Mucuna pruriens seed shells (MSS) were collected from Mucuna pruriens seed processing, Ogbete market, Enugu state, Nigeria and washed thoroughly with de-ionized water to remove dirt, dried in the oven at 105˚C. The MSS were ground, sieved to the desired particle size of 1 - 2 mm and soaked in 60% orthophosphoric acid (H₃PO₄) acid for 24 hours at room temperature and carbonized at 500˚C for 2 hours using muffle furnace (Model SX-2.5-10). The carbonized samples were washed with de-ionized water until pH 7, filtered and dried in the

oven at 105˚C for 8 hours. The dried sample was allowed to cool to room temperature, sieved to different particle sizes and stored in air tight container for adsorption studies.

The physicochemical properties of the adsorbent (AMSS) were determined using the standard methods described by [15] . X-ray Fluorescence (XRF) was used to determine the oxides present in the Mucuna pruriens seed shells (MSS) using XRF spectrometer (Munipal 4 model). The Fourier transforms infrared (FTIR) was used to determine the functional groups present in MSS and AMSS using Shimadzu S8400 spectrophotometer, with samples prepared by the conventional KBr disc method. X-ray diffraction (XRD) was used to determine the diffraction pattern and inter planar spacing of AMSS. XRD patterns of the carbon was obtained on a powder X-ray diffractometer (Stchmabzu model 6000) with CuKα radiation having a scanning speed of 8.000 deg/min and tested at 40.0 kV and 30 mA. The patterns were recorded over a 2-theta (2θ) range of 2.0000 - 60.0000 deg. Scanning Electron Microscopy (SEM) was used to determine the surface morphology of MSS and AMSS.

Congo red (CR) and malachite green (MG) of analytical grade were used without further purification. The reagents were of high grade. 0.1 g of each dye was dissolved in 1000 ml of de-ionized water to get a dye solution of 100 mg/l. Dye adsorption experiments were performed by taking 100 ml stock solution of dye and treated with a known dose of adsorbent at 120 rev/min. After a desired time of treatment, the dye solution was centrifuged and the concentration of the residue was determined using UV-Vis spectrophotometer (Model UV 754) at wavelength, λ_max of 498 and 617 nm for CR and MG respectively. The effects of particle size, pH, adsorbent dose, concentration and contact time on the process of adsorption were investigated in the range of 0.3 - 1.5 mm, 2 - 10, 0.2 - 2.0 g, 100 - 500 mg/l and 10 - 150 min respectively. The percentage adsorbed was investigated by varying a parameter and keeping the other parameters constant. The amount of dye adsorbed per unit mass of activated carbon (mg/g) and percentage adsorbed (%) were obtained as follows:

where C_o is the initial concentration of dye in the solution (mg/l); C_e is the final concentration of dye in the solution (mg/l); V is the volume of the solution (l); and W is the weight of activated carbon (g).

The correlation coefficient (R²) was used to determine the conformity, applicability and acceptance of the isotherm and kinetic models. The sum of the squares error (SSE %) was also used to determine the adequacy of fit of the isotherm models. The higher the R² value and the lower the value of SSE (%) are, the better the conformity and adequacy of fit of the model are. The SSE was estimated using the expression [16] :

where N is the number of the data points used in each kinetic model plot.

The physicochemical properties of the adsorbent are presented in Table 1. The AMSS was found to have a high surface area indicating its high adsorptive performance. The adsorbent was also found to have a high percentage fixed carbon and iodine number. The pH value is near neutral which will be helpful for the treatment of all cases of dye waste water and the carbons can also be used for drinking water purification [17] . The activated carbon

was found to have low ash content (5.38%). If the ash content is high, it will interfere with the pore structure development and hence adsorption will be less [18] . X-ray Fluorescence (XRF) was used to determine the oxides present in the MSS. The oxides present in the adsorbent which may be attributed to adsorption study are shown in Table 2. The oxides of calcium, potassium, iron and phosphorus are the major constituents of the adsorbents indicating that the Mucuna pruriens seeds can be a source of obtaining useful elements. The FTIR analysis was used to examine the surface functional groups of the adsorbent and to identify those groups responsible for dye adsorption. The FTIR spectra of MSS and AMSS are shown in Figure 2 and Figure 3 (plot of IR transmittance against wave number). Their frequencies of the bands and the corresponding functional groups are presented in Table 3 and Table 4. The O-H stretch in alcohols which is a very strong and broad bond can be detected in the carbons which are important adsorption sites. The modifications in the frequencies of the bands present in MSS may be as a result of chemical activation and reactions with O-H groups. Some of the bands and intensities were shifted or removed and new peaks were also detected after reaction with the activating agent (H₃PO₄) indicating a change in the crystalline structure of the carbon (MSS). Xray diffraction (XRD) has been used to determine the diffractive pattern present on AMSS. The XRD spectrum of the carbons showed broad peaks (Figure 4), which indicates the presence of high content of amorphous form of carbon and little amounts of crystalline materials in the adsorbents. The three strongest peaks present in AMSS, with their interplanar spacings (d) and 2-Theta value (2θ) are presented in Table 5. The SEM micrographs of MSS and AMSS are presented as Figure 5 and Figure 6 respectively. Both SEM figures seem showed that H₃PO₄ activation increased the surface area of MSS. The SEM of AMSS (Figure 6) seems to be threadlike with, some breakage in nature and dotted illuminations.

The equilibrium isotherm relationship between the concentration of the dyes in the liquid phase and the dyes in the adsorbent (AMSS) at a given temperature was studied. The results were analyzed using the Langmuir, Freundlich, Tempkin and Dubinin-Radushkevich isotherm.

The adsorption kinetics was studied in order to study the mechanism of the process of adsorption.

The activation energy was evaluated from the pseudo second-order kinetic rate constants obtained from adsorptive removals performed at 303, 313 and 323K. The Arrhenius equation is given as [30] :

where k₂ is the rate constant obtained from the pseudo second-order kinetic model (g/mg h), Ea is the Arrhenius activation energy of adsorption (kJ/mol) and A is the Arrhenius factor. In k₂ was plotted against 1/T and Ea was obtained from the slope. The activation energies of adsorption of CR on AMSS is 19.9182 K/Jmol which implies that the rate-limiting step might be a physically controlled process since which is less than 40 K/Jmol and MG adsorption on AMSS is 66.7789 K/Jmol which is greater than 40 K/Jmol, might be a chemically controlled process [31] .

Adsorption thermodynamics is the study of the effect of temperature on the process of adsorption. ∆G⁰ and ∆H⁰ were used to evaluate the spontaneity and nature of the adsorption process respectively [28] . The thermodynamic parameters free energy (∆G⁰), enthalpy (∆H⁰) and entropy (∆S⁰) were determined as follows [32] :

where R (8.314 J/mol K) is the universal gas constant, T (K) is the solution temperature and K_L (L/mg) is the Langmuir isotherm constant. The values of ΔH⁰ and ΔS⁰ were calculated, respectively from the slope and intercept of the Vant Hoff plot of ln K_C versus 1/T (Figure 20) are presented in Table 8. The values of ∆G⁰ were found to be negative which decreased with increased temperature showing that the adsorption was more favourable and spontaneous at higher temperature (Table 8). The positive value of ∆H⁰ indicates the endothermic nature of the process. The positive value of ∆S⁰ indicates an increase in the randomness at the solid/solution interface due to the redistribution of energy between dyes and the adsorbents [23] . The removal of MG was found to be more spontaneous than the removal of CR on AMSS.