THE REMOVAL OF SINGLE AND BINARY BASIC DYES FROM SYNTHETIC WASTEWATER USING BENTONITE CLAY ADSORBENT

. ABSTRACT In order to broaden the application of bentonite clay, an easily obtainable and bio-available low cost adsorbent, it was employed for the decolourization of synthetic wastewater consisting of single and binary basic dyes (malachite green and rhodamine b). The sorbent was used as obtained without any further modification and also characterized for its specific surface area, point of zero charge and its surface functional groups pre and post dyes sorption was determined using Fourier Transform Infrared Spectroscopy (FTIR). Batch adsorption methods were employed in order to study the effects of pH, Ionic strength and contact time in the single solute system. The parameters of sorption of rhodamine b (RDB) and malachite green (MG) were obtained and fitted to three isotherm models; Freundlich, Langmuir and Temkin. The Freundlich plot analysis indicated the process occurred via heterogeneous coverage of adsorbent by both dyes. The kinetics of adsorption data were analyzed using the; pseudo-first order, pseudo-second order, Intraparticle diffusion, film diffusion, and Boyd kinetic models. Over the study of these parameters, the film diffusion mechanism was found to predominate in the sorption process of the dyes. Competitive sorption studies was carried out by using both dyes as either the sorbate of interest or as the interfering specie and the competitive co-coefficient values obtained from interfering MG in RDB removal were significantly lower than those obtained from interfering RDB in MG removal, indicating that the presence of RDB in the aqua matrix had antagonistic effect on MG adsorption by bentonite.


INTRODUCTION
Years of increased industrial activities have resulted in the generation of large amount of wastewater containing a number of coloured toxic pollutants, which are polluting the available fresh water continuously. Having realized this basic fact that these coloured pollutants in aqueous systems adversely affect human and animal life, the control of pollution is now a high priority task. The availability of clean water for various activities is becoming the most challenging task for researchers and practitioners worldwide. Dye pollutants are a major source of environmental contamination and colour is the first contaminant that is recognized in wastewater (Banat et al., 1996). This water soluble dyes are recalcitrant, carcinogenic and offer considerable resistance to biodegradation due to their complex structures and high thermal/photo-stability, hence there removal is paramount. Industries such as the textile industries, dye manufacturing industries, paper and pulp mills, tanneries, electroplating factories, distilleries, food companies, and a host of other industries generate and discharge this coloured wastewater (McKay et al., 1998). The first synthetic dye was encountered in 1856 by William Perkin Henry in the accidental synthesis of Mauveine but presently over 100,000 dyes have been made and more than 7 x 10 5 tonnes are produced annually (McMullan et al., 2001). More than 10,000 dyes are commercially available and about 5-10% of these are somehow discharged as wastewaters by these industries directly or indirectly into our water bodies (Gong et al., 2005) The two conventional methods for the treatment of coloured wastewater are; the biological method and the physical/chemical method. Amongst the numerous techniques of dye removal, adsorption which is a physical/chemical method is the procedure of the choice and gives the best results as it can be used to remove different types of colouring materials (Derbyshire et al., 2001;Ho and McKay, 2003;Jain et al., 2003). The major advantage of an adsorption treatment over other methods is due to its cost-effectiveness and sludge free clean operation (Gupta et al., 2009). Over the past decade, bentonite clay has been successfully employed for the adsorption of dye molecules and metal ions (Tahir et al., 2006, Tahir et al., 2010. To date several conventional and non-conventional sorbents have been employed in the removal of these dyes from wastewaters while limited attention has been given to the fact that this wastewater usually contains more than one dye constituent, hence in the removal of one dye constituent the other may either possess an antagonistic or a non-interactive effect on the constituent of interest. This study particularly takes a case study of two different basic dyes (RDB and MG) in a binary system and examines the nature of the effect of one on the other. The effects of pH, Ionic strength and contact time were further evaluated and reported for each dyes in their single solute system.

EXPERIMENTAL Adsorbent characterization
The pH point of zero charge (pHPZC) of the adsorbent was determined via the solid addition method as described by Balistrieri and Murray (1981). This is used to determine the pH at which the surface of the sorbent exhibits total surface electrical neutrality. The Sear's method (1956) was employed in determining the surface area of the adsorbent as well, as surface area is one of the major factors the affects adsorption processes. The surface functional groups present on the adsorbent and there possible involvement in the sorption process was examined from the Fourier Transform Infrared spectrum (FTIR) of the bentonite which was obtained from the Fourier Transform Infrared Spectrophotometer (Buck Scientific Infrared Spectrophotometer) between 400 -4000 -cm .

Sorbate preparation and quantification
The dyes used in this present work are; Malachite green (chemical formula = C23H25N2CI, λmax= 621nm) and Rhodamine b ((chemical formula = C28H31CIN2O3, λmax= 543nm) were accurately weighed and dissolved in a double distilled-deionized water to prepare a stock solution (100mg/L) and different working solutions which ranged between 2.5mg/L -30mg/L were prepared from the stock via serial dilution.

Single solute system adsorption experiment Determination of equilibrium isotherm and kinetic parameters
Batch adsorption experiment studies were carried out as described by Nadi et al (2012). 0.1g of the adsorbent with 50 ml of the dyes (RDB and MG) solutions for each of the desired initial dyes concentration of 2.5-30 mg/L in a 100 ml capped conical flask and the mixture was agitated on a temperature controlled magnetic stirrer at a constant speed of 1000rpm until equilibrium is attained. Samples were withdrawn after the equilibrium time (3hrs) and centrifuged at 4000rpm for 20min for complete separation of the bentonite particles from the solution and the residual dye concentrations were calculated from the calibration curve. The kinetics of sorption of the dyes was studied by monitoring the uptake of the dyes from aqueous solution at different time intervals (1,3,5,10,20,30,60,90,120,180) (Olaseni et al., 2014). The amount of dye removed per unit mass of the adsorbent was calculated as; where Co (mg/L) is the initial concentration of the dye, Ce (mg/L) equilibrium concentration, m is the mass of the adsorbent, and V is the volume of the solution. Percent dye removed (%DR) was calculated using the equation;

Influence of pH
The effect of pH was examined between pH4 and pH12. The different pH (4-12) of the aqueous solution was adjusted by the addition of either 0.1M HCl or 0.1M NaOH solution via drop wise method when necessary to the dyes solutions before the introduction of the adsorbent. The adsorbent-adsorbate ratio was fixed at 0.1g to 50ml while the initial concentration was fixed at 30mg/L.

Influence of Ionic strength
The dyes solution ionic strength effect was also examined by using NaCl solution of varying concentrations; 0%, 0.1%, 0.5% and 1% which is equivalent to ionic strengths of 0.0 Mol/L, 0.017 Mol/L, 0.085 Mol/L and 0.17 Mol/L. The adsorbent-adsorbate ratio was also fixed at 0.1g to 50ml while the initial concentration was fixed at 30 mg/L.

Bi-solute system adsorption experiment
The competitive sorption studies was studied in a synthetic wastewater containing only the two solutes (RDB and MG) at different initial concentrations which was between 2.5mg/L and 30mg/L for each of the sorbate of interest. The concentrations of the sorbates of interest were varied between 2.5-30mg/L while the interfering dyes molecule was kept constant at 30mg/L (Sheindorf et al, 1981). The competitive sorption equilibrium parameters were obtained by contacting 50ml solutions of known concentrations of the sorbate of interest and interfering dye solution in a binary system. The mixture was agitated for 3hrs and samples withdrawn at the end of the sorption process, centrifuged and the supernatant concentrations of both the sorbate of interfering solution were determined using the UV -VIS Spectrophotometer at there're respective wavelengths.

Error Analysis.
In order to examine the best fit of the different kinetic models to the observed experimental data, an error function is required in the optimization procedure.
In the present study, the kinetic models were examined using the linear coefficient of determination, r 2 , and the non-linear chi-squared statistical method, χ 2 . The coefficient of determination r 2 , represents the percentage of variability in the dependent variables that has been explained by regression line (Oladoja and Akinlabi, 2009). The value of the coefficient of determination may vary from zero to one and was calculated with the aid of the equation; Where is the sum of squares of x, is the sum of squares of y and is the sum of squares of x and y. In order to evaluate the best of the kinetic models that fit the experimental data, the sorption process was also examined using nonlinear chi-squared (χ 2 ) statistical test. The chi-squared statistical test is basically the sum of the squares of the difference between the experimental data and the theoretical data obtained by calculating from models, with each squared difference divided by the corresponding theoretical data obtained by calculating from models. The equivalence of the mathematical statement is; If the data from model and experimental data are similar, then χ 2 value will be small and if they are different χ 2 will be a large value.

Results and Discussion. Characterization of adsorbent
The pH point of zero charge (pHZPC) is used to determine the pH at which the surface of the sorbent exhibits total surface electrical neutrality. The functional groups present on the sorbent surface may reside in the positive or negative charge state, depending on the pH of the system. A large relationship co-exists between the amount of sorbate adsorbed to the surface of the sorbent and the pHZPC of the adsorbent, which is that; adsorption of positively charged sorbates will be favoured at pH values higher than the pHZPC when the surface of the adsorbent is predominantly negatively charged while adsorption of negatively charged sorbent will be favoured at pH values lower than the pHZPC when the surface of the adsorbent is predominantly positively charged (Nomanbhay and Palanisamy, 2005). In the determination of the surface charge of bentonite used in this study, this showed that it possesses a pHZPC of 2.14 which was supported by literature to be between 2.0 and 3.0 (Hashen, 2012., Akpomie andDawodu, 2015., Kosmulski, 2009). This signifies that the sorbent is suitable for the removal of basic dyes over a large pH range above 2.14. Its specific surface area was also determined to be 21.5(m 2 /g). A BET surface area of 20(m 2 /g) and 31.5(m 2 /g) was reported by Shu-li et (2009) andXifang et al (2007) respectively. The difference in the surface area values reported could be ascribed to the method of determination, purity, method of preparation of sample and type of bentonite. The FTIR spectra of the sorbent pre and post dyes uptake is illustrated in FIG 1. The FTIR spectra of bentonite before adsorption of dyes showed absorption peak at 3456.80 cm -1 which is linked to the stretching vibration of H2O, the peak at 1649.60 cm -1 is linked or corresponds to the bending of H2O, the peak at 1047.20 cm -1 corresponds to the stretching of Si-O, the peak at 920.80 cm -1 denotes the bending vibration of Al-OH-Al. furthermore, FTIR peak at 620.00 cm -1 is associated with Al-O + Si-O out of plane vibrations while the peak at 537.00 cm -1 denotes Al-O-Si vibration bending. For the FTIR peaks of malachite green and rhodamine b, the peak at 1576 cm -1 and 1594 cm -1 respectively, indicates the presence of = + . The peaks at 1467 cm -1 and 1178 cm -1 for malachite green, peaks at 1468.8 cm -1 and 1176.8 cm -1 for rhodamine b denotes the vibration of the heterocyclic skeleton of the dye molecules. The peaks at 1178 cm -1 and 1176.8 cm -1 for malachite green and rhodamine b respectively is assigned to C-H3. It should be noted that the intensity of the peak at 1047.20 cm -1 present in the FTIR spectra of bentonite (denoting Si-O vibrations) in the absence of adsorbed dyes (FIG 1a) decreases considerably and consistently after the adsorption of each of the dyes to its surface and also in the binary system. A shift in this peak from 1047.2 cm -1 to 1046.40 cm -1 for the adsorption of MG, to 1052.80 cm -1 for the adsorption of RDB and to 1033.60 cm -1 in the presence of both dyes was observed. This may be due to the electrostatic attraction between Si-O group of the bentonite and the positively charged nitrogen moiety present on both dyes (malachite green and rhodamine b) signifying the possible involvement of the Si-O group in the sorption process while the shift in peaks confirms the presence of adsorption.

Sorption kinetics of MG and RDB unto bentonite from synthetic wastewater.
To investigate the adsorption process, monitoring of the adsorption kinetics is an important step. It was observed that equilibrium was reached at about 60mins and the amount of the dyes congregating unto the surface of the adsorbent was highly dependent upon initial concentration. The kinetic parameters of the adsorption of MG and RDB unto bentonite were evaluated by employing the pseudo-first order and the pseudo-second order kinetic models.
(a) Pseudo-First Order Kinetic Model. In cases where adsorption is preceded by diffusion through the boundary layer, the kinetics most have followed the pseudo-first order mechanism as described by lagergren. (Lagergren, S. 1898., Ho, Y. S. 2004. The linearized form of the pseudo-first-order equation of Lagergren is generally expressed as: where qe and qt are the sorption capacity at equilibrium and at time t respectively, (mg/g). k1 is the rate constant of pseudo-first-order adsorption. The plot of In (qe -qt ) versus t should give a linear relationship from which k1 and qe can be deduced from the slope and intercept of the plot, respectively. The pseudo-first-order constants, k1, the correlation coefficients, r 2 , and the chisquare test statistic, χ 2 , were deduced from the plot of In(qe -qt) versus time (t), and the respective values obtained are presented in Table 1 and 2. The correlation coefficients for the pseudo-first order plots for the adsorption of these dyes ranged between 0.812 to 0.976 for RDB (Table 1) while that of MG ranged between 0.871 to 0.994 (Table 2). The calculated qe values (qe.theo) possess a large disparity from the experimental qe values obtained for both dyes.
For the applicability of this kinetic model towards the description of the mechanism of the sorption of these dyes unto bentonite, the nonlinear chisquare test statistic was used to correlate the qe values obtained from the theoretical prediction (qe.theo) and the actual sorption process qe (qe,exp) (Tables  1 and 2). The results presented in Tables 1 and 2 showed that the χ 2 , values were very high, which is an indication that the pseudo-first order model cannot be employed in the description of the adsorption process. Hence, the process does not follow the pseudo-first order adsorption rate expression of Lagergren.  where qe , qt , and t have the same meaning as explained above and Where k2 is the pseudo second order rate constant (gmg -1 min -1 ) which is determined from the intercept and qe from the slope of the plot. The plot of t/qt should give a linear relationship if the pseudo-second order kinetic model is applicable. The initial sorption rate (h) can be obtained from the pseudo-second-order linear plots, as qt/t approaches zero: the initial sorption rate, h, the pseudo-second order rate constant, k2 , the amount of RDB and MG sorbed at equilibrium, qe , the linear coefficient value, r 2 , and the chi-square test statistic function, χ , obtained are presented in Table 3 and 4. The values of h and qe increased with the increase in the initial concentration of both dyes while the sorption rate constants decreased with increasing initial concentrations. The large values of the r 2 and negligible χ values is a pointer to the fact that the mechanism of adsorption of the dyes follows the pseudosecond order. Muhammad et al 2016Muhammad et al , 2015, have also reported the mechanism  Table 4: Malachite Green pseudo-Second order kinetic parameters

Intraparticle diffusion model.
Based on the theory proposed by Weber and Moris, Intraparticle diffusion model which has been widely applied for adsorption studies was employed to investigate the adsorption mechanism of RDB and MG unto bentonite. This model is mathematically expressed as; q t = K id t 0.5 + C … … … … … … … … . .8 Where C (mg/g) is the intercept and Kid (mg/gmin 0.5 ) is the intraparticle diffusion rate constant. The value of Kid was obtained from the slope of the linear plot of qt vs t 0.5 for both dyes (FIG 4 and 5). The value of C which is given in Table 5 and 6 represent the boundary layer effect (Kannan and Sundaram, 2001). The significance of the deviation from this theory is the difference between the rate of mass transport between the initial and final stages of adsorption (Panday and Singh, 1985) Furthermore, such deviation of straight lines from the origin indicates that the pore diffusion is not the sole ratecontrolling step (Poots et al, 1978). The results presented in Figure 4 and 5 show that two separate regions are found in the plot: the first straight portion of the plot is attributed to the macropore diffusion (surface sorption) and the second linear portion is attributed to micropore diffusion (Intraparticle diffusion) (Lakshmi et al, 2009). In order to actualize the rate controlling step between these two distinct stages by determining the rate of Intraparticle diffusion (Kid) which can be obtained from the slope of the Intraparticle diffusion plots. Drawing a comparison between the Kid values of the two stages, it can be observed that the rate-limiting step is the macropore diffusion stage for the adsorption of both dyes (table 5 and 6). REF

Liquid film diffusion model.
Sequel to the inability of the intraparticle diffusion model to adequately describe the sorption mechanism of Rhodamine B and Malachite green by bentonite, the adsorption dynamics was investigated utilizing the liquid film diffusion model. The possible migration of dye molecules from bulk solution to the exterior surface of the sorbent through the liquid film was investigated by utilizing the liquid film diffusion model (equation x) in order to determine the rate controlling step (Boyd et al, 1947).
In(1 − F) = −K fd t … … … … … … … … .9 Where Kfd (min -1 ) is the liquid film diffusion rate constants and F = t e q q . A linear plot of ln(1 ) F − vs t, for liquid film diffusion for concentration between 2.5 -30mg/l were tested for both dyes (RDB and MG). From these plots, the overall sorption process yielded linear plots with coefficients of correlation (R 2 ) ranging between 0.8121 to 0.9768 with intercepts between -1.7581 to -3.1278 for rhodamine b and 0.8717 to 0.9946 with intercepts ranging between -0.2403 to -1.0645 for malachite green. The rate constant for the liquid film diffusion Kfd was between 1.81 x 10 -2 -2.27 x 10 -1 for rhodamine b and between 6.14 x 10 -2 -8.87 x10 -2 for malachite green. This is pointer of the fact that as these plots do not pass through the origin (i.e. intercepts less than zero for rhodamine b and malachite green), the applicability of this model in describing the mechanism of the adsorption of these basic dyes by bentonite is limited.

Overall
First

Boyd Kinetic Model.
In order to actualize the rate controlling step involved in the sorption of rhodamine b and malachite green unto the bentonite, the kinetic data's were further analyzed using the Boyd kinetic model according to the equation (Boyd, et al. 1947); F = 1 − (6 tπ 2 ⁄ )exp(−B t ) … … … … … … … . .10 Where t B represents the mathematical function of F, and F is the solute adsorbed fraction at various time t. = ∞ … … … … … … .11 Where t q is the amount adsorbed at time t, q  represents the amount adsorbed at infinite time. (In this study is 180min). A linear form of the Boyd kinetics is written as: B t = −0.4978 − In(1 − q e q t ⁄ )……………..12 Hence, the values of Bt can be estimated for each value of F using equation 10. The calculated Bt values were plotted against time ( Figure 6 and 7), and the linearity test of the Bt versus t plot for different initial RDB and MG concentrations was employed to differentiate between the particle diffusion and film diffusion controlled adsorption process. If the Boyd plot is a straight line passing through the origin (zero intercept), the particle diffusion mechanism predominates. However, if the plots do not pass through the origin (i.e. intercept greater than zero), the film diffusion mechanism predominates. Fig 6 and Fig 7 shows that the Boyd plots for the removal of rhodamine b and malachite green by bentonite do not pass through the origin, indicating that the film diffusion mechanism predominates in the adsorption process

Adsorption isotherms
Adsorption equilibrium isotherm is expressed by relating the amount of adsorbate sorbed per gram of sorbent, qe (mg/g), to the equilibrium solution concentration, Ce (mg/L), at a fixed environmental conditions (Baek et al, 2010). In this present study, the Temkin, Langmuir and Freundlich isotherm models were employed in the description of the Adsorption process and and the isotherm parameters obtained using these models are presented in Table A. The linear form of Langmuir's isotherm model is expressed as; Where qm the maximum value of adsorptive capacity (mg/g), KL, is the Langmuir constant (L/mg) which is related to the energy of adsorption are obtained from the intercept and slope of the plot respectively (Alshabanat et al., 2013;Soni et al., 2012). The constants qL and KL were calculated from the slope and intercept of the linear plot of Ce/qe versus Ce. From Table 9, the low correlation coefficient values (r 2 ) obtained for Langmuir isotherm model, 0.465 for RDB and 0.042 for MG is an indication that this isotherm model did not express the experimental data for both dyes, suggesting that the surface of bentonite is heterogeneous and not homogeneous in nature. The essential characteristics of the Langmuir isotherm, used to predict the adsorption efficiency, was expressed in terms of a dimensionless equilibrium parameter RL, defined by , where Co (mg/l) is the highest concentration of adsorbate. The value of RL indicates if Langmuir isotherm is unfavourable (RL > 1), linear (RL = 1), favourable (0< RL >1) or irreversible (RL = 0). The values of RL at Co = 30mg/l in the present study were found to be 1.6058 X 10 -3 for Rhodamine b and 0.718 for malachite green at 298.15k, indicating that the adsorption of these basic dyes unto bentonite is favourable. The Freundlich adsorption isotherm is the first mathematical fit to an isotherm published (Freundlich, 1906). It is a purely empirical isotherm which describes the adsorption onto a heterogeneous surface with the linear equation; Inq e = InK f + 1 n InC e … … … … … … . .14 Where qe is the adsorptive capacity which is the quantity of the material adsorbed per unit gram of the adsorbent (mg/g), Ce is the equilibrium adsorbate concentration (mg/L); Kf, the Freundlich isotherm constant related to adsorption capacity (indicating the quantity of dye adsorbed onto the adsorbent) and n, the Freundlich isotherm constant related to adsorption intensity (which indicates the favourability of the adsorption process) can be obtained from the intercept and slope of the plot respectively (Alshabanat et al., 2013;Runping et al., 2008). Therefore, the plot of Inqe versus InCe gives a straight line of slope 1/n and intercepts logKF. It can be observed from Table 9 above that the Freundlich isotherm provides a very good fit to the experimental data possessing r 2 of 0.980 for RDB and 0.905 for MG. suggesting that the sorption of the dyes occurred via heterogeneous coverage of the surface of the adsorbent. The values of KF which denotes the sorption capacities of the adsorbent towards the dyes was found to be 0.386mg/g for RDB and 0.8055mg/g for MG. Rhodamine B and Malachite Green adsorption onto Bentonite clay is considered to be unfavourable because the Freundlich exponent values of n (which determines the intensity and feasibility of the adsorption process) were less than 1 (0.73 for Rhodamine B and 0.98 for Malachite Green). Generally, the value of 1/n less than 1 and the value of n in the range of 2-10 indicates that the adsorption process is favourable, 1-2 moderately difficult and below 1 poor adsorption characteristics (Rahman et al., 2012). The Temkin isotherm is based on the assumption that the free energy of adsorption is dependent on the surface coverage and takes into account the interactions between adsorbents and dye molecules. The linear form of the Temkin isotherm model equation is expressed as (Tempkin and Pyzhev, 1940):  Table 9. therefore the Temkin isotherm model do not describe the mechanism of sorption of these dyes unto bentonite.

Effect of pH
It must be emphasized that pH is one of the most important variable to be considered in adsorption studies. The pH of the aqueous medium alters the net charge present on the surface of the adsorbent, the ionization degree of the sorbate molecules and the dissociation extent of the functional groups present on the adsorbent (Nandi et al, 2009). The variations in the total percentage of the dyes removed at different pH and initial concentration of 30mg/l for RDB and MG are shown in FIG 8. For these dyes, the maximum percentage removal was attained at pH 4 (in excess of 70% for Rhodamine B, and excess of 90% for Malachite Green). The trend in the plot also shows that the percentage removal peaked at pH 4 and maintained a steady decline until pH 12. This variation in behavior of the adsorption can be explained on the basis of point of zero charge of the adsorbent (pHZPC = 2.14) as well as the structures of the dyes. At pH below the point of Zero charge of the adsorbent (2.14) the surface of the sorbent has a high positive charge density and under this condition the uptake of positively charged dyes would be low. With increasing pH i.e. beyond the point of zero charge, negative charge density on the surface of the adsorbent increases, resulting in an enhancement in the removal of these positively charged dyes. Also as a result of the acidic groups on these dyes that dissociates with increasing pH, giving rise to negative charge on the dyes which accounts for the reduction in the percentage removed in these dyes at higher pH. . This is because in solution of high ionic strength, the electrostatic attraction mechanism is suppressed due to the competition between the cationic dyes molecules with the Na + present for the active sites on the adsorbent surface, leading to reduction in electrostatic attraction between the dyes and the adsorbent surface. High ionic strength also enhances hydrophobichydrophobic interactions by compression of the electrical double layer that moves particles much closer together which leads to increase in dye adsorption as observed in FIG 9. Hu et al, 2013, also reported a similar observation for the sorption of methyl orange, methylene blue and neutral red. It has been reported that ionic strength greatly influences particle aggregation by altering electrostatic interactions between sorbates and sorbents. An increase in ionic strength of a solution has been described to suppress electrostatic repulsion thereby promoting particle aggregation (Mercer andTobiason, 2009., Piret andSu, 2008). For Malachite Green/Rhodamine B system q R = K M C M (a MR C R + C M ) n 1 −1 … … … … … … .18 Where Malachite Green is the sorbate of interest and Rhodamine B is the interfering ionic species. The linear form of the bicomponent isotherm equation can be written as;

R/M System
As expressed above, the competition coefficients were determined from the intercept of a straight line plot of CR against βR in equation 17, which is presented below; FIG 10: Experimental results presented in the linear form of the multicomponent adsorption isotherm: Rhodamine B in the presence of Malachite Green at constant concentration system. Fig 10 shows the extended Freundlich plot for Rhodamine B-Malachite Green system. The correlation coefficient was found to be 0.993 which shows a very good linearity of the plot and indicates the applicability of the extended Freundlich equation to describe the competitive adsorption behavior of the binary mixture. The values of the competition coefficients were determined from the intercept of the above plot (Fig.10). However, it should be noted that since by definition = 1 ⁄ (i.e. 12 = 1 21 ⁄ as described by Sheindorf et al, (1981 ), only one of the coefficients is required (Sheindorf et al., 1981 Table 10: Summary of competitive coefficients values for the binary-solute system of rhodamine b and malachite green in synthetic wastewater system. Deb et al., (1967) proposed that the value for the competitive coefficient range from zero (complete lack of competition) to greater than zero (normally less than 10) for a high degree of competition. An overview of the values of the competitive coefficients obtained showed that the values of the (Table 10) were far lower than that of the in the synthetic wastewater system. The values of the ranged between 0.251 and 0.318 while that of the ranged between 3.15 and 3.99. These showed that the competitive effects of Malachite Green in Rhodamine B sorption by Bentonite were insignificant, since the values of the competition coefficient were below unity (Wu et al., 2002). The values of the competitive coefficient of Rhodamine B in Malachite Green sorption by Bentonite were very high (Table 10). The values observed for the competitive coefficients of both Malachite Green and Rhodamine B, as interfering ionic specie, in their respective systems, signifies that the presence of Rhodamine B in the aqua matrix had antagonistic effects on the sorption of Malachite green by the sorbent while the presence of Malachite Green had noninteractive effects on the sorption of Rhodamine B by Bentonite (Oladoja et al., 2016).

CONCLUSIONS
This study evaluated the use of a readily bio-available and inexpensive bentonite as a low cost adsorbent for the removal of basic dyes in single and binary system. The FTIR characterization revealed the presence of silanol and aluminol groups on the surface of the bentonite which was responsible for the sorption of these basic dyes. The sorbent was found to possess a high adsorption potential for MG than RDB which is possibly due t the presence of the acidic group -COOHon the RDB which MG do not possess. The -COOHgroups stimulates an electrostatic repulsion between RDB and bentonite. The sorption of both dyes was found to be highly dependent on various environmental conditions such as; pH, Ionic strength, contact time and initial dye concentrations. The equilibrium data analysis revealed heterogeneous nature of the bentonite which was confirmed by the fitting of the experimental data to the Freundlich isotherm model. The extremely high values of R 2 and the predicted values of the equilibrium sorption capacity qe which was very much in agreement with experimental data for all initial RDB and MG concentrations confirms that the sorption process follows a pseudo-second-order mechanism. The competitive effects of the presence of Malachite Green in the sorption of Rhodamine B by Bentonite were insignificant (less than unity), thus, it could be concluded that Malachite Green has non-interactive effect in the sorption of Rhodamine B by Bentonite. Whereas the competitive effect of the presence of Rhodamine B in the sorption of Malachite Green were very high (greater than unity), thus it could be concluded that Rhodamine B is antagonistic in the sorption of Malachite Green by Bentonite.