Adsorption of Methylene Blue from aqueous solution using Senegal River Typha australis

In this work, batch adsorption experiments were carried out for the removal of Methylene Blue (MB) from aqueous solutions using Typha australis leaf as a low cost adsorbent. The effects of some variables governing the efficiency of the process such as adsorbent mass, pH, ionic strength, contact time and temperature were investigated. The adsorption kinetic data were analyzed using the Pseudo First Order (PFO) and Pseudo Second Order (PSO) models. The experimental equilibrium data were analyzed using Langmuir and Freundlich isotherm models. The results show that the PSO model is the best for describing the adsorption of MB by Typha australis for all initial MB concentrations. The equilibrium data fitted well with the Langmuir model with the monolayer adsorption capacity for MB-Typha australis leaf system was of 103.12 mg g. The values of activation parameters such as free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°) were also determined as 4.44 kJ mol, 55.13 kJ mol and 203.21 J mol K, respectively. The thermodynamics parameters of MB-Typha australis system indicate spontaneous and endothermic process. These results indicate that the Typha australis leaf can be feasibly employed for the eradication of MB from aqueous solution.


Introduction
The textile industry is one of the most waterconsuming industries in the world and produces large quantities of wastewaters which contain hazardous compounds such as dyes. These chemical species can have an important ecological impact on ecosystems due to their strong toxicity and environmental persistence 1 .
Among the dyes, Methylene Blue (MB) has been widely used as a colorant, an indicator and an antiseptic agent in clinical therapy 2,3 .
However, disposal of MB containing waters can cause severe damage to the environment 4,5 . Many human diseases have been reported to be closely related to MB, such as hemolytic anemia and acute renal failure 6 . Hence, the removal of MB is a very important task in the protection of our environment and health.
Some techniques have been applied for the elimination of dyes in aquatic media such as biodegradation 7 , electrochemical treatment 8 , electrochemical oxidation and aerobic biodegradation 9 , nanofiltration and reverse osmosis 10 , photo catalytic degradation 11 , degradation by Fenton and photo-Fenton processes 12 and adsorption 13 .
Solid-phase adsorption is one of the most efficient technologies for the treatment of variety of hazardous compounds in water [14][15][16][17][18][19][20][21][22][23] . However, the adsorption of dyes onto activated carbons has attracted many researchers, but its high cost inhibits its application on a large scale 24 .
For this reason, researchers have concentrated on finding alternative natural adsorbents to commercial activated carbon. Natural adsorbents are preferred for their biodegradable, non-toxic nature, low commercial value and highly cost-effective nature. A number of non-conventional and low cost agro wastes sorbents have been tried for removing MB from aqueous solution via adsorption process [25][26][27][28] .
In this work, Typha australis an abundant and available plant along the Senegal River was chosen to investigate its adsorption capacity for MB present in aqueous solution. The effects of various parameters such as adsorbent mass, pH, ionic strength, contact time and temperature on the adsorption efficiency of MB were studied using the batch technique. The adsorption kinetic data were analyzed by the pseudo-first-order (PFO) and pseudo-second-order (PSO) kinetic models using the nonlinear method. The experimental equilibrium data were examined by Langmuir and Freundlich isotherms using nonlinear method. The thermodynamics parameters, such as ΔG°, ΔH°, ΔS°, have been determined. So, the adsorption parameters obtained using the present Typha australis leaf adsorbent will be compared with the ones presented in the literature.

Adsorbate preparation and analysis
All chemicals used in this study were of analytical reagent grade. All the solutions are prepared using pure MB and distilled water. The stock solution is prepared by adding 1 g of the MB to 1 L of distilled water. Other concentrations are prepared by dilutions of the stock solution and used to develop the standard curves using the Spectrophotometer UV1800 Ray Leigh.

Collection, preparation and characterization of Typha australis
Biomass of Typha australis growing along the Senegal River was collected from the city of Rosso, Wilaya of Trarza, from the south of Mauritania. The collected materials were washed thoroughly with distilled water to remove dirt. The biomass was then air dried for 3 days followed by drying in an oven at 105°C for 24 h. The dried biomass was ground, sieved to obtain particle sizes below 0.5 mm and stored in a dessicator before use. The physicochemical characteristics of the Typha australis leaf are given in Table 1 29 . 3.9 Ash (%) 9.9 Total surface acidity (meq g -1 ) 0.744 Total surface basicity (meq g -1 ) 0.376 The Brunauer-Emmett-Teller (BET) surface of the Typha australis was obtained using a micromeritics® TriStar II Plus device, based on N2 adsorption isotherms determination. The BET surface for Typha australis was found to be 0.91 m 2 g −1 . The value of the surface area of Typha Australis without any thermal or chemical process is satisfactory when compared with other species of Typha studied in the literature whose preparations required excessive heat and chemical inputs and especially with the use of toxic acids and corrosive 30 . At the end of each experiment, the stirred solution mixture was centrifuged and the residual concentration of MB was analyzed by Spectrophotometer UV1800 Ray Leigh at 655 nm wavelength. The adsorption uptake at equilibrium time qe (mg g -1 ), is expressed by following equation (1):

Batch adsorption studies
The percentage of the MB removed (%) from the solution was calculated using the equation (2): Where qe is the MB concentration in adsorbent (mg g -1 ), Ci is the initial MB concentration (mg L -1 ); Ce is the MB concentration at equilibrium (mg L -1 ); V is the solution volume (L) and m is the mass of the Typha australis leaf as adsorbent used (g). All batch experiments were conducted in triplicate and the average values are reported.

Kinetics and equilibrium adsorption modelling
The mechanism of the adsorption process was evaluated using PFO and PSO models. We used a PFO equation of Lagregren, based on solid capacity with the assumption that the adsorption mechanism is rate limiting 31 . The non-linear kinetic PFO model may be expressed by equation (3): Where qt is the amount of MB adsorbed per unit mass of Typha australis leaf adsorbent (mg g -1 ) at time t, k1 is the PFO rate constant (L min -1 ), and t is the contact time (min). PSO equation based on solid phase adsorption was used with the assumption that the rate limiting step may be chemical sorption (chemisorptions) involving valence forces through sharing or exchange of electrons between adsorbent and the adsorbate 32 . The non-linear kinetic PSO model may be expressed as in equation (4): Where k2 (gmg -1 min -1 ) is the rate constant for adsorption, qe (mg g -1 ) the amount of MB adsorbed at equilibrium and qt (mg g -1 ) is the amount of MB adsorbed at time t.
The Langmuir and Freundlich isotherms had been used to evaluate the equilibrium characteristics of the adsorption processes. The Langmuir isotherm model assumes that the adsorption is localized on a monolayer and all adsorption sites at the adsorbent are structurally homogeneous 33 . The non-linear Langmuir isotherm model may be expressed as in equation (5): Where qm and KL are Langmuir constants related to adsorption capacity and affinity of the binding sites, respectively. The factor of separation of Langmuir, RL, which is an essential factor characteristic of this isotherm is calculated by using the relation (6): Where Co refers to the initial concentration of the MB. The RL value implies the adsorption to be defavourable (RL>1), linear (RL=1), favourable (0<RL<1), or irreversible (RL=0).
The Freundlich adsorption isotherm is based on the assumption that the adsorption occurs on heterogeneous surfaces of non-identical sites with different energy of adsorption. The Freundlich isotherm model employed to describe multilayer adsorption with interaction between the adsorbed molecules 34 . The non-linear Freundlich isotherm model may be expressed as in equation (7): Where KF and n are Freundlich constants representing adsorption capacity and the energy of adsorption effectiveness, respectively.
The R 2 analysis was used to fit experimental data with adsorption kinetic and isotherm. The fit appreciation was assessed by the coefficient of determination R 2 which is given by the expression (8): Where qexp (mg g -1 ) is equilibrium capacity from the experimental data, qavr is equilibrium average capacity from the experimental data and qmod is equilibrium from model. So that R 2 ≤ 100the closer the value is to 100, the more perfect is the fit.

Effect of adsorbent mass
Biomass dosage is an important parameter in adsorption studies, as it gives the optimum mass at which maximum adsorption occurs. The effect of the amount of adsorbent on the efficiency of adsorption was studied. Variation of mass in the range 0.01-0.6 g at a fixed MB concentration (10 mg L -1 ) for MB removal by Typha australis leaf adsorbent is shown in Figure 1.
The results suggest that the increase in the mass of Typha australis results in an increase in adsorption, probably due to increase in the retention surface area. However, further increase after a certain mass does not improve the adsorption; perhaps due to the interference between binding sites of the Typha australis at different mass. The optimal Typha australis adsorbent mass obtained is 0.2 g.

Effect of pH
In biosorption, pH is an important parameter as it affects both the ionization degree of the adsorbate and the surface charge of the adsorbent during the biosorption process 35 .
The adsorption of MB under different pH (2.5, 7 and 11.5) is determined for 10 mg L −1 of MB solution as shown in Figure 2. The highest removal efficiency of MB adsorption obtained at pH 11.5 is evaluated at 92 %. In addition, the pHPZC of the Typha australis adsorbent was found to be 6.36. For values less than pHPZC, the Typha australis surface was positively charged, which would result in an electrostatic repulsion and therefore a decrease in MB adsorption (42.55 % at pH 4). At pH > pHPZC the Typha australis surface was negatively charged, which would cause an electrostatic attraction and therefore an increase in MB adsorption. Some authors have reported that MB adsorption usually increases as the pH is increased 36-38 .

Effect of ionic strength
The effect of inorganic salt (NaCl) on adsorption of MB on Typha australis is presented in Figure 3. As seen in Figure 3, the presence of inorganic salt has influenced the percentage of the MB removed. The MB adsorption increases with the increasing NaCl concentration.
Our results show that higher concentration of salts promotes the adsorption of MB on Typha australis leaf. Similar results have been reported for MB adsorption onto hazelnut shell 36 .

Effect of temperature on MB adsorption
Temperature is anticipated to have an influence on the dye adsorption properties of Typha australis leaf adsorbent with MB. Figure 5 shows the effect of temperature on MB removal by varying the temperature in the range 20-30°C at MB concentration of 10 mg L -1 . The observed results in Figure 5 indicate that the adsorption process of MB onto Typha australis leaf was favoured at higher temperature, in agreement with an endothermic adsorption process 40 .

Kinetic and thermodynamic study
Contact time is an important issue in adsorption and determining the equilibrium time is of real importance. The sorbed MB (5 and 10 mgL -1 ) at equilibrium qe was plotted against time for the Typha australis (Figures 6 and 7).  Figures 8 and 9 show the experimental equilibrium data and the predicted theoretical kinetics for the sorption of MB onto Typha australis for 5 and 10 mg L -1 , respectively. The values of model parameters qe, k1, k2 and the correlation coefficient R 2 are presented in Table 2.
The correlation coefficient R 2 showed that the PSO model was the more suitable for sorption MB behaviour onto the Typha australis adsorbent. In addition, the qe calculated by the PSO kinetic model are close to those obtained from the experiments at all initial MB concentrations, indicating that the PFO kinetic model did not properly describe the adsorption process of MB on Typha australis adsorbent.
These results suggest that the adsorption data are well represented by PSO and the rate-limiting step of MB onto Typha australis leaf adsorbent may be chemisorption. Similar phenomena have been described for MB adsorption on wheat shells 41 , perlite 42 , cedar sawdust and crushed brick 43 , sepiolite 44 and coir pith carbon 45 .   Figure 10 shows the experimental equilibrium data and the predicted theorical isotherms for the sorption of MB onto Typha australis leaf. The calculated adsorption parameters and the correlation coefficient R 2 for Langmuir and Freundlich for the adsorption of MB onto Typha australis are summarized in Table 3.

Sorption isotherms
The results compiled in Table 3 Table 4 for comparison. As listed in Table 4, that the adsorption capacity of Typha australis adsorbent is found substantially superior or comparable with many reported low-cost adsorbents. This proves the viability of Typha australis as one of the most superior adsorbents for removal of MB from aqueous solution.

Conclusion
The Typha australis biomass, collected from Senegal River bank, exhibited great potential as low cost adsorbent for effective removal of MB from aqueous solution. The optimum Typha australis mass was 0.2 g and highest removal efficiency of MB adsorption was obtained in solution pH 11.5. A very good agreement with experimental data obtained indicates that a PSO kinetic model is favorable for the MB adsorption on Typha australis adsorbent. The equilibrium data fitted well with the Langmuir model with the monolayer adsorption capacity for MB-Typha australis leaf system was of 103.12 mg g -1 .
From the thermodynamic calculations ΔG° values for Typha australis leaf (-4.44 kJ mol −1 ) being negative revealed that the mechanism of MB adsorption from the aqueous solution is feasible and shows spontaneity. The positive value of ΔH° (55.13 kJ mol −1 ) confirms the endothermic process, meaning the reaction consume energy. The positive value of ΔS° (203.21 JK −1 mol −1 ) showed the increased randomness of the adsorbate molecules on the solid surfaces than in the solution for MB. The Typha australis showed greater adsorption capacity towards MB than other agro wastes adsorbents and can be easily prepared without any physical and /or chemical treatment. Adsorption of cationic MB dye on Typha australis leaf can be considered as a simple, fast and economic method for its removal from aqueous solutions. For future studies, the usability of Typha australis for dyes removal from real wastewater will be tested and as comparison, a fixed bed column will be employed to investigate the effect of reactor design.

Acknowledgements
The author would like to thank the Cooperation and Cultural Action Service of the French Embassy in Mauritania for a mobility grant.