Sawdust Derivative for Environmental Application : Chemistry , Functionalization and Removal of textile dye from aqueous solution

The adsorption of Violet Remazol 5R (VR 5) on wood sawdust modified with succinic anhydride (SSA) as a function of contact time, pH, and initial dye concentrations was investigated using a batch technique under ambient conditions. The SSA obtained was confirmed by IR spectroscopy, thermogravimetry and C NMR, and degrees of substitution (DS) were calculated. A study on the effect of the pH on the adsorption of VR 5 showed that the optimum pH was 2.0. The interactions were assayed with respect to the pseudofirst-order and pseudo-second-order kinetic models, and were found to follow closely the pseudo-secondorder. The isotherm was adjusted to the Langmuir, the Freundlich and the Temkin sorption models. SSA is a promising material for the removal of dye textile from aqueous solutions, and under conditions studied the removal percentage achieved was 51.7%.

to several restrictions (Sulak et al. 2007, Gupta andSuhas 2009).Among these treatment technologies, adsorption technology is currently under application and investigative research because it provides a simple, fast, efficient, and economical means to restore polluted areas and to treat wastewater as well (Crini 2006,Yagub et al. 2014).
Most commercial systems currently use activated carbon as a sorbent to remove dyes from wastewater due to its excellent adsorption ability.However, although activated carbon is a preferred sorbent, its high cost prevents a widespread use (Crini 2006, Yagub et al. 2014).Alternatively, agricultural wastes have been reported as adsorbents to dye removal (Crini 2006, Gupta and Suhas 2009, Rafatullah et al. 2010, Salleh et al. 2011, Sharma et al. 2012, Bharathi and Ramesh 2013, Sanghi and Verma 2013, Kyzas and Kostoglou 2014).The materials tested as dye adsorbents include sphagnum moss peat (Ho and Mckay 1998), wheat bran (Çiçek et al. 2007), silkworm pupa (Noroozi et al. 2007), babassu coconut mesocarp (Vieira et al. 2009), pistachio hull waste (Moussavi and Barikbin 2010) and babassu coconut epicarp (Vieira et al. 2011); however, in some situations the sorption capacity of native agricultural by-products was found to be low.In order to improve the sorption capacity of these biomaterials, agricultural by-products have been chemically modified with different reagents and methods.Most materials obtained from chemical modification have been used to remove metals (Randall et al. 1978, Okieimen and Okundaye 1989, Laszlo and Dintzis 1994, Wartelle and Marshall 2000, Vieira et al. 2010), while dye removal is covered by few reported studies.Some studies have reported modification of lignocellulosic materials to remove cationic dyes (Gong et al. 2006, 2007, Mao et al. 2010).
The objectives of this work were to convert wood sawdust into a new anionic sorbent with high sorption capacity, and to examine the potential application of this new sorbent in dye wastewater treatment.Chemical modifications of wood sawdust were conducted by reaction with succinic anhydride (Melo et al. 2011).After characterization of the resulting material, its ability to remove the Violet Remazol 5R (VR 5) dye from aqueous solution was investigated.The influence of pH, time, and dye concentration in the isothermal kinetic modeling has been considered.

mATERiAlS AnD mEThoDS
The VR 5 dye was provided by Indústria de Toalhas São Carlos, from the state of São Paulo, Brazil.The dye was a chemical product of analytical grade and was used without further purification.The wood sawdust was crushed to result in particle sizes in the 0.088-0.177mm range.N,N'-dimethylacetamide (DMA) (Synth) and the succinic anhydride (SA) (Aldrich), ethanol (Synth), hydrochloric acid (Synth), and sodium hydroxide (Nuclear) were all of analytical grade.

PreParation of the Sorbent
The wood sawdust was chemically modified by reaction with succinic anhydride in the absence of solvent (Melo et al. 2011).A sand bath was previously stabilized at a temperature above 391 K.A reaction flask containing 31.0 g of succinic anhydride was immersed in a sand bath and maintained under magnetic stirring, until the merger of the same.After the merger, 5.0 g of wood sawdust were dumped into the flask and left to react for 20 min.Throughout the procedure the balloon was closed by a silica gel drying tube to prevent hydrolysis of the anhydride.At the end of the reaction, N,N'-dimethylacetamide (DMA) was added to the mixture at a 6:1 ratio between DMA and anhydride to remove non-reacted anhydride, which solidifies when cooled during filtration.Then, it was filtered hot and washed with distilled water.The material was dried for 24 h at 298 K and denominated SSA.

MeaSureMentS
Infrared spectra were obtained by accumulating 32 scans in the 4000-400 cm -1 range on a MB-Bomem FTIR spectrophotometer using KBr pellets with a resolution of 4 cm -1 .Solid state 13 C NMR spectra of the samples were obtained on a Bruker AC 300/P spectrometer.The CP/MAS technique was used.The measurements were obtained at 75.47 MHz frequencies, with magic angle spinning of 4 kHz with pulse repetition of 5 s and contact times of 1 ms.
Thermogravimetric curves (TG) were carried out with a Shimadzu TGA-50 instrument under a nitrogen flow rate of 0.50 cm 3 s −1 , in the temperature interval from 298 to 1000 K, with a heating rate of 0.167 K s −1 for a sample mass of about 10 mg.The experiments of the visible spectrophotometry were carried with VARIAN-AA 50 instrument.

Degree of SubStitution
The amount of carboxylic functions attached to the SSA surface was determined through retrotitration.For this purpose, 0.1000 g of each material was treated with 0.100 dm 3 of 400.0 mg dm -3 sodium hydroxide solution for 1 h under constant magnetic stirring.The solid was separated by filtration and three aliquots of 0.020 dm 3 of each solution obtained were titrated with 365.0 mg dm -3 aqueous hydrochloric acid (Gurgel et al. 2008).The concentration of the carboxylic function was calculated by Eq. ( 1): where C NaOH and C HCl are the concentrations of hydroxide and acid (mg dm -3 ), V NaOH and V HCl are the volumes of initial hydroxide and hydrochloric acid (dm 3 ) used in the titration of the excess of non-reacted base, and m mat (g) is the mass of the final chemically modified material.

SorPtion
The sorption tests were performed in batch, from the mechanical agitation of flaks containing 0.0250 g of the sorbent SSA in 10.0 cm 3 of VR 5 solution.After the sorption process, the suspension was centrifuged and the dye concentration in the supernatant was determined by spectrophotometry in the uv-vis.All tests were made under pressure and constant agitation at a temperature of 298 K.
In order to verify the pH influence of the sorption process, the experiment was performed using buffer solution of pH 1.0 to 6.0 (prepared with KCl, HCl, NaOH, and KHC 8 H 4 O 4 ), and dye solution at a concentration of 100 mg dm -3 .The dye solution was in contact with the sorbent under constant stirring for 24 h, and then the suspension was centrifuged and the supernatant was separated to determine the amount of dye sorbed.
The amount sorbed per gram of sorbent was obtained by the equation: THAIS where C i and C f represent the initial and final concentration of the dye, respectively, w is the mass of sorbent (g), V is the volume of dye solution used (dm 3 ), and q is the amount sorbed per gram of sorbent (mg g -1 ).
The kinetic study was performed at pH 2.0, which was verified by a pH study as the one in which maximum sorption occurs.The procedure used was similar to that used in the pH study, varying only the contact time between 5 and 120 min.The experimental data were fitted to the kinetic models of pseudofirst order (Eq.3) and pseudo-second order (Eq.4) (Lagergren 1898, Ho andMcKay 1998) as expressed in equation ( 3) and (4) below: ) where q e and q t (mg g -1 ) are the amounts of sorbed metals at equilibrium and at any time, t (min), respectively, and k f (min −1 ) and k s (g mmol -1 min -1 ) is the sorption rate constant.
Similarly, the experiments were conducted to obtain the sorption isotherm, but varying the concentration of the dye from 20 to 200 mg dm -3 at pH 2.0 and the sorption equilibrium time.The experimental data were fitted to the isothermal models of Langmuir, Freundlich and Temkin (Langmuir 1918, Freundlich 1906, Temkin and Pyzhev 1940), as expressed in equations ( 5), ( 6) and ( 7) below: where C s is the supernatant concentration after the equilibrium of the system (mg dm -3 ), K L the Langmuir affinity constant (dm 3 mg -1 ), and q m is the maximum sorption capacity of the material (mg g -1 ), assuming a monolayer of the sorbate uptake by the sorbent.K F is the Freundlich constant related to the sorption capacity [mg g -1 (mg dm -3 ) -1/n ] and n is the dimensionless Freundlich exponent.K T (dm -3 mg -1 ) and β are Temkin constants.

CharaCterization
The amount of carboxylic groups introduced on the surface of wood sawdust was determined by titration and is given in the Table I.The results observed are comparable with those found in literature (Gurgel et al. 2008).It can be verified that there was a considerable increase in the carboxylic group of the modified material, confirming the surface modification of WS, stating that carboxylic groups were introduced to the matrix through reactions with succinic anhydride.
The infrared spectra of wood sawdust (a) and sawdust modified with succinic anhydride (b) are shown in Figure 1.As can be seen, the main bands that appear in the spectra of wood sawdust (a) are: an intense and broad band in the 3000-3700 cm -1 range attributed to the OH stretching vibrations; the peaks around 2904 cm -1 represented C-H stretching vibrations and CH 3 ; the band around 1730 cm -1 might be assigned to the C=O stretching vibration, and the strong peaks at 1115-1056 cm -1 represented C-OH stretching vibration.The spectra (b) of chemically modified wood sawdust (SSA) is very close to that of the preceding WS, except for the band at 1730 cm -1 , which became more intense and is attributed to stretching of the ester carbonyl formed during the modification reaction with succinic anhydride (Gurgel et al. 2008).The nonappearance of the band at 1850 cm -1 , which corresponds to anhydride-free, demonstrates the purity of the sorbent (Tserki et al. 2005).
Figure 2 shows the comparison between the 13 C NMR spectra of the SSA (b) and cellulose (a), where we can observe the signs that represent the chemical shifts assigned to six carbon atoms present in a monomer unit of cellulose.Signs related to the parts assigned to the crystalline and amorphous carbon 6 are respectively 64 and 55 ppm, and suggest that the hydroxyl groups of complex carbohydrates have been esterified by succinic anhydride (Safou-Tchiama et al. 2007).The signals at 88 and 83 ppm refer to the C4.The region between 75 and 70 pm refers to cabon C2, C3, and C5.The signal at 105 ppm refers to the C1.In addition to signals common to cellulose, the spectrum (b) of modified wood sawdust still, SSA, shows   peaks that appear in the region at 172 ppm, related to the carbonyl carbon 7 and 10, and the methylene group at 32 ppm, carbon 8 and 9, shows the occurrence of reaction with succinic anhydride (Chang and Chang 2001).
The thermogravimetric curves of wood sawdust (a) and SSA (b) are shown in Figure 3. Compared to the precursor, the modified material showed similar behavior regarding decomposition, and the SSA presented a stage of mass loss higher than the wood sawdust.The first stage of mass loss, which occurs at 363 K, is attributed to the release of water physically adsorbed on the surface of the material, which corresponds to 1.23% of the weight of the modified material (Han et al. 2010).The second stage of decomposition in this material occurs at temperatures between 423 and 593 K, and is associated with the decomposition of lignocellulosic materials, which consist of mixture of cellulose, hemicelluloses, and lignin that corresponded to mass of 44.68% (Tserki et al. 2005).The third stage of decomposition occurs at temperatures between 593 and 645 K, where there is a mass loss of 18.47% that can be attributed to the output of the carboxylic group.The decomposition is not complete, and the residue formed was not characterized, but may correspond to carbon formation.Degradation process occurring in these reactions, such as condensation of the hydroxyl carbons 2 and 3, the conversion of acids and esters directly to carbon dioxide and also cellulose fiber loss, is due to decrease of the polymer chains.The chemical modification of wood sawdust kept the resistance to thermal degradation as expected.

Point zero Charge (Ph PzC ) anD Ph influenCe of the SorPtion
The study of point zero charge (pH PZC ) of a material is an important parameter to be considered because it provides useful information about the behavior of charges on the surface of sorbents as a function of pH. Figure 4a shows the graph of pH=pH i -pH f as a function of pH i , which describes the behavior of the surface charge of the SSA.As shown, the pH PZC of the SSA is 6.4, which is the pH where the surface positive and negative charges are equivalent.It can be stated that the sorption of positive charge is favored when the pH is higher than the pH PZC of the material, while the sorption of negative charge, on the other hand, is favored when the pH is lower than the pH PZC (Vieira et al. 2009(Vieira et al. , 2011)).So it is expected that the sorption of the  VR 5 dye by SSA is favored at pH values lower than the pH PZC of the sorbent.This trend was confi rmed by studying the infl uence of pH on sorption, as shown in Figure 4b.The SSA sorption capacity for the VR 5 dye is higher at pH 2.0, since at this pH the surface of the SSA is protonated and is therefore favorable for sorption of anionic species.

KinetiCS anD eQuilibriuM
Figure 5a describes the behavior of sorption as a function of time at pH 2.0, which was defi ned as optimal for sorption.From these isotherms it is clearly evidenced that the equilibrium in aqueous solution is reached after 90 of contact between VR 5 and the suspended SSA.
Experimental data to establish the time required for the sorption equilibrium were fi tted to non-linear mathematical equations of the pseudo-fi rst and peudo-second order to examine the mechanism that controls the sorption process.A good correlation of kinetic data explains the sorption mechanism of the dye in the solid phase (Wu et al. 2001).The data obtained from kinetic models are listed in Table II.As observed,

TABlE ii
Parameter obtained by non-linear fi t of the kinetic models.

Parameter
Pseudo-fi rst order Pseudo-second order q e /mg g -1 24.7 26.1  the pseudo-first order model presented low coefficients of determination (R 2 ) and is also followed by a high chi-square value (χ 2 ), compared to the experimental data.In contrast, the pseudo-second order model showed a high coefficient of determination (R 2 ) and yet low chi-square value (χ 2 ), which indicates that the model is satisfactory to explain the kinetic behavior of this sorption system.Usually the sorption fits this model when the initial concentration of the sorbate is not excessively high (Azizian 2004).Other studies also show a tendency for kinetic models of pseudo-second order to explain the mechanism of dye sorption by lignocellulosic material (Vieira et al. 2009(Vieira et al. , 2010(Vieira et al. , 2011)).
As shown in Figure 5b, the values of amount adsorbed (q) increase with increasing supernatant concentration of dye, which indicates an exponential distribution without a tendency to saturation of the surface.According to the Giles classification, this isotherm is of type H3, which implies large slope at low concentrations indicating strong preferential sorption of the solute (Giles et al. 1960).The experimental results were used to fit the Langmuir, Freundlich, and Temkin models, and the parameters found are shown in Table III.The determination coefficients (R 2 ) calculated show that the three adsorption models tested relatively represented well the adsorption on SSA.

ConCluSionS
The surface modification of WS through succinic anhydride was evidenced by characterization techniques.The analysis of IR, 13 C NMR and titration of the acidic groups showed that carboxylic groups were introduced onto the surface of wood sawdust.The thermogravimetric analysis indicated that there was little change to the thermal resistance of the material, which remains stable up to temperatures close to 393 K.The material obtained can be used as a sorbent for the VR 5dye.
The sorption of the dye proved to be pH dependent, being favored under acidic conditions.At pH 2.0, the active sites existing in the SSA are protonated, since pH PZC for this solid is 6.4.The most favorable sorption at pH 2.0 suggests that the mechanism involves the interaction between the sulfonic groups of dye and the protonated active sites of the sorbent.The system obeys the kinetic q m /mg g -1 55.6 --K L /dm 3 mg -1 0.02 --K F /mg g -1 (dm 3 mg -1 )

Figure 2
Figure 2 -13 C NMR spectra of cellulose (a) and wood sawdust modified with succinic anhydride, SSA (b).

Figure 4 -
Figure 4 -(a) pH zpc and (b) Influence of pH on the sorption of the VR 5 dye by wood sawdust modified with succinic anhydride, SSA.

Figure 5 -
Figure 5 -Effect of contact time (a) and concentration (b) on sorption of the VR 5 dye by wood sawdust modifi ed with succinic anhydride, SSA.Temperature = 323 K, pH 2.0.

TABlE iii Parameter obtained by non-linear fit of the isotherm models. Parameter langmuir Freundlich Temkin
of pseudo-second order and was well adjusted to the adsorption models ofLangmuir,  Freundlich and Temkin. model