Synthesis of adipic acid dihydrazide-decorated coco peat powder-based superabsorbent for controlled release of soil nutrients

Abstract

Superabsorbent composites based on agricultural byproducts represent a promising strategy for recycling exploitable waste resources and relieving environmental stress, but their development is usually hindered, because the traditional pretreatment technology for lignocellulosic raw materials is intricate and time-consuming, and always generates pollution-causing effluents. In this research, we firstly employ adipic acid dihydrazide as an outstanding decorating substrate for the pretreatment of CP, and then synthesize a novel superabsorbent (MA-CP@ADH-g-PAA) through grafting poly(acrylic acid) onto an MA-modified CP@ADH surface. Compared with conventional acid and alkali pretreatment approaches, CP@ADH, without the need for any harsh reaction conditions or complex equipment, has avoided possible pollution during the process and simultaneously introduced plenty of amine groups onto the inert CP surface. The CP@ADH had a higher degree of substitution than that of pristine CP, and the grafting efficiency reached 75.2%. The swelling equilibrium of the MA-CP@ADH-g-PAA reached about 470.02 g g⁻¹ in distilled water, and the swelling behavior under load and load-free both fitted well with the pseudo-second-order model. Owing to their excellent water absorbing and holding capacity, and slow release behavior of soil nutrients, it is believed that the MA-CP@ADH-g-PAA superabsorbent product could have potential industrial applications, especially in agricultural and horticultural usage.

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1. Introduction

Superabsorbent polymers have the ability to absorb large volumes of water and retain a swollen state even under pressure,^1,2 and have been widely used in hygiene products,³ drug-delivery systems,⁴ wastewater treatment,⁵ and agriculture.⁶ The synthesis of superabsorbent polymers is mainly concentrated on petroleum-based polymers, such as poly-acrylic acid and/or acryl amide,^7–10 which are obviously non-renewable, costly to produce, difficult to degrade, and harmful to the environment.¹¹ Seeking to solve this problem, some renewable natural resources, such as wheat,¹² corn,¹³ carrageenan,¹⁴ and sea buckthorn branches¹⁵ have been proposed to partially substitute conventional petroleum-based polymers. Coco peat powder (CP), another resource-rich lignocellulosic material, is a byproduct of coconut palm, which represents one of the most widely distributed food crops in the world.¹⁶ Just like other natural agricultural byproducts, CP materials have prominent advantages, including: (i) they are inexpensive, and completely biodegradable and renewable; (ii) the dominating chemical composition of CP is cellulose, hemicellulose, and lignin,¹⁷ which contain hydroxyl, ketone, aldehyde, or carboxylic groups,¹⁸ providing the prerequisite hydrophilicity and a versatile platform for further chemical modification with other desired molecules; (iii) there are many capillaries inside lignocellulosic crops to deliver essential moisture and nutrients; the inherent capillary structure of coco peat can serve as reservoirs to store water. As seen from the above highlights, CP can be an ideal candidate to be employed in the fabrication of a superabsorbent composite. Regretfully, the abundant CP is mainly treated as agricultural waste, which is characterized by serious resource waste and significant environmental pollution.^19,20 For developing waste resources and relieving environmental stress resulting from burning, burying, or discarding coco peat powder outside, the construction of a CP-based superabsorbent composite by encapsulating coco peat powders into an interpenetrative structure of a polymer matrix is urgently needed.

For preparing a superabsorbent composite based on lignocellulosic material, free radical grafting copolymerization is the most widely applied method, which depends heavily on the hydroxyl groups of cellulose as radical acceptors.^21,22 In this case, if more cellulose-containing hydroxyl groups are exposed to radicals, the grafting reaction occurs much more easily. Therefore, prior to grafting, it is always necessary to perform pretreatment to eliminate hemicellulose, lignin, and other various ashes covering the cellulose to increase the reactivity of raw lignocellulosic materials. In fact, these removed substances are most probably a treasure stored in the wrong places, since they are harmless, biodegradable, and renewable, and as good as cellulose. Currently, the popular pretreatment approaches include steam explosion,²³ acid and base,²⁴ and organic solvent processes.²⁵ It has been verified that such pretreatment tactics have several limitations, including the fact that they are somewhat intricate and tedious, require harsh reaction conditions in some cases, and often generate unpleasant and pollution-causing effluents, making them difficult to apply in practical applications.²⁶ In brief, the pressing question is how to boost the reactivity of raw materials and simultaneously take advantage of all the components of lignocellulosic materials through a simple and feasible method, rather than tedious pretreatment. To solve the above problem, a simple and versatile pretreatment method based on adipic acid dihydrazide (ADH) was proposed. Adipic acid dihydrazide, as an important non-toxic and non-mutagenic covalent crosslinking molecule,^27,28 possesses two symmetrical amido groups containing carbonyl and terminal amino groups. The amino groups within the ADH molecular structure could react with free carboxylic groups to form an amide linkage.²⁹ Relying on its unique bi-functional acylhydrazine groups, ADH has been used as a bi-functional linker for the functionalization of hydrogels,^30,31 membranes,³² and drug carriers.³³ For example, Agrahari et al. tested microbicide release from hyaluronic acid nanoparticles cross-linked with ADH and confirmed the amide bond formation between hyaluronic acid and ADH after cross-linking.³⁴ Wu et al. functionalized graphene oxide with ADH to introduce amine moieties, and then hyaluronic acid was further grafted to graphene oxide by the formation of amide bonds.³⁵ Ito et al. synthesized novel injectable hydrogels by modifying carboxymethyl dextran with ADH for peritoneal adhesion prevention.³⁶ In view of these characteristics, we hypothesize that the ADH-connecting technology can be applied to functionalize CP by forming an amide bond with the carboxylic group of lignin, eliminating the negative effects of traditional pretreatment processes and making secondary chemical reactions with the desired monomers easier. It should be noted that this is the first application of ADH in decorating lignocellulosic materials to the best of our knowledge.

Herein, we proposed a facile and robust strategy for the formation of superabsorbent composites containing degradable and renewable CP; that is, the CP is firstly decorated by ADH, and further modified with maleic anhydride (MA), followed by grafting poly(acrylic acid) (PAA) to yield the MA-CP@ADH-g-PAA superabsorbent. The structure and morphology of the sample were characterized by FTIR and SEM, respectively. The surface characteristics in terms of degree of substitution (DS) and grafting efficiency were also studied for further affirmation of the synthesis procedure. Additionally, the swelling behaviors in various media, swelling kinetics under load and load-free, water-retaining ability, and the release profile of soil nutrients were systematically investigated. The present research provides a more eco-friendly and simpler pretreatment approach that can be extended to the simple synthesis of other natural resources with similar structures, and simultaneously help to significantly address the multiple issues created by petroleum-based superabsorbents and excess agricultural wastes.

2. Experimental section

2.1. Materials

Coco peat powder (CP) was produced from the hard shell of coconut fruit, processed in a blender and sieved to 200 mesh (provided by the Hainan fruit malls, China). Adipic acid dihydrazide (ADH) was obtained from Tokyo Kasei Kogyo Co., Ltd. Maleic anhydride (MA) was supplied by Nanjing Branch Chemical Industry Co., Ltd. Acrylic acid (AA), N,N-methylene-bis-acrylamide (MBA), ammonium persulfate (APS), and N,N-dimethyl formamide (DMF) were purchased from Tianjin Tiancheng Chemical Co., Ltd. Acrylic acid was purified by distillation under vacuum prior to use.

2.2. Preparation of CP@ADH and MA-CP@ADH intermediates

CP (1.0 g) was ultrasonically dispersed in 100 mL of N,N-dimethyl formamide (DMF) for 30 min, and ultrasonication was allowed to proceed for 2 h after 5.0 g ADH was added. The produced ADH-decorated CP (denoted as CP@ADH) was separated by centrifugation and rinsed repeatedly with water. Then, 1.1 g MA was introduced into the mixture and stirred at 50 °C for another 1 h. After the reaction, the resulting product, MA-modified CP@ADH (denoted as MA-CP@ADH), was filtered, and washed with distilled water and ethanol, followed by drying in a vacuum at 30 °C. In this procedure, the MA content ranged from 30% to 90%. To further demonstrate the properties of MA-CP@ADH and for comparison, MA-modified CP without ADH decoration (denoted here as MA-CP) was also prepared by employing similar synthetic conditions to those described above, and studied.

2.3. Preparation of MA-CP@ADH-g-PAA superabsorbent composite

The MA-CP@ADH-g-PAA superabsorbent was synthesized using the following procedure: typically, a quantity of dried MA-CP@ADH powder and distilled water were put in a 250 mL three-necked flask equipped with a stirrer, a condenser, and a thermometer. The slurry was heated to 55 °C and, after 30 min, the initiator APS (80 mg dissolved in 25 mL of distilled water) was added into the slurry under nitrogen atmosphere. Then, various amounts of AA with a degree of neutralization of 60% were added into the slurry with continuous stirring. After that, 48 mg of MBA was dissolved in 25 mL of distilled water and added into the flask. After reaction at 55 °C with continuous stirring for 4 h, the resulting product was washed with distilled water and ethanol and then dried at 70 °C to a constant weight.

2.4. Determination of degree of substitution

The degree of substitution of samples, including MA-CP@ADH and MA-CP, was determined by a chemical titration method.³⁷ Briefly, 1.0 g MA-CP@ADH or MA-CP was dissolved in 10 mL of benzyl alcohol, and then 10 mL 0.5 M NaOH was added to the slurry and the solution was occasionally shaken over 72 h to complete saponification at room temperature. The excess alkali in the solution that leached from the sample was titrated with 0.5 M HCl, using phenolphthalein as an indicator. Blank runs were carried out for correction. The degree of substitution (DS) was estimated using the following equations:where H_ma, content of MA group; W, mass of samples; c, HCl concentration (mol L⁻¹); V₀, volume (mL) of HCl used to titrate blank; V, volume (mL) of HCl used to titrate sample; 162, molecular weight of the anhydroglucose unit; 98, molecular weight of MA.

2.5. Grafting efficiency

A dry MA-CP@ADH-g-PAA sample was immersed in 300 mL distilled water and stirred continuously for 10 h at room temperature.³⁸ After that, the mixture was centrifuged to separate the undissolved graft copolymer. During this process, the water was periodically changed to ensure complete removal of the water-soluble component. The remaining MA-CP@ADH-g-PAA obtained after water extraction was dehydrated with methanol and dried overnight in an oven at 60 °C. The dried MA-CP@ADH-g-PAA was weighed to determine the amount of the water-soluble component, consisting of free PAA. The grafting efficiency of MA-CP@ADH-g-PAA was calculated using the following equation:where W₀, W₁, and W₂ denote, respectively, the final weight of the product after water extraction, the original weight of CP and ADH, and the original weight of the monomers used.

2.6. Water adsorption capacity

The MA-CP@ADH-g-PAA obtained after water extraction and drying treatment was immersed in distilled water and allowed to soak at 25 °C for 120 min. At a fixed interval, the swollen sample was filtered with a mesh, drained fully, and weighed. The swelling capacity of each product was calculated according to the following equation:where Q₁ and Q₂ are the weights of the dry and swollen samples, respectively. The water absorbency behavior of MA-CP@ADH-g-PAA in NaCl, CaCl₂, and FeCl₃ solution for 100 min was similarly investigated as defined in eqn (4).

2.7. Water absorbency under load (AUL)

A fixed amount of dried MA-CP@ADH-g-PAA wrapped in gauze was evenly distributed on a hollow macroporous Plexiglas cylinder with a diameter of 3.0 cm. Next, the cylinder was placed in a 100 mL beaker containing 60 mL distilled water. A cylindrical solid load (varied in weight) that slipped freely in the cylinder was used to apply the load to the dry superabsorbent samples (8.3 and 13.2 g cm⁻²). The MA-CP@ADH-g-PAA sample was allowed to soak in the solution at room temperature for a defined duration (1–120 min). The entire setup was covered to prevent surface evaporation. Then, the swollen sample was separated from the unabsorbed water at set intervals and weighed. The absorbency under load (AUL) at different time intervals was calculated according to eqn (5):³⁹where m₀ and m₁ represent the weights of the dry superabsorbent sample and cylinder, and m₂ is the weight of the glass cylinder with the swollen sample.

2.8. Water retention capacity

The dried MA-CP@ADH-g-PAA sample was soaked in distilled water at 25 °C to achieve saturation. Then, the swollen MA-CP@ADH-g-PAA was placed at 35 °C and 65 °C, under a centrifugal force of 2100 rpm and 4200 rpm, respectively. At fixed intervals, the sample was weighed. The water retention capacity R_i under various conditions was determined by the following equation:where M₀ and M_i are the weights of the swollen and desorbed samples (g), respectively, and R_i is the water retention rate per gram of swollen sample (g g⁻¹).

2.9. Loading of NPK and controlled release

The NPK-loading system was prepared according to the method described previously.⁴⁰ Briefly, an appropriate amount of the MA-CP@ADH-g-PAA superabsorbent was added to 500 mL of distilled water containing 0.03 mol L⁻¹ of both KH₂PO₄ and NH₄NO₃ for 6 h to reach an equilibrium state, and then dried at 40 °C for 3 days. The fertilizer loading efficiency was calculated according to the following equation:where M_u and M_L are the weights of unloaded and loaded dry samples, respectively.

To study the controlled release behavior, 1.0 g of the NPK-loaded products was put into 500 mL of distilled water. The released N, P, and K contents in the solution were measured using an elemental analyzer instrument (Elementar Vario MAX CN, Germany), using a colorimetric method with the aid of a spectrophotometer (Shimadzu-18A, China) and atomic absorption spectrometer (Z-2000, Japan).^41,42 Comparison samples without the MA-CP@ADH-g-PAA superabsorbent, containing the same amount of NPK, served as a blank control group, and the released contents of soil nutrients were evaluated as above.

2.10. Characterization

Fourier transform infrared (FTIR) spectra of samples were recorded on a Nicolet Nexus 670 FTIR spectrometer in the 4000–400 cm⁻¹ region by use of KBr pellets, with samples extracted in distilled water for 72 h at room temperature. Micrographs of samples were examined with a Hitachi S-4800 scanning electron microscope (SEM).

3. Results and discussion

3.1. Synthesis and characterization of MA-CP@ADH-g-PAA superabsorbent

The detailed mechanism of grafting-copolymerization of AA monomers onto the CP surface with the aid of an ADH linker and MA modifier, as well as the formation of intermediates, is illustrated in Scheme 1.

Scheme 1 Proposed mechanism for the formation of MA-CP@ADH-g-PAA superabsorbent.

The fabrication of the MA-CP@ADH-g-PAA superabsorbent composite began with the synthesis of CP@ADH. In the current work, when CP was dropped into ADH solution, the carboxylic groups of CP lignin could couple with the amino moieties to yield amide linkages,⁴³ implanting numerous ADH molecules onto the surface of CP (step 1). Afterwards, the primary amine moieties in CP@ADH were allowed to react with MA through an amide linkage on the other side (step 2). Then, under heating conditions, the binding MA molecules were despoiled of hydrogen atoms and changed into MA-CP@ADH macroradicals through reaction with sulfate anion-radicals generated during the decomposition of the APS initiator (step 3).⁴⁴ The AA monomer adjacent to the reaction sites became an acceptor of the resultant MA-CP@ADH macroradicals, leading to the propagation of a new polymeric chain.⁴⁵ In this way, actively dissociative monomers, groups, and chains can be spontaneously involved in the polymerization reaction and bring growth to the grafted chain. Meanwhile, the polymeric chains reacted with the ends of vinyl groups of the cross-linker MBA during the chain propagation (step 4),^46,47 forming an interpenetrating network structure and consequently enhancing the hydrophilicity and adsorption ability.

On the basis of the above analysis, it is obvious that the introduction of ADH molecules is pivotal to the construction of the MA-CP@ADH-g-PAA composite. Firstly, with the use of ADH, the pretreatment step of raw CP becomes facile, economical, and time-saving, avoiding the segregation of valuable lignin components and possible pollution. Secondly, the ADH linker supplies a powerful platform for facile occurrence of the graft copolymerization reaction, since the ADH linker can seize both MA and AA molecules and further react with them in the presence of cross-linker MBA and initiator APS. Moreover, the MA modifier can introduce a number of carboxylic groups onto the CP@ADH, which is good for enhancing the swellability of the CP@ADH; the double linkage of MA can be attacked by sulfate anion radicals and transformed to free radicals, facilitating AA grafting onto the CP@ADH and favoring a higher polymerization reaction rate.³⁷ In brief, integration of the ADH linker and MA modifier onto the CP surface, with each component acting in a coordinated way, has directed a critical middle step for the synthesis of the MA-CP@ADH-g-PAA superabsorbent.

FTIR spectroscopy was employed to confirm the above formation mechanism of the MA-CP@ADH-g-PAA superabsorbent, and the results are shown in Fig. 1.

Fig. 1 The FTIR spectra of CP (a), CP@ADH (b), MA-CP@ADH (c), and MA-CP@ADH-g-PAA superabsorbent (d).

The absorption bands observed at 3320 cm⁻¹ (hydroxyl stretching vibration), 2927 cm⁻¹ (methyl and methylene), and 1041 cm⁻¹ and 890 cm⁻¹ (β-1,4-glycosidic bond) were the characteristic absorptions of the cellulose structure (Fig. 1a). The band at 1650 cm⁻¹ was attributed to C=O stretching from ketones, aldehydes, or carboxylic groups of lignin.⁴⁸ Compared with the spectrum of CP, the appearance of absorption bands at 3365 cm⁻¹, 3083 cm⁻¹, and 1633 cm⁻¹ in the spectrum of CP@ADH (Fig. 1b) may be ascribed to the characteristic absorptions of –NH₂, –NH, and –C=O, respectively.⁴⁹ Furthermore, additional absorption peaks at 1428 cm⁻¹ (–CN stretching vibration), 2861 cm⁻¹, and 1466 cm⁻¹ (–CH stretching vibration and bending vibration) were observed, providing evidence of ADH successfully coating onto the CP surface. Fig. 1c displays the FTIR spectrum of the MA-CP@ADH; the absorption band at 1428 cm⁻¹ disappeared after the reaction, and the emergence of new absorption bands at 1728 cm⁻¹ and 1251 cm⁻¹ corresponded to the carboxylic acid of MA, indicating that MA has reacted with CP@ADH.^50,51 Fig. 1d represents the spectrum of the MA-CP@ADH-g-PAA superabsorbent, which displayed the following bands. Namely, the band at 2927 cm⁻¹ indicated the existence of CP scaffolds in the superabsorbent and the peak at 1510 cm⁻¹ was ascribed to asymmetric –COO⁻ stretching. Moreover, a new absorption peak appearing at 1656 cm⁻¹ may be attributed to the reaction between PAA and MA. On the basis of the analysis of FTIR, the conclusion can be drawn that MA was fixed on the CP surface through ADH linking, and subsequently AA grafting polymerization gave rise to the MA-CP@ADH-g-PAA superabsorbent composite.

The formation procedure of MA-CP@ADH-g-PAA, prepared by grafting PAA on the surface of MA-modified CP@ADH, can be further demonstrated by their SEM images. The micrographs of the original CP, CP@ADH, MA-CP@ADH, and MA-CP@ADH-g-PAA are depicted in Fig. 2a–d, respectively.

Fig. 2 SEM images of CP (a), CP@ADH (b), MA-CP@ADH (c), and MA-CP@ADH-g-PAA superabsorbent (d).

As can be seen from Fig. 2a and b, the primitive CP clearly displayed a virgulate structure with a smooth and loose flat ribbon, while the CP@ADH samples showed a coarse and wrinkled surface containing some small pores and fine particles. In comparison, the water contact angle of CP@ADH in the inset image of Fig. 2b was measured to be 37°, which was obviously decreased compared to that of primitive CP (68°). The changes in morphology and water contact angle further supported the occurrence of successful coverage and fixation of ADH molecules on the surface of the CP scaffolds. After subsequent modification with maleic anhydride, the water contact angle of MA-CP@ADH was changed to 0° (the embedded image in Fig. 2c), providing assertive evidence that many MA molecules had been introduced onto the CP@ADH scaffolds. Meanwhile, numerous rugged folds and irregular micro-sized aggregates were formed on the MA-CP@ADH surface. In the visual area, the MA-CP@ADH-g-PAA (Fig. 2d) presented an undulant and coarse surface structure with many microporous holes, implying that many CP scaffolds were implanted into the polymer matrix as unique fillers. Such a unique surface morphology could increase the water-absorbing channels and boost the penetration of moisture into the network, thereby resulting in a relatively high water absorbency.⁵²

Complementary to the SEM images and FTIR spectra, the degree of substitution (DS), which is a quantitative indicator of the carboxylation reaction efficiency between MA molecules and CP substrates, provided additional evidence of successful immobilization of ADH molecules on the CP surface. Herein, the DS of MA-CP@ADH and MA-CP was determined by a chemical titration method, in order to further illustrate the response of the carboxylation reaction efficiency to the fixation of ADH. The results are shown in Table 1.

Table 1 The influence of ADH on the degree of substitution

Samples	Influence of MA content on degree of substitution^a
	30%	40%	50%	60%	70%	80%	90%
a The MA content was based on the mass ratio of the CP.b Difference value means the extra values of the DS of MA-CP@ADH relative to that of MA-CP on the corresponding mass ratio of CP to MA.
MA-CP	3.4	10.3	14.4	16.2	16.8	17.5	18.0
MA-CP@ADH	6.2	13.5	17.3	19.0	19.9	20.6	21.0
Difference value^b	2.8	3.2	2.9	2.8	3.1	3.1	3.0

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As shown in Table 1, the degree of substitution of both MA-CP and MA-CP@ADH increased rapidly as the MA amount increased from 30% to 60%, while it leveled off with a further increase in the MA content. As the MA content continuously rose, the superfluous anhydride from the MA molecules could react with amino or hydroxyl groups of CP to produce larger branched chains. The presence of larger branched chains implied that a higher degree of substitution was prevented because of their steric hindrance effect.⁵³ In contrast, after fixation of ADH molecules, the DS values of MA-CP@ADH were a little higher than those of MA-CP under identical reaction conditions. This is because, when the overall surface of CP was eventually decorated by ADH, the CP@ADH possessed an excellent active surface with both amino and hydroxyl groups and consequently, the subsequent reaction procedure between CP@ADH and the desired MA became easily feasible compared with the bare CP substrate. Moreover, the amino groups of ADH partly replaced the hydroxyl groups of CP in the carboxylation reaction to improve the reactive performance.⁵⁴ It is obvious that the reactive properties of CP are significantly improved by the early decoration with the ADH linker, since many amino active groups from ADH molecules were introduced onto the CP surface.

3.2. Water absorbency of the MA-CP@ADH-g-PAA superabsorbent with different AA contents

Water absorbency is one of the biggest advantages of a superabsorbent, and the application of a superabsorbent composite depends largely on its water-absorbing capacity. The swelling properties of the MA-CP@ADH-g-PAA superabsorbent with various AA ratios were tested in distilled water, tap water, and 0.9% NaCl solution. As shown in Table 2, when the ratio of MA-CP@ADH to AA changed from 1 : 3 to 1 : 7, the water absorbency increased to 470.02 g g⁻¹ in distilled water, 350.19 g g⁻¹ in tap water, and 61.03 g g⁻¹ in 0.9% NaCl solution and then slumped. With an increase in the AA content, the molecular mass and grafting efficiency of poly(acrylic acid) chains were increased, resulting in the introduction of more hydrophilic groups (–COOH) onto the polymer chains, and consequently the water-absorbing capacity was increased.⁵⁵ When the amount of AA exceeded the ratio of 1 : 7, the water absorbency began to decrease. This phenomenon may be due to the formation of AA homopolymer and an increase in the reaction medium viscosity, impeding the movement of free radicals and AA monomer.⁵⁶ In addition, the effect of the MA-CP@ADH to AA ratio on grafting efficiency was also investigated, and the average grafting efficiency at a ratio of 1 : 7 reached 75.2% (Table 2). The grafting efficiency should be attributed to a combination of the ADH and the MA molecules captured on the CP surface, which provide more active sites for grafting more AA monomer.

Table 2 Water absorbency of MA-CP@ADH-g-PAA superabsorbent with different contents of AA

The ratio of MA-CP@ADH to AA	Grafting efficiency (%)	Water absorbency (g g^−1)
		Distilled water	Tap water	0.9% NaCl solution
1 : 3	70.9 ± 0.9	445.61	311.03	55.28
1 : 5	72.1 ± 1.1	461.39	329.61	58.01
1 : 7	75.2 ± 1.3	470.02	350.19	61.03
1 : 9	69.5 ± 0.8	458.34	334.21	57.38
1 : 11	68.3 ± 1.2	416.32	308.07	50.31

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3.3. Swelling kinetics of the MA-CP@ADH-g-PAA superabsorbent under load

Analysis of the swelling kinetics of a superabsorbent under a given pressure is beneficial to clarify the mechanism of the swelling process and to further evaluate the response of the MA-CP@ADH-g-PAA composites to pressure. To study the effect of pressure on the water-absorbing kinetics, the swelling behaviors of MA-CP@ADH-g-PAA composites under applied loads of 8.3 and 13.2 g cm⁻² (Fig. 3(d)–(i)) were also investigated, and the load-free MA-CP@ADH-g-PAA composites were used as control groups (Fig. 3(a)–(c)). The swelling rate of the MA-CP@ADH-g-PAA superabsorbent composite under a load was a little slower than that of the load-free composite. This is because the load pressure weakens the electrostatic repulsion and further brings about shrinkage of the network, hindering the permeation of water into the polymeric network.⁵⁷ Under load-free conditions (Fig. 3(a)–(c)), all the samples presented higher water uptake rates with premier swelling progress and gradually leveled off until swelling equilibrium was reached within 100 min. The initial swelling properties of the samples mainly depended on the capillary effect and the capturing ability of the hydrophilic groups on the surface of the molecular structure of the MA-CP@ADH-g-PAA superabsorbent composite.⁵⁸ When the water entered into the interior network of the superabsorbent composite, it could accelerate the dissociation of ionic groups. Thus, the electrostatic repulsion caused by the ionization groups promoted the expansion of the MA-CP@ADH-g-PAA network. Also, such electrostatic repulsion in the network is conducive to strengthening the osmotic pressure difference,⁵⁹ which could consequently promote the movement of more water into the superabsorbent. As many more water molecules diffused into the network and gradually weakened the osmotic pressure difference, the swelling rate became slower, and the swelling ability finally reached equilibrium.⁶⁰

Fig. 3 Water uptake kinetics of MA-CP@ADH-g-PAA superabsorbent with various AA contents under a load and load-free.

The swelling kinetics are one of the most fascinating characteristics responsible for the efficiency of water absorption. In this section, pseudo-first-order and pseudo-second-order models are often used to describe the kinetics of water absorption of the superabsorbent polymer. The models can be delivered in the following equations:^61,62

ln(q_e − q_t) = ln q_e − k₁ (8)

where q_e (g g⁻¹) is the equilibrium water absorbency, q_t (g g⁻¹) is the water absorbency at contact time t (min), and k₁ (min⁻¹) and k₂ (g g⁻¹ min⁻¹) are the rate constants of the pseudo-first-order and pseudo-second-order models, respectively. The calculated values of k₁ and k₂ and their corresponding regression coefficients (R²) and chi-square (χ²) values are presented in Table 3. On the basis of the closed values of R² and χ², the experimental data fitted well with the pseudo-second-order kinetics. Furthermore, theoretical values (q_e,cal) obtained from the pseudo-first-order kinetic model seemed to be remarkably less than the experimentally observed data (q_e,exp), while the values from the pseudo-second-order model were very close to the q_e,exp. Therefore, the swelling behavior of MA-CP@ADH-g-PAA, with or without load, followed the pseudo-second-order swelling kinetic model.

Table 3 Kinetic parameters of pseudo-first-order and pseudo-second-order kinetic models

MA-CP@ADH : AA	Under the load	qe,exp (g g^−1)	Pseudo-first-order kinetic model				Pseudo-second-order kinetic model
			k1 (min^−1)	qe,cal (g g^−1)	R^2	χ^2	k2 (g g^−1 min^−1)	qe,cal (g g^−1)	R^2	χ^2
1 : 3	0	445.61	0.0305	404.78	0.9823	20.65	0.004	477.12	0.9975	53.98
	8.3	163.51	0.0319	151.09	0.9721	25.52	0.007	173.24	0.9944	14.12
	13.2	133.97	0.0298	120.70	0.9833	15.53	0.006	145.33	0.9957	7.95
1 : 7	0	470.02	0.041	425.48	0.9768	85.17	0.011	500.08	0.9965	68.08
	8.3	225.54	0.047	208.95	0.9728	39.58	0.019	233.32	0.9963	13.58
	13.2	192.27	0.039	179.93	0.9876	55.92	0.007	196.54	0.9932	35.53
1 : 11	0	416.32	0.025	400.86	0.9882	29.58	0.002	427.75	0.9986	29.77
	8.3	102.74	0.024	90.37	0.9733	8.91	0.006	110.85	0.9943	7.04
	13.2	78.02	0.026	72.85	0.9724	6.19	0.005	80.98	0.9941	3.99

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3.4. Water absorbency of MA-CP@ADH-g-PAA superabsorbent in different salt solutions

In order to reflect the swelling behavior of the MA-CP@ADH-g-PAA superabsorbent under practical swelling conditions, for example in the presence of mineral salts,⁶³ such as Na⁺, Ca²⁺, and Fe³⁺, samples soaked in certain applied concentrations were studied, as shown in Fig. 4.

Fig. 4 Effect of saline solution on the water absorbency of the MA-CP@ADH-g-PAA superabsorbent.

As depicted in Fig. 4, the water absorption capacity of MA-CP@ADH-g-PAA in NaCl, CaCl₂, and FeCl₃ solutions was markedly lower than that in distilled water. This was because, for ionic hydrogels, the additional cations can shield the carboxylate anions and destroy the anion–anion electrostatic repulsion, consequently leading to a reduced osmotic pressure difference between the external solution and the polymer network, resulting in a decreased swelling capacity.⁶⁴ Moreover, the water absorbency appreciably decreased as the concentration of the salt solutions increased from 0 to 50 mmol L⁻¹ and gently decreased at concentrations > 50 mmol L⁻¹. This phenomenon was ascribed to the fact that the high salt concentration decreased the differential osmotic pressure between the polymeric network and the swelling medium, resulting in collapse of the swollen superabsorbent.⁶⁵ Besides, the effects of these cations on the swelling property were in the order: Na⁺ > Ca²⁺ > Fe³⁺. This was because the polyvalent metal ions can form complexes with the carboxylate groups, and the solutions have higher ionic strength.⁶⁶ So, the sensitivity of the MA-CP@ADH-g-PAA superabsorbent composite to monovalent saline was more than that to multivalent saline.

3.5. Water retention properties of MA-CP@ADH-g-PAA superabsorbent

The water retention capacity is one of the key properties of a superabsorbent in various applications, such as in agricultural and horticultural areas. In this section, we describe the influence of the temperature and centrifugal force on the water-retention capacity of the optimized MA-CP@ADH-g-PAA sample; the experimental results are collected in Fig. 5.

Fig. 5 Water-retaining capacity of the MA-CP@ADH-g-PAA superabsorbent centrifuged at 2100 rpm and 4200 rpm and dried at 35 °C and 65 °C.

As can be seen in Fig. 5, the retained water gradually decreased as the time increased, whether under conditions of high-temperature drying or centrifugal separation. The difference in water retention between these two different conditions is that the MA-CP@ADH-g-PAA superabsorbent maintained 15% after centrifugation for 40 min at 2100 rpm and 27 min at 4200 rpm, whereas the retained water was completely evaporated after 48 h and 40 h when the sample was incubated in an oven heated at 35 °C and 65 °C, respectively. The reason for this phenomenon was the van der Waal's forces and hydrogen-bonding interactions between the superabsorbent and water molecules, which directly affected the water retention performance of the absorbent resin.⁶⁷ Additionally, judging from the centrifugation curve, it can be seen that in the first 18 min, the curve showed a sharp decline, and after 18 min, the water retention decreased slightly. This may be because, initially, more weakly absorbed water contained in the gel was more easily broken free, and with the passage of the centrifugation time, the relative proportion of strongly absorbed water increased, resulting in relatively slow dehydration.⁶⁸ And yet, the gradual reduction in the drying curve indicated that the change in the rate of water loss was not obvious. According to the above analysis, the water-retaining performance of the MA-CP@ADH-g-PAA superabsorbent was more sensitive to pressure than to temperature. In brief, the MA-CP@ADH-g-PAA superabsorbent is promising for potential applications in agricultural areas, especially as a reservoir to store water or a nutrient in arid regions.

3.6. Slow release of soil nutrients

As well as water, NPK fertilizer is another main factor in the sustainable development of agriculture. However, the low use efficiency of fertilizers and subsequent nutrient release gives rise to serious damage to the environment, ecology and economy.^69,70 Superabsorbent materials with the ability to prolong the release time of soil nutrients represent an attempt to couple irrigation and fertilizer applications together in a single system, emerging as an ideal tool to overcome these problems.⁷¹ The MA-CP@ADH-g-PAA superabsorbent, with excellent water absorption and retaining ability may be used as an agricultural water-retaining agent for effective utilization of water resources, and as a promising vehicle for delivery of NPK fertilizer. In addition, the embedded and dispersed CP scaffolds inside the network of the MA-CP@ADH-g-PAA superabsorbent are natural lignocellulosic materials and can be completely degraded to non-toxic small molecules without harm to the environment. By virtue of these benefits, the MA-CP@ADH-g-PAA superabsorbent composite was designed to gradually deliver soil nutrients and moisture to targeted crops over time. To further investigate the slow release properties of the MA-CP@ADH-g-PAA superabsorbent, the release trend of NPK from the superabsorbent containing 16.8% of fertilizers was examined. The experimental results are depicted in Fig. 6.

Fig. 6 Release behaviors of N, P, and K from (a) loaded superabsorbent and (b) free NPK fertilizer without superabsorbent.

It is obvious that the cumulative release efficiency of NPK from the as-prepared superabsorbent (Fig. 6a) increased more slowly as a function of time in comparison with the free forms (Fig. 6b). A similar result was reported in previous research.⁷² The slower release rate from the loaded superabsorbent results from the interaction between the loaded NPK nutrients and the many active ionic sites within the network structure of the MA-CP@ADH-g-PAA superabsorbent. Moreover, the nutrient release from the free NPK was very fast and immediately reached equilibrium within 20 min, while this was prolonged to around 4 h from the loaded MA-CP@ADH-g-PAA superabsorbent. To be more specific, the N, P, and K released steadily from the superabsorbent system was of the order of 30.7%, 30.0%, and 30.2% within 30 min; 45.4%, 41.5%, and 44.3% within 50 min; and 84.1%, 84.4%, and 80.7% within 120 min. Interestingly, for the free NPK sample, the release rate of N was apparently faster than that of P and K. However, the liberation of all components from the superabsorbent system is more controlled and simultaneous, which is considered to be beneficial to plant growth. These promising results reveal that MA-CP@ADH-g-PAA has great potential for agricultural and horticultural applications, i.e., the superabsorbent, with the ability to control the release of the loaded soil nutrients, can serve as an efficient fertilizer and water carrier to couple irrigation and fertilizer applications together and increase the use efficiency of fertilizers.

4. Conclusion

In summary, a novel superabsorbent polymer (MA-CP@ADH-g-PAA) was synthesized through grafting PAA onto the surface of MA and ADH-functionalized CP scaffolds. FTIR and SEM analyses confirmed that the ADH was first fixed onto the surface of CP and then participated in a graft polymerization reaction. After decoration with ADH molecules, a higher DS was observed and more MA molecules were introduced onto the surface of the CP@ADH substrate. The swelling equilibrium of the MA-CP@ADH-g-PAA samples can reach about 470.02 g g⁻¹ in distilled water, 350.19 g g⁻¹ in tap water, and 61.03 g g⁻¹ in 0.9% NaCl solution. An external load pressure has a significant negative effect on the water-absorbing ability. The swelling behaviors of the superabsorbent fitted well with the pseudo-second-order kinetic model and were significantly sensitive to the concentration of salt solution. Moreover, the as-synthesized superabsorbent exhibited excellent water-retaining ability and slow release of soil nutrients, which represent an attempt to couple irrigation and fertilizer applications together when applied in agricultural and horticultural areas.

Acknowledgements

This work was supported by Shaanxi Provincial Natural Science Foundation of China (No. 2015JM2071), the National Natural Science Foundation of China (No. 21176031) and Fundamental Research Funds for the Central Universities (No. 2014G3292007).

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