Radiation Synthesis of Green Nanoarchitectonics of Guar Gum-Pectin/Polyacrylamide/Zinc Oxide Superabsorbent Hydrogel for Sustainable Agriculture

In the current study, the performance of superabsorbent hydrogel composites comprised of Guar gum-Pectin/Polyacrylamide/ZnO crosslinked with gamma irradiation (10 kGy) has been investigated for sustainable agriculture. The claimed composites (GG/PC/PAAm/ZnOx) were characterized by FTIR, TGA, and AFM. The swelling capacity data reveal that the equilibrium water swelling (EW) of the composites was increased by increasing the ZnO content from 600 to 1050 g/g for zero to the highest concentration of ZnO, respectively. Furthermore, the physical properties of the soil mixed with the hydrogels were improved; water holding capacity (WHC) increased to 66% and water retention (WR) kept at 15% after 20 days. The composites showed a good degradability in the biodegradation test. They also portrayed super-absorption capacity at three swelling/deswelling cycles. This advancement is important for reducing water consumption through the irrigation of arid lands. The prepared composites were proved as excellent candidates in sustainable agriculture applications.


Introduction
One of the most important bases of bioeconomy is replacing fossil-based materials with environmentally friendly sustainable resources [1,2]. In this regard, efforts are directed to employ different biopolymers in industrial, agricultural and biomedical applications [3]. Moreover, different biomass finds great importance in many novel applications such as electrode material for super capacitors [4], capacitive deionization perfromance [5], electromangetic wave absorber [6] and as electrocatalyst [7].
Plant-derived polymer such as cellulose, starch, pectin and guar gum are more advantageous than those of animal origin due to their abundancy, higher functionalities and ease of extraction and handling [8].
On the other hand, the continuous depletion of freshwater resources accompanied by an incessant increase in world's population is a serious global problem especially due to climate change. Water is the principal demand for agriculture, cultivation and crop production [9,10]. One of the most effective solutions to reduce water consumption is using materials with high water absorption and good retention capacities called superabsorbent hydrogels [2]. Superabsorbent hydrogels consist of three-dimensional (3D) hydrophilic polymer networks that have the potential to absorb and hold a large amount of water and aqueous solution [11,12]. They are being used for water management aimed at agricultural development [13], as they can absorb and store water many times more from its particular volume and perform the function of reservoirs with water and increase water retention. This feature helps reduce water stress in plants and ensure high plant productivity, which leads to increased growth and yield [14]. The use of polymeric hydrogel can improve water and fertilizer efficiency. Also, superabsorbent polymers improve some physical properties of the soil. Synthetic hydrogels based on acrylates and acrylamides show high mechanical strength and the potential to absorb significant quantities of water. Due to the problem of their biodegradability, they are being blended with biopolymers such as alginate, agar, cellulose, chitosan, gum and starch [15]. Blending of natural polymer with a synthetic one has received increasing attention in last decades as it can reduce pollution of the synthetic polymers and also has a wide range of potential applications in different fields [16].
Natural polymers, especially the polysaccharides, have been widely used particularly in environmental applications due to their attractive issues such as low-cost, abundance, renewability, and biodegradability [17]. Guar gum (GG) is a galactomannan polysaccharide with comprised of main chain of mannose units joined by β-d (1 → 4) linkages having β-d galactopyranose units attached to this linear chain by (1 → 6) linkages. The guar molecule possesses numerous hydroxyl groups that can form extensive intra and inter hydrogen bonding which make guar material one of the best-known natural polymer as gel forming material with excellent gelation property [18]. Pectin (PC) is another highly functional natural polymer [19]. At present, pectin is believed to consist mainly of linear d-galacturonic acid units, joined by a-(1-4) glycosidic linkage. These uronic acids have carboxyl groups, some of which are naturally present as methyl esters [20]. Combining these two natural polymers in a three-network structure fulfills maximum benefits from the advantages of both polymers. Moreover, incorporating an inorganic core such as zinc oxide reserves the cohesion of the gel matrix and keeping the gel consistency while absorbing huge amount of water. Therefore, the main target is to constitute a series green superabsorbent hydrogel for water retention and soil amendment. This was achieved by a green synthetic methodology based on irradiation polymerization and crosslinking of guar gum/Pectin grafted with polyacrylamide with a core of zinc oxide. The gamma route fulfills several advantages such as clean simple technique, high-productivity, low energy, and low-cost process that leads to less contamination and fewer byproducts [21,22]. Radiation treatment can also introduce better surface crosslinking between natural and synthetic polymers by enhancing the formation of free macroradicals which finally lead to formation of crosslinked structure [23]. Moreover, the hydrogel composites were characterized via different techniques including infrared spectroscopy, Atomic force microscopy, and thermal analysis. The swelling properties of the prepared hydrogel composite GG-PC/Pam/ZnO x were verified in different saline solutions. Water retention and degradation rate of prepared superabsorbent composites were also evaluated. The swelling performance of the optimum hydrogel was investigated at three swelling/deswelling cycles.

Materials
Pectin powder from apple [Poly-d-galctouronic acid methyl ester (PC)] with degree of esterification (DE) of 50-75% was purchased from Sigma-Aldirch (China), The viscosity average molecular weight (Mv) of this pectin grade was informed to be 1.538 × 10 5 g/mol [24].Ultra-pure Guar gum powder (GG) with viscometric molecular weight 2.5 × 10 5 g/ mol was provided from Sigma-Aldrich and used without further purification. Acrylamide (AAm) of purity 99.9%, and zinc oxide nanopowder < 50 nm particle size molecular weight 81.39 surface area 10-25 m 2 /g were supplied from Sigma Aldirch, UK. Double-distilled water was used for the preparation of all solutions in this study. All other reagents were in analytical grade and used without any further purification.

Sandy Soil
The sandy soil sample was collected from different points at Suez Road to the direction of Sokhna, Cairo, Egypt. Physical and chemical properties of the studied soil samples, were determined as described by Hasija et al. [25] (The data are presented in Table 1).

Preparation of GG-PC/PAAm Hydrogel
A simple protocol was applied. Briefly, 1.3 g Guar gum and 0.7 g of pectin were separately dissolved in 46.2-and 46.8-ml deionized water, respectively and stirred at 50 °C till complete solubility. Then, the two solutions were added together. Afterwards, 5 g of Acrylamide monomer was added to the GG-PC solution then the solution was stirred at ambient temperature until complete homogeneity, where the total polymer and monomer content was 7 wt%. Then 20 mL of the resulted solution was poured into glass test tubes each and they were exposed to 60 Co-gamma irradiation of irradiation doses 10 kGy; (this irradiation dose was sufficient to form a gel% ≈ 99.9%), the resultant hydrogels were coded as GG-PC/PAAm/ZnO 0 . The prepared hydrogel was sliced into almost identical discs and incubated overnight in distilled water at 80 °C, to remove the unreacted monomer then dried at room temperature.

Preparation of GG-PC/PAAm/ZnO x Composites
The hydrogel composites were prepared via a facile methodology. ZnO solution (0.1 gm in 50 mL of deionized water) was sonicated at 25 °C for 30 min then, different volumes of ZnO solution (0,1, 3, 5 &7 mL) were added to 20 mL of GG-PC/AAm solution with continuous stirring for 10 min to assure complete homogeneity. The resulted solutions were then transferred into glass test tubes and subjected to 60 Co-gamma rays at 10 kGy irradiation dose for synthesis of the GG-PC/PAAm/ZnO 0 , GG-PC/PAAm/ZnO 1 , GG-PC/ PAAm/ZnO 3 , GG-PC/PAAm/ZnO 5 , and GG-PC/PAAm/ ZnO 7 respectively.

Characterization
Fourier transform-infrared spectroscopy (Bruker, Unicom infra-red spectrophotometer, Germany) was used to identify the functional groups at 400-4000 cm −1 wavelength range. The KBr was ground and mixed with the dried sample, and the mixture was pressed into thin plates for infrared testing. The surface morphology and topography of the composites were monitored by AFM, Flexaxiom Nanosurf, C3000 at the dynamic mode (non-contact) to confirm chemical modification and to detect the changes accompanying the swelling process. The AFM examinations were conducted at room temperature using a NCLR silicon cantilever with rectangular shape and a resonance frequency of 9 kHz. The thermal behavior of the prepared composites was verified via a thermogravimetric analyzer (Perkin-Elmer Co., USA) at a heating rate of 15 °C min −1 from 30 to 600 °C under a nitrogen atmosphere.

Swelling Behavior in Different Media and Equilibrium Water Absorbance of GG-PC/ PAAm/ZnO x Superabsorbent Composites
The equilibrium water absorbance and swelling of synthesized hydrogel composites were measured by immersing a circular disc of dried hydrogel in 250 ml of distilled water to attain maximum swelling equilibrium state (Q eq ) [26]. The swollen gel was taken off the water and gently dried with a tissue paper to remove the excess water, then it was weighed. The Equilibrium water absorbance was calculated according to the Eq. (1): where Q eq is the equilibrium water absorbance (g/g) (EW) at time t (min), W s is swollen hydrogel (g), and W d is dried hydrogel (g). The swollen gels were filtered, and the Equilibrium water absorbance was calculated according to the above Eq.

Water Retention (WR) and Water Holding Capacity (WHC) of Soil with GG-PC/PAAm/ZnO x Composite
The ability of the prepared composites to modify the soil holding and retaining capacity was monitored by calculating the WHC% and WR% of soil mixed with different concentrations of GG-PC/PAAm/ZnO x . They were measured by taking 100 g of dry soil (A) as blank, and 100 g of dry soil mixed with (1, 2 and 3%) g of GG-PC/PAAm/ZnO 0 and GG-PC/PAAm/ZnO 7 as (B) and (C) respectively. Each sample was placed in a sealed tube with 200-mesh nylon fabric and weighed (W O ). The three soil samples (A), (B), and (C) samples were soaked with distilled water until saturation, and then the samples were weighed again (W 1 ). The WHC % of the soil was calculated using Eq. (2). The WR % of the soil with the GG-PC/PAAm/ZnO x composite was calculated by Eq. (4), and the soil samples were kept under the same conditions and weighed every 2 days (W i ) for 20 days.

Biodegradation Rate (BR) of GG-PC/PAAm/ZnO x Composite in Soil
The where: W o and W i are the weights of the hydrogel composites before and after degradation (g), respectively.

Swelling/Deswelling Study
The reusability study was conducted at three swelling/ deswelling cycles. A fixed weight of the composite (0.1 gm) was placed in an excessive amount of fresh water and allowed to reach maximum swelling at ambient temperature. Then, it is picked up and weighed. Afterwards, the swelled hydrogel is placed in an oven at 100 °C until reaching its original weight. This test is repeated for three times. The maximum water absorbance is calculated according to Eq. 5.
where Q max is the maximum water absorbance (g/g) (EW) at time t (min), W s is swollen hydrogel (g), and W d is dried hydrogel (g).

Synthesis of Green Composites
A green gamma irradiation is applied to generate macroradicals on the reactive groups of guar gum and pectin. These macroradicals are crosslinked with interchain of acrylamide to form the three-dimensional structure that can hold and retain water molecules. Zinc oxide particles are incorporated to keep the consistency of the hydrogel during swelling/deswelling process. The proposed reaction is given in Scheme 1. The initial step is the formation of guar gum and pectin macroradicals initiated by the gamma irradiation. The acrylamide radicals are also formed by the same mechanism. The radical formation step is followed by crosslinking and network formation. Finally, zinc oxide is chelated with the amine groups of polyacrylamide. This mechanism is proposed by other investigators for irradiation crosslinking of different carbohydrate polymers [27,28]. The photochemical initiation and crosslinking is highly favoured over thermochemical process, particualry at adjusted dose and controlled conditions.

Fourier Transform Infrared spectroscopy (FT-IR)
FT-IR spectra of GG, PC, GG-PC/PAAm/ZnO 0 and GG-PC/ PAAm/ZnO 7 are shown in Fig. 1. The spectrum of GG showed a broad band at 3363 cm −1 due to the stretching vibration of-OH groups. The band at 1637 cm −1 is due to ring stretching of mannose. The bands at 2897 cm −1 , 1395 cm −1 , 1117 cm −1 and 1004 cm −1 correspond to C-H stretching mode, -C-H asymmetric bending, C-O-C stretching and C-O stretching vibrations respectively. [29]. The FTIR spectrum of PC also showed the broad band of-OH groups at 3334 cm −1 . The asymmetrical stretching band of C-H appeared at 2931 cm −1 . The bands of C = O and C-O of glycosidic bond was observed at 1731 cm −1 and 1008 cm −1 [30], respectively. The FTIR spectrum of GG-PC/ PAAm/ZnO 0 hydrogel showed shifting and reduction the intensity of the band of-OH group due to the formation of the crosslink structure in the hydrogel. The band of C = O group is shifted to 1603 cm −1 and become more intense due to the presence of amide group of PAAm. Almost the band of C-O disappeared, which also confirmed the formation of the hydrogel crosslinking structure. The FT-IR spectrum of PC-GG/PAAm /ZnO 7 showed that the C = O stretching band of pectin has shifted from 1603 cm −1 to 1627 cm −1 because of interaction with ZnO [31]. The other bands were also slightly shifted due to bonding between ZnO and polymeric matrix. This confirmed the successful formation of the nanocomposite.

Atomic Force Microscopy (AFM)
AFM is a modest tool for topography charcaterization of different material. It is used in describing the surface of hydrogels in previous works [10,32]. In this work, the AFM is used to study the surface topography, the porous structure, and the height of GG-PC/PAAm/ZnO 0 , GG-PC/PAAm/ ZnO 3 , and GG-PC/PAAm/ZnO 7 (Fig. 2a-c) to investigate the effect of the addition of zinc oxide on the porosity and the pore volume of the prepared hydrogel composites. The images revealed that the height increases by the inclusion of zinc oxide in the composite matrix, which reflected the growth in the pore volume. The height measurements were found 0.51 nm, 0.92 nm, and 31.8 nm for GG-PC/PAAm/ ZnO 0 , GG-PC/PAAm/ZnO 3 , and GG-PC/PAAm/ZnO 7 , respectively. However, very wide pores are not suitable for retaining the water absorbed by the hydrogel. Therefore, it is expected from the AFM data that the maximum swelling will be achieved by GG-PC/Pam/ZnO 7 hydrogel composite. Moreover, the humpy surface morphology resulting from the inclusion of zinc oxide indicated the increase in the effective Scheme 1 Proposed reaction for hydrogel composite synthesis surface area, which guarantees better absorption and swelling [23].

Thermogravimetric Analysis (TGA)
The TGA thermograms of GG-PC/PAAm/ZnO 0 , GG-PC/ PAAm/ZnO 3 and GG-PC/PAAm/ZnO 7 are provided in Fig. 3. It is observed that the thermal degradation pattern includes three decomposition stages. The first decomposition stage was observed at 175 °C due to the elimination of the hydrated water. The second decomposition stage was observed at 295 °C due to the elimination of side groups. The third decomposition stage is due to decomposition of the backbone matrix 300-558 °C and the weight remaining (%) about 25% at 600 °C. On the other hand, the incorporation ZnO nanoparticle (GG-PC/PAAm/ZnO 3 and GG-PC/PAAm/ ZnO 7 ) induced a slight improvement in thermal behavior compared with GG-PC/PAAm/ZnO 0 which may indicate strong interfacial interactions between ZnO and function groups of GG and PC the similar behavior was reported by Priyadarshi et al. with the addition of nZnO nanoparticles [33].

Effect of ZnO Concentration
Various concentrations of ZnO (wt%) are incorporated into GG-PC/PAAm/ZnOx hydrogel composite during preparation by gamma polymerization irradiation at a total exposure dose 10 kGy. The swelling performance of the prepared hydrogel composites versus zinc oxide concentration is represented in Fig. 4a. It can be depicted that there is a tremendous improvement in swelling performance as the percentage of incorporated zinc oxide increases. This improvement may be attributed to that the presence of ZnO nanoparticles causes penetration of more water molecules to balance the build-up ion osmotic pressure, which leads to swelling of the gel [34]. Furthermore, the presence of ZnO could expand the hydrogel network and increase the pores and free spaces within the networks, and as a consequence adsorbs more water [18]. the anion-anion electrostatic repulsion is responsible for a decrease in the osmotic pressure difference between gel network and the external solution [35]. Therefore, the osmotic pressure difference between the gel network and the external solution reduced with increase in the ionic strength of the saline concentration [36]. It can also be concluded that the hydrogel composite has high salt tolerance and can hold huge amount of water at relatively high salt concentration which fits the requirement of farming in salty and sandy soils.

Effect of pH
The swelling capacity of GG-PC/PAAm/ZnO x hydrogel composite was studied at various pH solutions ranged from 1.0 to 11.0 as seen in Fig. 4c. The swelling capacities of both GG-PC/PAAm/ZnO 0 and GG-PC/PAAm/ZnO 7 hydrogel composites increase with increasing pH of the medium to get the maximum value at pH 9. Above this value a drop in the swelling capacity was observed. The carboxylic groups on the polymeric chains deprotonated with increasing the pH of the solution above the pKa value, which is 4.6. When the pH of the solution increased (5-9), some of -COOH groups are ionized and the electrostatic repulsion among negatively charged -COO-groups was increased which facilitates the relaxation of polymeric chains [37]. In addition, the hydrogen bonding interactions among the -COOH groups was partially broken because which widens the size of the network pores and thus enhances the swelling capacity due to less crosslinked network structure. However, at much higher pH (> 10), the 'charge screening effect' of excess Na + in the swelling medium restricts the net charges of -COO-anions and prevents effective anion-anion repulsion so swelling decreased [37].

Effect of Temperature
The swelling behavior of GG-PC/PAAm/ZnO 0 and GG-PC/ PAAm/ZnO 7 was verified at temperature range 30, 45, and 55 °C, Fig. 4d. It was found that increasing temperature resulted in increasing the swelling capacity of the investigated hydrogel composites. This may be explained by that the increase in temperature causes an increase in the diffusion and penetration of water into the pores existing within the hydrogel matrix.

Swelling Kinetics
The swelling capacity of GG-PC/PAAm/ZnO 0 and GG-PC/ PAAm/ZnO 7 was investigated as a function of time as shown in Fig. 5a. It is observed that the swelling capacity of all hydrogels increases progressively within the time of swelling up to 500 min, thereafter it levels off. The swelling kinetic parameters GG-PC/PAAm/ZnO 0 hydrogel, and PC-GG/PAAm/ZnO 7 hydrogel composite were investigated. The mechanism of the water transportation through the prepared hydrogel and nanocomposite was obtained by applying the following relationships [38,39]. k is the diffusion constant (dependent on hydrogel type and swelling medium) incorporating characteristic of the polymeric network system., n is the diffusion exponent (which provides information regarding the transport mechanisms that drive the sorption of a particular solute), and M t and M eq are hydrogel masses at a swelling time t and at equilibrium, respectively.
The Fick law (Eq. 6) was applied on GG-PC/PAAm/ ZnO 0 and PC-GG/PAAm/ZnO 7 composite hydrogels as shown in Fig. 5b and the obtained data are summarized in Table 2. The nature of the water diffusion process depends on the value of n. When n less than 0.5, the swelling mechanism is controlled by diffusion, when the value of n is between 0.5 < n < 1, the system is controlled by diffusion and relaxation. According to the values of n, GG-PC/PAAm/ZnO 0 hydrogel is Fickian diffusion while PC-GG/PAAm/ZnO 7 hydrogel composite is non-Fickian diffusion. This means that ZnO controls in the mechanism of swelling.

Water Holding Capacity (WHC)
The WHC% of soil with various amounts of the PC-GG/ PAAm/ZnO 7 hydrogel composite (1, 2, and 3%) are represented in Fig. 6a. The WHC% of soil without composite (0%) is 15%, whereas the WHC% of the treated soil is 25%, 49% and 61%for PC-GG/PAAm/ZnO 7 of 1%, 2% and 3%, respectively. The soil can catch and keep a significant amount of water as the amount of added composite material is increased. As a result, soil moisture is amended, and water consumption is diminished. This finding confirms the outstanding efficiency of the claimed hydrogel composite in modifying the physical nature of the sand soil used in this study. This result can be discussed on the basis of the hydrophilic nature of PC-GG/PAAm/ZnO 7 [34]. Moreover, the highly porous structure and high surface area of PC-GG/ PAAm/ZnO 7 as proved by the AFM images enhance water holding of the soil by capturing a huge amount of water within its voids. The water holding capacity of the prepared hydrogels was compared over similar materials in terms of maximum amount of absorbed water, the data are given in Table 3. Figure 6b depicts the effect of different concentrations of the superabsorbent hydrogel composites PC-GG/PAAm/ZnO 0 and PC-GG/PAAm/ZnO 7 on the water retention of the sandy soil. It can be seen that WR of blank soil decreases, with all the irrigated water lost over 7 days, while the WR% of the soil mixed with any concentration of the prepared composite was down after 18 and 20 days for 3% of PC-GG/PAAm/ ZnO 0 and PC-GG/PAAm/ZnO 7 , respectively. Therefore, the water retention rate of the soil was improved by adding 3% of the composite. It is also clear that soil amendment achieved by PC-GG/PAAm/ZnO 7 is more pronounced than that of PC-GG/PAAm/ZnO 0 . This behavior may be attributed to that the PC-GG/PAAm/ZnO x hydrogel composites possessing a high hydrophilic structure with numerous hydrophilic

Biodegradation in Sandy Soil
The rate of biodegradation of GG-PC/PAAm/ZnO 0 and GG-PC/PAAm/ZnO 7 is illustrated in Fig. 6c. It is clear that the GG-PC/PAAm/ZnO 0 exhibited fast degradation, i.e., about 45% of the mass of the incubated hydrogel was degraded after 30 days of the experiment due ot its high content of the green polymers. On the other hand, GG-PC/PAAm/ZnO 7 showed high resistance to the biodegradation due to the antimicrobial activity of ZnO [44].

Reusability Study
The maximum water absorption of GG-PC/PAAm/ZnO 0 and GG-PC/PAAm/ZnO 7 was estimated at three swelling/ deswelling cycles in order to investigate the reusability of these hydrogels. The data are presented in Fig. 7. It is clear that the composite hydrogel GG-PC/PAAm/ZnO 7 can be repeatedly used with reasonable swelling capacity when compared with PC/PAAm/ZnO 0 . This observation may be due to the role of ZnO in reserving the hydrogel structure and protecting the gel matrix during repeated swelling/deswelling cycles [45]. Repeated use of hydrogel with acceptable performance is very important for economic efficiency and sustainable agriculture.

Conclusions
Green superabsorbent hydrogel composites were prepared and employed for sustained agriculture. The hydrogels comprised of guar gum, pectin and acrylamide crosslinked together with gamma irradiation and fabricated with different weight ratios of zinc oxide. The chemical structure of the composites was proved by the FT-IR and their topography was scanned by the AFM. The data reveal that the composite GG-PC/PAAm/ZnO 7 depicted maximum swelling, high water retention and the lowest degradation due to the presence of zinc oxide which possess wellestablished antibacterial effect. Moreover, the swelling kinetics reveal that GG-PC/PAAm/ZnO 0 hydrogel follows Fickian diffusion while PC-GG/PAAm/ZnO 7 hydrogel composite obeys non-Fickian diffusion. This proves that ZnO controls in the mechanism of swelling. The results also confirmed the high efficiency of the prepared composites in amending the physical properties of sandy soil. So that they can be employed as excellent candidates in sustained agriculture. Funding Open access funding provided by The Science, Technology & Innovation Funding Authority (STDF) in cooperation with The Egyptian Knowledge Bank (EKB). The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Data Availability
The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.

Conflict of interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work presented in this manscript.
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