Adsorptive Removal of Methyl Red from Aqueous Solution onto Charred and Aminated Sugarcane Waste

Adsorptive removal of Methyl Red (MR) from aqueous solution onto chemically modified charred sugarcane waste (CSW) and aminated sugarcane waste (ASW) has been investigated. The surface modification was characterized by FTIR, SEM, elemental analysis and Boehm titration. The effect of pH, contact time and MR concentrations were studied by batch equilibrium method. Maximum dye removal was observed at pH 2 onto CSW while that for ASW at pH 7. The dye can be quantitatively removed onto the surface of these adsorbents at a contact time of 3 h. Maximum adsorption capacity (qmax) for the CSW and ASW were found to be 125.0 mg/g and 142.85 mg/g, respectively. Adsorption kinetic data were tested using pseudo-first order, pseudo-second order and intra-particle diffusion models. Kinetic studies revealed that the adsorptive removal of the dye onto the adsorbents followed pseudo-second order kinetics model. The obtained results indicated an excellent alternative for the treatment of dye contaminated wastewater using such chemically modified sugarcane waste at low cost with better efficiency.


INTRODUCTION
Rapid progress in industrial activities during recent years has led to the discharge of unprecedented amount of waste water containing different pollutant such as organic surfactants, heavy metal ions, acids, alkalis, solvents and dyes. Among these pollutants dyes are also main constituents of pollutant of natural water which causes harm to human and other living organisms [1,2]. A majority of dyes are bright colored due to the presence of one or several azo (-N=N-) groups associated with substituted aromatic structures [3]. Effluents from textile, leather, food processing, cosmetics, paper and pulp mill, electroplating factories, dyes manufacturing industries, distilleries, tanneries are some examples of the sources of azo dyes [1,4,5]. Dyes and their byproducts are toxic to living organisms [7]. Most of the commercially used dyes are resistant to biodegradation, photo degradation and even oxidizing agents [1,7]. Unless otherwise properly treated, these dyes significantly affect the photosynthetic activity in aquatic life due to reduced light penetration and also are toxic to human and some aquatic life by its mutagenic and carcinogenic effect [10,11]. As a matter of fact, the discharge of such effluents is worrying for toxicological, environmental and esthetical reasons [6,10,11].
Methyl red is a commonly used monoazo dye in laboratory assays, textiles and other commercial *Address correspondence to this author at the Central Department of Chemistry, Tribhuvan University, Kirtipur, Kathmandu, Nepal; Tel: 0977-9841830466; E-mail: knghimire@yahoo.com products; however, it may cause skin and eye sensitization, pharyngeal and digestive tract irritation if inhaled or swallowed [12]. Further, methyl red is mutagenic under aerobic condition; it undergoes biotransformation into 2-aminobenzoic acid and N,Ndimethyl-p-phenylene diamine [3,[7][8][9].
A number of treatment methods for the removal of dyes from aqueous solution have been reported. It includes mainly physicochemical, chemical, biological methods and flocculation [14]. Some other processes are ultra filtration, electro-chemical adsorption, ozonization, fungal decolonization [15] and photoxidation [11,13]. Among them, adsorption has gained importance in recent years due to excellent efficiency in the removal of pollutant from effluents [13]. Although activated carbon is most widely used as adsorbent for removal of dyes from the effluents of different concentrations [16], however, the process has not been used by small and medium scale industries because of its high manufacturing cost [17]. For this reason, the use of low cost materials as adsorbent is still in progress. Efforts have been made to develop new adsorbent as an alternative to activated carbon. These materials range from industrial waste such as rubber tyres, agricultural wastes and other biomasses [18,29].
Sugarcane waste are abundantly available in many countries, at almost free of cost [14,39,43]. Sugarcane waste has around 50% cellulose, 27% polyoses and 23% lignin [41][42][43]. These polymers have abundant hydroxyl functional groups that can be chemically modified with new properties [43]. They can be modified by chemical activation, steam activation and thermal activation for the development of large surface area onto the product [21]. Despite creating large surface areas, materials with certain functional group has gained importance for the complexsation / adsorption of the host molecules [34].
Under such circumstances, we have explored a new process of chemical modification which revealed to have better adsorption properties with the inclusion of amine functional moiety onto the surface of the adsorbent.

Preparation of Charred Sugarcane Waste (CSW)
Raw sugarcane waste was collected from the local juice vending centre and thoroughly washed with distilled water to remove the remaining sugar. Then the wastes were sun dried to remove moisture. The dried waste was ground into powder form by mechanical grinder and sieved into sizes of 125 micrometer. The powdered waste was treated with concentrated sulphuric acid in 1:1 ratio and left for 48 h. It was further washed with distilled water almost to neutrality. Then it was dried at 70 0 C for 6 h in hot air oven. The dried powder was sieved to the particle size 125 micrometer which is referred to as Charred Sugarcane Waste (CSW).

Preparation of Aminted Sugarcane Waste (ASW)
CSW was soaked with pyridine in three necked flask in ice bath at 0 C, then thionyl chloride was added drop wises so that the temperature of the bath does not exceed. The suspension was stirred for 10 minutes in fume cup hood and heated for 2.5 h. The unreacted thionyl chloride was washed with water for several times. Finally it was washed with acetone. It was dried in an oven at 70 0 C. The dried powder was further treated with dimethyl sulphoxide (DMSO), Thermo Fisher Scientific, India Pvt. Ltd., purity = 99.5%,, Na 2 CO 3 and ethylene diamine at temperature of 70 0 C for 48 h. After cooling down to room temperature it was washed with 0.1N HCl. Again, it was washed with distilled water till almost neutrality and acetone to remove water then dried at 70 0 C. The main aim of using acetone for washing purpose is to remove any remaining low molecular weight organic compounds that are present in suspension, which could not be removed by washing with water.

Chemicals
Methyl Red, a cationic dye (purity = 99.5%) is a product of Thermo Fisher Scientific, India Pvt. Ltd. It was used without further purification and the structure of dye is shown in Figure 1 A stock solution of Methyl Red (1000mg/L) was prepared by dissolving 1g MR in 250 mL of absolute alcohol and was diluted up to the mark by adding distilled water. It was further diluted to get working solutions of desire concentration.

Instrumentation
A calibration curve for MR was constructed by using UV Spectrophotometer (WPA, Type S104, Linton, Cambridge, U.K.) at maximum wavelength of 530 nm and 430 nm at pH 2 and pH 7, respectively. The initial and residual concentration of MR was determined by spectrophotometric measurement. Replicate analyses of the experimental results obtained from spectrophotometric measurement have been carried out for each sample for four times. Only the data within 95% confidence interval and of within 1.25 standard deviation have been presented. Similarly, the pH of the solution was monitored using pH meter, Elico, EL 613, Hyderabad, India.

Experimental Procedure
Batch adsorption experiments were carried out in a thermostatic shaker maintaining temperature of 25 0 C. 25 mL of MR solution of known concentration (10-350 mg/L) was shaken at constant agitation speed (100 rpm) with constant adsorbent dose 25 mg for specific period of contact time (10 minutes -24 hrs) in mechanical shaker. The pH of the solution was adjusted by contacting with HCl or NaOH solution. After equilibrium, the final concentrations (C e ) were measured and the amount of MR adsorbed in mg/g at equilibrium was computed by the equation.
where, C i and C e are the initial and equilibrium concentration of MR in mg/L, respectively. V is volume of MR solution in liter and W is the weight of adsorbent in gram.
And the percentage adsorption of dye was calculated using the following expression, Where A% is the percentage of dye adsorption from the solution.

Determination of Surface Functional Groups
The amount of total titrable surface functional group of Raw Sugarcane Waste (RSW) and Charred Sugarcane Waste (CSW) were evaluated by Boehm titration. The total surface functional group was found to be 3.2 and 4.6 meq/g respectively. Similarly phenolic group for RSW and CSWare 0.5 and 2.1 meq/g which is depicted as a bar diagram in Figure 2.
The above result shows that on treating concentrated H 2 SO 4 with RSCW , acidic functional groups increase and basic functional groups decrease and the total functional group has increased by 42.5%. Another significance of charring with concentrated H 2 SO 4 is that the phenolic group in CSW has increased by 32%. The function of the phenolic group is to provide a suitable medium for the complexation/adsorption of cationic dye. This is why increased phenolic sites onto the adsorbents are important in the adsorption of dye molecules. Furthermore, larger the phenolic sites available in the biomaterials, easier would be the substitution with suitable functional group. Thus, it can be concluded that the increased phenolic groups may act as good sorption sites for MR.

SEM Images of Adsorbents
The SEM images of RSCW, CSW and ASW are shown in Figure 3a, 3b and 3c respectively. The SEM image of RSCW appears to be rough whereas those of CSW and ASW appear to be micro porous, showing high surface area. This could be the reason why CSW and ASW appear to possess higher sorption capacity.

Infrared Spectra Analysis
The Fourier Transform Infrared (FTIR) spectra of charred and aminated sugarcane waste are shown in Figure 4. A strong, broad band in the 3300-3700 cm -1 region is due to OH stretching and thus confirms the presence of polyphenols in both the adsorbents studied [40].The peak observed at 3096 cm -1 can be assigned to stretching vibration of the C-H group. The peak around 1605 cm -1 corresponds to the C=C stretching that may be attributed to the lignin aromatic group. The strong band at 1009 cm -1 due to C-O group vibration indicates the presence of -OCH 3; it also indicates the presence of lignin structure in the sugarcane waste. Similarly, the broad absorption band at 600 cm -1 can be assigned to bending mode of aromatic compound.
The absorption peak near 1217 cm -1 represents the C-N stretching that is depicted in Table 1. The adsorption peak nearer to 3355 cm -1 may be due to the overlapping of both hydroxyl group and -NH 2 group in ASW which confirmed the introduction of amine group into ASW.

Elemental Analysis
Elemental analysis reveals the percentage and types of elements present in the adsorbents which are shown in the Table 2. The increase percent of nitrogen onto ASW as compared to RSW and CSW confirms the amine fictionalization onto CSW to form ASW.

Adsorption Kinetic Studies
Kinetic studies for the adsorption of MR onto CSW and ASW were studied using pseudo-first order, pseudo-second order and intra-particle diffusion model [18]. Different parameters were calculated from the kinetic plots for the adsorption of MR onto CSW and ASW and the values are included in Table 3.

The pseudo first order kinetic equation is given by:
Log(q e -q t ) = log q e k 1 2.303 t (3)    Where, q e and q t are the amount of MR adsorbed per gram at equilibrium and at time t in mg/g. k 1 is the rate constant of pseudo first order adsorption (min -1 ). The plot of log (q e -q t ) versus t should give a linear relationship which is shown in Figure 5, from which k 1 and q e can be determined from the slope and intercept of the plot, respectively.
The pseudo-second order kinetic model is given by: Where K 2 is the rate constant for pseudo second order adsorption in (g/mg/min), q t and q e are the amount of MR adsorbed (mg/g) at any time and equilibrium time, respectively. It was observed from the experimental data that the correlation coefficient (R  2 ) for pseudo second order was found to be close to unity for all cases. On the other hand, the correlation coefficient for the pseudo-first order kinetics was lower than that of pseudo second order, which indicates the pseudo second order model is fitted better than the pseudo-first order model.
If the initial adsorption rate is 0 (mg/g min), The equation (4) can be rewritten as, Experimentally, the value of q e and K 2 can be determined from the linear plot of t/q t versus t with the help of slope and intercept respectively, as shown in Figure 6.  According to Poots et al. [17] during the batch mode of experiments, there was a possibility of intra-particle pore diffusion of MR, which often may be the ratelimiting step. The intra-particle diffusion varies with square root of time as introduced by Weber and Morris as: Where q t is the amount adsorbed (mg/g) at time t (min) and k id (mg g -1 min -1 ) is the rate constant of intraparticle diffusion. If the rate limiting-step is the intraparticle, a plot of q t versus t 1/2 gives straight line passing through the origin and k id can be calculated from the slop of the plot. The intra-particle diffusion plot is shown in Figure 7 and the value of k id is found to be 0.833 (mg g -1 min -1 ) for ASW and 0.875 (mg g -1 min -1 ) for CSW, respectively. It was observed that the straight line do not pass through the origin, which indicates that intra-particle pore diffusion is not solely the rate limiting-step [17].

Effect of pH
The removal of dyes from aqueous solutions by adsorption is dependent on pH of solution as it affects the adsorbents surface charges, the degree of ionization and the species of adsorbates. The effect of pH on adsorption of MR was presented in Figure 8. The adsorption of MR on CSW and ASW was found to be pH 2 and pH 7, respectively. As the pH o the system increases, the number of available positively charged sites decreases while the number of negatively charged sites increase. The negatively charged sites favor the adsorption of dye cation due to electrostatic  attraction. There is competence between the MR cation and H + ion for adsorption sites. At low pH there is high H + ion concentration which leads to low adsorption of MR cation but at high pH there is low concentration of H + concentration and leads to high adsorption of MR due to decreased competition. But beyond the neutral pH there is also decrease in the percentage of adsorption due to the formation of aqua complexes thereby retarding the dye sorption.

Adsorption Isotherm Studies
The Langmuir Eq.7 and Freundlich Eq.8 isotherms were studied by adsorption experiments as a function of MR concentrations in aqueous solution under equilibrium time and optimum pH condition and the plots are shown in Figures 9 and 10, respectively. The results of MR adsorption for Langmuir and Freundlich isotherm and different isotherm parameters were evaluated which is shown in Table 4.
Where, q e is the amount of adsorbate per unit mass of adsorbent at equilibrium in (mg/g), C e is the equilibrium concentration of adsorbate in (mg/L), q m is the maximum adsorption capacity ( mg/g) and b is the Langmuir adsorption equilibrium constant (L/mg).
log q e = logk + 1 n logC e (8) Where, q e is the amount adsorbed per unit mass of adsorbent (mg/g), C e is the equilibrium concentration of the adsorbate (mg/L). k and n are Freundlich equilibrium coefficients, which are considered to be the relative indicators of adsorption capacity and adsorption intensity. The value of 1/n varies between 0.1 and 1.0 indicates the favorable adsorption of dyes.
The high correlation coefficient R 2 for Langmuir plot indicates that the data obtained from the experiment is well fitted to this model by both adsorbents (CSW and ASW). The Langmuir isotherm parameter q max indicates maximum adsorption capacity of the adsorbents. By comparing the R 2 values of these two models, it shows that the sorption of MR on these two adsorbents follow the Langmuir isotherm model. Since aminated and charred sugarcane waste has a very low specific surface area (about 25 m 2 /g), physical adsorption alone cannot contribute to the higher MR uptake capacity, so the predominant mode of adsorption is the chemisorptions, which signified monolayer is formed due to the chemisorptions. The q max values of different bio-adsorbents given in Table 5 are compared with CSW and ASW and found to be higher adsorption capacity. Due to the high values of q max , these two adsorbents would be an alternative to activated charcoal for the adsorption of dyes.

CONCLUSIONS
Based on the present investigation, removal of methyl red from an aqueous solution by using chemically modified aminated and charred sugarcane waste as biosorbents were found to be effective and superior as compared to previous works. The uptake of MR is pH dependent. The equilibrium adsorption data was good fitted with Langmuir adsorption isotherm and followed the pseudo second order kinetic model which agreed with chemisorptions as rate limiting and intraparticle diffusion was not only the rate-controlling. The results show that aminated and charred sugarcane waste can be effectively applied for the removal of MR from the aqueous solution.