The Delonix regia pods were washed extensively with distilled water to remove soil and dirt particles. They were sundried for 7 days and then crushed into smaller pieces, mechanically [23] . They were later carbonized at 300˚C for 2 h in an electric furnace and the carbonized samples were ground into granules before activation with steam in a fabricated steam activation reactor. The resultant steam activated Delonix regia carbon (SADRC) was dried inside electric oven at 105˚C and cooled at room temperature. They were ground further and sieved to 2 mm mesh size, before storage in a desiccator to avoid moisture absorption, until further use.

The adsorbate was prepared by weighing 1 g of Aniline Blue dye powder and dissolving it in small quantity of distilled water in 1000 mL capacity volumetric flask. Distilled water was then added to make up to the mark of the 1000 mL volumetric flask and then stirred rigorously for 5 min to ensure homogeneity and this makes the stock solution (1000 mg/L). Varying concentrations (50 - 200 mg/L) of the stock solution were prepared by serial dilution using Equation (1)

where C₁ is the initial dye concentration; C₂ is the final dye concentration; V₁ is the initial volume of solution and V₂ is the final volume of solution.

Batch adsorption was conducted by weighing (0.5 - 2.0 g) of the activated carbon into sample bottles and 100 mL of various concentrations (50 - 200 mg/L) of stock solution maintained at pH 3, 4, 5, 6, 8, 9, 10 and 11 were poured into different sample bottle. Each sample bottle was shaken in a water bath shaker (Uniscope SM 101) at varied time (0 - 100 min). The solutions were removed from water bath shaker, filtered and the residue kept for further analysis such as scanning electron microscope, while the amount of dye adsorbed (q_e) was quantified according to Equation (2) and the removal efficiency (RE) was determined from Equation (3).

where q_e (mg/g) is the quantity of Aniline Blue adsorbed per unit mass of activated carbon, C_O (mg/L) is the initial Aniline Blue concentration, C_e (mg/L) is the Aniline Blue concentration after adsorption, V (mL) is the volume of the solution and W (g) is the mass of activated carbon.

FTIR analysis, (Perkin-Elmer Spectrum GX, Kuala Lumpur, Malaysia) was employed to identify functional groups responsible for adsorption on the surface of the SADRC produced.

The FTIR spectra of SADRC before and after Aniline Blue adsorption onto the activated carbon developed from the Delonix regia pod (SADRC), were compared. The FTIR spectra before adsorption were in the range of 418 to 2370.81 cm⁻¹ (Figure 1), while spectra after adsorption ranged from 426.49 and 1560 cm⁻¹ (Figure 2), and these changes suggested that Aniline Blue adsorbed onto the SADRC. The stretching vibration band at

2169.54 cm⁻¹ may be due to strong CN, while the stretching vibration band at around 1580 - 1650 cm⁻¹ may be due to C=C stretching vibration. The bands around 1350 and 426.49 cm⁻¹ are due to C-N and -SO₃H group respectively and this further suggests that some functional groups may be present on the surface of the carbon due to the low temperature of carbonization (300˚C) of the adsorbent.

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

The effect pH on the removal of Aniline Blue from aqueous solution was studied by using 0.5 g dosage of the SADRC, 100 mg/L initial concentration, and 30 min. contact time. Figure 3 shows that percentage removal of Aniline Blue increased from 97.1% - 97.5% as the pH increased from pH 3 - 10, but decreased to 97.44% at pH 11. This is because pH affects the solubility of the dye, concentration of the counter ions on the functional groups of the adsorbent (SADRC) and the degree of ionization of the adsorbate during reaction [24] . When the pH of the adsorbing medium increased from pH 3 - 10, there was a corresponding increase in deprotonating of the adsorbent surface leading to a decrease in the H⁺ ion on the surface of the adsorbent [25] . This creates more negative charges on the adsorbent surface, thus, favouring adsorption of positively charged species onto the adsorbent (SADRC) surface [26] . The optimum removal efficiency (97.53%) ranged between pH 9 and 10.0.

The adsorption of the initial concentration (100 - 200 mg/l) of Aniline Blue was investigated at various temperatures (20˚C - 50˚C) under agitation time ranging from 0 - 100 mins (Figures 4(a)-(d)). Generally, the adsorption of the dye increases as the initial concentrations of the dye increased at all the temperature. Similarly, equilibrium was attained after first 60 min for all the concentrations studied except 100 and 200 mg/l at 20˚C (Figure 4(a)) as well as 200 mg/l at 40˚C (Figure 4(c)), where the equilibrium were attained in contact time less than 60 min. The equilibrium state indicates that there is no change in the amount adsorbed, thus, given rise to constant value where the amount of dye desorbed is proportional to the amount of dye on the adsorbent infers dynamic equilibrium [27] . The time required to attain this state of equilibrium is referred to as equilibrium time and

the amount of Aniline Blue adsorbed at equilibrium indicates the maximum adsorption capacity of the adsorbent under the operating conditions [28] .

The adsorption of Aniline Blue on SADRC was studied at pH 10 with varying amount (0.5 - 2.0 g) of the SADRC while keeping the initial concentration of the Aniline Blue solution constant, since it has been established above that the adsorption of the dye increases as the initial concentrations of the dye increased at all the temperatures. The result (Figure 5) shows that the removal efficiency increases with increase in adsorbent dosage and this may be because increase in the adsorbent dosage increases the number of available adsorption sites and consequently increase the amount of Aniline Blue adsorbed. This further justifies that the effectiveness of an adsorption process is function of the amount of adsorbent used [29] .

The solute uptake rate is usually described by kinetic of sorption and, consequently governs the residence time or sorption. The kinetic of Aniline Blue adsorption on Delonix regia pod was analyzed using pseudo first order and pseudo second order kinetic models. The correlation coefficient (R²) and sum of error squares (SSE%) were employed to express and validate conformity between experimental and the calculated data, respectively. Relatively high R²-values and low SSE (%) characterize the model that describes, successfully, the kinetics of Aniline Blue adsorption.

where N is the number of data points.

The thermodynamic of the adsorption of Aniline Blue onto SADRC at different temperatures carried out using the vant Hoff equation (Equation (10)) and the Gibbs free energy was determined from Equation (11).

where (equilibrium constant) is equivalent to, T is temperature in K, and R is the gas constant.

A plot of against gave a straight-line graph from which and were evaluated as the slope and intercept, respectively (Figure 10). The positive values of and (Table 4) suggest endothermic and non-spontaneous nature of sorption process of the adsorption of Aniline Blue onto the SADRC at all temperatures investigated. On the other hand, the negative values of indicate the existence of decreasing randomness of the solid/solution interface during the sorption of Aniline Blue onto the SADRC [32] .