Synthesized Superabsorbent Based on Cellulose from Rice Straw for Controlled-Release of Urea

Rice straw was used as a-cellulose sources for synthesizing a novel a -cellulose-g-poly(acrylic acid)superabsorbent by graft polymerization between a-cellulose rice straw powder and acrylic acidin aqueous solution. The synthesis and controlled-release of fertilizer from superabsorbent wascarried out.The super absorbent composite was characterized by FTIR, SEM and TGA. The results of FTIR spectra showed OH of a-cellulose rice straw participated in graft polymerization with acrylic acid. The equilibrium water absorbency of super absorbent was investigated by calculating the swelling ratio. The super absorbent composite was swollen in aqueous solution of urea to load urea. Furthermore, the release of urea from loaded super absorbent was analyzed in water. Super absorbent with the highest water absorbency and the slowest profile of release controlled were obtained when the temperature of polymerization reaction was 70 oC. keywords: superabsorbent, rice straw, a-cellulose, controlled-release, urea.


INTROdUCTION
Super absorbent is a kind of polymer that able to absorb and hold large amount of water 1 .Superabsorbent polymers have been developed and used in daily life, such as infant diapers 2 , feminine hygiene products 1 , also have been used in specific area, such as in agriculture to prohibit the large amount of evaporation [3][4] .Usually for synthesis and develop super absorbent, acrylate and its derivate was used for the main ingredient.Super absorbent based only on poly(sodium)acrylate has several disadvantages such as, relatively high in production cost, not biodegradable, and not suitable to water and to soil rich with mineral contents [5][6] .
Natural polymer, such as cellulose, is possible to develop as superabsorbent polymer through grafting modification to the main backbone of cellulose, where as the techniques of grafting polymerization process are classified into such as photo-grafting, irradiation, and chemical initiation.The development super absorbent polymer based on cellulose support by some previous researches, such as Chen Y. et al 7 provided cellulose super absorbent through copolymerization and crosslinked between cellulose and vinyl's monomer; Princi et al. 8 developed super absorbent cellulose through copolymerization between cellulose and derivate of acrylate's monomer (methyl methacrylate and ethyl acrylate); and Khan et al. 9 used okra fiber as cellulose's source through grafting technique to acrylonitrile's monomer with K 2 S 2 O 8 and FeSo 4 as initiator and catalyst, respectively.There have been manyreports on introducing cellulose, inorganic clays and chitosan into pure polymeric super absorbents in order to improve swelling property, reduce production costs and ensure biodegradability [10][11][12] .However, the researches that focus on super absorbent polymer based on crops residuesare rare.Rice straw is one of the source of cellulose with so many advantages, such as abundantly available, biodegradable, renewable, since these are waste biomass.The composition of rice straw consists of cellulose (36,5%), hemicellulose (33,8%), lignin (12,3%), extractive compound (3,8%), ash (13,3%), and silica (70,8%) 13 .
The utilization of super absorbent in agriculture areas can provide several advantages, such as decrease the consumption of water irrigation and the death of plant, increase the water availability in soil, decrease the probability of soil hardens, prohibit the erosion, and increase the fertilizer efficiency [14][15][16] .The combination between superabsorbent and fertilizer in its application can increase the plant nutrition also decrease the negative impact of solvation of fertilizer 17 .The aim of this research focused on two points (i) synthesize of cellulose super absorbent based on cellulose from rice straw and (ii) absorption and controlled-release of urea in water.

Material
Rice straw is obtained from several paddy fields in Tanjung Morawa subdistrict, regency of Deli Serdang, North Sumatera.Acrylic acid, K 2 S 2 O 8 , hydrochloric acid, sodium sulfite, sodium nitrite, sodium hypochlorite, hydrogen peroxide, sodium hydroxide, ethanol, and methanol are ordered from Merck Chemical.N, N-methylene-bisacrylamide is ordered from Sigma Aldrich.Those commercial chemicals are used without further treatments.

Isolation of a-cellulose
The a-cellulose from rice straw was isolated using method Ohwoavworhua, F.O dan T.A. Adelakun 18 with some modifications.Rice straw powder (30 g) was first refluxed with 3,5% of nitric acid solution and 10 mg sodium nitrite at 90 o C for 2 hours.The resulting residues were washed and continued for further treatments with 2% of sodium hydroxide and 2% of sodium sulfite, at 50 o C for 1 hour.Residues from digestion process were washed tillneutrality and the residues were hydrolyzed with 17,5% of sodium hydroxide at 80 o C for 30 minutes then continued by treating the residues in 10% of hydrogen peroxide at 60 o C for 15 minutes.The residues from bleaching process were washed tillneutrality and dried in oven at 60 o C. The dried powder was characterized by FTIR, SEM, and TGA.

preparation cellulose hydrogel
The cellulose from rice straw (800-100 mesh) and aquadest were reacted in the fournecks flask equipped with a magnetic stirrer bar, the flask is heated in a water bath at 95 o C under nitrogen atmosphere.After 30 minutes of cellulose treatment, the temperature was adjusted to 60 o C and several supported components, such as potassium persulfate, acrylic acid, and NMBA were added to the flask.The temperature then adjusted to 70-80 o C until the white color gel was obtained.The white gel was washed with methanol for 30 minutes, continued with ethanol for 5 minutes and dried in oven at 60 o C until the constant weight was obtained.The white gel then characterized, including the microstructural morphologyby SEM, functional group by FTIR, and swelling ratio also controlled-release urea.

Swelling Capacity
Certain amountof dry sample, 0.1000 g, was put into a 200-mesh fastened nylon bag.Then, the bag was immersed into distilled water for 24 h to reach the swelling equilibrium at room temperature.Subsequently, the bag was lifted from water, while theresidual water was wiped up with filter paper, and weighed the bag.The swelling ratio was calculated using Eq. ( 1)19: where Ms, M 1 and M o refer to the weight of the swollen, dried cellulose hydrogel, and wet nylon bag, respectively.

Measurement of urea absorbency
Certain amount of cellulose hydrogel was immersed into a 100 ppm of urea solution (based on N content) and allowed to soak at room temperature for 24 hours.The swollen cellulose hydrogel was filtered to remove non-absorbed water and weighted.The swollen cellulose hydrogel was dried in oven at 60 o C until the constant weight was reached.The remaining N content was measured by spectrometry UV/Vis technique.

Controlled-release of urea
To study the slow-release behavior of cellulose hydrogel in water, the following experiment was carried out: certain amount of cellulose hydrogel/urea was immersed in 250 mL of water and incubated for different periods atroom temperature.After 15, 30, 45, 60, 90, 120, 150, and 180 minutes, the releasing urea in the solution was measured by spectrometry UV/vis technique.

Measurement of N content
N content of urea was measured follow Watt and Crisp method 20 .With the volume of 5 mL of the urea solution was added to vial bottle, then the 2 mL of coloring agent was added to the solution till the equilibrium was reached and the absorbance was measured at 420 nm.

RESULT ANd dISCUSSION
Isolation a-cellulose from rice straw a-cellulose was successfully isolated from rice straw powder by following Ohwoavworhua dan  Adelakunmethod 18 .Delignification process was completely done by addition of 3,5% of HNO 3 solution and NaNO 2 , those reagents caused conversion of lignin to become nitro lignin, this process provided 25, 70% of yield.On the second step, hydrolysis process, the reaction was completely done by addition of 2% of NaOH solution and 2% of Na 2 SO 3 , the aim of those solution is to remove hemicellulose, mineral, silica, and ash.On this step was followed by bleaching process with 1,75% of NaOCl and provided 15,84% of yield.The yield of each process was shown in Table 1.The color of the rice straw reduced in each step of processes, from brown to be white powder.
a-cellulose white powder from those processes was confirmed by FT-IR analysis (Fig. 1) and it was followed by characterizing the reference standard of a-cellulose standard.
In Fig 1 .the a-cellulose from rice straw gave absorbency of OH vibration at 3411 cm -1 , the stretching of C-H is shown at 2900 and 894 cm -1 while in reference standard a-cellulose the -OH peak is appeared at 3410 cm -1 and the C-H is appeared at 2916 and 894 cm -1 .The details of wave number of a-cellulose absorbency were shown in Table 2.
The microstructural morphology analysis of a-cellulose was shownup in Fig. 2. The morphology of a-cellulose fibers gave the appearance like fiber bundle.This result is close to the a-cellulose's morphology reported in the literature 21 .Fig. 3 showed the TG curve analysis of a-cellulose standard and from rice straw.The thermal degradation of a-cellulose consists of a series of degradation reaction, such as dehydration and pyrolysis.The dehydration reaction occurred at 100 o C, this weight loss was mainly due to the evaporation of water that binding on a-cellulose.While the pyrolysis reaction occurred at 338, 93 o C for a-cellulose standard and 350, 01 o C for a-cellulose from rice straw.Therefore, the rice straw a-cellulose is superior to that of a-cellulose standard.Obviously, the presence of other substituent (substituted group) on that cellulose skeleton is expected to influence thermos-stability.Those results were supported by the higher residue content in a-cellulose from rice straw than standard, 10, 77% and 8,25%, respectively.

preparation and characterization of cellulose hydrogel
The synthesized of cellulose hydrogel was done by polymerization reaction between a-cellulose and poly sodium acrylate in the presence K 2 S 2 O 8 as initiator and NMBA as crosslinked agent.Fig. 4 displayed the proposed mechanism reaction of crosslinked superabsorbent between á a-cellulose and poly sodium acrylate.
The polymerization of cellulose and poly sodium acrylate followed the rule of polymerization step, such as initiation, propagation, and termination.During the polymerization, NMBA with two pieces of reactive double bonds can enter the chains of polymer simultaneously and form the permanent      bonding between each chain (crosslinking).The degree of crosslinking influences the swelling ability of cellulose hydrogel.The swelling ability of hydrogel decrease with the increasing of degree of crosslinking.In the end of the reaction cellulose hydrogel has white color and chewy texture.
Swelling ratio has linear correlation to the absorbency capacity, the highest the swelling ratio the highest the absorbency capacity of hydrogel,in this research the hydrogel was immersed for 24 hours.The swelling ratio data is showed in Table 3 and Fig. 5.
The rate of swelling of cellulose hydrogel progresses slowly because when the hydrogel contact with water, the outer side of hydrogel is dry.The impact is the hydrogel in water will form two phases, such as the swollen hydrogel and dried polymer.The process of water enters into the hydrogel occurred through diffusion process 22 .Table 3 showed the cellulose hydrogel that prepared at 70 o C of temperature has the highest swelling ratio, 225, 2226.These differentiate are influenced by the degree of crosslinking of hydrogel.The increasing of degree of crosslinking of hydrogel can decrease the swelling ability but it can increase the mechanic properties of hydrogel 23 .Those results indicated, the cellulose hydrogel that prepared at 70 o C has the lower of degree of crosslinking than cellulose hydrogel that prepared at 80 o C.Other than degree of crosslinking, the swelling ratio of hydrogel is influenced by some factors, such as (1) interaction of polymer-solvent, (2) elasticity interaction, (3) osmotic pressure, and (4) electrostatic repulsion 24 .
The cellulose hydrogel is characterized by SEM to provide the surface morphology of hydrogel (Fig. 6).The SEM microphotograph of cellulose hydrogel C 70 and C 80 showed on the surface of hydrogel there some part of a-cellulose fiber.It indicated the a-cellulose content only reacted partially with the sodium acrylate and NMBA as crosslinked agent.On the hydrogel surface no pores observed, this result is close to the a-cellulose's morphology reported in the literature 22 , generally the pores on the surface of super absorbent polymer are random close to semi open cells.Because of the pores system of cellulose hydrogel is random close cells the urea absorbency occurred by diffusion process than capillarity 25 .To identify the cellulose hydrogel has been synthesized, the FT-IR spectra analysis gave the data of functional group of hydrogels (Fig. 7 and Table 4).In this research, hydrogel that will be chosen for further application is the hydrogel that provide the slow rate in the releasing urea to the water.In analogy, the releasing of urea to the soil when it applied will provide controllable of hydrogel in the release of urea also it will provide the stable rate of the release urea.To measure controlled-release of urea of hydrogel, the dried one is immersed in water and the water was measured by UV-vis technique in range of 15-150 minutes at 420 nm.The reaction that occurred in the UV-vis measurement was shown in Fig. 8. Fig. 9 showed the calibration curve of urea standard that provided regression formula, y = 0.0019x -0.0027 with R 2 = 0.9953.In Fig. 10 provided data of the absorbance curve of controlledrelease of urea in 15-150 minutes.The conversion of absorbance value will provide other data, that is N% content value by substituting the absorbance value to the regression formula.After substituting those value, the N% content of each treatment was displayed in Fig. 11.
The hydrogel that prepared at 70 o C of temperature has the slowest controlled-release of urea, which showed the first released of urea held in minutes of 90 with N% content of 0.3684 ppm and at minutes of 180 this hydrogel also gave the low value of N% content of 4.05263 ppm.In other side, the hydrogel that prepared at 80 o C the first released of urea held in minutes of 30 with N% content of 1.4210 ppm and at minutes of 180 this hydrogel also gave the low value of N% content of 8.2631 ppm.
The objective of this research is to obtain the model of hydrogel with two properties, where it was related to each other.Both are high absorbency capacity and slow in the release of urea.Therefore, the suited model of hydrogel that has both properties is the hydrogel which prepared at 70 o C of reaction temperature.It is altogether reasonable to conclude that the hydrogel which prepared at 70 o C is the suited model of hydrogel for applied in slow-release of urea, because of this hydrogel has the slowest number in the release of urea.Besides that, this hydrogel has the highest absorbency capacity, its means it can load much number of urea.The highest absorbency capacity is linear to the ability of hydrogel to expand itself in the urea solution, that calculated as swelling capacity.The highest the swelling ration and absorbency capacity, the lowest the number of crosslinking unit in the hydrogel that prepared in 70 o C of reaction temperature.

CONCLUSION
The a-cellulose-g-poly(acrylic acid) super absorbent composite was synthesized by graft copolymerization reaction of a-celluloserice straw and AA in aqueous solution.The highest water absorbency and the slowest profile of release controlled were obtained when the temperature of polymerization reaction was 70 o C.

Fig. 1 :
Fig. 1: Spectra FTIR of a-cellulose from rice straw and reference standard

Fig. 7
Fig. 7 showed, there are peak at 3446 and 3600, that indicate the presence of hydroxyl (O-H) and amino (N-H) group from cellulose and NMBA, respectively.The other peak that can prove the has been occurred are the presence of

Table 1 :
Yield of a-cellulose