2.3. Batch adsorption

All batch adsorption experiments with single as well as binary dye solution were carried out in an Erlenmeyer flask by taking 100 mL specimen solution. The test flasks were agitated at 200 rpm by fixing them on an orbital shaker. The effect of various parameters like MWCNTs dosage (0.025 g to 0.2 g), contact time of pollutant solution with adsorbent (1 min to 40 min), initial solution pH (2 to 9), initial concentration of dye solution and temperature (293 K to 323 K) were observed. For the co-adsorption of dyes from binary dye solution, the experiments were carried out by fixing the concentration of one dye and vary the concentration of other dye (100–400 ppm) and vice versa. After the equilibrium was achieved in a particular run, the solution was filtered out to separate the adsorbent from solution and filtrate was analyzed using a UV–vis spectrophotometer (JASCO Model V-6700). For the single dye systems, the remaining concentration of dye was evaluated by comparing the absorbance of solution (corresponding to λ_max of each dye) with calibration curve recorded earlier for pre-determined dye concentrations. For binary dye systems, absorption value was measured against both wavelength and unknown concentrations in solutions were determined by the following equations [25].

where k₁₁, k₂₁, k₁₂ and k₂₂ are the cross-calibration constants for components CR (1) and RhB (2) at the wavelength λ_1,max, λ_2,max, and their values are 0.039, 0.039, 0.026, and 0.217 respectively. A₁ and A₂ are the absorbance at wavelength λ_1,max and λ_2,max, respectively. The adsorption capacity of each dye per unit weight of adsorbent at equilibrium, q_e (mg dye/g adsorbent) can be calculated by using the equation:

where q_e is the adsorption capacity of a single or binary system and C_o and C_eq are the initial and the final equilibrium concentration of dye. V is the volume of the dye-containing solution (L) and w is the mass of the adsorbent (g). Moreover, thermodynamic selectivity of MWCNTs in the single and binary solution can be calculated by the following equation:

where Q_i,s and Q_i,b represents the amount of Congo Red adsorbed, i, in single and binary solution respectively. However, Q_j,s and Q_j,b are the amount of Rhodamine B adsorbed, j, in single and binary solutions respectively.