Adsorption of Azo-Anionic Dyes in a Solution Using Modified Coconut (Cocos nucifera) Mesocarp: Kinetic and Equilibrium Study

Abstract

The effect of adsorbent dose and initial concentration on removing the azo-anionic dyes Congo Red andtartrazine present in a synthetic aqueous solution was studied using natural cellulose (CC) and modified cationic cellulose (MCC) from coconut mesocarp. Three levels of adsorbent dosage (5, 8 and 12 mg/L) and initial concentration (40, 70 and 100 mg/L) were used. Cetyl trimethyl ammonium chloride (CTAC) was used as a modifying agent. TGA and DSC showed that the extracted cellulose was of good quality, composed mostly of cellulose with lignin and hemicellulose traces, and 8% moisture. The FTIR spectrum showed the effectiveness of the modification in the structure of the material with symmetric deformation of the C6H6-Cl group in 1472 cm−1 present in the CTAC. It was found that decreasing the adsorbent dosage and increasing the initial concentration favored the dyes’ adsorption capacity on the two bioadsorbents. Tartrazine removals of 5.67 mg/g on CC and 19.61 mg/g on MCC were achieved, and for CR of 15.52 mg/g on CC and 19.99 with MCC with removal percentages over 97% with the quaternized biomass in all cases. The kinetic and equilibrium study was carried out to identify the mechanisms involved in the adsorption process. The Freundlich model can describe the equilibrium isotherm data of tartrazine on CC and MCC. In contrast, those of CR is defined by the Langmuir and Dubinin–Radushkevic models for CC and MCC, respectively Adsorption kinetics showed that equilibrium was reached at 30 min, with rapid adsorption in the initial minutes with the removal of about 97% of the contaminant in the first 5 min; fitting to kinetic models showed that the kinetics of tartrazine on CC was fitted by Elovich (R2 = 0.756), and on MCC the Elovich (R2 = 0.887) and pseudo-second-order (R2 = 0.999) models. Removing CR on CC was fitted by pseudo-first-order, pseudo-second-order and Elovich models (R2 > 0.98), and when using MCC, all models show a good fitting with R2 = 0.99 in all cases.