Synthesis of Biodegradable Aloevera-acrylic acid based hydrogel with enhanced water 1 retention capacity and its impact on agriculture 2

16 The present work deals with the synthesis of biodegradable hydrogel using a natural 17 polysaccharide Aloevera and vinyl monomer acrylic acid. In this synthesis, ammonium per 18 sulphate was used as an initiator and glutaraldehyde as cross-linker, acrylic acid was used as 19 monomers and Aloevera as backbone. Grafting was confirmed by different techniques like 20 SEM, FT-IR, XRD and EDS. Maximum percentage swelling of synthesized hydrogel was 21 found to be 796 %. Biodegradation behavior of Av-cl-poly(AA) was studied by soil burial, 22 compositing and vermicompositing method. Maximum biodegradation was found to be 90%, 23 94% and 93% in case of soil burial, composting and vermicomposting methods, respectively. 24 Biodegradation of Av-cl-poly(AA) was confirmed by FTIR and SEM techniques. Water 25 retention capacity was prolonged from 11days to 20 days using synthesized Av-cl-poly(AA). Water content of clay soil and sandy loam soil was increased to an extent of 6.1% and 5.79%, 27 respectively.


Measurement of water-retention capacity of soil by Av-cl-poly(AA)
Water retention capacity of soil using Av-cl-poly(AA) was studied in two types of soils: clay soil and sandy loam soil. Five samples 0.5 g, 1.0 g, 1.5 g, 2.0 g and 2.5 g of each synthesized hydrogel Av-clpoly(AA) were mixed thoroughly with 50 g of dry soil in disposable container separately. Then 100 mL of water was added and the initial weight of the sample was taken. Container having soil and water, without hydrogel was referred as control, while the containers with hydrogels were taken as test samples. Regularly upto 25 days weights of the samples were taken and water retention capabilities of the soil with and without hydrogel were studied (Danahue et al., 1971). Residual water (g/g) was calculated from the equation given below: Residual water = (Wf -Wi/Wi) (2.4) Where, Wf and Wi are the final and initial weights of the samples, respectively

Measurement of water uptake by clay and sandy loam soils using swollen hydrogel
Two different soil samples clay and sandy loam were taken for this experiment. Soil samples were collected from the upper layer (0-50 cm). Then soils were air dried. Known amount of dried Av-cl-poly(AA) were kept in tea bags and immersed in 100 mL of water, so as to soak water for 24 h. After 24 h tea bags were taken out from the water and gently wipe with tissue paper, so that excess of water can be removed. Then these tea bags were kept in dry clay and sandy loam soils. because of C-O stretching of tertiary alcohol, 1237 cm -1 because of C-C deformation in the methyl group ( Figure 1a).

Av-cl-poly(AA)
On the other hand, IR spectrum of Av

Scanning electron microscopic (SEM)
The surface morphology of the synthesized candidate polymer on grafting was studied through SEM. Scanning was synchronised with the microscopic rays upholding a small size over a large distance. SEM was performed on JSM-6100. The surface morphology of the Aloevera and Av-cl-poly(AA) were studied by the SEM technique. SEM images of both the samples exhibited morphological differences.

Aloe vera
Smooth surface was observed in SEM of backbone, Aloevera ( Figure. 2a). The surface morphology of backbone did not exhibited any kind of ridges, grooves or pits as observed in figure.

Av-cl-poly(AA)
Rough surface bearing depressions and pits were observed extending throughout the surface of Av-cl-poly(AA), which is not seen in backbone (Figure 2b). The occurrence of such coarse and porous structures in Av-cl-poly (AA) proves the morphological difference by chemical modification of the backbone through grafting as well as cross-linking brought about by covalent bonding in-between polymeric chain on cross-linking through MBA.

X-ray diffraction (XRD)
XRD of Aloevera and Av-cl-poly(AA) has been explored by X' Pert Pro, showed the disparity in the anisotropy of Aloevera and Av-cl-poly(AA)

Aloevera
The coherence length of the samples was calculated by Scherrer equation (Mittal and Kaith, 2010). The coherence length of Aloevera was found to be 0.542. L= 0.9 λ / β 1/2 x cos Ɵ where L=coherence length, λ= wavelength, Ɵ =diffraction angle and β 1/2 =full width half maximum. XRD of backbone is shown in Figure 3a.

Av-cl-poly(AA)
In Av-cl-poly(AA), the rationality (coherence) length is found to be 0.8092 which was higher than the backbone, which showed that the backbone become more crystalline by grafting and cross-linking it with acrylic acid. This is because of the influence of increased cross-linker concentration, thereby enhancing the cross-linking density in-between polymeric chains and hence resulting in greater aligned crystalline lattices. Anisotropy increased with increase in coherence length. It proves that acrylic acid has been grafted on backbone demonstrating successful grafting. XRD of Av-cl-poly (AA) is shown in Figure 3b.

Figures 3a,b: XRDs of (a) Aloevera and (b) Av-cl-poly(AA)
Electron dispersive X-ray spectroscopy (EDS): EDS is used for analytical examination of the present elements as well as for the estimation of its relative abundance. It includes an interactive communicative interaction in-between Xray source as well as sample. A high-energy beam of electrons is focused onto the sample under study. The incident beam causes the excitation of electrons, ejecting them and creating a hole. The electron from higher energy level, shifts to the hole, consequently difference in energy of higher and lower energy shells gets escaped in the form of X-ray. The quantity and energy released is noted by the ED spectrometer. Line intensities of various elements present in the samples are measured.

Aloe vera
Carbon, oxygen and nitrogen were found to be the main elements present in aloe vera backbone. It was found that the atomic % of carbon in backbone was 53.02 %. The atomic percentages of oxygen in backbone, was determined to be 42.77 % and nitrogen in backbone was 4.22%, correspondingly ( Figure 4a).

Av-cl-poly(AA)
Carbon, oxygen and nitrogen were found to be the main elements present in Av-cl-poly(AA).
It was found that the atomic % of carbon were 55.22 %. The atomic percentage of oxygen was 42.35 % and nitrogen was 2.43% ( Figure 4b). The decrease in atomic % of oxygen is because of the exclusion of H2O molecule.

Biodegradation studies of Av-cl-poly(AA) by soil burial, composting and vermicomposting method
Biodegradation behavior of synthesized Av-cl-poly(AA) was studied by soil burial, composting and vermicomposting method. The soil utilized for biodegradation studies were taken from garden of CT Institute, Jalandhar in case of soil burial method. The compost for the biodegradation study was taken from the waste water release from CT campus, Jalandhar and vermicompost was purchased from nursery, respectively. The water level was maintained to avoid drying of the soil by evaporation. The compost rich in microbial species was regularly fed to the samples contained in the pot to enrich and nourish the medium with lot of microbes. Synthesized hydrogel was placed at a distance of 3cm apart from each other, while placing them in the respective soil sample. Samples were weighed after regular interval of 7 days. Biodegradation of the samples were validated by FT-IR and SEM characterization.

Results and discussion
Optimization of reaction parameters of Av-cl-poly(AA)

Impact of reaction time
It is clear from the (Figure 5a

Impact of reaction temperature
Maximum Ps (426 %) was obtained at 60ºC, but further rise in temperature decrease in Ps

Impact of pH
Maximum Ps (312%) under the Impact of microwave radiations was scrutinised under neutral medium (Figure 5c). Ps was observed to diminish in acidic and basic medium that might be because of premature cessation of polymerization reaction. It might also be owing to repression in the formation of per sulphate ion.

Impact of initiator ratio
It is noted that percent swelling increases with increase in the amount of APS and reached to optimum value 796% (Figure 5d). Initially SO4 -* and *OH ion concentration increased

Impact of cross-linker concentration
The optimum amount of glutaraldehyde for the reactions was 0.0139 molL -1 (459%) ( Figure   5g). Increase in cross-linker concentration above optimum concentration leads to decrease in Ps. Initially, Ps increases because surface area of the grafted cross-linked moiety increases.
But with further increase in the concentration of cross-linker, leads to compacting of polymeric chains. Thereby, decreasing pore size, which further leads to desorption and thus, reduced the percentage swelling (Tokuyama et al., 2007).

Impact of monomer concentration
Maximum Ps, 318% was observed at acrylic acid concentration of 4.1632 mol L -1 . Increasing acrylic acid concentration above optimum concentration decreases Ps (Figure 5f). It may be due to the fact that further increase in the amount of monomer predominate the homopolymerisation over grafting. Moreover the cross-linking density increases and Av-clpoly(AA) becomes more compact. Moreover, the accessibility of water molecules towards functional group present in the polymeric chain also decreases, thereby decreasing the Ps (Anupama et al., 2002).

Swelling studies of Av-cl-poly (AA) in de-ionised water
Swelling behavior of Av-cl-poly(AA) was studied as a function of time, temperature and pH of the swelling medium.

Evidences of biodegradation through FT-IR
A great distinction in peak positions of FT-IR of biodegraded hydrogel is observed because of hydrolysis and enzymatic reactions occurred at the time of biodegradation. At the initial stage biodegradation was slow, but with the passage of time it was increased, this can be explained on the basis in the initial stage surface of the synthesized hydrogel was hard and it exposed to moisture and it became soft and easily assessable to the microorganism (Mittal et al., 2018).

FT-IR of various biodegradation stages of Av-cl-poly(AA)
It was found that the peaks, which were seen in Av-cl-poly(AA) before biodegradation ( Figure 1a) on 2128-1864 cm -1 were missing during first stage of biodegradation and some of the peaks 2931-2659 cm -1 and 1699-1051 cm -1 were shifted as seen in (Figures 7a,d,g). In the second biodegradation phase peaks observed at 3427 cm -1 and 1453 cm -1 in first stage were found missing and some of the peaks at 3088-1921 cm -1 were shifted (Figures 7b,e,h). This may be due to the degeneration of cross-linking between Aloevera and poly acrylic acid chains. In the third stage of biodegradation, as seen in, peaks observed at 3870-3388 cm -1 , 3068-2898 cm -1 and peak at 2321 cm -1 were missing and peaks corresponding to 3152 cm -1 , 2656-2575 cm -1 , 2114-1404 cm -1 were shifted as seen in (Figures. 7c,f,i), this may be due to the fact that intensity of peak due to -OH group present over backbone was found to decrease progressively because of biodegradation. The missing and the shifting of the peaks showed that the synthesized hydrogel is biodegradable as well as eco-friendly in its nature.  8a,d,g). In the second phase of biodegradation big crevices with further superficial pits were observed (Figures 8b,e,h). SEM images clearly showed that in the 3 rd stage of biodegradation, there is complete disintegration of Av-cl-poly(AA) occurred (Figures 8c,f,i).

Impact of biodegradation of synthesize hydrogel on soil
Biodegraded hydrogel impact on soil fertility was studied by carrying-out the soil analysis before and after biodegradation of hydrogel. Organic carbon, phosphorus, potassium contents and pH of the soil sample was analyzed.
The results revealed that pH of the control soil was pH 6.9 and organic carbon was 0.31%. Phosphorus and potassium content was 12.5g/m 2 and 59 g/m 2 , respectively ( Table 2).
As it is well known that carbon is the one of the constituent for the photosynthesis process and its low value act as a limiting factor for the plant growth  (Table 2).
It was clear from the results that there was only small change in the pH of the control and test soil sample. The pH was 6.9 in the control sample and it was 6.7 in the soil with degraded hydrogel, which is in the permissible limit of the soil pH (Table 2).

Conclusion
It was clear from the foregone discussion that a novel Aloevera-acrylic acid based hydrogel was synthesized. The maximum swelling capacity of the synthesized hydrogel was found to be 796%.
Synthesized hydrogel was eco-friendly in nature. It was biodegradable and has no harse impact on the fertility of the soil, rather it enhanced the soil fertility. Maximum biodegradation was found to be 90%, 94% and 93% in case of soil burial, composting and vermin-composting methods, respectively. It enhanced the water retention and water holding capacity of clay and sandy loam soil. The water retention periods of soil using synthesized hydrogel prolonged from 11 to 20 days. Thus, it can be concluded that synthesized hydrogel is very efficient from agriculture as well as environmental view point.