Preparation, Swelling Behaviors, and Slow-Release Properties of a

superabsorbent polymer,8 many methods have been attempted to improve the ... capabilities of the PAA-AM/SH superabsorbent composite. 2. Experimental ...
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Ind. Eng. Chem. Res. 2006, 45, 48-53

Preparation, Swelling Behaviors, and Slow-Release Properties of a Poly(acrylic acid-co-acrylamide)/Sodium Humate Superabsorbent Composite Junping Zhang,†,‡ Ruifeng Liu,†,‡ An Li,†,‡ and Aiqin Wang*,† Center of Ecological and Green Chemistry, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China, and Graduate School of the Chinese Academy of Sciences, Beijing 100049, P. R. China

A novel functional superabsorbent composite with slow-release fertilizer properties from acrylic acid (AA), acrylamide (AM), and sodium humate (SH), PAA-AM/SH, was prepared by aqueous solution polymerization, using N,N′-methylenebisacrylamide (MBA) as a cross-linker and ammonium persulfate (APS) as an initiator. In this article, the effects of the SH content on water absorbency and swelling rate were studied. In addition, the water absorbency in various saline solutions and the reswelling capacity of the superabsorbent composite were also systematically investigated. The results show that the water absorbency, salt resistance, and reswelling capacity of the superabsorbent composite are improved by introducing SH into the PAA-AM polymeric network. The functionality of the superabsorbent composite in practice was investigated experimentally by studying the release of SH and by testing the water-retention capability of the composite in sand soil. The superabsorbent composite shows slow-release fertilizer behavior after SH is introduced. Compared to sand soil without the superabsorbent composite, 33.45 wt % water was still retained on the 20th day when the sand soil was mixed with 1.0 wt % composite. These results indicate that the efficiency of SH utilization and water-retention capability of the superabsorbent composite are greatly enhanced at the same time by introducing SH into the PAA-AM polymeric network. 1. Introduction Superabsorbents are widely used in many fields, such as hygienic products, agriculture, horticulture, and gel actuators, as well as drug-delivery systems and water-blocking tapes.1-5 Since the U.S. Department of Agriculture reported the first superabsorbent polymer,8 many methods have been attempted to improve the absorbing properties and to expand the application fields of superabsorbents.6,7 Recently, research on the use of superabsorbents as water-managing materials for the renewal of arid and desert environments has attracted great attention,9 and encouraging results have been observed as these materials can reduce irrigation water consumption, improve fertilizer retention in soil, lower the plant death rate, and increase the plant growth rate.10 However, the application of superabsorbents in this field has met some problems because most polymeric superabsorbents are based on pure poly(sodium acrylate), and so, they are too expensive and not suitable for saline-containing water and soils.11,12 In the past, research on superabsorbents focused on reducing production costs and improving salt resistance.13,14 Little information is available about superabsorbents with slow- or controlled-release fertilizer properties other than just saving water.15 In fact, fertilizers are as important as water in agriculture and horticulture; however, about 40-70% normal fertilizer is lost to the environment and cannot be absorbed by crops and trees when mixed with soil directly, resulting in large resource losses and serious environmental pollution.16,17 Thus, incorporating fertilizers into a superabsorbent polymeric network is an effective method for enhancing the utilization efficiency of water and fertilizer. Sodium humate (SH) is composed of multifunctional aliphatic components and aromatic constituents, and so, it contains a large * To whom correspondence should be addressed. Tel.: +86-9314968118. Fax: +86-931-8277088. E-mail: [email protected]. † Lanzhou Institute of Chemical Physics. ‡ Graduate School of the Chinese Academy of Sciences.

number of functional groups, such as carboxylates and phenolic hydroxyls.18 SH can regulate plant growth, accelerate root development, enhance photosynthesis, improve soil cluster structures, and enhance the absorption of nutrient elements. On the basis of the above background and our previous work on superabsorbent composites,14,19-21 it is expected that a multifunctional superabsorbent composite could be obtained by introducing SH into the pure poly(acrylic acid) (PAA)acrylamide (AM) system. As a result, a multifunctional and costefficient superabsorbent composite with improved water absorbency and slow-release fertilizer properties will be obtained. This report discusses systematically the preparation, swelling behaviors, slow-release behavior, and practical water-retention capabilities of the PAA-AM/SH superabsorbent composite. 2. Experimental Section 2.1. Materials. Acrylic acid (AA; chemically pure, Shanghai Wulian Chemical Factory, Shanghai, China) was distilled under reduced pressure before use. Acrylamide (AM; analytical grade, Shanghai Chemical Factory, Shanghai, China) was purified by recrystallization from benzene. The initiator, ammonium persulfate (APS; analytical grade, Xi’an Chemical Reagent Factory, Xi’an, China), was recrystallized from water before use. The cross-linker, N,N′-methylenebisacrylamide (MBA; chemically pure, Shanghai Chemical Reagent Corp., Shanghai, China), was used as purchased. Sodium humate (SH; Shuanglong Ltd., Xinjiang, China) with an average molecular weight of 1020 was used as received. Other agents used were all analytical grade, and all solutions were prepared with distilled water. 2.2. Preparation of PAA-AM/SH Superabsorbent Composites. A series of superabsorbent composites containing different amounts of SH were synthesized according to the following procedure. Typically, 3.55 g of AM and 4.32 g of AA with a fixed neutralization degree of 50% (neutralized at 5 °C with 2 M sodium hydroxide solution) were introduced into 35 mL of distilled water in a 250 mL four-neck flask equipped

10.1021/ie050745j CCC: $33.50 © 2006 American Chemical Society Published on Web 12/02/2005

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with a stirring rod, a reflux condenser, a thermometer, and a nitrogen line. MBA (8.8 mg) was added to the above monomer solution, and then the appropriate amount of SH was dispersed in the mixed solution. After being purged with nitrogen for 30 min to remove the oxygen dissolved from the solution, the mixed solution was heated to 50 °C gradually, and then the initiator (APS, 35.2 mg) was charged into the flask. The solution was stirred vigorously under nitrogen atmosphere for 3 h to complete the polymerization. After polymerization, the product was washed with ethanol and water, and then dried in an oven at 70 °C until the weight of the product was constant. The dark products were milled, and all samples used for tests had a particle size in the range of 40-80 mesh. 2.3. Characterization of PAA-AM/SH. FTIR spectra of SH, PAA-AM, and PAA-AM/SH were recorded as KBr pellets using a Thermo Nicolet NEXUS TM spectrophotometer. SEM micrographs of samples were recorded using a JSM5600LV instrument (JEOL, Ltd.). Before SEM observations, all samples were fixed on aluminum stubs and coated with gold. 2.4. Measurement of Water Absorbency and Swelling Rate. A weighed quantity of superabsorbent composite (0.05 g) powder was immersed in excess distilled water (500 mL) or various saline solutions (250 mL) at room temperature for 4 h to reach the swelling equilibrium. The swollen sample was then separated from the unabsorbed water by filtering through a 100mesh screen. The water absorbency of the superabsorbent composite, QH2O, is calculated using the equation

QH2O )

m2 - m1 m1

(1)

where m1 and m2 are the weights of the dry sample and the swollen sample, respectively. QH2O is calculated as the number of grams of water per gram of sample. Swelling rate was measured according to the previously reported method.22 2.5. Measurement of Reswelling Capacity. The specimen (0.05 g) was immersed in a certain volume of distilled water to ensure that swelling equilibrium was achieved. The swollen gel was placed in an oven at 100 °C until the gel was thoroughly dried. An equal volume of water was added to the dried gel, and the sample was returned to the oven. A similar procedure was repeated, and then the saturated absorbency of the sample after several times of reswelling was obtained. 2.6. Measurement of the Slow-Release Behavior of the PAA-AM/SH Superabsorbent Composites. The dry sample (0.20 g) was immersed in 400 mL of distilled water in a beaker at 25 °C. Exactly 5.00 mL of solution was transferred from the beaker into a 10-mL test tube at each fixed time interval. The test solution was oxidized at 100 °C by adding 3.0 mL of K2Cr2O7 solution [C(1/6K2Cr2O7) ) 0.8 M] and 15 mL of concentrated H2SO4 (eq 2). The residual K2Cr2O7 was titrated with (NH4)2Fe(SO4)2 solution (0.1 M) (eq 3). The release of SH was expressed using carbon content [C (mg/5 mL)] in the solution (eq 4).

2K2Cr2O7 + 3C + 8H2SO4 f 2K2SO4 + 2Cr2(SO4)3 + 3CO2 + 8H2O (2) K2Cr2O7 + 7H2SO4 + 6FeSO4 f K2SO4 + Cr2(SO4)3 + 3Fe2(SO4)3 + 7H2O (3) C (mg/5 mL) ) 12.01CFe2+(V0 - V1)/4

(4)

where CFe2+ is the Fe2+ concentration of the (NH4)2Fe(SO4)2

Figure 1. FTIR spectra of (a) SH, (b) PAA-AM/SH composite containing 30 wt % SH, and (c) PAA-AM. Table 1. Effect of SH Content on Water Absorbency of PAA-AM/ SH Superabsorbent Composites SH content (wt %) QH2O Q0.9wt% NaCl

0

5

10

20

30

40

678 47.3

879 52.1

971 54.3

1068 56.1

1184 57.2

831 38.1

solution (M), V0 is the volume of (NH4)2Fe(SO4)2 solution used to titrate residual K2Cr2O7 in a blank solution (lixivium of PAA-AM) (mL), V1 is the volume of (NH4)2Fe(SO4)2 solution used to titrate the residual K2Cr2O7 in the test solution (mL), 12.01 is the molar weight of carbon (g mol-1), and 4 is the molar ratio of Fe2+ to carbon according to eqs 2 and 3. 2.7. Water-Retention Capability in Sand Soil. In this test, 0.30-, 1.50-, and 3.0-g samples of the PAA-AM/SH superabsorbent composite were well mixed in cups with sand soil to a total weight of 300 g, and then 60 g tap water was slowly added into the cups. The temperature was kept at 25 °C, and the cups were weighed every day, so that the practical water-retention capacity of the composite could be obtained. A control experiment without the PAA-AM/SH composite was also carried out. 3. Results and Discussion 3.1. FTIR Analysis of the PAA-AM/SH Superabsorbent Composite. The FTIR spectra of SH, PAA-AM, and PAAAM/SH superabsorbent composite containing 30 wt % SH are shown in Figure 1. Compared to the spectrum of SH (Figure 1a), the absorption bands of SH at 3214 and 3072 cm-1 (N-H stretch), as well as those at 2362 cm-1 (O-H stretch of the carboxylic group) and 1707 cm-1 (CdO stretch of the carboxylic group), are absent in the spectrum of the PAA-AM/SH composite (Figure 1b). In addition, the intensity of the absorption band at 1609 cm-1 (COO- asymmetric stretch) is decreased, and the absorption band at 1240 cm-1 (phenolic C-O stretch) shifts to 1231 cm-1. Compared to the spectrum of PAA-AM (Figure 1c), the absorption band at 1665 cm-1 (CdO stretch of CONH2) shifts to 1647 cm-1, and the intensity of the absorption band at 1565 cm-1 (COO- asymmetric stretch) is decreased in the spectrum of the PAA-AM/SH composite. It can be concluded from the FTIR information that some chemical reactions occurred between SH and PAA-AM during the polymerization process. 3.2. Effect of SH Content on Water Absorbency and Swelling Rate. The effect of the SH content in the PAA-AM/ SH superabsorbent composite on water absorbency is indicated in Table 1. It can be seen that the water absorbency increases with increasing SH content in the weight range of 0-30 wt % in the feed and then decreases with further increasing SH

50 Ind. Eng. Chem. Res., Vol. 45, No. 1, 2006

Figure 2. Swelling rates of PAA-AM/SH superabsorbent composites containing different amounts of SH in distilled water.

content. The composite containing 30 wt % SH acquired the highest water absorbency values of 1184 and 57.2 g g-1in distilled water and in 0.9 wt % NaCl solution, respectively. The water absorbency in distilled water for the composite is still higher than that of pure PAA-AM superabsorbent even when 40 wt % SH is introduced, which significantly reduces the production costs of the superabsorbent composite. The improvement in water absorbency for the PAA-AM/SH superabsorbent composite owing to the introduction of SH can be attributed to the fact that SH contains a large number of functional groups, such as carboxylates, OH groups (enolic, phenolic, and alcoholic), and amino groups.23,24 Therefore, SH can chemically bond with and improve the polymeric network during the polymerization process. The apparent decrease in water absorbency when the SH content is above 30 wt % is attributed to the facts that excess SH only acts as filler and the amount of hydrophilic groups on the polymeric backbone decreases with increasing SH content, which causes a decrease in the osmotic pressure difference and thus the shrinking of the superabsorbent composite. It is well-known that the swelling rate of a superabsorbent material is significantly influenced by factors such as the composition of the superabsorbent, the particle size, and the surface area. The swelling rates for PAA-AM/SH superabsorbent composites containing different amounts of SH are shown in Figure 2. The results show that the composites prepared with greater amounts of SH exhibit lower swelling rates and require more time to reach absorption equilibrium. The composite containing 5 wt % SH requires about 40 min to reach absorption equilibrium, whereas that containing 40 wt % SH requires about 2 h. It is known that the swelling rate is controlled by the diffusion of the water penetrating into the inside of polymeric network. The percentage of hydrophilic groups (such as -COOH and -COO-) on the polymeric network in the PAAAM/SH composite decreases with increasing SH content, so the diffusion rate of water penetrating into the inside of the superabsorbent composite decreases. Therefore, the superabsorbent composite with more SH exhibits a lower initial swelling rate and needs more time to reach equilibrium water absorbency. 3.3. Water Absorbency in Various Saline Solutions. Considering the great impact of external saline solutions on the water absorbency of superabsorbents and the expanding of their applications especially for agriculture and horticulture, the interaction of various saline solutions with the superabsorbent composite was investigated. The water absorbencies for PAA-

Figure 3. Water absorbencies of PAA-AM/SH superabsorbent composite in NaCl(aq) solutions with different concentrations.

Figure 4. Water absorbencies of PAA-AM/SH superabsorbent composite in CaCl2(aq) solutions with different concentrations.

Figure 5. Water absorbencies of PAA-AM/SH superabsorbent composite in FeCl3(aq) solutions with different concentrations.

AM and PAA-AM/SH in NaCl(aq), CaCl2(aq), FeCl3(aq), Na2SO4(aq), and Na3PO4(aq) were measured and are shown in Figures 3-6, respectively, as a function of concentration. It can be seen from Figures 3-5 that the water absorbency in various external saline solutions of the same concentration is improved when SH is introduced into the PAA-AM polymeric network. This phenomenon can be attributed to the

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Figure 6. Water absorbencies of PAA-AM/SH superabsorbent composites in NaCl, Na2SO4, and Na3PO4 aqueous solutions.

Figure 7. Variation in water absorbency of PAA-AM/SH superabsorbent composites containing different amounts of SH versus reswelling time.

fact that the introduced SH improves the PAA-AM polymeric network. The water absorbencies in saline solutions of the PAA-AM/SH composite containing 10 wt % SH are higher than those of the composite containing 30 wt % SH, although the latter has a higher equilibrium water absorbency in distilled water. This indicates that only the appropriate amount of SH can improve the salt resistance of the PAA-AM superabsorbent to the highest extent. It can also be seen that the water absorbencies for the three superabsorbents in various saline solutions decrease with increasing concentration. This phenomenon is related both to the polyelectrolyte nature of the polymeric network and to the ionic strength in various external saline solutions. Fixed charges on the polymer network are related to “Donnan-type” absorption, which promotes the osmotic pressure difference between the polymeric network and the external saline solutions, and the electrostatic repulsion between the polymer chains, which contributes to the expansion of the polymeric network. With increasing concentration of the external saline solution, the osmotic pressure difference decreases, and the screening effect of penetrating counterions (Na+, Ca2+, and Fe3+) on carboxylate group, which restricts the expansion of the polymeric network, is more evident. Furthermore, the water absorbencies for these three superabsorbents decrease in the order NaCl(aq) > CaCl2(aq) > FeCl3(aq). This is because of the difference in the ability of the carboxylate group on the superabsorbent network to complex with various cations. The complexing abilities of the carboxylate group to these three cations are in the order Na+ < Ca2+ < Fe3+ according to their formation constants with ethylenediamine tetraacetic acid (EDTA; the logarithms of the formation constants of EDTA with these cations are 0, 10.69, and 25.10 for Na+, Ca2+, and Fe3+, respectively). The effect of cations on the water absorbency of superabsorbents is obvious according to many studies.25,26 To investigate the effect of different anions on the water absorbency of the PAA-AM/SH superabsorbent composite, the concentrations of NaCl, 1/2(Na2SO4), and 1/3(Na3PO4) solutions are equal, which means that the contribution of Na+ to the whole ionic strength is equal regardless of the valence of the anions. Figure 6 shows the effects of Cl-, SO42-, and PO43- on the saturated water absorbency of the PAA-AM/SH composites. It can be seen from Figure 6 that the valence of the anion has an effect on the water absorbency of the superabsorbent composite. The water absorbencies of the superabsorbent composite containing 10 wt % SH in various sodium salt solution are in the order PO43- > Cl- > SO42-. This result indicates that the water absorbency

of the superabsorbent composite in a salt solution containing trivalent anions (PO4-) is higher than that in salt solutions containing monovalent anions (Cl-) and divalent anions (SO42-) at the same concentration. Similar results were found in the study of the swelling behavior of poly(acrylic acid)/attapulgite in various anionic salt solutions (aqueous NaNO3, Na2SO4, and Na3PO4).19 3.4. Effect of SH Content on Reswelling Capability. Reswelling capability is an important property of superabsorbents just as the saturated absorbency and swelling rate are, especially in the fields of agriculture and horticulture. Figure 7 shows water absorbencies of the PAA-AM/SH composites with different SH contents as a function of reswelling time. It can be seen that the resulting dry samples, after several instances of reswelling, still retain a degree of water-absorbing capability. These superabsorbent composites might prove useful as recyclable superabsorbent materials. It can also be seen that the influence of the reswelling time on the saturated absorbency is more significant for the composites containing greater amounts of SH. The saturated absorbency of the sample containing 30 wt % SH decreases from 1184 to 366 g g-1 after five times of reswelling, and only 30.9% of its initial saturated water absorbency is retained, whereas the composite doped with 10 wt % SH retained 59.1% of its initial saturated water absorbency after reswelling five times. This result indicates that introducing a moderate amount of SH is helpful for the improvement of the reswelling capability of the superabsorbent composite. 3.5 Slow-Release Behavior of the PAA-AM/SH Superabsorbent Composites. The release of SH from the PAAAM/SH superabsorbent composite in distilled water is shown in Figure 8. As can be seen, the release rate is higher during the first 2 days, and the carbon contents are 0.097, 0.172, and 0.483 g/5 mL on the second day for the composites containing 5, 10, and 30 wt % SH, respectively. The higher release rates in this period can be attributed to the fact that the SH released in this period mainly exists on the surface or is freely incorporated in the composite network, so that it can dissolve in water rapidly. Two days later, the release rate decreased, and the carbon contents increased gradually to 0.134, 0.243, and 0.772 g/5 mL, respectively, on the 36th day. The slow release rate might be because the SH released during this period mainly bonds with the superabsorbent polymeric network and then needs more time to diffuse from the hydrogel granule and dissolve in the water. The release behavior of SH in the composite was similar to the behavior of other conventional slow-release fertilizers.27 Thus, the superabsorbent composite

52 Ind. Eng. Chem. Res., Vol. 45, No. 1, 2006 Scheme 1. Schematic Structures of PAA-AM/SH Superabsorbent Composite: (a) in the Dry State, (b) in the Swollen State, (c) upon Release of SH on the Surface or Freely Incorporated in the Composite Network, and (d) upon Release of SH Bonded with the Polymeric Network

Figure 8. SH release curve of PAA-AM/SH superabsorbent composite in distilled water.

is endowed with a slow-release property. In addition, the strength of the swollen gel after releasing SH connected to the network decreases. This means that some of the cross-linkages in the network have disappeared, and this might cause the PAA-AM material to undergo gradual dispersion into the environment. Micrographs of PAA-AM, fresh PAA-AM/SH, and PAAAM/SH after 30 days of SH release are shown in Figure 9a-c, respectively. It can be seen from these images that PAA-AM has a leprose and porous surface, but the introduction of SH leads to an even and tight surface (Figure 9b). This result indicates that the introduced SH blocks the pore canals of the PAA-AM polymeric network. As the PAA-AM/SH composite is dipped in water, some of the SH in the composite is released and dissolves in the water first, and the pore canals are partly dredged, and then water molecules penetrate into the polymeric network. This is in accord with the decrease in swelling rate of the composite as the amount of SH introduced is increased (see Figure 2). After 30 days of SH release, the composite shows a porous structure again. This is because much of the SH on the polymeric network has been released and dissolved in water

with the prolongation of the release time, and the pore canals of the composite reppear. Scheme 1 shows the schematic structures of the PAA-AM/SH superabsorbent composite in the dry state, in the swollen state, and after the release of SH. 3.6. Practical Water Retention of the PAA-AM/SH Superabsorbent Composite in Sand Soil. The practical water-retention capability of superabsorbent in soil is highly significant, as there are intensive investigations into using superabsorbents as water-managing materials in agriculture and horticulture. Figure 10 shows the water-retention capability of the PAAAM/SH superabsorbent composite containing 30 wt % SH in sand soil. It can be seen that the weight of water remaining in the sand soil decreases with time. The sand soil containing the PAA-AM/SH superabsorbent composite has a higher waterretention capability. The control sample lost almost all of the absorbed water by the 15th day, whereas the sand soil containing

Figure 9. SEM images of (a) PAA-AM, (b) PAA-AM/SH (10 wt % SH), and (c) PAA-AM/SH (10 wt % SH) after 30 days of SH release.

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Figure 10. Practical water-retention capability of PAA-AM/SH superabsorbent composite containing 30 wt % SH in sand soil.

0.1 wt % PAA-AM/SH superabsorbent composite lost all of its absorbed water by the 20th day. For the sand soil samples containing 0.5 and 1.0 wt % PAA-AM/SH superabsorbent composite, 24.75% and 33.45%, respectively, of the initial absorbed water was still retained on the 20th day. This result indicates that the PAA-AM/SH superabsorbent composite can enhance the water-retention capability of sand soil. 4. Conclusions A novel functional superabsorbent composite with slowrelease fertilizer properties was prepared from AA, AM, and SH by aqueous solution polymerization, using N,N′-methylenebisacrylamide as a cross-linker and ammonium persulfate as an initiator. The superabsorbent composite acquired its maximum water absorbency of 1184 g g-1 when the amount of SH was 30 wt %. As the pore canals of the PAA-AM polymeric network were blocked with SH, the swelling rate of the composite decreases with increasing SH content. The water absorbencies of PAA-AM and PAA-AM/SH composites containing different amounts of SH in various saline solutions were investigated, and the PAA-AM/SH composite always had a higher water absorbency. The functionality and practical waterretention capability of the superabsorbent composite were also investigated experimentally by studying the release of SH and by testing the water-retention capability of the composite in sand soil. The results of the present work indicate that the PAAAM/SH superabsorbent composite is a kind of multifunctional water-managing material with slow-release fertilizer properties. This superabsorbent composite might find application in agriculture and in the renewal of arid and desert environments. Acknowledgment This work was financially supported with the Western Action Project of CAS (No. KGCXZ-SW-502) and the “863” major project of the Ministry of Science and Technology, P. R. China (No. 2005AA2Z4030). Literature Cited (1) Ende, M.; Hariharan, D.; Pappas, N. A. Factors influencing drug and protein transport and release from ionic hydrogels. React. Polym. 1995, 25, 127. (2) Dzinomwa, G. P. T.; Wood, C. J.; Hill, D. J. T. Fine coal dewatering using pH- and temperature-sensitive superabsorbent polymers. Polym. AdV. Technol. 1997, 8, 762.

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ReceiVed for reView June 22, 2005 ReVised manuscript receiVed October 24, 2005 Accepted November 4, 2005 IE050745J