Journal of Colloid and Interface Science 454 (2015) 200–209

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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

Fast dye removal from water by starch-based nanocomposites Raelle F. Gomes a, Antonio C. Neto de Azevedo a, Antonio G.B. Pereira b, Edvani C. Muniz c, André R. Fajardo d,⇑, Francisco H.A. Rodrigues a,c a

Coordenação de Química, Universidade Estadual Vale do Acaraú (UVA), Campus da Betânia, 62040-370 Sobral, CE, Brazil Universidade Tecnológica Federal do Paraná (UTFPR), 85660-000 Dois Vizinhos, PR, Brazil c Departamento de Química, Universidade Estadual de Maringá (UEM), 87020-900 Maringá, PR, Brazil d Centro de Ciências Químicas, Farmacêuticas e de Alimentos, Universidade Federal de Pelotas (UFPel), 96010-900 Pelotas, RS, Brazil b

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 29 March 2015 Accepted 13 May 2015 Available online 22 May 2015 Keywords: Hydrogel Hydrogel nanocomposite Starch Cellulose nanowhiskers Adsorption Dye removal

a b s t r a c t Robust and efficient methylene blue (MB) adsorbent was prepared based on starch/cellulose nanowhiskers hydrogel composite. Maximum MB adsorption capacity of 2050 mg per g of dried hydrogel was obtained with the composite at 5 wt.% of cellulose nanowhiskers and at pH 5. Adsorption capacity varied from 1450 mg/g to 2050 mg/g with increasing the initial MB concentration from 1500 mg/L to 2500 mg/L, respectively. For all the concentrations studied ca. 90% of MB was removed by the adsorbent. Optimal conditions were obtained at pH P 5 due to the generation of negatively charged groups (ACOO) in the adsorbent, which can strongly interact with the positive charges from MB. The main advantage of this system over other reported adsorbents, besides the fact of being synthesized from biodegradable polymers (starch and cellulose), is its fast adsorption kinetics that follows the pseudo-second order model, which is based on chimisorption phenomenon. Saturation condition was reached as fast as 1 h of experiments owing to the formation of an adsorbed MB monolayer as suggested by the Langmuir isotherm model. Desorption experiments showed 60 wt.% of MB loaded can be removed from the adsorbent by immersing it in a pH 1 solution, showing its feasibility to be reused. Therefore, starch/cellulose nanowhiskers hydrogel composite presents outstanding capacity to be employed in the remediation of MB contaminated wastewaters. Ó 2015 Elsevier Inc. All rights reserved.

⇑ Corresponding author. Fax: +55 53 3275 7354. E-mail address: [email protected] (A.R. Fajardo). http://dx.doi.org/10.1016/j.jcis.2015.05.026 0021-9797/Ó 2015 Elsevier Inc. All rights reserved.

R.F. Gomes et al. / Journal of Colloid and Interface Science 454 (2015) 200–209

1. Introduction

2. Experimental

Methylene blue (MB) is a fully synthetic chemical, officially named as 3,7-bis(dimethylamino)-phenothiazin-5-ium chloride, first used in the treatment of malaria [1,2]. Since its first report in 1876, MB has been investigated in many medical applications ranging from biomedical dyes for cell staining, antidepressant agents, antimicrobial chemotherapy, cancer phototherapy, cyanide, and carbon oxide poisoning, methemoglobinemia treatment to Alzheimer’s disease, among others [3]. The presence of delocalized electrons from the conjugated p-bonds (chromophore group), in the MB structure, is responsible for its characteristic blue color while in aqueous solution and red color in toluene (with equimolar amounts of hydroxide) [4]. MB has found applications as dye and staining in the textile and paper industry due to its vivid color. According to United States Environmental Protection Agency (US EPA) and United States National Library of Medicine (US NLM), the production of MB in 1998 was ca. 230 tons (500  103 pounds), mostly to supply the textile and paper industry demands [5]. It is estimated that ca. 30% of synthetic dyes are lost during manufacturing or are released into the environment as wastewater from industries [6]. MB is not regarded as an extremely hazardous compound considering its oral LD50 of 3500 mg/kg (in mice) [7]. However, high doses of MB (>7.0 mg/kg) may cause vomiting, abdominal pain, nausea, mental disorder, high blood pressure, and other issues [8]. Chronic exposure to MB has been reported to be mutagenic and cause reproductive disorder [8]. In addition, MB easily interacts with cell membranes due to its cationic or lipophilic nature in the oxidized or reduced forms, respectively, which may induce cell death or inactivation [9]. More than visual concerns, releasing MB wastes into water bodies represent risk to aquatic life due to large amounts discharged annually. MB wastes decreases the water quality by increasing the organic matter and as a consequence the chemical oxygen demand. Besides, photosynthesizing species (e.g. phytoplankton and other organisms in the base of the trophic level) are also harmed due to the restriction in light penetration in dye-contaminated water [10]. Furthermore, MB can easily transpose the cellular membrane and is readily photosensibilized by white light generating highly reactive singlet oxygen (1O2) species, which at high concentrations can damage DNA structures or induce phototoxic response on aquatic living organisms spreading, eventually, to all food chain [11]. Therefore, efficient devices for removing MB from wastewaters are urged required. So far, different approaches for removing dyes and pigments from water including chemical (oxidation, coagulation, etc.), physical (adsorption, filtration, ion-exchange, irradiation, etc.) and biological (microbial discoloration) have been investigated. Some review papers discuss rationally the pros and cons of each of the abovementioned methods [12–14]. It is consensual the use of affordable and efficient method is desirable in remediation of large amounts of contaminated effluents [12–14]. In this sense, adsorption technique stands out as promising process due to its simplicity, the variety of available adsorbents especially those based on polysaccharides as well as the effectiveness in removing non-biodegradable compounds from wastewaters, such as dyes [15]. It has been proved polysaccharide-based hydrogels are excellent dye adsorbents and a robust method for purifying wastewaters [16,17]. Hydrogels are defined as crosslinked polymer networks that can absorb aqueous fluids and solutes neither dissolving nor losing the 3D structure [18]. This contribution is an extension of a previously published paper [19] regarding to an original starch/cellulose-whiskers hydrogel nanocomposite and explores the feasibility of such system work as an efficient adsorbent for MB removal from aqueous environment.

2.1. Materials

201

Acrylic acid (AAc), N,N,N0 ,N0 -tetramethylethylenediamine (TEMED), potassium persulfate (K2S2O8) and N,N0 -methylenebisac rylamide (MBA) were purchased from Sigma–Aldrich (St. Louis, MO, USA). Cassava starch kindly supplied by INPAL S.A. (São Tomé, PR, Brazil) presented Mw of 5.4 kDa determined by GPC/SEC, as described elsewhere [20]. Methylene blue (MB, basic blue 9) spectroscopic grade was purchased from Fluka (Buchse, Switzerland). All other chemicals of analytical grade were used as received without further purification. 2.2. Extraction of cellulose nanowhiskers (CNWs) from cotton CNWs were extracted by acidic hydrolysis from cotton fibers according to the methodology previously described by Spagnol et al. [19]. Briefly, fibers were washed in a NaOH solution (2 wt.%), under mechanic stirring for 1 h at room temperature, soaked in deionized water for 2 h at 60 °C and then dried in a circulating air oven for 5 h at 50 °C. The pre-treated fibers were immersed in concentrated HCl solution (cellulose:HCl ratio of 1:20 g/mL) at 45 °C for 1 h under vigorous magnetic stirring. Finally, the CNWs were recovered by centrifugation (10,000 rpm for 5 min), washed thoroughly with deionized water to neutral pH, and then lyophilized at 57 °C for 48 h. 2.3. Synthesis of Starch-g-PAAc/CNWs nanocomposites Starch was added to 30 mL of distilled water at 85 °C and stirred for 30 min under N2 dynamic atmosphere (to remove oxygen). The solution was cooled to 60 °C and K2S2O8 (1 wt.%) was added to the reaction system, followed by stirring at 60 °C for 15 min to generate free radicals. Then, adequate amounts of AAc (partially neutralized by NaOH solution), MBA and CNWs (0, 5, 10, 15 and 20 wt.%) were added to the system, which was slowly heated up to 70 °C and kept for 3 h to complete the grafting reaction. Samples were cooled to room temperature, followed by extensive washing to remove unreacted chemicals. Finally, all the samples were oven-dried at 70 °C up to constant weight, ground and sieved (9–24 mesh or 2.0– 0.7 mm) for further assays. The samples were labeled as follows: Starch-g-PAAc and Starch-g-PAAc/CNWs(5–20%). 2.4. Characterization Samples were characterized using Fourier Transform Infra-Red spectroscopy (FTIR) and X-ray diffraction (XRD) techniques. The FTIR spectra were recorded in a FTIR-BOMEM-100 spectrometer with a detector of 4 cm1 and 128 scans per sample. For this, samples were smashed and blended with KBr as pellets. XRD patterns were obtained through a powder diffractometer, SHIMADZU model XRD 6000, with Cu Ka radiation source (k = 0.154178 nm) at 30 kV and 20 mA. The scanning range was 5–50° with a scanning rate of 1 °/min. The maximum water uptake capacity (equilibrium swelling – Weq) was evaluated at different pH conditions and constant ionic strength (I = 0.1). For this, known amounts of each dried sample (ca. 100 mg) were weighed and put in a glass vial, to which was added 100 mL of aqueous solutions with different pH (range 2– 12). Each vial was kept under constant slow stirring at room temperature for 24 h to reach the swelling equilibrium. The swollen samples were taken from the vials and drained to remove the water excess. Afterward, those samples were weighed and the Weq parameter was calculated from the following equation [19]:

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W eq ðg water =g absorbent Þ ¼

ws  wd wd

ð1Þ

where ws is the swollen sample weight (g) and wd is the dry sample weight (g). The Weq data are average of triplicates (n = 3). The point of zero charge (PZC) [21,22] analysis was performed using 100 mg of each sample, which was taken in a 100 mL flasks filled with 0.1 mol/L NaCl solutions. The initial solution pH (pH0) was adjusted from 2 to 12 by adding 0.1 mol/L HCl or NaOH. The experimental set (sample-loaded solution) was kept under stirring for 24 h to reach equilibrium. The final solution pH (pHf) was measured and then the difference between the initial and final pH (DpH = pH0  pHf) was plotted against pH0. The point of intersection of the curve at DpH equal to zero (or pH0 = pHf) was assigned to PZC for the sample [21,22]. 2.5. Adsorption and desorption experiments Dye removal capacity of the hydrogel nanocomposites was evaluated by adsorption measurements. Briefly, in a series of glass vials 50 mg of adsorbent were put with 50 mL of methylene blue (MB) solution. The experimental set was kept under stirring (ca. 100 rpm) at controlled temperature (33 ± 1 °C). At desired time intervals, a supernatant aliquot was collected and analyzed at 670 nm using a Micronal BS82 UV–Vis spectrophotometer in order to monitoring absorbance changes. The concentrations of MB solution before and after adsorption were determined using a linear regression equation (R2 > 0.998) by plotting a calibration curve. The adsorption capacity of the adsorbent samples for MB was calculated from the follow equation:

qe ¼

ðC 0  C e ÞV m

ð2Þ

where qe is the adsorption capacity of MB-loaded adsorbent (mg/g), C0 is the initial MB concentration (mg/L), Ce is the MB concentration (mg/L) at equilibrium, m is the mass (mg) of adsorbent, and V is the volume (L) of MB solution. The qe data are average of triplicates (n = 3). Similar procedures were performed in order to evaluate the effect of some parameters on the adsorption capacity: contact time (0–180 min); pH values (2–10), percentage of CNWs incorporated into hydrogel matrix (0–20 wt.%); and initial concentration of MB solution (1500–2500 mg/L). The samples utilized in the adsorption experiments performed with an initial concentration of MB solution of 2000 mg/L were collected by centrifugation and then thoroughly washed with distilled water. The MB loaded adsorbents samples were put in glass vials, to which was added 100 mL of aqueous solutions with different pH. Such experimental set was kept under slow stirring at 33 ± 1 °C for 1 h. After, the amounts of MB desorbed from the samples were determined by UV–Vis spectrophotometry as described above. Finally, the percentage of MB desorbed in each pH condition was calculated from the follow equation:

%¼

C adsorbed  100 C desorbed

ð3Þ

where Cadsorbed is the amount of MB adsorbed onto the samples and Cdesorbed is the amount desorbed from the MB loaded samples. 3. Results and discussion 3.1. Nanocomposites characterization FTIR spectra of cotton fibers and CNWs show similar absorption bands (Fig. 1a and b), which demonstrate the acid hydrolysis did not affect the chemical nature of cellulose. Both spectra show a broad band in the 3700–3300 cm1 region assigned to

Fig. 1. FTIR spectra of (a) cotton fibers, (b) CNWs, (c) raw starch, (d) Starch-g-PAAc, and (e) Starch-g-PAAc/CNWs5%.

hydrogen-bonded OH groups and bands originated from the CAH asymmetric stretching and bending (2900 cm1 and 1310– 1290 cm1), CAO stretching (1200–950 cm1), and CAOAC, CACAO and CACAH deformation modes and stretching (898 cm1) [23]. Fig. 1c displays the starch FTIR spectrum, in which is observed bands originated from hydrogen-bonded OH groups at 3365 cm1 and from CAO stretching in the 1160–1010 cm1 wavenumber regions. The Starch-g-PAAc spectrum (Fig. 1d) shows the characteristics bands of starch, a shoulder-type band at 1723 cm1 assigned to carboxylate groups stretching and bands at 1572 and 1406 cm1 assigned to C@O symmetric and asymmetric stretching. Additionally, the absorption bands at 1647, 1421 and 1368 cm1 assigned to CAOH deformation, which proceeds from raw starch, disappeared due to grafting of AAc monomers in the starch backbone. Such FTIR data and discussion corroborate with other similar works previously published in the literature [24]. Finally, the Starch-g-PAAc/CNWs5% spectrum (Fig. 1e) despite being quite similar to that recorded for the bare hydrogel, shows bands in the 1170–1060 cm1 region assigned to CAOAC asymmetric vibration, CAC and CAO stretching and deformation, regarding to CNWs. The appearance of these bands confirms the successful incorporation of CNWs into the hydrogel matrix. X-ray diffraction (XRD) provides substantial solid-state structural information of polymers, hydrogels, and hydrogels composites. Herein, XRD analysis was applied to investigate structural changes on cellulose resulting from acid hydrolysis of cotton fibers as well as to confirm the incorporation of CNWs into hydrogel matrix. Cotton fibers and CNWs presented diffraction peaks corresponding to the crystalline planes of cellulose I (Fig. 2a and b) [25]. Such peaks were observed at 2h = 14.5° (1 0 1), 16.3° (1 0 10 ), 22.4° (0 0 2), and 34.1° (0 0 4). According to Elazzouzi-Hafraoui et al. [26], the crystallinity index (Icr) of cotton fibers and extracted CNWs were determined from XRD data by the ratio between the crystalline and amorphous regions present in each material. For this, the follow equation was utilized:

Icr ð%Þ ¼

½I002  IAM   100 I002

ð4Þ

where I002 is the maximum diffraction intensities corresponding to 002 crystalline plane of cellulose I (peak at 2h = 22.4°) and IAM is the intensity of the halo at 2h = 18° related to the amorphous phase. Icr value determined from Eq. (4) is given in percentage. Cotton fibers presented Icr of 80% while CNWs 91%. Acidic hydrolysis, the most common method to extract CNWs from plant cellulose fibers and fibrils, increases the crystallinity of the final material due to the preferential hydrolysis of amorphous regions. In other words,

R.F. Gomes et al. / Journal of Colloid and Interface Science 454 (2015) 200–209

Fig. 2. X-ray diffractograms of (a) cotton fibers, (b) CNWs, (c) Starch-g-PAAc, and (d) Starch-g-PAAc/CNWs5%.

strong acids such as HCl and H2SO4 are able to disrupt the amorphous regions of cellulose fibrils preserving the crystalline domains due to the distinct hydrolysis kinetic of amorphous and crystalline phases [27]. The CNWs Icr value of 91% is in agreement with other reports for similar approaches [28,29]. The Starch-g-PAAc XRD (Fig. 2c) reveals the hydrogel did not exhibit any crystalline pattern. Amorphous hydrogels are commonly obtained when synthesized by chemical crosslinking pathway [19]. In contrast, Starch-g-PAAc/CNWs5% diffractogram exhibited the characteristic CNWs crystalline pattern (Fig. 2d). This result confirms the incorporation of CNWs into Starch-g-PAAc hydrogel matrix and corroborates with FTIR data. One of the most important hydrogel properties is the fluid uptake capacity (or swelling ability), which determines potential applications in agriculture, health care, pharmaceutics, among others. Herein, we investigated the water uptake capacity of both hydrogel and hydrogel nanocomposite in aqueous solutions at different pH. Weq values calculated were plotted against pH and the Starch-g-PAAc and Starch-g-PAAc/CNWs5% swelling profiles are displayed in Fig. 3. The hydrogel composite showed uptake capacity higher than that observed for the bare hydrogel independent of pH. However, pH condition played a role in the water absorption capacity. Starch-g-PAAc/CNWs5% Weq values were 10 and 24 units larger than that of Starch-g-PAAc at pH 2 and pH 12, respectively. The

Fig. 3. Swelling profile versus pH: (a) Starch-g-PAAc and (b) Starch-g-PAAc/ CNWs5%.

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slight uptake capacity enhancement showed by the nanocomposite was a clear effect of CNWs incorporation into the hydrogel matrix. Cellulose nanowhiskers possess on the surface a considerable number of hydroxyl groups, which favors self-association. As reported, water molecules display hydrogen-bonding capacity, in the swollen hydrogel [27] and can interact with cellulose nanowhiskers mitigating agglomeration and facilitating CNWs dispersion into the hydrogel matrix [27]. Furthermore, the compatibility and strong interaction between CNWs fillers and the hydrogel matrix enhanced the dispersion as well the interfacial adhesion, thus preventing whiskers agglomeration. Carboxylate groups present into the hydrogel matrix establish strong hydrogen bonds with the hydroxyl group in the nanowhiskers surface [27]. Such well-desired characteristic is emphasized as the major factor in the enhancement of composite mechanical properties. The pH had a paramount effect in the swelling profile of both hydrogel and hydrogel nanocomposite. At low pH values the absorption capacity was restricted while favored at higher pHs. Such pH responsive behavior can be associated with the ratio between the uncharged and charged groups present on the hydrogel matrix. In this case, carboxyl groups (ACOOH) proceeding from the PAAc chains grafted onto starch may be charged (carboxylate form) as function of pH. The balance between [ACOO]/[ACOOH] forms can be determined from the Weq data [30]. Frequently, the swelling curves built as a function of pH show sigmoidal profile, which is reliable to determinate the [ACOO]/[ACOOH] balance [30]. Utilizing the ionization constant equation (Eq. (5)) the [ACOO]/[ACOOH] balance was determined for each pH:

Ka ¼

½ACOO ½Hþ  ½ACOOH

ð5Þ

where Ka is the ionization constant, [ACOO] and [ACOOH] indicates the concentrations of ionized and non-ionized carboxyl groups in the hydrogel matrix and [H+] is the concentration of protons. As indicated in Fig. 3, at low pH values (pH 6 4) [ACOOH] is higher than [ACOO]. Thus the matrix remains stable and the swelling was low. At pH 4, [ACOOH] is almost equivalent to [ACOO] and a slight swelling increasing was observed. The pKa of the PAAc carboxyl groups is ca. 4.5, but it can be higher as the PAAc polymerization degree increases [31]. The ionization of the carboxyl groups increases significantly at pH above pKa. For instance, at pH P 6, the amount of absorbed water remarkably increased due to the high number of ionized groups that expanded the hydrogel as a consequence of anion-anion electrostatic repulsion among carboxylate moieties (ACOO). Other than affecting the swelling profile, the solution pH caused an effect on the sorption properties of hydrogels. Therefore, the pH in which the point of zero charge (PZC) occurred was determined for the Starch-g-PAAc and Starch-g-PAAc/CNWs5% samples. The methodology utilized in this work assumes that the sample functional groups are able to interact with H+ and/or OH specimens according to their physical/chemical properties and solution pH. Therefore, the active sites in the sample matrix can be protonated (at low pH) or deprotonated (at high pH) [32]. The initial solution pH (pH0) may change to (pHf) in the presence of functionalized hydrogel/hydrogel composites resulting in a pH variation (DpH). Such DpH caused by the Starch-g-PAAc and Starch-g-PAAc/ CNWs5% presence was plotted against pH0 as showed in Fig. 4a. The pH of the solution at which DpH is zero is the point of zero charge (labeled here as pHPZC). At the pHPZC the adsorbent’s surfaces is electrically neutral, this means the number of positive charges is the same as negative ones [21]. On the other hand, at pH < pHPZC the sample’s surface was positively charged while at pH > pHPZC was negatively charged [21]. pHPZC values found for Starch-g-PAAc and the composite at 5 wt.% were 6.10 ± 0.05 and

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Fig. 4. PCZ of (a) Starch-g-PAAc and Starch-g-PAAc/CNWs5% and (b) as function of CNWs.

5.90 ± 0.05, respectively (Fig. 4a). Such data show that the incorporation of 5 or 10 wt.% of CNWs decreased this parameter value. In contrast, for the nanocomposites filled with amounts of CNWs higher than 10 wt.% the pHPZC values became higher than that determined to the bare hydrogel (see Fig. 4b). Therefore, in any condition of pH < pHPZC (5.9–6.3, according to the sample) the adsorption sites would be positively charged; thus favoring the adsorption of anionic specimens. Instead, at pH > 5.9–6.3, the adsorption sites would be negatively charged; favoring the adsorption of cationic specimens. The physical adsorption process, which is observed for functionalized hydrogel composites, is directly impacted by the nature of electrical charges present in the adsorbent and adsorbed specimens. The electrical charges present in these two components have to be opposite to assure effective interaction. Hence, the pHPZC gives precious information on the favorable adsorption of an adsorbent material at specific conditions of pH [33]. 3.2. Adsorption experiments 3.2.1. Effect of contact time All the samples presented fast MB adsorption rate during the first 15 min, as shown in Fig. 5a. Then, the adsorption capacity slightly increased with lower rate reaching equilibrium conditions around 60 min. The maximum adsorption capacity for all samples was ca. 90%. The distinct adsorption rates at the beginning and at

Fig. 5. Effects of (a) contact time, (b) CNWs amount and (c) pH on MB adsorption capacity. (Experimental parameters: (a) C0 2000 mg/L; pH 5; T = 33 ± 1 °C. (b) C0 2000 mg/L; pH 5; contact time 60 min; T = 33 ± 1 °C. (c) C0 2000 mg/L; contact time 60 min; T = 33 ± 1 °C).

any time later were attributed to the availability and accessibility to the adsorption sites (ACOO; AOH; ACOOH; vacancies; among others) in the hydrogels. In other words, at the beginning (t  0 min) the most part of the adsorption sites are available to interact with MB molecules and, as result, fast adsorption rates were verified. As those sites became occupied with longer contact times, the adsorption rates gradually decreased reaching a constant value, i.e. the process is in a dynamic equilibrium. Therefore, after 60 min the Starch-g-PAAc and Starch-g-PAAc/CNWs(5–20%) adsorptive capacities were saturated, and that time was considered

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R.F. Gomes et al. / Journal of Colloid and Interface Science 454 (2015) 200–209 Table 1 Comparative chart: adsorption capacities of different hydrogels and hydrogels composites for MB at different experimental conditions.

a

Adsorbent

pH

T (°C)

Contact time (h)

Adsorption capacities (mg/g)

Ref.

Alginate/polyaspartate beads Chitosan-g-PAAc Chitosan-g-PAAc/vermiculite10% Chitosan-g-PAAc/halloysite TiO2/Poly(acrylamide-co-acrylic acid) Starch-g-PAAc Fe3O4-PAAc-co-Poly(sodium acrylate) Sodium alginate-g-Poly(sodium acrylate-co-styrene) Arabic gum-g-Poly(acrylamide-co-acrylic acid) Carbon nanotubes-Xylan/Polymethylacrylate Chitosan/Fe–hydroxyapatite beads Polyacrylamide/sodium alginate/montmorillonite Starch-g-PAAc Starch-g-PAAc/CNWs5%

7 7 7 6.6 – – 7 – 8 – – – 5 5

25 30 30 41 25 25 25 30 25 25 20 28 33 33

24 4 4 24 0.4 16 24 3 5 72 48 – 1 1

472.0 1571.3 1612.3 1316.1 21.3 630.0 246.1 1797.3 48.0 170.0 1324.0 2639.0 1872.9 1918.8

[37] [38] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] (a) (a)

Present work.

optimal. The adsorption curves presented a single and continuous profile suggesting the formation of MB monolayer at the adsorbent surface. Similar results were observed for different adsorbent materials applied in MB removal [34,35]. Further experiments were performed at fixed contact time of 60 min considering the maximum adsorption capacities were obtained at that optimal time. 3.2.2. Effect of CNWs CNWs showed remarkably effect on MB adsorption capacity. The incorporation of 5 wt.% of CNWs in the hydrogel matrix enhanced adsorption capacity from 1872.90 mg/g to 1918.81 mg/g (Fig. 5a). Low CNWs amounts were well dispersed into the hydrogel matrix increasing the composite hydrophilicity and favoring the MB adsorption. Further, the Starch-g-PAAc hydrogel presented lower porosity than the composite, as reported in our previous work [19]. High hydrogel porosity in adsorption experiments is desirable because it allows high volume influx as well as more functional groups are exposed on the wall pores to interact with the dye. However, the adsorption capacity decreased with further increase of CNWs amount. Two factors explain this behavior: (i) high CNWs amounts favor the interaction between hydrogel matrix-filler. The hydroxyl groups on the nanowhiskers surface interact with the carboxylate groups (ACOO) of PAAc resulting in additional crosslinking points that prevents matrix expansion and consequently, solution uptake [36]; (ii) decreasing in carboxylate groups, which are responsible to interact with the MB molecules [27]. From these results, the sample Starch-g-PAAc/CNWs5% was utilized in further experiments. The hydrogels and composites synthesized in this work presented outstanding adsorption capacities in significantly shorter contact time (only 1 h) as compared to other adsorbents. Table 1 presents the adsorption capacities of different hydrogels and hydrogels composites for MB. Therefore, the Starch-g-PAAc/CNWs hydrogel nanocomposite can be use as active and fast adsorbent material for the adsorption of MB from water. 3.2.3. Effect of pH MB adsorption capacity for both Starch-g-PAAc and Starch-g-PAAc/CNWs5% samples increased significantly as function of pH increasing as a result of changes in the electrostatic and dipolar interactions between adsorbent and adsorbed molecules (Fig. 5c). Distinct adsorption behaviors were observed in the curves according to pH range. At low pH, the excess of protons (H+) competes with the MB cationic groups for the adsorption sites. Additionally, at this pH the ionization of carboxyl groups was suppressed, which weakened the electrostatic attraction between adsorbent and MB molecules [48]. The pH increasing from 2 to 5

promoted an adsorption burst. For instance, the adsorption capacity of Starch-g-PAAc/CNWs5% duplicated, e.g. it increased from 960.21 to 1894.45 mg/g. At pH conditions close to the pKa of carboxyl groups, the negative charge density within the hydrogel/hydrogel nanocomposite matrixes increased and the adsorption capacity was enhanced. This is associated to the fact that at pH 5 the Starch-g-PAAc and Starch-g-PAAc/CNWs5% water uptake increased allowing higher contact between the MB molecules and the negatively charged groups favoring the adsorption process. Further pH increasing did not promoted any significant variation in the adsorption capacity. Therefore, pH 5 was established as optimal for further experiments. Moreover, the hydrogel nanocomposite adsorption behavior at different pH levels allows its application as MB adsorbent in a wide pH range. 3.2.4. Adsorption kinetics The study of adsorption kinetics is mandatory taking into consideration the developing of adsorbent materials for wastewater treatment. Adsorption kinetics gives important information concerning the mechanism of adsorption and allows comparing different adsorbent under distinct operational conditions for similar applications [49]. Herein, to investigate the MB adsorption in the hydrogels nanocomposites, experimental data were treated by pseudo-first-order and pseudo-second-order models, Eqs. (6) and (7), respectively [50]:

logðqe  qt Þ ¼ log qe  t 1 t ¼ þ qt k2 q2e qe

k1 2303

ð6Þ

ð7Þ

where qe and qt are the MB adsorption capacity (mg/g) in the nanocomposite sample at equilibrium and at any time t, respectively. k1 is the rate constant of the pseudo-first-order, which was estimated from the slope of the linear plots of log(qe  qt) versus t (Fig. 6a), and k2 is the rate constant of the pseudo-first-order that was estimated from the intercept of the linear plots of t/qt versus t (Fig. 6b). The parameters calculate from these kinetic models (qe, k1 and k2) as well the linear correlation coefficients (R2) are presented in Table 2. The theoretical values calculated for qe using the pseudo-first-order model were not in agreement with the experimental qe values (Table 1). In contrast, the qe(th) values calculated using the pseudo-second-order model are consistent to the experimental data. The correlation coefficient (R2) values calculated for that last kinetic model were higher than 0.999; strong evidence that MB adsorption process in the adsorbent samples followed a

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Fig. 7. Effects of initial MB concentration on the adsorption capacity (experimental parameters: pH 5; contact time 60 min; T = 33 ± 1 °C).

Fig. 6. (a) Pseudo-first-order and (b) pseudo-second-order models for MB adsorption in the Starch-g-PAAc and Starch-g-PAAc/CNWs(5–20)% samples (experimental parameters: adsorbent mass 50 mg; C0 2000 mg/L; pH 5; T = 33 ± 1 °C).

pseudo-second-order kinetics. This model assures that the MB adsorption process was driven by chemisorption phenomenon, which results from valence forces through the sharing or exchange of electrons between the adsorbent and the adsorbed specimen [43,51]. Additionally, according to this model the adsorption rate of the hydrogels nanocomposite is given in the order of the amount of CNWs incorporated 5 > 10 > 15 > 20 > 0 (wt.%). The adsorption capacity increases from 1439.35 to 1856.90 mg/g for the bare sample and from 1455.76 to 1918.81 mg/g for the nanocomposite sample, when the initial MB concentration increases from 1500 to 2000 mg/L (Fig. 7). Above such concentration the adsorption profile showed slight increment until constant values, for both systems. Such results are consistent with other previously reported in the literature for similar systems [43,51]. According to some authors, high initial concentrations of

MB can generate a concentration gradient, which affects the diffusional process of MB molecules into the hydrogel matrix [43,44]. Moreover, above a certain concentration (>2000 mg/L), the MB molecules interacted with almost the totality of adsorption sites, and the saturation of adsorption was reached [43,44]. Generally, adsorption isotherm models are employed to fit the adsorption data, in order to describe the equilibrium relation between the quantity of the adsorbed material and the concentration in the bulk fluid phase at constant temperature [21,43]. Further, isotherm models are very powerful tools to optimize the adsorption system. The Langmuir and Freundlich isotherms are the mostly employed models to this purpose. The Langmuir isotherm describes adsorption process that occurs in a monolayer under the surface where there are a fixed number of adsorption sites. In this case, each site is able to adsorb only one molecule [52]. The Freundlich isotherm describes the adsorption process on an amorphous surface and considers the possible formation of various monolayers [52]. The Langmuir and Freundlich isotherms models are expressed by the following Equations:

Ce 1 Ce ¼ þ qe bqm qm log qe ¼ log kF þ

ð8Þ 1 log C e n

ð9Þ

where qe is the MB adsorption capacity (mg/g) at equilibrium and qm is the monolayer adsorption capacity per unit mass of adsorbent (mg/g) and Ce is the MB concentration at equilibrium (mg/L). The parameter b is the Langmuir constant (L/mg), kF is the Freundlich constant (L/g) and the parameter 1/n (dimensionless) correlates the adsorption intensity of the surface heterogeneity. The experimental data were fitted to the Langmuir and Freundlich isotherms

Table 2 Pseudo-first-order and pseudo-second-order models constants for MB adsorption onto the Starch-g-PAAc and Starch-g-PAAc/CNWs(5–20)% samples. CNWs (wt.%)

0 5 10 15 20 a b

Experimental values. Theoretical values.

qe(exp)a (mg/g)

1872.90 1918.81 1855.18 1837.34 1819. 97

Pseudo-first-order

Pseudo-second-order

qe(th)b

k1 (min1)

R2

qe(th)b

k2 (104 g/mg min)

R2

703 780 728 632 644

7.15 8.46 8.91 6.51 6.30

0.9694 0.9933 0.9720 0.9511 0.9347

1899.50 1920.99 1875.97 1856.23 1846.29

3.58 4.23 4.17 4.12 3.70

0.9999 0.9999 0.9999 0.9999 0.9999

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and the plots are presented in Fig. 8. Through the isotherms plots, all the parameters for both models were calculated and are displayed in Table 3. For both adsorbent samples, the correlation coefficient (R2) of the Langmuir isotherm was significantly higher than that calculated from the Freundlich isotherm (Table 3). This suggests that the Langmuir isotherm is the more adequate to explain the MB adsorption than the Freundlich isotherm. Therefore, the MB adsorption occurred through a monolayer formation in the adsorbent. Such behavior was also observed for MB adsorption in hydrogels nanocomposites based on chitosan-g-PAAc filled with different clay types (e.g. montmorillonite, attapulgite, and vermiculite) [38,53,54]. Two other parameters calculate from the Langmuir isotherm are worthy of discussion; the Langmuir constant (b) and qm. The parameter b represents the interaction intensity between the adsorbent and the adsorbate. As can be seen in Table 3, the hydrogel nanocomposite showed higher b value than

that calculated to the bare hydrogel, indicating the incorporation of CNWs into the hydrogel matrix enhanced the adsorbent-MB interaction. As a consequence, the monolayer adsorption capacity per unit mass of adsorbent (qm) was higher for the nanocomposite. These data show clearly that the incorporation of CNWs is a suitable approach to enhance the adsorption capacity of conventional hydrogels. Finally, from the Langmuir isotherm is possible to predict if the adsorption of MB in the adsorbent synthesized in this work were favorable or not. For this, a separation factor (RL) can be calculated using the following equation [55]:

RL ¼ 1=ð1 þ bC 0 Þ

ð10Þ

RL value reveals the type of the isotherm to be favorable (0 < RL < 1), unfavorable (RL > 1), linear (RL = 1) or irreversible (RL = 0). Herein, the RL values calculated for the Starch-g-PAAc and Starch-g-PAAc/CNWs5% samples were 0.05 and 0.008, respectively. Thus, the MB adsorption on both adsorbent samples is favorable. 3.2.5. Adsorption mechanism FTIR spectra of the adsorbents, Starch-g-PAAc and Starch-g-PAAc/CNWs5%, were recorded before and after the adsorption experiments and are shown in Fig. 9. Such approach is suitable to understand the MB adsorption mechanism by different hydrogels matrices [56]. FTIR recorded for the loaded nanocomposite showed the hydrogel characteristic absorption bands, bands ascribed to MB and some peculiarities. For example, the broad band assigned to the hydrogen-bonded OH groups slightly shifted from 3454 to 4420 cm1 accompanied by intensity decreasing. We suggest the formation of hydrogel bonds between the sulfur atoms in the heterocycles of MB and the OH groups present in the nanocomposite matrix [57]. Moreover, bands at 1723, 1599 and 1435 cm1 originated by the stretch of the carboxyl groups (ACOOH) and the asymmetric and symmetric stretches of the carboxylate groups (ACOO) shifted to low wavenumber region (bands at 1712, 1574 and 1408 cm1, respectively) and changed intensity. Such

Fig. 8. (a) Langmuir and (b) Freundlich isotherms plots for MB adsorption onto Starch-g-PAAc and Starch-g-PAAc/CNWs5% samples (experimental parameters: adsorbent mass 50 mg; pH 5; T = 33 ± 1 °C).

Fig. 9. FTIR spectra of (a) Starch-g-PAAc/CNWs5% and (b) MB-loaded Starch-gPAAc/CNWs5%.

Table 3 Parameters derived from the Langmuir and Freundlich isotherms plots for MB adsorption. Adsorbent

Starch-g-PAAc Starch-g-PAAc/CNWs5%

T (°C)

33 33

qe (mg/g)

1998 2055

Langmuir model

Freundlich model

qm (mg/g)

b (L/mg)

R2

kF (L/g)

n

1/n

R2

2043 2236

0.0098 0.0590

0.9996 0.9982

1094 914

9.98 6.70

0.100 0.149

0.6338 0.6270

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Fig. 10. Proposed scheme for the adsorption mechanism of MB onto Starch-g-PAAc/CNWs hydrogel nanocomposite.

behavior is originated due to formation of hydrogen bonds between the negatively charged groups in the nanocomposite matrix and unsaturated dimethylamino groups of MB [57]. Finally, the band assigned to the vibration modes ascribed to methylene groups (at 2945 cm1) shifted to 2923 cm1 suggesting that other than electrostatic interactions between MB and the nanocomposite, hydrophobic interactions were also involved. FTIR spectra recorded for Starch-g-PAAc and MB-loaded Starch-g-PAAc showed to be similar and were not included. Therefore, it is suggested the adsorption mechanism of MB in the Starch-g-PAAc/CNWs is driven by combined electrostatic interactions, hydrogen bonds and hydrophobic interactions [58,59]. Fig. 10 displays an illustrative scheme for the proposed adsorption mechanism. 3.2.6. Desorption experiment Desorption experiments of MB from the loaded Starch-g-PAAc and Starch-g-PAAc/CNWs5% were performed to investigate the economic feasibility and reusability of both adsorbents (data not shown here). Maximum desorption (ca. 60%) for both adsorbents was verified at pH 1. Afterward, desorption decreases drastically (pH 1–4) and then did not vary significantly at pH P 4. This behavior can be attributed to the strong interaction between the MB adsorbed and the adsorbent material [56–59]. Further studies are being performed at different experimental conditions (for example, different temperatures and swelling media) in order to enhance the desorption capacity and to investigate the optimum desorption conditions. 4. Conclusions This contribution reports a set of experiments assessing the feasibility of hydrogel composites based on starch and cellulose nanowhiskers (CNWs) in the remediation of methylene blue-contaminated wastewaters. Nanocomposites formation was

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