Chemically modified sugarcane bagasse as a biosorbent for dye removal from aqueous solution

1 Abstract—The use of adsorbent prepared from sugarcane bagasse, an agro waste from sugar industries has been studied as an alternative substitute for activated carbon for the removal of dyes from aqueous solution. Adsorbents prepared from sugarcane bagasse modified with citric acid was used as a low-cost biosorbent for removal of dyes from the aqueous solution. Adsorption parameters such as initial pH values, dyes concentrations, adsorbent dosages and contact times were investigated by the batch experiments. The Freundlich and Langmuir adsorption isotherm models were used to evaluate the experimental data. The results showed that the adsorption process of dyes onto the modified sugarcane bagasse leaned towards Langmuir model for MSB and Freundlich for SB. Maximum adsorption capacity of MSB was found to be 8.40 mg/g at pH 9. The results showed that the modified sugarcane bagasse with citric acid could be a potential low-priced adsorbent for removal of the color from the aqueous solution. Keywords—sugarcane bagasse, adsorption, dyes, chemical modified, isotherm models


INTRODUCTION
ye production industries and many other industries which use dyes and pigments generate wastewater, which is characteristically high in color and organic content [1]. Dyes are widely used in industries such as textile, rubber, paper, plastic, cosmetic etc. Among these various industries, textile ranks first in usage of dyes for coloration of fiber [1,2].
The textile industry plays an important role in Viet Nam economy. In fact, this industry has been significantly the received investment. But, its fast growth leads to concerns in environmental pollution. It was well known that the textile industry consumes a large volume of water and generates a significant amount of wastewater. In textile industries, about 1000 L of water is used for every 1000 kg clothes processed. It is estimated about 10-15% of the dyes were lost in industrial effluent [3,4]. Discharge of such coloured effluents impart the colour to the receiving water bodies (rivers, stream and lakes) and causes many significant problems such as increasing the toxicity and chemical oxygen demand. It also reduces light penetration which is a derogatory effect on photosynthesis phenomena [4,5]. Therefore, wastewater with dyes is very difficult to treat, since the dyes are recalcitrant organic molecules, resistant to aerobic digestion and stable to light, heat and oxidizing agent [1,6].
Many methods have been studied to remove the color from the textile wastewater and other industries using dyes such as adsorption, coagulation/ flocculation, flotation, oxidation, ozonation, membrane filtration, oxidation and physical methods like membrane filtration, ion exchange and electrochemical techniques. Among them, adsorption has been known as an efficient method to removal the colour [1,4,7]. Adsorption techniques were used easily with available low cost adsorbents mainly of biological origin. Therefore, many researches have been conducted to find out alternative low-cost adsorbents. Agricultural waste materials are receiving much more attention as adsorbents for the removal of dyes from wastewater due to its low cost and good availability. Many studies have been undertaken for the removals of pollutants by using a variety of materials used as adsorbents such as apple pomace [8], sugarcane bagasse [9], Neem leaf powder [10], banana Pith [11], wheat shell [12], palm tree flower [13], Indian rose wood sawdust [14], rice husk [15], peanut shell [16], coir pith [17], sunflower seed husk [18], orange peel [19] and eucalyptus bark [20] for the removal of different dyes from aqueous solutions at different operating conditions. Sugarcane bagasse, a waste from food, sugar and beverage industries, is a potential adsorbent to remove dyes. Many investigations on bagasse as an effective adsorbent for adsorption of organic pollutants [21,22] and metals. It is usually modified to enhance its adsorption capacity or used to prepare cheap activated carbon [7,23]. The objectives of our investigation were to investigate the potential of using modified sugarcane bagasse as a lowcost adsorbent to remove dye from aqueous solutions. Due to their low cost, after these materials have been expended, they can be disposed of without expensive regeneration.

Preparation of adsorbent
The clean bagasse pith selectively collected from the canteen in the University of Technology and Education was washed 2 times with distilled water, then left to dry in the oven at 60 o C overnight. After that, the bagasse was ground and sieved. The fraction of 0.20-0.45 mm sizes were collected for experiment then the material was boiled with distilled water at 100 o C to removal all the sugar in its structure and was labeled as SB.

Citric acid modified bagasse
The sugarcane bagasse (SB) was treated with NaOH 0.1 M (solid SD/solution = 1/20 w/v) to remove lignin in raw sugarcane bagasse. The mixture was agitated at 250 rpm in 2 hours at room temperature. The mixture was filtered and washed with deionized water to neutral and dried at 55 o C in the oven.
In order to enhance the adsorption capacity of sugarcane bagasse, citric acid was used to modify the SB. 20 gram bagasse was mixed with 500 mL of citric acid 0.8 M and then was shaken at 250 rpm for 24 h at room temperature. The bagasse was then separated and put in the oven at 60 o C overnight. Again, the bagasse was washed several times with distilled water until neutral. The material was dried in an oven at 60 o C. The material which modified by citric acid was labeled as MSB and which was used for this investigation [24]. The products obtained were characterized by FTIR spectra using FTIR system 8400 Model. The surface morphology of the adsorbent was visualised via scanning electron microscopy [SEM] JEOL-5333 (Japan).

Dye solution preparation
In this study the Direct Fast Turquoise Blue (FBL) was used and it was obtained from the chemist supplier. An accurate weighed quantity of the dye was dissolved in double distilled water to prepared the stock solution (500 mg/L). Experimental solution of the desired concentration was obtained by successively dilutions. Dye concentration was determined by using absorbance values measured before and after the treatment, at 610 nm, with Hitachi UV Visible Spectrophotometer (model no.: UH5300).
Experiments were carried out at initial pH values which were controlled by the addition of 1,0 M or 0,1 M of sodium hydroxide, NaOH or hydrochloric acid, HCl

Experimental methods
All the experiments were performed in batch mode at room temperature (30 0 C ± 1). For the initial pH investigation, a wide ranging from 1 to 11 was used. In each adsorption experiment, 50,0 mL of the dye solution of known concentration and pH was added to 100 mg of adsorbents in 250 mL round bottom flask at room temperature (30 0 C ± 1) and the mixture was stirred on a rotary orbital shaker at 250 rpm. The sample was withdrawn from the shaker at the predetermined time intervals. The experiment was done by varying the amount of absorbents (0.1 to 1.

Characteristics of adsorbents
The surface morphology of SB and MSB visualized via scanning electron microscopy, are shown in Fig.1 and Fig. 2.  The spectra of raw bagasse (SB) before (a) and (b) after modification with citric acid (MSB) are shown in Fig. 3. When comparing the FTIR spectrum of MSB to the one of SB, the higher intensity of the peak at 1730 cm -1 confirmed by the presence of carboxyl groups in citric acid, leading to the increase the adsorption capacity of MSB for dyes.

Effect of pH
The removal efficiency of the sugarcane bagasse (SB) and the modified sugarcane bagasse (MSB) at different pH is shown in Fig. 4. The results showed that the percentage of dye removal decreased with increasing pH values. At the low pH region, the pH range of 1 to 4, the same adsorption efficiency of SB and MSB became meaningless. When the pH reached to 5, the removal efficiency of two SB and MSB started to be different. The percentage of colour removal of MSB was higher comparing with those of SB. As shown in Fig. 4, at pH 1-2, the dye was removed completely for both SB and MSB. At pH 9, the maximum percentage efficiency of dye removals were observed, 18% for SB and 64% for MSB. the cellulose structure of the materials. However, the adsorption process at very low pH was not useful due to a large amount of acid must be used to neutralize, leading to high cost of the treatment.
Therefore, at pH 9 was used for the investigation of the adsorption process.  At any given adsorbent dosage, the final pH in the presence of dye was higher compared to those in the absence of dye. This was attributed to the anion-exchange reaction between dye anion and surface-active groups on the adsorbent [1].  This result from the surface active sites of the adsorbent was limited since the dose was constant. When the color increased, there was not enough the space to the adsorption which led to the decline in the efficiency.

Effect of the contact time
The contact time effect on the adsorption was shown in Fig. 7. As shown in the results, from 15 minutes to 4 hours, the adsorption percentage of removal increased from 72.5% to 91.5%. The longer shaken time was, the more contact between the adsorbent and the adsorbate, which meant the dye had more chances to attach onto the material's cellulose chains. Hence, much dye adsorbed, made the colour removal efficiency higher. After 15 mins, the efficiency still rose, but not much. So, 15 mins could be considered as the optimum contact time.

Adsorption isotherm
Freundlich and Langmuir adsorption isotherm models were used to evaluate the experimental data. Glass powder 4.03 [26] Coir pith 6.72 [27] Sugarcane bagasse 5.78 [28] Rice husk 40.59 [29] Walnut bark 15.1 [30] Cherry saw dust 39.00 [31] Banana peel 20.08 Turquoise Blue (FBL) followed the Langmuir and the Freundlich isotherm models in the order of Freundlich < Langmuir. The results showed that the bagasse pith had the potential to be a competitive alternative natural adsorbent.