongsheng Material Co. Ltd. (Shenzhen, China). Deionized water was used throughout the experiment. All other chemicals were analytical-grade and used as received.

Characterization

The absorbance change of the TA solutions at λ = 350 nm was recorded by a UV–vis spectrophotometer (UV-3200PCS, MeiPuDa Company, Shanghai, China). The contact angle of the substrate was tested by a water contact angle tester (DSA100, Kruss, Germany). The morphology of PTA coating deposited on the PC substrate was studied by scanning electron microscopy (SEM) (JSM-6490LV, Japan Electronics Co. Ltd.). The compositions of coating were analyzed by Fourier transform infrared spectroscopy (FT-IR, Nicolet iS10, Thermo Fisher Scientific, USA), energy dispersive X-ray (EDX) spectra, and elemental mapping (SEM with EDS detector). Atomic force microscopy (AFM, MultiMode 8, Bruker, Germany) was used to test the flatness and uniformity of coating. The thickness of the layers on the substrates was measured with an ellipsometer (L116s, Gaertner Scientific Corporation, USA).

PTA Coating Formation by Spraying Technology

The plane substrates were cut into 2 × 2 cm, washed with Milli-Q, sonicated in ethanol for 30 min, and then dried at room temperature. Then the substrates were prepared and fixed on an iron stand. The TA, NaIO4, and CuSO4 + H2O2 solutions were prepared freshly with DI water. TA and oxidation solutions were placed in an atomizer device in equal amounts. The angle and the distance between the atomizer nozzle and the substrate were fixed at 90° and 20 cm, respectively. Each spraying lasted for 5 s, which may be repeated to obtain suitable coating thickness. The substrates were washed with DI water and dried under a stream of air.

Preparation of PTA-Coated Urea

TA (15 or 25 mg/mL), PVA (1 mg/mL), and oxidation (CuSO4 + H2O2) solutions were prepared freshly. Then the PTA-coated urea granules were prepared by a single spout fluidized bed (WBF-2G, Chongqing Aibote Electromechanical Co. Ltd. China). The process parameters are as follows: the temperature was 45–50 °C, spray rate was 2–10 rpm/min, atomization pressure was 1.8–2.0 MPa, and the coating was ∼20 g per 500 g of raw urea granules. The PTA-coated SRFs were dried at 54–60 °C for 10 min in the spout fluidized bed.

Release Property of PTA-Coated Urea in Water

1 g of SRF was enclosed in a dialysis bag (MWCO = 100), placed in a conical bottle with 200 mL of DI water (pH = 6.8 ± 0.2), and incubated at 25 ± 0.5 °C in an oscillator (100 rpm/min, ZHEY-100D, Shanghai Zhicheng Analytical Instrument Manufacturing Co., Ltd., China). 2 mL of the solution was withdrawn at certain time periods, and 2 mL of fresh medium was added to maintain a constant volume of solvent. The release of coated urea was evaluated by the para-dimethylamino-benzaldehyde colorimetric method.30 Each sample has three parallel experiments.

Release Property of PTA-Coated Urea in Soil

A 300 mesh sieve was placed at the bottom of a PVC pipe having a diameter of 5 cm and a height of 30 cm. 15 cm (275 g) of soil was filled, then 2 g of fertilizer was added dispersedly, and 5 cm (125 g) of soil was covered to the top. A 250 mL container was attached to the bottom of the PVC tube. Initially, the soil was wet with a certain amount of water, and then 50 mL of water was added for a leaching cycle, which may be repeated until the urea was released completely. The released urea was measured with the Kjeldahl method by Kjeldahl apparatus (Foss 2300, Foss Tecator AB, Denmark). All the experiments were carried out in triplicate.

Results and Discussion

Oxidation Kinetics of TA in Solution

It is found that the phenolic derivatives could be oxidized by different oxidation solutions, such as Tris–HCl (pH = 8.5), H2O2, CuSO4, and NaIO4. First, the oxidation process of TA in various oxidation solutions was monitored by UV–vis spectroscopy, as shown in Figure 1. The increase of absorbance at around λ = 350 nm is attributed to the oxidation of phenolic hydroxyl of tannin to quinone31 (Figure S1). Similar results have been reported on the oxidation of dopamine.32 Among these results, the solution contains both CuSO4 and H2O2 and shows the fastest oxidation rate and the greatest absorbance increase at around λ = 350 nm, indicating the fastest TA oxidation. Such a phenomenon may be ascribed to a large number of active oxidants (O2– and HO2·) by using CuSO4 and H2O2.33 The concentrations of CuSO4 and H2O2 were optimized, as shown in Figure 1B,C. The optimum concentrations for CuSO4 and H2O2 are 17.8 and 19.6 mM, respectively. The solution contains NaIO4 and shows the second-fastest oxidation rate, which is optimized and selected as a comparison condition for further testing.

Figure 1.

Figure 1

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UV–vis absorbance curves as a function of time at 350 nm of TA (8.9 mM) at 26 ± 0.5 °C with (A) various oxidation solutions, their corresponding photographs, and a certain amount of TA; different proportions of (B) CuSO4, (C) H2O2, and (D) NaIO4.

PTA Coating on the Planar Substrate by Spraying Technology

PTA is a potential coating material to prepare SRF. It has been reported that catechol derivatives are easily deposited on the substrates by the dipping method.34,35 However, it requires a longer time (480 min), while the thickness of the coating is hard to control. In addition, urea, which is soluble in water, cannot be coated with PTA by dipping in aqueous solution. Spraying is an alternative method for PTA coating, as shown in Scheme 1. A solution containing TA was mixed with oxidation solution during spraying, which was sprayed on the substrate through a spraying gun with airflow. Such a process may be repeated until the desired thickness was achieved. Uncoated materials can be removed by rinsing at a predetermined time. After drying by vacuum, substrates with PTA coating were obtained, which was used for further studies. In this study, four kinds of substrates including PTFE, PU, PC, and glass were selected with different hydrophilicity.

Scheme 1. Deposition of PTA on the Substrate by Spraying Technology.

Scheme 1

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PTA coating can be easily and rapidly deposited on substrates by the spraying method, resulting in the surface color to turn into brownish, which are exhibited by digital photographs in Figure 2A. This color change indicates the formation of PTA coating. Figure 2B shows the FT-IR spectrum of PTA coating. The peaks at 3415, 1720, and 1617 cm–1 are attributed to the stretching vibrations of the phloroglucin of TA, C=O, and C=C of quinone formed by tannic acid oxidation, respectively. PTA coatings are able to efficiently induce the reduction of Ag+ and form Ag nanoparticles, leading to the color of the substrates to become blackish (Figure S2). This phenomenon is attributable to the reduction of PTA, indicating that the PTA was successfully coated on the surface of the substrates. After PTA deposition, the surface hydrophilicity may be different and caused a change of water contact angle. The difference in the water contact angle (Figure 2C) indicates that the interface becomes more hydrophilic. The thicknesses of the PTA on PC and silica substrates are 1511 and 516 nm for NaIO4 and 1309 and 547 nm for CuSO4 + H2O2 (Figure 2D), respectively.

Figure 2.

Figure 2

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(A) Digital photographs, (B) FT-IR spectra, (C) static water contact angles, and (D) thickness analysis of PTA on various substrates.

SEM was employed to investigate PTA morphology on the PC substrate, which is presented in Figure 3. The plain PC substrate was used as a control group. A rough surface with globular sediments was formed on the PC after treatment with NaIO4, while a smoother coating when using CuSO4 + H2O2 was formed. PTA coating on the substrates may induce Ag formation. After treatment with silver nitrate (AgNO3) solution, a large amount of Ag particles was found on the PC surface (Figure S3), indicating the successful PTA deposition. Ag was dispersed uniformly on PTA, as shown in the mapping analysis (Figure S3). EDX shows similar results that C, O, and Ag elements are present evenly on the coating. Besides, I and Cu were found by EDX in the coating deposited on the PC by NaIO4 and CuSO4 + H2O2, respectively. AFM and ellipsometry were used to measure the roughness (Figure 3) and thickness (Figure 2D and Figure S4) of PTA coating, which was consistent with SEM results. The average surface roughness (Rq) increases from 0.903 (plain PC) to hundreds of nanometers for these coatings deposited by NaIO4 and CuSO4 + H2O2 spraying, respectively. The coating prepared by CuSO4 + H2O2 shows a better uniformity, which may be attributed to the rapid polymerization of TA and the fast homogenous nucleation of PTA. The stability of PTA coating on the PC substrate surface was tested by treatment with strong base, acid, and acetone (Figure S5). The coating is more stable in acid and organic solvent than that in base. After 2 h of immersion, 2 mL of solution was withdrawn and diluted three times, and then its absorbance was recorded (Figure S5). In the same condition, the elution solution of PTA coating prepared by CuSO4 + H2O2 spraying shows a lower absorbance, indicating less PTA leakage. This spraying process simulates spraying technology of the single spout fluidized bed. Therefore, PTA-coated SRF was prepared by using CuSO4 + H2O2 as an oxidation solution in a spout fluidized bed.

Figure 3.

Figure 3

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AFM images of PTA coating on Si substrates (left), SEM images (middle), and mapping (right) of PTA coating on PC substrates.

Preparation of PTA-Coated Urea in the Spout Fluidized Bed

As shown in Scheme 2, the urea granules were suspended in hot airflow, which were coated by atomized droplets containing TA and oxidases in the spout fluidized bed. Coating thickness can be controlled by spraying times and cycles, while solvent evaporation can be used to control urea agglomeration. PTA-coated urea (PTA@urea) was successfully prepared by using CuSO4 + H2O2 to oxidize TA (Figure S6). The PTA@urea has a black coating, which is consistent with the results of PTA coating on the flat substrates. SEM was used to examine the surface and cross-sectional morphology of PTA coating on urea (Figure 4). The urea particle has a rough and irregular surface (Figure 4A,D). As shown in Figure 4B, PTA@urea a good core–shell structure where the urea prills were coated with PTA (Figure 4B). The PTA coating has a smoother surface (Figure 4E) in which thickness is 50 ± 5 μm (three spraying cycles, Figure 4B inset). However, some cracks and holes were found on the PTA coating. Poly(vinyl alcohol) (PVA), a water-soluble synthetic polymer, has been widely used in a variety of coatings due to its excellent film-forming, emulsifying, and adhesive properties.36 PVA was used as an additive during the preparation of coated urea, which was defined as PVA + PTA@urea. The SEM image of PVA + PTA@urea (Figure 4C,F) is similar to PTA@urea, which presents that the PTA layer was well coated on urea particles. Interestingly, there are no pores and cracks on the PVA + PTA coating (Figure 4F). The PVA may serve as a cross-linking site during the PTA formation by the oxidation of CuSO4 + H2O2.

Scheme 2. Preparation of PTA-Coated Urea in the Spout Fluidized Bed.

Scheme 2

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Figure 4.

Figure 4

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SEM images of (A) urea, (B) fracture surfaces of PTA@urea and (C) PVA + PTA@urea, and surfaces of (D) urea, (E) PTA@urea, and (F) PVA + PTA@urea; insets are the high-magnification images of fracture surfaces of the corresponding SRFs.

Release Behavior of PTA-Coated Urea

The release property is the most important parameter for coated urea. Urea release from PTA@urea and PVA + PTA@urea in water and soil is shown in Figure 5. Urea was quickly dissolved in water and almost completely released within 30 min. However, 32% urea was released from PTA@urea and 27% from PVA + PTA@urea at the same predetermined time (Figure 5A). This result is in agreement with SEM studies. The defect of PTA coating may cause a faster release, while a denser coating made by PVA and PTA may slow the urea release rate. Urea could be released completely from PTA@urea and PVA + PTA@urea within 240 and 420 min, respectively. Urea formed SFRs in soil shows similar results (Figure 5B), which indicated that the urea is released slower from PVA + PTA@urea than that from PTA@urea.

Figure 5.

Figure 5

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Urea release from PTA@urea and PVA + PTA@urea in (A) water and (B) soil. Urea used as control.

Pot Experimental Analysis

Pot experiments were conducted for the effectiveness of coated urea on plant growth. The digital photos show the 30 days’ growth of the cotton plant (Figure 6). Compared with control and plant growth with urea, the plant grew faster with coated urea. Details are listed in Table 1. The plant fertilized with PVA + PTA@urea has shown better parameters than those with water and urea. The improvement of cotton production may be attributed to the following reasons. First, urea is released from coated urea in a slow and sustained manner, which has a positive effect on the growth of the cotton plant. Second, PTA may induce nitrogen fixation,32 promoting and accelerating plant growth. Finally, the biodegradable coating may be used as a carbon source for beneficial bacteria and microbes to enhance soil respiration.

Figure 6.

Figure 6

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Digital photos of cotton plant growth by treatment with water (control), urea, and PVA + PTA@urea for 30 days.

Table 1. Germination Rate and Plant Growth Parameters.

fertilizer germination rate (%) plant height (cm) root length (cm) fresh weight (mg) dry weight (mg)
control 83 ± 4.3 8.8 ± 3.9 3.9 ± 0.2 635 ± 70 41 ± 1
urea 83 ± 2.4 9.9 ± 0.5 5.8 ± 0.4 760 ± 40 53 ± 1
PVA + PTA@urea 90 ± 3.6 13.4 ± 0.6 7.1 ± 0.2 858 ± 30 68 ± 3
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Conclusions

In this work, PTA-coated urea was developed as a kind of environment-friendly SRF using a spout fluidized bed. In planar substrate PTA coating studies, PTA was rapidly formed on various substrates using a simple spraying technology by mixing of TA and oxidation solutions. PTA prepared by the oxidation of CuSO4 and H2O2 showed a thicker and smoother coating, which was selected to prepare PTA-coated urea. The thickness of PTA on the urea core was controlled by the spraying cycles. The PTA-coated urea release is significantly slower than raw urea prills. The PTA-coated urea shows a positive effect on cotton plant growth in the pot experiments. Such environment-friendly SRFs prepared by a spout fluidized bed with simple spraying technology might facilitate the development of new SRFs and broaden their application in modern green and sustainable agriculture and horticulture.

Acknowledgments

This study was supported by the National Natural Science Foundation of China (U1703351, 51663021), Bingtuan Excellent Young Scholars (CZ027205), and Bingtuan Science & Technology Nova Program (2018CB012).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.9b03310.

  • Experimental procedures, characterization, and other experimental details (PDF)

Author Contributions

§ Y.W. and H.G. contributed equally to this work. The manuscript was written through contributions of all the authors. All the authors have given approval for the submission of the manuscript.

The authors declare no competing financial interest.

Supplementary Material

ao9b03310_si_001.pdf (617.9KB, pdf)

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