Kinetics, equilibrium and thermodynamics studies of congo red dye adsorption from aqueous solution onto activated cowpea (Vigna unguiculata) husk

Adsorption of Congo red (CR) dye from aqueous solution onto activated cowpea husk (ACPH), a low-cost agricultural waste material in a batch process was investigated. Adsorption was studied as a function of amount of adsorbent, pH, initial dye concentration and time. It was found that adsorption capacity varied linearly with the amount of adsorbent, initial CR concentration with time. The results show that maximum adsorption capacity was obtained at the optimum levels of contact time (24.3157mg/g at 60 minutes), adsorbent dose (24.532mg/g at 0.1g), initial dye concentration (407.2787mg/g at 500mg/L) and pH (24.26mg/g at 1.5).  Adsorption equilibrium data were represented by isotherm, kinetics and thermodynamics models. Three isotherm models namely Langmuir, Freundlich and Temkin were tested and adsorption was found to fit well into Langmuir model relatively better than others. The maximum loading capacity (q_m) of the adsorbent for Congo red obtained from the Langmuir isotherm model is 263.16 mg/g. The kinetic data was well described by the pseudo-second order kinetic model with the correlation coefficients (R²) value of 0.994. The adsorption process was found to be thermodynamically endothermic and spontaneous. The negative value of ΔS (-0.00053J/mol.k) infer that the randomness decreases at the adsorbent/adsorbate interface during the adsorption process.  FTIR and SEM analyses of the adsorbent suggest that adsorption of the dye was through an electrostatic interaction between the functional groups present in the dye and those on the surface of the adsorbent.