rice husk ash hydrogel composites. II. Temperature effect on rice husk ash obtention

rice husk ash hydrogel composites. II. Temperature effect on rice husk ash obtention

Composites: Part B 51 (2013) 246–253 Contents lists available at SciVerse ScienceDirect Composites: Part B journal homepage: www.elsevier.com/locate...

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Composites: Part B 51 (2013) 246–253

Contents lists available at SciVerse ScienceDirect

Composites: Part B journal homepage: www.elsevier.com/locate/compositesb

Poly(acrylamide-co-acrylate)/rice husk ash hydrogel composites. II. Temperature effect on rice husk ash obtention Jean de S. Cândido a, Antonio G.B. Pereira b, André R. Fajardo b, Nágila M.P.S. Ricardo c, Judith P.A. Feitosa c, Edvani C. Muniz b, Francisco H.A. Rodrigues a,b,⇑ a b c

Coordenação de Química, Universidade Estadual Vale do Acaraú, Av. da Universidade, 850, Campus da Betânia, 62040-370 Sobral-CE, Brazil Departamento de Química, Universidade Estadual de Maringá, Av. Colombo, 5790, 87020-900 Maringá-PR, Brazil Departamento de Química Orgânica e Inorgânica, Campus do Pici, Universidade Federal do Ceará, 60455-760 Fortaleza-CE, Brazil

a r t i c l e

i n f o

Article history: Received 13 October 2012 Received in revised form 15 February 2013 Accepted 10 March 2013 Available online 22 March 2013 Keywords: A. Polymer–matrix composites (PMCs) A. Smart materials D. Thermal analysis

a b s t r a c t Superabsorbent hydrogel composites based on poly(acrylamide-co-acrylate) and rice husk ash (RHA) were prepared by free-radical copolymerization in aqueous media, using N,N-methylenebisacrylamide (MBA), as crosslinker and potassium persulfate (K2S2O8), as initiator. The effect of calcination temperatures (400–900 °C) for obtaining RHA was evaluated. FTIR, WAXS, SEM–EDS and TGA were applied to characterize a series of hydrogels filled with RHA. The hydrogels composites were formed with constant amounts of RHA (10 wt.%) and crosslinking agent (0.1 mol-%) in relation to the relative to the total mass of acrylamide (AAm) and potassium acrylate (KAc) monomers (50–50 mol-%). A blank sample of poly(acrylamide-co-acrylate) hydrogel without RHA was used as control. A superabsorbent hydrogel composite, with a maximum absorption Weq > 1000 gwater/gabsorbent, was obtained with the RHA calcined at 900 °C. In addition, the hydrogel composite showed to be sensitive to the pH variation and to the presence of salts in the swelling media. From the results, it is possible to infer that the poly(acrylamide-coacrylate)/RHA hydrogel composites presented good characteristics to be applied as soil conditioner for using in agriculture. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Searching for technologies considered ecologically correct has become of major concern in recent years. This new paradigm of economic development is directed to provide an improvement in welfare of further generations, incorporating in their conception the needing of new production methodologies able to reduce the pollution and the environmental impact. The large amounts of materials that present low degradation rate at ambient conditions have caused severe environmental, economic, social and juridical issues. The rice husk ash (RHA) is an agro-industrial residue resulting from the thermochemical conversion of rice husk, and as a waste, the majority of RHA generated is discarded, causing pollution. The physical and chemical properties of RHA are determined according to the methodology applied to its production (pyrolysis, gasification and/or combustion, for instance) and by some variables, such as, type of equipment employed, burning temperature and time. Regardless the process of burning, the resultant ash has silica con-

⇑ Corresponding author at: Coordenação de Química, Universidade Estadual Vale do Acaraú, Av. da Universidade, 850, Campus da Betânia, 62040-370 Sobral-CE, Brazil. Tel./fax: +55 85 3611 6342. E-mail address: [email protected] (F.H.A. Rodrigues). 1359-8368/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2013.03.027

tent around 74–97% [1–3]. On the other hand, temperature achieved during calcination is the determining factor for the appearance of silica in amorphous or crystalline states [4,5]. Several researchers have utilized RHA to prepare zeolites and mesoporous materials [6,7], as an adsorbent for metal ions such as Cd2+, Zn2+ and Ni2+ [8,9], and heavy metal such as lead and mercury from aqueous solution [10], as substitute for cement [11], as alternative source for active silica production [12,13], as fillers for natural and synthetic rubbers [14–16] and for acrylamide–acrylate copolymer hydrogels [17]. Some organic matters, such as Congo red and vacuum pump oil [18], palmytic acid [19], Indigo Carmine dye [20], Brilliant Green dye [21], Methylene Blue dye [22], can also be absorbed by RHA. Superabsorbent hydrogels are insoluble in water and can absorb and retain large amounts of aqueous fluids even under pressure. Therefore, superabsorbents have great advantages over traditional water-absorbing materials. Due to their excellent properties, superabsorbent hydrogels have raised considerable interest and they have been widely used in several fields, such as, hygienic products, horticulture, gelactuators, drug delivery systems, as well as water blocking tapes and coal dewatering [23–29]. Based on above description, this paper is a second of a series of papers in which superabsorbent hydrogels composites based on

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poly(acrylamide-co-acrylate) filled with RHA were formed. The superabsorbent hydrogels composites were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Scanning electron microscopy coupled to energy dispersive spectroscopy X-ray (SEM–EDS) and Thermogravimetry (TGA). In this paper the focus is on the temperature effect of RHA obtention as well as on the swelling–drying capabilities of hydrogels. 2. Experimental 2.1. Materials Acrylamide (AAm), acrylic acid (AAc), N,N,N0 ,N0 -tetramethylethylenediamine (TEMED), as catalyst, and potassium persulfate (K2S2O8), as initiator, were purchased from Sigma Aldrich (USA). N,N0 -methylenobisacrylamide (MBA), as crosslinker, was obtained from Pharmacia Biotech (USA). The acrylate salt (KAc) was obtained by the neutralization of acrylic acid with potassium hydroxide. The used rice husk originates from Mucambo/CE, Brazil. Ashes were produced through calcination in a muffle furnace at temperatures ranging from 400 to 900 °C. The ashes, labeled as RHAT (where T is the temperature applied during calcination), were previously ground, and sieved through a 325 mesh (644 lm) sieve prior using in hydrogel formation. All reactants were of analytical grade and were used without further purification. 2.2. Poly(acrylamide-co-acrylate) hydrogel formation 2.1 g of AAm and 3.25 g of KAc were added to 30 mL of distilled water, bubbled with nitrogen gas (to reduce the inhibiting effect of the oxygen in the radical polymerization). After 10 min, 16.2 mg of K2S2O8 were added. Thus, MBA (0.05, 0.1 and 0.2 mol-% related to the amount of monomers) and 100 L of TEMED solution 0.57 g L1 were also added. The system was maintained under stirring and flowing nitrogen until the hydrogel formation (up to 30 min), after it was left to rest for further 15 h at room temperature. The as-obtained material was cut in small pieces and washed in distilled water to remove non-reacted monomers. The hydrogels were oven-dried at 70 °C. The size distribution of particles spreads from 9 to 24 mesh (2–0.71 mm). The poly(acrylamide-co-acrylate) hydrogel was labeled as PAMACRYL. 2.3. Poly(acrylamide-co-acrylate)/RHA hydrogel composite formation The poly(acrylamide-co-acrylate)/RHA hydrogel composites were synthesized as described to the same methodology applied to form the PAMACRYL hydrogel. However, the monomers were dissolved in a dispersion of RHA obtained at different calcinations temperature (10 wt.% related to the total amount of monomers). The hydrogel composites were labeled as RHAG400, RHAG500, RHAG600, RHAG700, RHAG800 and RHAG900 (where the number subscript is referent to the temperature of rice husk calcination).

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2.4.3. Scanning electron microscopy coupled to energy dispersive spectroscopy X-ray (SEM–EDS) The morphology of the hydrogels (with and without RHA) was analyzed through scanning electron microscopy coupled to Energy Dispersive X-ray Spectroscopy (SEM–EDS/Shimadzu, model SS 550) operating at 10 keV. The hydrogels were immersed in distilled water at room temperature until the equilibrium swelling has been reached (approximately 24 h). Next, the samples were removed and immediately frozen by immersion in liquid nitrogen. Thereafter, the frozen hydrogels were fractured and freeze-dried (Christ, Alpha 1–2 LD Plus) at 55 °C for 24 h. Then, the hydrogels were gold-coated by sputtering before observation by SEM–EDS. 2.4.4. Thermogravimetry (TGA) TGA was performed in a Simultaneous Thermal Analysis System, Netzsch (Model STA 409 PG/4/G Luxx) with a scanning rate of 10 °C min1 under flowing N2(g) at 20 mL min1 in a temperature range from 22 to 1000 °C. 2.5. Study of physical properties 2.5.1. Swelling experiments Initially, the swelling tests were employed to determine water uptake capacity of the materials. In this way, 15 mg of dried gels were placed in 30 mL filter crucibles (porosity no. 0) pre-moistened and with a dried outer wall. This set was inserted in water in such a way that the gel was completely submerged. The crucible/hydrogel composite sample sets were removed at various time intervals, with the external wall of the set dried and the system weighed. For each sample, 3 assays were performed (n = 3). The swelling capacity of the hydrogel composites was determined by Eq. (1), where Weq is the gained water mass (in grams) per gram of composite hydrogel (absorbent), m is the mass of the swollen absorbent and m0 is the mass of the dry material [17,30]. The kinetics of swelling in each studied medium was evaluated. The size distribution of the hydrogel composites remained in the 9–24 mesh range.

W eq ¼ ½m=m0   1

ð1Þ

2.5.2. The effect of the ionic strength and type of metal ions on the salt The hydrogels were immersed in distinct salt aqueous solutions at different concentrations (0.001, 0.01, 0.05 and 0.10 M) and the swelling capabilities of gels were determined through the previously described procedures [17,30]. NaCl and NaHCO3 solutions were used for studying the anion effect while NaCl and CaCl2 solutions were used for evaluating the cation effect on swelling properties. 2.5.3. Evaluation of pH effect on swelling capability of gels The effect of the pH on the swelling was also verified in buffer solutions (pHs 2–12). The procedures were the same as described above. The ionic strength of the buffer solutions was kept constant (I = 0.1 M).

2.4. Characterization 3. Results and discussion 2.4.1. Fourier transformed infrared spectroscopy (FTIR) The FTIR spectra were obtained using Shimadzu FTIR-8300 equipment. The dried material was blended with KBr powder and pressed into tablets before spectrum acquisition. 2.4.2. Wide angle X-ray scattering (WAXS) The WAXS profiles of RHA and hydrogels were obtained through a powder diffractometer Shimadzu model XRD 6000; with Cu Ka radiation source at 30 kV and 20 mA.

3.1. FTIR and WAXS techniques The FTIR spectra of RHA obtained at different temperatures are shown in Fig. 1a. The bands assigned to the main vibrational modes of SiAOASi bonds at 1100 cm1 and 800 cm1 and 471 cm1 appear in the spectra independent on temperature used for calcination. These bands are attributed to the asymmetric and symmetric stretching and angular deformation, respectively [31–

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33]. The broad band at 3548 cm1 is assigned to the vicinal silanol with hydrogen-bonded water. The intensity of band at 621 cm1 can be observed on spectra obtained for RHA calcined at T P 700 °C and intensifies as the temperature used for calcinations is increased. This band is characteristic of crystalline cristobalita [34,35]. The FTIR spectra of hydrogels formed using the ash obtained at different temperatures are shown in Fig. 1b. These spectra show the appearance of the bands at 1100, 800 and 471 cm1, assigned to SiO2 and SiAOASi bonds [31–33]. Furthermore, a weak band at 621 cm1 can be observed on FTIR spectra of hydrogel obtained using ashes burnt at T P 700 °C. Thus, the presence of crystalline cristobalita [6,34] in hydrogel can be assigned. In addition, the bands at 1670 cm1 and at 1564 cm1, related to the [email protected] stretching and to the NAO stretching, respectively, due to the formation of copolymers. These results indicate that copolymers and RHA component coexisted in superabsorbent hydrogels composites. The WAXS patterns of RHA calcined at different temperatures for 2 h and cooled to room temperature (within the muffle for 24 h) are shown in Fig. 1c. Such profiles provide information relative to the main mineral constituint and structural parameters of RHA. It can be noticed from WAXS profiles that RHA calcined at different temperatures presented different structures corroborating the FTIR data. The ashes obtained at T < 700 °C showed no defined diffraction peaks, but rather an amorphous halo that appears at ca. 2h = 22°, which indicates the absence of crystalline domains, being

predominant the non-crystalline form of silica [13,35,36]. On the other hand, RHA obtained at 900 °C showed peaks of cristobalite, indicating that the material is predominantly crystalline [6,34,37]. Therefore, is possible to infer that when the rice husk is calcined at high temperatures the resulting RHA presents crystalline structure. The characteristics of amorphous and crystalline silica from RHA calcined in different temperature are maintained in the hydrogel composites (Fig. 1d) but the diffraction peak assigned to the crystalline portion is enlarged due to the presence of copolymer. The WAXS patterns and FTIR data evidence the formation of the hydrogel composite and suggest the presence of RHA into the polymer matrix, as sketched in Fig. 2. 3.2. SEM coupled to energy dispersive X-ray scattering (SEM–EDS) Fig. 3 shows the SEM micrographs of freeze-dried hydrogel filled with rice husk ashes (RHA400, RHA600 and RHA900) after being swelled at equilibrium. All samples exhibit porous structure characteristic of hydrogels, however changes in structure of material due to different types of ash (crystalline or not) can be evidenced. For instance, the micrograph of RHAG400 hydrogel shows homogeneous porous distribution, but average size smaller than those of RHAG900, while RHAG600 is in an intermediate condition. This phenomenon could be responsible for the difference in the superabsorbent properties among RHAG900, RHAG600 and RHAG400 composites. EDS tech-

Fig. 1. (a) FTIR spectra of the RHA calcined in different temperatures; (b) FTIR spectra of the hydrogels composites; (c) WAXS patterns of the RHA calcined in different temperatures and (d) WAXS patterns of the hydrogels composite.

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Fig. 2. Proposed model for the structure of hydrogel composite network (particle size 644 lm).

nique was utilized to evaluate the filler dispersion into polymer matrixand the possible existence of silicate particles diffusion outward the hydrogel. Signal relative of silicon (from ashes) were evidenced on images generated by EDS mapping technique, indicating the homogeneous dispersion of Si in whole matrix. 3.3. Thermal stability TGA and DTG curves of PAMACRYL and hydrogels composites (RHAG400, RHAG600 and RHAG900) filled with 10 wt.% of RHA calcined at different temperatures are shown and compared in Fig. 4. The decomposition curve of PAMACRYL and hydrogel composites could be divided into three steps. The first stage is in the range of 50–200 °C due to a loss of moisture present in the samples. Following, the weight losses within the temperature of 300–450 °C, which are attributed to the thermal decomposition of the carboxylate and amide side-groups of the copolymers, and also MBA moieties in the network, leading to the evolution of ammonia and other gases [38,39]. During this period, the onsets of the PAMACRYL and hydrogels composites filled with RHA400, RHA600 and RHA900 are similar and stability increases with increasing temperature in which the ashes were obtained, 391 °C (33.5%), 386 °C (31.4%), 395 °C (33.4%) and 405 °C (33.3%), respectively. The third stage was attributed to the breakage of copolymer chains, in which it was observed a displacement for higher temperatures. It can be concluded from TGA data that the temperature in which the RHA was calcined has an influence on thermal stability of corresponding superabsorbent composites. The hydrogel RHA900 could enhance the thermal stability to the highest degree among the hydrogel samples investigated. The properties of RHA in the superabsorbent composite polymeric network may be the main reasons for the difference in TGA result of this system.

constituent chains of the matrix. Swelling is primarily due to the penetration of water into the hydrophilic polymer matrix by capillarity and diffusion. On the other hand, the swelling rate of a superabsorbent is significantly influenced by swelling capacity, size distribution of powder particles, specific surface area, and apparent density of polymer. Fig. 5a shows the kinetics of swelling in distilled water of RHAG400, RHAG600 and RHAG900 composites and PAMACRYL. The observed trends in the swelling kinetics of four different samples are very similar. The degree of swelling increased quickly during the first 20 min of immersion for all samples and ca. 90% of the swelling equilibrium value was reached in this time range. Then, a slower swelling process took place up to the equilibrium (Weq), around 30 min. The Weq values and the speed for reach the equilibrium depend on the gel formulation (see the insertion on Fig. 5a). The presence of RHA greatly improved the water absorbency at equilibrium. Besides, RHAG900 composites took longest time to reach equilibrium. The PAMACRYL hydrogel presented water absorption capacity at equilibrium (Weq) of 645 gwater/gabsorbent while RHAG400, RHAG600 and RHAG900 composites presented higher water absorption capabilities of 781, 802 and 1077 gwater/gabsrobent, respectively. For evaluating the mechanism of the swelling process and the temperature effect of RHA obtention on swelling kinetics of the superabsorbent hydrogels composites, the second order swelling kinetics model [40,41] was adopted to test the experimental data. The second order swelling kinetic model can be expressed as follows:

t=W ¼ A þ Bt

ð2Þ

where



3.4. Kinetics of swelling

and

The swelling process of a hydrogel is controlled by chemical and physical forces as well as by the consequent elastic response of the



1 ks w2t

ð3Þ

1 wt

ð4Þ

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Fig. 3. SEM and EDS diffractograms of (a) RHAG400, (b) RHAG600, and (c) RHAG900 hydrogels composites.

The A parameter corresponds to an initial swelling rate [(dW/dt)0] of the hydrogel, ks is the constant rate for swelling, Wt is a theoretical swelling value at equilibrium. Wt and ks were calculated by fitting experimental data shown in Fig. 5b to Eqs. (2)–(4) and the results are given in Table 1. For all the straight lines, the correlation coefficients (R2) were higher than 0.999, indicating that the swelling processes of the superabsorbents obey the model utilized for predict the swelling. The swelling rate constant (ks) for RHAG900 – was smaller than that of PAMACRYL. Although the introduction of RHA900 into PAM-

ACRYL network greatly improved the equilibrium water absorbency, the necessary time to reach the equilibrium was higher leading to a slower swelling rate. The relationship between water uptake capability and the temperature applied for obtaining the RHA is shown in Fig. 6. According to the swelling assays it is verified the RHAG900 presented highest water uptake because RHA900 has virtually no residue. RHA900 showed high crystallinity degree and the preferable intra-interactions among its silanol groups set free new sites on the hydrogel matrix that could interact to water increasing their water uptake capacity.

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Fig. 4. (a) TGA and (b) DTG curves of PAMACRYL and hydrogels composites (RHAG

400,

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RHAG600 and RHAG900) filled with 10 wt.% of RHA.

Fig. 5. (a) Degree of swelling and (b) plot of t/W versus t as a function of immersion time.

Table 1 Effect of temperature of obtaining the RHA swelling kinetics of hydrogels composites formed.

a b c d

Hydrogel

Weqa

Wtb

teqc

ksd

PAMACRYL RHAG400 RHAG500 RHAG600 RHAG700 RHAG800 RHAG900

645 ± 21 781 ± 27 773 ± 25 802 ± 19 939 ± 24 1031 ± 26 1077 ± 32

649 791 789 800 952 1056 1124

24 ± 3 12 ± 2 13 ± 4 10 ± 1 24 ± 4 29 ± 5 35 ± 4

6.54  104 1.15  103 1.91  103 1.42  103 2.49  104 1.19  104 7.40  105

Experimental equilibrium swelling (gwater/gabsorbent). Theoretical equilibrium swelling (gwater/gabsorbent). Equilibrium time (min). Swelling rate constant [(gabsorbent/gwater)/min].

3.4.1. Effect of salt solution on water absorbency It has been confirmed, through theoretical and experimental considerations, that the presence of ions has great effect on the hydrogels swelling behavior [42–44]. In this work, the influence of ions in the swelling capability of hydrogels was tested by the addition of NaCl (0.001–0.1 M) or NaHCO3 (0.1 M) or CaCl2 (0.1 M) in the hydrogel-surrounding solution.

Fig. 6. Effect of temperature for obtaining the RHA in the maximum swelling value (Weq) of hydrogels composites.

Table 2 presents the data collected in swelling measurements for the hydrogels in NaCl solution. The sensitivity of hydrogel to

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Table 2 Weq (gwater/gabsorbent) as function of the ionic force for the hydrogels. Hydrogel

RHAG400 RHAG600 RHAG900 PAMACRYL

H2O

NaCl (0.001 M)

NaCl (0.01 M)

NaCl (0.05 M)

NaCl (0.1 M)

Weq

Weq

f

Weq

f

Weq

f

Weq

f

781 ± 27 802 ± 19 1077 ± 32 645 ± 21

445 ± 6 459 ± 4 638 ± 7 361 ± 4

0.43 0.43 0.41 0.44

249 ± 3 253 ± 2 345 ± 4 195 ± 2

0.68 0.68 0.68 0.70

118 ± 2 123 ± 2 169 ± 3 85 ± 1

0.85 0.85 0.84 0.87

84 ± 1 87 ± 1 114 ± 2 63 ± 1

0.89 0.89 0.89 0.91

presence of salts can be related by the dimensionless factor f defined as [45]:

f ¼1

  W saline W water

ð5Þ

in which Wsaline and Wwater are, respectively, the swelling capacity in saline solution and in deionized water. According to Eq. (5) the f values range between 1 and zero. As close the value is of unit (1.0) higher is the sensitivity of hydrogel to the presence of salt. The opposite is valid: for f equal to zero the hydrogel would not possess any sensitivity to presence of salt. The f values (Table 2) indicate that the hydrogel composites undego slightly less influence to the presence of salt than PAMACRYL. The increase in the ionic strength reduces the difference in the concentration of movable ions between the polymer matrix and the external solution (osmotic swelling pressure) and leads to an immediate contraction of hydrogel network. The presence of divalent and trivalent cations in swelling solution drastically reduces the swelling capacity of the hydrogels. This is due to the complexation ability of carboxymide or carboxylate groups and to the formation of inter and intramolecular complexes [46,47]. The hydrogel composites presented greater Weq values in saline solution (Table 3) as compared to PAMACRYL one. Moreover, based on Weq values the hydrogel composites were not sensible to the type (and size) of anion, because the values of Weq were similar for Cl and HCO 3 salt counter ions. An analogous observation was recently reported for poly(acrylamide-co-acrylate) and cellulose nanowhiskers superabsorbent composites [48]. 3.4.2. Equilibrium swelling in buffer solutions at various pH The swelling behavior of PAMACRYL and composites (RHAG400, RHAG600 and RHAG900) at several pH conditions was observed with the use of buffer solutions at pHs 2–12 at constant ionic strength (Fig. 7). An increase in Weq value was observed as the pH of the external solution is increased [49]. In acidic medium, the carboxylate anions are protonated and the anion–anion repulsive forces vanishes; this leads to a minimum swelling of the hydrogel. As swelled in buffer with higher pH values, the carboxylate groups of hydrogels composites become ionized and the electrostatic repulsion between ACOO groups causes expansion of matrix and, consequently, increases Weq [50]. At pH > 4 the hydrogel swells much more due to the repulsion of ACOO groups, while at pH 2 the hydrogel network fast collapses due to the shielding effect from excess of cations. The hydrogel nanocomposites based on starch-g-poly(sodium acrylate) matrix filled with cellulose nanowhiskers studied by Spagnol

Table 3 Weq (gwater/gabsorbent) as function of the type of anion and cation. Hydrogel

RHAG400 RHAG600 RHAG900 PAMACRYL

H2O

NaCl

Weq

Weq

f

Weq

NaHCO3 f

Weq

f

781 ± 27 802 ± 19 1077 ± 32 645 ± 21

85 ± 1 87 ± 2 114 ± 3 63 ± 1

0.89 0.89 0.89 0.91

83 ± 2 79 ± 2 117 ± 2 69 ± 1

0.89 0.90 0.89 0.89

26 ± 1 24 ± 1 38 ± 2 17 ± 1

0.97 0.97 0.96 0.97

Fig. 7. pH effect on the degree of swelling.

et al. [51] presented similar responsive behavior in relation to pH. This behavior makes the hydrogels composites strong candidates to be utilized in controlled release systems. Hydrogels with responses to variations of pH and temperature have been extensively studied within the referred class of ‘‘smart materials’’ which have been applied in wastewater and industrial effluents, controlled drug release, separation membranes among others [52–54]. 4. Conclusions Poly(acrylamide-co-acrylate)/RHA hidrogel composites presenting relevant properties were successfully synthesized, as observed by FTIR and WAXS techniques. RHA proceeding from rice husk was calcined in different temperatures from 400 to 900 °C. WAXS patterns showed that that RHA calcined at different temperatures presented different structures confirming the FTIR data. The ashes obtained at T 6 700 °C showed no defined diffraction peaks, i.e., presents silica with certain crystallinity. This study also evaluated the effect of crystalline or amorphous RHA on the water uptake capability of the hydrogel. The presence of RHA900 on acrylamide–acrylate polymeric matrix improved the water-absorption properties of material, providing an increase of 67% in the Weq values as compared to hydrogel without RHA particles. The RHA in crystalline form induces higher water uptake capacity (Weq) of composites hydrogels due to the intra-interactions among silanol groups on RHA make available new sites in the polymer matrix, which could interact to water. The hydrogels composite poly(acrylamide-co-acrylate) filled with RHA proved to be adequate for the use as soil conditioner. These preliminary results indicate that the hydrogels composites have great potential for their utilization in the agronomic area.

CaCl2

Acknowledgements The authors would like to thank the financial support by FUNCAP (BPI 0280-106/08 and PIL – 139.01.00/09), by CNPq (Proc. 507308/2010-7) and to COMCAP-UEM.

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