A urea-loaded hydrogel comprising of cellulose nanofibers and carboxymethyl cellulose An effective slow-release fertilizer

Prévia do material em texto

Journal of Cleaner Production 434 (2024) 140215
Available online 22 December 2023
0959-6526/© 2023 Elsevier Ltd. All rights reserved.
A urea-loaded hydrogel comprising of cellulose nanofibers and 
carboxymethyl cellulose: An effective slow-release fertilizer 
Priya E a, Akash Jha b, Sudipta Sarkar a,b,*, Pradip K. Maji c,** 
a Centre for Nanotechnology, Indian Institute of Technology Roorkee, Roorkee, 247667, Uttarakhand, India 
b Department of Civil Engineering, Indian Institute of Technology Roorkee, Roorkee, 247667, Uttarakhand, India 
c Department of Polymer & Process Engineering, Indian Institute of Technology Roorkee, Saharanpur Campus, Saharanpur, 247001, Uttar Pradesh, India 
A R T I C L E I N F O 
Handling Editor: Jian Zuo 
Keywords: 
Biodegradable 
Cellulose nanofibers 
Slow-released urea 
Seed germination 
Water absorption 
and retention 
A B S T R A C T 
Conventional fertilizers used worldwide to increase crop yield have several environmental and economic issues 
resulting from their quickly soluble nature, causing most of the fertilizers to be lost through agricultural field 
runoff and leachate. Utilizing slow or controlled-release fertilizers may reduce the issues associated with con-
ventional fertilizers. However, many commercially available variants of slow-release formulations are ecologi-
cally unsustainable as they are made of non-biodegradable polymers. In the present study, a hydrogel composed 
of cellulose nanofibers (CNF) and carboxyl methyl cellulose (CMC) was synthesized for use as a biodegradable 
and environmentally sustainable host for carrying urea within its matrix and released the same slowly over a long 
period of time. The swelling measurements showed that the thermally stable urea-loaded CNF/CMC hydrogel 
(UCNF) had a high water absorption capacity of 147 g/g in distilled water owing to its porous morphology and 
the presence of various polar groups. When exposed to a dry environment, it released the absorbed moisture 
slowly over a period of more than 2 weeks. The urea release rate was also relatively slow; it took about 30 days to 
release the urea loaded in UCNF. The results indicated that urea diffusion from the material was non-Fickian in 
character and fit well with the pseudo-second-order kinetics model. The hydrogel demonstrated excellent 
biodegradability; more than 80% by weight of the hydrogel disappeared through biodegradation by soil bacteria 
within 3 months. The plant study revealed that adding UCNF to the soil effectively enhanced the seed germi-
nation and growth of the plant. Also, plants treated with urea-loaded hydrogel had better growth than the pure 
urea- and control plant samples. The study found that crosslinked and functionalized cellulose nanofiber may 
effectively transport water and fertilizer slowly and sustainably. 
1. Introduction 
Population growth is driving the growing food demands around the 
globe. Additionally, dwindling land fertility compels farmers to use 
excessive mineral fertilizers to enhance crop yield and meet demand 
(Guo et al., 2021; Rop et al., 2018; Wen et al., 2017). Urea is arguably 
the most widely utilized fertilizer, often used for nitrogen supplemen-
tation due to its high nitrogen content (Lin et al., 2021; Shaghaleh et al., 
2022). Despite its distinct advantages, urea has disadvantages - it has 
low thermal stability, high solubility, and low molecular weight, 
resulting in its tendency to vaporize quickly to the atmosphere and to be 
lost through runoff and leachate into aquatic systems (Zhou et al., 2018). 
Excessive use of fertilizers in agricultural fields can result in 
environmental degradation and increased production costs caused by 
the leaching of nutrients beyond the plant’s requirements (Dhanapal 
et al., 2021; Guo et al., 2021; Zonatto et al., 2017). Researchers have 
used superabsorbent hydrogels (SAHs) as a slow-release fertilizer (SRF) 
system in agriculture to overcome these drawbacks. SAHs are 
three-dimensional crosslinked polymer networks. They are insoluble in 
water and can absorb and retain large volumes of aqueous solutions or 
biological fluids within their structure, even under high-pressure con-
ditions (Bao et al., 2011; Tally and Atassi, 2015). Due to their cross-
linked structure, SAHs are stable in various media and environmental 
conditions (Pourjavadi et al., 2004). They have gained significant 
attention as hydrophilic materials suitable for agricultural applications 
in arid and semi-arid climatic conditions (Bora and Karak, 2022). 
* Corresponding author. Centre for Nanotechnology, Indian Institute of Technology Roorkee, Roorkee, 247667, Uttarakhand, India. 
** Corresponding author. 
E-mail addresses: sudipta.sarkar@ce.iitr.ac.in (S. Sarkar), pradip@pe.iitr.ac.in (P.K. Maji). 
Contents lists available at ScienceDirect 
Journal of Cleaner Production 
journal homepage: www.elsevier.com/locate/jclepro 
https://doi.org/10.1016/j.jclepro.2023.140215 
Received 1 August 2023; Received in revised form 29 November 2023; Accepted 14 December 2023 
mailto:sudipta.sarkar@ce.iitr.ac.in
mailto:pradip@pe.iitr.ac.in
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Journal of Cleaner Production 434 (2024) 140215
2
Superabsorbent hydrogels have been synthesized using diverse mate-
rials, such as inorganic constituents and synthetic and natural polymers. 
Notwithstanding their advantages, SAHs encounter various chal-
lenges. The fertilizers often take a long time to degrade under natural 
conditions; the synthesis process suffers from problems such as poly-
merization inhibition; excessive use of methanol calls for the by-product 
removal before the fertilizer can be used in the field. Moreover, often, 
there is a quicker release of nutrients (Salimi et al., 2021). These unfa-
vorable factors necessitate exploring alternative environmentally 
friendly, biodegradable SRFs that can be synthesized using environ-
mentally sound, sustainable resources and production methods (Man-
zoor Ghumman et al., 2022). Polysaccharides are a crucial ingredient in 
the hydrogels industry, supporting the polymer network and imparting 
other qualities like biodegradability. Polysaccharide-based slow-release 
fertilizers have been found to tackle environmental and economic issues 
effectively. These SRFs facilitate the efficient utilization of fertilizers by 
steadily releasing mineral nutrients readily absorbed by crops (Di Mar-
tino et al., 2021; Liu et al., 2022). In other words, the 
polysaccharides-based hydrogel disassembles if the chains of poly-
saccharides are broken, and this is due to the polysaccharide’s suscep-
tibility to biodegradation by microorganisms. This property makes the 
polysaccharide-based hydrogels suitable for soil usage as a biodegrad-
able system for regulating nutrient release (Baldrian and Valášková, 
2008). Furthermore, the incorporation of polysaccharide materials has 
been reported to enhance various properties of superabsorbent mate-
rials, such as swelling capacity, gel strength, acid or alkaline resistance, 
temperature sensitivity, nutrition release, and thermal (Hua and Wang, 
2009; Liang et al., 2009). In contrast, synthetic superabsorbents based 
on polymers are produced through the synthesis of chemical substances 
such as poly (acrylic acid), poly (vinyl alcohol), and poly-N-isopropyl 
acrylamide (Madduma-Bandarage and Madihally, 2021). The synthetic 
materials in question have poor biodegradability, implying that hydro-
gels made from them do not readily undergo degradation processes. 
Several studies have examined the application of environmentally 
friendly cellulose-based hydrogels for slow-release fertilizers due to the 
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https://doi.org/10.1021/acssuschemeng.7b01721
https://doi.org/10.1016/j.indcrop.2020.112558
https://doi.org/10.1016/j.carbpol.2017.02.049
https://doi.org/10.1016/j.carbpol.2017.02.049
https://doi.org/10.1016/j.scitotenv.2017.09.084
https://doi.org/10.1016/j.scitotenv.2017.09.084
https://doi.org/10.1016/j.ijbiomac.2017.07.051
https://doi.org/10.1016/j.ijbiomac.2017.07.051
	A urea-loaded hydrogel comprising of cellulose nanofibers and carboxymethyl cellulose: An effective slow-release fertilizer
	1 Introduction
	2 Experimental methods
	2.1 Materials
	2.2 Synthesis of cellulose nanofibre(CNF)
	2.3 Preparation of urea-loaded CNF/CMC hydrogel (UCNF)
	2.4 Material characterization
	2.5 Water absorbance capacity of the synthesized hydrogel
	2.6 Water retention analysis
	2.7 Urea loading and release study
	2.8 Swelling kinetics
	2.9 Biodegradation test
	2.10 Soil burial method of biodegradation
	2.11 Plant growth experiment
	2.12 Cost analysis
	3 Result and discussion
	3.1 Characterization of slow-release hydrogel containing urea (UCNF)
	3.1.1 Fourier transform infrared Spectroscopy(FTIR)
	3.1.2 Field-emission scanning electron micrograph (FESEM) and X-ray diffraction (XRD) analysis
	3.1.3 Thermo-gravimetric analysis (TGA)
	3.1.4 Discussion: property, structure, and morphology of CNF and UCNF
	3.2 Water absorption and retention behavior of the UCNF hydrogel
	3.3 Biodegradability study
	3.3.1 Biodegradability test with a pure bacterial strain
	3.3.2 Soil burial test
	3.4 Studies on the urea release by UCNF
	3.5 Plant study
	3.6 Cost analysis
	4 Conclusion
	CRediT authorship contribution statement
	Declaration of competing interest
	Data availability
	Acknowledgments
	Appendix A Supplementary data
	Referencesimproving soil quality 
(Kabiri et al., 2010; Lin et al., 2021). Cellulose is a naturally occurring 
crystalline polymer that exhibits favorable mechanical properties as a 
carrier/host. It can be obtained from diverse natural sources such as flax, 
straw, cotton, bamboo, wood, bacteria, and algae. Compared to con-
ventional cellulose, cellulose nanofibers (CNF) present distinct advan-
tages as a promising and environmentally-friendly constituent at the 
nanoscale due to their nanometer-scale diameter and increased specific 
surface area (Shaghaleh et al., 2021; Wang et al., 2021). Also, cellulose 
nanofiber-based hydrogels exhibit exceptional physical properties due 
to their complex three-dimensional nanofiber networks. Due to their 
high absorptive capacity, they can serve as reservoirs for carrying 
additional water and nutrients to agricultural fields (Abe and Yano, 
2011). According to the standard definition, hydrogels are complex 
structures consisting of polymeric networks in three dimensions (3D) 
established through physical or chemical crosslinking of polymer 
(Ahmed, 2015). The formation of nanoscale 3D networks of nanofibers 
within the CNF-based hydrogel offers unique chemical and physical 
properties due to the formation of hydrogen bonds. The material ex-
hibits essential characteristics such as high strength and stiffness, hy-
drophilic properties, and a surface that can be readily modified (Alhaj 
Hamoud et al., 2023). The exceptional physicochemical characteristics 
of CNFs make them a highly favorable constituent for constructing 
advanced functional hydrogels with superior performance. Cellulosic 
hydrogels have emerged as a promising option for large-scale applica-
tions for cost-effective feedstock and environmentally friendly fertilizer 
formulations (Alhaj Hamoud et al., 2023; Guo et al., 2021). 
In this article, we report the synthesis, characterization, water up-
take, and urea and water release characteristics of a biodegradable urea- 
loaded CNF/CMC hydrogel (UCNF) and validation of the UCNF hydrogel 
as a slow-release plant growth-promoting fertilizer. The hydrogel has 
been synthesized through green processing of pine wood scraps to pro-
duce cellulose nanofibers and crosslinking with epichlorohydrin in the 
presence of carboxymethyl cellulose used as a hydrophilic filler in the 
hydrogel matrix to improve its water uptake capacity. During the gela-
tion process, the hydrogel imbibed the urea solution within its matrix, 
and the process did not use any catalyst, nor was there any requirement 
for any energy-intensive post-purification or post-processing step. This 
article also presents the swelling characteristics of UCNF, water reten-
tion capability, and urea release characteristics. We also report the re-
sults of the study on the biodegradability of the material and on 
validation as a plant germination and growth-promoting fertilizer. 
2. Experimental methods 
2.1. Materials 
Waste wood scraps, the starting material for the preparation of cel-
lulose nanofiber, were collected from a timber manufacturing company 
located in Saharanpur, Uttar Pradesh, India. All other chemicals used in 
this study were of reagent grade and were procured as follows: dimethyl 
sulfoxide (DMSO), toluene from Avantor Performance Materials India 
Limited, ethanol from Changshu Hongsheng Fine Chemical Co., Ltd., 
sodium chlorite (NaClO2), potassium hydroxide (KOH), urea, sodium 
hydroxide (NaOH), and carboxymethyl cellulose sodium salt (CMC) 
with 1500–3000 cp (1% in water at 25 ◦C) from HiMedia Laboratories 
Pvt. Ltd. And glacial acetic acid from Sisco Research Laboratories Pvt. 
Ltd. These reagents were used without further purification. 
2.2. Synthesis of cellulose nanofibre (CNF) 
Cellulose fibers (CF) and cellulose nanofibers (CNF) were obtained 
following a process described by (Chhajed et al., 2019; Yadav et al., 
2017) with some modifications. Briefly, wood fibers were first dewaxed 
using a soxhlet extraction apparatus at 50 ◦C for 8 h to remove impu-
rities like waxes, pectin, and resin, followed by cleaning and drying, and 
bleaching with a 6% solution of acidified sodium chlorite at 75 ◦C for 4 h 
to obtain hollocellulose, which is cellulose fibers with white color. The 
bleaching step was repeated twice. Next, the fibers were de-lignified by 
subjecting them to two rounds of treatment with a 4% KOH (potassium 
hydroxide) solution at 90 ◦C for 2 h each, after which the pure CFs were 
obtained. A 0.2 wt percent CF suspension was then defibrillated to the 
nanoscale through homogenization using a REMI (RQ-140-D) homoge-
nizer for 3h, followed by sonication using a probe sonicator (UP400S 
Hielsher Ultrasonics GmbH, Germany, 24 kHz) 20 min while main-
taining an ice water bath to prevent sample burning from elevated 
temperatures. After sonication, the milky suspension of CNF was 
centrifuged at ambient temperature for 15 min using a Laby T-70 B L 
centrifuge (India) to obtain a concentrated solid content of 2 wt% (w/w) 
of CNF. Finally, the synthesized CNF was stored at 4 ◦C for subsequent 
use. A suspension with a solid content of about 0.2% was subjected to 
centrifugation at a speed of 4500 revolutions per minute (rpm) for 20 
min to separate the nanofibrillated material. The yield of the produced 
nanofibres was calculated using Equation (1) (Besbes et al., 2011; 
Yadav et al., 2017). 
Yield (%)=
(
1 −
weight of dried sediment
weight of diluted samples × % solid content
)
× 100 (1) 
2.3. Preparation of urea-loaded CNF/CMC hydrogel (UCNF) 
Cellulose nanofibers (CNF) and carboxymethyl cellulose (CMC) were 
mixed at 1:1 ratio to make 5% (w/v) concentration in a solution con-
taining 4% NaOH and 4% urea under continuous stirring to ensure the 
formation of a homogeneous solution. 3 mL of epichlorohydrin was 
mixed into the homogeneous solution under gentle stirring at 30 ◦C for 
30 min to form the crosslinking. Subsequently, the solution was 
P. E et al. 
Journal of Cleaner Production 434 (2024) 140215
3
subjected to a temperature of 45 ◦C for 5 h to facilitate the gelation 
process to form hydrogel of CNF/CMC with urea imbibed. The resulting 
hydrogels were washed with water to remove excess reagents and im-
purities. A schematic representation of the synthesis process of the UCNF 
hydrogel is provided in Fig. 1. From the material balance calculations 
performed during several synthesis campaigns, it was observed that 
there was on average 0.375 g urea/g of UCNF. The same was confirmed 
through accelerated desorption experiments using DI water. 
2.4. Material characterization 
The UCNF hydrogel samples were dried and crushed into tiny beads 
before casting them into KBr pellets for analysis through Fourier trans-
form infrared spectroscopy (FTIR) (PerkinElmer FTIR spectrum two, 
USA). Each spectrum was acquired by performing 16 scans on each 
sample within a wavenumber range of 4000-400 cm− 1, with a resolution 
of 4 cm− 1. The structural morphology of the hydrogel was evaluated 
using Field Emission Scanning Electron Microscopy (Carl-Zeiss Gemini 
FESEM), operated at an accelerating voltage of 5–10 kV and a working 
distance of 10 mm. The thermal decomposition characteristics of the 
hydrogel were analyzed using a TGA apparatus (NETZSCH TG209 F3 
Tarsus, Germany) with a consistent heating rate of 10 ◦C/min in the 
presence of nitrogen gas with a flow rate of 10 mL/min. The XRD pattern 
was recorded at a 4◦/min scanning rate using Cu-k radiation as the X-ray 
source over an angular range of 5–80◦ and 40 kV (XRD, Rigaku Ultima 
IV, Japan). The samples were dried in an oven before testing. 
2.5. Water absorbance capacity of the synthesized hydrogel 
0.5 g of dried UCNF hydrogel was submerged in deionized water at 
ambienttemperature. The hydrogels absorbed water and swelled. The 
swollen hydrogels were periodically extracted from the aqueous solu-
tion, gently cleansed with filtered paper, and subsequently weighed, 
following the method reported by (Bora and Karak, 2022). The weighing 
and measuring process continued until the swollen hydrogel reached a 
constant weight. The water absorption capacity was calculated using 
Equation (2). 
Water absorption (g / g)=
[
Ms− M0
Mo
]
(2) 
Where Mo and Ms are masses of the dry hydrogel and swollen hydrogel 
after time t, respectively, all the experiments were performed in tripli-
cate, and the mean values were used for analysis. 
Additionally, the swelling capacity of UCNF was also evaluated in 
different salt concentrations, specifically 0.01% and 0.05% for NaCl, 
KCl, NH4Cl, and CaCl2 (Rodrigues et al., 2012). The swelling test was 
conducted in triplicate, and the result shown is the average of the three 
values measured. Additionally, the dimensionless salt sensitivity factor 
(f) was calculated (Alam and Christopher, 2018) as per Equation (3). 
f = 1 −
Ws
Ww
(3) 
Where Ws is the equilibrium swelling capacity in salt solution, and Ww is 
the equilibrium swelling capacity in distilled water. 
2.6. Water retention analysis 
The water retention behavior of the synthesized hydrogel material 
was analyzed using the gravimetric technique, following the method 
described by (Kiran et al., 2019). Dry UCNF weighing 0.5 g was allowed 
to absorb water at room temperature under ambient conditions of 
neutral pH until it reached constant equilibrium weight, at which the 
water imbibition was maximum. The swollen UCNF hydrogel was then 
exposed to ambient conditions (25 ◦C and 60% RH) for drying and 
weighed until a stable mass was achieved. The water retention per-
centage was calculated using Equation (4), where WR, Mo, and Mt stand 
for the water retention ratio, dry hydrogel mass, and mass of the 
hydrogel at time t. Water retention was measured thrice, and the 
calculated average of the three outcomes is presented in the result. 
WR(%)=
[
Mt− M0
Mo
]
× 100 (4) 
2.7. Urea loading and release study 
Urea concentration was determined by a colorimetric method pre-
viously reported (Dutta and Karak, 2018; Sarmah and Karak, 2020). The 
colorimetric reagent of p-dimethylaminobenzaldehyde (P-DMABH) re-
acts with urea to produce a complex that makes a greenish-yellow color 
in the acidic environment by Ehrlich reaction, which was used to mea-
sure the urea concentration in water. The p-DMABH and urea complex 
absorbs light at 430 nm in the visible spectrum. Initially, a standard 
calibration curve was plotted using absorbance values obtained using a 
UV–visible spectrophotometer with urea solutions of six known con-
centrations (0.25M, 0.125M, 0.062M, 0.031M, 0.016M, and 0.008M). 
For the urea release study, the 5 g of UCNF (containing 1.8 g urea) 
was submerged in a beaker with 1000 mL of sterile DI water kept under 
Fig. 1. A schematic showing the synthesis process of the hydrogel loaded with urea (UCNF). 
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Journal of Cleaner Production 434 (2024) 140215
4
insulated conditions under a nitrogen atmosphere with gentle stirring. 
At regular intervals, 1 mL of the solution was collected to determine urea 
concentration, and 1 mL of sterile water was added to the beaker, 
thereby maintaining a constant volume of the solution. The urea con-
centration in the collected sample was determined using the above 
method and against the calibration curve obtained. The percentage of 
urea released was calculated using Equation (5). 
Urea release (%)=
Mt
M1
× 100 (5) 
Where, Mt is the mass of urea released from hydrogel at time t, and M1 is 
the total mass of urea loaded on the UCNF hydrogel. 
2.8. Swelling kinetics 
The UCNF hydrogel’s water swelling capacity and absorption rate 
were examined using swelling kinetics. In order to conduct this experi-
ment, 0.5g of hydrogel was submerged for various time periods (10, 20, 
30, 40, 50, 60, 90, 120, and 240 min) in excess distilled water. After the 
specified period, the swollen hydrogel was filtered out to remove the 
remaining water. The mass of the hydrogel after water absorption was 
then measured, and Equation (2) was used to calculate the swelling 
capacity of water at a specific time. The general sorption-time re-
lationships for a polymeric hydrogel can be modeled using the following 
Equations 6 and 7. 
Wt
Weq
= ktn (6) 
log
(
Wt
Weq
)
= logk + n log t (7) 
Where Wt stands for the weight of the hydrogel at a particular time t, and 
Weq is the equilibrium weight of the swollen hydrogel, k and n are 
constants. The n and k parameters were determined from the slope and 
intercept of the linear regression curve, resulting from plotting the 
logarithm of the ratio (Wt/Weq) against log(t), respectively. The value of 
n obtained in this study can indicate the mechanism of water diffusion 
during the swelling process. The Fickian mechanism of diffusion is 
observed when the value of nwas quantified by observing the weight loss of the UCNF hydrogel over 6 
weeks. This was achieved by removing the sample from the medium, 
rinsing it with water, and drying and weighing it. The weight remaining 
was thus measured, and the weight loss was calculated using following 
Equation (9). 
Weight loss(%)=
[
Wo− Wt
Wo
]
× 100 (9) 
Where, W0 and Wt are the weights of the swollen UCNF hydrogel 
initially and after time t, respectively. 
2.10. Soil burial method of biodegradation 
The biodegradability of the UCNF hydrogel was determined at actual 
conditions by using the soil burial test. For this purpose, 0.5 g of the 
dried hydrogel sample was introduced into paper cups, each holding 60 
g of the soil sample. The soil samples obtained from the field were 
subjected to a sun-drying process for three days, followed by manual 
removal of unwanted dirt particles. Subsequently, 40 mL of DI water was 
introduced into each cup, then their submersion into the soil at a depth 
ranging from 8 to 9 cm. After a specific pre-determined time, the 
hydrogel sample was recovered from the soil and cleaned using water to 
eliminate any adhered soil particles. It was dried, and the remaining 
weight was measured. The following Equation (10) determined the 
percentage biodegradation of the UCNF hydrogel: 
Percentage hydrogel remaining (%)=
[
Wi− Wf
Wi
]
× 100 (10) 
Where, Wi represents the initial weight, and Wf denotes the weight after 
biodegradation within the soil. 
2.11. Plant growth experiment 
The performance validation of the slow-release UCNF hydrogel as an 
effective fertilizer was done by observing the impact of the hydrogel on 
the germination and growth of wheatgrass plants in a pot experiment 
performed under a controlled environment. The soil that was used in the 
experiments was a mixture of silt and sand particles with a pH value of 
7.08. The carbon content of the soil was analyzed to be 0.42%, with a 
total content of organic matter of 0.73% and an electrical conductivity of 
0.174 mS/cm. The plant growth studies were conducted using five 
groups, each with three pots as replicates. 50 number seeds of Triticum 
aestivum (wheatgrass) were sown in each pot at a depth of 3 cm below 
the soil surface. The pots in the control group had untreated soil, which 
was used as the benchmark for comparison purposes. The test groups 
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Journal of Cleaner Production 434 (2024) 140215
5
consisted of two groups: a) plants grown in pots having soil with 1 and 3 
g of UCNF, which had approximately 0.4 and 1.2 g of urea loaded on 
them, respectively (1 g of UCNF contained approximately 0.4 g of urea) 
and b) plants grown in pots treated with 0.4 and 1.2 g of pure urea. The 
UCNF hydrogel was buried at a depth of 5 cm below the soil surface. 
After 14 days, the wheatgrass plants were taken out, and a total of 40 
plants were randomly selected from the three pots in each group. 
Further, their fresh weight and the length of their shoots and roots were 
measured and recorded. The collected grasses were dried, and their dry 
weights were recorded. All the results were statistically analyzed. 
2.12. Cost analysis 
Evaluation of cost associated with material and energy is important 
for finding a competitive market price for a new product. Material and 
energy balance analysis was conducted on the synthesis of UCNF from a 
20 g batch of raw fiber. The tentative material and energy balance cal-
culations are mentioned in the supplemental data. 
3. Result and discussion 
3.1. Characterization of slow-release hydrogel containing urea (UCNF) 
The present study involved the extraction of cellulose nanofibers and 
the subsequent synthesis of a urea-loaded CNF/CMC (UCNF) slow- 
release hydrogel. Epichlorohydrin (ECH), a commonly used crosslinker 
in various biopolymers, cellulose, and starch, has been used here (Chang 
et al., 2008; De Miguel et al., 1999). The yield of CNF from the raw fibers 
and the extracted CF was 41% and 31%, respectively. After a second 
processing of sedimented fibers, the yield of CNF from raw fibers 
increased to 73%. CNF plays a crucial role in maintaining the structural 
integrity of the synthesized hydrogel, whereas the addition of CMC aids 
in gelation and binding in the material and also helps in facilitating 
improved water absorption capabilities. Therefore, it is essential to 
maintain an appropriate balance of both constituents to ensure the 
hydrogel’s structural stability and water absorption properties. Among 
the number of variants tested, the UCNF hydrogel with a 1:1 ratio of CNF 
and CMC showed the most favorable results in terms of structural 
strength and water absorption properties, and therefore it was chosen 
for further studies. The photographs of UCNF hydrogel with different 
ratios of CNF and CMC are shown in Fig. S1. 
3.1.1. Fourier transform infrared Spectroscopy(FTIR) 
FTIR spectroscopy was conducted to gain information about the 
possible chemical structures of the hydrogel. The spectra of raw fiber 
(RF), chemically purified cellulose fibers (CF), and cellulose nanofiber 
(CNF) were analyzed and are presented in Fig. 2a. All three samples - RF, 
CF, and CNF showed an absorbance peak at around 3400 cm− 1, asso-
ciated with the stretching vibration of free–OH groups. The presence of 
this group signifies that all three samples were hydrophilic. The ab-
sorption bands at 898 and 1636 cm− 1 were linked to the glycosidic 
linkage (–C1–O–C4) between the cellobiose unit and the bending of O–H 
due to water adsorption, respectively (Maiti et al., 2013; Mandal and 
Chakrabarty, 2011; Yadav et al., 2017). In the spectra of RF, a carbonyl 
(− C=O) stretching vibration of acetyl and uronic ester group from 
pectin/hemicellulose was observed at 1735 cm− 1 (Chhajed et al., 2022). 
The same was absent in the spectra of CF and CNF, confirming that the 
purification step during the synthesis process has successfully elimi-
nated these impurities. The band at 1509 cm− 1 represents the –C=C 
stretching vibration of the aromatic ring present in lignin, while the 
band at 1462 cm− 1 is associated with –CH3 asymmetric bending, and the 
band at 1263 cm− 1 belongs to C–O stretching vibration, indicating the 
presence of aryl groups in the lignin structure of the raw fiber (Chhajed 
et al., 2022; Maiti et al., 2013; Mandal and Chakrabarty, 2011; Yadav 
et al., 2017). The FTIR spectra of CF and CNF did not show all the bands 
mentioned above, indicating the effective elimination of components 
such as lignin and hemicellulose. Fig. 3a shows the comparative FTIR 
spectra of CNF and urea-loaded CNF (UCNF). The FTIR of CNF showed 
the presence of a peak at 2925 cm− 1, indicating that C–H stretching 
vibration and peak at 1424 cm− 1 confirmed the carboxyl group as a salt 
(Mondal et al., 2015). The peak at wave numbers 1675 cm− 1 and 790 
cm− 1 of the FTIR spectra of UCNF confirmed the presence of C=O 
stretching vibration and C=O and NH bending vibration, respectively 
(Ski, 1969) arising out of the presence of urea inside the hydrogel. 
3.1.2. Field-emission scanning electron micrograph (FESEM) and X-ray 
diffraction (XRD) analysis 
The surface morphology of RF, CF, and CNF was investigated using 
FESEM analysis, presented in Fig. 2b. It may be observed that the 
diameter of the RF was approximately 150–200 μm. The morphological 
characteristics of the extracted CFs show that the extracted CF had a 
symmetrically dense structure attributed to hydrogen bonding. The fiber 
dimensions of the extracted CFs were 10–25 μm. The cellulose structure 
Fig. 2. (a) FTIR spectra and (b) SEM images at different magnifications of raw fiber (RF), chemically purified cellulose (CF) and cellulose nanofibers (CNF). 
P. E et al.Journal of Cleaner Production 434 (2024) 140215
6
with high-density CF was defibrillated from a micro to a nano range by 
applying high-intensity sonication waves, and the SEM of the resulting 
CNF may also be observed in Fig. 2b. The high magnification image of 
CNF reveals that most of the fibers had a diameter in the nanoscale 
range. Fig. 3c shows the SEM of the surface morphology and the cross- 
section of the UCNF. The porous nature of the UCNF can be readily 
observed. The X-ray diffraction (XRD) pattern presented in Fig. S2 shows 
the urea’s well-defined peaks at approximately 2θ = 22.8◦, 25.18◦, 
30.1◦, 32.32◦, and 35.9◦, along with two minor peaks at around 2θ =
45.60◦ and 55.60◦(Arafa et al., 2022). XRD pattern of the UCNF 
hydrogel exhibited a significantly wider peak at approximately 2θ = 22◦, 
which can be attributed to the presence of urea. Additionally, two sig-
nificant peaks at 2θ = 32◦ and 35.8◦ along with two minor peaks at 
around 2θ = 45.80 and 56.80◦ were observed, which are also associated 
with the presence of urea. The observed peaks in the UCNF are com-
parable to those found in pure urea, suggesting that the urea has been 
effectively incorporated into the synthesized hydrogel. 
3.1.3. Thermo-gravimetric analysis (TGA) 
The thermal decomposition experiment was carried out to examine 
the thermal degradation of CNF, CMC, CNF/CMC, pure urea, and UCNF 
at different temperatures. Fig. 3b shows the thermograms and their 
corresponding first derivatives (DTG) of CNF, CMC, CNF/CMC, pure 
urea, and UCNF. The TGA graphs indicate weight losses at 800 ◦C for 
CNF, CMC, CNF/CMC, pure urea, and UCNF as 100%, 86%, 94%, 100%, 
and 83%, respectively. These values represent the weight loss observed 
after heating the materials up to 800 ◦C. The primary breakdown of 
cellulose nanofibers (CNF) takes place at temperatures above 250 ◦C, 
which can be linked to the degradation and decomposition of the 
chemical structure of cellulose. At temperatures below 250 ◦C, the pri-
mary mechanisms responsible for degradation processes involve the 
dehydration of water and the generation of peroxides, which function as 
catalysts for the degradation of cellulose. Degradation at temperatures 
above 250 ◦C exhibits faster rates and distinct characteristics than 
degradation at lower temperatures. Within this specific range of tem-
perature, the hydrogen bonds are destroyed, resulting in a modification 
of the crystalline structure. This process is responsible for generating 
free radicals, carbonyl, and carboxyl groups, thus enhancing cellulose 
degradation (Borsoi et al., 2016). Carboxymethyl cellulose (CMC) has 
higher heat stability than CNF. Above 160 ◦C, the temperature at which 
carboxymethyl cellulose (CMC) shows a 5% initial weight loss. The 
subsequent reduction in weight was observed between the temperature 
range of 250 ◦C to 300 ◦C, followed by the end of the third stage, 
occurring approximately between 560 ◦C to 620 ◦C. The subsequent 
weight loss steps were ascribed to the dehydration of saccharide rings 
and the disintegration of the primary cellulose chain (El-Sakhawy et al., 
2019; Shang et al., 2023). The decomposition process of urea took place 
in three distinct steps, as can be observed from the TGA and DTG curves. 
The first stage of thermal decomposition exhibited a substantial mass 
reduction of approximately 79%, which took place in the temperature 
range from 135 ◦C to 260 ◦C. The phenomenon can be described as the 
process of urea decomposition, which is further superposed by the for-
mation and decomposition of biurets (Xiang et al., 2020). The second 
stage of decomposition, which had a mass loss of about 16%, involves 
breaking down cyanuric acid in the temperature range from 260 ◦C to 
328 ◦C (Brack et al., 2014; Eichelbaum et al., 2010). Also, some 
ammelide, ammeline, and melamine might have been created. These 
small amounts break down in the final step, and the mass loss is around 
5% (Brack et al., 2014; Eichelbaum et al., 2010). On the other hand, the 
TGA/DTG curve of UCNF clearly displays four distinct stages of weight 
loss when it undergoes thermal degradation. The dried UCNF lost about 
10 % of its weight at a temperature of 179 ◦C, which can be attributed to 
the evaporative loss of water molecules inside it. Additionally, the UCNF 
hydrogel undergoes thermal degradation at temperatures between 
180 ◦C and 260 ◦C, resulting in a weight loss of approximately 22%. The 
weight loss occurred because the urea in the hydrogel broke down, and 
the bonds between the polymer and crosslinker in the hydrogel network 
were destroyed (Mohammadi-khoo et al., 2016). The disintegration of 
carboxymethyl cellulose, cellulose, and degradation of the remaining 
urea was attributed to the subsequent reduction in weight during the 
third phase, which occurred at a temperature range of 257 ◦C–340 ◦C 
and resulted in a weight loss of approximately 32% (Chang et al., 2010; 
Fig. 3. (a) FTIR spectra of (a) CNF/CMC and UCNF; (b) TGA and DTG curve of CNF, CMC, CNF/CMC, pure urea, and UCNF; (c) SEM images of UCNF hydrogel 
showing surface and cross-sectional morphology. 
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Journal of Cleaner Production 434 (2024) 140215
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Shang et al., 2023). The broad fourth peak was observed due to the 
degradation of complex molecules, which might have been formed by 
the reaction of CNF, CMC, and epichlorohydrin in the solution, and it is 
degraded over the range of temperature between 357 ◦C and 536 ◦C, 
resulting in a weight loss of approximately 9 %. At a temperature of 
800 ◦C, a residual amount of 17% of the UCNF hydrogel remained, 
which may be due to the presence of CMC within the hydrogel. The 
TGA/DTG results showed that the UCNF was thermally more stable than 
pure urea. 
3.1.4. Discussion: property, structure, and morphology of CNF and UCNF 
The SEM images Fig. 2b indicate that the nanofibrils of CNF were 
successfully extracted from pine wood scraps. During the chemical 
treatment, the primary and secondary cell walls of the wood scrap or 
raw fiber underwent breakdown, leading to the dissolution of non- 
cellulosic components such as lignin, hemicellulose, and pectin, leav-
ing behind the pure cellulose fibers (CF). Micro-nano-dimensioned CNFs 
can form complex structures through hydrogen bonding, resulting in an 
ultrafine and uncharged entangled structure. The FTIR results of RF, CF, 
CNF, and UCNF reveal some essential aspects of the structural frame-
work of these materials. First, the results indicate that the chemical 
cleaning procedure was able to eliminate impurities present in RF like 
pectin, hemicellulose, and lignin; almost impurity-free CF thus produced 
could be nano-fibrillated to create the UCNF through further cross-
linking using epichlorohydrin (ECH) as a crosslinker. The addition of 
CNF and CMC in the NaOH/urea solution leads to hydrogen-bond- 
induced inclusion complex formation. Epichlorohydrin is added to 
create a crosslinked structure, and urea remains within the final product, 
a network-like structure with all the components chemically bonded 
together. The hydrogel created has chemical stability, and the urea ex-
hibits favorable interactions with carboxymethyl cellulose (CMC) and 
cellulose nanofibers (CNF). The gelation phenomenon in poly-
saccharides such as cellulose mainly occurs through the reaction be-
tween the ECH and hydroxyl (OH) groups on the polymer chain. As a 
crosslinking agent, ECH provides many beneficial properties to the 
resultant material - hydrogels have increased pore size, improved water 
retention ability, enhanced chemical stability, and superior mechanical 
strength (Kittipongpatana and Kittipongpatana,2013). Fig. 4 depicts the 
suggested pathway as to how ECH crosslinks cellulose and CMC in an 
alkaline medium. The hydroxyl group on CF or CMC reacts with NaOH 
to form an alcoholate anion in a basic medium. The alcoholate anion 
(RO− ) from cellulose or CMC attacks the epichlorohydrin molecule to 
start the reaction. The alcoholate anion attacks the carbon atom 
attached to chlorine in epichlorohydrin. Alcoholate anion attacks 
epichlorohydrin, displaces chloride ion (Cl− ), and forms an ether link-
age. More epoxide forms when the chloride ion (Cl− ) reacts with water 
(H2O) after being displaced from the monoether. The water hydroxyl 
group replaces the chloride ion, forming a new epoxide molecule. Then, 
the new epoxide reacted with another alcoholate anion and complete 
the crosslinking process (Chang et al., 2010). Furthermore, the proposed 
reaction suggests that cellulose in cellulose nanofiber form within the 
hydrogel provides robust structural support, thereby preserving its ri-
gidity. Cellulose nanofibers have unique qualities, such as high me-
chanical strength and a large surface area. The urea is trapped within the 
hydrogel during the gelation process. The interconnected crosslinked 
structure in the hydrogel hinders the quick dissolution and release of 
urea from the hydrogel. Crosslinking produces a homogenous porous 
framework in the hydrogels, which is advantageous for more water 
absorbance and subsequent retention. The porous nature allows for easy 
diffusion and storage of water molecules, increasing swelling behavior. 
The porous hollow structure allows the urea solution to stay within the 
hydrogel only to be released slowly through diffusion along with the 
degradation of the hydrogel structure. The FTIR results confirm the 
presence of urea within the UCNF hydrogel in pure and unaltered form. 
3.2. Water absorption and retention behavior of the UCNF hydrogel 
The UCNF hydrogel contains hydrophilic –OH and ionizable 
carboxylate (-COOH) groups, making the material hydrophilic. There-
fore, it can absorb water and swell when placed in a polar solvent such as 
an aqueous solution. The dry hydrogel was immersed into different 
aqueous solutions containing deionized water and different amounts of 
common salts, such as 0.01% and 0.05% NaCl, KCl, NH4Cl, and CaCl2 
solution, to check for the water absorption capacity of the hydrogel. 
Fig. 5a provides the UCNF’s water absorption capacity over time when 
placed in solutions with different salinity at neutral pH. It may be 
observed that the highest degree of swelling occurred when the UCNF 
hydrogel was placed in DI water. The maximum water absorption ca-
pacity was 147 g/g, about 150 times greater than the initial weight of 
the dry UCNF. The swelling capacity decreased when placed in solutions 
Fig. 4. Proposed mechanism of the crosslinking reaction of epichlorohydrin (ECH) with CNF and CMC to produce UCNF hydrogel. 
P. E et al. 
Journal of Cleaner Production 434 (2024) 140215
8
of higher salinity. This phenomenon of decreased water absorption of 
UCNF hydrogel in media with higher salinity indicates that the water 
absorption depends on the osmotic pressure differential between the 
UCNF hydrogel and the surrounding media. The effect of different ion 
types on the swelling ability of the UCNF is shown in Table 1. The type of 
salt present in the water significantly impacts the hydrogel’s water ab-
sorption capacity. Among the salt solutions, a higher degree of swelling 
occurred when the UCNF was placed in NH4Cl salt and lowest for the 
CaCl2 salt solution. Monovalent cations possessing a lower charge and 
exhibiting a larger ionic radius have a favorable impact, resulting in the 
attainment of maximum swelling. Conversely, cations possessing a 
divalent charge and exhibiting a smaller ionic radius exert a relatively 
adverse influence, leading to less swelling. The sequence in which the 
cations exhibit swelling is as follows: ammonium (NH+
4 ) > potassium 
(K+) > sodium (Na+) > calcium (Ca2+) (Alam and Christopher, 2018; 
Rizwan et al., 2021). Moreover, this phenomenon can be explained by 
considering the exchange capabilities of several cations with varying 
valencies. There is a positive correlation between the valency of a cation 
and its exchange capacity, whereby an increase in cation valency leads 
to a corresponding increase in exchange capacity. The observed phe-
nomenon can be attributed to the enhanced complexing ability resulting 
from the coordination between multivalent cations, carboxylate, and 
hydroxyl groups present in UCNF. The process of ionic crosslinking 
primarily occurs on the surface of particles, resulting in their rubbery 
and very rigid nature upon swelling in solutions having Ca2+ ions 
(Cândido et al., 2013; Mahdavinia et al., 2004). Additionally, The ab-
sorption capacity of the hydrogel has an inverse relationship with the 
salt sensitivity factor (f) (Alam et al., 2019). 
In order to learn the process of water transportation in agriculture, it 
is vital to investigate the swelling kinetics of CNF/CMC hydrogel loaded 
with urea. Fig. S3 shows a log (Wt/Weq) versus log (t) plot for the water 
absorption data obtained for the UCNF hydrogel when soaked in DI 
water. An excellent fit was observed, with the correlation coefficient 
value (r2 = 0.98) close to 1. The values of n and k obtained from the 
fitted model were 0.651 and 0.019, respectively. Further, swelling ki-
netics parameters are indicated in Table S1. The Fickian diffusion 
mechanism is found to be the rate-controlling step when the value of n 
non-Fickian diffusion, reaching 
saturation may be considerably prolonged. The phenomenon of mois-
ture sorption can be observed in two separate stages, usually known as 
dual-stage sorption. The formation of hydrogen bonds between water 
molecules and hydrophilic polymer chains can also account for the 
occurrence of non-Fickian behavior. This binding and molecules’ sub-
sequent expansion and deformation can result in non-Fickian diffusion 
behavior. Conversely, unbound water molecules within micro- and 
macro gaps are responsible for generating Fickian diffusion behavior. A 
plot of t/Wt versus t with the experimentally obtained data presented in 
Fig. S4 helped to determine the kinetic parameters. A linear plot with a 
high correlation coefficient (r2 = 0.99) was obtained, signifying that the 
water absorption and swelling followed pseudo-second-order kinetics. 
The Weq value of UCNF hydrogel was theoretically determined to be 147 
g/g, almost the same as the experimentally obtained equilibrium 
swelling capacity of the hydrogel. The ks value was also 7.64 × 10− 5 g g−
1min− 1 and is indicated in Table S1. 
Fig. 5c shows the water retention behavior of the fully swollen UCNF 
hydrogel. The graph depicts almost a linear decline in the quantity of 
imbibed water over 16 days when kept under room conditions (25 ◦C 
and 60% RH). Inside the soil matrix, the temperature would be lower 
than the room temperature, and RH values shall be higher than the 
ambient. Hence, in practice, the CNF hydrogel would retain water longer 
than that observed in the laboratory experiment. Therefore, when used 
in arid and semi-arid regions, the UCNF hydrogel shall release water 
slowly so that there is no water shortage in the plants’ root zone for an 
extended period. Thus, the UCNF would benefit sustainable agriculture 
in arid and semi-arid regions. Fig. 5d shows the photographs of a dry 
UCNF and a fully swollen UCNF. The extent of swelling can be quickly 
figured out. 
3.3. Biodegradability study 
The material’s biodegradability is an essential virtue that makes 
UCNF hydrogel a more sustainable SRF than other commercial variants. 
We employed two methods to test its biodegradability – a biodegrad-
ability test with pure strains of bacteria under controlled conditions and 
a natural biodegradability test by keeping UCNF under soil burial. 
3.3.1. Biodegradability test with a pure bacterial strain 
The biodegradability of the UCNF hydrogel was tested using two 
pure species of bacteria, namely Bacillus subtilis and Pseudomonas aeru-
ginosa. Each species of bacteria was allowed to grow in a nutrient me-
dium containing inorganic micronutrients, in which a specific weight of 
UCNF hydrogel was added so that the UCNF hydrogel was the only 
source of organic carbon. Without any other organic material present, 
the biodegradation of UCNF would provide the carbon source required 
for the growth of the bacteria within the medium. A control experiment 
was also run where UCNF hydrogel was added only to a sterilized 
nutrient medium without containing any bacteria. The bacterial growth 
was indirectly measured by observing changes in the optical density 
(OD) of the medium at 600 nm wavelength with respect to the control. 
Fig. 6a shows the gradual increase in the turbidity as measured by the 
OD of the bacterial medium, indicating a gradual increase in bacterial 
populations over 6 weeks. Fig. 6b shows the percentage of the initial 
weight remaining over time for the UCNF hydrogel placed within the 
bacterial medium. There was a gradual decrease in the weight of the 
UCNF hydrogel, indicating that the material underwent biodegradation 
over time. At the end of the 6 weeks, there was a significant reduction in 
the weight of the UCNF. The SEM of the UCNF hydrogel after being kept 
for 6 weeks within the control and in the two types of bacterial media, 
are shown in Fig. 7. It is evident that compared to the control, the UCNF 
has undergone significant fragmentation inside the bacterial media. 
Figs. 6 and 7 show that B. subtilis strain was more effective in the 
biodegradation of the hydrogel than the P. aeruginosa strain. 
3.3.2. Soil burial test 
The natural biodegradability of UCNF hydrogel was tested through 
the soil burial experiments conducted by putting a known weight of fully 
hydrated and swelled UCNF within the soil for varying time intervals. A 
biodegradable substance has the ability to be broken down into simpler 
compunds by naturally occurring decomposers. The material under 
consideration should possess nontoxic characteristics and can undergo 
decomposition within a short period. Within soil, the process of 
biodegradation can occur in three distinct stages: biodeterioration, 
biofragmentation, and assimilation. The process of biodeterioration in 
materials is a consequence of several degradative factors, including 
thermal degradation, mechanical degradation, and degradation result-
ing from the presence of oxygen, moisture, environmental pollutants, 
and ultraviolet radiation (Arshad et al., 2014). As a consequence of the 
Fig. 6. (a) Growth kinetics of B. subtilis and P. aeruginosa on UCNF hydrogel; (b) Weight residual percentage of UCNF hydrogel due to bacterial breakdown vs. 
incubation period (c) Weight loss of UCNF after 3 months (biodegradation by soil burial method). 
P. E et al. 
Journal of Cleaner Production 434 (2024) 140215
10
aforementioned circumstances, a substantial quantity of microbes ad-
heres to the surface of biodegradable material. Biofragmentation is a 
biological phenomenon characterized by the proliferation of microor-
ganisms which subsequently release enzymes and free radicals to facil-
itate the degradation of complex macromolecules into smaller units such 
as oligomers, monomers, and dimers. During the process of assimilation, 
microbes generate energy, new biomass, and different metabolites while 
simultaneously releasing simple gaseous molecules and mineral salts 
into the surrounding environment (Arshad et al., 2014; Falkiewicz-Dulik 
and Michalina, 2010). The UCNF hydrogels were taken out at specific 
time intervals and weighed so that the percentage weight loss of the 
hydrogel over various burial times could be calculated. Fig. 6c shows the 
percentage weight loss of the swollen UCNF over time after it was placed 
under the soil. It may be observed that after 30 days, there was a loss of 
only about 34% of the initial weight of the material due to biodegra-
dation. After 90 days, a weight loss of about 81% was observed. 
3.4. Studies on the urea release by UCNF 
The measurements performed during the synthesis indicated that the 
UCNF had 92 wt% of urea loaded on the hydrogel. To observe the 
dissolution kinetics of urea from the UCNF, a known weight of UCNF 
was placed in sterile water under a nitrogen atmosphere with gentle 
stirring, and a small volume of a sample of the solution was taken 
regularly to determine urea content. Each time a sample was taken, it 
was replaced by the same amount of sterile water to keep the total so-
lution volume constant. Fig. 8 shows the time history of the percentage 
of initial urea content in the UCNF hydrogel released in the solution. It 
may be observed that during the initial phase, up to 5 days, there was 
only a 15% release of urea. Between days 5 and 20, over 60% of the urea 
was released, whereas, between days 20 and 30, nearly 90% was 
released. UCNF being a hydrogel, the increased water absorption results 
in the diffusion of a larger quantity of water into the hydrogel network, 
which subsequently leads to the discharge of a greater quantity of urea 
molecules. In addition, a densely interconnected structure can effec-
tively retain urea molecules, leading to a gradual releasefrom the 
hydrogel matrix. The change in osmotic pressure between the inside and 
outside of the gel is the cause of the urea being released by the hydrogel 
and diffusing throughout the porous network structure. Table 2 is a 
comparison between the percentage of urea release from UCNF and 
other slow-release fertilizers at specific points in time. 
3.5. Plant study 
Once the slow-release properties of UCNF hydrogel have been 
established, it is customary to validate the UCNF slow-release fertilizer’s 
performance through real-life plant studies. We undertook a study to 
observe the growth of wheatgrass plants in pots under controlled envi-
ronmental conditions using different amounts of UCNF and pure urea as 
a fertilizer. All the experiments were performed in triplicate, with each 
pot containing plants germinated out of 50 seeds (Fig. S5). The study 
groups had one control group without any fertilizer and four groups 
containing fertilizer in the form of either UCNF or urea. Among the four 
groups containing fertilizer, two groups had UCNF as fertilizer con-
taining 1g and 3g of UCNF. The other two groups had 0.4 g and 1.2 g of 
pure urea, the same as the amount of urea contained in 1g and 3g UCNF, 
respectively. The average root and shoot length values of plant growth 
were used for further analysis. Fig. 9a shows a photograph of the growth 
of the wheat plants after two (2) weeks of germinating the seeds. It may 
be observed that the wheatgrass grown with UCNF exhibited more 
growth in terms of size and height compared to the control group and 
pure urea. Fig. 9b shows the photograph of the shoot and root of the 
plants grown in pots treated with different amounts of UCNF and pure 
urea. 
Fig. 9c presents a quantitative comparison of the effect of UCNF and 
pure urea on the root and shoot length of the plant samples. Using the 
UCNF as a fertilizer effectively imparted higher growth in the wheat-
grass plant. Fig. 9d shows the fresh and dry weight of the plants after 14 
days of germination. The effect of UCNF on the plants’ length and weight 
is evident. Thus, the results indicate that the use of UCNF could poten-
tially enhance the process of seed germination and subsequent growth. 
Additionally, the quality of the wheatgrass was enhanced, as indicated 
by the increased plant height with a higher amount of UCNF. 
Fig. 7. SEM images of UCNF hydrogel after 6 weeks of residence in an aqueous medium with inorganic micronutrients when: (a) the medium was sterile; (b) 
inoculated with B. subtilis; and (c) inoculated with P. aeruginosa. 
Fig. 8. Urea release profile of the UCNF hydrogel over time when placed in 
DI water. 
P. E et al. 
Journal of Cleaner Production 434 (2024) 140215
11
3.6. Cost analysis 
Cost assessment is essential before the commercialization of any 
product synthesized in the laboratory. The cost analysis of UCNF 
hydrogel was carried out only on the basis of material and energy bal-
ance applied on a batch size synthesized from 20 g of raw wood fiber. 
Table S2 shows the prices of chemicals and electrical energy in India, 
based on which the cost of production of UCNF has been calculated in 
Table S3 using energy consumption and material balance. Results of the 
study reveal that the estimated cost of the UCNF per kg without labor 
cost and profit would be $7.23, taking into account the subsidized rate of 
urea in India. The cost marginally increases to around $7.38 per kg when 
considering the international price of urea. Slow-release fertilizers are 
currently available in the Indian market. Some examples include 
Nutricote 13-11-11 type 180, Basacote plus6M, and Wonder Cote 90, 
priced at $15.56, $13.36, and $12.02 per kg, respectively. Slow-release 
fertilizers such as Osmocote Plus 15-9-12, Osmocote Smart-Release, and 
Jack’s nutrient bloom fertilizer 10-30-20 are available internationally at 
prices of $7.48, $22.16, and $25.94 per kg, respectively. UCNF contains 
only urea, whereas slow-release fertilizers normally encompass nitrogen 
(N), phosphorous (P), and potassium (K). If P and K had been incorpo-
rated, UCNF would have an estimated cost of INR 633.67 per kg, 
approximately equivalent to $7.62/kg. Thus, it can be concluded that 
the UCNF would have a price likely to be comparable and competitive 
Table 2 
The percentage urea release of UCNF compared to other slow-release fertilizers. 
Type of fertilizer Nutrient Nutrient release performance in 
water 
biodegradation References 
UCNF urea 33 % released in 10 days After 90 days, a weight loss of about 
81% 
This study 
CCDEUs-g-poly(AA)/urea slow-release fertilizer urea 40.1% after 10 h. Superabsorbent biodegradability was 
39.14% after 40 days. 
Mohammadbagheri et al. 
(2021) 
CSt-g-PAA/NR/PVA semi-IPN hydrogel (BHWCU/ 
9:1) 
urea 41.5% released in 7 days Biodegradation of the material is 77.8 
after the 90 days 
Tanan et al. (2021) 
NR/Cassava Starch (W-IPN-CUB) urea 100 % released in 72 h 48% of the biodegradability after 90 
days 
Vudjung and Saengsuwan 
(2018) 
SC-g-PAA/PAM/Urea urea 82 % released in 8 h – Nomura and Terwilliger 
(2020) 
carboxymethylcellulose and 
hydroxyethylcellulose (H15CA) 
urea 80 % released in 5 days 87.5 % degradation in 20 days Durpekova et al. (2021) 
CNFs were loaded with nitrogen-based fertilizer 
(ammonium chloride) in 1:1 
Ammonium 
chloride 
58% cumulative release of 
ammonium ions within 8 days 
– Sharma et al. (2023) 
P(AM-co-NHMA) hydrogel Synthetic (P(AM-co- 
NHMA)/U 4) 
urea Around 43 % were released within 
3 days 
– Kiran et al. (2019) 
Fig. 9. (a) Photograph of plants after two weeks of seeding under different doses of UCNF fertilizer and pure urea; (b) photographs of the shoot and root of the plants 
grown with different amounts of UCNF and pure urea; (c) the lengths of shoots and roots and (d) the fresh and dry weight of plants after two weeks of seeding under 
different dose of UCNF fertilizer and pure urea. 
P. E et al. 
Journal of Cleaner Production 434 (2024) 140215
12
with other commercially available slow-release fertilizers, but being 
completely biodegradable would provide excellent environmental sus-
tainability over other products. 
4. Conclusion 
This study indicates the synthesis of a biodegradable UCNF hydrogel 
via solution polymerization utilizing water as the reaction medium. The 
FTIR, XRD, and SEM techniques were employed to determine the 
structural morphology of the synthesized CNF and UCNF hydrogel. TGA 
data indicates that the synthesized UCNF hydrogel is thermally stable. 
The hydrogel synthesized demonstrated potential for agricultural 
application due to its optimal blend of natural polysaccharides cellulose 
and carboxyl methylcellulose, which met the necessary criteria, 
including biodegradability. The synthesized hydrogel exhibited a 
remarkable capacity for water absorption and demonstrated an excellent 
ability to retain water. The synthesized hydrogel shows a water trans-
port mechanism following non-Fickian-type diffusion. The investigation 
of the pseudo-second-order kinetics model suggests that the theoretical 
water swelling capacity closely approximates the observed experimental 
values. The loading capacity of urea was demonstrated to be 92% in 
terms of efficiency. One significant outcome of this study was the 
development of UCNF for the slow release of urea in agricultural ap-
plications. Moreover, it exhibits exceptional urea encapsulation perfor-
mance and enables slow regulation of urea discharge. The incorporation 
of UCNFinto soil exhibited a significant positive impact on both seed 
germination and plant growth. The cellulose nanofibers can also be 
derived from agricultural residues, thereby addressing the waste mate-
rial’s upcycling and reusability. Thus, this study would usher in a new 
direction toward synthesizing biodegradable hydrogel fertilizer exhib-
iting exceptional water retention and controlled nutrient release 
capabilities. 
CRediT authorship contribution statement 
Priya E: Conceptualization, Data curation, Formal analysis, Inves-
tigation, Methodology, Writing – original draft. Akash Jha: Data cura-
tion, Investigation. Sudipta Sarkar: Conceptualization, Supervision, 
Validation, Visualization, Writing – review & editing. Pradip K. Maji: 
Conceptualization, Supervision, Validation, Visualization, Writing – 
review & editing. 
Declaration of competing interest 
None. 
Data availability 
No data was used for the research described in the article. 
Acknowledgments 
This work was supported by the Department of Science and Tech-
nology, Govt of India [DST/TDT/AGRO-39/2020]; and the Council of 
Scientific and Industrial Research, Govt of India [09/143(0971)/2019- 
EMR-I]. 
Appendix A. Supplementary data 
Supplementary data to this article can be found online at https://doi. 
org/10.1016/j.jclepro.2023.140215. 
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