The natural low cost material, Lantana camara, was obtained from Stadium Area, Ogbomosho, and OyoState, Nigeria. The stems were separated from the seed and flower and were extensively washed with tap water to remove soil and dust particles. It was then sundried, and further crushed into smaller size. The Lantana camara was carbonized using an electric furnace at 300˚C for 2 h. The carbonized sample was ground into granular form and the carbon was activated using steam activating reactor. The resultant activated carbon was oven-dried at 105˚C for 2 h and cooled to room temperature in a desiccator. It was then ground with mortar and pestle and sieved to 2 mm mesh size. It was later stored inside a desiccator to avoid moisture absorption. The Infrared spectroscopic analysis was conducted on the Steam-Activated Lantana camara Stem (SALCS) to investigate the functional groups present on its surface using Fourier Transform Infrared Spectrometer (Perkin-Elmer Spectrum GX, Kuala Lumpur, Malaysia). The morphologies of the steam activated carbon were characterized using a scanning electron microscope (SEM, JEOL JSM-6480 LV).

The adsorb ate was prepared by dissolving 1 g of methylene blue powder in small quantity of distilled water in 1000 ml volumetric flask. More distilled water was added to make up to the mark. It was then shaken vigorously for five minutes to ensure complete dissolution and homogeneity; this makes the stock solution of concentration 1000 mg/L. Different concentrations were prepared by serial dilution [19] . Batch adsorption method was employed to determine the effects of contact time, adsorbent dose, dye concentration and pH solution.

100 ml of (50 mg/L) dye solution was poured into a conical flask with adsorbent dosage 0.5 g and place inside a shaker (environmental orbital shaker, Deneb Instruments). The samples were withdrawn from the shaker at predetermined time intervals and the dye solution was separated from the adsorbent by the help of a micropipette.

The absorbance of the solution was then measured. The dye concentration was measured after 50, 100, 150, and 200, until the equilibrium was reached. To study the effects of initial pH of the solution on the adsorption process, 100 ml of 50 mg/L dye solution was poured into a conical flask with adsorbent dosage of 1 g. The pH of the dye solutions was adjusted with dilute HCl (0.05 N) or KOH (0.05 N) solution by using a pH meter (EUTECH Instrument, pH 510). 10 ml of dye solution was prepared taking dye and the pH of solution was changed from 3 to 10.

The adsorbent dosage was varied as 0.5, 1.0, 1.5, and 2.0 g/150ml. Effect of concentration 100 ml of dye solution was prepared in conical flask with dye concentration 50 mg/L and adsorbent dose (1 g/L) and place the shaker. The temperature was maintained at 20˚C. The final dye concentration readings were taken at 50, 100, 150, and 200. Dye concentration was estimated, spectrophotometrically, at the wavelength corresponding to maximum absorbance, λ max using a spectrophotometer (JASCO UV/Vis-550). The samples were withdrawn from the shaker at predetermined time intervals (30, 45, 60 and 120 min) and the dye solution was separated from the adsorbent by the help of a micropipette. The absorbance of the solution was then measured. The dye concentration was measured after 50, 100, 150 and 200, until the equilibrium was reached. The q_e is expressed as

where, q_e = amount of dye adsorbed per unit mass of adsorbent (mg/g), C_o = initial dye concentration (mg/L), C_e = final dye concentration (mg/L), V = volume of dye solution and M = mass of adsorbent (g/L).

The FTIR spectra of SALC were taken before and after the adsorption of MB to ascertain the possible involvement of the functional groups on the surface of SALC in the adsorption of MB (Figure 1 and Figure 2). The broad band at 3500 - 2500 cm⁻¹ can be attributed to -OH stretching vibration while the band at 2196 cm⁻¹ can be assigned to the C≡C and/or C≡N stretching vibrations. The peak at 3500 cm⁻¹was strong and broad due to a carbonyl group stretching vibrations superimposed on the side of hydroxyl group band, which has been reported to occur at 3500 - 2500 cm⁻¹. The peaks at 1709.05 - 1682 cm⁻¹ may be attributed to C=O stretching, -CH bending vibrations of carbonyl compound, respectively. The bands at 3030 cm⁻¹ and 2822 cm⁻¹ can be linked to C-H stretching Figure 3 reveals that some new bands appeared, some remained and some were shifted after MB sorption. The bands at 3500, 2500, 2196 and 1682 cm⁻¹ were shifted to 3646.3, 3030, 2822, 1709.05 cm⁻¹ after MB adsorption. This is an indication that OH, C-H, C=C and C=O, carbonyl group could be involved in the adsorption of MB onto SALC.

Plates 1(a) and (b) shows the SEM micrographs of SALC samples before and after dye adsorption. The SALC exhibits a caves-like, uneven and rough surface morphology. The surface of dye-loaded adsorbent, however, shows that the surface of SALC is covered with dye molecules: (a) fresh SALC and (b) dye adsorbed SALC.

The pH of the dye solution plays an important role in the whole adsorption process and particularly on the adsorption capacity [20] . The effect of each individual solution pH (3 - 11) on the equilibrium uptake capacity of

MB dye was studied at different initial dye concentrations (50 - 200 mg∙L⁻¹) at room temperature. The dye uptake was found to increase with an increase in pH (Figure 4). At lower pH, the surface charge may be positive,

thus making (H⁺) ions compete effectively with cationic dyes causing a decrease in the amount of dye adsorbed [21] . And at higher pH, the surface of SALC may be negatively charged which enhance the positively charged on MB dye through electrostatic force attraction [22] . Similar observations were reported for adsorption of dyes indicating that the adsorbent has a net positive charge on its surface [23] [24] .

The adsorption of MB dye at initial concentration of 50 mg∙L⁻¹was studied at different contact time (30 - 120 min). The dye adsorption uptake was increased as contact time increased, and reaches equilibrium at 60 minutes (Figure 5). The result suggests that, adsorption takes place rapidly at the initial stage on the external surface of the adsorbent followed by a slower internal diffusion process, which may be the rate determining step [25] [26] . In addition, the fast adsorption at the initial stage may be due to the fact that a large number of surface sites are available for adsorption but after a lapse of time, the remaining surface sites are difficult to be occupied. This is because of the repulsion between the solute molecules of the solid and bulk phases, thus, taking long time to reach equilibrium [27] .

The solid/solution ratio is an important factor determining the capacity of adsorbent in a batch adsorption. The effect of adsorbent dosages on the percentage removal of MB dye has been shown in Figure 6. It followed the predicted pattern of increasing percentage adsorption as the dosage was increased [28] . This is probably because of the resistance to mass transfer of dye from bulk liquid to the surface of the solid, which becomes important at high adsorbent loading in which the experiment was conducted [29] .

It can be clearly seen that the removal of MB dye increased with increasing the amount of SALC. However the amounts of adsorbed dye per unit weight (Q_e) of the SALC decreased with increasing the solid/solution ratio (Figure 6). The removal of the dye increased when the dosage was changed from 0.5 - 2.0 g∙L⁻¹, at different dye concentrations (50 - 200 mg∙L⁻¹). As expected, at constant initial concentration of dye, increasing the sample dose provides a greater surface area and larger number of adsorption sites and hence enhancement of dye uptake [30] . The primary factor explaining this characteristic is that adsorption sites remain unsaturated during the adsorption reaction whereas the number of sites available for adsorption site increases by increasing the adsorbent dose [31] .

The effect of the initial dye concentration on the dye adsorption capacity was investigated in the concentration range of 50 - 200 mg/L⁻¹ at room temperature without changing the initial pH of the medium. The results represented in Figure 7 shows that the percentage of removal decreased with increasing initial dye concentration for SALC. The lower uptake at higher concentration resulted from an increased ratio of initial adsorption number of moles of the dye to the available surface area; hence fractional becomes dependent on initial concentration. The initial dye concentration provides an important driving force to overcome the mass transfer resistance of the dye between the aqueous and solid phases. Therefore, at higher initial dye concentration, the number of

ions competing for the available sites on the surface of SALC was high, hence, resulting in higher MB adsorption capacity [27] . Similar results were also reported by other researchers [32] .

The adsorption isotherm indicates how the adsorption molecules distribute between the liquid phase and the solid phase when the adsorption process reaches an equilibrium state. The analysis of equilibrium adsorption data, by fitting them to different isotherm models, is an important step to find the suitable model that can be used for design purpose [33] . Adsorption isotherm study was carried out on two well-known isotherms, Langmuir and Freundlich. The applicability of the isotherm equation was compared by judging the correlation coefficients, R².

It is important to be able to predict the rate at which contaminants is removed from aqueous solution in order to design adsorption treatment plant. In order to investigate the mechanism of adsorption and potential rate controlling steps such as mass transfer and chemical reaction, the kinetics of MB sorption onto SALC was investigated using two different models: the pseudo-first-order kinetic [42] , and the pseudo-second-order kinetic [43] models. The conformity between experimental data and the model predicted values was expressed by the correlation coefficients (R², values close or equal to 1 and SSE = sum of square of errors) [41] . A relatively high R² value indicates that the model successfully describes the kinetics of MB adsorption [44] .

The rate constant of adsorption was determined from the pseudo-first-order equation given by

where: q_e and q_t are the amounts of methylene blue adsorbed (mg/g) at equilibrium and at time (min), respectively and K₁ the rate constant of adsorption (2 h). Values of K₁ were calculated from plots of vs t at different concentrations of methylene blue (MB). The correlation coefficient values are higher than 0.90, the experimental q_e values also agree slightly different of about (4.7%) with the calculated ones, obtained from the linear plots (Figure 10 and Figure 11). This shows that the adsorption of MB onto SALC is pseudo second order kinetics.

Sum of square of errors is a test of kinetic models. Beside the value of R², the applicability of both kinetics models were verified through the sum of error squares (SSE, %). The adsorption kinetics of MB on adsorbent prepared from Lantana camara stem (SALC) was tested at different initial concentration. The validity of each

model was determined by the sum of error squares (SSE, %) given by;

where: N is the number of data points:

The higher the value of R² and the lower the value of SSE, the better the goodness of the fit. Table 4 lists the calculated results. It was found that the adsorption kinetics of MB on adsorbent prepared from Lantana camara stem can be described by the second-order kinetic model.