Methylene Blue Adsorption from aqueous solution by low cost vine-wood biomass.

In this work, the sawdust of vine wood (VW) was treated with sulfuric acid and used to 8 adsorb methylene blue (MB) from aqueous solutions via a batch adsorption process. The 9 characteristics of the adsorbent were determined by various analytical techniques such as 10 Fourier-transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD) and scanning 11 electron microscopy (SEM) and Brunauer−Emmett−Teller (BET) N 2 adsorption−desorption 12 isotherms. The effects of various experimental parameters including sulfuric acid 13 concentration, particle size of the adsorbent, pH of the solution, contact time, initial 14 concentration, adsorbent dosage and temperature on adsorption of MB by activating sawdust 15 were systematically investigated. The experimental results showed that the adsorption 16 efficiency was increased with contact time and adsorbent dosage. The maximum removal 17 efficiency was found after 180 min of solid/liquid contact with adsorbent doses of 1 g/l for 18 sawdust. The isotherm and kinetic experimental data for MB adsorption on VW sawdust were 19 best-fitted by Langmuir models and Pseudo-second-order, respectively. The calculated values of the entropy (ΔS°), enthalpy (ΔH°) and Gibbs energy (ΔG°) indicated 21 that the adsorption process was exothermic in nature. These results suggest that the activated 22 sawdust can be employed as a low-cost and environmentally friendly adsorbent for the 23 treatment of wastewaters containing dyes.


Introduction 26
Wastewater pollution by toxic dyes is an emerging concern because of potential health 27 impacts. They are mainly discharged into the environment through various industrial activities 28 like textile, printing, leather, paper-making, plastics etc. there complex aromatic molecular 29 structure make them stable and more difficult to be removed from the effluents. Therefore, it 30 is necessary to remove dye from wastewater before releasing them into the environment. 31 Methylene blue (MB) is a cationic dye used in different industries, especially in the textile 32 and dyeing industries (Kumar et al. 2004). However, the existence of MB in the environment 33 can cause eye and skin irritation, dyspnea, gastritis and mental confusion. Therefore, the 34 removal of MB from polluted water is an important concern in wastewater treatment. Vine wood, which is composed of natural polymers such as lignin, cellulose, hemicellulose, 47 and other extractives, is available in large quantities and is formed as waste product.
Herein, vine wood is used as raw material to prepare activated sawdust adsorbent, and the 49 obtained material was used for the adsorption of cationic MB from aqueous solutions. 50 The effects of adsorbent dosage, pH, contact time and initial dyes concentration (MB) were 51 Studied. Moreover, the adsorption was analyzed using isotherms, kinetic models, 52 thermodynamics and adsorption mechanism. The results show that modified vine wood is an 53 effective adsorbent for removal of cationic dyes from aqueous solutions. 54 2 Experimental 55

Materials 56
All chemicals used were analytical reagents grade and prepared in distilled water. Vine wood 57 samples (VW) were received from a local company. The methylene blue was chosen in this 58 study because of its known strong adsorption onto solids. A solution of 10 -5 mol L -1 59 methylene blue [3, 7-bis (dimethylamino) phenothiazin-5-ium ion] was prepared in distilled 60 water as the stock solution (Fig. 1). Methylene blue (MW =319.85 g mol -1 termed as MB) 61 shows an intense absorption peak in the visible region at 664.5 nm (Fig. 2). This wavelength 62 corresponds to the maximum absorption peak of the methylene blue monomer. The pH 63 adjustments were carried out using dilute NaOH and HCl solutions. 64

Determination of methylene blue concentration 70
Concentrations of methylene blue (MB) in the supernatant solutions were estimated by 71 measuring absorbance at maximum wavelengths of the dye (λmax=664.5nm) using the 72 calibration curve shown in Fig. 3. The calibration curve of absorbance against MB 73 concentration was obtained by using standard MB solutions at pH 6.9 (initial pH). The 74 calibration curve shows that Beer , s law (A= εbc) is obeyed in concentration range (2×10 -6 -75 10×10 -6 mol L -1 ). 76

Preparation of vine wood biosorbent. 78
Vine wood samples were collected from the Ain Témouchent region (western Algeria). They 79 were then cleaned and dried at a temperature of 105 °C for 24 h. Samples were ground and 80 sieved to particle sizes of 80 μm to1mm. Then, they were refluxed in sulfuric acid solution of 81 wt 10% at 100°C for 2 h to open VW surface pores. The biosorbent was washed several 82 times with double-distilled water and dried for 24 hours after filtering. 83 . EDS analysis of activated VW (Fig. 7(b) ) showed that the activated 142 VW sawdust contained higher contents of oxygen, which caused by oxidation process, 143 which might be to form the oxygen-containing functional groups on VW surface. It is also 144 observed that the reduction of contents of carbon, which is due to the degradation of 145 hemicelluloses, lignin and the elimination of extracts, following the modification chemical of 146 cellulosic material.This result was in good agreement with XRD. 147   . Data were taken for from 2θ angles ranging from 10 to 80 degrees. It is found that 152 the diffraction patterns of the VW untreated have low crystallinity (amorphous). The 153 amorphous characteristic of raw VW is due to the high lignin content (amorphous compound) 154 in its structure (Rosa et al. 2010). Significant 2θ angle peaks at 18° and 22° are observed in 155 XRD patterns of the treated sawdust, which are associated with the crystalline cellulose 156 structure. This is due to the partial reduction of hemicelluloses and lignin during the chemical 157 treatment. This observation was also reported by (Alemdar and Sain (2008); Shahabi et al.

Effect of particle size 195
The effects of adsorbent particle size on the adsorption kinetics of activated VW in aqueous 196 solution are depicted in Figure 12. It is noted that the dye adsorption capacity of the activated 197 VW was decreased with increase in particle size. This is because bigger particle sizes have a 198

Effect of adsorbent dosage 205
The effect of adsorbent dosage on the removal of MB is shown in Figure 13.

Effect of Contact Time 222
The effect of contact time of 0.1 g VW sawdust to remove the MB dye in particle size 223 125/250 µm with agitation speed 500 rpm, 100 ml, and 10 -5 to 3010 -6 mol L -1 dye solution 224 was studied from 1 -210 min. Fig. 14 showed that the adsorption was quick during the initial 225 30 min and steadily reached equilibrium in 50-120 min. The maximum amount of dye 226 adsorbed was found to be 9.88.10 -3 -29.95.10 -3 mmol/g (98.88% -99.83% removal 227 efficiency) at 180 min, beyond which saturation was seen suggesting the adsorption-228 desorption equilibrium. Furthermore the adsorption of MB became difficult due to the 229 repulsion of solutes between solid (saturation of the active sites) and bulk phase (Malik. 230 2003). 231

Effect of solution pH 246
The effect of the solution pH was examined in the range of pH 2.5 to 11 at a fixed adsorbent 247 dosage of 100 mg, and 10 -5 mol/L of MB concentration. As clearly indicated in Figure 16, as 248 the solution pH increases, the dye adsorption increases due to the increasing electrostatic 249 force of attraction between positive charged adsorbate and negatively charge adsorbent 250 surface (pH > pH pzc) in alkaline condition due to the ionization of VW. While in acidic 251 pH, the adsorbent surface is positively charged (pH < pH pzc), the dye adsorption is lower, 252 this is mainly due to the presence of higher concentration of H + ions on the VW sawdust surface hindered the adsorption of cationic MB dye. Nevertheless, it was also observed that 254 the despite surface of activated VWS being positive, the adsorption of MB remains high 255 (84.50 to 89.15%. ), when pH was within the range of 2.5 to 4, suggesting that not only 256 electrostatic mechanism but rather also by chemical reaction between the adsorbent and dye 257 molecules.Therefor, the normal pH of MB solution was selected to be the best for further 258 studies.This is in concord with the results of ( Han X et al. 2011). 259

Effect of Temperature 261
The temperature-dependent adsorptions were performed using 0.1g of the adsorbent in 100 262 mL of the MB solution (10 -5 mol/l) at 20, 30, 40 and 50°C (Fig. 17). It was found that with 263 rising the temperature, the percent removal decreased (from 99.85% to 97.71 %.).This may be 264 due to the decreased surface activity, suggesting that adsorption between MB and VWS was 265 an exothermic process. This phenomenon is in agreement with the Arrhenius rule. Similar

Effect of shaker speed 270
The shaker speed is also a very significant factor to interfere the adsorption efficiency. Its

Kinetic study of adsorption of the BM on the sawdust of vine wood. 280
The kinetics modelling was conducted using 100 mL of 10 -5 -3.10 -5 mol L -1 of dye solution 281 with an initial pH (pH= 6.9), then contacted with 100 mg VW sawdust and shell agitated at 282 500 rpm for 180 min. This kinetics were analyzed by kinetic rate equations, ie pseudo-first 283 order, pseudo-second order, and intra-particle diffusion models. The model parameters of all 284 the three models were estimated by plotting graphs Ln(Qe-Qt) versus t, t /Qt versus t and Qt 285 versus t 0.5 , for pseudo-first order, pseudo-second order, and intra-particle respectively, and 286

Pseudo-first-order kinetic model (Lagergren model) 289
The adsorption kinetics data were evaluated using Lagergren's pseudo-first order (Lagergren 290 1898), it is the first speed equation established to describe the adsorption kinetics in a system 291 (solid-liquid). This model is introduced by the following relation: 292 The integration form of the equation (3) applying the initial conditions t = 0 to t = t and Q = 0 294 to Q = Qt becomes: 295 Where Qe and Qt are theamount of dye adsorbed at equilibrium and at contact time t, 297 respectively (mmo l / g); t is the time of contact (min); K1 is the equilibrium rate constant of 298 pseudo-first-order kinetics (min -1 ). The values of K1 and Qe were obtained from the slope and 299 intercept respectively of plots of Ln (Qe -Qt) versus t (Fig. 19). Values of K1 , Qe 300 (experimental and calculated) and correlation coefficient (R 2 ) subsequently given in Table 2. 301 Fig. 19 Pseudo-first-order kinetics plots for the adsorption of the MB on VWS.
According to the plots Ln (Qe -Qt) vs t, as shown in Fig. 19 Table 2. The constant k2 is used to calculate the initial 318 sorption rate h, at t→0, as follows: 319 The verification of the models proposed to describe the adsorption kinetics and adsorption 321 isotherms is based on the correlation coefficients (R 2 ) and "average relative error, ARE" was 322

intra-particle diffusion model ( IPD) 341
The application of the equation (9) of the intra-particle diffusion model proposed by Weber 342 and Morris (1963) to the experimental data was applied to examine the adsorption 343 mechanism of the adsorption system.The linear form of the intraparticle equation is shown as 344 follows: 345 Where Ki is the intra-particle diffusion rate constant (mmol. g -1 .min -1/2 ); C is the constant that 347 gives an idea about the thickness of the boundary layer (mmol /g) and Qt is the amount of dye 348 adsorbed at time t (mmol /g) . 349 The slope of the graph Qt versus t 1/2 makes it possible to evaluate the intra-particle diffusion 350 rate constant (K i) and correlation coefficient (R 2 ) indicate the fitness of this model . The

values of intercept inform about the thickness of the boundary layer (C). If the plots of Q t 352
versus t 1/2 yield straight lines do not passing through the origin, then the adsorption process 353 is not the only limiting mechanism of the adsorption kinetics and this is confirm the imply of 354 some other mechanisms (Weber and Morris 1963). The obtained results, for the various initial 355 concentrations are represented in Fig. 21 & 22 and are summarized in Table 2.  Table 2, the R 2 values of this diffusion model were 368 closer to 1 (0.94.95-0.9954) and the linear lines of the second stage at each concentration did 369 not pass through the origin. These indicate that the intra-particle kinetics of diffusion into the 370 pores is implied in the mechanism of adsorption process but is not the only rate controlling 371

step. The increases in the initial concentration of the MB entrain an increase in the constant C 372
which is proportional to the thickness of the boundary layer. 373

Adsorption Isotherms of MB Adsorption of VW Adsorbent 378
The adsorption process was fitted using the Langmuir (eq 11), Freundlich (eq 14), and 379 Temkin (eq 16) models for the different initial dye concentration from 10 -5 mol/L to 30.10 -6 380 mol/L after 180 min in the presence of 100 mg of VWS at room temperature. 381

Langmuir Adsorption Isotherm 382
It is the model more used. Langmuir supposes that adsorption is done into monolayer surface, The linear form of this model is given by the following equation: 396 Where Ce is the equilibrium concentration of solution dyes (mol/L), Qe is the amount of dye 398 adsorbed at equilibrium(mmol/g), Qm is the maximum adsorption capacity (mmol/g), K L 399 is the Langmuir equilibrium constant related to the affinity of the binding sites or the free 400 energy of adsorption (L/mol). 401 The values of the constant Qm and KL can be determined from the intercept and the slope of 402 the linear plot of 1/Ce versus 1/Qe (Fig. 23) and are presented in Table 3. 403 The essential characteristics of the Langmuir isotherm was expressed in terms of 404 dimensionless constant separation factor RL of Hall (without dimension) is given by the 405 following equation (Hall et al. 1966). 406 Where C0 is the initial concentration of dye (mol/L), K L is the Langmuir constant (L/mol). 408 The parameter RL indicates the type of the isotherm and whether it is favorable (0 < RL< 1), 409 unfavorable (RL > 1), linear (RL =1), Irreversible (RL = 0). 410 The linear form of this model is given by the following equation: 417

Freundlich Adsorption Isotherm
Where K F is Freundlich equilibrium constant related to the adsorption capacity of adsorbent 419 (mmol/g(L/mol) 1/n ), n is the adsorption intensity of adsorbent, Ce is the equilibrium 420 concentration of solution dyes (mol/L) , Qe is the amount of dye adsorbed (mmol/g). 421 From the slope and intercept obtained by plotting Ln (Qe) versus Ln (Ce) (Fig. 24), the values 422 of n and KF were calculated. The Freundlich parameter 1/n relates to the surface 423 heterogeneity gives us the intensity of adsorption and the shape of the isotherm. 424 .When 0 < 1/n < 1, the adsorption is favorable; 1/n =1, the adsorption is homogeneous and 425 there is no interaction among the adsorbed species; 1/n > 1, the adsorption is unfavorable. 426

(Rauf et al. 2008) 427
When 1 / n = 1the isotherm is linear type C; 1 / n> 1 the isotherm is S-type convex; 1 / n <1 428 the isotherm is type L concave; 1 / n << 1 the isotherm is type H.   to unity (R 2 = 0. 9846) and lower value of average relative error, suggesting that a mainly 458 monolayer adsorption behavior on the prepared biosorbent. It is obvious from Table 3 and 459  Table 4. 467

Thermodynamic analysis 473
To explore the thermodynamic characteristics of the VW sawdust adsorbent, the adsorption 474 process was examined at different temperatures (293, 303, 313, and 323 K) (Fig. 27). The 475 thermodynamic parameters such as free energy change (ΔG 0 ), enthalpy change (ΔH 0 ), entropy 476 change (ΔS 0 ), etc. for adsorption were calculated via the Gibbs free energy and the Van't Hoff 477 equation (Eqs. 19 & 20). 478 The change in free energy is related to the equilibrium constant by the following relationship: 479 According to the Gibbs' free energy equation: 481 Combining equation 18 and 19, we get: 483 where T is the absolute temperature (Kelvin), R is universal gas constant (8.314 Jmol −1 K −1 ), 485 Kd is the distribution coefficient for adsorption. The ΔH 0 and ΔS 0 values obtained from the 486 slope and intercept of Van't Hoff plots are given in Table 5. 487

Conclusions 500
This work demonstrated the successful preparation of a low-cost biomass adsorbent based on 501 vine wood sawdust with sulfuric acid treatment for methylene blue dye removal. The 502 optimum adsorption condition was achieved by analyzing the influencing parameters such as 503 adsorbent dose, initial dye concentration, pH, temperature, contact time, agitation speed and 504 particle size. The maximum adsorption of MB was found at the normal pH of the MB 505 solution, temperature 20°C, and after 3 h of equilibration time. The kinetic experimental data 506 conformed to the pseudo-second-order model, with a high correlation coefficient of 0.999. 507 The adsorption of MB followed the Langmuir isotherm model with maximum adsorption 508 performance of 0.4579 mmol / g (146.46 mg g-1). This study provided a practical method for 509 preparing an economical, efficient bioadsorbent from a wide variety of biomass. 510 Declarations 511

• Ethics approval and consent to participate 512
Not applicable 513

• Consent for publication 514
Not applicable 515

• Availability of data and materials 516
Not applicable 517

• Competing interests 518
We have no competing interests 519