**1. Introduction**

Fertilizers play a vital role to supplement the nutrients required for plant growth [1]. However, plants require varying amounts of nutrients during different phases of growth. They need lesser amounts at the infancy and higher quantities at the development of roots, stalk and stem. Fertilizer demands therefore change intermittently throughout the plant growth. However, uncoated fertilizers are susceptible to losses from volatilization, leaching and fixation, and researchers have found that only about 30% of the fertilizers are used by plants while the rest is lost [2]. Such losses have adverse environmental impacts due to the addition of excess nutrients to air

and water [1, 3]. Similarly, excess fertilizer spoils seedlings as only a small quantity of fertilizer is needed during sprout development [4–6].

Although coated urea provides a much longer release time and higher utility rate, it is mostly used in developed countries. It has not been popular in developing countries because of its higher cost [7]. It is noteworthy to say that developing countries, specifically Nigeria, consume more and more nitrogen fertilizer and yet have only 20–35% efficiency of nitrogen use.

Controlled release fertilizer (CRF) is a prominent green technology that helps to reduce the adverse effect of fertilizer on the environment. A controlled-release fertilizer (CRF) is a granulated fertilizer that releases nutrients gradually into the soil. The low solubility of the chemical compounds in the soil moisture determines the slowness of the release of the CRF. As conventional fertilizers dissolves in aqueous medium, the nutrients diffuse as quickly as the fertilizer dissolves. However, as controlled-release fertilizers are not readily water-soluble, their nutrients diffuse into the soil more slowly. The fertilizer granules may possess an insoluble substrate or a semi-permeable jacket that hindered the dissolution while allowing nutrients to flow out. Therefore, it is necessary to develop new types of fertilizers (CRF) that can improve nitrogen (N) use efficiency, sustain crop production, and protect the environment. Among newly developed commercial fertilizers, waterborne starch biopolymer-coated urea has great potential [8].

Starch is among the cheap, biodegradable, and abundantly available natural polysaccharide [9]. Starch alone is not realistic to be used as coating material due to its profound hydrophilic nature and poor mechanical properties [10]. Therefore, starch can be modified with some suitable additional agents to obtain coating materials devoid of these discrepancies. There is a plethora of studies for the use of starch as a coating material to produce controlled release devices [11, 12].

Coating film thickness plays a significant role for better controlled release properties [13]. The nutrient release time is a function of the diffusional path that the dissolved nutrient has to pass through and release from inside of the coating shell to the bulk water it is immersed in [14]. Critical review of literature reveals that viable controlled release properties of CRU are achieved mostly at the cost of nonbiodegradability of coating material, process complexity, and elevated price [15].

Nutrient release from coated CRFs is generally controlled by diffusion through the coated layer [1, 16] and numerous recent studies have focused on predicting nutrient release behaviour. The simplest approach is regression reported and used in [17–21]. Kochba et al. [17] employed a semi-empirical model where the release of CRF was found to be a first-order process. However, the effects from geometry and size were ignored. Moreover, they failed to account for the lag period. Gandeza et al. [18] developed a semi-empirical model to investigate the effect of soil temperature on nutrient release from CNR-polyolefin-coated urea using a quadric equation: *CNR* <sup>=</sup> <sup>α</sup> <sup>+</sup> <sup>β</sup>(*CT*) <sup>+</sup> <sup>γ</sup>(*CT*<sup>2</sup> ). Wang et al. [22] also studied the effect of temperature on the release rate by using a regression model that reduced experimental time from days to hours. However, each of these models only related to a specific coating material. Recently, Azeem et al. [7] studied the effect of coating thickness on waterborne starch biopolymer coated urea, the study did not account for the lag, constant and decay release characteristics, expressly. In another account, Lubkowski [23] established that sigmoidal equation best correlate constant and decay release kinetics of chitosan coated fertilizer. A model developed by Than et al. [24] only simulates the second stage of nitrogen release from a coated urea.

From the foregoing, a clear understanding of lag, constant and decay characteristics of coated urea is still lacking. **Figure 1** illustrates the dissolution model of a spherical urea particle surrounded by water defined as the fluid zone. Urea particles comprise two parts: the urea core and the coating layer.

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(India).

**Figure 1.**

*Lag, Constant and Decay Release Characteristic of St-PVOH Encapsulated Urea as a Function…*

In this research, it is presumed that the coating layer was saturated with water at the time (t0) of initial release i.e. lag period. Water inside the core initiates dissolving of the urea granules, where the concentration of the urea is kept constant at a saturated level provided that the solid urea is still inside the core. Nitrogen, from the urea, starts its release via the coating layer by diffusion at a constant rate and it is called "constant release" stage. When urea granule in the core is completely dissolved, urea concentration decreases, and a "decay release" stage starts and then proceeds to the end of the process. From on Raban's experiments, Shaviv et al. [16] suggested that the release profile from a single polymer coated granule had a sigmoidal shape comprising of these three stages. The cost of the coated controlled release fertilizer has made its usage limited and a paradigm shift has been observed on the frontiers to; use cost effective, environmentally friendly material to produce CRF and their nutrient release characteristics (lag, constant and decay). This research, therefore, covers the production of CRU using polyvinyl alcohol modified starch biopolymer as encapsulation material. The effect of coating thickness on the nitrogen release time and characteristics using different empirical models (power,

Fertilizer granules were procured from Eke Awka market in Awka, Anambra state. The granular fertilizer was subjected to sieve analysis to get a uniform range of the granules. For this study, granules of 2–2.8 mm were selected. The granules were carefully sealed to evade leaching on exposure to the atmosphere. Analytical grade polyvinyl Alcohol (PVOH) was obtained from CDH® and acetic acid were procured from JHD®. Refined starch was gotten from Burgoyne Burbidges & Co

exponential and sigmoidal law) were also investigated.

*Dissolution model of a spherical urea particle in water environment.*

**2. Materials and methods**

**2.1 Materials and pre-treatments**

*DOI: http://dx.doi.org/10.5772/intechopen.83729*

*Lag, Constant and Decay Release Characteristic of St-PVOH Encapsulated Urea as a Function… DOI: http://dx.doi.org/10.5772/intechopen.83729*

**Figure 1.** *Dissolution model of a spherical urea particle in water environment.*

In this research, it is presumed that the coating layer was saturated with water at the time (t0) of initial release i.e. lag period. Water inside the core initiates dissolving of the urea granules, where the concentration of the urea is kept constant at a saturated level provided that the solid urea is still inside the core. Nitrogen, from the urea, starts its release via the coating layer by diffusion at a constant rate and it is called "constant release" stage. When urea granule in the core is completely dissolved, urea concentration decreases, and a "decay release" stage starts and then proceeds to the end of the process. From on Raban's experiments, Shaviv et al. [16] suggested that the release profile from a single polymer coated granule had a sigmoidal shape comprising of these three stages. The cost of the coated controlled release fertilizer has made its usage limited and a paradigm shift has been observed on the frontiers to; use cost effective, environmentally friendly material to produce CRF and their nutrient release characteristics (lag, constant and decay). This research, therefore, covers the production of CRU using polyvinyl alcohol modified starch biopolymer as encapsulation material. The effect of coating thickness on the nitrogen release time and characteristics using different empirical models (power, exponential and sigmoidal law) were also investigated.

## **2. Materials and methods**

#### **2.1 Materials and pre-treatments**

Fertilizer granules were procured from Eke Awka market in Awka, Anambra state. The granular fertilizer was subjected to sieve analysis to get a uniform range of the granules. For this study, granules of 2–2.8 mm were selected. The granules were carefully sealed to evade leaching on exposure to the atmosphere. Analytical grade polyvinyl Alcohol (PVOH) was obtained from CDH® and acetic acid were procured from JHD®. Refined starch was gotten from Burgoyne Burbidges & Co (India).

*Microencapsulation - Processes, Technologies and Industrial Applications*

of fertilizer is needed during sprout development [4–6].

have only 20–35% efficiency of nitrogen use.

biopolymer-coated urea has great potential [8].

second stage of nitrogen release from a coated urea.

comprise two parts: the urea core and the coating layer.

and water [1, 3]. Similarly, excess fertilizer spoils seedlings as only a small quantity

Although coated urea provides a much longer release time and higher utility rate, it is mostly used in developed countries. It has not been popular in developing countries because of its higher cost [7]. It is noteworthy to say that developing countries, specifically Nigeria, consume more and more nitrogen fertilizer and yet

Controlled release fertilizer (CRF) is a prominent green technology that helps to reduce the adverse effect of fertilizer on the environment. A controlled-release fertilizer (CRF) is a granulated fertilizer that releases nutrients gradually into the soil. The low solubility of the chemical compounds in the soil moisture determines the slowness of the release of the CRF. As conventional fertilizers dissolves in aqueous medium, the nutrients diffuse as quickly as the fertilizer dissolves. However, as controlled-release fertilizers are not readily water-soluble, their nutrients diffuse into the soil more slowly. The fertilizer granules may possess an insoluble substrate or a semi-permeable jacket that hindered the dissolution while allowing nutrients to flow out. Therefore, it is necessary to develop new types of fertilizers (CRF) that can improve nitrogen (N) use efficiency, sustain crop production, and protect the environment. Among newly developed commercial fertilizers, waterborne starch

Starch is among the cheap, biodegradable, and abundantly available natural polysaccharide [9]. Starch alone is not realistic to be used as coating material due to its profound hydrophilic nature and poor mechanical properties [10]. Therefore, starch can be modified with some suitable additional agents to obtain coating materials devoid of these discrepancies. There is a plethora of studies for the use of

). Wang et al. [22] also studied the effect of temperature on the

release rate by using a regression model that reduced experimental time from days to hours. However, each of these models only related to a specific coating material. Recently, Azeem et al. [7] studied the effect of coating thickness on waterborne starch biopolymer coated urea, the study did not account for the lag, constant and decay release characteristics, expressly. In another account, Lubkowski [23] established that sigmoidal equation best correlate constant and decay release kinetics of chitosan coated fertilizer. A model developed by Than et al. [24] only simulates the

From the foregoing, a clear understanding of lag, constant and decay characteristics of coated urea is still lacking. **Figure 1** illustrates the dissolution model of a spherical urea particle surrounded by water defined as the fluid zone. Urea particles

starch as a coating material to produce controlled release devices [11, 12]. Coating film thickness plays a significant role for better controlled release properties [13]. The nutrient release time is a function of the diffusional path that the dissolved nutrient has to pass through and release from inside of the coating shell to the bulk water it is immersed in [14]. Critical review of literature reveals that viable controlled release properties of CRU are achieved mostly at the cost of nonbiodegradability of coating material, process complexity, and elevated price [15]. Nutrient release from coated CRFs is generally controlled by diffusion through the coated layer [1, 16] and numerous recent studies have focused on predicting nutrient release behaviour. The simplest approach is regression reported and used in [17–21]. Kochba et al. [17] employed a semi-empirical model where the release of CRF was found to be a first-order process. However, the effects from geometry and size were ignored. Moreover, they failed to account for the lag period. Gandeza et al. [18] developed a semi-empirical model to investigate the effect of soil temperature on nutrient release from CNR-polyolefin-coated urea using a quadric equation:

**112**

*CNR* <sup>=</sup> <sup>α</sup> <sup>+</sup> <sup>β</sup>(*CT*) <sup>+</sup> <sup>γ</sup>(*CT*<sup>2</sup>

#### **2.2 Preparation of starch-polyvinyl alcohol (St-PVOH) bio-polymer**

Preparation of St-PVOH was done using two round bottom flasks. 10grams of polyvinyl alcohol was dissolved in 100 ml of deionized water at 90°C while aqueous solution of 10grams of starch dissolved in 100 ml of deionized water is added. A constant temperature of 90°C was maintained and stirred for 90 minutes to obtain a homogeneous hydrogel. The mixture was then allowed to cool to room temperature and an aqueous solution of acetic acid was added and stirred for another 90 minutes. A 25 grams by mass of starch was added to the resultant mixture to enhance its handling ability. The synthesized biopolymer was properly covered, kept at room temperature and consequently used for the coating of the granular urea.

#### **2.3 Synthesis of St-PVOH encapsulated urea (St-PEU)**

The St-PVOH coated urea was developed by creating balls of the synthesized biopolymer with the hands and fingers. Thereafter, the urea granule was encapsulated into each balls such that, the urea forms the core and the biopolymer, the sheath. These were repeated for five different coating thicknesses and then dry with the oven. A Constant diameter of the five different coating thicknesses was obtained by sieve analysis.

#### **2.4 Determination of coating thickness of St-PEU granules**

Vanier calipers were used to obtain the external diameter of the St-PVOH coated fertilizer. The average diameter of the urea granules were then subtracted from the measure diameter to obtain the thinking coating.

#### **2.5 Determination of nutrient (nitrogen) release time from St-PEU granules**

The dissolution of St-PEU in distilled water was studied by determining the time it took for the nitrogen present in the sample to be completely released (i.e. 100% release). As described in [7], two grams of each sample was immersed in a 200 ml of distilled water in a carefully sealed beaker. The period taken for the entire nitrogen released from the different coating thickness per time by using the Kjeldahl method was recorded. The experimental data related to release degree of mineral components from the starting fertilizer and from all prepared materials were described and interpreted with three kinetic equations: power, exponential and sigmoidal (Eqs. (1)–(3)):

$$\frac{M\_t}{M\_{\text{ov}}} = k\_{p^\*} t^n \tag{1}$$

$$\frac{M\_t}{M\_m} = \mathbf{1} - \exp\left(-k\_\varepsilon \cdot t\right) \tag{2}$$

$$\frac{M\_t}{M\_{ov}} = \frac{a-b}{\mathbf{1} + \exp\left(\frac{t-d}{c}\right)} + b \tag{3}$$

**115**

*Lag, Constant and Decay Release Characteristic of St-PVOH Encapsulated Urea as a Function…*

Starch is hydrophilic in nature. The mechanical properties of starch are improved when it is blended with PVOH. Nevertheless, according to Izhar et al. [25], since both starch and PVOH are polar substances with hydroxyl group in their chemical structure, the blend of starch and PVOH exhibits poor water retaining properties. Therefore, acetic acid was introduced as a cross linker which enables the formation of intermolecular and intramolecular hydrogen bonds between hydroxyl groups of PVOH and starch to improve the integrity of St-PVOH blends. The chemical structure of St-PVOH blends can further be explained by the FTIR analysis as shown in **Figure 2**. The FTIR spectra of starch and PVOH were studied and compared with the spectra of the St-PVOH blend film. **Figure 2** shows the occurrence

) in the starch granules. For PVOH, a broad band at 3400–3100 cm<sup>−</sup><sup>1</sup>

tallinity-dependent; this is in agreement with the work of Jayasekara et al. [26]. The FTIR spectra displays the following three sets of changes in the film: (a) the peak

peak of PVA weakened; and (c) the absorption peak of 922 cm<sup>−</sup><sup>1</sup>

The coated urea was characterized using SEM to compare their morphologies to the original urea obtained. Morphology refers to the study of form and structure of a material and in this case, CRCU. SEM images were taken from the samples to present the morphology. As stated earlier, it must be noted that the starch is hydrophilic in nature due to the structure and composition. The SEM image of uncoated and coated urea (CRCU) is shown in **Figure 3** (a and b, respectively). The optical image for the uncoated urea is observed to appear rough and have fine openings where water can penetrate so as to dissolve the urea granule. An asymmetric structure and porosity are clearly seen at the magnification of ×1000. A seemingly decrease in membrane porosity, ordered and uniform layer, however, were noticed in **Figure 3**. This may be as a result of encapsulation with the biopolymer (St-PVOH). The slower the penetration time, the longer it will take for the urea to dissolve and consequently escape to the outer surface of the sample. This characteristic is needed for increasing the encapsulation efficacy of the released material, which in this research is urea. The elementary nanolayers of the bio-composite can be observed in the SEM image of **Figure 3a**. Using the image, it is difficult to determine the actual interlayer spacing of the biopolymer. In order to determine the d-spacing, highresolution transmission electron microscopy would be needed. From **Figure 3b**,

to be sensitive to the degree of crystallinity in PVOH. The peak at 1147 cm<sup>−</sup><sup>1</sup>

 shifted. The weakness, vanishing, and shifting of the characteristic absorption bands may be as a consequence of the interaction of different -OH groups in the starch and PVA molecular chains. It can be concluded therefore from these results that the starch was linked with PVA by chemical bonding introduced by acetic acid. This type of linkage has significant effect on the enhancement of compatibility.

region characterizes a number of modes, which have been shown

of absorption bands weakened; (b) the crystallinity-dependent

), the intramolecular hydrogel (1646 cm<sup>−</sup><sup>1</sup>

) as well as C-O-C ring vibration (928,

assigned as C-H stretching

denote C-C stretching. The

)

due to

is crys-

of PVA

*DOI: http://dx.doi.org/10.5772/intechopen.83729*

**3.1 Chemical formation of St-PVOH**

of -OH stretching vibration (3426 cm<sup>−</sup><sup>1</sup>

vanished and the peak of 854 cm<sup>−</sup><sup>1</sup>

**3.2 Scanning electron microscope (SEM)**

858 cm<sup>−</sup><sup>1</sup>

1147 cm<sup>−</sup><sup>1</sup>

1200–1100 cm<sup>−</sup><sup>1</sup>

at 3300–3400 cm<sup>−</sup><sup>1</sup>

and -CH2OH stretching vibration (1260 cm<sup>−</sup><sup>1</sup>

O-H stretching vibration and another band at 2936 cm<sup>−</sup><sup>1</sup>

vibration were seen. The absorption points of 922 cm<sup>−</sup><sup>1</sup>

**3. Results and discussion**

where *M*t*/M*∞ is the fraction of nutrients released at time *t*, *k*p and *n* are the power equation constants, *k*e is the exponential equation constant, and *a, b, c, d* are the sigmoidal equation constants.

*Lag, Constant and Decay Release Characteristic of St-PVOH Encapsulated Urea as a Function… DOI: http://dx.doi.org/10.5772/intechopen.83729*
