Equilibrium, kinetics, and thermodynamic studies of biosorption of Reactive Red-120 Dye by biomass of Enterobacter sp. strain MM05

Water pollution caused by organic dyes poses a significant global threat to human and marine life. Reactive Red-120 dye is one of the textile dyes that is widely used for dyeing in many countries. The dye and its metabolites have been shown also to be toxic, carcinogenic, and mutagenic. A screening result of several bacteria for the dye sorption has yielded the best isolate, Enterobacter sp. strain MM05. Then, the bacterial biomass morphological structure was characterized using Fourier Transform Infrared (FTIR) and Scanning Electron Microscopy (SEM) techniques. The adsorption kinetics, isotherm, and thermodynamic parameters for RR-120 dye adsorption were determined in the study. Twelve (12) different bacterial isolates were screened against RR-120 dye. Of the 12 isolates screened, Enterobacter sp. strain MM05 biomass was found to have a higher percentage of dye removal (67.86%). Optimization techniques such as One-factor-at-a-time (OFAT) and Response Surface Methodology (RSM) were used to improve the adsorption process. The OFAT optimum conditions found for the adsorption of the reactive red-120 dye were; 60 min (contact time), 50 ppm (dye concentration), 7.0 (pH), 150 rpm (agitation speed), 1.5ml (adsorbent amount), and 45 °C (temperature). The RSM optimisation provided the most ideal condition for the removal of the RR-120 dye, which increased the percentage removal of the dye to 73%. The models' R2 was determined to be 0.9563. Different non-linear mathematical models were used to study the adsorption kinetics of the bacterial biomass on RR-120 dye. The dye exhibited a pseudo-second-order kinetics reaction with the lowest AICc value of -39.09 and adjR2 that is closest to 1.0 (0.998). Langmuir, Henry Freundlich, BET, Toth, Sips, Fritz-Schlunder IV, and Fritz-Schlunder V were the best-fitting mathematical models for the RR-120 dye in the isotherm studies. Except for the Henry model, all of the nine (8) isotherm models tested on the RR-120 dye provided significant fitting results. The Feundlich model was found to be the best isotherm model after statistical analysis using the corrected Akaike Information Criterion (AICc), root-mean-square error (RMSE), adjusted coefficient of determination (adjR2), accuracy factor (AF), and bias factor (BF). The model had the lowest AICc value of -51.54. Thermodynamic parameters for the adsorption of reactive red-120 dye were studied at different temperatures (30, 35, 40, 45, and 50 °C), starting with the rate constants derived from the pseudo-second- order kinetic (k2) and moving on to the isotherm constant (KL) determined from the Langmuir model. By converting the KL constant, the dimensionless thermodynamic constant KC was obtained. The thermodynamic parameters such as standard Gibbs free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) were determined using non-linear regression of the van't Hoff plot. The RR-120 dye adsorption thermodynamic results showed that the reaction was endothermic having the positive ΔH° value of 52.91 kJ/mol. However, the result equally showed that the adsorption process was spontaneous and feasible having negative Gibb's free energy. Furthermore, the positive value of the ΔS° (301.30 kJ/mol/K) indicated the affinity of the bacterial biomass (adsorbent) toward an adsorbate (RR-120 dye). In other word, the positive ΔS° value suggested an increased randomness at the solid/liquid interface with the structural changes in the adsorbent and adsorbate. The presence of many functional groups, including hydroxyl, alkyl, and carbonyl, which are essential in the process of dye adsorption, particularly reactive red-120 dye, is revealed by Fourier transform infrared (FTIR) analysis. The heterogeneous, non-porous, smooth, and tightly packed surfaces following the dye adsorption of RR-120 dye are revealed by the analysis of the SEM data on the biomass of Enterobacter sp. strain MM05. This finding has concluded that, the biomass of Enterobacter sp. strain MM05 is essential in the adsorption of reactive red-120 dye and could be employed in bioremediation processes especially in the removal of azo dyes from the environment.

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