Sorption of Congo red Dye from Aqueous Solution by Mercerized and Benzoylated Sugarcane Bagasse: Isotherm, Kinetic and Thermodynamic Studies

: The use of large quantity of synthetic dye in process industries to colour products amongst other things has given rise to aqueous effluents containing high level of this pollutant. This paper therefore evaluates the use of isotherm, kinetic and thermodynamic to study the sorption capacities of congo red dye by mercerized sugarcane bagasse (MSB) and benzoylated sugarcane bagasse (BSB) in aqueous solution using batch experimental protocol. Data obtained show that MSB was the most effective adsorbent material, with equilibrium sorption (q e = 13.64 mg/g) after 50 minutes. The optimum pH and dosage for dye uptake were 3.0 and 3.5 g respectively. Sorption of dye by the modified surfaces decreases with increasing temperature and ionic strength. The 4% NaOH presented the best desorption of dye on the adsorbents. Kinetic studies showed pseudo second-order model fit for the experimental data. Adsorption of the anionic dye on the modified surfaces fits the Langmuir isotherm model based on the R 2 values. Thermodynamic results showed ∆G o values (-12.42 to -4.16 KJ/mol) for MSB and (-15.13 to - 7.55KJ/mol) for BSB, consistent with the spontaneity and feasibility of the sorption process. The uptake of dye by the modified surfaces depicts a random nature at the solid-solution interface and showed that the sorption is exothermic as evidenced by value (0.256-0.334KJmol/K) and (-18.41 to -10.17 KJ/mol) for ∆S o and ∆H o respectively. The results present MSB and BSB as effective sorbent materials for removal of anionic dye from waste effluents.

Advancement in technological knowledge has given rise to large scale industrial activities, resulting in the upsurge of organic and inorganic pollutants from process industries.Contamination of water bodies by these pollutants has provoked worldwide interest in remediation methods to mitigate the harmful and environmental health hazards pose by these pollutants in eco-systems Cheng et al., (2015).Textile, leather, rubber latex, food industries churn out immense volume of waste water containing reactive dye, disposal of these polluted waste waters into the ecosystem without remediation could result in deleterious effects on human being Muhammad et al., (2013).
Hence, the sorption of dye from these colored waste effluents is imperative before discharging them into aquatic streams.Coloured dyes are classified namely; anionic, cationic and non-ionic dyes Ngulube et al., (2017).Congo red dye is a dipolar benzidine-based diazo anionic molecule, which largely constitutes waste water effluents from dying, paper, textile, plastics, printing and rubber industries Yusef et al., (2017).Stemming from its aromatic ringed structure, congo red dye is very stable and non-biodegradable Cheng et al., (2015); Yusef et al., (2017); Chatterjee et al., (2010).Several physicochemical methods notably: electrochemical techniques, chemical oxidation, coagulation-flocculation, ion-exchange, membrane separation and adsorption have been employed for remediation of congo red dye from waste streams Muhammad et al., (2013) and Yusef et al., (2017).Considerably, adsorption method has been given more attention owing to its high sorption capacity, eco-friendliness and efficiency.Because of its cellulosic nature, large surface area and low-cost, sugarcane bagasse has been used for the sorption of congo red dye in the form of bagasse fly ash Mall et al., (2005), Ball-milled sugarcane bagasse Zhang et al., (2011) and milled sugarcane bagasse Muhammad et al., (2013).Similarly, glucose-amine copolymers and their composites: Chitosan Ngulube et al., (2017), Chitosan/Montmorillonite nanocomposite Wang and Wang (2007) and clay-based materials namely; Kaolin Vimonses et al., (2009), montmorillonite Yusef et al., (2017), have been reported for the remediation of congo red dye from waste water and aqueous media.Sugarcane bagasse, a pulp or dry refuse left after juice has been extracted from sugarcane or sugarbeets, find uses as fuel, cattle feeds or starting material for paper making.This cellulosic based polymer is a low-cost, non-toxic, eco-friendly and high surface area material.This study investigates the use of isotherm, kinetic and thermodynamic studies on the sorption capacities of congo red dye by benzoyl peroxide and sodium hydroxide modified sugarcane bagasse in aqueous solution.

Preparation of Adsorbent:
The sugarcane bagasse used for this study was obtained from a sugarcane processing section of Zuru market Kebbi State, brought into the laboratory, washed under running water and dried (105 o C) in a thermostatic oven for 6 h.The dried sugarcane bagasse was milled using a jar mill (JM 4055) loaded with ceramic pebbles for 72 h and then fractionated (mesh: 75 micron) to obtained the fine sugarcane bagasse powdered material (SB).Mercerization of Sugarcane Bagasse: 100 g sugarcane bagasse (SB) powder was weighed into a 500 ml beaker and 100 ml of 10% (w/v) NaOH solution was added.The mixture was equilibrated on a magnetic stirrer set at 150 rpm and 50 o C for 1 h.The beaker was cooled to room temperature and then filtered.The residue was then washed repeatedly with deionised water until neutral to litmus.The mercerized sugarcane bagasse (MSB) was then dried in an oven at 105 o C for 6h, cooled and packed into a polyethylene bag for sorption studies.
Benzoylation of Sugarcane Bagasse: 100 g of sugarcane bagasse powder was weighed into a 500 ml conical flask and 10 ml of toluene was added, the flask and its content were homogenized (100 rpm for 30 min.) on a flask shaker.The flask was then charged with 1% benzoyl peroxide compared to the weight of sugarcane bagasse powder (Sharma and Kundu, 2008).The flask and its content was then placed on a magnetic stirrer and heated at 120 o C with continous stirring (200 rpm) for 30 min.the benzoylated sugarcane bagasse (BSB) was then filtered and washed repeatedly with deionised water until neutral to litmus.
Effect of Initial Dye Concentration: 1.0 g MSB and BSB were studied separately with 20 ml of dye solutions (100, 150. 200, 250 and 300 ppm) in a 250 ml stopped conical flask.The flasks were agitated in a shaker at 200 rpm for 50 min., each flask was then filtered and the filtrate were analysed at a maximum wavelength of 490 nm using UV-Visible spectrophotometer for residual dye concentration.The amount of dye on the adsorbent material which represents the dye uptake was calculated from the equation Soldatkina and Zavrichko (2019).Effect of Contact Time: 1.0 g of MSB was weighed into a 250 ml conical flask; 20 ml of 300 ppm dye solution was aggregated into the flask.The flasks were agitated (200 rpm) at different equilibration time (20, 30, 40, 50, 60, 70, 80 and 90 min.).The content of the flask was then filtered and the filtrate analyzed using UV-Visible spectrophotometer.The same experimental protocol was carried-out for the BSB sorbent material.
Effect of Adsorbent Dosage: MSB and BSB in various weights (1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5 and 6.0g) were weighed separately into 250 ml conical flask.20 ml of 300 ppm dye solution was aggregated into each flask and agitated (200 rpm) for 50 min.the content of flask was then filtered and the residual dye concentration determined.The amount of dye on the sorbent material at equilibrium time (qe) was evaluated.
Effect of Temperature 1.0g of each sorbent material were weighed and aggregated with 20 ml of 300 ppm dye solution into a 250 ml conical flask.The flask was then mounted on a magnetic stirrer (200 rpm at 30 o C) for 50 min.this experiment was repeated at the same stirring speed and time, but with variation in temperature: 35, 40, 45 and 50 o C respectively.The residual dye concentration was determined and the value of qe evaluated.
Effect of pH: 3.5g of adsorbent materials was weighed into a 250 ml conical flask, 20 ml of 300 ppm dye solution was added to each flask, and the pH was adjusted to 2. The content of the flask was agitated at 200 rpm for 50min.the mixture was then filtered, residual dye concentration determined and value of qe evaluated.The same experimental procedure repeated, but with adjustment in pH to 3, 4, 6, 8 and 10, using 0.5 M HCl and 0.5 M NaOH as required.

Effect of Ionic Strength:
The study was carried out by adding different amounts of NaCl (25 to 125 ppm of Na + ) into 20 ml of 300 ppm dye solution in conical flask containing 1.0g adsorbent material.The flask was agitated at 200 rpm for 50 min.the residual dye concentration were determined and qe was subsequently calculated.

Desorption of Dye from Sorbent Material:
The method Soldatkina and Zavrichko (2019) was used for the desorption studies.1.0g of sorbent material was weighed into a 250 ml conical flask; 100 ml of 300 ppm dye solution was aggregated into the flask.The pH was adjusted to 3 and the flask was allowed to stand for 150 min.at room temperature.The sorbent material loaded with congo red dye was washed repeatedly with deionised water and dried at 50 o C. 0.1g of dye loaded material was weighed into a 250 ml conical flask, 10 ml of deionised water, 10 ml of 4% NaOH, 10 ml of 4% H2SO4 and 10 ml of methanol respectively were aggregated into each flask and agitated (200 rpm) at room temperature for 50 min.The efficiency of dye removal was calculated.
Where, Cd is the dye concentration in solution after desorption, Vd is the volume of eluent, m is the mass of dye loaded material and qe is equilibrium amount of dye adsorbed.

Isotherm studies:
The data obtained in this study was fitted into three isotherms; Langmuir, Freundlich and Temkin.
Where, kL, is the Langmuir constant, qmax is the quantity of adsorbate required to form a single monolayer per unit mass of adsorbent and Qe is the amount adsorbed on unit mass of the adsorbent (Cheng et al., 2015).Where, kT is the Temkin equilibrium constant corresponding to the maximum binding energy and b, is the heat of adsorption (Soldatkina and Zavrichko, 2019).
Kinetic studies: Dye uptake rates, is best described using kinetic models which determines the residence time of adsorbate and the controlling mechanism of the adsorption of the adsorbate on the adsorbent material.The pseudo-first order kinetic can be expressed (Adegoke et al., 2017) as

6
A plot of ln (qe-qt) vs t generated a straight line.The amount of dye sorbed per gram of adsorbent at equilibrium (qe) was determined from the intercept ln qe and the slope gave the rate constant k1.The pseudo-second order kinetic model can be expressed Su et al., (2014) as.
Where, qe is the maximum sorption capacity, t is the time, qt is the amount adsorbed at time t and k2 is the equilibrium rate constant.Plot of t/qt vs t, gives qe 2 and k2 from the slope and intercept respectively.
Thermodynamic Study: Fundamental thermodynamic parameters; Gibbs free energy (∆G o ), enthalpy change (∆H o ) and entropy change (∆S o ) derivable from the study could help fully understand the controlling process and will largely determine the feasibility of dye sorption by the modified sugarcane bagasse.The following equations Akpomie and Dawodu (2015) and Soldatkina and Zavrichko (2019) described the thermodynamic parameters Where, T, is the temperature in Kelvin, R is the ideal gas constant, kD is the thermodynamic equilibrium constant.Ca and Ce are the concentrations of dye sorbed and the equilibrium concentration of the dye in solution.A plot of lnkD vs 1/T yields ∆H o and ∆S o from the slope and intercept respectively.

RESULTS AND DISCUSSION
Table 1 below presents some physicochemical properties of the unmodified and modified adsorbent materials.The highest swelling ratio and bulk density values shown by MSB may not be unconnected with the largest surface area due to the decrystallization of glucose chain occasion by the breaking and sodium hydroxide salvation of glycosidic linkages.The high acidity as represented by BSB corroborates the combination of abstracted proton from the cellulosic backbone of SB by phenyl radical and the subsequent reaction of this proton with benzoyl radical resulting in the formation of benzoic acid Samuel et al., (2019).The presence of this carboxylic acid on the surface of BSB is evidenced in the low pHpzc value recorded for the adsorbent material.The slight acidity and alkalinity showed by SB and MSB presumably may be attributed to oxidation of fermented sugar on SB and presence of residual NaOH on MSB respectively.The FT-IR spectra for the sugarcane bagasse (SB), mercerized sugarcane bagase (MSB) and benzoylated sugarcane bagasse (BSB) are shown in figures 1, 2 and 3 respectively.The O-H stretching vibration at 3324 cm -1 presents a stronger and broader peak than 3287 cm -1 for BSB, therefore substantiates the larger surface and presence of more OH adsorption site on MSB.The C-H (methylene group) stretching vibration at 2914 cm -1 were present in MSB and BSB, suggesting the increase in aliphatic carbon chain in modified materials.The stronger peak at 1718 cm -1 for BSB compared to 1595 cm -1 for MSB may be ascribed to stretching vibration from C=O (carbonyl group) of cellulose and hemicelluloses in the native SB and contributory stretching vibrations from COOH.The peak at 1625 cm -1 and the weak peak at 1155 cm -1 may be due to the deformation vibration for H-O-H bonds for surface adsorbed water as a result of the hygroscopic nature of the surface carboxylic acid and the presence of aromatic ring on BSB respectively.
Figure 4 below presents the effect of sorption of initial dye concentration by MSB and BSB.The equilibrium amount of dye adsorbed by the modified materials increased with increase in dye concentration.This is in agreement with the sorption of congo red dye by sugarcane bagasse and bambara shell Muhammad et al., (2013), and adsorption of congo red dye with bagasse fly ash Rao et al., (2006).However, the equilibrium quantity sorbed by MSB was slightly greater than BSB.The marginal increase may be attributed to the delignification of the lignocelluloses structure of sugarcane bagasse owing to alkaline hydrolysis, resulting in increased internal area and porosity of MSB adsorbent material.The result of the batch experiment showing the effect of contact time is represented in figure 5.An increase in time brought about a concomitant increase in the equilibrium amount sorbed by the materials.
Comparatively, at the equilibrium time of 50 min.for both materials, MSB and BSB showed equilibrium amount of 13.04 and 10.87 mgg -1 respectively.The relative sorption of adsorbate by MSB, suggest, that the alkaline treatment of the sugarcane bagasse resulted in decrystallization of glucose monomer and scissions of α-1,4 and β-1,6 glycosidic linkages present in the lignocellulosic material.Besides, the treatment of sugarcane bagasse with benzoyl peroxide may results in proton abstraction by the highly reactive benzoyl radical from cellulosic material and the subsequent formation of benzoic acid Penezek et al., (2005) and Samuel et al., (2019).The presence of this compound with aromatic ring and their preoccupation of active site, results in decreased dye sorption by BSB material.The binding site available for sorption is determine, by the amount of sorbent material used Adegoke et al., (2017).Figure 6, shows the effect of adsorbent dosage on the sorption of congo red dye by MSB and BSB.
The percentage sorption increases with increase in adsorbent dosage, showing a peak at 3.5 g for both materials.Thereafter, incremental dosage of adsorbent results in decrease in the percentage of adsorbate uptake.This may be attributed to occupation of available binding sites by the dye molecules, such that a further increase in the quantity of MSB and BSB may not bring about any appreciable sorption of the adsorbate Zafer et al., (2007) and Adegoke et al., (2017).This result compares well with 4.0 g maximum adsorbent dosage reported for sugar cane bagasse and bambara shell (Muhammad et al., 2013).Figure 7 shows the effect of pH on the sorption of congo red dye by MSB and BSB.pH of the dye solution is the most contributory factor that determines the surface chemistry of the adsorbent material and therefore, affects the adsorption of adsorbate by an adsorbent material Ozacar and Sengil (2003).The equilibrium quantity of the dye uptake increases from pH 2 to 3 and decreases thereafter.Congo red dye is a dipolar molecule, which exists as anionic molecule at basic pH and as cationic moieties at acidic pH Zhang et al., (2011) and Muhammad et al., (2013).At lower pH protonation of the nucleophilic (OH -, COO -) moieties on the adsorbent surfaces, enhances their affinity for the adsorbate, due to electrostatic attraction with their opposite charged groups present in the anionic dye molecule.
The increase in pH of the dye solution concomitantly increases OH -in the dye solution resulting into competition for adsorptive sites on the adsorbent surfaces.Besides, the electrostatic repulsion between the negatively charge dye molecule and the negatively charge adsorbent surfaces contributes significantly to the decreased sorption of the dipolar molecule Malik, (2003), Tor and Cengeloglu, (2006) ; Muhammad et al., (2013) The effect of temperature on the sorption of dye molecule is presented in figure 8.The result showed that for both materials, the amount sorbed at equilibrium time decreases as temperature increases from 30 to 50 o C. Suggesting, an increase in temperature results in swelling of the biopolymer materials with associated desorption and possible diffusion of the dye molecules into the dye solution.
The result from this study affirms the report of sorption of light green onto peanut husk modified by hexadecylpyridinium bromide Zhao et al., (2017) and adsorption of methyl orange on wheat straw modified by cetyltrimethylammonium bromide Su et al., (2014).The effect of the strength of electrolyte in dye solution is shown in figure 9. Sorption capacity of sorbent materials decreases marginally with a corresponding increase in strength of electrolyte.The dye uptake for MSB was higher than BSB this may be attributed to delignification and decrystalization of the cellulosic backbone by Sodium Hydroxide, resulting into a filamentous fibrous material with greater flexibility and improved sequesting of dye molecules.The decrease in dye uptake as the concentration of Na + increases may be attributed to the blocking of vacant sites by the electrolyte at a higher salt concentration Yusef et al., (2017).This is also in agreement with reports on removal of navy blue from aqueous solution by combined coagulation Nourmoradi et al., (2016) and adsorption of congo red dye from aqueous solution by montmorillonite Yusef et al., (2017).The slight decrease in dye uptake capacity as salt concentration increases suggest, that the modified materials could be   Zavrichko (2019).Besides, desorption of the anionic dye in excess by methanol more than distilled water and HCl may be attributed to entanglement of the dye molecules on the modified surfaces by ion-exchange or electrostatic attraction Su et al., (2014).The pseudo-first-order and pseudo-second-order kinetic model parameters of sorption of congo red dye onto MSB and BSB are presented in Table 2.The low correlation value of R 2 as determined in pseudo-first-order model implies non applicability of this model.However, the high correlation values i.e.R 2 = 0.995-0.999for MSB and R 2 =0.992-0.999for BSB in the pseudo-second-order model indicates that chemosorption is the rate limiting step Soldatkina and Zavrichko (2019), and controls the mechanism for the sorption of congo red dye onto the modified surfaces.The results from this study affirm previous report on sorption of Acid Green and Acid Red on sawdust modified by cetyltrimethylammonium bromide Zhao et al., (2017).
As shown in Table 3 above, the Langmuir, Freundlich and Temkins isotherms were used to describe the sorption of congo red dye onto MSB and BSB respectively.The highest correlation values of R 2 = 0.993-0.999for MSB and R 2 = 0.996-0.999for BSB as evidence in the Langmuir model, suggest better fit of the experimental data, indicating that the occupation of adsorption sites by the dipolar molecule was in form of a monolayer coverage Adegoke et al., (2017).The Freundlich parameters 1/n and KF decreases with increase in temperature for both sorbent materials, with correlation coefficient less than unity.The Temkins model shows a good linearity, judging by the correlation values of R 2 = 0.957-0.977for MSB and R 2 = 0.955-0.979for BSB.The good fitting of the data to Temkins isotherm suggest, a strong adsorbateadsorbent interaction.In addition, the decreasing KT value with temperature and positive value of b, which also increases with a corresponding, increase in temperature shows that adsorption of the dye molecule, was an exothermic process Soldatkina and Zavrichko (2019).This result is consistent with report from sorption of anionic dye on corn stalks modified by cetylpyridinium bromide Soldatkina and Zavrichko (2019), removal of congo red from aqueous solution by adsorption onto acid activated red mud Tor and Cengeloglu (2006), and biosorption of methyl orange from aqueous solutions using cationic surfactantmodified peanut husk in batch mode Su et al., (2014).-10.17 -7.55 -8.16 -11.11 -13.44 -15.13 Table 4 represents the thermodynamic parameters for the adsorption of congo red dye by MSB and BSB adsorbent materials.The negative values of the enthalpy change (∆H o ), for all the adsorbents suggest, the exothermic nature of the sorption process.The positive values of the change in entropy (∆S o ), indicates an increase randomness of the dye uptake process, with increase in temperature, suggesting spontaneity of the sorption process Adegoke et al., (2017) and Soldatkina and Zavrichko (2019), consistent with decrease in dye uptake as temperature of the dye solution increases.

Conclusion:
Optimum conditions for the adsorption of the adsorbate by the modified surfaces were determined.The kinetic data described by the pseudo second-order model, fit better with the isotherms in the following order: Langmuir > Temkins > Freundlich.
The modified surfaces demonstrate strong affinity for the dye molecule and showed that the sorption of the anionic dipolar molecule was spontaneous and feasible.This study shows that MSB and BSB could find practical applications in the remediation of congo red dye from waste streams.
and Ce are the initial and final concentrations of dye solution, v is the volume of dye solution and m is the mass of sorbent materials.
is the Freundlich constant related to the adsorption capacity, n -1 is the adsorption intensity.

Fig 6 :
Fig 6: Effect of adsorbent dosage on dye sorption

Fig 7 :Fig 8 :
Fig 7: Effect of dye solution pH on sorption capacity

Fig 9 :
Fig 9: Effect of ionic strength on sorption capacity Figure 10 represents the desorption of congo red dye using different solvents.The 4% H2SO4 showed the lowest desorption of the anionic dye from the sorbent materials.The highest desorption of dye by 4% NaOH indicates that reduction in positively charged adsorption sites in alkaline solution resulted in electrostatic repulsion of the dipolar molecule from the surfaces of the adsorbent materials Soldatkina and

Fig 10 :
Fig 10: Effect of different eluents on dye desorption

Table 1 :
Physicochemical properties of unmodified and modified adsorbent materials

Table 2 :
Kinetic of dye sorption on the adsorbent materials

Table 3 :
Isotherm parameters of dye uptake by adsorbent materials

Table 4 :
Thermodynamic parameters for the adsorbent materials